WO1991005073A1

WO1991005073A1 - Thin film fabrication method and device

Info

Publication number: WO1991005073A1
Application number: PCT/US1990/005431
Authority: WO
Inventors: Ian G. Brown; Robert A. Macgill; James E. Galvin; David F. Ogletree; Miquel Salmeron
Original assignee: The Regents Of The University Of California
Priority date: 1989-10-02
Filing date: 1990-09-28
Publication date: 1991-04-18
Also published as: AU6716390A

Abstract

A device (10) for layering metallic thin film or films on a substrate (18) utilizing a metal plasma beam generator (10) including a unit (12) having an anode (16), a cathode (14) of the film material, and trigger electrode (32). A control is used for accurately determining the rate of metal plasma emanating from the plasma beam generator (10). A support (70) is employed to hold the substrate (8) at a predetermined place apart from the plasma beam generator (10). The vacuum chamber (75) envelopes the plasma generator (10) and the substrate (18).

Description

THIN FILM FABRICATION METHOD AND DEVICE

STATEMENTS AS TO RIGHTS OF INVENTION MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The United States government has rights in this invention pursuant to contract No. DE-AC03-76SF00098 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

The present invention relates to a novel device for creating thin metallic films on a substrate.

Thin film fabrication is very important in the scientific research and commercial applications. For example thin films may be applied to epitaxy, integrated circuit fabrication, x-ray optics, magnetic recording media, mechanical property studies, to name a few in general, the term "thin film" is understood to mean a film of a solid substance of a thickness less than about 1 micron which is formed on a substrate material. In general, the film material is different from the substrate material. Metallic thin films constitute a large portion of existing thin film applications.

Prior art means for preparing metallic thin films have included evaporation from a source of the required material that is heated by any number of different means. Also, plasma or ion beam sputtering from a target has been used. However such methods of thin film fabrication are generally large and expensive.

Physical vapor deposition (PVD) has been employed to coat tools with a metal vapor. In the PVD process, a steady state cathode or group of cathodes vaporize metal which is transported onto a tool surface by biasing the same. In general, the PVD methods provide thick coating and require heating of the tools which are coated.

United States Patents 4,714,860 and 4,785,220 to Brown et al and a paper entitled Miniature High Current Metal Ion Source by Brown et al describe metal vapor vacuum arc ion sources which my also be used to implant ions into surfaces of objects. Such metal vapor vacuum source requires ion beam extraction from a plasma before implanting.

A simple, efficient, and accurate device for forming metallic thin film layers on a substrate would be a great advance in at least the metallurgical, electronic, and transportation fields.

SUMMARY OF THE INVENTION

In accordance with the present invention a novel device and method for producing metallic thin films is hereinafter described.

The device of the present invention utilizes a metal plasma beam generator which includes a unit having an anode, a cathode of the film material to be deposited on the substrate, and a trigger electrode. Controls means, such as a pulse generator, repeatedly fires the trigger electrode and successive metal plasma streams of cathodic material of distinct quantity and energy are produced. The arc or discharge forms between the cathode and anode to produce the plasma stream which is directed through the anode orifice toward the substrate. Such orifice may be sized to focus the plasma beam on the substrate.

Holding means is employed to support the substrate at predetermined place apart from the plasma beam generator unit. Means is also included for directing the successive metal plasma streams of cathodic material onto the substrate. Such directioning means is often achieved by axial alignment between the substrate anode, which may include a orifice, and the cathode. A vacuum chamber envelope the plasma beam generator unit as well as the substrate found therein.

The device of the present invention may also include meaπsfor applying a magnetic field to the plasma streams leaving the metal plasma beam generating unit. Such magnetic field may externalize in a multiplicity of electrical conduits wrapped or turned about the exterior of the metal plasma beam generator. The magnetic field formed by this structure may also be pulsed with the arc current.

The anode which forms the metal vapor vacuum arc positions a certain distance from the cathode. The anode also may include an anode plate having a orifice which would determine the transverse dimension of the beam.

In certain embodiments of the device of the present invention, two or more units may be placed side-by-side, each having cathodes of different materials. In this aspect of the invention multiple units may be pulsed in turn or alternately to produce interleaved thin layers of different materials originating with the cathodes of the multiple units. It may be apparent that a novel and useful device for creating a metallic thin ilm on a substrate has been described.

It is therefore an object of the present invention to provide a device for creating thin films on a substrate which is simple an inexpensive to produce and operate.

It is another object of the present invention to provide a device for creating a metallic thin film on a substrate having a thickness which is accurately predetermined and is easily reproducible.

It is yet another object of the present invention to provide a device which inexpensively and accurately produces multiple metallic thin films on a substrate.

A further object of the present invention is to provide a device for creating metallic thin films on a substrate which are of very high quality and usable for integrated circuits.

