U.S. patent application number 14/162802 was filed with the patent office on 2015-07-30 for high-voltage energy-dispersive spectroscopy using a low-voltage scanning electron microscope.
The applicant listed for this patent is Keysight Technologies, Inc.. Invention is credited to Scott W. Indermuehle, Dimitri Klyachko, Lawrence P. Muray, James P. Spallas, Ying Wu.
Application Number | 20150213995 14/162802 |
Document ID | / |
Family ID | 53679667 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150213995 |
Kind Code |
A1 |
Muray; Lawrence P. ; et
al. |
July 30, 2015 |
HIGH-VOLTAGE ENERGY-DISPERSIVE SPECTROSCOPY USING A LOW-VOLTAGE
SCANNING ELECTRON MICROSCOPE
Abstract
A scanning electron microscopy (SEM) and energy dispersive
spectroscopy (EDS) apparatus that includes a scanning electron
microscope, an x-ray detector, and an auxiliary acceleration
voltage source. The scanning electron microscope includes a sample
holder, and a layered electron beam column arranged to output an
electron beam towards the sample holder at an initial beam energy.
The auxiliary acceleration voltage source is to apply an auxiliary
acceleration voltage between the sample holder and the layered
electron beam column to accelerate the electron beam to a final
beam energy. At the final beam energy, the electron beam is capable
of generating x-rays at multiple wavelengths from a larger range of
atomic species than the electron beam at the initial beam
energy.
Inventors: |
Muray; Lawrence P.; (Moraga,
CA) ; Indermuehle; Scott W.; (Danville, CA) ;
Spallas; James P.; (San Ramon, CA) ; Wu; Ying;
(Sunnyvale, CA) ; Klyachko; Dimitri; (Campbell,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Keysight Technologies, Inc. |
Minneapolis |
MN |
US |
|
|
Family ID: |
53679667 |
Appl. No.: |
14/162802 |
Filed: |
January 24, 2014 |
Current U.S.
Class: |
250/305 ;
250/311 |
Current CPC
Class: |
H01J 2237/1205 20130101;
H01J 2237/2442 20130101; H01J 2237/202 20130101; H01J 37/04
20130101; H01J 37/05 20130101; H01J 37/28 20130101; H01J 37/244
20130101; H01J 2237/2445 20130101; H01J 37/20 20130101; H01J
2237/04735 20130101; H01J 2237/2007 20130101 |
International
Class: |
H01J 37/05 20060101
H01J037/05; G01T 1/36 20060101 G01T001/36; H01J 37/28 20060101
H01J037/28 |
Claims
1. A scanning electron microscopy (SEM) and energy dispersive
spectroscopy (EDS) apparatus, comprising: a scanning electron
microscope comprising a sample holder, and a layered electron beam
column arranged to output an electron beam towards the sample
holder at an initial beam energy; an x-ray detector; and an
auxiliary acceleration voltage source to apply an auxiliary
acceleration voltage between the sample holder and the layered
electron beam column to accelerate the electron beam to a final
beam energy, the electron beam at the final beam energy capable of
generating x-rays at multiple wavelengths from a larger range of
atomic species than the electron beam at the initial beam
energy.
2. The spectroscopy apparatus of claim 1, in which the x-ray
detector comprises a silicon drift detector.
3. The spectroscopy apparatus of claim 2, in which the silicon
drift detector is part of a silicon drift detector die mounted on
the layered electron beam column.
4. The spectroscopy apparatus of claim 3, in which the silicon
drift detector die is mounted on a surface of the layered electron
beam column facing the sample holder.
5. The spectroscopy apparatus of claim 3, in which: the layered
electron beam column comprises a detector mounting chamber defined
therein, the detector mounting chamber comprising an inclined
detector mounting surface; and the silicon drift detector die is
mounted on the inclined detector mounting surface.
6. The spectroscopy apparatus of claim 5, in which: the layered
electron beam column comprises one or more additional inclined
detector mounting surfaces defined therein; and the spectroscopy
apparatus additionally comprises: a respective additional silicon
drift detector die mounted on each of the additional inclined
detector mounting surfaces, and a summing circuit to sum x-ray
detection signal components generated by the silicon drift detector
dies to generate an x-ray detection signal having a higher
signal-to-noise ratio than the x-ray detection signal
components.
7. The spectroscopy apparatus of claim 3, additionally comprising:
additional silicon drift detector dies mounted on the surface of
the layered electron beam column facing the sample holder, and a
summing circuit to sum x-ray detection signal components generated
by the silicon drift detector dies to generate an x-ray detection
signal having a higher signal-to-noise ratio than the x-ray
detection signal components.
8. The spectroscopy apparatus of claim 2, in which the silicon
drift detector and a silicon photodiode electron detector are
integrated on a common multi-detector die mounted on a surface of
the layered electron beam column facing the sample holder.
9. The spectroscopy apparatus of claim 2, in which: the silicon
drift detector and a silicon photodiode electron detector are
integrated on a common multi-detector die; and the layered electron
beam column comprises a detector mounting chamber defined therein,
the detector mounting chamber comprising an inclined detector
mounting surface on which the multi-detector die is mounted.
10. The spectroscopy apparatus of claim 2, in which: the x-ray
detector additionally comprises thermoelectric cooler comprising a
cold face; and the silicon drift detector is mounted on the cold
face of the thermoelectric cooler.
11. The spectroscopy apparatus of claim 2, in which the x-ray
detector additionally comprises an amplifier electrically connected
to the silicon drift detector.
12. The spectroscopy apparatus of claim 1, in which the auxiliary
acceleration voltage source sets the sample holder to a voltage
more positive than the layered electron beam column.
13. The spectroscopy apparatus of claim 1, in which the auxiliary
acceleration voltage source comprises a high-voltage power
supply.
14. The spectroscopy apparatus of claim 1, in which the auxiliary
acceleration voltage source comprises an electrical connector to
receive the auxiliary acceleration voltage from an external
high-voltage power supply.
15. The spectroscopy apparatus of claim 1, additionally comprising
a controller to correct x-ray image distortion resulting from the
auxiliary acceleration voltage.
16. The spectroscopy apparatus of claim 1, additionally comprising
an electrical connector in series between the auxiliary
acceleration voltage source and the sample holder, the electrical
connector automatically disconnecting upon removal of the sample
holder from the spectroscopy apparatus.
17. The spectroscopy apparatus of claim 1, in which: the auxiliary
acceleration voltage is additionally to divert electrons away from
the x-ray detector; and the x-ray detector lacks an electron
trap.
18. A scanning electron microscopy (SEM) and energy dispersive
spectroscopy (EDS) apparatus, comprising: a scanning electron
microscope comprising a sample holder, and an electron beam column
arranged to output an electron beam towards the sample holder, the
electron beam column subject to a voltage limitation that limits
the electron beam to an initial beam energy incapable of generating
x-rays at multiple wavelengths from atomic species having atomic
numbers greater than a threshold atomic number; an x-ray detector;
and an auxiliary acceleration voltage source to apply an auxiliary
acceleration voltage between the sample holder and the electron
beam column to accelerate the electron beam to a final beam energy,
the electron beam at the final beam energy capable of generating
x-rays at multiple wavelengths from atomic species having atomic
numbers greater than the threshold atomic number.
19. The spectroscopy apparatus of claim 18, in which the x-ray
detector comprises a silicon drift detector die mounted on the
electron beam column, the silicon drift detector die comprising a
silicon drift detector.
20. The spectroscopy apparatus of claim 18, in which the x-ray
detector comprises a multi-detector die mounted on the electron
beam column, the multi-detector die comprising a silicon drift
detector and additionally comprising an electron detector.
Description
BACKGROUND
[0001] Layered electron beam columns are described in U.S. Pat.
