Thin film SIMS depth profiling

Thin Films. SIMS depth profiles are also used In the development of new thin film processes. These processes are being Investigated to produce higher speed and greater device density In VLSI applications. Molybdenum metal gates are being Investigated... 

Figure 5. SIMS depth profiles of Ti/thin Cu films on polyimide as deposited, showing O18 from polyimide and ambient humidity exposure exposed to H2018 and annealed in nitrogen at 350 C...

Figure 8. SIMS depth profiles of Cr/thin Cu films on polyimide (a) as deposited, showing O 8 from ambient humidity exposure (normalized for isotopic abundance) (b) exposed to H2018 and (c) exposed to H,018 and annealed in forming gas at 350 C (200 W, 30 min RF) and (d) exposed to H20 8 and annealed in forming gas at 350 (50 W, 10 min RF).

Indispensable and continue to be Important In the development of models for range statistics In Ion Implantation. SIMS depth profiles are also used to monitor and develop an understanding of the diffusion of dopants during laser and thermal annealing processes. Metallization and thin films have also been Investigated by SIMS. In addition SIMS depth profiles are useful for failure analysis and problem solving. 

In order to understand the mechanism of improved oxidation resistance of nanocrystalline Fe-lOCr alloy, the composition, including Cr content of the thin oxide films developed on the nanocrystalline and microcrystalline alloys was characterized. The thin oxide films formed over nanocrystalline Fe-lOCr and microcrystalline Fe-10wt%Cr alloys at 300,350 and 400°C in air were characterised by SIMS depth profiling [12,39]. 

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

In contrast to SIMS, in SNMS - where the evaporation and ionization processes are decoupled -the matrix effects are significantly lower, because the composition of sputtered and post-ionized neutrals corresponds more closely to the composition in the solid sample (compared to the sputtered secondary ions in SIMS), which means the RSCs of elements vary by about one order of magnitude. Consequently, a semi-quantitative analysis by SNMS can also be carried out if no suitable matrix matched CRM is available. This is relevant for thin film analysis, especially for the determination of elemental concentration profiles in depth, for studying the stoichiometric composition of thin films and interdiffusion effects. 

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

In dynamic SIMS, the primary ion current density is very much higher (usually >10 p,A cm ) so that the surface is rapidly eroded. The intensity of elemental ions is followed as a fnnction of time (i.e. eroded depth) to provide composition depth profiles. This techniqne is particnlarly valuable for studying buried interfaces and thin film stmctnres. 

Gastel, M., Brener, U., Holzbrecher, H., Becker, J.S., Dietze, H., Wagner, H. (1997) Depth profile analysis of thin film solar cells using SNMS and SIMS. Fresenius Journal of Analytical Chemistry, 358,207-210. 

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. 

A practical definition could finally also be derived from the capabilities of the instrumentation in use. For instance, SIMS, the most widespread MS technique, applied to surface and thin films can be operated in static mode (giving information from the first atomic layers of a nearly undamaged surface) or dynamic mode (depth profile of the layer). When the material to be analyzed is sputtered, this sputtering could be very slow, providing a practical limit (often in the micrometer range for SIMS) to the thickness range achievable in a reasonable amount of time. 

By using dynamic mode SIMS the lateral distribution of phases in three dimensions can be resolved (Fig. 31). Thin films (thickness ca. 500 nm) of binary mixtures of deuterated or partially brominated PS, polyisoprene and poly(vinylpyridine) were investigated with a lateral resolution of approximately 120 nm and composition versus depth profiles with a resolution better than 10 nm [208]. The brominated PS formed continuous phase-domain structures in the interior of the films whereas they were encapsulated by deuterated PS layers at the interfaces. Moreover a very thin layer (ca. 3 nm) of polyisoprene covered the surface of a binary mixtme of poly(isoprene)/deuterated PS [208]. 

Both of the above sectioning techniques have an important place in the measurement of film thickness. For films that are much thicker than 1 pm, polishing seems to be the preferred approach. However, for thinner corrosion films (even as thin as a few hundred nm) fracturing at cryogenic temperatures can often produce thickness values that are more accurate than depth profiling using AES, XPS or SIMS, and is certainly faster. 

The elemental composition of a film can be important to the film properties and is an indication of process reproducibility. In many cases, the elemental composition can change with thickness and some technique must be used that allows depth profiling of the elemental composition. Depth profiling can be accomplished using sputter etching and the surface spectroscopies of AES, ISS, SIMS, and XPS, as discussed in Secs. 2.4.1 and 2.4.3. Several techniques are available to non-destructively analyze the elemental composition of a thin film. 

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