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Description  |
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The invention relates to an inductive coil, and, more particularly, to an
inductive coil for an inductively coupled plasma production apparatus.
BACKGROUND OF THE INVENTION
Plasma system have come into use in the processing of semiconductor
materials. Inductively coupled system are increasingly being employed for
producing the plasma in such systems. One type of inductive coil being
used in such inductively coupled systems is a flat or pancake coil such as
shown in U.S. Pat. No. 4,948,458. The flat coil is mounted externally of
the vacuum chamber in which the plasma is generated and is inductively
coupled into the plasma through an insulating window. Although this coil
configuration has a number of attractive features, including simplicity of
design, it also has a number of limitations. One limitation is that it
requires an insulating window to allow the RF. field to pass therethrough
into the vacuum chamber. A second limitation is that the radial plasma
density profile, in the plane of the coil, produced by a flat coil is
relatively non-uniform. Such a window can be expensive to manufacture and
is subject to being damaged by the plasma. Also, the inductive coupling
provided by the flat coil has two components, a capacitive (E) field
component and the inductive (H or B) field component. This is undesirable
since it is only the inductive component that is desired. Furthermore,
with the flat coil, the plasma density drops off rapidly away from the
window.
SUMMARY OF THE INVENTION
The invention is directed to an inductive coil having a planar coil portion
and a tubular coil portion extending from the outer edge of the flat coil
portion. The axis of the tubular coil portion is substantially
perpendicular to the flat coil portion.
The invention is also directed to a plasma production apparatus which
includes a chamber having walls and adapted to be placed under a vacuum.
An electrode is in the chamber, and a window extends into the chamber from
a wall thereof. An induction coil is in the well formed by the window. The
induction coil has a flat portion at the bottom of the window and a
tubular portion extending from the flat portion.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of one form of a plasma generating apparatus
having the inductive coil of the invention;
FIG. 2 is an enlarged view of a portion of the apparatus shown in FIG. 1;
FIG. 3 is a perspective view of a form of the inductive coil of the
invention;
FIG. 4 is a graph showing the density of a plasma across a vacuum chamber
for a flat coil and the coil of the invention; and
FIG. 5 is a schematic view of another form of a plasma generating apparatus
having the induction coil of the invention.
DETAILED DESCRIPTION
Referring initially to FIG. 1, there is shown a plasma generating apparatus
10 having the inductive coil 12 of the invention. Plasma generating
apparatus 10 comprises a chamber 14 in which there can be generated a
vacuum. The chamber 14 has top and bottom walls 16 and 18, and side walls
20. Within the chamber 14 and adjacent the bottom wall 18 is an electrode
22. The electrode 22 is electrically connected to an RF power supply 24
outside the chamber through a matching network circuit 26. On the
electrode 22 is a table 28 for supporting an object, such as a
semiconductor wafer, which is to be treated in the plasma. The top wall 16
has an opening 30 therethrough. In FIG. 2, window 32, preferably
cup-shaped, extends through the opening 30 and into the chamber 14. The
window 32 has a rim 34 around its upper edge which seats on and is
hermetically sealed to the top wall 16 around the opening 30. The window
32 may be of an insulating material, a conductive material or an
insulating material coated on its outer surface with a layer of a
conductive material. The coil 12 is within the well formed by the window
32.
A pipe 36 extends through the bottom wall 18 of the chamber 14 and is
connected to means, such as a vacuum pump, for drawing a vacuum in the
chamber 14. A door 38 is in a side wall 20 of the chamber 14 to allow
objects to be inserted and removed from the chamber 14. An inlet pipe 39
extends through a side wall 20 of the chamber 14 to allow for the
admission of a plasma forming gas. The coil 12 is electrically connected
to an RF power supply 40 through a matching network circuit 42. The RF
power supply 40 for the coil may be the same as or separate from the RF
power supply 24 for the electrode 22.
In FIG. 3, the coil 12 has a flat or pancake portion 44 and a tubular
portion 46 extending substantially perpendicularly from the outer edge of
the flat portion 44. The coil 12 is formed of a solid or hollow conductor
which is first wound in a spiral to form the flat portion 44 and then
wound in the form of a tube to form the tubular portion 46. The tubular
portion 46 may be substantially cylindrical or can be conical. As will be
explained, the tubular portion 46 is shaped to provide a desired shape of
the plasma formed in the chamber 14. A terminal 48 is provided at each end
of the coil 12 whereby the coil 12 can be connected to the RF power supply
40. A coil 12 formed of a hollow conductor is preferred to allow cooling
of the coil by flowing a cooling liquid therethrough.
The dimensions and aspect ratio (the ratio of the size of the flat section
44 to the height of the tubular section 46) of the coil 12 can vary
depending on the size of the plasma apparatus 10, and in order to shape or
modify the plasma profile for a specific purpose. However, for a plasma
apparatus which is between about 20 centimeters (cm) and 25 cm in
diameter, a typical coil 12 may be formed of 0.6 cm diameter copper tubing
with the flat section 44 having between 1 to 3 full turns and an outer
diameter of between about 6.3 and 8.9 cm. The tubular portion 24 is a
continuation of the last turn of the flat section 22 and thus has a
diameter of between about 6.3 and 8.9 cm. The length or height of the
tubular portion can be approximately 8.9 cm.
