[labnetwork] TMAH Working Group - TMAH Alternatives - Summary of Findings

Tue May 5 11:14:54 EDT 2026

Thanks, Mac, for summarizing the group’s work so nicely.

Just a quick plug, I will be speaking at One Nano’s<https://onenano.org/> Midwest’s inaugural chapter meeting on this topic next week, May 12, at The University of Michigan. If you are interested in topics like this, please join us in lovely Ann Arbor. I’m from OSU so it must be important for me to speak highly of our “sworn enemy!”

Midwest Spring 2026 - OneNano<https://onenano.org/midwest-spring-2026/>

We are also planning on submitting our work to JVST B by the end of this month. I would love to hear from others who are interested in this topic. Our main goal at OSU is to find a safer option for high contrast HSQ development that doesn’t include sodium or potassium. Like Gary Bordanaro mentioned, the 25% TMAH option is high risk, and I’ve been steering my users away from it. We have a TMAH chemical SOP that we require all our cleanroom users to read and understand. It covers the risks as currently understood by the industry (ever evolving) and our PPE requirements for all volumes, concentrations, and associated actions (using versus pouring, transporting, etc.) with TMAH solutions that might be available in our cleanroom. While I cannot share that in an open forum, I’d be happy to share the appropriate details to anyone who wants them.

Best,
Aimee Bross Price

Manager, Nanofabrication
The Ohio State University
Nanotech West Lab
Institute for Materials and Manufacturing Research
1381 Kinnear Road
Suite 100
Columbus, OH 43212
614-292-2753
Price.798 at osu.edu<mailto:Price.798 at osu.edu>
Nanotech.osu.edu

Pronouns: she/her/hers

From: labnetwork <labnetwork-bounces at mtl.mit.edu> On Behalf Of Mac Hathaway
Sent: Tuesday, April 28, 2026 3:34 PM
To: Lab Network (labnetwork at mtl.mit.edu) <labnetwork at mtl.mit.edu>
Subject: [labnetwork] TMAH Working Group - TMAH Alternatives - Summary of Findings

Hello Labnetwork: The recent discussion about PPE for lithography work seems an appropriate moment to present the findings of the TMAH Working Group. The TMAH Working Group is an informal ad-hoc group formed a couple of years ago amongst primarily

Hello Labnetwork:

The recent discussion about PPE for lithography work seems an appropriate moment to present the findings of the TMAH Working Group.

The TMAH Working Group is an informal ad-hoc group formed a couple of years ago amongst primarily academic nanofabrication facility folks concerned about students handling TMAH and the relatively poor awareness of its special hazards. As some of you will be aware, its primary hazard is associated not with its extremely corrosive nature, but rather with the fact that it is a potent neurotoxin, on par with HF but without any known antidote.  The goal of the TMAH Working Group was to find viable and less toxic alternatives to TMAH.  What follows is a summary of our findings.

TMAH is used in our labs primarily in 3 ways:

  1.  Developer for novolac photoresist. The concentration is low (2.38% typically) and so the toxicity is lower than it is for full strength (25%).  There has been one fatality ascribed to an extensive exposure to 2.3% TMAH as well as other cases with less catastrophic outcomes.

  1.  Developer for HSQ-type e-beam resist. The concentration for this application is higher (25%) and the motivation to find an alternative is therefore greater. While relatively few substantial (greater than 5-7% of body area) exposures to 25% TMAH  have occurred, the current fatality rate seems to be around 75%. These findings are part of what inspired the IBM study mentioned in a recent labnetwork post.

