U.S. patent application number 09/824936 was filed with the patent office on 2001-09-27 for plasma reactor for the treatment of large size substrates.
This patent application is currently assigned to UNAXIS BALZERS AKTIENGESELLSCHAFT, FL-9496 Balzers, Furstentum Liechtenstein. Invention is credited to Schmitt, Jacques.
Application Number | 20010023742 09/824936 |
Document ID | / |
Family ID | 4211146 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010023742 |
Kind Code |
A1 |
Schmitt, Jacques |
September 27, 2001 |
Plasma reactor for the treatment of large size substrates
Abstract
A radiofrequency plasma reactor (1) for the treatment of
substantially large sized substrates is disclosed, comprising
between the electrodes (3, 5) of the plasma reactor a solid or
gaseous dielectric layer (11) having a non planar-shaped
surface-profile, said profile being defined for compensating a
process non uniformity in the reactor or generating a given
distribution profile.
Inventors: |
Schmitt, Jacques; (La Ville
du Bois, FR) |
Correspondence
Address: |
NOTARO & MICHALOS P.C.
Suite 110
100 Dutch Hill Road
Orangeburg
NY
10962-2100
US
|
Assignee: |
UNAXIS BALZERS AKTIENGESELLSCHAFT,
FL-9496 Balzers, Furstentum Liechtenstein
|
Family ID: |
4211146 |
Appl. No.: |
09/824936 |
Filed: |
April 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09824936 |
Apr 3, 2001 |
|
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09401158 |
Sep 22, 1999 |
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6228438 |
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Current U.S.
Class: |
156/345.47 ;
118/723E |
Current CPC
Class: |
H01J 37/32541 20130101;
H01J 37/32559 20130101; H01J 37/32532 20130101; H01J 37/32348
20130101; H01J 37/32091 20130101; H01J 37/32238 20130101; H01J
37/32192 20130101; C23C 16/509 20130101; C23C 16/4583 20130101 |
Class at
Publication: |
156/345 ;
118/723.00E |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 1999 |
CH |
1999 1466/99 |
Claims
1. A capacitively coupled radiofrequency plasma reactor (1, 20)
comprising: at least two electrically conductive electrodes (3, 5)
spaced from each other, each electrode having an external surface
(3a, 5a), an internal process space (13) enclosed between the
electrodes (3, 5), gas providing means (7) for providing the
internal process space (13) with a reactive gas, at least one
radiofrequency generator (9) connected to at least one of the
electrodes (3, 5), at a connection location (9a), for generating a
plasma discharge in the process space (13), means (8) to evacuate
the reactive gas from the reactor, at least one substrate (15)
defining one limit of the internal process space, to be exposed to
the processing action of the plasma discharge, said at least one
substrate (15) extending along a general surface (15a) and being
arranged between the electrodes (3, 5), characterized in that said
plasma reactor (1, 20) further comprises at least one dielectric
layer (11) extending outside the internal process space, as a
capacitor electrically in series with said substrate (15) and the
plasma, said dielectric layer (11) having capacitance per unit
surface values which are not uniform along at least one direction
of said general surface (15a), for generating a given distribution
profile, especially for compensating a process non uniformity in
the reactor.
2. A capacitively coupled radiofrequency plasma reactor comprising:
at least two electrically conductive electrodes (3, 45) spaced from
each other, each electrode having an external surface (3a, 5a), an
internal process space (13) enclosed between the electrodes (3, 5),
gas providing means (7) for providing the internal process space
with a reactive gas, a radiofrequency generator (9, 91) for
geneating a plasma discharge in the process space (13), said
generator connected to at least one of the electrodes (3, 45) at a
connection location, preferably centrally arranged on said
electrodes, an additional radiofrequency generator (93) connected
to at least one of the electrodes (3, 45), for increasing the ion
bombardment on said substrate, means (8) to evacuate the reactive
gas from the reactor, the at least one substrate (35) defining one
limit of the internal process space to be exposed to the processing
action of the plasma discharge, said at least one substrate
extending along a general surface and being arranged between the
electrodes, characterized in that said plasma reactor (1, 20)
further comprises at least one dielectric layer (95) extending
outside the internal process space, as a capacitor electrically in
series with said substrate (35) and the plasma, said dielectric
layer (11) having capacitance per unit surface values which are not
uniform along at least one direction of said general surface (15a),
for generating a given distribution profile, especially for
compensating a process non uniformity in the reactor.
