WO1998013532A1

WO1998013532A1 - A multiple target arrangement for decreasing the intensity and severity of arcing in dc sputtering

Info

Publication number: WO1998013532A1
Application number: PCT/US1997/016723
Authority: WO
Inventors: Norman L. Boling; Byron A. Wood
Original assignee: Deposition Sciences Inc
Current assignee: Deposition Sciences Inc
Priority date: 1996-09-24
Filing date: 1997-09-18
Publication date: 1998-04-02
Anticipated expiration: 1999-03-24

Abstract

An arrangement of targets (50, 51) simultaneously sputtering material on a substrate (52) within a sputtering chamber in which multiple targets (50, 51) are individually connected to separate power supplies (41, 42) having arc suppression systems (45, 46). The arrangement uses a shaped magnetic field to provide enhanced arc suppression capability.

Description

A MULTIPLE TARGET ARRANGEMENT FOR DECREASING THE INTENSITY AND SEVERITY OF ARCING IN A DC SPUTTERING

TECHNICAL FIELD OF THE INVENTION The present invention is a method for reducing the frequency and severity of arcing during a DC reactive sputtering process. Arcs cause material to be ejected from the site of the arc, and some of this material travels to the substrate where it forms defects in the sputtered coating on the substrate surface. Arcs also disrupt the functioning of the sputtering power supply, causing interruptions in sputtering which can result in unacceptable loss of process control and instability of process parameters.

Arc suppression is therefore a critical factor that must be considered when designing any sputtering system and arc suppression features are normally incorporated into the target power supply of such a system. This invention enhances the arc suppression performance of prior art sputtering power supplies which employ voltage interruption or reversal as a means for arc suppression together with the use of a shaped magnetic field.

BACKGROUND OF THE INVENTION

Electrical arcing during the coating of insulating material onto a substrate by a process known as reactive DC sputtering is often a severe problem. In such a process, particularly one involving reactive DC sputtering, the onset of arcing as increasing electrical power is applied to the sputtering target sets an upper limit on the electrical power that can be applied to the target, and consequently the rate at which material can be sputtered. Consider a particular arrangement which has been set up for the purpose of performing reactive DC sputtering of insulating material. As the electrical power being applied to the sputtering target in such an arrangement is increased, a level of power is reached above which arcing causes disruption of the process. This level will be called the arc threshold power. To increase the arc threshold power, arc suppression features are incorporated into the sputtering power supply. Typically, these features cause the power supply voltage, which is negative during sputtering, to become zero or to attain a positive value for periods of short duration during the operation of the process. During the periods of voltage interruption or reversal, potential arc sites situated on an insulating layer that covers the sputtering target are discharged by electrons within the plasma that occupies the space adjacent the target. This plasma begins to dissipate rapidly as soon as interruption or reversal is initiated.

Arc suppression systems known by the trade names SPARC™ , SPARC -LE™ and STARBURST ™ manufactured by Advanced Energy Systems of Fort Collins, Colorado serve as examples of the current art of arc suppressing power supplies. These operate by replacing the negative sputtering voltage with a positive voltage that is generated by a separate supply. They can operate in an arc triggered mode in which voltage reversal occurs when a detector in the power supply senses the beginning of an arc, or in a self triggered mode in which the arc detector is not used and voltage reversals of fixed duration occur at regular intervals. The arc triggered mode can be combined with the internally triggered mode so that the systems will trigger either upon detection of an arc or detection of an internally derived signal. SPARC-LE™ ,

SPARC™ and STARBURST ™ in the order listed are designed for use with power supplies that deliver successively higher sputtering power. The designation "SPARC™ -type" will be used to refer to any of the three arc suppression systems just listed, or to other arc suppression systems that provide periodic voltage reversals in an internally or externally triggered mode.

U. S. Patent Application No.08/437,816, (now approved) the disclosure of which is hereby incorporated by reference, teaches a method for reducing the frequency and intensity of arcs which occur during reactive DC sputtering processes. In the practice of the above-cited invention, at least one plasma generating device is provided within the sputtering chamber. The device or devices are located adjacent the sputtering target and generate a large volume, high density plasma in the vicinity of the target. The action of this plasma enhances the arc suppression system of the power supply.

