WO2011073937A2

WO2011073937A2 - System and method for doping semiconductor materials

Info

Publication number: WO2011073937A2
Application number: PCT/IB2010/055871
Authority: WO
Inventors: Rico Böhme; Lars Hartwig; Robby Ebert; Mathias Müller
Original assignee: Roth & Rau Ag
Priority date: 2009-12-17
Filing date: 2010-12-16
Publication date: 2011-06-23
Also published as: WO2011073937A3; KR20120112586A; TW201137952A; DE102009059193B4; WO2011073937A9; CN102763194A; DE102009059193A1

Abstract

The invention relates to a system and to a method for doping substrates using a laser, wherein in the method at least one doping agent is in contact with the substrate surface and the substrate surface is locally heated by a laser beam. It is the aim of the present invention to provide a laser doping process, which allows for substrates to be doped at high speed, while generating a low dislocation density on the substrate surface, achieving good electrical activation of dopants, and moreover providing the option of deliberately doping certain regions to a higher degree. Said aim is achieved by a system for doping substrates using a laser, wherein the system comprises at least one fiber laser having a laser beam with a round beam cross-section and a scanner unit, by which a laser beam can scan the substrate surface, wherein the emitted light of the fiber laser has a wavelength in the range of 750 nm to 3000 nm. The aim is further achieved by a method for doping substrates, in which at least one doping agent is in contact with the substrate surface and the substrate surface is locally heated by a laser beam, wherein a fiber laser having a laser beam with a round beam cross-section is generated, which is guided over the substrate surface by a scanner unit, wherein the fiber laser emits light having a wavelength of 750 nm to 3000 nm.

Description

Plant and method for doping semiconductor materials

The invention relates to a system and a method for doping substrates with laser, wherein in the method at least one dopant is in contact with the substrate surface and a local heating of the substrate surface is effected by a laser steel.

Plants of the type mentioned are usually in industrial practice batch ovens or continuous doping systems in which the solar cell substrates are heated to required for dopant diffusion temperatures of more than 800 ° C. When doping the emitter of solar cells, as described for example in the document DE 10 2007 035 068, partially contrary requirements for the doping. In the optically open regions, the doping must not be too high, since very high doping atom concentrations favor the recombination of electron-hole pairs and thus stand in the way of a desired high efficiency of the solar cells. On the other hand, a very high doping is particularly necessary in the contact region of terminal electrodes for the production of a required ohmic contact. The contrary requirements have recently led, especially in laboratory samples, to the realization of more heavily doped areas in the contact areas and the so-called selective emitters. The preparation of these selective emitters often requires an uneconomically high cost when realized with conventional methods.

In recent years, laser systems for doping semiconductors have also been developed. The document DE 10 2004 036 220 B4 describes laser doping processes as a replacement for conventional furnace doping processes, whose main advantage is that the processing time and process logistics are more favorable than in furnace processes and that the energy efficiency in laser processes is better. However, the methods described in this document generally have the problem that the laser-doped substrates have high dislocation densities and low quality.

The document solves the problem of high dislocation density after melting of the surface and recrystallization in that the laser beam to a line with a Width of 10 μιη and a length of 100 μιη is focused and this line focus scans the surface of the substrate. The disadvantages of this method are the low processing speed and the technical complexity for realizing a reliable autofocus system. The desired characteristics for the doping are based on the characteristics of the dopants, which are produced in furnace processes. That is, as the depth of the doping, a range up to one micrometer is desired. From the known depth of penetration of light into silicon as a function of the wavelength, it is concluded that laser radiation having a wavelength of 600 nm or less must be used.

Batch systems for doping are currently standard in silicon solar cell production. Disadvantages are a relatively long processing time on these systems and large mechanical dimensions. The controllable temperatures are limited to less than 1000 ° C, and the heating of the entire substrates poses the problem of introducing contaminants across the back of the substrate. Another problem with conventional doping methods is that although the dopants diffuse into the silicon crystal, they are largely not incorporated into lattice sites and are electrically activated.

It is the object of the present invention to provide a laser doping process which allows substrates to be doped at high speed, thereby producing a low dislocation density at the substrate surface, achieving good electrical activation of dopants and, moreover, offering the option to dope certain areas targeted higher.

This object is achieved by a plant of the above-mentioned type, which has at least one fiber laser with a laser beam with a circular beam cross section and a scanner unit, through which the substrate surface can be latched to the laser beam, wherein the emitted light of the fiber laser has a wavelength in the range of 750 nm up to 3000 nm.

