Method and apparatus for periodic polarity reversal during an active state

Apparatus for depositing a film of material on a substrate may comprise sputter deposition apparatus for transferring target atoms from a target to the substrate and a servo control circuit operatively associated with the sputter deposition apparatus. The servo control circuit operates the sputter deposition apparatus in an alternating manner between an "active" state and a "quiescent" state so that a power density on the target during the "active" state is at least about 400 watts per square inch of target area. During the active state, target atoms are transferred from the target to the substrate. During the quiescent state, substantially no target atoms are transferred from the target to the substrate. A polarity reversing circuit operatively associated with the servo control circuit periodically reverses a polarity on the sputter deposition apparatus during the "active" state.

FIELD OF INVENTION 
The present invention relates to power supplies in general and more 
specifically to power supplies for plasma processing systems. 
BACKGROUND 
Plasma deposition refers to any of a wide variety of processes in which a 
plasma is used to assist in the deposition of thin films or coatings onto 
the surfaces of objects. For example, plasma deposition processes are 
widely used in the electronics industry to fabricate integrated circuits 
and other electronic devices, as well as to fabricate the magnetic tapes 
and disks used in audio, video, and computer applications. Plasma 
deposition processes may also be used to apply coatings to various objects 
to improve or change the properties of the objects. For example, plasma 
deposition processes may be used to apply wear resistant coatings to 
machine tools, while other types of coatings may be used to increase the 
corrosion resistance of other items, such as bearings, turbine blades, 
etc, thereby enhancing their performance. In still other applications, 
plasma deposition may be used to apply coatings to various types of 
surfaces in the optics and glass industries. 
In most plasma deposition processes the plasma is created by subjecting a 
low-pressure process gas (e.g., argon) contained within a vacuum chamber 
to an electric field. The electric field, which is typically created 
between two electrodes, ionizes the process gas, creating the plasma. If 
direct current (DC) is used to produce the electric field, the negatively 
charged electrode is usually referred to as the cathode, whereas the 
positively charged electrode is referred to as the anode. Thus, in the 
case of a DC sputter deposition plasma process, the material to be 
deposited on the object or substrate is usually connected as the cathode, 
whereas some other element, typically the vacuum chamber itself, is 
connected as the anode. Ionized process gas atoms comprising the plasma 
are accelerated toward the negatively charged cathode which also includes 
a target containing the material to be deposited on the substrate. The 
process gas atoms ultimately impact the target material and dislodge or 
sputter atoms from the target, whereupon the sputtered atoms subsequently 
condense on various items in the chamber, including the substrate that is 
to be coated. The substrate is usually positioned with respect to the 
target so that a majority of the sputtered target atoms condense onto the 
surface of the substrate. 
While sputter deposition processes of the type described above may be used 
to deposit a wide variety of metals and metal alloys onto various 
substrates, they may be used to deposit compound materials as well. 
Reactive sputter deposition is the name usually given to sputtering 
processes which involve the sputtering of the target in the presence of a 
reactive species (e.g., oxygen or nitrogen gas) in order to deposit a film 
comprising the sputtered target material and the reactive species. A wide 
variety of compounds, such as SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 
N.sub.4, and TiO, can be deposited by reactive sputter deposition 
processes. 
The film deposited by such plasma deposition processes may be characterized 
by certain properties, such as adhesion; stress (i.e., compressive or 
tensile); stoichiometry; microstructure, including morphology, grain size, 
grain orientation, and epitaxy; hardness; abrasion resistance; density; as 
well as overall film thickness, just to name a few. Certain of these 
properties or characteristics may be of greater or lesser importance 
depending on the particular film and the type of application. 
Unfortunately, while sputter deposition apparatus of the type described 
above are relatively easy to operate in a basic sense, the problem of 
operating sputter deposition apparatus to produce high quality films 
having the desired characteristics and properties on a repeatable basis is 
by no means trivial. Indeed, a significant portion of the current research 
efforts in sputter deposition technology are directed to developments and 
refinements of the sputter deposition apparatus and methods in order to 
improve the qualities of the deposited films. 
While much remains to be learned about the mechanisms associated with film 
growth and the production of films having certain characteristics, certain 
mechanisms have been discovered that have predictable effects on film 
properties. For example, it has been found that the irradiation of the 
growing film with ions strongly affects film nucleation and growth, 
adhesion, film microstructure, and chemistry. Accordingly, many sputter 
deposition apparatus have been provided with separate ion sources (e.g., 
ion beams) to produce surface coatings having the desired properties. 
Unfortunately, however, the ion beam produced by a typical ion source is 
relatively small, typically a few millimeters in diameter, which limits 
the use of ion beam sputtering to applications involving like-sized 
substrates. 
Partly in an effort to solve this problem, some operators have replaced the 
conventional narrow beam ion sources with ion thrusters of the type 
developed for space propulsion systems. Such ion thruster devices 
generally produce ion beams having relatively large cross sections, on the 
order of tens of millimeters, which allows larger substrates to be coated 
with the ion beam sputtering process. Unfortunately, problems still remain 
with regard to the maximum size of the substrate that may be effectively 
coated with such ion beam sputtering processes. Also, the provision to the 
sputtering system of an additional component (i.e., the ion beam source), 
increases the overall complexity of the system and may cause other 
problems. 
Another method for controlling certain film properties that has been used 
with some degree of success is bias sputtering. In bias sputtering, the 
substrate is biased negatively with respect to the plasma potential. The 
negatively biased substrate attracts ions from the plasma, thereby 
providing low-energy ion bombardment of the growing film. With proper 
control of the low-energy ions, bias sputtering process may be used to 
achieve or control certain film properties. Unfortunately, however, bias 
sputtering is also not without its problems and limitations. For example, 
bias sputtering techniques cannot be used if the substrate is an 
electrical insulator. Also, the sputter deposition system must be provided 
with apparatus suitable for controlling the charge placed on the substrate 
to ensure that the ions bombard the substrate at the proper energy levels. 
In summary, while the ion beam and bias sputtering processes described 
above are useful in certain applications, a need still exists for a film 
deposition system that will provide for the convenient control of certain 
specified film properties. Ideally, such a film deposition system should 
provide the operator with the desired degree of control of the specified 
film properties but without the need to resort to separate ion sources, 
with all their associated complexities and shortcomings. Additional 
advantages could be achieved if the desired film properties could be 
realized without the need to bias the substrate, as is required in bias 
sputtering processes. 
SUMMARY OF THE INVENTION 
Apparatus for depositing a film of material on a substrate may comprise 
sputter deposition apparatus for transferring target atoms from a target 
to the substrate and a servo control circuit operatively associated with 
the sputter deposition apparatus. The servo control circuit operates the 
sputter deposition apparatus in an alternating manner between an "active" 
state and a "quiescent" state so that a power density on the target during 
the "active" state is at least about 400 watts per square inch of target 
area. During the active state, target atoms are transferred from the 
target to the substrate. During the quiescent state, substantially no 
target atoms are transferred from the target to the substrate. A polarity 
reversing circuit operatively associated with the servo control circuit 
periodically reverses a polarity on the sputter deposition apparatus 
during the "active" state. 
Also disclosed is a method for depositing a film of material on a substrate 
which comprises the step of providing sputter deposition apparatus for 
transferring target atoms from a target to the substrate; operating the 
sputter deposition apparatus in an alternating manner between the active 
state and the quiescent state so that a power density applied to the 
target during the "active" state is at least about 400 watts per square 
inch of target area; and periodically reversing a polarity on the sputter 
deposition apparatus during the "active" state.

DETAILED DESCRIPTION OF THE INVENTION 
Film deposition apparatus 10 according to one preferred embodiment of the 
present invention is shown in FIG. 1 as it could be used to deposit a film 
12 onto a surface 14 of a substrate 16 contained within a vacuum chamber 
18. The material comprising the film 12 may be derived by sputtering a 
target 42 (FIGS. 2 and 3) associated with a cathode/target assembly 20. 
