FLOW-THROUGH ION BEAM SOURCE

TECHNICAL FIELD

This invention relates to deposition of thin film coatings by means of ion

bombardment.

This invention was made with government support under Contract No. W-

7405-ENG-36 awarded by the U.S. Department of Energy. The government has

certain rights in the invention.

BACKGROUND ART

Much research has been done to develop better methods of applying coatings,

or thin films, to various types of articles. Purposes for adding a coating on an article

include improving wear resistance, reducing friction, improving heat resistance, and

enhancing or providing electrical conductivity. Thin film deposition is an important

procedure in manufacture of microelectronics components. Thin film deposition is

presently done using ion-beam-assisted deposition, which has limited deposition rates

and requires large processing chamber space and two separate ion guns at large angles to the normal of the substrate to produce oriented films.

An object of this invention is to provide an apparatus for depositing a highly adherent coating.

Another object of this invention is to provide an apparatus for rapidly

producing thin films of superior smoothness.

A further object is to provide an apparatus for thin film deposition on

electrically non-conductive substrates.

A still further object of this invention is to provide an apparatus capable of

producing coatings of crystalline materials having specific crystal orientations rather

than randomly oriented structures.

An additional object of this invention is to provide an ion gun capable of

depositing a coating comprised of any material which can be vaporized.

Yet another object of this invention is to provide an ion source which can be

used to clean a substrate before the same source is used to apply a coating.

Another object of this invention is to provide a charge neutral ion beam.

DISCLOSURE OF INVENTION

A method and an apparatus for forming a charge neutral ion beam which is

useful in depositing thin films of material on electrically conductive or non-

conductive substrates are provided. The present invention is a flow-through ion gun

and a process for forming an ion beam which is more versatile than prior art methods

and apparatuses for depositing thin films.

More particularly an apparatus is provided for forming a charge neutral ion

beam. The apparatus comprises:

(a) a means for isolating a plasma from its surrounding environment having a

delivery aperture through which an ion beam passes, wherein said ion beam is formed

of ions produced in said plasma;

(b) a plasma chamber having a receiving end and a discharge end, said plasma

chamber being disposed within said isolating means in such manner that ions which

pass out of said plasma chamber discharge end pass through said delivery aperture;

(c) a means for establishing a positive electrical potential between said plasma

chamber and a reference location, whereby the magnitude of said electrical potential is

effective to form a plasma;

(d) a means for providing plasma gas to said receiving end of said plasma

chamber;

(e) a means for providing electrons to said receiving end of said plasma

chamber;

(f) a means for providing a magnetic field within said isolating means, such

that lines of force of said magnetic field are normal to an electric field formed as a

result of said electrical potential applied to said plasma chamber;

(g) an accelerating grid disposed across said ion beam in a location adjacent to

said plasma chamber and within said isolating means;

(h) a means for maintaining a negative electrical potential between said

accelerating grid and said reference location;

(i) a means for providing electrons to said ion beam in a sufficient quantity to

substantially neutralize said ion beam, wherein said electron providing means is located downstream of said accelerating grid;

(j) a retarding grid disposed adjacent to said plasma chamber receiving end;

(k) a means for maintaining said retarding grid at a positive electrical

potential about equal to the value of said plasma chamber electrical potential; and

(1) a means for providing a feed material to said receiving end of said plasma

chamber.

Also in another more particular aspect, there is provided a method of forming

a charge neutral ion beam comprising:

(a) establishing a positive electrical potential effective to cause formation of a

plasma between a plasma chamber and a reference location;

(b) establishing a magnetic field within said plasma chamber, such that lines

of force of said magnetic field are substantially normal to an electric field

formed as a result of said electrical potential applied to said plasma

chamber;

(c) providing electrons to a receiving end of said plasma chamber;

(d) providing a plasma gas to said plasma chamber receiving end, thereby

establishing a plasma and forming an ion beam comprised of plasma ions which pass

out of a discharge end of said plasma chamber;