Fiber lasers are a recent development in the field of laser technology. These novel lasers are characterized by a very high performance and very high beam quality at the same time low price. Due to the high performance of this new In the laser class, it is possible to break new ground in the construction of laser doping systems. The energy required for processing can now be provided not only locally in one focal point, but it can be used with a laser beam whose round beam cross-section dimensions of 20 to 500 μιη and this cross section is available in a comfortable beam length range of about 1 mm , The performance of these lasers is suitable for processing complete substrate surfaces of photovoltaic substrates.

In comparison of the energy efficiency of the laser doping system according to the invention with a conventional furnace doping system Laserdotieranlage invention is advantageous. In fiber lasers, the light beam is generated with high efficiency, and the light energy in the laser doping process is used efficiently because only the surface of the substrate and not the entire substrate is heated. The sparing of the substrate from thermal stress in the doping process also opens up new possibilities for the overall technology of the solar cells. Thus, unlike furnace doping, laser doping may occur in a later step in overall technology, where the substrate already has temperature sensitive elements. This advantage is particularly important in more complex technologies and, for example, in bifacial cell concepts. With Laserdotieranlagen more doping processes in sequence and on both surfaces of the substrates can be performed.

In comparison to conventional furnace doping systems not only the energy is used more efficiently but also the dopants are more efficiently fed to their task of semiconductor doping. The dopants are not incorporated undesirably at interstices on the surface, but they are installed by the higher temperatures advantageous and efficient on lattice sites. Because of the more efficient use, a smaller dopant application is sufficient.

For moving the laser beam over the substrate surface, it is also possible according to the invention to use suitable commercially available scanner units, which can move the beam with the necessary precision and speed. The speed of the scanners can be varied so that in addition to uniform processing over the entire surface, locally differentiated processing is also possible. The laser process is very flexibly controllable, controllable and optimized, since both the scanner unit and the fiber laser itself are very flexible to operate. Due to the properties of the fiber laser and scanner unit system, the depth of the doped region in the substrate can be flexibly controlled. Doping profiles and electrical resistances are precisely adjustable. As a result, the use of laser doping systems according to the invention can lead to improved products, for example improved solar cells.

In addition to the advantages of the actual doping process, the use of laser doping also leads to further advantages on the edge of the processing. Since the substrate surface heats up very quickly by the impinging laser beam, no preheating phase and no cooling phase are necessary for the substrates. This makes a faster overall process possible. Furthermore, the laser doping system according to the invention can be realized with smaller spatial dimensions as a doping furnace. The space savings when using laser doping systems leads to a cost advantage in the construction of manufacturing facilities.

According to the invention, the laser doping system uses a fiber laser which emits light having a wavelength in the range of 750 to 3000 nm. Sophisticated fiber lasers exist in this wavelength range. For the processing of silicon substrates, however, this wavelength range initially seems to be unsuitable, since silicon is transparent in this area. However, due to the high power density in a fiber laser beam, the surface of the substrate is heated rapidly, with the effect of thermal accumulation of volume excitation. Due to the material heating, the optical properties of the substrate change. Silicon increases the absorption. This reduces the penetration depth. The penetration depth is even by setting suitable parameters such. As the power, the scanner speed, the pulse energy, the pulse duration or the repetition rate to a desired range adjustable. With a laser doping system in this wavelength range, silicon substrates can be doped deeper than with furnace processes. In the production of solar cells and especially the phosphorus doping of the emitter of solar cells, for example, doping depths in the range of 1 to 10 μm can be realized hereby. Due to the higher doping depth and at the same time homogeneous doping over the depth, the contacting and the overall efficiency of the solar cell can be improved. In principle, it is also possible to use a fiber laser emitting light in a wavelength range of 500 to 600 nm. Fiber lasers in this wavelength range have only recently become commercially available. Systems operating in this shorter wavelength range are of particular interest for applications where low doping depth is desired.

In a preferred embodiment of the present invention, the fiber laser used is a continuous wave laser or a pulsed laser with relatively long pulse lengths in the range of 80 ns to 10 με. Under these conditions, a doping of substrates is possible without superficially damaging their crystalline structure. Furthermore, the dopants are well incorporated into the substrate crystal and electrically activated. In continuous wave operation, particularly high powers of, for example, 5 kW are available. The use of continuous wave lasers is therefore particularly advantageous for rapid and cost-effective processing. In pulsed operation, only lower powers of, for example, 300 W are available. Advantages in pulse mode are additional available parameters. Thus, for example, via the pulse shape, the pulse duration, the repetition rate and the spatial pulse-to-pulse overlap a stronger influence on the dopant distribution can be taken.

