Abstract:
An electrostatic clamp includes a heating block for heating a substrate, the heating block having a first surface disposed toward the substrate and a second surface opposite the first surface. A base is arranged to adjoin at least a portion of the second surface of the heating block. The adjoined base and heating block may mutually define an inner gap between a first portion of the heating block and the base. An outer gap is arranged concentric with the inner gap between a second portion of the heating block and the base, the inner and outer gaps being isolated from one another by a first sealing surface formed between the second surface of the heating block and the base.

Description:
FIELD 
       [0001]    This invention relates to substrate holders, and more particularly, to controlling leaks in electrostatic clamp systems. 
       BACKGROUND 
       [0002]    Substrate holders, such as electrostatic clamps, are widely deployed in apparatus that impart heat into substrates, which may require controlled heat transfer into or out of the substrate holder to maintain the proper substrate temperature. The heat may be imparted from a process itself or by deliberate heating of the substrate. In resistively heated electrostatic clamps, gas may be provided between a heating block and a cooled base in order to aid thermal transfer. Because the heating block and base may comprise dissimilar materials, such as a ceramic and a metal, respectively, it may be necessary to avoid bonding the two components together to avoid excessive thermal mismatch strains when the block is heated. The use of gas to transfer heat from the heating block may therefore be necessary since thermal transfer may be very low in a low pressure ambient if the base is not bonded to the heating block. The temperature mismatch between the ceramic and base may be reduced by using a high enough pressure of gas to rapidly transfer heat away from the ceramic. However, the gas supplied between ceramic and metal may leak along the interface between base and heater block and into a process chamber containing the electrostatic clamp. The unwanted gas leakage may lead to poor process control or substrate contamination in processes that depend on control of the gas ambient in the process chamber, including plasma or beamline implantation processes in ion implanters. 
         [0003]      FIG. 1   a  depicts a prior art ESC configuration  10  in which a base  12  and heating block  14  are joined together. ESC  10  includes a heater (not shown), which may be used to resistively heat substrates that are supported by the heating block  104 . ESC  10  may operate as a substrate holder in a process chamber, such as a low pressure chamber for performing one or more processes on the substrate. Examples of such low pressure chambers include plasma and ion beam tools, which may be evacuated to a pressure of 10 −7  Torr or less before substrate processing and may operate in an ambient gas pressure in the range of 10 −7 -10 0  torr, for example. 
         [0004]    During processing, substrates  16  may be heated to a fixed temperature using heating block  14 . In order to maintain process control, base  12  may act as a heat sink to maintain proper heat flow out of heating block  14 , and thereby more accurately control substrate temperature, as well as temperature in the heating block. In order to provide appropriate heat conduction between heating block  14  and base  12 , a gas may provided through an inlet (not shown) into a narrow gap (chamber)  18  formed between heating block  14  and base  12 . The gas may aide in thermal conduction to maintain a rapid heat flow into base  12 . This configuration also helps avoid thermal mismatch problems between base  12  and heating block  14  that may occur between the base and heating block, as noted above. 
         [0005]    However, the prior art ESC configuration of  FIG. 1  may result in gas leaks into the process chamber  24  outside of ESC  10 . For example, gas may leak along interface  20  located between heating block  14  and base  12  that is located towards the outside of gap  18 . Because heating block  14  may be a ceramic and base  12  may be a metal, the interfaces may move with respect to one another during heating. In addition, the dissimilar materials may not form an intimate contact at their mutual interface, leading to appreciable leakage of gas in the direction  22 . For example, the pressure in gap  18  may be several tens or of Torr or higher and the pressure outside ESC  10  may be in the mTorr range or less, which large pressure differential, combined with the imperfect seal at interface  22 , may cause a large leak rate of gas into the substrate processing chamber  24 . 
