WO2015109678A1

WO2015109678A1 - Uncooled focal plane array for ir and thz imaging

Info

Publication number: WO2015109678A1
Application number: PCT/CN2014/076239
Authority: WO
Inventors: Xiaomei Yu; Wei Ma; Yongzheng WEN; Shuyang Wang
Original assignee: Xiaomei Yu
Priority date: 2014-01-22
Filing date: 2014-04-25
Publication date: 2015-07-30
Also published as: CN104792420A

Abstract

An uncooled focal plane array (FPA) for imaging at full-band infrared and terahertz regime is provided. The FPA utilizes bimaterial cantilever array as the pixels (101) and are fabricated on a visible light transparent substrate (103). The bimaterail cantilever (101) comprises an absorber (102), two bimaterial legs (202), two thermal isolation legs (203), two anchors (102), and a mirror. The radiation to be detected casts on the front side of bimaterial cantilevers (101), while the transparent substrate (103) is illuminated by visible light to readout the deformation of the bimaterial cantilevers (101) caused by the energy absorption by the absorber (102). The FPA is fabricated by surface sacrificial layer processes, especially by using metamaterial to enhance the function of the absorber and polyimide as the sacrificial layer. The advantages of the uncooled focal plane array include multiband work,high sensitivity and resolution, good reliability and uniformity, low cost and easy fabrication.

Description

UNCOOLED FOCAL PLANE ARRAY FOR IR AND THZ IMAGING BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an uncooled focal plane array (FPA) for imaging systems which operate at terahertz and full band infrared regime. More particularly, this FPA is fabricated on a transparent substrate based on Micro-electromechanical Systems technique, and the responses of FPA are read out by optical methods through the substrate.

2. Discussion of Related Art

FPA is a kind of device that detects the energy of radiation and converts it into other readable signals for imaging purpose. Electromagnetic wave in the range from terahertz to infrared finds a broad scope of application in both civil and military field. Infrared detectors have undergone a booming development since the Second World War because they can help the soldiers overcome the limitation of night and greatly increase their fighting ability in dark battle field. Commercially available infrared imaging systems have been used in automobile industry, environmental monitoring, security surveillance and so on. In military realm, infrared detectors are used in applications such as night vision device for soldiers, laser warning system and weapon guidance. The detection of terahertz radiation is a burgeoning technology in recent years. Electromagnetic wave in THz band shares many characteristics as infrared waves, but has some unique features such as low photon energy and good biological compatibility, and thus especially suitable for applications in medical diagnosis, security check, astronomical observation and explosive detection.

Generally, radiation detectors can be classified into two categories by the mechanism they detect the radiation energy, namely thermal detectors and photon detectors. Thermal detectors senses the temperature change caused by the absorption of incident radiation energy and converts the temperature difference to resistance difference (bolometric), voltage difference (thermoelectric) or mechanical deformation (bimaterial). Photon detectors normally employ semiconductor materials, the electrons in which will be excited to higher energy level upon incident photon and results in an induced current or a modulation of conductivity. Photon detectors outperform the thermal detectors by response time and sensitivity, but need ponderous cryogenic equipment to suppress noise, thus limiting their applications for civil use. Thermal detectors have the advantage of being light and cheap, which is suitable for commercial products, especially handheld devices.

In recent years, uncooled focal plane array (FPA) based on bimaterial cantilevers have gained more and more attention. Theses detectors employ arrays of bimaterial cantilevers as the sensor in each pixel. The bimaterial cantilever is a stacked layer of two materials, the thermal expansion coefficients of which have a great difference and will bend when temperature changes. The deformation of the bimaterial cantilever can be readout through the mirror attached to it by an optical system. Two approaches are commonly employed to fabricate such bimaterial cantilever, which are surface sacrificial layer process and bulk silicon process. In conventional surface sacrificial layer process, the bimaterial cantilevers are fabricated on silicon substrate and the radiation to be detected is incident from the back side of substrate while visible light is incident from front side of bimaterial cantilever for readout. The silicon substrate will cause a considerable dissipation of the radiation energy because of the absorptive characteristics of silicon, so the sensitivity of the device will deteriorate. Bulk silicon process selectively removes the substrate under the bimaterial cantilever to avoid the energy absorption caused by the substrate. However, this approach needs time consuming etching process, which makes the reliability and uniformity of the FPA poor.

