It is anticipated that a new generation of advanced electronic and optical devices will involve the synthesis of diverse materials in single or multielement thin-film form, or in layered heterostructures. These devices will most likely involve diverse materials such as high-temperature superconductors, ferroelectric, electrooptic, and optical materials; diamond; nitrides; semiconductors; insulators; and metals in the form of ultra-thin layers with sharp interfaces in which the layer thickness may reach atomic dimensions. Therefore, it becomes increasingly important to be able to monitor the deposition process in situ and in real time, particularly for complex multicomponent oxides or nitrides, in which the production of the desired phase is a highly sensitive function of the growth conditions, often requiring relatively high-pressure oxygen or nitrogen environments up to several hundred mTorr, and in some cases, several Torr. Consequently, the growth environment for many of these materials is incompatible with conventional surface-analytic methods, which are typically restricted to high-or ultrahigh-vacuum conditions. New deposition and analytical methods, or adaptation of those already established, will be required.

Since thin-film growth occurs at the surface, the analytical methods should be highly surface-specific, although sub-surface diffusion and chemical processes also affect film properties. Sampling depth and ambient-gas compatibility are key factors which must be considered when choosing in situ probes of thin-film growth phenomena. In most cases, the sampling depth depends on the mean range of the exit species (ion, photon, or electron) in the sample.

Low-energy (5–15 keV) ion scattering spectroscopy (ISS) and direct recoil spectroscopy (DRS) are well-established analysis techniques that have been used for several years to characterize surfaces of materials. ISS utilizes a beam of ions in the 1–15 keV range which are scattered by a surface atom at an angle θ1 relative to the direction of incidence. In this energy range, the collisions are elastic and are described by classical two-body collision kinematics. The kinetic energy E1 of the scattered primary ion is given by

where E0 is the primary ion kinetic energy and α = M2/M1. Note that Equation 1 has real values only if M2 > M1, and ISS is not able to detect surface atoms which are lighter than the primary ion beam.

The kinetic energy of the backscattered primary particle may be measured either by using an electrostatic energy analyzer (ESA) or by pulsing the beam and using a time-of-flight (TOF) detection scheme. The ESA only detects the ion fraction (typically 10 2−10−4) of the scattered primary beam and transmits only a narrow portion of the energy distribution of these ions, whereas the TOF detection scheme simultaneously detects the full energy range of both ions and neutrals. Consequently, the required beam dose for TOF detection is 3–4 orders of magnitude smaller than that required by the ESA. A typical analysis beam dose for TOF detection is ∼1011−1012 ions/cm2, removing or displacing one part in 103−104 of the near-surface atoms and making the TOF scheme essentially nondestructive.

Ellipsometry has been used in microelectronics since the early 1960s when it became necessary to measure the thickness of transparent films on reflective substrates. Ellipsometry is an old technique but with the increasing technological importance of thin films, particularly in microelectronics, and with the availability of faster computers for calculations, ellipsometry is in a renaissance. In principle it is a simple technique (don't confuse simple with easy!) that relies primarily on the physical optics of stratified media. Excellent treatments of ellipsometry exist (e.g., a book, recent reviews), and it is not the intent here to try to improve upon them. Rather, this article is intended to briefly discuss the essential ideas and then to show a few examples of in situ ellipsometry applied to modern materials-research problems. The emphasis here is on in situ ellipsometry, because it is one of the few techniques that can accurately monitor thin-film processes while they occur, yielding both fundamental properties and process information about the system under measurement. In addition it is now possible to use feedback to affect control of thickness, composition, and temperature, for example.

We first consider some basic questions: What is ellipsometry, what does it measure, and how well? Next, we discuss hardware for in situ ellipsometry, and finally we show a few examples.

The remarkable properties of diamond, including its hardness, chemical inertness, high thermal conductivity, low coefficient of friction, optical transparency, and semiconducting properties, have led to considerable research in the area of diamond thin-film deposition. Diamond films have been characterized ex situ by a large number of diagnostic techniques including Raman spectroscopy, x-ray diffraction, SEM, and TEM. In situ diagnostics, which can provide information in real time as the film is growing, are less common.

