Patent Publication Number: US-6902625-B2

Title: Multiple pocket electron beam source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 09/584,940, filed Jun. 1, 2000, entitled “Multiple Pocket Electron Beam Source” by Bruce G. Ramsay, now U.S. Pat. No. 6,342,103, which is incorporated herein in its entirety by this reference. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to crucible covers for multiple pocket electron beam sources used in vacuum coating systems in which individual crucible pockets containing coating materials are heated to produce coating vapors. The invention further relates to cooperating cover and crucible combinations that isolate heated pockets from all other pockets by forming a line-of-sight blocking baffle. 
     BACKGROUND OF THE INVENTION 
     Vacuum coating or deposition of materials is a process commonly used in the manufacture of a variety of products including, but not limited to, semiconductors, optics, and the formation of thermal barriers on high temperature components. One method for coating materials having very small vapor pressures, such as metals or ceramics, is by placing the coating material and a workpiece to be coated in a vacuum chamber, reducing the pressure (typically in the range of 10 −7  to 10 −4  Torr), and heating the material. The resulting coating vapor streams outwards from the material and coats the surface of the workpiece. Due to the low vacuum chamber pressure, vaporized coating materials travel nearly unimpeded, following an approximately line-of-sight trajectory. The most common sample heating method uses directed electron beam energy. 
     Many manufacturing steps involve depositing multilayered coatings of a multitude of materials. Generally it is important to be able to control the coating purity and deposition thickness to achieve desired results. In addition, many of the benefits of producing multiple coating layers on a workpiece are achieved when the coating steps are carried out sequentially under vacuum, and the trend in coating technology has been towards obtaining purer, more uniform and controllable coating thickness of multiple materials. An apparatus for carrying out sequential coating is disclosed in U.S. Pat. No. 2,482,329, which describes a mechanism for sequentially moving crucible pockets in line with a single heating source in a single vacuum chamber. While the mechanism of that patent was used in conjunction with electric resistance heating, the same mechanism works just as well with other sources, such as electron beams (see, for example, U.S. Pat. No. 4,632,059 to Flatscher et al.). The use of electron beams, which can be positioned using controllable magnetic fields, allows for the option of keeping the samples in place while deflecting the heating source, as disclosed in U.S. Pat. No. 5,792,521. 
     One major problem with the open material containers disclosed in the previously referenced patents is cross-contamination between crucibles. Although the majority of coating materials is directed away from its source, some material invariably diffuses back towards the source, contaminating the other source materials. This reduces the purity of the coating material and limits the usefulness of multiple coating chambers. One solution to this contamination problem is to incorporate a noncontacting crucible cover or movable lid with an opening that allows vapor from the heated sample to coat the workpiece, while shielding all of the other samples contamination. A typical cover arrangement is shown in U.S. Pat. No. 4,748,935 to Wegmann, which discloses an electron beam heated vapor source with such a cover. The arrangement is that of a flat, fixed cover and a multiple pocket crucible that rotates the samples through a heating region, allowing each pocket in turn to become a heated pocket. The cover and crucible are separated by a small gap that limits, but does not eliminate, line-of-sight contamination. In this configuration coating deposits that build up on the lid, interfering with the desired operation of the cover and requiring much maintenance. In addition, this type of cover can result in a new path for contamination—cover deposits can be scraped off or flake off into crucibles containing different materials. Nearly all covers known in the art rely on close tolerances to reduce, but not eliminate, line-of-sight contamination, and thus they still have the disadvantage of providing a path for contamination. 
     Some of the limitations of noncontacting covers were addressed in U.S. Pat. No. 4,944,245 to Stoessl et al. In Stoessl et al., a cover contacts the crucible surface in the vicinity of the heated sample to hinder the flow of material from the heated sample to any unheated samples. The cover and crucible are connected so that crucible rotation lifts the cover, allowing for sample selection. Although this approach reduces contamination by vapors migrating from heated to unheated samples, it is well known by those in the art that coating material rapidly accumulates on cover and crucible surfaces located within line-of-sight and particularly on those surfaces that are also near heated samples. In addition, the lifting mechanism described by Stoessl et al. occupies a large fraction of the crucible surface area, limiting the number of samples which can be accommodated. The coupled rotation and lifting mechanism of Stoessl et al. also allows for multiple contacts per material selection if the rotation is not between adjacent crucibles, which may increase the opportunity for cross-contamination. 