Another problem that the uncooled radiation detector encountered is the absorption efficiency. At present, bimaterial cantilever FPA mainly utilizes silicon nitride as the absorber and achieve good results in long wavelength infrared band. The absorption of silicon nitride has a peak around 10 μπι but diminish rapidly towards both shorter and longer wavelength. Especially in THz regime, no natural materials have been found yet to have a good absorbing property suitable for the uncooled detectors. Meanwhile, many applications require multiband or selective band detection, which makes more strict demands on the absorbing materials. Nowadays, metamaterial has come into researchers' sight as a good alternative of conventional absorbing materials. Metamaterial s are artificial materials arranged in period configuration the same as atoms in normal materials. The electromagnetic property of metamaterial is not determined by its composition, rather by the geometry and arrangement of the periodic structure. With precisely design, metamaterials can be applied in an absorber for certain frequency and the absorption wavelength and bandwidth can be tuned conveniently.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a high-performance focal plane array that operates in THz and full band IR regime at room temperature. The FPA utilizes arrays of bimaterial cantilever as the temperature sensitive pixel and is readout by an optical system. Each bimaterial cantilever comprises: bimaterial legs which bend in response to temperature change, thermal isolation legs which minimize the heat exchange between the bimaterial cantilever and the substrate, mirrors for optical readout, absorbers that convert radiation energy to temperature rise, and anchors that support the bimaterial cantilevers on the substrate. The number and configuration of bimaterial cantilevers can vary depending on practical need and fabrication process.

According to one preferred embodiment of the present invention, the bimaterial cantilevers are fabricated on transparent substrate, such as glass and quartz wafers. The radiation to be detected casts on the front side on which the bimaterial cantilevers are fabricated, while the back side of the device is illuminated by a beam of visible light through the transparent substrate for optical readout. The FPA fabrication employs surface sacrificial layer processes, and the surface sacrificial layer material can be semiconductor dielectric or organic materials, such as polyimide, which is easily removed by oxygen plasma dry etching technique.

According to another preferred embodiment of the present invention, the FPA utilizes metamaterials to enhance the absorbing property of traditional simple and narrow band absorbing materials like silicon nitride. The metamaterial is arrays of periodic metal structures, together with a dielectric layer in the middle and a metal ground plane at bottom, forming a three-layered configuration as the absorber. The geometry and arrangement of the metamaterial on top can be designed to achieve high absorption from short wavelength infrared to terahertz regime with significant broadband responses; therefore the devices can realize the detections at different frequencies. In addition, the ground plane of metamaterial absorbers can function as the mirror for optical readout at the same time.

Other advantages of present invention include: 1) the detected radiations incur almost no energy loss because they reach the absorber directly from the front side of bimaterial cantilevers other than the substrate, and therefore the sensitivity of FPA is increased greatly compared with previous works using silicon wafer as the substrate. 2) The surface sacrificial layer processes provide much better reliability and uniformity compared with the bulk silicon process, which can easily cause damage to the suspended bimaterial cantilevers in fabricating the FPA. The organic surface sacrificial layer processes used in present invention will decrease the cost greatly at the same time. 3) The glass and quartz wafers are good thermal isolation material which will prevent the heat conduction between bimaterial cantilever and substrate through the legs. 4) The FPA is more convenient for vacuum packaging with the substrate as the readout window for visible light and another cap wafer that is transparent to detected radiation as the incident widow. 5) It is very easier to realize big arrays for the optical readout FPA since it avoids the readout circuits of electrical readout FPA.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The aforementioned aspects of the present invention will be more clearly explained in the following descriptions in connection with accompanying drawings wherein:

FIG. 1 is the top view of the proposed focal plane array. FIG. 2A illustrates the top view of detailed bimaterial cantilever of FIG. 1; FIG 2B is the bimaterial cantilever with metamaterial to enhance absorption; FIG 2C is another bimaterial cantilever structure with folded legs.

FIG. 3 is the cross section view of the proposed focal plane array.

FIG. 4 illustrates the cross section of a bimaterial cantilever with metamaterial to enhance absorption.

