A Comprehensive Review of MEMS-Enabled Dual-Mode Pressure and Temperature Sensors

: This paper offers an examination of MEMS-enabled dual-mode sensors, emphasizing key concepts, technologies, and developments in the area. Micro-Electro-Mechanical Systems, or MEMS, have revolutionized sensor design, enabling the creation of small, effective, and multipurpose devices for simultaneous monitoring of temperature and pressure. The review begins with how MEMS and Integrated Circuits (ICs) are integrated into sensor systems and explores MEMS incorporation into dual-mode sensors, clarifying crucial manufacturing processes and design approaches. Furthermore, the paper classifies and examines basic MEMS sensor types, such as Surface Acoustic Wave (SAW), Barometers, Differential Pressure Sensors


INTRODUCTION
Micro-Electro-Mechanical Systems (MEMS) have transformed the field of sensor technology by allowing for the development of small, effective, and adaptable devices for simultaneous temperature and pressure monitoring.By combining MEMS and ICs, MEMS sensors are able to overcome conventional size limitations and achieve unprecedented levels of performance and versatility.The review begins with a thorough analysis of MEMS integration into dual-mode sensors, clarifying important production procedures and design techniques.We provide a thorough taxonomy of dual-mode sensing technologies by breaking down different MEMS sensor types, among them Force Sensors, Accelerometer-Based Pressure Sensors, Magnetometer-Based Pressure Sensors, Surface Acoustic Wave (SAW), Barometers, Differential Pressure Sensors, Absolute Pressure Sensors, Strain Gauge Pressure Sensors, Accelerometer-Based Pressure Sensors, and Fiber Bragg Grating (FBG) Temperature Sensors.The review aims to deconstruct each type of MEMS sensor to reveal its intrinsic operating principles, sensor topologies, and compensating mechanisms.The importance of temperature compensation is highlighted in particular, as it guarantees the precision and dependability of dual-mode measurements under a variety of operating circumstances.

SYSTEM DETAILS A. Objectives
• Provide a thorough understanding of MEMS-enabled dual-mode pressure and temperature sensors, covering integration methods and sensor technologies.

MEMS INTEGRATION A. Need
For external electrical connectivity and intelligence, which are essential for signal processing and system control, MEMS packages need to be integrated with ICs (Integrated Circuits).Miniaturization, performance, and functionality are all improved by integration.Shorter signal pathways, more reliable systems, and compact system designs are made possible by it.For advanced sensor systems to be realized in a variety of applications, MEMS and IC integration is essential.

B. Methods
System-on-a-chip (SoC) or multi-chip solutions are used for integration.MEMS and ICs are manufactured independently and then hybridized in multi-chip solutions.Making MEMS and IC components on the same substrate is a requirement for SoC solutions.MEMS and IC integration methods are based on either hybrid multi-chip solutions or system-on-chip solutions as depicted in fig 4

Fig 4 MEMS Integration methods
The Hybrid integration of MEMS and ICs being one among the MEMS integration methods entails : • Independently Manufactured Components: The design, manufacturing, and testing operations for MEMS and IC wafers are carried out independently of one another.• Packaging and Integration: After processing, the wafers are divided into individual chips.After that, these chips are assembled and incorporated into systems with several chips.Usually, this integration takes place at the package or board level.
• Methods: MEMS and IC chips are frequently bonded together via wire bonding or flip-chip bonding and they are positioned side by side in a package.While flip-chip bonding uses solder balls or bumps to attach chips upside down, wire bonding uses metal wires to build electrical connections.• Benefits: Modularity, versatility, and very cheap development costs are some of the benefits that come with multi-chip modules.They make it possible to quickly change product specifications and complete development cycles.To further reduce complexity, MEMS and IC chips can be verified independently prior to packing.• Cons: Nevertheless, multi-chip solutions have limits, including thicker layers, bigger footprints, and fewer integration densities.Extended electrical connections among chips may result in lower electromagnetic compatibility and parasitic effects, particularly for devices like as capacitive transducers.Furthermore, it might be difficult to trace individual chips over the course of a product's lifecycle, especially in sectors like the automotive industry where quality standards are stringent.