The above-cited patent application contains a discussion of the prior art of arc suppression and points out the advantages of the use of a separately generated plasma over the prior art in which no additional plasma was supplied. The application presents the theory of arc suppression by voltage reversal using an auxiliary plasma, indicating that its operation depends on the presence of electrons in the auxiliary plasma. These electrons are spatially disposed so as to be able to travel to potential arc sites on the target during periods of voltage interruption or reversal that are created by the power supply. Furthermore, in the practice of the invention cited, the plasma does not dissipate at the beginning of the voltage reversal because it is supplied by a generator that operates during the reversal period. Therefore, the practice of the cited invention provides a more complete discharge of potential arc sites than had been provided by the prior art, and has been found to allow an increase in the arc threshold power by an amount such that arcing is no longer the factor that limits the rate of sputtering . In the practice of the above invention, the source of the auxiliary plasma was a separate plasma applicator. Particular embodiments were disclosed in which the sources were microwave-activated plasma applicators. These plasma applicators were driven by microwave generators so that the power delivered to them during the sputtering process could be controlled without changing the sputtering power supply settings. The source of plasma did not provide material that was sputtered onto the substrate.

It is possible to use a second sputtering target as a source of auxiliary plasma, thereby obtaining improved arc suppression while deriving the additional benefit of having a second source of sputtering material. Suzuki in Japanese Patent 5-148644 and Shimizu et al. in European Patent EP639655-A1 describe such arrangements.

FIG. 1 depicts the essential features of these prior art approaches.

In FIG. 1 two identical sputtering power supplies 1,2 are connected to identical sputtering target assemblies which are placed side by side in a sputtering chamber. Each target assembly consists of a sputtering cathode 4 and the sputtering anode 5. The cathode contains a sputtering target not shown and presumably also contains magnets. The cathodes are each connected to the power supplies by means of electrical connections 6 which pass through electrical standofFs 7 in the chamber wall 8. The arc suppression capability of the arrangement derives from the ability of the power supply to periodically interrupt or reverse the voltage on the sputtering target and operate in what we have called a self triggered mode; however, the voltage interruptions or reversals are timed in such a way that they never occur in both supplies at the same time. To emphasize the importance of the voltage modifying feature of the invention, we show each power supply being comprised of two boxes, a box 9 which performs the arc suppression function, and a box 10, which is the source of sputtering power. When the two sputtering targets are operated simultaneously, the plasmas from each target form in the regions 1 1 and 12 above the targets and bring about an increase in the sputtering rate which can be achieved before the sputtering process is disrupted by arcing.

In a sputtering process magnets are almost always used to contain the plasma within a small volume above the sputtering target and thereby to increase the rate of sputtering. A sputtering target assembly which contains such magnets is called a magnetron sputtering target. FIG. 2 shows schematically a plan view of and a section through a magnetron target. The target assembly 20 is elongated, typically 5 to 7 inches wide and 15 to 40 inches long and is shown with a cut through it to emphasize the variability of its length. It has within it a sputtering target 21 with a magnet assembly 22 underneath. The magnet assembly is shown by dotted lines in the plan view. An anode structure 23, shown schematically in the plan view by a rectangle, surrounds the magnet assembly. The view of Section AA shows the edge magnets 24 disposed in lines around the perimeter of the target and the center magnets disposed along the target center line. The polarity of the magnets in the center is opposite to that of the magnets on the perimeter, so that a large portion of the magnetic field lines, represented by the dotted lines 26 loop through both the center and edge magnets.

The magnet assembly 22 may contain a soft iron plate 27 to provide a closed path for magnetic flux passing between the magnets. Electrical standoffs 28 electrically isolate the magnet assembly and the target from the anode.

As is well known, the details of the design of the magnet assembly are important in determining the behavior of the target during a sputtering process. In particular, the ratio of the magnetic flux passing through the center magnets to the flux passing through the peripheral magnets is known to be important in determining the characteristics of the ion and electron flow from the target. See for example, B. Window and N. Savides, J. Vac. Technol. A4, (2), Mar/ Apr. 1986. Page 196. FIG. 3a, 3b, and 3c are based on FIG. 1 of Window and Savides. Each of these figures shows a section through the magnet assembly 22 of FIG. 2. In FIG. 3 a the magnet assembly has been designed such that the total magnetic flux passing through the center magnets is nearly equal to the flux passing through the edge magnets. Regions in the space surrounding the magnet assembly are shown as circles containing identifying numbers.

