Spreading Resistance Technique Research Articles

Spreading resistance:A quantitative tool for process control and development

During the last five years much work has been done in developing the spreading resistance technique. This work has led to a much clearer understanding of the strengths and weaknesses of the technique. It has been shown that it is capable of producing carrier concentration profiles which are accurate enough to be used in process control and development. A review is presented which highlights the three steps necessary to obtain such data. These are the optimization of the acquisition of raw data, the conversion of the raw data to carrier profiles, and finally the interpretation and use of the final profiles. A great emphasis is placed on the need to acquire the most accurate and reproducible data and on the methods which are used to achieve this objective. For the accurate profiling of ultrashallow layers such procedures are shown to be of paramount importance.

Diffusion, solubility, and thermodynamic properties of gold in solid germanium studied by means of radiotracer and spreading-resistance analysis

Diffusion profiles of gold in germanium were measured using the radiotracer as well as the spreading-resistance technique. Diffusion coefficients from both experimental methods show a good agreement and are described by an activation energy of 147±2 kJ mol−1 and a preexponential factor in the range (1–2)×10−6 m2 s−1. The results are interpreted within a generalized dissociative model allowing for interstitial-substitutional exchange with the aid of vacancies or interstitial Au-vacancy pairs. Solubility data taken from the boundary concentration of the penetration profiles reveal nonsignificant differences between the total Au concentration and that of substitutional Au atoms. Basic thermodynamic quantities were calculated from the temperature dependence of the solid solubility using data from the Au-Ge phase diagram.

Diffusion and solubility of zinc in dislocation-free and plastically deformed silicon crystals

Floating-zone Si crystals enclosed in quartz ampoules were exposed to Zn vapour released by an elemental diffusion source. Penetration profiles of Zn in Si were recorded using the spreading-resistance technique or neutron activation analysis. Both the erfc-type distributions observed in plastically deformed specimens and the non-erfc profiles determined on dislocationfree wafers are consistently interpreted within the framework of the kick-out model. As an implication, Si self-interstitials generated in excess by interstitial-to-substitutional transitions of in-diffusing Zn atoms annihilate not only at the surface but also at dislocations. On the other hand, dislocation-induced segregation of Zn appears to be rather minor, as revealed by transition electron microscopy. Combining the Zn incorporation rate in dislocation-free Si with solubility data from saturated specimens yields the self-interstitial contribution to the Si self-diffusion coefficient.

Diffusion and solubility of copper, silver, and gold in germanium.

Diffusion profiles of Cu, Ag, and Au were recorded on Ge crystals with various dislocation densities using the spreading-resistance technique. The penetration rate of Cu in effectively highly dislocated samples proves to be much faster than in virtually dislocation-free material. In the case of intermediate dislocation densities, two-stage Cu profiles are found. Diffusion coefficients of Ag and Au do not depend on the presence of dislocations. For all three elements, equilibrium solubilities were taken from the boundary concentration of the penetration profiles. A combined analysis of diffusivity and solubility data reveals that all features can be explained within the framework of the dissociative mechanism involving interstitial-substitutional exchange with the aid of vacancies. Whereas transport of interstitial Cu atoms proceeds faster than that of vacancies, for Ag and Au the opposite case appears to be true.

Limitations of the spreading resistance technique for ion implant profile measurements

The main problem with the spreading resistance technique is that it yields a net carrier concentration profile measured on a beveled sample and this may be quite different from the corresponding dopant profile due to carrier spilling. We have made theoretical and experimental studies of this phenomenon to evaluate the accuracy of the technique to measure ion-implanted profiles. We see that errors are important when a pn shallow junction is formed with low implant doses.

Control of gold concentration profiles in silicon by ion implantation

Diffusion of ion-implanted gold in a silicon single crystal has been studied by the spreading resistance technique. In this work we investigate both unidimensional and bidimensional diffusion across the wafer and along the wafer surface, respectively, by using limited or unlimited gold sources. We will show that by using ion implantation of gold it is possible to produce unique concentration profiles through the fine control of the amount of gold atoms in the diffusion source; both depth and surface profiles can be tailored. All the measured profiles are in good agreement with the theoretical prediction of the kick-out mechanism for gold diffusion in silicon.

The Nature of Electrically Inactive Implanted Arsenic in Silicon after Rapid Thermal Annealing

AbstractThe electrical activation of As implanted Si has been investigated on rapid thermal annealing timescales using sheet resistance, spreading resistance and Hall Effect techniques. For high dose implants (>1015 As cm-2) differential Hall Effect and spreading resistance profiles confirm the existence of a temperature dependent electrical solubility limit. However for low dose implants, annealing schedules chosen such that the electrical solubility limit is not exceeded reveal electrical deactivation which is not accounted for in the clustering theory. Hall Effect measurements performed as a function of temperature have enabled us to reveal directly electrically inactive As which is not observable at room temperature using standard electrical techniques. The results indicate that As atoms in Si introduce deep trapping levels within the bandgap which are responsible forremoving As from the conduction process at room temperature. This temperature activated process is characterized with an activation energy of 0.4eV.

