The interaction of matter and electromagnetic radiation is studied during spectroscopic analysis to obtain useful information regarding the physical structure, composition, and morphology of the substance being studied. Thin films, ranging from several nanometers to micrometers, are studied via spectroscopy. Traditionally, spectroscopy methods such as Infrared spectroscopy (IRS) and Raman spectrometry are preferred for the analysis of thin films, but recently, thin film desorption spectroscopy has been the center of attention.
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Thermal Desorption Spectroscopy for Thin Films: An Introduction
Thermal Desorption Spectroscopy (TDS) is a method employed to analyze gases emitted from materials during heating. This technique is crucial for investigating desorption kinetics and surface reactions of materials, particularly in thin film and surface science studies.
Molecules that are bound to the surface of any material with any specific energy can be released by increasing the temperature. During TDS, the molecules that desorb, either entirely or partially, are detected using a mass spectrometer (QMS) while the surface is heated at a defined rate. This method is useful for the thermal stability analysis of the molecular species, especially thin films.
History and Methodology
An article published in the Journal of Alloys and Compounds states that thermal Desorption Spectroscopy (TDS), initially developed in the field of surface science, was first applied to study hydrogen desorption kinetics in bulk hydrides around 1980.
In this technique, a non-isothermic study of desorption kinetics is conducted. A sample, previously loaded with hydrogen, undergoes continuous heating following a predefined temperature profile (usually a linear ramp) while recording the amount of gas desorbed from the material.
The resulting plot of gas desorption flow versus temperature constitutes a TDS spectrum. TDS spectra typically exhibit multiple desorption peaks, each associated with a distinct kinetic process. The primary objective of a TDS experiment is to identify the rate-limiting step and determine the kinetic parameters related to the process.
Utilization of TDS for the Study of Thin Films
The molecular solids research group at Philipps-Universität Marburg is using TDS to characterize organic thin films, which are either evaporated onto various substrates or formed through immersion.
The temperature required to release molecules from the surface serves as a direct measure of desorption energy. In this process, the system must be capable of detecting low amounts of molecules, and precise regulation of temperature over time is essential. High and stable vacuum conditions, along with an extremely sensitive measurement technique, are necessary for accurate results.
The research group members use TDS to detect the characteristic mass and perform analysis of the desorption temperature and peak shapes of the spectrum. This provides valuable information regarding the nature of bonding of molecules to the surface i.e., whether the bonding is covalent or the molecule is just physisorbed.
The thermal spectrum is also useful in determining the number of layers of molecules in a thin film, and the comparison of thin film desorption spectra at different times with varying masses allows the researchers to comment on the changes taking place in the thin film over time.
Utilization of TDS for Semiconductor Thin Films and Microelectronics
Thermal desorption spectroscopy (TDS) has played an essential role in the development of dielectric films in microelectronics, particularly with the industry aiming to reach the nanoscale.
The introduction of new materials for interconnects, such as copper (Cu) and low-k materials, and for the gate, such as high-k materials, necessitates precise understanding and characterization of their thermal properties. TDS allows researchers to study the desorption kinetics and surface reactions of these materials upon heating, providing valuable insights for optimizing their performance in microelectronic applications.
The semiconductor industry began employing Thermal Desorption Spectroscopy (TDS) in the early 2000s to gain insights into the chemical and structural changes associated with the surface modification of porous low-k thin films.
TDS analysis of entire semiconductor wafers offers low detection limits for pore volumes and residues of metal-organic precursors in high-k films. In fact, these detection limits are comparable to or even better than those achieved by alternative techniques. This application of TDS contributes to the optimization of high-k materials used in semiconductor manufacturing.
TDS: The Preferred Choice for Organic Thin Films Study
Interest in organic thin films has surged, particularly due to their potential applications in organic electronics. Understanding the initial stages of organic film growth is crucial for advancing applications in organic electronics.
In the article published in Surface Science, the researcher highlighted the importance of TDS for thin films by employing it for the study of organic films, which were deposited on various types of substrates such as gold and silicon dioxide.
Conventionally, it was believed that organic molecules adhere with a sticking coefficient of one on most substrates at room temperature and below. However, results from TSD indicated that for flexible molecules like phenylenes, the initial sticking coefficient was nearly one, while for rigid molecules such as rubicene and pentacene, the initial sticking coefficient was considerably less than one. As the coverage increased, the sticking coefficient approached the value of one in all cases.
Thermal desorption spectroscopy (TDS) in the study of organic molecule adsorption and desorption offers the potential to identify potential cracking processes during sample heating. For instance, in the adsorption of 4P and 6P on gold surfaces, observations revealed not only complete decomposition but also partial decomposition. This partial decomposition was followed by cyclo-dimerization, resulting in more stable polycyclic hydrocarbons, which eventually decomposed at even higher temperatures.
Thermal Desorption Spectroscopy (TDS) plays a crucial role in the analysis and development of thin films. Its capability to offer detailed insights into surface chemistry makes it indispensable in both materials science research and industrial applications. As technology progresses, TDS is expected to become even more versatile and insightful, fostering innovations in thin film technologies and other related fields.
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References and Further Reading
Philipps-Universität Marburg, (2023). Thermal Desorption Spectroscopy (TDS). [Online]
Available at: https://www.uni-marburg.de/en/fb13/molecular-solid-state-physics/inside-the-lab/methods/spectroscopy/tds
Carbonell, Laureen & Vereecke, Guy & Jehoul, Christiane & Gallagher, M & Gronbeck, D & Van Elshocht, S. & Caymax, Matty & Maex, K & Mertens, Paul. (2003). Characterization of Advanced Semiconductor Materials by Thermal Desorption Mass Spectrometry with Atmospheric Pressure Ionization. Proceedings of SPIE - The International Society for Optical Engineering. 5133.
Castro, F. et. al. (2002). Thermal desorption spectroscopy (TDS) method for hydrogen desorption characterization (I): theoretical aspects. Journal of alloys and Compounds, 330, 59-63. Available at: https://doi.org/10.1016/S0925-8388(01)01625-5
Winkler, A. (2016). Initial stages of organic film growth characterized by thermal desorption spectroscopy. Surface science, 643, 124-137. Available at: https://doi.org/10.1016/j.susc.2015.06.022
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