RTP Refines Dielectric Processing
-- Semiconductor International, 6/1/2001
Rapid thermal processing (RTP) has long been an enabling technology for shallow junction formation and contact silicidation. The technology is establishing its role in the production of many dielectric films, including tunnel oxides for non-volatile memory devices and various oxynitride gate dielectrics for advanced CMOS technologies. These are just some of the applications that will be discussed at the ninth annual IEEE RTP Conference, to be held Sept. 25-29 in Anchorage, Alaska. The conference will be preceded by a one-day workshop entitled "Rapid Thermal Annealing of Silicon Implanted Layers: the Manufacturing Recipe Level."A review of the most recent RTP conference papers reveals several interesting developments in dielectric processing using rapid thermal CVD, oxidation, nitridation and annealing. For example, engineers from Texas Instruments Inc. (TI, Dallas) and Applied Materials Inc. (Santa Clara, Calif.) evaluated a remote-plasma nitrided silicon dioxide (RPNO) gate dielectric process that allows the user to start with a relatively thick oxide film (thereby easing process control requirements), and control the equivalent oxide thickness, teq, by varying the degree of nitridation.
In the research study, the engineers grew the oxide layer using either in situ steam generation or standard thermal oxidation and varied nitrogen incorporation in the film by controlling N2 flow rate or N2 and He flow rates, yielding concentrations of 0.3-5x1015 atoms/cm2. The best results showed that nitridation reduced the starting oxide thickness of 32 Å to a teq of 28 Å, with a substantial reduction in leakage and comparable or better thickness uniformity relative to the starting oxide film.
Also targeting the improvement of gate dielectrics, engineers from Infineon (Dresden, Germany) explored the use of pyrogenic steam oxidation as a method of reducing thermal budget when the gate oxide is grown following source/drain and channel implantation steps. The steam-containing ambient (90% H2O) grows a 10 nm film in 15 sec at 1000°C.
The steam process allowed a significant reduction in thermal budget and low electrical defect density values, but initially worse intrinsic breakdown behavior and less stability against carrier injection than dry oxide films. However, the engineers found that a post-oxidation anneal (1000°C) removed the defects effectively, leading to oxides with equal or better quality (characterized by electrical defect density, charge to breakdown and stability against electrical stress tests) than those grown by RTO or furnace oxidation. The engineers recognized a trade-off between oxide quality and reduced thermal budget because a wet oxidation with 1100°C anneal gives about the same diffusion length (thermal budget) as a dry oxidation.
In a very different yet also critical oxidation process, engineers from Atmel Corp. (Colorado Springs, Colo.) analyzed manufacturing issues that arise in the processing of flash memory devices. Using data collected from production or short-loop lots, the engineers processed wafers in either an SVG Thermco TMX 10k horizontal furnace or Applied Materials' RTP Centura system. Tunnel oxides used in flash memory devices, expected to remain in the 60-70 Å range for the next few device generations, must conduct Fowler-Nordheim or HCI (hot carrier injection) current during high-voltage programming and erasing cycles. These oxides are grown using a pre-oxidation clean, oxidation, and post-oxidation anneal, and tested by charge to breakdown (QBD) or time to breakdown (JT).
After growing oxides with a target thickness of 110 Å under varying conditions, they found that the RTO films with light N2O anneal or in situ steam generation compared well with furnace oxides, whereas RTO films with higher nitrogen content (NO or high N2O flow anneal) were degraded. Pure RTO films also did not perform as well as the furnace oxides. Compatibility with LOCOS isolation proves critical, and better active/gate interface characteristics resulted from a 900°C tunnel oxide process than a 1050°C oxide process. The engineers concluded that lower temperature and time of processing leads to better charge-to-breakdown characteristics.
Finally, engineers from IBM Microelectronics (Essex Junction, Vt.) evaluated the replacement of furnace-based processes with RTP as a means of achieving faster cycle time, a particularly important metric to foundry customers. Over a 20-month period, they realized a 6% cycle time reduction on a current-generation CMOS technology, with added benefits of faster response to process deviations, faster yield learning, reduced work in progress and more rapid introduction of new technologies.
- Laura Peters
