A tutorial review of the application of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy to the characterization of semiconductors is presented. The PL technique provides a contactless, non-destructive means for the identification of certain chemical impurities in semiconductors, and comparison of the spectral sharpness of PL features associated with the recombination of free and bound excitons provides a qualitative measure of crystal quality. PL and tunable dye laser PLE spectroscopy of shallow acceptors and acceptor excited states are discussed briefly, but particular emphasis is placed on PL studies of deep impurity states. The 3d transition metals are the most common inadvertent deep impurities in III-V and II-VI semiconductors and PL techniques have been applied to the study of the details of their incorporation in these materials. The role of Cr and Fe dopants in the production of semi-insulating GaAs and InP has been studied extensively by PL techniques. Comparison of PLE spectra of radiative d-d transitions associated with transition metal impurities with published absorption spectra for the same dopants allows identification of the luminescent charge state. PL techniques have also proven useful in the characterization of ion implanted semiconductors. The assessment of lattice reconstruction during annealing, the optical activation of implanted impurities, and the study of the redistribution of impurities such as Mn, Fe, and Cu and surface degradation resulting from implantation and annealing procedures are examples of the application of PL techniques to the evaluation of the ion implantation technology in III-V semiconductors.

Sophisticated characterization capabilities are essential in order to satisfactorily analyze high purity semiconductor materials. The electrical properties of semiconductors have a long history of extensive investigation. As the investigations of semiconductors were extended to wider band gap materials electrical measurements were not as readily applicable. This coupled with the understanding of excitons and their contribution to the elucidation of materials properties in the 1960's lead to a wide application of optical studies to semiconductor materials. It was found that these materials reflect, absorb, disperse, scatter and radiate light and in general interact strongly with the electromagnetic radiation field. Because of this strong interaction many of the fundamental properties of these materials such as their energy band gaps, activation energies of defects and foreign impurities, effective mass parameters, refractive indicies, dielectric functions, exciton binding energies and lattice vibration frequencies can be determined from optical experiments. The technique of high resolution optical absorption, reflection and photoluminescence spectroscopy has been extensively used to analyze the intrinsic energy band parameters of semiconductors, as well as their impurity and defect states. Intrinsic or free exciton formation is observed in most well formed crystal structures when optically excited with the proper energy and at cryogenic temperatures. The free excitons have been applied with a great deal of success in probing the intrinsic band structure of semiconductors. Bound excitons have been applied successfully in probing the impurity and defect structure of many of these same materials. The free exciton is the probe in this case, becoming bound to various chemical impurities, lattice defects and complexes to form bound states whose sub-sequent radiative decay yields information concerning the electronic states of the impurities and defects in these materials. Magnetic field splittings of the bound exciton transitions make it possible to differentiate between neutral and ionized donor and acceptor impurities. In conjunction with systematic impurity doping experiments, specific donor and acceptor impurities can be identified. Upon application of stress fields, in conjunction with a knowledge of the energy band structure of the host lattice, it is also possible to differentiate between simple substitutional donors and acceptors and complexes comprising combinations of impurities and/or defects.

We report the application of photoluminescence to the characterization of defects induced by thermal annealing at low temperatures around 500 °C, which is very important in the fabrication processes of LSI. In annealed CZ-Si wafers with the high content of oxygen, twiddle 10 18 cm -3, new broad luminescence bands have been observed together with the usual bound -V exciton luminescence lines of dopant impurity. Analysis of Hall-effect data has shown that the intensity of such broad luminescence bands correlates with the density of the so-called thermally induced oxygen donor. The broad luminescence bands in annealed CZ-Si are strongly influenced by the presence of carbon. In photoluminescence spectra of annealed CZ-Si containing a high concentration of carbon, %lx1017 cm -3, many sharp luminescence lines have been found, whose energy positions and temperature behaviors are quite different from those of the usual bound-exciton luminescence lines. Thermal behaviors of these lines indicate that such sharp luminescence lines might be originated from isoelectronic traps possibly related to carbon and oxygen. Furthermore, photoluminescence measurements have shown that these sharp luminescence lines and broad luminescence bands coexist for annealed CZ-Si wafers with the moderate contents of oxygen and carbon. Thus it has been found that the photoluminescence technique is very useful for characterizing thermally induced defects in CZ-Si wafers commonly used in LSI.

