Research Request
"Can you quote 25 pieces of 4-inch DSP (100) silicon with 100 nm low-stress silicon nitride? Please also quote 25 undoped bare DSP (100) silicon wafers and the same wafers with a 100 nm low-stress silicon nitride coating."
Undoped Silicon Wafers for Anisotropic Etching
Undoped silicon wafers are commonly used for anisotropic etching because they provide predictable etch rates and excellent dimensional control. Researchers and semiconductor manufacturers frequently choose undoped (100) silicon wafers with double-side-polished (DSP) surfaces for MEMS fabrication, microfluidic devices, pressure sensors, and silicon micromachining.
The following request was submitted by a semiconductor facilities manager.
Typical Wafer Specifications
- Material: Undoped Silicon
- Diameter: 4 inch (100 mm)
- Orientation: (100)
- Surface Finish: Double-Side Polished (DSP)
- Coating: Optional 100 nm Low-Stress Silicon Nitride
- Application: KOH & TMAH Anisotropic Etching
- Reference: #123402
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Why Boron-Doped Silicon Can Affect Anisotropic Etching
Anisotropic etching is commonly used in MEMS, microfluidics, sensors, and silicon micromachining to create precise cavities, trenches, membranes, and angled sidewalls. Etchants such as KOH and TMAH remove silicon at different rates depending on crystal orientation, doping level, temperature, and masking material.
Boron-doped silicon can create challenges during anisotropic etching because heavily doped p-type regions may etch much more slowly than lightly doped or undoped silicon wafers. This behavior is useful when an intentional etch-stop layer is needed, but it can cause problems when the goal is a uniform etch through the wafer.
Common Issues with Boron-Doped Silicon
- Reduced etch rate: Heavy boron doping can slow the etch rate in KOH or TMAH, making depth control more difficult.
- Etch-stop behavior: Highly doped p++ silicon can act as an etch stop layer in MEMS and silicon micromachining.
- Surface roughness: Differences in dopant concentration can affect etched surface quality and uniformity.
- Crystal-orientation effects: Silicon {111} planes etch more slowly than {100} planes, creating the angled sidewalls used in anisotropic etching.
Why Undoped Silicon Is Preferred for Anisotropic Etching
For many KOH and TMAH etching processes, researchers choose undoped or lightly doped silicon because it provides more predictable etch behavior. This is especially important when fabricating MEMS devices, microchannels, cavities, cantilevers, membranes, and through-wafer structures.
Benefits of Undoped Silicon Wafers
- Uniform etch rates: Undoped silicon helps produce consistent etch depths across the wafer.
- Cleaner surface finish: Fewer dopant-related variations can lead to smoother etched surfaces.
- No unintended etch stop: The etch can proceed more continuously without heavily doped regions blocking progress.
- Better process control: Etch time, temperature, and mask design are easier to optimize.
- Suitable for MEMS: Undoped silicon wafers are widely used for precise silicon micromachining.
Best Wafer Options for Anisotropic Etching
For anisotropic etching, customers often request double-side-polished silicon wafers with a (100) orientation. When a mask layer is required, low-stress silicon nitride is commonly used because it can withstand wet etching processes better than many standard films.
UniversityWafer supplies undoped silicon, DSP silicon, low-stress nitride coated wafers, and custom wafers for KOH etching, TMAH etching, MEMS fabrication, and other anisotropic etching applications.