Abstract:
A target for a physical vapor deposition system includes a top, a bottom, and a base. The base essentially is defined by the surface of the target to be sputtered. A first, inner ring and a second, outer ring extend from the base. Each ring has an inner side and an outer side, wherein sputtering is concentrated on the outer sides by means of a magnet arrangement adjacent to the target.

Description:
This application claims the benefit of U.S. provisional patent application Ser. No. 61/307,077 filed Feb. 23, 2010, the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     The present invention relates to a physical vapor deposition (PVD) target. In particular, it relates to a target that is shaped like a truncated cone, a dome, or a Fresnel lens. 
     Discussion of Prior Art 
     Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by ejecting or “sputtering” material from a target onto a substrate, such as a silicon wafer. 
     Sputter coating apparatuses are generally known. In a typical apparatus, an energy discharge is used to excite atoms of an inert gas, e.g. argon, to form an ionized gas or plasma. Charged particles (electrons) from the plasma are accelerated toward the surface of a sputter target by application of a magnetic field. The sputter target typically is provided in the form of a rectangular slab, sheet, or plate. The plasma bombards the surface of the target, thus eroding that surface and liberating target material. The liberated target material then can be deposited onto a substrate, such as metal, plastic, glass, or a silicon wafer, to provide a thin-film coating of the target material on the substrate. 
     Sputtering sources can be magnetrons that utilize strong electric and magnetic fields to trap electrons close to the magnetron surface or target. These magnetic fields can be generated by an array of permanent magnets arranged behind the target, thus establishing a magnetic tunnel above the target surface. The electrons are forced to follow helical paths caused by the electric and magnetic fields and undergo more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. This results in a closed plasma loop during operation of the magnetron. At the location of the plasma loop on the surface of the target, a “racetrack” groove is formed, which is the area of preferred erosion of material. In order to increase material utilization, it is known in the prior art to use movable magnetic arrangements to sweep the plasma loop over larger areas of the target. 
     In order to decrease the racetrack groove formation and achieve more efficient utilization of the target, non-flat targets are known in the prior art. In general, it is a well known practice to increase the target thickness in the regions of main erosion. For example, U.S. Pat. No. 4,842,703 to Class et al. and U.S. Pat. No. 5,688,381 to Grünenfelder et al. disclose targets with a concave surface. 
     Several PVD applications require using a long distance between the target and the substrate. This is known as long target-to-substance distance (TSD) sputtering. Long TSD sputtering narrows the angular profile of the material sputtered from the target, making the sputtered material easier to direct. Long TSD sputtering is required in order to produce layers of film with low sidewall coverage, such as to enable lift-off processing or to avoid unwanted fencing of sidewall material when a photo resist is removed. 
     A disadvantage of a long target-to-substrate distance (TSD) is the resulting poor uniformity of the deposited material on the substrate. This effect can typically only be compensated by increasing the diameter of the target. However, increasing the diameter of the target can be burdensome and impractical. For example, if the substrate is a 300 mm wafer, a very big and uneconomic target size would be required. 
     Another disadvantage of long TSD sputtering is a dramatically reduced sputtering rate. In addition, because gas is scattering over an increased distance, the effect of narrowing the angular distribution is alleviated. In fact, the effect of directional sputtering almost disappears at realistic pressures of 1-2 mTorr, even at target-to-substrate distances as low as 150 mm. 
       FIGS. 1 to 5  illustrate the aforementioned issues with current sputtering targets. In particular,  FIG. 1  illustrates a sputtering erosion profile of at a prior art target. In the graph of  FIG. 1 , the Y-variable is percent erosion of the target center area, and the X-variable is target radius at a point on the target in cm. The calculations are based on a target diameter of 400 mm and a substrate diameter of 300 mm at varying target substrate distances. The center of the target has a percent erosion of 10%. At a target radius between 10 and 15 cm, the percent erosion begins to increase until it eventually reaches 100%. This increased erosion represents the racetrack groove.  FIG. 1  shows that while the material around the racetrack groove is completed eroded, plenty of useful material remains near the target center. 