     In both the Wegmann and Stoessl et al. configurations, deposits have a tendency to rapidly accumulate thick layers of coating material that can easily flake or be scraped off and contaminate adjoining samples during rotation of the crucible. In addition, the close tolerances between cover and crucible in both of those references can be violated by deposits, requiring equipment maintenance limited by the ability of the cover mechanism to operate properly. It is desired to have an improved crucible cover that can accommodate deposits while reducing contamination. 
     SUMMARY 
     The prior art solution to reducing cross-contamination between pockets in multiple pocket sources relied on either closely spaced flat covers or covers that sealed with the crucible. Those solutions rely on maintaining close tolerances in regions where tolerances are difficult to maintain due to rapid build up of unwanted deposits. The present invention circumvents the problems in prior art systems by relying on a noncontacting baffle arrangement that is unaffected by unwanted deposits to a much greater degree. The present invention comprises a cover and crucible with matching features that, in combination, form a baffle to block the line-of-sight between heated and unheated crucible pockets, thereby reducing contamination. The rotation of the multiple crucible and lifting of the cover are controlled by separate mechanisms to provide flexibility in the design and operation of the source. Selection of any of the multiple pockets is achieved by rotating the crucible to the pocket of interest with the cover clear of the crucible. Cross-contamination protection is achieved by lowering the cover over the crucible to form a line-of-sight baffle after the selected crucible is properly aligned. In a first embodiment, the portion of the baffle nearest the heated pocket has a trough or grooves in the crucible with a cover lip that protrudes into the groove. This configuration allows the normal build up of coating materials on the cover top surface without causing degradation in performance from contamination or interference of moving parts. In a second embodiment there is a barrier that extends upwards from each of crucible pockets, while the cover protrudes below the barrier level, blocking line-of-sight between heated and unheated crucibles. In addition, the covers of the present invention lift away from the crucible prior to crucible rotation. In contrast to the prior art cam-based crucible lid combinations of Stoessl et al., in which the cover is lifted as the result of crucible rotation, the present invention reduces the amount of crucible surface area needed to implement the cover arrangement by providing a mechanism to lift the cover that does not depend on the rotation of the crucible. 
     The noncontact baffle arrangement of the present invention is an improvement over prior art covers are configured to either a) limit the line-of-sight between heated and unheated pockets with a close tolerance between cover and crucible, or b) forming a seal between cover and crucible. Both of those prior art covers have problems resulting from the build up of coating materials on the cover that tend to reduce their performance or that require frequent maintenance. In addition, some prior art covers place the edge of the cover near the top of the crucible pocket, thus allowing for the possibility of contamination due to deposits dropping directly into crucible pockets. 
     By providing a light tight, line-of-sight blocking baffle with clearance to allow for the build up of coating material, the present invention circumvents many of the problems with the prior art, allowing the cover to provide more contamination reduction for longer periods of time. 
     It is an advantage of the invention that a crucible cover for a multiple pocket evaporative source can be operated to greatly reduce cross-contamination of coating materials and that materials coating back onto the source not reduce the efficacy of the cover to reduce contamination or become a source for contamination. 
     It is yet another advantage of the invention that a multiple pocket crucible forms a line-of-sight blocking baffle in cooperation with a movable crucible cover, reducing cross-contamination between pockets without crucible-to-cover contact between heated and unheated crucible pockets. 
     It is an advantage of the invention that contamination is reduced with a crucible cover that can be lifted away from a crucible prior to rotation. 
     It is an advantage of the invention that it can be used for coating of materials that cold weld on contact by providing a contamination barrier that will not allow deposits to weld the crucible to the cover. 
     It is another advantage of the invention that materials that coat the cover do not hinder the movement of a turreted crucible. 
