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NDSCS Tech Transfer - July 2011
The transfer of the UNM MTTC Pressure sensor process to NDSCS facility is going well. This article is meant primarily for the SCME and NDSCS folks as a place to keep some of the critical information so that we can refer back to this.
Backside Pattern/Nitride etch process
We started with wafer prime (liquid HMDS) 1813 resist, 3000rpm, 1min PEB at 125C which gave us a thickiness of approximately 1um. We also coated a wafer twice - HMDS, coat 1, coat 2, PEB 1min at 125C.
We ran these wafers through the RIE etcher with two versions of the etch recipe. The first one had the mfc set at a very low sccm for O2 and CF4 and the power at 250W. The plasma color was a pink/purple hue. The etch rate of the resist was about 30A/sec or 1800A/min. The second recipe was using an O2 flow rate of 10sccm and CF4 flow rate of 30sccm based on a paper by B. E. E. Kastenmeier, "Chemical dry etching of silicon nitride and silicon dioxide using CF4/O2/N2 gas mixtures". This paper shows that a 1/3 gas flow ratio of O2/CF4 works well. The resist etch rate for this second recipe was about 40nm/sec or 2400A/min.
We ran a bare nitride wafer at both recipe 1 and 2. Recipe 1 - 100A/min, Recipe 2 - 390A/min
Note: Recipe 2 plasma was a blue color, and we settled on using recipe 2.
We rant a test wafer, double coated with 1813, patterned (60sec exposure instead of the standard 30sec), 1min PEB at 125C, 1min develop. Patterned well, areas clear that were supposed to be clear. Used the chrome on glass cantilever mask NDSCS has (the PS mask was not availble).
We decided to run a 20min etch, 10min, measure, rotate 180 degrees and run another 10min.
Observations: this is too long! After the first 10 minutes, there was little resist left and we measured only 300nm of nitride in the open areas. The etch looked uniform (but we may of been already cleared after the 10minutes). The etch rate of nitride was about 70nm/min (700A/min). Much higher than expected. After thinking about this, we decided that it may be higher since the open area is lower!
Day 2 - ran a double 1813 coat at 2000rpm, softbake at 1min 90C. Exposed 60sec using transparency mask, develop 45sec. Etched with Recipe 2 for 12 min - all on wafer backside. Looks clear. Etch in 40% KOH solution at about 90-100C (was heating up).
KOH etch rates can be found on the BYU site. These data show that the highest rate is at 20% KOH (gms/100ml) but ther is a note that it produces a rough etch. There is also a note that IPA is often added to smooth the etch out.
There is a good reference from Virginia Semiconductors on etching and cleaning of silicon wafers.