Abstract:
A method of mounting an optical device having a step on the surface opposing to a mounting substrate favorably by face-down bonding which enables a decrease in the number of components or integrate additional components on one identical substrate and, accordingly, is useful for reducing the size and the thickness of an optical head using a light source, the method typically includes the step of making the area ratio of each electrode to a solder pattern different for every wiring electrode portions upon mounting the electrodes on the substrate for mounting the optical device, in which the optical device having the step can be mounted favorably to the substrate by the control for the height of solder upon melting, and the volume of the solder is previously controlled depending on the wettability of a region of the substrate covered by the solder.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of mounting an optical device having a step on an outer profile of an element on a substrate by face down bonding and a mounting structure. The present invention more particularly relates to an optical integrated module device mounting an optical device by the mounting method and the mounting structure described above and an optical device such as an integrated optical head device using the same.  
           [0003]    2. Related Art  
           [0004]    A nitride semiconductor laser device capable of oscillating a blue laser light at a wavelength of about  410  nm has the following constitution. That is, a nitride semiconductor material comprising InGaN as an active layer and AlGaN as a clad layer are crystallographically grown on a gallium nitride (GaN) substrate by a crystal growing method  
           [0005]    When such a blue semiconductor laser is put to practical use, the recording capacity of existent DVD apparatus using red laser light at a wavelength of about 650 nm will be increased by about four times. In view of the foregoing, sooner practical use of the blue semiconductor laser is expected. However, since no GaN substrate of a large diameter is available, crystal growth is conducted by using a sapphire (A 1   2   0   3 ) substrate having a lattice constant substantially identical with that of the GaN substrate. A typical example of an optical device having a step on the outer profile of the element is the GaN series semiconductor laser element. Accordingly, a P-electrode and N-electrode of a GaN series semiconductor laser formed on sapphire as an insulator are formed on one identical surface, while the P-electrode on the side of the active layer is made higher than the N-electrode, for example, by about 3 μm. For the purpose of improving the characteristic and the life of the optical device, mounting is preferably conducted by the face-down bonding. In this case, however, it is necessary to mount the device with the surface having the step on the outer profile of the element being opposed to the element mounting substrate. Accordingly, for the sake of electrically connecting the P-electrode and the N-electrode having a step on the outer profile of the element as in the existent GaN series semiconductor laser element, a mounting method different from the prior art is required. That is, in most of existent elements, the P-electrode or N-electrode can be formed on one of the surface and rear face of the outer profile of the element. Accordingly, there was no problem that occurs in the nitride semiconductor laser device formed on an insulative substrate.  
           [0006]    [0006]FIG. 7 is a cross sectional view illustrating an example of mounting a gallium nitride series semiconductor laser element on a substrate. A laser diode chip  108  has a structure in which a gallium nitride series compound semiconductor layer  102  is laminated on the sapphire substrate  101 . In the laser diode chip  108 , a positive electrode  104  and a negative electrode  103  are formed on one identical surface of a crystal laminate. Then, as can be seen from FIG. 7, the outer profile of the optical device has a step, and the positive electrode  104  and the negative electrode  103  are formed on the surfaces of different levels that form the step.  
           [0007]    On the other hand, an insulative heat sink  105  is formed with a positive electrode  106  and a negative electrode  107  being metallized on one identical surface of the heat sink. Each of the electrodes on the heat sink and each of the electrodes  104 ,  103  of the laser diode chip  103  corresponding to them respectively are connected with each other. Such a connection method is referred to as mounting by face down (also referred to as junction down) bonding since connection is made such that the heat sink  105  is covered not on the side of the crystal growing substrate but at the junction surface constituting the active layer of the laser diode chip  108 . Reference numeral  109  denotes connection solder. Such a constitution is disclosed in, for example, Japanese Patent Laid-Open No. Hei 7-235729.  
         SUMMARY OF THE INVENTION  
         [0008]    An primary object of this invention is to provide a method of mounting an optical device having a step on the surface opposing to a mounting substrate favorably by face down bonding. This invention also intends to attain reduction in the size and the thickness of an optical head device using the optical device. Technical subjects of the invention are to be detailed.  
           [0009]    When mounting is conducted by face down bonding, the following problems are present. FIG. 8A and FIG. 8B are cross sectional views illustrating an example of a method of mounting an optical device  110  having a step on the mounting surface of an element by face down bonding in accordance with the procedures.  
           [0010]    [0010]FIG. 8A shows a state before mounting by face down bonding. A P-electrode  111  and an N-electrode  112  of an optical device are formed on an optical device at a surface opposing to a mounting substrate, while a P-side wiring pattern  113  and an N-side pattern  114  are formed on a mounting substrate  119  at positions facing the P-electrode  111  and the N-electrode  112 , respectively. Further, connection solder  115  and  116  are formed at the same height both for the P-side and the N-side.  
           [0011]    [0011]FIG. 8B shows a state in which connection is made by melting solder. After positioning the optical device  110 , a load of about 10 g is applied from above the optical device  110 and it is heated to a temperature equal to or higher than the melting point of the solder. The solders  115  and  116  are melted and the molten solder is in contact with the P-electrode  111  and the N-electrode  112  of the optical device  110 . Then, by reaction of the molten solder and each of the electrodes, solder connection with the mounting substrate is completed.  