In a favorable development of the invention, the system has a plurality of fiber lasers and / or at least one beam splitter for producing a plurality of laser partial beams while forming a multi-beam arrangement. The use of multiple lasers operating in parallel can be beneficial to increase plant throughput. Thus, a high-power laser beam whose entire power is not required in one beam can be separated into a plurality of laser sub-beams using the beam splitter, thereby enabling simultaneous processing of the substrate with a plurality of laser beams or partial laser beams, thereby enabling a higher processing speed.

In a specific embodiment of the present invention, the laser doping system uses a diffractive element for holographic beam splitting in 50 to 400 laser partial beams. This beam splitter is tuned to the respective task. For example, with such a split beam on solar cell substrates, a plurality of selective emitter fingers can be written. The beam splitter is present preferably structurally adapted to the arrangement of the fingers. The contact electrode of a solar cell comprises not only the fingers but also main lines, so-called "bus bars", on which the charges of the individual fingers are collected.The selective emitter of solar cells is used both in the areas of the individual fingers and in the areas of the "bus bars". educated. The doping depth can be changed in the laser doping process, which can be used, for example, in individual steps in the production of a selective emitter, a deeper doping train.

In a further advantageous variant of the invention, the scanner unit has at least one fast scanner, which may be, for example, a galvo scanner, a polygon scanner and / or a resonance scanner. With such a fast scanner, a sufficiently high processing speed is possible on large-area substrates that meet the requirements of production. The selection of the scanner components depends on the laser used and on the planned laser beam movements. Standard values for currently achievable process speeds are 20 m / s when using pulsed lasers with repetition rates up to 1 MHz and 400 m / s when using continuous lasers.

In a favorable embodiment of the system for doping substrates, the scanner unit has a scanning region which extends over at least two substrates. If smaller substrates, such as wafers, are being processed in the plant and, for example, 7 × 8 wafers are being processed on a substrate carrier, it is particularly favorable if the scanner unit has a scanning area which is as large as the substrate carrier, since in this case only a fiber laser and a scanner unit per system must be installed. However, the processing area can also be divided into two or more laser beams, each laser beam then having a scanning area which is smaller than the substrate carrier.

In a development, the system according to the invention has at least one positioning device for the substrates in the scanning region of the laser and / or at least one control device for the substrates. If you not only process substrates over the entire surface, but experience locally defined special processing It is necessary that an exact positioning of the substrates is carried out relative to the scanner unit. To ensure this high positioning accuracy in the laser doping system, a positioning device known from the prior art can either be integrated in the laser doping module, or the positioning device can be arranged in the transport direction in front of the laser doping module, in which case a transport system with exact substrate position transmission must be used.

Furthermore, in a favorable development, a control device can be provided for the doping that has taken place. This control device may be a known from the prior art measuring device for optical or electrical surface characterization. For the correct assignment of the measured values in their position, an exact substrate position transmission is necessary again.

The stated object is further achieved by a method for doping substrates in which at least one dopant is in contact with the substrate surface and a local heating of the substrate surface by a laser beam takes place. In this case, a fiber laser is used which generates a round beam cross section and which is guided with a scanner unit over the substrate surface, wherein the fiber laser emits light having a wavelength of 750 nm to 3000 nm.

A prerequisite for the realization of the method described is that the laser technology has undergone rapid development in recent years, which has led, inter alia, to high-performance and low-cost fiber lasers. Essential features that lead to a realization of a technically functioning and economically meaningful Laserdotierverfahrens are the low price of the fiber laser, the good beam quality with gau ßförmiger power density distribution in the beam cross section, a round beam cross section and the sophisticated scanner units, which allow the scanning of the substrate surface.

In the method according to the invention, the fiber laser emits light with a wavelength of 750 nm to 3000 nm. Silicon is transparent to low power light in this wavelength range. However, at high power densities, such as occur in the beam of a fiber laser, the substrate surface is heated rapidly and there is a change in optical properties. This will reduce the penetration depth of the Tes reduced in the silicon to the order of 1 μιη. The exact penetration depth can even be adjusted by the choice of process parameters.

