Sputtering apparatus with a magnet array having a geometry for a specified target erosion profile

A rotatable magnet configuration for use in a magnetron sputtering system for obtaining a desired sputter target erosion profile and for obtaining a sputtered film on a substrate having a desired film characteristic, such as uniformity of thickness, and a method of designing such a magnet configuration are disclosed. The disclosed design and method compensate for the discrepancy between the between the actual position of the magnet and the "effective" position of the magnet as measured by a static erosion profile obtained holding the magnet stationary. The method shown describes how to adjust the actual shape of the magnet to obtain a desired effective shape that will produce a predetermined erosion profile in the sputter target. In addition, the disclosure describes how to determine an optimal erosion profile to produce a sputtered film having a desired characteristic.

FIELD OF THE INVENTION 
This invention relates to sputtering apparatus and, in particular, to a 
magnetron sputtering apparatus using a generally heart-shaped, closed-loop 
rotatable magnet array. 
BACKGROUND OF THE INVENTION 
Physical vapor deposition by sputtering is a well known process that has 
found widespread application in the fabrication of integrated circuit 
semiconductor devices. In semiconductor device fabrication, a large number 
of integrated circuit devices are normally formed on a thin, generally 
circular semiconductor substrate referred to as a wafer. Integrated 
circuit device fabrication involves a large number of processing steps, 
with sputtering typically being used to provide metallization layers and 
interconnects between device layers. Most commonly, sputtered aluminum is 
the material used for these purposes. Modern semiconductor processing has 
also seen the increased use of sputtered tungsten, tungsten silicide, 
titanium, titanium nitride and other films. 
A magnetron sputtering source is capable of high rate sputtering and 
represents an enormous improvement over devices forming thin films based 
on diode sputtering or evaporative techniques. Magnetron sputtering 
sources are routinely used by the semiconductor processing industry to 
coat semiconductor wafers during the manufacture of integrated circuits. 
In magnetron sputtering a plasma is formed in a low pressure inert gas by 
the application of a suitable voltage. The plasma is confined to a region 
near a sputter target, which is made of the material to be sputtered and 
which usually serves as the cathode of the system. A magnetic field, 
typically having field lines which loop through the sputter target 
surface, restricts the trajectories of the electrons in the plasma, 
thereby intensifying and confining the plasma. Ions in the plasma bombard 
the sputter target dislodging atoms of the target material which are then 
deposited on a substrate. 
In recent years wafer sizes have continually increased, and now the use of 
eight-inch diameter wafers is common in the industry. Large wafer sizes 
permit a larger number of integrated circuit devices to be grown on a 
single substrate. However, larger wafer sizes impose greater demands on 
sputtering systems. For example, one requirement of a sputtering system 
used in semiconductor processing is that it deposit a layer of uniform 
thickness over the entire wafer surface. (Hereinafter the term uniformity 
will be used in connection with the thickness of the deposited film unless 
the context suggests otherwise.) Lack of uniformity may result in lowered 
device yield (i.e., the percentage of devices which meet operating 
specifications) and/or variations in device performance. Larger wafer 
sizes make it more difficult to achieve very demanding levels of 
uniformity. Likewise, the trend towards ever smaller integrated circuit 
device geometries has required that even greater levels of sputtered film 
uniformity be achieved. 
Other sputtered film characteristics are also quite important to integrated 
circuit device manufacturers. For example, as noted above, sputtered 
conductive material is frequently used to form interconnects between 
device layers. Forming interconnects involves uniformly filling small 
diameter holes, called vias, in the surface of the wafer. As integrated 
circuit device geometries have shrunk, the difficulty in filling vias with 
sputtered material has increased appreciably. Step coverage, or the 
ability of the sputtered film to evenly conform to angular features on the 
wafer surface is, likewise, another important film characteristic. 
An earlier approach to improving the uniformity and step coverage 
characteristics of a sputtering system is to sputter from two concentric 
targets. For an example of this approach see U.S. Pat. No. 4,606,806 which 
describes a sputtering source sold by the assignee of the present 
invention under the trademark ConMag.RTM. II. In the ConMag.RTM. II 
sputtering source each of the sputter targets has a unique shape and its 
own separate power supply enabling separate control over the sputtering 
rate from each target. 
A number of commercially available sputtering sources use planar sputtering 
targets. Early designs, wherein the planar magnetron sputtering device 
used a stationary magnet had practical shortcomings, the most serious of 
which is that the plasma discharge is localized and erodes a narrow groove 
in the target in the vicinity of the greatest magnetic field strength. 
This localized erosion generates a non-uniform distribution of sputtered 
atoms from the target and a film with non-uniform thickness on the 
semiconductor wafer. The non-uniform erosion of the sputter target leads 
to inefficient target utilization. Given the high cost of the sputter 
targets used in semiconductor manufacture, it is important to obtain the 
greatest possible target utilization that is consistent with the need for 
sputtered film uniformity and other required sputtered film 
characteristics. 
Numerous attempts, some partially successful, have been made to modify the 
planar magnetron source to extend the target erosion and to make the 
distribution of sputtered atoms more uniform. Attempts have been made to 
spread out the erosion over a larger surface area using extended magnetic 
fields. The magnets required for such an approach are large and 
complicated, and it is difficult to assure that the properties of the 
magnetron do not change as the target erodes away. The resulting erosion 
pattern is thus difficult to predict. 
U.S. Pat. No. 4,444,643, which is incorporated herein by reference, 
describes a sputtering device which includes a mechanically rotated 
annular permanent magnet assembly. The rotation of the permanent magnet 
assembly causes erosion over a wider area of the target. A version of the 
sputtering source described in the '643 patent has been sold commercially 
by the assignee of the present invention under the trademark VersaMag.TM.. 
This source relies on a rotating magnet mounted behind the target for 
moving the plasma over the face of the target. Rotation of the plasma was 
introduced for the purposes of improving uniformity and step coverage, as 
well as improving the uniformity of target erosion so that targets are 
more efficiently utilized. 
The VersaMag sputtering source, while a significant improvement over planar 
magnetron sources employing stationary magnets, nonetheless did not 
produce truly uniform sputtered film nor uniform target utilization. Thus, 
efforts have been made to develop improved rotating magnet designs for use 
with planar targets. (The term "planar target" is intended throughout this 
specification to be descriptive of the sputter target surface before it is 
eroded. Those skilled in the art will recognize that after the target has 
been eroded it may no longer have a planar surface.) 