     For the erosion profile of  FIG. 1 ,  FIG. 2  plots uniformity of the layer deposited on the substrate for target-to-substrate distances (TSDs) from 120 mm to 800 mm. The Y-variable is percent uniformity of the layer on the substrate, and the X-variable is substrate radius in mm. The graph of  FIG. 2  shows that the flattest, and therefore most ideal, deposition profile is achieved for a TSD of 150 mm. For lower distances, such as 120 mm, the profile has a concave shape. For higher distances, such as 175 mm, 200 mm, 300 mm, 400 mm, or 500 mm, the profile gets convex. However for very long distances, such as 800 mm, the convexity decreases since the target can be seen more and more as a point source. 
     In  FIG. 3 , the behavior of  FIG. 2  is plotted as function of TSD. The Y-variable is percent surface fluctuation of the deposited layer on the substrate, and the X-variable is TSD in mm. Similar to  FIG. 2 ,  FIG. 3  indicates the best uniformity around a TSD of 150 mm. 
       FIG. 4  depicts sputtering efficiency based on the same conditions as  FIGS. 1-3 . In  FIG. 4 , the Y-variable is percent deposition efficiency, and the X-variable is TSD in mm. As the TSD increases, the deposition efficiency decreases. 
     It should be appreciated that these calculations have been done for very low pressures of 0.1 mTorr. When the pressure is increased, which may be necessary to sustain stable plasma, the uniformity gets much worse.  FIG. 5  depicts this pressure effect on uniformity for a TSD of 500 mm. The Y-variable is percent surface fluctuation of the deposited layer on the substrate, and the X-variable is substrate radius in dm. 
     Thus, there is need for improvements in sputtering targets in order to more evenly erode the target while having positive uniformity and efficiency characteristics. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The following summary presents a simplified summary in order to provide a basic understanding of some aspects of the systems and/or methods discussed herein. This summary is not an extensive overview of the systems and/or methods discussed herein. It is not intended to identify key/critical elements or to delineate the scope of such systems and/or methods. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
     In accordance with one aspect, the present invention provides a target for a physical vapor deposition system. The target includes a base with a center and a rim, an inner ring extending from the base, and an outer ring extending from the base. 
     In accordance with another aspect, the present invention provides a sputter chamber. The sputter chamber includes an enclosure, a substrate support member, a sputter target that faces the substrate support member within the enclosure, and a magnetron. The sputter target has a base, an inner ring extending from the base, and an outer ring extending from the base. 
     In accordance with still another aspect, the present invention provides a target for a physical vapor deposition system. The target includes a top, a bottom, and a sloped edge connecting the top and the bottom. The sloped edge has a first portion extending from the top a first vertical distance. The first portion and the top define a first obtuse angle. The sloped edge also has a second portion extending from the first portion a second vertical distance. The second portion and the top define a second obtuse angle. The first vertical distance is greater than the second vertical distance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other aspects of the invention will become apparent to those skilled in the art to which the invention relates upon reading the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a sputtering erosion profile of a prior art target; 
         FIG. 2  is a deposition uniformity profile for the prior art target; 
         FIG. 3  is a uniformity profile similar to  FIG. 2 , but plotted as a function of TSD; 
         FIG. 4  is a graph of sputtering efficiency for a prior art target; 
         FIG. 5  is a uniformity profile similar to  FIG. 2 , but calculated at a higher pressure; 
         FIG. 6  is a schematic illustration of a sputtering chamber; 
         FIG. 7 a    is a prospective view of a target according to one aspect of the present invention; 
         FIG. 7 b    is a cross section of the target in  FIG. 7   a;    
         FIG. 7 c    is an enlarged view of the target in  FIGS. 7 a    and  7   b;    
         FIG. 7 d    is a table of the specific dimensions of the target in  FIGS. 7 a   - 7   c;    
         FIG. 8 a    is cross sections of targets with different sloped profiles; 
         FIG. 8 b    is a uniformity profile for the targets in  FIG. 8   a;    
         FIG. 8 c    is an erosion profile of the target of  FIGS. 7 a   - 7   c;    
         FIG. 9  is a schematic illustration of emission characteristics for the target of  FIGS. 7 a   - 7   c;    
         FIG. 10  is a cross section of a target according to another aspect of the present invention; 
         FIG. 11 a    is a cross section of a target according to still another aspect of the present invention; 
         FIG. 11 b    is an enlarged view of the right half of this cross-section of  FIG. 11   a;    
         FIG. 12  is a schematic illustration of emission characteristics for the target in  FIG. 11   a;    
         FIG. 13 a    is an erosion track for the target in  FIGS. 11 a    and  11   b;    
         FIG. 13 b    is an erosion profile of the target in  FIGS. 11 a    and  11   b;    
         FIG. 14 a    is a cross section of a target according to yet another aspect of the present invention; and 
         FIG. 14 b    is an enlarged view of the cross section of  FIG. 14 a   ; and 
         FIG. 15  is a uniformity profile for the target in  FIGS. 14 a    and  14   b.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example embodiments that incorporate one or more aspects are not intended to be overall limitations on the invention. For example, one or more aspects of the invention can be utilized in other embodiments and even other types of devices. Moreover, certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. Still further, in the drawings, the same reference numerals are employed for designating the same elements. 
