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Early stages of IBAD-film growth: Differences between (1 0 0) and polycrystalline Mo substrates
Nuclear Instruments and Methods in Physics Research B 148 (1999) 98±103
Early stages of IBAD-®lm growth: Dierences between (1 0 0) and polycrystalline Mo substrates Jacqueline C. van der Linden, Leon J. Seijbel, Barend J. Thijsse
*
Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
Abstract The eects of 250 eV Ar ion assistance on the growth of ultrathin molybdenum ®lms on Mo (1 0 0) and on wellannealed polycrystalline molybdenum substrates (largely (110)-oriented) are investigated by concurrent thermal desorption spectrometry of incorporated Ar and He, the latter species brought into the ®lm after deposition as probe particles to decorate the point defects. Ar entrapment, defect concentration, and the transition from planar to columnar growth are quantitatively determined. The overall defect creation rate of 250 eV Ar assistance is 0.1 defects per ion, including self-trapping. The dierence between the substrate orientations shows up mainly in the growth morphology, the defect concentration, and the immediate subsurface trapping of Ar and He. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72; 81.15.J; 68.55 Keywords: Film growth; IBAD; Defects; Molybdenum; Thermal desorption spectrometry
1. Introduction Ion beam assisted deposition (IBAD) is a technique commonly used for manipulating the composition or microstructure of thin ®lms. Most phenomena have been observed for energy-to-atom ratios ranging between 1 and 100 eV/atom [1], by applying ion beams with energies exceeding 100 eV. A consequence of the ion assistance is that, depending on the beam energy, part of the ions will be incorporated in the ®lm. In this paper, we present thermal desorption measurements of in-
* Corresponding author. Tel.: +31 15 2782221; fax: +31 15 2786730; e-mail: [email protected]
corporated argon in thin molybdenum ®lms deposited under argon ion assistance. In addition to direct argon desorption measurements, thermal helium desorption spectrometry (THDS) is applied as a tool to determine the total number of defects in the ®lms: vacancies, vacancy clusters and incorporated argon atoms. A comparison is made with PVD ®lms, i.e., ®lms grown without argon assistance, and with results from molecular dynamics (MD) simulations [2]. 2. Experimental The ®lms are deposited in an UHV apparatus (1 ´ 10ÿ10 mbar), described in more detail elsewhere
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 7 8 - 7
J.C. van der Linden et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 98±103
[3]. The system consists of a deposition chamber for producing IBAD ®lms connected to a chamber in which the ®lms are analyzed by THDS. The molybdenum atoms arrive at 15° o-normal incidence from an electron-beam evaporator, with a typical deposition rate of 1 A/s. A Kaufman source (normal incidence) is used for Ar ion assistance. The THDS technique has been described more extensively elsewhere [4,5]. It begins with ÔdecoratingÕ the defects in the ®lm by lowenergy helium ions as probe particles. After thermalization and diusing through the ®lm, some of the He atoms are trapped by defects. During heating of the sample the helium desorption ¯ux L is monitored as a function of the sample temperature T. The L vs. T data form the Ôthermal desorption spectrumÕ: a collection of peaks, each originating from the dissociation of helium at a particular energy, i.e. from a particular type of defect in the material. In all measurements we have used a helium ¯uence of U 1 ´ 1014 He/cm2 and a helium energy of 100 eV, which is below the threshold energy for defect creation in bulk molybdenum. We have grown thin Mo ®lms on a (1 0 0)Mo single crystal and on a well-annealed polycrystalline Mo substrate (with a large grain size of order 10 lm) at room temperature. Additional test measurements on a (1 1 0) single crystal have shown that (1 1 0) spectra are very similar to those for the polycrystal (an example is shown in Fig. 3(b)), indicating that our poly-Mo largely consisted of (1 1 0) grains. The ®lms were grown both with normal PVD and under ion assistance. The energy of the Ar ions used was 250 eV. The ion assisted ®lms were grown with an ion-to-atom ratio (IAR) of 0.019 for (1 0 0)Mo and 0.034 for poly-Mo (there are only irrelevant historical reasons for this dierence). The purpose of these measurements is to determine the concentration of native defects in the deposited ®lms for the dierent growth conditions. ÔDefectsÕ in the current experimental context are limited to trapping sites for He: crystal point defects and incorporated Ar-atoms. However, it will turn out that additional information on ®lm morphology can also be obtained.
