Tuesday, January 15, 2013

Polystyrene Bead Etching and E-beam Optimization

It has been some time since I have posted, mainly due to holidays and equipment malfunctions but I have new images and new plans to report!  On the pillar or "trough" side of the project we have successfully completed an etching trial which displayed some interesting results.  Five samples of 100 nm polystyrene beads on silicon were run under different conditions using CF4 and O2 etching.  From figures 1 through 5 below, it can be seen that there was some etching of the silicon and in a few instances there were cylindrical formations, but the beads are being etched by the O2 which causes them to either adhere to the silicon and melt, which causes the final structure to be malformed, or as the bead is eaten it shrinks causing a cone or rounded shape hemisphere shape.  From literature I am finding that shorter etching times with higher power (W) seems to give more anisotropic etching profiles (following performance optimization) and the use of a passivation step (usually flowing a polymer) was used in some, but not all, cases.  It seemed though that in some of the cases that a reactor had been specially ordered or built for such etching and may not be feasible for our purposes because alternative means of achieving near comparable results are possible.  Our next step may be to shorten the etching time for each pulse from 45-60 seconds to somewhere around 5-15 seconds to allow the sample to cool.  Also, in the literature the lowest power used in the RIE (reactive ion etching) step was 600 W and we use at the most 60 W and I am interested to see if we increased the power and shortened the pulse time if our etch might become more anisotropic.  We will also have to use a different plasma (perhaps argon) and remove all O2 etching steps as they degrade the beads quickly.
Figure 1. Polystyrene Bead Etching on Silica

Figure 2. Polystyrene Bead Etching on Silica

Figure 3. Polystyrene Bead Etching on Silica


Figure 4. Polystyrene Bead Etching on Silica

Figure 5. Polystyrene Bead Etching on Silica
The other side of the project is the "trench" or "trough" formation in PMMA using e-beam lithography.  After a few setbacks (new source installation and a cooling pump break down) I was able to return and etch a few samples to continue to try and achieve 20 nm lines.  So far I can get 50 nm almost every time and still have a few things that I can adjust to see if they shrink that number a bit.  Some of the most recent images can be seen in figures 6, 7 and 8.  Next I will try to reduce the scope current and see if a smaller dosage per length might help reduce the beam interaction with the sample, creating a smaller line width overall.
Figure 6. Line width of e-beam wheel on PMMA

Figure 7. Line width of e-beam wheel on PMMA

Figure 8. Wheel Array with Different dosage per wheel