Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
1999-07-08
2001-01-09
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C118S725000, C438S795000
Reexamination Certificate
active
06172337
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 a system and method for thermally processing a semiconductor substrate using a stable temperature heat source.
2. Background
Diffusion furnaces have been widely used for thermal processing of semiconductor device materials (such as semiconductor wafers or other semiconductor substrates). The furnaces typically have a large thermal mass that provides a relatively uniform and stable temperature for processing. However, in order to achieve uniform results, it is necessary for the conditions in the furnace to reach thermal equilibrium after a batch of wafers is inserted into the furnace. Therefore, the heating time for wafers in a diffusion furnace is relatively long, typically exceeding ten minutes.
As integrated circuit dimensions have decreased, shorter thermal processing steps for some processes, such as rapid thermal anneal, are desirable to reduce the lateral diffusion of dopants and the associated broadening of feature dimensions. Thermal process duration may also be limited to reduce forward diffusion so the semiconductor junction in the wafer does not shift. As a result, the longer processing times inherent in conventional diffusion furnaces have become undesirable for many processes. In addition, increasingly stringent requirements for process control and repeatability have made batch processing undesirable for many applications.
As an alternative to diffusion furnaces, single wafer rapid thermal processing (RTP) systems have been developed for rapidly heating and cooling wafers. Most RTP systems use high intensity lamps (usually tungsten-halogen lamps or arc lamps) to selectively heat a wafer within a cold wall clear quartz furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. Rapid wafer cooling is also easily achieved since the heat source may be turned off instantly without requiring a slow temperature ramp down. Lamp heating of the wafer minimizes the thermal mass effects of the process chamber and allows rapid real time control over the wafer temperature. While single wafer RTP reactors provide enhanced process control, their throughput is substantially less than batch furnace systems.
FIG. 1
is a graph illustrating a desired heating profile for a wafer during rapid thermal processing in a lamp heated RTP system. In particular, the solid line in
FIG. 1
is a plot of the temperature of the center of a wafer over the duration of a rapid thermal annealing process. As shown in
FIG. 1
, the wafer may be heated at a rapid rate as indicated at
102
in FIG.
1
. Lamp radiation may be rapidly adjusted as a desired processing temperature is approached in order to achieve a constant processing temperature, as indicated at
104
. At the end of the processing step, the lamp radiation may be quickly reduced to allow cooling as indicated at
106
.
While RTP systems allow rapid heating and cooling, it is difficult to achieve repeatable, uniform wafer processing temperatures using RTP, particularly for larger wafers (200 mm and greater). The temperature uniformity is sensitive to the uniformity of the optical energy absorption as well as the radiative and convective heat losses of the wafer. Wafer temperature nonuniformities usually appear near wafer edges because radiative heat losses are greatest at the edges. During RTP the wafer edges may, at times, be several degrees (or even tens of degrees) cooler than the center of the wafer. At high temperatures, generally greater than eight hundred degrees Celsius (800° C.), this nonuniformity may produce crystal slip lines on the wafer (particularly near the edge). To minimize the formation of slip lines, insulating rings are often placed around the perimeter of the wafer to shield the wafer from the cold chamber walls. Nonuniformity is also undesirable since it may lead to nonuniform material properties such as alloy content, grain size, and dopant concentration. These nonuniform material properties may degrade the circuitry and decrease yield even at low temperatures (generally less than 800° C.). For instance, temperature uniformity is critical to the formation of titanium silicide by post deposition annealing. In fact, the uniformity of the sheet resistance of the resulting titanium silicide is regarded as a standard measure for evaluating temperature uniformity in RTP systems.
Temperature levels and uniformity must therefore be carefully monitored and controlled in RTP systems. Optical pyrometry is typically used due to its noninvasive nature and relatively fast measurement speed which are critical in controlling the rapid heating and cooling in RTP. However, accurate temperature measurement using optical pyrometry depends upon the accurate measurement of the intensity of radiation emitted from the wafer and upon the wafer's radiation emitting characteristics or emissivity. Emissivity is typically wafer dependent and depends on a range of parameters, including temperature, chamber reflectivity, the wafer material (including dopant concentration), surface roughness, and surface layers (including the type and thickness of sub-layers), and will change dynamically during processing as layers grow on the surface of the wafer. In addition, radiation from heat sources, particularly lamps, reflect off the wafer surface and interfere with optical pyrometry. This reflected radiation erroneously augments the measured intensity of radiation emitted from the wafer surface and results in inaccurate temperature measurement.
Increasingly complex systems have been developed for measuring emissivity and for compensating for reflected radiation. One approach uses two optical pyrometers—one for measuring the radiation from the lamps and one for measuring the radiation from the wafer. The strength of the characteristic AC ripple in radiation emanated from the lamp can be compared to the strength of the AC ripple reflected from the wafer to determine the wafer's reflectivity. This, in turn, can be used to essentially subtract out reflected radiation in order to isolate the emitted radiation from the wafer for determining temperature using Planck's equation. See, e.g., U.S. Pat. No. 5,166,080 to Schietinger et al. However, such systems may require complex circuitry to isolate the AC ripple and perform the calculations that effectively eliminate reflected radiation. Such systems also require an additional optical sensor and other components.
Another approach for measuring wafer temperature and compensating for the effects of emissivity uses an infrared laser source that directs coherent light into a beam splitter. From the beam splitter, the coherent light beam is split into numerous incident beams which travel to the wafer surface via optical fiber bundles. The optical fiber bundles also collect the reflected coherent light beams as well as radiated energy from the wafer. In low temperature applications, transmitted energy may be collected and measured as well. The collected light is then divided into separate components from which radiance, emissivity, and temperature may be calculated. See, e.g., U.S. Pat. No. 5,156,461 to Moslehi et al. It is a disadvantage of such systems that a laser and other complex components are required. Such systems, however, are advantageous because they may provide measurements of wafer temperature at multiple points along the wafer surface which may be useful for detecting and compensating for temperature nonuniformities.
In order to compensate for temperature nonuniformities, a heater with multiple independently controlled heating zones may be required. One approach is to use a multi-zone lamp system arranged in a plurality of concentric circles. The lamp energy may be adjusted to compensate for temperature differences detected using multi-point optical pyrometry. However, such systems often require complex and expensive lamp systems with separate temperature controls for each zone
Johnsgard Kristian E.
Mattson Brad S.
McDiarmid James
Zeitlin Vladimir J.
Mattson Technology Inc.
Pelham Joseph
Wilson Sonsini Goodrich & Rosati
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