Oxide etching

[Mary Tang]

Hi Houbi --

I would suggest looking into HCl.  It's similar in many ways to HF and I 
think has some etching effect on oxide, but I am not sure how much -- I 
think it is quite slow.  But if you are only removing native oxide, then 
time is less of a concern.  The nice thing about it is that it also is 
pretty effective at decontaminating.  The normal prediffusion clean 
includes a 5:1:1 water:H2O2:HCl dip -- I suspect the H2O2 might oxidize 
the silicon surface, so an additional HCl dip without H2O2 might be in 
order.

I would suggest checking in the literature -- there was a lot published 
in the 60's-70's on various cleans (most notably by "Kern" of "Vossen 
and Kern" fame published the definitive articles in the "RCA Review" in 
1970 -- the library has bound copies.)  They investigated a lot of 
different chemicals before deciding on the famous RCA clean.

If you do decide to change the clean, please submit a SpecMat request, 
so we can ensure that your process is compatible with the wet bench and 
furnace you will be using.

Thanks --

Mary

Houbi Nguyen wrote:
> Hi Mary.  Yes, I may have mentioned this to you before, but I wasn't 
> sure of my analysis plans back then either.
>
> Now, my process is to clean the native oxide off new wafers, grow 
> roughly 10 nm of oxide on the surface, then go through a very simple 
> photolithography procedure resulting in a chrome or chrome/aluminum 
> grid pattern on the surface of the wafer.  At the end of my 
> processing, I'll have a 5 x 5 mm substrate with ~10 nm of oxide 
> exposed on the surface and divided into squares, where the lines of 
> the squares are the chrome or chrome/aluminum (I try to evaporate on 
> 5-10 nm of metal using the Innotec).
>
> I typically use Tylan 1 or 2.
>
> The only requirements for my interface are a very thin layer of oxide 
> (although not as thin as native oxide) and as little fluorine as I can 
> possibly have on the surface.  I've been told that fluorine, once it 
> gets on the surface and stays on the surface, can become embedded in 
> the oxide layer grown on the surface of wafers and I would like to 
> prevent that from happening.
>
> The reason why I even care about how much fluorine is on the surface 
> is that I am conducting mass spectrometry experiments on organic 
> samples that I will later deposit onto these substrates.  Fluorine is 
> a potential label for some of the molecules I'm looking at, but the 
> instrument I'm using is so sensitive that it is giving me background 
> counts of fluorine, even in the very minute amounts coming from an HF 
> dip during pre-diffusion cleaning.  I know this because I've examined 
> an empty substrate (no sample deposition yet) and I am getting 
> fluorine counts (and counts that can vary by quite a bit from one 
> substrate to another).  I'd like to minimize these counts as much as 
> possible and right now, for lack of a better source of contamination 
> or background, I think the HF dip is responsible for the presence of 
> fluorine on the wafers.
>
> It's my understanding that the HF dip removes chemical oxides on the 
> wafer surface during pre-diffusion cleaning.  If I could find an 
> alternative to HF or any solvent normally used for cleaning, that 
> would be excellent.  Thanks for the help Mary.
> Houbi
>
>
> On Jul 8, 2008, at 8:24 AM, Mary Tang wrote:
>
>> Hi Houbi --
>>
>> I think we talked about this before, but I don't remember the 
>> outcome...   I remember you did not want any fluorine on your wafer 
>> surface.  What is on your wafer at this point?  And what is your next 
>> step in your process?  (i.e., which furnace?)  And what is critical 
>> for your interface, other than excluding fluorine?  What kind of 
>> analysis are you planning to do?
>>
>> I ask, because there are alternatives to HF -- not so much for 
>> removing native oxide, but for decontamination.  So, it would be good 
>> to get a better idea of what your objective or experimental plan is....
>>
>> Thanks,
>>
>> Mary
>>
>> Houbi Nguyen wrote:
>>> Hello everyone.  I have a question regarding native oxide etching.  
>>> Does anyone know of a method by which I can strip off native oxide 
>>> (i.e., for a pre-diffusion cleaning) without using a 
>>> fluorine-containing mixture, specifically HF?  Thanks for the help.
>>> Houbi
>>
>>
>> -- 
>> Mary X. Tang, Ph.D.
>> Stanford Nanofabrication Facility
>> CIS Room 136, Mail Code 4070
>> Stanford, CA  94305
>> (650)723-9980
>> mtang at stanford.edu
>> http://snf.stanford.edu
>>
>


-- 
Mary X. Tang, Ph.D.
Stanford Nanofabrication Facility
CIS Room 136, Mail Code 4070
Stanford, CA  94305
(650)723-9980
mtang at stanford.edu
http://snf.stanford.edu