Defects and thermal stability of nanothin Cu films on Mo and Ta

Defects and thermal stability of

nanothin Cu films on Mo and Ta

Defects and thermal stability of

nanothin Cu films on Mo and Ta

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen

op maandag 10 oktober 2005 om 10.30 uur

door

Vinay VENUGOPAL

Master of Science in Physics,

Indian Institute of Technology, Madras, India,

geboren te Bangalore, India

Dit proefschrift is goedgekeurd door de promotor:

Prof.dr. B.J. Thijsse

Samenstelling promotiecommissie:

Rector Magnificus

voorzitter

Prof.dr. B.J. Thijsse

Technische Universiteit Delft, promotor

Prof.dr. J.H.W. de Wit

Technische Universiteit Delft

Prof.dr. H.W. Zandbergen

Technische Universiteit Delft

Prof.dr.ir. A.H.M. Verkooijen

Technische Universiteit Delft

Prof.dr.ir. G.J. Kramer

Technische Universiteit Eindhoven en

Shell Research and Technology Centre

Amsterdam

Dr.ir. W.G. Sloof

Technische Universiteit Delft

Dr.ir. J. van der Kuur

SRON, Stichting Ruimteonderzoek

Nederland

ISBN: 90-9019791-5

Copyright © 2005 by Vinay Venugopal

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This work is part of the research programme of the 'Stichting voor Fundamenteel

Onderzoek der Materie (FOM)', which is financially supported by the

Contents

Introduction 1

1.1 Background 1

1.2 Objective of this thesis 2

1.3 Structure of this thesis 5

Chapter 2 Experimental methods 9

2.1 Introduction 9

2.2 Principles of IBAD and THDS 9

2.3 Apparatus for combined IBAD and THDS 12

2.3.1 The main chamber 12

2.3.2 The sample holder 13

2.3.3 THDS chamber 13

2.3.4 Calibration of the mass spectrometer 15

2.3.5 Deconvolution and smoothing of the measured desorption flux 16

2.3.6 Data analysis 17

2.4 Other techniques 18

2.4.1 X-ray diffraction 18

2.4.2 SEM, EDS and AES 19

2.4.3 Variable energy positron beam analysis 20

Chapter 3 Ultrathin Cu films on Mo(110) characterized by helium implantation 23

3.1 Introduction 23

3.2 Experimental details 23

3.3 Results and discussion 24

3.3.1 Bare Mo(110) substrate 24

3.3.2 Effect of film thickness 25

3.3.3 Effect of annealing 25

3.3.4 Implantation with 75 eV helium 27

3.3.5 Effect of annealing: 75 eV helium implantation 28

3.3.6 Effect of overlayer 28

3.4 Conclusions 29

Chapter 4 Defects and morphological changes in nanothin Cu films on Mo(100) studied by thermal helium desorption spectrometry 31

4.1 Introduction 31

4.2 Experimental details 32

4.3 Results and discussion 34

4.3.1 X-ray diffraction 34

4.3.2 Bare Mo(100) substrate 36

4.3.3 Effect of Cu film thickness 36

4.3.4 Effect of annealing 38

4.3.5 Helium release from Cu 40

4.3.7 Effect of substrate temperature during deposition 43

4.3.8 Effect of helium fluence 44

4.3.9 Effect of helium implantation energy 45

4.3.10 Effect of heating rate 46

4.3.11 Effect of substrate orientation 46

4.4 Conclusions 47

Chapter 5 Defects and morphological changes in nanothin Cu films on polycrystalline Mo analyzed by thermal helium desorption spectrometry 51

5.1 Introduction 51

5.2 Experimental details 52

5.3 Results and discussion 53

5.3.1 Effect of annealing 53

5.3.2 Effect of film thickness 55

5.3.3 Variation of helium decoration fluence 59

5.3.4 Variation of helium implantation energy 59

5.3.5 Role of the substrate surface 60

5.3.6 Effect of substrate temperature during deposition 60

5.3.7 Effect of ion bombardment during deposition 61

5.3.8 Effect of substrate orientation 63

5.4 Conclusions 64

Chapter 6 Defects and thermal stability of thin Cu films on Ta studied by thermal helium desorption spectrometry 69

6.1 Introduction 69

6.2 Experimental details 70

6.3 Results and discussion 71

6.3.1 Tantalum 71

6.3.1.1 Variation of helium fluence 71

6.3.1.2 Variation of helium implantation energy 73

6.3.1.3 Variation of heating rate 74

6.3.2 Cu films on Ta 76

6.3.2.1 Effect of film thickness (1000 eV helium implantation) 76

6.3.2.2 Effect of annealing 78

6.3.2.3 Effect of film thickness (75 eV helium implantation) 81

6.3.2.4 Effect of overlayer 82

6.4 Conclusions 83

6.4.1 Ta 83 6.4.2 Cu films on Ta 84

6.4.3 Comparison of Cu/Ta with Cu/Mo 85

Summary 87

Samenvatting 91

Parts of this thesis have been published or have been submitted for publication. ‰ Chapter 3:

‘Ultrathin Cu films on Mo(110) characterized by helium implantation’, V. Venugopal and B. J. Thijsse, Mat. Res. Soc. Symp. Proc. 792, 351 (2004). ‰ Chapter 4:

‘Defects and morphological changes in nanothin Cu films on Mo(100) studied by thermal helium desorption spectrometry’, V. Venugopal, L. J. Seijbel, N. M. van der Pers, and B. J. Thijsse, J. Appl. Phys. 96, 4463 (2004).

‰ Chapter 5:

‘Defects and morphological changes in nanothin Cu films on polycrystalline Mo analyzed by thermal helium desorption spectrometry’, V. Venugopal, L. J. Seijbel, and B. J. Thijsse, J. Appl. Phys. (accepted).

‰ Chapter 6:

‘Defects and thermal stability of thin Cu films on Ta studied by thermal helium desorption spectrometry’, V. Venugopal and B. J. Thijsse, J. Appl. Phys. (submitted).

Introduction

Chapter 1

Introduction

1.1 Background

Thin films have chemical, mechanical, thermal, electrical, magnetic and optical properties that are different from those of bulk materials. This difference is a result of thin films being:

 Not fully dense  Under stress

 Not free from lattice defects  Quasi-two dimensional

 Strongly influenced by surface and interface effects.

Thin films find extensive applications in microelectronics, optics (anti-reflection coatings), opto-electronics, sensors (gas, magnetic), data storage, corrosion

protection, catalysis, energy generation and conservation, coatings for thermal barrier, wear resistance, coatings for biomedical applications, materials conservation, etc.

Often thin films are deposited under conditions that are quite far away from thermodynamic equilibrium. This not only leads to the incorporation of defects of various types in the film but it can also make the film morphologically metastable. Defects also arise due to film-substrate lattice mismatch. The defects can be in zero dimension (vacancies, interstitials, impurities), one dimension (dislocations), two dimensions (stacking faults, grain boundaries) or three dimensions (voids, bubbles). The presence of defects significantly modifies the properties of thin films. Hence by understanding the parameters that determine the defects in thin films one can in principle control their properties for the desired application. A typical evaporated noble metal film deposited at room temperature contains [1] ~ 1 at. % vacancies and 1010-1012dislocations/cm2. The concentration of other point and line defects is

normally insignificant due to either their high energies of formation or low energies of migration. The role of structural defects (vacancies, dislocations) in electron transport properties (resistivity, Hall coefficient, mobility, temperature coefficient of resistance, thermoelectric power) of Cu films on glass and mica has been investigated by others [2]. The reason for quoting the work on Cu films is that defects in thin Cu films is a subject of this thesis. Vacancies play a major role in processes related to solid state diffusion which includes recrystallization, grain growth, sintering and phase transformations [3].

