Patent Publication Number: US-2016230269-A1

Title: Radially outward pad design for electrostatic chuck surface

Description:
FIELD 
     Embodiments disclosed herein generally relate to electrostatic chucks; more specifically, embodiments disclosed herein generally relate to a pattern for an electrostatic chuck surface. 
     BACKGROUND 
     Electrostatic chucks are widely used to hold substrates, such as semiconductor substrates, during substrate processing in processing chambers used for various applications, such as physical vapor deposition (PVD), etching, or chemical vapor deposition. Electrostatic chucks typically include one or more electrodes embedded within a unitary chuck body, which comprises a dielectric or semi-conductive ceramic material across which an electrostatic clamping field can be generated. Semi-conductive ceramic materials, such as aluminum nitride, boron nitride, or aluminum oxide doped with a metal oxide, for example, may be used to enable Johnsen-Rahbek or non-Coulombic electrostatic clamping fields to be generated. 
     Variability of the chucking force applied across the surface of a substrate during processing can cause an undesired deformation of the substrate, and can cause the generation and deposition of particles on the interface between the substrate and the electrostatic chuck. These particles can interfere with operation of the electrostatic chuck by affecting the amounts of chucking force. When the substrates are subsequently moved to and from the electrostatic chuck, these deposited particles can also scratch or gouge the substrates and ultimately lead to breakage of the substrate as well as wear away the surface of the electrostatic chuck. 
     Additionally, conventional electrostatic chucks may experience a sudden spike in temperature as a backside gas is introduced during deposition processes. Non-uniform or excessive heat transfer between a substrate and the electrostatic chuck can also cause damage to the substrate and/or chuck. For example, an over chucked substrate may result in an excessively large area of contact or an excessively concentrated area of contact between the substrate and chuck surfaces. Heat transfer occurring at the area of contact may exceed physical limitations of the substrate and/or chuck, resulting in cracks or breakage, and possibly generating and depositing particles on the chuck surface that may cause further damage or wear. 
     Thus, there is a need for a better electrostatic chuck which reduces damage to the substrate and/or chuck. 
     SUMMARY 
     An electrostatic chuck assembly and processing chamber having the same are disclosed herein. In one embodiment, an electrostatic chuck assembly is provided that includes a body having an outer edge connecting a frontside surface and a backside surface. The body has chucking electrodes disposed therein. A wafer spacing mask is formed on the frontside surface of the body. The wafer spacing mask has a plurality of elongated features. The elongated features have long axes that are radial aligned from the center to the outer edge. The wafer spacing mask has a plurality of radially aligned gas passages defined between the elongated features. 
     In another embodiment, a processing chamber is provided that includes an electrostatic chuck assembly disposed in a processing volume of the processing chamber. The electrostatic chuck assembly includes a body having an outer edge connecting a frontside surface and a backside surface. The body has chucking electrodes disposed therein. A wafer spacing mask is formed on the frontside surface of the body. The wafer spacing mask has a plurality of elongated features. The elongated features have long axes that are radial aligned from the center to the outer edge. The wafer spacing mask has a plurality of radially aligned gas passages defined between the elongated features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic sectional side view of a physical vapor deposition (PVD) chamber within which an exemplary electrostatic chuck may be operated. 
         FIG. 2  is a schematic cross-sectional detail view of electrostatic chuck assembly shown in  FIG. 1 . 
         FIG. 3  is a schematic cross-sectional detail view of a wafer spacing mask on a frontside surface of an electrostatic chuck assembly. 
         FIG. 4  illustrates a top view of a top surface of the electrostatic chuck assembly, having an arrangement of minimum contact area features. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     As described above the application of a non-uniform chucking force across a substrate, as well as an uneven or excessive heat transfer between the substrate and the chuck, can cause particle generation to occur at the substrate-chuck interface, which can result in damage or increased wear to the substrate and chuck. Therefore, reducing particle generation at the interface of an electrostatic chuck and a substrate may directly lead to reduced wear and the longer operational life of both elements, and may provide a more consistent and desired operation of the chuck. 
