An apparatus

Abstract

The invention relates to an apparatus for subjecting a surface (100a) of a substrate (100) to successive surface reactions of at least a first precursor and a second precursor according to the principles of atomic layer deposition. The apparatus comprising a reaction chamber (1), a gas inlet (4) arranged to supply precursor gases into the reaction space (3) of the reaction chamber (1); a gas outlet (5) arranged to discharge gases from the reaction space (3) of the reaction chamber (1), the gas outlet (5) being arranged spaced apart from the gas inlet (4); and a plasma discharge electrode (6). The apparatus further comprises at least one substrate rack (7) being arranged to support two or more separate substrates (100) such that a substrate batch is formed. The substrate holders (8) are arranged to support the substrates (100) in a vertical position.

Description

[0001] AN APPARATUS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an apparatus for subjecting a surface of a substrate to successive surface reactions of at least a first precursor and a second precursor according to the principles of atomic layer deposition and more particularly to an apparatus as defined in the preamble of the independent claim 1.

[0004] BACKGROUND OF THE INVENTION

[0005] In a typical plasma-enhanced ALD (PE-ALD) apparatus two electrodes are placed within a small distance from each other, one of the electrodes is connected to an RF power supply and the other is grounded. Thus, plasma is ignited between the electrodes. In plasma mode an electric discharge is subjected to one of the precursors such that active precursor radicals are formed from the precursor. The active precursor radicals react on the surface of the substrate during an ALD cycle. This is so called direct plasma in which the substrate is arranged between the plasma electrodes and the plasma discharge is arced through the substrate. In this case the plasma is ignited in the reaction space between the plasma discharge electrode coupled to RF power supply and the substrate. This enables forming of the active precursor radicals close to the substrate such that the active precursor radicals do not become deactivated before reaching the substrate.

[0006] However, the disadvantage of the above described typical PE-ALD apparatuses is that one substrate is coated at a time. There are some batch PE-ALD tools, but the plasma source is typically located on the side. In our case the plasma source is at the top which allows for a different substrate configuration.

[0007] BRIEF DESCRIPTION OF THE INVENTION

[0008] An object of the present invention is to provide a more effective plasma- enhanced ALD apparatus which alleviates the problems of the prior art.

[0009] The objects of the invention are achieved by an apparatus which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.

[0010] The invention is based on the idea of coating in a simple but effective way multiple substrates in a plasma-enhanced ALD apparatus.

[0011] An apparatus according to the invention for subjecting a surface of a substrate to successive surface reactions of at least a first precursor and a second precursor according to the principles of atomic layer deposition comprises a reaction chamber having side walls, a top wall and a bottom wall defining a reaction space inside the reaction chamber; a gas inlet arranged to supply precursor gases into the reaction space of the reaction chamber; a gas outlet arranged to discharge gases from the reaction space of the reaction chamber, which the gas outlet being arranged spaced apart from the gas inlet; and a plasma discharge electrode for generating an electric discharge to the reaction space for forming active precursor radicals. The apparatus further comprises at least one substrate rack being arranged to support two or more separate substrates such that a substrate batch is formed, the substrate rack is arranged in the reaction space of the reaction chamber between the gas inlet and the gas outlet. The apparatus further comprises substrate holders provided to the substrate rack for receiving the substrates to be supported in the substrate rack during the atomic layer deposition. The substrate holders are arranged to support adjacent substrates in the substrate rack at a distance from each other to form a flow path between the adjacent substrates and through the substrate rack for the precursor gases to flow from the gas inlet to the gas outlet. The substrate holders are arranged to support the substrates in a vertical position.

