WO2010100561A1

WO2010100561A1 - Process and device for the thermal conversion of metallic precursor layers into semiconducting layers using a chalcogen source

Info

Publication number: WO2010100561A1
Application number: PCT/IB2010/000472
Authority: WO
Inventors: Immo KÖTSCHAU; Reinhard Lenz; Dieter Schmid; Robert Michael Hartung
Original assignee: Centrotherm Photovoltaics Ag
Priority date: 2009-03-06
Filing date: 2010-03-08
Publication date: 2010-09-10
Also published as: TWI509107B; TW201038770A

Abstract

The present invention relates to a process for the thermal conversion of metallic precursor layers on flat substrates into semiconducting layers with a recovery of chalcogen, as well as a device for carrying out the process. The aim of the invention is to provide a rapid and readily executable process for the thermal conversion of metallic precursor layers, as well as a device suitable for carrying out the process with as small as possible primary consumption of chalcogens..This is achieved by forming an inlet-side and outlet-side gas lock (4.1, 4.2, 4.3, 4.4) for closing a furnace chamber (1) in an oxygen-tight manner, introducing one or more substrates (6) prepared with at least one metallic precursor layer into the furnace chamber (1), introducing a chalcogen vapour /carrier gas mixture (10) above the substrates (6) at a pressure close to atmospheric pressure, said mixture being distributed as uniformly as possible over the width of the substrates (6), heating the substrates (6) in the chalcogen vapour/carrier gas atmosphere (10) to a final temperature with the metallic precursor layers being transformed into semiconducting layers, removing the chalcogen vapour that has not been consumed in the reaction, cooling the substrates (6) and removing the latter from the furnace chamber (1). It is advantageous to heat the substrates (6) in a protective gas atmosphere to a temperature at which as far as possible no condensation of chalcogens can take place.

Description

Process and device for the thermal conversion of metallic precursor layers into semiconducting layers using a chalcogen source

The present invention relates to a process and a device for the thermal conversion of metallic precursor layers on flat substrates into semiconducting layers.

For an inexpensive and as far as possible environmentally friendly generation of power by converting sunlight into elec- trical energy, it is necessary to produce highly efficient solar cells using the smallest possible quantity of materials and energy. One highly promising prospect in this regard is thin layer solar cells, in particular solar cells based on compound semiconductors such as, for example, copper indium gallium selenide (CIGS) .

The production of semiconducting layers takes place in a multi-stage process. The metallic precursor layers may contain copper (Cu) , gallium (Ga) and indium (In) , which can be ap- plied to the substrate, which may be a glass substrate with a molybdenum (Mo) layer, by means of known technologies such as sputtering for example. In a second step, the metallic precursor layers are transformed in a heating process in a chalco- gen-containing atmosphere, preferably consisting of selenium and/or sulphur, into semiconducting layers, preferably into a CuInGaSe (CIGS) layer. The chalcogens assume a solid aggregate state at room temperature, i.e. approximately 20⁰C.

Such substrates prepared with a semiconducting layer can then be further processed to form solar modules. For good efficiency, it is essential that the metallic precursor layers are converted as completely as possible. into a semiconducting lay- er with a regular layer thickness across the surface of the substrate.

The prior art discloses processes for the thermal conversion of such prepared precursor layers into semiconducting layers which take place under vacuum. The problem with vacuum processes is the long conversion time, also referred to as process time. This leads to problems in industrial use because long process times are always accompanied by low productivity. One solution would be on the one hand to use many machines simultaneously, but this would mean high investment costs, or on the other hand to accelerate the processes. However, the prior art does not give any indications in respect of this.

Furthermore, the prior art also discloses processes for the thermal conversion of such prepared precursor layers into semiconducting layers which take place under atmospheric conditions and under a supply of hydrogen-containing gases, for example hydrogen selenide (EP 0 318 315 A2) . However, the use of toxic gases, such as hydrogen selenide for example, is problematic.

EP 0 662 247 Bl discloses a process for producing a chalcopy- rite semiconductor on a substrate, in which the substrate pre- pared with metals, such as copper, indium or gallium, is heated in an inert process gas to a final temperature of at least 350⁰C at a heating rate of at least 10°C/second. The final temperature is maintained for a time period of 10 seconds to 1 hour, during which the substrate is exposed to sulphur or se- lenium as a component in excess relative to the components copper, indium or gallium. To this end, a covering is located above the layer structure on the substrate at a distance of less than 5 mm, in the sense of an encapsulation. As described in the patent specification, the partial pressure of sulphur or selenium lies above the partial pressure that would form over a stoichiometrically exact composition of the starting components copper, indium or gallium and selenium or sulphur in a ratio of 1:1:2. However, no description is given of a furnace segmented into different temperature regions which is suitable for a continuous process.

The subsequently published international patent application PCT/EP2008/007466 discloses a simple to produce, rapid con- tinuous process for the thermal conversion of metallic layers on any substrates into semiconducting layers, and also a device suitable for carrying out the process.

This is achieved by a process in which the substrates prepared at least with one metallic precursor layer are heated in several stages at approximately atmospheric ambient pressure in a furnace which is segmented into different temperature regions, in each case to a predefined temperature up to the final temperature of between 400⁰C and 600⁰C, and are transformed into semiconducting layers whilst maintaining the final temperature in an atmosphere consisting of a mixture of a carrier gas and chalcogen vapour.

In this way, good semiconducting layers can be obtained at heating rates considerably less than 10°C/second.

According to the prior art it must be ensured that, when the final temperature is reached, there are enough chalcogens for the metallic precursor layers to be transformed as completely as possible into semiconducting layers.

