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Ion implantation is widely used for doping semiconductors at low concentration, but, with the advent of a new generation of high current implanters, synthesizing new materials rather that simply doping them has become feasible. This technique has been successfully applied to fabricating silicon-on-insulator (SOI) structures with oxygen and nitrogen for several years. Since we are interested in understanding the mechanisms of formation of these layers, we have concentrated on sub-stoichiometric implantation doses of oxygen where it is easier to observe the coalescing layer. In order to determine whether this process of compound formation is more general, our studies were expanded to include implantation of the transition metals. Here, elevated substrate temperatures are necessary to minimize Si surface damage. The resulting disilicide layers are of remarkably high quality: they are single crystals in registry with the silicon wafer and they have better residual resistivities than comparable UHV-reacted suicides.
The bombardment of solid surfaces with ions in the energy range below about 150 eV, depending on the ion-substrate combination, results in a net growth of material on the surface. An ion beam facility capable of producing highly uniform, low energy beams of current densities in the range 10-1 to 1 Am-1 has been developed to study the potential of this growth technique for the fabrication of thin epitaxial films at low temperatures.
The energy deposition associated with ion bombardment, which is considered to be responsible for the low temperature epitaxy capability, can also cause atomic displacements on the surface and near-surface regions of the substrate during initial growth and in the growing film. A study of the growth processes thus requires investigation of the damaging effects of low energy ion bombardment. In the present paper, fundamental aspects of the implantation and deposition of materials using very low energy ions will be discussed.
Recent results on silicon on insulator structures formed by implantation of oxygen and subsequent high temperature annealing will be discussed. The resulting silicon on insulator structure has sharp interfaces and a dislocation density of less than 105 cm -2 in the top silicon film. This density of defects is several orders of magnitude lower than previously reported values. The relation between the microstructure after implantation and this relatively low defect density will be discussed. Silicon point defects will be shown to play an important role in the establishment of the microstructure during implantation. Relations between implantation conditions, point defect concentrations and microstructure will be discussed to come to the formulation of the boundary conditions for the formation of high quality silicon on insulator material by this method.
A technique for dislocation density reduction in SIMOX (Separation by IMplantation of OXygen) substrates by using multiple implant and anneal cycles is described. This scheme produces SIMOX material with dislocation defect densities less than 1 x 105/cm2. This dislocation density value is three to four orders of magnitude lower than that of comparable single implant slices having the same total dose and anneal conditions.
A cross-sectional transmission electron microscopy study has been performed on SIMOX wafers prepared using three sequential low dose (6 x 1017 cm2) oxygen implantations. After each implant the wafers were annealed using rapid thermal processing at temperatures up to 1360°C for times of 1 to 5 minutes. The TEM results show that, although low dislocation densities are obtained, oxygen precipitate dissolution is incomplete for these conditions. Therefore, longer annealing times will be required. In addition, lower increments of oxygen dose are recommended to approach dislocation-free superficial silicon layers.
The creation of SIMOX material by multiple step substoichiometry oxygen ion implantation of silicon wafers followed by high temperature annealing has already been demonstrated by different groups [1-4] This paper reports on the formation of SIMOX wafers at temperatures well below the critical temperature (500-550°C) specified for oxygen implantation of the SIMOX process. A multiple step procedure has been devised, each step consisting of oxygen ion implantation at doses of 2.5 and 3 x 1017 O+/cm2 followed by solid phase epitaxy at a temperature of 950°C for two hours. Non-destructive optical analysis and XTEM investigation of the wafers indicates the formation of a continuous buried oxide with good quality single crystal silicon on the surface after accumulated dose of 1.1x1018 O+/cm2 following high temperature annealing at 1300°C for six hours.
The processing of SIMOX material at a lower temperature will enable the utilization of a wide variety of ion implanters, will simplify the design of the end station of the new generation high current ion implanters, and will have an impact on the availability and economics of SIMOX wafers.
We have studied the formation of buried oxide in MeV oxygen implanted Si. A continuous oxide layer is formed in the samples implanted with 2x1018/cm2 oxygen and annealed at 1300° C. The microstructures are studied by cross-sectional transmission electron microscopy and high resolution electron microscopy. Chemical information was obtained by electron energy loss spectroscopy. The effects of implantation temperature are studied. Implantation at a low substrate temperature leads to a well-defined buried SiO2 layer, inhibits the formation of oxide precipitates in the silicon, and reduces silicon inclusions in the SiO2.
We have investigated the effect of different implantation parameters on cavity formation in the top Si layer in SIMOX structures. Cavities were found to occur in the temperature range between 600 and 675°C. The nucleation and growth kinetics of cavities could be reasonably explained using classical theory, and showed a behavior similar to that of irradiation-induced voids in metals. A similar dependence on instantaneous current and beam scanning frequency was also observed. Post implantation annealing at a temperature of 1150°C for 80 min showed cavities starting to facet, and a threading dislocation density of < 105 cm2. SIMOX structures formed in (111) silicon are also presented.
A method of achieving total dielectric isolation (TDI) of device islands (or larger areas) using ion beam synthesis to form a continuous but non planar layer of SiO2 is described. The technique involves implantation through a patterned masking layer in which windows have been opened to define the dimensions and location of the islands. TDI structures have been successfully formed in annealed (1300°C, 5 hours) wafers implanted with a dose of 2.2 x 1018 O+ cm2 at 200 keV, using a thermal oxide mask of thickness 4750 Å.
