US 3501393 A
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March 17, 1970 wE-H ET AL 3,501,393
APPARATUS FOR SPU'I'TERING WHEREIN THE PLASMA IS CONFINED I BY THE TARGET STRUCTURE Filed May 5. 1967 A 2 FIG 4 Sheets-Sheet i Attorney March 17, 1970 K, W H ET AL 3,501,393 APPARATUS FOR SPUTTERING WHEREIN THE PLASMA 1s CONFINED BY THE TARGET STRUCTURE 4 Sheets-Sheet 2 Filed May 5. 1967 INVENTORS G.S. ANDERSON G. K. WEHNER March 17, 1970 Filed May 5. 1957 ION CURRENT T0 PROBE BIASED AT 2000 V0lI$(mA) ca. K. WEHNER ET AL 3,501,393 APPARATUS FOR SPUTTERING WHEREIN THE PLASMA IS CONFINED BY THE TARGET STRUCTURE 4 Sheets-Sheet 3 3AMP MAIN DISCHARGE CURRENT ZAMP MAIN DISCHARGE CURRENT L5 AMP MAIN DISCHARGE CURRENT c l I 1 FILM THICKNESS (A) NEGATIVE TARGET CAGE VOLTAGEIWiIh respect IoanodeITlMES I0 (VOLTS 4OOOA &
I l 0 IO 20 30 DISTANCE ALONG SUBSTRATE (gm) INVENTORS G.S. ANDERSON G.K.WEHNER A "or March 17, 1970 G. K. WEHNER ETAL 3,501,393
APPARATUS FOR SPUTTERING WHEREIN THE PLASMA IS CONFINED BY THE TARGET STRUCTURE Filed May 5, 1967 4 Sheets-Sheet 4 f f2 Fla INVENTORS G.S. ANDERSON G.K.WEHNER away.
Attorney United States Patent 3,501,393 APPARATUS FOR SPUTTERING WHEREIN THE PLASMA IS CONFINED BY THE TARGET STRUC- TURE Gottfried K. Wehner, Minneapolis, and Gerald S. Anderson, St. Paul, Minn., assignors t0 Litton Systems, Inc., Beverly Hills, Calif., a corporation of Maryland Filed May 5, 1967, Ser. No. 636,331 Int. Cl. C23c /00 US. Cl. 204298 22 Claims ABSTRACT OF THE DISCLOSURE The sputtering apparatus consists of an anode electrode and a cathode electrode mounted at spaced locations in an airtight chamber. A target cage mounted between the anode and the cathode consists of a plurality of spaced sections which define a given volume in a central part of the chamber and an outer region between the target cage and the walls of the chamber. The cage may be open at the top and bottom and is made from parallel rods or turns of a spiral member which define the given volume.
The method of using the target cage includes establishing a gas discharge plasma in the chamber and applying a negative voltage (with respect to the plasma) to the target cage. The plasma enters the given volume within the target cage and upon increasing the negative voltage applied to the cage, ion sheaths which form around the spaced sections overlap and confine the plasma to the given volume. Positive ions in the confined plasma bombard the spaced sections to sputter material. The sputtered material passes between the sections on substrates located in the outer region.
BACKGROUND OF THE INVENTION This invention relates to methods of and apparatus for sputtering, and more particularly, to an improved target structure and sputtering process in which a given voltage applied to the target structure is effective to confine a gas discharge plasma within a volume defined by the target structure and to cause positive ions to bombard the target to remove atoms therefrom.
The art of sputtering has been the subject of varying degrees of interest since its initial development in 1852. One form of sputtering, cathode sputtering, is a process which results in the injection of atoms from the surface of the cathode as a result of the impact of ions against the surface of the cathode. The sputtering process by which the ions strike the cathode and eject atoms is performed in an enclosed chamber, maintained at pressures of from 10" to about 50- torr, in which electrodes, such as an anode and the cathode, are mounted in spaced relationship. A source of DC potential is applied between the anode and the cathode to establish a potential difference of from 200 to several thousand volts, for example, the negative potential being applied to the cathode. The
' electrode that is sputtered is generally designated the target, and a substrate having a surface upon which the ejected atoms are collected may be positioned on the anode. The potential applied between the anode and the cathode produces positive ions in a gas discharge plasma in the chamber. The positive ions are acclerated toward the target, ejecting atoms'or molecules therefrom. The major part of the applied voltage appears as a voltage drop across the cathode fall adjacent to the cathode, thus the surface of the target is under steady bombardment by ions, the impact of which causes atoms of the target ma terial to leave the surface and to move away from the target. A percentage of the atoms reach the substrate surface upon which the film is formed. In this form of sputtering, the density of the plasma and the energy with which the ions bombard the target are directly related because both factors are governed by the gas pressure in the chamber.
Separate control over such factors as the plasma density and the target potential can be achieved in so-called triode sputtering systems in which an anode and an electron emitting cathode are used to control the plasma density, and a third separate electrode, the target, is biased with a potential for accelerating positive ions from the plasma toward the target to remove atoms from the target. In one commercial triode sputtering system, the substrate which is to be coated is mounted in the system parallel to a planar surface of the target for receiving a film having a uniform thickness. In such a system, a magnetic field having longitudinal field lines substantially parallel to the parallel surfaces of the substrate and the target is used in an effort to achieve uniform ionization of the plasma between the surfaces of the substrate and the target. Such use of a magnetic field within the sputtering chamber for the purpose of controlling the ionization of the plasma and therefore film thickness uniformity, can become a significant limitation when one attempts to deposit ferromagnetic films, for example, because the only magnetic field permissible in the region around the substrate is one which is used to control the magnetic properties of the ferromagnetic film.
