A. A. Istratov

Physics of Copper in Silicon

This article reviews the progress made in the studies of copper in silicon over the last several years and puts forward a comprehensive model of the behavior of copper in silicon. Technical aspects of this model are discussed in detail. It is shown that many important aspects of the behavior of copper in silicon are not shared with the other 3d transition metals. The positive charge state of interstitial copper makes its defect reactions Fermi-level-dependent, and results in a noticeable difference in the out-diffusion and precipitation behavior of copper in n-Si and p-Si. The extremely high diffusivity of copper in silicon, which is a consequence of the small ionic radius of copper and its relatively weak interaction with the silicon lattice, makes it highly mobile at room temperature and impacts the stability of copper complexes. Large lattice strains and electrostatic effects in p-Si make the formation of copper-silicide precipitates in the hulk energetically unfavorable, unless the chemical driving force for precipitation is high enough to overcome the nucleation and precipitation harrier. Literature data on the effect of copper on minority carrier lifetime and device yield are analyzed using our improved understanding of the physics of copper in silicon. Finally, the impact of the physics of copper in silicon on the development and characterization of copper diffusion barriers is discussed.

Electrical and Recombination Properties of Copper‐Silicide Precipitates in Silicon

Copper-silicide precipitates in silicon obtained after copper diffusion and quench in different liquids were studied by transmission electron microscopy and capacitance spectroscopy techniques. A correlation between the quenching rate, geometric size, and deep level spectra of the copper-silicide precipitates was established. The unusually wide deep level spectra are shown to be due to a defect-related band in the bandgap. The parameters of the band are evaluated using numerical simulations. a positive charge of copper-silicide precipitates in p-type and moderately doped n-type Si is predicted by simulations and confirmed by minority carrier transient spectroscopy measurements. Strong recombination activity of the precipitates due to attraction of minority carriers by the electric field around the precipitates and their recombination via the defect band is predicted and confirmed by the experiments. The pairing of copper with boron is shown to be an important factor determining the precipitation kinetics of the interstitial copper at room temperature.

Structural and Electrical Properties of Metal Silicide Precipitates in Silicon

This paper summarizes current understanding of structural and electronic properties of metal silicide precipitates in silicon and their interrelation. Combined studies of high-resolution transmission electron microscopy and deep level transient spectroscopy together with numerical simulations show that the bounding dislocation of nickel silicide platelets is the key to understand their rapid growth and electrical properties. Different misfit relaxation phenomena govern the structural evolution of copper silicide precipitates from their early stages to the well-known colony growth. This evolution involves different types of secondary defects indicating that the deep band-like states observed throughout this process are associated with the silicide precipitates themselves.

Recombination activity of copper in silicon

The carrier recombination activity of copper in n-type and p-type silicon has been investigated. The minority carrier diffusion length has been found to decrease monotonically with increasing copper concentration in n Si and to exhibit a step-like behavior in p-type silicon at Cu concentrations above a certain critical level. It is suggested that the impact of copper on the minority carrier diffusion length is determined by the formation of copper precipitates. This process is retarded in perfect silicon due to the large lattice mismatch between Cu3Si and the silicon lattice and even more retarded in p Si, due to electrostatic repulsion effects between the positively charged copper precipitates and interstitial copper ions. Comparison of the impact of Cu on minority carrier diffusion length obtained with p-Si samples of different resistivity confirmed the electrostatic model. Studies of the impact of copper on minority carrier diffusion length in samples with internal gettering sites indicated that they p...

Direct strain measurement in a 65 nm node strained silicon transistor by convergent-beam electron diffraction

Using the energy-filtered convergent-beam electron diffraction (CBED) technique in a transmission electron microscope, the authors report here a direct measurement of the lattice parameters of uniaxially strained silicon as close as 25nm below the gate in a 65nm node p-type metal-oxide-semiconductor field-effect transistor with SiGe source and drain. It is found that the dominant strain component (0.58%) is compressive along the source-drain direction. The compressive stress is 1.1GPa along this direction. These findings demonstrate that CBED can serve as a strain metrology technique for the development of strained silicon device technology.

