Optical module

An electrode of a laser diode (LD) is joined with a Si substrate via a solder layer. A cavity region is formed directly under the LD in the solder layer. The solder layer spreads outside the cavity region. Since there arises no stress in a space of the cavity region, the internal stress of the active layer, which is formed directly over the cavity region, is relaxed as compared with that in a conventional optical module, which is provided with no cavity region in the solder layer. Accordingly, a highly reliable optical module can be obtained.

FIELD OF THE INVENTION
 The invention relates to an optical module mounting an optical device, such
 as a semiconductor laser diode or a photodiode.
 BACKGROUND OF THE INVENTION
 An optical module, which optically couples an optical transmission line,
 such as an optical fiber, with an optical device, such as a semiconductor
 laser (LD, hereinafter) or a photodiode (PD, hereinafter), plays an
 important role in constructing an optical communication system.
 In a method for mounting an optical device on a substrate, the optical
 device is mounted on a Si substrate in most cases. For example, such an
 optical module is reported by K. Kurata and others in a paper "A surface
 mount type optical module for subscriber network", Proceeding of the 1995
 Electronics Society Conference of IEICE, SC-1-12.
 In the optical device mounted on the substrate by soldering, an internal
 stress remains behind in the active layer or the photoabsorption layer of
 the soldered optical device, because of the difference in the thermal
 expansion coefficients between the optical device and the substrate, hence
 the optical device is short-lived and reliability of the optical module
 deteriorates. Especially, in an array optical device of multi-channels,
 since the amount of thermal contraction of the optical devices is larger
 than that in the optical device of single channel, there has been further
 apprehension for deterioration of reliability.
 SUMMARY OF THE INVENTION
 Accordingly, it is an object of the invention to provide an optical module,
 which is highly reliable and contributes for integration and
 miniaturization of an optical circuit by decreasing an internal stress in
 an active layer or a phtoabsurption layer of an optical device.
 According to the feature of the invention, an optical module comprises:
 an optical device provided with a first electrode formed on a bottom
 surface thereof,
 a substrate provided with a second electrode formed on a top surface
 thereof,
 a solder layer region for joining the first and second electrodes, and
 a cavity region formed directly under the optical device in the solder
 layer region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the embodiment of FIG. 1, the electrode 5 of a LD and an electrode 2
 formed on a Si substrate 1 are joined with solder. The electrode 5 is
 formed on the bottom surface of the LD 4 near the active layer 6. In a
 cavity region 3-1 formed under the active layer 6, there is no solder, and
 the solder layer region 3-2 spreads in parallel with both the electrodes 2
 and 5 outside the cavity region 3-1.
 According to such a structure, there arises no stress in the space of the
 cavity region 3-1, which is formed directly under the active layer 6.
 Consequently, a stress in the active layer 6 formed directly overhead the
 cavity region 3-1 is relaxed as compared with that in a conventional
 structure, in which no cavity region is provided in the solder layer
 region.
 Next, explanation will be given to the concrete embodiment referring to
 FIG. 1. The LD 4 is an InP semiconductor laser of edge emission type, and
 the length, the width and the depth of its cavity (lasing cavity) are
 respectively 200 .mu.m, 250 .mu.m and 1000 .mu.m. The Au electrode 5 near
 the active layer 6 is joined with the Au electrode 2 formed on the Si
 substrate 1 by the AuSn solder layer region 3-2. The width and the
 thickness of the active layer are respectively 1 .mu.m and 0.2 .mu.m, the
 thickness of the Au electrodes 2 and 5 are respectively 1.mu.m, the
 thickness of the AuSn solder layer is 2 .mu.m, and the width of the cavity
 region 3-1 is 60 .mu.m. 0.1 .mu.m thick SiO.sub.2 layers 7 are
 respectively formed on the inner surfaces of the Au electrodes 2 and 5 in
 the cavity region 3-1 in order to prevent penetration of AuSn solder into
 the cavity region 3-1.
 FIG. 2 shows a relation between the width W of the cavity region 3-1 and
 the internal stress of the active layer 6, after the LD 4 is soldered to
 the Si substrate 1. In case that the width W of the cavity region 3-1 is
 zero, in other words, in case that a conventional method for soldering, in
 which the cavity region 3-1 is not provided, is used, there arises a large
 stress in the active layer. However, the stress decreases, as the width W
 of the cavity region increases.
 According to the embodiment mentioned in the above, in case that the width
 W of the cavity region 3-1 is 60 .mu.m, the stress is reduced to
 one-fourth of the conventional one. In the space of the cavity region 3-1
 formed directly under the active layer, there arises no stress.
