METHOD OF REMOVING UNDESIRED PARTICLES FROM A SURFACE OF A SUBSTRATE

Abstract: A method of removing particles (3) from a surface (2) of a substrate (1), in which by means of a liquid (11) a removing force is exerted on the particles (3). According to the invention, this force is exerted by moving an interface (10) of the liquid (11) over the surface (2) of the substrate (1). Thus, it is achieved that particles (3) having a diameter which is smaller than 0.3 µm can also be effectively removed from the surface (2) of the substrate (1). This is of great impor­ tance in the manufacure of integrated circuits with submicron details, in which contamination with particles having said dimensions and larger may have destructive consequences for the satisfactory operation of the circuit.

Description: The invention relates to a method of removing undesired particles from a surface of a substrate, in which a removing force is exerted on the particles by means of a liquid. Such a method is particularly suitable for removing undesired particles from a surface of a semi-conductor substrate. In the manufacture of integrated circuits (IC's), a substantial part of the rejected IC's is due to the unfavourable influence of particles which have adhered to the substrate in one of the processing steps. During the ultimate operation of the IC, these particles may give rise to unstabilities, voltage breakdowns or shortcircuits, which render the whole IC unserviceable. During the manufacture of IC's, about 60 % of the rejected IC's is due to such a contamination with small particles, which causes these particles to be an important factor for the yield of the production. US-PS No. 4,118,649 discloses a method of the kind mentioned in the opening paragraph, in which the force removing the particles is produced by megasonic waves. In this case, the substrate is located in a tank containing a cleaning liquid, in which megasonic waves are generated. The frequency of these waves is 0.2 to 5 MHz. Thus, particles having a diameter of 0.3 µm and larger can be effectively removed from the surface of the substrate. The known method described has the disadvantage that the removal of the particles requires expensive and complicated equipment and that it is not sufficient to remove particles having a diameter smaller than 0.3 µm effectively from the surface of the substrate. In IC's, particles having a diameter of more than 10 to 20 % of the minimum detail size in the IC can give rise to an ultimately unsatisfactory operation of the IC. This means that in the modern large-scale integration of components in IC's, in which details of the order of magnitude of 1 µm occur, particles having a diameter smaller than 0.3 µm can exert a destructive influence. Since the scale on which components are integrated in IC's gradually increases, the unfavourable influence of particles having a diameter of 0,3 µm and smaller will in future play an increasingly larger part. The invention has for its object to provide a method which permits of removing in a simple manner undesired particles from a surface of a substrate, more particularly also particles having a diameter smaller than 0.3 µm. The invention is based on the following recognition. In the known method, the force by which the particles are removed from the surface of the substrate depends upon the cross-section of the particle to be removed and hence is proportional to the square value of its radius. The force by which the particle adheres to the substrate on the contrary is directly proportional to the radius of the particle. This means that the cleaning effect of the known method described strongly decreases with a decreasing particle size. Thus, it is not possible to effectively remove by means of the known method particles having a diameter smaller than a given value from the surface of the substrate. In order to achieve the object aimed at, the method mentioned in the opening paragraph is characterized according to the invention in that the removing force is exerted by moving an interface of the liquid over the surface of the substrate. The term "an interface of a liquid" is to be understood to mean herein not only the surface of a liquid, but also a differently obtained phase boundary between the liquid and a gas as well as the phase boundary between the liquid and another liquid. It has been found that the removing force which can be exerted by an interface of a liquid on a particle is a force which is caused by the surface tension of the interface and which, like the force by which the particle adheres to a substrate, is directly proportional to the radius of the particle. For the removal of particles from a surface of the substrate by means of the method according to the invention, the particle size consequently does not play a part. Thus, it is achieved that particles having a diameter smaller than 0.3 µm can also be effectively removed from the surface of the substrate. As will appear below, this can be realized in a simple manner. According to the invention, the method is carried out efficiently if the interface of the liquid is moved over the surface of the substrate at a speed of at most 10 cm/sec. Experiments have shown that the speed at which the interface is moved over the surface of the substrate plays an important part. Too high a speed does not lead to effective removal of particles adhered to the surface of the substrate, while, if this speed is lower than 10 cm/sec, this may result in the major part of the particles being removed. It is presumed that the interface requires some time to be adjusted. Moreover, the particle must