One direction that has been taken by those seeking to improve the design of 
rotating magnets used with planar magnetron sputtering sources has been 
the used of closed-loop, generally heart-shaped magnet configurations. 
Such magnet configurations typically employ an array of magnets which are 
centered along a line defining a heart-shaped, closed loop. 
One such arrangement is described in U.S. Pat. No. 4,872,964, entitled 
"Planar Magnetron Sputtering Apparatus And Its Magnetic Source", issued 
Oct. 10, 1989 to Suzuki, et al. Suzuki, et al., review the shortcomings of 
a sputtering source of the type described in the '643 patent and describe 
a heart-shaped rotating magnet array which is said to produce more uniform 
target erosion. However, the Suzuki, et al., patent overly simplifies the 
mathematics of the situation and, therefore, does not fully teach how to 
obtain truly uniform target erosion. In apparent recognition of this 
shortcoming, Suzuki, et al., describe the need to adjust the magnet array, 
after it has been laid out in accordance with their mathematical analysis, 
"to get more uniform erosion after a test run of the sputtering 
apparatus." (Col. 5, lines 27-28.) Unfortunately, Suzuki, et al., do not 
teach any methodology for making the necessary adjustments. The teachings 
of Suzuki, et al., are directed to how to obtain uniform erosion of the 
target. While uniform target erosion is important, the characteristics of 
the sputtered film, such as uniformity, are of greater importance to 
integrated circuit device manufacturers. In many instances, as will be 
described below, a non-uniform target erosion pattern improves the 
uniformity of the sputtered film. 
Another sputtering source having a heart-shaped magnet arrangement is 
described in Japanese Patent Application Publication (Kokai) No. 
62-211,375 entitled "Sputtering Apparatus", published Sept. 17, 1987. That 
patent prescribes the use of a heart-shaped closed loop magnet having a 
curve defined by the equation r=l-a+2a.vertline..theta..vertline./.pi., 
(for -.pi.23 .theta..ltoreq..pi.); where the center of the sputter target 
is located at the origin of a polar coordinate system, r is the distance 
between the origin and a point on the curve defining the magnet 
centerline, l is the distance between the center of the heart and the cusp 
of the heart, and a is the distance between the center of the heart and 
the center of the target. No derivation is given as to how the inventors 
arrived at this equation, and it appears to be a compromise between the 
annular-shaped magnet of the '643 patent and the heart-shaped magnet of 
the '764 patent. As discussed in the '375 application, a magnet having the 
prescribed curve will not produce uniform erosion. Moreover, the '375 
application does not teach how to obtain any arbitrarily selected erosion 
profile. 
U.S. Pat. No. 4,995,958, entitled "Sputtering Apparatus With A Rotating 
Magnet Array Having A Geometry For Specified Target Erosion Profile", 
issued Feb. 26, 1991, to Anderson, et al., also assigned to the assignee 
of the present invention, describes another generally heart-shaped, 
closed-loop magnet array for use in a planar magnetron sputtering source. 
The Anderson, et al., patent, which is hereby incorporated by reference, 
includes a rigorous mathematical analysis to show how to construct a 
closed-loop rotating magnet to realize a predetermined erosion profile to 
thereby achieve, for example, highly efficient target material utilization 
and high deposition rates. It is noted that the invention of the '958 
patent is readily adapted to use in a VersaMag.TM. sputtering source. 
Among other things, the '958 patent describes the shortcomings of the 
aforementioned Suzuki, et al., patent and the teaching of the '375 
Japanese patent application, showing how each reference fails to provide a 
teaching which truly enables one to obtain uniform erosion of a planar 
sputter target. Importantly, FIGS. 12A-12E of the '958 patent, and the 
related text, clearly show that minor changes in the shape of a 
heart-shaped magnet may lead to very dramatic differences in the resulting 
erosion pattern of the sputter target. (It is believed that this is also 
shown by the '375 application.) Given the demonstrated fact that minor 
perturbations of the shape of a heart-shaped magnet may cause significant 
changes in the resulting target erosion profile, it becomes quite 
difficult to optimize the shape empirically. Thus, Anderson, et al.'s, 
mathematical analysis is a highly significant teaching in making 
heart-shaped, closed-loop magnets practically useful. 
A closed-loop magnet configuration of the type described in the '958 patent 
has the additional advantage of being easily adjustable so that the shape 
of the magnet array, and therefore the characteristics of the sputtering 
source, can be changed without great difficulty or expense. As described 
in that patent, a plurality of magnets are held in position by two iron 
keepers, or pole pieces, which define the shape of the closed loop. 
Replacement and/or adjustment of the iron keepers to provide a different 
closed-loop configuration is a relatively simple matter. In this manner it 
is possible to use one source for different purposes, or to adjust the 
source as needs change. 
A prime objective of the closed-loop rotating magnet of the '958 patent was 
to achieve better target utilization efficiency, normally an important 
objective given the high cost of sputter targets, and to achieve high 
deposition rates, another important factor due to the demand for ever 
greater system "throughput". As noted above, the need for greater 
sputtered film uniformity generally outweighs the need for efficient 
target utilization and deposition rate. Accordingly, the Anderson, et al., 
patent provides the basis for obtaining any arbitrary target erosion 
profile. It is noted, however, that the Anderson et al., patent provides 
no instruction as to how to determine what erosion profile to use under a 
given set of conditions to maximize sputtered film uniformity or other 
sputtered film characteristics. 
As described therein, the mathematical analysis provided by Anderson, et 
al., is inapplicable at two areas of the heart, i.e., in the area near the 
"tip" of the heart, which is defined herein to mean the generally convex 
portion of the loop farthest away from the axis of rotation, and in the 
area near the "cusp" of the heart, which is defined herein to mean the 
generally concave portion nearest the axis of rotation and which lies 
between the two lobe-shaped portions of the heart. As a result of the 
inapplicability of the Anderson, et al., teaching to the region of the 
cusp of the heart, the designs they show leave the very center of the 
target unused, and are not optimized for best utilization of the sputter 
target edge. Moreover, the analysis of the '958 patent is based on the 
assumption that the magnet has uniform strength at all point along the 
loop, i.e., the sputtering intensity is the same at all points. In other 
words, the total quantity of material sputtered per unit length of the 
magnet is a constant. It has been observed that this assumption is not 
correct. 
