Title: Electron beam inspection system and inspection method and method of manufacturing devices using the system
Abstract: An electron beam inspection system of the image projection type includes a primary electron optical system for shaping an electron beam emitted from an electron gun into a rectangular configuration and applying the shaped electron beam to a sample surface to be inspected. A secondary electron optical system converges secondary electrons emitted from the sample. A detector converts the converged secondary electrons into an optical image through a fluorescent screen and focuses the image to a line sensor. A controller controls the charge transfer time of the line sensor at which the picked-up line image is transferred between each pair of adjacent pixel rows provided in the line sensor in association with the moving speed of a stage for moving the sample.
Patent Number: 6,992,290 Issued on 01/31/2006 to Watanabe, et al.
| Inventors:
|
Watanabe; Kenji (Kanagawa, JP);
Sobukawa; Hirosi (Kanagawa, JP);
Noji; Nobuharu (Kanagawa, JP);
Satake; Tohru (Kanagawa, JP);
Yoshikawa; Shoji (Tokyo, JP);
Karimata; Tsutomu (Kanagawa, JP);
Nakasuji; Mamoru (Kanagawa, JP);
Hatakeyama; Masahiro (Kanagawa, JP);
Murakami; Takeshi (Tokyo, JP);
Yamazaki; Yuichiro (Tokyo, JP);
Nagahama; Ichirota (Ibaraki, JP);
Nagai; Takamitsu (Kanagawa, JP);
Sugihara; Kazuyoshi (Kanagawa, JP)
|
| Assignee:
|
Ebara Corporation (Tokyo, JP);
Kabushiki Kaisha Toshiba (Tokyo, JP)
|
| Appl. No.:
|
985317 |
| Filed:
|
November 2, 2001 |
Foreign Application Priority Data
| Jan 10, 2001[JP] | 2001/002722 |
| Mar 16, 2001[JP] | 2001/075865 |
| Mar 28, 2001[JP] | 2001/092748 |
| Apr 24, 2001[JP] | 2001/125349 |
| Jun 22, 2001[JP] | 2001/189325 |
| Current U.S. Class: |
250/310 |
| Current Intern'l Class: |
H01J 37/26 (20060101) |
| Field of Search: |
250/306,310
|
References Cited [Referenced By]
U.S. Patent Documents
| 4912052 | Mar., 1990 | Miyoshi et al.
| |
| 5030908 | Jul., 1991 | Miyoshi et al.
| |
| 5315119 | May., 1994 | Komatsu et al.
| |
| 5359197 | Oct., 1994 | Komatsu et al.
| |
| 5362968 | Nov., 1994 | Soloman.
| |
| 5363968 | Nov., 1994 | Soloman.
| |
| 5479535 | Dec., 1995 | Komatsu.
| |
| 6038018 | Mar., 2000 | Yamazaki et al.
| |
| 6184526 | Feb., 2001 | Kohama et al.
| |
| 6365897 | Apr., 2002 | Hamashima et al.
| |
| 6555815 | Apr., 2003 | Feuerbaum et al.
| |
| 6670602 | Dec., 2003 | Kohama et al.
| |
| Foreign Patent Documents |
| 09-311112 | Dec., 1997 | JP.
| |
| 10-012684 | Jan., 1998 | JP.
| |
| 11-242943 | Sep., 1999 | JP.
| |
| 11-345585 | Dec., 1999 | JP.
| |
| 2000/-100369 | Apr., 2000 | JP.
| |
| 2000/-113848 | Apr., 2000 | JP.
| |
| 2000/-113848 | Apr., 2000 | JP.
| |
| 2000/-356512 | Dec., 2000 | JP.
| |
| WO99/50651 | Oct., 1999 | WO.
| |
Other References
"LSI Testing Symposium/1996; Meeting Minutes" (Nov. 7-8, 1996) with English Translation.
International Search Report dated Feb. 12, 2002.
|
Primary Examiner: Lee; John R.
Assistant Examiner: Gurzo; Paul M.
Attorney, Agent or Firm: Westerman, Hattori, Daniels & Adrian, LLP
Claims
What is claimed is:
1. An electron beam inspection system comprising:
an electron irradiation gun for irradiating a primary electron beam to a surface
of a sample;
stage for supporting and moving the sample in a direction;
a secondary optical system for magnifying and projecting secondary electrons
generated from the sample by irradiating the primary electron beam to form an image
of the surface of the sample;
a TDI (Time Delay Integration)-CCD (Charged Coupled Device) for detecting the
secondary electrons and integrating electric charges converted from the detected
secondary electrons in a surface array direction of the TDI-CCD; and
a magnetic lens for rotating an image formed by the secondary electrons passing
the magnetic lens such that a scanning direction of the sample as scanned by moving
the stage coincides with an integration direction for integrating the electric
charges in the surface array direction of the TDI-CCD by controlling the intensity
of magnetic field which is produced by the magnetic lens and applied to the secondary electrons.
2. An electron beam inspection system according to claim 1, wherein the magnetic
lens is positioned between a lens in a final stage of the optical system and a
microchannel plate placed in a preceding stage of the TDI-CCD.
3. An electron beam inspection system according to claim 2, wherein the magnetic
lens is disposed at a crossover position closest to the microchannel plate.
4. An electron beam inspection system according to claim 1, wherein the magnetic
lens is disposed at an image-formation position closest to a final-stage lens of
the optical system on a side of the final-stage lens remote from the microchannel plate.