Laser-reflectance interferometry (LRI) has been used to monitor the growth of diamond films in situ. The technique involves measuring the intensity of a laser beam reflected from the substrate surface on which the film is growing. The reflected beam is the sum of beams reflected by the gas-diamond interface and the diamond-silicon interface. Oscillations in the reflectivity are observed as the film grows because of interference between the reflected beams. Each oscillation indicates an increase in film thickness of λ/2n, where λ is the laser wavelength and n is the index of refraction of the film. If the index of refraction of the film is known, the thickness and growth rate can be determined in situ. For LRI measurements with 632.8-nm-wavelength HeNe lasers, the index of refraction of diamond films has been found to be within 10% of the bulk diamond value of 2.4. Each oscillation therefore indicates an increase in film thickness of 0.13 μm.

The reflectivity measured by LRI is also affected by scattering because of surface roughness.

The majority of sophisticated ultra-high-vacuum (UHV) systems for deposition of thin films, such as molecular beam epitaxy (MBE) machines, contain some kind of electron diffraction apparatus which is used to scrutinize the surface structure of the film while it is grown. Reflection high energy electron diffraction (RHEED) is probably the most frequently employed configuration. An excellent introduction to RHEED, including a treatment of electron diffraction, reciprocal-space description, reflection from imperfect surfaces, etc., was recently published in the MRS Bulletin by Lagally and Savage. Hence, we will not review these basics here. Rather, we will assume the reader to be familiar with that article, and will refer to it as LS.

The materials discussed in LS include Si, Ge on Si, and GaAs. The applications of MBE for synthesis of semiconductor thin films and heterostructures are widely recognized. MRS has recently bestowed its greatest honor, the Von Hippel Award, to Alfred Y. Cho for his pioneering work on MBE synthesis of GaAs and its application to new devices.

In contrast, here we focus on complex oxides—cuprates, titanates, manganates, etc. This is a relatively new area of application for both MBE and RHEED. Actually, it was only after the discovery of high-temperature superconductivity (HTS) that many MBE systems were designed and built specifically for metallic oxide deposition. More recently, some of these machines have also been employed to synthesize other interesting oxides, for example, ferroelectrics and ferromagnets. In all these studies, RHEED has been the principal diagnostic tool and source of information about the growing film surface.

For many thin-film applications it is useful or necessary to accurately monitor and control the film's physical properties. Particularly important are composition and structure, which determine most of the material's properties. Examples include the semiconducting properties of SixGe1−x, the ferroelectric properties of BaTiO, and the superconducting properties of Bi2Sr2CaCu2Oy, all of which depend strongly on composition. The development of in situ analyses not only provides the possibility of feedback control during deposition, but also assists in a more efficient evaluation of the best processing conditions to achieve particular properties. Control of the film structure and composition during film growth has not been easy, and in situ analyses during film nucleation and growth has been a goal of many investigators.

Our interest in in situ monitoring of film deposition was motivated by sputter growth of Bi2Sr2CaCu2Oy from a composite target. These studies found that the film composition was a strong function of deposition parameters, particularly the substrate temperature. Investigating these complex relationships required the growth and ex situ analysis of many films. It was clear that a technique to determine the composition of films during growth could efficiently unravel the complex relationships between composition and processing parameters.

An important consideration for an in situ analysis technique is that it not interfere with the growth process either by altering the film surface or by changing the deposition dynamics. During deposition, the substrate temperature, gas flow, and deposition rate all must be maintained; any interruptions for the analysis may alter the growth irreversibly.

The engineering of advanced micro-electronic circuits, optoelectronic devices, and integrated optical circuits requires precise control of the lateral dimensions and thicknesses of device features and of the stoichiometry and doping of epitaxial semiconductor regions. This is preferably achieved by real-time monitoring and control of the individual deposition and etching processes that constitute the processing sequence. The use of optical probe techniques for the real-time monitoring of etching and/or growth processes is favored because of their nondestructive character and their potential use in real-time feedback control. Some of these methods are ideal in monitoring the overall growth process and/or substrate temperature in industrial applications, requiring low cost and maintenance. For example, in situ reflectance-spectroscopy methods, such as dynamic optical reflectivity (DOS), spectral-resolved normal incidence reflectance spectroscopy (MRS), or pyrometric interferometry (PI), are successfully applied to various deposition processes and provide information on both the growth rate and the composition of the deposits. However, small changes in the reflectance (because of chemical interactions at the surface of the films with the reactants supplied from the vapor phase) are of the order of 10−3 to 10−4 and are hardly observable with normal-incidence reflectance techniques because of the high reflectivity of substrate/film interface, which is typically of the order of 40%–60% for many semiconductors.

In order to increase the sensitivity to surface- and interface-related growth properties, alternative optical-observation methods such as reflectance difference spectroscopy (RDS), surface photoabsorption (SPA), and spectral ellipsometry (SE) have been developed.