Apparatus and method for supplying electric power to a CVD-reactor

An apparatus and method for applying a voltage across silicon rods in a CVD reactor has a series connection wherein the silicon rods may be inserted as resistors A first power supply unit has first transformers connected with one silicon rod. A second power supply unit has second transformers connected to the same number of silicon rods as the first transformers in parallel to one or more of the first transformers. The second transformers have an open circuit voltage lower than the first transformers and a short circuit current higher than the first transformers. A third power supply unit has outputs connected with the silicon rods in parallel to the first and second transformers. The third power supply unit is capable of providing a current in a voltage range below the open circuit voltage of the second transformer and higher than the short circuit current of the second transformer.

The present invention relates to a method and an apparatus for applying a voltage across a plurality of silicon rods in a CVD-reactor which silicon rods are connected in series.

It is known in the semiconductor technology and the photovoltaic industry to produce silicon rods having high purity, e.g. in accordance with the Siemens method in deposition reactors, which are also referred to as CVD-reactors (CVD=chemical vapour deposition). In this process, initially thin silicon rods are received in the reactors, onto which silicon is deposited during a deposition process. The thin silicon rods are received in clamping and contacting apparatuses, which hold the thin silicon rods in a desired orientation and which also provide electrical contacting thereof. At their respective free ends, the thin silicon rods are typically connected via electrically conducting ridges, in order to allow closing of an electrical circuit via contact elements, which are arranged on the same side of the reactor. Alternatively, it is also possible to contact the thin silicon rods are their opposite ends, i.e. from above and from below, in order to enable conducting a current through the thin silicon rods. A pair of thin silicon rods, which is connected via an electrically conducting bridge, as well as thin silicon rods, which are contacted at their opposite ends, are referred to as silicon rod for reasons for simplicity.

The silicon rods are heated to a predetermined temperature, at which deposition of silicon from the vapour or gas phase onto the silicon rods occurs. Heating is achieved during the process by a flow of current at a substantially predetermined voltage by means of resistance heating. The deposition temperature is typically in the range of 900-1350 degree Celsius and in particular, at about 1100 degrees Celsius, but may also be at other temperatures.

Since the silicon rods initially have a high resistance, which lowers at higher temperatures, it is necessary to first apply a high initial voltage across the silicon rods, in order to initiate an initial flow of current, which is also referred to as igniting the silicon rods. After initial heating of the silicon rods by the flow of current and thereby reducing the resistance of the silicon rods, the voltage applied across the silicon rods can be reduced to an operating voltage. Further temperature regulation of the silicon rods can be primarily controlled via the current.

DE 10 2009 021403 A describes an apparatus and a method for a two-step application of voltage for thin silicon rods in a CVD-reactor. In particular, in the apparatus described, a first supply unit having a plurality of transformers is provided, wherein each transformer supplies a voltage to a respective pair of the thin silicon rods during a start phase. Furthermore, a second supply unit is provided, which is capable of supplying pairs of serially connected thin silicon rods during an operating phase with a voltage. The transformers of the first supply unit are connected with a three-phase supply network on their primary side. Furthermore, between each transformer and the three-phase supply network, a choke and a power regulator in the form of a bidirectional thyristor power regulator are provided. The power regulators are operable for adjusting and maintaining a voltage level on the secondary side of the respective transformers and also for limiting the flow of current therethrough. The power regulators are specially used to counteract a decrease of the voltage at increased current flow, which typically occurs in transformers, by maintaining the voltage level at the secondary side. Such power regulators, however, are expensive and require a respective control, which leads to involved controlled electronics.

It is therefore an object of the present invention to provide an apparatus and a method for applying a voltage across a plurality of silicon rods in a CVD-reactor which provide, in an easy and cost effective manner, a multi-step voltage supply in accordance with respective requirements.

In accordance with the present invention, the object is solved by an apparatus according to claim1and a method according to claim9. Further embodiments of the invention are disclosed in the dependent claims.