           [0012]    However, as can be seen in FIG. 8B, the P-electrode  111  on the side of the active layer is formed on a surface at a high position from the surface opposite to the surface facing the substrate (hereinafter simply referred to as a convex portion). Accordingly, the molten solder is crushed by the application of the load through the convex portion. Then, surplus solder flows to the outside of the P-side wiring pattern  113 . This state is shown by arrows in FIG. 8B. There was a worry of short-circuit between the P-electrode  113  and the N-electrode  114  by the solder  117  flowing to the N-side wiring pattern  114 . FIG. 8C shows an example of forming a groove  118  for receiving the flowing solder between the wiring pattern  113  on the P-side and the wiring pattern  114  on the N-side of the mounting substrate. However, a groove forming process has to be provided additionally. Further, there is a drawback that the substrate is caused to crack at a groove-formed portion. This invention solves such a problem.  
           [0013]    Recently, an optical disk apparatus capable of using various types of optical disks of CD, CD-ROM, CD-R, and CD-rewritable specifications at a wavelength of 780 nm and DVD, DVD-ROM, DVD-RAM specifications at a wavelength of 650 nm has been provided. In the light source for such apparatus, a light source and an optical detector are disposed separately for every semiconductor lasers of different wavelength. Further, short wavelength lasers at wavelength of blue color, purple color or shorter further improved with the recording density are expected to be used in the feature. Accordingly, it is expected that an increase in the number of parts for the optical head is inevitable. Therefore, the drawbacks concerned with the reduction of the size and the reduction of the thickness of the entire apparatus cannot be solved completely by the existent art. This invention also satisfies such a demand.  
           [0014]    The gist of this invention is as described below. That is, a substrate having at least a first electrode and a second electrode, and an optical device having a third electrode and a fourth electrode corresponding to the first electrode and the second electrode respectively are prepared. In this optical device, the surfaces of the device for mounting the third electrode and the fourth electrode have a first height and a second height different from each other from the surface of the device on the side opposite to the surfaces on which the third and the fourth electrodes are disposed. This invention intends to favorably mount such an optical device on the substrate.  
           [0015]    Each of the first electrode and the second electrode has at least a solder-underlying region and is connected with a conductor layer therebelow. The solder is mounted directly to the solder-underlying region. Then, the volume of the solder to be prepared is controlled before melting in accordance with the position for each of the electrodes of the optical device. In a practical measure, the area ratio of the area of the solder-underlying region to the area of the solder layer formed thereon is determined such that the ratio is different for every electrodes. The way of disposing the solder is to be described specifically later.  
           [0016]    The corresponding electrodes of the substrate and the optical device corresponding to each other are opposed and connected electrically by melting of solder. In this case, the height and the area of the solder corresponding to at least one of the first electrode and the second electrode are controlled, and each of the electrodes of the optical device and each of the electrodes of the substrate corresponding thereto are electrically connected favorably to each other.  
           [0017]    In this case, two cases may be considered depending on the nature of the surface of the region that is in contact with and covered with the solder. That is, there are a material having such a nature of increasing the height of the solder by the melting of the solder to higher than the initial height of the solder and a material having such a nature of decreasing the height of the solder by the melting of the solder to lower than the initial height. The material having the nature of increasing the height of the solder by the melting of the solder to higher than the initial height is the so-called less wetting material, whereas the material having the nature of decreasing the height of the solder by the melting of the solder to lower than the initial height is the so-called highly wetting material with respect to solder.  
           [0018]    In a case where the surface of the substrate in the region in contact with and covered by the solder is formed of a material having the nature of increasing the height of the solder by melting to higher than the initial height, the area of the solder corresponding to an electrode disposed at a lower height from the surface of the optical device opposite to the surface on which the electrode disposed, for the first height and the second height at which each of the third electrode and the fourth electrode is formed, is made larger than the area of the solder-underlying region below the solder. Briefly, it is preferred to use the surface having the nature described above for the solder-underlying region of the electrode corresponding to a portion of a larger distance between the substrate and the opposing surface of the optical device among the electrode of the substrate. Then, it is preferred that the height of solder does not change greatly for the other electrode. For this purpose, it is practical that the area is made substantially identical between the area for the solder-underlying region and the area for the solder to be prepared.  
           [0019]    On the other hand, in a case where the surface of the substrate in the region in contact with and covered by the solder is formed of a material having the nature of decreasing the height of the solder by melting lower than the initial height, the area of the solder corresponding to an electrode disposed at a position higher than the surface of the optical device opposite to the surface on which the electrode is disposed, for the first height and the second height at which each of the third electrode and the fourth electrode is formed, is made smaller than the area of the solder-underlying region below the solder. Briefly, it is preferred to use the surface having the nature described above for the solder-underlying region of the electrode corresponding to a portion of a smaller distance between the substrate and the opposing surface of the optical device among the electrode of the substrate. Then, it is preferred that the height of solder does not change greatly for the other electrode. For this purpose, it is practical that the area is made substantially identical between the area for the solder-underlying region and the area for the solder to be prepared.  
           [0020]    The gist for the concept of this invention is as has been described above and the details for the method of providing the solder and each of the electrodes are to be explained specifically. In the following explanations. FIG. 2 through FIG. 5 are referred to while citing reference numerals in the figures for the purpose of easy understanding of the invention but they are cited merely as example. It will be apparent that this invention may adopt various other embodiments than the concrete examples described above.  
           [0021]    Areas for solder-underlying regions ( 127 ,  128 ) of a first electrode and a second electrode formed on the surface of a substrate  121  for mounting a semiconductor element  120  are defined as aS 1  and aS 2 , respectively. Volumes of solder ( 124 ,  125 ) disposed on the solder-underlying regions for the first electrode and the second electrode are defined as v 1  and v 2 , respectively. In this invention, v 1 ≠v 2 .  
