There have been recent innovations to reduce the cost of packaging for MEMS devices, without deteriorating their performance. One such novel design for device-level encapsulation (self-packaged) of uncooled infrared (IR) microbolometers is documented here. Device-level vacuum encapsulation has the potential to eliminate some major problems associated with the bolometer performance such as high thermal conductance of the ambient atmosphere, the high cost associated with conventional vacuum packaging, and the degradation of optical transmission at different wavelengths through a conventional package window. The device-level encapsulated bolometers can also be fabricated with flexible substrates, which have the advantage of conforming to non-planar surfaces compared to Si or other rigid substrates. In addition, a flexible superstrate with low shear stress has applications in robotics, aerospace, defense and biomedicine as a "Smart skin", a name given to multisensory arrays on conformal substrates to emulate human skin functions on inanimate objects. Self-packaged uncooled microbolometer arrays of 40x40 μm2 and 60x60 μm2 are fabricated on top of Si wafer with a sacrificial layer using semiconducting Yttrium Barium Copper Oxide (YBCO) as the infrared sensing material. A two-layer surface micromachining technique in conjunction with a resonant cavity and a reflecting mirror are used for the sensor structure. The devices have demonstrated voltage responsivities of 7.9x103 V/W with a temperature coefficient of resistance of -2.5% K-1, and thermal conductivity of 2.95x10-6 W/K. The device performance was similar in air and vacuum, demonstrating vacuum integrity and a good device-level encapsulation.
This is a review paper summarizing the recent reports on low-frequency noise in Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) with high dielectric constant (high-k) gate oxides. Although several such publications have appeared in the literature recently, the only unified theme among them is that the noise in these high-k gate stacks is considerably higher than that observed on conventional SiO2 gate oxide transistors. In addition, interface- and bulk-dielectric trap-induced correlated carrier number and mobility fluctuations (Unified Model) seem to be the commonly accepted cause of these fluctuations. This report attempts to compile the published data, make comparisons between different high-k dielectrics with respect to 1/f noise characteristics and reach preliminary conclusions. Since there is still room for improvement in processing of high-k materials for MOSFET applications, the review represents merely a slice in time of the progress made, and not meant to be a fundamental, theoretical review.
In order to replace the conventional SiO2 in MOSFETs and minimize gate tunneling currents, high permittivity dielectric materials have been proposed as alternatives. These materials have successfully resolved the gate leakage problem with thicker oxide dielectric. However, other issues such as lower effective mobility and increased low frequency noise due to higher oxide trap density, limit its further development. Among these candidates, HfSiON offers many advantages compared to other high-k devices such as suppression of Boron penetration, remaining amorphous during high temperature annealing, and offering better thermal stability and interface quality. In addition, the extracted oxide trap density from measured 1/f noise shows lower values compared to other high-k MOSFETs. This paper presents low frequency noise characteristics of MOSFETs with HfSiON and SiON gate dielectrics of varying gate length dimensions and effective oxide thickness. The measured noise spectra as well as DC parameters will be compared between HfSiON and SiON MOSFETs. The noise parameters are extracted from the measured noise data using the interface-generated, correlated number and mobility fluctuation model.
The flicker noise characteristics of MOSFETs with HfO2, HfAlOx, Al2O3 / HfO2 gate stacks have been studied, where both n and p type devices having 10 Å or 40 Å SiO2 interfacial layer (IFO) were considered. The corresponding equivalent oxide thickness (EOT) values were 23Å, 53Å for HfO2; 28.5Å, 58.5Å for HfAlOx; and 33Å, 63 Å for Al2O3 / HfO2. Gate leakage currents were (~ 2-5x10-5 A/cm2) which are 2 orders of magnitude lower compared to SiO2 devices with similar oxide thickness. The positive and negative threshold shifts as derived from the threshold voltages of n, p devices, respectively, suggested the presence of interface states between poly-gate and dielectric. Normalized power spectral density plot vs. gate overdrive shows that the devices with thin interfacial oxide (IFO) layer exhibit about an order of magnitude higher noise than those with thicker IFO owing to higher number of traps in the high-k and to the fact that carrier tunneling distance is ~20 Å. The Unified Model representing the carrier number and correlated mobility fluctuations was used to extract the oxide trap density and Coulomb scattering coefficient values. In general, n-type HfO2 devices yielded low noise, trap density and best mobility values compared to the other two gate stacks.
