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Advanced Manufacturing

Choices MAE 165-265

Spring 2012, Dr. Marc MadouClass 7

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• Lithography definitions• Resist tone• Introduction to the lithography

process• Surface Preparation• Photoresist Application• Soft Bake• Align & Expose• Develop• Hard Bake• Inspection• Etch Layer or Add Layer• Resist Strip• Final Inspection• Clean- Room, Wafer Cleaning

Content•CD and Tg•Making a Mask•Moore’s ‘Law’

Photolithography -- Definitions

Photolithography is used to produce 2 1/2-D images using light sensitive photoresist and controlled exposure to light.

Microlithography is the technique used to print ultra-miniature patterns -- used primarily in the semiconductor industry.

Photolithography is at the Center of the Wafer Fabrication Process

*

Thin Films

Implant

Diffusion Etch

Test/Sort

Polish

PhotoPatterned

wafer

Photolithography -- Definitions

Resist ToneNegative: Prints a pattern that is opposite of the

pattern that is on the mask.Positive: Prints a pattern that is the same as

the pattern on the mask.

Negative Lithography

Island

silicon substrate

oxide

photoresist

Window

Areas exposed to light become polymerized and resist the develop chemical.

Resulting pattern after the resist is developed.

photoresistoxide

silicon substrate

Ultraviolet Light

Exposed area of photoresist

Shadow on photoresist

Chrome island on glass mask

Resist Tone

silicon substrate

oxide

photoresist

Positive Lithography

Island

Window

Areas exposed to light become photosoluble.

Resulting pattern after the resist is developed.

Shadow on photoresist

Exposed area of photoresist

Chrome island on glass mask

photoresist

silicon substrate

oxide

Ultraviolet Light

Resist Tone

Ten Basic Steps of

Photolithography

1. Surface Preparation2. Photoresist Application3. Soft Bake4. Align & Expose*

5. Develop6. Hard Bake7. Inspection8. Etch9. Resist Strip10. Final Inspection

* Some processes may include a Post-exposure Bake

Introduction to the Lithography Process

1. Surface Preparation (HMDS vapor prime)

• Dehydration bake in enclosed chamber with exhaust

• Clean and dry wafer surface (hydrophobic)

• Hexamethyldisilazane (HMDS)

• Temp ~ 200 - 250C• Time ~ 60 sec.

HMDS

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1. Surface Preparation (HMDS vapor prime)

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1. Surface Preparation

(HMDS vapor prime)

2. Photoresist Application• Wafer held onto

vacuum chuck• Dispense ~5ml of

photoresist• Slow spin ~ 500 rpm• Ramp up to ~ 3000 -

5000 rpm• Quality measures:

– time– speed– thickness– uniformity– particles & defects vacuum chuck

spindleto vacuum

pump

photoresist dispenser

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• Resist spinning thickness T depends on:– Spin speed– Solution concentration– Molecular weight (measured by

intrinsic viscosity)• In the equation for T, K is a calibration

constant, C the polymer concentration in grams per 100 ml solution, the intrinsic viscosity, and the number of rotations per minute (rpm)

• Once the various exponential factors (, and ) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system

2. Photoresist Application

3. Soft Bake• Partial evaporation of

photo-resist solvents• Improves adhesion• Improves uniformity• Improves etch resistance• Improves linewidth control• Optimizes light absorbance

characteristics of photoresist

4. Alignment and Exposure

• Transfers the mask image to the resist-coated wafer

• Activates photo-sensitive components of photoresist

• Quality measures: – linewidth resolution– overlay accuracy– particles & defects

UV Light Source

Mask

ResistResist

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• Alignment errors (many different types)

• Mask aligner equipment• Double sided alignment

especially important in micromachines

4. Alignment and Exposure

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4. Alignment and Exposure

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• Contact printing• Proximity printing• Self-aligned• Projection printing : R = 2bmin

= 0.6/NA

4. Alignment and Exposure

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• The defocus tolerance (DOF)

• Much bigger issue in miniaturization science than in ICs

http://www.newport.com/tutornew/optics/Optics_Reference_Guide.html

4. Alignment and Exposure

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4. Alignment and Exposure

Photolithography-DOF The defocus tolerance (DOF)

Much bigger issue in miniaturization

science than in ICsA small aperture was used to ensure the foreground stones were as sharp as the ones in the distance.

What you need here is a use a telephoto lens at its widest aperture.

Photolithography-DOF

5. Develop• Soluble areas of

photoresist are dissolved by developer chemical

• Visible patterns appear on wafer– windows– islands

• Quality measures:– line resolution– uniformity– particles & defects

to vacuum pump

vacuum chuck

spindle

developerdispenser

6. Hard Bake Evaporate remaining

photoresist Improve adhesion Higher temperature

than soft bake

7. Development Inspection

• Optical or SEM metrology • Quality issues:

– particles– defects– critical dimensions– linewidth resolution– overlay accuracy

8. Plasma Etch-Or Add Layer

• Selective removal of upper layer of wafer through windows in photoresist: subtractive

• Two basic methods:– wet acid etch– dry plasma etch

• Quality measures:– defects and particles– step height– selectivity– critical dimensions

• Adding materials (additive)• Two main techniques:

– Sputtering– evaporation

Plasma

CF4

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8. Plasma Etch-Or Add Layer

9. Photoresist Removal (strip)• No need for photoresist

following etch process• Two common methods:

– wet acid strip– dry plasma strip

• Followed by wet clean to remove remaining resist and strip byproducts

O2

Plasma

10. Final Inspection• Photoresist has been

completely removed• Pattern on wafer

matches mask pattern (positive resist)

• Quality issues:– defects– particles– step height– critical dimensions

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Clean-rooms, Wafer Cleaning

• Yellow light and low particle size/density curves

• Cleaning steps– RCA1-peroxides and NH3-

removes organics– RCA2-peroxide and HCl-

removes metals• Dry vs. wet cleaning• Supercritical cleaning-no liquid

phase

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Clean-rooms, Wafer Cleaning

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Clean-rooms, Wafer cleaning

• Yield is the reason for the clean-rooms-the smaller the features the more important the cleanroom

• In the future people will work outside the cleanroom and only wafers will be inside the clean environment

• At universities, modularity (many different materials and processes) is more important than yield

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CD and Tg

• CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process)

• Glass transition temperature, above Tg the resist picks up dirt quite readily and the profile might get degraded

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Making a Mask• Software Mask

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Moore’s ‘Law’• Observation and self

fulfilling prophecy --not a physical law

• Is it running out of steam?