Piezo-resistive pressure sensor

Introduction:

Sensor is defined to be a device that can be responded to any type of signal and can receive those signals. The physical property of sensor is that it can convert any input into electrical signals with in electronic circuits. A sensor does not function itself because it is such larger system and consist of many detectors, signal processors and memory devices. In every device sensor is placed in either intrinsic or extrinsic states. Sensors are of two kinds, one is passive that can directly generate electric signal and responds to external factors. Another is active sensors these needed some external power of excitation signal for operation of the device. Sensors can be classified into many ways according to the usage. The classification scheme arranged from simple to complex. Sensors are divided into physical, chemical and biological type. The physical sensors consist of thermoelectric, photoelectric, electro elastic, photo elastic and pressure sensors. Sensor is the one of the MEM’S application. Among different types of physical sensors iam discussing in this essay about the Piezo-Resistive pressure sensors.

         The Piezo-Resistive sensors are used to measure the strain on a silicon diaphragm. Piezo-Resistive sensor devices are widely used in bio-medical field. These types of sensors always need temperature sensors for calibrating the device. Piezo-Resistive pressure sensor is the commercial product that is successful in MEM’S technology. For increase the progress in integrated circuits the sensors are combined with the Piezo-Resistive effect. These types of sensors are widely used in many applications like in pressure switches, pressure gauges and in automobile parts. In this essay in below the Piezo-Resistive pressure sensors physical principle, fabrication process and the design system and the applications of this type of sensors is explained.

Physical principle:

The Piezo-Resistive pressure sensor main principle is linearity and sensitivity. These two are the main principles involved in this type of sensor.

Piezo-Resistive principle: An elastic material is taken and due to some source the strip of this material tends to move and if there is increase in longitudinal dimension then there will be decrease in lateral dimensions then cross section area will be decreased. If this is positive strain means there will be change in resistance value due to Piezo-Resistive effect. The pressure sensor consists of Silicon diaphragm, Piezo-Resistive in a wheat stone bridge and Silicon diaphragm is used to convert pressure into mechanical stress. The Piezo-Resistors converts stress into resistance and then finally resistivity changes into output voltage. Subsystems here are divided into pressure sensor with high sensitivity and good linearity and this entire setup.

1. Linearity: Analysis of linearity begins with small deflection theories and deflections are small compared to thickness of diaphragm. If the error in linearity error is less than 0.3% then linearity error decreases as the length of resistivity changes. As the length of resistor increases then linearity error will be decreased. There are some steps to be followed in linearity. First the magnitude error must be lower than linearity error then shape of curves varies as the length of the materials changes. Then error moves from negative to positive applied pressure changes. In final step linearity error is no more symmetric and it will be irregular. If the diaphragm thickness increases then linearity error also reduces then error shifts from positive to negative sign. A best linearity error observed at a diaphragm with a thickness of 2.2µm.The linearity error decreases if the thickness of diaphragm increases. When compared to linearity error in square and circular diaphragm means in circular diaphragm occupies less area then square. Then large deflections are reduced in this case.
2. Sensitivity: Sensitivity analysis is based on small deflection theories of plates. The pressure deflection relationship of plates is fabricated from isotropic and homogenous materials. The location and shape of resistors are also the effects of pressure sensors. Resistors are usually placed where there will be increase in stress larger to increase the sensitivity. The parameters are length L, width W, for the shape and the distance between in outer parallel resistor and the distance between in outer parallel resistor and diaphragm is 2dXt and distance between perpendicular resistor and diaphragm id dy.Sensitivity is approximated if all resistors are exactly same and have no Zero offset. In circular shape diaphragm the sensitivity is high at the edge and resistors are placed in radical directions. In top or bottom of diaphragm the sensitivity is high.

Fabrication process: The pressure sensor chips are packaged individually for pre-moulded-housing packing techniques leading to low packaging throughout a large body. The packaging steps are shown in below and here top-down fabrication process takes place. A lithographic dam-ring approach is used to develop for fabricating the Piezo-Resistive pressure sensors.

