Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1998-07-16
2001-06-26
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06250161
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
The present invention relates generally to an ultrasonic photoresist process monitor and method and more particularly to an in situ monitor which monitors the processing of photoresist film, particularly pre-exposure bake conditions and film thickness.
BACKGROUND OF THE INVENTION
In the manufacture of integrated circuits the various circuit components are defined by using a photoresist layer which is applied to the surface of a semiconductor wafer, exposed and developed. The photoresist is applied to the surface of the wafer by applying a few drops of liquid resist onto a spinning wafer to obtain a photoresist film of predetermined thickness. The photoresist liquid film is prebaked to remove any excess solvent that is initially added to the resist as well as to anneal the polymer film, allowing the resin molecules to relax into their lowest energy state. This thin film of resist is typically on the order of 1-2 &mgr;m thick. The film thickness changes about 0.5 &mgr;m during prebake.
If the solvent is not fully evaporated or if the prebake time is too long, then the feature size of the developed film may not be as small as expected. It is important when heating the resist for prebake that the temperature of the film reach and exceed the glass transition temperature (T
g
) in order to facilitate the removal of solvent and formation of polymer chains. It is important that the bake time be controlled. If the glass transition temperature is measure then the processing time may be easily controlled. In practice, a calibration run will provide the time T, after T
g
is reached, required to complete the prebake cycle. It is also important to monitor the postbake or cure process after exposure and before development so that the wafer is not heated above the softening temperatures where features would be altered.
Several techniques are available to measure T
g
but none of them have been applied in situ for endpoint detection of photoresist softbake. There has been research in endpoint detection of the prebake process. T. E. Metz et al. “Process Module Metrology Control and Clustering”, SPIE, Vol. 1594, 1991, pp. 145-152, performed real-time measurement of resist film thickness on silicon wafers using multi-wavelength reflection interferometry. They determined resist thickness vs. spin and bake time. This method was used to monitor non-uniformities for statistical process control.
There is a need for in situ photoresist monitoring as feature size surpasses sub-quarter micron size since it is expected that the resist materials will be more sensitive to process conditions such as temperature and time.
The development of the film after it has been laid down, prebaked and exposed involves the selective dissolution of exposed areas of positive tone photoresist (unexposed areas of negative resist) by applying a basic solution to the wafer and spinning. It is important to determine when all of the exposed resist is removed so that the wafer can be immediately advanced to the next processing step, thereby saving time and increasing throughput.
OBJECTS AND SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an in situ photoresist process monitor and method for monitoring photoresist process conditions.
It is another object of the present invention to provide an ultrasonic photoresist process monitor.
It is a further object of the present invention to provide a photoresist process monitor which has fast response and provides repeatable process conditions.
It is a further object of the present invention to provide a monitor which measures glass transition temperature of a photoresist during its pre-exposure bake.
It is a further object of the present invention to provide an ultrasonic monitor which measure the glass transition temperature of photoresist during its pre-exposure bake (prebake), and also during post-exposure baking.
The monitor of the present invention determines the glass transition temperature, T
g
, of a photoresist film during prebake processing by measuring the change of phase of a high frequency ultrasonic pulse as it is reflected from the interface between the wafer and the photoresist film.
It is a further object of the present invention to provide a photoresist monitor which measures the photoresist film thickness during development and determines when the exposed film has been removed.
The photoresist process monitor includes a transducer which transmits ultrasonic waves into the underside of a wafer and receives ultrasonic waves reflected from the interface between the wafer and a photoresist film carried on the other surface of the semiconductor wafer. The monitor provides an output signal representative of the phase difference between two successive reflected waveforms from the silicon photoresist interface. The output signal is processed to provide an indication of glass transition temperature and/or film photoresist thickness.
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Ballantine, David S., Jr. et al.; “Use of Saw Devices to Monitor Visco-Elastic Properties of Materials”,IEEE1988, pp. 559-562.
Ballantine, D.S. Jr., et al.; “Acoustic Wave Sensors: Theory, Design, and Physico-Chemical Applications”,Academic Press, Chapter 4, pp. 154-167.
Degertekin F. Levent
Khuri-Yakub Butrus T.
Morton Susan
Board of Trustees of the Leland Stanford Junior University
Flehr Hohbach Test Albritton & Herbert LLP
Moller Richard A.
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