  1.  Etchant for Si. The traditional etchant for this application is KOH, but for cases where metal ions are problematic, TMAH is the default. This application is the most hazardous as it typically requires larger volumes, longer times, and higher temperatures
The TMAH Working Group has identified two primary chemical suppliers who have made an effort to develop and market TMAH-alternative quaternary ammonium salts. SACHEM Chemical is selling some more obvious alternatives like tetraethyl-, ethyltrimethyl-, and tetrabutyl-ammonium hydroxide under the brand NOVO-SAFE, distributed by Transene (https://transene.com/tmah-substitutes/<https://urldefense.proofpoint.com/v2/url?u=https-3A__transene.com_tmah-2Dsubstitutes_&d=DwMF-g&c=WO-RGvefibhHBZq3fL85hQ&r=TEMLD8-VsxCGtcVzmvpT5GFNSczskEKHzW6aYlttmIY&m=byprFfYOOKk_Ho4LCBXGTSIneDLMb-MumT0G2i4B9pwQnqlo6887VnIvGL9YMrnu&s=fBK-owS68YfMmHfTua1jlFdSyGUYpOp1RsNNUHmtpwo&e=>). Huntsman Chemical produces some more unconventional quaternary ammonium hydroxides like choline-OH and THEMAH (https://www.huntsman.com/products/detail/550/tmah-alternatives<https://urldefense.proofpoint.com/v2/url?u=https-3A__www.huntsman.com_products_detail_550_tmah-2Dalternatives&d=DwMF-g&c=WO-RGvefibhHBZq3fL85hQ&r=TEMLD8-VsxCGtcVzmvpT5GFNSczskEKHzW6aYlttmIY&m=byprFfYOOKk_Ho4LCBXGTSIneDLMb-MumT0G2i4B9pwQnqlo6887VnIvGL9YMrnu&s=jOCJCGKlfcQRtfZS83iCQ_uiogAH7ceQkA_LFAZbBkM&e=>). Several labs, including Queens Univeristy in Canada, Ohio State and Purdue, have tested products from both companies in standard tests for at least one of the applications above. There is quite a bit of data collected at this point, but the general observation is that all of the products tested are safer (in many cases dramatically safer) than TMAH and either equivalent or slightly slower than TMAH for etching and developing applications.  Further testing is required to determine the suitability of any given product for any given application, but we do have some general recommendations and confidence that something will work as long as you are willing to sacrifice some aspect of performance.

Here is summary of the work performed at Queens University by Graham Gibson:
A systematic study was done at Queen’s University to characterize photoresist developing normally done by 2.38 wt% TMAH. A typical positive and negative Novolac-based photoresist were developed with TMAH and 8 other potential alternative quaternary ammonium hydroxide solutions, after screening out more than 10 others. Each UV-patterned sample was developed for a specific time, with the amount of resist dissolved measured by profilometry. At least 3 development times gives a linear plot, where the slope is the developing rate.
Among the candidate quats were ETMAH and TEAH from SACHEM and Choline-OH and XHE-125 from Huntsman. The SACHEM products were purchased from Transene and the Huntsman products were obtained as samples from the manufacturer (note that XHE-125 is an experimental product not yet commercially available). Each candidate was prepared at 5 different concentrations, each having a developing rate as described above.
During screening, it was evident that some quats were not suitable developers, either because they were a poor solvent for the soluble part of the resist or too good a solvent for the insoluble part of the resist.
In all cases, the standard TMAH developer was the fastest developer at a given wt% concentration. Based on the results, all the candidate developers in the full concentration study were able to develop the photoresist satisfactorily at some concentration. Focusing on the four highest performing candidates listed above, the concentration at which each had equivalent developing rate to 2.38% TMAH for the positive photoresist was:

-        TMAH:                      2.38%

-        ETMAH:        3.25%

-        Choline-OH:  4.30%

-        XHE-125:       5.70%

-        TEAH:                       7.50%
The selectivity of TMAH was also better than any of the candidates, although all the above 4 candidates still had good selectivity. The relative erosion rate (rate of loss of insoluble resist divided by developing rate) for these candidates for the positive resist at the concentration of equivalence were as follows (all selectivities were better for the negative resist):

-        TMAH:           0.003

-        XHE-125:       0.007

-        ETMAH:        0.007

-        TEAH:                       0.017

-        Choline-OH:  0.022
Effectively, then, a lab could use any one of these quats at the concentration indicated as a replacement for 2.38% TMAH with some success. The exact equivalent concentration depends on the resist, so some experimentation is required.
Here is a link to an abstract regarding work done at Ohio State and Purdue on  Novosafe Developer SE-44-26 and Novosafe SE-44 from Transene:   https://urldefense.proofpoint.com/v2/url?u=https-3A__eipbn.org_abstracts_2024_papers_9A-2D3.pdf&d=DwMFAw&c=WO-RGvefibhHBZq3fL85hQ&r=TEMLD8-VsxCGtcVzmvpT5GFNSczskEKHzW6aYlttmIY&m=1_54H_dFFYNv-wcmLpnX3rpvsgl5GrSCHKKSuBptlAD_JLPboiw9os5SiHW9-I43&s=rRzF1J0kBIyVqCC3oyl_AYzFIyaOJfOqiGCKaIUwarA&e=

We welcome your thoughts and comments.  Thanks to all 20 members of the TMAH Working Group, who have offered lots of hard work and thoughtful discussion to the rendering of these findings.

Mac Hathaway                        Center for Nanoscale System - Harvard University
Aimee Price                Institute for Materials and Manufacturing Research - The Ohio State University
Dave Hollingshead     Institute for Materials and Manufacturing Research - The Ohio State University
Graham Gibson           NanoFabrication Kingston  - Queens University
Justin Wirth                Birck Nanotechnology Center - Purdue University

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