Mahmoud Almasri

Self-supporting uncooled infrared microbolometers with low-thermal mass

A new micromachined microbolometer array structure is presented that utilizes a self-supporting semiconducting yttrium barium copper oxide (Y-Ba-Cu-O) thin film thermometer. The Y-Ba-Cu-O thermometer is held above the substrate only by the electrode arms without the need of any underlying supporting membrane. This represents a significant improvement in the state-of-the-art for microbolometers by eliminating the thermal mass associated with the supporting membrane. The reduced thermal mass permits lowering the thermal conductance to the substrate to obtain increased responsivity or having a shorter thermal time constant to allow for higher frame rate camera. The simple structure does not suffer from warping problems associated with stress imbalances in multilayer microbolometer structures that utilize a supporting membrane such as Si/sub 3/N/sub 4/. Devices were fabricated by growing Y-Ba-Cu-O films on a conventional polyimide sacrificial layer mesa. Subsequent etching of the sacrificial layer provides the air gap that thermally isolates the microbolometer. Y-Ba-Cu-O possesses a relatively high temperature coefficient of resistance of 3.1%/K at room temperature. The 400-nm-thick Y-Ba-Cu-O film exhibited absorptivity of about 30%. The responsivity and detectivity approached 10/sup 4/ V/W and 10/sup 8/ cm Hz/sup 1/2//W to filtered blackbody infrared (IR) radiation covering the 2.5 to 13.5 /spl mu/m band. This extrapolates to noise equivalent temperature difference (NETD) less than 100 mK. The micromachining techniques employed are post-complementary metal-oxide-semiconductor (CMOS) compatible, allowing for the fabrication of focal plane arrays for IR cameras.

Amorphous silicon two-color microbolometer for uncooled IR detection

This paper describes the modeling and design of two-color microbolometers for uncooled infrared (IR) detection. The goal is to develop a high resolution IR detector array that can measure the actual temperature and color of an object based on two spectral wavelength regions. The microbolometer consists of high temperature amorphous silicon (a-Si:H) thin film layer held above the substrate by Si/sub 3/N/sub 4/ bridge. A thin NiCr absorber with sheet resistance of 377 /spl Omega//sqr is used to enhance the optical absorption in the medium and long IR wavelength windows. A tunable micromachined Al-mirror was suspended underneath the detector. The mirror is switched between two positions by the application of an electrostatic voltage. The switching of the mirror between the two positions enables the creation of two wavelength response windows, 3-5 and 8-12 /spl mu/m. A comparison of the two response wavelength windows enables the determination of the actual temperature of a viewed scene obtained by an IR camera. The microbolometer is designed with a low thermal mass of 1.65/spl times/10/sup -9/ J/K and a low thermal conductance of 2.94/spl times/10/sup -7/ W/K to maximize the responsivity R/sub v/ to a value as high as 5.91/spl times/10/sup 4/ W/K and detectivity D/sup */ to a value as high as 2.34/spl times/10/sup 9/ cm Hz/sup 1/2//W at 30 Hz. The corresponding thermal time constant is equal to 5.62 ms. Hence, these detectors could be used for 30-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 2.34 mK for the 8-12 /spl mu/m window and 23 mK for the 3-5 /spl mu/m window. The calculated absorption coefficients in the medium and long IR wavelength windows before color mixing are 66.7% and 83.7%. However, when the color signals are summed at the output channel, the average achieved absorption was 75%.

Fabry-Pérot cavity sensors for multipoint on-column micro gas chromatography detection

We developed and characterized a Fabry-Pérot (FP) sensor module based micro gas chromatography (microGC) detector for multipoint on-column detection. The FP sensor was fabricated by depositing a thin layer of metal and a layer of gas-sensitive polymer consecutively on the endface of an optical fiber, which formed the FP cavity. Light partially reflected from the metal layer and the polymer-air interface generated an interference spectrum, which shifted as the polymer layer absorbed the gas analyte. The FP sensor module was then assembled by inserting the FP sensor into a hole drilled in the wall of a fused-silica capillary, which can be easily connected to the conventional gas chromatography (GC) column through a universal quick seal column connector, thus enabling on-column real-time detection. We characterized the FP sensor module based microGC detector. Sensitive detection of various gas analytes was achieved with subnanogram detection limits. The rapid separation capability of the FP sensor module assembled with both single- and tandem-column systems was demonstrated, in which gas analytes having a wide range of polarities and volatilities were well-resolved. The tandem-column system obtained increased sensitivity and selectivity by employing two FP sensor modules coated with different polymers, showing great system versatility.