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3. Results 3.1. Argon incorporation in molybdenum ®lms grown with ion beam assistance The purpose of the ®rst experiment is to measure the concentration of Ar-atoms incorporated in IBAD ®lms. In Fig. 1 the Ar desorption spectra are shown for various ®lm thicknesses. In the ®gure, v is the normalized desorption ¯uence de®ned as the desorbing ¯ux of Ar atoms L divided by the Ar ¯uence during growth and the heating rate. Integration of v directly gives the trapping probability (Ftr ) of the argon atoms. Peak labels [6,7] are indicated in the ®gure. The ArB -peak is sub stitutional argon desorbing from deeper than 6 A
Fig. 1. Normalized argon desorption spectra of thin molybdenum ®lms of various thicknesses d grown with 250 eV argon ion assistance on a (1 0 0)Mo single crystal (dashed curves) and on a polycrystalline Mo substrate (full curves). The ion-to-atom ratio is 0.019 for (1 0 0)Mo and 0.034 for poly-Mo. The heating rate is 40 K/s. Peak labels are explained in the text.
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below the surface. The position and shape of the peak are in agreement with Mo self-diusion, showing that argon atoms desorb via the diusion of thermal vacancies. ArI and ArII peaks are assigned to argon atoms incorporated in the ®rst and second atomic layer of the ®lm, respectively, as shown in Ref. [7]. If we compare the spectra of (1 0 0)Mo and poly-Mo, we see that the ArI peak is larger for the (1 0 0)Mo spectra. This means that the number of argon atoms close to the surface is larger for (1 0 0)Mo than for poly-Mo. When the desorbed ¯uence is plotted against the ®lm thickness (d), one can ®nd the concentration of Ar atoms in the ®lm (CAr ): CAr
1 oUtr nMo od
1
3 , the number density of with nMo 0.064 atoms/A molybdenum, and Utr the ¯uence of desorbing argon atoms and thus the number of trapped argon atoms per unit area. Fig. 2 shows Utr as a function of the ®lm thickness. From these data we ®nd that the argon concentration in the (1 0 0)Mo ®lm is 1.7 ´ 10ÿ3 argon atoms per molybdenum atom. For the polycrystalline ®lms we ®nd an argon concentration of 2.3 ´ 10ÿ3 Ar/Mo. These numbers are also listed in Table 1. The concentration of argon atoms in the ®lms is much lower than could be expected from IAR 0.019 for (1 0 0)Mo and IAR 0.034 for poly-Mo: for (1 0 0)Mo the incorporation probability is 9%, for poly-Mo 7%. The latter value is in agreement with a trapping probability of 6.5% found earlier [8] for argon implantation measurements in poly-Mo without ®lm growth. Ar saturation concentrations are almost reached for these IAR values. For 250 eV Ar they are 3 ´ 10ÿ3 in (1 0 0)Mo and 4 ´ 10ÿ3 in poly-Mo [7].
Fig. 2. Number of argon atoms per unit area (Utr ) desorbed from the IBAD ®lms, obtained from Fig. 1 by integration. The slopes of the ®tted straight lines give the concentrations of argon atoms in the ®lms, Eq. (1). For the polycrystalline substrate (D) the concentration is found to be 2:3 10ÿ3 argon per molybdenum atom and for the (1 0 0) crystal (h) 1.7 ´ 10ÿ3 argon per molybdenum atom.
3.2. Thermal helium desorption spectrometry The second experiment is performed to ®nd the defects in the deposited ®lms by He decoration. Figs. 3(a) and (b) show the normalized helium desorption spectra for (1 0 0)Mo and poly-Mo, respectively for IBAD ®lms and PVD ®lms. The spectra are normalized by dividing L by the implanted He-¯uence and the heating rate. Earlier work [5,7,8] has shown the following. The peaks in the spectra are labeled S for helium atoms trapped by defects immediately under the surface, H for one helium atom trapped by a monovacancy (note that for (1 0 0) the peak in fact occurs 50 K earlier),
Table 1 Argon concentration (CAr ), total trap concentration (Ctr ), initial ®lm thickness before column growth (D0 ), and average column size (L) for the dierent ®lms. The Ar energy is 250 eV. The poly-substrate largely consists of (110)-oriented grains of size 10 lm Substrate
Growth mode
CAr (Ar/Mo)
Ctr (trap/Mo)
D0 (A)
L (A)
(1 0 0) (1 0 0) Poly Poly
PVD IBAD, IAR 0.019 PVD IBAD, IAR 0.034
0 1.7 ´ 10ÿ3 0 2.3 ´ 10ÿ3
3 ´ 10ÿ4 2 ´ 10ÿ3 1 ´ 10ÿ4 4 ´ 10ÿ3
10 10 25 15
15 20 50 20
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Fig. 3. Normalized helium desorption spectra for dierent Mo ®lm thicknesses d for PVD ®lms (full curves) and IBAD ®lms (dashed PVD ®lm is combined curves). The heating rate is 40 K/s. (a) (1 0 0)Mo single crystal (b) polycrystalline Mo substrate. In (b) the 30 A IBAD ®lm. As an example, the spectrum of a 50 A ®lm on a (1 1 0)Mo single crystal is also shown (dotted curve indicated with the 35 A by the arrow).