FIG. 5A demonstrates the top view of the metamaterial absorber; FIG. 5B shows the cross section of the unit cell in FIG. 5A; FIG. 5C-5H gives some other metamaterial as examples.

FIG. 6A-6I gives the whole fabrication process of the presented FPA.

FIG. 7A and 7B shows a proposed wafer level packaging and an intermediate wafer design of the presented FPA.

FIG. 8 is the schematic diagram of the optical readout system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is contrived to detect a wide range of radiation from infrared to terahertz for uncooled imaging applications. The structure and working mechanisms will be described in detail with reference to accompanying drawings.

FIG. 1 shows the top view of the FPA with bimaterial cantilevers as the pixels 101 repeated in both directions. The number of the bimaterial cantilevers can have different values like 64x64, 128x 128, 256x256, 512x512 or 1024x 1024 for different applications. The bimaterial cantilevers in an FPA are of the same structure and arranged in any compact manners to increase the fill factor, the bimaterial cantilevers shown in FIG.l share a common anchor 102 with the neighboring bimaterial cantilever on a transparent substrate 103.

FIG. 2A is a zoomed-in top view of a single bimaterial cantilever in FIG. 1. It comprises an absorber 201, two bimaterial legs 202, two thermal isolation legs 203, two anchors 102 and a mirror. The absorber 201 is in sufficient big size and made of materials to absorb the radiation as much as possible. More specifically as shown in FIG. 2B, metamaterials 204, which are arrays of periodic metal structures, can be fabricated on top of the absorber 201, and together with the mirrors as the ground plane, enhance the absorbing property as a whole called metamaterial absorber. The bimaterial leg 202 employs a stacked layer of two materials, the thermal expansion coefficients of which differ by more than one order. The bottom layer of the bimaterial leg 202 is normally dielectric materials like silicon nitride or silicon dioxide, which have a relatively low thermal expansion coefficient. The top layer of the bimaterial leg 202 is usually metal such as gold or aluminum with higher thermal expansion coefficient. The principal of choosing the two materials and the thickness of each material is to achieve the deformation as large as possible under same temperature change. The thermal isolation leg 203 contains only the dielectric material of the bimaterial leg 202. The thermal isolation legs 203 are anchored to the substrate at two anchors 102 to suspend the whole structure and minimize the heat exchange between the bimaterial cantilever 101 and the substrate 103 due to their low thermal conductivity. The number and configuration of bimaterial legs and thermal isolation legs have a variety of choices. They can be straight, folded or multi-folded. FIG. 2C shows another bimaterial cantilever with folded legs in which the thermal isolation leg is longer to achieve better isolation effect.

Referring to FIG. 3, the FPA is fabricated on a substrate 103 that is transparent to visible light, such as glass, quartz and polymers. The bimaterial cantilevers 101 are suspended from the substrate and supported by the anchors 102. Totally different from other work, in present invention, the radiation to be detected casts on the FPA from front side of bimaterial cantilevers, while the transparent substrate is illuminated by a beam of visible light from back side for readout. Therefore, the radiation energy loss and heat conduction brought by the silicon substrate can be avoided.

