Jim J. Wortman

First spatio-temporal results from the LDEF interplanetary dust experiment

Abstract The LDEF Interplanetary Dust Experiment was unique in providing a time history of impacts of micron-sized particles on six orthogonal faces of the vehicle over a span of nearly a full year. Over 15000 hits were recorded, representing a mix of zodiacal dust, meteor stream grains, orbital debris, perhaps beta-meteoroids, and possibly interstellar matter. Although the total number was higher than predicted, the relative panel activity distribution was near expectations. Detailed deconvolution of the impact record with orbital data is underway, to examine each of these populations. Very preliminary results of the fairly crude “first look” analysis suggest that debris is the major particle component at 500 km. The data show clear evidence of some known meteor streams as sharp, tightly-focused events, unlike their visible counterparts. Some apparent debris events show similar signatures. Data from the leading and trailing edges suggest a detection of beta-meteoroids, but the analysis is not yet conclusive. Absolute fluxes and flux ratios are not yet known, since the detector status analysis is yet incomplete.

Estimation of debris cloud temporal characteristics and orbital elements

Abstract The Interplanetary Dust Experiment (IDE), one of the 56 experiments carried by LDEF, was designed to detect impacts of extra-terrestrial particles and orbital debris. The IDE detectors (which covered about one square meter of the surface of LDEF) were sensitive to particles ranging in size from about 0.2 μm to 100 μm. The IDE experiment recorded the location in orbit and approximate origin direction for each impact. The resulting dataset represents perhaps the most extensive record ever gathered of the number and location of impacts due to small particles on a spacecraft in Earth orbit. The IDE dataset shows that impacts often occurred in “bursts” reoccurring each orbit. Such events, which we have designated Multiple Orbit Events, can occur only if the impacting particles are in an Earth orbit that intersects that of LDEF. We have examined several such event sequences in the LDEF IDE dataset and have found orbital debris concentrations distributed in both moderate inclination (ca. 25°–35°) and high inclination (> 60°) orbits. A peak flux of at least 12 impacts/second/sq. meter was detected in one such cloud. This is nearly 4 orders of magnitude above the mean flux observed over the LDEF 5.77 year mission duration.

Review of rapid thermal processing: system design and applications

Issues of rapid thermal processing (RTP) system design and process applications are reviewed. Temperature measurement is the most important and limiting factor in current RTP systems. Problems related to the temperature measurement and control and potential solutions are discussed. Process uniformity control is another important issue in RTP system design. Reactor chamber design, selection and arrangement of heat source, as well as issues related to dislocation generation, patterned and doped wafer, and ramp-up (down) thennal cycle are considered. Experimental results (RTO, RTCVD silicon nitride and polysilicon) based on an in-house RTP system developed in our laboratory are taken as examples to demonstrate the process applications and system requirements for single wafer processing.

Selective deposition of polycrystalline SixGe1-x by rapid thermal processing

Low pressure chemical vapor deposition (LPCVD) of polycrystalline SixGei. . x alloys in a cold-wall lamp heated rapid thermal processor was studied. SiGei. . alloys were deposited using the reactive gases GeHz and SiH2C12 in a hydrogen carrier gas. The depositions were performed at a total pressure of 2. 5Ton and at temperatures between 500C and 800C using GeH : SiH2C12 ratios ranging from 0. 025 to 1. 00. An enhancement in the deposition rate due to the addition of GeH was observed in agreement with earlier reports. The activation energy for deposition in the surface reaction limited regime varied from 20-30 Kcal/mole with the gas flow ratios used in this study. Results showed that SiGei. . alloys could be deposited selectively on silicon with no nucleation on Si02. Selective depositions were obtained when the GeH:SiH2Cl2 gas flow ratio was greater than 0. 2 regardless of the deposition temperature corresponding to a Ge content of 20 or higher in the films as determined by Auger Electron Spectroscopy (AES). Enhancement of the selectivity was attributed to the formation of highly volatile GeO. It was also shown that selectively deposited alloys could be used as diffusion sources to form very shallow ( 1000 A) pLn junctions in silicon by ion-implantation and rapid thermal annealing. 1.

Low-Resistivity Contacts to Silicon Using Selective RTCVD of Germanium

Isotype heterojunctions using low pressure rapid thermal chemical vapor deposition (RTCVD) of germanium on silicon have been fabricated and incorporated into a Kelvin structure to study the contact resistivity of the stacked structure. Germanium films were deposited at 400C and 3 Torr in a tungstenhalogen lamp heated rapid thermal processor utilizing a water cooled stainless-steel process chamber. In this work germanium is considered as a buffer layer to prevent silicon consumption over source/drain regions of MOS transistors during silicidation anneals to form low resistivity silicided contacts. We report here the results of our work with a germanium/silicon heterostructure and the potential advantages of this approach. Experimental results were compared against theoretical calculations and show that the Al/Ge/Si structure can lead to substantially lower contact resistivities than the Al/Si structure especially at lower substrate doping levels. This is especially pronounced in p-type contacts where 19 improvement in the contact resistance to silicon has been observed. Native oxide removal during the initial stages of germanium growth may play a key role in this reduction in contact resistivity.

IMPA:Ct--in-situ monitors of the particulate ambient: circumterrestrial--an international consortium of instrument suppliers

The Industry and University participants listed above have joined together to form the IMPA:Ct consortium, which offers a broad range of flight qualified technologies for real time monitoring of small particles, 0.1 micron to 10 cm, in the space environment. Instruments are available in 12 months or less at costs ranging from 0.5 to 1.5 million dollars (US) for the total program. Detector technologies represented by these groups are: impact-induced capacitor-discharge (MOS, metal-oxide-silicon), cratering or penetration of electroactive thin film (polyvinylidene fluoride), impact-plasma detection, acoustic detection, ccd tracking of optical scatter of sunlight, and photodiode detection of optical scatter of laser light. The operational characteristics, general spacecraft interface and resource requirements (mass/power/telemetry), cost and delivery schedules, and points of contact for 7 different instruments are presented.