It is noted that all of the heart-shaped designs shown by Anderson, et al., 
Suzuki, et al., and the '375 application are symmetrical about a line 
which passes through the tip, the cusp, and the axis of rotation of the 
heart. The symmetry of the Anderson, et al., designs is due to the fact 
that their method of generating a heart-shaped magnet is by forming a 
spiral-like shape over 180.degree. (i.e., over one half of a polar 
coordinate system) and then mirroring this shape to close the loop over 
the remaining 180.degree.. However, as used herein, the term heart-shaped 
does not require that there be two strictly symmetrical halves. As will be 
described below, there may be circumstances when an asymmetrical 
heart-shaped magnet is desired. Likewise, as used herein, the term 
heart-shaped, does not require that the heart have a noticeable "tip". It 
has been found that there are advantages to using a design wherein the 
region farthest from the axis of rotation and generally opposite the cusp 
forms an arc of a circle. As used herein, the term "heart" implies that 
there is a cusp-like transition between two lobes. The cusp-like 
transition may be smoothed for design convenience. 
Finally, it has been discovered that there are discrepancies between the 
position of the magnetic field adjacent to the sputter target surface as 
predicated by Anderson, et al., and as empirically measured. As noted 
above, even minor changes in the shape of the magnetic field generated by 
a heart-shaped magnet may result in significant variations in the erosion 
profile obtained. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to improve and extend 
the teachings of the Anderson, et al., patent to obtain better sputtered 
film characteristics and better utilization of the target in a sputter 
source when using a generally heart-shaped, closed-loop rotating magnet 
array in a planar magnetron sputtering device. 
Another object of the present invention is to provide a method for defining 
the shape of a heart-shaped, closed-loop rotating magnet array used in a 
planar magnetron sputtering system so as to produce a desired sputter 
target erosion profile at positions that are nearest and farthest from the 
axis of rotation of the magnet array. 
Still another object of the present invention is to provide a method for 
determining the erosion profile on a sputter target which results in 
desired sputtered film characteristics, such as a desired level of 
uniformity, on a wafer being coated by sputtering. 
Yet another object of the present invention is to provide a method for 
determining the shape of a generally heart-shaped, closed-loop rotating 
magnet for producing a desired erosion profile in a sputter target that 
takes into account variations between the predicted position of the 
magnetic field and the measured position of the corresponding static 
erosion track. 
Another object of the present invention is to provide a method for 
designing a generally heart-shaped, closed-loop rotating magnet for 
producing a desired erosion profile in a sputter target, which is not 
symmetrical. 
These and other objects of the present invention that will be apparent to 
those skilled in the art upon reading this specification are realized by a 
method and apparatus for providing a closed-loop rotating magnet array for 
use in a sputtering source having a planar target. In one aspect of the 
present invention, the method of determining the shape of the magnet is 
determined by starting with a heart-shaped magnet array, preferably one 
which is believed to be close to the desired shape. A magnet array having 
this starting shape is placed in a sputtering system and sputtering is 
performed while holding the magnet stationary. A static erosion profile is 
generated in this manner. The relationship between the location of this 
static erosion profile and the magnet centerline is mapped and this 
information is used to adjust the shape of the magnet so that a desired 
erosion profile is obtained. 
By using the foregoing method it is possible to construct a magnet that 
results in a predetermined erosion profile on the surface of a planar 
magnetron sputtering target, where the static shape of the erosion profile 
differs from the shape of the underlying magnet and takes into account the 
discrepancy between the two. 
In another aspect of the present invention, the desired shape of the 
erosion profile is established by determining a distribution function 
defining, to at least the first order, the angular distribution of atoms 
ejected from the sputter target under the operating conditions of the 
sputter source; determining the spacing between the surface of the sputter 
target and the substrate being coated; determining the sizes of the 
substrate and the sputter target and calculating the erosion profile that 
will result in uniform sputtered film deposition on the substrate.

DETAILED DESCRIPTION 
FIG. 1A is a schematic plan view of a prior art heart-shaped, closed-loop 
magnet array 5, comprising a plurality of individual permanent magnets 7a, 
7b, 7c . . . 7n, for use in a planar magnetron sputtering system, built in 
accordance with the teachings of U.S. Pat. No. 4,995,958. As taught by the 
'958 patent, the centerline 10 of magnet array 5 lies on a curve defined 
by the equation: 
##EQU1## 
where .xi.(u) is a function defining a preselected erosion profile and C 
is a selected constant. 
As described in the '958 patent, each of the magnets 7i is uniformly 
disposed on a centerline 10, which lies between spaced apart inner and 
outer iron keepers 12 and 14, respectively. The spacing between inner and 
outer keepers 12 and 14 is uniform at all points except in the vicinity of 
the tip 11 and the cusp 12 of the heart. Iron keepers 12 and 14 are 1/16" 
thick in accordance with a preferred embodiment of the present invention. 
Magnet array 5 is mounted on plate 17 which is connected to means (not 
shown) for rotating it about an axis of rotation 60. 
While the preferred embodiment is described in connection with a magnet 
array of the type depicted in FIG. 1A having iron keepers which serve as 
pole pieces, it will be recognized by those skilled in the art that other 
magnet arrangements are possible. For example, segmented magnets of the 
type shown and described in the aforementioned Suzuki, et al., patent will 
be recognized as being equivalent. A segmented magnet array offers the 
advantage of being easier to adjust from one configuration to another and, 
if desired, fine tune. 
FIG. 1B shows the theoretically calculated erosion profile 19 for the FIG. 
1A magnet array 5. In other words, centerline 10 of magnet array 5 is laid 
out, per the above equation, to produce the erosion profile 19 shown in 
FIG. 1B. Stated equivalently, in FIG. 1B the function .xi.(u) is a 
constant so as to produce uniform erosion between the limits of 
integration. It is noted, however, that for r.ltoreq.1 the equation is not 
solvable. Accordingly, when the centerline 10 of magnet array 5 is laid 
out in accordance with the teachings of the '958 patent, there is little 
or no erosion of the center of the target, as shown in FIG. 1B. 
