UDK621.3:(53+54+621 +66), iSSN0352-9045 Informacije MIDEM 27(1997)1, Ljubljana FOR MATERIALS USED IN MICROELECTRONICS Marta Kianjšek Gunde National Institute of Chemistry, Ljubljana, Slovenia Keywords: materials in microelectronics, infrared spectroscopy, semiconductor silicon, Ga-As, gallium arsenide, oxygen, carbon, hydrogen, shallow-level impurities, free carriers, surface analysis. Abstract: The application of infrared spectroscopy to analyse the semiconductor substrates used for microelectronic devices has been reviewed. For the quantitative determination of various impurities involved in the bulk materials, absorption spectroscopy is used. Infrared absorption in monocrystal silicon due to oxygen, carbon, hydrogen and shallow-level impurities is examined extensively. The informations about free carriers in doped semiconductors could be obtained from bulk reflectance spectra. Some methods for characterising the semiconductor surfaces are described also. The recently published literature on this field is reviewed. Uporaba infrardeče spektroskopije pri analizi materialov za mikroelektronsko industrijo 1. Polprevodniški substrati Kjučne besede: materiali v mikroelektroniki, spektroskopija infrardeča, silicij polprevodniški, Ga-As galijev arzenid, kisik, ogljik, vodik, primesi donorske in akceptorske, nosilci naboja prosti, analiza površinska. Povzetek: Članek predstavlja pregled metod infrardeče spektroskopije, ki se najpogosteje uporabljajo za analizo polprevodniških substratov v mikroelektronski industriji. Infrardeča absorpcijska spektroskopija omogoča analizo vsebnosti nečistoč. Podrobneje je opisana infrardeča absorpcija zaradi nečistoč v monokristalnem siliciju, kot so kisik, ogljik, vodik, ter donorske in akceptorske primesi. Spekter odbojnosti dopiranega polprevodnika vsebuje podatke o prostih nosilcih naboja. Opisane so tudi nekatere metode za analizo površin polprevodniških substratov. Tematika je predstavljena v luči novejših publikacij. I. INTRODUCTION Continuos improvements in technology of making microelectronic devices on the surface of a semiconductor requires special and controllable properties of all applied materials on each stage of the production /1/. Infrared (IR) spectroscopy is a powerful tool for the characterisation of these materials. Interaction of IR radiation with matter as a function of frequency provides a relatively direct probe of molecular structure via the excitation of vibrational states in molecules. This is one of the fundamental analytical techniques for obtaining qualitative and quantitative information about a substance. The advantages of Fourier-Transform IR spectroscopy together with the development of highly sensitive, rapid-response and low-noise detectors create a probe that is rapid, noncontact, nondestructive, and highly precise /2,3,4/. This paper will consider the most frequently used applications of IR spectroscopy to analyse the fundamental microelectronic materials with special attention to the recent literature. In the first part, the characterisation of bulk materials is discussed. Mostly monocrystal silicon is considered whereas other compounds (e.g. Group lll-V) are mentioned only briefly. II. ABSORPTION SPECTROSCOPY The most usual way to obtain the absorption spectrum of a sample is to use the logarithm of measured normal incidence transmittance (Figure 1). For IR radiation with energies bellow the band gap semiconductors become transparent. In this region the /o mmrn^^^^^^m sample L Figure 1. Normal incidence transmittance (T) and ab-sorbance (A) measurement. absorption spectroscopy play an imporlant role, it enables to detect even low impurity concentrations in the host semiconductor material, if vibrations due to these impurities are IR active. There are two different mechanisms governing absorption of IR radiation due to impurities involved in bulk semiconductor material: (a) the local vibrational modes caused by the distortion of the semiconductor crystal lattice due to electrically active or inactive impurities. The impurity species may be located either on lattice or on interstitial sites, bounded to the neighbouring host lattice atoms, and exhibiting a dipole moment. (b) the electronic bands caused by the excitation of excited states of electrically active impurities exhibiting mostly shallow levels in the forbidden band-gap of the respective semiconductor. The absorption peaks caused by vibration of impurities are not the only feature of the IR spectrum of a real semiconductor sample. The superposition of various kinds of absorptions may more or less seriously complicate detection of impurity vibrations. a) lattice vibration of the host semiconductor is the main additional spectral feature. In homopolar crystals (silicon, germanium) it appears due to nonvanishing second order electric dipole moment that causes a multiphonon IR spectrum. Corresponding absorptions are the same order of magnitude as the local vibrational modes due to impurities. The vibration of polar crystal lattice (e.g. in GaAs) leads to reststrahlen bands in the far IR spectral region, representing the low-frequency boundary of the transparency region in polar semiconductors. b) high charge carrier concentrations. Free carriers absorption increases continuously with increasing wavelength. The high-resistivity wafers (above 2 Qcm) are transparent up to the far-IR spectral region whereas low-resistivity wafers (0.02-0.05 Qcm) reflect almost all radiation in the whole IR spectral region. c) additional absorptions due to surface layers such as oxides, nitrides, unintentional contamination or even fingerprint. d) highly scattering rough surfaces cause a loss in transmittance. The radiation scattered by surface irregularities cannot reach the IR detector. Radiation scattering can be hardly distinguished from real absorption in a spectrum. e) high reflectivity of semiconductor surface reduces the transmittance. This reduction is due to