Note: Descriptions are shown in the official language in which they were submitted.
~48~
HIGH CAPACITY ETCHING APPARATUS
Background of the Invention
This invention relates to high-precision patterning
and, more particularly, to an apparatus in which fine-line
patterns are delineated by dry etching processes.
Considerable interest exists in employing dry
processing techniques for patterning workpieces such as semi-
conductor wafers. The interest in dr~ processing techniques
stems from their generally bett~r resolut.ion and improved
dimensional and shape control capabiliti.es relative to standard
wet etching. Thus, dry etching is being utilized increasingly
for, for example, pattern delineation in the processing of
semiconductor wafers to form large-scale_integrated (I.SI)devices.
Various dry etching processes that involve radio-
frequency-generated plasmas are known. These processes include
sputter etching which is described, for example, in J. Vac.
Sci.Technol., Vol. 13, NO. 5, pp. 1008-1022, Sept./Oct. 1976,
and reactive sputter etching which is described, for example,
in Proc. 6th Int'l. Vacuum Congr. 1974, Japan. J.Appl.Phys.,
suppl. 2, pt. 1, pp. 435-438, 1974.
Heretofore, so-called parallel-plate reactors have
been utilized for sputter etching or reactive sputter etching
of workpieces such as semiconductor wafers. In many cases of
practical importance, however, it has been recognized that
the throughput characteristics of available reactors have
not been adequate for large-scale production of LSI devices.
Accordingly, efforts have been directed at trying to devise
high-throughput etching equipment that would be capable of
simultaneously processing a relatively large number of wor]c-
pieces. It was recognized that such
4~
2.
equipment, if available, could substantially decrease the
cost of devices processed therein~
Summary of the Invention
Hence, an object of the present invention is an
improved etching apparatus. More specifically, an object
of this invention is an etching apparatus exhibiting a
high-throughput characteristic.
In accordance with one aspect of the invention
there is provided apparatus for etching workpieces
comprising a cylindrical chamber adapted to be connected
to a point of reference potential, a workpiece holder
centrally positioned within said chamber and adapted to be
capacitively coupled to a source of a-c power, and means
~or establishing a specified gaseous atmosphere within
said chamber so that in response to a-c excitation of said
apparatus a dark space is formed in the immediate vicinity
of said holder and a plasma is formed between said dark
space and the inner wall of said chamber.
; In accordance with another aspect of the
invention there is provided a dry etching process for
delineating fineline patterns in multiple workpieces
simultaneously by sputter etching or reactive sputter
etching in a conductive cylindrical anode chamber adapted
to be connected to a point of reference potential, said
process comprising the steps of mounting the workpieces
on a longitudinally extending cathode holder centrally
positioned within said chamber, establishing a pre-
determined gaseous atmosphere at a specified pressure
within said chamber in the space between the holder and
the inner wall of said chamber, and capacitively coupling
a-c power to said cathode holder to form a dark space in
the immediate vicinity of said holder and to ~orm a plasma
between said dark space and the inner wall of said chamber
to cause etching of said workpieces to occur.
Briefly, these and other objects of the present
. .
. .
2a.
invention are realized in a speci~ic illustrative em~odi-
ment thereof that comprises a multi-faceted workpiece
holder centrally disposed within a cylindr;cal chamber.
A source of radio-frequency (r-~) power is capacitively
coupled to the holder and the cylindrical chamber is
grounded. A gaseous atmosphere is established within the
chamber. In response to r-f excitation of the apparatus,
a dark space is formed in the immediate vicinity of the
holder and a plasma is formed between the dark space and
the inner wall of the chamber. In such an embodiment,
uniform high-throughput sputter etching or reactive
sputter etching of workpieces such as semiconductor wafers
can be carried out in a reliable and low-cost way.
In accordance with a feature of the present
invention, workpieces whose top surfaces are to be etched
are respectively mounted on conductive portions of the
workpiece holder The workpieces are secured in place on
the holder to establish electrical contact between the
conductive portions and the bottom surfaces of the
workpieces. The instrumentality utilized to secure the
workpieces in place exposes the workpiece surfaces to be
etched to the gaseous atmosphere. All other surfaces
exposed to the etching atmosphere in the vicinity of the
workpieces are designed to be nonconductive.
