We have developed an in situ method to measure the change in thickness
of photoresist during development. The phase of a high frequency ultr
asound signal is monitored as it is reflected from the silicon/photore
sist interface during resist development. The method was tested using
a 1.5 mu m film of Shipley 1811 resist. The total phase change during
development of 19 degrees was consistent with theoretical calculations
at 280 MHz, and this change was used to obtain the resist thickness d
uring development. The method was used to find the development rate of
this positive-tone resist as a function of exposure dose in the 20-68
mJ/cm(2) range. As expected, there was an increase in development rat
e as the exposure time increased; this continued up to about 40 s of e
xposure, beyond which the rates were essentially unchanged. Measuremen
ts on a wafer with microelectronic devices ranging in topography from
0.10 to 1.0 mu m show that the method is applicable to wafers with typ
ical circuit topography. (C) 1998 American Institute of Physics. [S000
3-6951(98)03341-5].