Novel circuit design approach makes pressure sensors smaller and more sensitive

This article is a part of our University Technology Exposure Program 2022. The program aims to recognize and reward innovation from engineering students and researchers across the globe.

About the project

Microelectronics always tries to solve the task of balancing the performance and size of semiconductor devices. This is one of the main challenges for R&D Engineers in the field of microelectromechanical systems (MEMS) too. The chip size reduction can bring cost reduction and open up the possibilities for finding new applications. For more than 50 years, piezoresistive pressure sensors have used classic Wheatstone bridge circuits. This circuit dominates the pressure sensor market. 

Technology continuously improves by changing sensor topology or fabrication processes. But there can be a different approach for this problem. By changing the electrical circuit on the chip, improvements can be brought in. This is different from the combination of MEMS structures and Application Specific Integrated Circuit (ASIC) on a single chip, as it uses some advanced technologies. This is a way to increase the number of piezoresistive elements on the deformable part of the sensor while maintaining the power supply. This project presents a pressure sensor chip with a novel electrical circuit utilizing piezo sensitive differential amplifiers on Bipolar Junction Transistors (BJT) with a negative feedback loop (PDA-NFL) (Fig. 1). 

By increasing the number of sensitive elements in the circuit with the correct balance to change the output signal from mechanical stress (pressure), it is possible to either increase the sensitivity with the same dimensions, reduce the dimensions with the same sensitivity, or find a middle point with other benefits.

Fig. 1. The electrical circuit, top and back view of the PDA-NFL chip

Conceptualization and Technical mechanisms 

Researchers tried to solve this task by changing chip mechanical structure in the different forms of membrane and piezoresistors  arrangement. There are many attempts to combine wet etching (by Potassium hydroxideor Tetramethylammonium hydroxide) or/and Deep Reactive Ion Etching (DRIE) to create a novel diaphragm, but in time, we understand that there are some limits for low and middle-pressure ranges. The creation of a chip by Bipolar CMOS (BiCMOS) and MEMS technology helps us amplify the output signal, change it from analog to digital, and for example, decrease some numbers of errors introduced by temperature, nonlinearity, and others. 

But we will not be able to achieve a certain threshold sensitivity for different pressure ranges if there are size restrictions, or we will not be able to get uncompensated errors in temperature & mechanical hysteresis, long-term stability, and mechanical overload. These issues can be solved only by MEMS chip. 

Initially, this project was inspired by the idea of the reaction of various transistor structures to mechanical stress. The transistors are smaller than resistors and have a good response to mechanical stress (piezojunction effect for BJT). But after years of development and several wafer batches (epitaxial structures with the crystallographic plane) with different topologies for the pressure sensor chip, the role of BJT became clear. 

It’s difficult to arrange BJT and resistors in a small area between the stress concentrators (in this case, it was about 20x500 microns) for the pressure sensor, PDA-NFL circuit. All work on increasing the sensitivity is performed by piezoresistors with different values. Non-deformable BJT with specific operating points of vertical structure with npn conductivity allows all resistors with p-type conductivity to be connected together, balance the response in a much better way than the Wheatstone bridge circuit, and compensate for temperature errors from BJT by piezoresistors . All data on an operation of pressure sensor chip PDA-NFL from pressure and temperature (the main error) and a technological route for semiconductor structures were simulated before production (Fig. 2). 

Performance and Technical Specifications

Achieved development benefits can be shown in various comparative analyses for pressure sensor chips with the Wheatstone bridge circuit. For example, it can be seen in  Fig. 3a how the chip area can be reduced by 2.4 times while maintaining the same sensitivity (S = 1.9 mV/V/kPa), circuit supply voltage (5 V), and membrane geometry (its topology, production method, and thickness of the thin part). Additionally, the pressure sensor chip PDA-NFL increases the burst pressure by more than 5 times (up to 1.6 MPa) relative to its analog.

