Patent Publication Number: US-2010107974-A1

Title: Substrate holder with varying density

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to semiconductor substrate handling systems and, in particular, to systems and methods for supporting a substrate during material deposition processes. 
     2. Description of the Related Art 
     High-temperature ovens, or reactors, are used to process substrates for a variety of reasons. In the electronics industry, substrates, such as semiconductor wafers, are processed to form integrated circuits. In a reaction process, a substrate, typically a circular silicon wafer, is placed on a substrate holder. In some processes, the substrate holder helps to attract radiation and more evenly heat the substrate. These substrate holders are sometimes referred to as susceptors. The substrate and substrate holder are enclosed in a reaction chamber, typically made of quartz, and heated to high temperatures, typically by a plurality of radiant heat lamps placed around the quartz chamber. 
     In an exemplary high temperature process, a reactant gas is passed over the heated substrate, causing the chemical vapor deposition (“CVD”) of a thin layer of the reactant material onto a surface of the substrate. As used herein, the terms “processing gas,” “process gas,” and “reactant gas” generally refer to gases that contain substances, such as silicon-containing gases, to be deposited on a substrate. As used herein, these terms do not include cleaning gases. Through subsequent processes, the layers of reactant material deposited on the substrate are made into integrated circuits. The process gas flow over the substrate is often controlled to promote uniformity of deposition across the top or front side of the substrate. Deposition uniformity can be further promoted by rotating the substrate holder and substrate about a vertical center axis during deposition. As used herein, the “front side” of a substrate refers to the substrate&#39;s top surface, which typically faces away from the substrate holder during processing, and the “backside” of a substrate refers to the substrate&#39;s bottom surface, which typically faces the substrate holder during processing. 
     As mentioned above, a typical substrate to be processed is comprised of silicon. In the production of integrated circuits, it is sometimes desirable to deposit additional silicon, for example via CVD, onto the substrate surface(s). If the additional silicon is deposited directly onto the silicon surface of the substrate, the newly deposited silicon maintains the crystalline structure of the substrate. This type of deposition is known as epitaxial deposition. However, the surfaces of the original substrate to be processed are typically polished on both sides. When brought into contact with an oxygen environment, a native oxide layer, such as SiO 2 , is formed on the substrate. A deposition of silicon onto the native oxide layer forms polysilicon deposits. In order to conduct epitaxial deposition, it is ordinarily necessary to remove the native oxide layer from each of the substrate&#39;s top and/or bottom surfaces onto which new silicon is to be deposited. The native oxide layer is typically removed by exposing it to a cleaning gas, such as hydrogen gas (H 2 ), at a sufficiently high temperature, prior to the deposition of additional silicon. As used herein, the term “cleaning gas” is different than and does not encompass reactant gases. 
     There are a large variety of different types of substrate holders for supporting a substrate during processing. A typical substrate holder comprises a body with a generally horizontal upper surface that receives and/or underlies the supported substrate. A spacer or spacer means is often provided for maintaining a small gap between the supported substrate and the horizontal upper surface of the substrate holder. This gap prevents process gases from causing the substrate to stick to the substrate holder. The substrate holder may include an interior portion that supports the substrate from below and an annular shoulder that closely surrounds the supported substrate. One type of spacer or spacer means comprises a spacer element fixed with respect to the substrate holder body, such as an annular lip, a plurality of small spacer lips, spacer pins or nubs, etc. An alternative type of spacer element comprises a plurality of vertically movable lift pins that extend through the substrate holder body and are controlled to support the position of the substrate above the upper surface of the substrate holder. Often, the spacer element is positioned to contact the substrate only within its “exclusion zone,” which is a radially outermost portion of the substrate within which it is difficult to maintain deposition uniformity. The exclusion zone is typically not used in the manufacturing of integrated circuits for commercial use, due to the non-uniformity of deposition there. A processed substrate may be characterized, for example, as having an exclusion zone of five millimeters from its edge. 
