Marta Klanjšek Gunde National Institute of Chemistry, Ljubljana, Slovenia Kevwords- materials for modern technologies, infrared spectroscopy, microelectronics, thin fiims. IR beams, infrared beams, LO modes, Longitudinal Optical modes, Berreman effect, IRE, Internal Reflection Element, ATR, Attenuated Total Reflection, MNOS, Metal/Si-Nitride/Oxide/Semiconductor materials, Metal/lnsulator/Semiconductor material, PECVD, Plasma-Enhanced Chemical Vapour Depositions epitaxial layers, Si02, silicon oxides, silicon nitrides, a-Si, amorphous silicon, PSG, phospho-silicate glasses, BPSG, boro-phospho-silicate glasses, fluorinated-silica glasses, semi-insulating polycrystaiine silicon, aluminium oxides, aluminium nitrides, strontium titanates, gallium nitride, superiattice, multilayer materials Abstract- The application of infrared spectroscopy to analyse thin solid films used in modern technology devices has been reviewed. A variety of problems related to chemical bonding and morphology are possible to solve for amorphous, polycrystalline, and crystalline films as well as for low-dimensional structures Determination of the sort and amount of dopants is now routinely applied to control the technological process. Together with relevant scientific hypothesis and with others appropriate characterisation methods, infrared spectroscopy may be efficiently used in research and development in material science. Kliučne besede- materiali v modernih tehnologijah, spektroskopija infrardeča, mikroelektronika, plasti tanke, IR zarki infrardeči, LO nacini valovanja optični longitudinalni TO načini valovanja optični transverzalni, Berreman efekt, IRE elementi notranji refleksijski, ATR refleksija tota na dušena MNOS materiali kovina/Si-nitrid/oksid/polprevodnik, MIS materiali kovina/izolator/polprevodnik, PEVCD nanosi OVO plazemsko izbol|sani, plasti epitaksialne SiOz oksidi silicijevi, nitridi silicijevi, a-Si silicij amorfni, PSG stekla fosfor-silikat, BPSG stekla bor-fosfor-silikat, stekla fluor-silikat, silicij polikristalni pol-izolatorski, oksidi aluminijevi, nitridi aluminijevi, titanati stroncijevi, nitriti galijevi, superkristali, materiali večplastni Povzetek- Članek predstavlja pregled uporabe metod infrardeče spektroskopije za analizo tankih plati, ki se uporabljajo v modemih tehnologijah. Mogoče ie rešiti različne probleme, ki so povezani s kemijskimi in strukturnimi lastnostmi v amorfnih, polikristalnih m monokristalnih tankih plasteh, pa tudi v strukturah s kvantnimi dimenzijami. Določanje vrste in vsebnosti dopantov se že rutinsko uporablja za spremljanje m morebitne korekcije v tehnološkem procesu. Z ustreznimi znanstvenimi hipotezami in skupno z drugimi analiznimi metodami lahko infrardeča spektroskopija nudi odlično pomoč praktično na vseh nivojih raziskav in razvoja materialov. I. INTRODUCTION In this paper the application of infrared (IR) spectroscopy to analyse most frequently used microelectronic materials is considered with special attention to the recent literature. In the first part, the semiconductor substrates were discussed /1/. in the second part (present article), IR spectroscopy of thin solid films deposited or formed on such substrates is reviewed. The literature published in 1990 and later is considered mostly. Thin films are one of the crucial components of the microelectronic and other modern technology devices. To ensure high yield of devices, the films must satisfy a large set of chemical, structural, and electrical requirements. Film composition and thickness have to be strictly controlled. Defects and imperfections present in these films are critical for small device geometries, therefore their densities have to be low. Excellent adhesion, low stress, conformable step coverage, and appropriate surface topography are demanded /2/. A large variety of films are used for this purpose. They can be metals, semiconductors, or insulators. They may be crystalline, polycrystalline, or amorphous. They may have high density or they may be porous. The formation of such films is accomplished by a large variety of techniques that can be divided into two groups; (a) film growth by interaction of a vapour-deposited species with the substrate material; and (b) deposition of film without causing changes to the substrate. The first category includes thermal oxidation and nitridation of single crystal silicon, formation of silicides by direct reaction of a deposited metal and substrate /2/, as well as electrochemical etching that forms porous layers (e.g. porous silicon, /3/). The second group includes three subclasses; chemical vapour deposition (CVD), physical vapour deposition (PVD) and coating of the substrate with liquid, which is then dried to form the solid thin film. By the CVD process single crystal thin films (epitaxy), amorphous, or polycrystalline films may be formed. The PVD includes sputtering, evaporation, and molecular beam