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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sputtering apparatus and method for
fabricating durable dielectric thin films such as those used for optical,
semiconductor, tribological, appearance and other applications such as
quarter wave stacks for laser mirrors.
2. Prior Art
Thin films are often fabricated by condensing vapors upon a collecting
surface within a vacuum vessel. Vapors of the desirable thin film
materials are produced by heating or providing energy in some other
manner. Thin films made in this way are used to enhance the properties of
the substrates upon which they are deposited, as in the case of
anti-reflection coatings on optical lenses.
Thin films are also used for their own intrinsic properties apart from the
substrate upon which they rest. Chrome coatings on automobile grills is an
example of one such use. Thin beryllium windows for x-ray instrumentation
are fabricated using thin film deposition. After deposition, the thin film
is parted from the substrate and used alone.
The family of vapor phase processes of which the current method is a member
are generally referred to as physical vapor deposition methods. Two
methods that are used to convert the parent material into a vapor form are
represented as evaporation and sputtering.
The evaporation process involves raising the temperature of the parent
material by resistance, inductive or electron beam heating. The material
for deposition vaporizes directly, or after melting. A major drawback of
this method is that a compound material will usually chemically
disassociate allowing the components with highest vapor pressure to
vaporize first.
Chemical disassociation produces a thin film whose properties and chemical
composition vary throughout the film thickness. For this reason, the
evaporation method is primarily limited to the coating of single elements
such as copper, aluminum and gold.
By using multiple sources, clustered together, it has been possible to
condense more than one element onto a substrate simultaneously, thereby
forming alloys. It is also possible to form alloys by holding several
elemental components in the liquid state in a common bath, and by
controlling the liquid inventory of each, to maintain a vapor cloud
composed of these elemental components in a specified ratio.
The sputtering process involves accelerating a population of ions,
extracted from a plasma, toward a parent material usually referred to as a
target. Under preferred conditions, as the ions collide with the target
surface, a significant number of surface atoms are ejected which form a
vapor cloud. The pressure in the vessel is held at a level that results in
few collisions between the accelerated ions and the ejected vapor atoms.
The vapor moves away from the target surface with considerable kinetic
energy. If the parent material is a chemical compound or alloy, atoms of
each of the constituents are found in the vapor phase and condense upon
the substrate. Generally, because all the atomic species necessary to
reform the compound or alloy of the target are present in the condensate,
the original compound or alloy is reformed in the thin film.
Sputtering is most commonly practiced by filling the vacuum vessel with an
inert gas which is then ionized forming a low energy plasma. A negative
electrical potential is applied to the target material which then attracts
positive ions from the plasma which bombard the target surface causing the
sputtering of atoms into the vapor phase. The vapor which is electrically
neutral, condenses upon the nearby substrate forming the thin film.
An improved method uses a large area ion beam source to bombard the target
surface causing a vapor flux to be generated by sputtering action. The gas
pressure in the chamber, e.g. at the substrate surface, using this
approach can be in the tenths or hundredths of a millitorr range. This is
a great advantage since the finished film tends to contain fewer gas atoms
and have an improved grain structure and atomic packing density. An
extension of this method uses an elemental target to produce a vapor flux
by ion beam bombardment as described above, plus the introduction of a
chemically reactive gas into the chamber in order to form a thin film upon
the substrate which is a chemical compound of the reaction between the
target vapor atoms condensing on the substrate with the reactive gas atoms
introduced separately.
This application uses the method described above but introduces a second
ion beam source which is used to bombard the substrate directly wtth a gas
mixture partly composed of the required reactive gas species. By
controlling the parameters associated with this second ion beam, it is
possible to improve the physical, electrical, crystal morphology and
stoichiometry pooperties of the thin films which are formed. By.
controlling certain parameters of the first ion beam and the target and
substrate motion, it is possible to improve the uniformity, homogeneity
and purity of the thin films.
This application also relates to U.S. Pat. No. 4,142,958, titled "Method
for Fabricating Multilayer Optical Films", issued Mar. 6, 1979 for
inventors David T. Wei and Anthony W. Louderback and assigned to Litton
Systems Inc. of Woodland Hills, Calif. This application provides a novel
improvement to the method of the Litton patent of Wei and Louderback.
The authors of the Wei and Louderback '958 patent provided a prior art
discussion of quarter wave stacks. A part of that discussion is repeated
here for the reader's convenience.
