Adjusting DC bias voltage in plasma chambers

A method of adjusting the cathode DC bias in a plasma chamber for fabricating semiconductor devices. A dielectric shield is positioned between the plasma and a selected portion of the electrically grounded components of the chamber, such as the electrically grounded chamber wall. The cathode DC bias is adjusted by controlling one or more of the following parameters: (1) the surface area of the chamber wall or other grounded components which is blocked by the dielectric shield; (2) the thickness of the dielectric; (3) the gap between the shield and the chamber wall; and (4) the dielectric constant of the dielectric material. In an apparatus aspect, the invention is a plasma chamber for fabricating semiconductor devices having an exhaust baffle with a number of sinuous passages. Each passage is sufficiently long and sinuous that no portion of the plasma within the chamber can extend beyond the outlet of the passage. By blocking the plasma from reaching the exhaust pump, the exhaust baffle reduces the deposition of unwanted particles on exhaust pump components. The exhaust baffle also reduces the cathode DC bias by reducing the effective surface area of the electrically grounded chamber wall which couples RF power to the plasma.

FIELD OF THE INVENTION 
This invention relates generally to plasma chambers used for fabricating 
semiconductor devices. More specifically, this invention relates to 
apparatus and methods of adjusting the DC bias voltage on one or more 
chamber electrodes to which RF power is applied. 
BACKGROUND OF THE INVENTION 
In the fabrication of semiconductor devices, plasma chambers commonly are 
used to perform various fabrication processes such as etching, chemical 
vapor deposition (CVD), and sputtering. Generally, a vacuum pump maintains 
a very low pressure within the chamber while a mixture of process gases 
continuously flows into the chamber and an electrical power source excites 
the gases into a plasma state. The constituents of the process gas mixture 
are chosen to effect the desired fabrication process. 
In essentially all etching and CVD processes, and in many sputtering 
processes, the semiconductor wafer or other workpiece is mounted on a 
cathode electrode, and a radio frequency (RF) electrical power supply is 
connected, through a DC blocking capacitor, between the cathode electrode 
and an anode electrode in the chamber. Most commonly, the walls of the 
chamber are metal and are connected to the RF power supply to function as 
the anode electrode. When the chamber walls are the anode, they typically 
are connected to electrical ground. 
The body of the plasma has a positive charge such that its average DC 
voltage is positive relative to the cathode and anode electrodes. Because 
the RF power supply is connected to the cathode and anode electrodes 
through a DC blocking capacitor, the respective DC voltages at the cathode 
and anode can be unequal. Specifically, because the cathode's surface area 
facing the plasma is much smaller than the anode's surface area facing the 
plasma, the cathode is much more negative than the anode. In other words, 
the voltage drop between the plasma body and the cathode is much greater 
than the voltage drop between the plasma body and anode. This voltage 
asymmetry is a widely observed phenomenon, although its physical cause is 
complex and not completely understood. (See M. A. Lieberman et al., 
"Principles of Plasma Discharges and Materials Processing," pub. John 
Wiley & Sons, 1994, pages 368-372.) The negative DC voltage at the cathode 
relative to the anode commonly is referred to as the "cathode DC bias". 
The negative DC bias voltage at the cathode accelerates ions from the 
plasma to bombard the semiconductor wafer with a kinetic energy 
approximately equal to the voltage drop between the cathode and the plasma 
body. The kinetic energy of the bombarding ions can be beneficial in 
promoting the chemical or physical reactions desired for the semiconductor 
fabrication process. 
However, bombarding ions having excessive kinetic energy can damage the 
device structures being fabricated on the semiconductor wafer. Therefore, 
it often is desirable to reduce the cathode DC bias. 
A known method of reducing the cathode DC bias is to reduce the level of RF 
power applied to the cathode electrode. However, reducing the RF power 
undesirably reduces the rate of dissociation of molecules in the plasma, 
thereby undesirably reducing the rate at which the fabrication process is 
carried out (i.e., increasing the time required to fabricate a 
semiconductor device). Therefore, a need exists for an apparatus and 
method for reducing the cathode DC bias other than by reducing the RF 
power supplied to the cathode. 
Certain semiconductor fabrication processes require more highly energetic 
ion bombardment than other processes. It is desirable for a single plasma 
chamber to be adaptable to a number of different processes. Therefore, a 
need exists for a method of adjusting the cathode DC bias in a given 
chamber other than by adjusting the RF power supplied to the cathode. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention is a method of adjusting the DC bias 
on one chamber electrode relative to another electrode by interposing a 
dielectric shield between one of the electrodes and the plasma. Adjusting 
any property of the shield which alters the capacitance between the plasma 
and the electrode covered by the shield can be used to adjust the DC bias. 
Specifically, the DC bias is adjusted by any of the following adjustments: 
(1) changing the thickness of the dielectric in the shield; (2) 
substituting a dielectric material having a different dielectric constant; 
(3) changing the size or shape of the dielectric to change the surface 
area of the electrode which is covered thereby; or (4) changing the gap 
between the shield and the covered electrode. 
In particular, the method is useful to adjust or reduce the DC bias voltage 
at the cathode electrode in a plasma chamber in which the anode electrode 
includes an electrically grounded chamber wall. The dielectric shield is 
positioned between the plasma and a selected portion of the electrically 
grounded components of the chamber, such as the chamber wall. The method 
permits reducing the magnitude of the negative DC bias voltage at the 
cathode (relative to the anode) without reducing the RF power applied to 
the cathode. 
In another aspect, the present invention is a plasma chamber for 
fabricating semiconductor devices having an exhaust baffle which reduces 
the cathode DC bias by reducing the effective surface area of the 
electrically grounded chamber wall which couples RF power to the plasma. 
Specifically, the exhaust baffle has a number of sinuous passages, and the 
baffle overlies the exhaust port of the plasma chamber so that chamber 
gases exhausted from the chamber by the vacuum pump pass through the 
sinuous passages. Each passage is sufficiently long and sinuous that no 
portion of the plasma within the chamber can extend beyond the outlet of 
the passage. 
The exhaust baffle of the invention electrically isolates the plasma from 
the portion of the chamber wall behind the baffle, thereby reducing the 
effective surface area of the grounded chamber wall which couples RF power 
to the plasma. This reduces the ratio of the effective chamber wall 
surface area facing the plasma to the cathode surface area facing the 
plasma, thereby advantageously reducing the magnitude of the negative DC 
bias at the cathode. 
To further reduce the cathode DC bias, the exhaust baffle can include 
dielectric material. Preferably, the dielectric isolates any electrically 
conductive elements in the exhaust baffle from the grounded chamber wall. 
