Coherent light generators – Particular beam control device – Control of pulse characteristics
Reexamination Certificate
2001-05-15
2004-07-20
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Control of pulse characteristics
C372S026000, C372S028000, C372S005000, C372S018000, C372S022000
Reexamination Certificate
active
06765934
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to repetition rate multiplication of pulsed light beams in general, and of mode locked lasers in particular. An important application of the invention is in the multiplication of the repetition rate of mode locked lasers for use in the production of printed circuit boards.
BACKGROUND OF THE INVENTION
In applications such as printed circuit board (PCB) production, a laser may be used to expose a pattern on a photoresist coating on a copper coated substrate. In a typical exposure system such as the DP 100 of LIS of Germany, a CW UV laser beam is scanned across the PCB surface, while its intensity is modulated in accordance with a pattern to be generated. The modulating device receives electronic pixel data supplied by control circuitry. In modern PCB production it is desirable to operate at high data rates to increase production speed. The practical data rate is limited by the modulation rate and/or the available laser power.
In the production of PCBs utilizing UV sensitive photoresists an Argon Ion laser is often used. Although they are widely utilized as UV light sources, Argon lasers, being gaseous lasers, have a number of drawbacks such as complicate and delicate to operate, poor maintainability and/or high price.
Methods for producing UV laser radiation are known. For example, one such method utilizes an IR mode locked laser to generate high repetition rate laser light pulses. The frequency of the laser light is doubled to UV by passing the light through a non linear medium. However, the utilization of such lasers to photoresist exposure is severely limited by an inherent paradox: practical UV power drops rapidly with an increase in the repetition rate. This is because the frequency doubling process is extremely non linear and thus, its efficiency increases with increasing peak power. For a given average IR power, peak power drops as pulse repetition rate increases, leading to reduced UV generation efficiency.
U.S. Pat. No. 5,462,433 describes a device for use in electronic warfare for delaying coherent RF signals with an adjustable time delay element. The RF signals are divided into multiple signal paths, some of the paths are delayed relative to the other paths, and then the delayed and non delayed paths are recombined to increase the pulse repetition rate.
SUMMARY OF THE INVENTION
One broad aspect of some preferred embodiments of the present invention deals with the multiplication of the pulse repetition rate of optical signals. In preferred embodiments of the invention utilizing this aspect of the invention, the average power is substantially preserved. That is, the conversion process is practically lossless.
A second broad aspect of some preferred embodiments of the invention deals with the use of amplitude modulation of a pulsed light beam to transfer information and in particular to expose a photosensitive surface. In preferred embodiments of the invention, the pulse rate of the light pulses is at least several times the frequency of the modulation. Herein, such a combination of modulation of pulsed radiation is termed “quasi-CW” modulation. Thus, one aspect of some preferred embodiments of the invention deals with using quasi-CW modulated pulsed light to expose a resist on a PC board.
Quasi-CW modulation results when the pulse repetition rate is made high enough compared with the modulation data rate, so as to minimize the timing errors resulting from the lack of synchronization between the two. The pulse repetition frequency should be at least 2-3 times the pixel data rate to assure a smooth, error free written pattern.
A third broad aspect of the invention deals with the use of a pulsed laser, synchronized in time with the data modulation and having the same repetition rate in a PCB writing system. From a practical point of view however, the requirement that the pulse rate be exactly the same as the data rate and that the pulses be individually turned on and off make such a device more difficult to implement than the quasi-CW embodiments of the invention.
The present invention is generally described in the context of repetition rate multiplication of pulsed UV lasers used in direct writing on photoresists in PCBs production. However, as it will become evident, the present invention is applicable to the repetition rate multiplication of any pulsed light beam such as chopped, coherent and/or non coherent, monochromatic and/or non-monochromatic light beams, and/or stroboscopic light. To emphasize this broader applicability of the invention, the term “pulsed light beam” (PLB) is used herein to refer to light beams other than lasers and the terms “laser beam” or “pulsed laser beam” is used when referring to pulsed lasers.
It is an object of some preferred embodiments of the present invention to provide a system and method for increasing the repetition rate of pulsed light beams. Preferably, the system will not substantially reduce the average power output of the light.
Another object of some preferred embodiments of the present invention, is to provide a laser system and a method for the use in direct writing on photoresists in the production of PCBs. Preferably, the system comprises a pulsed UV laser and a pulse rate multiplication device which multiplies the pulse repetition rate of the laser. In one aspect, some preferred embodiments of the present invention, allow for data rates higher than the laser's pulse repetition rate prior to the multiplication.
In a preferred embodiment of the invention, a laser writing system, for example, for PCBs, is provided. This system utilizes a high power pulsed laser at a relatively long wavelength and low repetition rate, for example an IR mode locked laser operating at about 80 MHz and preferably having an average power of about 1 W. The system transforms this laser light to UV, preferably using a non linear optical medium. The pulsed UV light is amplitude modulated and used to scan and expose a PCB coated with a UV sensitive resist to form a pattern. Preferably, the UV light comprises a quasi-CW train of pulses such that methods known in the art may be used to modulate the UV light and utilize it to scan the area to be exposed.
It is an object of some preferred embodiments of the present invention, to provide a repetition rate multiplication apparatus and method for use in the multiplication of the repetition rate of pulsed light beams. Preferably, the repetition rate multiplication is obtained external to a generator that produces the pulsed light beam generator. In a preferred embodiment of the invention, the pulsed light beam generator is a pulsed laser. Preferably, the pulse rate conversion process is substantially lossless such that the average power in the light beam is preserved.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication method and apparatus which do not affect the PLB and/or laser beam generator initial functional specifications/characteristics such as efficiency, resonant cavity length, heat removal scheme, etc.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus which is retrofitted to an existing pulsed light source preferably, a pulsed laser.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus that is composed of passive components not requiring external power for their operation. Preferably, the passive components are optical components which do not require to be mechanically displaced in order for the apparatus to be operated.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus which will aim the beam pulses onto one target or simultaneously onto more than one target.
Another object of some preferred embodiments of the present invention, is to provide a repetition rate multiplication apparatus and method which allows for variable delay of individual pulses, relative
Fenster & Company
Jr. Leon Scott
Orbotech Ltd.
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