Method and apparatus for localized liquid treatment of the surface of a substrate

<u>Field of the invention</u>

0001The present invention is related to a method of localized liquid treatment of the surface of a substrate, like cleaning or etching. The invention is also related to an apparatus used to perform said treatment. Application of the invention is possible in a number of wet processing steps which are frequently used, e.g. in the fabrication of integrated circuits or liquid crystal displays.

<u>State of the art</u>

0002In the fabrication of micro-electronic devices such as integrated circuits or liquid crystal displays, a substrate has to go through a number of fabrication steps, including wet etching, wet cleaning or rinsing.

0003In the documents US-A-5271774 and JP-A-07211686, methods are described to remove a liquid from the surface of a substrate by applying a gaseous tensio-active substance to the substrate, which, when mixed with the liquid, reduces the surface tension of the liquid, so that it can be easily removed from the surface by a centrifugal force, i.e. by subjecting the substrate to a rotary movement. However, these methods are only applicable to the substrate as a whole, and cannot be used for local treatment of the substrate. Document EP-A-817246 is describing an apparatus and method for wet cleaning or etching of flat substrate surfaces, whereby a substrate is moved through a stationary amount of liquid. This method is also related to a treatment of the substrate as a whole, not of a local zone of said substrate.

0004Document EP-A-905747 describes a method and apparatus for removing a wet processing liquid from a surface of at least one substrate. According to the method described, a liquid is supplied on a surface of a substrate. Simultaneously, or thereafter, a gaseous tensio-active substance can be supplied, thereby creating a sharply defined liquid-vapour boundary. The substrate is subjected to a rotary movement at a speed to guide said liquid-vapour boundary over said substrate thereby removing said liquid from said substrate.

0005The method and apparatus described in EP-A-905747 have the purpose of applying and removing a liquid from the whole of the surface of a substrate. The apparatus does not allow a local liquid treatment of a substrate.

0006During some processing steps, an annular edge area of the substrate is treated, for example for the removal of certain layers, like a resist or a metal film (e.g. Cu). Sometimes, it is only the outer rim of the substrate which is treated, while the top and bottom surfaces of the substrate must remain untouched.

0007Techniques exist whereby a beam of liquid is directed to the edge area or the rim of a rotating substrate in order to perform these process steps. However these existing techniques offer a poor protection of the substrate surface against the cleaning liquid. It would therefore be advantageous to find a way of protecting the substrate surface while treating said annular edge area or its outer rim with a liquid. Another disadvantage of existing techniques is that the rotational speed of the substrate during processing must be relatively high, which is particularly problematic for large substrates.

0008Some processing steps require the removal of larger sized features from the substrate surface, e.g. for revealing underlying zero markers, which are used for the accurate lateral positioning during processing, e.g. during photographic exposure. Although the definition of the zero markers themselves requires high accuracy, the accuracy requirements of the window for removing a layer on top of the zero markers can be fairly relaxed. So far in the state of the art, the use of a photo resist step is mostly used to accomplish these areas. By using a photo resist step, a patterned protective resist layer is obtained on the substrate surface. The pattern is such that the areas to be etched are not covered with resist. After this, the substrate is etched and the photo resist layer is removed. This is however an expensive and time-consuming effort.

0009An alternative way of producing such larger sized areas which are free of film, consists of a local shielding by shielding plates during film deposition. However, this technique leads to an increased risk of particle contamination and scratch formation on the substrate. Furthermore, this shielding technique is not generally applicable. It requires compatibility of the shielding plates with the deposition process of interest. To produce these larger sized features, it would therefore be advantageous to find a method that allows less accuracy but lower cost and higher processing speed, without creating any particle contamination.

0010Document JP-A-11166882 describes a preconcentration or collection technique, used in contamination measurements for semi-conductors. It is a technique, whereby a droplet is moved over the substrate surface, in order to collect contaminants and subsequently analyze their concentrations. Currently, this technique is mainly used on silicon surfaces, by rendering the surface hydrophobic by way of an HF-treatment. This way, the water-based droplet is contained. The technique is however not confined to silicon surfaces. In some cases, it has been observed that the contact angle between the droplet and the substrate surface is insufficient. Furthermore, it would be advantageous to find a method whereby, in the case of silicon surfaces, the extra processing step of providing an HF-treatment would become unnecessary.

