What is an SEM Examination?

History

The scanning electron microscope was first constructed by von Ardenne in 1938 when he added scan coils to a transmission electron microscope (TEM), producing a scanning transmission electron microscope (STEM). The first scanning electron microscope to employ secondary electron detection was developed by Zworykin and his team in 1942. He and his team recognized that secondary electron emission could be used to generate an image showing the topographic contrast of a sample, or specimen as it is usually referred. The collector was biased to +50 volts to capture the secondary electrons and the voltage drop across a connected resistor was used to generate the image. The initial spatial resolutions were not very good, on the order to 200 nm. Within several years, Zworykin and his colleagues were able to reduce the beam spot size and obtain images with a resolution of 50 nm. In the late 1940's and early 1950's researchers Oatley and McMullin introduced several notable improvements to the scanning electron microscope including the electromagnetic lens, the stigmator, and signal amplification (? processing). Everhart and Thornley came up with the concept of attaching the scintillator directly to the face of the photomultiplier. This produced much improved signal-to-noise ratios.

The two most notable developments to the scanning electron microscope in the 1960's were the development of the LaB6 (Lanthanum-hexaboride) electron cathode and the revival of the field emission tip electron source. The LaB6 provides a high-brightness electron gun, helping to improve resolution. The field emission tip electron source produces current densities on the order of thousands of amps per cm2, yielding even higher resolution. As a result, today's commercial SEMs can obtain resolutions on the order of 10-20Å. In 1968, Fitzgerald demonstrated the addition of an energy-dispersive x-ray detector to an SEM, moving the SEM into the analytical probe arena.

SEM Basics

Electron Optics

There are basically three types of electron guns on today's scanning electron microscopes: 1. the tungsten cathode, 2. the lanthanum hexabordie (LaB6) cathode, and the field emission gun. The three electron guns are described briefly below.

Tungsten cathode

The tungsten cathode is a fine wire approximately 100mm in diameter that has been bent into the shape of a hairpin with a V-shaped tip. The tip is heated by passing current through it; normally, the tip is heated to around 2400°C. At this temperature, one can expect a current density of approximately 1.75 A/cm2. The electrons will have a potential distribution of 0 to 2 volts. With a bias voltage between 0 and 500 volts, the electrons can be accelerated toward the anode. At an emission of 1.75 A/cm2, the lifetime of the tungsten filament is approximately 50 hours. A schematic diagram of a tungsten cathode is shown below (see Figure 1).

Figure 1.  Schematic of a tungsten filament (self-biased) electron gun (after Goldstein et. al.).

Lanthanum hexaboride (LaB6) cathode

As the need for higher resolution imaging increased, so did the need for brighter filaments. The most straightforward method to achieve this goal is to find a material with a lower work function Ew. A lower work function means more electrons at a given temperature, hence a brighter filament and higher resolution. Lanthanum hexaboride, commonly known as LaB6, has been the best material developed to date for this application. The LaB6 filament operates at approximately 2125°C, resulting in a brightness on the order of five times brighter than a tungsten filament under the same conditions. LaB6 filaments tend to be an order of magnitude more expensive than tungsten filaments. A schematic of the LaB6 filament is shown in Figure 2.

Figure 2.  Gun configuration for a LaB6 cathode (after Goldstein et. al.).

Field Emission Gun

Another method for generating electrons is the field emission gun. When the cathode forms a very sharp tip (typically 100 nm or less) and the cathode is placed at a negative potential with respect to the anode, so that the local field at the tip is very strong (greater than 107 V/cm), electrons can tunnel through the potential barrier and become free. Although the total current is lower than either the tungsten or the LaB6 emitters, the current density is between 103 and 106 A/cm making it hundreds of times brighter than a thermionic emission source. Furthermore, since the electrons are field generated rather than thermally generated, the tip remains at room temperature. Tips are usually made from tungsten, etched in the <111> plane to generate the lowest work function. Because a native oxide will quickly form on the tip, even at moderate vacuum levels (10 µPa), a high vacuum system (10 nPa) is needed. To keep the tip diameter sufficiently small the cathode can be kept warm (800-1000 °C ) or rapidly heated to approximately 2000 °C for a few seconds to blow off material. A schematic of a field emission tip is shown in Figure 3.

