Note: Descriptions are shown in the official language in which they were submitted.
CA 02336369 2000-12-29
WO 00/67289 PCT/i1S00/40017
APPARATUS AND METHOD FOR REDUCING CHARGE
ACCUMULATION ON A SUBSTRATE
BACKGROUND
1. Field of the Invention
This invention relates to charged particle beam columns,
and more specifically to techniques for reducing surface
charge on a target substrate.
2. Description of The Related Art
Charged particle beam columns and microcolumns are well
known in the arts of lithography and electron microscopy
imaging, i.e., using a charged particle beam (e.g. of
electrons or ions) to measure feature dimensions and view a
surface sensitive to a charged particle beam. See, e.g.,
"Electron-Beam Microcolumns for Lithography and Related
Applications," by T.H.P. Chang et al., Journal of Vacuum
Science Technology Bulletin 14(6), pp. 3774-81, Nov./Dec.
1996, incorporated herein by reference in its entirety.
FIG. 1 depicts a conventional charged particle beam
column 100 that is well known in the art for, e.g., electron
beam lithography. A conventional charged particle beam
column 100 includes, e.g., a charge particle (electron)
source 102 that outputs a charged particle beam 114; a
limiting aperture 104 positioned downstream with respect to
the direction of charged particle beam 114 from charged
particle source 102 (hereafter "downstream" means downstream
with regard to a charged particle beam direction from charged
particle source): a transfer lens 106 positioned downstream
from limiting aperture 104, where the transfer lens 106
controls the focal point of the charged particle beam 114; a
blanking system 108, positioned downstream from transfer lens
106, that includes blanking deflectors 116 and blanking
aperture 118, where blanking deflectors 116 cause charged
particle beam 114 to intersect blanking aperture 118: a
-1-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
deflection system 110 positioned downstream from blanking
system 108, where deflection system 110 controls the location
that charged particle beam 114 intersects surface 120: and an
objective lens 112 positioned downstream from deflection
system 110 that focuses and controls the cross section size
of charged particle beam 114 on surface 120.
FIG. 2 depicts a conventional microcolumn 200 that is
well known in the prior art. Microcolumn 200 includes, e.g.,
a beam emitter 202 which emits a charged particle beam 204: a
source lens 206 positioned downstream from beam emitter 202;
a deflection system 208 positioned downstream from source
lens 206, where deflection system 208 controls a location
that charged particle beam 204 hits surface 212; and an
einzel lens 210 positioned downstream from deflection system
208.
When a primary charged particle (electron) beam from a
column, e.g., charged particle beam column 100 or microcolumn
200, is incident on a substrate, e.g., surface 120 or surface
212, that is constructed of an insulative or semiconductive
material, a variety of charged particles are generated, e.g.,
secondary electrons, backscattered electrons, and so-called
Auger electrons. The primary electrons create electron-hole
pairs in the substrate material. Electrons created within a
few nanometers of the surface escape and leave behind a
positive charge, resulting in a positive surface potential.
On a local scale, a significant level of charging can be
detected, although on a global scale, charge is balanced.
Such charging effects, both local and global, present a
significant problem for both lithography and imaging. In
particular, charging effects interfere with accurate
placement of the charged particle beam on the substrate.
Therefore what is needed is a method and apparatus for
controlling the undesirable charging effect in such columns.
-2-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
SUMMARY
An embodiment of the present invention reduces surface
charge on a substrate surface that is the target of a charged
particle beam using an apparatus including a beam column that
outputs a charged particle beam towards the substrate
surface; and a charge reducing device positioned between the
surface and the beam column, where the charge reducing device
emits charged particles to neutralize charge on the surface
induced by the particles. In one embodiment, the charge
reducing device includes: a MOS device and a voltage source,
where the voltage source is coupled to provide a voltage
across the MOS device to cause the MOS device to emit the
charged particles (electrons). In another embodiment, the
charge reducing device includes multiple MOS devices mounted
on a mechanical mount and a voltage source, where the voltage
source is coupled to provide a voltage across the MOS devices
to cause the MOS devices to emit the charged particles.
Thereby, an associated method for reducing surface
charge includes the outputting the charged particle beam
towards the target surface and emitting charged particles to
neutralize the resulting charge on the surface.
An embodiment of the present invention provides an
associated method for reducing the surface charge on a
surface, including: outputting a charged particle beam
towards the surface and emitting charged particles to
neutralize the resulting charge on the surface. In an
embodiment, an additional act includes repelling stray
charged particles towards a central region that the charged
particle beam intersects on the surface.