Gold implantation in n-type silicon: Entropy factor and diffusion studies

Rutherford backscattering spectrometry and spreading resistance techniques have been used to determine the concentration profiles of gold implanted and diffused in n-type silicon. Diffusion has been performed at 1243 K for times ranging between 1 and 20 h in a dry-nitrogen flux. Resistivity profiles were transformed into concentration profiles by solving the charge balance equation. The entropy factor for the ionization of gold acceptor level was determined to be 50±5. By using this value and solving the charge balance equation we have calculated the silicon resistivity versus gold concentration (and therefore minority-carrier lifetime) curves as a function of the resistivity of starting material. Experimental gold concentration profiles were compared to the profiles obtained by numerical solution of the diffusion equation for gold in silicon. The measured diffusion coefficients coincide with the values determined for p-type material.

The spreading resistance technique applied to mercury cadmium telluride heterojunctions

The spreading resistance technique has been used to characterize beveled heterojunctions of mercury cadmium telluride grown by molecular beam epitaxy. We show that important information on the growth axis profiles can be recovered when probing both at room temperature and cryogenic temperature. Examples of in situ and ex situ doped structures as well as CdHgTe/CdTe and CdHgTe/HgTe heterojunctions are described. In the latter case the measurements seem to indicate the presence of an inversion layer between the n-type narrow bandgap material and the semimetal.

Recent Developments in the Interpretation of Spreading Resistance Profiles for VLSI‐Technology

The interpretation of spreading resistance depth profiles for submicron devices is seriously affected by the fact that the spreading resistance technique measures an electrical on‐bevel carrier profile which may be quite different from the underlying atomic dopant profile. A one‐dimensional Poisson model has been implemented for simulating the carrier spilling effects responsible for these differences. The model is able to explain anomalies such as the absence of a junction peak for certain types of abrupt junctions, the observed discrepancies between implanted and measured doses for low‐dose implants, and the substantial differences in junction depth found between identical implants in same and opposite type material. The model has further led to the construction of a new correction procedure for spreading resistance measurements which corrects spreading resistance data into a dopant profile taking into account all the effects of beveling, carrier spilling, and the correct mobility values.

Kick-out diffusion of zinc in silicon at 1262 K

Penetration profiles of Zn in Si were recorded with the aid of the spreading-resistance technique after in-diffusion from the vapour phase at 1262 K. The conversion of resistance data into concentrations of substitutional Zn is based on an acceptor level at EV+0.24 eV detected by Hall effect measurements on the authors' Zn-diffused samples. The diffusion behaviour is found to be affected by surface conditions. In dislocation-free Si crystals with damaged surfaces the profile shape as well as the incorporation rate of Zn reveal a self-interstitial-limited diffusivity which depends on C-2. Together with the much larger diffusion constant in highly dislocated samples this provides strong evidence that Zn in Si migrates by the kick-out mechanism.

The current understanding of epitaxial CVD silicon layer doping in the light of modeling and theory development (VII)

AbstractExtension and maximum concentration of autodoping profile are discussed for both lateral and vertical autodoping phenomena and, additionally, bearing in mind the typical course of autodoping profile as obtainable by spreading resistance technique, on the background of previously published theoretical concepts of dopant incorporation. It is shown that the “improved” (three‐step mechanism) as well as the “consequent” dopant incorporation concepts (two‐step mechanism) are suited for theoretically explaining autodoping phenomena. If no additional supposition will be stated, however, it is a consequence of the former that the concentration maximum in the profile of vertical autodoping equals the buried‐layer surface dopant concentration in agreement with non‐steady state layer doping behaviour after dopant source flow has been immediately interrupted. In the contrary the latter conception simply identifies lateral and vertical autodoping effect since autodoping above as well as beyond buried layer is controlled in the same way by parasitie dopant partial pressure in the gas.

Fabrication of CMOS retrograde wells by doping compensation with ion implantation

A process for a retrograde well obtained by partial compensation of doping in the surface region is described. This structure is useful to reduce the latch-up susceptibility in CMOS technology. The process has been designed using the SUPREM program. Sample characterization has been carried out by the spreading resistance technique and by the four-point probe method. Good agreement has been obtained between simulated and experimental values.

Electrical inhomogeneity in 1" diameter partially dislocation-free undoped LEC GaAs

The influence of dislocations on the electrical properties of 1" partially dislocation-free LEC GaAs has been studied as a function of the fraction of melt solidified (g). Microscopic electrical characterisation was performed using spreading resistance and anodisation imaging techniques, and Hall measurements at 294-400 K were employed for macroscopic characterisation. The zero-dislocation-density seed end regions are microscopically uniform with consistently high mobility, resistivity and thermal activation energy. However, dislocated regions are microscopically non-uniform with lower mobilities and activation energies. At g>0.4, carrier concentrations increase due to the presence of a shallow donor, possibly at Ec-0.42 eV, at the dislocations, and this causes large microscopic fluctuations in resistivity. Radial dependences of the electrical parameters vary with dislocation distribution and sub-structure along the crystals, with the mobility having an 'M' shaped radial variation at g=0.5 where dislocations lie in slip bands and a 'W' shape at g approximately 0.97 where the resistivity is low and dislocations form cellular networks. Good correlation of electrical inhomogeneity with slip bands, cell structure and lineage on a microscopic scale is obtained.