0011Document US-A-5492566 describes a way of holding a substrate to a flat surface by way of the Bernoulli effect. By supplying a gas at a high speed between said substrate and said surface through an annular nozzle, a pressure drop will result, under said substrate, thus holding said substrate to said surface. Substrates held in this way can be subjected to various wet treatment steps, like cleaning or etching. However, a danger exists of liquid attaching itself to the rim or the backside of the substrate.

<u>Aim of the invention</u>

0012The present invention aims to propose a method and apparatus to perform a liquid treatment of a part of a substrate, such as a cleaning or etching step, while another part of said substrate is protected from said liquid by the use of a gaseous substance.

<u>Summary of the invention</u>

0013The present invention relates to a method of dispensing liquid on a part of a substrate, e.g. for cleaning or etching purposes, while another part of said substrate is prevented from contacting said liquid, said method comprising the steps of : <ul id="ul0001" list-style="dash" compact="compact"><li>supplying a liquid on a part of said substrate,</li><li>simultaneously with said liquid, supplying a gaseous tensio-active substance to said surface, said gaseous substance being at least partially miscible with said liquid and when mixed with said liquid yielding a mixture having a surface tension lower than that of said liquid.</li></ul>

0014A first embodiment of the invention is a method comprising the steps of : <ul id="ul0002" list-style="dash" compact="compact"><li>subjecting a circular shaped substrate to a rotational movement, preferably in a horizontal plane, the axis of rotation being perpendicular to the substrate surface and comprising the center of said substrate,</li><li>supplying at least one stream of a liquid so that said stream hits a flat surface of said substrate in an area of said surface, said area being adjacent to the outer rim of said substrate,</li><li>simultaneously supplying at least one stream of a gaseous tensio-active substance to the same flat surface so that said stream hits said surface in an area which is adjacent to the area hit by said liquid stream, and closer to the center of rotation.</li></ul>

0015An additional stream of liquid may be supplied to the opposite surface of said substrate, in order to treat the whole of said opposite surface.

0016According to another embodiment of the invention, a stream of liquid may be directed at the outer rim of the substrate, while the flat surfaces are protected from said liquid by a stream of a gaseous tensio-active substance.

0017Another embodiment of the invention is a method comprising the steps of : <ul id="ul0003" list-style="dash" compact="compact"><li>holding a circular shaped substrate,</li><li>supplying a stream of liquid to the whole of an annular edge area of at least one flat surface of said substrate,</li><li>simultaneously supplying a stream of a gaseous tensio-active substance to the central area of said surface, said central area comprising the whole of said surface, except said annular area.</li></ul>

0018In this method, the substrate may be subjected to a rotational movement, the axis of rotation being perpendicular to the substrate surface and comprising the center of said substrate.

0019An additional stream of liquid may be supplied to a surface which is opposite to a surface of which the annular edge area is treated, in order to treat the whole of said opposite surface.

0020Another embodiment of the present invention is a method comprising the steps of : <ul id="ul0004" list-style="dash" compact="compact"><li>holding a circular shaped substrate in a preferably horizontal plane,</li><li>providing a means of bringing an annular edge area of both flat surfaces of said substrate, and the outer rim of said substrate into contact with an amount of liquid,</li><li>supplying streams of a gaseous tensio-active substance to the border areas between said amount of liquid and said flat surfaces.</li></ul>

0021In this method, the substrate may be equally subjected to a rotational movement, the axis of rotation being perpendicular to the substrate surface and comprising the center of said substrate.

0022Another embodiment of the present invention is a method comprising the steps of : <ul id="ul0005" list-style="dash" compact="compact"><li>placing a substrate on a flat, rotating and preferably horizontal surface, said surface containing an annular channel, so that said substrate is covering said channel,</li><li>supplying a stream of a gaseous tensio-active substance through said annular channel and in the direction of said substrate, so that said substrate is held on said surface by the Bernoulli effect,</li><li>supplying a stream of a liquid on the surface of said substrate opposite to the surface placed on said annular channel.</li></ul>

0023Another embodiment of the present invention is a method comprising the steps of : <ul id="ul0006" list-style="dash" compact="compact"><li>supplying a continuous stream of liquid to a part of a flat surface of a preferably horizontally placed substrate through a first channel,</li><li>draining said stream of liquid from said surface through a second channel, concentrically placed around the first channel,</li><li>simultaneously supplying to said surface a stream of a gaseous tensio-active substance around said second channel, to prevent said liquid from making contact with the rest of the surface of said substrate.</li></ul>

0024In this embodimemnt, the gaseous tensio-active substance may be drained from the substrate surface through an additional channel.