The apparatus, in particular comprises a series connection in which the silicon rods may be inserted as resistors, at least one first current supply unit, at least one second current supply unit, at least one third current supply unit and at least one control unit, which is capable of applying a voltage across the silicon rods in the series connection via the first, the second or the third supply unit. The respective current supply units are preferably transformers and, in particular preferably transformers for transforming single phase line voltages, which have a plurality of inputs on their primary side for supplying at least one external line voltage and on their secondary side, a plurality of outputs for applying the at least one transformed line voltage with ohmic and/or inductive loads. Under a plurality of inputs/outputs in the following one or more inputs/outputs are to be understood. The first current supply unit comprises a plurality of first transformers, the outputs of which are each serially connected to at least one silicon rod, wherein the first transformers have a first open circuit voltage and a first short circuit current. The second current supply unit comprises a plurality of second transformers, the outputs of which are connected in series with at least the same number of silicon rods as the first transformers and which may be arranged parallel to one or more the first transformers, wherein the second transformers have a second open circuit voltage and a second short circuit current and wherein the second open circuit voltage is lower than the first open circuit voltage and wherein the short circuit current is higher than the first short circuit current. The third current supply unit comprises outputs which are connected with the series of silicon rods and may be arranged parallel to the first and second transformers, wherein the third current supply unit is capable of providing a current at a voltage which is below the open circuit voltage of the second transformer, the current being larger than the short circuit current of the second transformers. Such an apparatus enables in an easy manner, a multi-step application of different voltages across the silicon rods. By providing different transformers in the first and second power supply units, which may selectively apply voltages to the silicon rods under control of the control unit, in combination with the third current supply unit, it is possible to omit for example, power regulators and a corresponding control electronics on the primary side of transformers. Furthermore, more simple transformers may be used.

Preferably, the second transformers are each connected in series to a larger number of silicon rods than the first transformers are connected to. In so doing, the number of transformers in the second current supply unit may be reduced over the number of transformers in the first current supply unit, which may lead to a reduction of cost and/or space for the second current supply unit. In particular, the second transformers are each connected in series to twice the number of silicon rods compared to the number of the first transformers. In so doing, the number of transformers in the second current supply unit is halved. Furthermore, the outputs of the third current supply unit are preferably connected in series to at least twice the number of silicon rods compared to the second transformers. In so doing, there is an increasing serial connection of the silicon rods.

For simplifying the required shielding or safety techniques, the first and/or second transformers, which are connected in series with adjacent silicon rods, are oppositely wound. This allows that the outer ends of the silicon rods in the series are respectively at one potential, the absolute value of which is the lowest and is preferably around zero volt.

In one embodiment of the invention, at least a fourth power supply unit is provided comprising at least one fourth transformer, the outputs of which are connected in series to more silicon rods than the outputs of the respective second transformers, and said fourth transformer having a fourth open circuit voltage, which is lower than the second open circuit voltage, and a fourth short circuit current, which is higher than the second short circuit current. In this case, the at least one control unit is capable of alternatively applying a voltage to the silicon rods in the series connection also via the fourth current supply unit, and the third current supply unit is capable of providing a current which is larger than the fourth short circuit current at a voltage which is below the fourth open circuit voltage. The fourth current supply unit provides further increments of the voltage supply for the silicon rods, thereby, the third voltage supply may provide a lower maximum voltage level, which may reduce the cost for the overall apparatus.

Preferably, the transformers of the first power supply unit have a steeper current/voltage characteristic than the transformers of the second power supply unit, and the transformers of the second power supply unit have a steeper current/voltage characteristic than the third power supply unit. In so doing, it is easy to adjust to a temperature dependent resistance change of the silicon rods. The specific resistance of the silicon rods initially decreases steeply as the temperature increases and then goes into a saturation state. The different current/voltage characteristics of the power supply unit reflects this trend in the different gradients.

In order to avoid overloading the transformers of the first and second power supply unit, the control unit is capable of switching to a different power supply unit depending on a flow of current through the respective first and second transformers. This enables, in a simple manner, a switching between different voltages in accordance with the requirements at the time.