           [0022]    Then, the semiconductor element mounted by face down bonding on a substrate has an outer profile having the following cross section. A third electrode  122  is formed on a surface at a height ah 1  from one surface  140  of the semiconductor element, while a second electrode  132  is formed on a surface at a height ah 2  from the surface  140 , where ah 1 &gt;ah 2 . An area for the third electrode  122  is defined as aS 3  and an area for the fourth electrode  103  is defined as aS 4 .  
           [0023]    The first electrode  127  is connected with the third electrode  122 , while the second electrode  128  is connected with the fourth electrode  123 . It is practical that the areas for the pair of electrodes to be connected with each other are of an identical area. That is, aS 1 ≈aS 3  and aS 2 . The areas may of course be different from each other.  
           [0024]    According to the mounting method of this invention, each of the values for ah 1 , ah 2 , aS 1 , aS 2 , v 1  and v 2  are defined such that the height for each of the opposing surfaces of the semiconductor element has a substantially constant value H relative to each of the electrode surfaces of the substrate when the semiconductor element  1  is face-mounted on the substrate. Among the six values described above, at least two values are predetermined and each of the remaining values is determined corresponding thereto. That is, each of the values is determined and prepared such that the constant height H constitutes a height that is in proportion to (ah 1 +v 1 /aS 1 ) or (ah 2 +v 2 /aS 2 . In determining each of the values, the nature of the surface of the region in contact with and covered with the solder is of course taken into consideration. While the area for the solder-underlying region is expressed as aS 1  and aS 2  in the foregoing explanation, they are expressed as aS′ 1  and aS′ 2  in FIGS. 4 and 5 since volumes of the solder to be prepared are different. However, in the general description, such areas of the solder-underlying regions are typically expressed as aS 1  and aS 2 .  
           [0025]    The solder-underlying region of the substrate comprises a multi-layered film having three layers in most cases. Preferably, a first layer is formed of titanium (Ti) or chromium (Cr), and a second layer is formed of any of platinum (Pt), molybdenum (Mo) and tungsten (W), while a third layer (uppermost layer) is formed of gold (Au) or silver (Ag). In a typical example, the solder comprises Au and Sn. A composition of the solder comprising 20% by weight to 33% by weight of Sn and the balance of Au is often used.  
           [0026]    Silicon is used usually for the substrate. A typical example of the material with less wettability to solder on the surface of the substrate is an inorganic material such as a silicon oxide film or silicon nitride film, or an organic material such as polyimide. Further, as the material of less wettability, metal such as chromium (Cr), platinum (Pt) or molybdenum (Mo) may also be used.  
           [0027]    Preferred embodiments in this specification are described with reference to a cross sectional view of a set of electrodes but this invention can of course be practiced, for example, in a case where there are plural regions of electrodes to be connected with solder. For example, in the example of FIG. 1, a portion of an N-electrode  123  to be connected with solder includes three separated portions, while the P-electrode  124  includes an electrode of one region. Further, each of the three separated portions may have a further different number of sub-separated regions, or the electrode may be constituted as plural separated regions in one region. Such embodiments are selected in accordance with the requirement of products to be practiced.  
           [0028]    The optical head device according to this invention has a light source for irradiating a disk substrate with light to conduct at least one of writing and reading of information, and a driving circuit for driving the light source to output light and uses the optical device as the light source by applying the mounting method explained so far. Since the mounting method according to this invention can favorably mount also a semiconductor laser device in which plural electrodes electrically connected with plural electrodes on the mounting substrate are opposed to the substrate surface and formed at the positions of different levels, plural optical devices of different forms, for example, semiconductor lasers, photoreceiving devices, or semiconductor devices, for example, driving circuits or amplifiers can be formed by integration. The devices of different forms mean herein those having electrodes with the levels of the electrodes to be mounted on the mounting substrate relative to the substrate surface being different. Accordingly, it is required to use a mounting method different from that for the element in which electrodes are disposed on a substantially identical plane.  
           [0029]    In this case, the light source portion of the optical head apparatus can be mounted with required components such as plural semiconductor laser devices, optical detectors for automatic focus detection and optical detectors for detecting tracking, amplifiers for amplifying signals from both of the detectors noted above or, optionally, semiconductor devices. Further, it will be apparent that desired elements can be incorporated monolithically in a case of using a semiconductor substrate for the substrate.  