Nos. 7,045,794, 7,109,486, 7,332,729, and 7,335,895, now assigned
to the assignee of this disclosure, and in U.S. Pat. Nos.
8,003,952, 8,106,358, 8,110,801, and 8,115,168, assigned to the
assignee of this disclosure. A layered electron beam column is
composed of a stack of layers of insulating materials such as
ceramic, glass and undoped semiconductor. Each layer supports a
respective miniature component capable of extracting, accelerating,
collimating, focusing, blanking, or steering, etc., an electron
beam. The use of a layered electron beam column allows a scanning
electron microscope (SEM) to be reduced in size from a room-sized
instrument to a benchtop instrument. Scanning electron microscopes
similar in size to a typical laser printer are now commercially
available, for example, the model 8500 FT-SEM sold by Agilent
Technologies, Inc., Santa Clara, Calif.
[0002] Energy dispersive spectroscopy (EDS) is described by Joseph
Goldstein et al. in Chapter 7 of Scanning Electron Microscopy and
X-ray, Microanalysis, 3rd ed., (Springer US 2003). Energy
dispersive spectroscopy can be used for material identification and
quantification of the constituents of a sample. To uniquely
identify a particular atomic species in a sample, at least two
X-ray lines need to be identified. The electron beam energy needed
to generate x-rays at at least two wavelengths from a given atomic
species increases with atomic number. For example, to uniquely
identify atomic species with atomic numbers greater than 14
requires an electron beam energy greater than 2 keV. Conventional
SEMs use beam energies substantially greater than 2 keV, and can
therefore offer full-spectrum EDS as an auxiliary feature. However,
voltage maxima in the layered electron beam column of current
layered electron beam column benchtop SEMs limit the electron beam
energy to less than that needed to generate x-rays at multiple
wavelengths from a full spectrum of atomic species. Consequently,
current layered electron beam column benchtop SEMs offer only a
part-spectrum EDS capability.
[0003] Accordingly, what is needed is an SEM with a layered
electron beam column that has a full-spectrum EDS capability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a block diagram showing an example of a scanning
electron microscopy and energy dispersive spectroscopy
apparatus.
[0005] FIGS. 2A and 2B are schematic block diagram showing examples
of an auxiliary acceleration voltage source.
[0006] FIGS. 3A and 3B are a schematic front view and a schematic
side view, respectively, of a sample holder assembly that
constitutes the sample holder in some embodiments of the
spectroscopy apparatus shown in FIG. 1.
[0007] FIGS. 3C and 3D are enlarged and exploded versions of FIGS.
3A and 3B, respectively.
[0008] FIGS. 4A-4D are schematic drawings showing examples of the
effect of the auxillary acceleration voltage on the trajectories of
the electrons of the electron beam and the range over which the
electron beam can be scanned in the spectroscopy apparatus shown in
FIG. 1.
[0009] FIG. 5 is a flowchart showing an example of a method for
calibrating the spectroscopy apparatus shown in FIG. 1.
[0010] FIG. 6 is a flowchart showing an example of a method for
generating the energy-dispersive spectrum of a feature of interest
of a sample using the spectroscopy apparatus shown in FIG. 1.
[0011] FIGS. 7A-7C, 8 and 9 are cross-sectional views showing
examples of an x-ray generation and detection system that
constitutes part of some embodiments of the spectroscopy apparatus
shown in FIG. 1.
[0012] FIG. 7D is a cross-sectional view showing the x-ray
generation and detection system shown in FIG. 7A operating with a
tilted sample holder.
DETAILED DESCRIPTION
[0013] FIG. 1 is a block diagram showing an example 100 of a
scanning electron microscopy and energy dispersive spectroscopy
apparatus in accordance with this disclosure. Spectroscopy
apparatus 100 includes a scanning electron microscope (SEM) 110,
and x-ray detector 120, and an auxiliary acceleration voltage
source 130. SEM 110 includes a sample holder 140, and a layered
electron beam column 150. Layered electron beam column 150 is
arranged to output an electron beam 152 towards sample holder 140
at an initial beam energy. Layered electron beam column 150 has a
column axis 156 along which electron beam 152 in its un-steered
state is output.
[0014] In the example shown, SEM 110 additionally includes an
electron source 160 and an electron detector 170. Electron source
160 is located on the column axis 156 of layered electron beam
column 150 on the side of the layered electron beam column remote
from sample holder 140. Electron source 160 provides electrons 162
to layered electron beam column 150. A voltage applied between
electron source 160 and layered electron beam column 150 defines
the initial beam energy of electron beam 152. In the example shown,
electron detector 170 is mounted on a surface of layered electron
beam column 150 facing sample holder 140, and generates an electron
detection signal ES in response to electrons incident thereon.
Layered electron beam column 150 and sample holder 140 are arranged
such that electron beam 152 in its un-steered state is incident at
the center of sample holder 140 with the sample holder at its home
position.
[0015] SEM 110 additionally includes a controller 190 that applies
column control signals CC to layered electron beam column 150.
Column control signals CC, at least some of which are in the
kilovolt range, cause the layered electron beam column to perform
such functions as extracting, accelerating and collimating
electrons 162, and focusing, blanking and steering electron beam
152. Controller 190 additionally receives electron detection signal
ES from electron detector 170.
[0016] The thinness of the layers constituting layered electron
beam column 150 imposes limitations on the voltages of column
control signals CC that can be applied within the electron beam
column. These voltage limitations in turn impose a limitation on
the initial beam energy of electron beam 152. The highest initial
beam energy of electron beam 152 output by an example of layered
electron beam column 150 is about 2 keV.
[0017] To identify a constituent atomic species of a sample using
EDS requires that electron beam 152 be incident on the sample with
a beam energy sufficiently high to generate x-rays at multiple
wavelengths, but at least at two different wavelengths. Electron
beam 152 at its initial beam energy of, for example, about 2 keV is
capable of generating x-rays at multiple wavelengths from only the
first 14 atomic species of the periodic table, i.e., hydrogen
through nitrogen. Detecting and quantifying atomic species with
atomic numbers greater than 14 is also of interest. Accordingly,
spectroscopy apparatus 100 additionally includes auxiliary
acceleration voltage (AAV) source 130 that provides spectroscopy
apparatus 100 with the capability to perform EDS on samples
containing atomic species with an atomic number greater than the
atomic number corresponding to the initial beam energy of electron
beam 152.
[0018] Auxiliary acceleration voltage source 130 applies an
auxiliary acceleration voltage between sample holder 140 and
layered electron beam column 150. Specifically, auxiliary
acceleration voltage source 130 sets sample holder 140 to a more
positive voltage than layered electron beam column 150. The
auxiliary acceleration voltage accelerates electron beam 152 to a
final beam energy. At its final beam energy, electron beam 152 is
capable of generating x-rays at multiple wavelengths from a larger
range of atomic species than electron beam 152 at its initial beam
energy. A range of atomic species includes the atomic species with
consecutive atomic numbers between hydrogen and the atomic species
with the highest atomic number from which the electron beam at its
final beam energy is capable of generating x-rays at multiple
wavelengths. The auxiliary acceleration voltage is not subject to
the maximum voltage limitations of layered electron beam column
150, and can therefore be made as large as is necessary for the
range of atomic species from which electron beam 152 at its final
beam energy is capable of generating x-rays at multiple wavelengths
to include a highest atomic weight atomic species of interest.
[0019] In an example, a final beam energy of 15 keV is needed to
generate x-rays at multiple wavelengths from the highest atomic
weight atomic species of interest, and the initial beam energy of
electron beam 152 is 2 keV. In this example, auxiliary acceleration
voltage source 130 applies an auxiliary acceleration voltage of 13
kV between sample holder 140 and layered electron beam column 150.
