US3223493A - Method of crucible-free zone-melting of semiconductor material - Google Patents

Description

Dec. 14, 1965 w. KELLER ETAL 3,223,493

METHOD OF CRUCIBLE-FREE ZONE-MELTING 0F SEMICONDUCTOR MATERIAL Filed June 9. 1961 United States Patent 3,223,493 METHOD OF CRUClBLE-FREE ZONE-MEETING 0F SEMICONDUCTOR MATERIAL Wolfgang Keller, Pretzfeld, and Herbert Kramer, Forehheim, Germany, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation of Germany Filed June 9, 1961, Ser. No. 115,974 Claims priority, application Germany, June 11, 1960, S 68,896; Nov. 2, 1960, S 71,088 3 Claims. (Cl. 23-301) Our invention relates to a method of producing or processing hyperpure semiconductor material by a crucible-free or floating zone-melting method, such hyperpure simiconductor material being applicable in the production of rectifiers, transistors, photo-diodes, four-layer devices and other electronic semiconductor components.

As a rule, the crucible-free zone-melting is performed by mounting a rod of the semiconductor material, preferably vertically, between two holders which engage the respective ends of the rod. A ring-shaped heating device, in most cases an induction coil, surrounds the semiconduction rod and, during zone-melting operation, travels axially along and over the entire exposed length of the rod while melting an axially narrow zone of the rod located within, and travelling with, the heater coil. In the travelling direction of the molten zone, new semiconductor material is continuously melted at the leading side of the zone, whereas previously melted semiconductor material solidifies at the lagging side. Due to the crystalline structure of the material, the progressing melting and recrystallizing has a purifying effect which eliminates impurities from the rod material and causes it to migrate toward one end of the rod where it can subsequently be romoved by cutting a piece of the rod. The rod is thereafter sliced into Wafers for use in respective semiconductor devices.

Another application of crucible-free zone-melting methods is the growing of monocrystals. For this purpose, a monocrystalline seed is fused to one end of a polycrystalline semiconductor rod, and a melting zone, starting from the fusion point, is passed through the entire length of the semiconductor rod toward the other end, this method being repeated several times if desired.

The crucible-free zone-melting is also applicable for zone levelling. In contrast to the purifying or monocrystal-growing methods, the molten zone is repeatedly passed through the entire length of the rod in both directions so that the concentration of certain impurities, contained in the semiconductor material or added thereto, is uniformly distributed over the entire rod length.

FIG. 1 shows apparatus for crucible-free zone-meltmg.

As a rule, the crucible-free zone-melting is performed within a protective gas atmosphere or in an evacuated vessel. In the latter case, the above-described phenomena are accompanied by an additional purifying effect due to evaporation of foreign impurities from the rod into the vacuum.

While our invention is more specifically described with relation to crucible-free zone-melting of silicon, other semiconductor material, such as germanium and llIV compounds e.g. gallium arsenide are also improved by our invention, which fundamentally involves the above-mentioned principles of the known methods, namely the use of an evacuated vessel in which a rod-shaped semiconductor body is mounted, preferably in vertical position, and in which the molten zone is passed several times along, and over the entire exposed length, of the rod, the passes being either all in the ICC same direction or in opposite directions depending upon the particular purpose.

According to the figure, apparatus for crucible-free zone-melting comprises a vessel composed of hell 31 of relatively great Width which is vacuum-tightly seated upon a bottom plate 32. The bell 31 may consist of steel or other sheet metal and is preferably provided with an observation window 31a of glass. The processing space within the vessel can be evacuated through a nipple 31b to be connected to a vacuum pump. When desired water vapor containing gas may also be supplied through the nipple, or through a separate inlet (not shown) to the vessel. Mounted on the bottom plate 32 of the vessel is a frame structure 21 which carries a holder 22 vertically above another holder 23 that is mounted on the bottom plate 32 and may be fastened to a shaft 25 passing through a seal 33 in the bottom plate in order to permit imparting rotation to the semiconductor material 24 during the zone-melting operation, as is desirable for some purposes.

The semiconductor rod 24, for example of silicon, is held in and between the two holders 22 and 23 and is surrounded by an inductive heating coil 26. During operation of the device, with rod 24 inserted as shown and the processing space evacuated, the coil 26 is energized by electric current and causes a narrow horizontal zone of the rod 24 to melt, while being slowly shifted upwardly or downwardly along the rod thus gradually passing the melted zone through the entire body for the purpose of purifying it, levelling it and/or converting it to monocrystalline condition.

Coolant, such as water, passes through the turns of the heater coil. The entrance and exit of the flowing coolant are indicated in FIG. 1 by arrows 39.

According to our invention, we introduce into the processing vessel for a short interval of time, a water-containing gas, for example air from the environment or steam, prior to commencing the last pass of the melting zone.

We have discovered that by performing the cruciblefree zone-melting in the manner just described, the resulting semiconductor material exhibits greatly improved properties, due to the fact that the lifetime of the minority carriers in the product is considerably increased. We have been unable to fully explain this phenomenon. However, We consider it reasonable to assume that a slight amount of oxygen is built into the resulting crystal lattice and thereby eliminates lattice disturbances, particularly dislocations.

It is impossible to perform the crucible-free zonemelting in the air of the normal atmosphere because at the high processing temperatures (germanium melts at 930 C., silicon at 1430 C.) a disturbance in the course of the process and in the resulting crystal lattice takes place by formation of oxides and other compounds. However, if the process is carried out in high vacuum, using a high-vacuum vessel connected to continuously operating pumps that preserve the necessary vacuum, then any impurities emerging from the semiconductor material are continuously exhausted from the processing space.

