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

[Mac Hathaway]

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://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= 
<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 HollingsheadInstitute 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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