1/f noise was investigated in a complementary polysilicon emitter bipolar process. Noise measurements were carried out for variable base bias resistance (RS) to analyze how the contribution of each noise source changes as RS is varied. Two noise measurement setups were used to identify different noise sources in the transistors: noise from the base current (SIB), collector current (SIC), and internal resistances (SVr). The coherence for transistors measured in both measurement setups were close to unity, implying a single dominant noise source. SIB had the dominant contribution at lower bias currents. In this case, RS was relatively larger than the input resistance of the transistor. Higher current measurements with a smaller RS showed a dominant contribution from SVr. SIB was modeled as a combination of the minority carrier diffusion fluctuations in the monosilicon and polysilicon emitter, and tunneling fluctuations through the interfacial oxide. A combination of the number and diffusion fluctuations of the minority carriers in the base was used to model SIC. It was concluded that mainly originates from the fluctuations in the internal emitter resistance, which was ascribed to the tunneling fluctuations of the majority carriers through the interfacial oxide.
One common requirement of microbolometers fabricated on both rigid and flexible substrates is the need for vacuum packaging to eliminate the thermal conductivity of air and achieve high performance. However, vacuum packaging of microbolometers is expensive and is a limiting factor in achieving truly low-cost uncooled infrared detection. Vacuum packing of microbolometers on flexible substrates requires a novel approach unless flexibility is to be sacrificed. This paper explores the vacuum packaging of microbolometers through self-packaging. In this case, the micromachined encapsulation in a vacuum cavity is investigated through computer simulation of microbolometers in flexible polyimide films and through the encapsulation of microbolometers on rigid Si substrates with a Si3N4 shell. In this manner, self packaged uncooled microbolometers were fabricated on a Si wafer with semiconducting yttrium barium copper oxide (YBCO) as the infrared sensing material. The self-packaged structure is designed such that it can be covered with a superstrate, yielding low stress in the flexible skin sensors and better detection figures of merit. The devices have demonstrated voltage responsivities over 103 V/W, detectivities above 106 cm Hz1/2/W and temperature coefficient of resistance around -3.3% K-1. Computer simulations using CoventorWare and MEMulator have been used to determine suitable materials for the process, the optimum design of a vacuum element and a streamlined process flow.
This paper reports progress on the development of micromachined infrared microsensors on flexible polymer substrates. The flexible substrates were formed by spin-coating polyimide films (HD Microsystems PI-5878G) on silicon wafer carriers. Semiconducting Yttrium Barium Copper Oxide (YBCO) was used as the thermistor. The microbolometer was fabricated using a polyimide (HD Microsystems PI2737) sacrificial mesa and titanium electrode arms. The YBCO thermistor was suspended above the substrate by the electrode arms after the sacrificial layers have been removed by micromachining. The polyimide substrate was then removed from the silicon wafer carrier to complete the fabrication of the infrared microsensors on a flexible polyimide substrate. The measured thermal conductance of the microbolometers ranged from 9.07 x 10-6 W/K for a non-micromachined to 4.0 x 10-7 W/K for a micromachined sensor. As a result, the responsivity and detectivity were measured to be as high as 6.1 x 104V/W and a 1.2 x 108 cmHz1/2/W, respectively, with a 970 nA current bias. This level of performance is comparable to similar miromachined detectors fabricated on silicon substrates.