1. Initially a pressure sensor wafer with Pyrex glass combination is taken and the thickness of layer is up to 150µm.
2. The ultra thick layer of 150µm with negative-tone positive resist is spin coated on the surface at a level up to 4inches.
3. Then photolithography process is taken place to use dam-ring approach around the silicon membrane surface of the pressure surfaces.
4. Then dicing process is used to separate the wafer and then splits into multiple pressure chips as observed. Then an adhesive material is placed on die pads on the substrate and then a dam-ring is then picked and placed on the die pads of organic substrate. Then it is heated to cross link the adhesive material and it will combine both pressure sensor and organic substrate.
5. Then a wire bonding takes place between the aluminium bonding pads of the pressure sensors and the electrode pads of organic substrate.
6. Then organic panel substrate is attached with pressure sensors and placed into a transfer molding and encapsulate the pressure sensors and organic panel substrate. Because the top surface is moulded with inner wall surface in a closed position then inner space of dam-ring is not fulfilled by the fluid epoxy moling compound (EMC) during molding process.
7. Finally a pressure with a sensing channel space is separated from the organic substrate by using a saw machine after the EMC process.

Dam-ring deposition: In photo resist model to achieve a wide operation window a specific coating thickness is required. For this high film thickness a photo resist with high viscosity is taken. A spin wafer and a hot plate are used for spin coating process of the dam-ring material. To produce a ultra thick sacrifice layer a two stage spin coating process is employed. Lithographic process is introduced to achieve a double layer of photo resist in dam-ring method.

Transfer molding:As the pressure sensor is attached to organic substrate then substrate is placed in a transfer molding.To reduce the wrapage of encapsulated product the molding compound must be carefully chosen so that thermal expansion is close to that of organic substrate. To eliminate the wrapping of organic panel substrate a low molding temperature of 165° is utilized. The silicon membrane of pressure sensor and pressure loading of environment is reserved by the dam-ring.

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Design of the system: The majority available of micro-machined pressure sensors are bulk micro-machined Piezo-resistive devices. The Piezo-resistors are arranged in such a way by selectively doping portions of the diaphragm to form junction-isolated resistors. In an anisotropic material in silicon is defined by a tensor that relates the three directional components of the electric field to the three directional components of current flow. In a tensor general it has nine elements and expresses in a 3*3 matrix as they reduce to six independent values.

Where Ei and Ji are electric field and current density components and ?i is the resistivity component. If the Cartesian axis is aligned to the (100) axes in a cubic crystal structure then ρ1, ρ2, ρ3 are equal along the (100) axes denoted by ‘ρ’.The remaining components of matrix and then cross axis resistivity’s will be zero due to unstressed silicon is electrically isotropic. Finally the change in the components in the matrix leads to six stress components by a 36 element tensor. This tensor is finally populated by three non-zero components as shown in below.

Here ΠIJ co-efficient have units of Pa-1 and this can be either positive or negative. The Π11 have the resistivity in any direction to stress in same directions. The equation (1) is derived along the (100) co-ordinate axes and convenient to apply. The fractional change is represented as ΔR/R = ΠLσL+ΠTσT. Where ΠL and σL are Piezo-resitive co-efficient and these are parallel to the direction of current flow and Πt and σt are values in transverse direction.

Combining the equations by using a transformation of the co-ordinate system in (100) axes the equations can be stated as

ΠL = Π11+2(Π44+Π12-Π11) * (L12m12+L12n12+n12m12)

Πt = Π12-(Π44+Π12-Π11) * (L12L22+m12m22+n12n22)

Where L1, m1, n1 are the directions cosines of a vector that are parallel to the current flow and L2,m2,n2 are unit length vector perpendicular to the resistor. By combining and neglecting terms in above two equations (2 & 3) the Piezo-Resitive co-efficient is varied with doping level and operating temperature then p-type is placed in equation(4).

Π (N, T) =P (N, T) Πref.

The longitudinal and transverse Piezo-resitive co-efficient in the surface of a (100) silicon wafer is observed. Observe that each figure in silicon wafer (100) splits into two halves as ΠL & Πt for both p-type and n-type in silicon. Then for p-type silicon both ΠL & Πt the peak is along (100) and for n-type silicon also peak is along (100).

         If the length of resistor decreases means then resistance also decreases then increase in power consumption is not favourable. If the width decreases then variations are observed along the non-ideal lithography. From the above it concludes that size of diaphragm reduces as the resistor have a large area between its perimeter and the centre.

Advantages of Piezo-Resistive pressure sensors:

         At present today pressure sensors are used in a variety of applications in industries in overall MEMS market.Piezo electrical is used to measure high pressure with a diaphragm and widely used highly in pressure sensors. Piezo-Resistive force sensors have high applications that are fabricated using MEM’S processes. The Piezo-Resistive pressure sensors are used for direct mounting on printed circuit boards. Piezo-Resistive are used to measure the cell consists of a glass back plate and the silicon chip with diffused resistor bridge.

