Tomonori Mukaigawa

A new type of dielectric bolometer mode of detector pixel using ferroelectric thin film capacitors for infrared image sensor

Abstract We have developed a new type of detector pixel circuit operated in an infrared image sensor of dielectric bolometer mode. The detector pixel consists of capacitors of ferroelectric BST (Ba1−xSrxTiO3) thin film, whose dielectric constant changes drastically with temperature. Our proposed circuit is a serially connected capacitor–capacitor, where one capacitor is composed of a BST ferroelectric thin film irradiated by infrared light and the other is nonirradiated one. BST film has been prepared on Si membrane structure by Pulsed Laser Deposition method (PLD). Dielectric constant of the BST film, which is about 450 at 25°C, changes by about 1 to 10%/K at ambient temperature. As a result of on-board evaluation of the assembled circuit with a source-follower output, the output level is about 40 mV when a relative capacitance change in the capacitor is about 3%. On the other hand, in PSPICE circuit simulations, the output level is about 25 mV when a relative capacitance change in the capacitor of about 100 pF is 1%. The simulated relationship between the output voltage of the assembled circuit and capacitance change of the BST film agrees well with that in the experimental results. It is considered that the circuit has enough output signal level for input of conventional operational amplifier. Voltage responsivity Rv, and specific detectivity D* estimated from temperature change of dielectric constant are 50 kV/W and 6.5×109 cm·Hz1/2·W−1, respectively, which means high-sensitivity compared to the other type of IR sensors. The pixel structure also shows a simple configuration, and is very effective in reducing their pixel size and then increasing the pixel density.

Si monolithic microbolometers of ferroelectric BST thin film combined with readout FET for uncooled infrared image sensor

Abstract A silicon monolithic ferroelectric thin-film bolometer coupled with a readout FET has been developed for uncooled infrared imaging applications by means of Si-bulk micromachining and pulsed-laser-deposited (PLD) barium strontium titanate Ba 1− x Sr x TiO 3 (BST) thin films. It is a new type of dielectric bolometer (DB) mode based on the strong temperature dependence of capacitance upon the ferroelectric phase transition. The pixel circuit is a serial pair of capacitors where a sensing capacitor is fabricated on a thermally isolated membrane and a reference capacitor on bulk area. And a new pulse-biased-operation mode has been implemented to sense the voltage change at the interface node between sensitive and the reference capacitors. In order to avoid crack and deformation on the thermally insulated structure, a stress-balanced structure by multi-layered membrane has been adopted, where the ferroelectric capacitor is formed on a triple layer of NSG/SiN/SiO 2 -stacked films. A BST (75/25) film on membrane is found to show positive temperature-coefficient of dielectric constant (TCD) ranging from 1 to 6%/K. Upon infrared irradiation on the membrane part, a capacitance difference arising from a temperature rise is significantly induced within the two capacitors. Thermal signals have been confirmed with one pixel bolometer tested under an infrared light irradiation. A new monolithic process flow is developed to combine an n-MOSFET process and a Si-bulk micromachining process, and ferroelectric capacitors on the stress-balanced membrane are able to be formed monolithically with MOSFETs for source-follower output buffer. The pixel structure also shows a simple configuration, and is very effective in reducing their pixel size and then increasing the pixel density. Finally, it is especially noted that the operation in the detector pixel in the DB mode is confirmed on the monolithically integrated device structure.

Development of Si monolithic (Ba, Sr)TiO3 thin film ferroelectric microbolometers for uncooled chopperless infrared sensing

We have been developing a total monolithic microbolometer technology for uncooled thermal sensing along the route from fabricating pixels of thin-film ferroelectric bolometers on micromachined Si substrates. Toward achieving this objective, sensor material of (Ba0.75Sr0.25)TiO3 (BST) has been prepared into thin-film form and been investigated to obtain a large temperature coefficient of dielectric constant (TCD) within the ambient temperature region. Operated in our proposed dynamic pulse-biased mode, the infrared responsivity (Rv) of sensor pixels is analyzed to reflect how those materials properties of BST film dominate the ultimate array performances. This new ferroelectric bolometer is expected to provide value-added merits of chopperless operation and high sensitivity enhanced by pulsed bias. In this paper, pixels of C-C balanced BST thin-film microbolometers have been fabricated by integrating Si-bulk micromachining and ferroelectric thin-film processing. Both pulsed laser deposition (PLD) and metal organic decomposition (MOD) methods have been employed in preparing BST films on those micromachined silicon substrates. Both the films show similar insufficient bolometric behavior with TCD-values smaller than 1%/K. Under pulsed bias, chopperless operation of pixels of so- fabricated microbolometers was confirmed. PLD enabled low- temperature preparation of high-quality films at 520 degree(s)C, so renders it for cutting-edge investigations to attain the TCD-value that demonstrated in BST ceramic plate by preparing large-grained, stress-free, micrometers -thick films. Meanwhile, MOD provides us those advantages of low-cost, large-area deposition and good uniformity compared to PLD method, films with TCD-value about -0.3%/K have been developed by MOD and are being geared to fabricate arrays. Finally, the future direction towards prototyping ferroelectric arrays was formulated based on the practical view of our development expeditions.