The invention possesses further objects and advantages especially as concerns particular characteristics and features thereof which will become apparent as the specification continues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a metal plasma beam generator usable with the device of the present invention.

FIG. 2 is an enlarged end view of the device of the present invention taken along line 2-2 of FIG. l and further illustrating a portion of the support structure in phantom. FIG. 3 is a front elevation view of an embodiment of the present invention using outlined multiple ion beam generator units.

FIG. 4 is a schematic view of the electrical system employed to pulse the ion beam generator units of the present invention.

FIG. 5 is a schematic view of implantation of ions in a substrate.

FIG. 6 is a schematic view of the formation of single or multiple thin film layers on a substrate.

FIG. 7 is a schematic view of the electrical biasing of the substrate used in the present invention.

FIG. 8 is a graph of an experiment resulting in a multiple layer film of yttrium and cobalt on a silicon substrate, described in detail in EXAMPLE II hereinafter.

For a better understanding of the invention reference is made to the following detailed description of the preferred embodiments thereof which should be taken in conjunction with the hereinabove described drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Various aspects of the present invention will evolve from the following detailed description of the preferred embodiments which should be further understood together with the prior described drawings.

The invention as a whole is depicted in the drawings by reference character 10, FIG. 3. The plasma generator 10 includes a one of its elements a plasma source or unit 12. Unit 12 is provided with a cathode 14 which is spaced from an anode 16. Cathode 14 is typically composed of the metallic material which is to be deposited on substrate 18, FIG. 3, which will be described in detail hereinafter. For example, cathode may be a metallic element such as cobalt, yttrium. Suffice it to say, cathode 14 may also be composed of a metallic compound or alloy as long as such composition is electrically conductive. Rod 20 threads into cathode 14 and is held to connector block 22 by set screw 24. Connector 26 threads into connector block 22 and serves as the terminus for negative electrical biasing of cathode 14. Insulator tube 28, which may be formed of alumina, fits over cathode 14. It should be noted that cathode 14 and rod 20 slip in and out of insulator tube 28 to facilitate the replacement of cathode 14. Tube 28 also fits within cavity 30 of connector block 22. Trigger electrode 32 is formed generally concentrically relative to insulator tube 28 an cathode 14, FIG. 2. Insulator bushing 34 and conductive support collar 36 complete the formation of plasma source unit 12. Support collar 36 serves as a base for the mounting of anode 16 by the way of plurality of set screws 38. It should be noted that anode 16, collar 36, trigger electrode 32, and rod 20 may be formed of stainless steel or other similar material.

Cavity 40 is formed by cathode 14, anode 16, and insulator bushing 34. The plasma generated by unit 12 forms within cavity 40 before passing through orifice 42 of anode 16. Electrical fitting 43 serves as the electrical terminal for trigger electrode 32.

With further reference to FIG. 1, means 44 for applying a magnetic field to any plasma beam or stream emanating from orifice 42, is also included. Means 44 is depicted as being several turns of 46 of insulative metallic wire. Turns 46 lie on the exterior of collar 36. Slot 48, FIG. 2, on anode 16 permits the magnetic field generated by means 44 to enter the plasma region within cavity 40.

Referring now to FIG. 3, it may be observed that plasma source unit 12 is mounted on a support 50. Support 50 is electrically conductive and serves as a electrical conduit for the potential placed on anode 16 through conductive collar 36. Support 50 is fastened to insulative base block 52 by the use of fastener 54 which also serves as a terminal for electrical conduit 56. Conduit 58 feeds trigger electrode 32 while conduit 60 electrically connects to rod 20 of cathode 14. A second plasma source unit 62 is also shown in FIG. 3 as being supported to base block 52 by electrically conductive support 64. Plasma source unit 62 is essentially the same as plasma source unit 12, except that the cathode of plasma source unit 62 could be of different metallic material, the function of which will be described hereinafter. FIG. 3 also illustrates plasma streams 66 and 68 passing from plasma source units 12 and 62, respectively, and onto substrate 18. Z-shaped electrically conductive support 70 also fixes to base block 52 by plurality of fasteners 72. Electrical conduit 74 may also be connected to one of plurality of fasteners 72 to permit biasing of substrate 18. A vacuum enclosure 75 envelopes plasma source units 12 and 62, as well as substrate 18 during operation of plasma generator 10.

With reference to FIG. 4, an electrical schematic is represented depicting the firing circuit for trigger electrode 32. A trigger pulse generator 76 feeds pulse transformer 78. Trigger pulse generator 76 may include power supply 80, resistor 82, electron tube 84, capacitor 86, feeding a pulsed power supply to trigger electrode 32. The metal vapor vacuum arc between cathode 14 and anode 16 is thereby caused to discharge from arc power supply 88. It should be noted that the arc current to anode 16, also flows through means 44, resulting in a pulsed magnetic field.