(e) preventing ions from passing out of said receiving end of said plasma

chamber by placing a retarding grid adjacent to said receiving end, where said

retarding grid is maintained at a positive electrical potential about equal to the value

of said plasma chamber electrical potential;

(f) accelerating ions of said ion beam by means of an accelerating grid

disposed across said ion beam, where said accelerating grid is maintained at a

negative electrical potential effective to ac elerate ions;

(g) neutralizing said ion beam by providing electrons to said ion beam at a

location downstream of said accelerating grid; and

(h) providing feed material to said plasma chamber receiving end, whereby

said feed material is ionized and ions formed from said feed material become a part of

said ion beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic of the apparatus of this invention showing the

electrical circuitry.

Figure 2 is a graph of composition as a function of time in samples given a

light sputter cleaning before coating, as determined by an Auger probe beam.

Figure 3 is a graph of composition as a function of time in a sample given a

15-minute etch prior to coating, as determined by an Auger probe beam.

Figure 4 is a graph of composition as a function of time of gold deposited on a

machined aluminum surface in accordance with the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

It has been discovered that charge neutral ion beams useful for deposition of

thin films onto electrically conductive or non-conductive substrates can be produced

by an ion beam source (gun) having two open ends with electrically charged grids

which allows for both a plasma-forming gas feed and a feed for normally solid or liquid materials.

The arrangement of an electromagnetic coil and the electrically charged grids

ensures that electrons formed in the ion beam source of this invention flow outward

with a spiraling movement from a plasma chamber in a single direction. This

arrangement is shown with more particularity in Figure 1.

Referring to Figure 1 , a charge neutral ion beam indicated by arrow L7 passes

out of an enclosure 44, which is comprised of a hollow cylinder 3 of electrically

conductive material with grids j_6 and 40 covering the ends of the cylinder 3. The

enclosure 44 can be from about 2" to about 30" in diameter and from about 2" to

about 10" long. It is presently believed that for most applications larger sizes are

preferable because the ionization efficienty is higher for the energy input to the plasma.

A plasma is lit, or established, in a plasma chamber I in order to produce ions

which comprise the charge neutral ion beam. The plasma chamber I is a hollow

cylinder of electrically conductive material typically having a diameter from about

1.5" to about 25" and a length from about 2" to about 8".

A discharge filament 25 is located at the lower end of the plasma chamber 1

and is connected to a power supply 24 by wires 30 and 3L The discharge filament 25

is comprised of a material, such as tungsten, which emits electrons when it is heated

to a sufficient temperature (thermionic emission). Electrical current to heat the

discharge filament 25 is provided by means of the discharge filament power supply

24. A second power supply 22 provides an electrical potential between the plasma

chamber 1 and the discharge filament 25 in order to light and sustain the plasma. The

positive terminal of the second power supply 22 is connected to the plasma chamber 1

by wire 23 and the negative terminal is connected to wire 30 or to the negative

terminal of power supply 24 by means of a wire 32, so that power supply 22 floats on

the discharge filament power supply 24.

Plasma gas is provided to the lower end of the plasma chamber 1 through a

conduit 4 from a convenient source, as indicated by arrow 5_. Electrons emitted from

the discharge filament 25 ionize the feed gas to create a plasma.

A large positive electrical potential is applied to the plasma chamber 1 by

5 means of a third power supply 7 and wire 8. The third power supply 7 is grounded by

means of a wire 36. The purpose of biasing the plasma chamber 1 by means of the

third power supply 7 is to impart energy with respect to ground to ions created in the

plasma chamber L After the plasma is lit, it also flows into a floating chamber 45.

The floating chamber 45 is comprised of a hollow cylinder 2 of electrically

l o conductive material with its ends covered by grids 26 and 29. The floating chamber

45 is connected to the positive side of the third power supply 7 by a wire 9. A resistor

33 is provided between the third power supply 7 and the floating chamber 45 in order

to limit current flow. The floating chamber 45 serves to partially confine and control

the plasma. An outer enclosure 44 and the floating chamber 45 are means for

15 isolating the plasma from the environment surrounding it.

