Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2002-04-12
2004-03-16
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C257S417000
Reexamination Certificate
active
06706549
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor.
FIELD OF THE INVENTION
This invention is in the field of micro electromechanical devices or related materials.
BACKGROUND OF THE INVENTION
Strain gages have been bonded on metal diaphragms to produce pressure sensors or accelerometers. Because these transducers are made of materials with dissimilar properties, they suffer from coefficient of thermal expansion (CTE) mismatch, which leads to fatigue and early failure. In addition the production process is time consuming since each strain gage must be placed on the diaphragm one at a time.
I am a named inventor of U.S. Pat. No. 5,637,905 to Carr et al. and it discloses a high temperature pressure and displacement microsensor made from a Si substrate. A first coil structure is positioned within a recess in the Si and a pressure diaphragm is glass bonded about the periphery to the rim of the semiconductor substrate. A second coil structure is positioned on the underside of the pressure diaphragm and is electrically isolated from the first coil structure. The coils are inductively coupled together and provide an output indicative of changes in the coupling between the coils.
My U.S. Pat. No. 6,248,646 discloses a process for making an array of SiC wafers on standard larger industry sized wafers. This patent discusses the operating conditions for SiC and SiC-On-Insulator technology and cites the need for sensors made from SiC.
U.S. Pat. No. 5,447,067 to Biebl et al. discloses an acceleration sensor constructed on Silicon-On-Insulator substrate. Piezoresistors are disclosed for use in conjunction with a proof mass suspended by one or more resilient elements. These sensors are not useable in harsh environments. U.S. Pat. No. 5,576,250 to Diem et al. discloses a process for the production of accelerometers using Silicon-On-Insulator technology. The '250 patent discloses an accelerometer with moving elements consisting of one or more flexible beams supporting a seismic mass. Further, the '250 patent discloses packaging of accelerometers and the driving circuit by multichip module technology.
Sensors manufactured from 3C-SiC, 4H-SiC and 6H-SiC are used in harsh environments, for example high temperature environments, high vibration environments, radiation environments and corrosive environments. “H” means hexagonal and “C” as used in “3C” means Cubic and both refer to the crystalline structure of SiC.
SiC is a wide band gap semiconductor. Semiconductors are materials whose electrical conductivity is between that of a conductor and that of an insulator. “If an electron in an atom happens to be in an energy level which overlaps a higher, empty level, that electron proves to be essentially free from its original atom. It is then capable of moving freely through the solid, and the material will be a conductor, i.e., a metal. However, if the electron in the highest energy state of the atom exists in a level which does not overlap higher energy levels, this electron will be firmly held to its atom. Such a material will be a nonconductor of electricity. An intermediate case exists if the energy levels do not overlap but are close enough so that the energy gap between them is of the order of thermal energies. These materials are called semiconductors.”
Introduction To Physics For Scientists And Engineers
, Copyright 1969, McGraw-Hill, Inc., Library of Congress Catalog Card Number 69-13598, ISBN 07-008833-0, pgs. 804-805.
The semiconductor SiC is known as a wide band gap semiconductor meaning that electrons in the valence band must traverse an energy gap of several electron Volts (eV) at 300 K to reach the conduction band. SiC is operable at temperatures up to 873 K without substantial leakage current. Leakage current, for example that is due to the temperature of the operating environment, is kept to a minimum in SiC.
Batch fabrication of a single function type SiC sensors, namely, pressure sensors, has been demonstrated and has piqued the interest of many who desire stable sensors operable in harsh environments. SiC is, however, a very expensive material with wafer costs much greater than conventional silicon semiconductor for a two inch diameter wafer. One such wafer can produce between 100-400 pressure sensors.
There is not enough demand, however, for batch production of pressure sensors alone. Unlike silicon based sensors, silicon carbide sensors have a low volume specialized market The current process for fabricating silicon carbide sensors is limited to producing only one type of sensor per wafer at a time and, as such, the commercial viability of silicon carbide is greatly reduced. Further, there is no known process for simultaneously making different devices (sensors) having different functionality at the same time. Several different types of sensors exist such as accelerometers having proof masses suspended therein and pressure sensors having diaphragms.
There is a need for SiC accelerometers having suspended proof masses. Presently, such devices are not manufactured and are not believed to exist. Further, there is a need for the batch fabrication of multifunctional, multistructural sensors and other devices manufactured from SiC.
Although batch fabrication of SiC pressure sensors has been demonstrated, the economic viability of SiC sensors heretofore has been in doubt because there is no need for the mass production of one type of sensor, i.e, a pressure sensor. Industry remains reluctant to devote its resources to commercial production of SiC sensors for the following reasons:
(1) unlike Si sensors, SiC sensors of any one particular type have a low volume, specialized market;
(2) SiC has an inherently high material and capital cost when only one sensor is made in bulk from a single wafer and as a result the profitability incentive does not exist to encourage commercial production; and,
(3) the current process for fabricating these devices is limited to producing only one type of device at a time therefore doubling the fabrication cost for making two different devices.
One major problem in the batch manufacturing of SiC multistructural sensors is that some of the sensors such as accelerometers require the construction of apertures or annular recesses in the substrate. An aperture or a recess is three dimensional. Sensors such as accelerometers desirably include a suspended mass in the substrate from which they are manufactured. This mass is made from SiC and the piezoresistance of the n-type or p-type SiC which connects the mass to the remainder of the substrate is measured. Mathematical analysis of the piezoresistance determines the value and direction of acceleration. The suspension of the mass requires that the substrate be etched very precisely.
It is not possible to precisely construct the apertures or annular recesses in the SiC substrate before metallization because they interfere adversely with the remaining fabrication/manufacture of the SiC sensor. SiC sensors precisely measure the piezoresistance of specific areas of n-type SiC and, therefore, it is necessary that the contact metallization be precisely located and engage the n-type SiC in those specific areas. Positioning of the contact metallization is controlled by a masking process where photoresist is spun onto the wafer that is held under suction on a chuck. Therefore, if the wafer was perforated prior to application of the photoresist it would not be possible to create a suction due to the perforations. Further, the suction from the chuck through the perforations in the wafer would disturb the uniform application of the photoresist. SiC wafers are rotated between 1000 to 7000 revolutions per minute as photoresist is applied to the center of the wafer. As photoresist is spread radially it will impact whatever three dimensional apertures or recesses exist and will not spread evenly in those areas thereby resulting in l
Nelms David
Stone Kent N.
The United States of America as represented by the Administrator
Vu David
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