       FIG. 6  schematically illustrates an example sputtering chamber  10  according to one aspect of the invention. The sputtering chamber  10  includes a chamber enclosure wall  20  having at least one gas inlet  30 . A substrate  40  and substrate support pedestal  50  are disposed at the lower end of the chamber, and a target  60  is received at the upper end of the chamber. 
       FIG. 7 a    illustrates a simplified perspective view of target  60  according to a first embodiment of the present invention. It should be appreciated that the target in this view (and subsequent views except  FIGS. 9 and 12 ) is inverted compared to  FIG. 6 . The target is generally disk-shaped with sloped outer edges, such that it is in the shape of a truncated cone or conical frustum. The target has a top or substrate side  61  and a bottom or wall side  62 . The sloped area or “mantle”  65  connects the top  61  and the bottom  62 , and is where the main erosion takes places. 
       FIG. 7 b    is a cross section of target  60 . In  FIG. 7 b   , the top line is the initial cross section of the target, and the subsequent lines represent the cross section after erosion due to sputtering. The Y-variable is target distance from the substrate in mm, and the X-variable is target radius to a point on the target in mm. As will be discussed in detail below, the target&#39;s specific mantle shape results in the best uniformity of material on the substrate at a high deposition rate. 
     In  FIG. 7 c   , an enlarged view of the cross section of  FIG. 7 b    is depicted. As in  FIG. 7 b   , the Y-variable is target surface height, with reference to the top surface, in mm, and the X-variable is target radius in mm. The mantle has a height of 10 mm, which is 1/20 or 5% of the target radius, and comprises two sloped sections. A first section  66  is closest to the target&#39;s center and sloped at a first angle A. At a location  67  between a surface height of 6 mm and 7 mm, a second section  68  begins. The second section  68  is sloped at a second angle B, and the second angle B is greater than the first angle A. The first section  66  has a steep slope, and the second section  68  has a less steep slope. The transition from the top of the target to sloped section  66  is rounded, as is the transition from sloped section  66  to sloped section  68 . 
     The specific dimensions of the target are indicated in the table of  FIG. 7   d.    
     The basis for the target shape will now be described.  FIG. 8 a    illustrates targets with differently sloped mantles and their corresponding idealized erosion profiles. The measurements of 0 mm to 30 mm refer to the target surface height, which is the vertical distance from the top to the bottom of the mantle. A target with a 30 mm surface height has the steepest slope, and a target with a 0 mm surface height has the least steep slope because it is a flat disk. Similar to  FIG. 7 b   , the top line of each target is its initial cross section, and the subsequent lines represent the cross section after erosion due to sputtering. These targets are opposed to a substrate at a distance of 120 to 150 mm. 
     The uniformities resulting from the targets of  FIG. 8 a    are plotted in  FIG. 8 b   . In  FIG. 8 b   , the Y-variable is percent uniformity, and the X-variable is radius of the substrate in mm. An ideal target would have 100% uniformity. The graph shows that profiling the target edge by 10 mm is able to improve uniformity at a substrate radius of 140 mm significantly. As such, the present invention utilizes this dimension. 