In principle, however, it is also conceivable to use fiber lasers which emit light having a wavelength of 500 nm to 600 nm. Such methods are mainly used when a low penetration depth of the laser into the substrate or a low doping depth is desired.

In carrying out the laser doping method according to the invention, it has proven to be advantageous to use continuously operating continuous wave lasers or pulsed lasers having relatively long pulse lengths in the range from 80 ns to 10 με. The use of these long pulses causes a relatively long temperature reaction time on the substrate surface, which therefore does not have to be melted or only slightly melted and therefore no crystal damage to the substrate surface can be recorded after the doping. Sufficient dopant introduction is achieved despite the relatively low temperature for laser doping processes due to the relatively long temperature action time.

In a favorable embodiment of the method according to the invention, the pulse shape of the pulsed laser is designed to appear in a time-power diagram an approximately rectangular shape with at least a short rise time and pulse lengths in the range of 80 ns to 10 με and without significant power peaks. Unlike other pulsed lasers, where the available power is concentrated on very short power peaks, a fiber laser makes it possible to generate very long pulses with uniform pulse power. These long pulses enable gentle substrate processing, which, even when the substrate is melted, leads to low crystal damage and thus to efficient solar cell products. For rapid processing of the substrates, it is advantageous if the front pulse edge is steep, that is, that the pulse power of the laser is reached quickly. On the other hand, the falling flank can be made flatter to increase the temperature response time and to enable a slower and less stressful recrystallization.

In an advantageous variant of the method according to the invention is carried out so that the substrate surface is heated only so far that their solid state preserved. In this case, the laser doping according to the invention takes place at a relatively low temperature, that is to say below the melting temperature of the substrate and, in return, with a relatively long doping time; This allows a very gentle processing of the substrates. By avoiding melting and recrystallization, failure mechanisms associated with recrystallization are completely avoided. For the processing of solar cell substrates whose surface is structured to increase the photosensitivity, it is also essential that doping methods are used which completely preserve the surface structures produced and do not melt.

In a favorable embodiment of the method according to the invention, the method is used for a full-area doping of substrates. The gentle substrate processing with the continuous wave laser or the pulsed with long pulses fiber laser allows both technically and economically the doping of large substrate surfaces such as the optically active regions on solar cells.

In an advantageous development of the method according to the invention not all areas of the substrate surface are processed the same, but defined substrate areas are more thermally stressed and more heavily doped. An important process parameter for realizing a stronger doping is a reduced speed of the scanner movement. Alternatively or as a support thereto, however, other parameters of the laser doping method can also be varied.

In a possible variant of the method according to the invention, work is carried out in such a way that the substrate surface is melted locally. If the surface morphology of the substrates need not be obtained, laser doping with reflow and recrystallization of the substrate surface is also possible. Due to the higher temperature and the liquid state of aggregation at the surface, the production of a very high doping is possible. When using very high energy densities, it is also possible to open on the substrate existing insulating layers, if necessary to remove or diffuse into the substrate melt. When using solar cell substrates with already existing emitter doping and antireflection coating, the method variant according to the invention can be used, for example, to open the antireflection coating in the areas of the subsequent contacting and at the same time to carry out high doping in these selective emitter regions. Since sem method only the surface areas intended for the contacts are electrically conductive, a particularly simple, self-aligned electrodeposition of the contacts is possible.

In a further embodiment of the method according to the invention, the processing speed is increased by several lasers simultaneously irradiating different spatial regions of the substrate surface and multiplying the processing speed by the parallel processing.

In a special variant of the laser doping method according to the invention, the laser beam is split by a beam splitter into laser partial beams. Such a method is especially interesting if not the entire substrate is to be processed, but only local areas. For example, such a method may be used when 50 to 400 finger structures of a selective emitter are to be written on a solar cell substrate. In the doping process, the laser beam may be split by a holographic beam splitting diffractive element into 50 to 400 laser sub-beams, which may then simultaneously write 50 to 400 finger lines of the selective emitter.

In a favorable embodiment of the method according to the invention, the scanner unit moves the laser beam in at least one spatial direction over at least two substrates. When using the Laserdotierverfahrens invention is often the case that multiple substrates are arranged in a row in the scanning direction of a laser. The fact that the laser beam is moved directly over several substrates, the process is particularly simple. Depending on the design of the system, the laser beam movement can take place over several substrates in one or in two spatial directions.

