FAULT LOCALIZATION TECHNIQUES III

EMT 361

SCHOOL OF MICROLELECTRONIC

ENGINEERING

KUKUM

Shell model

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nucleus = positively charged protons and uncharged neutrons. ~ same size and weight. like charges repel~ "nuclear glue" that holds the nucleus together. There must usually be more neutrons than protons for a nucleus to be stable, so it has been thought that neutrons had something to do with this nuclear glue. Present theories state that there is a sub-nuclear particle called a gluon that holds the nucleus together.

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Orbits & electrons

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He 1s2

Ne 1s22s22p6

Ar 1s22s22p63s23p6

More orbits…

shell # (code) set of orbitals letter 1 (K) s2 (L) s,p 3 (M) s,p,d 4 (N) s,p,d,f 5 (O) s,p,d,f,g 6 (P) s,p,d,f,g,h ... s,p,d,f,g,h,i...

s, p, d, f, g, h, i... The original designation of those orbitals was done according to their spectral appearance, namely: sharp, principal, diffuse, fundamental, hence the letter code with the remaining letters being ordered alphabetically. To cause even greater confusion, orbitals always come in groups. There is one s-orbital, three p-orbitals, five d-orbitals, seven f-orbitals

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spectroscopy

Stage 1: Ionisation atom is ionised by knocking one or more electrons off to give a positive ion. Mass spectrometers always work with positive ions.

Stage 2: Accelerationions are accelerated - same kinetic energy.

Stage 3: Deflection ions are deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.

The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.

Stage 4: Detection beam of ions passing through is detected electrically.

Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on: the mass of the ion. Lighter ions are deflected more than heavier ones. the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge. These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z (or sometimes m/e). For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio of 28. In the last diagram, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio.

SYLLABUS

• FTIR

• NMR

• AFM

• STM

• AES

• FIB

FTIR

ftir

FTIR (Fourier Transform Infrared) Spectroscopy = FTIR Analysis,

= failure analysis technique that provides information about the chemical bonding or molecular structure of materials, whether organic or inorganic.

It is used in failure analysis to identify unknown materials present in a specimen, and is usually conducted to complement EDX analysis.

principle

bonds and groups of bonds vibrate at characteristic frequencies .

molecule that is exposed to infrared rays absorbs infrared energy at frequencies which are characteristic to that molecule.

FTIR analysis- spot on specimen subjected to modulated IR beam.

specimen's transmittance and reflectance of the infrared rays at different frequencies is translated into an

IR absorption plot consisting of reverse peaks.

resulting FTIR spectral pattern is then analyzed and matched with known signatures of identified materials in the FTIR library

• FTIR spectroscopy does not require a vacuum - oxygen nor nitrogen absorb infrared rays.

• minute quantities of materials, whether solid, liquid , or gaseous - single fibers or particles are sufficient

• no library of FTIR spectral patterns - individual peaks in the FTIR plot may be used to yield partial information about the specimen.

• Organic contaminants in solvents may also be analyzed by first separating the mixture into its components by GC, and then analyzing each component by FTIR.

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NMR

• compass needle normally lines up with the Earth's magnetic field with the north-seeking end pointing north.

• twist the needle around so that it points south - lining it up opposed to the Earth's magnetic field.

• let it go again, it will flip back to its more stable state.

• Hydrogen nuclei ~ little magnets • a hydrogen nucleus can also be aligned with an external

magnetic field or opposed to it - alignment where it is opposed to the field is less stable (at a higher energy).

• possible to flip it from the more stable to less stable by supplying exactly the right amount of energy. - usually in the range of energies found in radio waves - at frequencies of about 60 - 100 MHz. (FM radio 88 - 105 MHz!)

• detect this interaction between the radio waves of just the right frequency and the proton as it flips from one orientation to the other as a peak on a graph. This flipping of the proton from one magnetic alignment to the other by the radio waves is known as the resonance condition.

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• Spectroscopy - interaction of electromagnetic radiation with matter. • NMR phenomenon to study physical, chemical, and biological properties of

matter - study chemical structure using simple one-dimensional techniques. Two-dimensional techniques are used to determine the structure of more complicated molecules.

• Time domain NMR spectroscopic techniques are used to probe molecular dynamics in solutions.

• Solid state NMR spectroscopy is used to determine the molecular structure of solids, measuring diffusion coefficients.

AFM

Atomic Force Microscopy

• AFM = form of scanning probe microscopy (SPM)- measuring a local property of the surface being inspected, such as its height, optical absorption, or magnetic properties.

• probe or tip that's positioned very close to surface to get measurements.

• AFM operates in two modes, namely, contact mode imaging and non-contact mode imaging.

Contact mode imaging

• a soft cantilevered beam - sharp tip at its end - brought in contact with the surface of the sample.