The low-frequency noise observed on advanced junction bipolar transistors consist of 1/f noise as well as Random telegraph Signals (RTS). In relatively small emitter-base junction areas, RTS is seen in the spectra which can be differentiated from the typical generation-recombination (gr) noise through time domain analysis. For most cases, the 1/f noise can be modeled primarily with a current noise source in the base SIB. There are cases, however, the noise originates primarily in the collector side due to SIC, which has been neglected in modeling equations. We have designed two different measurement and analysis systems where the effect of SIC and SIB can be differentiated and separately modeled through correlated noise measurements that are performed at the collector and base or collector and emitter. Cross-power spectral density, as well as coherence is used to extract different noise components. Variable temperature low-frequency noise measurements to extract the different components of SIB revealed that the diffusion noise due to mobility fluctuation, fluctuations in the recombination at the surface of the emitter/base depletion region, and fluctuations in the interfacial oxide tunneling barrier height, (and thus the tunneling probability of the carrier) are the components that need to be modeled. The results of the experimental data as well as modeling equations and techniques will be discussed.
A gated lateral bipolar transistor is a bulk lateral BJT in parallel with a MOSFET at the surface. The base current components such as surface recombination and space charge recombination currents are two of the dominant noise sources in the lateral BJT. If the gate is biased such that the MOSFET is in the off-state by accumulating carriers underneath the oxide in the base surface, the noise contribution by these two base current (Ib) components can be better understood. The carrier accumulation in the base surface can be modulated with different gate bias, which in turn will affect the fluctuation of the surface recombination current component. In this paper, noise power spectral density of gated lateral PNP transistors, fabricated in Texas Instruments Standard Bipolar Process, has been discussed. The base current noise power spectral density (SIb) was extracted from the cross-correlation noise spectrum measured between the base and the collector circuits for different gate biasing conditions. Based on the frequency exponent dependence of the noise power spectral density, it was found that the noise in the low frequency range is in the form of 1/f noise. SIb was found to be the dominant noise source for these devices as the coherence between the base and collector power spectral density was very close to 1. SIb was extracted for a base current range of 8 nA to 1microA for a gate bias range of 0V to 40V. The SPICE noise model parameters, AF and KF were also determined for each case from the dependence of SIb on Ib. The noise was measured on devices with different base width values.
Infrared microbolometer thermal detectors have been fabricated on a flexible substrate, a polyimide that shows very similar characteristics to Kapton , when cured. The polyimide is spin-coated on a regular Si wafer with a release layer. Low temperature fabrication techniques are employed to minimize the thermal cycling of the polyimide substrate as well as to maintain compatibility with CMOS circuitry. Infrared microsensors on flexible substrates showed a Temperature Coefficient of Resistance (TCR=(1/R)(dR/dT)) of -3.03%, at room temperature. The microbolometers reached the responsivity and detectivity of 3.5x103 V/W and 1x107 cm.Hz1/2 /W, respectively, at 2.88 μA of current bias even though at this time no micromachining has been performed. Some devices were encapsulated between polyimide layers in order to provide protection and passivation. These bolometers demonstrated a responsivity and detectivity of 1.6x103 V/W and 4.9x106 cm.Hz1/2/W, respectively at 1 μA of current bias. This is the first attempt towards a smart skin that incorporates flexible microsensors on a cured polyimide-substrate.
This paper describes the modeling, design, fabrication and testing of advanced uncooled thermal detectors, based on semiconducting YBaCuO. The aim is to provide NASA with advanced broad-band infrared (IR) detectors to replace the current CERES (Clouds and the Earth's Radiant Energy System) hardware that utilizes three channels, each housing a 1.5 mm X 1.5 mm thermister bolometer with 1 X 4 array of detectors in each of the three channels, thus yielding a total of 12 channels. A double mirror structure is used to obtain uniform spectral response from 0.3-100 μm wavelength. Double absorbers are utilized to further flatten the spectral response and to enhance the absorption of infrared radiation. The devices were fabricated using a polyimide sacrificial layer to achieve thermal isolation of the detector. A low thermal conductivity to the substrate enables the detector to integrate the energy from the incident radiation. An air gap was created by ashing the polyimide sacrificial layer from underneath the thermometer. A passivation layer was used to protect YBaCuO during ashing process and maintain a relatively high temperature coefficient of resistance of around 2.8%. These devices have successfully demonstrated voltage responsivities over 103 V/W, detectivities above 108 cm Hz1/2/W, NEP per root Hertz bandwidth less than 4 X 10-10 W/Hz1/2 and thermal time constant less than 15 ms. Several specific designs were fabricated and tested. Relatively uniform response in the wavelength range of 0.6 to 15 μm was measured.
