request to use PDMS in P5000/gryphon

Mary Tang mtang at stanford.edu
Wed Jan 4 15:37:40 PST 2006


Hi Alissa --

I'd be reluctant to approve it,  basing the call on my often-faulty 
memory.  I'm poring through my files, but can't seem to find the info 
that I thought I'd had.  I've emailed Claudia, on the off-chance she was 
the person who provided the info.  I'm a little concerned that I might 
be actually thinking of one of the other silicone elastomers which are 
made for electronic purposes (there are lots of them -- even UV curable 
varieties)...  Silicone elastomers are potting materials -- some are 
used in intimate contact with die, and so are ion-free.  Check out the 
Dow website at:  http://www.dowcorning.com/content/etronics.   I would 
suggest calling up Dow Corning and asking them (I can't seem to retrieve 
datasheets - I get an error message.)

As for PDMS -- yes, a do's-and-don't's for PDMS has been in the works 
for ages.  I'm hoping to post it some day (a how-to-use-PDMS in our 
lab).  It's my favorite stuff.  There are actually a couple of websites 
that have already done a really good job of this -- just do a google 
search and you'll see for yourself.

By the way, I think PDMS decomposes at high temperature.  For the 
monomer, it's about 150C, according to the MSDS (you get formaldehyde.)  
Intuitively, it seems to me that the polymer would have a higher 
decomposition temperature, but I don't know what it is.  This could be a 
concern for the P5000 etch, if substrate surface temperatures get high 
(for thick PDMS).

My suggestion is that we hold off on approving things, just yet -- let's 
see what we can do in terms of finding purity information for PDMS or 
other silicone elastomers.  Meanwhile, I would suggest building into 
your project plan (if PDMS or other silicone elastomer is approved for 
use in the P5000 metal etch chamber) some considerable margin to 
accomodate plasma etch characterization work.

Mary

Alissa M. Fitzgerald wrote:

>Hi Mary, Jim,
>
>Thanks for the info.  Based on your information, and Jim's most recent
>email, does this mean it is approved for use in the P5000?  (Also, as a side
>note, based on the number of inquiries I get regarding PDMS, SpecMat may
>want to consider and publicize a general policy with regards to this
>material.  I think it's an important material that is gaining popularity in
>MEMS, esp. with regards to medical and biotech applications.)
>
>The process is aggressive and experimental.  Honestly, I am not sure this it
>is going to work, but the customer is interested in trying it out. We may
>need to start with different PDMS thickness, bigger CD's, etc. 
>
>We will need permission to put PDMS in the gryphon, too.  We need to use
>aluminum.
>
>Regards,
>Alissa
>
>  
>
>>-----Original Message-----
>>From: Mary Tang [mailto:mtang at stanford.edu] 
>>Sent: Wednesday, January 04, 2006 7:47 AM
>>To: Jim McVittie
>>Cc: Alissa M. Fitzgerald; SpecMat at snf.stanford.edu
>>Subject: Re: request to use PDMS in P5000
>>
>>Hi Alissa, Jim --
>>
>>I think I have this information somewhere, from a previous 
>>request.  As I vageuly recall, PDMS (Dow Corning Sylgard 182 
>>or 184) is based on a Pt-catalyzed reaction, although very 
>>little Pt is actually present.  
>>Other metals and impurities, such as sulfur, will prevent 
>>polymerization.  So, other than the Pt, PDMS is actually 
>>pretty clean -- although perhaps not by electronics-grade 
>>standards, it's cleaner than your ordinary plastics.  I'll 
>>see if I can find the info.  I think that Claudia Richter 
>>provided it, so I'll also check with her.
>>
>>Just on a side note, I'm personally less concerned about the 
>>potential contamination than the process flow itself (Alissa, 
>>perhaps you've got experience or references on this already.) 
>> 500 microns of PDMS is pretty thick...  It's got a high 
>>thermal expansion coefficient, so I'm not entirely sure that 
>>you could put 0.5 microns of Al on it without having it peel 
>>off due to stress differences, even with an adhesion 
>>layer (although having thin lines might help).   I think Claudia or 
>>Neville Mehenti may have experience in depositing metals on 
>>PDMS in our lab (although I'm pretty sure they would have 
>>used metalica or
>>innotec.)  By the way, does your request entail using gryphon 
>>for Al deposition?
>>
>>Also, PDMS is a darn good insulator -- I think the Al etch 
>>rates and profiles may be very different than they would be 
>>on silicon due to differences in plasma behavior (at least, I 
>>understand that P5000 etching of films on quartz is very 
>>different from etching on silicon.) I would suggest that if 
>>you have problems, a thinner PDMS layer (tens of microns -- 
>>you may have to dilute and spin coat) might help.  
>>Constrained PDMS (by adhesion at the Si/PDMS interface) won't 
>>expand as much and electronic effects on plasma *might* be reduced.
>>
>>Again, I'll if I still have the purity info, and if not, I'll 
>>drop a note to Claudia.  I think that Dow provided this info 
>>to Claudia (or whomever it was who gave it to me) so you 
>>might try asking them.
>>
>>Mary
>>
>>Jim McVittie wrote:
>>
>>    
>>
>>>Hi Alissa,
>>>
>>>During the overetch, the Al etch chamber will be contaminated by the 
>>>decomposition products of the PDMS. So the important 
>>>      
>>>
>>question what is 
>>    
>>
>>>in PDMS and is it a problem to other users of the chamber. 
>>>      
>>>
>>My concern 
>>    
>>
>>>is what metals at in PDMS and at what level. Can you find a purity 
>>>analysis for PDMS?
>>>
>>>   Thanks, Jim
>>>
>>>"Alissa M. Fitzgerald" wrote:
>>>
>>> 
>>>
>>>      
>>>
>>>>  Part 1.1    Type: Plain Text (text/plain)
>>>>          Encoding: 7bit
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>>>>
>>>>        
>>>>
>>> 
>>>
>>>      
>>>
>>--
>>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





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