WO2014083241A1

WO2014083241A1 - Method for fabricating a passivation film on a crystalline silicon surface

Info

Publication number: WO2014083241A1
Application number: PCT/FI2013/051091
Authority: WO
Inventors: Shuo Li
Original assignee: Beneq Oy
Priority date: 2012-11-29
Filing date: 2013-11-21
Publication date: 2014-06-05
Also published as: FI20126253L

Abstract

A method for fabricating a passivation film (1) on a crystalline silicon surface (2), the passivation film (1) comprising aluminium oxide AIO_x, comprises providing an existing crystalline silicon surface (2) in a reaction space; depositing in the reaction space a first passivation layer (3) on the existing silicon surface, wherein the existing silicon surface with the deposit already formed thereon is alternately exposed to surface reactions of at least one first precursor for aluminium and at least one first precursor for oxygen; and depositing in the reaction space a second passivation layer (4) on the first passivation layer, wherein the first passivation layer with the deposit already formed thereon is alternately exposed to surface reactions of at least one second precursor for aluminium and at least one second precursor for oxygen. According to the invention, the at least one first precursor for oxygen comprises ozone O₃.

Description

METHOD FOR FABRICATING A PASSIVATION FILM ON A CRYSTALLINE SILICON SURFACE

FIELD OF THE INVENTION

The present invention relates generally to thin film passivation of semiconductor surfaces, in particular to passivation of crystalline silicon surfaces of e.g. photovoltaic cell structures.

BACKGROUND OF THE INVENTION

Photovoltaic (hereinafter abbreviated as "PV") cells are becoming more and more important means for gener^¬ ating electrical energy. Especially solar cells, i.e. photovoltaic cells designed to convert sunlight di^¬ rectly into electrical energy, are considered as one of the most promising candidates for renewable energy production .

Among the various PV and solar cell technologies which typically are classified according to the light ab^¬ sorbing semiconductor material (s) of the cell, crys^¬ talline silicon, i.e. "c-Si" is one of the major mate^¬ rials in commercial PV modules and devices.

As in any energy converting device, the efficiency of conversion of the primary form of energy, i.e. light, into the secondary form of energy, i.e. electricity, is of crucial importance in commercial PV cells. There are numerous factors each having an impact to the overall conversion efficiency. These include e.g. the reflectance of the cell surface and the thermodynamic configuration of the cell structure affecting the ex^¬ ternal efficiency of the cell. On the other hand, there are also many factors related to the internal mechanisms within the cell structure. One particular internal mechanism related to the semiconductor mate^¬ rial as the active material, i.e. the material where the absorption of incident light and the generation of free charge carriers take place, is surface recombina^¬ tion. By surface recombination is meant undesired re^¬ combination of generated charge carriers at or in the vicinity of the surface of the active semiconductor material of the cell. Surface recombination in semi^¬ conductors is a result of many different mechanisms leading to trapping of charge carriers in specific en^¬ ergy states at or close to the surface of a semicon^¬ ductor. These energy states, or surface states as they are often called, may originate from different sources, such as impurities at the surface or the in^¬ evitable disruption of periodicity of a semiconductor crystal at the surface. In a photovoltaic cell the quantum efficiency, and therefore the overall effi^¬ ciency, decreases as charge carriers generated by the absorption of photons in the semiconductor recombine with the surface states and therefore cannot be col^¬ lected in the cell electrodes to contribute to the cell current.

As a measure of the passivation performance is typi^¬ cally used the minority charge carrier lifetime at the surface to be passivated. The higher the carrier life^¬ time, the lower the surface recombination velocity, and thus the better the passivation performance. In practice, the actual parameter most often used as the measure of the passivation performance is the effec^¬ tive lifetime x_eff which is affected both by the sur^¬ face lifetime T_surf at the passivated silicon surface and by the bulk lifetime T_buik in the bulk silicon.

In the case of c-Si PV cells, the surface recombina^¬ tion can be reduced by a particular passivation film on either or on both of the silicon surfaces. Nowadays, the most promising passivation film material used for commercial c-Si PV cells is aluminium oxide ΑΙ Οχ . In the ALD A10_x, the x typically lies in the range of 1.5...2.0, thus possibly deviating from the stoichiometric composition A1₂0₃. For example, with TMA and 0₃ as the precursors for aluminium and oxygen, re^¬ spectively, x can be up to 1.69.