Simple detector pixel of dielectric bolometer mode and its device structure for an uncooled IR image sensor

A new type of simple pixel circuit has been developed in an IR image sensor of dielectric mode (DB). The detector pixel consists of capacitors of ferroelectric BST thin film prepared by laser ablation, whose dielectric constant changes drastically with temperature. Our new circuit is a serially-connected capacitor-capacitor circuit with a source-follower output buffer, where one capacitor is an IR detecting BST thin film capacitor formed on a membrane and the other is the same capacitor as reference but on Si bulk and nonirradiated. In order to avoid crack and deformation on the thermally insulated structure, a stress-balanced structure by multi-layered membrane has been newly developed, where the ferroelectric capacitor is formed on a triple layer of SiO2/SiNx/SiO2 films. A BST film on membrane is found to show positive TCD ranging from 1 to 6 percent K in our experiment. A new monolithic process flow is developed to combine an n-MOSFET process and a Si-bulk micromachining process, and ferroelectric capacitors on the stress-balanced membrane are able to be formed monolithically with MOSFETs for source-follower output buffer. Finally, it is especially noted that the operation in the detector pixel in the DB mode is confirmed on the monolithically integrated device structure.

New type of dielectric bolometer mode of detector pixel using ferroelectric thin film capacitors for an infrared image sensor

We have developed a new type of detector pixel circuit operated in an infrared image sensor of dielectric bolometer mode. The detector pixel consists of capacitors of ferroelectric BST (Ba1-xSrxTiO3) thin film, whose dielectric constant changes drastically with temperature. Our proposed circuit is a serially connected capacitor-capacitor, where one capacitor is composed of a BST ferroelectric thin film irradiated by infrared light and the other is nonirradiated one. BST film has been prepared on Si membrane structure by laser ablation. Dielectric constant of the BST film, which is about 450 at 25 degrees Celsius, changes by about 1 to 10%/K in ambient temperature. The maximum change is as large as about 100/K, and more than about 10%/K relative change in the dielectric constant or the corresponding capacitance has been induced. As a result of on-board evaluation of the assembled circuit with a source-follower output, the output level is about 40 mV when a relative capacitance change in the capacitor is about 3%. On the other hand, in PSPICE circuit simulations, the output level is about 25 mV when a relative capacitance change in the capacitor of about 100 pF is 1%. The simulated relationship between the output voltage and capacitance change of the BST film in the assembled circuit agrees well with that in the experimental results. It is considered that the circuit has enough output signal level for input of conventional operational amplifier. Calculated thermal responsivity Rv, and specific detectivity D* are 50 kV/W and 6.5 X 109 cm (DOT) Hz1/2/W, respectively, which means high-sensitivity compared to the other type of IR sensors. The pixel structure also shows a simple configuration, and then is very effective in reducing their pixel size and then increasing the pixel density.

A new dielectric bolometer mode of detector pixel for uncooled infrared image sensor with ferroelectric bst thin film prepared by metal-organic decomposition

Abstract A Ba1−xSrxTO3(Bi/Sr=75/25) ferroelectric thin film for detector pixel of uncooled infrared (IR) image sensor was prepared by metalorganic decomposition (MOD) with final annealing at 700 to 800°C. The films electrical characteristics such as temperature dependence of capacitance and insulation are very good from the viewpoints of spatial uniformity and stability against thermal cycling. The MOD film was applied to our proposed dielectric bolometer (DB) mode cf IR detector that has merits in 1) room temperature operation, 2) chopperless, 3) low power dissipation, and 4) high sensitivity. Finally, the DB-mode operation in the detector pixel was confirmed on the integrated device structure, and the resultant voltage sensitivity (Rv) and specific detectivity (D*) were observed to be 0.4 kV/W and 9.8×107 cmHzl/2/W with noise voltage (Vn) of 100 nV, respectively, where the detector size was 200μm2. Excellent output linearity against IR power down to 0.5 mW/cm2 and an IR image from IR source with average power of 6 mW/cm2. were also obtained.

Chopperless-operated dielectric bolometer mode of infrared image sensor with ferroelectric BST film using improved operation

Proposes a new type of simple detector pixel circuit and device structure for application of dielectric bolometer mode (DB) of IR image sensor, which has merits in uncooled operation, chopperless operation, low power dissipation, and high-sensitivity. We have successfully developed techniques of supply-voltage to the detector and a stable Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST) thin film with large temperature coefficient of dielectric constant (TCD) as an IR detecting material, in order to stabilize the output level and improve the IR sensitivity. Also, a stress-balanced membrane for thermally isolated structure as an IR detector and resultant sensor arrays monolithically integrated with Si readout were developed. As a result, the stabilities in both TCD and output level have significantly improved and enabled us to obtain a fairly large IR detectivity, where best R/sub v/ and D* were about 1.2 kV/W and 2.9 /spl times/ 10/sup 8/ cmHz /sup 1/2//W, respectively.