Various other heart-shaped, closed-loop magnet shapes designed to 
theoretically produce different sputter target erosion profiles wherein 
the function .xi.(u) is not a constant are shown in FIGS. 12A-12E of the 
aforementioned '958 patent. It will be noted from an inspection of these 
figures, and the accompanying text, that minor variations in the shape of 
a heart-shaped magnet will produce significant differences in the shape of 
the resulting target erosion profile. It follows that, while use of a 
properly configured rotating heart-shaped, closed-loop magnet in 
connection with a planar magnetron sputtering device ca result in improved 
target erosion uniformity, it is not obvious how to empirically arrive at 
the proper shape necessary to produce any arbitrary erosion profile. 
Each of the heart-shaped magnet arrays shown in the '958 patent, or in any 
of the other prior art of which the inventor is aware, is symmetrical 
about a line 18 which runs though the tip 11, the cusp 12 and the axis of 
rotation 60 of the magnet. Moreover, each of the prior-art magnets is 
"pointed" at its tip 11; in other words the centerline is not smooth at 
the point the curve is reflected, i.e., the derivative of the centerline 
is discontinuous at this point. 
EPC Patent Application Serial No. 91-300565.8, published Jan. 15, 1992, 
(hereinafter the '565 application) also assigned to the assignee of the 
present invention, describes magnet configurations based on the teachings 
of the '958 patent but extended to correct for the limitations of the '958 
patent so that uniform erosion may be obtained in the center region of the 
target. 
FIG. 2 is a schematic plan view of a prior art heart-shaped, closed-loop 
magnet array 5' for use in a planar magnetron sputtering system, that has 
been sold commercially by the assignee of the present invention under the 
trademark "Quantum S.TM.". For clarity the magnets within the array have 
been omitted and only inner and outer iron keepers 20 and 30 are shown. A 
plurality of individual permanent magnets is distributed between the iron 
keepers in a manner to similar to what is shown in FIG. 1. 
The magnet array 5' of FIG. 2 is based on the teachings of the '958 patent 
as extended by the '565 application. In contrast to the heart-shaped 
designs of the '958 patent, the design of FIG. 2 includes magnets 
positioned in the vicinity of the axis of rotation 60 of magnet array 5'. 
In the FIG. 2 embodiment, the placement of the magnets near the axis of 
rotation is not strictly based on the teachings of the '565 application 
but also includes a combination of designer intuition and empirical 
results. It is noted that near the lobes and cusp 12 of the heart the iron 
keepers are not evenly spaced, and the departure from uniform spacing of 
the keepers in the vicinity of cusp 12 is far greater than the slight 
departure present in the FIG. 1A embodiment. The lack of uniform spacing 
makes placement of the magnets more difficult in this vicinity. Moreover, 
as described in the '565 application, the magnets in the central region 
may be different in strength. 
While the magnet design of FIG. 2 represents an improvement over the design 
of FIG. 1A, insofar as it produces better target utilization near the 
center of the sputter target, the mathematical analysis that is required 
is cumbersome, making it difficult to obtain predictable control over the 
sputtering from the center with changes in the design. In addition, it has 
been determined that neither the FIG. 1A nor the FIG. 2 design precisely 
results in the erosion profile predicted by the teachings of the '958 
patent or of the '565 application respectively. 
FIG. 3A is a schematic plan view of one embodiment of a heart-shaped, 
closed-loop magnet array 5" for use in a planar magnetron sputtering 
system built according to the present invention. Again, in the interest of 
clarity, the individual magnets which comprise the array are not shown. 
Rather, only inner and outer iron keepers 40 and 50 are shown in relation 
to the axis of rotation 60 and the plate 17 that magnet array 5" is 
mounted on. 
Before describing the method used to arrive at the shape of the FIG. 3A 
magnet array 5" , a description of its shape vis-a-vis the prior art is 
given. Starting at cusp 12 of the heart, it is noted that inner keeper 40 
crosses the axis of rotation 60 of the magnet array, as in the FIG. 2 
embodiment, while maintaining nearly even spacing between inner and outer 
keepers 40 and 50 in the vicinity of the cusp. Thus, this portion of the 
magnet achieves the benefits of the FIG. 2 embodiment, without sacrificing 
the benefits of maintaining uniformly spaced keepers, and without the 
design complexities of other embodiments shown in the '565 application. 
It will also be noted that, just beyond the lobes of the heart, the shape 
of the magnet array has two inward inflections 76 and 77. In other words, 
while the shape of the prior art heart-shaped magnets all are such that 
the curve is at all points convex in relation to the interior of the loop, 
in the embodiment of FIG. 3A, there are two portions of the curve, 76 and 
77, that are concave in relation to the interior of the loop. 
Finally, it should be noted that the FIG. 3A embodiment has no "tip". While 
the FIG. 3A embodiment is symmetrical about line 18 running through cusp 
12 and axis of rotation 60 of the heart, the portion of the curve that is 
farthest away from axis of rotation 60, and which lies on either side of 
the axis of symmetry 18, lies on an arc of a circle. Accordingly, the 
curve at this point is smooth, i.e., the derivative of the curve is 
continuous at this point. Moreover, in the FIG. 3A embodiment, a major 
portion of the heart, perhaps as much as one-forth or more of the curve, 
lies on this arc. 
FIG. 4 shows the magnet array of FIGS. 2 and 3A in juxtaposition, so that 
the differences in shape may be more clearly seen. While the overall 
shapes appear to be quite different, the departure is not great at any 
given point around the loop. 
FIGS. 3B and 3C show the calculated and observed target erosion profile, 
respectively, for the magnet of FIG. 3A when used with a titanium target. 
Note that the two curves are nearly identical, except in the center 
region, where the actual erosion was greater than calculated. This 
discrepancy at the center may be related to the fact that the actual 
position of the magnet differed slightly from the position used to make 
the calculated erosion profile. It also may reflect the relative paucity 
of measured static erosion data points near the very center of the target. 
In arriving at the magnet configuration of FIG. 3A it was determined that 
a non-uniform target erosion profile would produce the best uniformity of 
sputtered film. 
The method by which the shape of the magnet array depicted in FIG. 3A, and 
by which other shapes can be constructed to produce a selected erosion 
profile, will now be described. An initial heart-shaped, closed-loop 
magnet array is first constructed. In the preferred manner of performing 
the method of the present invention, the initial closed-loop magnet may 
either be constructed in accordance with the principles described in the 
'958 patent or in the manner used to create the Quantum S.TM. magnet, so 
that a desired erosion profile will be approximated by the initial magnet 
shape. It is also possible to start with other heart-shaped designs, 
including those known in the prior art. 