0025Another embodiment of the present invention is a method comprising the steps of : <ul id="ul0007" list-style="dash" compact="compact"><li>bringing an amount of liquid into contact with a part of a flat surface of a preferably horizontally placed substrate,</li><li>supplying to said surface a stream of gaseous tensio-active substance around said amount of liquid, thereby preventing said liquid from making contact with the rest of said surface.</li></ul>

0026In this embodiment, the gaseous tensio-active substance may be drained from the substrate surface through an additional channel.

0027The present invention is also related to an apparatus for subjecting a substrate to a localized liquid treatment such as cleaning or etching, said apparatus comprising : <ul id="ul0008" list-style="dash" compact="compact"><li>a means for holding said substrate,</li><li>at least one liquid supply system, able to supply said liquid on a part of the surface of said substrate,</li><li>at least one supply system of a gaseous substance, able to supply said gaseous substance to another part of said substrate, adjacent to the part which is treated by said liquid.</li></ul>

0028In such an apparatus, a means may be provided for rotating said substrate around an axis which is perpendicular to said substrate and which comprises the center of said substrate.

0029According to one embodiment, an apparatus is proposed for treating an annular edge area of at least one flat surface of a circular shaped, preferably horizontally placed substrate, comprising at least one fixed pair of nozzles, a nozzle being defined as an apparatus able to supply a contiguous stream of liquid. Of this pair, one nozzle is used to supply a stream of liquid on said annular edge area, while the other is used to dispense a gaseous tensio-active substance on an area of said flat surface adjacent to said annular edge area, and closer to the center of said substrate.

0030Said pair of nozzles may be positionable on any location along a fixed radius of said substrate.

0031A nozzle may be added on one of the flat surfaces of said substrate which is opposite to a surface of which the edge area is treated, said nozzle being used to dispense a stream of liquid on the whole of said opposite surface.

0032According to another embodiment, an apparatus is proposed for treating an annular edge area of a flat surface of a circular shaped substrate, comprising : <ul id="ul0009" list-style="dash" compact="compact"><li>a centrally placed channel used to supply a gaseous substance to the central part of said substrate,</li><li>a first annular channel, used to supply the liquid to the whole of an annular edge area of the surface of said substrate,</li><li>a second annular channel placed concentrically with respect to the first channel and closer to the center of said substrate, said second channel being used to guide the gaseous substance coming from the central part of said substrate, in order to prevent said liquid from touching said central part.</li></ul>

0033Said apparatus may rotate around an axis which is perpendicular to said substrate and which comprises the center of said substrate.

0034A nozzle may be added on one of the flat surfaces of said substrate which is opposite to a surface of which the edge area is treated, said nozzle being used to dispense a stream of liquid on the whole of said opposite surface.

0035A sealing device may be added between said substrate and the outer wall of said second annular channel.

0036According to another embodiment, an apparatus is proposed for treating an annular edge area of both flat surfaces and the outer rim of a circular shaped substrate, placed preferably in a horizontal plane, comprising : <ul id="ul0010" list-style="dash" compact="compact"><li>a container filled with an amount of treatment liquid so that a pressure is maintained above the surface of said amount of liquid, said pressure being less than or equal to the ambient pressure, working on the substrate surface, said container having a narrow gap in one side, into which said circular substrate is partially inserted, so that said edge areas and said outer rim of said substrate is immersed in said liquid,</li><li>at least one pair of nozzles, one nozzle of said pair on each side of said substrate, directing a stream of a gaseous substance at the border area between said container and said substrate.</li></ul>

0037Said apparatus may rotate around an axis which is perpendicular to said substrate and which comprises the center of said substrate.

0038According to another embodiment, an apparatus is proposed for treating a local zone of a preferably horizontally placed substrate, said apparatus comprising : <ul id="ul0011" list-style="dash" compact="compact"><li>a central channel used to supply a stream of liquid to the surface of said substrate,</li><li>a second channel, concentrically surrounding the first channel, and draining said stream of liquid from the surface of said substrate,</li><li>a third channel, concentrically surrounding the second channel, and used to supply a stream of a gaseous tensio-active substance.</li></ul>

0039According to the same embodiment, a fourth channel may be concentrically placed with respect to said third channel, said fourth channel being used to drain said gaseous tensio-active substance from the substrate surface.

0040A sealing device may be added between said substrate and the outer wall of said second channel. In case of a fourth channel, an additional sealing device may be placed between said substrate and the outer wall of the apparatus.