         [0006]    Concomitant with gas leakage, the gas pressure may vary across the gap  18 , as illustrated in  FIG. 1   b , leading to temperature non-uniformities across ESC  10 . The pressure may be highest near the center of the ESC at point I, which may be located near an inlet of gas (not shown) provided to the gap. The pressure may steadily drop toward the outer portion of the gap  18  (R C ) and then rapidly drop across the nominal sealing surface  22  to the outside edge Ro of the ESC, as gas leaks out of gap  18 . This varying pressure may result in a temperature gradient along the x-direction as the rate of heat conduction from heater block  14  to base  12  varies. 
         [0007]    It will be apparent therefore that improvements are desirable over known ESC configurations used for heating substrates. 
       SUMMARY 
       [0008]    In one embodiment, an electrostatic clamp includes a heating block for heating a substrate, the heating block having a first surface disposed toward the substrate and a second surface opposite the first surface. A base is arranged to adjoin at least a portion of the second surface of the heating block. The adjoined base and heating block may mutually define an inner gap between a first portion of the heating block and the base. An outer gap is arranged concentric with the inner gap between a second portion of the heating block and the base, the inner and outer gaps being isolated from one another by a first sealing surface formed between the second surface of the heating block and the base. 
         [0009]    In another embodiment, an electrostatic clamp includes a base portion having a first surface and an outer block for supporting a substrate in a process chamber, the outer block having a second surface arranged to adjoin at least a portion of the first surface of the base portion. The electrostatic clamp may further include a first annular portion coupled to an outer surface of the outer block, the first annular portion having a first sealing surface, and a second annular portion coupled to the outer surface of the outer block and having a second sealing surface. The first annular portion may define an inner compartment and the first and second annular portion together may define an outer compartment concentric with the inner compartment when the substrate is placed on the first and second sealing surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0011]      FIG. 1   a  is a cross-section of a known electrostatic clamp arrangement; 
           [0012]      FIG. 1   b  depicts gas pressure variation as a function of position in the electrostatic clamp of  FIG. 1   a;    
           [0013]      FIG. 2   a  is a cross-section of an electrostatic clamp embodiment; 
           [0014]      FIGS. 2   b - 2   c  depict gas pressure variations for different operating conditions as a function of position in the electrostatic clamp of  FIG. 2   a;    
           [0015]      FIGS. 3   a  and  3   b  are a respective plan view and cross-section of an exemplary base of an electrostatic clamp; 
           [0016]      FIG. 4  is a cross-section of another electrostatic clamp embodiment; and 
           [0017]      FIG. 5  is a schematic of an exemplary ion implantation system. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout. 
         [0019]    In various embodiments, a heated electrostatic clamp may be provided in process equipment including ion implantation systems, plasma etchers, and deposition systems, among other systems. Referring to  FIG. 5 , a block diagram of an ion implanter  100  including an ion source chamber  102  is shown. A power supply  101  supplies the required energy to source  102  which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes  104  (extraction electrodes) and formed into a beam  95  which passes through a mass analyzer magnet  106 . The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer. Ions of the desired species pass through deceleration stage  108  to corrector magnet  110 . Corrector magnet  110  is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to provide a ribbon beam targeted toward a work piece or substrate positioned on support (e.g. platen)  114 . In some cases, a second deceleration stage  112  may be disposed between corrector magnet  110  and support  114 . The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. 
         [0020]    In one embodiment of ion implantation system  100 , substrate platen  114  may be an electrostatic clamp, such as the electrostatic clamp  200  depicted in  FIG. 2   a . The electrostatic clamp embodiment of  FIG. 2   a  may be employed to provide substrate heating or to support an unheated substrate during an implantation process, for example. In other embodiments, the electrostatic clamp  200  may be used to provide substrate heating in other processing apparatus. 