Turning to FIG. 10 it may be observed that substrate 18 may also be biased by DC power supply V-2 and parallel capacitor C-2. Resistor R-2 senses the current delivered to the substrate 18.

In operation, plasma source unit is assembled using a particular material for cathode 14 which is determinative of the material of the thin film to be placed on substrate 18. The trigger pulse line is shown in FIG. 4 is then set as to the desired number of pulses per second and the arc current on trigger electrode 32. The amperage through resistor R-2 is also calculated. Comparison of the arc current through trigger electrode 32 and the plasma current to substrate 18 reveals the efficiency of the plasma generating system. For example, an efficiency of one percent would be acceptable in the embodiments show in the drawings. The number of ions per shot or pulse (N) is then calculated using the following formula:

N/εhot= ^X _ ^u eQ

where I = bias current on substrate 18, tau - pulse length, e = electrode charge, Q = the mean charge state of an ion of cathodic material. The number of atoms per cubic centimeter (N) is then calculated utilizing the following formula:

N/cc=

A(1.6 x 10"²⁴) where p is the density of the cathodic material, and A is the atomic weight of one atom of cathodic material. The term, (N/cc)²⁷³ equals the number of atoms per cm² for one monolayer of cathodic material. Thus, the thickness of one shot or pulse of plasma exiting plasma generator unit 12 may be determined. By counting the number of pulses or shots, particular thickness or number of monolayers may be predetermined With further reference to FIG. 3, plasma generator units 12 and 62 may be alternated to produce multiple layers on substrate 18. Controlling the duration of the succession of pulses from either plasma source unit 12 or 62 will also result in successive layers or films on substrate 18 of predetermined thicknesses.

With reference FIG. 5, substrate 18 is depicted as having a surface 90. High energy plasma ions 92 (shaded circle) are shown as being implanted within substrate 18. Such a result derives from the prior art implantation device noted herebefore. It should be noted that the ions implanted within substrate 18 are normally found in a zone 94 generally between 0.1 and 1.0 microns below surface 90 of substrate 18.

Turning now to, FIG. 6, the results obtained by the present invention is shown where the production of a film or layer 96 take place on surface 90 of substrate 18 utilizing ions 100 of plasma stream 66 ("x" within circle) . The thickness of film 96 determined by regulating the number of pulses or shots of the plasma stream 6£ sent through opening or orifice 42 of anode 16. It should be mentioned that plasma stream 66 is directed onto substrate 18 by aligning axis 102 of unit i; and cathode 14 with the surface of substrate 18. The transverse dimension of ion stream 66 may also be regulated by the size of orifice 42 of anode 16. In other words, the spread of plasma stream 66 may be widened or narrowed by widening or narrowing orifice 42 of anode 16. FIG. 6 also depicts a second layer 98 placed atop layer 96 by utilizing ions 104 (slant line within circle) originating from plasma stream 68 of unit 62 illustrated in FIG. 3. Of course, layer 98 may also be produced by unit 12 after simply changing cathode 14, therewithin, to a different material.

In the processes depicted in FIG. 6, it should be understood that the ions 100, and 104, gain electrons from substrate 18 to form the atoms of a layer atop surface 90 of substrate 18. For the sake of convenience, the symbols employed in FIG. 6 for such atoms are the same as the source ions shown therein.

The following is a table of components typically used in the circuitries shown in FIGS. 4 and 10.

ITEM IDENT DATA SOURCE

Pulse TR 136 B E.G.&G. , Transformer 78 Boston, MA

Arc Power Pulse & Digital Millman and Taub Supply 88 Cir. Chapt. 10 McGraw Hill 1956 Pages 291-304 (L-C pulse line)

Power Supply 80 HV-1584R Power Designs Co. Palo Alto, CA

Resistor 82 1 MOHM Ohmite Co. 10 Watt Skokie, IL

Electron tube 5C22 Thyratron I.T.T. 84 Easton, PA Capacitor 86 0.1 micro f, 15 G.E. kv Schnectady, NY

Capacitor C-2 10 micro f G.E. Schnectady, NY

Voltage Source 25V. DC Battery Duracell Inc. , V-2 Bethel, CT

Resistor R-2 1 ohm Ohmite Co. Skokie, IL

In order to describe the invention more completely the following examples are given without intending to limit the invention to the specific examples set forth therein, except as such as they appear as limitations in the appended claims.