With such an auxiliary acceleration voltage applied between sample
holder 140 and layered electron beam column 150, the landing energy
of electron beam 152 at the sample is 15 keV and the range of
atomic species from which electron beam 152 can generate x-rays at
multiple wavelengths is comparable with that of a conventional SEM
operating with a beam energy of 15 keV.
[0020] In an example, SEM 110 additionally includes an armature
(not shown) to which electron source 160, layered electron beam
column 150, sample holder 140, and x-ray detector 120 are coupled.
The armature defines the spatial relationship among the electron
source, the layered electron beam column, the sample holder, and
the x-ray detector. In the example shown, sample holder 140
includes a sample platform 142 that is electrically insulated from
the armature, and, hence, from the remaining components of SEM 110,
by an insulator 144 interposed between the sample platform and the
armature. In the example shown, sample holder 140 is mounted on a
positioning stage 146. In an example, positioning stage 146 is an
XY stage that operates in response to stage control signals SC
output by controller 190 to move sample holder 140 in the x-y plane
relative to layered electron beam column 150. Positioning stage 146
moves sample holder 140 over a greater range of motion in the x-y
plane than the range of motion obtained by layered electron beam
column 150 steering electron beam 152. In another example,
positioning stage 146 is an XYZ stage that operates in response to
stage control signals SC additionally to move sample holder 140 in
the z-direction parallel to column axis 156. In yet another
example, positioning stage 146 additionally operates in response to
stage control signals SC to rotate sample holder 140 about an axis
parallel to the column axis and/or to tilt the sample holder about
an axis parallel to the x-y plane. In other examples, sample holder
140 is mounted on the armature in a fixed position relative to
layered electron beam column 150.
[0021] SEM 110 and x-ray detector 120 are housed within a vacuum
chamber 180. In an example, a wall (not shown) divides the vacuum
chamber into a ultra high vacuum (UHV) section (not shown) and a
high vacuum (HV) section (not shown). The wall includes an
isolation valve (not shown) located on column axis 156. Electron
source 160, layered electron beam column 150, and electron detector
170 are located within the UHV section, and x-ray detector 120 and
sample holder 140 are located within the HV section. Vacuum chamber
180 is differentially pumped to maintain a pressure of typically
10.sup.-9-10.sup.-10 Torr within the UHV section, and to maintain a
pressure of typically 10.sup.-4-10.sup.-5 Torr within the HV
section during scanning electron microscopy and/or energy
dispersive spectroscopy operations. The isolation valve can be
moved into position to seal the UHV section, which allows the HV
section to be vented to the atmosphere to exchange samples while
maintaining the ultrahigh vacuum within the UHV section. The HV
section is then evacuated to high vacuum prior to spectroscopy
apparatus 100 being used to perform scanning electron microscopy
and/or energy dispersive spectroscopy operations. Because of the
small dimensions of SEM 110, the dimensions of vacuum chamber 180
are correspondingly small and only a few minutes to are needed to
evacuate the HV section of vacuum chamber 180 to its operating
pressure.
[0022] In some embodiments of spectroscopy apparatus 100, an
electron beam column lacking the layered structure of layered
electron beam column 150, but subject to a voltage limitation that
limits the electron beam output by the electron beam column to an
initial beam energy incapable of generating x-rays at multiple
wavelengths from atomic species having atomic numbers greater than
a threshold atomic number is substituted for electron beam column
150. In such an embodiment, auxiliary acceleration voltage source
130 applies an auxiliary acceleration voltage between the electron
beam column and sample holder 140 to accelerate the electron beam
to a final beam energy at which the electron beam is capable of
generating x-rays at multiple wavelengths from atomic species
having atomic numbers greater than the threshold atomic number.
[0023] FIGS. 2A and 2B are schematic block diagram showing examples
of auxiliary acceleration voltage source 130. In the example shown
in FIG. 2A, auxiliary acceleration voltage source 130 includes a
high-voltage power supply 132. In the example shown, high-voltage
power supply 132 is located outside vacuum chamber 180. In another
example, high-voltage power supply 132 is located within vacuum
chamber 180. Power supplies that convert line-voltage AC or
low-voltage DC to DC voltages in a range from 10,000 V to 20,000 V
are commercially available and may be used. Alternatively, power
supply circuits for converting line-voltage AC or low-voltage DC to
DC voltages in this range are known and may be implemented.
[0024] In the example shown in FIG. 2B, auxiliary acceleration
voltage source 130 includes a connector 134 to which a high-voltage
power supply 32 external to spectroscopy apparatus 100 can be
connected to provide the auxiliary acceleration voltage. In some
examples, connector 134 is mounted on spectroscopy apparatus 100.
In other examples, connector 134 is at the distal end of a cable
that extends from spectroscopy apparatus 100. Connectors for
connecting voltages in a range from 10,000 V to 20,000 V are
commercially available and may be used.
[0025] FIGS. 3A and 3B are a schematic front view and a schematic
side view, respectively, of a sample holder assembly 200 that is
used as sample holder 140 in some embodiments of spectroscopy
apparatus 100. FIGS. 3C and 3D are enlarged and exploded versions
of FIGS. 3A and 3B, respectively. In the example shown in FIGS.
3A-3D, sample holder assembly 200 includes a base 210, a
positioning stage 220, a kinematic base 230, a sample carrier 240,
and a sample holder 250. Positioning stage 220 is mounted on base
210 and operates in response to stage control signals SC received
from controller 190 (FIG. 1) to position sample holder 250 in
translation and/or in rotation relative to column axis 156.
Kinematic base 230 is mounted on positioning stage 220. Sample
carrier 240 is detachably mounted at a defined location on
kinematic base 230. Sample holder 250 is mounted on sample carrier
240. Sample holder 250 and sample carrier 240 collectively
constitute a sample carrier assembly 242. In an example,
positioning stage 220 moves kinematic base 230 and sample carrier
assembly 242 in the x-, y-, and z-directions. In another example,
positioning stage 220 moves kinematic base 230 and sample carrier
assembly 242 in the x- and y-directions. In other examples,
positioning stage 220 additionally rotates kinematic base 230 and
sample carrier assembly 242 about an axis parallel to the
z-direction and/or tilts kinematic base 230 and sample carrier
assembly 242 about an axis parallel to the x-y plane. Base 210
connects sample holder assembly 200 to the armature (not shown) of
spectroscopy apparatus 100 in a position such that, when
positioning stage 220 is at its home position, sample holder 250 is
centered on the column axis 156 of layered electron beam column
150.
[0026] Sample carrier assembly 242, composed of sample holder 250
mounted on sample carrier 240, is easily removable from kinematic
base 230 located in vacuum chamber 180 to enable a sample to be
placed on sample holder 250. The sample carrier assembly with the
sample on sample holder 250 is then replaced on the kinematic base
in the vacuum chamber. The electrical connection between auxiliary
acceleration voltage source 130 (FIG. 1) and sample holder 250 runs
through part of sample holder assembly 200. The electrical
connection is broken automatically when sample carrier assembly 242
is removed from kinematic base 230, and is restored automatically
when the sample carrier assembly is replaced on the kinematic
base.
[0027] Sample holder assembly 200 operates in response to stage
control signals SC received from controller 190 (FIG. 1) to move
sample holder 250 relative to layered electron beam column 150 in
the x-y plane over a greater range of motion than the range of
motion obtained by steering electron beam 152. Sample holder
assembly 200 additionally operates in response to the stage control
signals to move sample holder 250 in the z-direction to maintain
the surface of the sample at the location of the focal point of
electron beam 152 notwithstanding variations in the z-direction
dimension of the sample.
[0028] Referring now to FIGS. 3C and 3D, and in particular to FIG.
3D, in the example of sample holder assembly 200 shown, sample
holder 250 includes a sample platform 252, a platform mount 254, an
insulator tower 256 and a spring-loaded contact assembly 258.