In accordance with a further feature of this
invention, an embodiment of the type described above
adapted to carry out reactive sputter etching includes a
grid element interposed between the workpieces and the
inner surface of the cylindrical chamber. In such an
Mayd~n-16
embodiment, the difference in etching rates between those
portions of the workpiece surfaces designed to be etched
and those portions thereof designed not to be etched can be
significantly increased.
5 Brief Description o~ the Drawing
A complete understanding of the present invention
and of the above and other features thereof may be gained
from a consideration of the following detailed description
presented hereinbelow in connection with the accompanying
10 drawing, in which:
FIG. 1 is a partially broken away depiction of a
specific illustrative etching apparatus made in accordance
with the principles of the present invention;
FIGS. 2 and 3 show respective portions of a
5center member included in the FIG. 1 apparatus;
FIG. 4 is a top view of the center member;
FIG. 5 illustrates the manner in which workpieces
are secured in place in the depicted apparatus;
FIG. 6 is a schematic top view of a portion of
20another specific illustrative etching apparatus made in
accordance with the principles of this invention;
FIG. 7 is a depiction similar to FIG. 1 but
showing, additionally, a grid element included in a
reactive sputter etching apparatus made in accordance with
25thiS invention;
and FIG. 8 is a schematic top view of the
apparatus of FIG. 7.
Detailed Description
The specific illustrative system depicted in
30FIG. 1 comprises a main cylindrical etching chamber 10 made
of an electrically conductive material such as, for
example, aluminum or stainless steel. By any standard
affixing means such as screws 14, a member 12 is secured to
an upper flange portion 13 of the cylindrical chamber 10.
3sIn addition, the member 12 is sealed to the top of the
chamber 10 by a conventional 0-ring 16.
Illustratively, the flange portion 13 of the
M~ydan-16
'I .
chamber 10 of FIG. 1 includes an extension 18 that is
mechanically attached to a supporting arm 20~ In turn, the
bottom end of the arm 20 is attached to an extension 21 of
a lower Elange portion 22 of -the chamber 10. sy means of
5 the arm 20, the chamber 10 can be raised upwards to provide
access by an operator to a workpiece holder 24 that is
centrally mounted within the depicted apparatus. In one
particular illustrative example, the arm 20 is a component
part of a standard hydraulic lifting mechanism 26.
When the chamber 10 of FIG. 1 is raised fully
upward by means of the arm 20, the holder 24 is rendered
accessible for mounting workpieces thereon. The particular
illustrative holder 24 shown in FIG. 1 includes six flat
surfaces or facets. By way of a specific example, each
15 such surface indicated in FIG. 1 is designed to have four
6-inch wafers mounted thereon. One illustrative way in
which the wafers are so mounted on the holder 24 will be
specified in detail later below.
When the chamber 10 is lowered into the position
20 shown in FIG. 1, the lower flange portion 22 rests on the
upper rim 28 of a metallic base member 30. A seal is
achieved between the chamber l0 and the base member 30 by
interposing a conventional 0-ring 32 therebetween. With
the apparatus of EIG. 1 in its depicted position,
25 observation of the sealed interior of the chamber l0 can be
made via a viewing port 34.
A bottom plate 42 of the base member 30 of FIG~ l
is mechanically supported by columns 34, 36, 38 ... on one
of the top surEaces of auxiliary equipment 40. Two
30 conduits 46 and 48 extend from the equipment 40 through
respective openings in ~he bottom plate 42. As will be
evident later below in connection with the description of
FIG. 2, the conduit 46 contains therein two fluid-carrying
pipes and a conductive bus. The fluid carried in the pipes
35 is utilized to cool the workpiece holder 24, and the bus is
for the purpose of capacitively coupling a high-frequency
potential to the holder 24. The conduit 48, which is
-
Mayd~n-16
connected to a standard vacuum pump in the equipment 40,
serves to establish a prescribed low-pressure condition in
the sealed chamber lOo In addition, an inlet pipe 50 is
utilized to introduce a specified gas or mixture of gases
5 into the depicted chamber from the equi~ment 40.
