Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-03-29
2003-09-16
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201800, C356S609000, C356S631000
Reexamination Certificate
active
06621060
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an autofocus feedback method and system for laser processing of materials.
BACKGROUND OF THE INVENTION
Laser processing, such as micromachining, is a technique that offers precise, non-contact and accurate machining of very small components, and is an emerging advanced manufacturing technology that is being adapted to widely diverse industrial applications. Conventional mechanical machining can produce work-pieces and assemblies with typical feature sizes larger than a few hundred microns. However, the steadily increasing demand for smaller sizes requires new tools and processes, of which laser micromachining is one.
When laser processing or micromachining a work-piece, the beam from a laser is focused onto the work-piece with a lens assembly. Typically, it is at the focal point of the lens, where the laser beam is the smallest and hence the most concentrated, that the machining is meant to occur. The focus of the beam is usually where the laser is most efficient at cutting, drilling or otherwise modifying the work-piece. The laser beam is considered to be “at focus” over a working distance called the confocal parameter, or working focal range, which is the distance around the focus along the laser beam where the energy density of the laser beam is close to that at the focus, and therefore good enough to do the work and/or to achieve required machining properties. The distance between the lens and its focal point is fixed by the dimensions and specification of the lens, and is constant for any given lens and any given laser beam. During the machining process, the work-piece is moved under the fixed laser beam such that the features are machined according to the wanted design. Alternatively, the work-piece can be stationary but then the laser focus has to move. Of critical importance for the best possible machining parameters is to have the work-piece at a constant distance from the lens to keep its surface within the working focal range. In most cases, laser micromachining is performed on flat work-pieces. If the work-piece has deformations along the laser beam axis that are greater than the working focal range of the laser, these deformations will cause the laser to go out of focus on the work-piece, such that the laser will not properly machine in that area. Even though the work-piece is fastened to a holder or substrate, deformations due to the heat damage of the laser or preexisting deformations may result in that region of the work-piece being unmachinable. Similarly, it becomes difficult to machine work-pieces that have a curved surface by design. Sometimes, it is preferred to process the work-piece with the laser focus intentionally below or above the work-piece surface, but the problem of keeping the focus at a constant distance with respect to the work-piece surface remains the same.
During laser machining, it thus becomes necessary to map the surface of the sample accurately and to correct the focusing of the laser beam on the fly. There exist several commercially-available autofocusing systems for laser machining features on large work-pieces (on metal sheets, for example), where the depth of focus is usually long. These typically are based on capacitance sensing, on lasers, on ultrasound ranging or even on touch. None of these are best suited for the breadth of applications typical of laser micromachining, which often involve very short depth of focus, and fragile, often non-metallic materials.
Capacitive sensors use changes in the electric field caused by a change in the relative dielectric coefficient of the material being sensed. The amount of material in close proximity to the sensor dictates the dielectric coefficient, as compared to air, which has a dielectric coefficient close to that of free space. While these sensors are capable of high reproducibility, they only work for metallic work-pieces, the sensing distance is very limited, and its function is usually limited to a proximity switch. In addition, capacitive sensors are not adequate for use in high-precision applications because the sensing region or spot may be large, of the order of centimeters.
Ultrasonic sensors work on ultrasonic waves being reflected from the surface of the work-piece and are not material dependent. A drawback to these types of sensors is they suffer from low precision (a resolution of 200 &mgr;m), and require near vertical positioning.
Fiber-optic lever-displacement transducers use two optical fibers, one to emit light and the other to receive light. The light intensity detected by the receiver increases with distance from the surface at first as the reflected amount increases (from a geometrical consideration having to do with the positions of the two fibers), but then decreases after a certain point (due to a distance-squared intensity drop-off relationship). These transducers offer very high resolution, but are very expensive, and may be sensitive to the plasma flame produced by the laser-matter interaction causing interference thus necessitating a protective shroud. In addition, incorrect readings can be obtained if the work-piece is not very smooth.
Laser sensors emit a light beam from a laser diode which strikes the object's surface reflecting a small spot onto a position-sensitive detector. Signal processing electronics translate the detector output into a voltage proportional to displacement. While laser sensors have a very high resolution of 10 &mgr;m, and a visible beam for easy alignment, the use of a highly-specialized position-sensitive detector makes them somewhat expensive. Also, if the small laser diode spot falls in a precut hole, the measurement is rendered completely unusable.
Contact sensors are mechanical in nature, and usually consist of a switch, potentiometer or transducer detection of change in magnetic field (primary winding and two secondary windings). The resolution is essentially infinite, these sensors are simple and relatively inexpensive, and they are not dependent on the surface being cut. On the other hand, these types of sensors are problematic in that they may damage the cutting surface since they are in physical contact. In many laser-processing applications, it is necessary to have a non-contact sensor since the material may be very delicate and require high accuracy because the consistency of the cut is very sensitive.
In addition to their particular limitations, all the above sensors suffer from geometrical limitations. Most involve positioning a piece of equipment (a capacitance sensor, an ultrasound system, a fiber assembly or a contact sensor) at the measurement site. By finding the work-piece position at a spot more or less near the machining spot, these methods are adequate to the general laser machining of large features with limited precision. But since laser micromachining concerns itself with micron-precision resolutions, it requires an autofocus system that measures the work-piece surface position at the very machining spot while not being sensitive to precut features in the work-piece. Due to the close proximity of the focusing lens and gas assist nozzle to the work-piece, this is not usually possible with the currently available methods mentioned above.
It would be very advantageous to provide an autofocusing system that will measure the position of the work-piece at the machining spot relative to the focal spot of the machining laser and provide constant adjustment to maintain the focal spot either at the surface of the work-piece or, in the case where a specific and constant vertical offset of the focal point from the surface is desired when working “off-focus”, it would be advantageous for such a system to be able to hold the focal point at a pre-programmed distance from the surface of the work-piece. Such a system would allow the laser micromachining of work-pieces which are not flat and have surface variations, or develop surface variations during machining, with the autofocusing system being able to compensate for these variations.
SUMMARY OF THE INVENTION
The
Grozdanovski Dejan
Nantel Marc
Hill & Schumacher
Photonics Research Ontario
Pyo Kevin
Schumacher Lynn C.
Sohn Seung C.
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