Method and apparatus for isolating defects in an integrated circuit near field scanning photon emission microscopy

An apparatus for isolating defects in an integrated circuit using near field scanning photon emission microscopy comprises a photon collector 10 which receives emitted photons 16 from a surface 14 of an energized or biased integrated circuit 12, a CCD camera 20 for converting the photons into an emission image 22, and an optical fiber 18 coupling the CCD camera 20 to the photon collector 10, so that the optical fiber transmits photons from the collector to the CCD camera. As a result, defects in integrated circuits can be isolated with greater resolution than currently available using conventional far field photon emission microscopy.

TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to the field of integrated circuit 
manufacture and more particularly, to a method and apparatus for isolating 
defects in an integrated circuit using near field scanning photon emission 
microscopy. 
BACKGROUND OF THE INVENTION 
During the manufacture of integrated circuits, it is important to be able 
to detect and isolate very small defects caused by leakages, latch-ups, 
and other problems. Far field photon emission microscopy has been used in 
the past to detect photon emissions of very low energy from integrated 
circuits, thereby helping to isolate these types of defects. Although the 
sensitivity of far field photon emission microscopy is very high, its 
spatial resolution, being about 0.5 microns, is inadequate to detect many 
defects, given the fact that integrated circuits are becoming increasingly 
small and are already in the low sub-micron range. Thus, it is very 
difficult to obtain adequate resolution beyond 0.5 microns using far field 
photon emission microscopy. 
The ability to improve resolution using far field photon emission 
microscopy is limited because resolution is dependent on the wavelength of 
the emitted light and the numerical aperture of the microscope. The 
resolution of far field photon emission microscopy can only be improved 
two ways--either by detecting shorter wavelength photons or by increasing 
the numerical aperture of the microscope. However, most photons emitted by 
integrated circuit defects have fixed wavelengths. To detect the shorter 
wavelengths, the integrated circuits can be coated by special materials 
such as rare earth chelates capable of emitting short wavelength photons, 
as discussed in Jerry M. Soden, et al., "IC Failure Analysis: Techniques 
and Tools for Quality and Reliability Improvement," Proceedings of the 
IEEE, Vol. 81, No. 5, May 1993, p. 707. However, this kind of material 
poses radiation problems and is limited in use. In addition, the numerical 
aperture of a camera is limited by the physical size and focal length of 
the lens, making it impractical and difficult to increase the numerical 
aperture to the desirable range. 
Laser tips as a source of light have been used in the past for far field 
collection of light reflected off of the surface of integrated circuits, 
as discussed in T. D. Harris, et al., "Super-Resolution Imaging 
Spectroscopy," Applied Spectroscopy, Vol. 48, No. 1, January 1994, p.19A. 
The light path in such devices may be reversed to perform collection mode 
near field scanning microscopy. This technique merely collects reflected 
light, giving a topographical image of the sample. It does not detect 
defects located below the surface of the integrated circuit. 
Integrated circuits have also been analyzed in the past using a multi-step 
process as described in KLA Instruments Corporation, San Jose, Calif., 
1620 EMMI (Emission Microscope For Multilayer Inspection) Operator's 
Manual, Revision A, June 1990. In one step of this process, the integrated 
circuit is energized or biased using the proper electrical stimulus, and 
emitted photons are captured by a conventional lens using far field 
techniques. In another step, light is shined on the surface of the 
integrated circuit and the reflected light is captured by a camera. In the 
final step, the two sets of data are combined. However, the resolution 
available using this technology is insufficient, being only about 0.5 
microns. 
SUMMARY OF THE INVENTION 
Therefore, a need has arisen for an improved method and apparatus for 
detecting and isolating defects, including subsurface defects, in an 
integrated circuit, which has greater spatial resolution than that 
available using far field photon emission microscopy. In accordance with 
the present invention, an apparatus is provided that uses near field 
scanning photon emission optical microscopy to isolate defects in an 
integrated circuit, including defects located below the surface of the 
sample. The invention features a near field photon collector which 
receives photons emitted from the surface of a biased integrated circuit 
following application of an electrical stimulus to the circuit, a charged 
coupled device such as a charge coupled device camera (CCD camera) to 
convert the photons into an emission image, and an optical fiber coupling 
the CCD camera to the photon collector so that the photons are transmitted 
from the photon collector to the CCD camera. 
In another embodiment, a method for isolating defects in an integrated 
circuit comprises four steps. These steps are: energizing the circuit with 
electrical energy, collecting photons emitted from a surface of the 
integrated circuit, transmitting the photons to an imaging device capable 
of converting the photons into an emission image, and converting the 
photons into an emission image. In an optional fifth step the emission 
image is combined with a topographical image. 