The schematically depicted equipment 40
represented in FIG. 1 is conventional in nature. It
includes, for example, a vacuum system, gas sources, a
variable high-frequency alternating-current (a-c) power
10 supply adjustable to operate in, for example, the range 8
kilohert~ to 50 megahertz, a pumped source of cooling
fluid, and associated standard controls and gauges by means
of which specified operating conditions of the type set
forth later below are established in the chamber 10.
15 ~Herein, for purposes of a specific illustrative example,
r-f excitation of the etching apparatus at a freuency of
13.5 megahertz will be assumed.)
In accordance with the principles of the present
invention, the aforementioned bus is connected to the
20 workpiece holder 24 (FIG. 1) and the chamber 10 is
connected to a fixed point of reference potential such as
electrical ground. Moreover, the a~c driven holder 24 is
electrically insulated froln both a top conductive
element 50 and a bottom conductive element 52. Inturn,
25 the top element 50 is electrically connected to the inner
surface of the cha~nber 10 via conductive strips 54, 55.
The bottom element 52 contacts a conductive collar 56 and
an apertured conductive member 58. In turn, the bottom
edge of the member 58 electrically contacts the base
30 member 30. Thus/ the elements 50 and 52, the collar 56 and
the member 58 are in effect all electrically connected to
the same ponit of reference potential as is the chamber 10.
In FIG. 1, the workpiece 24 constitutes the
cathode and the chamber 10 constitutes the anode of the
35 depicted apparatus. In accordance with the invention, the
anode-to-cathode area ratio (Aa:Ak) is designed to exceed
unity. Illustratively, this ratio is selected to be in the
'; - ~ ' . ' ~
Maydan-16
6.
range 1.5 to 10. In one specific advantageous system of
the type represented in FIG. 1 r this ratio was designed to
be approximately 2.6. ~ore generally, Aa:Ak=Da:~k, where
Da is the diameter of the chamber 10 and Dk is the diameter
5 o~ the holder 24.
The area of the cathode of the apparatus depicted
in FIG. 1 is approximately the sum of the surface areas of
the six facets of the workpiece holder 24. The area of -the
anode thereof is approximately the area of the cylindrical
10 band of the inner surface of the chamber 10 that is
directly opposite and equal in height to the facets of the
holder 24.
Cooling of the holder 24 of the FIG. 1 apparatus
is advantageous. Otherwise, heat generated during the
15 etching process may cause material included on the
workpiece to flow and thereby deleteriously alter the
geometry of the device being fabricated. Moreover, by
controlling the temperature of the holder 24 to maintain a
specified optimal temperature on the surEaces thereof, a
20 relatively uniform and eficient etching action is achieved
for materials whose etch rates are temperature dependent.
In an apparatus made in accordance with the
principles of the present invention, a plasma of the type
typically utilized ~n conventional sputter etching or
25 reactive sputter etching is established in the sealed
chamber 10 (~IG. 1). In particular, a symmetrical dark
space is formed in the immediate vicinity of the workpiece
holder 24 and a plasma is formed between the dark space and
the inner wall of the chamber 10.
In FIG. 4, which is a top sectional view of the
workpiece holder 24 shown in FIG. 1 through 3, a dark
space 60 i5 schematically represented as enveloping
workpieces 61 mounted on the facets of the hexagonal
holder 24. In turn, a radio-frequency-generated plasma 62
35 is depicted as enveloping the dark space 60. As specified
before, this plasma fills the entire space between the dark
space and the inner surface of the chamber 10.
,
'' ,. . .
''
Maydan~16
Additionally, since the elements 50, 52, the collar 56 and
the me~ber 58 (see FIG. 1~ are electrically connected to
the chamber 10, the aforementioned plasma extends to the
surfaces of these components also. Hence, the dark space,
5 which in effect defines the regions where etching can
occur, is confined in the depicted apparatus to the
immediate vicinity of the facets of the holder 24.
Wafer-containing assemblies for each mounting
four wafers to be etched on the respectlve facets of the
10 holder 24 are shown in FIG. 1 ~see, for example,
assembly 64). The details of a portion of the assembly
are illustrated in a sectional view in FIG. 5.
