POLYPROPYLENE WAFER CARRIER

Abstract: Shipping containers for silicon wafers, the containers being of polypropylene from a double deashed random copolymer with minimal amounts of ethylene and additives in minimal amounts of high molecular weight hindered phenolics as the antioxidant/antiozonant, and minimum amounts of clarifier compound and flow enhancers. The test method of determining the hydrophobicity effect of polypropylenes on stored semiconductor wafers including cleaning organic contaminants from specimen wafers, from the inside of and cover of a glass jar and stand for holding the wafer; exposing the wafer to a quantity of polypropylene resin in the jar for a length of time at advanced temperatures; and measuring the sessile drop contact angle of DI water on the faces of the silicon wafers which have been exposed to various polypropylenes; minimum drop contact angle producing indications of the hydrophobicity effect of the polypropylene on the wafer.

Description: POLYPROPYLENE WAFER CARRIER This invention relates to polypropylene containers used in shipping silicon wafers which are to be processed into circuit chips, and more particularly to the nature of the polypropylene used in such shipping containers. BACKGROUND OF THE INVENTION Such containers, known as shippers, have been utilized in the past, and have been molded of polypropylene, and the particular polypropylene which was previously selected for use was selected because of its high clarity. The previous polypropylene used was thought to be very clean, based upon the total amount of volatile material given off, as was determined by well-known testing methods as set forth in Designation: F 1227 in the Annual Book of ASTH Standards, volume 15.03, as published by the American Society for Testing and Materials, Philadelphia, Pennsylvania. However, users of such shippers determined that previous polypropylene shippers outgassed organic impurities which would be collected on the wafers. Ordinarily, when no organic impurities are present on the face of such a wafer, the oxide layer on the wafer is very hydrophilic such that deionized (DI) water will sheet across the wafer and will not bead up on the wafer. It has been experienced that wafers contained in such prior art shippers of polypropylenes previously used are hydrophobic because of the organic contamination derived from exposure to the polypropylene of the shipper. The polypropylene utilized in the prior shippers causing such hydrophobicity was made from a random copolymer originating with Hi ont U.S.A. Inc. of Wilmington, Delaware. The random copolymer had monomers and a high activity catalyst added, was polymerized, and then after polymerization, a clarifying agent, calicum stearate, was added as a flow enhancer and an antioxidant including phosphites and hindered phenolics. Neither the catalyst nor the atactic material produced in the polymerization was removed prior to use of the polypropylene for molding such shippers. It is noted that it is common practice to keep the exact nature of polypropylene compounds as a trade secret, and accordingly, the nature of such compounds used in other commercial shippers is not public knowledge, or known. Further, double deashed polypropylenes have been previously known, but have not been known to be used in shippers or containers in which wafers of silicon or gallium arsenide or memory disks of magnetic media are stored as for shipping. Such previously known double deashed polypropylenes have used large concentrations of such things as clarifiers and flow enhancers and have used certain high molecular weight hindered phenolic antioxidants/antiozonants, all resulting in polypropylene which, if it had been used for silicon wafer shippers, would have significant and largely unacceptable levels of outgassing and contamination of stored wafers causing the wafers to be unacceptably hydrophobic, i.e., requiring cleaning prior to use. It has been quite common practice to determine the hydrophobicity problem by simply wetting the surface of a wafer with DI water and if the water beaded up as distinguished from sheeting across the wafer, it was readily determined that the wafer had been contaminated and had to be cleaned before use. Of course, such cleaning involved extra steps in processing the wafer, which are undesirable because of cost and because of a likelihood of generating additional particles which would further contaminate the faces of the wafers. Wafer fabricators are also aprehensive of the hydrophobic contamination because of the possibility that the contamination may have affected the wafer in some other way. A somewhat crude and rough test has been used for determining whether such polypropylene shippers cause hydrophobicity of the wafers carried thereby. Although the method previously used has been very subjective, it did show that the amount of hydrophobicity produced on the wafers was different for different polypropylenes. The testing procedure previously used involved storing wafers inside such polypropylene shippers in an oven at 50 β C for 24 hours. At the end of the 24 hour storage, the wafers were removed from the shipper, wet with DI water