EP2061915A2

EP2061915A2 - Apparatus and method of forming thin silicon nitride layers on surfaces of crystalline silicon solar wafers

Info

Publication number: EP2061915A2
Application number: EP07801317A
Authority: EP
Inventors: Birte Dresler; Volkmar Hopfe; Ines Dani; Rainer MÖLLER; Milan Rosina
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2006-09-01
Filing date: 2007-08-29
Publication date: 2009-05-27
Also published as: DE102006042327A1; WO2008025353A2; WO2008025353A3; DE102006042327B4

Abstract

The invention relates to an apparatus and a method of forming thin silicon nitride layers on surfaces of crystalline silicon solar wafers. It is an object of the invention to provide possibilities allowing thin silicon nitride layers to be produced on surfaces of crystalline silicon solar wafers, exhibiting a defined layered-material formation having desired properties. The apparatus of the invention is designed so that at a reaction chamber region, above a silicon solar wafer surface to be coated, there is a supply for at least one precursor containing gaseous silicon, this precursor contributing to layer formation. Moreover, a source emitting electromagnetic radiation, which is a plasma source, is disposed in such a way that the electromagnetic radiation emitted effects photolytic activation of atoms and/or molecules of the precursor(s). The plasma source ought to be so disposed, and is also to be operated in such a way, that there is no direct influence of the plasma on the silicon solar wafer surface and on the precursors that lead to layer formation, and it is exclusively the electromagnetic radiation emitted which acts.

Description

Device and method for forming thin silicon nitride layers on surfaces of crystalline silicon solar wafers

The invention relates to an apparatus and a method for forming thin Siliziumnitridschichten on surfaces of crystalline silicon solar wafers.

In the production of solar cells, it is customary to provide the surface exposed to the useable radiation with a passivating and optical properties improving thin coating. This is usually formed from silicon or titanium nitride. In particular, under the aspect of efficiency, the optical properties play an important role in order to reflect as little as possible on electromagnetic radiation on the outer surface and to absorb it in the layer. Furthermore, the layers should be suitably bound to water. fabric included. This serves to saturate (passivate) defect centers in the interior and at the surface of the solar cells and thus ultimately leads to a further improvement in cell efficiency over the service life increase of the free charge carriers.

In the thin-film technique, a variety of methods are known to form thin films on silicon solar wafer surfaces. The training takes place predominantly under vacuum conditions, for example by thermal CVD, PVD or PECVD techniques. It is obvious that the production cost is considerable and only limited silicon solar wafer surfaces are so coatable.

Frequently, only low coating rates can be achieved or problems due to coating defects (for example, droplets or inhomogeneous layer formation) can be accepted. Certain properties of a coating can not be achieved, which may also apply to other methods to be mentioned. Particularly in the case of a thermal CVD formed by the required high temperatures an impairment of already advantageously set electronic states or surface textures occur.

It is also known to use electromagnetic radiation in vacuum as activation for CVD coatings. In this case, emitted by a light source electromagnetic radiation is directed from the outside through a window in a coating chamber. Such windows must be cleaned very expensive and there are transmission losses. Layers can also be formed in SoI gel technique. However, not all desired coating materials can be realized and very high temperatures are required for forming and curing the layers.

A layer formation by means of plasma sources under atmospheric pressure conditions is known from DE 102 39 875 A1, DE 10 2004 015 216 B4 and the formation of thin layers of silicon nitride from DE 10 2004 015 217 B4. In these solutions, a plasma source is used, which is supplied to a gas or gas mixture for plasma formation. The plasma gas also contains at least one component which is also used for layering. But it can also be at least one

Precursorgas be additionally introduced into the plasma or in the outflowing plasma gas flow and used for the film formation ("remote plasma activation") .In any case, however, plasma is directly applied to the silicon to be coated.

Solar wafer surface directed and effective for the reactive formation of layers on silicon solar wafer surfaces directly and actively. As plasma sources, arc or microwave plasma sources can be used.

It has been found that layers can be formed with these known technical solutions, as is explicitly known for silicon nitride from DE 10 2004 015 217 B4. Certain properties and

However, coating material compositions can not be achieved in this form.

It is therefore an object of the invention to provide possibilities with which thin silicon nitride layers can be deposited on surfaces of crystalline silicon oxide. Solar wafers can be produced, which have a certain coating material formation with desired properties.

According to the invention, this object is achieved with a device having the features of claim 1. It can be worked with a method according to claim 12. Advantageous embodiments and further developments of the invention can be achieved with features described in the subordinate claims.

In this case, the device according to the invention is designed so that a feed for at least one gaseous silicon-containing precursor is present at a reaction chamber area above a silicon solar wafer surface to be coated, which contributes to layer formation. In addition, a source emitting electromagnetic radiation, which is a plasma source, is arranged such that a photolytic activation of atoms and / or molecules of the precursor (s) takes place with the emitted electromagnetic radiation. The plasma source should be arranged in this way and should also be operated in such a way that no direct influence of the

Plasmas on the silicon solar wafer surface and leading to the layer formation precursors occurs and only the emitted electromagnetic radiation acts.

