Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR REMOVING COATING
FROM SUBST~ATE
The present invention relates to a method and
apparatus for removing a layer of coating material from a
substrate.
Layers of coating material, such as polymer
photoresist, are commonly applied to substrates, such as
semiconductor wafers, during the manufacture of semiconductor
devices. It is known to remove such layers of coating
material from the substrates by activating a gas and
impinging the activated gas on the layer of coating material
to break down the coating material and strip it from the
substrate. The activated gas, for example oxygen, reacts
chemically with the coating material to form gaseous
products, thereby stripping the coating material from the
substrate.~
When such known techniques are used to remove
polymer photoresist from substrates, accumulation of
positively charged species of the activated gas at the outer
surface of the polymer photoresist layer sets up an electric
field which drives mobile positively charged ions ~for
example sodium, potassium or lithium ions) present in the
polymer photoresist layer into the substrate, thereby
contaminating the substrate.
The present invention seeks to provide a method and
apparatus for removing a layer of coating material from a
substrate which reduces positive ion contamination of the
substrate.
According to one aspect of the present invention,
there is provided a method for removing a layer of coating
material from a substrate, comprising: activating a gas;
removing positively charged species from the activated gas;
and impinging the activated gas on a layer of coating
material after removal of positively charged species from the
activated gas to break down the coating material and strip it
from a substrate.
According to another aspect of the invention, there
is provided apparatus for removing a layer of coating
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material from a substrate, comprising: means for supporting
a substrate in a support position; means for activating a
gas; and means for removlng positively charged species from
the activated gas, said removing means being disposed between
said activating means and said support position, having at
least one opening to permit flow of activated gas from the
activating means to the support position, and being operable
to remove positively charged species from said flow of
activated gas.
By removing positively charged ions from the
activated gas before it is impinged on the coating material,
the method and apparatus of the present invention avoid
accumulation of positively charged species at the outer
surface of the coating layer, thereby avoiding the creation
of electric fields which would drive mobile positively
charged ions into the substrate.
The positively charged species may be removed from
the activated gas by bringing the activated gas into contact
with a grounded surface to discharge ionized species in the
activated gas. This may be done by passing the activated gas
through a diffuser comprising electrically grounded
conducting walls defining a chamber, one of the walls having
at least one inlet aperture for permitting activated gas to
flow into the chamber and another of the walls having at
least one outlet aperture for permitting discharged activated
gas to flow out of the chamber and onto the coating.
Preferably, the inlet apertures and outlet apertures are
arranged in offset patterns to induce turbulence in the
activated gas as it flows through the chamber. The
turbulence ensures adequate contact of the activated gas with
the conducting walls of the chamber, and provides a
relatively uniform flow of discharged activated gas onto the
coating material.
Thus, according to yet another aspect of the
present invention, th~re is provided a diffuser for
discharging and diffusing an activated gas, comprising
electrically conducting walls defining a chamber, one of said
walls having at least one inlet aperture and another of said
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walls having at least one outlet aperture, said inlet and
outlet apertures being arranged in offset positions, and
means for mounting the diffuser within a reaction vessel.
Alternatively, positively charged species may be
removed from the activated gas by bringing the activated gas
into contact with a negatively charged surface which attracts
and discharges the positively charged species.
Embodiments of the invention will now be described,
by way of example only, with reference to the accompanying
drawings, in which:
Figure 1 is a cross-sectional view of a substrate
carrying a layer of coating material;
Figure 2 is a cross-sectional view of part of a
conventional plasma stripping apparatus;
Figure 3 is a cross-sectional view of part of a
plasma stripping apparatus according to a first embodiment;
Pigure 4 is a plan view of a diffuser used in the
first embodiment;
Figure 5 is an elevational view, partly in cross-
section, of the diffuser of Figure 4, the section taken onsection line V-V of Figure 4;
Figure 6 is a cross-sectional view of part of a
plasma stripping apparatus according to a second embodiment;
Figure 7 is a plan view of a filter used in the
second embodiment; and
Figure 8 is an elevational view, partly in cross-
section, of the filter of Figure 7, the section taken on
section line VII-VII of Figure 7.
Referring to Figure 1, a layer lO of coating
material in the form of polymer photoresist is commonly
applied to a substrate 12 in the form of a semiconductor
wafer during the manufacture of semiconductor devices. The
layer 10 may extend over all or part of a surface 14 of the
substrate 12, and the su~strate may include layers of oxide,
nitride, metal or polycrystalline semiconductor material in
addition to monocrystalline semiconductor material.
It is frequently necessary to remove such a layer
from such a substrate and for this purpose a known plasma
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stripping apparatus 20, shown in part in Figure 2, is
commonly used. The plasma stripping apparatus 20 comprises a
reaction vessel 22 having an inlet 24 and an exhaust outlet
26. Within the vessel 22 a support 28 is provided for
supporting the substrate 12 in a support position between the
inlet 24 and the exhaust outlet 26. The support 28 is heated
by means of a heater 29, in thermal contact with the support,
so as to heat the substrate 12. The MTI DPR Afterglo
machine, manufactured by Machine Technologies, Inc. of
Parsipanny, New Jersey, is one such plasma stripping
apparatus. (Afterglo is a trade mark of Machine
Technologies, Inc.)
In use of the plasma stripping apparatus, the inlet
2~ is connected to a plasma generation means (not shown)
which applies a microwave frequency electric field to a gas
to generate a plasma of the gas. The resulting activated gas
is drawn through the inlet 24 by means of a pressure
- differential and into the vessel 22 to impinge upon a coaking
layer 10 on a substrate 12 placed on the support 28 and
heated by the heater 29. The gas may be a reactive gas, such
as 2~ CF4, SF6 or a freon gas. Such gases, when activated,
chemically react with polymer photoresist to break it down
and form gaseous products. The gaseous products are drawn
from the vessel 22 through the exhaust outlet 26.
