Specific Process Knowledge/Etch/KOH Etch/ProcessInfo

QC

Quality Control (QC) for the KOH Si etching baths.

Quality Control (QC) for Si Etch 01, and Si Etch 02

The QC procedure for Si Etch: 01 The QC procedure for Si Etch: 01 The newest QC data for KOH2 The newest QC data for KOH3 QC Recipe:   Solution 28 wt% KOH Temperature 80°C Time 90 min Substrate Si (100) Masking No masking QC limits Si Etch 01 Si Etch 02 Etch rate in Si(100) 1.3 ± 0.1 µm/min 1.29 ± 0.06 µm/min Roughness not measured not measured Nonuniformity < 3% < 3%

Theory

Definition of structures

Due to the almost inert (111)-planes it is possible by KOH etching to realize high aspect ratio structures in sigle crytalline silicon using the (111)-planes as sidewalls. In Si(100) these sidewalls are inclined - 54.7^o with respect to the (100) surface - whereas in Si(110) the sidewalls are vertical (see figures below).

• Anisotropic wet silicon etch: dependency on crystal orientation
• 
  Etched profile when etching Si(100).
• 
  Etched profile when etching Si(110).

For Si(100), the relation between the width of the bottom of the etched groove (W_b) and the width of the opening (W_o) at the wafer surface in a groove etched to the depth l is given by:

$W_{b}=W_{o}-2lcot(54.7^{o})=W_{o}-{\sqrt {2}}l$

Definition of <110> alignment structures

The etch rate dependence on the crystallographic planes can be used to determine the <110> crystal directions with high precision (better than +/- 0.05 ^o). A fast method for doing this, using the symmetric under-etching behavior around but not at the <110>-directions, was described by Vangbo and Bäcklund in J. Micromech. Microeng.6 (1996), 279-284. High-precision control of the <110>-direction during alignment can be necessary in order to control the dimensions of KOH-etched structures (e.g. precise control of V-groove dimensions). A dedicated mask (MASK NAME) has been designed for this purpose.

Etch rates: Empirical formula (Seidl et al)

The following empirical formula can be used for concentrations in the range of 10-60 wt%:

R = k₀ [H₂O]⁴ [KOH]^0.25 e^-E_a/kT,

where k₀ = 2480 µm/hr (mol/l)^-4.25, E_a = 0.595 eV for Si(100)

and k₀ = 4500 µm/hr (mol/l)^-4.25, E_a = 0.60 eV for Si(110)