Direct pattern writer

Details Direct pattern writer Direct pattern writer

2002-01-03

2004-08-03

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S559290

Reexamination Certificate

active

06770866

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to devices for writing patterns using pulsed light beams in general, and of mode locked lasers in particular. An important application of the invention is 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 and available from Orbotech Ltd. of Yavne, Israel, a CW UV laser beam is scanned across the PCB surface, while its intensity is modulated in accordance with a raster 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 a CW 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 their being complicated and delicate to operate, poor maintainability and/or high price.
Various methods for producing UV laser radiation are known. For example, one such method utilizes an IR solid state laser oscillator to generate mode locked high repetition rate laser light pulses. The wavelength of the IR mode locked laser light is converted to UV by passing the mode locked IR light pulses through a non linear medium. However, the utilization of such lasers to expose photoresist at high data rates is severely limited by a paradox inherent in the frequency conversion process, which is extremely non-linear. Frequency conversion becomes more efficient as power increases.
Although mode locking is useful to obtain laser pulses each of which have a high peak power as is necessary to promote high efficiency frequency conversion, as the repetition rate of the laser pulses increases, for example in order to achieve increased data rates, peak power in individual pulses decreases and the resulting average UV power drops rapidly. Thus, for a given average IR power, the average peak power after frequency conversion drops as the repetition rate of the laser pulses increases, leading to reduced UV generation efficiency.
Practically, using pulsed lasers to write raster patterns is problematic for various reasons. Modulating data at a rate exactly equal to the pulse repetition rate of a mode locked laser is problematic due to difficulties in high speed pulse and data synchronization. Conversely, modulating data at a rate different than the pulse repetition rate of a mode locked laser is problematic due to timing errors in which a pulse is not available at exactly the time required to write or expose a pixel which is supposed to be written or exposed. This latter problem is particularly prevalent when data rates for writing pixels approach or exceed the pulse rate of an exposing radiation source—such as a mode locked laser.
In addition, in a system, such as the DP-100 system, in which feedback of light reflected from a scale is used to control the data flow, a “clean” signal, modulated only by the scale was considered to be necessary, whether the signal was used directly to control the data or when, as in the DP-100 system it provided the input to a locked (PLL) pulse generator.
SUMMARY OF THE INVENTION
One broad aspect of some embodiments of the invention deals with the use of modulation of a first light beam, such as a pulsed UV laser beam, to transfer information and in particular to expose a photosensitive surface. In some embodiments of the invention, the surface is exposed in a raster pattern and information modulation is asynchronous with the pulse repetition rate of the light pulses. In some embodiments of the invention, the first light beam is not pulsed.
In some embodiments of the invention, a second pulsed beam scans the surface together with the first light beam. The position of the second beam is determined and used to control the modulation of the first light beam.
In some embodiments of the invention the first and second beams are pulsed at a high repetition rate. Herein, a high pulse repetition rate light beam, such as is provided by a solid state laser diode pumped mode locked laser, is termed “quasi-CW”. Asynchronously modulating pulsed radiation, such as by high pulse repetition rate mode locked laser radiation, is termed “quasi-CW modulation.”
In embodiments of the present invention employing a quasi-CW modulating system, the pulse repetition rate may be less than one laser pulse per increment of the modulation data rate. In others it may be higher than the modulation data rate.
One broad aspect of the invention relates to systems and methods to determine the presence of a scanned pulsed laser beam at a plurality of locations along a scan path. A marked scale is provided and an optical clock is generated as the reflected from the scale. In some embodiments of the invention the instantaneous location of the pulsed beam is determined to a greater precision than the distance between markings. Additionally, data for modulating a second laser beam may be controlled in response to the reflection of the first beam.
Another broad aspect of the invention relates to systems and methods to determine the presence of a pulsed data-modulated scanned laser beam at a plurality of locations as it is scanned along a scan path, and then modulating data at least partly in response to determined positions of the beam. A determination of the presence of the beam is made at least at three locations in the scan path, however such determination may be made at any number of locations.
According to some embodiments of the invention, the system comprises a pulsed UV laser light source and a pulse rate multiplication device which multiplies the pulse repetition rate of the UV laser light. Such pulse rate multiplication device, is however not absolutely required. In one aspect, some embodiments of the present invention allow for data rates higher than the laser's pulse repetition rate prior to the multiplication. Additionally and alternatively, portions of the UV laser light are independently and spatially modulated at a data rate that is suitable to be quasi-CW modulated, and the data rate used to modulate each portion is lower than the overall data rate.
In some embodiments of the invention, a laser writing system, for example, for directly imaging PCBs, is provided. This system optionally utilizes a high power solid state pulsed laser at a relatively long wavelength and low repetition rate, for example an IR mode locked laser oscillator operating at about 80 MHz and optionally having an average power of at least 1 W. The system transforms this laser light to UV, for example using a non linear optical medium. The medium may be located external to the laser cavity. 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.
In some embodiments of the invention, the UV light comprises a quasi-CW train of pulses as described above, 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.
There is thus provided, in accordance with an exemplary embodiment of the invention, apparatus for scanning a beam across a surface, comprising:
a scanner that scans a pulsed laser beam across a surface; and
a position indicator receiving an input from said pulsed laser beam at a plurality of locations across said surface, and outputting position indications indicating a position of said pulsed laser beam along said surface.
In an embodiment of the invention the surface includes a plurality of spaced markings, such that a modulated pulsed laser beam is reflected from said surface.
In an embodiment of the invention, the position indicator includes a detector, wherein said detector receives said modulated pulsed lase

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