According to an advantageous development of the present invention, the substrate is held or transported during doping by a carrier, wherein the carrier has a surface reflecting the laser beam in the direction of the substrate. That is, the carrier has a specular surface that reflects the laser beam that has penetrated the substrate. In this case, the reflected laser beam again can be used to heat the surface of the substrate and thus to allow an improved diffusion of the dopant into the substrate.

In this case, a doping layer can be provided both on the front side of the substrate and on the substrate rear side, through which dopants can penetrate into the substrate.

The present invention will be explained in more detail below with reference to figures, wherein

Figure 1 shows a possible basic structure of a Laserdotieranlage invention based on a schematic diagram;

Figure 2 shows schematically a further possible embodiment of the present invention, in which a division of a laser beam by means of a beam splitter into partial beams takes place;

Figure 3 shows schematically a possible pulse shape of a fiber laser;

Figure 4 shows a possible embodiment of a system according to the invention as

Outlined component of an in-line substrate processing equipment;

FIG. 5 schematically shows that the scanning length of the laser is greater than several

Can extend substrates;

Figure 6 schematically outlines another possible embodiment of a system according to the invention and a possible procedure for laser doping; and

Figure 7 shows schematically an application variant of the method according to the invention for the doping of substrates.

Fig. 1 shows schematically a possible embodiment of a Laserdotieranlage invention. The individual, shown in Fig. 1 elements of the laser doping system illustrate only their operating principle and are therefore not to scale and drawn in detail, the arrangement of the elements in the drawing is due only to the representability. It does not reflect the concrete arrangement of the elements in the laser doping system. A fiber laser 1 emits a laser beam 2, which is guided by a scanner unit 3 on a defined path D over a substrate surface 4 of a substrate 8. The individual components of the laser doping system will now be described below.

The laser doping system has at least one fiber laser 1, which in the illustration shown is a high-performance laser whose power is sufficiently high to process the substrate 8 in an acceptable time. The wavelength of the fiber laser 1 used in the embodiment of Fig. 1 is in the near infrared spectral range, in a wavelength range between 750 nm and 3 μιη. The wavelength range and the power of the fiber laser 1 determine the penetration depth of the laser beam 2 in the substrate 8. In a silicon substrate, the penetration depth of the laser beam 2 is preferably adjusted to a favorable depth range between 1 μιη and 10 μιη.

The fiber laser 1 or its actual wavelength used depends on the nature of the substrate 8 and on the desired penetration depth. Thus, shallower wavelengths than 750 nm can also be used for flat dopants in silicon or for other substrate materials. The choice of laser wavelength used also depends on the availability of fiber lasers in the market. Currently, in addition to fiber lasers operating in the near infrared, fiber lasers are available that produce green light in the spectral range of 500 nm to 600 nm by frequency doubling infrared light. In embodiments where low doping depth is desired, these short wavelength lasers are preferred.

Depending on the laser development and of new substrates or other applications than the doping of silicon solar cell emitters, however, other wavelength ranges between ultraviolet light having wavelengths of 150 nm and infrared light having wavelengths of 11 μιη can also be used according to the invention.

For the application of the laser doping of solar cell substrates, it has proven to be advantageous to introduce the thermal budget into the substrate 8 in a relatively long time. The substrate surface 4 should be melted by the laser processing as little as possible in order to avoid dislocations. Therefore, the fiber laser 1 used in Fig. 1 is a continuously operating laser, that is, a cw laser. In another embodiment, not shown, the fiber laser 1 is a pulsed laser with relatively long pulse lengths between 80 ns and 10 με. When using pulsed lasers, the doping profile can additionally be adjusted via the pulse shape. Be particularly favorable rectangular pulses have proven with steep edges at the beginning of the pulse. However, the pulse shape is only one parameter that must be considered and adjusted together with other parameters. Thus the laser power, the beam diameter, the beam quality, the intensity distribution in the beam cross section, the relative movement speed of the laser beam, the pulse-to-pulse overlap, the line overlap, the substrate material, its texture, crystallinity and quality, the dopant used, ambient temperature and a number of other parameters.

The laser beam 2 has a simple circular beam cross-section with preferably Gaussian-shaped intensity distribution over the cross section. Such a beam has the advantages that a simple optical system is sufficient for beam shaping and that the beam has a large focal depth of about 1 mm at a focus diameter in a preferred range between 20 μιη and 500 μιη. This long focus area results in a robust and unproblematic process. A complex and expensive autofocus system can thus be dispensed with in the laser beam 2.

