While any list of technology trends in sensors would have to include increased stability, MEMS technology, functional integration, wireless technology and the development of new materials, one of the most consistent—and ongoing—trends is the need for miniaturization. In most cases and across most applications, this miniaturization typically affords not just smaller scale, but often dramatic energy-consumption improvement as well.
Sensor miniaturization encompasses more than just the chip itself; it also impacts packaging, assembly and bonding techniques, and the robustness requirements of the sensor.
Because of the need to downsize, chip-scale packaging for surface-mount designs is often only 20 percent greater than the underlying chip. In automotive applications, surface-mount monolithic sensors based on CMOS technology are sometimes stacked to integrate the sensors in system-on-package or system-in-package constraints.
There are several potential stressors accompanying miniaturization. For example, when the sensor assembly experiences mechanical stress, it is thermally induced. Yet the expansion behavior of the sensors and assembly materials can be quite different. Other stressors include the presence of moisture, mechanical disturbance and, in automotive and space applications, the effects of substantial acceleration.
Bonding techniques are currently fairly stable and wire bonds are stable at temperatures of 200 degrees Celsius or more. While the techniques are stable in both ball-wire bonding and the newer wedge-wedge bonding with aluminum wire in non-IC applications, it is common practice to accommodate shrinking diameters of less than 20 μm.
Finally, flip-chip assembly and lead-free solder are both evolving and leading to greater temperature stability and lead-free sensor technologies.
A wide array of applications
Whether the intended application is robotics, cell phones, proximity detection, medical, automotive or industrial, sensors are becoming substantially smaller than their predecessors.
In the temperature sensor segment, the Rohm Semiconductor BDE1250GTR-ND is a small, high-accuracy temperature sensor IC series with low quiescent current (16 µA) and high-accuracy thermostat (temperature switch) IC, as seen in Figure 1. It features a tiny package (typically 2.90 x 2.80 x 1.25 mm) for use in applications such as thermal protection for electrical equipment, including notebooks, cell phones, FPD-TV and fan control.
While Honeywell has offered its TruStability sensors for a while, the recent NSC series of uncompensated and unamplified board-mount pressure sensors shown in Figure 2 target a variety of industrial and medical applications.
The series offers many package sizes, including a miniature 10 x 10-mm (0.39 x 0.39-in.), which is very small compared to most board-mount pressure sensors. It occupies less area on the PCB and typically allows for easy placement on crowded PCBs or in small devices. The extremely low power consumption, with an operating supply voltage as low as 1.8 VDC reduces power consumption, provides extended battery life and promotes energy efficiency. Frequency response is limited only by the end user’s system. The sensor will operate as specified from -40-85 degrees Celsius (-40-185 degrees Fahrenheit).
The NSC series is intended for use with non-corrosive, non-ionic gases, such as air, and other dry gases for non-corrosive and non-ionic liquids.
In addition to the obvious trend toward smaller, low-power electronics requirements, several application areas have their own unique demands for miniature sensors. One of the most compelling examples is within the medical industry, where miniature intelligent sensor systems will increasingly be used to monitor patients remotely. These sensors will have the ability to monitor vital data around the clock.
Advances in miniaturization, signal process and data processing will be required to ensure medically valid information is available. Not only for remote monitoring of patients, but also to improve safety in respiratory and neonatal applications, motion sensors for the elderly, the measurement of range of motion, and more.
Miniature sensors also are finding their way into aircraft, aquatic, e-nose, imaging, motes, lunar locomotion, gas, homeland security and other advanced applications. Major efforts to continue to improve stability, packaging, manufacturability, and design of these sensors can be expected long into the future.