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PHOTORESIST APPLICATIONBY ROLLER COATING

Published August 1976

Project Leader:L. E. SchantzDepartment 842

...

Project Team:J. H. Pusch

PDO 6984726Topical Report

-NOTICE Thb report was prepared as an account of work isponsored by the United SweS Government. Neitherthe United States nor the United States EnergyResearch and Development Administration, nor any oftheir employees, nor any of their contractors,subcontracto/, or their employees, makes any 1

' wmnly, express of impied, or assumes any legal I liability or responsibility for the accuracy. completeness |1

or uxfulnez of any information, apparatus, product or I| process dkclosed, or represents thatits use wodd not j infringe privately owned rights.- ---- ]-1

DISTRIBUTION OF THIS DOCUM'ENT IS UNILIMIT D

Technical Communications Bendixi Kansas Cityb-/mi Division./01/.al:.7.

DISCLAIMER

This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.

DISCLAIMER

Portions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.

PHOTORESIST APPLICATION BY ROLLER COATING

BDX-613-1288, Published August 1976U.

Prepared by L. E. Schantz, D/842, under PDO 6984726

Application of AZ1350J photoresist by roller coating was investi-gated as an economical means of coating 3.75- by 4.50-inch (95 by I114 mm) substrates for multiple hybrid microcircuit fabrication.The most important technical aspect of the resist coating is thethickness uniformity. The large rectangular substrate made photo-resist application by spinning technically unsatisfactory; spraycoating., although technically acceptable, represented a capitalequipment investment 10 to 15 times that of roller coating costs.Resist viscosity, doctor bar pressure, roller/substrate inter-ference, and roller thread configuration were optimized to achievemaximum thickness uniformity. The correlation between resistthickness and dimensional control of the photoresist image wasestablished, and a predicted distribution of thin-film line widthswas made and verified.

4

THE BENDIX CORPORATION

This report was prepared as an account of work sponsored KANSAS CITY DIVISIONby the United States Government. Neither the United States P.O. BOX 1159 -nor the United States Energy Research and DevelopmentAdministration, nor any of their employees, nor any of their KANSAS CITY, MISSOURI 64141

contractors, subcontractors, or their employees, makesany warranty, express or implied, or assumes any legal lia-bility or responsibility for the accuracy, completeness orusefulness of any information, apparatus, product or processdisclosed, or represents that its use would not infringe pri- A prime contractor for the Unitedvately owned rights States Energy Research and

Development AdministrationContract Number E(29-1)-613 USERDA

2

CONTENTS

Section Page

SUMMARY. .. . . . . . 7

DISCUSSION . . . . . 9

SCOPE AND PURPOSE. 9

PRIOR WORK . . . . 9

ACTIVITY . . . . . 10

Photoresist Coating Process Selection. . . . . . 10

General Operating Description of Roller Coater . 10

Methods of. Resist Application . . . . . . . . . 16

Resist Viscosity . . . . . . . . . . 20

Doctor Bar Pressure Adjustments. . . . . . . . . 30

Roller Size. . . . . . . . . . . . . . . . . . . 33

Roller/Substrate Interference. . . . . . . . . . 35

* Thin-Film Surface Effects on Resist Thickness. . 43

Edge Effects and Substrate Orientation . . . . . . . 43

Resist Thickness and Expected Line Width Deviation . 49

Thickness Uniformity Distributions . . . . . . . . . 49

Line Width Distributions . . . . . . . . . . . . . . 52

Recommended Operating Procedure. . . . . . . . . . . 53

ACCOMPLISHMENTS. . . . . . . . . . . . . . . . . . . . 61

FUTURE WORK . . . . . . . . . . . . . . . . . . . . . 61

APPENDICES

A. START UP AND SHUTDOWN PROCEDURES.... 62

B. MAINTENANCE AND FACILITY REQUIREMENTS. 64

C. MICROCOATER TROUBLESHOOTING GUIDE . 65

DISTRIBUTION 66

3

ILLUSTRATIONS

Figure Page

1 Gyrex Micro 9 Roller Coater (P-82611) . . . . . . . 11

2 Resist Reservoir and Pump (P-82616) . . . . . . . . 12

3 Threaded Rubber Roller and Doctor Bar (P-82613) . . 13

4 Effects of Doctor Bar Pressure . . . . . . . . . . 15

5 Substrate Carrier (P-82615) . . . . . . . . . . . . 17

6 Resist Coating Thickness Versus Resist Viscosity. . 21

7 Resist Viscosity Versus Resist Temperature. . . . . 23

8 Doctor Bar Pressure and Resist Coating Thickness(Viscosity 21, Roller E-10) . . . . . . . . . . 25

9 Doctor Bar Pressure and Resist Coating Thickness(Viscosity 55, Roller E-10) 26

10 Doctor Bar Pressure and Resist Coating Thickness$ (Viscosity 21, Roller E-6). . . . . . . . . . . 27

11 Doctor Bar Pressure and Resist Coating Thickness(Viscosity 35, Roller E-6). . . . . . . . . . . 28

12 Adjustment for Uniform Doctor Bar Pressure(P-8 2 6 0 8) . . . . . . . . . . . . . . . . 31

13 Roller Coater Controls (P-82610). . . 32

14 Roller Size and Thread Configuration. 34

15 Resist Coating Thickness Versus Interference(Viscosity 21, Roller E-10) . . . . . . . . 36

16 Resist Coating Thickness Versus Interference(Viscosity 55, Roller E-10) . . . . . . . . 37

17 Resist Coating Thickness Versus Interference(Viscosity 22, Roller E-6). . . . . . . . . 38

18 Resist Coating Thickness Versus Interference(Viscosity 35, Roller E-6). . . . . . . . . 39

19 Interference and Doctor Bar Pressure (Viscosity 21,Roller E-6) (P-89011) . . . . . . . . . . . . . . 41

4

20 Interference and Doctor Bar Pressure(Viscosity 35, Roller E-6) (P-89012). 42

21 Gold Versus Tantalum-Nitride Effects on ResistThickness . . . . . . . . . . . . . . . . . . 44

22 Resist Thickness by Positional Measurements(Straight Substrate Entry) . . . . . . . 45

23 Resist Thickness by Positional Measurements(Angle Substrate Entry) . . . . . . . . . . 46

24 Resist Thickness Histogram (Angle SubstrateEntry) , . . . . . .

47

25 Resist Thickness Histogram (StraightSubstrate Entry). . . . . . . . . . 48

26 Line Width Change Versus Resist Thickness(5-mil Lines) . . . . . . . . . . . . . 50

27 Line Width Change Versus Resist Thickness(3-mil Lines) . . . . . . . . . . . . . 51

28 Resist Thickness Versus Exposure Energy 52

29 Resist Thickness Distributign Measured at9 Positions, Run-to-Run (X >3.0 Um) . . 53

30 Resist Thickness Distribution Measured atCenter of Substrate, Run-to-Run(X>3. .0 jim ) ': · - - -7 0 - 7 ... .. .... 54

31 Resist Thickness Distribution Measured at9 Positions, Run-to-Run ( R <2.0 um) 0 0 55

32 Resist Thickness Distribution Measured atCenter of Substrate, Run-to-Run(X<2.0 U m) . . . . . . . . . . . . . . . 56

33 Line Width Distribution Measured at 9 Positions,Run-to-Run (>3.25 pm Resist Thickness). . . . . 57

34 Line Width Distribution Measured atCenter of Substrate, Run-to-Run(>3.25 Um Resist Thickness) . . . 58

35 Line Width Distribution Measured at 9 Positions,Run-to-Run (<2.0 um Resist Thickness) . . . . . 59

5

36 Line Width Distribution Measured at Center ofSubstrate, Run-to-Run (<2.0 Um Resist

-

T h i c k n e s s) . . . . . . . . . . . . . . . . . . . 60

TABLES

Number Page

1 Roller Thread Size Versus Coating Thickness. . . . 14

2 Reflow Philosophy. . . . . . . . . . . . . . . . . 18

3 Limited-Reflow Philosophy. . . . . . . . . . . . . 19

:

6 -

- SUMMARY

- Roller coating of AZ1350J photoresist onto 3.75- by 4.5-inch(95 by 114 mm) substrates was investigated as an integral partof an overall, cost-effective hybrid microcircuit (HMC) photo-lithography process. This process involved making several HMCson a single substrate and was accordingly termed Mu Ztip le HMCProcessing. New equipment was needed to process these multipleHMCs with sufficient parameter control to achieve the designedelectrical characteristics. The roller coater study centered aroundthe investigation and t4e gontrql of thgse parameters which influencethe dimensional quality of the thin-film, resistor-conductorpattern. General system operation, orientation to other process-ing equipment, and maintenance-facility considerations were alsostudied.

The most important technical aspect of the photoresist coating isthe thickness uniformity followed by the overall photoresistthickness. Three equipment variables: doctor bar pressure,roller/substrate interference, and roller thread configurationwere found to control the overall resist thickness and thicknessuniformity. Photoresist viscosity was also identified, with itsrelation to the roller thread configuration, to control thethickness and thickness uniformity.

Increasing the doctor bar pressure was found to decrease theresist thickness on the substrate. The resist thickness wasdiscovered to be less dependent upon the interference than thedoctor bar pressure. The interference primarily controls thecontinuity and the uniformity of the resist coating. The largerthread configurations were found to transfer larger amounts ofresist to the substrate and therefore increase the thickness.The resist viscosity was shown to affect both thickness andthickness uniformity and can be effectively manipulated tooptimize each. The viscosity was also linked to the edge build-up of resist and bubbles in the coating. Fortunately theviscosity values which achieve the optimum uniformity also affectreduced edge build-up and bubbles. It therefore becomes importantto maintain control on the resist viscosity.

The doctor bar pressure is the most effective operational adjust-ment for the control of resist coating thicknesses; however, therelation between doctor bar pressure and resist thickness is not --

- consistent. Doctor bar pressure values are relative numbers andcannot be used universally as they depend upon the initialset-up procedures. Roller size and viscosity are fixed, measur.able parameters. The viscosity is conveniently measured witha Zahn cup and the roller size is fixed by the manufacturer.

7

There are four roller sizes available, representing successivelylarger thread sizes and therefore coating thicknesses.

