Abstract:
This invention includes a method for heating a deposition material to form a vapor that may be deposited on a polymer film and thereby create a holographic film or similar material. Included within this method is a vaporizer that has an adjustable aperture. The vaporizer may have (i) a flexible wall that defines a cavity in which a deposition material is heated (ii) a first lip extending outward from a first side of the flexible wall and (iii) a second lip extending outward from a second side of the flexible wall. The aperture size can be adjusted to a desired cross-sectional opening by adjusting the distance of separation between the first lip and the second lip. By adjusting the size of the aperture, the flow rate of the vapor from the vaporizer can be adjusted to achieve a desired flow rate. Heat from a heat source in thermal communication with the vaporizer may be employed to heat the deposition material to form a vapor. The vaporizer may be disposed within a vacuum chamber, which has a feed roll and a take up roll. Film may be transferred from the feed roll to the take up roll so that, the vapor is deposited on the film.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of application Ser. No. 08/869,076 filed Jun. 4, 1997 now U.S. Pat. No. 5,951,769, issued Sep. 14, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of manufacturing a high refractive index (HRI) film and an apparatus and a system for accomplishing such a method. 
     Typically, high refractive index film refers to a film that is semi-transparent to the human eye. It may be transparent when viewed from one angle, but not from another angle. Included within the uses of this type of film are decorative wrapping paper, diffraction gratings and holograms. Holograms create the appearance of a three-dimensional image from a two dimensional object. Among the many uses of holograms are identification cards, trading cards, such as those picturing sports players, credit cards, and artistic uses. 
     Conventionally, high refractive index film is manufactured by depositing a layer of material onto a transparent plastic film. This may be accomplished by exposing the film in a vacuum chamber to a vapor which deposits on the film to create a layer. The deposited layer causes the refraction. This layer may be deposited as the film is fed from a feed spool to a take up spool. This method of creating a high refractive index film is generally described in U.S. Pat. No. 5,107,791 issued to Hirokawa et al. and U.S. Pat. No. 5,351,142 issued to Cueli. 
     The material heated to form the deposition layer varies. For example, Hirokawa teaches heating a composition composed mainly of a combination of silicon and silicon oxide or silicon oxide alone. However, zinc sulfide may also be used. Conventionally, this material is placed in a crucible or bowl like structure and heated through conduction. This is explained in detail in Hirokawa. A crucible like structure has several disadvantages. For instance, since the top of a crucible is generally open and the walls of the crucible are generally perpendicular to the top of the crucible, the vapor produced from heating the deposition material can diffuse out of the crucible in an uncontrolled fashion. More particularly, as the vapor is formed it diffuses upward and then exits the crucible. Upon exiting the crucible, the vapor can diffuse in almost any direction. Experience with crucibles shows that the vapor diffuses outward in addition to upward. Since the film is disposed above the crucible, it is preferable that the vapor diffuse upward and not outward so that the vapor contacts the film and deposits on it. Vapor that diffuses outward slows the deposition rate onto the film. This slows the rate at which film can be transferred from the feed roll to the take up roll and the overall rate of production of HRI film. 
     Another drawback of bowl shaped crucibles is that when they are used in conjunction with zinc sulfide, the zinc sulfide tends to form a crusty like layer across the top of the crucible when it is heated. This occurs because the zinc sulfide pellets or tablets at the top of the crucible form a layer and become supported by the zinc sulfide pellets beneath the layer. This layer limits the flow rate of vapor from the crucible. As described above, this then slows the feed rate of film and the rate of production. 
     Conventional crucibles also have a fixed opening at the top. Because of this, the flow rate of vapor produced by the crucible cannot be adjusted easily. Rather, assuming the same amount of heat and deposition material, a new crucible would have to be manufactured with a different size opening at the top in order to produce the same flow rate. This is inefficient and costly, particularly since the flow rate may have to be varied depending on the specific characteristics of the vacuum chamber and system employed. 
     In addition to bowl like crucibles, continuous vaporizers have also been designed. Hirokawa, for example, teaches a vaporizer that has an opening at either axial end through which deposition material can be continuously fed. At one end, the deposition material is inserted into the vaporizer. It is pushed through the vaporizer and is heated. As it is heated, vapor is produced and exhausts through a fixed opening in the top of the vaporizer. Such a system also has its disadvantages. For instance, the opening in the vaporizer taught by Hirokawa is fixed and cannot be adjusted to vary the flow rate of vapor. In light of the prior art, an improved method of heating a deposition material is needed that is simple, yet allows the flow rate of vapor to be varied by adjusting the vaporizer. Furthermore, an apparatus and a system for accomplishing such a method is needed. 
