Specific Process Knowledge/Etch/KOH Etch/ProcessInfo

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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 and Si Etch 02: KOH The QC procedure for Si Etch: 03 QC Recipe:   Solution 28 wt% KOH Temperature 80°C Time 90 min Elevator On Substrate Si (100) Masking Thermal SiO2 QC limits Si Etch 1 Si Etch 2 Si Etch 3 Etch rate in Si(100) Latest QC Data on Si Etch 1 Info page (under Documents) Latest QC Data on Si Etch 2 Info page (under Documents) Latest QC Data on Si Etch 3 Info page (under Documents) Roughness Only evaluated visually Only evaluated visually Only evaluated visually Nonuniformity < 3% < 3% < 3%

Mixing KOH

Nanolabs standard solution in the KOH baths has a concentration of 28 wt% and is mixed of 1 liter of a premade 50wt% KOH solution to 1.2 l of water.

For 1 l of water you can calculate the required amount of KOH pills (X) to achieve the desired concentration in wt% (Y) by:

If you use the 50wt% premade KOH solution you can calculate the amount of solution X you need to add 1 liter of water to achieve a concentration of Y wt% by:

${\frac {(0.5\cdot 1.509g/ml\cdot X)}{(X\cdot 1.509g/ml+1000g)}}=Y$

$X={\frac {(Y\cdot 1000g)}{(1.509g/ml\cdot (0.5-Y))}}$

where 1.509 g/ml is the density of a 50wt% solution KOH at 20 °C.

So for a 10 wt% solution you will need 165 ml of KOH (50wt%) to 1 l of water.

Backside protection

It is possible to protect the backside of the wafer against the KOH etch. Two different solutions have been used:

• Using special wafer holder. File:AMMT PI WETandem.pdf

• KOH Backside Protection
• 
  Tandem4 wafer holder.
• 
  Tandem4 holder with wafer.

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)