     Particle generation may be reduced by adjusting several design or process parameters. For example, the chuck surface may be designed to reduce or minimize the deformation of a chucked substrate, thereby reducing the probability of generating particles due to deformation of the substrate. In accordance with other physical design parameters (e.g., heat transfer gas flow), the chuck surface may employ particular arrangement(s) of contact points with the substrates, and/or may use particular material(s) having desired properties. 
       FIG. 1  illustrates a schematic sectional side view of a PVD chamber  100  within which an exemplary an electrostatic chuck assembly  120  may be operated, according to one embodiment. The PVD chamber  100  includes chamber walls  110 , a chamber lid  112 , and a chamber bottom  114  defining a processing volume  116 . The processing volume  116  may be maintained in a vacuum during processing by a pumping system  118 . The chamber walls  110 , chamber lid  112  and the chamber bottom  114  may be formed from conductive materials, such as aluminum and/or stainless steel. A dielectric isolator  126  may be disposed between the chamber lid  112  and the chamber walls  110 , and may provide electrical isolation between the chamber walls  110  and the chamber lid  112 . The chamber walls  110  and the chamber bottom  114  may be electrically grounded during operation. 
     The electrostatic chuck assembly  120  is disposed in the processing volume  116  for supporting a substrate  122  along a contact surface  158 . The electrostatic chuck assembly  120  may move vertically within the processing volume  116  to facilitate substrate processing and substrate transfer. A chucking power source  132  may be coupled to the electrostatic chuck assembly  120  for securing the substrate  122  on the electrostatic chuck assembly  120 , and may provide DC power or RF power to one or more chucking electrodes  150 . The chucking electrodes  150  may have any suitable shape, such as semicircles, “D”-shaped plates, disks, rings, wedges, strips, and so forth. The chucking electrodes  150  may be made of any suitable electrically conductive material, such as a metal or metal alloy, for example. 
     A target  124  may be mounted on the chamber lid  112  and faces the electrostatic chuck assembly  120 . The target  124  includes materials to be deposited on the substrate  122  during processing. A target power source  138  may be coupled to the target  124 , and may provide DC power or RF power to the target to generate a negative voltage or bias to the target  124  during operation, or to drive plasma  146  in the chamber  100 . The target power source  138  may be a pulsed power source. The target power source  138  may provide power to the target  124  up to about 10 kW, and at a frequency within a range of about 0.5 MHz to about 60 MHz, or more preferably between about 2 MHz and about 13.56 MHz. A lower frequency may be used to drive the bias (thereby controlling the ion energy), and a higher frequency may be used to drive the plasma. In one embodiment, the target  124  may be formed from one or more conductive materials for forming dielectric material by reactive sputtering. In one embodiment, the target  124  may include a metal or an alloy. 
     A shield assembly  128  may be disposed within the processing volume  116 . The shield assembly  128  surrounds the target  124  and the substrate  122  disposed over the electrostatic chuck assembly  120  to retain processing chemistry within the chamber and to protect inner surfaces of chamber walls  110 , chamber bottom  114  and other chamber components. In one embodiment, the shield assembly  128  may be electrically grounded during operation. 
     To allow better control of the materials deposted onto the substrate  122 , a cover ring  123  may be positioned about the perimeter of the substrate  122  and rest on a portion of the shield assembly  128  during processing. The cover ring  123  may generally be positioned or moved within chamber  100  as the electrostatic chuck assembly  120  moves vertically. The cover ring  123  may be shaped to promote deposition near the edge of the substrate while preventing edge defects. The cover ring  123  may prevent deposition material from forming in and around the bottom of the processing chamber  100 , for instance on the chamber bottom  114 . 
     A process gas source  130  is fluidly connected to the processing volume  116  to provide one or more processing gases. A flow controller  136  may be coupled between the process gas source  130  and the processing volume  116  to control gas flow delivered to the processing volume  116 . 
     A magnetron  134  may be disposed externally over the chamber lid  112 . The magnetron  134  includes a plurality of magnets  152 . The magnets  152  produce a magnetic field within the processing volume  116  near a front face  148  of the target  124  to generate a plasma  146  so that a significant flux of ions strike the target  124  causing sputter emission of the target material. The magnets  152  may rotate or linearly scan the target to increase uniformity of the magnetic field across the front face  148  of the target  124 . As shown, the plurality of magnets  152  may be mounted on a frame  140  connected to a shaft  142 . The shaft  142  may be axially aligned with a central axis  144  of the electrostatic chuck assembly  120  so that the magnets  152  rotate about the central axis  144 . 