In FIG. 3 a almost all of the lines of magnetic flux pass through both center and edge magnets and very few lines close outside the magnet assembly. A strong magnetic field exists in the regions 30 and 31 between the inner and outer portions of the magnet assembly and only a very small field exists in the regions 32, 33, 34, and 35 which are adjacent to and slightly removed from the target assembly. The magnet configuration of FIG. 3 a is called "balanced". Note that the center magnet is shown larger that the edge magnets, since it must carry approximately twice as much flux.

In FIG. 3 b the edge magnets are "stronger" than the center magnets; that is, they generate more flux than the edge magnets and accordingly they are drawn larger in FIG. 3b than the edge magnets in FIG. 3a In this case the stronger magnets on the edge generate many field lines that do not loop through the center magnets. Some of the additional lines from each edge magnet which would have closed by passing through the center magnet travel toward the center of the target. These lines are repelled by lines from the other edge magnet which has the same polarity and are bent away from the center causing an extended magnetic field to exist in the regions 33', and 34'. This field is directed more or less parallel to the vertical direction in the figure with a small inwardly directed component. Additional lines that travel outward from the edge bend sharply outward and downward, passing back through the edge magnet. Few lines of flux pass through regions 32' and 35'. Inspection of FIG. 3b shows that many lines of flux from the edge magnets travel inward toward the center of the figure; thus, the configuration of FIG. 3b is called "inward directed". FIG. 3 c shows the situation which occurs when the center magnets are stronger as shown in the figure by their larger size. In this case, lines of force from the center magnet cannot close through the edge magnets and loop out beyond the magnet assembly, so that the general direction of the field lines is outward from the center of the target. There is an appreciable field in the region 32" which points up and to the left and a field in region 35" which points up and to the right. Since the general configuration of the flux is such that the lines point outwardly from the center, this unbalanced field configuration is referred to as "outward directed". Window and Savides investigated the diffusion of electrons and ions in the plasma within the region of the sputtering target. They found that the motion was highly dependent on the magnetic field configuration, the electrons spiraling along the lines of magnetic field and carrying the ions with them by electrostatic attraction. Thus the inward directed magnetron tends to produce a beam of plasma which is concentrated toward the center of the target and impinges on the substrate when it is opposite the target. On the other hand, the outward directed magnetron projects the plasma that leaves the target into a region which is adjacent the periphery of the target.

SUMMARY OF THE INVENTION

The present invention is an improvement upon the prior art or arc suppression shown in FIG. 1, the improvement comprising tailoring the magnetic field configuration existing in the vicinity of multiple sputtering targets to facilitate transport of electrons between the multiple targets and thereby to facilitate the discharge of potential arc sites during the periods of voltage reversal.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic of a prior art sputtering arrangement in which two targets with arc suppressing power supplies are used. FIG. 2 is a simplified drawing showing a plan view and section through an elongated target for magnetron sputtering.

FIG. 3a is a section through a magnetron sputtering target showing the magnetic field lines in a balanced configuration.

FIG. 3b is a section through a magnetron sputtering target showing the magnetic field lines in an unbalanced "inward directed" configuration.

FIG. 3c is a section through a magnetron sputtering target showing the magnetic field lines in an unbalanced "outward directed" configuration.

FIG, 4 is a diagram showing the configuration of the components in the practice of the present invention. FIG. 5 is a section through a sputtering target illustrating the magnetic field configuration of the present invention under conditions of weak magnetic interaction between targets.

FIG. 6a is a section through as sputtering target illustrating the preferred magnetic field configuration when the magnetic interaction between targets is significant. FIG. 6b is a section through a plurality of sputtering targets operating in a similar manner to the targets of FIG. 6a.

FIG. 7 is a section through an embodiment of the invention where the sputtering targets are of the type known as cylindrical magnetron targets.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with prior art at least two sputtering power supplies that employ arc suppression are separately connected to sputtering targets disposed adjacent one another in a sputtering chamber for the purpose of reactive sputtering. Such targets may be comprised of various materials such as silicon, niobium or titanium, which react with a gas, such as oxygen, within the sputtering chamber to form an insulating coating. FIG. 4 shows a configuration in which two separately connected targets are used and the power supplies are previously described SPARC ™ type systems.