5. A device fabrication method using the electron beam inspection system according
to claim 1.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an inspection system for inspecting an object,
e.g. a wafer, by using an electron beam to detect a defect or the like in a pattern
formed on the surface of the sample under inspection. More particularly, the present
invention relates to an inspection system and inspection method wherein an electron
beam is irradiated to the surface of an object under inspection, and image data
is obtained from the number of secondary electrons emitted from the sample surface,
which varies according to the properties of the sample surface, and a pattern or
the like formed on the sample surface is inspected with high throughput on the
basis of the image data, as in the case of detecting defects on a wafer in the
semiconductor fabrication process. The present invention also relates to a method
of fabricating devices at high yield by using the inspection system.
Semiconductor fabrication processes are going to enter a new era where
design rules are 100 nm. The form of production is shifting from limited large-lot
production, represented by the fabrication of DRAMs, to diverse small-lot production
as is the case with SOC (Silicon On Chip). As a result, the number of production
steps has increased, and it has become essential to improve the yield for each
production step. Consequently, the inspection for process-induced defects has become
important. Conventionally, a wafer defect inspection is performed after each step
of the semiconductor manufacturing process. With the progress of the technology
to fabricate high-integration semiconductor devices and to form small and fine
patterns, a high-resolution and high-throughput defect inspection system has been
demanded. The reason for this is that a resolution of 100 nm or below is required
to detect the defects on a wafer substrate fabricated with 100 nm design rules.
In addition, as the degree of integration of semiconductor devices increases, the
number of production steps increases, resulting in an increase in the number of
inspections to be performed. For this reason, high throughput is demanded. Further,
as the number of layers constituting semiconductor devices increases, it is required
that the defect inspection system should have the function of detecting contact
failures (electrical defects) of vias for connection between wiring patterns on
different layers.
As this type of defect inspection system, an optical defect inspection system
has heretofore been used. However, the optical defect inspection system is limited
in its resolution capability. That is, the resolution is limited to ½ of the
wavelength of light used. In a practical example using visible light, the resolution
is of the order of 0.2 μm. Thus, the optical defect inspection system cannot
meet the resolution requirements. Further, the optical defect inspection system
cannot perform inspection for electrical conduction failures (opens, shorts, etc.),
that is, contact failures occurring in semiconductor devices.
Under these circumstances, a defect inspection system using an electron beam
has recently been developed for use in place of the optical defect inspection system.
In such an electron beam type defect inspection system, a scanning electron microscope
system (SEM system) has generally been put to practical use. The inspection system
exhibits a relatively high resolution, i.e. 0.1 μm, and is capable of inspection
for electrical defects (disconnection and conduction failures of wiring patterns,
conduction failures of vias, etc.). In the defect inspection system making use
of SEM, however, the amount of beam current and the response speed of the detector
are limited. Therefore, a great deal of time is required to perform defect inspection.
For example, it takes 8 hours to inspect one wafer (20-cm wafer). Thus, the inspection
time is extremely long. Accordingly, the throughput (the number of wafers inspected
per unit time) is unfavorably low in comparison to other process systems such as
optical defect inspection systems. In addition, the electron beam type defect inspection
system is very costly. Accordingly, it is difficult to use it after each step of
the semiconductor fabrication process. In the present state of the art, the electron
beam type defect inspection system is used after an important process step, e.g.
after etching, film deposition (including copper plating), or CMP (Chemical/Mechanical
Polishing) planarization treatment.
The defect inspection system using the scanning electron microscope system (SEM
system) will be described below in more detail. In the defect inspection system,
an electron beam is focussed (the focussed beam diameter corresponds to the resolution)
and a sample, e.g. a wafer is linearly irradiated so as to be scanned with the
focussed electron beam. Meanwhile, a stage having the wafer placed thereon is moved
in a direction perpendicular to the electron beam scanning direction, whereby an
observation region on the wafer is irradiated planarly with the electron beam.
The scanning width of the electron beam is generally several 100 μm. By the
irradiation with the focussed electron beam (referred to as "primary electron beam"),
secondary electrons are emitted from the wafer. The secondary electrons are detected
with a detector (a scintillator+a photomultiplier) or a semiconductor type detector
(a PIN diode type detector), for example. The coordinates of the position of irradiation
with the electron beam and the number of secondary electrons (signal intensity)
are combined to produce an image. Image data thus obtained is stored in a storage
unit. Alternatively, the image can be output onto a CRT (Cathode-Ray Tube). The
foregoing is the principle of the SEM (Scanning Electron Microscope). From the
image obtained by this method, possible defects on the in-process semiconductor
(Si, in general) wafer are detected. The inspection speed (corresponding to the
throughput) is determined by the amount of the primary electron beam (electric
current value), the beam diameter, and the response speed of the detector. The
present maximum values of these factors are as follows. The beam diameter is 0.1
μm (this may be regarded as equal to resolution). The electric current value
is 100 nA. The response speed of the detector is 100 MHz. In this case, it takes
about 8 hours to inspect one wafer having a diameter of 20 cm, as has been stated
above. Thus, the scanning electron beam type defect inspection system suffers from
a serious problem that the inspection speed is extremely lower ( 1/20 or less)
than those of other process systems such as optical defect inspection systems.
The present invention was made in view of the above-described problems. Accordingly,
an object of the present invention is to improve the inspection speed for detecting
defects on a sample, e.g. a wafer.
SUMMARY OF THE INVENTION
The present invention relates to a defect inspection system utilizing an image
projection system using an electron beam as a method for improving the inspection
speed of the scanning electron beam (SEM) type defect inspection system. The image
projection system will be described below.
