Chapter 1

Study of Molecular Bonding or Adhesion by Inelastic Electron Tunneling Spectroscopy, with Special Reference to Microelectronics

Robert R. Mallik

Department of Physics, The University of Akron, Akron, Ohio USA

E-mail: mallik@uakron.edu

Abstract

This chapter presents an outline of the principles, methods, applications, and scope of Inelastic Electron Tunneling Spectroscopy (IETS) with emphasis placed on the study of molecular adsorption on metal oxide and semiconductor surfaces. Strengths and limitations of the technique are highlighted, with particular attention being paid to applications in adhesive systems comprised of materials pertinent to microelectronics device fabrication including epoxy resins, polyimides, and silanes. A brief description of how IETS may be used to investigate adsorption and conduction mechanisms for self-assembled monolayers of molecules adsorbed on photovoltaic semiconductor materials is given as a segue into an examination of how IETS and related techniques are being developed for the study of molecules of interest in the rapidly developing field of molecular electronics.

Keywords: Adhesion, microelectronics, molecular electronics, photovoltaics, thin-films, vibrational spectroscopy, IETS.

1.1 Introduction

Inelastic Electron Tunneling Spectroscopy (IETS) is a relatively new technique in the toolbox of surface scientists. It was discovered by Jaklevic and Lamb in 1966 while investigating the superconducting bandgap of lead in metal/insulator/superconductor tunnel junctions [1]. Specifically, these workers were recording current-voltage (I-V) curves of aluminum/ aluminum oxide/lead tunnel junctions at a temperature of 4.2 K by immersing them in liquid helium. The first metal electrode of an IET junction is usually referred to as the base electrode and the (usually) superconducting top electrode is the cover electrode. In tunneling experiments, derivatives of I-V curves (i.e., plots of quantities proportional to dI/dV and d2I/dV2 with respect to bias voltage) are often recorded to reveal superconducting structure more clearly. Jaklevic and Lamb noticed that, in addition to low-bias structure present due to the superconducting energy gap, associated with the lead cover electrode (which appears at bias voltages V = ±Δ / e ≈ ±2.15mV where, e, is the electronic charge), additional fine structure was evident at higher bias voltages. This additional structure was best revealed as peaks in the second derivative plots, and the peaks appeared at bias voltages in the range of vibrational modes of molecules (approximately 0–500 mV or, equivalently, 0–4000 cm−1). It transpired that the peaks were due to the presence of minute amounts of pump oil which had inadvertently been adsorbed onto the aluminum oxide surface of the tunnel junction base electrode. The peak energies corresponded closely to IR vibrational mode energies of the hydrocarbons present in the pump oil and recognition of this fact led to the birth of IETS. Since then, IETS has been used to study a wide variety of adsorbates on metal oxide and semiconductor surfaces. The purpose of this chapter is to briefly highlight the principles of IETS in order to illustrate the strengths and limitations of the technique with particular attention given to applications relevant to adhesive bonding, particularly in the area of microelectronics and its potential in the developing area of molecular electronics. For readers wishing to learn more about the theory, experimental procedures, and scope of IETS several books on the technique are available [2] but, as a starting point, the reader is referred to the excellent review article by Hipps and Mazur [3].

