Television – Mechanical optical scanning – By acoustic wave
Reexamination Certificate
2000-03-08
2003-03-25
Kostak, Victor R. (Department: 2611)
Television
Mechanical optical scanning
By acoustic wave
C359S305000, C385S007000
Reexamination Certificate
active
06538690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to acousto-optic scanners, and is particularly directed to an arrangement and method for controlling pixel clock and scan timing of an acousto-optic scanner, in accordance with the acoustic velocity of an acoustic wave lens (ATWL) traveling through the scanner's acousto-optic waveguide.
BACKGROUND OF THE INVENTION
A number of industrial systems for conducting extremely high resolution optical scanning of a workpiece, such as a semiconductor substrate, may employ an acousto-optic Bragg cell-based scanner. Critical to success of operation of such systems is the need for extreme precision in the alignment of the light beam and the workpiece. An acoustic traveling wave lens (ATWL) scanner is capable of providing such position accuracy while scanning at very high speed. This positioning accuracy of the ATWL scanner is derived from the fact that the scan progresses with a traveling acoustic wave in a highly stable material, such as fused silica.
Fused silica has excellent dimensional stability due to its low thermal expansion coefficient of 0.6×10
−6
per degree Centigrade. However, the variation in the acoustic velocity of fused silica is much higher. The temperature coefficient for scan velocity is near 1×10
−4
per degree Centigrade. As a consequence, the principal placement error in an ATWL scanner arises from the change in acoustic velocity with temperature. It is common practice to vary the sampling time during a scan, in order to compensate for scanner placement errors. Namely, the time of taking or exposing samples is varied in such a way as to cause the samples to occur in the proper place on spatial sampling grid.
SUMMARY OF THE INVENTION
The present invention is directed to a new and improved apparatus and method for generating a pixel clock for an ATWL-based optical scanner, wherein the pixel clock is varied in such a manner to provide a uniform and constant sampling grid, independently of small acoustic velocity variations in the ATWL propagation medium. As will be described in detail below, the pixel clock is derived as a function of the propagation velocity in the ATWL medium in such a manner to render each pixel spatially invariant to propagation velocity changes in the ATWL medium. This means that as changes in temperature retard or increase the speed of the pressure-induced lens traveling from the excitation transducer to the end of the ATWL cell on each scan, the pixel clock is correspondingly slowed down or speeded up by the same proportional amount, so as to maintain registration in time and space. The compensation mechanism employed by the present invention measures the time it takes for the pressure induced lens to travel the length (or large portion thereof) of the ATWL cell. It then forces the pixel rate to produce a desired number of pixels within the same time interval.
In accordance with the invention, scanner system timing is governed by an acoustic velocity-driven, phase locked loop containing an adjustable voltage controlled pixel clock generator (VCXO), which is controlled by a detector that produces delayed and attenuated replica of the excitation waveform applied to an ATWL scanner used to scan a light beam across a workpiece. Pursuant to a first embodiment of the invention, an end-of-cell transducer converts the pressure induced traveling lens into an electrical signal replica of the excitation input. In a second embodiment, an end-of-scan optical pick-off monitor is employed to detect the scanned optical spot as it crosses its field of view.
The pixel rate clock signal is used to locate spatially repeatable time instances along the optical scan of the ATWL scanner. The pixel rate clock signal is coupled to a subharmonic rate generator, which outputs a relatively low rate clock signal having pixel registration edges, so as to facilitate scan cycle timing events through use of commercially available logic devices. The reduced rate clock signal is used to clock a cycle timing generator, programmable delay line, a set of cascaded flip-flops, and an up/down counter that drives a digital-to-analog converter (DAC) whose output of which is used to adjust the clock rate of the VCXO.
Using the low rate clock signal produced by the subharmonic rate generator, the cycle timing generator initiates a scan cycle and all subsequent scan events, in response to an externally sourced scan request strobe. This programmable delay line provides for fine tuning of the pixel rate VCXO about its nominal center of range of operation, and thereby allows the full VCXO range of to be applied to pixel rate compensation due to temperature-induced propagation velocity changes experienced by the ATWL scanner under normal operating conditions. Also the small fixed propagation delays associated with various other devices, cables, filters, etc., are readily removed using the programmable delay line.
The output of the delay line is coupled to a direct digital synthesis (DDS) based five-cycle burst generator that is enabled for a prescribed number of subharmonic cycles (beginning at a first selected ATWL gate clock count terminating at a second clock count of the output of the cycle timing generator. This produces a scanner excitation waveform comprised of a fixed plurality of cycles of a reduced clock signal. This excitation waveform is low pass filtered to produce a filtered burst signal that is amplified and applied to the ATWL scanner by way of an input transducer.
The acoustic traveling wave lens scanner cell may comprise of section of optical material that supports the propagation of a pressure wave (or series of acoustic waves) with low attenuation along its length from the input transducer to an output detector (end-of-cell output transducer in the first embodiment, end-of-scan optical monitor in the second embodiment). The traveling pressure wave creates a lens that provides a relatively high spot resolution of the deflected optic beam aligned with the lens as it travels the length of the ATWL cell.
A buffer amplifier and zero-crossing comparator coupled to the scanner detector output amplify the output signal derived from the ATWL scanner and convert the attenuated, delayed replica of the scanner's excitation signal into logic levels that are sampled for polarity. The first of the two cascaded flip-flops monitors the output of the zero-crossing comparator, while the second flip-flop monitors the output of the first flip-flop. The first flip-flop determines if a selected (e.g., third negative-going) zero-crossing of the delayed excitation replica of the scanner's input burst signal occurs early or late, relative to a particular pixel subharmonic cycle's leading edge, and samples and stores this decision. The second flip-flop reduces the probability of a metastable output being coupled to the up/down counter.
The up/down counter increments on a relatively later (e.g. 298th) subharmonic cycle produced by the cycle timing generator, if the digital input to the counter from the second flip-flop is a logical “zero”—indicating that third negative-going zero crossing of the delayed replica of the scanner's excitation burst had already occurred prior to a slightly earlier (e.g., 296th) subharmonic cycle sampling the first flip-flop. Conversely, loading “one” is loaded into the first flip-flop on the 296th subharmonic cycle indicates that the pixel clock rate is too fast, since the leading edge of the 296th subharmonic cycle occurred before the third cycle's negative going zero-crossing of the excitation replica (i.e., the subharmonic cycle was early) and the up/down counter is decremented by one. The DAC produces an analog voltage proportional to the digital count stored in the up/down counter. At a minimum count, the DAC generates a voltage to drive the VCXO to its minimum output frequency (300 MHz−200 ppm), while at a maximum count it generates a voltage to drive the VCXO to a maximum output frequency (300 MHz+200 ppm).
The servo-mechanism of the inventi
Montgomery Mark T.
Montgomery Robert M.
Otten Thomas
Ward Reeder N.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Kostak Victor R.
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