The assembly 64 shown in FIG. 5 comprises
a conductive base plate 66 made, for example, of aluminum.
15 Four wafer-holding recesses are formed in the plate 66.
Illustratively, these recesses are cylindrical and just
slightly larger in diameter than the respective wafers
designed to be placed therein. The depth of the recesses
is approximately the same as the thickness of the wafers.
20 One such recess 67/ having a wafer 68 therein, is indicated
in E'IG. 5. A nonconductive top plate 70 made, for example,
of fused silica (or of aluminum oxide, or of silicon, or
comprising a dielectric material deposited on a metallic
plate) is secured to the base plate 66 by screws (one of
25 which, designated 72, is shown in FXG. 5). The top
plate 70 contains ~our apertures therethrough in aligned
registry with the recesses in the plate 66. The diameter
of each aperture is slightly less than the diameter of the
wafer contained in the recess immediately thereunder.
30 ~ccordingly, the plate 70 ser~es to retain the workpieces
to be e~ched in place in the base plate 66. A major
portion of the top surface of each retained workpiece is
thereby exposed through the respective aperture in the
plate 70. When the waEer-containing assemblies are screwed
35 in place on the facets of the holder 24 r the top or exposed
surfaces of the retained workpieces are mounted in place
for etching in the apparatus shown in FIG. l. When so
:
' ' '. ' ~ ' :
- , ~
hlaydan-16
3~
mounted, the bottom surfaces of the workpieces are
maintained in electrical contact with the base plate 66
which, in turn, is in electrical contact with the
holder 24. Maintaining good electrical contact between the
5 workpieces and the cathode holder in this manner has been
determined to be particularly important when carrying out,
for example, anisotropic etching of doped polysilicon.
Before describing typical operating conditions
for the FIG. 1 apparatus, further details of the holder 24
10 will be described with the aid of FIG. 2 and 3.
In FIG. 2, the previously specified conduit 46 is
shown extendiny through the bottom plate 42 of the base
member 30. Contained within the conduit 46 are two
nonconductive pipes, an inlet pipe 74 and an outlet
15 pipe 76, for carrying cooling fluid to and from the
workpiece holder 24.
The conduit 46 (FIG. 2), which is made, for
example, of stainless steel, constitutes the main
structural support for the holder 2~o The upper end of the
20 conduit 46 is attached by any standard means to the
conductive element 52. In turn, the element 52 is secured
by screws 80 to ring member 82. Another ring member 84 is
fastened by screws 86 into the bottom of a metallic
block 88 that constitutes a major component of the
25 holder 24. A nonconductive ring element 90 made, for
example, of glass is clamped between element 52 and
member 82 and between member 84 and block 88. It is
evident, therefore, that the block 88 is electrically
insulated from the depicted mechanical supporting structure
30 therefor.
An r-f bus 91 is also contained within the
conduit 46 shown in FIG. 2. Illustratively, the top enc] of
the bus 91 is electrically connected to the conductive
block 88 by a screw 92. In that way, the holder 24 can be
35 electrically driven by the aforementioned a-c power source
contained in the equipment 40 of FIG. 1 to establish a
plasma in the herein-considered etching apparatus.
.
May~lan-16
The block 88 of FIG. 2 has a centrally located
cylindrical bore formed therein. As indicated in FIG. 2
and 3, the bore extends through the top of the block 88 but
ends short of the bottom of the block 88. Positioned at
5 -the bottom of the bore is a cylindrical member 94 which is
made, for example, of aluminum. Openings 95 and 96
respectively aligned with the fluid-carrying pipes 74 and
76 are formed in the member 94. Further, a cylindrical
sleeve 97 is centrally positioned within the specified bore
10 in the block 88 in snug engagement with the member 94. An
opening is formed at the bottom of the sleeve 97 in
alignment with the opening 95 in the member 94.