One of the most common deposition technologies for manufacturing passivation thin films is atomic layer deposition ALD and its different variations. ALD pro^¬ duces a very conformal and uniform passivation film which can be produced by reasonable costs at moderate temperatures. According to the current common under^¬ standing in the art, the actual passivation function of an ALD aluminium oxide is based on at least two functions. First, hydrogen atoms present in the ALD aluminium oxide layer passivate the Si dangling bonds, thereby reducing the interface state density D_it at the Si surface (chemical passivation) . Second, an oxygen- rich ΑΙ Οχ passivation film produces a high density of fixed negative charges at the Si surface, thereby cre^¬ ating a built-in electrical field which reduces the minority charge carrier density at the Si surface (field effect passivation) . In addition to hydrogen H and oxygen 0, also other impurities such as carbon C and chlorine CI may affect the passivation perfor^¬ mance, though the detailed mechanisms of different im^¬ purities are not fully known.

As stated above, one key parameter affecting the pas^¬ sivation performance is the amount of H atoms at and close to the Si/A10_x interface. This, as well as the composition of the A10_x passivation film in general, is affected by the actual process type and the pro- cesss parameters used, such as the deposition tempera^¬ ture and the precursors, particularly the precursor (s) for oxygen. In addition to the actual composition of the film, including the impurities therein, and so the functional characteristics of the passivation film, the selection of the precursors also affect the depo^¬ sition process e.g. via the different growth rates for different precursors. Thus, the precursor selection is in a key role in overall optimization of the PV cell and its manufacturing. For example, it is clear that water H₂0 as a precursor for oxygen may produce a higher growth rate than ozone 0₃. On the other hand, ozone has other advantages, such as high excess oxygen content in the deposited film, which affects positive^¬ ly the field effect passivation via the high amount of fixed negative charges.

In US 2012/0255612, the advantages of different oxygen precursors are proposed to be combined by depositing a two-portion passivation film. The first portion is deposited using water or some other "milder" oxidant than ozone as the oxygen precursor. The second portion is deposited using ozone or oxygen plasma as the oxygen precursor. The reason for water or other "mild" oxidant as the oxygen precursor for the first portion is that the major problem of using ozone (0₃) as a precursor for oxygen results in undesirably high interface state density (D_it) of the Si surface. However, according to US 2012/0255612, the advantages of ozone, such as the high negative charge density, can be exploited via the use of ozone as the oxygen precursor in the deposition of the second portion of the passivation film.

In addition to the surface passivation performance itself, another key feature of a passivation film for passivation c-Si surfaces in Si PV cells is the stability of the passivation film, particularly the thermal stability, i.e. the stability of the passivation film under high temperatures. During the further steps of a PV cell manufacturing process, the passivation film deposited on the Si surface (s) typically first experiences a thermal treatment of tens of minutes at an elevated temperature of about 350 - 450°C. This can occur e.g. during deposition of silicon nitride SiN_x by plasma enhanced CVD (PECVD) . Further, during the formation of the metal contacts, the cell structure together with the passivation film(s) and contact pads formed of silver or aluminium (for front and back surfaces, respectively) pastes thereon is exposed to so called firing, during which the temperature is held at about 800 - 900°C from some seconds to some tens of seconds. During the firing step, the metal of the contact pads interact with the underlying layers, thereby forming ohmic contacts to the semiconductor layers of the cell .

During annealing and/or firing, the interface between the passivation film and c-Si is restructured. Structural and compositional changes may take place in the "bulk" of the passivation film also. These mechanisms may result in several problems. First, with some precursors and process parameters, a dramatic reduce of the minority carrier lifetime has been observed after firing.

Another severe problem seen in some passivation films after high temperature treatment is blistering, i.e. formation of blister or bubbles on the interface of the A10_x passivation film and the c-Si. This phenomenon can have many causes, but the main reason is believed to be the excess hydrogen in the bulk and at the interface of the very dense amorphous A10_x passivation film, which excess hydrogen under certain circumstances bubbles on said interface in form of pure hydrogen or as water (H₂0) . Apart from greatly reducing the passivation performance, a "blistered" structure also has undesired parasitical local contacts between the contact metal and silicon after annealing and/or firing of the cell. This is due to interdiffusion of the silicon in the contact metal at the locations of the blisters. This results, for example, in shunts and increased leakage current, and thus decreased overall performance of the PV cell. In addition to the amount of hydrogen in the bulk and at the interface, and the time and temperature of the thermal treatment to which the passivation film is exposed, blistering is also shown to be dependent on the passivation film thickness. In general, increasing the passivation film thickness increases the occurrence of blisters.

