Page 45 - Sami Franssila Introduction to Microfabrication

24 Introduction to Microfabrication

2.12 OTHER METHODS 2.13 ANALYSIS AREA AND DEPTH
Unfortunately, most methods are limited to certain Analysis methods differ fundamentally in their analy-
elements only. The only exception is SIMS, which sis depth:
can detect every element from hydrogen to uranium.
Auger spectroscopy cannot detect H, He or Li because – surface-sensitive methods
of fundamental limitation of the three-electron Auger – bulk methods
process, but all other elements that are detectable. X-ray – micrometre methods.
methods are insensitive to light elements: depending on
X-ray window design, boron (m = 11) can be detected, Surface-sensitive methods probe only the topmost
but sometimes fluorine (m = 19) or sodium (m = 23) is atomic layers, a nanometre or two.
the lightest detectable element. Methods that analyse low-energy electrons are
Infrared spectroscopy measures absorption due to surface-sensitive because the escape depth of low-
molecular vibrations that are around 10 µm wavelength. It energy electrons is just a few nanometres. Auger elec-
gives information about chemical bonds, because infrared tron spectroscopy and X-ray Photoelectron Spectroscopy
vibrations are typically bond stretching and bending are examples.
vibrations. Si–O bonds are desirable in silicon dioxide, Diffusion depths and film thicknesses are often of
but Si–H bonds indicate unwanted atomic arrangements the order of one micrometre. Analysis techniques that
and potential reliability problems. Si–F bonds on an extend this deep would be very useful, but only a
etched surface hint at polymeric residue formation few exist. Rutherford backscattering spectrometry (RBS)
mechanism and help in designing the removal process. has a typical analysis depth of around micron (for
Infrared spectroscopy is most often practiced using an helium ion energy of 2 MeV). Electron beam–induced
interferometric measurement set-up known as FTIR, for X-ray fluorescence also probes at ca. micron depth.
Fourier-transform IR. It is used to measure oxygen and The combination of sputter erosion and surface-sensitive
carbon concentrations in silicon wafers, as revealed by analysis is commonly adopted for top micrometre
optical absorption in 8 to 17 µm wavelength range. analysis: ion-beam sputtering removes material and the
Bulk wafers can be analysed by charge-carrier excita- newly formed surface is probed by, for example, Auger
tion methods such as microwave photoconductive decay or SIMS.
(µPCD) and surface photovoltage (SPV). In µPCD, the Optical beam spots are micrometre-sized and they
sample is excited by a laser beam that creates excess- can be used to measure within a real device structure.
charge carriers. The amount of these carriers over time However, some optical methods such as ellipsometry
is measured in a non-contact arrangement by microwave require ca. 100 µm analysis area. Because X-rays cannot
reflection. Charge-carrier lifetime can be correlated with be focussed, X-ray methods require typically rather
impurities and defects in the semiconductor material. large areas, in the millimetre range. Ion beams can be
Neutron activation analysis (NAA) detects gamma focussed to submicron spots in focussed ion beam (FIB)
quanta that have been excited by neutrons. NAA equipment, but most applications use broad beams, in
can detect selected elements at concentrations as low the millimetre range.
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as 10 cm −3 (Cu, Ag, Au) and many others at Analysis must be done not only on microfabricated
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concentrations <10 cm −3 (Fe, Zn, Ni). structures themselves but also on defects and non-
X-ray tomography (XRT) images full wafers with idealities that are smaller than the device dimensions.
micron resolution. This is not enough for most crys- If the chemical composition or structure of defects
tallographic defects as such, but local stresses around has to be identified, it is even more demanding than
defects often extend to many microns, so the method analysis of regular microstructures. Contaminants often
can indirectly see small defects. come in quantities too small for even the best ana-
If the material to be analysed can be extracted lytical methods. Vacancies and other point defects are
from the wafer, a much larger repertoire of analytical smaller than the resolution of even the best microscopic
methods can be used. Thermal desorption spectroscopy methods. Indirect methods, such as carrier lifetime mea-
(TDS) analyses desorption products upon heating. If the surements (defects act as traps for charge carriers),
material can be dissolved in acid, atomic adsorption positron annihilation spectroscopy (PAS) (positron life-
spectroscopy (AAS) and other methods of standard time is longer in material with voids) or photolumines-
chemical analysis become available. cence (identification of defects by their recombination