 Accordingly, after the LD is soldered to the Si substrate 1, the internal
 stress in the active layer 6 formed directly overhead the cavity region
 3-1 is relaxed as compared with that in the conventional structure, in
 which the LD 4 is directly soldered to the Si substrate 1 via a solder
 layer region provided with no cavity region. Consequently, the reliability
 of an optical device, which is mounted on the Si-substrate by means of the
 solder layer region according to the invention, is improved.
 In case that the cavity region 3-1 is formed directly under the active
 layer 6, it is apprehended that conduction of heat generate in the active
 layer 6 is obstructed by the cavity region 3-1 and diffusion of heat is
 disturbed. However, since the cavity region 3-1 is thin, heat in favorably
 conducted therethrough and no problem occurs on diffusion of heat.
 Although FIG. 2 shows the relation between the stress of the active layer 6
 and the width W of the cavity region 3-1, a similar relation is observed
 in one between the stress of the photoabsorption layer of a photodiode and
 the width W of the cavity region. That is to say, the stress in the
 photoabsorption layer of the photodiode can be relaxed by forming the
 cavity region formed directly under the photoabsorption layer in the
 solder layer region, and reliability of the photodiode can be improved.
 In the aforementioned embodiment, the bottom surface of the LD, which is
 joined with the solder layer, is flat. However, the bottom surface of the
 LD 4 may be formed into a mesa like shape in order to improve the
 characteristic of the LD 4. In such a case, if a cavity region is formed
 in the solder layer as in the case of the aforementioned embodiment, the
 effect of the invention can be exhibited.
 In the aforementioned invention, the 0.1 .mu.m thick SiO.sub.2 layers 7 are
 respectively formed on the electrode 5 of the LD 4 and the electrode 2
 formed on the surface of the Si substrate 1 in order to prevent
 penetration of solder into the cavity region 3-1. However, the SiO.sub.2
 layer 7 may be formed on an electrode selected from the electrodes 5 and
 2. In such a case, one of the top and bottom surfaces of the cavity region
 3-1 is covered with solder film, but the effect on the invention can be
 exhibited.
 Moreover, in the aforementioned embodiment, the width W of the cavity
 region is selected to be 60 .mu.m. However, the width of the cavity region
 can be selected at will, so long as the stress in the cavity layer 6 is
 kept within a tolerance limit and the mechanical strength of joining is
 sufficient. In the aforementioned embodiment, the LD of a single channel
 is mounted on the substrate. However, the invention can be applied to an
 array LD, which is composed of plural LDs arranged along a line. In this
 case, the respective LDs maybe provided with the cavity regions formed
 directly under the respective LDs in the solder layer region.
 Furthermore, the LD is used as an optical device in the aforementioned
 embodiment, but the optical device is never restricted to the LD. The
 invention can be applied to other kinds of optical devices, such as a
 photodiode of a waveguide type, which receives a light incident on its
 edge surface, a semiconductor laser of distributed feed back type (a
 DFB-LD, hereinafter), a semiconductor amplifier and an array optical
 device. For example, in a case of the PD of the waveguide type, the cavity
 region can be provided under the photoabsorption layer in the solder layer
 region.
 In the case of the DFB-LD, since a grating is formed in the active layer,
 reliability of the active layer can be improved, uncertainty of the
 characteristic of the grating caused by internal stress is reduced, and
 its operation characteristic can be stabilized, by forming the cavity
 region directly under the active layer.
 In a optical module according to the invention, the cavity region, which is
 formed directly under the active layer or the photabsorption of the
 optical device, is a space with no solder, and there is the solder layer
 region outside the cavity region. According to this structure, there
 arises no stress in a space in the cavity region. Consequently, the
 internal stress in the active layer or the photoabsorption layer, which is
 formed directly overhead the cavity region, is relaxed as compared with
 that in the conventional optical module, in which no cavity region is
 provided, hence reliability of the optical device is improved.
 Since the invention can be applied the array optical device, in which
 plural optical devices are arrange along a line, the integrated and
 miniaturized optical module with high reliability can be provided. Since
 the electrode near the active layer or the photoabsorption layer of the
 optical device can be joined with the substrate and pedestals for
 adjusting the heights of the optical device can be further formed on the
 substrate, regulation of the heights of the optical device, which is
 indispensable for mounting the optical device in a way of passive
 alignment, can be made to be easy, which contributes to reduction of time
 and labor necessary for alignment of optical axes. Accordingly, the
 optical module with high reliability can be provide at low a price.
 Moreover, in the case of the DFB-LD, since uncertainly of its
 characteristic caused by inner stress of the grating formed therein is
 reduced, the DFB-LD with excellent characteristic can be provided at a low
 price.
 Although the invention has been described with respect to specific
 embodiment for complete and clear disclosure, the appended claims are not
 to be thus limited but are to be construed as embodying all modification
 and alternative constructions that may be occurred to one skilled in the
 art which fairly fall within the basic teaching here is set forth.