           [0030]    The thus obtained light source portion make it possible that an optical path from the light source to the disk substrate that passes the optical light source, beam splitter and objective lens is made into a single constitution in the optical head apparatus. Other practical constitutional examples are to be explained below in the column for the embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 is a perspective view for explaining a mounting structure for mounting an optical device having a step on a mounting surface on a substrate;  
         [0032]    [0032]FIG. 2A is a diagram for explaining a first mounting method according to this invention for mounting an optical device having a step on the mounting surface and a substrate on a substrate;  
         [0033]    [0033]FIG. 2 is a plan view for explaining a first mounting method according to this invention for mounting an optical device having a step on the mounting surface and a substrate on a substrate;  
         [0034]    [0034]FIG. 3A is a cross sectional view showing a first mounting method of this invention for mounting an optical device having a step on the mounting surface and a substrate on a substrate in the order of manufacturing steps;  
         [0035]    [0035]FIG. 3B is a cross sectional view showing a first mode of the invention;  
         [0036]    [0036]FIG. 4A is a diagram for explaining a second mounting method according to this invention for mounting an optical device having a step on the mounting surface and a substrate on a substrate;  
         [0037]    [0037]FIG. 4B is a cross sectional view showing a second mode of the invention;  
         [0038]    [0038]FIG. 5A is a cross sectional view showing a second mounting method of this invention for mounting an optical device having a step on the mounting surface and a substrate on a substrate in the order of manufacturing steps;  
         [0039]    [0039]FIG. 5B is a cross sectional view showing a third mode of the invention;  
         [0040]    [0040]FIG. 6A is a cross sectional view showing an example of a method of locally forming solder in the order of manufacturing steps;  
         [0041]    [0041]FIG. 6B is a cross sectional view showing an example of a method of locally forming solder in the order of manufacturing steps;  
         [0042]    [0042]FIG. 6C is a cross sectional view showing an example of a method of locally forming solder in the order of manufacturing steps;  
         [0043]    [0043]FIG. 6D is a cross sectional view showing an example of a method of locally forming solder in the order of manufacturing steps;  
         [0044]    [0044]FIG. 7 is a cross sectional view showing an existent example of mounting a gallium nitride series compound semiconductor laser element on a substrate;  
         [0045]    [0045]FIG. 8A is a cross sectional view for explaining a drawback in the method of mounting an optical device having a step on the mounting surface on a substrate;  
         [0046]    [0046]FIG. 8B is a cross sectional view for explaining a drawback in the method of mounting an optical device having a step on the mounting surface on a substrate;  
         [0047]    [0047]FIG. 8C is a cross sectional view for explaining a drawback in the method of mounting an optical device having a step on the mounting surface on a substrate;  
         [0048]    [0048]FIG. 9 is a schematic explanatory view showing an example of an optical head apparatus having a single optical path mounting an integrated light source module according to this invention;  
         [0049]    [0049]FIG. 10A is a plan view for explaining a structure of a light source according to this invention;  
         [0050]    [0050]FIG. 10B is a cross sectional view of the semiconductor substrate taken along broken line A-A′ in FIG. 10A;  
         [0051]    [0051]FIG. 11 is a diagram showing an example of a substrate of an integrated light source, a positioning index, a solder pattern and electrodes according to this invention;  
         [0052]    [0052]FIG. 12 is a diagram showing an example of an index pattern for positioning provided for a semiconductor laser;  
         [0053]    [0053]FIG. 13 is a diagram showing a method of positioning a semiconductor laser light source with an index and an integrating substrate with a corresponding index pattern;  
         [0054]    [0054]FIG. 14 is a cross sectional view taken along line A-A′ in FIG. 10A;  
         [0055]    [0055]FIG. 15 is a cross sectional view showing an example of an integrating substrate provided with a layer for promoting heat dissipation from a semiconductor laser light source;  
         [0056]    [0056]FIG. 16 is a plan view showing an example of mounting three kinds of semiconductor laser light sources on an integrating substrate according to this invention; and  
         [0057]    [0057]FIG. 17 is a plan view showing an example of monolithically integrating amplifiers and optical detectors on an integrating substrate according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]    [0058]FIG. 1 is a perspective view showing an embodiment of a mounting structure according to this invention. An optical device  120  is opposed to an optical element mounting substrate  121 . The optical device  120  has a step at a surface opposing to the substrate  121  for optical element mounting. A P-electrode  122  and an N-electrode  123  of the optical device are disposed on the surface at a first level and the surface at a second level, respectively, which form the step. A typical example of the optical device is, for example, a blue semiconductor laser (blue Laser Diode: hereinafter simply referred to as blue LD). Generally, in view of the restriction of the device manufacturing method, the blue LD has a structure in which the N-electrode is formed at a position lower by about 3 μm compared with the P-electrode having an active layer that emits light.  
         [0059]    [0059]FIG. 1 shows a mounting structure in a case of mounting the blue LD  120  by face down bonding with an aim of improving the characteristics and reliability of the device. The surface on which the P-electrode  122  is mounted has a larger distance from the other surface of the device than the surface on which N-electrode  123  is mounted. The thickness of solder  124  on a mounting substrate  121  corresponding to the P-electrode  122  is 3 μm, while the thickness of solder  125  on the side of the N-electrode is 6 μm. Thus, the step of 3 μm between the P-electrode  122  and the N-electrode  123  of the blue LD  120  can be accommodated by the difference of the solder thickness and the blue LD and the substrate can be favorably electrically connected and secured.  
         [0060]    [0060]FIGS. 2 and 3 are views for specifically explaining a first method of face down mounting of the LD according to this invention. FIG. 2A is a cross sectional view of a substrate and an optical device before mounting the LD. FIG. 2B is an upper plan view of the substrate. FIGS. 3A and 3B are cross sectional views succeeding to the state shown in FIG. 2 in the order of manufacturing steps.  
         [0061]    An element mounting substrate  121  for mounting the blue LD  120  is made of silicon (Si) and a silicon oxide film  126  with less wettability to solder is formed on the surface of the substrate  121 . The silicon oxide film  126  serves also an insulative protective film. Further, a solder-underlying pattern  127  for the P-side and a solder-underlying pattern  128  for the N-side are formed at positions opposing to the P-electrode  122  and the N-electrode  123  of the blue LD  120 , respectively. In the figure, reference  131  denotes an active region of the optical device and reference numerals  129  and  130  denote lead ends for the respective electrodes, being connected with the solder-underlying regions. Incidentally, the connection portion thereof is not illustrated in the upper plan view.  
         [0062]    The solder mounted on the substrate are prepared as described below. As shown in FIG. 2B, the area (aS 1 ) for the solder-underlying metal pattern  127  for the P-side and the area (SS 1 ) for the solder pattern  124  are made substantially identical. The thickness (t 1 ) is 3 μm. On the other hand, on the side of the N-electrode, a solder pattern  125  is formed at an area (SS 2 ) twice the area (aS 2 ) of the solder-underlying metal pattern  128  for the N-side. The thickness (t 2 ) is 3 μm.  