3. The reactor of claim 1 or claim 2, characterized in that said
dielectric layer has a thickness (e.sub.1) along a direction
perpendicular to the general surface of the substrate, said
thickness being non uniform along said dielectric layer, so that
the reactor has said location dependent capacitance per unit
surface values.
4. The reactor according to claim 3, characterized in that: the
said dielectric layer (15) is the thickest in front of the location
in the process space (13) which is the farest away from said
connection location (9a) where the radioirequency generator is
connected to said at least one electrode, and said thickness
decreases from said process space location as the distance between
the process space location and the connection location on the
corresponding electrode decreases.
5. The reactor according to anyone of claims 1 to 4, characterized
in that said dielectric layer (15) has at least one non
planar-shaped external surface.
6. The reactor according to anyone of claims 1 to 5, characterized
in that at least one of said electrodes has a non planar-shaped
surface facing the substrate.
7. The reactor of anyone of claims 1 to 6, characterized in that:
said one dielectric layer is locally delimited by a surface of one
of said electrodes (5a, 41b, 51b), and said delimitation surface of
said one electrode is curved.
8. The reactor according to anyone of claims 1 to 7, characterized
in that said dielectric layer comprises at least one of a solid
dielectric layer and a gaseous dielectric layer, or a combination
of the both mentioned.
9. The reactor according to anyone of the preceding claims,
characterized in that the at least one substrate comprises a plate
having a non planar-shaped external surface.
10. The reactor of anyone of the preceding claims, characterized in
that the at least one substrate (65) has a curved shape.
11. The reactor according to anyone of the preceding claims,
characterized in that spacing members are arranged between said
substrate (35', 65) and one of the electrodes (25, 45), said
spacing members having elongations being non uniform.
12. The reactor according to claim 11, characterized in that the
spacing members (89) at the non-substrate-end being surrounded by a
space (91), for at least partially compensating the electromagnetic
perturbation induced by the contact between the spacing member and
the substrate.
13. A process for treating at least one substrate (15, 35', 65) in
a radiofrequency plasma reactor (1, 20), comprising the steps of :
locating the at least one substrate (15, 65) between two electrodes
(3, 5), the at least one substrate extending along a general
surface (15a), having a circulation of a reactive gas within the
reactor, so that such a gas is present in an internal process space
(13) arranged between the electrodes, having a radiofrequency
generator (9) connected to at least one of the electrodes (3, 5),
at a connection location (9a), having a plasma discharge in at
least a zone of the internal process space (13) in such a way that
said substrate is exposed to the processing action of the plasma
discharge, characterized in that it further comprises the steps of
creating an extra-capacitor electrically in series with said
substrate and the plasma, said extra-capacitor having a profile,
and defining the profile of the extra-capacitor in such a way that
it has location dependent capacitance per unit surface values along
at least one direction of the general surface of the substrate, for
generating a given distribution profile, especially for
compensating a process non uniformity in the reactor.
14. The process according to claim 13, characterized in that the
radiofrequency discharge is generated at a frequency higher than
for example 1 MHz, preferably higher than 19 MHz, the at least one
substrate has a surface larger than 0.5 m.sup.2, and the largest
dimension of the substrate surface exposed to the plasma discharge
is higher than 0.7 m.
15. The process of claim 13 or claim 14, characterized in that the
step of defining the profile of the extra-capacitor comprises the
step of defining such a profile having a non planar-shape along a
surface, in such a way that said extra-capacitor is materially
defined by at least one dielectric layer having a non uniform
thickness along said surface.
Description
[0001] The invention relates to a capacitively coupled
radiofrequency (RF) plasma reactor and to a process for treating at
least one substrate in such a reactor. Especially, the present
invention applies to a large size capacitive capacitively coupled
(RF) plasma reactor.
[0002] Often, such a reactor is known as a "capacitive" RF glow
discharge reactor, or planar plasma capacitor or parallel plate RF
plasma reactor, or as a combination of the above named.
[0003] Capacitive RF plasma reactors are typically used for
exposing a substrate to the processing action of a glow discharge.
Various processes are used to modify the nature of the substrate
surface. Depending on the process and in particular the nature of
the gas injected in the glow discharge, the substrate properties
can be modified (adhesion, wetting), a thin film added (chemical
vapour deposition CVD, diode sputtering) or another thin film
selectively removed (dry etching).
[0004] The table shown below gives a simplified summary of the
various processes possibly performed in a low pressure capacitive
discharge.
