Short pulse plasma immersion ion implantation of oxygen into silicon: determination of the energy distribution

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Surface and CoatingsTechnology93 (1997) 238-241

Short pulse plasma immersion ion implantation of oxygen into silicon: determination of the energy distribution N . P . B a r r a d a s a,,1, A . J . H . M a a s b, S. M~indl °, R. Gt~nzel ° School of Electronic Engineering, Information Technology and Mathematics, University of Surrey, Cadldford, Surrey GU2 5XH, UK b Eindhoven University of Technology, Department of Applied Physics, POB 513, 5600 MB Eindhoven, The Netherlands Research Center Rossendorf Inc., Institute for Ion Beam Physics and Materials Research, POB 510119, 01314 Dresden, Germany

Abstract

Plasma inamersion ion implantation was used to implant oxygen ions into silicon with applied voltage pulses of 40 kV and 2.5 gs total length. Positive ions from the plasma, O~ and O ÷, with a continuous energy distribution between 0 and 40 keV were implanted with nominal doses between 2 x 1016 and 2 x 10~/cm2. The resulting oxygen depth profiles were measured with elastic recoil detection analysis using 13.4 MeV c~particles. The obtained depth profiles were simulated using a linear superposition of single-energy profiles calculated with TRIM, in order to determine relevant parameters of the accelerated ions. The energy distribution of the incident ions is derived from the results obtained and compared with theoretical models. The agreement found is very good. The plasma is found to be composed of 35(8)% O~ and 65(8)% O + ions. An Fe contamination in the plasma is observed using Rutherford backscattering. © 1997 Elsevier Science S.A. Keywords: Plasma applications; Ion implantation; Energy distribution of ions; Silicon oxide

1. Introduction

Plasma immersion ion implantation (PIII) is a versatile method for implanting ions [1,2]. The sample is immersed in a plasma and negative high voltage pulses are applied to it, resulting in the expansion of the plasma sheath and the acceleration of ions onto the surface of the sample. As the sheath conformably surrounds the target, all surfaces are implanted at the same time, leading to significantly reduced implantation times and cost-effective implantations. PIII processes have been demonstrated for semiconductor and metallurgical applications [3,4]. The total allowed charge per pulse to avoid oxide breakdown in semiconductor processing is a limiting factor in the implantations [5], necessitating short pulses. During the rise time the ion energy increases continuously from 0 to Z . V0, where Z is the ionic charge and -V0 is the applied voltage. However, for short pulses, * Correspondingauthor. Fax: +44 148334139, e-mail:[email protected] 1Permanentaddress: Centro de Fisica Nuclear da Universidadede Lisboa, Av. Prof. Gama Pinto 2, 1699 Lisboa Codex, Portugal. 0257-8972/97/$17.00 © 1997ElsevierScienceS.A.All rights reserved. PII S0257-8972 (97) 00052-2

the finite rise time of the high voltage of the order of 1 gs is not negligible compared with the pulse length. This results in implantation profiles that differ significantly from single-energy implantations. Furthermore, as there is no mass separation, all positively charged ion species present in the plasma are implanted. This includes, for example, O~- and O + ions in an oxygen plasma, and also contaminant species present in the plasma. Energy distribution measurements have been reported for PIII with secondary-ion mass spectrometry [6] and resistance measurements [7]. In this report, we present oxygen PIII profiles in silicon substrates obtained by elastic recoil detection analysis (ERDA) measurements. Four samples were implanted to nominal doses from 2 x 1016 to 2 x 10~7/cm2, at 40 keV and pulse length of 2.5 gs. The measured depth profiles are fitted with theoretical profiles based on the computer code T R I M [8], and are used to deduce the initial energy distribution of the impinging ions and the O~/O ÷ ion ratio in the plasma. Additionally, Rutherford backscattering measurements (RBS) were performed to determine co-implanted contaminations. The energy distribution is compared with the predictions of a theoretical model [9].

