Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
2000-07-20
2001-10-23
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C118S7230ER, C118S7230AN, C156S345420, C315S111510
Reexamination Certificate
active
06305316
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to Plasma Immersion Ion Implantation (PII) systems, and more specifically to a system and method for providing plasma ignition within a plasma chamber via an integrated power oscillator RF source.
BACKGROUND OF THE INVENTION
In a Plasma Immersion Ion Implantation (PII) process, a semiconductor wafer is placed in a plasma chamber (generally by a wafer handling system), a plasma is ignited, and wafer implantation occurs by pulsing the wafer at a negative potential. This process is repeated for each wafer. A significant challenge associated with PII is related to the repeatability of the process, and notably, one of the primary sources that may introduce variability into the process is related to the plasma ignition phase.
Referring initially to prior art
FIG. 1
, a conventional PII system
10
is shown. An RF power plasma source (not shown) is generally inductively or capacitively coupled to a plasma chamber
20
. Plasma ignition is achieved when sufficient power is injected into the system
10
via an RF antenna
30
(shown as an inductor). Conventionally, power is injected into the system
10
from a fixed frequency (13.56 MHz) RF generator
40
through a 50 ohm coaxial cable
42
via a matching network
50
. The matching network
50
is required to provide maximum power to the load by matching the 50 ohm output impedance of the RF generator
40
and a complex impedance established by the power antenna
30
and resultant plasma impedance
60
within the plasma chamber
20
. The matching network
50
includes mechanically variable high voltage vacuum capacitors
50
a
and
50
b
. The tunable capacitors
50
a
and
50
b
account for variations in the antenna impedance caused by changes in plasma impedance
60
before, during and after plasma ignition. Capacitors
50
a
and
50
b
are employed to minimize “reflected power” back to the RF generator
40
. The reflected power is monitored by a power meter
70
, and a reflected power measurement is provided as an input
70
a
to an RF control
72
. Based on the reflected power input
70
a
, the controller
72
directs a control output
72
a
to one or more motor drives
74
for adjusting the tunable capacitors
50
a
and
50
b
in order to minimize reflected power from the load. It is noted, that if the reflected power becomes too high, the RF generator
40
may fault. An external inductance
76
is depicted between the matching network
50
and the plasma chamber
20
and represents stray inductances associated with the system
10
.
Generally, the antenna
30
impedance varies significantly during the plasma ignition phase versus the steady state phase due to the changes caused by the plasma impedance
60
. As shown, the plasma impedance
60
may be roughly modeled as a parallel network containing an imaginary component (X)
60
a
and a real component (R)
60
b
. During the changes between plasma ignition and steady state, large adjustments of the tuning capacitors
50
a
and
50
b
are generally required to account for large values of reflected power due to changes in plasma impedance
60
during ignition. Even though tunability is achieved by capacitors
50
a
and
50
b
, the delivered power is often limited to a fraction of the RF generator
40
output capability, and in many cases, plasma ignition is achieved only by increasing the pressure in the plasma source or chamber.
The process of increasing and subsequently reducing pressure, in conjunction with varying the tuning capacitors
50
a
and
50
b
, may require more than 10 seconds to complete. This lengthy period of time may enable substantially large voltages to be induced on the antenna
30
and may result in substantial electric fields at the wafer—possibly endangering the devices on the wafer. It is noted that until the plasma is ignited wafers are exposed to the unshielded antenna fields. Furthermore, even before pulsing of the wafer, deposition may occur producing a surface concentration of dopant. Thus, variability in ignition times, source pressures, and voltage transients may result in variations in resultant implant characteristics—making tightly controlled repeatability exceedingly difficult to achieve. Still further, if the control system
72
, and/or any of the related circuits
50
,
70
and/or
74
fail, the plasma will be lost. Even if the control system
72
performs flawlessly, the system
20
is slow to react and move due to the tuning requirements discussed above.
Another conventional approach to solving the problem of matching a variable impedance plasma source to an RF generator, is by varying the frequency of the generator to maintain a resonant condition. However, this approach also requires a control loop which varies generator frequency to minimize reflected power. The control is generally not fast enough, however, to prevent fault conditions during large and rapid impedance variations as a result of plasma ignition. Thus, power must still be limited. Additionally, this approach generally only matches reactive load changes, and therefore a mechanically variable capacitor may still be required to match resistive load changes.
Consequently, there is a strong need in the art for a system and/or method to provide repeatable and reliable plasma ignition. Moreover, there is a strong need for a PIII system providing a substantially faster, repeatable and more economical plasma ignition process to alleviate the aforementioned problems associated with conventional PIII systems and/or methods.
SUMMARY OF THE INVENTION
The present invention is directed to an integrated power oscillator in a Plasma Immersion Ion Implantation System (PIII) which incorporates a plasma source antenna in the tank circuit of the power oscillator—resulting in generally automatic or immediate passive tracking of the antenna circuit resonant frequency. This enables virtually instant ignition of the plasma at pressures to about 0.5 mTorr. By integrating the oscillator and plasma antenna, conventional system components such as controls, tuning capacitors, coupling cables and power feedback meters are eliminated. As a result, substantially higher repeatability and performance is achieved over conventional systems. Moreover, since the oscillator is integrated with the plasma source housing and requires only a DC power supply (no RF generator), the present invention substantially reduces the complexity and parts count of the power system and thus provides lower cost and greater reliability over conventional systems.
More particularly, the present invention utilizes characteristics of the plasma source antenna (e.g., antenna inductance) and associated system parameters (e.g., plasma impedance, external system inductance) and incorporates these factors within a power oscillator tank circuit. Since plasma ignition causes significant parametric changes (e.g., plasma impedance changes affecting antenna impedance), the tank circuit and associated power supply are designed to operate across the variable parametric conditions within the plasma chamber. By incorporating the oscillator with the plasma source housing, load reflection and matching problems associated with conventional systems are substantially eliminated.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
REFERENCES:
patent: 3832648 (1974-08-01), McDowell
patent: 3958883 (1976-05-01), Turner
patent: 4667111 (1987-05-01), Glavish et al.
patent: 5212425 (1993-05-01), Goebel et al.
patent: 5643364 (1997-07-01), Zhao et al.
patent: 5654043 (1997-08-01), Shao et al.
patent: 5
Divergilio William F.
Kellerman Peter L.
Ryan Kevin T.
Axcelis Technologies Inc.
Eschweiler & Associates LLC
Hassanzadeh P.
Mills Gregory
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