Light receiving module

A light receiving module of the present invention includes a semiconductor substrate formed with a guide groove and a mount groove for receiving an optical fiber and a light receiving device, respectively. The mount groove includes a mount surface to which the light receiving device is affixed. The mount surface is contiguous with the output end of the guide groove. The light receiving device is inserted into the accurately formed guide groove and then affixed to the mount surface of the groove. The fiber and light receiving device can therefore be accurately positioned relative to each other. This, coupled with the fact that the optical coupling length is reduced because of the close contact of the fiber and light receiving device, implements adjustment-free mounting and therefore high optical coupling.

BACKGROUND OF THE INVENTION 
The present invention relates to a light receiving module and, more 
particularly, to a light receiving module of the type having a light 
receiving device and an optical fiber affixed to a semiconductor substrate 
together. 
A light receiving module of the type described is taught in, e.g., Japanese 
patent laid-open publication No. 7-174944. The module taught in this 
document includes a light receiving device portion accurately mounted to a 
submount substrate formed with an accurately etched V-shaped projection. 
An optical fiber portion is positioned on a silicon substrate having 
V-shaped grooves formed by anisotropic etching. 
The above conventional light receiving module has some problems left 
unsolved, as follows. Because the light receiving device and optical fiber 
are mounted on different substrates, it is likely that they are 
positionally deviated from each other during assembly. Further, mounting 
the light receiving device to the subcarrier results in additional 
capacity. Because a preamplifier or similar high speed module is easily 
effected by an input portion, the extra capacity obstructs the application 
of the above structure to a high speed module. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a light 
receiving module capable of solving the above problems and allowing a 
light receiving device and an optical fiber to be accurately coupled to 
each other without any positional deviation. 
It is another object of the present invention to provide a light receiving 
module capable of reducing parasitic capacity and therefore operating at 
high speed. 
A light receiving module of the present invention has a semiconductor 
substrate, and a light receiving device and an optical fiber affixed to 
the semiconductor device together. A guide groove is formed in the 
semiconductor substrate and fixedly receives the optical fiber. A mount 
groove is also formed in the semiconductor substrate contiguously with the 
output end of the guide groove and fixedly receives the light receiving 
device. A mount surface included in the mount groove and to which the 
light receiving device is mounted is contiguous with the output end of the 
guide groove.