This is ensured by providing an excess of chalcogens. Any excess chalcogen not consumed during the reaction is transported away together with the carrier gas via an exhaust gas line of the furnace. According to the prior art, the chalcogens can and must be filtered out of the discharged chalcogen vapour/carrier gas mixture, also referred to as exhaust gas, and disposed of as waste.

Furthermore, according to the prior art, the chalcogens for the thermal conversion of the metallic precursor layers are preferably provided via at least one layer of chalcogens on the metallic precursor layers, which then vaporise in the fur- nace and are available for the process. A rapid and inexpensive coating process for chalcogens and also a device suitable for carrying out the process have been specified in the subsequently published patent application PCT/EP2008/062061.

In this coating process, only some of the chalcogens condense on the substrates. The chalcogens that do not condense are transported away in a controlled manner and disposed of as waste .

Reprocessing for the purpose of reuse is hardly ever possible. The chalcogen vapour/carrier gas mixture should therefore be fed to the process as far as possible according to what is needed, and losses during the process should as far as possible be avoided.

The object of the invention is to provide a process and a device for the thermal conversion of metallic precursor layers into semiconducting layers with the best possible quality and optimal use of the process gases, wherein the chalcogen waste from the process as a whole is intended to be considerably reduced.

The object on which the invention is based is achieved by a process which comprises forming an inlet-side and outlet-side gas lock for closing a furnace chamber in an oxygen-tight manner, introducing one or more substrates prepared with at least one metallic precursor layer into the furnace chamber, introducing a chalcogen vapour/carrier gas mixture above the sub- strates at a pressure close to atmospheric pressure, said mixture being distributed as uniformly as possible over the width of the substrates, heating the substrates in the chalcogen vapour/carrier gas atmosphere to a final temperature with the metallic precursor layers being transformed into semiconduct- ing layers, removing the chalcogen vapour/carrier gas mixture that has not been consumed in the reaction, cooling the substrates, and removing the substrates from the furnace chamber.

By virtue of the invention, the quantity of waste is reduced since optimal use is made of the process gases and since process gas losses are virtually avoided. This leads to a simplified production process and to a reduction of the costs, since less chalcogen is used initally.

In one embodiment variant of the invention, the chalcogen vapour/carrier gas mixture is introduced and chalcogen vapour from the chalcogen vapour/carrier gas mixture is reacted directly from the gas phase with the metallic precursor layers. Directly from the gas phase means here that chalcogen vapour from the chalcogen vapour/carrier gas mixture is not converted into a different aggregate state prior to the reaction with the metallic precursor layers.

In another embodiment variant, chalcogen vapour from the chal- cogen vapour/carrier gas mixture is firstly condensed at least partially on substrates introduced into the furnace chamber, and a chalcogen layer is formed thereon which is then reacted with the metallic precursor layers in order to form semiconducting layers. The condensation of chalcogen vapour on the substrates can be brought about by bringing the substrates in the cold state, that is to say at a temperature below the condensation temperature of the chalcogen vapour, into contact with the chalcogen vapour/carrier gas mixture.

A further improvement of the process according to the invention is achieved if the substrates are heated in a protective gas atmosphere to a temperature at which as far as possible no vaporisation of chalcogens takes place. This heating prefera- bly takes place prior to the introduction of the chalcogen vapour/carrier gas mixture.

Furthermore, a flow of the chalcogen vapour/carrier gas mixture can be generated across the surface of the substrates be- tween the gas locks, as a result of which on the one hand the quality of the semiconducting layers to be produced is improved. On the other hand, the quantity of chalcogen vapour/carrier gas mixture to be supplied can thus be reduced in comparison to a still atmosphere.

The chalcogen vapour/carrier gas mixture is preferably produced by carrier gas which flows past molten chalcogens and thereby picks up chalcogen vapour.

Preferably, the chalcogen used is selenium and the protective/carrier gas used is an inert gas, such as nitrogen for example .

After the metallic precursor layers have been transformed into semiconducting layers and before the chalcogen vapour that has not been consumed in the reaction is removed, the substrate can be maintained at a predefined temperature in the chalcogen vapour/carrier gas atmosphere and can then be cooled. In one further development of the invention, the substrates are heated in several stages to a respectively predefined temperature in a furnace chamber segmented into several temperature regions .

In this case, the substrates located in the furnace chamber are transported simultaneously and in stages from segment to segment, wherein the predefined dwell time in the individual segments is identical.

The dwell time may be, for example, 60 seconds.

The heating of the substrates may be carried out in stages from room temperature to, for example, approximately 150⁰C, 450⁰C and 550°C, wherein as the final temperature the 55O⁰C mark does not have to be exceeded.

The substrates can then be cooled to room temperature in at least one stage.

In order to provide the necessary chalcogen vapour for transforming the metallic precursor layers into semiconducting layers, the substrates can already be provided with at least one chalcogen layer before being introduced into the furnace. The chalcogens on the substrate then vaporise completely in the furnace in the case of thin layers and are available in the furnace, in addition to the direct feeding in of the chalcogen vapour/carrier gas mixture, for the transformation process.

In the case of thick chalcogen layers, it is also possible for just some of the chalcogens to vaporise, and the metallic precursor layers can in part also be transformed directly by molten chalcogens into semiconducting layers. The chalcogen layers are preferably applied to the metallic precursor layers through vapour deposition of chalcogens. This may take place under atmospheric conditions in a continuous process .

The invention can furthermore be characterised in that the metallic precursor layers are produced by successively sputtering copper/gallium and indium.

For this purpose, substrates made from glass for example are firstly provided with a molybdenum layer by sputtering, onto which a second layer of copper/gallium is then sputtered from a combined copper/gallium target and finally a third layer of indium is applied by sputtering from an indium target under a high vacuum. Typically, the coating with molybdenum takes place in a first sputtering system, and the coating with copper/gallium and indium takes place in a second sputtering system.