FIG. 4 illustrates the cross-section of a single bimaterial cantilever. The absorber 201 is connected to the bimaterial legs 202 and anchored to the substrate through the thermal isolation legs 203. The mirror 401 is under the absorber 201 for optical readout from back side through the transparent substrate 103. The mirror 401 also functions as the ground plane of the metamaterial 204 at the same time. Referring to FIG. 5A and 5B, the absorber 201 is designed to absorb the detected radiation as much as possible. It can be dielectric films such as SiN_x and Si0₂, or stacked structure of dielectric films and nano-metal films or black metal, or metamaterial absorber. Specifically, metamaterials 204 can be fabricated on top of a layer of absorber 201, backed by a thick metallic ground plane 401 to form a three-layered structure called metamaterial absorber. The top metamaterial 204, referred as electric resonator, consists of arrays of patterned metallic sub -wavelength structure and is responsible for the electric response of the absorber. The metallic ground plane 401 is thicker than the penetration depth of the incident wave to eliminate any transmission. The coupling of two metallic layers and the dielectric spacer determines its magnetic response. Therefore, by altering the geometry of the metamaterial 204 and changing the thickness of the absorber layer 201, the effective permittivity ε and permeability μ can be tuned independently, resulting in an impedance match to the free space, and thus perfect absorption of the incident wave at certain frequencies. The geometry of the metamaterial 204 has a wide range of varieties. Apart from the square resonators in FIG. 5A, some other commonly used metamaterial patterns are depicted as examples, wherein a square ring resonator in FIG. 5C; a split ring resonator in FIG. 5D, a cross resonator in FIG. 5E, an H-shape resonator in FIG. 5F, a double-split-ring resonator in FIG. 5G, and a Jerusalem cross in FIG. 5H. Moreover, these metamaterials can be configured in nested, multiplexed or stacked manner to create a plenty of interesting absorption spectra like multiband or broad band absorption. In addition, the material chosen for the ground plane 401 and metamaterial 204 is not limited to metals. They can be doped semiconductors like doped silicon or doped germanium, or metal silicide like cobalt silicide, titanium silicide or tungsten silicide. The size and period of the metamaterials 204 is determined by the target wavelength to detect. The material for the absorber layer 201 can be silicon based dielectric material such as silicon nitride and silicon dioxide, or polymers such as polyimide or Parylene-C, the thickness of which is optimized to achieve near unity absorption according to its dielectric constant.

The present invention can be fabricated using any surface sacrificial layer processes on transparent wafers, such as glass and quartz wafer. The materials used as surface sacrificial layer is dielectric film or polymer film. After coating the surface sacrificial layer on transparent wafer and patterning the anchors, the bimaterial cantilever array is fabricated on it. At last the suspended bimaterial cantilevers are released by removing the sacrificial layer. A proposed fabrication process using polyimide as sacrificial layer for the FPA is illustrated in FIG. 6A to 6H, where each drawing shows the cross section of a single bimaterial cantilever at a specific step. Referring to FIG. 6A, a layer of polyimide is first spin coated on the transparent substrate and baked, to form a sacrificial layer 601. A thin layer Au/Cr and is deposited, wherein Cr is the adhesion layer between gold and dielectric layers, and Au servers as the mirror for visible light as well as the ground plane for the metamaterial absorber. The first photolithography step is carried out to define the anchor pattern with the Cr/Au as the hard mask for the polyimide layer 601, which is later etched by oxygen plasma, as in FIG. 6B. Referring to FIG. 6C, a second photolithography step is adopted followed by wet etching of Cr/Au to form the mirror as well as the metamaterial ground plane. Referring to FIG. 6D, a layer of low stress silicon nitride is deposited by Low Pressure Chemical Vapor Deposition with a thickness designed to guarantee the strength of the main structure while satisfy the requirement of the optimal dielectric thickness in the metamaterial absorber. Next, a layer of aluminum is deposited with the certain thickness ratio to the silicon nitride, to meet the requirement of detectable amount of deformation of the bimaterial legs. The aluminum is patterned by a third photolithography step and wet etched to form the top layer in the bimaterial legs, as in FIG. 6E. Referring to FIG. 6F, after the fourth photolithography step, a layer Au/Cr is deposited and patterned by lift-off technique to form the periodic resonant structure in the top layer of the metamaterial structure. Referring to FIG. 6G, after the fifth photolithography, the silicon nitride is selectively etched to form the main structure, including the absorber, the bottom layer of the bimaterial legs and the thermal isolation legs. At last, the whole suspending structure is released by isotropic etching of the polyimide sacrificial layer in oxygen plasma etching system, as shown in FIG. 6H. The FPA is vacuum packaged in wafer level or chip level to minimize the heat exchange with the atmosphere. Referring to FIG. 7A, a method for wafer-level packaging is realized by soldering bonding technique of three wafers. The bottom wafer is the transparent substrate 103 with the FPA fabricated on it, allowing optical readout from backside, while the cap wafer 701, which is used for the radiation incident widow, adopts materials such as germanium, silicon, polymer according to the wavelength to be detected. An intermediate wafer 702 is utilized as a cushion between the cap wafer 701 and the substrate 103. The intermediate wafer 702 has hollow areas 703 on it, which is in alignment with the FPA chip on the substrate 103 to add space for the movement of bimaterial cantilevers in the package cavity. After adhesion and barrier metal are deposited at the bonding area of the wafers, the three wafers are then bonded using soldering bonding technique, thus avoiding high temperature process in fusion bonding or static electric force in anodic bonding, which may damage the device. The solder is normally metal alloy which will melt and bond the wafers at certain temperature, such as Au-Sn alloy with a bonding temperature of 330 ^°C .

