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ETCHANTS FOR USE IN
MICROMACHINING OF CMOS
AND METHOD OF MAKING THE SAME
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of three-dimensional microstructures. and in particular, to accelerometers compatible with CMOS technology.
2. Description of the Prior Art
The marriage of semiconductor fabrication technology with electromechanical microstructures has given rise to the theoretical possibility of a vast array of microsensors, microtransducers and microactuators that can potentially be made in large numbers at low cost to perform a wide variety of functions, particularly when integrally combined with integrated circuits.
However, the development of an application in micro electromechanical systems (MEMS) is often restricted by process-related barriers. Even conceptual designs can take a substantial amount of process development time. The development of a design for a device is typically coupled to process parameters, so that the process designer and the application designer must be in close communication, if not in fact, be the same person.
This is sharply contrasted with the state of integrated circuit design, in which it is possible to design good circuits without expert knowledge of the fabrication processes. Integrated circuit fabrication has produced scaleable CMOS design rules so that digital circuits can be designed and fabricated using standardized design assumptions and so that the design can be translated and used with substantially different design rules and hence device parameters.
Several research groups in Europe and North America have demonstrated that it is possible to mieromaehine commercial CMOS wafers using maskless post processing. This is done by stacking an active area, both metal contact cuts, and over-glass cuts, to leave bare silicon exposed when the chips are returned from the foundry. See for example J. Marshall et al„ "Realizing Suspended Structures on Chips Fabricated by CMOS Foundry Processes Through MOSIS Service," NISTIR 5402. U.S. National Institute of Standards and Technology. Gaithersburg, Md. 20899 (1994). The chips are then etched in EDP or some other anisotropic etchant. This technique was demonstrated in the late 1980's and has been used to produce a variety of micromachined components, including: infrared emitting arrays for thermal scene simulation, M. Parameswaran et al„ "Commercial CMOS Fabricated Integrated Dynamic Thermal Scene Simulator" IEEE Int. Elec. Dev. Mtg.. San Francisco, Calif., Dec. 13-16, 1991. at 753-56; thermopile converters, M. Gaitan et al., "Performance of Commercial CMOS Foundry Compatible Multifunction Thermal Converters," in Proc. 7th Int. Conf. on Solid State Sensors and Actuators (Transducers '93) Yokohama, lun. 7-10, 1993, at 1012-14; thermal actuators, M. Parameswaran et al.. "CMOS Electrothermal Microactuator," in Proc. IKKR Microelectro Mechanical Systems Workshop, Napa. Calif.. Feb. 11-14. 1990, at 128-31. flow sensors. D. Moser. "CMOS Flow Sensors," Ph.D. Thesis ETH Zurich, Physical Electronics Laboratory, Swiss Federal Institute of Technology, Zurich. Switzerland, and resonant hygrometers. T. Boltshauser et al., "Piezoresistive Membrane Hygrometers Based on IC Technology," Sensors and Materials, 5(3): 125-34 (1993) and H. Baltes.
"CMOS as a Sensor Technology," Sensors and Actuators A37-38:51-6 (1993).
Additionally, CMOS foundry has been used to make high voltage devices. Z. Parpia et al., "Modeling with CMOS
5 Compatible High Voltage Device Structures," in Proc. Symp. High Voltage and Smart Power Devices at 41-50 (1987). Absolute Temperature Sensors, P. Krummenacher et al.. "Smart Temperature Sensor in CMOS Technology," Sensors and Actuators. A21-23:636-38 (1990). Invisible
10 Light Imaging Arrays. This impressive list of sensors and actuators comes packaged along with all the analog and digital electronics available in the foundry process. Nevertheless, two limitations of this approach has been the constraints imposed by etching and limited structural com
Therefore, what is needed is a micromachining process useful in a wide variety of applications and devices, including but not limited to accelerometers. that will allow engineers and scientists from many different disciplines to create 20 new micro electromechancial systems and applications, particularly using post process foundry CMOS wafers.
BRIEF SUMMARY OF THE INVENTION
25 The invention is an improvement in a method for fabricating microelectromechanical (MEM) devices. The improvement comprises the steps of providing a substantially completed MEM device except for the need of at least one unmasked etch to complete the device. The MEM
30 device is etched with a generally isotropic etchant selected from the group of noble gas flourides and tetramethyl ammonium hydroxide. Xenon difluoride is the preferred gas phase etchant. but other flourides of xenon and of other noble gases could be equivalently substituted.
35 In the embodiment where the etching of the MEM device is performed with a gas phase xenon difluoride. the etching the MEM device is at ambient temperature without external heating, and is performed under a partial vacuum.
In an alternative embodiment the etching the MEM device
40 is performed in a aqueous solution of tetramethyl ammonium hydroxide with silicic acid.
Somewhat more specifically the invention is a method for fabricating microelectromechanical (MEM) devices, using a
4J standard integrated circuit (IC) process, comprised of the steps of providing a substantially completed MEM device except for the need of a single unmasked etch to complete the device, and etching the MEM device with xenon difluoride in a gas phase.
50 The invention is also a microelectromechanical (MEM) accelerometer defined in a semiconductor substrate comprising a proof mass fabricated on the substrate using integrated circuit processes. At least one and preferably two oxide beams couple the proof mass to the substrate. The
55 oxide beam is at least in part unsupported and extending from the proof mass to the substrate. At least one poly silicon piezoresistor is disposed in the oxide beam. As a result, the MEM accelerometer is inexpensively manufactured using standard integrated circuit technology.
60 Preferably the accelerometer comprises at least two unsupported oxide beams coupling the proof mass to the substrate. Each of the oxide beams has at least one polysilicon piezoresistor disposed therein.