The determination of the proper resist thickness and thicknessuniformity is largely empirical_ and related to the proper exposureenergy and prebake conditions. However, the following generalrules can be used as a guide:

• Increasing doctor bar pressures decreases the resist thicknesson the substrate;

• Resist thickness is not predictably dependent upon the inter-ference except at high interference levels, and any contributionit makes to overall thickness is decreased at higher doctorbar pressures;

• Increasing the roller thread size increases the overallresist thickness on the substrate;

• Increased resist viscosity provides for more consistent butgenerally'thicker resist coatings; and

• Consecutively smaller size rollers, which produce thinneroverall resist coatings, are more consistent in repeatillga specific coating thickness when all other parameters

# are held constant.

8 -

DISCUSSION

-1

SCOPE AND PURPOSE

Roller coating of AZ1350J photoresist onto thin-film mirrors*was investigated as an integral part of a larger, cost-effectivehybrid microcircuit (HMC) production philosophy, termed MuZtipteHybrid Microcircuit Processing. Multiple processing, the fabri-cation of more than one HMC on a single ceramic substrate, wasinitiated to support HMC production schedules at economicalproduction costs. Since more than one HMC was to be made froma single substrate, the substrate must be larger than thosepreviously used [3.75- by 4.5-inch versus 1.5- by 1.5-inch(95 by 114 mm versus 38 by 38 mm)], which required a differentphotoresist coating technique than that used on single HMCsubstrates. Roller coating was chosen as an expedient andeconomical processing technique capable of coating the 3.75-by 4.5-inch multiple HMC substrates.

The identification and necessary control of roller coatingparameters which influenced the dimensional quality of the finalHMC resistor-conductor network was the subject of this investiga-tion. General system operation, orientation to other processingequipment, maintenance, and facility considerations were alsostudied. The characterized technology was to be transferred toProcess Engineering and implemented into production.

PRIOR WORK

There has been no prior work towards multiple HMC processing on3.75- by 4.5-inch substrates except to confirm that the presentphotoresist application method, spinning, was not satisfactoryin either a technical or an economical sense. Literature searchesand contacts within the HMC industry identified two possiblephotoresist coating techniques for the larger substrate size--roller coating and spray coating. Roller coating was recommendedat the close of PDO 6984719** because of substantially lower cap-ital equipment costs.

*A ceramic substrate with thin-film metallization on one or both --surfaces used in the fabrication of resistive and conductiveelectrical networks. The metallized surface has a mirror-likefinish.**Lee E. Schantz, Hybrid Microcircuit PhotoZ€thographic Process(Topical Report). UNCLASSIFIED. Bendix Kansas City: BDX-613-678, July 1973.

9

ACTIVITY

.4Photoresist Coating Process Selection

Multiple HMC processing was proposed as an overall cost-effectivephotolithographic approach to the production of thin-film HMCs.Using the industry standard 3.75- by 4.5-inch, unglazed, aluminasubstrate required a complete changd-over in the photolithographyprocessing equipment. Photoresist coating was only one of sevenphotolithographic process steps, each of which required a uniquepiece of equipment to define the thin-film, resistor-conductornetwork. Two resist coating techniques, roller coating and spraycoating, were determined capable of coating the larger substrates.

Roller coating was chosen for initial development and investiga-tion because of its substantially lower capital equipment cost($5000 versus $30000).* 1

However, it was never proposed that .one coating method was technically superior for production thanthe other, but only that an initial purchase cost advantageexisted for the roller coater**. Both techniques have beensu-(cd-sslully employed by various thin-film HMC manufacturers.

The Gyrex Micro 9 roller coater (Figure 1) was chosen for thedevelopment activity because it was the most widely acceptedroller coating system in the HMC industry and was the leastexpensive. Three equipment variables -- doctor bar pressure,roller/substrate interference, and roller thread configuration --were described by the manufacturer to control the final photo-resist thickness on the substrate. Photoresist viscosity wasalso identified with its relation to the roller thread configura-tion. Photoresist thickness and thickness uniformity were theprincipal criteria by which the coating quality was judged.

General Operating Description of Roller Coater

The photoresist is supplied by a one-half gallon (0.0019 m3)reservoir located beneath the roller coater (Figure 2). Theresist is pumped from the reservoir through a 5.0 um sinteredsilver filter to the doctor bar on the roller coater (Figure 3).The resist flows along the doctor bar from one end to the other

- *Not including installation costs.**Lee E. Schantz, BDX-613-678.

Text continued on page 14.

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Figure 3. Threaded Rubber Roller and Doctor Bar

13

and then gravity returns it to the reservoir where thecycle. is repeated. The continuous flow provides a rela-tively constant resist viscosity and temperature at thedoctor bar where it is applied to the roller.

The doctor bar contacts the threaded rubber roller (Table 1)along its entire length as shown in Figure 3 and transfers theresist from the doctor bar to the roller in an amount proportionalto the doctor bar pressure on the roller. Figure 4 is a linedrawing of the doctor bar and roller interface which shows therelative amounts of resist transferred as a function of thedoctor bar pressure. In general, the greater the doctor barpressure, the less resist is tranferred to the roller. With lessresist on the roller, less resist can be transferred to the sub-strate. Hence the first rule:

• Increasing doctor bar pressures decreases the resist thicknesson the substrate.

Table 1. Roller Thread Size Versus Coating Thickness

Manufacturer's PublishedRoller Coating Thickness RangeSize Threads/Inch (mm) (um)

E-6 166 (25.4) 0.5 to 1

E-10 100 (25.4) 1 to 2

E-16 62 (25.4) 2 to 3

E-22 45 (25.4) 3 to 5

Increasing doctor bar pressure flattens the threads and squeegeesa fixed amount of resist onto the roller. The thickness ofresist on the roller is theoretically equal to the thread depthminus the portion of thread depressed by the doctor bar; however,there is always some residual amounts of resist left on thethread walls and crest connecting each thread.

The roller then rolls into contact with the substrate and depend-ing upon the pressure of the roller on the substrate (termed inter.ference and measured in thousandths of an inch) the resist istransferred in a reverse fashion to the substrate. The substrateis conveyed under the roller at the tangential velocity of the

- roller; therefore, no slipping exists between the substrate androller. The dependence of resist thickness on the substrate"isnot as strongly dependent upon the interference as upon the doctor

14

4CONVEYOR

A.- THREADED _ FLOWING RESIST

COATING ROLL 1£ _X F DOCTOR BARDOCTOR BAR4 li-1 SUBSTRATE-1 f 7 7 PRESSUREC ) \C ®112-1- APPLIED- PHOTORESIST - ENLARGED n

B VIEWS 3 (THREADED RUBBER ROLLER

\/ \-/

A'+|1 ROLLERCONVEYOR SUPPORT ROLL

SUPPORTCONVEYOR + TRAVEL

VIEW AA'

»v

ZERO DOCTOR BARPRESSURE ALLOWS FOR

VIEW 81 ROLLER THREAD TO BECOMPLETELY FILLEDWITH RESIST AND

DOCTOR BAR WITH RESISTALLOWS FOR SOMERESIST LEAKAGE.

rw4,0-c<<U»>u, lil/r \- -7 (VIEWS AT RIGHT SHOWF n 111-1111-1 THE RESIST/THREAD

i ROLLER THREADS CONDITION JUST PRIOR

l____L - _ll11111111- J TO CONTACTWITH (EACH THREAD FULL)

SUBSTRATE.)

VIEW 82PARTIAL DOCTOR BARPRESSURE ALLOWS FOR PHOTORESISTDOCTOR BAR WITH RESISTROLLER THREADS TO BEPARTIALLY FILLED WITH

E-el-VV-RK=Pley-Vip RESIST BY FLATTENINGTHE TOPS OF EACHTHREAD AGAINST THE

-.-------- DOCTOR BAR.(EACH THREADPARTIALLY FILLED)

VIEW 83

FULL DOCTOR BAR

DOCTOR BAR WITH RESIST PRESSURE ALMOSTCOMPLETELY FLATTENS

- -1331-171THREADS AND ALLOWS

ROLLER ONLY A SMALL AMOUNTOF RESIST TO ENTER (THREADS NEARLY VOID

-- THE THREADS. OF PHOTORESIST)

Figure 4. Effects of Doctor Bar Pressure

15

bar pressure, and the interference becomes even less effec-tive in the control of overall resist thickness as the doctor barpressure increases. The interference primarily controls thecontinuity and uniformity of the resist coating. The secondrule can now be stated:

• Resist thickness is not predictably dependent upon theinterference except at high interference levels, and anycontribution it makes to overall thickness is decreasedat higher doctor bar pressures.

There is an obvious dependence of total resist thickness on thethread size. The thickness range for each roller, as stated bythe manufacturer, is given in Table 1. The third rule can bestated as follows:

• Increasing the roller thread size increases the overallresist thickness on the substrate.

Substrates, regardless of size, are placed on a substrate carrierto be passed beneath the roller (Figure 5). Since the optimuminterference is nearly equal to the substrate thickness[- 25 mils (635 um)], the area on the roller, not in contact withthe substrate, would contact the conveyor and transfer part ofits resist to the conveyor if the carrier were not present. Itis highly undesirable to get resist on the conveyor; therefore,the substrate carrier is used. Any unwanted resist transferwill be to the carrier which is easily cleaned and reused.

Figure 3 shows a seam in the conveyor. If the substrate-carrieris placed on this seam, the seam imprint will be visible in the - -resist coating. Consequently the seamed area is avoided.

Methods of Resist Application

There are two general resist coating philosophies:

• Reflow Technique,.applying the photoresist in a sufficientlyfluid state as to allow the resist to reflow to a self-leveling condition and therefore help eliminate anynon-uniformities in the coating thickness; and

• Limited-Reflow Technique, applying the resist. in a higher _ --viscosity condition and controlling the coating parametersto achieve satisfactory thickness uniformity.

There are several other factors to consider besides the thicknessuniformity in both techniques and there must be some trade-offon each. Tables 2 and 3 list several advantages and disadvantagesof each philosophy and some of the trade-off considerations.

