Rapid thermal anneal (RTA) is a process used in semiconductor device fabrication which consists of heating a single wafer at a time in order to affect its electrical properties. Unique heat treatments are designed for different effects. Wafers can be heated in order to activate dopants, change film-to-film or film-to-wafer substrate interfaces, densify deposited films, change states of grown films, repair damage from ion implantation, move dopants or drive dopants from one film into another or from a film into the wafer substrate. Rapid thermal anneals are performed by equipment that heats a single wafer at a time using lamp based heating that a wafer is brought near. Unlike furnace anneals they are short in duration, processing each wafer in several minutes. Rapid thermal anneal is a subset of processes called Rapid Thermal Process (RTP).
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 ~200-1250°C with ramp rates typically 20-200°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 , 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.
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.
Recently, RTP-like processing has found applications in another rapidly growing field — solar cell fabrication. RTP-like processing, in which an increase in the temperature of the semiconductor sample is produced by the absorption of the optical flux, is now used for a host of solar cell fabrication steps, ncluding phosphorus diffusion for N/P junction formation and impurity gettering, hydrogen diffusion for impurity and defect passivation, and formation of screen-printed contacts using Ag-ink for the front and Al-ink for back contacts, respectively.
Some solar cell companies have successfully applied our advanced Rapid Thermal Processing (RTP) technology to its process for creating highly efficient and durable CIGS solar cells. This eliminates a key process bottleneck found in many state-of-the-art process implementations and enables the use of low-cost substrates in ways that were not considered possible before.
In Rapid Thermal Processing, a layer is heated for a very brief period only in a highly controlled way. For instance, RTP techniques can flash-heat a layer for just several picoseconds and put energy just into the top several nanometers of a layer in a highly controlled way -- while leaving the rest of the layer unaffected.
RTP has a secondary benefit of reducing the energy payback time of their solar cells to less than two months (for the full panel). By comparison, a typical silicon solar panel has an energy payback time of around three years, and a typical vacuum-deposited thin-film cell has one of 1-2 years. The energy payback time is the time that a solar panel has to be used in order to generate the amount of energy that it required to be produced.
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