P. Asoka‐Kumar

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.

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.

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.

Point defects in Si thin films grown by molecular beam epitaxy

Depth profiles of vacancylike defects have been determined by positron annihilation spectroscopy in 200‐nm‐thick Si films grown by molecular beam epitaxy on Si(100) substrates at growth temperatures Tgrowth=200–560 °C. The line shape of the radiation emitted from implanted positrons annihilating in the near‐surface region of a solid gives quantitative, depth‐resolved information on defect concentrations in a nondestructive way. In particular, the method is sensitive to vacancylike defects in a concentration range inaccessible to electron microscopy or ion scattering, but important for electrical device characteristics. The sensitivity limit for these defects in the present experiments is estimated as 5×1015 cm−3. Films grown at Tgrowth≥475±20 °C are indistinguishable from virgin wafers. So are samples with Tgrowth=220±20 °C, subjected to a 2 min, TRTA≳500 °C rapid thermal anneal (RTA) after every ≊30 nm of Si growth. If TRTA=450±20 °C, part of the film contains a concentration of vacancylike defects on th...

DETECTION OF HYDROGEN-PLASMA-INDUCED DEFECTS IN SI BY POSITRON ANNIHILATION

We report a positron annihilation study of defects created in Si by rf hydrogen‐plasma exposure at 275 °C. Analysis of positron annihilation spectroscopy data indicates voidlike structures in a defective ayer extending to ≊14 nm from the surface at a concentration of 1.9±0.5×1020 cm−3. The Doppler broadening parameter for the annihilation gamma rays is strongly correlated to the hydrogen coverage of the void surfaces, voids remain in the Si to at least 800 °C while the hydrogen is desorbed from their surfaces between 600 and 800 °C.

Kinetics of hydrogen interaction with SiO2‐Si interface trap centers

The effects of low temperature (≤700 °C ) annealing on the thermal dissociation of hydrogen‐passivated interface trap centers of a SiO2‐Si(100) system is studied using positron annihilation spectroscopy. The Si—H bonds dissociate with an activation energy of 2.60±0.06 eV. Assuming that the anneal generates trap centers with a single charge, positron measurements indicate that ∼4.5×108 trap centers/cm2 are created by a 600 °C anneal.

Study of SiO2‐Si and metal‐oxide‐semiconductor structures using positrons

Studies of SiO2‐Si and metal‐oxide‐semiconductor (MOS) structures using positrons are summarized and a concise picture of the present understanding of positrons in these systems is provided. Positron annihilation line‐shape S data are presented as a function of the positron incident energy, gate voltage, and annealing, and are described with a diffusion‐annihilation equation for positrons. The data are compared with electrical measurements. Distinct annihilation characteristics were observed at the SiO2‐Si interface and have been studied as a function of bias voltage and annealing conditions. The shift of the centroid (peak) of γ‐ray energy distributions in the depletion region of the MOS structures was studied as a function of positron energy and gate voltage, and the shifts are explained by the corresponding variations in the strength of the electric field and thickness of the depletion layer. The potential role of the positron annihilation technique as a noncontact, nondestructive, and depth‐sensitive ...

Annealing of low-temperature GaAs studied using a variable energy positron beam

The annihilation characteristics of monoenergetic positrons implanted in a molecular beam epitaxy layer of low‐temperature (LT) GaAs annealed at temperatures from 300 to 600 °C were measured. A gallium vacancy concentration of approximately 3×1017 cm−3 is inferred for the as‐grown material. The S parameter increased significantly upon anneal to 500 °C. The dominant positron traps in samples annealed at and below 400 °C are distinct from those acting for samples annealed to 500 or 600 °C. The change in S parameter for the 600 °C annealed sample compared to the GaAs substrate, SLT,600=1.047Ssub, is consistent with divacancies or larger open volume defects.

Implantation profile of low‐energy positrons in solids

A simple form for an implantation profile of monoenergetic, low‐energy (1–10 keV) positrons in solids is presented. Materials studied include aluminum, copper, molybdenum, palladium, and gold with atomic number ranging from 13 to 79. A simple set of parameters can describe the currently used Makhov profile in slow positron studies of solids. We provide curves and tables for the parameters that can be used to describe the implantation profiles of positrons in any material with atomic number in between 13 and 79.