and tipped on edge in a vertical position. After ten seconds, the percentage of the wafer that was no longer wet was recorded as the percent hydrophobicity. SUMMARY OF THE INVENTION An object of the invention is to produce storage devices or containers, suitable for storing and shipping wafers, of polypropylene of such a nature as to minimize the possibility of producing organic contamination and the related hydrophobicity of the wafers to be contained in the shipper. Another object of the invention is to create an improved method for checking the hydrophobicity produced in wafers from polypropylenes which may be used in such shippers. A feature of the invention is a polyproplylene container, especially adapted for storing and shipping silicon wafers and other similar wafers and magnetic disks, and molded of a polypropylene which is cleaned of materials, such as atactic polypropylene and oligomers which outgas from a container onto silicon wafers stored therein, causing contamination and hydrophobicity of such wafers. A preferred polypropylene comprises a double deashed, random copolymer polymerized with an old style catalyst containing minimal amounts of a clarifying agent, antioxidant, and a flow enhancer. Such a polypropylene in the shipper minimizes outgassing organic contaminants so as to allow the silicon wafers to remain clean during shipping and storage, thereby avoiding the need to clean the wafers after they have been stored in the container, prior to further processing. A polypropylene from a polymerization process using a high activity catalyst, and with such additives included, may be utilized if the materials are removed which otherwise would cause contamination of the wafer surfaces, resulting in hydrophobicity of the wafer surfaces. This container of the double deashed polypropylene is impact resistant and sufficiently clear to permit visual inspection of stored wafers without opening the closed container. Elimination of the clarifier from the double deashed polypropylene is satisfactory in many instances because in thin sections the panels of the container will be sufficiently clear to permit the needed inspection. The preferred polypropylene utilized to mold the container utilizes a random copolymer with a minmal ethylene content for clarity and impact strength, which are important properties for the shipping container. Incorporating a minimal amount of clarifier into the polyproylene is important because random copolymers do not have enough clarity on their own for the shipping container, unless portions of the container are especially thin in cross section. The clarity is needed to be adequately transparent in its normal use as to allow checking of the silicon wafers contained in the container. Alternately, the polypropylene may comprise a double deashed homopolymer to which is added minimal amounts of an antioxidant/antiozonant and flow enhancer. Ordinarily, a homopolymer does not have clarifier added. Such a double deashed polypropylene from a homopolymer will have adequate clarity if molded in very thin sections in certain panel areas. Although a polypropylene comprised of a homopolymer may not have the high impact resistance of a polypropylene comprised of a random copolymer, the former will have adequate impact resistance for many uses because polypropylene shippers loaded with wafers are very carefully handled due to the high value of the stored wafers. The principal difference of the homopolymer polypropylene is the elimination of ethylene from the monomer; whereas the random copolymer does have the ethylene content. The antioxidant/antiozonant is important to the polypropylene in order to prevent the polymer chains from breaking down and to prevent the material from becoming brittle. Preferably, the antioxidant used is Ethanox 330, a hindered phenolic compound with low outgassing characteristics, which results in less hydrophobicity of the stored wafers. A minimal amount of flow enhancer in the polypropylene is important to improve the mold filling and ejection characteristics of the polypropylene, without causing undesired hydrophobicity of the wafers. It is considered important that the polypropylene be double deashed to remove the oligomers, the atactic polypropylene, and the polymerization catalyst. The double deashing involves running the polypropylene through the solvent washing twice so that the molded container will not outgas unacceptable levels of organic contaminants to the wafers as to require further cleaning after storage. Although the double deashed material will still have some volatiles left in them from the solvent washing, these compounds have little enough molecular weight and are volatile enough that they will not recondense on the wafer and cause hydrophobicity. It is also important that the double deashed polypropylene will have a lower level of trace metals which are also considered contaminants if any are collected on the surfaces of the wafers. Wherever the word wafers appears herein, it is meant to include any of a variety of wafers or disks to be, or being, processed into circuit chips and made of silicon or gallium arsenide, or epitaxial wafers, or disks being