US3268301A

US3268301A - Method of pulling a semiconductor crystal from a melt

Info

Publication number: US3268301A
Application number: US327556A
Authority: US
Inventors: Rummel Theodor; Dorner Jorg
Original assignee: Siemens AG
Current assignee: Siemens and Halske AG; Siemens AG
Priority date: 1962-12-03
Filing date: 1963-12-02
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23
Also published as: NL301226A; DE1244113B; GB1013064A; CH406160A; SE310356B

Description

Aug. 23, 1966 RUMMEL ETAL 3,268,301

METHOD OF PULLING A SEMICONDUCTOR CRYSTAL FROM A MELT Filed D60- 2, 1963 2 Sheets-Sheet 1 Al lg. 23, 1966 RUMMEL ETAL METHOD OF PULLING A SEMICONDUCTOR'CRYSTAL FROM A MELT 2 Sheets-Sheet 2 Filed Dec. 2, 1963 Fig.3

United States Patent 3,268,301 METHOD OF PULLXNG A SEMICONDUCTOR CRYSTAL FROM A MELT Theodor Rummel, Munich, and Jiirg Dorner, Dachau,

Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin, Germany, a corporation of Germany Filed Dec. 2, 1963, Ser. No. 327,556 Claims priority, application Germany, Dec. 3, 1962, S 82,693 6 Claims. (Cl. 23-301) Our invention relates to the method of producing semiconductor crystals by pulling them from a molten mass of the semiconductor material.

As a rule, such methods are performed by contacting a monocrystalline seed of the semi-conductor material with a melt heated to a temperature a few degrees above the melting point and having the tip of the seed immersed in the melt until an equilibrium between melt and crystal has come about, whereafter the seed is pulled out of the melt at such a speed that the semiconductor material adhering to the seed will crystallize .onto the seed. According to a different method of this general type, the seed is pulled out of a melt while the latter is being suspended in a crucible-free or floating manner. To this end, the top of a vertically mounted rod of semiconductor material is meltedby induction heating and thereafter contacted by a monocrystalline seed. As soon as the seed is wetted by the melt, the molten zone is caused to travel, starting from the seed and progressing in the direction of the rod axis. As a result, molten semiconductor material will freeze and crystallize at the seed.

When performing such methods, it is often found that the crystals thus pulled leave much to be desired as regards the perfection required for electronic semiconductor techniques and devices. Such faults as lattice dislocations and tothe disturbances in crystalline structure are observed and may greatly impair the subsequent fabrication or use of the crystals needed for transistors, diodes and other semiconductor devices.

Essential for securing crystalline perfection of the pulled crystals are the symmetry of the heating zone, as well as a certain after-heating zone, and the shape of the solidifying front, that is, the shape of the isotherms in .the region of the still plastic portion adjacent to the solidifying front in the just frozen material.

It has been proposed to better approach crystalline perfection by passing the pulled crystal through a zone that reduces the radial temperature gradient in the rod, and to thereafter pull the crystal through a zone that increases the axial temperature gradient.

According to another proposal, the shape and position of the isotherms in the pulled crystal are influenced by adjusting for a given rod diameter the pulling speed to a given value. This expedient, however, cannot be employed if the rod is to possess a given constant dopant concentration over its entire length because in this case the pulling speed is already fixed by the desired dopant concentration. I

It is an object of our invention to devise a possibility of improving the crysal perfection also in cases of the latter type, namely regardless of whether or not the pulling speed is predetermined by other requirements or desiderata.

According to our invention, we perform the pulling of a semiconductor crystal from a melt with the aid of a crystal seed by proceeding in the following manner. During the pulling operation we pass an electric direct current through the melt and the crystal being pulled and thereby subject the freezing region of the growing crystal to the so-called Peltier effect dependent upon the currentflow direction and current intensity. We have discovered 3,268,301 Patented August 23, 1966 ice that in this manner the lattice disturbances in the pulled crystal can be considerably reduced. This method is applicable with melts held floating, that is, suspended without the use of a crucible, as well as with melts located in a crucible. However, the invention is preferably and most readily practiced with a crucible-free crystal-pulling or monocrystal-forming zone-melting operation.

