Dynamic SIMS, sputtering techniques

Compensation of Preferential Sputtering. The species with the lower sputter yield is enriched at the surface. This effect is called preferential sputtering and complicates, e. g.. Auger measurements. The enrichment compensates for the different sputter yields of the compound or alloy elements thus in dynamic SIMS (and other dynamic techniques in which the signal is derived from the sputtered particles, e.g. SNMS, GD-MS, and GD-OES), the flux of sputtered atoms has the same composition as the sample. 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

In ion implantation the gaseous dopant is ionized and accelerated into the wafer. SIMS has been used extensively to obtain dopant concentration depth profiles because it is the most sensitive of the surface analytical techniques and because sputtering is an intrinsic part of the dynamic SIMS process (40). AES, combined with ion sputtering, has also been used to obtain depth profiles for high dose implants (41). 

There are several MS-based techniques that can provide chemical information for thin and thick layers [12]. For very thin layers (sub to 1-2 monolayers), nondestructive techniques such as static SIMS [13], ion scattering MS [14], or MS of recoiled ions [15] are suitable. These techniques are also the best adapted for examining surface contamination. They are all based on surface interactions of an ion beam with the solid surface. For depth profiling of thin and thick layers, MS is associated with a destructive source of neutrals or ions dynamic SIMS, secondary neutron mass spectroscopy (SNMS), glow discharge mass spectroscopy (GD-MS), matrix-enhanced SIMS, laser desorption/ionization MS, and desorption electrospray ionization (DESI) MS [16]. Ions are either desorbed from the solid surface or generated by postionization of neutrals sputtered off the surface. 

There are two varieties of SIMS - static and dynamic. Static SIMS (often referred to as time-of-flight SIMS, TOF-SIMS) is often the method-of-choice, used for elemental analysis and imaging of the top two to three monolayers of a sample in comparison, dynamic SIMS is used to determine elemental concentrations of the sample, as a function of depth. As such, dynamic SIMS is a destructive technique primarily used for depth profiling, whereas TOF-SIMS does not appreciably deteriorate the surface being analyzed. For instance, due to a slow, controllable sputtering rate, the entire analysis may be performed without removing less than one-tenths of an atomic monolayer. 

In dynamic SIMS, a primary ion beam of energy, ranging from 0.5 to 20 keV, is used to sputter-remove successive layers of the sample in a well-defined area ranging in size from, typically, 1x1 mm to 10x 10 pm. This yields elemental information on the surface region from a few nanometres to several hundreds micrometres in depth. The detection limits of the technique are in the ppm-ppb range. Unfortunately, quantification by SIMS is bedevilled by matrix effects. They arise because the particle emission and ionisation processes take place simultaneously . If we were able to decouple sputtering from ionisation (e.g. ionisation occurring after the neutrals had been moved away from the surface), the ion yield would be independent of the matrix and quantification would be easier. 

Dynamic SIMS has long been an established technique for the detection of low concentration species in bulk samples (e.g. dopant levels in semiconductors). While the sample is being sputtered (see Sec. 2.3), a quadrupole mass analyzer detects the resulting fragments in real time, resulting in a depth profile of the desired species in the sample or, with calibration, an indication of the concentration of the desired species in the bulk of the sample. The method is fast and simple, but a quadrupole mass analyzer can only tune into one particular mass-to-charge ratio at a time, or... 

A. 10.3.3 SNMS and RIMS Secondary Neutral Mass Spectrometry (SNMS), also referred to as Sputtered Neutral Mass Spectrometry, is a destructive technique primarily used for examining elemental constituents within solid samples. This technique is closely related to Dynamic SIMS in that an ion beam is used to sputter the solid of interest. The difference lies in the fact that the sputtered neutral population, once ionized, is passed through a mass spectrometer. Ionization is induced via the action of a laser, an electron beam, or plasma (ionization yields vary from 10% for lasers to 1% for plasmas). As the greatest fraction of the sputtered population departs in the neutral state, this methodology provides the advantage of improved detection limits and reduced matrix effects relative to SIMS. Depth resolution can extend to 1 nm. Spatial imaging is generally not carried out. No prior sample preparation is needed, but HV or better conditions are required. 

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. 

Information derivable from these techniques is of vital importance in the understanding of the atomic, molecular, ionic, solid-state, and electronic processes that occur at the surface and within the bulk of materials. The dynamics of ion and neutral species, as they approach, penetrate, charge-exchange, diffuse, dissociate (molecules and molecular ions), and sputter some of the substrate atoms, molecules, ions, and clusters, provides a wealth of information. Some of this information cannot be obtained by any other means. Although SIMS is now moving out of adolescence into a stage of matnrity, there is stiU much to be learned about the mechanisms and applications of SIMS and SNMS. 

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