A magnetic field is provided by an electromagnetic coil 21 which surrounds a

portion of the floating chamber 45, as depicted by reference number 2L Power for the

electromagnetic coil 21 is provided by a fourth power supply H) by means of wires JJ.

and L2. Alternatively, an equivalent magnetic field could be provided by permanent

20 magnets. The purpose of the magnetic field is to cause electrons emitted by the

discharge filament 25 to follow spiral paths in an inward direction rather than travel

directly to the walls of the plasma chamber 1. Increased electron path lengths resulting from providing a magnetic field having a direction perpendicular to that of

the electric field increase the likelihood of a collision between an electron and a

molecule of plasma gas.

An accelerator grid 18 is placed above the floating chamber 45 to increase the

number of ions accelerated from the floating chamber 45. This grid 18 is maintained

at a negative voltage by means of a fifth power supply 20, which is connected to the

accelerator grid 18 by means of a wire 19. The accelerator grid power supply 20 is

grounded by means of another wire 39.

The ion beam is neutralized by electrons emitted from a neutralizing filament

13, which is a material, such as tungsten, that emits electrons upon being heated. A

sixth power supply J_5 provides electrical current to heat the neutralizing filament J_3

by means of a wire 14. The filament circuit is completed by means of a wire 34 to a

ground. Electrons emitted by the neutralizing filament 13 do not combine with the

positive ions of the beam to neutralize the ions, but instead become part of the beam

as a result of attraction of the electrons by the positive ions. The beam is a stream of

positive ions and electrons and may be termed a charge neutral ion beam when equal

numbers of positive and negative charges are present.

A retarding grid 27 is placed below the floating chamber 45 in order to prevent

positive ions from moving downward. The retarding grid 27 is maintained at the

same positive electrical potential as the plasma chamber 1 by means of being

connected to the plasma chamber biasing power supply 7 by wire 28. The outer enclosure 44, which is grounded by means of a wire 3_5, provides an electrical

reference for the apparatus and prevents electrical coupling with the environment

surrounding the apparatus. The plasma chamber end grid 40 prevents electrons from

being attracted to the retarding grid 27.

Both the accelerator grid 18 and the retarding grid 27 have square openings

from about one-fourth inch to about one-half inch. Generally, the accelerator grid 18

should have larger openings than the retarding grid 27. These grid openings are larger

than those useful in the Kaufman-type ion beam guns in which the top grids are used

for aperture lenses and there are no retarding grids. In the present invention, the

accelerator grid U8 does not function as an aperture lens.

A crucible 37 contains feed material 38, which is vaporized by any convenient

source of heat and flows upward into plasma chamber 1, as depicted by arrow 6.

Alternatively, a feed conduit for a gaseous feed could be used in the stead of the

crucible for normally liquid or solid feed material. Once in the plasma chamber 1, the

feed material 38 is ionized by means of two mechanisms. The first is collisions with

electrons emitted by the discharge filament 25. The second mechanism is Penning

ionization, which occurs if the excited levels of the plasma gas are deep enough to

cause ionization of the feed material. Penning ionization is caused by collisions of

molecules of feed material with plasma gas ions. Thus, the ion beam indicated by

arrow J_7 contains ions of the feed material in addition to ions of the plasma gas.

Generally a charge neutral ion beam is preferred, particularly when non-conductive

materials are being coated. However, if electrically conductive material is being coated, an ion beam having a positive charge can be employed.

The plasma can be monitored by any suitable means such as a probe having negative potential which measures the ion current. The information from the probe

can be used to make needed adjustments in the plasma gas pressure, the magnetic

field, or the feed gas pressure by monitoring the extracted ion current density.

