We've been learning the various processes required to fabricate silicon cantilevers following Zach's recipe.
The last step involves etching cantilevers out of the silicon wafer using KOH. Specifically, Zach's etch recipe for a 500 um wafer coated with 400 nm SiNx on both sides is:
I tried to follow this recipe last Wednesday, with the following modifications:
Because the wafer is 3/5 the thickness but the etch rate is also 3/5 as fast, I anticipated that a 6 hour etch would be appropriate to produce something for scribing and breaking... however, after 4.5 hours, the entire wafer was almost entirely dissolved. All that remained were thin, fragile sheets of Si or SiNx. What's going on? Some possibilities
I have since perused the KNI wiki for more resources on etch rates through Si. The most extensive data seem to be from the BYU page on KOH etching. It suggests that 30% KOH at 80 C etches through 100 Si at 80 um/hour (compared to 60 um/hour that Zach was using), while 50% KOH etches through 100 Si at 45 um/hour. This doesn't really explain the results. Even at 45 um/hour, we should have been left with 100 um of Si after 4.5 hours, or 1/3 of the initial material. If we take Zach's 60 um/hour at face value and apply the data from BYU as a 'relative rate of etching,' we would be scaling by a factor pretty close to that suggested in Silicon Micromachining.
I wasn't able to find good data on the rate of KOH etching through SiNx depending on temperature or concentration.
We are using doped silicon, but I found reference online to boron doped silicon etching more slowly.
The bath temperature could also have been systematically higher than 80 C, or just not well controlled around 80 C.
To test this, I'm planning to do a shorter etch of a sacrificial piece of Si under the same conditions as before (30% KOH, 80 C). I'll remove any oxide layer with HF beforehand, and check the bath temperature with a thermometer.
I used the standard SiNx deposition recipe on both sides of the wafer, but did not check the resulting mask with an ellipsometer. We should in the future do this after most previous steps: PECVD, optical lithography, DRIE etching, etc.
I'm being trained on the ellipsometer this afternoon, and plan to measure the thickness of SiNx on some 4" wafers Zach had left over.
Our cleaning process involves a few minutes in an HF bath, and we use DRIE etching during the optical lithography step. Either of these processes could thin the wafer. In particular, during lithography I noticed that my photoresist was a bit thinner than I'd intended. Perhaps the I etched through the exposed photoresist more quickly than anticiapted, allowing the DRIE etch to reach the underlying silicon for longer.
In the future I'll measure the thickness of the edges of the wafer (where there is no cantilever) with a micrometer before etching.
This afternoon, I tried to measure the etch rate of the KOH bath. I did the following:
I had intended to remove the wafer and measure its thickness again, so unfortunately can only place a lower bound on the etch rate. Nonetheless, the implied rate of etching is >16.5 um/minute (etched through at least 290 um in 35 minutes, from both sides of the wafer). This is more than an order of magnitude faster than expected, even allowing for the increased bath temperature. Clearly I am missing something -- is the KOH actually being diluted due to some additional DI water in the pumping system? Is the boron doping really increasing the etch rate by that much? Did the wafer just fall off of its holder and get lost in the murky KOH (I did fish around for several minutes, no sign of a wafer in the bath)?