Silicon (Si) photonic micro-electro-mechanical systems (MEMS), with its low-power phase shifters and tunable couplers, is emerging as a promising technology for large-scale reconfigurable photonics with potential applications for example in photonic accelerators for artificial intelligence (AI) workloads. For silicon photonic MEMS devices, hermetic/vacuum packaging is crucial to the performance and longevity, and to protect the photonic devices from contamination. Here, we demonstrate a wafer-level vacuum packaging approach to hermetically seal Si photonic MEMS wafers produced in the iSiPP50G Si photonics foundry platform of IMEC. The packaging approach consists of transfer bonding and sealing the silicon photonic MEMS devices with 30 μm-thick Si caps, which were prefabricated on a 100 mm-diameter silicon-on-insulator (SOI) wafer. The packaging process achieved successful wafer-scale vacuum sealing of various photonic devices. The functionality of photonic MEMS after the hermetic/vacuum packaging was confirmed. Thus, the demonstrated thin Si cap packaging shows the possibility of a novel vacuum sealing method for MEMS integrated in standard Si photonics platforms.
Silicon photonics is the study and application of integrated optical systems which use silicon as an optical medium, usually by confining light in optical waveguides etched into the surface of silicon-on-insulator (SOI) wafers. The term microelectromechanical systems (MEMS) refers to the technology of mechanics on the microscale actuated by electrostatic actuators. Due to the low power requirements of electrostatic actuation, MEMS components are very power efficient, making them well suited for dense integration and mobile operation. MEMS components are conventionally also implemented in silicon, and MEMS sensors such as accelerometers, gyros, and microphones are now standard in every smartphone. By combining these two successful technologies, new active photonic components with extremely low power consumption can be made. We discuss our recent experimental work on tunable filters, tunable fiber-to-chip couplers, and dynamic waveguide dispersion tuning, enabled by the marriage of silicon MEMS and silicon photonics.
We present a method to characterize the temperature dynamics of miniaturized thermal IR sources. The method circumvents the limitations of current IR photodetectors, by relying only on an electrical measurement rather than on optical detection. Thus, it enables the characterization of the light emission of IR sources over their full operation frequency range. Moreover, we develop a model of thermal IR sources allowing simulations of their thermal and electrical behavior. By combining measurements and modeling, we achieve a comprehensive characterization of a Pt nanowire IR source: the reference resistance R0 = 17.7Ω, the TCR α = 2.0 × 10-3 K-1, the thermal mass C = 2.7 × 10-14 J/K, and the thermal conductance G = 1.3 × 10-6 W/K. The thermal time constant could not be measured, because of the frequency limitation of our setup. However, the operation of the source has been tested and proved to function up to 1 MHz, indicating that the thermal time constant of the source is smaller than 1 μs.
Most of today's commercial solutions for un-cooled IR imaging sensors are based on resistive bolometers using either
Vanadium oxide (VOx) or amorphous Silicon (a-Si) as the thermistor material. Despite the long history for both
concepts, market penetration outside high-end applications is still limited. By allowing actors in adjacent fields, such as
those from the MEMS industry, to enter the market, this situation could change. This requires, however, that
technologies fitting their tools and processes are developed. Heterogeneous integration of Si/SiGe quantum well
bolometers on standard CMOS read out circuits is one approach that could easily be adopted by the MEMS industry.
Due to its mono crystalline nature, the Si/SiGe thermistor material has excellent noise properties that result in a state-ofthe-
art signal-to-noise ratio. The material is also stable at temperatures well above 450°C which offers great flexibility
for both sensor integration and novel vacuum packaging concepts. We have previously reported on heterogeneous
integration of Si/SiGe quantum well bolometers with pitches of 40μm x 40μm and 25μm x 25μm. The technology scales
well to smaller pixel pitches and in this paper, we will report on our work on developing heterogeneous integration for
Si/SiGe QW bolometers with a pixel pitch of 17μm x 17μm.
In this paper we present a comprehensive calculational model for the noise equivalent temperature difference (NETD) of infrared imaging systems based on uncooled bolometer arrays. The NETD model is validated and benchmarked using published performance data of state-of-the-art uncooled infrared bolometer arrays. The calculational model is used to evaluate possible infrared sensor and system design tradeoffs that allow optimization for low-cost infrared systems with improved reliability and lifetime, while still achieving a NETD of about 150 mK, required for pedestrian injury mitigation systems. We propose an approach in which high performance crystalline semiconductor materials with very low 1/f-noise properties and a temperature coefficient of resistance (TCR) of 3 %/K are used as thermistor material for the bolometers. The resulting increased bolometer performance can be used to operate the infrared imaging arrays in a vacuum atmosphere with increased gas pressure while still achieving useful NETD levels. The proposed calculational model suggests that a NETD on the order of 150 mK can be reached with uncooled infrared bolometer arrays operating in vacuum pressures on the order of 6 mbar. Such specifications for the bolometer vacuum package dramatically simplify wafer-level vacuum packaging and ease long-term reliability issues, contributing to lowering the vacuum packaging and thus, the overall infrared imaging chip costs.
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.
