Binbin Jiao

A novel uncooled substrate-free optical-readable infrared detector: design, fabrication and performance

A novel substrate-free uncooled IR detector based on an optical-readable method is presented and fabricated successfully. The detector is composed of a bi-material (BM) cantilever array, without a silicon substrate, which is eliminated in the fabrication process. Compared with the generally used sacrificial layer cantilever, the loss of incident IR energy caused by the reflection from and absorption by the silicon substrate is eliminated completely in the substrate-free structure. The IR radiation reaching the IR detector surface increases by over 80% in the case of the novel substrate-free detector array structure, compared to the sacrificial layer structure. Moreover, the substrate-free structure has less heat loss than the sacrificial layer structure. The results of thermal imaging of the human body show the detector is able to sense objects at room temperature. The experimental NETD was estimated to be 200 mK.

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.

Optical readout sensitivity of deformed microreflector for uncooled infrared detector: theoretical model and experimental validation

The authorss group proposed an optical-readout uncooled infrared detector. Primarily because of the bilayer structure of the usual such detector, deformation of the reflector is often unavoidable and seriously degrades the optical readout sensitivity. According to the theoretical analysis and experimental validation, an optical solution to this problem was established, and it was found that for the specific curvature radius, there are many characteristic reflector lengths and filter positions corresponding to the sensitivity peaks. When employing this solution, the sensitivity loss induced by the deformed reflector would be reduced to a minimum level. The strategy of this solution may also be suitable for other micromechanical devices that experience the same problem.

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...

Optical sensitivity analysis of deformed mirrors for microcantilever array IR imaging

Optical sensitivity is a major issue to improve the sensor responsivity and the spatial resolution of uncooled optomechanical focal plane arrays (FPA). The optical sensitivity is closely related to the mirror length and the undesired mirror deformation induced from the imbalanced residual stresses in different layers. In this paper, the influences of mirror length and deformation on the optical sensitivity are discussed by Fourier Optics. Theoretical analysis and experiments demonstrate that the optical sensitivity is seriously degraded by undesired mirror deformation, and that there exists an optimal mirror length which makes the optical sensitivity achieve its maximum under a certain mirror deformation. Based on the results, an optimized mirror configuration is presented to increase the optical sensitivity of substrate-free bi-material microcantilever array (SFBMA).

Optical sensitivity analysis of a bent micro reflector array in uncooled infrared imaging

In an optomechanical infrared (IR) detection system, the optical detection sensitivity, a key factor that affects the noise equivalent temperature difference (NETD), is influenced by the length and bend of micro-cantilever reflector of a focus plane array (FPA). The influence on optical detection sensitivity by such a bend due to the bulk micromachining techniques is discussed based on Fourier optics. In this study, the reflectors of the fabricated FPAs with pixel sizes 200 µm, 120 µm and 60 µm have the curvature radii of 23 mm, 11 mm and 8 mm, respectively. Theoretical analysis and experiments show that the sensitivities are degraded by the undesired bend of the micro-cantilever reflector. Moreover, the study demonstrates that the sensitivity reaches the maximum value when the reflector length L and the curvature radius R of the micro-cantilever reflector satisfy the relation: L2/R = λ (λ is the wavelength of illumination light) and this result provides an approach for optimizing the optical sensitivity as the diminishing of the pixel size of FPA. Finally, the methods of optimizing the sensitivity are discussed.

Design of a Novel Substrate-Free Double-Layer-Cantilever FPA Applied for Uncooled Optical-Readable Infrared Imaging System

This paper describes the design and performances of a novel focal-plane array (FPA) containing pixels of double bimaterial-layer cantilevers without silicon (Si) substrate for being applied in the uncooled optical-readable infrared (IR) imaging system. The top layer of the cantilever pixels is made of two materials with large mismatching thermal expansion coefficients: silicon nitride (SiNx) and gold (Au), which convert IR heat into mechanical deflection. The bottom layer is SiNx cantilever, which partially serves thermal isolation legs. The top and bottom pads form the resonant cavity, which can dramatically enhance the absorption of incident IR irradiation, and the substrate-free configuration enables reducing the loss of incident IR energy. Responding to the IR source with spectral range from 8 to 14 mum, the IR imaging system may receive an IR images through visible optical readout method. A thermal-mechanical model for such cantilever microstructure is proposed, and the thermal and thermal-mechanical coupling field characteristics of the cantilever microstructure are optimized through numerical analysis method and simulation by using the finite-element method. The thermal-mechanical deflection simulated is 7.2 mum/K, generally in good agreement with what the thermal-mechanical model and numerical analysis forecast. The analysis suggests that the detection resolution of current design is 0.03 K, whereas the noise analysis from FPA indicates the current resolution to be around 100 muK and the limit noise-equivalent temperature difference (NETD) of the IR imaging system can reach to 7 mK.

Elimination of initial stress-induced curvature in a micromachined bi-material composite-layered cantilever

Micro-devices with a bi-material-cantilever (BMC) commonly suffer initial curvature due to the mismatch of residual stress. Traditional corrective methods to reduce the residual stress mismatch generally involve the development of different material deposition recipes. In this paper, a new method for reducing residual stress mismatch in a BMC is proposed based on various previously developed deposition recipes. An initial material film is deposited using two or more developed deposition recipes. This first film is designed to introduce a stepped stress gradient, which is then balanced by overlapping a second material film on the first and using appropriate deposition recipes to form a nearly stress-balanced structure. A theoretical model is proposed based on both the moment balance principle and total equal strain at the interface of two adjacent layers. Experimental results and analytical models suggest that the proposed method is effective in producing multi-layer micro cantilevers that display balanced residual stresses. The method provides a generic solution to the problem of mismatched initial stresses which universally exists in micro-electro-mechanical systems (MEMS) devices based on a BMC. Moreover, the method can be incorporated into a MEMS design automation package for efficient design of various multiple material layer devices from MEMS material library and developed deposition recipes.

The optimizing designing of bi-material micro cantilever with adhesive layer in between and its application in an uncooled MEMS IR FPA

Bi-material cantilever is an important basic structure in MEMS device. Most of the materials with thermal property fit for bi-material are not adhering together steadily. An adhesive layer in between is needed. In this paper, based on the thermal stress and combined deformation in Mechanics of Materials, a model related to the physics properties, structure dimension, and the tilt angle caused by thermal stress is set up. A research of how to select the materials and how to determinate the thickness and other size of a bi-material cantilever is carry out by this model, further more, an optic read out IR image chip pixel is designed that shows this model is simple and practical.

Design of a Novel Uncooled Infrared Focal Plane Array

This paper presents the optimized design of a novel Focal Plane Array (FPA) structure for opt-mechanical uncooled infrared imaging system. The FPA structure is a bi-material micro-cantilever array which without Si substrate and with thermal isolation organ. In the paper, we build up series of model to describe the structures IR absorb, heat exchange and thermal-mechanical characters. To optimize the parameter for a given pixel size (200 mum times 200 mum ) and certain materials, we have studied the number of deforming cantilever and given out an optimal value. The sensitivity of the optimized structure is calculated out to be 12.2 times 10-3deg/K. which means with the optical readout system we have reported [Zheying Guo, et al.,2005], the NETD can get 1.6 mK