DE102009059193B4 - Process for doping semiconductor materials - Google Patents

Abstract

Method for doping silicon substrates (8), in which a fiber laser (1) generates a laser beam (2) with a wavelength that is in the range from 750 nm to 3000 nm and, on the other hand, meets the condition that the silicon substrate is at the wavelength used is transparent, and generates the laser beam with a round beam cross section, the laser beam being guided over the substrate surface (4) with a scanner unit (3), in which at least one dopant is in contact with the substrate surface (4), in which a local heating of the Substrate surface (4) is carried out by the laser beam (2) for laser doping and in which the laser doping takes place below the melting temperature of the silicon substrate and the laser beam (2) is split into partial laser beams (9) using a beam splitter (5) and the partial laser beams (9) simultaneously produce locally defined structures in different substrate regions, the substrate surface being in contact with a dopant source in a gaseous state from which the dopant is provided.

Description

The invention relates to a method for doping silicon substrates with a laser, wherein in the method at least one dopant is in contact with the substrate surface and the substrate surface is locally heated by a laser beam.

In industrial practice, systems of the type mentioned are usually batch ovens or continuous doping systems in which the solar cell substrates are heated to temperatures of more than 800 ° C required for dopant diffusion. The doping of the emitter of solar cells consists, for example, in the publication DE 10 2007 035 068 A1 described, sometimes contradictory requirements for the doping. In the optically open areas, the doping must not be too high, since very high doping atom concentrations promote the recombination of electron-hole pairs and thus stand in the way of the desired high efficiency of the solar cells. On the other hand, a very high doping is necessary to produce the required ohmic contact, particularly in the contact area of connection electrodes. The conflicting requirements have recently led to the realization of more highly doped areas in the contact areas and the so-called selective emitters, especially in laboratory samples. The production of these selective emitters often requires an uneconomically high level of effort when implemented using conventional methods.

In recent years, laser systems have also been developed for doping semiconductors. In print DE 10 2004 036 220 A1 Laser doping processes are described as a replacement for conventional oven doping processes, the advantage of which is mainly that the processing time and process logistics are cheaper than in oven processes and that the energy efficiency is better in laser processes. However, the processes described in this publication generally have the problem that the laser-doped substrates have high dislocation densities and low quality. Further state of the art can be found in the following publications: US 2007 / 0 054 443 A1 , US 2005 / 0 181 566 A1 , US 2009 / 0 253 273 A1 , US 2004 / 0 137 731 A1 , WO 81/00 789 A1 , EP 2 058 842 A2 , DE 10 2008 047 611 A1 .

The publication solves the problem of high dislocation density after melting of the surface and recrystallization by focusing the laser beam into a line with a width of 10 µm and a length of 100 µm and this line focus scans the surface of the substrate. The disadvantages of this process include the low processing speed and the technical effort required to implement a reliable autofocus system. The desired characteristics for the doping are based on the characteristics of the dopings that are generated in oven processes. This means that the desired depth of doping is a range of up to one micrometer. From the known penetration depth of light into silicon depending on the wavelength, it is concluded that laser radiation with 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 temperatures that can be controlled are limited to less than 1000°C, and the heating of the entire substrate creates the problem of contamination entering via the back of the substrate. Another problem with conventional doping methods is that the dopants diffuse into the silicon crystal, but are largely not incorporated into lattice sites and electrically activated.

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

This task is solved by the method according to claim 1. The dependent claims indicate advantageous developments of the method according to the invention.

Below, among other things, a laser doping system is described with which the method according to the invention can be carried out. The system itself is not used.

Fiber lasers are a recent development in the field of laser technology. These new lasers are characterized by very high performance and very high beam quality combined with a low price. The high performance of this new laser class makes it possible to take new paths in the construction of laser doping systems. The energy required for processing can now no longer only be provided locally in a focal point, but can be worked with a laser beam whose round beam cross section has dimensions of 20 to 500 µm and this cross section is available in a comfortable beam length range of approximately 1 mm . The Leis This laser is suitable for processing entire substrate surfaces of photovoltaic substrates.

