Lecture 4 Fundamentals of Multiscale Fabrication Multiscale fabrication III: Silicon surface micromachining, lithography Kahp-Yang Suh Associate Professor SNU MAE sky4u@snu.ac.kr

Surface Micromachining Conventional Silicon Technology Material Deposition Etching Lithography

Bulk vs. Surface Micromachining Typical surface micromachining Bulk + surface

Surface Micromachining

Advantages and limitations

Thin Film Deposition (1) The transformation of vapors into solids, frequently used to grow solid thin film and powder materials Physical Vapor Deposition (PVD) Direct impingement of particles on the hot substrate surface Electron-beam Evaporation, Sputtering Chemical Vapor Deposition (CVD) Convective heat and mass transfer as well as diffusion with chemical reactions at the substrate surfaces More complex process than PVD More effective in terms of the rate of growth and the quality of deposition LP/AP CVD, Thermal/PE/Ph/La CVD

Thin Film Deposition (2) Type Materials Physics Considerations Thickness Time Uniformity Cost Damage Contamination Adhesion Material Choice Spin-on Organic Cathode Sputtering Metals Plasma Anode Vapor Chemical Vapor Deposition Silicon Compounds Heated Plate Doping Conductors

Step Coverage Profile (1) A: Rapid surface migration process (before reaction), yielding uniform coverage since reactants adsorb and move, then react B: Long mean free path process and no surface migration, with reactant molecule arrival angle determined location on features (local “field of view” effects are important) C: Short mean free path process with no surface migration, yielding nonconformal coating It is favorable to performed in high temperature, low pressure but cost problem should be considered.

Step Coverage Profile (2) Key Parameters Mean Free Path Surface Migration Energy ( E ∝ Temperature) Arrival angle For conformal step coverage α < l (mean free path) where α = arctan (w/z) High Surface Mobility Process tendency A: Low Pressure CVD B: Plasma Enhanced CVD Evaporated & Sputtered Metal

Step Coverage Profile (3) Step coverage profile example Good Bad

Step Coverage Profile (4) Mean free path Electron’s & ion Mean Free Path ( Considering mean speed of mutual Approach in Ion Mean Free Path) ( Unless T is extremely high , p is the main determinant of l ) Knudsen number :

Photolithography

Overview

General photoresist Positive resist is suppressing-melt type: Lighted parts melt Negative resist is linked-structure type: Lighted parts left

Procedure

Positive PR (1) Poly(methyl methacrylate) or PMMA

Positive PR (2)

Negative PR (1)

Special photoresist

Lithography process (1)

Lithography process (2)

Lithography process (3)

Resolution To get higher Resolution (small R) l Resolution(R) = K1 NA 1. increase NA 2. shorten wavelength  3. decrease K1 To get higher Depth of Focus 1. decrease NA 2. lengthen  3. increase K2 l Resolution(R) = K1 NA l DOF = K2 NA2 Here,  is wavelangeth of light, NA is numerical aperture of lens, K1 and K2 are proportional constant of resist process NA = D/2f (D: diameter of the lens, f: focal length)

Projection: Key Parameters

Photomask

Mask to wafer alignment

Exposure technique

Exposure source

Problems of photolithography (1)

Problems of photolithography (2)

Problems of photolithography (3)

Lift-off process