Power supplies 41 and 42, shown as dotted boxes are individually comprised of main supplies 43,44 and SPARC™ type units 45, 46. The output of each main supply is a negative voltage (nominally -500 volts) which passes through its associated

SPARC™ type device, whereupon the SPARC™ type device, when triggered by an arc, or commanded by an internally or externally supplied signal, interrupts the negative voltage and inserts in its place a positive voltage of short duration, after which the original sputtering voltage is restored. The voltage from each power supply, so modified, is conveyed by an electrical lead 47 or 48 through an insulating feedthrough

49 in the wall of the sputtering chamber to a separate sputtering target 50 or 51. Material is sputtered from each of the targets so that it travels to the adjacent substrates 52 which are being conveyed past it in the direction of the arrow 53 by the transporting means 54. The two targets acting together form a single sputtering station such that plasma from each target overlaps the other target.

The power supply for each of the targets is separately controllable, so that in the practice of the present invention the rate of sputtering from each of the targets may separately adjusted.

The present invention consists of the use of a shaped magnetic field to enhance the arc suppression process. In the practice of the invention, the method used depends upon the distance between the sputtering targets in the coating machine, which distance may be predetermined by the configuration of the machine and therefore not easy to change. FIG. 5 shows the improvement of the present invention on the prior art of FIG. 2. This embodiment is applicable when the distance between targets is large enough so that the magnetic field from each magnetron target is very small at the location of the other target. In this case the behavior of electrons diffusing out from one target is independent of the field of the other. In the configuration of FIG. 5 the magnetic field has been designed to facilitate diffusion transport of the plasma between targets by allowing the plasma to escape more readily from the target region.

Referring to FIG. 5, it is possible to understand the relation between the magnetic field configuration of this invention and the arc suppression process. The Figure shows a section through one of the two elongated magnetron sputtering target assemblies mounted on the wall 58 of a sputtering chamber. Each target contains a magnet with central magnets 61 and peripheral magnets 62 Coupling between the magnets is improved by means of the soft iron base plate 68. Each target assembly is connected to a power supply 59 of the SPARC™ type, and is isolated from the wall by insulating spacers 60. The anode structure on the target is not shown. During sputtering, material is removed from the target in the sputtering zones 63 and travels to substrates, not shown, that move in the direction of the arrow 64 which is transverse to the long axis of the target. A layer 65 of reacted insulating sputtered material builds up on the conducting surface of the target. Deposition of positive charge on this surface by ion bombardment occurs during sputtering and is responsible for causing the voltage breakdown across the layer 65 which results in arcing. The voltage reversing action of the power supply causes electrons to be drawn from the plasma to the positively charged surface of the insulating layer and, if there is sufficient charge available in the plasma, neutralizes the charge that had built up during the sputtering period since the last reversal and prevents an arc from occurring.

The present invention provides a magnetic field configuration which facilitates transport of plasma between targets, making more plasma available for discharge of arc sites in the practice of the invention. According to the present invention both targets are operated in an unbalanced outwardly directed mode in which the central magnets 61 are stronger than the outer magnets 62 as in FIG. 3 c. FIG. 5, which depicts one of the targets therefore shows the central magnets having a larger size than the peripheral magnets.

The design of the second sputtering target is the same as that of the first target shown in FIG. 5. The first target is situated next to the second and the long axes of the two targets are parallel. Each of the two target helps suppress arcs in the other target by providing a plasma that persists during the voltage reversal period of the other. This enables a more complete discharge than is possible when only one target is present.

The present invention provides a magnetic field configuration around the targets which facilitates passage of plasma from one target into the region 66 above the other. The unbalanced outward directed field accomplishes this. Plasma escapes from the space above the sputtering region by traveling along the outwardly directed field lines 67 and eventually reaches a field free region. It then diffuses into the region of the other target and during voltage reversal contributes electrons which discharge the insulating layer on the other target. The outward directed field lines from the other target provide a path on which the electrons from the original target can travel to the insulating layer of the other. Thus the use of an outwardly directed unbalanced magnetic field according to the practice of the present invention materially improves the arc-suppression capability of multi-target configuration.

