2.3 Rapid Thermal Oxidation

The decreasing size of the semiconductor devices demands very short high-temperature oxidation steps, because thermal oxidation influences the distribution of impurities in the bulk of silicon and at the Si/SiO$_2$ interface. Since the movement of impurities affects the device size and its electrical properties, it is important to control and minimize the effects of oxidation on the impurity profile. This can be achieved by precisely controlling the oxidation temperature and reducing the thermal budget of the heat cycle required for an oxide film growth.

Unfortunately, for such applications there is a limitation of the conventional furnace oxidation due to its inertia to temperature transitions, which results in a higher thermal budget than required for oxidation. The thermal budget can be reduced considerably by decreasing the duration of these transitions. As shown in Fig. 2.5, a smaller thermal budget can be achieved by rapid thermal processing (RTP) [30].

During RTP, the wafer is rapidly heated from a low to a high processing temperature (T > 900$^{\circ }$C). It is held at this elevated temperature for a short time and then brought back rapidly to a low temperature. Typical temperature transition rates range from 10 to 350$^{\circ }$C/s, compared with about 0.1$^{\circ }$C/s for furnace processing. So RTP reduces the ramp-up and ramp-down durations. The RTP durations at high processing temperatures vary from 1s to 5min. This makes RTP very suitable to grow thin oxide films (< 40 nm), where a precise temperature control and short oxidation times are important.

A schematic RTP system is shown in Fig. 2.6. The heat source is typically an array of lamps in an optical system. In contrast to conventional furnaces, where a batch of wafers is introduced into the furnace and oxidized at the same time, RTP systems are single-wafer machines, and only one wafer is in the chamber and processed. However, due to the high processing temperature (T > 900$^{\circ }$C), the processing time required for oxidation is in RTP systems reduced.

One of the difficult problems in an RTP system is to know exactly the wafer temperature. These systems usually support the wafer on a small thermal mass in order to heat the wafer rapidly. This makes it very difficult to use thermocouplers for temperature measurement as is done in a furnace. Another technique is to measure with an infrared pyrometer from the back side of the wafer. Precise temperature measurement is rather difficult with this method, because the ``energy reading'' depends mainly on the surface condition of the back side. Furthermore, the wafer temperature can change by approximately 1000$^{\circ }$C in a few seconds, which also complicates an accurate temperature measurement.

Figure 2.5: Reduction of the thermal budget with rapid thermal processing.

Figure 2.6: Cross section view of a RTP system.