Annealing effects in samples of silicon implanted with helium by plasma immersion ion implantation

Annealing effects in samples of silicon implanted with helium by plasma immersion ion implantation

Nuclear Instruments and Methods in Physics Research B 240 (2005) 219–223 www.elsevier.com/locate/nimb Annealing effects in samples of silicon implante...

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Nuclear Instruments and Methods in Physics Research B 240 (2005) 219–223 www.elsevier.com/locate/nimb

Annealing effects in samples of silicon implanted with helium by plasma immersion ion implantation J.C.N. Reis a

a,*

, A.F. Beloto a, M. Ueda

b

Laborato´rio Associado de Materiais e Sensores (LAS), Instituto Nacional de Pesquisas Espaciais (INPE), CP 515, 12245-970 S.J. Campos, SP, Brazil b Laborato´rio Associado de Plasma (LAP), Instituto Nacional de Pesquisas Espaciais (INPE), CP 515, 12245-970 S.J. Campos, SP, Brazil Available online 3 August 2005

Abstract Silicon samples were implanted with helium using plasma immersion ion implantation (PIII). The effects of implantation were analyzed by Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and reflectance measurements before and after PIII, and after annealing (325 C, for 30, 60 and 90 min and 450 C for 30 min, in nitrogen atmosphere). After annealing, large bubbles were observed from SEM images and a connection between surface microstructure and materials properties was analyzed through AFM measurements. It was observed a reduction of the reflectance and an increase of the peak intensity of the photoluminescence (PL) with the increasing of the annealing time.  2005 Published by Elsevier B.V. PACS: 52.77.Dq; 78.55. m; 78.55.Mb; 87.64.Je Keywords: PIII; Ion implantation; Photoluminescence; Annealing

1. Introduction Among countless plasma surface modification techniques that have been studied recently, plasma immersion ion implantation (PIII) stands out be-

*

Corresponding author. Tel.: +55 12 39456570; fax: +55 12 39456717. E-mail address: [email protected] (J.C.N. Reis). 0168-583X/$ - see front matter  2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.06.119

cause it is a non-line-of-site technique, allowing the high energy ion treatment of three-dimensional complex shaped substrates. Metal, polymer, ceramic and semiconductor materials have been treated with a variety of gas plasmas for wear, corrosion and oxidation resistance [1,2]. For semiconductor processing, PIII has been explored for many features, including fabrication of high-quality p+/n diodes with junction depths below 100 nm [3], conformally doped sidewalls of silicon

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trenches [4], hydrogenation of polycrystalline silicon thin film transistors [5] and studies related to surface modification of silicon and porous silicon by nitrogen PIII implantation [6–8]. In this way, silicon nitride formation occurs after nitrogen implantation and could be used as antireflective coatings or devices passivation. In the present work, silicon samples have been implanted with helium by PIII at low energies. At this time, the major studies of noble-gas implantation into silicon have been reported using the conventional implantation processes based on a target exposed to a controlled ion beam, for example: investigation of formation of cavities which have been found to be effective gettering sites for metallic impurities and the effect of annealing conditions [9], ‘‘Smart-Cut’’ process based on proton implantation and wafer bonding [10], hydrogen implantation to silicon on insulator material technology [11] and formation of helium bubbles in silicon as a function of implantation temperature [12]. Using PIII process, helium implantation was obtained for higher energies, between 20 and 100 keV [13,14]. In PIII, the ions to be implanted into the near surface of materials are extracted directly from the plasma in which the samples or industrial components to be processed are immersed without the need of extraction or acceleration grids. The threedimensional ion implantation is achieved by applying repetitively a negative high voltage pulse (typically 10–100 kV, 10–50 ls duration, 10– 3000 Hz repetition frequency) to the sheath formed between the plasma and the sample holder or the component itself [1].

the PS samples were placed for treatment. The high voltage pulser was run at the peak voltage of 5 kV, with square shaped pulses of about 50 ls duration and 300 Hz repetition frequency. The silicon samples were loaded onto a supporting holder made of stainless steel (SS) AISI304 and PIII implanted with helium for 30 min. The implantation dose obtained from the collected current and the sheath propagation model were 1.2 · 1017 cm 2. After implantation, the samples were annealed at 325 C for 30, 60 and 90 min and 450 C for 30 min in a tubular quartz furnace in nitrogen atmosphere. Before and after annealing, measurements were carried out in order to determine changes in photoluminescence, reflectance and structural modifications using Raman spectroscopy, SEM, AFM and spectrophotometer. Micro-Raman measurements were carried out using a Raman Renishaw microscopy system 2000. The Raman spectra were recorded in the backscattering configuration at room temperature employing an argon-ion laser excitation line (514.5 nm). The laser beam was focused on the sample using a spot size of 5.0 lm in diameter. In order to avoid sample inhomogeneities and to improve the statistics of Raman data, the summation method was used. Scanning electron microscope, JEOL, JSM 5310, with secondary electron contrast was used for SEM images. A SPM2- Shimadzu atomic force microscopy was used to analyze the surface modifications. The total reflectance was measured between 200 and 800 nm using a Hitachi U-3501 Spectrophotometer equipped with an integrating sphere.