       FIG. 8 c    depicts the erosion profile of target  60 . The Y-variable is erosion of the target center area in mm, and the X-variable is target radius at a point on the target in mm. As in  FIG. 1 , there is increased erosion near the target edge. While the corresponding magnet is optimized to erode mainly near the edge, some erosion of 10 to 20% is actually beneficial to keep the target clean in the center area. 
     The operation of the target of the present invention will now be described with reference to  FIG. 9 . The emission characteristics of the sputtered particles are often cosine or somewhat broader than cosine,
 
 f (θ)=cos n  θ
 
where the exponent n describes the directionality of the emission. Values of n from 0.5 to 1.0 are often reported for experimental emission characteristics. The emission characteristics are sketched as an ellipse in  FIG. 9 .
 
     For a flat target, emissions from the target can reach anywhere on the substrate. Emissions near the target edge will largely deposit on the nearest substrate locations across from the edge. However, at a reduced rated, the edge emissions will deposit near the center of the substrate. Some edge emissions may deposit at distal edge locations of the substrate, but the rate is extremely reduced due to quadratic decrease with distance, combined with high angles of emission and incidence. 
     These deposition properties change with a sloped target edge. As with a flat target, emission from the target edge contributes to deposition across from the edge. However, deposition on the opposite side of the wafer is zero due to shadowing. Even deposition to the central locations of the wafer is reduced, due to high emission angles. Detailed calculations show that decrease of the deposition rate in central locations of the substrate may be more pronounced than on the substrate edge, resulting in improved film uniformity. 
       FIG. 10  illustrates a second embodiment of a target according to the present invention.  FIG. 10  is a cross-section of the target, which is similar in shape to a hollow dome or an upturned bowl. The top line is the initial cross section of the target, and the subsequent lines represent the cross section after erosion. The Y-variable is target distance from the substrate in mm, and the X-variable is target radius to a point on the target in mm. Similar to the truncated cone target, the dome-shaped target has several sloped sections. Because the sloped sections in this embodiment are larger and more plentiful, there is more shadowing than in  FIG. 9 . This results in an even more uniform deposition of sputtered material on the substrate. 
     The dome-shaped target of  FIG. 10  requires an arrangement of magnets in three dimensions. In comparison, the truncated cone target only requires magnets in two dimensions. In addition, the dome-shaped target may be more difficult to manufacture, leading to higher costs. 
       FIG. 11 a    illustrates a third embodiment of a target according to the present invention.  FIG. 11 a    is a cross-section of the target, and  FIG. 11 b    is an enlarged view of the right half of this cross-section. As in  FIG. 10 , the top line is the initial cross section of the target, and the subsequent lines represent the cross section after erosion. The Y-variable is target distance from the substrate in mm, and the X-variable is target radius to a point on the target in mm. The target has concentric rings with sloped sidewalls directing outwards, and is similar in shape to a Fresnel lens.  207  denotes the surface side of the target, whereas  208  is the material (bulk) side of the target. Reference numeral  205  refers to a reference plane or base of the target, independent of any surface structure of the target. A first ring  200  and a second ring  210  are concentric and the same height. A first groove  220  is formed inside the first ring  200 , and a second groove  230  is formed between the first ring  200  and the second ring  210 . A flat rim  240  is formed around the edge of the target. 
     The first ring  200  has a first side  201  sloped at an angle C from the bottom of groove  220 . The first ring  200  also has a second side  202  sloped at an angle D from the bottom of groove  230 . Angle D is slightly larger than angle E. 
     Similarly, the second ring  210  has a first side  211  sloped at an angle E from the bottom of groove  220 . The second ring  210  also has a second side  212  sloped at an angle F from rim  240 . Angle F is larger than angles C, D, and E. Accordingly, angle E is smaller than angles C and D. Angles C-F are all 90 degrees or greater. 
       FIG. 12  is a schematic illustration of emission characteristics for the target in  FIGS. 11 a  and 11 b   . As in  FIG. 9  which depicts the emission characteristics of the truncated cone target, there is shadowing from the sloped regions. However, because there are more sloped regions in this embodiment, there is more shadowing. 