Specific and targeted detection of viable Escherichia coli O157:H7 using a sensitive and reusable impedance biosensor with dose and time response studies.

A gold interdigitated microelectrode (IME) impedance biosensor was fabricated for the detection of viable Escherichia coli O157:H7. This sensor was fabricated using lithography techniques. The surface of the electrode was immobilized with anti-E. coli IgG antibodies. This approach is different from other studies where the change in impedance is measured in terms of growth of bacteria on the electrode, rather then the antibody/antigen bonding. The impedance values were recorded for frequency ranges between 100 Hz and 10 MHz. The working range of the dose response for this device was found to be between 2.5×10(4) CFU ml(-1) and 2.5×10(7) CFU ml(-1). The time response studies indicated that antibody/antigen binding is not a function of time, but can decrease if excess times are allowed for binding. It was observed that the impedance values for 60 min antibody/antigen binding were higher than the impedance values for 120 min binding time. The main advantages of the reported device are that, it provides for both qualitative and quantitative detection in 3h while other impedance sensors reported earlier may take up to 24h for detection. If enrichment steps are required then it may take 3-4 days to infer the results. This sensor can be used to detect different types of bacteria by immobilizing the antigen specific antibody. Most of the sensors are not reusable since they either use enzymes or enrichment steps for detection but this device can be reused, following a cleaning protocol which is easy to follow. Each device was used at least five times. The simplicity of this sensor and the ease of fabrication make this sensor a useful alternate to the microfluidics and enzyme based impedance sensors, which are relatively more difficult to fabricate, need programmable fluidic injection pumps to push the sample through the channel, suffer from limitation of coagulation and are difficult to clean.

Design of Dual-Band Uncooled Infrared Microbolometer

Abstract-This paper describes the design and modeling of a smart uncooled infrared detector with wavelength selectivity in the long-wavelength infrared (LWIR) band. The objective is to enhance the probability of detecting and identifying objects in a scene. This design takes advantage of the smart properties of vanadium dioxide (VO2): it can switch reversibly from an IR-transparent to an IR-opaque thin film when properly triggered. This optical behavior is exploited here as a smart mirror that can modify the depth of the resonant cavity between the suspended thermistor material and a patterned mirror on the substrate, thereby altering wavelength sensitivity. The thermistor material used in the simulation is vanadium oxide (VOx). The simulation results show that, when VO2 is used in the metallic phase, it reflects IR radiation back to the suspended VOx and enhances IR absorption in the 9.4-10.8-μm band. When the film is switched to the semiconductor phase, it admits most IR radiation, which is then reflected back to the suspended VOχ by a patterned gold thin film under an SiO2 spacer layer. The spacer layer is used to increase the resonant cavity depth underneath the microbolometer pixel. Thus, the peak absorption value is shifted to 8-9.4 μm, creating the second spectral band. The detector is designed with a relatively low thermal conductance of 1.71 X 10-7 W/K to maximize responsivity (Rv) to values as high as 1.27 X 105 W/K and detectivity (D*) to as high as 1.62 x 109 cm-Hz1/2/W, both at 60 Hz. The corresponding thermal time constant is equal to 2.45 ms. Hence, these detectors could be used for 60-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 14 and 16 mK for the 8-9.4- and 9.4-10.8-μm bands, respectively. The calculated absorption coefficients in the two spectral bands were 59% and 65%, respectively.

Note: Mechanical study of micromachined polydimethylsiloxane elastic microposts.

This paper reports the detailed statistical measurement of Youngs modulus (E) and spring constant of micromachined three-dimensional polydimethylsiloxane microposts with various sizes using atomic force microscope. The paper also describes the design and fabrication of these microposts. The micropost array was fabricated with a height to diameter aspect ratio of up to 10. We have found that posts with different sizes have different E values, and posts that are cured at room temperature have smaller Youngs modulus than the ones that are cured at 65 °C for the same duration.