C. Techniques
Bulk or surface micromachining can be used for MEMS-first processing, MEMS-last processing, or interleaved MEMS and IC processing.These are examples of monolithic MEMS and IC integration approaches.Based on the needs of the application, a particular technique is chosen from those that offer significant advantages.

MEMS INTEGRATION IN DUAL-MODE SENSOR
The research by Chen et.al. presents a novel kind of sensor that uses a single chip to sense temperature and pressure.The sensor is constructed on a silicon-on-insulator (SOI) wafer, which is renowned for its reliable insulating qualities and constant thickness.In order to detect pressure, sensors were traditionally made by etching holes into silicon wafers.But this procedure made the wafer weaker and necessitated a bigger sensor area.In order to overcome this, the scientists employed an alternative etching technique known as ICP (Inductively Coupled Plasma) etching, which enabled them to make smaller caverns and lower the price per chip.They additionally made use of PN junctions to incorporate two temperature sensors onto a single chip.These junctions use extremely little power and are capable of measuring temperature precisely.There are no additional processes or costs involved in the manufacturing process of these temperature sensors because it is compatible with the manufacture of pressure sensors.All things considered, the sensor satisfies the design specifications well thanks to its great sensitivity to temperature and pressure.It's a major development in sensor technology, particularly for tire pressure monitoring systems (TPMS) applications where temperature and pressure data are essential.

SURFACE ACOUSTIC WAVE MEMS
Temperature-sensing Surface Acoustic Wave (SAW) MEMS (Microelectromechanical Systems) pressure sensors provide a flexible option for a range of applications needing simultaneous temperature and pressure readings.To detect pressure changes, SAW devices use acoustic waves that propagate along the surface of a piezoelectric substrate, such quartz.The gadget changes the frequency of the acoustic wave in response to pressure, giving a precise measurement of the pressure.By accurately accounting for the impacts of temperature on sensor performance, the integration of temperature sensing into SAW MEMS pressure sensors improves their usefulness.If temperature changes are not appropriately adjusted for, they can have a considerable impact on sensor readings and result in mistakes.SAW MEMS sensors can provide real-time temperature compensation by combining temperature detecting elements with pressure sensing components, guaranteeing accurate and dependable results.

Fig 4 SAW based MEMS
Microelectromechanical systems (MEMS) technology is the foundation for differential MEMS pressure sensors that have integrated temperature sensing capabilities.These sensors have tiny diaphragms that shift in response to variations in pressure between two spots.They also have temperature sensors to keep an eye on variations in temperature.The diaphragm moves in response to pressure, changing capacitance or resistance, which is then measured to ascertain the pressure differential.Accurate pressure readings under a range of situations are guaranteed by the integrated temperature sensors, which monitor temperature variations.For applications requiring accurate pressure monitoring in dynamic situations, these sensors' combined capacity to sense temperature and pressure makes them highly valuable.The suggested microsensor has a pressure-sensitive diaphragm on the handle layer that is coupled to both side and center resonators on the device layer.The intrinsic frequencies of these resonators are engineered to produce a differential output (f1−f2) that is responsive to variations in pressure.Thermal simulations demonstrate that a Pt film temperature sensor on the handle layer precisely uses silicon's strong thermal conductivity to monitor the temperature of the resonators in order to offset temperature fluctuations.Moreover, the effect of external disturbances is lessened by a static pressure sensor based on an Hshaped resonator on the device layer.Due to its increased sensitivity, it can be utilized for static pressure compensation and offers more stability even if it is less sensitive to temperature and differential pressure.For the purpose of detecting differential pressure, the microsensor has side and central resonators.A differential output is produced by the similar but opposing frequency vibrations of these resonators.An H-shaped resonator on the device layer measures static pressure, while a Pt film-based sensor on the handle layer monitors temperature.After combining these parts, the differential pressure (DP) adjusted model is expressed as follows: where f1−f2, R, and f3 stand for the differential output, temperature sensor, and static pressure sensor outputs, respectively.Applying pressure causes the pressure-sensitive diaphragm to distort, changing the resonators' frequencies.The sensors in question are used to reduce measurement errors by mitigating the effects of temperature and static pressure disturbances.

