Application of MEMS Technology in Uncooled Infrared Detectors

As an emerging microfabrication technology, micro-electromechanical system (MEMS) technology has begun to be applied in various fields. It can integrate functions such as information acquisition, processing and execution, and has the advantages of being small, intelligent, executable, integrable, good process compatibility and low cost. It also has very broad application prospects in the field of infrared detection technology, and will provide a newer approach for research in this field. The use of MEMS technology in the development of uncooled infrared detectors can enable the devices to develop in the direction of high reliability, miniaturization, intelligence, high-density array integration, low cost and mass production, and it is possible to use this technology to manufacture uncooled infrared detectors with a completely new mechanism.

Infrared detectors are the most basic and key components of infrared instruments and the heart of infrared devices. The development level of infrared technology is mainly marked by the development of infrared detectors. Before the 1960s, infrared detectors were mainly unit detectors, and two-dimensional optical scanning was required to achieve infrared imaging; in the 1970s, linear multi-element infrared detectors appeared, and only one-dimensional optical scanning was required to achieve infrared imaging; after entering the 1980s, focal plane devices were developed, which can be stared directly without optical scanning. However, since quantum infrared systems have always required low-temperature cooling to obtain the desired system application performance, this requirement has brought about problems such as system reliability and high cost, which has greatly restricted their application. In recent years, with the development of various new technologies, especially the application of MEMS technology, the overall performance and reliability of uncooled infrared detectors that can work at room temperature have been greatly improved. In addition, the system is small and easy to carry, convenient and flexible to use, and low in cost, which further promotes the application and development of uncooled infrared detectors. This paper reviews the process and main characteristics of MEMS technology, and introduces in detail the specific application and process structure of its representative uncooled infrared detectors. A detailed comparison is made on their performance and cost, and a brief introduction is given to the latest achievements in the application of MEMS technology in uncooled infrared detectors.

2. Introduction to HEHS Technology

MEMS technology is gradually developed by absorbing and integrating other processing technologies on the basis of microelectronics manufacturing technology. It is an emerging special micro-processing technology that integrates micro sensors, micro actuators, micro energy and electronic circuits (r3), and is a device system composed of smaller movable sub-elements of 0.5 to 500 gm. In the early 1960s, the important technology of MEMS was developed, namely crystal anisotropic corrosion and anodic bonding technology; in the late 1980s, LIGA technology was developed and initial results were achieved, and gears, cranks, springs, micro electrodes and more complex MEMS were developed; in the 1990s, MEMS technology has entered practical applications, such as acceleration sensors used in automobile anti-collision airbags, which cost only about US$5.

Generally, MEMS technology can be divided into bulk micromachining (corrosion, coating, doping, bonding), surface micromachining, high aspect ratio micromachining and ultra-fine precision machining, etc., and also uses some mature semiconductor processes, such as lithography, oxidation, diffusion, ion implantation, sputtering, epitaxial growth and deposition. At present, the processing materials are mainly silicon-based, and the research on metals, glass, ceramics, plastics and III, V compounds is also increasing.

Microelectronics technology is an important foundation of MEMS technology, and its processing method is one of the important processing methods of MEMS technology. MEMS also has its own characteristics, such as diversified processes, and can produce beams, diaphragms, grooves, holes, sealing holes, cones, needle tips, springs and complex mechanical structures. At the same time, it is compatible with microelectronics processes, devices can be mass-produced, and costs are reduced. MEMS technology can be applied to almost every field, especially high-tech fields that require small size, high precision, high reliability and low power consumption.

3 Application of MEMS technology in uncooled infrared detectors

In recent years, MEMS technology has developed rapidly, and there have been relatively successful examples of its application in uncooled infrared detectors, opening up a new path for the miniaturization of existing unit devices and high-density array integration.