processed into memory disks, or magnetic media, without limitation. Another feature of the invention is to provide a simple method of determining the suitability of a polypropylene for use in a shipper without contaminating the silicon wafers and causing hydrophobicity. The cleaning of a specimen wafer of organic material must be done first so that hydrophobicity later created can be determined. After a wafer is suitably cleaned, the wafer is stored in a jar which has also been cleaned to be free of organic contamination; and then the wafer is stored in the sealed and clean jar along with a quantity of a polypropylene resin for approximately 20 hours at a temperature, such as 50 β C, which is representative of the high temperature to which the wafer and shipper may be exposed during storage of the wafer in the shipper. Hydrophobicity of the wafer is determined, after such storing and exposure is completed, by a technique called sessile drop contact angle measurement which produces a measurement of hydrophobicity which is very accurate except where the organic contamination is very low and wherein the surface of the wafer is highly hydrophilic. Preferably the jar and its cover, the wafer and an aluminum stand to support the wafer in the jar, are cleaned with the use of an oxygen plasma cleaning apparatus which is commercially available and well-known to persons of skill in this art. Otherwise, the jars may be cleaned by soaking in a strong oxidizing acid such as concentrated sulfuric acid with nochromix additive or chromium trioxide in sulfuric acid. After such acid cleaning of the jars, the jars would be rinsed with DI water and dried over an open flame. Organic contaminates may be cleaned off the wafers, and from the aluminum stand for supporting the wafers, by heating them over an open flame until the wafers glowed red. Alternately, a UV ozone cleaner may be used for cleaning the jar, stand and wafer, but preferably the oxygen plasma cleaning is used. In utilizing the sessile drop contact angle measurement technique for determining the hydrophobicity of the wafer surface, a small drop of DI water is placed on the wafer using a micro-syringe. When the drop stops spreading, the angle between the face of the wafer and a line drawn tangent to the base of the drop is measured. This is called the advancing angle. A receding angle is also measured, either by tipping the wafer until the drop runs, or by pulling the water back up into the micro-syringe. When the edge of the drop starts to recede, water removal is stopped and the angle is measured between the face of the wafer and a line drawn tangent to the base of the drop. The receding angle is very difficult to measure when the angle becomes very low, but the low angle indicates that the surface is highly hydrophilic, and very low hydrophobic. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 a diagrammatic view of a polypropylene shipping container for silicon wafers according to the present invention. Figure 2 is a flow chart diagrammatically illustrating the polymerizing and washing of the polymer used in molding the polypropylene shipping container of Figure 1. Figure 3 is a perspective view of one of the silicon wafers with its surface cleaned and hydrophilic to cause the deionized water thereon to sheet across. Figure 4 is a side elevation view of the wafer of Figure 3 showing the deionized water uniformly dispersed across the hydrophilic surface of the wafer. Figure 5 is a perspective view of a silicon wafer with a surface that is at least partially hydrophobic showing deionized water beaded up on the surface. Figure 6 is an elevation view of the wafer of Figure 5 and showing the water beaded on the surface of the wafer. Figure 7 is a diagrammatic illustration of an oxygen plasma cleaning apparatus useful for cleaning specimen wafers and other apparatus used in the improved method for checking the hydrophobicity effect of polypropylene resins. Figure 8 is a diagrammatic view of a closed and clean jar used in exposing a wafer to polypropylene resins to determine the hydrophobicity effect of the resins on the stored wafer. Figure 9 is a diagrammatic view illustrating the sessile drop contact angle measurements used in the improved method of checking hydrophobicity effect of polypropylenes on silicon wafers. DETAILED SPECIFICATION The wafer storage device 9 comprises a polypropylene outer container 10 for shipping wafers. The important components of the outer container are a receptacle 11 and a cover 12 which fit tightly together at a joint 13 which may be sealed as by a length of tape with adhesive on it so that the interior of the container is hermetically sealed and isolated from the ambient conditions at the exterior of the container. Alternatively, the cover 12 may be held on the lower portion 11 by a latching mechanism. The container 10 is entirely formed of polypropylene, as by injection molding. The polypropyelene is quite clear and accordingly, the objects found within the interior of the container may be visible through the container walls. The container 10 may confine an inner container or wafer carrier 14 which may store a multiplicity, typically 