Photolithography

PEOPLE ProgramJuly 8, 2013

• Computer chips are made using photolithography

• Instead of drawing with a sharp tip, it uses light to transfer a tiny pattern from a photomask to a light-sensitive chemical

Packaging

Central Processing Unit (CPU)

How is Lithography Related to Computers?

Apple II

An early personal computer

ENIAC (Electronic Numerical Integrator and

Computer)

First general-purpose electronic computer

Apple MacBook Air

Modern laptop computer weighing

only 3 pounds!

65 years… and many advances in photolithography!

How is Lithography Related to Computers?

Photolithography• Role of the Resist:

Light exposure changes solubility and allows mask formation

• Applications: Making tiny, detailed stuff! For example, microelectronics, microfluidics, MEMS, bioanalytics, sensing, and many more

• Main Benefit: High throughput!Geissler, et al. Adv. Mater. 2004, 16, 1249-1269

Silicon wafer Transparency with desired pattern

Transferring the Pattern: Make a Mask!

Substrate that will “receive” the pattern

Mask that controls the passage of light

Transferring the Pattern to the Silicon Wafer

Silicon WaferPhotoresist

1. Coat silicon wafer with photoresist

2. Tightly place mask on top

3. Expose to UV light

4. Develop and Rinse

Depends on the type of photoresist?

Mask

Silicon wafer

Components of Conventional Photoresist

Additives: chemicals that control other aspects of the resist material

Solvent: gives the resist its flow characteristics

Sensitizers: sensitive to light; these will react when exposed to light

Resin: mix of polymers that hold the resist together; gives the resist its mechanical and chemical properties

What it mean if something can dissolve?

What does it mean if something can’t dissolve?

What could be some What could be some examples of things that examples of things that can can

dissolve dissolve in Hin H22O?O?

What could be some What could be some examples of things that examples of things that cannot dissolve cannot dissolve in Hin H22O?O?

Our definition

*Dissolving can take other forms, as well. For example a gas can dissolve into another gas, or a gas can dissolve into a liquid.

Dissolve: A solid loses its structure and becomes dispersed into a liquid.*

Examples:•Table salt (NaCl) dissolves in water.•Sand (SiO2) does not dissolve in water.•Gasoline does not dissolve in water.•A breath-strip dissolves in your saliva.

• Positive Photoresist: Exposed areas become able to dissolve in the developer (so the exposed portion is dissolved by the developer)

• Negative Photoresist: Exposed areas become not able to be dissolved in the developer (so the unexposed portion is dissolved by the developer)

– We will be using SU-8, a common negative photoresist

Positive vs. Negative Photoresist

3. Expose to UV light

4. Develop and rinse 4. Develop and rinse

6. Remove residual photoresist 6. Remove residual photoresist

5. Etch patterns into wafer 5. Etch patterns into wafer

NEGATIVE Photoresist

POSITIVE Photoresist

Positive vs. Negative Photoresist

3. Expose to UV light

4. Develop and rinse 4. Develop and rinse

6. Remove residual photoresist 6. Remove residual photoresist

5. Etch patterns into wafer 5. Etch patterns into wafer

NEGATIVE Photoresist

POSITIVE Photoresist

Make sure you get it! Fill in the blanks:In a _____ photoresist, _____ light reacts with the photoresist to make it unable to be dissolved, so it is rinsed away by the _____.

A Common Photoresist: SU-8

• “Traditional” epoxy resin negative photoresist

• UV sensitized with a cationic polymerization initiator

Lee, et al. JVSTB 1995, 13, 3012-3016; Bertsch, et al. Sens. Actuators 1999, 73, 14-23; Lin, et al. J. Micromech. Microeng. 2002, 12, 590-597; Liu, et al. Adv. Mater. 2007, 19, 3783-3790

EPON® Resin SU-8 (Shell Chemical)

• Low near-UV optical absorption high aspect ratio (>15) features

Hg “I-line”, 365 nm

Review!What is cross-linking?

Do you think that a substance that has a lot of cross-links would be more easily or less easily dissolved? Why do you think so?

