Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
1999-09-10
2001-02-13
Bueker, Richard (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S093000
Reexamination Certificate
active
06187091
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Area of the Art
The invention relates generally to an apparatus and a process for growing a silicon epitaxial layer, and specifically to an apparatus and a process for growing a silicon epitaxial layer which has an epitaxial surface with a consistent resistivity profile.
2. Description of the Prior Art
In an epitaxial reactor, a phenomenon known as autodoping occurs on substrate silicon wafers which do not have a backside film barrier. Autodoping occurs when the dopant used in the substrate wafer (such as boron for P-type, or phosphorus or antimony for N-type) outgasses from the backside of the wafer, gets caught in the flow of the gases used in the epitaxial reactor, and is redeposited on the front side of the wafer toward the peripheral edge of the wafer. The result of this deposition of dopant causes the resistivity profile of the epitaxial surface to significantly vary radially from the edge to the center of the wafer. The center of the wafer will theoretically have the desired resistivity, then the resistivity will decease following the wafer radially outwardly from the center when boron is used, and will increase following the wafer radially outwardly from the center when antimony or phosphorus is used.
Since autodoping is a very undesirable phenomenon and the resulting variation in resistivity is unacceptable, present technology calls for deposition of a film barrier of a silicon nitride, silicon carbide, or more commonly silicon dioxide, on the backside of the wafer. The film barrier on the backside of the wafer prevents the dopant used in the substrate from outgassing and becoming airborne in the reactor chamber. Often, however, the backside film barrier has to be removed after epitaxial deposition. The removal of the backside film barrier after epitaxial deposition increases the cost of the wafer because of the added steps of both deposition and removal of the film.
Several apparatuses and methods have been used to control the effects of autodoping without the use of a backside film barrier. For example, U.S. Pat. Nos. 5,421,288 and 5,487,358 disclose an apparatus and a process for growing a silicon epitaxial layer. Both patents discuss controlling the effects of autodoping without the use of a backside film barrier. According to the patents, the thickness and the resistivity profile of the epitaxial layer are controlled through the use of independent mass flow controllers (MFC) on center injectors and peripheral injectors.
Another example of epitaxial reactors is the one manufactured by Applied Materials Technology Corporation (AMT). This type of epitaxial reactor uses a mass flow controller (MFC) to control an auxiliary dopant supply that is deposited only into the injectors that effect epitaxial deposition on the center of the wafer. The auxiliary dopant added is such that it matches the autodoping dopant on the peripheral edges of the wafer, theoretically giving a smooth resistivity profile.
The limitation of the MFC design relates to the sensitivity of the MFC. For example, when the AMT epitaxial reactor is used, the reliability of the auxiliary dopant is dependent upon a consistent and well-controlled gas pressure on both sides of the MFC. During normal operation of the reactor, there are some pressure fluctuations in the gas flow line leading to the injector. This results in varying auxiliary dopant concentrations being emitted, and thus an unstable resistivity profile in the epitaxial layer of the wafer.
Therefore, a need exists to provide an apparatus and a process for growing silicon epitaxial layer having a consistent dopant concentration and resistivity profile.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus and a process for solving the above-mentioned problems, and particularly to provide an apparatus and a process for growing a silicon epitaxial layer having a consistent dopant concentration and a resistivity profile.
These and other objects are achieved in an epitaxial reactor system having a horizontal chamber containing a susceptor mounted within the chamber for holding the silicon substrate wafer in a horizontal position on the top surface of the susceptor so that the main surface of the silicon substrate wafer is facing upward. The system comprises a series of injectors arranged transversely to the horizontal axis of the chamber. The injectors comprise a central injector passing a flow of a reactive gas past a central part of the chamber and peripheral injectors passing flows of the reactive gas past a peripheral part of the chamber. The reactive gas consists of a silicon source, a dopant and hydrogen. The system also comprises a gas feeder system connected to the central injector and the peripheral injectors and feeding the reactive gas to the injectors; an auxiliary dopant feeder system connected to the central injector passing flows of dopant gas past the central part of the chamber, wherein both the gas feeder system and the auxiliary dopant feeder system are connected to the central injector through a central flow line; and means connected between the auxiliary dopant feeder system and the central injector for absorbing pressure variations in the central flow line to prevent the pressure variations from reaching the auxiliary dopant feeder system.
Such an arrangement has been found to provide a number of advantages. As explained in greater detail below, it has been found that by providing means for absorbing the pressure variations in the central flow line between the auxiliary dopant mass flow controller (MFC) and the central injector, the pressure variations therein may be prevented from reaching the auxiliary dopant MFC, and therefore a consistent resistivity profile can be achieved on the epitaxial layer without a protective seal on the back surface of the wafer.
The system is well suited for use in connection with an epitaxial reactor system, and particularly in an epitaxial reactor system using an auxiliary dopant mass flow controller to control the auxiliary dopant supply that is deposited only into the injectors that effect epitaxial deposition on the center of the wafer. In this use of the epitaxial reactor system, wherein an epitaxial layer is grown on a main surface of a silicon wafer which is held in a horizontal type of chamber, parallel flows of reactive gas from a plurality of positions which are arranged in a width direction of the flows are concurrently fed into the chamber so that an epitaxial layer is formed in the vicinity of the center of the silicon wafer and in the vicinity of the peripheral portions of the wafer. The reactive gas comprises a silicon source, a dopant and hydrogen. The concentration of the dopant in the reactive gas which is fed from a center side in the width direction of the flows is adjusted so that the resistivity of the epitaxial layer formed in the vicinity of the center of the silicon wafer is approximately the same as that of the vicinity of the peripheral portions thereof. In a preferred embodiment, the adjusting step comprises a first adjusting step to provide a predetermined amount of an auxiliary dopant to pass into the reactive gas which is fed from the center side, and a second adjusting step to reduce the concentration variation of the auxiliary dopant in the reactive gas. Preferably, the second adjusting step is carried out by a bellows metering valve.
The invention is defined in its fullest scope in the appended claims and is described below in its preferred embodiments.
REFERENCES:
patent: 5421288 (1995-06-01), Ohta et al.
patent: 5487358 (1996-01-01), Ohta et al.
patent: 58-156593 (1983-09-01), None
Alston & Bird LLP
Bueker Richard
SEH America Inc.
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