     It is a further advantage of the invention that the cover is lifted away from the crucible by an external lifting mechanism. 
     It is yet a further advantage of the invention that improved reliability and length of time between servicing will reduce the amount of scrap material and reduce the cost of operating coating equipment. 
     It is an advantage of the invention that it is simple and inexpensive to manufacture and service and that it is easy to use. 
     All patents cited herein are hereby incorporated by reference in their entirety for all purposes. Additional objects, advantages, aspects and features of the present invention will become apparent from the description of preferred embodiments, set forth below, which should be taken in conjunction with the accompanying drawings, a brief description of which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic of a vacuum deposition system; 
         FIG. 2  is a perspective view of a first embodiment of the invention; 
         FIG. 3  is an exploded perspective view of a first embodiment of the invention; 
         FIG. 4   a  is a perspective view of the heated pocket of a first embodiment of the invention with the cover in the “down” position; 
         FIG. 4   b  is a perspective view of the heated pocket of a first embodiment of the invention with the cover in the “up” position; 
         FIG. 5  is a perspective view from the bottom of a first and second embodiment of the invention; 
         FIG. 6  is a side view of a first embodiment of the invention with showing cover motion; 
         FIG. 7  is a perspective view of a second embodiment of the invention; 
         FIG. 8  is an exploded perspective view of a second embodiment of the invention; 
         FIG. 9  is a top view of a second embodiment of the invention; 
         FIG. 10   a  is cross-sectional view of  FIG. 9  showing the heated pocket and an adjacent unheated pocket of a second embodiment of the invention with the cover in the “down” position; and 
         FIG. 10   b  is cross-sectional view of  FIG. 9  showing the heated pocket and an adjacent unheated pocket of a second embodiment of the invention with the cover in the “up” position. 
       Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. The reference symbols used herein are not to be confused with any reference symbols used in the items that have been incorporated herein by reference. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     An example of a vacuum coating system  101  configured to coat workpieces such as semiconductor wafers is shown schematically in FIG.  1 . Vacuum coating system  101  includes a vacuum chamber  103 , an evaporative source  105  producing vapors of coating material  107 , and a framework  111  generally positioned to uniformly coat a plurality of optical components, flat panel display panels, semiconductor wafers or other workpieces  109  that may be located approximately equidistant from source  105  or on a spherical surface of approximately constant coating rate. Vacuum chamber  103  is opened, and one or more substrates  109  are loaded onto framework  111 . Using an electron beam evaporative source, materials with high boiling points or low vapor pressure, include many metals, semiconductors and refractory materials, can be coated or deposited onto workpieces. At the low pressures under which coating from evaporative and reactive evaporative processes occur, from about 10 −7  to about 10 −4  Torr, the amount of background material in vacuum chamber  103  is very small, and material  107  travels nearly collision free, and in nearly a straight line, from source  105  to workpiece  109 , coating surfaces within line-of-sight of the source. 
     The present invention is directed to an evaporative source that can hold many different materials for coating while reducing contamination between materials. As such it is applicable to a wide variety of coating systems, and this description is not meant to limit its use to the configuration of FIG.  1 . Other applicable configurations include: fixing the framework to vacuum chamber, rotating the framework with a motor or executing more complex motion with a planetary gear system, moving the source within the chamber, and rotating the workpiece relative to the source. 
     Details of a first embodiment of the present are shown in  FIGS. 2  to  6 . Details of an evaporative source  105  are shown in the perspective views of  FIGS. 2 and 3 . A turreted crucible or turret  301  with six crucible pockets  201  is mounted inside of source housing  217 . A variety of crucible materials and construction techniques are known to those skilled in the art. The present invention can be used in conjunction with any of these crucibles, including but not limited to: crucibles with high thermal conductivity, low melting point materials, such as copper or aluminum, that incorporate water cooling; crucibles constructed out of high melting point materials such as graphite, tungsten, or molybdenum; and crucibles with graphite liners or those that use surface oxidation to limit thermal conductivity from the evaporant to the crucible surface. 