Sample platform 252 includes a sample plate 260 having a planar
major surface 262 on which a sample (not shown) can be placed for
analysis. A pedestal 264 extends orthogonally from the center of
the major surface of sample plate 260 opposite major surface 262.
In the example shown, sample plate 260 is circular in shape,
pedestal 264 is cylindrical in shape and insulator tower 256 is
substantially cylindrical in shape. Other shapes of these elements
are possible and may be used. A blind bore 270 extends into
insulator tower 256 from at or near the center of one end surface
272 thereof, and an off-center bore 274 extends through the
insulator tower from end surface 272 to an end surface 276 opposite
end surface 272. Spring-loaded contact assembly 258 is accommodated
within off-center bore 274. A hollow, cylindrical insulator 278
extends axially from end surface 276 around off-center bore 274 to
protect the spring contact 280 of spring-loaded contact assembly
258. Lugs 282 extend radially from insulator tower 256 adjacent end
surface 276.
[0029] Platform mount 254 includes a mounting plate 284 having a
bushing 286 at or near its center and having an off-center through
hole 288 extending between its major surfaces. Platform mount 254
is attached to insulator tower 256 with bushing 286 located in
blind bore 270, mounting plate 284 in contact with end surface 272,
and the end 281 of spring-loaded contact assembly 258 remote from
spring contact 280 engaged with through hole 288. In an example,
platform mount 254 is affixed to insulator tower 256 by machine
screws (not shown) passing through holes (not shown) in mounting
plate 284 into threaded holes (not shown) in insulator tower 256.
Sample platform 252 is mounted on insulator tower 256 by inserting
the end of the pedestal 264 remote from sample plate 260 into the
bushing 286 of platform mount 254.
[0030] Sample carrier 240 includes a substantially L-shaped
armature 300, that can be regarded as having a mounting portion 302
and a handle portion 304. Handle portion 304 facilitates the
manipulation of sample carrier assembly 242 as the sample carrier
assembly is removed from, and placed on, kinematic base 230.
Mounting portion 302 has opposed, parallel, planar major surfaces
306, 308. A through hole 310 extends through mounting portion 302
between major surfaces 306, 308. Alignment holes 312 extend into
mounting portion 302 from major surface 308. In the example shown,
alignment holes 312 are blind holes that extend partway into
mounting portion 302. In another example, alignment holes 312
extend all the way through mounting portion 302.
[0031] When sample holder 250 is mounted on sample carrier 240 to
form sample carrier assembly 242 (FIGS. 3A, 3B), insulator tower
256 is mounted on the mounting portion 302 of sample carrier 240
with the end surface 276 of the insulator tower abutting major
surface 306 and cylindrical insulator 278 and spring contact 280
located within through hole 310. In an example, insulator tower 256
is affixed to mounting portion 302 by machine screws (not shown)
passing through lugs 282 into threaded holes (not shown) in
mounting portion 302. When mounted as described, the planar major
surface 262 of the sample plate 260 of sample holder 250 is
parallel to the major surface 306 of mounting portion 302.
[0032] Best seen in FIG. 3C, kinematic base 230 includes a base
plate 320, a fixed contact 322, an insulated cavity 324, a
conductor 326, a connector 328 and alignment protrusions 330. Base
plate 320 has opposed, parallel, planar major surfaces 332, 334,
and an end surface 336 that extends between the major surfaces. An
L-shaped cavity 338 extends into base plate 320 in the minus
z-direction from major surface 332, and in the minus x-direction
from end surface 336. Conductor 326 runs along the axes of cavity
338 from fixed contact 322 to connector 328. Cavity 338 is filled
with an electrical insulator 340 to form insulated cavity 324 in
which conductor 326 is insulated from base plate 320. Additionally,
insulator 340 is shaped to define a hollow, cylindrical insulator
342 surrounding, and extending in the z-direction relative to,
fixed contact 322, and to define a cavity 344 around the fixed
contact. A flexible cable (not shown) having on one end a connector
(not shown) configured to mate with connector 328 is used to
connect connector 328 to the positive output terminal of auxiliary
acceleration voltage source 130 (FIG. 1). Alignment protrusions 330
extend in the z-direction from major surface 332 in alignment with
the alignment holes 312 of sample carrier 240.
[0033] When sample carrier assembly 242 is mounted on kinematic
base 230, the major surface 308 of the mounting portion 302 of
sample carrier 240 abuts the major surface 332 of kinematic base
230 to define the location of sample carrier assembly 242 in the
z-direction, and alignment holes 312 engage with alignment
protrusions 330 extending from the major surface 332 of the
kinematic base to define the position of sample carrier assembly
242 in the x-y plane relative to the kinematic base. Sample carrier
assembly 242 is retained in position on kinematic base 230 by
gravity, but can be secured, for example, using suitable fasteners.
Other ways of defining the alignment of sample carrier assembly 242
relative to kinematic base 230 are known and may be used.
[0034] Additionally, cylindrical insulator 278 protruding from
insulator tower 256 receives cylindrical insulator 342 protruding
from the major surface 332 of kinematic base 230, and, within the
volume defined by the cylindrical insulators, spring contact 280
extending from spring-loaded contact assembly 258 electrically
contacts fixed contact 322. This forms a robust electrical
connection via pedestal 264, platform mount 254, spring-loaded
contact assembly 258, spring contact 280, fixed contact 322,
conductor 326 and connector 328 between the sample plate 260 of
sample holder 250 and the flexible cable (not shown) connected to
auxiliary acceleration voltage source 130. As noted above, the
electrical connection is automatically broken when sample carrier
assembly 242 is removed from kinematic base 230, and is
automatically restored when the sample carrier assembly is placed
on the kinematic base.
[0035] Referring again to FIG. 1, during operation of spectroscopy
apparatus 100, controller 190 initially controls SEM 110 to perform
a conventional scanning electron microscopy (SEM) operation to
identify the location of a feature of interest on a sample (not
shown) placed on sample holder 140. No auxiliary acceleration
voltage is applied between sample holder 140 and led electron beam
column 150 during the SEM operation. In the SEM operation,
controller 190 provides column control signals CC to layered
electron beam column 150 to cause the layered electron beam column
to steer electron beam 152 to perform a raster scan of the surface
of the sample. For large samples, controller 190 additionally
provides stage control signals SC to positioning stage 146 to cause
the positioning stage to move the sample in steps in the x- and
y-directions relative to the column axis 156 of layered electron
beam column 150. In an example, the steps in which the positioning
stage moves in the x- and y-directions are equal to the x- and
y-dimensions, respectively, of the area of the sample holder
scanned by the electron beam with the sample holder in a static
position. In this disclosure, this scanned area will be referred to
as the field of view of the electron beam, and the length of the
field of view in the x-direction or in the y-direction will be
referred to as the scan length of the electron beam. At each step
of the movement of the positioning stage, controller 190 provides
column control signals CC to layered electron beam column 150 to
cause the layered electron beam column to perform a raster scan of
the portion of the sample centered on column axis 156. Electron
detector 170 detects the backscattered electrons and secondary
electrons stimulated by electron beam 152 at its initial beam
energy to generate electron detection signal ES. Controller 190
associates values of electron detection signal ES with the X and Y
coordinates of the location at which the electron beam is incident
on the sample to generate an image signal.
[0036] Controller 190 then activates auxiliary acceleration voltage
source 130 to apply the auxiliary acceleration voltage between
layered electron beam column 150 and sample holder 140, and
provides column control signals CC to layered electron beam column
150 to cause the layered electron beam column to steer electron
beam 152 to the location of the feature of interest on the sample.