Quarter wave stacks and their design are explained in detail in the
Military Standardization Handbook entitled, "Optical Design," MIL-HDBK141,
Oct. 5, 1962. Each layer or thin film dielectric coating in a quarter wave
stack has a thickness of about one quarter of a wavelength of the light
which it is designed to reflect. The number of layers which comprise the
quarter wave stack depends on the degree of desired reflectance and the
differences in refractive indices of the layers. To increase reflectance,
the number of layers and/or the differences in refractive indices may be
increased. For mirrors used in ring lasers, the quarter wave stacks
generally consist of 17 to 25 quarter wave thin film optical layers
deposited on a substrate. Each layer is typically from 500 to 800
Angstroms thick. The layers alternate between a material of high index of
refraction and a material of low index of refraction. Typically, the high
index material is tantalum pentoxide (Ta.sub.2 O.sub.5) or titanium
dioxide (TiO.sub.2) and the low index material is silicon dioxide
(SiO.sub.2, i.e., quartz).
The principal method of fabricating quarter wave stacks for ring laser
mirrors has been to use an electron beam evaporation technique. A
substrate on which a reflective stack is to be coated is located inside of
a vacuum chamber with a sample of the bulk or target material which is to
be deposited. An electron beam focused on the sample material causes
localized heating of the material to a point where molecules are
evaporated off. These molecules then condense on the other surfaces
located in the interior of the vacuum chamber, including the substrate
which is being coated.
The process of electron beam evaporation as a means of coating is
thoroughly explained in the text, "Physical Vapor Deposition," distributed
by Airco Temescal, 2850 7th Street, Berkeley, Calif., 1976.
One problem encountered in using the electron beam evaporation technique is
that molecules of the target material condense on the substrate in such a
manner that voids are left between them. The resulting coating is less
dense than the bulk target material, which results in a difference in the
layer's index of refraction making it difficult to control the refractive
indices of the stack.
The electron beam technique has the added disadvantage of producing
localized high heat concentrations that result in small explosions which
throw out larger chunks of the melt including impurities which condense in
the layer.
SUMMARY OF THE INVENTION
It is a major object of this invention to provide a method for making
multiple layer ion beam sputtered coatings having predetermined physical,
electrical, crystal morphology and stoichiometry properties.
It is another object of the invention method to allow the user to control
the thickness, stoichiometry, and morphology of each separate layer formed
by the method.
The method comprises the steps of forming a first ion beam from an inert
gas; bombarding the target surface with the first ion beam in a vacuum
chamber to generate a vapor cloud composed of the target material atoms by
the process of sputtering; adjusting the respective positions of the
substrate surface for coating and the target surface to promote the
condensation of the vapor upon the substrate surface for coating; forming
a second ion beam, the second ion beam's ions being formed from a mixture
of inert and reactive gases; bombarding the substrate surface
simultaneously with the second ion beam to promote a chemical reaction
between the target vapor atoms and the chemically reactive gas ions as the
target vapor atoms and the chemically reactive gas ions impinge upon the
substrate surface for coating, the resulting chemical compound building up
in thickness as a homogeneous thin solid film layer; positioning an
alternative target material in the place occupied by the previous one; and
repeating the above method steps to produce each successive thin solid
film layer of different material formed upon each previous layer.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cross-sectional view of a vacuum chamber apparatus described
in PRIOR ART U.S. Pat. No. 4,142,958.
FIG. 2 shows a cross-sectional schematic representation of a chamber
apparatus for practicing the invention method.
FIG. 2a is a front elevational view of the target cube in FIG. 2.
FIG. 2b is a front sectional view of the drive mechanism housing showing
substrate holder angle from rotation on AXIS 2.
FIG. 3 is a partial side elevation, partial sectional view of the TARGET
CUBE, rotational support, sealing and cooling system.
FIG. 4 is a schematic diagram of the gas supply and control mechanism as
well as the pressure sensor and throttle plate used to set chamber
pressure for the process.
FIG. 5 is a plan view of the cover for the substrate holder.
FIG. 5a is a side sectional view of the cover for the substrate holder.
FIG. 6 is a plan view of the substrate holder.
FIG. 6a is a side sectional view of the substrate holder.
FIG. 7 is a schematic representation of the large area Kaufman type ion
beam source.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a cross-sectional view of a vacuum chamber apparatus described
in U.S. Pat. No. 4,142,958, issued Mar. 6, 1979 to D. T. Wei and A. W.
Louderback for "Method for Fabricating Optical Films" and assigned to
Litton Systems of Woodland Hills, Calif. This patent teaches a method used
for fabricating interference optical films by ion beam sputtering. Vacuum
chamber 2 contains argon at about 1.5.times.10.sup.-4 torr. Argon enters
the chamber through tube 3, the argon is released in the chamber in the
area of the ion beam gun 4.