As shown in FIG. 2, the diameter of the sleeve 97
is less than that of the depicted bore in the block 88~ As
15 a result, passageways 98 and 99 are defined wi~hin the
holder 24. Cooling fluid directed through the inlet
pipe 74 flows upwards through the annular passageway 9~ to
the top of the bore (see FIG~ 3) and returns via the
cylindrical passageway 99 to the outlet pipe 76 shown in
20 FIG. 2.
FIG. 3 indicates the manner in which the top of
the holder 24 is constructed. An electrically insulating
plate 100 is attached to the block 88 with screws 102. In
turn, an electrically conductive plate 104 is secured to
25 the plate 100 with screws 106. In addition, portions of
the aforedescribed conductive strips 54 and 55 are shown
connected to the plate 104 by means of the screws 106.
The specific illustrative apparatus described
herein is adapted to simultaneously etch twenty-four
30 workpieces. A number of particular examples of sputter
etching or reactive sputter etching in the depicted
apparatus are set forth below.
Various gases are suitable for introduction into
the apparatus of FIG. 1 to carry out sputter etching
35 therein. Thus, for example, substantially pure gases such
as aryon, helium, neon, nitrogen, xenon, krypton or
mixtures thereof, or other gaseous atmospheres known in the
,
- : :
~l~ydan-l6
10 .
art to be suitable for sputter etching, can be utilized for
sputter etching in the depicted apparatus. In one
particular illustrative example, a gold layer on each of
multiple wafers was selectively sputter etched within the
5 chamber 10 using a titanium or tantalum maskiny layer in an
atmosphere comprising ~0 percent argon and 20 percent dry
air by volume. In this example, the holder 24 was driven
by an r~f source operating at 13.5 megahertz to provide
power at a density of approximately 0~3 watts per square
10 centimeter at the surface of tlle layers to be etched.
Etching of the gold layers occurred at a rate of about 500
Angstrom units per mintue when the gas flow into the
apparatus was approximately 5-to-20 cubic centimeters per
minute and the pressure within the etching apparatus was
15 established in the range 5-to-10 microns.
In an atmosphere of substantially pure argon and
under operating c~nditions that were otherwise the same as
those specified above for gold, multiple layers of
permalloy were simultaneously sputter etched in the FIG. 1
20 apparatus at a rate of approximately 300 Angstrom units per
minute .
Reactive sputter etching can be carried out in
the FIG. 1 apparatus utilizing a variety of gases. Gases
such as, for example, substantially pure oxygen, chlorine,
25 C2F6, CHE3, ClF3, BC13, SiF4, any one of the freon gases,
or mixtures thereof, or mixtures of the aforespecified
gases with helium, argon, nitrogen, hydrogen, xenon, neon
or Icrypton, or other gaseous atmospheres known in the art
to be suitable for reactive sputter etching, can be
30 utilized for reactive sputter etching in the depicted
apparatus~ In one particular illustrative example, a
thermally grown silicon dioxide layer on each of multiple
wafers was selectively etched within the chamber 10 using a
photoresist masking layer in an atmosphere comprising
35 substantially pure CHF3. In this example, the holder 24
was driven by an r-f source operating at 13.5 megahertz to
provide power at a density of approximately 0.5 watts per
ll ~
squar~ centimeter at the surface of the layers to be etched.
- Etching of the oxide layers occurred at a rate of about
500 Angstrom units per minute when the gas flow into the
apparatus was approximately 5-to-50 cubic centimeters
per minute and the pressure within the etching apparatus
was established at a~out 5 microns.
Finally, it is to be understood that the above-
described arrangements are only illustrative of the
principles of the present invention. In accordance with
these principles, numerous modifications and alternatives
may be devised by those skilled in the art without
departing from the spirit and scope of the invention. For
example, it is advantageous in some cases of practical
importance to modify the inner surface of the chamber lO of
FIG. l to include thereon an instrumentality for capturing
material sputtered from the holder 24. A honeycomb
structure or simply an array of vertically extending vanes
108 (FIG. 6) affixed to the inner surface of the chamber
lO is effective for this purpose. Such a modified
structure is also characterized by an especially uniform
dark field around the holder 24, whereby highly uniform
etching of the multiple workpieces mounted on the holder 24
is achieved. Moreover, because in a structure of the type
shown in FIG. 6 the effective anode area is substantially
increased, the surface area of the cathode or holder 2~ may
be correspondingly increased while still maintaining an
optimal anode-to-cathode area ratio. Accordingly, the
wafer-holding capacity of such a modified structure can be
designed to be particularly high.
