Background of the Invention
An important use of gas discharge excimer lasers is to provide high quality light sources for integrated circuit fabrication. These light sources are used by stepper machines and scanner machines for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the stepper and scanner machines are designed for a particular laser beam with a narrow band of wavelengths. The output beam of the excimer laser is typically comprised of a very narrow band of wavelengths distributed about a xe2x80x9ccentralxe2x80x9d wavelength (referred to as the xe2x80x9cline centerxe2x80x9d wavelength) approximately in a gausian distribution. For krypton fluoride (KrF) and argon fluoride (ArF) excimer lasers the output beam is xe2x80x9cline narrowedxe2x80x9d to produce the desired narrow band of wavelengths. The laser center line wavelength may drift over time and, thus, a feedback network is typically employed to detect the wavelength of the laser and to control the wavelength within a desired range.
In one type of feedback network used to detect and adjust the wavelength of a laser, a grating and an etalon each receive a small portion of the emitted light from the laser. The spatial position of a band of light reflected from the grating determines the center line wavelength of the laser beam to a coarse degree. The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The concentric bands surround a center bright portion. The diameter of one of the concentric bands is used to determine the center line wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm.
Various feedback methods are well known for wavelength tuning of lasers using the measured wavelength values. Typically the tuning takes place in the same device that line narrows the laser output. This device is referred to as a line narrowing package (LNP) or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the laser""s discharge cavity through which a portion of the laser beam passes into the LNP. There, the portion of the beam is expanded with a prism beam expander and directed to a grating which reflects a narrow selected portion of the laser""s broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment in the beam path. The wavelength of the beam is measured for each pulse and an error signal is calculated and used as a feedback signal to position the grating or the mirror to minimize the error. These prior art wavelength control techniques are very effective in maintaining average wavelength values within a desired range over relatively long time periods. However, they have not been very effective in controlling wavelengths over short time periods of about 3 to 30 milliseconds or very short time periods of about 1-3 milliseconds or less. Prior art wavelength feedback control techniques had response times of a few milliseconds which was the time required to detect a shift in the wavelength and adjust the illumination angle.
A typical mode of operation for an excimer laser used as a light source for integrated circuit fabrication is known as xe2x80x9cburstxe2x80x9d mode operation. In this mode, the laser beam illuminates a dye spot on a silicon wafer with a xe2x80x9cburstxe2x80x9d of, for example, 250 pulses at a pulse rate of 2000 Hz (in this case) in 0.125 seconds. The laser is then xe2x80x9coffxe2x80x9d for about 0.2 seconds while the lithography machine moves optical components so as to cause the next burst to illuminate the next dye spot. This sequence continues until all of the dye spots in the wafer have been illuminated after which the laser is off for about 1 minute while a new wafer is loaded. FIG. 1 is a graph 10 which illustrates the center line wavelength shift during a burst of pulses from a laser operating at 1000 Hz. In particular, FIG. 1 indicates a wavelength shift of about +0.1 pm to about xe2x88x920.09 pm from a desired wavelength output over a time period of about 35 milliseconds. Wavelength shifts of this type are referred to as wavelength xe2x80x9cchirpxe2x80x9d. These chirps often are very predictable, coming at the same time (always at the beginning of the burst) during each of many bursts of pulses. As shown in FIG. 1, after the wavelength chirp, the wavelength output settles down to wavelength shifts occurring rapidly and seemingly randomly but with a maximum magnitude of less than about 0.05 pm. Applicants believe that this wavelength chirp near the start of a burst of pulses is primarily due to changing acoustic disturbances within the discharge region of the laser. The pattern of change is affected by the temperature of the laser gas. These shifts occur over time periods in the range of a few to several milliseconds. Conventional wavelength correction techniques do not adequately correct these large and sudden wavelength shifts near the beginning of each burst of pulses.
Conventional prior art wavelength correction techniques also are not adequate to correct the small very rapidly occurring (high frequency) wavelength shifts that occur throughout the burst. The high frequency wavelength shifts are believed to be caused primarily by vibrations of the laser optical components including those in the LNP itself. Most of the vibration type shifts are believed to be primarily attributable to laser""s rotating fan and its motor drive and to the periodic electric discharges of the laser. Vibration modes may be amplified by resonant conditions of various laser structures including the LNP and its components.
Excimer lasers operating in a burst mode also produce a pulse energy chirp similar to the wavelength chirp. Prior art methods have been disclosed to minimize the pulse energy chirp. One such method is described in an article by the inventors"" co-workers, xe2x80x9cAdvanced Krypton Fluoride Excimer Laser for Microlithography, SPIE Vol. 1674, xe2x80x9dOptical/Laser Microlithography V, (1992) 473-484, see page 480.
What is needed is equipment to control the wavelength of gas discharge lasers over short and very short time periods in the range of a few microseconds to about 5 milliseconds.
The present invention provides an electric discharge laser with fast wavelength correction. Fast wavelength correction equipment includes at least one piezoelectric drive and a fast wavelength measurement system and fast feedback response times. In a preferred embodiment, equipment is provided to control wavelength on a slow time frame of several milliseconds, on an intermediate time frame of about one to five milliseconds and on a very fast time frame of a few tens of microseconds. Preferred techniques include a combination of a relatively slow stepper motor and a very fast piezoelectric driver for tuning the laser wavelength using a tuning mirror. A preferred control technique is described (utilizing a very fast wavelength monitor) to provide the slow and intermediate wavelength control with the combination of a stepper motor and a piezoelectric driver. Very fast wavelength control is provided with a piezoelectric load cell in combination with the piezoelectric driver. Preferred embodiments provide (1) fast feedback control based on wavelength measurements, (2) fast vibration control, (3) active damping using the load cell and an active damping module, (4) transient inversion using feed forward algorithms based on historical burst data. A preferred embodiment adapts the feed forward algorithms to current conditions. Another preferred embodiment measures tuning mirror position to permit wavelength pretuning and active wavelength tuning.
FIG. 1 is a prior art graph of measurements of wavelength drift over a burst of pulses from a laser.
FIGS. 2A-D are prior art graphs of measurements of wavelength drift over four sequential bursts of pulses from a laser.
FIG. 3 is a graph of measurements of wavelength drift over a burst of pulses from a laser that has its wavelength output corrected using a slow response stepper motor.
FIGS. 4, 4A and 4B show a proposed technique for providing fast and finer wavelength control.
FIG. 5 is a drawing of a wavemeter.
FIGS. 5A and 5B show how wavelength is calculated.
FIG. 6 is a drawing depicting the surface of a photo diode array.
FIG. 6A shows how grating and etalon images appear on the surface of the FIG. 6 photo diode array.
FIG. 7 shows the arrangement of wavelength calculating hardware.
FIG. 8 and 8A show a fast mirror driver and a control module.
FIG. 9 shows a feedback control algorithm flow chart.
FIG. 10 shows test results.
FIG. 11 shows a feedforward control algorithm flow chart.
FIG. 12 shows test results.
FIGS. 13A and 13B show test results.
FIGS. 14A, 14B and 14C show features of a preferred embodiment.
FIG. 15 show an embodiment with a mirror position detector.