Photodiode module

Details Photodiode module Photodiode module

1999-09-17

2001-07-10

Dang, Hung Xuan (Department: 2873)

Optical waveguides

With disengagable mechanical connector

Optical fiber to a nonfiber optical device connector

C385S088000, C257S432000, C257S433000

Reexamination Certificate

active

06257772

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a planar type photodiode (PD) module as a receiving device for optical communication. The meaning of the planar type is explained. Prior PD modules have a three dimensional structure mounting a PD chip on a round stem, sustaining a lens above the PD chip by a cylindrical holder standing on the stem, supporting a fiber just above the lens and the PD chip by a cylindrical sleeve standing on the stem. The lens converges rays from the fiber into the top surface of the PD chip. The rays propagate in free space from the fiber to the PD in the package. Here, lines of individual partial light are called rays. An assembly of rays is called a beam. The beam axis is vertical to the PD chip and the stem. Since the end of the fiber is distanced from the PD, the rays disperse in the free space. Thus, the lens is required for converging the rays onto the PD. The prior PD module is a cylinder having pins at the bottom and a fiber at the top. Such a cylindrical device is not suitable for handling. Mounting on a print circuit board requires soldering the pins to holes of the print circuit board and inclining the cylinder into a horizontal posture by bending the pins at right angles. When a plurality of print circuit boards are piled in a vertical direction, it is determined that the pitch between the neighboring boards should be shorter than 9 mm. The pitch is a sum of the thickness of the board and the distance between the neighboring boards. Prior tall cylindrical PD modules cannot satisfy the requirement.
The planar type module is a contradictory to the prior bulky, tall modules. The planar type signifies the device having the optical fiber being parallel with and fixed to the substrate and dispensing with a lens. Since the optical fiber lies on the substrate and the beam axis is parallel to the surface, the device is called planar type. Since the fiber is stuck to the substrate, the planar type device requires a cylindrical package no more. The planar type device can make use of a flat package by laying the fiber, the PD and so forth on a flat substrate and molding the whole by a resin. The short distance between the fiber and the PD can afford to eliminate the lens.
The omission of the lens allows the device to exclude the optical adjustment among the fiber, the lens and the PD. Elimination of the lens lowers the parts cost and the assembly cost. Low, flat packages are convenient for mounting the planar type device on a print circuit board. Suppressing the cost of the optical devices ardently requires planar type devices.
In general, an end of an optical fiber faces directly to an LD, an LED or a PD without a lens in the planar type optical devices. Elimination of the lens requires more rigid tolerances for mounting. Some contrivances are suggested for exact mounting of PDs, LEDs or LDs in the planar type devices. None of the proposals have been prevailing owing to the drawbacks.
This application claims the priority of Japanese Patent Applications No. 10-283416 (283416/1998) filed on Sep. 18, 1998 and No.10-274670 (274670/1998) filed on Sep. 29, 1998, which are incorporated herein by reference.
2. Description of Prior Art
A typical planar type PD module has a Si substrate having a structure for fixing a PD chip and an end of an optical fiber at a determined position. Anisotropic etching of Si forms a V-groove on the Si substrate for adapting a fiber. The PD module guides the light going out of the fiber to the PD chip by reflecting the light by a mirror made in the V-groove. Anisotropy of etching signifies that the etching rate on {100} planes are far faster than the etching rate of {111} planes for special etchants in a silicon single crystal. Some etchants reveal such anisotropic etching speeds for Si. The anisotropy allows the etchant to make holes enclosed by {111} planes.
The anisotropic etching makes a V-groove having a (1-11) plane and (11-1) plane by painting a (100) Si single crystal substrate with a resist, removing a resist in a stripe extending along [011], making a striped window in the direction and etching the Si substrate by the etchant which has a faster {100} plane etching rate than a {111} plane etching rate. Fortunately, a (111) plane appears at the end of the V-groove. The angles held between the surface (100) and the V-groove side walls (1-11) and (11-1) are 126 degrees. The bottom angle of the V-groove is 71 degrees. The angles between the V-groove walls (1-11) and (11-1) and the end wall (111) are 108 degrees. The angle between the end wall (111) and the surface (100) is not 135 degrees but 126 degrees.
Direction indexes and plane indexes have been defined in crystallography as follows. Individual direction is denoted by square bracketed numerals [ . . . ] Collective direction is denoted by edged bracketed numerals< . . . >. Individual plane is designated by round bracketed numerals ( . . . ). Collective plane is designated by wavy bracketed numerals { . . . }. The above explanation is directed to a [011] striped V-groove. A [0±1±1] striped V-groove can also be made in a similar way. The Si substrate has a good conductivity, which is inconvenient as a base. The Si substrate is preparatively coated with a SiO
2
film of a 0.5 &mgr;m to 3 &mgr;m thickness which is made by oxidization of the Si or sputtering SiO
2
. Thus, the Si substrate consists of a bulk Si single crystal and a thin SiO
2
insulating film. The Si substrate is a SiO
2
/Si substrate in a rigorous meaning. But it is simply called a Si substrate in brief hereafter.
An optical fiber is adapted into the V-groove on the Si substrate for producing the planar type PD module. The rays emitted from the fiber are reflected upward by the (111) mirror plane at the end of the V-groove. The rays enter the PD from the bottom. The V-groove, the end mirror and the PD above the mirror build up a planar PD module which dispenses with a lens.
The planar PD module has a fundamental structure explained above. Improvements have been proposed for the planar type PD modules in addition to the fundamental structure. Three of the proposals are described here. None of them have been brought into practice yet owing to difficulties.
[Prior Art 1: PD Riding on the Fiber End (
FIG. 1
to FIG.
4
)]
{circle around (1+L )} German Patent Publication DE 35 43 558 C2 (Inventors: Hillerich Bernd, Rode Manfred, Filing date, Dec. 10, 1985)
{circle around (2+L )} B. Hillerich & A. Geyer, “SELF-ALIGNED FLAT-PACK FIBER-PHOTODIODE COUPLING”, Electronics Lett., vol.24, No. 15, 1988, p918-919
These documents reported a method of making a V-groove on a Si plate by anisotropic etching, placing a PD at an end of the V-groove, inserting an end of a fiber beneath the PD into the V-groove and fixing the fiber with an adhesive having a refractive index similar to the fiber. These proposals have a feature of placing the fiber end into a small hole formed by the V-groove and the PD chip. The novel structure is explained by
FIG. 1
to FIG.
4
.
FIG. 1
is a section of the PD module structure.
FIG. 2
is a plan view of a part of the V-groove made on a Si platform (substrate).
FIG. 3
is a plan view of the V-groove sustaining an end of a fiber.
FIG. 4
is a plan view of the Si substrate having the V-groove with the fiber end covered with a PD chip.
A V-groove
2
is made by coating a (100) Si single crystal substrate
1
with a resist, opening a [011] directing striped window by mask-based exposure and development, and etching the masked Si substrate by anisotropic etchant, which can reveal a (1-11) plane and a (11-1) plane on the Si by the difference of the etching rates. In addition to side walls of (11-1) plane and (1-11) plane, a (111) plane is formed at an end of the V-groove
2
. The (111) plane is assigned to a slanting mirror surface
4
. A PD chip
5
is mounted above an end of a fiber
3
upon the (100) surface. An electrode i

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