1 Industry Substrate type Process Inlet gas nature Semi- wafer
Surface Cleaning Ar conduc- up to 30 cm PECVD SiH.sub.4, . . . tor
diameter Dry Etching CF.sub.4, SF.sub.6, Cl.sub.2, . . . Ashing
O.sub.2, Disks Polymer or glass Diode sputtering Ar + others for up
to 30 cm PECVD Organometallics memory diameter Surface activation
O.sub.2, etc . . . Flat Glass Same as for Same as for display up to
1.4 m diagonal semiconductors semiconductors Win- Glass up to 3 m
Cleaning/ Air, Argon - dow width, foil, plastic activation,
Monomer, Nitrogen, pane or metal Nitriding, polymer . . . Web PECVD
coaters
[0005] The standard frequency of the radiofrequency generators
mostly used in the industry is 13.56 MHz. Such a frequency is
allowed for industrial use by international telecommunication
regulations. However, lower and higher frequencies were discussed
from the pioneering days of plasma capacitor applications.
Nowadays, for example for PECVD applications, (plasma enhanced
chemical vapour deposition) there is a trend to shift the RF
frequency to values higher than 13.56 MHz, the favourite values
being 27.12 MHz and 40.68 MHz harmonics of 13.56 MHz).
[0006] So, this invention applies to RF frequencies (1 to 100 MHz
range), but it is mostly relevant to the case of higher frequencies
(above 10 MHz). The invention can even be applied up to the
microwave range (several GHz).
[0007] An important problem was noted especially if the RF
frequency is higher than 13.56 MHz and a large size (surface)
substrate is used, in such a way that the reactor size is no more
negligible relative to the free space wave length of the RF
electromagnetic wave. Then, the plasma intensity along the reactor
can no longer be uniform. Physically, the origin of such a
limitation should lie in the fact that the RF wave is distributed
according to the beginning of a "standing wave" spacial oscillation
within the reactor. Other non uniformities can also occur in a
reactor, for example non uniformities induced by the reactive gas
provided for the plasma process.
[0008] It is an object of the invention to propose a solution for
eliminating, or at least notably reducing, an electromagnetic (or a
process) non uniformity, in a reactor. Thus, according to an
important feature of the invention, an improved capacitively
coupled RF plasma reactor should comprise:
[0009] at least two electrically conductive electrodes spaced from
each other, each electrode having an external surface,
[0010] an internal process space enclosed between the
electrodes,
[0011] gas providing means for providing the internal process space
with a reactive gas,
[0012] at least one radiofrequency generator connected to at least
one of the electrodes, at a connection location, for generating a
plasma discharge in the process space, and potentially an aditional
RF generator for increasing the ion bombardment on the
substrate,
[0013] means for evacuating the reactive gas from the reactor, so
that said gas circulates within the reactor, at least in the
process space thereof,
[0014] at least one substrate defining one limit of the internal
process space, to be exposed to the processing action of the plasma
discharge, said at least one substrate extending along a general
surface and being arranged between the electrodes,
[0015] characterized in that it further comprises at least one
dielectric "corrective" layer extending outside the internal
process space, as a capacitor electrically in series with said at
least one substrate and the plasma, said at least one dielectric
layer having capacitance per unit surface values which is not
uniform along at least one direction of said general surface, for
compensating a process non uniformity in the reactor or to generate
a given distribution profile.
[0016] In other words, the proposed treating process in the reactor
of the invention comprises the steps of
[0017] locating the at least one substrate between at least two
electrodes, the substrate extending along a general surface,
[0018] having a reactive gas (or gas mixture) in an internal
process space arranged between the electrodes,
[0019] having a radiofrequency generator connected to at least one
of the electrodes, at a connection location,
[0020] having a plasma discharge in at least a zone of the internal
process space facing the substrate, in such a way that said
substrate is exposed to the processing action of the plasma
discharge,
[0021] creating an extra-capacitor electrically in series with the
substrate and the plasma, said extra-capacitor having a profile,
and
[0022] defining the profile of the extra-capacitor in such a way
that it has location dependent capacitance per unit surface values
along at least one direction of the general surface of the
substrate.
[0023] It is to be noted that such a solution is general. It is
valid for all plasma processes, but only for a determined RP
frequency.
[0024] The "tailored extra-capacitor" corresponding to the
above-mentioned said (substantially) "dielectric layer" acts as a
component of a capacitive divider.
[0025] Advantageously, the capacitive variations will be obtained
through a non uniform thickness of the layer. Thus, the
extra-capacitor will have a profile having a non planar-shape along
a surface.