Figure 3.  Gun configuration for field emission (after Goldstein et. al.).

Electron Lenses

The electron beam is focused using electromagnetic lenses. The condenser lens system, which is composed of one or more lenses, determines the beam current which impinges on the sample. The probe-forming lens, often called the objective lens, determines the final spot size of the electron beam. Conventional electromagnetic lenses are used and the electron beam is focused by the interaction of the electromagnetic field of the lens on the moving electrons. A schematic depiction of the scanning electron microscope column is shown below (see Figure 4). The a angles drawn in Figure 4 are exaggerated on purpose to more clearly show the effect of the lenses on the electron optics. The probe lens can be adjusted to focus the beam depending on the working distance. The working distance is defined as the distance between the bottom pole piece of the objective lens and the sample surface. It is typically 5 to 25 mm. As the working distance is increased, the spot size increases on the sample, given the same final aperture lens. There is a second tradeoff between spot size and beam current. The spot size is directly controlled by minimizing di (see Figure 4). However, by minimizing di, the beam current falls off by the ratio (aa/ai)2. The operator must make a conscious decision as to whether the analysis requires minimizing the beam spot size or maximizing the beam current.

Figure 4. Schematic of ray trace in a typical scanning electron microscope column (after Goldstein et. al.).

There are several aberrations that occur in an electron optical column. These include: spherical aberration, chromatic aberration, diffraction, and astigmatism. The first three are depicted schematically in Figure 5. Spherical aberration results from nonuniformity of the lenses. Electrons which pass through the lens further off the optical axis are pulled more strongly than those that pass through near the center of the lens. To reduce this effect, the final aperture can be reduced, but this results in a lower beam current. Chromatic aberration results from differences in electron velocity through the lenses. Electrons with higher velocity or energy will be bent more strongly by the magnetic lenses, resulting in a dull or blurred image. There is no method for correcting this problem other than to use a more expensive LaB6 or field emission instrument. Tungsten filament systems typically have a 2 eV spread when leaving the cathode; LaB6 systems have a 1 eV spread, and field emission systems have a 0.2 to 0.5 eV spread. Diffraction occurs because of the wave nature of electrons and the aperture size of the final lens. The only way to reduce diffraction problems is to increase the final aperture size. Astigmatism results from the fact that magnetic lenses to not have perfect symmetry. Machining errors can cause lens systems to be slightly elliptical, rather than perfectly circular, and irregularities in the iron windings can cause variations in the magnetic field. Most instruments contain a stigmator in the final lens system to help compensate for this effect. The stigmator usually has two controls, one to correct for the magnitude of the asymmetry and one to correct for the direction of the asymmetry of the main field. The stigmator can only correct for asymmetry in the final lens. It cannot correct for things such as a dirty aperture or filament misalignment.

Figure 5.  Schematic drawing showing spherical and chromatic aberrations, as well as diffraction at a lens aperture (adapted from Hall, Oatley, and Goldstein et. al.).

Figure 6 shows the relationship between probe current and beam diameter for both tungsten and LaB6 systems. These values were taken with the tungsten gun current density at 4.1 A/cm2 and the LaB6 gun current density at 25 A/cm2.

Figure 6.  Relationship between probe current and the size of the electron beam.  Calculations use the W hairpin filament and the LaB6 gun operating at 15 and 30 kV (after Goldstein et. al.).

Electron Beam/Specimen Interactions

Electron beam interactions can be classified into two types of events: elastic interactions and inelastic interactions. Elastic scattering occurs when the energy of the scattered electron is the same as the energy of the incident electron, i.e. there is no energy transferred from the beam into the specimen. Elastic scattering causes the beam to diffuse through the sample. Inelastic scattering results when the incident electron loses energy in its interaction with the sample. There are a number of different processes that cause this. They include: plasmon excitation, excitation of conduction electrons leading to secondary electron emission, ionization of inner shells, Bremsstrahlung or Continuum x-Rays, and excitation of phonons. Inelastic scattering then, slows the electrons as they penetrate into the sample.