The dielectric should be substantially thicker than the width of the 
plasma sheath or, alternatively, of sufficient thickness to substantially 
impede the coupling of RF power from the chamber wall to the plasma via 
the exhaust baffle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
1. Mechanical Description of Preferred Embodiment 
Before describing the operation of the invention, the structural details of 
a vacuum chamber incorporating the invention will be described. 
FIG. 1 shows the presently preferred embodiment of the invention as 
implemented in a vacuum chamber used for plasma etching of silicon oxide 
dielectric layers on a silicon wafer. The principal components of the 
invention are a dielectric anode shield 10 and an anodized aluminum 
cathode shield 12. Each of the two shields 10 and 12 is generally 
cylindrical in shape; more specifically, each is symmetrical about the 
longitudinal axis of the chamber cathode 30. 
The anode shield 10 and the cathode shield 12 respectively include 
overlapping annular protrusions 14 and 16, respectively, which function in 
combination as an exhaust baffle. 
The sides and bottom of the vacuum chamber are bounded by an aluminum wall 
18. The side portion 20 of the chamber wall 18 is essentially cylindrical. 
The bottom portion 22 of the chamber wall is annular, with a central 
opening in the center to accommodate a cathode pedestal to be described 
below. (The side portion 20 and the bottom portion 22 of the chamber wall 
18 are referred to below as the chamber side wall 20 and the chamber 
bottom wall 22, respectively.) The top of the vacuum chamber is bounded by 
a circular aluminum lid 24. An annular aperture or slit 26 in the chamber 
side wall 20 allows a workpiece (e.g., a silicon wafer) to be transferred 
into and out of the chamber. The anode shield 10 also has an aperture 23 
coincident with the slit 26. A vacuum valve (not shown), known as a slit 
valve, maintains a vacuum seal over the wafer transfer slit 26 while a 
workpiece is being etched during operation of the chamber. 
An annular aluminum spacer 25 is attached to the chamber bottom wall 22 by 
bolts (not shown), and an O-ring 27 provides a vacuum seal between the 
spacer and the wall. A quartz dielectric spacer 28, having an annular 
shape with an L-shaped cross-section, rests atop the aluminum spacer. An 
O-ring 29 provides a vacuum seal between the two spacers. 
A disc-shaped aluminum cathode electrode 30 has a flat, circular top 
surface 32 on which a silicon wafer to be etched is placed by a robot (not 
shown). The cathode 30 rests atop the lower, inwardly-extending portion of 
the L-shaped dielectric spacer 28. An O-ring 31 provides a vacuum seal 
between the cathode and the dielectric spacer. The O-rings 27, 29, 31 
permit the region beneath the cathode to remain at atmospheric pressure 
while the interior of the vacuum chamber is operated at a vacuum. 
An RF transmission line (not shown) connects the cathode to the ungrounded 
output terminal of an RF power supply 60 (shown only in FIG. 3), the other 
power supply output terminal being connected to the electrically grounded 
chamber wall 18. The lower, inwardly-extending portion of the L-shaped 
dielectric spacer 28 insulates the cathode from the metal spacer 25, which 
is electrically grounded through its attachment to the chamber bottom wall 
22. The side portion of the L-shaped dielectric spacer surrounds the 
cathode and prevents any electrical discharge between the cathode and the 
adjacent portion of the chamber side wall 20. 
A quartz dielectric ring 38 rests atop the cathode and protects the top 
perimeter of the cathode from exposure to the plasma. The remaining top 
surface 32 of the cathode is covered by a semiconductor wafer during 
chamber operation. 
The upper periphery of the anode shield 10 includes an outwardly projecting 
annular lip. The lip rests on the upper edge of the chamber side wall 20 
and supports the weight of the anode shield. An O-ring 37 provides a 
vacuum seal between the upper lip of the anode shield 10 and the upper 
edge of the chamber side wall 20. Another O-ring 39 provides a vacuum seal 
between the upper lip of the anode shield 10 and the chamber lid 24. 
The lower end of the cathode shield 12 is a horizontal annular flange 13 
which rests on the chamber bottom wall 22. The flange 13 covers the entire 
exposed surface of the lower chamber wall between the metal spacer 25 and 
the anode shield 10. The cathode shield 12 is held in place by the weight 
of the anode shield 10. Specifically, the axial length of the anode shield 
10 is sufficient to compress an O-ring 15 on the lower flange 13 of the 
cathode shield 12. The O-ring 15 enables the weight of the anode shield 10 
to press the flange 13 of the cathode shield downward against the chamber 
bottom wall 22, thereby improving heat transfer between the flange 13 and 
the bottom wall 22, for reasons to be explained below. 
In the preferred embodiment, the bottom surface of the flange 13 is bare 
aluminum, whereas all other surfaces of the cathode shield 12 are 
anodized. The bare aluminum provides consistently good electrical contact 
between the cathode shield 12 and the grounded chamber bottom wall 22, 
thereby providing more consistent performance of the semiconductor 
fabrication process performed in the chamber than would be possible if an 
anodized bottom surface provided inconsistent electrical contact. However, 
as explained below, it may be desirable to fabricate the cathode shield of 
a dielectric. In that case, there would be no risk of inconsistent 
electrical contact between the cathode shield and any electrode. 
We found our O-ring 15 tends to stick to the two shields 10 and 12, making 
it difficult to separate the shields when they are being replaced. 
Therefore, the chamber preferably includes a thin, aluminum ring (not 
shown) placed between the O-ring 15 and the bottom of the anode shield 10. 
Additionally, two smaller dielectric shields 40 and 42 cover the upper and 
lower surfaces, respectively, of the wafer transfer slit 26 through which 
the workpiece is carried into and out of the chamber. Each of the slit 
shields 40 and 42 is attached to the chamber side wall by a number of 
bolts 44. The anode shield 10 covers these bolts so that they are not 
exposed to the plasma in the chamber. 
A perforated, circular, gas distribution plate 44, commonly called a 
showerhead, is mounted on the underside of the chamber lid 24 and is 
coaxial with the cathode 30. The gas distribution plate is composed of a 
dielectric material such as quartz. The anode shield 10 surrounds the 
perimeter of the gas distribution plate so as to cover the exposed area of 
the chamber lid 24. One or more gas lines connect to fittings in the 
chamber lid and convey process gases to the inlet manifold area above the 
gas distribution plate. The process gases then flow through the gas 
distribution plate into the interior of the chamber. 
