Building a Free Standing Structure with KOH Etching | Expert Tips

[mocaliber]

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!

 

[OhulahanBass]

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?

 

[mocaliber]

oh, sorry for my ambiguous description! The free-standing structure I meant is like this picture.

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

Thanks for your reply!

 

[OhulahanBass]

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.

 

[Mapes]

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.