U.S. patent application number 15/526676 was filed with the patent office on 2017-11-02 for sample stage.
The applicant listed for this patent is Phenom-World Holding B.V.. Invention is credited to Adrianus Franciscus Johannes Hammen, Ton Antonius Cornelis Henricus Kluijtmans, Gerhardus Bernardus Stamsnijder, Sander Richard Marie Stoks, Paul Cornelis Maria van den Bos, Karel Diederick van der Mast.
Application Number | 20170316913 15/526676 |
Document ID | / |
Family ID | 52355148 |
Filed Date | 2017-11-02 |
United States Patent
Application |
20170316913 |
Kind Code |
A1 |
Stamsnijder; Gerhardus Bernardus ;
et al. |
November 2, 2017 |
Sample Stage
Abstract
Sample stage, e.g. for use in a scanning electron microscope.
The sample stage includes a base, a sample carrier, and an actuator
assembly arranged for moving the sample carrier in at least one
direction substantially parallel to the base. The actuator assembly
is arranged so as not to contribute to the mechanical stiffness of
the sample stage from the sample carrier to the base.
Inventors: |
Stamsnijder; Gerhardus
Bernardus; (Eindhoven, NL) ; van den Bos; Paul
Cornelis Maria; (Eindhoven, NL) ; Kluijtmans; Ton
Antonius Cornelis Henricus; (Eindhoven, NL) ; Stoks;
Sander Richard Marie; (Eindhoven, NL) ; Hammen;
Adrianus Franciscus Johannes; (Eindhoven, NL) ; van
der Mast; Karel Diederick; (Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Phenom-World Holding B.V. |
Eindhoven |
|
NL |
|
|
Family ID: |
52355148 |
Appl. No.: |
15/526676 |
Filed: |
November 11, 2015 |
PCT Filed: |
November 11, 2015 |
PCT NO: |
PCT/NL2015/050790 |
371 Date: |
May 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/28 20130101;
H01J 2237/204 20130101; H01J 2237/20228 20130101; H01J 2237/2006
20130101; H01J 2237/28 20130101; H01J 2237/20221 20130101; H01J
2237/0216 20130101; H01J 37/18 20130101; H01J 2237/20235 20130101;
H01J 37/185 20130101; H01J 37/20 20130101 |
International
Class: |
H01J 37/20 20060101
H01J037/20; H01J 37/18 20060101 H01J037/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 12, 2014 |
NL |
2013783 |
Claims
1-35. (canceled)
36. A sample stage including a base, a sample carrier, and an
actuator assembly arranged for moving the sample carrier in at
least one direction substantially parallel to the base.
37. The sample stage of claim 36, wherein the actuator assembly
does not contribute to the mechanical stiffness of the sample stage
from the sample carrier to the base.
38. The sample stage of claim 36, including a two-dimensional slide
bearing arranged for allowing the sample carrier to slide in two
different directions in a plane parallel to the base.
39. The sample stage of claim 36, wherein the actuator assembly is
connected to the sample carrier through a connection that is
flexible in a direction orthogonal to the base.
40. The sample stage of claim 39, wherein the sample carrier is
positioned against the base.
41. The sample stage of claim 40, wherein the sample carrier is
slidingly positioned against the base.
42. The sample stage of claim 41, wherein the sample carrier
includes at least one sliding surface.
43. The sample stage of claim 36, wherein the actuator assembly is
further arranged for rotating the sample carrier about an axis that
is orthogonal to the base, and wherein the slide bearing is further
arranged for allowing the sample carrier to rotate about the axis
orthogonal to the base.
44. The sample stage of claim 36, wherein the connection of the
actuator assembly to the sample carrier is substantially rigid in
the direction for moving the sample carrier.
45. The sample stage of claim 36, wherein the actuator assembly is
positioned beside the sample carrier.
46. The sample stage of claim 36, wherein the sample carrier
includes a sample holder for holding a sample.
47. The sample stage of claim 36, wherein the sample carrier is
positioned within a vacuum chamber having a loading door, and
wherein the sample carrier includes a sealing element positioned
such that the sample carrier can be sealingly pressed against a
wall of the vacuum chamber so as to allow access to the sample
carrier via the loading door.
48. The sample stage of claim 36, wherein the sample carrier
includes a bottom and a circumferential wall enclosing a cavity for
holding a sample, and a sealing member, wherein the sealing member
is positioned on the circumferential wall.
49. The sample stage of claim 36 wherein the sample stage is
positioned within a vacuum chamber having a loading door, the
sample carrier being movable in a positioning direction
substantially parallel to the base, the sample carrier further
being movable towards the loading door in a loading direction,
different from the positioning direction.
50. The sample stage of claim 49, wherein the loading direction is
orthogonal to, or has a component orthogonal to, the base.
51. A vacuum system including a vacuum chamber having a loading
door and a sample stage according to claim 36, wherein the sample
carrier is positioned within the vacuum chamber.
52. The vacuum system of claim 51, wherein the actuator assembly is
positioned inside the vacuum chamber.
53. The vacuum system of claim 53, wherein the vacuum chamber
includes a pushing device for sealingly pressing the sample carrier
against the wall of the vacuum chamber.
54. The vacuum system of claim 53, wherein the pushing device
includes bellows.
55. The vacuum system of claim 53, wherein the pushing device is
operated by vacuum and/or ambient air pressure.
56. A scanning electron microscope including a vacuum system
according to claim 51.
57. The scanning electron microscope of claim 56, designed as a
desktop scanning electron microscope.
58. The scanning electron microscope of claim 56, further including
an electron optical column connected to the vacuum chamber, wherein
the base is or is connected to a wall of the vacuum chamber
opposite the electron optical column.
