Building a Free Standing Structure with KOH Etching | Expert Tips

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Discussion Overview

The discussion revolves around the fabrication of free-standing structures using KOH etching, specifically focusing on the challenges and techniques involved in etching SiO2 and silicon. Participants explore the use of KOH as an etchant and the conditions necessary for achieving desired structural outcomes.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about the process of creating a free-standing structure using KOH etching, expressing uncertainty about additional steps such as coating photoresist.
  • Another participant clarifies that KOH is typically used to etch silicon, while SiO2 or Si3N4 can serve as mask materials, and questions the specific type of free-standing structure desired.
  • A participant emphasizes that KOH cannot etch SiO2 and suggests that BOE might be a theoretical alternative, but notes significant challenges associated with deep wet etching of SiO2.
  • It is mentioned that to achieve a specific structure with KOH etching, the material must be silicon rather than SiO2, and that deep etching of SiO2 is particularly difficult.
  • Another participant supports the previous comments by stating that KOH etches crystalline silicon planes more effectively than amorphous SiO2, which lacks crystalline structure.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of using KOH for etching SiO2, with some asserting it is not possible while others discuss theoretical alternatives. The discussion remains unresolved regarding the best approach for achieving the desired free-standing structure.

Contextual Notes

Participants highlight limitations associated with etching techniques, including the challenges of maintaining mask integrity during long-term etching and the difficulty of achieving sharp angles with BOE.

mocaliber
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Hi,

Though this may be a basic technique, I would like to know how to make a free standing structure.

Most of the papers use KOH as etchant to etch SiO2 and then get such structure.

And I think besides putting SiO2 into the etchant, there should be some other things work with this fabrication. Coating the photo resist? I am not sure..

So, it would be a great help if anyone can help me solve this stupid question.

Thank you!
 
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KOH is used to etch Si, SiO2 or Si3N4 is often used as a mask material. Not sure what you want as a free standing structure. If you wanted a cantilever structure in KOH than the correct mask can achieve this.

If you want to undercut a Si structure by etching SiO2 bellow you want to use HF or a more controllable BOE (buffered HF). If you where using SOI (silicon on insulator) chips than BOE/HF is the way to go.

What wafers are you using?
 
oh, sorry for my ambiguous description! The free-standing structure I meant is like this picture.
etching.png


My problem is that how can I get such structure with KOH as etchant. And my wafer is SiO2.

Thanks for your reply!
 
If your wafer is SiO2 you CAN NOT accomplish that etch with KOH. In theory you could with BOE, but deep wet etches in SiO2 present many problems and there is not a 111 plan to etch at a sharp angle as with KOH. BOE would leave rounded edges, would under cut your mask and it is difficult to keep a mask on SiO2 while under long term BOE etches.

If you want the structure you posted from a KOH etch, the green material must be Si not SiO2. The gold material should be SiO2 or Si3N4. Remember, deep etching of SiO2 is a very hard problem, one I have been working out for months, KOH etching Si is pretty standard.
 
Just wanted to second OhulahanBass's comments. KOH etches certain crystalline planes faster than others in Si, but amorphous SiO2 has no crystalline planes, so it would be impossible to get that characteristic pyramidal shape.
 

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