IR Absorption Spectroscopy

Most materials absorb electromagnetic radiation in the IR spectral region at wavelengths (between 0.8 µm and 14 µm) that are characteristic of the material's molecular structure. IR absorption spectroscopy is a common chemical analysis tool that measures absorption of an IR beam that has been passed through a sample. The position of the absorption peaks in an IR spectrum (Figure 1) are characteristic of the sample's chemical composition or purity and the intensity of an absorption peak is proportional to the concentration of the species for which that peak is a characteristic.

IR spectroscopy can be used for both qualitative and quantitative non-destructive analysis of gases, liquids, pastes, powders, films, and surfaces. The absorption spectrum of a molecule provides a unique "fingerprint" of absorbances that can be used to deduce a sample's chemical composition and species concentrations. Figure 1 shows IR absorption spectra for some of the gases that are routinely measured in stack emissions monitoring applications. Each of the gas species labelled can be seen to exhibit a unique pattern of IR absorption.

IR spectrometers typically consist of a broadband IR light source, a wavelength separating device and a detector, as shown in Figure 2. Liquid or gas samples are typically contained in a sample cell. Solid samples can be analyzed using absorption or reflectance spectroscopy, in situ or in a standoff measurement system, or as a pressed disc of powder diluted by an IR transparent material or diluted in pastes commonly referred to as mulls.

Popular IR spectrometers used for routine chemical identification and quantitative measurement include: grating-based/dispersive IR spectrometers, FTIR spectrometers, and filter-based or non-dispersive IR (NDIR) instruments. The choice of spectrometer for a particular application is driven by requirements such as sensitivity, matrix complexity, packaging, and cost requirements. Dispersive spectrometers (Figure 3) use a grating to disperse and separate the wavelengths of broadband light. There are two types of dispersive spectrometers: monochromators and spectrographs. The former uses a single-element photodetector and a rotating grating assembly, while the latter uses a fixed grating assembly and a photodetector array. The advantage of dispersive spectrometers lies in their simplicity, which enables hardware miniaturization while retaining the ability to scan relatively wide spectral ranges. However, compared to FTIR or NDIR instruments, the throughput (or optical etendue) is limited since only a small portion of the source light ends up on the photodetector. Because of this, dispersive spectrometers are typically used for VIS and NIR spectral regions rather than for the MIR region where the radiation has lower photon energy.

An FTIR spectrometer (Figure 4) generates a spectrum by modulating the IR radiation in the time domain using interference to produce an interferogram that is then subjected to a Fourier transform. In a Michelson interferometer, the most common interferometer used in FTIR, the incoming beam of light is split into two identical beams using a beamsplitter (a partially reflecting mirror). Each of these beams travels a different route and they are recombined before arriving at a detector. The path difference, the difference in the distance travelled by each beam, creates a phase difference between them. This recombined beam is the interferogram, a modulated signal as a function of the path difference. Performing a Fourier transform on the interferogram produces the spectrum of the incoming beam.

The FTIR spectrometer has several advantages over a traditional dispersive spectrometer. First, it provides fast measurements over a wide wavelength range. A modern FTIR instrument such as the MKS MultiGas™ FTIR analyzer can perform a scan in as little as 200 ms. With each scan, it covers the entire MIR wavelength region between approximately 2 µm and 16 µm, depending on the material of the optics and the type of photodetector employed in the instrument. Second, and perhaps most important, is the fact FTIR spectrometers have high optical throughput or etendue. An FTIR does not use a slit to control the wavelength resolution of the instrument. As a result, the spectra produced by FTIR spectrometers are generally much "sharper" than those produced by dispersive spectrometers under the same conditions. This is important in quantitative analysis where SNR generally determines the sensitivity of the measurement. Another advantage of the FTIR spectrometer is its wavelength precision and stability. An FTIR spectrometer normally uses a laser to control the position and velocity of the moving mirror and to trigger the collection of data points throughout the scan. A well-designed FTIR instrument provides a very high unit-to-unit repeatability, eliminating the need for cumbersome and expensive individual unit calibration. NDIR analyzers are filter-based instruments designed for specific measurement applications. For example, NDIR analyzers are the industry standard method for measuring the concentrations of CO and CO₂ in stack emissions monitoring. Instead of scanning the wavelength and generating spectra, NDIR instruments generally capture the absorption of discrete wavelengths relevant to the chemical species being measured. They do so by employing optical filters that transmit light at selected narrow-band regions. Figure 5 shows the basic concept of an NDIR instrument.