Various embodiments of the present invention will be
more fully understood in light of the following detailed
description taken together with the accompanying drawings.
-3-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 depicts a conventional charged particle beam
column 100 that is well known in the prior art.
FIG. 2 depicts a conventional microcolumn 200 that is
well known in the prior art.
FIG. 3 depicts schematically system 300 that includes
beam column 302 and charge reducing device 309, in accordance
with an embodiment of the present invention.
FIG. 4A depicts a top plan view of charge reducing
device 304A in accordance with an embodiment of the present
invention.
FIG. 4B depicts a cross sectional view of charge
reducing device 304A of FIG. 4A along line A-A in accordance
with an embodiment of the present invention.
FIG. 4C depicts a top plan view of charge reducing
device 304B in accordance with an embodiment of the present
invention.
FIG. 4D depicts a cross sectional view of charge
reducing device 304B of FIG. 4C along line B-B in accordance
with an embodiment of the present invention.
FIG. 5 illustrates emission of charged particles from a
MOS device when a voltage Vb is applied.
FIGs. &A and 6B each depict a side view of an
implementation of system 300 respectively having charge
reducing device 304A and charge reducing device 304B, each in
accordance with an embodiment of the present invention.
FIG. 7 depicts schematically an implementation of system
700 that includes microcolumn 200 and modified charge
reducing device 702, in accordance with an embodiment of the
present invention.
FIG. 8A depicts modified charge reducing device 702 in
more detail, in accordance with an embodiment of the present
invention.
FIG. 8B depicts a plan view of modified charge reducing
device 702, in accordance with an embodiment of the present
invention.
-4-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
DETAILED DESCRIPTION
FIG. 3 depicts schematically system 300 that includes
beam column 302 and charge reducing device 304. Beam column
302 can include, for example, either a conventional charged
particle beam column 100 or a conventional microcolumn 200,
both described above. Beam column 302 outputs a charged
particle beam 308, being, e.g., charged particle beam 114 or
charged particle beam 204, towards surface 306. Surface 306
is, for example, a substrate that beam column 302 writes onto
(lithography) or examines (electron microscopy). Charge
reducing device 304 is positioned between beam column 302 and
surface 306 and is coaxial with charged particle beam 308.
Charge reducing device 304 controls the charging effect on
surface 306.
FIG. 4A depicts a top plan view of charge reducing
device 304A, an embodiment of charge reducing device 304. In
this embodiment, charge reducing device 304A includes a metal
oxide semiconductor (MOS) device having an opening 402 that
charged particle beam 308 passes through. In this
embodiment, opening 402 is circular although it can be other
shapes, such as a square. Suitable dimensions X and Y of
charge reducing device 304A are respectively 10 mm and 10 mm.
FIG. 4B depicts a cross sectional view of charge
reducing device 304A of FIG. 4A along line A-A. As shown in
FIG. 4B, charge reducing device 304 includes three layers:
silicon substrate layer 404, silicon dioxide layer 406, and
metal layer 408. In one embodiment, silicon substrate layer
404 is approximately 2 to 300 ~Zm thick, silicon dioxide (Si02)
layer 406 is approximately 2 to 10 nm thick, and metal layer
408 is approximately 2 to 20 nm thick.
A suitable process to fabricate charge reducing device
304A of FIG. 4A follows. To form silicon substrate layer
404, a surface of an approximately 300 um thick crystalline
silicon substrate wafer is implanted with n-type donor ions
so that the wafer becomes n+ doped or n++ doped. A suitable
-5-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
resulting implant level of the wafer is 1x1019/cm3. Next,
silicon dioxide layer 406 is formed by, e.g., thermal growth,
over silicon substrate layer 404, thereby having a thickness
of 5 to 10 nm. Next, metal layer 408 being, e.g., aluminum,
palladium, chromium, or platinum, is formed over silicon
dioxide layer 406 by, e.g., a conventional thermal
evaporation or electron beam sputtering process to have a
thickness of 3 to 20 nm. For the charge reducing device 304A
of FIG. 4A, a circular opening, corresponding to opening 402,
with a diameter of 1 to 3 mm is next etched through the
combination of metal layer 408, silicon dioxide layer 406,
and silicon substrate layer 404.
