by Ed Korczynski, Senior Technical Editor, Solid State Technology
May 20, 2008 – Novellus Systems has released two new variations on its Gamma multi-station sequential processing (MSSP) architecture, targeting high volume memory and logic/foundry fabs.
Kevin Jennings, VP and GM of Novellus’ surface integrity division, in an exclusive interview with WaferNEWS, explained the product evolution that has led to >400 wafers/hour (wph) bulk stripping and >100wph for crusty implanted wafers — and thus to the need for the two types of Gamma tools.
The key to ash rate is generating high-power and high-temperature conditions, he explained. Novellus’ tools can deliver up to 10kW of power, and the company has thermal mass heaters which rapidly raise and lower to deliver fast therms. With independent control of the processing parameters in each MSSP station, the tool can be configured for high-dose implant strip (HDIS) such that two of the five stations are used to break through the tough HDIS crust, followed by bulk removal, final removal, and then clean (see figure).
Novellus claims to have gained some market share to hold a strong #2 position in the $400M-$500M global market for resist ashing, citing data from Dataquest. Recent sales have mainly been of the Gamma hardware platform, and most of these systems are field-upgradable to either of these new-and-improved versions. The single largest sub-market is transistor formation in mega memory fabs. “We’re now seeing fabs with 140k wafer starts/month, and we’ve heard rumors of 200k wafer starts/month fabs,” Jennings told WaferNEWS. “The margins are so thin, these single-part-number lines have to increase volumes to try to make money.” While discussions among logic customers tend to focus more on the technology, “when you talk to a mega-memory fab it’s all about productivity and cost-of ownership, in that order,” he said. “At 400wph its’ a FOUP every few minutes.”
Front-end of line (FEOL) is probably over 3/4 of the total ash market. Logic customers are using higher-dose implants FEOL, which creates more crust, more residues, and more residues-post-strip. Despite the challenges in removing tougher resist, the specification for silicon loss during stripping is extremely challenging. “Silicon loss is key for the technology driven logic and foundry segments; they’d ideally like zero silicon loss,” said Jennings. For these tough processes at an advanced foundry, the GxT version provides wider temperature range heaters, and a new gas delivery system which allows any gas to be delivered to any station (within limits of safety and reason). A strip process at a very advanced foundry it might run at 100-150wph in this tool. Looking at end-point such as clearing-crust, with computing power and control over heating, each station is almost a mini-chamber, Jennings noted.
The maximum throughput version — the Gamma G400 — is claimed to be reliable enough to easily process over 120,000 wafers/month, Jennings said. Mechanical handling limits are at least 420wph, but unlike single-wafer tools the Novellus MSSP can still achieve processing targets with just 5-6 sec. at each of the five stations (indexing in just 2-2.5sec). Another productivity trick is keeping the RF plasma on continuously such that some ashing occurs during wafer transport.
The Gamma Express will continue to be supported, but the company expects that most new fabs will order either the G400 or the GxT. Jennings noted that the company has run >250K wafers though the G400 lab tool, so it is ready for prime time. The GxT has already shipped to multiple advanced foundry customers, winning business with claimed 30%-100% throughput improvements over the competition. — E.K.