[0026] For compensating a non uniform voltage distribution across
the process space of the reactor, said thickness will preferably be
defined in such a way that:
[0027] the so-called "corrective layer" is the thickest in front of
the location in the process space (where the plasma is generated)
which is the farest away from the connection location where the
radiofrequency generator is connected to said at least one
electrode, the distance being measured by following the electrode
external surface,
[0028] and said thickness preferably decreases from said process
space location, as the distance between the process space location
and the connection location on the corresponding electrode
decreases.
[0029] Of course, it is to be understood that the above-mentioned
"distance" is the shortest of all possible ways.
[0030] So, if the electromagnetic travelling waves induced in the
process space combine each other near the center of the reactor to
form a standing wave having a maximum of voltage in the vicinity of
the reactor center, the thickness of the so-called "corrective
layer" will be larger in the vicinity of the center thereof, than
at its periphery.
[0031] One solution in the invention for tailoring said "corrective
layer" is to shape at least one surface of the layer in such a way
that the layer has a non planar-shaped external surface, preferably
a curved concave surface facing the internal process space where
the plasma is generated. Various ways can be followed for obtaining
such a "non planar shaped" surface on the layer.
[0032] It is a priviledged way in the invention to shape at least
one of the electrodes, in such a way that said electrode has a non
planar-shaped surface facing the substrate, and especially a
generally curved concave surface.
[0033] It is another object of the invention to define the
composition or constitution of the so-called "corrective
layer".
[0034] According to a preferred solution, said layer comprises at
least one of a solid dielectric layer and gaseous dielectric
layer.
[0035] If the layer comprises such a gaseous dielectric layer, it
will preferably be in gaseous communication with the internal
process space where the plasma is generated.
[0036] A substrate comprising a plate having a non planar-shaped
external surface is also a solution for providing the reactor of
the invention with the so-called "corrective layer".
[0037] Another object of the invention is to define the arrangement
of the substrate within the reactor. Therefore, the substrate could
comprise (or consist in) a solid member arranged against spacing
members located between said solid member and one of the
electrodes, the spacing member extending in said "corrective layer"
along a main direction and having, each, an elongation along said
main direction, the elongations being non uniform along the solid
member.
[0038] A difficulty induced by such spacing members relates to a
local perturbation relative to the-contact between the solid member
and the substrate.
[0039] So, the invention suggests that the spacing members
preferably comprise a solid end adapted to be arranged against the
solid member, said solid end having a space therearound.
[0040] Below, the description only refers to a capacitively coupled
RF plasma reactor in which the improvements of the invention
notably reduce the electromagnetic non uniformity during the plasma
process.
[0041] First of all, for most processing plasmas, the
electromagnetic propagation brings really a limitation in RF plasma
processing for substrate sizes of the order, or larger than 0.5
m.sup.2 and especially larger than 1 m.sup.2, while the frequency
of the RF source is higher than 10 MHz. More specifically, what is
to be considered is the largest dimension of the substrate exposed
to the plasma. If the substrate has a substantially square surface,
said "largest dimension" is the diagonal of the square. So, any
"largest dimension" higher than substantially 0.7 m is
critical.
[0042] A basic problem, which is solved according to the present
invention, is that, due to the propagative aspect of the
electromagnetic wave created in the plasma capacitor, the RF
voltage across the process space is not uniform. If a RF source is
centrally connected to an electrode, the RF voltage decreases
slightly from the center to the edges of said electrode.
[0043] As above-mentioned, one way to recover a (substantially)
uniform RF voltage across the plasma itself, is the following:
[0044] a capacitor is introduced between the electrodes, said
capacitor being in series with the plasma (and the substrate) in
the reactor,
[0045] this extra-capacitor acts with the plasma capacitor itself
as a voltage divider tailoring the local RF power distribution, to
(substantially) compensate a non uniformity of the process due, for
example, to gas compositional non uniformity, to edge effects or to
temperature gradient.
[0046] Below is a more detailed description of various preferred
embodiments according to the invention, in reference to drawings in
which:
[0047] FIGS. 1 and 2 are two schematic illustrations of an improved
reactor according to the invention (FIG. 1 is a section of FIG. 2
along lines I-I),
[0048] FIGS. 3, 4, 5, 6, 7 and 8 show alternative embodiments of
the internal configuration of such a reactor.
[0049] FIGS. 9, 10, 11, 12 and 13 show further schematic
embodiments of typical processes corresponding to the
invention.
[0050] FIG. 14 illustrates the "tailoring" concept applied to a
variation of thickness.