be brought outside the range of action of the force by which it adheres to the substrate, which also requires some time. According to the invention, a preferred embodiment of the method is characterized in that the interface of the liquid is formed by its surface, which is moved over the surface of the substrate, by moving the substrate into the liquid. In this case, a particle adhered to the surface of the substrate is confronted with an advancing liquid. Thus, in a simple manner, particles which are wetted "not too well" by the liquid can be removed from a substrate, which is wetted "satisfactorily" by the same liquid, which in practice is the case for a substantial part of all the particles. During the step of wetting a surface (in this case the surface of the substrate or the surface of the particle) by a liquid, the interface of the liquid and this surface enclose an angle located in the liquid, which is designated hereinafter as "wetting angle". This angle has a value in the range of from 0 to 180°, while, according as the angle is smaller, the wetting is said to be more satisfactory. The term "the wetting of a surface is satisfactory" is to be understood to mean that the wetting angle is smaller than 90°, while the term "the wetting is not too good" is to be understood to mean that the wetting angle is larger than 30°. Another preferred embodiment of the method according to the invention is characterized in that the interface of the liquid is formed by its surface, which is moved over the surface of the substrate, by moving the substrate out of the liquid. In this case, a particle adhered to the surface of the substrate is confronted with a retracting liquid. Thus, in a simple manner, particles can be removed from a substrate, which is "poorly" wetted by the liquid, which particles are wetted "not too poorly" by the same liquid, which in practice is also the case for a substantial part of all the particles. The term "the wetting of a surface is poor" is to be understood to mean that the wetting angle is larger than 90°, while the term "the wetting is not too poor" is to be understood to mean that the wetting angle is smaller than 150°. This embodiment of the method has the additional advantage that, after having been effectively removed, the undesired particles are left in the liquid, while after the cleaning operation the substrate is situated outside the liquid so that a recontamination with removed particles cannot occur. A further preferred embodiment of the method according to the invention is characterized in that the interface of the liquid is formed by its phase boundary with a gas bubble which is moved over the surface of the substrate, the substrate being immersed into the liquid. Thus, a particle adhered to the substrate is confronted successively both with an advancing liquid and with a retracting liquid and, irrespective of the extent of wetting of the substrate, most of the particles can be removed from the surface thereof. An additional advantage of this embodiment of the method according to the invention is that the efficiency can be simply increased by moving several gas bubbles at a time over the surface of the substrate. Such a gas bubble can be formed very readily if the gas bubble is a vapour bubble which is formed by irradiation of the substrate immersed into the liquid by a beam of laser radiation and which is moved over the surface of the substrate by moving the beam of laser radiation over said surface. It has been found that the vapour bubble during its movement over the surface is not constantly equally large, but has a constantly varying diameter. Thus, on the movement of the interface caused by the movement of the laser beam, an additional movement is superimposed. A still more efficient method according to the invention is characterized in that the surface of the substrate and of the undesired particles adhered thereto are previously treated by a surface-active substance, as a result of which the surfaces of the substrate and of the particles will exhibit a less satisfactory wetting. Thus, it can be achieved that the wetting of particles which are wetted "too good" by the liquid and have adhered to a surface of a substrate which is wetted "satisfactorily" by the same liquid so that they cannot be effectively removed by one of the preceding embodiments of the method according to the invention, is reduced so that this becomes possible indeed. Furthermore, it can be achieved by this measure that the same wetting of a substrate which is wetted "satisfactorily" by a liquid and consequently cannot be freed from undesired particles adhered to its surface by moving it out of the liquid, is reduced to such an extent that this is possible indeed so that recontamination with removed particles can thus be counteracted. The treatment of the surfaces of the substrate and of the particles, as a result of which their wetting decreases, can be carried out efficiently if the surface-active substance is a substance chosen from the group of compounds comprising silanes, alcohols and alkyl lithium. The invention will be described more fully hereinafter, by way of example, with reference to a drawing and a few embodiments. In the drawing: Fig. 1 shows diagrammatically a substrate having a surface on which a particle is present, Fig. 2 shows diagrammatically a particle which has adhered to an interface of a liquid in a state of equilibrium, Fig. 3 shows diagrammatically a substrate having a surface on which a particle is present which is moved into a liquid, Fig. 4 shows diagrammatically a substrate having a surface on which a particle