In the operation of the plasma generating apparatus 10, an object to be
treated, such as a semiconductor wafer, is placed in the chamber 14 on the
table 28. The chamber 14 is the evacuated to a desired vacuum. A suitable
gas is then admitted to the chamber 14. The power to the coil 12 and the
electrode 22 is turned on to create a field across the gas in the chamber
14. This results in the formation of a plasma in the chamber.
The coil 12 of the invention has a number of advantages over the flat coil
heretofore used. The coil 12 has a greater inductive component than the
flat coil and therefore generates a greater ion density. The coil 12
permits a lower pressure of operation than the flat coil. The coil 12 will
produce a more intense plasma for the same RF power resulting in greater
gas and electrical (ionization) efficiencies. Also, the coil 12 provides
for greater ease of plasma ignition. Since the coil 12 provides a greater
inductive component, the window 32 can be made of a conductive material or
of an insulating material coated on its outer surface with a conductive
material. A conductive material is less subject to being etched by the
plasma in the chamber 14. Thus, a window formed of a conductive material
or having a conductive coating on its outer surface which is subjected to
the plasma, is less subjected to being etched and therefore has a longer
life. However, the most important advantage of the coil 12 of the
invention is that it provides a more uniform plasma density profile across
the chamber 14. Referring to FIG. 4, there is shown a graph of the plasma
density across one-half of a chamber 14. The distance starts with 0 being
directly under the coil. It should be understood that a similar density
curve is obtained across the other half of the chamber 14. The curve
having the solid dots is for a flat coil. The other three curves are for
the coil 14 of the invention (indicated as a hybrid coil) at different
power levels. It can be seen from FIG. 4 that the curves for the coil 14
of the invention is much flatter than the curve for the flat coil. This
shows a more uniform density across the chamber 14. The shape of the
tubular portion 46 of the coil 14 can be varied to alter the plasma
density profile. Thus, the plasma density profile can be made more uniform
by varying the tubular portion 46 between cylindrical and conical.
In FIG. 5, there is shown another form of a plasma generating apparatus 100
having the induction coil 112 of the invention. Plasma generating
apparatus 100 comprises a chamber 114 having end walls 116 and 118 and
side walls 120. Within the chamber 114 and adjacent an end wall 118 is an
electrode 122. The electrode is electrically connected to an RF power
supply, not shown, similar to the power supply 24 of FIG. 1. An elongated
cup-shaped window 132 extends through an opening 130 in the end wall 116
and across substantially the full length of the chamber 114. The window
132 has a rim 134 around its open edge which seats on and is hermetically
sealed to the end wall 116. The window 132, like the window 32 of the
apparatus 10 of FIG. 1, may be of an insulating material, a conductive
material or an insulating material coated on its outer surface with a
layer of a conductive material.
The coil 112 is in the window 132. The coil 112, like the coil 12, has a
flat or pancake portion 144 and a tubular portion 146. The only difference
between the coil 112 and the coil 12 is that the tubular portion 146 of
the coil 112 is much longer than the tubular portion 46 of the coil 12.
Thus, the coil 112 extends completely along the window 132 with the flat
portion 144 being at the closed end of the window 132. This provides the
coil 112 along the entire length of the chamber 114. Although not shown,
the coil 112 has terminals which are electrically connected to a RF power
supply, such as the power supply 40 of FIG. 1.
Although not shown, the chamber 114 has a pipe which is connected to means,
such as a vacuum pump, for drawing a vacuum in the chamber, 114, a door by
which articles to be treated can be placed in and removed from the chamber
114, and an inlet pipe to allow for the admission of a plasma forming gas.
In the chamber 114, the articles 150 to be treated, such as semiconductor
wafers, are mounted in the chamber 114 around the window 132. This allows
for a larger number of the objects to be treated at one time. The
apparatus 100 operates in the same manner as the apparatus 10 of FIG. 1
except that it permits a greater number of articles to be treated at one
time. Although the chamber 114 is shown as being vertical, it can also be
in a horizontal position.
Thus, there is provided by the invention an inductive coil 14 having a flat
portion 44 and a tubular portion 46 extending from the flat portion. When
this coil is used in a plasma generating apparatus 10, it produces greater
inductive component than a flat coil and amore uniform plasma density
profile across the chamber 14 of the apparatus 10. If desired, a magnet
system may be used in the chamber 14 to make the plasma density profile
more uniform. However, the coil 14 of the invention does not necessarily
need such a magnet. Also, although the coil 14 has been described as being
preferably used in a plasma generating apparatus, it can be used in other
devices, such as electrodeless high efficient lamps.
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