25, of silicon wafers 15. The carriers 14 are also molded of the same polypropylene from which containers 10 are molded. Other similar wafer carriers 14 may be molded of Teflon PFA, otherwise known as perfluoroalkoxy, or of other materials such as polyethylene, but will probably not be used with the polypopylene containers 10. The individual wafers are arranged in side-by-side relation to each other, and in alignment, and are separated from each other by a plurality of separating teeth or webs 16 which are formed integrally of the carrier 14, in the sidewalls thereof. The webs or teeth 16 function to confine each wafer 15 against engaging adjacent wafers and minimize movement of the wafers in the carrier. The carrier 14 is typically provided with webs 16 and other facilities to accommodate handling of the carrier by means of robots. The top 17 of the carrier 14 is open and unobstructed for facilitating insertion and removal of wafers when the carrier is removed from the shipping container 10, and the bottom 18 is also open to accommodate flow of processing liquids for the wafers 15. Typically, the sidewalls 19 of the carrier also have slots to provide access for liquids and gases in the processing of the wafers. Wafer carrier 14 may have the particular configuration of the wafer carriers shown in U.S. Patents 4,949,848 or 3,961,877 or other similarly commerically available carriers. It should be recognized that the lower portion or container 11 and the cover 12 are entirely closed and do not allow fluids or gases to enter or escape from the interior of the container 10. During storage of the wafers 15, the wafers are exposed to the interior surfaces of the container 11 and whatever outgassing may exist from the container walls. The shipping container may have cushioning devices 20 formed or attached to the cover 12 to engage and hold down the wafers 15 and hold them more stationary in the carrier 14. The preferred material used in molding the containers 10 and 14 is a double deashed, random copolymer polypropylene containing an antioxidant/antiozonant and a flow enhancer, and preferably a clarifier. Alternately, the material may be a double deashed homopolymer containing an antioxidant/antiozonant, and a flow enhancer. A double deashed random copolymer or double deashed homopolymer polypropylene is found to be least likely to cause wafer hydrophobicity which is the most important characteristic. The random copolymer polypropylene has inherent clarity and impact strength which are important properties for the shipping container. These features are considerably better as compared to a homopolymer which is more rigid and has less clarity and impact resistance, but in many instances are not of critical importance. The random copolymer has ethylene monomers polymerized randomly into the polypropylene backbone. The random copolymer has the following repeat units (CH 2 -CH 2 ) y (CH 2 -CH) χ CH- where "x" comprises 90% or more of the links. The ethylene content of the random copolymer resin has an ethylene content of 2.2 to 2.8%. The double deashed polypropylenes are considered to be among the cleanest polypropylene resins because the deashing process removes oligomers, atactic polypropylenes and the polymerization catalyst, leaving the desired propylene known as isotactic polypropylene which has the following steric orientation: The removed oligomers are compounds having few monomer units. The removed atactic polypropylene is a very soft, sticky material often used as glue and has monomer units with a particular steric orientation to one another, as follows: In both foregoing steric orientations, for polypropylene, the R respresents CH 3 groups. The deashing process also removes the polymerization catalyst which is the old style catalyst as contrasted with the recently developed high activity catalyst used in polymerizing certain polypropylenes. The high activity catalyst polypropylenes may be suitable if cleaned or washed after being polymerized to remove undesirable catalysts, atactic material, and other materials, such as oligomers that outgas. The double deashed random copolymer polypropylene used herein will still have certain volatiles left in it from the solvent washing, but these compounds are low enough in molecular weight and volatile enough that they will not recondense on the wafer, causing hydrophobicity. The washing of the polymers is called deashing because the catalyst being removed is an oxidized metal ash of sorts. The deashing must be used in the process of developing the polypropylene which is polymerized because of the high ash and atactic content. Double deashing involves running the polypropylene through the washing process twice. In addition to having lower levels of hydrophobic compounds, the deashed polypropylenes have lower levels of trace metallics, which is beneficial because metals are considered contaminants in the semiconductor industry. The clarifying agent used in the polypropylene molded into container 10 is desirable because random copolymers, although fairly clear, may not have enough clarity for use in this environment. The clarifying agent will crystallize the polypropylene in such