Rapid Thermal Processing, as the name implies, involves heating and cooling a single silicon wafer as quickly as possible. Several factors are key:

1. Speed. It is critical for small geometry devices to minimize the movement of the dopants placed in the wafer by ion implantation. This can only be accomplished by reducing the time the wafer (or the device layers within the wafer) spends at the highest temperatures (reducing “thermal budget”). Speeding the “ramp-up” and “cool-down” rates and providing the fastest possible transition from heating to cooling, or “turn around” is therefore important.
2. High Electrical Activation. Advanced devices need high conductivity as well as shallow junctions to meet performance targets. As many of the dopant atoms as possible must be activated. The process for doing this is complex, but higher temperatures lead to higher activation. Although this seems to imply more diffusion of the junction, the two processes act in such a way that high temperatures for very short times will give better electrical activation with less diffusion.
3. Uniformity. A 300mm silicon wafer can contain hundreds of integrated circuits. A temperature difference of only a few degrees across the wafer can cause stress that will damage some of the devices or lead to variations in the performance of the circuits. It is therefore vital that the entire heating cycle be carried out with great uniformity.
4. Abruptness. To achieve the high performance of advanced logic devices, the junctions must be very abrupt laterally.

Current leading RTP machines use banks of many small tungsten halogen lamps to heat the wafer. Not only are the control problems formidable, but the use of a filament lamp means that there is a physical constraint on how rapidly the system can be switched off and the wafer cooled down. It is generally accepted that this technology has now reached its capability ceiling.

Vortek RTP systems (Impulse, iRTP Flash Assist, fRTP) employ the very high power arc lamp developed by Vortek that has been used in various scientific applications for more than 20 years. Its extremely high power and spectral match to silicon provides very fast ramp up. The fact that it is an arc means that switch off is virtually instantaneous. Finally, the wafer cools faster than in todayís industry leaders as i/fRTP operate in an effectively black chamber, absorbing all radiated energy. The system, through use of the Vortek lamp and advanced optics, is inherently uniform. fRTP is targeted at USJ formulation down to the 40nm node. iRTP is designed to optimize other anneal processes such as silicidation, with superior processing capability and cost of ownership, within a common tool family.

ADVANCED ANNEALING TECHNOLOGIES

The process technologies that are being actively pursued for Ultra-Shallow Junction (USJ) anneal by Vortek are Impulse™ Anneal (iRTP), and Flash Assist (fRTP). These stand in comparison to conventional “spike anneal” as is currently used. All of these processes in principal anneal semiconductor junctions by quickly raising the wafer to high temperatures, spending a minimal amount of time at peak, followed by rapid cooling.

Of the USJ anneal types, tungsten-halogen spike anneals are the current industry state of the art. Spike anneal has the longest time at peak temperature where undesirable diffusion processes rapidly degrade semiconductor performance and yield. Vortek iRTP is faster, spending only a few milliseconds at peak temperature and accumulating up to 7 times less effective process time for the undesired diffusion.

Beyond iRTP, laser annealing (LTP) has been viewed as a possible path forward for advanced annealing. A laser is employed to scan the front surface of a wafer (much as a lithography stepper). The intensity of the radiation heats the front surface of the wafer almost instantaneously to melting point, with equally fast cooling due to the heat sink effect of the rest of the wafer material. Despite its initial attraction, it turns out that the method does not appear viable – at least in the foreseeable future. Reasons include:

1. Process integration problems – LTP causes extreme thermal gradients on the wafer, and thus damage. In addition, processing is “metastable.” The physical results of processing can be undone by later even modest heating of the wafer for other processes.
2. Additional process steps are required to lay down absorbing layers that diffuse the impact of the laser energy. These layers need to be removed before further processing.
3. Assuming it is even possible to make a production laser tool, the technology will be very expensive – probably twice as costly as current RTP tools, with lower throughput.

Flash Assist RTP (fRTP) has been developed by Vortek to provide “laser like” performance without disadvantages such as integration problems and excessive costs. In simple terms, it is an augmented form of iRTP, where a wafer heated to an intermediate temperature of several hundred degrees received a large pulse, or flash of energy to the entire upper surface. The flash is of the order of a millisecond long.