26 Pressure Sensor

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

In the present era, MEMS based sensors are being used in every field of life, industry, commercial sectors as well as in the defense sector. Due to the latest advancements in micro scale fabrication technology, micro pressure sensors are being fabricated utilizing the MEMS based diaphragms for a wide pressure range varying from ultra low pressure to very high pressures. With the advent of silicon based devices, traditional metal diaphragms are being replaced by silicon/silicon dioxide/silicon nitride based membrane structures. These technologically derived sensors reduce the cost to a great extent. The pressure sensors used in market for commercial applications are based on different principles such as piezoresistive, capacitive, optical, piezoelectric and many more. Amongst all these principles, piezoresistive and capacitive based pressure sensors are widely used. There are various advantsges being offered on integtating pressure sensor with functional piezoelectric thin film. The advantages of utilizing piezoelectric film for pressure sensors include high-pressure sensitivity, low power consumption, low noise levels, large dynamic range, low temperature sensitivity compared with the piezoresistive pressure sensors etc.

2. Working Principle:

The schematic of pressure sensors based on MEMS structure using piezoelectric thin film as shown in figure 1. It consists of two parallel plates (top and bottom electrodes) separated by a piezoelectric thin film in between. With the application of the applied pressure, either the thickness of the piezoelectric layer or its dielectric constant changes which is directly related to the applied pressure. The schematic shown in figure 1 gives the design of a pressure sensor based on change in dielectric constant of the piezoelectric layer on application of the applied pressure. As pressure is applied from the top side, due to the movement of the diaphragm, the capacitance of the piezoelectric layer changes. The complete sensor is bonded on a pyrex glass which gives strength to the fabricated sensor besides providing a pressure compensation using a specialized design process.

The three type of pressure sensors based on the design modifications are explained below:

Absolute pressure sensor: This sensor measures the applied pressure relative to perfect vacuum (0 psi) encapsulated within the sensor. In this case, the fabricated MEMS pressure sensor chip having a diaphragm is bonded to the glass in vacuum such that there is vacuum between the cavity and the glass. Pressure sensor shown in Figure 1 is an absolute pressure sensor. Such devices are used for atmospheric pressure measurement, as manifold absolute pressure sensor, cabin pressure control, in launch vehicles and satellites etc.

Gauge Pressure sensor: This sensor measures the pressure relative to atmospheric pressure. There is no vacuum between the glass pate and the sensor chip fabricated over the diaphragm. A tire pressure gauge is an example of the gauge pressure sensor. A zero pressure measured using a gauge pressure sensor means that the applied pressure over the diaphragm from the top side is same as that of the atmospheric pressure. Gauge pressure sensors are used for Blood pressure (BP) measurement, intra-cranial pressure (ICP), gas cylinder pressure and most of ground-based pressure measurements. Vacuum sensors are also gauge sensors designed to operate in the negative pressure region.

Differential Pressure sensor: This sensor measures the difference between the two pressures, on each side of the diaphragm, one from the top and other one from the cavity side of the MEMS structure. The bonded glass consists of a hole through which the pressure may be applied. Technically speaking, most pressure sensors are really differential pressure sensors; for example a gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere. Differential pressure sensors are used in high pressure oxidation systems to maintain the oxygen pressure ranging from 1 to 10 atmospheres inside the quartz tube during the oxidation of silicon. In such systems, the outer ambient of the tube is maintained at slightly high pressure of nitrogen and this pressure difference is monitored using differential pressure sensors. Differential pressure sensors can be used the pressure drop across an oil filter. They are also used to measure the flow or level in pressurized vessels which is the major requirement in defense.

So depending on the application, one can use anyone of the above mentioned pressure sensors.

Pressure sensors are used in number of applications like gas sensing, brake boosters, diesel engines, Blood pressure measurement, and Heart Beat measurement system, in space propulsion applications, Flight control systems, hydraulic systems Manifold Air Pressure, Tire Pressure Management System and many more. It has wide range of applications in the field of aerospace and defense. It is also used for pilot input force measurement and servo-loop feedback. Moreover, the pressure sensing products can be exploited for the applications including military trucks, unmanned vehicles, armored vehicles, fuel monitoring, compressors, transmissions and hydraulic pumps and motors for variety of government vehicles and test equipments.

3. MEMS based Pressure Sensor

The two types of pressure sensors are:

(1) Capacitive pressure sensor

(2) Piezoelectric pressure sensor

Capacitive Pressure Sensor: Capacitive sensors employ two thin membranes enclosing a cavity forming a capacitor where the deformation on one of the membranes results in change in capacitance. The schematic of the pressure is shown in figure 2.

