In the past few years, there has been rapid growth in the positron annihilation spectroscopy (PAS) of overlayers, interfaces, and buried regions of semiconductors. There are few other techniques that are as sensitive as PAS to low concentrations of open‐volume‐type defects. The characteristics of the annihilation gamma rays depend strongly on the local environment of the annihilation sites and are used to probe defect concentrations in a range inaccessible to conventional defect probes, yet which are important in the electrical performance of device structures. We show how PAS can be used as a nondestructive probe to examine defects in technologically important Si‐based structures. The discussion will focus on the quality of overlayers, formation and annealing of defects after ion implantation, identification of defect complexes, and evaluation of the distribution of internal electric fields. We describe investigations of the activation energy for the detrapping of hydrogen from SiO2−Si interface trap centers, variations of interface trap density, hole trapping at SiO2−Si interfaces, and radiation damage in SiO2−Si systems. We also briefly summarize the use of PAS in compound semiconductor systems and suggest some future directions.

Characterization of defects in Si and SiO2−Si using positrons

Boron diffusion in ion‐implanted and annealed single‐crystal and amorphized Si is compared to determine the effect of amorphization on the initial transient boron motion reported for single crystal. The boron was implanted at 20 keV and at doses of 1×1015 and 3×1015cm−2. The Si was either preamorphized or postamorphized to a depth of 320 nm by implantation of Si ions at three different energies. In the amorphized samples the entire boron profile was always contained within this distance. The samples were annealed by furnace or rapid thermal annealing to 900–1100 °C with or without a preanneal at 600 °C. The initial rapid diffusion transient in the tail region of the boron profile was observed in all the crystal samples. This transient was totally absent in the amorphized samples. This is manifest by careful comparison of boron concentration profiles determined by secondary ion mass spectrometry of single‐crystal and amorphized samples after annealing. For anneals where significant motion occurs, the profi...

Transient boron diffusion in ion-implanted crystalline and amorphous silicon

The fabrication of ultra large-scale integrated (ULSI) circuits with ever shrinking feature size requires a continued reduction of diffusion lengths of dopants in Si. However, boron implants undergo an “anomalous” diffusion upon annealing, which is detrimental to the ultra-shallow junction formation. Over the last decade, boron transient diffusion has been much better understood. This implantation-induced transient-enhanced diffusion is known to be driven by the large supersaturation of self-interstitial silicon atoms emitted from the extended defects. The form and evolution of implantation-induced defects with respect to implant species, energy, dose, and annealing temperature have been systematically studied. The improved understanding has led to the development of new doping techniques, such as carbon co-implant, multiple step annealing and point defect engineering (PDE) using co-implantation of high-energy ions. In the PDE approach, high-energy ion bombardments inject vacancies near the surface region and create excessive interstitials near the end of projected range deep inside the substrate. Such manipulation of point defects can retard boron diffusion and enhance the activation of boron. The control of boron diffusion could be used to form ultra-shallow junctions for next generation Si device. In this article, we review the history and recent progress in PDE.

Boron diffusion in silicon: the anomalies and control by point defect engineering

Shallow p/sup +/-n and n/sup +/-p junctions were formed in germanium preamorphized Si substrates. Germanium implantation was carried out over the energy range of 50-125 keV and at doses from 3*10/sup 14/ to 1*10/sup 15/ cm/sup -2/. p/sup +/-n junctions were formed by 10-keV boron implantation at a dose of 1*10/sup 15/ cm/sup -2/. Arsenic was implanted at 50 keV at a dose of 5*10/sup 15/ cm/sup -2/ to form the n/sup +/-p junctions. Rapid thermal annealing was used for dopant activation and damage removal. Ge, B, and As distribution profiles were measured by secondary ion mass spectroscopy. Rutherford backscattering spectrometry was used to study the dependence of the amorphous layer formation on the energy and dose of germanium ion implantation. Cross-sectional transmission electron microscopy was used to study the residual defects formed due to preamorphization. Complete elimination of the residual end-of-range damage was achieved in samples preamorphized by 50-keV/1*10/sup 15/ cm/sup -2/ germanium implantation. Areal and peripheral leakage current densities of the junctions were studied as a function of germanium implantation parameters. The results show that high-quality p/sup +/-n and n/sup +/-p junctions can be formed in germanium preamorphized substrates if the preamorphization conditions are optimized. >

Optimization of the germanium preamorphization conditions for shallow-junction formation

RTA studies of B, BF 2 and As implantations in silicon are reviewed. The optical coupling properties of the wafer are considered as a motive for obtaining accurate temperature assessment. Optical pyrometer techniques with feedback control are now commonly used. Enhanced diffusion of boron is widely reported for the channel tail in the several second time frame at ∼1050°C. Boron diffusion in preamorphized and crystalline silicon are compared, and dopant interactions (gettering) with residual disorder located just beyond the amorphous/crystalline (α/c) interface are discussed. The removal of residual α/c disorder from a 5 × 10 15 As/cm 2, 100 keV implant is shown to occur with ∼5.0 eV energy, while a “normal” concentration enhanced diffusion with ∼4.0 eV activation energy is obtained including data in the several second (RTA) time frames. Shallower defect free arsenic junctions are made using high temperature-short time cycles, rather than longer times at lower temperatures. The arsenic diffusion issues are discussed.

A review of rapid thermal annealing (RTA) of B, BF 2 and As ions implanted into silicon

Depth distributions, measured by secondary ion mass spectrometry (SIMS), and carrier profiles, measured by differential capacitance‐voltage (C‐V) profiling, of boron and fluorine implanted as B, F, BF, or BF2 ions into random and channeling orientations of crystalline silicon, and into silicon amorphized by silicon ion implantation are reported. Low boron energies of 8 and 10 keV and the corresponding energies of 36 and 45 keV for BF2 ions are emphasized because of their use for high resolution device and circuit applications in silicon and silicon‐on‐sapphire. Amorphizing crystalline silicon prior to boron implantation eliminates the significant channeling tails on 8‐ or 10‐keV boron profiles. Fluorine penetrates more deeply into crystalline silicon than boron does. Both boron and fluorine redistribute during annealing at 925 °C/20 min for B, F, BF, or BF2 implants, but with quite different characteristics as illustrated, and depend on the implantation fluence (5×1014 and 2×1015 cm−2 reported here). The fluorine redistribution profiles are strongly influenced by the magnitude and distribution of damage that remains after annealing. Fair agreement is shown between boron atom depth distributions measured by SIMS and C‐V electrical profiles measured for a fluence of 1.5×1012 cm−2. Differential C‐V profiles indicate that the entire ion spectrum from BF3 can be implanted and electrically activated (for a fluence of 1.5×1012 cm−2), as can a BF2 implant. Implantation through a 20‐nm layer of SiO2 has no significant effect on the boron depth distribution in crystalline silicon. Pearson IV moments are given for the low energy boron profiles. The use of these profiles for modeling calculations is discussed. The suprem model of an exponential for the channeling tail of boron implants in crystalline silicon is fairly good for fluences greater than about 1015 cm−2, but poorer for lower fluences, but the slope and matching to the random portion of the profiles are difficult to predict. In order for modeling calculations to reasonably represent boron profiles, either the silicon substrate should be amorphized prior to boron implantation, or the modeling should be modified to use experimental data measured for the implant and silicon conditions.

R. G. Wilson

Papers

Boron, fluorine, and carrier profiles for B and BF2 implants into crystalline and amorphous Si