Zone Refining

Summary

Zone refining is a method for producing ultrapure materials that relies on the same chemical principles as the purification of compounds by recrystallization from the melt. The method utilizes a basic process in which a zone of melted material moves relative to the solid phase. This can be accomplished either by slowly translating the melted zone along the length of a solid bar, as is done for most metals, or by slowly withdrawing a solid phase from the liquid melt, as is done with silicon to produce the ultrapure silicon single crystals for integrated circuits. Zone refining can produce materials containing impurities at levels of only a few parts per billion.

Definition and Basic Principles

Zone refining is a procedure for the purification of materials. When a material containing impurities is melted and allowed to resolidify, the first material to become solid again, or recrystallize, does so without incorporating the impurities. Instead, the impurities remain preferentially dissolved in the liquid state. Zone refining makes use of this principle.

If a relatively small liquid state is maintained and made to travel through the solid mass of the material, like a band of liquid between two regions of solid material, it will accumulate and transport impurities. In practice, the material to be purified is placed in a controlled environment, such as a tube. Beginning at one end of the tube and progressing toward the other, a relatively small segment of the material is heated to melting. The heat source is moved along so that the melted region continuously moves along the length of the sample, which freezes again behind the molten region. A cooling system may be employed to assist in refreezing the material.

By repeating the process, impurities are collected in the molten region and transported to the terminal end of the sample. Each iteration of the process increases the purity of the sample in a mathematically predictable way.

Background and History

The original process of zone melting was developed at Bell Telephone Laboratories, and the basic principles of the methodology were published by William Gardner Pfann in 1952. The basic method was further developed to employ temperature gradients rather than the original hot-cold melt-freeze zones. The details of this methodology were published by Pfann in September 1955.

The basic principle is a modification of the long-used practice of producing crystalline compounds of high purity by melting and cooling a suitable material, rather than allowing the crystals to reform from a solvent-based solution. Both solvent and melt crystallization can be thought of as basic methods to isolate pure compounds from impurities.

In one case, the separate solvent represents and contains the entrained impurities, while in the other case the molten material acts as the solvent to contain the entrained impurities. The method was developed to provide high-purity germanium for semiconductor research following the invention of the semiconductor junction transistor in 1947. It has since been developed into different variations, most notably the floating-zone method used to produce high-purity silicon single crystals.

How It Works

Purification by crystallization is one of the first practical laboratory methods learned by beginning chemists, and it has remained essentially unchanged since it was used by alchemists in the Middle Ages. In the process, an impure material is dissolved in the minimum amount of a heated solvent that will dissolve the material. The key to the success of recrystallization is that the material should have a fairly steep solubility-temperature gradient in the particular solvent being used. That is, it should dissolve well in the heated solvent but only poorly as the solvent cools. Allowing the resulting solution to cool slowly permits highly pure crystals to form while the original impurities remain dissolved in the solvent.

A variation on this method calls for melting the material and allowing the liquid to cool slowly. This method has the same effect as solvent-based crystallization in that the first crystals to form in the molten mass will have the highest purity, while the impurities that were present will be entrained in the last material to solidify. In essence, the molten material becomes the recrystallization solvent. Separating the pure crystals from the impure melt presents a technical difficulty, however, as strict temperature control is required to maintain the molten material without re-melting the newly formed crystals or allowing the impure melt to solidify. The melting process is, thus, typically used to produce high-quality crystals from material that is already pure rather than as a means of separating pure from impure.

Purification by zone refining relies on the same principles of crystal formation as the solvent and melt methods of crystallization. Zone refining was developed as crystallization from the melt with the restriction that only a portion of the material is molten at any given time. In its original form, the method called for placing a material in a relatively long tube. Beginning at one end of the tube, the material at that location is heated to melt in a narrow band. The heating source is moved along the tube, followed by a cooling mechanism. In this way, the molten band travels the length of the solid material in the tube.

Within the molten band, crystallization of higher-purity material takes place while the impurities remain dissolved and accumulate in the liquid phase. After the process has been repeated the required number of times to produce the desired material purity, the impurities will have been transported by the molten phase to one end of the material in the tube. The methodology can achieve purities of just a few parts per billion, or greater than 99.9999995 percent purity.

The success of the method depends on the relative motion of the liquid and solid phases. From this point of view, there is no distinction between a molten phase traveling through a solid phase or a solid phase being drawn out of a liquid phase. Both cases have the same effect on the composition of the material. Floating zone refining uses the method in which a solid phase of extreme purity is drawn out of a liquid phase that retains the impurities present in the material. This is the method now used for the production of silicon single crystals.

Two methods can be used to produce high-purity silicon. One is to distill volatile silicon chlorides to high purity and then react them with hydrogen gas, producing hydrogen chloride and pure silicon. The other is to melt a relatively high-purity form of silicon called polysilicon and slowly draw out a uniform single crystal that forms when a seed crystal is introduced and withdrawn.

The behavior of the material is described by the phase rule—a physicochemical principle that relates the composition of material solutions to their melting points. The purity of a material is characterized by a sharp, distinct melting point. At that specific temperature, the material changes phases from solid to liquid or liquid to solid, almost as one. The higher the purity of the material, the sharper and more distinct its melting or freezing temperature. However, materials that contain impurities melt in a range of temperatures, the span of which increases according to the number of impurities entrained within the material. Some solutions have compositions that are eutectic. That is, at a specific composition, the solution exhibits a sharp melting or freezing point as though it were a single pure compound. Phase theory is fundamental to understanding the process of zone refining.