FIG. 4C depicts a top plan view of charge reducing
device 304B, another embodiment of charge reducing device
304. Charge reducing device 304B includes four distinct, MOS
devices 410A-410D mounted on a mechanical support 420 by, for
example, clamping or glue. The mechanical support 420
includes an opening 430, through which charged particle beam
308 passes. A suitable shape of opening 430 is a circle,
although other shapes such as a square are suitable. A
suitable diameter of opening 430 is approximately 100 um,
where beam column 302 includes microcolumn 200, or
approximately 1 to 2 mm, where beam column 302 includes
charged particle beam column 100.
A structure of each of MOS devices 410A-410D is similar
to charge reducing device 304A. A suitable process for
fabricating each of MOS devices 910A-410D is described
earlier with respect to charge reducing device 304A, except
no opening is formed through a MOS device. A suitable shape
of each of MOS devices 410A-41OD is square having a side
length S of approximately 1 to 10 mm. The shape of each of
MOS devices 410A-410D can be varied to be, for example,
circular or rectangular. A suitable thickness of each of MOS
devices 410A-410D is approximately 300 um. A suitable
distance D (FIG. 4C) between each MOS device is approximately
0.5 to 2 mm. The MOS devices 410A-410D should be mounted as
-6-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
close as possible to the opening 430, so that any
neutralizing charge 312, discussed in more detail below,
emits close to the area on surface 306 that charged particle
beam 308 intersects.
A suitable material of mechanical support 420 is, for
example, aluminum, or a metal. The dimensions M and N of the
mechanical support 420 are respectively 30 mm and 30 mm.
FIG. 4D depicts a cross sectional view of charge
reducing device 304B of FIG. 4C along line B-B.
By comparison, the MOS devices of charge reducing device
304B may operate more reliably than charge reducing device
304A because charge reducing device 304A may suffer from
defects incurred from the formation of opening 402.
Referring to FIG. 5, when a voltage Vb is applied between
metal layer 408 and silicon substrate layer 404 so that
silicon substrate layer 404 is biased more negatively than
metal layer 408, an electric field forms that forces
electrons 502 from silicon substrate layer 404 into silicon
dioxide layer 406 by Fowler-Nordheim tunneling. The majority
of the tunneling electrons scatter inelastically in the metal
layer 408, although a small fraction of the electrons,
approximately 10-3 to. 10-', tunnel through and out of metal
layer 408 (electrons 504). For example, if metal layer 408
and surface 306 are both biased at ground potential and the
silicon substrate layer 904 is biased to approximately -5 to
-10 V, where metal layer 408 faces surface 306, emission of
low energy electrons from metal layer 408 towards surface 306
is likely. Referring to FIG. 3, the emitted low energy
electrons correspond to neutralizing charge 312 and are
injected into the region between charge reducing device 304
and surface 306.
The neutralization of charge on surface 306 can be
achieved by at least two different mechanisms. When surface
306 charges positively, the accumulation of positive charge
on surface 306 creates an electric field which attracts
neutralizing charge 312, i.e., the low energy electrons from
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
charge reducing device 304. Absorption of these low energy
electrons into surface 306 eliminates or minimizes the
positive charge buildup.
Alternatively, the cloud of low energy electrons
establishes surface 306 as the potential of the source of low
energy electrons, e.g., approximately 0 V, and locks the
surface potential within the range of the energy spread of
the low energy electrons, e.g., 0.2 eV to 1 eV. When a high
energy primary electron beam impacts surface 306 locked to a
uniform potential of approximately 0 V, any placement or
imaging errors are minimized, because no electric field,
which could distort the path of incident charged particle
beam 308, can be created at surface 306.
FIGS. 6A and 6B each depict in side view implementations
of system 300 respectively including charge reducing device
304A and charge reducing device 3048. In the embodiments
depicted in FIGs. 6A and 6B, the metal layers 408 of both
charge reducing device 304A and MOS devices 410A-410D of
charge reducing device 3048 face surface 306. A voltage Vb is
applied between each metal layer 408 and silicon substrate
layer 404 so that silicon substrate layer 404 is biased more
negatively than metal layer 408 to cause either charge
reducing device 304A or 3048 to emit neutralizing charge 312.
Surface 306 is biased to the same voltage as metal layer 408.
