Optical system

Optical system provides a uniform, well-collimated beam particularly for the exposure of photoresist on integrated circuit boards. The well-collimated beam is produced in a short beam path through the use of a series of mirrors, including concave mirrors and an integrator mirror to produce reliable exposure characteristics at the exposure focal plane. Preferably, a beam splitter is provided so that the focal plane is illuminated on both sides for concurrent exposure of both sides of the board.

BACKGROUND 
This invention is directed to an optical system which provides 
well-collimated light of excellent uniformity of intensity so that it is 
particularly useful for an exposure system and, with a beam splitter, it 
is useful for concurrent both-side exposure of printed circuit boards or 
other systems having photosensitive material thereon. As an exposure 
system, the optical design is useful for either single-sided exposure or 
double-sided exposure. Furthermore, the characteristics of the beam make 
the optical system particularly suitable for use as a solar simulator. The 
optical design provides good collimation even with a short optical path 
and good uniformity of the intensity of the beam for uniform exposure. 
Modern electronic design includes the reduction in physical size of 
assembly by employing systems of smaller size. Concurrent with this, the 
interconnection of electronic systems has been made more compact by 
employing printed circuit boards of continually more compact arrangement. 
The other arts have improved, including the exposure definition of the 
photosensitive material, the making of photo-exposure masks of continually 
finer definition, and the control of the chemical etching procedure 
itself. In order to take advantage of these advances and to maximize the 
advantages of these advances in the other arts, advances in the optical 
exposure of the photosensitive resist must be made. The exposure must be 
substantially uniform over this exposure area to provide substantially 
uniform development characteristics. Furthermore, the illumination 
provided for the exposure must be well-collimated to provide a sharp 
exposure edge at each edge in the mask. 
The exposure system of this invention is designed to satisfy those needs 
and also to provide for large area exposure and for double-sided exposure 
for those cases where photosensitive material and masks are provided on 
both sides of the board. Furthermore, the exposure system is well-suited 
for being incorporated into an automated system. 
SUMMARY OF THE INVENTION 
In order to aid in the understanding of this invention, it can be stated in 
essentially summary form that it is directed to an optical system wherein 
an optical design provides a substantially collimated, substantially 
uniform photoresist exposure beam which is provided over a short exposure 
path. The optical structure includes at least one concave mirror and at 
least one integrator mirror to provide substantial focus, substantial 
collimation, and substantial uniformity of intensity. 
It is, thus, an object of this invention to provide a substantially 
collimated, high resolution exposure system particularly useful for 
providing exposure to printed circuit boards and similar structures. It is 
a further object of this invention to provide an optical design in an 
exposure system where the optical path length is quite short, but with 
substantial collimation, substantial uniformity, and a substantial 
increase in the exposure intensity over the focal plane at which the 
exposure is made. It is a further object to provide an exposure system 
wherein material to be exposed can be readily handled and placed at the 
focal plane by manual or automated systems so that work can be quickly, 
easily and accurately exposed. 
Other objects and advantages of this invention will become apparent from a 
study of the following portion of this specification, the claims and the 
attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The exposure system of this invention includes an optical system 10 
schematically indicated in FIGS. 2 and 3. The optical system 10 is 
employed in housing 12 which contains the control equipment for the 
optical exposure as well as the structure on which the device to be 
exposed is moved into the focal plane. In view of its compact physical 
arrangement in housing 12, exposure system 10 is best understood by 
reference to the two views of FIG. 2. 
Light source 14 must be of a high-intensity character in order to provide 
the required illumination. It is preferably a mercury-xenon arc lamp with 
a short arc to provide the smallest spot available for focal purposes. The 
lamp must be vented in order to prevent its overheating, and housing 12 is 
provided with suitable ducting for air flow across the lamp or light 
source 14. One specific example is a 5,000-watt mercury-xenon short arc 
lamp, and with such a lamp, a cooling air flow of about 500 cubic feet per 
minute is required across the lamp. Parabolic reflector 16 contains lamp 
14 at its focal point and directs the lamp output upward to focus mirror 
18. Central ray 20 is indicated for this light path. Focus mirror 18 is 
concave and redirects the optical energy generally downward with central 
ray 22. The spherical configuration of focus mirror 18 condenses the 
energy. In the path from parabolic reflector to spherical mirror 18, the 
optical energy with its central ray 20 comprises substantially parallel or 
collimated energy in view of the parabolic character of mirror 16. The 
spherical figure of focus mirror 18 causes the energy beam to converge as 
it reaches integrator mirror 24 on a path indicated by the central ray 22. 
The focal point of concave spherical mirror 18 is beyond integrator mirror 
24, and thus the optical energy bundle is of smaller diameter at 
integrator mirror 24. 
