Method of producing optical integrated circuit

Abstract: There is provided a method of producing efficiently custom IC optical integrated circuits with convenient connections between the optical devices with less loss of light and with different connections but similar configuration. The optical integrated circuit comprises a plurality of ridge-type active optical devices formed of a variable wavelength laser section (1), a light modulator (2) and a ridge-type passive Y-branching optical waveguide (4) for connecting the ridge-type active optical devices with each other or with output waveguides. The ridge sections of the ridge-type active optical device and the ridge-type passive optical waveguide are formed by the selective crystal growth.

Description: The present invention relates to a method of producing an optical integrated circuit in which a plurality of waveguide-type optical devices are integrated on a substrate, and more particularly to a method of producing an optical integrated circuit capable of optical wiring between the waveguide-type optical devices even after formation of the waveguide-type optical devices with different connections but a similar configuration. In recent years, to cope with advent of the widespread computerisation in society, the research, development, and practical use of optical fiber communications for transmitting information with a larger capacity has rapidly been promoted. Conventionally, the use of optical communication was essentially for communicating from one point to another point, so called point-to-point communication. Accordingly, only a single photodiode or a transmitter has satisfactorily been used as an optical communication device. However, the following considerations will be significant problems in the near future, namely (1) in a photonic switching technique, the light signals are processed without conversion into electrical signals, (2) in a wavelength division multiplexing transmission, a plurality of rays emitted from a plurality of semiconductor laser sources are transmitted together through a single optical fiber, and (3) as in a coherent light transmission technique, complicated processing of signals is required at the receiver side. In such a system necessitating complicated processing, a method of combining each single unit of optical device into hybrid junction with each other may not be considered as a satisfactory method in viewpoint of cost and reliability. Thus, research and development have just started for so called an optical integrated circuit in which the optical devices to be required are monolithically integrated on a substrate and in particular for an optical integrated circuit in which a plurality of different optical devices are integrated on the same substrate. A method of optical wiring for connecting between the optical devices has not yet been proposed. Electronics Letters, vol 26, no 2, pages 142-143 discloses a method of forming an optical integrated circuit in which a buried ridge DFB layer is formed on a semiconductor substrate with integrated strip loaded passive waveguides. US-A-4 066 482 discloses a method of forming such waveguides by a selective epitaxial growth technique. The object of this invention is to provide a method of producing an optical integrated circuit which is integrated with a plurality of different waveguide type optical devices with easier connection between the optical devices and low loss and capable of effectively producing a custom IC optical integrated circuit, which is of general use in the Si LSI field providing different connections but the same configuration. Accordingly, the present invention provides a method of fabricating a custom optical integrated circuit, said method comprising the steps of : forming a structure including an optical guiding layer on a surface of a semiconductor substrate; forming an active layer over a portion of said optical guiding layer to form an active region of said structure; and forming on said structure both an array pattern of ridge-type cladding layers in said active region, thereby defining plurality of ridge-type active optical devices, and an optical interconnection pattern of ridge-type cladding layers in a passive region, thereby forming a ridge-type passive optical waveguide for connecting the ridge-type active optical devices with each other and/or with output waveguides, characterised in that at least said optical interconnection pattern is formed by a single selective growth process on windows of a custom-made single dielectric mask provided over said passive region, said optical interconnection patterns on said passive region being defined in a final fabricating step for facilitating a customization of said optical interconnection pattern. Thus in the method of producing an optical integrated circuit in accordance with the present invention, a ridge-type active optical device may be formed before a passive optical waveguide for optical wiring is formed by the growth of a ridge section with a selective growth technique. Such a ridge-type optical waveguide formed by the selective growth technique enables easier fine machining with far superior accuracy of dimension as compared with that of processing the optical waveguide by the wet chemical etching. The selectively grown ridge-type optical waveguide which is free of damage provides a