a way that it is optically clear. The clarifying agent is preferably a sorbitol based material having a sugar backbone and benzaldehyde derivative appendages. Preferably, the clarifying agent is known as Mitsui NC4, which is a very clean material. Preferably, the clarifying agent is used at minimum low levels, preferably at a level of 1500 ppm, plus or minus 15%. This minimal load level of the clarifier is considered to contribute to the lack of hydrophobicity effect of the polypropylene on the stored wafers. This minimal loading level is appreciated when it is recognized that although clarifier loading levels may be in the 100 ppm to 3,000 ppm range, they are most often at the 2,500 ppm level. The Mitsui NC4 material has a structure as follows: Sorbital Sugar - Back Bone The antioxidant/antiozonant (hereinafter referred to as an antioxidant) is added into the polypropylene so as to prevent the polymer chains from breaking down and preventing the material from becoming brittle. Although there are a number of antioxidants to choose from, the preferred antioxidant is Ethanox 330, manufactured by Ethyl Corporation. Technically, The Ethanox 330 is constituted: 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxl benzyl) benzine This antioxidant is a high molecular weight hindered phenolic compound, without any phosphites. It is preferred that the phosphites be eliminated because phophorous is a strong semiconductor dopant which would be a distinctly disadvantageous antioxidant by-product if it were outgassed onto the wafers. The preferred loading level of the preferred antioxidant is approximately 1,000 ppm plus or minus 15%, which is a minimal range of loading levels for the antioxidant in the production of polypropylene. It will be understood that typically antioxidants are loaded in the range level of 1,000 to 2,500 ppm, although in some circumstances, load levels may be in a wider range of 250 to 4,000 ppm. Another antioxidant that may be suitable, but not preferred, is Irganox 1010, manufacturing by Ciba Geige, technically described as : Tetrakis(methylene) [3,5-di-tert-butyl-4- hydroxyhydrocinnamate]) methane The polypropylene utilized in molding container 10 has a flow enhancing additive, as do most polypropylenes. Calcium stearate, being the most common flow enhancer, is used in the polypropylene of container 10. The loading level of the flow enhancer in the present polypropylene for the container 10 is set at approximately 100 ppm, plus or minus 15%, as to minimize contamination potential of the material outgassed from the polypropylene while still giving sufficient flow characteristics. This loading level is appreciated to be minimal when it is understood that a typical loading level is about approximately 250 ppm and otherwise load levels may range up to 1,000 ppm of flow enhancer. Calcium stearate is considered to be the preferred form of flow enhancer in the present polypropylene compound. Preferably, other additives that might be added to the polypropylene are eliminated in the polypropylene for the container 10. Such other additives might be in the category of UV and heat stabilizers, as to increase the useful life of the materials exposed to UV light. The most common sources of UV radiation are the sun and fluorescent lights, and transparent or semitransparent polypropylenes that do not have UV stabilizers in them may embrittle when exposed to UV light for long periods of time. This type of use of containers 10 is not expected, and accordingly, additives which may be potential contaminants are avoided. In preparing a homopolymer polypropylene, the ethylene is eliminated from the monomers supplied to the polymerization reactor, but all of the process steps indicated in Figure 2 are followed except that no clarifier is added. The quantities are as indicated for the random copolymer polypropylenes. In summary, no polypropylene having the extremely low hydrophobic effect upon wafers stored in polypropylene containers has been previously used in such containers. In addition, the polypropylene is considerably improved as compared to other known polypropylenes because of the extremely minimal loading of the clarifier and flow enhancer and the selection of the preferred Ethanox 330 used in the new resin to produce improved hydrophilic characteristics and reduced hydrophobic effect at the surfaces of the wafers stored in the container. Figure 2 illustrates the general process of polymerizing, deashing, drying and pelletizing the polypropylene used in molding the shippers or shipping containers 10. Monomers including ethylene and propylene, along with the catalyst, and propane as a solvent, are supplied into a polymerization reactor 25 wherein the polymers are suspended in a solvent. The propane is flashed off at 26 after the polymer suspended in the propane is drawn off from the reactor. Polymer solids flashed off a solvent, preferably heptane, for the deashing are supplied into a deashing apparatus 27 from which the deashed solids suspended in solvent are drawn and directed into the filters 28; first a bag filter and then a drum filter, to separate the liquids from the solid. It is at the filters where