[0012] The substrates arranged in the substrate rack and supported by the substrate holders are either planar substrates placed vertically at the substrate rack, or the substrates are provided to planar substrate carriers which are placed vertically at the substrate rack and which hold the substrates during deposition. The latter means that the substrates are, for example, lenses or other non-planar substrates that are removably attached to the planar substrate carriers. The planar substrates or planar substrate carriers that are supported by the substrate holders form the flow path through the substrate rack. The surfaces of planar substrates or substrate carriers form the edges of the flow path through the substrate rack. Planar meaning in this context that the surface of the substrate or the substrate carrier is substantially flat and extends two-dimensionally in all directions. Substantially flat does not mean that the surface is completely even. The planar substrate may also mean that the substrate comprises several planar sections, which together form an essentially planar surface. The planar substrate may comprise grooves, minor protrusions, or other forms, but the overall appearance is planar. Thereby in the context of this application every substrate is either a planar substrate or a planar substrate carrier that holds non-planar substrates during the deposition. In other words, the apparatus comprises at least one substrate rack which is arranged to hold substrates during the plasma-enhanced atomic layer deposition process in the reaction chamber. The substrates are not part of the apparatus since the apparatus is provided separately the substrates will only be coated on the apparatus and thus constantly change during the lifetime of the apparatus. However, the substrate rack which is arranged to hold the substrates to be coated is part of the apparatus. The substrate rack comprises substrate holders which are able to support the substrate during the plasma-enhanced atomic layer deposition in the substrate rack. The substrate holders are arranged spaced apart in the substrate rack so that when substrates are placed on the substrate holders, there will remain a gap between adjacent substrates. The gap between adjacent substrates supported on the substrate holders form the flow path for the precursor gases such that the gases can flow from the gas inlet of the reaction chamber to the gas outlet of the reaction chamber across the surfaces of the substrates, thereby flushing the surfaces of the substrates. The flow path is therefore formed between adjacent substrates so that surfaces of the adjacent substrates form boundaries of the flow path and this means that the precursor gases flowing through the flow path contact the surfaces of the substrates. The gas inlet and the gas outlet are arranged to the reaction chamber such that the reaction chamber is a cross flow reaction chamber. This means that the precursor gases supplied into the reaction space of the reaction chamber from the gas inlet flow across the reaction space to the gas outlet, where the precursor gases are discharged out from the apparatus. The precursor flow through the reaction chamber is thus arranged as a cross flow. The substrate holders are provided to the substrate rack such that the substrates are placed vertically in the substrate holders. The substrate holders are preferably such that the substrates can be hung from them or supported at the lower and upper ends to the substrate rack. Vertically meaning in this context that one end of the substrate is toward the top of the reaction chamber and the opposite end of the substrate is toward the bottom of the reaction chamber and the surface of the substrate extending between the opposite ends faces toward the side wall of the reaction chamber. Therefore, vertically does not mean an absolute verticality. In other words, the substrates are placed in the substrate rack such that the surfaces of the substrate face toward the side walls of the reaction chamber, or in still other words, the surfaces of the substrate face away from the top wall of the reaction chamber or from the bottom wall of the reaction chamber.

[0013] According to the invention the substrate rack is arranged in the reaction space of the reaction chamber such that a direction of surfaces of the substrates to be supported on the substrate holders of the substrate rack are transverse to the plasma discharge electrode.

[0014] In other words, the substrate holders of the substrate rack are provided to the substrate rack such that the direction of surfaces of the substrates to be supported on the substrate holders are transverse to the plasma discharge electrode. In other words, the surfaces of the substrates face away from the plasma discharge electrode and preferably such that the surfaces of the substrates is in an angle between 80 - 110° to the plasma discharge electrode, and preferably in an angle between 85 - 95° to the plasma discharge electrode, and most preferably in an angle of 90° to the plasma discharge electrode. The plasma discharge electrode is arranged preferably at the top wall, above the substrates which are placed in vertical position so that the coating can be applied on both sides of the substrates simultaneously.

[0015] According to the invention the substrate rack is arranged to remain stationary in the reaction space during the atomic layer deposition.

[0016] In other words, the substrate rack is arranged to keep its position in the reaction chamber during the plasma-enhanced atomic layer deposition. This means that the substrate rack is not movable in the reaction chamber during the plasma-enhanced atomic layer deposition, which means that it cannot be rotated, for example.

[0017] According to the invention the substrate rack is arranged to be movable into the reaction chamber and out from the reaction chamber.