     One problem associated with CVD is the phenomenon of “backside deposition.” Many substrate holders are unsealed at the substrate perimeter so that process gases can flow down around the peripheral edge of the substrate and into the gap between the substrate and the substrate holder. These process gases tend to deposit on the substrate backside, both as nodules and as an annular ring at or near the substrate edge. This undesirable deposition creates non-uniformities in substrate thickness, generally detected by local site flatness tools. Such non-uniformities in substrate thickness can adversely affect chucking down of the substrate, and thus make impossible subsequent processing steps, such as photolithography. 
     One method for reducing backside deposition involves the use of a purge gas that flows upwardly from between the substrate holder and substrate and around the substrate edge to reduce the downward flow of cleaning or process gases. Conventional purge gas systems typically include gas flow channels to allow for the flow of purge gas through the substrate holder. 
     Another problem in semiconductor processing is known as autodoping. Autodoping can cause undesired variations in dopant concentration on the substrate, particularly in high-temperature epitaxial deposition processes. The formation of integrated circuits involves the deposition of dopant material, such as doped silicon, onto the front side of the substrate. Autodoping is the tendency of dopant atoms to diffuse downwardly through the substrate, emerge from the substrate backside, and then travel between the substrate and the substrate holder up around the substrate edge to redeposit onto the substrate front side, typically near the substrate edge. These redeposited dopant atoms adversely affect the performance of the integrated circuits, particularly semiconductor dies from near the substrate edge. Autodoping tends to be more prevalent and problematic for higher-doped substrates. 
     One method of reducing autodoping involves a susceptor that has a plurality of holes that permit the flow of gas between the regions above and below the susceptor. Autodoping is reduced by directing a flow of inert gas horizontally underneath the susceptor. Some of the gas flows upwardly through the holes of the susceptor into a gap region between the susceptor and a substrate supported by the susceptor. As diffused dopant atoms emerge at the substrate backside, they become swept away by the gas downwardly through the holes in the susceptor. In this way, the dopant atoms tend to get drawn down into the region below the susceptor. 
     SUMMARY 
     In one aspect, a substrate support system has a substrate holder for supporting a substrate of a particular size in a supported position above an upper surface of an interior portion of the substrate holder. The upper surface of the interior portion has a substrate center alignment point configured to vertically align with a center of the substrate when the substrate is in the supported position on the substrate holder. The substrate center alignment point of the upper surface of the interior portion is configured to be spaced further apart from the substrate than an outer perimeter of the interior portion when the substrate is in the supported position on the substrate holder. A mass density of the interior portion varies along one or more radial lines extending from the substrate center alignment point of the interior portion. 
     In another aspect, a substrate support system includes a substrate holder for supporting a substrate. The substrate holder has a mass density that varies along a radius from a center of the substrate holder to an outer perimeter of the substrate holder. The substrate holder is formed of a porous material having a porosity between about 10%-40% and configured to allow gas flow therethrough. 
     In another aspect, a substrate support system comprises a substrate holder for supporting a substrate of a particular size in a defined supported position. The substrate holder comprises holes extending to and between upper and lower surfaces of the substrate holder. The substrate holder has a point configured to vertically align with a center of the particularly sized substrate when the substrate is in the supported position. The substrate holder has a mass density that decreases along a radius from the point to an outer annular location of the substrate holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be readily apparent to the skilled artisan in view of the description below, the appended claims, and from the drawings, which are intended to illustrate and not to limit the invention, and wherein: 
         FIG. 1  is a schematic, cross-sectional view of an exemplary reaction chamber with a substrate supported on a substrate holder. 
         FIG. 2  is a schematic representation of a substrate supported on an embodiment of a substrate holder with a varying density. 
         FIG. 3  is a top view of a substrate holder according to one embodiment, wherein the mass density of the substrate holder varies radially by varying a hole density from a substrate center alignment point of the substrate holder. 
         FIG. 4  is a top view of a substrate holder according to another embodiment, wherein the substrate holder has three regions with different hole densities. 