We review performance and physical characteristics of yttrium barium copper oxide (YBCO) compound as an infrared (IR) photodetector. YBCO has been used as the IR detector material in both superconducting (oxygen-rich) and semiconducting (oxygen-depleted) phases. YBCO in its crystalline, Yba2Cu3O6+x phase with x>0.95 is a high-temperature superconducting material with the superconducting transition Tcapproximately equals 90K. The superconducting YBCOIR detectors operate as either nonequilibrium (quantum) or bolometric (thermal) devices. The nonequilibrium devices are characterized by very short, single-picosecond photoresponse times and are expected to find applications in optoelectronics and imaging, as well as ultrafast optical-to-electrical transducers for digital input applications. The bolometric mechanism results in relatively slow but very sensitive detectors with possible applications in astronomy. In addition to superconducting IR sensors, interest in uncooled YBCO devices is growing very rapidly. Despite somewhat lower sensitivity and significantly reduced speed of response, as compared to the superconducting counterpartners, the uncooled IR detectors are characterized by much lower operating cost and weight due to lack of cooling cryogens and are compatible with existing silicon-based processing and fabrication. The last point is of paramount importance if the IR-sensitive pixels are to be integrated with CMOS read-out circuitry for monolithic focal plane arrays and infrared cameras. Amorphous uncooled YBCO photodetectors operate as either photoconductive bolometers of unbiased pyroelectric devices.
Amorphous semiconducting Y-Ba-Cu-O is attractive as the temperature sensitive element for uncooled IR bolometers and pyroelectric detectors. Thin films can be easily fabricated by RF magnetron sputtering at room temperature from a composite target. It is compatible with micromachining techniques for the fabrication of thermally isolated structures. As a bolometer, Y-Ba-Cu-O possesses a relatively high temperature coefficient of resistance of 3.5% K-1 near room temperature. This paper will present the IR characteristics of 40-micrometer X 40-micrometer microbolometer arrays fabricated in thermal isolation structures. These detectors are aimed at thermal imaging at 10-micrometer wavelength. Recently, self-supporting YBaCuO pixels have been developed. In this case, the Y-Ba-Cu-O thin film pixel requires no underlying bridge material to provide structural support. The Y-Ba-Cu-O thin film is supported solely by the electrode arms. Responsivity and detectivity greater than 4 X 103 V/W and 108 cmHz1/2/W respectively have been measured in these detectors. The development of large area 1.5-mm and 0.4- mm square YBaCuO bolometers for NASA's global warming studies in low-orbiting satellites will also be presented. These large area detectors require large optical bandwidths covering the 0.3-micrometer to 100-micrometer wavelength band.
