Czochralski Crystal Puller Research Articles

Effect of necking on Czochralski-grown LiF crystals and its influence on crystalline perfection and the correlated physical properties

A bulk single crystal of lithium fluoride (LiF) was grown using an in-house-developed Czochralski crystal puller. The effect of necking on crystalline perfection, which in turn was found to influence the optical and dielectric properties, was studied. The crystalline perfection of the grown crystal was assessed along the length of the boule using high-resolution X-ray diffraction (HRXRD), and it was found that the grain boundaries that were present in the seed crystal were stopped gradually from propagating into the bulk crystal by necking. The UV–vis transparency of the crystal region having best crystalline perfection was found to be higher. Photoluminescence (PL) spectra revealed that a crystal region that was found to contain vacancy (point) defects by HRXRD yielded the maximum PL intensity because of color centers at the vacancies. The dielectric properties were also studied over a wide range of frequency.

Visualization of Large-scale Atomic Interactions During the Melting and Crystallization Process

Atomic-scale material model capable of melting, crystallization and amorphization has been developed to examine the defect formation and crystal growth processes from melted silicon (Si) based on the ordinary Langevin equations of motion. The developed computer system consists of simulation and visualization part. Simulation supports the large-scale molecular-dynamics (MD) clusters with solid/liquid interface responding interactively to the control parameters such as the temperature gradient and pulling speed. Material behaviour simulation is limited to 104 particle objects representing different atoms. A particle in proposed dynamic system interacts through attractive covalent forces and short-range repulsion forces in all three dimensions. This research was conducted to understand the processes that can control the quality of single-crystal Si grown from the melt by Czochralski crystal puller.

Full-scale experiments on solid-pellets feed continuous Czochralski growth of silicon crystals

Two long-term solid-pellets feed continuous Czochralski growth experiments were performed in an industrial Czochralski crystal puller as an extension to our previous work [7]. The goals of these experiments were to examine how polysilicon pellets would melt in a standard Cz system, to discover the thermal effects the pellets would have on the overall melt, and to find if pellet addition could be an effective melt replenishment technique. These experiments demonstrate that the quality of the melt for the CCz growth is based heavily on the surface temperature of the melt. A novel characterization method (“impact severity”) is developed to characterize the quality of the CCz melt. Stable feed rate and melt conditions were achieved for three different pull rates. These experiments demonstrate that the process is technically feasible, and can be retrofitted to the existing industrial systems. Several critical issues that need to be addressed to develop a successful CCz process are also discussed.

Measurement of flow phenomena using the ultrasonic velocity profile method in a simulated Czochralski crystal puller

An experimental investigation of flow phenomena in a simulated Czochralski (CZ) crystal puller was conducted using an ultrasound velocity profile (UVP) probe to measure fluid velocities. To isolate the various forces that influence the fluid motion, only the crystal was rotated and a water-glycerol mixture was used as the test fluid in this experiment. Measurements of the velocity components, V z and V r , under the initial transient from zero rotation to a set rotation rate as well as under steady-state conditions were made. The corresponding Reynolds number based on the crystal radius and rotation varied from Re Ω , x ≈ 660–5000 (Ω x ≈ 110–2000 rpm). The measured velocity profiles, V z and V r , were processed using a simple color image scheme based on the magnitude and direction of flow with respect to the probe. These profiles were studied to postulate flow patterns in the simulated CZ device. The measurements revealed three major regions of flow: (1) one beneath the crystal, (2) one near the free surface and crucible wall and (3) one beneath the crystal and occupying the extent of the crucible. In the initial transient, spatially symmetric outward flow beneath the crystal developed until the recirculating flow from below disturbed this pattern. At this point the apparent shear between the flow beneath the crystal and that circulating upwards from the bottom of the crucible induced flow instabilities and a non-axisymmetric flow pattern. The non-axisymmety was verified by the color-coded velocity profiles and a power spectrum analysis. The analysis revealed the critical Reynolds number for the onset of the primary instability to be Re Ω crit ≈ 669. The linear variation in the power of the primary peak with the Reynolds number indicates that the onset of the axisymmetry-breaking instability is due to Hopf bifurcation.

Melt motion in a Czochralski crystal puller with a non-uniform axisymmetric magnetic field: isothermal motion

The use of magnetic fields during the growth of semiconductor crystals from the melt in a Czochralski (CZ) crystal puller has shown promise in controlling the heat and mass transport to the growth interface. The magnetic field suppresses turbulence and thermal convection in the melt in which large thermal gradients are present, thus improving the quality of the crystal. In this paper, analytical solutions are presented for the isothermal melt motion and electric current density driven by the differential rotation of the crystal and crucible about their common vertical axis. There is an applied, non-uniform, axisymmetric magnetic field with only radial and axial components which are independent of the azimuthal coordinate. The melt motion with a uniform axial magnetic field represents a singular limit of the flow considered here: as the radial magnetic field component goes to zero, the radial and axial (meridional) velocity components decrease in magnitude by a factor of M-1, where M is the large Hartmann number. The uniform axial field is a singular limit because the centrifugal acceleration due to the azimuthal velocity is exactly perpendicular to the magnetic field. Since the radial isothermal motion near the growth interface controls the radial distributions of dopants and impurities in the crystals, a non-uniform axisymmetric magnetic field is better than the uniform axial field. In addition, the axisymmetric field avoids the detrimental deviations from axisymmetric heat and mass transport associated with a uniform transverse (horizontal) magnetic field.Two classes of shaped fields are considered, with only one class leading to the presence of the large meridional flow driven by differential rotation. The small electrical conductivity of the crystal plays an important role in determining the behaviour of the melt's angular velocity, which is constant along each magnetic field line. Results for two simple field configurations are presented in order to illustrate the effect of the field configuration on the nature of the meridional circulation and the potential for flow tailoring with the shaped field.