A scanner unit 3 realizes the guidance of the laser beam 2 over the substrate surface 4. The scanner unit 3 can, as shown in FIG. 1, comprise various scanner components 3a, 3b. As a rule, each scanner component 3a, 3b is responsible for the movement of the laser beam 2 in a spatial direction. However, it is also possible that the scanner unit 3 only has a scanner for moving the laser beam 2 in a spatial direction. In this case, not shown here, a substrate transport device, not shown here, ensures a relative movement between substrate 8 and laser beam 2 in a second spatial direction.

In the representation in FIG. 1, the scanner component 3 a is a polygon scanner, which is rotatable about its axis of rotation A and can thus cause a laser beam movement in the x-direction on the substrate 8. As scanner component 3b, a galvo scanner is sketched. ed. This scanner is movable about its axis of rotation B in both directions C from its rest position and can thus cause a laser beam movement in the y-direction on the substrate.

The control of the scanner components 3a, 3b leads to the movement of the laser beam 2 on a defined path D over the substrate surface 4. The path D can be defined so that a uniform substrate doping takes place, but it can also be defined so that a locally higher doping is reached. The selection of the scanner components 3a, 3b takes place depending on the fiber laser 1 used and on the desired path on the substrate 8. In addition to the scanner types mentioned, resonance scanners or other scanners could also be used.

The substrate surface 4 is in the laser doping process in contact with a dopant source, which may be in the solid, liquid or gaseous state. However, the dopants may also be present in the form of aerosols, and especially in connection with laser doping, other special forms may also be used. Thus, a liquid precursor can be transferred under the influence of the laser beam 2 in a steam bell, from which then the doping takes place. As dopants, it is possible to use all compounds known in the prior art. For doping of silicon are the compounds of elements from the second, the third, the fifth or the sixth main group, often, for example, compounds of boron and phosphorus are used. Particularly preferred for the doping of silicon are the phosphoric acid for an n-doping and the boric acid for a p-doping. The dopants can be applied in a manner known in the art, for example liquid dopants by spraying, rolling or spin coating.

For the processing of liquid dopants, dilution with an organic or inorganic solvent may be favorable; it is particularly common to use water as the solvent. For example, when using phosphoric acid, concentrations between 0.001% and 85% are used. In particular, when using gaseous precursors also inert or active diluent or purge gases, such as noble gases, nitrogen, hydrogen or oxygen are used. ever After the dopant used, it may be favorable to heat the dopant or the substrate 8 to temperatures up to 200 ° C. This may be necessary for procedural reasons or because of a desired side effect such as an etching activity.

FIG. 2 shows a further embodiment of the laser doping system according to the invention. The same elements as in Fig. 1 are denoted by the same reference numerals. In the course of the laser beam 2, a beam splitter 5 is arranged, which divides the laser beam 2 into a plurality of partial beams 9. The fiber laser 1 used here is a very powerful laser whose power can be divided into a plurality of sub-beams 9, which then form a multi-beam arrangement and allow a simultaneous processing of different substrate areas faster overall processing. A multi-beam arrangement can also be formed by a simultaneous use of multiple lasers.

In a particularly preferred embodiment, which is not shown, however, the multi-beam arrangement comprises a fiber laser 1 and a diffractive element for holographic beam splitting. With such a beam splitter, splitting into 50 to 400 laser partial beams 9 is possible. Such a fanned beam can be used, for example, on solar cells to produce a selective emitter structure.

A selective emitter consists of higher doped regions of the substrate surface 4, on which later contact electrodes are formed. The selective emitter structure comprises, as schematically illustrated in Fig. 2, a multiplicity of thin fingers 6 and a few perpendicularly arranged main lines 7, which are referred to as "bus bars" in the English term the laser beam 2 of the fiber laser 1 is divided so that the individual partial beams 9 can be used to write the individual fingers 6 of the selective emitter structure.

According to the invention, the production of a selective emitter structure can be carried out in a general doping system, which performs both the formation of a planar basic doping and a local selective emitter doping. But it can also be built specialized equipment, either only for the training of areal Basic doping or specialized only for the training of local or selective doping. A special plant for producing a selective emitter structure could also be combined with a conventional furnace doping process. In this case, as a dopant source, a highly doped surface layer, such as the so-called "dead layer" may be used, which is present as an unwanted layer after a furnace doping.