In some embodiments, with large samples, controller 190
additionally provides stage control signals SC to positioning stage
146 to cause the positioning stage to align a portion of the sample
containing the feature of interest with the column axis 156 of
layered electron beam column 150. In response to electron beam 152
at its final beam energy, the feature of interest on the sample
emits x-rays at wavelengths that depend on the atomic species
constituting the feature of interest. X-ray detector 120 detects
the x-rays and in response thereto generates x-ray detection signal
XS. X-ray detector 120 outputs x-ray detection signal XS to
controller 190. Controller 190 processes the x-ray detection signal
to obtain a spectrum from which the atomic species constituting the
feature of interest can be identified.
[0037] However, application of the auxiliary acceleration voltage
between layered electron beam column 150 and sample holder 140
changes the relationship between column control signals CC that
control the steering of electron beam 152 by layered electron beam
column 150 and the location at which electron beam 152 is incident
on the sample. Thus, a calibration operation should be performed
prior to using layered electron beam column 150 to steer electron
beam 152 to the location of the feature of interest on the sample
with the auxiliary acceleration voltage applied.
[0038] FIGS. 4A-4D are schematic drawings showing examples of the
effect of the auxiliary acceleration voltage on the trajectories of
the electrons of electron beam 152 (FIG. 1) and the range over
which layered electron beam column can scan electron beam 152. The
examples shown in FIGS. 4A-4D are highly simplified and idealized.
Factors such as edge effects, surface roughness or topography,
stray fields, and the geometry of the sample that can affect the
trajectories of electron beam 152 are ignored in the following
description on the assumption that the effect of these factors on
the trajectories is relatively small compared with the
below-described effect of the auxiliary acceleration voltage. Each
of FIGS. 4A-4D shows layered electron beam column 150, column axis
156, sample platform 142 and a sample S on the surface of the
sample platform. Sample platform 142 is centered in the x-y plane
on the column axis 156 of layered electron beam column 150 in FIGS.
4A and 4B. Sample platform 142 is off-center in the x-direction
relative to column axis 156 in FIGS. 4C and 4D. No auxiliary
acceleration voltage is applied in FIGS. 4A and 4C. The auxiliary
acceleration voltage is applied between sample platform 142 and
layered electron beam column 150 in FIGS. 4B and 4D. Each figure
additionally shows the 2-D image resulting from each scan. The
image is a scanning electron image in FIGS. 4A and 4C, and an x-ray
image in FIGS. 4B and 4D.
[0039] In FIG. 4A, SEM 110 is used to generate a scanning electron
microscopy image of the surface of sample S with sample platform
142 grounded. Sample S has a first feature F1 and a second feature
F2 on its surface. Feature F2 is located further off-center
relative to the sample and feature F1. Lines 350, 351 indicate the
trajectories of electron beam 152 at the extreme deviations of the
scan of the electron beam from column axis 156.
[0040] In FIG. 4B, SEM 110 is used to generate an energy dispersive
spectroscopy image of sample S with the auxiliary acceleration
voltage applied between sample platform 142 and layered electron
beam column 150. The auxiliary acceleration voltage applied to
sample platform 142 attracts the electrons of electron beam 152
causing the electrons to be deflected, in this example, towards the
sample platform. This results in the electrons in the electron beam
152 having altered trajectories (indicated at the extreme
deviations of the scan by lines 352, 353), and the field of view on
sample S typically having reduced dimensions in the x-y plane.
However, the coordinates of feature F1 in the field of view,
relative to the center of the field of view, scale proportionally
to the reduced dimensions of the field of view. In case of the
cylindrical symmetry resulting from sample platform 142 being
centered on column axis 156, the reduction in the scan length of
the electron beam is symmetric with respect to column axis 156.
Such a symmetrical arrangement, however, is not required for
calibration.
[0041] Conditions in FIG. 4C are the same as those in FIG. 4A,
except that sample platform 142 has been displaced laterally in the
-x-direction so that feature F2 can be imaged. Consequently, sample
platform 142 is no longer centered in the x-y plane on column axis
156. Despite this asymmetry, the trajectories of electron beam 152,
indicated by lines 350, 351, and the generated image are the same
as the electron beam trajectories and the generated image,
respectively, shown in FIG. 4A.
[0042] Conditions in FIG. 4D are the same as those in FIG. 4B,
except that sample platform 142 has been displaced laterally in the
-x-direction, similar to FIG. 4C. The off-center position of sample
platform 142 relative to column axis 156 modifies the trajectories
of electrons of electron beam 152 (indicated at the extreme
deviations of the scan by lines 354, 355), shifting them in the
direction of displacement of the sample platform. The coordinates
of feature F2 in the field of view, relative to the center of the
field of view, again scale proportionally to the reduced dimensions
of the field of view, but feature F2 is additionally shifted in the
x-direction relative to the center of the field of view.
[0043] The effect of moving sample platform 142 laterally can be
modeled by adding a virtual piece 356 of sample platform to the
sample platform to restore the symmetry of the sample platform
relative to column axis 156. If the auxiliary acceleration voltage
were applied to virtual piece 356, the trajectory of electrons of
electron beam 152 would be exactly the same as that shown in FIG.
4B. This assumes that the distance between layered electron beam
column 150 and sample platform 142 is much smaller than the size of
the sample platform in the x-y plane. However, the net charge on
virtual piece 356 is zero. This condition can be achieved by
offsetting the positive charge resulting from the auxiliary
acceleration voltage with an equal and opposite negative charge.
The additional negative charge moves the field of view in the
-x-direction, the direction in which sample platform 142 is
shifted.
[0044] FIG. 5 is a flowchart showing an example 400 of a process
for calibrating scanning electron microscopy and energy dispersive
spectroscopy apparatus 100. In an embodiment, calibration process
400 is typically performed by controller 190 (FIG. 1). The
calibration process calculates a scale factor between an image
generated by spectroscopy apparatus 100 in scanning electron
microscopy mode (SEM mode), i.e., with auxiliary acceleration
voltage turned off, and an image generated by the spectroscopy
apparatus in energy dispersive spectroscopy mode (EDS mode), i.e.,
with the auxiliary acceleration voltage turned on. The calibration
process additionally calculates an image shift between an image
generated by spectroscopy apparatus 100 in SEM mode and an image
generated by spectroscopy apparatus 100 in EDS mode. While the
calibration method could be refined to produce calibration data
with an accuracy sufficient to enable the location of a feature of
interest on a sample determined in SEM mode to be translated
directly to stage control signals SC and column control signals CC
that achieve the same relative positioning between the electron
beam and the sample in EDS mode, the calibration method described
below produces calibration data with only with the accuracy needed
to enable an indirect positional translation. The accuracy of the
calibration data is sufficient to ensure that the feature of
interest is located within the field of view of the electron beam
in EDS mode. With the sample positioned relative to the electron
beam such that the feature of interest is within the field of view
of the electron beam, an EDS-mode image is generated, the position
of the feature of interest in the EDS-mode image is determined, and
the electron beam is then steered to the determined position of the
feature of interest to perform the energy-dispersive
spectroscopy.
[0045] In calibration process 400, in block 402, a periodic test
structure is placed on sample holder 140, and SEM 110 is operated
to generate images of the periodic test structure. An SEM-mode
image is generated in SEM mode, i.e., with auxiliary acceleration
voltage turned off, and an EDS-mode image is generated in EDS mode,
i.e., with the auxiliary acceleration voltage turned on. Sample
holder 140 remains static during this operation.
[0046] In block 404, a scan length of electron beam 152 with SEM
110 in SEM mode, i.e., with auxiliary acceleration voltage turned
off, and a scan length of the electron beam with SEM 110 in EDS
mode, i.e., with the auxiliary acceleration voltage turned on, are
calculated in real-world units, such as millimeters. As noted
above, the scan length of electron beam 152 is the x-direction or
y-direction dimension of the field of view of the electron beam,
and the field of view of the electron beam is the area of sample
platform 142 scanned by the electron beam. In an example, the
respective scan length is calculated by determining the number of
periods of the periodic test structure in the direction
corresponding to the scan length in each image. The number of
periods in each image is then multiplied by the known pitch of the
periodic structure to generate the respective scan length.