     The physical vapor deposition chamber  100  may be used to deposit a film onto substrate  122 .  FIG. 1  schematically illustrates the physical vapor deposition chamber  100  in a processing configuration to deposit a film onto substrate  122 . During deposition, a gas mixture including one or more reactive gases and one or more inert gases may be delivered to the processing volume  116  from the gas source  130 . The plasma  146  formed near the front face  148  of the target  124  may include ions of the one or more inert gases and the one or more reactive gases. The ions in the plasma  146  strike the front face  148  of the target  124  sputtering the conductive material, which then reacts with the reactive gases to form a film onto the substrate  122 . 
     Depending on the material to be formed on the substrate  122 , the target  124  may be formed from a metal, such as aluminum, tantalum, hafnium, titanium, copper, niobium, or an alloy thereof. The reactive gases may include an oxidizing agent, a nitriding agent, or other reactive gases. According to one embodiment, the reactive gases may include oxygen for forming a metal oxide, or nitrogen for forming a metal nitride. The inert gases may include argon. 
     While PVD chamber  100  was described above with respect to the operation of an exemplary electrostatic chuck assembly to treat a substrate  122 , note that a PVD chamber having the same or a similar configuration may also be used to deposit materials to produce a desired surface on the electrostatic chuck assembly  120 . For example, the PVD chamber  100  may use a mask to produce the electrostatic chuck surface shown in  FIG. 4 . 
       FIG. 2  illustrates a schematic cross-sectional detail view of the electrostatic chuck assembly  120  shown in  FIG. 1 . As shown, two chucking electrodes  150  are embedded into a body  202  the electrostatic chuck assembly  120 . The body  202  may be fabricated from a dielectric material, such as a ceramic such as aluminum nitride and the like. The body  202  alternatively may be fabricated from plastic materials, such as from sheets of polyimide, polyether ether ketone, and the like. The body  202  has a backside surface  204  and a frontside surface  205 . The frontside surface  205  is utilized to support the substrate  122 . 
     A wafer spacing mask  210  is formed on the frontside surface  205  to minimize the contact area between the substrate  122  and the electrostatic chuck assembly  120 . The wafer spacing mask  210  may be integrally formed from the material comprising the body  202 , or may be comprised of one or more separate layers of material deposited on the frontside surface  205  of the body  202 . 
     The wafer spacing mask  210  may have a top surface  208  and a bottom surface  206 . The bottom surface  206  may be disposed directly upon the frontside surface  205  of the electrostatic chuck assembly  120 . A thickness  260  of the wafer spacing mask  210  may be preferentially selected and spatially distributed across the frontside surface  205  to form features such as a plurality of mesas  215  and, optionally, an outer peripheral ring  225 . The mesas  215  are generally configured to support the substrate  122  along the top surface  208  during processing. Gas passages  220  are formed between the mesas  215 , allowing backside gas to be provided between the substrate  122  and the frontside surface  205  of the electrostatic chuck assembly  120 . The outer peripheral ring  225  may be a solid ring or segments in a structure similar to the mesas  215  on the top surface  208  of the electrostatic chuck assembly  120 , and utilized to confine or regulate the presence of the flow of backside gas from under the substrate  122  through the gas passages  220 . In one embodiment, the outer peripheral ring  225  is similar to the mesas  215  in shape and configuration. Alternately, the outer peripheral ring  225  may be utilized to center the substrate  122  on the electrostatic chuck assembly  120 . 