In use of the experimental apparatus of this invention, voltage applied to the

discharge filament 25 is in the range from about 200 to about 600 volts. The electrical

potential between the discharge filament 25 and plasma chamber 1 is normally from

about 50 to about 100 volts. The positive biasing voltage applied by means of the

plasma chamber power supply 7 is in the range of about 200 to 600 volts. The

potential applied to the accelerator grid 18 is from about 25 to about 100 volts

negative, relative to the reference potential, or ground. The neutralizing filament

power supply 1_5 provides from about 5 to about 20 volts to the neutralizing filament

13. The resistor 33 _. through which the floating chamber 45 is connected to the biasing

voltage power supply has a rating of 10,000 ohms. It has been determined that

electrically connecting the retarding grid 27 to the plasma chamber 1 is optimal,

although, in theory, it could be slightly more positive than the reference supply

applied to the discharge filament 25. Current applied to the electromagnetic coil 2J. is

in the range of about 0.5 to 3.5 amperes, providing a magnetic field having a strength

of about 20 to 100 gauss.

Coatings comprised of any material which can be vaporized can be deposited

by use of the charge neutral ion beams produced by the ion gun of this invention.

Surfaces can be coated with a large variety of pure materials, including metals with

high melting points.

The primary application of this ion gun is to deposit thin films comprised of

materials which can be vaporized. When an inert plasma gas is used, a coating

consisting only of atoms of the feed material is formed. With a chemically reactive

plasma gas, a coating consisting of ions from both the plasma gas and the feed

material is produced. For example, titanium may be evaporated from a crucible

placed below the plasma chamber while nitrogen is provided to the receiving end of

the plasma chamber, thereby forming a beam of ions of both species which produces

titanium nitride when it impinges on a substrate.

Examples of most commonly used plasma gases include, but are not limited

to, argon, nitrogen and oxygen. Mixtures of gases can be used for the plasma gas.

Examples of liquid and solid materials which can be evaporated and deposited

onto a substrate by the apparatus of this invention include, but are not limited to,

aluminum, titanium, gold, platinum, zirconium oxide, indium tin oxide, magnesium

oxide and mixtures thereof.

Examples of gases which can be used in the stead of solid or liquid feedstock

include, but are not limited to, hydrogen, methane, argon, chlorine, oxygen, and

mixtures thereof.

Choice of feed materials for deposition will depend upon choice of material

for the substrate, the choice of plasma gas feed, and desired end product.

The ion beam source of this invention was designed to produce a wide variety

of coatings of various types. Coatings in a single layer, multilayered coatings, pure coatings, compound coatings and graded coatings can be produced using the apparatus

of this invention.

Coatings with multiple layers can be produced by first evaporating atoms

through the ion beam source with the plasma source voltage turned off, then applying

a pulse to the plasma source to produce an ion beam. When the plasma is off, the

deposited species will be atoms from the evaporating feed stock; when the plasma is

on, the deposited species will be a mixture of atoms from the evaporating feed stock

and the plasma gas. Thus alternating layers of different species will be deposited to

form a multilayer coating. The mechanical or other properties of multilayer coatings

can be adjusted by changing the composition and thickness of each layer.

The apparatus and methods of this invention can be used to produce compound

semiconductor films. For example, silicon can be evaporated through the flow-

through ion beam source using a plasma consisting of hydrogen and methane. This

mixture of plasma gases would allow alpha-silicon carbides to be produced with

varying compositions and hydrogen content as the ratio of the hydrogen to methane is

controlled concurrently with independent control of the silicon evaporation. The band

structure, and hence the electrical properties can be tailored by changing the mix

during the coating operation.

Being able to control independently the flow of each of the feed components

enables a wide range of materials to be deposited. Modulated electronic structures

can be produced simply by changing the ratios of amounts of plasma gas feed during a

coating operation using the invention of this application.

The ion beam source of this invention can be used for the production of

chemically modulated nano layers. The ion beam gun can be turned on and off at

selected intervals as the film grows so that a film of alternating compositions is

produced. The plasma forces the production of compound formation that does not

occur by chemisorption. The plasma then controls the reaction rate by controlling the

arrival rate of the reactive gas relative to the condensation rate of the film. By

switching a potential at the gun, the plasma can be quickly turned on and off, allowing

a modulated structure to be formed. This technique allows the production of nitrides

and carbides at room temperature, for example. X-ray mirrors could be produced

5 using this method by using modulated tungsten carbide and carbon layers on a glass

substrate by modulation of the tungsten evaporation from the electron beam source.

