Design And Fabrication Of State Of The Art Uncooled Thermopile Infrared Detectors With Cavity Coupled Absorption

Author : Fabian Purkl
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (115 download)

DOWNLOAD NOW!

Download or read book Uncooled Infrared Detectors Based on Nanometer-thin Metal Films written by Fabian Purkl and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: Infrared detectors allow the remote and contactless measurement of the temperature of an object through radiometry. By using an array of multiple IR sensors assembled into a camera, a thermal image of a scene can be collected without the need for illumination. The applications for early infrared photon detectors were limited, because their requirement for cryogenic cooling resulted in bulky systems. The evolution of micromachining technology enabled the development of uncooled bolometric detectors, which found more widespread use in both military and civilian settings. Typical use cases include vision enhancement, navigation and collision avoidance, process control, search and rescue operations, surveillance, building inspection, process control and medical diagnostics. However, the relatively high costs of infrared cameras have prevented a more widespread use in consumer-oriented applications. Part of the challenge is the complex set of requirements for materials used in resistive bolometers, which measure the incident IR radiation through the absorption-induced temperature change of a free-standing and thermally-isolated thermistor. The desired properties include a high IR absorption, a large magnitude and an excellent linearity of the temperature coefficient of resistance (TCR), a high electrical and thermal resistance as well as a low amount of intrinsic noise sources. Typically, a stack of layers is required to fulfill all the mentioned requirements, which results in a complex fabrication process. In this work, we instead developed a bolometer based on a single functional layer of sub-10 nm thin platinum. Reducing the thickness and number of layers has the immediate advantage of allowing for a better thermal isolation and a smaller thermal capacitance. Both measures help to improve the sensitivity, the temperature resolution and the thermal time constant, which represent the most important figures-of-merit of IR detectors. An additional benefit of this approach is a simple and cost-effective fabrication process, the complexity of which commonly scales with the number of mask layers and required lithography steps. Platinum has attractive properties and a long history of use as a thermistor material for temperature sensors. However, to achieve an effective absorption of IR radiation, a thickness in the nanometer-range is required to match the metal film's impedance to that of free space. A surface micromachining process was developed at the Stanford Nanofabrication Facility that enables the reliable fabrication of the resulting delicate detector structures with aspect ratios in excess of 8000:1. The process is compatible with post-CMOS integration and wafer level packaging. Plasma enhanced atomic layer deposition (PE-ALD) was selected for a repeatable and homogeneous deposition of the functional layers. A careful investigation of the thickness dependence of the relevant material properties was performed in order to establish the required knowledge for the design and modeling of the detectors structures. A steep increase in both the electrical and thermal resistivity was measured for a reduction of the Pt film thickness below 10 nm. The TCR of thin-film Pt was found to decrease simultaneously. The characterization of the optical properties of Pt showed an increase in both the real and imaginary part of the complex refractive index for smaller layer thicknesses. The risk of damage from high current densities in the Pt thin film due to electromigration was also evaluated and found to be unproblematic. Through the characterization of the mechanical properties of nanometer-thin PE-ALD layers, a reduction of the Young's modulus of Al2O3 of up to 50% compared to bulk values was determined, whereas only a slight degradation was found for Pt. All layers exhibited tensile residual stresses with an average value around 131 MPa. A detailed performance model was compiled based on these measured material properties and allowed the accurate prediction of the detector's characteristics as well as an understanding of trade-offs involved in its design. In order to fully exploit the unique features of a bolometer with a single functional layer, an improved geometry based on a self-supporting serpentine structure was subsequently developed, which provides an increase in the sensitivity by a factor of four. The properties of fabricated demonstrator structures were characterized using a vacuum chamber setup. Depending on the employed layer thicknesses, the thermal time constant of the detectors was found to be in the range of 0.7 ms to 4.5 ms. These values are up to an order of magnitude faster compared to state-of-the-art bolometric detectors thanks to the comparatively low thermal mass of the free-standing structure. The sensitivity to IR radiation improved drastically with a reduction in Pt thickness and responsivities of 4×108 V/WA could be achieved for 6 nm thin Pt layers. This value corresponds to a temperature resolution (NETD) of 70 mK, which is based on the measured 1/f noise characteristics of Pt and the assumption of infrared optics with an F-number equal to one. While this new detector technology still leaves room for improvement, the achieved temperature resolution is only about a factor of two away from the performance of commercially available bolometers, which exhibit a significantly slower reaction time and have to rely on a more complicated fabrication process. In summary, we demonstrated an uncooled IR detector based on a single free- standing, sub-10 nm thin Pt layer for the first time. Its performance profits significantly from the reduction in film thickness enabled by the use of PE-ALD. The presented results and the lessons learned from this work can assist in the use of released, nanometer-thin layers for MEMS devices in other domains that can profit equally from such a development