Chapter 4: Wet Bulk Micromachining

Introduction

Historical Note

Silicon Crystallography

Introduction

Miller Indices

Crystal Structure of Silicon

Geometric Relationships between Some Important Planes in the Silicon Lattice

Introduction

[100]-Oriented Silicon

[110]-Oriented Silicon

Selection of [100]- or [110]-Oriented Silicon

Examples of Wet Etched Structures in Si

Silicon as Substrate

Silicon as a Mechanical Element in MEMS

Introduction

Stress-Strain Curve and Elasticity Constants

Residual Stress in Si

Yield, Tensile Strength, Hardness, and Creep

Piezoresistivity in Silicon

Bending of Thin Si Plates

Silicon as a Mechanical MEMS Material: Summary

Other Si Sensor Properties

Thermal Properties of Silicon

Silicon Optical Properties

Wet Isotropic and Anisotropic Etching

Introduction

Isotropic Etching

Usage of Isotropic Etchants

Simplified Reaction Scheme

Iso-Etch Curves

Arrhenius Plot for Isotropic Etching

Masking for Isotropic Silicon Etchants

Dopant Dependence of Silicon Isotropic Etchants

Electrochemical Isotropic Silicon Etch-Etch Stop

Preferential Etching

Problems Associated With Isotropic Etchants

Anisotropic Etching

Introduction

Anisotropic Etchants

Arrhenius Plots For Anisotropic Etching

Selected Anisotropic Etchant Systems

Overview

Potassium Hydroxide (KOH)

Ethylenediamine Pyrocatechol (EDP)

Ammonium Hydroxide-Water (AHW) and Tetramethyl Ammonium Hydroxide-Water (TMAHW)

Hydrazine

Si Surface Roughness

Masking for Anisotropic Etchants

Back-Side Protection

Etch Rate and Etch Stops

Alignment Patterns

Chemical Etching Models

Introduction

Seidel et al.’s Model

Elwenspoek et al.’s Model

Isotropic versus Anisotropic Etching of Silicon

Etching with Bias and/or Illumination of the Semiconductor

Introduction

Electropolishing and Microporous Silicon

Electropolishing

Porous Silicon

Introduction

Microporous Silicon

Macroporous Silicon

Etch-Stop Techniques

Introduction

Boron Etch Stop

Electrochemical Etch Stop

Photo-Assisted Electrochemical Etch Stop (for n-type silicon)

Photo-Induced Preferential Anodization (for p-type silicon)

Etch Stop at Thin Insoluble Films

Problems with Wet Bulk Micromachining

Introduction

Extensive Real Estate Consumption

Introduction

Real Estate Gain by Etching from the Front

Real Estate Gain by Using Silicon Fusion-Bonded Wafers

Corner Compensation

Underetching

Undercutting

Computer Simulation Software

Examples

4.1 Dissolved Wafer Process

4.2 An Electrochemical Sensor Array Measuring pH, CO₂, and O₂ in a Dual Lumen Catheter

4.3 Disposable Electrochemical Valves

4.4 Self-Aligned Vertical Mirrors and V-Grooves for a Magnetic Micro-Optical Matrix Switch

Problems