The passage of electric current produces heating or cooling by the Peltier effect not only at the boundary face between different contact materials but also at the boundary face between the solid and liquid phase of one and the same conducting material, particularly semiconductor material. For example silicon and germanium are known to exhibit a positive Peltier effect; that is, heat is absorbed at the boundary face between the solid and liquid phases of these substances if the solid phase is conected to the positive pole and the liquid phase to the negative pole of a direct-voltage source, whereas the reverse poling causes the Peltier effect to produce heating at the other boundary face. Consequently, when a germanium crystal, for example, is subjected to zone melting with the melt held between two solid crystal pieces, and a direct current is passed between these two pieces, the boundary face between one of the crystal pieces and the melt is subjected to cooling when positive poling is applied, whereas with negative poling of the crystal piece the other boundary face becomes heated.

When a crystal is being pulled out of a melt, a radial temperature gradient develops in the pulled crystal, the solidifying front and the isotherms being curved in the crystal being pulled. The molten zone, produced in most cases either by an induction coil or by radiation from the outside, possesses a higher temperature in the marginal zone of the crystal than in the center region, the solidification of the crystal commences from the center region of the rod, and the solidifying front is curved from the seed toward the melt. At some distance from the solidifying front, the heat radiation of the crystal surface preponderates over the original temperature difference between the marginal zone and the center region, so that here the thermal conditions are reversed; the isotherms are then curved toward the seed. In the pulled crystal, therefore, the isotherms, starting from the concavely curved solidifying front, first become shallow and then, at some distance from the front, convert to a convex curvature.

The radial temperature gradient in the crystalliz-ing semiconductor material as it is being pulled out of the melt is the larger, the more strongly the isotherms are curved. A planar or weakly convex solidifying front, therefore, is particularly favorable if dislocations in the resulting crystal are to be prevented. This is because in this case the slightest thermal tensions occur in the immediate vicinity of the solidifying front and hence in the region in which the semiconductor material is still plastic.

The shape of the solidifying front and of the isotherms can also be modified by adjusting the pulling speed. For each crystal there can be found a pulling speed at which the most favorable shape of the solidifying boundary face occurs; however, as mentioned, the pulling speed in most cases is already fixed by the desired constant degree of doping. In this case the invention provides for a new controlling parameter. That is, the heating or cooling occurring as a result of the Peltier effect in dependence upon current intensity and current direction, can in any case, and consequently for any pulling speed and any desired crystal diameter as well as for any material, be so adjusted that the solidifying front assumes the most favorable shape, and the occurring Joules heat simultaneously reduces the occurring tensions. If it is desired to still further increase the resulting tension-reducing heating, it is in some cases of advantage to superimpose an alternating current upon the direct current. The alternating current does not result in a Peltier effect, but it heats by Joules heat the semiconductor material, particularly the solid crystal piece. That is, by employing a superimposed alternating current, the amount of Joules heat can be given a desired dosage independently of the Peltier heat.

The invention will be further described with reference to the accompanying drawings in which FIG. 1 shows schematically a portion of a semiconductor rod which is being processed by crucible-free zone melting without utilization of the Peltier effect;

FIG. 2 shows a corresponding portion of a semiconductor rod also processed by floating-zone melting but simultaneously subjected to the Peltier effect in accordance with the present invention; and

FIG. 3 shows schematically and in section a device for performing the method of the invention.

As shown in FIG. 1, a molten zone 3 is located between an upper rod portion 1 and a lower rod portion 2, the zone 3 travelling in the downward direction so that the upper rod 1 is grown in form of a monocrystal out of the melt. The direction of crystal growth is indicated by the arrow 4, this direction being identical with the downward travel direction of the molten zone 3. Shown schematically in the crystallizing rod portion 1 are a group of lines of which the one denoted by 5 represents schematically the solidifying front, whereas the other lines indicate isotherms. The isotherms denoted by 6 have the shape most favorable for the solidifying front as well as for the isotherms adjacent to the solidifying front, if the resulting crystal is to be as free of dislocations as feasible.

By virtue of the invention, namely by subjecting the growing crystal as well as the melt to the Peltier effect with the aid of direct current, the position of the solidifying front relative to the heating device can be changed. That is, the solidifying f-ront can be displaced as desired in the direction of the crystal axis, and this can be done in the sense promoting the desired reduction in dislocation density. That is, by heating one of the solid-liquid boundary faces, for example the solidifying front, with the aid of the Peltier effect, requiring, for example for germanium or silicon, a negative poling of the solidified crystal and hence a current-flow direction opposed to the zone-pulling direction, the solidifying front becomes displaced in the direction of the crystal being pulled. By applying a given current intensity or density, readily ascertainable by pretesting for each given diameter and each pulling speed, the solidifying front can thus be displaced up to the isotherm face at which the smallest thermal tensions will occur.