59. The scanning electron microscope of claim 56, further including
an electron optical column connected to the vacuum chamber, wherein
the base is or is connected to a wall of the vacuum chamber to
which the electron optical column is connected.
60. A method for positioning a sample carrier including the steps
of: providing a base; positioning a sample carrier positioned on
the base; and moving the sample carrier in a direction
substantially parallel to the base using an actuator connected to
the sample carrier through a connection that is flexible in a
direction orthogonal to the base and/or using a two-dimensional
slide bearing arranged for allowing the sample carrier to slide in
two different directions in a plane parallel to the base.
Description
FIELD OF THE INVENTION
[0001] The present invention concerns a sample stage. More
specifically the invention relates to a sample stage for use in an
electron microscope. The invention also relates to a vacuum system,
such as for use in an electron microscope. The invention also
relates to an improved load lock.
BACKGROUND TO THE INVENTION
[0002] Sample stages are commonly used for moving a sample relative
to a reference position. The reference position can e.g. be related
to a field of view of a microscope such as a scanning electron
microscope. The reference position can e.g. be an optical axis or a
beam position. The sample stage allows positioning and
repositioning of the sample relative to the reference position,
e.g. to allow inspection of certain features of the sample, or to
allow inspection of a surface area larger than the field of
view.
[0003] A load lock forms a port between the inside atmosphere of an
apparatus, such as an electron microscope, and the outside
atmosphere. Load locks are commonly used when samples are inspected
under modified atmosphere, e.g. in a vacuum, for allowing multiple
samples to be inspected sequentially while minimizing upsetting of
the modified atmosphere during loading and unloading of the
subsequent samples.
SUMMARY OF THE INVENTION
[0004] In general it is desirable to be able to position a sample
relative to a reference position, e.g. by moving a sample carrier
relative to a base by means of a sample stage. The base can be
fixedly positioned relative to the reference position. The sample
can be positioned in X direction, and optionally in Y direction,
and optionally in Z direction. It is also possible to,
additionally, position the sample in rotational directions, such as
a rotational direction R around an upright axis, and tilt
directions T1 and T2. Herein X and Y represent two orthogonal
directions parallel to the plane of the base, and Z is a direction
orthogonal to X and Y. The tilt directions T1 and T2 normally
represent rotations around axes that are orthogonal to the upright
axis of rotation R, and that are mutually orthogonal.
[0005] When positioning a sample accuracy of the position and
stability of maintaining the position is of great importance. It is
often undesirable that the sample moves, e.g. vibrates within the
field of view during observation of the sample. Thereto, the
ability of the sample stage to maintain the position of a sample,
positioned on a sample carrier of the sample stage, is of great
importance. One important aspect herein is the mechanical stiffness
of the sample stage from the sample carrier to the base. It is
known that displacement mechanisms, e.g. guide rails (e.g. for
positioning the sample carrier in X, Y and/or Z directions), or
rotation mechanisms (e.g. for rotating the sample carrier in R, T1
and/or T2 directions), can reduce the overall mechanical stiffness
of the sample stage. In the past much effort has been put into
designing displacement mechanisms and rotation mechanisms with high
mechanical stiffness.
[0006] Common sample stage designs use an X-stage, i.e. a
displacement mechanism for linear displacement in the X direction,
mounted to the base, a Y-stage, i.e. a displacement mechanism for
linear displacement in the Y direction stacked on top of the
X-stage, and a sample carrier stacked on top of the Y-stage. The
inventors realized that such stacked design can significantly
reduce the overall stiffness of the sample stage. This is
exacerbated when the sample stage includes movement mechanisms for
more degrees of freedom, e.g. X, Y, Z, R, T1 and T2 in stacked
design. In accordance with an aspect of the invention there is
provided a sample stage including a base, a sample carrier, an
actuator assembly arranged for moving the sample carrier in two
different directions substantially parallel to the base. The sample
stage includes a two-dimensional slide bearing arranged for
allowing the sample carrier to slide in two different directions in
a plane parallel to the base. This provides the advantage that the
two different movements of the sample carrier relative to the base
are made possible with a single two-dimensional slide bearing,
which significantly increases stiffness of the sample stage from
the sample carrier to the base. The two different directions
substantially parallel to the base can e.g. be a translation and a
rotation, or two orthogonal translations. It will be clear that
moving the sample carrier in two orthogonal directions
substantially parallel to the base can be done by using two
orthogonal actuators, e.g. a first actuator acting in the X
direction and a second actuator acting in the Y direction. It is
also possible that moving the sample carrier in two orthogonal
directions substantially parallel to the base is done by using two
non-orthogonal actuators, e.g. a first actuator acting in the
X-direction, and a second actuator at 45 degrees to the X-axis. It
is also possible that moving the sample carrier in two orthogonal
directions substantially parallel to the is base done by using a
linear and a rotational actuator, e.g. a first actuator acting in
the R-direction, and a second actuator acting in the radial
direction. Optionally, the actuator assembly is further arranged
for rotating the sample carrier about an axis that is orthogonal to
the base, and the two-dimensional slide bearing is further arranged
for allowing the sample carrier to rotate about the axis orthogonal
to the base.
