US3382114A - Method of manufacturing semiconductor plate using molten zone on powder support - Google Patents

Description

May 7, 1968 c, BEAUZEE ET A1. 3,382,114

METHOD OF MANUFACTURING SEMICONDUCTOR PLATE USING MOLTEN ZONE ON POWDER SUPPORT Filed Jan. '7. 1964 INVENTORJ CLAUDE BEAUZEE FRANCOIS DESVIGNES United States Patent METHOD OF MANUFACTURING SEMICONDUC- TOR PLATE USllN G MOLTEN ZONE 0N POWDER SUPPORT Claude Beauze, Evreux, and Francois Desvignes, Bourgla-Reine, France, assignors to North American Philips Company, Inc, New York, N.Y., a corporation of Delaware Filed Jan. 7, 1964, Ser. No. 336,297 5 Claims. (Cl. 148186) ABSTRACT OF THE DISCLOSURE A method of manufacturing thin semiconductor plates using a zone melting treatment, in which the semiconductor is provided at a powder layer on a support, and the upper surface is zone melted while separated from the support by part of the powder layer to reduce contamination.

The invention relates to a method of manufacturing slabs of semiconductor material, and to a method of manufacturing barrier-layer cells.

It has already been suggested to produce layers of semiconductor material on a support by sintering or fusing this material on to such a support. This method has a limitation in that the material may be contaminated by the underlying support. According to another method the material is deposited on a support from the vapour phase. In this method, however, crystallites are frequently produced which have many lattice imperfections, which may adversely affect the electrical properties of such a layer. It is true that the structure of the layer may be improved by subsequently melting the layer, however, this again may give rise to the above-mentioned disadvantage of contamination by the underlying support. It has also been proposed to improve the structure of a layer provided on a support, particularly for obtaining large crystallites or even a single crystal, by passing a molten zone through such a layer. However, in the latter process also the semiconductor material may be contaminated by the underlying support.

For a plurality of purposes semiconductor material of high purity and great crystalline perfection is required. To this end, a bar of a semiconductor material is purified by a multiple zone melting process, for example a floatingzone melting process, after which from this material a barshaped single crystal is made, for example, by pulling or by zone-melting from a seed crystal. The generally monocrystalline bars may subsequently be subdivided into small wafers, which may be used for manufacturing semiconductor electrode systems. The said treatments render the material comparatively expensive and, since the bars have limited diameters, limits are set to the dimensions of the slabs made therefrom, and the manufacture of slabs having comparatively large surface areas from such bars involves appreciable losses of material.

It is an object of the invention to provide a method of manufacturing slabs or plates of semiconductor material which does not have the aforementioned disadvantages. A- further object of the invention is to provide a comparatively simple and cheap method of manufacturing such slabs which, although they are polycrystalline, satisfy fairly exacting requirements with respect to their structure and purity for many uses. According to the invention, a method of manufacturing slabs or plates of semiconductor material is characterized in that a layer of powdered semiconductor material is arranged on a support having a fiat upper surface, after which a molten zone is produced to a depth smaller than the thickness of the layer of powder by heating from above so that the 'ice molten zone is supported by the underlying solid powdered semiconductor material, after which the molten zone is laterally passed through the powder layer. Since semiconductor materials in the molten state have a high surface tension, the molten semiconductor material of the zone does not penetrate through the underlying powder down to the supporting surface and consequently the melt cannot be contaminated by the material of the support. In addition, by the passage of the molten zone a purification of the material is obtained, especially by removal of undesirable impurities having low segregation coeflicients. In principle, for improved purification, more than one molten zone may be passed through the material either simultaneously or successively. It has, however, been found that for many purposes the passage of only a single molten zone may be sufficient.

It should be noted that it is known to produce a melt of silicon in powdered silicon contained in a cooled crucible while maintaining solid powdered silicon between the wall of the crucible and the melt so that the molten silicon cannot be contaminated by the material of the crucible. A crystal may be pulled from the melt. However, the resulting products are not slabs of semiconductor material, but bars.

