In Rapid Thermal Processing (RTP) of semiconductor substrates, such as wafers or the like, it is advantageous to perform processes so that the wafer is exposed to the high temperature environment for a precise amount of time, and not exposed for too long to lower temperatures during ramping. Among such processes are included the ion implant annealing processes, the metal-silicide formation and growth processes, and the surface reaction and some thermal CVD processes. There are other requirements for these processes, including that the wafer temperature be very uniform during the process and that its temperature also be independent of the emissivity of its front and back surfaces. Further, it is important that there be no plastic deformation of the silicon in the wafer at any time, which means the wafer temperature must be kept uniform to a reasonable degree even after the process is done, as long as the wafer is above a critical temperature of approximately 950 degrees Centigrade. Unless all of these requirements are met at reasonable cost and with high reliability the RTP processing chamber and method is not suitable for many of these high temperature semiconductor manufacturing processes.
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 lamp RTP systems allow rapid heating and cooling, it is difficult to achieve repeatable, uniform wafer processing temperatures using such 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 non-uniformities 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.degree. C.), this non-uniformity 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. Non-uniformity 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.degree. 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.
In order to compensate for temperature non-uniformities, a heater with multiple independently controlled heating zones may be required. One approach is to use a multizone 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 of lamps. For instance, U.S. Pat. No. 5,156,461 to Moslehi et al. discloses a multi-zone heater with sixty five tungsten-halogen lamps arranged into four heating zones. In addition, a light interference elimination system is disclosed which uses light pipes in seven dummy lamps to measure lamp radiation as well as five or more light pipes for measuring radiation across the surface of the wafer. The light interference elimination system uses the radiation of the dummy lamps to determine the fraction of total radiation from the wafer surface that is reflected from the lamps as opposed to emitted from the wafer surface. The emitted radiation can then be isolated and used to detect temperature across the wafer surface, which in turn can be used to control the lamp heating zones.
While multi-zone lamp systems have enhanced wafer temperature uniformity, their complexity has increased cost and maintenance requirements. In addition, other problems must be addressed in lamp heated RTP systems. For instance, many lamps use linear filaments which provide heat in linear segments and as a result are ineffective or inefficient at providing uniform heat to a round wafer even when multi-zone lamps are used. Furthermore, lamp systems tend to degrade with use which inhibits process repeatability and individual lamps may degrade at different rates which reduces uniformity. In addition, replacing degraded lamps increases cost and maintenance requirements.
In order to overcome the disadvantages of lamp heated RTP systems, a few systems have been proposed which use a resistively heated plate. Such heated plates provide a relatively large thermal mass with a stable temperature. In particular, one such RTP system is described and claimed in co-pending application Ser. No. 08/499,986 filed on Jul. 10, 1995 in the names of Kristian E. Johnsgard, Brad S. Mattson, James McDiarmid and Vladimir J. Zeitlin as joint inventors, titled "System and Method for Thermal Processing of a Semiconductor Substrate", assigned of record to the assignee of record of this application, and which is hereby incorporated herein by reference. The RTP system described in such co-pending application uses a heavily insulated large thermal mass heater. A wafer is placed on pins and lowered perpendicularly onto or near the heater for processing. During processing, the heater and wafer are enclosed within an insulated cavity at vacuum pressure. The insulation and low pressure reduce non-uniform heat losses and provide for a stable thermal processing environment. This system minimizes thermal gradients within the chamber to provide uniform thermal processing, and therefore, does not contain a separate cooling station within the chamber to allow rapid cooling of the wafer before removal from the chamber. Rather, the wafer is removed after processing and transported to a separate cooling station.
For some processes, however, it may be beneficial to cool the wafer within the chamber before removing it. In particular, such cooling may be useful for high temperature processes where slip may occur if the wafer is removed before cooling. Throughput may also be improved in some circumstances if a cooling station can be provided within the same chamber. In providing such a cooling station, however, it is important not to introduce unacceptable thermal gradients, heat loss, and non-uniformities into the processing chamber.
One approach for providing heating and cooling within a single chamber is described in U.S. Pat. No. 5,252,807 issued to Chizinsky ("Chizinsky"). Chizinsky describes a system using a vertically elongated chamber with opposing hot and cold surfaces. The hot surface may comprise a resistively heated plate and the cold surface may comprise a surface coated with a radiation absorbing material. In Chizinsky, the wafer is moved longitudinally from proximity to the hot surface for processing followed by proximity to the cold surface for cooling.
While such a configuration provides for heating and cooling within the same chamber, a variety of disadvantages may be encountered when designing a commercial RTP chamber using such a configuration. First, the wafer temperature during processing may be dependent on the front and backside emissivities of the wafer because the wafer can radiate to the unheated side walls of the chamber and the opposing cold surface even when the wafer is near the hot surface. Second, the upper cold surface and the chamber wall may be exposed to vapors coming from the wafer during processing. Such surfaces may become coated with condensible materials from the vapors, and thus require frequent cleaning so as not to cause particulate or cross contamination on succeeding wafers when processed. Third, if a raised wall is used to prevent excessive heat loss from the edge of the wafer as it is raised (as suggested in Chizinsky) two problems may be encountered: (1) the chamber wall is exposed to direct high radiant heat from the raised wall (which results in inefficient heat loss from the raised wall) because it extends vertically well above the reflector insulating dish (which is used to protect the walls from radiation from the heating surface); and (2) the raised wall temperature is less than that of the hot surface, which is the source of its heat, and therefore detracts from the temperature uniformity of the hot surface and wafer. Thus, the wafer, hot surface, and any raised wall may radiate directly to cooled chamber walls and the opposing cold surface which results in inefficient heat loss and potential thermal gradients and non-uniformities in the thermal processing environment. Fourth, Chizinsky contemplates both heating and cooling of a wafer before the next wafer is introduced into the chamber. This results in decreased throughput relative to a system that allows the next wafer to be heated while the first wafer is being cooled.
Therefore, what is needed is a system and method for rapid thermal processing that provides (i) a stable thermal processing environment with uniform heating; (ii) a closely adjacent cooling environment insulated from direct radiation from the heating environment; and (iii) a configuration that allows stable and uniform heating and cooling in the same chamber with high throughput. In particular, such a system and method should preferably allow one wafer to be cooled while another wafer is being heated within the same chamber.
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