Amorphous semiconducting Y-Ba-Cu-O has shown promise as the temperature sensitive element for uncooled IR detectors as both a bolometer and pyroelectric material. Thin films can be easily fabricated by RF magnetron sputtering at room temperature from a composite target. As a bolometer, Y-Ba- Cu-O possesses a relatively high temperature coefficient of resistance of 3.5% K-1 near room temperature. As a pyroelectric detector, pyroelectric coefficients as high as 20 (mu) C/cm2-K have been measured yielding a pyroelectric figure of merit of 0.065 (cm3/J)1/2. In Y-Ba-Cu-O, the oxygen concentration has been shown to determine the hole concentration and mobility. However, the anion stoichiometry plays an equally important role in determining the electronic characteristics. In this work, we have explored the effects of substitution for Cu and the corresponding changes on the electronic properties affecting the performance as an IR detector. Further, we have fabricated micromachined 1 X 10 arrays in which utilize a self-supporting Y-Ba-Cu-O thin film geometry. In this case, the Y-Ba-Cu-O film is held above the substrate only by the electrode arms, without the need of any underlying bridge material. These detectors posses a low thermal mass and have yielded detectivities as high as 108 cm-Hz1/2/W, which extrapolates to NETDs less than 20 mK.
This paper reports the fabrication of microbolometers using semiconducting YBaCuO as the IR sensing material. The detectors are operable at room temperature and thus are suitable for lost-cost and high performance imaging applications. Semiconducting YBaCuO is promising as a bolometric material as it has a thermal coefficient of resistance near 3% and relatively low noise. Two different bolometer structures will be reported here. First generation YBaCuO microbolometers were built on micromachined SiO2 bridges using wet etching techniques to undercut the silicon. The second generation structures were processed upon micromachined Si3N4 membranes with sputtered MgO films used as sacrificial layers. The membrane structures are the first of its kind to incorporate MgO as a sacrificial layer, and they offer a fabrication technique that is fully CMOS compatible, with all processing at ambient temperatures. Detectivities in the order of 108 cm Hz1/2/W were measured at 30 Hz chopping frequency in both structures. The thermal conductance of the suspended membranes was on the order of 10-7 W/K, which is desirable as low thermal conductance yields high responsivities. There are realizable optimizations for both applications to yield detectivities over 109 cm Hz 1/2/W. All measurements reported here were performed at ambient temperature with no temperature stabilization.
Linear arrays of 40 X 40 micrometers 2 to 60 X 60 micrometers 2 microbolometers utilizing a semiconducting YBaCuO thin film as the IR sensitive material were fabricated and tested. This material displays a high temperature coefficient of resistance at room temperature, which makes it very attractive as an uncooled IR detector. The compatibility of semiconducting YBaCuO with Si micromachining techniques and CMOS technology and its ease of fabrication into thin films is attractive to the fabrication of inexpensive, high performance infrared detectors and imaging arrays. The room temperature responsivity, Rv, of the detectors was measured using 100 Hz chopped broad band infrared radiation to be as high as 104 V/W at 0.8 (mu) A current bias, providing a detectivity of 107 cm Hz1/2/W. The thermal conductance, G, of the isolation structure was measured to be a relatively high 10-5 W/K which limited the responsivity of the detector. Since Rv varies as 1/G, the responsivity and hence detectivity may be increased by up to two orders of magnitude by improving the thermal isolation. Zero bias responsivity was also observed on the structures, which was interpreted as pyroelectric IR detection. This was confirmed with independent pyroelectric current measurements.
Semiconducting YBaCuO thin films are a candidate for infrared bolometers operating at room temperature without the need for temperature control. Semiconducting YBaCuO thin films were fabricated by rf magnetron sputtering onto silicon substrates at room temperature. Room temperature deposition provides compatibility with CMOS technology to allow for the fabrication of low cost infrared imaging arrays with focal plane signal processing. The temperature coefficient of resistance (TCR) of the thin films was measured to be as high as 4% K-1 over a wide temperature range near 300 K. In this respect, amorphous YBaCuO thin films provided a higher TCR than epitaxial, tetragonal YBa2Cu3O6+x films. The noise voltage at 30 Hz was measured to be typically less than 1 (mu) V/Hz1/2. If the thin films are integrated into a typical air-gap thermal isolation structure, the projected responsivity Rv would be as high as 3.8 multiplied by 105 V/W with 1 (mu) A of current bias providing an estimated detectivity D* of 1.6 multiplied by 109 cm Hz1/2/W at a frame frequency of 30 Hz.
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