refractive index of applied semiconductor material. In silicon, it is about 50% and is almost independent on the wavelength of the impinging radiation. These effects are shown in Figure 2, where normal incidence transmittance spectra of three silicon wafers are shown. Spectral features of high-resistivity, high purity (float zone, FZ), double-side polished sample (Figure 2, curve a) are caused by lattice vibration of the silicon crystal lattice. The wavelength-almost-independent reduction of transmittance is caused by high reflectivity of silicon. The origin of this effect is the high value of refractive index of silicon (n=3.42). Throughout the mid IR region of high resistivity silicon, this value varies only on the fourth decimal place, practically only at frequencies of lattice vibrations /5/. The commercial wafers for microelectronic devices are cut from Czochralski (CZ) grown ingots and polished only on one side. Typical distribution of surface irregularities of the rough rear surface causes spectral dependent loss of intensity in the measured direction for the wavelengths in the mid IR spectral region. This additional spectral feature changes the baseline of transmittance (Figure 2, curve b). When the concentration of free carriers increases, the absorption edge due to free carrier or plasma absorption moves to shorter wavelengths. In low-resistivity silicon wafers, the free-carrier absorption dominates thus making detection of absorption due to impurity vibrations hardly or even non possible. At sufficiently high carrier concentration, the wafer is almost opaque throughout all mid IR spectral region (Figure 2, curve c). c 05 4000 3000 2000 wavenumbers, cm-i 1000 Fig. 2. Absolute (air reference) transmittance spectra of silicon samples (room temperature measurements): (a) high resistivity (-16 Qcm), high purjfy (float-zone, <2x10 oxygen atoms/cm ), both side polished wafer, thickness 502 pm, (b) high resistivity (~10 ilcm), low purify (Czochralski, 7.4x10^^ oxygen atoms/cm-^), one side polished wafer, thickness 480 ,Lim, (c) low resistivity (0.01-0.02 Qcm), low purity (antimony doped, carrier concentration -2x10^^ /cm^), both side polished wafer, thickness 420 |.im. There are some ways to avoid superposition of the above mentioned unwanted absorptions. Most simple and widely used are selection and preparation of suitable sample as well as application of known reference specimen to detect only the differences between the investigated sample and the reference. To obtain information about the inherent impurity in a sample, the respective absorption bands have to be known. If quantitative results are wanted, the calibration factor of this absorption is necessary. Another possibility is to measure corresponding quantity of same semiconductor samples with known concentration of the same type of impurity species (calibration samples). To determine the calibration factor, detailed investigations have been performed on the commercially important semiconductor silicon, GaAs and GaP. One sort of impurity species has various possible modes of vibration, what gives rise to various local mode absorption bands. The spectral positions of these bands shift with the sample temperature, with the respective isotopes involved, possibly even due to some entirely optical effect. Getting the most of these multitude of data offers the possibility to detect also some details about impurity species considered. The electronic absorptions also exhibit a variety of bands due to the transitions to various excited states. The corresponding absorption coefficient and the respective full-width at half-maximum depend on the sample temperature. Detection of electronic bands requires cryogenic sample temperatures. The main application of IR absorption spectroscopy in the semiconductor industry is the quantitative determination of the impurity concentrations in single crystal semiconductor material. For such analysis, the intensity of the IR absorption bands produced by the selected impurity is used as a measure of its concentration. In contrast to much more expensive techniques such as charged particle activation analysis, SIMS or the vacuum fusion method, IR spectroscopy is quite simple and fast to apply for this purpose. The former techniques were used only to calibrate the IR absorption for quantitative analysis. The net concentration of dopants can be determined from the electrical resistivity measured at room temperature, whereas the chemical nature of impurities can be analysed by IR absorption spectroscopy. They may be classified as element impurities, pairs, complexes, and precipitates. Some of them are electrically neutral, the other are electrically active. For detection of electrically neutral impurity species, IR spectroscopy is one of the few techniques available. The presence of electrically active impurities can be detected also by means other than IR spectroscopy. These impurities generally determine the free carrier concentration and thus can be detected by electrical measurements. This technique is not very species selective, however. Different impurities give very similar results in the electrical properties, particularly in the Group ill-V compounds. By IR absorption spectroscopy, on the other hand, the specific species of impurity present in the semiconductor can be identified. 11.1. IR absorption in silicon The Czochralski grown silicon is the most widely used semiconductor grade material for production of microelectronic devices. The major impurities present in it are oxygen and carbon. They are introduced during the growth process with maximum solubility at melting point 2x10^0 atoms/cm^ and 5x10"!^ atoms/cm^, respectively. Both impurities are electrically inactive, but can seriously affect the carrier recombination process. The oxygen atoms occupy interstitial sites in the lattice and give rise to Si-O-Si bonds. Other oxygen species such as complexes and precipitates (SiOx, 0