E AMPLE I

The plasma source 12 of FIG. 1 was fitted with an yttrium cathode. The pulse line provided an arc current of 40 amps, for a pulse duration of 250 micro seconds. The silicon substrate 18 was biased to (-)25 volts D.C. , FIG. 10, using a c ne ohm resistor R^*2 and a 10 microfarad capacitor C-2. The vacuum on the plasma source and substrate was approximately 8xl0'⁷ Torr. Ascertaining the voltage across resistor R-2, the plasma ion current measured 0.3 amps. Thus, the efficiency of the system, comparing the plasma ion current to the arc current, was about 0.75%. Through time-of-flight diagnostics the mean charge state of the yttrium ions was measured as 2.4. The number of ions per shot or pulse was calculated as being 2.34X10¹⁴. Using the yttrium specific gravity as 4.47, 1560 shots was calculated to equal a layer thickness of 1000 angstroms. The plasma source was run through 1560 shots. A layer of yttrium of about 980 angstroms deposited on the silicon substrate 18 (Some oxygen combined with yttrium in the deposited layer, resulting from the presence of oxygen within the vacuum chamber surrounding the plasma source and substrate) . The thickness of the deposited yttrium layer was confirmed by Rutherford Backscattering Spectrometry (RBS) . The film was found to be of high quality.

EXAMPLE II The plasma source shown in FIG. 3 was operated with an yttrium cathode in source unit 12 and a cobalt cathode in source 62. Under calculations similar to those of Example I and employing (30) amps of arc current, 1 pulse per second, and a vacuum of l.δxlO"⁶ Torr, 250 shots of each source 12 and 62 were alternately applied to a silicon substrate, such as substrate 18. Thus, three layers of yttrium and four layers of cobalt, were alternately applied or interleaved. The thickness each of the cobalt layers and each of the yttrium layers were measured to be 45 angstroms and 200 angstroms, respectively. Such measurements were obtained by sputtering 1 KV argon ions in conjunction with Auger Electron Spectroscopy (AES) , and are plotted on FIG. 11. Oxygen was also detected in the layers, primarily as yttrium oxide, due to the presence of oxygen within the vacuum chamber surrounding the plasma source units 12 and 62 substrate 18. While in the foregoing embodiments of the present invention have been set forth in considerable detail for the purposes of making a complete disclosure of the invention, it may be apparent to those of skill in the art that numerous changes may be made in such detail without departing from the spirit and principles of the invention.

Claims

WHAT IS CLAIMED IS

1. A device for layering at least one metallic thin film on a substrate, comprising: a. a metal plasma beam generator including a unit having anode, a cathode of the film material, and a trigger electrode; b. control means for pulsing said metal plasma beam generator to produce successive metal plasma stream of cathodic material therefrom and toward said anode, each of said metal plasma streams possessing a certain mass; c. holding means for supporting the substrate at a predetermined place apart from said plasma beam generator; d. a vacuum chamber for enveloping said plasma beam generator and said substrate; and e. means for directing said successive metal plasma streams of cathodic material from said anode onto the substrate for deposition as a thin metallic film.

2. The device of claim 1 inwhich said control means includes a pulsing circuit for repeatedly firing said trigger electrode to produce said successive metal plasma streams.

3. The device of claim 1 in which said metal plasma beam generator anode lies a certain distance from said cathode and includes an orifice for determining a transverse dimension of any of said plasma streams.

4. The device of claim 1 in which said metal plasma beam generated by said ion beam generator possesses a flux as high as 10¹⁷ ions per second.

5. The device of claim 1 which additionally comprises means for applying a magnetic field to any of said plasma streams produced by said means for generating metal plasma beam.

6. The device of claim 5 in which said means for applying a magnetic field to said plasma streams includes at least one turn of conductive material surrounding said means for generating a metal plasma beam.

7. The device of claim 6 in which said metal plasma beam generator anode includes an anode plate spaced from said cathode, said anode plate further including an orifice there at the central region thereof, and a channel passing from said central region to the periphery of said anode plate, said periphery of said anode plate lying in the vicinity of said means for applying a magnetic field.

8. The device of claim 1 which layers at least a pair of metallic films on a substrate and said metal plasma beam generator includes at least a pair of units each having an anode, a cathode a particular film material, and a trigger electrode.

9. The device of claim 8 in which said control means for pulsing said metal plasma beam generator to produce successivemetal plasma streams therefrom includesmeans for producing successive metal plasma streams from each of said units.

10. The device of claim 9 in which said particular material of said cathode of one of said pair of units differs from said particular material of said cathode of another one of said units.

11. A method of creating a metallic thin film on a substrate comprising a. generating a metal plasma beam utilizing a metal plasma beam generator including a unit having a anode, a cathode of the metallic film material, and a trigger electrode; b. pulsing said metal plasma beam generator to produce successive metal plasma streams of cathodic material of certain mass toward said anode, c. supporting the substrate a predetermined place apart from said plasma beam generator. d. providing a vacuum envelope for said plasma beam generator and said substrate. e. directing said successive metal plasma streams of cathodic material from said anode onto the substrate for depositing as a thin metallic film.