16

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Figure 5. Substrate Carrier

17

Table 2. Reflow Philosophy*

Advantages Trade-Off Remarks

No initial adjustment of resistviscosity required

Less sensitive to changes inresist viscosity

Disadvantages

Larger edge build-up effect HMC must be recessed fromedge of substrate

Requires leveling time Increase dwell time betweencoating and IR prebaking

fl.2 um resist uniformity Loosen tolerance on thin-film(E-10 roller) resistor values

Increased minimum thickness Maintain tighter control onlimit (greater overall exposure light energy andthickness) resist thickness uniformity

for acceptable line widthcontrol

Requires that substrate go Resist thickness uniformitythrough coater at an angle is made worseto minimize resist build-up Special handling requiredon substrate edges Reduces capacity of coater

More susceptible to entrap- Greater incidence of pinholeping airborne dust and a decreased yield

*20 Zahn Number 2 cup seconds at 72°F (0.053 Pa·s at 22'C)

The prime responsibility of the photolithographic. operation isto faithfully reproduce the photomask image in the thin-filmcoating with sufficient dimensional control to maintain thedesigned electrical circuit characteristics. Thinner resistcoatings minimize the effect of non-uniformities in resist

--

18

Table 3. Limited-Reflow Philosphy*

Advantages Trade-Off Remarks

Greater resist thickness uni- Provides for greater controlformity (E-6 and E-10 on line widthsrollers)

No edge build-up Allows HMC patterns to extendto substrate edge

Allows for thin, consistent Minimizes the effect of non-resist coatings uniform resist coating on

line width deviation

Disadvantages

Requires initial thickening 24-hour thickening period withof resist minimal operator effort

More difficult to filter Use larger (5 to 10 Um filter)

20 percent increase inresist usage costs

*35 to 40 Zahn Number 2 cup seconds at 72'F (0.105 tO.003Pa·s at 22'C)

thickness in consideration of the proper exposure energy* fortwo reasons. First, the resist thickness deviation from theaverage is relatively constant at f0.6 Um for average thicknessesbetween 2 and 5 um, but only about t0.2 um at lower averagethicknesses (<2.0 um). Second, changes in resist thicknessesat lower thicknesses are less dependent upon exposure energy thanat greater thicknesses. It is the decreased dependence uponthe proper exposure energy and tighter thickness uniformitiesthat afford the increased dimensional control for a fixed resistthickness uniformity. The ability to obtain _thin and consistentresist coatings justifies the limited-reflow techniques with itsrequirement for an initial increase in resist viscosity.

*The proper exposure energy is defined as the amount of lightenergy in J/cm2 in the 350 to 450 nm region required to maintainthe photoresist line width deviation (ALW) within the specifiedtolerance. Generally ALW must be t2 percent of the line widthor t0.1 mil (2.54 um) whichever is smaller. ALW = photoresistline width minus the mask line width.

19 -

Resist Viscosity

Shipley AZ1350J photoresist has a normal viscosity of 20 t2Zahn Number 2 cup seconds (0.055 tPa·s). The resist can be usedat this viscosity using an E-6 or E-10 roller; however, sig-nificant difficulties are encountered in edge build-up ofresist*, bubbles in the coating, and non-uniformity in theresist thickness. If the resist is thickened to approximately35 to 40 Zahn Number 2 cup seconds (0.092 to 0.105 Pa·s), near-1yall of the above difficulties can be minimized or avoided entirely.With all other parameters held constant, the higher viscosityresults in thicker resist coatings as shown in Figure 6.

The general increase in coating thickness as the viscosityincreases is expected. However, the measured** resist coatingthickness is not constant as indicated by the 1 Um range invalues of Figure 6. The cause for the variation is not wellunderstood, but appears to be related to variations in one ora combination of the following parameters:

• Interference,

• Substrate Thickness,

• Substrate Surface Conditions (cleanliness, roughness),

• Resist Temperature, and I

• Resist Age.

In general, the coating thicknesses are not highly consistent atlower viscosities, but can be made more consistent with highviscosities and higher doctor bar pressures. The fourth rulecan now be stated as follows:

• Increased resist viscosity provides for more consistentbut generally thicker resist coatings.

At first thought, rule 4 indicates that resist viscosity anddoctor bar pressures are the only variables which affect theresist thickness and variations in each cause a corresponding

*Gross changes in the resist thickness near the substrate edgewhich is not removed during developing and this prevents pattern

- definition in that area. The buildup may extend 0.125 to 0.250inch (3.175 to 6.35 mm) into the substrate from the edge.-**Measurements were made at 9 positions across the 3.75- by4.5-inch (95 by 114 mm) substrate.

20

A5 - A E-10 ROLLER

-DOCTOR BAR PRESSURE = 5E

14 - INTERFERENCE = 20

- AA = UNCONTROLLABLE5 BUBBLES APPEAR IN

3- COATING AT

= 1 VISCOSITIES BELOWA THIS LIMIT

A A'S 2- E-6 ROLLERS-

i 1 w I / i I I I I .20 24 28 32 36 40 44

RESIST VISCOSITY (ZAHN NUMBER 2 CUP SECONDS)

Figure 6. Resist Coating Thickness Versus ResistViscosity

variation in the thickness uniformity. However, this deductionis not necessarily valid in all cases. It appears that eachcan be a secondary variable which interacts with the primaryvariable causing thickness variation. This is not to imply thatvariations in the resist viscosity do not cause changes in thecoating thickness, but that they are not independently responsiblefor all measured variations·in the coating thickness.

Such an example would be a non-uniform thread pattern on the rolleror an out-of-round roller. As the doctor bar pressure is increased,it tends to nullify irregularities in the threads and minor out-of-roundness. Therefore, the primary cause for thickness variationscould be the roller and not the doctor bar.

A second example is the cohesion between the substrate surfaceand the wet resist. The cohesion variation is primarily dependentupon substrate surface conditions. Again, increasing the resistviscosity tends to nullify the variations in substrate surfaceconditions. Therefore, manipulation and control of resist vis-cosity becomes the control for a distinctly different parameter.

Figure 6 shows the change in coating thickness to be 0.006 Um -

per Zahn Number 2 cup second. If the average coating thicknessis to be maintained to t2 Um, then the resist viscosity must beheld to +3.3 Zahn Number 2 cup seconds. Viscosity control tot3 seconds at the doctor bar is not difficult provided.the fol-lowing four conditions are met:

21

• First, resist viscosity is adjusted to the nominal valueprior to starting the roller coater;

• Second, the volume of resist in the reservoir. is at least0.4 gallons (0.0015 m3);

• Third, resist flow along the doctor bar is sufficient toprevent resist scumming on the doctor bar or roller; and

• Fourth, resist temperature is maintained to t3.5°C fromthe resist temperature when the first condition was met(usually room temperature).

At a constant resist temperature, the resist viscosity doesnot change in excess of the tolerance limits in a 4-hour operat-ing period. The resist does show a general trend towardhigher viscosities with continued use as the solvent graduallyevaporates. Therefore, the viscosity should be checked prior toeach time the roller coater is used and adjusted as necessary.

Experience has shown that the resist temperature may not remainconstant as pump heat can raise the resist temperature by 5 to15°F (2.8 to 18.3'C), over an 8-hour operating period, depending upon the volume of resist in the reservoir. Figure 7 shows thechange in viscosity as a function of resist temperature.

If the resist temperature is raised sufficiently above the nominal,the overall resist coating thickness will change and coatingparameter adjustments will be required to hold the coating thick-ness within tolerance. A second problem exists at resisttemperatures which are significantly higher than the nominal.As the resist flows across the doctor bar it may cool-off forminga resist temperature gradient and therefore a viscosity gradient.This produces uneven wetting of the roller and an uneven resistcoating on the substrate. Coating parameter adjustments will notcompensate for this situation; however, the resist temperaturegradient problem is not often encountered if the second andthird conditions are maintained.

It is desirable, in terms of proper exposure energy, to maintainthe most consistent thickness uniformity possible. Figure 6shows that higher viscosities result in greater thickness uni-formity but generally thicker overall coatings. It is known thatthin coatings (.- 2.0 um) are best for close dimensional control

- on the circuit image, therefore it seems there must be some -1

trade-off on uniformity and overall thickness. However, usinga smaller roller thread configuration affords the benefits ofboth a thinner coating thickness and higher resist viscositiesto be realized. Such an example is shown in the E-6 rollercurve of Figure 6. It can be seen that a 1.7 Um overall thick-ness tO.2 Um can be obtained at 35 Zahn Number 2 cup seconds using

22

RESIST TEMPERATURE (0C)18 20 22 24 26 28 30 32 34 36 38 40

50 1 1 1 1 1 1 1 I i- 80

48 -

46 -11 1

_. " f l 1 1 1 1 1 lilli

42 +1111111111111- 68

M 'l i l i t t i l l i t i l l040-

& lilil'll'll'llilli 14ilill ilitillill

.-I

38 -111'11'11111'11111 8

36- ililillilillilillililli'

- 60.5 5tl

1111111111.11'lilll' 11'111'11.11'11111 1

-56 3z 34 -

15 Illiilillijljilli i

= 11111'llililililill I- 53.5 5

-32 - ''1'1'1'1'1'1'1'11 81

11111'1111111 1,11 -50 -= 1lililill'11111:.1111111 -47 "g 30-

1 111!lili lili lili - 43.5 -I I i ililili fililill 1 63 28 -1

111'llillillillilll I - 41.5 -1- 1 111 1111111 1 1

2 26 -5 1 li Illillililill I

W 11'j'iliiiiiglilll'1

- 39.5

24 - 1

1 111111111111 1 - 37.51 1 1,1 1 1 11

' 1 't'i'*11111i 35. 622 -

20

tIIIllIII65 68 73 75 79 82 86 90 93 97 100 105

RESIST TEMPERATURE (oF)

Figure 7. Resist Viscosity Versus Resist Temperature

l\DCO

the E-6 roller. Furthermore, changes in overall coating thick-nesses are less dependent upon changes in the resist viscosityusing the smaller roller.

All difficulties involving changes in resist viscosity, whateverthe cause, can be avoided by efficient and intelligent productionscheduling of parts through photolithography. The roller coaterhas a sufficiently large production capacity that it only needsto be operated for a minimum time period per shift. (See sectionon General Operating Procedure.) If all substrates can bescheduled or held for resist coating in a single, 2-houroperating period at the beginning of each shift, then viscosityproblems during roller coating can be minimized. Scheduling the2-hour coating period at the beginning of each shift allowsample time for completion of all remaining photolithographyoperations.

