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

Wed Apr 29 09:46:02 EDT 2026

Great information, Mac, thanks for sharing it.  Glad to see people actually looking into this.

Just to follow up, in our facility there is a lot of beaker development done.  This is in ventilated clean wet benches, with eye protection and face shields, with nitrile gloves.  There is MF312 available, which is >5% TMAH, but rarely used except in dilution.  Everything in general is 2.38% or lower.

We DO NOT ALLOW HSQ development in 25% TMAH AT ALL.  We see that as too high a risk.  It is only used for Si etching in fume hoods using full PPE, and there are strong warnings regarding its use.

Replacement of TMAH will need to be made cost-comparative for acceptance, and more testing, including contamination/defects/device performance, needs to be done.  This means that the HVM folks need to be on-board, and I have seen little evidence so far.  Hopefully soon.

Garry J. Bordonaro

Microlithographic Engineer

Cornell NanoScale Facility

250 Duffield Hall

343 Campus Road

Ithaca NY 14853-2700

(607) 254-4936

bordonaro at cnf.cornell.edu <mailto:bordonaro at cnf.cornell.edu> 

http://www.cnf.cornell.edu/

Please acknowledge CNF in your publications:

"This work was performed in part at the Cornell NanoScale Science & Technology Facility (CNF), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant NNCI-2025233)."

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 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.  

2.	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.

3.	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://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=> https://transene.com/tmah-substitutes/). Huntsman Chemical produces some more unconventional quaternary ammonium hydroxides like choline-OH and THEMAH ( <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=> https://www.huntsman.com/products/detail/550/tmah-alternatives). 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 <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=> &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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