METHOD AND APPARATUS FOR PROTECTING ULTRACLEAN SURFACES

Abstract: A method for protecting an ultraclean surface of an object includes the steps of forming, from an ultraclean fluid, a laminar flow moving over the surface, wherein, in order to enhance the reliability of protection, the velocity in the layer of the laminar flow from the ultraclean fluid is varied according to the height thereof in a cross-section perpendicular with respect to the direction of propagation of the flow.

Description: METHOD AND APPARATUS FOR PROTECTING ULTRACLEAN SURFACES The invention relates to a method and apparatus for * protecting a surface and, in particular, thought not exclusively, to a method and apparatus employing a flow of I 5 a protective fluid. The invention may be used, for example, in the manufacture of electronic equipment, in precise mechanical engineering and instrumentation, and medical engineering for the treatment of abacterial chambers. Modern industrial production is characterized by the 10 use of materials having a high uniformity of composition and accuracy of machining (Clean Rooms. Ed. By Hayakawa. 1990. Mir Publishing House, Moscow, pp. 44-58) . It is impossible to manufacture such products without creating a special production environment which is capable of ensuring 15 predetermined conditions in production rooms for long periods of time. It is most efficient to maintain an ultraclean environment in production rooms with laminar air flow (laminar flow is a flow in which the intensity of transfer processes is close to a level determined by 20 molecular movement) . The velocity of contaminant particles in a laminar flow is determined by molecular movement, and such velocity is several times lower than the velocity of the flow * proper. For this reason the particles can penetrate only to *' 25 a small depth into the flow because they are very soon washed off the flow. In such a case, the degree of contamination of a surface to be protected depends on the degree of cleaning of the laminar flow proper. In addition, the direction of air flows carrying fine particles can be predicted with the laminar flow so as to prevent them from getting to the surface being protected. For this reason this circulation method is most suitable and is widely used for producing high-purity fluids. Vertical or horizontal circulation has been used so far for maintaining a high cleanliness of the environment within clean rooms. Two methods for protecting surfaces are known. One method resides in washing off, with a laminar air flow, fine particles released from a source of contamination to reduce the contamination down to such as level as to prevent the penetration of the particles into a zone near to the surface being protected. The second method involves forming a flow of highly clean air in a zone near to the surface being protected in order that it moves in parallel with the surface so as to prevent fine particles from the interior space of the room getting to the surface of an object being protected. This method of protection is efficient, but it has a number of disadvantages and, in particular, it calls for substantial investment, i.e. the special equipment of the rooms and a high operating cost. With this method, gases and vapors released at various production steps propagate throughout the entire room, and a high degree of cleaning of the incoming flow is necessary. This results in the need to have multiple cleaning filters, and the release of dust in the room is to be prevented so that requirements imposed upon clothing, equipment and operating procedures are rather stringent. In the manufacture of miniature products which require especially clean conditions (Clean Rooms. Ed. by Hayakawa. 1990. Mir Publishing House. Moscow, pp. 58, 59), e.g. in making extra ELSIs, the effective protection of the surfaces of products against dust can be ensured within a relatively small space with an ultraclean environment. The method involves producing a laminar flow of an ultraclean fluid to flow over the surface being protected. This method allows power consumption to be reduced, since the size of ventilation and filtering devices is smaller; and the requirements imposed upon the cleanliness of the whole room are lower. The system does not call for protective chambers so that it can be integrated into a continuous automatic production process. The method for forming a flow of an ultraclean fluid does not, however, allow turbulence to be avoided, hence it is not reliable enough. The above-described method may be carried out by means of an apparatus for providing a local clean space (Clean Rooms. Ed. by Hayakawa. 1990. Mir Publishing House. Moscow, pp. 58, 59) . The apparatus has an air cleaning unit connected through a pipeline to a laminar flow former accommodated in a casing. The flow, which is thus formed. is directed to a zone in which an object being protected is located. This apparatus has the same disadvantages as the above-described method. The provision of such flow is also important in applications where it is desired to protect the surface of an object not only against contaminants, but also against the air environment. In this case a steady laminar flow, which does not mix with the air environment and which is formed from a gas neutral with respect to the surface of an object being protected, constitutes a reliable screen against the negative effect of the air environment. Depending on the condition of the air environment, i.e., on the velocities of air flows, the variation of the