In addition to the foregoing triode sputtering system, the use of a magnetic field parallel to a surface of a substrate to confine a plasma in a region around a target and the substrate in a sputtering chamber is shown in G. S. Anderson Patent No. 3,291,715. This patent also discloses a second chamber which extends through and outside of the sputtering chamber for confining the plasma. Openings in the second chamber permit positive ions from the plasma in the second chamber to enter the region around the target and to sputter particles from the target onto the substrate. While this arrangement confines the plasma to a volume that is not in significant heat transfer relationship with the sputtering chamber, it requires a special sputtering chamber which renders the arrangement impractical from a commercial standpoint.
A later development in triode sputtering apparatus employed a conventional hot cathode, substrate and target configuration, but housed these elements in a plasma tube to confine the flow of incoming gases to the volume around the substrate and the target. In this arrangement the size of the plasma tube is limited. This limits the size of a substrate which may be coated because the susbstrate is mounted in the plasma tube. Accordingly, substrates having large areas cannot be coated in such apparatus.
In an effort to provide a sputtering system capable of coating substrates having large areas and to provide an improved way of confining the plasma in a sputtering system, consideration has been given to prior target structures used in sputtering systems. For example, in one prior system, a plurality of spaced, parallel metal rods surrounded a sheet-like member which was to be coated. Alternate ones of the rods were electrically connected as cathodes which disintegrated to coat the member. However, such apparatus was not designed to confine a gas discharge plasma to a central volume away from the substrate to be coated. Rather, the apparatus achieved the opposite result in that the substrate was centrally located. Moreover, the spacing of the rods was not related to the possibility of confining the plasma. Finally, the electrical connection of alternate rods as cathodes indicated that it was not intended to use the rods as a way of confining the plasma to a central volume away from the substrate to be coated.
In addition, attempts have also been made to uniformly coat large area substrates by using a negatively biased target in the form of a single rod located along the central axis of both a chamber and a plasma maintained in a gas filled chamber by node and cathode electrodes. This arrangement was found to be deficient because the plasma tends to become non-uniform and unstable. In particular, slight unsymmetries cause the plasma density to increase in portions of the chamber. As a consequence, the gas is heated in this portion, the density thereof becomes lower and the plasma moves to another location. As a result, the main plasma shifts its position and rotates around the target, rendering it difficult to control deposition rates and uniformity of film thickness on a substrate adjacent to the inner circumference of the chamber.
Further, reference was made to conventional hollow cathode configurations where the cathode is a solid tube or hollow cylinder which is placed in a gaseous atmosphere. The cylinder is biased negatively relative to a plasma which forms inside the tube. Positive ions bombard the inside wall of the cylinder and sputter particles therefrom onto an article to be coated which is received within the cylinder. Because the cylinder is solid and the article to be coated must be received within the cylinder, this type of cathode configuration was not available for use in coating substrates having large areas.
THE PRESENT INVENTION In view of the lack of disclosure in the prior art of suitable ways to confine the plasma ina sputtering system in a practical way, research was conducted to develop an apparatus and method which would not substantially limit substrate size or require additional structure, such as a plasma tube or secondary chambers in a primary enclosure, and which would be effective to confine the plasma without the use of a magnetic field. As a result of such research, a target structure and process of sputtering using such structure have been developed by which problems of plasma shifting have been overcome. One aspect of such structure and process permits the plasma within a sputtering chamber to be confined without the concomitant disadvantages of prior art attempts to confine the plasma. In particular, the target structure of the present invention consists of an element having a plurality of spaced sections which form a target cage or cage-like structure which defines a given volume. In one embodiment, a cylindrical target cage includes a plurality of elongated, spaced members disposed substantially parallel to one another and arranged, for example, so that the given volume is cylindrical.
In general, and in a triode sputtering configuration, the cylindrical target cage is mounted in a chamber parallel to the longitudinal axis of the chamber and spaced therefrom between spaced anode and cathode electrodes so that a plasma established by such electrodes at least partially passes through the cylindrical target cage. In another, and more particular embodiment, the members are spaced relative to each other and to the cathode and the anode such that'when a selected sputtering voltage is applied to the target cage, ion sheaths which form around the members at least touch, and preferably overlap each other. In this manner, the gas discharge plasma within the chamber is confined within the given cylindrical volume.
Advantages resulting from the target cage and methods of using the target cage are that the gas discharge plasma does not shift its position or rotate within the chamber because it is confined within the target cage. Further, the spacing between the members of the target cage permits neutral sputtered atoms to move outwardly from the given volume for deposition on an article located in a region outside such volume. Because the article need not be within the given volume, substantially increased power may be used without proportionately increasing the substrate temperature.
In addition, the substrates may be as large as can be accommodated within the space between the members of the target and the walls of the chamber. Because chambers presently used for evaporative coating are fabricated having diameters up to ten feet, for example, the substrate size is not limited from a practical standpoint in the present invention.
A further advantage of the present invention is that a percentage of the ions which are accelerated toward the target members have a curved trajectory and strike the members of the target cage at an oblique angle. As is well known, the sputtering yield is increased when ions strike a target at an oblique angle, rather than at a normal angle. Moreover, even though the members of the target cage may, in one embodiment, define a cylindrical volume, films having improved uniformity of thickness are deposited on the surface of the substrate.