The cathode/target assembly 20 is connected to a negative (-) terminal of 
a power supply 36. A positive (+) terminal of power supply 36 may be 
connected to the vacuum chamber 18, in which case it serves as the anode. 
As will be described in greater detail below, the power supply circuit 36 
produces a modulated output signal 66 (FIG. 4) which alternately operates 
the sputter deposition apparatus (i.e., the cathode/target assembly 20) 
between an "active" state and a "quiescent" state. When the cathode/target 
assembly 20 is operated in the active state, material (not shown) is 
sputtered from the cathode/target assembly 20. The sputtered material 
subsequently condenses on the surface 14 of substrate 16, whereupon it 
forms the film 12. When operated in the quiescent state, substantially no 
material is sputtered from the cathode/target assembly 20 or is 
transferred to the substrate 16. Operating the sputter deposition 
apparatus (i.e., the cathode/target assembly 20) in such an alternating 
manner (i.e., between the "active" state and the "quiescent" state) 
provides for improved control of certain film properties, as will be 
described in greater detail below. 
The vacuum chamber 18 may comprise a generally closed, gas-tight chamber to 
which are connected a variety of ancillary systems and devices that may be 
necessary or desirable in order to perform the desired plasma process. For 
example, in one preferred embodiment, the vacuum chamber 18 may include a 
process gas supply 22 which contains a supply of a suitable process gas 
(indicated by arrow 26). The process gas supply 22 may be connected to the 
vacuum chamber 18 via a process gas valve 24 which regulates the flow of 
the process gas 26 into the interior 32 of chamber 18. The vacuum chamber 
18 may also include a vacuum pump assembly 28 and a vacuum valve assembly 
30 to maintain the interior 32 of the vacuum chamber 18 at a pressure 
suitable for carrying out the desired process. A cooling system 34 may 
also be provided to cool the cathode/target assembly 20 and/or the vacuum 
chamber 18. Finally, if reactive sputtering processes are to be performed, 
the vacuum chamber 18 may also be provided with a supply of a suitable 
reactive gas (not shown), along with the appropriate devices (e.g., 
valves, regulators, etc., also not shown) for introducing the appropriate 
quantity of reactive gas into the process chamber 18. 
As was mentioned above, the cathode/target assembly 20 provides the 
material that will be ultimately deposited as the film 12. Specifically, 
the material originates from a separate target 42 that is associated with 
the cathode/target assembly 20. See FIGS. 2 and 3. If non-reactive 
sputtering processes are to be performed, then the cathode/target assembly 
20 will be operated in the absence of a reactive material (e.g., a 
reactive gas). Alternatively, if reactive sputtering processes are to be 
performed, then the cathode/target assembly 20 will usually be operated in 
the presence of a reactive material in order to produce a compound film 
comprising the material sputtered from the target and the reactive 
species. 
Regardless of whether the cathode/target assembly 20 is used to accomplish 
non-reactive or reactive sputtering, the cathode/target assembly 20 may 
comprise a conventional magnetron sputtering "source" of the planar type. 
Alternatively, the cathode/target assembly could comprise other types of 
sources, such as non-planar magnetrons, or even non-magnetron (i.e., 
diode) sources. Referring now to FIGS. 2 and 3, the cathode/target 
assembly 20 that is used in one preferred embodiment may comprise a planar 
magnetron having a backing plate 40 to which is mounted a generally 
circular target member 42. A pole piece 44 may also be also mounted to the 
backing plate 40, generally between the backing plate 40 and the target 
42. Pole piece 44 may include a plurality of magnets 48, 50, and 52 which 
produce a closed loop magnetic field (not shown) over the front surface 46 
of target 42. The closed-loop magnetic field confines the electrons over 
the front surface 46 of target 42, which generally increases the 
ionization of the process gas and improves the sputtering rate and overall 
target utilization. 
The power supply 36 produces the modulated output signal 66 (FIGS. 4 and 5) 
which is then applied to the cathode/target assembly 20. The modulated 
output signal 66 operates the sputter deposition apparatus (i.e., the 
cathode/target assembly 20) in an alternating manner between the active 
state and the quiescent state. The power supply 36 may include a rectifier 
circuit 80 for rectifying or converting alternating current (i.e., AC) 
from an AC source 82 to a direct current (i.e., DC). A servo control 
circuit 62 connected to the direct current terminals (i.e., the (+) and 
(-) terminals) of the rectifier circuit 80 modulates the DC current 
produced by the rectifier circuit 80 to produce the modulated output 
signal 66 (FIG. 4), which may then be applied directly to the 
cathode/target assembly 20. The power supply circuit 36 is also provided 
with a polarity reversing circuit 64 which superimposes onto the modulated 
output signal 66 a plurality of polarity reversal signals 86, as best seen 
in FIG. 5. The polarity reversal signals 86 briefly reverse the polarity 
on the cathode/target assembly 20, thereby discouraging the formation of 
arcs within the chamber 18. 
Referring now primarily to FIG. 4, the modulated output signal 66 comprises 
a periodic signal having a plurality of negative periods or pulses 68 each 
of which is separated by a quiescent period 70. When the modulated output 
signal 66 is applied to the cathode/target assembly 20, material will be 
sputtered from the cathode/target assembly 20 during the negative period 
68, but will not be sputtered during the quiescent period 70. Put in other 
words, the modulated output signal 66 produced by the power supply 
assembly 36 alternates the state of operation of the cathode/target 
assembly 20 between the active state and the quiescent state. 
The modulated output signal 66 comprises several significant parameters 
that are of importance in achieving the objects and advantages of the 
present invention. One significant parameter is the "turn-on" time 84, 
i.e., the time required for the voltage to reach substantially the 
operational voltage V.sub.1. Another significant parameter is the 
"turn-off" time 91, i.e., the time required for the voltage to reach 
substantially the quiescent or zero voltage point. Also of importance are 
the durations of both the negative pulse (i.e., period 68) and the 
quiescent period 70. In one preferred embodiment, the turn-on time 84 may 
be in the range of about 5 microseconds (.mu.s) to 200 .mu.s (&lt;30 .mu.s 
preferred). The turn-off time 91 may also be in the range of about 5 .mu.s 
to 200 .mu.s (&lt;30 .mu.s preferred). The duration 72 of the negative pulse 
68 may be in the range of about 0.5 milliseconds (ms) to 10 ms (1-2 ms 
preferred), whereas the duration 74 of the quiescent period 70 may be in 
the range of about 0.5 milliseconds (ms) to 50 ms (5-10 ms preferred). 
The polarity reversing circuit 64 superimposes on the modulated output 
signal 66 a plurality of short duration polarity reversal signals 86, as 
best seen in FIG. 5. The polarity reversal signals 86 are provided during 
the negative period 68 of the modulated output signal 66 (i.e., during the 
"active" state). Note that the duration 88 of each polarity reversal 
signal is considerably shorter than the duration of the negative pulse 68. 
Consequently, each negative pulse period 68 may comprise a large number of 
polarity reversal signals 86. The advantages associated with the polarity 
reversal signals 86 will be described in greater detail below. 
The film deposition apparatus 10 according to the present invention may be 
operated as follows to deposit the film 12 onto the surface 14 of 
substrate 16. Assuming that proper pressure and atmosphere have been 
established within the chamber 18, the sputter deposition process (i.e., 
either non-reactive or reactive) may be initiated by activating the power 
supply 36. Thereafter, the power supply 36 places the modulated output 
signal 66 on the cathode/target assembly 20. During the negative pulse or 
period 68, an electric field is established between the cathode/target 
assembly 20 and the anode (e.g., chamber 18) which initiates or "ignites" 
the glow discharge (i.e., the plasma 38). The plasma 38 causes sputtering 
of the target portion 42 of the cathode/target assembly 20. That is, atoms 
(not shown) sputtered from the target 42 are released into the interior 32 
of chamber 18, whereupon a portion of the sputtered atoms subsequently 
condense on the surface 14 of substrate 16, thereby forming the film 12. 
