Hi All,
I have been doing e beam lithography for couple of months to make ellipsoidal nano magnets (250nm by 150nm . 350nm by 275nm etc.). Usually I use e beam evaporation for a thin (5-8nm) Ti adhesion layer and on top of that I sputter FeGa (ferromagnetic with thickness 10 to 14 nm).
Initially I did lithography on Silicon and got nano magnetic structures by warm acetone liftoff process (with Sonicator) using the deposition technique I mentioned above. Now I am trying to make it on a different substrate like PMN-Pt (piezoelectric substrate). But now the lift off is not working properly. The PMMA is either not at all lift off or washed away completely with the magnets.
I know that sputtering is not directional like evaporation but on silicon I finally got to make the structures by lowering pressure (1 militorr) and power (45 watt) and deposition time. I did not find any literature to evaporate FeGa (probably it is not possible since Ga has very low melting point).
Can anyone please give me any type of suggestion or methods which I can apply to have a better liftoff while depositing with sputtering machine. (There is no way to change any set up of the sputtering machine other than playing with power, time and pressure).
Hei Jiayu Li,
Thanks for the reply. Can you explain a little bit more about the nozzle tip.
This type of problem I have faced earlier when I tried to apply e-beam lithography recipe (calibrated on conducting Silicon substrate ) on thick silicon-dioxide coated conducting silicon substrate. Because during e-beam lithography conductivity of the substrate matters severely due to the back-scatter electrons. So you might have to recalibrate your recipe for bigger pattern on that substrate.
For well lift-off people generally use lower molecular weight polymer as bottom layer and higher molecular weight polymer as top layer. In our case we use PMMA-450K and PMMA-950K.
n response to Tillmann Krauss,
yes Ti adheres to the PMN-Pt substrate. and we have made nanomagnetic structures using evaporation of Ti and Cobalt.
But I think my problem is that my deposition technique is sputtering which may have cured the top layer of PMMA while depositing. Can anyone give any suggestion how to resolve the issue?
If you cannot lift the PMMA then it may be that the undercut is less good (different distribution of SE and BSE in new substrate or thinner resist). If you wash everything away it also indicates that the resist is too thin and you bridge it with the deposition (or bad adhesion of both resist and Ti seed layer).
If I were you, I would try using a thicker bilayer (reduce the spinning speed or dilution), while decreasing the total thickness of your deposit. You may also want to try different adhesion layers, that will react with the top surface of your substrate. You can also try using O2 plasma in case there are resist traces left after your development, or develop longer (for PMMA up to 60s in MIBK1:3IPA).
Another option would be to sputter first your magnetic film on the surface and only afterwards do the lithography. The advantage is that you can use a much thinner Ti layer as mask (ideally by e-beam evaporation), but it requires Ar milling to remove the materials (eventually with a mass spectrometer if you need to remove all the Ti on the top). You can certainly use also negative resist (like ma-N2403, ma-N2410, AR-N, etc) instead of PMMA+Ti if your structures are above 100nm.
I think you have part answer in the above replies but here is what I think would solve your problem. The piezoelectric substrate would be electrically insulator so e-beam would not work as it work on the Si substrate as the charge has to be spread. So you should use the PMMA as normal you did on Si wafer then before e-beam exposure deposit 10nm of Au as the charge spreading layer on top of PMMA, then expose using e-beam. After e-beam exposure remove the charge spreading layer using wet etch in Au etch, rinse and blow dry and then develop PMMA as normal and I hope every thing would work fine. You can use Al charge spreading layer if you prefer but Au works fine and can be removed very easily just 10-15 seconds. I have done this many times on LiNb wafers. For lift off under sputtering can be improved if PMMA is thicker or using a bi-layer also helps but I think if the process had already worked on Si then it would work again as soon as you solve the charge spreading issue. Goog luck, let me know if you need help regarding bi-layer PMMA process.
Really useful information from Mihai Gabureac and Ata H Khalid.
Right now, I am trying to make a thicker resist profile and see what happens but I am really interested in the Au as a charge spreading layer.
#Ata H Khalid, will it increase the undercut? and what is a good Au etchant?
#Mihai Gabureac, I am developing for 70s in 1:3 (MIBK:IPA) solution. and can you tell me what should be the power, pressure and time for oxygen plasma etching to remove the residue from the substrate.
I let my samples soak in acetone overnight at 60 degrees to loosen the coating, then sonicate them in fresh acetone the following day. This gives me a much cleaner lift-off than sonicating straight away. If time is an issue, soaking for just an hour should show some effect.
And as an alternative to evaporating gold to dissipate the charge, you can spin coat e-spacer onto the resist (link). It's a conductive polymer you spincoat onto your resist, which washes off in water before development and saves you time using an evaporator.The downside is it cost £3k, but I figure this is less than what 100 g of gold will cost you. It depends how often you plan on doing this.
http://www.sds.com.sg/products/electronics/espacer/
In addition to Bhim’s suggestion of a thicker resist you could also consider a MMA/PMMA double-layer stack which creates a nice undercut profile and/or try different developer solutions which might offer different undercuts due to the difference in clearing dose and/or contrast coming with different developers.
You can use Bilayer of PMMA 2% and PMMA 4%. First spin PMMA 950K 2% and then PMMA 950K 4%.
Good question.. Please share me the best answer might you trust...
Regards…
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