Negative PhotolithographyAreas exposed to light become crosslinked and don’t dissolve in the developer chemical.

Unexposed areas remain able to be dissolved by developer chemical.

Pre-exposure- photoresist

Post-exposure- photoresist

Post-develop- photoresist

UV

OxidePhotoresist

Substrate

Crosslinks

Unexposed Exposed

Soluble

• Bucky masks have been created for you

• Glass slides have been primed and coated SU-8 photoresist

• You will be performing photolithography using these masks

PEOPLE Program 2013 Your Name

Your Mask:

Today’s Activity: Photolithography

Today’s Activity: Photolithography

8) Inspect5) Post-exposure bake

6) Develop 7) Hard bake

UV Light

Mask

4) Alignment and Exposure

Resist

2) Spin coat 3) Soft bake1) Vapor prime

Step 1: Vapor Prime

The First Step of Photolithography:

• Wafer “primed” for photoresist deposition by cleaning with isopropanol and acetone

• Promotes good photoresist-to-wafer adhesion

• Followed by dehydration bake (200 °C, 5 min)

Ensures wafer surface is clean and dry

Silicon Wafer

1.Wafer secured to vacuum chuck of spin coater

2.Dispense ~3 mL of photoresist onto wafer

3.Slow spin: 500 rpm for 15 s

4.Fast spin: 6000 rpm for 60 s

• Quality measures:– time– speed– thickness– uniformity– particles and defects

Vacuum chuck

Spindle connected to spin motor

To vacuum pump

Photoresist dispenser

Step 2: Spin Coat Photoresist on Wafer

Steps of Photoresist Spin Coating

3) Spin-off 4) Solvent evaporation

1) Resist dispense 2) Spin-up

Step 3: Soft Bake• Improves photoresist-to-

wafer adhesion

• Promotes resist uniformity on wafer

• Drives off most solvent in photoresist

• Improves linewidth control during subsequent development/etching steps

• Our soft bake procedure:– T = 95 °C on hot plate– t = 2 min– Cool to room

temperature

Silicon wafer coated with photoresist

Hot plate (T = 95 °C)

Step 4: Alignment and Exposure

• Exposure transfers the mask pattern to the photoresist-coated wafer

• Light activates the photosensitive components of photoresist

• Quality measures: – linewidth resolution– overlay accuracy– particles and

defects

UV light source

Mask

Resist

• Performed immediately after exposure

• Completes the photo reaction initiated during exposure

• Same procedure as the pre-exposure soft bake:– T = 95 °C on hot plate

– t = 2 min

– Cool to room temperature

Step 5: Post-Exposure Bake

Silicon wafer coated with exposed photoresist

Hot plate (T = 95 °C)

• Soluble areas of photoresist are dissolved by developer solvent

• Can be performed on the spin coater or simply by immersing in the developer

• Visible patterns appear on wafer: windows, islands, lines, text, images, …

• Quality measures:- line resolution- uniformity- particles and defects

Vacuum chuck

Spindle connected to spin motor

To vacuum pump

Developer dispenser

Step 6: Photoresist Development

Steps 7 and 8: Hard Bake and Inspect

• Hard bake step increases thermal, chemical, and mechanical stability of developed resist features.

• Developed resist features can be inspected (usually with magnification) to assess the quality of pattern transfer.

Summary

Silicon WaferPhotoresist

1. Coat wafer with photoresist

2. Tightly place mask on top

3. Expose to UV light

4. Develop and rinse

5. Etch patterns into wafer

6. Remove residual photoresist

Today’s Activities

Group 1

• Jake & Zahmere• Ruby & Tyrice• Roselena & Manuel• Paola & Shealyn• Terry & Mario

Group 2

• Champange & Jazmin

• Alvaro & Karen• Tashiana & Tabitha• Mariah & Zantasia• Alma & Sara

Today’s Activities

Si Wafers(Diane, Ben &

Kelly)

Photolithography

(Matt & Alfonso)

Session 1

Group 1 Group 2

Session 2

Group 2 Group 1