is present which is moved out of a liquid, Fig. 5 shows diagrammatically a gas bubble which can be moved over a surface of a substrate on which a particle is present and which can be formed by means of laser radiation. The Figures are schematic and not drawn to scale, while for the sake of clarity the dimensions are greatly exaggerated. Fig. 1 shows diagrammatically a substrate 1 having a surface 2 to which a particle 3 has adhered. An attracting force F A indicated diagrammatically by an arrow 6 is exerted on the particle 3 by a substrate 1, for which it holds that: in which R is the radius 5 of the particle 3, z is the distance 7 of the surface 4 of the particle 3 from the surface 2 of the substrate 1 and A is the so-called Hamaker constant. This force is directly proportional to the radius 5 of the particle 3. When removing such a particle 3 from the surface 2 of the substrate 1 by means of a liquid, this liquid must exert on the particle 3 a force which is larger than and is oppositely directed to the force F A 6 by which the particle 3 is adhered to the surface 2 of the substrate 1. Removal of undesired particles from the surface of a substrate is of major importance in the manufacture of integrated circuits (IC's), in which particles having a diameter of more than 10 to 20 % of the minimum detail size in the IC may lead to this IC operating unsatisfactorily. In the modern integration of components in IC's, details of the order of 1 µm occur. Particles having a diameter which is smaller than 0.3 µm can therefore already be very harmful. In the known method, by means of megasonic waves a liquid exerts on a particle a force which is proportional to the cross-section of the particle and hence to the square value of its radius. The value of this force consequently decreases with a decreasing particle size much more rapidly than the attracting force F A , which is in fact directly proportional to the radius R of the particle. With very small particles, this force can be smaller than F A so that these particles cannot be removed by means of the known method. It has been found that this is the case with particles having a diameter which is smaller than 0.3 µm. According to the invention, undesired particles 3 are removed from a surface 2 of a substrate 1 with a force exerted on the particles 3 by an interface of a liquid which is moved over the surface 2 of a substrate 1. The term "an interface of a liquid" is to be understood herein to mean not only the surface of a liquid, but also a differently realized phase boundary between the liquid and a gas and the phase boundary between the liquid and another liquid. Fig. 2 shows diagrammatically a particle 3 adhered to an interface 10 of a liquid 11. A number of forces act upon the particle and among these forces only that force caused by the surface tension of the interface is of importance for the very small and light particles which are concerned here. ϑ is the angle 12 located in the liquid which the interface 10 encloses with the surface 4 of the particle 3 and which is designated as wetting angle of the particle 3, while ω is the angle 13 which represents the position of the interface 10 with respect to the particle 3. In the case shown in Fig. 2 - a state of equilibrium-, ϑ 12 is equal to ω 13. If the particle 3 is moved into the liquid 11, this movement is counteracted by the interface 10 of the liquid 11 with a force F₁ which has at most a value F₁, max = 2πRγsin²( ϑ 2 ) , R being the radius 5 of the particle 3 and γ being the surface tension of the interface 10. If the particle 3 is moved out of the liquid 11, this movement is counteracted by the interface 10 of the the liquid 11 with the force F₂ which has a maximum value F₂, max = -2πRγsin²(90°+ ϑ 2 ) . The force F₁ or F₂ is exerted according to the invention on the particle 3 by the interface 10 of the liquid 11. Both the force F₁ and the force F₂ are directly proportional to the radius 5 of the particle 3. Particles having a diameter which is smaller than 0.3 µm can also be effectively removed from the surface of a substrate by means of the method according to the invention. According to the invention, the interface 10 of the liquid 11 is moved over the surface 2 of the substrate 1 at a speed of at most 10 cm/sec. If this speed is higher, the particles 3 are not effectively removed. Apparently, it requires some time before the interface 10 of the liquid 11 has been adjusted and before the particle 3 has been brought out of the attractive action of the substrate 1. Fig. 3 shows diagrammatically a step at a given instant of the method according to the invention, in which the interface 10 of the liquid 11 is formed by its surface 10, which is moved over the surface 2 of the substrate 1, by moving the substrate 1, as indicated diagrammatically by an arrow 16, into the liquid 11. For the sake of clarity, only that part of the surface 10 of the liquid 11 is shown which is of importance in this case and which is located in the immediate proximity of the surface 2 of the substrate 1. Thus, the particle 3 is confronted with an advancing liquid 11. The force F A 6 by which the particle 3 is adhered to the substrate 1 is counteracted by the component F₁ (→) of the aforementioned force F₁ (indicated diagrammatically by an arrow 17) which is directed transversely to the surface 2 of the substrate. This component, F₁(→) is indicated diagrammatically by an arrow 18 and has a maximum value of : F₁, max (→) = 2πRγsin²( ϑ 2 )cosα, where α is the angle 19 located in the liquid, which is enclosed by the surface 10 of the liquid 11 and the surface 2 of the substrate 