It is well known to fabricate an IR Emitter based on silicon process. Such devices typically consist of a resistive micro-heater embedded within a thin membrane and supported on a silicon substrate. When current is passed through the heater, it heats up to a high temperature (which can be as much as 700° C. or even higher), and at this high temperature the device emits Infra Red radiation.
A large number of designs with IR emitters have been reported. For Example, Parameswaran et. al. “Micro-machined thermal emitter from a commercial CMOS process,” IEEE EDL 1991 reports a polysilicon heater for IR applications made in CMOS technology, with a front side etch to suspend the heater and hence reduce power consumption. Similarly, D. Bauer et. Al. “Design and fabrication of a thermal infrared emitter” Sens & Act A 1996, also describes an IR source using a suspended polysilicon heater. U.S. Pat. No. 5,285,131 by Muller et al. and patent US2008/0272389 by Rogne et. al, San et. al. “A silicon micromachined infrared emitter based on SOI wafer” (Proc of SPIE 2007) also describe similar devices using a polysilicon heater.
Yuasa et. al “Single Crystal Silicon Micromachined Pulsed Infrared Light Source” Transducers 1997, describe an infrared emitter using a suspended boron doped single crystal silicon heater. Watanabe, in patent EP2056337 describes a suspended silicon filament as an IR source. The device is vacuum sealed by bonding a second substrate.
Many designs based on a platinum heater have also been described. For example, Hildenbrand et. al. “Micromachined Mid-Infrared Emitter for Fast Transient Temperature Operation for Optical Gas Sensing Systems”, IEEE Sensor 2008 Conference, reports on a platinum heater on suspended membrane for IR applications.
Similarly Ji et. Al. “A MEMS IR Thermal Source For NDIR Gas Sensors” (IEEE 2006) and Barritault et. al “Mid-IR source based on a free-standing microhotplate for autonomous CO2 sensing in indoor applications” (Sensors & Actuators A 2011), Weber et. al. “Improved design for fast modulating IR sources”, Spannhake et. Al. “High-temperature MEMS Heater Platforms: Long-term Performance of Metal and Semiconductor Heater Materials” (Sensors 2006) also describe platinum based as well as other emitters.
Some other IR Emitter designs are given by U.S. Pat. No. 6,297,511 by Syllaios et. al., U.S. Pat. Nos. 5,500,569, 5,644,676, 5,827,438 by Bloomberg et. al, and WO 02/080620 A1 by Pollien et. al.
One limitation of many of these devices is that their emissivity is not optimal. There is also no control over the emission at specific wavelengths. For this purpose, the devices are often coated with different materials to improve the emissivity. Some materials used are metal blacks, carbon, carbon nanotubes and other thin film interference structures. These structures can be difficult to deposit and require additional processing steps. In addition they may degrade over time at high temperatures, and hence limit the operating temperature of the IR emitter.
There have been several reports in literature that suggest that the emissivity of devices can be varied at particular wavelengths by using plasmonic structures, which are periodic structures created on a surface. For example these are described in V. Shklover et. al., “High-Temperature Photonic Structures, Thermal Barrier Coatings, Infrered Sources and Other Applications,” Journal of Computational and Theoretical Nanoscience, Vol 5, 2008, pp. 862-893.
There are also several reports of IR emitters with plasmonic structures. For example, M. Tsai et. al., “Two Color Squared-Lattice Plasmonic Thermal Emitter,” Proceedings of Sixth IEEE Conference on Nanotechnology, Vol 2, pp 746-748, describes a silver/silicon dioxide/silver sandwich structure on a silicon substrate, where the top silver and/or silicon dioxide layer have a periodic pattern. The emission spectrum of the device shows peaks near 4 μm and 6 μm. Heat is provided by passing current through a gold and chromium layer at the back of the substrate. However, as there is no membrane, the device would consume a lot of power. Very similar devices are also described in Y. Jiang, “Enhancement of thermal radiation in plasmonic thermal emitter by surface plasmon resonance,” Proceeding of IEEE conference on Nanotechnology 2008, pp. 104-107, and in H. Fu, “A thermal emitter with selective wavelength: Based on the coupling between photonic crystals and surface plasmon polaritions,” Journal of Applied Physics 105, 033505 (2009). S. Huang, “Triple peaks plasmonic thermal emitter with selectable wavelength using periodic block pattern as top layer,” Proceedings of IEEE International Conference on Nanotechnology 2011 pp. 1267-1270, also describe a device based on silicon dioxide and silver layers, but using block shapes in different patterns at the top surface. Another device is described by J. Daly et. al. “Nano-Structured Surfaces for Tuned Infrared Emission for Spectroscopic Applications,” Micro-Nano-photonic Materials and Devices, Joseph W Perry, Axel Scherer, pp. 80-89, which has a plasmonic structure made of gold with chromium as an underlying layer, and this structure is tested by putting on a graphite sheet ont a hotplate.