[0007] In accordance with an aspect of the present invention there
is provided a sample stage including a base, a sample carrier
positioned on the base and an actuator assembly arranged for moving
the sample carrier in a direction substantially parallel to the
base. The actuator assembly is positioned such that it does not
contribute to the mechanical stiffness of the sample stage from the
sample carrier to the base. The actuator assembly can be connected
to the sample carrier through a connection that is flexible in a
direction orthogonal to the base. The inventors realized that
rather than trying to increase the stiffness of the actuator
assembly, such as an X-stage and Y-stage, it is beneficial to
connect the actuator assembly, such as the X-stage and/or the
Y-stage, to the sample carrier through a connection that is
flexible in a direction orthogonal to the base. This flexibility
allows for the sample carrier to freely move relative to the
actuator in the direction orthogonal to the base. This in turn
allows the sample carrier to abut against the base, e.g. by gravity
or another biasing force, such as a spring force, a magnetic force,
a pneumatic force, or the like. This also allows the position
actuator assembly, such as the X-stage and the Y-stage, to be not
interposed between the sample carrier and the base. Hence, the
mechanical stiffness of the actuator assembly, such as the X-stage
and the Y-stage, effectively plays no role in the mechanical
stiffness of the sample stage from the sample carrier to the
base.
[0008] It will be appreciated that herein the flexibility of the
connection can be achieved by resilience: the connection can e.g.
include a leaf spring extending substantially parallel to the base
or a living hinge. It will be appreciated that herein the
flexibility of the connection can alternatively, or additionally,
be achieved by articulated connection: the connection can e.g.
include a hinged connection. It will be appreciated that herein the
flexibility of the connection can alternatively, or additionally,
be achieved by play: the connection can e.g. include play in a
direction orthogonal to the base.
[0009] According to an aspect of the invention, the connection of
the actuator to the sample carrier is substantially rigid in the
direction for moving the sample carrier. Optionally, the connection
of the actuator to the sample carrier is substantially rigid in the
direction for moving the sample carrier in both senses (e.g.
forward and backward, left and right, positive X direction and
negative X direction, positive Y direction and negative Y
direction). Hence, the actuator assembly can accurately position
the sample carrier in the moving direction. The person skilled in
the art can easily determine the required rigidity for achieving a
desired positioning accuracy, taking into account positioning
speeds, accelerations, friction and the like. Alternatively, or
additionally, the sample stage may include a position sensor for
determining a position of the sample carrier relative to the base.
The position sensor may e.g. determine a position in the X
direction and in the Y direction. It will be appreciated that when
the position of the sample carrier is being determined it can be
allowed that the connection of the actuator assembly to the sample
carrier in the direction for moving the sample carrier is less
rigid since positioning accuracy can be achieved through closed
loop control on the basis of the determined position.
[0010] Optionally, the actuator assembly is positioned beside the
sample carrier. This provides the advantage that the actuator
assembly is not interposed between the sample carrier and the base.
Moreover, this allows for simple mechanical layout of the sample
stage.
[0011] Optionally, the sample carrier is positioned against the
base. When the sample carrier abuts against the base, a high
mechanical stiffness of the sample stage from the sample carrier to
the base can be realized.
[0012] Optionally, the sample carrier is slidingly positioned
against the base. This provides the advantage that the sample
carrier can easily be moved relative to the base in directions
parallel to the base, while maintaining a high mechanical stiffness
of the sample stage from the sample carrier to the base. Thereto,
the sample carrier can include at least one sliding surface, e.g.
sliding feet. Also the base may be provided with a sliding surface,
such as a smooth surface.
[0013] The sample carrier can e.g. be slidingly moved in a first
direction substantially parallel to the base. Possibly the sample
carrier can also be slidingly moved in a second direction
substantially parallel to the base, wherein the second direction is
different from the first direction, e.g. orthogonal to the first
direction. Possibly the sample carrier can be slidingly rotated
around an axis that is orthogonal to the base. More in general, the
actuator assembly is arranged for moving the sample carrier in a
first direction substantially parallel to the base and optionally
in a second and/or third direction substantially parallel to the
base, wherein the first direction is different from the second
and/or third direction.
[0014] According to an aspect of the invention the sample carrier
includes a sample holder for holding a sample. Optionally the
sample holder is a replaceable sample holder. This provides the
advantage that the sample can be prepared and mounted to the sample
holder while the sample holder is not in or on the sample carrier.
Hence a plurality of sample holders can be prepared and exchanged
at will.
[0015] According to an aspect of the invention the sample carrier
is positioned within a vacuum chamber having a loading door. This
provides the advantage that sample can be loaded onto or off the
sample carrier through the loading door. This allows for rapid
exchange of the sample by another sample. It is for instance
possible to unload a first sample holder holding a first sample
through the loading door and to load a second sample holder holding
a second sample through the loading door into or onto the sample
carrier.
[0016] The invention also relates to an improved load lock. Thereto
according to the invention is provided a sample stage arranged to
be positioned within a vacuum chamber having a loading door. The
sample stage includes a base, a sample carrier, and an actuator
assembly arranged for moving the sample carrier in a positioning
direction substantially parallel to the base. Hence the sample
carrier can be moved substantially parallel to the base, e.g. for
moving a sample into and out of a field of view, and/or for
positioning a sample relative to a reference position. The sample
carrier is further movable towards the loading door in a loading
direction, different from the positioning direction. The loading
direction can be orthogonal to the base or have a component
orthogonal to the base. The sample carrier can be pressed against a
wall of the vacuum chamber, in sealing engagement, so as to allow
access to the sample carrier via the loading door. It is possible
that the actuator assembly for moving the sample carrier in a
direction substantially parallel to the base is connected to the
sample carrier through a connection that is flexible in a the
loading direction, e.g. in a direction orthogonal to the base.
[0017] Optionally, the sample carrier includes a sealing element
positioned such that the sample carrier can be sealingly pressed
against a wall of the vacuum chamber so as to allow access to the
sample carrier via the loading door.
[0018] Optionally, the sample carrier includes a bottom and a
circumferential wall enclosing a cavity for holding a sample or a
sample holder, wherein the sealing member is positioned on the
circumferential wall, e.g. on a leading edge of the circumferential
wall. The sealing member can e.g. be an O-ring.