These two phenomena give rise to the interaction volume (see Figure 7). The interaction volume is a teardrop shape and can be experimentally produced by spotting the electron beam in polymethylmethacrylate. The interaction volume can also be predicted through the use of Monte Carlo simulation. The shape of the interaction volume is dependent on three main parameters: the atomic number or density of the material, the beam energy, and the tilt angle. Monte Carlo simulation data indicate that the linear dimensions of the interaction volume tend to decrease with increasing atomic number or density when the accelerating voltage is kept fixed. This is due to the fact that the effect cross section for inelastic scattering increases, causing the electrons to slow more rapidly, preventing them from significant lateral travel. The size of the interaction volume is a strong function of the beam energy because the effective cross section decreases as a square of the energy, Q ~ 1/E2. The effect of tilt on the interaction volume can be mainly attributed to forward scattering. As the sample is tilted more off axis, more forward scattering takes place, reducing the depth of the interaction volume. The range of the interaction volume, or rather its statistical depth, has been reduced to equation format by several researchers. The most common is the Kanaya-Okayama range, which is given below.

where RKO is expressed in µm, E0 is given in keV, A in g/mol, ? in g/cm3, and Z is the atomic number of the target.

Figure 7.  Electron Beam Physics Interaction Volume (after Cole).

Backscattered Electrons

Approximately 30% of the incident electrons escape the sample by scattering. These electrons are known as backscattered electrons. Backscattered electrons provide a useful signal for SEM imaging. The fraction of backscattered electrons emitted from a sample is dependent on the atomic number of the elements in the sample. A sample with a high atomic number emits a greater fraction of backscattered electrons than a sample with a low atomic number. The backscatter coefficient e is defined as follows:

where nBS is the number of backscattered electrons and nB is the number of beam electrons incident on the target. While the ratio of backscattered electrons are dependent on the atomic number of the sample, the ratio is almost independent of beam energy (see Figure 8). The ratio of backscattered electrons to incident electrons is also dependent on tilt angle. As the tilt angle increases, the ratio of backscattered electrons also increases.

Figure 8.  Variation of the backscatter coefficient as a function of atomic number at E0 = 10 keV (blue) and E0 = 49 keV (red) (after Heinrich).

Signals from Inelastic Scattering

There are basically four signals that are the result of inelastic scattering. These include: (1) secondary electrons, (2) characteristic x-rays, (3) bremsstrahlung or continuum x-rays, and (4) cathodoluminescense radiation.

Secondary electrons comprise the electrons in region III shown in Figure 9. The upper portion of the distribution, region I, is the broad hump of electrons which have lost less than 40% of their incident energy due to inelastic scattering. A smaller fraction of beam electrons lose greater than 40% before escape the specimen, forming region II. The increase in electrons which forms region III is due to the process of secondary electron emission. Secondary electrons are defined as those electrons emitted that have an energy of less than 50 eV. Secondary electrons come from the top 1 to 10 nm of material in the sample, with 1nm being more characteristic for metals, and 10 nm being more characteristic for insulators. The secondary electron coefficient tends to be insensitive to atomic number. The secondary electron coefficient is, however, dependent on beam energy, as shown in Figure 10. Starting at zero energy, the secondary electron coefficient rises with increasing energy, reaching unity around 1 keV. The curve then peaks at just over 1 for metals and as high as 5 for insulators and then falls below unity between 2 and 3 keV. This region above unity tends to be a good beam energy for performing voltage contrast.

Figure 9.  (a) Energy distribution of electrons emitted from a target over the entire energy range including backscattered electrons (regions I and II) and secondary electrons (region III). (b) Energy distribution both measure (blue) and as calculated (magenta).  After Goldstein et. al.

 

Figure 10.  Schematic illustration of the total electron coefficient as a function of beam energy (after Goldstein et. al.).