     A heat transfer gas source  230  is coupled through the electrostatic chuck assembly  120  to the frontside surface  205  to provide backside gas to the gas passages  220  defined between the mesas  215 . The heat transfer gas source  230  provides a heat transfer gas (i.e., the backside gas) that flows between the backside of the substrate  122  and the electrostatic chuck assembly  120  in order to help regulate the rate of heat transfer between the electrostatic chuck assembly  120  and the substrate  122 . The heat transfer gas may flow from outwards from a center of the electrostatic chuck assembly  120  and through the gas passages  220  around the mesas  215  and over the outer peripheral ring  225  into the processing volume  116  (shown in  FIG. 1 ). In one example, the heat transfer gas may comprise an inert gas, such as argon, helium, nitrogen, or a process gas. The heat transfer gas, such as argon, may be a process gas, and wherein a flow rate into the chamber volume is measured to obtain predictable results. The heat transfer gas may be delivered to the gas passages  220  through one or more inlets  222  in the electrostatic chuck assembly  120  that are in fluid communication with one or more gas passages  220  and the heat transfer gas source  230 . The outer peripheral ring  225  contacts the substrate near its edge and may be preferentially designed to control the amount of heat transfer gas that escapes from between the substrate  122  and the electrostatic chuck assembly  120  into the processing volume. For example, the outer peripheral ring  225  and mesas  215  may be configured to provide a resistance to flow the transfer gas such that a pressure of the gas present between the substrate  122  and electrostatic chuck assembly  120  does not exceed a predetermined value. 
     Temperature regulation of the body  202 , and ultimately the substrate  122 , may further be monitored and controlled using one or more cooling channels  245  disposed in a cooling plate  240  disposed in contact with the backside surface  204  of the body  202 . The cooling channels  245  are coupled to and in fluid communication with a fluid source  250  that provides a coolant fluid, such as water, though any other suitable coolant fluid, whether gas or liquid, may be used. 
     The wafer spacing mask  210  may be formed by depositing material through a mask onto the frontside surface  205 . The use of a mask may allow better control of the size, shape, and distribution of features in the wafer spacing mask  210 , thereby controlling the both the contact area of the mesas  215  and the conductance of the gas passages  220  defined between the mesas  215 . 
     While depicted as having a flat top surface  208 , each individual mesa  215  may generally have any suitable shape and height, each of which may be preferentially selected to fulfill particular design parameters (such as a desired chucking force and/or heat transfer). In one embodiment, the top surface  208  of the mesas  215  of the wafer spacing mask  210  may form a planar surface. In other embodiments, the top surface  208  of the mesas  215  of the wafer spacing mask  210  may form a non-planar surface, for example, a concave or convex surface. Generally, mesas  215  may have a mesa height  262  of about 1 micron to about 100 microns, or more preferably between about 1 micron and 30 microns. In one embodiment, the surface of the mesas  215  that supports the substrate  122  may have a small rounded bump-like shape to minimize total contact area between the mesas  215  and the substrate  122 . In another embodiment, mesas  215  may include a small bump or protrusion atop a generally flat surface. In yet another embodiment, the frontside surface  205  itself may vary between relative high and low points (similar to mesas  215  and gas passages  220 ), and wafer spacing mask  210  may be formed on this non-uniform surface. 
     In one or more embodiments, a non-uniform mask profile may be used to form the wafer spacing mask  210 . Generally, the non-uniform mask profile may permit the height of each mesa  215  or depth of each gas passage  220  to be controlled individually or in combination. A wafer spacing mask  210  created using the non-uniform mask profile may advantageously provide a more uniform chucking force across a substrate. 
       FIG. 3  illustrates a schematic cross-sectional detail view of a wafer spacing mask deposited onto an electrostatic chuck assembly, according to one embodiment. In this example, the height of mesas  215  increase with lateral distance from a centerline  360  of the electrostatic chuck assembly  120 , so that a maximum mesa height occurs at the outermost mesa  325 , corresponding to outer peripheral ring  225 . Likewise, the heights of the mesas  215  may be at a minimum at mesas  315  most proximate the centerline  360 . As described above, individual mesas  215  may have any suitable shape and the mask profile may be selected to provide mesas  215  having different sizes and/or shapes. The mask profile may provide for lateral symmetry so that corresponding mesas  215  at a particular lateral distance from centerline  360  have the same height and/or shape. 