FIG. 1 shows the ion beam emanating from gun 4 being directed at surface A
of the target turret 18, as indicated by the arrows. Atoms of the target
material on surface A are dislodged and are coated onto the surfaces of
ceramic substrate 20 inside of the chamber 2. The ceramic substrate 20
comprises a base on which dielectric coatings are to be deposited. This
substrate is mounted on the disk at the end of the shaft 22. The substrate
typically forms a base for a ring laser gyro mirror. It is typically
located near the target 18 inside of the chamber so that it will be in the
main stream of the atoms dislodged from target 18. The shaft 22 has a
joint 24 so that the angle of the substrate MAY BE varied by moving the
bar linkage 26 in and out, i.e. to the right and to the left in FIG. 1
while horizontal in the chamber. The bar linkage is arranged to slide in
and out of the vacuum chamber by use of a hermetic seal. By sliding the
shaft 22 further into the chamber and adjusting the bar linkage 26
accordingly, substrate 20 is placed directly in the stream of the ion beam
and tilted such that the beam strikes the surface of the substrate. This
substrate position is illustrated by dashed lines 28.
Sleeve 30 is provided to connect bar linkage 26 to the shaft 22. This
sleeve permits the shaft to rotate while being supported by the bar
linkage 26. During the coating process, the shaft is rotated at about 60
revolutions per minute.
Oxygen is also present inside of the chamber to insure the proper
stoichiometry of the layers deposited onto the base 20.
The target surfaces A, B, C, and D become hot due to the sputtering
process. The sputtering rate is temperature dependent, the higher the
temperature of the target 18, the higher the rate of the deposition onto
the substrate 20 being coated. To control the temperature of the targets
A, B, C and D, water is circulated via lines inside of the turret 18 to
cool the targets.
Targets A, B, C and D are flat slabs of material at whose surface the ion
beam is directed. Atoms of the target material are dislodged from the
target surface to form a vapor which condenses onto the substrate 20.
A glass shield 102 is provided inside of the chamber. The shield is rotated
into the position shown in FIG. 1 by the apparatus 103 located outside of
the chamber. The vacuum chamber to remain sealed while allowing the shaft
104 to rotate the shield.
Operation
FIG. 2 is a cross-sectional schematic representation of the apparatus for
fabricating multilayer optical films on substrates. In particular, FIG. 2
shows the vacuum chamber apparatus used in performing the invention method
of fabricating multiple layer solid thin films on a surface to be coated.
The vacuum chamber has an end cover or door that is removable and which
serves as a means for obtaining access to the chamber interior, and which
is capable of sealing the vacuum chamber. This door or cover is not shown
in FIG. 2. The invention method provides control over the thickness and
stoichiometry and crystal morphology of each layer formed. The method
fabricates mirrors by depositing optical interference film stacks onto
optical quality surfaces by ion-beam sputtering technique in high vacuum.
ION BEAM GUN 1, 210 is used to form a first ion beam 212 from an inert gas
and bombard a predetermined sequence of targets, such as targets 214, 216,
218 and 220 in a vacuum chamber 200. The first ion beam 212 strikes a
target material 214 with high mass inert ions with high predetermined
energy, thereby vaporizing the target material. The target vapor moves as
a cloud (not shown) to deposit onto a set of substrates which are optical
quality surfaces 231, 232 and 233 held within a substrate holder 230. The
process is referred to as sputter deposition.
The substrate holder 230 is continuously rotated by substrate holder motor
240 to adjust the respective positions of the substrate surfaces exposed
to the vapor. The target surface 214 is also oscillated or partially
rotated about AXIS 3 to promote a more uniform vapor cloud which enhances
the uniformity of the condensation of the vapor on the substrate surfaces
231, 232, 233 for coating.
As shown in FIG. 2, ION BEAM GUN 2, 250 is used to form a second ion beam
252. The second ion beam's ions are formed from a mixture of inert and
reactive gases (not shown in FIG. 2). Oxygen is a typical reactive gas
useful in forming multiple layer thin films.
The second ion-beam 252 bombards the substrate surfaces 231-233 directly
with high density, low energy, chemically reactive oxygen ions to promote
a chemical reaction between the target vapor atoms and the chemically
reactive gas ions as the target vapor atoms and the chemically raactive
gas ions impinge upon the substrate surfaces for coating. The resulting
chemical compound builds up in thickness as a homogeneous thin solid film
layer.