Another advantageous embodiment of the principles
of the present invention is shown in FIG. 7 which
Maydan-16
represents a modification of the FIG. 1 apparatus.
Identical elements in the two figures are designated with
the same reference numerals. As wlll be specified
hereinbelow, the FIG~ 7 equipment constitutes an
5 advantageous reactive sputter etching apparatus.
In FIG. 7, a grid elernent 120 is shown mounted in
the chamber 10 surrounding the workpiece holder 24.
Illustratively, the element 120 is mounted therein by means
of plural identical assemblies each of which comprises
10 rigid metallic rod members 121, 122 having an insulating
spacer 123 therebetween. In that way, the grid element 120
is, illustratively, maintained in a so-called electrically
floating condition with respect to both the anode 10 and
the cathode 24 of the depicted apparatus.
By way of a specific illustrative example, the
particular grid element 120 shown in FIG. 7 is represented
as having six sides each of which is spaced apart from and
parallel to a correspond;ng facet of the hexagonal
workpiece holder 24. Other geometries for the element 120
20 are also feasible. Thus, for example, the element 120 may,
alternatively, comprise a conductive cylinder spaced apart
from the holder 24 and centrally mounted within the
cilamber 10.
For the particular hexagonal grid element shown
25 in FIG. 7, six of the aforedescribed mounting assemblies
are utilized to hold the grid element 120 in place. Two o~
these assemblies, which extend between the back side of the
element 120 and the inner wall of the chamber 10, are not
shown in FIG. 7. The rod members 121 and 122 of the
30 depicted assemblies are spot welded, or otherwise suitably
attached in a secure mechanical fashion, to the element 120
and the chamber 10. Accordingly, when the chamber 10 is
fully raised upward by means of the arm 20~ as described
earlier hereinabove, the grid element is also moved upward.
35 In that way, the holder 24 is rendered accessible for
mounting workpieces thereon.
The schematically represented grid element 120 of
-
.
. , .
Maydan-16
3~l
13.
FIG. 7 comprises, illustratively, a lattice of orthogonally
disposed overlapping conductive members made, for e~ample,
of aluminum or stainless steel strips or wires or rods.
But other alternative constructions for the element 120 are
5 also suitable for inclusion in a reactive sputter etching
apparatus made in accordance with the principles of the
present invention. For example, the element 120 may be
formed simply by making apertures in a hexagonal metallic
member or in a sheet of metal formed in the shape of a
10 cylinder.
In one particular illustrative embodiment of the
FIG. 7 apparatus~ the grid element 120 was formed such that
the openings therein constituted approximately S0 percent
of the tota~ surface area thereof. But other embodiments
15 in which the opening percentage ranges from approximately
zero to 90 are also feasible.
In one specific embodiment of applicantls etching
apparatus, the distance between each face of the centrally
positioned workpiece holder 24 (FIG. 7) and the inner
20 surface of the chamber 10 was approximately 5 inches. In
that specific embodiment, the grid element 120 was
centrally positioned in the depicted apparatus such that
each of its faces was about 3 inches from the inner surface
of the chamber 10. But various other spacings are
25 suitable. In general, the spacing between the holder 24
and the grid element 120 should be at least twice the width
wds (see FIG. 4) of the aforespecified dark space
established in the chamber during etching.
As shown in FIG. 7, an r-f generator 132 and an
30 r-f tuning network 134 are electrically connected via a
capacitor 136 to the workpiece holder 24. By means of a
filter 138 anda meter 140, an indication is provided of
the peak r-f voltage applied to the holder 24. In one
specific illustrative case in which reactive sputter
35 etching was carried out, the potential indicated on the
meter 140 was -300 volts.