To prevent blistering, some useful approaches are known. First, particular high-temperature treatment at about 600 °C have been found to prevent formation of blisters with film thicknesses of less than 10 nm. The high temperature helps to degas the hydrogen from the film. However, such a high temperature requires time and thus affects adversely the production time and costs of the passivation. A long time annealing at such high temperatures also degrade the passivation quality of the A1₂0₃ film. Blistering can be avoided also by using TMA as the precursor for aluminium and H₂0 or plasma 0₂ as the precursor for oxygen and a deposition temperature of higher than 250°C. However, the effective lifetime as the measure of the passivation performance remains then lower in comparison to the case of a lower deposition temperature of e.g. 200°C.

To summarize, despite the intense development in the art during the recent years, there is still a strong need for further high quality passivation films and the manufacturing methods thereof.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a novel method for fabricating a high quality A10_x passivation film on a crystalline silicon surface.

The present invention is characterized by what is presented in claim 1.

The present invention is focused on a method for fabricating a passivation film on a crystalline silicon surface, the passivation film comprising aluminium oxide A10_x, for reducing the surface recombination of charge carriers on the silicon surface .

By passivation film is meant here a passivation coating in the form of a thin film. By thin film is meant here generally a film with a thickness of less than or equal to 1000 nm, often less than or equal to 200 nm. Typically, the total thickness of the thin film passivation film is clearly below these values, e.g. about 20-30 nm.

By crystalline silicon surface is meant here a surface of single crystal silicon, multicrystal silicon or mono-like (quasi mono) crystal silicon.

The passivation film, i.e. passivation material deposited on the silicon surface, comprises aluminium oxide ΑΙ Οχ , wherein the "x" denotes the composition possibly deviating from the stoichiometric A1₂0₃. Said passivation material preferably consists mainly of ΑΙ Οχ , though it may also contain some other material (s) . Aluminium oxide A10_x, in its turn, typically comprises some impurities such as hydrogen H, oxygen 0, carbon C, and chlorine CI.

As described in the background section above, the main general purpose of the passivation film is to reduce the undesired surface recombination of charge carriers on the silicon surface, which might cause deterioration of the optoelectronic device, to which said silicon surface belongs.

The method of the present invention comprises providing an existing crystalline silicon surface in a reaction space; depositing in the reaction space a first passivation layer on the existing crystalline silicon surface, wherein the existing crystalline silicon surface with the deposit possibly already formed thereon is alternately exposed to surface reactions of at least one first precursor for aluminium and at least one first precursor for oxygen; and depositing in the reaction space a second passivation layer on the first passivation layer, wherein the first passivation layer with the deposit possibly already formed thereon is alternately exposed to surface reactions of at least one second precursor for aluminium and at least one second precursor for oxygen .

Said alternate exposure of the deposition surface, i.e. the surface on which the material is to be deposited, to the precursors for aluminium and oxygen refers to a chemical vapour deposition method often called atomic layer deposition ALD. Sometimes, also other names such as atomic layer epitaxy (ALE) , atomic layer chemical vapor deposition (ALCVD) , and corresponding plasma enhanced variants can be used to refer to this kind of "ALD-type" process. In ALD or ALD-type process, the material is grown on the deposition surface, typically controlled by self- limiting surface reactions, by a monolayer basis, producing thus a very uniform, conformal film with very accurately controllable film thickness.

The alternate or sequential exposure of the deposition surface to different precursors can be carried out in different manners. In a batch type process, the precursors for the different elements of the composition to be deposited are supplied to the reaction space in gaseous form as separate pulses, between which the residuals of the precursor (s) for the previous element are removed from the reaction space during a separate purging step. When pulses of precursors for both elements, i.e. aluminium and oxygen, have been supplied to the reaction chamber so as to expose the deposition surface to the precursors, the same cycle is usually started again and repeated as many time as required for the desired thickness of the deposit.

As an alternative, in spatial atomic layer deposition systems, the exposure to different precursors is based on spatial separation of the precursor gases. The different precursors can be confined in specific process areas or zones within a reactor space, through which the deposition surface is passed. In this kind of continuous ALD-type process, constant gas flow zones separated in space and a moving substrate are thus used to provide the required sequential exposure to different precursors. In continuous ALD-type process, the cycle time depends on the speed of movement of the substrate through and between the gas flow zones . "At least one" precursor for aluminium or oxygen, respectively, means that one, two, or more different precursors for a single element can be used during a single cycle, supplied to the reaction space either simultaneously or sequentially. In the latter case, a purging step can be performed between the different precursors to remove the residuals of the previous precursor from the reaction space before supplying there the next precursor. In different cycles of deposition of each layer, however, the at least one precursors for oxygen and aluminium are kept the same. In other words, the precursors are changed only when starting the deposition of the next layer.

