Abstract:
An optical assembly structure includes, a semiconductor element generating a large amount of heat and a high impedance optical element which are to be mounted, with low optical loss on the same semiconductor substrate which has an optical waveguide formed thereon.The element generating a large amount of heat is mounted on a terrace of the semiconductor substrate directly or through an insulating layer having a thickness of submicron order, while the high impedance element is mounted in a groove which is etched into the semiconductor substrate to such an extent that the optical axis of the high impedance element mates with the optical axis of the optical waveguide layer formed on a recess in the semiconductor substrate. The optical axes can be adjusted independently for the respective elements. Alternatively, with a potential on the semiconductor substrate set equal to a supply voltage or a ground potential, a crystal substrate for a forward biased heat generating element is given the polarity opposite to that of a crystal substrate for a reverse biased element which may imply a problem of stray capacitance.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a low cost optical module structure which is applied to optical access networks, optical exchange systems, optical interconnects, and so on. 
     There are several reports on hybrid integration of laser diodes and/or photodiodes on optical platforms. For example, H. Tabuchi, et al. reported &#34;alignment-free photodetector single-mode fiber coupling using a planarized Si platform&#34; (paper 15B1-3) at the fifth optoelectronics conference held at Chiba, Japan in 1994. At the same conference, Y. Yamada reported &#34;silica-on-terraced-silicon platform for optical hybrid integration&#34; (paper 15B1-3). 
     Further, there is a paper by Yamada et al entitled &#34;A Hybird Integrated Optical WDM Transmitter/Receiver Module for Optical Subscriber Systems Utilizing a Planar Lightwave Circuit Platform&#34;, pages PD12-1 to PD12-5, reported at the Optical-fiber Communication Conference in 1995 . 
     FIG. 1A is a top plan view illustrating the structure of a prior art optical assembly, and FIG. 1B is a cross-sectional view taken along a line I-I&#39; in FIG. 1A. The illustrated structure includes a semiconductor laser 2 and waveguide-type photodiodes 3, each formed on an n-type substrate, which are mounted on a silicon substrate 10 having a silica waveguide 1 formed on the upper surface thereof. The optical waveguide 1 includes an under cladding layer 12 made of silica embedded in a recess ( portion) of the silicon substrate 10, an adjusting layer 13 made of silica formed on the under cladding layer 12, an optical waveguide core 14 having a refractive index larger than that of the under cladding layer 12, and an upper cladding layer 15 made of silica. The semiconductor laser 2 and the waveguide-type photodiodes 3 are fixed opposite to a common port for optical input and output of the optical waveguide 1 on electrodes 720, 730 on a place for mounting 5 formed by selectively etching glass on a terrace ( portion) 11 of the silicon substrate 10. 
     Since the optical waveguide 1 and the optical elements 2, 3 are required to be optically connected to each other with low loss, techniques such as a fine positioning control using index marks, self-alignment utilizing surface tension of solder bump, and so on have been proposed as low cost mounting methods in order to adjust the optical axes between the optical waveguide core 14 and the mounted elements in the plane of Si bench. In addition, several designs for reducing the influence of misalignment have been included in the optical assembly, for example, a mounted optical element is selected such that its spot size is close to that of a silica waveguide. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical assembly structure which is capable of mounting an element generating a large amount of heat and a high impedance optical element on the same optical waveguide substrate with the optical axes thereof mated with the optical axis of an optical waveguide at a low cost. 
     To achieve the above object, an element generating a large amount of heat is fixed on a terrace of a semiconductor substrate directly or through an insulating layer having a thickness of submicron order, so that the element generating a large amount of heat can be mounted with a low heat resistance. In the case of laser diodes, the optical axes of the laser and the silica optical waveguide should be adjusted for the best optical coupling. This adjustment can be attained physically by setting the waveguide axis height measured from the silicon terrace surface to the sum of the active layer depth of the laser diode and the thicknesses of an electrode and solder. When the high impedance optical element is mounted in a groove which is etched into the semiconductor substrate to such an extent that the optical axis of the high impedance optical element mates with the optical axis of the optical waveguide formed on a recess of the semiconductor substrate, the high impedance optical element can be mounted substantially free from any electrical influence of the conductive silicon substrate. 
     As another approach, it is also possible to equivalently reduce a stray capacitance of the high impedance optical element, including a stray capacitance caused by a connection with electric circuits. Specifically, the semiconductor substrate is set at the same potential as a supply voltage or a grounding potential, and signal lines of two or more mounted elements are kept away from the semiconductor substrate. This structure can be realized by mounting a forward biased heat generating element and a reverse biased high impedance optical element, causing a problem of a stray capacitance, with their polarities opposite to each other. 