and A and B for, respectively, one and two or more helium atoms trapped by substitutionally incorporated argon atoms. Obviously A and B peaks are not found for the PVD ®lms. Helium desorbing between B and H peaks is attributed to multiple ®lling of a monovacancy but also to lattice distortions caused by incorporated argon atoms. Helium desorbing at higher temperatures than the H peak is attributed to clusters of vacancies. A number of observations can be made by looking at the spectra. (i) Ion assistance: Films grown under argon assistance contain more defects than PVD ®lms. From the viewpoint of ®lm quality improvement IBAD is not successful in this case. Although we have seen that Ar incorporation plays a role (in these spectra the A and B peaks form the direct evidence), the number of other lattice defects is also aected. For example, the vacancy cluster concentration increases as a result of the argon assistance, most notably in ®lms with thickness
Also, more monovacancies are formed as P 20 A. a result of the argon ions. This is especially the case for ®lms thinner than 50 A. (ii) Substrate orientation: A notable dierence between the two substrates is the presence of an S peak in the spectra of poly-Mo. The He-®lled defects causing this peak are also found in annealed, defect free substrates, both poly-Mo and (1 1 0)Mo, and therefore must be created by the 1 0 0 eV He ions. Apparently the dierences between the (1 1 0) and (1 0 0) surfaces prevent the formation of these defects in the latter case. In the S peak is reduced by terestingly, above 5 A ion assistance. As mentioned earlier, the spectrum PVD ®lm grown on a (110)Mo single for a 50 A crystal is included in Fig. 3(b). Apart from a small dierence in the amount of clusters the similarity to the poly-Mo spectrum is striking. Overall, poly-®lms contain more defects than (1 0 0) ®lms. However the situation is more complicated than it appears; this is discussed in the following sections.
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3.3. Defect concentration and grain size In ®lms with a constant defect concentration the trapping probability Ftr for helium should increase with ®lm thickness until a certain saturation value [8]. In Fig. 4 Ftr is plotted against the ®lm thickness (the S peak is excluded, since these defects are not native). We see that in all cases Ftr goes through a maximum. It is tempting to explain this by a defect concentration decreasing to Ctr < 10ÿ5 as the ®lm grows. However, there is no apparent reason for this; also, we have seen that the fraction of trapped Ar atoms in IBAD ®lms is constant over the thickness (about 2 ´ 10ÿ3 ). These atoms act as trapping sites and thus the trap concentration could not decrease that much. A more realistic explanation for the decrease in trapping probability is the formation of fast diffusion paths. When a ®lm grows in a columnar way, the maximum distance to the surface is decreased to the radius of a column, because the open space between the columns acts as a fast diusion path. This increases the probability for
helium to escape to the surface before trapping. Evidence for this growth mode was also found by electron microscopy and MD simulations [2]. To model this quantitatively, we have used a 3dimensional diusion and trapping model to calculate Ftr of the helium ions in the ®lms. The variables are the trap concentration (Ctr ), the average size of the columns (L), and the ®lm thickness at which column formation begins (D0 ). The results are included in Fig. 4. and summarized in Table 1. To keep the model simple, the trap concentration was kept constant over the whole thickness of the ®lm. Although the accuracy of the results in Table 1 is not better than about one signi®cant digit, some conclusions can be drawn. We see for (100)Mo are formed that columns of average size L 15 A Ion assisafter an initial ®lm thickness D0 10 A. tance does not change D0 and L, or only slightly. Poly-®lms have larger columns, starting in a later stage of the growth. The eects of ion assistance are much larger here: D0 and L are both signi®cantly reduced. Contrary to what appears from glancing over Figs. 3(a) and (b), the defect concentration in PVD ®lms is higher for (1 0 0)Mo than for poly-Mo. This is consistent with MD results [2]. Ion assistance increases the defect concentration in all cases. Table 1 shows that under the present conditions, an average Ar ion creates approximately 0.1 point defects. The majority of these are substitutionally trapped Ar atoms. 4. Summary and conclusions
Fig. 4. Helium trapping probability in the ®lm (Ftr ) versus ®lm thickness. IBAD ®lms on a polycrystalline substrate (h) have the highest trapping probability, followed by IBAD ®lms on a (1 0 0) substrate (}). PVD ®lms have lower trapping probabilities, (1 0 0)(s), poly (D). The curves are obtained from model calculations described in the text.