epitaxy (MBE). For the coating of substrate with liquid, most common the spin-coating technique is applied /2/. The crystaliographic structure of prepared tiiin films can vary from a highly disordered (amorphous) state to a well-ordered (e.g. epitaxial growth on a single crystal substrate). Dielectrics are frequently observed in amorphous state, while most metals result in polycrystaiiine structure. Silicon films can be amorphous, polycrystaiiine, or single crystal, depending on the deposition temperature and on the substrate material /2/. The properties of thin films are closely related to their chemical composition, materia! structure, order, and method of preparation and may be substantially different from those of the corresponding material in bulk. For extensive characterisation of films, a non-destruc-tive method is desired to apply. Such demands are fulfilled by optical techniques. Among them, IR spectroscopy is potentially the ideal one for characterising thin films. It is especially essential if the nature of chemical bonds at the surface or at some interface have to be understood, where any sample preparation may induce serious artificial effects. IR spectroscopy has been used to solve a variety of structural problems for amorphous, polycrystaiiine, and crystalline films and to investigate the dependence of the film structure upon the deposition parameters. The greatest volume of work was performed for silicon and silicon-containing films /4-7/. il. THEORETICAL BACKGROUND The spectroscopic characterisation of thin solid films and surfaces includes analysis of their chemical composition, concentration of impurities, film thickness, or/and concentration profiles. For conducting layers the properties of free electrons or holes in some cases can be determined also. The analyses are based on spectroscopic measurements of the contribution of vibronic excitations and of free carriers to the dielectric properties of the sarr)ple: the frequency dependent dielectric function includes informations about molecular vibrations and about conduction electrons. IR spectroscopy is used mainly for films that contain metal-hydrogen or metal-oxygen bonds that absorb in the mid IR spectral region. Structures of films with metal-hydrogen or metal-metal bonds or heavier oxides have IR absorptions that fall in the far-IR and can be studied by far-IR spectroscopy and Raman spectroscopy. The goal of the successful spectroscopic analysis is to apply the optimised optical conditions of the sampling technique to obtain unambiguous data also on very thin films or on multilayered structures. Therefore, many diversified techniques to observe spectra for many configurations of the sample have been developed. Polarised and unpolarised beams may be applied with optionally chosen incidence angle. The interpretation of obtained spectra has become more and more complex especially if extracted data are expected to be very precise and as complete as possible. Samples required in modern technology applications usually include silicon in monocrystalline, polycrystaiiine, or amorphous forms. Their high refractive indices cause a considerable reflection of the IR beam on each surface boundary. In thin films (d=X) the observed spectral features are not only due to the nature of the dielectric function, but interferences ofthe partial waves reflected atthe surface and/or interfaces ofthe layers also occur. The so-called Fabry-Perot fringes carry informations about optical constants and thickness of the layer. The interference fringe patterns obscure the baseline, impede detection of weak peaks, and alter the intensity. The broad region where anomalous dispersion in refractive index of dielectric materials takes place, shifts the position of IR absorption bands and additionally changes their shapes and intensities. All effects act simultaneously and cause strongly pronounced optical effects in IR spectra of such samples. It is well known that optical theory can predict or eliminate these effects from certain spectra /8-11/. ill. SAMPLING TECHNIQUES Avariety of techniques are available for obtaining IR spectra of different thin solid supported films. The choice of sampling technique depends on the type of the sample and on the information desired from the analysis. In the following, some more pertinent techniques for thin film analysis are described briefly. Essentially the same techniques are used as described in the previous paper /1/. To apply them on thin films and multilayers, some modifications have to be done. III.1. Transmission The transmission experiment is the most familiar to anyone who uses IR spectroscopy on a routine basis. During measurements, the light passes through the sample, and its absorption from the entire thickness of the film (multilayer, superlattice) and of the substrate is measured. The substrate must be transparent or low-absorbing so that