A metastable film is thermally stable in only a limited temperature range above which, it agglomerates into 3D crystals. The driving force for this

agglomeration [4] process is generally attributed to minimization of surface (of the film) and interface (film-substrate) free energies and stress relaxation of the film. Agglomeration process is also referred to as island formation and dewetting. In all the technological areas where integrity and continuity of films are essential

Introduction

agglomeration is a problem [5,6]. Pt/Ti [5] and NiSi/Si [6] are the film/substrate combinations. However, the agglomeration process has been used to form self-organized nanoscale Ni islands [7] and nanocatalyst sensitizers [8]. The

thermodynamics and kinetics of thin film agglomeration has been reviewed by Srolovitz and Goldiner [4]. Stoyanov and Markov [9] have described a nucleation theory approach to explain 2D-3D transition in epitaxial thin film during growth at elevated substrate temperature and on annealing a continuous film. Agglomeration of thin Cu films has been studied experimentally on sapphire [10], TiO2[11], and SiO2 [12]. As far as we know the kinetics of agglomeration of Cu films on metal substrates has not been studied extensively.

1.2 Objective of this thesis

The main aim of this thesis is to characterize defects and thermal stability of nanothin Cu films (5-200 Å) on Mo and Ta substrates. This thickness range is not only important to understand the onset of growth in thin films but also has

applications in the emerging field of nanotechnology. The films are deposited by electron beam evaporation in ultrahigh vacuum and in some cases by ion beam assisted deposition (IBAD). In IBAD [13-15], a growing film is bombarded with energetic ions, which results in the modification of the properties of the film. These include improved adhesion, densification, modification of grain size and morphology, modification of hardness and ductility, etc. Defects and thermal stability of the Cu films are characterized using in-situ thermal helium desorption spectrometry (THDS) [16,17]. THDS was initially used in the study of point and volume defects in metals [18,19]. It is a very powerful technique for quantitative measurement of small

concentrations of defects in the subsurface region. In addition it gives information on structural transformation in the film like agglomeration, phase transformation process like sublimation of the film, film coverage, and ion damage effects. Argon desorption spectra gives information on argon incorporation in IBAD films, mechanism of argon release, and penetration depth of argon.

In earlier work on thin films, THDS was applied to analyze defects in Mo thin films (5-800 Å) on Mo substrates (homoepitaxy) deposited with and without ion assistance [20-22]. A few significant findings by THDS include the following. The defect concentration for Mo thin films
30 Å was found to be 1.3  10-4. For thicker films, columnar growth was observed and the grain boundaries between the columns acted as a fast diffusion path for helium, which decreased the trapping probability and mean penetration depth of helium. The activation energy for monovacancy mobility was found to be 2.4 eV. For thin Mo films deposited with 250 eV Ar+_bombardment argon incorporation was at substitutional positions. Net rate of defect creation by argon was found to 0.7  10-3vacancy/argon atom. The growth mode was similar to that of non-IBAD films except that columnar diameter decreased under 25 eV argon ion assistance. The defect concentration of films deposited with 5 and 100 eV Ar+ (ion/atom = 0.1) was 1.3  10-3.

For the present work, Cu films on Mo and Ta were chosen. Cu/Mo and Cu/Ta are model heteroepitaxial systems that are being characterized for the first time by THDS. Cu has an fcc structure and a low melting point while Mo and Ta have a bcc structure and high melting points. They are also model system for studying noble-metal-layer on transition-metal-substrate interactions. Cu is immiscible in both Mo

Introduction

and Ta, a fact that has an important advantage for the current work, since at the end of the so called desorption run the substrate is free from the film and can be used for the next experiment. This avoids the need to replace the substrate after each experiment. Table 1.1 lists some properties of Cu, Mo, and Ta.

Table 1.1 Some properties of Cu, Mo and Ta. The data are taken from http://www.webelements.com/.

Property Cu Mo Ta Atomic number Atomic weight Crystal structure Lattice constant (Å) Electronic configuration First ionization energy (kJmol-1₎ Atomic radius (calculated) (pm) Density (kgm-3₎

Youngs modulus (GPa) Rigidity modulus (GPa) Bulk modulus (GPa) Poisson’s ratio

Brinell hardness (MPa) Vickers hardness (MPa) Electrical resistivity (10-8 _Wm) Melting point (K)

Superconducting temperature (K) Thermal conductivity (Wm-1_K-1₎

Coefficient of linear thermal expansion (10-6 K-1₎

Enthalpy of vaporization (kJmol-1₎

29 63.546 fcc 3.6149 [Ar]3d10_4s1 745.5 145 8920 130 48 140 0.34 874 369 1.7 1357.77 - 400 16.5 300 42 95.94 bcc 3.147 [Kr]4d5_5s1 684.3 190 10280 329 20 230 0.31 1500 1530 5 2896 0.915 139 4.8 600 73 180.9479 bcc 3.3013 [Xe]4f14_5d3_6s2 761 200 16650 186 69 200 0.34 800 873 13 3290 4.47 57 6.3 735

Molecular dynamics simulations have been used by our group to study the growth of thin Cu films on Mo(110) [23] and on Ta [24]. Some of the salient results include the following. In the case of Cu/Mo(110) [23] the first Cu monolayer (ML) was pseudomorphic and fcc(111) film with intrinsic stacking faults evolved. The effects of 75 eV helium ion implantation on the films included the clustering of helium-vacancy complexes. For Cu/Ta(110) and Cu/Ta(100) [24] initial growth mode was Stranski-Krastanov and latter part was layer-by-layer. Texture of the Cu film was fcc(111) in both the cases. No point defects were present after deposition in both the cases.

Cu on Mo has been used in the study of nanowires on stepped metal surfaces [25], beaded thin films [26], fabrication of Fibonacci metallic superlattices [27], multilayers [28], superconducting transition–edge microcalorimeters [29] and metastable alloy thin films [30].

Cu on Ta has assumed technological significance for the latest interconnect fabrication where Cu is the interconnect material and Ta is a candidate for diffusion barrier [31, 32]. The control and reduction of the agglomeration of the Cu seed layers employed in the Cu interconnection [33,34] is an important technological challenge for the integrated circuits (IC) industry.

The important questions addressed in this thesis are

1. What is the effect of the substrate type and orientation on the defects and thermal stability of the Cu films?

Introduction

2. What are the effects of the deposition parameters, like substrate temperature, ion bombardment during deposition, film thickness, and the post-deposition conditions, like annealing, on the defects and the thermal stability of the Cu films?

3. What are the types (including their concentration and distribution) of defects that can be identified by THDS?

4. What is the thermal stability of the Cu film as measured by THDS? 5. What is the effect of defects on the thermal stability of the Cu films? 6. How do defects in thin Cu films differ from those in bulk Cu?

7. What is the effect of THDS parameters, such as helium location in the film, helium implantation energy, helium dose and heating rate during desorption on the defects and the thermal stability of the Cu films?

Table 1.2 shows deposition, post-deposition, and THDS parameters and the kind of information one can obtain from varying them.

Table 1.2 Film deposition, post-deposition, and THDS parameters and the kind of information that can

be obtained by varying them.

Parameter Phenomenon studied

a. Film thickness

b. Helium location in the film c. Substrate temperature

d. Argon ion assistance during deposition

e. Pre-implantation annealing temperature f. Helium implantation energy

g. Helium fluence

h. Heating rate during desorption

Effect of thickness on defects and thermal stability

Surface roughness, helium retrapping Effect of substrate temperature on defects and thermal stability

Effect of Ar ion assistance during deposition on defects and thermal stability. Ar spectrum gives information on Ar content in the film, mechanism of Ar release from the film, etc

Effect of annealing on defects, thermal stability Range of implanted helium and defect production Multiple filling of defects and trap mutation Activation energy of dissociation (valid independent of the order of the desorption process) and jump frequency (valid only for first order desorption)

Work on defects in thin Cu films carried out by others includes the following. Open volume defects were observed at the Cu (50 ML)/W(110) interface using variable energy positron (VEP) beam [35]. Helium diffraction was used to quantify the defects (adatoms, clusters, step edges and vacancies) in Cu films (
9 ML) on W(110) during growth [36]. Vacancy clusters in electroplated Cu films (0.5-2 µm) on Ta(20 nm)/SiO2(100 nm)/Si was probed by VEP beam [37]. Some of these are

Introduction

1.3 Structure of this thesis

Chapter 2 describes the experimental techniques used to characterize nanothin Cu films on Mo and Ta. The main experimental technique used in this thesis is

THDS. The ultrahigh vacuum experimental setup for performing combined IBAD and THDS experiments is explained, as well as desorption data acquisition and processing and data analysis. Other techniques used in this thesis are x-ray diffraction (XRD) to find the orientation relation between the film and the substrate, scanning electron microscopy (SEM) to characterize agglomerated Cu films and variable energy positron (VEP) beam analysis for probing open volume defects in Cu films. These techniques will be explained briefly. Chapters 3, 4 and 5 consider the characterization of Cu films on Mo (Mo(110), Mo(100) and Mo(polycrystalline) respectively) while Chapter 6 deals with Cu films on Ta (Ta(110) and Ta(100)). The visual outline of the thesis is shown in Fig. 1.1.