         FIG. 5  is a top view of a substrate holder according to another embodiment, wherein the mass density of the substrate holder varies radially by varying a hole size from a substrate center alignment point of the substrate holder. 
         FIG. 6  is a top view of a substrate holder according to another embodiment, wherein the substrate holder has three regions with different hole sizes. 
         FIG. 7  is a cross-sectional view of an embodiment of a substrate holder wherein the mass density of the substrate holder varies radially by varying a recess density from a substrate center alignment point of the substrate holder. 
         FIG. 8  is a cross-sectional view of an embodiment of a substrate holder wherein the mass density of the substrate holder varies radially by varying the size of recesses from a substrate center alignment point of the substrate holder. 
     
    
    
     The drawings are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following detailed description of the preferred embodiments and methods describes certain specific embodiments to assist in understanding the claims. However, one may practice the present invention in a multitude of different embodiments and methods as defined and covered by the claims. 
     Referring more specifically to the drawings for illustrative purposes, the present invention is embodied in the devices generally shown in the figures. It will be appreciated that the apparatuses may vary as to configuration and as to details of the parts, and that the methods may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein. 
     Two problems to avoid in a substrate processing system are crystallographic slip and backside damage. Slip refers to the formation of crystal defects in the substrate, and is caused primarily by temperature variations across the substrate surface. Temperature variations can be reduced by minimizing the gap between the substrate and the substrate holder, particularly at the substrate&#39;s center. The thermal mass of the substrate holder is typically much larger than that of the substrate, such that the substrate holder temperature tends to be more uniform than the substrate temperature. Thermal gradients across the substrate are remedied to an extent by reducing the aforementioned gap between the substrate and the substrate holder so as to boost the thermal coupling of the two components. 
     Backside damage refers to damage that is caused by contact between the substrate backside and the substrate holder. As noted above, the substrate is typically supported on several spacers, which isolates and minimizes the contact between the substrate and the substrate holder. Typically, the spacers are located near the edge of a supported substrate, because the edge portion of the substrate (sometimes referred to as the “exclusion zone”) is often not used in the formation of integrated circuits. Unfortunately, the substrate often tends to bow or warp slightly when supported by the substrate holder, for example, when the substrate is being heated after loading, due to temperature gradients across the substrate surface. Notwithstanding the use of the spacers, the substrate&#39;s bowing or warping can cause it to contact the upper surface of the substrate holder, particularly at or near the center of the substrate. One approach to preventing consequent backside damage is to increase the size of the gap between the substrate and the substrate holder by increasing the height of the spacers. Another approach is to use a substrate holder with a concave upper surface, and to use a concavity depth that is sufficient to avoid contact between the substrate and the substrate holder caused by bowing or warping of the substrate. Substrate holders with concavities often still include spacers that support the substrate. 
     Unfortunately, these approaches to preventing crystallographic slip and backside damage oppose one another. That is, increasing the gap between the center of the substrate and the substrate holder decreases the risk of backside damage but increases the risk of crystallographic slip due to temperature gradients across the substrate. In substrate holders with concave upper surfaces, temperature gradients occur because the edge region of the substrate is closer to the substrate holder than the center of the substrate. Regardless of whether the substrate holder has a concavity, temperature gradients are also caused due to the contact between the substrate and the spacers. The heightened gap reduces the thermal coupling between the substrate holder and the substrate, which makes it easier for temperature gradients to exist. 
     Hence there is a need for reducing both crystallographic slip and backside damage simultaneously. One way of doing this is with a substrate holder having a concave shape, so as to avoid backside damage, while also varying the thermal coupling between the substrate holder and the substrate  16 , so as to reduce the risk of temperature gradients within the substrate. One way of varying the thermal coupling is by varying the thermal mass density of the substrate holder. As used herein, thermal mass is related to how quickly or slowly a material or structure reacts to temperature variations. Hence, a substrate holder with a high thermal mass will react slowly to temperature variations. As used herein, thermal mass density is a measure of thermal mass per unit volume of the substrate holder. The thermal mass of the substrate holder may depend on, among other factors, the mass density of the substrate holder. Accordingly, the present application discloses several embodiments of substrate holders whose mass density varies to compensate for variations in surface geometry of the substrate holder, such as a concavity, in order to provide a substantially uniform thermal coupling between the substrate holder and a substrate supported thereon. 
