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July 2000 |
Lasers
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Quality-assurance
applications increasingly call upon Process control and on-line QC/QA applications place stringent demands on laser performance, especially on its output stability, reliability, and short mean time to repair (MTTR). The latter factor is especially important because production-line down time often represents a far greater portion of the total cost of ownership for process-control machinery than the equipment's original purchase price. To this end, laser manufacturers have continued to enhance and optimize established laser technologies, as well as develop entirely new design concepts. This article examines two representative on-line QC applications and briefly reviews some of the significant laser advances that have contributed to their success. Darkfield wafer inspection is used for detecting submicron-sized defects and particulate contaminants at several points in the fabrication of semiconductor wafers. The technique is particularly useful as an adjunct to chemical mechanical planarization (CMP), which involves using a polishing pad and abrasive slurry to planarize the wafer. CMP delivers better planarization than prior technology, enabling the production of circuits with a larger number of layers. Unfortunately, CMP can scratch the wafer, as well as leave behind residue from the polishing slurry.
In a darkfield inspection system produced by Inspex of Billerica, MA, a laser illuminates a small area of the wafer under test from an oblique angle. Collection optics and a CCD camera, placed perpendicular to the surface of the wafer, are used to detect any scattered light (Figure 1). In the absence of any defects, all the light is specularly reflected, and nothing reaches the camera. A defect-free wafer therefore appears completely dark to the camera. Any scratches or contaminants will scatter some of the laser light into the detection optics, causing them to appear as bright features. By automated translation of the wafer, the entire surface can be rapidly inspected. Once the wafer has been patterned, periodic circuit structures will diffract the laser light, and can also show up as bright features. There are several techniques for distinguishing this diffracted light from the light scattered by defects. The most flexible and effective of these is to adjust the illumination geometry so as to direct the brightest diffracted orders away from the collection optics. In the Inspex instrument, this is accomplished by varying the laser's angle of incidence (altitude), as well as by rotating the plane of incidence (azimuth).
The source for this application was developed by Spectra-Physics Lasers, in cooperation with Inspex, specifically to meet the needs of darkfield wafer inspection. The result was a frequency-doubled (532-nm) diode-pumped solid-state Nd:YVO4 laser, which delivers 200 mW of continuous-wave output power in a TEM00 beam (M2<1.1). All cavity optics in this laser are mounted on a monolithic I-bar structure: this approach delivers excellent mechanical rigidity and makes optical alignment insensitive to temperature changes. The entire cavity itself is completely sealed to prevent outside contaminants from entering. The total elimination of epoxy adhesives or rubber seals within the laser prevents internal contamination from outgassing. The result is a laser that never needs cleaning or adjustment of its cavity optics.
MTTR is minimized in this design by locating the pump lasers in the power supply and coupling them into the laser head via fiber optics. This enables diode replacement to be performed without disturbing the optical alignment of the laser head. This is very important in the Inspex system, which requires accurate alignment of the laser beam with the instrument's optics. Extended lifetime and reliability are insured by running the laser diodes at derated levels. Testing has proven that after 18,000 hours of operation less than 8 percent of laser diode current headroom has been consumed (Figure 2). Sorting is used throughout the food industry to eliminate spoiled products, unwanted plant or animal parts, and foreign objects. Typical examples include separating shrimp meat from peel, legs, eyes, and heads, and sorting nut meat from shells. Barco Machine Vision of Aarschot, Belgium, is a leading producer of automated sorters for the food, recycling, tobacco, and textile industries. Their equipment utilizes a wide variety of illumination and vision technologies, such as CCD cameras, x-rays, LEDs, and lasers.
Figure 3 shows a simplified schematic for a typical laser-based sorter from Barco. The product to be examined falls in a wide, thin sheet from a vibrating platform or conveyor belt. A rotating polygon scanner sweeps one or more laser beams perpendicular to the direction of product motion. The combination of rapid beam raster scanning and product motion enables all of the product to be sampled. A second identical optical system is used to view simultaneously the other side of the product stream. Beamsplitters and other optics separate off light returned from the product, which is then split into three separate beams and sent to detectors. The first detector looks at the center of the returned beam, the second blocks the center and looks at the periphery of the returned light, and the third detects all the returned light. By combining and comparing the output from these detectors, the system can differentiate specular reflection from scattered light. The intensity of the specular reflection provides information on the color of the product. This enables the system, for example, to differentiate a French fry with a dark spot on it from an unblemished one. The intensity of the scattered light enables the system to sense differences in texture, even in the absence of a color difference. This, then, might be used to separate a white rock from a white bean. Once an unwanted object is identified by the system, it is blown out of the product stream with an array of air nozzles.
Multiple lasers at various wavelengths can be used to allow the detection of several different colors. Multiple beams must be rendered colinear through the use of dichroic filters before entering the scan optics; colinearity is necessary to insure that each laser is examining exactly the same point. The first step in selecting a laser source for food sorting is to determine the wavelength required to make the necessary color differentiation. After wavelength, the most important parameters are output stability, beam quality, and reliability. Output stability insures consistent measurement, and beam quality is required to maintain the proper focused spot size. The air-cooled argon ion laser has proved to be an advantageous source for Barco, for many reasons. It can simultaneously supply two wavelengths (488 and 514 nm) that cannot be obtained from any other source, and that are perfectly colinear, thus eliminating the need for beam-combining optics. Argon ion lasers also provide the requisite amplitude stability, noise characteristics, beam profile, and polarization characteristics. The air-cooled ion laser currently being supplied by Spectra-Physics for this application has also been specifically engineered for long lifetime, typically performing within specification for an average of 10,000 hours. These lifetimes have been achieved through the use of high-efficiency mirror coatings, which allow efficient operation with a low tube current. Spectra-Physics has also designed this laser to minimize MTTR. The most involved repair task involves putting a new plasma tube into the laser head. These tubes are prealigned at the factory and precisely positioned in the head in the field using locating pins. Thus only minimal system realignment is required after tube replacement. Maturing laser technology has resulted in products with the reliability and performance characteristics necessary to meet the rigorous demands of many on-line process control and inspection applications. Continued development of more rugged and long-lived lasers, especially based on all-solid-state technology, promises to push the laser into ever more diverse uses. The authors of this article are Michael Watts, product manager, OEM business unit, and Donna Berns, marketing manager, commercial lasers division, Spectra-Physics Lasers Inc.; Joe Danko, vice president of R&D and engineering, and Rob Simpson, director of sales and marketing, Inspex; and Johan Calcoen , R&D optics manager, and Marc van Gerven, marketing manager, Barco Machine Vision. For more information contact Watts at Spectra-Physics Lasers, 1305 Terra Bella Ave., Mountain View, CA 94043; (650) 966-5761; mwatts@splasers.com; or Berns at (650) 966-5808; dberns@splasers.com.
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