[0051] In FIGS. 1 and 2, the reactor is referenced 1. Reactor 1
encloses two metallic electrodes 3, 5 which have an outer surface,
3a, 5a, respectively. The electrodes are spaced from each
other.
[0052] A gas source 7 provides the reactor with a reactive gas (or
a gas mixture) in which the plasma is generated through a
radiofrequency discharge (see the above table). Pumping means 8 are
further pumping the gas, at another end of the reactor.
[0053] The radiofrequency discharge is generated by a
radiofrequency source 9 connected at a location 9a to the upper
electrode 3. The location 9a is centrally arranged on the back of
the external surface 3a of the electrode.
[0054] These schematic illustrations further show an
extra-capacitor 11 electrically in series with the plasma 13 and a
substrate 15 located thereon.
[0055] The plasma 13 can be observed in the internal space (having
the same numeral reference) which extends between the electrode 3
and the substrate 15.
[0056] The substrate 15 can be a dielectric plate of a uniform
thickness e which defines the lower limit of the internal process
space 13, so that the substrate 15 is exposed to the processing
action of the plasma discharge. The substrate 15 extends along a
general surface 15a and its thickness e is perpendicular to said
surface.
[0057] The extra-capacitor 11 interposed between the substrate 15
and the lower electrode 5 induces a voltage modification in such a
way that the RF voltage (V.sub.P) across the plasma (for example
along line 17, between the electrode 3 and the substrate 15), is
only a fraction of the radiofrequency voltage (V.sub.RF) between
the electrodes 3, 5.
[0058] It is to be noted that the extra-capacitor 11 is materially
defined as a dielectric layer (for example a ceramic plate) having
a non uniform thickness e.sub.1 along a direction perpendicular to
the above-mentioned surface 15a.
[0059] Since the location of the RF source on the electrode 3 is
central, and because of the arrangement (as illustrated in FIGS. 1
and 2) of the above-mentioned elements disposed in the reactor, the
thickness e.sub.1 of the dielectric plate 11 is maximal at the
center thereof and progressively decreases from said center to its
periphery, in such a way to compensate the electromagnetic non
uniformity in the process space 13. So, the presence of said
relatively thick series capacitor 11 reduces the effective voltage
across the plasma. Hence, for the compensation of electromagnetic
effects in a large surface reactor as illustrated in FIGS. 1 and 2,
the series capacitor 11 has to be a bit thicker in the center of
the reactor and must be thinned down toward the periphery
thereof.
[0060] The schematic illustrations of FIGS. 3 to 8 show various
possible configurations allowing such a compensation of non
uniformity in a capacitively coupled radiofrequency plasma reactor,
of the type illustrated in the above FIGS. 1 and 2. It will be
noted that combinations of the basic options illustrated in FIGS. 3
to 8 are possible.
[0061] In FIG. 3, a flat, planar ceramic plate 21 of a uniform
thickness e.sub.2 is attached to the upper electrode 23. There is a
tailored spacing 31 between the metal electrode 23 and the ceramic
plate 21. Above the other electrode 25 is arranged a substrate 35
which can be either dielectric or metallic (or electrically
conductive on at least one of its surface).
[0062] In FIGS. 3 to 8, the location of the connection between the
power source (such as the RF source 9 of FIGS. 1 and 2) and the
corresponding metallic electrode is supposed to be centrally
arranged on said electrode, and the general geometry of the reactor
is also supposed to be as illustrated, so that, in such conditions,
the tailored layer 31 has a back surface 31a which is curved with a
concave regular profile facing the process space 13.
[0063] Thus, the corresponding upper electrode 23 (the internal
limit of which, facing the process space 13, is defined by surface
31a) has a variable thickness e.sub.3. The dimension e.sub.3 is the
thinnest at the center of the electrode and the thickest at its
periphery.
[0064] The second opposed electrode 25 is generally parallel to the
first electrode 23 and has a uniform thickness e.sub.4.
[0065] It will be noted that the connection between the solid
dielectric plate 21 and the tailored gap 31 is not a gas-tight
connection. So, the reactive gas introduced within the process
space 13 can circulate in the gap 31 which will preferably have a
thickness adapted for avoiding a plasma discharge therein.
Providing the "corrective gap" 31 with complementary means for
avoiding said plasma discharge therein is also possible.
[0066] In FIG. 4, the electrode 23 has the same internal profile 31
a as in FIG. 3.
[0067] But, the "corrective layer" is presently a ceramic plate 41
having a variable thickness e.sub.5.
