Lecture 4 Photolithography

Introduction The aim is to produce the features on the mask with the highest possible resolution using photoresist onto the wafer surface.

Photolithography Process Steps Dehydration Bake HMDS Priming and Cool Down Wafer Photoresist Coating Softbake Align and Expose Post Exposure Bake and Cool Down Wafer Photoresist Development Hardbake

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

To promote adhesion of the photoresist to the wafer surface Hexamethyldisilazane Parameters: Temperature Time Step 2: HMDS PRIME

To cool down a wafer to the ambient temperature after HMDS baking process and before resist coating process Parameters : Cool down rate Temperature uniformity Substrate temperature Step 3: Cool plate

Step 4: PHOTORESIST COATING To coat a resist onto the Si wafer for patterning with a required resist thickness and uniformity Parameters : Resist viscosity Resist dispense method Resist dispense velocity Resist dispense volume Resist dispense time Nozzle position Spin speed Spread time Acceleration / deceleration Final spin speed Final spin time Chuck diameter Chuck vacuum Controlled exhaust Ambient atmosphere (RH & Temperature) Cup ambient (RH & Temperature) Wafer centering Machine and chuck leveling Resist temperature control

Step 5: SOFTBAKE To remove a solvent from photoresist and activates photoactive compound To remove stress in the photoresist To improve photoresist adhesion Parameters Type of heat transfer (Hot plate or convection) Temperature Temperature uniformity Time Exhaust Cool down rate

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.

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 : Type of plate (hot plate or convection oven) Temperature Temperature uniformity Time Exhaust Cool down rate

Step 8: Cool plate To cool down a wafer to the ambient temperature before development process Parameters : Cool down rate Temperature uniformity Substrate temperature

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 : Development technique (Immersion/Spray/Puddle) Developer type (Metal Ion/metal ion free) Developer concentration Developer temperature Development time Carbon dioxide absorption Agitation Time between exposure and develop Time between develop and rinse Resist thickness Post exposure bake temperature Exposure energy

Step 10: Hardbake To increase the resistance of the resist To remove any residual solvent Parameters : Type of plate (conduction or convection) Temperature Temperature uniformity Time Exhaust Cool down rate

Photolithography equipment: Exposure tools

Photolithography equipment: Printing methods

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

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

Reticle Chrome Pattern Quartz Substrate Pellicle

Mask vs. Reticles Mask 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.

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

Photoresist Performance Factors Resolution 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 Pinhole count Pinhole increase –layer thickness decrease Negative resist has fewer pinholes than negative photoresist

Negative PR vs. Positive PR

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