The process gases exit the chamber through a circular exhaust port 50 in 
the chamber bottom wall 22. A vacuum pump (not shown) coupled to the 
exhaust port pumps process gases out of the chamber and maintains a 
desired level of vacuum within the chamber. A throttle valve 52 mounted 
between the exhaust port 50 and the vacuum pump regulates the gas pressure 
within the chamber by regulating the impedance to gas flow from the 
exhaust port 50 to the vacuum pump. The annular flange 13 at the bottom of 
the cathode shield 12 includes an arcuate aperture 51, coinciding with the 
circular exhaust port 50, to permit gases to exit the chamber through the 
flange aperture 51 and through the exhaust port 50. 
A radio frequency (RF) power supply 60 (not shown in FIG. 1) produces an RF 
voltage across two output terminals which are ungrounded and grounded, 
respectively. The ungrounded output is connected through a DC blocking 
capacitor 62 to the cathode electrode 30. (The RF power supply and 
capacitor are shown in FIG. 3, which depicts an alternative embodiment of 
the exhaust baffle.) The grounded output of the RF power supply connects 
to all components of the chamber which are electrically grounded, 
including the chamber wall 18, the chamber lid 24, and the throttle valve 
52. Because the RF power supply is connected between the cathode electrode 
and the grounded chamber components, these grounded components 
collectively function as the anode electrode. 
The RF electrical power applied between the cathode electrode 30 and the 
grounded chamber wall 18 excites the process gases into a plasma state. A 
significant proportion of the gas molecules in the plasma are dissociated 
into their constituent atoms, ions, and free electrons. These particles 
interact with each other and with the surface material of the 
semiconductor workpiece to perform a desired semiconductor device 
fabrication process. 
2. Reducing or Adjusting Cathode DC Bias 
a. Overview of Cathode DC Bias 
As stated in the earlier section entitled "Background of the Invention", 
the body of the plasma has a positive charge such that its average DC 
voltage is positive relative to the cathode and anode electrodes. Because 
the RF power supply 60 is connected in series with a DC blocking capacitor 
62, the respective DC voltages at the cathode and anode can be unequal. 
Specifically, because the cathode's surface area facing the plasma is much 
smaller than the anode's surface area facing the plasma, the cathode is 
much more negative than the anode. In other words, the voltage drop 
between the plasma body and the cathode is much greater than the voltage 
drop between the plasma body and anode. The average negative DC voltage 
V.sub.bias at the cathode electrode relative to the (typically grounded) 
anode electrode commonly is referred to as the "cathode DC bias". 
The negative DC bias voltage at the cathode accelerates ions from the 
plasma to bombard the semiconductor wafer with a kinetic energy 
approximately equal to the voltage drop between the cathode and the plasma 
body. The kinetic energy of the bombarding ions can be beneficial in 
promoting the chemical or physical reactions desired for the semiconductor 
fabrication process. 
However, bombarding ions having excessive kinetic energy can damage the 
device structures being fabricated on the semiconductor wafer. Therefore, 
it often is desirable to reduce the cathode DC bias. 
In our invention, the magnitude of the negative DC bias voltage at the 
cathode (relative to the anode) is reduced by interposing a dielectric 
shield between the plasma and a portion of the anode electrode surface 
facing the plasma, thereby reducing the effective surface area of the 
anode electrode through which power from the RF power supply 60 is 
capacitively coupled to the plasma. In the illustrated preferred 
embodiment, the anode shield 10 covers almost all of the exposed surface 
of the electrically grounded chamber wall 18 which otherwise would 
capacitively couple RF power to the plasma within the chamber. If the 
resulting decrease in cathode DC bias is more than desired, decreasing the 
size of the dielectric shield so as to decrease the portion of the anode 
surface covered by dielectric will increase the cathode DC bias. 
b. Modelling DC Bias in a Conventional Plasma Chamber 
The physics of how the RF power is coupled to the plasma is complex and not 
completely understood. (See Lieberman, supra, pp. 368-372.) We believe the 
operation of the invention can be understood by reference to the 
electrical models shown in FIGS. 2A and 2B. FIG. 2A represents a 
conventional plasma chamber without the dielectric shield of the present 
invention, and FIG. 2B represents the same chamber with the addition of 
our dielectric shield. The modelling of physical elements by resistors and 
capacitors as shown in FIG. 2 is only a rough approximation, because the 
actual behavior of these elements is non-linear. 
In FIG. 2A, the plasma body is modelled as an impedance Z.sub.plasma having 
resistive and inductive components. The plasma is surrounded by a sheath 
or dark space which is highly depleted of free electrons and which can be 
modeled as a vacuum, that is, as a dielectric having a dielectric constant 
of unity (Lieberman, supra, p. 95, equation 4.2.25a). Capacitor 
C.sub.CathodeSh models the capacitance between the cathode 30 and the 
plasma body, i.e., the capacitance across the plasma sheath adjacent the 
cathode. (The subscript "CathodeSh" is an abbreviation for "Cathode 
Sheath", i.e., "the plasma sheath adjacent the cathode".) Although the 
chamber geometry is not planar, to a first approximation C.sub.CathodeSh 
is like a parallel plate capacitor in which the cathode electrode 30 and 
the plasma body are the two plates, and the plasma sheath is the 
dielectric between the two plates. 
The capacitance C.sub.CathodeSh between the plasma body and the cathode 30 
is proportional to the area A.sub.cathode of the portion of the surface of 
cathode 30 which faces and contacts the plasma sheath. (In general, 
A.sub.cathode equals the area of the cathode top surface 32, plus some 
upper portion of the cathode side wall, depending on how far the plasma 
extends down the side of the cathode. However, in the preferred 
embodiment, the L-shaped dielectric spacer 28 decouples the side of the 
cathode from the plasma, so A.sub.cathode simply equals the area of the 
cathode top surface 32.) Furthermore, the capacitance C.sub.CathodeSh is 
inversely proportional to the width W.sub.CathodeSh of the plasma sheath 
or dark space adjacent the cathode. Hence, C.sub.CathodeSh 
.varies.A.sub.cathode /W.sub.CathodeSh, where the symbol ".varies." means 
"is proportional to". 
Similarly, the capacitor C.sub.AnodeSh models the capacitance between the 
anode electrode and the plasma body, i.e., the capacitance across the 
plasma sheath adjacent the anode. The capacitance of C.sub.AnodeSh is 
proportional to A.sub.anode divided by W.sub.AnodeSh, where A.sub.anode is 
the surface area of the portion of the anode electrode which faces and 
contacts the plasma sheath, and W.sub.AnodeSh is the width of the plasma 
sheath or dark space adjacent the anode electrode. The surface area 
A.sub.anode includes the inner surface of the chamber side wall 20 and the 
portion of the chamber lid 24 outside the perimeter of the quartz gas 
distribution plate 44. Aanode additionally may include the upward-facing 
surfaces of the chamber bottom wall 22 and the grounded exhaust throttle 
valve 52, to the extent the plasma sheath extends to contact such 
surfaces. 
