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Scanning Electron Microscope, A valuable

NDT Method
Paul Dick
USA
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I. BACKGROUND

Scanning Electron Microscope (SEM) is becoming a valuable new tool for the nondestructive
inspection, examination and evaluation of materials, both metallic and non-metallic, as well as
assemblies and surfaces. The item to be examined and evaluated is placed on a specimen stage
inside a vacuum enclosure of the SEM station and is irradiated with a finely focused electron beam
that can be static or swept in a cyclic fashion over the specimen's surface. The resulting signals that
are produced when the scanning electron beam impinges on the surface of the specimen include
both secondary emission electrons as well as backscattered electrons. These signals vary as the
result of differences in the surface topography as the scanning electron beam is swept across the
specimen surface.

The secondary emission of electrons from the specimen surface is usually confined to an area near
the beam impact zone that permits images to be obtained at a relatively high resolution. These
images as seen on a Cathode Ray Tube provide a three dimensional appearance due to the large
depth of field of the Scanning Electron Microscope (SEM) as well as the shadow relief effect of the
secondary electrons contrast. A typical SEM has a working magnification range of from 10 to
100,000 diameters. A resolution can be attainable of 100 Angstroms, and a depth of field (focus)
300 times that of an optical microscope and having good working distances. The large depth of field
available with a SEM makes it possible to observe three-dimensional objects in Stereo. The three-
dimensional images produced allow different morphological features to be correctly interrelated and
correctly analyzed.

One of the unique advantages of Scanning Electron Microscopy is the fact that many specimens can
be examined with minimal specimen preparation activity. The thickness of the specimen is not a
consideration. Therefore bulk specimens can be examined in a SEM with a size only limited by the
dimensions of the test specimen compared to the dimensions of the SEM's specimen stage within the
vacuum enclosure. For the examination and evaluation of a metallic material surface, the only usual
amount of specimen preparation is to be sure that the specimen surface to be examined is clean. If
there appears to be some undesired surface condition that could mask the SEM work, it may be
required to conduct a light cleaning of the surface of interest with alcohol, toluene or acetone. In
any event should cleaning by required, the technique employed must be non-damaging or degrading
to the specimen.

The scanning electron microscope (SEM) is becoming one of the most unique and also versatile
instruments available for the nondestructive inspection, evaluation, examination or analysis of the
microstructural surface condition, configurational and point-to-point characteristics of solid objects.
SEM's great advantage to the NDT Technologist is the ultra high resolution, which can be achieved
on the test object. Some commercially available SEM installations provide resolutions down to 10
nanometers, which equates to 100 Angstroms. The high resolution ability of the SEM coupled with
the three dimensional resulting appearance of the test objects image presentation on the SEM screen
provide valuable pieces of information that help the NDT Technologist to determine the quality
status of the item under test.

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II. BASIC CONSTRUCTION

The basic parts of a typical Scanning Electron Microscope Station include (See Figure 2):
Fig 2: Tyical SEM Station.
Fig 1a: Typical defects that SEM will find on metallization runs on
integrated circuits, transistor and diodes.
Fig 1b: Cross section view (view A-A) of typical defects in a above
figure.
Vacuum Enclosure with specimen mounting stage to hold the test object
Electron gun to produce the scanning electron beam
Collector circuits - for Emitted Secondary electrons
Electronic Elements such as Signal Amplifier and Signal Conditions, etc.
Cathode Ray Tube (CRT) Display Unit and Lens Elements
Positioning Controls including Tilt for movement and offset of the test object
Additions such as Camera, Video Recorder and additional CRT's

As already noted, a possible limitation to the use of a Scanning Electron Microscope is the largest
test specimen size that can be placed on the specimen stage within the vacuum enclosure. That test
specimen size varies from a major dimension of 1-centimeter (0.39 inches) to 20 centimeters
(approximately 8 inches) depending on the particular SEM construction and the size of the Vacuum
Enclosure and its internal specimen stage dimensions.

III. PRESENT APPLICATIONS

Before we speak of SEM as a valuable visual inspection tool for NDT work, the use of Scanning
Electron Microscopy has already found its place in various disciplines such as: Process control,
quality assurance, and quality improvement programs as well as a tool for failure analysis and
problem resolution. The Semiconductor Electronics Industry has made extensive use of SEM both
as a Process Control Tool and Product Quality evaluator and as a Failure Analysis diagnostic tool.
As a Process Control and Product Quality evaluator SEM is used to examine the topography of
complex Integrated Circuit Chips (IC's) to assure that areas of circuit metallization, silicon islands,
bond wires and other critical zones and parts of the Integrated Circuit Chip conform to the design
and production requirements for these high technology imaging products.

