1. GOALI: Systems Integration of Uncooled YBaCuO IR Detectors

Funding Source: National Science Foundation

Funding Amount: $199,908

Project Dates: September 1, 1998 to August 31, 2000

Infrared imaging has demonstrated itself as a vital aspect of modern weapons systems. Infrared (IR) imaging has the potential to play an equally important role in commercial applications in medicine and transportation. Automobiles equipped with infrared imaging capabilities have been envisioned for the near future. This technology has the potential to tremendously improve personal safety by enabling good vision at night and under adverse weather conditions. Infrared imagers in automobiles may also be an enabling technology for "intelligent super-highways". However, IR imaging systems currently used by the military are too costly for consumer applications. This necessitates the development of inexpensive, uncooled infrared imaging systems that possess high detectivity for night vision applications.

This proposal concerns the investigation and development of high detectivity pyroelectric infrared imagers operating at room temperature. Previous work supported by the National Science Foundation and Army Research Office has shown that semiconducting, thin film YBaCuO possesses a high pyroelectric coefficient at room temperature, two hundred times greater than other thin film materials. This enables high detectivity pyroelectric thermal detectors to be fabricated on micromachined thermal isolation structures integrated on CMOS read-out circuitry, since the amorphous semiconducting phase of YBaCuO, used in these imaging devices, is deposited at ambient temperature with no need of high temperature post-annealing. It has been demonstrated by researchers at Southern Methodist University (SMU) that surface-micromachined Nb/YBaCuO/Nb capacitor structures are capable of exhibiting pyroelectric coefficients in the range of 65 nC/K cm² without an externally applied electric field and in the absence of prior poling. When poled, the pyroelectric coefficient was found to increase to 18 mC/cm²-K. If integrated with state-of-the-art thermal isolation structures, background limited performance is achievable as defined by high specific detectivity, in the order of 10¹⁰ cm-Hz^1/2/W, and low noise equivalent temperature difference of around 2 mK. This performance would surpass all other uncooled IR cameras.

YBaCuO is best known as a high temperature superconductor. The optical and electronic properties of YBa₂Cu₃O_6+x are determined by its oxygen stoichiometry. For x» 1, YBaCuO possesses an orthorhombic crystal structure, exhibits metallic conductivity, and becomes superconductive upon cooling below its critical temperature. As x is decreased to 0.5, the crystal undergoes a phase transition to a tetragonal structure and it exhibits semiconducting conductivity characteristics as it exists in a Fermi glass state. As x is decreased further below 0.3, YBaCuO becomes a Hubbard insulator with a well defined energy gap on the order of 1.5 eV. This work utilizes the semiconducting phase of the material. SMU researchers have done extensive investigations into the electrical and optical properties of the material and have over 15 publications and presentations, and three patents on the subject.

In this work, high detectivity pyroelectric imagers will be fabricated by combining the SMU pyroelectric devices with the state-of-the-art thermal isolation structures and read-out circuitry produced by Raytheon/TI Systems. Formerly, a part of Texas Instrument Inc., the Defense Systems and Electronics Group has recently been acquired by Raytheon to form Raytheon/TI Systems (RTIS). Both the RTIS and SMU are in Dallas, Texas, in close proximity with each other, making such collaboration very convenient. Two groups in RTIS, in conjunction with SMU principal investigators, will integrate the device concepts developed at the University with TI’s imaging systems that were originally developed for their Barium Strontium Titanate pyroelectric detectors.

Various issues at the systems level must be addressed and solved including process compatibility, electrical compatibility, and system figures of merit. To solve the electrical compatibility issue, the resistive loss of YBaCuO will be decreased by adjusting its oxygen content. Process compatibility issue will be addressed by finding an appropriate electrode metal for YBaCuO that is CMOS compatible and designing a fabrication process that incorporates the principles of YBaCuO deposition and lithography with micromachining technology developed for the thermal isolation structures at Raytheon/TI Systems. Several figures of merit, such as pyroelectric coefficient, pyroelectric figure of merit, responsivity, specific detectivity, noise, will be used to assess performance at the focal plane array level. The pyroelectric properties and the noise mechanisms which limit the detectivity and are uniquely characteristic of the high degree of thermal isolation, will be investigated. Upon successful completion of integration at the focal plane array level, a systems integration will be attempted towards a complete IR camera. Systems level figures of merit, such as noise equivalent temperature difference, will be used as a measure of performance.

The basic material and device level stages of this work have been sponsored by a previous grant from NSF and ARO. The proposed work will realize a full range of engineering processes from basic research to device concepts to systems integration, thereby developing a complete uncooled IR imager, unsurpassed in performance and cost.

Funding Source: Midwest Superconductivity Inc.