[0047] In block 406, a ratio of the scan lengths calculated in
block 404 is calculated to provide the scale factor calibration. In
an example, the scan length of the EDS-mode image is divided by the
scan length of the SEM-mode image to generate the scale factor
calibration. In another example, an x-direction scan length of the
EDS-mode image is divided by an x-direction scan length of the
SEM-mode image to generate an x-direction ratio, a y-direction scan
length of the EDS-mode image is divided by a y-direction scan
length of the SEM-mode image to generate a y-direction ratio, and
the x-direction ratio and the y-direction ratio are averaged to
provide the scale factor calibration.
[0048] In block 408, a non-periodic test structure containing
distinct features is placed on sample holder 140, and SEM 110 is
operated to image the non-periodic test structure with sample
holder 140 located in a number of different positions in the x-y
plane relative to column axis 156. In an example, the positions of
the sample holder are offset from one another in the x-direction or
the y-direction by distances equal to the EDS mode x-direction scan
length and the EDS mode y-direction scan length, respectively. In
another example, a substantially larger offset is used. In each
position of the sample holder, a respective pair of images is
generated. Each pair of images consists of one SEM-mode image taken
with the auxiliary acceleration voltage turned off, and one
EDS-mode image taken with the auxiliary acceleration voltage turned
on.
[0049] In block 410, for each position of sample holder 140, from
the respective pair of images taken at that position, a feature in
the respective SEM-mode image is mapped to the same feature in the
EDS-mode image using the scale factor calculated in block 406, and
an image shift between the feature in the mapped SEM-mode image and
the feature in the EDS-mode image is calculated to provide a
respective image shift.
[0050] In block 412, a mathematical model is generated that
represents the image shifts between the features in the mapped
SEM-mode images and corresponding features in the EDS-mode images
as functions of the position of the sample holder. In an example,
the mathematical model is a mathematical equation that, for any
position of sample holder 140, quantifies the image shift that
occurs when SEM 110 is switched from SEM mode to EDS mode. In
another example, the mathematical model is a table that, for any
position of sample holder 140, quantifies the image shift that
occurs when the SEM is switched from SEM mode to EDS mode. Table
data for positions of sample holder 140 intermediate between those
at which images are generated in block 408 can be calculated by
interpolation.
[0051] In embodiments of spectroscopy apparatus 100 in which
positioning stage 146 is capable of tilting sample holder 140 about
an axis parallel to the x-y plane, the symmetry shown in FIG. 4B
does not exist except when sample holder 140 is parallel to the x-y
plane. However, the calibration process described above with
reference to FIG. 5 can be used to generate calibration data for
such embodiments. The calibration process is performed at a number
of discrete tilt angles of sample holder 140 to generate respective
scale factors and image shifts. Scale factors and image shifts for
tilt angles intermediate between the tilt angles at which the
images are generated in blocks 402 and 408 can be calculated by
interpolation
[0052] FIG. 6 is a flowchart showing an example 420 of a method for
generating the energy-dispersive spectrum of a feature of interest
of a sample using spectroscopy apparatus 100. In an embodiment,
method is performed by controller 190 (FIG. 1). In block 422, SEM
110 is used to image the sample in SEM mode with scaled positioning
signals. In this, the positioning functions of SEM 110 are
temporarily operated as if the SEM were in EDS mode. Specifically,
the beam steering signals that constitute part of the column
control signals CC generated by controller 190 (FIG. 1) are scaled
by the scale factor generated in block 406 of calibration process
400 described above with reference to FIG. 5 prior to being output
to layered electron beam column 150. Scaling the beam steering
signals makes the scan length of electron beam 152 in SEM mode
equal to the scan length in EDS mode. Additionally, the stage
control signals SC generated by controller 190 are scaled by the
scale factor prior to being output to positioning stage 146.
Scaling the stage control signals makes the positioning steps of
positioning stage 146 in the SEM mode equal in size to the
positioning steps of the positioning stage in EDS mode, and
compensates for the reduced field of view of electron beam 152
caused by the scaled beam steering signals. The beam steering
signals that constitute part of the column control signals CC and
stage control signals SC are referred to herein collectively as
positioning signals.
[0053] In block 424, the states of the original (un-scaled)
positioning signals generated by controller 190 when electron beam
152 is incident on the feature of interest are memorized. In block
426, spectroscopy apparatus 100 is switched to EDS mode. To switch
SEM 110 to EDS mode, the auxiliary acceleration voltage is applied
between sample holder 140 and layered electron beam column 150, and
the scaling is removed from the positioning signals. In block 428,
the memorized states of the beam steering signals are input to
layered electron beam column 150 and the memorized states of stage
control signals SC are input to positioning stage 146. In block
430, the image shift corresponding to the current position of
sample holder 140 is obtained from the mathematical model generated
in block 412 of calibration process 400 and the positioning stage
is operated in response to the image shift to move sample holder
140 a distance equal and opposite to the image shift. This locates
the feature of interest within the field of view of electron beam
152. Alternatively, when the image shift is small, e.g., less than
one half of the scan length of electron beam 152, the electron beam
is steered a distance equal and opposite to the image shift to
compensate for the image shift.
[0054] In block 432, a test is performed to determine whether the
feature of interest is small. A small feature of interest is
smaller than the field of view of electron beam 152 in EDS mode. A
NO result in block 432 causes execution to advance to block 440,
where the EDS spectrum of the feature of interest is generated, as
will be described below. A YES result in block 432 causes execution
to advance to block 434, described next.
[0055] In block 434, column control signals CC are provided to
layered electron beam column 150 to cause the layered electron beam
column to perform an initial EDS-mode scan. In the initial EDS-mode
scan, a raster scan of the electron beam is performed while
detecting x-rays of specific energies at x-ray detector 120. The
x-ray energies are dependent on the material of the feature of
interest.
[0056] In block 436, the position of the feature of interest in the
initial EDS-mode scan is determined. Then, in block 438, column
control signals CC are provided to layered electron beam column 150
to cause the electron beam column to steer electron beam 152 to the
position of the feature of interest determined in block 436.
Finally, in block 440, the energy-dispersive spectrum of the
feature of interest is generated by detecting the x-rays generated
by electron beam 152 at its final beam energy incident on the
feature of interest.
[0057] Referring briefly to FIG. 1, in the example of spectroscopy
apparatus 100 shown therein, the close proximity of layered
electron beam column 150 and sample holder 140 results in x-ray
detector 120 being located substantially off-axis relative to
column axis 156. With the off-axis location of the x-ray detector,
the signal-to-noise ratio of the x-ray detection signal XS output
by the x-ray detector may be less than optimum.
[0058] X-ray detectors are typically highly sensitive to electrons.
Electron beam 152 incident at its final beam energy on a sample
(not shown) placed on sample holder 140 generates not only x-rays
but also backscatter electrons and secondary electrons that are
emitted towards x-ray detector 120. Conventionally, x-ray detector
120 would include an electron trap to filter out electrons that
would otherwise impair the signal-to-noise ratio of x-ray detection
signal XS. As well as increasing the range of atomic species from
which x-rays can be generated at multiple wavelengths, the
auxiliary acceleration voltage acts as an inherent electron trap.
The auxiliary acceleration voltage accelerates the backscatter
electrons and secondary electrons towards sample holder 140, and,
hence, away from x-ray detector 120. This allows a simpler x-ray
detector that lacks a separate electron trap to be used as x-ray
detector 120. The simpler x-ray detector is small enough to be
integrated with layered electron beam column 150, which allows the
x-ray detector to be located much closer to column axis 156.
[0059] FIGS. 7A-7D, 8 and 9 are cross-sectional views showing
examples of an x-ray generation and detection system that may be
used as layer electron beam column 150 and x-ray detector 120 in
spectroscopy apparatus 100 described above with reference to FIG.
