In the days of the dinosaurs, there was the batch furnace. The user places quartz boats of wafers on a sliding arm and pushes them into the tubular furnace to be processed. This was to anneal the wafers and relieve the crystalline stresses. This method wasn¡¯t very thermally uniform and the thermal exposure of devices would be too long. This allowed the development of the single wafer thermal processing. Single wafer rapid thermal processing (RTP) is widely used today as the preferred method for:
Ion implant activation
Oxidation and Nitridation
Crystallization and Densification
Other heat treatment processes
Vacuum RTP systems are a relatively new machine. Actually, they are a strip-down version of a RTCVD. Most vacuum RTP systems being produced today started not more than 10 years ago.
Vacuum RTP systems are inherently more complex and larger than their atmospheric RTP counterpart. They need vacuum control systems to control the vacuum in the process chamber and possibly the load-lock. They also need a vacuum pump and the facilities to accommodate the vacuum system.
If using a vacuum RTP system for R&D, the process cannot be transferred to production. To date, there are no vacuum RTP systems for the mass production of wafers. There are, however, cassette-to-cassette RTCVD production systems; this is much more machine than is needed. These systems are inherently slow: pump?down, process, vent, pump-down, process, vent, etc. Each pump-down and vent can take from 10 to 60 seconds or more. This can take an additional 25 minutes to process a cassette of 25 wafers. One third of the wafer¡¯s time out of the cassette is for pump-down and venting. Therefore, the vacuum RTP system can only be used for low volume runs or the fab has to purchase multiple systems, thus, taking up additional floor space. In production, ¡°time is money¡± and ¡°floor space is expensive¡±.
The cost of a vacuum RTP system is greater in the short term and in the long term. In purchasing a RTP system, the cost of the system must be considered. There is basically no difference between a vacuum RTP system and an atmospheric RTP system. They both can have a pyrometer and/or thermocouple, gas flow is controlled by MFC, and both have an air-tight chamber. The only difference is a vacuum RTP system has to have a vacuum system and be designed to withstand ambient atmospheric pressure of 14.7 psi.
The vacuum RTP system has a quartz window that allows the radiant heat from the tungsten-halogen lamps into the process chamber to heat the wafer. This window has to be cleaned periodically. It can only be cleaned about 3 times before it has to be replaced. It is recommended to clean this window about once every month. If the process is dirty and out-gases, the quartz window has to be cleaned more frequently. This becomes a very expensive disposable item, in excess of $300 each.
When the quartz window has to be cleaned, the chamber should also be cleaned. A vacuum RTP system does not have a self-cleaning ability like a PECVD. The chamber has to be removed and cleaned. The down-time of the vacuum system would be from several hours to several days.
The vacuum RTP system, in addition, consists of gate valves, throttle valve, purge valve, pressure manometer, vacuum pump, and vacuum lines. All of these items are an additional cost when compared to an atmospheric RTP system. Then the vacuum pump oil needs to be changed periodically. This oil is very expensive, $200-$300 per liter.
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