[labnetwork] Piranha processing vessels

Fri Apr 29 14:59:49 EDT 2016

Teflon cassette story. We did a phosphoric acid/reflux operation at 180 degrees C for the removal front and backside of siicon nitride for a CMOS process we were doing. There was around 500 angstroms of silicon dioxide under the nitride to begin with at the start of the process. We dedicated new Teflon cassettes for the operation and immediately found out that the outgassing of fluorine from the cassettes etched away all of the silicon dioxide film. It took several hundred wafers before action was taken to eliminate the problem. If I remember right it took 8 hours of soaking in hot phosphoric to deplete the outgassing of the fluorine for the new cassettes. After that only those cassettes were used for the nitride removal operation with little of no etching of the oxide underneath the nitride. Bob Henderson

From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Keith Franklin
Sent: Friday, April 29, 2016 8:19 AM
To: Mary Tang
Cc: Labnetwork
Subject: Re: [labnetwork] Piranha processing vessels

Thank you all for the feedback.  Very helpful!  Please keep them coming should anyone else be willing to contribute.

Best,

Keith

Keith Franklin

Operations Manager

University of Alberta - nanoFAB

W1-060 ECERF Building

9107 - 116 Street

Edmonton, Alberta

Canada T6G 2V4                     Ph: 780-492-0170

www.nanofab.ualberta.ca                 

On Fri, Apr 29, 2016 at 7:29 AM, Mary Tang <mtang at stanford.edu> wrote:

Thanks Bob!

A wonderful example of how "contamination" can be managed by means other than the paranoia -- something we struggle with as we try to serve a broader community while trying to keep our electronics and detector researchers happy.  I would hazard a guess that this works in industry because distillation is vacuum rather than heated and that an additional RCA-equivalent clean is used before high temperature processing steps, thus minimizing the risk of driving in mobile ions.

That said, at Stanford, we do promote paranoia when it comes to dedicated teflon cassettes in MOS stations because of their propensity to absorb whatever chemical they are exposed to.  I've not seen any data that shows cross-contamination due to this, but have seen the results of off-gassing on wafers stored or heated in chemical bath cassettes.  The risk of significant carryover of HF into piranha may be very low, but might be worth considering dedicated cassettes when using pyrex.

Mary

-- 
Mary X. Tang, Ph.D.
Stanford Nanofabrication Facility
Paul G. Allen Bldg 141, Mail Code 4070
Stanford, CA  94305
(650)723-9980 <tel:%28650%29723-9980> 
mtang at stanford.edu
http://snf.stanford.edu

On 4/28/2016 1:21 PM, Robert M. HAMILTON wrote:

Keith, 

I'll stick my neck out!  

The UC Berkeley NanoLab uses fused silica tanks with encased heaters and ground-fault detection for our piranha baths. I am guessing your suggestion of Pyrex or an alternative borosilicate glass will raise the hackles of some Labnetwork readers. And, I cannot claim experience with the impact of borosilicate glasses, used for piranha cleans, for MOS devices. 

However, I can point to a patent issued to Alameda Instruments that made H2SO4 reclaim/reprocessing systems for a number of major semiconductor manufacturers. I also have known, since boyhood the research glassblower who made their vacuum stills. The glass used was Pyrex or an alternative Duran, which is Schott's equivalent. This leads me to the conclusion H2SO4 from a pyrex still was good enough for Intel, Pyrex is probably good enough for you.

Having said this if there is accidental contamination via HF your Pyrex will contribute alkali and your devices will be DOA. We have seen HF contamination in our baths on a few occasions.

For reference see:  https://www.google.com/patents/US4980032

Bob Hamilton

PS Caveat emptor! Corning now uses the term Pyrex for a broader number of glasses than their traditional 7740.

Robert Hamilton

University of CA, Berkeley
Marvell NanoLab Equipment Manager

Rm 520 Sutardja Dai Hall, MC 1754

Berkeley, CA 94720

Phone 510-809-8618 (desk - preferred)

Mobile 510-325-7557 (my personal mobile)

E-mail preferred: bob at eecs.berkeley.edu
http://nanolab.berkeley.edu/

On Thu, Apr 28, 2016 at 10:59 AM, Keith Franklin <keithf at ualberta.ca> wrote:

Good day all, 

We're currently revisiting what vessels or tanks we use for processing with hot piranha.  Our most common application is for cleaning a partially filled cassette of 4" Si wafers.  We typically purchase Pyrex 6944 vessel and have our glass shop cut them down to a more reasonable height.  In our last purchase cycle, we noticed that the ID has changed ever so slightly and it's now challenging to squeeze our standard Entegris cassette into this vessel.  We're currently looking into custom machined PVC, PP,  Teflon & Quartzware, as well as off the shelf small volume tanks, etc.

I'm hoping to better understand what other similar labs are using for this application (& potentially 6" wafers as well).  Any advice or feedback would be greatly appreciated.

Kind Regards,

Keith

Keith Franklin

Operations Manager

University of Alberta - nanoFAB

W1-060 ECERF Building

9107 - 116 Street

Edmonton, Alberta

Canada T6G 2V4 Ph: 780-492-0170

www.nanofab.ualberta.ca 

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