Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1999-07-28
2002-06-04
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06399945
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to systems for detecting backscattered electrons propagating from an object being irradiated with an electron beam or other charged particle beam. Such detection systems are utilized, for example, in charged-particle-beam microlithography apparatus and methods requiring accurate alignment of a reticle pattern with a desired projection location on a substrate. More specifically, the invention pertains to backscattered-electron detection systems that produce a high signal-to-noise (S/N) ratio for greater detection (and hence alignment) accuracy.
BACKGROUND OF THE INVENTION
A conventional backscattered-electron (BSE) detector system is shown in FIGS.
9
(
a
)-
9
(
b
). Such BSE detector systems are typically included with electron-beam microlithography apparatus for use in, e.g., aligning the silicon substrate prior to the substrate being exposed by electron-beam irradiation.
In FIG.
9
(
a
), a sample
107
comprises a substrate
104
made of a relatively “light” (low mass) element such as silicon. Pattern features
105
are formed on the surface of the sample
107
. The pattern features
105
are made of a relatively “heavy” (high mass) element such as tantalum. An electron beam EB incident to the surface of the substrate
107
is scanned in the direction of the “scan” arrow along the dashed line
108
. As the incident electrons in the beam EB penetrate into the sample
107
, they experience many scattering events that produce backscattered electrons
103
. The backscattered electrons
103
are detected by BSE detectors
101
and
101
′.
The signal produced by the BSE detectors
101
,
101
′ as the electron beam EB passes over the feature elements
105
is profiled in FIG.
9
(
b
). FIG.
9
(
b
) also depicts a representative relationship between the specific location on the sample
107
being irradiated by the electron beam EB versus the amplitude of the electrical signal produced by the BSE detectors
101
,
101
′.
Conventional BSE detectors
101
,
101
′ commonly comprise PN-junction or PIN-junction semiconductors for detecting backscattered electrons. The detector junction is biased and, as backscattered electrons impinge on the detector junction, corresponding changes in current or voltage flowing through the junction are produced to generate a detector signal. The amplitude of the detector signal is a function of the energy of the corresponding incident backscattered electrons. The detector signal is normally amplified.
The energy of electrons backscattered from the sample
107
differs according to the specific material on which the electron beam EB is incident (e.g., on a pattern feature
105
versus on the surface of the substrate
104
). Hence, as the electron beam EB encounters different materials on the sample
107
, the amplitude of the signal produced by the BSE detectors
101
,
101
′ changes. This phenomenon is exploited for detecting, using the BSE detectors
101
,
101
′, the presence and position of a pattern on the sample
107
. For example, prior to performing a microlithographic exposure of the substrate
104
using the electron beam, the location of an alignment mark on the substrate
104
is detected, using the BSE detection system of FIGS.
9
(
a
)-
9
(
b
), so as to positionally align the substrate for exposure.
SUMMARY OF THE INVENTION
A general object of the invention is to increase the accuracy with which a pattern feature on a sample can be detected, compared to the performance of the conventional BSE detector system summarized above. The initial approach was to increase the differences in energy exhibited by backscattered electrons from various regions on the sample. As noted above, the energy of electrons backscattered from a substance is characteristic of the substance and is normally different for different substances. Another variable that influences the energy of backscattered electrons is the thickness of the substance on which the beam is incident. Hence, in order to increase the difference in energy of electrons backscattered from various materials, increasing the thickness of pattern features (e.g., elements of an alignment mark) was initially considered.
However, modern methods for fabricating semiconductor devices typically include one or more planarization steps, generally performed by chemical-mechanical polishing or an analogous technique. This has made it difficult to provide a suitably thick alignment-mark pattern, for example, for obtaining a desired large difference in energy of backscattered electrons as the electron beam passes over an alignment mark on a planarized surface.
In view of the above, an object of the present invention is to provide BSE detection systems capable of producing and detecting large changes in detection-signal amplitude as the beam is incident on various materials on the sample surface. The resulting detection signals produced by such systems have an increased signal-to-noise ratio and increased position-detection accuracy.
According to a first aspect of the invention, detection systems are provided for detecting electrons backscattered from a locus on a sample surface irradiated with an electron beam. A representative embodiment of such a system comprises first and second BSE detectors. The first BSE detector has a first prescribed “energy-sensitivity band” (i.e., the first BSE detector is sensitive to electrons having respective energies within a first energy range), and is configured and situated so as to receive a first group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam. The first BSE detector produces, from the received backscattered electrons, a first electrical signal. Similarly, the second BSE detector has a second prescribed energy-sensitivity band (i.e., the second BSE detector is sensitive to electrons having respective energies within a second energy range) that is different from the first energy-sensitivity band. The second BSE detector is configured and situated so as to receive a second group of backscattered electrons propagating from the locus due to irradiation of the locus with the electron beam, and to produce from the received backscattered electrons a second electrical signal. The system includes a signal combiner connected to the first and second BSE detectors. The signal combiner is configured to combine the first and second signals together yielding a composite output-signal waveform having an amplitude that differs depending upon a characteristic of the locus.
A typical characteristic of the locus is the specific material of the locus. For example, the sample can comprise a substrate surface including a pattern feature. In such an instance, the output signal produced by the signal combiner has a waveform corresponding to irradiation of the substrate surface and the pattern feature with the electron beam.
According to a first example embodiment of a detection system according to the invention, the first and second BSE detectors are solid-state detectors, in which each first and second BSE detector comprises a PN or PIN junction with a respective depletion layer. Desirably, at least one of a depth and width of the respective depletion layer is adjustable so as to adjust the energy-sensitivity band of each BSE detector. If the output signal waveform of the first BSE detector is denoted S
A
and the output signal waveform of the second BSE detector is denoted S
B
, then the signal combiner produces a composite signal according to the expression: S
A
−&agr;S
B
.
Multiple first and second solid-state BSE detectors can be used. For example two first BSE detectors can be used, designated “A” and “A′”, and two second BSE detectors can be used, designated “B” and “B′”. In such an instance, the combiner produces a composite output signal waveform according to the expression:
(
S
A
+S
A′
)−&agr;(
S
B
+S
B′
)
wherein S
A
and S
A′
denote respective output waveforms from the first solid-s
Nguyen Kiet T.
Nikon Corporation
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