         [0063]    Then, as shown in FIG. 3A, when solder is melted by heating, since the solder  124  on the side of the P-electrode has the area identical with the underlying metal pattern  127 , the height of the molten solder is substantially identical with the initial solder height (h 1 ). On the other hand, on the side of the N-electrode, since the area for the solder pattern  125  is larger than the area for the N-side underlying metal pattern  128  on the substrate  121  opposing to the electrode at a lower position from the surface of the optical device opposite to the mounting substrate, the height increases to (h 2 ) which is higher than the initial height of the solder. This is because the molten solder  125  on the silicon oxide film (SiO 2 )  126  made of a material with less wettability to the solder detaches from the surface of the silicon oxide film  126  and gathers on the electrode of good wettability due to the surface tension, so that it is higher (h 2 ) than the initial height of the solder. Thus, as shown in FIG. 3B, solder connection is conducted preferably also including the electrode at a lower position from the surface of the step of the optical device opposite to the mounting substrate.  
         [0064]    [0064]FIGS. 4 and 5 show a second face down mounting method according to this invention. FIG. 4A is a cross sectional view of each of components before mounting an LD (Laser Device: hereinafter simply referred to as LD). In this LD, the P-electrode  122  of the LD  120  forms a higher region in view of one surface  140  of the crystal growing substrate, while the N-electrode  123  forms a lower region. A substrate mounting the LD  120  is made of silicon (Si)  121 , and a solder-underlying pattern  127  for P-side and a solder-underlying pattern  128  for an N-side are formed at positions opposing to the P-electrode  122  and the N-electrode  123  of the LD  120 . Portions identical with those in FIG. 2 carry the same reference numerals.  
         [0065]    The method of forming solder is as described below. As shown in the plan view of FIG. 4B, a solder pattern  125 ′ 0  of an area (indicated as area (aS′ 2 )) identical with the area (aS′ 2 ) of a solder-underlying metal pattern  128  for soldering is formed. The thickness (t′ 2 ) is 6 μm. On the other hand, a solder pattern  124 ′ at an area (S′S 1 ) one-half of the area (S′ 1 ) for the solder-underlying metal pattern  127  for the P-side is formed at a thickness (t′ 1 ) of 6 μm.  
         [0066]    Then, when the solder  124 ′ and  125 ′ are melted by heating, since the solder  125 ′ on the side of the N-electrode has an area identical with that of the underlying metal pattern  128 , the height of the molten solder is substantially identical with the initial solder height (h′ 2 ). On the other hand, on the P-electrode side, since the area for the solder pattern  124 ′ is smaller than the area for the underlying metal pattern  127  on the substrate opposing to the stepped convex portion on the surface of the optical device, the height of the solder becomes lower than the height at the initial stage (h′ 1 ) (FIG. 5A). This is attained by the wetting and spreading of the molten solder on gold (Au) metal which is a material having high wettability to the solder over the gold metal. Thus, as shown in FIG. 5B, the solder is favorably connected and mounted including the step of the optical device.  
         [0067]    Then, an example of locally forming a solder layer is shown. FIGS. 6A through 6D are cross sectional views for explaining the method of forming the pattern.  
         [0068]    An SiO 2  insulative film  126   a  is deposited by a thickness of about 200 Å on the surface of a silicon (Si) substrate  121  by a thermal oxidation method. Then, a P-side wiring layer and an N-side wiring layer are formed on the SiO 2  insulative film  126   a  at positions opposing to the P-electrode and the N-electrode of a semiconductor laser chip to be mounted by using the usual photo-etching technique and vapor deposition process. Each of the wiring layers is a laminated film comprising, for example, a Ti layer (1000 Å thickness), a Pt layer (1000 to 2000 Å thickness) and an Au layer (about 5000 Å thickness).  
         [0069]    Further, a silicon oxide film  126  is formed by a plasma CVD process. Then, the silicon oxide film at predetermined portions of the silicon oxide film  125  is selectively removed. Metal layers  127  and  128  for solder-underlying patterns are formed covering the openings after removal. The metal layer is made of a stacked film comprising, for example, a Ti layer (1000 Å thickness), a Pt layer (1000 Å to 2000 Å thickness) and Au layer (about 2000 Å thickness). This state is shown in FIG. 6A.  
         [0070]    A resist film  140  is formed on the thus prepared substrate. Then, a window  141  of an area identical with that of the solder-underlying metal pattern  127  for the P-side is disposed in the resist film  140  by using the usual photoresist technique. On the other hand, a window  142  of an area twice the area of the solder-underlying metal pattern  128  for the N-side is formed on the side of the N-electrode (FIG. 6B).  
         [0071]    In the state of depositing the resist film, an AuSn solder layer  143  is deposited to a thickness of about 3 μm on the surface (FIG. 6C).  
         [0072]    Then, the specimen is immersed in an organic solvent capable of dissolving in the resist (for example, acetone), supersonic vibration is applied to the solvent to solve the resist film and form the solder layers  144  and  145  selectively (FIG. 6D). Simultaneously, the metal film on the resist film is naturally removed. This method is referred to as “lift-off method”.  
         [0073]    While only one element is shown in the figure, plural elements are usually formed in one Si wafer in the mass production. Accordingly, the Si substrate is finally cut by using a dicing saw in accordance with the pitch of patterning to manufacture optical element mounting wiring substrates  121 .  
         [0074]    For the method of depositing the metal, a description has been made of an example of using the vapor vacuum deposition method, but it will be apparent that the sputtering method, ion plating method or plating method is also applicable. Further, for the method of selectively forming the metal layer, an etching method utilizing the photoresist technology, a lift-off method, a selective plating method, as well as a selective vapor deposition method by using a metal mask or the like can be adopted.  