[0068] In FIGS. 5 to 8, the substrates 35' are dielectric
substrates.
[0069] In FIG. 5, the above electrode 33 is a planar metallic
electrode having a uniform thickness e.sub.4. The lower electrode
45 corresponds to the upper electrode 23 of FIG. 3. The electrode
45 has an internal upper surface 51b which defines a rear limit for
the curved concave gaseous "corrective layer". Above said layer 51
is arranged a dielectric planar horizontal plate 21. The ceramic
plate 21 of a uniform thickness e.sub.2 is connected at its
periphery to the lower electrode 45 (counterelectrode). The
substrate 35' is arranged on the ceramic plate 21.
[0070] Since the pressure of the reactive gas adapted to be
introduced within the reactive space is typically between 10.sup.-1
Pa to 10.sup.3 Pa, the pressure within the gaseous corrective gap
can be substantially equal to said injected gas pressure.
Typically, the reactive gas pressure within the plasma discharge
zone 13 will be comprised between 1 Pa and 30 Pa for an etching
process, and will be comprised between 30 Pa and 10.sup.3 Pa for a
PECVD process. Accordingly, the pressure within the corrective gap
(31, 51 . . .) will typically be a low pressure. So, such a gaseous
dielectric gap could be called as a "partial vacuum gap".
[0071] In FIG. 6, the substrate 35' (of a uniform thickness) is
laying on a solid dielectric plate (surface 41a) which can
correspond to the ceramic plate 41 of FIG. 4 in an inverted
position. The front, inner surface 41a of the plate 41 is flat,
while its back surface 41b is convex and directly in contact with
the lower metallic electrode 45, the inner surface of which is
presently concave. So, the plate 41 is a sort of "lens".
[0072] The electrodes 33, 45 illustrated in FIG. 7 correspond to
the electrodes of FIG. 5. The substrate 35', which has a uniform
thickness, is planar and parallel to the upper metallic electrode
33. Substrate 35' is laying on small posts 47 which are erected
between the electrode 45 and the substrate. The non planar internal
upper surface 51b of the electrode 45 gives a non uniform thickness
e.sub.6 to the gaseous gap 61 between the electrode 45 and the
substrate 35'. Thus, the space 61 acts as a corrective dielectric
layer for compensating the process non uniformity and enables the
substrate 35' to be uniformly treated by the plasma discharge.
[0073] In FIG. 8, the two opposed electrodes 25, 33 have a uniform
thickness, are planar and are parallel from each other. The
tailored layer 71 is obtained from a non planar substrate 65
arranged on erected posts 57. The elevations of such "spacing
elements" 57 are calculated for giving the substrate 65 the
required non planar profile.
[0074] The design of FIG. 8 should be mechanically the most
attractive, because both electrodes 33, 25 remain flat and the
profile of the small gap 71 is defined by the inserts 57.
[0075] For any purpose it may serve, it will be noted that the
radiofrequency power can be fed either on the electrode on which
the substrate is attached, or on the opposite electrode.
[0076] In the examples of arrangements illustrated in FIGS. 1 to 8,
it will further be noted that the tailored layer (11, 31, 41, 51,
61, 71) will preferably have a thickness calculated as a Gaussian
bell-shape for the electrode to electrode distance (on the basis of
the above-mentioned "central" arrangement). Then, said tailored
layer itself will be deduced from a truncation of the bell-shape,
what is left, namely the pedestal of the bell-shape after
truncation is the space for the plasma gap (internal process space
13), and the substrate.
[0077] FIGS. 9 to 15 show other embodiments of an improved
capacitively coupled radiofrequency plasma reactor, according to
the invention.
[0078] FIG. 9 shows the most straightforward implementation of the
invention. The radiofrequency power source 9 is centrally connected
to an upper electrode 3 called "shower head electrode" having holes
83 through its lower surface facing the plasma process space 13,
within the inner chamber 81 of the reactor 10. The
counter-electrode 30 is defined by the metallic external wall of
the chamber 81. The admission of the reactive gas is not
illustrated. But the pumping of said reactive gas is made through
the exhaust duct 85.
[0079] It will be noted that all the mechanical (material) elements
arranged within the reactor 10 and illustrated in FIG. 9 are kept
flat (electrodes and substrate 135, notably). However, the
substrate 135 (which has a uniform thickness e.sub.7) is curved by
laying it on series of spacing elements 87 erected between the
substrate and the counter-electrode 30. The spacing supports 87
have variable height. The substrate 135 is curved due to its own
flexibility. The average distance between the supports is defined
by the substrate thickness and its Young modulus.