Fig. 1.1 Visual outline of the thesis.

Introduction Chapter 1 Experimental Methods Chapter 2 Cu/Mo _Cu/Ta Cu/Mo(110) Chapter 3 * Film thickness * Annealing * Helium location in the film * Helium implantation energy Cu/Mo(100) Chapter 4 *Film thickness * Substrate temperature * Annealing * Helium implantation energy * Helium fluence * Heating rate during desorption Cu/Mo(poly) Chapter 5 * Film thickness * Substrate temperature * IBAD * Annealing * Helium location in the film * Helium implantation energy * Helium fluence * Heating rate during desorption

Cu/Ta(110) & Ta(100)

Chapter 6 * Ta substrate * Film thickness * Annealing * Helium location in the film * Helium implantation energy * Helium fluence * Heating rate during desorption

Introduction

References

1. K. L. Chopra, Thin Film Phenomena (MacGraw-Hill, New York, 1969). 2. K. L. Chopra, R. Suri, and A. P. Thakoor, J. Appl. Phys. 48, 538 (1977). 3. M. Ohring, Materials Science of Thin Films, 2nd ed. (Academic Press, San

Diego, 2002).

4. D. J. Srolovitz and M. G. Goldiner, J. Metal 47, 31 (1995). 5. S. L. Firebaugh, K. F. Jensen, and M. A. Schmidt, Journal of

Microelectromechanical Systems 7, 128 (1998).

6. C. Detavernier, A. Özcan, C. Lavoie, J. J. Sweet, J. M. E. Harper, Mat. Res. Soc. Symp. Proc. 745, 135 (2003).

7. J. D. Carey, L. L. Ong and S. R. P. Silva, Nanotechnology 14, 1223 (2003). 8. J. Mizsei, L. Pirttiaho, M. Karppinen, V. Lantto, Sensors and Actuators B 65,

195 (2000).

9. S. Stoyanov and I. Markov, Surface Science 116, 313 (1982). 10. C. M. Kennefick and R. Raj, Acta Metallurgica 37, 2947 (1989).

11. D. L. Carroll, M. Wagner, M. Rühle, and D. A. Bonnell, J. Mater. Res. 12, 975 (1997).

12. J. Y. Kwon, T. S. Yoon, and K. B. Kim, J. Appl. Phys. 93, 3270 (2003). 13. F. A. Smidt, International Materials Reviews 35, 61 (1990).

14. J. K. Hirvonen, Materials Science Reports 6, 215 (1991). 15. J. K. Hirvonen, Mat. Res. Soc. Symp. Proc. 792, 647 (2004). 16. A. van Veen, Materials Science Forum 15-18, 3 (1987).

17. A. van Veen, Fundamental Aspects of Inert Gases in Solids, edited by S. E. Donnelly and J. H. Evans, (Plenum Press, New York, 1991), p. 41.

18. L. M. Caspers and A. van Veen, phys. stat. sol. (a) 68, 339 (1981). 19. G. J. van der Kolk and A. van Veen, Physica Scripta T13, 53 (1986).

20. J. van der Kuur, E. J. E. Melker, T. P. Huijgen, W. H. B. Hoondert, G. T. W. M. Bekking, A. van den Beukel, and B. J. Thijsse, Mat. Res. Soc. Symp. Proc.

396, 587 (1996).

21. J. van der Kuur, B. Korevaar, M. Pols, J. van der Linden and B.Thijsse, Mater. Res. Soc. Symp. Proc. 504, 57 (1998).

22. J. C. van der Linden, L. J. Seijbel, and B. J. Thijsse, Nucl. Instrum. Methods Phys. Res. B 148, 98 (1999).

23. B. S. Bunnik, C. de Hoog, E. F. C. Haddeman and B. J. Thijsse, Nucl. Instrum. Methods Phys. Res. B 187, 57 (2002).

24. T. P. C. Klaver, PhD thesis, Delft University of Technology, 2004. 25. F. J. Himpsel, T. Jung and J. E. Ortega, Surf. Rev. Lett. 4, 371 (1997). 26. Yu. S. Kaganovskii, D. L. Beke and S. P. Yurchenko, Surf. Sci. 319, 207

(1994).

27. K. Kyuno, T. Kaneko, M. Sakuda, S. Tianfu and R.Yamamoto, J. Magn. Magn. Mater. 126, 158 (1993).

28. S. Luby, E. Majkova, M. Jergel, E. D. Anna, G. Leggieri, A. Luches, M. Martino and J. Valicek, Appl. Surf. Sci. 106, 243 (1996).

29. K. D. Irwin, G. C. Hilton, J. M. Martinis, S. Deiker, N. Bergren, S. W. Nam, D. A. Rudman and D. A. Wollman, Nucl. Instrum. Meth. A 444, 184 (2000). 30. B. Zhao, D. M. Li, F. Zeng and F. Pan, Appl. Surf. Sci. 207, 334 (2003). 31. T. Laurila, K. Zeng, J. K. Kivilahti, J. Molarius, I. Suni, Thin Solid Films 373,

Introduction

32. K. M. Latt, H. S. Park, S. Li, L. Rong, T. Osipowicz, W. G. Zhu, Y. K. Lee, Journal of Materials Science 37, 1941 (2002).

33. T. Hara, K. Sakata, A. Kawaguchi and S. Kamijima, Electrochemical and Solid-State Letters 4, C81 (2001).

34. C-Y. Yang and J. S. Chen, Journal of The Electrochemical Society 150, G826 (2003).

35. P. J. Schultz, K. G. Lynn, W. E. Frieze and A. Vehanen, Phys. Rev. B 27, 6626 (1983).

36. H. Xu, Y. Yang and T. Engel, Surface Science 255, 73 (1991).

Experimental Methods

Chapter 2

Experimental Methods

2.1 Introduction

Thermal helium desorption spectrometry (THDS) is the primary experimental technique used in this thesis to characterize defects in and thermal stability of thin films. The technique works as follows. First, low energy helium is ion-implanted into the sample (film + substrate). After collisional slowdown and thermalization, these helium atoms diffuse interstitially until they either leave the sample or encounter a defect where they may be trapped. Subsequently, when the sample is given a temperature ramp, each helium atom is released from its defect at a temperature depending on the characteristic helium-defect binding energy. After the helium-defect dissociation, helium becomes mobile and diffuses to the surface and desorbs into the vacuum, where it is detected by a mass spectrometer. On the way to the surface, re-trapping in another defect may occur. It is clear that only those defects that can trap helium are detected by this technique. These include vacancies and vacancy clusters while the examples of non-trapping defects are dislocations, grain boundaries, and open pores. Helium can also be released when the film undergoes structural transformation and when the film evaporates from the substrate.

This chapter describes the full details of the THDS technique. It explains how this technique (which is generally used for bulk metals) is applied to the case of thin films. A description of the experimental setup combining thin film deposition (with possible ion assistance during deposition) and THDS is given. The processing and analysis of THDS data are also considered. To explain and support the outcome of some of the THDS experiments it was inevitable to use other experimental techniques. These techniques include x-ray diffraction (XRD), scanning electron microscopy (SEM) and variable energy positron (VEP) beam analysis. These techniques will be briefly explained.