Although unimportant to a qualitative understanding of the present 
invention, an additional consideration complicates a quantitative analysis 
of the invention. The proportionality between the surface area of an 
electrode and the capacitance between that electrode and the plasma body 
is non-linear, because the capacitance and the plasma sheath width 
adjacent that electrode are interdependent. Specifically, the plasma 
sheath width W.sub.AnodeSh decreases in response to a decrease in the DC 
sheath voltage drop V.sub.AnodeSh, and, for reasons explained below, the 
DC sheath voltage drop V.sub.AnodeSh decreases in response to an increase 
in the capacitance C.sub.AnodeSh. Therefore, increasing the anode surface 
area A.sub.anode will increase the capacitance C.sub.AnodeSh, which will 
decrease the voltage drop V.sub.AnodeSh across the plasma sheath adjacent 
the anode, which will decrease the plasma sheath width W.sub.AnodeSh 
adjacent the anode, which will further increase the capacitance 
C.sub.AnodeSh. 
Consequently, the capacitance C.sub.AnodeSh between the anode and the 
plasma body is roughly proportional to the anode surface area A.sub.anode 
raised to a power "q", where "q" is a function of the geometry of the 
cathode and anode electrodes, the gas pressure in the chamber, and other 
factors. (Lieberman, supra, pp. 368-372). Similarly, the plasma 
body-to-cathode capacitance C.sub.CathodeSh is roughly proportional to the 
total cathode surface area raised to the power "q". 
The RF power supply 60, is connected between the cathode electrode and the 
anode electrode through a series-connected capacitor C.sub.0 which 
functions to block DC voltage. In general, it does not matter which point 
in the circuit is connected to electrical ground, or whether the DC 
blocking capacitor C.sub.0 connects to the cathode or the anode. In 
practice, when the anode electrode includes the chamber wall 18, it is 
most convenient to connect the anode electrode and one of the two RF power 
supply output terminals to electrical ground as shown in FIG. 2, and to 
connect the DC blocking capacitor C.sub.0 between the cathode and the 
ungrounded output terminal of the RF power supply. 
Conventionally, the capacitance selected for the DC blocking capacitor is 
C.sub.0 is much greater than both C.sub.AnodeSh and C.sub.CathodeSh, so 
that the RF voltage drop across the DC blocking capacitor is negligible. 
The plasma impedance Z.sub.plasma is much smaller than the impedances of 
both C.sub.AnodeSh and C.sub.CathodeSh, so that the RF voltage drop across 
the plasma body also is negligible (Lieberman, supra, p. 96, paragraph 
following equation 4.2.25b). Consequently, the RF voltage produced by the 
RF power supply is divided between the plasma body-to-cathode capacitance 
C.sub.CathodeSh and the plasma body-to-anode capacitance C.sub.AnodeSh in 
inverse proportion to their respective capacitances. 
Furthermore, there is a DC voltage drop across the plasma sheath which 
approximately equals 0.83 times the RF voltage drop across the sheath. 
(Lieberman, supra, equation 11.2.22, pp. 342-344, and p. 368). Therefore, 
the ratio of the DC voltage V.sub.CathodeSh across the plasma sheath 
adjacent the cathode to the DC voltage V.sub.AnodeSh across the plasma 
sheath adjacent the anode approximately equals the ratio of the plasma 
body-to-anode capacitance C.sub.AnodeSh to the plasma body-to-cathode 
capacitance C.sub.CathodeSh. That is, (V.sub.CathodeSh 
/V.sub.AnodeSh).apprxeq.(C.sub.AnodeSh /C.sub.CathodeSh), where .apprxeq. 
means "approximately equals". Therefore, the DC bias voltage V.sub.bias on 
the cathode electrode relative to the grounded anode electrode is: 
EQU V.sub.bias =(V.sub.AnodeSh -V.sub.CathodeSh).varies.-(C.sub.AnodeSh 
-C.sub.CathodeSh) (Eqn. 1) 
The cathode DC bias is negative because the anode's surface area facing the 
plasma is much greater than the cathode's surface area facing the plasma, 
hence C.sub.AnodeSh &gt;&gt;C.sub.CathodeSh. 
c. Reducing or Adjusting DC Bias According to Our Invention 
In our invention, a dielectric shield is positioned between the plasma and 
a selected portion of either the cathode electrode or the anode electrode 
for the purpose of making the DC bias voltage on that electrode more 
negative, or, equivalently, for the purpose of making the DC bias voltage 
on the opposite electrode more positive. In a typical plasma chamber 
having electrically grounded components such as an electrically grounded 
chamber wall, the dielectric shield preferably is positioned between the 
plasma and a selected portion of the electrically grounded components for 
the purpose of making the DC bias on the opposite, ungrounded electrode 
more positive, i.e., less negative. 
In the preferred embodiment shown in FIG. 1, the electrically grounded 
chamber wall 18 is the anode electrode, and the semiconductor workpiece is 
mounted on the top surface 32 of a cathode electrode 30 connected to the 
ungrounded output terminal of an RF power supply 60. In this embodiment, 
the dielectric shield 10 reduces the negative DC bias voltage at the 
cathode relative to the grounded anode. 
As will be explained below, the DC bias voltage V.sub.bias at the cathode 
relative to the grounded anode can be adjusted by adjusting one or more of 
the following parameters: (1) the surface area of the chamber wall or 
other grounded components which is blocked by the dielectric shield; (2) 
the thickness of the shield; (3) the gap between the shield and the 
chamber wall; and (4) the dielectric constant of the shield material. 
FIG. 2B shows an electrical model of the preferred plasma chamber shown in 
FIG. 1 which incorporates a dielectric shield 10 covering most of the 
anode surface. (To simplify the discussion, the smaller dielectric shields 
40, 42 over the slit 26 are not discussed here, but their function is the 
same as the dielectric anode shield 10.) The dielectric shield 10 has the 
effect of interposing capacitor C.sub.shield between the chamber wall 18 
and the perimeter of the plasma sheath. This capacitor C.sub.shield 
represents the capacitance across the dielectric shield 10. The 
capacitance C.sub.AnodeSh between the perimeter of the plasma sheath and 
the plasma body--i.e., the capacitance across the plasma sheath adjacent 
the anode--remains approximately the same as in FIG. 2A. (More precisely, 
as mentioned earlier, the capacitance C.sub.AnodeSh across the plasma 
sheath is somewhat greater in FIG. 2B than in FIG. 2A due to a decrease in 
sheath width W.sub.AnodeSh in response to a decrease in sheath voltage 
drop V.sub.AnodeSh.) 
