Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
2000-08-28
2003-08-05
Beck, Shrive P. (Department: 1763)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S024000, C216S051000, C216S057000, C216S087000, C216S099000
Reexamination Certificate
active
06602427
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is related to a wavelength division multiplexing (WDM) transmitters receiver module, more particularly, is related to a WDM transmitter/receiver module that utilizes a micromachined optical mechanical modulator as its transmitter.
Fiber optic communication, particularly fiber-to-the-home (FTTH), drives the demand for low cost, highly reliable transmitter/receiver modules which fulfill standards of successfully implemented technologies such as WDM. At least two types of WDM transmitter/receiver modules have been developed. One type needs an additional semiconductor laser resident at the home and used as the transmitter for the home user. The laser can be directly modulated up to several G bit/s, but is expensive, temperature-sensitive, and power hungry, driving up system cost and affecting performance. The other type is a ring-back architecture that features a single laser in the system, located at the central office, while the home terminus only requires a micromachined optical mechanical modulator. Data to the home is sent in the conventional way; while the data to the office is produced through modulating the light beam from the central office using the micromachined optical mechanical modulator and then sending it back.
For many applications the data to the office may be much slower than the data to the home, i.e., several M bit/s would be sufficient. Such a slow data can be achieved using micromachined optical mechanical modulators. In addition, the micromachined optical mechanical modulator presents the advantage of low cost and good optical performance including low insertion loss, high contrast and polarization insensitivity.
Research on machined optical modulators goes back several decades. These early mechanical optical modulators are of the phase shifting or scanning mirror type. In recent years micromachined optical mechanical modulators based on the interference effect of Fabry-Perot cavity have gained great attention. Microfabrication technologies allow an entire system of electronics integrated in a silicon chip, a few millimeters in size, which are mass-produced from wafers of single crystalline silicon. The same basic fabrication concept and materials, is now being adopted to make low cost, small size, high performance optical components and systems.
Several micromachined optical mechanical modulators have been reported. FIG.
1
A and
FIG. 1B
show a micromachined optical mechanical modulator consisting of a single crystalline silicon substrate
101
, an Aluminum supporting frame
102
, an air gap
103
, and a silicon nitride membrane
104
. The silicon nitride membrane
104
is defined as the area released from the single crystalline silicon substrate
101
and consists of the central plate
106
suspended by thin support beams
107
. An opening in the electrode material on the central plate
106
of the device defines an optical window
108
. To operate the device a voltage is applied to the device through a top electrode
105
and a bottom electrode
109
. When the voltage is zero, the gap
103
between the membrane
104
and the substrate
101
is m&lgr;/4 (m is odd number) and the reflectivity of the Fabry-Perot cavity comes to maximum. When a voltage is increased so that the membrane
104
is deformed downward to the substrate
101
and the gap
103
is (m−1)&lgr;/4, the reflectivity of the Fabry-Perot cavity comes to minimum. In this way an incident light beam can be reflected by the Fabry-Perot cavity and the intensity of the reflected light beam can be modulated through the voltage applied to the device.
The Aluminum/silicon nitride structure is convenient but does present some problems. When deposited thickness approaches 1 &mgr;m, the surface of the Aluminum layer can become quite rough and hillocks. Coupled with the surface of the Aluminum layer, the pinhole density of the silicon nitride becomes a greater issue. The existence of pinholes in the silicon nitride or Aluminum hillocks under the silicon nitride can lead to shorting of the electrodes and failure of the device. In addition, as with any micro-machining process, etch selectivity is a concern. This is especially true in forming the Fabry-Perot cavity, since any change in the dimension of the cavity would change the optical properties of the device. Unfortunately, it is impossible to control the lateral dimension of the cavity using an Aluminum layer as the sacrificial material.
In an alternate prior art design, as shown in
FIG. 2
, the Fabry-Perot cavity consists of a deformable mirror
205
, an air gap
206
, and a fixed mirror
203
formed on the surface of a single crystalline silicon substrate
201
coated with a silicon dioxide layer
202
. The fixed mirror
203
comprises a polysilicon layer formed by low-pressure chemical vapor deposition (LPCVD) and an anti-reflection layer
204
. The anti-reflection layer
204
consists of a wet silicon dioxide layer and a LPCVD silicon nitride layer. A 1.6 &mgr;m thick phosphorus-doped oxide (PSG) layer is deposited on the surface of the fixed mirror used as a sacrificial layer. A second polysilicon layer is deposited on the surface of the PSG layer. The air gap
206
is formed using selective etching of the PSG layer. A hole
207
with a slanted wall is formed on the backside of the substrate
201
. An optical fiber consisting of grading layer
209
and core
208
is inserted into the hole
207
so that the core
208
is aligned with a small hole under the fixed mirror
203
.
There are several problems with this design: (1) it is difficult to make a stress free material, or a low tensile stress material for the use of Fabry-Perot cavity. Actually there is a notable compressive stress existed in the polysilicon membrane that makes the membrane uneven and therefore dramatically affects the performance of the Fabry-Perot cavity; (2) the surface roughness of the polysilicon membrane is up to 140 Å due to the rough surface of the underlying thicker sacrificial PSG layer which results in a higher insertion loss; (3) the applied voltage is up to 70 V due to the thicker sacrificial PSG layer or a wider gap between the two mirrors; (4) the hole for receiving an optical fiber is not precisely aligned with the Fabry-Perot cavity and cannot be used for passive alignment between the Fabry-Perot cavity and the optical fiber; and (5) the Fabry-Perot cavity is out of the plane of the substrate which makes the device easy to break and the production yield difficulty to increase.
It is, therefore, an object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the Fabry-Perot cavity of which is formed by structuring a homo-junction layer stack instead of a hetero-junction layer stack to eliminate any intrinsic stress caused by the mismatch in the thermal conductivity and lattice parameters between the different layers.
Another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, in which the membrane and its supporting beams are finally released by dry etching instead of wet etching to eliminate any stiction caused by the liquid capillary forces occurring during the wet release process.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the sacrificial layer of which can be quite thin so that the air gap of the Fabry-Perot cavity formed by removing the sacrificial layer is narrower and therefore the needed operating voltage for changing the distance of the air gap is relatively low.
Still another object of the present invention is to provide a micromachined optical mechanical modulator based transmitter/receiver module, the sacrificial layer of which again can be quite thin, so that the unevenness of the membrane and its supporting beams, which is caused by the rough surface of the thicker sacrificial layer, is relatively low and therefore the insertion loss of the Fabry-Perot cavity i
Beck Shrive P.
Johnsonbaugh Bruce H.
Olsen Allan
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