Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal
Reexamination Certificate
2003-05-08
2004-06-01
Tsai, H. Jey (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
C438S053000
Reexamination Certificate
active
06743653
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a micromachine such as an optical switching element used for communication, measurement, display, and the like which utilize switching.
There is a MEMS element which is fabricated by a micromachine technique of performing three-dimensional micropatterning by etching based on thin film formation or photolithography. Of MEMS elements serving as micromachines, one is comprised of a fine fixed structure and movable structure, and controls the operation of the movable structure by an electrical signal. Such MEMS element is an optical switching element whose movable structure has a reflecting surface (reference 1: Japanese Patent Laid-Open No. 2001-198897, reference 2: Japanese Patent Laid-Open No. 2002-189178, reference 3: Japanese Patent Laid-Open No. 11-119123, reference 4: “MEMS Micro Technology, Mega Impact” Circuit & Device, pp. 14-25 (2001), reference 5: Renshi Sawada, Eiji Higurashi, Akira Shimizu, and Tohru Maruno, “Single Crystalline Mirror Actuated Electrostatically by Terraced Electrodes With High-Aspect Ratio Torsion Spring,” Optical MEMS 2001, pp. 23-24 (Okinawa Japan), 2001, and reference 6: Renshi Sawada, Johji Yamaguchi, Eiji Higurashi, Akira Shimizu, Tsuyoshi Yamamoto, Nobuyuki Takeuchi, and Yuji Uenishi, “Single Si Crystal 1024ch MEMS Mirror Based on Terraced Electrodes and a High-Aspect Ratio Torsion Spring for 3-D Cross-Connect Switch,” Optical MEMS 2002, pp. 11-12 (Lugano Switzerland), 2002).
The optical switching device is comprised of, e.g., a fixed structure and movable reflecting structure. The reflecting structure has a support member and movable member, and the movable member is coupled to the support member by a spring member such as a torsion spring. The optical switch with this arrangement performs switching operation of switching the optical path by moving the reflecting structure by the attractive force or repulsive force between the fixed structure and the movable reflecting structure.
As an optical switching element manufacturing method, a method using an SOI (Silicon On Insulator) substrate is proposed. A process of fabricating a mirror (movable portion) by this method will be explained. As shown in
FIG. 4A
, grooves
401
a
are formed by known photolithography and etching such as DEEP RIE on a side (major surface: SOI layer) of an SOI substrate
401
on which a buried oxide
402
is formed, thereby forming a mirror
404
from a single-crystal silicon layer
403
on the buried oxide
402
.
In DEEP RIE, for example, SF
6
gas and C
4
F
8
gas are alternately introduced in dry-etching silicon. Etching and formation of a side wall protective film are repeated to form a groove or hole with an aspect ratio of 50 at an etching rate of several &mgr;m/min.
A resist pattern which is open In the formation region of the mirror
404
is formed on the lower surface of the SOI substrate
401
. Silicon is selectively etched from the lower surface of the SOI substrate
401
by using an etching solution such as an aqueous solution of potassium hydroxide. In etching, the buried oxide
402
is used as an etching stopper layer. As shown in
FIG. 4B
, an opening
401
b
is formed at a portion of the lower surface of the SOI substrate
401
that corresponds to the formation region of the mirror
404
. The opening
401
b
is a region corresponding to the pixel of the optical switching element.
The region where the buried oxide
402
is exposed through the opening
401
b
is selectively removed with hydrofluoric acid, forming the pivotal mirror
404
supported by the SOI substrate
401
, as shown in FIG.
4
C. To increase the reflectance of the mirror
404
, a metal film of gold or the like may be formed on the surface of the mirror
404
on the opening
401
b
side.
A silicon substrate
411
is selectively etched with an aqueous solution of potassium hydroxide by using as a mask a predetermined mask pattern formed from a silicon nitride film or silicon oxide film, thus forming a recessed structure, as shown in
FIG. 4D. A
metal film is formed on the recessed structure by vapor deposition or the like. The metal film is patterned by photolithography and etching using known ultra-deep exposure, thereby forming an electrode
412
including a mirror driving electrode interconnection and the like, as shown in FIG.
4
E.
After that, the SOI substrate
401
and silicon substrate
411
are diced into chips, thus forming a mirror chip and electrode chip. The mirror chip and electrode chip are adhered into an optical switching element in which the mirror
404
can be moved by applying an electric field, as shown in FIG.
4
F. After each chip is diced, a metal film of gold or the like may be formed on the mirror surface in order to increase the mirror reflectance.
In a step after etching a buried oxide according to the conventional manufacturing method, the mirror portion is coupled by a pair of coupling members so as to be pivotal on a pivot shaft which extends through the coupling members. The coupling members are bar- or plate-like spring members such as torsion bar springs which elastically deform upon application of torsion.
For example, while being coupled by torsion bar springs, the mirror undergoes a wafer dry step after etching a buried oxide with a buffered hydrofluoric acid solution and cleaning the buried oxide with water, a wafer dicing step, a step of forming a metal film on a diced mirror surface, a step of adhering a mirror chip to a substrate bearing a mirror driving electrode interconnection, a step of bonding a die to a package, a wire bonding step, a potting step, and the like.
The optical switching element applies an attractive force to the mirror by an electric field generated by a voltage applied to the mirror driving electrode, and pivots the mirror through an angle of several degrees. For reduction in power consumption and the like, the mirror must be pivoted by applying a voltage of about 100 V to the mirror driving electrode. Thus, the coupling member is processed into a width of about 2 &mgr;m so as to easily pivot the mirror.
Since the SOI layer is about 10 &mgr;m thick, the coupling member is about 2 &mgr;m wide and 10 &mgr;m thick. For example, as shown in
FIG. 5
, a circular mirror
501
having a diameter of about 500 &mgr;m is coupled to a surrounding concentric mirror frame
502
via thin coupling members
511
having a width of about 2 &mgr;m. The mirror frame
502
is coupled to an SOI layer
503
via coupling members
512
.
In the above-mentioned steps, a water flow, a centrifugal force in drying a wafer, vibrations, or shocks are applied. This readily damages a coupling member or mirror, decreasing the manufacturing yield of the mirror substrate. Especially when even one mirror becomes defective on a mirror substrate on which many mirrors are arrayed in a matrix, the mirror substrate becomes a defective and cannot be used, resulting in a lower yield.
When a mirror substrate wafer is transported as a wafer or diced chip after the manufacture, the wafer or chip itself is protected by a vessel which stores it. However, a mirror and mirror frame which are coupled by thin coupling members are movable and vulnerable to the centrifugal force, vibrations, and shocks. The manufacturing yield of the mirror substrate may further decrease.
The manufacture of an optical switching element mirror substrate is completed when a mirror surface which reflects incident light is exposed. In dicing into a chip, small wafer shaving powder is attached to the mirror surface via the gap of the coupling member or the like. Dust is attached in safekeeping or handling till packaging, decreasing the optical reflectance.
SUMMARY OF THE INVENTION
It is a principal object of the present invention to manufacture a micromachine having a movable portion such as a mirror at a high nondefective yield.
To achieve the above object, according to one aspect of the present invention, there is provided a micromachine manufacturing method comprising at least the step of preparing a silicon substrate
Ishii Hiromu
Machida Katsuyuki
Tanabe Yasuyuki
Yagi Shouji
Blakely & Sokoloff, Taylor & Zafman
Nippon Telegraph and Telephone Corporation
Tsai H. Jey
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