         [0021]    As detailed below, electrostatic clamp (or “clamp”)  200  may facilitate control of process conditions during substrate processing by providing good thermal transfer of heat from a substrate heating block, while minimizing the introduction of gas contaminants into the ambient surrounding the substrate. As is well known, electrostatic clamps may provide sufficient gripping force on a substrate to be especially effective at low pressures (less than several tens of Torr, for example). Electrostatic clamp  200  may, for example, be particularly useful in low pressure apparatus, such as plasma processing systems or the aforementioned ion implantation system, in which it may be desirable to heat a substrate and may be important to control the composition of the gaseous species in the process chamber surrounding the substrate platen  114  ( 200 ). For example, in order to ensure that that substrate  16  is exposed only to the desired species, control of ambient  220  surrounding the substrate platen may be critical. To this end, embodiments of electrostatic clamp  204  reduce leaks  224  of gas that may be used internally within the clamp during its operation. 
         [0022]    In some embodiments, clamp  200  comprises a base  202  and heater block  204 , which may be detachable as depicted in  FIG. 2   a . Heater block  204  may be used to support a substrate  16  during processing. The heater block may be provided with heating apparatus (not shown), such as a thin film heater. During processing, the heater block  204  may be deliberately heated to a desired temperature to heat substrate  16 . In various embodiments, the heater block  204  may be a ceramic material capable of sustaining high temperatures, such as temperatures of at least several hundred degrees Celsius. In order to draw heat from the heater block to maintain the appropriate heater block temperature, the base  202  may be provided with cooling (not shown), such as a circulating liquid. In various embodiments, the base may be a metal having good thermal conductivity, such as aluminum. Because the base  202  and heater block  204  may be dissimilar materials, during heating the two materials may expand at different rates. Accordingly, in various embodiments the base  202  and heater block  204  may be slidably coupled, such that the interfaces  210 ,  214  between the base  202  and heater block  204  can slide with respect to each other. In the configuration of  FIG. 2   a , the base  202  and heater block  204  are mechanically coupled together using springs  218 . The springs  218  may exert a force normal to the common interface that joins base  202  and heater block  204  together to provide a seal along interfaces (sealing surfaces)  210 ,  214 . This seal may form without preventing the interfaces from sliding with respect to one another in the x-y plane (as indicated in  FIG. 2   a ). This type of coupling allows base  202  to expand or contract independently of heater block  204 . 
         [0023]    In various embodiments, an ESC may be provided with gaps (also referred to herein as “compartments” or “chambers”) between heating block  204  and base  202 . These gaps may constitute small gaps in the z-direction normal to the interface between heating block  204  and base  202 . In some embodiments, the ESC forms two gaps, for example, gaps  208  and  212  shown in  FIG. 2   a . In some embodiments, as illustrated in  FIG. 2   a , gap  208  is a large circular gap that is circumscribed by a concentric annular gap  212 . In some embodiments, the ESC gaps may be formed by providing recesses in the base  202 , as depicted in  FIGS. 3   a ,  3   b . As illustrated, a circular recess  208  and annular recess  212  are provided within the base  202 , such that the portion of base  202  that is to be joined to the heating block  204  comprises two annular surfaces  210  and  214 . 
         [0024]    In the embodiment of  FIG. 2   a , heating block  204  has a planar lower surface that forms sealing surfaces with base  202  at surfaces  210 ,  214 . However, in other embodiments, the heating block may be provided with recesses that serve to form interior gaps when joined with a base. 
         [0025]    Gap  208  may be coupled to an inlet (not shown) that provides a gas to gap  208  for use during substrate heating. During substrate processing, gas may be provided into gap  208  and pumped out through an outlet (not shown) such that the pressure in gap  208  is maintained in a desired range. In some embodiments, the gas pressure may be in the one-Torr, ten-Torr or hundred-Torr range, which may be sufficient to conduct heat from heating block  204  to base  202  at a desired rate. 
