Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-03-28
2003-11-11
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S559300, C250S216000
Reexamination Certificate
active
06646279
ABSTRACT:
The present invention relates to apparatus for exposing at least one face of a panel, in particular a printed circuit panel.
BACKGROUND OF THE INVENTION
Such apparatuses are used for manufacturing printed circuits from a panel coated in a photosensitive material that is to have artwork placed in front of it bearing the pattern of tracks to be generated on the printed circuit. A light beam passes through the artwork and thus serves to expose the panel.
Such exposure apparatuses are known, e.g. from European patents EP 618 505, EP 807 505, and EP 807 856, in which the light source and the panel to be exposed are both stationary and exposure is performed over the entire surface that is to be exposed without scanning.
However, in that type of apparatus where the entire surface is exposed simultaneously, the chemical reactivity of the photosensitive material is not optimum. The efficiency of the reaction is improved when exposure is stronger and instantaneous exposure time shorter.
Such apparatuses are also known in which the surface of the panel is exposed by scanning, with a light beam coming from a light source being reflected on a rotary mirror.
Unfortunately, the definition with which tracks are drawn, and the fineness of such tracks is directly related to the angle of incidence of the light beam on the artwork. Each light beam occupies a circular cone about an axis that is inclined at a greater or lesser angle relative to the surface that is to be exposed, and that is referred to as declination. The half-angle at the apex of the cone represents collimation, i.e. the degree of parallelism between light rays. It will thus be understood that the angle of incidence of a light beam depends both on its collimation and on its declination. Consequently, when the light is not collimated and/or when some of the light beams reach the surface for exposure at an angle of incidence that is too large, the size of the tracks and the paths they follow are generally modified relative to the artwork.
Similarly, when exposure is not performed uniformly, then tracks are formed unequally and the quality of the resulting printed circuit is poor.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the invention is to provide an exposure apparatus that makes it possible to improve exposure of a surface by scanning, in particular for manufacturing printed circuits, by providing a strip of light which presents both a good uniformity and a good angle of incidence relative to the panel.
Such apparatus makes it possible to make printed circuits having a conductor track density that is very high, and it ensures that the tracks are very fine and follow very accurate paths, i.e. it ensures that the surface for exposure is perfectly exposed. The size of the conductor tracks of printed circuits that are to be made using such apparatus lies in the range 25 micrometers (&mgr;m) to 50 &mgr;m, and they are spaced apart by approximately the same amount.
It will thus be understood that a light beam reaching the surface to be exposed at a poor angle of declination produces a parallax error causing the light to be offset on the surface to be exposed, thereby shifting the conductor tracks away from the design location. The same applies to the size of the tracks which increases and becomes less precise with worsening collimation of the beam relative to the surface to be exposed. Under extreme circumstances, those two phenomena taken together can lead to short circuits because tracks touch each other.
In addition, the quality of development, i.e. the quality of removal of that portion of the photosensitive material which is not to overlie future tracks during etching, and consequently the quality of that etching, depends on prior transformation of the photosensitive material, which transformation is itself related to the quantity of light energy received. It will thus be understood that when the light beam is not uniform, it gives rise to non-uniform transformation of the photosensitive material and thus to tracks following paths that are imprecise and in extreme cases this can lead to tracks being interrupted.
Throughout the description below, the term “light strip” is used to designate the set of light beams reaching the surface to be exposed, and the term “mean angle of incidence” corresponds to the angle measured in any plane substantially transversal to the plane of the surface to be exposed and within which half of the light flux reaches the surface, the other half reaching the surface at any angle.
Each light beam lies in a circular cone of axis substantially perpendicular to the plane of the surface to be exposed. Under such circumstances, the half-angle at the apex which represents collimation is less than or equal to the mean angle of incidence.
Since the length of the light strip is not less than the length of one of the sides of the panel to be exposed, it suffices to scan in a single direction in order to expose the entire surface of the panel. The light strip is moved relative to the surface to be exposed (or vice versa) in a direction extending transversely to the length of the light strip so as to scan the entire surface. The direction in which the panel is scanned corresponds to the direction of one of the sides of the panel. Thus, by generating a light strip in a first direction parallel to one of the sides of the panel, scanning is performed in a second direction substantially transversal to said first direction.
In addition, since the zone that is illuminated during exposure forms a strip, scanning time is reduced by using a narrow light pencil of flux density greater than when exposing the entire surface. For equal flux density, the greater the height of the quadrilateral constituted by the light strip, the shorter the scanning time. However, the greater the height of the quadrilateral, the more difficult it is to obtain light at a small angle of incidence which is collimated and uniform over the entire quadrilateral, and the lower the flux density. It is therefore necessary to find a compromise.
In addition, it must be possible to guarantee identical exposure throughout scanning, i.e. that the uniformity and the angle of incidence of the light beam remain constant throughout displacement of the strip of light.
In a first aspect, the present invention provides apparatus for improving the exposure of a surface, in particular for manufacturing printed circuits, by processing a light beam emitted from a single stationary light source.
In this first aspect, the invention provides apparatus comprising:
means for holding at least one artwork and said panel on a frame;
an optical system comprising a light source emitting a light beam, processor means for processing said light beam to generate a uniform and collimated light beam having a mean angle of incidence relative to the surface to be exposed of less than 2° and having illumination uniformity that departs by less than ±10% from a mean value, and shaper means enabling said uniform and collimated light beam to be transformed into a uniform and collimated light strip on the surface of the panel to be exposed and including said artwork, said uniform and collimated light strip being of length not less than the length of one of the sides of said surface to be exposed, said means for processing the light beam comprising a reflector and an integrator-collimator assembly;
displacement means for generating relative displacement between said light strip and said face to be exposed in a direction substantially transverse to the longitudinal direction of said light strip; and
matching means for matching the speed of relative displacement between said light strip and said face to be exposed to the illumination of the light strip and to the sensitivity of the surface to be exposed.
In the usual case of panels that are substantially rectangular, it will be understood that scanning is accelerated by being performed in a direction parallel to the width of the panel, i.e. when the light strip is parallel to the long direction of the panel. In this first circumstan
Sorel Alain
Vibet Gilles
Automa-Tech
James Ray & Associates
Le Que T.
LandOfFree
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