a portion of light lost due to absorption and/or scattering on them is small. Typical substrates for transmission work in the mid IR spectral region are polished crystal discs made of alkali halides (e.g. KBr, NaCI, KCl). For the modern technology applications it is of much importance, that low-level doped silicon and GaAs slices are also suitable for this purpose. Transmission studies are used for dielectric films with thickness on the order of |.i,m that have been deposited on transparent or semitransparent substrates. The thickness restriction is due to overdamping of intense absorption lines. Routine quantitative analysis of transmittance spectra of thin supported films are made from their negative logarithm (i.e. absorbance spectra). The intensity diminish due to absorption in the substrate is account for by subtraction ofthe corresponding absorbance spectrum (Figure 1). It is assumed that the absorption bands in the so obtained spectrum are proportional to the absorption coefficient of respective vibration, concentration of the corresponding species, and optical path length (Beer-Lambert law). The approxima-tion neglects the effects of refractive index as well as all optical effects. It is routinely used for organic materials where absorptions are small, although some errors are still introduced. In precise work especially with inorganic materials, the more complex analytical methods of chemometrics have to be applied /6/. Another possibility is to apply optical theories to account for optical effects that are strongly pronounced in spectra of these samples /8,9,11,12/. In some situations this is the only way to minimise the error of quantitative analysis. 4 4 r ■ " I. Isl • n layers, «>1 substrate I o I si L = -iog(r,) A,I - -iog(r/,) calculated absorbance Ai = A^i ~ A^ Figure 1. Normal incidence transmitiance (T) measurement of the substrate (index s) and of n layers deposited on them. Absorbance (A) of the unknown layer (index I) is obtained by subtraction of the corresponding data for the known composition. Example for one layer (n = 1). the semiconductor substrates are appropriate reflective substrates for external reflection spectroscopy to be performed. Additional possibility to apply external reflection for thin films analysis are the metal layers used in various types of MOS devices. For the impinging light, these layers act as reflecting mirrors. For that reasons, the external reflection is relevant analysing method for most films used in microelectronic industry. There are many ways to measure the external reflection of thin solid supported films. All of them measure the specular reflectance obeying Snell's laws. Optionally polarised beam impinges on the sample at a certain angle. The radiation passes trough the sample until it reflects on the reflective surface somewhere inside the sample. Then, the beam returns trough the sample towards the detector (Figure 2.). By increasing number of reflections on the upper surface of the sample, the sensitivity of measurement can be further increased (see/1/, Fig. 5.). For films thicker than the wavelength (2.5 - 25 |j.m for the whole mid IR), the intensity of the reflected light is similar to that of the transmitted light trough the sample with an twofold effective pathiength. In such cases the term reflection-absorption spectroscopy (RAS) is usually used to describe the experiment. For thinner films, the absorption of the radiation inside the sample will be strongly influenced by the reflecting surface, by the polarisation of the incoming radiation, by the incident angle, and by the orientation of dynamic dipoles inside investigated material with respect to the reflective surface. The polarisation of the incoming radiation influences the intensity of absorption. Most intensity is collected in the component that is polarised parallel to the plane of incidence, whereas the contribution of the perpendicular component is very small. The absorption of light inside thin film reaches its maximum when the incident angle is about equal to the Brewster's In spite of all sorts of problems, the most simple transmission technique is very important. Before the extensive use of computers, the studies reported in the literature were done more or less only by the simple transmission measurement and even today studies using this approximate method are often reported. It is used especially to identify the basic chemical composition of the film. Many times it is the most convenient method to follow the changes in sample materials after some process (e.g. thermal annealing) qualitatively. By this procedure, very recently some small changes in spectra of superlattices were measured /13, 14/. III.2. External reflection When a one- or multilayered structure is deposited on a reflective substrate, the reflection of the IR beam from the sample may bear worthy information about its optical