0041According to another embodiment, an apparatus is proposed for treating a local zone or for collecting impurities from the surface of a preferably horizontally placed substrate, said apparatus comprising : <ul id="ul0012" list-style="dash" compact="compact"><li>a central channel containing an amount of a liquid in such a way that said liquid is in contact with the surface of said substrate, and that a pressure is maintained above the surface of said amount of liquid, said pressure being less than or equal to the ambient pressure, working on the substrate surface,</li><li>a second channel, concentrically surrounding the first channel, and supplying a stream of a gaseous tensio-active substance on the surface of said substrate.</li></ul>

0042According to the same embodiment, a third channel may be placed concentrically with respect to said second channel, said third channel being used to drain said gaseous tensio-active substance from the substrate surface.

0043A sealing device may be added between said substrate and the outer wall of said central channel. In case of a third channel, an additional sealing device may be placed between the substrate and the outer wall of the apparatus.

<u>Short description of the drawings</u>

0044Figure 1a (front and top view) describes the method and apparatus according to a first embodiment of the invention, used to clean or etch an edge area of a rotating substrate.

0045Figure 1b (front view) describes a method and an apparatus for treating the outer rim of a rotating substrate.

0046Figure 1c (front view) describes the method and an apparatus according to the invention for performing a backside cleaning.

0047Figure 2 (front and top sectioned view) describes a second method and apparatus according to the invention, used to clean or etch an annular edge area of a substrate.

0048Figure 3a (sectioned front view and top view) describes a third method and an apparatus according to the invention, used to clean or etch an annular edge area and the outer rim of the substrate.

0049Figure 3b (sectioned front view and top view) represents an alternative form of the apparatus shown in figure 3a.

0050Figure 4 (front view) describes the method according to the invention, used to hold a substrate by way of the Bernoulli effect.

0051Figures 5a and b (front and top sectioned view) describes the method and an apparatus according to the invention, used to clean or etch a local area of a substrate by applying a continuous stream of a liquid.

0052Figures 6a and b (front and top sectioned view) describes the method and apparatus according to the invention, used to contain a small amount of a liquid on a part of the surface of the substrate.

<u>Detailed description of the preferred embodiments</u>

0053Figure 1a describes the method and apparatus according to a first embodiment of the invention, whereby a liquid, used for cleaning or etching, is supplied on a horizontally placed rotating circular shaped substrate 1 through a first nozzle 2, while a gaseous tensio-active substance is supplied through a second nozzle 3. The rotation of the substrate is indicated by the arrow. The rotation in this and all following embodiments (if a rotation is applied) is occurring around the axis perpendicular to the substrate and comprising the center of said substrate.

0054In the apparatus of figure 1a, both nozzles are placed such that the streams of liquid and tensio-active substance hit the substrate surface on two spots which are preferably lying along a radius of said substrate. The spot where the liquid stream hits the surface is lying on an annular edge area 4, the spot where the stream of tensio-active gas hits the surface is lying next to the first spot and closer to the center of the substrate. Both nozzles are fixed. The liquid, supplied through nozzle 2 can be used to clean or etch a well defined annular edge area 4 of the surface of the substrate, while the rest of said surface is protected from making contact with the liquid, by the gaseous tensio-active substance. This substance, when mixed with the liquid, reduces the surface tension of said liquid, so that it does not spread out over said surface, but becomes contained and is easily removed from the surface by the centrifugal forces, resulting from the substrate's rotational movement. The walls 5 of the compartment in which the substrate is placed are slanted, so that they can provide an efficient removal of liquid droplets 6, without splashing onto the substrate.

0055The angles between the nozzles and the substrate surface may differ from the exemplary case of figure 1a. It is especially important for the liquid nozzle to be placed at an angle which minimizes splashing. Both nozzles may be placed on a movable structure, so that they can be fixed at a variable position along the radius of said substrate. Several pairs of nozzles (2,3) may be placed on fixed positions on one or both sides of the substrate.

0056This embodiment represents an improvement on the state of the art in that it allows the treatment of a localized edge area of the substrate surface, without contacting the central part of said substrate. An additional advantage of the method according to the invention is that it allows a rotational movement of the substrate at a lower speed, compared to existing methods.

0057If said nozzles are directed at the outer rim 7 of the substrate, as in figure 1b, the method according to the invention is suitable for the treatment of the rim, whereby the surface of the substrate is protected from the liquid. A second pair of nozzles 8 (liquid) and 9 (gaseous tensio-active substance) may be added on the opposite side of the substrate, to improve the efficiency of the rim treatment process.

