Fourier Transform Infrared (FTIR) spectroscopy is a powerful analytical technique widely used in semiconductor research and industry for material characterization. By measuring the absorption of infrared light, FTIR provides critical insights into vibrational modes, impurity concentrations, and free-carrier behavior in semiconductors. The technique is non-destructive, highly sensitive, and capable of probing bulk and surface properties with minimal sample preparation. This article explores the principles of FTIR spectroscopy in semiconductor analysis, focusing on vibrational modes, impurity detection, and free-carrier absorption. Additionally, it discusses advanced FTIR methodologies such as Attenuated Total Reflectance (ATR)-FTIR and micro-FTIR, along with their applications in doping profiling and oxide characterization.

Semiconductors exhibit distinct vibrational modes that can be detected using FTIR spectroscopy. These modes arise from the interaction of infrared radiation with lattice vibrations, including transverse optical (TO) and longitudinal optical (LO) phonons. In polar semiconductors like GaAs or SiC, the interaction between infrared light and phonons results in Reststrahlen bands, which are highly sensitive to crystal quality and stoichiometry. For instance, in silicon, the absorption peaks near 610 cm⁻¹ and 1100 cm⁻¹ correspond to Si-O-Si stretching and bending vibrations, providing information about oxide layers or contamination. In compound semiconductors such as GaN, the E₁(TO) and A₁(LO) modes near 560 cm⁻¹ and 740 cm⁻¹, respectively, are indicators of crystallinity and strain. By analyzing these vibrational signatures, researchers can assess material purity, phase composition, and structural defects.

Impurity detection is another critical application of FTIR spectroscopy in semiconductor analysis. Impurities such as carbon, oxygen, and hydrogen introduce localized vibrational modes (LVMs) that appear as sharp absorption peaks in the infrared spectrum. In silicon, interstitial oxygen gives rise to a characteristic peak at 1107 cm⁻¹, while substitutional carbon produces a peak at 605 cm⁻¹. The intensity of these peaks correlates with impurity concentration, enabling quantitative analysis. For example, the ASTM F1188 standard uses the 1107 cm⁻¹ peak to measure oxygen concentration in silicon wafers with a detection limit as low as 1×10¹⁵ atoms/cm³. Similarly, hydrogen-related defects in GaN can be identified through peaks near 3120 cm⁻¹ and 3300 cm⁻¹, corresponding to N-H and Ga-H stretching modes. FTIR spectroscopy thus serves as a reliable tool for monitoring impurity levels during semiconductor manufacturing.

Free-carrier absorption is a key phenomenon studied using FTIR spectroscopy, particularly in doped semiconductors. When infrared light interacts with free carriers (electrons or holes), it induces intraband transitions that result in broad absorption features in the mid- to far-infrared range. The absorption coefficient is proportional to the free-carrier concentration and mobility, making FTIR a valuable technique for doping profiling. In silicon, heavily doped regions exhibit increased absorption at wavenumbers below 1000 cm⁻¹ due to free-carrier effects. By modeling the absorption spectrum using Drude theory, researchers can extract carrier density and scattering time. For instance, n-type GaAs with a carrier concentration of 1×10¹⁸ cm⁻³ shows a pronounced free-carrier absorption tail extending beyond 500 cm⁻¹. This capability is particularly useful for characterizing doping uniformity in epitaxial layers and ion-implanted substrates.

ATR-FTIR is a specialized technique that enhances surface and thin-film analysis by utilizing total internal reflection. In ATR-FTIR, the infrared beam penetrates a few micrometers into the sample, making it ideal for studying thin oxide layers or surface contaminants. For example, native oxide layers on silicon wafers can be characterized by their Si-O-Si stretching and bending modes at 1070 cm⁻¹ and 800 cm⁻¹, respectively. ATR-FTIR is also effective for analyzing organic residues or adsorbed species on semiconductor surfaces, with detection limits in the sub-monolayer range. The technique requires minimal sample preparation and is compatible with opaque or rough surfaces, offering advantages over traditional transmission FTIR.

Micro-FTIR combines FTIR spectroscopy with microscopy, enabling spatially resolved analysis with a resolution of approximately 10-20 µm. This capability is crucial for investigating localized defects, doping inhomogeneities, or patterned structures in semiconductors. For instance, micro-FTIR can map oxide thickness variations across a silicon wafer or identify contamination hotspots in device structures. In compound semiconductors like InP, micro-FTIR can detect spatial variations in carrier concentration by measuring free-carrier absorption differences. The technique is also valuable for failure analysis, where it can pinpoint impurities or structural anomalies in specific regions of a device.

In doping profiling, FTIR spectroscopy provides a non-contact method for measuring carrier concentrations in semiconductors. By correlating free-carrier absorption with calibration curves, researchers can determine dopant densities without electrical contacts. For example, in phosphorus-doped silicon, the free-carrier absorption at 300 cm⁻¹ can be used to estimate doping levels ranging from 1×10¹⁶ to 1×10¹⁹ cm⁻³. This approach is particularly useful for vertical doping profiling in epitaxial layers or diffused regions. FTIR can also detect activation anomalies in implanted wafers by comparing free-carrier absorption with expected doping profiles.

Oxide characterization is another area where FTIR spectroscopy excels. The technique can identify oxide stoichiometry, hydration, and interface quality through vibrational mode analysis. In silicon dioxide, the position and shape of the Si-O stretching peak near 1070 cm⁻¹ provide information about film density and stress. Additionally, FTIR can detect hydroxyl groups in oxides through O-H stretching modes near 3400 cm⁻¹, which are critical for assessing oxide reliability in MOS devices. For high-k dielectrics like HfO₂, FTIR reveals metal-oxygen vibrational modes that correlate with film crystallinity and interfacial reactions.

In summary, FTIR spectroscopy is a versatile tool for semiconductor analysis, offering detailed insights into vibrational modes, impurities, and free-carrier effects. Advanced techniques like ATR-FTIR and micro-FTIR extend its capabilities to surface and spatially resolved studies. With applications ranging from doping profiling to oxide characterization, FTIR remains indispensable for semiconductor research and quality control. Its non-destructive nature, high sensitivity, and compatibility with various sample types make it a cornerstone of modern semiconductor metrology.