[labnetwork] CMOS Clean in a MEMS Fab Facility

Tue Mar 19 12:59:20 EDT 2019

Hi Michael,

Our fab does not do a lot of traditional CMOS work, so I am by no means an expert in this area.  A lot of our work is concentrated in MEMS (including back-end processing on CMOS tapeout chips), magnetics, spintronics, photonics, 2D materials, functional oxides, bio interfaces, other emerging technologies.  However, I can hopefully offer a few comments from lessons learned or experiences we've had in the past, especially when working on devices where trapped charge or ion contamination were an issue.

Referencing your original question numbers:

1. We typically use PTFE petri dishes for this application.  We routinely process 100mm wafers in low profile  evaporating dishes.  While not cheap, a couple dishes won't typically set you back too much.

2. We separate glassware for metal ion free (MIF) and metal ion containing (MIC) containers (I'm assuming you are using MIF developers for CMOS).  Beakers are labeled MIF or MIC by etching the letters into the glass exterior of the beaker.  If I recall correctly most of the beakers are Type 1, Class A, 33 expansion Borosilicate glass (note that I'm not endorsing this one way or the other for CMOS).

2A. We do not have a dedicated spinner for CMOS, but we do limit which resists can go in which spinners (in the case where non-standard resists are used).

2B.  I would verify the type of glass used in the amber bottles.  Also, we buy droppers in clean, sterile packaging, as we have seen that droppers packaged and stored incorrectly can introduce contaminates.  In extreme cases, we have had some users request and move to glass pipettes.

2C.  The shared bath of NMP would be one of my biggest concerns in this whole process.  Manufacturers will list that NMP is safe on a lot of metals, including copper.  However, there is a caveat.  If the NMP bath collects or becomes contaminated with moisture, it makes the bath corrosive.  I have seen first-hand how NMP can corrode, or even etch through metals, such as copper.  If people are using the bath with such materials, it could have trace metals and other contaminants.

We do not do a lot in the way of furnace work, so I will default to others in the network that are much more of an expert in this area than me, but for what it's worth, the latter questions on quartz tube contaminants would be a concern in my opinion.  Even in simple annealing furnaces and our RTA, we keep "clean" and "dirty" tubes/chambers that we exchange depending on the materials being used.  In regards to potential vendors, we have purchased quartz products from Technical Glass Products in the past (https://technicalglass.com/), although, again, others who do a lot more work with furnaces will likely have more input than me.

Hope this helps in some capacity.

Best Regards,

Matt

--
Matthew T. Moneck, Ph.D.
Executive Manager, Claire & John Bertucci Nanotechnology Laboratory
Electrical and Computer Engineering | Carnegie Mellon University
5000 Forbes Ave., Pittsburgh, PA 15213-3890
T: 412.268.5430
F: 412.268.3497
www.ece.cmu.edu<http://www.ece.cmu.edu/>
nanofab.ece.cmu.edu

From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of Martin,Michael David
Sent: Monday, March 18, 2019 2:07 PM
To: labnetwork at mtl.mit.edu
Subject: [labnetwork] CMOS Clean in a MEMS Fab Facility

Hi,
   I'm trying to track down potential sources of contamination for a CMOS process we are trying to run through our predominantly MEMS fab here at U of Louisville.  Really the only pieces of equipment that are dedicated for CMOS type processes is our RCA bench, an older Technics sputterer, and our oxidation furnace (sort of, see below). So I have a few questions for those of you who have experience with this:
1) For HF etch/dips is there a particular polymer type or brand we should use for our containers that are known to be free of trace metals? Can I avoid PTFE as this is super expensive?
2) When you do litho do you have separate labware for developing? We currently use a Pyrex pan develop which I know is a No-No due to Na and other ions. What sort of container does your lab use (assuming pan develop)?
  2 a) Do you have a dedicated spinner for CMOS?
  2 b) Is there any danger that we are picking up contamination from the amber bottles we are temporarily storing our resists in? What about the polypropylene droppers we are dispensing resists with?
   2 c) What about resist stripping after etching? We typically use a big warm vat of NMP that is shared by all users.  We can also do a plasma etch but I worry about carry over from other folks as none of our plasma etchers are dedicated CMOS.
3) I presume quartz glassware works for my metal (usually aluminum) etching? Do you do regular aqua regia cleans on quartz-ware to scavenge other metals and potential contaminants?
4) We gravitate to peek tipped metal tweezers.  Are they okay? Do you regularly run the tips through a RCA clean?
5) Oxidation furnace: Before trying to transition to CMOS like devices the tube was used with non-RCA cleaned wafers and a pyrex bubbler. After moving to a quartz bubbler with DI water we cleaned the 4" tube with HF.  This is the one I'm really concerned about because I'm guessing that ionic contamination that might have been removed from the surface will readily diffuse back at 1000C.  So should we just bite the bullet and buy a new tube? Any vendor suggestions for a 4" Blue-M?
6) Any other suggestions other than buying a dedicated CMOS tool set?

I did find a very nice document from Stanford that has a lot of practical suggestions found here https://web.stanford.edu/class/ee410/cleaning.pdf )
Krishna Saraswa - Stanford University<https://web.stanford.edu/class/ee410/cleaning.pdf>
6 tanford University araswat 11! Cleaning - Surface Issues Contaminant * Organics - Skin oils - Resist - Polymers * Metals
web.stanford.edu

Thank you in advance,
   Michael

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