[0019] The movement of the sample carrier in the loading direction
can be effected by means of a pushing device for sealingly pressing
the sample carrier against the wall of the vacuum chamber. The
pushing device can e.g. include bellows. The pushing device can be
operated electrically, magnetically, hydraulically or
pneumatically, e.g. using compressed air. Optionally, the pushing
device is operated by vacuum and/or ambient air pressure.
[0020] The invention also relates to a vacuum system including a
vacuum chamber having a loading door and at least one sample stage
as described hereinabove, wherein the sample carrier is positioned
within a vacuum chamber. Optionally, the actuator assembly is
positioned inside the vacuum chamber.
[0021] The invention also relates to a scanning electron microscope
including such vacuum system. The scanning electron microscope can
e.g. be designed as a desktop scanning electron microscope.
[0022] The scanning electron microscope further includes an
electron optical column connected to the vacuum chamber, wherein
the base is or is connected to a wall of the vacuum chamber
opposite the electron optical column. Alternatively, or
additionally, the base is or is connected to a wall of the vacuum
chamber to which the electron optical column is connected.
[0023] The invention also relates to a method for positioning a
sample carrier. The method includes providing a base and
positioning a sample carrier positioned on the base. The method
includes moving the sample carrier in a direction substantially
parallel to the base using an actuator assembly connected to the
sample carrier through a connection that is flexible in a direction
orthogonal to the base.
[0024] The invention also relates to a method for loading a sample
into a vacuum chamber. The method includes moving a sample holder
towards a preloading position adjacent to a loading door of the
vacuum chamber in a first moving direction. The method includes
sealingly pressing the sample holder against the wall of the vacuum
chamber surrounding the loading door in a second direction, wherein
the second direction is different from the first direction. The
second direction can be orthogonal to, or have a component
orthogonal to, the first direction. The method includes opening the
loading door.
[0025] The invention also relates to a system for displaying a live
microscope image, such as a scanning electron microscope image. The
system includes a processor having a first input for receiving live
images. The system includes an output for outputting live images to
a display device. It will be appreciated that the output live
images may be slightly lagging behind the received live images. The
time lag is preferably sufficiently small to allow visual feedback
of sample manipulation to an operator. Preferably, the time lag is
less than 1 second, more preferably less than 0.5 second, most
preferably less than 0.1 second. The processor is arranged for
generating a live image of a first type from a first number of
recent images received at the first input. The first number is
preferably larger than one, e.g. two, four, eight, sixteen,
twenty-four, thirty-two, or any other number. The processor is
further arranged for outputting the live image of the first type to
the output.
[0026] The live image of the first type can e.g. be an averaged
image, averaged from the first number of most recent images
received at the first input. Such averaged image can be output as
improved live image.
[0027] Generating the live image of the first type from the first
number of recent images received at the first input provides the
advantage of providing sharper images, better contrast, less noise,
etc. For example, a moving average, using a predetermined number of
images to be averaged, can be used to allow the output averaged
image to adapt when a region of interest changes, e.g. when the
sample is moved relative to the microscope for observing a
different portion of the sample. The inventors found, however, that
such adapting of the moving averaged output image can be slow,
which upsets user experience and may inhibit searching for
artefacts on a sample surface.
[0028] In order to enhance the user experience, and to increase
reaction speed of the system when displaying live images during
changes to a region of interest, the inventors realized that it is
beneficial that the processor includes a second input for receiving
process information representative of changes to a condition of the
microscope, and that the processor is further arranged for
automatically, in response to receiving on the second input an
indication of a change to the condition of the microscope, switch
to outputting to the output a live image of a second type based on
a second number of recent images received at the first input, the
second number being smaller than the first number. Since the live
image of the second type is based on a smaller number of recent
images received at the first input effects of the change to the
condition of the microscope, such as motion artefacts, to the live
image of the second type will be smaller than to the live image of
the first type. Conversely, since the live image of the second type
is based on a smaller number of recent images received at the first
input image improvement of the live image of the second type will
be smaller than image improvement of the live image of the first
type.
[0029] In a special embodiment, the second number is one. In that
case each image of the live image of the second type is based on a
single image received at the first input. Hence, in that case the
live image of the second type substantially corresponds to the live
image received at the first input. It will be appreciated that
nevertheless the processor may apply image improvement techniques
such as for example speckle removal, contrast/brightness
enhancement or the like.
[0030] If the live image of the first type is a moving average
based on the first number of received images, then the live image
of the second type can be obtained by resetting the moving
averaging.
[0031] The live image of the first type may e.g. be obtained by
Kalman filtering, wherein the live image of the first type to be
output at the output is calculated by multiplying the image most
recently received at the first input by a Kalman gain, Kk,
(0<K.sub.k.ltoreq.1) and adding the immediately preceding output
image multiplied by 1-K.sub.k. The live image of the second type
can then be obtained by Kalman filtering using a larger Kalman gain
Kk than for the live image of the first type, e.g. a Kalman gain of
1.
[0032] The change to the condition of the microscope can relate to
a change of region of interest. The change to a region of interest
can relate to one or more of a change in sample position (e.g. X, Y
and/or Z), a change in sample orientation (e.g. R, T1 and/or T2), a
change in focal depth, and a change in magnification M. The change
to the condition of the microscope can also relate to one or more
of a change in electron acceleration voltage, a change in electron
beam current, a change in beam tilt, a beam shift, a change in scan
rotation, a change in electron gun tilt, an electron gun shift, a
change in astigmatism correction, a change in vacuum pressure, and
a change in temperature.
[0033] Hence, the outputting of the live image of the first type is
automatically, temporarily, disabled during changes to the
condition of the microscope. The outputting of the live image of
the first type can automatically be resumed as soon as the changes
to the condition of the microscope stop.