FIG. 8 illustrates the readout system of the present invention. The radiation was cast on the front side of the FPA through a wavelength selective lens 801 to filter as well as concentrate. The radiation source can be either objects at different temperatures, or lasers with different radiation wavelength from short-wavelength infrared to terahertz. The back side of the FPA was illuminated by a beam of parallel visible light from a light source 802 and a visible light lens 803, and the light reflected by the mirrors of the FPA was filtered by a pin hole or knife edge filter 804 before reaching a CCD sensor 805. At last, the deflection of the bimaterial cantilever was converted to a gray scale image.

Claims

1. An uncooled focal plane array (FPA) for imaging at full-band infrared and terahertz regime, comprising: array of bimaterial cantilevers serving as the pixels of FPA and a substrate supporting the bimaterial cantilevers. All the bimaterial cantilevers are in the same configuration and arranged in any compact manners to increase the fill factor of the FPA. Each bimaterial cantilever consists of an absorber, two bimaterial legs, two thermal isolation legs, two anchors, and a mirror. The bimaterial legs connect the absorber at one end and connect the thermal isolation legs at the other end. The thermal isolation legs are attached to the substrate at the anchor to support the whole bimaterial cantilever. The number of the bimaterial cantilevers can have different values like 64x64, 128 128, 256x256, 512x512 or 1024x 1024. . . for different applications. The infrared and terahertz wave casts directly on the front side on which the bimaterial cantilevers are fabricated, the radiation energy are converted into heat energy by the absorber, and cause the bimaterial cantilevers deformation due to the bimaterial effect.

2. The FPA according to claim 1, wherein the substrate is transparent to visible light for optical readout. The substrate can be glass wafer, quartz wafer, polymer wafers, and wafers made of some other material that is transparent to visible light. When the FPA works, a beam of visible light illuminates the mirror through the transparent substrate, the different deformations of all bimaterial cantilevers are detected by an optical readout system. Then, the targets of infrared and terahertz are imaged.

3. The bimaterial cantilever according to claim 1, wherein the absorber can be dielectric films such as SiN_x and Si0₂, or multilayer structure of stacked dielectric films, nano-metal films or black metal, or metamaterial absorber. Wherein the absorber is designed with predetermined thickness to absorb the radiation as much as possible and holes are opened on it for the later bimaterial cantilever releasing.

4. The absorber according to claim 3, wherein a typical structure of the metamaterial absorber is a three-layered structure, comprising a bottom layer of ground plane, a middle absorber layer and a top metamaterial with periodic resonant structures. The metamaterial absorber can also be multiple layered structures of dielectric and metamaterial stacked sequentially, aimed at achieving selective band, multi-band or broad band absorption of incident infrared and terahertz wave.

5. The metamaterial absorber according to claim 4, wherein the ground plane can be made of metals like gold, aluminum, etc, or some other materials that reflect infrared and terahertz and block the transmission through the metamaterial absorber, such as metal silicide like cobalt silicide, titanium silicide or tungsten silicide, The thickness of the ground plane must meet the requirement of reflecting and blocking infrared and terahertz as much as possible, with a range from 10 nm to Ιμπι.

6. The metamaterial absorber according to claim 4, wherein the absorber layer is on top of the ground plane. The material of the absorber layer can be silicon-based dielectric, such as silicon dioxide and silicon nitride, or polymers like polyimide, parylene-C. The thickness of absorber layer should be adjusted referring to its dielectric constant and detection requirements of different wavebands, with a range between lOOnm and ΙΟμιη.