In order to improve the structure of the material of the slab, the material formed in the solid state behind the molten zone may be cooled down at a low rate to a temperature lower than the softening temperature of the semiconductor material by after-heating. The term softening temperature is to be understood to mean herein the highest temperature at which dislocations may be formed in or removed from the crystal lattice. At a temperature higher than this softening temperature any stresses in the structure of the newly crystallized material may be reduced or eliminated by movement of atoms or ions. Once the temperature has fallen to a value lower than the softening temperature of the semiconductor material, there is no longer any objection to increasing the rate of cooling down. It has been found that the method is particularly suitable for semiconductor materials having a high melting point at which most materials suited for use as an underlying support are likely to be attacked by the molten semiconductor material. The method according to the invention is particularly suited to the manufacture of slabs consisting of silicon. Silicon has a melting point exceeding 1400 C. and in the molten state attacks substantially all refractory materials. However, the invention is not restricted to the manufacture of silicon slabs but may also be applied to other semiconductor materials, for example, germanium, and to compounds of the type A B When silicon is used, the material crystallizing from the molten zone is preferably cooled gradually to a temperature of at most 1200 C. by after-heating. The duration of the afterheating treatment may be dilferent with different materials. The material which has crystallized out behind the molten zone is preferably after-heated for at least 3 minutes and at most 8 minutes.

The semiconductor material used may already contain doping material, for example, at least one donor and/or acceptor material. In principle one or more donor and/ or acceptor materials may be added during the zonemelting process, for example, from the gas-phase. For the manufacture of a slab of p-type silicon the starting material preferably is a powder consisting of silicon which contains an appreciable amount of boron. The basic materials for the production of silicon, especially quartz, generally contain boron which, as is known, is diflicult to bd separated from the silicon either by physical or chemical processes. For this reason silicon which is substantially free from boron, is comparatively expensive. It has now been found that silicon which has not been subjected to so high a degree of purification as to be free from boron, is suitable as a basic material for manufacturing slabs according to the invention which consist of p-type silicon having a specific resistance of less than 1 ohm-cm. and are suitable for use in manufacturing barrier-layer cells, more particularly barrier-layer photocells, for example for use as solar-cells.

The heating of the molten zone may be effected in a plurality of manners. A heat radiation source may be used in the shape of a narrow strip disposed at a small distance above the powder, while beside it a wider strip may be disposed which is heated to a lower temperature. Heating may be effected by the passage of current. Alternatively, high-temperature radiation sources, for example, an electric are or the sun may be used in combination with suitable reflectors and/ or lenses. As a second alternative, the material may be subjected to bombardment by electrically charged particles, preferably by electrons. According to the invention for this purpose a device may be used which comprises a vessel adapted to be evacuated and containing a support having a flat surface for the semiconductor powder, a thermionic cathode and a system of control electrodes for accelerating and directing the electron beam emitted by the thermionic cathode on to a narrow strip of the flat surface, while means are provided to move the support relatively to the thermionic cathode and the control electrodes in a manner such that the narrow strip is displaced along the surface in the direction of its width. Such a device preferably includes, within the vessel, a second thermionic cathode and a second system of control electrodes which accelerate the electron beam emitted by the second cathode and directs it on to a second strip of the surface of the support which is wider than and adjacent to the first strip. If necessary to prevent the molten zone from penetrating to the surface of the support, suitable cooling measures may be taken. It is not necessary for the support itself to be directly cooled. Generally it is sufficient to provide means for cooling the wall of the vessel.

The invention further relates to a method of manufacturing a barrier-layer ce-ll, more particularly a barrierlayer photocell. For this purpose an active impurity is introduced, for example, by diffusion, into one large surface of a slab manufactured by the method according to the invention, the said impurity changing the conductivity type of a region of the semiconductor material situated immediately under the surface, and ohmic contacts are provided on this layer and on the material of the original conductivity type. In comparison to barrier-layer cells including material obtained from a monocrystalline bar the electric properties of the barrier-layer cell made by the method according to the invention are slightly less satisfactory, however, its manufacture is much simpler and, in particular, large-size slabs may be produced. For many uses the barrier-layer cell has to satisfy less stringent requirements, for example, when used as a power diode in motor-cars and as a barrier-layer photocell where it is not essential for the photo-response per unit of surface area of the cell to be a maximum. Barrierlayer photocells are known which may be used for convetting radiation energy from the sun into electric energy. Such cells generally are comparatively expensive. Due to the simple method of manufacturing large slabs of semiconductor material it is in principle possible to produce such solar cells which are less expensive per watt of their output.