Distinguished from targets which are small and are mounted opposite to the substrate to be coated, the target cage of the present invention has a large surface area. As a result, sputtered material is projected onto all surfaces of the chamber. This enhances the gettering of impurities and promotes clean discharge conditions. The present target cage further permits the development of a high degree of ionization of the gas in the chamber. With a high degree of ionization, there is a pumping action in a direction toward the given volume because ions can leave such volume only as neutrals. Thus, despite the spaced relationship of the members of the target cage, a lower gas pressure results in the substrate region than in the given volume. The lower gas pressure in the substrate region is often desirable if one wants to reduce the noble gas content of the films deposited onto the substrate.
In the event a long substrate is to be coated or if the substrate is a sheet which is wound on a reel and has a width of many feet, for example, the target cage can easily be lengthened within the teachings of the present invention and still retain the plasma receiving and confining capabilities. Further, as will now be obvious, the confinement of the plasma is achieved by the target cage of the present invention without using a magnetic field. As a result, ferromagnetic films, for example, may be deposited onto the substrate.
It is an object of the present invention to provide new and improved methods and apparatus for sputtering.
Another object of the present invention resides in the provision in a sputtering chamber of a target cage having a plurality of sections which are spaced for receiving a gas discharge plasma and for confining such a plasma to a given volume upon the application of a suitable potential to the target cage.
A further object of the present invention is to provide a method of using a target cage for confining a gas discharge plasma to a given volume in a coating chamber without the use of a magnetic field.
DESCRIPTION OF THE DRAWINGS These and other objects of the present invention will become apparent upon reference to the following description of the preferred embodiments of the present invention and to the appended drawings illustrating such preferred embodiments, in which:
FIG. 1 is a cross-sectional view of a triode sputtering apparatus including a target cage adapted to perform the process of the subject invention for confining a gas discharge plasma within a given volume defined by the target cage;
FIG. 2 is a plan view taken along line 2-2 of FIG. 1 showing the target cage as including an element having a plurality of spaced sections, wherein the sections are formed by a series of parallel, elongated rods which define a cylindrical volume;
FIG. 3 is an elevational view of another embodiment of the present invention showing a sputtering apparatus in which the cathode is in the form of the target cage shown in FIGS. 1 and 2;
FIG. 4 is a plan view taken along line 44 of FIG. 3 showing substrates to be coated mounted outside of the volume defined by the target cage;
FIG. is an enlarged, partial plan view of the disclosure of FIG. 2 showing the parallel, elongated rods spaced such that upon the application thereto of a selected potential an ion sheath formed around each rod forms a substantially continuous ion sheath which is effective to substantially confine the plasma within the given volume defined by the target cage;
FIG. 6 is a plan view of a single electrode introduced into a plasma and having a negative potential (with respect to the plasma) applied thereto for forming an ion sheath therearound; and
FIG. 7 is an elevational view of a target cage in the form of a mesh for confining the plasma according to the present invention;
FIG. 8 is a graph illustrating the effect of various target potentials on the amount of plasma confinement achieved;
FIG. 9 is a graph showing improved film thickness uniformity produced by the target cage shown in FIG. 10;
FIG. 10 is an elevational view of a further embodiment of the target cage designed to produce films having improved thickness uniformity;
FIG. 11 is an elevational view taken along line 1111 in FIG. 3 showing substrates to be coated;
FIG. 12 is an elevational view of a spiral target cage; and
FIG. 13 is a partial plan view showing the target cage formed from hollow rods to permit cooling of the target cage.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2 of the drawings, performance of the process of sputtering may be generally understood to require an apparatus 10 including an airtight chamber 12 mounted on and sealed to a base 14. A vacuum pump (not shown) is provided to evacuate the chamber 12. A supply 18 of suitable gas, such as argon, is introduced to the chamber 12 through a port 20 so that the environment within the chamber 12 is maintained at a pressure of 10" to 10* torr, for example. In a triode sputtering arrangement, the chamber 12 encloses an anode or anode electrode 22 and hot cathodes or cathode electrodes 24. The cathodes 24 are mounted in a standard manner in a cathode housing 25 having a neck 26 which opens upwardly toward the anode 22. The anode 22 and the cathodes 24 are connected, via respective positive and negative leads 6 and 28, to a voltage source (not shown).
In operation, the pump exhausts the chamber 12 and gas, such as argon, is introduced to the chamber 12 through the port 20 to a pressure of about 10- to 10 torr, for example. A potential of about 300 volts applied in series with a resistor (not shown) across the anode 22 and the cathodes 24 is effective to ionize the argon and to establish a gas discharge plasma including electrons and positive ions in the chamber 12. When the discharge is in operation, the voltage between the anode 22 and the cathodes 24 is about 30 volts, with the remaining 270 volts representing the voltage drop across the resistor (not shown).
Referring now to FIG. 6, when an element, such as an electrode 30, is introduced into a gas discharge plasma, such as that described above with respect to FIGS. 1 and 2, and when a negative potential (with respect to the plasma) is applied to the electrode 30 (by means of a conductor 31, for example), it is known that almost the entire voltage applied to the electrode 30 is dropped across an ion sheath 32 that surrounds the electrode 30. The potential applied to the electrode 30 accelerates the ions toward the electrode 30-. The impact of the ions against the electrode 30 removes atoms or particles from the surface of the electrode. The atoms removed are free to move away from the electrode 30. Part of the atoms strike a substrate 34 to form a film or a deposit 36 thereon.
An ion sheath which forms in the foregoing manner on an electrode, such as the electrode 30, may be defined as a region between the plasma and an electrode from which electrons from the plasma are repelled by the negative potential on the electrode 30. The ion sheath is dark because the absence of the electrons prevents the gas in the region from being excited. Ions which arrive at the edge of the ion sheath by random or thermal movement are accelerated by the potential applied to the electrode 30 so that the ions bombard the electrode with high energy.