When comparing the energy efficiency of the method according to the invention with a conventional furnace doping system, the method according to the invention has an advantage. 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. Protecting the substrate from thermal stress during the doping process also opens up new possibilities for the overall technology of solar cells. Unlike oven doping, laser doping can take place in a later step in the 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. Laser doping systems can also be used to carry out several doping processes in sequence and on both surfaces of the substrates.

In comparison to conventional furnace doping systems, not only is the energy used more efficiently, but the dopants are also used more efficiently for their task of semiconductor doping. The dopants are not installed in an undesirable manner at interstitial sites on the surface, but rather they are advantageously and efficiently incorporated into lattice sites due to the higher temperatures. Because of the more efficient use, a smaller application of dopant is sufficient.

According to the invention, suitable commercially available scanner units can also be used to move the laser beam over the substrate surface, which can move the beam with the necessary precision and speed. The speed of the scanner can be varied, so that in addition to uniform processing across the entire surface, locally differentiated processing is also possible.

The laser process can be controlled, regulated and optimized very flexibly, as both the scanner unit and the fiber laser itself can be operated very flexibly. Due to the properties of the system consisting of fiber laser and scanner unit, the depth of the doped area in the substrate can be flexibly controlled. Doping profiles and electrical resistances can be precisely adjusted. As a result, the use of laser doping systems can lead to improved products, for example improved solar cells.

In addition to the advantages in the actual doping process, the use of laser doping also leads to further advantages at the edge of the processing. Since the substrate surface heats up very quickly due to the incident laser beam, no preheating phase or cooling phase is necessary for the substrates. This makes a faster overall process possible. Furthermore, the laser doping system can be implemented with smaller spatial dimensions than a doping furnace. The space savings when using laser doping systems results in a cost advantage when setting up production facilities.

According to the invention, the laser doping system with which the method is carried out uses a fiber laser which emits light with a wavelength in the range of 750 to 3000 nm. There are sophisticated fiber lasers in this wavelength range. However, this wavelength range initially appears to be unsuitable for processing silicon substrates because silicon is transparent in this range. However, due to the high power density in a fiber laser beam, the surface of the substrate is heated quickly, with the effect of thermal accumulation of the volume excitation occurring. The optical properties of the substrate change as the material heats up. With silicon the absorption increases. This reduces the penetration depth. The penetration depth can even be adjusted by setting suitable parameters such as: B. the power, the scanner speed, the pulse energy, the pulse duration or the repetition rate can be regulated to a desired range. With a laser doping system in this wavelength range, silicon substrates can be doped deeper than with oven processes. When producing solar cells and especially the phosphorus doping of the emitter of solar cells, doping depths in the range of 1 to 10 µm can be achieved. 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.

An alternative laser doping system for semiconductor substrates, not according to the invention, uses a fiber laser that emits light in a wavelength range of 500 to 600 nm. Fiber lasers in this wavelength range have only recently become commercially available. Systems that operate in this shorter wavelength range are particularly interesting for applications where a 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 from 80 ns to 10 µs. Under these conditions, substrates can be doped without damaging their crystalline structure on the surface. Furthermore, the dopants are well incorporated into the substrate crystal and electrically activated. In continuous wave operation Particularly high outputs of, for example, 5 kW are available. The use of continuous wave lasers is therefore particularly advantageous for fast and cost-effective processing. In pulsed operation, only lower powers of, for example, 300 W are available. Advantages in pulse operation are additional available parameters. For example, the pulse shape, the pulse duration, the repetition rate and the spatial pulse-to-pulse overlap can have a greater influence on the dopant division.