One further development of the process according to the invention provides that, after the metallic precursor layers have been transformed into semiconducting layers, the substrates are cooled in an atmosphere consisting of a mixture of at least one carrier gas and chalcogen vapour, to a temperature at which the semiconducting layers formed are chemically stable. It has been found that in this way the quality of the semiconducting layers can be improved and the efficiency of the solar modules produced therefrom can be improved. This further development of the process according to the invention, along with the further developments and embodiment variants of this process described below, can be combined with further developments of the process according to the invention which have been described previously or which are described in the claims . When cooling the substrates in the atmosphere consisting of a mixture of the carrier gas and chalcogen vapour, which may be formed for example of the chalcogen vapour/carrier gas mixture described above, the substrates should be cooled to a temperature at which no further chalcogens or chalcogen compounds can vaporise from the semiconducting layers or can be released in any other way. Obviously the substrates can also be cooled to lower temperatures. Advantageously, the substrates are cooled to a temperature below 350 °C.

The substrates can then be cooled to room temperature in a further step. This cooling should take place in the absence of chalcogen vapour so as to prevent any condensation of chalco- gens on the semiconducting layers.

A gas flow of carrier gas _.and chalcogen vapour which takes place across the substrates and which may be formed for example by the above-described flow of the chalcogen va- pour/carrier gas mixture is preferably maintained at least until there is a drop below the temperature below which the formed semiconducting layers, in particular the formed chalcopy- rite semiconductors, are chemically stable.

One embodiment variant of the described further development provides that the gas flow of the gas stream consisting of carrier gas and chalcogen vapour is controlled in such a way that, during the cooling of the substrates, enough chalcogen vapour is present at least until a temperature is. reached which lies below a transformation temperature at which the transformation of chalcogens and metallic precursor layers to semiconducting layers takes place. To this end, it may be advantageous to supply additional chalcogen vapour during the cooling of the substrates. In practice, it has proven useful to tailor the rate of gas flow in the furnace chamber from the end of the heating zone to the exhaust gas line to the transport speed of the sub- strates.

The object is also achieved by a device which consists of a furnace with a furnace chamber segmented into several temperature regions, which furnace chamber has an opening for intro- ducing the substrates and an opening for removing the substrates, said openings having the smallest possible dimensions, with a gas lock at the opening for introducing the substrates, with a gas lock at the opening for removing the substrates, with a transport means for flat substrates for trans- porting in stages and simultaneously all the substrates located in the furnace chamber to the respective next segment, and with an exhaust gas line for removing a chalcogen vapour/carrier gas mixture, wherein means for generating a flow of a chalcogen vapour/carrier gas mixture along the substrates are arranged in a wall of the furnace chamber between the gas locks .

The flow along the substrates is preferably formed in or counter to the transport direction.

The means for generating the flow consist of at least one opening in a wall of the furnace chamber for the feed of a chalcogen vapour/carrier gas mixture and at least one opening in the wall of the furnace chamber for removing the chalcogen va- pour/carrier gas mixture by suction. The at least one opening for removing the chalcogen vapour/carrier gas mixture by suction is advantageously formed by an opening into which the suction duct opens. The dimensions of the openings should be almost the same as those of the substrates to be passed through them; at least the height of the openings is only marginally greater than the thickness of the substrates. As a result, losses of process gases in connection with the gas locks can be considerably reduced.

In such a device according to the invention, an internal pressure close to atmospheric pressure is set in the furnace cham- ber, namely due to the dimensions of the openings for passing through the substrates, and also the associated gas curtains, the quantity of supplied gases, the removal of the gases or of the chalcogen vapour/carrier gas mixture by suction, and the temperature. The precise pressure can be set via the ratio be- tween the supplied gases and the gases removed by suction.

In one embodiment of the invention, the opening for the feed of the chalcogen vapour/carrier gas mixture is located in the wall of the furnace chamber which lies opposite the coated surface of a substrate located in the furnace chamber.

Preferably, the chalcogen used is selenium.

The temperatures in the different segments can be adjusted in- dependentIy of one another for example by means of heating and cooling systems.

In one further development of the invention, each segment is thermally insulated from the others. This means that adjacent segments can be brought to considerably different temperatures. Furthermore, each segment can itself be thermally insulated in order to reduce the consumption of energy for heating the segment .

In one embodiment of the invention, the walls of the furnace chamber are made from graphite.

Due to the fact that the substrates are transported in stages and simultaneously from segment to segment, the dwell time of the substrates in the individual segments is identical and may be for example approximately 60 seconds.

Several segments at the start of the furnace chamber form a heating zone and several segments at the end of the furnace chamber form a cooling zone. The segments between the heating zone and the cooling zone form the zone in which the thermal transformation takes place, the reaction zone.

The heating zone is provided with the inlet-side gas lock at the inlet of the heating zone. An additional gas lock may be provided at the outlet of the heating zone.

The gas locks at the start and at the end of the heating zone separate the atmosphere of the heating zone from the atmos- phere outside the furnace chamber and from the atmosphere of the reaction zone. A heating of the substrates to a predefined temperature can thus take place in a protective gas atmosphere, in particular with the exclusion of oxygen but also in the absence of chalcogen vapour.

In one embodiment of the invention, the opening for the feed of the chalcogen vapour/carrier gas mixture is located directly after the additional gas lock at the outlet of the heating zone. If no additional gas lock is provided, the open- ing for the feed of the chalcogen vapour/carrier gas mixture is preferably located directly after the gas lock which is ar^¬ ranged at the opening for introducing the substrates.