The Langmuir space charge equation (for the plane case) is discussed in chapter VI of Gaseous Conductors, by I. D. Cobine, 1941, and may be used to relate the thickness d (FIGS. 5 and 6) of the ion sheath to the plasma conditions in the chamber 12 as follows:
The random ion current j+ in the plasma is defined as: i where:
n the number of ions/cm? (ion density), v=average velocity of the ions in the plasa, and e=the charge on a single ion.
The ion current density j+, the voltage U across the ion sheath, and the ion sheath thickness d are related by:
U=the voltage applied between the target and the plasma.
and d=the thickness of the ion sheath around the target.
Substituting Equation 2 into Equation 1 a/z V FN/HWB e is, of course, always constant, and v changes only very little with the discharge conditions. Thus, if U is kept constant, and n is increased by increasing the discharge current, d must become smaller. With the discharge current kept constant, then d increases in proportion to U It is noted that the gas pressure is not a major factor in Equation 3. Thus, :at least within certain limits, the gas pressure is not considered to be a critical factor in the range of pressures to which Equation 1 is applicable. However, from the standpoint of obtaining maximum sputtering rates, at pressures in the torr range the mean free path of the atoms and ions is shorter and as a result, the average bombarding energies of the ions are significantly less than that in the milli-torr range because the ions lose much of their energy in collision. Consequently, the sputtering process becomes very ineificient. With these considerations in mind, it is preferable to use gas pressures much below 1 torr, such as in the 10" to 10 torr range.
Equation 3 is also of interest in achieving maximum sputtering rates. Such rates may in general be achieved by using high discharge currents because this increases the bombarding ion current density j+. From Equation 3, if the ion current density j+ increases, the target voltage must be increased in order to keep the ion shetth thickness constant. The increase in target voltage is elfectively limited by the available cooling of the target. When a noble gas discharge is established in a triode sputtering configuration, such as that shown in FIGS. 1 and 2, the discharge current is limited by the capability of the thermionic cathodes 24 to supply the necessary current.
To determine specific ion sheath thicknesses d for various noble gas plasmas, plasma densities, and target voltages, reference may be made to appropriate graphs at page 508 of 'M. Von Ardenne, Tabellen der Electronenphysik, Ionenphysik and 'Ubermikroskopie, VEB Deutscher Verlag der Wissenschaften, Berlin, 1956.
Referring again to FIGS. 1 and 2, the chamber 12- is shown having a longitudinal axis 38. The anode 22 and the neck 26 of the cathode housing 25 are shown mounted within the chamber 12 in spaced relationship along the axis 38. In the space between the anode 22 and the cathodes 24, a new and improved target electrode 40 is depicted. In general, the target electrode 40 may be considered to consist of an element 42 having a plurality of spaced section 44. The sections 44 may be arranged as shown in FIGS. 2 or 12, for example, for defining a target cage or cage-like structure 45 (FIG. 1) or 146 (FIG. 12) which encompasses a given volume V.
As shown in FIG. 1, a shield plate 47 is mounted on the neck 26 for shielding the cathode housing 25 and the feed-through connections, such as the port 20, from sputtered material. The shield plate 47 also supports a glass ring 49.'An annular support member 51 is received in the ring 49 for mounting three rods 52 which are secured to three of the spaced sections 44 for mounting the target cage 45 between the anode 22 and the neck 26 of the cathode housing 25. The outer circumferential portion of the member 51 extends outwardly sufficient to protect the outer wall of the glass ring 49 from material sputtered from the target cage 45 so that the target cage 45 remains electrically insulated from the shield plate 47.
The target cage 45 further includes supports 53, such as rings, which are provided with apertures for receiving the sections 44 at the top 54 and at the bottom 55 thereof. The sections 44 are provided with tabs 56 on opposite sides of the ring 53 to maintain the ring in position while permitting the sections to expand and contract. The target cage 45 is open at the top 54 and at the bottom 55.
As shown in greater detail in FIG. 5, in one embodiment of the target cage 45, the rings 53 mount the sections 44 in parallel, spaced relationship with respect to each other at distances equal to, at most, about two times the thickness d of ion sheaths 57 which are formed around the spaced sections 44 upon impression of a negative potential thereon by wayof an insulated conductor 58 (FIG. 1) connected to a voltage source (not shown). With a plasma established in the chamber 12, and with a relatively low negative potential (with respect to the plasma) applied to the sections 44, the plasma will at least partially pass through or extend into the given volume V defined by the target cage 45. The plasma is thus at least partially received within the target cage '45.
As higher negative potentials are applied to the target cage, i.e., as the target potential becomes more negative relative to the plasma, the ion sheaths 57 which form around each section 44 will at least touch or, as shown in FIG. 5, will overlap to form a substantially continuous ion sheath 59, which is effective to define substantially the given volume V defined by the target cage 45. The continuous ion sheath 59 is effective to confine the plasma to the given volume V so that a region R between the walls of the chamber 12 and the target cage 45 is substantially plasma-free.
As shown in FIG. 5, the continuous ion sheath 59 may have portions 60 thereof which tend to extend between the spaced sections 44 of the target cage 45. Positive ions from the plasma are accelerated from all points along the boundary between the continuous ion sheath 59 and the plasma. Such ions cross the continuous ion sheath 59 and bombard the spaced sections 44 with high energy. Ions accelerated from the portions 60 of the continuous ion sheath 59 between the spaced sections 44 have a curved path or trajectory (shown by arrows 61) as they proceed toward the surfaces of the spaced sections 44. As a result of the curved trajectories, the outside surfaces, as well as the inside surfaces, of the sections 44 are sputtered. Moreover, significant numbers of ions bombard the spaced sections 44 obliquely and increase the number of atoms sputtered per incident ion (sputtering yield).