In the invention, the system has at least one beam splitter for generating several partial laser beams to form a multi-beam arrangement. The use of several lasers that are operated in parallel can be beneficial in order to increase the throughput of the system. A powerful laser beam, the entire power of which is not required in one beam, can be split into several partial laser beams using the beam splitter, which enables simultaneous processing of the substrate with several laser beams or partial laser beams and thereby a higher processing speed.

In a special embodiment of the present invention, the laser doping system uses a diffractive element for holographic beam splitting into 50 to 400 partial laser beams. This beam splitter is tailored to the respective task. For example, a large number of selective emitter fingers can be written on solar cell substrates with such a split beam. The beam splitter is preferably structurally adapted to the arrangement of the fingers. In addition to the fingers, the contact electrode of a solar cell also includes main lines, so-called “bus bars,” on which the charges from the individual fingers are collected. The selective emitter of solar cells is formed both in the areas of the individual fingers and in the areas of the “bus bars”. The laser doping process is divided into several work steps that are used to write the individual fingers and the “bus bars”. The doping depth can be changed in the laser doping process; this can, for example, be used to create a deeper doping in individual work steps when producing a selective emitter.

In a further advantageous variant of the invention, the scanner unit has at least one fast scanner, which can 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 production requirements. The selection of the scanner components depends on the laser used and the planned laser beam movements. Guide values for currently achievable process speeds are 20 m/s when using pulsed lasers with repetition rates of 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 area that extends over at least two substrates. If smaller substrates, such as wafers, are processed in the system and, for example, 7 × 8 wafers are processed on a substrate carrier, it is particularly advantageous if the scanner unit has a scanning area that is as large as the substrate carrier, since in this case only a fiber laser and a scanner unit must be installed per system. However, the processing area can also be divided into two or more laser beams, with each laser beam then having a scanning area that is smaller than the substrate carrier.

In a further development, the system has at least one positioning device for the substrates in the scanning area of the laser and/or at least one control device for the substrates. If the substrates are not only to be processed over the entire surface, but are to undergo special processing in a locally defined manner, it is necessary that the substrates are positioned precisely 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 front of the laser doping module in the transport direction, in which case a transport system with exact substrate position transfer must be used.

Furthermore, in a favorable further development, a control device for the doping that has taken place can be provided. This control device can be a measuring device known from the prior art for optical or electrical surface characterization. In order to correctly assign the measured values to their position, an exact substrate position transfer is again necessary.

The object is achieved by a method for doping silicon substrates, in which at least one dopant is in contact with the substrate surface and the substrate surface is locally heated by a laser beam. A fiber laser is used, which generates a round beam cross section and which is scanned across the substrate surface using a scanner unit surface is guided, the laser beam being split into partial laser beams using a beam splitter and the laser partial beams simultaneously producing locally defined structures in different substrate regions, and the substrate surface being in contact with a dopant source in a gaseous state from which the dopant is provided.

A prerequisite for the implementation of the described process is that laser technology has undergone rapid development in recent years, which has led, among other things, to powerful and inexpensive fiber lasers. Essential features that lead to the realization of a technically functioning and economically sensible laser doping process are the low price of the fiber laser, the good beam quality with a Gaussian power density distribution in the beam cross section, a round beam cross section and the highly developed scanner units, which allow 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. The wavelength range is chosen so that silicon is transparent to low-power light. However, at high power densities, such as those found in the beam of a fiber laser, the substrate surface is heated quickly and the optical properties change. This reduces the penetration depth of light into the silicon to the order of 1 µm. The exact penetration depth can even be adjusted by selecting process parameters.

However, the fiber laser doping process is not limited to the near infrared spectral range. In an alternative, non-inventive variant of the invention, fiber lasers can also be used that emit light with a wavelength of 500 nm to 600 nm. Such methods are used primarily when a low penetration depth of the laser into the substrate or a low doping depth is desired.

