Patent Publication Number: US-2015062772-A1

Title: Barrier Layer For Electrostatic Chucks

Description:
Embodiments of the present disclosure relate to an electrostatic chuck, and more particularly, to an electrostatic chuck having a barrier layer for use in substrate processing systems. 
     BACKGROUND 
     Ion implanters are commonly used in the production of semiconductor workpieces. An ion source is used to create an ion beam, which is then directed toward the workpiece. As the ions strike the workpiece, they dope a particular region of the workpiece. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these workpieces can be transformed into complex circuits. 
     As a workpiece is being implanted, it is typically clamped to a chuck. This clamping may be mechanical or electrostatic in nature. This chuck traditionally consists of a plurality of layers. The top layer, also referred to as the dielectric layer, or dielectric top layer, contacts the workpiece, and is made of an electrically insulating or semiconducting material, such as alumina with embedded metal electrodes, since it produces the electrostatic field without creating a short circuit. Methods of creating this electrostatic field are known to those skilled in the art and will not be described herein. 
     A second layer, also referred to as the base, may be made from an insulating material. To create the required electrostatic force, a plurality of electrodes may be disposed between the dielectric top layer and the insulating layer. In another embodiment, the plurality of electrodes may be embedded in the insulating layer. The plurality of electrodes is constructed from an electrically conductive material, such as metal. 
       FIG. 1  shows a top view of a chuck  10 , specifically showing the plurality of electrodes  100   a - f  of the chuck  10 . As shown, each of the electrodes  100   a - f  is electrically isolated from the others. These electrodes  100   a - f  may be configured such that opposite electrodes have opposite voltages. For example, electrode  100   a  may have a positive voltage while electrode  100   d  may have a negative voltage. These voltages may be DC, or may vary with time to maintain the electrostatic force. For example, as shown in  FIG. 1 , the voltage applied to each electrode  100   a - f  may be a bipolar square wave. In the embodiment shown in  FIG. 1 , three pairs of electrodes are employed. Each pair of electrodes is in electrical communication with a respective power source  110   a - c,  such that one electrode receives the positive output and the other electrode receives the negative output. Each power source  110   a - c  generates the same square wave output, in terms of period and amplitude. However, each square wave is phase shifted from those adjacent to it. Thus, as shown in  FIG. 1 , electrode  100   a  is powered by square wave A, while electrode  100   b  is powered by square wave B, which has a phase shift of 120° relative to square wave A. Similarly, square wave C is phase shifted 120° from square wave B. These square waves are shown graphically on the power supplies  110   a - c  of  FIG. 1 . Of course, other numbers of electrodes and alternate geometries may be used. 
     The voltages applied to the electrodes  100   a - f  serve to create an electrostatic force, which clamps the workpiece to the chuck. 
     In some embodiments, it may be desirable to implant the workpiece at an elevated temperature, such as above 300° C. In these applications, impurities may migrate or diffuse from the dielectric top layer in the electrostatic chuck to the workpiece. The introduction of these impurities to the workpiece may affect yield, performance or other characteristics of the workpiece. Therefore, it may be advantageous to have a system whereby material contained within an electrostatic chuck does not diffuse or migrate to the workpiece during the hot implant process. 
     SUMMARY 
     An electrostatic chuck for implanting ions at high temperatures is disclosed. The electrostatic chuck includes an insulating base, with electrically conductive electrodes disposed thereon. A dielectric top layer is disposed on the electrodes. A barrier layer is disposed on the dielectric top layer so as to be between the dielectric top layer and the workpiece. This barrier layer serves to inhibit the migration of particles from the dielectric top layer to the workpiece, which is clamped on the chuck. In some embodiments, a protective layer is applied on top of the barrier layer to prevent abrasion. 
     According to one embodiment, an electrostatic chuck is disclosed. The electrostatic chuck comprises an insulating base; 
     one or more electrically conductive electrodes disposed on the insulating base; a dielectric top layer, having a top surface and an opposite bottom surface, such that the electrodes are disposed between the insulating base and the dielectric top layer; and a barrier layer disposed on the top surface, wherein the barrier layer inhibits migration of particles from within the dielectric top layer to a workpiece clamped on the electrostatic chuck. 
