Reactive Sputtering to Increase Sheet Resistance of WSiN Thin FilmsRaymond Chen, Antonio Cruz, Jack Lam, Niteesh Marathe, Camron Noorzad, Yongsheng Sun, Cheng Lun Wu, Disheng Zheng

Outline1. Problem Identification2. Design Approach3. Evaluation4. Conclusion and

Recommendations

A. Project BackgroundB. Problem ScopeC. Technical ReviewD. Design Requirements

Project Motivation Keysight Technologies has interest of expanding into new markets:

1. Develop new platforms 2. MMIC (High-frequency monolithic microwave integrated circuit)3. TFRVH (thin film resistor very high)4. Students Research and Development5. Sell products and make profit

Example Product

MMIC

Project Goals Develop a fabrication process for WSiN TFRVHs:

1. Produce TFRVHs with desired specifications: 2000 /sq sheet resistance 750 ~1500 thickness 10% Standard Deviation and Uniformity

2. Demonstrate our results were consistent and repeatable

Problem Scope Concern of produce TFRVHs on Silicon Wafer

Use appropriate deposition method Determine parameter input Achieve priority specification Maintain consistent output

http://project-planners.com/wp-content/uploads/the_project_triangle1.jpg

Technical Review: Reactive Sputtering Method of introducing reactive gas into

sputtering to fabricate thin film resistor Easy to control deposition properties PVD Target is bombarded by energetic ions, In

this case Argon ions (Ar) Collisions knock and sputter atoms from

the target Sputtered atoms flow to be deposited

onto the substrate

magnets

http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.htm

Technical Review: Advantages of Sputtering Wide range of possible sputtered materials High deposition rates High purity thin films (vacuum, low pressure) Good adhesion Good step coverage and uniformity Allow various parameter control Available in both DC and RF power Magnetron sputtering uses magnets behind

target to attract electrons to facilitate electron-Argon collision

http://dir.indiamart.com/impcat/sputtering-systems.html

Technical Review: Disadvantages of Sputtering

Deterioration of equipment and target material High sheet resistance uniformity percentage

Bad yield percentage Possible sputter gas incorporation into film

Technical Review: Why we use RF power Power oscillated at radio frequencies sustains the Argon plasma

If not. The negative charge applied to target can be neutralized by Ar Ions will not be attracted to target

Ions are too heavy and slow to follow this frequency Electrons can follow this frequency and build up a negative self bias on the

target

Technical Review: Why Ar Big gas ion Inert to WSiN Produce high sputtering yield

manufacturing process to be timely and efficient

Relatively inexpensive and available in high purity

Source: [9]

Technical Review: Tungsten Silicon Nitride Ability to reduce the local atomic ordering when sputtered due to argon ion

bombardment High melting point of around 3000 oC Applications:

Lower power consumption of a capacitive touch screen Mask material for x-ray lithography Hard coating Printer heads

Technical Review: Target Processing Composite from hot pressing

Tungsten powder and Silicon Nitride powder

Because of this, we suspect that the sputter result will be silicon nitride and tungsten.

Technical Review: Substrate and Chamber Silicon substrate (100) orientation with approximately 100 nm silicon dioxide on

top Negative substrate bias: -60 V

Better guidance of WSiN movement to substrate and substrate adhesion, increasing nitrogen content

Low cost substrate for experimental purpose One patterned + one non-patterned

Real substrate will be GaAs and InP High vacuum chamber: 10 mTorr

lower sputter rate increase the mean free path of sputtered target

Technical Review: WSiN Thin Film Would cause crystallization and loss of nitrogen content around 800oC Nitrogen atoms bonded to silicon atoms of the Tungsten and Silicon

Amorphous Network increase the resistivity Coefficient of thermal expansion of WSiN is 6.37 X 10^-6 C1,

The coefficient of thermal expansion of Si is 3.45 106 C1 This difference can result in significant thermal stresses if the Si

substrate is heated during deposition. Amorphously deposited on the substrate Very effective at blocking atom diffusion Chemical Inertness

Technical Review: Sheet Resistance Measure of resistance for thin film materials

instead of a bulk material Sheet resistance is defined as: Rs=(/t)

is materials resistivity and t is thickness

Has unit of ohm but usually use ohm/square

Only need to specify length and width of the resistor to define value.

