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·Methods and Instrumentation
Scanning Electron Microscope, A valuable NDT MethodPaul Dick
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.
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.
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.
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.
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 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 over Oxide Step (at 5520x)
Fig 3: Oxide Layer Defect in Circuit (at 4.960x)
Fig 4: Separations and Microcracks (at 8000x)
Fig 5: Microcracks in Over Glass and Aluminium (at 12.400)
Fig 6: Meld Anode in Power Diode (at 25x)
Fig 7: Meld Gold Wires (1.5 Mil Dia) in a Diode Array (at 1.230x)
Fig 8: Microcracks in Over Glass and Aluminium (at 12.400x)
Fig 9: Overheat Condition has Caused Melting Aluminium and Silicon(at 6.510x)
Fig 12: Cracking of Metal 2 over Metal 1 (at 8.400x)
Fig 13: Defects in Aluminium and Nitride Presence (at 19.500x)
Fig 14: Electrostatic Discharge Damage (at 825x)
Fig 15: Crack in Metal Run (at 9.500x)
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.
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.
|Table 1: NDT Methods Used in the Electronics Industry for Failure Analysis|
|Visual||Examination of conditions both external and Internal to the device that could have been the cause of the failure or contributed toward the failure. Use of items such as Microscopes, Borescopes and Metallographs.|
|Radiography||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 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.|
|Leak Tests||To determine if the hermeticity of the package has been compromised or degraded. Electronic packages are usually charged with inert atmospheres of Nitrogen Argon or Helium. Loss of hermeticity can be a cause or contribute to a failure.|
|Acoustic Emission||To determine if the package has any loose particles or debris that could have caused the failure or contributed to it. This method is called "PIND" (Particle Impact Noise Detection)|
|Neutron Radiograph||To detect an internal defects or particles of a nature not detected by X-Ray but visible to N-Ray. Wide use is made during inspection of Relays looking for non-metallic and carbon based debris that can cause or contribute to failure, due insulating effects.|
|Die Penetrants||Is used to detect cracks, voids or passageway in the seal (solder, weld, etc.) between the package and lid. Usually used as an alternate to Leak Tests.|
|Liquid Crystal||Used to monitor thermal profiles of current flow in the device that could have contributed to or been the cause of failure.|
|SEM||To observe, present and then photo document various anomalies beyond the 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.
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 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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