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Advanced Manufacturing Choices

MAE 165-265 Spring 2012, Dr. Marc Madou Class 7

• 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 4/28/2020

Content

•CD and T g •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 -- Definitions

Thin Films Polish Patterned wafer Diffusion Photo Test/Sort Implant Photolithography is at the Center of the Wafer

*

Fabrication Process Etch

Resist Tone

Negative:

Prints a pattern that is opposite pattern that is on the mask.

of the

Positive:

on the mask.

Prints a pattern that is the same as the pattern

Chrome island on glass mask Shadow on photoresist

Resist Tone

Ultraviolet Light photoresist oxide silicon substrate

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

Island Exposed area of photoresist Window photoresist oxide silicon substrate

Resulting pattern after the resist is developed.

Negative Lithography

Resist Tone

Chrome island on glass mask Exposed area of photoresist Ultraviolet Light photoresist oxide silicon substrate Shadow on photoresist

Areas exposed to light become photosoluble.

Island Window photoresist oxide silicon substrate

Resulting pattern after the resist is developed.

Positive Lithography

Introduction to the Lithography Process Ten Basic Steps of Photolithography

1. Surface Preparation 2. Photoresist Application 3. Soft Bake 4. Align & Expose * 5. Develop 6. Hard Bake 7. Inspection 8. Etch 9. Resist Strip 10. Final Inspection

* Some processes may include a Post-exposure Bake

• Dehydration bake in enclosed chamber with exhaust • Clean and dry wafer surface (hydrophobic) • Hexamethyldisilazan e (HMDS)

1. Surface Preparation

• Temp ~ 200 - 250 °

(HMDS vapor prime)

C

HMDS

• Time ~ 60 sec.

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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 photoresist dispenser vacuum chuck to vacuum pump spindle

2.

Photoresist Application • 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, h the intrinsic viscosity, and w the number of rotations ( per minute (rpm) • Once the various exponential factors a , b and g ) 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 4/28/2020

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

UV Light Source

• Transfers the mask image to the resist coated wafer • Activates photo sensitive components of photoresist • Quality measures: – linewidth resolution – overlay accuracy – particles & defects l

Mask Resist

4. Alignment and Exposure

• Alignment errors (many different types) • Mask aligner equipment • Double sided alignment especially important in micromachines 4/28/2020

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4. Alignment and Exposure

4. Alignment and Exposure

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

min 0.6

l

/NA =

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4.

Alignment and Exposure • The defocus tolerance (DOF) • Much bigger issue in miniaturization science than in ICs 4/28/2020 http://www.newport.com/tutornew/optics/ Optics_Reference_Guide.html

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4. Alignment and Exposure

Photolithography-DOF

 The defocus tolerance (DOF)  Much bigger issue in miniaturization science than in ICs

A 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 developer dispenser vacuum chuck spindle

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

CF 4

• 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

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8. Plasma Etch-Or Add Layer

9. Photoresist Removal

• No need for photoresist following etch process

O 2

(strip)

• Two common methods: – wet acid strip – dry plasma strip • Followed by wet clean to remove remaining resist and strip byproducts

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

Clean-rooms, Wafer Cleaning

• Yellow light and low particle size/density curves • Cleaning steps – RCA1-peroxides and NH 3 removes organics – RCA2-peroxide and HCl removes metals • Dry vs. wet cleaning • Supercritical cleaning-no liquid phase 4/28/2020

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Clean-rooms, Wafer Cleaning

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 4/28/2020

• CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process) • Glass transition temperature, above T g the resist picks up dirt quite readily and the profile might get degraded

CD and T g

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• Software Mask

Making a Mask

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• Observation and self fulfilling prophecy --not a physical law • Is it running out of steam?

Moore’s ‘Law’

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