       FIG. 4  illustrates a top view of the frontside surface  205  of the electrostatic chuck assembly  120 . The frontside surface  205  of the electrostatic chuck assembly  120  has the wafer spacing mask  210  of deposited thereon. Thus, the frontside surface  205  of the electrostatic chuck assembly  120  can be characterized as having raised areas  402  defined by the wafer spacing mask  210  and unmodified areas  404  defined by the portions of the frontside surface  205  substantially uncovered by the wafer spacing mask  210 . The unmodified areas  404  of the frontside surface  205  may include a layer of the same materials deposited to form the wafer spacing mask  210  which remains below the top surface  208  of the mesas  215  and defines the gas passages  220 . 
     The wafer spacing mask  210  may also include elongated features  406  that correspond to the mesas  215  of  FIG. 2 . The wafer spacing mask  210  may also include cylindrical features  408 , and  410 , and center tap features  414 . The top surface  208  may also have lift pin hole openings  416 . The cylindrical features  410  may be formed inward of the lift pin hole openings  416  in place of an elongated feature to locally reduce the substrate contact area and allow more gas flow to compensate for thermal non-uniformities caused by presence of the lift pin hole openings  416  extending through the body  202  of the electrostatic chuck assembly  120 . The long axis of the elongated features  406  of the wafer spacing mask  210  may generally be radially aligned from a centerline  460  to an outer edge  462  of the electrostatic chuck assembly  120 . Additionally, the rounded features  408 , and  410  may also be radially aligned the elongated features  406  from the centerline  460  to the outer edge  462 . An outermost ring  418  of mesas  215  may define the outer peripheral ring  225 . Gas passages  220  are defined between the top surfaces  208  of the mesas  215  defining the wafer spacing mask  210 . The gas passages  220  may also radially aligned from the centerline  460  to the outer edge  462  of the electrostatic chuck assembly  120 , or may also extend in different directions, such as concentrically from the centerline  460  of the electrostatic chuck assembly  120 . 
     The elongated features  406  may be arranged in concentric rows  409  emanating from the center. In one embodiment, each concentric row  409  has the same number of elongated features  406 . In another embodiment, the number of elongated features  406  in each of the concentric rows  409  may increase from the centerline  460  to the outer edge  462 . For example, the number of elongated features  406  in the row  409  nearest the outer edge  462  is greater than the number of elongated features  406  in the concentric row  409  nearest the centerline  460 . In yet another embodiment, the number of elongated features  406  may double in one or more subsequent concentric row  409 . For example, the number of elongated features  406  in a first row  413  may be half of the number of elongated features  406  in a second row  415 . The number of elongated features  406  in the second row  415  may be half of number of elongated features  406  in a fourth row  417 . The number of elongated features  406  in the fourth row  417  may be half of number of elongated features  406  in a sixth row  419 . That is, the number of elongated features  406  may double in every other row  409  starting from the centerline  460  to the outer edge  462 . In this manner, a spacing  440  between elongated features  406  in the rows  409  remains fairly consistent. The spacing  440  between adjacent elongated features  406  in a row  409  may have a lateral distance of about 0.1 inches to about 0.5 inches. The radial length of long axis of the elongated feature  406  may be within a range of about 0.1 inches to about 0.5 inches. The spacing between radially aligned elongated features  406  in adjacent rows  409  may be within a range of about 0.1 inches to about 0.5 inches. 
     To provide further reduce particle generation and wear of the top surface  208  of the electrostatic chuck assembly  120 , the material composition of the wafer spacing mask  210  may be preferentially selected based on several properties. For example, the material composition for an improved top surface  208  may be selected to exhibit one or more of high hardness, a high modulus of elasticity, low coefficient of friction, and/or a low wear factor. In one embodiment, the wafer spacing mask  210  may be fabricated from titanium nitride. In another embodiment, the wafer spacing mask  210  may be fabricated from diamond-like carbon (DLC) compositions, such as DYLYN™ (a trademark of Sulzer Ltd.) and the like. 
