Peter L. F. Hemment

SiGe HBTs on bonded SOI incorporating buried silicide layers

A technology is described for fabricating SiGe heterojunction bipolar transistors (HBTs) on wafer-bonded silicon-on-insulator (SOI) substrates that incorporate buried tungsten silicide layers for collector resistance reduction or buried groundplanes for crosstalk suppression. The physical structure of the devices is characterized using cross section transmission electron microscopy, and the electrical properties of the buried tungsten silicide layer are characterized using sheet resistance measurements as a function of bond temperature. Possible contamination issues associated with the buried tungsten silicide layer are investigated by measuring the collector/base reverse diode tics. A resistivity of 50 /spl mu//spl Omega/cm is obtained for the buried silicide layer for a bond anneal of 120 min at 1000/spl deg/C. Collector/base reverse diode tics show a voltage dependence of approximately V/sup 1/2/, indicating that the leakage current is due to Shockley-Read-Hall generation in the depletion region. Fitting of the current-voltage tics gives a generation lifetime of 90 ns, which is as expected for the collector doping of 7 /spl times/ 10/sup 17/ cm/sup -3/. These results indicate that the buried tungsten silicide layer does not have a serious impact on junction leakage.

Low‐temperature properties and phototransport in silicon‐on‐insulator films synthesized by oxygen implantation

Silicon‐on‐insulator material formed by oxygen implantation (SIMOX) is analyzed by comparing conductivity and Hall‐effect measurements performed at low temperature, under darkness, illumination, and substrate biasing. The quality of SIMOX films annealed at high temperatures above 1300u2009°C is found to be greatly enhanced in comparison with that obtained after conventional annealing at 1150–1200u2009°C. The analysis is fulfilled by discussing the carrier mobility and lifetime behavior, impurity ionization, doping compensation, film contamination, and scattering mechanisms. Hopping conduction is found to prevail below 60 K.

Mass‐dispersive recoil spectrometry studies of oxygen and nitrogen redistribution in ion‐beam‐synthesized buried oxynitride layers in silicon

Mass‐dispersive recoil spectrometry has been employed to study the influence of annealing conditions in ion‐beam‐synthesized silicon oxynitride structures prepared by implantation of 1.8×1018 and 1×1017 200 keV 16O+ and 14N+ ionsu2009cm−2, respectively, at approximately 600u2009°C. Subsequent annealing at 1200 and 1300u2009°C leads to redistribution of the implanted oxygen to form a buried oxide layer with nitrogen segregation to the buried SiO2/Si interfaces. Implantation with N+ subsequent to O+ followed by annealing at 1200u2009°C for 2 h was found to lead to both a lower oxygen content and lower channeling detectable defect concentration in the overlying silicon film than if the order of implantation was reversed. No significant dependence on order of implantation was observed after annealing at 1300u2009°C for 5 h.

Leakage current mechanisms in SiGe HBTs fabricated using selective and nonselective epitaxy

SiGe heterojunction bipolar transistors (HBTs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p/sup +/ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor. In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe extrinsic base. In these transistors, the Gummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through of the base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakage is shown to be due to a Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p/sup +/ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole-Frenkel and band to band tunneling. These results are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter.

Photoluminescence and paramagnetic defects in silicon-implanted silicon dioxide layers

Thermally grown SiO 2 layers on Si substrates implanted with Si‘ ions with a dose of 6]1016 cm~2 were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO 2 and non-bridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at „’1100iC, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment of the samples in a low-temperature RF plasma. ( 1999 Elsevier Science B.V. All rights reserved.