[0034] According to an aspect is provided a method for displaying a
live microscope image, such as a scanning electron microscope
image. The method includes receiving a stream of live images,
generating a live image of a first type from a first number of most
recent received images, and outputting the live image of the first
type. The method includes in response to receiving an indication of
a change to the condition of the microscope, outputting a live
image of a second type based on a second number of recent images
received at the first input, the second number being smaller than
the first number.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Embodiments of the present invention will now be described
in detail with reference to the accompanying drawings in which:
[0036] FIG. 1 is a schematic representation of a system having
aspects according to the invention;
[0037] FIG. 2 is a schematic representation of a system having
aspects according to the invention;
[0038] FIG. 3 is a schematic representation of a system having
aspects according to the invention;
[0039] FIG. 4 is a schematic representation of a system having
aspects according to the invention;
[0040] FIG. 5 is a schematic representation of a system having
aspects according to the invention;
[0041] FIG. 6 is a flow chart of a process according to an aspect
of the invention.
DETAILED DESCRIPTION
[0042] FIG. 1 shows a schematic representation of a system having
aspects according to the invention. FIG. 1 shows, by way of
non-limiting example a schematic representation of a scanning
electron microscope. FIG. 1 shows a sample stage 1 positioned
inside a vacuum chamber 14. The sample stage 1 includes a sample
carrier 4. In this example the sample carrier 4 includes a bottom
4A and a circumferential wall 4B enclosing a cavity 18 for holding
a sample or a sample holder. In this example the cavity 18 holds an
exchangeable sample holder 5. The exchangeable sample holder 5
holds a sample 5A.
[0043] The sample stage 1 further includes a base 2. The sample
carrier 4 is positioned on the base 2. In FIG. 1 the sample carrier
4 includes sliding feet 12 for allowing sample carrier 4 to
slidingly move along the surface of the base 2. Since the sample
carrier 4 directly abuts against the base 2, and here is pressed
against the base by gravity, the mechanical stiffness of the sample
stage 1 from the sample carrier 4 to the base 2 can be made very
high. The sliding feet can be chosen of a material having a
suitable Young's module, and can have dimensions (surface area and
thickness) to provide high stiffness in the upright direction. This
construction also provides a high stiffness of the sample stage 1
from the sample carrier 4 to the base 2 in directions parallel to
the surface of the base 2.
[0044] The sample stage 1 further includes an actuator assembly. In
FIG. 1 the actuator assembly includes a first actuator 6 arranged
for moving the sample carrier 4 in a direction substantially
parallel to the base 2. In this example the first actuator 6
includes a screw spindle for moving the sample carrier from left to
right in the Figure, henceforth X direction. The actuator assembly
further includes a second actuator 8 arranged for moving the sample
carrier 4 in a direction substantially parallel to the base 2 and
substantially orthogonal to the moving direction of the first
actuator 6. In this example the second actuator 8 includes a screw
spindle for moving the sample carrier in a direction into and out
of the paper in the Figure, henceforth Y direction. In this example
the second actuator 8 is connected to the actuated part of the
first actuator 6. Thus, the "steady" part of the second actuator 8
can be moved in X direction by the first actuator 6. Hence, the
actuated part of the second actuator 8 can be moved in X and Y
direction. The actuated part of the second actuator 8 is connected
to the sample carrier by means of connection 10. Thus, together the
first and second actuators 6, 8 are arranged for moving the sample
carrier 4 in X and Y direction. Thus, in effect the sample stage 1
includes a two-dimensional slide bearing arranged for allowing the
sample carrier 4 to slide in two different, in this example
orthogonal, directions in a plane parallel to the base 2. It will
be appreciated that it is also possible that the actuator assembly
includes a third actuator for rotating the sample carrier about an
axis that is orthogonal to the base 2, hereinafter referred to as
rotation in the R direction. The slide bearing is also arranged for
allowing the sample carrier 4 to rotate about the axis orthogonal
to the base 2.
[0045] The connection 10 is flexible in a direction orthogonal to
the base 2. In this example, the connection 10 includes a leaf
spring. The flexibility in the direction orthogonal to the base 2
takes care that the first and second actuators 6, 8 cannot force
the sample carrier in the direction orthogonal to the base. Any
residual forces exerted by the first and second (and third)
actuators 6, 8 in the direction orthogonal to the base 2 are
dissipated by the flexible connection 10. Thus, the mechanical
stiffness of the sample stage 1 from the sample carrier 4 to the
base 2, i.e. in FIG. 1 the mechanical stiffness in vertical
direction, is independent of any mechanical stiffness of the first
and second (and third) actuators 6, 8. It will be appreciated that
in this example the first and second actuators are not interposed
between the sample carrier 4 and the base 2. This too renders the
mechanical stiffness of the sample stage 1 from the sample carrier
4 to the base 2 independent of any mechanical stiffness of the
first and second actuators 6, 8. In this example the first and
second actuators 6, 8 are positioned beside the sample carrier 4,
outside the contour of the sample carrier 4 when viewed from above.
It will be appreciated that it is also possible that the second
actuator 8 is interposed between the sample carrier 4 and the base
2. In such case, preferably, the sample carrier 4 still abuts
against the base 2 by sliding feet straddling the second actuator
8, so as to exclude the second actuator 8 from determining the
mechanical stiffness of the sample stage 1 from the sample carrier
4 to the base 2.