         [0075]    In the foregoing explanations, the first metal layer constituting the metal patterns  127  and  128  for the solder-underlying layer is a contact layer which is used for enhancing adhesion with the SiO 2  insulative layer  126  deposited on the Si substrate  121 . Accordingly, Cr or the like can also be used in addition to Ti for such a layer.  
         [0076]    The second metal layer constituting the metal patterns  127  and  128  as the solder-underlying layer functions as a diffusion barrier layer for preventing inter-diffusion between the third Au layer and the first Ti layer. Accordingly, other materials than Pt, for example, Cr, Mo or W may also be used for the layers. The third metal layer constituting the metal patterns  127  and  128  as the solder-underlying layer has a role of preventing oxidation on the upper surface of the barrier layer and ensuring the wettability with the solder. Accordingly, Ag or like other material can also be used in addition to Au for such a layer.  
         [0077]    Further, as a material with less wettability to the solder, an inorganic material such as a silicon nitride film or an organic material such as polyimide can also be used in addition to the silicon oxide film for the surface on the substrate  121 . Further, as the material with less wettability to the solder, metals such as chromium (Cr), platinum (Pt) and molybdenum (Mo) may also be used.  
         [0078]    The solder in this embodiment comprises Au and Sn but other solder may also be used. Examples of other solder can include PbSn, In and SnAg.  
         [0079]    Then, a concrete example of applying the optical device according to this invention is shown. This is an example of an optical head and FIG. 9 is a schematic explanatory diagram showing the constitution of the optical head.  
         [0080]    In constituting a light source of the optical head, an integrated module  100  having a semiconductor laser portion, a reflection mirror  5 , and optical detectors  7 ,  8  and  9  is prepared. The semiconductor laser portion has a structure in which a blue semiconductor laser chip  4   a  and a red semiconductor laser chip  4 b are mounted on a semiconductor substrate  1 . In the figure, reference numeral  2  denotes an attaching surface of the laser chip. The blue semiconductor laser chip  4   a  has an outer profile explained so far. That is, it has both of a P-electrode and an N-electrode on the surface for mounting on the substrate and both of the electrodes are formed with surfaces at two levels respectively on the crystal growing substrate. On the other hand, the red semiconductor laser chip  4   b  has a structure in which electrodes are led out from the surface and the rear face of the chip. When the blue semiconductor laser chip  4   a  is mounted on the substrate  1 , the mounting method according to this invention is adopted. The concrete structure thereof can adopt any of the embodiments explained so far. Accordingly, a detailed explanation of the mounting method is to be omitted.  
         [0081]    Blue and red laser light emitted from the integrated module  100  form beams  6   a  and  6   b,  respectively, which are arranged into parallel light through a collimator lens  10 . Then, they pass through an upwarding mirror  11 , a diffraction grating plate  12  and reach an objective lens  13 . The laser light is applied by way of the objective lens  13  as spots  15 ,  16  onto the surface of an optical disk  14 . Depending on the semiconductor laser wavelength, the objective lens  13  comprises plural members, or a single member capable of collecting light of plural wavelength. The lens is focused on a recording surface by an actuator  17  in accordance with the motion along with the rotation of the optical disk and conducts tracking, that is, follows the recording track  18  on the surface of the disk. Thus, signals are recorded as a row of pits on the optical disk in accordance with the driving state, that is, the ON-OFF state of the semiconductor laser, or the signals are used for reading already recorded pits.  
         [0082]    As described above, when plural semiconductor lasers are integrated in the integrated module  100 , the collimator lens  10 , the objective lens  13  and the upwarding mirror  11 , etc. are arranged in one piece to make the optical paths of the optical head into a single constitution.  
         [0083]    In other words, the following optical apparatus can be realized by using this optical head. The example can include, for example, (1) those capable of recording and reproducing DVD of 1.2 mm in thickness by a semiconductor laser  4   b  at a wavelength of 650 nm or (2) those capable of recording and reproducing DVD-RAM super DVD of 0.6 mm in thickness by a semiconductor laser  4   a  at a wavelength of 410 nm.  
         [0084]    As described above, this invention provides a breakthrough for reducing the size and the thickness of the entire driver apparatus capable of recording and reproducing various types of optical disks. Various types of semiconductor lasers of different wavelengths and optical detectors corresponding to such different wavelengths are positioned at an order of mask accuracy and plural semiconductor lasers are hybridized and integrated so as to reduce the number of components comparable with that of monolithic. Then, plural optical paths in existent optical heads can be reduced into a single optical path.  
         [0085]    Then, an example of the constitution of a DVD integrated substrate is to be explained. FIG. 10A specifically shows the surface of the semiconductor substrate  1  as viewed from the side of the collimate lens  10 . Eight solid quadrants shown by reference numeral  32   a  each denotes a laser beam at a wavelength of λa separated by the diffraction grating  23 , while eight bland quadrants shown by reference numeral  32   b  correspond to laser beams at a wavelength of λb separated by the diffraction grating  12 . Optical detector elements for obtaining out-of-focus detection signal are indicated by reference numeral  7 . The region  7  comprises eight rectangular optical detector elements  7   a  for receiving laser beams  32   a  at a wavelength of λa and eight rectangular optical detector elements  7   b  for receiving laser beams  32   b  at a wavelength λb. The out-of-focus detection method employs a knife edge method or Foucault&#39;s method by quadrant beam and when they are wired by a conductive thin film  33  as shown in FIG. 10A, a differential signal can be obtained from a terminal A and terminal B of the wire bonding pad  34 . The conductive thin film  33  comprises a Ti/Pt/Au laminate or Al for instance.  