Oxygen in Czochralski silicon crystals grown under an axial magnetic field

Results are reported of an experimental study of axial magnetic field effects (including the effect of a weak magnetic field) on oxygen in 105–155 mm diameter silicon crystals grown in a conventional Czochralski crystal puller and in a puller with a double-crucible arrangement. The oxygen concentration and distribution in the crystals depend upon the conditions of oxygen-rich melt transport from the crucible wall to the melt-crystal interface. A regime of low oxygen (down to 2×10 17 cm -3), large diameter crystal growth has been found. The features of oxygen distribution in the crystals are qualitatively explained.

A thermal model for czochralski silicon crystal growth with an axial magnetic field

This paper presents a thermal model for molten silicon in a Czochralski crystal puller system with an applied uniform axial magnetic field. The melt depth is treated as continually decreasing, which affects the thermal environment of the melt and crystal. The radiative heat loss and the input heat flux are treated as functions of time, with a constraint placed on the heat lost to the crystal from the melt. As the melt motion reaches a steady state rapidly, the temperature and flow fields are treated as instantaneously steady at each melt depth. The heat transport is a mixture of conduction and convection, and by considering the crystal and crucible to be rotating with the same angular velocity, the flows driven by buoyancy and thermocapillarity are isolated and provide the convective heat transport in the melt for the range of magnetic field strengths 0.2 ≤ B ≤ 1.0T.

Mass transport in a Czochralski crystal puller with an axial magnetic field: Melt mouton due to crystal growth and buoyancy

The mass transport of dopants and impurities in a silicon melt is considered for a Czochralski crystal puller with a uniform axial magnetic field. For magnetic field strengths 1≤ B O ≤5 T, the dominant flow fields are those driven by buoyancy and the growth of the crystal (the pulling of the crystal and the raising of the crucible bottom). The mass transport is intrinsically unsteady, and the concentration of a crystallizing particle depends on the entire history of the particle. The buoyancy driven flow provides a mechanism for the recirculation of fluid particles. However, the character of the flow in the diffusion boundary layers and the convection dominated cores depends on the competition between the crystal growth and buoyancy driven velocities, which in turn depends on the magnetic field strength and depth.

Melt motion in a Czochralski crystal puller with an axial magnetic field: motion due to buoyancy and thermocapillarity

Melt motion in a Czochralski crystal puller with an axial magnetic field: motion due to buoyancy and thermocapillarity

Modelling of melt motion in a Czochralski crystal puller with an axial magnetic field

Abstract The use of matched asymptotic expansions provide analytical solutions for the bulk flow in a Czochralski crystal puller in a strong axial magnetic field. Treating the crystal as a slight electrical conductor alters the radial and axial flows driven by centrifugal pumping. The motion due to buoyancy and thermocapillarity are found by considering the temperature as a known function and solving the non-linear heat equation numerically for different magnetic field strengths and melt depths. This note presents a summary of the analysis and results that are detailed in two papers.

Melt motion in a Czochralski crystal puller with an axial magnetic field: isothermal motion

A magnetic field suppresses turbulence and thermal convection in a Czochralski crystal puller. The amounts and distributions of dopants and oxygen in the crystal are determined by the motion of the molten silicon during crystal growth. This paper presents analytical solutions for the melt motion in a Czochralski puller with a strong, uniform, axial magnetic field. The relatively small electrical conductivity of the crystal plays a key role in determining the flow. Certain combinations of crystal and crucible rotation rates lead to flow patterns with a large volume of almost stagnant fluid under most of the crystal face. The values of these rotation rates depend on the magnetic field strength.

Purification of metallurgical-grade silicon by slagging and impurity redistribution

The purification of metallurgical-grade silicon (MGSi) was carried out by slagging and impurity redistribution through repeated melting and pulling. A Czochralski crystal puller was used for performing both functions. The slags consisting of CaOSiO 2 and CaOMgOSiO 2 were found to be effective for extracting aluminum from the MGSi melts. The repeated melting and pulling method was effective for the removal of other metallic impurities. When slagging and multiple pulling were combined, the thrice-pulled ingots contained an impurity concentration equivalent to that of semiconductor-grade silicon with the exception of boron and phosphorus.

A Czochralski crystal puller automated by the weighing method

AbstractThe automated crystal growing equipment makes use of a commercial electronic balance equipped with a microprocessor. The mode of operation is explained and experiences got on the occasion of crystal growth experiments are presented.

Origin and Control of the Dominant Impurities in High-Purity Germanium

Techniques have been developed to consistently purify germanium to the 1010 atoms/cm3 range. The major impurities are the acceptors boron and aluminum and the donor phosphorous. Formation of nonsegregating compounds of boron and aluminum make these elements difficult to remove by conventional zone refining whereas phosphorous and all other electrically-active impurities segregate normally and are therefore removed in the zone refining process. The only significant impurity introduced in our Czochralski crystal puller is phosphorous whose source has been traced to the quartz crucible. To reliably reduce impurity concentrations to acceptable levels it has been necessary to develop techniques for determining the types and concentrations of impurities in both the final single crystals and in the polycrystalline zone-refined ingots and to relate these results to the use of various boat and crucible materials. Photothermal Ionization Spectroscopy and electrical measurements have been used to identify and measure the impurities.

Pulling of gallium phosphide crystals by liquid encapsulation

A Czochralski crystal puller has been built to grow crystals of gallium phosphide by the liquid encapsulation technique. With suitable starting material large crystals can be easily grown. Such crystals are only lightly contaminated by the crucible and boric oxide encapsulant and have electrical properties similar to those of epitaxial vapour grown material.