     SUMMARY OF THE INVENTION 
     A system and an apparatus for heating a deposition material to form a vapor to be deposited on a film and thereby create a high refractive index film includes a heat source in thermal communication with a vaporizer. The heat source may be conventional electrical resistance. The vaporizer preferably has an adjustable aperture and a deposition material may be deposited therein. Upon heating the deposition material, it produces a vapor which flows through the aperture at the top of the vaporizer. 
     This aperture is preferably adjustable so that the flow rate of vapor from the vaporizer can be varied. Before assembling the vaporizer to the system, the size of the aperture can be varied to achieve the requisite flow rate. In a preferred embodiment of this invention, the vaporizer is flexible and can be adjusted to have an aperture of the desired size. After flexing the vaporizer to achieve the desired size of the aperture, the vaporizer can be assembled to the system. In order to achieve a higher flow rate the aperture may be increased in size, and conversely to decrease the flow rate the aperture can be made smaller. 
     The vaporizer also preferably has a width below the aperture greater than the width or size of the aperture. Furthermore, the width of the vaporizer may narrow, similar to a funnel, to direct the flow of vapor to the aperture. By funneling the flow of vapor and limiting the size of the aperture, the vapor is directed to flow generally in an upward direction. This should be contrasted with a heating element that is a conventional bowl like crucible or similar structure. In this structure the vapor can diffuse outward in addition to upward. 
     Zinc sulfide may be employed as the deposition material. The apparatus of this invention prevents or limits crusting of zinc sulfide across the cross-section of the apparatus, which occurs as described above in prior art heating elements. Specifically, since the heating element preferably has a circular cross-section, the zinc sulfide at the top tends to fall to the center of the heating element as it becomes crusty. 
     The apparatus of this invention may be employed in a system of this invention for forming high refractive index film and similar materials according to a preferred method of this invention. Included within this system and method may be a vacuum chamber in which the vaporizer is disposed below film wrapped on a feed roll and a take up roll. The film is transferred from the feed roll to the take up roll. As this occurs, the depositing material is heated to form a vapor. The vapor diffuses upward through the aperture and deposits on the film. Conventionally, the rate of transferring film, while depositing the layer on the film, was about 40 feet per minute (fpm) and the maximum rate has proven to be about 600 fpm. Through the use of this invention, the rate of production has improved drastically. Feed rates of about 1,500 fpm have been achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatical sketch of a system and an apparatus according to a preferred embodiment of this invention; 
     FIG. 2 is an isometric view of a component of the systems and apparatuses depicted in FIGS. 1 and 4; 
     FIG. 3 is a schematic diagram of a cooling system for the systems and apparatuses depicted in FIGS. 1 and 4; 
     FIG. 4 is a diagrammatical sketch of a system and an apparatus according to another preferred embodiment of this invention; and 
     FIG. 5 is a schematic diagram of another cooling system for the systems of FIGS. 1 and 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals designate corresponding structure throughout the views there is shown in FIG. 1 a preferred embodiment of a system and apparatus for manufacturing high refractive index film, diffraction gratings or similar materials. As described above, high refractive index film generally includes a multi-layer semi-transparent structure. Furthermore, the refraction index varies through the layers. The varying of the refraction index through the film creates the appearance of a three-dimensional object from a two dimensional image. 
     The system  10  according to a preferred embodiment includes the vacuum chamber  12 , a feed roll  14  and a take up roll  16 . Attached to the vacuum chamber  12  may be a vacuum pump  62  that operates to create the requisite pressure in the vacuum chamber  12 . Pressure within the vacuum chamber will vary depending on the specific operation. In a preferred embodiment, the pressure is about 10 −5  Torr. Film  18  is generally loaded on the feed roll  14 , and the feed roll  14  is then rotated by a conventional manner to transfer the film  18  to the take up roll  16 . Depending on the requirements of the system, a multitude of transfer rollers  15  may be employed in order to transfer the film  18  from the feed roll  14  to the take up roll  16 . Additionally, the system  10  may have a plurality of mounts  17 . The mounts may be detents or similar structure that are used to direct or guide the film as it is transferred between rolls. The film may be an embossable film, such as mylar. 
     A viewing window  11  may be disposed on the periphery of the vacuum chamber  12 . The viewing window  11  permits an operator to visually check the operation of the system  10 . 
