Apparatus for enhanced inductive coupling to plasmas with reduced sputter contamination

A shield for shunting capacitive electric fields generated by an RF coil away from a gas plasma process chamber's dielectric window and toward ground. The shield comprise an electrically conducting, substantially planar body section having a periphery and adapted to be located between the RF coil and the dielectric window during plasma treating of a workpiece. A central opening in the body section and gaps about the periphery permit RF magnetic fields to inductively couple with the plasma and return around the coil, respectively. The shield substantially reduces interference by capacitive electric fields generated by the coil with inductive coupling between the coil and the gas plasma, thus substantially eliminating contamination from sputtering of the dielectric window by the capacitive electric fields.

TECHNICAL FIELD 
This invention relates generally to the field of semiconductor device 
manufacturing, and specifically to plasma-based processes with reduced 
sputter contamination. 
BACKGROUND ART 
The uniform and rapid processing of materials using induction generated, 
plasma-based processes (also referred to as inductive-coupled plasma 
processes) is important in the fields of semiconductor device 
manufacturing, packaging, optics, and the like. Many plasma processes are 
extensively used for the depositing or reactive etching of layers during 
semiconductor device fabrication. However, the radio frequency (RF at 
about 13.56 MHz) induction plasma source is known to produce high electron 
density (n.sub.e &gt;10.sup.11 cm.sup.-3) plasmas, thus providing high 
processing rates. 
One conventional apparatus described in U.S. Pat. No. 3,705,091 to Jacob, 
produces a high density plasma which consists of a helical coil energized 
by 13 MHz RF radiation. The plasma is generated inside a low pressure 
cylindrical vessel within the helical coil. The coil structure induces 
electric fields within the plasma region which drive the discharge. High 
RF potentials on the coil cause capacitive coupling with the vessel walls. 
The capacitive coupling accelerates charged particles (electrons and ions) 
from the plasma into the dielectric vessel walls causing process 
contamination due to sputtering of the dielectric vessel walls. In 
addition, capacitive coupling is much less efficient than inductive 
coupling. 
M. C. Vella et al. in Development of R.F. Plasma Generators for Neutral 
Beams, (Journal of Vacuum Science Technology, Vol. A3(3), pp. 1218-1221 
(1985)), describe an inductive-coupled plasma process having a coil 
immersed in a plasma that is confined by permanent magnets. This apparatus 
also exhibits a degree of capacitive coupling to the discharge since the 
coil is in contact with the plasma. 
D. K. Coultras et al. in European Patent Application 0 379 828 and Ogle in 
U.S. Pat. No. 4,948,458, describe inductive-coupled plasma process using a 
spiral coil separated from the plasma by a planar dielectric called a 
window. Again, high potentials on the coil cause some degree of capacitive 
coupling, and thus contamination of the process due to sputtering of the 
dielectric window. 
In U.S. Pat. No. 4,918,031, Flamm et al. describe a helical resonator with 
a coil similar to that of Jacob in which a split cylindrical ground shield 
is placed between the coil and the vacuum vessel such that high fields 
from the coil are shorted to ground. Capacitive coupling is essentially 
eliminated in this configuration. However, the cylindrical geometry of 
this device does not allow efficient use of the ions and reactive species 
on large area substrates such as semiconductor wafers. Additionally, the 
cylindrical geometry can not be scaled for use with very large area 
substrates such as liquid crystal displays. 
What is desired is a technique for both eliminating capacitive coupling to 
reduce contamination, and maintaining high inductive coupling between the 
coil and the plasma for improved processing rates as well as a reactor 
geometry which is scalable to large areas. 
DISCLOSURE OF THE INVENTION 
The present invention is directed to an apparatus for enhanced inductive 
coupling to plasmas with reduced sputter contamination. The present 
invention eliminates sputtering of the dielectric window by shunting to 
ground capacitive electric fields generated by high potentials on the 
adjacent spiral-like or helical coil. This is achieved by adding grounded 
conducting elements, called shields, between the dielectric window and the 
coil. 
The shields of the present invention are designed so that they do not 
interfere with the inductive coupling of the coil to the plasma, but guide 
capacitive electric fields generated by the coil away from the 
plasma-window interface and toward ground. 
