Grahame C. Rosolen

Performance Evaluation of a Passive Millimeter-Wave Imager

A cross-correlating 186-GHz passive millimeter-wave imager has been built. The key components in the signal processing hardware are two 186-GHz receivers and a broadband complex correlator. To evaluate the performance of this imager, its point-spread function, beam pattern, baseline vector, and their variations with the scanning direction have been experimentally measured and derived. Some of these results are needed for optimizing the imagers parameter settings. Others are required for implementing the modulated-beam and modulated-scene algorithms proposed in a previous paper dealing with the imagers fringe in its point-spread function. These results will also reveal any problems in the construction process of the imager. The theoretical bases for these measurements are analyzed. Novel algorithms for deriving each antennas point-spread function and beam pattern, as well as the imagers baseline vector from the measurement results of the imagers point-spread function and beam pattern are proposed and successfully applied in the measurements. Experimental results are presented and discussed.

Automatically aligned electron beam lithography on the nanometre scale

A scanning electron microscope (SEM) has been modified to enable it to be used for electron beam lithography. This involved modifications to the electron optics and sample stage of the SEM, as well as adding an image correlation system and pattern generator to provide the alignment and lithography functions. The instrument has been used for direct-write electron beam lithography to fabricate devices with dimensions on the nanometre scale. The standard SEM stage has been motorized and is used to move to each device. The location of each device is found by using a hierarchical image correlation technique and electron beam deflection system to achieve a device alignment accuracy of 80 nm. The main application is to write the gate electrodes in gallium arsenide high electron mobility transistors (HEMTs). A variety of HEMT designs have been written with exposed linewidths of 130 nm. The instrument has also been used to write large arrays of closely spaced electrodes as part of a semiconductor detector. Each detector contains 80 electrodes which are 160 nm wide, 60 μm long and separated by gaps of 370 nm. The full electron microscope functionality of the instrument is maintained to enable the results of the electron beam lithography to be analysed.

A 190GHz active millimeter-wave imager

The design and testing of a 190 GHz imaging system is presented. The system features two beam-scanning antennas; the first transmits a horizontal fan beam and the second receives a vertical fan beam. By correlating the signals from the antennas, an estimate of the millimeter-wave reflectivity at the intersection of the fan beams is obtained. Each fan beam is scanned by rotating a small subreflector within the antenna; this simple rotation motion allows rapid scanning. The system is portable, currently approximately 0.6m × 0.6m × 2m high; the key size constraint is imposed by the 450 mm aperture length of the antennas. The imager has an angular resolution of 0.25° and a field of view of 14°×14°, resulting in a raw image of approximately 50 × 50 pixels. The raw image is processed using super-resolution techniques. Images will be presented which show the capability of the system to image metallic and ceramic objects beneath clothing. These images were obtained by illuminating the scene with signals from a frequency-doubled Gunn oscillator. While this paper focuses on active imaging, the system can also operate in passive mode with reduced sensitivity.

Design and testing of an active 190-GHz millimeter-wave imager

The design and testing of an active 190-GHz imaging system is presented. The system features two beam-scanning antennas, one of which transmits a vertical fan beam, and the other which receives a horizontal fan beam. By correlating the transmitted and received signals, an output is obtained that is proportional to the millimeter-wave reflectivity at the intersection of the two fan beams. Beam scanning is obtained by rotating a small subreflector within each antenna, allowing rapid scanning. The system has an angular resolution of 0.3 deg, a field of view of 14×14 deg, and operates at a standoff distance of 5 m.

Combined optical and electron beam lithography for integrated circuit fabrication

A direct-write electron beam lithography system has been developed for use in conjunction with optical lithography for device fabrication. The electron beam lithography system comprises a scanning electron microscope to provide the electron beam, an automated pattern alignment system, for accurately aligning the electron beam with the features defined by optical lithography, and a dedicated pattern generator to control the electron beam during exposure. The system is primarily used for fabricating the gate electrodes in high electron mobility transistors which are the active components in monolithic microwave integrated circuits. The high frequency performance of these devices is determined by the size and cross-section profile in the top layers to reduce the gate resistance. The system is able to automatically align the gate electrodes by initially acquiring an image to accurately place the beam prior to exposure. Registration marks are not required and the system is able to automatically compensate for stage positioning errors and fabrication tolerances associated with the placement of the optically defined features.

