Etching Processes for Microsystems Fabrication

Overview Micro Systems Micro Devices Etching Process Etching Parameters Qualitative Reasoning

Presented by: Arman Ur Rashid

Microsystems

Micro devices: Neural Probes 100um 10nm

Micro Devices: Microgear and Alignment Pin Substrate Gear [Courtesy of Sandia National Laboratories]

Microsystems Etch Processes Wafer Surface Below the Surface Within the substrate

Etching Process Deposition Photolithography Etch Pattern Transfer

Etch Parameters

Etch Rate= ETCH RATE

Etch Profile Isotropic Etching Anisotropic Etching Isotropic Etch Profile Anisotropic Etch Profile

Poor selectivity -> 1:1 Good selectivity -> 100:1 Ef = etch rate of the film undergoing etch Er = etch rate of the photoresist Poor selectivity -> 1:1 Good selectivity -> 100:1

Etch Process: Wet Etching Dry Etching Chemical Process Chemical or Physical Process

Overview of Dry Plasma Etch Substrate Etch process chamber Exhaust Gas delivery RF generator Cathode Anode 1) Etchant gases enter chamber Electric field l l Anisotropic etch Isotropic etch

Chemical Versus Physical Dry Plasma Etching

Down Stream Reactor

Ion Beam Etcher _ + + Screen grid Accelerating grid Hot filament emits electrons Gas inlet (Argon) To vacuum system Neutralizing filament Accelerating grid Screen grid Electromagnet improves ionization Plasma chamber (+anode repels +ions) Wafer can be tilted to control etch profile _ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Redrawn from Advanced Semiconductor Fabrication Handbook, Integrated Circuit Engineering Corp., p. 8-12. Figure 16.18

Deep Reactive-Ion Etching (DRIE): The Bosch Process The Cryogenic Process

Illustration of The Bosch Process: SF6 plasma F+ions Mask SF6 plasma SiF4 C4F8 plasma Si Substrate

Illustration of The Bosch Process(Cont): Oxide Hard mask SF6 plasma C4F8 plasma C4F8 plasma SF6 plasma Passivation in First Cycle Etch Stage in First Cycle Etch C4F8 plasma C4F8 plasma C4F8 plasma SF6 plasma SF6 plasma Second Etch Passivation Removal After 4th Cycle Etch

SEM Graph:

Characteristics of The Bosch Process: Roughness of Sidewall Aspect Ratio 90 Degree Edge

Challenge of The Bosch Process: Multiple Deposition Parameters Multiple Etching Parameters Variation of Etch Rate depth Notching Effect Si SiO2 x min Etching Notching Si SiO2 x min + overetch time

Notching in SEM Graph: Notching

Illustration of The Cryogenic process: SiF4 F+ions SF6 plasma Mask Ultra thin layer of SiO2 Si

SEM Graph:

Characteristics of The Cryogenic Process: Low Ion Energies Little Physical Etching on The Mask Surface High Selectivity. Low Sidewall Roughness High Etch Rate

Why Cryogenic Temperature: Condensation on Surfaces Spontaneous Chemical Reaction Etch Rate of the Mask Material

Challenges of The Cryogenic Process: Cracking of Masks Sensitive to Heat Path Variations Etch Rate Varies with Depth Shape and Depth Depends on Multiple Parameter Notching effect Reduction in Etch Rate Due to Aspect Ratio Low Etch Rate High Etch Rate

Comparison of The Bosch and Cryogenic Process: Sidewall Etch Rate Selectivity Cracking of Mask

Summary Etching Process Parameters Dry Plasma Etching Techniques Deep Reactive Ion Etching DRIE Procedures and Challenges