properties. If the incident and reflected beams lie outside the sample, the method is called the external reflection. Due to high refractive indices, silicon as well as lll-V compound materials have considerable reflectance even in low-level doped form. Increasing doping, the reflectance of slices is increased further. Therefore, (- v iii| - »1 fiä^i iiiiii Siiiiiffife^ , J ^reflective surface 1 calculated absorbance 4s-log(-—) ■«0 Figure 2. External reflection measurement of substrate (index s) and layers deposited on them (index I). Any incident angle and polarisation of the incident beam may be applied. Absorbance of the unknown layer is obtained as negative logarithm of layer reflectance that was normalised by the reference reflectance. angle for the reflective material. For metals, this angle is very large (-90°), whereas for silicon smaller incidence angle is preferable (-60°). The largest angle that can be obtained experimentally, is about 85°. When the experiment is performed at this largest angle, the term near grazing incidence angle reflection spectroscopy (NGIA) is used. External reflection spectroscopy of molecules that were adsorbed on metal surface was formulated 30 years ago /15/. It gained full advantage of increasing sensitivity by parallel polarisation and near grazing incidence angle. The experimental conditions together with the optical consideration of the three-phase system leads to the so-called metal surface selection rules that can be used to determine the orientation and molecular ordering at the surface /16/. A small blue shift of the adsorbate band maxima respect to the corresponding bands in transmission was thought to be due to optical effects inherent in the experiment. For inorganic thin films, that are extensively used in modern technologies, the most important is the so-called "Berreman effect" /17/. He made an experiment with very thin (<2 |am) crystal films, deposited on metallic substrates. The component of the beam polarised normal the film (parallel to the plane of incidence), causes absorption bands at frequencies of the zeros of film dielectric function. These are the frequencies of the longitudinal optical (LO) polar modes. They depend strongly also upon intrinsic material properties, e.g. the effective charge and mass of all species participating the corresponding mode, dielectric properties of the surroundings etc. /18/. The Berreman effect is an macroscopic phenomenon of electrodynamics and offers the possibility of an precise analysing tool for very thin dielectric layers. However, the interpretation of the results is substantially different from measurement in transmission. In polar materials the band frequencies appearing in both experimental configurations are much apart so that the difference can not be described as a "small blue shift" as it is relevant for organic adsorbates. Furthermore, only some factors affecting absorption frequencies obtained in transmittance and in external reflection of parallel polarised light are the same. For all these reasons, the full understanding and interpretation of measured spectra also for thicker layers, other polarisation, incidence angles and substrates was not performed until recently when various spectral simulations with extensive computer support were accomplished /19-21/. III.3. internal reflection In internal reflection spectroscopy, the incident radiation is reflected from the interface between a material with higher refractive index and a material with lower refractive index as shown in Figure 3. The higher refractive index material is called internal reflection element (IRE) and is cut to a specific geometry. In microelectronic applications, the semiconductor sub-strate (low-level doped silicon or lll-V compound materials) usually represent an IRE. The material to be sampled (single layer or layered structure) is coated directly on IRE. For total internal reflection, the incidence angle must exceed the critical angle. In this circumstances, the lower refractive index material is probed by that portion of radiation (called the evanescent wave) that passes trough the boundary of the IRE crystal surface. The evanescent field is characterised by refractive indices of the two materials, incident angle, state of polarisation, and upon the wavelength of the beam in the IRE crystal. However, the decay of this field inside the optically rarer medium (its penetration depth) is independent upon the polarisation. The obtained spectrum is similar to a transmission spectrum. The method is often called attenuated total reflection (ATR). (a) ^ metal (optional) -n layers, n>l -IRE (Si wafer) (b) - metal (optional) ^—n layers', «>1 IRE (Si-wafer) Figure 3. Internal reflection experiments with one reflection (a) and two reflections (b). For more experimental configurations, see Hi, Fig. 6. Internal reflection spectroscopy is especially useful for films that