The reaction space can be part of any deposition chamber suitable for ALD-type deposition of aluminium oxide. The details of the reaction space or other equipment used in the deposition are not essential for the core principles of the present invention, and they can be selected according to principles as such known in the art. Suitable process equipment are commercially provided e.g. by Beneq.

As is clear for a person skilled in the art, the above description does not specify all details of an ALD- type deposition process. Naturally, there can be e.g. various pre-treatment steps of the deposition surfaces. Such details of the process can be selected according to the principles as such known in the art.

By "layer" is meant in this specification generally a volume of deposited material extending along the direction of the crystalline silicon surface so that its thickness is substantially lower than the lateral dimensions of the deposited material along the direction of the crystalline silicon surface. The different layers and the boundaries thereof in the passivation film are not necessarily easily observable in the completed structure. In this sense, though different precursors naturally produce different compositions of the deposited layers, "layer" primarily refers here to the different stages of the manufacturing method and to the different precursors used in those stages. On the other hand, a layer does not necessarily extend over the entire surface area of the surface to be passivated.

The basic principle of depositing at least two passivation layers, as defined above, enables one to combine the advantageous features of different precursors to adjust the accurate composition of the deposited passivation film. By the composition is meant here the actual composition A10_x, particularly the proportion x of oxygen, but also, which is very important for the passivation performance of the passivation film, the amounts and distributions of the impurities such as H, 0, C, and CI in the Si/A10_x interface and within the bulk passivation film (there is typically a silicon oxide layer formed at this interface, so the interface is actually Si/SiO_y/A10_x) . These impurities are essential factors affecting e.g. the carrier lifetime, the passivation film stability, and the occurrence of blistering.

According to the present invention, the at the least one first precursor for oxygen comprises ozone 0₃. In other words, during deposition of the first passivation layer, ozone is used as a precursor for oxygen. This can be implemented by using ozone, possibly supplied along one or more carrier/diluting gases, as the only precursor for oxygen when depositing the first layer. Alternatively, in the case of more than one first precursors for oxygen, ozone is used as one of them. For example, ozone and water can be used as the first precursors supplied to the reaction space simultaneously or sequentially during a single deposition cycle. However, when using both water and ozone as the first precursors for oxygen, it may be beneficial to continue the deposition of the first passivation layer so as to form a relatively thin first "layer" only, e.g. about 1 nm, in order to ensure efficient reduction of blistering in comparison to the case of using water as the only first precursor for oxygen.

The key principle of using ozone as the first precursor for oxygen in the process as defined above is based on the surprising observation by the inventor that this way the blistering can be greatly reduced in comparison to the case of depositing a passivation film by using water as the only precursor for oxygen. This is believed to be a consequence of a lowered H content at the Si/A10_x interface, though the accurate mechanisms are not fully known. Reduced blistering greatly enhances the overall passivation performance and decreases the number of parasitical local contacts between the contact metal and silicon. Further, reduced blistering allows one to increase the overall thickness of the layered passivation film, which further increases the passivation performance via the increased amount of hydrogen affecting positively the chemical passivation.

In particular, when comparing the present invention to the process and teachings of US 2012/0255612, the inventor has found that the alleged problem of high interface state density (D_it) of the Si resulted when using ozone as a precursor for oxygen is actually not any dramatic problem in a completed solar cell. This is due to the fact that during the firing step, the the Si/A10_x interface is restructured, wherein the interface state density decreases strongly. Thus, in contrast to the approach proposed in US 2012/0255612, the present invention fully exploits the advantages of using ozone as a precursor for oxygen when depositing the first passivation layer on the existing crystalline Si surface. One of these advantages is the high negative fixed charge density. When ozone is used in the deposition of the first passivation layer, the fixed negative charges (due to the oxygen-rich composition of A10_x) are generated at or close to the Si/A10_x interface, and they do not need to migrate there through any other passivation layer as is the case e.g. in the approach by US 2012/0255612.

