Abstract:
An optical module including a substrate, an optical waveguide formed on the substrate, and an optical element mounted on the substrate so as to be optically coupled to an end of the optical waveguide, for performing conversion between light and electricity. The optical module further includes a transparent resin for covering at least an optical coupling portion between the optical waveguide and the optical element, and a thermoplastic resin for covering only the optical element and its periphery including the transparent resin to seal the optical element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical module and a sealing method therefor. 
     2. Description of the Related Art 
     In recent years, the development of an optical access network has been actively pursued. For realization of an optical access network, it is greatly important to reduce the cost of an optical device and/or an optical module. Also in sealing an optical element for performing opto-electric conversion or electro-optic conversion in an optical module, further cost reduction is required, and a method of easily sealing the optical element with a sufficient reliability is demanded. 
     In many optical modules at present, a metal package or ceramic package is used to ensure reliability and the package is hermetically sealed by welding or soldering, resulting in a very high cost of optical module. For cost reduction of an optical module, it is now important to simplify the sealing method for an optical element. As an example of simplification of the sealing method for an optical element, there has been proposed a method of applying a resin to the entire surface of a substrate on which an optical element is mounted and next curing the resin (Mitsuo Fukuda et al., “Plastic Packaging of Semiconductor Laser Diode”, Electronic Components and Conference, 1996, pp1101-1108). 
     In the case of applying a resin to the entire surface of a substrate on which an optical element is mounted and next curing the resin to seal the optical element as described in the above literature, there is a possibility that separation or cracking of the resin may occur because of a large difference in coefficient of thermal expansion between the substrate and the resin, or the substrate may be broken by a residual stress in the resin. The larger the thickness of the resin coating covering the optical element and the wider the range of spread of the resin coating, the larger the residual stress in the resin. Accordingly, the residual stress can be relaxed by thinly applying the resin to a minute area surrounding the optical element. However, the resin is generally in the form of gel, so that when dropped onto the substrate, it spreads widely on the substrate and it is difficult to apply the resin to only the minute area surrounding the optical element. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a low-cost high-reliability optical module by sealing only an optical element and its periphery with a resin. 
     It is another object of the present invention to provide a sealing method for a low-cost high-reliability optical module. 
     In accordance with an aspect of the present invention, there is provided an optical module comprising a substrate; an optical waveguide formed on said substrate and having a first end; an optical element mounted on said substrate so as to be optically coupled to said first end of said optical waveguide, for performing conversion between light and electricity; a transparent resin for covering at least an optical coupling portion between said optical waveguide and said optical element; and a thermoplastic resin for covering only said optical element and its periphery including said transparent resin to seal said optical element. 
     In accordance with another aspect of the present invention, there is provided an optical module comprising a substrate; an optical waveguide formed on said substrate and having a first end; an optical element mounted on said substrate so as to be optically coupled to said first end of said optical waveguide, for performing conversion between light and electricity; and an ultraviolet-curing resin for covering only said optical element and its periphery including an optical coupling portion between said optical waveguide and said optical element to seal said optical element. 
     In the optical module according to the present invention, only the optical element and its periphery are sealed with the resin, so that the residual stress in the resin can be reduced. Therefore, separation and cracking of the resin can be prevented to thereby ensure the cost reduction and reliability of the optical module. 
     In accordance with a further aspect of the present invention, there is provided a sealing method for an optical module including an optical waveguide formed on a substrate and having a first end, and an optical element mounted on said substrate so as to be optically coupled to said first end of said optical waveguide, for performing conversion between light and electricity, comprising the steps of applying a transparent resin to an optical coupling portion between said optical waveguide and said optical element; curing said transparent resin; applying a thermoplastic resin to only said optical element and its periphery; and curing said thermoplastic resin by cooling to thereby seal said optical element. 