X-rays can be generated by two different mechanisms. One is the interaction of the electron with the coulombic field of the atom core. This is the bremsstrahlung ("braking radiation") or x-ray continuum. The other mechanism is the interaction of the electron with an inner shell electron that results in an ejection of the inner shell electron. During the subsequent deexcitation, an outer shell electron will fall into the vacancy, resulting in the release of a characteristic x-ray.

Another product from inelastic scattering is the Auger electron. The Auger electron has an energy which is characteristic of the atom because the electron transitions occur between sharply defined energy levels. While characteristic x-rays have a longer mean free path, Auger electrons typically have a very short mean free path, on the order of 0.1 - 2 nm. This is because Auger electrons have a fairly low energy range (50 eV to 2 keV).

Cathodoluminescence occurs when certain materials are bombarded with energetic electrons and emit long-wavelength photons in the ultraviolet and visible regions of the em spectrum. This occurs in materials that have a full valence band and an empty conduction band, and a non-zero bandgap. When an energetic beam electron scatters inelastically in such a solid, electrons from the filled valence band can be promoted to the conduction band leaving "holes". At some later point, if there is no external bias, the electron and hole will recombine and produce a photon.

Image Formation

As the electron beam strikes the sample surface, it produces detectable signals ranging from backscattered electrons, secondary electrons, absorbed electrons, characteristic and continuum x-rays, and cathodoluminescent radiation. By measuring these signals with suitable detectors, information regarding the topography and composition can be made at a single location. In order to study more than one single point, the beam must be scanned. Scanning is accomplished by driving electromagnetic coils arranged in sets consisting of two pairs, one pair each for deflection in the X and Y directions. A typical system, shown in Figure 11, has two sets of scan coils located in the final aperture.

Figure 11. Schematic illustration of scanning system of the scanning electron microscope (after Goldstein et. al.).

The SEM image is constructed by creating a map of intensities from the detector of interest with respect to the x-y location of the electron beam. This information can be used to create two principal types of displays: a line scan and an area scan. A line scan is generally formed by allowing the x position on the CRT correspond to the x scan of the electron beam, while the y position on the CRT is used to display intensity of the signal. An area scan is generally formed by allowing the x and y position on the CRT to correspond to the x and y position of the scanned electron beam. The intensity of the beam is represented by the intensity of the signal on the CRT, sometimes referred to as "Z modulation."

Magnification in the SEM image is accomplished by adjusting the scale of the map on the CRT. Because the size of the CRT screen is fixed, an increase in magnification is accomplished by decreasing the length of the area scanned. Magnification is dependent only of the excitation of the scan coils and not on the objective lens. Finally, the image does not rotate as the magnification is increased. The image rotates when the working distance is changed (the distance between the sample surface and the pole piece).

Another concept in SEM imaging is that of picture point versus picture element. Picture point is the region that constitutes a single spot on the CRT. On a digital SEM, the picture point would be the size of a single pixel. The picture element is the region on the sample surface with which the beam interacts. For example, if the beam diameter is 50 nm and the resultant interaction volume for the secondary image is 100 nm, then the picture element is 100 nm. The minimum picture element is related to magnification by the following equation

This gives rise to the effect of hollow magnification. The magnification on the SEM can be set higher than the picture element diameter, leading to a fuzzy image.

Depth of field is a term expressing over what range of depth an image appears to be in focus. Depth of field D can be determined by the equation D = 2r/a where r is the maximum radius of the beam that is tolerable and a is the beam divergence. The beam divergence a can be expressed as a function of final aperture radius R and working distance D, where a = R/WD. Table 1 lists several combinations of magnification and aperture sizes, giving the resulting working distances.

