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:

1. Submerge wafer in 30% KOH solution at 80 C for 6 hours
2. Remove and rinse the wafer, then scribe and break along the lines between the cantilevers
3. Return the separated pieces to the KOH bath for an additional 2 hours
4. Rinse, dry, and briefly HF etch the resulting cantilevers

something wrong with the KOH etch

I tried to follow this recipe last Wednesday, with the following modifications:

• Our wafer is only 300 um thick
• KNI now stocks 50% KOH instead of 30% KOH. This is probably good for us, since higher molarity KOH results in more specular etched surfaces. But it does change the etch rate
  • According to Fig 2.28 in Silicon Micromachining (fig from Seidel et al), The etch rate for Si in KOH should decrease from 50 um/hour in 30% KOH to about 30 um/hour in 50% KOH

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

possible explanations

Expected rate of etching was off

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. 

We didn't really have 400 nm of hard SiNx on both sides of the wafer

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. 

Wafer was thinner than 300um

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.

Update

This afternoon, I tried to measure the etch rate of the KOH bath. I did the following:

• Auto-tuned the PID parameters of the KOH bath's temperature controller. The settings ended up being unchanged from before
• Heated the bath to a nominal temperature of 80 C. I put a thermometer in the bath and found the true temperature to be 85 C. After finishing for the day, I set the calibration offset of the temperature controller such that the thermometer reads the true temperature. 
• Tested the rate of etching with a wafer I'd recently RCA cleaned
  • The wafer is from the same batch as the previous wafer: 3" diameter, boron doped with resistivity 1-10 Ohm-cm, and double side polished
  • Before starting, I used a micrometer to measure the thickness of the wafer at 0.30 mm. 
  • Prepared a solution of 2.5% HF, and etched the wafer for 2 minutes then rinsed with DI water
    • After the HF etch, I measured the thickness of the wafer at 0.29 mm. The purpose of the HF etch was to remove any SiO2 from the surface of the wafer before bathing in KOH.
  • Place the wafer in KOH bath (85 C true temperature) for 35 minutes. The wafer was completely dissolved in this time.

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)?