Rochester Institute of Technology (RIT) and its collaborators at the University of Rochester and Harris Corporation are developing a room-temperature imaging Terahertz (THz) frequency detector using Si-MOSFET (Silicon Metal Oxide Semiconductor Field Effect Transistor) CMOS devices. They are implemented into a focal plane imaging array for use in many applications, such as transmission or penetration imaging and spectroscopy. Technology for THz detection is often extremely costly, due to either expensive detector materials or cryogenic cooling systems. However, the devices tested here are low-cost due to the use of conventional room temperature silicon CMOS technology. The devices operate from 170 to 250 GHz with an additional detector design has been fabricated for 30 THz (10 microns wavelength). Results are presented for the initial testing of single test structure FETs. These devices were designed with several different antenna configurations and a range of MOSFET design variations for evaluation. The primary goal of the work presented here is to determine the optimized detector design for the subsequent focal plane array implementation based on the largest responsivities and lowest noise-equivalent power (NEP). Transmission testing of the devices yields responsivities of about 100 to 1000 V/W and a NEP of about 0.5 to 10 nW·Hz<sup>-1/2</sup>. Through this evaluation and by utilizing signal amplification on the chip, signal modulation at higher frequencies, and smaller process sizes the performance of these devices will continue to improve in future designs.
Exelis Geospatial Systems and its CEIS partners at the University of Rochester and Rochester Institute of Technology are developing an active THz imaging system for use in standoff detection, molecular spectroscopy and penetration imaging. The current activity is focused on developing a precision instrument for the detection of radiation centered on atmospheric windows between 200 GHz and 400 GHz (available sources). A transmission imager is developed by raster scanning through a semi-coherent non-ionizing beam, where the beam is incident on a NMOS FET detector. The primary goal of the initial system is to produce a setup capable of measuring responsivity and sensitivity of the detector. The Instrumentation covers the electromagnetic spectral range between 188 GHz and 7.0 THz. Transmission measurements are collected at 188 GHz in order to verify image formation, responsivity and sensitivity as well as demonstrate the active imager’s ability to make penetration images.
Collaboration between Exelis Geospatial Systems with University of Rochester and Rochester Institute of Technology aims to develop an active THz imaging focal plane array utilizing 0.35um CMOS MOSFET technique. An appropriate antenna is needed to couple incident THz radiation to the detector which is much smaller than the wavelength of interest. This paper simply summarizes our work on modeling the optical characteristics of bowtie antennae to optimize the design for detection of radiation centered on the atmospheric window at 215GHz. The simulations make use of the finite difference time domain method, calculating the transmission/absorption responses of the antenna-coupled detector.
Exelis Geospatial Systems and its CEIS partners at the University of Rochester and Rochester Institute of Technology
are developing an active THz imaging focal plane for use in standoff detection, molecular spectroscopy and penetration
imaging. This activity is focused on the detection of radiation centered on the atmospheric window at 215.5 GHz. The
pixel consists of a direct coupled bowtie antenna utilizing a 0.35 μm CMOS technology MOSFET, where the plasmonic
effect is the principle method of detection. With an active THz illumination source such as a Gunn diode, a design of
catadioptric optical system is presented to achieve a resolution of 3.0 mm at a standoff distance of 1.0 m. The primary
value of the initial system development is to predict the optical performance of a THz focal plane for active imaging and
to study the interaction of THz radiation with various materials.
Interest in array based imaging of terahertz energy (T-Rays) has gained traction lately, specifically using a CMOS process due to its ease of manufacturability and the use of MOSFETs as a detection mechanism. Incident terahertz radiation on to the gate channel region of a MOSFET can be related to plasmonic response waves which change the electron density and potential across the channel. The 0.35 μm silicon CMOS MOSFETs tested in this work contain varying structures, providing a range of detectors to analyze. Included are individual test transistors for which various operating parameters and modes are studied and results presented. A focus on single transistor-antenna testing provides a path for discovering the most efficient combination for coupling 0.2 THz band energy. An evaluation of fabricated terahertz band test detection MOSFETs is conducted. Sensitivity analysis and responsivity are described, in parallel with theoretical expectations of the plasmonic response in room temperature conditions. A maximum responsivity of 40 000 V/W and corresponding NEP of 10 pW/Hz<sup>1/2</sup> (±10% uncertainty) is achieved.
Scientists are interested in using digital micromirror devices (DMD) as slit-masks in multiobject spectrometers on future space missions. A favored orbit is at the second Lagrangian point (L2). A requirement for mission planning is to determine how long such microelectrical mechanical systems devices would remain operational given the L2 radiation environment, which is primarily composed of solar protons and cosmic rays. To this end, we initiated DMD proton testing. Three DMDs were irradiated with high-energy protons (35 to 50 MeV) at the Lawrence Berkeley National Laboratory 88 in. Cyclotron. Assuming a typical spacecraft shielding of 100 mils of aluminum, our tests imply that DMDs remain fully operable in a five-year mission at L2 with a margin of safety of 4.5.