The filter used in NDIR instruments is usually an interference type, called an "etalon", which is essentially a Fabry-Perot interferometer. It is typically made of a thin-film spacer separating two thin-film reflectors. Wave interference occurs in the etalon, and the waves that are in phase constructively interfere and are transmitted through the filter. The rest of the waves interfere destructively and are therefore "blocked". The broadband IR source is generally blackbody radiation produced by a heated filament such as tungsten or Kanthal.

The main advantage of an NDIR instrument is the simplicity of its hardware. This makes NDIR instruments both low-cost and rugged, making them ideal for industrial applications. MKS Process Sense™ is an example of an NDIR analyzer used in semiconductor process applications.

TFS™ spectrometer refers to an MKS NDIR instrument that provides wavelength scanning capability. The wavelength scanning is generally achieved by adjusting the gap distance between the two thin-film reflectors in the Fabry-Perot element. MKS' TFS™ spectrometer adjusts this gap distance by adjusting the incident angle of the light by rotating the filter element. As depicted in Figure 6, the filter transmission wavelength is altered as the incident angle is adjusted. As the incident angle increases, the transmission scans to lower wavelengths.

MKS Instruments offers several analytical tools for emissions and process monitoring applications based on IR spectroscopy. Gas emissions monitoring is normally required at emission sources such as power plant stacks and chemical manufacturing facilities. Accurate monitoring data is a critical part of regulatory compliance and environmental protection programs designed to control and regulate airborne pollutants such as carbon monoxide, nitrogen oxide, sulphur dioxide, and ozone, as well as greenhouse gases such as methane and carbon dioxide. MKS' MultiGas™ 2030 system is a high-speed, high-resolution FTIR-based gas analyzer designed to simultaneously monitor more than 30 emission gases in under a second. There is a growing demand in the safety and security industries for rapid and reliable detection of chemical agents and toxic industrial chemicals. MKS Instruments' FTIR-based AIRGARD^® system can detect parts per billion (ppb) levels of most chemical warfare agents (CWA) and toxic industrial compounds within 20 seconds. It is the industry standard, a fixed monitoring of CWAs in critical infrastructure buildings. Semiconductor chemical-vapor deposition process chambers must be periodically cleaned to remove deposited build-up on the chamber walls and internal components. Optimal cleaning times for different processes depend on a complex relationship between variables such as the thickness of the build-up, the interior temperature of the chamber components, deposition/sputter ratios, and the chemical composition of the materials to be removed. MKS' Process Sense™ endpoint sensor monitors the effluent from the chamber cleaning process in real-time using an NDIR method. For example, Process Sense™ monitors are used with fluorine-based chamber cleaning processes to monitor the by-product silicon tetrafluoride (SiF₄) from chambers employed for silicon-based deposition processes (including poly silicon, silicon dioxide, and silicon nitride). The sensor reports the SiF₄ level remaining in the effluent from the chamber in real-time, allowing the user to rapidly detect the endpoint of the cleaning process and avoid over-etching of the chamber components that might lead to damage or other maintenance issues. Knowledge of the heating value (BTU content) of a fuel is an important parameter for optimal control of processes such as power generation, petrochemical manufacturing, and flare control. Traditionally, gas chromatography (GC) has been the analytical technique most commonly used to determine the heating value of fuels. However, GC analyses are slow (updates require minutes) and GC instrumentation typically requires significant maintenance. MKS' Precisive^® Gas Analyzer is the industry's first all-optical analyzer that performs quantitative speciation of hydrocarbon gases. Using patented TFS™ analyzer technology, the Precisive analyzer is a simple and rugged NDIR instrument that provides high accuracy BTU measurements in often dirty and hazardous industrial environments.