     In accordance with a still further aspect of the present invention, there is provided a sealing method for an optical module including an optical waveguide formed on a substrate and having a first end, and an optical element mounted on said substrate so as to be optically coupled to said first end of said optical waveguide, for performing conversion between light and electricity, comprising the steps of applying an ultraviolet-curing resin to the entire surface of said substrate; laying a mask having an opening for exposing said optical element and its periphery over said substrate at a given height; directing ultraviolet radiation through said mask onto said ultraviolet-curing resin to thereby cure only a part of said ultraviolet-curing resin exposed to said opening; removing said mask; and removing the remaining uncured part of said ultraviolet-curing resin by using an organic solvent. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a laser diode module; 
     FIG. 2 is a perspective view showing a general configuration of an apparatus used for resin sealing according to the present invention; 
     FIG. 3 is a partially cutaway, perspective view of a hot dispenser; 
     FIG. 4 is a sectional view of the laser diode module after resin sealed; 
     FIG. 5 is a plan view of an optical module for bidirectional transmission; and 
     FIGS. 6A to  6 D are perspective views showing a sealing process according to a second preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a perspective view of a laser diode module (LD module)  2  suitable for sealing of a laser diode by a sealing method according to a first preferred embodiment of the present invention. An SiO 2  glass layer  6  is formed on an Si substrate  4  by a CVD process, for example. An optical waveguide  10  doped with germanium (Ge) or titanium (Ti), for example, is formed in the SiO 2  glass layer  6 . A laser diode (LD)  12  is mounted on the substrate  4  so as to be opposed to an end of the optical waveguide  10 . Reference numerals  14  and  15  denote feed electrodes for the LD  12 . The feed electrode  14  is connected to the LD  12  by bonding through a gold wire  18 . 
     Referring to FIG. 2, there is shown a general configuration of an apparatus used for resin sealing according to the present invention. Reference numeral  20  denotes a stage assembly for mounting the LD module  2 . The stage assembly  20  includes a base  22 , an X table  24  movable in an X-axis direction on the base  22 , and a Y table  28  movable in a Y-axis direction on the base  22 . By rotating a knob  26 , the X table  24  is moved in the X-axis direction on the base  22 , whereas by rotating a knob  30 , the Y table  28  is moved in the Y-axis direction on the base  22 . 
     A stage  32  is mounted on the Y table  28 . A sheath heater  36  is inserted in the stage  32 , and the temperature of the stage  32  can be measured by a thermocouple  38 . The stage  32  is surrounded by a protective cover  34 . The stage  32  is formed with a vacuum hole  40  for attraction of the LD module  2  under vacuum. Reference numeral  41  denotes a dispenser containing a silicone resin. The dispenser  41  is provided with a hose  43  connected to a compressed air source (not shown). Reference numeral  42  denotes a hot dispenser containing a thermoplastic resin. The hot dispenser  42  is provided with a hose  45  connected to a compressed air source (not shown). Reference numeral  44  denotes a cooling air blowing nozzle. 
     Referring to FIG. 3, there is shown a partially cutaway, perspective view of the hot dispenser  42 . The hot dispenser  42  has a plastic container  46  in which the thermoplastic resin is contained. A preferred example of the thermoplastic resin is Staystick  383 , a trade name of Alphametals Inc., USA. The common name for Staystick  383  is polyoxyether. A Nichrome wire  48  is wound around the plastic container  46 . The Nichrome wire  48  is connected through a Variac  56  to a power source  54 . By adjusting the Variac  56 , a current flowing in the Nichrome wire  48  is controlled to thereby adjust the temperature of the container  46 . The Nichrome wire  48  is covered with a heat insulator  50 . The temperature of the container  46  is measured by a thermocouple  52 . 
     The LD module  2  is placed on the stage  32 , and adjacent two sides of the LD module  2  are made to abut against adjacent two sides of a recess formed on the upper surface of the stage  32  to thereby position the LD module  2  just over the vacuum hole  40 . Then, the LD module  2  is fixed to the upper surface of the stage  32  by suction through the vacuum hole  40 . The stage  32  is next heated to about 150° C. by the heater  36 . This temperature of 150° C. is a curing temperature of the silicone resin as a thermosetting resin, and is also a curing temperature of the Staystick  383  as a thermoplastic resin. 