0058An additional nozzle 10 may be added on the opposite side of the substrate, as is shown in figure 1c. This setup is then suitable for the process of backside cleaning or etching, in which the backside 11 of the substrate is treated as well as a limited area 12 of the front side, and the outer rim 7. Using the method of invention, the central part 13 of the front side of the substrate can be effectively shielded from the treatment liquid.

0059Figure 2 describes the method and apparatus according to another embodiment of the invention, in which a liquid is dispensed on an annular edge area 20 of a circular shaped substrate 1. The substrate may be stationary or rotating. An apparatus 19 is placed above the surface of the substrate, consisting of two concentric annular channels 21 and 22. Said apparatus may also be rotating around the central perpendicular axis of the substrate. Through the outer channel 21, a liquid is supplied on the edge of the rotating substrate. At the same time a gaseous substance which is preferably tensio-active, is supplied on the central part of the substrate, through a central nozzle 23. The gaseous tensio-active substance is flowing outward from the center of the substrate to the edge, where it is guided through the inner concentric channel 22. Said gaseous substance reduces the surface tension of the liquid in such a way that the liquid is prevented from making contact with the central part of said surface. In case the substrate is rotating, the centrifugal forces acting on the liquid will further facilitate the removal of the liquid from the surface.

0060This embodiment allows also to treat an edge area of the substrate, presenting the added advantage that the liquid can be applied to the whole of the edge area, which is also protected as a whole by the centrally supplied gaseous tensio-active substance. The same structure may be placed on the opposite side of the substrate, in order to treat both edge areas simultaneously. Also, in case one apparatus 19 is applied, a nozzle may be directed at the opposite side, supplying liquid on said opposite side, analogous to the action of nozzle 10 in figure 1c. The distance between the apparatus 19 and the substrate surface must be sufficiently small, to optimize the separation between the liquid and the gas.

0061In case both the apparatus 19 and the substrate 1 are stationary or rotating at the same speed, a sealing device, e.g. an O-ring, may be placed between the substrate and the outer wall of channel 22, to separate the central part from the treated edge area.

0062Figure 3a describes the method and apparatus according to another embodiment of the present invention, in which a substrate 1 is subjected to a rotational movement. To one side of the substrate is a container 30, containing a liquid 31, to be used for the treatment of the edge areas 32 and the outer rim 33 of the substrate 1. The gap 34 is made so that it can receive the rotating substrate's edge, with very small clearances. The pressure inside said container 30 is preferably lower than or equal to the pressure outside said container. Nozzles (35, 36) are present on both sides of the substrate, supplying streams of a gaseous substance which is preferably tensio-active, to the border area between the container and the substrate, thereby preventing the liquid from touching the central part of the substrate.

0063In a derived case of this embodiment, the container may be rotating around the substrate's central perpendicular axis, at the same or a different speed as the substrate. This way, the centrifugal forces will contribute to the efficient separation between the liquid and the central area of the substrate. Also, the substrate may be stationary, while the container is rotating.

0064The container may even be of an annular form, so that the whole of the edge area to be treated is immersed in the treatment liquid. Also in this case, said annular-shaped container may rotate at a speed, equal to or different from the speed of said substrate. In this case also, the substrate may be stationary. If the container has an annular form, as is shown in figure 3b, it should consist of two parts : a bottom part 100 in which to place the substrate, and a top part 101 which is placed onto the first part, after which the compartment between both parts is filled with a treatment liquid, e.g. via at least one opening 102 so that the substrate's annular edge area is totally immersed in said liquid. Along the outer perimeter of both parts of said container, a suitable sealing device 103 should be provided between said parts so that no treatment liquid can escape from the container. Annular shaped nozzles 104 may be used to supply an equal amount of the gaseous tensio-active substance to the border areas between the container and the substrate.

0065This embodiment presents the added advantage that the edge areas of both sides and the outer rim of the substrate can be treated simultaneously.

0066For holding a substrate and subjecting it to a rotational movement in one of the previous embodiments of the present invention, a planetary drive system can be used. In such a system, a set of rotating grooved wheels located around the perimeter of the round substrate, hold and directly drive the substrate. This offers an efficient removal of edge beads. Also, there is a free access to both sides of the substrate. Alternatively, a vacuum chuck may be used to hold the substrate.

0067Figure 4 describes the method according to another embodiment of the invention, in which a stream of a gaseous tensio-active substance is used to hold a substrate 1 by way of the Bernoulli effect. The substrate 1 is placed on a flat rotating surface 40 in which an annular nozzle 41 is provided. The rotation of said surface 40 is indicated by the arrow. Through this nozzle, a tensio-active gaseous substance is supplied at a certain speed. This speed is responsible for a pressure drop between the substrate and the flat surface, causing the substrate to be held to said surface. Supports 42 are mounted on the surface 40 to ensure that the substrate is kept in place.

