TMOS is a new type of thermal sensor consisting in a micromachined thermally isolated transistor fabricated using CMOS-SOI(Silicon on Insulator) MEMS(Micro electro-mechanical system) technology. It has been developed in the last decade by the Technion - Israel Institute of Technology. A thermal sensor is a device able to detect the thermal radiation emitted by an object located in the FOV(Field Of View) of the sensor. Infrared radiation ( IR ) striking the sensor produces a change in the temperature of the device that as a consequence generates an electric output signal proportional to the incident IR power. The sensor is able to measure the temperature of the object radiating thanks to the information contained in the impinging radiation, exploiting in this sense Stefan - Boltzmann law. TMOS detector has two important characteristics that make it different from others: it's an active and uncooled sensor.
Fabrication process
A TMOS detector consists in a mosaic structure composed of several sub-pixels, which are electrically connected in parallel or in series or in a mixed combination, and are thermally isolated. In each sub-pixels the sensitive element is the TMOS sensor, that is suspended in vacuum, fabricated in CMOS - SOI technology and dry released. The mosaic structure includes: the pixel frame, the suspended transistor, that absorbs IR radiation and that could also be embedded in an absorbing IR membrane which determine the thermal capacitance of the sensor, and two folding arms that determine the sensor thermal conductance.
TMOS fabrication is based on built - in masks and dry bulk micromachining. In TMOS fabrication to the standard CMOS - SOI technology, used to produce MOS transistor, is added a MEMS post process necessary to realize the folded arms and the suspension of the transistor. In standard CMOS process there are several metallization layers. In TMOS production the upper ones, made in aluminum or copper, are used as built - in masks. Both metals are not affected by the fluorine plasma, used to dry etch silicon and interlevel dielectrics. The use of built- in mask grants high alignment accuracy and resolution while reducing fabrication costs. Final step of MEMS post process is the metal mask removal. This step is performed using standard wet etchant of aluminum or copper.
At present 130 nm CMOS - SOI technology implemented on 8 inch wafers is used to produce TMOS sensors, employing wafer level processing in standard CMOS facilities, allowing cost reduction and large production volumes.
Packaging
To improve sensor's performance and to protect it from the surrounding environment, especially from moisture, TMOS sensor are packaged under vacuum. The wafer-level production enables also wafer-level packaging, allowing the possibility to integrate optical windows and filters to improve their efficiency and widening their applicability.
TMOS package contains two devices: one "active", that sense and is exposed to external radiation, and another one "blind", that is shielded from the outside through an aluminum mirror deposited on the package.
Operating principle
The working principle of TMOS sensor provides that when thermal IR radiation is absorbed in the sensitive area heats up the TMOS causing a variation in its temperature. The temperature change produces a current or a voltage output signal proportional to the absorbed radiation.
TMOS performance depends on the transistor operating region and configuration: two terminals component, diode-like configuration, or three terminals component. Two terminals configuration is characterized by a grater thermal isolation. On the other side the three-terminal configuration has an higher internal voltage gain, given by the higher output resistivity.
Subthreshold region is the preferred one because avoids self heating effects and leads to higher sensitivity. Another reason to work in subthreshold region is that TMOS is an active device so requires a bias, however in this operating region the power consumption is lower than in other ones.
From a circuit point of view the produced TMOS signal can be modelled as a temperature dependent current source in parallel with the generator for small signal equivalent circuit. The value of is directly proportional to the drain source current variation with respect to TMOS operating temperature and to the temperature variation induced on the TMOS by the radiation absorbed from target object. This temperature has a direct dependence on the absorbing efficiency, the incident radiation power and on the thermal conductance of the sensor.