Electric heating – Metal heating – By arc
Reexamination Certificate
2002-01-14
2004-05-25
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121570, C219S121430, C315S111510, C156S345450, C118S7230IR
Reexamination Certificate
active
06740842
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the generation of inductively coupled plasmas in apparatus for performing etching and deposition processes.
A variety of semiconductor fabrication operations involve deposition and etching processes performed on a semiconductor substrate mounted within a process chamber. Such processes typically involve the use of a low pressure, high density discharge wherein a plasma is generated by the interaction of an ionizable gas with a radio frequency (RF) electromagnetic field. The coupling of RF power to a plasma in semiconductor process chambers can be categorized as either predominantly capacitive or predominantly inductive. Many examples of each can be found in the prior art.
In the case of capacitive coupling, RF power is coupled to the bottom plate and/or the top plate of a parallel plate process chamber. In general, the top plate also serves as the ionizable gas feed, the bottom plate serves as the wafer holding chuck and the remainder of the chamber is grounded.
Inductive coupling generally employs a planar geometry, or a cylindrical geometry, or a combination of the two geometries. Furthermore, low RF power is usually applied to a bottom electrode, or chuck, to provide a RF bias.
FIGS. 1A
,
1
B and
1
C present some examples of the inductive discharge geometries.
FIG. 1A
illustrates an example of planar geometry in which a planar multi-turn coil is located at the top of a process tube, or process chamber.
FIG. 1B
shows an example of cylindrical geometry in which a multi-turn cylindrical coil is wound around a process tube, while
FIG. 1C
shows a modified version of cylindrical geometry in which the cylindrical coil is surrounded by a conductive shield. The structure shown in
FIG. 1C
is an example of a helical resonator. In each of the illustrated arrangements, the coil is connected to receive a RF current and, thence, to induce an electromagnetic (EM) field parallel to the longitudinal axis of the cylindrical geometry. This resultant RF EM field, that is a manifestation of the RF current in the coil, consists primarily of radially propagating EM waves proximate to the plasma volume when polarized by an electrostatic shield (to remove the azimuthally propagating field). The radially propagating waves interact with a small thin surface layer of the bulk plasma. The thickness of this thin layer is often referred to as the skin depth. This interaction ultimately leads to energized electrons and subsequent gas ionization, and the formation of a plasma. In general, a process tube acts as a protective barrier and delineates the inner plasma volume from the external structure. At least in the structures of
FIGS. 1B and 1C
, the process tube is made of a dielectric material that is transparent to the electromagnetic energy emanating from the coil. It will be understood that these figures are schematic. Actual equipment can take a variety of forms in practice.
The coupling of RF power to a plasma in semiconductor processing is conventionally at a drive frequency of 13.56 MHz, using a 50 &OHgr; RF power generator. This frequency is conveniently located within a RF band designated for industrial use. However, the frequency of operation is not limited to this value in the prior art and, in fact, multiple frequencies are employed typically when using multiple coupling electrodes.
RF power is typically supplied to the coil by an oscillator having at least one active component that may be a solid state, or semiconductor, component, or a vacuum tube.
As is known in the art, energy can be inductively coupled into a process chamber through a helical resonator as described in Lieberman & Lichtenberg, Chapter 12 (
Principles of plasma discharges and materials processing,
John Wiley & Sons, Inc., 1994). With a helical resonator, the coil (or helix) has a length equal to an integral number of quarter waves of the RF input. The coil surrounds the plasma chamber and is encased within a cylindrical container that is grounded.
FIG. 1C
shows the basic structure of such a helical resonator including the coil, an electrostatic shield enclosed by the coil to minimize capacitive coupling of the RF field with the plasma, a dielectric process tube that is enclosed by the electrostatic shield and separates the helical coil from the plasma, an outer conductor, or shield, surrounding the coil and an RF input line connected to a tap of the coil. As shown in
FIG. 1C
, the coil tap to which the RF input is applied is spaced from one end of the coil which is grounded. The portion of the coil between the coil tap and ground effectively serves as part of the matching circuit, thus the tap position can be selected to achieve a match condition. Under a given set of conditions, proper definition of the tap point location can provide impedance matching for the circuit.
However, the load impedance on a RF power generator is a function of the intrinsic impedance of the coil and the impedance presented by the plasma, the latter impedance being a function of the properties of the plasma. Therefore, fluctuations in the process conditions can lead to fluctuations in the impedance as seen by the RF power generator. Furthermore, the impedance of the process chamber, in which the plasma is established, varies significantly between the condition prior to plasma ignition and the run condition. In order to maintain efficient energy transfer from the RF power generator to the plasma, proper matching of the power supply output impedance to the load impedance is required.
One technique used in the prior art is a variable frequency power supply. The frequency is determined by a phase mag detector that determines the match conditions at the input of a fixed match network coupling to the tap of the coil. However, systems of this type can be very expensive, and hence a fixed frequency power supply is generally employed in conjunction with a match network.
An example of a fixed frequency RF oscillator coupled to the coil of a helical resonator via an impedance matching network is shown in FIG.
2
. The matching network is a &pgr;-filter composed of a series connected inductor, L, and two shunt connected variable capacitors C
1
and C
2
. The matching network compensates for differences between the variable load impedance represented by the coil and the plasma, and the output impedance of the RF power generator. For example, as shown in
FIG. 2
, when the source impedance Z
s
is equal to the load impedance Z
MNi
, this impedance including the impedances of the match network, the helical coil and the plasma load, then the power transfer can be maximized. In this particular case, the input impedance to the match network-load circuit Z
MNi
is the complex conjugate of the source impedance Z
s
, and the output impedance of the match network Z
MNo
, as seen by the load, is the complex conjugate of the load impedance Z
L
. Under this special condition, the coupling between the RF source and the combination of the match network and the plasma loaded coil can be represented as equivalent to a purely resistive circuit. Hence, the matching network is designed to maximize power transmission from the RF power generator to its load.
Given feedback of the power transfer state (reflected/transmitted power levels using special detector circuits whose outputs approximate the difference in phase between the forward and reflected signals and the magnitude of the reflected signal), matching networks have been developed to respond to changes in the load impedance. In particular, during plasma ignition and run conditions, the variable capacitors are adjusted to tune the load circuit, which includes the impedance match network, the coil and the plasma load, to a resonant condition for the fixed frequency power supply. When the circuit impedances are matched, power reflected to the source at the match network juncture is minimized, or even zero, depending upon the accuracy of the match, thus reducing damage to the power supply, which must ultimately absorb this reflected power. It is known,
Johnson Wayne L.
West Leonard G.
Paschall Mark
Pillsbury & Winthrop LLP
Tokyo Electron Limited
LandOfFree
Radio frequency power source for generating an inductively... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radio frequency power source for generating an inductively..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radio frequency power source for generating an inductively... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3229262