Abstract:
The present invention is a laser light source device having: a silicon substrate having a first flat surface and a second flat surface which is formed at a position lower than the first flat surface by a level difference in the thickness direction; a first junction having a microbump structure comprising Au formed on the first flat surface; a second junction having a microbump structure comprising Au formed on the second flat surface; a first optical element and a second optical element for emitting laser light, which are joined to the first junction by a surface activation technique; a reflective member for reflecting the laser light from the first optical element toward a multiplexer, the reflective member being joined to the second junction by the abovementioned technique; and a multiplexer for directly receiving the laser light from the second optical element and multiplexing the laser light from the first optical element and the laser light from the second optical element, the multiplexer being joined to the second junction by the abovementioned technique; a configuration being adopted whereby the distance between the first optical element and the reflective member is different from the distance between the second optical element and the multiplexer, and the length of the optical path from the first optical element to the multiplexer is equal to the length of the optical path from the second optical element to the multiplexer.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a laser light source device and a method for manufacturing the laser light source device, and more particularly to a laser light source device constructed by mounting optical elements and optical combiners on a single substrate, and a method for manufacturing such a laser light source device. 
       BACKGROUND ART 
       [0002]    It is known in the art to provide a projector which modulates light emitted from a light source unit by using a spatial light modulator and which projects the modulated light in an enlarged form onto a screen through a projection optical system such as a projection lens, etc. Traditionally, a metal halide lamp or a halogen lamp has been used as the light source unit for such a projector. However, in recent years, for such purposes as downsizing the light source unit and the projector while at the same time achieving extended service life and enhanced image quality, a display apparatus has been proposed that uses a light source constructed from laser diodes (LDs) of three primary colors (RGB) (for example, refer to patent document 1). 
         [0003]    The laser light source used in the projection-type display apparatus disclosed in patent document 1 has a can-type structural stem. More specifically, an LD chip is rigidly fastened onto a common terminal that is connected to the stem by vertically passing therethrough, and the common terminal and a terminal to which the other electrode of the LD chip is connected by a wire are brought out to the back side of the stem; then a cylindrically shaped metal cap having a transparent window is mounted on the stem to seal the LD chip therein. In patent document 1, a laser unit which includes three such can-type laser light sources, one for each of the RGB colors, and which combines the laser light from the respective laser light sources by using a plurality of dichroic mirrors and outputs the combined laser light through a lens, is proposed as the light source for the display apparatus. 
       PRIOR ART LITERATURE 
     Patent Literature 
       [0004]    Patent document 1: Japanese Unexamined Patent Publication No. 2011-175268 
       SUMMARY 
       [0005]    In recent years, the need for tiny projectors called pico projectors designed for use in portable devices has been increasing. However, the laser unit incorporated in the display apparatus disclosed in patent document 1 is of the type in which each can-type laser light source is fitted into a hole opened in a wiring substrate and is mounted thereto by soldering. It is therefore difficult to achieve size reductions; in fact, it is not possible to achieve a laser unit smaller in thickness than the diameter of the laser light source. There is thus a limit to how much the can-type laser light source disclosed in patent document 1 can be reduced in size and thickness. 
         [0006]    Further, in the laser unit disclosed in patent document 1, the laser light sources to be soldered, the plurality of dichroic mirrors, and the lens must be aligned relative to each other in order to adjust the light path for the laser light to be emitted. This has led to the problem that the adjustment process of the laser unit is complex and the light path adjustment is difficult to accomplish. Furthermore, since there are many parts to be adjusted, the light path may vary for each laser light, resulting in the problem that the light loss increases and the coupling efficiency drops. 
         [0007]    An object of the present invention is to provide a laser light source device wherein provisions are made to solve the above-outlined problems, and a method for manufacturing such a laser light source device. 
         [0008]    Another object of the present invention is to provide a laser light source device that is compact and thin in design and makes light path adjustments easy by integrating optical elements, etc., on a single substrate, and a method for manufacturing such a laser light source device. 
         [0009]    There is provided a laser light source device includes a silicon substrate having a first planar surface and a second planar surface, the second planar surface being formed lower than the first planar surface by forming therebetween a step in a thickness direction of the silicon substrate, a first bonding portion having a micro-bump structure made of Au formed on the first planar surface, a second bonding portion having a micro-bump structure made of Au formed on the second planar surface, first and second optical elements for emitting laser light, each optical element being bonded to the first bonding portion by surface activated bonding, a reflecting member, bonded to the second bonding portion by surface activated bonding, for reflecting the laser light received from the first optical element toward the optical combiner, and an optical combiner, bonded to the second bonding portion by surface activated bonding, for directly receiving the laser light from the second optical element and thereby combining the laser light from the second optical element with the laser light from the first optical element, and wherein the distance between the first optical element and the reflecting member is made different from the distance between the second optical element and the optical combiner so that the light path length from the first optical element to the optical combiner becomes equal to the light path length from the second optical element to the optical combiner. 
         [0010]    Preferably, in the laser light source device, the step formed in the thickness direction of the silicon substrate is formed in a staircase shape horizontally across the first planar surface, and the first and second optical elements are arranged along staircase-shaped edge faces of the step formed in the thickness direction. 
         [0011]    Preferably, in the laser light source device, the first and second optical elements are laser devices, and the optical combiner is a dichroic mirror prism. 
         [0012]    Preferably, the laser light source device further includes a driver IC, mounted on the silicon substrate, for driving the first and second optical elements. 
