[labnetwork] pmma cracking issue

[Espen Rogstad]

Hi John,
We've also had issues with cracking and even bubbling of pmma during metallization. The problem was most severe for Pt deposition. We haven't gotten totally rid of the problem but the following helped in limiting it:
- Increasing the deposition rate from 5?/s normally used to ~10?/s.
- Increasing the source-sample distance. We have around 40cm distance in one of our e-beams and it mostly works fine.
- Attaching samples to a 1cm thick aluminum disc (will act as a heat sink) using Mung II vacuum grease (we normally attach samples to a 4" Si wafer using kapton tape).
- Putting the source material directly in the Cu pocket without using a crucible liner

This does not explain why you suddenly see this problem though. Maybe your process cooling water pressure has changed?
If you use a carbon liner for gold you normally get a crust of carbon on top which will deflect a lot of electrons which in turn will expose the pmma during metallization. We use tungsten liners (with a small tungsten disc under to minimize contact with Cu pocket) for gold and it works very well.

Anyway, as I said we still see this problem occasionally and are very interested in what others have to say about this. Thanks for raising the question John!

Regards,
Espen Rogstad
NTNU NanoLab

-------------------------
Sem Saelands vei 14, K1-113
NO-7491 Trondheim, Norway
Tlf: +47 95285642
http://www.ntnu.no/nanolab

Den 17. nov. 2015 kl. 21.56 skrev John Watson - TNW <J.D.Watson at tudelft.nl<mailto:J.D.Watson at tudelft.nl>>:

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


<resist cracking.jpg>
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