7. The metamaterial absorber according to claim 4, wherein the metamaterial is on top of the dielectric layer. The metamaterial is designed to resonate at certain frequencies in infrared and terahertz regime, with the dimension and period in the sub -wavelength scale of detection wavelength, the geometry of which can be square patch, cross, square ring, circle, split ring, one dimensional or two dimensional gratings, or some other plasmonic structures such as H-shape resonator, double-split-ring resonator, Jerusalem cross, etc. Moreover, these metamaterials can be configured in nested, multiplexed or stacked manner to create a plenty of interesting absorption spectra like multiband or broad band absorption. The material used to fabricate the metamaterial can be metals like gold, aluminum and copper, doped silicon or germanium, or metal silicide like cobalt silicide, titanium silicide or tungsten silicide.

8. The bimaterial cantilever according to claim 1, wherein the mirror is at the bottom of the absorber and is made of a thin layer of metals, such as gold, aluminum, etc., or some other materials that reflect visible light for the optical readout system. The mirror could share the same layer of the ground plane of metamaterial absorber with the same thickness.

9. The bimaterial cantilever according to claim 1, wherein the two legs contain two sets of bimaterial legs and thermal isolation legs located symmetrically on either side of the absorber. The bimaterial leg consists of a stacked layer of two materials, where one material has low thermal conductivity and low thermal expansion coefficient, normally semiconductor dielectric materials as silicon dioxide or silicon nitride, and the other material has high thermal expansion coefficient as metals or polymers, to deform under temperature change. The thermal isolation leg consists of one or more materials with low thermal conductivity, and can be the same as one material of the bimaterial leg like silicon dioxide or silicon nitride, to minimize the thermal exchange through legs to the substrate. The bimaterial legs have one end attached to the absorber and the other end connected to the thermal isolation legs, which are connected to the substrate at the anchors. The configuration of the legs can be straight, folded, double folded or multi-folded. When infrared and terahertz wave is incident, the absorber converts the energy into heat, the heat transmit to bimaterial legs and result in the bending of bimaterial legs and further the deformation of the whole bimaterial cantilevers.

10. The FPA according to claim 1, wherein its fabrication can adopt any surface sacrificial layer processes on the transparent substrates. The possible materials used as the sacrificial layer is semiconductor dielectric and polymer films, such as Si0₂, polyimide, which is first deposited or coated on the transparent substrate. After patterning the anchors, the bimaterial cantilevers, including the absorbers, legs and mirrors, are fabricated on top of sacrificial layer. At last the bimaterial cantilevers are released by wet or dry etching of sacrificial layer through releasing holes and suspend on the transparent substrate.

11. The FPA according to claim 10, wherein a fabrication method comprises the steps of:

(a) , spin coating a layer of polyimide on glass wafers, bake at 350^°C for 2 hours to form the sacrificial layer of 2.0μπι thick.

(b) . sputtering a layer of 20/100 nm Cr/Au which serves as the hard mask of dry etching of polyimide, ground plane and mirror. After the first photolithography, the anchors are formed by etching the polyimide with oxygen plasma;

(c) . etching the Cr/Au after photolithography to form the ground plane for the metamaterial absorber as well as the mirror for optical readout;

(d) . depositing low stress silicon nitride with a thickness of 500 nm by plasma enhanced chemical vapor deposition technique, which serves as material for the main structure of the bimaterial cantilever;

(e) . sputtering 300 nm thick aluminum and wet etching after photolithography to form the top material of the bimaterial legs;

(f) . dry etching the silicon nitride after photolithography to form the dielectric layer of metamaterial absorbers, the bottom layer of bimaterial legs, and thermal isolation legs of the bimaterial cantilevers;

(g) . dice the wafer into individual FPAs;

(h) . dry etching polyimide sacrificial layer isotropically using oxygen plasma to release the bimaterial cantilevers.

12. The FPA according to claim 1, wherein the FPA is packaged by a wafer-level or a chip level packing technique. Specifically in the wafer-level packing technique, three wafers are utilized. The transparent substrate with FPA suspended on it serves as a bottom optical readout window; the top cap wafer adopts materials such as germanium, silicon, polymer which is transparent to IR and terahertz radiation and is used as the incident window, and the intermediate wafer with hollowed areas in alignment with the FPA chip on the substrate is between the cap wafer and the substrate to increase the space for the package. The three layers of wafers are bonded using soldering bonding technique in a vacuum environment after the metallization of bonding area is finished.