         [0013]    Preferably, the laser light source device further includes a third optical element for emitting laser light and a second optical combiner, wherein the second optical combiner directly receives the laser light from the third optical element, and combines the laser light from the third optical element with the laser light from the second optical element combined by the optical combiner with the laser light from the first optical element. 
         [0014]    There is also provided a method for manufacturing a laser light source device, includes the steps of forming a first planar surface and a second planar surface on a silicon substrate, the second planar surface being formed lower than the first planar surface by forming therebetween a step in a thickness direction of the silicon substrate, forming a first bonding portion having a micro-bump structure made of Au on the first planar surface; forming a second bonding portion having a micro-bump structure made of Au on the second planar surface, bonding first and second optical elements to the first bonding portion by surface activated bonding, the first and second optical elements being arranged to emit laser light, bonding an optical combiner to the second bonding portion by surface activated bonding, the optical combiner being positioned to directly receive the laser light from the second optical element and to combine the laser light from the second optical element with the laser light from the first optical element, and bonding a reflecting member to the second bonding portion by surface activated bonding, the reflecting member being positioned to reflect the laser light received from the first optical element toward the optical combiner, and wherein the distance between the first optical element and the reflecting member is made different from the distance between the second optical element and the optical combiner so that the light path length from the first optical element to the optical combiner becomes equal to the light path length from the second optical element to the optical combiner. 
         [0015]    Preferably, the laser light source device manufacturing method further includes the steps of causing the first and second optical elements to emit the laser light, and adjusting the position of the reflecting member so that the light path of the laser light from the first optical element and the light path of the laser light from the second optical element overlap each other. 
         [0016]    Preferably, in the laser light source device manufacturing method, the laser light source device further includes a third optical element for emitting laser light and a second optical combiner for combining the laser light from the third optical element with the laser light from the second optical element combined by the optical combiner with the laser light from the first optical element, wherein in the step of adjusting the position of the reflecting member, the position of the reflecting member is adjusted so that the light path of the laser light from the second optical element combined by the optical combiner with the laser light from the first optical element and the light path of the laser light from the third optical element overlap each other. 
         [0017]    Preferably, in the laser light source device manufacturing method, the step formed in the thickness direction of the silicon substrate is formed in a staircase shape horizontally across the first planar surface, and the first and second optical elements are arranged along staircase-shaped edge faces of the step formed in the thickness direction. 
         [0018]    Preferably, in the laser light source device manufacturing method, the first and second optical elements are laser devices, and the optical combiner is a dichroic mirror prism. 
         [0019]    Preferably, the laser light source device manufacturing method further comprises the step of mounting on the silicon substrate a driver IC for driving the first and second optical elements. 
         [0020]    A laser light source device is provided which is constructed by mounting on a silicon substrate a plurality of optical elements and a plurality of optical combiners for combining the laser light emitted from the plurality of optical elements, the laser light source device includes the silicon substrate includes a bonding portion having a micro-bump structure made of Au, the plurality of optical elements and the plurality of optical combiners are bonded to the bonding portion by surface activated bonding, the silicon substrate includes a first planar surface and a second planar surface which is formed lower than the first planar surface by forming therebetween a step in a thickness direction of the silicon substrate, the plurality of optical elements are bonded to the first planar surface, the plurality of optical combiners are bonded to the second planar surface in such a manner as to correspond to respective ones of the plurality of optical elements, the step formed in the thickness direction of the silicon substrate is formed in a staircase shape horizontally across the first planar surface, and the plurality of optical elements are arranged along edge faces of the staircase-shaped step formed in said thickness direction, thereby arranging the respective optical elements at different distances with respect to their corresponding optical combiners so that the light path length becomes substantially equal for all the laser light emitted from the plurality of optical elements. 
         [0021]    A manufacturing method is provided for a laser light source device which is constructed by mounting on a silicon substrate a plurality of optical elements and a plurality of optical combiners for combining the laser light emitted from the plurality of optical elements, the manufacturing method includes the steps of a stepped substrate forming step for forming a step in a thickness direction of the silicon substrate and thereby forming on the silicon substrate a first planar surface and a second planar surface which is formed lower than the first planar surface, a bonding portion forming step for forming a bonding portion having a micro-bump structure made of Au on the silicon substrate; an optical element bonding step for bonding a plurality of optical elements to the first planar surface by surface activated bonding; and an optical combiner bonding step for bonding a plurality of optical combiners to the second planar surface by surface activated bonding in such a manner as to correspond to respective ones of the plurality of optical elements, and wherein in the stepped substrate forming step, the step is formed in a staircase shape horizontally across the first planar surface and, in the optical element bonding step, the plurality of optical elements are bonded along edge faces of the staircase-shaped step, thereby arranging the respective optical elements at different distances with respect to their corresponding optical combiners so that the light path length becomes substantially equal for all the laser light emitted from the plurality of optical elements. 
         [0022]    In the laser light source device manufacturing method, the optical combiner bonding step includes the sub-steps of bonding a first one of the plurality of optical combiners, causing two of the plurality of optical elements to emit laser light, adjusting the position of a second one of the plurality of optical combiners so that the light path of one laser light and the light path of the other laser light, when combined by the second optical combiner, overlap each other, bonding the second optical combiner, causing all of the plurality of optical elements to emit laser light; adjusting the position of a third one of the plurality of optical combiners so that the light paths of all the laser light, when combined by the third optical combiner, overlap each other; and bonding the third optical combiner. 
         [0023]    Since the optical elements, optical combiners, and reflecting member can be bonded to the silicon substrate by surface activated bonding and thus integrated on the substrate, the laser light source device can be made compact and thin in design. 
