U.S. patent number 3,708,418 [Application Number 05/016,720] was granted by the patent office on 1973-01-02 for apparatus for etching of thin layers of material by ion bombardment.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Richard Edward Quinn.
United States Patent |
3,708,418 |
Quinn |
January 2, 1973 |
APPARATUS FOR ETCHING OF THIN LAYERS OF MATERIAL BY ION
BOMBARDMENT
Abstract
A method of etching thin layers of material by bombarding a
target with accelerated ions in the presence of a magnetic field
parallel to the path of the accelerated ions and including
apparatus to carry out the method. A voltage is applied to glow
discharge electrodes in a vacuum environment causing plasma
formation between the electrodes and accelerating the resultant
ions toward a target to be etched. A magnetic field parallel to the
accelerating electric field is provided to condense the ionic
stream and cause more ions to strike the target. Some form of
masking is utilized to define the desired pattern of holes in the
surface layer of the target.
Inventors: |
Quinn; Richard Edward
(Willingboro, NJ) |
Assignee: |
RCA Corporation (N/A)
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Family
ID: |
21778601 |
Appl.
No.: |
05/016,720 |
Filed: |
March 5, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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612908 |
Jan 31, 1967 |
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Current U.S.
Class: |
204/298.37;
204/192.34; 257/E21.332 |
Current CPC
Class: |
H01J
37/32009 (20130101); H01L 21/2633 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); H01L 21/263 (20060101); H01L
21/02 (20060101); C23c 015/00 () |
Field of
Search: |
;204/192,798 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kay, J. of Appl. Phys, Vol. 34 No. 4, April '63, pgs. 760-68 .
Davidse, Vacuum Vol. 17, No. 3, 8-26-66.
|
Primary Examiner: Kaplan; G. L.
Assistant Examiner: Kanter; Sidney S.
Parent Case Text
This application is a continuation-in-part of an application filed
Jan. 31, 1967, Ser. No. 612,908, now abandoned, which is assigned
to the assignee of the instant invention.
Claims
I claim:
1. Apparatus for etching a surface of a target, comprising, in a
vacuum:
a. upper and lower insulating housings each having an opening which
communicates with the other opening;
b. an insulating gasket between said housings;
c. an insulating member extending through said gasket and into said
openings, said member having a central aperture therein;
d. a first electrode disposed in said upper housing and
comprising
a metallic plate having a central aperture therein, said plate
being carried by said insulating member,
an iron bar having legs carried by said plate, and an iron slug
centrally disposed in said iron bar;
e. a non-magnetic conductive spacer having a central aperture,
disposed in said upper housing and carried by said plate;
f. an apertured mask centrally disposed in said upper housing and
carried by said spacer
g. said target interposed between said mask and said iron slug;
and
h. a second electrode in said lower housing and comprising a
metallic cup and a magnet centrally disposed in said cup, said
magnet being in alignment with said iron slug, said target, said
mask, and said central apertures.
Description
BACKGROUND OF THE INVENTION
This invention relates in general to improved methods of, and means
for, etching thin layers of material supported on a substrate. It
relates particularly to improved methods of and means for etching
thin film devices such as thin film polycrystalline or single
crystal semi-conductor devices, including active and inactive
circuit components, or assemblies thereof, such as thin film
transistors and integrated circuits.
Often it is desirable to remove selected areas of a thin film
surface during the fabrication of various devices. One method of
accomplishing thin film removal is to bombard a surface with ions.
This method, used in cleaning operations, requires a source of ions
and a means for accelerating them toward the surface to be cleaned.
Selective removal of small areas of a thin film is difficult to
obtain using this technique.
Ion bombardment can be used to obtain selective removal if an
apertured mask is placed between the target and the ion source.
This will not solve the whole problem, however, because the ions
are too diffused to provide high pattern resolution. Some ions near
the mask edge will cut in under the mask, causing the pattern to
become blurred. This will be an even greater problem if the thin
film device is not completely flat, since gaps will then exist
between the mask and the target surface allowing the ions to
undercut the mask to a greater extent.
The use of a magnetic field parallel to the accelerating electric
field in an ion bombardment apparatus, has been reported by E. Kay,
in J. of Applied Physics, vol. 34, No. 4 (Part I), April, 1963, pp.
760-768, at page 761. This magnetic field produces a "magnetic
wall" which prevent ions from reaching the container wall, which
would otherwise cause contamination of the ions and recombinations
deleterious to some types of target substrates.
Electron bombardment can be used to remove surface areas of
material and to define holes in various materials; but because
electrons have little mass, they require acceleration voltages
which are about one hundred times higher than those required for
ions, and also require the use of a better vacuum. These additional
requirements greatly increase the cost of building electron film
removal apparatus, without providing practical advantages in many
cases.
The photoresist process uses chemical means to remove a thin film.