high evenness of the side wall of the ridge section with less loss as compared with the processing of the light waveguide by dry etching. Further, according to the present invention, after a ridge-type active optical device is provided, a passive optical waveguide for optical wiring is formed by the selective growth. Thus, a mask for the active optical devices may be commonly utilized even for producing the custom IC optical integrated circuit with different connection but similar configuration. Therefore, by modifying the optical wiring mask only depending on its application, the custom IC optical integrated circuit may be produced with higher efficiency. Preferred embodiments of a method of producing an optical integrated circuit according to the present invention will hereinafter fully be described in detail with reference to the accompanying drawings. FIG. 1 is a perspective view of a first embodiment of a method of producing an optical integrated circuit according to the present invention, showing a configuration of an integrated light source for wavelength multiplexing; FIG. 2 is a pictorial view of the shows production processes (a) to (e) of the integrated light source for wavelength multiplexing of FIG. 1; FIG. 3 is a pictorial view of the production processes (a) to (d) continued to these of the integrated light source for wavelength multiplexing of FIG. 2; FIG. 4 is a pictorial view of the production processes (a) to (e) of forming a ridge-type optical waveguide by selective growth when the integrated light source for wavelength multiplexing of FIG. 1; FIG. 5 is a top view of a wafer showing a state before a ridge-type optical waveguide is formed by the selective growth with respect to an optical coupler unit of the integrated light source for wavelength multiplexing of FIG. 1; FIG. 6 is a top view of a wafer of an embodiment showing a ridge-type optical waveguide formed by the selective growth with respect to an optical coupler unit of the integrated light source for wavelength multiplexing of FIG. 1; FIG. 7 is a top view of a wafer of another embodiment showing a ridge-type optical waveguide formed by the selective growth with respect to an optical coupler unit of the integrated light source for wavelength multiplexing of FIG. 1; FIG. 8 is a top view of a wafer of further another embodiment showing a ridge-type optical waveguide formed by the selective growth with respect to an optical coupler unit of the integrated light source for wavelength multiplexing of FIG. 1; FIG. 9 is a perspective view of a second embodiment of a method of producing an optical integrated circuit according to the present invention, showing a configuration of an integrated light source for wavelength multiplexing; FIG. 10 is a pictorial view of the production processes (a) to (d) of the integrated light source for wavelength multiplexing of FIG. 9; FIG.11 is a pictorial view of the production processes (a) to (d) continued to those of the integrated light source for wavelength multiplexing of FIG. 10; and FIG. 12 is a top view of a wafer of an embodiment showing a ridge-type optical waveguide formed by the selective growth with respect to an optical coupler unit of the integrated light source for wavelength multiplexing of FIG. 9. In an integrated light source for wavelength multiplexing of FIG. 1, an InP substrate is integrated thereon with a variable wavelength distributed Bragg reflector (DBR) type laser section 1 formed of four ridge-type variable wavelength type DBR lasers, a light modulator unit 2 formed of four ridge-type Franz-Keldysh-type light modulators, and a ridge-type optical coupler unit 3 of four inputs and one output. The four DBR lasers are then allowed to oscillate in the continuous wave (cw) by a respective different optical wavelength. Each resultant cw-oscillated light may be modulated with a smaller spectrum chirping at a high speed by the light modulators on a tip of each DBR laser. The respective modulated light is wavelength-multiplexed by the optical coupler unit 3 of the four inputs and one output and is emitted from devices. FIG. 1 shows an example for a multiplex-wavelength light source emitted by multiplexing four wavelengths, notwithstanding the number of multiplexing may conveniently be increased. Accordingly, since the integrated multiplex-wavelength light source as shown in the present embodiment provides a high speed modulation with a smaller spectrum chirping, then it may be utilized for an optical communication light source of a very large capacity with a high density of wavelength multiplexing. In FIG. 1, the variable wavelength distributed Bragg reflector (DBR) laser section 1 is comprised of an active region 5 which employs an InGaAs/InGaAsP Multiple Quantum Well (MQW) as an active layer, a phase control region 6, and a DBR region 7 with a diffraction grating 104. In FIGS. 2 and 3, the diffraction grating 104 is formed only at the portion of the DBR region on an n + -InP substrate 101 (see FIG. 2(a)). Thereafter, by a metal organic vapor phase epitaxy (MOVPE) method, an entire substrate is continuously grown thereon with an n-InGaAsP guiding layer 102 (wavelength composition is 1.3 micro meters, thickness of the layer is 1,000 angstroms), an InGaAs/InGaAsP MQW active layer 103 and a p-InP cladding layer 106 (layer thickness is about 1,000 angstroms) (see FIG. 2(b)). A