In the method according to the invention, the steps mentioned herein below are carried out sequentially. First, a first voltage is applied across the silicon rods via a plurality of first transformers. The term first voltage refers to an average effective value of the voltage, which is averaged over the time during which the current is applied via the first transformers. Subsequently, a second voltage is applied across the silicon rods via the plurality of second transformers, wherein the second voltage is lower than the first voltage. The term second voltage refers to an average effective value of the voltage, which is averaged over the time during which the voltage is applied via the second transformers. With an increase in temperature in the silicon rods which is caused by a current flow initiated by the first and/or second voltage, the respective voltage may decrease over time, while at the same time, the current flow increases. Finally, a third voltage is applied to the silicon rods using a power supply unit, wherein the third voltage is lower than the second voltage. The term third voltage refers to an average effective value of the voltage which is averaged over the time during which the voltage is applied via the power supply unit. Again, with an increase in temperature of the silicon rods, the voltage may decrease over time while the current may further increase. The method allows the above mentioned advantages of a simple and multi-step application of voltage across silicon rods in a CVD reactor.

Preferably, the third voltage is applied across all of the silicon rods which are connected in series, while the first and/or second voltage is applied across a single silicon rod or groups of silicon rods. The first voltage may be applied across each single silicon rod and the second voltage may be applied across two silicon rods which are connected in series. In so doing, a step-wise increase in the series connection of silicon rods is achieved.

In order to avoid overloading the first and second transformers, the current flow through the silicon rods is determined and at least one switch between different voltages may be controlled in accordance with the thus determined current flow. The position reactors may have a different number of silicon rods which are connected in series. The circuit disrupt path may be used several times in a single reactor.

In accordance with one embodiment of the invention, a force voltage is applied across the silicon rods after applying the second voltage and before applying the third voltage, wherein the fourth voltage is applied by at least one third transformer and may be between the second and third voltages. The term fourth voltage refers to an average effective value of the voltage, which is averaged over the time at which the voltage is applied via the at least one third transformer.

When applying the first and/or the second voltage via the first and second transformers, respectively, the voltage is applied in such a manner, that at the ends of the series connection, the voltage level is the lowest in absolute value. In so doing, the required insulation or safety technique may be simplified.

FIG. 1shows a schematic side view of an arrangement of silicon rod pairs1in a CVD reactor which is not shown in detail. In order to simplify the representation, only two rod pairs1are shown in one plan standing next to each other. It is noted, however, that in a CVD reactor, more such rod pairs1may be received which are not oriented with respect to each other in one plane. Typically, four to twenty four of such rod pairs1are provided in a CVD reactor while obviously, more or less rod pairs1may be provided.

FIG. 1shows two rod pairs1which each consist of two thin silicon rods3and a connecting bridge4. The right rod pair inFIG. 1shows the initial configuration prior to a gas phase deposition. The left rod pair shows the configuration after a gas phase deposition of silicon on the thin silicon rods, which are still indicated in the left rod pair by a dashed line.

The thin silicon rods are each arranged in a known manner in electrode arrangements6at the floor7of the CVD reactor. The connecting bridge4of the rod pairs1connects the free ends of the thin silicon rods3. The connecting rods4also consist of silicon and preferably have the same electrical characteristics as the thin silicon rods3. In particular, the connecting bridges4may be manufactured from thin silicon rods and may be connected in any appropriate manner to the free ends of the silicon rods3in the arrangement shown inFIG. 1.

Such an arrangement of rod pairs1allows electrical contact of the thin silicon rods3exclusively via the electrode arrangement6at the floor7of the CVD reactor. In the following, for simplification of the description, both silicon rod pairs1in accordance withFIG. 1as well as single thin silicon rods which are connected at both ends with the electrodes, are referred to as silicon rods.

FIG. 2shows a schematic circuit arrangement10for applying different voltages across silicon rods S1to S4, which may be received in a CVD reactor (not shown) and the manner shown inFIG. 1.

