Depth profiling limited applicability

The complexity of the apparatus needed for ion implantation makes this method of case hardening of limited application. Further, the case depth is considerably lower than that produced by carburizing or nitriding. The depth of implantation of nitrogen in a steel is about 0.00006 cm (19), ie, so thin that it is difficult to measure the hardness profile by conventional microhardness measurements. 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

In principle GD-MS is very well suited for analysis of layers, also, and all concepts developed for SNMS (Sect. 3.3) can be used to calculate the concentration-depth profile from the measured intensity-time profile by use of relative or absolute sensitivity factors [3.199]. So far, however, acceptance of this technique is hesitant compared with GD-OES. The main factors limiting wider acceptance are the greater cost of the instrument and the fact that no commercial ion source has yet been optimized for this purpose. The literature therefore contains only preliminary results from analysis of layers obtained with either modified sources of the commercial instrument [3.200, 3.201] or with homebuilt sources coupled to quadrupole [3.199], sector field [3.202], or time-of-flight instruments [3.203]. To summarize, the future success of GD-MS in this field of application strongly depends on the availability of commercial sources with adequate depth resolution comparable with that of GD-OES. 

Q.) is based on the ejection of the recoiled particles out of the sample in the forward direction by an energetic heavy ion beam. The measured energy spectra of these recoiled atoms can be related to their concentration profiles. The use of range foil in front of the energy detector to permit selective absorption of the various recoils introduces a few limitations in the application of the technique, e.g. deterioration of the energy resolution and hence the depth resolution, the limitation on the accessible depth in the depth profile information, etc. Indeed, the practical utility of the experimental set-up is enormously reduced in the region where overlapping spectra of various atoms are difficult to separate. 

In this review the various modes of SIMS and examples of their applications are discussed. SIMS depth profiles are widely used to study dopant profiles and Intermetallic diffusion. The extreme surface sensitivity and low concentration detection limits of SIMS make It useful for Investigation of substrate and metallization cleaning processes. SIMS elemental Imaging Is also used In contamination studies. The ability of SIMS to provide Isotopic Information has allowed elegant mechanistic studies. The Identification and determination of the relative abundance of various molecular or elemental species by SIMS Is applicable to the development characterization and understanding of microelectronic processing. The capability of SIMS In the area of quantitative analyses Is also discussed. 

SIMS is used for quantitative depth profile determinations of trace elements in solids. These traces can be impurities or deliberately added elements, such as dopants in semiconductors. Accurate depth prohles require uniform bombardment of the analyzed area and the sputter rate in the material must be determined. The sputter rate is usually determined by physical measurement of the crater depth for multilayered materials, each layer may have a unique sputter rate that must be determined. Depth prohle standards are required. Government standards agencies like NIST have such standard reference materials available for a limited number of applications. For example, SRM depth profile standards of phosphorus in silicon, boron in silicon, and arsenic in silicon are available from NIST for calibration of SIMS instmments. P, As, and B are common dopants in the semiconductor industry and their accurate determination is critical to semiconductor manufacture and quality control. 

The use of an intermediate barrier layer to vary the penetration depth in an ATR experiment was investigated. Both theoretical and experimental results suggested that this method could be useful in probing concentration profiles away from the interface, particularly in the case of thin films, where variable-angle methods had limitations. Application of this technique in the study of PMMA/PDMS graft copolymers deposited onto a copper oxide surface indicated a preferential presence of the siloxane component at the oxide interface, a concentration which tended to decrease with increasing distance into the bulk of the sample film. 20 refs. 

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