         [0024]    Since the surface activated bonding technique that uses micro bumps and that does not require heating for bonding is used in the manufacture of the laser light source device, no thermal stress is applied and no functional degradation of the components occurs, which serves to prevent the occurrence of distortion due to the difference in thermal expansion coefficient. 
         [0025]    In the laser light source device, since the optical elements and optical combiners are each bonded by surface activated bonding, misalignment during component mounting is minimized, and highly accurate alignment can be achieved. 
         [0026]    In the laser light source device, the relationship between the height of the optical element and the height of the optical combiner or reflecting member is optimized, and the optical combiner can be mounted in the optimum position by considering the spreading of the light path of the laser light emitted from the optical element. The laser light source device thus achieves low optical loss and a high coupling efficiency with an external optical circuit or a light modulator (not shown). Furthermore, since some of the thickness of the optical combiner is accommodated within the silicon substrate by forming the step on the silicon substrate, the laser light source device can be made extremely thin. 
         [0027]    In the laser light source device, the light path length can be made equal for all the laser light emitted from the plurality of optical elements. As a result, since the light path diameter can be made equal for all of the laser light without having to insert a lens in the light path, the laser light source device does not require the provision of a lens. 
         [0028]    In the laser light source device, when the optical element is a laser device, the light source is compact in size, has long service life, and achieves high brightness. Further, in the laser light source device, when the optical combiner is a dichroic mirror prism, the bottom face of the prism is substantially triangular in shape; therefore, by forming an Au film on the bottom face, the prism can be reliability bonded to the silicon substrate. 
         [0029]    In the laser light source device, when the driver IC for driving the optical element is mounted on the silicon substrate, the driver IC need not be provided outside the substrate but can be integrated on the substrate, and the laser light source device can be made extremely compact in size. 
         [0030]    In the laser light source device manufacturing method, since the optical elements and optical combiners are bonded to the silicon substrate by surface activated bonding, component misalignment is minimized, and highly accurate alignment can be achieved. 
         [0031]    In the laser light source device manufacturing method, when the number of optical elements mounted is three, the light path of the light to be emitted outside can be adjusted by just adjusting the positions of two optical combiners, and thus the light path adjustment can be accomplished extremely easily. 
         [0032]    In the laser light source device manufacturing method, when the optical element is a laser device, and the optical combiner is a dichroic mirror prism, component misalignment is minimized, and highly accurate alignment can be achieved. 
         [0033]    In the laser light source device manufacturing method, when the step of mounting a driver IC for driving the optical elements is included, the step of providing the driver IC outside the substrate can be eliminated, and the manufacturing process can thus be simplified. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a diagram showing in schematic form the configuration of a laser light source device  1 . 
           [0035]      FIG. 2  is a cross-sectional view taken along line AA′ in  FIG. 1 . 
           [0036]      FIG. 3  is a top plan view of the laser light source device  1 . 
           [0037]      FIG. 4  is a process diagram illustrating the manufacturing steps of the laser light source device  1 . 
           [0038]      FIG. 5  is a process diagram illustrating the details of a bonding portion forming step. 
           [0039]      FIG. 6  is a perspective view showing in enlarged form a portion of a bonding portion  40  formed in the bonding portion forming step (ST 2 ). 
           [0040]      FIG. 7(   a ) is a perspective view for explaining an optical element bonding step, and  FIG. 7(   b ) is a perspective view for explaining the bonding of a first optical combiner. 
           [0041]      FIG. 8(   a ) is a perspective view for explaining the bonding of a second optical combiner, and  FIG. 8(   b ) is a perspective view for explaining the bonding of a third optical combiner. 
           [0042]      FIG. 9  is a diagram showing in schematic form the configuration of an alternative laser light source device  60 . 
       
    
    
     DESCRIPTION 
       [0043]    A laser light source device and a method for manufacturing the laser light source device will be described below with reference to the drawings. It will, however, be noted that the technical scope of the present invention is not limited by any particular embodiment described herein, but extends to the inventions described in the appended claims and their equivalents. 
         [0044]      FIG. 1  is a diagram showing in schematic form the configuration of a laser light source device  1 . 
         [0045]    The laser light source device  1  will be described below with reference to  FIG. 1 . The basic structure of the laser light source device  1  is characterized in that a plurality of laser devices ( 21 ,  22 ,  23 ) and dichroic mirror prisms ( 31 ,  32 ,  33 ) are bonded to a stepped substrate  10  by surface activated bonding. 
         [0046]    The laser light source device  1  comprises the silicon substrate  10  as a platform, and the laser devices  21 ,  22 , and  23  as a plurality of optical elements and the dichroic mirror prisms  31 ,  32 , and  33  (hereinafter referred to as the dichroic prisms  31 ,  32 , and  33 ) as a plurality of optical combiners mounted on the silicon substrate  10 . In the description hereinafter given, the term laser device  20  will be used when referring to the plurality of laser devices collectively, and the term dichroic prism  30  will be used when referring to the plurality of dichroic prisms collectively. 
         [0047]    The silicon substrate  10  has a first planar surface  11  and a second planar surface  12 , the second planar surface  12  being formed lower than the first planar surface  11  by forming therebetween a step  13  in the thickness direction of the silicon substrate. The step  13  is formed in a staircase shape horizontally across the first planar surface  11 , and the laser devices  20  are bonded to the first planar surface  11  so as to be arranged along the edge faces of the staircase-shaped step  13 . 