It is a common practice in the art that when using ozone as a precursor for oxygen, the ozone concentration of the deposition atmosphere is kept high. As high ozone concentrations as 300 g/Nm³ and higher at the output of the ozone generator are often used. (With a typical case of N₂ as a carrier gas flow with 0.5 Standard Litre per Minute SLM, 300 g/Nm³ makes an ozone molar concentration of 16 % of the total flow.) However, the inventor has surprisingly found that the advantageous features of using ozone as a precursor for oxygen during the deposition of the first passivation layer can be further intensified by lowering the ozone concentration from those values used in prior art. Thus, in one embodiment of the present invention, during depositing the first passivation layer, the existing crystalline silicon surface with the deposit possibly already formed thereon is exposed to a deposition atmosphere comprising ozone 0₃ with a mole ratio of (kT+b)%, wherein k lies in the range of -0.09 to -0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temperature in Celcius (put in the formula as plain number without the unit thereof) . By deposition atmosphere is meant the gaseous atmosphere in the reaction space, with which atmosphere the deposition surface is in contact during the exposure to a precursor for oxygen. This deposition atmosphere can comprise e.g. nitrogen N₂ as a carrier gas, and ozone 0₃ and oxygen 0₂ supplied by an ozone generator.

Typically, precursor doses or concentrations are given in grams of ozone per normal cubic g/Nm³ produced by the ozone generator used, or supplied to the reaction space. However this is actually not very informative unit because the actual dose of ozone to which the deposition surface is exposed depends largely e.g. on the carrier gas flow with which the ozone flow is mixed, and also the specific reactor type used. Thus, here the ozone dose or concentration is defined via the mole ratio, i.e. the molecular ratio of ozone of the overall deposition atmosphere:

(0₃ mole/ (O3+O2+N2) mole) x 100.

As can be seen in the formula of the ozone mole ratio above, the optimal ozone concentration depends on the deposition temperature T. This is due to the simple fact that also the deposition temperature affects the growth and the accurate composition of the deposited passivation film. As explained in the background section, it has been generally understood in the art that with the high ozone concentrations typically used, there is an optimal deposition temperature at about 200°C. Deviating from this optimum decreases the carrier lifetime and thus the passivation performance. Actually, similar behaviour has been now found by the inventor for the lower ozone concentrations according to the present invention also. Thus, for each deposition temperature, there is an optimal low ozone concentration defined by that formula above. Vice versa, for each such low ozone, there is an optimal deposition temperature. Thus, both the ozone concentration and the deposition temperature should be selected properly. For example, for T = 200°C, k = - 0.097, and b = 22.0, the ozone molar ratio according to said formula is 2.6 %. Changing the temperature to 150°C increases the ozone molar ratio to 7.45 %.

As is clear for a skilled person on the basis of the above formula for the mole ratio itself, the deposition temperature cannot be too high. For example, for deposition temperatures exceeding 250°C, said equation would produce a negative mole ratio, which is mindless. On the other hand, a skilled person also knows that there is also some lower limit of the deposition temperature, below which the surface reactions of the ozone, and thus the deposition process, no more works properly. On the other hand, a skilled person also understands that when lowering the deposition temperature too much, the ozone mole ratio produced by said formula approaches the self-ignition limits of ozone concentration. Therefore, it is clear for a skilled person that the deposition temperature T during the exposure of the existing crystalline silicon surface with the deposit possibly already formed thereon to surface reactions of the at least one first precursor for oxygen comprising ozone can lie e.g. in the range of 150°C to 200°C. On the other hand, the inventors have found that by using the claimed ozone mole ratio according to the above equation, as low deposition temperature as 100°C can be used. Thus, an overall deposition temperature range in which the above formula can be used to determine the appropriate ozone mole ratio can be as wide as from about 100°C to about 200°C. Even more preferably, the factors k and b of the formula above lie in the range of -0.097 to -0,098 and in the range of 21.9 to 22.1, respectively.

The at the least one second precursor for oxygen may comprise water H₂0. Water is a preferable second precursor for oxygen in particular due to the possibility to adjust thereby the amount of hydrogen in the passivation film so as to achieve high effective carrier lifetime x_eff . Again, it is also possible to use also some other second precursor for oxygen, e.g. ozone.

To further enhance the adjustability of the properties of the passivation film, the method may further comprise depositing in the reaction space a third passivation layer on the second passivation layer, wherein the second passivation layer with the deposit possibly already formed thereon is alternately exposed to surface reactions of at least one third precursor for aluminium and at least one third precursor for oxygen. In this kind of three-layer process, advantageous features of at least three different precursors for both oxygen and aluminium can be combined in a single passivation film and its manufacturing process.