The energy of the ion beam leaving the ion beam source of this invention can

be adjusted without affecting the flux of evaporated atoms entering the back of the

gun. This control of ion beam energy, and the fact that all of the ions in the beam

l o strike the substrate at the same angle, enables the production of coatings of crystalline

materials having specific crystal orientations rather than randomly oriented structures.

The usual orientation of a film that is cubic in nature is an orientation with the 11 1

plane normal to the surface of the substrate. This is because the highest packing

planes are then exposed to the vapor. However, films deposited by the ion gun show

15 orientation of the 200 perpendicular to the substrate. This then may allow a simple

method to produce highly oriented or single crystal films on amorphous substrates.

Deposition onto substrate surfaces which already have an oriented crystalline

structure or which have been coated with oriented crystalline materials can result in

forcing of the newly deposited material into oriented crystalline structures.

20 Oriented diamond-like films could be deposited with the apparatus and

methods of this invention. These would be of particular use for the field emitter

technology employed in the flat panel display development.

These oriented crystalline films can be tailored to obtain desired combinations

of properties in terms of chemical resistance, chemical diffusion and elevated

temperature properties.

Because substrates can be coated with the apparatus and methods of this

invention at ambient temperatures, differential thermal expansion between the films

being deposited and the substrate is minimized. In addition, the intrinsic stress of the

coatings is reduced. This prevents film spalling or cracking during or at the end of the

coating operation. Thus, the wear resistance of coatings applied in accordance with

this invention can be improved.

Substrate surfaces can be coated very rapidly using the apparatus and methods

of this invention because the ion source is only used to excite the vapor, not to

produce the vapor. Because the ion source of evaporated atoms and the region of the

ion gun where the plasma is formed and accelerated are electrically isolated and the

open structure of the grids, the ion flux produced by the gun is not controlled

exclusively by space-charge physics. Space-charge limits, which are determined by

the electric fields created by the grids that confine and accelerate the plasma, normally

restrict the rates at which ion guns can clean surfaces or deposit thin films. Without

this limitation, the ion beam source of this invention can independently control the

production of ions and evaporated source material. For example, using ion-beam deposition, deposition can be carried out at about 0.2 to 0.5 A/second. With the

invention, using the experimental power levels, deposition rates of 5 A/second were

accomplished. Deposition rates of 500 A/second are expected for a larger invention

device having a sufficient power supply.

The ion source of this invention can be used to clean and/or oxidize the surface

of a substrate before the same source is used to apply a coating. For cleaning prior to

deposition, the ion beam source is operated using only an inert plasma feed gas.

However, if desired, in addition to sputter cleaning the substrate, the low

energy ions of a chemically active gas, about 300 volts, can penetrate the surface of

the substrate about 0.5-1.5 nm, thereby promoting a mechanical/chemical mix of the

coating with the substrate during the coating operation.

As described here and as demonstrated by the following examples, the

apparatus of this invention can be used to rapidly deposit highly adherent and smooth

coatings and thin films on electrically conductive and electrically non-conductive

substrates.

Example I

This example demonstrates use of the flow-through ion beam source of this

invention for cleaning a substrate before the same ion beam source is used to deposit a

highly adhered coating to the substrate.

A substrate of aluminum foil was cleaned in a minimal manner by bombarding

it with argon ions formed by using argon as the plasma gas for about 5 minutes. Ion current, which was determined by a small biased disk, was about 1 to 2 mA/cm ² .

Voltage applied to the plasma chamber, which is the sum of the voltage settings of the

plasma chamber power supply 7, the discharge filament power supply 24, and the

power supply 22 which provides the electrical potential between the plasma chamber

and the discharge filament, was 500 volts. The magnetic field was 50 gauss.

After the cleaning step, platinum in the crucible was vaporized to deposit a

layer of platinum about 50 nm thick on the aluminum foil, as measured by a quartz

crystal monitor.

A second aluminum substrate was coated with platinum in the same manner,

except that it was cleaned for 30 minutes before starting platinum deposition.