The formation and structure of defects and precipitates in high-dose oxygen implanted silicon-on-insulator material was directly studied by weak beam and high resolution electron microscopy. In as-implanted material, the edge of the oxygen implant profile contained 1.5 nm diameter precipitates at a density of 1019 cm2. Defects, including micrctwins, stacking faults, and (311) defects, were present in as-implanted material but no threading or loop dislocations were observed. This suggests that threading dislocations are formed in the thermal ramping and annealing cycle. In material annealed for different times and temperatures precipitate size was much more dependent on peak temperature rather than time-at-temperature indicating that oxygen diffusion distance is less important than thermodynamic factors in controlling precipitate size. Annealing from 1150°C to 1250°C produced threading dislocations and possible dislocation dipoles which extended through the superficial layer. Transient annealing of very low dose oxygen implanted Si produced loop and threading dislocations. This suggests that a high heating rate during precipitation will generate excess Si interstitials at a rate high enough to create high stresses at precipitates and form dislocations. A qualitative model for dislocation formation is proposed and processing conditions for reducing dislocation density are suggested.
Silicon with buried oxides formed by ion implantation (SIMOX) or zone-melt recrystallization (ZMR) was exposed to deuterium gas at temperatures from 773 K to 1273 K, and the depth profile of the D was then determined by nuclear-reaction analysis. The D was localized within the buried oxide, with no measurable quantity in the Si phase. Uptake was controlled by permeation through the Si overlayer, and the permeability of D in Si was determined at 873 K. The sample dependence of D uptake indicated substantially fewer defect-trap sites in SIMOX oxide annealed at 1678 K as opposed to 1548 K, with still smaller defect densities in the ZMR oxide. Hydrogen exposure at 1273 K substantially disrupted the SIMOX structures.
Nitrogen was found in as-implanted SIMOX wafers. The redistribution of the nitrogen within the SIMOX structure was studied after the formation anneal for different temperatures (up to 1250 °C), times, ambients (nitrogen plus 1% oxygen and argon), and anneal with and without a capping layer. The nitrogen was found to be concentrated in the buried SiO2 layer and the adjacent damaged silicon regions in the as-implanted oxygen wafers. During the formation anneal, the nitrogen was found to collect(build-up) at the Si/SiO2 buried layer interface. It is postulated that an oxynitride is preferentially formed at the interface where the dangling silicon bonds can capture the soluble nitrogen.
Single crystal silicon films on top of a buried SiO2 layer were produced by implanting 1.7x10180+ions/cm2 at 150keV into (100) Czochralski silicon, followed by annealing at higher temperatures. The defect properties of the layers are studied after each processing step by low temperature photoluminescence measurements and transmission electron micrography (TEM). Dislocation-related photoluminescence signals correlate with their TEM observations in the same samples. The photoluminescence method proves to be a very versatile and convenient method for characterizing the quality of silicon-on-insulat or structures.
Epitaxial silicon layers of 5¼m were grown on SIMOX wafers. The dislocation density decreases by more than an order of magnitude as a function of distance away from the buried oxide. Shallow pits (about 0.5 urn deep and several um wide) are observed on the epitaxial layer with a density of 1-2 mm2. Their density did not change with various processing variations. A search for the origin of the pits by transmission electron microscopy reveals that they may be associated with regions of irregularly thin and sometimes missing buried oxide, which appear after the usual high temperature SIMOX annealing step. These defective regions in the buried oxide appear to initiate twinned growth in the epitaxial silicon, and are associated with pits at the top epitaxial silicon surface.
This paper reports on a study of the Silicon-On-Insulator (SOI) structures obtained by oxygen ion implantation (SIMOX) and subsequent thermal annealing. With Transmission Electron Microscopy (TEM) a novel defect structure is revealed in the case of low temperature annealings. Electrical measurements of test devices are performed and a correlation with impurity decoration of defects is investigated.
Infrared absorption and Raman scattering measurements of SIMOX structures implanted at various temperatures yield information on the structure and the strain in both the top silicon and the buried oxide layers. Both techniques can also be used to monitor the implant temperature after the implantation.
Single crystal (111) and (100) Ge wafers were implanted with 16O (180 keV, 2.0 x 1018/cm2, 14–28 ¼A/cm2 ) at substrate temperatures of 250, 330, and 500°C. Implanted samples were annealed at 350, 450, 550, and 650°C for 30–90 minutes in an Ar ambient. Rutherford backscattering channeling analysis and cross-sectional transmission electron microscopy indicate that an amorphous buried layer is formed by implantation and that the overlayer contains a dense network of precipitates. Electron spin resonance measurements indicate that the layer does not contain GeO2, but rather oxygen deficient GeO2. Annealing of samples up to 550°C showed no change in the morphology, however, after annealing at 650°C the buried layer was gone and all that remained was a damaged Ge substrate with little or no oxygen. Further annealing for 60 min left nearly virgin Ge.
RBS, SIMS, and IR measurements have been made on a SIMOX wafer, implanted with a second, low-temperature oxygen implant. These measurements indicate changes in the oxygen/silicon ratio in the buried oxide layer and differences in the annealing behaviour of the original layer and the double implant layer.
The Semiconductor Equipment Division of Eaton has, for the past year, supplied Simox wafers to various companies and laboratories throughout the world. A review will be presented of the operation in a production mode of the NV-200 Oxygen Implanter. Improvements in the production version will be related to both particulate counts and elemental contamination on the wafers. The addition of a polysilicon liner in the beamline has greatly reduced the heavy metal and carbon contamination levels in the machine. Historical Sims and Auger data will be presented to show these effects. The most recent production machine is operating on the manufacturing floor. Sims data from wafers implanted in October 1987 will be presented.