BAROMETER
When combined with integrated temperature sensors, MEMS barometers-an acronym for Microelectromechanical Systems-offer a compact and efficient way to measure ambient pressure and account for temperature variations.They come in particularly useful in situations that call for accurate pressure measurements, such as altitude sensing, weather forecasting, and interior and outdoor navigation systems.Pressure Sensing Mechanism: A tiny diaphragm or membrane in MEMS barometers responds to changes in atmospheric pressure by bending in the appropriate direction.This bending is converted into an electrical signal using a variety of techniques, including capacitive, piezoresistive, and piezoelectric sensing.Temperature Compensation: Variations in temperature can cause estimates of barometric pressure to be inaccurate.MEMS barometers incorporate temperature sensors directly onto the same chip or package in order to combat this.These sensors may be based on temperature-dependent diodes, thermistors, or RTDs, among other technologies.Algorithms for calibration and compensation are utilized to adjust the sensor's output and rectify temperature-related inaccuracies in pressure measurements.To ensure accuracy in a variety of settings, they adjust pressure measurements based on information from the embedded temperature sensor.Digital Interface: A lot of contemporary MEMS barometers have SPI or I2C digital interfaces, which make it simple to connect them to microcontrollers and other digital systems.This facilitates advanced signal processing techniques to increase accuracy and performance while also simplifying communication and setup.

ABSOLUTE MEMS
Temperature-sensing and pressure-sensitive components are combined to provide absolute MEMS pressure sensors with temperature correction.In order to guarantee reliable readings, they are made to monitor temperature changes in addition to pressure variations.The primary component of the sensor is a pressure-sensitive diaphragm or structure that bends or deforms in response to changes in pressure.The electrical or mechanical characteristics of the sensor, such as its capacitance, resistance, or resonance frequency, are changed by this bending.The design incorporates temperature-sensing components to counteract the effects of temperature variations on sensor performance.These components continuously sense the ambient temperature since they are frequently dependent on resistance or capacitance variations with temperature.Any mistakes in pressure readings resulting from temperature differences are subsequently corrected using the temperature data collected.The sensor is calibrated to determine the link between temperature variations and pressure measurements during manufacture or setup.In order to develop compensation algorithms or tables for modifying pressure readings based on temperature, this entails testing the sensor under known pressure and temperature settings.After signal processing, the sensor's output is applied to compensate for temperature-related mistakes in pressure measurements by using algorithms based on temperature data.As a result, accurate pressure measurements with consistent temperature adjustments are obtained.Absolute pressure sensors use a reference vacuum inside the sensor to detect pressure.These sensors are frequently used in automotive systems, such as manifold absolute pressure (MAP) sensors for airflow control and ignition, for measuring atmospheric pressure.Additionally, they are used in launch and cabin pressure management.

STRAIN GAUGE MEMS
When a force, such as pressure or tension, is applied, a strain gauge is a sensor that measures changes in resistance.It is frequently used to quantify mechanical stress by converting it into electrical impulses.Temperature and pressure monitoring is dependable with MEMS strain gauge pressure sensors, which are based on the concepts of metal resistance strain gauges.In order to precisely account for changes in both pressure and temperature, these sensors incorporate temperature sensors and strain gauges into their design.The strain gauge, temperature monitoring components, insulation, and signal wires are all part of the sensor's architecture.The resistance of the strain gauge is carefully adjusted to balance sensitivity with other considerations such as power consumption and electromagnetic interference.The output from these sensors is handled by circuits for amplification and conversion, just like with conventional strain gauges.Pressure and temperature variations are represented by the ensuing electrical signal, which can be utilized for display or analysis among other things.Tire pressure monitoring systems (TPMS) for automobiles frequently use MEMS strain gauge pressure sensors.By giving drivers access to real-time pressure data, they help ensure tire performance and safety by assisting them in identifying problems such as under-or over-inflation.These sensors compensate for temperature variations by measuring temperature as well, guaranteeing precise pressure readings under all circumstances.This lessens the likelihood of issues like tire blowouts and uneven tire wear.By supplying information for traction control and stability management algorithms, these sensors enhance vehicle safety.Precise monitoring of temperature and pressure aid in maximizing vehicle performance.The sensor is able to deliver precise pressure readings over a wide range.Data Fusion and Processing: To extract pertinent information, data from the temperature, magnetic field, and pressure sensors are combined and processed.The precision and dependability of the measurements can be increased by using sophisticated signal processing techniques, such as sensor fusion algorithms or Kalman filtering, particularly in dynamic environments with changing conditions.Digital Interface and Communication: For simple integration with microcontrollers and digital systems, a lot of MEMS sensor systems provide a digital interface, like SPI or I2C.This makes it possible for simple configuration, connectivity, and sensor data retrieval.Miniaturization and Low Power Consumption: Highly miniaturized sensors with low power consumption may be created using MEMS technology, which qualifies them for battery-operated and portable applications.