0068A nozzle 43 is placed on the opposite side to the annular nozzle and is used to supply a stream of treatment liquid. This embodiment is suitable for the process of thinning of the substrate, by applying an etching fluid on one surface, while the opposite surface is effectively protected by the tensio-active gas.

0069Figure 5a describes the method and apparatus according to another embodiment of the invention, in which a continuous stream of liquid is supplied on the surface of the substrate 1. An apparatus is used, consisting of three concentric channels. The central channel 50 is used to supply the liquid on the surface of the substrate. The surrounding channel 51 is used to drain the liquid from said surface. The third channel 52 is used to supply a gaseous tensio-active substance which reduces the surface tension of the liquid so that it is contained on a localized area of the surface. A sealing device, e.g. an O-ring 53, may be added between the substrate and the outer wall of the liquid draining channel 51. In this case, the stream of gaseous tensio-active substance makes sure that any liquid leaking through the seal is contained on the area to be treated.

0070Figure 5b reveals an additional fourth channel 54 surrounding channel 52, which may be placed in case the gaseous substance must be prevented from touching the rest of the substrate or from leaking out into the environment. The arrows indicate that the gaseous tensio-active substance is drained from the surface of the substrate through this fourth channel. However, it is equally possible for the gas to be supplied through the fourth channel 54 and drained through the third channel 52.

0071Whatever the direction of the stream of gaseous tensio-active substance in the apparatus of Figure 5b, an additional sealing device 55 may be placed between the substrate and the outer wall of the apparatus, in order to prevent the gaseous substance from leaking into the environment.

0072The device containing the three or four channels could be designed so that it covers the area to be treated exactly, or it could be made smaller and subsequently be scanned over the area to be treated.

0073The device containing the three or four channels may advantageously be placed under a horizontally placed substrate. In this way, a splashing of liquid on the substrate when removing the device may be avoided.

0074This embodiment can be used to clean local areas of the substrate surface, or to etch these areas, such as zero marker areas, without requiring a photo-resist pattern or shielding plates. For the production of such larger sized film-free areas, this method presents a quicker and cheaper way compared to the classic photo step method. Compared to the method using shielding plates, the method according to the present invention eliminates the danger of particle contamination or scratch formation.

0075Figure 6a describes the method and apparatus according to another embodiment of the invention, which is suitable for the technique of impurity collection. It may also be suitable for obtaining larger sized film-free areas, like zero-markers. An amount of liquid 62 is put into contact with the surface of the substrate 61 through a central channel 63. This channel is closed off on the top side. Advantageously, the pressure inside the channel 63 is lower than the ambient pressure acting on the substrate surface. The lower the pressure above the liquid, the higher the amount of liquid which can be contained. A channel 64 which is placed around the channel 63 is used to supply a gaseous tensio-active substance, which reduces the surface tension of the liquid so that said liquid is contained on a localized area of the surface.

0076In case a droplet of said liquid is used, a reduction of the pressure above said droplet is unnecessary. In this case, the central channel 63 may be open on top.

0077A sealing device such as an O-ring 65 may be added between the substrate and the outer wall of the channel 63. In this case, the stream of gaseous tensio-active substance would make sure that any liquid leaking through the seal is contained on the area to be treated.

0078In figure 6b, an additional channel 66 is added to the apparatus. This version of the apparatus is preferred when the gaseous tensio-active substance must be prevented from touching the rest of the substrate or from leaking into the environment. The arrows indicate that the gaseous tensio-active substance is drained from the surface of the substrate through this third channel 66. However, it is equally possible for the gas to be supplied through the third channel and drained through the second channel.

0079Whatever the direction of the stream of gaseous tensio-active substance in the apparatus of Figure 6b, an additional sealing device 67 may advantageously be placed between the substrate and the outer wall of the apparatus, in order to prevent the gaseous substance from leaking into the environment.

0080This embodiment represents an improvement compared to the existing techniques described in the state of the art, as it is capable of holding an amount of liquid, larger than a droplet, so that a larger amount of contaminants can be collected. In case a droplet is used, the flow of the gaseous tensio-active substance increases the contact angle between the droplet and the surface to be treated. Eventually, it may render the pre-treatment of the substrate surface of hydrophilic substrates unnecessary.