The sputtered atoms from the target structure 40 proceed through openings or spaces 62 between the spaced sections 44 into the plasma-free region R. While articles 63 (FIG. 5) to be coated with the sputtered atoms in the plasma free region R may be many and varied, there is shown in FIG. 1 a sheet 70, such as a foil or a film, which may form the article to be coated. The sheet 70 may be formed from a material such as Kapton foil (polyimide), and may have an indefinite length wound upon a supply reel 74 (FIG. 2) received within the chamber 12. The sheet 70 passes from the reel 74 over guide rollers 76 in a generally arcuate path through the plasmafree region R where it receives the sputtered atoms. The sputtered atoms form a film on the sheet 70 having a thickness governed by the sputtering rate and the rate of advancement of the sheet 70 from the supply reel 74 to a takeup reel 78.
An advantage of the present form of target cage 45 is that sheets 70 of different widths W may be easily accommodated by providing a chamber 12 having a height (where the axis 38 of the chamber 12 is vertical) or a length (where the axis 38 of the chamber 12 is horizontal) similar or larger (for uniformity of film) to that of the width W of the sheet 70. Uniform deposition of films onto such different width sheets 70 is obtained by providing the spaced sections 44 of the target structure 45 with a length substantially longer than the width W of the sheet 70.
As shown in FIGS. 1, 2 and 5, the spaced sections 44 may be provided in the form of rods having circular cross-sections, for example. The rods 44 may easily be designed so as to provide a relatively large supply of material to be sputtered. More particularly, if an ion current density of 10 ma./cm. is provided at the rod surface and a sputtering yield of 5 atoms/ion is achieved by applying to the rods a negative potential (with respect to the plasma) of 3000 volts, for example, about 300 atom layers per second would be sputtered form the rods. Under such conditions, a 1 mm. diameter rod 44 would be completely sputtered in 30 hours. If all the material to be sputtered from twelve such rods 44 were deposited on the inner wall of a 40 cm. diameter chamber 12, for example, a film having a thickness of about 10 microns would be deposited. Because a 30-hour life for the rods 44 is relatively short, larger rod diameters may be used to provide an increased supply of material to be sputtered.
While there is no definite lower limit for the diameter of the rods 44 from the material supply point of view, the rods 44 should have diameters which are large enough to provide sufiicient heat transfer therefrom so as to prevent the temperature of the rods from reaching the melting point. It may be understood that the thinner the'rod 44 becomes, the higher the bombarding ion current density per unit surface area of the rod, hence the rod becomes hotter. Therefore, the melting point of the material used to fabricate the rods 44 must be considered in selecting a rod diameter. For example, in the case of rods 44 fabricated from tungsten or tantalum, relatively high rod temperatures can be tolerated.
The maximum preferable diameter of the rods 44 is selected to permit most of the material sputtered from the rod surfaces which face the inside of the target structure to escape to the outside of the given volume V. In general, an open space between the rods 44 of about 5 times the roddiameter is preferred.
The diameter of the target cage 45 shown in FIG. 1 may be determined by the diameter of the chamber 12 or by the film uniformity requirements. For example, with a 40 cm. diameter chamber 12, a target cage 45 of not more than about 5.5 cm. in diameter is suitable. If the ratio of chamber diameter to target structure diameter is too small, such as when it is less than 3, the material deposited shows a marked periodicity in thickness. It has been determined that when this ratio is larger than about 8, such periodicity in film thickness on the inner wall of the chamber 12 is reduced to a few percent. In practice, as this ratio becomes higher, more uniform films are obtained. However, the ratio should not be so high as to cause very thin rods 44 to be used, because such thin rods are subject to overheating and may not provide enough supply of material to be sputtered.
It should be understood that with a chamber 12 of several meters diameter, the target cage diameter may be larger and larger diameter rods 44 may be used. As an xample, for a 3-meter diameter chamber 12, the rods may have a 1 cm. diameter and the diameter of the target cage 45 may be 20 cm., for example. Thus, a large supply of target material is available and the rods 44 may be hollow, as shown in FIG. 13, to receive. coolant, such as water.
With these rod diameter and target cage diameters considerations in mind, and in accordance with the present invention, effective confinement of the plasma may be achieved when the ion sheath 57 of the neighboring rods 44 at least touch and preferably overlap, i.e., the iron sheath thickness d should be at least one-half the distance between the rods 44.
Referring to FIGS. 3 and 4, the principal features of the present invention may be utilized in a hollow cathode type sputtering configuration wherein the cathode and the target functions are combined in one target cage 90 which is configured similar to the target cage 45 shown in FIGS. 1 and 2. In FIGS. 3 and 4, the target cage 90 is provided as a hollow cathode electrode which defines the given volume V. The cage 90 is connected to a negative terminal 92 of a voltage source (not shown). The base 14 forms an anode 94 spaced from the target cage 90 upon connection to a positive terminal 95 of the power supply (not shown).
