J. Thornton

The production of excess interstitials by pre-amorphisation

Single-crystal silicon has been amorphised by bombardment with Si+, Ge+ or Sn+ ions. After conventional annealing (900 degrees C, 30 min) some disorder remained. The amount was measured by ion channelling and transmission electron microscopy and compared with the quantity of excess interstitials predicted by the Monte Carlo computer program TRIM. Both the predicted and measured integral disorder fell as the atomic weight of the projectile increased. Because lattice strain also leads to a reduction in ion channelling this had to be taken into account when estimating residual damage from back-scattering analysis. The results indicate that there are six scattering centres per interstitial.

Predicted dose, energy and implantation temperature effects on the residual disorder following the annealing of pre-amorphised silicon

A band of disorder is observed following the annealing of surface amorphous layers produced by ion bombardment. The depth of the bands centre is just below the original amorphous/crystalline interface. The ion bombardment simulation computer program TRIM.CASCADE has been used in conjunction with ion channelling measurements to predict the areal density of atoms available for the formation of this band of disorder. The variation in the amounts of disorder expected due to changing the implantation parameters: ion dose, ion energy and implantation temperature are reported. The predictions are compared with the published transmission electron microscopy results and electrical measurements of other workers. The implications of this work for the technique of pre-amorphisation are also discussed.

Amorphisation of silicon by bombardment with group IV ions

An empirical recipe for the amorphisation of single crystal silicon by bombardment with the group IV ions: Si + , Ge + , Sn + and Pb + , has been developed. Samples amorphised using 29 Si + ions were implanted with boron (25 keV, 10 15 ions cm −2 ) and annealed using conventional furnace techniques. Ion channelling and sheet resistivity measurements indicate that good regrowth and complete activation of the shallow doped layers can be obtained provided amorphisation is complete. Similar results were also obtained using 70 Ge + and 118 Sn + implants.

Nuclear reaction studies of 15N implanted spheroidal graphite cast iron using an ion microprobe

Abstract The relative concentrations of the matrix and graphite nodules of a spheroidal graphite cast iron implanted with 15 N (200 keV, 2 × 10 7 ions/cm 2 , 440 K) have been studied by nuclear microprobe using the 15 N(p, αγ) 12 C reaction at 898 keV. A beam diameter of less than 20 μm enabled individual nodules to be investigated independently to the matrix for two samples of differing microstructure. The matrix of the first sample was fine pearlite and that of the second sample was martensite. Significant diffusion of the implanted nitrogen out of the matrix into the nodules had taken place in the pearlitic sample, resulting in broadening of the nitrogen distribution. This is believed to be due to the large number of grain boundaries present in the matrix. Diffusion in the hardened iron was limited to a migration towards the surface, whilst there was no appreciable migration into the nodule.

Helium microprobe analysis of nickel silicide diodes

Abstract In this paper we present RBS and channelling measurements made on microscopic nickel silicide diode structures. These were obtained by using the helium ion microprobe at the University of Surrey. We also show that the new pre-lens deflection system enables measurements to be made with a 10 μm diameter probe over a 2 × 2 mm area without significant dechannelling or increase in the probes diameter.

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.

Quantification of the separate matrix constituents of spheroidal graphite cast iron implanted with 15N by nuclear reaction analysis using an ion muprobe

The retained dose of nitrogen in a spheroidal graphite (SG) cast iron (4% carbon) implanted with 2 × 101715solNcm2 at 200 keV has been determined separately in the iron matrix and in the graphite inclusions of 30 μm average diameter randomly dispersed in the matrix, using the 15N(p, αγ)12C resonance at 898 keV and a proton muprobe focussed to less than 20 μm spot diameter. In normalised and tempered SG cast iron the retained doses were 1.09 × 1017 and 1.74 × 1017Ncm2 in the pearlitic matrix and graphite nodules, respectively, and in induction hardened SG cast iron the retained doses were 1.18 × 1015 and 0.97 × 1017Ncm2 in the martensitic matrix and nodules, respectively. The profile shapes are also quite different in both types of samples, and in both matrix and nodule.

Defects after preamorphisation and annealing

Abstract The control of implanted depth distributions by preamorphising silicon results in the production of defects in sensitive regions of devices fabricated in this fashion. The most stable — and therefore the most troublesome — defects are the dislocation loops that form at the original amorphous/crystalline interface on annealing. Our models suggest that the dislocation loops evolve from atoms injected into the underlying crystalline region during the amorphising implant. The idea was tested by annihilating these excess atoms with excess vacancies produced during the formation of a deep buried amorphous layer as our model predicted. No dislocation loops were then observed in this region.

Ion channelling analysis of pre-amorphised silicon diodes using a nuclear microprobe

Aligned and random ion channelling analysis was performed on p+n diode structures in (100) silicon, with the Surrey nuclear microprobe. Three different types of diode were investigated, each pre-amorphised by a different ion (Si+, Ge+ or Sn+) before the p+ region was formed by BF2+ implantation. The ion channelling measurements are presented and compared with previously published electrical measurements on these diodes. Relatively large residual disorder and junction leakage currents were found for the Si+ pre-amorphised diodes; however, all the diodes were leaky (>10-6 A cm-2). The results are consistent with dislocation loops within the depletion regions of the diodes causing both the residual disorder and the large leakage currents. Cross-sectional transmission electron microscopy studies support this model.

Nuclear microprobe ion-channelling analysis of total dielectric isolation structures in silicon

The production of isolated device islands by high dose oxygen implantation into silicon followed by high temperature annealing has been termed total dielectric isolation (TDI). Higher packing densities, higher frequency performance and radiation hardness are the three potential advantages of fabricating very large scale integrated circuits by this method. Because of the microscopic size of the silicon islands, they cannot be analysed using conventional ion-channelling apparatus. However, some nuclear microprobes are now available which produce small diameter, low convergence MeV ion beams suitable for microscopic ion-channelling analysis. Thus, a technique which has proved very useful in analysing the near surface region in bulk semiconductor samples can now be applied to semiconductor device structures. The authors report on a preliminary study of TDI structures using the Surrey scanning microprobe. Measurements of crystalline quality, silicon thickness and buried oxide thickness were obtained from isolated islands. Secondary electron and optical microscopy were used to assess beam damage, and the feasibility of nuclear microprobe analysis of TDI structures is discussed.