miércoles, 3 de febrero de 2010

Chemical Characterization

We now focus on chemical characterization of thin films. This includes identification of surface and interior atoms and compounds, as well as their lateral and depth spatial distributions. To meet these needs, we use an important subset of the analytical techniques Usted in Table 6-1. Space limitation will restrict the discussion to include only the most popular methods (EDX, AES, XPS, RBS, and SIMS) and variants based on these. The justifica-tion for selecting these and not others is that they, together with the SEM and TEM, form the core of the diagnostic facilities associated with all phases of the research, development, processing, reliability, and failure analysis of thin-film electronic devices and integrated circuits. In VLSI technology some of these methods have gained wide acceptance as support tools for manufacturing lines. In addition, all of the associated equipment for these techniques is now commercially available, albeit at high cost. All excellent film characterization laboratoríes are outfitted with the total complement of this equipment.
Table 6-3 will assist the reader to distinguish among the various chemical analytical methods. The capabilities and limitations of each are indicated, and the comparative strengths and weaknesses for particular analytical applications can, therefore, be assessed. The following remarks summarize several of these distinctions:
1.   AES, XPS, and SIMS are true surface analytical techniques, since the
detected electrons and ions are emitted from surface layers less than  ~ 15
O
A deep. Provisión is made to probé deeper, or depth profile, by sputter-etching the film and continuously analyzing the newly exposed surfaces.
2.    EDX and RBS generally sample the total thickness of the thin film (~ 1 ftm) and frequently some portion of the substrate as well. Unlike RBS with a depth resolution of ~ 200 A, EDX has little depth resolution capability.
3.          AES, XPS, and SIMS are broadly applicable to detecting, with few exceptions, all of the elements in the periodic table.
4.    EDX can ordinarily only detect elements with Z > 11, and RBS is re-stricted to only selected combinations of elements whose spectra do not overlap.
5.         The detection limits for AES, XPS, EDX, and RBS are similar, ranging from about — 0.1 to 1 at%. On the other hand, the sensitivity of SIMS is much higher and parts per million can be detected. Even lower concentra-tion levéis (~ 10~6 at%) are detectable in certain instances.
6.         Quantitative chemical anaJysis with AES and XPS is problematical with composition error bounds of several atomic percent. EDX is better and SIMS significantly worse ín this regard. Composition standards are essen-tial for quantitative SIMS analysis.
7.         Only RBS is quantitatively precise to within an atomic percent or so from first principies and without the use of composition standards. It is the only nondestructive technique that provides simultaneous depth and composition information.
8.         The lateral spatial resolution of the región over which analyses can be performed is highest for AES (~ 500 A) and poorest for RBS (~ 1 mm). In between are EDX (~ 1 /¿m), SIMS (several ¿un), and XPS (~ 0.1 mm). AES has the distinction of being able to sample the smallest volume for analysis.

9. Only XPS, and to a much lesser extent AES, are capable of readily providing information on the nature of chemical bonding and valence states.
The preceding characteristics earmark certain instruments for specifíc tasks. Suppose, for example, a film surface is locally discolored due to contamina-tion, or contains a residue, and it is desired to identify the source of the unknown impurities. Assuming access to all instruments at equal cost, AES and EDX would be the techniques of choice. If only ultrathin surface layers are involved, EDX would probably be of little valué. The presence of trace elements would necessitate the higher sensitivity of SIMS analysis. If prelimi-nary examination pointed to the presence of Cl from an etching process, then evidence of the actual chemical compound formed would be obtained from XPS measurements. In a second example, a broad-area, thin-film metal bilayer structure is heated. Here we know which elements are initially present, but wish to determine the stoichiometry of intermetallic compounds formed as well as their thicknesses. This information is without question most unambiguously provided by RBS methods.
In what follows, the various techniques are considered individually where additional details of instrumentation, aspects of particular capabilities and limitations, and applications will be presented. First, however, it is essential to appreciate the scientific principies underlying each type of analysis. 

6.4.2. Electrón Spectroscopy
We start with a discussion of atomic core electrón spectroscopy since it is the basis for identification of the elements by EDX, AES, and XPS techniques. Consider the electronic structure of an unexcited atom schematically depicted in Fig. 6-14a. Both the K, L, M, etc., shell notation and the corresponding ls, 2s, 2p, 3s, etc., electrón states are indicated. Through excitation by an incident electrón or photon, a hole or electrón vacancy is created in the K shell (Fig. 6-14b).
In EDX an electrón from an outer shell lowers its energy by filling the hole, and an X-ray is emitted in the process (Fig. 6-14c). If the electrón transition occurs between L and K shells, Ka X-rays are produced. Different X-rays are generated, e.g., K^ X-rays from M -► K, and La X-rays from M -> L transitions. There are two facts worth remembering about these X-rays.