In the above process, positively charged species of
the activated gas tend to accumulate at an outer surface 16
(see Figure 1) of the layer 10 of photoresist. The
accumulated positively charged species apply an electric
field across the layer 10 which drives mobile positively
charged ions (such as Na, Li or K ions) commonly found in
polymer photoresists through the interface 18 (see Figure 1)
between the photoresist and the underlying substrake, thereby
contaminating the substrate. The contamination remains when
the stripping operation has been completed. The
contamination is particularly significant where the substrate
12 includes an oxide layer immediately below the photoresist,
since such oxide layers are relatively receptive to the
contaminants and have physical properties which are
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significantly altered by the presence of the contaminants.
~ plasma stripping apparatus, according to a first
embodiment as shown in Figure 3, inhibits the above problem.
The apparatus is basically constructed in a manner similar to
the apparatus of Figure 2, and in the following description
the same reference numerals will be used for such parts as
were described with reference to Figure 2. The first
embodiment differs from the prior arrangement in that a
diffuser 30 is mounted within the vessel 22 between the inlet
24 and the position in which the support 28 supports the
substrate 12.
The diffuser 30, shown in more detail in Figures 4
and 5, comprises electrically conducting walls 52, 54, 56 of
aluminum defining a cylindrical chamber 58. The diffuser 30
1~ is provided with a pair of arms 42 extending outwardly of the
chamber 58 which act as means for mounting the diffuser in
the vessel 22.
An uppermost wall 52 has several openings in the
form of inlet apertures 44, and a lowermost wall 56 has
several openings in the form of outlet apertures 46. The
inlet apertures 44 are arranged in a first pattern, and the
outlet apertures 46 are arranged in a second pattern offset
from the first pattern so that there is no straight line path
from the vessel inlet 24 through an inlet aperture 44 and an
outlet aperture 46 and onto a substrate 12 mounted in the
support position within the vessel 22.
In use of the apparatus of the first embodiment,
the diffuser 30 is mounted in the vessel 22 and electrically
connected to ground. The inlet 24 is connected to plasma
generation means (not shown), and activated gas is drawn from
the plasma generation means through the inlet 24 and onto the
diffuser 30 by means of a pressure differential. The
activated gas enters the chamber 58 via the inlet apertures
44. Because the outlet apertures 46 are not aligned with
corresponding inlet apertures 44, turbulence is induced in
the ionized gas, which is thereby brought into contact with
the grounded electrically conductive surfaces of the diffuser
30. Such contact discharges charged species in the gas, to
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convert the charged species to activated uncharged gas atoms
or molecules. Conse~uently, the diffuser provides means for
removing positively charged species from the activated gas.
The uncharged, activated atoms or molecules pass through the
outlet apertures 46 and impinge upon the layer 10 ~f
photoresist. As in conventional plasma stripping of
photoresist, the activated gas atoms or molecules chemically
react with the photoresist to form gaseous products, thereby
stripping the photoresist from the substrate. Unlike
conventional plasma stripping, however, there is little or no
build up of positive charge on the exposed surface of the
photoresist, so fewer mobile positively charged ions are
driven from the photoresist into the underlying substrate.
As a result, contamination of the substrate is reduced.
A plasma stripping apparatus according to a second
embodiment as shown in Figure 6, is constructed in a manner
similar to the first embodiment, except that the diffuser 30
of the first embodiment is replaced with an electrostatic
filter 60 mounted within the vessel 22 between the inlet 24
and the position in which the support 28 supports the
substrate 12.
The filter 60, shown in more detail in Figures 7
and 8, comprises an annular aluminum frame 72 provided with a
pair of arms 74 extending outwardly of the frame. The arms
74 provide means for mounting the filter in the vessel 22.
The filter also comprises a plurality of parallel aluminum
plates 76 extending along chords of the annulus 72. The
plates 76 are spaced apart to provide openings therebetween.
In use of the apparatus of the second embodiment,
the filter 60 is mounted in the vessel 22 and provided with a
negative electric charge. The inlet 24 is connected to
plasma generation means (not shown) 7 and activated gas is
drawn from the plasma generation means through the inlet 24
and onto the filter 60 by means of a pressure differential.
Positively charged species of the activated gas are attracted
to the negatively charged surfaces of the plates 76 of the
filter 60. Because positively charged species of the
activated gas are removed by the filter 60, there is little
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or no build up of positive charge on the exposed surface of
the photoresist, so there is a reduced tendency to drive
mobile positively charged ions from the photoresist into the
substrate. Consequently, contamination of the substrate is
reduced. The substrate 12 may be provided with a positive
charge to attract negatively charged species of the activated
gas toward the substrate and to further discourage migration
of positive ions from the photoresist to the substrate.
The diffuser 30 and filter 60 described above may
be constructed of any suitable material so long as adequately
conducting surfaces are provided. For example, metal,
passivated metal or other materials coated with passivated
conducting or semiconducting material may be used. Where
metal is used, annealing following machining operations is
advantageous to prevent cracking and eliminate inherent
material faulting. The diffuser 30 may adopt any suitable
shape which provides adequate surface contact with the
activated yas for removal of positively charged species.
The reactive gas may be activated by methods other
than application of microwave frequency electric fields. For
example, the gas could be activated by exposure to
ultraviolet light, although if this method is employed care
should be taken to minimize exposure of the substrate to the
ultraviolet light since the ultraviolet light may damage the
surface of the substrate.