In a second embodiment of the invention, a different magnetic field configuration is preferred. The second embodiment is used when the targets are close enough to one another that a substantial magnetic field caused by the magnet assembly of one target exists at the location of the magnetic assembly of the other. In this case it is desirable to configure the magnets of the two targets so that the polarity of the magnets of one target is opposite to that of the other. This configuration is shown if FIG. 6a. Two elongated, unbalanced, outwardly directed sputtering target magnet assemblies 70 and 71 like the one shown if FIG. 5 are mounted adjacent one another on the wall 72 of a sputtering chamber with their long axes parallel. The targets have central magnets 73 and peripheral magnets 74 as in FIG. 5; however, the polarity of the magnets in each of the two targets is opposite from that of the other. Because of the opposite polarity of the magnets, many of these lines indicated by the arrows 77, now pass through magnets on both targets providing a path for the diffusion of plasma between the targets. Thus the reversal of the polarity of the magnets facilitates the transport of plasma between the targets with a resulting enhancement of the arc- suppression capability.

As in previous configurations each target assembly contains a magnet assembly with a soft iron base plate 75. It is preferred to improve the coupling between targets by providing a magnetically permeable connecting means, such as a soft iron plate 76 between the baseplates. Alternatively, a single continuous plate could be used for both targets so that the targets are mechanically integral.

The configuration of FIG. 6a can be adapted to accommodate a plurality of sputtering targets in which the individual targets are disposed adjacent one another is shown in FIG. 6b. Individual sputtering targets are shown so disposed with the target on the right side being cut to indicate that the sequence can be repeated as often as desired. The polarities of the magnets in the assemblies of adjacent targets is opposite. This opposite polarity together with the stronger outer magnets in each assembly facilitates transfer of plasma between targets and enhances arc suppression. As in FIG. 6a tighter coupling may be achieved by adding a magnetically permeable coupling means such as a soft iron plate between targets, of a single plate could connect all of the targets.

In a particular embodiment of the present invention, the sputtering process, incorporates two targets belonging to a class known to the art as cylindrical magnetron targets. Such targets are used in a wide range of sputtering applications and are often used to reactively sputter an insulating material such as silicon dioxide onto a variety of substrates.

Cylindrical magnetron devices are well known to the art. Examples of the cylindrical magnetron target assemblies employed in this embodiment are manufactured by Airco, Inc of Fairfield, CA. The main components of these devices, are the cylindrical target, which is comprised of the material, such as silicon, that is to be sputtered, the magnet pack, and a means for rotating the target. Cylindrical magnetron targets have the advantage that due to the rotation of the target during sputtering, material is removed uniformly from the entire cylindrical surface of the target. A common arrangement to which the present invention is particularly applicable incorporates two cylindrical magnetrons which are spaced a few inches apart, a configuration often referred to by those skilled in the an as a "dual C-mag." FIG. 7 shows the a section through a configuration in which the present invention has been applied to a sputtering process employing cylindrical magnetron targets. Two such targets 80 are shown mounted next to one another. Unbalanced, outwardly directed magnet assemblies 81 are mounted within each target so that magnetic lines of flux emanate from the targets. These lines facilitate the transport of plasma between targets, enhancing arc suppression. During operation of the process the targets rotate in the direction of the arrows 83 and substrates to be coated travel past the target in the direction of the aπow 83.

Referring to FIG. 1, the main power supplies 1,2 in these large processes are capable of delivering electrical power to the sputtering targets at a level of 50 kilowatts or more. Such main supplies, examples of which are manufactured by Halmar

Electronics, Inc. of Columbus, Ohio, may have various arc suppression features incorporated into them, such as the ability to detect the onset of an arc and temporarily shut down, or an arc diversion feature in which a reversed voltage that is stored on a capacitor is substituted for the supply voltage upon arc detection. In the practice of the present invention, the existing arc suppression features, if any, are replaced by the those of the previously described SPARC™ type units, and therefore these presently existing features are shut down or removed from the main supplies. SPARC™ type units then connected in series with the output of each main unit or incorporated into each main unit, providing the ability to create periodic voltage reversals as previously described and enhancing the arc suppression capability of the process.