Electron-beam induced emission from insulating oxides on semiconductors can be used to probe the oxide-semiconductor interface both by studying films of different thicknesses and by varying the penetration depth of the electron beam. In two important systems, anodic oxides on GaAs and thermal oxides on Si, major luminescence centers, possibly related to oxygen vacancies, are determined by these methods to be concentrated near the interface. Emission from native oxides can be observed on both these semiconductors and is similar to emission from thicker oxides, although there are modifications in the relative intensities and in the position of some lines. In the anodic oxide spacial inhomogeneities and changes in the sample with time are also demonstrated by cathodoluminescence.

The effect of semiconductor surfaces and interfaces on the electro-optical response of semiconductor structures has been investigated experimentally and theoretically. The dependence of photoconductance (PC), photoelectronagnetic (PEM) current, and photovoltage (PV), on the gate bias voltage in n-Si MOSFET-type structures has been calculated and com-pared with experiment. The characteristic feature is a minimum of the PEM current and of PC in depletion. Quantitative discrepancies occurring in depletion and inversion are tentatively explained by nonuniformities in the surface potential and by leakage current. The results, obtained for uniform illumination, are compared with the electro-optical response to point illumination and with measurements on unpassivated silicon samples. In bicrystals of GaAs and of InP, the PEM and PC response to a laser beam scanned across the grain boundary suggests that those grain boundaries are interior surfaces with properties similar to outer surfaces of the corresponding materials.

A contactless optical technique has been developed for the measurement of excess photogenerated carrier lifetime in Hg0.7Cd0.3Te in a raster scan mode. The technique consists of measuring the steady state mod-ulation AI in the transmitted intensity I of a probe beam (-11,a) < Eg) due to a modulated pump beam (Kw < Eg) incident on the same surface. The fractional change in the probe beam transmission AI/I is related to the excess carrier lifetime. Lifetimes in the 20 ns to 20 us range have been measured using pump photon fluxes on the order of 1018 photons/cm2-s.

This method, aimed at determining doping level and minority carrier lifetime of low-doped semiconductors, is based on pulsing the MOS device into deep depletion. A delayed photopulse is then applied to the sample, which causes partial collapse of the depletion region. The changes in fill time and in capacitance versus collected photocharge are measured. The minority carrier lifetime is computed through the dependence of fill time on the magnitude of the photoinjected charge. The doping level is determined by the change in capacitance following the photoinjection. The method is advantageous in: (a) independently supplying the doping level and lifetime, (b) being insensitive to edge injection, and (c) enabling the determination of the diffusion length.

Applications of a two-wavelength laser scanner for evaluating spatial variations in several material/device parameters WI semiconductors are described. Use of two-wavelength scanning (λ = 1.15 pm and λ = 6328 A) permits excitation and collection of carriers from different depths below the surface of a semiconductor, thereby allowing separation of bulk and near surface (such as a surface junction) characteristics. These characteristics include minority carrier diffusion length, junction recombination, and local variations in photovoltaic effects caused by localized defects, twins and grain boundaries. Laser scanning may be done in two different modes. 1. Carrier generation and collection along the same direction -- in this mode the signal is related to carrier recombination effects. It is shown that the signal due to long wavelength excitation is proportional to the bulk diffusion length. Measurements may be done either on unprocessed substrates using a transparent electrode and capacitive coupling, or following P-N junction formation at the surface. 2. Carrier excitation and collection normal to each other -- this mode is used in unprocessed substrates (single or polycrystalline) to determine lateral variations in the internal fields arising from such effects as impurity inhomogeneities and grain boundaries.

The application of surface reflection Raman scattering as an optical probe for monitor-ing the presence and growth of elemental deposits of group V (P,As,Sb) metalloids in native oxide films on III-V compound semiconductors is discussed. Selective data from the litera-ture concerning arsenic inclusions in native oxides on GaAs and Al_xGa_l__xAs and red phosphor-us deposits in thermally oxidized films on InP are used to illustrate the technique.

