Dissolved wafer fabrication process and associated...

Details Dissolved wafer fabrication process and associated... Dissolved wafer fabrication process and associated...

2000-08-24

2003-10-28

Flynn, Nathan J. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Physical deformation

C257S414000

Reexamination Certificate

active

06639289

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a microelectromechanical device and method for making the device. In particular, the invention relates to a microelectromechanical device having a support substrate that includes mesas for supporting the doped region of a partially sacrificial substrate which defines various mechanical and/or electro-mechanical members of the microelectromechanical device and a method for making the device.
BACKGROUND OF THE INVENTION
Traditionally, the miniaturization of mechanical and/or electromechanical systems has been frustrated by limitations on the manufacture of small lightweight mechanical or electromechanical parts. The intricacy of the parts made their manufacture on a small scale difficult and impractical. For instance, until recently, heavy and large gimbal systems were used for navigational guidance systems in the aerospace industry. These systems contained mechanical parts that were formed of metal and were generally large and heavy. However, the intricacy of the mechanical parts made miniaturization of the navigational guidance system difficult.
However, in recent years with the proliferation and increased precision in semiconductor fabrication procedures, many of the mechanical and electromechanical structures in a mechanical system may now be replaced by microelectromechanical structures (MEMS) that are fabricated by semiconductor fabrication techniques. For instance, some gimbal systems have been replaced by gyroscopes that include one or more MEMS devices. An example of these gyroscopes is described in U.S. Pat. No. 5,650,568 to Greiff et al., the contents of which are incorporated herein by reference.
The Greiff et al. '568 patent describes a gimballed vibrating wheel gyroscope for detecting rotational rates in inertial space. The typical gimbals of the traditional larger and heavier mechanical systems have been replaced by the lightweight, miniaturized MEMS devices. These MEMS devices that form the various mechanical and/or electromechanical parts of the gyroscope are fabricated with conventional semiconductor techniques. The electrical properties of the gyroscope are then used to provide power to these parts and to receive signals from the parts.
An important advantage in the use of MEMS devices for mechanical and electromechanical systems is the reduction of size and weight that can be achieved over the conventional mechanical systems that use metal parts. However, many mechanical and electromechanical systems, such as the gimballed systems described above, have many moving parts that must be accurately fabricated in order to operate properly with the requisite accuracy and precision. Thus, the ability to replace metallic parts with MEMS devices fabricated by semiconductor techniques is limited by the precision that can be achieved with the semiconductor fabrication techniques. Although current semiconductor fabrication techniques have been utilized to manufacture MEMS devices, these fabrication procedures still present several limitations as described below in conjunction with the Greiff et al. '568 patent.
In this regard,
FIGS. 1A-1D
illustrate a conventional method for manufacturing MEMS devices with conventional semiconductor fabrication techniques. The process illustrated in these figures is commonly known as a Dissolved Wafer Process (DWP) and is described in the Greiff et al. '568 patent.
In particular, with reference to
FIG. 1A
, a silicon substrate
10
and a support substrate
12
are shown. In a typical MEMS device, the silicon substrate is etched to form the mechanical and/or electromechanical members of the device. The mechanical and/or electromechanical members are generally supported above the support substrate such that the mechanical and/or electromechanical members have freedom of movement. This support substrate is typically made of an insulating material, such as Pyrex® glass.
As illustrated in
FIG. 1A
, support members
14
are initially etched from the inner surface of the silicon substrate. These support members are commonly known as mesas and are formed by etching, such as with potassium hydroxide (KOH), those portions of the inner surface of the silicon substrate that are exposed through an appropriately patterned layer of photoresist
16
. Preferably, the etching is continued until mesas
14
of a sufficient height have been formed.
With reference to
FIG. 1B
, the etched inner surface
18
of the silicon substrate is thereafter doped, such as with boron, to provide a doped region
20
of a predetermined depth such that the silicon substrate
10
has both a doped region
20
and an undoped sacrificial region
22
. Referring to
FIG. 1C
, trenches are then formed, such as by a reactive ion etching (RIE), that extend through the doped region
20
of the silicon substrate
10
. These trenches form the mechanical and/or electromechanical members of the MEMS device.
As shown in
FIGS. 1A-1C
, the support substrate
12
is also initially etched and metal electrodes
26
and conductive traces (not shown), are formed on the inner surface of the support substrate. These electrodes and conductive traces will subsequently provide electrical connections to the various mechanical and/or electromechanical members of the MEMS device.
Once the support substrate
12
is processed to form the electrodes and conductive traces, the silicon substrate
10
and the support substrate
12
are bonded together. With reference to
FIG. 1D
, the silicon and support substrates are bonded together at contact surfaces
28
on the mesas
14
, such as by an anodic bond. As a final step, the undoped sacrificial region
22
of the silicon substrate is etched away such that only the doped region that comprises the mechanical and/or electromechanical member of the resulting MEMS device remains. The mesas that extend outwardly from the silicon substrate therefore support the mechanical and/or electromechanical members above the support substrate such that the members have freedom of movement. Further, the electrodes formed by the support substrate provide an electrical connection to the mechanical and/or electromechanical members through the contact of the mesas with the electrodes.
As known to those skilled in the art, the step of reactive ion etching trenches through the doped region
20
of the silicon substrate
10
must be controlled to insure that the resulting trenches are accurately located on the inner surface of the partially sacrificial substrate and that the widths and walls of the trenches are formed with great precision. For example, the trenches formed by RIE must generally be formed to within a tolerance of 0.1 to 0.2 microns of a predetermined width. Further, it is important that the trenches extend completely through the doped region. As such, the inner surface of the silicon substrate must be planar and the doped region must have a predetermined thickness. While the silicon substrate
10
can be initially formed to have a planar inner surface, the inner surface of the silicon substrate is thereafter etched to form the mesas
14
. Unfortunately, an etching process will produce a surface that is no longer planar, but which, instead, has a number of surface irregularities. Accordingly, because the surface is non-planar, the subsequent RIE step cannot produce trenches that are accurately located on the inner surface doped region of the partially sacrificial substrate nor can the walls of the trenches be etched with great precision. Further, the subsequent RIE step may not produce trenches that extend completely through the doped region or may produce trenches that extend too far into the undoped sacrificial region of the silicon substrate.
Thus, a method is needed for manufacturing MEMS devices utilizing semiconductor fabrication techniques that separate the various mechanical and/or electromechanical members by means of RIE through the planar inner surface of a silicon substrate such that the trenches formed by RIE extend completely through the doped region of the sacrificial silicon substr

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