Integrator mirror 24 is of special construction. In integrator mirror is 
illustrated in D. D. Dourte, et al., U.S. Pat. No. 4,195,913, the entire 
disclosure of which is incorporated herein by this reference. The Dourte 
mirror has a plurality of facets, and each of the facets is optically 
configured to deliver radiation over the entire object field. In the 
Dourte construction, each facet is individually optically formed and 
secured in place. The integrator mirror 24 in the present optical system 
is functionally similar but structurally different. Integrator mirror 24 
has a plurality of reflecting facets 26, each of which is configured to 
optically deliver the beam over the the entire object area, in this case, 
the focal plane at the top surface of glass tray 50. Each of the facets 26 
is convex to expand the individual beam from each of the facets to the 
collimation mirrors 40 and 42 which have the same size as the focal plane. 
Considering just one reflecting surface 26 of one facet or one body of 
integrator mirror 24, the central ray 28 reflected from the spherical 
convex facet 26 on the central body is directed towards mirror 30. In the 
present structure, the mirror 30 is a flat mirror which redirects the 
central ray to the path of central ray 32. 
Beam splitter 34 is positioned on the beam containing central ray 32. Beam 
splitter 34 is planar and is dielectrically coated to transmit half the 
optical energy through the beam splitter 34 in a beam which contains 
central ray 36 and to reflect half of the energy in the beam on an 
upwardly directed path which contains central ray 38. Both beams are still 
diverging as a result of the convex configuration of the facets of 
integrator mirror 24. The beams are then respectively reflected upwardly 
by concave collimation mirror 40 and downwardly by concave collimation 
mirror 42. These mirrors are concave spherical mirrors of such focal 
length so that the beams respectively directed up and down, and 
respectively including central rays 44 and 46, are well-collimated, that 
is, the individual rays therein are substantially parallel. The focal 
lengths of the mirrors 40 and 42 are the same and are correlated with the 
divergency of the beams to those mirrors so that the output beams are 
parallel. Furthermore, the beams are directed at the top and bottom of 
focal plane 48, which is equidistant between the collimation mirrors 40 
and 42. The top surface of glass tray 50, see FIG. 1, is on the focal 
plane. Glass tray 50 receives the material to be exposed and slides from 
the inactive position of FIG. 1 where the sensitive material is placed on 
the tray to an active position at the focal plane indicated in FIG. 2. 
Drawer structure 52 is provided to move the tray 50 from the inactive 
position of FIG. 1 to the active position of FIG. 2. 
The description of integrator mirror 24 includes the description of 
individual facets on this mirror. The description of the central ray 28 
reflected from facet 26 includes the spreading of the beam from that facet 
to full size at mirrors 40 and 42 and the focus of the beam from that 
facet in collimated condition onto the focal plane. The complexity of the 
facet 26 thus provides the beam divergence to the mirrors 40 and 42. Each 
of the other facets of integrator mirror 24 is similarly shaped, and each 
is directed so that its central ray respectively impinges upon collimation 
mirrors 40 and 42 at the same point as the impingement of central rays 36 
and 38, in the theoretically accurate condition. The optically correct 
shape of each of the facets 26 of the integrator mirror 24 is 
computer-determined. Additionally, the orientation of each facet is 
determined by the same computer program so that the desired result is 
achieved. After computer determination, one master mirror is produced. For 
production, additional mirrors are replicated from it. Electro-forming the 
copies is a suitable present-day replication method. In this way, when 
more than one such optical system is to be manufactured, costs can be 
reduced. The integrator mirror 24 in the optical system of this invention 
achieves both collimation and uniform intensity of illumination across the 
focal plane. 
Lamp 14 is positioned within reflector 16 in a way that the arc in the lamp 
can be adjusted to the focal point of the reflector for maximum 
collimation. In these lamps, the illumination point does not substantially 
change, and thus, adjustment is required only once for each lamp. Thus, 
adjustment of the lamp can be manually achieved. 
Control system 54 is connected to lamp 14 to cut off the lamp when 
ventilation fails, as a safety precaution. In addition, control system 54 
is connected to shutters 56 and 58 which are respectively positioned in 
the lower and upper beam of illumination output from beam splitter 34. In 
some cases, exposure from only one side of the sensitive material is 
desired. In that case, the other shutter may remain closed. In addition, 
the length of the exposure is controlled by the open duration of shutters 
56 and 58. The photosensitivity information with respect to the material 
to be exposed is considered in establishing the exposure duration. The 
shutters are positioned so that they each cut off only one of the upper or 
lower exposure beams for individual control thereof. The shutters are 
preferably hinged or pivoted plate shutters to permit them to cover the 
entire beam area. 
This invention has been described in its presently contemplated best mode, 
and it is clear that it is susceptible to numerous modifications, modes 
and embodiments within the ability of those skilled in the art and without 
the exercise of the inventive faculty. Accordingly, the scope of this 
invention is defined by the scope of the following claims.