Machine for exposing a panel to laser radiation

Details Machine for exposing a panel to laser radiation Machine for exposing a panel to laser radiation

2000-10-16

2001-11-27

Adams, Russell (Department: 2851)

Photocopying

Projection printing and copying cameras

Step and repeat

C355S023000, C355S024000, C355S025000, C355S026000, C355S067000, C355S071000, C250S492200, C250S492220

Reexamination Certificate

active

06323936

ABSTRACT:

The present invention relates to a machine for exposing a panel, in particular a printed circuit panel, to laser radiation.
BACKGROUND OF THE INVENTION
There exist various known techniques that make use of laser beams for making printed circuits, and more precisely for implementing one of the steps of the method consisting in defining conductor tracks of the printed circuit using a layer of metallization formed on an insulating backing. Such techniques can consist either in transforming the state of a layer of resist with the help of the laser beam, or else in directly ablating the layer of resist so as to reveal the layer of metallization, or finally in not using a layer of resist at all and in proceeding directly with ablation of the layer of metallization with the help of the laser beam. The present invention relates to all three of those methods which make use of a laser beam.
Naturally, in such a technique, each point or pixel on the surface of the working zone of the printed circuit panel is scanned by the laser beam which is either in an active state or in an interrupted state depending on whether the corresponding point is to be printed or not. Scanning control is associated with computer means which contain information in memory serving, for each pixel associated with its coordinates, to specify the printed or unprinted state to be achieved.
In known techniques, scanning is performed by means of a single laser beam which must therefore scan the entire surface of the panel. More precisely, the laser beam makes successive scans in one of the directions of the panel and the scans are offset in the other direction.
It will be understood that when printed circuits are of large size, the operation of scanning by means of a single laser beam in application of known techniques can require a considerable length of time. It would therefore be advantageous to provide an exposure machine in which scanning is performed simultaneously by a plurality of laser beams, each laser beam scanning a respective zone of the printed circuit.
However, it will readily be understood that using a plurality of scanning laser beams simultaneously cannot be achieved merely by juxtaposing devices each of which uses a single scanning laser beam. It will be understood that to obtain continuity in the printed circuit from one scan zone to another scan zone, specific problems are encountered.
It should also be emphasized that in order to define the shape of the conductor tracks very accurately, laser beams are used having an impact diameter on the panels of 20 &mgr;m. However, because of the way energy is unevenly distributed within such a circle and also because of the way impacts overlap due to scanning, the net result is as though the surface of the panel that is printed by an impact is a square of 12.7 &mgr;m×12.7 &mgr;m. The length of 12.7 &mgr;m thus constitutes the unit of definition applied to the various patterns that are made on the panel in the two orthogonal directions thereof. To conserve the precision of such definition, it is necessary for the laser beam striking the printed circuit panel to be perpendicular thereto. In addition, it is necessary in each zone to have a certain amount of latitude available for adjustment in the scan direction of the laser beam in order to be able to accommodate positioning imperfections in the various components used in generating and controlling the laser beam.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to provide a machine for exposing panels to laser radiation that makes use of a plurality of laser beams operating simultaneously, while nevertheless obtaining a printed circuit having the same quality of definition as when only one laser beam is used.
To achieve this object, the invention provides a machine for exposing a panel to laser radiation, said panel having a working length L in a first direction Y, said length L being divided into N successive segments of length L/N, said machine comprising:
a fixed structure;
a horizontal panel support for receiving said panel;
displacement means for moving said support relative to the structure in a direction X that is orthogonal to the Y direction of the panel;
means for generating N main laser beams;
modulator means for modulating each main laser beam in time; and
N optical units that are fixed relative to said structure each having an optical axis in a direction that is substantially orthogonal to the Y direction, each optical unit comprising:
deflector means for periodically converting one of the main beams into a beam that is continuously deflected and that is modulated in time, said beam being directed along the optical axis of said optical unit; and
a deflector mirror for directing said deflected laser beam in a direction orthogonal to the X,Y plane towards said panel support, the length l
1
of said deflector mirror in the direction orthogonal to the optical axis of said unit and suitable for being struck by said beam during each period being greater than L/N;
said optical units being organized as two sets of optical units with each set having N/2 units if N is even and ½(N+1) and ½(N−1) units if N is odd, the units within each set being juxtaposed in the Y direction in such a manner that their deflector mirrors are substantially in alignment along the Y direction, the distance in the X direction between the deflector mirrors of the two sets being equal to D, the distance in the Y direction between the optical axes of two juxtaposed optical units in the same set being equal to 2L/N and the distance in the Y direction between the optical axis of a unit in one set and the optical axis of the nearest optical unit in the other set being equal to L/N, whereby the deflected laser beam from an optical unit is capable of scanning over a length l
1
that is longer than the length of the panel segment that needs to be scanned, the deflected laser beam thus being capable of being controlled to scan the panel segment from its first end to its second end.
Specifically because of the particular disposition of the optical units which are disposed in two parallel lines, the optical units of one assembly being offset by one “half-pitch” relative to those of the other set, using the term “pitch” to designate the distance between the optical axes of two juxtaposed units in the same set, it is possible to cause the scan zones of the various optical units to overlap. This possibility of causing the scan zones to overlap makes it possible, given the inevitable positioning inaccuracies of the various components constituting the optical unit, to make the corrections necessary to achieve the precision that is desired concerning the successive points of impact of the laser beams that actually scan each zone of the panel.
Preferably, the means for deflecting each optical unit comprise:
a rotary polygonal mirror having n facets for continuously deflecting said laser beam in the same plane during successive fixed periods corresponding to said facets; and
objective lens means to focus and direct the beam as deflected successively by each facet of the polygonal mirror along the direction of said optical axis and in a common plane containing the Y direction.
In a preferred embodiment, the modulator means comprise an acousto-optical modulator which receives the main beam and which outputs said beams that are successive in time, during said fixed periods.
It will be understood that controlling the transparent or non-transparent state of the acousto-optical modulator makes it possible, in addition to modulating the laser beam, to synchronize the state of the modulator with the rotation of the polygonal mirror and thus with the position of each of its reflecting faces. This makes it possible in particular to introduce corrections, as explained below, so as to ensure that the ends of each scan segment are positioned precisely.
In a preferred embodiment of the machine:
the displacement means for moving the panel support in the X direction move said support continuously at a const

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