Ultrasonic monitoring of photoresist prebake using TOF measurement

We have measured the time of flight (TOF) through a 4" silicon wafer with and without a 1.8 μm coating of Shipley 1813 positive photoresist. The TOF change from bare to coated wafer was measured three times on a single wafer; the average of the three experiments was a TOF of 14.3 ns (+/-2.5 ns), consistent with the calculated expected results. In order to increase the precision of the TOF measurement to the level needed for prebake monitoring, we have applied a least squares algorithm to estimate the transfer function between the echo and received waveforms. Results of pulsed data studies indicate that we can get a decrease in the standard deviation of the TOF measurement from 2.93 ns to 2.53 ns by determining the TOF from the estimated transfer function