Raman scattering has been used as material characterizations such as crystal orientation, imperfections, structural and compositional disorders, carrier concentration, and even mobility. In particular, specific applications for the study of alloy composition in GaAlAs and GaA1P; for GaAlAs-GaAs superlattice; and for laser annealing of ion-implanted amorphous Si and GaAs; will be presented in detail. Two examples on the determination of carrier concentrations; for polar crystals such as GaAs; and for non-polar crystals such as Si, are also presented.

Raman spectra were recorded in backscattering from (100) oriented, Cr-doped, semi-insulating crystalline wafers of GaAs to characterize the changes introduced by implan-tation and by encapsulation and thermal anneal15ing. The studies were carried out at 300 and 100K. Implant fluences of lx10¹² to 1x10¹⁵ S-ions/cm² at 120 keV were used. Annealing temperatures ranged from 750 to 950°C for 15 minutes. The encapsulation was by means of rf-plasma-deposited Si3N4 films. The characteristics of a fixed annealing pro-cedure as a function of fluence and a fixed fluence at different annealing temperatures were determined. Unprocessed samples and a laser-annealed sample were studied. A pulsed, doubled YAG laser at 532 nm was used with about 16 mW of incident power in a 0.12-mm-diameter spot. A "triple" spectrometer, gated photon counting and computer pro-cessing of data were used. Following are the results: (1) the intensity of the LO pho-non line strongly varied with both implant dosage and annealing including a five-fold enhancement at 10¹² S-ions/cm2 fluence, (2) the LO mode "softened" apparently due to implant-induced bond weakening, (3) polycrystalline and amorphous conditions of the implanted layer were identified, (4) thermal annealing itself was found to introduce disorder in the surface, at least for temperatures above 850°C, and (5) removal of amor-phous regions by laser annealing was observed.

Surface plasmons are surface electromagnetic waves. That is, they are travelling waves whose electric and magnetic fields are localized at the interface between a metal and a dielectric. Their propagation characteristics (dispersion) are functions of the optical properties of both the metal and the medium in contact with it. Since they are localized at the interface, they are also sensitive to thin films on the metal surface. Quantitative measurement of surface plasmon resonances can thus yield the optical constants of any one of these three regions. For instance, we can determine the thickness and the dielectric constant (real and imaginary parts) of metal films less than 70 nanometers thick; measure the thickness or refractive index of organic coatings in the 1 to 30 nanometer range; observe the orientation of molecules near a surface in a liquid crystal cell; detect submonolayer adsorption of ionic species from an electrolyte solution. While both prism and grating coupling can be used to excite surface plasmons, we will describe the prism coupling or attenuated total reflection method we commonly use and discuss the sensitivity and range of applicability of the technique.

The capabilities and limitations of infrared transmission as a characterization tool for p-type gallium-arsenide are being studied. Absorption spectra resulting from impurity ground state to excited state transitions have been analyzed to yield impurity activation energy and absorption cross section. These data allow determination of impurity type and concentration. Sensitivities achieved with copper, manganese, and magnesium are discussed.

The optical properties of semiconductor crystalline materials can be altered through ion implantation according to various physical mechanisms. In this paper we examine the optical changes in GaAs brought about by free carrier compensation. N-type GaAs wafers with a high free carrier concentration are implanted with protons having an energy of 300 keV. The infrared properties of the resulting altered surface layer are characterized by infrared reflectance measurements. A spectrophotometer is used to measure the reflectivity of the implanted wafers from 4000 cm ^-1 to 200 cm ^-1. Interference fringes typical of a thin layer/substrate structure is observed. From the location of the fringes and a classical dielectric model, the effective thickness and the average free carrier concentration of the altered layer are obtained. This data is used in computer simulations to achieve good agreement with the measured reflectivity over the entire spectral range.

The localized vibrational mode of ¹²C implanted into GaAs at 6 MeV and a fluence of 5xlO¹⁶ /cm² is studied as a function of annealing to 900°C. The feature sharpens and decreases in strength until 500°C and saturates thereafter. Electrical measurements indicate p-type behavior of the implant at 900°C, which is usual for implants of this nature.

In this paper is described the principles of an asymmetric Michelson interferometer for use in the far infrared region of the electromagnetic spectrum, for the direct determination of the optical constants of a solid. Further, an apparatus for realizing the determination of the optical properties is described, and the method and some recent developments are included. The instrument is useful for the material characterization of semiconductors and some examples are given.

