Photolithography PEOPLE Program July 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,

Silicon waferTransparency 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 Wafer Photoresist 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 examples of things that can dissolve in H 2 O? What could be some examples of things that cannot dissolve in H 2 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 (SiO 2 ) 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 6. Remove residual photoresist 5. Etch patterns into wafer NEGATIVE Photoresist POSITIVE Photoresist Positive vs. Negative Photoresist

3. Expose to UV light 4. Develop and rinse 6. Remove residual photoresist 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, ; Bertsch, et al. Sens. Actuators 1999, 73, 14-23; Lin, et al. J. Micromech. Microeng. 2002, 12, ; Liu, et al. Adv. Mater. 2007, 19, 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 Photolithography Areas 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 Oxide Photoresist 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

8) Inspect5) Post-exposure bake 6) Develop7) Hard bake UV Light Mask 4) Alignment and Exposure Resist 2) Spin coat 3) Soft bake 1) 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-off4) 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 Wafer Photoresist 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 1Group 1Group 2 Session 2Group 2Group 1

SU-8 Exposure Procedure Spin Coating:15 sec rpm 60 sec rpm 15 sec rpm Soft Bake: 2 min (95 °C) Exposure: 3 min Post Bake: 2 min (95 °C) Development: 1 min (immersion) Hard Bake: 1 min (95 °C)