Funding Amount: $25,826 with an option of additional $10,045 and extension to

December 31, 1998

Project Dates: May 1, 1998 to December 31, 1998

Project Summary:

Technology transfer will be realized from the SMU Infrared Detector research group to 4 MSI personnel in techniques associated with YBCO deposition, metallization, micromachining and characterization with regards to the fabrication of room temperature bolometers using an active sensor material based upon the compound YBCO. The characterization will be resistance vs temperature, responsivity, detectivity, and noise measurements. MSI will need about a total of 12 days of clean room time to be completed in a 4^1/2 month period.

Funding Source: Raytheon/TI Systems

Funding Amount: $46,630

Project Dates: July 15, 1998 to July 14, 1999

Project Summary:

SMU is going to measure, analyze and model different sources of noise in thermally isolated pyroelectric infrared detectors supplied by Raytheon/TI Systems. Fluctuations in the detected signal due to temperature fluctuations noise, Johnson noise and amplifier noise will be measured as a function of bias conditions, thermal conductance of the detector element to the substrate, and temperature to fully characterize the origin of these noise components. Recommendations will be provided to Raytheon/TI Systems to minimize these noise sources as well as the full analysis and modeling results.

There will be two phases. During the first phase, different sources of noise will be identified that contribute to the NETD. Pyroelectric current noise power spectral density due to detector Johnson noise and temperature fluctuations will be measured on the pyroelectric structures using a current pre-amplifier and a dynamic signal analyzer. This will enable the researchers to separate the detector noise from the amplifier-originated noise. Of special interest is the 1/f noise component, since it might have detrimental effects to the detection process for a camera operating at 60 Hz, due to its spectral form, which increase with decreasing frequency.

During the second phase, the detectors will be coupled to the read-out circuitry normally used in the camera. The noise power spectral density of the output voltage will be measured using a voltage preamplifier and a dynamic signal analyzer. This will measure the total noise including that contributed by the amplifier conductance Johnson noise and the amplifier shot and generation-recombination noise components. Together with information obtained in the first phase, this will fully characterize the noise sources that determine the NETD.

Funding Amount: $273,527

Project Dates: March 1995 to October 1998

Project Summary:

The principal investigators propose to explore the non-superconducting phases of YBa2Cu3Ox thin films for uncooled infrared detection applications. The long-term goal is to build thermal imaging arrays on micromachined silicon structures with high thermal isolation and therefore high responsivity. This would enable the fabrication of integrated circuits where radiation detection and signal processing are performed on the same chip. These infrared detectors will utilize the oxygen deficient phase of YBa2Cu3Ox (x» 6) and will be operated at or near room temperature. Two different thin film, thermal detector structures will be tried: semiconductor bolometers and pyroelectric capacitors. The bolometers will be made of the semiconducting YBa2Cu3Ox on SiO2 or Si3N4 and exhibit a high rate of resistance change with temperature dR/dT. The P.I.s’ initial investigations on this material have revealed dR/dT values as high as 105 W /K and temperature coefficient of resistance 3.5 % 1/°C over a 40°C temperature range around room temperature which implies an excellent bolometric response. Detectivities in the order of 5 x 1010 cm Hz1/2/W are predicted which is far superior to other uncooled materials and comparable to cooled HgCdTe.

The pyroelectric capacitors will be built in metal - YBa2Cu3Ox - metal geometry and will be used at zero electric field exploiting the classic pyroelectric effect. Investigators have shown capability of fabricating Nb - YBa2Cu3Ox - Nb and Nb - YBa2Cu3Ox - Al capacitors with low charge leakage, high permittivity (er» 100) and high dC/dT values, approximately 500 pF/cm2/K. YBa2Cu3Ox is a ferroelectric material with a perovskite structure. The oxygen content in YBa2Cu3Ox determines the electronic properties of the thin film, including the conductivity and permittivity. The oxygen content can be easily adjusted during the radio-frequency (rf) sputtering process.

The significance of the proposed research lies in the fact that proposed detectors operate at room temperature without refrigeration. They are relatively easy and inexpensive to fabricate and compatible with silicon technology since they can be deposited at ambient temperature by rf sputtering. It is projected that the proposed research will lead to a better understanding of the electronic properties of nonsuperconducting YBa2Cu3Ox and their dependence upon oxygen stoichiometry. This proposal will also be the first time where YBa2Cu3Ox IR detectors will be fabricated on air-gap bridge structures. Since the proposed devices do not require cooling for operation and can be fabricated using inexpensive techniques and materials, it will be less costly for defense applications and more suitable for civilian needs such as night-time navigation, security or biomedical thermal imaging. Their cryogenic counterparts, superconducting bolometers and HgCdTe photovoltaic or photoconductive devices are expensive to fabricate, costly to operate and unsuitable for certain medical applications due to the requirement for cooling.