     According to a second embodiment, an electrostatic chuck for use in high temperature ion implants is disclosed. The electrostatic chuck comprises an insulating base comprising a ceramic material; one or more electrically conductive electrodes disposed on the insulating base; a dielectric top layer, having a top surface and an opposite bottom surface, such that the electrodes are disposed between the insulating base and the dielectric top layer, and wherein the dielectric top layer comprises an oxide material having metal impurities introduced thereto; and a barrier layer, comprising silicon nitride, disposed on the top surface, wherein the barrier layer inhibits migration of metal particles from the dielectric top layer to a workpiece clamped on the electrostatic chuck. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  represents an electrostatic chuck of the prior art; 
         FIG. 2  shows an electrostatic chuck according to a first embodiment; and 
         FIG. 3  shows an electrostatic chuck according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an electrostatic chuck  200  in accordance with one embodiment. As described above, the electrostatic chuck  200  comprises an insulating base  210 , and a dielectric top layer  220 , with a plurality of electrodes  230  disposed between these two layers  210 ,  220 . The workpiece (not shown) may be clamped in place by the electrostatic forces created by the chuck  200 . 
     Furthermore, at elevated temperatures, such as above 300° C., or in some embodiments, above 500° C., it may be advantageous to heat the electrostatic chuck  200 . In some embodiments, heating elements, such as heat lamps, are used to heat the workpiece disposed on the electrostatic chuck  200 . Radiated heat serves to heat the electrostatic chuck  200 . In other embodiments, the electrostatic chuck  200  is directly heated, either through the use of resistive elements embedded in the insulating base  210 , or by passing a heated fluid through channels in the insulating base  210 . In each of these embodiments, one or more heating elements are used to raise the temperature of the workpiece during the ion implant process. 
     Because of the amount of heat generated in the electrostatic chuck  200 , it may be advantageous to utilize a heat-resistant material to create the insulating base  210 . For example, ceramic materials may be capable of withstanding the heat generated in the electrostatic chuck without deformation or cracking. The insulating base  210  may be constructed of, for example, alumina or some other ceramic material. In some embodiments, a heating mechanism may be embedded in the insulating base  210 . For example, the electrostatic and heating elements may be formed in the insulating base  210 . Alternatively, the surface electrical properties may be modified to create a Johnsen-Rahbek type (JR type) ESC, or elements may be sandwiched between plates attached by one by one of several methods, or layers of oxides or similar materials may coat or encapsulate electrical elements. 
     It may be advantageous, especially at these elevated temperatures, to utilize materials for the insulating base  210  and the dielectric top layer  220  that have functionally equivalent coefficients of thermal expansion (CTE). In this disclosure, the phrase “functionally equivalent” means that the CTEs of these two layers are such that the stress generated in these two layers due to thermal expansion can be tolerated without causing either layer to fracture. Furthermore, this phrase means that the CTEs are such that adhesion between these layers does not fail, causing the layers to separate. In some embodiments, these CTEs may be, for example, within 15% of each other over the intended temperature range. However, a larger or smaller percentage difference may be required to insure the above conditions are met. In another embodiment, these CTEs may be within 20% of each other over the intended temperature range. 
     At these elevated temperatures, it may be beneficial to create the dielectric top layer  220  from some type of oxide, such as silicon oxide, or other high temperature tolerant material, such as a ceramic material. To modify the CTE of the material used for the dielectric top layer  220 , impurities may be added to that material. For example, particles, such as magnesium, lead or zinc, may be added to the oxide or ceramic material to create a CTE that is functionally equivalent to that of the insulating base  210 . Thus, the dielectric top layer  220  may be an oxide material with impurities intentionally introduced to alter its thermal or dielectric properties. Alternatively, the dielectric top layer  220  may be a ceramic material with impurities intentionally introduced to alter its thermal or dielectric properties. 
     As described above, electrically conductive electrodes  230  are disposed on the insulating base  210  prior to the introduction of the dielectric top layer  220 . These electrodes  230  may be created by deposition of a metal on the insulating base  210 , or using other techniques known in the art. In some embodiments, these electrodes  230  are constructed of a conductive metal. The electrodes  230 , or the material coating the electrodes  230 , may contain trace materials, such as copper, for example, that may migrate to the top surface  221 . As described in  FIG. 1 , each electrode  230  is in electrical communication with a power source (not shown), as described above. 