[0046] The connection 10 is substantially rigid in the X and Y
directions (and in the R direction). It will be appreciated that
the connection 10 is rigid in both the positive X direction and the
negative X direction. Hence, the connection 10 is substantially
rigid in the X direction in both senses. It will be appreciated
that the connection 10 is rigid in both the positive Y direction
and the negative Y direction. Hence, the connection 10 is
substantially rigid in the Y direction in both senses. It will be
appreciated that the leaf spring has a high stiffness in the plane
in which it extends. Hence, the actuators 6, 8 can accurately
position the sample carrier 4 in the X and Y directions. It will be
appreciated that the rigidity for achieving a desired positioning
accuracy can easily be determined, taking into account positioning
speeds, accelerations, friction and the like. Alternatively, or
additionally, the sample stage 1 may include a position sensor for
determining a position of the sample carrier 4 relative to the base
2. The position sensor may e.g. determine a position in the X
direction and in the Y direction. It will be appreciated that when
the position of the sample carrier 4 is being determined it can be
allowed that the connection of the actuators 6, 8 to the sample
carrier 4 in the X and Y directions is less rigid since positioning
accuracy can be achieved through closed loop control on the basis
of the determined position.
[0047] The system shown in FIG. 1 further includes an electron
optical column 16. The electron optical column 16 includes an
electron gun for generating an electron beam, and lenses for the
beam, for forming an image of the sample 5A in a manner known per
se. In FIG. 1 the base 2 is formed by a wall of the vacuum chamber
14 opposite the electron optical column 16. It is also possible
that the base 2 is formed by or is positioned on a wall of the
vacuum chamber 14 to which the electron optical column 16 is
mounted. If this wall is the upper wall, the sample carrier 4 may
be biased towards the base 2 by a biasing force, such as a spring
force, a magnetic force, a pneumatic force, or the like.
[0048] The system shown in FIG. 1 further includes a loading door
20. The loading door 20 sealingly closes a loading aperture of the
vacuum chamber 14. In FIG. 1 the loading door 20 includes a sealing
member 22, e.g. an O-ring. The vacuum chamber is maintained at a
low pressure, of e.g. 0.3 mbar, when operating the electron optical
column 16. It will be appreciated that opening the loading door 20
in the system in the state of FIG. 1 for loading or unloading a
sample (holder) would allow ambient air (approximately 1000 mbar)
to enter the entire vacuum chamber 14. As a result a long pumping
time would be required to achieve a suitable vacuum for operating
the electron optical column again. Moreover, critical parts of the
electron microscope, such as the electron gun, could be damaged
when subjected to such high pressures. In order to reduce the pump
time, and protect the critical parts, a load lock can be used.
FIGS. 1-4 illustrate how the system can be used to provide load
lock functionality.
[0049] In FIG. 2, the sample carrier 4 is moved towards a
preloading position adjacent to the loading door 20 by the first
and second actuators 6, 8. This movement of the sample carrier 4
from the inspection position underneath the electron optical column
(FIG. 1) to the preloading position underneath the loading door 20
(FIG. 2) is substantially parallel to the base 2.
[0050] In FIG. 3 the sample carrier 4 is moved in a direction
parallel to the base 2. The sample carrier 4 is sealingly pressed
against the wall of the vacuum chamber 14 surrounding the loading
door 20. In this example the sample carrier 4 has a sealing member
28, e.g. an O-ring, for providing a sealing engagement between the
sample carrier 4 and the wall of the vacuum chamber 14, such that
the cavity 18 of the sample carrier can be accessed through the
loading door 20. The system includes a pushing device 24 for
sealingly pressing the sample carrier 24 against the wall of the
vacuum chamber 14.
[0051] In this example the pushing device 24 includes a bellows 26.
In the system state in FIG. 2 the space below the bellows 26 is
maintained at a low pressure, e.g. the same pressure as inside the
vacuum chamber 14. As a result a pusher 27 is maintained in a
retracted position. In the system state in FIG. 3 the space below
the bellows 26 is maintained at a high pressure, e.g. at ambient
pressure or at an elevated pressure. As a result, the pusher 27 is
brought in an extended position and pushes the sample carrier 4 in
sealing engagement with the wall of the vacuum chamber 14.
[0052] In FIG. 4 the loading door 20 is opened. The sample holder 5
and/or the sample 5A can be unloaded from the sample carrier 4. It
is also possible to load a sample holder 5 and/or sample 5A into
the sample carrier 4. While the loading door 20 is opened the
pressure inside the cavity will be ambient pressure. It will be
appreciated that the downward force on the sample carrier 4 due to
the ambient pressure inside the cavity 18 must be counteracted by
the upward force generated by the bellows 26 to maintain the
sealing engagement between the sample carrier 4 and the wall of the
vacuum chamber 14 surrounding the loading door 20. As can be seen
in FIGS. 1-4 in this example the surface area of the bellows is
chosen to be larger than the surface area of the sample carrier (in
top view). Such larger surface area of the bellows 26 will bias the
sample carrier 4 towards the sealing position.
[0053] As can be seen in FIGS. 3 and 4 the flexible connection 10
also allows for lifting of the sample carrier 4 towards the loading
door 20 while the sample carrier remains connected to the actuators
6, 8.
[0054] After loading a sample holder 5 or a sample 5A the loading
door 20 is closed again (see FIG. 3). Next, the pressure in the
cavity 18 is reduced. Thereto, a vacuum pump or prevacuum pump of
the system has a connection to the cavity 18. Once the pressure in
the cavity is reduced, for example to 4.5 mbar, the sample carrier
4 is lowered again (see FIG. 2). This causes the remaining air in
the cavity 18 to spread throughout the vacuum chamber 14. Since the
volume of the cavity 18 can be chosen to be small relative to the
volume of the vacuum chamber 14, this has a reduced effect on the
pressure inside the vacuum chamber 14. The pressure in the vacuum
chamber 14 may e.g. rise to 0.5 mbar. Next, the vacuum chamber 14
is pumped to the desired pressure for operation of the electron
optical column again, e.g. 0.3 mbar.