[0018] In other words, the substrate rack is arranged separately inside the reaction chamber so that it is not an integral part of the reaction chamber but can be removed from the reaction chamber and moved outside the reaction chamber and from outside the reaction chamber into the reaction chamber, into the reaction space of the reaction chamber.

[0019] According to the invention the flow path formed between the substrates supported on the adjacent substrate holders is arranged to extend in a direction between the gas inlet and the gas outlet.

[0020] In other words, the flow path which is formed when the substrates are placed on the substrate holders extend in the direction between the gas inlet and the gas outlet so that the gas supplied from the gas inlet can flow through the gap between adjacent substrates. The gas inlet and the gas outlet are arranged at a distance from each other so that the substrate rack is provided between the gas inlet and the gas outlet, so the flow path is formed so that the precursor gases can flow through the substrate rack holding the substrates. Thereby, the surfaces of the vertically placed substrates face away from that side wall where the gas inlet is provided, if the gas inlet is provided to the side wall, and the surfaces of the vertically placed substrates face away from that side wall where the gas outlet is provided, if the gas outlet is provided to the side wall. If the gas inlet and the gas outlet are provided to the bottom wall of the reaction chamber, the substrate rack is arranged on the bottom wall between the gas inlet and the gas outlet, and then the gas is supplied to the reaction space so that the gas flow from the gas inlet to the gas outlet is through the substrate rack holding the vertically placed substrates and the surfaces of the substrates extend in the direction of the flow direction from the gas inlet to the gas outlet. As the substrate rack is arranged to support multiple substrates simultaneously, there will be multiple flow paths formed for the precursor gases so that the gases can flow from the gas inlet to the gas outlet along multiple flow paths.

[0021] According to the invention the flow path formed between the substrates supported on the adjacent substrate holders is arranged to extend in a direction towards the plasma discharge electrode.

[0022] In other words, as the flow path is formed between the gas inlet and the gas outlet, it is also formed to extend in the direction towards the plasma discharge electrode such that when the plasma is ignited the active precursor radicals are enabled to be formed close to the substrate.

[0023] According to the invention the gas inlet and the gas outlet are arranged at opposite side walls of the reaction chamber.

[0024] In other words, when the gas inlet and the gas outlet arranged at opposite side walls of the reaction chamber the flow paths formed between adjacent substrates placed vertically on the substrate holders in the substrate rack form flow paths extending between said opposite side walls having the gas inlet and the gas outlet.

[0025] According to the invention the gas inlet and the gas outlet are arranged at the bottom wall of the reaction chamber at a distance from each other such that the substrate rack is placed between the gas inlet and the gas outlet.

[0026] This means that the precursor gases supplied from the gas inlet to the reaction space flow to a space in which the substrate rack is not placed and therefrom the precursor gases flow through the substrate rack along the flow paths formed between adjacent vertically placed substrates to the other side of the substrate rack where the gas outlet is provided. The gas outlet typically comprises a discharge pump so that the flow between the gas inlet and the gas outlet is formed.

[0027] According to the invention the plasma discharge electrode is arranged in connection with the top wall of the reaction chamber.

[0028] In other words, the plasma discharge electrode is at the top wall of the reaction chamber and the plasma is effective from the top wall towards the bottom wall such that as the substrates are vertically placed in the substrate rack and the flow paths, that are formed between adjacent substrates, are open toward the plasma discharge electrode enable the plasma discharge effect to the precursor gases flowing along the flow paths.

[0029] According to the invention the apparatus further comprises a plasma gas supply arranged to supply plasma gas into the reaction space of the reaction chamber, the plasma gas supply is arranged in connection with the plasma discharge electrode.

[0030] In other words, the plasma gas supply is arranged to supply plasma gas from a plasma gas source to the reaction space. The plasma gas supply is arranged in connection with the plasma discharge electrode such that the plasma gas is supplied toward the flow paths formed of adjacent substrates placed vertically at the substrate rack. The plasma gas supply is preferably arranged in connection with the plasma discharge electrode that the plasma gas supply extends through the plasma discharge electrode or is arranged to the side of the plasma discharge electrode.

