Substrate-free Focal Plane Array Research Articles

Bimaterial Cantilever Focal Plane Array for Uncooled Infrared Imaging Using Sandwich-Framed Structure

This paper reports a novel infrared focal plane array (FPA) using SiN x /SiO2/SiN x sandwiched structure as the supporting frames for bimaterial cantilever pixels. The device was fabricated by a bulk silicon process, where a trench backfill technique was employed to form the SiN x /SiO2/SiN x sandwiched frames. By replacing the silicon frame with SiN x /SiO2/SiN x sandwiched frame, the fill factor of the FPA is increased to 42%. Compared with other substrate-free FPAs with frames making of a thin layer of SiN x , the sandwiched frame provides more supporting strength for the cantilever pixels. Meanwhile, the reliability and the uniformity of the FPA were improved with a design of staggered arrangement of cantilever pixels. Thermal images of human bodies were captured successfully at room temperature. A complete analysis of device performance was also carried out both theoretically and experimentally, including thermomechanical sensitivity, noise equivalent temperature difference, and response time. [2015-0261]

A substrate-free optical readout focal plane array with a heat sink structure

A substrate-free optical readout focal plane array (FPA) operating in 8–12 μm with a heat sink structure (HSS) was fabricated and its performance was tested. The temperature distribution of the FPA with an HSS investigated by using a commercial FLIR IR camera shows excellent uniformity. The thermal cross-talk effect existing in traditional substrate-free FPAs was eliminated effectively. The heat sink is fabricated successfully by electroplating copper, which provides high thermal capacity and high thermal conductivity, on the frame of substrate-free FPA. The FPA was tested in the optical-readout system, the results show that the response and NETD are 13.6 grey/K (F / # = 0.8) and 588 mK, respectively.

Equivalent circuit model of the substrate-free focal plane array based on the optical readout uncooled infrared imaging system

与传统的有基底FPA(焦平面阵列)相比, 基于全镂空支撑框架结构的新型无基底FPA在热学特性上存在显著差异, 传统的基于恒温基底假设的热学分析模型不再适用, 因此, 通过电学比拟方法, 将无基底FPA的热响应特性等效为电学模型.通过该模型, 进一步分析了无基底FPA在非真空环境下的热学性能, 分析表明: 该无基底FPA具有在大气压下优良的红外成像性能, 其NETD(噪声等效温度差)值仅比真空环境下增加了数倍.;Thermal characteristics of new substrate-free FPA (focal plane array) with full hollow supporting frame structure are very different from that of traditional substrate FPA. The thermal analysis model which based on temperature-constant substrate assumption is not efficient any more. Therefore, an equivalent circuit model was presented to analyze the thermal response characteristics of substrate-free FPA with electrics and holistic approach. According to this model, the thermal characteristics of substrate-free FPA under non-vacuum environment were analyzed. The result shows that, the substrate-free FPA has excellent infrared imaging performances under atmospheric pressure, while its NETD (noise equivalent temperature difference) is just increased several times than in the vacuum environment.

Research of Infrared Imaging at Atmospheric Pressure Using a Substrate-Free Focal Plane Array

An equivalent circuit model to the substrate-free focal plane array (FPA) is established. Using this fast and effective model, the performance of infrared (IR) imaging at atmospheric pressure is investigated and it is found that the substrate-free FPA has the ability of IR imaging at atmospheric pressure, whereas it has a slightly degraded noise equivalent temperature difference (NETD) as compared with IR imaging under a high vacuum. This feature is also identified experimentally by a substrate-free FPA with pixel size of 50 × 50 μm2. The NETDs are measured to be 160 mK at 10−2 Pa pressure and 1.08 K at atmospheric pressure.

