Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate
Reexamination Certificate
2000-01-31
2002-08-20
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
C438S758000
Reexamination Certificate
active
06436796
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to systems and methods for chemical vapor deposition (CVD) and thermal processing, such as epitaxial deposition.
2. Background
A variety of semiconductor processes require uniform thermal processing at high temperatures. An example of such a process is called chemical vapor deposition (CVD) in which a layer of a material from the vapor phase is deposited onto a semiconductor substrate having been placed on a susceptor within a reactor. The susceptor is then heated either by induction or high intensity light radiation to high temperatures, typically between about 800 to 1250° C. Gases are then passed through the reactor and the deposition process occurs by chemical reaction, within the gas phase, but closely adjacent to the surface of the substrate. The reaction results in the deposition of the desired product onto the substrate.
One form of this type of processing is called epitaxy, in which a single-crystal layer of a substance is deposited onto a substrate that is also single-crystal in nature. As an example, silicon epitaxy is one of the first steps performed in the fabrication of an integrated circuit device, and in this process a layer of doped single crystal silicon is deposited onto a silicon wafer in order to have a layer of known and closely regulated resistivity into which transistors and other devices may be formed. Epitaxy offers a convenient method for controlling the thickness, concentration, and profile of the doping layer.
An important parameter that must be controlled during an epitaxial deposition is the temperature uniformity of the substrate. Non-uniformities in temperature of the substrate can lead to a process of plastic deformation called slip, in which the crystal relieves built-up stresses by allowing portions of its structure to move relative to other regions. Slip occurs in a crystal over certain crystallographic planes and along certain crystallographic directions, causing one portion of the material to be displaced relative to another. A common cause of slip in a crystal is a temperature gradient during film growth, but it can also be the result of the manner in which the substrate is supported, the mechanism by which the substrate is heated, and the time-temperature profile of the epitaxial process. Slip-related defects are most often found at the edges of a substrate and appear as short lines.
Thermal gradients in a substrate may arise as a result of a non-uniform thermal environment within the CVD reactor. Because there are gases flowing within a CVD reactor, heat transfer mechanisms involve conduction and convection as well as radiation. However, radiative heat transfer may well be the most important with regard to temperature uniformity. A substrate adjacent to a heated susceptor within a cold walled reactor will see a variety of thermal gradients in both axial and radial directions. These thermal gradients have a large effect because radiative heat transfer between two objects is a function of the two temperatures, each temperature taken to the fourth power.
In many CVD and epitaxial deposition systems, high intensity lamps such as tungsten-halogen lamps are used to selectively heat a wafer within a cold wall furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. However, it is more difficult to control the temperature of the semiconductor substrate using only low thermal mass lamp heating. Some reactors use a large thermal mass silicon carbide coated graphite susceptor to maintain temperature uniformity of the substrate during processing. The substrate to be processed is placed either on or adjacent to the susceptor, and because of the susceptor's high thermal conductivity, it can conduct heat laterally to maintain temperature uniformity and even out non-uniformities across the substrate. The susceptor is typically wider than the substrate which allows it to compensate for radiative heat loss at the edge of the substrate.
Alternatively, the susceptor may be heated by RF induction. This method takes advantage of the fact that an oscillating electric current passing through a conductor placed adjacent to the susceptor produces an oscillating magnetic field around the conductor, which in turn induces an oscillating current in the susceptor itself. Since the susceptor has an electrical resistance, the oscillating electrical current causes the susceptor to heat up. It should be noted that the current induced in the susceptor falls off non-linearly with distance from the conductor. The relationship is that the magnetic flux varies as the inverse square of the distance.
A typical configuration of the coil profile is shown in FIG.
1
. The distance between any one particular coil segment and the susceptor may be adjusted with standoffs (not shown in FIG.
1
). The coil in
FIG. 1
is profiled to compensate for the radiative heat losses that occur at the edge of the susceptor when the susceptor is at processing temperature, and thus, coil segment
120
is closer to the susceptor than coil segment
122
. It should be noted that the coil configuration shown in
FIG. 1
is optimal when the reactor is at processing temperature, but may not be not optimal during transient periods when the reactor is being heated or cooled.
What is desired is an improved apparatus and method for CVD and/or epitaxial processing of a semiconductor substrate. Preferably, such a system and method would provide a uniform substrate processing temperature such that temperature gradients in the substrate, and the resulting problems with defects such as crystallographic slip, are reduced or eliminated.
SUMMARY OF THE INVENTION
Aspects of the present invention provide a CVD reactor for epitaxial processing, the reactor configured to reduce thermal gradients in the substrates onto which epitaxial layers are being deposited. Reducing thermal gradients in a wafer diminishes slip. One type of epitaxial reactor comprises an RF induction coil positioned adjacent to a silicon carbide coated graphite susceptor. An alternating current through the coil segments produces an oscillating magnetic field around each segment, which in turn induces a current in the susceptor. Electrical energy associated with the induced current is converted into thermal energy, thereby heating the susceptor. The coil is supported by a number of support studs, and the different segments of the coil may be set at different heights, thus varying the distance separating coil segments from the susceptor. Conventional methods of addressing temperature uniformity in a susceptor comprise adjusting the coil segments such that they are closer to the susceptor at the susceptor's inner and outer edges than at the center in order to compensate for the greater amount of heat loss from the edges. Furthermore, the susceptor has to be rotated during heating and processing to minimize temperature gradients caused by the coil.
A problem with conventional methods of addressing temperature uniformity is that coil/susceptor separation profile is configured to provide optimum temperature uniformity in the susceptor at the processing temperature. This profile is not optimal for the transient portions of the process, for example, the heat ramp-up and cool down, where the closely spaced coil segments at the susceptor's edges cause the edges to overheat during ramping. Since the coil/susceptor separation profile is not easily re-configured, especially during processing, apparatus and methods for improving temperature uniformity in the susceptor during the transient portions of the process are needed, assuming the coil/susceptor separation profile is fixed.
An aspect of the present invention provides a mechanism for raising and lowering the susceptor while it is rotating within the reactor. Because the magnetic fields fall off non-linearly with distance from each coil segment, raising the susceptor de-couples
Evans Gary Lee
Johnsgard Kristian E.
Mailho Robert D.
O'Hara Mark J.
Pfefferkorn Glenn A.
Mattson Technology Inc.
Mulpuri Savitri
Wilson Sonsini Goodrich & Rosati
LandOfFree
Systems and methods for epitaxial processing of a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Systems and methods for epitaxial processing of a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Systems and methods for epitaxial processing of a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2934451