When the modulated output signal 66 enters the quiescent period 70, the 
plasma 32 is extinguished, which terminates the sputtering process. 
Consequently, substantially no material is transferred from the target 42 
to the substrate 16 during the quiescent period 70. 
While no significant activity occurs on the cathode/target 20 during the 
quiescent period 70 (at least insofar as the properties of the film 12 are 
concerned), the same cannot be said with regard to the surface 14 of 
substrate 16. During the quiescent period, previously deposited atoms 
(i.e., adatoms) of the target material may be rearranging themselves on 
the surface 14 of substrate 16. This phenomenon is referred to herein as 
"surface mobility." The alternating sputtering process according to the 
present invention significantly increases the surface mobilities of the 
deposited adatoms over those which would be associated with the same 
sputtering apparatus but operating in a continuous manner, i.e., without a 
quiescent period 70. 
The increased surface mobilities of the adatoms are due primarily to two 
factors. The first factor is the quiescent period 70. Essentially, the 
quiescent period 70 provides the adatoms with additional time to migrate 
over the surface 14 of substrate 16. Therefore, even though the adatoms 
may possess substantially the same energies as those which would be 
associated with the same apparatus but operating without a quiescent 
period 70, the additional migration time afforded by the quiescent period 
70 means that the adatoms will generally have greater surface mobilities. 
Moreover, even relatively low-energy adatoms may have significantly 
greater surface mobilities due to the additional migration time afforded 
by the quiescent period 70. 
The second factor that tends to increase the surface mobilities of the 
adatoms is that the deposited adatoms themselves may have relatively high 
energies. For example, the present invention will generally allow the 
target to be sputtered at much higher power levels (e.g., at least about 
400 watts/in.sup.2 of target surface area) during the active state, but 
without sacrificing process control, as will be described in greater 
detail below. Such higher power levels provide the incoming adatoms with 
correspondingly higher energies which generally increases their surface 
mobilities. 
The increased adatom mobility provided by the quiescent period 70 and the 
increased adatom energies associated with the present invention provides 
several distinct advantages and may be used to enhance and control many 
film properties, including microstructure, density, adhesion, and 
chemistry. For example, the increased adatom mobility generally allows 
more adatoms to migrate to low energy sites where nucleation and growth 
occurs, thereby increasing the nucleation density in many instances. The 
increased nucleation density may promote more interfacial reactions and 
allow for the deposition of adherent films on substrates where normally 
the adhesion of the film is poor. 
The increased mobility may also result in improved film morphology. For 
example, prior to this invention, the use of high material deposition 
rates typically resulted in amorphous films having low density, columnar 
microstructures. It is generally believed that the formation of such 
columnar microstructures is due in large part to the fact that the adatoms 
do not have sufficient time to diffuse across the surface and find low 
energy sites before being buried by subsequently deposited adatoms. The 
present invention solves this problem by periodically operating the 
sputter deposition apparatus in the quiescent state to allow the adatoms 
additional time to migrate to low energy sites before being covered by 
subsequently deposited adatoms. Consequently, films produced by the 
present invention tend to have improved surface coverage with a consequent 
reduction in the number of interfacial voids which may result in the easy 
fracture and poor adhesion of the film. Such increased film density may be 
reflected in film properties such as better corrosion resistance, lower 
chemical etch rate, higher hardness, lower electrical resistivity (in 
metals), and increased index of refraction for optical coatings. 
Yet another advantage associated with the present invention is that the 
increased energies and mobilities of the adatoms results in improved "step 
coverage" of surfaces that are oblique, or even normal to, the incoming 
adatoms. Such improved step coverage provides for significant advantages 
in the processing of electronic components and integrated circuits, just 
to name a few. 
Still yet another advantage of the present invention is that the high 
surface mobilities of the adatoms does not come at the expense of 
effective control of other film properties, such as film thickness. For 
example, while prior to this invention high power levels could be used to 
achieve high deposition rates, thus achieve high material throughput, it 
proved difficult to precisely control the film thickness. Indeed, in many 
applications where film thickness was a critical parameter it was not 
uncommon to reduce the power level, thus deposition rate, of such devices 
in order to achieve a satisfactory degree of control over film thickness. 
The present invention, by providing for relatively fast switching between 
the active and quiescent states, allows the process to be immediately 
terminated once the film has reached the desired thickness. 
Still other advantages are associated with the film deposition apparatus 10 
according to the present invention when used to perform reactive 
sputtering. For example, in addition to increasing the surface mobilities 
of the deposited adatoms, the quiescent period 70 also allows for the 
improved diffusion of the reactive species adjacent the substrate surface 
14. The quiescent period 70 also provides additional reaction time. The 
improved diffusion and reaction time tends to enhance the film chemistry 
and other properties of the deposited compound film. 
Having briefly described the film deposition apparatus 10 and method of 
using the same, as well as some of its more significant features and 
advantages, the preferred embodiments of the film deposition apparatus and 
method according to the present invention will now be described in detail. 
Referring back now to FIG. 1, the film deposition apparatus 10 according to 
one preferred embodiment of the present invention is shown and described 
herein as it may be used to perform non-reactive sputter deposition to 
deposit a film 12 onto the surface 14 of an object or substrate 16. 
Alternatively, the film deposition apparatus 10 could be used to perform 
reactive sputter deposition, as will be described in greater detail below. 
In either case, the sputtered material may be provided by a suitable 
sputter deposition apparatus that is alternately operated between an 
"active" state and a "quiescent" state. Accordingly, as used herein, the 
term "sputter deposition apparatus" refers to apparatus that includes a 
process chamber (e.g., vacuum chamber 18), a sputtering source (e.g., 
cathode/target 20), an anode (e.g., vacuum chamber 18), as well as any 
associated systems and devices required for either non-reactive or 
reactive sputter deposition (e.g., process gas supply 22, vacuum pumping 
system 28, etc). 
In accordance with its use in non-reactive sputter deposition, the process 
or vacuum chamber 18 utilized in one preferred embodiment may include a 
process gas supply 22 which contains a supply of a suitable process gas 
(e.g., argon, although other gases may also be used). A process gas valve 
24 connected between the process gas supply 22 and the vacuum chamber 18 
may be used to control the flow of process gas (arrow 26) into the 
interior 32 of chamber 18. Vacuum chamber 18 may also include a vacuum 
pumping system 28 and a vacuum valve assembly 30 to maintain the interior 
region 32 of the vacuum chamber 18 at a pressure suitable for carrying out 
the desired process. For example, in most magnetron sputter deposition 
processes it is usually desirable to maintain the interior 32 of the 
process chamber 18 at or below a pressure of about 1 milliTorr (mTorr), 
although other pressures could also be used depending on the type of 
process being performed and on other extrinsic factors. 
It should be noted that process chambers (e.g., vacuum chamber 18), as well 
as the various ancillary devices and systems (e.g., process gas supply 
systems, pumping systems, etc.) associated therewith, are also well-known 
in the art and could be easily provided by persons having ordinary skill 
in the art after having become familiar with the teachings of the present 
invention. Therefore, the particular process chamber 18 and related 
ancillary systems and devices utilized in one preferred embodiment of the 
invention will not be described in further detail. 
