Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
2001-05-09
2003-04-22
Nguyen, Matthew (Department: 2838)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
Reexamination Certificate
active
06552892
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to ion implantation systems, and more specifically to an electrostatic clamp having a discharge path associated with a dielectric layer thereof for bleeding off charge which may accumulate during workpiece processing.
BACKGROUND OF THE INVENTION
Ion implanters are used to implant or “dope” silicon wafers with impurities to produce n or p type extrinsic materials. The n and p type extrinsic materials are utilized in the production of semiconductor integrated circuits. As its name implies, the ion implanter dopes the silicon wafers with a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers. If p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium will be implanted.
The ion implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The ion implanter includes beam forming and shaping structure extending between the ion source and the implantation station. The beam forming and shaping structure maintains the ion beam and bounds an elongated interior cavity or region through which the beam passes en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
For some types of ion implanters, the wafer or workpiece at the implantation station is mounted on a surface or support pedestal. For serial type implantation systems, a rotating support is not used and therefore a mechanism is typically employed to secure the workpiece on the support pedestal. One type of securement mechanism is an electrostatic chuck or clamp. While electrostatic chucks or clamps may vary in design, they are based primarily on the principle of applying a voltage to one or more electrodes embedded in the chuck so as to induce opposite polarity changes in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece in its position in a secure manner.
A typical electrostatic chuck or clamp includes an electrode covered by an insulator or dielectric layer. When the electrode of the chuck or clamp is electrically biased with respect to the substrate or workpiece by a voltage, an attractive electrostatic force is generated that holds the substrate or workpiece to the chuck. In monopolar electrode type chucks, an electrically charged plasma above the substrate induces electrostatic charge in the substrate that electrostatically holds the substrate to the chuck. A bipolar electrode chuck comprises bipolar electrodes that are electrically biased relative to one another to provide the electrostatic attractive force.
Referring initially to prior art
FIG. 1
, a simplified electrostatic chuck or clamp
10
is illustrated, wherein the chuck includes a dielectric or insulating region
12
overlying an electrode
14
. A workpiece
16
, for example, a silicon wafer undergoing implantation, overlies the dielectric region or cover
12
. In operation, a voltage potential
18
is applied across the wafer
16
via the electrode
14
. Due to the presence of the dielectric layer
12
which exhibits a large electrical resistance, an accumulation of electrostatic charge in the wafer
16
and the electrode
14
results in a coulombic electrostatic force characterized by the equation:
F=
(½)&egr;
o
&egr;
r
A
(
V/t
)
2
,
wherein &egr;
o
and &egr;
r
are the dielectric constants associated with a vacuum and the dielectric layer
12
, respectively, A is the area of the electrode, V is the voltage applied to the electrode
14
via the source
18
, and t is the thickness of the dielectric layer
12
.
Another type of electrostatic clamp or chuck (not shown) employs Johnsen-Rahbek electrostatic attraction forces, which are a function of charge accumulation across an interfacial contact resistance such as an air gap. In any event, regardless of the particular type of clamp or chuck employed within the system, electrostatic forces work to secure the wafer in position on the chuck without need of a mechanical or physical mechanism touching the workpiece. The lack of physical clamping advantageously reduces particulate frontside contamination which may potentially result when a mechanical clamp mechanism contacts a workpiece.
One problem associated with electrostatic chucks or clamps such as the chuck
10
of prior art
FIG. 1
is caused by positive charge accumulation on the workpiece
16
during implantation. As stated above, since positively charged ions are typically employed to dope a workpiece, positive charge may accumulate thereon, for example, as illustrated in prior art
FIG. 2
, and designated at reference numeral
20
. As the charge
20
accumulates, it naturally seeks a path to ground, and when such a path is identified, such a substantial charge accumulation may result in an arcing to ground in an uncontrolled manner, which undesirably may damage the workpiece
16
. Therefore mechanisms have been developed to address the problem by attempting to discharge any accumulated charge in a controlled manner.
Prior art
FIGS. 3-5
illustrate one conventional method of discharging accumulated charge from the implantation workpiece. Prior art
FIG. 3
is a plan view of an implantation pedestal
30
having one or more grounded, conductive electrodes
32
formed on a surface
34
thereof. Prior art
FIG. 4
is a cross sectional view of the pedestal of prior art
FIG. 3
, taken along dotted line
4
—
4
. The conductive electrodes
32
are grounded and thus readily provide a discharge path for any charge which may accumulate on the workpiece during ion implantation, thereby preventing a substantial accumulation of charge on the workpiece and protecting the workpiece from damage associated with an uncontrolled discharge.
As illustrated in prior art
FIG. 4
, the conductive electrodes
32
are formed on the pedestal surface
34
and thus represent raised portions which impact the topography of the pedestal
34
. More particularly, the raised conductive portions
32
negatively impact the planarity of the pedestal surface
34
. A resultant condition is illustrated in prior art
FIG. 5
, wherein the wafer or workpiece
16
overlies the pedestal surface
34
and makes contact with the grounded, conductive electrodes
32
. Note that due to the conductive portions
32
being raised, the wafer
16
does not make good thermal contact to the chuck surface
34
. As is generally known in the art, ion implantation, particularly in serial batch type systems, causes a significant amount of workpiece heating. The poor thermal contact caused by the electrodes
32
may cause the workpiece to overheat during implantation, thereby causing thermal damage to the workpiece or negatively impacting the implantation characteristics.
In addition to the poor thermal contact caused by the conductive raised portions
32
, the physical contact caused at the wafer/electrode interface may generate particulate contamination in the ion implantation chamber, which undesirably may result in defects. Therefore although the raised, grounded electrodes
32
of
FIGS. 3-5
are an effective means of discharging positive charge that would otherwise accumulate on the workpiece during implantation, the prior art solution negatively impacts the effective conduction of heat away from the workpiece and may contribute to increased particulate contamination.
Therefore there is a need in the art for an apparatus and method of addressing charge accumulation on the wafer during ion implantation which overcomes the disadvantages associated with the prior art.
SUMMARY OF THE
Carroll James C.
Klesel Dennis M.
Lagos Bryan C.
Petry Klaus
Axcelis Technologies Inc.
Eschweiler & Associates LLC
Nguyen Matthew
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