The selective emitter structures which have been produced by the laser doping method according to the invention are distinguished by particularly favorable properties. The higher doping depth producible with the laser doping method according to the invention has already been mentioned. Another positive characteristic, which results from a deep and homogeneously doped emitter, is a better barrier against the diffusion of contact materials, such as copper in the substrate material. A further feature to be emphasized of the selective emitter structure produced in the laser doping method according to the invention is the sharp lateral delineation of the more highly doped regions in relation to adjacent regions. In addition, neighboring areas are not damaged by selective emitter doping. With the laser doping method according to the invention, high dopant concentrations of more than 10 ²⁰ at / cm ³ in the surface region and low film resistances of <20 Ω / sq. will be realized.

Fig. 3 shows schematically a preferred substantially rectangular pulse shape of the fiber laser according to the invention. The fiber laser beam quickly reaches its high maximum power after a short rise time compared to the pulse length. As a result, a rapid substrate heating is achieved. The high power is then maintained for a relatively long time, which may be between 80 ns and 10 με. In this case, the laser pulse has no appreciable power peaks, even at the beginning of the laser pulse on. By the energy input within a relatively long period of time, a good diffusion of the dopants and a gentle substrate processing is achieved. In principle, it is possible in this case for an at least partial melting of the substrate surface 4 to occur. In a preferred embodiment of the invention, however, the substrate surface 4 is heated by the fiber laser 1 only to the extent that their solid state of aggregation is maintained. Thus, unwanted melting and recrystallization effects can be avoided and still very good doping results can be achieved. Fig. 4 shows a detail of a possible inline system, which has a component according to the invention a Laserdotieranlage in the form of a Laserdotiermoduls. The inline system has other components, such as lock chambers, but the other components are not shown. The laser doping system 10 comprises the process chamber 11 and components outside the process chamber 11. A fiber laser 1 generates a laser beam 2 which radiates through a window 12 into the process chamber 1 1. In the illustrated embodiment, the Laserdotieranlage 10 only a scanner, which is specifically the polygon scanner 3a, on. The scanner can move the laser beam 2 in a spatial direction; For adjusting the position of the laser processing on the planar substrate surface 4 in a second spatial direction, the substrate is moved in the illustrated embodiment with the substrate transport device 15. In other non-illustrated embodiments of laser doping modules, discontinuous transport devices and two-dimensional scanner units are used. The laser doping system further comprises a media supply 13, which can provide controlled liquid and gaseous media, and a media disposal unit 14, which serves for the discharge of spent liquids, gases and auxiliary gases.

FIG. 5 schematically illustrates, in addition to the preceding figures, that the scanner unit 3 can move the laser beam 2 on a path E over a plurality of substrates 8. In the illustrated example, the laser beam 2 is moved in the y direction. For controlling the laser path in a second spatial direction x on the substrates 8 no moving means is shown. For movement, the beam or wafer may be moved, or both the wafer and the beam may be moved.

FIG. 6 outlines another possible embodiment of a laser doping system according to the invention and a method sequence for laser doping. One or more substrates 8 are in a first step by a transport system 15, which may be, for example, a belt transport system, an air cushion transport system or a moving Chuck transport system, moved in a transport direction T in the system. In a first method step E, an application of a dopant, which may be, for example, a liquid, which is applied to the substrates 8, for example, by a spraying or rolling technique. In a next process step F further transport of the substrates 8 to a position detection system 17, where the substrates 8 are picked up by a chuck 16, the exact substrate position is detected and then the substrates 8 on the chuck 16 with a high spatial position accuracy in the scanning of a in the above Embodiments explained fiber laser to be transported. In a next method step G, the laser doping of the substrates 8 takes place with the laser beam 2 of the fiber laser schematically illustrated here, wherein the high positioning accuracy also allows a locally defined doping of the substrates 8. In a further method step H, a further transport of the substrates 8 takes place, likewise under high positioning accuracy, under a control device 18, where the result of the laser doping can be controlled. After checking, the further transport of the substrates 8 is carried out with a normal transport system 15. In a next method step K, the substrates 8 are cleaned, for example by applying a rinsing liquid.