Entropy factor of donor level in gold implanted silicon

Rutherford backscattering spectrometry and spreading resistance techniques have been used to determine the concentration profiles of gold implanted on and diffused in p-type silicon. The measurement of the gold concentration is possible by the solution of the charge neutrality condition and thus it requires the determination of the entropy factor X associated with the ionization of the gold donor level. The value of 28±2 compares well with the latest determination of X obtained by using rather complex techniques.

A Study of Atomic and Molecular Arsenic Ion‐Implanted Silicon

The properties of silicon implanted with atomic (As+) and molecular arsenic ions at the equivalent energy of 33 keV/atom are compared using transmission electron microscopy, Rutherford backscattering, secondary ion mass spectrometry, and spreading resistance techniques. Both As+ and amorphize the silicon continuously from the surface to an identical depth, but the projected range, , and range straggling, , are found to be larger for silicon implanted with than for silicon implanted with As+. This suggests that the energy loss of the ion is low with respect to the dissociation energy of the molecule. After furnace annealing, the silicon retains a lower concentration of substitutional arsenic and shows less carrier activity at the surface than the As+‐implanted silicon. The higher concentration of nonsubstitutional arsenic in the silicon enhances diffusion and causes the metallurgical junction to be deeper than the electrical junction. Junction diodes formed by rapid thermal annealing either or As+‐implanted silicon show similarly good I‐V characteristics.

Dopant redistribution in silicon-on-sapphire films during thermal annealing

Arsenic depth distributions in silicon-on-sapphire films, after annealing at 900 °C for a range of times, have been studied by Rutherford backscattering and spreading resistance techniques. A continuum pipe diffusion model is developed which accounts satisfactorily for the rapid diffusion and segregation effects observed. In addition, the same model predicts two-dimensional dopant profiles with several novel features relevant to microelectronic device processing.

Diffusion of gold in dislocation-free or highly dislocated silicon measured by the spreading-resistance technique

The diffusion of Au in dislocation-free or plastically deformed Si (1011 to 1013 dislocations/m2) was measured with the aid of the spreading-resistance technique. The Au profiles produced indislocation-free Si slices by in-diffusion from both surfaces possess nonerfc-type U shapes as predicted by the so-called kick-out diffusion model. This model is used to calculate the contribution of self-interstitials to the (uncorrelated) Si self-diffusion coefficient,D =0.064×exp(−4.80 eV/kT)m2 s−1, from the present and previous data on the diffusivity and solubility of Au in Si in the temperature range 1073–1473 K. Inhighly dislocated Si the diffusion of Au is considerably faster than in dislocation-free Si. From the erfc-type penetration profiles found in this case, effective Au diffusion coefficients were deduced and combined with data on the solubility of Au in Si. ThusC D i=0.0064 ×exp(−3.93 eV/kT)m2 s−1 was obtained in the temperature range 1180–1427 K, whereC andD i are the solubility and diffusivity of interstitial Au in Si.

Diffusion and solubility of copper in germanium

Diffusion profiles and the solubility of Cu in Ge were measured in the temperature interval 850–1200 K by means of the spreading-resistance technique. From these data it is concluded that the diffusion of Cu in Ge involves the interchange between a highly mobile interstitial configuration, Cui, and a practically immobile substitutional configuration, Cus, with the aid of vacancies, V, via the so-called dissociative mechanism, Cui+V⇄Cus. The excellent agreement of the values of the vacancy contribution to the tracer self-diffusion coefficient in Ge, as calculated from our diffusivity and solubility data on Cu in Ge, with directly measured values of the 71Ge tracer self-diffusion coefficient from the literature demonstrates that self-diffusion in Ge occurs via vacancies. A comparison with the mechanisms of Au and self-diffusion in Si is presented.

In-depth spreading resistance characterization of buried insulating layers synthesized by high dose implantation of nitrogen or oxygen ions

In this paper, we present several electrical profiles obtained by the spreading resistance technique on bevelled samples which were implanted with high doses of nitrogen or oxygen ions. These profiles clearly show, in some cases, a surface layer which was not damaged by the ion bombardment and a high resistivity buried layer. The formation of high resistance buried layer was studied by varying experimental parameters such as: — the implanted ion (N + or O + ), — the total implanted ion dose from 1E17 cm −2 to 2E18 cm −2, — the sample temperature during the ion bombardment by varying the current density, — the influence of the annealing after the ion implantation (1200°C; 5 h; N2).