         [0048]    The dichroic prisms  30  are bonded to the second planar surface  12  at positions corresponding to the respective laser devices  20 . That is, the dichroic prism  31  is bonded at a position to which the laser light emitted from the laser device  21  is directed, the dichroic prism  32  is bonded at a position to which the laser light emitted from the laser device  22  is directed, and the dichroic prism  33  is bonded at a position to which the laser light emitted from the laser device  23  is directed. 
         [0049]    A bonding portion  40  having a micro-bump structure made of Au (gold) for bonding the laser device  20  thereon is formed on the first planar surface  11  of the silicon substrate  10 . The bonding portion  40  is electrically connected to the bottom electrode (not shown) of the laser device  20 , and is electrically connected to an external laser device driving circuit (not shown) via a plurality of electrode patterns  46  formed on the first planar surface  11 . 
         [0050]    A bonding portion  50  having a micro-bump structure made of Au (gold) for bonding the dichroic prism  30  thereon is formed on the second planar surface  12 . The details of the bonding portions  40  and  50  will be described later. The laser device  20  may be a laser device that directly emits laser light of a designated color, or may be a second harmonic generation (SHG) laser device or the like. In the case of the SHG laser device, there is a need to add a wavelength conversion device that receives laser light from the laser device  20  and converts it into light at the second harmonic wavelength. 
         [0051]    The operation of the laser light source device  1  will be briefly described below. When a drive current is supplied from the external laser device driving circuit via the electrode patterns  46  and the bonding portion  40 , the laser device  20  emits laser light of a designated color. For example, the laser device  21  is a red laser device and emits red laser light  21   a  (hereinafter simply the R laser light  21   a ), the laser device  22  is a green laser device and emits green laser light  22   a  (hereinafter simply the G laser light  22   a ), and the laser device  23  is a blue laser device and emits blue laser light  23   a  (hereinafter simply the B laser light  23   a ). 
         [0052]    The R laser light  21   a  emitted from the laser device  21  enters the corresponding dichroic prism  31 ; the dichroic prism  31  is a reflecting member and reflects the R laser light  21   a  toward its adjacent dichroic prism  32 . The dichroic prism  32  selectively transmits the entering R laser light  21   a,  and selectively reflects the G laser light  22   a  incident from the laser device  22 . In this way, the dichroic prism  32  combines the R laser light  21   a  with the G laser light  22   a,  and directs the combined RG laser light  24  to its adjacent dichroic prism  33 . 
         [0053]    The dichroic prism  33  selectively transmits the entering RG laser light  24 , and selectively reflects the B laser light  23   a  incident from the laser device  23 . In this way, the dichroic prism  33  combines the RG laser light  24  with the B laser light  23   a,  and outputs the RGB laser light  25 . That is, the RGB laser light  25  is produced by combining the R laser light  21   a,  the G laser light  22   a,  and the B laser light  23   a.  A high-quality full color image can be displayed by converting the RGB laser light  25  into image light using an external spatial light modulating means (not shown) and by projecting the light onto a screen (not shown) through a projection optic means (not shown). 
         [0054]      FIG. 2  is a cross-sectional view taken along line AA′ in  FIG. 1 . 
         [0055]    The relationship between the height of the laser device and the height of the dichroic prism in the laser light source device  1  will be described below with reference to  FIG. 2 . As shown in  FIG. 2 , the laser device  22  is bonded via the bonding portion  40  to the silicon substrate  10  along an edge face of the step  13  formed across the first planar surface  11  of the silicon substrate  10 . Preferably, the laser device  22  is bonded so that its light-emitting face  22   b  slightly protrudes from the edge face of the step  13 . 
         [0056]    The G laser light  22   a  can then be emitted from the light-emitting face  22   b  without being vignetted by the first planar surface  11 . The light-emitting face  22   b  has a light-emitting port  22   c  at its lower end, from which the G laser light  22   a  is emitted. As shown, the G laser light  22   a  emitted from the light-emitting port  22   c  gradually spreads out at a prescribed angle θ as it travels forward. 
         [0057]    The dichroic prism  32  for the laser device  22  is bonded via the bonding portion  50  to the silicon substrate  10  at the second planar surface  12  thereof which is formed lower than the first planar surface  11  by the height of the step  13 . Thus, the dichroic prism  32  is located at a prescribed position lower than the laser device  22 . 
         [0058]    The height H of the step  13  is determined so that the optical axis  22   d  defining the center of the light path of the G laser light  22   a  passes through the position located approximately halfway through the thickness of the dichroic prism  32 . With this arrangement, if the light path of the G laser light  22   a  enters the dichroic prism  32  while spreading out at the angle θ as shown, the dichroic prism  32  can accommodate the entire light path of the spreading out G laser light  22   a  and can reflect the light in its entirety for output. 
         [0059]    The silicon substrate  10  has the first planar surface  11  and the second planar surface  12 , the second planar surface  12  being formed lower than the first planar surface  11  by the height H of the step  13  formed in the thickness direction, and the laser device  20  is bonded to the first planar surface  11  while the dichroic prism  30  is bonded to the second planar surface  12 . With this structure, the relationship between the height of the laser device  20  and the height of the dichroic prism  30  is optimized so that the dichroic prism  30  can accommodate the spread out light path of the laser light without any loss. The laser light source device  1  thus achieves low optical loss and a high coupling efficiency with an external optical circuit or a light modulator (not shown). 
