Semiconductor device manufacturing: process – Chemical etching – Combined with coating step
Reexamination Certificate
2001-04-27
2002-10-15
Utech, Benjamin L. (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Combined with coating step
C438S699000, C438S700000
Reexamination Certificate
active
06465355
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of fabricating a suspended microstructure. More specifically, the present invention relates to a method of fabricating a suspended microstructure that eliminates the need to form through holes in the suspended microstructure that reduce a useful surface area of the suspended microstructure.
BACKGROUND ART
Suspended platforms are useful in applications where the advantages of microelectronic fabrication techniques can be used to form microstructures such as accelerometers, pressure sensors, actuators, fluidic devices, biochemical devices, and miniature machines. Micro-Electro-Mechanical Systems (MEMS) are one example of a system that can incorporate a suspended platform.
For instance, MEMS can integrate micromechanical elements and electronic elements on a common substrate material such as a silicon wafer. Microelectronic fabrication techniques can be used to fabricate the electronic elements such as CMOS circuits, for example. On the other hand, the micromechanical elements can be fabricated using micromachining techniques that deposit layers of materials to form mechanical and electromechanical devices or that selectively etch one or more layers of material such as a layer of silicon or a layer of silicon oxide to form mechanical and electromechanical devices.
A prior method for fabricating a suspended structure from single crystal silicon (Si) is illustrated in
FIGS. 1
a
through
1
d
.
FIG. 1
a
illustrates a prior structure
100
including an upper wafer
108
having a platform
102
that is suspended by flexures
104
. The platform
102
includes several holes
114
that extend all the way through the platform
102
. That is, the holes
114
perforate the platform
102
.
FIGS. 1
b
through
1
d
are a cross-sectional view taken along line AA of
FIG. 1
a
that illustrate a process for fabricating the prior structure
100
. The process begins with a bonded silicon-on-insulator wafer
106
that includes the upper wafer
108
that is chemically bonded to a lower wafer
110
by a thin silicon oxide (SiO
2
) layer
112
(i.e. a layer of dielectric material). The process for forming the bonded silicon-on-insulator wafer
106
are well understood in the microelectronics art.
Next, a top surface
124
of the upper wafer
108
is patterned using conventional photolithography techniques and then the upper wafer
108
is etched to form trenches
103
that define the flexures
104
and the platform
102
as well as a regularly spaced array of the holes
114
as illustrated in
FIG. 1
c
. The trenches
103
extend through the upper wafer
108
.
The holes
114
are through holes (i.e. they extend all the way trough the platform
102
) and are required to allow the silicon oxide (SiO
2
) layer
112
to be removed from beneath the platform
108
in a subsequent etching step.
In
FIG. 1
d
, the bonded silicon-on-insulator wafer
106
is exposed to a selective etch material such as hydrofluoric acid (HF). The etch material flows through the holes
114
and dissolves the silicon oxide layer
112
that is beneath the platform
102
thereby freeing the platform
102
from the silicon oxide layer
112
. The holes
114
are required in order to reduce the distance an etch front of the etch material must travel to free the platform
102
. The silicon oxide layer
112
surrounding the platform
112
is undercut by a distance
122
that is approximately equal to one-half a hole-to-hole spacing
120
(i.e. the space between holes
114
, see
FIG. 1
a
).
A major disadvantage to the prior structure
100
is that the holes
114
reduce the surface area available on the platform
102
. For instance, in ultrahigh density data storage applications, the platform
102
may include one or more layers of a storage medium that stores data as an alterable state of the storage medium. Optical or electron emission means (i.e. a laser or an electron beam) can be used to read and/or write data to the storage medium. It is undesirable to have the platform
102
perforated with the holes
114
because the holes
114
reduce the surface area of the platform
102
available for the storage medium. Moreover, an addressing scheme for reading or writing data to the storage medium must take into account the locations of the holes
114
to prevent reading or writing to an area in which the storage medium is non-existent. Because the holes
114
only serve to facilitate the removal of the silicon oxide layer
112
from beneath the platform
102
, they are a non-functional feature of the platform
102
. Accordingly, it is desirable to eliminate the holes
114
as they serve no useful purpose once the platform
102
has been formed.
Another prior method for fabricating a suspended structure from single crystal silicon (Si) is illustrated in
FIGS. 2
a
through
2
c
.
FIG. 2
a
illustrates a prior structure
200
including an upper wafer
208
having a platform
202
that is suspended by flexures
204
.
FIGS. 2
b
through
2
c
are a cross-sectional view taken along line AA of
FIG. 2
a
that illustrate a process for fabricating the prior structure
200
. The process begins with a bonded silicon-on-insulator wafer
206
that includes the upper wafer
208
that is chemically bonded to a lower wafer
210
by a thin silicon oxide (SiO
2
) layer
212
. In
FIG. 2
b
, prior to bonding the upper wafer
208
to the lower wafer
210
, the thin silicon oxide layer
212
is patterned and then etched to form a well. The well becomes a sealed cavity
216
after the upper and lower wafers (
206
,
208
) are bonded to each other.
Next, in
FIG. 2
c
, a top surface
220
of the upper wafer
208
is patterned and then etched to form trenches
203
that define the flexures
204
and the platform
202
. The trenches
203
extend through the upper wafer
208
to the sealed cavity
216
. As a result of the etching, an upper surface
222
of the lower wafer
210
is exposed to the etching material and is subsequently etched to form shallow pits
218
that extend inward of the upper surface
222
as illustrated in
FIG. 2
c.
Consequently, one disadvantage of the method for fabricating the prior structure
200
is that the fabrication results in damage to the upper surface
222
of the lower wafer
210
. In some applications the lower wafer
210
may contain buried components such as electrodes, interconnect structures, circuitry, or some other element that is essential to the functioning of the structure
200
. Therefore, it is desirable to protect those components during the fabrication process. Conversely, the method for fabricating the prior structure
200
can result in damaging those components because the upper surface
222
is not protected from the etch material during the fabrication process.
Accordingly, there exists a need for a method for fabricating a suspended microstructure that does not require etch holes to remove a layer of material from beneath the suspended microstructure.
There is also a need for a method of fabricating suspended microstructures that protects (i.e. does not damage) an upper surface of a lower wafer from etch materials during the fabrication process so that components that are buried in the lower wafer are not damaged by the etch materials.
SUMMARY OF THE INVENTION
A method of fabricating a suspended platform of the present invention solves the aforementioned needs. The method of fabricating a suspended platform according to the present invention does not require etch holes in the suspended platform to remove a layer of material beneath the suspended platform. As a result, the suspended platform is non-perforate and substantially all of the surface area of the suspended platform is available for use.
Moreover, the method of fabricating a suspended platform according to the present invention prevents pitting of an upper surface of a lower wafer by covering the upper surface with a thin layer of material that protects the upper surface during etching.
Broadly, the present invention is embodied in a method for fabricating a suspended pl
Chen Kin-Chan
Denny III Trueman H.
Hewlett--Packard Company
Utech Benjamin L.
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