The circuit arrangement10in accordance withFIG. 2is designed for four silicon rods S1to S4, but it may also be provided for a different number, wherein four of an integer multiple thereof is currently the preferred number of silicon rods for a circuit arrangement10. Depending on the number of silicon rods in a CVD reactor, a corresponding number of circuit arrangements10may be provided. The silicon rods S1to S4are electrically connected in series. The circuit arrangement10has a first power supply unit12, a second power supply unit14, a third power supply unit16and a control unit which is not shown.

The first power supply unit12has altogether four transformers21-24which may be connected on their primary side via respective switches26-29to a one phase alternating voltage of for example, 400 Volts. The transformers21-24have, on their secondary side, an open circuit voltage of approximately 8,000 Volts and a short circuit current of approximately 6 Amperes. InFIG. 3, a current/voltage characteristic K1of the transformers21-24is shown. As shown, the transformers21-24each have a steep gradient of the current/voltage characteristic, i.e. the voltage on the secondary side quickly decreases when the flow of current increases.

Each of the transformers21-24is assigned to one of the silicon rods S1-S4and the outputs at the secondary side of the transformers21-24are each connected to opposite ends of one of the silicon rods S1-S4. Thus, each of the transformers21-24is capable of applying a voltage of 8,000 Volts across a respective one of the silicon rods S1-S4in the series connection. The secondary windings of the transformers21,23are oppositely wound to the secondary windings of the transformers22,24, such that transformers22,24having the primary and secondary side wound in the same direction alternate with the transformers21,23which have their primary and secondary sides wound oppositely with respect to the row of adjacent silicon rods. In particular, the transformers21-24are wound in such a manner and connected to the silicon rods S1-S4such that the outer ends of the silicon rods S1-S4which are connected in series are at the lowest level with respect to the absolute value and are in particular at approximately 0 Volts when voltage is applied across the silicon rods S1-S4via the transformers21-24.

The second power supply unit14has two transformers31,32, which may be connected via respective switches34,35to a one phase alternating voltage of for example, 400 Volts on their primary side. The transformers31,32as shown each have an open circuit voltage of 4,000 Volts and a short circuit current of 20 Amperes on their secondary side. InFIG. 3, a current/voltage characteristic K2of the transformers31,32is shown. As shown, the transformers31,32have a less steep gradient of the current/voltage characteristic compared to the transformers21-24of the first power supply unit12.

Each of the transformers31,32is assigned to two adjacent ones of the silicon rods S1-S4and the outputs at the secondary side of the transformers31,32are each connected to the opposite ends of the group of two adjacent ones of the silicon rods S1-S4which are connected in series. Thus, each of the transformers31,32is capable of applying a voltage of approximately 2,000 Volts across a respective one of the silicon rods S1-S4in the series connection (the silicon rods connected in series act in substance as a voltage divider). The secondary sides of the transformers31,32are oppositely wound. In particular, the transformers31,32are wound in such a manner and connected to the silicon rods S1-S4such that the outer ends of the silicon rods S1-S4of the series connection are each at the lowest level in absolute value and are in particular at approximately 0 Volts when a voltage is applied across the silicon rods S1-S4via the transformers31,32.

The third power supply unit16is any regulated current supply, which is capable of providing a voltage in a range below the open circuit voltage of the transformers31,32of the second current supply unit and a current flow above the short circuit of the transformers31,32of the second power supply unit14. In the embodiment as shown, the third power supply unit16is for example capable of providing a voltage at its outputs between 2,500-50 volt and a current of 10-3,400 Amperes. InFIG. 3, the current/voltage characteristic K3of the third power supply unit16is shown. As shown, the third power supply unit has a less steep gradient than the current/voltage characteristic of the transformers of the second power supply unit14.

The third power supply is connected to the silicon rods S1, S2, S3, S4which are connected in series such that the voltage drops along the row of silicon rods S1-S4which are connected in series, i.e. it is connected to the outer ends of the silicon rods S1and S4of the silicon rods connected in series.

The control device, which is not shown, is capable of selectively and sequentially applying a voltage across the silicon rods S1-S4via the first, second or third power supply unit12,14,16. In so doing, the respective transformers21-24of the first power supply unit12and the transformers31,32of the second power supply unit14may be switched on and switched off individually, in groups or altogether, as is shown by the respective switches26-29and35,36.

