Patent Application
20120183009
The present application claims priority from Japanese patent application JP 2011-006630 filed on Jan. 17, 2011, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a semiconductor laser device used in optical communications, and relates to, for example, a horizontal cavity surface emitting laser and a manufacturing method thereof, and a photodiode.
2. Description of the Related Arts
With the development of information technologies, the transmission of data using optical fiber communications has been rapidly developed. An optical fiber communication technology has heretofore mainly been used in long-distance high-speed data transmission typified by a land trunk line or a submarine optical communication, an access communication network typified by FTTH (Fiber To The Home), and a metro network that connects trunk lines and an access communication network. With an increase in the transmission capacity with the proliferation of Internet, the optical fiber communications have recently been spreading even to a storage network (SAN), Ethernet (Registered Trademark) (LAN) that connects among in-bureau high-speed network devices, etc. A communication protocol of a 100-Gbit Ethernet for transmitting 100 Gbits has been established on June, 2010 as an example of a next-generation optical LAN.
In recent years, per-device throughput of a high-end router used in a trunk line network has reached 1Tbps. A further capacity expansion has been expected form this time forward. With this capacity expansion, optical fiberization of wiring has been prospected to efficiently process large capacity data even at data transmission of an extremely short distance as in the case of between these transmission devices (a few m to a few hundred of m) or within a device (a few cm to a few tens of cm). Thus, while an increase in the capacity of a system has been advanced using light, a low cost technology will increasingly be an important challenge to provide data transmission using these at lower cost.
Under such a background, an improvement in high-speed performance and simple/high-density mountability becomes an important issue for a semiconductor optical element that performs transmission/reception of signals. This reason will be explained as below. While the speeding-up/increase in capacity of the system is progressing, the speeding up of a single optical device is facing a physical limit. Therefore, there arises a need to perform the transmission that has heretofore used a 1-channel signal, using plural channels. For example, it is assumed that such a configuration that 40 channels are used at 25 Gbs per channel and 1Tbps is transmitted, and the like are taken as throughput per board in a next-generation 10Tbps-class router. Thus, the high-speed semiconductor optical device excellent in high density and simple mountability, becomes one key device at a large-capacity system in the future. As a strong candidate of such an optical device, may be mentioned an array type device in which plural channels are monolithically integrated. The array device is advantageous in terms of a mounting area and the number of man-hour for work as compared with the case in which a device of a single channel is independently mounted. Since the interval between adjacent elements in the array device can be controlled with the accuracy of a semiconductor process, narrow pitching that exceeds a limit where the single channel is independently mounted, is made possible. As an index of a pitch interval, for example, the core interval between adjacent fibers is 250 μm in a commercialized ribbon fiber. In an organic substance polymer waveguide that has actively been developed in recent years, the core interval between respective channels is made possible with less than or equal to 120 μm or so. Since the narrow pitching is equivalent to a reduction in the device width, the utilization efficiency per wafer unit area can be increased. It can therefore be said that this is advantageous even in terms of device mass productivity.
Semiconductor laser devices each corresponding to this signal transmission light source are classified into three types according to how to combine their cavity directions (vertical resonance, horizontal resonance) and emitting surfaces of laser light (end face emission, plane or surface emission). The first type is of a horizontal cavity end face emitting laser device, the second type is of a vertical cavity surface emitting laser device, and the third type is of a horizontal cavity surface emitting laser device.
The first horizontal cavity end face emitting laser is formed with an optical waveguide in a substrate in-plane horizontal direction and emits laser light from end faces divided by cleavage of a substrate. Since the cavity length can be taken long up to a few hundred of μm in this laser structure, a high output of a few tens of mW is obtained even under a high temperature. It is however necessary to install an optical member for receiving the laser light adjacent to the laser device within the surface of a printed circuit board. This is not adequate to multichannel high-density mounting or miniaturization of the entire module.
