Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Lecture 4

Photolithography

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Introduction

The aim is to produce the features on the

mask with the highest possible resolution

using photoresist onto the wafer surface.

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Photolithography Process Steps

1. Dehydration Bake

2. HMDS Priming and Cool Down Wafer

3. Photoresist Coating

4. Softbake

5. Align and Expose

6. Post Exposure Bake and Cool Down Wafer

7. Photoresist Development

8. Hardbake

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 1: Dehydration bake

• To remove moisture from the wafer surface

• Moisture on wafer surface reduces resist adhesion

• Wafer on an oxide surface will allow wet etchants to penetrate easily between the resist and the wafer surface

• Parameters : Temperature

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

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Step 2: HMDS PRIME• To promote adhesion of the photoresist to the wafer

surface• Hexamethyldisilazane• Parameters:

1. Temperature

2. Time

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 3: Cool plate• To cool down a wafer to the ambient

temperature after HMDS baking process and before resist coating process

• Parameters :

1. Cool down rate

2. Temperature uniformity

3. Substrate temperature

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 4: PHOTORESIST COATING• To coat a resist onto the Si wafer for patterning with a required resist thickness and

uniformity• Parameters :

1. Resist viscosity2. Resist dispense method3. Resist dispense velocity4. Resist dispense volume5. Resist dispense time6. Nozzle position7. Spin speed8. Spread time9. Acceleration / deceleration10. Final spin speed11. Final spin time12. Chuck diameter13. Chuck vacuum14. Controlled exhaust15. Ambient atmosphere (RH & Temperature)16. Cup ambient (RH & Temperature)17. Wafer centering18. Machine and chuck leveling19. Resist temperature control

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 5: SOFTBAKE• To remove a solvent from photoresist and activates

photoactive compound• To remove stress in the photoresist• To improve photoresist adhesion• Parameters

1. Type of heat transfer (Hot plate or convection)2. Temperature3. Temperature uniformity4. Time5. Exhaust6. Cool down rate

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Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 6: Align and exposure

• Si wafer with a layer of photoresist on its top will be aligned to the mask and exposed to the UV light.

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 7: POST EXPOSURE BAKE

– To reduce the effect of standing waves which occur during exposure

– To increase the resistance of the resist prior to the etching process

– Parameters :1. Type of plate (hot plate or convection oven)2. Temperature3. Temperature uniformity4. Time5. Exhaust6. Cool down rate

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 8: Cool plate• To cool down a wafer to the ambient

temperature before development process

• Parameters :

1. Cool down rate

2. Temperature uniformity

3. Substrate temperature

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 9: Development process– To remove area of resists that have been exposed/unexposed

to form a pattern– The development rate for positive resist much less than

negative resist– Parameters :

1. Development technique (Immersion/Spray/Puddle)2. Developer type (Metal Ion/metal ion free)3. Developer concentration4. Developer temperature5. Development time6. Carbon dioxide absorption7. Agitation8. Time between exposure and develop9. Time between develop and rinse10.Resist thickness11.Post exposure bake temperature12.Exposure energy

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Step 10: Hardbake– To increase the resistance of the resist– To remove any residual solvent– Parameters :

1. Type of plate (conduction or convection)

2. Temperature

3. Temperature uniformity

4. Time

5. Exhaust

6. Cool down rate

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Photolithography equipment: Exposure tools

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Photolithography equipment: Printing methods

Taklimat UniMAP

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Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

MASK

– A glass plate with a thin film of chromium

– Contain complete pattern for the whole wafer

– Same feature size on the wafer

– The pattern is transferred by using mask aligner

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Taklimat UniMAP

Universiti Malaysia Perlis

WAFER FABRICATION

Hasnizah Aris, 2008

Taklimat UniMAP

Universiti Malaysia Perlis

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RETICLE– A glass plate with a thin film of

chromium

– Contains multiple image fields (two or three) and each field contains numerous die patterned

– Images on the reticles are protected by a pellicle

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Reticle

Chrome Pattern

Quartz Substrate

Pellicle

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Mask vs. ReticlesMask Reticles

Chrome glass has an image that covers the entire wafer

Chrome glass image covers only a part of the wafer

A mask normally transfers the image to the wafer surface in 1:1 ratio

A reticle has a larger image and feature size than image it projects on the wafer surface (usually with 4:1. 5:1 or 10:1 reduction ratios)

Exposure systems such as projection printers, proximity printers, and contact printers

Exposure systems need to expose several times to cover the whole wafer. The step and repeat processes need an exposure system called steppers.

Taklimat UniMAP

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Photoresist Components

Polymer• Phenol-formaldehyde• React when exposed to energy (Photosolublization)• Dark

Solvent• Vehide for polymers and sensitizers• Ethoxyethylacetate & methoxyethylacetate• -Determine the thickness of resist

Sensitizers• Control / modify chemical reaction

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Universiti Malaysia Perlis

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Photoresist Performance FactorsResolution• Smallest opening can be resolved in photoresistlayer• Positive resist > negative resist

Adhesion• Ability to adhere to the variety of surface• Negative resist > positive resist

Exposure Speed• The speed with which resist react to exposure• Negative resist > positive resist

Pinhole count• Pinhole increase –layer thickness decrease• Negative resist has fewer pinholes than negative photoresist

Taklimat UniMAP

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Negative PR vs. Positive PR

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Storage and Handling of Photoresist

– Photoresist should be contained in the dark bottle

– Photoresist should be operated in the yellow or gold lighting area

– Photoresist must be stored under constant temperature condition

– Photoresist bottles should be tightly closed– Photoresist should be filtered before use