The first-mentioned variant has the advantage that the substrates are heated in the heating zone in the absence of oxygen but also of chalcogen vapour to a temperature at which as far as possible no condensation of chalcogen vapour can take place. When the substrates are then passed through the addi- tional gas lock at the outlet of the heating zone, chalcogen vapour for the entire surface of the substrates is provided through the opening for the feed of the chalcogen vapour/carrier gas mixture.

The cooling zone is provided with the outlet-side gas lock at the end of the cooling zone. An additional gas lock may be provided at the inlet of the cooling zone.

The gas locks at the start and at the end of the cooling zone separate the atmosphere of the cooling zone from the atmosphere outside the furnace chamber and from the atmosphere of the reaction zone. A cooling of the substrates can thus take place in a protective gas atmosphere, in particular with the exclusion of oxygen.

Furthermore, the additional gas locks at the end of the heating zone and at the start of the cooling zone prevent both the escape of chalcogen from the reaction zone into the surroundings and also the penetration of oxygen and hydrogen for exam- pie into the reaction zone.

For the better exclusion of oxygen or hydrogen for example from the furnace chamber, the furnace chamber may be sur- rounded by a housing with an opening for introducing the substrates and an opening for discharging the substrates.

The housing may be for example a stainless steel case.

Furthermore, the housing may have a separate housing extraction, and a flushing with a protective gas may be provided.

In one embodiment of the invention, the housing has a separate cooling system. This makes it possible to dissipate the heat emitted by the furnace chamber.

Finally, an oxygen sensor and/or an H₂Se sensor may be fitted in the housing.

The oxygen sensor makes it possible to detect any penetration of oxygen into the space between the housing and the furnace chamber .

The H₂Se sensor serves a safety purpose for detecting in good time any production of hydrogen selenide and alerting the operator accordingly.

In one further development of the device, the gas flows on both sides of the gas locks can be adjusted independently of one another.

The gas locks of the furnace chamber may to this end consist of in each case at least two gas curtains. Additional extrac- tion may also be provided between the gas curtains. This reliably prevents any process gases from being able to leave the furnace chamber in an uncontrolled manner. Preferably, the protective/carrier gas used is an inert gas, such as nitrogen for example.

The opening for introducing the substrates, the opening for removing the substrates and the gas locks make it possible to operate the device in a continuous process, at a pressure close to atmospheric pressure and under defined residual gas conditions, in particular with the. exclusion of oxygen and hydrogen. As a result, it is also possible to prevent the uncon- trolled loss of process gases.

The transport means, the opening for introducing the substrates and the opening for removing the substrates make it possible to introduce the substrates into the furnace chamber, to transport the substrates through the furnace chamber and to remove the substrates from the furnace chamber after the metallic precursor layers have been transformed into semiconducting layers.

The device is arranged in such a ,way that the chalcogen vapour/carrier gas mixture flows mainly over one surface of the substrates Located in the furnace chamber.

In order to supply the chalcogen vapour/carrier gas mixture through the opening for the feed of the chalcogen vapour/carrier gas mixture, this opening is connected to an evaporation chamber via one or even several lines.

In one further development of the invention, the opening for the feed of the chalcogen vapour/carrier gas mixture is a slot-shaped opening which is oriented perpendicular to the transport direction and which extends over the entire substrate located in the furnace chamber. This has the advantage of a more uniform distribution of the chalcogen vapour in the furnace chamber.

The lines from the evaporation chamber to the slot-shaped ope- ning may end in a manner distributed along the slot-shaped opening for the feed of the chalcogen vapour/carrier gas mixture, in order to obtain a uniform distribution of the chalcogen vapour/carrier gas mixture over the entire opening.

In one further development of the invention, the lines which lead to a slot-shaped opening can assume the shape of the slot-shaped opening in the lower part of the lines. This allows an almost uniform distribution of the chalcogen vapour/carrier gas mixture over the entire slot-shaped opening.

Furthermore, the lines may be provided in the lower part with constrictions followed by expansion zones.

This means that the chalcogen vapour/carrier gas mixture which flows through these constrictions is backed up and thus compressed and then expanded again. This process is repeated. As a result, the chalcogen vapour/carrier gas mixture is distributed over the desired length and allows a homogeneous distribution of the chalcogen vapour.

The evaporation chamber is a chamber with at least one outlet for discharging the chalcogen vapour/carrier gas mixture, to which the lines are connected. The evaporation chamber may have at least one additional inlet for supplying solid chalcogen and an access for feeding in a carrier gas.

The solid chalcogen melts in the heatable evaporation chamber and forms a chalcogen pool. For metering of the chalcogens, the evaporation chamber may be equipped with a filling level sensor .

The carrier gas which enters the evaporation chamber and which has preferably been heated beforehand picks up chalcogen vapour in the evaporation chamber and the chalcogen vapour/carrier gas mixture is fed via one or more lines to the opening for the feed of the chalcogen vapour/carrier gas mixture .

In order to control the flow rate at which the carrier gas flows into the evaporation chamber, it may be monitored by a flow meter and adjusted via a fine-control valve. It will be understood that the control can also take place in an auto- mated manner.

In order to prevent any backward flow to the evaporation chamber from the opening for the feed of the chalcogen vapour/carrier gas mixture, an additional carrier gas feed may be provided in each line between the evaporation chamber and the feed opening. The so-called additional process gas which is introduced by these additional feeds generates a suction which sucks the chalcogen vapour/carrier gas mixture out of the evaporation chamber.

In order to control the flow rate at which the additional process gas flows in a line between the evaporation chamber and the opening for the feed of the chalcogen vapour/carrier gas mixture, each line may be monitored by a flow meter and ad- justed via a fine-control valve.

