Not sure you need the reductants suggested by Carlos. Silica/quartz dissolution is slow though, even at temperature. Do you know how thick the layer is?

To selectively etch SiO2 with regards to Si without using HF, and preferring NaOH or KOH as etchants, I would suggest hot (to boiling) diluted aq. sol. of either KOH or NaOH, possibly with some added reducing agent, such as K2SO3 or Na2SO3 ― to better prevent silicon etching, what could readily occur in oxidant solution. Heating in autoclave, while avoiding contact with air or oxidants, may be preferable. Further addition of fluoride salts (e.g. NaF, KF, NH4F) could be also considered.

Not sure you need the reductants suggested by Carlos. Silica/quartz dissolution is slow though, even at temperature. Do you know how thick the layer is?

You'll get the best selectivity at high concentrations. At 60% the selectivity is about 7:1 SiO2:Si110. The etch rate is slower at high concentrations for both materials which could give you better control of the etch.

Thank you for your answers,

Carlos : Thank you for this suggestion, I will be definitively trying these KOH or NaOH solutions.

Frank : The layer is thin, maybe 10 to 50 nm. That's why I need a realy slow etching process in order to get some control. If we are talking about µm/min, it may be too fast to attack only the SiO2 layer and not the silicon below I guess.

Ron : Thank you for this suggestion. 7:1 seems ok if I can control the speed of the etch. What about 60% KOH but at low temperature such as 25°C ?

Julien

Hello Julien

There are some tables in the appendix of following paper:

http://www.lmm.uni-saarland.de/content/stuff/lehrstuhl/publikationen/papers/JES003612.pdf

In general I've always found BYU's website very informative:

http://www.cleanroom.byu.edu/KOH.phtml

Kind regards,

Jules

Hi Jules,

Thank you for your answer,

I have read these data from "Anisotropic Etching of Crystalline Silicon in Alkaline Solutions" and it seems to indicate that it will be very hard not to etch the silicon... Etch speeds are more than 1000 times higher for Si than for SiO2, at the opposite of what Ron said previously. 

But anyway, if we use the right amount of KOH or NaOH, the etch should stop at some point right ? If I can roughly calculate the amount of SiO2 I want to etch, then it should be fine ?

You can use a simple chemical method, you have to do an immersion of your substrate in diluted 10% HF(48%) solution for 2 minutes.

This paper is quite illustrative

Anisotropic etching of silicon in KOH + Triton X‑100 for 45°

micromirror applications

The Si samples were etched in 2 M (10 wt%) potassium

hydroxide (KOH) aqueous solution containing 50 ppm (by

volume) of Triton X-100 surfactant, supplied by Sigma-

Aldrich. The KOH solution was prepared from potassium

hydroxide pellets (supplied by Chempur, Poland) and

deionized water (1 MΩ cm). During etching, the samples

were situated vertically in a Teflon holder immersed in

the etching solution of the volume 1.5 dm3, contained in a

glass vessel. The etching processes were carried out under

atmospheric pressure and at stabilized, elevated temperatures

(60–90 °C).

note :During the KOH etch,

the areas were bare Si is present (no SiO2) small hydrogen bubbles will

appear.

lower temp is better

check these tables

http://www.cleanroom.byu.edu/KOH.phtml

good luck