Lithography

Lithography Lithography Lithography comes from two Greek words : “lithos " means stone, and "graphein" means write. "writing a pattern in stone" Lithography is the process of imprinting a geometric pattern from a mask onto a thin layer of material called a resist, which is sensitive to photons, electrons or ions. Pattern transfer

Lithography Optical Lithography System

Lithography Types of Photolithography Processes Photoresists are photosensitive polymeric materials and are of two types: positive and negative. Negative: Prints a pattern that is opposite of the pattern that is on the mask. Positive: Prints a pattern that is the same as the pattern on the mask. Negative Photoresists Positive Photoresists

Lithography Negative Photoresists Areas exposed to light become polymerized and resist the develop chemical. Prints a pattern that is opposite of the pattern that is on the mask. Resulting pattern after the resist is developed.

Lithography Positive Photoresists Areas exposed to light become photosoluble. Prints a pattern that is the same as the pattern on the mask. Resulting pattern after the resist is developed.

Lithography Basic Steps of Photolithography Spin coat a thin layer of photoresist on surface. Soft bake : - Partial evaporation of photo-resist solvents - Improves adhesion, uniformity, etch resistance, linewidth control Soft Bake

Lithography Precise alignment of mask to wafer.

Lithography Exposure of photoresist. Negative resist is polymerized. Development : Removal of unpolymerized resist . Hard bake

Lithography Etch : Oxide layer is removed through opening in resist layer. Photoresist strip : remove photoresist layer from wafer. Final inspection

Lithography Radiation Sources The pursuit of submicron feature sizes has motivated extensive work in four major lithography technologies : optical lithography e-beam lithography x-ray lithography ion-beam lithography

Lithography Optical Lithography (Photolithography) UV - Mid-1950s - Typically 300~450 nm - Mercury spectrum g(436nm), h(405nm), i(365nm) - 0.5 to 3 um film thickness(thin films) - Resolution : 0.5 um

Lithography Deep UV - Mid-1980s - Typically 200-300 nm - 0.5 to 3 um film thickness - Resolution : 0.25 um - Necessary to use masks made of quartz. Two types of UV exposure sources - Mercury arc lamp - Excimer laser 13. 5 nm

Lithography Photomask Substrate High transmission at exposure wavelength Small thermal expansion coefficient High degree of flatness Low non-linear effect Common material : quartz, fused-silica or borosilicate glasses Opaque Material No transmission at exposure wavelength Good adhesion to the substrate High degree of durability Common material : Chrome, emulsion and ion oxide Chrome-on quartz masks - Formed by fused silica plates - Plates are coated with a thin chrome layer(~800A). Emulsion masks

Lithography Wafer Exposure System Three optical lithographic techniques : - Contact printing - Proximity printing - Projection printing 1:1 exposure systems

Lithography Typical contact or proximity printing system

Lithography Projection Printing

Calculating Resolution for a given , NA and k Lithography Performance of Lithographic Exposure Tool Resolution - The minimum feature size that can be transferred with high fidelity to a resist film on the surface of the wafer. Registration Calculating Resolution for a given , NA and k - A measure of how accurately pattern of successive masks can be aligned with respect to the previously define pattern on a wafer. Throughput - The number of wafers that can be exposed per hour for a given mask level.