Semiconductor substrate with an opto electronic element and an optical waveguide

Abstract: An optical assembly structure is disclosed, wherein a semiconductor element (2) generating a large amount of heat and a high impedance optical element (3) can be mounted with low optical loss on the same semiconductor substrate (10) having an optical waveguide (1) formed thereon.The element (2) generating a large amount of heat is mounted on a terrace (11) of the semiconductor substrate (10) directly or through an insulating layer having a thickness of submicron order, while the high impedance element (3) is mounted in a groove which is etched into the semiconductor substrate (10) to such an extent that the optical axis of the high impedance element (3) mates with the optical axis of the optical waveguide layer (1) formed on a recess in the semiconductor substrate (10). The optical axes can be adjusted independently for the respective elements (2, 3). Alternatively, with a potential on the semiconductor substrate (10) set equal to a supply voltage or a ground potential, a crystal substrate (20) for a forwardly biased heat generating element (2) is given the polarity opposite to that of a crystal substrate (30) for a reversely biased element (3) 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 "alignment-free photodetector single-mode fiber coupling using a planarized Si platform" (paper 15B1-3) at the fifth optoelectronics conference held at Chiba, Japan in 1994. At the same conference, Y. Yamada reported "silica-on-terraced-silicon platform for optical hybrid integration" (paper 15B1-3). Further, there are papers PD12-2 by Yamada at 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' 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. An optical assembly in accordance with the invention is defined in claim 1. To achieve the above object in a preferred embodiment, 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 a 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 is aligned 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. In additon, it is also possible to equivalently reduce a stray capacitance of the high impedance optical element, including a stray capacitance caused by the manner of connection with associated 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 forwardly biased heat generating element and a reversely 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 illustrating the structure of a prior art optical assembly; Figs. 2A, 2B are a top plan view and a cross-sectional view illustrating a structure associated with a first embodiment 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 illustrating a structure not in accordance with 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 illustrating a structure not in accordance with 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' 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 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 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 as a port for connecting to the semiconductor laser 2 on the place for mounting 52 and a port for connecting to the waveguide type photodiode 3 on the 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 fix the semiconductor laser 2 thereon. Then, with a similar method, the waveguide type photodiode 3 is fixed on the place for mounting 53. 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 an Au wire 9 to establish electric connections of the waveguide type photodiode 3. After the trial manufacturing of the optical assembly, a fiber was 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 was driven, a fiber optical output of 1 mW was 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 mounted. Also, for a light receiving sensitivity, a sufficient sensitivity characteristic of 0.31 A/W was obtained. A positioning error of the waveguide type photodiode 3 was also within 1 µm. In addition, from the result that the thermal resistance of the semiconductor laser was below 40°C/W, it is confirmed that a sufficient heat releasing characteristic can be ensured. The capacitance of the photodiode was not more than 1 pF, including a capacitance due to the mounting thereof. Fig. 4A is a top plan view illustrating an assembly and Fig. 4B is a cross-sectional view taken along a line IV-IV' 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 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 as a port for connecting to the semiconductor laser 2 on the place for mounting 52 and a port for connecting the waveguide type photodiode 3 on the 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, with a similar method, the waveguide type photodiode 3 is fixed on the place for mounting 53. 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 damages during a fiber connection process and enables stable trial manufacturing of the optical assemblies. Unlike the assembly of Figure 2 embodiment, an n-type InP crystal is used for the semiconductor laser 2, while p-type InP crystal is used for 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. After the trial manufacturing of the optical assembly 100, a fiber 104 was 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 was 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 were 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. Fig. 6A is a top plan view illustrating a further assembly, and Fig. 6B is a cross-sectional view taken along a line VI-VI' in Fig. 6A. Instead of using the lid 61 for covering the mounted elements 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 were 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, any of the 11 modules did not exhibit deteriorated characteristics. On the other hand, when the same test was conducted on an optical module without filled resin for protection, the results showed that after 200 - 500 hours, a dark current in the photodiode 3 abruptly increased, thus confirming the validity of the filled resin. In the assembly of Figure 6, the material for the optical waveguide is changed from silica used in the assembly of Figure 4 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. In the present invention, each of the hights 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 aligned 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.

