<!DOCTYPE html><html><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman";color:black">Hello Labnetwork:</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman";color:black"> </span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman";color:black">The recent discussion about PPE
for lithography work seems an appropriate moment to present the
findings of the TMAH Working Group.</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman";color:black"><br>
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.<span style="mso-spacerun:yes"> </span>The goal of
the TMAH Working Group was to find viable and less toxic
alternatives to TMAH.<span style="mso-spacerun:yes"> </span>What
follows is a summary of our findings. <br>
</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman";color:black"><br>
</span><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman""></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
line-height:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black">TMAH is
used in our labs primarily in 3 ways: </span><span style="font-size:12.0pt;
font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman""></span></p>
<ol type="1" start="1">
<li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;line-height:normal;mso-list:l2 level1 lfo1;tab-stops:list .5in"><b style="mso-bidi-font-weight:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman"">Developer for
novolac photoresist</span></b><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman"">.
The concentration is low (2.38% typically) and so the toxicity
is lower than it is for full strength (25%).<span style="mso-spacerun:yes"> </span>There has been one
fatality ascribed to an extensive exposure to 2.3% TMAH as
well as other cases with less catastrophic outcomes.<span style="mso-spacerun:yes"> </span></span><span style="font-size:12.0pt;
font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman""></span></li>
</ol>
<ol type="1" start="2">
<li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;line-height:normal;mso-list:l0 level1 lfo2;tab-stops:list .5in"><b style="mso-bidi-font-weight:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman"">Developer for
HSQ-type e-beam resist</span></b><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman"">. 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 <span style="mso-spacerun:yes"> </span>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.</span><span style="font-size:
12.0pt;font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman""></span></li>
</ol>
<ol type="1" start="3">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
line-height:normal;mso-list:l1 level1 lfo3;tab-stops:list .5in"><b style="mso-bidi-font-weight:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black">Etchant
for Si</span></b><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman";
color:black">. 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 </span><span style="font-size:
12.0pt;font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman""></span><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black"></span><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black"></span><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black"></span></li>
</ol>
<span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black">The
TMAH Working Group has identified two primary chemical suppliers
who have made an effort to develop and market TMAH-alternative
quaternary ammonium salts<b style="mso-bidi-font-weight:normal">.
SACHEM Chemical</b> is selling some more obvious alternatives
like tetraethyl-, ethyltrimethyl-, and tetrabutyl-ammonium
hydroxide under the brand NOVO-SAFE, distributed by Transene (</span><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman""><a href="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="><span style="font-size:11.0pt;color:blue">https://transene.com/tmah-substitutes/</span></a></span><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman";
color:black">). <b style="mso-bidi-font-weight:normal">Huntsman
Chemical</b> produces some more unconventional quaternary
ammonium hydroxides like choline-OH and THEMAH (</span><span style="font-size:12.0pt;font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman""><a href="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="><span style="font-size:11.0pt;color:blue">https://www.huntsman.com/products/detail/550/tmah-alternatives</span></a></span><span style="font-family:"Times New Roman","serif";mso-fareast-font-family:"Times New Roman";
color:black">). 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.
<span style="mso-spacerun:yes"> </span>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.</span><br>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
line-height:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black"><br>
</span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
line-height:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black">Here is
summary of the work performed at Queens University by Graham
Gibson:</span></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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.</span></b></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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.</span></b></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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.</span></b></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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:</span></b></p>
<p class="MsoListParagraphCxSpFirst" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">TMAH:<span style="mso-tab-count:
2"> </span>2.38%</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">ETMAH:<span style="mso-tab-count:
1"> </span>3.25%</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">Choline-OH:<span style="mso-tab-count:1"> </span>4.30%</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">XHE-125: <span style="mso-spacerun:yes"> </span><span style="mso-tab-count:1"> </span>5.70%</span></b></p>
<p class="MsoListParagraphCxSpLast" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">TEAH: <span style="mso-tab-count:
2"> </span>7.50%</span></b></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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):</span></b></p>
<p class="MsoListParagraphCxSpFirst" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">TMAH: <span style="mso-tab-count:
1"> </span>0.003</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">XHE-125:<span style="mso-tab-count:1"> </span>0.007</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">ETMAH: <span style="mso-tab-count:
1"> </span>0.007</span></b></p>
<p class="MsoListParagraphCxSpMiddle" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">TEAH:<span style="mso-tab-count:
2"> </span>0.017</span></b></p>
<p class="MsoListParagraphCxSpLast" style="text-indent:-.25in;mso-list:l3 level1 lfo4"><b><span style="font-size:12.0pt;line-height:107%;font-family:"Aptos","sans-serif";
mso-fareast-font-family:Aptos;mso-bidi-font-family:Aptos" lang="EN-CA"><span style="mso-list:Ignore">-<span style="font-style: normal; font-variant: normal; font-stretch: normal; font-size: 7pt; line-height: normal; font-family: "Times New Roman"; font-size-adjust: none; font-kerning: auto; font-language-override: normal; font-feature-settings: normal;">
</span></span></span><span style="font-size:12.0pt;
line-height:107%;font-family:"Garamond","serif"" lang="EN-CA">Choline-OH:<span style="mso-tab-count:1"> </span>0.022</span></b></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;line-height:115%;font-family:
"Garamond","serif"">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.</span></b></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
line-height:normal"><span style="font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";color:black">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: <a href="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="><span style="color:blue">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=</span></a></span><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-font-family:
"Times New Roman""></span></p>
<p class="MsoNormal"><span style="font-family:"Times New Roman","serif""><br>
</span></p>
<p class="MsoNormal"><span style="font-family:"Times New Roman","serif"">We
welcome your thoughts and comments.<span style="mso-spacerun:yes"> </span>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. </span></p>
<p class="MsoNormal"><span style="font-family:"Times New Roman","serif""> </span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif"">Mac
Hathaway <span style="mso-tab-count:1"> </span><span style="mso-tab-count:1"> </span>Center for
Nanoscale System - Harvard University</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif"">Aimee
Price <span style="mso-spacerun:yes"> </span><span style="mso-tab-count:1"> </span><span style="mso-spacerun:yes"> </span><span style="mso-tab-count:1">
</span>Institute for Materials and Manufacturing Research - The
Ohio State University</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif"">Dave
Hollingshead<span style="mso-tab-count:1"> </span>Institute
for Materials and Manufacturing Research - The Ohio State
University</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif"">Graham
Gibson <span style="mso-spacerun:yes"> </span><span style="mso-tab-count:1"> </span>NanoFabrication
Kingston - Queens University</span></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-family:"Times New Roman","serif"">Justin
Wirth <span style="mso-tab-count:1"> </span><span style="mso-tab-count:1"> </span>Birck
Nanotechnology Center - Purdue University</span></p>
<div class="moz-signature"><span style="font-size:12.0pt;line-height:115%;font-family:"Times New Roman","serif""><o:p></o:p></span>
<div class="WordSection1">
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:
normal"><span style="font-size:12.0pt;font-family:"Times New Roman","serif""><o:p> </o:p></span></p>
</div>
</div>
</body>
</html>