If it is desired to configure the film deposition apparatus 10 to 
accomplish reactive sputter deposition, the process chamber 18 may also be 
provided with a reactant gas source (not shown) that contains a supply of 
a suitable reactant gas. Examples of suitable reactant gases include, but 
are not limited to, oxygen (O.sub.2), nitrogen (N.sub.2), and hydrogen 
sulfide (H.sub.2 S). The reactant gas source (not shown) may also include 
a valve assembly (also not shown) to regulate the flow of the reactant gas 
into the interior 32 of process chamber 18. However, since such additional 
systems and devices required to accomplish reactive sputter deposition are 
well-known in the art and could be easily provided by persons having 
ordinary skill in the art after having become familiar with the present 
invention, the additional systems and devices that may be required or 
desired to accomplish reactive sputter deposition also will not be 
described in further detail herein. 
The cathode/target assembly 20 utilized in one preferred embodiment of the 
present invention is best seen in FIGS. 2 and 3 and may comprise a planar 
magnetron cathode/target assembly of the type shown and described in U.S. 
Pat. No. 5,262,028, which is incorporated herein by reference for all that 
it discloses. Alternatively, other types of cathode/target assemblies, 
such as non-planar magnetrons or even regular (i.e., non-magnetron) diode 
sources, could also be used. Therefore, the present invention should not 
be regarded as limited to the cathode/target assembly 20 shown and 
described herein. 
The circular cathode/target assembly 20 used in one preferred embodiment 
and described in U.S. Pat. No. 5,262,028 will now be briefly described. 
Referring now to FIGS. 2 and 3, the circular cathode/target assembly 20 
may comprise a generally circular pole piece member 44 which supports a 
plurality of permanent magnets 48, 50, and 52. Specifically, pole piece 44 
supports an inner magnet 48, a plurality of outer magnets 50, and a 
plurality of intermediate magnets 52. The north poles (N) and south poles 
(S) of the inner and outer magnets 48 and 50 are generally "vertically" 
oriented and may have the polarities indicated in FIG. 3. The intermediate 
magnets 52 may be positioned adjacent inner magnet 48 and may have their 
north poles (N) and south poles (S) oriented generally "horizontally" and 
may have the polarities shown. Alternatively, the various magnets 48, 50, 
and 52 may be positioned on the pole piece 44 so that they have opposite 
polarities. 
In any event, the various magnets 48, 50, and 52 produce a plasma-confining 
magnetic field (not shown) over the front surface 46 of target 42. The 
magnetic field confines the electrons (also not shown) over the surface 46 
of the target 42, which generally increases the ionization of the process 
gas and improves the sputtering rate and overall target utilization. 
The magnets 48, 50, and 52 preferably comprise rare-earth 
neodymium-iron-boron (NdFeB) magnets having magnetic field energy products 
of about 35 megagauss-oersteds. However, other types of rare earth 
magnets, such as samarium cobalt (SmCo) magnets, may also be used. Good 
results can also be obtained with ceramic magnets, such as barium or 
strontium ferrite magnets, although their magnetic field energy products 
are generally lower. Alternatively, the magnets 48, 50, and 52 may 
comprise an electro-magnet or electromagnets. 
It is generally preferred that the pole piece 44 comprise a ferromagnetic 
material, such as magnetic stainless steel. As used herein, the term 
"ferromagnetic" refers to those metals, alloys, and compounds of the 
transition (iron group) rare-earth and actinide elements in which the 
internal magnetic moments spontaneously organize in a common direction, 
giving rise to a magnetic permeability considerably greater than that of 
vacuum and to magnetic hysteresis. Ferromagnetic materials may include, 
without limitation, iron, nickel, cobalt, and various alloys thereof. 
The pole piece 44 may be mounted to a backing plate 40, which in turn 
supports the target material 42, as best seen in FIG. 3. The backing plate 
40 may also include a plurality of coolant passages 54 therein for 
allowing a liquid coolant 56, such as water, to circulate adjacent the 
back surface 45 of pole piece 44. The various passages 54 in the backing 
plate 40 are connected to an inlet pipe 76 and an outlet pipe 78. The 
coolant 56 may be provided by a suitable cooling system 34, as best seen 
in FIG. 1. The coolant 56 is used to keep the various magnets 48, 50, and 
52 from exceeding their respective Curie temperatures, i.e., from losing 
their magnetism. As used herein, the term "Curie temperature" refers to 
that temperature below which a magnetic material exhibits ferromagnetism 
and above which ferromagnetism is destroyed and the material is 
paramagnetic. The Curie temperature of a magnetic material may be 
different depending on whether the magnetic material is placed in a 
"closed loop" magnetic field or an "open loop" magnetic field. Generally 
speaking, the Curie temperature of a given magnetic material is lower when 
the material is in an open loop magnetic field. 
The backing plate 40 may be made from a non-ferromagnetic material, 
preferably having good thermal conductivity. For example, in one preferred 
embodiment, the backing plate 40 comprises copper, although other 
materials could also be used. 
The entire cathode/target assembly 20 may be surrounded by a shield 
assembly 58. The shield assembly 58 includes an aperture 60 which exposes 
the front surface 46 of target 42 to the interior 32 of the process 
chamber 18. The shield assembly 58 should be electrically isolated from 
the cathode/target assembly 20. Preferably, shield assembly 58 should be 
grounded. In one preferred embodiment, the shield assembly 58 may comprise 
a non-ferromagnetic material, such as type 304 stainless steel. 
The power supply 36 produces the modulated output signal 66 (FIGS. 4 and 5) 
which is applied to the cathode/target assembly 20. The modulated output 
signal 66 comprises a periodic signal having a plurality of negative 
pulses or periods 68 each of which is separated from the others by a 
plurality of quiescent periods 70. When the modulated output signal 66 is 
applied to the cathode/target assembly 20, material from the target 42 
will be sputtered during the negative period 68 but will not be sputtered 
during the quiescent period 70. Put in other words, the modulated output 
signal 66 produced by the power supply assembly 36 operates the 
cathode/target assembly 20 in an alternating manner between an "active" 
state and a "quiescent" state. During the active state, i.e., during the 
time when a negative voltage pulse 68 is applied to the cathode/target 
assembly 20, material is sputtered from the target 42 and is transferred 
from the target 42 to the substrate 16. During the quiescent state, 
substantially no sputtering of the target 42 occurs and substantially no 
material is transferred to the substrate 16. 
The modulated output signal 66 comprises several significant parameters 
that are of importance in achieving the objects and advantages of the 
present invention. While the various significant parameters are described 
in detail below, the order in which they are described is not related to 
their importance and no single parameter should be regarded as of 
paramount importance in any particular application. 
One significant parameter is the "turn-on" time 84. As used herein, the 
"turn-on" time 84 refers to the time required for the voltage on the 
cathode/target assembly 20 to decrease from substantially zero volts 
(e.g., ground potential) to about 90% of the steady-state operating 
voltage V.sub.1 (V.sub.1 being a negative voltage). Since the steady-state 
operating voltage V.sub.1 can vary significantly depending on the target 
material, the strength of the plasma-confining magnetic field, the 
pressure in the process chamber 18, the nature of the process being 
performed, and other extrinsic factors, the steady-state operating voltage 
V.sub.1 should not be regarded as being limited to any particular voltage 
or range of voltages. However, by way of example, the steady-state 
operating voltage V.sub.1 in one preferred embodiment may be in the range 
of about -100 to -1,000 volts. 
The turn-on time 84 should be made as fast as possible to promote effective 
control of the process being performed within the vacuum chamber 18. In 
one preferred embodiment, the turn-on time 84 may be in the range of about 
5 microseconds (.mu.s) to about 200 .mu.s, with a turn-on time 84 less 
than about 30 .mu.s being preferred. Since short turn-on times are 
advantageous, the shorter the turn-on time 84, the better. Accordingly, 
the critical limit on the turn-on time 84 is usually the high end of the 
range (i.e., corresponding to longer turn-on times 84). Therefore, it is 
preferred that the turn-on time 84 be no longer than about 200 
microseconds (.mu.s). 