Depth of Field at 10-mm Working Distance
Magnification	100-µm aperture (a = 5 x 10-3 rad)	200-µm aperture (a = 10-2 rad)	600-µm aperture (a = 3 x 10-2 rad)
10 x	4 mm	2 mm	670 µm
50 x	800µm	400 µm	133 µm
100 x	400 µm	200 µm	67 µm
500 x	80 µm	40 µm	13 µm
1000 x	40 µm	20 µm	6.7 µm
10,000 x	4 µm	2 µm	0.67 µm
100,000 x	0.4 µm	0.2 µm	67 nm
Table 1.  Depth of field at 10 mm working distance

Detectors

There are basically two types of detectors: electron detectors and cathodoluminescent detectors. In failure analysis applications, electron detectors are of the most interest. There are four types of electron detectors: the scintillator-photomultiplier detector, the scintillator backscatter detector, the solid state detector, and the specimen current detector (where the specimen itself is the detector).

The scintillator-photomultiplier detector is the most common detector used in scanning electron microscopy. The basic detector was developed by Everhart and Thornely around 1960. Basically, the scintillator is made of a material that will emit photons when electrons strike it. Typically, scintillators are made of a doped glass or plastic target or from a compound such as CaF2 that has been doped with europium. The photomultiplier then amplifies the signal on the order of 105 to 106. The resulting signal can then be used to create the image on the CRT. The scintillator is biased positively to approximately 10 keV to attract the secondary electrons. To eliminate unwanted electrical fields, the scintillator is surrounded by a faraday cage that is grounded. The faraday cage can be biased to a lower voltage, say +300V, to enhance the collection of electrons. Because of the nature of the electron beam specimen interaction, most electrons are emitted at normal beam incidence. The emittance of electrons falls of by the cosine of the angle, resulting in poor electron collection if the detector is at a large angle to the sample or if topographical features block the path.

Scintillator backscatter detectors are based on the same principle as the scintillator-photomultiplier detectors, but are placed such that they collect the backscattered electrons. There are several types of backscatter detectors including large angle scintillator detectors, multiple scintillator arrays, and conversion detectors attached to the pole piece that convert backscattered electrons into secondary electrons for collection.

Solid state detectors are usually silicon-based p-n junctions. Typically, they are not used in failure analysis applications, but the concept does form the basis for techniques like EBIC. Basically, when an electron strikes the detector, it will randomly recombine. However, if a bias is placed across the p-n junction, the electrons and holes will be swept apart, creating a current. For silicon, approximately 3.6 eV is expended per electron-hole pair. For a 10 keV electron striking the detector, a current of up to 2800 electrons will flow from the detector. This signal can be amplified to produce a signal on the CRT. It should be noted that this type of detector has a narrow bandwidth because of the capacitance of the silicon. This prevents its use at fast (TV) scanning rates.

The specimen current detector is the basis for several FA techniques, including RCI, CIVA and EBIC. The specimen is a junction with currents flowing into and out of it. The beam electrons represent current into the specimen and the backscattered electrons, secondary electrons, and specimen current represent the current flowing out of it. In order to make use of the specimen current, it must be routed through a current amplifier on its way to ground. Typically, these amplifiers are very high gain amplifiers, with gains on the order of 109 or better.

Die

An SEM examination would be a careful examination of the entire die surface, looking for signs of mechanical damage, processing defects, dendrites, contamination, chipouts (portion of die missing) and/or cracks using a scanning electron microscope. The material below describes the basics of scanning electron microscopy.

Bond Wires

An SEM bond wire examination would be a careful examination of the bond wire, the bond pads and all surrounding areas for any anomalies, particulate or contaminates. The SEM would be used to provide a high mag view of any questionable areas detected during optical evaluation.

IC Package

An SEM package examination is a careful examination of an IC package which would include the lid seal area, the leads or pins, the lid or the overall package.

Cross Section Examination

An SEM examination after cross sectioning is the examination (with a scanning electron microscope, of course) of cross sectional information exposed by mechanically cross sectioning the IC or wafer or cross sectional information exposed by using a focused ion beam to cross section the IC. The SEM can yield important information concerning the processing of the IC, including feature sizes, thicknesses of layers, step coverage, etc.

Why Perform an SEM Die Examination?

Die

An SEM die examination should be performed whenever there is any questions about the die condition which cannot be verified using high mag optical examination.

Bond Wires

An SEM bond wire examination should be performed whenever a possible bond wire anomaly is suspected but cannot be verified with optical microscopy. The SEM can provide greater spatial resolution and greater depth of field than an optical microscope.