the undesirable materials including the atactic polypropylene, catalyst, oligomers, etc. , are carried away with the solvent separated from the solids which are the isotatic polypropylene flakes which are drawn off and supplied into a second stage of deashing or washing 29 at which additional solvent, i.e., heptane, is added. Again, the solvent, carrying polymer flakes, is drawn off and delivered to the filters 30; first a bag filter and then a drum filter, which separates the liquids and solids, and again, solvents carrying the undesirable materials including atactic polypropylene, catalysts, oligomers, etc. are drawn off. Polymer flakes are drawn from the filters 30 and supplied into the ovens 31, which drive off the remaining solvent from the polymer flakes and tumble the flakes to complete the drying process. Additives including the antioxidant, clarifier and flow enhancer, are then added to the dry polymer flakes to complete the processing as the flakes are supplied into an extruder and pelletizer 32. The pellets comprise the desired polypropylene to be used in the containers 10 which are molded in the molding machines 33 from the pellets produced as described. An improved method has been devised for determining the hydrophobicity effect on silicon wafers produced by shipping containers of polypropylene without requiring molding of shipping containers of all the various polypropylenes that may be under consideration. In general, a silicon wafer, cleaned of organic contaminants, is placed in a covered glass jar which has also been cleaned, at least in its interior, of organic contaminants; and the wafer is supported in the jar, preferably on an aluminum stand which has also been cleaned of organic contaminants; and a quantity of polypropylene resin is placed in the jar and stored for a time and at a temperature as may be encountered in shipping or storing wafers in the container 10 of Figure 1. At the end of the exposure time, the wafer is removed from the jar and a sessile drop of deionized water is placed upon the surface of the wafer; and both advancing and receding contact angles of the profile of the drop of water are measured and the angles provide an indiciation of the hydrophobicity of the surface of the wafer. Higher contact angles indicate greater hydrophobicity and lesser hydrophilicity; and smaller contact angles indicate lesser hydrophobicity and greater hydrophilicity Thereafter, the polypropylenes tested which produce minimal hydrophobicity effect upon the wafer surface may be molded into shipping containers and used for that purpose. More specifically, a silicon wafer such as the wafer 35 seen in Figures 3 and 4 and a glass jar 36, together with its cover 37 and an aluminum stand 38, may all be subjected to oxygen plasma cleaning in a commercially available plasma cleaner of the type shown diagrammatically in Figure 7 and indicated in general by the numeral 39. The plasma cleaner has an RF frequency generator indicated by the numeral 40 and a vacuum pump 41 for evacuating the process chamber 42. Placing the wafer 35 and the jar 36 and the cover 37 separately, along with the aluminum stand 38 for the wafer, will cause all of the organic contaminants on the surfaces of these objects to be cleaned. When the wafer has been cleaned in this manner, a small quantity of deionized water, if applied to the face of the wafer 35, will spread itself out across the face of the wafer in a thin sheet or film 43 as indicated in Figures 3 and 4. After the wafer has been cleaned, but without wetting it, the clean stand 38 with the clean wafer 35 is placed in the jar 36 and the cover 37 is applied after a quantity of polypropylene resin is supplied into the lower portion of the jar. The closed jar is then stored for an appropriate length of time such as 20 hours, and at a typical temperature of approximately 50°C so that the wafer 35 and the resin 44 are subjected to typical conditions under which the wafers may be subsequently stored in the containers 10. The resin 44 in the jar will outgas and the surface of the wafer 35 may accumulate organic contaminants on its surface, tending to cause hydrophobicity (lacking affinity for water) . When the wafer 35 is removed from the jar 36, after having been exposed to the polypropylene resins for a period of time, one or more drops of deionized water 45 are applied onto the surface of the wafer 35. If the surface of the wafer is hydrophobic, the water will collect in beads as illustrated in Figures 5 and 6. The contact angles of the sessile drop are then measured and the contact angle under advancing and receding conditions will provide a direct indication of the hydrophobicity of the surface of the wafer. Measuring of contact angles of a sessile drop has been a well-known technique, in general, in the past and has been accomplished in any of a number of ways. The contact angle is the angle between the profile of the drop at its edge portion in relation to the surface upon which the drop lies. An advancing contact angle is the contact angle at the edge of the drop as the drop tends to travel or advance across the face of the wafer, as in the situation where the size of the