Another significant parameter is the "turn-off" time 91. As used herein, 
the "turn-off" time 91 refers to the time required for the voltage on the 
cathode/target assembly 20 to increase from substantially the steady-state 
operating voltage V.sub.1 to the ground potential (e.g., zero volts). In 
one preferred embodiment, the turn-off time 91 is in the range of about 5 
.mu.s to 200 .mu.s, with a turn-off time 91 less than about 30 .mu.s being 
preferred. 
Two other critical parameters are the duration 72 of the negative pulse 68 
and the duration 74 of the quiescent period 70. For example, excessively 
long durations 72 of the negative pulse 68, while serving to increase the 
overall deposition rate, may tend to decrease the surface mobilities of 
the adatoms due to their becoming covered with additional incoming adatoms 
before they have sufficient time to migrate to low energy sites. Such 
decreased surface mobilities may well result in an increase in the number 
of interfacial voids and may lead to the formation of columnar 
morphologies, which are not generally preferred. Excessively long 
durations 74 of the quiescent period 70 tends to decrease the overall 
deposition rate which, again, may be undesirable in many applications. 
In view of the foregoing considerations, then, the optimal trade-off 
between effective control of film properties and high effective deposition 
rates can usually be achieved if the duration 72 of the negative period 68 
is between about 0.5 milliseconds (ms) and about 10 ms (1-2 ms preferred). 
The duration 74 of the quiescent period 70 should be between about 0.5 
milliseconds (ms) and about 50 ms (5-10 ms preferred). However, these 
optimal durations may vary somewhat depending on the particular process, 
the material being deposited, and other extrinsic factors. 
As was mentioned above, the power supply 36 includes a polarity reversing 
circuit 64 which superimposes upon the negative pulses 68 a plurality of 
polarity reversal signals 86, as best seen in FIG. 5. The polarity 
reversal signals 86 discourage the development of arc conditions within 
the chamber 18, thereby significantly reducing the occurrences of arcs, 
which can be deleterious to the process being performed in the chamber 18. 
Referring now specifically to FIG. 5, each polarity reversal signal 86 may 
comprise a short duration period 88 wherein the voltage on the 
cathode/target assembly 20 is briefly made positive with respect to the 
anode (i.e., the chamber 18). That is, the voltage on the cathode/target 
assembly 20 is momentarily increased to a positive potential V.sub.2 with 
respect to the anode. As was the case for the steady-state operating 
voltage V.sub.1, the positive potential V.sub.2 required to provide a 
satisfactory degree of arc suppression may vary depending on a wide 
variety of factors including, for example, the type of material being 
sputtered, the strength of the plasma confining magnetic field, the 
pressure in the process chamber, and the nature of the process being 
performed. Therefore, the positive potential V.sub.2 should not be 
regarded as being limited to any particular voltage or range of voltages. 
However, by way of example, the positive potential V.sub.2 may be in the 
range of about 20 volts to about 200 volts (100 volts preferred). 
The duration 88 of each polarity reversal pulse 86 required to provide a 
satisfactory degree of arc suppression may also vary depending on a 
variety of extrinsic factors, such as the type of material being 
sputtered, the strength of the plasma confining magnetic field, and the 
pressure in the process chamber. In one preferred embodiment, the duration 
88 of each polarity reversing pulse 86 may be in the range of about 2 
microseconds (.mu.s) to 5 .mu.s (3 .mu.s preferred) and the pulses 86 may 
occur at a frequency in the range of about 20 kilohertz (kHz) to about 80 
kHz (30 kHz preferred). However, other durations 88 and frequencies may 
also be used, again depending on the particular material being sputtered, 
the pressure within the chamber, and other extrinsic factors. 
One example of a power supply 36 suitable for producing the modulated 
output signal 66 is shown in FIGS. 1 and 6-9, and will be described below. 
However, other types of power supplies may be available now or in the 
future that may be capable of producing a modulated output signal 66 that 
meets the critical parameters discussed above. Accordingly, the present 
invention should not be regarded as limited to the particular power supply 
36 shown and described herein. 
Referring now to FIGS. 1 and 6-9, the power supply 36 used to produce the 
modulated output signal 66 may comprise a rectifier circuit 80 connected 
to a suitable supply of electrical power, such as an alternating current 
source 82. The rectifier circuit 80 converts the alternating current (AC) 
produced by the alternating current source 82 to a direct current (DC) 
suitable for use by the servo control circuit 62. As was mentioned above, 
the servo control circuit 62 produces the modulated output signal 66 that 
comprises a plurality of negative pulses 68 and quiescent periods 70. The 
power supply 36 is provided with a polarity reversing circuit 64 to 
generate a plurality of polarity reversing signals 86 which are then 
superimposed on each negative pulse 68 produced by the servo control 
circuit 62. See FIG. 5. 
The servo control circuit 62 utilized in one preferred embodiment may 
comprise a servo control circuit of the type shown and described in U.S. 
patent application Ser. No. 08/966489 filed Nov. 7, 1997 now U.S. Pat. No. 
5,910,866 published Jun. 10, 1999 and entitled "Phase-Shift Power Supply", 
which is incorporated by reference herein for all that it discloses. 
However, in order to provide a better basis for understanding the present 
invention, the servo control circuit 62 will now be briefly described. 
Referring specifically to FIG. 6, the servo control circuit 62 may comprise 
a first inductor or choke 90 and a first capacitor 92 connected in series 
across the positive (+) and negative (-) terminals of the rectifier 
circuit 80. See also FIG. 1. The first choke 90 and first capacitor 92 add 
inductive and capacitive reactance to the circuit and help dampen the 
power fluctuations resulting from the operation of the phase shift 
converter circuit 94. The values of the first inductor or choke 90 and the 
first capacitor 92 are not particularly critical and any of a wide range 
of values may be suitable depending on the design voltage and power levels 
of the particular power supply 36, as well as on other extrinsic 
considerations. Accordingly, the present invention should not be regarded 
as limited to any particular values or ranges of values for the choke 90 
and capacitor 92. By way of example, in one preferred embodiment the choke 
90 may have an inductance in the range of about 0.3 millihenrys (mH) to 
100 mH (1 mH preferred), and the capacitor 92 may have a capacitance in 
the range of about 100 microfarads (.mu.F) to 10,000 .mu.F (1500 .mu.F 
preferred). 
A converter circuit 94 is connected across the first capacitor 92 and 
comprises a first pair of switching devices 95, 96 connected across 
capacitor 92 and a second pair of switching devices 97, 98 connected in 
parallel with the first pair of switching devices 95, 96. A second choke 
99 and the primary winding 49 of a transformer 100 are connected in series 
across circuit node 81 (located between the first and second switching 
devices 95 and 96) and circuit node 83 (located between the third and 
fourth switching devices 97 and 98). The converter circuit 94 allows the 
voltage and current produced by the rectifier circuit 80 to be controlled 
as necessary to produce the modulated output signal 66 (FIGS. 4 and 5). 
The converter circuit 94 also provides other advantages, as will be 
described in greater detail below. 
The secondary winding 101 of transformer 100 is connected across a diode 
bridge formed by diodes 102, 103, 104, and 105. A third choke 107 and a 
second capacitor 108 are connected in series across the DC side of the 
diode bridge, i.e., between the cathodes of diodes 102 and 103 and the 
anodes of diodes 104 and 105. The positive (+) and negative (-) output 
terminals are connected across the second capacitor 108. The positive (+) 
and negative (-) output terminals may be connected to the polarity 
reversing circuit 64. Alternatively, if no polarity reversing circuit 64 
is used, then the positive (+) and negative (-) output terminals may be 
connected directly to the anode (e.g., vacuum chamber 18) and 
cathode/target assembly 20, respectively. A current sensor 106 connected 
between third choke 107 and the cathodes of diodes 102 and 103 senses the 
average output current of the servo control circuit 62. The function of 
the current sensor 106 will be described in greater detail below. 