         [0086]    Reference numeral  8  denotes optical detector elements for obtaining track deviation detection signal and information reproducing signal. In the optical detector elements, output signals from four optical detector elements  8  are allowed to pass through amplifiers  35  formed on the semiconductor substrate and outputted from the terminal D, the terminal E, the terminal F and terminal G of the pad  34 . Reference numeral  9  denotes an optical detector element for monitoring the amount of light emitted from the semiconductor laser chips  4   a  and  4   b.  The output signal from the optical detector element  9  is outputted from the terminal C of the pad  34 . Points  31   a  and  31   b  show the reflection positions of the laser beams  6   a  and  6   b  emitted from the semiconductor laser chips  4   a  and  4   b  on the surface of the semiconductor mirror  5 . When the emission point distance D between the semiconductor laser chips  4   a  and  4   b  as the distance between the points  31   a  and  31   b  is defined substantially as: D≈fc×(λb-λa)/P, the light collection position for the laser beam at the wavelength λa and the light collection position for the laser beam at the wavelength λb are allowed to substantially coincide with each other. In this way, the optical detector elements and amplifiers can be used in common for the beams of different wavelength, which can economize the surface of the semiconductor substrate  1 , and additionally the number of wire bonding pads and output wires can be reduced. Accordingly, it has also an effect of reducing the size of the package for containing the semiconductor substrate  1 .  
         [0087]    [0087]FIG. 10B shows a cross sectional structure of the semiconductor substrate  1  taken along broken line A-A′ in FIG. 10A. The semiconductor mirror  5  is formed preferably at an angle of 45° relative to the laser chip attaching surface  2 . The fabrication is attained sufficiently by the so-called anisotropic etching. The anisotropic etching utilizes a phenomenon, for example, in the fabrication of a mirror face for the silicon substrate that when the silicon (100) face is etched with an aqueous potassium hydroxide solution, a concave of frustum of pyramid having a flat (111) face as a slope is formed since the etching rate for the (111) face is slower by about two digits relative to (100) face. In this case, the angle of (111) face relative to (100) face of the silicon crystal is about 54.7°. Accordingly, in order to form a semiconductor mirror at 45°, it is necessary to use a silicon substrate at an off angle of about 9.70° with the crystallographic axis being slanted relative to the surface. However, it is necessary that the off angle is determined also considering the adaptability of the semiconductor process for forming the optical detector element or electronic circuit, and the semiconductor mirror  5  is sometimes offset from 45° and the emitting direction of the laser beam  6   a  or  6   b  may sometimes be offset from the vertical direction of the semiconductor substrate  1 .  
         [0088]    Then, a method of mounting plural semiconductor lasers with high accuracy on a silicon conductor substrate is to be explained. FIGS. 11, 12 and  13  are views for explaining the mode of aligning optical elements to the integrating substrate.  
         [0089]    In a first example, positioning is conducted by attaching positioning index marks on both of a silicon substrate formed with an optical detector and a semiconductor laser, performing the irradiation of visible light or infrared light, focusing the images thereof on an opto-electronic conversion surface, for example, of CCD, which are input into a computer and calculating the center of gravity for each of the marks. In view of the calculation for the center of gravity, the positioning accuracy can be attained at the order of sub-micron. Further, plural semiconductors and monolithically integrated silicon can be hybridized and integrated with higher positional accuracy by index marks.  
         [0090]    [0090]FIG. 11 is a plan view showing a state in which index patterns  400  are attached to a silicon substrate  1 . Reference numeral  401  denotes a solder pattern, on which a semiconductor laser is solder bonded. An electrode pattern  402  is formed in connection with the electrode pattern  402 . On the other hand, FIG. 12 shows a solder pattern  501  and a positioning index pattern  502  formed on the rear face of corresponding semiconductor lasers  4   a  and  4   b.    
         [0091]    [0091]FIG. 13 explains a method of positioning an index pattern  400  on a substrate  102  and an index pattern  502  on a rear face of semiconductor lasers  4   a  and  4   b.  The outline of the method is as described below. That is, the substrate  1  and the semiconductor laser  4   a  (or  4   b ) are irradiated from the surface or the rear face with infrared rays  600 , the reflected light or transmission light is received by a microscope  601  and the index pattern attached there is enlarged and projected on a video monitor  602 . Then, the center position for each of the index patterns  400  and  502  is calculated by a computer  603  and the substrate  1  or the semiconductor laser is moved slightly till the positional displacement of two centers is reduced to zero. When the positioning is completed, they are applied with tact bonding and treated in a reflow furnace to complete solder bonding.  
         [0092]    The mode of forming the reflection mirror on the silicon substrate formed with the optical detector is highly practical. That is, this comprises providing an off substrate at about 9.7° and forming a reflection mirror at an angle of about 45° by anisotropic etching of silicon, for reflecting the beam from the semiconductor laser on the mirror and deflecting the beam substantially at an normal angle relative to the surface of the silicon substrate.  
         [0093]    [0093]FIGS. 14 and 15 show examples of adding a structure for heat dissipation or stress relaxation of an integrating substrate. FIG. 14 is a cross sectional view in a case of solder mounting semiconductor lasers  4   a  and  4   b  on a substrate  1  with a mirror. This is a cross section taken along line A-A′ in FIG. 10A. In this example, a mirror  5  is formed being integrated with a silicon substrate for instance. An electrode  700  and a positioning index pattern  502  are formed at the rear face of a semiconductor laser and they are soldered on the substrate  1  formed with an electrode  701  and solder  702 . Positioning for the semiconductor laser and the substrate are conducted between the index patterns  502  and  703 . Beams from the semiconductor lasers  4   a  and  4   b  are emitted from a light emission point  704 , reflected at the mirror  5  and then reaches a beam splitter, an objective lens and then an optical disk. A base  750  is formed on the substrate  102  so that the beam from the light emission point  704  is not interfered at the bottom of the substrate.  