IETS is a technique particularly useful for the investigation of adsorption and conduction mechanisms of ultra-thin layers. While advances in more widely used and well-established surface vibrational spectroscopies, for example multiple reflection/absorption methods in IR [4] and surface enhancement effects in Raman [5, 6], have allowed for surface-specific measurements on a variety of systems [7], the sensitivity of both of these techniques nevertheless decreases with sample layer thickness. In contrast to this, IETS sensitivity actually increases as the layer thickness decreases and becomes optimal for adsorbed layers close to monolayer coverage. The reason for this is that whilst IR and Raman require greater sample volume for increased interactions between the sample and excitation energy source (photons), quantum tunneling by its very nature is intrinsically sensitive to thin layers at length scales corresponding to nanometer thickness. This is because the wavefunction of the tunneling electrons (the excitation source) is greatly attenuated when traversing the thin layer. Indeed, the probability of electrons tunneling through a thin layer, which constitutes a potential barrier, decreases exponentially with the height and width of the barrier. For a rectangular barrier of width, d, and height, φ, the electron tunneling probability, P, is given approximately [8] by the expression P = exp(−2αd), where α2 ≈ 2me φ/ħ2 (where me is the electronic mass and ħ = h/2π, h being Planck’s constant). This highly sensitive dependence of the tunneling probability on barrier height and thickness is why IETS is ideally suited to probe species at, or in close proximity to, the surfaces of the tunnel barrier. Another important feature of IETS alluded to above is that, since the technique is based on measurement of thin layers I-V curves, information can be extracted regarding conduction mechanisms through said films. This information is not available via IR and Raman spectroscopies. In practice, information from all three techniques is mutually valuable. IETS can provide complimentary information, inaccessible via the other two techniques, which may lead to a more thorough characterization of the surfaces and interfaces of the system under investigation than by the use of any one of these techniques in isolation [9].

The potential of IETS in various, and seemingly diverse, research areas such as surface chemistry, heterogeneous catalysis, analytical chemistry, environmental pollution monitoring, adhesion science, radiation damage, biological chemistry and electronic energy level studies was recognized early in the development of the technique [10]. Of particular interest for the present chapter, as will be shown, is early work illustrating the usefulness of IETS in the study of numerous adhesive systems, for example silane coupling agents on alumina [11, 12, 13, 14, 15, 16, 17], and other adhesive, or adhesive-related, systems on alumina including epoxides [18, 19, 20], polymers [21, 22], and phenolics [23]. Since aluminum is the base electrode of choice for most IETS work, and because aluminum is a widely used material for adhesive bonding in aerospace, automotive, and packaging applications, it is perhaps not surprising that IETS was deemed particularly appealing for such adhesion studies. Since these early studies, it should be noted that IETS has also been applied to other adhesive systems for example systems of adhesion promoters on glassy substrates [24]. More recently, IETS has been used to study adsorption on materials other than the native oxide of the base electrode metal, i.e., on so-called artificial tunnel barriers. In 1989, Barner and Ruggiero reported the material and electron tunneling properties of thin radio-frequency magnetron sputtered alumina films supported on copper base electrodes [25]. Around the same time, Mazur and Cleary demonstrated the potential utility of an aluminum nitride artificial barrier deposited by reactive ion-beam sputtering [26], and two years later, it was reported that sputtered amorphous silica formed a viable artificial tunnel barrier when deposited onto gold base electrodes [27]. These and subsequent studies on artificial tunnel barriers were significant in that they demonstrated that the technique of IETS could be extended to investigate a wider range of systems than just those involving adsorption studies on alumina. IETS has now been successfully performed on other semiconductor barriers [28, 29], including photovoltaics [30], and it has also been used to the study molecular adsorption on photovoltaics [31] illustrating how the technique may be applied to materials of interest in the area of microelectronics.

1.2 Principles of IETS

1.2.1 General Overview

As outlined in the Introduction, IETS relies on the quantum mechanical phenomenon of electron tunneling between two metal electrodes through a sufficiently thin potential barrier. It allows one to measure the vibrational energies of molecular species constituting the barrier when excited by these tunneling electrons. Barriers are incorporated in metal/insulator/metal tunnel junctions fabricated sequentially on insulating substrates by conventional vacuum deposition techniques. They must be uniformly thin (of the order of 2–3 nm) and continuous if sufficient tunnel current is to flow. Monolayers of compounds of interest may be introduced onto the barriers if desired. Resulting IET spectra yield information regarding the nature of molecular bonding at the interface so formed. IR, Raman, and other modes in the barrier and metal electrodes are detectable, and IET peak intensities may be correlated to surface coverage, bond angles, and the location of bonds within the tunnel barrier [2, 32].

A description of how IETS is applied in practice, describing the above items in more detail, is given in sections 1.2.2,...