In the image projection system, an observation region of a sample is irradiated
with a primary electron beam by one shot (i.e. a predetermined area is irradiated
with the electron beam without performing scanning), and secondary electrons emitted
from the irradiated region are collectively focused onto a detector (a microchannel
plate+a fluorescent screen) as an electron beam image by a lens system. The image
thus formed is converted into an electric signal by a two-dimensional CCD (Charge-Coupled
Device; solid-state image pickup device) or a TDI (Time Delay Integration)-CCD
(i.e. a line image sensor) and output onto a CRT or stored in a storage unit as
image information. From the image information, possible defects on the sample wafer
[in-process semiconductor (Si) wafer] are detected. In the case of CCD, the travel
direction of the stage is the minor axis direction (or may be the major axis direction),
and the stage is moved in a step-and-repeat manner. In the case of TDI-CCD, the
stage is moved continuously in the integration direction. Because it allows images
to be obtained continuously, TDI-CCD is used to perform defect inspection continuously.
The resolution is determined by the magnification, accuracy and so forth of the
image projection optical system (secondary optical system). In a certain experimental
example, a resolution of 0.05 μm was obtained. In the experimental example,
when the resolution was set to 0.1 μm and electron beam irradiation conditions
were set so that the size of an inspection region on a wafer was 200 μm by
50 μm and the amount of the primary electron beam (electric current value)
was 1.6 μA, the inspection time was of the order of 1 hour per 20-cm wafer.
In other words, the image projection system provides an inspection speed 8 times
as high as that obtained by the SEM system. It should be noted that the specifications
of TDI-CCD used in the experimental example were as follows. The number of pixels
was 2048 pixels×512 rows, and the line rate was 3.3 μs (line frequency:
300 kHz).
The irradiation area (planar dimension) in this example was set in conformity
to the specifications of the TDI-CCD. However, the irradiation area may be changed
according to the object to be irradiated.
The following is the outline of an electron beam inspection system utilizing
the image projection system.
The electron beam inspection system includes a primary electron optical system
for shaping an electron beam emitted from an electron gun into a desired configuration
(e.g. a rectangular or elliptical configuration) and irradiating the shaped electron
beam to the whole area of an observation region on the surface of a sample (e.g.
a wafer or a mask; hereinafter occasionally described as a wafer) to be inspected.
The electron beam inspection system also includes a secondary electron optical
system directs secondary electrons emitted from the wafer toward a detector. The
detector converts the secondary electrons into an optical image and forms an image
of the wafer. The electron beam inspection system further includes a controller
for controlling the detector. The primary electron optical system has an electron
gun for emitting an electron beam, and a primary electrostatic lens system for
shaping the electron beam into a beam having a predetermined sectional configuration.
The primary electron optical system is disposed at a predetermined angle to a direction
perpendicular to the surface of the wafer. The constituent elements of the primary
electron optical system are placed in series with the electron gun positioned at
the top. Between the primary electron optical system and the secondary electron
optical system, an E×B deflector (also known as "Wien filter" or "E×B
separator") is disposed along a direction perpendicular to the surface of the wafer
to deflect the electron beam and to separate the secondary electrons from the wafer
by a field where an electric field and a magnetic field perpendicularly intersect
each other. The secondary electron optical system has a secondary electrostatic
lens system disposed to extend in a direction perpendicular to the wafer surface
along the optical axis of the secondary electrons from the wafer, which are separated
by the E×B separator, from the primary electron optical axis to deflect and
focus the secondary electrons.
As the electron gun, a thermal electron beam source type is used, in which electrons
are emitted by heating an emissive material (cathode). Lanthanum hexaboride (LaB
6)
is used as the emissive material (emitter) as a cathode. It is also possible to
use other materials, provided that they have a high melting point (the vapor pressure
at high temperatures is low) and a low work function. A cathode of lanthanum hexaboride
(LaB
6) having a truncated conical tip is used. It is also possible to
use a frustoconical cathode, i.e. a truncated cone-shaped cathode. The diameter
of the truncated tip of the cathode is of the order of 100 μm. There are
other electron beam sources available, i.e. a field emission type electron beam
source and a thermal field emission type electron beam source. However, a thermal
electron beam source using LaB
6 is most suitable for use in a system
in which a relatively wide region (e.g. 100 by 25 to 400 by 100 μm
2)
is irradiated with a large electric current (of the order of 1 μA) as in
the case of the present invention. It should be noted that the SEM system generally
uses a thermal field emission type electron beam source. It is a matter of course
that a field emission type electron beam source or a thermal field emission type
electron beam source may be used in this embodiment in place of thermal electron
beam source. The thermal field emission type electron beam source is a system in
which electrons are emitted by applying a high electric field to an emissive material,
and the emission of electrons is stabilized by heating its electron beam emitting part.
The primary electron optical system constitutes a part that forms a primary electron
beam emitted from an electron gun and shapes the primary electron beam into a desired
configuration, e.g. a rectangular or circular (elliptical) configuration, and further
irradiates the rectangular or circular (elliptical) primary electron beam to the
wafer surface. The beam size and current density of the primary electron beam can
be controlled by controlling the conditions of lenses provided in the primary electron
optical system. The E×B filter (Wien filter) provided at the joint between
the primary and secondary electron optical systems can change the course of the
primary electron beam so that it is incident perpendicularly or normally on the
wafer surface.
The electron gun is further provided with a Wehnelt, triple anode lens and a
gun aperture. Thermal electron emitted from the cathode formed from LaB
6
are focused through the Wehnelt and triple anode lens onto the gun aperture as
a crossover image.
The primary electron optical system is further provided with a field aperture
for optimizing the area of the primary electron beam on the wafer, together with
an NA aperture. The primary electron beam whose angle of incidence on the lens
has been optimized by the field aperture is focussed by the primary electrostatic
lens system to form a light crossover image at the NA aperture before being planarly
irradiated to the wafer surface. The second stage of the primary system electrostatic
lens comprises quadrupole lenses (QL) arranged in three stages and one aperture
aberration correcting electrode. Quadrupole lenses require strict alignment accuracy
but exhibit strong focussing action in comparison to rotationally symmetric lenses.
