Bosch DRIE Aspect Ratio Etch Calculator
MEMS & Semiconductor Micromachining: Model Bosch process Deep Reactive Ion Etching (DRIE), aspect-ratio-dependent etch lag (ARDE), scallop depth, and mask selectivity.
Feature Geometry & Cycle Timing
Etch Rate, Scallop & Process Time
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Bosch Deep Reactive Ion Etching (DRIE) Mechanics
Invented and patented by Robert Bosch GmbH in 1996, the Bosch time-multiplexed process enables vertical, high-aspect-ratio (up to 50:1 and beyond) etching of bulk silicon for MEMS sensors, gyroscopes, microfluidic channels, and 3D Through-Silicon Vias (TSVs).
1. Aspect Ratio Dependent Etching (ARDE) Model
The local silicon etch rate as a function of depth $z$ and aspect ratio $AR = z / W$ is modeled by:
ER(z) = ER_0 / (1 + β · (z / W))
where $\beta$ is the radical transport resistance coefficient (typically 0.05 to 0.12).
Frequently Asked Questions
What causes Aspect Ratio Dependent Etching (ARDE / RIE Lag) in the Bosch process?
As microstructures become deeper and narrower, neutral reactive radicals (fluorine atoms from SF6) must navigate deep trenches via Knudsen diffusion with numerous wall collisions. Depletion of radicals at the trench bottom causes the local etch rate to drop dramatically compared to open, wide features.
What is the origin of sidewall scalloping in Bosch DRIE?
The Bosch process alternates between an isotropic chemical etching cycle (SF6 plasma) and a conformal passivation cycle (C4F8 fluorocarbon polymer deposition). Directional ion bombardment removes the polymer from horizontal trench floors while leaving sidewalls protected, creating distinct wavy scalloped ripples corresponding to each cycle.
How can sidewall scallop depth be minimized?
Scallop height is directly proportional to the duration of the etch step. Ultra-fast gas switching (sub-second cycles ~0.5 to 1.5 s), ramped process parameters (decreasing etch times as the trench deepens), and cryogenic DRIE (-110°C) eliminate or drastically reduce scallops below 20 nm.