An infrared (IR) sensor may have many applications. For example, an IR sensor may be used for automatic lighting control, intrusion detection, and thermal imaging for medical, security, and automotive purposes. There has been a trend of moving away from ‘cooled’ IR sensors towards ‘uncooled’ approaches, primarily for cost effectiveness, and improving the versatility and robustness of the sensor, however trading off some performance parameters.
One approach to passive infrared detection is a pyroelectric detector, for example. A pyroelectric detector is typically constructed from lead zirconate titanate (PZT) using a sol-gel deposition method. While a pyroelectric detector may have relatively good sensitivity (V/W) to infrared radiation, it has several performance shortcomings. In particular, a pyroelectric detector typically requires constant motion, has Z-directional insensitivity, is prone to false alarms, and may have a relatively low response time. Moreover, a pyroelectric detector may be relatively expensive.
Another approach to passive infrared detection is a microbolometer. A microbolometer is used particularly for thermal imaging applications due to cooling requirements. Current cooling approaches are generally implemented using a cryogenic system, for example, which make an IR sensor relatively bulky and less robust. While an un-cooled version of a microbolometer exists, which is cheaper than its cooled counterpart, it has still not found entry into the commercial market because of its relative fabrication complexity.
One cost-effective approach may be a thermopile, which, for example, can detect immobile warm bodies, has reduced Z-directional insensitivity, has lower false alarms, and a relatively quicker response. However, one major drawback comes in terms of the sensitivity which could be a little lower than half that of a pyroelectric sensor, for example. However, this parameter is a trade-off between physical device size and performance requirement based upon an end application.
One problem common to all three above-noted uncooled sensors may be noise due to self-heating. Self-heating may be caused by incident infrared rays being absorbed by the housing, current consumed by the application specific integrated circuit (ASIC), or the device elements themselves, for example.
One approach for improving sensitivity of an IR sensor involves the use of high vacuum level packaging. However, high vacuum level packaging may be problematic with respect to product lifetime. Another approach for improving sensitivity of an IR sensor involves the use of IR filters with an anti-reflective coating (ARC).
Yet another approach for improving sensitivity of an IR sensor is the use of cooling, especially for a microbolometer. In some applications, an on-board temperature sensor may also be used to improve self-heating.
U.S. Patent Application Publication No. 2008/0171249 to Arana et al. discloses a thermally efficient micromachined device. More particularly, Arana et al. discloses a micromachined device that processes at least one fluid stream and includes at least one fluid conducting tube having at least a region with wall thickness of less than 50 μm. The device optionally includes one or more thermally conductive structures in thermal communication with first and second thermally insulating portions of the fluid conducting tube. The device also may include a thermally conductive region, and at least a portion of the fluid conducting tube is disposed within the region. A plurality of structures may be provided projecting from a wall of the fluid conducting tube into an inner volume of the tube. The structures enhance thermal conduction between a fluid within the tube and a wall of the tube.
Several references disclose a Peltier cooler integrated into the same plane as thermocouples. However, this may lead to increased noise in thermocouple readout as the Peltier coolers are current consuming devices, and the heat generated may interfere with the IR signals read by the thermocouples.
Further improvements to an IR sensor may be desired. More particularly, it may be desirable to reduce self-heating, for example, at the package level, to thus increase operational and cost efficiency.