MEMS FORCE SENSOR
MEMS force sensors with temperature adjustment function by employing several techniques, such as piezoresistive, capacitive, or piezoelectric sensing, to translate mechanical force into electrical signals.For example, capacitive sensors assess changes in capacitance resulting from force-induced deflection, while piezoresistive sensors use specific resistors that alter resistance when force is applied.An electrical charge is produced by piezoelectric sensors in direct proportion to applied force.Temperature compensation circuitry is built into sensor designs to counteract the impact of temperature variations on sensor accuracy.To monitor temperature changes, this circuitry may include temperature-sensitive parts like resistors or diodes.The sensor output is subsequently modified by algorithms based on temperature information to preserve accuracy throughout a broad temperature range.The sensor's response to force at various temperatures is characterized during the production process through calibration methods.Using this information, compensation algorithms are developed to fix mistakes in the sensor output caused by temperature changes.These algorithms can be applied in software, firmware, or hardware, based on the application and design of the sensor.For convenient communication with external devices, MEMS force sensors are equipped with a digital interface such as SPI or I2C.Techniques for digital signal processing can enhance sensor accuracy and dependability even more.These strategies include linearization techniques to enhance linearity, averaging techniques to lower measurement errors, and filtering algorithms to eliminate noise.Compact sensors with minimal power consumption may be created thanks to MEMS technology, making them perfect for portable battery-operated devices.Additionally, because of their compact size, they can be integrated into systems with limited space, where larger sensors would not fit.

MEMS FIBER BRAGG GRATING
MEMS Fiber Bragg Grating (FBG) temperature sensors with integrated pressure sensing capabilities are advanced devices that combine FBG technology for temperature measurement with the ability to sense pressure.These sensors are highly useful in industries like aerospace, automotive, oil and gas, and industrial process control, where monitoring temperature and pressure simultaneously is crucial.FBG Temperature Sensing: Special optical fibers known as FBGs have a refractive index that varies periodically across their length.A portion of light that travels through an FBG is reflected back at a wavelength known as the Bragg wavelength.With temperature variations, this wavelength shifts, providing a precise temperature reading.Pressure Sensing Mechanism: MEMS FBG sensors detect pressure in a number of ways.A typical method involves including a structure that moves in response to pressure, such as a membrane or diaphragm.The Bragg wavelength shifts as a result of this movement's impact on the strain on the optical fiber that houses the FBG.On the other hand, variations in the pressure inside the surrounding medium may also impact the Bragg wavelength.Temperature and Pressure Sensing Integration: These sensors' MEMS construction enables the placement of both temperature and pressure sensing components on a single chip or substrate.The proximity of the pressure sensing element and the FBG temperature sensor allows for simultaneous measurement of pressure and temperature, improving the sensor's performance.Optical Interrogation: Techniques such as wavelength demodulation and optical spectrum analysis are used to examine the optical signal from the FBG sensor.Temperature and pressure readings are obtained from changes in the Bragg wavelength caused by changes in these parameters.Benefits of optical interrogation include the capacity to use a single optical fiber for many sensors and immunity to electromagnetic interference.Signal Processing and Calibration: The sensor's output signals, representing temperature and pressure measurements, may be processed further to correct for any inaccuracies, cross-sensitivity, or drift.Advanced algorithms and calibration techniques ensure precise and dependable sensor performance across different conditions.

