Patrick J. McMarr

Fabrication of nanometer‐scale side‐gated silicon field effect transistors with an atomic force microscope

The fabrication of nanometer‐scale side‐gated silicon field effect transistors using an atomic force microscope is reported. The probe tip was used to define nanometer‐scale source, gate, and drain patterns by the local anodic oxidation of a passivated silicon (100) surface. These thin oxide patterns were used as etch masks for selective etching of the silicon to form the finished devices. Devices with critical features as small as 30 nm have been fabricated with this technique.

A study of Si implanted with oxygen using spectroscopic ellipsometry

Si(100) wafers were implanted with O+ at an energy of 180 keV to a dose of 2.3×1018/cm2 in the separation by implanted oxygen process. Following implantation, one wafer was annealed at 1275 °C for 2 h. Spectroscopic ellipsometry measurements were then performed on these samples. Effective medium modeling of the measurements was used to nondestructively depth profile the samples. These results show that the superficial Si layer for the unannealed sample includes noncrystalline and crystalline components. In addition, the optical properties of the buried oxide for the unannealed sample were found to be different from those of bulk fused silica or thermal oxides of Si. The superficial Si layer for the annealed sample was crystalline Si, but the buried oxide consisted of a phase‐separated mixture of noncrystalline SiO2 and crystalline Si. These results were further substantiated by selective chemical etch‐back studies and additional spectroscopic ellipsometry measurements, and by other techniques, including R...

RELATIONSHIP BETWEEN OXIDE DENSITY AND CHARGE TRAPPING IN SIO2 FILMS

Spectroscopic ellipsometry was used to determine the density of oxides thermally grown on Si substrates as a function of the oxidation temperature, and the time and temperature of postoxidation anneals. All the oxides were found to be denser than fused silica. The density of the as-grown oxides was found to decrease as the growth temperature was increased. Postoxidation anneals were found to reduce the oxide density; high temperature or long-time anneals caused the greatest reduction in density. Holes alone, or holes and electrons, were injected into the oxides by irradiating with vacuum ultraviolet light or x rays under electric field bias. Using capacitance–voltage measurements, it was found that low-density oxides trap charge more efficiently than high-density oxides. Electron spin resonance measurements indicated that, for most of these oxides, the number of paramagnetic defects was substantially smaller than the number of trapped charges. It is hypothesized that the additional, nonparamagnetic, charg...

Hole and electron trapping in ion implanted thermal oxides and SIMOX

Optically assisted methods of injecting either electrons or holes into SiO/sub 2/ layers were used to determine the effect of ion implantation on charge trapping in oxides. Dry-grown thermal oxides and the buried oxides of material grown by the SIMOX (separation by implantation of oxygen) process were studied. Al, Si, and P ions were implanted into the oxides at doses of 1/spl times/10/sup 13/ to 1/spl times/10/sup 16/ and the oxides were annealed at 700, 900, or 1050/spl deg/C after implantation. High dose implantations were found to create electron traps having high capture cross sections, the density of which depends on the implant species, suggesting that electron trapping is related to chemical aspects of the implanted ion. This was supported by measurements on an oxide implanted with a large dose of Ar, which showed no increase in electron trapping. It was found that the shift in the flatband voltage resulting from hole trapping could be reduced by high dose implantations, and that this effect is only weakly dependent on implant species. The hole trapping results are explained in terms of the effect of implantation on the oxide structure.

The role of nanoclusters in reducing hole trapping in ion implanted oxides

At the 2000 IEEE Nuclear and Space Radiation Effects Conference, it was shown that the negative shift in flatband voltage that results from hole injection is reduced in oxides that have been implanted with large doses of Al, Si, or P ions. In the present paper, we study the basic mechanism responsible for this reduced shift in the flatband voltage in more detail by comparing electron and hole trapping in Si and Ar implanted oxides. We find that in Si implanted oxides, the reduction in the shift of the flatband voltage is accompanied by the formation of entities in the oxide that have a large electron capture cross section, and that can become positively charged by photoemitting electrons. Photoluminescence studies indicate that these entities are Si nanoclusters. Oxides implanted with large doses of Ar do not form clusters, and these oxides show neither a reduction in the shift of the flatband voltage nor the formation of large capture cross-section electron traps. We show evidence that the nanoclusters reduce the shift of the flatband voltage by trapping protons formed during hole injection.

Total dose hardening of SIMOX buried oxides for fully depleted devices in rad-tolerant applications

A total dose hardening treatment is applied to SIMOX buried oxides. Total ionizing dose radiation testing is performed on fully-depleted transistors Fabricated on both hardened and non-hardened substrates. At 200 krads X-ray dose, the front gate shift is reduced from -0.7 to -0.2 V for FETs built on the hardened wafers.

Radiation induced charge in SIMOX buried oxides: lack of thickness dependence at low applied fields

Commercially prepared Separation-by-IMplantation-of-OXygen (SIMOX) wafers having buried-oxide (BOX) thicknesses ranging from 80 to 400 nm, and thermal oxides of similar thicknesses, were exposed to various doses of 10 keV X-rays. The net positive charge trapped in the buried-oxides during the radiation was determined by dual capacitance-voltage (C-V) and point-contact current voltage (I-V) measurements. For the thermal oxides, using metal-oxide-semiconductor (MOS) CV techniques, the amount of trapped charge was found to have the expected linear dependence on oxide thickness. For the SIMOX buried oxides, however, the amount of net trapped charge was found to be independent of BOX thickness when the oxides were biased at 0.05 MV/cm (a typical operating field). The SIMOX results are explained in terms of bulk-oxide hole and electron trapping.

Improvement in electrical properties of buried SiO2 layers by high‐temperature oxidation

The density of defects in the buried oxide of implanted oxide silicon‐on‐insulator material which cause low resistance paths between the substrate and top silicon layer has been greatly reduced by high temperature oxidation. The mechanism for this is the diffusion of oxygen through the top silicon layer to the buried oxide, where it oxidizes chains of silicon atoms.

Roughness measurements of Si and Al by variable angle spectroscopic ellipsometry

Rough surfaces of silicon and aluminum have been studied by rotating analyzer spectroscopic ellipsometry (RASE). The roughness of a silicon sample similar to that used for the RASE measurements was also studied by cross-sectional transmission electron microscopy. Total integrated scattering was measured on the aluminum specimens to obtain numerical estimates of the rms roughness. The ellipsometry measurements on these specimens were carried out at a number of angles of incidence in the 30-80 degrees range and at a number of discrete wavelengths in the 300-650-nm spectral range. The RASE results were hen analyzed using the Bruggeman effective-medium theory for the Si sample and scalar diffraction theory for the Al samples. This study shows that 70 degrees is the optimum angle of incidence for characterizing the roughness of these Al surfaces using RASE. It also demonstrates the self-consistency of the Bruggeman theory with angular variation for the Si sample. The need for a vector diffraction theory for the retation of the rms roughness using ellipsometric angles Delta and Psi is discussed.

The use of spectroscopic ellipsometry to predict the radiation response of SIMOX

We have studied SIMOX (Separation by Implantation of Oxygen) material using spectroscopic ellipsometry to determine the structure of the buried oxide and C-V measurements to determine the radiation response of the buried oxide. Our ellipsometric measurements indicate that the buried oxide is best described as a layer of stoichiometric SiO/sub 2/ which is more dense than bulk vitreous (v-) SiO/sub 2/. We find that the radiation response of the buried oxide is determined primarily by its density. We also find that small variations in the conditions used to prepare the SIMOX wafer can significantly affect the oxide density and its radiation response. The density of the buried oxide is also found to affect how it etches. >