     The radial aligned gas passages  220  and mesas  215  reduce the pressure of the backside gas flowing through the gas passages  220 . The radial aligned gas passages  220  and mesas  215  promote the flow of the backside gas by reducing the conductance of the gas flow. For example, the radial aligned gas passages  220  and mesas  215  may reduce the backside gas pressure at the outer edge  462  from non-radial aligned gas passages and mesas from about 50% to about 70%, such as about 64% at less than 10 SCCM flow rates on a 300 mm electrostatic chuck assembly  120  as compared to conventional electrostatic chuck assemblies not having radially aligned elongated features. Thus, where the backside gas having a pressure of about 3 Torr and 3 SCCM at the inlet, such as inlet  222 , and a pressure of about 7 Torr on the outer edge of a conventional ESC, having non-radial aligned mesas, may have the pressure reduced to about 4 Torr on the ESC  120  having radial aligned gas passages  220  and mesas  215 . The reduced pressure beneficially increases the velocity of the backside gas by about 100%. Similarly, where the backside gas having a pressure of about 3 Torr and 0.1 SCCM at an inlet, such as inlet  222 , and a pressure of about 4 Torr on the outer edge of a conventional ESC, having non-radial aligned mesas, may be able to reduce the pressure to about 2 Torr on the ESC  120  having radial aligned gas passages  220  and mesas  215 . The reduced pressure beneficially increases the velocity of the backside gas by about 100%. The improved backside gas pressure and velocity promotes thermal uniformity of the substrate  122  disposed on the wafer spacing mask  210 . Since the backside gas flows more freely, the backside gas is better able to regulate the temperature of the substrate  122  as heat is be transferred from the substrate  122  more readily. For example, sudden temperature spikes from deposition when the backside gas is introduced and the heat transfer from the electrostatic chuck assembly  120  to the substrate  122  upon process termination is reduced by the freely flowing backside gas which does not further promote rapid heating of the substrate  122 . Additionally, the improved backside gas pressure and velocity negates the need to tune the flow of the backside gas to promote thermal uniformity. In one embodiment, the radial aligned gas passages  220  and mesas  215  produce a backside gas pressure between about 2.5 Torr and about 8 Torr, such as 2.5 Torr, at the outer edge  462  when flowing about 0.1 SCCM of backside gas through the inlet  222  at a pressure of about 3 Torr. In another embodiment, the radial aligned gas passages  220  and mesas  215  produce a backside gas pressure of about 4 Torr at the outer edge  462  when flowing about 3 SCCM of backside gas through the inlet  222  at a pressure of about 3 Torr. 
     The maximum velocity of the backside gas at the outer edge  462  is between about 6 mm/s and about 1 mm/s, such as about 5.77 mm/s when flowing about 3 SCCM of backside gas through the inlet  222  into the gas passages  220 . In one embodiment, the maximum velocity is 4 mm/s when a rate of 3 SCCM of backside gas is flowed into the inlet  222  at 3 Torr. In another embodiment, the maximum velocity is 1.31 mm/s when a rate of 21 SCCM of backside gas is flowed into the inlet  222  at 3 Torr. The maximum velocity of the backside gas at the outer edge  462  is between about 6 mm/s and about 1 mm/s, such as about 4 mm/s when flowing about 0.1 SCCM to about 1 SCCM of backside gas through the inlet  222  into the gas passages  220 . In one embodiment, the maximum velocity is 2.1 mm/s when a rate of 0.1 SCCM of backside gas is flowed into the inlet  222  at 3 Torr. In another embodiment, the maximum velocity is 4.7 mm/s when a rate of 0.1 SCCM of backside gas is flowed into the inlet  222  at 3 Torr. 