Claims: An optical assembly comprising: a semiconductor substrate (10) having a base portion and a terrace portion (11) projecting from the base portion, first and second dielectric layers (12, 15) formed over said substrate, an optical waveguide having a core (14) surrounded by and having a larger refractive index than said first and second dielectric layers (12, 15), a semiconductor light emitting element (2) mounted on or above said terrace portion (11), and a semiconductor light receiving element (3), wherein said optical waveguide (1) is Y-shaped with one branch having its optical axis aligned to the optical axis of the semiconductor light emitting element (2) and another branch having its optical axis aligned to the optical axis of the semiconductor light receiving element (3) for transmission and reception of an optical signal by said optical assembly, characterised in that said semiconductor light receiving element (3) is mounted above said base portion over a portion of said first dielectric layer (12). An optical assembly according to claim 1, wherein said optical waveguide (1) is formed on said base portion; and said semiconductor light receiving element (3) is mounted in a recess in said dielectric layer (12). An optical assembly according to claim 1, including means arranged to apply said semiconductor light receiving element (3) with a predetermined voltage such that said semiconductor light receiving element (3) is reversely biased. An optical assembly according to claim 1, wherein said semiconductor light emitting element (2) is mounted so as to be biased in a forward direction with respect to said semiconductor substrate (10), and said semiconductor light receiving element (3) is mounted to be biased in a reverse direction with respect to said semiconductor substrate (10). An optical assembly according to claim 1, wherein an insulating film is formed on or above said terrace (11). An optical assembly according to claim 1, wherein said semiconductor light receiving element (3) has an optical waveguide structure, and the height of the optical axis of said semiconductor light receiving element (3) is adjusted to the height of the optical axis of said optical waveguide (1) with an offset equal to or less than 3 microns. An optical assembly according to claim 1, wherein a material for said semiconductor substrate (10) is silicon having a crystal orientation of (100) with an offset of less than 5 degrees. An optical assembly according to claim 1, wherein a dielectric material constituting said optical waveguide (1) contains silicon oxide as a main component. An optical assembly according to claim 1, wherein said semiconductor light emitting element (2) has an optical waveguide structure, and the height of the optical axis of said semiconductor light emitting element (2) is adjusted to the height of the optical axis of said optical waveguide (1) with an offset equal to or less than 3 microns. An optical assembly according to claim 1, wherein said optical waveguide (1) connected to said semiconductor light emitting element (2) is optically coupled to said optical waveguide (1) connected to said semiconductor light receiving element (3). An optical assembly according to claim 2, wherein said recess in which said semiconductor light receiving element (3) is mounted is covered by a lid. An optical assembly according to claim 2, wherein the interior of said recess in which said semiconductor light receiving element (3) is mounted is filled with a resin to bury an optical path. An optical transmission module using the optical assembly according to claim 1. An optical assembly according to claim 1, wherein material (12, 83, 730) between said semiconductor light receiving element (3) and said semiconductor substrate (10) has a larger thickness than material (82, 720) between said semiconductor light emitting element (2) and said semiconductor substrate (10). An optical assembly according to claim 2, wherein said recess has been formed by partially etching said first dielectric layer (12). An optical assembly according to claim 1, wherein said semiconductor light emitting element includes a semiconductor laser (2) and said semiconductor light receiving element includes a photodiode (3).

Inventors: AOKI SATOSHI, KIKUCHI SATORU, SHISHIKURA MASATO, TAKAHASHI RYUTA, TSUJI SHINJI, AOKI, SATOSHI, KIKUCHI, SATORU, SHISHIKURA, MASATO, TAKAHASHI, RYUTA, TSUJI, SHINJI
Applicants: HITACHI CABLE, HITACHI LTD, HITACHI TOBU SEMICONDUCTOR LTD, NIPPON TELEGRAPH & TELEPHONE, HITACHI CABLE, LTD., HITACHI TOHBU SEMICONDUCTOR,LTD., HITACHI, LTD., NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1996
Main classification: G02B6
IPC codes: G02B6, H01S5, H04B10, H01L27, H01L31