       FIG. 13 a    depicts an example erosion track design for the Fresnel lens target embodiment of  FIGS. 11 a  and 11 b   . A rotating magnet arrangement follows the closed erosion track. The main erosion has to be concentrated underneath the sloped sidewalls in order to benefit from an emission facing outwards ( FIG. 12 ). The erosion is held primarily on the radii R 1  and R 2 , wherein R 1  corresponds to the radius of the first ring  200  and R 2  corresponds to the radius of the second ring  210 . The erosion has to jump between these partial concentric tracks and streak the center part of the target in order to decrease re-deposition. 
       FIG. 13 b    depicts an erosion profile for the Fresnel lens target of  FIG. 11 a   . The Y-variable is percent erosion, and the X-variable is target radius at a point on the target in mm. The percent erosion reaches three peaks corresponding to erosion racetracks, with the greatest between a target radius of 170 and 180 mm. 
       FIG. 14 a    illustrates a fourth embodiment of a target according to the present invention.  FIG. 14 a    is a cross-section of the target from the radius to the rim, and  FIG. 14 b    is an enlarged view of this cross-section. As in the previous figures, the top line is the initial cross section of the target, and the subsequent lines represent the cross section after erosion. The Y-variable is target distance from the substrate in mm, and the X-variable is target radius to a point on the target in mm. Similar to the third embodiment, the target of the fourth embodiment has concentric rings with sloped sidewalls directing outwards, and is similar in shape to a Fresnel lens. A first ring  300 , a second ring  310 , a third ring  320 , a fourth ring  330 , and a fifth ring  340  are concentric and the same height. A first groove  350  is formed inside the first ring  300 , a second groove  360  is formed between the first ring  300  and the second ring  310 , a third groove  370  is formed between the second ring  310  and the third ring  320 , a fourth groove  380  is formed between the third ring  320  and the fourth ring  330 , and a fifth groove  390  is formed between the fourth ring  330  and the fifth ring  340 . A rim  400  is formed around the edge of the target. 
     The first ring  300  has a first side  301  sloped at an angle G from the bottom of groove  350 . The first ring  300  also has a second side  302  sloped at an angle H from the bottom of groove  360 . Angle H is larger than angle G. 
     The second ring  310  has a first side  311  sloped at an angle I from the bottom of groove  360 . The second ring  310  also has a second side  312  sloped at an angle J from groove  370 . Angle J is larger than angle I. 
     The third ring  320  has a first side  321  sloped at an angle K from the bottom of groove  370 . The third ring  320  also has a second side  322  sloped at an angle L from groove  380 . Angle L is larger than angle K. 
     The fourth ring  330  has a first side  331  sloped at an angle M from the bottom of groove  380 . The fourth ring  330  also has a second side  332  sloped at an angle N from groove  390 . Angle N is larger than angle M. 
     The fifth ring  340  has a first side  341  sloped at an angle O from the bottom of groove  390 . The fifth ring  340  also has a second side  342  sloped at an angle P from rim  400 . Angle P is larger than angle O. 
     Overall, angle O is the smallest and angle P is the largest. The angles on the center-facing side of each ring decrease from the center of the target, i.e. angle G of first ring  300 , toward the rim of the target, i.e. angle O of fifth ring  340 . As such, angle G is larger than angle I, angle I is larger than angle K, angle K is larger than angle M, and angle M is larger than angle O. Similarly, the angles on the rim-facing side of each ring increase from the center of the target, i.e. angle H of first ring  300 , toward the rim of the target, i.e. angle P of fifth ring  340 . As such, angle H is smaller than angle J, angle J is smaller than angle L, angle L is smaller than angle N, and angle N is smaller than angle P. Angles G-P are all 90 degrees or greater. 
     For the embodiment depicted in  FIGS. 14 a  and  b   ,  FIG. 15  plots uniformity of the layer deposited on the substrate at a TSD of 200 mm. The Y-variable is percent uniformity of the layer, and the X-variable is substrate radius in mm. The film is almost completely uniform on the entire substrate, and only tapers off slightly toward the edge of the substrate.