The requirements for SEM inspection and examination of Semiconductor Chips are detailed in
various Manufacturer Specifications and Standards and user requirements citing acceptance and
rejection limits.

IV. PROCESS CONTROL AND QUALITY ASSURANCE OF SEMICONDUCTOR

In today's world of semiconductor chips in computers as well as the literally thousands of other
integrated circuit chips that find their way into our everyday lives: in the house, in the automobile,
cell phones, in all of our never-ending new appliances, toys, games, security systems, navigational
systems, communication systems etc, one must ask how do we know that these tiny semiconductors
have the required quality and reliability.

Semiconductors (transistors, diodes, IC's) are very small ranging in cross-section size from 30 to 200
thousands of an inch. Others of higher density can range to about one-half inch in major dimensions,
while Hybrid Circuits that can contain literally hundreds of those chips can be interconnected on a
substrate that is about 2 inches or more square. Within the small size but dense configuration of IC's
are many smaller complex structures such as: Bond Pads, Interconnections, Glassivation, Via's, and
Barrier Materials.

The chip has its original home location on a Semiconductor Material Wafer (Silicon, germanium,
etc.) that is the product of the process controls, production equipments, design rules and leading

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edge technology of the chip supplier. Both military and commercial application producers of
integrated circuits have stringent requirements for the acceptance of the Wafers themselves before
any of its colony of integrated circuits are removed.

For military, space and other very high reliability applications, SEM is the Nondestructive Process
Control, Inspection and Evaluation method used to determine whether the Wafer is acceptable for
further processing or is to be rejected. Wafers that are accepted can continue processing to yield
chips.

Many standards and specifications require that the apparatus for these inspections shall be a
Scanning Electron Microscope having a resolution of 250 Angstroms or less as measured on the
screen or resulting photographs and a variable magnification of 1,000X to 20,000X and a viewing
angle of between 0° and 85°. It is also required that SEM station magnification and resolution
calibration shall be traceable to and verified per National Institute Standards. It is further noted that
Operator Certifications and Recertifications shall be documented and made available to the
qualifying activity for review or to a designated representative of the procuring activity. The method
requires a minimum recertification period of one year.

At some future date when SEM becomes more established within the NDT communities of the
world, certification requirements should be defined for those product areas beyond those presently
noted in Military, Space and High Technology areas. Perhaps SEM should be included as a subset of
Visual NDT certification methods or stand alone as an individual separate method in much the same
manner as Neutron Radiography is not a subset of Radiographic Methods.

Some typical Acceptance Requirements used for high reliability IC's are as follows for sample chips
removed from Wafers. Evidence of poor metallization is reason for rejection. Defects such as voids;
cracks; separation; notches; depressions or tunnels in the metallization that singly or in combination
significantly reduce the cross sectional area of the metallization are causes for rejection. See Figures
2 - 17 for examples of acceptable conditions and examples of rejectionable cases.

Fig 2: Notching of Metal Fig 3: Oxide Layer Defect Fig 4: Separations and Fig 5: Microcracks in Over
over Oxide Step (at 5520x) in Circuit (at 4.960x) Microcracks (at 8000x) Glass and Aluminium (at
12.400)

Fig 6: Meld Anode in Fig 7: Meld Gold Wires Fig 8: Microcracks in Over Fig 9: Overheat Condition
Power Diode (at 25x) (1.5 Mil Dia) in a Diode Glass and Aluminium (at has Caused Melting
Array (at 1.230x) 12.400x) Aluminium and Silicon(at
6.510x)

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Fig 12: Cracking of Metal 2 Fig 13: Defects in Fig 15: Crack in Metal Run
over Metal 1 (at 8.400x) Aluminium and Nitride Fig 14: Electrostatic (at 9.500x)
Presence (at 19.500x) Discharge Damage (at
825x)

Fig 16: Open in Metal

Line(at 15.000x) Fig 17: Defecive
Metallization (at 11.000x)

The results of SEM examination of sample chips from the Wafer under consideration are
documented in photographs taken of the SEM images seen on the Cathode Ray Tube. The following
information should be traceable to each SEM photograph, some of which will be noted directly on
the photograph and other data documented traceable to that photograph.

Part Number (Drawing Number, Date Code, Serial Number, etc)

Date of SEM Photograph and SEM Operator Name
Magnification and Viewing Angle
Electron Beam Voltage

The electronics industry, which includes not only Integrated Circuit Chips but also involves
Transistors, Diodes, Resistors, Capacitors, Relays, Filters, Magnetics, etc. also uses SEM for failure
analyses activities to "determine" the Cause(s) of a device failure as well as to "establish" what
Corrective Action(s) are required to prevent future such failures or anomalous performance.

