MEMS Technology and Working Principle of Accelerometers

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Modern world technology has advanced to such an extent, sometimes we wonder if we are in a science –fiction world.

To a large extent, semiconductor technology has made things possible, things which used to be unimaginable or considered science fiction in the past. One of the famous science fiction television series Star Trek predicted many of the innovations that have turned into reality. In Star Trek, replicators can create things from food to human organs to machine parts. But in today’s non-fictional world, NASA has invented a 3D printer. A London based restaurant know as Food Ink is the world’s first 3D printing restaurant.

Technology that allows our smartphones to detect the motion and inkjet printer to print, these technologies all rely on the emerging technology, which is known as Micro electro-mechanical systems.

Micro electro-mechanical systems or MEMS are tiny integrated devices that combine mechanical and electrical components, traditional manufacturing techniques such as milling, turning, and molding becomes nonfeasible at a small scale level.

MEMS devices are fabricated using the same process technology used in integrated circuits. Design engineers must be expertise in engineering and manufacturing domains. MEMS devices are a hybrid of mechanical and electronic mechanism and manufactured using a combination of traditional integrated circuit technologies and more sophisticated methods that manipulate both silicon and other substrates which exploit the mechanical properties.

When we look into traditional integrated circuits, the manufacturing process is known as photolithography. Photolithography is the process that transfers shapes from a photomask to the surface of a silicon wafer using light. A wafer is first coated with a substrate of silicon and its surface is oxidized, the oxide surface is then coated with an ultraviolet sensitive polymer called photoresist.

Image Credit: Pixabay

A pattern of UV light projected onto the photoresist, which is then chemically developed into a mask on the surface. Positive photoresist becomes removable by developing agents, while the negative resist is removable in areas exposed to UV light atoms radiated on the exposed region and changing the conductive properties. The unmasked oxide layer is chemically etched away, and the remaining photoresist chemically washed away.

MEMS fabrication is similar to integrated circuit fabrication, it involves the addition or subtraction of two-dimensional layers on a substrate through photolithography and etching process.

 MEMS device manufacturing fall under the following classification

  1. Bulk micromachining
  2. Surface micromachining
  3. High aspect ratio micromachining

MEMS technology is used in many applications such as accelerometer, gyroscope, and pressure sensors. Accelerometers are extensively used in the wearables, automotive airbag sensors, etc. They measure the rapid deceleration of a vehicle upon hitting an object by sensing a change in voltage. Based on the rate of this voltage change, the circuitry subsequently sends a signal to trigger the airbags.

Working Principle of an accelerometer Let us understand the working principle of accelerometer, based on the science of acceleration measurement using a spring and mass system as shown in Figure1.

Figure1:  Spring and mass system

A body of mass M is attached to a wall by a spring, and the spring coefficient is k. When force F is applied to mass m, there is a displacement towards the direction of force F, by x with acceleration a.

F=m.a=FS

S represents the spring force, the force due to spring tension kx, the displacement of the body for the initial rest position defined as x.

Equating the above two equations we get, m.a=kx

Thus acceleration is a function of displacement a=f(x)

If we measure displacement, we can calculate the acceleration on the body. There are three different methods to sense displacement ‘x’.

  1. Resistive technique
  2. Capacitive technique
  3. Inductive technique

According to basic science,

C=f(A/d)

d is the displacement between the plates forming the capacitance. By varying the distance “d” between the plates, we change the capacitance C. When we compare the movable plates to the spring and mass system, the movable plate is attached to mass m, and the other plate is at the initial position as shown in Figure 2.

Figure 2:  Movable plates with spring mass system.

When the body accelerates due to force F, it causes a change in displacement x. Due to the displacement, capacitance C across the fixed and movable plate changes. When we measure C, we can calculate the value of x, which helps to determine the acceleration. Inside the accelerometer as shown in Figure 3, there are a series of fixed plates on the outer assembly and internal movable assembly connected to the spring outer assembly with the help of a spring contact.

Figure 3: Anatomy of MEMS Capacitive Accelerometer.

The accelerometer mechanism is composed, of a capacitive finger proof mass this compliance structure deflects under acceleration or deceleration, this creates a differential capacitance. When the system moves due to acceleration, the internal assembly also moves due to displacement, and there will be a change in the value of capacitance. By measuring the value change of capacitance, we can deduce the acceleration acting on the body.

Image Credit: Bosch Sensortec

Bosch Sensortec manufactures MEMS accelerometer designed for low power applications optimized for devices such as smartwatches, smart clothes and wearables. BMA456 is an ultra-small high-performance acceleration sensor in a 2 x 2 x 0.65 mm³ package and consumes around 14 microamps in low power mode. BMA456 sensors have applications across smartwatches, wearables and hearables.

As we step into the future, we will find many MEMS devices integrating into our day to day activity, which would benefit the human race.

About the Author

Avinash N Bhat is a supplier marketing specialist at Mouser Electronics. Before joining Mouser Electronics, Avinash worked as a Senior Application Engineer. He carries a work experience of 4 years in Electronic design automation and device simulation tools. His previous work involves working on technologies, such as triple-junction solar cells, FinFET, and CMOS technology.