The invention also includes a method of making three
65 dimensional structures using aluminum micro hinges. In one embodiment, the hinge is used to create accelerometers with sensitivity along orthogonal axes. The substrate is generally
planar and the hinge is deformed so that the proof mass is oriented in a predetermined position out of the plane of the substrate. In one embodiment the accelerometer further comprises at least three of the proof masses fabricated from the substrate using integrated circuit processes and at one hinge. Each of the hinges couples a proof mass, support bean and peixoresistor to the substrate. At least one and possibly all three of the proof masses are oriented in a different plane relative to the substrate.
The hinges are comprised of aluminum. The hinges are fabricated using xenon difluoride as a gas-phase ambient temperature etchant to define the hinges apart from the substrate and proof mass. Alternatively, the hinges are fabricated using a tetramethyl ammonium hydroxide aqueous solution with silicic acid to define the hinges apart from the substrate and proof mass. Specifically, the proof mass, hinges, and polysilicon piezoresistor are formed using CMOS processes.
The invention is also an improvement in a method of mass fabricating a microelectromechanical device. The improvement comprises providing a device having portions which must be oriented to predetermined three dimensional positions in order assume a final configuration for said device, selected portions of said device having electrically isolated structures for receiving and holding charge for at least a temporary period. Selected amounts of electric charge are selectively deposited on said electrically isolated structures to generate electrostatic forces therebetween to move selected portions of said device to assume said final configuration according to well understood principles of mechanics and electrostatics specifically applied to each device topology. In the illustrated embodiment a scanning electron microscope is used to deposit electron charges on the electrically isolated structures.
The invention can still further be defined as an improvement in a microelectromechanical (MEM) device having a substrate and a movable part separate from the substrate comprising a derformable hinge coupling the substrate and part. The hinge is deformed so that the substrate and separate part form a three dimensional structure.
The improvement further comprises a sensing element for generating a position signal indicative of the spatial orientation of the part, such as a piezoresistor, capacitive, magnetic or electron tunneling device.
The improvement further comprises source of a force disposed on the substrate and part for deforming the hinge to form the three dimensional structure, such as a source of electrostatic charge, magnetic fields or thermally generated forces.
The improvement further comprises a circuit disposed either on the substrate, part or both, which circuit is coupled to the sensing element for processing the position signal, such as sensing, amplifying, filtering controlling or otherwise interfacing to the sensing element or an actuator.
In some embodiments the part is fabricated in a form or out of a substance so that it interacts with electromagnetic radiation, such as particle or x-ray radiation, visible, infrared, ultraviolet or other portions of the electromagnetic spectrum, to achieve a defined function.
The invention may be better visualized by now turning to the following drawings wherein like elements are referenced by like numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified plan view of a first embodiment of a two beam accelerometer made according to the invention.
FIG. la is a longitudinal cross-sectional of the two beam accelerometer of FIG. 1.
FIG. 2 is a schematic of the circuit for the accelerometer shown in FIG. 1. 5 FIG. 3 is a simplified plan view of a second embodiment of a two beam accelerometer including aluminum hinges, made according to the invention.
FIG. 4 is a graph of the percentage change in resistance 10 of the polysilicon strain gauges as a function of the vertical displacement of the surface of the accelerometer of FIGS. 1 and 3.
FIG. 5 is a graph of the percentage change in resistance of the polysilicon strain gauges as a function of the hori15 zontal displacement of the surface of the accelerometer of FIG. 3 after rotation to a plane orthogonal to the substrate.
FIG. 6 is an idealized perspective view of an MEM three dimensional accelerometer.
FIG. 7 is an idealized diagram showing one etching 20 system in which the etehant of the invention is used.
FIG. 8 is an enlarged view of a plurality of plates supported by aluminum hinges made according to the invention.
FIG. 9 is a side cross-sectional view of an aluminum
hinge running between oxide plates and a polysilicon piezoresistor.
The invention as described in the context of the illustrated embodiment as well as various other embodiments can now 30 be understood by turning to the following detailed description.
DETAILED DESCRIPTION OF THE
35 What is described in the present specification are accelerometers using tiny proof masses and piezoresistive force detection. Conventional wisdom would indicate that this approach would not yield useful sensors. However, in fact, according to the invention, such devices are suitable in a
40 wide range of applications.
In the illustrated embodiment, the accelerometers are fabricated using a standard CMOS foundry, such as 2 micron double poly, double metal p-well service, provided by Orbit Semiconductor from MOSIS service. When an additional
45 etching step is performed on the chips after they are received from the CMOS fabricator to free the acceleration sensing element without harming the existing electronics on the chip. Using standard process and existing fabrication facilities, fabrication costs for tested chips is well below one
50 dollar in comparison to currently existing accelerometers having a comparable performance, which accelerometers typically cost between 30 and 300 dollars.
A typically accelerometer is shown diagrammatically in FIG. 1 with piezoresistors 34 and each support beam 32. As
55 shown in the schematic of FIG. 2, piezoresistors 34 typically perform two opposing arms of wheatstone bridge in combination with on chip resistors 46. The wheatstone bridge circuit typically provides low temperature sensitivity and good power supply rejection. The change in resistance is
60 given by the product of the gauge factor of the piezoresistive polysilicon and the strain which the gauge experiences. The result is that the percentage change in output voltage for the wheatstone bridge is one-half the gauge factor times the strain in the gauges. In a simplified model, the strain at any
65 point in the beam is given by the equation below where M is the bending moment which is being sensed, X is the position along the beam, z is the vertical distance from the