FIG. 7 schematically shows a possible application variant of the method according to the invention for doping substrates 8. In the exemplary embodiment shown, the substrate 8 to be doped has a doping layer 19 or 20 both on the substrate surface 4 and on the substrate rear side 24. In other variants of the invention, a doping layer 19 or 20 may also be provided only on the substrate surface 4 or only on the substrate rear side 24. The substrate 8 is held during the doping by a carrier 22 or also transported by the carrier 22 through a doping system. The carrier 22 is, for example, a chuck or carrier suitable for holding or transporting substrates 8. The carrier 22 has a reflecting surface 23 through which a laser beam 2 impinging on the carrier 22 through the substrate 8 is reflected back by the substrate 8 as a reflected laser beam 21. In the return reflection, not only the substrate surface 4 but also the substrate backside 24 is thermally activated. This results in a diffusion of dopants from the doping layer 19 and the doping layer 20 into the substrate 8. It is thus possible with the method shown in Figure 7 to dope the substrate 8 both front and back.

As already mentioned above, the doping layer 20 on the substrate back 24 is optional, that is, it does not have to be provided. The method shown in FIG. 7 is suitable also to accelerate the doping of the substrate front side of the substrate 8, and / or to increase the penetration depth of the dopants into the substrate 8.

As shown in the example of FIG. 7, an air gap 25 may optionally be provided between the doping layer 20 on the substrate rear side 24 and the carrier 22.

Claims

claims

1 . Plant for doping substrates (8) with a laser, characterized in that the installation has at least one fiber laser (1) with a laser beam (2) with a circular beam cross-section (2) and a scanner unit (3) through which the substrate surface (4) with the laser beam (2) can be latched, wherein the emitted light of the fiber laser (1) has a wavelength in the range of 750 nm to 3000 nm.

2. Plant according to claim 1, characterized in that the fiber laser (1) is a continuous wave laser or a pulsed laser with pulse lengths in the range of 80 ns to 10 με.

3. Installation according to one of claims 1 or 2, characterized in that the system comprises a plurality of fiber laser (1) and / or at least one beam splitter (5) for generating a plurality of laser partial beams (9) to form a multi-beam arrangement.

4. Plant according to claim 3, characterized in that the multi-beam arrangement has a diffractive element for holographic beam splitting in 50 to 200 laser partial beams (9).

5. Installation according to one of claims 1 to 4, characterized in that the scanner unit (3) has at least one galvo scanner (3b), a polygon scanner (3a) and / or a resonance scanner.

6. Installation according to one of claims 1 to 5, characterized in that the scanner unit (3) has a scanning region which extends over at least two substrates (8).

7. Installation according to one of claims 1 to 6, characterized in that the system comprises at least one positioning device for the substrates (8) in the scanning region of the laser 1 and / or at least one control device for the substrates (8).

8. A method for doping substrates (8), wherein at least one dopant in contact with the substrate surface (4) and a local heating of the substrate surface (4) by a laser steel (2), characterized in that a fiber laser (1 ) generates a laser beam (2) with a round beam cross section, which is guided with a scanner unit (3) over the substrate surface (4), wherein the fiber laser (1) emits light having a wavelength of 750 nm to 3000 nm.

9. The method according to claim 8, characterized in that the fiber laser (1) radiates as continuous wave laser continuously or as a pulsed laser with pulse lengths in the range of 80 ns to 10 με.

10. The method according to claim 9, characterized in that the pulsation of the fiber laser (1) is carried out in a time-power diagram with a rectangular pulse shape without power peaks.

1 1. Method according to one of claims 8 to 10, characterized in that the laser doping takes place below the melting temperature of the substrate (8).

12. The method according to any one of claims 8 to 1 1, characterized in that the method is used for a full-surface doping of substrates (8).

13. The method according to any one of claims 8 to 12, characterized in that in locally limited areas of the substrate surface (4), the speed of movement of the scanner unit (3) is reduced.

14. The method according to any one of claims 8 to 10, 12 or 13, characterized in that the substrate surface (4) is locally melted.

15. The method according to any one of claims 8 to 14, characterized in that a plurality of laser (1) at the same time irradiate different spatial areas of the substrate surface (4).

16. The method according to any one of claims 8 to 15, characterized in that at least one laser beam (2) with a beam splitter (5) in laser partial beams (9) is decomposed and the laser partial beams (9) simultaneously generate locally defined structures.

17. The method according to any one of claims 8 to 16, characterized in that the scanner unit (3) moves the laser beam (2) in at least one spatial direction over at least two substrates (8).

18. The method according to any one of claims 8 to 17, characterized in that the substrate (8) during the doping of a carrier (22) is held or transported, the one the laser beam (2) in the direction of the substrate (8) reflecting surface (23).

19. The method according to claim 18, characterized in that the dopant in the form of a doping layer (19) on the substrate surface (4) and a doping layer (20) on the substrate back side (24) is provided.