Piezoresistive Pressure and Temperature Sensor Cluster

The sensing material in a piezoresistive pressure sensor is a diaphragm formed on a silicon substrate, which bends with applied pressure. A deformation occurs in the crystal lattice of the diaphragm because of that bending. This deformation causes a change in the band structure of the piezoresistors that are placed on the diaphragm, leading to a change in the resistivity of the material. This change can be an increase or a decrease according to the orientation of the resistors.

In this project, our aim is to produce low-cost piezoresistive pressure sensors with variuos sensitivities for industrial applications. This variation in the sensitivity of the sensor will enlarge the application areas of the sensors. Those sensors will also include temperature sensors. With those, tem perature information can be obtained and can be used for temperature compensation purposes.

APPLICATION AREAS

• Household Appliances: Washing machines, dishwashers, vacuum cleaners;
• Automotive Applications: Oil level, gas level, air pressure detection;
• Biomedical Applications: Blood pressure measurement, etc...

ADVANTAGES

• Low-cost sensor fabrication opportunity.
• Mature processing technology.
• Different pressure levels can be achieved according to the application.
• Also, various sensitivities can be obtained.
• Read-out circuitry can be either on-chip or discrete.

BASIC PROCESSING STEPS

• Diffusion (n-type and p-type)
• Metallization
• Anisotropic back-side etching (by using well known chemicals as TMAH or KOH)

For a typical piezoresistive pressure sensor, the piezoresistive elements (i.e., the diffused resistors) are located on an n-type epitaxial layer of typical thickness 2-10m m.  The epitaxial layer is held by a p-type substrate.  The pressure sensitive diaphragm is formed by silicon back-end bulk micromachining.  For this process, anisotropic etchants like TMAH and KOH are be used.

FABRICATION OF THE SENSOR

• The piezoresistive pressure sensor is produced at METU-MET.
• The back-side etching is performed and the diaphragms are formed.
• For the packaging of the sensor, alumina substrates are used, and a cap is made for the input pressure port.

Figure on the left shows the alumina substrate with a hole at the middle.  This hole is required for differential pressure measurements and the air pressure is always applied to the back side of the sensor via this hole.  Figure on the right shows the sensor and the substrate connected together.  also, the wire bonding is done so that the sensor is ready for connection with the discrete readout circuitry.

FUTURE GOALS

• Different pressure sensors with different sensitivities.
• Increase in the wafer-to-wafer uniformity of the sensors.
• Capability of performing advanced pressure, temperature, and vibration test for better characterization of the final product.
• Improvement in the packaging of the sensor.