IC Package

This examination would usually be performed after the high mag optical examination where a possible abnormally was detected but could not be verified.

Cross Section Examination

An SEM examination of the cross sectioned surface is important because the SEM is the best tool normally, at the failure analysts disposal, for examining a cross sectioned surface. The spatial resolution of the SEM, coupled with its large depth of field and capability to perform elemental analysis, make it the tool of choice for most cross sections. The Focused Ion Beam (FIB), while potentially providing better contrast for some cross sectional information, is not always available and doesn't have the spatial resolution of a standard SEM (not to mention the fact that the FIB is a much more expensive tool than the SEM).

An SEM examination of the cross sectioned surface will allow the analyst to identify such defects as poor step coverage, misalignment of various layers, voiding, incorrect thicknesses, and a host of other potential failure mechanisms.

How is an SEM Die Examination Performed?

Die

Select an accelerating voltage which will allow the least amount of charging while obtaining the best resolution. If the device is to be tested further after the SEM examination, care selecting the accelerating voltage must be used so as not to damage the device with the electron beam. Scan the die, if step coverage is of concern, then the sample should be tilted at an angle which will allow easy observation of the metal steps. If metal voiding is of concern, then the use of the backscattered electron detector would be best. (note: If a barrier metal is used under the conductors than the backscattered electron detector should not be used.) If contamination is observed or suspected the use of the EDX detector to identify the contamination can be used.

Bond Wires

Using a beam energy of 20 to 25 KV to start a careful examination, of the suspect bond wire from the bond pad to the package lead, would be performed. Attention should be paid to any anomalies such as a open wire, particles or contamination, scribe line short, excessive wire height, reverse bonded, or previously bonded wires.

IC Package

Select an accelerating voltage which will allow the least amount of charging. The packages are usually a non-conducting material so e-beam charging will be a major problem. If charging will not allow a complete analysis, coating the package with carbon or Au/Pd should be considered.

Cross Section Examination

For help on how to operate a scanning electron microscope, consult your operator's manual. To examine a cross sectioned sample, be sure to use an SEM where you can achieve a fairly large tilt angle ( >45o ). If the sample is mounted in an epoxy resin (from mechanical cross sectioning), be sure to clean the sample and vacuum bake to remove any contamination/moisture. This will facilitate a faster pump-down. Before placing the sample in the SEM, you may want to place fiduciary marks near the area of interest so that you can quickly locate the defect. This is especially important on large memory ICs where the structures are repetitive and difficult to follow. If you are using a field emission SEM, try examining the IC at a low accelerating voltage (~ 1kV) without coating the sample. The spatial resolution should be good enough for you to see most everything. If you are using a standard tungsten or LaB6 system, you will probably need to coat the sample and use a higher accelerating voltage (~ 10kV or higher) to resolve everything.

When is an SEM Examination Performed?

Die

An SEM die examination would usually be performed after all other testing of the device is complete. The SEM analysis is considered to be non-destructive, however the SEM will deposit a thin carbon layer on the sample when the beam scans the surface.

Bond Wires

An SEM bond wire examination should be performed anytime that a lead or bond wire anomaly is suspected but cannot be verified by high mag optical examination.

IC Package

An SEM package examination can be performed whenever an abnormally such as contamination, cracks, chips or any defect which could cause the device to fail has been detected.

Cross Section Examination

An SEM examination of the cross sectioned surface should be performed immediately after you have finished the cross section and have determined optically that you have sectioned to the appropriate location. If you have placed the IC in the SEM and discovered you have not yet reached the area of interest, you can certainly continue polishing on a mechanical cross sectioning wheel or remove additional material using the FIB.

Photographs

Die

Figure 1 SEM image showing contamination bridging between a bond wire and the substrate. (Photo courtesy DM Data).

Figure 2 SEM image showing corrosion on the bond pad. (Photo courtesy DM Data).

Figure 3 SEM image showing dendritic growth. (Photo courtesy Analytical Solutions).