         [0026]    Because the heating block  204  and base  202  may be dissimilar materials, and may slide with respect to one another along their sealing surfaces  210 ,  214 , the sealing surfaces may fail to seal sufficiently to prevent gas from leaking out of gap  208 . When gas leaks out of gap  208  along interface  210 , the gas may enter gap  212  that surrounds gap  208 . Gap  212 , in turn, is isolated from ambient  224  by sealing surface  214 . Accordingly, any gas leaking into gap  212  from gap  208  may be hindered from entering the ambient surrounding the ESC  200  by the presence of outer sealing surface  214 . 
         [0027]    In some embodiments, gap  212  is coupled to a pumping port  216  such that gas can be pumped (evacuated) out of gap  212 . In the embodiment shown in  FIGS. 2   a  and  3   a ,  3   b , the pumping port is provided in the base  202 , but may be provided in the heating block  204 . This may help maintain the average pressure in gap  212  at a much lower value than that in gap  208 . 
         [0028]    In various embodiments, the evacuation rate or pressure partial pressure of gap  212  can be tailored according to processing requirements.  FIGS. 2   b  and  2   c  present two different pressure curves  220  and  230 , respectively, which may represent the pressure in various portions of ESC  200  as a function of radial position for two different embodiments. Curve  220  may represent the case in which a rapid pumping rate of chamber  212  takes place, while curve  230  represent the case in which a very low (or no) pumping rate takes place. 
         [0029]    Under processing conditions in which it is critical to minimize any gas leaks into ambient  220 , a high pumping rate of gap  212  may be useful. As illustrated in  FIG. 2   b , during operation of ESC  200  in which gas is provided into gap  208 , the inlet may be near the center causing a maximum in gas pressure towards the center at point I. Because gap  212  is evacuated, gas may leak out of gap  208  and into chamber  212 , thereby causing a drop in gas pressure at R C1  as compared to point I. In gap  212 , gas  222  leaking into the gap may be rapidly pumped out, such that the average pressure in gap  212  is much lower than that in gap  208 . For example, the pressure in gap  208  may be in the 10 Torr range, while the pressure in gap  212  may be in the 100 mTorr range. In addition, the pressure may rapidly drop from the inner radius R c2i  to the outer radius R c2o  such that the pressure of gas at interface  214  is much lower than at interface  210 . Accordingly, interface  214 , though forming an imperfect seal, may be sufficient to reduce the gas leak rate out of ESC  200  to an acceptable level, such as a level where impacts upon a substrate process are undetectable. If the pumping rate is high enough, the gas pressure at the outer radius R c2o  of gap  212  may be so low that little, if any, gas escapes into ambient  220  at the outside edge R 0  of ESC  220 , as depicted in  FIG. 2   b.    
         [0030]    Accordingly, in one example, if the process window for successfully processing substrates may tolerate a drop in gas pressure across inner gap  208 , a system may be arranged to evacuate gap  212  to a low gas pressure to avoid gas contamination in the ambient  220 . For example, for a given heating condition, the substrate temperature may be constant or within an acceptable temperature process window over a range of different gas pressure in the gap  208 , which gas pressure range may fall within that exhibited by curve  220 . 
         [0031]    In the case a small gas leak into ambient  220  is of less concern, ESC  200  provides the ability to maintain a more uniform gas pressure in gap  208 , thereby providing a uniform thermal conduction towards base  202  as a function of radial position. In some embodiments, the gas pressure differential between gaps  208  and  212  may be arranged such that the drop in pressure across gap  208  is much less than in the single gap configuration of prior art ESC  10 . This may be accomplished, for example, by reducing or eliminating evacuation of gap  212 , such that the pressure in gap  212  is similar to that in gap  208 , as illustrated by curve  230  in  FIG. 2   c . However, the relatively high pressure of gas in gap  212  may cause a more significant gas leak into ambient  220  as illustrated. Accordingly, in some embodiments, by varying the pressure in gap  212 , substrate processing using ESC  200  may be tuned to emphasize more uniform pressure (therefore thermal conduction in the ESC) in central gap  208  on the one hand or a lower gas leak rate into the processing ambient  220  on the other hand. 