An increased flow rate of the additional process gas leads to a greater chalcogen vapour/carrier gas flow in the associated line between the evaporation chamber and the slot-shaped opening for the feed of the chalcogen vapour/carrier gas mixture.

By virtue of the independent control of the additional process gas flows, the homogeneity of the chalcogen vapour/carrier gas mixture perpendicular to the transport direction can be further improved.

Since, as already described, not all of the chalcogen vapour is consumed in the reaction, this chalcogen vapour/carrier gas mixture must be discharged as exhaust gas via the exhaust gas line. In order to limit the loss of chalcogen, this exhaust gas can be divided by means of a flow volume divider. Some is then still discharged as exhaust gas, referred to as residual exhaust gas, while the other part can be made available again to the process for example via the additional carrier gas feeds or via the carrier gas feed of the evaporation chamber. In this case, the temperature of the recirculated chalcogen vapour/carrier gas mixture should never fall below the conden- sation temperature of the chalcogen, so as to avoid any condensation of the chalcogen in the recirculation flow.

The residual exhaust gas can be filtered and then discharged. The chalcogen waste must be disposed of or supplied for re- processing.

In one further development of this exhaust gas recirculation, chalcogen is removed from the residual exhaust gas and the gas recirculated to the coating operation is enriched with chalco- gen.

In order to supply the solid chalcogen to the evaporation chamber, at least one feed device may be incorporated in the device. A feed device may consist of a container, which may be funnel-shaped and provides a supply of chalcogen, a metering device, which makes it possible to introduce a predefined quantity into an evaporation chamber via at least one pipeline, and a valve for each pipeline. The valves are open dur- ing the feed of the chalcogen and otherwise remain closed, so that any escape of chalcogen vapour from the evaporation chamber to the feed device is largely prevented.

In the case of large installations, a uniform distribution in an evaporation chamber can be ensured via several inlets for supplying solid chalcogen. In this case, a feed device can distribute solid chalcogen uniformly between the different inlets of the evaporation chamber, or else a separate feed device may be provided for each inlet.

An escape of chalcogen vapour from the evaporation chamber to a feed device can be further prevented by a carrier gas feed in the pipeline between the valve and the evaporation chamber. The flow rate of this carrier gas feed can be detected for ex- ample via a flow meter and adjusted via a fine-control valve.

In one embodiment of the invention, components which can be brought into contact with the chalcogen vapour or the chalcogen vapour/carrier gas mixture are preferably made from a ma- terial which is resistant to this mixture, such as graphite for example.

Furthermore, components which come into contact with chalcogen vapour should be heated so as to prevent any condensation of the chalcogen on these components. This prevents complicated cleaning and maintenance.

One possibility for achieving this is to accommodate parts of the device in a block, for example made from graphite, and to heat this to the desired temperature by means of an integrated heater.

In one further development of the invention, the opening for the feed of the chalcogen vapour/carrier gas mixture is accommodated in an evaporation head which is inserted in a recess in the wall located opposite the transport means. It therefore forms part of the evaporation head.

The evaporation head is preferably made from a material which, is resistant to the chalcogen vapour, in particular from graphite for example.

The evaporation head may be brought to a desired temperature for example via an integrated heater.

Furthermore, a thermal decoupling of the evaporation head from the furnace chamber may be provided. This makes it possible to" keep the evaporation head at a different temperature than the temperature of the corresponding segment of the furnace chamber.

In one further development of the invention, -the evaporation head may be placed beneath a sealed cover with an integrated cooling system. In one special embodiment, the interior space of the cover can be flooded with a protective gas and subjected to suction.

In one further development of the invention, components which come into contact with chalcogen vapour, for example the evaporation chambers and the lines between the evaporation chamber and the opening for the feed of the chalcogen vapour/carrier gas mixture, and also nitrogen gas feeds are also accommodated in the evaporation head. The accommodation of the nitrogen gas feeds in the evaporation head has the advantage that the nitrogen gas is already heated in the evaporation head.

One example of embodiment of the invention will be explained with reference to the appended drawings.

In the drawings :

Fig. 1 shows a schematic drawing of a furnace chamber for the thermal conversion of metallic precursor layers into semiconducting layers,

Fig. 2 shows an evaporation head, and

Fig. 3 shows a schematic drawing of a feed device for se- lenium.

Fig. 1 shows a furnace chamber 1 with walls made from graphite, which is surrounded by a coolable stainless steel case

(not shown) . The space between the furnace chamber and the stainless steel case can be subjected to suction and flushed with nitrogen. The furnace chamber is divided into segments Sl

... S7.

In each segment Sl ... S7, a selected temperature can be pre- defined by a heating system 2 or cooling system 3. Here, the segments Sl ... S5 are each equipped with a heating system 2 and the segments Sβ, S7 are each equipped with a cooling system 3.

The heating zone Sl and the cooling zone S7 are equipped with inlet-side and outlet-side gas locks 4.1, 4.2; 4.3, 4.4. Ni- trogen is used as the protective or carrier gas. The gas locks 4.1, 4.2; 4.3, 4.4 are such that the gas flows on both sides of the gas locks 4.1, 4.2; 4.3, 4.4 can be adjusted independently of one another.

The gas locks 4.1, 4.2; 4.3, 4.4 in each case consist of two gas curtains. Furthermore, the gas locks 4.1, 4.2; 4.3, 4.4 in each case have an extraction between the two gas curtains.

The gas locks 4.1, 4.2; 4.3, 4.4 make it possible to transport substrates 6 by means of a transport means 5 through the individual segments Sl ... S7 of the furnace chamber 1 in a continuous process, at atmospheric pressure and under defined residual gas condition, in particular with the exclusion of oxy- gen and hydrogen.