Figure 4 SEM image showing dendritic growth at higher magnification. (Photo courtesy Analytical Solutions).

Figure 5 SEM image showing a cracked die. (Photo courtesy Analytical Solutions).

Figure 6 SEM image showing contamination on a bond pad. (Photo courtesy Analytical Solutions).

Figure 7 SEM image showing particulate contamination on the die surface. (Photo courtesy Analytical Solutions).

Figure 8 SEM image showing a cracked die. Crack propagates into the substrate. (Photo courtesy Analytical Solutions).

Figure 9 SEM image showing voiding the metal. (Photo courtesy Analytical Solutions).

Figure 10 SEM image showing a metal to silicon contact. (Photo courtesy Analytical Solutions).

Figure 11 SEM image showing galvanic corrosion. (Photo courtesy Analytical Solutions).

Figure 12 SEM image showing subtle electrical overstress damage. (Photo courtesy Analytical Solutions).

Figure 13 SEM image showing Fig. 12 at higher magnification. (Photo courtesy Analytical Solutions).

Figure 14 SEM image showing bond pad cratering. (Photo courtesy DM Data).

Figure 15 SEM image showing damage caused by a blunt mechanical object. (Photo courtesy Analytical Solutions).

Figure 16 SEM image showing cracks in the glass caused by electromigration. (Photo courtesy DM Data).

Figure 17 SEM image showing a pin hole in the top glass layer. (Photo courtesy Analytical Solutions).

Figure 18 SEM image showing thinning at steps, poor step coverage. (Photo courtesy DM Data)

Figure 19 SEM image showing an electron channeling pattern (Photo courtesy Analytical Solutions):sem00001.gif

Figure 20 SEM image showing crack in glass layer over metal line (Photo courtesy Analytical Solutions):sem00002.gif

Figure 21 SEM image showing copper leaching out on aluminum metal line as a result of moisture (Photo courtesy Analytical Solutions):semgls01.gif

Figure 22 SEM image showing copper leaching out on aluminum metal line - higher magnification (Photo courtesy Analytical Solutions):semgls02.gif

Figure 23 SEM image showing gate oxide short at edge of poly line (Photo courtesy Analytical Solutions):sm1pyd01.gif

Figure 24 SEM image showing gate oxide short at edge of poly line (Photo courtesy Analytical Solutions):sm1pyd02.gif

Figure 25 SEM image showing gate oxide short at edge of poly line (different angle) (Photo courtesy Analytical Solutions):sm1pyd03.gif

Figure 26 SEM image showing gate oxide short sites after poly removal (Photo courtesy Analytical Solutions):spoly101.gif

Figure 27 SEM image showing metal 1 bridging short after metal2 - metal1 interlevel dielectric layer removal. (Photo courtesy Sandia Labs):semm1003.gif

Figure 28 SEM image showing titanium-tungsten particle bridging several metal 3 lines (Photo courtesy Sandia Labs):semm3001.gif

Figure 29 SEM image showing a gate oxide defect site before poly removal (Photo courtesy Sandia Labs):sm1pyd07.gif

Bond Wires

Photo 1 . SEM image of magnetic particle bridging bond wires in a hybrid microcircuit (photo courtesy DM Data).

Photo 2 . SEM image of properly bonded wires in a Static RAM (photo courtesy Analytical Solutions).

Photo 3 . SEM image of contamination on a bond wire (photo courtesy DM Data).

Photo 4 . SEM image of bond wire that is not making connection to the bond pad as evidenced by bond wire charging positively, blurring the image (photo courtesy Analytical Solutions).

Photo 5 . SEM image of contamination on a bond wire in an operational amplifier (photo courtesy DM Data).

Photo 6 . SEM image of extra bond wire material in the package cavity (Courtesy DM Data).

Photo 7 . SEM image of contamination on a bond wire in an operational amplifier (Courtesy DM Data).

Photo 8 . SEM image of bond wire with a low take off angle, most likely reverse bonded (Courtesy DM Data).

Photo 9 . SEM image of a bond wire shorting to the die scribeline (Courtesy Analytical Solutions).