Main Piezo-Resistive pressure sensors applications are:

1. It is used in barometric, small airplanes and used in robotic, sanitary and meteorology, air conditioning.
2. These are used to improve sensitive, small size and are less in cost. Mainly some silicon sensors that use Piezo-Resistivity effect use a four element Wheatstone bridge configuration.
3. Piezo-Resistive pressure sensors are low in cost and having small size and these have high resolution and have high sensitivity.
4. Use of this element also removes four resistors that form during wheat stone bridge design. A cost effective current mode circuit is used to operate with a single Piezo-Resistive element.
5. Ultra miniaturized 0.69 French Piezo-Resistive pressure sensors are designed for fabrication in bio-medical applications. These ultra-miniaturized Piezo-Resistive pressure sensors have many users in biomedical fields like in intra cranial pressure monitoring during nano-surgery. And also used for air monitoring in respiratory diseases, observing the blood pressure during surgery then for monitoring obstetrics and used for monitoring the urinary pressure in diagnosis for respective disorders.

         The continuous development is reducing the catheter size leads to develop in ultra-miniaturized pressure sensors.

6. The micro-maching technology in both industrial and automotive pressure sensors is very good. Due to these factors Piezo-Resistive sensors have some advantages. Like it has high tensile strength, it is low cost and has high elasticity also have good elastic properties and compatible in microcircuit technology.
7. Pressure sensors and accelerometers both of these use Piezo-Resistive sensors with piezo electric effect. These Piezo-Resistive sensors also used in automotive applications it has up to now 40 applications in future they can be increased.

         For controlling the engine some sensors are used for controlling the engine and some are used as for good fuel economy and for controlling the engine. Accelerometers here are used for anti-skid braking, air bags and also in case of antiskid braking. Many transmissions are continuously developed that requires use of sensors and electronics to optimize transmission rations and power demand.

8. These type of pressure sensors are also used in various industrial applications such as automobiles and also in biomedical applications. Pressure sensors are used in measurement of mechanical quantities like force, stress and other. Then biomedical applications are used to measure the control flow of blood pressure and force generated within heart cells.
9. In capacitive sensors it has major problems in massive commercialization is sealing of capacitive cavity and the electrical lead transfer between vacuum-sealed cavity and outside world. To overcome this problem recently piezo resistive sensors are invented and these became major choice.To overcome so many number of difficulties these type of sensors are used.A nano crystalline silicon is used to achieve a high sensitive has been proposed.
10. The silicon piezo resitive pressure sensors are used to increase the sensitivity by introducing stress in regions in the formation of elliptical holes.For continous monitoring on operational temperature and for signal to noise ratio the piezoresistor is taken into account.In biomedical applications the addition of sensitivity and temperature effect and the signal to noise ratio should laso be considered in order of design the system.
11. The piezo resitive pressure sensor represents has one of MEMS applications that used in domestic appliances and used in various applications like laundary,washing machines and in vaccum cleaners.Differntial pressure sensors are used to measure the pressure difference in between two environments.
12. Silicon has excellent properties in piezo resistive material as it changes the bulk resistivity when stress is apllied.
13. For manufacturing new devices like electronic devices and maintaining high yield level is a challenge task that depends used for identifying the mechanism.Electronic sensors are mainly used in industrial automotion applications.Due to direct connection and profitability the manufacturing of yield is important.As is yield is 100% means it constantly improving the techniques.
14. In all aircrafts pressure sensors are used in engine,fuel,hydraulics and in enviromental applications.By using these type of piezo resistive sensors increase in demand on weight,size,cost and in signal processing.These type of piezo-electric resitive sensors are used to built a small size,light weight and the low cost of value.

These are the advantages of Piezo-Resistive pressure sensor that gives an idea why this sensor is more effective than compared to other sensors and why this type of Piezo-resistive pressure sensors are used.

Conclusion:

Sensors are been developed from past 20 years and widely been used in industrial and in biomedical. These sensors also offers a many type of sensors among them magnetic sensor are one type. By observing all the factors in above we can conclude that Piezo-Resistive pressure sensor is one type of sensor that have excellent properties in electrical and this sensor is fabricated in a very small size. The Piezo-Resistive pressure sensor has many advantages that mentioned in above essay. The fabrication process, main principle involved and the Design of the system of Piezo-Resistive pressure sensor is explained above. The output voltage of this Piezo-Resistive pressure sensor is small in magnitude. So due to this the output must be amplified to increase the S/N ratio and provides an output that is used in microprocessor system.Fianlly an Piezo-Resistive pressure sensor with an high output voltage with low fabrication cost must been developed.

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