Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
2000-04-28
2003-02-18
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
C361S233000
Reexamination Certificate
active
06522519
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to improved electrostatic chucking devices used to secure a sample such as a semiconductor wafer in a microlithography apparatus. More specifically, the invention is directed to such devices that prevent decreases in microlithographic exposure accuracy and precision due to leakage of current from the chucking device to the wafer, thereby reducing wafer heating and consequent thermal distortion of the wafer.
BACKGROUND OF THE INVENTION
A conventional electrostatic chucking device
51
is depicted schematically in FIG.
2
. The device
51
is constructed of electrically insulative materials with interior electrically conductive electrodes
67
,
69
. The electrodes
67
,
69
are electrically connected to a controlled power source
79
that supplies a voltage to each of the electrodes
67
,
69
. More specifically, the device
51
comprises a substrate
71
(normally made of a ceramic material) and a dielectric layer
61
. The electrodes
67
,
69
are situated between the substrate
71
and dielectric layer
61
. The dielectric layer
61
includes a sample-contacting surface
60
parallel to the electrodes
67
,
69
.
The dielectric layer
61
typically has a thickness of a few hundred micrometers. Whenever a sample
3
(e.g., silicon wafer) is placed on the sample-contacting surface
60
and voltage is applied to the electrodes
67
,
69
, an electrostatic force, either a Coulomb force or a Johnsen-Rahbek force, is generated between the device
51
and the wafer
3
. The force causes the wafer
3
to be electrostatically attracted to and thus held fast by the device
51
.
Generally speaking, the Coulomb force dominates whenever the dielectric layer
61
has a large volume resistivity (e.g., greater than 10
14
&OHgr;-cm for Al
2
O
3
), and the Johnsen-Rahbek force dominates whenever the dielectric layer
61
has a small volume resistivity (e.g., within a range of 10
10
to 10
12
&OHgr;-cm for Al
2
O
3
).
A Coulomb chucking force is a function of and substantially affected by the dielectric constant of the dielectric layer
61
. Hence, to obtain a large Coulomb chucking force, the dielectric constant of the dielectric layer
61
should be correspondingly large. A Johnsen-Rahbek chucking force, in contrast, is a function of and substantially affected by the width of any gaps (microscopic or otherwise) between the sample-contacting surface
60
and the wafer
3
. Hence, to obtain a large Johnsen-Rahbek chucking force, the sample-contacting surface
60
normally has a particular surface roughness.
Any of various problems can occur whenever a conventional electrostatic chucking device
51
is used to hold a sample (e.g., semiconductor wafer) in an electron-beam or other charged-particle-beam (CPB) microlithography (projection-exposure) apparatus. As noted above, if the volume resistivity of the dielectric layer
61
is relatively small, then the Johnsen-Rahbek force dominates and a strong chucking force can be obtained. However, whenever the volume resistivity of the dielectric layer
61
is relatively small, a voltage applied between the electrodes
67
,
69
and the wafer
3
causes electrical current to flow to the wafer (e.g., 500 &mgr;A current to 300-mm diameter wafer). The magnetic field generated from such a current can have a deleterious effect on a charged particle beam used for projection-exposure of a pattern to the wafer, which can result in a decreased accuracy of the exposed pattern.
On the other hand, as noted above, if the volume resistivity of the dielectric layer
61
is relatively high, then the Coulomb force dominates. Whenever the volume resistivity of the dielectric layer
61
is relatively high, very little current flows to the wafer
3
. However, the ceramic material normally used to make the dielectric layer
61
generally has a relatively small dielectric constant, so a strong chucking force cannot be obtained. As a result, there is a substantial risk of the wafer
3
being displaced on or from the chucking device during microlithographic exposure. A principal cause of such detachment is thermal deformation of the wafer
3
caused by CPB-mediated heating of the wafer.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art summarized above, an object of the invention is to provide electrostatic chucking devices, especially for use in microlithography, that exhibit improved resistance to decreases in exposure accuracy and precision due to current leakage to the sample (wafer) and/or displacement of the sample from thermal expansion.
To such end, and according to a first aspect of the invention, electrostatic chucking devices are provided to which a sample can be secured by an electrostatic chucking force. A representative embodiment comprises a chucking surface that comprises a dielectric layer. The dielectric layer is divided into at least a first region and a second region each formed of a respective dielectric material having a different respective dielectric property. The chucking device also comprises a respective electrode associated with each region. The first region desirably has a volume resistivity of 10
14
&OHgr;-cm or higher, and the second region desirably has a volume resistivity of 10
13
&OHgr;-cm or less. In addition, the first region desirably occupies at least 20% of the surface area of the chucking surface, and the second region desirably occupies no more than 30% of the surface area.
Alternatively, the first region can have a dielectric constant of 20 or higher, and the second region can have a dielectric constant of less than 20.
Each of the first and second regions of the dielectric layer can have multiple portions. The portions of both regions can be, for example, arranged concentrically with each other in ring-shaped configurations. Alternatively, for example, the second regions can be arranged as multiple point loci dispersed in the first region(s).
According to another embodiment, an electrostatic chucking device comprises an electrically insulative substrate, an electrostatic electrode situated on an upstream-facing surface of the insulative substrate, and a dielectric layer defining a sample-contacting surface. The dielectric layer is situated on the electrostatic electrode such that the electrostatic electrode is sandwiched between the insulative substrate and the dielectric layer. The dielectric layer comprises at least two coplanar portions at the chucking surface, wherein each planar portion is formed of a respective dielectric material having a different dielectric property than the other dielectric material.
In one configuration of this embodiment, the first region has a volume resistivity of 10
14
&OHgr;-cm or higher, and the second region has a volume resistivity of 10
13
&OHgr;-cm or less. In this configuration, the first region desirably occupies at least 20% of the surface area, and the second region desirably occupies no more than 30% of the surface area.
Alternatively, the first region can have a dielectric constant of 20 or higher, and the second region can have a dielectric constant of less than 20.
Further desirably, each electrode is configured so as to be energized with a voltage that can be independently controllable for each electrode. For example, the electrodes can be connected to a power source configured to energize the electrodes such that the sample held by the chucking device has a ground electrical potential.
The chucking surface can be roughed so as to have at least one indentation that is no more than 20 &mgr;m deep relative to the chucking surface.
According to another aspect of the invention, methods are provided for holding a microlithographic sample. In a representative embodiment of such a method, an electrostatic chuck is provided that comprises a dielectric layer divided into at least a first and a second region each formed of a respective dielectric material having a different respective dielectric property. A respective electrode is provided for each region. The electrodes are independently energized with respective v
Jackson Stephen W.
Klarquist & Sparkman, LLP
Nikon Corporation
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