J.R. Davis

Improved subthreshold characteristics of n-channel SOI transistors

It has been found that certain n-channel MOSFETs fabricated on silicon-on-insulator (SOI) substrates formed by oxygen implantation can have\log (I_{d}): V_{gs}, characteristics with very steep slopes in the subthreshold region. In contradiction to normal models for short-channel transistors on bulk silicon, the slope becomes steeper for shorter gate lengths or higher drain voltages. This effect is shown to be related to the kink in the output characteristics of transistors with floating islands.

Formation of buried layers of β‐SiC using ion beam synthesis and incoherent lamp annealing

It is demonstrated that well‐defined buried layers of β‐SiC can be grown epitaxially within a silicon substrate. This structure is formed by implanting high doses of carbon ions (>3×1017 C+ cm−2) at 200 keV into a (100) single‐crystal silicon which is maintained at a temperature of approximately 550 °C. During the subsequent anneal at 1405 °C for 90 min redistribution of the implanted species occurs, enabling the formation of a buried layer of β‐SiC overlain by high‐quality single‐crystal silicon (χmin=4.1%).

A transmission electron microscope investigation of the dose dependence of the microstructure of silicon‐on‐insulator structures formed by nitrogen implantation of silicon

Transmission electron microscope studies have been made of (100) silicon wafers implanted at 500 °C with 200‐keV 14N+ ions to doses of either 0.25, 0.75, or 1.4×1018 cm−2. For all of these specimens, the as‐implanted wafers contained a buried amorphous layer with a damaged upper single‐crystal silicon layer. For the 1.4×1018 cm−2 specimen, the amorphous layer contained bubbles. Wafers subsequently annealed at 1200 °C in order to form silicon‐on‐insulator structures showed the following. For the 0.25×1018 cm−2 specimen, there was a buried discontinuous polycrystalline α‐Si3N4 layer, and an upper silicon layer with no observable defects. For the 0.75×1018 cm−2 specimen, there was a buried continuous polycrystalline α‐Si3N4 layer containing small silicon islands, and an upper silicon layer either without defects or with microtwins adjacent to the nitride/silicon interface. For the 1.4×1018 cm−2 specimen, there was a buried multilayer structure with the middle layer consisting of substantially single‐crystal ...

Novel dielectric/silicon planar structures formed by ion beam synthesis

Abstract Multilayer SiO 2 Si planar structures with abrupt interfaces have been formed in single crystal silicon by ion beam synthesis, using multiple energy implants and high temperature annealing. SIMS, RBS and cross-section TEM techniques have been used to follow the evolution of these structures. Provided an implantation sequence from high to low energy is followed, the synthesised buried SiO 2 layers do not interact during the implantation, and the Si and SiO 2 thicknesses are independently defined by the ion energy and dose, respectively. The formation, during high temperature annealing, of planar structures with abrupt interfaces, is driven by chemical forces, which cause the excess oxygen in the supersaturated silicon matrix to segregate at the peaks of the oxygen concentration profiles. The mechanism is qualitatively similar to diffusion controlled precipitate growth in bulk silicon.

Buried layers of silicon oxy-nitride fabricated using ion beam synthesis

Abstract Ion beam synthesis (IBS) has been used to fabricate buried compound layers in silicon. These layers were produced by implanting combinations of O + and N + ions or NO + ions at 200 keV/atom into 100 single crystal silicon maintainted at a temperature of between 510°C and 580°C. The specimens were then annealed at 1200°C for two hours. It is found that the types of structure formed are highly dependent on the sequence in which the ions are implanted. When oxygen is implanted prior to nitrogen, the nitrogen segregates to the wings of the oxygen distribution where it forms an oxy-nitride. The presence of nitrogen, during the subsequent anneal is found to improve both the crystalline quality of the silicon overlayer and to reduce the impurity concentration therein. A similar improvement in the crystallinity of the silicon overlayer is also observed when oxygen and nitrogen are implanted together as NO + . Conversely if nitrogen is implanted prior to oxygen the redistribution of the impurities, during annealing, leads to a degradation in the structure, when compared to a specimen only implanted with oxygen. In this case the portion of the silicon overlayer adjacent to the buried layer, is highly defective with a high impurity concentration. Models based upon self diffusion and impurity interactions within the matrix are advanced to describe the evolution of the buried structures.

SIMS analysis of buried silicon nitride layers formed by high dose implantation of 14N and 15N

In order to obtain more information about the formation of buried layers of insulating material, we have undertaken a series of experiments in which we added incremental doses of the stable isotope, 15N, as a tracer, during the ion beam synthesis of the buried Si3N4 layers. The atomic processes causing tracer redistribution are discussed with particular reference to those occurring during post implant annealing.

High-performance SOI-CMOS Transistors in oxygen-implanted silicon without epitaxy

CMOS transistors with channel mobilities within a few percent of the equivalent bulk values have been produced in silicon-on-insulator (SOI) substrates formed by oxygen implantation. By performing the implantation at high energy (200 keV) and annealing the wafers at 1300°C, the thickness and quality of the resulting silicon film is such that the expensive and difficult to control step of epitaxial growth is not needed. The transistors have very low junction leakage. The lack of anomalous lateral diffusion of the source-drain dopants allows 1-µm gates to be used without excessive channel shortening.

High-performance thin-film silicon-on-insulator CMOS transistors in porous anodized silicon

Ultrathin-film silicon-on-insulator (SOI) CMOS transistors, produced in silicon islands 100 nm thick, formed by oxidation of porous anodized silicon, are described. Both n-channel and p-channel mobilities are similar to equivalent bulk values. Subthreshold slopes are less than 80 mV/decade and junction leakages are approximately 0.1 pA/ mu m. No kink is seen in the output characteristics of the n-channel transistors as the silicon film is fully depleted. A ring-oscillator gate delay of 161 ps has been achieved, at a power dissipation of 270 mu W/stage, for 1.5- mu m gate length.<<ETX>>

Dependence of silicon-on-insulator transistor parameters on oxygen implantation temperature

Silicon‐on‐insulator films have been formed by high‐dose oxygen implantation. Thermal donors, resulting from the residual oxygen content of the single crystal silicon region of the films, have been found to influence the electrical performance of transistors fabricated in them. The amount of oxygen in the active region of the silicon layer is strongly dependent on the oxygen implantation temperature. As the temperature is decreased below 500 °C an increasing thickness of oxygen‐rich polycrystalline silicon is formed between the single crystal region and the buried oxide, causing the oxygen concentration in the single crystal region (and hence the thermal donor activity) to fall. As well as providing thermal donors, the residual oxygen also causes a lattice strain which increases the electron mobility.

Nonplanar and noncontinuous buried layers of SiO2 in silicon formed by ion beam synthesis

Abstract The synthesis of buried layers of SiO2 by the implantation of high doses (1.8 × 1018O+ cm−2) of O+ ions through thick oxide masks is described. Due to the rapid sputter erosion of the SiO2 mask (1.1 atoms/particle) these layers grow, unlike conventional SIMOX structures, by the preferential growth (“internal oxidation”) of the lower SiO2/Si interface. It is shown that improved TDI structures may be formed by implantation into a thick (1.8 μm) oxide mask.