2.2 Principles of IBAD and THDS

Ion bombardment of a growing film has been found to significantly affect a number of properties of the film [1-3]. The modified properties include adhesion, density, residual stress, texture (orientation), grain size and morphology, number of voids, hardness and ductility. This process of concurrent deposition of a film and directed ion bombardment is called ion beam assisted deposition (IBAD) [4-7]. The modification of the properties is due to the physical and sometimes chemical processes that occur when energetic ions are incident on the surface of a growing film. These processes are sputtering and desorption of atoms from the substrate, cluster nucleation and dissociation, enhancement of surface diffusion, atomic displacements in the bulk, and energy and momentum transfer to the atoms in the growing film. IBAD provides additional deposition parameters like ion to atom

Experimental Methods

arrival ratio (i/a), ion energy deposited per arriving atom, angle of incidence of the ion, and ion energy. In this thesis Cu films (5-200 Å) were deposited with 250 eV Ar ion assistance with an i/a = 0.1 on polycrystalline Mo (see Chapter 5). Ar was used since it is chemically inert while 250 eV Ar with i/a = 0.1 was used to unambiguously identify the effects of IBAD.

THDS is a technique that is conventionally used for quantitative analysis of small concentration of defects in metals [8,9]. The technique is based on the

properties of helium in metals viz. – helium is mobile at room temperature, migrates interstitially until it is trapped by a lattice defect, undergoes no surface adsorption and obeys first order desorption kinetics from simple defects. Fundamental aspects and technical requirements for quantitative thermal desorption spectrometry of ionically implanted inert gases have been first explained by van Gorkum and Kornelsen [10,11].

The THDS technique usually involves the following steps:

1. Damage production: The metal crystal (defect free) is implanted with keV ions to create defects.

2. Defect annealing: On annealing the crystal at a temperature, some defects created in step 1 may disappear either because they become mobile or helium dissociates from the defect.

3. Defect decoration: On sub-threshold (below the energy required for Frenkel pair creation in the crystal) implantation of low energy helium ions, the remaining defects after Step 2 may trap the mobile helium.

4. Helium desorption yielding the helium desorption spectrum: Monitoring of the helium release rate while heating the sample with a constant heating rate dT/dt (typically 40 K/s), usually to 2/3 Tm (Tm = melting temperature), which

removes all the helium and defects. Helium-defect complexes formed in step 3 dissociate. The desorption spectrum obtained consists of a number of peaks, each one being related to a specific defect, and, in the simplest case, the peak area is a measure of the number of those defects. Few complications may occur which includes the following. Trap mutation- a defect (vacancy or substitutional atom) that provides room for the binding of n helium atoms creates room for further added helium by emitting a self interstitial. Defects may become mobile during the temperature ramp. If the trap concentration is high re-trapping of helium may occur. Non-uniform trap profile will make it difficult to calculate the trap concentration values.

In this work, defects are already present in thin films due to film-substrate lattice mismatch and deposition conditions, which are most often far away from equilibrium. In order to detect intrinsic defects in the films one can directly start with Step 3. To investigate the effect of annealing on native defects in the film one should start with Step 2. For metastable films it will be shown in this thesis that with Steps 2,1 and 4 one can observe the helium release due to agglomeration of the film. In many instances Step 1 followed by 4 is used to detect the evaporation of the film from the substrate. In this work, deposition of the film itself is a step and some times to vary the location of the helium in the film it is done in two steps. The film/substrate systems considered in this work do not alloy on heating and hence the substrate is clean at the end of the step 4. For the films deposited with argon ion assistance, both the argon and helium desorption signal have to be monitored. For this the quadrupole mass spectrometer is switched between helium and argon channels with a sample time of 25 ms.

Experimental Methods

10 cm

QMS

Fig. 2.1. Schematic diagram of the apparatus for IBAD and in-situ THDS. The upper and the lower parts

of the main chamber are separated by a copper shield (CS). The arrows indicate the locations of the turbo pumps. The electron beam evaporator (E) is located in the lower part of the main chamber. Excess heat is removed by a water-cooled shield (W). Pneumatic shutter S1 controls the vapor beam. A Kaufman source (K) is used for ion assistance. Its output current is measured by a Faraday cup (F), located beneath the sample at sample position (1). Pneumatic shutter S2 controls the ion beam from the Kaufman source (K). An electron gun (EG) is used to heat the substrate during deposition. A fixed

Experimental Methods

aperture protects the surroundings of the sample from the ion and vapor beams. A magnetic linear motion drive (M) is used to move the sample. At position (2), the sample is rotated by 90º before it is moved into the THDS chamber. Helium implantation is done at sample position (3). Helium ion gun is not shown. At position (4) the sample is heated (desorption) by an electron gun (not shown). Valve (V) and load lock (L) are used to change the sample without breaking the vacuum. Needle valves (N) regulate the gas flow to the helium ion gun (IG) and the Kaufman source (K). To calibrate the mass spectrometer (QMS), gas stored between valves (V1) and (V2) supplied from a calibrated storage volume (Vb) is released in the THDS chamber. A capacitance pressure gauge (P) is used to calibrate the pressure in Vb(Figure reproduced from Ref. 10 with permission).

2.3 Apparatus for combined IBAD and THDS

The construction of the apparatus for ion beam assisted (thin film) deposition (IBAD) and thermal helium desorption spectrometry (THDS) by T. P. Huijgen et al. of our group was completed in 1991 [12]. In view of several shortcomings [13] of this design, it was modified by J. van der Kuur et al. [13,14]. The schematic of this

modified apparatus is shown in Fig. 2.1, has been described in detail by J. van der Kuur [13]. For convenience most of the details are given in the forthcoming sections. All the THDS experiments described in this thesis were carried out in this apparatus.

2.3.1 The main chamber

by

The main stainless steel chamber (Fig. 2.1) consists of a lower (‡ 444 mm u 318 mm) and an upper part (‡ 250 mm u 352 mm) separated

a copper shield (CS). This shield maintains the residual pressure in the upper part, where the sample is placed during deposition (1), to a low level and hence avoids impurity

incorporation in the film. There is a 9 mm aperture in front of the sample. Fig. 2.2 shows the schematics of the pumping scheme of the apparatus. In order to achieve ultrahigh vacuum conditions, the apparatus is baked at 250 qC for 48 hours after each vacuum break (on average after 100 spectra), using a dismountable oven. The final base pressure obtained after bake out is 1 u 10-10 torr. A load lock (L) is used to increase the bake out cycle time. An electron beam evaporator (E) and the Kaufman ion source [15-17] (K) are located in the lower part of the main chamber. The 4 kW e-beam evaporator (ESV4, Leybold-Heraeus GmbH) is the film deposition source; the crucible-sample

p p C S S D R R CS TP TP T u eff,u u l l l 1 2

Fig. 2.2. Schematic drawing of the pumping

scheme in the apparatus. The system is pumped by two turbo molecular pumps (TPu and TPl) to

obtain differential pumping between the upper and lower part of the system. A copper shield (CS) is also installed for this purpose. Each turbo pump has a separate roughening pump (R1

and R2). To enhance the compression rate for

light gases (He, H) an oil diffusion pump (D) can be connected between the turbo pumps and the roughening pumps. A titanium sublimation pump (T) provides the possibility to enhance the vacuum after changing a sample with load lock (Figure reproduced from Ref. 10 with permission).

Experimental Methods

distance is 46 cm and the angle of incidence of the vapor flux on the sample is 15q off-normal. A water cooled shield (W) between the copper shield and the crucible acts as a heat sink for the evaporation source. The film deposition rate is measured using a quartz crystal oscillator; the sensor head is placed at a larger distance from the

crucible than the sample due to geometrical constrictions. However, the angle of incidence of the vapor flux is the same for both the sample and the sensor head. The extraction grid of the 3 cm Kaufman ion source is located 23 cm from the sample and the ion beam is incident normally on the sample. During the ion source operation, the residual pressure in the upper part of the chamber has to be kept minimum to

minimize charge exchange between the energetic ions and neutral molecules in the residual atmosphere, which minimizes the energetic neutrals reaching the sample. This has been achieved by the replacement of the 3cm grid by an 1 cm grid and extending the Kaufman source by a 28 mm diameter hollow chimney. The Kaufman source was usually operated at an Ar partial pressure of 4 u 10-5 _{mbar. The ion flux at} the sample was determined by measuring the current (PA range) on the sample.

