Formation of polycrystalline silicon films on glass substrates at low-temperatures by a direct negative Si ion beam deposition system

Formation of polycrystalline silicon films on glass substrates at low-temperatures by a direct negative Si ion beam deposition system

Journal of Crystal Growth 191 (1998) 718—722 Formation of polycrystalline silicon films on glass substrates at low-temperatures by a direct negative ...

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Journal of Crystal Growth 191 (1998) 718—722

Formation of polycrystalline silicon films on glass substrates at low-temperatures by a direct negative Si ion beam deposition system D.J. Choi!, Y.H. Kim!, D.W. Han!, H.K. Baik!,*, S.I. Kim" ! Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, South Korea " SKION Corporation, 612 River Street, Hoboken, NJ 07030, USA Received 27 February 1998

Abstract We have constructed a direct metal ion beam deposition (DMIBD) system, which consists of a primary cesium ion source and a negative ion source. The Si~ ion current and deposition energy can be controlled independently and precisely, which are the major advantages of DMIBD system. Poly-Si films were deposited on a glass substrate at a temperature of 200—500°C with ion beam energy from 10 to 100 eV by a direct metal ion beam deposition (DMIBD) system. The deposition of poly-Si films at low temperatures can be explained by a kinetic bonding process. Transmission electron microscope images show the capability of grain size control by adjusting the ion beam energy. The resistivity is very low and it is also considered that in situ doping was performed during the deposition of the silicon films by this technique. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 68.55.!a; 68.55.Eg; 73.60.Gx Keywords: Polycrystalline silicon thin films; Metal ion beam deposition; Negative ion beam

1. Introduction Polycrystalline silicon (poly-Si) formation has been intensively investigated because of its wide applications such as TFT-LCD and solar cell. So far, most poly-Si films widely used in semiconductor device technology, have been deposited by

* Corresponding author. Fax: #82 2 312 5375; e-mail: [email protected]

chemical vapor deposition (CVD) which requires processing temperature above 600°C [1]. This temperature is higher than the glass transition temperature, so the glass substrate cannot be used to deposit the silicon films in the CVD process. Ion beam assisted deposition (IBAD) has been used to deposit the crystalline silicon films at a low temperature. In the IBAD process, the kinetic energy of ion beams can be transferred to the bonding process only when the ion beam and deposition particles have coherence in both time and geometry.

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 7 0 - 4

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However, the bombarding energy is either not effective or the energy is dissipated very locally. For that reason, the increase of ion beam power in the IBAD process is not effective in enhancing the thin film properties. Direct metal ion beam deposition (DMIBD) is currently being studied as a new approach to the synthesis and the modification of materials. The major difference of DMIBD to the conventional CVD or IBAD techniques is that the thin film formation energy of DMIBD is supplied by a controlled ion beam energy (10—100 eV), which is energetic compared to thermal energy [2]. In the DMIBD particles of the materials being deposited bombard the substrate in the form of ions and are deposited with kinetic energy. The energy and flux of the ion particles can be independently and precisely controlled to obtain the optimum condition, so using the DMIBD the materials can be deposited at a much lower processing temperature than used for conventional chemical or physical vapor deposition techniques. In DMIBD the primary cesium ions sputter the silicon target and produce negative silicon ion beams. Because of low surface charge up voltage and an internal-potential energy, negative-ion beam deposition is a prospective technology for both application and research. So DMIBD has been used in the formation of a-D films, formation of carbon nitride films, surface treatment for the diamond cold cathode, and surface modification for diamond epitaxial films [3]. In addition, silicon doped with boron or arsenic is used as a target material in DMIBD, so in situ doping can be obtained during the deposition of the film. Dopant atoms are ionized at the same time as the silicon atoms and accelerated into the substrate with kinetic energy. Thus, these dopants also participate in the atomic bonding formation process on the substrate and are located at the substitutional sites. So the post activation process can be eliminated. We designed the ion source and developed a direct metal ion beam deposition system. The silicon films were deposited using the DMIBD at various deposition energies and temperature. We investigated the silicon film properties in a wide range of deposition energies and temperatures. These results are presented here in detail.