The proposed research will be in collaboration with Texas Instruments Inc. and University of North Texas.

Funding Source: Army Research Office

Funding Amount: $36,769

Project Dates: May 15, 1995 to October 1998

Project Summary:

The Army Research Office has identified in its Annual Broad Agency Announcement for 1998-2000, as one of the priorities for the Physics Division, the investigation of novel imagers, including multiple wavelength detectors and new sensors. Infrared imaging has demonstrated itself as a vital aspect of modern weapons systems. Infrared imaging also has the potential to play an equally important role in commercial applications in medicine and transportation. Automobiles equipped with infrared imaging capabilities have been envisioned for the near future. This technology has the potential to tremendously improve personal safety by enabling good vision at night and under adverse weather conditions. Infrared imagers in automobiles may also be an enabling technology for "intelligent super-highways". However, infrared imaging systems currently used by the military are too costly for consumer applications. This necessitates the development of inexpensive uncooled infrared imaging systems that possess high detectivity for night vision applications. This proposal concerns the investigation and development of high detectivity pyroelectric infrared detectors.

Previous work supported by the NSF and ARO has shown that semiconducting, thin film YBaCuO possesses a high pyroelectric coefficient at room temperature, two hundred times greater than other thin film materials. Since the amorphous semiconducting phase of YBaCuO, used in these imaging devices, is deposited at ambient temperature with no need of high temperature post-annealing, this enables high detectivity pyroelectric thermal detectors to be fabricated on micromachined thermal isolation structures integrated on CMOS read-out circuitry. It has been demonstrated by researchers at Southern Methodist University (SMU) that surface-micromachined Nb/YBaCuO/Nb capacitor structures are capable of exhibiting pyroelectric coefficients in the range of 65 nC/K cm² without an externally applied electric field and in the absence of prior poling. When poled, the pyroelectric coefficient was found to increase to 18 mC/cm²-K. If integrated with state-of-the-art thermal isolation structures, background limited performance is achievable as defined by high specific detectivity, in the order of 10¹⁰ cm-Hz^1/2/W, and low noise equivalent temperature difference of around 2 mK. This performance would surpass all other uncooled IR cameras. SMU currently holds two patents and one is pending on the infrared detection using YBaCuO thin films.

Well known for its superconductive (x ³ 0.5) properties at cryogenic temperatures, YBa₂Cu₃O_6+x is a low conductivity semiconductor for x < 0.5 at 300 K. YBa₂Cu₃O_6+x is also ferroelectric and has intrinsically low noise when compared to other IR detection materials such as HgCdTe and amorphous silicon. The oxygen content of the YBa₂Cu₃O_6+x thin film can be precisely adjusted to obtain the desired conductivity and dielectric constant. In spite of the aforementioned advantages, no detailed investigation has been done on the dielectric and pyroelectric properties of semiconducting YBaCuO thin films. It is the aim of this proposed project to fill in this gap. During the project, the following objectives will be fulfilled:

• The physical mechanisms leading to dielectric, transport, pyroelectric and noise characteristics of semiconducting YBaCuO will be studied.
• The physics of metal-semiconducting YBaCuO contact junctions will be investigated, including noise, transport, work function, material interactions, and interface layers.
• The effect of micromachining procedures, physical geometry and thickness on these physical mechanisms as measured by observed electrical characteristics will be investigated. The role of grain boundaries versus crystallinity on pyroelectricity and ferroelectric domains (if they exist) will be studied.
• The dependence of the aforementioned physical mechanisms on temperature, bias conditions and IR radiation will be examined.

The proposed work will be done in four phases: Phase I will consist of measurements, analysis, and modeling of dielectric, pyroelectric, transport and noise mechanisms in semiconducting YBaCuO. This is the most comprehensive phase of the project and therefore is expected to take five quarters. During the second phase, the effect of element substitutions for Yttrium and Barium will be explored to investigate the effect of contribution by copper oxide planes to transport and dielectric properties of copper oxide perovskite structures. Pyroelectricity in Presidium Barium Copper Oxide (PrBa₂Cu₃O_6+x), or Lanthanum Strontium Copper Oxide (La_(2-x)Sr_xCuO_4-d), for example, might exceed that of YBaCuO and should be investigated. Therefore, although it is not envisioned to be the main thrust of the research, the pyroelectric effect and the dielectric properties of some of the other CuO structures will also be studied. Phase III will include research on the effect of micromachining procedures, different electrode materials, physical geometry and thickness on the physical mechanisms as measured by observed electrical characteristics. It is envisioned to take five quarters. The last phase is the final wrap-up and report-writing. Due to the overlap in these activities, the total project duration is two years.