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2. Experimental details The implantation chamber was spherical with a diameter of 40cm and a base pressure better than 10-6mbar. The main impurities are nitrogen and carbon. The operating pressure was 2 x 10 -3 mbar. The plasma was generated using an electron cyclotron resonance (ECR) source at a power of 350 W on top of the chamber. The plasma diffuses from the source down into the chamber, where, just above the sample, a plasma density no of 6 x 109/cm3 and an electron temperature kTe of 2 eV were measured with a Langmuir probe. The polished 1 inch2 crystalline silicon wafer samples were clamped onto the 2 inch target holder with a stainless steel guard ring for good thermal and electrical contact. The negative high voltage of 40 kV was pulsed with a tetrode hard tube switch, resulting in a rise time of 1 las and a voltage plateau of 1.5 gs for a total pulse length of 2.5 gs. The fall time is determined by the plasma density and the charging resistor, resulting in an exponential decay with a time constant of 1 gs in our experiments. A repetition rate of 200 Hz was used, resulting in a target temperature that can be calculated to be between 100 and 150 °C [2]. The measured temperature was below the 200 °C detection iimi( of the pyrometer. The oxygen profiles of the samples were measured using ERDA with g (He 2+) particles. A detailed description of the method has been given elsewhere [10]. The depth resolution in this experiment was about 18 nm. The sensitivity for oxygen is 1 x 10 I4 at/cm 2. The samples were also measured with RBS, using a 1.0 MeV He ÷ beam.

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Depth (nm) 3. Results The oxygen depth profiles measured with ERDA are shown in Fig. 1. In sample 1, for the lowest implanted dose, a peak is observed at the surface, below 20 rim, and a tail extends up to a depth of about 120 nm. In samples 2 and 3, for the intermediate dose values, the surface peak is still visible, but the tail has a much larger intensity than in sample 1. In sample 4, with the highest implanted dose, the surface peak is barely visible, and the signal comes practically only from the tail. Implantation of 40 keV O + into Si at doses lower than 3 x 10~7/cm= leads to an implantation range of about 100-140nm [11], which compares very well with the deepest depth where oxygen is present in our samples, that thus should correspond to the fraction of oxygen that was implanted with 40 keV. An expanded part of the RBS spectra taken is shown in Fig. 2. It is observed that Fe is present in all the 2 1 inch=2.54 cm.

Fig. I. Oxygen depth profiles of the four samples measured: (a) 1 (nominal dose 0.2x101~/cm2), (b) 2 (0.5x10~7/cm2), (c) 3 ( 1.0 x 10t~/cm 2) and (d) 4 (2.0 x 101~/cm2). The fitted theoretical functions are also shown.

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Channel Fig. 2. Part of the RBS spectra taken corresponding to the Fe peak. The amount of Fe in each sample scales with the nominal implanted dose.

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N._P.Barradaset aL / Surface and Coatings Technology93 (1997) 238-241

samples, and that its amount is larger in the samples where more oxygen was implanted.

4. D i s c u s s i o n

To determine the energy distribution of the incident ions, the experimental depth profiles Ozxp(X) are fitted with theoretical functions based on Monte Carlo TRIM [8] calculations of the depth profiles of O implanted into Si with different energies. The O~ ions break down into O atoms when colliding with the sample surface, and each of the resulting O atoms has half the energy of the original O~- ion. The theoretical depth profile O~(E,x) due to ions with energy E is then

Oe. (E,x) = [ 2( 1 --f)OTRIM(E/2,X) +flOTRIM(E,x)]/(2 --f) (1) where f is fraction of O + ions, and the total theoretical depth profile Ozh¢o~y(E,x) is N~ OTheory(X)= 2 Ci(Ei)OE(Ei,x) (2) i=t The c~(E.O parameters define the energy distribution of the incident ions. The sputtering effect must also be taken into account. From the sputter yield Y, the thickness x~v~t of the sputtered layer can be estimated, and a surface layer of thickness x~v~t is then subtracted from OTh¢o~y(X).The result is convoluted with the experimental depth resolution F(x) to obtain the final theoretical oxygen depth profile O(x): O (X) = f OTheory (/' + Xsput )/'(X -- r) dr

