Understanding MEMS
Understanding the Use and Function of MEMS Piezoresistive Pressure Sensors
Pressure sensors are vital components that play a crucial role in various industries. The pressure sensor market is projected to see remarkable growth, as indicated by recent research, and it is expected to reach nearly 24.5 billion dollars by 2028.
They find applications in automotive, industrial automation and process control, consumer products like diving, e-bikes, and, importantly, in the medical field.
Pressure sensors work based on the principle of the force of a fluid divided by area. To put it into perspective, consider a syringe – a smaller one can generate more pressure than a larger one with the same force applied. Understanding this concept is crucial when dealing with pressure sensors.
Atmospheric pressure is a term we often hear...
Atmospheric pressure is essentially the weight of the air pressing down on us. As you ascend in an airplane or spacecraft, the air becomes less dense, with fewer molecules and less oxygen. It is important to consider atmospheric pressure when measuring the pressure in your application, because it will determine the type of pressure sensor needed.
Not just automotive
Pressure sensors find applications across a diverse spectrum of industries, expanding far beyond their traditional use in the automotive sector. One of the sectors where pressure sensors play a critical role is the diving industry. In this field, pressure sensors are employed to monitor the depth of divers underwater, enabling them to calculate ascent and descent times accurately. The hostile underwater environment necessitates reliable pressure measurements to ensure diver safety.
Pressure sensors have made their way into the consumer product market. For instance, in the realm of bicycles, especially the emerging category of e-bikes, pressure sensors are integrated into various components. These sensors may be utilized on bike shocks, tires, and other critical parts to enhance the overall performance and user experience.
What are the typical applications of piezoresistive pressure sensors, and how do they work?
Piezoresistive pressure sensors are a subtype of pressure sensors known for their versatility and use in various applications. Piezoresistive sensors function on a principle involving doped, semiconductor silicon crystal, which allows them to measure pressure more repeatably than other technologies.
To understand their typical applications, it is worth highlighting that piezoresistive pressure sensors are not constrained to a single field. They are used in an array of industries, including the medical sector. In medical procedures like angioplasty, where surgeons inflate balloons inside arteries, pressure sensors play a crucial role. In these procedures, the pressure sensor's output helps the surgeon monitor the level of inflation inside the balloon, and assess the overall status of the procedure.
What MEMS technology is and its advantages
MEMS stands for Micro-Electrical-Mechanical Systems, and there are many different types of MEMS. The deposition, ion implantation, and diffusion steps are all fundamental to semiconductor manufacturing, as well as the photolithography and etching of MEMS pressure sensors. MEMS pressure sensors feature an elastic silicon diaphragm, which means they are free from hysteresis and creep. This elasticity benefits the sensor because it allows it to undergo repeated pressure cycling without altering its properties. When things tend to creep or change, it is almost always a result of how they are packaged, not necessarily the silicon chip itself. Putting thousands of pressure sensors onto one wafer also significantly helps to reduce costs.
Merit Sensor has its wafer fab in Utah. There are many advantages to working with a vendor that has its wafer fab. When it comes to developing a new product, having your wafer fab is a huge advantage, as it allows you to control the supply chain. Keeping the design in-house is often considered the key to the successful development and launch of any product.
The key design characteristic of a MEMS pressure sensor is the Wheatstone bridge diffused into the silicon diaphragm. The change in output from this bridge corresponds to a change in applied pressure. When a customer needs a higher pressure range, the sensor requires a thicker diaphragm to handle the increased pressure. Conversely, for measuring low pressures, like inches of water or low pascals, a thin diaphragm suffices.
After the manufacturing process, the silicon is bonded to a piece of glass. The glass may have a hole to create a vent for various pressure applications, or it can be sealed without a hole. In the latter case, the glass and silicon are bonded together in a vacuum.
When there is no hole in the glass, it is known as an absolute sensor because the space between the silicon and glass represents zero pressure absolute.
There are two types of absolute sensors made from MEMS dies. The traditional type has no hole in the back glass, creating a sealed vacuum reference for absolute pressure. However, this design requires protection for the circuitry on the top side to prevent corrosion and shorts from humidity or moisture.
Alternatively, there is the absolute sensor with backside pressure. In this design, a piece of glass is added to the top of the silicon, creating a sealed vacuum reference on the top side and allowing for backside pressure on the MEMS element. This type is commonly used in automotive and high-temperature applications, as well as with liquids.
MEMS dies are common in pressure transducer or pressure transmitter applications where the MEMS die is on a header. The header gets welded into a stainless-steel housing with a stainless-steel diaphragm. Stainless steel is an excellent choice since it is very media-friendly, and most people are usually very familiar with stainless steel's capabilities. This package is suitable for many industrial applications.
After welding the stainless-steel diaphragm and package together, there's a back-filling of oil into this housing. A clean silicon oil surrounds the MEMS die, transmitting pressure between the stainless-steel diaphragm and the silicon diaphragm of the MEMS element.
In the HVAC industry, the MEMS die can be purchased separately or packaged into our LP series and placed on a control board. These control boards are found in large buildings, either atop the building near the air intake or in the building's utility room, where heat exchangers and air airflow systems are located.
Another common application is in transportation. Pressure sensors are widely used in various types of automobiles. This area continues to grow as legislation drives the demand for higher efficiency and cleaner emissions.
(source: https://www.azosensors.com/ extract from the interview to Scott Sidwell, Engineering Manager at Merit Sensor Systems)