A holistic approach performance analysis of substrate-free focal plane array

Based on a bi-material micro-cantilever thermal deformation principle, an uncooled infrared focal plane array (FPA) with optical readout has been developed from a substrate-based structure to a substrate-free structure. Infrared imaging of the substrate-free FPA indicates that this structure does not satisfy temperature-constant substrate conditions when the FPA unit size decreases from 200 μm to 60 μm. To evaluate the performance of the FPA, this paper puts forward an analytical model of heat transfer in the substrate-free FPA, by using a holistic approach and an electrical circuit analogy. The analytical model provides a fast and convenient way to calculate the temperature gray response and the response time of the substrate-free FPA. A substrate-free FPA with a unit size of 50 μm was fabricated. Infrared imaging experiments validate the model and indicate a noise equivalent temperature difference value of 170 mK has been achieved.

Analysis of theoretical model of thermal infrared imager based on the substrate-free focal plane array

Based on thermal deformation of bi-material microcantilever, the focal plane array (FPA)of uncooled optical readout infrared(IR) imaging system has undergone a development from substrate array to substrate-free array. The experimental imaging result and finite element method (FEM) analysis indicated that the substrate-free focal plane array (FPA) did not accord with the condition of constant frame temperature. This paper proposed a new theoretical model on thermal transmission of substrate-free FPA with electrical analogy method. Considering the system as a whole, the analysis of complex thermal interaction of adjacent elements could be averted while the heat absorption and transmission of frame could be considered. The temperature of outer frame was set to be equal to the ambient temperature as a boundary condition. Although it was not so flexible compared with FEM analysis when dealing with the boundary condition, the theoretical model was proved to correspond with the experimental result, and could be used as an approximate formula in thermal response calculation of substrate-free FPA. The model avoids the complication of FEM analysis, especially for large arrays. Further more, the model can be used for substrate-free FPA dimensions design and optimization under certain response target.

Thermal and mechanical characterizations of a substrate-free focal plane array

We propose a substrate-free focal plane array (FPA) in this paper. The solid substrate is completely removed, and the microcantilevers extend from a supporting frame. Using finite element analysis, the thermal and mechanical characterizations of the substrate-free FPA are presented. Because of the large decrease in thermal conductance, the supporting frame is temperature dependent, which brings out a unique feature: the lower the thermal conductance of the supporting frame is, the higher the energy conversion efficiency in the substrate-free FPA will be. The results from the finite element analyses are consistent with our measurements: two types of substrate-free FPAs with pixel sizes of 200×200 and 60×60 μm2 are implemented in the proposed infrared detector. The noise equivalent temperature difference (NETD) values are experimentally measured to be 520 and 300 mK respectively. Further refinements are considered in various aspects, and the substrate-free FPA with a pixel size of 30×30 μm2 has a potential of achieving an NETD value of 10 mK.

Analysis of Optical Readout Sensitivity for Uncooled Infrared Detector

An optical readout uncooled infrared detector, employing a substrate-free focal plane array with pitch size 60 μm, is established. The reflector deformation induced by the stress mismatching of the bi-layer structure is discussed and, in turn, a universal solution to determine both the optical readout sensitivity and the optimal filter position is found. By applying this solution, the optical readout sensitivity for the ideal plane reflector could theoretically increase by 80% as compared with the conventional operation, and the sensitivity loss caused by the reflector deformation can also be reduced to a reasonable level.

Performance of an optimized substrate-free focal plane array for optical readout uncooled infrared detector

This note presents an optimized substrate-free focal plane array (FPA), which is implemented in an optical readout uncooled infrared (IR) detector. The supporting frame of such FPA is a temperature-variable one due to the large decreases in both the heat capacity and the thermal conductance. This brings a unique thermal characteristic: the supporting frame functions as a “thermal isolation” frame which reduces the thermal conductance and therefore increases the temperature change and also functions as a “thermal diffusion” frame which certainly results in the temperature prechange in the ones not absorbing IR radiation. This characterization could significantly increase the temperature change of microcantilevers and therefore improve the performance of the substrate-free FPA. In the proposed IR detector, the fabricated 160×160 FPA has an average noise equivalent temperature difference (NETD) and a response time of 330 mK and 16 ms, respectively. The performance of the IR detector theoretically increases by about 5.5 times compared with the one using a substrate FPA. Here, the geometry of the substrate FPA is just the same as the fabricated FPA, but the supporting frame is assumed to be a temperature-constant one. If the optical readout sensitivity can be increased enough using an enhanced IR absorber, the fabricated FPA thus has the potential to achieve a NETD value of 70 mK.