         [0060]    The dichroic prism  30  must have a sufficient thickness to accommodate the laser light. Since the dichroic prism  30  is bonded to the lower second planar surface  12  of the silicon substrate  10 , some of the thickness of the dichroic prism  30  is accommodated within the silicon substrate  10 , which means that the laser light source device  1  can be made extremely thin. While  FIG. 2  has shown the relationship between the height of the laser device  22  and the height of the dichroic prism  32 , the above description also applies to the relationship between the height of the laser device  21  and the height of the dichroic prism  31  and the relationship between the height of the laser device  23  and the height of the dichroic prism  33 . 
         [0061]      FIG. 3  is a top plan view of the laser light source device  1 . 
         [0062]    The light path length of the laser light in the laser light source device  1  will be described below with reference to  FIG. 3 . In  FIG. 3 , the bonding portions  40  and  50  and the electrode patterns  46  are omitted from illustration. In  FIG. 3 , the step  13  of the silicon substrate  10  is formed in a staircase shape horizontally across the first planar surface  11 , as previously described, and the laser devices  20  are bonded to the first planar surface  11  so as to be arranged along the edge faces of the staircase-shaped step  13 . 
         [0063]    The R laser light  21   a  emitted from the laser device  21  enters the dichroic prism  31 , is reflected at right angles by the dichroic prism  31  and transmitted through the dichroic prisms  32  and  33 , and enters an external lens  2 . On the other hand, the G laser light  22   a  emitted from the laser device  22  enters the dichroic prism  32 , is reflected at right angles by the dichroic prism  32  and transmitted through the dichroic prism  33 , and enters the external lens  2 . Further, the B laser light  23   a  emitted from the laser device  23  enters the dichroic prism  33 , is reflected at right angles by the dichroic prism  33 , and enters the external lens  2 . 
         [0064]    The external lens  2  is an optical component for transmitting the combined RGB laser light  25  to an external spatial light modulating means (not shown). Each laser light spreads out while traveling along the light path, but the light path here is shown as being a straight line for ease of understanding. 
         [0065]    As shown, the light paths for the R laser light  21   a,  G laser light  22   a,  and B laser light  23   a  emitted from the respective laser devices  20  are spaced apart from each other in a horizontal direction by a distance  11 . The shorter the distance  11 , the smaller the size of the silicon substrate  10  can be made, but a distance of a certain length must be provided because of constrains such as the patterns of the bonding portions  40  (see  FIG. 1 ) and the tool (not shown) used for bonding the laser devices  20  by surface activation. Because of the presence of this distance  11 , if all the laser devices  20  were arranged horizontally in a row, the light path length from the dichroic prism  30  to the lens  2  would be different for each laser light. 
         [0066]    If all the laser devices  20  were arranged horizontally in a row, the light path length of the R laser light  21   a  emitted from the laser device  21  located farthest from the lens  2  would be longer by twice the distance  11  than the light path length of the B laser light  23   a  emitted from the laser device  23  located nearest to the lens  2 . If the light path length thus differed, the light path diameter (beam diameter) of the R laser light  21   a  traveling the longer light path length would become larger than the light path diameter (beam diameter) of the B laser light  23   a  traveling the shorter light path length, because the laser light spread out at the angle θ as earlier described. As a result, the RGB laser light  25  output from the dichroic prism  33  would reach the lens  2  as a beam produced by combining the laser light of different colors having different light path diameters. Therefore, if the thus combined RGB laser light  25  were modulated, a problem may occur due to such flaws as image color nonuniformity. 
         [0067]    To address the above problem, in the laser light source device  1 , the step  13  is formed in a staircase shape between the first and second planar surfaces  11  and  12  horizontally across the width thereof, and the laser devices  20  are bonded to the first planar surface  11  so as to be arranged along the edge faces of the step  13 . Then, the per-step distance  12 , the distance from one step to the next, of the staircase-shaped step  13  is made equal to the distance  11  between the light paths. Thus, in the laser light source device  1 , the difference in light path length due to the distance  11  is accommodated by the per-step distance  12  so that the light path length becomes equal for all of the laser light. 
         [0068]    The laser device  21  located farthest from the lens  2  is bonded at the position corresponding to the top of the staircase-shaped step, that is, at the position nearest to the dichroic prism  31  and not spaced by the distance  12 . The laser device  22  located at an intermediate position from the lens  2  is bonded at the position corresponding to the first stepped portion of the staircase-shaped step, that is, at the position spaced farther away from the dichroic prism  32  by the distance  12 . The laser device  23  located nearest to the lens  2  is bonded at the position corresponding to the second stepped portion of the staircase-shaped step, that is, at the position spaced farther away from the dichroic prism  33  by twice the distance  12 . 
         [0069]    In this way, by forming the step  13  in a staircase shape horizontally across the surface, and by bonding the laser devices  20  along the edge faces of the step  13 , the distance between each laser device  20  and the dichroic prism  30  corresponding to the laser device  20  is made different so that the light path length becomes equal for all the laser light. As a result, since the light path diameter can be made equal for all the colors of the RGB laser light  25  entering the lens  25 , a high-quality image free from flaws such as color nonuniformity can be obtained. 
         [0070]    If a lens is inserted in the light path between the laser device  20  and the dichroic prism  30 , and control is performed so that the laser light does not spread out, the light path diameter can be prevented from varying even if the light path length differs for each laser light. However, arranging such lenses on the silicon substrate  10  involves additional steps for mounting and adjusting the lenses, and can lead to the problem that the size of the silicon substrate  10  increases because of the space for mounting the lenses. In view of this, in the laser light source device  1 , the light path length of each laser light is made equal, eliminating the need for mounting such lenses on the silicon substrate  10 , and thus offering an enormous effect in simplifying the manufacturing process and reducing the size and thickness of the light source unit. 
