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Engineered Bolometer Leg Materials Towards Physics-Limited Thermal Infrared Imaging Arrays


OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics, Advanced Materials


The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.


OBJECTIVE: Demonstrate an engineered material system able to be deposited and patterned with semiconductor foundry techniques with very low thermal conductivity and reasonable electrical conductivity for use as a bolometer leg.


DESCRIPTION: Bolometer technology is used in nearly all uncooled longwave imaging sensors worldwide; these sensors are widely used for both commercial and military applications on ground, personnel-carried, and air platforms.  This technology supports targeting, autonomy, situational awareness, security, and many other application spaces with its inclusion into numerous Army Programs of Record (PoR) and are increasingly used as inputs to AI/ML-powered algorithms.  This is because bolometer-based sensors are uniquely positioned to provide low-cost imaging in the longwave infrared (thermal) band.


Domestic U.S. industry has historically held strong advantages in both performance and number of sensor units manufactured.  However, high levels of investment by foreign companies and countries has eroded this advantage.  This topic seeks to extend the advantage of U.S. industry.

A bolometer-based imaging sensor is comprised of a focal plane where each pixel is a bolometer structure, a read-out integrated circuit, supporting electronics, optics, and a mechanical housing.  The basic bolometer structure itself, a microelectromechanical structure (MEMS), is comprised of a transducing body and two legs.  The legs serve to mechanically support the body and thermally isolate it while passing an electric current to read out the transducing body’s signal.

The leg, and the materials that comprise it, are key to a highly sensitive and manufacturable (high yielding) pixel.  Higher thermal isolation results in a more sensitive pixel.  However, the typical way to increase thermal isolation is to make the leg longer and thinner, often wrapping around the pixel many times.  This, in turn, decreases the pixel manufacturability and sensor robustness in operational use.  This is especially important as bolometer pixels get smaller in support of higher resolution devices.


Therefore, this topic seeks a new material or engineered material system which is inherently more thermally isolating while maintaining electrical conductivity and mechanical robustness.  This would enable a shorter, wider leg and push bolometers closer to their physical performance limit and away from the practical structural limits imposed today. This necessarily requires a material system which breaks the usual Wiedemann–Franz relationship between electrical and thermal conductivity.


To be applicable to high-rate bolometer fabrication, the material must be capable of being deposited and lithographically patterned by standard CMOS foundry equipment used in bolometer fabrication and be compatible with other portions of the fabrication and packaging processes.  A low-noise ohmic contact must be formed with the substrate and bolometer body material (commonly vanadium oxide, but sometimes α-silicon, titanium oxide, or other materials).  Overall total leg thermal mass must be low to avoid degrading sensor performance.

Note that proposals will not be considered for material systems for other bolometer components (e.g., the body/transducing material), alternate sensing technologies, other components of the camera module, or anything else that is not a bolometer leg capable of mechanical support, thermal isolation, and electrical conductivity.


PHASE I: Describe one or more material systems and propose a method of fabricating the material compatible with the constraints of a semiconductor fabrication facility.  Through a combination of modeling, theory, and/or experimental evidence, demonstrate that the system meets all requirements to act as a bolometer leg and is superior to materials used in current production devices.  The material system will be evaluated based on it having low thermal conductivity (< 250 pW/K, ideally approaching 0), low overall thermal mass (<0.3 pJ/K), moderate electrical conductivity (ideally resistance < 250 kΩ, but up to 2 MΩ for certain readouts), ability to form low-noise ohmic contacts, low deposition thermal budget (<200°C), and overall thermal and mechanical robustness (withstand 300°C, mechanical shock and vibration).  This shall be delivered in a final technical volume.


PHASE II: Further develop, fabricate, and characterize the material system.  Show that the material system is capable of meeting the requirements of bolometer legs.  No particular physical form is required for this demonstration, but it is desired that the final material system demonstration be as high fidelity as possible in replicating its end use as a bolometer leg (though a complete bolometer is not necessary).

Formulate a fabrication process flow fully compatible with bolometer fabrication flows used by U.S. industry to promote transition of the material system.  Collaboration with industry is desired to show buy-in of the material system and compatibility with production flows.  Demonstrate or otherwise show that this fabrication method is low-cost, high yield, and high uniformity.


PHASE III DUAL USE APPLICATIONS: Work with a U.S.-based bolometer fabricator to transition the material system to a high-rate production environment.  Support the bolometer fabricator in developing an imaging demonstration prototype LWIR bolometer sensor system, perhaps based on an existing camera/sensor product, to prove the viability and benefits of the material system for increasing performance and/or manufacturing yield.  Such a material system is useful to all domestic bolometer manufacturers and could be used to improve any existing or future bolometer product (domestic or commercial use) as a 100% drop-in replacement.  The enhanced sensors could then be qualified for use in any COTS or Program of Record acquisition program for operational use.  Since sensors utilizing this material system would be a 100% drop-in replacement, it could be used in existing or future programs utilizing uncooled thermal technology.



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KEYWORDS: Bolometer, microbolometer, longwave, LWIR, sensor, thermal, conductivity, MEM

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