Issue



Advanced devices using low-temperature NiSi formation


09/01/2003







Low-temperature (<500°C) formation of electrodes with low contact resistance is one of the key technical challenges in fabricating advanced devices with small features — particularly for conventional lamp-based RTP systems.

Among the many silicides, nickel silicide is considered to be one of the most suitable materials for deep submicron, self-aligned silicide (salicide) applications. Key benefits of nickel silicide over other silicides include: lower reaction temperature for silicide formation; less linewidth sensitivity; less Si consumption; lower specific resistivity; and smaller contact resistance [1, 2]. Additionally, there is reduced risk for bridging due to very slow Ni diffusion during formation. As silicide films have lower morphological and thermal stability >550°C, it is easy to form the higher-resistivity NiSi2 phase (which has a rough surface) of the resulting silicide films if the annealing temperature during silicide formation is not precisely controlled.

Nickel silicide has at least six stable phases (Ni3Si, Ni31Si12, Ni2Si, Ni3Si2, NiSi, and NiSi2) at room temperature among eleven phases in the Ni-Si phase diagram [3]. Studies on nickel silicide formation using isothermal anneals or rapid thermal anneals (RTA) mainly reveal the sequential phase transition of Ni2Si, NiSi, and NiSi2 from 200–700°C. Several nickel silicide phases are often found in resulting films. Since each phase has different crystallographic and electrical properties, it is important to form nickel silicide films with a homogeneous phase by controlling the thermal environment of the wafer.

Reflectivity changes during nickel silicide formation as well as pattern-induced localized heating effects make the formation of homogeneous-phase nickel silicide films difficult using lamp-based RTA systems. In a hot wall system (furnace-like system), the wafer is the coldest object in the process chamber at all times. As long as the temperature of the process environment is uniform and the thermal mass is sufficiently large, temperature uniformity within the wafers and wafer-to-wafer temperature repeatability should not be a problem. Temperature repeatability is less dependent on local and global emissivity distribution on the wafer, because uniform heat is supplied from surroundings. Temperature control requirements in the range of 200~550°C make conduction and convection heating of the wafer more efficient than radiation. Selection of appropriate annealing equipment for low-temperature (200~550°C) applications is a very important task from the advanced device manufacturer's point of view.


Figure 1. Sheet resistance change of Ni films (9nm thick) on Si as a function of annealing temperature.
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Stacked annealing oven

A hot plate-based, stacked oven was designed for low-temperature-annealing applications in the temperature range of 100~550°C and is capable of processing five wafers simultaneously under controlled process gas environments [4, 5]. The design of the stacked hot plates — which are made of aluminum and have an embedded heater for temperature control — allows gradual heating of wafers for low-temperature-annealing and baking applications without sacrificing productivity.

Aluminum was chosen because of its thermal stability, high thermal conductivity, and ease of machining. The individual hot plates have three standoffs to accurately maintain the distance between the wafer and the hot plate surfaces. The wafers are heated by natural convection and conduction through ambient gas as well as by radiation. Hot plate temperature and process gas pressures are controlled and accurately determine the wafer temperature profile. [6, 7]

NiSi formation

Sputtered Ni films (9nm thick) on Si wafers were annealed in the temperature range of 200~550°C to form nickel silicide (Fig. 1). The sheet resistance of nickel silicide was measured before and after annealing. Ni2Si formation was observed as low as 200°C. The sheet resistance was increased from 25.8–36.0Ω/sq. after annealing at 200°C for 5 min due to Ni2Si formation. As the annealing temperature increased above 300°C, the sheet resistance sharply decreased to ~10Ω/sq. by forming a low-resistivity NiSi phase. The uniformity change before and after annealing was also <1%, 1σ. At 600°C, a high-resistivity, NiSi2 phase started to form; the film broke up and peeled off at nickel silicide formation temperatures >650°C.

Spectral reflectance of nickel silicide films formed at different temperatures in the wavelength region of 220~820nm is shown in Fig. 2. As the annealing temperature increases, the reflectance of nickel silicide films in the short wavelength region (250~300nm) increases monotonically. There was a very strong correlation between the reflectance and sheet resistance (or stoichiometry) of nickel silicide films. Metal-rich nickel silicide (Ni2Si) formed at low temperatures (<350°C) shows lower reflectance <400nm and moderate reflectance in the 400~800nm wavelength region. As the Ni2Si-NiSi-NiSi2 phase transformation progresses, the reflectance at <400nm in wavelength increases from 0.3 to 0.6. The reflectance between 400nm and 800nm is maximized and stable when the low-resistivity NiSi phase is formed in the temperature range of 350~550°C. Above 550°C, the reflectance suddenly drops as the Si-rich NiSi2 phase starts to mix.

The reflectance change during nickel silicide formation in lamp-based annealing makes it more difficult to control wafer temperature for repeatable process results in a mass production environment. The stacked annealing system provides production-worthy process solutions for low-temperature annealing applications such as nickel silicide formation, copper annealing, and spin-on-dielectrics, and photoresists [4–7].


Figure 2. Spectral reflectance of nickel silicide films formed at different temperatures.
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Surface roughness/thermal stability

The surface roughness (Ra) of nickel silicide films was measured using scanning probe microscopy (SPM). Nickel silicide films with a TiN capping layer generally showed lower Ra values (0.10~0.13nm) than those values (0.19~0.35nm) for nickel silicide films without a TiN capping layer. The TiN capping layer was found to be effective in suppressing surface roughening during nickel silicide formation. An SPM image of a nickel silicide film with a TiN capping layer is shown in Fig. 3. Lower sheet resistance of 8.7Ω/sq. and surface roughness of 0.13nm were obtained after 5 min of annealing at 450°C.


Figure 3. Surface roughness of nickel silicide film formed at 450°C.
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Nickel silicide films formed at <450°C resulted in smooth surfaces. Thermal treatment at higher temperatures (>500°C) not only increases surface roughness, but also increases the chance of forming a high-resistivity nickel silicide phase in NiSi films. High-quality, low-resistivity nickel silicide (NiSi) can be formed at temperatures between 350~550°C without affecting the electrical characteristics obtained by previous thermal process steps.

Conclusion

A production-worthy NiSi formation process (200~500°C) was shown using a stacked hot plate-based, low-temperature annealing system. The sheet resistance and reflectance of nickel silicide showed good correlation as a function of annealing temperature.

Low-temperature annealing and/or a TiN capping layer were effective in forming nickel silicide with smooth surface conditions.

Tomomi Murakami, WaferMasters Service Factory, Kumamoto, Japan; Véronique Carron, CEA-DRT – LETI/DTS – CEA/GRE, Grenoble, France; Woo Sik Yoo, WaferMasters Inc., San Jose, California

References

  1. Q. Xiang, Abst. of 201st Electrochem. Soc. Meeting, 2002-1, 583, 2002.
  2. Y. Hu, S.P. Tay, J. Vac. Sci. Tech. (B), 19, 2, 376, 2001.
  3. C. Lavoie et al., Electrochem. Soc. Proc., PV2002-11, 455, 2002.
  4. W.S. Yoo et al., European Semi., 129, April 2001.
  5. W.S. Yoo et al., SST, 44, No. 6, 152, 2001.
  6. W.S. Yoo, T. Fukada, Electrochem. Soc. Proc., PV 2000-9, 355, 2000.
  7. T. Murakami, T. Fukada, W.S. Yoo, Electrochem. Soc. Proc., PV 2002-11, 253, 2002.

Acknowledgments

The authors thank Benoit Froment, STMicroelectronics Srl, Crolles, France, for his contributions to this article.

For more information, contact Woo Sik Yoo, [email protected].