The target atoms that condense on the substrate surfaces 231-233 are able
to form chemical bonds with the reactive gas ions. The second gun 250
provides control over the arrival rate of reactive ions at the substrate
and also the reactive energy available to this process. The ionizing
process delivers monoatomic, energetic reactive gas to the growing film,
thereby enhancing reactivity at the substrate surfaces 231-233.
FIG. 2 shows horizontal tube 206. Reactive gas, such as oxygen, can be
introduced into the chamber via horizontal tube 206. The flow rate of gas
passing into the chamber via tube 206 is controlled using a gas source and
control means similar to those to be described in connection with FIG. 4.
The interference film stacks typically consist of alternating layers of the
oxides of silicon and titanium. The targets 214-220, that are sputter
vaporized, are fabricated of high purity elemental silicon and titanium.
Compound formation occurs at the solid surface in conjunction with film
growth. Control of the ion density from beam 252 and the energy level
provides control over critical film characteristics including refractive
index, morphology, which includes molecular structure and density,
stoichiometry and film purity.
The apparatus described in the FIGS. 2, 2a, 3, 4, 5, 5a, 6, 6a, and 7 is
suitable for ion beam sputtering of any solid target material. The process
described here will be found useful for deposition of thin film layers
onto ceramic substrates to produce highly reflective laser mirrors of
quarter wave thick layers of alternately high and low index of refraction
materials. This process is useful for the fabrication of highly reflective
laser mirrors or layers to reflect laser beams or other narrow bandpass
light energy. However, the process is not necessarily limited to the
fabrication of highly reflective laser mirrors.
The chamber is capable of high temperature bake and ultra high vacuum
evacuation i.e. to the low 10.sup.-10 torr region. This is the primary
means used to reduce residual gas levels in the chamber and thus in the
film formation to below 1 PPM.
As shown in FIG. 2, the invention method is practiced using a unique
optimization of the physical positions of the two ion beam sources 210,
250, the target 219 and substrate holder 230, coupled with control of the
substrate holder's angle with the horizontal as shown in FIG. 2b. The
control of the substrate holder 230 rotation about vertical axis No. 1,
the target angle and dither range and rate contribute to optimizing the
invention method. The process is capable of extreme uniformity across the
substrate holder 230 diameter, as well as optimizing the rate of film
formation thereby minimizing residual gas content incorporation in the
film structure.
The first and second ion beam guns 210, 250 are both equivalent
commercially available ion emitting apparatus generally known in the art
as a Kaufman type ion beam source such as those available from the ION
TECH, INC. company at Ft. Collins, Colo., 80522.
FIG. 7 is a detailed schematic representation of the Kaufman broad beam ion
source showing the relative position of the guns' internal components. The
source's cathode 706 is a thermonic emitter. It emits electrons
represented by arrows 704 by passing an electric current through a wire
within cathode 706 (not shown) to heat it to incandescence. A low work
function material within cathode 706 is heated by the wire and emits
electrons which are accelerated towards a return electrode at the source
cathode window within cathode 706 (not shown but near inert gas inlet
705). The electrons are accelerated from cathode 706 to the several anodes
708. Permanent bar magnets 710, attached to anode 708, introduce a
magnetic field into the area between the cathode 706 and the anode 708
which cause the electrons, traveling towards the several anodes, to
spiral. This spiral motion effectively increases the distance which the
electrons travel in reaching any anode surface and thereby increases the
probability of an atom ionization through electron collision.
In the space between the cathode 706 and several anodes 708, electron and
atom collisions cause ionizations to occur which results in a glowing
plasma in region 711. The difference in voltage between the two grids 712
ano 714 is typically adjusted to be in a range of from 500 to 1500 volts
so that the gas ions represented by arrows 716 passing between the two
grids are accelerated at a high velocity away from the plasma and out of
the gun. These ions form a collimated ion beam 722.
A hollow cathode electron emitter, NEUTRALIZER 718 is positioned near the
ion beam 722 at the bottom of FIG. 7 and provides neutralizing electrons
720 so that both ion beams carry no net charge.
The COLLIMATED ION BEAM 722 has an intensity of approximately 100
milliampere. The ions have an average potential of typically 1000 volts
with respect to the chamber walls. Beam intensity depends upon the cathode
current, the voltage between anode 708 and cathode 706, an the partial
pressures of the gases inside of the chamber. The cathode heater current
is typically 5 amps within the cathode while the voltage between the anode
708 and cathode 706 ranges from 40 to 80 volts. The cathode to anode
current is typically 300 nano-amps. The inert gas partial pressure is
4.times.10.sup.-4 torr. In addition, oxygen may be introduced into the gun
via reactive gas inlet 715 with a partial pressure of about
4.5.times.10.sup.-4 torr.