The capacitor C.sub.x in FIG. 2B represents the capacitance between the 
plasma body and the whatever portion of the anode electrode is left 
uncovered (i.e., exposed to the plasma) by the dielectric shield. It is 
proportional to the surface area of such uncovered portion which faces the 
plasma. Initially, we will assume the dielectric shield 10 completely 
covers the anode electrode, so that the capacitor C.sub.x can be ignored. 
The capacitance C of a parallel plate capacitor is the dielectric constant 
.epsilon. of the dielectric which separates the two plates, multiplied by 
the mutually opposing surface area A of the two plates, divided by the 
width W of the dielectric separating the two plates. C=.epsilon.A/W. The 
capacitance C.sub.shield across the dielectric shield has the same surface 
area A.sub.anode as the capacitance C.sub.AnodeSh across the corresponding 
portion of the plasma sheath. As stated earlier, the dielectric constant 
.epsilon..sub.sheath of the plasma sheath is essentially unity. Therefore, 
the ratio of the shield capacitance to the plasma sheath capacitance is: 
EQU C.sub.shield /C.sub.AnodeSh =.epsilon..sub.shield (W.sub.AnodeSh 
/W.sub.shield) (Eqn. 2) 
Since these two capacitances are effectively in series between the anode 
and the plasma body, the resultant or effective capacitance between the 
anode and the plasma body is: 
EQU C.sub.AnodeEff =(C.sub.shield .multidot.C.sub.AnodeSh)/(C.sub.shield 
+C.sub.AnodeSh) (Eqn. 3a) 
EQU .apprxeq.C.sub.shield if C.sub.AnodeSh &gt;&gt;C.sub.shield (Eqn. 3b) 
The dielectric constant .epsilon..sub.shield of materials of which the 
dielectric shield may be fabricated, such as silicon carbide, ceramics or 
quartz, is in the range of about 2 to 5. The width W.sub.AnodeSh of the 
plasma sheath typically is 2 mm or less, whereas the width W.sub.shield of 
the dielectric shield in the preferred embodiment is about 5 to 20 mm. 
Because the ratio between dielectric widths W can greatly exceed the ratio 
between dielectric constants .epsilon., the capacitance C.sub.shield 
across the dielectric shield can be much smaller than the capacitance 
C.sub.AnodeSh across the plasma sheath. For example, suppose the 
dielectric shield has a width W.sub.shield equal to 15 mm and a dielectric 
constant of 3, and suppose the plasma sheath width W.sub.AnodeSh is 0.8 mm 
(with a dielectric constant of 1). Then the capacitance C.sub.shield 
across the dielectric shield equals 3.times.(0.8 mm/15 mm)=0.16 times the 
capacitance C.sub.AnodeSh across the plasma sheath. Consequently, the 
resultant or effective capacitance C.sub.AnodeEff between the anode 
electrode and the plasma body equals 0.16/(1+0.16)=0.14 times the 
capacitance across the plasma sheath C.sub.AnodeSh. 
EQU C.sub.AnodeEff =0.14C.sub.AnodeSh (Eqn. 4) 
In other words, the dielectric shields reduce the capacitive coupling 
between the plasma body and the portion of the anode covered by the 
shields to only fourteen percent (14%) of what the capacitive coupling 
would be without the dielectric shields. This is equivalent to reducing 
the effective surface area of the shielded portion of the anode electrode 
to only 14% of its actual surface area. 
So far we have disregarded the capacitor C.sub.x, which represents the 
capacitance between the plasma body and any portions of the anode 
electrode, including any grounded chamber components, which are not 
covered by the dielectric shield 10 and therefore are directly exposed to 
the plasma. The capacitance C.sub.x is proportional to such exposed 
surface area of the anode. The total capacitance C.sub.AnodeEff between 
the anode and the plasma body equals the sum of this capacitance C.sub.x 
and the capacitance previously calculated in Equation 3b from the series 
connection of C.sub.shield and C.sub.AnodeSh : 
EQU C.sub.AnodeEff .apprxeq.C.sub.x +C.sub.shield (Eqn. 5) 
In the present invention, the earlier Equation 1, expressing the cathode DC 
bias as a function of the plasma body-to-cathode capacitance and the 
plasma body-to-anode capacitance, becomes: 
EQU V.sub.bias =(V.sub.AnodeSh -V.sub.CathodeSh).varies.-(C.sub.AnodeEff 
-C.sub.CathodeSh) (Eqn. 6) 
Broadly speaking, our invention is a method of adjusting the DC bias on one 
electrode relative to another electrode by interposing a dielectric shield 
between one of the electrodes and the plasma. The preceding equations 
indicate that adjusting any property of the shield which alters the 
capacitance between the plasma and the electrode covered by the shield can 
be used to adjust the DC bias. Specifically, such capacitance is decreased 
by (1) increasing the thickness of the dielectric in the shield; (2) 
substituting a dielectric material having a higher dielectric constant; 
(3) changing the size or shape of the dielectric to increase the surface 
area of the electrode which is covered thereby; or (4) increasing the gap 
between the shield and the covered electrode. Decreasing the capacitance 
between one electrode and the plasma will make the DC bias voltage on that 
electrode less positive (or more negative) relative to any other 
electrode, or, equivalently, will make the bias voltage on any other 
electrode less negative (or more positive) relative to the electrode 
covered by the dielectric shield. 
More specifically, with reference to the preferred embodiment in which the 
grounded anode electrode is covered by dielectric shield 10 in order to 
reduce the cathode DC bias V.sub.bias, the cathode bias can be adjusting 
downward or upward by decreasing or increasing, respectively, the 
capacitance C.sub.AnodeEff. In particular, Equation 5 implies that 
increasing the portion of the chamber wall covered by the dielectric 
shield 10 will reduce C.sub.x and thereby reduce the magnitude of the 
negative cathode bias V.sub.bias. Equation 2 implies that choosing a 
material for the shield having a lower dielectric constant, or increasing 
the thickness of the shield, will reduce C.sub.shield and thereby reduce 
the magnitude of the negative cathode bias voltage V.sub.bias. 
Because a vacuum has a dielectric constant (i.e., unity) less than the 
dielectric constant of any solid dielectric material, another method for 
reducing the magnitude of the negative cathode bias is to space the 
dielectric shield 10 a small distance from the chamber wall 18 so as to 
create a small vacuum gap between them that functions as a dielectric. 