drop is growing larger which tends to move the edge outwardly or where the drop tends to move across the surface under the effect of gravity caused by tilting the wafer on which the drop lies. A receding angle is the contact angle of the drop, the volume of which is decreasing and tending to move the edge of the wafer inwardly, or tending to follow the drop in the event the wafer is tilted. In Figure 9, the drop 45 is greatly enlarged; and a tangent line 46 is tangent to the profile of the drop 45 at an advancing edge of the drop, relative to the surface of the wafer 35. The advancing contact angle is indicated by numeral 47. A receding contact angle is indicated by arrows 48 between the tangent 49 of the drop 45a after a quantity of liquid has been removed from the drop as by microsyringe 50. The receding angle 48 is measured just as the edge portion of the drop tends to move inwardly across the face of the wafer as the volume of the drop is reduced so that the profile assumes the dotted line position 45a seen in Figure 9. The contact angle may be easily measured by an optical comparator or a camera with a macro lens which enlarges the profile of the drop and allows the contact angle to be measured. In other circumstances, the contact angle may be measured by projecting the image of the drop profile or by using a telescope fitted with a goniometer eyepiece; or the contact angle may also be determined by measuring the dimensions of the drop, i.e., its diameter and height. Of course, in measuring the advancing or receding contact angles, vibration and distortion of the drop must be avoided. The sessile drop contact angle of DI water on silicon wafers exposed to various polypropylenes have been measured, as follows: By measuring the sessile drop contact angle of water on silicon wafers after being exposed to these various materials, all of the materials showed rather minimal hydrophobic effects and substantial hydrophilic effects on the wafers. The material AA as described in this specification as the preferred material from which the containers 10 are molded is a random copolymer double deashed utilizing old style catalyst. The copolymer had an ethlyene content of 2.2 to 2.8%; Ethanox 330 was used as the antioxidant/antiozonant with a loading level of approximately 1,000 ppm. The clarifier compound as specified was used at a loading level of 1,500 ppm; and a flow enhancer, calcium stearate was used with a loading level of 100 ppm. Polypropylenes made of this preferred material showed considerable advantage compared to the other materials tested. Material A is a homopolymer material which makes up into a more rigid and brittle polypropylene than the random copolymer material. This homopolymer was double deashed and used 1,000 ppm of hindered phenolic as the antioxidant/antiozonant; and approximately 100 ppm of calcium stearate was added as a flow enhancer. The material F indicated in Table I is a random copolymer material which is double deashed with additives of approximately 2,500 ppm of sorbitol clarifier; 1,000 ppm hindered phenolic as the antioxidant/antiozonant; and calcium stearate as the flow enhancer at an estimated loading level of 250 ppm. It will be seen that the present invention has involved an improved shipping container for silicon wafers of polypropylene with a minimum hydrophobicity effect upon the wafers stored. The invention also includes an improved method for checking the hydrophobicity effect of various polypropylenes on silicon wafers that may be stored in carriers made of polypropylene. The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.

Claims: I CLAIM: 1. A wafer storage device for wafers comprising a wafer confining container comprising means for supporting at least one wafer therein, and the container being formed of molded polypropylene comprising a double deashed polypropylene to which is added an antioxidant/antiozonant and a flow enchancer. 2. A wafer storage device according to claim 1 wherein said double deashed polypropylene comprises a random copolymer polypropylene. 3. A wafer storage device according to claim 1 wherein said double deashed polypropylene comprises a homopolymer polypropylene. 4. A wafer storage device according to claim 1 wherein said double deashed polypropylene comprises a random copolymer polypropylene to which a clarifying agent is also added. 5. A wafer storage device according to claim 1 wherein said means comprises a wafer carrier portion formed of molded polypropylene comprising a double deashed polypropylene to which is added an antioxidant/antiozonant and a flow enhancer. 6. A wafer storage device according to claim 1 wherein the double deashed, random copolymer is twice run through the washing process. 7. A wafer storage device according to claim 1 wherein the ethylene content of the random copolymer is in the range of 2.2 to 2.8%. 8. A wafer storage device according to claim 1 wherein the polypropylene is polymerized utilizing an old style catalyst and is thereafter twice deashed. 9. A wafer storage device according to claim 4 wherein the clarifier compound is a sorbitol based material having a sugar backbone and benzaldehyde derivative appendages. 