[0080] In this arrangement, there are two layers in the space
between the electrodes that are not constant (uniform) in
thickness: the plasma process space 13 itself and the "corrective
space" 89 behind the substrate. Although this example is not a
straightforward solution, this configuration is effective, because
the RF power locally generated in the plasma depends far more on
the little variation of the thin "gaseous" capacitive layer behind
the substrate, than the small relative variation of the thickness
e.sub.8 of the plasma process space 13 (along the direction of
elongation of electrode 3).
[0081] The "corrective" tailored layer 89 is, in that case, behind
the substrate. It is a gaseous (or partial vacuum) tailored layer,
such a wording "vacuum" or "gaseous" being just used to stress the
fact that this layer has a dielectric constant of 1. The layer can
contain gases (the dielectric constant is not affected).
[0082] There is a danger that the supports 87, whether they are
metallic or dielectric, introduce a local perturbation of the
process.
[0083] Indeed, just at the support level where the series capacitor
of the tailored "corrective" layer 89 is not present, the RF field
is locally going to be larger. The perturbation, as seen by the
plasma, is going to spread over a given distance around the
support. This distance scales as the substrate thickness e.sub.7
plus the "plasma sheath thicknesses" (typically 2-4 mm) referenced
as 13a and 13b in FIG. 9.
[0084] FIG. 9a shows a potential way to reduce to a bearable level
the perturbation due to a support. The solution consists in
surrounding each spacing member 89 by a small recess 91. At the
recess level, the capacitive coupling is reduced. By adjusting the
recess to make an exact compensation, the local perturbation should
be practically eliminated.
[0085] In relation to the invention, such an arrangement shows that
the tailored "corrective" layer proposed in the invention should
follow the tailored profile, on the average: very local
perturbations on the profile could be accepted as long as the
capacitive coupling, remains substantially continuous and properly
tailored, when averaged over a scale of a few millimiters.
[0086] In the arrangement of FIG. 9, the substrate 135 is a
dielectric member. This is important, since any tailored dielectric
layer (such as 89) must absolutely be within the space defined by
the two extremely opposed metallic layers defining the "process
gap". If a substrate is metallic (electrically conductive), it
screens off the effect of any underlying tailored capacity. Then,
the substrate must be considered as one of the electrode.
[0087] In FIG. 10 is illustrated a rather common design in the
process industry. The reactor 20 is fed with two different driving
energy sources: a RF high frequency source (higher than 30 MHz) and
a RF bias source 93 (lower than 15 MHz). The upper "shower head"
electrode 3 is connected to the high frequency source 91 and the
low electrode 45 is connected to the RF bias source 93.
[0088] One of the sources is meant to provide the plasma (in that
case, we assume that it is an RF driving frequency with a rather
high frequency, through source 91). The other source 93 is
presently used as an additive to provide an extra ion bombardment
on the substrate 35. Typically, such an extra input (93) is plugged
on the "susceptor" side and is driven at 13.56 MHz.
[0089] Such a RF bias feature is often used in etching systems to
provide the reactive ion etching mode. It has been used in
combination with many types of plasma (such as microwave, or
electron cyclotron resonance).
[0090] In the example of FIG. 10, there are two electrodes (3, 45)
facing each other. None of them is actually grounded. However, even
in that particular configuration, the tailored capacitor of the
invention (layer 95 of a non uniform thickness) is appropriate. In
the case of FIG. 10, the configuration of FIG. 5 is implemented. An
important feature is that the active part of the reactor 20 (plasma
process space 13, substrate 35, flat planar dielectric plate 21 of
a uniform thickness and tailored gaseous gap 95 of a non uniform
thickness) is between two metallic plates (electrodes 3, 45). The
fact that one is grounded or not and the fact that one or several
RF frequencies are fed on one and/or the other electrode, are
irrelevant. The most important fact is that there is an RF voltage
difference propagating between the two metallic plates 3, 45. In
the example of FIG. 10, two RF frequencies are used. The drawing
shows two injections (up and down) for the two RF waves. It is
arbitrary. They could be injected from the top together, or from
the bottom (upper electrode 3 or lower electrode 45). What is
important here is that there are two different frequencies, one
high frequency and one low frequency. Both propagate in the
capacitive reactor.