This paper gives an in-depth description of two recent projects at the Royal Institute of Technology (KTH) which utilize MEMS and microsystem technology for realization of components intended for specific applications in medical technology and diagnostic instrumentation.
By novel use of the DRIE fabrication technology we have developed side-opened out-of-plane silicon microneedles intended for use in transdermal drug delivery applications. The side opening reduces clogging probability during penetration into the skin and increases the up-take area of the liquid in the tissue. These microneedles offer about 200µm deep and pain-free skin penetration. We have been able to combine the microneedle chip with an electrically and heat controlled liquid actuator device where expandable microspheres are used to push doses of drug liquids into the skin. The entire unit is made of low cost materials in the form of a square one cm-sized patch.
Finally, the design, fabrication and evaluation of an integrated miniaturized Quartz Crystal Microbalance (QCM) based "electronic nose" microsystem for detection of narcotics is described. The work integrates a novel environment-to-chip sample interface with the sensor element. The choice of multifunctional materials and the geometric features of a four-component microsystem allow a functional integration of a QCM crystal, electrical contacts, fluidic contacts and a sample interface in a single system with minimal assembly effort, a potential for low-cost manufacturing, and a few orders of magnitude reduced in system size (12*12*4 mm3) and weight compared to commercially available instruments. The sensor chip was successfully used it for the detection of 200 ng of narcotics sample.
Pedestrian fatalities are around 15% of the traffic fatalities in Europe. A proposed EU regulation requires the automotive industry to develop technologies that will substantially decrease the risk for Vulnerable Road Users when hit by a vehicle. Automatic Brake Assist systems, activated by a suitable sensor, will reduce the speed of the vehicle before the impact, independent of any driver interaction. Long Wavelength Infrared technology is an ideal candidate for such sensors, but requires a significant cost reduction. The target necessary for automotive serial applications are well below the cost of systems available today. Uncooled bolometer arrays are the most mature technology for Long Wave Infrared with low-cost potential. Analyses show that sensor size and production yield along with vacuum packaging and the optical components are the main cost drivers. A project has been started to design a new Long Wave Infrared system with a ten times cost reduction potential, optimized for the pedestrian protection requirement. It will take advantage of the progress in Micro Electro-Mechanical Systems and Long Wave Infrared optics to keep the cost down. Deployable and pre-impact braking systems can become effective alternatives to passive impact protection systems solutions fulfilling the EU pedestrian protection regulation. Low-cost Long Wave Infrared sensors will be an important enabler to make such systems cost competitive, allowing high market penetration.
In this paper we present the design, fabrication and characterization of arrays of boron doped polycrystalline silicon bolometers. The bolometer arrays have been fabricated using CMOS
compatible wafer-level transfer bonding. The transfer bonding technique allows the bolometer materials to be deposited and optimized on a separate substrate and then, in a subsequent integration step to be transferred to the read-out integrated circuit (ROIC) wafer. Transfer bonding allows thermal infrared detectors with crystalline and/or high temperature deposited, high performance temperature sensing materials to be integrated on CMOS based ROICs. Uncooled infrared bolometer arrays with 18x18 pixels and with 320x240 pixels have been fabricated on silicon substrates.
Individual pixels of the arrays can be addressed for characterization purposes. The resistance of the bolometers has been measured to be in the 50 kΩ range and the temperature coefficient of resistance (TCR) of the bolometer has been measured to be -0.52%/K. The pixel structure is designed as a resonant absorbing cavity, with expected absorbance above 90%, in the wavelength interval of 8 to 12 μm. The measured results are in good agreement with the predicted absorbance values.
In this paper we present a new membrane transfer bonding technology for fabrication of uncooled infrared focal plane arrays (IRFPAs). The technology consists only of low temperature processes, thus, it is compatible with standard integrated circuits (ICs). In the future this technology may allow infrared detectors with high temperature annealed, high performance thermistor materials to be integrated in CMOS based uncooled IRFPAs. The infrared detectors and the ICs are processed and optimized on different wafers. The wafer with the detectors (sacrificial detector-wafer) is bonded to the IC wafer (target wafer) using low temperature adhesive bonding. The detector-wafer is sacrificially removed by etching or by a combination of grinding and etching, while the detectors remain on the target wafer. The detectors are mechanically and electrically contacted to the target wafer. Finally, the adhesive bonding material is sacrificially removed. One of the unique advantages of this technology is the ability to integrate small, high temperature annealed detectors and ICs. We have applied membrane transfer bonding to the fabrication of arrays of infrared bolometers with polycrystalline silicon thermistors. In principle, membrane transfer bonding can be applied to the fabrication of any type of free-standing transducer including bolometers, ferroelectric detectors and movable micro-mirrors.
A novel and simple technology for making robust 3D silicon structures with small radii of bending has been developed and investigated. The proposed self-assembling method of bending 3D structure out-of-plane, without the use of interlocking braces is based on thermal shrinkage of polyimide in V-grooves. The static bending angle for the permanent out-of-plane rotated structure can be chosen and well controlled over a wide range by varying the curing temperature of the polyimide and the number of V-grooves in the joint.
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