Scientists conceiving future space missions are interested in using DMDs as a multi-object spectrometer (MOS) slit mask. The main uncertainties in utilizing DMDs in a space-based instrument are associated with their operational longevity given the exposure to high levels of proton radiation and their ability to operate at low temperatures. Since a favored orbit is at the second Lagrangian point (L2), it is important to determine how long such Micro-Electrical Mechanical Systems (MEMS) would remain operational in the harsh L2 radiation environment, which primarily consists of solar protons and cosmic rays. To address this uncertainty, we have conducted DMD proton testing at the Lawrence Berkeley National Laboratory (LBNL) 88” Cyclotron. Three DMDs were irradiated with high-energy protons (20- 50MeV) with energies sufficient to penetrate the DMD package’s optical window and interact electrically with the device. After each irradiation step, an optical test procedure was used to validate the operability of each individual mirror on the DMD array. Each DMD was irradiated to a wide range of dosage levels and remained 100% operable up to a total dose of 30 krads. In addition, a few single event upsets were seen during each irradiation dose increment. To determine the minimal operating temperature of the DMDs, we placed a DMD in a liquid nitrogen dewar, and cooled it from room temperature to 130 K. During this test, the DMD was illuminated with a light source and monitored with a CCD camera. Additionally, the temperature was held constant at 173 K for 24 hours to test landing DMD patterns for long periods of time. There was no indication that extended periods of low temperature operation impact the DMD performance. Both of these results point to DMDs as a suitable candidate for future long duration space missions.
A division of focal plane (DoFP) micro grid polarizer array (MGPA) has been characterized. The MGPA under test is a
commercial device available from Moxtek Inc. These wire grid style polarizers use aluminum lines fabricated on a glass
substrate and have opaque regions surrounding individual pixels. Our approach to testing the MGPA has been to reimage
them onto a detector by placing the MGPA at an intermediate focal plane. For the purposes of characterizing the MGPA,
a high magnification reimaging optical system was assembled. The oversampled MGPA pixels were examined by using
an adjustable analyzing polarizer. The effects of pixel throughput and cross talk are examined as a function of both
wavelength and illumination f/#. A calibration procedure has been determined for the use of such devices. The MGPA
array was also examined using a scanning electron microscope (SEM). From these SEM measurements, the pitch, fill
factor, and aluminum thickness were measured. In preparation for attaching the MGPA directly to a CCD, an alignment
tolerance analysis was completed. The results indicate that 0.5 μm alignment of MGPA pixel center to image sensor is
required to get a system with significantly low crosstalk for useful polarization imaging.
An optical tracking sensor that produces images containing the state of polarization of each pixel can be implemented using individual wire-grid micropolarizers on each detector element of a solid-state focal plane array. These sensors can significantly improve identification and tracking of various man-made targets in cluttered, dynamic scenes such as urban and suburban environments. We present electromagnetic simulation results for wire-grid polarizers that can be fabricated on standard imaging arrays at three different technology nodes (an 80-, 250-, and 500-nm pitch) for use in polarization-sensitive detector arrays. The degradation in polarizer performance with the larger pitch grids is quantified. We also present results suggesting the performance degradation is not significant enough to affect performance in a man-made vehicle-tracking application.
The DMD (Digital Micromirror Device) has an important future in both ground and space based multi-object
spectrometers. A series of laboratory measurements have been performed to determine the scattered light properties of a
DMD. The DMD under test had a 17 μm pitch and 1 μm gap between adjacent mirrors. Prior characterization of this
device has focused on its use in DLP (TI Digital Light Processing) projector applications in which a whole pixel is
illuminated by a uniform collimated source. The purpose of performing these measurements is to determine the limiting
signal to noise ratio when utilizing the DMD as a slit mask in a spectrometer. The DMD pixel was determined to scatter
more around the pixel edge and central via, indicating the importance of matching the telescope point spread function to
the DMD. Also, the generation of DMD tested here was determined to have a significant mirror curvature. A maximum
contrast ratio was determined at several wavelengths. Further measurements are underway on a newer generation DMD
device, which has a smaller mirror pitch and likely different scatter characteristics. A previously constructed instrument,
RITMOS (RIT Multi-Object Spectrometer) will be used to validate these scatter models and signal to noise ratio
predications through imaging a star field.
The DMD™ (Digital Micromirror Device) has become an integral part of the instrumentation for many applications.
Prior characterization of this device has been focused on its use in DLP™ (TI Digital Light Processing)
projector applications where a collimated wavefront impinges on the DMD. The results of such investigations
are not applicable to using DMDs at the focal plane of an optical system where it is used as a slit mask (e.g.
in a multi-object spectrometer). In order to study the DMD scattering function in this second case, a subpixel
spot scanning system has been assembled. The scattered light collected from this system allowed a subpixel
scattering function to be determined for the DMD when illuminated by a converging beam.
A novel multi-object spectrometer (MOS) is being explored for use as an adaptive performance-driven sensor that tracks
moving targets. Developed originally for astronomical applications, the instrument utilizes an array of micromirrors to
reflect light to a panchromatic imaging array. When an object of interest is detected the individual micromirrors imaging
the object are tilted to reflect the light to a spectrometer to collect a full spectrum. This paper will present example
sensor performance from empirical data collected in laboratory experiments, as well as our approach in designing optical
and radiometric models of the MOS channels and the micromirror array. Simulation of moving vehicles in a highfidelity,
hyperspectral scene is used to generate a dynamic video input for the adaptive sensor. Performance-driven
algorithms for feature-aided target tracking and modality selection exploit multiple electromagnetic observables to track
moving vehicle targets.