Ge quantum dots on silicon for terahertz detection
Terahertz has attracted attention from both millimeter and infrared technologies. Currently, solutions consist of expensive sensor systems limiting the amount of applications. To obtain a low cost solution, one approach is to use thermistor material based on Ge on Si quantum dots on microbolometers. It has been demonstrated that SiGe monocrystalline materials have superior properties to amorphous silicon or VOx regarding the temperature coefficient of resistivity and 1/f noise. Unfortunately, the calculated critical thickness for strained SiGe material show unreasonable layer thickness for high Ge amounts. To avoid this, Ge dots instead of SiGe layers are preferred. This article focuses on optimizing the thermistor properties of Ge-dots. All samples were grown on Si (100) substrates in a temperature range of 500-650 C at 20 torr by RPCVD. They were characterized (XRD, HRSEM and AFM etc.) determining the diameter, thickness, density and uniformity. Fourier transform photoluminescence was performed to find the spatially direct (EA) and indirect (EB) bandgap transitions.
Low-cost far infrared bolometer camera for automotive use
A new low-cost long-wavelength infrared bolometer camera system is under development. It is designed for use with an automatic vision algorithm system as a sensor to detect vulnerable road users in traffic. Looking 15 m in front of the vehicle it can in case of an unavoidable impact activate a brake assist system or other deployable protection system. To achieve our cost target below €100 for the sensor system we evaluate the required performance and can reduce the sensitivity to 150 mK and pixel resolution to 80 x 30. We address all the main cost drivers as sensor size and production yield along with vacuum packaging, optical components and large volume manufacturing technologies. The detector array is based on a new type of high performance thermistor material. Very thin Si/SiGe single crystal multi-layers are grown epitaxially. Due to the resulting valence barriers a high temperature coefficient of resistance is achieved (3.3%/K). Simultaneously, the high quality crystalline material provides very low 1/f-noise characteristics and uniform material properties. The thermistor material is transferred from the original substrate wafer to the read-out circuit using adhesive wafer bonding and subsequent thinning. Bolometer arrays can then be fabricated using industry standard MEMS process and materials. The inherently good detector performance allows us to reduce the vacuum requirement and we can implement wafer level vacuum packaging technology used in established automotive sensor fabrication. The optical design is reduced to a single lens camera. We develop a low cost molding process using a novel chalcogenide glass (GASIR®3) and integrate anti-reflective and anti-erosion properties using diamond like carbon coating.
Wafer-level membrane transfer bonding of polycrystalline silicon bolometers for use in infrared focal plane arrays
In this paper we present a new, innovative technology for fabrication and integration of free-hanging transducers. The transducer structures are processed on the original substrate wafer (sacrificial device wafer) and then transferred to a new substrate wafer (target wafer). The technology consists only of low-temperature processes, thus it is compatible with integrated circuits. We have applied the new membrane transfer bonding technology to the fabrication of infrared bolometers for use in uncooled infrared focal plane arrays (IRFPAs). In the future this may allow bolometers to be integrated with high-temperature-annealed, high-performance thermistor materials on CMOS-based uncooled IRFPAs. Membrane transfer bonding is based on low-temperature adhesive bonding of the sacrificial device wafer to the target wafer. The device wafer is sacrificially removed by etching or by a combination of grinding and etching, while the transducer structures remain on the target wafer. The transducer structures are mechanically and electrically contacted to the target wafer and the adhesive bonding material is sacrificially removed. The free-hanging transducers remain on the target wafer. One of the unique advantages of this technology is the ability to fabricate and integrate free-hanging transducers with very small feature sizes. In principle, membrane transfer bonding can be applied to any type of free-hanging transducer including ferroelectric infrared detectors, movable micro-mirrors and RF MEMS devices.