The primary advantage of the present shielding invention is the reduction 
or elimination of sputtered contaminates from the dielectric vacuum 
window. 
The shields also guide the induction electric field through the plasma in a 
way such that plasma generation uniformity is improved when helical coils 
are used. 
Another advantage is improved generation of ions in the plasma. This 
improved generation of ions in the plasma causes increased etch rates 
compared to the rate achieved using conventional plasma-based processes. 
The foregoing and other features and advantages of the invention will be 
apparent from the following more particular description of preferred 
embodiments of the invention, as illustrated in the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows a schematic cross-section of a plasma-based semiconductor 
device manufacturing apparatus 100 of the present invention. 
A general operational description of an inductive coupled plasma 
low-pressure chemical vapor deposition (CVD) or reactive ion etching (RIE) 
apparatus is found in Coultras, et al., and Ogle, the disclosures of which 
are incorporated herein by reference. 
Referring now to FIG. 1 of the present invention, a low-pressure plasma 
process chamber 102 comprises a substrate holder 104 for supporting a 
workpiece 106. Other applicable processes include: plasma etching, CVD, 
surface treatment, atom and radical source, ion beam source and light 
source (visible, UV, vacuum UV). More specifically, the workpiece 106 is 
one or more semiconductor wafers, or the like. The process chamber 102 has 
a process gas inlet 108 through which a process gas is pumped according to 
conventional techniques. A plasma 110 is generated inside the low-pressure 
process chamber 102, as will be discussed further below. The process 
chamber 102 also includes permanent magnets 112 which are used for shaping 
the plasma 110 during processing. 
Attached to the top of the process chamber 102 is an RF housing 114, which 
is commonly referred to as a "matchbox". Housed within the matchbox 114 is 
a spiral-shaped coil 116 and an RF impedance-adjusting circuit 118. The RF 
impedance-adjusting circuit 118 is powered by an RF power input 120. A 
quartz vacuum window (dielectric window)122 separates the RF coil 116 from 
the process chamber 102 and (during operation) the plasma 110. Also 
separating the RF coil 116 from the process chamber 102 is a conductive 
shield 124 and insulating layer 126 (such as air, gas, vacuum, or the 
like) sandwiched between the RF coil 116 and the conductive shield 124. 
The spiral coil 116 is generally planar, and is therefore also referred to 
as a planar coil, but for purposes of this description will be simply 
referred to as the "coil 116." As evident by inspection of the drawing, 
coil 116 is located outside of the process chamber and inside of the 
matchbox 114. The coil 116 is positioned proximate the dielectric window 
122, but is separated therefrom by the insulating layer 126 and conductive 
shield 124. As discussed in Ogle, for example, the planar geometrical 
shape of coil 116 produces a planar plasma 110 for more even processing of 
the workpiece 106. Thus, the plane in which the coil 116 lies is 
substantially parallel to the dielectric window 122 and conductive shield 
124. The conductive shield 124 is grounded by connection directly to the 
matchbox 114, which in turn is connected to process chamber 102. 
During operation of the apparatus the conductive shield 124 guides 
capacitive electric fields generated by the coil 116 away from the 
dielectric window 122 and to the grounded matchbox 114. This grounding of 
the capacitive electric fields substantially reduces interference by the 
capacitive electric fields with the inductive coupling between the coil 
116 and the plasma 110. 
The basic geometry of conductive shield 124 will now be discussed in 
connection with FIG. 2. In a preferred embodiment of the present 
invention, the conductive shield 124 comprises four shield elements 230. 
The shield elements 230 are made of conductive metal such as copper, 
aluminum, or the like, having a thickness on the order of about 0.01-1 mm. 
The shield elements 230 each comprise a ground lead 232 for connection to 
an inside wall of the matchbox 114. Other equivalent techniques for 
grounding the conductive shield 124 should become evident to those skilled 
in the art. 
Each shield element 230 has an inner edge 234, two side edges 236, and an 
outer edge 238. The side edges 236 and an outer edges 238 define a 
periphery of the shield 124. A center opening in the conductive shield 124 
is defined by the extremities of the four inner edges 234, as shown 
generally in FIG. 2. In addition, radial gaps 242 are defined by the side 
edges 236 of adjacent shield elements 230. Finally, outer gaps 244 are 
defined by the outer edges 238 and the inside wall of the matchbox 114 
represented by a dashed line 246. 