are too thin for the spectrum to be measured in transmission. In microelectronic and photovoltaic systems as metal/oxide/semiconductor (MOS), met-al/Si-nitride/oxide/semiconductor (MNOS), and metal/insulator/semiconductor (MIS) are of interest. In those systems a very thin oxide interlayer between M and S has to be characterised. In an IR experiment, the interlayer is illuminated via polished areas of the border of the Si-wafer that acts as an IRE (Fig. 3). In the oxide film, absorption of the IR light due to vibronic resonances is only possible for p-polarisation (parallel polarisation) where it is still amplified by the metal film (M). By this configuration, the 1.3 nm thick oxide on silicon within a MIS structure was measured /22/ (see also Fiq 3). Various experimental configurations of internal reflection spectroscopy appearing in the recent literature /23-25/ are essentially ATR experiments. Among them, there is also excitation of surface polaritons, called also surface electromagnetic waves (SEW). The echo from the back of the thin layer can be avoided by residual vacuum gap of nm range between a sample and IRE (Otto geometry). The later method is not always possible to perform. By the ATR measurements, the optical depth of the material may be profiled. The method makes use of angle-dependent penetration depth of evanescent wave /26/. The procedure represents one possibility to detect and analyse the depth-dependent composition of films. IV. EXAMPLES A large variety of films used in modern technologies represent relevant measurable samples for IR spectroscopy. Metal layers can not be analysed but they may play an important role in using some of the reflection techniques. All other type of films can in principle be analysed in terms of their intrinsic properties such as chemical composition, structure, order, free charge carriers, thickness, etc. The examples of IR spectroscopy application for thin films are divided in representative groups as epitaxial layers, silicon oxide films, silicon nitride films, amorphous silicon, various glass-type films, other films, multilayers, heterostructures, and superlattices. IV.1. Epitaxial layers Epitaxial layers are grown on a surface of a single crystal substrate whereas its crystallographic orientation is retained. Therefore, epitaxial layer is an extension of a substrate. In homoepitaxiai layers, the layer and the substrate are the same material such as silicon on silicon. Heteroepitaxial layers are made from different materials, such as silicon on sapphire or rnercury-cad-mium-telluride on cadmium telluride /27/. The most interesting property of epi-layers is that they can be doped to a different extent than the substrate. Most epi-layers used in production environment are produced by CVD process involving silane halides, whereas MBE growth technique is used mostly for the research and development. Many factors of a finished devices depend upon the exact thickness of the epi-layer involved. The measurement of epi-layer thickness by the IR interference method is known from 1961 by Spitzer and Tanenbaum /28/ where the basic requirements for the interference fringes to be seen in the external reflection spectrum are defined. There has to be a suitable spectral range in which the epi-layer is transparent. Provided that on the interface between the epi-layer and the substrate the impurity concentration suddenly changes, the incident radiation will be reflected both at the surface of the epi-layer and at the interface. Since the waves transverse the layer, these so-called Fabry-Perot fringes carry information about layer thickness as well as about its optical constants. The onset of the fringes will occur at a wavelength governed by the carrier concentration of the heavily doped substrate. The spectra are measured by external reflection technique using unpolarised IR beam at normal incidence /27, 28/. To increase the sensitivity of the method, oblique incidence of polarised beam may be applied /5/. For the method to be applied for homoepitaxiai Si layers, the substrate resistivity has to be less than 0.02 Ocm (at 23°C) and the resistivity of the epi-layer more than 0.2 fžcm (at 23°C). In these circumstances, the technique is capable of measuring the thicknesses of both n- and p-type layers greater than 2 |im thick. With reduced precision the technique may also be applied to both n- and p-type layers from 0.5 to 2 |am thick. The external reflection spectra of some homoepitaxiai Si layers are shown in Figure 4. (U o c « o (U 0.8-, 0.6- 0.4- 0.2- 0.0 2500 2000 1500 1000 500 wavenumbers, cm-i Figure 4. External reflection spectra of two silicon homoepitaxiai layers. full line: substrate: 0.015 Qcm, Sb-doped, n~type, epi-layer: P-doped, n-type, 1-6 Llcm, measured thickness 23.4 ixm dashed: substrate: 0.002-0.004 Qcm, As-doped, n-type, epi-layer: P-doped, n-type, 2.0-2.4 Llcm, measured thickness 7.2 ^.im By the same technique, the thickness of the heterostructures of 111-V semiconductors epitaxially grown on GaAs substrate were measured. Simultaneously, the diffusion profile in epitaxial Si-layers were studied /5/. IV.2. Silicon oxide films Silicon oxide films are one of the most crucial films for the microelectronic industry. Their ability to form highly passivating, stable, and insulating films represents the basis for planar processing of silicon integrated circuits. Silicon oxide is the simplest form of glass and appears also in most mixed glasses. Therefore, the study of its properties may help in understanding the whole group of materials. Silicon oxides fabricated in the laboratory condense into amorphous solids noted by SiOx. If x=0 we have amorphous silicon and if x=2 the silicon dioxide was formed. The structure of amorphous layers compose of statistical mixture of Si-(Siy04-y), y=0,1,2,3,4 tetrahedra. The final structure represents a three-dimensional random network where some of the tetrahedra joins to each other by an oxygen ion. This ion is called bridging oxygen and is shared between two touching polyhedra. In the crystalline 8102, all oxygens are bridging oxygens. The greatest the ratio of bridging to non-bridging oxygens in amorphous form, the better the cohesiveness of the glass /2,29/. IR spectra of silicon oxide films can be obtained as absorbance spectra recorded in normal incidence transmission (see Figure 1). As deposited silicon oxide films are different by the stoichiometry, as well as by the porosity and bonded impurities. In Figure 5, absorbance spectra of thermal oxide and of two differently formed CVD silicon oxide layers are shown /30/. Beside strongly pronounced vibration of bridging oxygen in Si04' random network tetrahedra, also vibrations due to other species in films (e.g. SiOH, Si203, H2O) are obtained. By thermal annealing at temperatures above 800°C for 10-15 min, densification of layers is complete and absorbance spectra of all silicon oxide films are indistinguishable from each other and from the thermal oxide which is SiOg /30/. This simple form of IR analysis was frequently used to study the properties of various silicon oxide films /31,32/ also if they were very thin /33,34/. For precise analysis and to extract more data from measured spectra, optical effects have to be taken into account /35,36/. Applying optical analysis it was shown that the position of absorption peak due to asymmetrical stretching vibration of Si-O-Si bond may acts as an detector of differences among films: increasing the annealing temperature yields to stronger bonds in the oxide structure /37,38/. To increase the sensitivity for very thin films (d< 10Onm), oblique incidence angles or some of the reflection techniques using polarised IR beam may be applied. This way the structural nature of the Si/Si02 interface was studied /39-43/. These analyses are important to understand the properties of the so-called conduction channel formed at this interface. IV. 3. Silicon nitride films in recent years, special attention has been devoted to the use of silicon nitride in semiconductor device technology, since it can be used safely for interlayer isolation, device passivation, and mechanical protection /44/. The most important method to produce silicon nitride thin films is plasma-enhanced CVD (PECVD) that enables the formation of thin film dielectrics at low temperatures (200-500°C). PECVD silicon nitride films tend to be nonstoichiometric what is chemically represented as SixNyHaOb with a and b being much less than x and y. The IR absorbance spectrum of such film obtained by transmission measurement (see Fig. 1) is represented in Figure 6. The main absorption region (750-950 cm-1) is dominated by the antisymmetrical stretching mode of the Si-N-Si molecular group. The symmetrical stretching of the same molecular group leads to the much smaller band at 490 cm-1. An appreciable amount of hydrogen in the form of Si-H and N-H bonds is recorded at 2170 and 3340 cm-i, respectively. A small band due to Si-0 vibration is found at 1160 cm-i. The changes in position, shape, and intensities of absorption bands were observed at various preparation ways as well as after heat treatments. These changes are possible to relate to the properlies of local environment of the constituent atoms and hence to the molecular structure of the layer. Such data may lead to conclusions about important intrinsic properties as follows: stoichiometry, identification of densification or structural loosening of the film, passivating the dangling bonds of silicon and nitrogen (these are the principal source for the localised gap states), reduction of mechanical strain within the network etc. /44/. The studies may be performed applying the normal incidence transmission /45/ or some of reflection techniques /46, 47/. The ultrathin silicon nitride film (0.4