A technical advantage of the present invention is that much better 
resolution of integrated circuit defects caused by leakages, latch-ups and 
other problems, including those located below the surface, can be achieved 
than the resolution available using conventional far field photon emission 
microscopy. Resolution less than or equal to 50 nanometers is possible 
using the present invention. Another technical advantage of the present 
invention is that it allows the use of a photon collector or scanning 
probe tip in the size of nanometers to scan very close to the specimen 
surface and gather data emitted from the surface due to defects, including 
defects below the surface. This emitted data may then be used to form an 
image. Thus, the spatial resolution of the present invention is much 
greater than far field photon emission microscopy, as it is limited only 
by the dimension of the tip, which can be in tens of nanometers or 
smaller. Another technical advantage of the present invention is that in 
integrated circuit applications, one can get more detail by using the 
present invention to observe metal to metal shorts, to view the details of 
hot electrons penetrating a gate oxide, to observe photons generated by 
shorts such as a Vcc to Vss short in an integrated circuit, and to isolate 
other defects such as P-N junction leakages, latch-ups, process and/or 
structure induced failures, saturated transistors, substrate related 
failures, junction spiking, electrostatic discharge damages, hot electron 
effects, oxide defects, impact ionization, junction breakdown, gate 
pinholes and current leakages. Another technical advantage of the present 
invention is that the emission image can be saved and overlapped with a 
topographical image obtained using conventional collection mode near field 
scanning optical microscopy, in order to better pinpoint the emission site 
or defect.

DETAILED DESCRIPTION OF THE INVENTION 
Preferred embodiments of the present invention and its advantages are best 
understood by referring to FIGS. 1 through 4 of the drawings, like 
numerals being used for like in corresponding parts of the various 
drawings. 
FIG. 1 is a schematic of a preferred embodiment of the present invention. 
The apparatus comprises a photon collector 10, such as a laser tip or 
scanning probe tip available from TopoMetrix of Santa Clara, Calif., or 
any other device capable of collecting photons. Scanning probe tips 
currently available, such as those available from TopoMetrix, may be as 
small as 0.05 microns in diameter. The photon collector 10 is used to scan 
an integrated circuit 12, and more particularly, a surface 14 of the 
integrated circuit being evaluated. Alternatively, the present invention 
could be used to evaluate any kind of semiconductor device. 
Photon collector 10 picks up photons 16 emitted from surface 14 of biased 
integrated circuit 12. These photons are emitted from defects or biased 
transistors in the circuit, including subsurface defects, when an 
appropriate electrical stimulus is supplied to the circuit. Subsurface 
defects can be detected because silicon and silicon-related substances, 
such as those used in passivation layers, are transparent to the emitted 
photons. Defects capable of detection include, but are not limited to, 
shorts, P-N junction leakages, latch-ups and process and/or structure 
induced failures, saturated transistors, substrate related failures, 
junction spiking, electrostatic discharge damages, hot electron effects, 
oxide defects, impact ionization, junction breakdown, gate pinholes and 
current leakages. The electrical stimulus may be applied to the circuit 
using any conventional means. 
Photon collector 10 is coupled to an optical fiber 18. Optical fiber 18 is 
used to transmit the emitted photons from the photon collector to a CCD 
camera 20 using fiber optics. Suitable optical fibers are available from 
3M; however, any optical fiber having a good band width in the 500 to 1100 
nanometer range may be used. Suitable CCD cameras include Model CH 270 
from Photomatrics of Tucson, Ariz. In an alternative embodiment, the 
photons may be transferred to any CCD device for conversion into usable 
data. 
The CCD camera 20 converts the transmitted photons into emission images 22. 
These images provide information as to the presence and location of many 
kinds of defects, such as those caused by leakages, latch-ups, and other 
problems in the integrated circuit. Photon collector 10 may be mounted on 
a platform 24 which may be moved perpendicular to or vertically with 
respect to surface 14 of the integrated circuit 12 being tested. As 
depicted in FIG. 1, the sample or integrated circuit being tested may be 
mounted on an end of a piezoelectric scan tube 26. The piezoelectric scan 
tube 26 may be capable of motion in all three axes, x, y and z, where x 
and y are the axes lying in a plane parallel to the surface of the 
integrated circuit, and z is the axis perpendicular to the x/y plane, or 
vertical with respect to horizontal surface 14. Piezoelectric scan tubes 
suitable for use in the present invention are commercially available from 
a number of commercial sources, including Digital Instruments of Santa 
Barbara, Calif., or TopoMetrix of Santa Clara, Calif. In a preferred 
embodiment, piezoelectric scan tube 26 moves the integrated circuit so 
that the photon collector 10 scans surface 14 of integrated circuit 12 in 
raster fashion. 
