Coating apparatus – Gas or vapor deposition – Crucible or evaporator structure
Reexamination Certificate
2000-09-22
2003-04-22
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
Crucible or evaporator structure
C414S217000, C414S939000
Reexamination Certificate
active
06551405
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is in the field of tools and methods for use in connection with maintained environments found in vapor phase deposition (VPD) processes, such as but not limited to vacuum environments maintained in molecular beam epitaxy (MBE) machines. In particular, the invention is directed to tools and methods for reloading source materials and performing maintenance in VPD environments, such as maintained in an MBE machine, without the necessity of breaking the vacuum of the environment to perform the reload or maintenance operation.
MBE is a technology that was developed in the early 1970's for the purpose of growing high-purity crystals, particularly epitaxial layers of compound semiconductors. Numerous types of crystal materials may be grown in MBE machines, but the most widely-used application today is III-V compound semiconductors (so called because the two elements used in forming the semiconductor are found in Groups IIIB and VB, respectively, of the periodic table of elements). Gallium Arsenide (GaAs) and other III-V compound semiconductor materials are widely used in optoelectronic components found in cellular telephones, lasers, microwave equipment, and other electro-optical applications.
In the MBE process, the elements that the semiconductors are made from are deposited onto a heated crystalline substrate wafer in the form of “molecular beams” to form thin epitaxial layers. The molecular beams are formed from thermally evaporated elemental sources. To obtain the necessarily high purity in the epitaxial layers, the material sources must be extremely pure and the entire crystalline growth process must take place in an ultra-high vacuum environment. Also, in order to finely control the deposition of material, the flow of the molecular beams must be precisely controlled. This is generally accomplished using shutters that can open and close in a fraction of a second. Fast shuttering and slow effluence rates makes possible the transition of one material to another at levels which only partially complete an atomic layer. The abrupt epitaxial transitions which can thus be achieved with MBE can be alternated to achieve superlattice structures, wherein some anomalous and highly desirable electrical, optical and magnetic properties may appear.
Most commercial and research MBE machines include at least two major components: a growth chamber and a load chamber. The load chamber is used to bring substrate wafers into and out of the machine while maintaining the vacuum integrity of the growth chambers. The load chamber is also used for the preparation, manipulation, and storage of substrate wafers. The growth chamber is where the MBE process is performed upon the substrate wafers. Effusion cells containing source materials are generally attached to and extend outward from the growth chamber. For certain source materials with very high melting points, electron beam cells may replace effusion cells.
One of the initial problems that was overcome in the design of MBE machines is the transport, storage, and manipulation of substrate wafers. A considerable length of time is required to purify and reestablish a vacuum within the growth chamber once the chamber is opened to the ambient atmosphere. Far less time is required to reestablish a vacuum in the load chamber section once it is opened, provided that the growth chamber remains closed. This is achieved by the use of an air lock between the growth chamber and load chamber. MBE machines have thus been designed so that wafers can be loaded and unloaded without the loss of vacuum in the growth chamber. Wafers are first loaded into the load chamber with the air lock between the load chamber and growth chamber closed. Once the load chamber again reaches a vacuum state, the air lock is opened so that wafers can be moved into the growth chamber. A transport system may be used to move the wafer or wafers through the load chamber. One example of such a system is a chain-driven cart that travels the length of the load chamber. Once the cart reaches the end of the load chamber nearest the growth chamber entrance, a magnetically coupled transfer arm can be used to carry the wafer or wafers lying in the cart from the load chamber into the growth chamber. The arm may be magnetically manipulated by a user situated near the MBE machine. Generally, the arm travels just into the load chamber when retracted, but may be extended into the growth chamber when pushed inward.
The load chamber may also include a vertically oriented manipulation arm, commonly referred to as a “wobble stick,” which is generally used to move wafers to and from the load chamber transport means and the magnetically coupled arm. Once the wobble stick has transferred the wafer from the transport means to the magnetically coupled arm, the air lock between the load chamber and growth chamber is opened, and the magnetically coupled arm is extended into the growth chamber to load the wafer into the wafer holder.
The wafer holder in the growth chamber must be heated to maintain the proper temperature at the crystalline substrate wafer. The exact temperature required will depend upon the materials being used. In some systems the wafers are loaded into the growth chamber in an orientation that faces away from the material sources initially, so the wafer holder must flip around to face the wafer toward the material sources before material deposition may begin. The substrate wafer holder and other components that are to be heated must be made of materials that do not decompose or outgas impurities even when heated to high temperatures; such materials as Tantalum (Ta), Molybdenum (Mo), and pyrolytic boron nitride (PBN) have been used for these applications.
MBE machines may be either solid-source or gas-source. Gas-source machines have the advantage of easier reloading of the source material, since all that is required is the replacement of a pressurized bottle holding the gas source material. For a variety of reasons, including output quality and suitability, for some applications solid-source machines are preferred over gas-source machines. There are, therefore, certain MBE applications for which only solid-source materials are used. The solid source material is sublimated within the effusion cell by applying heat. The flux travels out of the effusion cell into the growth chamber as the shutter to the effusion cell opens. In gas-source MBE machines, the sublimation step is of course unnecessary, since a pressure regulator forces the flow of material which is already in the gaseous state to the substrate.
In solid-source MBE machines, the material sources are generally held in PBN crucibles contained within the effusion cells. Each effusion cell may be heated independently to reach the desired flux of the particular material located in that cell. Small changes in flux can significantly affect the epitaxial layer deposition process for some materials, and thus highly accurate thermostats must be used on the effusion cell heaters. Also, the control shutters that open and close the flow of flux from the effusion cells may be computer controlled to allow the cells to be opened and closed quickly and precisely. It should be noted that most MBE machines may accommodate a number of different effusion cells attached to the growth chamber housing, such that numerous types of source materials may be evaporated simultaneously. Since the properties of each material differ, the specific design of each effusion cell must be different in order to properly handle the particular material in question.
Generally, commercial effusion cells may only be heated to a temperature of approximately 1300° C. before the PBN crucibles begin to disintegrate. For this reason, some materials cannot be placed in effusion cells because the temperature required to vaporize such materials is too great. Iron (Fe) is an example of one such material. When such materials are to be used, electron beam cells replace the effusion cells. Electron beam cells vaporize source material by exposing the s
Bullock Daniel W.
LaBella Vincent P.
Thibado Paul M.
Dougherty J. Charles
Mills Gregory
Moore Karla
The Board of Trustees of the University of Arkansas
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