Summarizing, the growth of Mo ®lms on a polyMo substrate (which can be considered (1 1 0)-ori ented here) starts in a planar fashion, but after 25 A start to deextremely ®ne columnar grains (50 A) velop. In the grains the point defect concentration is 1 ´ 10ÿ4 . Mo ®lms on a (1 0 0) substrate grow in a similar way, but the columns begin earlier (10 A), and contain more and the grains are smaller (15 A) point defects (3 ´ 10ÿ4 ). This last point may be connected with the fact that surface diusion on (1 0 0) is slower than on (1 1 0) [2], thus allowing more surface vacancies to develop into ÔbulkÕ vacancies. 250 eV Ar assistance during Mo ®lm
J.C. van der Linden et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 98±103
growth leads on average to a 8% incorporation probability of Ar in the ®lm, up to an Ar saturation concentration of 3:5 10ÿ3 Ar/Mo. These numbers are almost independent of orientation. For (1 1 0)®lms Ar assistance reduces the values for grain development to those for (1 0 0)-®lms. Apart from self-incorporation the Ar ions also create additional defects: monovacancies especially in the thinner ®lms (planar stage), vacancy clusters in thicker ®lms (columnar stage). This is also seen in MD simulations [2], which suggest that Ôgrain boundariesÕ are in fact interconnected vacancy clusters; other work also shows that vacancy clusters do not develop in very thin ®lms [8]. The overall defect creation rate of a 250 eV Ar ion is about 0.1 defect, including self-trapping. Finally the substrate orientation is found to play a role for the immediate subsurface trapping of the noble gas ions. Whereas that of Ar is more pronounced in (1 0 0) than in (1 1 0), that of He is totally absent in (1 0 0). This is not yet clearly understood in terms of surface energies, but it illustrates how the subtleties of atomic coordination near a surface can lead to pronounced eects in experimental observations. The fact that ion assistance reduces subsurface He trapping is in agreement with the MD result that IBAD reduces surface roughness (and therefore surface area). Acknowledgements Edwin F.C. Haddeman is acknowledged for assisting in the computation of the 3-dimensional
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diusion problem. Jan van der Kuur is acknowledged for providing us with the results of the measurements on polycrystalline molybdenum. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research of Matter), and was made possible by ®nancial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek NWO (Netherlands Organization for Scienti®c Research). References [1] F.A. Smidt, Int. Mat. Rev. 35 (1990) 61. [2] T.P.C. Klaver, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 504 (1998) in print.; T.P.C. Klaver, E.F.C. Haddeman, B.J. Thijsse, paper presented at the Fourth International Conference on Computer Simulation of Radiation Eects in Solids, Okayama, 1998, preprint. [3] J. van der Kuur, E.J.E. Melker, T.P. Huijgen, W.H.B. Hoondert, G.T.W.M. Bekking, A. van den Beukel, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 396 (1996) 587. [4] A.A. van Gorkum, E.V. Kornelsen, Vacuum 31 (1981) 89. [5] A. van Veen, A. Warnaar, L.M. Caspers, Vacuum 30 (1980) 109. [6] A. van Veen, W.T.M. Buters, G.J. van der Kolk, L.M. Caspers, T.R Armstrong, Nucl. Instr. and Meth. 194 (1982) 485. [7] J. van der Kuur, B. Korevaar, M. Pols, J.C. van der Linden, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 504 (1998), in press. [8] J. van der Kuur, Defects in thin ®lms deposited with and without ion assistance, 1998, Ph.D. Thesis, Delft University of Technology.