Increasing the gap will reduce the capacitance C.sub.shield and thereby 
reduce the magnitude of the negative cathode bias V.sub.bias. This method 
preferably should include providing a seal between the edge of the 
dielectric shield and the chamber wall to keep the plasma from entering 
the gap. Alternatively, the distance between the shield and the wall can 
be small enough to prevent the formation of a plasma in the gap. 
The invention enables the cathode DC bias to be reduced without reducing 
the RF power applied to the cathode. Therefore, the RF power can be set to 
any level desired to obtain a desired process reaction rate and 
throughput, while the cathode DC bias can be set to a desired voltage by 
adjusting any of the above-mentioned parameters of the dielectric shield. 
If the dielectric shield 10 is readily replaceable, the present invention 
enables a single plasma chamber to be configured to perform different 
semiconductor fabrication processes requiring different optimum values of 
cathode DC bias voltage. The plasma chamber can be provided with a number 
of interchangeable dielectric shields which differ: (1) in their 
thicknesses, (2) in their dielectric constants by using different material 
compositions, or (3) in their axial lengths or other dimensions so as to 
cover different amounts of the surface area of the chamber wall. 
For example, in a plasma etching chamber, low cathode DC bias typically is 
desirable for etching metal or silicon features on a semiconductor 
substrate, whereas high cathode DC bias typically is desirable for etching 
dielectric features. Our invention enables the chamber to be optimized for 
metal or silicon etch processes by inserting a dielectric shield having a 
high thickness, low dielectric constant, or large surface area compared to 
a dielectric shield employed in the same chamber for a dielectric etching 
process. Alternatively, for dielectric etching, the dielectric shield 
could be eliminated altogether, or it could be replaced by a shield 
composed primarily of electrically conductive material such as anodized 
aluminum. 
The preferred implementation of the dielectric shield 10 as shown in FIG. 1 
fulfills the need for a readily replaceable shield as stated in the 
preceding paragraph. The cylindrical shield 10 is installed simply by 
lowering it into the chamber, and then closing the chamber lid 24 to 
secure the shield in place. The shield is removed by the reverse process 
of simply opening the chamber lid, and then lifting out the shield. The 
shield is not secured by any bolts. Because the slit 23 in the shield must 
be aligned with the slit 26 in the chamber wall, the shield preferably 
includes an alignment pin (not shown) which mates with an alignment slot 
in the upper edge of the chamber wall to ensure consistent angular 
orientation. 
Another option for adjusting the area of the chamber wall covered by the 
dielectric shield is to fabricate the shield in separate segments, so that 
selected ones of the segments can be installed in the chamber to cover 
selected portions of the chamber wall. Installing more shield segments to 
cover more of the chamber wall will correspondingly reduce the cathode DC 
bias. 
If plasma enters the gap between the dielectric anode shield 10 and the 
chamber wall 18, the plasma will effectively bypass or short-circuit the 
shield by electrically contacting the chamber wall which was intended to 
be shielded. In the preferred embodiment shown in FIG. 1, there is no path 
by which the plasma could enter the gap between the dielectric shield and 
the chamber wall. O-ring 15 between the anode shield and the cathode 
shield is not intended to provide a vacuum seal, but, for reasons 
explained below, there is no plasma present near this O-ring because the 
exhaust baffle 14, 16 blocks the plasma from penetrating downstream of the 
baffle. Although there are no O-rings between the anode shield 10 and the 
slit shields 40 and 42, any gap between the anode shield and the slit 
shields is much smaller than the plasma sheath width, and therefore is too 
small to admit the plasma. 
The invention has been described in the context of a plasma chamber having 
two electrodes for capacitively exciting the plasma. The invention is 
equally applicable to chambers having three or more electrodes. In such 
case, covering a portion of one of the electrodes with a dielectric shield 
10 will make the DC bias voltage on that electrode less positive (or more 
negative) relative to the other electrodes, or, equivalently, will made 
the bias voltage on the other electrodes less negative (or more positive) 
relative to the electrode covered by the dielectric shield. As described 
earlier, the change in DC bias can be adjusted by adjusting the dielectric 
constant, thickness, surface area, or spacing of the dielectric shield. 
The invention also is applicable to chambers in which the plasma is excited 
inductively or remotely, provided an RF power supply also is connected 
between two electrodes within the chamber to provided some capacitive 
coupling of RF power to the plasma. 
3. Chamber Cleaning 
The ease of replacing the dielectric shield 10 affords another advantage 
unrelated to the issue of cathode DC bias, namely, chamber cleaning. The 
process gases used for most semiconductor fabrication processes decompose 
within the plasma into reactive species which react at any exposed 
surfaces in the chamber to form deposits on such surfaces. Over time, such 
deposits may accumulate to the extent that they alter the process 
conditions within the chamber, or they may flake off the chamber surfaces 
in the form of microscopic particles which can lodge upon and contaminate 
the semiconductor workpiece. Accordingly, in most semiconductor processes 
it is necessary to periodically clean deposits off of all surfaces in the 
chamber which are exposed to the process gases. 
Because our removable anode shield 10 covers essentially all exposed 
surfaces of the chamber wall 18, deposits which otherwise would accumulate 
on the chamber wall will accumulate on the shield instead. When the 
accumulation of deposits on the shield is excessive, the shield readily 
can be removed and replaced more quickly than a conventional chamber wall 
could be cleaned, thereby improving the productivity of the plasma 
chamber. 
The reason for including the anodized aluminum cathode shield 12 in the 
preferred plasma chamber is to facilitate cleaning as described above with 
respect to the anode shield 10. Specifically, deposits which otherwise 
would accumulate on the dielectric spacer 28 or the metal spacer 25 will 
instead accumulate on the cathode shield. When the deposits accumulate to 
an undesirable level, the cathode shield can be replaced in less time than 
otherwise would be required to clean such deposits off of the cathode side 
wall. After removing the anode shield 10 as described above, the quartz 
ring 38 and the cathode shield 12 can be likewise removed by simply 
lifting them up through the top of the chamber. 
We have found that keeping the anode and cathode shields cool is important 
to achieving a low level of particulate contamination in the chamber. We 
believe that cycling of the shield between high and low temperatures tends 
to cause deposits on the shields to flake off, so that they can 
contaminate the semiconductor workpiece. To keep the shields cool, our 
preferred embodiment includes channels (not shown) surrounding the chamber 
side wall 20 through which cool water is pumped so as to maintain the 
chamber wall at a temperature of about 65.degree. C. The 
outward-projecting lip of the anode shield 10 makes good thermal contact 
with the chamber lid 24 and the chamber side wall 20, thereby conducting 
heat from the anode shield to the chamber wall. The bottom of the flange 
13 makes good thermal contact with the chamber bottom wall 22, thereby 
conducting heat from the cathode shield to the chamber wall. 