Next, the second vertical cavity surface emitting laser is of a laser having a structure in which a resonator cavity is formed in the direction normal to a semiconductor substrate. Therefore, the layout of placing a photodetecting member on the device's top surface is possible. This is advantageous in higher densification within the surface of a printed circuit board. The present structure has, however, a problem that the cavity length is very long since it is determined by a crystal growth thickness, and it is essentially difficult to obtain a high optical output.
It can be said that the third horizontal cavity vertical surface emitting laser is of a laser structure which combines excellent points of the two lasers. In the present structure, a resonator cavity is formed in a substrate in-plane horizontal direction. In addition to this, the present structure has a structure in which a reflection mirror tilted to 45° is integratedly formed to emit laser light from the surface of the substrate or its back surface.
The present invention relates to the third horizontal cavity surface emitting laser. As an example of such a conventional horizontal cavity surface emitting laser, a horizontal cavity surface emitting laser having an active region of 10 through 100 μm, a distributed bragg reflector and a tilt mirror has been disclosed in Japanese Patent Application Laid-Open No. 2007-5594. Japanese Patent Application Laid-Open No. 2007-5594 has also disclosed an example in which a lens is integrated onto a light emitting surface.
Further, a configuration of a small module of a horizontal cavity surface emitting laser having an integrated lens has been disclosed in Japanese Patent Application Laid-Open No. 2010-147197. In the present known example, the shapes of the lens and mirror portion are contrived to make it possible to three-dimensionally place a monitor PD and a laser. A compact module is therefore possible. As a third known example, the normal-temperature continuous oscillation characteristics of a horizontal cavity surface emitting laser equipped with an optical waveguide including an InGaAsP active layer formed on an InP substrate, a reflector formed with an angle of 45° at the end of the optical waveguide, and a circular lens formed at a position opposite to the 45° reflector on the back surface of the InP substrate have been reported to IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 9, 1991, pp. 776-778.
As disclosed in these documents, in the horizontal cavity surface emitting laser in which light is emitted from the substrate surface in particular, the lens can be monolithically integrated on its corresponding light emitting surface with relative ease. Integrating the lens and the device as one makes it possible to reduce the number of parts like an external lens and members for supporting it. It is also possible to reduce the areas for placing these members in addition to the above. It can thus be said that an optical system for connecting an optical device and a photodetecting part can be brought into less size and hence this is a structure suitable for high-density packaging. This laser is advantageous in terms of alignment accuracy since the lens and the light emitting position can be controlled with the accuracy of a semiconductor process.
Further, a simple and small module configuration is made possible by setting such a lens-integration type horizontal cavity surface emitting laser to the array type, thus enabling higher densification. It can thus be said that the narrow pitch array type horizontal cavity surface emitting laser having the integrated lens is of a device suitable for the next-generation optical communications in terms of both transmission capacity and cost.
Any of the above Patent Documents does not however disclose a description about arraying of a lens integration type horizontal cavity surface emitting laser.
As an example of a lens integration structure to a horizontal cavity surface emitting laser, there is considered either a structure integrated on the bottom of a recess (concave portion) of a circular form circular concentrically with such a lens as disclosed in Japanese Patent Application Laid-Open No. 2007-5594 (hereinafter referred to as Patent Document 1) and Japanese Patent Application Laid-Open No. 2010-147197 (hereinafter referred to as Patent Document 2), or a structure in which a lens disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 3, NO. 9, 1991, pp. 776-778 (hereinafter referred to as Non-Patent Document 1) protrudes against a substrate surface. The structure disclosed in the Non-Patent Document 1 has however a problem in that during a semiconductor process and when a device is mounted to a module, the protruded lens may break due to its hit to an object or the like, and hence the yield is degraded at both device fabrication and module assembly. On the other hand, such a structure as disclosed in each of the Patent Documents 1 and 2 does not cause such a problem since the substrate surface around the lens protects the lens. Accordingly, the structures disclosed in the Patent Documents 1 and 2 are more suitable. A structure example of such a conventional horizontal cavity vertical surface emitting laser as disclosed in each of the Patent Documents 1 and 2 will be explained using
As is understood from
In the conventional lens integration type horizontal cavity surface emitting laser disclosed in each of the Patent Documents 1 and 2, when it is set as a narrow pitch array structure, i.e., each device width is reduced, the interval between the adjacent lenses becomes small, and the intervals among the concave portions at which the lenses are formed as shown in
Incidentally, the present problem mainly relates to one peculiar to an optical device using a GaAs substrate and an InP substrate. If a sapphire substrate is taken as a typical substrate used in another optical device by way of example, this substrate is hard and hard to break as compared with GaAs, InP or the like in terms of its crystal property. Therefore, the cleavage for originally generating a crystal plane becomes difficult regardless of the presence or absence of a concavo-convex structure. In a device using silicon, the thickness of a substrate is about 600 μm or more, and the substrate is thick and hard to break with respect to 100 to 200 μm of a device using GaAs or InP. Further, devices using crystal planes by cleavage are almost none. In general, a chip division by dicing has been has been carried out.