The expressions "with the deposit already formed thereon", in the context of the "existing crystalline silicon surface", the "first passivation layer", and the "second passivation layer" refers to the fact that only during the very first precursor pulse, the exposed deposition surface is actually the existing crystalline silicon surface or the free surface of the first or the second passivation layer, respectively. After this first pulse, the deposition surface at issue comprises some deposit already formed thereon. Thus, the expression "with the deposit already formed thereon" actually means "with the deposit possibly already formed thereon", and covers both the surface at issue itself before any deposition thereon, and the same surface after one or more deposition cycles, when there is already some deposit formed thereon.

In one particular three-level process found to produce high quality passivation performance, the at least one second precursor for oxygen comprises water H₂0 and the at least one third precursor for oxygen comprises at least one of water H₂0, ozone 0₃, and dihydrogen dioxide H₂0₂. For example, water and ozone can be used as two precursors for oxygen for depositing the third layer. Thus, in this method, both water and ozone are used as precursors for oxygen when depositing the third passivation layer. In one specific advantageous alternative, they are supplied to the reaction space sequentially during a single cycle.

The passivation film thickness can be selected according to the principles known in the art. For example, the passivation film can be fabricated so as to have a total thickness of 1 to 200 nm.

Suitable compounds for use as the precursors for aluminium in the methods according to the present invention are e.g. trimethylaluminium TMA and dimethylaluminium chloride DMAC1. Thus, at least one of the first, second, and possible third precursor for aluminium may comprise at least one of TMA and DMAC1. For example, TMA and DMAC1 can be used as first and second precursors for aluminium, respectively. In addition to the precursors for oxygen, also the precursors for aluminium affect the deposited passivation film composition and thus the passivation performance. For example, it has been found that the presence of CI in DMAC1 makes the H content of the deposit produced by using DMAC1 as a precursor for aluminium to differ from the H content in the case of TMA as the precursor for aluminium.

The basic principles of the present invention and the various embodiments thereof apply to passivation of any crystalline silicon surface. Passivation enhances, via the decreased surface recombination, the performance of any optoelectronic device. The advantages of the present invention and the embodiments thereof are particularly clear when the silicon surface, on which the passivation film is deposited, is a surface in a photovoltaic cell. The method can be used to passivate both the rear and the front surface of a photovoltaic cell.

The embodiments of the invention described above may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention. In general, a method according to the present invention may comprise one or more of the preferred features of the embodiments above in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, some embodiments of the present invention are explained with reference to the accompanying drawings, wherein

Fig. 1 is a flow chart illustrating the steps of a method according to an embodiment of the present invention,

Fig. 2 is a block diagram illustrating a passivation film deposited according to an embodiment of the present invention, and

Fig. 3 illustrate one of the problems in prior art. DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 illustrates a method for fabricating a thin film A10_x passivation film on a crystalline silicon surface .

This exemplary embodiment of Figure 1 begins by bringing an existing crystalline silicon surface into the reaction space of an ALD system (step A) . The existing crystalline silicon surface can be e.g. a rear or back surface of a prefabricated silicon solar cell structure. The ALD reactor can be a part of an ALD system configured for a batch type or a spatial process. The reaction space is subsequently pumped down to a pressure suitable for forming a passivation film, using e.g. a mechanical vacuum pump. Alternatively, in the case of atmospheric pressure ALD systems and/or processes, protection flows are typically set to protect the deposition zone from the ambient atmosphere. The substrate is heated to a temperature suitable for forming the passivation film by the selected process.

After the existing crystalline silicon surface and the reaction space have reached the targeted temperature, e.g. 150 - 200°C, and other conditions suitable for deposition, e.g. a deposition pressure of about 1 mbar, the surface of the silicon substrate is typically pre-treated so as to condition it such that the passivation film may be deposited thereon. This conditioning of the silicon surface typically includes chemical purification of the surface from impurities and/or oxidation. Especially removal of oxide is beneficial when the silicon surface has been imported into the reaction space via an oxidizing environment, e.g. when transporting the exposed silicon surface from one deposition tool to another. In some alternative embodiments, the conditioning can be done ex-situ, i.e. outside the reaction space before introducing the existing crystalline silicon surface therein. An example of an ex-situ conditioning process is etching for 1 min in 1 % HF solution followed by rinsing in Dl-water.