     Then, the X table  24  and the Y table  28  are moved to adjust the position of the LD module  2  mounted on the stage  32  so that an optical coupling portion between the optical waveguide  10  and the LD  12  comes to a position just under the dispenser  41 . Thereafter, the silicone resin is dropped from the dispenser  41  to a gap between the optical waveguide  10  and the LD  12 , thereby filling the gap as shown by reference numeral  58  in FIG.  4 . Because the stage  32  is heated at about 150° C., the silicone resin  58  dropped is cured in the gap. Accordingly, the optical coupling portion between the LD  12  and the optical waveguide  10  is filled with the cured silicone resin  58  which is transparent, thus ensuring an optical path between the LD  12  and the optical waveguide  10 . 
     In the next step, the X table  24  and the Y table  28  are moved again to adjust the position of the LD module  2  mounted on the stage  32  so that the LD  12  comes to a position just under the hot dispenser  42 . In the case that Staystick  383  is used as the thermoplastic resin, the temperature of the lowermost end of the hot dispenser  42  is preferably set to about 200° C. In this case, the temperature of the container  46  is adjusted to about 230° C. to about 240° C. by adjusting the current flowing in the Nichrome wire  48 . The higher the temperature of the hot dispenser  42 , the lower the viscosity of Staystick  383  as the thermoplastic resin contained in the hot dispenser  42 , thereby allowing smoother dropping of the thermoplastic resin. However, an excessive increase in temperature of the hot dispenser  42  is not preferable because the resin becomes turbid in yellow. 
     After thus relatively positioning the hot dispenser  42  to the stage  32 , a given amount of thermoplastic resin is dropped from the hot dispenser  42  onto the LD  12 . The amount of thermoplastic resin to be dropped is controlled by controlling the compressed air to be introduced through the hose  45 , e.g., by controlling the time of connection of the hose  45  to the compressed air source and/or the pressure of the compressed air in the compressed air source. Not only the control of the amount of thermoplastic resin to be dropped, but also the control of the viscosity of thermoplastic resin to be dropped is important. That is, it is necessary to control both the amount and viscosity of thermoplastic resin to be dropped so that the thermoplastic resin dropped from the hot dispenser  42  onto the LD  12  as shown by reference numeral  60  covers only the LD  12  and its periphery. 
     Because the stage  32  is heated at about 150° C., the thermoplastic resin  60  dropped onto the LD  12  spreads to the periphery of the LD  12  and solidifies. To accelerate the solidification of the thermoplastic resin  60 , a cooling air may be sprayed from the nozzle  44  to the thermoplastic resin  60  spread. In the case that Staystick  383  is used as the thermoplastic resin  60 , however, the spraying of the cooling air from the nozzle  44  is unnecessary because Staystick  383  dropped onto the LD  12  spreads to the periphery of the LD  12  and immediately solidifies in spite of the fact that the stage  32  is heated at about 150° C. 
     The heating temperature of the stage  32  is important in controlling the viscosity of the thermoplastic resin  60  dropped to adjust the spread range thereof, so that the temperature of the stage  32  is preferably set to about 150° C. If the temperature of the stage  32  is a low temperature such as room temperature, the thermoplastic resin  60  dropped does not spread, but immediately solidifies, so that a necessary sealing area cannot be covered with the thermoplastic resin  60 . 
     According to the LD module  2  of this preferred embodiment, only the optical coupling portion between the LD  12  and the optical waveguide  10  is covered with the transparent silicone resin  58 , and only the LD  12  and its periphery are covered with the thermoplastic resin  60 . Accordingly, a residual stress in the thermoplastic resin  60  can be reduced. As a result, separation, cracking, etc. of the thermoplastic resin  60  can be prevented to thereby ensure the reliability of the LD  12 . It is not preferable to cover the LD  12  and its periphery with only the transparent silicone resin  58 , because the silicone resin  58  is insufficient in moisture resistance or the like, causing a problem that a long-term reliability of the LD  12  cannot be ensured. 