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2. Direct metal ion beam deposition (DMIBD) system A DMIBD system consists of a primary cesium ion source and a negative ion source (Fig. 1). The primary cesium ion source developed by Kim et al. [4,5] consists of a cylindrical solid-state cesium ion source heated by a tungsten filament and a Piercetype electrode system. Solid-state cesium ion source is alumino-silicate based zeolite which contains cesium. Cesium is stably stored in this material and can be extracted by applying an electric field across the electrolyte. Since the ion gun requires no gas supply, it can operate in high vacuum using a moderate speed pumping system without differential pumping and associated hardware. The primary Cs` ion currents are independently controlled by bias voltage. The bias voltage is applied across the cesium solid electrolyte source and is supplying the cesium to the emitter surface of the Cs` ion gun. The important feature for Si~ ion source is that the primary Cs` ion currents can be controlled in a manner independent of the extraction potential, which in turn, controls Si~ ion currents without changing the extraction potential. This feature is very important because the extraction potential of

Fig. 1. Schematic diagram of the direct metal ion beam deposition system.

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the Si~ ion beam is correlated to the deflection potential of Cs` ion beam. Thus, we can control Si~ ion beam currents independently by controlling the primary Cs` ion beam current which can be controlled by the bias voltage. The operational details of the Cs` ion gun are described in detail elsewhere [6,7]. Electrodes of the Si~ ion source are designed by PC based ion trajectory codes, “SIMION” [8]. Fig. 2 shows the ion trajectories as computed with the SIMION code. The geometry of the extraction electrode (top electrode) and the beam forming electrode (bottom electrode) of Si~ ions are designed in a way that the electrodes are to extract the collimated Si~ ion beam as well as the deflection of

the primary Cs` ion beam to the silicon target. Thus, the extracting field (potential difference between the extraction electrode and the beam forming electrode) is determined by the necessary deflection field for the primary Cs` ion beam to be properly aimed at the silicon target as seen in Fig. 2. The deposition energy of Si~ ion beam is independently controlled by the energy control electrode and the Si~ ion beam current is controlled by the primary Cs` ion current which is controlled by the biasing supply current independently from the electrode potential of the ion source. The independent controllability of Si~ ion current and deposition energy is one of the major advantages of the DMIBD system. So DMIBD is a most prospective system for studying the effect of ion energy and flux on the properties of films.

3. Experimental details The silicon films were deposited by Si~ ion beam deposition using the DMIBD system. The energy of the deposited negative ion beam was varied from 10 to 100 eV. The thickness of films was 2000—5000 A_ . The base pressure and the working pressure were 1]10~7 and 5]10~6 Torr, respectively. The deposition temperature was 500°C and the substrates were Corning 2947 glass. The target for Si~ ion beam is B doped silicon single crystal with a (1 0 0) plane and the resistivity of the target was 4—10 ) cm. Transmission electron microscopy was used to investigate the microstructure of silicon films. Cross-section TEM specimens were prepared by tripod polishing and low angle ion milling. The specimens were examined in a Philips CM 30 electron microscope at the acceleration voltage of 300 kV. The resistivity of an as-deposited film was measured by the four-point probe technique.

4. Results and discussion Fig. 2. Computer calculation of the deflection of the primary Cs` ion beam and emission of the secondary Si-ion beam (SIMION code).

Figs. 3 and 4 show the bright field image of poly-Si films deposited at 500°C with Si~ ion beam energy of 10 and 100 eV, respectively. It was

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Fig. 3. TEM image of poly-Si grown at 500°C with Si-energy of 10 eV.

Fig. 4. TEM image of poly-Si grown at 500°C with Si-energy of 50 eV.

demonstrated that poly-Si film could be grown at a substrate temperature of 500°C, which is below the glass transition temperature. Figs. 3 and 4 also illustrate the capability of grain size control by adjusting the silicon ion beam energy from 10 to 100 eV. Poly-Si film widely used in semiconductor device technology, has been deposited by Chemical Vapor Deposition (CVD) which requires processing temperature above 600°C. In CVD the silicon atoms, whose thermal energy is greater than the formation barrier energy, approach the position corresponding to the lowest interatomic potential. But at the temperature below 600°C, the thermal energy is not sufficient to overcome the formation barrier energy. So the silicon films are amorphous. But the silicon films deposited by DMIBD are polycrystalline even below 500°C and this can be explained by the kinetic bonding and negative ion beam process [9]. Recently, Ishikawa suggested the kinetic-bonding process. In the case of an atom which has a kinetic energy comparable to the

