K. G. Lynn

Characterization of defects in Si and SiO2−Si using positrons

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.

Effect of different preparation conditions on light emission from silicon implanted SiO2 layers

Visible light emission from Si+ implanted SiO2  layers as a function of different annealing conditions (temperature, time and ambient) is studied. It is shown that a 560 nm band, present in as implanted samples, increases its intensity for increasing annealing temperatures and is still observed after annealing at 1000 °C. The emission time is fast (0.5–2 ns). A second band centered at 780 nm is detected after annealing at 1000 °C. The intensity of the 780 nm band further increases when hydrogen annealing was performed. The emission time is long (1μs–0.3 ms). Based on the annealing behavior and on the emission times, the origin of the two bands is discussed.

Defects in MeV Si‐implanted Si probed with positrons

Vacancy‐type defects produced by implantation of MeV doses of Si ions (1011–1015 atoms/cm2) at room temperature have been probed using depth‐resolved positron annihilation spectroscopy. The defect (divacancy) concentration increases linearly with dose for low doses (<1012 Si/cm2). In situ isochronal annealing was followed for oxygen‐containing Si (10 ppm) and oxygen‐‘‘free’’ Si implanted to doses (5×1012 and 5×1014 Si/cm2). Two main annealing stages were observed at the same temperatures in the studied samples in spite of significant differences in doses and oxygen content. In the first stage (∼200 °C) a significant fraction of divacancies was observed to form large vacancy clusters. These clusters were removed in the second stage (∼675 °C) after which the oxygen‐free samples returned to pre‐irradiation conditions, whereas oxygen‐defect complexes were formed in the oxygen‐containing samples.

Detection of current‐induced vacancies in thin aluminum–copper lines using positrons

In situ depth‐resolved positron annihilation spectroscopy (PAS) is used to show dynamic formation of vacancies in 1 μm×1 μm Al‐0.5 wt % Cu lines under current flow. We show that the number of vacancies in these lines increases when a dc current (8×104 A/cm2) is applied. This increase in vacancy concentration is substantially greater than that due to thermal vacancy generation alone (4×1018 cm−3 versus 3×1017 cm−3). Isothermal measurements (with no current flow) yield a vacancy formation energy of 0.60±0.02 eV. These results show that PAS can be used to examine the initial stages of interconnect damage due to electromigration.

Development of transmission positron moderators

The conversion from fast to slow positrons by several thin (0.3–7 μm) film moderators has been studied. Positrons from a radioactive 22Na source enter the film where some are thermalized, consequently diffuse to the surface opposite to the source and are emitted with low energies (1–4 eV). The experimental data show that the conversion efficiency to slow positrons ranges from 1×10−4 to 9.1×10−4 for good moderators. The moderator efficiency was found to be very sensitive to annealing temperatures.

SiO2/Si interface probed with a variable‐energy positron beam

The annihilation characteristics of a monoenergetic beam of positrons, after implantation in Si with a 350‐nm overlayer of SiO2, were measured as a function of mean implantation depth. Positrons implanted overlapping the SiO2/Si interface were observed to decay from a state with properties distinctively different from the state in bulk Si and the thermally grown SiO2, i.e., a positron interface state. The momentum distribution of the annihilating positron‐electron pair, as observed in the Doppler broadening of the annihilation line, is much broader for this state than for either bulk Si or SiO2, in contrast to previously observed localized positron states in solids and at surfaces which show a narrower distribution.

Improved slow-positron yield using a single crystal tungsten moderator

A well-annealed W(110) single crystal was used as a fast-to-slow positron moderator. The measured moderator efficiency at room temperature using a58Co positron source in the backscattering geometry isɛ =(3.2±0.4)×10−3, roughly a factor of three better thanɛ for the best previously reported Cu(111)+S moderator. We find a stable positron moderation efficiency over a period of several weeks when maintained at pressures around 10−9 Torr and an energy spreadΔE = 0.7 eV of the emitted slow positrons. An initial attempt was made to fabricate a hybrid Cu on W(110) moderator, which yieldedɛ of about 1.2×10−3 after annealing.

Vacancy defects in (Pb, La)(Zr, Ti)O3 capacitors observed by positron annihilation

A study of vacancy-related defects in ferroelectric capacitors was performed using a variable energy positron beam (VEPB). Heterostructures of (Pb0.9La0.1)(Zr0.2Ti0.8)O3 (PLZT) ferroelectric with La0.5Sr0.5CoO3 (LSCO) electrodes were deposited by pulsed laser deposition and the effects of oxygen deficiency studied using structures grown with 760 and 1×10−5 Torr oxygen. The VEPB depth profile showed an increase in vacancy-related defects with increased oxygen nonstoichiometry. A study of LSCO and PLZT thin films was also performed. The formation of vacancy clusters in the LSCO top electrode, and VPb−VO defects in the PLZT layer, with increased oxygen deficiency is inferred.

Role of implantation‐induced defects in surface‐oriented diffusion of fluorine in silicon

Open‐volume defects introduced in Si(100) crystals during fluorine implantation were investigated by variable‐energy positron beam depth profiling. The behavior of the implantation‐induced lattice defects upon high temperature annealing and their role in the surface‐oriented diffusion of F impurities were examined. The defects become mobile and undergo recovery at temperatures below 550 °C, i.e., well before the onset of fluorine diffusion as seen by secondary ion mass spectroscopy (SIMS) profiling. This behavior suggests that after irradiation and annealing the fluorine occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two‐step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by a rapid diffusion of the interstitial F and the vacancy through the crystal to the surface.

Improvement of rare-gas solid moderators by using conical geometry

A slow positron beam with narrow energy spread can be obtained by moderating the β+ decay of a radioactive source. We report here the improvement in the efficiency of the rare‐gas solid positron moderators by using a different geometry. The efficiency for slow positrons, e, was measured for the cylindrical and the cone configurations of the moderator with the latter yielding e of (4.6±0.2)×10−3 for solid neon deposited on an encapsulated radioactive 22Na source deposited on a 5 μm Cu98/Be2 window. No corrections were applied for the attenuation caused by the window. The e for the conical configuration is (2.7 ± 0.2) times higher than that for the cylindrical configuration. Other rare‐gas solids (e.g., Ar, Xe, Kr) yielded lower e as compared to that for the solid neon in agreement with the earlier measured efficiencies of Mills and Gullikson [Appl. Phys. Lett. 49, 1121 (1986)].