Early stages of IBAD-®lm growth: Dierences between (1 0 0) and polycrystalline Mo substrates Jacqueline C. van der Linden, Leon J. Seijbel, Barend J. Thijsse
*
Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
Abstract The eects of 250 eV Ar ion assistance on the growth of ultrathin molybdenum ®lms on Mo (1 0 0) and on wellannealed polycrystalline molybdenum substrates (largely (110)-oriented) are investigated by concurrent thermal desorption spectrometry of incorporated Ar and He, the latter species brought into the ®lm after deposition as probe particles to decorate the point defects. Ar entrapment, defect concentration, and the transition from planar to columnar growth are quantitatively determined. The overall defect creation rate of 250 eV Ar assistance is 0.1 defects per ion, including self-trapping. The dierence between the substrate orientations shows up mainly in the growth morphology, the defect concentration, and the immediate subsurface trapping of Ar and He. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72; 81.15.J; 68.55 Keywords: Film growth; IBAD; Defects; Molybdenum; Thermal desorption spectrometry
1. Introduction Ion beam assisted deposition (IBAD) is a technique commonly used for manipulating the composition or microstructure of thin ®lms. Most phenomena have been observed for energy-to-atom ratios ranging between 1 and 100 eV/atom [1], by applying ion beams with energies exceeding 100 eV. A consequence of the ion assistance is that, depending on the beam energy, part of the ions will be incorporated in the ®lm. In this paper, we present thermal desorption measurements of in-
* Corresponding author. Tel.: +31 15 2782221; fax: +31 15 2786730; e-mail: [email protected]
corporated argon in thin molybdenum ®lms deposited under argon ion assistance. In addition to direct argon desorption measurements, thermal helium desorption spectrometry (THDS) is applied as a tool to determine the total number of defects in the ®lms: vacancies, vacancy clusters and incorporated argon atoms. A comparison is made with PVD ®lms, i.e., ®lms grown without argon assistance, and with results from molecular dynamics (MD) simulations [2]. 2. Experimental The ®lms are deposited in an UHV apparatus (1 ´ 10ÿ10 mbar), described in more detail elsewhere
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 8 7 8 - 7
J.C. van der Linden et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 98±103
[3]. The system consists of a deposition chamber for producing IBAD ®lms connected to a chamber in which the ®lms are analyzed by THDS. The molybdenum atoms arrive at 15° o-normal incidence from an electron-beam evaporator, with a typical deposition rate of 1 A/s. A Kaufman source (normal incidence) is used for Ar ion assistance. The THDS technique has been described more extensively elsewhere [4,5]. It begins with ÔdecoratingÕ the defects in the ®lm by lowenergy helium ions as probe particles. After thermalization and diusing through the ®lm, some of the He atoms are trapped by defects. During heating of the sample the helium desorption ¯ux L is monitored as a function of the sample temperature T. The L vs. T data form the Ôthermal desorption spectrumÕ: a collection of peaks, each originating from the dissociation of helium at a particular energy, i.e. from a particular type of defect in the material. In all measurements we have used a helium ¯uence of U 1 ´ 1014 He/cm2 and a helium energy of 100 eV, which is below the threshold energy for defect creation in bulk molybdenum. We have grown thin Mo ®lms on a (1 0 0)Mo single crystal and on a well-annealed polycrystalline Mo substrate (with a large grain size of order 10 lm) at room temperature. Additional test measurements on a (1 1 0) single crystal have shown that (1 1 0) spectra are very similar to those for the polycrystal (an example is shown in Fig. 3(b)), indicating that our poly-Mo largely consisted of (1 1 0) grains. The ®lms were grown both with normal PVD and under ion assistance. The energy of the Ar ions used was 250 eV. The ion assisted ®lms were grown with an ion-to-atom ratio (IAR) of 0.019 for (1 0 0)Mo and 0.034 for poly-Mo (there are only irrelevant historical reasons for this dierence). The purpose of these measurements is to determine the concentration of native defects in the deposited ®lms for the dierent growth conditions. ÔDefectsÕ in the current experimental context are limited to trapping sites for He: crystal point defects and incorporated Ar-atoms. However, it will turn out that additional information on ®lm morphology can also be obtained.
