Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
2000-10-27
2004-01-13
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S797000, C257S712000, C257S685000, C257S719000, C257S727000, C361S802000, C361S801000, C361S728000, C361S729000, C361S730000, C361S732000
Reexamination Certificate
active
06677673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to systems and devices for use in securing semiconductor elements of solid state devices. More particularly, embodiments of the present invention relate to a mechanical clamping assembly which facilitates uniform application of clamping forces to the semiconductor elements of high voltage solid state devices so as to ensure substantial thermal and electrical communication between the semiconductor elements, and thereby contribute to safe, reliable, and effective operation of the solid state device.
2. The Relevant Technology
Semiconductor materials, and devices that employ them, have been in use for some time. The physical and chemical structure of semiconductors makes them especially well suited for use in a variety of applications. In particular, it is well known that some materials such as silicon and germanium are arranged into regular atomic patterns or structures, often referred to as crystals. One characteristic of such materials however is that they are not particularly effective in conducting electricity, nor are they well suited for use as electrical insulators. Because such materials are not particularly useful for electrical conduction, or for impeding electrical conduction, they are generally referred to as “semiconductors.”
It is well known however, that by adding various amounts of certain other atoms to the crystal structure of materials such as silicon and germanium, in a process sometimes referred to as “doping,” these materials can made to assume certain desirable characteristics. In particular, the doping of materials such as silicon and germanium allows manufacturers to produce materials that have particular desired electrical properties. For example, some semiconductor materials can be doped so that they possess substantially improved electrical conductivity in some situations. Conversely, other semiconductor materials can be doped in such a way that they are substantially resistant to conduction of electricity in certain situations. Notwithstanding the desirable electrical characteristics of doped materials however, both doped and undoped materials are referred to generally as semiconductors.
Thus, semiconductors possess a number of desirable properties. The ability of semiconductors to be modified in a variety of ways to facilitate achievement of particular results or effects makes useful in a variety of applications. Further, because the semiconductors comprise solid crystals, they are resistant to rough handling and vibration.
Because the semiconductor material takes a solid form or “state,” electronic parts, components, and devices employing semiconductor materials are often referred to as “solid state” devices. Solid state devices are embodied in a virtually endless variety of forms. Examples of common solid state devices include relays, thyristor switch assemblies, transistors, and diodes. Furthermore, there are numerous fields of application for solid state devices. For example, solid state devices are commonly employed in lasers, radar systems, x-ray tubes, and the like.
Solid state devices are constructed in any of a variety of different ways. One common construction method involves stacking a plurality of semiconductor elements. For example, in the case of high voltage solid state devices, a sufficient number of semiconductor devices must be connected together in series to obtain the required voltage rating. Thus, to operate such a solid state device at 10,000 volts, three semiconductor devices rated at 4,500 volts each would be connected in series to obtain an aggregate rating for the solid state device of 13,500 volts.
Typically, the stack of semiconductor devices, or “stack” elements, are subjected to large compressive forces, sometimes as high as 10,000 pounds, in order to enhance their operational characteristics. Application and maintenance of the compressive force are important to the overall operation of the solid state device in that the compression facilitates substantial contact between the individual stack elements. This substantial contact, in turn, facilitates a high degree of thermal and electrical communication between the various stack elements. Because some of the stack elements typically comprise heat sinks or the like, a high level of thermal communication between the stack elements facilitates effective removal of heat from the solid state device. In similar fashion, good electrical communication between the stack elements facilitates safe, reliable, and effective operation of the solid state device.
In order to supply the large compressive forces necessary to facilitate effective and reliable operation of the solid state device, a variety of clamps and clamping devices have been devised. As discussed below, however, many known clamping devices suffer from a variety of shortcomings that render them ineffectual and/or cause damage to the stack elements. Such shortcomings are further aggravated when attempts are made to use known devices in high voltage environments.
Some of the problems and shortcomings with known clamping devices relate to the materials used to construct those devices. In general, known clamping devices typically employ upper and lower clamping plates joined together by a number of compression bolts. Typically, the stack is disposed between the upper and lower compression plates and nuts on the compression bolts are then tightened as required to move the upper and lower clamping plates together, and thereby compress the stack elements together.
In high voltage applications, in particular, the voltages involved are so large that the spacing between the stack and the compression bolts is insufficient to prevent arcing between the stack elements and the compression bolts. Accordingly, many known clamping devices that have been developed for use in high voltage environments employ compression bolts comprising fiberglass or other electrically non-conductive material. While the fiberglass material represents an improvement in that it substantially prevents arcing between the stack elements and the compression bolts, it has certain inherent shortcomings.
A significant problem with the fiberglass compression bolts relates to the relative softness of the fiberglass material. Specifically, clamping forces as high as ten thousand pounds are required in some high voltage applications to establish and maintain the contact between the stack elements that is necessary for effective and reliable operation of the solid state device. Thus, the nuts must be tightened to a high degree to produce such clamping forces. As a result of the high clamping forces that they are required to impose, the relatively soft threads of the fiberglass compression bolts are vulnerable to stripping.
A related problem with compression bolts constructed of fiberglass and like materials concerns the difficulty of tightening the nuts on the compression bolts in a symmetric fashion. That is, the nuts must be tightened so that the compression force exerted by the clamping device is distributed evenly across the upper surface of the stack. This arrangement is necessary to ensure consistent and substantial contact between the stack elements and thus, effective and efficient operation of the solid state device. As discussed below, this result is difficult to achieve in practice, and it is generally the case that the nuts on the compression bolts are tightened at least somewhat asymmetrically.
A typical consequence of even slightly asymmetric or unbalanced tightening of the nuts on the compression bolts is that the magnitude of the mechanical stresses and strains between the nut and corresponding compression bolt in the region of relatively greater stack compression is materially higher than the magnitude of the mechanical stresses and strains between the nut and corresponding compression bolt in the region of relatively lesser stack compression. The high stresses and strains that typically result from asymmetric tightening of the nuts on the compression bol
Varian Medical Systems Inc.
Williams Alexander O.
Workman Nydegger
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