Viscosity Adjustment

The as-received AZ1350J photoresist viscosity is increased byallowing the resist solvents to evaporate. This is accomplishedby filling a flat stainless steel pan with about 0.7 gallonfO·0026 m3) of photoresist and allowing_the solvents to evaporatein a class 100 exhaust hood for approximately 24 hours. Theexact evaporation time depends upon the room temperature andthe final viscosity desired. The viscosity can be convenientlymonitored using a Zahn Number 2 cup viscometer. The resist isthen transferred to the reservoir for use on the roller coater.

The solvent continues to evaporate during usage and the viscositywill gradually increase. Therefore, the viscosity should bemeasured prior to starting the roller coater each shift and ad-justed as necessary. Adjustments are made by adding sufficientquantities of as received resist (which.also helps replace usedresist) to bring the viscosity down to the desired value. Insome cases, viscosity adjustments may require too great a quantityof as received resist and AZ thinner is used to decrease theviscosity. In either case, the added ingredient must be thoroughlystirred into the bulk of resist.

Doctor Bar Pressure

The doctor bar pressure is the single, most effective operationaladjustment for the control of resist coating thicknesses. Thefirst rule states the relation between the doctor bar pressure ..

and coating thickness as: increased doctor bar pressures pro-duce decreased coating thicknesses. The exact relation is notwell controlled for individual cases as shown in Figures 8

---- · ·through 11. The decreasing resist thickness is net ·a · smooth- . --·-·· ·- -····- ---··· -function of doctor bar pressure at lower interference levels,and there is not a uniform family of curves for each inter-


24

4.5

I 0

4.05

I 5

203.5

'' ./

3.0 25 r \\ INTERFEREhCE = 15\

INTERFERENCE = 10

.-'en

\\

2.5

\\:0

0 \- kif ...042.0

./

3;

INTERFERENCE := 5 LU

= INTERFERENCE = 20

1.5 INTERFERENCE = 25

1.0

RESIST = AZI 350J

VISCOSITY = 21 NUMBER 2 CUP SECONDS

ROLLER = EIO0.5

11111111110 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

DOCTOR BAR PRESSURE (RELATIVE)

Figure 8. Doctor Bar Pressure and Resist Coating Thickness(Viscosity 21, Roller E-10)

25-

7.0

6.5

6.0

5.5

\\

5.0 \

\\ INTERFERENCE = 15

4.5 k ---'-' - -E -A *-4.0 --6---i-

-53 -

f INTERFERENCE = 25 -0 3.5

ff INTERFERENCE = 20 -,- INTERFERENCE = 10-2 3.0LLI-

2.5

2.0

1.5

RESIST = AZI 350J1.0 VISCOSITY =55 +3·ZAHN NUMBER 2 CUP SECONDS

ROLLER = EIO

0.5 1.5 BY 1.5 INCH (38 BY 38 mm) GOLD MIRRORS

1 1 1 1 1 1 1 ..-

0 1 2 3 4 5 6 7 8 9 10



26

ROLLER = E6

RESIST = AZI 350J



3.5

3.0

'-'E=.

.-.

. 2.50=0

. 2.0fi-

INTERFERENCE = 201.5

INTERFERENCE =5

0 INTERFERENCE = 15 IL...

1.0 ....

.... 1 -.- 4 -- -

I../

O.5 INTERFERENCE_"_9--\-- -INTERFERENCE = 10

llllllIIll

0 1 2 3 4 5 6 7 8 9 10 11



27

3.0

ROLLER = E62.8 RESIST = AZI 350J.


2.6 1.5 BY 1.5 INCH (38 BY 38 mm) GOLD MIRROR

2.4

2.2

2.0 5

.-'

E 20 INTERFERENCE = 104, 1.8.-' 15

INTERFERENCE = 20- 10w INTEPPERENCE = 5E 1.60

3 1.4 25

t;0- -

-0--./ - /

1 --0 -' -O- Sw 1.2 30- .. -0-- -\

\-0- \ \-- \ \ I-- \ \

1.0 0- .:--\./\0.8 ,

INTERFERENCE = 1 5 ", ';.\/\

0.6 INTERFERENCE = 25 \'

b,0.4 INTERFERENCE = 30

0.2

II1IIIllll

0 1 2 3 4 5 6 7 8 9 10 11

DOCTOR BAR PRESSURE

Figure 11. Doctor Bar Pressure and Resist Coating Tkickn6ss(Viscosity 35, Roller E-6)

28

ference level. As the roller/substrate interference increases,the relation becomes smoother and at high interference levels,the curves become relatively flat indicating a decreased influenceof doctor bar pressure on the coating thickness. The previousterms of low, medium, and high interference levels are relativeto a single resist viscosity and roller size as are the doctorbar pressure settings.

Doctor bar pressures, although indicated in exact numericalsettings, are relative numbers. The particular doctor barpressure ranges shown in Figures 8 through 11 are germane onlyto the set of conditions shown with each figure. With all otherconditions equal and the initial pressure adjustments made inthe same manner, the numerical pressure settings may be con-sidered essentially equal for purposes of comparing its effect.Such circumstances exist between.Figures 8 and 9, and 10 and 11where the only difference is the resist viscosity. Therefore,Figures 8 and 9, and 10 and 11 not only demonstrate the dependence .. -.

of resist thickness on doctor bar pressure, but also upon theresist viscosity at equal doctor bar pressure settings.

Increasing doctor bar pressures generally decreases the resistcoating thickness. Some irregularity in the generally decreasingthickness trends can be accounted for in terms of the interferencesetting, roller size, and resist viscosity. For instance, theoperating pressure range becomes less influential on resistthickness as the interference increases, as the viscosityincreases, and as the roller size becomes smaller (compare.Figures8 through 11 for each case).

As the interference becomes large, as compared to the thread con-figuration, the interference becomes the dominant control onthickness and changing doctor bar pressures have a diminishedeffect. The magnitude of the interference value where it becomesdominant over the doctor bar is a function of the thread depthand resist viscosity. In the same manner as the dynamic controlon resist thickness by the doctor bar decreases with smallerthread depths (comparison of Figures 8 and 10, and 9 and 11), sodoes the interference. However, as will be discussed ·later, -the ··- - ---- -interference is not used to control the overall resist thicknessbut rather the thickness uniformity and continuity. The doctorbar remains the dynamic controller of overall resist thickness.

As the doctor bar pressure becomes large (upper end of pressurerange) the interference becomes less effective on the overall -

resist thickness. High doctor bar pressures transfer only limitedquantities of resist on to the roller and regardless of how muchinterference is used, only a portion of the same limited quantityof resist can be transferred to the substrate. At most allinterference settings between 5 and 30 mils (127 to 762 um) the

29

interference exceeds the thread depth and essentially all avail-able resist is transferred to the substrate. The term essentiaZZyaZZ ava€Zabte resist is primarily dependent upon the doctor barpressure and the resist viscosity.

There is a measurable increase in coating thickness for higherviscosity resists at the same doctor bar pressure and inter-ference settings. The cause for the increased resist coatingthickness is believed to be linked to increased transfer ofresist from the doctor bar to the roller, from the roller to thesubstrates, or both. It has not been possible or necessary todetermine the exact mechanism. It remains that high doctor barpressures provide more consistent, substrate-to-substrate, over-all thickness; therefore, the second rule can be stated.

• Resist thickness is not predictably dependent upon the inter-ference except at high interference levels, and any contributionit makes to the overall thickness is decreased at higher doctorbar pressures.

Doctor Bar Pressure Adjustments

The doctor bar pressure must be periodically adjusted to achievea uniform pressure along the roller. - Such periodic adjustmentis necessitated by roller wear, roller swelling, shrinkingcaused by absorption, desorption of resist solvents, and rollerreplacement. Regardless of the cause the adjustment procedureis the same.

Figure 12 shows the procedure for adjusting the doctor bar toachieve a uniform pressure. The pressure is adjusted by turningtwo screws, located on each side of the conveyor, while checking_the pressure along the bar with a 0.002-inch (50.8 Um) feelergage. T le_--r.eller ·is thoroug#ly wetted with-resist solvent andthe pressure control (Figure 13) is adjusted until one end orthe other of the doctor bar is almost contacting the roller.The parallelity adjustment screws (Figure 12) are adjustedalternately until the entire doctor bar is in near contact withthe roller. The near contact determination is made visuallysuch that a small uniform gap. exists between the doctor bar andthe roller. The pressure indicator is then set at zero byloosening the allenhead screw in the center of the dial (Figure13) and moving the dial to indicate zero pressure. This ina sense caZibrates the indicator for each roller.

The feeler gage is placed between the roller and the doctor barat one end of the bar and the pressure control is turned untila slight drag is felt on the shim. The shim· is moved from oneend of the roller to the other and the relative drag is determined.The parallel adjustment screws are again adjusted alternatelyuntil the drag is the same-at each end of the roller. The

30

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AW' i.f r -11 1 1- r 4=1' ,1-44 1. i

1 -., 4+ .lk *- =1 7

Elfit' - 4 1.:ER=*=**-1

. k.143 PARALLELITi' AIID DOCTOR BAll PRESSUIi[t-:: melim**, 1 ADJU.,TIIEIIT JIREW :2-_ '4- &.mail//6244/I =6/m#=r -I - k ·3 *5|L*1 *al' 1 ...4HMIL'.ri-.'I -t,I IA - 1 Ill &_ 4 1.

eN,46 1 - 4 -12 4P 11-Ji-1/

1 8 =" 4 ' '44=' 11 /It -4 1-11 1 Ill

. 5,&

1#41'_. '- lf/.' I<1Re'-11*i= 13 tir_ - . 4

A E ,-,4 =e,_=-1

Figure 12. Adjustment for Uniform Doctor Bar Pressure

CA)

»6

1 Ar' Jit11,014# 1 N f *A 17 : .3

Ip2.It. - - - - E91 ,*r , ,- ,--* , f . ---, . E . 6:..7

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1 :- S : . -2/IJ*14-":'lurga.UA121*-1'*11,*64- 'i it'I 't.1, .. INTERFERENCE i4:'iNIZEW:FI"me/Mip'le,r *Ii& 2

, '-1 »fl_. ' '4 · ST I . 1 .3

0125*J'B. i.al"/1/1 - ' A =---3, •} -- 1.4 * ADJUSTMENT *ba * . i-:.'.- t· . il 6 1

1111,2/631mtimp,W#flmginililleaBS' - 4 - Cl ...1 .:,4=lilt- fj.,E IA..Ily'ke"& =r / 228''i.'.'' 1 '. I , 4' «. ., - 3.'fl-fiti' -:,r"

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t.#2 I. TS, -5, "..'.-+* „8 ! .1- CONVEYOR BELT '

TENSIONADJUSTMENT SCREWS ,1- + ...: 1 I

E·b .0 / 6 :"r -6- FM h

;64+3- 1 1,...IINTERFERENCE DIAL ZERO

f "13/4 5 .'42

/ SET POINT ADJUSTMENT , n,-i -· :1 2 1 . Ja I. -id

*t DOCTOR BAR ----4)MMkF£ i;-, 6PRESSURE M 1,1 '5 .4

r.4ADJUSTMENT #44.6,.i44 9 -2-FF· r ' 4

1* S':1* (J , C -i - dPRESSURE DIAL

; is-re3 4 4 elt:NER :**$' -- - INTERFERENCE READING

TOTAL F*5*,=sr (111LS) ZERO SET PO INT ADJUSTMENT f *1 fi8F» 21, *162'p f e.- 4.SUBSTRATE =-4&- : Ii"/2/4*6:4

1:iC: :::ER -' . <. 41 6 4 . PRESSURE &/PA .'L-' * 38*14:38- . / . - .1' .