velocity of the laminar flow can be effected by various means in arrangements to be described. If an external flow moves at a low velocity, the velocity of the laminar flow is monotonously or continuously decreased according to the height of its layer in the direction away from the surface of an object being protected. A more stable laminar flow is thus produced. If the external flow moves at a substantial velocity, it is preferred, in order to produce a more stable laminar flow, to monotonously increase the velocity of the laminar flow according to the height of its layer in the direction away from the surface of an object being protected. This lowers perturbations in the laminar flow since the velocities of the external flow and laminar flow are identical. If the temperature of the surface of an object differs from the temperature of the environment, or from the temperature of the laminar flow, the flow velocity is to be increased to prevent temperature flows or to avoid perturbations that can arise in the laminar flow. However, if the velocity of a laminar flow increases, the flow becomes less stable, and a perturbation on the part of an object being protected can cause turbulence. To avoid this, the velocity of the laminar flow is monotonously increased from zero and then monotonously decreased to zero according to the height of the layer of the laminar flow. A method according to the invention can be carried out by means of an apparatus for producing a flow of a protective fluid, comprising a hollow casing having an inlet for admitting the fluid into the casing and a laminar flow former provided inside the casing, with a partition wall pervious to the protective fluid being provided downstream of the laminar flow former, the partition having a monotonously varying resistance to the flow in at least one of orthogonal directions in a plane drawn perpendicularly with respect to the direction of the laminar flow. Apparatus to be described below for producing a flow of a protective fluid allows the above-described method to be carried out in the most simple way. The pervious partition may be designed in various ways in apparatus according to the invention. Thus, the partition may have parallel passages of identical cross-sectional areas equally spaced over the whole surface area of the partition, the lengths of the passages increasing in the plane of the longitudinal section of the partition at least in one direction according to the height thereof. The monotonously or continuously increasing lengths of the passages offers increasing resistance to the protective fluid. The velocity of the laminar flow that has passed through such a partition monotonously decreases according to the height of the flow layer thus formed, and this gradual variation of velocity of the laminar flow reduces the rate of growth of perturbations in the flow, i.e., it rules out the appearance of turbulence and, as mentioned above, reduces the thickness of the layer containing particles from the environment, whereby reliability of the protection is enhanced. The pervious partition may be in the form of a plate having equally spaced rows of holes. In one embodiment, the holes of one and the same diameter are arranged with a spacing of the holes monotonously increasing in each successive step. In another embodiment the diameter of the holes monotonously increases in each successive row, the spacing remaining unchanged, it is easier to make this pervious partition, than a partition in which the lengths of the holes varies, and the thickness of such a partition may be small, so that the apparatus is more compact. A pervious partition, which is most easy to make, can be manufactured from an open-porosity material which has an impervious screen on the side of the laminar flow former which partly covers the laminar flow, the thickness of the pervious partition being at least equal to the height of its portion free from the impervious screen. Embodiments of the invention will now be described, by way of example, with reference to specific non-limiting embodiments illustrated in the accompanying drawings, in which: Figure 1 shows an apparatus having a pervious partition in the form of a plate having holes; Figure 2 shows an apparatus having a pervious partition with passages of different lengths; Figure 3 shows a first embodiment of an apparatus having a pervious partition made of an open-porosity material Figure 4 shows a second embodiment of an apparatus having a pervious partition made of an open-porosity material. Reference will first be made to Figure 1, in order to enable there to be a better understanding of a method for producing a flow of a protective fluid using apparatus according to one aspect of the invention and the gist of methods according to the invention to be made clearer. The apparatus shown in Figure la has a hollow casing 1 having an inlet 2 for admitting a protective fluid. The casing 1 accommodates a laminar flow former 3 in the form of a set of woven nets in a frame with a mesh size of 0.2 to 0.3 mm fastened inside casing 1 by means of spacers 4 and 6. A pervious partition 6 having a support holder 7 is intimately pressed against spacer 5. The configuration of the support holder 7 conforms to the shape of the casing 1. The support holder 7 is inserted into the casing 1 and has a shoulder around its external perimeter, the outside dimensions of the shoulder correspond to the inside dimensions of the casing 1, the thickness of the shoulder being sufficient for pervious partition 