The transport means 5, which is not specified in any greater detail in the drawing, consists of rotatably mounted graphite • rollers, on which the substrates 6 are pushed along through the furnace chamber 1 in stages from segment to segment. To this end, displaceable and rotatable push rods provided with transport protrusions are provided.

In order to transport all the substrates 6 at the same time, the transport protrusions are brought into engagement with the substrates 6 before each transport stroke by rotating the push rod upwards, and all the substrates 6 are accelerated at the same time. At the end of each transport stroke, the substrates 6 are braked, wherein the front edge thereof comes into en- gagement with the transport protrusion of the respective preceding substrate 6. Once the transport stroke has taken place, . the transport protrusions are pivoted out of the way again, whereupon the transport rod is moved back into the starting position. The dwell time of the substrates 6 in the individual segments Sl ... S-7 is in each case identical and is 60 seconds, it also being possible for other times to be set.

In the process, a layer stack consisting of molybdenum on a glass substrate 6, a copper/gallium and indium layer 7 and a thin selenium layer 8 is introduced into the segment Sl. The molybdenum, copper/gallium and indium layers are applied by sputtering in a vacuum chamber, whereas the selenium layer is applied by vapour deposition at atmospheric pressure.

The gas locks 4.1, 4.2; 4.3, 4.4 and the heating system 2 ensure in Sl a first heating of the substrate 6 to approximately 150⁰C in the absence of oxygen and hydrogen for example.

After 60 seconds, the substrate 6. is transported through the second gas lock 4.2 into the second segment S2, in which the substrate 6 is heated to approximately 450⁰C.

According to the invention, selenium vapour is fed to the process in S2 via a selenium source directly after the gas lock 4.1, 4.2. To this end, a recess in the wall of the furnace chamber located opposite the transport means is provided in S2. An evaporation head 9, which is shown in detail in Fig. 2, is inserted in this recess. Via the evaporation head and the opening 10.1 located therein, a selenium vapour/nitrogen gas mixture 10 is introduced into the furnace chamber 1.

Furthermore, the selenium on the substrates 6 starts to melt in S2 and then vaporises completely. The selenium, which is now in vapour form, mixes with the selenium vapour/nitrogen gas mixture 10 from the source and the nitrogen gas of the gas locks 4.1, 4.2; 4.3, 4.4 in the furnace chamber 1 to form a selenium vapour/nitrogen gas mixture 10. By controlling the gas flows through the furnace chamber 1, this gas mixture is transported across the substrates located therein to an exhaust gas line 11 in the wall 1.1 of the furnace chamber 1 lo- cated opposite the transport means 5.

The gas flow is controlled in such a way that, when the final temperature at which the reaction takes place is reached in the third segment S3, during the maintaining of the tempera- ture in S4 and during a cooling in S5, enough selenium vapour is present and flows, over the entire surface of the substrates 6 at least in the sections S3 and S4. Selenium which is not consumed in the reaction is transported away via the exhaust gas line 11 of the furnace chamber 1 between S5 and S6.

In the following segments S6 and S7, the substrate 6 is cooled to approximately 250 °C and 50 °C respectively and then is passed out of the furnace chamber 1 again.

The control of the gas flow is made possible by the fact that the gas flows on both sides of the gas locks 4.1, 4.2; 4.3, 4.4 and in the exhaust gas line 11 are adjustable independently of one another. The rate of the gas flow in the furnace chamber 1 from the segment S2 to the exhaust gas line 11 must in this case be tailored to the transport speed of the substrates 6.

The transformation of the precursor layers into semiconducting CIGS layers (12) takes place already in S3 at a temperature of approximately 550⁰C. Thereafter, the temperature of the substrate 6 in S4 is maintained at approximately 500 °C and in S5 is cooled to approximately 350⁰C. Since the exhaust gas line 11 is located between the segments S5 and S6, this cooling takes place in an atmosphere which contains selenium. A re- vaporisation of selenium during this cooling is prevented by the vapour pressure of the selenium in the selenium vapour/carrier gas mixture 10. This makes it possible to produce semiconducting layers of high quality. In particular, solar modules with good levels of efficiency can be produced therefrom.

In order to further protect against selenium escaping into the surroundings, the evaporation head 9 is placed beneath a sea- led cover 13 with an integrated cooling system. The space between the cover 13 and the evaporation head 9 can be flooded with nitrogen and flushed and subjected to suction.

Fig. 2 shows the evaporation head 9 in detail. The selenium vapour/nitrogen gas mixture 10 is introduced into the furnace chamber 1 through a slot-shaped opening 14 in the evaporation head 9 which is oriented perpendicular to the transport direction of the substrates 6.

In order to obtain a uniform supply of selenium perpendicular to the transport direction, the selenium vapour must be introduced into the furnace chamber 1 uniformly over the entire width of the substrates 6.

-To this end, a line 15 for the selenium vapour/carrier gas mixture 10 has in the lower part the shape of a slot-shaped opening 14. This is achieved via the slot 15 for feeding in. the selenium vapour/carrier gas mixture 10 having several constrictions in the evaporation head 9. The feed slot 15 runs parallel to the slot-shaped opening 14 and ends at this open- ing. In this case, the mixture backs up and can then expand again in an expansion zone. This process is repeated a number of times. As a result, the selenium vapour/carrier gas mixture' 10 is distributed over the entire slot-shaped opening. Fig. 3 shows a schematic drawing of a feed device for selenium.

Selenium is commercially available as spheres in a solid aggregate state. The spheres have a typical diameter of 3-5 mm. The selenium spheres are poured into a funnel-shaped container 16. The funnel has' an opening at its lower end, through which the selenium spheres can fall vertically downwards. Further- more, the funnel is equipped with a filling level meter.