Photo 10 . SEM image of intermetallics on bond pad (Courtesy DM Data).

Photo 11 . SEM image showing intermetallic growth on two bond wire/bond pad interfaces (Courtesy Analytical Solutions).

Photo 12 . Higher magnification image of intermetallic growth (Courtesy DM Data).

Photo 13 . SEM image of bond wire sheared off at heel as a result of improper operation of the wire bond tool (Courtesy DM Data).

Photo 14 . SEM image of bond wire crack just above ball (Courtesy DM Data).

IC Package

Photo 1 SEM image showing cracking at lead interface to package (Courtesy DM Data).

Photo 2 SEM image showing contamination bridging two leads on a package (Courtesy DM Data).

Photo 3 High magnification SEM image showing cracking at lead interface to package (Courtesy DM Data).

Cross Section Examination

Figure 1 SEM image showing etching pit into the Si substrate as a result of a breach in the overlying oxide layer. Photo courtesy Analytical Solutions.

Figure 2 SEM image showing a different etching pit into the Si substrate. Note that oxide and Al were both deposited into the pit indicating that the etched pit occurred early in processing. Photo courtesy Analytical Solutions.

Figure 3 SEM image showing oxide whiskers due to incorrect processing and composition. Photo courtesy Analytical Solutions.

Figure 4 SEM image showing poor step coverage (breadloafing) of metal 1 into a silicon contact. Photo courtesy Analytical Solutions.

Figure 5 SEM image showing overetched contact windows that undercut the field oxide. Photo courtesy Analytical Solutions.

Figure 6 SEM image showing damage resulting from a collector to emitter short on a bipolar IC. Photo courtesy Analytical Solutions.

Figure 7 SEM image showing an open metal 1 - metal 2 contact. Oxide not fully removed from the contact area. Photo courtesy Analytical Solutions.

Figure 8 SEM image showing an overetched metal 1 - Si contact and poor oxide coverage. Photo courtesy Analytical Solutions.

Figure 9 SEM image showing a metal to substrate short on a capacitor in a bipolar IC. Photo courtesy Analytical Solutions.

Figure 10 Higher magnification image of the metal to substrate short. Photo courtesy Analytical Solutions.

Figure 11 SEM image showing breach in the field oxide, resulting in a metal to silicon short. Photo courtesy Analytical Solutions.

Figure 12 SEM image showing open metal interconnect at a step due to lack of step coverage. Photo courtesy Analytical Solutions.

Figure 13 SEM image showing processing defect over the p isolation region, resulting in a short. Photo courtesy Analytical Solutions.

Figure 14 SEM image showing excess copper in metallization system causing short. Photo courtesy Analytical Solutions.

Figure 15 Lower magnification image showing location of shorts due to excess copper in metallization system. Photo courtesy Analytical Solutions.

Figure 16 SEM image showing void in die attach. Photo courtesy Analytical Solutions.

Figure 17 SEM image showing gap between lead and leadframe (potential entry point for moisture or other volatiles. Photo courtesy Analytical Solutions.

Figure 18 Lower magnification image of the lead and leadframe. Photo courtesy Analytical Solutions.

Figure 19 SEM image showing collector to emitter damage on a pnp transistor on a bipolar IC. Photo courtesy Analytical Solutions.

Figure 20 SEM image showing blown fusible link on a read only memory. Photo courtesy Analytical Solutions.

Figure 21 SEM image showing aluminum inclusions in a polysilicon gate resulting in a shorted transistor. Photo courtesy Analytical Solutions.

Figure 22 Higher magnification image of aluminum inclusions into the polysilicon gate. Photo courtesy Analytical Solutions.

References on SEM Die Examination

1. Microelectronics Failure Analysis Techniques: A Procedural Guide, eds. Ed Doyle Jr. and Bill Morris, IITRI, 1980

2. John R. Devany, Gerald L. Hill, and Robert G. Seippel, Failure Analysis Mechanisms, Techniques and Photo Atlas, Failure Recognition and Training Services, Inc. Monrovia CA. 1986.