Metal Silicides by Ion Implantation with a MEVVA Ion Source

The progress in integrated-circuit (IC) technology since the late 1950s has enabled the realization of many brilliant electronic systems. These systems, such as computer equipment and peripherals, telecommunication devices, consumer electronics, and industrial electronics, have significantly improved the quality of life.

The fact that electronic equipment has increased in functionality and decreased in cost over the years is largely due to the successful miniaturization of semiconductor devices in IC chips. Such miniaturization is in turn dependent on the metallization process in IC fabrication.

Metallization, which is the process to form metal or metal-like layers in IC structures, provides electrical connection between internal devices as well as contacts of the IC to the outside world, and has great impact on IC performance. When the device dimensions go down to the deep-submicron domain, the delay time attributed to interconnections will gradually take over as the dominant factor in the operating speed of the IC. The delay time will be reduced if the electrical resistance of the interconnections is lowered. Lower resistance will also lead to less heat dissipation. This allows devices to be packed more densely, hence, the size of the IC can be minimized.

To enhance the continued evolution of smaller and smaller devices in VLSI/ULSI (very large-scale integration/ultra large-scale integration) circuits, new and better metallization schemes must be introduced. The realization of such schemes requires a large R & D effort to find out the right materials and the right processes. The results of a research project conducted by Prof. S.P. Wong and Prof. Ian H. Wilson in this area, supported by a grant of HK$568,000 from the Research Grants Council, are expected to contribute to the advancement of the metallization technology.

What Are Metal Silicides?

While pure metals have good electrical conductivity, they are not suitable, for different reasons, for VLSI/ULSI applications.1 Metal silicides, on the other hand, are compounds formed by metals and silicon. Many of them exhibit metallic conduction behaviour and have attracted much attention in the past three decades in the context of metallization applications because of their low and metal-like resistivity, high temperature stability, and high electromigration resistance.1

What Is Ion Implantation?

Though metal silicides have already been employed in a number of metallization schemes in the production of modern IC, the formation of silicides by ion implantation is a relatively new technique that has been under investigation only during recent years.

Ion implantation is an indispensable technique in modern IC industry to introduce impurity atoms into semiconductors to change their electrical conductivity in a controlled manner. It is a technique by which electrically charged ions are accelerated under the action of an electric field and implanted into the solid target. The advantages of forming metal silicides by ion implantation include precision in the number and depth of the metal ions introduced, excellent reproducibility and uniformity, and the possibility of forming particular silicides and silicide structures that are very difficult or impossible to produce using conventional processes.

A New Ion Source: MEVVA

In the mid-80s, Brown et al.2 in Lawrence Berkeley Laboratory developed a new type of metal ion source, namely the metal vapour vacuum arc (MEVVA) ion source. The MEVVA source makes use of the principle of vacuum arc discharge between the cathode and the anode to create a dense plasma from which an intense beam of metal ions of the cathode material is extracted. This new metal ion source operates in a pulse mode. A broad beam of high peak beam current of the order of about one ampere and a mean beam current of tens of milli-amperes can be obtained. Due to its high-current and broad-beam capabilities, the MEVVA ion source is employed to solve the throughput problem arising from the high implantation dose required to form silicides.

Fig. 1 The MEVVA implantation system in the Department of Electronic Engineering

Forming CoSi2by MEVVA Implantation

Prof. Wong and Prof. Wilson first chose to study the formation and properties of CoSi2layers by MEVVA implantation under various conditions, using a number of analytical techniques including atomic force microscopy, cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry, X-ray diffraction, spectroscopic ellipsometry, electrical resistivity measurements, and Hall effect measurements. CoSi2is known to have the lowest electrical resistivity among all the silicides at room temperature. And with its small lattice mismatch of -1.2% with silicon, it can form a good-quality epitaxial layer in silicon.

The researchers have shown that a continuous buried single-crystalline CoSi2layer in silicon can be formed with appropriate thermal treatment. They have also studied in detail3-7 the dependence of the structural and electrical properties of the buried CoSi2layers on the processing conditions. The use of MEVVA ion source in their project has enabled a detailed study of the ion beam synthesis of metal silicides. Prof. Wong said, 'Such a study is very difficult, if not impossible, by conventional ion implantation because of the high dose and long time required for the sample preparation.'