Raman scattering and photoluminescence analysis of silicon on insulator structures obtained by single and multiple oxygen implants

An analysis of silicon on insulator structures obtained by single and multiple implants by means of Raman scattering and photoluminescence spectroscopy is reported. The Raman spectra obtained with different excitation powers and wavelengths indicate the presence of a tensile strain in the top silicon layer of the structures. The comparison between the spectra measured in both kinds of samples points out the existence in the multiple implant material of a lower strain for a penetration depth about 300 nm and a higher strain for higher penetration depths. These results have been correlated with transmission electron microscopy observations, which have allowed to associate the higher strain to the presence of SiO2 precipitates in the top silicon layer, close to the buried oxide. The found lower strain is in agreement with the better quality expected for this material, which is corroborated by the photoluminescence data.

Hidden depths to devices

Silicon is the dominant semiconductor used in commercial and consumer electronics. This is due to its particular combination of physical properties – wide band gap, high carrier mobility, good thermal and chemical stability and the ease with which it can be converted into a stable high quality dielectric (SiO2) Ironically more than 99% of all the silicon used in integrated circuits (ICs) is present merely to provide mechanical support for the fragile transistors and components, fabricated in a thin surface layer typically 1μm thick (figure 1a). For ICs the ideal substrate for volume production would be simply a thin silicon film (figure 1b). Now, after almost two decades of research, this goal may, at last, be realised through silicon-on-insulator (SOI) technology which can achieve a thin film of high-quality single crystal silicon albeit supported on a dielectric slab (figure 1d). SOI substrates could then become the base material for new circuits being designed for the 1990s and beyond.

Improved facilities for ion beam surface analysis at the University of Surrey

Abstract Additional, facilities, which are being installed on the 2 MeV Van de Graaff accelerator at the University of Surrey, are described. These include improvements to the microbeam equipment and optical viewing system using an image intensifier, modifications to a 3-axis goniometer to provide batch processing, a goniometer control system and new software for data collection and analysis.

Characterisation of emitter/base leakage currents in SiGe HBTs produced using selective epitaxy

Abstract SiGe heterojunction bipolar transistors have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by non-selective epitaxy for the SiGe base and Si emitter cap. E/B leakage currents are compared with cross-section TEM images to identify sources of leakage currents associated with the epitaxy. In addition, the influence of the position of the extrinsic base implant with respect to the polysilicon emitter on the leakage currents is studied. The emitter/base leakage currents are modelled using Shockley–Read–Hall recombination, trap-assisted tunnelling and Poole–Frenkel (PF) generation. The position of the extrinsic base implant is shown to have a strong influence on the leakage currents. The PF effect dominates the emitter/base leakage current in transistors in which the collector area is smaller than the polysilicon emitter. This result is explained by penetration of the emitter/base depletion region into the p + polysilicon extrinsic base at the perimeter of the emitter. These leakage currents are eliminated when the collector area is increased so that the extrinsic base implant penetrates into the single-crystal silicon at the perimeter of the emitter.

Non-destructive characterization of thin film SIMOX structures using microscope spectrophotometry

Abstract Separation by implanted oxygen (SIMOX) substrates suitable for the fabrication of thin film fully depleted devices ( i.e. with silicon overlayer thicknesses of less than 1000 A) have been produced by low energy (70 keV) oxygen implantation (to doses of 3 × 10 17 , 7 × 10 17 and 10 × 10 17 O + cm −2 ) followed by annealing at 1405 °C for 30 min. Detailed structural analyses of the specimens were carried out, non-destructively, by microscope spectrophotometry (MSP) and, destructively, by cross-sectional transmission electron microscopy (XTEM). The results obtained from the MSP evaluation showed that the structures (produced with doses of 3 × 10 17 , 7 × 10 17 and 10 × 10 17 O + cm −2 ) consist of a thin silicon film overlying a region which varies in structure from discrete oxide precipitates, interwoven by silicon (for the lowest dose), to a continuous oxide layer containing a few silicon islands (for the highest dose). For all of the samples, extremely good fits were obtained between the measured and simulated MSP data, without the need for unjustified fitting parameters. In all cases, the thickness and physical composition of the various layers were found to agree extremely well with those determined, destructively, by XTEM.