     The system also preferably includes a vaporizer  22 , as depicted in FIGS. 1 and 2. Connected to the vaporizer  22  may be a heating supply  20 , and disposed within the vaporizer  22  may be a depositing material  44 . Although numerous heating supplies  20  may be employed, conventionally either electrical resistance is preferred. As is shown in FIG. 1, the vaporizer  22  is generally disposed in the vacuum chamber  12  below the film  18 . In operation, heat is supplied from the heating supply  20  to the vaporizer  22 . Heat is then conducted, as discussed in further detail below, to the depositing material  44 . The depositing material  44  is heated and vaporizes. Upon vaporizing the depositing material  44  diffuses in an upward direction to the film  18 , as it is being transferred from the feed roll  14  to the take up roll  16 . The depositing material  44  then deposits on the film  18  to form a refraction layer. 
     A variety of materials may be used as the depositing material  44 . In a most preferred embodiment, zinc sulfide pellets comprise the depositing material. These zinc sulfide pellets or tablets are available from EM Industries of Damstadt Germany. As is shown in FIG. 2 the vaporizer  22  is mounted within a support structure  28 . The vaporizer  22  is preferably constructed from a molybdenum alloy or another material that has sufficient thermal transfer characteristics, as well as sufficient strength and flexibility. A substantial portion of the periphery of the vaporizer  22  is thermally insulated with a conventional material, such as that used in fire boxes. The insulation  30  is disposed between the vaporizer  22  and the support structure  28 . The support structure  28  is preferably constructed of a material with good heat transfer characteristics, such as copper. Included with the support structure  28  are two retainers  24 . The retainers  24  may each include contact blocks  25  and bolts  26 . Each retainer functions to hold a lip  34  of the vaporizer  22  and is also used to vary the width of the aperture  32  of the vaporizer  22 . More particularly, the lip  34  is disposed between the contact blocks  25  and the support structure  28 . A fastening means then is employed to fasten the contact blocks  25  to the support structure  28 . In doing so the lips  34  are clamped between the contact blocks  25  and the support structure  28 . 
     As is depicted, a portion of the lips  34  of the vaporizer  22  are clamped under the retainers  24 . By varying the amount of the lip  34  that is in the retainers  24  the size of the aperture  32  of the vaporizer  22  can be adjusted. For instance, as mentioned above the vaporizer  22  is constructed from a material that is flexible, yet strong. Therefore, in order to create a larger aperture  32 , the vaporizer  22  can be flexed or bent outward before it is inserted in the retainer  24 . By flexing the vaporizer  22 , in the outward direction, the size of the aperture  32  increases. The vaporizer can then be held in this position and inserted against the insulation  30 . Since the insulation  30  is generally compressible, it can be compressed to conform to the shape of the vaporizer  22 . While still holding the vaporizer  22  in the desired shape with the desired size of the aperture, the lips  34  of the vaporizer can then be clamped by the retainers  24 . Since the vaporizer  22  has been flexed outward it will be appreciated, that a larger amount of the lips  34  will be clamped by the respective retainers  24 . 
     In order to decrease the cross-sectioned area of the aperture  32 , the vaporizer can be bent in an inward direction at the top. As the vaporizer  22  is bent inward, it will move away from the insulation  30 , and a larger amount of insulation  30  may have to be inserted prior to placing the vaporizer  22  in place. After the vaporizer  22  has been deformed and inserted, the lips  34  of the vaporizer  22  can be clamped with their respective retainers  24 . 
     The adjustment of the size of the aperture  32  may occur before operation of the system  10 . After adjusting the size of the aperture  32 , the vaporizer  22  can then be installed and clamped by the retainers  24 . In order to adjust the flow rate after assembly, the vaporizer  22  can be removed from the system  10 , adjusted and reinstalled in the system  10 . 
     In summary, in order to create a larger aperture  32  the amount of the lips  34  retained within the retainers  34  can be increased and thereby increase the size of the aperture  32 . Conversely the amount of the lips  34  in the retainers  24  can be decreased and the size of the aperture  32  is thereby decreased. In a preferred embodiment of this invention, the aperture is set between 1.25 inches and 2 inches. 
     The vaporizer  22  has a cross section that is substantially circular with the exception of the aperture  32 . The aperture  32  preferably extends from one axial end of the vaporizer  22  to the other, as shown in FIG.  2 . However, other shapes may be encompassed by this invention. Including those having a plurality of apertures disposed on the top, as opposed to a single aperture. By having a circular cross section, the width of the vaporizer  22  narrows as it approaches the aperture  32 . By narrowing the width of the vaporizer  22 , the vapor formed from the depositing material  44  is funneled as it transfers upward towards the aperture  32 . Because the vapor is funneled as it exits the aperture  32 , it diffuses in a general upward direction towards the film and deposits on the film  18 . 