         [0071]      FIG. 4  is a process diagram illustrating the manufacturing steps of the laser light source device  1 . 
         [0072]    First, the first and second planar surfaces  11  and  12  such as shown in  FIG. 1  are formed by forming the step  13  by deep etching the surface of the silicon substrate  10  manufactured in accordance with an LSI manufacturing process (Step ST 1 : Stepped substrate forming step). The height H of the step to be formed is determined by the relationship between the height of the laser device and the height of the dichroic prism, as earlier described, and is about 100 to 500 μm. 
         [0073]    Next, the bonding portions  40  and  50 , each having a micro-bump structure, are formed on the first and second planar surfaces  11  and  12 , respectively, of the silicon substrate  10  (Step ST 2 : Bonding portion forming step). 
         [0074]      FIG. 5  is a process diagram illustrating the details of the bonding portion forming step.  FIGS. 5(   a ) and  5 ( f ) are enlarged cross-sectional views showing a portion of the first planar surface  11  of the silicon substrate  10  of  FIG. 1  cut across the thickness thereof. 
         [0075]    First, an Au film  41  of gold as a metal material is formed on the first planar surface  11  of the silicon substrate  10  (see  FIG. 5(   a )). 
         [0076]    Next, a resist film  42  is formed in order to leave the Au film  41  as an electrode in a region  40   a  where the bonding portion  40  is to be formed (see  FIG. 5(   b )). That is, the region  40   a  is eventually formed as the bonding portion  40 . 
         [0077]    Next, the electrode is formed by etching away the Au film  41  everywhere except the portion thereof covered by the resist film  42  (see  FIG. 5(   c )). In this way, the Au film  41  is formed as the electrode in the region  40   a.    
         [0078]    Then, after removing the resist film  42 , a resist film  44  for micro-bump formation is formed on the surface of the Au film  41  left as the electrode (see  FIG. 5(   d )). The resist film  44  is formed, for example, in a pattern in which a larger number of tiny dots substantially circular in shape are arranged when viewed from the top. 
         [0079]    Next, half etching is performed to form a groove  41   a  to a prescribed depth in the Au film  41  exposed through each interstice of the dot-like pattern of the resist film  44  (see  FIG. 5(   e )). 
         [0080]    Thereafter, the resist film  44  is removed, and the region  40   a  containing a large number of micro bumps  45  is formed as the bonding portion  40  (see  FIG. 5(   f )). In this way, the large number of micro bumps  45  arranged in a dot-like pattern defined by the grooves  41   a  are formed on the surface of the Au film  41  left in the region  40   a.  Since the Au film  41  in the spacing between each micro bump  45 , that is, in the bottom of each groove  41   a,  is left unremoved so that the lower parts of the micro bumps  45  are connected to each other by the Au film  41 , the entire region  40   a  can be made to conduct and act as an electrode. The process so far described also applies to the formation of the bonding portion  50  to be formed on the second planar surface  12 . 
         [0081]    When forming other electrode patterns  46 , etc. (see  FIG. 1 ) than the micro bumps on the surface of the silicon substrate  10 , the resist film  42  formed in the step of  FIG. 5(   b ) is patterned to match the electrode patterns  46 . Then, by etching the resist film  42  patterned to match the electrode patterns  46  (see  FIG. 5(   c )), the electrode patterns  46 , etc. can be formed. According to the bonding portion forming step described above, the bonding portions  40  and  50 , each having a micro-bump structure formed from an Au film, and the electrode patterns  46 , etc. can be formed efficiently in a collective manner on the surface of the silicon substrate  10 . 
         [0082]      FIG. 6  is a perspective view showing in enlarged form a portion of the bonding portion  40  formed in the bonding portion forming step (ST 2 ). 
         [0083]    As shown in  FIG. 6 , the micro bumps  45  formed from Au are substantially cylindrical in shape; as an example, each micro bump  45  is formed with a diameter of about 8 μm and a height of about 2 μm. Since the Au film  41  is left in the spacing between each micro bump  45 , that is, in the bottom of each groove  41   a,  as described above, the micro bumps  45  are mechanically and electrically connected together by the Au film  41 , and the structure is thus formed as an integral one-piece electrode. 
         [0084]    An outline of the surface activated bonding technique used in the optical element bonding step and optical combiner bonding step performed on the bonding portions  40  and  50  of the micro-bump structure will be described below. 
         [0085]    The surface activated bonding technique is a technique that activates material surfaces by removing inactive layers, such as oxides, dirt (contaminants), etc. covering the material surfaces by plasma or other means, and that bonds the surfaces together by causing atoms having high surface energy to contact each other and by utilizing the adhesion forces acting between the atoms. However, in the case of flat bonding surfaces, surface activated bonding cannot be accomplished unless the surfaces are heated to a certain temperature (100 to 150° C.). In the laser light source device  1 , in order to lower the bonding temperature, the micro bumps  45  are formed from Au, a material that easily deforms plastically, on one side of the bonding surface, that is, on the bonding portions  40  and  50  of the silicon substrate  10  so that the bonding can be accomplished at normal temperatures. 