     After deposition of the electrodes  230 , the dielectric top layer  220  is applied. For example, the dielectric top layer  220  may be applied using silk screening, spin coating or using a vapor deposition process. The dielectric top layer  220  has a bottom surface  222  which is in contact with the electrodes  230  and an opposite top surface  221 . It has been discovered that, unexpectedly, at elevated temperatures, material contained within the dielectric top layer  220 , such as the metal particles, diffuses or migrates toward the top surface  221  of the dielectric top layer  220 . At these elevated temperatures, after reaching the top surface  221 , unless otherwise prevented from doing so, these materials may diffuse or migrate into the surface of the workpiece proximate the top surface  221 . Thus, when the workpiece is removed by the electrostatic chuck  200 , these materials become attached or embedded in the workpiece, thereby impacting the performance or utility of the workpiece. 
     These effects do not appear to occur at lower temperatures, such as room temperature, and thus have never been previously addressed. 
     Specifically, testing has shown that particles of zinc, magnesium, lead and copper are considered to be those most likely to diffuse or migrate from the dielectric top layer  220  into the workpiece. These particles may be the impurities added to the oxide or ceramic material used to create the dielectric top layer  220 , which were introduced to create the desired thermal and dielectric properties. Therefore, the removal of these particles from the dielectric top layer  220  may not be advisable or even possible. In other embodiments, these particles may come in contact with the electrostatic chuck  200  during the manufacturing process. Changes to the manufacturing process to eliminate contact with these particles may be impractical. In addition, these particles may have been used in the fabrication of the electrodes  230 . For example, copper, used in the fabrication of the electrodes  230 , may comprise one of these particles. Thus, these particles may not be easily removed from the dielectric top layer  220 . Therefore, it may be necessary to devise a system and method by which these particles, which are known to migrate toward the surface  221 , are kept away from the workpiece. 
     In a first embodiment, a barrier layer  240  is applied to the top surface  221  of the dielectric top layer  220 . This barrier layer  240  serves to stop the migration of particles from the dielectric top layer  220  to the workpiece that is clamped on the chuck  200 . Thus, the composition of the barrier layer  240  may be a material that inhibits the migration of these particles. In other embodiments, the composition of the barrier layer  240  may be such that it impedes the migration of these metal particles. In some embodiments, a nitride, such as silicon nitride, may be used. 
     This barrier layer  240  may be applied to a thickness of, for example, less than 10 microns. This thickness may be selected based on the time required to apply the barrier layer  240  and its effect of the electrostatic forces. This thickness may have minimal effect on the electrostatic forces created by the chuck  200 . Similarly, at this thickness, the CTE of the barrier layer  240  may be of little importance. This barrier layer  240  may be applied to the top surface  221  of the dielectric top layer  220  using, for example, chemical vapor deposition (CVD), although other deposition processes may also be employed. Optionally, the barrier layer  240  may also be applied to the sides of the dielectric top layer  220 . 
     Additionally, nitrides, such as silicon nitride, are very hard materials, and therefore may be resistant to mechanical abrasion between the chuck  200  and the workpiece being implanted on the chuck  200 . 
     Thus, particles from within the dielectric top layer  220  may still migrate to the top surface  221  of the dielectric top layer  220 . However, their further migration is inhibited by the presence of barrier layer  240 . Thus, the workpiece clamped on the barrier layer  240  is protected from these potentially harmful particles. 
       FIG. 3  shows an electrostatic chuck  300 , according to a second embodiment. This embodiment is similar to that of  FIG. 2  and similar components are given consistent reference designators, and will not be described again. As before, the barrier layer  240  may be a nitride, such as silicon nitride. The thickness of this barrier layer  240  may be, for example, less than 1 micron thick. In some embodiments, it may be hundreds of nanometers in thickness. In this embodiment, an additional protective layer  250  is applied on top of the barrier layer  240 . This protective layer  250  may be, for example, hundreds of microns in thickness. In other embodiments, the protective layer  250  may be as thick as 1 mm. The protective layer  250  is intended to protect the electrostatic chuck  300 , and particularly the barrier layer  240  from abrasion, which may result from contact with the workpieces. In one embodiment, the protective layer  250  is comprised of borosilicate glass (BSG). Other suitable materials may be used which are insulating, and do not affect the electrostatic fields being created. 
     Thus, a high temperature ion implant may be performed by clamping a workpiece on an electrostatic chuck  200  having the barrier layer  240  described herein. The barrier layer  240  serves to inhibit the migration of metal particles from the dielectric top layer  220  to the workpiece, thereby maintaining the integrity of the workpiece. As described above, these particles may be impurities added to the dielectric top layer  220  to alter its thermal or dielectric properties. These particles may be materials used in the fabrication of the electrodes  230 . To perform the high temperature ion implant, heating elements may be used to raise the temperature of the workpiece to about 300° C. during the ion implant process. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.