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.
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 10xe2x88x927 to 10xe2x88x924 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 contaminationxe2x80x94cover 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.
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.