2.3.2 The sample holder

The schematic of the movable part of the sample holder is shown in Fig. 2.3. The sample is typically 2 mm in thickness and 10 mm in diameter. The sample (1) is mounted in the center of a square hole (30 u 30 mm2) cut from a 85 u 50 mm2 ceramic plate (2), with 4 support rods (3) made of tungsten. Each rod (Ø 0.5 mm) is connected to the sample by inserting it into a Ø 0.6 mm hole, 3 mm deep, located on the rim of the sample. The rods are mounted on the ceramic plate using small stainless steel blocks (4). A W-Re thermocouple (5) is spot welded to the sample between the holes in order to measure the temperature of the sample. The thermocouple wires (Ø 0.25 mm) are shielded by ceramic cylindrical jackets. The sample holder has been constructed with 6 independent contacts (6). Two bayonet connectors (7) and (8) have been fitted to both ends of the ceramic base plate in order to facilitate the docking of the sample holder in the load lock and also to help the positioning of the sample in the THDS chamber.

2 cm 1 2 3 4 5 6 7 8

Fig. 2.3. Schematic diagram of the movable part of the sample holder.

See text for the meaning of the labeled numbers (Figure reproduced from Ref. 10 with permission).

2.3.3 THDS chamber

Helium implantation and desorption are carried out in the THDS chamber. The sample is moved from position (1) to position (3) using a magnetic linear motion drive (Fig. 2.1). Before inserting the sample into the THDS chamber it is rotated by 90q as shown at position (2). At position (3), the sample is implanted with helium ions having a well-defined energy using a modified version of ion source designed by van Veen et al. [18]. This source is usually operated at a helium partial pressure of 5u10-8

Experimental Methods

torr. The source consists of an ionization chamber, a Pierce extractor and two electrostatic Einzel lenses separated by a Wien filter. The Wien filter selects ions according to their e/m ratio. The ion energy is controlled by the potential between the last Einzel lens and the target. Filtering of the neutral energetic helium atoms is achieved by deflecting the ions leaving the ion source by 20q. There is a 4 mm aperture in front of the sample. The helium ion flux on the sample is determined by measuring the current on the sample (~10-30 nA), using a nano ampere meter; the reading is integrated by the computer using a calibrated timing card and an A/D converter. Helium fluence has to be corrected for backscattering and secondary electron generation. The ions reaching the vicinity of a target are neutralized by the electrons extracted from the target. Some ions backscatter, yet the electrons given up by the target are counted. Hence backscattering leads to an overestimation of the number of implanted ions. Impinging ions excite electrons in the target a certain percentage of which will have sufficient energy to escape from the target resulting in overestimation of fluence. 2.5 kV I(v 300 V + -+ -v v A A I I + -v v v ampiso D/A-1 D/A-2 t,m t,s 2 1 I_f,s I_f f I_f) t e

Fig. 2.4. Sample heating control circuit. A high voltage source (2.5 kV) delivers heating

power to the sample proportional to Ie. A voltage controlled current source I supplies the filament current If necessary to obtain the electron gun emission current Ie needed to hold the sample temperature at the setpoint value Ts. The control computer generates increasing values of Ts with time (according to the desired heating rate) and delives them in the form of thermocouple voltage vt,s to the converter D/A-2, using calibration tables provided by the thermocouple manufacturer. The differential amplifier A1 generates the temperature error signal, i.e. a signal proportional to the difference between the measured thermocouple voltage vt,m and the setpoint value vt,s corresponding to Ts. The differential amplifier A2 controls If by adding this error signal to the estimated control voltage vI f,s supplied by the computer through D/A-1(Figure reproduced from Ref. 10 with permission).

After being implanted with helium, the sample is moved to position (4) (Fig. 2.1) where it is heated linearly with time (usually 40 K/s) typically to 2000 K. The heating source is an electron emitter wire held at +300 V, and the sample is held at +2500 V. The thermocouple voltage is measured using an isolation amplifier for galvanic separation. A 12-bit A/D-converter is applied to read this voltage. The output power of the electron gun is controlled by a feedback circuit in order to achieve the desired heating rate control during desorption. This circuit is shown in Fig. 2.4. Monitoring the partial pressure of helium as a function of time using a quadrupole mass spectrometer (Balzers QMS 420) yields the helium desorption rate. For

experiments involving argon bombardment, the partial pressure of argon as a function of time yields argon desorption rate. A QMS consists of ion source, a mass filter and an ion detector (secondary electron multiplier, electrometer). The output is available

Experimental Methods

as an analogue voltage. This voltage is converted into counts/second (R(t)) using a voltage to frequency converter. A computer controlled counter measures R(t). During the desorption of an IBAD sample, QMS is operated in switching mode, switching between helium and argon mass channels. The detector efficiency of a QMS (G) for a given gas is the ratio of the output current I(t) and the partial pressure pp(t) of the gas I(t) = eR(t) { Gpp(t) (2.1)

The sensitivity of the system (E in counts/atom) is given by [13]: E kTGW

eV_d

kTG

eS (2.2) wherek is the Boltzmann constant, T the temperature,W the mean residence time of a gas molecule in the desorption chamber, e the electron charge, Vd the desorption volume and the S pumping speed. There is a door system between the IBAD and THDS chambers by which the sensitivity in the THDS chamber can be increased by closing the doors. The value of Vd# 3 l and W for He and Ar were 0.21s and 0.75 s respectively.

2.3.4 Calibration of the mass spectrometer

Vcal V V S p T T T QMS He T b cap d 1 2 3 4

Fig. 2.5. Schematic diagram of the quadrupole mass spectrometer calibration setup

(Figure reproduced from Ref. 10 with permission).

The sensitivityE is calibrated by measuring the total number of counts generated when a known number of gas atoms are introduced into the desorption chamber volume (Vd). The schematic of the setup is shown in Fig. 2.5. The number of atoms in the desorption chamberN(t) and the count rate of the QMS R(t) are related by [13]:

N(t) WR(t)

E (2.3) The gas is introduced near the position (4) (Fig. 2.1) since R(t) is dependent on the gas inlet location. A calibration container with volumeVcal (1.60 cm3) (Fig. 2.5) is filled with a pressure of 10-2 mbar. A capacitance manometer (pcap) is used for accurate pressure measurement. After filling Vcal, the gas is expanded into a second container with volumeVb (991.9 cm3), thereby decreasing the pressure by a factor of approximately 600; this is done because the manometer cannot measure pressures of as low as 10-5 mbar (1012 atoms).Vb acts as a storage facility in order to calibrate the spectrometer after each measurement.W is determined by measuring the exponential decrease of the QMS signal after a sudden closure of the helium or argon gas inlet. By knowingW,N(t), R(t) and using relation (2.3) E can be calculated.

Experimental Methods

2.3.5 Deconvolution and smoothing of the measured desorption flux

In a desorption measurement, the number of helium atomsN(t) in the chamber is measured as the sample is heated linearly with time.N(t) is the convolution of the desorption flux L(t) and the residence function of the chamber. Correcting L(t) for the delay effect of W is called the deconvolution of the desorption flux. For a volumeV pumped with speed S:

dN(t)

dt AL(t)  SN(t)

V_d (2.4) whereL(t) is the desorption flux from the sample (He/cm2/s) and A the implanted area (0.1257 cm2). The first term on the right hand side represents the number of helium atoms entering the desorption volume from the sample and the second the number of helium atoms removed by the pumps. Substituting forN(t) using eq. (2.3), one obtains

L(t) 1 EA R(t) W dR(t) dt § © · ¹ (2.5) L(t) is calculated using Savitzky-Golay smoothing and differentiation [19]. The choice of the smoothing is such that a fourth degree polynomial is locally fitted to the data point of interest plus the data points on either side within a time range of 1.8 residence times. In most cases this comes down to an 11- data point fit.

QCO control PC RS232 evaporator shutters ADC ADC nA sample F.C. iso-amp thermo-couple DAC DAC e-gun heat control D/D R/S 24-bit counter

discriminator _amplifierfast _{mass spectrometer}quadrupole

DAC

Fig. 2.6. Overview of the layer growth control system and data acquition (Figure

reproduced from Ref. 10 with permission).

