Thin and thick film analysis

The surface of a solid sample interacts with its environment and can be changed, for instance by oxidation or due to corrosion, but surface changes can occur due to ion implantation, deposition of thick or thin films or epitaxially grown layers. There has been a tremendous growth in the application of surface analytical methods in the last decades. Powerful surface analysis procedures are required for the characterization of surface changes, of contamination of sample surfaces, characterization of layers and layered systems, grain boundaries, interfaces and diffusion processes, but also for process control and optimization of several fihn preparation procedures. 

SIMS and SNMS are versatile analytical techniques for the compositional characterization of solid surfaces and interfaces in materials research. As one of the most important applications, both surface analytical techniques allow depth profile analysis (concentration profile as a function of the depth analyzed) to be performed in materials science and the semiconductor industry with excellent depth resolution in the low nm range. For depth profiling in materials science, dynamic SIMS and SNMS using high primary ion beam doses are applied. Both techniques permit the analysis of light elements such as H, O, C and N, which are difficult to measure with other analytical techniques. 

Quantitative depth profiling using polyatomic MCs+ and MCS2 ions instead of atomic ions M ions is well estabhshed in surface analysis using SIMS. The MCs technique, which reduces matrix effects significantly, was proposed by Gao in 1988. The formation of MCs+ has been explained by the recombination of sputtered neutral atoms (M) with 

Depth scale calibration of an SIMS depth profile requires the determination of the sputter rate used for the analysis from the crater depth measurement. An analytical technique for depth scale calibration of SIMS depth profiles via an online crater depth measurement was developed by De Chambost and co-workers. The authors proposed an in situ crater depth measurement system based on a heterodyne laser interferometer mounted onto the CAMECA IMS Wf instrument. It was demonstrated that crater depths can be measured from the nm to p-m range with accuracy better than 5 % in different matrices whereas the reproducibility was determined as 1 %. SIMS depth profiling of CdTe based solar cells (with the CdTe/CdS/TCO structure) is utilized for growing studies of several matrix elements and impurities (Br, F, Na, Si, Sn, In, O, Cl, S and Te) on sapphire substrates.The SnOj layer was found to play an important role in preventing the diffusion of indium from the indium containing TCO layer. 

The quantitative determination of Ge in SiGe quantum well structures (Sii j.Ge layers and interfaces) by SIMS is compared to low energy RBS. The thickness of the analyzed quantum well was about 12nm and it was situated at a depth of about 60 nm below the surface. The SIMS measurements were performed using an oxygen primary ion beam over an area of 350 x 350 jxm . 

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The semiconductor industry has largely driven the development of mass spectrometry (MS) instrumentation for thin film characterization, and secondary ion mass spectrometry (SIMS) is the reference technique for sensitive, quantitative depth profiling of implanted species in semiconductors [1], Applications of MS techniques to thin and thick film analysis are now found in many fields as the spectral information obtainable, both elemental and molecular, helps to address the most complex problems. 

After a tentative definition of thin and thick films, we present a rapid survey of several MS techniques that indicate the main features and benefits of each and the key issues. Detailed information on MS techniques are generally available in surface analysis books or journals [2-4] indicating the most common focus. A variety of examples have been selected from previous work performed by our colleagues or from our own work to give the reader an illustration of applications related to sur-... 

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is generally used for direct measurement of solids. The technique however could be applied to analysis of thin and thick films. 

The combination of various ionization sources with the inherently sensitive MS provide a powerful and flexible analytical platform for the analysis of layered materials. When applied to analysis of thin and thick films, these tools contribute to material research at its forefront and to the control of the most elaborate technology processes. 

For concentrated or bulk samples a transmission experiment is both the simplest and the most effective. In essence, one measures the X-ray intensities incident and transmitted through a thin and uniform film of the material. Careful analysis of signal-to-noise ratio considerations indicates that optimal results are obtained when the sample thickness is of the order of 2.5 absorption lengths. Since in this case a simple Beer s law applies, the data are usually plotted as In(7//0) versus E. The intensities are measured using ionization chambers in conjunction with high-gain electrometers (see Fig. 11). 

A laser-induced ToF mass spectrometer (LIMA-2A) was manufactured by Cambridge Mass Spectrometry Ltd., Cambridge, UK, for micro local analysis and was used to analyze thin sections of biological samples in the transmission mode or bulk material in the reflection mode.150,151 Typical LIMA applications in microelectronics include identification of impurities in dielectrics, microlocal analysis, depth profiling, thick film analysis and investigations on hybrid circuits. 

Finally, dynamic mechanical analysis (DMA) for PS in thin and ultrathin films is discussed. Studies over the past 15 years have shown that the Tg of nanometer thick polymer films can vary significantly with the film thickness K) when h falls below 50 nm [61-65]. Although the Tg has been found to both increase [5, 66, 67] and decrease [4, 10, 63, 68, 69] with decreasing h, the latter has drawn vastly more attention because of the significantly bigger size of the effect. 

Dorkenoo, K. D., and Pfromm,P.H., Experimental evidence and theoretical analysis of physical aging in thin and thick amorphous glassy polymer films, J. Polym. Sci. B, 37, 2239-2251... 

Since the 1950s XRF has been used extensively for the analysis of solids, powders, and liquids. The technique was extended to analyze thin-film materials in the 1970s. XRF can be used routinely for the simultaneous determination of elemental composition and thickness of thin films. The technique is nondesuuctive, rapid, precise, and potentially very accurate. The results are in good agreement with other elemental analysis techniques including wet chemical, electron-beam excitation techniques, etc. 

The empirical parameters method uses simple mathematical approximation equations, whose coefficients (empirical parameters) are predetermined from the experimental intensities and known compositions and thicknesses of thin-film standards. A large number of standards are needed for the predetermination of the empirical parameters before actual analysis of an unknown is possible. Because of the difficulty in obtaining properly calibrated thin-film standards with either the same composition or thickness as the unknown, the use of the empirical parameters method for the routine XRF analysis of thin films is very limited. 

The principle application of XRF thin-film analysis is in the simultaneous determination of composition and thickness. The technique has been used for the routine analysis of single-layer films since 1977 and multiple-layer films since 1986. Two main sources of publications in the fields are the annual volumes of Advances in X-Ray Analysis by Plenum Press, New York, and the Journal of X-Ray Spectrometry by Heyden and Sons, London. Typical examples on the analysis of single-layer films and multiple-layer films are used to illustrate the capabilities of the technique. 

With respect to the UHV-based techniques capable of providing chemical analysis, such as ESCA, AUGER, etc., several such studies have been performed. However, these studies were, by and large, performed on very thick oxide layers, formed after anodic oxidation of the Pt for many hours. Results from these studies thus have little bearing on the nature of the oxides formed on potential cycling. Part of the reason why these studies used such thick films lies in the considerable difficulty of detecting the thin oxide films formed during a potential sweep, even with relatively sensitive techniques. 

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