Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-05-27
2002-07-30
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S252100
Reexamination Certificate
active
06426501
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a defect-review SEM (scanning electron microscope) best adapted to inspect contact holes formed during fabrication of semiconductor devices.
DESCRIPTION OF THE PRIOR ART
It is quite important to enhance the yield in fabricating semiconductor devices. It is known that factors deteriorating the manufacturing yield include random factors and process factors.
Random factors are associated with various semiconductor process factors concerned with semiconductor fabrication environments, such as the cleanliness of the cleanroom, generation of dust from the wafer conveyance system and from steppers, removal of substances peeled from etchers, and insufficient washing by wafer cleaning equipment. Therefore, the random factors are uncertain and attributed to dust produced by factors that are difficult to forecast.
Process factors are produced by deviations of actually fabricated patterns and structures from ideal chip linewidths, chip arrays, and chip structures based on semiconductor designs. To solve the problems of the process factors, tools for inspecting patterned wafers are installed in semiconductor cleanrooms for individual process steps.
Among inspections of the process factors, inspections of contact holes have attracted attention recently. For these inspections, optical inspection systems and defect-review SEMs have been developed.
An optical inspection system can inspect dust, foreign matter, and defective metallization patterns up to a feature size of 0.2 &mgr;m by various methods. For example, scattering of light is employed. In another method, an electrical signal is obtained by detection of light and passed through a spatial filter to process the information. In a further method, features are extracted from optical images by pattern matching. In recent years, inspection equipment capable of inspecting contact holes having diameters of 0.1 to 0.05 &mgr;m has been announced and attracted attention. However, the minimum contact hole diameter which can be inspected precisely is not obvious in practice.
Users utilize wafer pattern dimension metrology and inspection tools that are normally used to inspect contact holes using an electron beam. In this case, it has been reported that aspect ratios of about 3 to 5 could be inspected at a diameter of 0.2 &mgr;m. However, defective apertures of contact holes smaller than 0.2 &mgr;m and having aspect ratios of 10 to 15 cannot be inspected.
In recent years, a dedicated contact hole inspection system capable of inspecting contact holes having diameters of up to 0.1 &mgr;m by sharply focusing the electron beam of a scanning electron microscope has been developed.
FIG. 1
schematically shows this inspection system having an electron gun (not shown) emitting an electron beam
1
that is accelerated. The beam
1
is sharply focused onto a semiconductor wafer sample
3
having a diameter of 8 to 12 inches by an electromagnetic lens
2
. The beam is scanned by an electromagnetic deflector
4
. A negative bias voltage (e.g., 19 kV) is applied to the sample to decelerate the beam down to 0.5 to 1.5 kV. The beam
1
is focused by the electromagnetic lens
2
to a diameter of less than 0.05 &mgr;m on a pattern formed on the wafer sample
3
.
The electromagnetic deflector
4
scans the electron beam
1
across the pattern on the sample
3
at a high rate in the direction indicated by the arrow. The broken line indicates the state during the blanking period of the electron beam. The deflector
4
scans the electron beam across 512 pixels in one excursion, 100 MHz per pixel.
The sample
3
is placed on a stage
5
. The velocity of movement of the stage
5
and its position are controlled at an accuracy of tens of nanometers, using a linear encoder or a laser interferometer.
Because the sample
3
is irradiated with the electron beam
1
, secondary electrons having energies from 0 to less than 10 eV are produced. Since a bias voltage is applied between the sample
3
and the electromagnetic lens
2
, the secondary electrons are accelerated to 19 keV and enter a Wien filter (not shown) mounted over the electromagnetic lens
2
. This filter does not deflect the incident electron beam
1
but deflects the secondary electrons from the sample through about 90°.
The secondary electrons deflected by the Wien filter are detected by a secondary electron detector
6
. A Schottky barrier diode detector that is a high-speed secondary electron detector responding at 100 MHz per pixel is used as the detector
6
. An amplifier
7
amplifies the signal at a response frequency of 100 MHz.
The output signal from the amplifier
7
is supplied to an A/D converter
8
and converted into a digital signal. The output signal from the A/D converter
8
is fed to a signal distributor
9
. The stage
5
is continuously moved in the Y-direction. The electron beam
1
is scanned only in the X-direction perpendicular to the Y-direction. The resulting signal representing 512×512 pixels provides a SWATH image in synchronism with a sync signal from the linear encoder or laser interferometer in the control system for the beam deflector
4
and for the stage
5
. The SWATH image is distributed to plural high-speed image processing circuit boards
10
.
Each of the high-speed image processing circuit boards
10
has an image distributor
11
, an image memory
12
, a defect feature extractor
13
, and a defect detector
14
. The image distributor
11
stores the SWATH images in the image memory
12
according to given image position information. The SWATH images are collected at the timing of image collection in synchronism with the sync signal from the linear encoder or laser interferometer in the control system for the beam deflector
4
and for the stage
5
.
The defect feature extractor
13
is loaded with an algorithm specialized by the signal stored in the image memory
12
. Features of the image are extracted according to the signal stored in the image memory
12
in accordance with the algorithm. The defect detector
14
compares defect features of two images extracted in succession (cell comparison) or compares different chip patterns (die comparison). Alternatively, the detector compares defect features with CAD design data about a chip pattern already collected from good products or chip patterns (data comparison). Defect sizes and their coordinates are detected by a specially prepared algorithm.
A defect inspection result processor
15
integrates defect sizes, coordinates, and wafer information (e.g., wafer names, lot numbers, and wafer recipe) processed in parallel at high speed by the high-speed image processing circuit boards
10
and stores the results.
The defect inspection system shown in
FIG. 1
has been developed only to focus the electron beam of the prior art scanning electron microscope as finely as possible. Therefore, where a contact hole not only has a small aperture diameter but also has a much larger depth than the aperture diameter, difficulties take place. For example, it is impossible to detect a contact hole with defective aperture if the aperture diameter is less than 0.05 &mgr;m and the aspect ratio is 10 to 15.
If a contact hole in SiO
2
film formed on a silicon (Si) substrate or a contact hole in a resist film is inspected with an electron beam, the accelerating voltage of the electron beam incident on the sample is set less than 1 kV where the secondary electron emission rate &dgr;≧1 to avoid charging of the oxide film or resist film. Also, it is known that the accelerating voltage is set to 0.5 to 0.8 kV in use to minimize the damage to the sample due to the electron beam bombardment.
In some inspection tools, the sample is scanned with an electron beam at an accelerating voltage of 0.8 kV at a high frequency of 100 MHz to suppress charging of the sample. In other inspection tools, the DC voltage between an objective lens and a wafer sample is adjusted to suppress charging of the sample.
The electron beams used in the method of preventing charging are finely focused to diameters of
Jeol Ltd.
Nguyen Kiet T.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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