[labnetwork] pmma cracking issue

[Price, Aimee]

Hi John,
We had a "cracking" issue with PMMA in two different time frames and labs here at Ohio State.  Many years ago the first problem was also sporadic and we did a similar analysis to your analysis below.  We even sent some samples (and a student) to another university for spinning and/or ebeam lithography.  In the end, it was found that certain areas of our evaporator were hotter than others.  That problem was a gross problem however and was visible immediately after removal from the evaporator.  The metal was Cr/Au.

Do you have any images of the metal before the liftoff?  You are welcome to email me directly if you wish.

Our most recent instance was also a heating issue in a completely different, and much newer evaporator, but with Ni.

In both cases they were temperature related.  We solved them by either slowing down the deposition rate, so using lower power in the electron beam evaporator, or changing the soak times.

Just looking at your process, I would also take a look at the actual temperature of your hotplate(s).  We have several of one type and one attached to a coater.  The one type has the thermocouple under the stage and the actual temperature varies as much as 10deg Celsius from the setpoint.  Some of them also vary across the stage itself.  The actual temperature may be an issue, since you want the resist to actually see 175C not 160C etc.

Also, I'd take a look at the resist under SEM after all of the steps post develop if you have time/enough samples.  If you are concerned about the spinner verify the exhaust has not and is not changing.  We've had issues in the past with the exhaust changing and causing trouble with other resist systems.

Best of luck to you.

Regards,
Aimee

From: labnetwork-bounces at mtl.mit.edu [mailto:labnetwork-bounces at mtl.mit.edu] On Behalf Of John Watson - TNW
Sent: Tuesday, November 17, 2015 12:47 PM
To: labnetwork at mtl.mit.edu
Cc: Jakob Kammhuber - TNW
Subject: [labnetwork] pmma cracking issue

Dear colleagues,
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:

PMMA 950K A4 spun at 4000 RPM (~200nm thickness)
Bake 175C 10 min
Expose 1100-1300 uC/cm^2 at 100kV
Develop 60s MIBK:IPA I:3, rinse in IPA 30s
60s O2 ashing (sample sits in Faraday cage, pmma etch rate ~1-3 nm/min)
Ammonium polysulfide passivation of III-V surface (sample sits in heavily diluted, super-saturated polysulfide solution at 60C for 30 min)
Quick load into evaporator followed by short in-situ He ion milling to remove residual sulfur layer
Evaporation of 10/120nm Cr/Au at 0.5/1.5 A/s. Vacuum level typically high 10^-8 Torr throughout deposition
Liftoff in acetone ~50C

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.

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,

John


-----------------
John Watson
Postdoctoral Fellow - Kavli Institute of Nanoscience
Delft University of Technology
Lorentzweg 1, 2628 CJ Delft
The Netherlands

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