     With the foregoing approaches, the element generating a large amount of heat and the high impedance element can be mounted on the same optical waveguide substrate with their optical axes mated with the optical axis of the waveguide at a low cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B are a top plan view and a cross-sectional view respectively illustrating the structure of a prior art optical assembly; 
     FIGS. 2A, 2B are a top plan view and a cross-sectional view respectively illustrating a structure associated with a first and a fourth embodiments of the present invention; 
     FIG. 3 is a graph showing an optical output characteristic associated with the first embodiment of the present invention; 
     FIGS. 4A, 4B are a top plan view and a cross-sectional view respectively illustrating a structure associated with a second embodiment of the present invention; 
     FIG. 5 is a top plan view illustrating an example in which the present invention is applied to a second transmission board; 
     FIGS. 6A, 6B are a top plan view and a cross-sectional view respectively illustrating a structure associated with a third embodiment of the present invention; and 
     FIGS. 7A, 7B are a top plan view and a cross-sectional view respectively illustrating a structure associated with a fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 2A is a top plan view illustrating the structure of a first embodiment of the present invention, and FIG. 2B is a cross-sectional view taken along a line II-II&#39; in FIG. 2A. On a silicon substrate 10 having a terrace 11 on the upper surface thereof, an under cladding layer 12 made of silica is formed in a thickness sufficiently larger than the height of the terrace 11 (20-30 m) using an electron beam deposition. Here, a silicon having a crystal orientation of (100) with an offset less than 5 degrees is used such that the terrace 11 has a symmetric shape. Thereafter, the under cladding layer 12 is polished until the upper surface of the terrace of the silicon substrate 10 is completely exposed to planarize the terrace 11 of the silicon substrate 10 and the under cladding layer 12. On the planarized upper surfaces of the terrace 11 and the under cladding layer 12, silica containing titanium, germanium, and so on is deposited by electron beam deposition and patterned to form an optical waveguide core 14. Next, silica free from additives is deposited by electron beam deposition on the optical waveguide core 14 to form an upper cladding layer 15. Then, the upper cladding layer 15 and the optical waveguide core 14 are partially dry-etched using fluorine-based gas until the upper surface of the terrace 11 of the silicon substrate 10 is exposed. Since the etching is stopped when the surface of the silicon substrate 10 is exposed, excessive etching would not cause any problem. Rather, the dry etching may be stopped at the time the depth of a recess serving as a second place for mounting 53 reaches a point at which the optical axis 31 of a waveguide type photodiode 3, to be mounted in the recess, mates with the optical axis of the optical waveguide core 14. Since the etching rate can be controlled at approximately 0.1 -1 μm per minute, the depth of the recess can be controlled with good reproducibility by controlling etching time. Since the length from the upper surface of the second place for mounting 53 to the silicon substrate 10 is 10 micron or more, an increase in stray capacitance in the waveguide type photodiode 3 through the silicon substrate 10 is 0.02 pF or less. 
     Next, since the optical axis of the optical waveguide core 14 in a first place for mounting 52 does not mate with the optical axis 21 of a semiconductor laser 2, the silicon substrate 10 is selectively etched to adjust only the height of the first place for mounting 52 without affecting the second place for mounting 53. In this way, the optical axes of the two places for mounting 52, 53 can be adjusted independently of each other. Next, electrode layers made of titanium, platinum, and gold are formed on the respective places for mounting 52, 53 by electron beam deposition and patterned in compliance with the shapes of electrodes to form electrodes 720, 721, 722, 730, 731, 732, thus fabricating an optical waveguide substrate 100. In the first embodiment, the optical waveguide 1 has a Y-shape in the top plan view, where one terminal is utilized as a common port for optical input and output and two branched terminals are utilized as a port for connecting to the semiconductor laser 2 on the first place for mounting 52 and a port for connecting to the waveguide type photodiode 3 on the second place for mounting 53, respectively. Index patterns for mounting the elements are made simultaneously with the formation of the electrode pattern. In addition, a crystal substrate 20 of the semiconductor laser 2 and a crystal substrate 30 of the waveguide type photodiode 3, used for the mounting, are both made of n-type InP crystal. 
     Next, the semiconductor laser 2 is mounted on the electrode 720 through an AuSn thin-film solder 82 (having a thickness ranging from 1 to 6 μm) by detecting index patterns using transmission infrared light illuminated from the backside of the silicon platform. An index pattern stamped on the semiconductor laser 2 and an index pattern stamped on the upper surface of the silicon substrate forming the first place for mounting 52 are simultaneously transmitted by infrared light, observed by an infrared TV camera to detect relative misalignment between the patterns, and aligned. After the alignment, the solder 82 is melted by heating to thereby fix the semiconductor laser 2 thereon. Then, 64 a similar method, the waveguide type photodiode 3 is fixed on the second place for mounting 53 using solder 83. The electrode 720 is connected to the electrode 721 by an Au wire 9 while the semiconductor laser 2 is connected to the electrode 722 by an Au wire 9 to establish electric connections of the semiconductor laser 2. Similarly, the electrode 730 is connected to the electrode 731 by an Au wire 9 while the waveguide type photodiode 3 is connected to the electrode 732 by n Au wire 9 to establish electric connections of the waveguide type photodiode 3. 