[0091] If, as proposed, a tailored capacitor such as 95 is
introduced to compensate for the high frequency non uniformity, it
will make the "low frequency" non uniform. The "low" frequency wave
amplitude will then provide a slightly hollow electric power
profile due to the extra tailored capacitor in the center. In other
words, applying the "tailoring" concept of the invention here makes
sense only if the "high" frequency local power uniformity is more
important for the process than the "low" frequency power
uniformity.
[0092] In FIG. 11, the tailored capacitive layer 105 is a gaseous
space between a ceramic liner 105 and the metallic electrode 109
which has been machined to have the smooth and tailored recess
(because of its non planar internal surface 109a) facing the back
part of the ceramic plate 107. The ceramic liner 107 has many small
holes 107a which transmit the reactive gas provided by the holes
109b in the backing metal electrode 109. The reactive gas is
injected through ducts 111 connected to an external gas source 113
(the pumping means are not illustrated). The RF source 115 is
connected to the electrode 109, as illustrated.
[0093] The design of the backing electrode 109 could have been a
traditional "shower head" as electrode 3 in FIG. 10. Another option
is the cascaded gas manifold design which is shown in FIG. 11.
[0094] In FIG. 12, a microwave capacitive plasma reactor 40 is
diagrammatically illustrated. The illustration shows a possible
design according to which a rather thick tailored layer generally
referenced as 120 (the thickness of which is designated as e.sub.9)
is used to compensate for the drastic non uniformity due to
electromagnetic propagation. The illustrated reactor 40 is a
reactor for etching a rather small wafer. The microwave comes from
a coaxial wave guide 121 which expands gradually at 122 ("trumpet"
shaped) to avoid reflection. Then, the microwave reaches the
process zone 13 where the wave should converge to the center of the
reactor (which is cylindrical).
[0095] For the dimensions, the substrate 35 arranged on a flat
counter-electrode 126 has a diameter of about 10 cm, and an 1 GHz
wave is generated by the microwave generator 123 (30 cm free space
wave length). The central thickness of the tailored layer 120 (if
made of quartz) should be about the same as the space 13 of the
free plasma itself.
[0096] It is presently proposed that the tailored layer 120 be
obtained from three dielectric plates defining three steps (discs
120a, 120b, 120c). The discontinuity of the steps should be
averaged out by the plasma. The tailored layer is preferably very
thick and it would actually make sense to call it "a lens". The
number of disks used to constitute the lens could be four or higher
if the ideally smooth shape of the lens must be reproduced with a
better approximation.
[0097] In said FIG. 12, it will be noted that the reactive gas is
introduced through the gas inlet 124, said reactive gas being
pumped via a series of slits (preferably radially oriented) through
the counter-electrode 126 and ending into a circular groove 125.
The exhaust means for evacuating the reactive gas injected in the
reactive space between the electrodes are not illustrated.
[0098] In FIG. 13, the reactor 50 corresponds to the reactor 40 of
FIG. 12, except that, in this case, the step variation of the
"corrective" dielectric layer 130 is not due to a change of
thickness, but to a change of material constituting said layer 130
which has a uniform thickness along its surface. In other words,
layer 130 is a variable dielectric constant layer having a uniform
thickness e.sub.10. The low dielectric constant layer is the
central plate 131 which is concentrically surrounded by a second
plate 132 having a medium dielectric constant layer. The third
external concentric plate 133 has the highest dielectric
constant.
[0099] Hence, the equivalent thickest part of the tailored layer
130 is made of the lowest dielectric material (quartz for example),
whereas the intermediate layer 132 can be made of a material such
as silicon nitride, the highest dielectric constant material at the
periphery 133 being presently made of aluminum oxide.
[0100] The example of FIG. 13 clearly shows that the dielectric
layer of the invention having a capacitance per unit surface values
which are not uniform along a general surface generally parallel to
the substrate can be obtained through a variation of the dielectric
constant of said layer, while the thickness thereof remains uniform
along its surface.
[0101] From the above description and the illustration of FIG. 14
(based on the embodiment of FIG. 1), it must be clear that, in any
case in which the thickness of the "corrective layer", such as 140,
is used to compensate the process non-uniformity, as observed, the
corrective layer(s) will be the thickest in front of the location
in the process space (or on the facing electrode, such as 3) which
is the farest away from the electrode connection (9a). It is to be
noted that the "way" (referenced as 150) for calculating said
"distance" must follow the external surface (such as 3a) of the
corresponding electrode.
[0102] Said thickness will be the lowest at the corresponding
location where the above "distance" is the smallest, and the non
planar profile of the layer will follow said distance
decreasing.
* * * * *