[0031] According to the invention the apparatus further comprises a plasma gas supply arranged to supply plasma gas into the reaction space of the reaction chamber, the plasma gas supply is arranged in connection with one of the walls of the reaction chamber or in connection with the top wall of the reaction chamber.

[0032] In other words, the plasma gas supply is arranged to supply plasma gas from a plasma gas source to the reaction space. The plasma gas supply is arranged in connection with one of the side walls of the reaction chamber such that the plasma gas is supplied toward the flow paths formed of adjacent substrates placed vertically at the substrate rack, and the flow path extends in the direction between the gas inlet and the gas outlet of the reaction chamber. Alternatively, the plasma gas supply is arranged in connection with the top wall of the reaction chamber such that the plasma gas is supplied in vertical direction toward the flow paths formed of adjacent substrates placed vertically at the substrate rack. According to the invention the plasma discharge electrode comprises multiple outlets forming the plasma gas supply to the reaction space.

[0033] In other words, the plasma gas supply is in a preferred embodiment of the invention arranged such that the plasma discharge electrode comprises multiple outlets through which the plasma gas supply is arranged. The plasma discharge electrode is formed in the form of a perforated plate, where the holes serve as openings for the plasma gas supply.

[0034] According to the invention the plasma gas supply is directed towards the flow paths formed between the adjacent substrates supported on the substrate holders.

[0035] In other words, the substrates are arranged vertically in the substrate rack such that the surfaces of the substrates extend vertically, thereby the flow paths formed between adjacent substrates extend vertically in the reaction chamber. The plasma gas supply is arranged in the top wall of the reaction chamber, and in connection with the plasma discharge electrode, therefrom the plasma gas supply is towards the flow paths extending vertically in the reaction space. The flow paths extend in the direction between the bottom wall and the top wall of the reaction chamber, and the flow paths simultaneously extend in the direction between the side walls of the reaction chamber where the gas inlet and the gas outlet are provided. Or if the gas inlet and gas outlet are arranged in the bottom wall on opposite sides of the substrate rack, the flow paths extend in the direction of the side walls between which the gas inlet and outlet and the substrate rack are located.

[0036] According to the invention the size of the plasma discharge electrode is such that the substrate rack arranged in the reaction space is entirely within the area of influence of the plasma discharge electrode.

[0037] In other words, the size of the plasma discharge electrode is larger than the extent of the outer circumference of the substrate rack or larger than the substrate to be placed in the substrate rack if the substrate extends outside the substrate rack.

[0038] According to the invention the substrate rack comprises an open top toward the top wall of the reaction chamber such that the flow path is open to toward the top wall, an open front toward the gas inlet and an open back toward the gas outlet such that the flow path is open through the substrate rack between the gas inlet and the gas outlet.

[0039] In other words, the substrate rack has on open top facing towards the top wall of the reaction chamber such that the flow paths that are formed by the vertically arranged substrates are open towards the top wall of the reaction chamber. The substrate rack also comprises an open front toward the gas inlet and an open back toward the gas outlet such that the flow paths that are formed by the vertically arranged substrates are open towards the gas inlet and the gas outlet, and such that the flow path is open through the substrate rack.

[0040] According to the invention the apparatus comprises two or more substrate racks, and the reaction chamber extends in the direction between the gas inlet and the gas outlet, the two or more substrate racks are arranged in a row assembly in the reaction chamber in the direction between the gas inlet and the gas outlet.

[0041] In other words, the reaction chamber can accommodate more than one substrate rack at a time such that the multiple substrate racks are arranged in a row assembly so that the flow paths formed by the vertically placed substrates extend through said multiple substrate racks. The flow paths are thus formed between the vertically placed substrates arranged in successive substrate racks, so that the precursor gases can flow through the substrate racks in the direction between the gas inlet and the gas outlet.