The cathode shield 12 preferably is composed of anodized aluminum because 
aluminum is an excellent thermal conductor, thereby keeping the cathode 
shield cool. Because the anode shield is dielectric, keeping it cool is 
more difficult. Suitable dielectric materials for the dielectric shield 10 
include silicon carbide, aluminum oxide (alumina), aluminum nitride, 
quartz (silicon dioxide), and various resins. One advantage of silicon 
carbide is that it is a much better heat conductor than most dielectrics, 
so it will remain cooler. Aluminum nitride also is a good heat conductor, 
but it has two disadvantages: it is difficult to obtain with impurity 
concentrations low enough to avoid contaminating the semiconductor 
fabrication process, and aluminum compounds are undesirable in etch 
processes which use fluorine, because fluorine is highly reactive with 
aluminum. 
Alternatively, the dielectric shield 10 can be fabricated of a dielectric 
which is a poor heat conductor, such as quartz, if the dielectric is 
sandwiched together with another material which is a good heat conductor, 
such as aluminum. For example, the dielectric shield can be a sheet of 
quartz affixed to a sheet of aluminum. 
4. Sinuous Exhaust Baffle 
The plasma chamber shown in FIG. 1 includes an additional inventive 
feature, namely, an exhaust baffle 14, 16 which reduces the magnitude of 
the negative DC bias voltage at the cathode (relative to electrical 
ground) by reducing the capacitive coupling between the plasma and 
electrically grounded exhaust components. 
In general, the exhaust baffle according to our invention has a number of 
sinuous passages, and the baffle overlies the exhaust port of the plasma 
chamber so that chamber gases exhausted from the chamber by the vacuum 
pump pass through the sinuous passages. Each passage is sufficiently long 
and sinuous that the plasma within the chamber cannot extend beyond the 
outlet of the passage. Consequently, the plasma is electrically isolated 
from electrically grounded components downstream of the exhaust baffle, 
such as the vacuum pump (not shown) and the throttle valve 52. 
In addition to reducing the cathode DC bias, our exhaust baffle has the 
advantage of reducing or eliminating the deposition of polymers and other 
undesirable reaction compounds on the throttle valve 52, vacuum pump, and 
other components downstream of the baffle. The gases exhausted from a 
semiconductor process chamber typically include reactive chemical species 
(reactive molecules, atoms, and ions) which tend to react and form 
deposits on surfaces which they contact. A common problem is that film 
deposits accumulate on the throttle valve 52 and the vacuum pump, which 
eventually requires shutting down the plasma chamber for cleaning or 
replacement of these components. 
In our invention, the sinuous passages of the exhaust baffle markedly 
increase the rate of collisions between gas atoms and the walls of the 
passages. Such collisions promote the reactions which deposit the 
undesirable films, so that more film is deposited in the exhaust baffle 
passages. This depletes the concentration of reactive species in the 
exhaust gases passing through the exhaust baffle, so as to reduce or 
eliminate the deposition of unwanted films on the throttle valve 52, 
vacuum pump, and other downstream components. 
FIG. 1 shows our preferred implementation of the exhaust baffle 14, 16. As 
described earlier, the process gases exit the chamber through a circular 
exhaust port 50 in the chamber bottom wall 22. A vacuum pump (not shown) 
coupled to the exhaust port pumps process gases out of the chamber and 
maintains a desired level of vacuum within the chamber. A throttle valve 
52 mounted between the exhaust port 50 and the vacuum pump regulates the 
gas pressure within the chamber by regulating the impedance to gas flow 
from the exhaust port 50 to the vacuum pump. 
All gases exhausted from the chamber must pass through the annular gap or 
cavity 54 which functions as an exhaust manifold. This gap 54 is bounded 
on the inside by annular cathode shield 12 which covers the radially outer 
surface of the dielectric spacer 28 surrounding the cathode 30 and the 
radially outer surface of the metal spacer 25, and it is bounded on the 
outside by annular anode shield 10 which covers the radially inner surface 
of the chamber side wall 20. Although the gap 54 is annular and completely 
encircles the cathode 30, the aperture through which the exhaust gases 
exit the chamber does not completely encircle the cathode. Instead, the 
bottom of the exhaust manifold is bounded by the bottom flange 13 of the 
cathode shield 12. The exhaust gases exit the chamber through the arcuate 
aperture 51 in the portion of the flange 13 which overlies the exhaust 
port 50 in the chamber bottom wall 22. 
The sinuous exhaust passage of our invention is defined by annular 
protrusions 14 and 16 which protrude into the exhaust manifold cavity 54 
from the anode shield 10 and the cathode shield 12, respectively. Because 
the protrusions overlap each other in the radial dimension, the chamber 
gases cannot travel a straight path through the exhaust manifold cavity 
54. Instead, the upper protrusion 14 forces the downward-flowing exhaust 
gases to turn radially inwardly toward the cathode shield 12, and then the 
lower protrusion 16 forces the exhaust gases to reverse direction and flow 
radially outwardly toward the anode shield 10. This sinuous path directs a 
high proportion of the molecules, atoms, and ions in the exhaust gas to 
collide with the boundary walls of the exhaust manifold, i.e., the 
surfaces of the anode shield 10 and the cathode shield 12. The high rate 
of collision with the boundary walls promotes the rate of reactions by 
which reactive species in the exhaust gas form stable molecules, generally 
including molecules which deposit as films on the boundary walls. 
The high reaction rate of reactive species within the sinuous exhaust 
passage enables two beneficial results. 
The first beneficial result is that reactive species which otherwise would 
form undesirable deposits on the throttle valve 52 or the vacuum pump will 
instead form such deposits on the boundary walls of the sinuous exhaust 
passage, i.e., on the surfaces of the anode shield 10 and the cathode 
shield 12 adjacent the protrusions 14 and 16. Consequently, reactive 
species will be significantly depleted from the exhaust gas downstream of 
the exhaust baffle 14, 16. If the shields 10 and 12 are readily removable 
for cleaning or replacement as in the preferred embodiment described 
above, it is highly desirable to promote such deposits on the shields in 
preference to the throttle valve and vacuum pump, which are more difficult 
to clean and maintain. Almost complete depletion of reactive species 
downstream of the exhaust baffle is possible if the exhaust passage is 
sufficiently long and sinuous. 
The second beneficial result is a reduction in the cathode DC bias. 