Formation and Properties of Copper Silicide Precipitates in Silicon

We report results of a detailed study of structural and electrical properties of copper silicide precipitates in silicon. Using conventional and high-resolution transmission electron microscopy: we observe that metastable platelets surrounded by extrinsic stacking faults form upon quenching from high temperatures. By ripening experiments at low temperatures as well as by a variation of cooling rates it is shown how homogeneous copper precipitation merges into the heterogeneous precipitation mode of colony growth. The application of recently developed criteria for the interpretation of deep level transient spectra from extended defects allows to conclude that deep electronic states associated with the precipitates have bandlike character.

Diffusion, solubility and gettering of copper in silicon

Abstract The feasibility of quantitative predictive modeling of gettering of Cu in silicon, which requires quantitative understanding of its diffusivity and precipitation behavior, is discussed. Investigations of diffusion of Cu at low temperatures enabled us to determine the pairing constants of copper with boron, re-evaluate its diffusivity at room temperature in p-Si, and to predict its diffusivity in p + -Si substrates. We demonstrate that copper may either precipitate in the bulk of the wafer or diffuse to its surface, depending on the position of Fermi level in the sample. It is suggested that the Fermi level position determines the sign and magnitude of the electrostatic charge on the growing copper precipitates, and thus enhances or suppresses precipitation of interstitial copper ions. Modeling of p/p + segregation gettering of copper shows that while the copper can be gettered in p + layer during or after high-temperature anneals, it eventually will be released and will precipitate in the device region within the first few months of operation, unless more stable gettering (precipitation) sites for copper are utilized. An n-type layer is predicted to be an effective gettering site.

Gettering of iron by oxygen precipitates

In order to better understand and model internal gettering of iron in silicon, a quantitative investigation of iron precipitation in silicon containing different oxygen precipitate densities was performed. The number of iron precipitation sites was obtained from the iron precipitation kinetics using Ham’s Law. At low temperatures, the iron precipitate density corresponded to the oxygen precipitate density. A strong temperature dependence of the iron precipitate density was observed for the samples with larger oxygen precipitate densities. These data were used to simulate iron precipitation during a slow cool. From those simulations, optimal cooling rates were obtained for different silicon materials assuming various iron precipitation site densities in the epitaxial layer.

Gettering simulator: physical basis and algorithm

The basic physical principles and mechanisms of gettering of metal impurities in silicon are well established. However, a predictive model of gettering that would enable one to determine what fraction of contaminants will be gettered in a particular process and how the existing process should be modified to optimize gettering is lacking. Predictive gettering of transition metals in silicon requires development of a robust algorithm to model diffusion and precipitation of transition metals in silicon, and material parameters to describe the kinetics of defect reactions and the stable equilibrium state of the formed complexes. This paper describes the algorithm of a gettering simulator, capable of modelling relaxation and segregation gettering of interstitially diffusing transition metal impurities in silicon wafers. The basic physical equations used to model gettering are differential equations for diffusion, precipitation and segregation. These equations are solved using the implicit finite-difference algorithm, based on the underlying physics of the problem. The material parameters required as input for the gettering simulator such as segregation coefficient, precipitation site density and precipitation radius, which need to be obtained experimentally, are briefly discussed.

Interstitial copper-related center in n-type silicon

n-type silicon samples were measured by deep level transient spectroscopy (DLTS) immediately (within one hour of storage at room temperature, required for the preparation of Schottky-diodes) after copper diffusion and quench. A donor level at Ec-(0.15±0.01) eV with a concentration of up to 1013 cm−3 was detected. The amplitude of the DLTS peak decreased with the time of storage at room temperature, and stabilized at a concentration (4 to 7)×1011 cm−3 after 15–20 h. The activation energies and prefactors of the decay of the DLTS peak in n-type Si and the reactivation of copper-compensated boron in p-type Si concur. This correlation suggests that the deep level is interstitial copper itself or a complex of interstitial copper.