The values for the various components described above are not particularly 
critical and may vary depending on the particular application and circuit 
design, as well as other extrinsic factors. Consequently, the present 
invention should not be regarded as limited to any particular values or 
ranges of values for the various components. However, by way of example, 
in one preferred embodiment wherein the voltage across the output 
terminals of the rectifier circuit 80 is in the range of about 100 volts 
to about 1,000 volts and wherein the expected output power of the power 
supply 36 is expected to be in the range of about 1 kilowatt (kw) to about 
15 kw, the third choke 107 should have an inductance in the range of about 
0.1 millihenrys (mH) to about 1 mH (0.3 mH preferred) and the second 
capacitor 108, a capacitance in the range of about 0.1 microfarads (.mu.F) 
to about 100 .mu.F (1.0 .mu.F preferred). The transformer 100 may have a 
turns ratio commensurate with the voltage produced by the rectifier 
circuit 80 and the desired output voltage. In one preferred embodiment, 
the transformer 100 has a turns ratio of 6:28. That is, the primary 
winding 49 comprises 6 turns, whereas the secondary winding 101 comprises 
28 turns. Alternatively, other turns ratios could also be used. The 
rectifying diodes 102, 103, 104, and 105 should be rated at 1,200 volts 
and 30 amperes. The current sensor 106 may comprise a model no. LT-100P 
current sensor available from LEM, USA of Milwaukee, Wis., although other 
devices could also be used. 
In one preferred embodiment, the switching devices 95, 96, 97, and 98 of 
the converter circuit 94 comprise MOSFETs rated at 500 V and 71 A , such 
as type APT50M60JN available from Advanced Power Technology, Inc., of 
Bend, Oreg. Alternatively, other types of switching devices may also be 
used, provided such devices are capable of switching the anticipated 
voltages and currents at the speeds required. 
Each of the switching devices 95, 96, 97, and 98 are connected to a pulse 
generator 109 which produces output signals suitable for switching each of 
the switching devices 95, 96, 97, and 98 between a non-conducting or open 
state and a conducting or closed state. Pulse generator 109 controls the 
various switching devices 95, 96, 97, and 98 based on a current control 
signal 11 (which may be provided by a suitable control circuit 13) and an 
average current feedback signal 15 (which is provided by current sensor 
106). Specifically, the pulse generator 109 produces a plurality of 
control signals 17, 19, 21, and 23 which control the respective switching 
devices 95, 96, 97, and 98. The control signals 17, 19, 21, and 23 are 
phase shifted with respect to one another to allow each of the switching 
devices 95, 96, 97, and 98 to be switched between the conducing and 
non-conducting states when the potential across each respective switching 
device is substantially zero volts. Such zero voltage switching minimizes 
power losses due to dissipation within the various switching devices 95, 
96, 97, and 98. 
While such zero voltage switching methods are well-known in the art and 
could be easily implemented by persons having ordinary skill in the art 
after having become familiar with the details of this invention, the zero 
voltage switching method used in one preferred embodiment will now be 
briefly described in order to provide a better basis for understanding the 
advantages of the servo control circuit 62 as they relate to the film 
deposition method and apparatus 10 according to the present invention. 
Basically, the zero voltage switching method of servo control circuit 62 
involves the out-of-phase (i.e., phase-shifted) switching of the switching 
devices 95, 96, 97, and 98. That is, instead of driving both of the 
diagonal switching devices (e.g., 95 and 98 or 96 and 97) together, in 
phase, the diagonal switching devices are turned on and off in a 
phase-shifted manner. Thus, only one of the diagonal switches is on before 
the other is activated. 
The operation of the converter circuit 94 may be better understood by 
considering a hypothetical example. Referring now to FIG. 7, the converter 
circuit 94 is operated by the various control signals 17, 19, 21, and 23 
which are used to control the switching devices 95, 96, 97, and 98, 
respectively. At a time t.sub.0, both control signals 17 and 23 (which 
control switching devices 95 and 98, respectively) are "on" (i.e., 
switching devices 95 and 98 are both conducting), while both diagonal 
switching device control signals 19 and 21 (which control switching 
devices 96 and 97) are "off," (i.e., switching devices 96 and 97 are 
non-conducting). Then, at a time t.sub.1, control signal 23 (i.e., 
switching device 98) is turned off, however control signal 17 (i.e., 
switch 95) is still "on." A short time later (t.sub.2), while control 
signal 17 (i.e., switch 95) is still on, control signal 21 (i.e., switch 
97) is turned on. Switches 95 and 97 both remain on for a time before 
control signal 17 (i.e., switch 95) is turned off at time t.sub.3. Then, a 
short time later, at time t.sub.4, control signal 19 (i.e., switch 96) is 
turned on, so that both diagonal switches (i.e., 96 and 97) are turned on. 
At time t.sub.5, control signal 21 (i.e., switch 97) is turned off, 
leaving on only switch 96. Again, after a short time, at time t.sub.6, 
control signal 23 (i.e., switch 98) is again turned on. Switches 96 and 98 
both remain on for a short time before control signal 19 (i.e., switch 96) 
is turned off at t.sub.7. Finally, control signal 17 (i.e., switch 95) is 
again turned on at time t.sub.8 and the cycle is repeated. 
The foregoing phase shifted switching method allows two of the switching 
devices in series with the primary winding 49 of transformer 100 to be on 
while the applied voltage to the primary winding 49 is zero, thus 
facilitating the zero voltage switching of the switching devices 95, 96, 
97 and 98. One advantage of the zero voltage switching technique is that 
it minimizes the power dissipation in the switching devices. Another 
advantage is that the zero voltage switching technique is that it 
eliminates the "pulse stretching" phenomenon that is typically associated 
with conventional pulse width modulated converter circuits. That is, in 
conventional pulse width modulated converter circuits, extraneous 
capacitance introduced by the switching devices and other circuit elements 
slows the decay of the pulse, thus lengthening or "stretching" the length 
(i.e., time duration) of the pulse beyond the desired turn-off time. 
Consequently, conventional pulse width modulated converter circuits will 
not generally be capable of the relatively fast turn-on and turn-off times 
required by the film deposition method and apparatus of the present 
invention. 
To sum up, the phase-shifted method of operating the converter circuit 94 
allows superior control of the output pulse width, which allows for the 
more precise control of the plasma process occurring in chamber 18, 
particularly at higher plasma impedances and at lower minimum power 
levels. The phase-shifted converter circuit 94 also allows the modulated 
output signal 66 to be produced which has a turn-on time 84 (FIGS. 4 and 
5) that meets the parameters set forth above. 
The phase-shifted control technique set forth above may be provided 
relatively easily by the pulse generator 109. In one preferred embodiment, 
the pulse generator 109 may comprise a model no. UC3875 pulse generator 
available from Unitrode Integrated Circuits Corporation of Merrimack, 
N.H., which is specifically designed for phase-shifted control. 
Alternatively, other pulse generators could also be used. 
Another significant feature of the servo control circuit 62 is that 
converter circuit 94 is controlled based on the average current output of 
the circuit, as sensed by the current sensor 106. Most conventional plasma 
power supply circuits embody two or more current and/or voltage feedback 
loops in order to control the output signal. The single average current 
feedback loop associated with the circuit topology shown and described 
herein substantially reduces the response time of the servo control 
circuit 62, which again affords superior control over the modulated output 
signal 66, thus the plasma process being carried out in chamber 18. 