On the other hand, the problem related to the present invention resides in that points easy to break up on the wafer newly occur due to the close integration of the lenses on GaAs or InP substrate, and therefore cleavage at a cleavage position desired to generate a crystal plane becomes difficult. That is, the problem about the hardness to break that occurs in sapphire and silicon results from the property of hardness that the original substrate has. This is different essentially and structurally from the problem related to the present invention that the hardness to break at a predetermined position occurs because the structure is provided.
As described above, the problems to be addressed by the present invention do not essentially arise in a device using sapphire or a silicon substrate. Therefore, the present invention mainly relates to an optical device using a GaAs or InP substrate.
Therefore, an object of the present invention is to provide a horizontal cavity surface emitting laser which is high in fabrication yield and excellent in high-density simple mountability.
A typical means for achieving the above object is shown below.
A horizontal cavity surface emitting laser of the present invention is equipped with a cavity structure portion which includes a stacked structure of a first conduction type clad layer, an active layer for generating light and a second conduction type clad layer stacked over a semiconductor substrate in this order, and which causes the generated light to be reflected or resonated in an in-plane direction; an optical waveguide layer which is provided at least part of the semiconductor substrate and guides the light generated from the active layer; a reflector provided at a part of the optical waveguide layer, for reflecting the light radiated from the cavity structure portion and emitting the light from the back surface of the semiconductor substrate; and a condensing lens which is provided in a light emitting region that corresponds to the back surface of the semiconductor substrate and causes the light to be emitted therefrom, and which focuses the light reflected by the reflector, wherein the back surface of the semiconductor substrate has a groove with the condensing lens provided at its bottom, and a terrace-like portion provided along the direction of cleavage of the semiconductor substrate, and wherein the terrace-like portion is disposed within a range in which a forming region of the cavity structure portion is extended downward, and has an open end on the lateral end side having a crystal plane formed by cleavage of the semiconductor substrate, a sidewall provided on the side opposite to the open end, and a terrace shape with the cleavage direction taken as a longitudinal direction thereof.
With such a configuration, a horizontal cavity surface emitting laser having a lens disposed with a narrow pitch is also capable of cleavage with satisfactory yields. Further, since a device width can be made small, wafer utilization efficiency per area can also be enhanced. It is therefore also possible to reduce fabrication costs. Since the groove formed in the cleavage position designation region can be formed simultaneously with the lens, it is possible to easily enhance yields without increasing a new process.
According to an aspect of the present invention, a groove is provided in advance directly below a desired cleavage position, and the thickness of a wafer is made thin at this position. There is thus provided a structure in which the wafer is easier to break at the desired cleavage position than concave regions densely disposed in row form, which occur with narrow pitching of a device interval. It is therefore possible to easily generate cleavage at a desired position and improve device's fabrication yields. Further, since the yield can be ensured even where the narrow pitching is done, the wafer utilization efficiency per are can be enhanced.
Exemplary embodiments of the present invention will hereinafter be explained in detail using the accompanying drawings.
