The invention relates to an arrangement for infrared emitters having at least one emitter tube.
The operation of infrared emission elements in a vacuum, in vacuum processes with reactive atmospheres, or in corrosive or reactive atmospheres, as for example, coating processes, Chemical Vapor Deposition, Physical Vapor Deposition, etching in the gas phase, the fabrication of thin-film solar cells in CIS technology, and RTP processes, in which a considerable quantity of heat is to be introduced into a substrate in a very short time, and thus a combination of a vacuum or an atmosphere of hot, corrosive gases having high quantities of released heat and cyclical loading, represents a particular demand on the components and materials being used.
Even with the use of IR heating elements made of the quartz-tube type, in which the outer sleeve of the emitter is made of a tube of quartz glass, which is resistant to heat and to nearly all atmospheres, nearly all of the technical hurdles remain. These are, among other things, the corrosion of the electrical feed lines to the emitters when these are carried out in a corrosive atmosphere or in a vacuum.
Sparkovers between the electrical feed lines among each other or to the chamber wall occur in certain pressure regions, when the feed lines are constructed in the chamber. With use of heat emitters in sleeve tubes, wherein the sleeve tubes represent parts of the wall of the processing chamber, a problem of heat accumulation is quickly generated, which leads to the destruction of the emitter or at least limits the maximum output of the emitter being used. With use of the sleeve tube solution, the maximum output to be realized is limited both thermally by the heat accumulation in the sleeve tube and also geometrically by the necessarily large distances between the sleeve tubes.
European Patent EP 1 228 668 B1 describes an arrangement in which at least one infrared emitter is arranged in a sleeve tube. The sleeve tube is here sealed relative to the vacuum chamber and also protects the emitter from reactive gases, which possibly appear in the chamber. A disadvantage, however, is that the emitters in such a sleeve tube quickly overheat and can be destroyed, because the sleeve tube must already have a considerable temperature, in order to be able to discharge heat to the surroundings via radiation.
The possibility indicated in the publication of adequately cooling the emitter by an air flow through the sleeve tube can usually not be used in real, technical applications, because an electrical connection would no longer be allowed at the outlet of the hot air—this connection would overheat. Only twin-tube emitters with one-side electrical connection come into question, which also reduces the maximum possible output. Air cooling also leads to a temperature gradient along the sleeve tube. Because the sleeve tubes also act as secondary IR emitters in the far IR and thus contribute to the application of energy into the substrate, a gradient of the temperature of the sleeve tube can be noted as the gradient of the incoming power onto the substrate, which cannot be tolerated in many processes.
Due to the narrow installation spaces and dimensions that are needed, in order to obtain homogeneous radiation sources and radiation fields of higher surface-area power, there is only a narrow gap between the wall and emitter in the sleeve tube. Therefore, the wall temperature of the emitter is considerably higher than that of an emitter located directly in the vacuum or that of an emitter cooled by convection.
If water is used as the cooling agent, then the problem of the temperature gradient in the sleeve tube can indeed be avoided. The use of water, however, can be realized only in a separate tube, because the electrical feed lines should not be constructed lying in water. Here, water absorbs a minimum of approximately 50% of the total emitter power, wherein this water is usually arranged in the gap between the emitter and sleeve tube or at a comparable position. In addition, water can be used only in those cases where the wall temperature of the sleeve tube may be low and where the additional heating of the sleeve tube is not needed for the process.
German published patent application DE 10 2004 002 357 A1 likewise describes active cooling, which is, however, technically very expensive.
A symmetrically arranged air guide (in which, e.g., air is blown centrally into the sleeve tube via an additional tube or even at many positions over the length of the sleeve tube) can move only a small quantity of air and thus can achieve only minimal cooling. In addition, it is very expensive in terms of energy to use compressed air. Typically, air is blown economically via fans, whereby initial pressures up to about 0.3 bars can be achieved. With compressors, air can be blown at a few bars of overpressure (the quantity of air is then limited by effects of compressibility). Nevertheless, inhomogeneities would also not be able to be ruled out here. In addition, complicated and expensive devices are used for controlling the cooling.
The assembly and electrical contacting of infrared emitters directly in the processing chamber also rarely appears advantageous. For example, in order to avoid voltage sparkovers or discharging in the chamber, the voltage must be kept lower than 80 volts, especially in the presence of an ion source of a plasma or in the pressure range on the order of magnitude of 10 Pascals to approximately 10,000 Pascals. Indeed, the maximum possible voltages that can be read from the Paschen curves of the processing gases are somewhat higher than this 80V, but experience has shown that sparkovers are actually avoided only below this limit. Such a low operating voltage considerably limits the possible electrical output of the infrared emitter, because depending on the type of construction, the current possible for each emitter is also limited. Thus, many emitters of lower output are needed, which then must be operated at non-typical operating voltages. This means that, among other things, an expensive and heavy transformer is needed for the generation of the voltage.
With the use of infrared emitters in corrosive atmospheres, the electrical feed lines could be attacked, especially the molybdenum film, which is located in the pinched section and is extremely sensitive. Here, a use can be completely impossible.
In order to accommodate the emitter ends and the electrical lines, it is further necessary to provide additional, not insignificant space in the vacuum chamber, which is usually limited or expensive, if all of the emitters with feed lines are to be accommodated in the vacuum chamber.
The simple passing of the emitter through the wall and the direct sealing of the emitter requires very strong cooling of the seal, because the seal is exposed to extreme thermal loading due to the high radiation powers found in the emitter tube. In quartz tubes, a considerable radiation power is transported in the axial direction of the tube, similarly as in an optical fiber.
The formation of flanges directly on the emitter tube is therefore extremely complicated. Such flanges must also be supported so that they move in the direction of the emitter axis against the chamber wall, in order not to convert slight thermal expansions into a tensile stress that is destructive for the emitter tube. Because the thermal expansion of the quartz glass is approximately one order of magnitude lower than that of the metallic chamber wall, even slight variations of the temperature of the chamber wall can lead to tensile loading that is destructive for quartz glass.