         [0086]    The principle of the surface activated bonding technique will be described. Films of oxides, contaminants, etc. remain adhered to the actual surface (including the bonding portions  40  and  50 ). Therefore, plasma cleaning or ion-beam sputter etching is performed, and the surfaces of the bonding portions  40  and  50  are activated, thus putting the surfaces of the bonding portions  40  and  50  in an activated condition in which the atoms having bonds are exposed on the surfaces. In this condition, interatomic bonding can be accomplished by just bringing the lower surfaces of the laser device  20  and the dichroic prism  30  into contact with the respective bonding portions  40  and  50 . 
         [0087]    Since this surface activated bonding does not require heating when bonding, there are the following advantages.
   1. Component breakage due to the residual stress arising from the difference in thermal expansion coefficient does not occur.   2. Since no thermal stress is applied to the components, functional degradation of the components does not occur.   3. Since the bonding is done in a solid phase without heating, misalignment does not occur during component mounting.   4. No thermal effects are caused to other components.   5. Since the atoms are directly bonded together, the bonded layers do not deteriorate over time.   
 
         [0093]    Next, each laser device  20  as an optical element is bonded to the bonding portion  40  on the first planar surface  11  of the silicon substrate  10  (step ST 3 : Optical element bonding step). The optical element bonding step will be described with reference to  FIG. 7(   a ). 
         [0094]    As shown, the bonding portion  40  is formed on the first planar surface  11  of the silicon substrate  10 , and the large number of micro bumps are formed on the bonding portion  40  in accordance with the earlier described bonding portion forming step. Each laser device  20  is bonded to the bonding portion  40  by using the above-described surface activated bonding technique. Preparatory to the bonding, the bonding portion  40  of the silicon substrate  10  and the lower surface of the laser device  20  are cleaned by argon plasma, and the respective surfaces are activated. An Au film (not shown) is formed as the electrode on the bonding surface on the underside of the laser device  20 . 
         [0095]    For example, when bonding the laser device  23 , the laser device  23  is held onto a pressing tool  3 , and is placed in a prescribed position on the bonding portion  40  of the silicon substrate  10 . At this time, the laser device  23  must be accurately positioned so that it can be bonded to the prescribed position at the edge face of the step  13 . The positioning of the laser device  23  can be accomplished, for example, by aligning it with an alignment marker (not shown) attached to the prescribed position on the bonding portion  40 . 
         [0096]    Once the laser device  23  is aligned and placed in the prescribed position, a prescribed load is applied to the laser device  23  by the pressing tool  3 . This causes the Au film on the underside of the laser device  23  to contact the micro bumps  45  (see  FIG. 6 ) formed on the bonding portion  40 , and the micro bumps  45  are slightly deformed in the thickness direction under pressure. Since the Au forming the micro bumps  45  and the Au film on the underside of the laser device  23  are both activated, the silicon substrate  10  and the laser device  23  are bonded together at normal temperature (surface activated bonding). The laser devices  21  and  22  are also bonded in the same manner. 
         [0097]    Next, each dichroic prism  30  as an optical combiner is bonded to the bonding portion  50  on the second planar surface  12  of the silicon substrate  10  (step ST 4 : Optical combiner bonding step). The optical combiner bonding step ST 4  comprises sub-steps ST 41  to ST 47 . The optical combiner bonding step ST 4  will be described with reference to  FIG. 8  in conjunction with  FIG. 7(   b ). 
         [0098]    As shown in  FIG. 7(   b ), the dichroic prism  33  as the first optical combiner is held onto the pressing tool  3 , and is placed in a prescribed position on the bonding portion  50  formed on the second planar surface  12  of the silicon substrate  10 . The dichroic prism  33  is mounted in the position corresponding to the laser device  23  so that the B laser light  23   a  (see  FIG. 1)  from the laser device  23  can be reflected. Since the dichroic prism  33  must also be accurately positioned, it is preferable to position it in reference, for example, to an alignment marker (not shown) attached to the prescribed position on the bonding portion  50 . 
         [0099]    Next, by applying a prescribed load K to the dichroic prism  33  using the pressing tool  3 , the dichroic prism  33  is bonded by surface activated bonding in the same manner as the laser device (ST 41 ). An Au film is formed on the bottom face of the dichroic prism  30 . A dichroic mirror which is functionally equivalent to the dichroic prism  30  may be used, but since such a mirror is thin, it is difficult to bond it to the silicon substrate  10 . By contrast, the bottom face of the dichroic prism  30  is a triangular face; therefore, by forming an Au film on the bottom face, the dichroic prism  30  can be reliability bonded to the silicon substrate  10 . 
         [0100]    Next, external drive current is applied to the laser devices  22  and  23  to cause them to emit the G laser light  22   a  and the B laser light  23   a,  respectively (ST 42 ). 
         [0101]    Then, as shown in  FIG. 8(   a ), while causing the laser devices to emit the G laser light  22   a  and the B laser light  23   a,  respectively, the dichroic prism  32  as the second optical combiner is held onto the pressing tool  3 , and is placed in a prescribed position on the bonding portion  50  formed on the second planar surface  12  of the silicon substrate  10 . Then, the GB laser light  26  output from the dichroic prism  33  (that is, the light produced by combining the G laser light  22   a  and the B laser light  23   a ) is detected by an external detector  4 . Here, the position of the dichroic prism  32  is determined by adjusting the position in the X- and Y-axis directions as well as the angle thereof using the pressing tool  3  so that the light path of the G laser light  22   a  and the light path of the B laser light  23   a  overlap each other within the prescribed position (ST 43 ). 
         [0102]    After positioning the dichroic prism  32 , a prescribed load K is applied to the dichroic prism  32  using the pressing tool  3  to accomplish the surface activated bonding (ST 44 ). 