         [0032]    Advantageously, the embodiment of ESC  200  thereby provides both the ability to obtain a more uniform gas pressure profile in a central gap region  108 , as well as the ability to obtain a lower gas leakage rate into a process chamber  220  than conventional ESC apparatus, even though a tradeoff between the two may exist as detailed above. 
         [0033]    Moreover, in some embodiments, the diameter of gap  208  may be arranged to approximate the substrate size, so that a more uniform thermal profile is experienced by the substrate. In  FIG. 2   a , for example, the gap  208  is nearly the size of active area A of substrate  16 . 
         [0034]      FIG. 4  depicts another ESC embodiment in which gas flow may be provided directly upon a substrate surface. In ESC  400 , a base  202  is coupled to a heating block  204  that is provided with annular features  414 ,  416  that protrude above main surface  410  of heater block  404 . In some embodiments, the annular features  414  and  416  are may be rings that are removable from heater block  404 . In other embodiments, the features may be integral to heater block  404 , wherein the upper portion of heater block  404  facing substrate  16  has a recessed structure similar to base  202  depicted in  FIGS. 3   a ,  3   b . The surfaces  418 ,  420  of respective annular features  414  and  416  may be arranged as sealing surfaces that are configured to grip wafer  16 . When wafer  16  is in contact with features  414 ,  416 , an inner compartment  408  and an outer compartment  412  are formed between substrate  16  and the main body of heater block  404 . 
         [0035]    In some embodiments, inner compartment  408  is provided with gas inlet(s) and outlet(s) (not shown), which may be disposed in heater block  404 . Flowing gas may be provided into compartment  408  during substrate processing to provide a good thermal conduction path for heat to transfer into or out of substrate  16 . During substrate processing, the pressure of flowing gas may be adjusted to provide a desired thermal conductivity, as discussed previously with respect to gap  208 . In some embodiments, the gas pressure in compartment  408  may be in the range of tens of Torr to hundreds of Torr. As in the case of mechanical coupling between heater block  204  and base  202 , the substrate  16  and annular surfaces  418 ,  420  may each form an imperfect seal such that gas leaks from compartments  408  and  412 . In various embodiments, compartment  412  may be provided with a pumping port  422  that serves to pump gas out of compartment  412  when a pump (not shown) is coupled to the port and turned on. 
         [0036]    Similarly to the situation depicted in  FIG. 2   b , the concentric-compartment configuration provided by heater block  404  may provide a much lower pressure in the outer compartment  412  than in the inner compartment  408 , thereby providing a much lower gas pressure at interface  420 , which is directly coupled to the ambient  430 . Accordingly, a relatively high pressure of gas may be provided in inner chamber  408  to facilitate heat transfer between substrate  16  and block  404 , without substantial gas leakage into ambient  430 . 
         [0037]    In embodiments of ESC  400 , the inner annular region  414  may be arranged at a radial position such that chamber  408  extends under a large portion of substrate  16 , for example, chamber  408  may extend under the active area of substrate  16 . 
         [0038]    In summary, embodiments of the disclosure provide ESC configurations that provide gas-filled gaps to manage thermal conduction between a heating block and cooling base and minimize gas leakage therefrom. In some embodiments a gap is provided between the heating block and base while in other embodiments a gas-filled gap is provided on both sides of a heating block, i.e., between the heating block and a respective substrate on one hand and a base on the other hand. 
         [0039]    In some embodiments the concentric gap arrangement of block  404  and substrate  16  depicted in  FIG. 4  may be used without heating of the substrate, such as in processing situations in which it may be desirable to supply a gas between substrate  16  and a block  404  for other purposes. 
         [0040]    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. In particular, embodiments involving substrate holders other than an electrostatic clamp are possible. In addition, embodiments are possible in which more than one annular gap are provided concentric to an inner gap. 
         [0041]    Thus, such other embodiments and modifications are in the tended 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. Thus, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.