Performance analysis of the substrate-free focal plane array in infrared imaging

Based on the proposed optical readout uncooled infrared (IR) imaging system，several batches of the substrate-free focal plane array (FPA) with different pitches have been successively fabricated，and by using these fabricated FPAs，the thermal images were obtained at room temperature. However，the theoretical analysis indicates that the deviation between the theoretical model assuming constant temperature of the supporting frames of the FPAs and the experimental results gradually increases with the decreasing pitch size of from 200μm to 60μm. Based on the finite element method，the thermal characteristic of those FPAs was simulated and the reason for the deviation is found to be that the temperature-constant assumption is not quite appropriate for the substrate-free FPA，and the absorption of the thermal radiation will obviously result in the temperature rise of the supporting frame，which would increase the temperature of the adjacent pixels. The imaging performance of the substrate-free FPA is also analyzed. Compared with the substrate FPA with the same pitch, the thermal detection sensitivity of the substrate-free FPA with 60μm pitch increases by about one order of magnitude.

Uncooled Infrared Imaging Using a Substrate-Free Focal-Plane Array

A substrate-free 160 times 160 focal-plane array (FPA) with a 60-mum times 60-mum pitch has been developed and used for an optical readout uncooled infrared (IR) detector. The supporting frame of the FPA is a temperature-variable one due to its large decreases in both heat capacity and thermal conductance. This thermal characteristic significantly increases the temperature change of the microcantilever, which depends on both the temperature change induced by its absorption of IR radiation and the linear superposition of the temperature prechange induced by other microcantilevers, and therefore improves the performance of the substrate-free FPA. In the proposed IR detector, the fabricated FPA had an average noise equivalent temperature difference (NETD) and a response time of 330 mK and 16 ms, respectively. Its performance increased by about 4.3 times compared with that of the substrate FPA that consists of the same microcantilevers.

The pressure-dependent performance of a substrate-free focal plane array in an uncooled infrared imaging system

Uncooled focal plane arrays (FPAs) are being developed for a wide range of infrared imaging applications. A substrate-free FPA for optical readout infrared imaging is fabricated with a pixel pitch of 120 μm. The pressure dependences of thermal conductance of a FPA with/without substrate are studied by modeling analysis. Infrared imaging experiments are performed to validate the modeling analysis. At atmospheric pressure the total thermal conductance of the substrate-free FPA is only 1/1000 time of the traditional FPA which has a 2 μm air gap between the cantilever beam and the substrate. Room temperature object’s thermal images are obtained even if the FPA is placed in the atmosphere. The air conductance of a substrate-free FPA at atmospheric pressure could be as small as that of a traditional FPA with a substrate in high vacuum (about 1 Pa). The experimental result also shows that the system noise keeps almost unchanged with the pressure. These characters will decrease the vacuum packaging request of the FPA and should be valuable for practical applications.

A novel MEMS-based focal plane array for infrared imaging

On the basis of opto-mechanical effect and micro electromechanical system (MEMS) technology, a novel substrate-free focal plane array (FPA) with the thermal isolated structure for uncooled infrared imaging is developed, even as alternate evaporated Au on SiN cantilever is used for thermal isolation. A human thermal image is obtained successfully by using the infrared imaging system composed of the FPA and optical detecting system. The experiment results show that the realization of thermal isolation structure in substrate-free FPA increases the temperature rise of the deflecting leg effectively, whereas the noise equivalent temperature difference (NETD) is about 200 mK.