Semiconductor technology requires probably more sophisticated characterization tech-niques than any other modern technology. For example, many applications require atomic sensitivities of 1 ppb or less and we may be interested in analyzing a volume 10^-16 cm³ (hopefully not at the same time). In addition to chemical analysis of very small volumes, determination of the chemical state (oxidation state, etc.) of the constituents is many times critical. Of equal importance, especially for the high density of devices contemplated for very large scale integration, is the detection and characterization of defects present in the wafer starting materials. In addition to the optical techniques, which are the subject of this conference, many other techniques including X-ray topography, electron beam induced current, Auger electron spectroscopy, X-ray photoelectron spectros-copy, Rutherford backscattering, and secondary ion mass spectrometry have been applied to these problems. This paper will be broken into two parts. First I will discuss some of our future needs in semiconductor characterization emphasizing requirements for sub-micron devices in silicon and potential materials problems in non-silicon technology. Next I will review the capabilities and limitations of some of the non-optical techniques mentioned above to indicate specifically where additional capabilities are necessary.

A recently developed optical reflectivity technique for monitoring laser-induced solid phase epitaxial crystal growth in real time is described, and examples which illustrate its use in studies of solid-state kinetics in ion-implanted and UHV-deposited films are presented. Data which show the dependence of epitaxial growth rate on the position of the crystal/amorphous interface, growth rate as a function of temperature, and deviations from ideal epitaxial growth due to competing crystallization processes are presented and discussed. The laser technique represents a significant advance in experimental capabilities for measuring details of solid phase epitaxy kinetics; crystallization rates can be accurately measured at higher temperatures, and with greater temporal and spatial resolution than had been previously possible.

Light scattering is a sensitive method for characterizing the topography of a smooth, reflecting surface. Particulate contaminants which may influence the yield of semiconductor devices are easily detected in scattered light. Surface irregularities which may influence the lifetime or mobility may be quantitatively evaluated by scattering. The total integrated scatter (TIS) fromoa silicon surface can be related to the rms surface roughness with a sensitivity of 10 Å. Some applications of light scattering to the char-acterization of silicon will be summarized. The TIS method for measuring the surface roughness will be presented in detail.

In modulation spectroscopy the optical spectra of a solid is modified in some manner by the periodic variation of the measurement condition. This modulated perturbation gives rise to sharp, differential-like optical features in the region of photon energies where optical excitation processes occur. Changes in reflectance or transmittance as small as 10^-6 - 10^-7 can be observed using phase-sensitive detection. The extensive fundamental experimental and theoretical work done in this area during the past 15 years has provided the necessary framework to develop the technique into a powerful tool for materials, device and processing characterization. In this paper we review a number of the applica-tions of modulation spectroscopy including determination of topographical variations in composition and carrier concentration, the nature of potentials and strains at hetero-junction interfaces, properties of the space-charge region including the semiconductor/ electrolyte interface, ion-implanted (including laser-annealed) semiconductors polariza-tion properties of ferroelectric materials, etc.

Internally reflected electroabsorption, a modulation spectroscopy tool, can be used as a nondestructive optical method to determine the gap energies and, hence, the composition of the epitaxial layers in heterostructures. At the same time, the presence of built-in inter-face potentials can also be determined. The method was used to study n-n GaAs_1-xSb_x hetero-structures grown on n^₊ GaAs substrates using a simple Schottky barrier contact.

A laser interferometric method was developed to detect end-of-etching of materials such as doped and undoped polysilicon, Si₃N₄, Si0₂ and metals used during different stages of IC and thin film device processing. For metal etching, a detector trace of constant magnitude is obtained until the underlying layers are exposed. At this point, a step change in re-flectivity occurs, signaling the end-point. For the other above mentioned films, a sinu-soidal waveform is obtained which changes its frequency once the film of interest is etched and the underlying layers become exposed. The method is applicable to all of the dry etch-ing processes and will be illustrated in some detail for polysilicon and silicon nitride etching applications using a barrel-type plasma reactor.

Plasma etching has become an important technology in the fabrication of integrated circuits. The importance of this technology will increase as the minimum geometry features continue to decrease to the submicron region. Process monitoring is a very desirable feature in maintaining the precise control that is required of plasma etching for VLSI circuits. This paper describes two optical methods of process monitoring and end point detection for plasma etching. Examples are presented for emission spectroscopy, and an optical reflection method. A direct comparison of these methods as end point detection monitors is also made.

