Profiles chemical depth

Figxire 7 A chemical Depth Profile of a thermal oxide on GaAs (after reference 19). 

Figure 9 A Chemical Depth Profile of 15 A SiOj on Si by sputtering.

The application of photoelectron spectroscopy to semiconductors euid semiconductor device structiires has been demonstrated through its application to the silicon dioxide-silicon interface, ni-V compound semiconductor metal jimctions, and plasma etching residues. Through the use of profiling methods, chemical depth profiles are obtained and are extremely useful to device structural studies. Many methods such as in-situ film growth, film deposition, air-lock mounted pretreatment chambers, etc., have been employed to study semiconductor surfaces and device structures. 

Used in conjunction with electron microscopy (T)SEM No chemical depth profile (2003)... 

The sputtering process is used deliberately in secondary ion mass spectrometry (SIMS) [48] not only to remove atoms from the surface and to identify them by measuring their mass but also to gradually erode the surface to probe the composition in depth (chemical depth-profiling). In LEIS, the goal is to make the measurement before damage or sputtering can alter the structure or composition of the surface. 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface teclmique, but also the most difficult one to quantify. SIMS is very popular in materials research for making concentration depth profiles and chemical maps of the surface. For a more extensive treatment of SIMS the reader is referred to [3] and [14. 15 and 16]. The principle of SIMS is conceptually simple When a surface is exposed to a beam of ions... 

SIMS is, strictly speaking, a destructive teclmique, but not necessarily a damaging one. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one monolayer per several hours, implying that the surface structure does not change during the measurement (between seconds and minutes). In this case one can be sure that the molecular ion fragments are truly indicative of the chemical structure on the surface. 

As in Auger spectroscopy, SIMS can be used to make concentration depth profiles and, by rastering the ion beam over the surface, to make chemical maps of certain elements. More recently, SIMS has become very popular in the characterization of polymer surfaces [14,15 and 16]. 

Destructive Chemical bonding Depth profiling Quantification Accuracy Detection limits Sampling depth Lateral resolution Imaging/mapping... 

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. 

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

The results shown in Figure 6 above are an example of this mode of analysis, but include additional information on the chemical states of the Si. The third most frequently used mode of analysis is the Auger mapping mode, in which an Auger peak of a particular element is monitored while the primary electron beam is raster scanned over an area. This mode determines the spatial distribution, across the surface, of the element of interest, rather than in depth, as depth profiling does. Of course, the second and third modes can be combined to produce a three-dimensional spatial distribution of the element. The fourth operational mode is just a subset of the third mode a line scan of the primary beam is done across a region of interest, instead of rastering over an area. 

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. 

Static SIMS is labeled a trace analytical technique because of the very small volume of material (top monolayer) on which the analysis is performed. Static SIMS can also be used to perform chemical mapping by measuring characteristic molecules and fiagment ions in imaging mode. Unlike dynamic SIMS, static SIMS is not used to depth profile or to measure elemental impurities at trace levels. 

Depth profiling by SALI provides quantitative information through interfaces and for extremely thin films, in the form of reliable chemical concentrations. 

In summary, the forte of SNMS is the measurement of accurate compositional depth profiles with high depth resolution through chemically complex thin-film structures. Current examples of systems amenable to SNMS are complex III-IV laser diode structures, semiconductor device metallizations, and magnetic read-write devices, as well as storage media. 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

Chemical effects are quite commonly observed in Auger spectra, but are difficult to interpret compared with those in XPS, because additional core levels are involved in the Auger process. Some examples of the changes to be seen in the KLL spectrum of carbon in different chemical environments are given in Fig. 2.24 [2.130]. Such spectra are typical components of data matrices (see Sect. 2.1.4.2) derived from AES depth profiles (see below). 

Sect. 2.1.6.6). The extent of interaction and interdiffusion can be established by AES with depth profiling, although chemical information is normally absent, but the development of the interface is usually studied by continuous recording of Auger spectra during film deposition. 

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

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