• force between tip and sample causes the cantilever to deflect in accordance with Hooke's Law, exhibiting a spring constant that typically ranges between 0.001 to 100 N/m - derive information about the surface of the sample.

• amount of deflection is measured by reflecting light from a laser diode off the back of the cantilever beam, and onto a pair or array of position-sensitive photodetectors.

• differences between reflected light received by individual photodetectors indicates amount of angular deflection of the cantilever at any given point on the sample.

• cheaper (but less sensitive) alternative - using piezoresistive AFM probes that serve as a strain gauge system.

• ability to monitor deflection allows the AFM to create an image of the sample non-destructively even if the tip is continuously in contact with the sample.

• prevent cantilever tip from damaging the surface of the sample- maintain at a constant angular deflection so that the force applied by the tip on the surface is also kept constant- feedback mechanism that adjusts the distance between the tip and the surface to keep the applied force constant .

• Applied forces between the tip and the sample typically range from 10 -11 to 10 -7 N.

• feedback mechanism controls piezoelectric elements that hold the sample and move it in all three axes relative to the tip. The z-movement is used for maintaining the force at the right level, while the x- and y- movements are used for the raster-scanning the tip over the sample. The resulting map s(x,y) of the tip's relative distance from the surface at different x-y values of the scan is then used to form a topographical image of the scanned area of the sample. ﾊ Thus, contact-mode imaging is primarily used for generating images of a sample's topography .

Non-contact imaging

• employs a small piezo element mounted under the cantilever - oscillate at its resonance frequency.

• oscillating cantilever is brought down to within 10-100 nm from the sample surface, the oscillation gets modified by interaction forces (Van der Waals, electrostatic, magnetic, or capillary forces) between the tip and the sample.

• changes in the oscillation usually involve a decrease in resonant frequency, a decrease in amplitude, and a phase shift.

• changes in oscillation characteristics used to generate a map that characterizes the surface of the sample - amplitude modulation = information about the sample's topography.-Phase shifts used to distinguish different surface materials from each other. - Frequency modulation used to get information about the sample's properties.

• One challenge in non-contact imaging is being able to keep the correct tip-to-sample distance while preventing the tip from touching the surface - since there is a maximum distance for the inter-atomic forces to become detectable. Furthermore, the tendency of most samples to develop a liquid meniscus layer in ambient conditions complicates this task.

advantages & disadvantages

• 1) it generates true 3-dimensional surface images;

• 2) it does not require special sample treatments that can result in the sample's destruction or alteration;

• 3) it does not require a vacuum environment in order to operate (it can operate in both air and liquid). ﾊ

• 1) the image size that it provides is much smaller than what electron microscopes can create; and

• 2) it is slow in scanning an image, unlike an electron microscope which does it in almost real-time.

• In the semiconductor industry, AFM is primarily used for the imaging of VLSI cross-sections. Materials that can be imaged by AFM include metals, polymers, photoresists, etc.

STM

Scanning Tunneling Microscopy

• STM = non-optical, very high-resolution microscopy for obtaining images of conductive surfaces at atomic scale level (~2 angstroms, i.e., 0.2 nanometer). ﾊ

• Just like AFM ~ scanning probe microscopy - employing an atomically sharp probe tip that is scanned over the sample surface in order to accomplish its imaging function.

• Aside from atomic level imaging, STM can also be used to alter the sample by manipulating individual atoms, initiating chemical reactions, and creating ions.

operation

• 'quantum mechanical tunneling.’• fact = very small current will flow between a sharp

metal probe tip and the surface of an electrically conductive material if:

1) a voltage is applied between the tip and the conductive material;

2) the tip is positioned just a few nanometers away from the sample

surface, i.e., no contact is made between the tip and the sample.

ooppeerraattiioonn• current produced = 'tunneling current' , ~ 1nA for an applied voltage of 1 V.

• amount of tunneling current = exponentially dependent on distance between the tip and the sample surface.

• sensitivity of tunneling current to the tip's distance from the sample is utilized by the STM in its operation.

• in constant current mode - the distance between the tip and the sample surface is kept constant in order to keep the tunneling current constant.- topography of the sample surface changes = microscope must move probe tip according to how the sample topography varies in order to keep the tip-to-sample distance constant.

• tip mounted on a piezoelectric tube which controls the position of the tip in three dimensions relative to the sample. The piezo element that moves the tip towards or away from the sample surface is controlled by a feedback circuit that monitors the tunneling current in order to determine whether the tip is too close or too far from the surface- feedback circuit supplies the electrode of the piezo element with a control voltage that moves the tip in the right direction to keep the distance of the tip from the sample constant.