When 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 with relatively short pulse lengths in the range from 80 ns to 10 μs. The use of these long pulses causes a relatively long temperature exposure time on the substrate surface, which therefore does not have to be melted and therefore no crystal damage is recorded on the substrate surface after doping. Adequate dopant introduction is achieved despite the relatively low temperature for laser doping processes due to the relatively long temperature effect time.

In a favorable embodiment of the method according to the invention, the pulse shape of the pulsed laser is designed such that it appears in a time-power diagram to have an approximately rectangular shape with at least a short rise time and pulse lengths in the range from 80 ns to 10 µs and without significant power peaks. In contrast to other pulsed lasers, where the available power is concentrated in very short power peaks, with a fiber laser it is possible to generate very long pulses with uniform pulse power. These long pulses enable gentle substrate processing, which results in minimal crystal damage and thus high-performance solar cell products. For rapid processing of the substrates, it is advantageous if the front pulse edge is steep, which means that the pulse power of the laser is reached quickly. The falling edge, however, can be made flatter in order to increase the temperature exposure time and enable slower and less stressful recrystallization.

In the method according to the invention, the substrate surface is only heated to such an extent that its solid state is retained. 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 enables very gentle processing of the substrates. By avoiding melting and recrystallization, error mechanisms associated with recrystallization are completely avoided. When processing solar cell substrates whose surface is structured to increase light sensitivity, it is also essential that doping processes are used that completely preserve the surface structures produced and do not melt them.

In a favorable embodiment of the method according to the invention, the method is used to dope substrates over the entire surface. The gentle substrate processing with the continuous wave laser or the fiber laser pulsed with long pulses allows the doping of large substrate areas, such as the optically active areas on solar cells, both technically and economically.

In an advantageous development of the method according to the invention, not all areas of the substrate surface are processed equally, but rather defined substrate areas are subjected to greater thermal stress and are more heavily doped. An important process parameter for realizing higher doping is a reduced speed of the scanner movement. Alternative or supportive However, other parameters of the laser doping process can also be varied.

In a variant that does not correspond to the invention, the substrate surface is melted locally. If the surface morphology of the substrates does not need to be preserved, laser doping with melting and recrystallization of the substrate surface is also possible. The higher temperature and the liquid state of aggregation on the surface make it possible to produce a very high doping. When using very high energy densities, it is also possible to open insulating layers present on the substrate, remove them if necessary or diffuse them into the substrate melt. This process variant not according to the invention can, for example, be used when using solar cell substrates with already existing emitter doping and anti-reflection coating to open the anti-reflection coating in the areas of later contacting and at the same time to carry out higher doping in these selective emitter areas. Since in this process only the surface areas intended for the contacts are electrically conductive, a particularly simple, self-adjusting galvanic deposition of the contacts is possible.

In a further exemplary embodiment of the method according to the invention, the processing speed is increased by several lasers simultaneously irradiating different spatial areas of the substrate surface and the parallel processing results in a multiplication of the processing speed.

In a special variant of the laser doping method according to the invention, the laser beam is split into partial laser beams by a beam splitter. Such a process is particularly interesting if the entire substrate is not to be processed, but only local areas. For example, such a method can be used if 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 can be divided into 50 to 400 partial laser beams by a diffractive element for holographic beam splitting, which can then write 50 to 400 finger lines of the selective emitter at the same time.

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 laser doping method according to the invention, it is often the case that several substrates are arranged in a row in the scanning direction of a laser. Because the laser beam is moved over several substrates at the same time, the process is particularly simple. Depending on the design of the system, the laser beam can move across several substrates in one or two spatial directions.