Representative dimensions of the shield elements 230 will now be discussed 
with reference to FIG. 2. These dimensions are merely representative of a 
preferred embodiment of the present invention. Modifications can be made 
to the general shape of the capacitive shield 124 and shield elements 230 
without departing from the spirit and scope of the present invention. 
The shield elements 230 are separated by the radial gaps 242 so that there 
does not exist a completed circular conducting path which would prevent 
magnetic inductive fields from reaching the plasma region. 
In a preferred embodiment, the distance between inner edge 234 and outer 
edge 238 is represented by the constant x. 
The lengths of the inner edges 234 and outer edges 238 are represented by 
the constants y and z, respectively. In this representative embodiment, y 
is equal to about 2x, and z is equal to about 2y. In this representative 
embodiment, an angle .alpha. between the inner edge 234 and side edge 236 
is approximately equal to 135.degree.. With .alpha. equal to approximately 
135.degree., the side edges 236 are approximately 1.4x. FIG. 2 also shows 
a 90.degree. bend at angle .beta. for attaching the ground leads 232 of 
the shield elements 230 to the interior wall of the matchbox 114. Again, 
these dimensions are only representative examples of the invention and are 
not limitations thereof. 
The present embodiment is intended for uniform treatment of square 
substrates. In the case of treatment of circular substrates the periphery 
of a shield would form a circle. The shape of the shielding modifies the 
geometry of inductive electric fields for optimal uniformity over various 
shaped substrates (workpieces). Further, one skilled in the art will 
recognize that the window, shielding, and coil need not be planar. In the 
instance of treating dome-like workpieces, it is advantageous to use a 
domed or hemispherical window. The shielding would then preferably be 
domed or hemispherical and conformal to the window, but may flat. The coil 
may then be conformal to the shielding or helical in shape. 
FIGS. 3A and 3B are diagrammatic representations of the capacitive shield 
124 and RF magnetic flux lines 302 produced by the coil 116. FIG. 3A is a 
cross-section of the coil 116 and the shield elements 230. This figure 
also shows the center opening 240, which permits the RF magnetic flux 
lines 302 to pass through the dielectric window 122 to generate the plasma 
110 (both not shown). The outer gaps 244 permit the RF magnetic flux lines 
302 to return to the coil 116, as also shown in the figure. The shield 
elements 230 are grounded, as shown schematically at 304. FIG. 3B shows a 
top view of the capacitive shield 124. Again, the RF magnetic flux lines 
302 are shown entering the center opening 240 and returning to the coil 
116 via the outer gaps 244 and the radial gaps 242. 
Because the inner end of the coil 116 is at ground potential, the RF 
potential on an inner turn of the coil 116 is very low. The shield's 
central opening 240 thus permits inductively coupling between the coil 116 
and the plasma 110 without concern for sputtering of the center of the 
dielectric window 122 by RF potentials. If, however, the central opening 
is made too small (less than approximately 1 inch), plasma ignition 
becomes difficult and a portion of the induction field is excluded from 
the plasma region by the shield. 
FIGS. 4A and 4B show photographs of polyimide-coated wafers which were 
etched under identical conditions both with and without the capacitive 
shielding of the present invention. The unshielded process shown in FIG. 
4A produces a wafer which is hazed and rough on the surface. This 
roughness is due to micromasking of the surface of the polyimide by 
silicon sputtered from the quartz window. With the grounded shield 
elements in place the polyimide on the wafer remains smooth and 
reflective, as shown in FIG. 4B. Polyimide removal rates are faster with 
the conductive shield in place. The average etch rate of the wafer etched 
with the conductive shield of the present invention was 0.75 .mu.m/min. 
Without shielding the etch rate under identical conditions is 0.55 
.mu.m/min. While this increased etch rate may be partly due to the 
elimination of micromasking, Langmuir probe measurements of the ion flux 
from the plasma also show an increased ion saturation flux density when 
shielding is used. 
