Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal
Reexamination Certificate
2001-11-16
2003-06-17
Zarabian, Amir (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
C438S007000, C438S016000, C438S048000, C438S057000, C438S065000
Reexamination Certificate
active
06579739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmitting and receiving device incorporating a light emitting device (LED) and a light receiving device into a substrate.
2. Description of the Related Art
Lately, although communication networks using optical fibers are being put into practice, so far only trunk route communication networks have been formed using optical fibers, and channels laid from trunk route networks to subscribers, that is, subscriber channels, still remain as electrical circuits. To further spread the use of optical communication networks, it is desired that all the subscriber channels also consist of optical channels. To meet this expectation a variety of research and development is actively being carried out.
The cost reduction of optical devices including a WDM filter constitutes a very important factor in the construction of an optical subscriber system using a wavelength multiplex technology called an “ATM-PON (Asynchronous Transfer Mode-Passive Optical Network). To reduce the cost, the mounting of a small number of compact optical devices is indispensable, and a device form in which an LED, a light receiving device and a WDM filter are hybrid-mounted on a substrate is expected to be developed. Furthermore, since the transmitting unit and receiving unit of a optical module operate asychronously in the ATM-PON system, it is necessary that the crosstalk between transmission and reception in the module is sufficiently small.
FIG. 1
explains the configuration of a conventional optical transmitting and receiving device.
As shown in
FIG. 1
, in the conventional optical device an LED
1004
, a light receiving device
1005
and a WDM filter
1002
are encapsulated as discrete devices, and the devices are connected to an optical network and with each other using optical fibers
1001
and
1003
, respectively. In the optical module with this configuration the CAN packages of the LED
1004
and the light receiving device
1005
are utilized as electrostatic shielding to suppress the crosstalk between transmission and reception signals.
Recently, although a miniature optical transmitting and receiving device hybrid-mounting an LED, a light receiving device and a WDM filter on a waveguide substrate is being developed, the application of this optical device is limited to a TCM (Time-Compression Multiplexing) transmission system for time-dividing transmitting time and receiving time. This is because the crosstalk from the transmitting unit to the receiving unit is difficult to suppress. In the transmitting unit, several tens of milliamperes of current are required to drive the LED, whereas the receiving unit requires only very little current, in the order of a microampere or less. For this reason, the current in the receiving unit is required to be in the order of 10 to 100 nA because of the crosstalk from the transmitting unit.
FIG. 2
explains how crosstalk is generated between an LED and a light receiving device in a hybrid-mounted optical transmitting and receiving device.
To simplify the description, only the minimum necessary component elements are shown in the diagram.
In an optical transmitting and receiving device hybrid-mounting an LED (laser diode: LD)
1100
and a light receiving device (photodiode: PD)
1101
, a silicon dioxide film (SiO
2
)
1104
is formed on a silicon (Si) substrate
1106
, and on the silicon dioxide film electrodes
1102
and
1103
are formed. Then, the electrodes
1102
and
1103
are connected to the LD
1100
and PD
1101
, respectively. A metallic film (not shown in the diagram) is provided to ground the back of the substrate
1106
. Although the silicon dioxide film
1104
is provided so that current may not flow in the electrode
1103
of the PD
1101
due to the voltage generated by the electrode
1102
, current leaks to the substrate
1106
by the effect of alternating voltage applied to LD
1100
since the insulation function of the silicon dioxide film
1104
is not complete and the silicon dioxide film
1104
itself has its own capacitance. At this moment, although much of the current flows out of the substrate
1106
since the back of the substrate
1106
is grounded, part of the current reaches the electrode
1103
through the inside of the substrate
1106
. Although the current reaching the electrode
1103
is small, significant noise appears on the signal generated by the PD
1101
due to the current reaching the electrode
1103
through the substrate
1106
, since there is a significant difference between the current for driving the LD
1100
and the current generated by the PD
1101
, as described before. Accordingly, the performance of the PD
1101
in detecting optical signals from the received light beans becomes lower because of this generated current.
In this way, as a result of the conventional configuration, significant crosstalk is generated between the transmitting side and receiving side through the substrate
1106
.
The silicon dioxide film
1105
is a heat-oxidized film generated during the substrate processing, and if the back of the substrate
1106
is left unprocessed, the thickness of this silicon dioxide film
1105
will grow to approximately 2 &mgr;m.
To realize a miniature optical device for an ATM-PON, it is necessary to hybrid-mount an LED, a light receiving device and a WDM filter on the same substrate, and to reduce the crosstalk between the transmitting side and the receiving side as described before. The crosstalk between the transmitting side and the receiving side is roughly classified into two groups; crosstalk due to an optical cause such as stray light, etc., and crosstalk due to an electrical cause such as free capacitance, etc.
As described before, the electrical cause is generated by a current flowing between the transmitting side and receiving side through the substrate, and this is a serious problem.
The stray light, etc. is generated by light beams emitted from the LD leaking out from an optical waveguide and generating a mode spreading over all the substrate. Accordingly, when the PD receives such stray light, it becomes impossible to accurately receive optical signals.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a miniature optical device in which transmission and reception can be simultaneously operated, by reducing the crosstalk between a hybrid-mounted LED and light receiving device.
The optical transmitting and receiving device of the present invention comprises a conduction layer formed on all or a part of the surface of a substrate, an insulation layer formed at least at the bottom of an LED mounting portion and a light receiving device mounting portion, an optical waveguide formed on the surface of the insulation layer, electric wiring patterns formed on the surface of the insulation layer, and an LED and a light receiving device connected to the electric wiring patterns so as to be optically coupled with the optical waveguide. The optical transmitting and receiving device is characterized in that the above-mentioned conduction layer is made electrically connectable to a constant potential portion.
The manufacturing method of the optical transmitting and receiving device of the present invention comprises the steps of forming a conduction layer by doping an impurity on the surface of the substrate, laminating an insulation layer on the surface of the conduction layer, providing an optical waveguide on the insulation layer and mounting an LED and a light receiving device.
The optical transmitting and receiving device in another aspect of the present invention is characterized in that in an optical transmitting and receiving device hybrid-mounting at least an LED and a light receiving device on the same substrate through the insulation layer, a conduction layer is located at least at the bottom of the above-mentioned LED and the above-mentioned light receiving device, and between the above-mentioned substrate and the above-mentioned insulation layer, and the conduction layer can be el
Tabuchi Haruhiko
Tanaka Kazuhiro
Terada Koji
Fujitsu Limited
Soward Ida M.
Staas & Halsey , LLP
Zarabian Amir
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