In an embodiment of the present invention, charge
reducing device 304 and an electrode layer of einzel lens 210
of conventional microcolumn 200 are combined. Specifically,
in this embodiment, an electrode layer of the einzel lens
acts as the silicon substrate layer of the charge reducing
device 304. FIG. 7 depicts schematically system 700, in
accordance with this embodiment, that includes a conventional
microcolumn, described in more detail earlier with respect to
FIG. 2, having a beam emitter 202 which emits a charged
particle beam 308, a source lens 206, a deflection system
208, and an einzel lens 210, having electrode layers 704,
706, and 802, positioned downstream from deflection system
_g_
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
208; and modified charge reducing device 702. In this
embodiment, electrode layer 802 is the substrate layer of
modified charge reducing device 702. FIG. 8A depicts a cross
sectional view of modified charge reducing device 702.
As discussed in "Electron-Beam Microcolumns for
Lithography and Related Applications," a suitable method to
construct an einzel lens is to fabricate each electrode layer
of the einzel lens separately and then assemble the electrode
layers. In accordance with this embodiment, a suitable
implementation of electrode layer 802 is either an n+ or n++
doped silicon substrate, where electrode layer 802 is
approximately 0.2 to 10 um thick. A suitable implant level
of electrode layer 802 is 1019/cm3. Electrode layer 802
includes a circular opening 710 formed, e.g., by etching. A
suitable diameter of opening 710 is approximately 100 um. In
this embodiment, each of electrode layers 704 and 706
includes a circular opening having the same diameter as
circular opening 710 and similarly located so that the
openings align when the electrode layers 704, 706, and 802
are assembled.
Electrode layer 802, used as the bottom electrode of the
einzel lens, i.e., closest to the surface 306, acts as the
substrate layer of modified charge reducing device 702.
Next, silicon dioxide layer 804 is formed by, e.g., thermal
growth, to a thickness of 5 to 10 nm over the bottom
electrode layer 802. Next, a metal layer 706 such as
aluminum, palladium, chromium, or platinum is formed over
silicon dioxide layer 804 by a conventional thermal
evaporation process so that metal layer 806 is 3 to 20 nm
thick. A circular opening 808 with a diameter of
approximately 100 to 300 um is next etched through only the
silicon dioxide layer 804 and metal layer 806. The diameter
of opening 808 defined in the silicon dioxide layer 804 and
metal layer 806 is larger than the diameter of opening 710
defined in electrode layer 802. Further, opening 808 is
-9-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
coaxial with opening 710. In other embodiments, opening 808
can be other shapes, such as a square.
FIG. 8B depicts a bottom plan view of modified charge
reducing device 702 shown in FIG. 8A from a direction
indicated by the arrow from C. FIG. 8B illustrates the
relationship between opening 710 in electrode layer 802 and
opening 808 in a combination of silicon dioxide layer 809 and
metal layer 806.
Subsequently, electrode layers 704 and 706 and the
electrode layer 802 of modified charge reducing device 702
are combined to form a modified einzel lens. For example, a
PyrexT" insulator can separate each electrode of the einzel
lens. In a microcolumn, metal layer 806 of modified charge
reducing device 702 is an outer surface. In system 700,
metal layer 806 faces surface 306.
To reduce the charging effect, neutralizing charge 312
must be concentrated near an area on surface 306 that the
charged particle beam 308 intersects (hereafter "central
region"). In an embodiment of the present invention, a
negatively charged, cylindrically shaped metallic barrier 800
surrounds but does not contact charge reducing device 304 and
extends towards but does not contact surface 306. The
metallic barrier 800 is aligned so that charged particle beam
308 passes through the opening. FIGs. 6A, 6B, and 7, each
depict barrier 800, in side view, in broken lines. In this
embodiment, barrier 800 is made of a metal such as aluminum,
copper, or stainless nonmagnetic steel.
When a bias voltage being more negative than the voltage
of metal layer 908 and surface 306, is applied to barrier
800, barrier 800 charges negatively relative to the surface
306. The negatively charged barrier 800 forces stray low
energy electrons 319 (FIGs. 6A, 6B, and 7) towards the
central region.
The above-described embodiments are illustrative and not
limiting. All parameters and dimensions herein are
illustrative. It will thus be obvious to those skilled in
-10-
CA 02336369 2000-12-29
WO 00/67289 PCT/US00/40017
the art that various changes and modifications may be made
without departing from this invention in its broader aspects.
For example, the shape and dimensions of the MOS devices
410A-410D and charge reducing device 304A, the materials of
the MOS devices 410A-4i0D and charge reducing device 304A,
the number of MOS devices 410A-410D, the range of bias
voltages can be varied. For example, the charge reducing
device 304 can be used to counter negative charge
accumulation on surface 306. Therefore, the appended claims
encompass all such changes and modifications as fall within
the scope of this invention.
-11-
