Inductively coupled plasma reactor with an inductive coil antenna having independent loops

An inductively coupled plasma reactor for processing a substrate has an inductively coupled coil antenna including plural inductive antenna loops which are electrically separated from one another and independently connected to separately controllable plasma source RF power supplies. The RF power level in each independent antenna loop is separately programmed and instantly changeable to provide a perfectly uniform plasma ion density distribution across the entire substrate surface under a large range of plasma processing conditions, such as different process gases or gas mixtures. In a preferred embodiment, there are as many separately controllable RF power supplies as there are independent antenna loops, and all the separately controllable power supplies receive their RF power from a commonly shared RF generator.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The invention is related to radio frequency (RF) plasma reactors for 
processing semiconductor wafers, flat panel display wafer or substrates 
generally, and in particular to inductively coupled RF plasma reactors and 
improvements increasing the uniformity of the plasma ion density across 
the entire surface of large substrates. 
2. Background Art 
RF plasma reactors are employed in performing various processes on 
semiconductor wafers, including etching processes and chemical vapor 
deposition processes, for example. An inductively coupled RF plasma 
reactor typically has an inductive coil antenna wound around the reactor 
chamber and connected to a plasma source RF power supply. Such an 
inductively coupled plasma reactor is particularly useful because it can 
achieve a very high plasma ion density for high production throughput, 
while avoiding a concomitant increase in ion bombardment damage of the 
wafer. 
One problem with inductively coupled plasma reactors is that the plasma ion 
density distribution can vary greatly, depending upon various parameters 
including the particular process gas or gas mixture introduced into the 
reactor chamber. For example, the plasma ion density may be high at the 
wafer center and low at the wafer periphery for one process gas, while for 
another process gas it may be the opposite pattern (i.e., low at the wafer 
center and high at the wafer periphery). As a result, the antenna design 
must be customized for each different process or process gas to provide an 
acceptable degree of plasma ion density uniformity across the wafer 
surface, a significant problem. 
If the plasma reactor is employed in processing a large flat panel display 
wafer (comprising a glass substrate), for example, the large surface area 
of the substrate prevents achievement of good plasma ion density 
uniformity across the substrate surface without extraordinary care in 
customizing the antenna design. 
It is therefore an object of the present invention to provide a uniform 
plasma ion density across an entire substrate surface in an inductively 
coupled plasma reactor without requiring installation of a completely new 
antenna in the plasma reactor. 
SUMMARY OF THE DISCLOSURE 
An inductively coupled plasma reactor for processing a substrate has an 
inductively coupled coil antenna including plural inductive antenna loops 
which are electrically separated from one another and can be connected to 
independent RF power supplies. The RF power level in each independent 
antenna loop is separately programmed and instantly changeable to provide 
a perfectly uniform plasma ion density distribution across the entire 
substrate surface under a large range of plasma processing conditions, 
such as different process gases or gas mixtures and/or absorbed RF power 
levels. In a preferred embodiment, there are as many independent RF power 
regulating circuit elements as there are independent antenna loops, and 
all the independent RF power regulating circuit elements receive RF power 
from a commonly shared RF generator. In this preferred embodiment, a 
variable reactance element is connected in series with each independent 
antenna loop, so that all antenna loops are separately and instantaneously 
controlled. Typically, the independent antenna loops are over the ceiling 
of the reactor chamber, while a wafer pedestal near the chamber floor 
supports the substrate being processed. In an alternative embodiment, a 
separate RF generator is provided for each independent antenna loop. 
In accordance with another aspect of the invention, the plasma reactor is a 
capacitively coupled reactor, having a pair of electrodes, including a 
cathode near the floor of the reactor chamber underlying the substrate 
being processed, and an anode at the top or over the ceiling of the 
reactor chamber, the anode consisting of plural independent electrode 
segments connected to separately controllable RF power sources.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the embodiment of FIGS. 1 and 2, an inductively coupled RF 
plasma reactor has a vacuum chamber 10 having a generally cylindrical side 
wall 15 and a dome shaped ceiling 20. A gas inlet tube 25 supplies process 
gas (e.g., chlorine for etch processing) into the chamber 10. A wafer 
pedestal 30 supports a substrate such as semiconductor wafer 35 near the 
floor of the chamber 10. A bias RF power supply 40 connected to the 
pedestal 30 through a conventional RF impedance match network 45 controls 
the plasma ion density at the top surface of the wafer 35. A plasma is 
ignited and maintained within the chamber 10 by RF power inductively 
coupled from a coil antenna 50 consisting of a pair of independent 
(electrically separate) antenna loops 52, 54 wound around different 
portions of the dome-shaped ceiling. In the embodiment of FIG. 1, both 
loops are wound around a common axis of symmetry coincident with the axis 
of symmetry of the dome-shaped ceiling 20 and the axis of symmetry of the 
wafer pedestal 30 and wafer 35. The antenna loop 52 is of the conventional 
type typically employed in an inductively coupled reactor and is wound 
around the bottom portion of the dome-shaped ceiling 20, leaving an 
aperture 60 surrounded by the antenna loop 52. The other antenna loop 54 
is placed over the ceiling 20 in the center of the aperture 60. Separately 
controlled RF signals RF1 and RF2 are applied through RF impedance match 
networks 70, 75 to respective ones of the independent antenna loops 52, 
54, so that RF power in each loop 52, 54 is separately controlled. The RF 
power signal RF1 applied to the outer antenna loop 52 predominantly 
affects plasma ion density near the periphery of the wafer 35 while the RF 
power signal RF2 applied to the inner antenna loop 54 predominantly 
affects plasma ion density near the center of the wafer 35. Thus, for 
example, where it is found in an etch process performed with the reactor 
of FIG. 1 that the etch rate at the wafer center is less than the etch 
rate at the wafer periphery, the power of the RF signal RF2 on the inner 
antenna loop 54 is increased until the center and periphery etch rates are 
at least nearly equal. Likewise, if the center etch rate is found to be 
higher than the periphery etch rate, then the RF signal RF2 on the inner 
antenna loop 54 is decreased (or the RF signal on the outer antenna loop 
52 is increased) until uniformity of plasma ion distribution is at least 
nearly achieved. 