[0042] An advantage of the invention is that the construction for the plasma- enhanced ALD apparatus for processing multiple substrates is very simple and reliable.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention is described in detail by means of specific embodiments with reference to the enclosed drawings, in which

[0045] Figure 1 shows the reaction chamber of the apparatus according to the invention as seen from the side;

[0046] Figure 2 shows the reaction chamber shown in figure 1 as seen from above;

[0047] Figure 3 shows the reaction chamber shown in figure 1 as seen from another side; and

[0048] Figure 4 shows an embodiment of the reaction chamber of the apparatus according to the invention as seen from the side.

[0049] DETAILED DESCRIPTION OF THE INVENTION

[0050] All the figures 1-4 show a plasma-enhanced atomic layer deposition apparatus and especially the reaction chamber 1 of said apparatus. The figures show the reaction chamber 1 as seen from different directions and before a more detailed description of the figures the common issues related to all the figures are reviewed so that they do not have to be repeated with each figure. The figures show the reaction chamber 1 of the apparatus, which the reaction chamber 1 comprises side walls 2a, 2a', 2b, 2b', a top wall 2c and a bottom wall 2d defining a reaction space 3 inside the reaction chamber 1. The plasma-enhanced atomic layer deposition is carried out in the reaction chamber 1. The reaction chamber 1 further comprises a gas inlet 4 arranged to supply precursor gases into the reaction space 3 of the reaction chamber 1 and a gas outlet 5 arranged to discharge gases from the reaction space 3 of the reaction chamber 1, which the gas outlet 5 is arranged spaced apart from the gas inlet 4 such that the substrate rack 7 is placed between the gas inlet 4 and the gas outlet 5. The gas inlet 4 which supplies precursor gases to the reaction space 3 is connected to corresponding precursor gas sources 13 of the apparatus. The precursor gases are supplied from the corresponding gas sources 13 through conduits to the gas inlet 4 wherefrom the precursor gases flow to the reaction space 3. The figures only show one precursor gas source 13, but naturally different precursor gases have their own precursor gas sources. The apparatus further comprises a discharge system 14 such that precursor gases are discharged from the reaction space 3 through the gas outlet 5 to a discharge conduit and finally to the discharge system 14. The precursor gas sources 13 and the discharge system 14 are not shown in every figure because of the angle of the view. The reaction chamber 1 further comprises a plasma discharge electrode 6 for generating an electric discharge to the reaction space 3 for forming active precursor radicals. The plasma discharge electrode 6 is connected to RF power 11. A plasma gas source 12 provides plasma gas to the reaction space 3 through plasma gas supply 9. The walls 2a, 2a', 2b, 2b', 2c, 2d of the reaction chamber 1 are connected to ground 15 or alternatively there is a separate ground electrode at the bottom wall 2d of the reaction chamber 1.

[0051] The apparatus also comprises a substrate rack 7 which is shown in every figure. The substrate rack 7 comprises substrate holders 8 through which the substrates 100 are supported on the substrate rack 7 during the atomic layer deposition. The substrate holders 8 carry the substrates 100 in a vertical position meaning that the substrates 100 are vertically oriented in the substrate rack 7. The substrate holders 8 are also arranged to the substrate rack 7 such that they are spaced apart from each other forming a gap between adjacent substrate holders 8. The gap is such that when the substrates 100 are placed on the substrate holders 8 the adjacent substrates 100 still have a gap between them. The gap is a flow path 10 for the precursor gases and for the plasma discharge.

[0052] Figure 1 shows the reaction chamber 1 of the apparatus according to the invention as seen from side, i.e., viewed from the wall opposite the side wall 2a having the gas inlet 4. This angle of view shows the flow paths 10 formed between adjacent substrates 100 placed on the substrate holders 8 of the substrate rack 7. The substrates 100 are in this embodiment of the invention placed on the substrate holders 8 such that they are suspended from the substrate rack 7. The lower ends of the substrates 100 may be detached or they may be supported on the substrate rack 7. The substrate rack 7 may comprise the substrate holders 8 at the upper end of the substrate rack 7 only or both at the upper end and at the lower end of the substrate rack 7. Figure 1 also shows that the flow paths 10 formed between adjacent substrates 100 placed on the substrate holders 8 extend in the direction between the gas inlet 4 and the gas outlet 5. The gas outlet 5 is provided opposite to the gas inlet 4 at the opposite side wall.