The polarity reversing circuit 64 used in one preferred embodiment may 
comprise the polarity reversing circuit described in U.S. patent 
application Ser. No. 08/667,417, filed on Jun. 21, 1996, now U.S. Pat. No. 
5,682,067, which is incorporated herein by reference for all that it 
discloses. However, in order to provide a better basis for understanding 
the present invention, the polarity reversing circuit described in the 
aforementioned patent will now be briefly described. 
Referring now to FIG. 8, the polarity reversing circuit 64 may comprise a 
first inductor 25 connected in series between the negative (-) terminal of 
the output of the servo control circuit 62 and the cathode/target assembly 
20. The first inductor 25 adds a substantial amount of reactance to the 
circuit, thereby allowing the plasma process to operate at substantially 
constant current, at least for short term transient impedance variations 
likely to occur in the plasma 38 (FIG. 1). The amount of inductance of the 
first inductor 25 is not particularly critical, and it is only necessary 
to provide sufficient inductance to allow the plasma process to be 
operated at substantially constant current for the typically expected 
transient impedance variations in the plasma 38. By way of example, the 
first inductor 25 may have an inductance of about 1.5 millihenries (mH), 
although other inductances may also be used. 
The cathode 27 of a diode 29 is also connected to the cathode/target 
assembly 20, whereas the anode 31 of diode 29 is connected to one plate of 
a first capacitor 33. The other plate of first capacitor 33 is connected 
to the negative (-) terminal of the servo control circuit 62. The series 
arrangement of the first capacitor 33 and diode 29 forms a voltage limiter 
or clamp to prevent excessive negative voltages from being imposed on the 
cathode/target assembly 20 when the switching device 35 is opened. The 
size of diode 29 will, of course, depend on the capacity of the particular 
power supply 36, the expected peak voltages and currents, as well as the 
values chosen for the other components in the circuit 36. However, in one 
preferred embodiment, the diode 29 may comprise a single diode rated for 
800 volts (V) and 6 amperes (A). Alternatively, a combination of diodes 
may be used, as would be obvious to persons having ordinary skill in the 
art. 
The value of the first capacitor 33 must be selected so that it will not 
discharge significantly during the longest conducting period of switching 
device 35 at the maximum load current, i.e., the duration 88 of each pulse 
86 (FIG. 5). Further, the LC resonant circuit that comprises the first and 
second capacitors 33 and 37 and the second inductor 39 should have a 
sufficiently long time constant so that the second capacitor 37 will be 
positively charged when the switching device 35 is again switched to the 
conducting state. The range of values for the first capacitor 33 will be 
described later. 
The second capacitor 37 and the switching device 35 are connected in series 
across the cathode/target assembly 20 and the anode (e.g., the chamber 
18). It is preferred, but not required, that a first resistor 41 be 
connected in series between the capacitor 37 and the cathode/target 
assembly 20, and a second resistor 43 be connected in series between the 
switching device 35 and the anode (e.g., chamber 18). Although both 
resistors 41 and 43 act as current limiters to protect their associated 
components, resistor 43 also provides current limiting information to the 
switch actuation circuit 47. In one preferred embodiment, the first 
resistor 41 may have a value in the range of 1 ohm (.OMEGA.) to 50 .OMEGA. 
(8.OMEGA. preferred). The second resistor 43 may have a value in the range 
of 10 milliohms (m.OMEGA.) to 100 m.OMEGA. (50 m.OMEGA. preferred). 
Finally, the second inductor 39 is connected between the first and second 
capacitors 33 and 37, respectively. 
The first and second capacitors 33 and 37, along with the second inductor 
39, form an LC resonant circuit. In order to provide good performance, the 
time constant of the resonant circuit should be selected so that it is at 
least four (4) times longer than the expected length of the non-conducting 
period of switching device 35 (i.e., the period 51 between adjacent pulses 
86, FIG. 5) at the slowest cycle rate. It is preferred, but not required, 
that the time constant of the LC resonant circuit be about one (1) order 
of magnitude (i.e., about 10 times) longer than the length of the 
non-conducting period (i.e., 51). Thus, in one preferred embodiment having 
a non-conducting time period 51 in the range of 20-60 microseconds 
(.mu.s), the time constant of the resonant circuit should be selected to 
be about 600 .mu.s. Therefore, any values for the second inductor 39 and 
the first and second capacitors 33 and 37 that will yield such a time 
constant usually will suffice. In one preferred embodiment, the first 
capacitor 33 has a value in the range of about 1 microfarad (.mu.F) to 10 
.mu.F (1 .mu.F preferred), the second capacitor 37 a value in the range of 
1 .mu.F to 10 .mu.F (1 .mu.F preferred), and the inductor 39 has a value 
of about 5 millihenries (mH) to 10 mH (10 mH preferred). 
The switching device 35 may comprise any number of semiconductor switching 
devices suitable for switching the anticipated currents at a suitable 
speed (e.g., 0.1 .mu.s). Examples of such semiconductor switching devices 
include, but are not limited to, bi-polar transistors, insulated gate 
bi-polar transistors (IGBTs), field-effect transistors (FETs), metal-oxide 
semiconductor field-effect transistors (MOSFETs), etc., that are readily 
commercially available and well-known to persons having ordinary skill in 
the art. Alternatively, non-semiconductor switching devices may also be 
used, provided such devices are capable of switching the anticipated 
voltages and currents at the speeds required. In one preferred embodiment, 
the switching device 35 may comprise an insulated gate bi-polar transistor 
of the type which are readily commercially available. 
The switch actuation circuit 47 may comprise any of a number of circuits 
well-known in the art for periodically actuating the switching device 35 
to change it from a non-conducting state to a conducting state. In one 
preferred embodiment, the switch actuation circuit 47 accomplishes the 
actuation of switching device 35 by applying the appropriate base current 
to the IGBT transistor comprising switching device 35 to switch it from a 
non-conducting state to a conducting state at the desired time intervals. 
That is, the switch actuation circuit 47 should be designed to provide a 
switch actuation signal 53 to the switching device 35 at any of a range of 
frequencies suitable for providing an acceptable level of arc suppression 
for the particular plasma process being performed. In one preferred 
embodiment adapted for a non-reactive sputter deposition process, the 
switch actuation circuit 47 activates (i.e., closes) the switching device 
35 for about 2-5 .mu.s (i.e., the duration 88 of the reverse polarity 
cycle 86 shown in FIG. 5), at a frequency of about 50 kilohertz (kHz), 
although other durations 88 and frequencies could also be used. 
In certain circumstances, it may be advantageous to enhance the arc 
suppression capabilities of the power supply circuit 36 described above by 
connecting an arc detection circuit 55 to the switch actuation circuit 47. 
The arc detection circuit 55 works in concert with the normal operation of 
the switch actuation circuit 47 to reverse the voltage potential on the 
cathode/target assembly 20 if an arc condition is detected during the time 
when the switching device 36 is in the non-conductive (i.e., open) state 
(e.g., during the time 51 between two successive pulses 86). While many 
different kinds of arc detection devices are known and may be used with 
the present invention, one preferred embodiment of the present invention 
uses an arc detection circuit 55 substantially as shown in FIG. 9. 
Essentially, arc detection circuit 55 may comprise first and second 
resistors 57 and 59 connected in series with a first capacitor 61 across 
the cathode and anode (e.g., the cathode/target assembly 20 and the 
chamber 18). See FIG. 1. A second capacitor 63 is connected in parallel 
with the first resistor 57. A third capacitor 65 is connected in series 
with the second capacitor 63 so that it is also connected in parallel with 
the series arrangement of resistor 59 and capacitor 61. The combination of 
resistors 57, 59 and capacitors 63 and 65 form a compensated voltage 
divider network which provides to the buffer amplifier 67 a 
reduced-voltage signal 69 that more accurately represents the actual 
voltage signal (i.e., wave-form) on the cathode (i.e., cathode/target 
assembly 20) and anode (i.e., chamber 18), than is possible with 
conventional, purely resistive voltage divider networks. A bias voltage is 
applied to resistor 59 to define the voltage level at which an arc is to 
be recognized. That is, an arc condition is assumed to exist and is 
recognized by the arc detection circuit 55 if the voltage potential 
between the electrodes starts to decrease and passes through a defined 
voltage level. The defined voltage level can be varied by adjusting the 
magnitude of the bias voltage applied to resistor 59. 