Applications and Products

Zone refining can be used with any material that exhibits suitable melting behavior. The material must melt without thermal decomposition and not have high volatility in the liquid phase, ideally remaining in the liquid phase (without boiling) over a broad temperature range. If one of these conditions exists, the material will decompose rather than purify or become subject to significant loss of mass to the vapor phase. This would, in turn, decrease the purity of the remaining material by effectively increasing the relative amounts of impurities in the remaining liquid phase. In addition, if the process is being carried out in a closed system, generating a significant vapor phase could have catastrophic results.

The various methods of zone refining are used for one purpose only—to provide materials of extreme purity for other applications. Zone refining is an energy-intensive methodology and tends to limit its general applicability to materials that are required only in small quantities or that have a high associated value. Therefore, the primary uses of zone refining are limited to producing research-grade compounds or highly desirable materials, such as silicon.

Research-Grade Materials. It is important to understand that the methodology of zone refinement is applicable at all temperatures. All that is required is that a small liquid phase be generated and made to travel in an opposite relative direction to the remaining solid phase. For the vast majority of research-grade materials, zone refinement procedures are a last resort.

Chemical compounds routinely used in laboratory procedures are simply not required to have the level of purity that can be achieved through zone refinement. In most cases, the desired purity is readily achieved through other means. High-purity materials such as spectroscopic grade solvents at 99.999 percent purity are readily obtained through repeated fractional distillation and other processes, while reagent grade materials are acceptable at 99.9 percent purity. The requirement for purities obtained through zone refinement only applies to materials of specialist interest and for reference data. For reference spectral data, it is desirable to have spectra that reflect only the compound of interest, with as little background interference from contaminants as possible. Similarly, the characterization data for the material should be influenced as little as possible by entrained impurities.

The physical property values of materials provide empirical data for the verification and refinement of theory. The greater the purity of a material, the more precisely its corresponding characteristic properties will refine foundational theory. For many materials, particularly elemental materials, zone refinement provides an entirely new avenue of study and research, one that focuses on the innate nature of the material.

Ironically, zone refinement is a method of choice for use with thermoplastic polymers. In the polymerization process, many separate polymerization chain reactions occur simultaneously. Each proceeds until the reactive end randomly encounters something that causes the chain reaction to terminate, usually an impurity or contaminant. This randomness produces polymer molecules whose chain lengths and molecular weights are randomly distributed around some average value. Thermoplastics have the property of melting when heated. Thermosetting polymers harden and solidify when heated. This makes thermoplastics amenable to zone refinement methods of isolating components with a narrower distribution of molecular weights and chain lengths.

High-Value Materials. Apart from certain chemical compounds that are required in high-purity form, the main application of zone refinement is the production of silicon single crystals for use in integrated-circuit chips. The process requires that a great deal of care be taken so that the resulting material meets the desired specifications.

The material known as polysilicon is typically stacked by hand into the furnace of a device called a crystal puller, along with a small, specified amount of a dopant material (to provide the desired electrical properties to the silicon). The unit is then sealed and purged to eliminate oxygen and other contaminant gases. The polysilicon mass is melted as the crucible is heated to the desired temperature. As the crucible rotates in one direction, a seed crystal attached to a counter-rotating armature is introduced into the melt to begin the formation of the single crystal. The growing crystal is slowly withdrawn from the melt, building a uniform cylindrical shape.

When completed, the crystal is allowed to cool, then checked by Fourier transform infrared spectroscopy for quality, and passed on for slicing into thin wafers, which will become the substrate of the integrated circuits etched onto their highly polished surfaces. Different manufacturers use minor variations of this procedure, such as the floating zone method, in which the crystal grows downward according to the flow of the melt rather than being pulled up from it. It should be noted that the purpose of the method in this application is not to purify the silicon but to produce a single crystal from a mass of silicon that has been purified by other means.

Social Context and Future Prospects

Silicon-based electronics are expected to continue reigning as the benchmark of modern technology. Zone refining of silicon to provide the basic material for that electronic technology will, therefore, continue to be an essential process across industries.

At the cutting edge of research are new materials that promise the development of new transistors that will displace the silicon-based transistor as the workhorse of modern electronics. With that change, the need to produce high-purity silicon and single crystals will decrease dramatically. Paradoxically, beginning in the mid-2010s, the demand for high-purity precious metals among traders and investors increased. Zone refinement is the most expedient way of obtaining metals greater than 99.99 percent purity, making it the preferred method for preparing these metals. As the demand for precious metals increases, it is also likely that the desired purity level will continually increase.

Zone refinement research and technology development are critical in meeting increased demand. Scientists aim to create higher levels of purity in shorter times, at lower costs, and through relatively straightforward processes. Combining zone refining with electromigration or vacuum degassing has been proposed as a way to achieve higher purity. To quickly and accurately measure such purity, trace element analysis technology must evolve to continuously or concurrently measure ultra-trace multi-elements.

In early 2024, German scientists used a novel zone-refining setup to refine germanium to the highest quality ever achieved using zone refining. Results were comparable to the Czochralski method. The low-temperature-gradient zone refining setup uses a single crystalline. A copper coil and a susceptive induction heater made of a ferric iron-chrome-aluminium alloy (Kanthal APM) are electrically isolated. The heater can withstand temperatures of 1250 degrees Celsius (2282 degrees Fahrenheit), and the copper coil allows scientists to better control temperatures—a great advancement in the field.

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