The invention is particularly suitable for the manufacture of silicon barrier-layer cells. As has been mentioned hereinbefore, slabs of p-type silicon may be obtained from powdered silicon which has not been highly purified and hence is comparatively cheap. To manufacture such a barrier-layer cell, more particularly a photocell, suitable for converting radiation energy from the sun into electric energy, a so-called solar cell, phosphorus is preferably introduced into a surface region, for example, by diffusion. The p-type material of the slab may have a comparatively low specific resistance, for example, a resistance between 0.01 and IQ-cnr, and hence may be made from powdered silicon purified to a comparatively low degree only.

In order that the invention may readily be carried out, an embodiment thereof, given by way of example, will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic vertical sectional view of a device for manufacturing slabs of semiconductor material, and

FIG. 2 is a plan view of the device of FIG. 1.

Referring now to the figures, a layer of powdered silicon 1 is disposed on a stainless steel plate 2 which forms part of a carriage 4 having wheels 5 and travelling on raiis 3. This assembly is arranged in a vessel 6 adapted to be evacuated, the carriage 4 being adapted to be moved by means of a pull-rod 7 which is passed through the wall of the vessel 6 and may be driven by a motor 8. The internal space of the vessel may be connected by a pipe 9 to a pump for evacuating the vessel. The vessel further contains an electron gun 10 comprising a thermionic cathode 11, an apertured electrode 12, accelerating electrodes 13 and a system of screen electrodes 14 for deflecting and focusing the beam on to the layer of silicon 1. The said electrodes are fed or biased by means of a low alternating-voltage transformer 15 and a high direct-voltage generator 16 respectively.

The cathode 11 may be a ribbon of tantalum having a thickness of 0.05 mm, a width of 5 mms. and an effective length of 20 mms., the said length influencing the formation of the beam current. At the normal operating temperature (about 2000 K.) this cathode consumes watts (5 v 15 a.). The electrode 12 has a rectangular aperture of about 5 X 20 mms. It is at a potential of 2 volts with respect to the cathode. The accelerating electrode 13 comprises four plates of molybdenum which form a frame open on both sides; the voltage set up at this electrode is +500 volts with respect to the cathode. The electrodes 14, the shapes, sizes and locations of which depend upon the geometrical arrangement of the other components in the vessel, are at cathode potential or at a slightly negative potential; the plate 2 acting as the support of the silicon is at a potential of +10,000 volts with respect to the cathode. For reasons of insulation and for the protection of the operator the plate 2 is connected to earth (zero potential) so that the electrode system of the electron gun is at a potential of about 10 kv.

At the beginning of the process, after the vessel has been evacuated to a high vacuum (to a pressure of less than 5 10- mms. of mercury), the carriage 4 is at the end of its path in the position shown in FIG. 1; the operating voltages are then applied to the electrodes of the gun 10 while the cathode is not yet heated. The motor 8 is then switched into circuit (the speed of the carriage 4 is about 0.3 mm./sec.) and simultaneously the cathode is heated rapidly but gradually to its normal temperature (within 2 to 3 seconds). The electron beam 17 is so focused that it reaches only the layer of silicon in a rectangular narrow region 18 with uniform current density; the longer sides of this rectangle are at right angles to the direction of movement of the carriage 4. The values of the said current density j (which is adjustable by means of the temperature of the cathode), of the acceleration potential V, of the speed w of the carriage 4, of the width 1 of the zone 18 and of the thickness e of the layer of powder 1 are so chosen empirically that the molten layer is given the desired thickness e. For example, the said values may be: j=l0 ma./cm.* V=l0 kv., w=0.3/ secf [=6 mms., 0:5 mms., c':=0.6 mm.

At the beginning of the process, due to the high specific resistance of the silicon and especially due to the poor contact between the powder grains, the potential drop in the layer may cause defocusing of the beam so that the current density is reduced and the commencement of the melting is retarded (for 4 to 5 secs.). In this case, it may be advisable to switch in the motor only 3 to 4 secs. after the beginning of the heating of the cathode in order to concentrate a slightly greater amount of energy in the area in which the molten zone is formed. The motor may be switched in, for example, when the molten zone has just been formed.