The reduced voltage signal 69 from the compensated voltage divider network 
is then fed into buffer amplifier 67. The output signal 71 from buffer 
amplifier 67 is fed via resistor 73 to a diode clamp circuit comprising 
diodes 75 and 77. Under normal circumstances, the output signal 71 from 
buffer amplifier 67 will be negative. Hence, current will flow via 
resistor 73 and diode 75 and a negative clamped voltage of about 0.6 volts 
will appear across the input leads 79 and 85 of comparator 87. Conversely, 
if the output signal 71 from amplifier 67 goes positive, diode 77 will 
conduct, again imposing a clamped voltage of about 0.6 volts across input 
leads 79 and 85 of comparator 87. The arrangement of diodes 75 and 77, 
along with resistor 73, forms a non-linear voltage divider network which 
substantially reduces the input impedance presented to the input of 
comparator 87, thus removing a substantial amount of the RF (i.e., radio 
frequency) noise present in the output signal 71. The comparator 87 
produces an output signal 89 when the voltage across its input leads 79, 
85 passes through zero. The output signal 89 from comparator 87 may then 
be fed into the switch actuation circuit 47 to trigger the switching 
device 35. It should be noted that in this application it will be 
desirable to design the arc detection circuit 55 so that it will ignore 
the low voltage on the cathode (i.e., cathode/target assembly 20) if the 
low voltage occurs during the normal reverse polarity cycle 86 of the 
switch actuation circuit 47, i.e., during normal pulsing. 
Before proceeding with the description of the operation of the film 
deposition apparatus 10 according to the present invention, it is useful 
to first define certain terms commonly used in the field of plasma 
processing and that are used herein. The term "plasma" (or, alternatively, 
"glow discharge") is usually defined as a region of high temperature gas 
containing large numbers of free electrons and ions. When used in the 
context of plasma processing technology, some persons consider the 
"plasma" as extending throughout the entire chamber. Others, however, 
consider the plasma as limited substantially to the region of the "glow 
discharge" (generally located adjacent the surface of the cathode/target) 
since the glow discharge region is readily visually discernable. As used 
herein, the term "plasma" shall mean the latter of the two definitions. 
That is, the term "plasma" shall refer to the region substantially 
coincident with the glow discharge. 
The film deposition apparatus 10 according to the present invention may be 
operated as follows to deposit the film 12 onto the surface 14 of 
substrate 16. Assuming that proper pressure and atmosphere have been 
established within the chamber 18, the sputter deposition process (i.e., 
either non-reactive or reactive) may be initiated by activating the power 
supply 36. Upon activation, power supply 36 places the modulated output 
signal 66 on the cathode/target assembly 20. During the negative pulse or 
period 68, an electric field is established between the cathode/target 
assembly 20 and the anode (e.g., chamber 18) which initiates or "ignites" 
the plasma 38. In accordance with the foregoing definition, the plasma 38 
is coincident with the glow discharge and is located adjacent the 
cathode/target assembly 20. Positively charged ions (not shown) contained 
in the plasma 38 are drawn to the negatively charged cathode/target 
assembly 20. Some of the incoming ions have sufficient energy to dislodge 
or sputter atoms of the target material. The sputtered atoms migrate 
through the interior 32 of the process chamber 18 and condense on various 
objects contained within the chamber, including the surface 14 of 
substrate 16, whereupon they form the film 12. When the modulated output 
signal 66 enters the quiescent period 70, the plasma 38 is extinguished, 
which terminates the sputtering process. Consequently, substantially no 
material is transferred from the target 42 to the substrate 16 during the 
quiescent period 70. 
As was briefly described above, there is no significant activity that 
occurs on the cathode/target 20 during the quiescent period 70, at least 
insofar as the properties of the film 12 are concerned. However, the same 
cannot be said with regard to the surface 14 of substrate 16. During the 
quiescent period, previously deposited atoms (i.e., adatoms) of the target 
material may be rearranging themselves on the surface 14 of substrate 16. 
This phenomenon is referred to herein as "surface mobility." The 
alternating sputtering process according to the present invention 
significantly increases the surface mobilities of the deposited adatoms 
over those which would be associated with the same apparatus but operating 
in a continuous manner, i.e., without a quiescent period 70. 
The increased surface mobilities of the adatoms are due primarily to two 
factors: The quiescent period 70 and the higher energy adatoms typically 
associated with the present invention. The quiescent period 70 provides 
the adatoms with additional time migrate over the surface. Therefore, even 
though the adatoms may not posses significantly greater energies compared 
with those which would be associated with the same apparatus but operating 
without a quiescent period 70, the additional migration time afforded by 
the quiescent period 70 means that the adatoms will have generally greater 
surface mobilities. In fact, even relatively low-energy adatoms may have 
significantly greater surface mobilities as a result of the increased 
migration time afforded by the quiescent period 70. 
The second factor that tends to increase the surface mobilities of the 
adatoms is that the adatoms produced by the present invention generally 
have relatively high energies. For example, the film deposition apparatus 
10 according to the present invention may be operated at relatively high 
power levels during the active state which tends to produce adatoms of 
considerably higher energies. In fact, in one preferred embodiment, the 
film deposition apparatus 10 may be operated at power densities of about 
400 watts per square inch of target surface area or higher, which is 
considerably greater than conventional film deposition apparatus. The 
higher energies of the adatoms produced by the present invention generally 
increases their surface mobilities. 
While the precise nature and mechanisms relating to the growth of the film 
12 are not yet fully understood, the increased adatom mobilities 
associated with the method and apparatus of the present invention provide 
operators with improved control of the process and the characteristics of 
the deposited film. For example, the increased adatom mobility associated 
with the present invention generally allows more adatoms to migrate to low 
energy sites where nucleation and growth occurs, thereby increasing the 
nucleation density in many instances. The increased nucleation density may 
promote more interfacial reactions, thereby improving adhesion. 
The increased mobility also provides increased operator control over film 
morphology. For example, if the film deposition apparatus 10 is operated 
at relatively high powers (e.g., 400 watts/in.sup.2) during the negative 
pulses or periods 68, the energies of the adatoms will be proportionally 
higher. The increased adatom energy, combined with the increased migration 
times afforded by the quiescent periods 70 generally results in the 
formation of relatively high density films free of the columnar type 
microstructures typically associated with prior high deposition rate 
processes. Put in other words, films produced by the present invention 
will tend to have improved surface coverage with a consequent reduction in 
the number of interfacial voids which may result in the easy fracture and 
poor adhesion of the film. Such increased film density may be reflected in 
film properties such as better corrosion resistance, lower chemical etch 
rate, higher hardness, lower electrical resistivity (in metals), and 
increased index of refraction for optical coatings. 
If the film deposition method and apparatus according to the present 
invention are used to perform reactive sputter deposition, then the 
quiescent period 70 provided by the present invention will provide 
additional time for the reactive species to diffuse to the regions where 
the reactions are taking place between the sputtered species and the 
reactive species. The increased diffusion time will result in improvements 
in film chemistry and other film parameters. 
It is contemplated that the inventive concepts herein described may be 
variously otherwise embodied and it is intended that the appended claims 
be construed to include alternative embodiments of the invention except 
insofar as limited by the prior art.