Defect type identification using hyper-extracting-field

Details Defect type identification using hyper-extracting-field Defect type identification using hyper-extracting-field

2002-07-10

2004-04-13

Zarneke, David A. (Department: 2829)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

Reexamination Certificate

active

06720779

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electron beam systems. The present invention more particularly relates to scanning electron microscopes in voltage contrast mode.
2. Description of the Background Art
A Scanning Electron Microscope (SEM) may be capable of Voltage Contrast inspection of a specimen (SEM-VC). SEM-VC inspection typically uses low landing energies and high electron current.
The specimen may be, for example, a semiconductor wafer with memory arrays thereon. The memory array may include, for example, three different types of contacts as illustrated in FIG.
1
.
FIG. 1
depicts examples of an N+ contact
102
, a P+ contact
104
, and a gate contact
106
. The N+ contact
102
comprises a tungsten (W) plug to polysilicon over a N+ contact well within a surrounding P well. The P+ contact
104
comprises a tungsten plug to a P+ contact well within a surrounding N well. The gate contact
106
comprises a tungsten plug to a floating gate that is separated from an N or a P well by a thin gate oxide layer. The examples in
FIG. 1
are formed on a P substrate. Alternatively, of course, such structures may be formed on an N substrate.
Consider, for example, the N+ contacts
102
. A conventional retarding voltage may be applied between an (extracting) electrode above the specimen and the specimen stage. The retarding voltage causes electrons to build up on those tungsten plugs that are floating due to the N+ contact
102
being a defective open circuit. This is because the open circuit prevents the electrons from “draining away” from those tungsten plugs. The excess electrons at the surface of those tungsten plugs make them appear brighter in the SEM-VC retarding mode image. An example of this is illustrated in FIG.
2
A. The brighter area in
FIG. 2A
is indicates the presence of open N+ contacts
102
in that area.
Alternatively, a conventional extracting voltage may be applied between the electrode above the specimen and the specimen stage. The extracting voltage causes electrons to be extracted from the tungsten plugs of normal N+ contacts
102
. The extraction of electrons from a normal N+ contact
102
, however, results in the N+ contact well becoming relatively positive charged. The positive charge of the N+ contact well causes the PN diode (from the surrounding P well to the N+ contact well) to be reversed biased. The reversed biased diode constrains and limits the flow of electrons out of the N+ contacts
102
. The lesser flow of electrons the normal N+ contacts makes their tungsten plugs appear somewhat dim in the SEM-VC image. On the other hand, a shorted N+ contact
102
may have a short circuit that goes directly from the polysilicon to the P well (bypassing the PN reverse biased junction). Such a shorted contact would not have a reversed bias diode to constrain the flow of extracted electrons. Thus, a plug associated with a shorted N+ contact would appear brighter in an SEM-VC extraction mode image than would a plug associated with a normal N+ contact
102
. An example of this is illustrated in FIG.
2
B. The brighter area in
FIG. 2B
indicates the presence of shorted N+ contacts
102
in that area. Note that the image in
FIG. 2B
is roughly of the same area from the same memory as the image in FIG.
2
A.
While conventional voltage contrast techniques are useful as described above, further improvement in defect detection techniques is desirable.


REFERENCES:
patent: 5523694 (1996-06-01), Cole, Jr.
patent: 5781017 (1998-07-01), Cole et al.

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