Optical modulator and photonic semiconductor device

Details Optical modulator and photonic semiconductor device Optical modulator and photonic semiconductor device

2001-03-27

2004-08-17

Glick, Edward J. (Department: 2882)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S014000, C385S002000

Reexamination Certificate

active

06778751

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator, a method for fabricating that optical modulator, and a photonic semiconductor device. More particularly, the invention relates to an optical modulator used in optical communication, a method for fabricating that optical modulator, and a photonic semiconductor device that combines such optical modulators.
2. Description of the Related Art
In order to promote widespread use of public communication networks with optical fibers, it is important to boost the performance of semiconductor laser devices and enhance their yield for less costly fabrication. In particular, improving semiconductor laser performance necessarily involves modulating laser emissions at higher speed so as to deal with growing quantities of information. Optical communication over long distances is implemented by minimizing wave fluctuations during high-speed laser modulation, whereas conventional setups having an injected current varied in single-mode semiconductor laser for direct modulation tend to suffer a pronounced wavelength chirping caused by fluctuating densities of injected carriers. For that reason, the direct modulation scheme cannot be used in long-distance high-speed modulated data transmissions at 10 Gbps or higher.
For 10-Gbps optical data transmission systems, the direction modulation scheme is typically replaced by an external modulation setup. External modulation involves keeping a semiconductor laser device oscillated at a constant level and having the emitted laser pass through optical modulators capable of turning on and off light transmission with a minimum of wavelength chirping in order to achieve light modulation.
Optical modulators used by the external modulation method are called electro-absorption modulators, abbreviated to EAMs hereunder. EAMs having a single optical absorption layer absorb light through the use of the Franz-Keldysh effect, and EAMs with a multiple quantum-well structure absorb light through absorption spectrum variations based on the Stark effect. Laser absorbency of an optical modulator varies depending on a backward bias voltage applied to the modulator in question. For that reason, if a modulation signal voltage is applied to a high-frequency electrical circuit connected to an optical modulator, a laser beam that is modulated in intensity reflecting the signal voltage is emitted from an emission face of the optical modulator.
In the field of high-speed communications at 20 Gbps or higher of the next generation, ultra-high-speed semiconductor optical modulators are drawing attention because they have a low-chirping characteristic, are small in size, and operate on low voltages. Implementing such ultra-high-speed semiconductor optical modulators faces an important challenge: how to minimize the capacity of optical modulator elements.
FIG. 25
is a perspective view of a conventional optical modulator, and
FIG. 26
is a cross-sectional view taken on line XXVI—XXVI across the optical modulator in FIG.
25
. In
FIGS. 25 and 26
, reference numeral
200
stands for an optical modulator;
202
for an n-type InP substrate (n-type conductivity is denoted by a symbol “n-” and p-type conductivity by “p-” hereunder);
204
for an n-InP clad layer;
206
for an optical absorption layer;
208
for a p-InP clad layer;
210
for a p-InGaAs contact layer;
212
for a surface protective film such as an SiO2 film;
214
for a polyimide layer;
216
for a p-type ohmic electrode;
216
a
for a bonding pad; and
218
for an n-type ohmic electrode.
A method for fabricating conventional optical modulators is outlined below.
FIGS. 27
,
28
and
29
are cross-sectional views of an optical modulator fabricated in sequence. On the n-InP substrate
202
, the n-InP clad layer
204
, the optical absorption layer
206
, p-InP clad layer
208
, and p-InGaAs contact layer
210
are first formed by epitaxial growth. An insulating film such as an SiO2 film is then formed over the surface on which is provided a stripe-shaped mask pattern
220
measuring 2 to 3 microns (&mgr;m) wide (see FIG.
27
).
With the mask pattern
220
used as a mask, dry etching is carried out to a depth beyond the optical absorption layer
206
, illustratively 2 to 3 microns deep, so as to form a ridge
222
(see FIG.
28
). The surface protective film
212
such as an SiO2 film is formed next. Polyimide
214
is applied over the film to flatten the surface. An opening
224
is formed on top of the ridge
222
for ohmic contact (see FIG.
29
).
The p-type ohmic electrode
216
and n-type ohmic electrode
218
are then formed, which completes the optical modulator shown in
FIGS. 25 and 26
. The element capacity of the optical modulator
200
thus fabricated is given as a sum of the capacity of the optical absorption layer
206
and the capacity of the bonding pad
216
a
. Because the capacity of the optical absorption layer
206
is determined by the performance of modulator elements complying with the dynamic range and extinction characteristic of the optical modulator
200
, the element capacity can only be reduced to a certain extent.
When the area for accommodating bonding wires is taken into consideration, the bonding pad
216
a
may be reduced in area to about 50 &mgr;m×50 &mgr;m at most; further reduction of the pad area is difficult to achieve. For that reason, the bonding pad
216
a
is formed on the surface of the insulating polyimide
214
in order to minimize the capacity of the bonding pad
216
a.
However, optical modulators designed to execute modulation at speeds as high as 40 Gbps or more are required to have an element capacitance of 0.1 pf or less. In the conventional optical modulator structure, the element capacitance is reduced using a thicker polymide layer
214
. This has posed a problem: the polymide layer
214
is difficult to form.
Japanese Patent Laid-open No. Hei 3-263388 discloses an optical modulator related to this invention. The disclosed optical modulator has a mesa stripe of a semiconductor multi-layer structure containing active layers, the mesa stripe being flanked by InP high-resistance layers. This optical modulator has its element capacity reduced by use of an air-bridge structure that connects the top of the mesa stripe with a bonding pad on a high-resistance semiconductor substrate. The disclosed optical modulator has a ridge structure different from that of the optical modulator of the invention, to be described below.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above circumstances, and a first object of the invention is to provide an optical modulator offering excellent high-frequency performance while being lowered in element capacity.
According to one aspect of the invention, there is provided an optical modulator comprising: a semi-insulating semiconductor substrate with a principal plane partially including an exposed surface; an optical waveguide ridge which is disposed on said semiconductor substrate and which includes a first clad layer of a first conductivity type, an optical-absorption layer, and a second clad layer of a second conductivity type, said optical waveguide ridge further having a side with a flat portion extending uniformly from a top of the ridge to said semiconductor substrate, the flat portion being in contact with the exposed surface of said semiconductor substrate; a dielectric film which covers said optical waveguide ridge and said semiconductor substrate and which has a first opening made at the top of said optical waveguide ridge and a second opening made in a region of said semiconductor substrate other than the exposed surface; a first electrode disposed on said dielectric film and mounted through said first opening on the top of said optical waveguide ridge, said first electrode further extending on the flat portion of said optical waveguide ridge while in close contact with a surface of said dielectric film, said first electrode further having one end thereof established on said semiconductor substra

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