Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2002-02-27
2004-06-29
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S739000
Reexamination Certificate
active
06756247
ABSTRACT:
BACKGROUND
The field of the present invention relates generally to microdevices and microstructures, and more particularly to a microfabrication process which enables the creation of a millimeter-scale, large area movable structure integral with and supported by, micron-scale micromechanical flexures, actuators and/or transducers.
The term microelectromechanical systems (“MEMS”) refers to a new technology in which electrical and mechanical devices are fabricated at substantially microscopic dimensions utilizing techniques similar to those well known in the manufacture of integrated circuits. Such devices will be referred to herein as MEMS devices or micromechanical devices for convenience, although it will be understood that present commercial applications of MEMS technology include microelectromechanical transducers such as pressure sensors, inertial measurement devices, electrostatic actuators, and the like, as well as a wide variety of nanometer-scale micromechanical support structures. For an introduction to the use of MEMS technology for sensors and actuators, see for example the article by Bryzek et al. in
IEEE Spectrum
, May 1994, pp. 20-31.
The application of this technology to inertial measurement devices has received a great deal of attention from the microelectromechanical community, as evidenced by the paper by Kuehnel and Sherman “A surface micromachined silicon accelerometer with on-chip detection circuitry,”
Sensors and Actuators A
45 (1994), pp. 7-16, and by U.S. Pat. Nos. 5,245,824, 5,563,343, 5,126,812 and 5,095,752. Microaccelerometers are available as commercial products, and most of these devices have been applied to the sensing required for deployment of airbags in automobiles. This application requires an accelerometer sensitive to accelerations in the range of 50 g (490 m/s
2
), and microaccelerometers offer size, cost and performance advantages over prior technologies, such as piezoelectric devices, for inertial sensing. There is, however, substantial interest it obtaining micromechanical accelerometers capable of sensing much smaller levels of acceleration, for example in the range of micro-g or even nano-g's (10
−5
to 10
−8
m/s
2
), but these low ranges of acceleration have eluded MEMS devices due to the inherent requirement for larger masses to sense smaller accelerations. Although MEMS fabrication techniques are versatile, they are inherently limited as to the surface area, the size and the mass of structures that can be produced.
One attempt to overcome the mass limitations of MEMS structures in accelerometers has been the use of electron tunneling transducers to provide extremely sensitive measurements of the very small displacements resulting from low levels of acceleration. The paper by Rockstad et al., “A miniature high-sensitivity broad-band accelerometer based on electron tunneling transducers,”
Sensors and Actuators A
43 (1994), pp. 107-114, discusses such a device, but the disadvantage of this approach is the complexity of the fabrication process required to obtain such an accelerometer. Furthermore, there are serious issues regarding the long term stability of the tunneling transducer, and accordingly such devices are not well suited to widespread commercial applications such as automotive and consumer products.
The use of wafer bonding techniques to create wafer-thick silicon structures which can serve as large masses, or the addition of layers of heavier materials such as gold as described in the paper by Roylance and Angell, “A Batch-Fabricated Silicon Accelerometer,”
IEEE Trans. Electron Devices
ED -26 (1979), pp. 1911-1917 have also been suggested. These approaches have the severe disadvantage of utilizing complex, and expensive, fabrication processes resulting in devices which are not competitive in the commercial marketplace. Therefore, it is desirable to find a cost-effective micromechanical fabrication technology, such as plasma micromachining, to fabricate improved high mass structures which can function as accelerometers in MEMS devices. What is needed is a novel approach to the design and manufacture of micromechanical accelerometers of arbitrary size and shape in which such high mass structures can be obtained to provide for high sensitivity accelerometers without the introduction of complex, low yield manufacturing steps.
The use of MEMS devices as actuators, is described, for example, in the papers by Hirano et al. “Design, Fabrication, and Operation of Submicron Gap Come Drive Microactuators,”
J. Microelectromechanical Sys.
1 (1992), pp. 52-59, and Jaecklin et al. “Comb actuators for xy-microstages,”
Sensors and Actuators A
39 (1993), pp. 83-89. Such actuators are used to effect switching functions, direct fluid flows, move valve assemblies, tilt mirrors, move microstages, and to carry out a wide range of other functions in various microstructures. However, these MEMS actuators have limited dimensions by reason of the process used to fabricate them, and there is a need for a reliable process for making large area structures for use in micromechanical devices. Such large area surfaces would have numerous applications in research as well as in commercial products such as high density data storage, optical deflectors, and the like. Thus, it is desirable to have large area, flat microstages which are capable of being controllably scanned along an axis or in two orthogonal directions (x and y). Moreover, it is necessary that such stages be capable of being scanned over relatively large distances, several tens of micrometers for example. Accordingly, there is a need for an effective process for fabricating large area, optically flat, micromechanical stages coupled with electrostatic actuators capable of large motion actuation in one or two directions.
What is required for both large mass accelerometers and large area microstages is a process for fabricating a large area structure having dimensions up to several millimeters, releasing that structure for motion, and integrating that structure with other micromechanical and microelectromechanical devices which may have dimensions in the range of 1-3 &mgr;m. It is further desirable that all of the structures be fabricated from a single crystal silicon substrate material. Moreover, substantially the same fabrication process should be utilized for the creation of the large area structure and other micromechanical and microelectromechanical devices, although it should be understood that there may be circumstances under which it is more effective or economical to utilize a different fabrication process for creation of the large area structure. Further, what is needed is the ability to readily integrate the large area micromechanical devices with microelectronic circuits which may be located on the same wafer, for such circuits are required for signal conditioning, for control of the actuation of the large area structure, and for sensing its motion.
SUMMARY
In order to achieve the foregoing and to overcome the problems inherent in fabricating large area released microstructures, the present invention is directed to a monolithic process for making silicon micromechanical devices in which large area, movable structures are integral with micromechanical flexible supports, or flexures, and microelectromechanical sensors and/or actuators.
A further aspect of the invention is a fabrication technique which permits the integration of a large area released structure with conventional micromechanical devices. The conventional micromechanical devices may be flexible supports, may be motion transducers (capacitive or otherwise) and/or electrostatic actuators, and may be comprised of released beam segments formed with substantially the same processing techniques, such as plasma micromachining, as the large area structure.
Another aspect of the invention is the provision of micron-scale, flexible silicon beam support members, or flexures, capable of supporting a large, millimeter-scale high mass structure and enabling its motion in a desired direction(s) while substantial
Adams Scott G.
Davis Timothy J.
Jones, Tullar & Cooper
Niebling John F.
Simkovic Viktor
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