well layer of the MQW active layer 103 has thickness of about 70 angstroms, and a barrier layer has wavelength composition of 1.3 micro meters and thickness of about 150 angstroms. Then, with use of the mask of SiO 2 film 112, the portion other than the active region 5 of the DBR laser is selectively removed by the etching up to the InGaAs/InGaAsP MQW active layer 103 (see FIG. 2(c)). The removed portion thus obtained is epitaxially grown thereon with, similarly employing the mask of SiO 2 film 112, an i-InGaAsP light absorption layer 105 (wavelength composition is 1.04 micro meters, layer thickness is 2,000 angstroms), and a p-InP clad layer 106 (layer thickness is 5,000 angstroms) (see FIG. 2(d)). After removing the SiO 2 film 112, with use of the mask of a SiO 2 film 113, the portion other than the DBR laser section 1 is selectively removed by the etching until the n + -InP substrate is exposed (see FIG. 2(e)). With use of the SiO 2 film 113 as a mask, an i-InGaAsP guiding layer 109 (wavelength composition is 1.15 micro meters, layer thickness is 3,000 angstroms) and an i-InP cladding layer 110 (layer thickness is 5,000 angstroms) are allowed to be epitaxially grown, and the SiO 2 film 113 is removed (see FIG. 3(a)). By using the SiO 2 film 115 as a mask having windows only at the portion corresponding to four regions, namely the active region 5 of the variable wavelength DBR laser section 1, the phase control region 6, the DBR region 7 and the modulator region 8, a p-InP cladding layer 107 and a P + -InGaAs capping layer 108 are selectively grown only at such windows to form a ridge section of a ridge-type variable wavelength DBR laser (see FIG. 3(b)). The mask windows of the SiO 2 film 115 has width of about 3.5 micro meters, and for respective length of the windows, the active region 5 has 600 micro meters, the phase control region 6 has about 100 micro meters, and the DBR region 7 has about 150 micro meters. The p-InP cladding layer 107 and the p + -InGaAs capping layer 108 have thicknesses respectively of about 5000 angstroms and about 2000 angstroms. The SiO 2 film 115 is removed to form a mask of an SiO 2 film 114 having a window only at the portion corresponding to a Y-branching optical waveguide 4, and then by the selective growth of an i-InP cladding layer 111 only on the windows, the ridge section of the optical coupler unit 3 is formed (see FIG. 3(c)). The i-InP cladding layer 111 has thickness of about 7000 angstroms and the mask of the SiO 2 film 114 has width of about 3.5 micro meters. Finally, the SiO 2 film 114 to be used for the selective growth is removed to form a low reflection film at the end surface of the optical coupler unit 3, ending to produce the device (FIG. 3(d)). The method of producing the integrated light source for wavelength multiplexing has been described. The optical integrated circuits produced by the method in accordance with the invention are capable of connecting the respective variable wavelength DBR laser output light to the output waveguide with less loss, and also the custom IC optical integrated circuits with different connection but similar configuration may effectively be produced as hereinafter described more in detail. The integrated variable wavelength light source of FIG. 1 is produced by the selective growth of the ridge-type optical waveguide as mentioned using FIGS. 2 and 3. The method of producing the ridge-type optical waveguide by the selective growth is described in detail as follows. In FIG. 4, a wafer of the n + -InP substrate 101 sequentially stacked with the i-InGaAsP guiding layer 109 and the i-InP cladding layer 110 is formed thereon with a mask of an SiO 2 film 401 having a window 402 only at the portion to be formed with the ridge (see FIG. 4(a)). Thereafter, with use of the MOVPE method and the like the i-InP clad layer 111 is selectively grown only at the window 402 of the mask of the SiO 2 film 401, a ride section 403 is thus formed (see FIG. 4(b)). Then the SiO 2 film 401 is removed to form a ridge-type optical waveguide with the selective growth (see FIG. 4(c)). A side-wall 404 of the ridge-type optical waveguide formed with the selective growth produces a certain smooth crystal face (for example, in the growth on (100) face with the MOVPE method, the side-wall of the ridge is (111) B face) absolutely without scattering loss to be caused by roughness of the side-wall of the ridge. In the method of forming the ridge-type optical waveguide by the selective growth according to the present invention, since no damage occurs by the etching, the waveguide loss is further reduced as compared with the method of forming the ridge-type optical waveguide by the dry etching. According to an experimentation by the present inventors, waveguide loss of the optical waveguide formed by the method of the invention is as low as 0.2 dB/cm. However, the integrated light source for wavelength multiplexing in FIG. 1 constitutes an optical coupler unit 3 by the optical waveguide with a very lower loss as hereinbefore described. Thus, an output light of the variable wavelength DBR laser may be introduced to an output waveguide of the devices almost without any loss with except a loss at the light modulator 2, a coupling loss between the optical modulator 2 and the Y-branching optical waveguide 4 and a combining loss at the Y-branching optical