At the end of the process the heating of the cathode is interrupted and when no more electrons are emitted the motor is switched off.

As set forth hereinbefore, the high temperature gradient in the cooling portion, which may be several degrees centigrade per centimetre, may give rise to internal stresses and dislocations which may adversely affect the properties of the material. With large slabs these phenomena may even give rise to cracking. To reduce or eliminate these disadvantages use may be made of an after-heating process enabling a lower temperature gradient to be obtained behind the solidification zone. This after-heating may be effected by means of a known tubular furnace into which the plate is inserted immediately after solidification. Alternatively, in the device described an electron bombardment may be carried out by means of a second electron beam 23 which is focused on to a wider zone 24 behind the molten zone 18. In manufacturing a slab consisting of silicon a temperature T to which the solidified material is slowly cooled by afterheating, of about 1200 C. is chosen while the melting temperature T, is about 1420" C. FIG. 1 shows an additional cathode and an additional apertured electrode 21 the sizes and locations of which with respect to the electron gun and the electrodes 13 and 14 are suitable for producing a more divergent after-heating beam. Obviously a separate electron gun co-operating with separate accelerating electrodes and screen electrodes may be used, which may be disposed directly above the after-heating zone.

In this embodiment a layer of powdered material is superficially melted in a very short time by means of a powerful directional heating system, after which the molten layer is solidified according to a predetermined temperature variation.

With a proper choice of the rates of the increase in temperature and of the cooling from the liquid phase to complete solidification, the thermal inertia and resistance of the underlying powder are capable of restricting the penetration depth of the liquid phase and hence the thickness of the molten layer. In addition, if the powder is disposed as a thin layer on a support having a low chemical reactivity with respect to the molten phase and having a great thermal inertia, a large part of the thickness of the layer or powder may be melted without the support itself being appreciably heated, that is to say, without the support being wetted by the melt and without the melt being influenced by the support.

In the embodiment given by way of example heating by electron bombardment only has been described. This method of heating does not restrict the invention, since other heating processes to produce superficial heating may also be used, for example, heating by means of an electric arc, by infra-red radiation of high density, by bombardment by atoms or ions, by gas discharge, etc. The method of heating may be chosen in relation to the temperatures to be reached, the physical and chemical properties of the materials to be heated, the rates of the desired increases and decreases in temperature and also to the cost.

The slabs obtained by the above described method are freely supported by the underlying support and may be removed after treatment. Any powder which may adhere to the lower surface of the slab may be brushed off. If desired, any powder particles which adhere to the melt may be removed, for example, mechanically, and the slab may be given a uniform thickness by grinding and, if required, etching.

The method of manufacturing slabs is not limited to the proportions given by way of example, by which slabs having a width of 2 cms. may be obtained. In principle slabs of larger size may be obtained having widths greater than 2 cms. With the aid of the preceding example the apparatus may readily be adapted to the desired sizes of the slabs to be made.

The manner in which slabs, made as described by way of example from p-type silicon powder having a grain size of less than 0.2 mm. and a specific resistance between 0.03 and 0.lt2=cm. may be used in the manufacture of of solar cells will now be discussed.

This manufacture may start from a slab of the aforementioned dimensions, from which any unevenly formed edge portions are removed. As an alternative, a large slab may in principle be subdivided into smaller slabs of the desired dimensions.

After the usual pre-treatments, such as etching, for purification of the surface, phosphorus is diffused into such a slab. For this purpose the slab is heated to 1100 C. while a gas stream consisting of nitrogen, which has previously passed over an amount of P 0 heated to a temperature of 240 C., flows along the slab at a rate of 1 m./min. The duration of this treatment is 4 hours.

In known manner the resulting n-type layer may be removed from the surface of the slab which during manufacture of the slab faces the support, hereinafter referred to as the lower surface, for example, by etching with concentrated hydrofluoric acid and, after masking the upper surface of the slab, that is to say, the surface which during manufacture of the slab was subjected to electron bombardment, with an etching resist, by etching with a mixture of concentrated hydrofluoric acid, nitric acid and glacial acetic acid, after which the resist may be removed by means of a known solvent. To the edges of the upper surface and to substantially the entire lower surface is applied a silver paste which is secured thereto by baking in known manner.

