Micromachining has emerged as a key technology in the miniaturization of sensors. By reducing the size of the sensing element, it enables significant reductions in overall system size through standard semiconductor fabrication techniques. The integration of signal processing further enhances the efficiency of the sensing element, allowing for a more compact system design and eliminating the need for external pin connections. The choice of micromachining technology can influence the extent of miniaturization, but this is often dictated by the type of sensor being used. For example, piezoelectric micromechanical components for pressure sensing can be built on CMOS silicon substrates, such as a suspended membrane, offering improved performance compared to traditional designs.
Figure 1 illustrates surface micromachined sensing elements. These sensors also help reduce sensitivity and performance issues caused by calibration, although innovative feature design can mitigate these effects. Such design approaches can also integrate multiple functions into a single sensor, which is particularly evident in 6-axis accelerometers. Well-designed elements can provide motion data across multiple axes, contributing to system miniaturization by replacing multiple devices with a single unit. However, this integration often comes at the cost of increased noise and reduced dynamic range. For instance, adding filtering and signal processing to distinguish between different forces on a composite sensing element may slow down the response time and limit the overall dynamic range. While acceptable for applications like wearable electronics, it may not meet the high accuracy demands of small unmanned aerial vehicles (UAVs).
Plasma treatment plays an important role in the development of advanced micromachined sensors. The Kionix KXCJ9 is a 3-axis micromechanical accelerometer that uses plasma micromachining technology. Its sensing element operates based on differential capacitance, with a common-mode design that minimizes errors from temperature and environmental pressure changes. The sensing element is hermetically sealed using a second silicon wafer, helping to reduce the device’s overall size.
Figure 2 shows the KXCJ9 housed in a closed cavity formed by bonding a second silicon wafer. A separate signal processing ASIC is integrated with the sensing element, further reducing the sensor's size. It fits into a compact 3 x 3 x 0.9 mm LGA package, operating on a 1.8–3.6 VDC power supply. The internal voltage regulator ensures stable operation over a wide input range, minimizing power-related errors and eliminating the need for external regulation.
Pressure sensing has also benefited from micromachining advancements. The ST LPS25H is an ultra-compact absolute pressure sensor featuring a monolithic piezoresistive element. It uses a single-crystal silicon substrate to suspend a thin film, resulting in smaller elements than traditional microfilm designs. Internal mechanical stops protect the diaphragm, and it measures absolute pressure ranging from 260 to 1260 hPa, suitable for applications like sports watches and weather stations.
Figure 3 shows the LPS25H in a 10-pin HCLGA package measuring 2.5 x 2.5 x 1 mm. The design allows external pressure to reach the piezoresistive sensing element. It uses a standard CMOS process for highly integrated designs, and its measurement chain includes a low-noise amplifier and a 24-bit ADC with selectable output data rates from 1 Hz to 25 Hz. The sensor can operate in a wide temperature range, from -30°C to +105°C.
Combined sensors are becoming increasingly popular for their ability to enhance user interaction and reduce system complexity. The Freescale MMA8451Q is a triaxial capacitive accelerometer that uses all three axes to provide accurate motion data in a compact 3 x 3 x 1 mm QFN package. It supports gesture recognition, tap detection, and other motion-based functions, making it ideal for wearables and mobile devices. The sensor features customizable timers, programmable thresholds, and dual filtering options (low-pass and high-pass) to improve accuracy and reduce noise.
In conclusion, micromachining technologies offer designers various options for creating compact, high-performance systems. From integrated signal processing to multi-axis sensor fusion, these techniques enable miniaturization while maintaining functionality. However, the underlying sensor technology can impose limitations on performance, so careful evaluation is essential when choosing the right approach for a specific application.
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