The aberration of the quadrupole lenses, which correspond to the spherical aberration
of rotationally symmetric lenses, can be corrected by applying an appropriate voltage
to the aperture aberration correcting electrode. Thus, a uniform planar beam can
be applied to a predetermined region on the wafer surface.
The secondary electron optical system has an electrostatic lens (CL) and an intermediate
lens (TL), which correspond to an projector, a field aperture (FA), and a second-stage
lens (PL) provided on the detector side of the field aperture position. Thus, a
two-dimensional secondary electron image generated by the electron beam applied
to the wafer surface is formed at the field aperture position by the electrostatic
lens (CL) and the intermediate lens (TL), which correspond to an projector, and
projected as a magnified image by the projection lens (PL). This image-forming
optical system is called "secondary electron optical system".
It is preferable that a minus bias voltage (decelerating field voltage) should
be applied to the wafer. The decelerating electric field has the effect of decelerating
the incident (irradiation) beam, which minimizes the damage to the sample. In addition,
the decelerating electric field accelerates secondary electrons emitted from the
sample surface by the electric potential difference between the electrostatic lens
(CL) and the wafer, thereby effectively reducing chromatic aberration. The electrons
converged by the electrostatic lens (CL) are focused through the intermediate lens
(TL) to form a secondary electron image at the field aperture (FA). The image is
projected as a magnified image on the microchannel plate (MCP) through the projection
lens (PL). In this optical system, a numerical aperture NA is provided between
the electrostatic lens CL and the intermediate lens TL. The numerical aperture
NA is optimized to form an optical system capable of minimizing off-axis aberrations.
Further, an electrostatic octapole stigmator (STIG) is provided to correct
errors in the manufacture of the electron optical systems and the astigmatism and
anisotropic aberration of magnification introduced into the image by passing through
the E×B filter (Wien filter). Misalignment is corrected by using a deflector
(OP) disposed between each pair of adjacent lenses. Thus, it is possible to attain
an image projection optical system providing a uniform resolution in the field
of view.
The E×B deflector is a unit of an electromagnetic prism optical system in
which electrodes and magnetic poles are disposed in orthogonal directions so that
an electric field and a magnetic field intersect each other at right angles. The
E×B deflector can produce conditions (Wien conditions) under which when an
electromagnetic field is selectively applied to the field, an electron beam entering
the field from one direction is deflected, whereas an electron beam entering the
field from the opposite direction is allowed to travel straight owing to the fact
that the influence of force applied to the electron beam from the electric field
and the influence of force applied thereto from the magnetic field cancel each
other. Thus, the primary electron beam is deflected to be irradiated perpendicularly
or normally to the wafer surface, while the secondary electron beam is allowed
to travel straight toward the detector.
The secondary electron image from the wafer formed by the secondary electron
optical system is first amplified by the microchannel plate (MCP) and then converted
into a light image through the fluorescent screen. The principle of the MCP is
as follows. A bundle of several millions of extremely short electrically conductive
glass capillaries having a diameter of 6 to 25 μm and a length of 0.24 to
1.0 mm is shaped into a thin plate. When a predetermined voltage is applied to
the plate, each capillary operates as an independent secondary electron multiplier.
Thus, the microchannel plate forms a secondary electron amplifier as a whole.
The light image produced through conversion by the fluorescent screen is projected
onto the TDI-CCD as a 1× magnified image by a relay optical system placed
in the atmospheric air through a vacuum transmission window or by a relay optical
system also serving as a vacuum feedthrough optical system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the general arrangement of an image projection type
electron beam inspection system according to an embodiment of the present invention.
FIG. 2 is a horizontal sectional view showing the detailed arrangement of an
E×B deflector (i.e. an electron beam separator) in the electron beam inspection system.
FIG. 3 is a sectional view showing a vertical sectional structure taken along
the line A—A in FIG. 2.
FIG. 4 is a diagram showing the general arrangement of an image projection type
electron beam inspection system designed to apply a plurality of primary electron
beams to an observation region of a sample surface while the region being two-dimensionally
scanned with the electron beams.
FIG. 5 is a diagram for describing a method of irradiating primary electron
beams in the system shown in FIG. 4.
FIG. 6 is a diagram schematically showing the arrangement of an electron beam
inspection system according to another embodiment of the present invention.
FIG. 7 is a block diagram showing in more detail a controller of the electron
beam inspection system shown in FIG. 6.
FIG. 8 is a diagram showing a wafer inspection procedure.
FIG. 9 is a diagram showing the array of pixels of a line sensor.
FIG. 10 is a diagram showing the arrangement of an image projection type electron
beam inspection system according to the related art.
FIG. 11 is a diagram showing the arrangement of an image projection type electron
beam inspection system according to still another embodiment of the present invention.
FIG. 12 is a diagram schematically showing an image projection type electron
beam inspection system according to a further embodiment of the present invention.
FIGS. 13(A) and 13(B) are diagrams illustrating the operating principle of
a magnetic lens shown in FIG. 12.
FIG. 14 is a diagram showing an example of placement of the magnetic lens shown
in FIG. 12.
FIG. 15 is a diagram showing another example of placement of the magnetic lens
shown in FIG. 12.
FIG. 16 is a diagram schematically showing the arrangement of an electron beam
inspection system according to a further embodiment of the present invention.
FIG. 17 is a diagram showing the layout of a multiple optical column as a modification
of the single optical column of the electron beam inspection system shown in FIG. 16.