The schematic of the experimental control and data acquisition is shown in Fig. 2.6. The experiments at positions (1), (3) and (4) (Fig. 2.1) are controlled by a 486-based DOS PC, equipped with a set of A/D and D/A converters and digital input and output interfaces. A Hewlett Packard multiprogrammer (HP 6940B) has been used as chassis for 14 independent I/O modules. This multiprogrammer is controlled by a GPIO interface.

The different types of measurements that were performed with this apparatus and their respective purposes were summarized in Table 1.2 (see Chapter 1).

Experimental Methods

2.3.6 Data analysis

The diffusion, trapping, and release of helium in a substrate can be treated as a one-dimensional problem assuming the average penetration depth of the helium ions to be much smaller than the lateral distance to surface sinks. The time-derivative of the interstitial helium particle concentrationni(x,t) at depth x below the surface depends on a diffusion term, the trapping of helium in defects, the release of helium from defects, and a source term indicative of the helium introduced by implantation. Assuming free diffusion of helium between interstitial positions, the following equations are obtained in the case of one type of trap [20],

wn_i(x,t)

wt Di(t)

w2n_i(x,t)

wx2 Qi(t)ctr(x,t)ni(x,t) Qtr(x,t)ntr(x,t)  S(x,t) (2.6)

withDi(t) the time-dependent diffusion coefficient for interstitial helium,Qi(t) and

Qtr(x,t) the frequency factors for interstitial jumps and release jumps from traps respectively,ctr(x,t) the trap concentration, and S(x,t) the concentration profile of implanted helium after thermalization (c and n are expressed in molar fractions).

The time-derivative of the trapped helium profile consists of a trapping and a release term,

wn_tr(x,t)

wt Qi(t)ctr(x,t)ni(x,t) Qtr(x,t)ntr(x,t) (2.7)

The diffusion coefficient Di(t) is related to the interstitial jump length Oi,

D_i(t) Oi

2_Q

i(t)

6 (2.8)

and the frequency factors Qi(t) QDexp 'Si k § © · ¹exp  E_HeM kT(t) § © ¨ · ¹ ¸ (2.9) Q_tr(x,t) Q_Dexp 'Str k § © · ¹exp  E_trD, He_(x) kT(t) § © ¨ · ¹ ¸ (2.10)

depend on the Debye frequency QD, the entropy difference 'S between the

configurations after and before a jump, the migration energy for interstitial jumps of helium E_HeM_{, the Boltzmann constant k, the time-dependent sample temperature T(t),}

and the dissociation energy of a helium atom from a trap E_trD,He_.Q

Dexp('S/k) is denoted by Q0.

For a first order desorption, interstitial diffusion is fast and the trap concentrationctr(x,t) is so small that the terms containing ctr can be ignored. The retrapping probability of a helium released from a trap is negligible. Ignoring the x-dependence of E_trD,He_{, removing the x-dependence of n}

tr by integrating with respect to

x, equation (2.7) reduces to L(t) dNtr(t) dt Ntr(t)Q0exp  E_trD, He kT(t) § © ¨ · ¹ ¸ (2.11)

whereNtr(t) is the number of trapped helium per unit area, Q0 is the rate constant (s-1),

E_trD,He _{is the dissociation energy of a helium atom from a trap, k is the Boltzmann}

constant and T(t) is the time-dependent sample temperature.

When the sample is heated linearly with time such that T(t) = T0 + Et, where

T0 is the initial temperature and E the heating rate, first order desorption results in a characteristic asymmetric peak shape. WhenE= 40 K/s with Q0 = 1013 s-1 release

Experimental Methods

peaks are observed with full width at half maximum on the temperature scale of ~ 8 % of the temperature at the peak maximum. The temperature Tm at which L(t) is maximal depends on the heating rate:

β T_m2 = ν₀k E_trD,Heexp − E_trD,He kT_m ⎛ ⎝ ⎜ ⎞ _⎠ ⎟ (2.12) assuming T0 « Tm. Taking the logarithm of both sides of equation (2.7) yields an Arrhenius plot. Plotting ln(β/Tm) versus 1/T

2

m for different β values and performing a

straight line fit, one can obtain EtrD and ν0 from the slope and the intercept respectively. Once E_trD and ν

0 are known, the peak shape can be calculated and compared with the measured peak. obtained by this method is valid independent of the order of the desorption while ν

Etr D

0 is valid only for a first order desorption [21, 22].

For a zero order desorption like for example sublimation of the Cu film L(t) is given by L(t) = −dN(t) dt =ν0exp − E_subl kT(t) ⎛ ⎝ ⎜ ⎞ _⎠ ⎟ (2.13) where N(t) is the number of Cu atoms per unit area and Esubl activation energy for sublimation of the Cu film.

Diffusive release is usually observed for the heavier noble gases like for example argon in Mo [23] (thermal vacancy assisted diffusion). In Au and in Al helium in a vacancy diffuses assisted by thermal vacancies before the temperature is reached where first order desorption should occur [24].

Helium release is also seen when a thin film undergoes agglomeration. In the case of Cu/Mo(100) the helium release peak due to film agglomeration is broader than a first order desorption for Cu films ≤ 70 Å and narrower than a first order desorption for Cu films ≥ 75 Å.

2.4 Other techniques

Apart from THDS several other characterizing techniques were used. These will be briefly explained here.

2.4.1 X-ray diffraction

X-ray diffraction measurements were done using a θ-2θ diffractometer with an incident beam monochromator and a position sensitive detector (CuKα1 radiation) set to an angular range of 30-110 °2θ. θ-2θ scans were done to determine the phases present in the film, the type of preferred orientation from the observed Cu reflections, and to investigate the out-of-plane crystallite size from the observed broadening of the diffraction lines. The sharpness of the texture in the Cu films was determined using rocking curves. These curves were obtained by plotting the net integrated intensity of the {200} reflection of Cu recorded by the position sensitive detector set on a fixed 2θ, as a function of the incident beam angle. The in-plane orientation relation between the film and the substrate was determined using a diffractometer with an Eulerian cradle (and CoKα radiation), by recording the diffracted intensity of the Cu{111}

Experimental Methods

reflection as a function of angle I, the rotation of the sample around its surface normal, at an appropriate tilt angle \ of the sample.

2.4.2 SEM, EDS and AES

Scanning electron microscopy (SEM) was used to study the surface

topography of agglomerated Cu films on polycrystalline Mo, Ta(110) and Ta(100). A high energy electron beam (15 keV) is scanned across the surface. The incident electrons cause low energy secondary electron to be generated, and some escape from the surface, which are detected by attracting them towards a phosphor screen. The intensity of the light is measured with a photomultiplier. Some of the incident electrons may strike an atomic nucleus and bounce back into the vacuum. These electrons are known as backscattered primaries and can be detected with a backscattered electron detector.

Energy dispersive spectroscopy (EDS) was carried out in the following way. A 15 keV electron beam strikes the surface of the sample, which causes X-rays to be emitted. The energy of the rays emitted is characteristic of the sample. These X-rays originate in a region 2 Pm in depth and hence EDS is not a surface technique, though it is used in conjunction with SEM. EDS was used to estimate the thickness of the Cu islands on Mo and Ta. This is based on the principle of attenuation of

monochromatic x-rays [25]:

ln(I0/I) (P1cscT1P2cscT2)Ud (2.14)

whereI0/I is the ratio of the corrected average counting rate for the bare substrate and that for the substrate covered with a film of thickness d. The mass absorption

coefficients of the film are P1, a mean value for the incident (polychromatic) beam, andP2, for the characteristic wavelength being counted. The angles T1 and T2 are those made by the incident beam and emergent beam with the sample, and U the density of the film.