The present invention is applicable to batch sputtering processes such as are commonly carried out using a rotatable drum or table to convey the substrates past a sputtering station. It is also applicable to continuous processes in which the substrates are transported on a substantially straight line past the sputtering station, as in a so called "in line" process, or to a roll coating process in which the substrate is a continuous strip which is conveyed from a feeding roll past the sputtering station to a receiving roll.

I I It is beneficial to large, in line industrial process such as are used in the fabrication of coatings for architectural glass. In such processes, the substrate dimension in the direction transverse to the substrate motion may be several meters. The sputtering target is elongated in the transverse direction in order to extend over the entire substrate as the substrate moves past it.

Claims

We Claim:

1. In a sputtering coating system comprising a chamber capable of maintaining a vacuum and accepting and maintaining a sputter gas, a first target of material to be sputtered, a first means for applying a sputtering voltage between the first target and said chamber and at least one substrate positioned with respect to said first target to be coated by sputtered material from said first target, said first means for applying a sputtering voltage between the first target and said chamber being a DC power supply which employs a voltage interruption or reversal feature in order to reduce energy or frequency of arcing from said first target, a second target of material to be sputtered, said second target being connected to a second means for applying a sputtering voltage between the second target and said chamber being a DC power supply which employs a voltage interruption or reversal feature in order to reduce energy or frequency of arcing from said second target, the improvement comprising providing an unbalanced, outwardly directed magnet assembly mounted within said first and second targets such that magnetic lines of flux are established emanating from said first and second targets which facilitate the transport of plasma between said targets enhancing arc suppression, whereby said first and second targets are positioned adjacent one another such that plasma from each target substantially overlaps the plasma of the other target.

2. The sputtering coating system of claim 1 wherein said two targets located adjacent one another, form a single station such that sputtered material from each target arrives substantially simultaneously at the at least one substrate.

3. The sputtering coating system of claim 1 wherein said first DC power supply is operated in either an arc triggered, self-triggered or externally triggered mode.

4. The sputtering coating system of claim 1 wherein said second DC power supply is operated in either an arc triggered, self-triggered or externally triggered mode.

5 The sputteπng coating system of claim 1 wherein said first DC power supply is operated simultaneously in both the arc tπggered and self-tπggered modes so that voltage reversals are applied to the output terminals of the power supply either after sensing the beginning of an arc, or after a fixed peπod of time has elapsed since the last reversal

6 The sputteπng coating system of claim 1 wherein said second DC power supply is operated simultaneously in both the arc tπggered and self-triggered modes to that voltage reversals are applied to the output terminals of the power supply either after sensing the beginning of an arc, or after a fixed peπod of time has elapsed since the last reversal

7 The sputteπng coating system of claim 1 wherein the DC power supply for each of the targets is separately controllable so that rates of sputteπng from each of the targets are separately adjustable

8 The sputteπng coating system of claim 1 wherein said first and second DC power supplies are operated in an externally tπggered mode wherein the external tπggers to the separate arc suppression systems are synchronized so that tπggers for voltage reversal are not applied to both DC power supplies at the same time

9 The sputtering coating system of claim 1 wherein said targets are cylindrical magnetron targets

10 The sputtering coating system of claim 1 wherein each target is comprised of a different material

1 1 The sputter coating system of claim 1 wherein each target comprises a member selected from the group consisting of silicon, niobium, aluminum, titanium, hafnium and zinc

12, The sputter coating system of claim 1 wherein suppression of arcs is enhanced by providing plasma from a first target that persists during voltage reversal period of the second target.

13. The sputter coating system of claim 1 wherein said targets are sufficiently close together such that the magnetic field generated from one magnet assembly of the first target exists at the location of the magnetic assembly of the second target.

14 The sputter coating system of claim 13 wherein the polarity of the magnets of the first target is opposite the polarity of the magnets of the second target.

15 The sputter coating system of claim 1 wherein said magnet assemblies further comprise soft iron baseplates.

16. The sputter coating system of claim 1 wherein said first and second targets comprise cylindrical magnetron targets.

17. The sputter coating system of claim 1 wherein each of said first and second targets are characterized as having a width and length and longitudinal axis along said length and wherein said longitudinal axis of each target is parallel to the longitudinal axis of the other.