EXECUTIVE OVERVIEW

Currently, nitride films formed using DCS and ammonia (NH₃) have reached the lower limit temperature boundary, which is ~630°C. At temperatures <630°C, low deposition rates and large amounts of ammonium chloride condensate-related particles makes the DCS nitride-based processes non-manufacturable. With even lower temperature requirements at the 45nm node and below, new chemistries are necessary to provide high quality dielectric films to maintain fundamental SiN film characteristics [1-3]. A new silicon nitride film process that meets low thermal budget requirements for sub-45nm devices is presented.

With the scaling of devices at the 45nm node and below, thermal budgets play an important role in maintaining good quality film properties in low pressure chemical vapor deposition (LPCVD) of SiN films. Such SiN films have been widely used to deposit spacer, etch stop layer, oxy-nitride-oxide (ONO) stacks or oxide masks for front-end-of-line (FEOL) applications [1, 2]. To achieve the lower thermal budgets needed for sub-45nm IC manufacturing, a silicon nitride (SiN) film was developed in a 300mm variable batch furnace. Deposition temperatures for the new nitride process were in the range of 510-630°C. The new film is a chlorine-free process that eliminates ammonium chloride (NH₄Cl) particle formation, which is common in dichlorosilane, SiH₂Cl₂ (DCS) and hexachlorodisilane (HCD) nitride processes. Wet-etch testing shows that the new nitride film, called SATIN, is a denser film than other conventional SiN films. Consistent process performance of this film is shown via passive data collection (PDC) data. An in situ dry-clean process was developed to maintain low and stable particle performance, while minimizing the frequency of costly hardware wet clean. Using the same precursor, oxide film is being developed for dual spacer oxide-nitride applications.

Background

Until recently, there have been two alternatives for “low temperature” nitride applications: hexachlorodisilane (Si₂Cl₆ or HCD) and bis- (tertiary-butylamino) silane (BTBAS). There are underlying process challenges associated with both HCD and BTBAS processes. The HCD process chamber and exhaust are plagued with the formation of NH₄Cl, which directly correlates to a large amount of particle formation. BTBAS has potential issues with film cracking for thickly deposited films. In addition, the BTBAS film has a propensity to incorporate significant amounts of hydrogen (H₂) [4]. Both HCD- and BTBAS-based nitrides result in various manufacturing challenges, including low deposition rate, high film nonuniformity, poor film conformality, and prohibitive chemical costs. In addition, HCD and BTBAS require frequent in situ dry-cleans to keep particle counts within specification, which can greatly reduce system availability and productivity, thus increasing overall cost of ownership.

Figure 1. HF (100:1) solution wet etch rate comparison of the new process with a DCS nitride film.Click here to enlarge image

The proprietary process developed by Aviza Technology offers an alternative for low temperature SiN processing targeted for 45nm and below pad, spacer, and liner applications [2]. This process is able to meet low thermal budget requirements and provide a SiN film-combating the inherent issues that most traditional nitride films face. As a chlorine-free precursor, the new process eliminates NH₄Cl formation, which is a source of large particle formation.

Experiment

A 300mm variable batch, vertical furnace system was used to deposit the SiN film, using the new precursor. The gases were introduced via a quartz injector inside a chamber designed to allow gases to flow parallel across the wafer surface from the injector to exhaust slot-simulating a single-wafer environment in a batch wafer process. The chamber has five temperature zones monitored by thermocouples and allows 25 to 100 wafer batch runs using the same process.

The new nitride process was optimized for the study. Various film thicknesses at different temperature set points were deposited using this process. Wet-etch rate test results were compared with DCS SiN for a film quality study. PDC for particles and film thickness uniformity was performed. Furthermore, an in situ dry-clean was developed and implemented during PDC to maintain consistent particle performance.

Results and discussion

To study the SATIN nitride film quality, Fig. 1 shows the wet-etch rate data comparison between the new film and DCS SiN films. The 100:1 HF solution study indicates that the new film is denser because of the lower wet-etch rate in comparison with a DCS SiN film deposited at similar temperatures. The new nitride can be deposited at 630°C or lower without compromising film characteristics. The wet-etch rate at 550°C is < 8.5Å/minute.