FIG. 18 is a graph showing an axial potential distribution when a voltage is
applied to each of electrodes and a sample.
FIG. 19 is a diagram showing an embodiment of a differential pumping structure
provided in a charged particle beam inspection system as an electron beam inspection
system according to the present invention.
FIG. 20 is a diagram showing a modification of the differential pumping structure
in which a high-purity inert gas outlet is directed toward the outer peripheral side.
FIG. 21 is a diagram showing another modification of the differential pumping
structure in which a vacuum chamber is provided in the differential pumping structure.
FIG. 22(
a) is a diagram showing another modification of the differential
pumping structure in which a member for height adjustment is provided on a stage,
the diagram illustrating a state where the optical column is positioned in the
vicinity of one end of the stage.
FIG. 22(
b) is a diagram illustrating the modification shown in FIG. 22(
a),
the diagram showing a state where the optical column is positioned in the vicinity
of the other end of the stage.
FIG. 23 is a diagram showing a modification of the charged particle beam system
according to the present invention in which a height-adjusting mechanism is provided
on the stage.
FIG. 24 is a diagram showing another modification of the differential pumping
structure in which the movable range of a sample moved by the stage is covered
with a container filled with an inert gas.
FIG. 25 is a diagram showing another modification of the differential pumping
structure in which the whole movable range of the stage is covered with a container
filled with an inert gas.
FIG. 26 is a diagram showing another modification of the differential pumping
structure in which a vacuum chamber is communicably connected to a container filled
with an inert gas.
FIG. 27 is a diagram showing another modification of the differential pumping
structure, the diagram illustrating an embodiment of an evacuation path.
FIG. 28 is a diagram showing another modification of the differential pumping
structure, the diagram illustrating a modification of the evacuation path.
FIG. 29 is a diagram showing another modification of the differential pumping
structure, the diagram illustrating an inert gas circulating path.
FIG. 30 is a diagram showing an arrangement in which a charged particle beam
system according to another embodiment of the present invention is applied to a
wafer defect inspection system.
FIG. 31 is a flowchart showing an embodiment of a semiconductor device fabrication
method according to the present invention.
FIG. 32 is a flowchart showing a lithography step, which is at the core of a
wafer processing step in FIG. 31.
FIG. 33 is a flowchart showing an inspection procedure according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An image projection type electron beam inspection system according to an embodiment
of the present invention will be described below specifically to clarify the relationship
between the principal functions of the image projection system and the overall
structure thereof.
FIG. 1 is a general view of the image projection type electron beam inspection
system according to this embodiment. It should be noted, however, that part of
the arrangement is omitted in the illustration.
In FIG. 1, the electron beam inspection system has a primary column
1,
a secondary column
2, and a chamber
3. An electron gun
4 is
provided in the primary column
1. A primary optical system
5 is disposed
on the optical axis of an electron beam (primary beam) emitted from the electron
gun
4. A stage
6 is installed in the chamber
3. A sample W
is placed on the stage
6.
Meanwhile, the secondary column
2 contains a cathode lens
8,
a numerical aperture
9, a Wien filter
10, a second lens
11,
a field aperture
12, a third lens
13, a fourth lens
14, and
a detector
15, which are disposed on the optical axis of a secondary beam
generated from the sample W. It should be noted that the numerical aperture
9
corresponds to an aperture stop, which is a thin plate of a metal (e.g. Mo) with
a circular hole. The numerical aperture
9 is disposed so that the aperture
portion thereof is coincident with the position where the primary beam is focused
and also coincident with the focus position of the cathode lens
8. Accordingly,
the cathode lens
8 and the numerical aperture
29 constitute a telecentric
electron optical system.
Meanwhile, the output of the detector
15 is input to a control
unit
16. The output of the control unit
16 is input to a CPU
17.
Control signals from the CPU
17 are input to a primary column controlling
unit
18, a secondary column controlling unit
19 and a stage driving
mechanism
7. The primary column controlling unit
18 controls the
voltage applied to the lenses of the primary optical system
5. The secondary
column controlling unit
19 controls the voltage applied to each of the cathode
lens
8 and the second to fourth lenses
11 to
14 and also controls
the electromagnetic field applied to the Wien filter
10.
The stage driving mechanism
7 transmits stage position information to
the CPU
17. The primary column
18, the secondary column
19
and the chamber
3 are connected to an evacuation system (not shown) and
evacuated by a turbo molecular pump or the like of the evacuation system so that
the inside of each of them is kept under vacuum conditions.
The primary beam from the electron gun
4 enters the Wien filter
10
while being subjected to the lens action of the primary optical system
5.
In this embodiment, the tip of the electron gun
4 is a rectangular cathode
formed from LaB
6, which allows a large electric current to be obtained.
The primary optical system
5 uses rotationally asymmetric quadrupole or
octopole electrostatic (or electromagnetic) lenses, which can cause convergence
and divergence along each of the X- and Y-axes. Such lenses are arranged in two
or three stages. By optimizing the conditions of each lens, the beam irradiation
region on the sample surface can be shaped into any desired rectangular or elliptical
configuration without loss of the incident electrons.
More specifically, when an electrostatic lens is used, four columnar rods (quadrupole)
are used. Mutually opposing electrodes are placed in an equipotential state and
given voltage characteristics opposite to each other.
It should be noted that the quadrupole lens is not necessarily limited to the
column-shaped lens but may be a lens having a configuration defined by dividing
a circular plate into four parts, which is generally used in an electrostatic deflector.
