Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal
Reexamination Certificate
1998-10-13
2001-05-01
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
C438S050000, C438S053000
Reexamination Certificate
active
06225140
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to pressure sensors and more particularly to pressure sensors formed by etching trenches in a substrate, undercutting the sidewalls of the trenches, filling the gaps between the trenches with dielectric, and forming piezoresistors in the region of the trenches and filled gaps.
(2) Description of the Related Art
Pressure sensors are widely used in a number of applications. Pressure sensors have been formed by depositing a layer of polysilicon on a sacrificial layer. Piezoresistors art then formed in the layer of polysilicon layer and the sacrificial layer is etched away leaving the layer of polysilicon as a flexible diaphragm. The resistance of the piezoresistors changes as the diaphragm is deflected by pressure changes. Pressure senors have also been formed by depositing piezoresistors on single crystal silicon and etching away the silicon from the back side of the wafer to form a flexible diaphragm.
U.S. Pat. No. 5,531,121 to Sparks et al. describes a method of forming a pressure sensor by forming an N+ region in the surface of a silicon substrate. A silicon layer, such as epitaxial silicon, is then grown from the surface. One or more trenches are then formed into the N+ region and the N+ region is selectively etched away forming a cavity beneath the surface the surface of the silicon layer. The flexible surface over the cavity is then used to form a pressure sensor.
U.S. Pat. No. 5,242,863 to Xiang-Zheng et al. describes a method of forming a pressure sensor by forming a buried low resistance N type layer under a predetermined diaphragm region of the substrate. One or more trenches which extend to the low resistance N type region are then formed at the edges of the diaphragm region. The low resistance N type material is then converted to porous silicon by anodization of silicon in a concentrated hydrofluoric acid solution. The porous silicon is then etched away leaving the diaphragm region over a cavity. The diaphragm is then flexible and can be used to form a pressure sensor.
U.S. Pat. No. 4,766,666 to Sugiyama et al. describes a method of forming a pressure sensor which uses an insulating diaphragm film formed of an etch resistant material on the surface of a substrate. The diaphragm film has at least one etching hole. A cavity is then etched in the substrate under the diaphragm film using the etching holes in the diaphragm film. The diaphragm film over the cavity then becomes a flexible diaphragm and can be used as a pressure sensor.
U.S. Pat. No. 4,853,348 to Tsubouchi et al. describes the formation of a cavity in a substrate in order to form a capacitor having increased capacitance.
This invention describes a method of forming a flexible diaphragm using different methods which can be used as a pressure sensor. The pressure sensor formed is protected from damage due to overpressure and is formed using processes which are compatible with processing of CMOS or other integrated circuit chips.
SUMMARY OF THE INVENTION
Many applications require pressure sensors to measure gas pressure, liquid pressure, or the touching pressure in specific direction of a solid. It is highly desirable that pressure sensors have low manufacturing cost, good sensitivity, a highly linear response, good stability, and protection from damage due to overpressure. Since these pressure sensors are frequently used in conjunction with semiconductor integrated circuits, particularly CMOS circuits, it is advantageous if the processes for fabricating pressure sensors are compatible with processes used to fabricate CMOS or other semiconductor integrated circuits.
Pressure sensors have been fabricated using piezoelectric resistors formed on a polysilicon thin film deposited on a sacrificial layer. The sacrificial layer is then removed and the polysilicon film becomes a suspended diaphragm which can deform under pressure changing the resistance of the piezoelectric resistor. Pressure sensors have also been fabricated on single crystal silicon substrates. After a piezoelectric resistor has been formed on the substrate surface the backside of the silicon substrate is etched away, using electrochemical etching or other chemical wet etching, leaving the piezoelectric resistor on a thin flexible membrane of silicon. While these methods of forming pressure sensors are compatible with silicon integrated circuit wafer processing they require expensive processing steps, such as etching the back side of the wafer. The pressure sensors fabricated in this manner do not have overpressure protection and can be easily damaged by higher than expected pressure.
It is a primary objective of this invention to provide a method of forming a low cost pressure sensor having good overpressure protection using methods which are compatible with integrated circuit processing.
It is another primary objective of this invention to provide a low cost pressure sensor having good overpressure protection which can be located on the same substrate as integrated circuits.
These objectives are achieved by forming a layer of first dielectric having a number of openings on a substrate, such as a silicon integrated circuit substrate. A number of trenches are then formed in the substrate using the dielectric layer with openings as a mask. Second dielectric spacers are then formed on the sidewalls of the trenches. Isotropic etching is then used to etch the bottom of the trenches thereby undercutting the sidewalls of the trenches and releasing them from the bulk substrate. A third dielectric is then deposited filling the gaps between the sidewalls of the trenches thereby forming a flexible membrane. Piezoresistors are formed on or in the flexible membrane to measure the deflection of the membrane due to pressure on the membrane. The piezoresistors can be implanted or diffused resistors formed in the silicon substrate prior to forming the layer of first dielectric. Alternatively piezoelectric material can be deposited on the flexible membrane and patterned after the deposition of the third dielectric. The distance between the flexible membrane and the bulk substrate can be carefully controlled and provides overpressure protection. The methods used to form the pressure sensor are low cost and compatible with semiconductor integrated circuit processing. The pressure sensor and method of forming the pressure sensor will now be described in detail with reference to the following drawings.
REFERENCES:
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patent: 4853348 (1989-08-01), Tsubouchi et al.
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Liu Lianjun
Wang Zhe
Ackerman Stephen B.
Institute of Microelectronics
Mulpuri Savitri
Prescott Larry J.
Saile George O.
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