(MILS) PRESSURE DIAL INDICATOR- 5:ammal.hal . - 2/,-le -1 - 1

- Pri#Mfmtwi .... 1., li , :

' .-*614#J

Figure 13. Roller Coater Controls

CA)

I\D

roller is kept thoroughly wetted with resist thinner during thepressure adjustment procedure. When the drag is equal at eachend of the roller, the final adjustment can be made while photo-resist is flowing through the system.

While photoresist is flowing along the doctor bar and transfer-ring to the roller, the roller surface will assume a sheenproportional to the amount of resist on the surface. Since theamount of resist on the roller is proportional to the doctor barpressure, any differences in sheen from one end to the otherindicates a non-uniform doctor bar pressure. Final adjustmentsto achieve uniform pressures are made by obtaining a uniformsheen along the entire roller. It may be found that substantialadjustments may still be required. Where a uniform sheen existsand then disappears (skips), the roller may be out-of-round ormay have other mechanical flaws. Experience has shown that minorskips in the roller sheen may be ignored and satisfactory coat-ing can be obtained. Once adjusted, the pressure uniformityshould be checked periodically by the sheen method.

Roller Size

The manufacturer makes four roller sizes for the Micro 9 rollercoater. The actual size of the roller does not change but thethread configuration does. Figure 14 shows the thread configura-tion and lists the thread dimensions for the E-6 through E-22roller sizes. It is obvious the greater the thread size thelarger amount of resist it will hold and the thicker the averageresist coating will be on the substrate. Figures 8 and 10 showthe overall coating thickness for an E-10 and E-6 roller respec-tively, where all other parameters were held essentially constantand equal. A 1 to 2 Ud difference in coating thickness is ap-parent between these two rollers, which is greater than thatspecified by the manufacturer (Figure 14). Also noticeable isthe smaller dynamic control the doctor bar has on the smallerroller as opposed to the larger one.

Average or overall resist coating thicknesses are, in the machinesense, controlled by the resist viscosity and the roller size.Since neither of these are considered as operator controlledvariables, their contributions are considered as non-dynamicsystem characteristics. Nevertheless, they are important factors.Differences in thicknesses between results obtained and thosespecified by the manufacturer are best explained by the resistviscosity, the substrate material inclusive of the type of thin-film on the substrate, and the machine parameters. By adjustingthe viscosity lower than 20 Zahn Number 2 cup seconds and coat-

- ing on very smooth substrates (glass or silicon), the specifiedthickness range can be achieved. However, the A1203 ceramic isrelatively textured as compared to glass or silicon and has atendency to puZ loff more resist from the rqller than the smoother -

33

-S T

1,1 f , E dX:L-...900

MANUFACTURER'S

SPECIFIED COATINGROLLER S d T B THICKNESS RANGE

6.0 mils 2.0 mils -,1.0 mils -1.0 mils 0.5 TO I BmE-6 (152.4 xm) (50.8 um) (25.4 Bm) (25.4 Km)

10.0 mils 4 mils -1.Omils 1.0 mils 1.0 TO 2.0 KmE-10 (254. Em) (100.16#m) (25.4#m) (25.4 um)

16.0 mils 7 mils -1.0 mils 1.0 mils 2.0 TO 3.0 KimE-16 (406.4 Bm) (177.8 um) (25.4 Bm) (25.4 gm)

22.0 mils 10 mils - 1.0 mils 1.0 mils 3.0.TO 5.0 BmE-22 (558#m) (254#m) (25.4 Bm) (25.4 Bm)

Figure 14. Roller Size and Thread Configuration

substrates. Furthermore, operating at an interference not toexceed the substfate thickness necessitates a higher Viscositywhich produces greater coating thidknesses. Therefore, it isnot unusual that an E-6 roller may result in greater than1.0 um resist coatings.

The dynamic thickness range achieved by varying the doctor barpressure is approximately 1.5 to 2.0 jim for the E-10 rollerand 0.4 to 0.8 Um for the E-6 roller. The result is in perfectagreement with the resist transfer and roller size statementsmade previously. It may also be stated that each consecutively

- smaller roller affords more consistent overall. resist coatingsthan the next larger size. Hence the fifth rule can be statedas:

• Consecutively smaller size rollers, which produce thinneroverall resist coatings, are more consistent inrepeating a specific coating thickness with all otherparameters held constant.

34

It was shown earlier that the same effect' is true for increasedviscosities.

Roller/Substrate Ihterference

The roller/substrate interference value (View AA' of Figure 4)is adjusted by the control shown in Figure 13. Also shown inFigure 13 is the interference indicator which is set by aligningthe total substrate and carrier thickness (dial on left) oppositethe desired interference level (dial of right). For example,the substrate is 27-mils thick (686 um) and the carrier is123 mils (3.12 mm), giving a total of 150 mils (3.8 mm). The150 mark is opposite the 22-mil interference value (Figure 13)indicating a 22-mil (558 Um) interference setting.

The effect of interference on overall coating thickness is visiblein Figures 8 through 11. The behavior of the interference valuesappears to be somewhat erratic; however, when plotting the resistcoating thickness at constant doctor bar pressures over varyinginterference levels and analyzing the results in terms of resisttransfer to and from the roller, this behavior becomes morereasonable, although not always predictable. Figures 15 through18 show the dependence of resist thickness on interference forvarious sets of conditions. Using Figure 15 as an example„it can be seen that the resist thickness varies at lower doctorbar pressures with increasing interference but becomes lessdependent on interference at higher doctor bar pressures.

At lower doctor bar pressures (<4) the roller threads are relativelyfull of resist if not overfilled depending upon the viscosity andnot a great deal of interference is required to retrieve theresist from the threads. As the interference increases, moreand more resist is transferred from the roller threads to thesubstrate until a point where the thread size is no longerconsequential to the transfer process and there begins to squee-gee the resist out between the roller and the substrate in alateral direction. On the E-10 roller at a viscosity of 21 ZahnNumber 2 cup seconds (Figure 15), the inflection point betweentransfer of resist by the threads to a squeegee action is approx-imately 20 mils (508 wm). This is shown by point B in Figure 15.

As the doctor bar pressure increases, the resist is deeper in thethreads and greater interference values are required to retrievethe resist. (Points A and A' of Figure 15 demonstrate this.) Fora doctor bar pressure of 6, a minimum of 10 mils (254 um) inter-ference is required before larger interferences retrieve moreresist; while 15 mils (381 um) interference is required at a

- doctor bar pressure of 8. (The point A for doctor bar pressure4 is off the graph to the left.) The initial decrease in resistthickness with increasing interference for pressures of 6 and 8


35

INTERFERENCE (um)

127 254 381 508 635

4.5 1 1 1 1

4.0

3.5

DBP = 4B

3.0

DBP = 6.-.E3

Bt& 2.5 ALU

tiU

- DBP = 8rE

A'56 2.0 DBP = 10iz=

1.5

1.0

ROLLER = EIO0.5

RESIST = AZI 350J


1 1 1 1 10 5.0 10.0 15.0 20.0 25.0

INTERFERENCE (MILS)

Figure 15. Resist Coating Thickness Versus Interference(Viscosity 21, Roller E-10)

36

INTERFERENCE (Bm)

127 254 381 508 635

6.5 1

ROLLER = EIO

RESIST = AZI 350J

6.0 - VISCOSITY = 55 NUMBER 2 CUP SECONDS

1.5 BY 1.5 INCH (38 BY 38 mm) GOLD MIRRORS

DBP = 2

5.5

-5.0E35 DBP = 4

. 5# DBP = 6

*1 0 1Sa=

4.0 DBP = 8

DBP = 10,.'- -/ - -

I - - -r --I3.5 /

8I

04

3.0

1 1 1 1

0. 5 10 15 20 25

INTERFERENCE (MILS)


37

INTERFERENCE (#m)

127· 254 381 508 635

3.0 j

ROLLER = E62.8 RESIST = AZI 350J



2.4

2.2

2.0

E 1.82·-

E 1.60

*E

4 -1.4- DBP = 2LU

0 1.2

DBP = 8

1.0 0 -O 'A- -. 0 DBP = 6-

-- ./ .0.8 -- ./

'f .. DBP = 4./

./ : - - - - -40.6 DBP = 10

0.4

0.2

1 1 1 1

0 5 10 15 20 25

INTERFERENCE (um)


38

INTERFERENCE (Km)

127 254 381 508 635 762

2.4

2.2

2.0

1.8

1.6

1.4 DBP = 8

2 \a\

1.2 ' DBP =6 -+0 00 \ DBP = 4/

1.0 .- -0 -00 DBP =2'4 . ./- 00/ 0& 0

0.8 I.\\\\

0.6 ,\

DBP = 10 I

0.4ROLLER = E6

RESIST = AZI 350J


1.5 BY 1.5 INCH (38 BY 38 mm) GOLD MIRROR0.2

1 1 1 1 1

0 5 10 15 · 20 25 30

INTERFERENCE (MILS)


39

can be attributed to the residual resist on the flat threadcrests (Figure 14) being forced back into the thread troughs orbeing spread thinner on the substrate because of the depressionof the threaded crests. At high doctor bar pressures (>10) theinterference point of retrieval (Point B) does not exist in thesame sense as in medium pressures (4 through 8) and the thicknessis relatively independent of the interference values.