This initial magnet array is then placed in a sputtering system and the 
system is operated while holding the magnet array stationary to generate a 
static erosion groove in the surface of the sputter target. When 
performing this step it is preferred that the sputter target be 
constructed of the material which will be used with the magnet array being 
designed, and that the operating parameters of the system coincide with 
the actual operating parameters that will be used by the system in 
production. For reasons that are not fully understood, it has been 
observed that the static erosion profile produced by a given magnet array 
will differ slightly depending upon the material that is being sputtered. 
As is to be expected, the resulting static erosion groove is also 
heart-shaped and forms a closed loop. Moreover, as expected from the prior 
art of planar magnetron sputtering using a stationary magnet array, any 
given cross section of the erosion groove around the loop has a 
valley-like appearance having a bottom region where erosion is greatest. 
However, for reasons that are not fully understood, the bottom of the 
erosion groove does not overlie the centerline of the magnet array. This 
discrepancy between the bottom of the erosion groove and the centerline of 
the magnet array, if not corrected or compensated for, will produce 
results that vary from the teachings of the '958 patent. 
An important premise of the '958 patent is that the region of greatest 
target erosion directly overlies the centerline of the magnet array. This 
premise is based upon the assumption that the magnetic field intensity 
adjacent to the target surface is greatest directly over the magnet 
centerline. It has now been empirically observed by the inventor that this 
premise is not fully accurate. As noted above, minor changes in the 
configuration of a heart-shaped, closed-loop magnet array can result in 
significant changes in the erosion profile produced by the magnet array. 
The discrepancy between the shape of the magnet array and the erosion 
profile it produces during static sputtering can be thought of as meaning 
that the magnet array being used has a different effective shape than is 
intended. This problem is not recognized in the '958 patent, nor does the 
'958 patent teach how to compensate for the discrepancy to produce a 
magnet with the proper effective shape. 
After a static erosion groove is made, the shape of the groove is carefully 
measured at a finite number of points around the groove and a plot is 
generated showing the mathematical relationship between position and depth 
of target erosion. In the preferred way of implementing the method of the 
present invention, these measurements are made in polar coordinates. For 
example, at a finite number of values of R, (R being the radial distance 
from the axis of rotation 60 of the magnet array 5,) the depth of target 
erosion is measured as a function of .theta.. 
An example of a plot created in this manner is shown in FIG. 5A. In FIG. 5A 
the horizontal axis is the angle .theta. and the vertical axis is the 
depth of erosion, E(.theta.). For R=K.sub.i, where K.sub.i is a given 
value of distance from the center of the coordinate system, i.e., the axis 
of rotation, it is seen that a typical erosion profile 100 includes two 
regions of erosion 110 and 120 as one rotates from 0.degree. to 
360.degree.. A set of similar plots are then created for a finite number 
of values of K.sub.i, for example, twenty such plots may be created, e.g., 
if the radius of the target is 5 inches, then plots could be created at 
each quarter inch interval between the origin and the edge of the target. 
The values of K.sub.i may be thought of as defining a set of concentric 
circles centered around the axis of rotation. The present technique does 
not require that the values of K.sub.i be evenly spaced. For example, one 
may select values of K.sub.i that correspond to the positions of the 
individual magnets in the array. In the example shown in FIG. 5A the 
selected value of K.sub.i is 3 inches. 
Each of the static erosion plots that has been empirically created using 
this technique is then integrated over a revolution of the magnet to 
produce a value of erosion depth E(R) for that particular value of 
K.sub.i. The erosion depth values for each K.sub.i are then plotted to 
generate an overall erosion profile for the magnet when it is rotated. An 
example of a curve 130 generated by this method is shown in FIG. 5B. The 
integration required by this step may be performed using standard 
numerical integration techniques that are well-know to those skilled in 
the art. In the graph of FIG. 5B, the vertical axis is, again, the depth 
of erosion, E(R), while the horizontal axis is the radial position on the 
target surface relative to the origin, i.e., the axis of rotation. Point 
140 represents the data point obtained from the plot of K.sub.i as shown 
in FIG. 5A, i.e., at R=3 inches. (Each of the calculated data points that 
comprise erosion profile 130 is represented by a solid square, and these 
points have been normalized to a maximum value of one.) 
It has been found that the calculated erosion profile 130 of FIG. 5B 
conforms to the observed erosion profile generated when the magnet is 
rotated. 
The static erosion data plotted to form the graph of FIG. 5A can be plotted 
in another manner, as is shown in FIG. 5C. Rather than plot the erosion 
depth at a certain radial distance as a function of angle, as in FIG. 5A, 
the erosion depth data can be plotted at a finite number of angles 
.theta.=.alpha..sub.i, as a function of distance, r. If the target in 
thought as being a wheel, the data points plotted on FIG. 5C may be 
thought of as the erosion profile taken along a "spoke" of the target, 
where each spoke is at an angle .alpha..sub.i. In the example of FIG. 5C, 
.alpha..sub.i =160.degree.. Again, the erosion profile shown in FIG. 5B 
can be generated from the data in the set of curves of FIG. 5C. 
It will be apparent to those skilled in the art that the static erosion 
profile data of FIGS. 5A and 5C can equivalently be represented in a 
three-dimensional system or numerical array wherein a finite number of 
data points on the surface of the sputter target each has a set of values 
associated with the angular position .theta., the erosion depth 
E(R,.theta.), and the radial distance R of that point on the surface of 
the sputter target from the origin, i.e., the axis of rotation. 
The curves of FIG. 5C have been conveniently used by the inventor in 
practicing the present invention to adjust the shape of the magnet array 
to correct for the discrepancy between the predicted erosion profile of 
the '958 patent and the static erosion profile observed. How this may be 
done will now be explained. On FIG. 5C the radial position of the 
centerline of the magnet array for .theta.=.alpha..sub.i is shown at 
dashed line 160. The positions of inner and outer pole pieces, 170 and 180 
respectively, are also shown. The discrepancy between the effective shape 
of the magnet and the actual shape is readily apparent from the offset 
between line 160 and the point of greatest erosion. For convenience, the 
position of the centerline of the magnet along the x-axis is arbitrarily 
defined to be at point x=0. (For illustrative purposes, an equivalent 
horizontal axis 190 is shown relating the erosion profile to the radial 
position on the target.) A fifth order polynomial is then derived, using 
known mathematical techniques, to fit the data points plotted in FIG. 5C. 