99
3. Results 3.1. Argon incorporation in molybdenum ®lms grown with ion beam assistance The purpose of the ®rst experiment is to measure the concentration of Ar-atoms incorporated in IBAD ®lms. In Fig. 1 the Ar desorption spectra are shown for various ®lm thicknesses. In the ®gure, v is the normalized desorption ¯uence de®ned as the desorbing ¯ux of Ar atoms L divided by the Ar ¯uence during growth and the heating rate. Integration of v directly gives the trapping probability (Ftr ) of the argon atoms. Peak labels [6,7] are indicated in the ®gure. The ArB -peak is sub stitutional argon desorbing from deeper than 6 A
Fig. 1. Normalized argon desorption spectra of thin molybdenum ®lms of various thicknesses d grown with 250 eV argon ion assistance on a (1 0 0)Mo single crystal (dashed curves) and on a polycrystalline Mo substrate (full curves). The ion-to-atom ratio is 0.019 for (1 0 0)Mo and 0.034 for poly-Mo. The heating rate is 40 K/s. Peak labels are explained in the text.
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below the surface. The position and shape of the peak are in agreement with Mo self-diusion, showing that argon atoms desorb via the diusion of thermal vacancies. ArI and ArII peaks are assigned to argon atoms incorporated in the ®rst and second atomic layer of the ®lm, respectively, as shown in Ref. [7]. If we compare the spectra of (1 0 0)Mo and poly-Mo, we see that the ArI peak is larger for the (1 0 0)Mo spectra. This means that the number of argon atoms close to the surface is larger for (1 0 0)Mo than for poly-Mo. When the desorbed ¯uence is plotted against the ®lm thickness (d), one can ®nd the concentration of Ar atoms in the ®lm (CAr ): CAr
1 oUtr nMo od
1
3 , the number density of with nMo 0.064 atoms/A molybdenum, and Utr the ¯uence of desorbing argon atoms and thus the number of trapped argon atoms per unit area. Fig. 2 shows Utr as a function of the ®lm thickness. From these data we ®nd that the argon concentration in the (1 0 0)Mo ®lm is 1.7 ´ 10ÿ3 argon atoms per molybdenum atom. For the polycrystalline ®lms we ®nd an argon concentration of 2.3 ´ 10ÿ3 Ar/Mo. These numbers are also listed in Table 1. The concentration of argon atoms in the ®lms is much lower than could be expected from IAR 0.019 for (1 0 0)Mo and IAR 0.034 for poly-Mo: for (1 0 0)Mo the incorporation probability is 9%, for poly-Mo 7%. The latter value is in agreement with a trapping probability of 6.5% found earlier [8] for argon implantation measurements in poly-Mo without ®lm growth. Ar saturation concentrations are almost reached for these IAR values. For 250 eV Ar they are 3 ´ 10ÿ3 in (1 0 0)Mo and 4 ´ 10ÿ3 in poly-Mo [7].
Fig. 2. Number of argon atoms per unit area (Utr ) desorbed from the IBAD ®lms, obtained from Fig. 1 by integration. The slopes of the ®tted straight lines give the concentrations of argon atoms in the ®lms, Eq. (1). For the polycrystalline substrate (D) the concentration is found to be 2:3 10ÿ3 argon per molybdenum atom and for the (1 0 0) crystal (h) 1.7 ´ 10ÿ3 argon per molybdenum atom.
3.2. Thermal helium desorption spectrometry The second experiment is performed to ®nd the defects in the deposited ®lms by He decoration. Figs. 3(a) and (b) show the normalized helium desorption spectra for (1 0 0)Mo and poly-Mo, respectively for IBAD ®lms and PVD ®lms. The spectra are normalized by dividing L by the implanted He-¯uence and the heating rate. Earlier work [5,7,8] has shown the following. The peaks in the spectra are labeled S for helium atoms trapped by defects immediately under the surface, H for one helium atom trapped by a monovacancy (note that for (1 0 0) the peak in fact occurs 50 K earlier),
Table 1 Argon concentration (CAr ), total trap concentration (Ctr ), initial ®lm thickness before column growth (D0 ), and average column size (L) for the dierent ®lms. The Ar energy is 250 eV. The poly-substrate largely consists of (110)-oriented grains of size 10 lm Substrate
Growth mode
CAr (Ar/Mo)
Ctr (trap/Mo)
D0 (A)
L (A)
(1 0 0) (1 0 0) Poly Poly
PVD IBAD, IAR 0.019 PVD IBAD, IAR 0.034
0 1.7 ´ 10ÿ3 0 2.3 ´ 10ÿ3
3 ´ 10ÿ4 2 ´ 10ÿ3 1 ´ 10ÿ4 4 ´ 10ÿ3
10 10 25 15
15 20 50 20
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Fig. 3. Normalized helium desorption spectra for dierent Mo ®lm thicknesses d for PVD ®lms (full curves) and IBAD ®lms (dashed PVD ®lm is combined curves). The heating rate is 40 K/s. (a) (1 0 0)Mo single crystal (b) polycrystalline Mo substrate. In (b) the 30 A IBAD ®lm. As an example, the spectrum of a 50 A ®lm on a (1 1 0)Mo single crystal is also shown (dotted curve indicated with the 35 A by the arrow).