• 1 einen nicht erfindungsgemäßen Grundaufbau einer Laserdotieranlage anhand einer Prinzipskizze zeigt;
• 2 schematisch eine weitere mögliche Ausführungsform der vorliegenden Erfindung zeigt, in welcher eine Aufteilung eines Laserstrahls mittels eines erfindungsgemäßen Strahlenteilers in Teilstrahlen erfolgt;
• 3 schematisch eine mögliche Pulsform eines Faserlasers zeigt;
• 4 eine nicht erfindungsgemäße Ausführungsform einer Anlage als Komponente einer Inline-Substratbearbeitungsanlage skizziert;
• 5 schematisch darstellt, dass sich die Scanlänge des Lasers über mehrere Substrate erstrecken kann, und
• 6 schematisch eine weitere nicht erfindungsgemäße Ausführungsform einer Anlage und einen nicht erfindungsgemäßen Verfahrensablauf zur Laserdotierung skizziert.

The present invention is explained in more detail below with reference to figures, where:

• 1 shows a basic structure of a laser doping system not according to the invention based on a schematic sketch;
• 2 schematically shows a further possible embodiment of the present invention, in which a laser beam is divided into partial beams by means of a beam splitter according to the invention;
• 3 schematically shows a possible pulse shape of a fiber laser;
• 4 an embodiment of a system not according to the invention is outlined as a component of an inline substrate processing system;
• 5 schematically shows that the scanning length of the laser can extend over several substrates, and
• 6 schematically a further embodiment of a system not according to the invention and a process sequence for laser doping not according to the invention are outlined.

1 shows schematically a possible exemplary embodiment of a laser doping system. The individuals, in 1 The elements of the laser doping system shown merely illustrate its functional principle and are therefore not drawn to scale or in detail; the arrangement of the elements in the drawing is only for the sake of representation. It does not reflect the specific 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 are now described below.

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

The specific fiber laser 1 used or its specific wavelength depends on the type of substrate 8 and the desired penetration depth. Shorter wavelengths than 750 nm could also be used (not according to the invention) for shallow dopings in silicon or for other substrate materials. The choice of laser wavelength used also depends on the availability of fiber lasers on the market. In addition to the fiber lasers that work in the near infrared range, there are currently fiber lasers available that generate green light in the spectral range from 500 nm to 600 nm from infrared light by doubling the frequency. In embodiments (not according to the invention) where a low doping depth is desired, these short-wave lasers are preferred.

Depending on further laser development and new substrates or applications other than the doping of silicon solar cell emitters, other wavelength ranges between ultraviolet light with wavelengths of 150 nm and infrared light with wavelengths of 11 μm could be used (not according to the invention).

For the application of laser doping of solar cell substrates, it has proven to be advantageous to enter the thermal budget into the substrate 8 in a relatively long time. The substrate surface 4 should be melted as little as possible during laser processing in order to avoid dislocations. That's why the fiber laser 1 used in FIG. 1 is a continuously operating laser, i.e. 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 μs. When using pulsed lasers, the doping profile can also be adjusted via the pulse shape. Rectangular pulses with steep edges at the beginning of the pulse have proven to be particularly beneficial. However, the pulse shape is only one parameter that must be viewed and adjusted together with other parameters. The laser power, the beam diameter, the beam quality, the intensity distribution in the beam cross section, the relative speed of movement of the laser beam, the pulse-to-pulse overlap, the line overlap, the substrate material, its texture, crystallinity and quality, the dopant used, and the ambient temperature all play a role and a number of other parameters play a role.

The laser beam 2 has a simple, round beam cross section with a preferably Gaussian intensity distribution across the cross section. Such a beam has the advantages that simple optics are sufficient for beam shaping and that the beam has a large focus depth of approximately 1 mm with a focus diameter in a preferred range between 20 μm and 500 μm. This long range of focus results in a robust and unproblematic process. A complex and expensive autofocus system can therefore be dispensed with in the laser beam 2.