6 to be pressed against the spacer 5. One of the walls of the support holder 7 has a recess for receiving a part to be protected, e.g., a silicon wafer 8. The depth of the recess corresponds to the thickness of the wafer 8. The pervious partition 6 (Figure lb) has several rows of holes 9 of identical diameter. The spacing of the holes 9 in each row increases from the lowest to the uppermost row. Accordingly, the resistance of the pervious partition 6 increases with the height of the partition. The apparatus is used in the following manner. Depending on a problem, either ultraclean air or ultraclean gas neutral with respect to an object being protected is admitted through the inlet 2 to the apparatus. The velocity of the flow admitted through the inlet 2 is higher than the velocity at the outlet of the apparatus by the same factor by which the cross-sectional area of casing 1 is greater than the cross-sectional area of inlet 2. For instance, if the velocity at the inlet 2 is 30 m/s, a velocity of 1 m/s, which is suitable to maintain the laminar character of the flow, can be obtained at the outlet of the apparatus with an appropriate ratio between the above-mentioned cross-sectional areas. When the flow passes through the former 3, perturbations in the flow are reduced, and the flow becomes uniform and laminar. Pervious partition 6, with a resistance that varies from its lower to its upper edge, reduces the velocity of the laminar flow according to the height thereof, i.e., the maximum value of the velocity is reached at the surface of the wafer 8 being protected and the minimum value obtains at the upper boundary of the flow. Maximum velocity of the flow at the outlet of the apparatus is as high as 1 m/s. In this embodiment it is very important that the wafer 8 does not protrude out of the recess. This allows eventual perturbations in the flow that might otherwise cause turbulence to be avoided. If the ambient environment were to have a substantial velocity it would be necessary to increase the velocity of the laminar flow in order to rule out perturbations in the laminar flow. In this case the velocity is at its maximum in the upper layer with respect to the wafer 8 and the velocity value is a minimum at the surface of wafer 8. With a velocity front which is thus formed, the thickness of the wafer 8 may be greater than the depth of the recess of the support holder 7. A low velocity of the laminar flow at the surface of a protruding wafer 8 does not cause perturbations in the flow and will not disrupt its laminar character. The pervious partition 6 (Figure lb) , with holes 9 of one and the same diameter, has spacing between the holes of a row which monotonously decreases row by row according to the height thereof. This arrangement of the pervious partition 6 allows the above-described velocity profile to be formed. As mentioned above, with a temperature difference between the environment or laminar flow and the wafer 8, convection flows arise that might break the laminar flow. To avoid the effect of convection flows, the maximum velocity of the laminar flow is increased. To prevent the eventual turbulence that might occur at high flow velocity, the velocity value is increased from zero up to its maximum and then again decreased down to zero. For that purpose the spacing between the holes 9 of the pervious partition 6 is decreased in an embodiment which is not illustrated in each successive row beginning with the second one to a certain value and is then increased in further rows back to the initial value. Figure lc shows a pervious partition 6 which can be used in the apparatus of Figure la to produce the same result as is obtained with the pervious partition 6 of Figure lb. The difference resides in the fact that the variation of the resistance of the partition to a protective fluid according to height is achieved by varying the diameters of the holes 9 in each successive row. Thus the holes 9 in the lower most row are of a larger diameter than the holes 9 in the uppermost row. In another embodiment of pervious partition 6, (not shown) the spacing between and the diameters of the holes 9 are both varied in each successive row of holes. It will be apparent from the above that a feature of a method according to the invention for protecting ultraclean surfaces of objects resides in producing a laminar flow of a protective fluid, with the velocity of the flow being monotonously varied within a layer of the flow according to the height thereof in a cross-section drawn perpendicularly with respect to the direction of propagation of the flow. The flow should move over the surface of an object being protected. This method of forming the laminar flow allows its properties to be maintained, e.g. over a length of flow of 1 m, with a thickness of the flow being formed of 0.4 m. Therefore, if an object is placed within such a space incorporating a protective fluid, reliable protection of a surface being protected against the penetration of contaminating particles from the environment, or against penetration of a gas towards the surface being protected from the environment can be achieved. Other embodiments of a pervious partition are possible which allow a steady laminar flow to be formed over a limited length so as to ensure reliable protection of the surface of an object. Parts of the apparatus shown in Figure 1 and identical parts of the apparatus shown in the following figures are indicated by the same reference numerals. Figure 2 shows an apparatus having a pervious partition 