For example, when producing a silicon monocrystal having a diameter of about 10.5 mm. from a floating melt at a pulling speed of approximately 2.5 mm./minute, a current density of approximately 100 amps per cm. has been found to be particularly favorable. Approximately the same current density is preferably employed for somewhat different crystal diameters and somewhat different pulling speeds, for example diameters of to 11 mm. and speeds of 2 to 3 mm. per minute. During the tests made, the crystal was kept in rotation about its own axis at the rate of 60 revolutions per minute. The smallest dislocation density is obtained by having the current flow in opposition to the pulling direction, that is, when the crystal piece being pulled out of the melt is connected to the negative pole of a direct-voltage source. The other boundary face of the molten zone is subjected to cooling by Peltier effect, so that here, too, the relative position of the solid-liquid boundary face is displaced in the direction of the rod axis toward the heating zone.

The resulting conditions are schematically shown in FIG. 2. The crystal 1 growing in the direction of the arrow 4 exhibits a particularly slight dislocation density. The isotherms 6 near the solidifying front 5, as well as the solidifying front itself, are slightly convex toward "the melting zone 3, so that only slight thermal tensions occur in the portion of the crystallizing rod that is adjacent to the solidifying front and is still plastic. The direct current is passed through the crystal in the direction of the arrow 7. For this purpose, the crystal piece 1 crystallizing out of the melt is connected to the negative pole of a direct-voltage source. The boundary face is subjected to heating, and freezing takes place only at a larger distance from the heating device 8. The conditions are reversed at the other boundary face. This boundary face is cooled by the Peltier effect so that the source rod 2 is melted later than would otherwise be the case.

FIG. 3 shows a rod 1, 2 and a heater 8 according to FIG. 2 in conjunction with suitable processing apparatus. While the molten zone 3 is being heated by the highfrequency induction coil 8, a direct current is passed longitudinally through the rod from a source 9 of constant voltage. The source 9 is connected through an inductance winding 10 and a current-control resistor 11 between

terminals

12 and 13 conductively connected with respective holders 14 and 15 of highly pure graphite in which the two ends of the rod are fastened by means of graphite clamping screws 19. Simultaneously impressed across the

terminals

12 and 13 is an alternating voltage from a source 16 through a control resistor 17 and a capacitor 18.

During operation, the semiconductor crystal 1 grows in the direction of the arrow 4 as the heater coil 8 moves downwardly. The direct current passes through the rod in the upward direction indicated by an arrow 7. That is, the rod portion 1 recrystallizing out of the melt 3 is connected to the negative pole of the direct-current source 9. The semiconductor rod and the holders 14, 15 are mounted in a tubular processing vessel 20 whose walls consist of quartz and which is provided at its respective ends with inlet and outlet ducts for protective gas such as argon, the gas flow being indicated by arrows 21. The current-supply leads pass through respective seals 26 at the top and bottom of the vessel. The high-frequency coil 8 is fastened to a vertically displ'aceable support 22 which passes through a glide seal to the outside of the vessel. A rack 24 on support 22 meshes with a pinion 25 which is slowly driven during operation for displacing the heater coil in the direction along the semi-conductor rod. The rod portion 1 crystallizing out of the melt can be kept in rotation by turning the upper holder 15 or its shaft from the outside of the vessel, this being indicated by an arrow 27.

As an example of applicable operating data, the following are mentioned. A rod of phosphorus-doped silicon of 0.5 ohm-cm. specific resistance, having a diameter of 0.9 cm., can be processed with good results according to the invention by passing through the rod a direct current of 47 amps/cm. density in the direction opposed to the zone-travel direction, the rod piece crystallizing out of the melt being negatively poled. Simultaneously superimposed upon the direct current is an alternating current of 65 amps/cm. density. The zone-pulling speed is 1.74 mm./min.

In the following comparative tests, the direct current passing through the crystal and the melt was changed, Whereas otherwise the test conditions were kept constant. Used in the tests were phosphorus-doped silicon rods of 10.5 mm. diameter. The zone melting was performed at a zone travel speed of 2.5 mm./min. Only direct current of amps/cm. density was employed. It was observed that in cases where the direct current was passed through the crystal and melt in the direction opposed to the zone-pulling direction, the dislocation density in the pulled crystal was about 5000/crn. Without applying the direct current, the dislocation density under otherwise the same test conditions was found to be approximately 50,000/cm. When the current was passed through the crystal and the molten zone in the zonepulling direction, the dislocation density was about 15,000/cm. under otherwise the same test conditions.