         [0103]    Next, external drive current is applied to all the laser devices  20  to cause them to emit the R laser light  21   a,  the G laser light  22   a  and the B laser light  23   a,  respectively (ST 45 ). 
         [0104]    Then, as shown in  FIG. 8(   b ), while causing the laser devices to emit the R laser light  21   a,  the G laser light  22   a  and the B laser light  23   a,  respectively, the dichroic prism  31  as the third optical combiner is held onto the pressing tool  3 , and is placed in a prescribed position on the bonding portion  50  formed on the second planar surface  12  of the silicon substrate  10 . Then, the RGB laser light  25  output after mounting the dichroic prism  31  (that is, the light produced by combining the R laser light  21   a,  the G laser light  22   a  and the B laser light  23   a ) is detected by the detector  4 . Here, the position of the dichroic prism  31  is determined by adjusting the position in the X- and Y-axis directions as well as the angle thereof using the pressing tool  3  so that the light path of the R laser light  21   a,  the light path of the G laser light  22   a,  and the light path of the B laser light  23   a  overlap each other within the prescribed position (ST 46 ). 
         [0105]    After positioning the dichroic prism  31 , a prescribed load K is applied to the dichroic prism  31  using the pressing tool  3  to accomplish the surface activated bonding (ST 47 ). By performing the sub-steps ST 41  to ST 47  as described above, the optical combiner bonding step is completed, completing the manufacture of the laser light source device in which the light path of the RGB laser light  25  to be emitted outside has been accurately adjusted. 
         [0106]    Since the laser devices and dichroic prisms are bonded by surface activated bonding and integrated on the silicon substrate, as described above, the laser light source device  1  is highly space efficient and is extremely thin and compact in size. Furthermore, since each laser device is directly bonded to the silicon substrate via Au having good thermal conductivity, the laser light source device  1  has excellent heat dissipation characteristics and is advantageous for a pico projector to be mounted in a portable device. 
         [0107]    Further, the laser devices and dichroic prisms are bonded to the substrate via micro bumps by using the surface activated bonding technique that does not require heating for bonding. Accordingly, in the laser light source device  1 , any distortion that may occur due to the difference in thermal expansion coefficient between the substrate and the laser device is suppressed and, since no thermal stress is applied, functional degradation of the components does not occur. Furthermore, since misalignment during component mounting is minimized, the laser light source device  1  can produce the highly accurately combined RGB laser light and can be used to achieve a high-performance projector free from flaws such as color nonuniformity. 
         [0108]    Moreover, in the laser light source device  1 , the light path adjustment is extremely easy because the light path of the RGB laser light to be emitted outside can be adjusted by just adjusting the positions of two dichroic prisms. 
         [0109]      FIG. 9  is a diagram showing in schematic form the configuration of an alternative laser light source device  60 . 
         [0110]    The laser light source device  60  will be described below with reference to  FIG. 9 . In the laser light source device  60 , the same component elements as those in the laser light source device  1  are designated by the same reference numerals, and the description of such component elements will not be repeated here. The basic structure of the laser light source device  60  differs from that of the laser light source device  1  in that an IC chip for driving the laser devices is mounted and integrated on the silicon substrate. 
         [0111]    The laser light source device  60  comprises the silicon substrate  10 , the laser devices  21 ,  22 , and  23  as a plurality of optical elements and the dichroic prisms  31 ,  32 , and  33  as a plurality of optical combiners mounted on the silicon substrate  10 , and the driver IC  61 . The laser devices  21 ,  22 , and  23  and the dichroic prisms  31 ,  32 , and  33  are identical in structure and operation to the corresponding components in the laser light source device  1 , and will not be further described herein. 
         [0112]    The driver IC  61  is bonded to the first planar surface  11  by surface activated bonding, is supplied with power from an external source, and drives the plurality of laser devices  20  to cause them to emit the R laser light  21   a,  the G laser light  22   a,  and the B laser light  23   a,  respectively. The driver IC  61  is connected to the respective laser devices  20  by interconnection patterns formed on the first planar surface  11 , but the interconnection patterns are not shown in  FIG. 9 . 
         [0113]    The step of bonding the driver IC  61  to the silicon substrate  10  can be performed simultaneously with the bonding of the laser devices  20  in the optical element bonding step (step ST 3 ) in the manufacturing process of the laser light source device  1 , and therefore will not be described herein. 
         [0114]    Since the driver IC  61  for driving the laser devices  20  is mounted on the silicon substrate  10  as described above, the driver IC  61  need not be provided outside the substrate but can be integrated on the substrate, and the laser light source device  60  can be made extremely compact in size. Furthermore, in the laser light source device  60 , since the number of interconnection lines to be wired to the laser light source device can be reduced by building the driver IC  61  into the laser light source device, not only can the electrical connecting means for the laser light source device be simplified, but the laser light source device can be easily installed. 
         [0115]    The configuration, manufacturing process, etc. to be employed for the laser light source devices  1  and  60  are not limited to those shown in the drawings given herein, but may be altered or modified as desired without departing from the spirit and scope of the present invention. Further, while the laser light source devices  1  and  60  have each been described by dealing with an example in which three RGB laser devices are mounted, it will be appreciated that the number of laser devices to be mounted may be two or, alternatively, four or more laser devices may be mounted. 
         [0116]    Since the laser light source devices  1  and  60  described above can be made extremely compact and thin in design, these laser light source devices can be used widely as light source units for pico projectors to be mounted in portable devices such as mobile phones.