FIG. 4 shows a schematic diagram of the gas controls used in the method.
Three gases: argon, xenon and oxygen are introduced through regulators
440, 441, 442, shut-off valves 430, 431, 432, and line filters 444, 445,
446 into flow control valves 422-427 and 412 and 413 and flow sensors
414-419, 433 and 434 filters, 435-438 and 468 to the vacuum vessel 410.
Five gas inlets 439, 468, 452, 451, 453 are used.
During the process, a portion of the gas entering the chamber from all five
inlets 451, 452, 453, 439, 468 enters the vessel proper, 410, as
electrically neutral gas atoms or molecules, unchanged in any way from its
form as supplied as a commercially available bottled gas. Ultra-pure gas
is preferred to minimize defects in the films formed. The term
"Ultra-pure" is an industrially accepted term used to describe the purest
gas available commercially in bottled form. Some of the gas which enters
gun 1, 463, gun 2, 455 and the neutralizer, 462, is changed in one or more
ways, and these changes and their controls constitute an important part of
the uniqueness of the method.
As described above, all five gas inlets are supplied from one or more gas
control valves 422, 427 and 420, 421 which regulate the quantity of gas
passing into the respective inlet 439, 468, 452; 453, 451. Each gas
control valve is controlled automatically and continuously by its
associated flow sensor 414-419, 433, 434 which is calibrated to sense the
quantity of gas of a specific atomic species passing that point in the gas
line. These two components, mass flow sensor and flow valve, are close
coupled so that they act as a unit to provide flow control for each gas.
As shown in FIG. 4, the argon and xenon gases are mixed by selecting a set
point for mass flow for both gas components and allows them to mix just
prior to the point of use filter 435-438, 468 at the neutralizer, 462 and
guns 1, 462 and 2, 455 The gas mixture is introduced into guns 1 and 2 via
the hollow cathode within the gun structure. This results in some gas
ionization. In the case of the neutralizer 462, the gas is also partially
ionized prior to its injection into the chamber vessel. Oxygen is also
introduced into gun 2, however not through the hollow cathode but directly
into the gun body. Because of the action of the Kaufman source upon
ionized gases within the gun, a portion of the ionized gases is constantly
accelerated from the gun in a directed, collimated beam of ions which
travels in straight lines to collide with the target surfaces in its path.
All gases entering the chamber vessel, whether as an electrically neutral
atom or molecule, or as a charged ion, are eventually removed by a high
vacuum pump 461 consisting of cryogenically cooled surfaces which capture
gases by condensation and cryosorbtion as they naturally impinge upon its
surfaces. Pumping action is continuous because natural diffusion causes
new gas molecules to move toward the pump to take the place of captured
atoms.
The throttle plate 460 is similar to a venetian blind and is used to set
the conductance at the throat of the pump so that chamber pressure may be
optimized for the process and also to control pressure transients.
The pressure sensor, 457, operates on the principle of the parallel plate
capacitor. Chamber pressure controls the space between the plates of the
capacitor. Its advantage is that it measures pressure without regard for
the atomic species involved. Therefore any ratio of gas mixtures can be
used without regard for calibration error. Pressure sensor 457 provides a
continuous external signal which is used to control the throttle plate 460
to maintain a constant chamber pressure.
Referring again to FIG. 2, the invention method proceeds as follows: the
substrate holder 230 is loaded with up to 10 substrates which are optical
surfaces, 231-233. The substrate holder 230 is mounted onto the drive
mecaanism, 240. The chamber is evacuated to the 10E-10 torr region using
high vacuum pump 280, thermal bakeout heaters on the outside skin of the
chamber (not shown) and interior radiant heaters (not shown). The
substrate holder 230 with its substrates 231-233 mounted in place, and the
drive mechanism housing 240 is rotated on horizontal axis 2 until the
substrate surfaces are at an oblique angle i.e. the substrate holder angle
262, with the ion beam 252. The substrate holder angle 262 is adjusted to
be approximately 50 degrees measured counterclockwise from horizontal. The
GUN 2 ion beam 252 is positioned to b fixed at approximately 45 degrees
measured counterclockwise with horizontal. At this time, substrate holder
230 rotation is started about vertical axis 1. Gun 2, 250 is energized and
stabilized, the neutralizer 208 is energized, the shutter 260 is opened by
rotation on vertical axis 4, and the substrate surfaces are cleaned and
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