Method of making thin film piezoresistive sensor

Details Method of making thin film piezoresistive sensor Method of making thin film piezoresistive sensor

1999-04-14

2001-11-20

Lebentritt, Michael (Department: 2824)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Physical stress responsive

C438S053000, C438S345000, C438S446000, 43, 43, 43, 43, 43

Reexamination Certificate

active

06319743

ABSTRACT:

This invention relates to semiconductor piezoresistive gauges and in particular piezoresistive sensors fabricated by a method wherein selected portions of a thin film of doped semiconductor material deposited on an insulated flexible substrate are activated by a laser annealing process.
BACKGROUND OF THE INVENTION
It is well known in the art that semiconductor material such as doped silicon possess piezoresistive characteristics. This simply means that the electrical resistance of the semiconductor material changes when the material is subjected to strains such as bending. The change in resistance, and hence the strain applied to the semiconductor material can be measured accurately. This material and its capabilities can be used as a piezoresistive sensor.
One method for making such a device is to simply take a piece of doped silicon and bond it to a strain receiving member by an adhesive. The strain-receiving member is typically a flexible metal sheet, bellows or diaphragm. The opposed side of strain receiving member is exposed to the media that is being measured. Bending of the membrane induces strain; hence resistance change, on the gauges. The major drawback to this glued sensor technology is its susceptibility to output drift. As the sensor ages the bond between the semiconductor material and strain-receiving member also changes.
U.S. Pat. No. 5,518,951 describes a sensor formed by applying two or more insulative silicon layers directly to a strain-receiving member such as a metal sheet, bellows or diaphragm. A layer of doped silicon is applied to the insulative layers. Metal contacts for connection between the resistive measuring device and the yet to be formed piezoresistive sensors are then formed at selected predetermined locations. The nonconductive doped layer is then selectively activated in specific locations between the metal contacts to form the resistive sensors. A laser of suitable wavelength is used to activate the doping agent in the layer into activation and conductivity. This causes the layer between the metal contacts to heat, anneal and recrystalize thereby causing the doping atoms into conductivity and form the piezoresistive sensors between the metal contacts.
The sensor fabrication technique of the '951 patent leaves much room for improvement. As the resistor is formed after placement of metal contacts, it can only be formed between the adjacent edges of the pads. As the activation only occurs where the laser can reach, the area of contact between the resistor and the metal pads is often no more than a thin contact line formed between the edge of pads and the resistor. Even placing the laser at an angle smaller than 90° to the surface of the doped layer has failed to provide any incremental area of contact between the pad and the resistor. Any thermal distortion can disrupt this thin connection causing the resistor to fail.
Accordingly, there is a need for a more robust design capable of withstanding thermal distortion and which can be formed in a method for high volume fabrication.
SUMMARY OF THE INVENTION
The difficulties and problems of prior art methods for fabricating piezoresistive sensors are addressed by the method of present invention to form thin film semiconductor piezoresistive sensors. A strain-receiving member in the form of a substrate of chosen material functions as a diaphragm that flexes in response to changes in the media to be measured. The flexion is typically cause by changes in pressure or temperature of the media. In a preferred embodiment of present invention, the substrate is a flexible diaphragm, the displacement of which is measured through a piezoresistive sensor on the diaphragm to provide an indication of pressure change, force change, temperature change, weight change, etc.
Using a deposition process, the nonconductive semiconductor layers are deposited on a cleaned surface of the diaphragm. Use of deposition technology accurately and consistently controls the desired thickness of all deposited layers thereby allowing for mass production of sensors exhibiting consistent design tolerances. First a thin dielectric insulated layer; either of silicon nitride or silicon oxide is deposited on the diaphragm surface. A layer of doped amorphous/polycrystalline silicon, to form the piezoresistive sensors, is vapor deposited over the dielectric layer. As deposited, the layer is highly resistive and has low piezoresistive qualities.
The nonconductive doped silicon layer is then activated in one or more selected locations to form the one or more piezoresistive sensors. A laser is used to activate the doping atoms present in the layer. The metal contacts for connection between the piezoresistive sensors are then placed at selected locations on the doped film over the piezoresistive sensors using either a sputtering or evaporation method. A shadow mask pattern for the contact location is placed over the doped layer to allow the metal to be deposited at the proper contact location. The mask is aligned with the piezoresistive sensors via the use of an alignment feature formed on the substrate. The alignment feature may be a mechanical device such as a notch, lug or mark relative to which the sensors and metal pads are formed. This ensures accurate and consistent location of the various layers and components on the substrate.
Optionally, a passivation layer may also be applied. This passivation layer also seals out any impurities that may corrupt the sensor and affect performance.


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