     Prior to describing certain embodiments of the substrate holder, an exemplary CVD reactor is disclosed.  FIG. 1  illustrates an exemplary CVD reactor  10 , including a quartz reaction chamber  12 . Radiant heating elements  14  are supported outside the transparent chamber  12  to provide heat energy to the chamber  12  without appreciable absorption by the chamber walls. Although the embodiments are described in the context of a “cold wall” CVD reactor, it will be understood that the substrate support systems described herein can be used in other types of reactors and semiconductor processing equipment. Skilled artisans will appreciate that the invention is not limited to use within the particular reactor  10  disclosed herein. In particular, one of ordinary skill in the art can find application for the substrate support systems described herein for other semiconductor processing equipment, wherein a substrate is supported while being heated, cooled, or processed. Moreover, while illustrated in the context of standard silicon wafers, the substrate holders described herein can be used to support other kinds of substrates, such as glass substrates which are subjected to treatments, such as CVD, physical vapor deposition (“PVD”), etching, annealing, dopant diffusion, or photolithography. The substrate holders are of particular utility for supporting substrates during treatment processes at elevated temperatures. Also, skilled artisans will appreciate that the embodiments described herein include substrate holders that are susceptors as well as those that are not susceptors. 
     The radiant heating elements  14  typically include two banks of elongated tube-type heating lamps arranged in orthogonal or crossed directions above and below a substrate holder holding a substrate  16 . Each of the upper and lower surfaces of the substrate can face one of the two banks of heating lamps  14 . According to an embodiment, a controller within the thermal reactor adjusts the relative power to each lamp  14  to maintain a desired temperature during substrate processing. 
     The illustrated substrate  16  includes a generally circular edge  17 , shown in  FIG. 1 , supported within the reaction chamber  12  upon a substrate support system  140 . The illustrated substrate support system  140  includes a substrate holder  100 , upon which the substrate  16  rests, and a spider  22  that supports the substrate holder  100 . Several embodiments of the substrate holder  100  are shown in greater detail in  FIGS. 2-8 , which are described below. The spider  22  can be formed of a transparent and non-metallic material. The skilled artisan will appreciate that the non-metallic aspect of the material helps to reduce contamination. The spider  22  may have arms  148  that are configured to support the bottom surface of the substrate holder  100 . In certain embodiments, the spider  22  can be hollow and capable of conveying a sweep gas upward through holes of the substrate holder  100 . Examples of hollow spiders used in conjunction with perforated substrate holders are disclosed in U.S. Patent Publication No. 2005-0193952 and in U.S. patent application Ser. No. 12/116,114, filed on May 6, 2008. 
     In an embodiment, the substrate holder  100  comprises a susceptor capable of absorbing radiant energy from the heating elements  14  and re-radiating such energy. The substrate holder  100  can be solid and formed of a single piece. Alternatively, the substrate holder  100  can be formed of multiple pieces that are assembled or attached together, such as pieces comprising an interior portion and one or more surrounding concentric annular portions, as described below. According to an embodiment, the spider  22  and the substrate holder  100  may be configured to rotate in unison about a vertical center axis during substrate processing. 
     Temperature sensors or thermocouples  28 ,  30  may be provided for sensing the temperature at the center of the substrate holder  100 . The thermocouples  28 ,  30  may be connected to a temperature controller (not shown), which controls and sets the power of the various radiant heating elements  14  in response to the temperature readings of the thermocouples  28 ,  30 . 
