Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1993-09-07
2001-02-20
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192130
Reexamination Certificate
active
06190512
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to reduction of device damage in plasma processes, including DC (magnetron or non-magnetron) sputtering, and RF sputtering.
A typical plasma processing apparatus is shown in FIG.
1
. The apparatus includes a plasma power supply
10
, which drives a cathode or target
12
to a large DC voltage (e.g., −400 Volts) relative to the walls of vacuum chamber
14
. The semiconductor substrate
16
(also known as the wafer) rests on a back plane
18
inside the chamber. The back plane may be driven by radio frequency (RF) AC voltage signals, produced by an RF power supply
20
, which drives the back plane through a compensating network
22
.
The AC and/or DC power supplies generate a plasma in the area above the wafer and between the wafer and the target, and cause material from the target to deposit on the wafer surface.
A typical DC power supply
10
includes a relatively sophisticated control system, designed to permit operation in constant power, constant voltage, or constant current modes. This control circuitry includes a damped control loop which, when the supply is engaged, produces a controlled ramping toward the desired output level. For example, as shown in
FIG. 2
, upon engagement of a typical DC power supply in an apparatus as shown in
FIG. 1
, the supply current (which represents the density of ionic transfer from the target due to sputter deposition on the wafer) ramps up to a constant value in a controlled manner with a small overshoot
24
.
Despite the otherwise carefully regulated output produced by typical power supplies, it is normal to observe a spike in the target voltage during process initiation. As shown in
FIG. 3
, the magnitude of the spike
26
at process initiation may exceed the normal DC voltage level by a factor of 2 or more (e.g., those shown in
FIG. 3
reach −1100 Volts). This phenomenon, known as the “break down” spike, is typically viewed as a necessary, isolated event associated with the creation of a plasma in the chamber
14
(otherwise known as “plasma ignition”). Furthermore, a large magnitude break down spike has been seen as necessary to improve process quality.
SUMMARY OF THE INVENTION
Overvoltage in the processing chamber deteriorates the quality of sputtered films in several ways: High voltage events electrically damage layers and/or devices on the processing substrate (wafer). Furthermore, arcing which can be produced by overvoltages can cause local overheating of the target, leading to evaporation or flaking of target material into the processing chamber and causing substrate particle contamination and device damage. These sources of wafer damage become increasingly significant as integrated circuits reach higher densities and become more complex. Thus, it is advantageous to avoid voltage spikes during processing wherever possible.
With this in mind, careful analysis has revealed that the so-called break down spike is not, in fact, an isolated event necessarily associated with the creation of a plasma in the chamber. The spike is not caused by the creation of a plasma per se, but rather by harmonic oscillations within the chamber.
As shown in
FIG. 4
, a gas-filled chamber generates sizable oscillations when driven by a DC voltage within a given voltage range. These oscillations are evident in regions
26
and
28
. Notably, however, the oscillations cease when the driving voltage exceeds a threshold voltage represented by line
30
. One explanation of this phenomenon is that complete plasma ignition occurs above threshold voltage
30
. When the power supply voltage is near to, but below this threshold voltage, unstable gas discharges, as well as related transitions between gas and plasma phases, occur in chamber
14
. (Similar effects have been observed in gaseous-discharge tubes.) As a result, the gas-plasma system begins unstable oscillation, producing brief, but very large magnitude voltage perturbations. This oscillation continues until the threshold voltage
30
is achieved, at which point the gas/plasma mixture fully transitions to a plasma, and oscillations cease.
Voltage
30
will be referred to as the “oscillation threshold voltage”. The value of the oscillation threshold voltage will depend on the target (cathode) material, process gas and pressure, chamber geometry, electrical characteristics of the external power wiring, and possibly the volt-ampere curve of the sputtering chamber.
Based on the preceding observations, the spike observed in region
26
of
FIG. 3
is now understood to be an oscillation caused when the output voltage of primary supply
10
lingers at a voltage just below the oscillation threshold. Furthermore, careful inspection of region
28
of
FIG. 3
also reveals oscillatory behaviors analogous to those which appear in region
28
of FIG.
4
. (The oscillations in region
28
have smaller magnitudes, in part because when the power supply is disabled, its output voltage drops relatively rapidly, whereas when the power supply is enabled its output voltage increases relatively slowly.)
It has been found that the oscillation spike observed in
FIG. 3
can be eliminated by elevating the target/cathode voltage above the oscillation threshold voltage before initiating gas flow into the chamber, and maintaining the cathode voltage above the oscillation threshold until processing is completed, gas flow is halted, and vacuum is restored. This technique prevents overvoltage during processing, and therefore can reduce device damage and particulate contamination.
In brief summary, this technique is implemented by a power supply circuit comprising two power supply sections: an essentially conventional primary power supply, which provides the primary power to electrically drive the cathode during the plasma process, and a secondary power supply which supplies an initial plasma ignition voltage sufficiently in excess of the oscillation threshold voltage. This secondary power supply “pre-ignites” the plasma so that when the primary power supply is applied, the system smoothly transitions to final plasma development and deposition. This design thereby avoids oscillations when the primary power supply is engaged and disengaged, and any corresponding device damage.
In preferred embodiments, a current limiting resistor, switch, and diode are connected in series between the secondary power supply and the cathode.
The current limiting resistor limits the current flowing from the secondary power supply into the cathode. Only a minimal current is needed to elevate the cathode voltage above the oscillation threshold and pre-ignite the plasma; by interposing a current-limiting resistor, the secondary power supply current is held at this minimum level, thus avoiding the need for a high power secondary supply, and also limiting the plasma current and deposition while the secondary power supply is enabled and the primary supply is disabled.
The diode automatically disconnects the secondary power supply from the cathode when the primary supply begins driving the plasma. To achieve this, the diode is connected so that it is “on”, i.e., current flows, when the magnitude of the secondary supply voltage exceeds the cathode voltage, and is “off” otherwise; thus, once the primary supply engages and begins driving the cathode, the diode turns “off” and the secondary supply is disconnected.
The switch is used to turn the secondary power supply voltage on and off; at the beginning of processing, this switch is closed and gas is introduced into the chamber. When the plasma process is completed, the gas flow is stopped, and once vacuum is restored, the switch is opened. Because the switch is opened and closed while the chamber is at full vacuum (when there is very little gas in the chamber), gas/plasma transition oscillations are substantially reduced.
REFERENCES:
patent: 4557819 (1985-12-01), Meacham et al.
patent: 4888088 (1989-12-01), Slomowitz
patent: 5288971 (1994-02-01), Knipp
patent: 59-222580 (1984-12-01), None
Mashiro Kazuhiko, “Discharge Triggering Method of Sputtering
Nguyen Nam
Tokyo Electron Arizona Inc.
Wood Herron & Evans L.L.P.
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