This fifth order polynomial is shown as curve 150. 
It is assumed that a minor perturbation of the centerline of the magnet at 
a given .alpha..sub.i will not affect the shape of curve 150, nor will it 
affect the offset between magnet centerline 160 relative to curve 150. It 
is also assumed that a minor perturbation of the centerline of the magnet 
at a given .alpha..sub.i will not affect the shape or offset of curve 150 
at any other value of .alpha.. These assumptions are quite reasonable if 
the displacement of the magnet at each .alpha. is small. In order that 
these assumptions remain valid, it is best to start with an initial magnet 
shape which is expected to be close to what the final shape will be, so 
that the perturbations of the positions of the individual magnets remain 
small. However, if the perturbations grow too large, the technique 
described herein may be done on an iterative basis. 
The effect of making minor adjustments to the position of the magnet 
centerline at selected values of .alpha..sub.i can readily be translated 
into a revised, calculated erosion profile that reflects the adjustments 
that have been made. For example, erosion profile 130' in FIG. 5B is shown 
to reflect the adjustment of several of the magnet centerline in a number 
of positions. (Each of the calculated data points that comprise erosion 
profile 130' is shown as a hollow square.) It is thus possible to 
calculate the perturbations in the position of the magnet means at 
selected points that are needed to produce a preselected erosion profile 
on the surface of the target. 
So long as the perturbations of the magnet positions are small enough that 
the assumptions described above remain reasonable, the technique that has 
been described is a very powerful tool for selecting a magnet shape that 
will produce a selected erosion profile. While based in principle on the 
teachings of the '958 patent, it will be seen that the technique of the 
present invention has several advantages over what is shown in the '958 
patent. These advantages include: 
(1) the ability of the present technique to obtain a magnet shape that will 
result in preselected erosion profile, accounting for the discrepancy 
between the actual and the effective shape of the magnet array; 
(2) the ability of the present technique to predict and adjust the shape of 
the erosion profile in the vicinity of the cusp of the magnet, and in the 
vicinity of the magnet closest to the edge of the sputter target; 
(3) the ability of the present technique to correct for variations in the 
erosion intensity over the length of the closed-loop; and, 
(4) the ability to make perturbations in the shape of the magnet to produce 
an asymmetrical heart-shaped design. 
The use of an asymmetrical design allows greater "fine-tuning" of the 
magnet shape. In a symmetrical design, any magnet centerline adjustment in 
a region lying on one half of the magnet is doubled because the same 
adjustment is automatically made to the symmetrical half. Using the 
present technique, it is possible to adjust one half of the magnet in a 
predictable way while holding the other half unchanged. This is because 
the technique uses data taken over the full 360.degree. of the coordinate 
system. 
As noted, by making suitable adjustments to the centerline of the magnet, 
it is possible to produce a magnet that has an effective shape that is in 
accordance with the teachings of the '958 patent. In other words, it is 
possible to configure a magnet that produces a static erosion pattern on 
the surface of the target a portion of which conforms to the equation: 
##EQU2## 
where .xi.(u) defines a preselected erosion profile and where the 
centerline of the magnet is displaced from the centerline of the static 
erosion groove to compensate for the discrepancy therebetween. 
Having shown how to obtain a magnet having a shape that results in a 
predetermined erosion profile on the surface of the sputter target, we now 
turn to a discussion of how to determine what is the optimal erosion 
profile. While many of the prior art patents emphasize the desirability 
and importance of uniformly eroding the surface of the sputter target, 
uniformity of erosion is actually a secondary consideration to most users 
of sputtering systems. The primary consideration is the need to 
consistently obtain a sputtered film having desired characteristics, for 
example, uniformity, on the wafers being coated. An aspect of the present 
invention is a technique that may be used to calculate the target erosion 
profile that will result in a film having the desired characteristics at 
the surface of the wafer. 
The present technique for predicting the uniformity of film deposition 
takes into account a number of variables that affect the rate at which 
sputtered material accumulates at any given point on the surface of the 
substrate. The important variables that enter into the technique of the 
present invention will now be discussed. 
First, it is necessary to know the angular distribution of atoms ejected 
from the surface of the sputter target. It is frequently assumed, for the 
sake of simplicity, that sputtered atoms are ejected in a cosine 
distribution. This assumption is reasonable in the case of aluminum, the 
most commonly used sputtered film in semiconductor integrated circuit 
fabrication, where the atomic weight of the material (Z=27) is 
significantly less than the atomic weight of argon (Z=40) which is 
typically used as the sputtering gas. It known, however, that when 
sputtering higher atomic weight materials such as titanium (Z=48) or 
tungsten (Z=184), the angular distribution of sputtered atoms does not 
conform to a cosine distribution. Moreover, in some cases the crystalline 
structure of the sputter target material may also affect the angular 
distribution of sputtered atoms. 
FIGS. 6A and 6B are plots, obtained from the scientific literature, showing 
the angular distribution of nickel (Z=59) and platinum (Z=195) atoms, 
respectively, ejected by bombardment with ions of mercury (Z=201). FIG. 6A 
clearly shows the influence of these factors in producing non-cosine 
distribution. This figure shows a major departure from a cosine 
distribution despite the fact that nickel has a much lower atomic weight 
than mercury. FIG. 6B shows the influence of the kinetic energy of the 
bombarding ions, i.e., as the ions become more energetic, the angular 
distribution increasingly approaches a cosine distribution. Conversely, it 
will be observed that at lower energies the distribution function has a 
"flatter" appearance, i.e., fewer of the ejected atoms leave at an angle 
that is nearly normal to the surface. This phenomenon may be explained by 
the fact that it takes more incident energy to cause an atom to be knocked 
loose in the normal direction than to knock it loose at a sharply acute 
angle. 
Accordingly, the initial step in determining a desired erosion profile is 
to establish the angular distribution of the atoms being sputtered under 
the operating conditions that will be used in the system. If the requisite 
information is not available from the literature, this may be accomplished 
by empirical measurement using the material to be sputtered in a system 
operated under similar conditions. After the distribution is empirically 
determined, the data is then fit to a mathematical function which 
approximates, to at least the first order, the measured distribution. In 
the preferred method of the present invention, it has been found useful to 
approximate the empirical data by using a distribution function which is a 
power of the cosine function multiplied by a second function having an 
adjustable parameter, as set forth in the following: 
##EQU3## 
where K.sub.1 is an adjustable parameter selected to fit the data and 
K.sub.2 is a scaling factor. 