The spheres fall into a metering device 17. The metering device 17 consists of a cylindrical housing and a likewise cylindrical rotary part, hereinafter referred to as the drum, mounted centrally therein in a rotatable manner. The housing has two holes, one on the upper side and one on the underside on^' the same reference diameter and offset in terms of their position by 180°. The inner rotary part has four holes which lie on the same reference diameter. In terms of their length, the two parts are configured in such a way that no selenium sphere fits between them.

When the holes of the parts are aligned with one another, the selenium spheres can fall into the drum. If the drum is ro- tated through 90°, the selenium spheres can exit from the drum and the housing, falling vertically downwards.

The number of selenium spheres, and thus the quantity of selenium provided, can be metered by way of the time period which elapses between the 90° rotations and the number of selenium spheres which fit into the hole of the drum..

The- selenium spheres then fall through a' vertical pipeline 18 into a heated chamber 19, also referred to as the evaporation chamber. A valve 20, designed as a ball valve with complete opening, is installed in the- vertical pipeline 18, which valve is open only in the metering time period. As a result, practically no vapour can escape into the feed device from the hea- ted chamber in which selenium vapour is present.

An escape of selenium vapour from the evaporation chamber 19 into the feed device is further prevented by a nitrogen gas feed in the pipeline 21 between the valve 20 and the evapora- ti^'on chamber 19. This nitrogen flow generates a flow into the evaporation chamber 19 and prevents any flow in the opposite direction.

As described, the selenium spheres fall into a chamber 19 from above. The chamber 19 is a simple horizontal bore in a block made from graphite, the evaporation head 9 in Fig. 2, and is closed at both end sides. The selenium collects in this chamber 19, which in addition to the selenium feed has an inlet 22 and an outlet 23. Said accesses are located on the upper side of the bore.

The block is heated together with the selenium via a heater. As a result of heating, the selenium is first melted and then vaporises as the temperature increases further. There is then a liquid selenium pool 24 in the lower half of the chamber, with selenium vapour 25 in the upper half.

In order to control the filling level in the chamber 19, a filling level sensor (not shown) is provided. The filling Ie- vel sensor outputs a signal to the described metering device when there is too little selenium in the chamber 19. The described procedure then begins, so that selenium drops into the chamber 19. Once a sufficient filling level is reached, the filling level sensor stops the metering process by way of a signal .

The selenium vapour which has formed in the chamber 19 must then be further conveyed to the substrates 6. In doing so, it must be ensured that the selenium vapour cannot cool and condense. The vapour is transported via a carrier gas. Nitrogen is used as the carrier gas. This must be heated beforehand to the same temperature as the temperature which prevails in the chamber 19. The nitrogen passes through the inlet 22 into the chamber 19 in which the selenium vapour is located. There, it picks up the selenium and conveys it through the outlet 23 of the chamber towards the substrate 6. The line for the feed 23 between the chamber 19 and the slot-shaped opening 14/15 must also be heated in order to prevent any condensation of selenium, which would entail complicated maintenance.

In order to achieve more easily this pre-heating of the nitrogen gas, the heating of the evaporation chamber 19 and of the transport paths, everything is accommodated in the evaporation head 9 made from graphite. The entire evaporation head 9 is heated. The nitrogen gas is passed through a meander (not shown) in the evaporation head 9. In the process, the gas is heated to the temperature of the evaporation head 9.

In addition, the flow of the nitrogen gas mixed with selenium vapour is controlled via a further nitrogen gas feed 26 which is located in the line between the evaporation chambe.r 19 and the slot-shaped feed opening 21. By virtue of this gas flowing in, which is referred to as additional process gas, a slight suction is generated which draws the gas mixture out of the chamber. The flow through the nitrogen gas feed for the additional process gas can be measured via a flow meter and adjusted via a. fine-control valve. In order to control the flow rate of the nitrogen feed for the additional process gas 26, the nitrogen feed into the pipeline 21 and the nitrogen feed into the evaporation chamber 22, these are in each case equipped with a flow meter and a fine- control valve. This makes it possible to measure and then to adjust the corresponding flows.

List of references

1 furnace chamber

1.1 wall

1.2 wall

1.3 opening/inlet

1.4 opening/outlet

2 heating system

3 cooling system

4.1 gas lock

4.2 gas lock

4.3 gas lock

4.4 gas lock

5 transport means

6 molybdenum on glass substrate

7 metallic precursor layers

8 Se layer

9 evaporation head

10 selenium vapour/nitrogen gas mixture

10.1 opening

11 exhaust gas line

12 CIGS layer

13 . cover

14 slot-shaped opening for feed

15 slot for feed

16 container

17 metering device

18 pipeline

19 chamber

20 valve

21 nitrogen gas feed into the pipeline

22 inlet

23 outlet

24 selenium pool selenium vapour nitrogen gas feed for the additional process gas

Claims

1. Process for the thermal conversion of metallic precursor layers on flat substrates into semiconducting layers in a furnace chamber, c h a r a c t e r i s e d b y

- forming an inlet and outlet side gas lock (4.1, 4.2, 4.3, 4.4) for the oxygen-tight closure of the furnace chamber (1) , - introducing one or more substrates (6) prepared with at least one metallic precursor layer into the furnace chamber (1) ,

- introducing a ^'chalcogen vapour/carrier gas mixture (10) above the substrates, said mixture being distributed as uniformly as possible over the width of the substrates (6),

- heating the substrates (6) in the chalcogen vapour/carrier gas atmosphere (10) to a final temperature and transforming the metallic precursor layers into semiconducting layers,

- removing the chalcogen vapour/carrier gas mixture (10) that has not been consumed in the reaction,

- cooling the substrates (6), and

- removing the substrates (6) from the furnace chamber (1).