     This structure should be contrasted with prior art structures that employ crucibles which are bowl like and do not funnel the vapor. In these structures the vapor diffuses outward in addition to upward, and therefore, the rate at which the vapor deposits on the film  18  is limited. This in turn limits the rate at which the film can be fed from the feed roll to the take up roll and the corresponding systems production rate of film in conventional systems. The feed rate of the film in conventional systems is about 40 fpm, and in some systems a feed rate of about 600 fpm has been achieved. Because in this system the vapor is funneled in an upward direction and the size of the aperture  32  can be varied to achieve the proper flow rate of vapor, feed rates of up to about 1500 fpm have been achieved. As is evident, this is a significant increase over the prior art systems. 
     Another advantage of the vaporizer  22  of this invention is that it prevents a crusty layer of zinc sulfide from being created, and thereby prevents or limits the flow of vapor from the vaporizer  22 . As the zinc sulfide is heated it tends to become crusty. In prior art bowl like crucibles with cylindrical shapes and straight sides, the sulfide tends to form a crusty layer across the top of these crucibles. As this layer forms, flow of vapor is limited, as is the feed rate of film. Production must then be halted and maintenance performed to remove the crusty layer and increase production rates. This is inefficient and costly. The formation of a crusty layer is prevented by the curved sides of the vaporizer  22  and its circular cross section. In this structure the zinc sulfide at the top falls towards the center of the vaporizer as it becomes crusty and thereby a crusty layer is prevented from forming across the width of the vaporizer  22 . 
     Upon heating zinc sulfide tablets they tend to pop and to be expelled from the vaporizer  22  through the aperture  32 . In order to prevent the tablets from being expelled from the vaporizer  22 , a screen  52  or similar structure may be disposed above the aperture  32 . Upon popping or exiting the vaporizer  22  the tablets will hit the screen  52  and be deflected back into the vaporizer  22 . A screen is preferred because it restrains the zinc sulfide, yet at the same time allows diffusion of vapor up to the film  18 . 
     Shutters  42  or a gate like structure may also be used in this invention. These shutters  42  can be opened and closed in a conventional manner and are disposed between the vaporizer  22  and the film  18 . Initially, upon starting this system  10  the shutters  42  are closed. The depositing material  44  is then heated. Once the depositing material  44  reaches the operational temperature and vapor is produced in sufficient quantity, the shutters  42  are then opened to allow the vapor to defuse upward and contact the film. Thus, the shutters ensure that the vapor does not begin to deposit on the film  18  until it is being produced at a sufficient rate relative to the rate at which film  18  is being transferred from the feed roll  14  to the transfer roll  15 . If the vapor defused upward before it is being produced at a sufficient rate, it would not produce a layer of the proper thickness along the film  18  at the operational feed rate. 
     Because the shutters  42  are disposed between the film  18  and the vaporizer  22 , they also accomplish two other functions. When the system is not operating and the film is not being transferred, the shutters  42  are closed to prevent heat from being applied continuously to the stationary film. If heat was applied continuously to the film, while it is not being transferred, it could ignite and burn. When the shutters  42  are open, they also assist in regulating the flow of vapor produced by the vaporizer. In particular, they assist in directing the flow of vapor up to the film and prevent if from diffusing outward and away from the film. By directing the flow in this manner, they assist in maximizing the vapor deposition rate and the feed rate of the film. 
     Shields  36  may also be disposed on the exterior of the vaporizer  22 . The shields function to prevent heat from being radiated from the support structure  28 . Furthermore, the shields may prevent any vapor that is defused outward from defusing down and out as opposed to an upward direction. The support structure  28  is preferably cooled with a typical cooling system  38 . Such a cooling system generally includes conduits and a coolant such as water, that continuously flows through the cooling structure and around the shields  36  to provide cooling. The coolant then travels through a heat exchanger where heat is transferred to another medium and continuously functions in this manner to provide cooling for the system. 
     The vaporizer  22  and supporting structure  28  may be formed into several assemblies commonly referred to as “boat assemblies.” Each of the assemblies includes the components discussed above, such as, a vaporizer  22 , a heating supply  20  and retainers  24 . The boat assemblies are then aligned axially, so that the cross sections of the vaporizer  22  are aligned. They may then be fasten together with bolts or other conventional fasteners. When fastened together, they form a vaporizer  22  that extends under the width of the film  18 . 