The transformers21,22,23,24,34and35of the power supply units12,14each have a “soft” characteristic and they may each be of the type having separate or internal choke, an air gap or a transductor. The power supply16adjusts the voltage and the current to the changing conditions during the deposition process.

In the following, operation of the circuit arrangement10during a CVD deposition process will be explained in more detail, wherein the description is limited to the four silicon rods, S1-S4according toFIG. 2. At the beginning of the process, all power supply units12,14,16are switched off. Subsequently, the first power supply unit12is actuated via the control unit to close switches26-29. A voltage of up to 8,000 Volts is then applied across the individual silicon rods S1-S4via the transformers21-24. Due to the high voltage, despite the high initial resistance of the silicon rods S1-S4, a current flow occurs within the silicon rods S1-S4. Due to this current flow, there will be heating of the silicon rods S1-S4, which leads to a reduction of the resistance thereof. The increased current flow due to the reduced resistance within the silicon rods S1-S4then leads to a reduction of the voltage applied via the transformers as shown in the characteristic K1inFIG. 3. InFIG. 3, it is also schematically shown by a dashed line, a graph for the current/voltage characteristic of a thin silicon rod pair as it is used in the CVD reactor for silicon deposition at the beginning of the silicon deposition. As can be shown by this graph, a very high initial voltage is required in order to initiate an initial current flow which is capable of affecting a sufficiently high temperature increase in the thin silicon rod pair which may lead to a significant lowering of the resistance of the thin silicon rod pair. At the beginning of the process, the initial voltage is therefore high and the initial current is very low. Alternatively, the silicon rods may also first be heated using for example, IR emitters, halogen lamps or other means for heating silicon to an initial temperature which is high enough that, at an applied initial voltage, an initial current may flow, which is high enough to achieve further heating of the silicon rod. The initial voltage first drops at a first steep gradient when there is an increase in temperature and the initial current increases correspondingly. In the further course, the graph becomes flatter and slowly approaches a saturation value, the resistance of the thin silicon thin rod pair is further reduced, the current flowing through the silicon increases since the temperature of the silicon increases and the deposition process starts.

The current/voltage characteristic K1of the transformers21-24is adjusted to the temperature dependent resistance change of the silicon rods S1-S4such that the voltage drops at a steep gradient with an increased current flow in order to avoid uncontrolled heating of the silicon rods in the start phase.

If the current flow through the silicon rods S1-S4reaches a certain level of for example, 4-5 Amperes due to the reduction of resistance of the silicon rods, the switches26-29of the first power supply unit12are opened and the switches35,36of the second power supply unit14are closed. A voltage of up to 4,000 Volts is then applied across pairs S1, S2; S3, S4of the silicon rods via the transformers31,32, wherein a maximum voltage of approximately 2,000 volts is applied across each individual silicon rod S1-S4.

The current flow through the silicon rods S1-S4increases as the temperature of the silicon rods increases and the resistance is further reduced. The current/voltage characteristic K2of the transformers31,32is more flat, corresponding to the current/voltage characteristic of the silicon rods, compared to the current/voltage characteristic of the transformers21-24. The transformers31,32are capable of providing a higher current compared to transformers21,22,23,24.

As the current flow through the silicon rods S1-S4reaches a third level of for example, 10-12 Amperes due to the reduction of resistance of the silicon rods, the switches35,36of the second power supply unit14are opened and the third power supply unit16is switched on in order to apply a voltage across all of the silicon rods S1-S4connected in series. At this point in time, ignition of the silicon rods S1-S4is finished, i.e. the silicon rods have a temperature which is high enough such that further temperature increases may be adjusted in substance via current regulation. Accordingly, the current/voltage characteristic K3of the third power supply unit is substantially more flat than the one of the transformers31,32of the second power supply unit.