The next parameter which needs to be understood before a calculation of a 
desired erosion profile may be made is the distance between the substrate 
and the sputter target. Normally, the distance between the sputter target 
surface and the wafer will be constant (ignoring the effects of target 
erosion), in a given system configuration. In most applications, it is 
desirable to space the substrate as close as possible to the sputter 
target to maximize the deposition rate and to minimize loss of target 
material. (From the foregoing discussion of the angular distribution of 
sputtered atoms, it should be apparent from the geometry that an atom 
which leaves the surface of the target at an acute angle has a greater 
chance of landing on the substrate if the substrate is closely spaced to 
the target. If a significant number of sputtered atoms do not land on the 
substrate, both the rate of film deposition will be lowered and the waste 
of sputtered material will be increased.) 
On the other hand, close coupling of the target and the substrate makes it 
more difficult to obtain adequate sputtered film uniformity. It is 
sometimes preferable to interpose shields, shutters, collimators, etc., to 
enhance the sputtered film characteristics or to control the sputtering 
process, and the present magnet designs and methods are applicable where 
the source to substrate spacing is increased to accommodate such 
structures. As a practical matter, sputter sources having a 
wafer-to-target distance of 2 to 10 cm are now in use. 
Another geometrical parameter that must be factored into the calculation of 
the desired erosion profile is the relative sizes of the sputter target 
and the wafer. If uniformly eroded, a target that is much larger than the 
substrate will result in considerable waste of sputtered material. On the 
other hand, if the target is similar in size or smaller than the wafer, it 
will be more difficult to achieve adequate deposited film uniformity. Such 
a configuration would also make it difficult to achieve adequate 
"step-coverage" in the deposited film, where step coverage is a well-known 
measure of the ability of the film to coat angled device features on the 
wafer surface. For example, close correspondence in size between the 
target and the wafer would result in a substantial difference in the 
ability of the sputtered film to cover angular features on the device 
depending on which way those features were facing. There would be a low 
flux of sputtered material arriving at the wafer at angles toward the 
perimeter and, thus, it is likely that outwardly facing features near the 
edge of the wafer would not receive an adequate coating of sputtered film. 
On the other hand, inwardly facing features at the same location would 
likely receive an adequate coating since the flux of material would be 
from angles corresponding to the center of the target. 
Most modern integrated circuit devices are now being made on eight-inch 
diameter wafers, and a sputter target of the type used in the present 
invention has a diameter of 11.64 inches, i.e., the edge of the target 
extends almost two inches beyond the edge of the wafer. 
Yet another parameter that should be considered in calculating the optimum 
sputter target erosion profile is the operating pressure of the sputtering 
system. While the sputtered atoms may leave the surface of the target with 
a certain angular distribution, collisions between gas molecules (or 
plasma ions) and sputtered atoms may alter the trajectories of the 
sputtered atoms before they reach the wafer surface. The scattering 
effects of gas collisions on the angular distribution of sputtered atoms 
reaching the surface of the wafer can be calculated. It has been found, in 
the context of the operating parameters used in the preferred embodiment 
of the present invention, that gas collisions may or may not be a 
significant factor depending upon the total pressure used during 
sputtering. The effects of gas scattering can be reduced with close 
coupling between the target and the substrate and low operating pressure 
of the sputtering system, i.e., 1 millitorr. 
Thus, using the method of the present invention for determining the desired 
target erosion profile involves first determining the geometry of the 
system. In particular, knowledge of the target-to-substrate spacing and 
the diameters of the target and the substrate are required. Next, one 
defines a mathematical function which approximates, to at least the first 
order, the distribution of sputtered atoms leaving the surface of the 
sputter target at the operating conditions of the sputter source. Finally, 
if necessary, an adjustment is made to the distribution function to 
account for gas scattering effects. 
With the foregoing information in hand, it is then possible to calculate a 
target erosion profile that will result in the application of a uniformly 
thick sputtered film on the surface of the wafer. The calculation is based 
on the fact that the erosion rate at a point on the surface of the target 
is a measure of the sputtering rate from that point. Such a calculation 
can be performed using a variety of computer modelling techniques of the 
type that are known in the art. It should be noted that there will be more 
that one erosion profile that will produce uniform sputtered film 
deposition. It is desirable to select an erosion profile which both 
produces a uniform film and, to the greatest extent possible, makes the 
best use of the target material, and low particulation by avoiding target 
areas of net build up. 
Having arrived at a desired erosion profile in the foregoing manner, it is 
then possible to configure the magnet, in the manner described above, to 
obtain the desired erosion profile. 
The technique of the present invention has, thus far, been described as two 
separate procedures, i.e., a procedure for calculating a desired erosion 
profile and a procedure for configuring a heart-shaped, closed loop magnet 
to obtain the desired erosion profile. These two procedures can be 
combined into a single method in which all of the necessary information is 
entered into a computer model. The effect on sputtered film uniformity due 
to minor perturbations in the position of the magnet centerline at one or 
more locations can then be directly calculated. One could use such a 
combined model to prescribe a suitable magnet configuration for a given 
set of parameters. 
FIG. 7 shows a flow chart for practicing a preferred method of the present 
invention wherein the sputtered film uniformity is directly calculated 
with changes in magnet position. As described above, the first step 710 is 
to form a static erosion groove on the surface of the sputter target. The 
depth of target erosion is then measured 720 at a finite number of points 
(r, .theta.) on the surface of the target. For selected values of .theta., 
fifth order polynomials are constructed 730 to fit the observed data, and 
the polynomials are entered into an array. For simplicity, the fifth-order 
polynomial for a particular .theta. is made relative to the centerline of 
the magnet used to generate the static erosion groove. 
The coordinates of the centerline of a new magnet shape to be tested are 
then entered into an array 740 and the distance from each value of (r, 
.theta.) used in the array to the new magnet centerline is calculated and 
entered into an array. Preferably, the position of the centerline of the 
new magnet does not vary significantly at any given location from the 
position of the centerline of the magnet used to form the static erosion 
groove. A static erosion depth calculation 750 is then made for each (r, 
.theta.) based on the new magnet positions. The calculated static erosion 
depth information is then integrated over a revolution of the magnet to 
obtain a calculated erosion profile associated with the new magnet 
positions 760. 