The silver contacts are connected by means of a lead-tin alloy to copper connecting leads. After masking the contacts and the lower surface, for example, with a polystyrene-base lacquer, the upper surface is briefly etched again with concentrated hydrofluoric acid to which 1 vol. percent of concentrated nitric acid has been added.

On irradiation with sunlight having an energy of 0.1 watt per sq. cm. of the free upper surface of the resulting cell the following results were obtained.

In a solar cell made from a slab of p-type silicon hav ing a specific resistance of 0.3Q-cm., in the manufacture of which no after-heating treatment was used, with a photo-terminal voltage of 140 mv. the photo-current is 5 ma. per sq. cm. of the irradiated surface. The efilciency of the cell is 0.7%.

In a solar cell likewise made from a slab of p-type silicon having a specific resistance of 0.39pm., the surface of which behind the molten zone was after-heated for 5 minutes, with a photo-terminal voltage of 220 mv. the photo-current is 10 ma. per sq. cm. of the irradiated surface. The efficiency of the cell is 2.2%.

In a solar cell made from a slice consisting of p-type silicon having a specific resistance of 0.6t'Z-cm. and obtained without the use of after-heating, with a photo-terminal voltage of 2 mv. the photo-current per sq. cm. of the irradiated surface is 4 ma. The efficiency of the cell is 1.1%.

In a solar cell likewise made from a slab of p-type silicon having a specific resistance of 0.6Q-cm. the surface of which behind the molten zone was after-heated for 5 minutes, with a photo-terminal voltage of 280 mv. the photo-current per sq. cm. of the irradiated surface is 10 ma. The efliciency of the cell is 2.8%

Although conventional commercially available silicon solar cells have an efiiciency of 6% to 7%, the manufacture of the required silicon slices owing to the preceding expensive treatment of the silicon and the large losses of the resulting material is so expensive that the advantages of the manufacture of the slabs according to the invention from a material not purified in so high a degree and involving smaller losses of material amply offset the losses in efficiency. It should also be noted that the efficiency of the solar cells manufactured according to the examples does not mean an upper limit of the efiiciency which may be reached in manufacturing solar cells according to the invention.

What is claimed is:

1. A method of manufacturing a thin plate of crystalline semiconductor material for use in semiconductor devices, comprising providing on the fiat upper surface of a solid support a layer of powdered semiconductor material having a given thickness, heating said powdered layer from above to form within the layer at its upper surface a molten zone whose depth is smaller than said given thickness such that said molten zone is supported on and separated from the support by a solid underlying layer of the powdered semiconductor material, and relatively moving the thusformed molten zone and the support to cause the molten zone to move laterally through the powder layer while remaining at all times spaced from the support.

2. A method as set forth in claim 1 wherein the molten zone is moved in a given direction through the powder layer, and the powder layer is heated from above to form an additional heated but non-melted zone behind the molten zone to after-heat the crystalline material which solidifies from the melt to cause it to cool slowly to a temperature below the softening temperature of the semiconductor material.

3. A method as set forth in claim 2 wherein the semiconductor material is silicon, the softening temperature is about 1200 C., and the cooling time to the softening temperature is between about 3 and 8 minutes.

4. A method as set forth in claim 2 wherein the powder layer is bombarded with electrons to form both the molten zone and the after-heated zone.

5. The method as set forth in claim 1 for manufacturing a barrier-layer cell, wherein into one surface of the 0 plate obtained after the zone-melting treatment is diffused an active impurity to form a surface zone of a conductivity type opposite to that of the plate.

References Cited UNITED STATES PATENTS 2,840,496 6/1958 Jenny 1481.6 2,793,282 5/ 1957 Steigerwald.

2,989,614 6/1961 Steigerwald 219-121 3,031,275 4/1962 Shockley 148-].6 X 3,066,052 11/1962 Howard.

3,218,154 11/1965 Sell l48l.6 X 3,228,753 1/1966 Larsen 1481.6 X

HYLAND BIZOT, Primary Examiner.

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