     The intersection of pockets  201  on the crucible surface  213  defines pocket edges  311  around each pocket. Turret  301  may have more or fewer pockets, the choice of six pockets being for illustrative purposes only. The crucible is mounted to the source such that it can rotate about the crucible axis of rotation  303 . An electron beam source (not shown) located inside the source has an exit  203 , from which an electron beam  205  emanates. The beam is directed from the exit to the material heating location  207  by magnets, also located inside the source (also not shown). The deflection of electron beams for evaporative heating, as well as sweeping the beam for controllable heating is well known in the art. Using such a deflection system, the heating location  207  comprises a finite area that is generally within the heated pocket edge. A crucible lid or cover  209  is positioned over the pockets has a cover opening  211  formed by protrusion or lip  215 . Rotation of turret  301  allows for any one of the pockets to be open at a time. The uncovered pocket is aligned with heating location  207  and is thus termed the “heated pocket,” while all of the other pockets are termed “unheated pockets.” Location  207  is fixed relative to electron beam  205 , while the each of crucible pockets  201  can be positioned near location  207  for sweepable heating of the exposed material. The area occupied by heating location  207  need not be larger than any one of pocket edges  311  to provide control of heating of the material in the pocket. 
     Details of the present invention are shown in the exploded perspective view of  FIG. 3 , in which the cover is removed from the source. Each of crucible pockets  201  has a trough, recess or groove  305  located on crucible surface  213  that is located between each pocket and all of the other pockets. The groove surrounding each pocket may either be isolated (not connected) to the other grooves, may be interconnected with the others, as in  FIG. 3 , and may by contained on surface  213  or extend beyond the edge of the surface. The three-dimensional shape of groove  305  matches that of lip  215  in the vicinity of the heated pocket, in matching pairs, that produces an interlocking, noncontacting baffle. Lip  215  and groove  305  thus situated cooperate to form a baffle, thereby blocking the line-of-sight between the heated and all of the unheated pockets. 
     Greater detail near the heated pocket of one embodiment is shown in FIG.  4 . Pocket edge  311  shown in  FIG. 4  contains the heating position  207 , and thus pocket  201  shown in  FIG. 4  is the heated pocket.  FIG. 4   a  shows the source configured for coating, with cover  209  in the “down” position. In the down position, lip  215  extends into groove  305  without making contact. The baffle thus formed extends around pocket  201  far enough to provide a baffle between the heated and unheated pockets.  FIG. 4   b  shows the cover “up” position, in which lip  215  is retracted from groove  305 , permitting rotation of turreted crucible  301 . To reduce the need to service the cover by removing deposit material, a vertical clearance between lip  215  and the adjacent groove  305  should exceed approximately 0.005″ for line-of-sight blocking, and may have a horizontal clearance between the lip edge closest to the heated pocket of as large as 0.050″ to accommodate deposits. 
     In this invention, the cover and crucible baffle arrangement blocks contaminants from propagating from heated to unheated pockets by eliminating the line-of-sight movement of coating vapors. Due to the low pressure, vapors travel nearly unimpeded until they strike one of the surfaces within system  101 . Upon impact, a substantial amount of incident vapor molecules will stick to the surface, resulting in an accumulating deposit. By allowing sufficient clearance between baffle surfaces, deposits can accumulate over some period of time without presenting a source for cross-contamination. Thus the clearance between cover and crucible allows for deposits to accumulate on those surfaces without decreasing the ability of the baffle to block contaminants, since deposits further limit the line-of-sight, or to allow deposits to easily flake or rub off and contaminate other pockets. If materials do become dislodged from the cover, they would fall into a groove instead of a pocket, containing the contamination. Also, some materials of interest, such as gold, weld when surfaces having deposits of those materials are brought into contact. Thus surfaces having deposits of materials that contact weld can, under the conditions present in a high vacuum chamber, weld when brought into contact. Welding results in either binding mechanisms within the system, or dislodging relatively large flakes of deposit if movement occurs. Welding is prevented in the present invention by preventing contact of surfaces and by providing regions for deposits that are away from possible contact points. 