The ratio L/W represents the number of unit squares of material in the resistor

Outline1. Problem Identification2. Process Design3. Evaluation4. Conclusion and Recommendations

A. Design Requirementa. Input Parameterb. Output Parameter

B. Design Approacha. Sputtering Systemb. Substrate Biasc. Justification of N2 gas flowd. Deposition Timee. Film Stressf. Thicknessg. Four-Point Probe

Design RequirementsSputtering Input Parameters

Fixed Parameters

RF Power 750 W

Substrate Bias -60 V

Total System Pressure 10 mTorr

Total Flow Rate 40 sccm

Controlled Parameters

Gas Ratio (N2 : Ar) 0.15 : 1

N2 Flow Rate 5.2 sccm

Ar Flow Rate 34.8 sccm

Deposition Time 1027 Seconds

Target Thin Film Parameters

Sheet Resistance 2000 ohm/square

Margin of Error 3%

Standard Deviation 10%

Uniformity 10%

Thickness (x) 750 < x

Design ApproachFlowchart of Design:

Design Approach CVC 611 Reactive Sputtering System

Older machine in the wafer fab Ion mill chamber to clean wafer before

deposition process [4] Rotating deposition to increase sheet

resistance uniformity [4]

Sputtering Target: WSi3N4

Front monitor and chamber of CVC 611 System. Source: [4]

Back of CVC 611 System with the RF Power Supply. Source: [4]

Design ApproachSubstrate Bias Negative bias allows for Ar ion

bombardment onto substrate, minimizes long range atomic order (amorphous thin film) [4]

Bias repels electrons from depositing onto the film [4]

Standard value for the CVC System in the wafer fab [4]

Diagram of RF Sputtering including Substrate Bias. Source: [4]

Design ApproachJustification to Incorporate N2 Gas into Film: Increasing sheet resistance = smaller mean

free path of electrons (more defects in film microstructure) [5]

Add atoms that bond to the amorphous network. Saturation point: atoms added as point defects [5]

Nitrogen already a part of the target in the CVC System chamber [5]

Diagram of RF Sputtering including Substrate Bias. Source: [4]

Design ApproachN2:Ar Gas Ratio

S.M. Kang, et al, showed that increased presence of N2 gas in chamber increases sheet resistance. Sheet resistance of thin film significantly increases above ~10%[1].

Gas flow rates calculated accordingly.

Design ApproachDeposition Time Keysight suggested deposition time of 20 minutes

Confirmed by Kang, et al, in their experiment [1]

Useful equation: Rs proportional to 1/t Rs = sheet resistance (ohm/sq.) t = thickness ()

Keep thickness in range Q * T = t Q = deposition rate (/s, assumed constant) T = deposition time (s)

Design ApproachMachine for Stress Measurements: Tencor P2 Long Scan Profiler

Stressed films bend substrates outward (compressive stress) or inward (tensile stress) [6]

Tencor P2 determines film stress by measuring samples change in curvature between two tests [6]

Measured wafers initial stress before deposition [6] After deposition, wafer is measured again to determine film stress, which is

calculated from wafers change in curvature [6]E = Youngs modulus of substratev = Poissons ratio of substratets = Thickness of substratetf = Thickness of filmr = Radius of curvatureL=Length of traceB=Maximum between chord and trace

Photo of Tencor P2 Long Scan Profiler. Source: [6]

=Ets2/6r(1-)tf

r=L2/8B, L>>B

Design ApproachMachine for Thickness Measurement: Tencor P12 Profilometer

Surface stylus profilometry determines change in height across sample. Patterned photoresist was applied onto silicon wafers before deposition. WSiN films were deposited onto patterned silicon wafers. After deposition, acetone was used to strip away photoresist.