The research team will consist of two principal investigators and two graduate students. The principal investigators are collaborating with the Uncooled IR Detection Group at Raytheon/TI Systems and have the support of DARPA for the proposed activities. Letters of support from Raytheon and DARPA are enclosed with this proposal.

Supplemental Grant I to Uncooled Infrared Detection with YBa2Cu3Ox Thin Films

Funding Source: National Science Foundation

Funding Amount: $9,994

Project Dates: November 1995 to October 1998

Supplemental Grant II to Uncooled Infrared Detection with YBa2Cu3Ox Thin Films

Funding Source: National Science Foundation

Funding Amount: $9,994

Project Dates: November 1996 to October 1998

Supplemental Grant III to Uncooled Infrared Detection with YBa2Cu3Ox Thin Films

Funding Source: National Science Foundation (Continuation)

Funding Amount: $3,250

Project Dates: January 1998 to October, 1998

Funding Source: National Science Foundation, Academic Research Infrastructure

Funding Amount: $155,000

Project Dates: September 1996 to August 1999

Project Summary:

The acquisition of a Laser Ablation Deposition system (LAD) is proposed. LAD has been demonstrated as a superior deposition technique for the fabrication of compound oxide thin films as compared to other available techniques such as sputtering and e-beam evaporation. The LAD will be placed in the Solid State Technology Laboratory which has the capability for semiconductor device fabrication including Si-MOS, Si micromachining, and semiconductor lasers. The proposed LAD system will become the only LAD system in the North Texas region. The LAD system will be primarily used to deposit semiconducting YBaCuO thin films for application as uncooled microbolometers and semi-insulating YBaCuO thin films for application as uncooled pyroelectric detectors. This work is supported by the NSF, ARO and Texas Instruments. The cost of the proposed LAD system project is $333,327 of which SMU will provide a 40% cost share.

YBaCuO is well known for its superconducting properties. However more recently, the non-superconducting phases of YBaCuO are demonstrating themselves useful in a number of applications including uncooled infrared (IR) detection, the second harmonic generation of light, and photoconductive switching. The materials are attractive since they are stable in composition and properties and easy to deposit. One of the phases of YBaCuO is YBa2Cu3O6+x. As x is increased from 0 to 1, YBa2Cu3O6+x evolves from an insulator with a well-defined energy gap to a Fermi glass, and then through a metal-insulator transition to a metal. Simultaneously, the crystal lattice changes from tetragonal to orthorhombic. The oxygen content thereby determines the electrical and optical properties of the material. YBa2Cu3O6+x is a ferroelectric material with pyroelectric properties. In the semi-insulating phase, it has a low leakage current and is therefore suitable for application as an uncooled pyroelectric detector. In the semi-conducting phase, YBa2Cu3O6+x has a narrow band-gap, inherently low 1/f noise and a large change in resistivity with temperature making it suitable as an uncooled bolometer. Our preliminary investigations have shown that semiconducting YBa2Cu3O6+x uncooled bolometers offer higher responsivities () and detectivities than cooled superconducting YBa2Cu3O6+x bolometers. In addition, their performance is superior to other uncooled IR detectors including VO2 and PbTiO3. To date, the semiconducting YBa2Cu3O6+x IR sensitive material has been fabricated by simple rf sputtering at room temperature onto Si, SiO2 and Si3N4 substrates showing material compatibility with a CMOS process towards the fabrication of micromachined, uncooled microbolometer arrays with focal plane signal processing resultant in a low cost IR imaging circuit. We believe that control over the stoichiometry of the YBa2Cu3O6+x thin films would be significantly improved by the use of LAD for the thin film fabrication. This would allow for greater reproducibility and allow for the stoichiometry to be optimized to produce even better performance. Therefore the acquisition of a LAD system would be a tremendous benefit to the project.

Further, an LAD system would improve the capability of the laboratory allowing for future research projects in the fabrication of compound oxide thin films with linear and nonlinear electro-optic properties. Work in this area would compliment the semiconductor laser group and allow for the fabrication of integrated optic devices. In addition, projects would be initiated on the application of ferroelectric compound oxides to Si devices such as the gate insulator in DRAMs.

Co-PI: Donald P. Butler (with Zeynep Çelik-Butler)

Funding Source: National Science Foundation

Funding Amount: $9,994

Project Dates: January 1997 to August 1999

Co-PI: Donald P. Butler (with Zeynep Çelik-Butler)

Funding Source: National Science Foundation (Continuation)

Funding Amount: $10,000

Project Dates: January 1998 to August 1999