V. FAILURE ANALYSIS

One may argue that failure analysis is not part of nondestructive testing, but in a sense, NDT
methods are being used to evaluate the failure mode and cause. And as such, the NDT method that
is used, must not alter, change or modify the failed condition but must survey the failure in a
nondestructive mode so as to not impact, change or further degrade the failure zone. The ultimate
mission of a failure analysis effort is to arrive at an accurate determination of the cause of the
failure. Many of the well known nondestructive inspection and evaluation and test methods are
utilized during failure analysis in the electronics industry (Table 1).

Table 1: NDT Methods Used in the Electronics Industry for Failure Analysis
NDT Method Application
Examination of conditions both external and Internal to the device that could
Visual have been the cause of the failure or contributed toward the failure. Use of
items such as Microscopes, Borescopes and Metallographs.
To detect an internal conditions could have been the cause of the failure or
contributed toward the failure. Metallic particles in electronic packages are of
Radiography particular concern due to the possibility of creating "Shorts". X-Ray
techniques (film or real time) are used coupled with appropriate
penetrameters designed for the electronics industry products.

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To determine if the hermeticity of the package has been compromised or

degraded. Electronic packages are usually charged with inert atmospheres of
Leak Tests
Nitrogen Argon or Helium. Loss of hermeticity can be a cause or contribute to
a failure.
To determine if the package has any loose particles or debris that could have
Acoustic
caused the failure or contributed to it. This method is called "PIND" (Particle
Emission
Impact Noise Detection)
To detect an internal defects or particles of a nature not detected by X-Ray
Neutron but visible to N-Ray. Wide use is made during inspection of Relays looking
Radiograph for non-metallic and carbon based debris that can cause or contribute to
failure, due insulating effects.
Die Is used to detect cracks, voids or passageway in the seal (solder, weld, etc.)
Penetrants between the package and lid. Usually used as an alternate to Leak Tests.
Liquid Used to monitor thermal profiles of current flow in the device that could have
Crystal contributed to or been the cause of failure.
To observe, present and then photo document various anomalies beyond the
SEM capabilities of optical items such as Microscopes, Borescopes or
Metallographs.
Table 1: NDT Methods Used in the Electronics Industry for Failure Analysis

As it turns out the use of Scanning Electron Microscope plays a major role in Failure Analysis, not
only at high magnification but also in the normal range of an optical microscope (10-100x). The
small size of a transistor, diode and integrated circuit contains within its own size, many unique
elements. These elements include Aluminum or Gold Pads, (2-5 mils square), Aluminum or Gold
wires as small as 1 mil in diameter, wire bonds formed in many different manners, metallization runs
in widths less than 5 mils as well as substrate material like Silicon, Germanium, and Gallium
Arsenide. An effective illumination of a sample for photographic documentation using conventional
optical devices (Microscope, Borescope, Metallograph) is difficult. Use of the SEM station because
of its unique characteristics of orientation, signal generation of the secondary electrons and its
collection and display methods overcome any illumination difficulties that would be encountered
with the standard visual methods.

It is for this reason that the SEM with its range to 100,000X has also become a widely used low
power camera in the range of 10 to 20X as well as intermediate ranges up to 100X. Figures 6 and 7
show such examples.

The failure analysis role of SEM is rapidly becoming a world-accepted technique.

VI. OTHER APPLICATIONS

Beyond the Electronics Industry, SEM has made significant progress as a tool on such varied
sampled materials as: Metals, Alloys, and Ceramics; Medical and Biological Particles; Fibers and
Polymers; Soils and Clays; as well materials from "living items" from plants to insects to trees thru
fish and animals as well as human tissue. It has also been used for such items as inspection of
Apollo Missions moon landings rocks and various famous artifacts and fossils.

The question now raised is how can SEM increase our ability for nondestructive visual inspection
and testing? The answer is by its very nature of its surface area resolution (as low as 100
Angstroms), extensive depth of filed (300 times that of an optical microscope), and stereotype
presentations. One possible negative is the maximum size of specimen that can be accommodated
on the specimen stage within the vacuum enclosure.

The optical microscope has long been a valuable and useful visual inspection tool to Nondestructive
Test personnel. The useful magnification possible with optical microscopes can range up to 1,500

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diameters. Use of oil immersion lens technique can double the limiting magnification with a possible
resolution of 1,000 Angstroms. A SEM installation in contrast has a magnification limit as high as
100,000 diameters with a resolution of down to 100 Angstroms.

I conclude this paper, predicting that Scanning Electron Microscope NDT will prove to be a
valuable addition to our current NDT methods finding its place in the upcoming new millennium.
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