Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
1999-05-03
2001-03-13
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S014000, C117S015000, C117S029000, C117S201000, C117S202000, C117S211000, C117S214000
Reexamination Certificate
active
06200383
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to crystal growth of semiconductor materials, and more specifically to continuous crystal growth of silicon sheet for use as solar cell substrate material.
BACKGROUND
To produce lower cost solar cells to facilitate large scale electrical applications of solar electricity, it is important to provide lower cost substrate materials for making the solar cells. A known method for achieving this objective is to grow a crystalline silicon using a continuous ribbon growth process as described in U.S. Pat. Nos. 4,661,200; 4,627,887; 4,689,109; and 4,594,229.
According to the continuous ribbon growth method, two high temperature material strings are introduced through holes in a crucible which contains a shallow layer of molten silicon. A crystalline silicon ribbon forms as the melt solidifies while being pulled vertically from the melt. The strings stabilize the position of edges of the growing ribbon. The molten silicon freezes into a solid ribbon just above the layer of molten silicon. To make this ribbon silicon process continuous, more silicon is added to the melt as the crystalline silicon is formed to keep the amount of melt constant. Keeping the amount of the melt constant during the growth process is important in order to achieve uniform and controllable growth of the crystalline silicon. It is also important to keep the thermal environment of the cooling ribbon constant. Slight changes in the depth of the melt and consequent changes in the vertical position of the solid-liquid interface can significantly change this thermal environment. For example, it has been found that variations in the melt depth of more than about one millimeter can result in a markedly different thickness and introduce a residual stress state of the grown silicon ribbon. For all of these reasons, a constant melt level is an important element in insuring uniform, controlled growth of silicon ribbon.
One way to control the depth of the melt is to continuously measure the depth of the melt and to control the rate at which silicon is added to the melt based on the measured depth. The depth of the melt must be measured in a way that is simple, cost effective, accurate, non-contaminating, and capable of being connected in a feedback loop to the feeder.
Several methods for measuring the melt depth are known. For example, one method uses an oscillating probe consisting of a thin graphite rod, with the making or breaking of electrical continuity as the probe touches the molten silicon giving the vertical position of the melt surface. This requires an actuator mechanism, a position sensor, and a clear vertical access to the melt. This method, however, presents a problem because one implementation of the growth technique relies on precisely positioned insulating shields to shape the cooling profile of the ribbon. Allowing room for a mechanical linkage is an undesirable constraint in the design and placement of these insulating shields.
Another known method utilizes a laser beam which is reflected from the melt surface. This technique is used in metal foundry work for melt depth measurement. The problem with this method, however, is that providing a clear optical path to the melt surface hampers the design of insulating components. In addition, it is difficult to maintain clean viewing ports in an environment where vapor deposited silicon oxide is common.
A third method is more indirect and involves measuring the weight and size of the grown crystal to track the loss of silicon from the crucible. This technique is described in U.S. Pat. No. 4,936,947. A major disadvantage of this method, aside from its complexity, is that it is applicable for growth of discrete crystals in a batch mode and is less applicable for continuous growth.
The above described methods are inadequate in satisfying the necessary criteria for making cost-effective solar cell substrate material. It is therefore an object of the present invention to provide a method which does satisfy these criteria.
SUMMARY OF THE INVENTION
The present invention features an apparatus and method for providing improved precision in the measurement and regulation of the depth of the molten semiconductor pool (the “melt”) in a continuous crystal growth process.
In one aspect, the invention features a method of continuous crystal growth. A crucible comprising a melt of a source material is provided, and an additional source material is continuously fed into the melt. To measure the depth, an input signal is passed through the crucible and the melt, and an output signal generated in response to the input signal is measured. The output signal relates to a depth of the melt. The depth of the melt is maintained at a substantially constant level by adjusting the rate of feeding the additional source material using the output signal. A crystalline ribbon is continuously grown by solidifying the melt at the solid-liquid interface. The source material can comprise a semiconductor material.
In one embodiment, to measure the depth of the melt, a current is passed through the crucible and the melt via a first pair of leads connected to the crucible, and a voltage generated in response to the current is measured via a second pair of leads connected to the crucible.
In another embodiment, the additional source material is continuously fed into the melt at a substantially constant rate. In still another embodiment, the depth of the melt is provided at a substantially constant level by maintaining the output signal to be substantially constant.
The invention also features a method of controlling a depth of a melt for crystal growth. An input signal is applied through a crucible and the melt disposed within the crucible, and an output signal generated in response to the input signal is measured. The output signal is related to the depth of the melt. An amount of a source material introduced into the melt is adjusted to maintain the depth at a substantially constant level using the output signal.
In one embodiment, to control the depth of the melt, a current is applied via a first pair of leads connected to the crucible, and a responsive voltage is measured via a second pair of leads positioned between the first pair of leads and connected to the crucible.
The invention further features a method of measuring the depth of a melt for crystal growth. An input signal is applied through a crucible and the melt disposed within the crucible, and an output signal generated in response to the input signal is measured. The output signal is related to the depth of the melt. In one embodiment, the response signal and the depth of the melt have a substantially linear relationship within an operating range.
In another aspect, the invention features a system for continuous crystal growth. The system includes a crucible comprising a melt of a source material, a power source electrically connected to the crucible through a first pair of leads, a feedback controller electrically connected to the crucible through a second pair of leads, and a feeder for continuously introducing the source material into the melt in response to the feedback controller. The source material can comprise a semiconductor material.
In one embodiment, the first pair of leads applies an input signal to the crucible and the melt, and the second pair of leads measures an output signal generated in response to the input signal. The input signal can comprise a current and the output signal can comprise a voltage.
In one detailed embodiment, the first pair and the second pair of leads are co-linear and attached to a bottom surface of the crucible. In another embodiment, the first pair of leads can be disposed on adjacent corners of the crucible and the second pair of the leads can be disposed on adjacent corners of the crucible.
The invention also features a system for measuring the depth of a melt for crystal growth. The system includes a crucible comprising a melt of a source material, a power source connected to the crucible through a first pair of leads for applying an input
Krauchune Richard C.
Wallace, Jr. Richard L.
Evergreen Solar Inc.
Kunemund Robert
Testa Hurwitz & Thibeault LLP
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