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The sputter yield as a function of energy was calculated with the SUSPRE [12] code. It is between 0.4 and 0.6 for the considered energy range. O(x) is automatically least-squares fitted to OExv(X), using f and the c~(EO as sole fit parameters. We used three implantation energies for the fits: 8, 27 and 40 keV, corresponding to the surface, intermediate and deeper depth ranges. The choice of which energy should be taken for each depth range is to some extent arbitrary. Using, for example, 24 instead of 27 keV for the intermediate range does not change the results significantly. The procedure described relies on the TRIM code. This is justified on the grounds that, (i) TRIM is based on a vast amount of experimental data on many different systems, and, (ii) it produces results that compare very well with measured data in many systems, as long as effects such as channeled implantation or temperatureenhanced diffusion are avoided, which was the case in this work. Vajo et at. [6] have used a similar, albeit less sophisticated, process to simulate the depth profile of N

implanted into Si with PIII, and found that the agreement was very good. The fits obtained are shown in Fig. 1. The theoretical partial profiles corresponding to the three energy values used to represent the energy distribution of the incident ions, 8, 27 and 40 keV, are also shown. The surface peak decreases from the lowest implanted dose, where it accounts for the most important part of the signal, to the highest implanted dose, where it has a reduced weight. On the other hand, the structure of the tail does not change much, and the relative weights of the 27 and 40 keV partial profiles are similar in all the samples. These features could be explained if the surface peak is due both to the implanted oxygen and to a surface oxide. The fraction f of O + ions is f=0.65(8), taking an average on the four samples. The measured O and Fe doses are shown in Fig. 3 as a function of the nominal implanted oxygen dose. The Fe measured dose extrapolates to zero at zero nominal dose, as expected. It probably arises from sputtering from the ring clamping of the target holder. As the angle of incidence gets more and more shallow near the edge of the target holder, the sputter yield increases. We do not expect the cause to be sputtering from the stainless steel chamber walls, since in experiments using different clamps, different elements (e.g. Cu) have been observed. The O measured dose does not extrapolate to zero at zero nominal implanted O dose, but to 2 x 1016/cm2. This means that this amount of oxygen, corresponding to an oxide layer of approximately 4 rim, is present in the implanted samples, independently of how much was actually implanted in each of them; it should then be due to oxidation of the samples. Hence, to obtain the real energy distribution of the incident oxygen ions, one should not take directly the ci(EO amplitudes of the three partial profiles considered, but subtract from the surface amplitude c1(8 keV) an amount equivalent to the 2 x 1016/cm2 due to oxidation. The resulting density function of the energy distribution of the O incident

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N.P. Barradas et aI. / Surface and Coatings Technology 93 (]997) 238-241

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distribution of energies, with a m a x i m u m value of 40 keV, and both O~ and O + ions are implanted. F r o m the data, we derived that the incident flux is composed of 35(8)% O~- and 65(8)% O + ions. Additionally, Fe ions originating from sputtering of the ring clamping the target holder were co-implanted. The results obtained for the energy distribution of the incident ions were compared with theoretical calculations, and the agreement found is good.

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Fig. 4. Density function of the energy distribution of the implanted O~ and O + ions for all the samples, as determined from the fits, compared with a distribution calculated with a theoretical model [8] (solid line). ions (pe), as derived from the automatic fits for the four samples, is shown in Fig. 4. As a high voltage pulse is applied to the sample, the plasma sheath between the sample and the plasma is expanding during the pulse and receding again after the voltage is switched off. A collisionless transit through the sheath, i.e. a mean free path larger than the sheath width, and a transit time of the ions through the sheath that is short compared with the voltage changes were obtained under the used experimental conditions. This allows the sheath width to be described by Child's law [13]. A theoretical model describing the plasma sheath evolution with a finite rise time [9], applicable to our experimental conditions with a rise time of 1 gs and a 1.5 gs long voltage plateau, was used to calculate the energy distribution of the ions. The ion flux during the fall time arises only from the drift current and can be neglected for our experimental conditions. A discrete energy distribution using these rise and plateau times calculated with the equations given in [9] is shown in Fig. 4 together with the experimentally obtained results, showing good agreement.

5. Conclusions Plasma immersion ion implantation was used to implant oxygen into silicon. The implanted ions have a

Acknowledgement N.P. Barradas acknowledges a European Union grant under the H u m a n Capital and Mobility program. A.J.H. Maas acknowledges Prof. M.J.A. de Voigt for his support. S. Mfindl is supported by Bundesministerium for Forschung und Technologie Research Contract No. 13N6443.

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