[0053] Figure 1 also shows that the flow path 10 is open toward the top wall 2c of the reaction chamber 1, where the plasma discharge electrode 6 is arranged. The substrate rack 7 has an open top which allows the plasma discharge to reach the substrates 100 placed in the substrate holders 8. The open top also allows the plasma gas supplied from the plasma gas supply 9 to reach the substrates 100. The plasma gas supply 9 is provided in connection with the plasma discharge electrode

[0054] 6 as supply holes in the plasma discharge electrode 6.

[0055] Figure 2 shows the reaction chamber 1 shown in figure 1 as seen from above from a space between the plasma discharge electrode and the substrate rack

[0056] 7 so that the plasma discharge electrode is not shown in this figure. The figure shows the open top of the substrate rack 7 and the flow paths 10 formed between adjacent vertically placed substrates 100 which are placed in the substrate holders

[0057] 8 of the substrate rack 7. The figure also shows that the flow paths 10 are open toward the gas inlet 4 and toward the gas outlet 5 and thereby form an open flow path 10 through the substrate rack 7. The substrate rack 7 is thus open toward the gas inlet 4 and toward the gas outlet 5. The substrate racj 7 can thus be a framelike rack with corner supports and possibly side walls towards the walls of the reaction chamber 1 where there is no gas supply or discharge. The arrows show the flow direction of the precursor gases in the reaction space from the gas inlet 4 through the substrate rack 7 to the gas outlet 5.

[0058] Figure 3 shows the reaction chamber 1 shown in figure 1 as seen from another side such that the surface 100a of the substrate 100 is shown. The flow paths 10 are formed between adjacent substrates 100 and from this angle of view they are not show since the flow paths 10 extend in the direction of the surface 100a of the substrate 100 from the gas inlet 4 to the gas outlet 5 through the substrate rack 7. The arrows show the flow direction of the precursor gases in the reaction space 3. The gas inlet 4 and the gas outlet 5 are arranged on opposite side walls 2a, 2b. The substrate holders 8 are arranged in the substrate rack 7 such that they are in a row preferably in a direction transverse to the flow direction which is the direction from the gas inlet 4 to the gas outlet 5.

[0059] Figure 4 shows an embodiment of the reaction chamber 1 of the apparatus according to the invention as seen from the side which differs only from the figure 3 in that the gas inlet 4 and the gas outlet 5 are arranged in connection with the bottom wall 2d of the reaction chamber 1. The gas inlet 4 and the gas outlet 5 are arranged to the reaction chamber 1 such that the substrate rack 7 can be placed between said gas inlet 4 and gas outlet on the bottom wall 2d of the reaction chamber 1, thereby the precursor gases flow through the substrate rack 7 along the flow paths formed between adjacent substrates 100 placed in the substrate holders 8 of the substrate rack 7. The flow paths 10 extend in the direction of the surfaces 100a of the substrates 100 and that is why the flow paths are not shown in this figure.

[0060] The invention has been described above with reference to the examples shown in the figures. However, the invention is in no way restricted to the above examples but may vary within the scope of the claims.

Claims

CLAIMS1. An apparatus for subjecting a surface (100a) of a substrate (100) to successive surface reactions of at least a first precursor and a second precursor according to the principles of atomic layer deposition, said apparatus comprising:- a reaction chamber (1) having side walls (2a, 2a', 2b, 2b'), a top wall (2c) and a bottom wall (2d) defining a reaction space (3) inside the reaction chamber (1);- a gas inlet (4) arranged to supply precursor gases into the reaction space (3) of the reaction chamber (1);- a gas outlet (5) arranged to discharge gases from the reaction space (3) of the reaction chamber (1), the gas outlet (5) being arranged spaced apart from the gas inlet (4); and- a plasma discharge electrode (6) for generating an electric discharge to the reaction space (3) for forming active precursor radicals, c h a r a c t e r i z e d in that the apparatus further comprises at least one substrate rack (7) being arranged to support two or more separate substrates (100) such that a substrate batch is formed, the substrate rack (7) is arranged in the reaction space (3) of the reaction chamber (1) between the gas inlet (4) and the gas outlet (5), and substrate holders (8) provided to the substrate rack (7) for receiving the substrates (100) to be supported in the substrate rack (7) during the atomic layer deposition, the substrate holders (8) are arranged to support adjacent substrates (100) in the substrate rack (7) at a distance from each other to form a flow path (10) between the adjacent substrates (100) and through the substrate rack (7) for the precursor gases to flow from the gas inlet (4) to the gas outlet (5), the substrate holders (8) are arranged to support the substrates (100) in a vertical position.