Interference Versus Coating Thickness

Even though the relation between coating thickness and interferencemay be explained in terms of resist transfer from the threads, theinterference level is not typically used to control the overallresist thickness. The interference has more obvious effects onthickness uniformity, trailing ed-gi-buildup, and bubbles inthe resist coating. Figures 19 and 20 are photographs of a matrixof 1.5- by 1.5-inch (38-by'38 mm) gold thin-film coated substratosat the noted interference and doctor bar pressure settings.It can be seen that at a constant doctor bar pressure there isan increase in the trailing edge buildup (dark area at the rightedge of each substrate) as the interference increases. Thiseffect begins to disappear as the doctor bar pressure increases.At lower interference levels the coatings have bubbles as can beseen in the left-hand part of the matrix; therefore, higher inter-ference values are necessary which in turn require higher doctorbar pressures. A second advantage of higher interference levelsis the greater thickness uniformity held across larger substrateareas. The higher interference and doctor bar pressure valuesresult in the most acceptable resist coatings, but have the dis-tinct disadvantage of wearing out the roller quicker as evidencedby the larger amounts of rubber in the resist filter.

An alternative to higher interference levels to eliminate bubblesin the coating and to decrease trailing edge buildup, is toincrease the resist viscosity. This minimizes some of the aboveproblems, but also .produces a thicker resist coating which isalso undesirable. The optimum results are obtained at viscosity,interference, and doctor bar pressure levels which produce coat-ings for good product yields at acceptable wear on the roller.This condition is generally an empirical derivation where therelations shown in Figures 8 through 11 and 15 through 18 are

·-··-- ·- -used only as guides to achieve the desired overall-thickness- - -level. The actual operating conditions are determined by measur-ing the resist thickness uniformity and associated line widthsafter proper exposure and adjusting the parameters to achieve a

--

distribution tight enough to meet the design criteria.

40

INTERFERENCE

5 10 15 20 25

-'......I-Y,9.-&. 1...,-r-- . : - , '4111'mIMP im N,1, ,re·· mlr'1'1,·"I,•mI'll,I./2.-

1 '1,4-- 40%/A':e'"' : idetti 9 ;ii=Ir·tc'kummt*isa s,t M"mmlt'ttl 4-,vw*FA,wag ww . N lel*h-=,-=p.=„„11.1'mIMM

441$2*' ·-

Af ! 4%

v,I ' M.1,1 ): . . * # 1 0,-E= .4*9#D'#M: +1 1 .131:'.1 1:.111' lili'",1 4 11, 1 1* 11 I,t, T, 1,- 1,11.1 1.i

- ..11 ':1:YP l. · . :I .V,'111.2Nb'.66494 Q E Zi' »=11t.,4-: 4 51*6.„:4 , 1

4 FL ;*Itli' N/F b 1157*18' 1.61 1'/."Triliti, 1,31,·'.11 8 ,. 'p I. 0 44 . .1 4 1" 1

f£& Au Li ii,In £18+F-' # ,",4,%, •rt' 7 " '. 4 . .P *Euv.

0-4,1 " 0 .4...... '1112 *'yumMEAA "tr..=......=.--"--=M--1. Adrfp -2.

t' '19''*,fe:· 'It=- .1#&1611£21-1 ., 2 1 1 1 i4'WB.C 6 t,+1£##1%1

a:: 1*'Pliel#· · ' ill te&*im'0 *jw",W#Mi *Aspi"m

9 'Pt/NUM* 1.,1,/1 1"'IW## 0.

44,:f' 22:FLF bj*,w -

8 , "|,-EWA-42- '* ki ..1, h' ," tqu·„ 1, Vi,:r

111*T :"'IL 112*£1,&'*'' I' 08wrbi ig' ,4

M, , MY# . -

I;,1,2 S i: ' I,tilt 1,111 ., .... 911. A

mim *'MAJ --

.th' I , 741,11'a

*»31, #M,lt, 1 1 9%1

&94#9*,-,1

ipi. .: · :..' .1 ;.1'.w

1511·.·:.6,'0111_14' it,$*4

9 11 Ir.. I..11'f

02' . M, , 1I 6 .9 ..1''l :3'i:1111.1;122 'H. 1,1 '•''... ,•.19&.Waw# 1

i' 11,1, *!0 FI . _*

p'"0-4/6· , 'I' · ·-2, '. r./. , I ,% i·,W 21limi'PS)/dmaelmtintiyeutle£32 3:1'10,2120$*thw'ir..Ii.-lilI ' 1- 74.4. m M, 9 AMUd 648 J:R -4- 1 ' , 11 ,Wittl-£*9.W.pm »71%fl'PZl 1.t

Figure 19. Interference and Doctor Bar Pressure(Viscosity 21, Roller E-6)

41

INTERFERENCE

5 tOi 15 20 25 30-

)4545#,1 - - H.' .=. -" , ·· .1 11

a a --lit -2,1:4- a - - . - - , -,

.*M

* I I. itati2-1.-1::442 3»,4 11 i=ie 7, 4 %9 4 1 %-: -/24 -,11 4

Sj#falj

:. *.

i -- ii - . i*i1 --'.1« I

t#flt: ul 1Isp-,/.5/F.. f.

8 ./.ti' t i ....41. .... . '.F:_ _ i ..|4 =r, '':.. 1 If#.-:S *9 .. :Fl,-: 9

, 4 /3-idi*i , 1 1/:.40Af---23 : iz 4u., iSi i 4 . 1 A Emi.kit

. *.,2 f

m 1 P3-4, 1 1 '. _ 1 L.. , ¥54-i,4th 44" ''Plit..I 1 +3 1 .. .. 4*J

f tivi *1= 6 84* M Ejefi.LE

., , /15.I / 1 1 %-*66&-.

mig*fim,1

a *14 .. r *

8 ,/1&.Pri

. . - .- . am 3#fikiN ..

'

64524»es - L. *%134.

1..- 1 64'ir0%2 06310 .

.IS.:.r"StiE;

18* ..90Et<fit'r.44Wip<2*%*,1. «tt NZAXi.i:*„.-"fh",2''l 1

Figure 20. Interference and Doctor Bar Pressure(Viscosity 35, Roller E-6)

42

Interference Dial Calibration

The interference dial is calibrated while resist is flowing throughthe system, at a medium to high doctor bar presdure, by passinga "part" through the coater and noting whether or not resist wastransferred to the part. The part, such as the glass substratecarrier of Figure 5, must be flat, rigid, and of constant thickness.The part is repeatedly passed through the coater, each timeincreasing the interference until the roller just barely touchesthe .part and transfers some resist to the part. The interference:is decreased until no resist is transferredto the part. (The partis cleaned between passes.) The thickness of the part in milsis found on the substrate dial (Figure 13) and set opposite thezero interference mark by loosening the allenhead setscrew shownin Figure 13. This process may be repeated several times toassure accuracy; however, such a calibration is probably nevermore accurate than t2 mils (50.8 um). It may be found that thecoating roller and backup roller (Figure 4) are not quite paralleland one side of the part may have some resist on it while therest may not. In such cases the difference between partially-coated and non-coated interference values should be split and thedial calibrated at that setting.

Thin-Film Surface Effects on Resist Thickness

It had been noted that different thin-film materials on thealumina ceramic "retrieve" the resist from the roller in differentamounts. Figure 21 shows the histogram of resist thickness measure-ments on gold and tantalum-nitride surfaces. The R for gold andtantalum-nitride differs by approximately 0.2 um because of anapparent difference in cohesion between the resist and the respec-tive thin-film materials. Although the difference does not appearlarge, when added to the normal fO.2 Um deviation, it becomesquite significant in terms of the proper exposure energy requiredto hold and acceptable tolerance in the image dimensions. Figure21 shows the difference for an E-6 roller. The difference inresist thickness for an E-10 roller is even greater.

Edge Effects and Substrate Orientation

To eliminate the resist buildup around the substrate edges, themost prominent of which is the trailing edge buildup, the sub-strates were run. through the roller coater at an angle; that is,with a corner contacting the roller first rather than the straightedge of the substrate. Although this practice was successfulin reducing the edge build-up, it also decreased the resist thick-ness uniformity in localized areas. Figures 22 through 25 showthe results of the angle versus the straight substrate orientation.Although the histograms of Figures 24 and 25 are not obviously


43

TANTALUM-NITRIDE MIRROR"GOLD MIRROR

ROLLER = E6

DBP = 8

INTERFERENCE = 30


0.50

1.21

X-BAR1.39

X-BAR

6 1

3 1§. 1Ef

ti

i

0.25 1

' 1 F-7l i l l I111

11

1 11

1 1 1l i l i1 11l i l IIll »--7111l i l l Il i l l I l i l l i

0 0.6 0.7 0.8 0.9 1.0* 1.1 1.2 1.3 1.4 1.5 1.6 1.7

RESIST THICKNESS (Bm)

Figure 21. Gold Versus Tantalum-Nitride Effects on ResistThickness

44

• 5.2ROLLER = EIO

RESIST = AZI350J ORIENTATION OF 3.75 BY

3.75 BY 4.50 INCH 4.50 INCH SUBSTRATE5.0

(95 BY 114 mm) GOLD MIRROR VISCOSITY = 40 NUMBER 2 - DIRECTION

CUP SECONDS OF TRAVEL.4.8 DBP =8 '

INTERFERENCE = 20

,+. ROLLER4.6

'-'

4.4ca

53

4.2#+ 0-

4.0

3.8

3.6

3.4

3.21

2 6 8- POSITIONS< 4 7

3 6 9

1 I l lilli

0 1 2 3 4 5 6 7 8 9POSITION

........

Fig te 22. Resist Thickness by Positional Measurements(Straight Substrate Entry)

45

6.0ORIENTATION OF3.75 BY 4.5 INCH

5.8. (95 BY 114 mm)

SUBSTRATE

5.6MEASUREMENT

POSITIONS

5.4 /147

M --- DIRECTION2 5 85.2 6 - OF TRAVEL

3 9 -

5.0 -.ROLLER

-2 4.83

4.6C.>. - -

ji 4.4A.-- - -

S. 4.2

-

4.0-

3.8

3.6

3.4ROLLER = EIO

RESIST = AZI 350J

INTERFERENCE = 20

3.2 DBP = 8

VISCOSITY = 42 NUMBER 2 CUP SECONDS . ......_ .._ ....