Auger Electron Spectroscopy (AES) was used to determine the composition of the top few layers of a surface. Incident electrons (3-20 keV) cause core electrons from atoms in the sample to be ejected. This results in a photoelectron and an atom with a core hole. Electrons with a higher energy drop to the core hole, which relaxes the atom. The energy released can be converted in to an x-ray (EDS) or electron emission (Auger electron). After Auger electron emission the atom is in a doubly ionized state. The energy of the Auger electron is characteristic of the element that emitted it. The short inelastic mean free path of Auger electrons in solids ensures the surface sensitivity of AES. In this work, AES was used to measure the thickness of the wetting layer in the case of 100 Å Cu/Mo(poly) annealed to 857 K for 10 s. Wetting layer is the Cu layer in between the Cu islands in the case of an annealed Cu film. The thickness of the wetting layer was calculated using the mean free path value for the Mo transition and the relation

I I0exp(d /O), (2.15)

whereI0 and I are the intensities of Mo signal without and with wetting layer,

respectively,d is the thickness of the wetting layer, and O the inelastic mean free path of Mo.

Experimental Methods

2.4.3 Variable energy positron (VEP) beam analysis

Positron annihilation has been used for studying defects in metals. With mono- energetic slow positron beams, the implantation profile of the positrons can be adjusted to the region of interest in the sample by varying the energy of the positrons [26]. A positron that is implanted in to the solid annihilates with an electron,

generating two ~511 keV J photons. The energy spectrum of these annihilation J rays is broadened by the momentum component of the annihilated electron-positron pair. The broadening is characterized by:

a) The S (shape) parameter that indicates the fraction of the positrons that annihilate with low momentum electrons (valence or conduction electrons), which is related to the open volume defects present in the sample.

b) The W (wing) parameter that indicates the fraction of the positrons that annihilate with high momentum electrons (core electrons), which is related to the chemical environment where annihilation takes place.

In this thesis, VEP was applied to obtain open volume defect concentration for 200 Å Cu/Mo(100) and 1000 Å Cu/Ta(110).

References

1. J. M. E. Harper, J. J. Cuomo, R. J. Gambino, and H. R. Kaufman, Nucl. Instrum. Methods Phys. Res. B 7/8, 886 (1985).

2. S. M. Rossnagel and J. J. Cuomo, Thin Solid Films 171, 143 (1989). 3. G. K. Wolf, Surface and Coatings Technology 43/44, 920 (1990). 4. F. A. Smidt, International Materials Reviews 35, 61 (1990). 5. J. K. Hirvonen, Materials Science Reports 6, 215 (1991). 6. B. Rauschenbach, Vacuum 69, 3 (2003).

7. J. K. Hirvonen, Mat. Res. Soc. Symp. Proc. 792, 647 (2004). 8. L. M. Caspers and A. van Veen, phys. stat. sol. (a) 68, 339 (1981). 9. G. J van der Kolk and A. van Veen, Physica Scripta T13, 53 (1986). 10. A. A. van Gorkum and E. V. Kornelsen, Vacuum 31, 89 (1981). 11. E. V. Kornelsen and A A van Gorkum, Vacuum 31, 99 (1981)

12. T. P. Huijgen, W. H. B. Hoondert, L. J. Seijbel, B. J. Thijsse and A. van den Beukel, Nucl. Instrum. Methods Phys. Res. B 59/60, 150 (1991).

13. J. van der Kuur, PhD thesis, Delft University of Technology. 1998.

14. J. van der Kuur, E. J. E. Melker, T. P. Huijgen, W. H. B. Hoondert, G. T. W. M. Bekking, A. van den Beukel and B. J. Thijsse, Mat. Res. Soc. Symp. Proc.

396, 587 (1996).

15. H. R. Kaufman, J. J. Cumo and J. M. E. Harper, J. Vac. Sci. Technol. 21, 725 (1982).

16. J. M. E. Harper, J. J. Cuomo and H. R. Kaufman, J. Vac. Sci. Technol. 21, 737 (1982).

17. H. R. Kaufman, J. Vac. Sci. Technol. A 4, 764 (1986).

Experimental Methods

19. W. H. Press, S. A. Teukolsky, W. H. Vetterling, and B. P. Flannery, Numerical Recipies in C, 2nd ed. (Cambridge University Press, Cambridge, 1992), pp. 650-655.

20. W. H. B. Hoondert, PhD thesis, Delft University of Technology, 1993. 21. H. E. Kissinger, Analytical Chemistry 29, 1702 (1957).

22. E. J. Mittemeijer, Journal of Materials Science 27, 3977 (1992).

23. A. van Veen, W. Th. M. Buters, G. J. van der Kolk, L. M. Caspers, and T. R. Armstrong, Nucl. Instr. Meth. 194, 485 (1982).

24. V. Sciani and P. Jung, Radiation Effects 78, 87 (1983).

25. H. A. Liebhafsky, H. G. Pfeiffer, E. H. Winslow, P. D. Zemany, X-ray Absorption and Emission in Analytical Chemistry (John Wiley & Sons, Inc,, 1960).

Cu on Mo(110)

Chapter 3

Ultrathin Cu Films on Mo(110) Characterized by Helium Implantation

Abstract

Point defects and thermal stability of ultrathin Cu films (3-200 Å) deposited on Mo(110) substrate at 300 K have been characterized using thermal helium desorption spectrometry (THDS). Implantation of the samples (10-100 Å Cu/Mo) with 1000 eV He+ aided in detecting He release from monovacancies in Cu films and desorption of the films (above 1200 K) from Mo substrate. A thickness dependent peak is identified in the helium desorption spectra, which is shown to be due to the process of island formation in the Cu films, using pre-implantation annealing treatments for the 40 Å film. Native defects in the films (3-200 Å) were probed using 75 eV He+ implantation. The effect of annealing on the native defects in a 20 Å Cu film was studied by 75 eV helium implantation. The surface roughness of a 5 Å Cu film is characterized by implanting it with 75 eV helium and subsequently depositing

overlayers (5-95 Å). This also yielded information on retrapping of helium released from the base layer in the overlayer

3.1 Introduction

Helium implanted into a thin film deposited on a substrate may get trapped in the defects present in the sample. On heating the sample, He is released from these defects at

temperatures that are characteristic of the He-defect dissociation energies. Helium may also be released if structural changes take place in the film. Hence the He desorption signal versus temperature data provides information on defects, thermal stability, film coverage, and ion damage effects. We have used this thermal helium desorption spectrometry (THDS)

technique [1] to characterize ultrathin Cu films (3-200 Å) on Mo(110) substrate. Cu/Mo(110) is a model for studying defects and thermally induced changes in a fcc film on a bcc

substrate. Cu/Mo has found applications in nanowires on metal surfaces [2], beaded thin films [3], Fibonacci superlattices [4], multilayers [5] and X-ray detectors [6]. Earlier work of Cu on Mo(110) [7-9] and THDS analysis of Cu(100) crystal [10] is relevant here. At room temperature Cu was found [7] to grow preferentially in the Frank-van der Merwe mode on Mo(110) while above 960 K [8] in the Stranski-Krastanov mode. The results presented in this paper are a part of an ongoing THDS study of Cu films on Mo [11] and Ta substrates.

3.2 Experimental details

The experiments were carried out in UHV (1.0 × 10-10 _{torr) [12]. Mo(110) (∅10.0 × 2.0} mm, purity 99.999 %, deviation of the surface from the (110) plane < 1˚) was annealed several times to 2100 K for 1 minute. Cu lumps (purity 99.999 %) in a graphite crucible was vaporized (incident 15° with respect to substrate normal) using e-beam evaporator with deposition rate 1 Å/s. The sample was implanted with 1000 eV He+ of fluence φ = 2.0 × 1014 He+/cm2 incident 20° with respect to the sample normal. Finally, the sample was heated to 2000 K at a rate β of 40 K/s, and the He desorption flux L (atoms/cm2_{/s) was monitored as a} function of the sample temperature T using quadrupole mass spectrometer. During heating Cu film sublimes from Mo at ~1350 K; sample heating is continued to 2000 K to anneal the Mo

Cu on Mo(110)

substrate before deposition of the next film. Desorption spectra have been corrected for the mean residence time (0.21 s) of He. He+fluence has not been corrected for ion backscattering and secondary electron yield. In the spectra shown here the normalized He desorption flux  is plotted as a function of T, where (T) = L(T) / (in K–1). Hence the integral area under a spectrum is the initially trapped fraction of He (ftr).