For example, when operating with high vacuum, for example of about 10" mm. Hg with a total of 10 passes of the molten zone along a silicon rod, the silicon product can be obtained with a specific resistance of 1000 ohm cm. and an eifective charge-carrier lifetime T (tau of about 300 to 500 seconds. These data relate to the use of starting material consisting of silicon already produced in hyperpure form, for example by pyrolytic precipitation of silicon from the gaseous phase onto a heated silicon core rod. All passes commence at the end of the silicon rod to which the crystal tube was fused,

Patented Dec. 14, 1965 and all phases terminate at the opposite end. Whereafter the heating is reduced to a low temperature at which the semiconductor material can solidify, but the material is still. in glowing condition, and the glowing zone is then passed in reverse direction back to the starting point of the next pass.

It now, according to the invention, prior to performing the last pass of the melting zone, the processing is interrupted and the vacuum vessel filled for a short interval of time with water-containing gas, for example atmospheric air, then the material resulting after completion ofthe subsequent last melting-zone pass exhibits a lifetime T about five timesgreater than otherwise obtained, all other essential properties being not discernibly afiected.

Inpractice, the process can be carried out for example as follows. After the ninth pass of the melting zone, the heating of the melting zone is completely discontinued, for example by de-energizingthe electric induction coil. Then the vacuum vessel is permitted to cool for a short period of time and rinsed with air. Thereafter the vacuum vessel is again closed, sealed'and evacuated. Thereafter the tenth and last pass of the melting zone is performed. Whenever the vacuum vessel is opened'in the course of the process, that is for the purpose of supplying air or upon completing the last pass, it is preferable to provide for a sufficient cooling time of the semiconductor material to prevent the formation of oxide skins. In practice, we found it to be sufiicient if after de-energizing the heating device, a waiting period of approximatelyone minute is observed before opening the vessel. The vessel may then be kept open for about 30 secondsin order to admit the ingress of air or other humid atmosphere after the penultimate zone-melting pass.

When using steam instead of air, the process can be carriedout in a similar manner. That is, after the melting zone has been passed through the rod the desired number of times, the melting zone is permitted to cool, steam is admitted into the vacuum vessel, whereafter the vessel is again evacuated, and ultimately a melting zone is again produced and is passed once through the entire length of the rod. The latter modification is sometimes preferable and simpler in performance because water can readily be produced in the high degree of purity suitable for semicondutor techniques.

The above-describedexamples can be modified in various respects. Thus, the number of zone passes is not critical and can be adapted to the other requirements or desiderata of the process; Critical only is the temporary admission of water-containing gas prior'to the last zonemelting pass. It is of course possible to repeat the justmentioned processing step by opening the vacuum vessel and admitting air or steam. For example, the vacuum vessel could be supplied with water-containing gas after each individual zone-melting pass. However, we have found that no further improvement is obtained in this manner. Hence, we found it to be sufilcient and most economical to admit water-containing air into the vacuum vessel only prior to performing the last zone-melting pass.

It should be noted that if water-containing gas were introduced during the zone-melting pass, there would be continuous electric discharges at the heater coil within the vacuum vessel, which is undesirable since it would' introduce uncontrolled amounts of impurities from the coil and make the rod unsuitable for semiconductor purposes.

We claim:

1. In the method of crucible-free zone-melting of silicon semiconductor material wherein arod-shaped semiconductor bodyis mounted within a vacuum vessel, said vessel being evacuated, and a heater within said' vessel serves to cause a molten zone to pass repeatedly through and along the length of said body being within said vessel, the improvement which comprises the steps of introducing humid air into said vessel after the penultimate zone-melting pass, re-evacuating the vessel and performing the last zone-melting'pass.

2. In the method of crucible-free zone-melting of silicon semiconductor material wherein a rod-shaped silicon body is mounted within a vacuum vessel, said vessel being evacuated, and a heater within said vessel serves to cause a molten zone to pass repeatedly. through and along the length of said body being within said vessel, the improvement which comprises the steps of introducing a water-containing gas into said vessel after the penultimate zone-melting pass, re-evacuating the vessel and performing the last zone-melting pass.

3.. In the method of crucible-free zone-melting of silicon semiconductor material wherein a rod-shaped silicon body is mounted within a vacuum vessel, said vessel beingevacuated, and a heater within said vessel serves to cause a molten zone to pass repeatedly through and along the length-of said body being within said vessel, the improvement which comprises the steps of introducing steam into said vessel after the penultimate zone-melting pass,

re-evacuating the vessel and performing the last zonemelting pass.

References Cited by the Examiner UNITED STATES PATENTS 2,901,325 8/1959 Theurer 23-301 X OTHER REFERENCES Burris et al.: Contribution to the Mathematics of Zone Melting, Argonne National Laboratory, July 7, 1954.

Wertheim et al.: Electron-Bombardment Damage in Oxygen-Free Silicon, Journal of Applied Physics, vol. 30, No. 8, August 1959; pp. 1232-1234.

NORMAN YUDKOFF, Primaly Examiner.

RAY K. WINDHAM, Ekaminer.

Claims (1)

1. IN THE METHOD OF CRUCIBLE-FREE ZONE-MELTING OF SILICON SEMICONDUCTOR MATERIAL WHEREIN A ROD-SHAPED SEMICONDUCTOR BODY IS MOUNTED WITHIN A VACUUM VESSEL, SAID VESEL BEING EVACUATED, AND A HEATER WITHIN SAID VESSEL SERVES TO CAUSE A MOLTEN ZONE TO PASS REPEATEDLY THROUGH AND ALONG THE LENGTH OF SAID BODY BEING WITHIN SAID VESSEL, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF INTRODUCING HUMID AIR INTO SAID VESSEL AFTER THE PENULTIMATE ZONE-MELTING PASS, RE-EVACUATING THE VESSEL AND PERFORMING THE LAST ZONE-MELTING PASS.

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