Convolution method for measuring laser bandwidth

#Spectrometers are well known devices for measuring the spectra of laser beams (i.e., the intensity of light in the beam as a function of wavelength). Ultraviolet laser light sources used for modem integrated circuit lithography are required to have very narrow bandwidth and operate within tight bandwidth specifications. Spectrometers used to measure the spectra of these lasers can be divided into two main categories: diffraction grating based spectrometers and Fabri-Perot etalon based spectrometers.

#A description of a typical KrF excimer laser used for lithography is provided in U.S. Pat. No. 5,991,324 which is incorporated herein by reference. There are two spectral bandwidth characteristics of these lasers which are referred to extensively in microlithography applications. These are the spectral bandwidth of the laser measured at 50 percent of the peak intensity, called its full width at half maximum bandwidth (abbreviated FWHM), and the spectral bandwidth, which contains 95% of laser energy called the 95% integral bandwidth (abbreviated 95% I). It is very important that the laser is always operating within specifications during microlithography chip manufacturing because spectral broadening causes blurring of the integrated circuit features being printed on silicon wafers which would result in yield problems. Therefore, it is very important to provide continuous accurate monitoring capabilities for the laser spectrum.

#A measure of the quality of a spectrometer is its slit function. This is the spectrum which is recorded by the spectrometer when measuring a very very narrow spectrum. For a spectrometer to accurately measure the spectrum of a laser beam, the slit function bandwidth of the spectrometer itself should be substantially smaller than the laser bandwidth.

#FIG. 1 is a flow chart showing the principal steps for practicing a preferred embodiment of the present invention.

#FIG. 2A is a drawing showing the principal components of a prior art spectrometer.

#FIG. 2B is a graph of the slit function of a prior art diffraction grating spectrometer.

#A preferred embodiment of the present invention may be described by reference to the drawings.

#Equipment

#This first preferred embodiment is preferably used to measure the bandwidths of a lithography gas discharge laser such as a line narrowed KrF laser, a line narrowed ArF laser or an F2 laser. A preferred spectrometer is a grating spectrometer known as the ELIAS spectrometer (an acronym for Emission Line Analyzing Spectrometer) which is supplied by Lasertechnik Berlin—GmbH with offices in Berlin, Germany. Preferably a personal computer running with Windows NT and a special algorithm described herein is used to calculate bandwidth parameters.