Preferably, the plasma source should be arranged within the reaction chamber area, wherein a window arranged therebetween can be dispensed with in order to avoid the disadvantages already mentioned in the introductory part of the description. The invention can be used under vacuum conditions but also at atmospheric pressure, atmospheric pressure being understood to mean a pressure range of ± 300 Pa around the respective ambient atmospheric pressure.

Particularly preferred is to be emitted with the plasma source electromagnetic radiation having wavelengths less than 230 nm. Electromagnetic radiation in the wavelength range of UV light and below is particularly suitable for the desired photolytic activation. This can be achieved with suitable gases and gas mixtures for plasma formation. The particular gas or gas mixture has an influence on the emission spectrum of the radiation and can therefore be adapted to the precursor (s) used for layer formation. For the formation of the plasma, the following gases can be used alone or as a mixture of at least two of these gases: argon, neon, helium, nitrogen, ammonia, hydrogen, oxygen, carbon dioxide, nitrogen dioxide and water vapor.

Various amorphous silicon nitride layers with certain stoichiometries and lattice structure or network structure can also be formed with the invention. The silicon nitride can also advantageously contain hydrogen in bound form, which in turn can favorably influence the properties of solar cells.

Organic silicon compounds can be used as precursors. Alternatively, or in a mixture, these may also be silanes or halosilanes, which may also be supplied as a gas mixture and photolytically activated for layer formation. By chemical reactions can then be formed each desired coating material as a thin layer on the silicon solar wafer surface.

Thus, for example, with SiH ₄ and ammonia, an amorphous hydrogen-containing silicon nitride layer can be formed as a layer on silicon wafers for solar cells, in order to improve the optical properties for this application compared to known solutions and at the same time to achieve a passivating effect against volume and surface defects ,

As a plasma gas for generating the electromagnetic radiation, argon nitrogen or an argon-ammonia mixture in the ratio of 100: 1 can be used. The ratio of layer-forming ammonia to silane is for example 4: 1. The temperature of the silicon solar wafer during the layer formation is about 150 ⁰ C, but can be increased to improve the coating properties up to 400 ⁰ C. The deposition rate is usually in the range of 1 to 10 nm / s. The refractive index of the layers can be adjusted within wide limits by the choice of the ratio of ammonia to silane and other process parameters between 1.7 and 2.3 (at

% 50 nm wavelength). The layers are transparent throughout the entire spectrum of sunlight.

The invention will be explained in more detail by way of example in the following.

Showing:

Figure 1 is a perspective view of an example of a device according to the invention and FIG. 2 is a sectional view of a device according to FIG. 1.

The device shown for the invention and in Figures 1 and 2 may be at least similar, as has already been addressed in the introduction to the description of the formation of layers at atmospheric pressure by means of plasma. Only a different arrangement and / or a different operation of the plasma source 2 has been selected.

Through a gap 7, a silicon solar wafer 1, which is to be coated on a surface, introduced and passed through the device. There is a relative movement between silicon solar wafer 1 and the device. Thus, the entire at least, but a large part of the surface, can be coated.

A plasma is formed with an arc formed between a cathode and an anode. The plasma source 2 is arranged in a windowless reaction chamber area 11. The arc is fed to a plasma gas.

In this case, a volume flow and also a pressure for supplied plasma gas is selected, which is used for plasma formation and thus for the emission of electromagnetic

Radiation sufficient but prevents plasma enters a region of the reaction chamber area 11, is present in the silicon-containing precursor for the formation of a thin amorphous silicon nitride layer. One or more gaseous precursor (s) are introduced via the feed 9 into the reaction chamber area 11. The activation of the atoms and / or molecules of the precursor (s) takes place exclusively photolytically by means of the electromagnetic radiation emitted by the plasma source 2. As a result of this activation, chemical reactions of the precursor (s) take place and the thin layer can be formed on the surface of the silicon solar wafer 1.

FIG. 2 is intended to further clarify how a device suitable for use under atmospheric pressure can be designed.

In this case, a sensor 10 is present at the supply for plasma gas, with the aid of which a control by a determination of pressure and / or flow rate of the supplied plasma gas can be done.

The correspondingly elongate arc plasma source 2 aligned in the plane of the drawing is arranged here in a slot-shaped reaction chamber region 11. Electromagnetic radiation emitted by the plasma impinges on the surface of the silicon solar wafer 1 to be coated and penetrates gaseous precursor (s) that enter the reaction chamber area 11 via the supply 9 just above the silicon solar wafer surface is / are introduced.

The superfluous reaction products can be removed as exhaust gas via an exhaust gas extraction 5 and 5 '. This can take place in the feed direction in front of and behind the reaction chamber area 11, but also circumferentially. For a seal with respect to the environment, an inert purge gas can be fed into a gap 7 via feeds 4 and 4 'formed around the reaction chamber area 11. The purge gas flows out of the device in one direction and into the reaction chamber region 11 in the opposite direction. However, purge gas can be removed again with the exhaust gas via the exhaust 5 and 5 ', so that no but at least the largest part of the purge gas supplied does not enter the reaction chamber region 11 and the layer formation process is not affected thereby.