Tencor P12 Profilometer. Source: [9]

Resist/Deposition/Strip sequence. Source:[17]

Design ApproachMachine for Rs Measurements: 4P Automatic Four Point Probe, 280C

Current passes through the outer two probes and film [7] Voltage across two inner probes is measured [7] Rs = 4.53 x V/I [7]

Measures at 25 points for average Rs value.

Schematic of Four Point Probe machine. Source: [7]280C Four Point Probe, Model 4D. Source: [8]

Design ApproachMachine for Surface Topography and Chemical Composition: FEI SCIOS Dual Beam FIB/SEM

Scanning Electron Microscope (SEM) Surface topography and composition at high

resolution Electron beam shoots at sample and interacts

Energy-Dispersive X-ray Spectroscopy (EDXS) Separates characteristic X-rays into elements Relative amounts of elements in sample

Electron Backscatter Diffraction (EBSD) Measures electrons diffracted from atomic planes If crystalline, gives crystal orientation and grain

size.

Photograph of a SEM. Source: [16]

Interaction of Electron Beam with sample. Source: [16]

Design ApproachMachine for Measuring Film Properties: PANalytical XPert PRO

X-Ray Reflectivity (XRR) Shoots X-Rays at film sample from a

range of small, grazing angles. X-rays reflect toward detector. Gives information about film

thickness, density, surface roughness, and degree of crystallinity.

Basic concept of XRR.

PANalytical XPert PRO.

Design ApproachFlowchart of Design:

Outline1. Problem Identification2. Design Approach

3. Evaluation4. Conclusion and

Recommendations

A. OverviewB. Testing Result

a. Sheet Resistance b. Thicknessc. Stressd. Morphology

C. Assessment + Cost AnalysisD. Future works/ Next steps

Overview of Results Graph compares our last 3

wafers using all of the same final parameters: Dep time= 1027 s 15% N to Ar ratio

Rs close to 2000 Good consistency Wafers 9 and 10:

sputtered simultaneously

Sheet Resistance 8: batch-to-batch comparison 9 and 10: wafer-to-wafer

comparison Standard engineering margin of

error = 3% Note: 9 and 10 only have same

RS, not std. dev. or uniformity.

Wafer Number #8 #9 #10

Rs (/sq) 1975 2060 2060

Margin of Error (%) 1.25 3.00 3.00

Std. Deviation

(%)5.64 5.86 6.05

Uniformity (%) 10.13 10.61 11.12

Causes of Variation Target condition affects

sputtering: Wear pattern directs

sputtered atoms Batch-to-batch variation

Old sputtering system

A used sputtering target (left) compared to a new target (right). Source: [4]

Nitrogen Gas Ratio Dependence Ratio test range: 10-20% N2/Ar Agrees with other experiments Exponential curve, just as Kang

et al Reinforces P. Homhuans work:

theory of N interstitials Shorter mean free path for

electrons

Film Thickness Dependence Dep time was altered after

viewing results of 15% N to fine tune RS

Thinner films yield higher RS (less is more)

RS1/t Left most point: prone to

statistical error, still within one standard deviation

Thickness Results

Wafer 8: 915 Wafers 9, 10: 974

Average of 5 measurements All thicknesses within

prescribed range 750 -1500 Some unexpected variation

WSiN Film

Si Substrate

Thickness

SEM micrographs Wafer 9 Cleaved through

the middle Edge-on view

Thickness Identical deposition

parameters Wafer 8: 915 Wafer 9: 974 Wafer 8

Wafer 9

Thickness

Wafer 8 Wafer 9

Variation Q=QAVG Assumed constant

Q Q=IC [11] Ion current I, sputtering

system constant C not expected to change

Sputtering yield must change Age of target

Thickness Variation

Q=QAVG Assumed constant

Q Q=IC [11] Ion current I, sputtering

system constant C not expected to change

Sputtering yield must change Age of target

A used sputtering target (left) compared to a new target (right). Source: [4]