After the possible pre-treatment of the crystalline silicon surface, in step B, an alternating exposure of the deposition surface to gaseous first precursors for aluminium and oxygen, wherein the first precursor (s) for oxygen comprises ozone, is started. Said alternating exposure means that the deposition surface is exposed alternately to one or more first precursors for aluminium and to one or more first precursors of oxygen, the latter one(s) comprising ozone. Subsequent exposures to precursors for aluminium and oxygen determines one deposition cycle. Preferably, during exposure to one or more precursors for oxygen, the deposition surface is exposed to an ozone concentration of e.g. 2.5 to 7.5. mole per cent of the deposition atmosphere, the optimal concentration depending on the deposition temperature used.

In each deposition cycle, there can be also more than only one first precursor for oxygen. TMA or DMAC1 are examples of the suitable precursors for aluminium. As usual in ALD, the reaction space is purged between the subsequent exposures of the deposition surface to different precursors and between the subsequent deposition cycles.

Cycles of exposure of the deposition surface to the alternating first precursors for aluminium and oxygen are repeated until the desired thickness of the first passivation layer, e.g. in the range of 1 to 5 nm, is achieved .

In step C, the crystalline silicon surface and the already formed deposit of the first passivation layer thereon is alternately exposed to gaseous second precursors for aluminium and oxygen in order to deposit a second passivation layer. Now, water H₂0 is preferably used as a second precursor for oxygen. The second precursor for aluminium can be the same as the first precursor for aluminium in step B. Again, the cycles of exposure of the deposition surface to the alternating second precursors for aluminium and oxygen are repeated until the desired thickness of the second passivation layer, e.g. in the range of 5 to 10 nm, is achieved

In the above exemplary method, all the details of the equipment as well as the process concerning the general arrangements and process steps can be selected and arranged according to the principles known in the art. The equipment used in the method can be based on a batch type reactor, a single wafer reactor, or a spatial reactor.

In the exemplary method illustrated in Figure 1, two passivation layers are deposited. Alternately, also one or more further passivation layers can be deposited on the second passivation layer in order to further increase the adjustability of the passivation film properties via the use of even more different precursors for oxygen and/or aluminium. Figure 2 shows an example of a passivation film this way deposited on a crystalline silicon surface 2.

The exemplary passivation film 1 of Figure 2 comprises three layers deposited by an ALD-type process. Such passivation film can be deposited by using TMA as a precursor for aluminium in deposition of the all three layers. In contrast, different precursors for oxygen are used on the three passivation layers. The first passivation layer 3 is deposited on a crystalline silicon surface 2 by using ozone as the precursor for oxygen. The concentration to which the deposition surface is exposed can be e.g. about 5 mole per cent, e.g. about 4.5 mole per cent or some less, of the deposition atmosphere, and the deposition temperature is, in this example, preferably about 200°C or less, e.g. about 190°C. The second passivation layer 4, instead, is deposited using water as the precursor for oxygen. The deposition temperature is preferably again the same 200°C. In the deposition of the third layer 5, both ozone (in a concentration of and 5 mole per cent) and water are used as precursors for oxygen so that the deposition surface is exposed to ozone and water sequentially during each single deposition cycle. The deposition temperature is preferably again about 200°C.

The first, the second, and the third passivation layers 3, 4, 5 of the passivation film of Figure 2 are preferably deposited so as to have the layer thicknesses of about 2, 5, and 3 nm, respectively.

As a part of a silicon PV cell structure, after deposition, a passivation film as that of Fig. 2 typically experiences annealing and firing, e.g. at about 400 for 30 min and at about 800°C for 3 s, respectively, during the later steps of manufacturing of the cell structure.

In the three-layer passivation film of Fig. 2, one of the main effects of the lowermost TMA+0₃ layer is to minimize the amount of H at the Si/A10_x interface to avoid blistering. In addition, the negative charge density is high due to the high oxygen content in the film, especially at the interface between Si and A10_x. TMA+H₂0 and TMA+H₂O+O3 layers are used to adjust the hydrogen content in the bulk A10_x, from where some hydrogen will diffuse into the interface, thereby improving the chemical passivation of the film.

A passivation film according to the example of Fig. 2 has very advantageous properties. Due to the use of ozone as the first precursor for oxygen, the SiO_x interface layer formed at the Si/A10_x interface is thicker than when using H₂0 as first oxygen precursor, e.g. 2-3nm. When using H₂0 as the only precursor for oxygen, the interface SiO_x layer is too thin to achieve a good bonding between Si and A1₂0₃. As another advantageous property, the H content at the Si/A10_x interface is lower, e.g. about 0.01%. As result of these properties, practically no blistering occurs in the passivation film.