First a layer of polymer is placed over the thin film. Then it is
removed from those areas of the thin film which are to be
eliminated. Finally the surface is treated with an etchant which
eats away the film in those areas not protected by the polymer.
Some thin films, such as the oxides of tin, aluminum, and chromium,
which form thin films on many electronic circuit devices, are
resistant to all but the strongest acids. If films of these
materials are as thick as one to two microns, acid must be used for
extended periods of time to obtain satisfactory results. Before
complete removal can be accomplished in these cases, the acids may
get beneath the protective polymer, enlarge or undercut the holes,
and greatly decrease resolution.
Frequently the substrate of the device should be preserved from
removal, since the substrate can provide a means of establishing
electrical contact with the device. This preservation is possible
if the removal can be halted before the substrate is damaged.
When other steps in the fabrication of a thin film device are
performed in a vacuum system, it would be quite useful to be able
to accomplish the pattern definition step without removing the
device from the vacuum. An efficient vacuum etching process would
save both time and money. Some substrate materials are prone to
rapid oxidation so that after a thin film of oxide has been removed
there is often insufficient time to perform necessary operations on
the device before a further thin film of oxide forms. Any method of
thin film removal in air is subject to this problem, which can best
be solved by vacuum removal of desired surface areas coupled with a
method of plating or otherwise applying contacts into the newly
defined holes while maintaining the vacuum.
SUMMARY OF THE INVENTION
A typical embodiment of the invention includes glow discharge
electrodes mounted in a vacuum chamber. A plasma is generated
between the glow discharge electrodes by a potential difference
which accelerates the random ions present causing collisions and
further ion formation. In general, ions may be produced in any
manner and accelerated toward the target by any type of electric
field. Permanent magnets, electromagnets or a combination of the
two are located so that they generate a magnetic field having lines
of force parallel to the accelerating electric field. A target thin
film device is placed between an apertured mask and the negative
accelerating electrode and is, if possible, in electrical and
physical contact with both the mask and the electrode. The mask,
apertured to provide a desired circuit configuration, is situated
between the thin film device and the ionic source.
In another embodiment of the invention a high frequency alternating
voltage is applied across the discharge electrodes causing a radio
frequency discharge to occur in the space between the electrodes,
thus generating an ionic plasma. This embodiment is useful for
pattern definition in insulating materials, since a target of
insulating material prevents current flow and plasma formation when
a direct potential is placed across the electrodes.
Whether alternating or direct potential is used, it is possible to
pulse the ion accelerating voltage rather than operate it
continuously. This slows down the surface removal rate, but can be
advisable if the electrodes overheat.
By using a magnetic field having lines of force parallel to the
electric field that accelerates the ion, the invention obtains high
pattern resolution in the target. This technique condenses the
ionic stream so that more of the ions strike the target causing
faster film removal and improving the efficiency of the apparatus.
Use of a mask permits selective removal of small areas of a target
surface and this is especially important for the fabrication of
electronic circuits and devices. The presence of the parallel
magnetic field also decreases the probability of ions undercutting
the mask edge and blurring the target pattern.
Most thin film material, including those which are difficult to
remove by the photoresist process, can be etched by this invention.
Masking allows areas of the target surface to be protected during
the film removal process even if an extended period of bombardment
is required for complete film removal. Areas of the target
substrate can be preserved by masking the substrate with a thin
film of a material which is resistant to ion bombardment. The
substrate can provide a means of establishing electrical contact
with the target device and the invention provides an easy,
inexpensive method of protecting it.
Because the ion bombardment etching occurs in a vacuum, it can be
easily integrated with other steps in the fabrication of a thin
film device since many of these steps also require a vacuum
environment. This will avoid the expense and problems involved in
removing the target from the vacuum for the etching step. Material
can be sputtered onto the target surface as well as removed from it
while the target remains in the apparatus. After a pattern of
apertures has been defined in the target, material is plated or
sputtered onto target surface through the mask without disturbing
the vacuum by reversing the polarity of the ion accelerating
electric field.
THE DRAWINGS
The invention will be described in greater detail by reference to
the accompanying drawings in which:
FIG. 1 is an elevational view of a typical structure for carrying
out the invention;
FIG. 2 is an elevational view of a structure for carrying out the
invention including an external source of high frequency
alternating voltage;
FIG. 3 is a perspective view of an electric circuit device which
has been selectively plated with a thin film of a material which is
resistant to the effects of ion bombardment; and
FIG. 4 is a side view of an electric circuit device in which the
substrate is fabricated of a material that is resistant to the
effects of ion bombardment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, the entire apparatus is mounted in a vacuum
chamber which is not shown but is indicated by legend. Hoses
connected to the chamber evacuate the apparatus until the pressure
is low enough to permit plasma formation in the area between the
electrodes 10, 12. The chamber communicates with this area through
gas ports 14.
