I am looking for a polished silicon wafer for microfluidic microfabrication. Essentially all I need is an ultra-smooth surface that can be used with heat and pressure to press thermoplastics against a mold in order to create as smooth a surface on the thermoplastic as possible. Ideally it would also be ~1mm in thickness to minimize any breaking during the process.
Would you be able to recommend me a type of wafer that would be sufficient for this, and its associated cost depdning on the size?
Substrates for Microfluidic Microfabrication
What Polished Silicon Wafer can be used for Microfluidic Microfabrication?
A PhD in microfluidics requested help with the following project.
UniversityWafer, Inc. Answered:
If you are doing micro-fluidic devices then presumably you etch in silicon wafers various channels, 
membranes, orifices, cantilevers, etc. The properties of the required wafers, depend on the etching process used.
Here are some parameters that you should consider and specify:
- Standard wafer diameter is 76.2±0.5mm - I you must have 75mm diameter then specify diameter and tolerance.
- Specify wafer thickness - 380±25µm is standard
- Specify Silicon dopant and resistivity - If you specify that the wafers be "undoped" that implies that they are of very high purity and resistivity (Resistivity is a measure of purity - the higher the resistivity, the purer the silicon, the higher the cost) - so do not do that. Generally, you should specify Resistivity as (1-100)Ohmcm. If your etching process depends on electro-chemical properties of the Silicon then you need to specify Boron doped Silicon (p-type) or Phosphorus doped Silicon (n-type). If your etching process is not sensitive to Silicon conductivity type then state "Doping immaterial" rather than "Undoped".
- If you use an etching process which which depends on p-type to n-type transition as the etch stop, then you may want to specify Silicon Epi wafers where the substrate is p-type and the Epi layer is n-type (or vice-versa). Epi layer can be anywhere form 5 to 150µm thick.
- If you use anisotropic etch (as is most common) then you need to specify Crystallographic orientation of the silicon wafer surface - (100) orientation is most commonly used, but you may need (110) orientation wafers for etching deep trenches with perpendicular walls.
- Generally Single-side polished wafers are adequate (but double-side-polished wafers are also available, if you intend to use both sides of the Silicon wafer). Surface roughness is normally about 1nm on rms basis. This is a result of the CMP polishing process used and is not measured so you should not attempt to specify it. Total Thickness Variation (TTV) relates to how well one can focus on the wafers and so how fine details you can etch. TTV<10µm is standard, TTV<5µm is extra quality, TTV<1µm is possible but very expensive. TTV relates to wafer thickness and presupposes that the wafer is used clamped to a flat reference surface (as is normal). If wafer is used "free-standing", so it rests on a flat reference surface touching it at just 3 points, then the Bow and Warp parameters are significant. For 3"Ø wafers Warp<30µm is standard (Bow is generally half of Warp although they are normally specified as the same, for example Bow/Warp<30µm), anything less incurs extra cost.
- Silicon is normally crystallized by the CZ process. This results in dissolved Oxygen content of about 20ppma. Some etch processes are sensitive to Oxygen content. In that case you may consider using FZ crystallized Silicon with dissolved Oxygen < 1ppma, but FZ Silicon is more expensive.
There are many other things that one can specify about Silicon wafers, but fhr MEMS and micro-fluidics above are probably most significant.
| Item | Material | Ori | Diam (mm) | Thick (μm) |
Pol | ResΩcm | Comment |
|---|---|---|---|---|---|---|---|
| 6413 | P/B | [100] | 3" | 380 | SSP | 5-10 | SEMI Prime, 2Flats, Empak cst, TTV<5μm |
Reference # 240748 for specs and pricing.