         [0094]    Further, it is useful to monolithically form an amplifier for electrically amplifying a photocurrent generated from an optical detector on the silicon substrate formed with the optical detector and interpose a high thermal conductive material between the semiconductor laser and the silicon substrate with an aim of widely diffusing heat generated from the semiconductor laser upon soldering the semiconductor laser onto the silicon substrate attached with an oblique mirror and positioning index marks.  
         [0095]    Further, it is also practically useful to monolithically form an amplifier for electrically amplifying photocurrent generated by the optical detector on the silicon substrate formed with the optical detector and interpose a material having a stress relaxation effect between the semiconductor laser and the silicon substrate for relaxing the stresses caused by difference of heat expansion efficients between them upon soldering the semiconductor laser on the silicon substrate attached with the oblique mirror and the positioning index mark.  
         [0096]    [0096]FIG. 15 is an example of interposing a high thermal conductivity material  800  in a layerous manner just below the semiconductor laser in order to improve the heat dissipation. This dissipates the heat generated from the active layer of the semiconductor laser just therebelow, to conduct heat through a larger area thereby lowering the heat resistance as far as the heat sink. Further, the layer  800  shown in the figure can be provided with a function of relaxing stresses generated due to the difference of the heat expansion coefficients between the semiconductor laser and the semiconductor substrate.  
         [0097]    [0097]FIG. 16 is a plan view showing an example in which in a multi-wavelength module three semiconductor lasers, for example, of blue color, red color and infrared light are mounted in an arranged manner. Since the basic constitution is identical with that in FIG. 9, only the portion for the semiconductor laser is to be explained. Such semiconductor lasers are a blue/purple semiconductor laser  810  at a wavelength near 410 nm, a red laser  306  of a wavelength near 650 nm and an infrared laser  307  of a wavelength near 780 nm as viewed from right on the drawing. Optical detectors  304 ,  303  and  811  corresponding to them respectively are formed each by three sets for tracking. Thus, this example shows a case where one set is used for tracking and reproducing signals in common. The three kinds of wavelength correspond to recording and reproducing optical disks for super DVD, DVD and CD for which standardization has now been under progress.  
         [0098]    A mode of monolithically forming an amplifier for electrically amplifying a photocurrent generated from an optical detector on a silicon substrate formed with an optical detector and incorporating an oblique mirror and positioning index marks is also practical.  
         [0099]    [0099]FIG. 17 is a plan view of an integrated module according to another embodiment of this invention. This is an example of monolithically integrating a photo-receiving element and an amplifier to one identical substrate. That is, an amplifier  900  for amplifying a photocurrent from optical detectors  32   a  and  32   b  is monolithically formed on a silicon or GaN substrate  102 . Thus, the degree of integration can be improved by the decrease in the number of components. As such an example, it will be apparent that other OEIC (Optoelectric Integrated Circuit) can be mounted also optionally.  
         [0100]    According to this invention, face down mounting of a blue LD chip comprising a gallium nitride series compound semiconductor and having a step on the mounting surface can be realized. Accordingly, it is possible to provide an optical disk apparatus for recording and reproducing use in super DVD, DVD and CD by multi-wavelength integrated module having an optical source of multiple wavelengths including a blue LD.  
         [0101]    The mounting method according to this invention allows an optical device having a step disposed on a mounting surface to be favorably mounted on a desired mounting substrate.  
         [0102]    Further, the optical head device according to this invention is capable of decreasing the number of components, thereby reducing the size and thickness of the optical head device extremely usefully.  
         [0103]    References numerals are as follows:  110 ,  120 : optical device,  121 : substrate,  111 ,  122 : P-electrode of LD,  112 ,  123 : N-electrode of LD,  113 : P-wiring of substrate,  114 : N-side wiring of substrate,  124 ,  130 ,  130 ′: P-side solder of substrate,  125 ,  129 ,  129 ′: N-side solder of substrate,  126 : insulating film,  127 : P-side solder-underlying metal of substrate,  128 : N-side solder-underlying metal of substrate,  140 : resist,  1 : semiconductor substrate,  2 : semiconductor laser mounting surface ,  4   a ,  4   b : semiconductor laser,  5 : reflection mirror,  6   a, b : beam from semiconductor laser,  7 : optical detector ,  8 : optical detector,  9 : optical monitoring detector,  10 : collimator lens,  11 : upwarding mirror,  12 : composite element of diffraction grating and wavelength plate,  13 : objective lens,  14 : light disk,  15 ,  16 : light spot ,  17 : actuator,  18 : track,  22 : diffraction grating,  23 : diffraction grating,  24 : quadrant wavelength plate ,  1   a ,  1   b : spot on mirror,  32   a ,  2   b : auto-focusing detecting light spot,  33 -wiring,  34 : electrode pad,  35 : amplifier,  00 : package base,  201 : conduction pin,  203 : cap,  204 : window,  41 : case,  42 : lead frame,  43 : base,  44 : window,  45 : reflective film,  400 : index mark,  401 : solder pattern,  402 : electrode pattern,  01 : semiconductor laser electrode pattern,  502 : semiconductor laser index mark,  00 : infrared ray,  601 : infrared ray camera,  602 : monitor,  603 : computer,  704 : emission point of semiconductor laser,  705 : base,  800 : high heat conductive material or stress relaxing material,  810 : blue-purple semiconductor laser,  32 C: optical detector,  900 : OEIC substrate with amplifier