and A and B for, respectively, one and two or more helium atoms trapped by substitutionally incorporated argon atoms. Obviously A and B peaks are not found for the PVD ®lms. Helium desorbing between B and H peaks is attributed to multiple ®lling of a monovacancy but also to lattice distortions caused by incorporated argon atoms. Helium desorbing at higher temperatures than the H peak is attributed to clusters of vacancies. A number of observations can be made by looking at the spectra. (i) Ion assistance: Films grown under argon assistance contain more defects than PVD ®lms. From the viewpoint of ®lm quality improvement IBAD is not successful in this case. Although we have seen that Ar incorporation plays a role (in these spectra the A and B peaks form the direct evidence), the number of other lattice defects is also aected. For example, the vacancy cluster concentration increases as a result of the argon assistance, most notably in ®lms with thickness
Also, more monovacancies are formed as P 20 A. a result of the argon ions. This is especially the case for ®lms thinner than 50 A. (ii) Substrate orientation: A notable dierence between the two substrates is the presence of an S peak in the spectra of poly-Mo. The He-®lled defects causing this peak are also found in annealed, defect free substrates, both poly-Mo and (1 1 0)Mo, and therefore must be created by the 1 0 0 eV He ions. Apparently the dierences between the (1 1 0) and (1 0 0) surfaces prevent the formation of these defects in the latter case. In the S peak is reduced by terestingly, above 5 A ion assistance. As mentioned earlier, the spectrum PVD ®lm grown on a (110)Mo single for a 50 A crystal is included in Fig. 3(b). Apart from a small dierence in the amount of clusters the similarity to the poly-Mo spectrum is striking. Overall, poly-®lms contain more defects than (1 0 0) ®lms. However the situation is more complicated than it appears; this is discussed in the following sections.
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3.3. Defect concentration and grain size In ®lms with a constant defect concentration the trapping probability Ftr for helium should increase with ®lm thickness until a certain saturation value [8]. In Fig. 4 Ftr is plotted against the ®lm thickness (the S peak is excluded, since these defects are not native). We see that in all cases Ftr goes through a maximum. It is tempting to explain this by a defect concentration decreasing to Ctr < 10ÿ5 as the ®lm grows. However, there is no apparent reason for this; also, we have seen that the fraction of trapped Ar atoms in IBAD ®lms is constant over the thickness (about 2 ´ 10ÿ3 ). These atoms act as trapping sites and thus the trap concentration could not decrease that much. A more realistic explanation for the decrease in trapping probability is the formation of fast diffusion paths. When a ®lm grows in a columnar way, the maximum distance to the surface is decreased to the radius of a column, because the open space between the columns acts as a fast diusion path. This increases the probability for
helium to escape to the surface before trapping. Evidence for this growth mode was also found by electron microscopy and MD simulations [2]. To model this quantitatively, we have used a 3dimensional diusion and trapping model to calculate Ftr of the helium ions in the ®lms. The variables are the trap concentration (Ctr ), the average size of the columns (L), and the ®lm thickness at which column formation begins (D0 ). The results are included in Fig. 4. and summarized in Table 1. To keep the model simple, the trap concentration was kept constant over the whole thickness of the ®lm. Although the accuracy of the results in Table 1 is not better than about one signi®cant digit, some conclusions can be drawn. We see for (100)Mo are formed that columns of average size L 15 A Ion assisafter an initial ®lm thickness D0 10 A. tance does not change D0 and L, or only slightly. Poly-®lms have larger columns, starting in a later stage of the growth. The eects of ion assistance are much larger here: D0 and L are both signi®cantly reduced. Contrary to what appears from glancing over Figs. 3(a) and (b), the defect concentration in PVD ®lms is higher for (1 0 0)Mo than for poly-Mo. This is consistent with MD results [2]. Ion assistance increases the defect concentration in all cases. Table 1 shows that under the present conditions, an average Ar ion creates approximately 0.1 point defects. The majority of these are substitutionally trapped Ar atoms. 4. Summary and conclusions
Fig. 4. Helium trapping probability in the ®lm (Ftr ) versus ®lm thickness. IBAD ®lms on a polycrystalline substrate (h) have the highest trapping probability, followed by IBAD ®lms on a (1 0 0) substrate (}). PVD ®lms have lower trapping probabilities, (1 0 0)(s), poly (D). The curves are obtained from model calculations described in the text.