Fig. 2 Typical atomic force micrographs showing the surface morphology of two as-implanted samples of Co implanted Si prepared at an extraction voltage of 70kV to a dose of 2 x 1017 ions/cm2 at substrate temperatures of (a) 210oC, and (b) 760oC

Other Silicides and Their Application Possibilities

In addition to CoSi2, Profs. Wong and Wilson have started studying other silicides formed by MEVVA implantation, including TiSi2and FeSi2. While TiSi2is similar to CoSi2and holds interest for metallization applications, b-FeSi2, a form of semiconducting FeSi2with a bandgap of 0.85eV, is promising for Si-based integrated optoelectronic device applications.8 The researchers have also studied the magnetic properties of Fe- implanted Si samples by MEVVA implantation and discovered a novel positive magnetoresistance effect.9

Given these results, Prof. Wong believes that MEVVA implantation is a technique worth further investigation. The formation of metal silicides will be increasingly important in IC technology of the next generation. It also has high potential for novel device applications. In fact, it has recently been proposed that metal silicides may provide a new approach to silicon nanoelectronics surmounting the limit of conventional silicon technology.10

Fig. 3 A cross-sectional transmission electron micrograph showing a buried CoSi2 layer in silicon formed by MEVVA implantation

Prof. S.P. Wong obtained his B.Sc., M.Phil., and Ph.D. in physics from The Chinese University of Hong Kong. He joined the University's Department of Electronics (now renamed Department of Electronic Engineering) in 1985 as lecturer and was promoted to senior lecturer rank in 1992. Prof. Wong's major research interest is in electronic materials and technology, especially in relation to ion implantation. He is a member of the Materials Research Society, USA and a founding council member of the newly established Hong Kong Materials Research Society.
Prof. Ian H. Wilson received his B.Sc. and Ph.D. degrees from the University of Reading, UK, in 1962 and 1966 respectively. Before joining The Chinese University as professor of electronic engineering in 1991, he was distinguished visiting professor to the Physics Department of Arizona State University, USA. His current research interest includes ion-assisted deposition of metal, dielectric and semiconducting thin films, scanning probe microscopy of electronic materials, and the fabrication of new devices by epitaxy and ion implantation. Prof. Wilson is a fellow of the Institute of Physics, a Chartered Engineer, and the founding president of the Hong Kong Materials Research Society.

Other Members of the Research Team
  • Mr. Qicai Peng and Mr. W.S. Guo, Ph.D. students of Prof. S.P. Wong
  • Prof. J.B. Xu and Dr. W.Y. Cheung, Department of Electronic Engineering, CUHK
  • Prof. S.K. Hark, Department of Physics, CUHK
  • Dr. N. Wang and Dr. K.K. Fung, Physics Department, HKUST
  • Dr. R. Morton and Prof. S.S. Lau, Department of Electrical and Computer Engineering, University of California at San Diego, USA

Reference
  1. Murarka, S.P., Silicides for VLSI Applications, New York: Academic Press, 1983; Metallization: Theory and Practice for VLSI and ULSI, Boston: Butterworth-Heinemann, 1993.
  2. Brown, I.G., Galvin, J.E. & MacGill, R.A., Appl. Phys. Lett., 1985, 47, p.358; Brown, I.G., Galvin, J.E., Gavin, B.F. & MacGill, R.A., Rev. Sci. Instrum., 1986, 57, p. 1069.
  3. Peng, Q., Wong, S.P., Wilson, I.H., Wang, N. & Fung, K.K., Thin Solid Films, 1995, 270, p. 573.
  4. Peng, Q. & Wong, S.P., MRS Symp. Proc., 1996, 402, p. 487.
  5. Peng, Q., Wong, S.P., Xu, J.B. & Wilson, I.H., MRS Symp. Proc., 1996, 396, p. 763.
  6. Wong, S.P., Peng, Q., Cheung, W.Y., Guo, W.S., Xu, J.B., Wilson, I.H., Hark, S.K., Morton, R. & Lau, S.S., MRS Symp. Proc., 1997, 438, p. 307.
  7. Peng, Q., Ph.D. Thesis, 1997, The Chinese University of Hong Kong.
  8. Derrien, J., Berbezier, I., Ronda, A. & Natoli, J.Y., Appl. Surf. Sci., 1996, 92, p. 311.
  9. Wong, S.P. & Cheung, W.Y., MRS 1997 Spring Meeting, paper M1.11, 31st March - 4th April 1997, San Francisco, USA; Wong, S.P. & Cheung, W.Y., MRS Symp. Proc., 1997, 475.
  10. Tucker, J.R., Wang, C. & Shen, T.C., Nanotechnology, 1996, 7, p. 275.