Figure 3 shows a picture of a prior art A10_x passivation film deposited on a c-Si substrate by using water H₂0 as a precursor for oxygen. The picture was made by FIB (Focused Ion Beam) and SEM (Scanning Electron Microscope) SEM. The picture shows a blister with a diameter of about 50 ym, at the location of which the passivation film does not contact the substrate. So there is no passivation at all at the location of the blister.

As is obvious to a person skilled in the art, with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention is thus not limited to the examples described above but may vary within the scope of the claims. In particular, it is to be noted that, except for the basic principle of the at least one precursor for oxygen comprising ozone, the other precursors used in methods according to the present invention may deviate from the examples above. The precursor (s) for aluminium and the precursor (s) other than ozone for oxygen can be selected e.g. from the table 1 below, wherein, "reactant A" and "reactant B" refer to precursors for aluminium and oxygen, respectively [Puurunen: Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process, J. Appl . Phys . 97, 121301] . As an exception, A1C1₃ may not be used as a precursor for aluminium in the same cycles where ozone is used as a precursor for oxygen. On the other hand, one possible precursor for aluminium, not listed in table 1, is AlMeCl₂.

2 Mai eriai Reactant A ^s Reactant B Refs.

AlMe₃ o₃ 236.244,245,

270-273

AlMe₃ <¼* 274-276

AlMe₃ M₂0 266

A Me₃ N0₂ 1 4

AlMe₃ N₂0₄ 151

AlMe₂Cl H O 27:8

AlMe₂0¾ H₂0 279.280

A!E†₃ H₂0 163

AlCOEt); H₂0 173

AliOEih o₂ 173

AI(0^HPr)₃ H₂0 173,181

Ai(0^KPr)3 o₂ 1 73

AlCimnph H₂0 2SI A1C13 ¾0 76,77,83,90,

91,91,171-

Ϊ t C ¾'^■ 4-

A1CI3 O2 185.186

A!Ch ROH ^iJ 173/181,187

.190-

262

AlMej H₂0? 263-269

Table 1.

Claims

1. A method for fabricating a passivation film (1) on a crystalline silicon surface (2), the passivation film (1) comprising aluminium oxide A10_x, for reducing the surface recombination of charge carriers on the silicon surface (2), wherein the method comprises

- providing an existing crystalline silicon surface (2) in a reaction space;

- depositing in the reaction space a first passivation layer (3) on the existing crystalline silicon surface, wherein the existing crystalline silicon surface with the deposit already formed thereon is alternately exposed to surface reactions of at least one first precursor for aluminium and at least one first precursor for oxygen; and

- depositing in the reaction space a second passivation layer (4) on the first passivation layer, wherein the first passivation layer with the deposit already formed thereon is alternately exposed to surface reactions of at least one second precursor for aluminium and at least one second precursor for oxygen;

characteri zed in that the at the least one first precursor for oxygen comprises ozone 0₃.

2. A method as defined in claim 1, wherein during depositing the first passivation layer (3) , the existing crystalline silicon surface with the deposit already formed thereon is exposed to a deposition atmosphere comprising ozone 0₃ with a mole ratio of (kT+b)%, wherein k lies in the range of -0.09 to - 0.10, b lies in the range of 21.0 to 22.5, and T is the deposition temperature in Celcius.

3. A method as defined in claim 2, wherein the deposition temperature T during the exposure of the existing crystalline silicon surface with the deposit possibly already formed thereon to surface reactions of the at least one first precursor for oxygen comprising ozone lies in the range of 100°C to 200°C.

4. A method as defined in claim 3, wherein the deposition temperature T during the exposure of the existing crystalline silicon surface with the deposit possibly already formed thereon to surface reactions of the at least one first precursor for oxygen comprising ozone lies in the range of 150 °C to 200 °C.

5. A method as defined in any of claims 2 to 4, wherein k lies in the range of -0.096 to -0.098, and b lies in the range of 21.9 to 22.1.

6. A method as defined in any of claims 1 to 5, wherein the at the least one second precursor for oxygen comprises water H₂0.

7. A method as defined in any of claims 1 to 6, wherein the method further comprises depositing in the reaction space a third passivation layer (5) on the second passivation layer (4), wherein the second passivation layer with the deposit already formed thereon is alternately exposed to surface reactions of at least one third precursor for aluminium and at least one third precursor for oxygen.

8. A method as defined in claims 6 and 7, wherein the at least one third precursor for oxygen comprises at least one of water H₂0, ozone 0₃, and dihydrogen dioxide H₂0₂.

9. A method as defined in any of claims 1 to wherein the existing crystalline silicon surface is a surface in a photovoltaic cell structure.