After the silicon rods have obtained a sufficiently high temperature, the deposition of silicon on the silicon rods S1-S4start within the CVD reactor. The silicon now continuously grows onto the rods until they have reached a predetermined thickness. At this point in time, the third power supply unit is switched off. The silicon rods S1-S4are no longer heated via a current flow and thus, cooled down and may then be taken from the CVD reactor and may be fed to another device for further processing.

The above given number of silicon rods and the corresponding number of transformers of the first and second power supply units are only given as an example and should not be seen in a limiting manner even though they are currently taken into consideration for a CVD reactor having an integer multiple of four silicon rods. Furthermore, the cited open circuit voltages and short circuit currents of the transformers are only given as examples by the fact that they are actually taken into consideration for use in a CVD reactor for igniting silicon rods and are indeed appropriate to do so.

FIG. 4shows a schematic circuit arrangement10for applying different voltages to silicon rods S1-S4in accordance with an alternative embodiment. InFIG. 4, the same reference signs are used as inFIG. 2inasmuch as the same or similar elements are described.

The circuit arrangement10according toFIG. 4has a first power supply unit12, a second power supply unit14, a third power supply unit16and a control unit which is not shown. These are in substance the same as the previously described units wherein the third power supply unit may provide a lower voltage range of for example, 2,000-50 Volts and a current between 20-3,000 Amperes. Additionally, however, a fourth power supply unit40is provided. The fourth power supply unit40has a transformer42which may be connected on its primary side via a switch44to a one face alternating voltage of for example, 400 Volts. The transformers as shown have an open circuit voltage of 3,000 Volts and a short circuit current of 40 Amperes on its secondary side. InFIG. 5, a current/voltage characteristic K4of the transformer42is shown.FIG. 5also shows the corresponding current/voltage characteristics K1-K3of the first to third power supply units. As shown, the current/voltage characteristic K4of the transformer42has a less steep gradient compared to the current/voltage characteristic K2of the transformers31,32of the second power supply unit14. The current/voltage characteristic K4, however, has a steeper gradient than the current/voltage characteristic K3of the third power supply unit16.

Despite the fact that the fourth power supply unit40is shown as comprising a single transformer42, it is noted that similar to the second power supply unit14, two transformers may be provided which may, for example, have an open circuit voltage of 1,500 Volts and a short circuit current of 40 Amperes. The solution of having a single transformer, however, is preferable due to cost and space reasons. Such a transformer may be provided with a center tap.

Operation of the circuit arrangement according toFIG. 4is substantially the same as the one described above, wherein after switching off the second power supply unit14, the fourth power supply unit40is switched on until a predetermined current flow of for example, 20-25 Amperes through the silicon rods S1-S4is achieved. Then, it is switched to the third power supply unit16.

FIG. 6shows a schematic circuit arrangement10for applying different voltages to silicon rods S1-S4in accordance with another embodiment. InFIG. 6, the same reference signs are used as inFIG. 2inasmuch as the same or similar elements are describes.

The circuit arrangement10according toFIG. 6has a first current supply unit12, a second current supply unit14, a third current supply unit16and a control unit which is not shown. These units are the same as the ones described with respect toFIG. 2.

However, the embodiment ofFIG. 6differs in that transformers21and22of the first current supply unit12are connected in series and may be connected on their primary side via a common switch50to the one phase alternating voltage of for example, 400 volts. Similarly transformers23and24of the first current supply unit12are connected in series and may be connected on their primary side via a common switch51to the one phase alternating voltage of for example, 400 volts.

Operation of the circuit arrangement according toFIG. 6is substantially the same as the one described above, wherein obviously transformers21and22on the one hand and23and24on the other hand are switched on and off by a common switch. Such a series connection is particularly useful when voltages above 8000V are applied the silicon rods S1to S4via the transistors21to24. When one of the silicon rods in the series connection ignites earlier than another one, the voltage may be pulled up when the transformers are connected in series. When the transformers are connected in parallel, the voltage may be limited.

The invention has been described herein above with reference to preferred embodiments without being limited to the specific embodiments. In particular, the invention is not limited to the numerical values given with respect to the number of elements as well as the voltage and current values.