2. Process according to claim 1, c h a r a c t e r i s e d i n t h a t , prior to the introduction of the chalcogen vapour/carrier gas mixture (10), the substrates (6) are heated in a pro- tective gas atmosphere to a temperature at which as far as possible no vaporisation of chalcogens takes place.

3. Process according to claim 1 or 2, c h a r a c t e r i s e d i n t h a t 2

33

a flow of the chalcogen vapour/carrier gas mixture (10) is generated across the surface of the substrates (6) between the gas locks (4.1, 4.2, 4.3, 4.4) .

4. Process according to one of claims 1 to 3, c h a r a c t e r i s e d i n t h a t the chalcogen vapour/carrier gas mixture (10) is produced by passing a flow ,of carrier gas across molten chalcogens.

5. Process according to one of claims 1 to 4, c h a r a c t e r i s e d i n t h a t , after the metallic precursor layers have been transformed into semiconducting layers and before the chalcogen vapour that has not been consumed in the reaction is removed, the substrates (6) are maintained at a predefined temperature in the chalcogen vapour/carrier gas atmosphere (10) and then are cooled.

6. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the substrates (6) are heated in several stages to a respectively predefined temperature in a furnace chamber (1) segmented into several temperature regions.

7. Process according to claim 6, c h a r a c t e r i s e d i n t h at the substrates (6) located in the furnace chamber (1) are transported simultaneously and in stages from segment to segment.

8. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the substrates (6) are heated in the protective gas atmos- phere to a temperature of approximately 150⁰C and then are heated in the chalcogen vapour/carrier gas atmosphere first to approximately 450⁰C and then to the final temperature of approximately 550⁰C.

9. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the substrates (6) are provided with at least one chalcogen layer before being introduced into the furnace chamber (1) .

10. Process according to one of the preceding claims, c h a r a c t e r i s e d i n t h a t the metallic precursor layers are produced on the substrate (6) by successively sputtering copper/gallium and indium onto a flat glass substrate coated with molybdenum.

11. Device consisting of a furnace with a furnace chamber (1) segmented into several temperature regions, which furnace chamber has an opening (1.3) for introducing the substrates (6) and an opening (1.4) for removing the substrates (6), with a gas lock (4.1) at the opening (1.3) for introducing the substrates (6), with a gas lock (4.4) at the opening (1-4) for removing the substrates (6), with a transport means (5) for flat substrates (6) for transporting in stages and simultaneously all the substrates (6) located in the furnace chamber (1) to the respective next segment (Sl, S2, S3, S4, S5, S6) , and with an exhaust gas line (11) for removing a chalcogen vapour/carrier gas mixture (10), c h a r a c t e r i s e d i n t h a t the openings (1.3, 1.4) for introducing and removing the substrates (6) have the smallest possible dimensions, and in that means for generating a flow of a chalcogen va- pour/carrier gas mixture (10) longitudinally over the sub- strates (β) are arranged in a wall (1.1) of the furnace chamber (1) between the gas locks (4).

12. Device according to claim 11, c h a r a c t e r i s e d i n t h a t the means for generating a flow longitudinally over the substrates (6) in or counter to the direction of transport thereof consist of openings (10.1) in a wall of the furnace chamber (1.1) for feeding in or discharging a chalcogen vapour/carrier gas mixture (10) or a carrier gas (10) .

13. Device according to claim 12, c h a r a c t e r i s e d i n t h a t the opening (10.1) for the feed of a chalcogen vapour/carrier gas mixture (10) forms part of an evaporation head (9) .

14. Device according to claim 12 or 13, c h a r a c t e r i s e d i n t h a t the openings (10.1, 11) for the feed and discharge of the chalcogen vapour/carrier gas mixture, or the carrier gas, are located in the wall (1.1) of the furnace chamber (1) which lies opposite the coated surface of a substrate (6) located in the furnace chamber (1) .

15. Device according to one of claims 11 to 14, c h a r a c t e r i s e d i n t h a t a heating zone is provided with an additional gas lock (4.2) at the outlet thereof.

16. Device according to claims 11 to 15, c h a r a c t e r i s e d i n t h a t the at least one opening (10.1) for the feed of the chalco- gen vapour/carrier gas mixture (10) is located directly after the gas lock (4.1) at the opening (1.3) for introducing the substrates (6) or directly after the additional gas lock (4.2) at the outlet of the heating zone.

17. Device according to one of claims 12 to 16, c h a r a c t e r i s e d i n t h a t the at least one opening (10.1) for the feed of the chalco- gen vapour/carrier gas mixture (10) is a slot-shaped opening which is oriented perpendicular to the transport direc- tion and which extends over the entire width of the substrates (6) located in the furnace chamber (1) .

18. Device according to one of claims 12 to 17, c h a r a c t e r i s e d i n t h a t the at least one opening (10.1) for the feed of the chalco- gen vapour/carrier gas mixture (10) is connected to an evaporation chamber (19) via at least one line.

19. Device according to claim 17 and claim 18, c h a r a c t e r i s e d i n t h a t the at least one line which leads to the slot-shaped open- ing assumes the shape of the slot-shaped opening in the lower part.

20. Device according to one of claims 18 to 19, c h a r a c t e r i s e d i n t h a t the at least one line is provided in the lower part with constrictions followed by expansion zones.

21. Device according to one of claims 18 to 20, c h a r a c t e r i s e d i n t h a t a feed device is provided for supplying a solid chalcogen into the evaporation chamber (19) , which feed device is preferably connected to the evaporation chamber (19) via at least one pipeline and preferably consists of a container, a metering device and a valve for each pipeline.