Fig. 2. The results of I-V characteristics for BJT and map of mechanical stress for creation of pressure sensor chip PDA-NFL

The next example is the inverse task. Fig. 3b shows the dependence of sensitivity on the thickness of the membrane thin part for two types of pressure sensor chips while maintaining the overall dimensions (4x4 mm2), circuit supply voltage, and also topology and production method for membrane. The new circuit PDA-NFL made it possible to increase the sensitivity by more than 3.5 times (all other conditions being equal). All types of pressure sensor chips were fabricated in the same production line.

 Fig. 3. Examples of the advantages of the new electrical circuit  PDA-NFL for pressure sensing Pressure sensor chip PDA-NFL showed significant advantages compared to piezoresistive analog circuits. 

More detailed comparative characteristics for chip PDA-NFL and analog circuits in terms of sensitivity, nonlinearity, mechanical hysteresis and repeatability, temperature characteristics, and other parameters for the same pressure ranges are given in Table 1.

Tab. 1. Output characteristics of pressure sensor chip PDA-NFL and advanced analogs with Wheatstone bridge circuit 

*  A.V. Tran, X. Zhang, B. Zhu, “Effects of temperature and residual stresses on the output characteristics of a piezoresistive pressure sensor,” IEEE Access, vol. 7, pp. 27668-27676, 2019.

** T. Xu, H. Wang, Y. Xia, Z. Zhao, M. Huang, J. Wang, L. Zhao, Y. Zhao, Z. Jiang.

Potential applications of pressure sensor chip PDA-NFL

The consumption market for pressure sensor chips like PDA-NFL is very wide and significantly depends on the conditions and requirements for different applications. Estimated by data from the reputable analytical company Yole Development, as it can be seen from Fig. 4, the pressure sensor market has 5 main directions for application fields: automotive, consumer, industrial, medical, and defense/aerospace. 

The market (after first-level packaging for MEMS pressure sensors) is currently growing with a CAGR of 5.1% and will reach $2.2B in 2026 (vs. $1.6B in 2020). This sharp rise is clearly related to the market's recovery from COVID-19. The presented pressure sensor chip PDA-NFL, or rather the type of the proposed electrical circuit can find application for all fields. Here is a list of some of them for mass fabrication:

• Automotive: Powertrain, Manifold Absolute Pressure (MAP), Barometric Air Pressure (BAP), particle filter, fuel tank, exhaust gas recirculation, engine oil, automatic transmission oil, Tire Pressure Monitoring System (TPMS), brake booster, side airbags, pedestrian protection.
• Consumer electronics: smartphones, watches, Internet of Things (IoT), smart home.
• Industrial systems: Heating, Ventilation, and Air Conditioning (HVAC), industrial process controls, transportation.
• Medical: Blood Monitoring (invasive), respiratory care, smart inhalers.
• Defense/Aerospace: Air data, Full Authority Digital Engine Control (FADEC), hydraulics.

Fig. 4 MEMS pressure sensor market dynamics. Source: Yole Development

Additionally, the pressure sensor chips PDA-NFL can be used in a variety of studies and research in hydromechanics, robots, biophysics, acoustics, geophysics, and a lot of other projects.

Conclusion

The project presented an opportunity for improvement in piezoresistive pressure sensing technology by on-chip integration of piezosensitive differential amplifier on BJTs. PDA-NFL showed significant advantages in terms of sensitivity and compactness compared to piezoresistive pressure sensors with the Wheatstone bridge circuit.

About the University Technology Exposure Program 2022

Wevolver, in partnership with Mouser Electronics and Ansys, is excited to announce the launch of the University Technology Exposure Program 2022. The program aims to recognize and reward innovation from engineering students and researchers across the globe. Learn more about the program here.

References:

[1] M. Basov, "Pressure Sensor with Novel Electrical Circuit Utilizing Bipolar Junction Transistor," 2021 IEEE Sensors, 2021, pp. 1-4, doi: 10.1109/SENSORS47087.2021.9639504.

[2] “Piezoresistive pressure sensor with high sensitivity for medical application using peninsula-island structure,” Front. Mech. Eng., vol. 12, pp. 546–553, 2017.