A two piece glow discharge containment arrangement 16, 18 provides
electrical isolation for the system. The domed portion 16 can be
removed to allow access to interior parts. Gaskets 20, also of
insulating material, separate the two halves of the glow discharge
chamber. A wire mesh screen 22 at anode potential supports the
apparatus.
The electrical circuit includes the two glow discharge electrodes
10, 12. The positive electrode or anode 10 includes an anode cup 24
and magnet 26 which is mounted in a depression 27 centrally
disposed within the cup 24. The negative electrode 12 includes a
circular cathode plate 28 having a centrally disposed circular
aperture 30, a rectangular iron bar 32 bent at the ends to form
legs which rest on the cathode plate 28 and having a centrally
disposed circular aperture 34, and a soft iron slug 36 which fits
within the circular aperture 34 of the bar 32. The cathode plate 28
rests on a circular piece of insulating material 38 which
physically separates the anode and cathode and provides isolation
between them. One terminal of an external direct voltage generator
is connected to the cathode 12 at slug 36 through insulated lead 40
and the other terminal is connected to wire mesh screen 22 which is
at anode potential.
The magnetic circuit includes a magnet 26 which is mounted in a
depression 27 centrally disposed within the anode cup 24 and may be
either a permanent iron magnet or an electromagnet. The iron bar 32
provides a return path for the magnetic lines of force. The soft
iron slug 36, mounted in the circular aperture 34 at the center of
the iron bar 32, completes the magnetic circuit.
In the embodiment of the invention shown in FIG. 1, a mask 44 is
provided, comprising a thin sheet of material having a pattern of
apertures 46 to etch a desired pattern of holes in a target 48. The
mask 44 has a surface of a material such as nichrome which is
resistant to the effects of ion bombardment. The mask 44 is spaced
with respect to the cathode plate 28 coaxially with the cathode
plate aperture 30 by a spacer 50 mounted in a centrally disposed
depression 52 in the cathode plate 28 and has a circular centrally
located aperture 53 which is directly beneath the target area
coaxial with apertures 30 and 34.
The spacer 50 preferably comprises a non-magnetic metal, such as
aluminum or iron. The use of a metallic spacer insures that the
mask 44 is maintained in electrical contact with the cathode 12 for
all types of targets. The target 48, in which a pattern of holes is
to be etched, is sandwiched between the soft iron slug 36 of the
cathode 12 and the apertured mask 44; the target 48 should be in
physical contact with both the slug and the mask.
A circular shield 54 of insulating material may be placed beneath
the cathode 12. The shield 54 has a circular centrally located
aperture 56 which has a slightly larger area than the target 48 and
is directly beneath the target area coaxial with apertures 30, 34
and 53. To be effective, this shielding should be placed close to
the cathode plate 28.
After holes have been etched in the target 48, the electrical
polarity of the apparatus can be reversed (the cathode 12 becomes
the anode and the anode 10 becomes the cathode) and material from
the surface of the anode 10 may be sputtered onto the exposed areas
of the target 48 through the apertures 46 in the mask 44. To
accomplish this sputtering step, the magnet 26 has on its flat
surface, facing mask 44, a layer of sputterable material 58.
FIG. 2 illustrates another embodiment of the invention. A high
frequency alternating voltage generator 60 is connected to the
cathode slug 36 through the insulated lead 40 and to the wire mesh
screen 22 which is electrically connected to the cup 24. In this
apparatus an ionic plasma is developed by radio frequency discharge
in the area between the electrodes 10, 12. This arrangement may be
used if the target 48 is an insulating material.
FIG. 3 illustrates an alternate method of masking selected areas of
the target 48. Areas 64 of the surface of the target 48 facing the
beam of ions, which areas are to be protected from the effects of
ion bombardment, are coated with a thin film 66 of a material such
a nichrome which is resistant to ion bombardment. The areas 68 to
be removed are left uncoated. In such an embodiment, the separate
mask 44 of FIG. 1 may be omitted. This type of masking may be used
when the target 48 does not present a planar surface to the
bombarding ions, since then the separate mask 44 is not readily
placed in contact with the surface of the target and ions would
tend to undercut the mask and blur the hole pattern, if the
separate mask were used.
FIG. 4 illustrates target 48 having a substrate 70 fabricated of a
material that is resistant to ion bombardment. When holes, such as
72, are produced in a surface thin film 74 by ion bombardment in
the arrangement of FIG. 1, the substrate 70, being resistant to the
ion bombardment, is not affected, even at the areas exposed by the
holes 72.
