ENGR 494C and 594C: Microsystems Principles
• Photolithography is an optical means for transferring
patterns onto a substrate.
It is essentially the same process
that is used in lithographic printing.
• Patterns are first transferred to an imagable photoresist
• Photoresist is a liquid film
that can be spread out onto a
substrate, exposed with a desired pattern, and developed
into a selectively placed layer for subsequent processing.
• Photolithography is a binary pattern transfer:
there is no
gray-scale, color, nor depth to the image.
Key Historical Events in Photolithography
Joseph Nicephore Niepce, in Chalon, France, takes
the first photograph using bitumen of Judea on a pewter
plate, developed using oil of lavender and mineral spiri
William Henry Fox Talbot, in England, develops
dichromated gelatin, patented in Britain in 1852.
Louis Minsk of Eastman Kodak developed the first
synthetic photopolymer, poly(vinyl cinnamate), the basis
of the first negative photoresists.
Otto Suess of Kalle Div. of Hoechst AG, developed
the first diazoquinone-based positive photoresist.
Louis Plambeck, Jr., of Du Pont, develops the
Dycryl polymeric letterpress plate.
Overview of the Photolithography Process
• Surface Preparation
• Coating (Spin Casting)
• Pre-Bake (Soft Bake)
• Post-Bake (Hard Bake)
• Processing Using the Photoresist as a Masking Film
• Post Processing Cleaning (Ashing)
Wafer Cleaning - 1
• Typical contaminants that must be removed prior to
• dust from scribing or cleaving (minimized by laser scribing)
• atmospheric dust (minimized by good clean room practice)
• abrasive particles (from lapping or CMP)
• lint from wipers (minimized by using lint-free wipers)
• photoresist residue from previous photolithography (minimized by
performing oxygen plasma ashing)
• bacteria (minimized by good DI water system)
• films from other sources:
– solvent residue
– photoresist or developer residue
Wafer Cleaning - 2
• Standard degrease:
– 2-5 min. soak in acetone with ultrasonic agitation
– 2-5 min. soak in methanol with ultrasonic agitation
– 2-5 min. soak in DI H
O with ultrasonic agitation
– 30 sec. rinse under free flowing DI H
– spin rinse dry for wafers; N
blow off dry for tools and chucks
• For particularly troublesome grease, oil, or wax stains:
– Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or
trichloroethylene (TCE) with ultrasonic agitation prior to acetone
– TCE is carcinogenic; 1,1,1-TCA is less so
– acetone is flammable
– methanol is toxic by skin adsorption