1. The difference in energy between the levéis involved in the electrón transition is what determines the energy (or wavelength) of the emitted X-ray.
2. The emitted X-rays are characteristic of the particular atom undergoing emission. Thus, each atom in the Periodic Table exhibits a unique set of K, L, M, etc., X-ray spectral lines that serve to unambiguously identify it. These characteristic X-rays are also known as fluorescent X-rays when excited by incident photons (e.g., X-rays and gamma rays).

The last equality indicates KL,L2 and KL2L, transitions are indistinguishable. Similarly, other common transitions observed are denoted by LMM and MNN. Since the K, L, and M energy levéis in a given atom are unique, the Auger spectral unes are characteristic of the element in question. By measuring the energies of the Auger electrons emitted by a material, we can identify its chemical makeup.
To quantitatively illustrate these ideas, let us consider the X-ray and Auger excitation processes in titanium. The binding energies of each of the core electrons are indicated in Fig. 6-15, where electrons orbiting cióse to the nucleus are strongly bound with large binding energies. Electrons at the Fermi level are far from the pulí of the nucleus and therefore taken to have zero binding energy, thus establishing a reference level. They would still have to acquire the work fünction energy to be totally free of the solid.

 X-Ray Energy-Dispersive Analysis (EDX)
 Equipment. Most energy-dispersive X-ray analysis systems are interfaced to SEMs, where the electrón beam serves to excite characteristic X-rays from the área of the specimen being probed. Attached to the SEM column is the liquid-nitrogen Dewar with its cooled Si(Li) detector aimed to efficiently intercept emitted X-rays. The Si(Li) detector is a reverse-biased Si diode doped with Li to créate a wide depletion región. An incoming X-ray generates a photoelectron that eventually dissipates its energy by creating electron-hole pairs. The incident photon energy is linearly proportional to the number of pairs produced or equivalently proportional to the amplitude of the voltage pulse they genérate when separated.
The pulses are amplified and then sorted according to voltage amplitude by a multichannel analyzer, which also counts and stores the number of pulses within given increments of the voltage (energy) range. The result is the characteristic X-ray spectrum shown for Ti in Fig. 6-15. Si(Li) detectors typically have a resolution of about 150 eV, so overlap of peaks occurs when they are not separated in energy by more than this amount. Overlap sometimes occurs in multicomponent samples or when neighboring elements in the periodic table are present.
Several variants of X-ray spectroscopy are worth mentioning. In X-ray wavelength-dispersive analysis (WDX), where wavelength rather than energy is dispersed, a factor of 20 or so improvement in X-ray linewidth resolution is possible. In this case, emirted X-rays, rather than entering a Si(Li) detector, are diffracted from single crystals with known interplanar spacings. From Bragg's law, each characteristic wavelength reflects constructively at different corresponding angles, which can be measured with very high precisión. As the goniometer-detector assembly rotates, the peak is swept through as a function of angle. The electrón microprobe (EMP) is an instrument specially designed to perform WDX analysis. This capability is also available on an SEM by attaching a diffractometer to the column. The high spectral resolution of WDX is offset by its relatively slow speed.
Characteristic X-rays can also be generated by using photons and energetic particles rather than electrons as the excitation source. For example, conven-tional X-ray tubes, and radioactive sources such as Am (60-keV gamma ray, 26.4-keV X-ray) and l09Cd (22.1-keV Ag-K X-ray) can excite fluorescent X-rays from both thin-film and thick specimens. Unlike electron-beam sources, they have virtually no lateral spatial resolution.


 Rutherford Backscattering (RBS) (Refs. 17, 20)
Physical Principies. This popular thin-film characterization tech-nique relies on the use of very high energy (MeV) beams of low mass ions. These have the property of penetrating thousands of angstroms or even microns deep into films or film-substrate combinations. Interestingly, such beams cause negligible sputtering of surface atoms. Rather, the projectile ions lose their energy through electronic excitation and ionization of target atoms. (For further discussion see Section 13.4.2.) These "electronic collisions" are so numerous that the energy loss can be considered to be continuous with depth. Sometimes the fast-moving light ions (usually 4He+) penétrate the atomic electrón cloud shield and undergo close-impact collisions with the nuclei of the much heavier stationary target atoms. The resulting scattering from the Coulomb repulsión between ion and nucleus has been long known in nuclear physics as Rutherford scattering. The primary reason that this phe-nomenon has been so successfully capitalized upon for film analysis is that classical two-body elastic scattering is operative. This, perhaps, makes RBS the easiest of the analytical techniques to understand.