In this case, it is possible to reduce the size of the lens. The primary beam passing
through the primary optical system
5 is bent by the deflecting action of
the Wien filter
10. The Wien filter
10 has a magnetic field and an
electric field arranged to intersect each other at right angles and allows only
charged particles satisfying the Wien condition E=vB to travel straight. In the
Wien condition E=vB, E is the electric field, B is the magnetic field, and v is
the velocity of the charged particles. The Wien filter
10 bends the path
of the other charged particles. For the primary beam, force FB due to the magnetic
field and force FE due to the electric field act to bend the beam path. For the
secondary beam, the force FB and the force FE act in the opposite directions and
hence cancel each other. Accordingly, the secondary beam travels straight as it is.
The lens voltage for the primary optical system
5 is preset so that a
electron beam crossover is focussed at the aperture portion of the numerical aperture
9. In other words, Koehler illumination used in optical microscopes is realized.
The numerical aperture
9 eliminates an electron beam undesirably scattered
in the system from reaching the sample surface, thereby preventing charge-up and
contamination of the sample W.
When the primary beam is irradiated to the sample W, secondary electrons, reflected
electrons or back-scattered electrons are emitted as a secondary beam from the
beam-irradiated surface of the sample W.
The secondary beam passes through the cathode lens
8 while being subjected
to the lens action thereof.
Incidentally, the cathode lens
8 comprises three electrodes.
The lowermost electrode is designed to form a positive electric field with respect
to the sample W so that the secondary electrons are efficiently drawn into the
cathode lens
8.
The lens action is produced by applying a voltage to the first and second electrodes
of the cathode lens
8 and placing the third electrode at zero potential.
Meanwhile, the numerical aperture
9 is placed at the focus position of the
cathode lens
8, that is, at the back focus position from the sample W. Accordingly,
a beam of electrons emitted from positions off the field center (i.e. off-axis
points) is also formed into a parallel beam and passes through the center of the
numerical aperture
9 without being eclipsed.
It should be noted that the numerical aperture
9 also serves to suppress
lens aberrations introduced into the secondary beam from the second to fourth lenses
11 to
14. The secondary beam passing through the numerical aperture
9 travels straight through the Wien filter
10 without being subjected
to the deflecting action of the Wien filter
10.
If the secondary beam is focused to form an image by the cathode lens
8
alone, chromatic aberration of magnification and distortion are likely to occur.
Therefore, the cathode lens
8 is combined with the second lens
11
to perform first image formation. The secondary beam forms an intermediate image
at the field aperture
12 through the cathode lens
8 and the second
lens
11. In this case, it is generally likely that the magnifying power
necessary for the secondary optical system will become insufficient. Therefore,
the third lens
13 and the fourth lens
14 are added to magnify the
intermediate image. The third and fourth lenses
13 and
14 are arranged
so that a magnifying image-forming operation takes place when the secondary beam
passes through each of the third and fourth lenses
13 and
14. That
is, a total of three image-forming operations take place in this optical system.
It should be noted that the third lens
13 and the fourth lens
14
may be arranged to perform one image-forming operation in combination (i.e. a total
of two image-forming operations take place in the optical system).
The second to fourth lenses
11 to
14 are all rotationally symmetric
lenses (or lenses symmetric with respect to the optical axis) known as "unipotential
lenses" or "Einzel lenses". Each lens comprises three electrodes. Normally, two
outer electrodes are placed at zero potential, and a voltage is applied to the
central electrode to effect lens action under control. The field aperture
12
is disposed at a point where the intermediate image is formed. The field aperture
12 limits the visual field to a necessary range as in the case of the field
stop of an optical microscope. The field aperture
12 also serves to eliminate
an unnecessary electron beam in cooperation with the third lens
13 and the
fourth lens
14 provided in the subsequent stage, thereby preventing the
generation of noise in the detector
15 and contamination thereof. It should
be noted that the magnifying factor is set by varying the lens conditions (focal
length) of the third and fourth lenses
13 and
14.
The secondary beam is projected by the secondary optical system to form a magnified
image on the detecting surface of the detector
15. The detector
15
includes a microchannel plate (MCP) for multiplying electrons and a fluorescent
screen for converting the electrons into light. The detector
15 further
includes a lens and other optical system for relaying and transmitting the optical
image from the vacuum system to the outside, together with an image pickup device
(e.g. a CCD). The secondary beam is focused onto the MCP detecting surface and
multiplied by the MCP. The multiplied electrons are converted into a light signal
by the fluorescent screen and then converted into a photoelectric signal by the
image pickup device.
The control unit
16 reads the sample image signal from the detector
15
and transmits it to the CPU
17. The CPU
17 inspects the pattern on
the sample W for defects on the basis of the image signal by template matching
or the like. The stage
6 is movable in the X- and Y-directions by the stage
driving mechanism
7. The CPU
17 reads the position of the stage
6
and outputs a drive control signal to the stage driving mechanism
7 to drive
the stage
6, thereby sequentially performing the detection and inspection
of the image are carried out.
Regarding the secondary beam, all the principal rays from the sample W
are incident perpendicularly or normally on the cathode lens
8 (i.e. in
parallel to the optical axis of the lens
8) to pass through the numerical
aperture
9. Therefore, marginal rays of light are not eclipsed. Hence, there
is no reduction in the image luminance at the peripheral portion of the sample
W. Usually, variations in the electron energy cause the image formation position
to differ. That is, chromatic aberration of magnification (chromatic difference
of magnification) occurs (in particular, the secondary electrons suffer large chromatic
aberration of magnification because of large variations in energy). However, the
chromatic aberration of magnification can be suppressed by placing the numerical
aperture
9 at the focus position of the cathode lens
8.