The results show that the Joules heat in conjunction with the cooling due to the Peltier effect with a corresponding poling, already affords a considerable improvement toward better crystal perfection, but that a further considerable approach to crystal perfection is achieved by reversing the polarity of the direct voltage and hence the current-flow direction.

A simple way of determining the dislocation density in crystals is to treat the crystal for a short period of time with an etching agent, for example a mixture of nitric acid and hydrofluoric acid in 1:1 ratio. The faults in the crystal lattice, mainly the lattice dislocation, then become manifest in the form of etch patterns. This method was employed in the foregoing tests.

We claim:

1. The method of pulling 'a semiconductor crystal from a melt of the semiconductor material with the aid of a crystal seed, which comprises pulling a seed crystal from a crucible-free melt zone passing during the entire pulling operation an electric direct current serially through the crystal being pulled and then the melt in 'a direction opposite to the direction of pull thereby developing a freezing region at the solid-liquid interface of the seed crystal and melt due to a Peltier effect whereby lattice faults in the pulled crystal are reduced due to said effect.

2. The method of pulling a semiconductor crystal from 'a melt of the semiconductor material with the aid of a crystal seed, which comprises holding the melt cruciblefree between a lower vertical supply rod of the semiconductor material and an upper vertically suspended crystal, pulling a seed crystal from a crucible-free melt zone, passing during the entire pulling operation an electric direct current in series through the crystal and then the melt in a direction opposite to the direction of pull thereby developing a freezing region at the solidliquid interface of the seed crystal and melt due to a Peltier effect whereby lattice faults in the pulled crystal are reduced due to said effect.

3. The method of pulling 'a semiconductor crystal from a melt of the semiconductor material with the aid of a crystal seed, which comprises holding the melt cruciblefree between a lower vertical supply rod of the semiconductor material and an upper vertically suspended crystal, pulling a seed crystal from a crucible-free melt zone, and impressing between the crystal and the rod a direct voltage by connecting positive potential to the rod and negative potential to the crystal to pass during the entire pulling operating a direct current serially through the crystal and then the melt in a direction opposite to the direction of pull thereby developing a freezing region at the solid-liquid interface of the seed crystal and melt due to a Peltier effect whereby reduction of lattice faults in the pulled crystal results.

4. The crystal pulling method according to claim 2, which comprises continuously turning the rod about its axis while passing the current through melt and crystal during pulling of the crystal.

5. The method of pulling a semiconductor crystal from a melt of the semiconductor material with the aid of a crystal seed, which comprises pulling a seed crystal from a crucible-free melt zone, passing during the entire pulling operation an electric direct current serially through the crystal being pulled and then the melt in a direction opposite to the direction of pull thereby developing a freezing region at the solid-liquid interface of the seed crystal and melt due to a Peltier effect and superimposing an alternating current upon the direct current, whereby the dislocation density in the pulled crystal is reduced.

6. In the semiconductor crystal pulling method according to claim 2, said rod consisting of silicon and having a diameter of about 10 to about 11 mm., the pulling speed being about 2 to about 3 mm. per minute, and said direct current having a density of approximately amps per cm.

References Cited by the Examiner UNITED STATES PATENTS 2,792,317 5/1957 Davis. 2,932,562 4/1960 Pfann 23-301 2,937,216 5/ 1960 Fritts et a1. 623 X 2,970,895 2/1961 Clark et al. 23-301 X 3,058,915 10/1962 Bennett 23273 X 3,152,022 10/ 1964 Christensen et a1. 23273 X NORMAN YUDKOFF, Primary Examiner.

G. HINES, Examiner.

Claims

1. THE METHOD OF PULLING A SEMICONDUCTOR CRYSTAL FROM A MELT OF THE SEMICONDUCTOR MATERIAL WITH THE AID OF A CRYSTAL SEED, WHICH COMPRISES PULLING A SEED CRYSTAL FROM A CRUICIBLE-FREE MELT ZONE PASSING DURING THE ENTIRE PULLING OPERATION AN ELECTRIC DIRECT CURRENT SERIALLY THROUGH THE CRYSTAL BEING PULLED AND THEN THE MELT IN A DIRECTION OP-