In the drawings, identical references denote identical structural elements. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1-3, a light receiving module embodying the present 
invention is shown and includes an optical fiber 4 and a light receiving 
device 5 having a light-sensitive surface 6. A silicon substrate 1 is 
formed with a guide groove 2 and a mount groove 3 for receiving the 
optical fiber 4 and light receiving device 5, respectively. The two 
grooves 2 and 3 are formed in the silicon substrate 1 by wet etching at 
the same time. The depth of the mount groove 3 is controlled in terms of 
etching time such that the center of the light-sensitive surface 6 of the 
device 5 aligns with the optical axis of the fiber 4. 
The mount groove 3 has a rectangular section and includes a mount surface 
7. The output end of the guide groove 2 and the mount surface 7 of the 
mount groove 3 are contiguous with each other. The bottom edge of the 
mount surface 7 has a width slightly greater than the width of the contact 
portion the light receiving device 5. 
The light receiving device 5 is of rear incidence type and implemented by 
an InP substrate. The device 5 is sensitive to light over a diameter of 30 
mm thereof. FIG. 3 shows opposite sides of the device 5. As shown, a p 
electrode 10 is formed on the front of the device 5 over the 
light-sensitive surface 6 in order to reduce parasitic capacity. An n 
electrode 9 and AuSn solder 13 for electrical connection are formed on the 
rear of the device 3. A light input window 8 is formed in the n electrode 
9. 
The tip of the fiber 4 is machined by the same angle as the mount surface 7 
of the mount groove 3 so as to be complementary in configuration to the 
rear of the light receiving device 5. 
FIG. 4 demonstrates how the above light receiving module is assembled. As 
shown, the light receiving device 5 is mounted to the silicon substrate 1 
along the mount surface 7 until the bottom of the device 5 contacts the 
bottom of the mount groove 3. Then, the device 5 is affixed to the mount 
surface 7. The resulting relation between the device 5 and the substrate 1 
is shown in FIG. 4. In this condition, the n electrode 9 of the device 5 
is brought into contact with, via the AuSu solder 13, electrodes 11 formed 
on the substrate 1. Subsequently, the fiber 4 is inserted into the guide 
groove 2 such that the tip of the fiber 4 is positioned complementarily to 
the rear of the device 5. Then, the fiber 4 is fixed in place in the guide 
groove 2. 
The above assembly implements an extremely short optical distance between 
the tip of the fiber 4 and the light-sensitive surface 6 because the fiber 
4 and light receiving device 5 are fixed in place in contact with each 
other. This successfully reduces the spread of light output from the fiber 
4 on the light-sensitive surface 6. As shown in FIG. 5, a refractive index 
buffering material 15 may be provided between the fiber 4 and the device 5 
in order to further reduce the radiation angle of light output from the 
fiber 4. The material 15 may be implemented by, e.g., silicon gel or 
optical adhesive. 
The positional relation between the light receiving device 5 and the fiber 
4 shown in FIG. 1 is implemented by the accurate mounting of the device 5 
to the substrate 1. In the illustrative embodiment, the device 5 and the 
bottom of the mount surface 6 are so dimensioned as to be coincident in 
configuration with each other. Therefore, the mounting accuracy of the 
device 5 is determined by the forming accuracy of the mount groove 3 and 
the cutting accuracy of the device 5. This embodiment reduces the 
positional deviation to about 35 1 mm. 
Reference will be made to FIGS. 6 and 7 for describing an alternative 
embodiment of the present invention. In this embodiment, the light 
receiving device 5 is mounted to the silicon substrate 1 by a visual 
alignment mounting system. As shown in FIG. 6, an alignment pattern 12 for 
visual alignment is formed on the mount surface 7 symmetrically with 
respect to the guide groove 2 as a part of a metallized pattern. The AuSn 
solder 13 is also formed on the mount surface 7 for affixing the device 5. 
The alignment pattern 12 is positioned such that a desired positional 
relation holds between it and the optical axis of the optical fiber 4. 
As shown in FIG. 7, an alignment pattern 21 is formed on the rear of the 
device 5 symmetrically with respect to the light-sensitive surface 6 as a 
part of a metallized pattern. The light input window 8 is also formed in 
the rear of the device 5. Two symmetrical portions of the alignment 
pattern 21 are spaced by the same distance from each other as two 
symmetrical portions of the alignment pattern 12 provided on the mount 
surface 7. 
FIG. 8 shows how the light receiving device 5 of the alternative embodiment 
is affixed to the mount surface 7. As shown, the substrate 1 is mounted to 
a heater 17. While infrared light 18 is projected from the bottom toward 
the top of the heater 17, the device 5 held by an arm 19 is laid on the 
substrate 1. At this instant, a camera 20 monitors the alignment pattern 
12 of the mount surface 7 and the alignment pattern 21 of the device 21. 
Image recognition and position processing are executed on the basis of an 
image being picked up by the camera 20. FIG. 9 shows a monitor image in 
which the substrate 1 and device 5 are superposed. The arm 19 holding the 
device 5 is adjusted such that the two portions of the alignment pattern 
12 are respectively positioned at the centers of the two portions of the 
alignment pattern 21, as illustrated. Thereafter, the heater 17 is caused 
to generate heat for melting the AuSn solder 13 on the substrate 1. As a 
result, the substrate 1 and device 5 are affixed to each other. 
The procedure shown in FIG. 8 allows the substrate 1 and device 5 to be 
affixed with an accuracy lying in the error range of the visual alignment 
mounting system, i.e., in the order of submicrons. As a result, a high 
coupling efficiently is achievable with ease. 
An optical coupling characteristic achievable with the fiber 4 and device 5 
affixed in the structure of FIG. 1 was estimated. The estimation showed 
that the optical coupling characteristic was only about 2% lower in 
quantitative efficiency than in the case where an optical fiber was 
adjusted to an optimal position. This indicates that highly accurate 
mounting is achievable without any adjustment. 
FIG. 10 shows a specific application of the embodiment shown in FIG. 1 to a 
preamplifier. As shown, the p electrode 10 of the light receiving device 5 
is directly connected to a bonding pad 23 by a wire 24. This configuration 
is extremely desirable for a preamplifier having high parasitic 
sensitivity because hardly any parasitic capacity is added to the input 
portion of the preamplifier 22. The electrodes 11 are used to feed a bias 
voltage to the substrate 1 and provided with bonding wires 25. The 
preamplifier 22 and the substrate 1 are arranged in a package (not shown) 
made of ceramics or metal. 
FIG. 11 shows another alternative embodiment of the present invention 
including a silicon substrate 11. As shown, the mount groove 3 formed in 
the substrate 11 is slightly greater in dimensions than the contour of the 
light receiving device 5. The mount groove 3 is formed by, e.g., dry 
etching the substrate 11 in the vertical direction. For the electrical 
connection of the device 5, bumps 28 and 29 respectively formed on the n 
and p electrodes are respectively brought into contact with electrodes 31 
and 30 formed on the substrate 11 and then connected together by being 
melted. 
In the structure shown FIG. 11, the positional relationship between the 
light receiving device 5 and the optical fiber 4 is determined by the 
accuracy with which the device 5 is received in the mount groove 3. 
Dimensional accuracy available with etching is generally on the order of 
several hundreds of angstroms and is determined by the cutting accuracy of 
the device 5. It follows that the positional deviation between the device 
5 and the fiber 4 can be reduced to about .+-.1 mm. 
The coupling characteristic of the device 5 and fiber 4 shown in FIG. 11 
was estimated. The estimation showed that the coupling characteristic was 
only about 2% lower quantitative efficiency than when the fiber 4 was 
adjusted to an optimal position. This indicates that highly accurate 
mounting is achievable without any adjustment. 
The configuration of FIG. 11 is extremely effective when the electrode of a 
light receiving device is too small to be directly bonded, although the 
signal lines formed on the substrate 11 would increase parasitic capacity, 
compared to the configuration of FIG. 1. 
In summary, in accordance with the present invention, a light receiving 
module allows a light receiving device to be accurately affixed to the 
mounting surface of a silicon substrate at a desired height aligning with 
the optical axis of an optical fiber. This, coupled with the fact that the 
optical coupling length and therefore the coupling loss is small, insures 
high optical coupling by adjustment-free easy mounting. Further, the light 
receiving device can be mounted without resorting to a carrier, so that a 
high frequency characteristic is improved due to a decrease in parasitic 
capacity.