10. A wafer storage device according to claim 9 wherein the clarifier comprises load levels of approximately 1500 ppm. 11. A wafer storage device according to claim 1 wherein the antioxidant/antiozonant consists in a high molecular weight hindered phenolic compound. 12. A wafer storage device according to claim 11 wherein the antioxidant/antiozonant comprises loading levels of approximately 1000 ppm. 13. A wafer storage device according to claim 1 wherein the antioxiant/antiozonant comprises Ethanox 330. 14. A wafer storage device according to claim 1 wherein the flow enhancer comprises calcium stearate. 15. A wafer storage device according to claim 14 wherein the flow enhancer comprises loading levels of approximately 100 ppm. 16. A shipper for wafers comprising an openable closed solid outer container portion and an inner container portion confined within said outer container portion and supporting at least one silicon wafer therein, and said inner and outer container portions being formed of molded polypropylene comprising a polypropylene monomer polymerized with the use of a catalyst and thereafter twice washing the resulting polymer to remove undesirable catalysts and other materials that may outgas and cause contamination and hydrophobicity of such a wafer, the polypropylene also having added an antioxidant and a flow enhancer whereby to minimize contamination and hydrophobicity of the wafer stored in the container. 17. A shipper according to claim 16 wherein said polypropylene monomer comprises a random polypropylene with ethylene added, and a clarifier also added to the polypropylene. 18. A shipper according to claim 16 wherein said polypropylene monomer comprises a homopolypropylene. 19. An improved method of determining the hydrophobicity of a surface of a silicon wafer, comprising conducting a sessile drop angle measurement for the wafer surface comprising, placing a drop of deionized water on the surface of the wafer, inducing a portion of the drop to advance across a portion of the surface whereby the portion of the drop assumes a profile having an advancing contact angle relative to the surface at the point of contact with the surface at a time immediately prior to advancing movement of said portion, and measuring said contact angle as an indication of the hydrophobicity of the surface. 20. An improved method of determining the hydrophobicity of a surface of a silicon wafer according to claim 19 wherein said inducing of said portion of the drop to advance across the surface comprises adding a quantity of water to the drop followed by allowing the drop to settle. 21. An improved method of determing the hydrophobicity of a surface of a silicon wafer according to claim 19 wherein a second portion of the drop is induced to recede away from the edge of the drop which contacts the surface of the wafer whereby the second portion of the drop assumes a profile of a receding contact angle relative to the surface at said edge of the drop at a time immediately prior to receding movement of said second portion, and measuring said receding contact angle as an indication of the hydrophobicity of the surface. 22. An improved method of determining the hydrophobicity effect of a polypropylene resin to be molded into closed containers on silicon wafers to be stored in such containers, comprising cleaning organic contaminants from a portion of the surface of a specimen silicon wafer, cleaning organic contaminants from the interior of a glass jar capable of receiving the wafer therein and a closure for the jar, inserting polypropylene resin particles into the jar and supporting the wafer in the jar and free of the polypropylene particles and closing the jar, heating the jars, wafer, and polypropylene particles for a length of time as to simulate storage conditions of the wafers within such closed containers, and thereafter conducting a sessile drop contact angle measurement of advancing and receding contact angles of a drop of deionized water on the wafer as an indication of the hydrophobicity effect of the polypropylene on the wafer. 23. An improved method of determining the hydrophobicity effect of a polypropylene resin as set forth in claim 22 wherein conducting said sessile drop contact angle measurement comprises placing a drop of deionized water onto a surface of the wafer, measuring the advancing and receding angles of the drop profile relative to the surface and at the point of contact of portions of the drop with the surface at the time said portions start to move across portions of the wafer. 24. An improved method of determining the hydrophobicity effect of a polypropylene resin as set forth in claim 22 wherein said cleaning of organic contaminants from the wafer surface and jar and closure comprises oxygen plasma cleaning. 25. An improved method of determining the hydrophobicity effect of a polypropylene resin as set forth in claim 18 wherein said cleaning of organic contaminants from the silicon wafer comprises a heating of the wafer.

Inventors: MIKKELSEN KIRK J, MIKKELSEN, KIRK, J.
Applicants: FLUOROWARE INC, FLUOROWARE, INC.
Application year: 1992
Main classification: H01L21
IPC codes: B65D85, H01L21