Next Generation MEMS thermopile IR sensor 2.0 for Internet of Things, smart homes, environment and energy technology

Rajesh Uppal June 2, 2019 Nanotech, Photonics Comments Off on Next Generation MEMS thermopile IR sensor 2.0 for Internet of Things, smart homes, environment and energy technology 998 Views

In the wake of increasing electronic gadget sales, the overheating problem for battery chargers has become an increasingly common safety issue. Overheating of chargers certainly can be caused by substandard chargers, faulty batteries, or by misuse of the chargers. In worst-case scenarios the devices melt or catch fire. Unsurprisingly, this kind of electrical fires have increased in recent years. When charging a smartphone or a tablet you provide a big boost of power into a small size battery, this in turn increases the risk of overheating since a considerable amount of energy is transferred into a smaller space (the battery).

Overheating problem for battery chargers has become an increasingly common safety issue – not the least around Christmas – causing fires at home at personal risk and dire costs for private persons as well as insurance companies. The U.S. Consumer Product Safety Commission reports that deaths, injuries, and property damage from consumer product incidents cost the U.S. more than $1 trillion annually.

Therefore, many have found it imperative to come up with solutions that can tackle this big safety issue in our homes and offices. A new kind of IR sensor – also referred to IR sensor 2.0 – has been found to have the properties required to cope with the overheating problems that cause so much damage. This new sensor is based on nanotechnology which provides it with several advantages, one of them allowing it to be easily integrated into chargers and function as an overheating alarm for chargers in mobiles, laptops and tablets.

JonDeTech’s sensors are very small and thin (thickness 0.17 mm), compared to conventional sensors, which enables them to be integrated into a variety of products at a low cost. IR sensor 2.0 can be produced at high volumes at a very low cost, making it suitable for consumer electronics, and other areas where there is a need for high-volume IR sensors. Due to the sensor’s unique properties it opens up for entirely new types of applications using infrared sensors, such as Internet of Things, buildings, smart homes, connected cities, environment and energy technology, consumer electronics, cars, clothing, MedTech and security solutions.

A new IR sensor can solve overheating issue

The overheating alarm consists of a small device that attaches to a charger and monitors the temperature using the sensor, a so-called thermal radiation sensor. This new generation of thermopiles (electronic devices that convert thermal energy into electrical energy) generates a direct, proportional voltage response when subjected to infrared radiation from the charger it is attached to. In more technical terms, the sensor registers wavelengths from 250nm – 22,5μm that is visual light reaching up to the mid-infrared region. The unique geometrical configuration where the thermocouples are vertically arranged enables the sensor to measure heat flux as well as temperature.

The sensor itself is based on nanotechnology, which allows many advantages. It only measures 0.17 millimetres in thickness and is almost entirely made of plastic. It is a very robust piece of plastic that can be used as is, without any metal casing – a protective housing – that is being used by many other producers.

The IR sensor 2.0 is extremely thin, bendable and can be manufactured in a cost efficient way in substantial volumes. This makes it suitable for all kinds of applications that require small sensors where temperature or heat flow is to be measured.

Thermopile sensors

Uncooled infrared (IR) imagers have attracted a great deal of interest due to their wide range of practical applications. Their greatest advantage is that they do not need expensive deep cooling systems in contrast to semiconductor quantum infrared photodetectors. In recent years, thermopile IR imager arrays have been actively developed on the base of MEMS technology, which is compatible with the silicon CMOS batch technology.

Thermopiles convert thermal energy (temperature differences) to electrical energy. Thermopiles are built from a series of so called thermocouples, i.e. two leads (also referred to as legs) of different/disparate materials (typically metal/alloy or semiconductor materials) attached to each other at two junctions. When the junctions are held at different temperatures, an electrical voltage is produced in the circuit. The physical principle of this is called the thermoelectric effect, or the Seebeck effect. This voltage is proportional to the applied temperature difference.

Hence, the thermoelectric circuit can be used as a heat flux sensor, a differential temperature sensor, or a thermal generator. They are also commonly used as thermal radiation sensors for infrared light. Depending on the application, other thermal radiation sensors, such as bolometers and pyrometers, can be used.

A thermopile infrared sensor has a number of thermocouples connected in series that have their ‘hot’ junctions attached to a thin infrared absorber, usually on a micro-machined membrane on a silicon chip.

These kinds of sensors can be used to measure the object’s temperature or relative temperature differences and can also measure presence and motion. Another attractive application for thermopile sensors is temperature monitoring of moving objects, where contact-based temperature sensors have serious drawbacks. These sensors use infrared (IR) radiation for heat transfer and are commonly used in portable devices such as infrared thermometers.

Thermopiles are the most useful sensor technology for several commercial applications such as smoke and gas detection, motion, absolute temperature measurements, heat measurement and control of heat sensitive parts and plumbing.

MEMS thermopile

A sensitive MEMS element comprises an IR high-absorbance dielectric membrane and micro-thermopiles, with the hot contact laying on the membrane and the cold one being in a good heat contact with the substrate. The output voltage of the thermopile, which is generated during the membrane heating (Seebeck effect) is read out by CMOS circuit, which is formed directly on the same chip. Good thermal isolation and low thermal capacity of thermopile-based MEMS elements are the good foundations for the development of uncooled thermal IR-image sensors with high sensitivity, linearity, low power consumption, and high responsivity.

SiNx layer may serve as an IR-absorbing material which provides quite effective IR absorption in the specified wavelength region of 8–14 lm. The thermopile is made of anisotype (p and n-doped) polysilicon buses which are characterized by sufficiently high Seebeck coefficient.

Micro-thermocouples are fabricated from polysilicon that ensures sufficiently high thermo-power while their conductivity and thermo-power can be adjusted by the proper doping. Besides, polysilicon is technologically well-proven and widely used material in CMOS technology, which makes it possible to form the sensitive elements and the electronic readout circuit in one chip, to produce efficient imagers and to reduce the production cost.

The process technology of this MEMS sensor is well compatible with standard CMOS batch technology. Vacuum-sealed package design is supposed to prevent from heat sink through the gas atmosphere.

Vertical Thermocouple IR sensor

The thermopile leads can be arranged in different manners; for most commercial sensors of today, typically, horizontal or “in-the-plane” architecture is used. However, this is not always the best sensor solution since the hot and the cold junctions are placed next to each other on the same surface. Therefore full surface of horizontally configured thermopiles cannot be used, neither can true “contact mode” be utilized.

The advantage of the vertical thermocouple lead arrangement (perpendicular to plane configuration) is that the hot and cold junctions are separated by the thickness of the substrate–the heat gradient is through the substrate. The object measured can be close to the sensor or actually come in contact with the sensor.

Vertical or “out-of-the-plane” configuration requires that the leads of the thermocouples are created through the substrate material, i.e. “via” interconnections are required. These vias have to be very small, which has been difficult to achieve previously. However, thanks to creative thinking and research at Uppsala University, an innovative idea with thermocouple lead structures based on nanotechnology has been realized, which is what JonDeTech AB offers.