1. This is the wetting angle of the substrate 1. Thus, in simple manner, particles 3 can be removed from a substrate 1 which is satisfactorily wetted by the liquid 11 (i.e. α < 90°), which particles are wetted not too well by the same liquid 11 (i.e. ϑ > 30°), which in practice is the case for a substantial part of all the particles 3. Fig. 4 shows diagrammatically a step at a given instant of the method according to the invention, in which the interface 10 of the liquid 11 is formed by its surface 10, which is moved over the surface 2 of the substrate 1, by moving the substrate 1, as indicated diagrammatically by the arrow 16, out of the liquid 11. For the sake of clarity, only that part of the surface 10 of the liquid 11 is shown which is of importance in this case and which is located in the immediate proximity of the surface 2 of the substrate 1. Thus, the particle 3 is confronted with a retracting liquid 11. The force F A 6 by which the particle 3 is adhered to the substrate 1 is counteracted here by the component F₂(→) of the aforementioned force F₂ (indicated diagrammatically by an arrow 20) which is directed transversely to the surface 2 of the substrate 1. This component, F₂(→), is indicated by an arrow 21 and has a maximum value of: F₂,max(→) = -2πRγsin²(90° + ϑ 2 )cosα Thus, particles 3 can be removed from a substrate 1 which is wetted poorly by the liquid 11 (α > 90°), which particles are wetted not too poorly by the same liquid 11 (ϑ < 150°), which in practice is also the case for a substantial part of all the particles 3. After having been removed, these particles 3 are left in the liquid 11 so that recontamination with these particles 3 is avoided. Fig. 5 shows a step at a given instant of the method according to the invention, in which the interface 10 of the liquid 11 is formed by its phase boundary 10 with a gas bubble 25, which, as indicated diagrammatically by an arrow 27, is moved over the surface 2 of the substrate 1, the substrate 1 being immersed into the liquid 11. Thus, the particle 3 is confronted successively with an advancing and a retracting liquid 11. Irrespective of the wetting of the substrate 1 (in Fig. 5 a poor wetting, i.e. α > 90°, is indicated) most of the particles 3 occurring in practice (30° ≦ ϑ ≦ 150°) can be removed. The cleaning operation can be carried out more efficiently by moving simultaneously several gas bubbles over the surface 2 of the substrate 1. Such a gas bubble 25 can be formed in a simple manner by irradiating the surface 2 of the substrate 1 with laser radiation indicated diagrammatically by arrows 28. In this case, the gas bubble 25 is a vapour bubble. In the following examples, a silicon wafer having a diameter of about 10 cm was cleaned with ethanol and was then covered with a suspension of particles having a known size, which was then dried up. Example 1 : In the manner indicated, palladium particles (ϑ ≈ 62°) having a diameter of 0.1 to 0.3 µm were provided on the silicon wafer (α ≈ 20°). The wafer was slowly moved at a speed of about 1 mm/s into a water-containing beaker, after which it was quickly drawn out of the water. After one immersion about 50 % of the particles proved to be removed; after five immersions this percentage was about 80 %. If such a wafer was moved into the water-containing beaker at a speed of more than 10 cm/s, an effective removal of the particles did not occur even after repeated immersions. Example 2 : In this example, rutyl particles (TiO₂) which were wetted "not too well" (ϑ > 30°) having a diameter of 0.1 to 1 µm were provided on the silicon wafer (α ≈ 20°) in the indicated manner. The wafer was moved slowly at a speed of about 3 µm/s into a water-containing beaker, after which it was quickly drawn out of the water. This embodiment resulted in an effective removal of about 85 % of the particles. However, if such a wafer was moved at a speed of more than 10 cm/s into the water-containing beaker, substantially no particles were removed after termination of this step. Example 3 : In the indicated manner, rod-shaped hematite particles (α Fe₂O₃) having a diameter of about 0.1 µm and a length of about 0.8 µm were provided on the silicon slice (α ≈ 20°). The slice was moved slowly at a speed of about 3 µm/s into a water-containing beaker, after which it was quickly drawn out of the water. All this resulted in an effective removal of about 97 % of the particles. Also in this case, substantially no particles were removed if such a wafer was moved at a speed of more than 10 cm/s into the water-containing beaker. In the following examples, the surface of the substrate and that of the undesired particles adhered thereto were previously treated by a surface-active substance, as a result of which the surfaces of the substrate and of the particles were wetted less satisfactorily. A suitable surface-active substance is a substance chosen from the group of compounds comprising silanes, alcohols and alkyl lithium. Example 4 : In the indicated manner, silicon oxide particles (ϑ ≈ 0°) having a diameter of about 0.7 µm were provided on the silicon wafer (α ≈ 20°). Before the wafer was moved at a speed of 3 µm/s into a water-containing beaker, the whole was treated by a vapour of CF₃(CH₂)₂SiCl₃ for 2 hours with exclusion of water at a pressure of 5 Pa and a temperature of about 100°C. By this pretreatment, a monomolecular layer of the said silane compound was deposited on the surface of the substrate and that of the particles, as a result of which the wetting with water of both surfaces was reduced to such an extent that for both surfaces the wetting angle assumed a value of