Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
1999-02-01
2001-08-28
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038070, C372S032000, C372S033000
Reexamination Certificate
active
06282218
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to laser control circuits and, in particular, to control circuits for controlling the laser wavelength.
BACKGROUND
Lasers are used for many applications. In one example, lasers are used in steppers for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the stepper are designed for a particular wavelength of the laser. The laser wavelength may drift over time and, thus, some means is typically employed to detect the wavelength of the laser and correct the wavelength as necessary.
In one type of feedback network used to detect and adjust the wavelength of a laser, an etalon receives a portion of the emitted light from the laser. 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 position of the bright center portion of the interference pattern is used to determine wavelength to a relatively coarse degree, such as to within 5 picometers (pm). The diameter of a light band is used to determine the wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm. The width of a light band is used to determine the spectral width of the laser output. The interference pattern is usually referred to as a fringe pattern.
In order to measure the light levels in the fringe pattern, the fringe pattern must be optically detected by a sensitive photodetector array and the resulting signal amplified. This signal usually contains errors due to manufacturing and temperature related variances in the components forming the feedback system. The amplified signal is then applied to an analog-to-digital (A/D) converter. Since the analog signals applied to the A/D converter have a relatively large dynamic range, the A/D converter must also have a large range, such as at least 12 bits of quantization, in order to adequately resolve small signals as well as large signals. Such a wide-range A/D converter and the processing circuits required to process this wide range are relatively expensive.
What is needed is a technique to lower the cost of the feedback path in such a laser control system without losing accuracy in the measurement of the wavelength.
SUMMARY
An automatic gain control circuit in the feedback path for a laser wavelength control circuit is described herein. This gain control circuit automatically adjusts the amplification of the analog signals output from a photodetector array, where the array detects a fringe pattern created by a laser beam.
A microprocessor, or other suitable circuit, in he feedback path determines a peak level of the fringe pattern signal and sets the gain of the amplifier so that the amplified peak signal, when converted by a downstream A/D converter, always results in a digital signal within a specified upper output range of the A/D converter. Thus, the A/D converter can have a much smaller dynamic range (e.g., 8 bits) than A/D converters (e.g., 12 bits) used in feedback paths having fixed gains. In the preferred embodiment, the automatic gain adjustment reduces the dynamic range of the analog signals to one-twentieth of the unadjusted signals, allowing the A/D converter range to be reduced by 4 bits. By only requiring a 1-byte A/D converter, downstream processing circuits can also be reduced in size.
Another feature of the preferred embodiment feedback circuit is the automatic setting of a DC offset voltage that compensates for errors in the feedback path and enables an accurate determination of a dark level signal in the fringe pattern signal. This dark level signal provides a reference for measuring the magnitude of the fringe pattern signal. Very small photodetector outputs may now be accurately measured. Errors corrected by this DC offset voltage may be due to manufacturing variances in the components as well as due to performance variations of the photodetector array and other components with temperature.
A microprocessor detects the minimum output by the A/D converter, assumes these minimum signals are dark level signals, and adjusts the DC offset voltage for an upstream amplifier as necessary to ensure the dark level signals output by the A/D converter are a predetermined value.
The preferred embodiment feedback circuit also employs a novel amplifier anti-saturation circuit. The center portion of the circular fringe pattern is very bright compared with the surrounding fringe pattern. As the photodetector array is scanned to read the center portion of the fringe pattern, the high magnitude signal may saturate the sensitive amplifier amplifying the photodetector array signals. Such saturation distorts the falling edge of the center signal in the fringe pattern, creating an erroneous reading of the position of the center portion. This creates error in the determination of the wavelength.
The distortion can also affect the determination of a dark level reference signal. The preferred embodiment of the feedback circuit incorporates an extremely fast anti-saturation circuit for the amplifier. Anti-saturation circuits are known which use conventional diodes or zener diodes. Applicant has discovered that unexpected results are obtained by using light emitting diodes (LED's) as the anti-saturation diodes. Using these LED's results in a faster clamping reaction time for the amplifier to prevent the amplifier from going into saturation. This faster reaction time allows the photodetector array to be scanned at higher speeds.
REFERENCES:
patent: 5867514 (1999-02-01), Anderson
Cymer Inc.
Font Frank G.
Ogonowsky, Esq. Brian
Rodriguez Armando
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