S. Tay et. al., “Plasmonic thermal IR emitters based on nanoamorphous carbon,” Applied Physics Letters 94, 071113 (2009), and U.S. Pat. No. 8,492,737B2 describe an IR emitter with nanoamorphous carbon patterned at the top as a hexagonal lattice of holes. K. Ikeda et. al, “Controlled thermal emission of polarized infrared waves from arrayed plasmon nano cavities,” Applied Physics Letters 92, 021117 (2008) describes a plasmonic IR emitter based on an epoxy substrate. Similarly I. E. Araci et. al. “Mechanical and thermal stability of plasmonic emitters on flexible polyimide substrates,” Applied Physics Letters 97, 041102 (2010) describes a plasmonic emitter based on a polyimide substrate. The use of epoxy or polyimide limits the maximum operating temperature of the device.
While these all these designs are made to optimise the emission spectrum of the surface, these devices do not have a proper mechanism for heating up. Either the heater is based on a metal layer at the back surface or need to be couple to an external heater, which can result in very high power consumption. Unlike conventional miniaturised IR emitters, none of these devices are based on a membrane to isolate the heat and reduce power consumption.
There are a number of reports on MEMS based IR emitters with plasmonic structures. X. Ji et. al. “Narrow-band Midinfrared Thermal Emitter Based on Photonic Crystal for NDIR Gas Sensor,” Proceedings of IEEE ICSICT 2010, pp. 1459-1461. describes a platinum heater on top of a silicon nitride/silicon dioxide/silicon composite membrane, where all these layers are patterned with an array of holes in a square pattern. F. Li et. al. “MEMS-based plasmon infrared emitter with hexagonal hole arrays perforated in the Al—SiO2 structure,” Journal of Micromechanics and Micro-engineering 21 (2011) 105023, describe an aluminium heater on an silicon dioxide/silicon membrane, and all these layers have circular holes in them in a hexagonal pattern. While these designs will have lower power consumption due to the use of a membrane for thermal isolation, making holes through most of the membrane layers requires extra steps, and can also structurally weaken the membrane as many of the layers including the silicon dioxide layers have holes in them.
Puscasu “Plasmonic Photonic Crystal MEMS Emitter for Combat ID,” Proc of SPIE Vol 8031, 80312Y, describes a plasmonic structure coupled with a MEMS platform. The plasmonic structure consists of circular holes in a hexagonal pattern, while the platform is a heater suspended on a micro-bridge type membrane. Similarly T. Sawada “Surface Plasmon Polarities Based Wavelength Selective IR Emitter Combined with Microheater,” proceedings of IEEE conference on Optical MEMS and Nanophotonics 2013, pp. 45-46, also describes an IR emitter which is suspended. Similarly the device described in U.S. Pat. No. 7,825,380 also describes a plasmonic structure on a suspended heater, which is held together by only two beams. Such suspended structures are less stable than full membrane structures.
M. Zoysa, “Conversion of broadband to narrowband thermal emission through energy recycling,” Nature Photonics 2012, 20.12.146, describe an IR emitter based on a gallium arsenide (GaAs) substrate and a membrane consisting of GaAs/Al—GaAs, with the membrane layers pattern into holes in a hexagonal pattern. Gallium arsenide is not as widely used as silicon, and so is more expensive.