[0055] The sample carrier 4 as described above can be positioned
relative to a reference position. The reference position can e.g.
be an optical axis or beam position of the electron optical column
16. Moving the sample carrier 4 relative to the reference position
allows different areas of the sample 5A to be brought into the
field of view of the microscope. The electron optical column 16
generates an image of the sample 5A within the field of view. This
image can e.g. be displayed at a computer screen of the system.
[0056] More in general, the microscope includes an image generator
30. The image generator 30 can be a CCD camera CMOS, or an other
type of image sensor, see FIG. 5. For displaying image, the system
includes a processor 32 having a first input 34 for receiving
images, such as live images. The images are fed to an output 36 by
the processor. The output communicates with a display unit 40 such
as a computer screen. The processor 32 is arranged for improving
the image quality of the live image fed to the output 36. Thereto,
the processor 32 can be arranged for generating a live image of a
first type from a first number of recent images received at the
first input.
[0057] The live image of the first type can for example be an
averaged image. The input images to be averaged may e.g. be
temporarily be stored in an image buffer 38. In the image buffer 38
the images can be processed prior to being displayed. In order to
providing sharper images, better contrast, less noise, etc. the
image output towards the display device may be obtained by
averaging the first number of images in the image buffer 38. It is
for instance possible to calculate a moving average of a
predetermined number of images: every image newly received at the
first input 34 then replaces the oldest image in the calculation of
the averaged image.
[0058] In an alternative embodiment, the live image of a first type
can be calculated on the basis of Kalman filtering. In this case an
image to be fed to the output 36 is calculated by multiplying the
image most recently received at the first input 34 by a Kalman
gain, Kk, (0<Kk.ltoreq.1) and adding the immediately preceding
output image multiplied by 1-Kk. The Kalman gain Kk represents the
proportional contribution of the image most recently received at
the first input 34 to the image output at the output 36. In an
example, the live image of the first type may be calculated by
setting the Kalman gain to 1/16. It is noted that the Kalman
filtering does not necessitate the use of the buffer 38.
[0059] Displaying the live image of the first type provides
increased image quality at the display unit while observing a
sample under constant conditions. However, when conditions change,
e.g. when the sample is moved to view another region, or when an
image magnification is changed, the live image of the first type
will at least partly be based on "old" images relating to the old
condition (e.g. old sample position), and partly on one or more
"new" images relating to the new condition (e.g. new sample
position). This will introduce unsharpness, such as motion blur,
into the displayed live image.
[0060] To resolve this, the processor 32 further includes a second
input 42. The second input 42 is in communication with a control
unit 44 of the microscope. The control unit 44 provides to the
second input 42 an indication of a change to a condition of the
microscope that can influence the obtained image. Such change of
condition can relate to a change of region of interest. Such change
of region of interest can include one or more of a change in sample
position (e.g. X, Y and/or Z), a change in sample orientation (e.g.
R, T1 and/or T2), a change in focal depth, and a change in
magnification M. Such change in condition can also include one or
more of a change in electron acceleration voltage, a change in
electron beam current, a change in beam tilt, a beam shift, a
change in scan rotation, a change in electron gun tilt, an electron
gun shift, a change in astigmatism correction, a change in vacuum
pressure, and a change in temperature.
[0061] When the processor, via the second input 42, receives a
signal indicating a change in condition of the microscope, the
processor 32 stops outputting the live image of the first type and
automatically switches to outputting a live image of a second type.
The live image of the second type is based on a second number of
recent images received at the first input 34, the second number
being smaller than the first number. The second number being
smaller than the first number, the live image of the second type is
less susceptible to artefacts such as motion blur. In a special
embodiment the second number is one, so that the live image of the
second type substantially corresponds to the live image received at
the first input.
[0062] When the live image of the first type is obtained by
averaging using the image buffer 38, the processor 32 in response
to receiving a signal indicating a change in condition of the
microscope can reset the image buffer 38. Hence, the averaging of
images is automatically temporarily disabled when a change to the
condition is detected. The resetting of the image buffer marks the
outputting of the live image of the second type. It is noted that
if no further change in a condition of the microscope occurs, and
thus no further signal indicating such change is received at the
second input 42, the averaging of images can automatically resumes
as the reset image buffer 38 can immediately start to include
multiple images which are again averaged. It will be appreciated
that the resetting of the image buffer can be achieved by deleting
images from the buffer, or by restarting calculation of the
averaged image from the latest image.
[0063] When the live image of the first type is obtained by Kalman
filtering, the processor 32 in response to receiving a signal
indicating a change in condition of the microscope can increase the
Kalman gain Kk. For example, the Kalman gain can be set to 1. A
Kalman gain Kk of 1 indicates that the image received at the first
input 34 is fed to the output 36 without adding historical image
data. The increasing of the Kalman gain Kk marks the outputting of
the live image of the second type. It is also possible that the
Kalman gain Kk for the live image of the second type is smaller
than 1, e.g. 0.5.ltoreq.Kk<1. It is noted that if no further
change in a condition of the microscope occurs, and thus no further
signal indicating such change is received at the second input 42,
the Kalman gain can be decreased again, e.g. abruptly, stepwise or
gradually. The reducing of the Kalman gain can be effected
automatically when the processor 32 determines that no further
signal indicating a change in a condition of the microscope is
received at the second input 42.
