Scanning electron microscopes are used to determine both user-induced damages and manufacturing defects subtle enough to be missed by conventional light microscopy. Method offers greater depth of field and increased working distances

Nicolas, D. P.

English

NASA Technical Reports Server

December 1975	 B75-10304 
NASA TECH BRIEF S Marshall Space Flight Center 	
- 4''r 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Microcircuit Testing and Fabrication, Using 
Scanning Electron Microscopes 
.
Scanning electron microscopes (SEM's) have been 
introduced into the nondestructive testing of micro-
circuits. They are used to determine both user-
induced damages and manufacturing defects that are 
subtle enough to be missed by conventional light 
microscopy. Because of the greater depth of field and 
increased working distances of the SEM's, micro-
circuit failures are examined more closely than with 
light optics. However, the SEM's do not replace the 
light optics; rather, they complement the data 
obtained from optical microscopy. For example, 
microcircuits undergo preliminary screening with 
light optics to observe discolorations and irregular 
geometries. These in turn are more closely examined 
with SEM's. 
With the SEM's the topology of the semiconductor 
devices is examined by using the standard secondary 
electron, or emissive, mode. This mode is used to 
observe various types of surface defects on the 
microcircuit. Examples are. metallization flaws which 
typically include voids in the metal around contact 
windows, severed interconnects, and lifted metal. 
These defects are usually detected by examining the 
questionable areas at various tilt and rotational 
angles. 
A more accurate technique called voltage contrast 
is used to confirm failures such as hairline cracks. 
With this technique a circuit is examined under static 
or dynamic conditions at TV scanning rates. A 
potential gradient on the surface of a material, alters 
the secondary emission by the polarity of localized 
areas. Hence, the normal topological contrast has a 
potential or voltage contrast superimposed upon it. 
This technique, in effect, provides an electrical 
continuity test of the device without physically 
probing the die.
Another area that is examined involves bonding 
problems usually associated with bonds that lift from 
the pad (or post) or with broken wires. Again, 
specimen orientation is very important in obtaining 
the right perspective to detect the fault. 
A solid-state X-ray spectrometer is attached to the 
SEM to detect contamination in the form of particles 
or surface film. The spectrometer detects fluorine and 
heavier elements. Although this spectrometer is less 
sensitive than visible-light spectrometers, it is more 
accurate on the nonpianar surfaces encountered in 
microcircuits. 
Further application of the SEM's is in microcircuit 
fabrication. The use of a SEM in lithography is 
comparable to the use of a light source in 
photolithography. An electron resist such as Incite is 
used instead of the photoresist. The electron beam is 
of great advantage where high resolution and/or high 
patterning density are required. With state-of-the-art 
techniques an order-of- magnitude gain in line 
separation (<1,000 A) can be achieved easily. This 
reduces an area by a factor of 100 which increases the 
packing density and high-frequency capability of 
future semiconductor devices. 
There are two general approaches to the electron-
beam control: the analog technique, such as a 
flying-spot scanner (FSS), and the digital technique, 
using a general-purpose computer. Essentially, the 
FSS technique is cheaper and easier to implement. 
The patterning density and resolution capabilities of 
this scheme are severely limited by the cathode-ray 
tube (CRT) and by the associated optics of the FSS. 
However, the FSS is well suited to producing simple- 
patterns such as the microbridges. The width of the 
bridge 0 produced experimentally is approximately 
1,000 A.
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19750000304 2020-03-19T20:15:58+00:00Z
In contrast, the digital-computer approach to 
electron-beam control is by far the most advanced. 
The computer can coordinate all of the operations 
required for microcircuit patterning, such as pattern 
exposure, pattern registration, and the step-and-
repeat operations. However, for simple patterns or 
prototype work the FSS technique is simpler.
Note: 
Requests for further information may be made in 
writing to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Code ATO1 
Marshall Space Flight Center, Alabama 35812 
Reference: B75-10304 
Patent status: 
NASA has decided not to apply for a patent. 
Source: D. P. Nicolas 
(MFS-23159)
[1 
. 
Categories: 01 (Electronics - Components 
and Circuitry) 
08 (Fabrication Technology) 
03 (Physical Sciences) B75-10304 .


Microcircuit testing and fabrication, using scanning electron microscopes

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