The two resulting coated samples were examined by Auger depth profiling

techniques.

The ion sputter beam on the Auger analysis system readily removed carbon

and oxygen from the outer surface of the platinum of each sample, as indicated by the

Auger profile plot of composition as a function of time, shown in Figure 2, where

time is proportional to depth. When the probe beam reached the platinum/aluminum

interface region, as indicated by a decreasing platinum signal and an increasing

aluminum signal, the carbon signal remained low and the oxygen signal went off the

scale, indicating that carbon was removed from the surface of the aluminum by the

short cleaning step, but that oxygen was not removed. The profile of the sample

subjected to 30 minutes of cleaning showed only a small amount of oxygen in the

interface region.

Effective cleaning of a substrate surface before deposition improved film

adherence.

Example II

This example demonstrates use of the invention apparatus to deposit a thin

film on the surface of pure aluminum which had micro-grooves due to machining.

A machined aluminum surface was lightly scrubbed using calcium carbonate

and water. The surface was then sputter-cleaned for 15 minutes with a beam of argon

ions.

A 500 nm thick gold coating was deposited on the surface of the machined

aluminum substrate in the same manner as described for the platinum deposition in

Example I. Argon was used as the plasma gas. Ion current was about 1 to 2 mA/cm ² .

Voltage applied to the plasma chamber was 500 volts. The magnetic field was about

50 gauss. The current density was about 1 mA/cm ² of argon gas and gold vapor.

Gold ingot was placed in the crucible as the feed material and vaporized by an

electron beam gun.

An Auger profile plot of the samples produced in this example showed some

oxygen at the gold/aluminum interface, but indicated that most of the oxygen and any

oxides from machining and cleaning with the calcium carbonate was removed by the

sputter cleaning step. This is shown in Figure 4. This was a more rigorous test than that of Example I because of the microscopic grooves in the surface of the aluminum

substrate as a result of the machining of the surface and since ion beam process is a

line-of-sight process.

Example III

A thin film of gold was deposited on a substrate of glass for the purpose of

testing the adhesion of the thin film to the glass.

A 500 nm thick gold coating was deposited on the surface of the glass

substrate in the same manner as described for the gold deposition in Example II.

Argon was used as the plasma gas. Ion current was about 1 to 2 mA/cm ² . Voltage

applied to the plasma chamber was 500 volts. The magnetic field was about 50 gauss.

Gold ingot was placed in the crucible as the feed material and vaporized by an

electron beam gun. The strength of the bond between the gold film and the glass was measured by

gluing the head of a test pin to the film and applying force to the pin in a direction

normal to the pin. The diameter of the test pin head was about 0.125 in. The pin

separated from the substrate at a stress of about 3350 psi with a portion of the film and

the substrate adhering to the pin. Thus, the glass fractured before the glue bond failed

and, more importantly, before the film/substrate bond failed.

In another sample, failure occurred by glass fracture at 3200 psi. When gold is

deposited on glass by prior art methods, the coating usually can be peeled from the

glass by applying Scotch tape to the coating and then peeling off the tape.

Example IV

Adhesion of a thin film of gold on aluminum was tested in the samples made

in this example.

A 500 nm thick gold film was deposited on a pure aluminum substrate using

the method detailed in Example II and the same equipment settings. Samples were

subjected to adhesion testing as in Example III. The aluminum surface had an about

25-micron surface finish due to the machine tool.

In one sample, the glue bond between film and substrate failed at about 6000

psi. In another sample, where failure occurred at about 5000 psi, about 75% of the

delaminated area was at the glue bond and 25% at the gold/aluminum interface.

INDUSTRIAL APPLICABILITY

The apparatus and process of this invention are useful for any application

requiring deposition of a thin film or coating of any vaporizable material onto a

conductive or non-conductive substrate. Parts used in the automotive, aerospace, and

aircraft industries can be coated using the methods and apparatus of this invention.

The invention methods and apparatus can be used in cleaning and coating processes in

the semiconductor industry.