PowerPoint Presentation

By: Pardis Palizvan Fateme BastiMask Fabrication Process

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Contents2IntroductionBrief ViewTypes Of Substrate Fused silicaFabrication process Plasma Cleaning HMDS Application Resist Application Pre-Bake

Contents3 Lithographic Patterning Laser writing E-Beam patterning Post Bake Develop Plasma De-scum Cr Etch Resist Removal and Mask Clean Final Inspection Diffraction Effects and Solutions

IntroductionTransparentfused silicablanks covered with a pattern defined with a chrome metal-absorbing filmDesigned to optically transfer patterns to wafers or other substrates Mask sizes range from three to nine inches square7x7 for 6-inch wafers(150mm wafers) 5 x 5 x 0.060 for 4-inch wafers(100mm wafers)4 x 4 x 0.060 for 3-inch wafersThey are used at wavelengths of 365nm,248nm, and 193nmThey have also been developed for other forms of radiation such as 157nm, 13.5nm (EUV),X-ray,electrons, andions4

Brief View5

Types Of Substrate1. Quartz - Ultra Low Thermal Expansion (ULTE)2. Borosilicate Glass Low Expansion3. Soda Lime Glass - Economical Substrate

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Types Of SubstratePhotographic emulsion(Silver Halide based) on soda lime glass (cheapest)Fe2 O3 on soda lime glassCr on soda lime glassCr on quartz glass (most expensive, needed for deep UV litho)

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Mask substrate: Fused silicaOr Fused quartzisglassconsisting of silica in amorphous formhas high working and melting temperaturesThe optical and thermal properties are superior to those of other types of glass due to its purityIt has betterultraviolettransmissionIt is produced byfusing(melting) high-purity silica sand, which consists ofquartzcrystals. Quartz contains only silicon and oxygenThe benefit of using a glass based photomask is that they are much easier to clean8

Fused silica

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Step 1: Plasma Cleaning

Photomasks may have a thin invisible layer of organic contamination on the Cr surface. Thisll cause adhesion problems between the Cr and the photoresist later It also may interfere with the Cr etch process after photomask imagingAggressive Oxygen plasma treatment before starting the photomask process would be usefulThis process uses the March Asher1 tool

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Step 1: Plasma CleaningMarch Asher tool

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Step 2: HMDS Application

In order to enhance the adhesion of the photoresist to the Cr we can add HMDS to itwe bake the mask in a special oven that applies a molecular monolayer of Hexamethyldisilizane (HMDS) [(CH3)3Si]2NH to the surface

12HMDS Oven

Step 3: Resist Application There are different kinds of resists which we can use to cover CrSome kinds of them are : S1813 ,AZ1518 ,AZ TFP650Its done by Spin-Coating Process : Dispensing a small puddle of fluid on or near the center of the substrate(range from 1 to 10 cc) Low speed spinning (about 500 rpm) High speed to thin the fluid to near its final desired thickness (about 1500-6000 rpm) This step can take from 10 seconds to several minutes

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Process Trend Charts

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Manual application of S1813 photoresist to a blank photomask in the CEE spinner

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Step 4: Pre-Bake

After the resist is coated it still contains a considerable amount of solvent that needs to be driven out This step is called pre-bake because it happens before exposureAir temperature and humidity play a large role in determining coated film propertiesThere are two distinct advantages to this system: slowed drying of the fluid resist minimized susceptibility to ambient humidity variations.16

Process Trend Charts

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Bake Parameters for S1813 Photoresist and CEE spinner hotplate with mask baking under exhaust lid

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Step 5: Lithographic Patterning Laser mask writer

The pattern information is created by the researcher in a drawing package(AutoCAD or L-Edit)The data is sent to the manufacturer by a variety of methods (such as email or ftp)then processed into internal CAD format (Gerber)Then transferred to a lithography toolwhich then exposes the design onto the photomask substrate(in DXF or GDSii format)One method for making photomask is the Heidelberg Instruments DWL-66 laser mask writer19

DWL-66 Laser Mask Writer:

20system used for direct writing patterns into UV sensitive mediaThe optical part is composed of a GaNbased laser diode generating 405 nm UV light with 40 mW optical poweraccuracy over 140 by 140 mm2area and a 5 mm/s stage speedAccepts DXF, GDS-II, CIF, Gerber, STL formats

Step 5: Lithographic Patterning E-Beam patterning

Another method is Electron Beam TechniqueMain problem with optical technique: light diffractionSystem resembles a scanning electron microscope + beam blanking and computer controlled deflectionThe electron beam changes the solubility of the resist

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E-Beam patterning

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Electron-beam resistIt is often a polymer dissolved in a liquid solvent

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Electron-beam resistExample of positive resist:25PMMA Resist