atomic-bonding energy, it can easily overcome the formation barrier and approach a position where an interatomic potential is equal to the kinetic energy. From this position, the atomic-bonding formation process starts and the atom settles down at a position corresponding to a minimum interatomic potential. In the DMIBD technique, in addition to the thermal energy, silicon atoms are deposited with kinetic energy. This provides enough surface mobility to diffuse into an equilibrium position, even at low temperatures, and the poly-Si films can be formed. By changing the kinetic energy of silicon ions, it is possible to control the structure and properties of silicon films. The conventional ion beam deposition techniques use the positive ion beams. But in DMIBD the negative silicon ion beams are formed. In the low kinetic energy range of positive ion beam deposition, an internal-potential energy, i.e. ionization potential, is comparable to the kinetic energy, so that the two effects induced by internal potential energy and kinetic energy are mixed in material synthesis.

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However, an internal potential energy of a negative ion, i.e. electron affinity, is very low and absorbable, so that the effect of kinetic energy can be independently investigated. A negative silicon ion has an extra electron with a weak binding energy, i.e. electron affinity, and easily releases it upon colliding with a surface of the substrate. Therefore, because of the precisely controlled kinetic energy and negative polarity of silicon ion beams, the poly-Si films can be grown even at low temperatures. The resistivity of an as-deposited film was measured by the four-point probe technique. Table 1 shows the electrical resistivity of the silicon films. The resistivity is very low. It is because in situ doping is obtained during the deposition of the film. Dopant atoms usually locate at the interstitial sites after conventional implantation doping process. Normally post-annealing is required to switch the location of dopants from the interstitial to substitutional sites. In the DMIBD technique, silicon doped with boron is used at a target material. Dopant atoms are also ionized at the same time as the silicon atoms and accelerated into the substrate with kinetic energy. These dopants participate in the atomic bonding formation process on the substrate and are located at the substitutional sites in the silicon matrix. But now the concentration of the boron atoms is under experiment using SIMS.

5. Conclusions We have constructed a direct metal ion beam deposition (DMIBD) system, which consists of a primary cesium ion source and a negative ion source. The Si~ ion current and deposition energy can be controlled independently and precisely, which are the major advantages of DMIBD system. Poly-Si films could be successfully grown on a glass substrate at a temperature below 500°C with ion beam energy from 10 to 100 eV. It can be explained by a kinetic bonding process and TEM micrographs illustrate the capability of grain size control by adjusting the ion beam energy. The resistivity is very low and it is also considered that in situ doping was performed during the deposition of the silicon films by this technique.

Acknowledgements This study was supported by the academic research fund of the Ministry of Education, Republic of Korea under contract 96-33-0118.

References Table 1 Electrical resistivity of the poly-Si film deposited at various temperatures and Si~ ion beam energies Substrate temperature (°C)

Si ion beam energy (eV)

Electrical resistivity () cm)

200 200 200 300 300 300 400 400

10 10 10 50 50 50 100 100

0.082 0.082 0.019 0.019 0.040 0.011 0.021 0.048

[1] H. Windishmann, J.M. Cavese, R.W. Collins, MRS Proc. 47 (1985) 187. [2] Y. Park, Y.W. Ko, M.H. Sohn, S.I. Kim, MRS Proc. 396 (1996) 623. [3] Y.W. Ko, Y.O. Ahn, M.H. Sohn, Y. Park, S.I. Kim, presented at 1995 MRS Fall Meeting, Boston, MA, 1995. [4] S.I. Kim, M. Seidl, J. Vac. Sci. Technol. A 7 (3) (1989) 5671. [5] S.I. Kim, M. Seidl, J. Appl. Phys. 67 (6) (1990) 2704. [6] S.I. Kim, M. Seidl, MRS Proc. 135 (1989) 95. [7] S.I. Kim, Y.O. Ahn, M. Seild, Rev. Sci. Instrum. 63 (12) (1992) 5671. [8] D.A. Dahl, S.E. Delmore, SIMION Version 4.0, Idaho Natl. Eng. Lab. EG&G Idaho Inc. [9] J. Ishikawa, in: P. Vincenzini (Ed.), New Horizons for Materials, 1995, p. 399.