Classical and new ellipsometric configurations and techniques are briefly reviewed. This includes null and photometric ellipsometry; azimumetry (ellipsometry based on azimuth measurements alone); film-substrate ellipsometry based upon detection of special values of ψ and Δ at certain angles of incidence; surface-modulated ellipsometry and AIDER (angle-of-incidence derivative ellipsometry and reflectometry).

The optical properties of a material in the near-ir--near-uv spectral range are predominantly determined by electronic polarizabilities. The electronic polarizabilities are determined in turn by composition, local order, and long-range order. We discuss methods developed to parametrize the information contained in optical properties of micro-scopically heterogeneous systems, and to obtain the values of these parameters. These methods are based on accurate bulk dielectric function values of constituents, effective medium theory, and linear regression analysis. Recent theories that establish limits on allowed values of the dielectric response of two-component systems, regardless of their microstructure, provide an indication of the reliability of these parameters.

The capabilities of automatic ellipsometry are demonstrated for semiconductor technology i.e. thin films processing on optical polished silicon and gallium arsenide wafers. Real-time evaluation of a film deposit is shown in the case of the plasma oxidation of a silicon substrate through a thin film of calcia stabilized Zr0₂ (CSZ). The trajectory in the (ψ , Δ) plane indicates that a Si0₂ layer is developed between CSZ and Si. Structural damages occur in this layer above a critical voltage, corresponding to large deviations of the ellipsometric trajectory. In the cases when in situ examination is not feasible, a non-destructive depth profiling of the thin film can be obtained by varying the energy of the incident light, using a spectroscopic ellipsometer. The Si₃N₄/Si interface is examined on Si₃N₄/Si02/Si structures (the active part of a MNOS device) as well as thermally nitrided silicon wafers. Optical absorption due to uncompletely nitrided Si tetrahedra is found in both cases. Also examined are implanted layers in GaAs. Because the incident ions locally transform the crystalline GaAs into an amorphous state, with a very distinct dielectric function, it is possible to derive from the spectroscopic ellipsometry data a depth profile of the ion induced damages.

Planar technology for today's electronic devices (ic's, p wave devices, IR detectors, etc.) are constructed of lamellae of metals, insulators and semi-conductors of varying thicknesses. Because thickness and material properties affect device parameters and influ-ence performance characteristics, their measurement and control is essential. Optical property measurements of these lamelliforms provide non-destructive techniques for acquiring information pertinent to device performance characteristics. A brief review of the charac-teristic matrix for the E-M wave equations is given, leading to the matrices involving the Fresnel relations. The relationships of reflectance, transmittance, and absorbtance (emit-tance) for both polarizations and all angles of incidence for a thick plane-parallel dielectric slab, having two differing series of layers on either side, will be given. A description of these relationships applied to a thin (d/λ > 2%) layer on an opaque substrate is presented. Included is a brief description of the two angle of incidence technique of AIRS (Angle of Incidence Reflection Spectroscopy) for studying such systems.

Internal and external reflection spectra of surfaces and surface layers show detailed structure caused by impurities. Various coatings on semiconducting surfaces have been examined for absorbing impurities and the offending species have been identified, located, and quantified. The impurity species are identified by com-parison with known absorption signatures, and their location in the film structure decided by comparing measurements made with varying angle of incidence and polarization. A causal transformation has been devel-oped that allows the calculation of the complex refractive index from the reflection spectrum of a single unknown layer in a known multilayer stack. If the bulk absorption coefficients are assumed for the impurities, then the amount of the species can be determined, within the accuracy of the model assumed. Water and hydro-carbon impurities typically are found at the surface or at other interface regions in monolayer or greater amounts for the production quality films measured. Water impurity concentrations as high as 5-107 have been observed in ThF₄ films deposited on ZnSe or Ag mirrors. These spectroscopic measurements are non-destructive and can be used to single out individual chemical species in situ. The techniques have been found to give accurate results for various multiphase systems without restrictions as to number of phases. These techniques emphasize use of continuous spectra as opposed to several point data, and make liberal use of adjustable parameters such as angle of incidence and polarization.

