Single-crystal – oriented-crystal – and epitaxy growth processes; – Apparatus
Reexamination Certificate
1999-02-02
2001-01-30
Utech, Benjamin L. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Apparatus
C117S206000, C117S208000, C117S214000
Reexamination Certificate
active
06179914
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the growth of doped semiconductor crystals and, more particularly, to the controlled delivery of dopant to a molten host material contained within a melt crucible in a crystal-growing furnace.
BACKGROUND OF THE INVENTION
Current methods of growing single crystal ingots in a Czochralski-type crystal-growing furnace typically involve melting a polycrystalline host material, such as silicon, and a measured amount of dopant together in a crucible to create a melt. Once the melt is prepared, a seed crystal is lowered into contact with the melt to begin the crystal-growing process. As the seed crystal is slowly extracted from the melt, a monocrystalline crystal or ingot is drawn from the melt. Unfortunately, the monocrystalline ingot does not necessarily include a proportionate share of the dopant in the melt. Instead, the percentage of dopant incorporated into the monocrystalline ingot depends on the applicable segregation coefficient and other parameters.
Typically, the ingot incorporates a smaller percentage of the dopant than the melt. As such, the dopant concentration in the melt will increase over the crystal-growing period as the ingot is drawn from the melt. Due to the increasing dopant concentration in the melt, the ingot will also gradually incorporate a larger amount of dopant as the growth process proceeds. Since the resistivity of the ingot is a function of the amount of incorporated dopant, ingots that incorporate a increasing amount of dopant over their length also have a resistivity that decreases over their length. As a result, the wafers into which an ingot is sliced will also have slightly different resistivities depending upon the relative lengthwise location from which each wafer was sliced. Since purchasers of the wafers typically specify an acceptable range of resistivity values depending upon the intended use of the wafer, only a subset of the wafers harvested from an ingot may satisfy the requirements imposed by a purchaser.
U.S. Pat. No. 5,406,905 to Yemane-Berhane et al. discloses a technique for doping the melt after the host material has been melted in the crystal-growing furnace. This technique involves casting the dopant around the seed crystal used to grow the ingot. When the furnace is prepared, the dopant-coated seed crystal is held in a relatively cool part of the furnace until the host material has melted and is ready for doping. The dopant-coated seed crystal is then lowered to a position just above the melt. Heat transferred from the melt to the dopant-coated seed crystal causes the dopant, in solid form, to slip off the seed crystal and into the melt, hopefully without splashing and without immersing the seed crystal in the melt.
However, the '905 Yemane-Berhane et al. patent does not address the problems associated with variations in the concentration of the dopant throughout the course of drawing an ingot from the melt. Instead, when the temperature of the seed crystal rises due to the heat from the melt, a point is attained where the dopant will slide off the seed. Here, all of the dopant is delivered to the melt before the seed crystal is immersed in the melt to begin the ingot growing process. Thus, this technique is still prone to the problem of the conventional techniques wherein, as the crystal is grown, the concentration of the dopant in the melt will continually change, thereby altering the resistivity profile of the ingot in a lengthwise direction.
U.S. Pat. No. 5,242,531 to Klingshirn et al. discloses a process for continuously recharging a melt crucible with additional molten host material and additional molten dopant. In this regard, the Klingshirn '531 et al. patent describes separate containers filled with the host material and the dopant that are positioned above the melt crucible. Feedlines connect the containers with an additional crucible or container in which the host material and the dopant are mixed and melted. This additional crucible includes an outlet for supplying additional molten semiconductor material to the melt in order to recharge the melt during the crystal-growing process. While the '531 Klingshirn et al. patent addresses some of the issues with respect to controlling the amount of dopant in the melt throughout the course of a crystal-growing process, the technique described by the '531 Kingshirn et al. patent requires multiple containers positioned above the melt crucible which may complicate the design of the crystal-growing furnace and limit access to the melt crucible during the crystal-growing process.
Therefore, a need still exists for improved techniques of controlling the amount of dopant in the melt throughout the course of the crystal-growing process without requiring significant modifications to the crystal-growing furnace and without incurring the attendant costs. Consequently, a need still exists for improved techniques for controlling the concentration of dopant incorporated into the ingot over the length of the ingot, thereby also permitting the resistivity of the resulting wafers to be controlled.
SUMMARY OF THE INVENTION
The dopant delivery apparatus and associated method of the present invention effectively control the amount of dopant in the melt throughout the course of a crystal-growing process such that the concentration of dopant incorporated into the resulting ingot can be controlled over the length of the ingot. As such, the resistivity of the resulting wafers is also controlled, thereby overcoming the shortcomings of conventional crystal-growing processes. According to the present invention, an apparatus is provided for controllably delivering dopant to a melt which includes a vessel disposed in the melt and defining an interior cavity for containing the dopant. The vessel also defines at least one output orifice through which molten dopant is released into the melt, albeit at a release rate limited by the configuration of the output orifice. By regulating the release rate of the molten dopant into the melt, the dopant delivery apparatus precisely controls the overall concentration of dopant during the course of the crystal-growing process. In this fashion, the concentration of dopant incorporated into the ingot is also controlled over the length of the ingot such that the resulting wafers can have precisely defined resistivity characteristics.
In one embodiment, the dopant delivery apparatus is a submergible vessel having a housing defining the interior cavity and the orifice through which the molten dopant is released into the melt. The submergible vessel of this embodiment also includes means for submerging the housing within the melt. For example, the means for submerging the housing can include a weight positioned within the housing. In one advantageous embodiment, the housing is a capsule having a substantially cylindrical body and two opposed closed ends. According to this embodiment, one of the enclosed ends is weighted such that the capsular housing stands substantially vertical when immersed in the melt. According to another embodiment, the housing of the submergible vessel is a truncated capsule having an open end and an opposed closed end. According to this embodiment, the closed end is weighted such that the truncated capsular housing stands substantially vertical when immersed in the melt. As such, the open end of the truncated capsular housing serves as the orifice through which the molten dopant is released. The truncated capsular housing can also include baffles affixed to an interior surface of the housing and extending into the interior cavity for controlling the mixing of the melt with the dopant.
According to another embodiment, the vessel is not only submerged within the melt, but is secured within the melt crucible. More particularly, the vessel can be at least partially defined by the melt crucible, preferably in a position that is aligned with the crystal drawn from the melt. The vessel of this embodiment can also include a lid which is removably attached to the melt crucibl
Alston & Bird LLP
Anderson Matthew
SEH America Inc.
Utech Benjamin L.
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