Step by step fabrication process is given below along with the schematic representation:

• Cavity and bottom electrode fabrication: The silicon needs to be cleaned thoroughly following the standard process of cleaning including RCA1, RCA2 and piranha cleaning. The cleaned wafers will be carried out for the photolithography for forming the cavity and bottom electrode. The photolithography process includes spin coating of photo-resist, baking, alignment, and exposure followed by developing of the pattern. The exposed photo-resist will be removed from the selective area of the wafer followed by the etching of silicon dioxide and silicon. The bottom electrode will be deposited over the patterned wafer of the desired thickness. After deposition, the residual photo-resist will be removed.
• The poly silicate glass (PSG) layer will be deposited (sacrificial layer) using ICP- chemical vapor deposition technique covering the entire cavity. The patterned wafer will be polished using chemical-mechanical polishing system such that the thickness of PSG matched with the thickness of silicon dioxide layer. Now the PSG layer is present only over the metal electrode in the cavity as shown in figure 3.
• Photolithography will be carried out again for the patterning of the processed wafer for the membrane layer. The membrane layer will be deposited using physical deposition technique over the patterned photo-resist coated wafer and extra photo-resist will be removed using acetone.
• A different set of photo mask will be used for patterning the wafer for removal of PSG. For removing of PSG from the cavity, some holes will be patterned in each cavity such that the sacrificial PSG layer will get removed easily.
• After the removal of PSG, the processed wafer will be patterned in such a way to deposit the top electrode over the membrane using conventional photolithography technique through desired photo mask as figure 3.
• Finally the fabricated devices will be characterized, diced and packaged in appropriate metallic headers.

Figure 3: Process flow for the fabrication of capacitive pressure sensor

There is another approach which may be adopted for the realization of capacitive pressure sensor i.e. using the wafer bonding technique and SOI wafers. Pressure sensors using both the techniques will be fabricated simultaneously. The schematic showing step wise process flowand the process adopted are explained below in detail:

• The silicon wafer will be cleaned thoroughly using standard wafer cleaning process. Silicon dioxide will be grown on silicon wafer using thermal oxidation technique.
• The conventional photolithography process will be used for patterning the silicon dioxide as shown in the process flow (figure 4). Then, the patterned wafer will be bonded with the SOI wafer using anodic bonding technique.
• Then, the silicon and silicon dioxide will be etched from SOI wafer from the front side as shown in the figure 4.
• The top electrode and the bottom electrode will be patterned in the same manner as mentioned in previous case.
• Once the devices are fabricated, characteristics will be studied using laser Doppler vibrometer and semiconductor characterization unit. Devices will be diced and packaged.

Figure 4: Process flow for the fabrication of capacitive pressure sensor using wafer bonding

Piezoelectric Pressure Sensor: Piezoelectric sensors use the piezoelectric effect to measure change in pressure. A schematic of the pressure sensor is shown in figure 5.

Figure 5: Schematic of piezoelectric pressure sensor

Fabrication steps of the piezoelectric pressure sensor are given below. Here ZnO is taken as the piezoelectric thin film. The fabricated devices will be characterized, diced and packaged. The steps of fabricating the piezoelectric pressure sensor are as follows:

• The n-type silicon wafer with <100> orientation will be taken for the fabrication of piezoelectric pressure sensor.
• Silicon dioxide will be grown on both sides of the silicon wafer using thermal oxidation technique. Photolithography will be carried out to open a window on one side of the oxidized silicon wafer.
• After photolithography, the silicon dioxide and silicon will be etched using wet etching technique. After successful etching, the remaining oxide layer present on the wafer will be removed as shown in figure 6.
• Fresh oxide layer of silicon will be grown on the processed silicon wafer using thermal oxidation technique. Bottom metal electrode will be deposited on the selective area on the silicon dioxide present over the silicon membrane using photolithography and physical deposition technique.
• Furthermore, a sequence of silicon dioxide, zinc oxide and silicon dioxide layers will be deposited over the bottom metal contact. Also, the top electrode will be deposited over it for taking the electrical connections. Prior to device fabrication individual SiO2 and ZnO layers will be deposited exhibiting desired properties.
• Finally, the processed wafer will be bonded on the glass substrate using wafer bonding technique to develop the desired piezoelectric pressure sensor.

Figure 6: Process flow for the fabrication of piezoelectric pressure sensor

4. Dicing and Packaging:

The MEMS based pressure sensors (capacitive or piezoelectric) can be fabricated employing photolithography technique as mentioned above. In order to make the whole process cost effective, large no of pressure sensors can be fabricated simultaneously on a single big Silicon wafer. Sensors fabricated in this way can be diced to seperete each sensor. Pressure sensor can be then packaged finally before using.

5. Summary

• Working principle of pressure sensor
• MEMS based pressure sensor
• Dicing and packaging of the sensor