MEMS piezoresistive flow sensors design and application: A review
There is lot of demand for fluid flow velocity and direction sensors in various medical, industrial and environmental applications. Though we have such critical demands on sensor of flow parameters (such as rate, velocity, direction and temperature), the properties of different target gases or liquids to be sensed pose challenges to the development of reliable, inexpensive and low powered sensors. This paper presents some recent advances in Micro-Electronic Mechanical System (MEMS) Pizoresistive flow sensors. These Piezoresistive flow sensors can be broadly classified as Cantilever and diaphragm structure. In this paper two cantilever structure and one diaphragm structure has been discussed in details. Additionally design, fabrication and application of these sensors have also been discussed. Some key challenges in MEMS piezoresistive flow sensor have been mentioned.
Piezoresistive Sensors are included among the initial Micro-electronic mechanical systems and contribute to a major share of MEMS sensor in the market today. Piezoresistive effect is the variation in electrical resistivity of a semi-conductor or metal when subjected to mechanical strain. Mechanical strain changes the internal atomic structure or lattice structures due to which there is a change in electrical resistivity. Piezoresistive effect has been widely used in flow sensing application according to the literature. The phenomenon of change in resistivity when subjected to mechanical strain makes it alluring for flow sensor application. In flow sensor variation in resistivity is converted to potential difference which changes with flow. Silicon piezoresistance is being used in most flow sensing application. Flow sensors are used to determine the rate and direction of liquid or gas flows in various applications. Flow sensing devices basically measure volume, mass, velocity by determining numerous physical variables. This paper reviews recently developed the micro scale piezoresistive flow sensors.
Piezoresistivity was first discovered by lord Kelvin(William Thomson) in 1856 . Telegraph wire signal transportation variation time dependent conductivity changes and noise in telegraph lead to further research in this field. Lord kelvin’s work was further carried by Tomlinson and reassured strain-induced change in conductivity of metals and made all the observations required . After the discovery of this phenomenon lot of further research was carried out. A step ahead in the research process was made after 94 years of the piezoresistive discovery with the finding of piezoresistive in silicon and germanium . In his seminal paper on seminal paper on semiconductor piezoresistance by C. S. Smith claimed of exceptionally large piezoresistive shear coefficient in silicon and germanium. In 1957, Mason and Thurston first reported silicon strain gauges for measuring displacement, force and torque . Semiconductor strain gauges with accuracy and sensitivity 50 times better than the conventional metal strain gauges was considered a big leap in sensing technology.
Using silicon and various polymers and by changing sensing materials and structural design numerous different types of piezoresistive flow sensors have been developed in literature. Piezoresistive flow sensors can be categorized in two sections Cantilever structure and diaphragm structure which will be discussed in further sections.
3. Piezoresistive fundamentals:
Let us consider a homogenous electrically conductive structure to which an electric field is applied. Assume that initial resistance of this material is
. Lets apply a tensile force F in the longitudinal direction there will be elongation in the structure the new resistance of the structure is
. And we know that Resistance
of structure is a function of resistivity
is length and a is the average cross sectional area of the structure. Through partial differentiation of the above equation we get.
And dividing by R we get
are the initial values of length, diameter and resistivity respectively, after elongation these values change to
. We also know that longitudinal strain(
), diametric strain and poisons ratio is given by
By further developing the equations we can also prove
are the volume and cross sectional area in initial condition. By considering above equations we can prove that
Now we can define the longitudinal gauge factor
This is the equation for gauge factor in which there are two factors that is the geometric effect and resistivity. In metals geometric effect (
) contributes about 1.4 to 2 and resistivity (
) provides 0.3. But in semi-conductor devices resistivity has 50 to 100 times more contribution then geometric effect. In semi-conductor material piezoresitivity is direction dependent.
4. Cantilever MEMS piezoresistive flow sensors
Cantilever structured MEMS Piezoresistive were the first kind of MEMS piezoresistive flow sensors that was developed made using silicon. Su et. Al fabricated a highly sensitive micro-cantilever MEMS flow sensor in . The schematic layout of the cantilever structure is shown in figure 2. A strain gauge resistor is fit at the base of each arm of cantilever. Strain gauge measures the deflection and twist in cantilever arm. The length of the strain gauge is 50µm along the cantilever. To compensate error in measurement due to temperature change one more strain gauge resistor of same dimension as the cantilever resistor is fit into the silicon chip this resistor is known as the reference resistor. The designed cantilever has the dimension ranging from 200to 550 µm, 25 to 40 µm in width with a upper square of L of 100 to 200 µm. The spring constant of the cantilever is calculated to be 0.2 to 2.7 N
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. P-type doped silicon-on-insulator (S.O.I buried with oxide) wafer is used to build the sensor. The fabrication of the sensor consists of four steps as shown in the figure 1. The first step is back etching of silicon in a KOH solution to create 20µm thick silicon membrane. The second step is infuse the strain gauge resistor on the front of the substrate. In the third step cantilever structure is made using titanium and aluminum using sputtering technique. They are further alloyed by keeping at
for 30 minutes. In the final step the cantilever structures are released. The cantilever structure and resistor is protected by coating it with photo-resist and the rest of material is removed from the back of the substrate by using a buffered hydro fluoric acid (BHF) solution.
Figure 1: steps in fabrication of cantilever flow sensor and schematic representation of cantilever structure .
Figure 2: A fabricated cantilever padel .