A structure of a horizontal cavity surface emitting laser according to a first embodiment will be explained using
The n-type InP substrate 2000 is formed with a concave-shaped step. Further, an integrated lens 2009 formed by etching the n-type InP substrate 2000 is integrated at the bottom of the step. The surface of the integrated lens 2009 is given reflection-free coating comprised of a thin film of alumina, for example. At this time, a p-type electrode 2005 is formed above the cavity. An n-type electrode 2008 is formed over the n-type InP substrate at a position opposite to the p-type electrode 2005. Since cleavage is done by applying the present invention, a terrace 2007 is formed at a cleavage end surface of the n-type InP substrate.
A detailed manufacturing method of the horizontal cavity surface emitting laser to which the present invention is applied, will next be explained using
An optical waveguide function is generated by holding the active layer 3002 with a clad layer lower in refractive index than it. The optical waveguide function is achieved by a stacked structure of a clad layer/active layer/clad layer. In a concrete form, however, light confinement layers are provided with a quantum well layer interposed therebetween to enhance light confinement at the active layer. As a matter of course, the refractive index of the clad layer is lower in value than that of the light confinement layer. Incidentally, in the present embodiment, the n-type InP substrate 3001 acts as a first semiconductor layer that functions as a clad layer.
The polarity of the diffraction grating layer 3003 is set as a p type. Such a structure is called refractive-index coupled DFB laser because only the refractive index periodically changes in an optical propagation direction. Incidentally, although the present embodiment has explained where the diffraction grating layer 3003 is uniformly formed over the whole region of the DFB laser, a so-called phase shift structure configured at part of its region with the phase of a diffraction grating being shifted, may be provided as needed. Although the present embodiment is configured by the DFB laser, a DBR laser may be adopted.
Manufacturing processes of the horizontal cavity surface emitting laser device according to the present embodiment will next be explained using
First, in order to form a structure of a laser portion as shown in
Next, a semiconductor multilayered body including a diffraction grating layer 3003 comprised of InGaAsP is formed above the active layer 3002. Further, a p-type semiconductor layer 3004 (clad layer) comprised of p-type InP is formed thereabove. Next, a contact layer 3005 comprised of p-type InGaAs is formed. A carrier concentration based on doping is set to 1018 cm−3 with respect to both n and p types. A silicon dioxide film is coated on an InP wafer having such a multilayer structure to function as a protective mask. Although not shown in the drawing, etching is done up to parts of the contact layer, p-type clad layer 3004, diffraction grating layer 3003, active layer 3002 and n-type InP substrate 3001 using the silicon dioxide mask to thereby form an optical waveguide (refer to
Next, as shown in
Next, as shown in
Thereafter, as shown in
Next, after removal of the silicon nitride film, a p electrode 3008 (p-type electrode) is evaporated onto the p-type InGaAs contact layer 3005. Further, the back surface of the substrate is polished to a thickness of 150 μm and thereafter a silicon nitride mask 3010 is formed on the back surface of the substrate.
Subsequently, as shown in
Subsequently, as shown in
Next, pattering is performed on the n-type InP substrate with a resist to evaporate an n-type electrode 3014. Two devices adjacent in the direction of the optical axis of the horizontal cavity surface emitting laser made up in accordance with the processes shown so far are illustrated in
Next, cleavage is done in a predetermined cleavage region f-f extending along the groove 3011, so that a bar-like formative body in which the devices are arranged in the direction orthogonal to the optical axis, is fabricated. At this time, an end surface formed in the cleavage is configured to be a (100) crystal plane of InP. A sectional view in the optical axis direction, of the bar-like formative body at this time is illustrated in
Thereafter, although not shown in the crystal plane formed by cleavage, a high reflective film comprised of a stacked structure of amorphous silicon and alumina is formed. Afterwards, chipping is performed every predetermined channel. A birds-eye view of a 4-channel array-type lens integrated horizontal cavity surface emitting laser fabricated by the above processes is shown in
The horizontal cavity surface emitting laser device of the present embodiment is capable of obtaining, by virtue of the effect of lens integration, a narrow outgoing beam whose beam expansion angle is 2° and which is formed as a circular beam spot having a diameter of 120 μm at a position of 100 μm as viewed from its laser backside. As described above, an array laser having a narrow pitch suitable for high density integration and a narrow beam expansion angle can be fabricated with satisfactory yields.