StressObserved decrease in film stress with increased N2/Ar

Stress Residual vs. thermally induced stresses

Thermal stress not significant [8] Residual stress due to

Ar+ contamination Densification effects

Stress reduction due to Change in microstructural regime [15]

Further characterization to confirm

Stress

SEM micrographs, wafer 9

No delamination or buckling

TOP MIDDLE FLAT

SEM micrograph, patterned wafer 9

Stress No delamination or buckling was observed Stress greater on GaAs substrates than on Si

substrates Lattice constants, CTE

Stress on GaAs can be reduced by annealing [3] Possible increase in resistivity [4]

Morphology Previous studies of

WSiN thin films suggested our film would be amorphous [3,4]

EBSD showed no crystallinity

XRD indicates degree of roughness

X-ray reflectivity curve

Composition Confounded by film dimensions

and interaction volume

EDXS data on patterned wafer 9

Assessment The TFR was successfully

fabricated at Keysight Technologies ~2000 /sq

Margin of Error = 3% 10% Uniformity Thickness>750

Cost Analysis - Materials WSi3N4 Target - ~$200 Silicon Wafers - $20/wafer

20 Wafers (10 patterned/10 not patterned) = $400 Fabrication/Testing Equipment Provided

Total Material Cost - ~$600

Cost Analysis - Characterization SEM - $58/hr

6 hrs = $348 XRD - $60/hr

4 hrs = $240 Optical Microscope - Free

Total Characterization Cost = ~$600

Cost Analysis - Labor Full Time Equivalent

(N2, Ar, CVC operation) Engineer - $15K/month ~ $90/hr

9 engineers x (98 hrs) = $80K Technician - $12K/month ~ $75/hr

1 technician x 18 hrs (2 days) = $1K

Total Labor Costs = ~$81K

Overall Cost

Investment Type Cost

Materials $600

Characterization $600

Labor $81K

TOTAL $82K

Previous Estimated Cost = ~220K Savings of 220K - 82K = $138K

Return on Investment Estimated Leverage Sales (Keysight Technologies) - $13M/year

$10M HBTs, $3M SFSs

Estimated Cost of Production is Half the Estimated Sales $13/2 = $6.5M Cost of Investment Total Cost = Production + Labor = $6.5M + $82K = $6.582M

Estimated Time of Return on Investment Based on Information Provided $6.582M/$13M/yr = Year

Outline1. Problem Identification2. Design Approach3. Evaluation

4. Conclusion and Recommendations

A. Conclusion

ConclusionIdentification

Review

Executed Design

Successful Evaluation

Identify the problem

Understand problem scope

Review process details

Research information

Plan design based on information

Controlled experiments

Procured results

All Values were within target requirements

ConclusionResult

2000 Ohm/sq. 3% difference in

range 10% standard dev.

Risks and Concern

There is a run to run variation which will affect the data

Must watch out for the life cycle of the target.

Recommendation

750W power -60V constant biasing 10 mTorr Total

Pressure 40 sccm flow rate A 15% nitrogen to

argon flow 1027 sec. deposition

time

Future Works 1. Possible pre-production for HBT/SFS

2. Use product substratesa. GaAs and InP

3. Further Characterization a. Determine film composition

i. Rutherford Backscattering Spectrometry, XPS (ESCA), Auger spectroscopy for impurities

b. Thermal Coefficient of Resistancei. Variety of carefully controlled experiments.

Acknowledgements The authors would like to thank:

Nick Kiriaze Rijuta Ravichandran Steven Zhang Ricardo Castro Michael Powers Vache Harotoonian Erkin Seker

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Thank You