Because the change of the magnifying power is made after the secondary electrons
have passed through the numerical aperture
9, even if the magnifications
of the third and fourth lenses
13 and
14, which have been set as
lens conditions, are varied, it is possible to obtain a uniform image over the
whole field of view on the detector side. Although a variation-free, uniform image
can obtained in this embodiment, when the magnifying factor is raised, usually,
the brightness of the image lowers unfavorably. To solve this problem, the lens
conditions of the primary optical system are set so that when the magnifying factor
is changed by varying the lens conditions of the secondary optical system, the
effective visual field on the sample surface, which is determined by the magnifying
power, and the electron beam irradiated to the sample surface have the same size.
More specifically, as the magnification is raised, the visual field becomes
narrower. However, if the electron beam irradiation energy density is increased
at the same time as the magnification is raised, the detected electron signal density
is kept constant at all times even if the secondary electrons are projected as
a magnified image by the secondary optical system. Thus, the brightness of the
image will not lower.
Although the electron beam inspection system in this embodiment uses the
Wien filter
10 that bends the path of the primary beam but allows the secondary
beam to travel straight, the present invention is not necessarily limited to the
described arrangement. The electron beam inspection system may use a Wien filter
that allows the primary beam to travel straight but bends the path of the secondary
beam. Further, in the foregoing embodiment, a rectangular beam is formed by using
a rectangular cathode and a quadrupole lens system. However, the present invention
is not necessarily limited to the described arrangement. For example, a rectangular
beam or an elliptical beam may be produced from a circular beam. Alternatively,
a rectangular beam may be formed by passing a circular beam through a slit.
The structure of the electron beam deflector
10 operating as a Wien filter,
i.e. an E×B deflector, will be described below in detail with reference to
FIG. 2 and FIG. 3, which shows a vertical section taken along the line A—A
in FIG. 2. As shown in FIG. 2, the field of the electron beam deflector
10
has a structure in which an electric field and a magnetic field are placed to intersect
each other at right angles in a plane perpendicular to the optical axis of an image
projection optical part (i.e. a part in which a one- or two-dimensional image of
secondary electrons and reflected electrons emitted according to the conditions
of the sample surface when an electron beam is applied to the sample is formed
on the electron beam detector). That is, the electron beam deflector
10
has an E×B structure.
In the electron beam deflector
10, the electric field is generated by
electrodes
10-
1 and
10-
2 each having a concavely curved surface.
The electric field generated by the electrodes
10-
1 and
10-
2
is controlled by control units
10a and
10d. Meanwhile,
electromagnetic coils
10-
1a and
10-
2a are
disposed to face each other in a direction perpendicular to the direction in which
the electric field-generating electrodes
10-
1 and
10-
2
face each other, thereby generating a magnetic field. It should be noted that the
electric field-generating electrodes
10-
1 and
10-
2
are in point symmetry (the electrodes
10-
1 and
102 may be
in concentric relation to each other).
In this case, in order to improve the uniformity of the magnetic field, pole
pieces
having a plane-parallel plate configuration are provided to form a magnetic path.
The behavior of the electron beam in the vertical section taken along the line
A—A is as shown in FIG. 3. The incident electron beam
1a is
deflected by the electric field generated by the electrodes
10-
1
and
10-
2 and the magnetic field generated by the electromagnetic
coils
10-
1a and
10-
2a. Thereafter, the
electron beam
1a is incident on the sample surface in a direction
perpendicular thereto.
The position through which the incident electron beam
1a enters
the electron beam deflector
10 and the incident angle of the electron beam
1a are uniquely determined when the electron energy is determined.
Further, the electric field generated by the electrodes
10-
1 and
10-
2 and the magnetic field generated by the electromagnetic coils
10-
1a and
10-
2a are controlled by the
control units
10a and
10d and the control units
10c
and
10b, respectively, so as to satisfy the condition for the
electric and magnetic fields, i.e. vB=E, so that secondary electrons
2a
travel straight. Consequently, the secondary electrons
2a travel
straight through the electron beam deflector
10 to enter the image projection
optical part. In the above condition vB=E, v is the velocity (m/s) of the electrons
2a, B is the magnetic field (T), and E is the electric field (V/m).
Next, another embodiment of the defect inspection system utilizing the image
projection system will be described.
The defect inspection system utilizing the image projection system involves the
following problems: {circle around (1)} because an electron beam is applied to
the whole observation region of the sample surface by one shot, charge-up is likely
to occur on the sample surface; and {circle around (2)} there is a limit to the
electron beam current obtained by this system (about 1.6 μA), which is an
obstacle to improvement in the inspection speed.
In this embodiment, a plurality of primary electron beams are used, and the primary
electron beams are applied to an observation region on the sample surface while
the observation region is scanned with the primary electron beams two-dimensionally
(X- and Y-directions; i.e. while performing raster scanning). Further, the image
projection system is adopted for the secondary electron optical system. With this
arrangement, the above-described problems can be solved. The defect inspection
system according to this embodiment has the above-described advantages of the image
projection system. In addition, scanning with a plurality of primary electron beams
makes it possible to solve the problems of the image projection system: {circle
around (1)} because an electron beam is applied to the whole observation region
of the sample surface by one shot, charge-up is likely to occur on the sample surface;
and {circle around (2)} there is a limit to the electron beam current obtained
by this system (about 1.6 μA), which is an obstacle to improvement in the
inspection speed. That is, because the electron beam irradiation point moves, it
is easy for the electric charge to escape. Consequently, charge-up reduces. The
required electric current value can be readily increased by increasing the number
of electron beams used. In this embodiment, when four primary electron beams are
used, for example, if the electric current of one electron beam is 500 nA (electron
beam diameter: 10 μm), an electric current of 2 μA is obtained in total.