waveguide 4. The invention may thus provide an integrated light source for wavelength multiplexing with a higher output than the conventional. The invention also provides a method of efficiently producing a custom IC optical integrated circuit with a different connection but similar configuration. FIG. 5 shows the upper surface of the wafer of the state before the ridge-type optical waveguide of the optical coupler unit 3 is formed by the selective growth or of the state after the SiO 2 film has been removed in the process of FIG. 3(b). Thus, the optical coupler unit 3 is formed thereon with a mask of the SiO 2 film having the windows only at the portion on which the Y-branching optical waveguide is formed and the the ridge-type optical waveguide of the optical coupler unit is formed by the selective growth to produce a desired integrated light source for wavelength multiplexing. In such an integrated light source for wavelength multiplexing, the number of wavelengths to be multiplexed is required to vary depending on customers. For example, a customer requires a number of light sources for multiplexing two wavelengths, another customer requires the multiplexing light source of four wavelength, and further another customer requires the multiplexing light source of eight wavelengths. To meet such requirements of the respective customers, the masks in response to the number of wavelengths to be multiplexed have to be prepared from the first time with the considerably uneconomical separate processes. As shown in FIGS 6, 7 and 8, however, the respective suitable multiplexing of wavelengths is achieved only by changing the optical wiring including a Y-branching optical waveguide 601, 701 and 801 of the optical coupler unit 3. This shows that multiplexing of two, four and eight wavelengths may conveniently be produced with an active optical device unit 501 of entirely the same configuration. Even in the customer's different requirement for the number of wavelengths to be multiplexed, according to the invention, a process up to forming of the ridge section of the optical coupler may completely be made into the commonly available form. The processes of FIGS 2 (a) to (e) and FIGS 3 (a) and (b) are irrespective of the number of multiplexing wavelengths of the customer's requirements, and those processes are possible to be made into the commonly available form. According to the invention, only the process of forming an optical waveguide pattern of the optical coupler may be changed by the later selective growth to meet the customer's requirement . In such a configuration, most of masks may commonly be available. On account of the common processes, the commonly produced wafers may always be stored to provide the respective optical wiring pattern of the optical coupler in accordance with the customer's orders. Thus, according to the invention, the customer's orders may conveniently and quickly be satisfied. In a second embodiment of FIGS. 9 to 12, a ridge-type optical waveguide of the optical coupler unit 3 is formed of the p-InP cladding layer 107 and p + -InGaAs capping layer 108 like the variable wavelength DBR laser section 1 and the light modulator unit 2 as compared with the first embodiment of FIG. 1. More specifically, in the first embodiment of FIG. 1, the ridge-type optical waveguide of the optical coupler unit 3 is formed of the i-InP cladding layer 111 with negligible free carrier absorption to be received by the light oozed on the cladding layer and with less loss of the light in an optical waveguide of the optical coupler 3. Since the variable wavelength DBR laser section 1, the ridge-type optical waveguide of the light modulator 2 and the optical coupler 3 are formed separately, a dimensional registration thereto with the ridge-type optical waveguide of the light modulator 2 is required to form the ridge-type optical waveguide of the optical coupler 3. Such registration accuracy entails variation of coupling loss between the optical modulator 2 and the coupler 3. In FIG. 9, since the variable wavelength DBR laser section 1, the ridge-type optical waveguide of the light modulator 2 and the optical coupler 3 are formed together by the selective growth at a time without such configuration, the optical modulator 2 and the optical coupler 3 are coupled always at less loss of the light. The processes of FIGS. 10(a) to 10(d) and of FIG. 11(a) and 11(b) are substantially the same as those of the first embodiment, so that the description thereabout was omitted. In FIG. 11(c) and 11(d), the embodiment is characterized by forming an active optical device and a passive optical device together without any dimension registration between respective optical waveguides. The optical integrated circuits according to the embodiment may far conveniently be produced as compared with the conventional method. In FIG. 12, the wafer of FIG. 11(b) is arranged thereon with a mask formed of SiO 2 film 116 to be used for the selective growth procedure of the ridge section. A ridge section of an active optical device 501 and another ridge section of a passive optical device 502 are of the same configuration so that they may be formed together. Accordingly, a mask to be used for selectively growing the active optical device 501 and another mask to be used for selectively growing the passive optical device 502 may be produced