3.1 Micromachined Infrared Thermopile Detector

The working principle of infrared thermopile detectors is the Seebeck effect. Earlier infrared thermopile detectors were obtained by depositing thermocouple materials onto plastic or ceramic substrates using a mask vacuum coating method, but the device size was large and difficult to mass produce. With the application of MEMS technology, micromechanical infrared thermopile detectors have emerged. K. D. Wise et al. first used MEMS technology to manufacture silicon-based infrared thermopile detectors in the early 1980s.

The basic structure of the micromechanical infrared thermopile chip is shown in Figure 1. Bulk silicon is generally used for device manufacturing. The anisotropic etching of silicon is used to obtain a pyramid-shaped etching hole from the back of the silicon wafer, and the side wall is a slow etching surface (111). The thermal insulation structure of the hot junction area and the cold junction area is now mainly realized by thin film structure. There are two types of applied thin film structures, namely closed film structure (Figure 1 (a)) and cantilever structure (Figure 1 (b)), in which the closed film refers to the support film of the thermal stack as a whole layer of composite dielectric film, generally silicon nitride film or silicon nitride and silicon oxide composite film. Cantilever refers to a membrane structure surrounded by an atmosphere medium, with one end fixed and the other end suspended. In terms of thermal insulation effect, cantilever beams have more advantages than closed films, because in closed film structures, heat can propagate along the dielectric support film, but not completely along the thermocouple, resulting in greater heat dissipation, low thermoelectric conversion efficiency, and low sensitivity. However, from the perspective of process manufacturing and yield rate, closed membrane has more advantages, because the advantage of this membrane structure is that the structure is stable. Since the membrane is connected to the substrate everywhere, it is less affected by stress. The membrane itself is not easy to break during the manufacturing process. The yield rate is high and it is easy to manufacture. The cantilever beam is connected to the substrate only by a fixed support at one end, and the other end is suspended in the air. Therefore, it is significantly affected by stress. The membrane is prone to warping or breaking during the manufacturing process, so the yield rate is low and it is not easy to manufacture.

Many research groups are now working on the research of micromechanical infrared thermopile arrays, and silicon-based thermopile is a hot research topic, such as polysilicon/gold thermocouple wire arrays, silicon/aluminum thermocouple wire arrays, and n-type polysilicon/p-type polysilicon thermocouple surface arrays. Compared with general infrared detectors, the advantages of micromechanical infrared thermopile detectors are: ① It has higher sensitivity, a relaxed working environment and a very wide spectrum response; ② It is compatible with standard IC processes, low cost and suitable for mass production.

3.2 Pyroelectric Uncooled Infrared Detector

The working principle of pyroelectric detectors is the pyroelectric effect of pyroelectric crystals. Since the performance of pyroelectric detectors decreases as the heat decreases, a good thermal insulation structure is the key to making high-performance pyroelectric detectors. The earliest insulation technology used was to connect the pyroelectric infrared detector or array to the Si signal processing circuit through a plastic metal (such as In) table, but the thermal insulation performance of In is very poor, which is not conducive to the production of high-performance large-area integrated pyroelectric infrared focal plane arrays. MEMS technology is now mostly used to make bridge structures or suspended membrane structures to improve the thermal insulation of the sensing unit. In this way, when infrared light is irradiated, each sensing unit can obtain a relatively large temperature rise value, which correspondingly improves the sensitivity of the detector. Figure 2 is a cross-sectional view of a sensing unit of a micromechanical pyroelectric infrared detector using a suspended membrane structure.

The ferroelectric materials currently used for uncooled infrared focal planes are mainly BST, PZT and PST. The following is a method for making a film-type insulating structure of a BST thin film infrared detector. First, Pt/Ti/pSi/n-Si is used as a substrate, and the BST film is deposited by the sol-gel method. Then, the BST film and the pt bottom electrode are etched into an array pattern using photolithography and ion beam etching technology. Then, photolithography and ion beam sputtering technology are used to sputter a Pt film as an upper electrode on each BST thin film detection unit. Then, double-sided photolithography technology is used to overlay the surface unit patterns that are not connected to each other on the back of the substrate, corresponding to the front detection unit. EDP is used to etch the Si substrate on the back of the detection unit, so that each detection unit is suspended to form a film-type insulating structure.

