Nobuyoshi Natsuaki

MeV-energy B+, P+ and As+ ion implantation into Si

Abstract Annealing behavior of secondary defects in 2 MeV B + and P + , and 1 MeV As + ion implanted (100) Si has been investigated in terms of their structure, nature and depth distribution. This is done mainly through cross-sectional TEM observations at doses of 2 × 10 13 , 1 × 10 14 and 5 × 10 14 ions/cm 2 . The critical dose for generating secondary defects is between 2 × 10 13 and 1 × 10 14 ions/cm 2 , independent of ion species. A characteristic of B + and P + ion implanted layers is that buried secondary defects are formed beneath the substrate surface, with their maximum densities at depths of around 3.2 μm (B) and 2.1 μm (P) below the surface. These defect peak positions in the crystal are constant under all annealing conditions (e.g., a temperature range of 700 to 1250°C, annealing time of up to 6780 min at 1000°C). In contrast, the defects in the layers implanted with As + ions have an almost constant density in their depth distribution and change their depth position with annealing time and temperature. The peaks (B and P) and the central position (As) of these secondary defects are positioned deeper than both the projected range and the primary defect peak of each ion species.

Advantages of Fluorine Introduction in Boron Implanted Shallow p+/n-Junction Formation

The advantages of fluorine introduction on fabrication of shallow p+/n-junctions have been demonstrated. This was done by implanting fluorine onto the boron implanted p+/n-junction area prior to annealing. By introducing optimized amounts of fluorine, (1) the boron redistribution after annealing is suppressed, (2) the concentration of activated boron becomes higher, and (3) the leakage current level of the p+/n-junction decreases. These behaviors may be due to interactions between fluorine and defects in the silicon substrate or at the SiO2/Si interface.

Observation of pn junctions on implanted silicon using a scanning tunneling microscope

Si pn junctions fabricated by photoresist masked As+ implantation were observed using current imaging tunneling spectroscopy (CITS) in a scanning tunneling microscope (STM). Using the CITS, a specific bias was chosen to define n‐type or p‐type areas according to whether or not current flowed. The pn junctions could be easily identified from the current image at this bias and in the STM topographic image. It also proved possible to find processing faults related to implantation. The STM images also identified the structure (corrugations) near the junctions, associated with volume expansion caused by implantation and annealing.

Low-energy mass-separated ion beam deposition of materials

Abstract A low energy mass-separated ion beam deposition system was developed. Ge+ ions of 100 μA were obtained at an acceleration energy of 100 eV with a beam spot size of 5 mm o at 5 × 10−8 Torr. Material buildup has been observed at the range of ion energy where the self-sputtering yield is below unity. The incident ion energy dependence of the reactions of the F+ and CFx+ ions with a silicon substrate has been measured, as well as the physical phenomena conventionally observed between impinging ions and a substrate material. Epitaxial growth of Ge films on Ge and Si single crystal substrates has been observed at 300°C with a Ge+ ion energy of 100 eV. Deposited Ge crystal properties were evaluated.

Change of the electron effective mass in extremely heavily doped n-type Si obtained by ion implantation and laser annealing

Abstract Infrared optical properties of extremely heavily doped n -type Si, obtained by ion implantation and laser annealing, were studied. A new relation between free carrier effective mass ( m ∗ ) and carrier concentration (10 19 −5 × 10 21 cm -3 ) was obtained. The value of m ∗ increases significantly with the increase of carrier concentration, when carrier concentration exceeds 10 21 cm -3 . The result is discussed in relation to the occupation of electrons in a new valley of the conduction band.

Depth distribution of secondary defects in 2‐MeV boron‐implanted silicon

Annealing behavior of secondary defects in 2‐MeV boron ion‐implanted (100) silicon has been investigated mainly through cross‐sectional TEM observations. The maximum defect density is located at a mean depth of 3.2 μm from the surface and the location is 0.3 μm deeper than that of the projected range of boron ions. This defect position in the crystal is constant under all annealing conditions (e.g., a temperature range of between 700 and 1000 °C, annealing time of up to 6780 min at 1000 °C), although the vertical distribution width of defects changes with both annealing temperature and time.

Nonequilibrium solid solutions obtained by heavy ion implantation and laser annealing

Nonequilibrium solid solubility of P, As, and B in single‐crystalline silicon annealed by Q‐switched ruby laser pulse irradiation was experimentally investigated for residual defects, lattice strain, and electrical activation of implanted impurities. The maximum solubility obtained without any macroscopically extended defect formation was 2–4 times higher than the thermal equilibrium solubility limit. Above this solubility, precipitates, dislocations, and surface cracks were observed. The highest full activation was realized by P implantation, with carrier concentration up to ∼5×1021/cm3 showing no such defects. Formation mechanisms of the defects are discussed and shown to be attributable to the rapid solidification process of the heavily doped layer and to large impurity‐induced misfit stress comparable to the fracture stress.Nonequilibrium solid solubility of P, As, and B in single‐crystalline silicon annealed by Q‐switched ruby laser pulse irradiation was experimentally investigated for residual defects, lattice strain, and electrical activation of implanted impurities. The maximum solubility obtained without any macroscopically extended defect formation was 2–4 times higher than the thermal equilibrium solubility limit. Above this solubility, precipitates, dislocations, and surface cracks were observed. The highest full activation was realized by P implantation, with carrier concentration up to ∼5×1021/cm3 showing no such defects. Formation mechanisms of the defects are discussed and shown to be attributable to the rapid solidification process of the heavily doped layer and to large impurity‐induced misfit stress comparable to the fracture stress.

A model for the segregation and pileup of boron at the SiO2/Si interface during the formation of ultrashallow p+ junctions

We have quantitatively investigated how boron segregates to regions close to the surface, and what controls this phenomenon, using x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and backside secondary ion mass spectrometry measurement techniques. We found that, contrary to the equilibrium segregation, the pileup of boron is mainly on and within 0.6 nm of the Si side of the interface, and that there is no difference between the kind of encapsulation. This also suggests that the pileup of boron is mainly on the Si side, and implies that the main factor in this segregation is the existence of the Si surface. From the viewpoint of device fabrication, this result seems to be useful in terms of the fabrication of sidewalls. The possibility of boron pileup to occurring in the interstitial state was also shown. Our results suggested a way of looking at dopant profiles by predictive computer modeling.

Local-field-enhancement model of DRAM retention failure

We have developed the local-field-enhancement model of the tail component of DRAM (dynamic random access memory) retention-time distribution. The model is in excellent agreement with experiments and proposes to control not the number but the energy-level distribution of traps and to reduce the space-charge-region-field variation together with the field itself to increase the retention time.

Improvement of SiO2/Si Interface Properties Utilising Fluorine Ion Implantation and Drive-in Diffusion

Thermal drive-in diffusion of ion-implanted F atoms has been employed to fluorinate SiO2/Si interfaces and thereby improve their electrical properties. The interface state density can be lowered with little fixed charge generation. Correspondingly, pn-junction surface leakage current decreases. Furthermore, the interfaces can be hardened against hot-electrons due to Fowler-Nordheim current injection and avalanche current at the junction surface. As a result, a fluorinated MOSFET shows higher hot-carrier immunity. It is pointed out that there is an optimal F dose for these improvements to be achieved.