Incidentally, although the present embodiment has shown an example applied to the 1.3 μm wavelength-band InGaAlAs quantum well type laser formed on the InP substrate, the material for the substrate, the material for the active layer and the oscillated wavelength are not limited to this example. The present invention is applicable similarly even to, for example, another material system such as a 1.55 μm-band InGaAsP laser or the like.
Although the embodiment having the BH structure has been shown above, the present invention is applicable even to a ridge wave guide (RWG) type structure.
The present embodiment is an example applied to a 1.3 μm-band InGaAlAs quantum well type horizontal cavity surface emitting laser having an RWG-type flip-chip mounted structure with a device width of 250 μm.
The present embodiment is a configuration example where an array-type lens integration horizontal cavity vertically emitting laser fabricated by applying the present invention thereto is applied to a small module.
In terms of the optical coupling, the diameter of the concave portion is set to 200 μm, and the diameter of the lens is set to 100 μm. In this case, the interval between the adjacent concave portions becomes as extremely small as 50 μm, but the device can be fabricated with satisfactory yields by application of the present invention. High-efficient optical coupling can be realized simultaneously on the four channels by using the so-fabricated device.
Using the present module enables transmission of a signal of 100 Gbps in total for the four channels constituted of 25 Gbps per channel. A small optical module suitable as for within a router device can be fabricated by using the laser array to which the present invention is applied.
The present embodiment is an example in which the present invention is applied to a vertical illuminated waveguide photodiode.
First, a first clad layer comprised of InAlAs, a first core layer comprised of InGaAlAs and an absorption layer 7001 comprised of InGaAs although not shown in the drawings, and a second core layer comprised of InGaAlAs, a second clad layer comprised of InAlAs and a contact layer comprised of InGaAs although not shown in the drawings are grown over an n-type InP substrate 7000 in this order using metal organic chemical vapor deposition. Next, etching is done up to parts of the first clad layer, first core layer, absorption layer, second core layer, second clad layer, contact layer and n-type InP substrate to thereby form a ridge shape having a length of 100 μm and a width of 10 μm. As the etching, for example, any technique may be used like dry etching such as reactive ion etching (RIE) using a chlorine gas, or wet etching using a bromine solution or the like, and a combination of the two. Subsequently, the peripheral portion of the ridge shape is buried with a buried semi-insulating layer 7003 comprised of Fe-doped InP. Thereafter, a reflector 7006 is formed ahead of the tip portion of the ridge shape by etching. Afterwards, although not shown in the drawing, a protective film 7005 comprised of SiN is formed in the surface of a wafer, and SiN of part of the upper portion of the ridge shape is removed. Thereafter, a p electrode 7004 is formed on a ridge stripe. At this time, a stud p electrode 7002 is simultaneously formed to prevent a tilt at mounting. Next, a lens 7009 is formed in the n-type InP substrate 7000.
As shown in
Incidentally, although
In the waveguide type photodiode fabricated as described above, light incident via the lens 7009 in the direction perpendicular to the n-type InP substrate 7000 is optical path-converted 90° in a substrate in-plane direction by the reflector 7006 and introduced into a stripe-like absorption layer. The waveguide type photodiode fabricated in accordance with the above procedures has achieved a conversion efficiency of 0.8 W/A, a modulation band of 30 GHz and an operation at 25 Gbps.
While the invention made above by the present inventors has been described specifically on the basis of the preferred embodiments, the present invention is not limited to the embodiments referred to above. It is needless to say that various changes can be made thereto within the scope not departing from the gist thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-006630 | Jan 2011 | JP | national |