Uncooled multimirror broad-band infrared microbolometers

A new generation of microbolometers were designed, fabricated and tested for the NASA CERES (Clouds and the Earths Radiant Energy System) instrument to measure the radiation flux at the Earths surface and the radiant energy now within the atmosphere. These detectors are designed to measure the earth radiances in three spectral channels consisting of a short wave channel of 0.3 to 5 /spl mu/m, a wide-band channel of 0.3 to 100 /spl mu/m and a window channel from 8 to 12 /spl mu/m each housing a 1.5 mm x 1.5 mm microbolometers or alternatively 400 /spl mu/m x 400 mm microbolometers in a 1 /spl times/ 4 array of detectors in each of the three wavelength bands, thus yielding a total of 12 channels. The microbolometers were fabricated by radio frequency (RF) magnetron sputtering at ambient temperature, using polyimide sacrificial layers and standard micromachining techniques. A semiconducting YBaCuO thermometer was employed. A double micromirror structure with multiple resonance cavities was designed to achieve a relatively uniform absorption from 0.3 to 100 /spl mu/m wavelength. Surface micromachining techniques in conjunction with a polyimide sacrificial layer were utilized to create a gap underneath the detector and the Si/sub 3/N/sub 4/ bridge layer. The temperature coefficient of resistance was measured to be -2.8%/K. The voltage responsivities were over 10/sup 3/ V/W, detectivities above 10/sup 8/ cm Hz/sup 1/2//W, noise equivalent power less than 4 /spl times/ 10/sup -10/W/Hz/sup 1/2/ and thermal time constant less than 15 ms.

Efficient and Rapid Detection of Salmonella Using Microfluidic Impedance Based Sensing

We present a low cost, easy to fabricate biosensor, which can quickly and accurately detect Salmonella typhimurium. This study also compares the advantages of the microfluidic biosensor over a nonmicrofluidic biosensor. High density interdigitated electrode array was used to detect Salmonella cells inside a microfluidic chip. Monoclonal anti-Salmonella antibodies were allowed to be immobilized on the surface of the electrode array for selective detection of Salmonella typhimurium. An impedance analyzer was used to measure and record the response signal from the biosensor. The biosensor provides qualitative and quantitative results in 3 hours without any enrichment steps. The microfluidic biosensor’s lower detection limit was found to be  CFU/mL compared to the  CFU/mL of the nonmicrofluidic biosensor, which shows that the microfluidic biosensor has 10-fold increased sensitivity. The impedance response of microfluidic biosensor was also significantly higher (2 to 2.9 times) compared to the nonmicrofluidic biosensor.

A micromachined impedance biosensor for accurate and rapid detection of E. coli O157:H7

An impedance biosensor based on interdigitated electrode (IDE) arrays was designed, fabricated and tested for detection of Escherichia coli O157:H7. The device consists of two sets of gold IDE arrays embedded in a SU8-PDMS microchannel. The first set of electrodes uses positive dielectrophoresis (p-DEP) force to focus and concentrate the E. coli into the centre of the microchannel, and direct it towards the detection zone microchannel which has dimensions of a third of the first channel. The bulk fluid keeps flowing toward the outer channel into the waste outlets. The second sets of electrodes are located in the centre channel and are used for impedimetric detection of the E. coli. A combination of standard photolithography, wet etching and plasma treatment techniques were used to fabricate the biosensor. The E. coli cells in the test solution were focused into the centre of the channel when an excitation signal of 5 Vp–p at 5.6 MHz was applied across the electrode arrays. Before injecting the E. coli cells, polyclonal anti-E. coli antibodies were non-specifically immobilized on the sensing electrode array. This ensures specific detection of E. coli O157:H7 bacterial cells. As the concentrated E. coli cells (antigen) reach the sensing electrode array, they bind to the immobilized antibody sites. This antigen–antibody binding causes a change in the impedance, which is measured using an impedance analyzer. The device performance was tested by measuring the impedance, between 100 Hz and 1 MHz frequency, before and after applying p-DEP on the focusing electrode array, and after applying p-DEP on both the focusing and sensing electrodes. The result shows clearly that the use of p-DEP on the focusing IDE array significantly increased the measurement sensitivity with the lower detection limit being 3 × 102 CFU mL−1. In addition, the use of p-DEP on both electrode arrays increased the measurement sensitivity by a factor of 2.9 to 4.5 times depending on the concentration.

Semiconducting YBaCuO microbolometers for uncooled broadband IR sensing

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 Earths 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.