1. FIG. 7A shows an example 500 of an x-ray generation and
detection system composed of a layered electron beam column 502 and
an integrated x-ray detector 504. An example of a solid state x-ray
detector that can be integrated with layered electron beam column
502 is a silicon drift detector (SDD). This type of detector has a
small anode capacitance compared to conventional x-ray detectors
such as Si(Li) and germanium x-ray detectors. The small anode
capacitance of the SDD leads to a higher energy resolution at
shorter shaping times compared with conventional x-ray detectors.
This is advantageous in high-count rate applications.
[0060] Silicon drift detector dies that include a silicon drift
detector and integrated amplifying electronics fabricated in and on
the die have been developed and are commercially available. Such
detectors provide adequate signal-to-noise ratios at room
temperature. In applications that require an increased
signal-to-noise ratio, the SDD can be mounted on the cold surface
of a cooling system to allow operation at temperatures down to
about -15.degree. C., as will be described below with reference to
FIG. 8. SDD dies are more robust than other types of x-ray
detectors for non-laboratory and environmental applications because
noise caused by mechanical vibrations is eliminated and electrical
pickup is significantly reduced.
[0061] In the example shown in FIG. 7A, layered electron beam
column 502 includes a stack 512 of insulating layers. An exemplary
insulating layer is shown at 514. Reference numeral 514 will
additionally be used to refer to insulating layers in general.
Insulating layer 514 has a planar major surface 518 facing sample
holder 140 when layered electron beam column 502 constitutes part
of SEM 110 (FIG. 1). Insulating layer 514 additionally has a planar
major surface 518 opposite and parallel to major surface 516, and a
bore 520 extending between major surfaces 516, 518 near the center
of the major surfaces. Insulating layer 514 has a functional
element 522 mounted on the major surface 516 and overlapping bore
520. Reference numeral 522 will additionally be used to refer to
functional elements in general. Functional element 522 has a
centrally-located electron path 524. The others of the insulating
layers are similar to insulating layer 514, but some have a
respective functional element mounted on each of its major
surfaces, some have no functional element mounted thereon, and some
have a respective functional element mounted on their major surface
518 instead of major surface 516. Insulating layers 514 are stacked
with the respective electron paths 524 of their functional elements
522 centered on column axis 156. Column control signals CC (FIG. 1)
applied to the functional elements and/or between the functional
elements enable layered electron beam column 502 to perform such
functions as extracting and collimating electrons 162 output by
electron source 160 (FIG. 1), and focusing, blanking and steering
electron beam 152 output by layered electron beam column 502.
[0062] In the example shown, integrated x-ray detector 504 is in
the form of a silicon drift detector (SDD) die 530. SDD die 530
includes a silicon drift detector 532 and integrated amplifying
electronics 536. Silicon drift detector 532 generates a detection
signal in response to x-rays 540 incident thereon. Amplifying
electronics 536 amplify the detection signal generated by silicon
drift detector 532 to generate a robust x-ray detection signal XS
for output to controller 190 (FIG. 1).
[0063] SDD die 530 is mounted on the insulating layer 534 of
layered electron beam column 502 closest to sample holder 140.
Specifically, SDD die 530 is mounted close to column axis 156 on
the major surface 538 of insulating layer 534. Major surface 538 is
the major surface of insulating layer 534 facing sample holder 140.
The SDD die can be mounted on major surface 538 using a
conventional die mounting technique commonly used in the
semiconductor industry. Mounting SDD die 530 on layered electron
beam column 502 facing sample holder 140 enables silicon drift
detector 532 to receive the higher intensity of x-rays that are
emitted at relatively small angles relative to column axis 156.
[0064] FIG. 7B shows another example 550 of an x-ray generation and
detection system composed of layered electron beam column 502,
described above with reference to FIG. 7A, and an integrated x-ray
detector 552. Elements of x-ray generation and detection system 550
that correspond to elements of x-ray generation and detection
system 500 described above with reference to FIG. 7A are indicated
using the same reference numerals and will not be described again
in detail. In the example of x-ray generation and detection system
550 shown, integrated x-ray detector 552 includes an SDD die 554,
an SDD die 556, and a summing circuit 558. SDD die 554 and SDD die
556 are mounted on the insulating layer 534 of layered electron
beam column 502 closest to sample holder 140. Specifically, SDD
dies 554, 556 are mounted on the major surface 538 of insulating
layer 534 close to column axis 156 and on opposite sides thereof.
Major surface 538 is the major surface of insulating layer 534
facing sample holder 140.
[0065] SDD dies 554, 556 are each similar to SDD die 530 described
above with reference to FIG. 7A. Each SDD die 554, 556 has a
respective x-ray detection signal output electrically connected to
a respective input of summing circuit 558. Summing circuit 558
additionally has an output. In response to x-rays 540 incident
thereon each SDD die 554, 556 generates a respective x-ray
detection signal component XS1, XS2. Summing circuit 558 sums x-ray
detection signal components XS1, XS2 to generate x-ray detection
signal XS having a higher signal-to-noise ratio than either of the
x-ray detection signal components.
[0066] In other examples of x-ray generation and detection system
550, integrated x-ray detector 552 includes one or more additional
SDD dies (not shown) mounted on major surface 538 around column
axis 156, and summing circuit 558 has a corresponding number of
inputs. The respective x-ray detection signal components generated
by the one or more additional SDD dies, when summed, further
increase the signal-to-noise ratio of x-ray detection signal
XS.
[0067] FIG. 7C shows another example 560 of an x-ray generation and
detection system composed of layered electron beam column 502,
described above with reference to FIG. 7A, and an integrated x-ray
and electron detector 562. Elements of x-ray generation and
detection system 560 that correspond to elements of x-ray
generation and detection system 500 described above with reference
to FIG. 7A are indicated using the same reference numerals and will
not be described again in detail. In the example of x-ray
generation and detection system 560 shown, integrated x-ray and
electron detector 562 includes the x-ray detector 120 and the
electron detector 170 of spectroscopy apparatus 100 described above
with reference to FIG. 1 implemented on the same multi-detector
die. X-ray and electron detector 562 includes a multi-detector die
564 in and on which are fabricated an x-ray detector 566, an
electron detector 568 and integrated amplifying electronics 570. In
an example, x-ray detector 566 is a silicon drift detector. Other
types of x-ray detectors are known and may be included in
integrated x-ray and electron detector 562. In an example, electron
detector 568 includes a silicon photodiode. Other types of electron
detector are known and may be included in integrated x-ray and
electron detector 562. Alternatively, the silicon drift detector
used as x-ray detector 566 may additionally serve as electron
detector 568. Multi-detector die 564 is mounted on the insulating
layer 534 of layered electron beam column 502 closest to sample
holder 140 in a manner similar to SDD die 530, described above.
[0068] When SEM 110 operates in SEM mode, electron detector 568
generates an electron detection signal in response to backscattered
electrons and secondary electrons stimulated by electron beam 152
and incident thereon. Amplifying electronics 570 amplify the
electron detection signal generated by electron detector 568 to
generate a robust electron detection signal ES for output to
controller 190 (FIG. 1). When SEM 110 operates in EDM mode, x-ray
detector 566 generates an x-ray detection signal in response to
x-rays 540 incident thereon. Amplifying electronics 570 amplify the
x-ray detection signal generated by x-ray detector 566 to generate
a robust x-ray detection signal XS for output to controller
190.
[0069] In other examples of x-ray generation and detection system
560, integrated x-ray and electron detector 562 includes one or
more additional multi-detector dies (not shown) mounted on major
surface 538 around column axis 156. Each of the multi-detector dies
is similar to multi-detector die 564. The integrated x-ray and
electron detector additionally includes respective summing circuits
to sum the electron detection signals and the x-ray detection
signals generated by all the multi-detector dies to generate
electron detection signal ES and x-ray detection signal XS each
having a higher signal-to-noise ratio than the electron detection
signals and the x-ray detection signals respectively generated by
the individual multi-detector dies. Some embodiments have
x-ray-only and/or electron-only detector dies mounted on major
surface 538 in addition to multi-detector dies.