In the embodiment of the invention shown in FIG. 1, a potential
difference between the electrodes 10, 12 with the anode positive,
causes the random ions present to be accelerated and to collide
with nearby atoms producing further ionization and plasma
formation. The resultant electric field causes positive ions to be
accelerated toward the slug 36 of the negative electrode 12 so that
a current flows across the air gap between the magnet 26 and the
slug 36. The target 48 is located at the center of the slug 36
where it is struck by the maximum number of accelerated ions. The
mask 44 limits removal of target material to those areas of the
target 48 exposed by the pattern of apertures 46 in the mask 44. By
changing masks, different patterns of holes and slits can be etched
in the target 48. To insure sharp pattern definition, the mask 44
should be in physical contact with all points of the target surface
facing the bombarding ions, otherwise some ions may undercut the
mask edge and blur the target pattern. Further, by maintaining the
mask 44 in electrical contact with the cathode 12, as previously
described, the interaction between the electrical (E) and magnetic
(H) crossed-fields at the edge of the aperture 46 of the mask
directs ions at the edge parallel to that edge, rather than
underneath it. Thus, the likelihood of mask undercutting is greatly
reduced.
By orienting the magnet 26 and the slug 36 as shown in FIG. 1, a
magnetic field is generated having lines of force parallel to the
accelerating electric field produced by electrodes 10, 12.
Electrons present in the plasma tend to follow the magnetic field
lines and are confined in a column between the electrodes 10, 12
having approximately the transverse cross-sectional area of the
magnet 26 and the slug 36. Plasma density is thereby increased, and
since the target 48 is located within this column, a greater number
of ions strike its exposed areas providing high efficiency. The
presence of this magnetic field also decreases the number of ions
which tend to undercut the edge of the mask 44 and which may cause
the target pattern to become blurred, because ions tend to follow
paths parallel to the magnetic field.
The apertured shield 54 of insulating material is placed between
the electrodes 10, 12 to improve efficiency. The shield 54
effectively shields most of the area of the cathode 12 causing the
ionic current to flow through the aperture 56 at the center of the
shield 54 which is located directly beneath the target 48 and mask
44. The aperture 56 is slightly larger than the area of the target
48 to insure uniform bombardment across the whole target area. In
order to be effective, the insulating sheet 54 should be placed
close to the cathode plate 28 so that the ionic stream will not
diverge before striking the target 48. Mask support 50 adds further
cathode shielding.
In the embodiment of the invention shown in FIG. 2, the ionic
plasma is produced by a radio frequency discharge occuring in the
area between the electrodes 10, 12. The radio frequency discharge
occurs when a high frequency voltage generator 60 is connected to
the electrodes 10, 12. The alternating voltage generator 60 of FIG.
2 produces a voltage of 5,000 to 10,000 volts within a range of
frequencies from 5 to 10 megacycles.
Ion bombardment will provide pattern definition in most metal,
insulating, and semiconducting materials including those which are
difficult to etch by the photoresist process. Some metals such as
aluminum, nichrome and chromium are not greatly affected by ion
bombardment, but are useful for masking target areas. Removal rates
for most materials are approximately 12 angstrom units per second
using the following example parameters: flux density of 1,000 gauss
over a 3/4 inch air gap, a pressure of 0.3mm of mercury, and an
anode cathode voltage drop of 800 volts with a current density of 5
ma/cm.sup.2. Increased removal rates can be achieved by utilizing
ions, such as fluorine, which react chemically with the target to
provide both ballistic and chemical action to remove the target
material.
The apparatus of FIG. 1 was used to cut a slot in a gold film 300
angstroms thick deposited on glass. A regular sputtering mask 48
with a thin coating of rhodium to protect it from the bombarding
ions was used to define the slot. Any suitable material which
resists ion bombardment such as those mentioned above could have
been used to protect the mask surface. Removal took place through
the mask 48 having an aperture 0.008 inch wide. A pressure of 0.2mm
of mercury was maintained in the apparatus. Bombardment for 75
seconds was required for complete film removal. The slot produced
was the same width as the mask aperture with well-defined edges.
The results were comparable with those obtainable by chemical
means.
The method and apparatus described offers the advantage of
potential savings in time and expense because much of the work of
fabricating thin film devices occurs in a vacuum and etching the
thin film by the means described herein is readily accomplished
without removing the device from the vacuum, thereby eliminating
the handling involved in other methods. Since this process does not
take place in the atmosphere, the reoxidation problems encountered
when some substrate, such as silicon, are exposed to air, are
greatly reduced.
By reversing the polarity of the accelerating electrical field in
the apparatus of FIG. 1, contacts can be sputtered into the newly
defined holes allowing a completed device to be made in a vacuum.
The top of the magnet 26 is coated with sputterable material 58 as
shown in FIG. 1. When the electrical polarity of the apparatus is
reversed, the material of the magnet coating 58 is deposited on the
exposed areas of the target 48 through the mask apertures 46. To
accomplish this sputtering step, the electrical spacing preferably
is decreased, but this can be done by an external control.
* * * * *