     After the trial manufacturing of the optical assembly, a fiber is connected to the common port for optical input and output of the optical assembly to evaluate the optical output characteristic and sensitivity characteristic of the optical assembly. FIG. 3 shows the optical output characteristics of three optical assemblies. When the semiconductor laser 2 is driven, a fiber optical output of 1 mW is generated with an operating current at 50 mA. It can be confirmed that a positioning error after mounting the semiconductor laser 2 can be controlled within 1 μm by comparing the optical output characteristic of the semiconductor laser 2 before being mounted. Also, for a light receiving sensitivity, a sufficient sensitivity characteristic of 0.31 A/W is obtained. A positioning error of the waveguide type photodiode 3 is also within 1 μm. In addition, from the result that the thermal resistance of the semiconductor laser is below 40° C./W, it is confirmed that a sufficient heat releasing characteristic can be ensured. The capacitance of the photodiode is not more than 1 pF, including a capacitance due to the mounting thereof. 
     Embodiment 2 
     FIG. 4A is a top plan view illustrating a second embodiment of the present invention, and FIG. 4B is a cross-sectional view taken along a line IV-IV&#39; in FIG. 4A. An under cladding layer 12 made of silica is formed on a silicon substrate 10 having terraces 11 on the upper surface thereof in a thickness sufficiently larger than the height of the terrace 11 (20-30 μm) using electron beam deposition. Here, a silicon having a crystal orientation of (100) with an offset less than 5 degrees is used such that each of the terraces 11 has a symmetric shape. Thereafter, the under cladding layer 12 is polished until the upper surface of the silicon substrate 10 is completely exposed to planarize the terraces 11 of the silicon substrate 10 and the under cladding layer 12. An adjusting layer 13 made of silica is deposited on the planarized upper surfaces of the terraces 11 and the under cladding layer 12, and then silica containing titanium, germanium, and so on is deposited by electron beam deposition and patterned to form an optical waveguide core 14. Next, silica free from additives is deposited by electron beam deposition on the optical waveguide core 14 to form an upper cladding layer 15. Thereafter, the upper cladding layer 15 and the optical waveguide core 14 are partially dry-etched using fluorine-based gas until the upper surfaces of the terraces 11 of the silicon substrate 10 are exposed. Since the etching is stopped when the surface of the silicon substrate 10 is exposed, excessive etching would not cause any problem. The adjusting layer 13 is formed to mate the optical axis of the optical waveguide core 14 with the respective optical axes 21, 31 of a semiconductor laser 2 and a waveguide type photodiode 3. 
     Next, after silicon oxide films 16 having a thickness equal to or less than 0.5 micron are formed on places for mounting 52, 53, respectively, electrode layers made of titanium, platinum, and gold are formed by electron beam deposition and patterned in compliance with electrode shapes to fabricate an optical waveguide substrate. Index patterns for mounting elements are stamped simultaneously with the formation of the electrode pattern. Also in the second embodiment, the optical waveguide 1 has a Y-shape in the top plan view, where one terminal is utilized as a common port for optical input and output and two branched terminals are utilized as a port for connecting to the semiconductor laser 2 on the first place for mounting 52 and a port for connecting the waveguide type photodiode 3 on the second place for mounting 53, respectively. 
     Next, the semiconductor laser 2 is mounted on the electrode 720 through an AuSn thin-film solder 82 (having a thickness ranging from 1 to 6 μm) by detecting index marks using transmission infrared light illuminated from the backside of the silicon platform. An index pattern stamped on the semiconductor laser 2 and an index pattern stamped on the upper surface of the silicon substrate forming the first place for mounting 52 are simultaneously transmitted by infrared light, observed by an infrared TV camera to detect relative misalignment between the patterns, and aligned. After the alignment, the solder 82 is melted by heating to fix the semiconductor laser 2 thereon. Then, by a similar method, the waveguide type photodiode 3 is fixed on the second place for mounting 53 using solder 83. After mounting the elements, a lid 61 made of glass is placed overlying grooves, and fixed by a resin 62. The use of the lid protects the elements from damage during a fiber connection process and enables stable trial manufacturing of the optical assemblies. 
     In the second embodiment, unlike the first embodiment, an n-type InP crystal is used for the crystal substrate 20 of semiconductor laser 2, while p-type InP crystal is used for crystal substrate 30 of the waveguide type photodiode 3. In this way, the silicon substrate 10 is set at the same potential as a power supply, and signal lines for the waveguide type photodiode 3 can be placed on the crystal substrate side, whereby an increase in stray capacitance can be reduced to 0.3 pF or less. 
