Electric heating – Inductive heating – With workpiece support
Reexamination Certificate
2000-04-28
2001-11-13
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
With workpiece support
C219S662000, C219S674000, C219S638000
Reexamination Certificate
active
06316754
ABSTRACT:
TECHNICAL FIELD
This invention relates to heating systems using high frequency energy to induce currents in objects to heat them directly or indirectly (“induction heating systems”). In particular it relates to heating systems for semiconductor processing equipment in which layers are formed on heated semiconductor wafers by Chemical Vapor Deposition.
Equipment for the formation of layers, such as epitaxial silicon or polysilicon, on semiconductor wafers is well known. One of the problems in forming layers of materials on a semiconductor wafer is ensuring that the temperature of the semiconductor wafer is kept uniform across the wafer during the deposition process, i.e., during heat-up, processing, and cool-down. Since the deposition rate of a layer of material upon the wafer is dependent on the temperature of the wafer, any temperature variations between the center and edge of a wafer will undesirably result in the deposition of a layer of non-uniform thickness on the wafer. In addition, non-uniform temperatures during heat-up, processing, and cool-down can cause stress on the wafer, undesirably resulting in slip. Accordingly, it is important during the deposition of a layer of material on a silicon wafer to minimize temperature variations across the surface of the wafer.
Some earlier systems developed to achieve temperature uniformity across a wafer teach applying heat in a uniform manner across all portions of a wafer. However, since heat loss is typically greater at the edge of a wafer than at the center, such earlier systems may result in significant temperature differences between the center and outer portions of the wafer and, thus, may be largely ineffective in minimizing temperature gradient across the surface of a wafer.
Some more recent heating schemes employ lamp assemblies having heating lamps arranged in a plurality of independently controlled zones to allow differing amounts of heat to be applied to the outer and center portions of the wafer. In this manner, variations of heat loss on a wafer can be compensated by applying differing amounts of heat to various portions of the wafer in order to achieve greater temperature uniformity across the wafer. Such techniques are typically able to maintain temperature differences of less than 10° C. across the wafer.
The heating lamps employed in such multiple-zone heating assemblies may result in several disadvantages. Each of these lamps, which is typically of the tungsten halogen type, has a finite useful life and thus must be periodically replaced. The replacement of such lamps, as well as the periodic adjustment of the lamps due to their aging, not only increases manufacturing cost but also decreases the throughput of the chemical vapor deposition (CVD) system.
Furthermore, the use of such heating lamps in a radiantly heated CVD system undesirably requires periodically cleaning of the quartz walls of the reaction chamber to remove deposited material from the chamber walls. Recall that since the walls in a lamp heated CVD reaction chamber become heated, layers of material deposit not only on the wafer but also on the chamber walls. The resulting film formed on the chamber walls then absorbs some of the radiant energy emitted from the heating lamps and thereby locally increases the temperature of the chamber walls. As a result, layers of material deposit on the chamber walls at an ever increasing rate, creating a “snowball” effect. It is therefore critical, in a lamp heated CVD chamber, that the walls be kept meticulously clean. This requires that the walls be etched frequently, in some cases after every run. Cleaning the walls typically takes between two and four minutes after depositing an epitaxial layer, and longer after depositing a polysilicon layer. Since the entire cycle in warm wall CVD systems may be between five and ten minutes, cleaning the walls accounts for a significant portion of the total cycle time and, therefore, can severely reduce throughput of the CVD system.
DISCLOSURE OF INVENTION
Accordingly, it is desirable to provide a heating system for chemical vapor deposition systems which uniformly and precisely heats a semiconductor wafer without the above mentioned deficiencies.
It is an object of the present invention, therefore, to permit control of the power delivered to each of a plurality of zones of an object to be heated by an induction heating system.
It is a further object of the invention to allow delicate and precise control over the temperature profile of an object being heated by an induction heating system.
It is another object of the invention to permit rapid temperature changes in an object being heated by an induction heating system while simultaneously maintaining the temperatures of a plurality of zones of said object in exact proportion to one another.
It is yet a further object of the invention to permit rapid temperature changes in an object being heated by an induction heating system while simultaneously holding the temperatures of a plurality of zones of said object precisely equal.
It is a further object of the present invention to maintain the temperatures of a plurality of zones of said object in exact proportion to one another as the object is cooled from a heated state.
It is another object of the invention to permit rapid cooling of an object in an induction heating system while simultaneously maintaining the temperatures of a plurality of zones of said object in exact proportion to one another.
It is yet another object of the present invention to eliminate the need for high current switches in a multi-zone induction heating system.
It is another object of the invention to permit the use of a single power supply in a multi-zone induction heating system.
It is a further object of the invention to eliminate or greatly reduce the interaction between zones of a multi-zone induction heating system.
It is yet a further object of the invention to obtain high power supply utilization in a multi-zone induction heating system.
It is still a further object of the present invention to permit rapid heating of a semiconductor wafer or other single crystal object without “slip”, or dislocation of the crystal lattice.
Accordingly, one embodiment of the present invention is directed to a system for producing heating in an object such that precise control is possible of the energy deposited in a plurality of zones of the object to be heated through precise control of the power delivered to sections of a nearby induction coil which induces circulating currents in either the object to be heated or a susceptor placed in close proximity to the object to be heated.
One embodiment accomplishes these objects by utilizing an induction coil split into sections (a “tapped” induction coil), and selecting resonating capacitors for each section such that the frequency of resonance is different for each section. A frequency agile power supply is connected to the coil and the frequency of the output of the power supply is switched from the resonance frequency for the first section to the resonance frequency of the second section, to the resonant frequency of the third section, and so on until power has been applied to all (or all desired) sections.
REFERENCES:
patent: 2669647 (1954-02-01), Segsworth
patent: 4093839 (1978-06-01), Moliterno et al.
patent: 4506131 (1985-03-01), Boehm et al.
patent: 4755648 (1988-07-01), Sawa
patent: 4788394 (1988-11-01), Vanneste et al.
Dorrenbacher John M.
Schatz Douglas S.
Advanced Energy Industries Inc.
Leung Philip H.
Santangelo Law Offices P.C.
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