     The system  10  may also include a cooling system  38 . Such a system  38  typically includes conduits  40  running through the support structure  28  and around the shields  36 , and is depicted schematically in FIG.  3 . The cooling system may also include a heat exchanger  46 . Water or another suitable fluid may be used as the coolant. As the coolant travels through the conduits, heat is transferred from the shields  36  and the support structure  28  to the coolant. The heated coolant than travels to the heat exchanger  46 , where heat is transferred from the coolant to another medium. The coolant continuously flows in this manner to provide cooling to the system  10 . 
     As mentioned above, the heating supply  20  is preferably either electrical resistance or inductance heat. Through a conventional electrical connection  50  and a resistance heating element  48 , the heating supply  20  is connected to the support structure  28 . As the electricity flows to the resistor  48  or similar structure, heat is generated. The heat is then conducted to the support structure  28 . Through thermal conductance, heat is then transferred through the retainers  24  to the vaporizer  22 . Through conductance and convection, the depositing material  44  is then heated. It will be appreciated that although only two electrical conductors  50  and heating elements  48  are illustrated in FIG. 1, the system  10  of this invention may have a plurality of electrical conductors  50  and heating elements  48  connected to the support structure  28 . In a preferred embodiment, the connections  50  and heating elements  48  are disposed axially on either side of the vaporizer  22 . 
     As shown in FIG. 1 is the system  10  may have a cooling drum  60  that is in communication with a film cooling system  64 . This cooling system is prior art, but it may be employed in the system  10  of this invention. The film cooling system  64 , shown schematically in FIG. 4, may have a heat exchanger  66 , a compressor  68 , a temperature sensor  70  and a controller  72 . Flowing through the film cooling system  64  may be a coolant, which in a preferred embodiment is Freon. The heat exchanger  66  may have water  71  or another fluid running though it and in thermal contact with the coolant to remove heat from the coolant. The compressor  68  pressurizes the coolant to cause it to flow from the heat exchanger to the cooling drum  60 . As can be seen in FIG. 1, the cooling drum  60  has film  18  disposed running around a portion of its circumference. The coolant runs through the cooling drum  60  and absorbs heat from the film  18 . From the cooling drum  60 , the coolant returns to the heat exchanger  66  where it is cooled. The coolant is then pressurized by the compressor  68  and flows in this closed loop system to continuously cool the film  18 . 
     This cooling system  64  may employ an automatic controller  72  and a sensor  70 . This sensor  70  is a conventional measuring device that preferably measures the temperature of the coolant exiting the heat exchanger and inputs this measured temperature to the controller  72 . The controller  72  may be a typical electrical controller that compares the measured temperature to a predetermined temperature to provide adequate cooling to the cooling drum  60 . After comparing these temperatures, the controller  72  responds in a conventional manner to vary the flow of heat absorbing fluid running through the heat exchanger  66  to obtain the desired temperature of the coolant. The temperature of the coolant may also be controlled through manual operation of valves and the like to vary the flow rate of the fluids through the heat exchanger. 
     FIG. 5 depicts another preferred embodiment of the system  10  of this invention. The preferred embodiment depicted in FIG. 5 is similar to that of FIG.  1 . However, in contrast to the preferred embodiment of the system  10  illustrated in FIG. 1, the cooling drum  60  is not disposed directly above the vaporizer  22  in the embodiment of this system  10  shown in FIG.  5 . Rather, the cooling drum  60  is disposed so that it receives film  18  after it has been fed across the top of the vaporizer  22 . Since the cooling drum  60  cools the film as is described above, the location of the cooling drum  60  may have an effect on the deposition rate and hence the feed rate of the film. 
     In addition, it has been found that the zinc pellets tend to pop out of the vaporizer when heated. If the pellets are not stopped by the screen  52 , they may contact the film  18 . Upon contacting the film, the relatively hot zinc pellets may cause thermal damage to the film. In order to minimize or prevent thermal damage, the cooling drum  60  is disposed, as shown in FIG. 1, directly over the vaporizer  22 . In this position, when a zinc sulfide pellet contacts the film disposed around the cooling drum  60 , the cooling drum  60  absorbs the heat and transfers it to the film cooling system  64 . 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention has been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made to detail, especially in matters of shape, size and arrangement of parts within the principals of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.