Electricity: electrical systems and devices – Electric charge generating or conducting means – Use of forces of electric charge or field
Reexamination Certificate
1997-08-29
2003-03-04
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Electric charge generating or conducting means
Use of forces of electric charge or field
Reexamination Certificate
active
06529362
ABSTRACT:
BACKGROUND
The present invention relates to electrostatic chucks useful for holding substrates during processing.
Electrostatic chucks are used to hold semiconductor substrates, such as silicon wafers, in a process chamber. A typical electrostatic chuck comprises an electrode covered by a dielectric layer. In monopolar chucks, an attractive electrostatic force is generated when the electrode of the chuck is electrically biased by a voltage and an electrically charged plasma in the chamber induces electrostatic charge in the substrate. A bipolar chuck comprises bipolar electrodes that are electrically biased relative to one another to generate the electrostatic attractive force.
The electrostatic attractive force generated by electrostatic chucks can also be of different types. As schematically illustrated in
FIG. 1
a,
a chuck
10
a
having a dielectric layer
11
with a high electrical resistance results in coulombic electrostatic forces where opposing electrostatic charges accumulate in the substrate
12
and in the electrode
13
of the chuck. The coulombic electrostatic force is described by the equation:
F
=
1
2
ϵ
0
ϵ
r
(
V
t
)
2
A
where ∈
0
and ∈
r
are the dielectric constant of vacuum and relative dielectric constant of the dielectric layer
11
, respectively, V is the voltage applied to the electrode
13
, A is the area of the electrode, and t is the thickness of the dielectric layer.
With reference to
FIG. 1
b,
Johnsen-Rahbek electrostatic attraction forces occur in the chuck
10
b
when an interface
14
between a low resistance or leaky dielectric layer
15
and the substrate
12
, has an interfacial contact resistance much greater than the resistance of the dielectric layer
15
, i.e., when the resistance of the dielectric layer
15
is typically from about 10
11
to about 10
14
&OHgr;/cm. Free electrostatic charge drifts through the dielectric layer
15
in the applied electric field, and accumulates at the interface of the dielectric layer
15
and the substrate
12
. The charge accumulated at the interface generates a potential drop represented by the equation:
F
=
1
2
ϵ
0
(
V
δ
)
2
A
where &dgr; denotes the contact resistance of the air gap
14
between the substrate
12
and the low resistance dielectric layer
15
. The Johnsen-Rahbek electrostatic attractive force is typically higher than that provided by coulombic forces, because polarization in the dielectric layer
15
, and free charges accumulated at the interface
14
combine to enhance electrostatic force. This provides a stronger electrostatic force that more securely holds the substrate
12
onto the chuck and also improves thermal transfer rates at the interface. Also, the lower voltages used in these chucks reduce charge-up damage to active devices on the substrate
12
.
The dielectric layers
11
,
15
covering the electrode
13
of these chucks typically comprise a thin polymer film, such as polyimide, adhered to the electrode, as for example disclosed in U.S. Pat. No. 5,745,331, patent application Ser. No. 08/381,786, entitled “Electrostatic Chuck with Conformal Insulator Film,” filed on Jan. 31, 1995, to Shamouilian, et al., which is incorporated herein by reference. However, the substrate held on the chuck often breaks or chips to form fragments having sharp edges that puncture the polymer film and expose the electrode. Exposure of the electrode at even a single pinhole in the dielectric layer can cause arcing between the electrode and plasma, and require replacement of the entire chuck. Polymers also have a limited lifetime in erosive process environments, such as processes using oxygen-containing gases and plasmas. Also, polymers or adhesives used to bond the polymer films to the chuck often cannot operate at elevated temperatures exceeding 1000° C.
Polycrystalline ceramics have also been used to form the dielectric layer to provide increased puncture resistance and higher temperature performance, as for example, described in U.S. Pat. No. 5,280,156 to Niori; Watanabe, et al., in “Relationship between Electrical Resistivity and Electrostatic Force of Alumina Electrostatic Chuck,”
Jpn. J. Appl. Phys.,
Vol. 32, Part 1, No. 2, (1993); or “Resistivity and Microstructure of Alumina Ceramics Added with TiO
2
Fired in Reducing Atmosphere,”
J. of the Am. Cer. Soc. of Japan Intl. Ed.,
Vol. 101, No. 10, pp. 1107-1114 (July 1993); all of which are incorporated herein by reference. The ceramic dielectric layers typically comprise a low conductivity polycrystalline ceramic, such as a mixture of Al
2
O
3
and TiO
2
, or BaTiO
3
. However, polycrystalline ceramics such as Al
2
O
3
doped with TiO
2
often have an electrical resistance that changes with temperature, and can exhibit low or insufficient electrical resistance at high temperatures. Also, polycrystalline ceramics comprise small grains or crystals that typically have a diameter of 0.1 to 50 microns, and have grain boundaries containing a mixture of glassy materials that hold the grains together. When such ceramic layers are exposed to erosive environments, such as a fluorine containing plasma, the plasma etches away the grain boundary regions causing the ceramic grains to loosen and flake off during processing of the substrate. Abrasion of the substrate against the chuck can also cause ceramic grains to flake off the chuck. These particulate ceramic grains contaminate the substrate and/or process chamber and reduce the yields of integrated circuit chips from the substrate.
Dielectric layers comprising a thin wafer of monocrystalline ceramic that is made of a few, relatively large, ceramic crystals have also been used to cover the electrode. For example, U.S. Pat. No. 5,413,360 to Atari, et al., describes an electrostatic chuck consisting of a monocrystalline ceramic wafer covering an electrode on a dielectric plate. Atari teaches that a bonding agent, or a high temperature joining method, is used to join the monocrystalline ceramic wafer to the electrode of the chuck. In another example, U.S. Pat. No. 5,535,090 to Sherman, filed Mar. 3, 1994, discloses an electrostatic chuck comprising small segments of monocrystalline ceramic wafers adhered to the surface of an electrode using a high temperature vacuum braze with a suitable brazing alloy. For example, a platinum layer can be sputtered onto the monocrystalline ceramic layer and a platinum paste used to adhere the monocrystalline ceramic layer to the metal electrode.
One problem with such chucks arises from their structure, which typically comprises a single relatively thin monocrystalline ceramic wafer bonded to the metal electrode with a layer of bonding material therebetween, and supported by a metal or dielectric plate made from another material. During the bonding process or during use of the chuck in an erosive process environment, the thermal expansion mismatch between the monocrystalline ceramic wafer and the electrode can result in failure of the bond. Also, the bonding material is typically a metal based material that thermally or chemically degrades during use of the chuck in reactive processes, causing failure of the chuck and movement or misalignment of the substrate during processing. The thin monocrystalline ceramic wafer and electrode can also separate from the supporting dielectric or metal plate at high temperatures due to stresses arising from the thermal expansion coefficient mismatches. Another problem arises because grooves, channels, and other hollow spaces which are used to hold coolant or to supply helium gas to the interface below the substrate, are difficult to form in the brittle, hard, and thin monocrystalline ceramic layers. During the series of machining or drilling steps that are used to form these hollow shapes, the brittle layers often crack or chip resulting in loss of the chuck. It is also difficult to precisely machine fine holes or grooves in the monocrystalline ceramic wafer.
Yet another problem with such conventional chucks arises from the
Applied Materials Inc.
Fleming Fritz
Janah & Associates
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