In the operation of the sputtering apparatus shown in FIGS. 3 and 4, the chamber 12 is evacuated and gas, such as argon, is introduced into the given volume V to establish a pressure of about 50 microns, for example. A potential of 5000 volts, for example, is applied across the anode 94 and the target cage 90. The spacing maintained by rings 97 between sections 96 of the target structure 90 is such that at the 5000 volt potential, a continuous ion sheath 98, such as that shown in FIG. 4 is formed. The plasma is confined by the continuous ion sheath 98 to decrease the plasma density in the plasma-free region R between the walls of the chamber 12 and the spaced sections 96. The positive ions from the plasma are accelerated, bombard the target cage 90 and sputter atoms from the target cage 90. Because the cage 90 is open rather than being a closed cylinder or tube, the sputtered atoms are free to move out of the cage 90 and form films 99 on substrates 100 which are mounted in substrate holders 101 in a circular path in the plasma-free region R. While not shown in FIGS. 3 and 4, the holders 101 may rotate relative to the anode 94, for example, around the axis 38 to provide increased film thickness uniformity on the substrates 190.
Referring to FIG. 7, a target cage 103 similar to the target cage 45 may be fabricated from a mesh material which also defines the given volume V. Sections 104 of the mesh material are spaced so that a continuous sheath 56 forms within the cage 103. In this embodiment, as well as in the other depicted embodiments, the top 54 o the cage 103 may include inwardly extending sections 105 which surround and enclose an anode 106 received within the given volume V. Similarly, at the bottom 55 of the cage 103, the cage 103 surrounds a thermionic cathode 107 which extends upwardly through the shield plate 47. In FIG. 7, the cage 103 is shown cut-away to reveal the extension of the thermionic cathode 107 into the cage 103. This construction of the cage 103 surrounding and enclosing the anode 106 and the cathode 107 is effective to achieve maximum confinement of the plasma.
In an experiment with triode sputtering apparatus of the 10 type shown in the FIGS. 1 and 2 of the drawings highly uniform, Well sticking, solderable films 99 of stainless steel were deposited on a Kapton (polyimide) foil substrate which was mounted against the cylindrical part of the inner wall of a 42 cm. diameter chamber 12. The dimensions of the substrate were cm. by 55 cm. The target cage 45 was fabricated from twelve stainless steel rods having a 55 cm. length and a 3 mm. diameter. The rods were equally spaced around a circle having a 5.5 cm. diameter such that the chamber diameter to target cage diameter ratio was about 7.6. The target cage 45 was mounted in the chamber 12 with its longitudinal axis coincident with the longitudinal axis 38 of the chamber 12.
The following operating conditions were used:
( 1) argon gas pressures: 1 micron;
(2) thermionic tungsten cathode with 7.5 volt filament voltage and 43 amps fila-ment current;
(3) anode-cathode voltage of 40 volts with a 4 amp discharge current;
(4) target voltage (with respect to the anode) of minus 1000 volts with a 1 amp target (ion) current.
Under these conditions, a deposition rate of about 60 A./min. was obtained. Those skilled in the art will realize that the operating conditions are indicated by way of example and may be varied to achieve greater deposition rates. For example, with the target cage 45 cooled, the discharge may be increased to 10 or more amps, thereby increasing the target current to more than 3 amps. With this increased power input, and with a 200() volt target voltage, a much higher deposition rate may be achieved.
The degree of plasma confinement with this arrangement was determined by measuring the ion current to a negative probe (not shown) mounted in the region R. Referring to FIG. 8, the probe current is shown plotted against the negative voltage (relative to the anode) applied t o the target cage 45. As shown, the ion current to the negative probe (biased at 200 volts) decreased from a maximum value as the target voltage became more negative. Also, with increased discharge current, the negative target voltage required to produce the same probe current increased. FIG. 8 also indicates that even at such high negative target as 800 volts, the probe current did not decrease to zero. The remaining probe current results from gamma electrons released by ion bombardment at the target and from ultraviolet radiation which provides a small amount of ionization in the region R. Becausethe remaining probe current is of minimal value relative to that present when the potential is not applied to the target cage 45, the plasma confinement is substantial.
These experiments indicate that to achieve best results in confining the plasma, the plasma should be clean, i.e., the background gases in the chamber 12 should be reduced to a minimum. A suitable way to achieve a clean plasma is to maintain the plasma in the entire chamber 12 for a certain period, such as 10 minutes, before applying the target voltage to the target cage 45.
Referring now to FIG. 13, a portion of the target cage 45 of the present invention is shown to indicate how the target cage 45 may be water cooled to permit use of higher input power, In this embodiment, the sections 44 may be fabricated from hollow tubing which is secured to the rings 53, which are also hollow. Ports 108 connect the rings 53 to the sections 44 to permit the passage of a suitable coolant, such as water, into the sections 44. In addition, in FIG. 13, an electrode 109 is shown extending through each hollow section 44. The electrodes 109 may be provided in this manner when the sections 44 are fabricated from insulating material, such as glass or quartz, to permit the application to the target cage 45 of a radiofrequency signal from a supply (not shown). The radio frequency signal is effective to sputter the insulating material during the negative portion of the signal.
The individual sections 44 of the target cage 45 may be fabricated from different target materials and may be connected, for example, to separate power supplies to facilitate application of different negative potentials of the different target materials. In particular, if it is desired to deposit a tungsten carbide film 99 on a substrate 100 (FIGS. 3 and 4) for example, alternate rods 44 in FIGS. 3 and 4 would be fabricated from tungsten and carbon. To deposit a film 99 of W C, for example, the carbon rods would be electrically insulated from the tungsten rods, and the target voltages applied to the tungsten rods 44 and to the carbon rods 44 would be adjusted such that the tungsten sputtering rate would be twice that of the carbon to produce the desired deposition of the W C film on the substrate 100. In accordance with the teachings of this invention, as long as the target potentials are sufficient to form the continuous ion sheath 59, the plasma is confined and the film 99 would be deposited in the substantially plasma-free region R. Using the target cage 45 in this manner, those skilled in the art can readily de velop other arrangements for depositing films 99 formed from different materials.