2. Apparatus according to claim 1, c h a r a c t e r i z e d in that the substrate rack (7) is arranged in the reaction space (3) of the reaction chamber (1) such that a direction of surfaces (100a) of the substrates (100) to be supported on the substrate holders (8) of the substrate rack (7) are transverse to the plasma discharge electrode (6).

3. Apparatus according to any previous claim, c h a r a c t e r i z e d inthat the substrate rack (7) is arranged to remain stationary in the reaction space (3) during the atomic layer deposition.

4. Apparatus according to any previous claim, characterized in that the substrate rack (7) is arranged to be movable into the reaction chamber (1) and out from the reaction chamber (1).

5. Apparatus according to any previous claim, characterized in that the flow path (10) formed between the substrates (100) supported on the adjacent substrate holders (8) is arranged to extend in a direction between the gas inlet (4) and the gas outlet (5).

6. Apparatus according to any previous claim, characterized in that the flow path (10) formed between the substrates (100) supported on the adjacent substrate holders (8) is arranged to extend in a direction towards the plasma discharge electrode (6).

7. Apparatus according to any previous claim, characterized in that the gas inlet (4) and the gas outlet (5) are arranged at opposite side walls (2a, 2a', 2b, 2b') of the reaction chamber (1).

8. Apparatus according to any of claims 1-6, characterized in that the gas inlet (4) and the gas outlet (5) are arranged at the bottom wall (2d) of the reaction chamber (1) at a distance from each other such that the substrate rack (7) is placed between the gas inlet (4) and the gas outlet (5).

9. Apparatus according to any previous claim, characterized in that the plasma discharge electrode (6) is arranged in connection with the top wall (2 c) of the reaction chamber (1).

10. Apparatus according to any previous claim, characterized in that the apparatus further comprises a plasma gas supply (9) arranged to supply plasma gas into the reaction space (3) of the reaction chamber (1), the plasma gas supply (9) is arranged in connection with the plasma discharge electrode (6).

11. Apparatus according to any of claims 1-9, characterized inthat the apparatus further comprises a plasma gas supply (9) arranged to supply plasma gas into the reaction space (3) of the reaction chamber (1), the plasma gas supply (9) is arranged in connection with one of the side walls (2a, 2'a, 2b, 2b') of the reaction chamber (1) or in connection with the top wall (2c) of the reaction chamber (1).

12. Apparatus according to claim 10 or 11, c h a r a c t e r i z e d in that the plasma gas supply (9) is directed towards the flow paths (10) formed between the adjacent substrates (100) supported on the substrate holders (8).

13. Apparatus according to any previous claim, characterized in that the size of the plasma discharge electrode (6) is such that the substrate rack (7) arranged in the reaction space (3) is entirely within the area of influence of the plasma discharge electrode (6).

14. Apparatus according to any previous claim, characterized in that the substrate rack (7) comprises an open top toward the top wall (2c) of the reaction chamber (1) such that the flow path (10) is open to toward the top wall (2c), an open front toward the gas inlet (4) and an open back toward the gas outlet (5) such that the flow path (10) is open through the substrate rack (7) between the gas inlet (4) and the gas outlet (5).

15. Apparatus according to any previous claim, characterized in that the apparatus comprises two or more substrate racks (7), and the reaction chamber (1) extends in the direction between the gas inlet (4) and the gas outlet (5), the two or more substrate racks (7) are arranged in a row assembly in the reaction chamber (1) in the direction between the gas inlet (4) and the gas outlet (5).

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