Summarizing, the growth of Mo ®lms on a polyMo substrate (which can be considered (1 1 0)-ori ented here) starts in a planar fashion, but after 25 A start to deextremely ®ne columnar grains (50 A) velop. In the grains the point defect concentration is 1 ´ 10ÿ4 . Mo ®lms on a (1 0 0) substrate grow in a similar way, but the columns begin earlier (10 A), and contain more and the grains are smaller (15 A) point defects (3 ´ 10ÿ4 ). This last point may be connected with the fact that surface diusion on (1 0 0) is slower than on (1 1 0) [2], thus allowing more surface vacancies to develop into ÔbulkÕ vacancies. 250 eV Ar assistance during Mo ®lm
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growth leads on average to a 8% incorporation probability of Ar in the ®lm, up to an Ar saturation concentration of 3:5 10ÿ3 Ar/Mo. These numbers are almost independent of orientation. For (1 1 0)®lms Ar assistance reduces the values for grain development to those for (1 0 0)-®lms. Apart from self-incorporation the Ar ions also create additional defects: monovacancies especially in the thinner ®lms (planar stage), vacancy clusters in thicker ®lms (columnar stage). This is also seen in MD simulations [2], which suggest that Ôgrain boundariesÕ are in fact interconnected vacancy clusters; other work also shows that vacancy clusters do not develop in very thin ®lms [8]. The overall defect creation rate of a 250 eV Ar ion is about 0.1 defect, including self-trapping. Finally the substrate orientation is found to play a role for the immediate subsurface trapping of the noble gas ions. Whereas that of Ar is more pronounced in (1 0 0) than in (1 1 0), that of He is totally absent in (1 0 0). This is not yet clearly understood in terms of surface energies, but it illustrates how the subtleties of atomic coordination near a surface can lead to pronounced eects in experimental observations. The fact that ion assistance reduces subsurface He trapping is in agreement with the MD result that IBAD reduces surface roughness (and therefore surface area). Acknowledgements Edwin F.C. Haddeman is acknowledged for assisting in the computation of the 3-dimensional
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diusion problem. Jan van der Kuur is acknowledged for providing us with the results of the measurements on polycrystalline molybdenum. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research of Matter), and was made possible by ®nancial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek NWO (Netherlands Organization for Scienti®c Research). References [1] F.A. Smidt, Int. Mat. Rev. 35 (1990) 61. [2] T.P.C. Klaver, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 504 (1998) in print.; T.P.C. Klaver, E.F.C. Haddeman, B.J. Thijsse, paper presented at the Fourth International Conference on Computer Simulation of Radiation Eects in Solids, Okayama, 1998, preprint. [3] J. van der Kuur, E.J.E. Melker, T.P. Huijgen, W.H.B. Hoondert, G.T.W.M. Bekking, A. van den Beukel, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 396 (1996) 587. [4] A.A. van Gorkum, E.V. Kornelsen, Vacuum 31 (1981) 89. [5] A. van Veen, A. Warnaar, L.M. Caspers, Vacuum 30 (1980) 109. [6] A. van Veen, W.T.M. Buters, G.J. van der Kolk, L.M. Caspers, T.R Armstrong, Nucl. Instr. and Meth. 194 (1982) 485. [7] J. van der Kuur, B. Korevaar, M. Pols, J.C. van der Linden, B.J. Thijsse, Mat. Res. Soc. Symp. Proc. 504 (1998), in press. [8] J. van der Kuur, Defects in thin ®lms deposited with and without ion assistance, 1998, Ph.D. Thesis, Delft University of Technology.