together by the mask forming procedure with the SiO 2 film at a time thereby to provide a window 901. Then, the opposite ridge sections described are formed by the selective growth, and a dimensional registration between the respective optical waveguides is not required in the embodiment. The custom IC optical integrated circuit with different connection but the same configuration may also be effectively produced by this embodiment. According to the embodiment, since a positional relationship between the respective active optical devices may be determined optionally when ridge-type optical waveguides are ultimately formed together, the embodiment is more suitable for producing the custom IC optical integrated circuit. It is to be understood that the present invention is not limited to the preferred embodiments as hereinbefore fully described. For example, configurations of the optical integrated circuits are not limited to the integrated light sources for wavelength multiplexing as specified in the preferred embodiments but may be of any configurations of the optical integrated circuits such as a waveguide-type matrix optical switch, a waveguide-type matrix optical switch integrated thereon with semiconductor laser, a light amplifier and a photo-detector, a coherent receiver integrated thereon with a semiconductor laser of a local oscillator, a waveguide-type 3dB coupler and a photo-detector, a node element to be used for the local area network which is integrated with a waveguide-type optical switch, a semiconductor laser of light sources and a photo-detector of a photo-receiver and the like. An InP system has been proposed in the embodiments, notwithstanding it is not limited to the InP system as a material system but a GaAs system may preferably be employed. For a crystal growth method, the MOVPE method essentially been described in the preferred embodiment, nevertheless the crystal growth method is not limited to the MOVPE method, but any of the growth method capable of employing the present invention may selectively be available. As hereinbefore fully described, according to the method of producing the optical integrated circuit of the invention, the substrate is integrated thereon with a plurality of different waveguide-type optical devices with a convenient connection therebetween and less loss of the light. Further, in the method of producing according to the invention, the custom IC optical integrated circuit with different connection but similar configuration may effectively be produced, bringing considerable effects in the fields of optical connection, photonic switching, optical information processing and the like.

Claims: A method of fabricating a custom optical integrated circuit, said method comprising the steps of : forming a structure including an optical guiding layer (102, 109) on a surface of a semiconductor substrate (101); forming an active layer (103) over a portion (102) of said optical guiding layer to form an active region of said structure; and forming on said structure both an array pattern (5, 6, 7, 8) of ridge-type cladding layers (107) in said active region, thereby defining a plurality of ridge-type active optical devices, and an optical interconnection pattern (4) of ridge-type cladding layers (111) in a passive region, thereby forming a ridge-type passive optical waveguide for connecting the ridge-type active optical devices with each other and/or with output waveguides characterised in that at least said optical interconnection pattern (4) is formed by a single selective growth process on windows (402; 901) of a custom-made single dielectric mask (114; 401; 116) provided over said passive region, said optical interconnection patterns on said passive region being defined in a final fabricating step for facilitating a customization of said optical interconnection pattern. The method as claimed in Claim 1, characterised in that, before forming said optical interconnection pattern, said array pattern (5, 6, 7, 8) is formed by a single selective growth process on windows of a custom-made single dielectric mask (115) provided over said active region, said array patterns on said active region being defined in a single fabricating step just before the final fabricating step for facilitating a customization of said active array pattern. The method as claimed in Claim 1, characterised in that both said array pattern (5, 6, 7, 8) and said optical interconnection pattern (4) are simultaneously formed, by a single selective growth process, on windows (901) of a custom-made single dielectric mask (116) provided over both said active and said passive regions, said patterns of said circuit being defined in said final fabricating step for facilitating a customization of said circuit. The method as claimed in Claim 1, characterized in that said optical interconnection pattern (4) comprises a plurality of Y-branching cladding layers (111) for wavelength-multiplexing a predetermined number of different light sources in said active region.

Inventors: KATO TOMOAKI, KOMATSU KEIRO, MITO IKUO, SASAKI TATSUYA, KATO, TOMOAKI, KOMATSU, KEIRO, MITO, IKUO, SASAKI, TATSUYA, KATO, TOMOAKI, C/O NEC CORPORATION, KOMATSU, KEIRO, C/O NEC CORPORATION, MITO, IKUO, C/O NEC CORPORATION, SASAKI, TATSUYA, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: G02B6
IPC codes: H01S5, G02B6, H01L27, G02F1