Figure 1. The normalized He desorption spectra of 10-40 Å (a) and 50-100 Å (b) Cu films deposited on

Mo(110) at 300 K and implanted with 1000 eV He+_{of fluence 2.0 10}14_He+_/cm2_{. The 0 Å spectrum in (a) was}

obtained for the bare Mo(110) substrate. The dashed horizontal line in the 75 and 100 Å spectra near the edge of the peak M in (b) indicates signal cutoff due to counter saturation. (c) He desorption flux of (b) multiplied by a factor of 60 in the temperature range 300-850 K.

3.3 Results and discussion

3.3.1 Bare Mo(110) substrate

The normalized He desorption spectrum obtained when the bare, well annealed Mo(110) substrate (0 Å) was implanted with 1000 eV He+of fluence 2.0  1014He+/cm2is shown in Fig. 1(a). The explanation of this spectrum will help in understanding some of the other results presented here. The threshold energy (Eth) for Frenkel-pair creation by He in Mo is

216 eV assuming a minimum displacement energy (Ed) value of 33 eV [13], calculated on the

basis of the maximum energy transferred in an elastic collision. Hence 1000 eV He+creates defects in Mo such as vacancies and vacancy clusters. These vacancies act as traps for the thermalized He atoms. Helium desorption from a Mo(110) crystal has been extensively studied [14]. The peaks H (1138 K), G (983 K) and F (908 K) are due to He released from a Mo monovacancy (V) containing 1, 2 and 3 He atoms respectively. The peaks I (1200-1450 K), J (1504 K) and K (1630-1870 K) are known to be He released from He-vacancy clusters (HenVm, n, m
2). The peak O (~400 K) has been attributed to He release from surface related trapping sites of Mo. The area under the total spectrum shows that the trapped fraction of the incident He is 0.40.

Cu on Mo(110)

3.3.2 Effect of film thickness

The normalized He desorption spectra of the Cu films (10-100 Å) deposited on the well annealed Mo(110) substrate at 300 K and implanted with 1000 eV He+ of fluence 2.0 ×1014 He+/cm2 are shown in Fig. 1(a) and (b). The calculated [15] projected range and straggle of 1000 eV He in Cu are 72 Å and 41 Å respectively. The Eth for Frenkel pair creation in bulk

Cu by He is 85 eV assuming Ed = 19 eV [13]. Therefore 1000 eV He+ creates defects in Cu

films. These defects add to the native defects already present in the film and at the interface. These native defects arise due to the deposition process, which allows point defects to be built into the film, and because of the film-substrate lattice misfit. The energy of He reaching Mo and hence the defect production in Mo decreases as the Cu film thickness over it is increased. The thermalized He may get trapped in the defects in the Cu film, at the interface and in Mo.

The helium release from Mo will be considered first. For the 10 Å film, the H peak of Mo is lowered by 55 K compared to the bare substrate. This could be due to the reduction of Mo surface energy by the Cu film. The intensities of the peaks I, J and K are seen to decrease with the increase in film thickness, confirming the reduction of the energy of the He reaching Mo. No He release from Mo is seen in the case of 100 Å spectrum.

Helium release from Cu is the main interest of this work. The labels S, HCu_{, L and M will} be used to denote the He release from Cu at different temperatures. The peak S (~450 K) for the 50-100 Å films (Fig. 1(c)) is the He release from trapping sites close to the surface of the Cu film. Such a helium release was also seen in the THDS study of perfect Cu(100) crystal [10]. The peak HCu seen for the 40 Å film at 608 K, shifts slightly to higher temperature with increase of film thickness and reaches 731 K for the 100 Å film (Fig. 1(c)). This peak is very likely due to the He released from monovacancies in the Cu film and the peak shift indicates a possible substrate-induced strain variation in the film as a function of film thickness. In [10], He release from monovacancy of bulk Cu occured at 780 K. Helium dissociation energy (Ed) derived from the HCu peak is 1.9 eV for the 100 Å film by applying first order desorption

with an attempt frequency (ν) of 1.0 × 1013 _s-1_{. This may be compared with the value E}

d = 2.0

eV obtained in [10]. The peak L for the 10 Å film (460 K) shifts to higher temperature with increase in film thickness and is at 1160 K for the 100 Å film. In order to understand this broad (when compared to a first order desorption peak) and strongly thickness dependent peak, pre-implantation annealing treatments were done for the 40 Å film, where the peak L lies at 942 K. This will be explained in the next section. The peak M is present for all (10-100 Å) the desorption spectra of Figure 1; at 1225 K for the 10 Å film and at 1283 K for the 100 Å film. This peak is due to the He released when the Cu film starts desorbing from the Mo(110) substrate. Note that the M peak temperatures are lower than the melting point of bulk Cu (1358 K). The defects in the Cu film that can trap He until the sublimation temperature of the film have to be He-vacancy clusters or small He bubbles. Fitting zero order desorption with ν = 6.63×1012 _s-1 _{(Debye frequency of the bulk Cu [8]), a desorption} energy of 2.63 eV was obtained for the M peak of the 40 Å film. This may be compared with the values 3.17 ± 0.06 eV/atom [8] (desorption of the second ML of Cu) and 3.0 eV/atom [9] (desorption of Cu overlayers) obtained by TDS.

3.3.3 Effect of annealing

In order to understand the mechanism of He release resulting in the L peak of Fig. 1, film annealing treatments prior to He implantation were carried out for 20 Å and 40 Å films. Only the 40 Å films are considered here, for the 20 Å films the results were analogous. Fig. 2(a) shows the normalized He desorption spectra of 40 Å films deposited on Mo(110) at 300 K,

Cu on Mo(110)

annealed to the indicated temperatures for 10 s, cooled to 300 K and implanted with 1000 eV He+of fluence 2.0  1014He+/cm2. When the film is annealed to 834 K the intensity of the L peak has become considerably smaller than that for an unannealed film, while the intensities of the I, J and K peaks of Mo have increased. The latter is only possible if the 1000 eV He+is able to create more defects in Mo. Annealing the film to 1122 K reduces the L peak intensity further; while the intensity of peaks I, J and K increases further. In fact most of the 1122 K spectrum (except the 1100-1200 K range) is virtually identical to the bare Mo spectrum. This clearly shows that from the point of view of 1000 eV He the thickness of the 40 Å Cu film annealed to 1122 K has decreased over most of its area. Since Cu film desorption up to this temperature is negligible, there must be a corresponding increase in the film thickness in the few remaining regions of the film. Hence annealing to 1122 K (which is close to the end of the L peak temperature) has caused the film to transform into a strongly islanded structure. This is confirmed by ex situ backscattered electron image (Fig. 2(b)) of 100 Å Cu film deposited on polycrystalline Mo (with {110} preferred orientation) at 300 K and annealed to 857 K for 10 s and cooled to 300 K [16]. The height of these islands is ~650 Å as determined by ex situ EDS. In between the islands ~ 2 ML of Cu was measured using ex situ AES. In annealing experiments of Cu/Mo(110) [7], only the first two monolayers of Cu were stable. Excess Cu was found to agglomerate into 3-D crystals at ~700 K for 3-4 ML of Cu film and at ~800 K for 10 ML Cu film [17]. A large-scale structural transformation like island

formation should invariably lead to the release of substantial amounts of trapped He and this is seen in the form of the L peak. The presence of the peak M even after annealing the film to 1122 K indicates that the defects responsible for this He release must be present in the island regions of the film. The theoretical model developed in [18] to estimate the time required for 2D-3D transformation of a monolayer deposit into 3D crystals upon annealing may be used to explain i) shift of L peak to higher temperatures with increase in film thickness and ii) L peak width. i) indicates the formation of critical nuclei with a certain number of edge atoms in a reasonable time requiring a critical temperature (which increases with film thickness) to be exceeded. ii) signifies the time for spontaneous growth of nuclei until the 2D-3D

transformation is completed.

Figure 2. The normalized He desorption spectra (a)

of 40 Å Cu film deposited on Mo(110) at 300 K annealed to the indicated temperatures for 10 s, cooled to 300 K and implanted with 1000 eV He+_of

fluence 2.0  1014_He+_/cm2_{. (b) Ex situ backscattered}

electron image of 100 Å Cu film deposited on polycrystalline Mo at 300 K, annealed to 857 K for 10 s and cooled to 300 K.