[0070] FIG. 7D is a cross-sectional view showing the operation of
x-ray generation and detection system 500, described above with
reference to FIG. 7A, with sample holder 140 tilted such that
electron beam 152 is incident on its surface at a non-zero angle of
incidence relative to the normal to the surface. In an example, an
implementation of positioning stage 220 described above with
reference to FIGS. 3A-3D having a tilting capability is used to
tilt the sample holder. Tilting the sample holder enables silicon
drift detector 532 on SDD die 530 to receive the higher-intensity
x-rays emitted at small angles relative to the normal, which can
increase the signal-to-noise ratio of x-ray detection signal XS.
The x-ray generation and detection systems described above with
reference to FIGS. 7B and 7C and to be described below with
reference to FIGS. 8 and 9 can also be operated with sample holder
140 tilted.
[0071] Locating the x-ray detector on a surface of layered electron
beam column 502 facing sample holder 140, as shown in FIGS. 7A-7D,
generates x-ray detection signal XS with a higher signal-to-noise
ratio than a similar x-ray detector located further from column
axis 156. FIG. 8 shows another example 600 of an x-ray generation
and detection system composed of a layered electron beam column 602
having an integrated cooled x-ray detector 604. X-ray generation
and detection system 600 is suitable for use in applications in
which a further increase in the signal-to-noise ratio of x-ray
detection signal XS is needed. Elements of system 600 that
correspond to elements of system 500 described above with reference
to FIG. 7A are indicated using the same reference numerals with 100
added. In the example shown in FIG. 8, layered electron beam column
602 includes a stack 612 of insulating layers. An exemplary
insulating layer is shown at 614. Reference numeral 614 will
additionally be used to refer to insulating layers in general.
Insulating layers 614 are similar to insulating layers 514
described above with reference to FIG. 7A, but each additionally
includes a heat-pipe bore 626 extending between major surfaces 616
and 618 at a location laterally offset from electron path 624.
Insulating layers 614 are stacked with the respective electron
paths of their functional elements 622 centered on column axis 156,
and with heat-pipe bores 626 arranged coaxially.
[0072] Integrated cooled x-ray detector 604 includes an x-ray
detector thermally coupled to a cooling system. In the example
shown, the x-ray detector includes silicon drift detector (SDD) die
530, and the cooling system includes a thermoelectric cooler 628,
such as a Peltier cooler, and a heat pipe 630. SDD die 530 is
mounted on the cold surface of thermoelectric cooler 628, and heat
pipe 630 is thermally coupled to the hot surface of the
thermoelectric cooler, opposite the cold surface. Thermoelectric
cooler 628 with SDD die 530 mounted thereon is mounted on the
insulating layer 634 of layered electron beam column 602 closest to
sample holder 140 with heat pipe 630 extending through heat-pipe
bore 626 to an external heat sink (not shown). Specifically,
thermoelectric cooler 628 with SDD die 530 mounted thereon is
mounted close to column axis 156 on the major surface 638 of
insulating layer 634. Major surface 638 is the major surface of
insulating layer 634 facing sample holder 140. Mounting SDD die 530
on layered electron beam column 602 facing sample holder 140
enables the silicon drift detector 532 on SDD die 530 to receive
the higher-intensity x-rays that are emitted at relatively small
angles relative to column axis 156. Supplying electric current to
thermoelectric cooler 628 extracts heat from SDD die 530 to improve
the signal-to-noise ratio of x-ray detection signal XS.
[0073] In other examples of x-ray generation and detection system
600, integrated cooled x-ray detector 604 includes one or more
additional SDD dies mounted on major surface 686 around column axis
156. Each of the SDD dies is similar to SDD die 530. In some
examples, each additional SDD die is thermally coupled to its own
cooling system. In other examples, all the SDD dies, or a subset of
the SDD dies, share a common cooling system. A summing circuit,
similar to summing circuit 558 described above with reference to
FIG. 7B, sums the x-ray detection signal components generated by
the SDD dies to generate an x-ray detection signal XS with a higher
signal-to-noise ratio than the x-ray detection signal components. A
multi-detector die on which are integrated an x-ray detector and an
electron detector, as described above with reference to FIG. 7C,
may be substituted for SDD die 530 and/or any of the additional SDD
dies.
[0074] Another way to increase the signal-to-noise ratio of x-ray
detection signal XS as is to tilt SDD die 530 such that x-rays 540
are incident on silicon drift detector 532 at a small angle of
incidence relative to the normal to the surface of the die. FIG. 9
is a cross-sectional view another example 650 of an x-ray
generation and detection system composed of a layered electron beam
column 652 having an integrated x-ray detector 654 suitable for use
in applications in which a further increase in the signal-to-noise
ratio of the x-ray detection signal XS is needed. Elements of
system 650 that correspond to elements of system 500 described
above with reference to FIG. 7A are indicated using the same
reference numerals with 150 added. In the example shown, layered
electron beam column 652 includes a stack 662 of insulating layers.
An exemplary insulating layer is shown at 664. Reference numeral
664 will additionally be used to refer to insulating layers in
general. Insulating layers 664 are similar to insulating layers 514
described above with reference to FIG. 7A, but respective portions
of at some of the insulating layers 664 are removed (or are not
initially formed) to define a cuboidal or cylindrical detector
mounting chamber 690 that includes a substantially planar detector
mounting surface 692. Insulating layer 684 is the insulating layer
closest to sample holder 140. Detector mounting surface 692 is
inclined relative to the major surface 688 of insulating layer 684
and is oriented such that a normal to the mounting surface and
column axis 156 intersect at sample holder 140 at the nominal
working distance of sample holder 140 from layered electron beam
column 652. SDD die 530 is mounted on inclined detector mounting
surface 692 using a conventional die mounting technique. The
removed or not initially formed portions of insulating layers 664
additionally define a cylindrical or cuboidal passageway 696
extending normally to detector mounting surface 692 from major
surface 688 to a region of detector mounting chamber 690 aligned
with the silicon drift detector 532 of SDD die 530. Mounting SDD
die 530 on inclined detector mounting surface 692 within detector
mounting chamber 690 enables silicon drift detector 532 to receive
the higher-intensity x-rays 540 that are emitted at relatively
small angles relative to column axis 156 at a substantially normal
angle of incidence at which reflection at the surface of the
detector is minimized. In some embodiments, a thermoelectric
cooler, similar to that described above with reference to FIG. 8,
is interposed between SDD die 530 and detector mounting surface
692.
[0075] In other examples of x-ray generation and detection system
650, insulating layers 664 are shaped to define one or more
additional detector mounting chambers and respective passageways
around column axis 156. The detector mounting chambers and
respective chambers are similar to detector mounting chamber 690
and passageway 696, respectively. In such examples, integrated
x-ray and electron detection system 650 additionally includes a
respective SDD die mounted on the detector mounting surface of each
additional detector mounting chamber. Each of the SDD dies is
similar to SDD die 530. A summing circuit, similar to summing
circuit 558 described above with reference to FIG. 7B, sums the
x-ray detection signal components generated by the SDD dies to
generate x-ray detection signal XS with a higher signal-to-noise
ratio than the x-ray detection signal components. A multi-detector
die on which are integrated an x-ray detector and an electron
detector, as described above with reference to FIG. 7C, may be
substituted for SDD die 530 and/or any of the additional SDD
dies.
[0076] This disclosure describes the invention in detail using
illustrative embodiments. However, the invention defined by the
appended claims is not limited to the precise embodiments
described.
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