Low temperature plasma Si or SiGe for MEMS applications

Details Low temperature plasma Si or SiGe for MEMS applications Low temperature plasma Si or SiGe for MEMS applications

2002-08-01

2004-08-03

Cuneo, Kamand (Department: 2829)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Reexamination Certificate

active

06770569

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to MEMS devices, and more particularly to low temperature methods for making MEMS devices out of silicon and silicon-germanium alloys.
BACKGROUND OF THE INVENTION
Advancements in micromachining and other microfabrication techniques and processes have enabled the fabrication of a wide variety of MicroElectroMechanical Systems (MEMS) and devices. These include moving rotors, gears, switches, accelerometers, miniaturized sensors, actuator systems, and other such structures.
One important application of microfabrication is in the fabrication of RF MEMS switches. Such devices have several advantages over their solid state counterparts. For example, RF MEMS switches provide lower insertion loss, higher isolation, better linearity, and lower power than solid state switches. RF MEMS devices are also useful in a variety of applications. Thus, they can be used as tunable preselectors and frequency synthesizers, and are also useful as components in a variety of telecommunications devices and systems, including signal routing devices, impedance matching networks, and adjustable gain amplifiers.
FIG.
1
and
FIG. 2
(the later of which is a side view of
FIG. 1
) depict a conventional RF MEMS switch
10
. The switch includes a cantilevered arm
20
which typically comprises an insulating material and which is attached to the substrate
12
by an anchor structure
14
. The anchor structure may be formed as a mesa on the substrate by deposition buildup or through the selective removal or etching away of surrounding material. A bottom electrode
16
, which is typically grounded, and a signal line
18
are also formed on the substrate. The bottom electrode and signal line typically comprise strips of a metal that is not easily oxidized, such as gold. A gap exists between the signal line and the bottom electrode.
The actuating part of the switch comprises the cantilevered arm
20
noted above. The cantilevered arm forms a suspended microbeam which is attached at one end to the top of the anchor structure and which extends over and above the bottom electrode and the signal line disposed on the substrate. An electrical contact
22
, which also typically comprises a metal such as gold that does not oxidize easily, is formed on the end of the cantilever arm that is removed from the anchor structure. The electrical contact is positioned on the bottom side of the cantilever arm so as to face the top of the substrate over and above the signal line.
A top electrode
24
, typically comprising a metal such as aluminum or gold, is formed atop the cantilever arm. The top electrode starts above the anchor structure and extends along the top of the cantilevered arm to end at a position above the bottom electrode. The cantilevered arm and top electrode are broadened above the bottom electrode (which is itself broadened) to form a capacitor structure
26
. The capacitor structure is provided with a grid of holes to reduce its mass.
In operation, the switch is normally in an “Off” position as shown in FIG.
2
. With the switch in the off-state, the signal line is an open circuit due to the gap between the electrical contact and the signal line. The switch is actuated to the “On” position by application of a voltage on the top electrode. With a voltage on the top electrode, electrostatic forces attract the capacitor structure (and cantilever arm) toward the bottom electrode. Actuation of the cantilevered arm toward the bottom electrode, as indicated by arrow
11
, causes the electrical contact to move against the signal line, thereby closing the gap and placing the signal line into the on-state (i.e., closing the circuit).
One problem encountered in devices of the type depicted in
FIGS. 1 and 2
relates to the mismatch in coefficients of thermal expansion (CTEs) between the materials used for certain components of the device. In particular, in the case of RF MEMS switches, a thermal mismatch typically exists between the top electrode (which, as noted above, is typically made out of a metal such as Au) and the cantilevered arm (which is usually made out of a material such as silicon oxynitride (SiON)). As a result, the movable portion of the switch tends to become permanently distorted during the thermal cycles that occur after release and during the packaging process, thus leading to changes in the operating characteristics of the switch and, in many cases, switch failure.
A variety of other materials have been used in MEMS fabrication processes, some of which have CTEs that more closely match the CTE of SiON. However, the use of many of these materials in the top electrode of an RF MEMS switch has been precluded by the processing considerations attendant to conventional fabrication methodologies. Thus, for example, silicon and silicon/germanium alloys have been used as structural elements in MEMS processes using LPCVD, and have a number of desirable properties. However, the maximum processing temperature for a typical RF MEMS switch is limited to about 350° C. (due primarily to the presence of the sacrificial layer, which is typically made out of a polyimide or a similar thermally sensitive material), which is well below the deposition temperatures of about 550° C. that are required for silicon or silicon-germanium alloys in an LPCVD or epitaxial process.
Processing temperature considerations have likewise precluded the use of materials such as silicon and silicon/germanium alloys in other MEMS applications, in spite of the desirable physical and electrical properties that these materials have. Such applications include, for example, the fabrication of MEMS devices integrated with CMOS (Complimentary Metal Oxide Semiconductor) structures such as sensors and actuators. CMOS structures are very effective device configurations for the implementation of digital functions, due to their low power consumption and dissipation and the minimization of their current in the off-state. With commercial CMOS-compatible micromachining, microstructures and support circuitry can coexist on the same substrate, and thus can be fabricated in an integrated process.
However, in order to ensure proper integration into a CMOS process and good portability between generations of CMOS, it is preferable to integrate MEMS fabrication into the backend of a CMOS process. This requires formation of the MEMS structures after the interconnect metal has already been deposited. However, the presence of the interconnect metal on the substrate requires that the substrate not be exposed to temperatures in excess of 450° C.; these temperatures are again well below the deposition temperatures of about 550° C. that are required for silicon or silicon-germanium alloys in an LPCVD or epitaxial process. Hence, the use of these materials in backend processing of a CMOS device is precluded. Although it may be possible in some process flows to circumvent this problem by integrating the MEMS fabrication into the beginning or middle of a CMOS process, this is undesirable in that it limits the portability of the process between generations of CMOS.
There is thus a need in the art for a low temperature method for making MEMS devices or components thereof out of silicon or silicon/germanium alloys. There is also a need in the art for a method of fabricating MEMS structures or components based on these materials which can be integrated into the backend of a CMOS process, and which can be used to fabricate sensors and actuators. There is further a need in the art for an RF MEMS device, and a method for making the same, in which the CTE of the top electrode and cantilevered arm are closely matched. These and other needs are met by the devices and methodologies disclosed herein.
SUMMARY OF THE INVENTION
In one aspect, a method for making a MEMS device is provided herein. In accordance with the method, a substrate is provided, and a MEMS structure or component thereof is created on the substrate through the Plasma Assisted Chemical Vapor Deposition (PACVD) of a material selected from the group consisting

Affiliated with

Also associated with

Rating

Low temperature plasma Si or SiGe for MEMS applications does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Low temperature plasma Si or SiGe for MEMS applications, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low temperature plasma Si or SiGe for MEMS applications will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3355910