Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2001-09-28
2004-03-30
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S317000, C430S318000, C257S522000, C257S644000, C257S752000, C438S619000, C438S624000, C438S626000, C438S785000, C438S787000
Reexamination Certificate
active
06713235
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for fabricating a thin-film substrate, and a thin-film substrate fabricated by the method, and more particularly, to a method for fabricating a thin-film substrate for use in the fabrication of a microstructure, and so forth, applicable to a semiconductor device, a liquid crystal light modulator, or a MEMS (microelctromechanical system), and a structure of a thin-film substrate fabricated by the method described.
BACKGROUND TECHNOLOGY
A silicon wafer, glass substrate, or quartz substrate has so far been in widespread use as a substrate for use in fabrication of a thin-film transistor, other thin-film devices, or a MEMS. It is more often than not that these substrates are formed to a thickness in a range of several hundred &mgr;m to several mm.
These substrates which are further ground are used as an extra-thin substrate at times, and for example, a silicon wafer on the order of 10 &mgr;m in thickness can be obtained from Virginia Semiconductor Inc. (1501 Powhaten Street, Fredericksburg, Va. 22401, USA), and so forth.
Further, there are cases where a film of a polymer such as polyimide, formed on a glass substrate by the spin cast method, is used as a thin-film substrate several&mgr;m or less in thickness.
Furthermore, there are also times when a thin-film of a metal oxide and so forth, formed directly on a silicon wafer or a glass substrate by the sputtering method or the chemical vapor deposition (CVD) method, is used as a thin-film substrate.
There has been adopted another approach wherein supports are provided on a support base, and on top of the supports, a thin-film serving as a functional structure or as a substrate of the functional structure is formed.
As a conventional technique of constituting a structure so as to be spaced away from the support base, there is available a method for fabricating a movable mirror of a spatial light modulator as disclosed in U.S. Pat. No. 4,956,619.
The conventional method for fabricating the structure is described hereinafter with reference to plan views of
FIGS. 44
to
47
, and the steps of the method for fabricating the structure are described with reference to cross-sectional views of
FIGS. 48
to
51
, respectively taken on line
48
—
48
in FIG.
44
.
As shown in
FIG. 48
, a spacer layer
203
a portion of which is to serve as a sacrifice layer is first applied to the top of a substrate
201
by the spin coater method, and a metal layer
205
is formed on top of the spacer layer
203
. The metal layer
205
is formed of an aluminum alloy with copper added thereto in order to enable it to function as a reflecting film.
Further, the metal layer
205
is patterned by photolithography in order to form a flap
101
which is to become a pixel of a light modulator from the metal layer
205
. Then, as shown in
FIGS. 44 and 48
, a plasma etch access gap
105
, and plasma etch access holes
103
for forming the flap
101
are provided in the metal layer
205
.
Subsequently, a cavity region
107
shown in
FIG. 49
is formed by plasma etching based mainly on oxygen.
FIG. 45
is a plan view of the flap with the cavity region
107
formed therein.
By introducing active species through the plasma etch access holes
103
and the plasma etch access gap
105
, positive photoresist of the spacer layer
203
is isotropically etched, thereby forming the cavity region
107
.
As etching proceeds further, the cavity region
107
expands as shown in
FIGS. 46 and 50
. Thereafter, as shown in
FIGS. 47 and 51
, etching is continued until the formation of a structure wherein the flap
101
is floatingly spaced away from the substrate
201
to be retained by the spacer layer
203
. As a result of such processing as described above, the structure can be formed wherein the flap
101
is floatingly spaced away from the substrate
201
.
As another method for implementing a similar structure for a light modulator, there is an example as disclosed in U.S. Pat. No. 4,592,628.
With this method, silicon (Si) is used for a spacer layer which is to serve as a sacrifice layer, and silica (SiO
2
) obtained by oxidizing silicon is used for a thin-film structure, removing portions of the spacer layer by the wet etching method wherein an etchant capable of selectively etching silicon only such as, for example, pyrocatechol ethylenediamine is introduced through openings.
Further, as another method wherein supports are provided on a support base, a sacrifice layer is made use of in order to form a thin-film serving as a functional structure or a substrate of the functional structure on top of the supports, and the sacrifice layer is removed by etching through openings, there is disclosed a method for removing the sacrifice layer by a vapor phase etching method in “JP, 10-107339, A”.
However, the conventional silicon wafer, glass substrate, or quartz substrate for use in the fabrication of the thin-film transistor, other thin-film devices or MEMS has often been on the order of several hundred &mgr;m to several mm in thickness.
Accordingly, in the case of using these substrates, for example, when applying heat treatment to a device or structure formed thereon, there has arisen a problem in that heat treatment can not be effectively applied thereto because an object being for heat treated on these substrates is often made up of a thin film of several &mgr;m or less in thickness, and heat is dissipated from the object for being heat treated to these substrates although heat capacity of the former is very small. Further, in the case of forming a thin metal film for forming wiring, another problem has been encountered in that the thin metal film needs to be insulated and retained by a dielectric film having dielectric constant at least several times greater than that of air or vacuum, and consequently, a large floating capacity is added to wiring in a high-frequency circuit.
The problems described above also occur to a substrate wherein an inorganic or organic thin film is formed directly on the silicon wafer, glass substrate, or so forth.
Further, as a particularly thin substrate, there is available a silicon wafer, for example, on the order of 10 &mgr;m in thickness, obtained by further grinding these substrates, but the silicon wafer is weak in mechanical strength, requiring special care in handling and transportation, and it is physically difficult to render the same further thinner, which limits a range of application thereof.
Accordingly, as a method for making the most of the function of a thin film, it is conceivable to make use of a process wherein supports are provided on a support base, and on top of the supports a thin-film serving as a functional structure or as a substrate of the functional structure is formed as practiced in a method for fabricating the MEMS having a space structure such as, for example, a DMD (digital micromirror device).
With the process proposed so far, however, there has been adopted a method wherein a thin film is formed on top of a spacer layer which is to serve as a sacrifice layer, and the sacrifice layer is removed by plasma etching, wet etching, or gas phase etching through openings provided in the thin film.
In such a case, since etching of the sacrifice layer proceeds mainly only in the traverse direction through the openings, a multitude of the openings are required. An area of the sacrifice layer, normally removable by one of the openings, is practically on the order of several thousand &mgr;m
2
owing to a low etching rate in the traverse direction.
There have recently been seen tendencies for a semiconductor integrated circuit (IC), a circuit board, and further, a MEMS, and so forth, formed on a silicon wafer, glass substrate, or so forth, to increase in scale and size, and to become higher in density, and consequently, a thin film having a relatively large area is often used as a substrate. However, a thin film provided with a multitude of openings will become unsuitable for such application.
It is therefore an object of the invention to enable a thin film substrate ha
Ide Masafumi
Sameshima Toshiyuki
Chacko-Davis Daborah
Citizen Watch Co. Ltd.
Huff Mark F.
Westerman Hattori Daniels & Adrian LLP
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