     The arrangement of the cover in this invention stands in contrast to prior art covers that relied on small clearances to reduce contamination, either by limiting (but not eliminating) line-of-sight contamination or by sealing the cover and crucible. It is important that the cover and crucible meet without contact, yet cooperate to form a baffle between heated and unheated pockets, and be movable between multiple pockets. It is also desirable to allow space for deposits in such a way that the deposits will not interfere with the noncontact line-of-sight baffle. There are many configurations of cover and crucible that will form the type of baffle to which this invention is directed, including, but not limited to, other cover lip and crucible groove shapes, a flat cover and a crucible with an edge that extends above the crucible, a cover with a groove and a crucible with a ridge, and combinations that result in more complex sinuous baffles. Additional contours or flat surfaces may also be included to separate deposits from crucible pockets. Other variations of cross-contamination prevention configurations within the scope of the invention include pockets having non-circular openings, such as regular or irregularly shaped two-dimensional shapes, multiple pockets of varying sizes, including shapes and or volumes, pockets arranged linearly or in multiple rows on the crucible surface, or arranged in a close packed configuration to increase the amount of usable crucible space on the crucible surface, or in multiple circular patterns. 
     OPERATION OF THE INVENTION 
     When selecting between the multiple pockets, the relative motion between turret and cover should proceed in a manner that does not allow for the crucible and cover to contact. From the described cooperation of the cover and crucible to form a baffle of the present invention, it would be obvious to one skilled in the art that crucible selection motion and cover motion must be controlled to provided the stated advantages. Thus contact between the baffle component, or between and any parts which have accumulated deposits should be minimized during pocket selection. Many mechanisms and control systems could be configured to achieve those advantages, and the descriptions and embodiments provided here are illustrative and are not meant to limit the scope of the present invention. 
     For the rotatable pocket configuration of the first embodiment, the cover and crucible form two halves of a baffle that would impede rotation of the turret  301 . The first embodiment thus includes a rotation mechanism (not shown) for rotating turret  301  to select from among multiple pockets  201 , and a lifting mechanism  300  to allow rotation without contact of cover  209  and turret  301 . The rotation mechanisms couples turret  301  to source housing  217 , and can be controlled by external devices not shown to rotate turret  301  about axis of rotation  303 . By controlling the rotational position of turret  301 , any of pockets  201  can be brought into heating location  207 . The rotation mechanism can consist of AC or DC motors or rotary or linear pneumatic actuators, and may also include sensors mounted on turret and housing to detect rotational position.  FIG. 5  shows a bottom perspective view shows the lifting mechanism  300 , including an actuator  307  and rods  309  that provide lifting motion to the cover  209  (not shown). Also shown is turreted crucible  301  and electron beam source exit  203 . In one embodiment actuator  307  is a single action pneumatic actuator with a spring return enclosed in a bellows, with the spring forcing the cover into a normally up position. In another embodiment, actuator  307  is double acting, providing both raising and lower operations. Other lifting mechanism that could be used with this invention include, but are not limited to, lead screws, piezo-electric actuators, bimetallic elements, magnetic solenoid, and linear motors. 
     The motion involved in lifting the cover is shown more clearly in the side view of FIG.  6 . The solid and dashed cover lifting components (actuator  307 , rods  309  and cover  209 ) shows the extreme points of motion. Cover is connected at its corners through rods  309  to actuator  307 . In the normally up, dashed position, lip  215  clears the obstructing crucible, and in the forced down, solid position, the cover  209  is fully engaged. The combination of a rotational and lifting mechanism is generally useful for rotational turrets as illustrated in the first embodiment, providing a way of covering and uncovering multiple pockets without allowing contact of surfaces which may have deposits, especially near pockets which may become contaminated. 