A scanner unit 3 guides the laser beam 2 over the substrate surface 4. The scanner unit 3 can, as in 1 shown include different 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 for the scanner unit 3 to have only one 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 1 the scanner component 3a is a polygon scanner that can be rotated about its axis of rotation A and can therefore cause a laser beam movement in the x direction on the substrate 8. A galvo scanner is sketched as scanner component 3b. This scanner can be moved about a rotation axis 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 occurs, but it can also be defined so that a locally higher doping is achieved. The scanner components 3a, 3b are selected depending on the fiber laser 1 used and the desired path on the substrate 8. In addition to the scanner types mentioned, resonance scanners or other scanners could also be used.

In the laser doping process, the substrate surface 4 is in contact with a dopant source, which can be in the solid (not according to the invention), liquid (not according to the invention) or gaseous state. However, the dopants can also be in the form of aerosols, and other special forms can also be used, especially in connection with laser doping. A liquid precursor can be transferred under the influence of the laser beam 2 into a vapor bell, from which the doping then takes place. As All compounds known in the prior art can be used as dopants. To dope silicon, these are compounds of elements from the second, third, fifth or sixth main groups; for example, compounds of boron and phosphorus are often used. Particularly preferred for doping silicon are phosphoric acid for n-doping and boric acid for p-doping. The dopants can be applied in a manner known in the art, for example liquid dopants by spraying, rolling or spin-coating.

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

In 2 A further embodiment of the laser doping system is shown, with which the method according to the invention can be carried out. The same elements are as in 1 designated with the same reference numerals. In the course of the laser beam 2, a beam splitter 5 according to the invention is arranged, which divides the laser beam 2 into several partial beams 9. The fiber laser 1 used here is a very powerful laser, the power of which can be divided into several partial beams 9, which then form a multi-beam arrangement and enable faster overall processing by simultaneously processing different substrate areas. A multi-beam arrangement can also be formed by using several lasers at the same time.

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

A selective emitter consists of more highly doped regions of the substrate surface 4, on which contact electrodes are later formed. The selective emitter structure includes, as in 2 Shown schematically, a large number of thin fingers 6 and some main lines 7 arranged perpendicular to them, which are referred to as “bus bars” based on the English term. In a favorable embodiment of the laser doping system, 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, a selective emitter structure can be produced in a general doping system, which carries out both the formation of a flat basic doping and a local selective emitter doping. However, specialized systems can also be built that are specialized either only for the formation of basic area dopings or only for the formation of local or selective dopings. A special system for producing a selective emitter structure could also be combined with a conventional oven doping process. In this case, a highly doped surface layer, such as the so-called “dead layer”, which is present as an undesirable layer after furnace doping could also be used as a dopant source.

The selective emitter structures that were produced using the laser doping process according to the invention are characterized by particularly favorable properties. The higher doping depth that can be produced with the laser doping method according to the invention has already been mentioned. Another positive characteristic that results from a deep and homogeneously doped emitter is a better barrier effect against the diffusion of contact materials, such as. B. copper in the substrate material. Another characteristic worth emphasizing of the selective emitter structure produced in the laser doping process according to the invention is the sharp lateral demarcation of the more highly doped regions from neighboring regions. In addition, neighboring areas do not experience any damage due to the selective emitter doping.

3 shows schematically a preferred essentially 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. This achieves rapid heating of the substrate. The high performance is then maintained for a relatively long time, which can be between 80 ns and 10 µs. The laser pulse does not have any significant power peaks, not even at the beginning of the laser pulse. Due to the energy input within a relatively long period Good diffusion of the dopants and gentle substrate processing are achieved over a period of time. In principle, it is possible that at least partial melting of the substrate surface 4 can occur. In a preferred embodiment of the invention, however, the substrate surface 4 is only heated by the fiber laser 1 to such an extent that its solid state is retained. In this way, undesirable melting and recrystallization effects can be avoided and very good doping results can still be achieved.