1 1'1 1 1 1 1 10 1 2 3 4 5 6 7 8 9 10

MEASUREMENT POSITION

Figure 23. Resist Thickngss by· Positional Measurements(Angle Substrate Entry)

46

0.50

N-90SIGMA 0.40 '

RESIST = AZI 350J

ROLLER = EIO


4.243.75 BY 4.50 INCH

X-BAR (95 BY 114 mm) GOLD MIRROR

.

1/ 0.25ti

tidC/

07 1, i, 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9

RESIST THICKNESS (Km)

Figure 24. Resist .Thickness Histogram (Angle Substrate Entry)

4-3

0.50N-90 ROLLER = EIOSIGMA 0.48 RESIST = AZI 350J

DPB = 8

4.03 INT = 20

X-BAR VISCOSITY = 42 NUMBER 2 CUP SECONDS

>-

lip

0.25>

tm/IMC.

0 1 1 1 13.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

RESIST THICKNESS (Kim)

Figure 25. Resist Thickness Histogram (Straight Substrate Entry)

ht:.

00

different, the positional measurements of Figures 22 and 23 showa sharp non-uniformity for position 7 on the angle-orientedscheme. As a result of the irregular thickness at position 7and better control of edge effects afforded by increasing theresist viscosity, the angle-orientation was not continued.

Resist Thickness and Expected Line Width Deviation

The expected line width of the photoresist image is a functionof the resist thickness and thickness uniformity. To achievea line image with near zero line width deviation from the designvalue, requires that the resist be exposed with the proper lightenergy for the particular resist thickness in use. Where thereare variations in the resist thickness across the substrate,there will be corresponding variations in the line widths at afixed exposure energy. Figures 26 and 27 show the expected changein 5.0-mil (127 Um) and 3.0-mil (76.2 Um) lines for a variableresist thickness and a constant exposure energy. These f igllres-consistently show a 0.2-mil (5.08 Um) change in line width permicrometer change in resist thickness. This is a rather signifi-cant change in line width; that is, 4__perceB-t _99-the 5.0-mil line.Obviously to minimize the variation in line widths it is necessaryto maintain the most uniform resist thickness possible. Aspreviously discussed this is done by higher doctor bar pressures,higher interference levels, and by higher viscosity levels.

A second approach to line width control is to operate at a thick-- ness level that is less sensitive to variations in the overallthickness. Figure 28 shows the required exposure energy forincreasing resist thicknesses. It can be seen that at higherresist thickness, more exposure energy is required to properlyexpose the resist and that the required exposure range is _

-strongly dependent upon small changes in the thickness. At lowerthickness values the opposite is true; that is, the proper energy

. required is only limitedly dependent upon the thickness. There-fore, resist thickness variations at the lower end of thethickness-energy curve have a far less significant effect onthe line width. It may be concluded from the contributions ofphotoresist thickness and thickness uniformity to the dimensionaldistribution on line widths, that a thinner resist coating willafford tighter dimensional distributions than thicker resistcoatings.

Thickhess Uniformity Distributions

Figures 29 and 30 show the resist coating thickness distributionon 3.75- by 4.5-inch (95 by 114 mm) gold-coated substrates fornominal thicknesses above 3.25 Vm. Comparing-F-igures 29 and--30--to Figures 31 and 32 which have a nominal thickness of less than2.0 Um, it can be easily seen that tighter distributions existfor the smaller overall thickness. Using the information on

Text continued on Page 52.

49

5.4 137.1

EXPOSURE SETTING = 8

5.3 134.6

5.2 132

5.1 129.4

T5 5.0

127 3

m E

4.9 3.45Bm

124.4 g

*

AVERAGE THICKNESSJ

J

4.8 121.9

6 LINE WIDTH= 0.19

4.7 -8 RESIST THICKNESS 119.3

4.6 116.8

lili 1 1 1

0' 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.0 4.25


Figure 26 . Line Width Change Versus Resist Thickness (5-Mil Lines)

g

3.5 88.9

3.4 3.0 MIL (76.2 Km) NOMINAL LINE WIDTH86.3

EXPOSURE SETTING = 8

3.3 83.8

3.2 81.2

S 3.178.4 -2

3 3

3.0- 76.2 61 1W W

3 2.9 73.6 33.45 Km Z

AVERAGE THICKNESS

2.8 71.16 LINE WIDTH CHANGE= 0.192.7 -j RESIST THICKNESS

68.5

2.666

1 1 1 1 1 1 1 1

0 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.0 4.25


Figur¢ 27. Line Width Change Versus; Resist Thickness (3-Mil Lines)

0, 1 1 1H

18.0

17.0 AZI 350J

16.0

15.0

14.0 -----

-13.0 ,- 12.0 , ' .EFFECT OF INCREASED PREBAKE..

/ TIME-TEMPERATURE PRODUCT. 11.0 *- .E 10.00 9.0 *

. 8.0

It; 7.0 '-

- 6.0I

5.0 / ...:

4.0 I3.0 /:2.0 /1.0 - /

1 1

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

EXPOSURE ENERGY (J/cm2)

Figure 28. Resist Thickness Versus Exposure Energy

expected line width deviation for changing resist thickness, itcan be reasoned that the thinner coating thicknesses would affordbetter process control on line dimensions.

Lihe Width Distributions

Figures 33 and 36 are the respective line width distributionsfor the resist thickness distributions shown in Figures 29 and32. As expected, the lesser resist thickness, by virtue of itstighter thickness distribution, produced a tighter line widthdistribution under proper exposure energy conditions for eachaverage thickness. It is also interesting to note the correlationbetween the thickness distributions and the respective linedistributions (Compare Figure 29 with 33 and Figure 30 with 34).

In Figure 29 the thickness is skewed to the high side of X as isthe line width distribution of Figure 33. For positive resistand exposure settings that are adjusted for proper exposure of theaverage resist thickness ( X ), the higher than - afd/RE- rifi- t ----thicknesses will be underexposed in proportion to the differencebetween the thickness and the R thickness. The underexposure

52

0.50N-236SIGMA 0.33

3.38X-BAR

il

2 0.25

t /dOC

O -7 F--72.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2

RESIST THICKNESS (#m)

Figure 29. Resist Thickness Distribution Measured at 9Positions, Run-to-Run (R >3.0 um)

condition will result in a line width proportionally higher thanthe X line width. The opposite is true for thicknesses lessthan the X thickness. This type of correlation can be easily seenwhen comparing Figure 29 with 33 and Figure 30 with 34 on a cell-for-cell basis. This effect is masked by thickness measurementresolution in Figures 31 and 35 and 32 and 36.

Recommended Operating Procedure

Machine set-up and start-up/shut-down procedures are given inAppendix A. This section will address only the technical andparts scheduling aspects of roller coating.

53·

0.50

N-100SIGMA 0.31

3.39X-BAR

iN

if 0.25

4*06=

0-- - 1 - 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0

RESIST THICKNESS ( Bm)

Figure 30. Resist Thickness Distribution Measured at Center ofSubstrate, Run-to-Run (R >3.0 Um)

The most important contribution of photolithography to HMCfabrication is the faithful reproduction of the photomask imagecontaining the electronic circuit with sufficient dimensionalcontrol to hold the circuits electrical characteristics withinthe designed tolerances. This philosophy applies principallyto thin-film resistors.

To maintain a k10 percent control on thin-film, tantalum-nitrideresistors whose sheet resistivity may vary by as much as +8. per-cent, requires a t2 percent control on the line width. Usinga minimal 5.0-mil (127 Um) line, this implies a dimensionalcontrol of &0.1 mil (5.08 um) maximum deviation from the nominalvalue. It is obvious that distributions like those of Figures 33

54

0.50N-81

ROLLER = E6SIGMA 0.18VISCOSITY = 35 NUMBER 2

CUP SECONDS1.62 RESIST = AZI350JX-BAR INT = 25 MILS (0.625.mm)

DBP = 83.75 BY 4.50 INCH

(95 BY 114 mm)GOLD MIRRORS

2

iu- 0.25Ae

tid(0

0 1 -7 1-1 1 - i l-

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4

RESIST THICKNESS ( Bm)

Figure 31. Resist Thickness Distribution Measured at9 Positions, Run-to-Run·.(R <2.0 um)

and 34 do not maintain, in all cases, the necessary tolerance.Therefore, it again becomes obvious that a thinner resist coatingis required.

The following conditions are recommended to achieve the tightestline width distribution possible:

• Resist thickness should be 1.8 +0.1 lim;

• Roller size should be E-6;

55

0.50

1.66

X-BAR

&3

38.ef

4

3ti

0.25

Ill lili

0 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2

THICKNESS (Bm)

Figure 32. Resist Thickness Distribution Measuredat Center of Substrate, Run-to-Run(X <2.0 Um)

56

- 0.50

N-760SIGMA 0.35 * ALL VALUES ARE

SLIGHTLY UNDEREXPOSED.

5.08PROPER EXPOSURE WOULD

X-BAR*SHIFT THE ENTIRE DIS-TRIBUTION TO THE LEFT

LLI 0.05 TO 1.0 MIL=D

( 1.2 7 TO 2 5.4#m) , BUTSTILL WOULD NOT MAKE

THE DISTRIBUTION ANY- TIGHTER./3a

23

---2 -0.4-+ 2-- TOLERANCE (PERCENT)

2 0.25titmd00

O-- --1---14.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

LINE WIDTH (Km)

Figure 33. Line Width Distribution Measured at 9 Positions,Run-to-Run (>3.25 Um Resist Thickness)

• Resist should be AZ1350J;

• Viscosity should be 35 t3.3 Zahn Number 2 cup seconds;

• Interference should be 25 to 30 mils (635 to 762 um);

• Doctor bar pressure should be sufficiently high to achievethe desired thickness and to eliminate trailing edge buildup;

• Filter size should be 5.0 Um maximum (sintered silver);

57

LINE WIDTH (#m)

109.2 114.3 119.2 124.5 129.5 134.6 139.7 145.1

111.8 116.8 122 127 132.1 . 137.2 142.2

0.501

1 1 l i l I 11

N-505SIGMA 0.55

5.13X-BAR

NOMINAL0

1LL. 0.25/2

--2-•++2-• TOLERANCE (PERCENT)

Ad

1-O --1--14.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7

LINE WIDTH (MILS)

Figure 34. Line Width Distribution Measured at Center of Sub-strate, Run-to-Run (>3.25 um Resist Thickness)

• Resist temperature should be 72 to 76'F (22 to 24'C); and

• Gloves should be worn at all times.