The target cage 45 of the present invention may also be designed to provide selected uniformity of film thicknesses deposited on the substrates 70 or 100. In particular, experiments were conducted with the following dimensions used for the target cage 45 shown in FIG. 2:
(1) target cage 45 configuration--cylindrical;
(2) rod material-stainless steel;
(3) rod 44 diameter-3 mm.;
(4) number of rods 4412;
(5 target cage diameter5 .5 cm.;
(6) rod 44 length37 cm.;
(7) rods 44 held together by two rings, one ring being 3 cm. from the top and one ring being 3 cm. from the bottom of the target cage 45;
(8) substrates2.5 cm. x 7.6 cm. flat glass slides;
(9) substrate location-(a) 14 cm. from the nearest rod 44, mounted in holders as shown in FIGS. 3 and 11 parallel to the axis 38 of the chamber 12; (b) with holders spaced along an arcuate path at a distance of 17 cm. from the rods 44 at a height of 16 cm. above the base plate 14.
The target cage 45 was sputtered and a film of stainless steel was deposited on each substrate 100. As shown in FIG. 11, a central section 110 was covered with an aquadag coating prior to deposition so that the film deposited thereon could be removed to provide a step for measuring the film thickness. Film thickness measurements were taken at vertically spaced locations along the substrates 100 and were plotted on a graph 120'shown in FIG. 9. Graph 120 shows that a maximum film thickness of about 6000 A. was deposited on the middle portion of the slides 100, where the 20 cm. value of the abcissa generally indicates the middle, the 6 cm. value generally indicates the bottom of the slides 100, and the 34 cm. value generally indicates the top of the uppermost slides. Over the vertical length of the slides 100, the film thickness varied from about 3700 A. at the bottom, to about 6000 A. in the middle, and to about 4300 A. near the top. Measuring at a given elevation the film thickness of the slides located circumferentially, the variation in film thickness did not exceed about The use of more accurate film thickness measurement equipment woud indicate less variation.
The decrease in film thickness at the top and bottom of the substrates 100 (as shown in graph 120) was substantially reduced using the target cage 130 shown in FIG. 10. The cage 130 is the same as the cage 45 described immediately above, except that additional rings 132, 134, 136, 138, 140, 142 and 144 were secured to the rods 44. The rings 132, 134 and 136 were spaced vertically at 1.5 cm. and 3 cm. and 6 cm., respectively, from the top ring 54. Similarly, the rings 144, 142 and 140 were spaced vertically at 24.5 cm., 27.5 cm. and 29.5 cm. from the top rings 54.
The substrates were arranged as described above, and the rods 44 were sputtered to provide the film 99 on the substrates 100. The film thicknesses measured at successive vertically spaced locations along the substrates 100 are shown in graph of FIG. 9. It is apparent that the maximum film thickness variation was reduced to about 1200 A. Also, along 16 cm. vertical distance on the sides 100, graph 150 indicates that there was only about a 200 A. variation in film thickness.
It is to be understood that neither the target cage 45 having only the rings 46, nor the target cage 130 having the rings 132, 134, 136, 138, 140, 142 and 144, were designed to produce the least possible variations in film thickness in the vertical direction. From FIG. 9, however, it is obvious that the addition of more rings, such as the rings 132 and 144 will be effective to produce films having substantially no variations in thickness over the entire vertical distance of the substrates.
In addition, those skilled in the art will now recognize that the teachings of the present invention may be also be used to provide a spiral target cage 146 which also defines a given volume V as shown in FIG. 12. The pitch 152 between turns 154 of the spiral may be varied to provide more available material to be sputtered at the top 156 and the bottom 1580f the target cage 146 so that maximum film uniformity results. In the use of the spiral target cage 146 to confine the plasma, the target potential should be such as to produce ion sheaths around the turns 154 which are equal to at least onehalf the value of the greatest pitch 152 separating the turns 154.
In addition, during the operation of apparatus 10 according to the present invention, those skilled in the art will recognize advantages resulting from the use of relatively high target potentials. When such high target potentials are used, sparks may appear adjacent to the cage 45 and the discharge may thereby be disturbed to such a degree that it will not fire and will, therefore, become extinguished. To maintain the plasma under such conditions, it is desirable to provide an auxiliary anode 160 (as shown tn FIG 10) below the target cage 45 to draw electrons into the target cage 45 and to provide sufficient plasma between the auxiliary anode 160 and the cathode for reestablishing the main gas discharge plasma to the anode 22.
Considering the method of the present invention, it may be understood that various approaches may be taken by one skilled in the art to effect confinement of the plasma. Thus, the sections 44 of the target cage 45 may be spaced by specific distances which are selected with a desired voltage in mind. On the other hand, the method of the present invention permits such spaces to be selected to facilitate ease of manufacturing, for example, with less regard for the target voltage. In the performance of the method, thetarget cage 45 is mounted in the chamber 12 and the plasma is established therein by applying a potential across the anode 22 and the cathode 24. Then, a suitable target potential is applied to the target cage 45 to cause the ion sheaths 57 to at least touch, and preferably, to overlap and form the continuous ion sheath 59.
It is to be understood that the above-described arrangements are simply illustrative of an application of the principles of the present invention. Numerous other arrangements may be devised by those skilled in the art which will embody the principles of the invention and will fall within the spirit and scope thereof.