As further depicted in FIG. 1, the piezoelectric scan tube 26 may be 
attached to a translation stage 28. In a preferred embodiment, the 
translation stage is fixed. Photon collector 10, integrated circuit 12, 
piezoelectric scan tube 26, and translation stage 28 are housed or 
enclosed in dark box 29 or any other container known in the art which is 
impervious to light. 
As depicted in FIG. 2, showing a block diagram of a photon collector being 
used to collect photons from a Vcc to Vss short, photon collector 10 may 
have very minute dimensions, i.e., 0.05 microns in diameter, and be placed 
in very close proximity to surface 14 of integrated circuit 12. Thus, the 
photon collector is able to pick up photons generated by such defects as a 
short 30 between a Vss 32 and a Vcc 34. 
In operation, the sample or integrated circuit 12, mounted on piezoelectric 
scan tube 26, is biased or energized using an appropriate electrical 
stimulus. The piezoelectric scan tube 26 moves the integrated circuit 12 
in raster or other fashion allowing the surface 14 to be scanned by photon 
collector 10. Defects in the integrated circuit will cause photons 16 to 
be emitted that are then picked up by photon collector 10. The photon 
collector then transmits the photons via optical fiber 18 to the CCD 
camera 20. The CCD camera 20 then creates an emission image 22, which 
isolates the defects in the circuit. The integrated circuit may also be 
rotated so that the reverse or backside surface can be analyzed or 
scanned. 
In another embodiment of the present invention, the position of the sample 
may be fixed, and the position of the photon collector varied with respect 
to the sample to allow scanning to occur. 
As depicted in FIG. 3, the present invention can be used in conjunction 
with known reflective imaging techniques, in which a reference optical 
image of surface 14 is obtained by shining light 36 on surface 14 of the 
integrated circuit 12 and capturing the reflected light. Specifically, 
after the emission image 22 has been captured and saved in a computer, the 
topography of integrated circuit 12 can be captured using collection mode 
near field scanning optical microscopy. As shown in FIG. 3, light 36 is 
focused on surface 14 of integrated circuit 12 using microscopic objective 
38, and the reflected light is picked by photon collector 10 and sent to 
CCD camera 20. The saved emission image 22 can then be overlapped in the 
computer with the topographical image or data to better pinpoint or locate 
the coordinates of the defect or emission site. Alternatively, the 
topographical image can be captured before obtaining the emission image. 
The present invention also relates to a method of isolating defects in an 
integrated circuit. Referring now to FIG. 4, the preferred method of the 
present invention is illustrated. In step 101, an integrated circuit is 
energized or biased by the application of appropriate electrical stimulus. 
This causes defects in the circuit to emit photons. In step 102, the 
emitted photons are collected from a surface of the integrated circuit 
using a photon collector such as a scanning probe tip. In step 103, the 
photons are transmitted to an imaging device such as a CCD camera using an 
optical fiber (preferably with a good band width in the 500 to 1100 
nanometer range) coupled to the photon collector and CCD camera. In Step 
104, the photons are converted into an emission image using the CCD 
camera. The photon collector may be mounted on a platform that can be 
moved perpendicular to or vertically with respect to the horizontal 
surface of the sample being evaluated. The integrated circuit being 
evaluated may be mounted on a piezoelectric scan tube which is capable of 
movement in all three axes, x, y and z, where the x and y axes lie in a 
plane parallel to the surface of the integrated circuit being tested and 
the z axis is perpendicular to the x/y plane. The piezoelectric scan tube 
may vary the position of the integrated circuit with respect to the photon 
collector so that the circuit may be scanned in raster or other fashion. 
The piezoelectric scan tube may be mounted on a translation stage. 
Preferably, the integrated circuit, photon collector, piezoelectric scan 
tube and translation stage may be housed or enclosed in a dark box or 
other container impervious to light. 
In an optional further step 105, the emission image is combined or 
overlapped with a topographical image or data which is obtained using 
collection mode near field scanning optical microscopy, in order to better 
pinpoint the emission site. 
As can be seen from the foregoing, the apparatus and method of the present 
invention can be used to detect and isolate minute defects in an 
integrated circuit or any kind of semiconductor device. The present 
invention is capable of much higher resolution than that available through 
conventional far field photon emission microscopy and can detect photons 
generated from sub-surface defects. 
While the invention has been particularly shown and described by the 
foregoing detailed description, it will be understood by those skilled in 
the art that various other changes in the form and detail may be made 
without departing from the spirit and scope of the invention.