<html>
<head>
<meta content="text/html; charset=windows-1252"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<div class="moz-cite-prefix">I have seen PMMA form stress cracks
like this, but not in 200nm thick films. It usually happens with
> 1 um thick films developed in MIBK/IPA, since MIBK swells the
resist film. A simple solution is to develop in IPA/water (3:1),
preferably at a low temperature (we use 7C). You can find a paper
about IPA/water development <a
href="http://nano.yale.edu/sites/default/files/files/pmma_develop_ipa_water.pdf">here.</a>
This is just a guess, but maybe the pmma is a lot thicker than you
think.<br>
<br>
<br>
<div class="moz-signature"> (Also, I suggest using hot NMP instead
of hot acetone. It's a lot safer.)<br>
<br>
<font size="-2">--------------------------------<br>
Michael Rooks <br>
Yale Institute of Nanoscience and Quantum Engineering <br>
<a href="http://nano.yale.edu">nano.yale.edu</a>
</font>
<br>
<br>
<br>
<br>
</div>
On 11/17/2015 12:46 PM, John Watson - TNW wrote:<br>
</div>
<blockquote
cite="mid:5033ABE112FA6B49ABEC13F5C6FCB14CF9CB4C@SRV366.tudelft.net"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=windows-1252">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
@page WordSection1
{size:8.5in 11.0in;
margin:70.85pt 70.85pt 70.85pt 70.85pt;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
<div class="WordSection1">
<p class="MsoNormal">Dear colleagues,<o:p></o:p></p>
<p class="MsoNormal">Our group has been having a troublesome and
seemingly random problem with pmma cracking during
metalization steps, and I am hoping to get the community’s
ideas on what might be causing the problem. Our typical
process (used to make ohmic contacts to III-V nanowire devices
on Si/SiO2 substrates) is as follows:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">PMMA 950K A4 spun at 4000 RPM (~200nm
thickness)<o:p></o:p></p>
<p class="MsoNormal">Bake 175C 10 min<o:p></o:p></p>
<p class="MsoNormal">Expose 1100-1300 uC/cm^2 at 100kV<o:p></o:p></p>
<p class="MsoNormal">Develop 60s MIBK:IPA I:3, rinse in IPA 30s<o:p></o:p></p>
<p class="MsoNormal">60s O2 ashing (sample sits in Faraday cage,
pmma etch rate ~1-3 nm/min)<o:p></o:p></p>
<p class="MsoNormal">Ammonium polysulfide passivation of III-V
surface (sample sits in heavily diluted, super-saturated
polysulfide solution at 60C for 30 min)<o:p></o:p></p>
<p class="MsoNormal">Quick load into evaporator followed by
short in-situ He ion milling to remove residual sulfur layer<o:p></o:p></p>
<p class="MsoNormal">Evaporation of 10/120nm Cr/Au at 0.5/1.5
A/s. Vacuum level typically high 10^-8 Torr throughout
deposition<o:p></o:p></p>
<p class="MsoNormal">Liftoff in acetone ~50C<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The problem that we see is illustrated in
the attached SEM image – thin filaments of metal extend from
the contacts (typically starting at sharp corners) and short
out devices. The fact that the cracks originate at sharp
corners suggests the issue is stress-related, but we have been
unable to track down the cause of this excess stress. Many
users (8-10) have seen this problem using different bottles of
resist, different viscosities and molecular weights of pmma,
different baking times and temperatures (as short/cold as 5
min/150C and as hot/long as 190C/30min), different hotplates,
3 different e-beam lithography systems, a number of
substrates/gate dielectrics (SiO2, SiNx, BN, InSb, sapphire),
different metalization systems (two different e-beam
evaporators and one sputtering system – the helium etch is
only done in one evaporator), different metalizations (Ti/Au,
Cr/Au, NbTiN), and varying waiting times between
spinning/exposure/developing/metalization (varying from 5
hours to 3 days for the whole process). The process I
described above had been stable for over a year until about a
month ago when the cracking problem came up. Since then it
has been sporadic, ruining a few (out of many) processing runs
per week, but it has not been repeatable enough to allow for
really systematic testing. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does anyone have any insight into what
could increase stress in the resist? For instance, is
temperature/humidity during spinning and baking particularly
critical? One theory we are currently testing is that a
recently instituted rule of cleaning the spinners with a
metal-ion-free photoresist developer following pmgi spinning
could be causing the problem (e.g. this would help explain the
random nature of the problem since it would depend on what the
previous user did with the spinner). Thanks,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">John<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;mso-fareast-language:EN-GB"
lang="DE">-----------------<br>
John Watson<br>
Postdoctoral Fellow - Kavli Institute of Nanoscience<br>
Delft University of Technology<br>
Lorentzweg 1, 2628 CJ Delft<br>
The Netherlands<br>
<br>
</span><span style="mso-fareast-language:EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
labnetwork mailing list
<a class="moz-txt-link-abbreviated" href="mailto:labnetwork@mtl.mit.edu">labnetwork@mtl.mit.edu</a>
<a class="moz-txt-link-freetext" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__www-2Dmtl.mit.edu_mailman_listinfo.cgi_labnetwork&d=AwICAg&c=-dg2m7zWuuDZ0MUcV7Sdqw&r=apnDUg1OD9ejswcjrIvVgS28NpQ7-FGy7Sl7_YPlupc&m=k00J_xGJbn6kSnBRzd-5iCwCVl8AjkSQ0Xp0-3Nqud0&s=CSD8dbI60PxdOmtcMQVPTZIv38xyKb-TkNiqaGIRpYw&e=">https://urldefense.proofpoint.com/v2/url?u=https-3A__www-2Dmtl.mit.edu_mailman_listinfo.cgi_labnetwork&d=AwICAg&c=-dg2m7zWuuDZ0MUcV7Sdqw&r=apnDUg1OD9ejswcjrIvVgS28NpQ7-FGy7Sl7_YPlupc&m=k00J_xGJbn6kSnBRzd-5iCwCVl8AjkSQ0Xp0-3Nqud0&s=CSD8dbI60PxdOmtcMQVPTZIv38xyKb-TkNiqaGIRpYw&e=</a>
</pre>
</blockquote>
<br>
</body>
</html>