Yu et. Al has presented a micro scale air flow sensor based on free-standing cantilever structure in . Figure 3 shows the schematic representation of the cantilever structure. In this sensor platinum is used as the piezoresistor material. Gold is used to connect the platinum resistor with the LCR circuit as gold has low resistance then platinum it reduces the overall error in calculation of physical parameter. When air flows through the beam or the cantilever structure it is pushed down, hence the cross sectional area of the platinum resistor changes and hence we get a chance in resistance. Air flow can be measured by determining the change in resistance measured by the external LCR circuit. Figure __ shows the procedure followed to micro-fabricate the free standing cantilever structure. The fabrication process starts with depositing 1 µm of silicon nitride layer on silicon wafer of 500 µm using low pressure chemical vapor deposition technique (LPCVD). After this chromium adhesion layer is deposited followed by platinum layer of thickness of 0.1 µm using electron-beam evaporation process technique. The same deposition technique is used to deposit gold on top of platinum which is used to connect the platinum and external LCR circuit. The free standing cantilever structures are released by patterning the upper and lower nitride layer using RIE plasma technique and then followed by back-etching process by KOH solution by protecting the front layer using Teflon coating. After etching it is observed that the cantilever structure bend upwards due to residual stress released during the fabrication process.To determine the effect of physical dimensions of the cantilever structure on sensitivity of the flow sensor three different dimensions of the cantilever structure are considered in this research paper. By changing the width of the cantilever structure Experiments are performed. Widths of cantilever considered for study were 400 µm, 1200 µm and 2000 µm. And according to the results of the experiments it was found that sensitivity of 2000 µm dimension cantilever showed higher sensitivity.
Figure 3: schematic illustration of gas flow sensor.
Figure 4: overview of fabrication process of flow sensor .
Figure 5: side view SEM image of Cantilever structure .
5. Diaphragm MEMS piezoresistive Flow sensor
Scientists have taken lot of efforts to use nature for problem solving and innovation is the past few decades. Lot of attempts is being made to understand nature’s principle for sensing mechanisms to increase the performance abilities of the artificial sensor. Nature’s mechanisms and sensing abilities are highly optimized. Numerous experiments are being conducted to understand the line mediated behavior of Blind cavefish. With the awareness of surrounding flow velocities, Pressure and variation in each of these it is able to perform rheotaxis, determine predator from prey and navigate in an efficient manner. Hence several biomimetic have employed a line of piezoresistive sensors mounted on a substrate to improve the efficiency of water flow measurement. The broad subject of diaphragm-based piezoresistive sensors were built through inspiration from biological flow sensors such as inner ear and outer ear hair cell and the lateral lines of blind cave fish. With the development of the micromachining techniques it has been easy to mimic the biological transduction technique.
In  chan et. Al have presented a nature inspired flow sensor. In which Cilium is located at the outer end of the silicon cantilever beam. At the base of the cilium on the silicon cantilever we have the silicon peizoresitive’s. SU-8 epoxy is the material of the cilium and is considered rigid. When a lateral force acts on the cilium in on-axis direction it creates a bending moment in the beam which is transferred to the base. At the base we have piezoresistive silicon whose resistance changes which is measured by the external circuitry. SOI wafers with a 2-μm-thick epitaxial silicon layer on top, 2 μm-thick oxide, and 300-μm thick handle wafer are used to fabricate the sensor. Piezoresistive strain gauges are fabricated by using ion implantation technique. After the ion implantation step, we perform a drive-in at 1100
for 13 min in oxygen and water-vapor mixture. The cantilever structures are obtained by using deep reactive ion etching [DRIE]. The complete Fabrication procedure is systematically explained in figure 6.
Figure 6: Overview of fabrication process .
Figure 7: SEM image of the actual Diaphragm MEMS sensor .
6. Application of MEMS flow sensors:
MEMS flow sensors have a very wide range of application ranging from biomedical flow sensing, marine hydrodynamic sensing, environmental flow sensing and industrial gas flow sensing. The small size, low cost, high sensitivity and batch fabrication makes very attractive for numerous application and commercilization. The demand for the MEMS flow sensors in medical field is increasing tremendously. In the medical field devices like ventilators, nebulizers and oxygen supply sytem need MEMS flow sensors. MEMS flow sensor which can measure flow velocities and flow rate and in huge demand in athletics and rehabilation field. MEMS flow sensor are also required in the field of architecture and building design to determine environmental wind flow and wind direction monitoring. Many MEMS flow sensors are integrated at various points of the building which ensures constant monitoring of air circulation around the building. Biomimetic flow sensor developed using diaphragm MEMS techniques is of great impostance in marine robotics application and underwater or surface water vehicles. Leakage monitoring of water, oil and other liquids is another potential application of MEMS flow sensor.
This paper summarizes the design fabrication and appliaction of three developing piezoresistive MEMS flow sensors namely Cantilever MEMS flow sensor, Cantilever free standing MEMS flow sensor and Diaphragm based MEMS piezoresistive flow sensor. A brief insight on the underlying (physics)principle of Piezoresistive is detailed. Piezoresistive flow sensors show excellent sensitivity at low flow velocity but they often require a power supply in order to bias the sensor in a wheatstone bridge configuration. Diaphragm MEMS flow sensor can measure flow velocity accurately up to 0.7 mm/sec in water . Compared to the traditional Macro scale flow sensors MEMS flow sensor are more sensitive, Low cost, small in size and easy integration with IC circuits. The performance of MEMS piezoresistive flow sensor is hugely affected by humidity and temperature. This is a very critical problem in case of piezoresistive flow sensors. There have been significant efforts in literature to overcome these problems.
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References , ,,, are journal articles.
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