Specifically, if the exhaust passage through the baffle 14, 16 is 
sufficiently long and sinuous, the concentration of ions in the exhaust 
gas downstream of the exhaust baffle will be below the level necessary to 
sustain a plasma. The exhaust baffle 14, 16 then can be said to block or 
quench the plasma. Consequently, the electrically grounded components 
downstream of the exhaust baffle (such as the chamber bottom wall 22 and 
throttle valve 52) will be electrically isolated or decoupled from the 
plasma, thereby reducing the magnitude of the negative DC bias at the 
cathode 30. 
The two stated benefits of the sinuous exhaust baffle 14, 16 are achieved 
whether the exhaust baffle is metal or dielectric. In addition, because 
the plasma is likely to contact to at least the upper part of the exhaust 
baffle 14, 16, the cathode DC bias can be further reduced by minimizing 
the capacitive coupling between electrical ground and the surfaces of the 
exhaust baffle facing the plasma. This can be implemented by incorporating 
in the exhaust baffle a dielectric material positioned between the plasma 
and any electrically grounded chamber components adjacent the baffle. 
For example, in the preferred embodiment shown in FIG. 1, the upper portion 
14 of the exhaust baffle is adjacent the electrically grounded chamber 
side wall 20. Therefore, the cathode DC bias is reduced either by 
fabricating the entire anode shield 10 of dielectric (as in FIG. 1), or 
else by including in the shield 10 a dielectric member interposed between 
the chamber side wall 20 and the surface of the protrusion 14 which is 
exposed to the plasma. 
To substantially reduce the capacitive coupling between the plasma and 
electrically grounded chamber components adjacent to the exhaust baffle, 
the dielectric preferably should be substantially thicker than the width 
of the plasma sheath or, alternatively, of sufficient thickness to 
substantially impede the coupling of RF power from the plasma to any 
adjacent grounded components via the exhaust baffle. 
In contrast, the lower portion 16 of the exhaust baffle is not adjacent an 
electrically grounded component; it is adjacent the dielectric spacer 28. 
(Even in a hypothetical embodiment which omits the dielectric spacer 28 
between the cathode shield 12 and the cathode 30, the cathode is 
electrically isolated from ground, so the lower portion 16 of the exhaust 
baffle would not be adjacent a grounded component in the hypothetical 
embodiment either.) Therefore, no further reduction in cathode DC bias 
would be achieved by incorporating dielectric material in the lower 
protrusion 16. Consequently, the lower protrusion 16, as well as the 
entire cathode shield 12, preferably is fabricated of anodized aluminum 
because of its superior thermal conductivity, for reasons described in the 
preceding section entitled "Chamber Cleaning". 
Although the portion of the cathode shield 12 below the exhaust baffle 14, 
16 does abut an electrically grounded component--namely, the metal spacer 
25--the plasma is quenched by the exhaust baffle so that no plasma reaches 
this lower portion of the cathode shield. Therefore, no reduction in 
cathode DC bias would be achieved by incorporating dielectric material in 
the portion of the cathode shield below the exhaust baffle 14, 16. 
The exhaust baffle initially was described as having a "number" of sinuous 
exhaust passages. However, the preferred embodiment just described has 
only one exhaust passage, an annular passage which encircles the cathode. 
FIG. 3 shows a plasma chamber including an alternative embodiment of an 
exhaust baffle 70 having a plurality of sinuous passages 72, 73. The 
exhaust baffle 70 is a annular assembly which occupies the gap between the 
chamber side wall 20 and an annular dielectric ring 28 which surrounds the 
cathode 30. As in the FIG. 1 embodiment, the chamber bottom wall 22 has a 
circular exhaust port or aperture 50 through which gases are exhausted 
from the chamber by a vacuum pump (not shown). 
As shown in FIGS. 3 and 4, the exhaust baffle 70 is constructed of three 
rings 74, 76, and 78 stacked upon each other coaxially. The top ring 74 
and the bottom ring 78 are identical, as shown in FIG. 5. The top ring and 
the bottom ring each have a plurality of axially oriented cylindrical 
holes 72 spaced around the circumference of the ring. The angular spacing 
between adjacent holes is defined as 2.DELTA.. If the integer N represents 
the number of holes 72, then 2.DELTA.=360.degree./N. The diameter of each 
of the holes 72 is less than .DELTA./2. The middle ring 76, as shown in 
FIG. 6, has a similar array of N circumferentially spaced holes 73 
extending axially between its top and bottom surfaces, except that the N 
holes 73 have an elongated cross section, with a circumferential angular 
width of .DELTA.. 
When the three rings 74, 76, and 78 are stacked as shown in FIG. 4, the 
holes 72 and 73 define zig-zag exhaust passages. Specifically, the 
counterclockwise end of each elongated hole 73 in the middle ring 76 is 
aligned with a hole 72 in the upper ring 74, and the clockwise end of each 
elongated hole 73 in the middle ring 76 is aligned with a hole 72 in the 
lower ring 78. Consequently, exhaust gases travel downward (i.e., axially) 
through a hole 72 in the top ring 74, then turn 90.degree. to travel 
horizontally (i.e., circumferentially) through a hole 73 in the middle 
ring 76, then turn 90.degree. again to travel downward through a hole 72 
in the bottom ring 78. 
The exhaust baffle 70 having zig-zag or sinuous exhaust passages 72, 73 
affords benefits similar to those of the sinuous exhaust passage in the 
FIG. 1 embodiment. Specifically, it promotes reactive species in the 
exhaust gas to react and form deposits in the exhaust baffle 70 rather 
than downstream, thereby minimizing the need to clean deposits from the 
throttle valve and vacuum pump. Additionally, it quenches the plasma so as 
to prevent capacitive coupling between the plasma and the chamber bottom 
wall 22, thereby reducing cathode DC bias. 
The exhaust baffle 70 can be composed of a dielectric material to further 
reduce capacitive coupling between the plasma and the portion of the 
chamber side wall 20 adjacent the exhaust baffle. For example, plastic 
resins are good dielectrics and are particularly easy materials in which 
to drill holes 72, 73. A disadvantage of some plastic resins is that they 
may visually resemble the deposits which accumulate on the exhaust baffle 
during operation of the plasma chamber, thereby making it difficult to 
visually recognize when the baffle is dirty and requires replacement. 
We found the stacked ring embodiment of FIGS. 3-6 particularly useful for 
quickly constructing a prototype to test the effectiveness of different 
zig-zap exhaust passage designs. For example, if the illustrated 3-ring 
design is found insufficient to quench the plasma for a particular chamber 
and process conditions, additional rings can be stacked to produce an 
exhaust passage with additional 90.degree. turns. The two ring designs 
shown in FIGS. 5 and 6, respectively, should occupy alternate positions in 
the stack. Additionally, the rings can be rotated relative to each other 
to test the effect of varying the length of the horizontal passage 73 in 
the middle ring.