• tip is scanned line by line over a small area of the sample surface in the x-y plane, topographic data based on the z-axis position of the tip - is collected by the computer

• image of topography of sample reconstructed from the collected data - Under the right conditions, high-quality STM's can produce images with sufficient resolution to show individual atoms.STM images are commonly presented in greyscale, with protrusions shown in white and depressions in black.

• important tool for surface physics and chemistry studies. • ability to show the structure of the uppermost layer of atoms or molecules, can

reveal surface defects, display the morphology of various depositions, or measure the surface roughness of a wafer in the angstrom-range.

• may also be used in the study of conduction or charge transport mechanisms. • can be used to move single atoms accurately, by pushing or dragging them with

the tip at low temperatures. Electrons emitted by the tip can also be used to alter the sample. The ability of STM to serve as a tool for 'rearranging' atoms has made it an important tool in nanosciences .

• STM does not need a vacuum in order to operate, although it is usually operated in an ultrahigh vacuum environment to avoid contamination or oxidation of sample surfaces when high-resolution imaging of metals or semiconductors is required.

• Surface oxidation reduces the conductivity of the sample's surface and affects the tunneling current, resulting in imaging problems.

• operates on the flow of the tunneling current, it cannot be used on non-conductive samples - possible to coat a non-conductive sample with a conductive layer such as gold to make it observable under an STM, but this coating step can mask hide certain features or degrade imaging resolution.

ﾊﾊ

AES

Auger Emission Spectroscopy

• AES = Auger Analysis= FA technique used in the identification of elements present on the surface of the sample.

• AES involves the bombardment of the sample with an energetic primary beam of electrons - generates, among other things, a certain class of electrons known as Auger electrons

operation• an electron ejected by the primary electron beam from its shell,

say, the K-shell. • Another electron from an outer shell (say, the L1-level) of the

same atom emits energy in the form of a photon in order to go down to the K-shell position vacated by the ejected electron

• photon released by the second electron will either get lost or eject yet another electron from a different level, say, L2.

• Auger electrons are electrons ejected in this manner, such as the third electron from L2 in the example - generation of an Auger electron requires at least three electrons, which in the example above are the K, L1, and L2 electrons.

• In this example, the emitted Auger electron is referred to as a KLL Auger electron.

• Hydrogen and Helium atoms have less than three electrons, and are therefore undetectable by AES.

• energy content of emitted Auger electron is unique to the atom where it came from.

• AES works by quantifying the energy content of each of the Auger electrons collected and matching it with the right element.

• energy of Auger electrons is usually between 20 and 2000 eV.• depths from which Auger electrons are able to escape from the

sample without losing too much energy are low, usually less than 50 angstroms - come from the surface or just beneath the surface.

• AES can only provide compositional information about the surface of the sample -to use AES for compositional analysis of matter deep into the sample, a crater must first be milled onto the sample at the correct depth by ion-sputtering.

• excellent lateral resolution, allowing reliable analysis of very small areas (less than 1 micron).

• offers satisfactory sensitivity, detecting elements that are less than 1% of the atomic composition of the sample.

• Coupled with ion-sputter milling capability and raster scanning, AES can even be used to generate 3-D maps of elemental distributions of a volume of the sample = scanning auger microscopy (SAM).

• Auger spectrum. - peaks at Auger electron energy levels corresponding to the atoms from which the auger electrons were released

in the semiconductor industry include but are not limited to the following:

• 1) identification of surface contaminants; • 2) detection of very thin SiO2 and other oxide layers on

surfaces; • 3) determination of contamination levels in barrier metals; • 4) analysis of corrosion failures; and • 5) detection of P, B, and As concentrations in SiO2 layers.

following limitations:

1) charging up of insulative surfaces when struck by the primary electron beam;

2) damage to certain materials, especially organic ones, when struck by the electron beam;

3) occurrance of matrix effects, i.e., signal alterations when some elements are present in particular matrices.

FIB

Focused Ion Beam

• FIB for failure analysis high magnification microscopy , die surface milling or cross-sectioning , and even material deposition .

• uses a finely focused beam of gallium (Ga+) ions rastered on the surface of the material to be analyzed.

• As it hits the surface, a small amount of material is sputtered or dislodged, from the surface.

• dislodged material - form of secondary ions ,atoms , and secondary electrons - collected and analyzed as signals to form an image on a screen as the primary beams scans the surface = high magnification microscopy

• The higher the primary beam current, the more material is sputtered from the surface.

• If only high-mag microscopy is intended, only a low-beam operation must be employed.

• High-beam operation is used to sputter or remove material from the surface, such as during high-precision milling or cross-sectioning of an area on the die.

• FIB system can also bombard an area on the die with various gases as it performs primary beam sputtering. Depending on the gases used, these gases can react with the primary beam to either etch material from or deposit material onto the surface.

Cross-section of a DRAM cell produced by FIB

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