While other SiN films show a high concentration of chlorine, carbon or hydrogen, the new SiN film is chlorine- and carbon-free with a low H concentration. Chlorine-free SiN film eliminates NH₄Cl formation and pitting or roughening of the underlying or adjacent silicon [5]. A carbon-free film also prevents wet-etch and conductivity issues due to carbon diffusion into the gate electrode. Low and consistent hydrogen concentration enables a stable nitride etch performance.

Figure 2. SATIN nitride PDC data collection. Process repeatability is demonstrated at two different process temperatures.Click here to enlarge image

Figure 2 shows the PDC performance from the optimized process at various temperatures at 550°C and 630°C. Deposition thickness and uniformity were stable and met the process requirement at lower thermal budget. Due to the elimination of chlorine, the particle adders were <50 at 0.12µm or above. The low particle performance level for a long period of time due to the absence of NH₄Cl formation reduces the requirement of frequent hardware wet cleans associated with BTBAS or DCS SiN deposition.

Figure 3. SATIN nitride in situ NF3 dry clean particle performance.Click here to enlarge image

An in situ dry-clean was also developed and implemented to maintain consistent particle performance while minimizing the hardware wet-clean frequency. An under-etch approach is used to minimize quartz over-etch. Figure 3 shows the particle performance and quick recovery after multiple NF₃ cleans without a coating step.

Using the same precursor, oxide film can be deposited for dual spacer oxide-nitride applications. Deposition rates of 1Å/minute and 5Å/minute were achieved at 550°C and 600°C, respectively, which provides an opportunity to perform in situ oxide/nitride deposition in the same furnace without breaking the vacuum for better film integrity.

Conclusion

The low temperature (~510-630°C) SiN film characteristics using a proprietary LPCVD process were discussed. Wet-etch rate tests shows that the new film is denser than a traditional DCS-based nitride film. PDC data confirms process and particle performance stability. Particle adders were <50 at >0.12µm due to the elimination of NH₄Cl formation with the chlorine-free nitride process. An in situ NF₃ dry-clean is used to minimize the frequency of hardware cleaning and maintain stable process performance. Furthermore, a SATIN oxide process is being developed that can be deposited within the same chamber for an in situ dual spacer application, thus providing an alternative solution to meet <550°C nitride and oxide applications.

Acknowledgments

The authors would like to thank their colleagues at Aviza Technology, Karl Williams, Sergio Luna, Helmuth Treichel, Thomas Qiu, Billy Cho, Jimmy Nguyen, and Jeff Bailey, for their research activities and simulation work. SATIN is a trademark of Aviza Technology Inc.

References

1. K. Ibrahim, T.Y. Teik, T.W. Teik, V. Umasangar, J.H. Joung, “Robust Silicon Nitride LPCVD Recipe Development,” Proc. ICSE, Penang, Malaysia, pp. 268-269, 2002.
2. M. Shenasa, M. Moninpour, B. O’Toole, Bill Stueve, R. Wilkes, J. Reece, et al., “Optimization of LPCVD Silicon Nitride Process in a Vertical Thermal Reactor: Use of Design of Experiments,” Proc. IEEE/SEMI Advanced Semiconductor Manufacturing Conference, pp. 216-219, 1992.
3. K. Williams, E. Brenniman, J. Su, H. Lee, X. Zhao, J. Sun, “A Variable Batch Size DCS Nitride Process Using Low-temperature LPCVD,” Solid State Technology, pp. 41-44, Aug 2007.
4. J. Gumpher, W. Bather, N. Mehta, D. Wedel, J. Electrochem. Soc., 151 (5) G353-G359, 2004.
5. C. Dussarrat, J-M Girard, T. Kimura, N. Tamaoki, Y. Sato, “The Cl & C-free TRISILYLAMINE (TSA): A Promising Solution For Low-temperature CVD of Silicon Nitride,” 2003 SEMI Technical Symposium, ISBN#1-892568-78-0.

Eddie Chiu is the process manager for the ALD and Thermal Business Unit at Aviza Technology Inc., 440 Kings Village Road, Scotts Valley, CA 95050 United States; e-mail eddie.chiu@aviza.com.

Alex Kolessov is a process engineer at Aviza Technology Inc.

Khalid Mohamed is a process engineer for the ALD and Thermal Business Unit at Aviza Technology Inc.

Jim Su is director of technology for the ALD and Thermal Business Unit at Aviza Technology Inc.