V.S. Dolat

CCD soft X-ray imaging spectrometer for the ASCA satellite

We describe the development of a charge-coupled device (CCD) array for use as a soft X-ray (0.4-12 keV) imaging spectrometer for the ASCA (formerly Astro-D) satellite. The CCDs are 420/spl times/420-pixel frame-transfer devices designed to be closely abutted to other chips on three sides of the imaging array. The imagers are made on 6500-/spl Omega//spl middot/cm p-type float-zone silicon for depletion depths of about 50 /spl mu/m under typical CCD bias conditions. The read noise of the CCD is typically 3-4 e/sup /spl minus// rms at data rates of 50 KHz resulting in an energy resolution E//spl delta/E/spl ap/50 at 5.9 keV. The complete focal-plane sensor consists of a 2/spl times/2 array of these devices mounted on a common substrate. Radiation damage from energetic protons is mitigated by the use of a narrow potential trough along the center of the CCD channel to confine the small X-ray event charge to a reduced volume and thereby minimize trapping effects. Charged-particle events from the non-X-ray space background are minimized by using a junction on the back of the chip to deplete most of the neutral bulk and draw background charge away from the CCD. Wafer-level device screening at low temperatures and the focal-plane packaging methods are also described. >

Surface acoustic wave devices and method of manufacture thereof

A phase and amplitude compensated surface acoustic wave (SAW) structure is described in which computer controlled compensation is achieved by laser chemical etching of selective portions of a compound chemical film deposited on the surface of a piezoelectric SAW substrate in the path of propagation. The compound film comprises a layer of amplitude attenuating cermet material formed on the substrate and a phase compensating layer of molybdenum formed over the cermet material and in contact with the substrate surface.

CCD soft x-ray imagers for ASCA and AXAF

We describe the development at Lincoln Laboratory of large-area CCD imager arrays for soft x-ray astronomy. One such array consists of four, closely abutted, 420 X 420-pixel CCDs for the ASCA (formerly Astro-D) satellite that was launched on February 20, 1993. The CCDs were fabricated on p-type 6500-(Omega) -cm material in order to attain the deep depletion depths needed for the higher-energy (> 4 keV) photons. The use of high- resistivity material and the effects of space-radiation are among the principal technical issues which will be discussed. We are also developing the next-generation CCD sensors for the Advanced X-ray Astrophysics Facility which is currently scheduled for launch in 1998. This mission will use two multichip focal planes comprising ten chips, each of a larger format (approximately 1000 X 1000 pixels). In addition to a new CCD, this program will require other technology developments such as an innovative packaging method for the nonplanar focal planes.

Reflective-Array Matched Filter for a 16-Pulse Radar Burst

Abstract : A new type of matched filter has been designed and fabricated for a doppler-sensitive burst waveform with 16 equally spaced linear-FM subpulses, each of which has a 60MHz bandwidth and is 3 microsec long. The interpulse period is 5 microsec, and the total duration of the waveform is 80 microsec. The filter consists of 16 reflective-array-compressor sections ion-beam etched in the surface of a 15.2-cm-long Y-Z LiNbO sub 3 substrate. The reflective-array section for each subpulse is depth-weighted according to a Hamming function for range-sidelobe suppression, and the peak responses of successive sections have a Hamming weighting for doppler-sidelobe suppression. A reduction of system complexity and an improvement in dynamic range is expected with filters of this type as compared to conventional doppler burst processors. We have fabricated on one substrate a matched filter for an entire burst, thus providing the full correlation gain inherent in the waveform within a single device. This yields a large dynamic range despite a relatively high insertion loss (43 dB CW at center frequency (200 MHz) for the central section). Within a given section, the phase deviations from quadratic are typically 5 deg. r.m.s. and corresponding range sidelobes are more than 30 dB down from the correlation peak. These phase deviations and the errors in delay between sections can be reduced by metal overlay patterns. A filter for the zero-doppler channel was built to operate at a temperature of 60 C. When the temperature is changed by 0.98 C, the peak response of this filter is shifted by an amount equal to the doppler resolution (18 kHz).

Self-Aligning Bilateral Chirp-Transform System

A SAW chirp-Fourier-tr ansform system is described which employs the bilateral property of dispersive delay devices to perform 100% duty-cycle Fourier transformtion using only two SAW devices. It is then shown how alignment of the complex Fourier outputs of a high-performance bilateral processor can be maintained with a digital system using a third SAW device, error-correction circui try and active cl osed-1 oop control to correct for errors at the output arising when device characteristics change due to temperature shifts and device aging. Three major adjustments, the timing of the i nput window over which the transform is taken, the timing of the output samples relative to the transform, and the phase used for in-phase (I) and quadrature (Q) demodulation are required. The input window and the output sample timing are aligned by independently controlling the timing of post and premultiplying chirps, r espectively. I and Q phase are adjusted by digitally controlling the phase of the impulse RF source. Adjustments are made under active closed-loop control utilizing algorithms that derive error corrections from special test signals. These techniques have been implemented in a system which accepts digital data at a continuous 10-Mword/s rate, D/A converts these data and delivers its 256-point Fourier transform to A/D converters. The system processes complex words consisting of 8 bits real and imaginary and maintains accuracy to 1%.

High-performance visible-UV CCD imagers for space-based applications

focusing on work on large device formats, improvements in quantum efficiency, and reduction of CCD degradation in the natural space-radiation environment. Research was based on a 420 x 420-pixel frame-transfer device and a new 1024 x 1024-pixel device. To obtain high quantum efficiency from the visible into the UV, a technology for making back-illuminated versions of these devices is being developed. Quantum efficiencies greater than 80 percent in the 500-800 nm band have been obtained with a SiO antireflection coating. Particular attention is given to the problem of charge-transfer inefficiency degradation caused by energetic protons in space-based systems. It is shown that CCDs can be significantly hardened to radiation effects by a combination of special buried channel potential profiles and operation at temperatures around 150 K, where the trap sites created by the protons have emission times much longer than the clock periods.

BGO Reflective-Array Compressor (RAC) with 125 micro seconds of Dispersion

Abstract : Linear FM pulse expanders and compressors in the reflective-array configuration have been fabricated on bismuth germanium oxide substrates. The low surface-wave velocity on this material and the folded RAC configuration allow 125 microseconds of dispersion over the 2.5-MHz bandwidth to be obtained in a compact device. The reflective arrays were depth weighted to provide a flat frequency response in the down-chirp expansion lines and a Hamming frequency response in the up-chirp compression lines. Maximum amplitude deviation from ideal was 0.5 dB. Midband (60 MHz) insertion loss was approximately 33 dB in both types of devices. Reflective arrays ion-beam etched with 500-eV argon ions showed no evidence of surface alteration or anomalous acoutic propagation loss. Phase compensation with Au-on-Cr films yielded a typical residual phase error of 2.0 degree and sidelobes below 33 dB were obtained in a subsystem comprising an expander, compressor, and assoicated electronics. Successful development of the devices depended on solving problems of angular accuracy of the reflective arrays and temperature sensitivity. These problems are especially severe in devices with small fractional bandwidth and large dispersion. (author)