As mentioned earlier, the roller coater is the first operationin a sdhiautomatic, multiple HMC photolithography process.The capacity of each of seven process steps is nearly matchedat - 400 substrates per hour. With the large capacity each

58

-' L I N E WIDTH (u m)

124.5 127 129.5 132.1

123.2 125.7 128.2 130.8 133,30..50 llllllll

N-807 ROLLER = E6LAJ

SIGMA 0.06 3 VISCOSITY = 35

91 5.04 NUMBER 2 CUP SECONDS

z X-BAR* INT = 35CD DBP = 7

E--2--=,-*.-- +2 r TOLERANCE (PERCENT)

.LL. 0.25%!

d0/

* ALL VALUES SLIGHTLYUNDEREXPOSED. PROPEREXPOSURE WOULD SHIFTTHE ENTIRE DISTRIBUTIONTO THE LEFT BY 0.05 MILS

( 1.27 B m) AND ABOUT 90PERCENT WOULD BE WITHINTOLERANCE.

0: r----- ,

4.85 4.9 4.95 5.0 5.05 5.1 5.15 5.2 5.25

LINE WIDTH (MILS)

Figure 35. Line Width Distribution Measured at 9 Positions,Run-to-Run, (<2.0 Um Resist Thickness)

59

LINE WIDTH (Bm)

123.2 124.5 125.7 127 128.2 129.5 130.8 132.1 133.3

0.50 1 1 111N-99

SIGMA 0.053

5.05

X-BAR

S

i1.610·25>

ti

rl0/

1 --3--i4.85 4.9 4.95 5.0 5.05 5.1 5.15 5.2 5.25

LINE WIDTH (MILS)

Figure 36. Line Width Distribution Measured at Center of Sub-strate, Run-to-Run (<2.0 um Resist Thickness)

system only needs to be operated at a fraction of its capacityto achieve production schedules. Since the capacity cannotactually be changed, each system, including the roller coater,should be operated only long enough to process the number ofparts required at the given time. The rest of the timethe process line will sit idle. This situation requires carefulparts scheduling by Production Control for·the efficient andeconomical use of the equipment and operating personnel. It ispossible to meet present production schedules with less thanhalf-shift operation. Therefore, scheduling should be arrangedsuch that all parts are made ready for photolithographicprocessing at the beginning of each shift.

60

' ACCOMPLISHMENTS

The roller coater has been fully characterized using ShipleyAZ1350J photoresist. The roller coater is a safisfactory systemfor coating photoresist on 3.75- by 4.50-inch (95 by 114 mm)substrates for the production of hybrid dicrocircuits with t10percent as-etched resistor tolerances. It is somewhat fragilein its structure and construction, but should withstand produc-tion use provided it is properly operated and maintained.

Dimensional control of HMC geometries requires a resist coatingthickness less than 2.0 um with a thickness uniformity of t0.1 Um.Control of doctor bar pressure, interference, and resist vis-cosity are critical in obtaining the desired thickness control.Determining the optimium doctor bar pressure, interference, andresist viscosity is largely empirical and settings may varyappreciably from system-to-system or roller-to-roller.

All planned activities involving the roller coater have beenaccomplished and roller coating technology has been transferredto Process Engineering. The roller coating system has beenimplemented and is being successfully used in HMC production.Process and material limits have been established which willmaintain a t2 percent dimensional control on the 5.0-mil (125 Um)photoresist image.

....---

FUTURE WORK

No future work on roller coating is required under present HMCproduct and processing requirements. I f product requirement-s,--- --processing requirements, or both are changed, then additionalprocess development efforts may be required.

A backup resist should be characterized on the roller coater incase the AZ1305J composition is changed and cannot be rollercoated or becomes unavailable. A short term process developmenteffort should be started to determine the roller coaters useful-ness with respect to vias. It is believed, under certain via-holecharacteristics (via corners with a smooth radius), the rollercoater could be used to protect via metallization.

It is recommended that spray resist coating be investigated asan alternative to roller coating to achieve smaller HMC geometries -and dimensional control. It is believed that spray coating maybe more durable and economical than roller coating in the long run.

61

Appendix A

START UP AND SHUTDOWN PROCEDURES

Gyrex Roller Coater Start Up

1. Remove plastic bottle of MEK and let drain.

2. Install plastic bottle of Shipley AZ1350J photoresist.

3. Turn control switch on coater to PUMP position.

4. Using a squeeze bottle filled with Shipley AZ thinner, com-

pletely wet coating roller.

5. Turn the doctor bar PRESSURE dial to 8.

6. Turn PUMP SELECTOR, on front of clean bench, to RESIST.

7. Open drain valve on bottom of filter and drain solvent.

Leave open until resist begins to flow into filter.

8. Adjust the flow control valve, on top of filter, to provide

adequate flow of resist, but not enough to flow over ·the

coating roller end-seals (about one-half turn open).

9. Turn the substrate INTERFERENCE DIAL, located on top of the

machine, until the substrate THICKNESS DIAL is set with(150) directly in line with (25) on the INTERFERENCE DIAL.

Substrate Loading and Coating

1. Place substrates on glass back-up plate.

2. Place back-up plate on conveyor belt.NOTE: Do not place back-up plate on the seam of the

conveyor.

3. Remove back-up plate with substrates from the other endof the conveyor.

NOTE: Avoid touching the coated surface of the substrates.

Flush Out and·Shutdown Procedure

1. Turn PUMP SELECTOR, on front of clean bench, to OFF.

2. Allow resist to drain into container. Scrape resist on

doctor bar down to return hole with Teflon scraper.

62

3. Remove the photoresist container, and install container ofMEK, approximately one-half full.

4. Turn PUMP SELECTOR, on front of clean bench, to RESIST.

5. Turn flow control valve all the way counterclockwise.

6. Open drain valve on bottom of filter and drain resist. Leaveopen until MEK begins to flow into filter.

7. Allow the machine to clean itself for approximately 5 minutes.

8. Turn PUMP SELECTOR to OFF.

9. Remove container of MEK and replace with a container offresh MEK.

10. Repeat steps 4 through 7 and then 8.

11. Using a squeeze bottle filled with MEK, clean photoresistfrom the Teflon seals and any spots on the machine.

12. Turn doctor bar control. counterclockwise to the end of itstravel.

13. Wet the coating roller using a squeeze bottle filled withShipley AZ thinner. Let the machine run until the rolleris dry,

14. Turn control switch off.

63

Appendix B

MAINTENANCE AND FACILITY REQUIREMENTS

Pnoper maintenance of the roller coater is important for successfuloperation. In particular, the system must be thoroughly cleanedafter every use to assure that no residual photoresist is lefton the roller, doctor bar, or in the resist supply lines. A dirtysystem will not produce good resist coatings, and in time willdamage the roller. Major maintenance such as replacing the con-veyor belt, replacing the roller, and lubricating the roller bearingshould be done when necessary. The rollers should never be touchedby hand, and there should never be any doctor bar pressure on theroller when the roller is not turning and wetted.

The roller coater should be in a class 100, vertical flow, exhausthood. The cleanliness requirements are necessary to preventforeign material from contacting the wet roller or the freshlycoated substrates. The exhaust portion is required to abstractall fumes when the system is cleaned. A spill pan should beplaced beneath the roller coater with a drain going to a wastesolvent pan. The pan used is to catch photoresist, cleaningsolvent, or both. The clean hood should have a hinged, full-faceshield for air control inside the hood. The exhaust volume shouldbe slightly less than the clean air volume during coating andmore than the clean air volume during the cleaning operation.Yellow lights should be provided in the hood.

The roller coater should be arranged so that it directly feedsinto the conveyor IR prebake oven either by hand or preferablyby an intermediate conveyor,

The Gyrex Micro 9 roller coater is fragile and should be maintainedand handled with care. The rollers will get out of alignmentif mishandled.

64

P I

Appendix Ctr -

MICROCOATER TROUBLESHOOTING GUIDE

The following are some possible coating deficiencies and theirremedies.

Coating too thin: Increase fluid viscosity or decrease doctorbar pressure.

Note: Multiple dry passes will increase thicknessby 100 percent. Multiple wet passes willincrease thickness by approximately 70 per-cent.

Coating too thick: Decrease fluid viscosity or increase doctorbar pressure.

Fine-line skips in Decrease backup to coating roller clearanceCoating: (increase interference) or decrease doctor

bar pressure.

Trailer edge bead Increase backup to coating roller clearanceor plugged holes (decrease interference) or increase fluidin substrate: viscosity.

Bubbles in Increase fluid viscosity or decrease backupcoating: to coating roller clearance (increase

interference), or allow further permea-tion of resist to coating roller.

65

g

.--,

DISTRIBUTION

Copy

R. Bulcock, ERDA-KCAO 1

V. C. Vespe, ERDA-ALO 2

J. C. Crawford, SLA 3

A. R. Engquist, SLA 4

B. L. Gregory, SLA 56R. E. Hampy, SLAJ. A. Hood, SLA 7C. B. McCampbell, SLA 8

G. W. Rodgers, SLA 9

J. T. Grissom, SLA 10

C. M. Tapp, SLA 11

R. K. Traeger, SLA 12

R. E. Wright, SLA 13

C. S. Selvage, SLL 14

H. R. Sheppard, SLL 15

R. F. Pippert, D/100 16

D. J. Stevens, D/141 17

J. S. Carson, D/142 18

J. D. Corey, D/554 19-20

L. Stratton, D/554 21-23

R. P. Frohmberg, D/800 24

D. H. Hax, D/800 25

F. A. Spies, D/800 26

B. T. Lampe, D/814 27

A. 0. Bendure, D/842 28

R. W. Hughs, D/842 29

D. P. Norwood, D/842 30

J. A. Peters, D/842 31

J. H. Pusch, D/842 32

L. E. Schantz, D/842 33-37

T. A. Wiley, D/842 38

D. L. Willyard, D/842 39

J. H. Lynch, D/845 40

D. D. Peterson, D/846 41

J. H. Swafford, D/863 42

R. E. Kessler, D/864 43

. --

66

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