For the question "Can the fabrication method of SiO2 make such a big
difference in etching rate?" the answer is "yes, it can make an enormous
difference."
Some methods of obtaining a layer of SiO2 are sputtering, thermal growth, ,
LPCVD with annealing, LPCVD without annealing, and PECVD.
These are listed in order of slowest-etching to fastest etching.
In addition to deposition method, the addition of other oxides such as
phosphorous oxide (as in PSG) or boron oxide (as in BSG) can increase or
decrease the etch rate.
Annealing (e.g., at 1000 C for 1 hour) can decrease the etch rate of CVD
oxides.
For tables of hundreds of measured etch rates, along with documentation of
preparation methods, see two papers from JMEMS:
http://microlab.berkeley.edu/labmanual/chap1/JMEMSEtchRates1(1996).pdf
http://microlab.berkeley.edu/labmanual/chap1/JMEMSEtchRates2(2003).pdf
    --Kirt Williams

----- Original Message -----
From: "Xiaoning Wang" 
To: 
Sent: Saturday, November 17, 2007 9:36 AM
Subject: [mems-talk] Another question about silicon dioxide wet etching


> Hello everyone!
>
> I am a undergraduate student in Peking University, China.
>
> I have been doing some experiments on silicon dioxide sacrificial layer
> etching lately. And I used 4 parts NH4F (40%), 2 parts glycerin and 1 part
> HF(40%) as the etchant. I noticed that the etching rate of silicon dioxide
> was about 5000A/min at room temperature, far above those rates reported in
> literatures even without heating. Can the fabrication method of SiO2 make
> such a big difference in etching rate? Or is there any other possible
> explanations for that?
>
> Also, I found that Aluminum's etching rate was lower when the etchant
> solution's temperature increased. Does that make any sense? Since normally
> the etching rate should go up when heated, right? I tried to explain this
> from a chemical kinetics' point of view, but was limited because of my
> major
> (electrical engineering). So was that I screwed up my experiments? (though
> I
> hope it's not likely) Or would anyone can tell me the reason, please?
>
> Thank you for your time!
>
> Best wishes!
>
> btw, are there any differences between glycerin and glycerol? Their
> Chinese meanings are exactly the same.
>