     As seen in FIG. 5, after, the trial manufacturing of the optical assembly 100, a fiber 104 is connected to the common port for optical input and output of the optical assembly 100 using a glass block 103 to evaluate the optical output characteristic and sensitivity characteristic. As a result, similar characteristics to those of the first embodiment were obtained. Then, the optical assembly 100 is mounted on a printed circuit board 101 together with a laser driving IC 105 and a pre-amplifier IC 106. FIG. 5 illustrates how the optical assembly is mounted on the printed circuit board 101. Two sets of such trial boards are prepared and placed opposite to each other to evaluate the transmission characteristic. With a transmission rate at 30 Mb/s and a transmission length extending 5 km, it is confirmed that the transmission can be performed with a bit error rate (BER) equal to or less than 10 to the minus ninth power(10- 9 ). 
     Embodiment 3 
     FIG. 6A is a top plan view illustrating a third embodiment of the present invention, and FIG. 6B is a cross-sectional view taken along a line VI-VI&#39; in FIG. 6A. Instead of using the lid 61 for covering the mounted elements as in the second embodiment, the third embodiment fills an epoxy resin 63 in grooves for protecting elements 2, 3. The filled epoxy resin 63 substantially completely prevents the characteristics of the elements from deteriorating. Eleven sets of modules each having the optical assembly of the third embodiment mounted thereon are left in a test chamber at a temperature of 85° C. and a relative humidity of 90% RH to evaluate deteriorations in the optical output characteristic and optical sensitivity characteristic. After a test over 2000 hours, none of the 11 modules exhibit deteriorated characteristics. On the other hand, when the same test is conducted on an optical module without the filled resin for protection, the results showed that after 200-500 hours, a dark current in the photodiode 3 abruptly increases, thus confirming the validity of the filled resin of the third embodiment. 
     Embodiment 4 
     In a fourth embodiment of the present invention, the material for the optical waveguide is changed from silica used in the third embodiment to polyimide. Fluorinated polyimide is used for cladding layers 12, 13, 15, and an optical waveguide is formed such that a refractive index difference with a core is limited to 1%. Since an organic material is used for the optical waveguide 1, the film formation can be completed in a shorter time by spin coating, thus facilitating a further reduction in cost. An epoxy resin is used to fill grooves, as is the case of the third embodiment. 
     Embodiment 5 
     FIG. 7A is a top plan view illustrating a fifth embodiment of the present invention, and FIG. 7B is a cross-sectional view taken along a line VII-VII&#39; in FIG. 7A. In the fifth embodiment, a waveguide type photodiode array 3 is optically connected to a waveguide array 110. A trial manufacturing process of an optical waveguide substrate used herein for mounting elements is similar to that of the first embodiment. In the fifth embodiment, a pre-amplifier IC array 4 having ten channels is mounted on a place for mounting 54 formed on a terrace 11 of a silicon substrate 10, and a waveguide type photodiode array 3 having ten channels is mounted on a place for mounting 52 formed by etching into a glass surface of a recess in the silicon substrate 10. The pre-amplifier IC array 4 is made on a trial basis using an Si bipolar process since the pre-amplifier IC array may operate at high speeds up to approximately 1 Gb/s. The waveguide type photodiode array 3 is connected to an optical waveguide core 14 having an array of ten channels. 
     Ten channels of fibers are connected to the optical assembly of the fifth embodiment to measure a light receiving sensitivity. As a result, the optical assembly exhibits a light receiving characteristic with small variations between channels at 0.85±0.02 A/ W. Also, the optical assembly is evaluated in terms of a receiver sensitivity at a rate of 200 bits/second for each channel over a transmission length extending 100 m. On each channel, error free operation continues for fifty hours or more. Since the application of the waveguide type photodiode facilitates the mating of an electric wire face with a light incident face, a simple module can be realized. 
     In the present invention, each of the heights of the optical axes of the semiconductor light receiving element 3 and semiconductor light emitting element 2 is adjusted to the height of the optical axis of the optical waveguide 1 with an offset equal to or less than 3 microns. 
     According to the present invention, an element generating a large amount of heat and a high impedance optical element can be mounted on the same optical waveguide substrate with the optical axes thereof mated with the optical axis of an optical waveguide. In this way, a hybrid optical circuit having a variety of optical elements and electric elements mounted on a waveguide can be mounted at a low cost. Thus, large capacity communications supporting multimedia environments can be accomplished by laying optical fibers to respective houses. In addition, since the optical assembly of the present invention allows for the introduction of optical fibers for wiring between apparatuses, larger scale parallel processing can be readily realized. 
     Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the claims.