MEMS ACCELEROMETER
Modern systems used for precise pressure measurements include MEMS accelerometer-based pressure sensors with temperature monitoring, particularly in environments where temperature variations may have an impact on accuracy.These sensors function by integrating temperature monitoring and pressure sensing into a single unit.MEMS accelerometers react to changes in pressure in addition to changes in acceleration utilizing capacitance or piezoelectric principles.The ambient temperature is continuously monitored by temperature sensors such as thermistors or resistance temperature detectors (RTDs).The MEMS accelerometer produces electrical impulses according to applied pressure when pressure varies.However, due to variations in sensor characteristics, temperature fluctuations might create mistakes in pressure readings.Temperature sensors offer exact correction of pressure readings caused by temperature variations by providing real-time temperature data.The MEMS accelerometer, temperature sensors, and signal conditioning circuitry are all part of the sensor's internal construction.The circuitry uses algorithms to compensate for temperature-induced mistakes in pressure measurements as it processes accelerometer and temperature data.Accurate pressure readings that have been compensated for temperature variations are provided by the resultant output, guaranteeing dependable performance in a variety of industries such as consumer electronics, automotive, and aerospace, where accurate pressure measurements are essential for efficiency and safety.Both moveable and permanent plates function as capacitor plates in the sensor's operation, and when acceleration takes place, the proof mass moves and modifies the capacitance between the plates.By measuring the separation between the capacitor plates, the sensor determines acceleration and outputs a voltage that is proportionate to acceleration.

Fig 8 MEMS Accelerometer
The movable plates and the fixed outer plates act as the capacitor plates.When acceleration is applied, the proof mass moves accordingly.This produces a capacitance between the movable and the fixed outer plates.When acceleration is applied, the distance between the two plates displace as X1 and X2, and they turn out to be a function of the capacitance produced.From the image above it is clear that all sensors have multiple capacitor sets.All upper capacitors are wired parallel to produce an overall capacitance C1 and the lower ones produce an overall capacitance of C2.If Vx is the output voltage of the proof mass, and V0 is the output voltage produced between the plates, The figure below shows the circuit that is used to calculate the acceleration, through change in distance between capacitor plates.The output obtained for different values of acceleration is also shown graphically.

Fig 9. capacitor type mems accelerometer
When no acceleration is given (a=0), the output voltage will also be zero.When acceleration is given, such as (a>0), the value of value of Vx changes in proportion to the value of V0.When a deceleration is given, such as (a<0), the signals Vx and Vy become negative.He demodulator produces an output equal to the sign of the acceleration, as it multiplies both the values of Vy and V0 to produce VOUT, which has the correct acceleration sign and correct amplitude.The length of the distance, d and the proof mass weigh is surprisingly very small.The proof mass weighs no more than 0.1 microgram and the output capacitance is approximately 20 aF and the plate distance is no more than1.3micrometers.MEMS accelerometers have various applications, including step counting in mobile phones, tilt sensing in cameras, stability control in camcorders, and protection of hard disk drives in laptops from impact damage.

CONCLUSION
A wide variety of sensing technologies and integration strategies are available for simultaneous pressure and temperature measurements, as demonstrated by the overview of MEMS-enabled dual-mode pressure and temperature sensors.When it comes to sensitivity, precision, and suitability for various applications, MEMS-based sensors such as piezoelectric, surface acoustic wave (SAW), resonant, thermal, optical, and capacitive offer distinct benefits.These sensors directly integrate temperature detecting elements onto the same chip or package, addressing the issue of temperature changes effecting pressure measurements.Under a variety of operating situations, MEMS sensors can produce measurements that are precise and dependable thanks to calibration, compensation algorithms, and signal processing techniques.Furthermore, their small size and low power consumption make them appropriate for battery-operated and portable devices across a range of industries, such as consumer electronics, automotive, aerospace, medical, and industrial automation.

•
Explore sensor working principles and architectures, focusing on accurate measurements across varying conditions.• Emphasize temperature compensation's role in enhancing sensor reliability in dynamic environments.• Discuss recent advancements, challenges, and future directions in MEMS dual-mode sensing for diverse applications.B. Block Diagrams The following figures fig 1, fig 2 and fig 3 depicts the block diagram of MEMS package, its manufacturing and a microsystem unit respectively.

Fig 1 Block
Fig 1 Block Diagram of MEMS Package

Fig 5 SAW
Fig 5 SAW based differential MEMS