Have you chosen the right temperature measurement method among the three methods in MEMS?

In the field of ambient temperature measurement for integrated MEMS chips, thermistors, thermopiles, and PN junctions are the three mainstream technologies. Thermistors utilize the constant temperature-resistance coefficient of a thermistor, such as platinum metal or implanted silicon, meaning that its resistance changes linearly with temperature. This resistance change directly corresponds to the absolute temperature and requires a constant current source. Thermopiles, based on the Seebeck effect, convert temperature differences into voltage using multiple pairs of thermocouples connected in series. The cold junction temperature must be independently measured to infer the target temperature. In MEMS chips, polysilicon/aluminum thermocouples are stacked on a silicon nitride suspension membrane. The hot junction absorbs infrared radiation, while the cold junction is anchored to the substrate. PN junction temperature measurement involves a semiconductor PN junction under a constant forward current, where the junction voltage drop is inversely proportional to the absolute temperature (approximately -2mV/°C). The voltage or digital code corresponds to the absolute temperature, eliminating the need for an external reference.

Platinum resistance thermometers are an option for high-precision industrial monitoring. Their advantages include a wide temperature range (-200°C to 800°C) and good long-term stability. However, caution is advised in power- and cost-sensitive applications, as errors caused by self-heating should be avoided. They are suitable for monitoring power equipment or high-temperature reactors. For example, the temperature inside chemical reactors typically ranges from -50°C to 500°C and requires precise control (e.g., polymerization reactions require an error within ±0.5°C). In this case, PT100 thermistors are an option, as they cover a temperature range of -200°C to 850°C, offer Class A accuracy (0.15°C + 0.002|t|), are vibration-resistant and corrosion-resistant (after packaging), and offer long-term stability.

For non-contact temperature measurement, thermopile temperature measurement is recommended. The advantage is that physical contact is eliminated, but an integrated cold-junction solution is required to improve real-time compensation. They are suitable for medical forehead thermometers and household appliance anti-scalding solutions (such as ceramic hobs). For example, surface temperature measurement of high-voltage motors is required. Since motors are electrically active during operation and cannot be touched, real-time monitoring of overheating (50°C to 150°C) is required. Thermopile sensors can be installed 10-30cm from the motor and quickly capture surface temperature using infrared radiation, minimizing the risk of electric shock.

Embedded systems prioritizing energy efficiency choose PN junction temperature measurement, offering advantages such as ultra-low power consumption and digital interfaces (I²C/single-bus), but are not suitable for high-temperature environments. For CPU temperature monitoring, computer CPUs operate between 40°C and 100°C, requiring real-time temperature feedback to adjust fan speed. PN junction sensors built into CPUs are micrometer-sized and can be directly attached to the core chip, responding to temperature changes within 0.01 seconds at an extremely low cost (integrating them within the chip requires no additional cost). For internal temperature measurement in lithium batteries, lithium batteries must be kept between -20°C and 60°C during charging and discharging to prevent overheating and fire. PN junction sensors can be embedded within the battery cell (due to their small size and structural integrity), capturing minute temperature changes (such as a 0.5°C fluctuation during charging and discharging) with high sensitivity, enabling timely power-off protection in conjunction with the BMS system.