Rapid Thermal Processing

Rapid thermal processing (RTP) provides a way to rapidly heat wafers to an elevated temperature to perform relatively short processes, typically less than 1-2 minutes long. Over the years, RTP has become essential to the manufacture of advanced semiconductors, where it is used for oxidation, annealing, silicide formation and deposition.

An RTP system heats wafers singly, using radiant energy sources controlled by a pyrometer that measures the wafer's temperature. Previous thermal processing was based on batch furnaces, where a large batch of wafers is heated in a tube. Batch furnaces are still widely used, but are more appropriate for relatively long processes of more than 10 minutes.

RTP is a flexible technology that provides fast heating and cooling to process temperatures of ~200-1300°C with ramp rates typically 20-250°C/sec, combined with excellent gas ambient control, allowing the creation of sophisticated multistage processes within one processing recipe. This capability to process at elevated temperatures for short time periods is crucial because advanced semiconductor fabrication requires thermal budget minimization to restrict dopant diffusion. Replacement of the slower batch processes with RTP also enables some device makers to greatly reduce manufacturing cycle time, an especially valuable benefit during yield ramps and where cycle-time minimization has economic value.

RTP systems use a variety of heating configurations, energy sources and temperature control methods. The most widespread approach involves heating the wafer using banks of tungsten-halogen lamps because these provide a convenient, efficient and fast-reacting thermal source that is easily controlled. In a typical RTP system (Figure ), the wafer is heated by two banks of linear lamps — one above and one below it. The lamps are further subdivided into groups or zones that can be individually programmed with various powers to maximize temperature uniformity. In RTP, the energy sources face the wafer surfaces rather than heating its edge, as happens in a batch furnace. Thus, RTP systems can process large wafers without compromising process uniformity or ramp rates. RTP systems frequently incorporate the capability to rotate the wafer for better uniformity.

In a typical rapid thermal processing furnace, the wafer is heated by two banks of linear lamps. (Source: Mattson Technology)

Today's state-of-the-art RTP systems can control temperature distribution across the wafer surface to a 3σ <2°C distribution. However, device patterns on the wafer surface impose a limit. Because RTP systems heat the wafer with radiant energy, process temperature can be affected by its optical properties. Thus, device patterns can induce temperature non-uniformities. Reduction of this "pattern effect" by optimizing the heating configuration is an important research area as device dimensions shrink and process uniformity requirements become more stringent. Different solutions can address the pattern effect, including dual-sided heating methods that reduce the lamp power incident to the patterned surface, and approaches that irradiate the patterned surface with a heat source close to the wafer's temperature.

Another critical RTP area is temperature measurement and control. The Figure shows a system controlled by a pyrometer that views the wafer's back surface. Early RTP systems suffered repeatability problems because variations in the spectral emissivity of coatings on the backs of wafers caused reading errors. Today's systems include sophisticated emissivity correction that allows repeatable processing.

An important RTP application is the activation of ion-implanted dopants to form ultrashallow junctions. This requires fast ramp and cooling capabilities because the wafer must be heated to ~1050°C to anneal out ion implantation damage and activate the implanted dopant species. However, the time at temperature must be reduced to minimize diffusion. This has led to the spike-anneal approach, where the wafer is ramped to a high temperature and then cooled immediately.

Another indispensable RTP application is in the formation of silicides. In this process, metal films react with the silicon on source/drain and gate regions to form silicides. In advanced logic processes, the metal is usually cobalt, but nickel is being explored for the 65 nm node. Silicide formation processes are usually performed at <500°C, and wafers must be kept in a very pure gas ambient because metal films can be sensitive to oxidation. RTP systems are ideal, because they have small chamber volumes easily purged with high-purity gas, creating a very clean environment.

RTP is also increasingly important in oxidation applications, where the capability to use short process times at high temperatures and a wide variety of gas ambients provides excellent quality films and superior process control. RTP-grown oxides are often used for gate dielectrics, tunnel oxides and shallow-trench isolation liners. The use of steam in the gas ambient has opened new RTP applications. One of special interest for advanced DRAM technology is the use of a hydrogen-rich steam ambient for selective oxidation of gate stacks that include tungsten.