3. Results and discussion 2. Experimental CZ silicon (1 0 0) n-type wafers with resistivity of 1 X cm were implanted with helium by PIII. The PIII treatments were carried out in a reactor where the helium plasma was produced by a DC glow discharge source. The plasma was obtained typically at 6.4 · 10 4 mbar pressure, with plasma densities of about 2.5 · 1010 cm 3 and temperatures of 5 eV. The plasma potential reached less than 100 V at the center of the chamber where

Fig. 1 shows the Raman spectra in the vicinity of the Si peak before and after annealing time at 325 C, 30 min. The spectrum corresponding to the crystalline silicon (square) is also plotted as a reference. After implantation the peak intensity decreased and after annealing it was partially recovered. The damaged crystalline structure produced only by PIII implantation has an important amorphous part that affects the Raman intensity. After annealing there was a structure modification

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Fig. 1. Raman spectra in the vicinity of the Si peak before and after annealing time at 325 C, 30 min.

Fig. 2. The evolution of PL intensity, with the increase of annealing time.

that could explain the increasing of the Raman peak intensity. Fig. 2 shows the evolution of PL intensity, with the increase of annealing time. The PL emission in the range of 200–8000 cm 1 increased substantially and a blue shift can be observed after 90 min of annealing. Fig. 3 shows SEM images, after PIII implantation, 30 min of annealing time at 325 C and at 450 C for 30 min. It can be noted that there is an increasing of bubbles formation. This fact had also been observed in other studies using ion beam implantation [10] and PIII implantation employing high energy [15] for smart-cut process. The PIII implantation produces bubbles on the silicon surface and also there is an interaction process between helium and vacancies produced or existing in the silicon crystal generating clusters that can work as absorption centers. On

the other hand, during annealing there is an increase of the bubbles quantity and bubbles size that can lead to a coalescence process producing more and more defects and consequently changes in the crystalline organization [12], that can contribute for increasing the photoluminescence. Fig. 4(a) shows the surface morphology observed by AFM with 2 · 2 lm2 scan area of the implanted silicon samples with no treatment and the Fig. 4(b) after 30 min of annealing time at 325 C. After implantation, two kinds of ridges with different average diameter (approximately 10 and 200 nm, with Z-Max of 13.55 nm) were detected and are distributed over the surface. After annealing there was release of He and probably a structural reorganization with the production of other kinds of defects that resulted in nodules formation on the surface, more uniformly distributed with

Fig. 3. EM images: (a) after PIII implantation, (b) after 30 min of annealing time at 325 C and (c) after 30 min of annealing time at 450 C. All bars represent 22 lm.

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diameter around 50 nm and Z-Max of 23.96 nm, resulting in an increasing of the PL intensity. Fig. 5 shows that the reflectance decreases with the annealing. It also suggests that the increase observed in the photoluminescence measurements is associated with the nanostructure (for instance crystallites, surface states or adsorbed species) presented by the samples after annealing (Figs. 3(b) and (c)). More light is absorbed for 514 nm and consequently higher is the PL.

4. Conclusions

Fig. 4. FM images of the implanted samples (a) before and (b) after 30 min of annealing time at 325 C.

The effect of helium plasma immersion ion implantation on silicon wafers was investigated. After implantation, the Raman peak intensity decreased in relation to polished silicon and after annealing it was partially recovered. The photoluminescence spectrum with a peak at around 3000 cm 1 increased and showed a blue shift after 90 min of annealing time. This behavior can be explained by the changes observed on the images obtained by SEM and AFM. SEM images showed the presence of bubbles and clusters on the silicon surface produced by PIII implantation that can work as absorption centers. The annealing provoked an increase of the bubbles quantity and bubbles size that can probably lead to a coalescence process producing another crystalline organization, and consequently the increase of the photoluminescence. AFM measurements also showed the surface morphology of implanted silicon samples with two different kinds of ridges with different average diameter (approximately 10 and 200 nm, with Z-Max of 13.55 nm). After annealing other kinds of defects contributed for nodules formation on the surface, with diameter around 50 nm and Z-Max of 23.96 nm, resulting in an increasing of the PL intensity.

References

Fig. 5. Reflectance measurements of implanted samples before and after annealing.

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