The number of primary electron beams can readily be increased to about 16 electron
beams. In this case, 8 μA can be obtained in theory. To scan a sample with
a plurality of primary electron beams, not only the above-described raster scanning
but also other types of scanning, e.g. Lissajous's figure scanning, can be used
as long as the plurality of primary electron beams can be applied so that the amount
of irradiation with the electron beams is uniform throughout the irradiated region.
Accordingly, the directing direction of the stage for scanning is not necessarily
limited to the direction perpendicular to the directing direction of the plurality
of electron beams for scanning.
As an electron beam source in this embodiment, it is possible to use a thermal
electron beam source (in which electrons are emitted by heating an emissive material).
In this case also, it is preferable to use LaB
6 as an emissive (emitter)
material. Other materials are also usable, provided that they have a high melting
point (the vapor pressure at high temperatures is low) and a low work function.
To obtain a plurality of electron beams, two methods are available. One is a method
wherein a single electron beam is obtained from a single emitter (with a single
projection) and passed through a thin plate (aperture plate) provided with a plurality
of holes, thereby obtaining a plurality of primary electron beams. The other is
a method wherein a plurality of primary electron beams are drawn out directly from
a plurality of tips formed on a single cathode material. Either of the methods
makes use of the property of the electron beam that it is readily emitted from
the tip of a projection. It is also possible to use other types of electron beam
sources, e.g. a thermal electric field emission type electron beam source. The
thermal electric field emission type electron beam source is a system in which
electrons are emitted by applying a high electric field to an emissive material,
and the emission of electrons is stabilized by heating its electron beam emitting part.
Next, the above-described embodiment, in which a plurality of primary electron
beams are irradiated to an observation region on the sample surface while the region
is scanned with the electron beam two-dimensionally (X- and Y-directions; i.e.
while performing raster-scanning), and the image projection system is adopted for
the secondary electron optical system, will be described in more detail with reference
to FIGS. 4 and 5.
In the following embodiment, a method wherein a plurality of primary electron
beams are drawn out directly from a plurality of tips formed on a single emitter
is adopted as a method of obtaining a plurality of primary electron beams.
As shown in FIG. 4, four electron beams
21 (
21-
1,
21-
2,
21-
3 and
21-
4) emitted from an electron gun
20
are shaped by an aperture
50-
1 and passed through lenses
22-
1
and
22-
2 arranged in two stages to form an elliptical image having
a size of 10 μm by 12 μm on the deflection center plane of a Wien filter
23. Raster-scanning is performed by a deflector
26 in a direction
perpendicular to the plane of the figure so that the four electron beams is formed
into an image so as to uniformly cover a rectangular region of 1 mm by 0.25 mm
as a whole. The electron beams deflected by the E×B deflector
23 serving
as a Wien filter form a crossover at the numerical aperture NA. The beams thus
formed are reduced to ⅕ by a lens
24 and projected onto the surface
of a sample W so as to cover a region of 200 μM by 50 μm and be applied
perpendicularly or normally to the sample surface. Consequently, four secondary
electron beams
25 carrying pattern image (sample image F) information are
emitted from the sample W. The electron beams
25 are magnified through the
lenses
24,
27-
1 and
27-
2 and focused onto an
MCP
28-
1 as a rectangular image (magnified projected image F′)
composed of the four electron beams
25. The magnified projected image F′
formed from the secondary electron beams
25 is intensified 10,000 times
by the MCP
28-
1 and converted into light by a fluorescent part
28-
2.
The light thus obtained is converted into an electric signal synchronized with
the continuously moving speed of the sample W by a TDI (Time Delay Integration)-CCD
29. The electric signal is acquired as a continuous image by an image display
unit
30 and output onto a CRT or the like.
The electron beam irradiation unit needs to irradiate the sample surface with
the electron beam in a rectangular configuration as uniformly as possible and with
minimized variations of irradiation. In order to increase throughput, it is necessary
to apply the electron beam to the region to be irradiated with an increased electric
current. Conventional systems suffer electron beam irradiation variations of the
order of ±10% and hence large image contrast variations. Further, in the conventional
systems, the electron beam current is as small as about 500 nA at the irradiated
region. Therefore, high throughput cannot be obtained. In addition, because in
this type of system a wide image observation region is entirely irradiated with
an electron beam by one shot, an imaging trouble due to charge-up is more likely
to occur than in the case of the scanning electron microscope (SEM) system.
The primary electron beam irradiation method according to this embodiment is
shown in FIG. 5. The combination of electron beams
21 consists of four electron
beams
21-
1,
21-
2,
21-
3 and
21-
4.
Each beam has an elliptical sectional configuration with a size of 2 μm by
2.4 μm and a rectangular region of 200 μm by 12.5 μm is raster-scanned
with each beam in such a manner that the raster-scanned regions do not overlap
each other. Thus, a rectangular region of 200 μm by 50 μm as a whole
is irradiated with the four electron beams
21-
1,
21-
2,
21-
3 and
21-
4. The beam at a point
21-
1
arrives at
21-
1′ in a finite time and then returns to a point
directly below
21-
1 (toward
21-
2), which is away from
21-
1 by a distance corresponding to the beam spot diameter (10 μm),
with substantially no loss of time. Then, the beam moves parallel to the path
21-
1
to
21-
1′, to a point directly below
21-
1′
(toward
21-
2′) again in the same finite time as the above.
This is repeated to scan ¼ (200 μm by 12.5 μm) of the rectangular
irradiation region indicated by the dotted line in the figure. Thereafter, the
beam returns to the starting point
21-
1. This operation is repeated
at high speed. The other electron beams
21-
2 to
21-
4
repeatedly are directed for scanning in the same way and at the same speed as the
electron beam
21-<