     ADDITIONAL EMBODIMENTS 
     Details of a second embodiment of the present invention are shown in  FIGS. 5 and 7  to  10 , in which  FIG. 5  shows a bottom view of the cover lifting mechanism,  FIG. 7  is a perspective view,  FIG. 8  is an exploded perspective view,  FIG. 9  is a top view, and cross-sectional views of the cover are shown in an up configuration ( FIG. 10   a ) and down configuration ( FIG. 10   b ). A crucible cover  701  with a lip  709  is positioned over turreted crucible  703  which includes multiple pockets  705  and is rotatably mounted in a housing using techniques well known to those in the art for mounting and controlling crucible rotation, as discussed in the first embodiment. Cover  701  is liftably mounted to the housing, using mechanism  300  as shown in FIG.  5 . as previously discussed. The main differences between the first and second embodiments is the manner in which a line-of-sight baffle is accomplished. Crucible  703  has a plurality of pockets  705 , each of which has a barrier  707  extending away from crucible  703 . Lip  709  is adapted to have approximately the same contour as barrier  707 , as is best seen in FIG.  9 . When cover  701  is aligned with crucible  703  as in  FIGS. 7  or  9 , one of pockets  705  is uncovered, with lip  709  near barrier  707  closest to the uncovered pocket. As with the first embodiment, the edge of uncovered pocket corresponds to the heating location  207 , and thus the uncovered pocket is the heated pocket. Extending away from cover  701  towards crucible  703  are protrusions  801  that fit into the space between barriers  707 . As with the first embodiment, cover  701  has an external lifting and rotation mechanism (not shown) that moves cover  701  away from crucible  703 , can rotate any of the pockets  705  within lip  709  such that protrusions  801  fit within barriers  707 , and then lowers the cover onto the crucible, coming close to but not touching the ends of the barriers. By lowering cover protrusions  801  below the tops of barrier  707 , the line-of-sight between the uncovered and all of the covered pockets is blocked. 
     Further details and the operation of the second embodiment is illustrated in  FIGS. 10   a  and b, which are sectional views of  FIG. 9  taken through heated pocket  1001  and an unheated pocket  1003 . Cover  701  includes protrusion  801  that forms a ledge  1005  nearlip  709 . In the view of the “down” position of  FIG. 10   a , the cooperation of barrier  707  and cover  701  form a baffle near ledge  1005 . Ray  1007  is illustrative of a glancing trajectory of vapors that might leave heated pocket  1001  and deposit on cover  701 . In the down position, cover  701  is situated below ray  1007 , thus reducing line-of-sight contamination of the cover. Contaminates migrating over barrier  707  from heated pocket  1001  towards unheated pocket  1003  will be accumulated on cover  701  at ledge  1005 . This will restrict the flow of coating vapors towards unheated pockets and reduce cross-contamination.  FIG. 10   b  shows cover  701  of the second embodiment in an “up” position, allowing for rotation of turret  711  without contact between barrier  707  and cover  701 . Line-of-sight blocking with the second embodiment is controlled independently of the shape of crucible contours, since blocking is not due to mutually mated surfaces as with the first embodiment. The degree of blocking can be varied by changing the protrusion  801  size and shape as well as the closest approach of cover  701  with crucible  703 . While there is a variation in dimensions that will provide for contamination control, preferred dimensions include barriers  707  that extends from approximately 0.1 inches to approximately 1 inch above crucible  703  towards cover  701 , protrusions  801  that extend at least approximately 0.02 inches down from lip  709  to form ledge  1005 , and a vertical clearance between barrier  707  and lip  709  of approximately 0.005 inches. 
     Although the embodiments illustrated include a mechanism to lift the cover off of the crucible, the necessity of lifting cover is not necessary in all embodiments. For example, an additional embodiments of the invention would have a fixed cover and a movable crucible, while other embodiments would be to have a cover that is hinged so that only part of the cover need be raised for turret motion, or the barrier may be designed such that a specific motion in a plane parallel to the crucible surface maneuvers the pockets into a selected position without a lifting motion. 
     Hence, although this invention has been described with respect to the embodiment discussed here, those embodiments are illustrative only. No limitation with respect to these embodiments is intended or should be inferred. It will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concept of the invention, and it is intended that the scope of the invention be defined by the claims appended hereto. 
     CONCLUSION 
     The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. The invention therefore should not be limited except as provided in the attached claims. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All patents cited herein are hereby incorporated by reference in their entirety for all purposes.