Lateral and depth resolution

The physical techniques used in IC analysis all employ some type of primary analytical beam to irradiate a substrate and interact with the substrate s physical or chemical properties, producing a secondary effect that is measured and interpreted. The three most commonly used analytical beams are electron, ion, and photon x-ray beams. Each combination of primary irradiation and secondary effect defines a specific analytical technique. The IC substrate properties that are most frequendy analyzed include size, elemental and compositional identification, topology, morphology, lateral and depth resolution of surface features or implantation profiles, and film thickness and conformance. A summary of commonly used analytical techniques for VLSI technology can be found in Table 3. 

By acquiring mass-resolved images as a function of sputtering time, an imaging depth profile is obtained. This combined mode of operation provides simultaneous lateral and depth resolution to provide what is known as three-dimensional analysis. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

A very important characteristic of laser radiation is the beam shape. So far most LA experiments have been performed with Gaussian laser beams. Lasers with uniform distribution of the beam cross-section have been used only recently to achieve high lateral and depth resolution. Specially designed beam homogenizers must be used for this purpose [4.226-4.228]. The Cetac LSX-200 system has a flat-top distribution of the laser beam. 

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Less pronounced thermal diffusion provides better lateral and depth resolution and is the basis of successful application of femtosecond pulses in material processing and microstructuring [4.231, 4.232]. All-solid-state femtosecond lasers with a pulse duration of 100-200 fs and a pulse energy of approximately 1 mj have recently become commercially available [4.233, 4.234]. 

Initial results prove the high potential of LA-based hyphenated techniques for depth profiling of coatings and multilayer samples. These techniques can be used as complementary methods to other surface-analysis techniques. Probably the most reasonable application of laser ablation for depth profiling would be the range from a few tens of nanometers to a few tens of microns, a range which is difficult to analyze by other techniques, e. g. SIMS, SNMS,TXRE, GD-OES-MS, etc. The lateral and depth resolution of LA can both be improved by use of femtosecond lasers. 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

Figure 8.3 Comparison of lateral and depth resolutions of various physical methods. For acronyms, see Appendix I. After Ramendik et al. [101], Reprinted from G. Ramendik et al., in Inorganic Mass Spectrometry (F. Adams et al., eds), John Wiley Sons, Inc., New York, NY, pp. 17-84, Copyright (1988, John Wiley Sons, Inc.) This material is used by permission of John Wiley Sons, Inc.

Three techniques with spatially resolved information capabilities have been selected here for some further explanation EPXMA, laser-induced breakdown spectroscopy (LIBS) and glow discharge optical emission spectrometry (GD-OES). Figure 1.15 summarises the lateral and depth resolution provided by the techniques described in this section. It is worth noting that the closer to the bottom left corner the technique is located, the higher (and so better) is the depth resolution. 

Figure 1.15 Comparison of the lateral and depth resolution allowed by different optical and mass spectrometric techniques used for direct solid analysis (A, IG, incident and emitted ions, respectively cT, electrons ho, photons). XPS and AES are included in the graph for comparison.

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. 

A higher order nonlinear dielectric microscopy technique with higher lateral and depth resolution than conventional nonlinear dielectric imaging is investigated. The technique is demonstrated to be very useful for observing surface layers of the order of unit cell thickness on ferroelectric materials. 

Under normal conditions (n= 1 for air, 1 = 632.8 nm, NA = 0.95), the typical lateral and depth resolutions would be about 400 and 700 nm, respectively. Using the smallest visible wavelength ( 400 nm) and a high numerical aperture (n = 1.515, NA 1.4) one can estimate the highest lateral resolution as 200 nm (based on Abbe criterion) and the smallest held depth as 400 nm. However, if a series of spectra are recorded at very close equidistant locations, a reduced spot size (considering the Gaussian prohle of the beam) is obtained through a convolution of the spot prohle. 

The use of TP initiated polymerization for 3D microfabrication has several advantages over OP initiated polymerization. A 3D resolution can be achieved with lateral and depth resolutions of 0.2 pm and 0.28 pm. This is fabrication at a... 

The main factor in beam analysis that affects the reliability of the analytical information is the reproducibility of the surfaces. When using scanning electron microscopy (SEM), the apparati are connected to the computer, which makes it possible to obtain quite a bit of information about the sample, especially by X-ray and AES. However, the apparati cannot assure the same length for beam penetration on the surface, which means that the analytical information can be uncertain. Because the beam analysis is rapid, it requires very fast detectors, e.g., Ge/Li or Si/Li. The LA can be successfully used in surface analysis. An automated system has been constructed, laser-induced breakdown spectrometry (LIBS).213 This is an alternative to other surface techniques — secondary ion mas spectroscopy (SIMS), SEM, X-ray photoelectron spectroscopy (XPS) — and it increases the lateral and depth resolution. 

Traditional mass spectrometric imaging (MSI) methods, such as matrix-assisted laser desorption ionization (MALDI) and secondary ion mass spectrometry (SIMS), have become important tools for the investigation of molecular distributions in tissues due to their high ionization efficiencies and excellent lateral and depth resolutions. Invasive sample preparation and the need for vacuum conditions, however, are incompatible with the analysis of live samples. 

Table 5.1. Lateral and depth resolution of different types of microscopes ...

Although, very often, the lower surface energy component tends to segregate to the surface, a number of other factors - such as solvent, specific interaction and crystallization - can also affect the blend s surface composition [4,5]. Consequently, whilst an understanding of the blend surface composition is of fundamental interest, it may not be easy to acquire. Implied is the simultaneous requirement for chemical speciation at both high lateral and depth resolution within the surface region. 

---