FIG. 5 plots the measured ion saturation current as a function of diagonal 
position in the plasma with and without shielding. The shielding increases 
the ion flux by 50%. FIG. 6 shows the data of FIG. 5 normalized to the 
same peak values to demonstrate that the ion flux uniformity is not 
adversely affected by these shields. 
It is evident by inspection of the physical results and data shown in FIGS. 
4-6, that the present invention is effective at reducing sputter 
contamination, while at the same time improving process rates in low 
pressure plasma processes. 
In another preferred embodiment of the present invention, the electrically 
conducting shield is used to shape the RF field geometry of a non-planar 
inductive coil such that uniformity is improved. Shaping of the conductive 
shield forces the induction field to be more uniform within the plasma 
even when a helical coil is used, for example. 
An RF induction plasma and ion source using a helical coil design together 
with the electrically conducting shield of the present invention are shown 
in FIG. 7. Rather than generating a plasma within the coil, as taught by 
Jacob, a planar plasma 110 below a helical coil 702 is generated using a 
grounded, electrically conducting shield 724 of the present invention. 
Planar plasmas are desirable for treating planar workpieces such as silicon 
wafers and multi-chip packages. To improve uniformity the shaped 
conducting shields are used between the plasma and the end of the coil 
such that the RF fields are modified in shape to generate a more spatially 
uniform plasma. Thus, the present invention can be used for uniform plasma 
processing of large area materials. The present embodiment was implemented 
for uniformity over square surface areas, and hence, this embodiment is 
optimized for square-shaped plasma excitation. However, the principal of 
this design is applicable to many other geometries, as those skilled in 
the art will recognize. 
FIG. 7 shows a low pressure (0.1-100 mTorr) plasma generated in a vacuum 
chamber 102. Radio frequency energy (13.56 MHz) is introduced by a helical 
coil 702 powered by a supply 706, to the discharge region through a quartz 
vacuum window 122 located at the top of the vacuum chamber 102. Both the 
chamber 102 and the end coil of helical coil 702 are grounded. An intense 
magnetic field is generated by the helical coil 702 which resides adjacent 
to the vacuum window 122 in the matchbox 114. The coil may consist of 1/4 
inch copper tubing wound about an 8 inch diameter coil form. FIG. 7 also 
shows that the substrate holder 104 for supporting the workpiece 106 may 
be protected by a shield 708. 
FIG. 8 shows electrically conducting shield 724 for use with the helical 
coil embodiment. The electrically conducting shield 724 is similar in 
operation and structure to the shield 124 described above. The 
electrically conducting shield 724 comprises shield elements 830, ground 
leads 832, inner edge 834, side edge 836, outer edge 838, center opening 
840, radial gaps 842 and outer gaps 844. 
Magnetic flux lines loop through the helix coil and pass through the plasma 
region inducing an electric field in the plasma. The fields generated by 
the coil alone are somewhat non-uniform. The uniformity is improved by the 
grounded conducting shield between the coil and the plasma. The RF 
magnetic flux generated by the helical coil 702 is forced through the 
center region of the plasma 110. The flux's return path is then within the 
plasma and around the outside of the shaped conducting pieces of the 
shield. The shape of the fields, and hence the uniformity of plasma 
generation, is controlled by the shape of the conducting pieces. The shape 
of the coil is secondary, and may assume many spiral-like geometries. For 
additional uniformity, those skilled in the art will recognize that 
magnetic confinement of the plasma may be used. 
The grounded end of the helical coil in the present embodiment is 
positioned near the plasma, thus capacitive electric fields between the 
coil and the plasma are very small compared to those generated by a spiral 
coil, thus grounding of the conductive shield 724 may not be necessary. 
The uniformity of plasma generated by the present embodiment is improved 
over conventional spiral couplers as shown in FIG. 9. The diagonal 
uniformity of the spiral coupler over about 20 cm is 19%, but, under 
identical conditions, a helical coil having a conducting shield of the 
present invention achieves 11% uniformity. The actual ion flux measured is 
approximately the same for both devices at (i.e., about 20 mA cm.sup.-2). 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that the foregoing and other changes in form and 
details may be made therein without departing from the spirit and scope of 
the invention. All cited articles and patent documents in the above 
description are incorporated herein by reference.