     The total area of top surface  208  of the wafer spacing mask  210  that is in contact with the substrate  122  is about 20 cm 2  to about 60 cm 2 , which is an increase in surface contact area of nearly three times greater than conventional wafer spacing masks. The increased contact area of the radial aligned mesas  215  increases the theoretical chucking force on the substrate from about 800 grams to about 3300 grams for the same chucking voltage. The addition contact area of the radial aligned gas passages  220  and mesas  215  with the substrate  122  reduce the overall stress on the substrate  122  significantly while the actual surface area of the electrostatic chuck assembly  120  in contact with substrate  122  is only between about 3% to about 15%. The radial aligned mesas  215  reduce the friction between the substrates  122  and the electrostatic chuck assembly  120 . The radial aligned mesas  215  reduce wear and particle generation due to greater surface contact between the substrate  122  and the electrostatic chuck assembly  120 . The greater contact area between the electrostatic chuck assembly  120  and the substrate  122  provides additional support to the substrate and thus lowers the overall stress across the substrate  122  from chucking the substrate  122 . For example, the electrostatic chuck assembly  120 , having radial aligned mesas  215 , may reduce the stress about 30% on the substrate  122  over a conventional electrostatic chuck assembly. Furthermore, the radial aligned mesas  215  reduce the temperature gradient from the centerline  460  to outer edge  462  of the substrate  122  as compared to a conventional electrostatic chuck assembly. The substrate  122 , especially along the outside perimeter, experiences a reduction in the stress, from the increased contact area, and temperature gradient, from the decrease pressure and increase velocity of the backside gas, which may damage (i.e., crack) the substrate. The stress on the substrate  122  is dependent on not only the thermal gradient but also the material. For example, a TTN film on the substrate  122  may be about 58 MPa at a time corresponding to the greatest temperature gradient in the film and then reach less than about 8 MPa after about 10 seconds. Similarly, a DLC film on the substrate  122  may be about 50 MPa at a time corresponding to the greatest temperature gradient in the film and then reach less than about 11 MPa after about 10 seconds. Where the substrate  122  stress is maximum at a time step of about 0 seconds to about 1 second due to a maximum difference in the temperature at the initial time step. The fatigue stress on the substrate during 0 to 3 seconds is very critical, which will result in fracture of the material in contact, hence preheating the substrate and controlled landing of the substrate on the Electrostatic chuck are both very critical. Convective heating of the substrate by increasing the inlet temperature is a possibility during the substrate transport in to the change. The blades of the heater can also be actively maintained at elevated temperature based on the process recipe +/−50 degree C. to reduce the thermal shock and thermal transient fatigue stress on initial 3 second contact. 
     Advantageously, the radially outward design of the mesas  215  and gas passages  220  on the frontside surface  205  of the electrostatic chuck assembly  120  improves thermal uniformity on substrates processed thereon. The radially outward design of the mesas  215  and gas passages  220  provide better control of backside gas for the electrostatic chuck assembly  120 . The radially outward design of the mesas  215  and gas passages  220  promote reduced wear characteristics due to more surface area contact between the substrate  122  and the electrostatic chuck assembly  120 . The radially outward design of the mesas  215  and gas passages  220  on the top surface  208  of the electrostatic chuck assembly  120  provides improved support to substrate backside due to improved contact area for reducing the stress, and subsequent damage, to the substrate  122 . Thus, the disclosed embodiments of the present invention provide a pattern of features for an electrostatic chuck assembly that are directed toward providing reduced particle generation and reduced wear of substrates and chucking devices. 
     In addition to the examples described above, some additional non-limiting examples may be described as follows. 
     Example 1 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein the radial aligned gas passages and mesas are arranged to maintain a pressure less than about 3 Torr at the outer edge when flowing about 3 SCCM of backside gas through the gas passages. 
     Example 2 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein a velocity of backside gas into the radial aligned gas passages is about 7 mm/s or less at the outer edge when flowing about 3 SCCM of backside gas through the gas passages. 
     Example 3 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein a velocity of backside gas into the radial aligned gas passages is about 4 mm/s or less at the outer edge when flowing at least 0.1 SCCM of backside gas through the gas passages. 
     Example 4 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein the radial aligned gas passages and mesas are arranged to maintain a pressure less than about 1 to 4 Torr at the outer edge when flowing at least 0.1 SCCM of backside gas through the gas passages. 
     Example 5 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein a velocity of backside gas into the radial aligned gas passages is about 1.31 mm/s or less at the outer edge when flowing about 3 SCCM of backside gas through the gas passages. 
     Example 6 
     An electrostatic chuck assembly, comprising: 
     a body having chucking electrodes disposed therein, the body having an outer edge connecting a frontside surface and a backside surface; and 
     a wafer spacing mask formed on the frontside surface, the wafer spacing mask having a plurality of elongated features, the elongated features having long axes that are radial aligned from the center to the outer edge, the wafer spacing mask having a plurality of radially aligned gas passages defined between the elongated features, wherein a velocity of backside gas into the radial aligned gas passages is about 2 mm/s to about 5 mm/s or less at the outer edge when flowing at least 0.1 SCCM of backside gas through the gas passages. 
     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.