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Fabrication of a Nanoimprint Lithography Mask for Improved Infrared Detectors

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Fabrication of a Nanoimprint Lithography Mask for Improved Infrared Detectors FABRICATION OF A NANOIMPRINT LITHOGRAPHY MASK FOR IMPROVED INFRARED DETECTORS Isha Datye Faculty Mentor: Dr. Sanjay Krishna Graduate Student Mentor: John Montoya The Center for High Technology Materials The University of New Mexico, Albuquerque, NM 87131 Undergraduate Student of The Department of Electrical and Computer Engineering The University of Illinois at Urbana-Champaign Urbana, IL 61801 ABSTRACT Infrared photodetectors will require new technologies on the pixel level to provide spectral information for the development of polarimetric and color images. Current infrared photodetectors have nearly identical pixels over a broad spectral range, resulting in black-and-white images. Scientists have been researching the idea of an infrared retina, which is similar in function to cones in the human eye, to produce multi-color images. A multi-color infrared camera system can be accomplished by tuning individual pixels to a specific infrared “color” with the aid of resonant structures patterned onto a photodetector’s surface. In addition, a resonant structure can also improve a detector’s detectivity (D*, a measure of the signal to noise ratio) or increase the operating temperature. One of the key limitations of present day technology is the difficulty in making deep subwavelength structures on a large scale. This research paper will focus on the fabrication of a nanoimprint lithography mask to pattern resonant structures on a scale that would make this multi-color technology ready for mass fabrication. Research has been conducted on nanoimprint lithography and it has proven to be a very efficient method to pattern structures on substrates with nanoscale precision. The goal for this project was to pattern complicated resonant structures on a substrate for the development of a multi-color infrared camera. 1. INTRODUCTION visible light, which has wavelengths from about 400 nm to 750 nm. Infrared detectors, The infrared region of the electromagnetic photodetectors that respond to infrared spectrum has wavelengths from 0.75 radiation, have made significant microns to 1000 microns, longer than that of improvements since they were first developed. Although there are several photodetectors. As one can see, these images different divisions within the infrared are based on the intensity of light to provide region, the mid-wavelength (MWIR) and a false color image. Next generation long-wavelength (LWIR) infrared regions photodetectors will be able to provide are most important for infrared detector spectral information based on the technologies, since they include the wavelength of light. wavelengths at which most objects emit radiation. For example, humans emit radiation at a wavelength of 10 microns, which is in the LWIR range [1]. There is an increased emphasis on obtaining hyperspectral and hyperpolarimetric sensitive detectors for night vision, missile tracking, medical diagnostics, and environmental monitoring applications [1- 3]. The development of frequency-selective surface technology on the pixel level has the a) b) potential to provide enhanced infrared detection for a desired wavelength of radiation. Since it was first developed in the Figure 1. (a) [18] Infrared images of the human 1960s, infrared imaging technology, body showing areas of muscle pain in the back especially in the area of focal plane arrays, and post-operative inflammation in the knee. has made significant improvements in (b) [19] Infrared detector images showing a jet producing an image. A focal plane array, a helicopter and jet engine. device that converts an optical image into an electrical signal that can then be processed 1.1 Background or stored, is the core of a long wavelength imaging sensor [14]. The first generation In the past decade, new infrared detector consisted of a single pixel or a one- technologies, such as quantum dot infrared dimensional array of pixels that required a photodetectors (QDIPs), quantum well mechanical sweep to produce a two- infrared photodetectors (QWIPs), quantum dimensional image [1]. The second dots-in-a-well infrared photodectors generation now consists of a two (DWELL), and superlattice structures (SLS), dimensional array of pixels to produce an have been developed with ever increasing image, eliminating any need for moving operating temperatures. Infrared parts [1]. Third generation infrared cameras photodetectors that can operate at room will consist of a two dimensional array of temperature can significantly reduce their pixels that can pass spectral information at cost of operation and therefore expand their room temperature, which is similar in use for everyday applications [2]. QWIPs, function to cones in the human eye [1]. generally made with GaAs materials [17], Although significant improvements have are already well-known and are available been made by the infrared detector commercially [3]. However, they have many community, infrared images that are truly shortcomings and are generally