     Referring to FIG. 5, there is shown a plan view of an optical module  62  for bidirectional transmission to which the resin sealing method according to the first preferred embodiment of the present invention is suitably applied. An SiO 2  glass layer  62  is formed on an Si substrate  64  by a CVD process, for example. Optical waveguides  68 ,  70 , and  72  doped with germanium (Ge) or titanium (Ti), for example, are formed in the SiO 2  glass layer  66 . The optical waveguide  68  is connected through a Y branch  74  to the optical waveguides  70  and  72 . An optical fiber  80  inserted and fixed in a ring  82  formed of ruby or the like is bonded to an end of the optical waveguide  68  by an optical adhesive such as an ultraviolet-curing optical adhesive. 
     A laser diode (LD)  76  for transmission is mounted on the substrate  64  so as to be opposed to an end of the optical waveguide  70 . The laser diode  76  has an excitation end  76   a  formed from a cleavage surface of a semiconductor. A photodiode (PD)  78  for reception is mounted on the substrate  64  so as to be opposed to an end of the optical waveguide  72 . The distance between the end of the optical waveguide  70  and the laser diode  76  is set to about 50 m, and the distance between the end of the optical waveguide  72  and the photodiode  78  is also set to about 50 m. 
     In applying the resin sealing method to the optical module  62 , the apparatus shown in FIG. 2 is used to cover an optical coupling portion between the LD  76  and the optical waveguide  70  and an optical coupling portion between the PD  78  and the optical waveguide  72  with a transparent resin such as a silicone resin to ensure an optical path. Then, a thermoplastic resin is dropped from the hot dispenser  42  onto the LD  76  and the PD  78  to seal them and their peripheries with the thermoplastic resin. Thus, the LD  76 , the PD  78 , and their peripheries only are sealed with the thermoplastic resin, so that a residual stress in the sealing resin can be reduced to thereby ensure a long-term reliability of the LD  76  and the PD  78 . 
     A sealing process according to a second preferred embodiment of the present invention will now be described with reference to FIGS. 6A to  6 D. Referring to FIG. 6A, reference numeral  84  denotes an optical module before resin sealed. An SiO 2  glass layer  88  is formed on an Si substrate  86  by a CVD process, for example, and a plurality of optical waveguides  90  doped with germanium (Ge) or titanium (Ti), for example, are formed in the SiO 2  glass layer  88 . A laser diode (LD)  92  is mounted on the Si substrate  86  so as to be opposed to an end of each optical waveguide  90 . 
     In this preferred embodiment, an ultraviolet-curing resin such as an acrylic ultraviolet-curing resin is used as the sealing resin. As shown in FIG. 6A, an adequate amount of acrylic ultraviolet-curing resin is dropped onto the LDs  92 , and is next spread over the entire surface of the substrate  86  by using a spinner (not shown) as shown by reference numeral  94 . Then, a mask  96  having a plurality of openings  98  respectively corresponding to the plural LDs  92  and their peripheries as shown in FIG. 6B is laid over the substrate  86  at a height of about 0.1 mm, and markers  100  and  102  of the substrate  86  and markers  104  and  106  of the mask  96  are respectively aligned with each other by using a commercially available mask aligner (not shown). The mask  96  may be formed by vapor deposition of aluminum or the like on a glass plate except the openings  98 . 
     As shown in FIG. 6C, ultraviolet radiation from an ultraviolet radiation source  108  is directed through the mask  96  onto the resin  94  applied to the substrate  86 . As a result, only a part of the resin  94  on the LDs  92  and their peripheries exposed to the openings  98  is cured by the ultraviolet radiation. For example, ultraviolet radiation having a wavelength of 350 nm is directed with a power of 3 joule/cm 2  Thereafter, the mask  96  is removed and the remaining uncured part of the resin  94  is next removed by an organic solvent such as acetone, thus sealing the LDs  92  with the cured resin as shown by reference numeral  110  in FIG. 6D, in which reference numeral  84 ′ denotes the optical module after resin sealed. In this preferred embodiment, local resin sealing at plural positions on the same substrate can be performed at a time. Accordingly, this preferred embodiment is effective for improvement in mass productivity of an optical module with many optical elements mounted on a substrate. 
     According to the present invention, only an optical element and its periphery can be sealed with a resin, thereby reducing a residual stress in the sealing resin. Accordingly, cracking and separation of the sealing resin can be prevented, and a long-term reliability of the optical module can be ensured by a simple resin sealing method.