NIVo - Novel isothermal sensor through front cavities


The market for thermopile sensors has grown in recent years and will continue to gain due to the current corona pandemic. To follow this trend, the aim in the current project is to combine two established principles in one sensor and thus to further develop the innovation capability in this field. For this purpose, on the one hand the isothermal principle and on the other hand the possibility to expose a membrane from the front side of the sensor shall be combined.

The isothermal principle is characterized by the fact that the electrical signal output by the thermopile depends only to a very small extent on the temperature of the chip frame. This can be achieved by various measures, such as the use of absorbers and reflectors.
In order to enable a front side release of the membrane, its surface must first be structured. Bulk material is removed through these relatively small openings by means of etchers, thus creating a free-standing structure. Since this structure is thermally insulated from the substrate (bulk material), a detectable temperature difference can be created in the presence of incident infrared radiation.
Exposing a structure from the front side has several advantages. The process for fabrication is simpler because all photolithographic masks need to be deposited on only one side of the wafer, increasing positioning accuracy. Another major advantage is the protected membrane due to the bulk material remaining on the bottom side. With the help of this technology, a higher stability of the chips can be achieved and thus the thermal coupling to the sockets can be improved. This makes thermopile sensors with a high dynamic range possible. For the wafers with the front-etched membrane, there is now the possibility of encapsulation at wafer level by means of a lid wafer. These measures provide optimum protection for the sensitive membrane and thus simplify further processing of the sensors.

The research and development work in the project Novel Isothermal Sensor by Front Cavities (NIVo) was funded by the German Federal Ministry for Economic Affairs and Energy.
FKZ: 49MF200129