6 which has passages 10 of one and the same diameter. The lengths of the passages 10 in each successive row, beginning with the lower row, increase monotonously so as to offer an increasing resistance to the protective flow. This partition 6 can also be made by suitably supporting capillary tubes arranged in a wall. Most noteworthy are embodiments of the apparatus shown in Figures 3 and 4. In these embodiments a pervious partition 6 is made of an open-porosity material and is partly covered by a screen which is impervious to the protective fluid. The thickness d of the pervious partition 6 should in these embodiments, be at least equal to the height h thereof which is free from the screen, i.e., d ≥ h. It is only this ratio between the thickness and height of the free portion of the pervious partition that allows the flow to move in both longitudinal (in the plane of the drawing) and transverse directions. The flow moving in the transverse direction encounters a greater resistance and its velocity decreases. Therefore, a smooth velocity profile of the laminar flow is formed in this apparatus. The difference between the apparatus shown in Figures 3 and 4 is in the construction of the impervious screen. In the apparatus of Figure 3 the impervious screen is in the form of a wall 11 of the hollow casing 1 which functions as a screen. In the apparatus of Figure 4 a liner 12 fills a part of the interior of the hollow casing 1 between the former and the pervious partition 6. The pervious partition 6 can be made of sintered balls, felt and cellular materials. Other structures for the pervious partition 6 includes a set of arrays, the spacing of the arrays being at least equal to the mesh size of the array. Such a construction of the previous partition 6 is of special interest since the resistance of the pervious partition 6 can be varied in both the longitudinal and transverse sections by increasing the spacing of the arrays so as to vary the thickness of the flow formed in the apparatus. With an increase in the spacing of the arrays the resistance in the transverse section deceases and the flow in this direction can reach the edge of the partition, i.e., maximum protective layer thickness can be obtained in this case. Preferred embodiments of the invention have been described above, and other embodiments and modifications can be used without going beyond the scope of the invention as defined in the appended claims.

Claims: CLAIMS 1. A method for protecting an ultraclean surface of an object including the steps of forming, from an ultraclean fluid, a laminar flow moving over the surface, wherein, in order to enhance the reliability of protection, the velocity in the layer of the laminar flow from the ultraclean fluid is varied according to the height thereof in a cross-section perpendicular with respect to the direction of propagation of the flow. 2. A method as claimed in claim 1, wherein the velocity of the laminar flow is decreased according to the height of a layer in the direction away from the surface being protected. 3. A method as claimed in claim 1, wherein the velocity of the laminar flow is increased according to the height of a layer in the direction away from the surface being protected. 4. A method as claimed in claim 1, wherein the velocity of the laminar flow is increased from zero and then decreased to zero according to the height of a layer of the laminar flow. 5. An apparatus for producing a flow of a protective fluid, including a hollow casing having an inlet for admitting a fluid thereto and a laminar flow former accommodated in the casing, wherein, in order to enhance the reliability of protection, a partition pervious to the protective fluid is provided downstream of the laminar flow former, the partition having a resistance to the flow which varies in at least one of orthogonal directions in a plane drawn perpendicularly with respect to the direction of movement of the laminar flow. 6. An apparatus as claimed in claim 5, wherein the pervious partition has a plurality of parallel passages of one and the same cross-sectional area which are equally spaced over the entire surface area, the length of the passages increasing in the plane of longitudinal section of the partition in at least one direction according to the height thereof. 7. An apparatus as claimed in claim 5, wherein the pervious partition is in the form of a plate having equally spaced rows of holes of identical diameter, the spacing between the holes increasing row by row. 8. An apparatus as claimed in claim 5, wherein the pervious partition is in the form of a plate having equally spaced rows of holes, the diameters of the holes increasing row by row. 9. An apparatus as claimed in claim 5, wherein the pervious partition is made of an open-porosity material and wherein on one side of the laminar flow former there is an impervious screen which partly covers the partition, the thickness of the pervious partition being at least equal to the height of that portion of the partition not covered by the impervious screen. 10. An apparatus as claimed in claim 9, wherein the pervious partition is made of an open-porosity material and is in the form of a set of arrays, the spacing between the arrays being greater than the mesh size of the arrays.

Inventors: PAVELIEV ANATOLY ABRAMOVITZ, PAVELIEV, ANATOLY ABRAMOVITZ
Applicants: OPA OVERSEAS PUBLISHERS ASS AM, OPA (OVERSEAS PUBLISHERS ASSOCIATION) AMSTERDAM, B
Application year: 1992
Main classification: H01L21
IPC codes: B08B17, F24F3, F24F13, H01L21