Active imaging at 187 GHz

Millimetre-wave imaging radars show great promise in a number of application areas. This paper briefly covers some of these application areas and describes two active coherent imaging systems that have been used by the authors to explore them. The architectures of these systems are explained along with a sample of measurements taken with the systems. The feasibility of these systems in the application areas is analysed.

Optically variable devices fabricated by electron beam lithography

A scanning electron microscope (SEM) has been modified to enable it to be used as an electron beam lithography instrument for fabricating optically variable devices (OVDs). The pattern data which describes the OVDs is divided into a number of fields which are tiled together. The pattern data for each field is decoded by a dedicated pattern generator which is interfaced to the electron optics of the SEM to control the electron beam deflection and beam blanking in order to expose the fields. The sample stage of the SEM is used to position each of the fields prior to exposure. An automatic focus compensation system has been developed to ensure that the beam is optimally focused for each stage position. The SEM controls are used to optimise the electron optics and to set the beam energy and beam current required for the exposure. One type of OVD comprises a series of diffraction gratings tiled to form a two dimensional array. The orientation and spacing of the lines in the grating pattern on each tile is chosen to produce optical effects that depend on the viewing angle. This method allows different images to be mixed in the same area, with each image revealed at specific viewing angles. Another OVD consists of stochastic arrays of pixels to reproduce a grayscale image. In this device the polarity of the image varies with the viewing angle. One application of these devices is to provide masters for the replication of OVDs for security and anti-counterfeiting purposes.

Binary mask scanning for THz imaging

We have designed a terahertz imaging system, built with electronic components and operating at a single tunable frequency. The system scans in hybrid mode, combining coarse mechanical positioning with a fine scan produced by perturbing the beam with a system of opaque masks, placed into the collimated beam. The mask set is based on a modified Hadamard design, which aims at minimizing the loss of power and noise effects. The image acquisition is performed in transmission mode, with the sample placed at the focal plane. We present several imaging results obtained using our technique.

Passive mm-wave imaging using two scanning fan-beam antennas

A prototype cross-correlating 190 GHz passive mm-wave imaging system has been developed. This system is based on the Mills Cross system used for radio astronomical imaging. It uses two pillbox antennas arranged in a T configuration. Each antenna generates a fan beam and the two fan beams are orthogonal to each other. By cross-correlating signals received from the two antennas, an output is obtained which is proportional to the millimeter-wave intensity radiated from the target at the intersection of the two fan beams. Beam scanning is generated by rotating a small sub-reflector inside each antenna. As a result, these relatively heavy antennas are stable during scanning and a high frame rate can be achieved. Another advantage of this approach is that only two receivers are required. The baseline (the displacement between phase centers of the two antennas) of this system is not zero, because the phase centers of the two antennas are not located at the same position. The baseline generates a fringe in the imaging system and its influence on the performance of the system is analyzed in this paper. The scanning speed of this system is also much faster than that of the Mills Cross imaging system and its influence on the resolution is also analyzed. It is found that the effect of the scanning speed is minimized when the beam scans along the equal-phase line of the fringe. This system can also be used as an active imaging system and this is discussed in another paper.

Direct-write electron beam lithography automatically aligned with optical lithography for device fabrication

A scanning electron microscope (SEM) has been modified for direct-write electron beam lithography. The instrument has the capability to automatically align with the features patterned by optical lithography and exposure the features requiring the finest linewidth with the electron beam. The main application for the instrument is one the process line for fabricating high electron mobility transistors (HEMTs) in GaAs monolithic microwave integrated circuits (MMICs). The high frequency performance of the HEMTs is critically dependent on the length of the gate electrode and the placement of this electrode between the source and drain electrodes. All of the mask layers for a MMIC except the gate layer are exposed using optical lithography, as it provides the required linewidth with high throughput. The sample containing the partially fabricated HEMTs is loaded in the instrument and is positioned at each HEMT in sequence in order to acquire an image of the source and drain electrodes. This image is correlated with a reference image of the HEMT to determine its precise location for subsequent exposure of the gate electrode by the electron beam. The instrument is able to achieve an alignment accuracy of 80 nm and has been used to expose features with linewidths less than 100 nm. As images of the device are used for alignment, the instrument does not require alignment marks on the sample and is able to automatically compensate for positional errors caused by same stage and mask tolerances. As the full SEM functionality of the instrument is retained, it may also be used to inspect the results of the lithography.