about 76°. After the substrate was quickly drawn out of the water, about 75 % of the particles proved to be removed. If the pretreatment was omitted, substantially no particles proved to be removed. Example 5 : In the indicated manner, silicon oxide particles (ϑ ≈ 0°) having a diameter of 0.7 µm were provided on the silicon wafer (α ≈ 20°). Subsequently, the whole was treated for 2 hours at a temperature of 100°C at a pressure of 5 Pa and with exclusion of water with a vapour of CF₃(CF₂)₇(CH₂)₂SiCl₃, which resulted in deposition of a monomolecular layer of the said silane compound on the surface of the substrate and that of the particles, as a result of which the wetting with water of both surfaces was reduced to such an extent that after termination of this step their wetting angle had a value of about 100°. Subsequently, the whole was quickly immersed into a water-containing beaker, after which it was drawn out of the water slowly at a speed of about 3 µm/s. After termination of the cleaning operation, about 60 % of the particles proved to be effectively removed. The removed particles were visible as a haze on the surface of the water in the beaker. Example 6 : Another silicon wafer was treated in the same manner as in the preceding example. After the silane layer had been provided, the whole was immersed into a water-containing beaker, after which a beam of monochromatic laser radiation having a wavelength of 514 nm and a cross-section of about 20 µm was directed by means of an argon laser onto the surface of the substrate, as a result of which vapour bubbles were formed in situ and in the proximity of the beam on the surface of the substrate. The beam was moved in lateral direction over the substrate at a speed of 16 µm/s. All this resulted in the removal of about 95 % of the particles. It has been found that during the movement over the surface of the substrate the vapour bubble was not constantly equally large, but had a constantly varying diameter. As a result, an additional movement was superimposed on the movement of the interface, which was produced by the movement of the laser beam, and this additional movement possibly accounts for the significant difference in efficiency with the preceding example. It should be noted that, if in the aforementioned examples a glass substrate should be used instead of a silicon substrate, corresponding results would be obtained. It would be appreciated that the invention is not limited to the examples given, but that many further variations are possible within the scope of the invention for those skilled in the art. For example, the wetting of the surfaces of the substrate and of the particles adhered thereto may be reduced by a treatment thereof with an alcohol or an alkyl lithium. The following table indicates a few examples thereof for a silicon substrate, of which the wetting angle α has a value 0° to 20°. In the second column of the table, the value of the wetting angle α of the silicon substrate is stated after the latter has been treated with the substance mentioned in the first column. It is also possible to facilitate the removal of particles from a surface of a substrate by increasing the setting of the surface and the particles adhered thereto with a surface-active substance. Thus, a substrate which is poorly wetted by a liquid (α > 90°) may be treated so that particles may be removed from its surface by moving the substrate into the liquid. Further, particles which are wetted too poorly by a liquid (ϑ > 150°) may be treated so that they may be removed from a surface of a substrate by moving the substrate out of the liquid.

Claims: A method of removing undesired particles from a surface of a substrate, in which by means of a liquid a removing force is exerted on the particles, characterized in that the removing force is exerted by moving an interface of the liquid over the surface of the substrate. A method as claimed in Claim 1, characterized in that the interface of the liquid is moved over the surface of the substrate at a speed of at most 10 cm/sec. A method as claimed in Claim 1 or 2, characterized in that the interface of the liquid is formed by its surface which is moved over the surface of the substrate by moving the substrate into the liquid. A method as claimed in Claim 1 or 2, characterized in that the interface of the liquid is formed by its surface which is moved over the surface of the substrate by moving the substrate out of the liquid. A method as claimed in Claim 1 or 2, characterized in that the interface of the liquid is formed by its phase boundary with a gas bubble which is moved over the surface of the substrate, the substrate being immersed into the liquid. A method as claimed in Claim 5, characterized in that the gas bubble is a vapour bubble which is formed by irradiating the substrate immersed into the liquid with a beam of laser radiation and which is moved over the surface of the substrate by moving the beam of laser radiation over said surface. A method as claimed in any one of the preceding Claims, characterized in that the surfaces of the substrate and of the undesired particles adhered thereto are previously treated with a surface-active substance, as a result of which the surface of the substrate and of the particles will be wetted less satisfactorily. A method as claimed in Claim 7, characterized in that the surface-active substance is a substance chosen from a group of compounds comprising silanes, alcohols and alkyl lithium.

Inventors: LEENAARS, ADRIAAN FRANCISCUS MARIA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: B08B3
IPC codes: H01L21, B08B3, B08B7