Wafer bias ring controlling the plasma potential in a...

Details Wafer bias ring controlling the plasma potential in a... Wafer bias ring controlling the plasma potential in a...

1998-11-16

2001-01-23

McDonald, Rodney (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Coating, forming or etching by sputtering

C204S192120, C204S192170, C204S192300, C204S298060, C204S298080, C204S298110, C204S298140, C204S298190, C204S298200

Reexamination Certificate

active

06176981

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to plasma processing of workpieces such as semiconductor integrated circuits. In particular, the invention relates to the physical vapor deposition (PVD) with sustained self-sputtering.
BACKGROUND ART
A critical part of any advanced semiconductor integrated circuit involves the one or more metallization levels used to contact and interconnect the active semiconductor areas, themselves usually residing in a fairly well defined crystalline silicon substrate. Although it is possible to interconnect a few transistors or other semiconductor devices, such as memory capacitors, within the semiconductor level, the increasingly complex topology of multiple connected devices soon necessitates another level of interconnect. Typically, an active silicon layer with transistors and capacitors formed therein is overlaid with a dielectric layer, for example, silicon dioxide. Contact holes are etched through the dielectric layer to particular contacting areas of the silicon devices. A metal is filled into the contact holes and is also deposited on top of the dielectric layer to form horizontal interconnects between the silicon contacts and other electrical points. Such a process is referred to as metallization.
A single level of metallization may suffice for simple integrated circuits of small capacity. However, dense memory chips and especially complex logic devices require additional levels of metallization since a single level does not provide the required level of interconnection between active areas. Additional metallization levels are achieved by depositing over the previous metallized horizontal interconnects another level of dielectric and repeating the process of etching holes, now called vias, through the dielectric, filling the vias and overlaying the added dielectric layer with a metal, and defining the metal above the added dielectric as an additional wiring layer. Very advanced logic devices, for example, fifth-generation microprocessors, have five or more levels of metallization.
Conventionally, the metallized layers have been composed of aluminum or aluminum-based alloys additionally comprising at most a few percent of alloying elements such as copper and silicon. The metallization deposition has typically been accomplished by physical vapor deposition (PVD), also known as sputtering. A conventional PVD reactor
10
is illustrated schematically in cross section in
FIG. 1
, and the illustration is based upon the Endura PVD Reactor available from Applied Materials, Inc. of Santa Clara, Calif. The reactor
10
includes a vacuum chamber
12
sealed to a PVD target
14
composed of the material to be sputter deposited on a wafer
16
held on a heater pedestal
18
. A shield
20
held within the chamber protects the chamber wall
12
from the sputtered material and provides the anode grounding plane. A selectable DC power supply
22
biases the target negatively to about −600VDC with respect to the shield
20
. Conventionally, the pedestal
18
and hence the wafer
16
is left electrically floating.
A gas source
24
of sputtering working gas, typically chemically inactive argon, supplies the working gas to the chamber through a mass flow controller
26
. A vacuum system
28
maintains the chamber at a low pressure. Although the chamber can be held to a base pressure of about 10
−7
Torr or even lower, the pressure of the working gas is typically kept between about 1 and 1000 mTorr. A computer-based controller
30
controls the reactor including the DC power supply
22
and the mass flow controller
26
.
When the argon is admitted into the chamber, the DC voltage ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target
14
. The ions strike the target
14
at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target
14
. Some of the target particles strike the wafer
16
and are thereby deposited on it, thereby forming a film of the target material.
To provide efficient sputtering, a magnetron
32
is positioned in back of the target
14
. It has opposed magnets
34
,
36
creating a magnetic field within the chamber in the neighborhood of the magnets
34
,
36
. The magnetic field traps electrons, and, for charge neutrality, the ion density also increases to form a high-density plasma region
38
within the chamber adjacent to the magnetron
32
.
With the continuing miniaturization of integrated circuits, the demands upon the metallization have increased. Many now believe that aluminum metallization should be replaced by copper metallization. Murarka et al. provide a comprehensive review article on copper metallization in “Copper metallization for ULSI and beyond,”
Critical Reviews in Solid State and Materials Science
, vol. 10, no. 2, 1995, pp. 87-124. Copper offers a number of advantages. Its bulk resistivity is less than that of aluminum, 1.67 &mgr;&OHgr;-cm vs. 2.7 &mgr;&OHgr;-cm for pure material, and any reduction in resistivity offers significant advantages as the widths and thicknesses of the metallization interconnects continue to decrease. Furthermore, a continuing problem with aluminum metallization is the tendency of aluminum atoms in an aluminum interconnect carrying a high current density to migrate along the interconnect, especially away from hot spots, in a process called electromigration. Any excessive amount of such migration will break an aluminum interconnect and destroy the integrated circuit. Copper-based alloys exhibit significantly reduced levels of electromigration.
Copper metallization is an unproven technology and is acknowledged to entail difficulties not experienced with the conventional aluminum metallization. However, it may afford ways to circumvent problems inherent in aluminum metallization.
One problem inherent in conventional sputtering is that it is performed in a fairly high pressure of the inert working gas, such as argon. However, the argon environment presents two problems. First, it is inevitable that some argon ions are deposited on the substrate and incorporated into the sputter deposited aluminum. Although the effect of these usually inactive argon ions is not precisely known, it is estimated that they reduce the conductivity of the sputter deposited aluminum by 50%.
Sputtering to fill holes relies at least in part on the sputtered particles being ballistically transported from the target to the wafer, that is, without scattering from the initial course. The ballistic trajectories allow the sputtered particles to arrive at the wafer nearly perpendicularly to the wafer's surface and thus to deeply penetrate into any aperture. However, the typical sputtering process is performed in an argon ambient of from 1 to 100 mTorr. Such a high pressure means that there is a significant probability that the aluminum sputter particles will collide with the argon atoms and thus be deflected from their ballistic paths. Accordingly, low-pressure sputtering is believed to provide better hole filling for deep vias. However, low pressure is generally equated with low deposition rates so that reducing the pressure is not a favored method for better directionality. Furthermore, a minimum pressure of about 0.2 mTorr is required to support a plasma in the usual configuration of FIG.
1
.
High-density plasma (HDP) sputter reactors are being actively developed and are approaching commercialization. One of the advantages of HDP sputtering is that a sizable fraction of the sputtered particles are ionized during their travel toward the substrate. Then, the pedestal supporting the wafer can be selectively biased by an RF source to create a DC self-bias with respect to the positively charged plasma. As a result, the wafer can be biased negatively with respect to the plasma (−20V being a typical value), and the positively charged sputtered ions are accelerated from the generally neutral plasma toward the substrate. The added velocity provides a highly directional flux normal to the plane of th

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