thought to multi-color are not readily available. A few be inferior to QDIPs [2]. For example, examples are given in figure 1 to QDIPs do not require diffraction gratings to demonstrate images taken with conventional couple normally incident light [2]. Because of this, there is one step less in the fabrication of QDIPs than in the fabrication are still interested in exploring new of QWIPs. QDIPs are similar to QWIPs in technologies that can take even better structure; the quantum well is substituted images and can incorporate more elements with a quantum dot [17]. QDIPs are such as color, polarization, and dynamic generally constructed with InAs dots on range. GaAs substrates [17]. An image of a QDIP structure is shown in figure 2. QDIPs can operate at higher temperatures as a result of having a lower dark current [2]. Figure 3. Image of a quantum dots-in-a-well structure, showing the InAs quantum dot in an InGaAs quantum well. Figure 2. [20] The image on the left shows a 10- layer InGaAs/GaAs QDIP structure, and the image on the right shows a diagram of a QDIP in Currently, all of the pixels in an infrared an electric field. camera are nearly identical, creating black- and-white images instead of images of different colors [15]. Scientists would like to The DWELL structure, a cross between change this by integrating multispectral QDIPs and QWIPs with InAs quantum dots capability on the pixel level. An example of in an InGaAs quantum well [3], has also multispectral imaging is shown in figure 4. been proven to have low dark currents, and They have been researching the concept of higher operating temperatures [3]. Similar to an infrared retina, which would act similarly QDIPs, these detectors allow normal to the cones in a human eye in the incidence, which ultimately provides better information conveyed in the images [1]. In control over the operating wavelength [16]. order to create this infrared retina, either A DWELL structure is shown in figure 3. plasmonics or matamaterials can be used. InAs/GaSb type-II strain layer superlattices Current infrared detector technology, can in also operate at higher temperatures, have a general, be improved with the aid of higher detectivity, high efficiency, and some resonant structures of a given design to multi-color capability [2], although not on a provide a higher operating temperature, large scale. This is the most promising spectral information on the pixel level, and a technology, but it also the most expensive. higher detectivity [3]. This project will focus on the development of a nanoimprint 2. MOTIVATION FOR PROJECT lithography mask for the fabrication of these resonant structures. Initially, a simple Although the current infrared detector nanoimprint lithography mask will be technologies have made many created to imprint an array of posts. These improvements in their images, researchers simple structures can aid in the fabrication of surface plasmon (SP) diffraction gratings, the beam and the surface normal to the which have been shown to enhance the sample. In electron beam lithography (EBL), signal received by an infrared photodetector a beam of electrons is scanned in a pattern [13]. A simple nanoimprint lithography across a surface with photoresist to create mask can be used to establish a fabrication small structures in the resist that can then be process for the creation of nanoimprint transferred to the substrate. masks with complicated features, such as metamaterials. Metamaterials are artificial materials that have properties usually not found in nature. They offer a huge advantage over SP structures because they can be created for pixels with a small size [5]. They can be engineered to have a negative index of refraction, meaning that the light is bent around the object rather than transmitted through or reflected away from (a) it [5]. Narrow-band perfect absorbers can be created because of the electric permittivity and magnetic permeability of metamaterials [5]. They can control and interact with infrared radiation only if their structures have wavelengths similar to those of the infrared light waves with which they interact [5]. (b) 3. RESEARCH OBJECTIVE Figure 4. (a) [21] In the top right picture, an object is detected that wouldn’t be seen with the human eye. (b) [22] In the bottom three pictures, different parts of This project focused on the fabrication of a a flame can be seen with an infrared detector. mask through different forms of lithography, such as interferometric and electron beam lithography, to pattern metamaterial EBL is generally used to make integrated structures. Lithography is a technique used circuits and masks [10, 12]. Although useful to transfer patterns onto a substrate. for creating interesting, repetitive patterns, Interferometric lithography is a process in electron beam lithography cannot be which a laser beam is split into two beams, performed on a large scale because it is one going directly to the sample and one extremely slow and expensive [10]. going to mirror and then reflected to the However, it can be used on a single sample sample, thereby creating an interference and then nanoimprint lithography can be pattern [8].
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