FIG. 8 is a schematic top view of a portion of
Maydan-16
14.
the EIG. 7 apparatus. As discussed above, the grid
element 120 is advantageously mounted in the depicted
apparatus in an electrically isolated way to constitute a
so-called floating grid. Alternatively, however, it is
5 feasible to carry out reactive sputter etching in such a
grid-equipped apparatus by electrically connecting the
element 120 directly to an external source of
potential 128O Thus, for example, lead 126 shown in FIG.
may be utilized to connect the element 120 to the
10 source 128. Illustratively, the lead 126 extends through
the chamber 10 via a standard insulating seal 130~
In accordance with a feature of the present
invention, the voltage applied to the grid element 120 by
the source 128 of FIG. 8 is selected to fall in a specified
15 range. In particular, the range extends, for example, from
slightly above ground to a value that is the negative of
the peak r-f voltage measured by the meter 140 (FIG. 7).
FIG. ~ schematically depicts the plasma
established in the chamber 10 during reactive sputter
20 etchiny, whether the aforespecified grid is floating or
connected to an external source. In the region between the
workpiece holder 24 and the grid element 120, a so-called
bright plasma is formed, whereas in the space between the
element 120 and the chamber 10 a so~called diffuse plasma
25 is established. Illustratively, the density of ions in the
holder-to-grid region is at least three times that in the
grid-to-cha~ber space.
A grid-equipped reactive sputter etching
apparatus made in accordance with the principles of the
30 present invention exhibits advantageous characteristics.
In particular, the difference in etching rate between
various materials is increased therein relative to the
difference exhibited in such an apparatus that does not
include a grid element.
In one specific example of practical importance,
a resist-masked layer of polysilicon overlying a layer of
silicon dioxide was selectively etched in a grid-equipped
Maydan-16
15.
reactive sputtering apparatus of the type sho~n in FIGS. 7
and 8. In a C~ atmosphere, at an r-f power input setting
of 150 watts, with the gas flow into the chamber set at
approximately 20 cubic centimeters per minute and the
5 pressure within the chamber established at about 6 micronsl
the polysilicon layer was etched at a rate of approximately
500 Angstrom units per minute. In that illustrative case,
any exposed silicon dioxide was etched at a rate of about
25 Angstrom units per minute. By contrast, under the same
lO conditions, out without a grid in the apparatus, the
polysilicon layer is etched at approximately the sarne rate
but, significantly, the silicon dioxide layer is etched at
the considerably higher rate Of approximately 80 Angstrom
units per minute. Moreover r in the grid-equipped
15 apparatus, the masking resist layer is eroded to a lesser
extent during etching than is the case when reactive
sputter etching is carried out in an apparatus without a
grid.
Similarly, the relative reactive sputter etching
20 rates of other materials are enhanced by utili~ing an
apparatus of the type shown in FIGS. 7 and 8. For example,
by employing a standard CHF3 atmosphere in such an
apparatus, the etch rate of a resist-masked silicon dioxide
layer relative to the etch rate Of an underlying
25 polysilicon layer can be thereby significantly increased.
In that case, erosion of the masking resist layer is also
reduced relative to the erosion thereof that occurs in a
gridless apparatus under similar operating conditions.
A complete and definitive theory Of operation for
30 the reactive sputter etching process that occurs in a
grid-equipped apparatus of the type shown in FIGS. 7 and 8
has not been formulated. But a tentative simplified
explanation for the improved results specified above has
been established. Although the validity and scope of
35 applicant's invention clearly do not depend to any extent
on the accuracy or comprehensiveness of that explanation,
it is instructive nevertheless to set it forth briefly
Maydan-16
8~3~
~ 6.
harein.
It is speculated that all etching reactions that
are primarily physically assisted chemical reactions remain
essentially the same for a given set of operating
5 parameters (e.g., power input, type of gas, gas flow,
pressure) whether the reactive sputter etching appara-tus
includes a grid or not. This is confirmed in practice by
the fact that the etching rates of the polysilicon layer in
the first example set forth above remain essentially the
10 same in the two types of equipment. In those examples,
polysilicon is removed during etching primarily due to
physically assisted chemical processes. But, in a grid-
equipped apparatus, the energy of ions incident on the
workpiece surfaces is relatively decreased due to the
15 presence of the grid. In turn, this causes a decrease in
the rate of removal of materials which are eroded primarily
by a physical etching action. This is confirmed in
practice by the observation that ~he rate of remova] of the
underlying silicon dioxide layer in the first example set
20 forth above is substantially reduced in a grid-equipped
apparatus.