The uniformity of the sputtered film is then calculated 770, using the 
target erosion profile calculated in step 760. As describes above, the 
calculation of film uniformity should take into account the geometry of 
the sputtering system as well as the characteristics of the material being 
sputtered and any significant gas scattering effects. Calculation 770 
involves a double integration so that the effects of the release of 
material at a given rate and at a given distribution from each point on 
the target is assessed for each point on the wafer. The double integration 
involves calculating, for each point on the surface of the wafer, the 
amount of material that will be deposited. This, in turn, involves 
integrating the flux from each point on the target both as a function of 
angular position and as a function of radial position. After the 
uniformity information is thus calculated, a judgment is made as to 
whether acceptable uniformity has been achieved, i.e., the calculated 
uniformity for each of the points on the wafer is compared to determine 
the variance in film thickness across the wafer surface. If acceptable 
film uniformity has been achieved, i.e., if the variance has been 
minimized, the method is complete and the magnet shape is constructed. If 
the film uniformity is not acceptable, or if it is thought that further 
improvement may be achieved, the magnet shape is further perturbed and 
entered at step 740 and the process is repeated again from that point. 
A systematic approach to perturbing the magnet at step 740 that has been 
used by the inventor is as follows. Starting with the magnet shape that is 
used to form the static erosion groove of step 710, a new magnet is 
defined wherein the position of the magnet is changed at only one point, 
which may be at any selected value of .theta.. After the sputtered film 
uniformity for this new magnet is calculated, the uniformity obtained is 
compared with the uniformity obtained by the prior magnet configuration. 
If the uniformity is improved, the same magnet position is further 
adjusted in the same direction and another uniformity calculation is made. 
If the uniformity is degraded, the magnet is moved in the opposite 
direction and a uniformity calculation is made per steps 740-780. The 
first magnet position is iteratively adjusted until no further improvement 
in uniformity is obtained. Thereafter, the next magnet position is 
adjusted and the same procedure is followed until no further adjustment of 
the second magnet produces improved uniformity. This process is repeated 
for all of the magnet positions in sequence, after which the entire 
process can be repeated as many times as is desired starting again at the 
first magnet position. This systematic approach lends itself quite well to 
automation, and can easily be implemented by a computer program. Other, 
equivalent, systematic approaches will be apparent to those skilled in the 
art. For example, one might limit the adjustments of any given magnet 
position to a set number during each "loop" around the magnet. 
Up until now, the method of the present invention has been described solely 
in the context measuring a static erosion groove to determine the 
"effective" magnet shape. While this is the preferred manner of 
implementing the present invention, other methods are possible. For 
example, it is possible to measure the "effective" magnet position by 
observing the position and intensity of the plasma created by a starting 
magnet. Modern techniques allow one to accurately measure the plasma 
intensity at a finite number of preselected points over the target, and 
this information could be used as a substitute for the information 
obtained from measurement of a static erosion groove. It is believed, 
however, the plasma intensity information would not be as accurate and is, 
therefore, less preferred. 
While the flow chart of FIG. 7 has been described solely in the context of 
sputtered film uniformity, other sputtered film characteristics can also 
be taken into account. For example, step 770 could, instead, involve a 
calculation of the step coverage produced by the new magnet configuration 
or of the via-filling properties. While the present techniques for 
via-filling involve steps which go beyond simple sputtering, the ability 
to deposit a suitably thick layer of film at the bottom of the via is an 
important aspect of the process. It should be apparent from geometrical 
considerations that the ability to fill the bottom of a narrow via is 
related to the angular distribution of the atoms of sputtered material 
striking the surface of the wafer. Thus, sputtered atoms which are 
incident at the wafer surface in a substantially perpendicular direction 
are likely to reach the bottom of a via, whereas atoms which are incident 
at a sharp angle will be intercepted by a wall of the via before reaching 
the bottom. 
Likewise, more than one film characteristic can be calculated, with the 
goal of optimizing the balance between them. For example, improved 
uniformity caused by a particular magnet perturbation may be offset by 
degraded step coverage. 
While the present invention has, thus far, been described solely in the 
context of a heart-shaped planar magnetron sputter source, it should be 
apparent to those skilled in the art that the methodology described herein 
is equally applicable to other magnet designs intended to produce a 
predetermine erosion profile in the surface of a sputter target. For 
example, the teachings of the '958 patent are not limited to heart-shaped 
magnet arrays, and several non-heart-shaped magnet array configurations 
are disclosed. To the extent there is a similar discrepancy between the 
actual and the effective shape of a magnet array built in accordance with 
one of the other embodiments, i.e., the static erosion groove does not 
overlie the magnet centerline, the methodology taught herein can be used 
to adjust the positions of the individual magnets in the array to achieve 
a desired effective magnet shape and a desired erosion profile. Likewise, 
according to the present invention, by appropriate computer modelling, the 
effect on sputtered film uniformity caused by adjustments in the positions 
of the individual magnet positions can be directly determined. 
U.S. patent application No. 471,898, abandoned, which is a 
continuation-in-part of the '958 patent, and which is hereby incorporated 
by reference, extends the teachings of the '958 patent so that magnet 
arrays can be constructed for use with non-planar sputter target surfaces 
to produce arbitrarily selected erosion profiles. A condition required by 
the '898 application is that the sputter target surface be a surface of 
revolution. The '898 application teaches that the centerline of the closed 
loop magnet array should lie on a curve defined by: 
##EQU4## 
where .xi.(u) is a preselected erosion profile to be generated in the 
curved target when the magnet is rotated and sputtering is performed, z(r) 
is a surface of revolution defining the surface of the sputter target, and 
C is a selected constant. 
Like the '958 patent, a premise of the '898 application is that the 
magnetic field intensity adjacent to the target surface is greatest 
directly above the centerline of the magnet array so that the shape of a 
static erosion groove would conform to the shape of the magnet. Again, it 
will be apparent to those skilled in the art that the methodology of the 
present application can be applied to magnets configured in accordance 
with the '898 application to compensate for discrepancies between the 
actual and effective magnet array shapes. 
While the above description has been directed to the preferred embodiments 
set forth herein, such is intended to be exemplary and not limiting. Many 
variations and substitutions will be obvious to those skilled in the art 
in view of the foregoing disclosure. Accordingly, it is intended that the 
scope of the patent be limited only by the following claims.