Elemental Information. All elements and their isotopes including Li and those above it in the periodic table are, in principie, detectable with He+ ions. The critical test is how well neighboring elements are resolved, and this ultimately depends on the detector resolution. With 2-MeV 4He+, isotopes with AM = 1 can general ly be separated for M below approximately 40. At valúes of M « 200 only atoms for which AM > 20 can be resolved. Thus,
209                      190
Bi and Os would be indistinguishable. The apparent advantage in separat-ing low-Z elements is offset by their low cross sections (o,) for scattering. The o¡ are a measure of how efficiently target atoms scatter incoming ions and depend on Z¡, particle energy E, the masses involved, and the angle of scattering. To a good approximation, their dependence on these quantities varíes as
where the term in brackets is an important correction for low-mass targets.

 Spatial and Depth Resolution. Since MeV ion beams can only be
focused to a spot size of a millimeter or so in diameter, the lateral spatial
resolution of RBS is not great. The depth resolution is commonly quoted to be
200 A. This can be improved to 20 A by altering the geometry of detection.
Grazing exit angles are employed to make the film appear to be effectively
thicker. For example, when 6 = 95
°, corresponding to an exit angle of 5 °, ion
scattering at a given film depth means that the energy loss path length ís an
order of magnitude longer. Implicit in the use of RBS is the desirability that the
specimen surface and underlying layered structures be precisely planar. Fortu-
nately, polished Si wafers are extraordinarily fíat. Films grown or deposited on
Si maintain this planarity and are thus excellently suited for RBS analysis.
Films with rough surfaces yield broadened RBS peaks.
The máximum film depth that can be probed depends on the ion used, its energy, and the nature of the matrix. Typically, ~ 1 ¡im is an upper limit for 2-MeV He+. On the other hand, 3H+ beams of 2 MeV penétrate ~ 5 /tm deep in Si.

Secondary Ion Mass Spectrometry (SIMS) (Ref. 18,21)
The mass spectrometer, long common in the chemistry laboratory for the analysis of gases has been dramatically transformed in recent years to créate SIMS appararus capable of analyzing the chemical composition of solid surface layers. A critical need to measure thermally diffused and ion-implanted depth profiles of dopants in semiconductor devices spurred the development of SIMS.  In typical devices, peak dopant levéis are about   1020/cm3   while background levéis are 1015/cm3. These correspond to atomic concentrations in Si of 0.2% to 2 X 10~6%, respectively. None of the analytical techniques considered thus far has the capability of detecting such low concentration levéis. The price paid for this high sensitivity is an extremely complex spectrum of peaks corresponding the the masses of detected ions and ion fragments. This necessitates the use of standards, composed of the specific elements and matrices in question, for quantitative determinations of composi-tion.
In SIMS, a source of ions bombards the surface and sputters neutral atoms, for the most part, but also positive and negative ions from the outermost film layers. Once in the gas phase, the ions are mass-analyzed in order to identify the species present as well as determine their abundance. Since it is the secondary ion emission current that is detected in SIMS, high-sensitivity analysis requires methods for enhancing sputtered-ion yields. Secondary ion emission may be viewed as a special case of (neutral atom) sputtering. However, a comprehensive theory to quantitatively explain all aspects of secondary ion emission (e.g., ion yields S+ and S~, escape velocities and angles, dependence on ion projectile and target material, etc.) does not yet exist. Reliable experiments to test proposed theories are difficult to perform. Experimentally, it has been found that different ion beams interact with the specimen surface in profoundly different ways. For example, the positive metal ion yield of an oxidized surface is typically cnhanccd 10-fold and frequently more relative to a clean surface. This accounts for the common practice of using 0¿" beams to flood the surface when analyzing positive ions. Similarly, the negative ion signáis can be enhanced by using Cs+ primary ion beams.
One of the theories that attempts to explain the opposing effects of O^ and Cs+ beams involves charge transfer by electrón tunneling between the target and ions leaving the target surface. Negative ion (0¿~) bombardment repels charge from the surface, in effect lowering its Fermi energy and raising its effective work function (<j>). Tunneling is now favored from the surface atom (soon to be ejected positive ion) into the now empty electrón states of the target. Similarly, positive ion (Cs+) bombardment lowers the target work function. Now electrons tunnel from the target into empty levéis of surface atoms, enhancing the creation of negative ions. Since these charge transfer processes depend exponentially on <j>, very large changes in ion yields with small shifts in <f> are possible.

Ronellys Flores---CRF---libro the materials science of thin films


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