Wafer clamp ring for use in an ionized physical vapor...

Details Wafer clamp ring for use in an ionized physical vapor... Wafer clamp ring for use in an ionized physical vapor...

1999-10-29

2001-01-23

Mills, Gregory (Department: 1763)

Coating apparatus

Gas or vapor deposition

C118S500000, C118S724000, C204S298010

Reexamination Certificate

active

06176931

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to improvements in a wafer clamp ring used in ionized physical vapor deposition (IPVD) apparatus, which are widely used in the fabrication of semiconductor devices.
BACKGROUND OF THE INVENTION
Physical vapor deposition (PVD or sputtering) is a well-known process for depositing thin solid films on substrates, and is widely practiced in the semiconductor industry. Ionized physical vapor deposition (IPVD), also referred to as ionized metal plasma (IMP) deposition, has been used more recently to deposit metal films (notably copper) in etched structures such as vias and contact holes. IMP deposition apparatus differs from earlier versions of PVD apparatus in that a dense plasma is used to ionize the metal atoms during the sputter deposition process.
A typical IPVD arrangement is shown schematically in FIG.
1
. In an IPVD apparatus
100
, a flux of metal atoms
101
is produced by sputtering from a target
102
. These atoms are ionized by passing through a plasma
103
, and are deposited on a sample 1 (typically a silicon wafer). The ions
104
are transported from the plasma to the wafer by means of a negative potential on the sample relative to the plasma. IPVD devices differ from conventional PVD devices in that during an IPVD process, the metal ions have additional kinetic energy because of the potential difference between the plasma and the sample (typically 5 to 15 volts).
In order to maintain a uniform potential difference between the plasma and the wafer, and to control the temperature of the wafer, the wafer is clamped against a cooled pedestal
111
using a clamp ring
110
. In one arrangement known in the art, the pedestal is mounted on an elevator mechanism, and is moved vertically upwards from an “idle” position to a “process” position where the weight of the clamp ring is borne by the outer edge of the wafer. (
FIG. 1
shows the wafer and pedestal in the “process” position.) The clamp ring is typically several pounds in weight and has an inside diameter slightly smaller than the diameter of the wafer.
The plasma potential is always more positive than the potential of the sample or the various fixtures in the process chamber, including the clamp ring
110
. The clamp ring is in mechanical and electrical contact with the wafer
1
; a flexible conductor
114
touching the pedestal and the underside of the clamp ring ensures electrical contact therebetween. Accordingly, the clamp ring is subject to ion bombardment from the plasma, which tends to heat the clamp ring. In addition, the sample
1
and pedestal
111
are often connected to a biasing power source (not shown); a portion of the bias power is dissipated in the clamp ring
110
, further adding to heating of the clamp ring. For a typical commercial 200 mm wafer IPVD system, the clamp ring might receive as much as 50% of the applied bias power of 500 W. Measurements using thermocouples attached to the clamp ring have shown that the clamp ring may reach a temperature of 300° C. during a deposition process.
Heating of the clamp ring is undesirable for several reasons. Thermal expansion of the ring, relative to the wafer, results in changes in the size of the shadowed edge region of the wafer. The clamp ring may warp due to thermal expansion, thus shortening its operating life.
Furthermore, since the clamp ring is subject to metal ion bombardment, a metal film is deposited on the clamp ring which typically has thermal expansion properties different from those of the clamp ring material. The heating of the clamp ring is cyclical (heating during a deposition process, cooling between deposition processes). This results in stresses in the films deposited on the clamp ring, which in turn may result in the film breaking and forming particles.
Active cooling of the clamp ring is difficult, since the IPVD process chamber is a clean, ultrahigh vacuum environment. The ring is moved within the process chamber to clamp and unclamp the wafer at the beginning and end of each deposition process. Accordingly, using water-cooling lines in contact with the clamp ring is not desirable, since those lines would flex with the movement of the clamp ring, and eventually leak or generate particles.
Another problem related to the clamp ring is that of clamp ring/wafer sticking. This effect has been observed in IPVD of copper and aluminum films. As shown in
FIG. 1
, a typical clamp ring has a portion
110
a
which overhangs the point of contact between the wafer and the clamp ring, thereby protecting this point from film deposition so that the wafer does not “weld” to the clamp ring. However, if the clamp ring does not properly contact the wafer (due to thermal expansion, eccentric wafer location on the pedestal, etc.) one side of the wafer may instead contact surface
110
b
on the overhanging portion of the clamp ring. In that event the wafer may stick to surface
110
b
. This usually leads to breakage of the wafer. Besides the loss of a valuable product wafer, the IPVD tool becomes contaminated with wafer debris and is unavailable for a substantial period of time.
There is a need for an improved wafer clamp ring arrangement in IPVD devices which reduces or avoids the problems of clamp ring heating and clamp ring/wafer sticking.
SUMMARY OF THE INVENTION
The present invention addresses the above-described need by providing a cooling structure for a wafer clamp ring, and a shield for a wafer clamp ring, so that thermal expansion and ion bombardment of the wafer clamp ring is reduced or avoided.
In accordance with a first aspect of the present invention, a cooling fixture is provided on which the wafer clamp ring is placed when the wafer clamp ring is not required for a deposition process. The fixture is annular in shape and has dimensions substantially identical to a bottom surface of the wafer clamp ring. The fixture is in close thermal contact with a circulating coolant and is thereby cooled below ambient temperature. The bottom surface of the wafer clamp ring is brought into close thermal contact with a top surface of the fixture when the wafer clamp ring is placed thereon, so that the wafer clamp ring is cooled below ambient temperature when not required for a deposition process.
Furthermore, the conductor for the coolant and the cooling fixture are fixed relative to the IPVD device, so that problems associated with flexible cooling lines are avoided. The top surface of the fixture may have a sloped portion with an inner diameter substantially identical to an outer diameter of the wafer clamp ring, which has the advantage of causing the wafer clamp ring to be centered relative to the fixture when the wafer clamp ring is placed thereon.
In accordance with a second aspect of the invention, a shield is provided for the wafer clamp ring, to protect the wafer clamp ring against ion bombardment during the deposition process. The shield is located between the plasma and the wafer clamp ring and is annular in shape; the inside diameter of the shield and the inside diameter of the wafer clamp ring are substantially identical. The wafer clamp ring is at a bias potential and the shield is at ground potential during the deposition process. This shield may be attached to a generally cylindrical shield which surrounds the wafer clamp ring during the deposition process.
In accordance with a further aspect of the invention, a wafer clamp ring is provided in which contact between the wafer and wafer clamp ring during a deposition process is minimized. The wafer clamp ring has an annular first portion with an inner diameter less than a diameter of the wafer, and a bottom surface in contact with the wafer during the deposition process. An annular second portion, with an inner diameter less than that of the first portion, is connected to the first portion so as to overhang the wafer during the deposition process. A ridge portion extends downwards from the second portion and tapers to a knife edge. This wafer clamp ring may also include a shield as described just above.
According to a further aspect of the invention

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