IR reflectance spectra of thin (>0.5μm) epitaxial layers on substrates containing n-type buried layers are investigated in the 5μm to 50μm wavelength range. From measurements on uniformly doped wafers it is found that the Drude model with a constant relaxation time should be used to compute the optical constants of the buried layers. The reflectance spectra can then be used to determine epitaxial layer thickness, peak concentration of the buried layer and the thickness of the buried layer. For our process conditions the optically measured epitaxial thick-ness and peak concentration are in close agreement to measurements performed with a secon-dary ion microprobe.

Spectroscopic ellipsometry can be used to assess in real time the effectiveness of etching and cleaning methods in reducing the amount of unwanted interface material (oxides, contamination, pits, damage, etc.) at the surface of a semiconductor. However, we show that an unambiguous response denoting removal of interface material occurs only in certain wavelength ranges. We have applied this technique to determine chemical procedures that yield the sharpest dielectric discontinuities (smoothest and/or cleanest surfaces) for Si, Ge, and some III-V compound semiconductors.

A unique differential spectrometer has been developed for the quantitative examination of surfaces. It combines a tuneable pulsed dye laser light source, a flexible precision sample holder, silicon photodiode-in-integrating sphere detectors, and a desktop computer controller/analyzer. This combination enables very precise measurements--twiddlel% systematic errors and twiddle.02% random errors in reflection and transmission measurements. The spectrome-ter has been applied to the investigation of the structure of various component films used in semiconductor devices and in a basic study of the molecular-molecular interactions of dyes adsorbed on surfaces. With only minor modifications, this device can make transient spectroscopy measurements with a resolution of twiddle 10 nsec. It can also be used for a variety of modulation spectroscopies. We will be exploring those areas in the future.

The change of polarization of light upon reflection from an optically isotropic film-substrate system is determined by the ratio of complex amplitude p and s reflection coeffi-cients, p = R_p/R_s. If the incident light is linearly polarized at 45 degree azimuth from the plane of incidene, p assumes the dual role of representing the elliptic vibration of the reflected light. We examine p as a function of angle of incidence φ and film thickness d for the SiO₂-Si system at wavelength λ = 6328 Å and present contours of φ = constant and d = constant in the complex p plane. Conversely, we also determine contours in the φ plane that represent ψ = arctan Ipl=constant and Δ=arg p=constant. Of particular interest are images in the φd plane of the real and imaginary axes (Δ=0, π and Δ=+π2) and of the unit circle(ψ=π4) of the complex p plane.

A computerized infrared microspectrophotometer has been designed which can measure over the infrared wave-length range 2.5 to 14.5 μm (4000-690cm^-1) from areas as small as 20 x 20 /um. The system can routinely measure the thickness of silicon dioxide, silicon nitride, photoresist or multiple layers of these and other non-metallic films from several hundred angstroms to several μm In addition, concentration of phosphorus over a wide range can be determined in phosphosilicate glass. The measurements are nondestructive and can be performed by unskilled operators in less than one minute. Since the instrument scans over the fundamental IR range, some contaminants on wafers can be identified by the interpretation of their spectra. The NanoSpec"/20 IR computerized infrared microspectrophotometer employs an all-reflecting optical system which allows viewing of small sample areas at 150X. Wavelength scanning or its inverse, frequency in waves per centimeter (cm-1), is accomplished by the computer controlled rotation of a circularly variable infrared narrow-band wavelength filter. This can also be positioned very quickly at a specific wavelength to select a narrow band (approximately 1% of wavelength) for specific analysis or monitoring purposes. Chopped infrared energy emitted by a heated ceramic rod is transmitted through the filter, the sample and a variable aperture which allows a specific sample area to be measured. The infrared radiation is then focused on the target of a liquid nitrogen-cooled HgCdTe photodetector. The amplified signal is digitized and the data processed by a microprocessor computer with large memory. CRT display of a large variety of interactive operating modes and convenient instructions is provided. Thickness and concentration measurements can be printed out on paper tape. Spectra are recorded from the computer on an X-Y recorder, linear in wavelength or cm-1, and in transmittance or absorbance. Up to eight complete working spectra can be stored in RAM memory and compared using a variety of arithmetic operations.