[0064] FIG. 6 shows a schematic representation of a flow chart
describing a possible operation of the processor 32. In step 100 an
image is received from the image generator 30 at the first input
34. In step 102 the second input is ready for receiving signals or
messages indicating a change in a condition of the microscope. In
step 104 the processor 32 check the second input 42 for receipt of
signals or messages indicating a change in a condition of the
microscope. If such signal or message has been received at the
second input 42 the processor resets the image buffer in step 106.
Next in step 108 the image received at the first input 34 is stored
in the image buffer that still contains historic images or has been
reset. If the predetermined number of images to be used for
averaging in the image buffer has been exceeded (check in step 110)
the oldest image is discarded from the buffer in step 112. It is
noted that the predetermined number of images to be used for
averaging can e.g. be set by a user, e.g. in a user interface of
the system. In step 114 the images in the buffer are averaged. In
step 116 the averaged image is output, e.g. to the display unit 40.
Then the procedure restarts at step 100 for the next image.
[0065] It will be appreciated that the processor 32 and the
associated process can be used in a scanning electron microscope,
but also in a tunneling electron microscope or in an optical
microscope. Alternatively, the processor 32 and the associated
process can also be used in other devices such as telescopes,
digital cameras, etc.
[0066] Herein, the invention is described with reference to
specific examples of embodiments of the invention. It will,
however, be evident that various modifications and changes may be
made therein, without departing from the essence of the invention.
For the purpose of clarity and a concise description features are
described herein as part of the same or separate embodiments,
however, alternative embodiments having combinations of all or some
of the features described in these separate embodiments are also
envisaged.
[0067] In the example of FIGS. 1-4 the pushing device 24 is
stationary relative to the vacuum chamber 14. It will be
appreciated that the pushing device can also be mobile relative to
the vacuum chamber, e.g. mounted to the sample carrier 4, the first
actuator 6, and/or the second actuator 8.
[0068] In the example the pushing device is operated by gas (e.g.
vacuum, ambient air or pressurized air). It will be appreciated
that it is also possible to operate the pushing device by means of
an electric motor, magnet(s), piezo-electric crystal, hydraulics,
manual force (e.g. via gears or levers), etc.
[0069] In the examples the load lock makes use of the flexible
connection 10. It will be appreciated that the load lock can also
be operated with the sample carrier vertically movably, but not
necessarily flexibly, mounted to an intermediate part that rigidly
couples to the actuator(s).
[0070] It will be appreciated that the sample stage with the
flexible connection 10 can also be put to use independent of the
load lock functionality.
[0071] In the example of FIGS. 1-4 the sample carrier is slidingly
positioned on the base. Alternative positioning and movement
mechanism are also possible. It is for instance possible to use a
gas bearing system, e.g. with a biasing force (e.g. magnetic force
or vacuum force). Such two-dimensional gas bearing system is herein
also referred to as two-dimensional slide bearing. In the example
of FIGS. 1-4 the sliding feet are static. It is also possible that
the sliding feet are formed by rollers allowing two-dimensional
sliding of the stage. Such two-dimensional rolling bearing system
is herein also referred to as two-dimensional slide bearing.
[0072] In the above examples the sample stage is used in the
context of a scanning electron microscope. It will be appreciated
that the sample stage can also be used in other inspection
apparatus, such as optical microscopes, tunneling electron
microscopes, atomic force microscopes, etc. It will be appreciated
that the sample stage can also be used in other apparatus, such as
milling machines, grinding machines, routing machines, etching
machines, (3D) printing machines, lithographic machines, component
placement machines, or the like. It will be appreciated that the
sample in such other apparatus can be an object being machined, a
semiconductor wafer, a printed circuit board , or the like.
[0073] It will be appreciated that the processor, first input unit,
output unit, image buffer, second input unit, control unit and
resetting unit can be embodied as dedicated electronic circuits,
possibly including software code portions. The processor, first
input unit, output unit, image buffer, second input unit, control
unit and resetting unit can also be embodied as software code
portions executed on, and e.g. stored in, a memory of, a
programmable apparatus such as a computer.
[0074] Although the embodiments of the invention described with
reference to the drawings comprise computer apparatus and processes
performed in computer apparatus, the invention also extends to
computer programs, particularly computer programs on or in a
carrier, adapted for putting the invention into practice. The
program may be in the form of source or object code or in any other
form suitable for use in the implementation of the processes
according to the invention. The carrier may be any entity or device
capable of carrying the program.
[0075] For example, the carrier may comprise a storage medium, such
as a ROM, for example a CD ROM or a semiconductor ROM, or a
magnetic recording medium, for example a floppy disc or hard disk.
Further, the carrier may be a transmissible carrier such as an
electrical or optical signal which may be conveyed via electrical
or optical cable or by radio or other means, e.g. via the internet
or cloud.
[0076] When a program is embodied in a signal which may be conveyed
directly by a cable or other device or means, the carrier may be
constituted by such cable or other device or means. Alternatively,
the carrier may be an integrated circuit in which the program is
embedded, the integrated circuit being adapted for performing, or
for use in the performance of, the relevant processes.
[0077] However, other modifications, variations, and alternatives
are also possible. The specifications, drawings and examples are,
accordingly, to be regarded in an illustrative sense rather than in
a restrictive sense.
[0078] For the purpose of clarity and a concise description
features are described herein as part of the same or separate
embodiments, however, it will be appreciated that the scope of the
invention may include embodiments having combinations of all or
some of the features described.
[0079] In the claims, any reference signs placed between
parentheses shall not be construed as limiting the claim. The word
`comprising` does not exclude the presence of other features or
steps than those listed in a claim. Furthermore, the words `a` and
`an` shall not be construed as limited to `only one`, but instead
are used to mean `at least one`, and do not exclude a plurality.
The mere fact that certain measures are recited in mutually
different claims does not indicate that a combination of these
measures cannot be used to an advantage.
* * * * *