4 shows a section of a possible inline system, which has a laser doping system as a component in the form of a laser doping module. The inline system has further components, such as lock chambers, but the other components are not shown. The laser doping system 10 includes the process chamber 11 and components outside the process chamber 11. A fiber laser 1 generates a laser beam 2 which radiates into the process chamber 11 through a window 12. In the exemplary embodiment shown, the laser doping system 10 has only one scanner, which is specifically the polygon scanner 3a. The scanner can move the laser beam 2 in a spatial direction; To adjust the position of the laser processing on the flat substrate surface 4 in a second spatial direction, the substrate is moved in the illustrated embodiment with the substrate transport device 15. In other embodiments of laser doping modules, not shown, discontinuous transport devices and two-dimensional scanner units are used. The laser doping system also includes a media supply 13, which can provide liquid and gaseous media in a controlled manner, and a media disposal unit 14, which serves to remove used liquids, gases and auxiliary gases.

5 In addition to the previous figures, schematically shows that the scanner unit 3 can move the laser beam 2 on a path E over several substrates 8. In the example shown, the laser beam 2 is moved in the y direction. No movement means is shown for controlling the laser path in a second spatial direction x on the substrates 8. For movement, the beam or the wafer can be moved, or both the wafer and the beam can be moved.

6 outlines another possible embodiment of a laser doping system and a process sequence for laser doping. In a first step, one or more substrates 8 are moved into the system in a transport direction T by a transport system 15, which can be, for example, a belt transport system, an air cushion transport system or a moving chuck transport system. In a first method step E, a dopant is applied, which can be, for example, a liquid, which is applied to the substrates 8, for example, using a spray or roller technique. In a next method step F, the substrates 8 are transported further to a position detection system 17, where the substrates 8 are picked up by a chuck 16, the exact substrate position is recognized and the substrates 8 are then placed on the chuck 16 with a high spatial positioning accuracy into the scanning area of a fiber laser explained in the above exemplary embodiments.

In a next process step G, the substrate is laser doped with the laser beam 2 of the fiber laser shown schematically here, with locally defined doping of the substrates 8 also being possible due to the high positioning accuracy. In a further method step H, the substrates 8 are further transported, also with high positioning accuracy, to a control device 18, where the result of the laser doping can be checked. After the check has been carried out, the substrates 8 are transported further using a normal transport system 15. In a next process step K, the substrates 8 are cleaned, for example by applying a rinsing liquid.

Claims (7)

Method for doping silicon substrates (8), in which a fiber laser (1) generates a laser beam (2) with a wavelength that is in the range from 750 nm to 3000 nm and, on the other hand, meets the condition that the silicon substrate is at the wavelength used is transparent, and generates the laser beam with a round beam cross section, the laser beam being guided over the substrate surface (4) with a scanner unit (3), in which at least one dopant is in contact with the substrate surface (4), in which a local heating of the Substrate surface (4) is carried out by the laser beam (2) for laser doping and in which the laser doping takes place below the melting temperature of the silicon substrate and the laser beam (2) is split into partial laser beams (9) using a beam splitter (5) and the partial laser beams (9) simultaneously produce locally defined structures in different substrate regions, the substrate surface being in contact with a dopant source in a gaseous state from which the dopant is provided. Procedure according to Claim 1 , characterized in that the fiber laser (1) emits continuously as a continuous wave laser or as a pulsed laser with pulse lengths in the range from 80 ns to 10 µs. Procedure according to Claim 2 , characterized in that the pulsing of the fiber laser (1) is carried out in a time-power diagram with a rectangular pulse shape without power peaks. Procedure according to Claim 1 , characterized in that the method is used for doping the entire surface of silicon substrates (8). Procedure according to one of the Claims 1 until 4 , characterized in that the speed of movement of the scanner unit (3) is reduced in locally limited areas of the substrate surface (4). Procedure according to one of the Claims 1 until 5 , characterized in that several lasers (1) simultaneously irradiate different spatial areas of the substrate surface (4). Procedure according to one of the Claims 1 until 6 , characterized in that the scanner unit (3) moves the laser beam (2) in at least one spatial direction over at least two silicon substrates (8).

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