For a regulation of SpülgasZuführung and exhaust of exhaust gas here more sensors 6 and 8 are present.

In contrast to the illustration, the reaction chamber region 11 can also be designed in such a way that, starting from the plasma source 11, it widens as conically as possible. As a result, a larger surface area can be used, since the emitted electromagnetic radiation propagates divergently anyway. Thus, at least the surface coating rate reduced in comparison with plasma-assisted process control can be compensated for again.

The device shown in FIGS. 1 and 2 has a further advantage over other devices which can also be used with the invention. It can be operated temporarily, if desired, even in conventional form. This is particularly favorable in the case of a layer formation with at least two layers which are arranged one above the other. For example, the silicon solar wafer 1, as with the Arrow indicated in Figure 1, are first passed from left to right through the device. The formation of the layer is carried out according to the invention alone by photolytic activation. Subsequently, an oppositely directed movement of the silicon solar wafer through the device takes place. In this case, pressure and / or volume flow of the plasma gas is increased so that the layer formation can be carried out in a conventional manner.

Of course, it is also possible to proceed in reverse order. The procedure can also be changed alternately, for example, to provide surface regions of the silicon solar wafer 1 with different thin layers.

Claims

claims

1. Device for Forming Thin Silicon Nitride Layers Layers on surfaces of crystalline silicon solar wafers, in which a feed for at least one gaseous silicon-containing precursor is present at a reaction chamber area above the respective surface and at least one source emitting electromagnetic radiation is arranged that by means of emitted electromagnetic radiation for the formation of a layer, a photolytic activation of atoms and / or molecules of the / the precursor follows, characterized in that the electromagnetic radiation emitting source is a plasma source (2).

2. Apparatus according to claim 1, characterized in that the plasma source (2) is arranged within the reaction chamber area (11).

3. Apparatus according to claim 1 or 2, characterized in that the plasma source (2) within the reaction chamber region (11) arranged and operable so that the plasma does not affect the / the precursor (s) directly.

4. Device according to one of the preceding claims, characterized in that at least within the reaction chamber region (11) is atmospheric pressure.

5. Apparatus according to claim 4, characterized in that a pressure in the range ± 300 Pa is maintained at the atmospheric pressure.

6. Device according to one of the preceding claims, characterized in that with the

Plasma source (2) electromagnetic radiation having wavelengths less than 230 nm is emitted.

7. Device according to one of the preceding claims, characterized in that silicon

Solar wafer (1) and reaction chamber area (11) with plasma source (2) are movable relative to each other.

8. Device according to one of the preceding claims, characterized in that an exhaust gas suction (5, 5 ') is connected.

9. Device according to one of the preceding claims, characterized in that a SpülgasZuführung (4, 4 ') is present.

10. Device according to one of the preceding claims, characterized in that purge gas in a gap (7) between the substrate surface and the reaction chamber region (11) can be fed and thereby a seal against the ambient atmosphere is formed.

11. Device according to one of the preceding claims, characterized in that the plasma source (2) is an arc or microwave plasma source.

12. A method for forming thin silicon nitride layers on surfaces of crystalline silicon solar wafers, wherein at least one precursor containing gaseous silicon is supplied above the respective surface into a reaction chamber area; a plasma source (2) is arranged and operated in the reaction chamber region (11) in such a way that the formation of a thin silicon nitride layer is exclusively due to photolytic activation of atoms and / or molecules of the precursor (s) by electromagnetic radiation emitted by the plasma source (2) is reached.

13. The method according to claim 12, characterized in that the formation of the layer is carried out at atmospheric pressure.

14. The method according to claim 12 or 13, character- ized in that the plasma is formed with a plasma gas, is emitted with the electromagnetic radiation having wavelengths less than 230 nm.

15. The method according to any one of claims 12 to 14, characterized in that a gaseous organic silicon compound is supplied as a precursor.

16. The method according to claim 15, characterized in that a silane and / or a halosilane is / are supplied.

17. The method according to any one of claims 12 to 16, characterized in that as a further pre- Cursor ammonia, nitrogen and / or hydrogen is supplied.

18. The method according to any one of claims 12 to 17, characterized in that for the Plasmabil- düng the volume flow of the plasma source (2) supplied plasma gas temporarily increased and thereby a different parameters having layer - education is achieved under these conditions.

19. The method according to any one of claims 12 to 18, characterized in that for the plasma formation, a gas which is selected from argon, nitrogen, ammonia and hydrogen is used.

20. The method according to any one of claims 12 to 19, characterized in that gaseous reaction products are removed by suction as exhaust gas.

21. The method according to any one of claims 12 to 20, characterized in that with a supplied inert purge gas, a seal between

Substrate surface, reaction chamber area (11) and ambient atmosphere is achieved.