There are two key issues to be addressed when using MEMS technology to make bridge structures or suspended membrane structures: one is to find a technology for depositing high-quality ferroelectric material films, which must also be compatible with the process temperature and etching technology of Si-CMOS circuits; the other is that there must be high thermal insulation between the deposited film and the silicon substrate.

T. Evans et al. used silica gel as a thermal insulation layer between the deposited film and the silicon substrate, which effectively solved the problem of thermal insulation between the deposited film and the silicon substrate. When the porosity of silica gel is 75% to 95%, it has a lower thermal conductivity than air (the comparison result is shown in Figure 3), and this method is fully compatible with standard IC processes (this technology has been maturely used in the production of ferroelectric memory). The array made with this technology is another choice for the production of low-cost uncooled infrared detectors.

3.3 Microbolometer

Microbolometers are made by using the sensitivity of the body resistance of an object to temperature. In order to increase the thermal insulation of the device as much as possible and reduce the thermal conductivity to improve the sensitivity of the device, most MEMS technology is now used to achieve a suspended microbridge structure to solve this problem. Figure 4 is a schematic diagram of the structure of a microbolometer using a microbridge structure. It uses a microbridge supported by two arms to achieve thermal insulation, Si, N, as a supporting film, the silicon substrate under the microbridge is hollowed out, polycrystalline silicon germanium Poly-Si07Ge03 is made on the bridge surface of the microbridge, and a thin film resistor is used as a thermal detection source. In order to improve the absorption of infrared, there is a SiO/SiN composite film on the surface as an infrared absorption layer.

Compared with micromechanical pyroelectric detectors, microbolometers have advantages in performance and low cost. They have taken the path of monolithic bridge-shaped thermal insulation detector structure, and they are more than ten years ahead of pyroelectric detectors. At present, the size of microbolometer arrays has reached 640X 480, and the pixel size can be 25um×25um, and the performance has reached the level of uncooled photon detectors.

Deniz Sabuncuoglutezcan et al. recently reported that a new microbolometer was made using a MEMS process that is completely compatible with IC technology. It uses anisotropic etching of silicon to hollow out the n-well of the CMOS structure to form a suspended structure (using electrochemical etching technology of TMAH solution), as shown in Figure 5. The microbolometer made by this method has a DC response rate of 9250V/W and a detection rate of 2 X 109cmHzla/W when the pixel unit is 74LLmX 74tm. Moreover, since this method no longer requires any photolithography or deposition of infrared sensitive materials after completing the CMOS structure, the cost of the detector is greatly reduced, almost equivalent to the cost of CMOS chips, so this method has a very bright future.

Compared with the above three types of detectors, the performance of thermopile detectors is at a disadvantage and there are relatively few studies on them. However, the performance and low cost of bolometric thermal detectors and pyroelectric detectors are relatively good. However, it is precisely because of the application of MEMS technology and IC process that the overall performance of detectors has been continuously improved and the cost has been continuously reduced.

3.4 Other uncooled infrared detectors

Due to the unique advantages of MEMS technology, the detector array elements are more integrated and have better performance. More and more research groups have developed other types of uncooled infrared detectors using MEMS technology. Figure 6 is a micro-mechanical electronic tunnel infrared detector made using the electron tunneling effect in the vacuum barrier. This detector uses a three-layer silicon structure: the first layer of silicon structure is used to make the tunnel silicon tip electrode and the electrostatic deflection electrode; the second layer of silicon structure is used to make the elastic sensitive film and half of the air cavity; the third layer of silicon structure is used to make the infrared transmission film and the other half of the air cavity. The sensitivity of this detector is relatively high, and the resolution of the electron tunneling displacement sensor part can reach 10-4nm/Hzl/2.

As a product of an emerging interdisciplinary subject, MEMS technology has played a breakthrough role in the development of uncooled infrared detectors. With the increasing integration of various new MEMS technologies, the development of uncooled infrared detectors will surely become more and more vibrant.