Thus, the same RF plasma reactor can be employed over a large range of 
different process parameters (including a large choice of process gases) 
tending to have different plasma ion density distribution patterns that 
must be compensated differently. The same plasma reactor is capable of 
compensating for the different plasma ion density distribution patterns 
characteristic of different plasma processes. 
Referring to FIG. 3, each antenna loop 52, 54 may be provided with its own 
RF power generator 80, 85 and RF impedance match network 90, 95 connected 
in series between the respective RF power generator 80, 85 and antenna 
loop 52, 54. In accordance with one aspect of the invention, the RF 
impedance match network (e.g., the RF impedance match network 95) may be 
coupled to the antenna loop (e.g., the inner antenna loop 54) through a 
transformer, such as the transformer 100 shown in FIG. 3. In the 
embodiment of FIG. 3, the transformer 100 has a primary winding 102 
connected to the output of the impedance match network 95 and a secondary 
winding 104 connected across the inner antenna loop 54. To provide a 
potential reference for the inner antenna loop 54, a ground tap 106 is 
connected to the center of the secondary winding 104. The advantage of 
this method of powering the antenna loop 54 is that it minimizes 
capacitive coupling to the plasma by reducing the peak voltage (with 
respect to ground) on the antenna loop 54. A general advantage of the 
embodiment of FIG. 3 is that the RF frequencies at each of the independent 
antenna loops 52, 54 are separately controllable so that the same RF 
frequency may be applied to both loops 52, 54 or else different RF 
frequencies may be applied to the loops 52, 54. 
Preferably, a common RF generator powers both antenna loops 52, 54, the RF 
power in each loop 52, 54 being separately controlled, as in the 
embodiment of FIG. 4. This feature has the advantage of simplicity and 
cost effectiveness. In FIG. 4, the single RF generator 110 is coupled 
through an RF impedance match network 115 to a pair of variable reactive 
circuits 120, 125 which in turn coupled RF power to the respective outer 
and inner antenna loops 52, 54. Each one of the variable reactive circuits 
120, 125 consists of an inductor 130 and variable capacitor 135 connected 
in parallel, a second variable capacitor 140 being connected in series 
with the combination of the parallel inductor and capacitor 130, 135. The 
variable capacitors in each variable reactive circuit 120, 125 permit the 
RF power level to be separately adjusted in each of the independent 
antenna loops 52, 54. 
There may be more than two independent antenna loops. Moreover, the plural 
independent antenna loops may be either symmetrically wound relative to a 
common axis of symmetry, as in the embodiment of FIGS. 1-4, or may be 
wound around separate axes and thus centered at different points 
distributed across the ceiling, as in the embodiment of FIGS. 5 and 6. 
Referring to FIGS. 5 and 6, the reactor may have a flat ceiling 20'. In 
this example, there are eight outer antenna loops 150a-150h centered at 
uniform intervals along an outer radius and three inner antenna loops 
150i-150k. Each loop 150 has a diameter which only a small fraction (e.g., 
1/10) of the diameter of the wafer 35. The inner end of each antenna loop 
150 is grounded while the outer end is connected through a respective one 
of a bank of variable capacitors 160a-160k and through respective reactive 
networks 165a-165k to a single commonly shared RF generator 170. The 
reactive networks may, for example, be RF impedance match networks. 
Control over each individual variable capacitor 160 in the bank of 
variable capacitors 160a-160k is exercised by a source power distribution 
controller 180. The user governs the RF power levels in each one of the 
independent antenna loops 150a-150k through the controller 180. A greater 
of lesser number of antenna loops may be employed than illustrated here. 
The reactor ceiling 20' is flat in this example, but it may also be domed, 
cylindrical or some other shape. Preferably, the independent loops are 
arranged symmetrically with respect to the reactor ceiling and substrate 
pedestal or processing substation. 
Referring to the alternative embodiment of FIG. 7, each of the independent 
loops 150a-150k may be replaced by small disk-shaped independent 
electrodes 150'a-150'k of the approximately the same diameter as the 
independent loop which it replaces. In FIG. 7, the RF plasma reactor is a 
capacitively coupled reactor. For this purpose, the wafer pedestal may be 
either grounded or connected to the separate RF power generated 40 as 
illustrated in FIG. 1. The independent electrodes together function as an 
anode electrode array while the wafer pedestal functions as the cathode of 
the capacitively coupled RF plasma reactor. 
While the invention has been described in detail by specific reference to 
preferred embodiments, it is understood that variations and modifications 
thereof may be made without departing from the true spirit and scope of 
the invention.