Characteristics:Positive toneVery high resolution, low contrastPoor dry etch resistanceSeveral dilutions available, allowing a wide range of resist thicknessNo shelf life or film life issuesNot sensitive to white lightDeveloper mixtures can be adjusted to control contrast and profile

PMMA :

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Electron-beam resist:

Example of negative resist:27

NEB-31:

Characteristics:Negative toneVery high resolution (40 nm demonstrated), high contrastDry etch resistance comparable to most photo resistsSeveral dilutions available, allowing a wide range of resist thicknessNo shelf life issues for resist solution if stored at room temperatureFilm life issuesSensitive to white light

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NEB-31 :

Step 6: Post Bake

The second photoresist bake is called PEB One method of reducing the standing wave effectSome resists require a short bake after exposure to activate the exposureThe two photomask resists, S1813 and AZ1518 do notThe AZand TI resists distributed by MicroChemicalsdo not belong to chemically amplified resists, and therefore donotrequire a PEB for this purpose.29

Step 7: Develop Each photoresist has its own developer Here we have automatic programs set up in the CPK spin developer tools to develop our mask For optical resists, the developer chemical is basically a solution of Tetra Methyl Ammonium Hydroxide (TMAH)

Photo of the CPK Spin Develop/Etch tool

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E-beam developerPMMA Resist :For low resolution features: MIBK:IPA 1:1, 1-2 minutes. For Higher resolution features:MIBK:IPA 1:3, 1-2 minutes

P(MMA-MAA):1:1 MIBK:IPA, 1-2 minutes. (1:3 MIBK:IPA is an option, offering higher contrast, but lower sensitivity ie. higher dose.)

NEB-31:MF-321; 10 seconds / 100nm resist thickness.

EBR-9 Resist:3:1 MIBK:IPA, 4 minutes. (Note that this is not 1:3 MIBK:IPA ! )

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Step 8: Plasma De-scumIf the inspection goes well, you need to prepare for the Cr etchThe effect of this light etch is to clean out the bottom of the developed features in the resistThis can be done if you do a very light O2 de-scum etch in the March AsherMarch Etcher Descum Parameters:

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Step 9: Cr EtchThe Cr etchant is: Ceric ammonium nitrate ((NH4)2Ce(NO3)6) + Nitric acid (HNO3)Hydrochloric acid (HCl)This etch can be accomplished on the CPK developer/etcherThe actual operation is very similar to the CPK develop program in Step 7

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Step 10: Resist Removal and Mask CleanThis is done to avoid polymer-to-polymer contact at the contact print step between the old resist on the mask and the new coat of resist on a waferIt can usually be done in the solvent CPK spinnerThis process uses puddles of Acetone Finally rinses the mask with Isopropyl AlcoholMask cleaning can be done in the UltraTech Mask Washer

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Step 11: Final InspectionIt is better to find errors and poor quality at this stage than after a bad pattern that can potentially ruin a multi-level substrate Modern technologies for this purpose are automated systems that involvescanning electron microscopyand other advanced toolsSEM is a type ofelectron microscopethat produces images of a sample by scanning it with a focused beam ofelectrons35

Mask Inspection SystemMATRICS X810HiT SeriesInspection MethodMulti-Die Mode, Single-Die ModeSensitivity (minimum defect size)20nmInspection Scan Time70 minutes per 100x100mm areaMask typeCr,MoSi,OMOG,EUVMask Size6 inch

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Diffraction EffectThe central issue of COG masks is the diffraction of the light on edgesThe light will be deflected into areas which must not be exposed.The intensity of the diffracted light should be reduced.

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Diffraction Solution 1.attenuated Phase Shift Mask (AttPSM)Uses a patterned layer of molybdenum silicide (MoSi) MoSi has a thickness which causes a phase shift of the transmitted light of 180It is easier to dry etch than chrome

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Diffraction Solution 1.attenuated Phase Shift Mask (AttPSM)

The phase shifted light and the radiation which transmits through glass only interfere destructivelyThis photomasks are named tritone masksOn the one hand the light is attenuated and on the other hand the light waves which are in opposite phase39

Diffraction Solution 2. Alternating Phase Shift Mask (AltPSM)Uses trenches which are etched into the glass substrate alternating to non-etched areasThere are areas which are covered with a chrome layer to decrease the intensity of radiation in this regions

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Diffraction Solution 3.Optical Proximity CorrectionSharp features are lost because higher spatial frequencies are lost due to diffractionThe image distortions typically consist of increased corner rounding, line-end shortening, changes in linewidth,41

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Diffraction Solution 3.Optical Proximity Correction42

S1813 resist

Dispersion curve

Interference curves

Absorbance spectrum43

AZ 1518 resist

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AZ 1518 resist

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THANKS46