What is claimed is:
1. Apparatus for depositing coatings of materials on the surfaces of substrates by sputtering, comprising:
an enclosure having a longitudinal axis;
means for evacuating said enclosure and providing an ionizable atmosphere therein;
electron receiving anode means mounted in said enclosure;
target means made from said material to be sputtered;
power supply means for applying to said anode means a positive electrical potential with respect to said target means for establishing a plasma in said enclosure;
said target means consisting of a plurality of spaced sections forming a generally cylindrical cage which is open at both ends and which defines a given volume, one of said ends being mounted adjacent to said anode means such that said plasma at least partially passes through said cage;
said power supply means being effective to apply a negative potential to said target cage to cause attraction of positive ions to said target cage so that said material is sputtered by ion bombardment; and
means for mounting said substrates in said enclosure outside of said target cage for receiving said material sputtered from said target cage.
2. Apparatus according to claim 1, in which:
said target cage surrounds said anode means.
3. Apparatus according to claim 1, in which:
said target cage and said anode means are mounted along longitudinal axis.
4. Apparatus according to claim 17, in which:
said target cage has a longitudinal axis and consists of parallel rods held in spaced relationship by a plurality of rings.
5. Apparatus according to claim 17, in which:
said means for mounting said substrates is arranged to position said substrates concentrically in said enclosure and outside of said target cage for receiving uniform deposits.
6. Apparatus according to claim 17, in which:
said means for mounting said substrates is sheet supporting means for guiding said substrates in a circular path adjacent to the inner wall of said enclosure.
7. Apparatus according to claim 17, in which:
said means for mounting said substrates is a holder which moves said substrates along a path between said target cage and said enclosure.
8. Apparatus according to claim 17, in which:
said target cage has a longitudinal axis and consists of a spiral member having at least one turn.
9. Apparatus according to claim 8, in which:
said turns of said spiral member are spaced by pitch distances adjusted for obtaining uniform deposits of said material on said substrates outside of said target cage, said deposits being uniform in a direction parallel to said longitudinal axis.
10. Apparatus according to claim 8, in which:
said turns of said spiral member are spaced by pitch distances such that ion sheaths which form around said turns overlap and confine said plasma essentially inside said target cage.
11. Apparatus for depositing a film of material on the surface of a substrate by sputtering, comprising:
an enclosure having a longitudinal axis;
means for evacuating said enclosure and providing an ionizable atmosphere therein;
electron emitting cathode means and electron receiving anode means mounted in a physically separated relationship in said enclosures;
means for applying to said anode means a positive electrical potential with respect to said cathode means to establish a plasma in said enclosure;
a separate target made of said material to be sputtered,
said target consisting of a plurality of spaced sections forming a cage which defines a given volume and having openings at both ends thereof, said cage being mounted between and closely spaced to said cathode means and said anode means so that said plasma at least partially passes through said cage; and
power supply means for applying a negative potential to said target cage to cause attraction of positive ions thereto for sputtering said target cage by ion bombardment.
12. Apparatus according to claim 11, in which:
said material to be sputtered is an electrical insulator;
said spaced sections are hollow;
electrode means are received within said hollow sections; and
said power supply means is a radio frequency power supply connected to said electrode means for applying a radio frequency signal to said hollow sections to cause said insulated material to be sputtered and deposited in the form of an electrically insulating film on said substrate.
13. Apparatus according to claim 11, including:
means for mounting said substrate in said enclosure outside of said target cage for receiving said material sputtered from said target cage.
14. Apparatus according to claim 13, in which:
said target cage surrounds and encloses said anode means.
15. Apparatus according to claim 13, in which:
said target cage surrounds both said anode means and said cathode means.
16. Apparatus according to claim 13, in which:
said cathode means is a thermionic cathode.
17. Apparatus according to claim 13, in which:
said target cage, said cathode means and said anode means are mounted in spaced relationship along said longitudinal axis of said enclosure.
18. Apparatus according to claim 13, in which:
said target cage is fabricated in the form of a spiral having pitch distances thereof selected for obtaining uniformly thick deposits of said sputtered material on said substrate.
19. Apparatus according to claim 13, in which:
said target cage includes a plurality of parallel rods spaced substantially equally around a generally circular path for defining a generally cylindrical volume.
20. Apparatus according to claim 13, in which:
said target cagepinclude a plurality of parallel rods mounted parallel to said longitudinal axis, said rods being held in spaced relationship by a plurality of rings.
21. Apparatus according to claim 20, in which:
said rings are mounted perpendicular to said axis and are spaced in the direction of said axis to obtain uniform deposits of said sputtered material on said substrates.
22. Apparatus according to claim 20, wherein:
said rings space said rods substantially equally along a circular path having a first diameter; and
said enclosure has a second diameter, the ratio of said second diameter to the first diameter being at least 3.
No references cited.
JOHN H. MACK, Primary Examiner S. KANTER, Assistant Examiner NITED STATES lATENT oEEICE CERTIFICATE o-F CORRECTION Patent No. 3,501,393 March l7, 19.70
Gottfried K. Wehner et al.
It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:
Column 1, line 47, "injection" should read ejection Column 3, line 2, "node" should read anode Column 6, line 22, "plasa" should read plasma Column 8, line 38, "form" should read from Column 10, line 43, after "target" insert voltages Column 12, line 42, "tn" should read in In the claims, in Claims 4, 5, 6, 7 and 8, delete "l7" and insert l Signed and sealed this 15th day of' September 1970.
Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.
I Attesting Officer Commissioner of Patents