     A slip ring  32  may be configured to absorb radiant heat during high temperature processing. The heated slip ring  32  helps to reduce heat loss at the substrate edge  17 . As illustrated, the dividers  36  divide the reactor  10  into an upper chamber  2  designed for the flow of reactant or process gases, for example for CVD on the substrate surface, and a lower chamber  4 . The dividers  36  and other elements of the reactor  10  can substantially prevent fluid communication between the chambers  2  and  4 . However, because the substrate holder  100  can be rotatable about a vertical center axis, a small clearance typically exists between the substrate holder  100  and the slip ring  32  or other elements. Thus, it is often difficult to completely prevent fluid communication between the upper chamber  2  and the lower chamber  4 . This problem is typically addressed by creating a pressure differential between the chambers  2 ,  4 , such that pressure is higher in the lower chamber  4  to inhibit downward flow of gases from the upper chamber  2  to the lower chamber  4 . While  FIG. 1  depicts the substrate  100  within an exemplary CVD reactor, the various embodiments of substrate holders disclosed herein may apply to rapid thermal annealing systems and other non-deposition applications where control of heating is desirable. 
       FIG. 2  depicts a schematic cross section of a substrate holder  200  with a varying density, for supporting a substrate  16 . In various embodiments, the substrate holder  200  has a thermal mass density that varies across the substrate holder  200 . For example, in some embodiments, the thermal mass density may vary along one or more radial lines extending from the center  210  of the substrate holder  200 . This makes it possible to compensate for temperature gradients that would otherwise occur across the substrate surface, such as gradients caused by the concave upper surface of the illustrated holder  200 . The ability to compensate for temperature gradients in turn makes it possible to increase the gap between the substrate  16  and the substrate holder  200  at the center of the substrate, thereby reducing the risk of backside damage to the substrate. Hence, as illustrated in  FIG. 2 , the substrate holder  200  may be configured to be spaced further apart from the substrate  16  at the center of the substrate  16  than at the outer perimeter  220  of the substrate  16  when the substrate  16  is supported by the substrate holder  200 . 
     For example, substrate holder  200  may comprise an interior portion  230  that underlies a supported substrate  16 . The interior portion  230  may be configured to support a substrate  16  from below. The substrate holder  200  may also comprise one or more spacers or supports  240  that contact the backside  236  of the substrate  16  from below the substrate  16 . There may be three such supports  240 , each angularly spaced about 120° apart from the other (and hence only one is shown in the cross section of  FIG. 2 ), however, other configurations are possible. For example, the supports  240  may comprise an annular lip formed near the outer perimeter  220  of much of the interior portion  230  of the substrate holder  200 . The top surface  250  of the interior portion  230  may be generally concave in shape. In alternative embodiments, the top surface  250  can be substantially conical, with a lower vertex at the center  210  (or substrate center alignment point  265 , described below, if it is different than the center  210 ) of the substrate holder  200 . As illustrated in  FIG. 2 , the top surface  250  is substantially concave, although other surface profiles are possible. For example, the top surface  280  may comprise curved surfaces of varying curvature or the top surface  280  may comprise different annular regions of different frustoconical shapes. 
     In some embodiments, a substrate center alignment point  265  of the substrate holder  200  may be configured to substantially vertically align with a center  215  of the substrate  16  when the substrate is supported by the substrate holder  200  in a substantially horizontal position of the substrate. The location  265  can be the center  210  of the substrate holder  200 , as in the illustrated embodiment, or alternatively offset from the center  210 . 
     As illustrated in  FIG. 2 , the interior portion  230  and the substrate holder  200  have a common center  210 . However, this is not necessary. The thermal mass density may vary along paths extending radially from the center  210  to the outer perimeter  220  of the interior portion  230 . In some embodiments, the thermal mass density may vary radially from the center  210  along the entirety of the interior portion of the substrate holder  200 , i.e., along each radial direction. The thermal mass density profile may be axisymmetric about the center  210  or substrate center alignment point  265 . That is to say that the thermal mass density may be substantially uniform at any given radial distance from the center of the interior portion  230 . In other embodiments, the thermal mass density may be non-axisymmetric. The thermal mass density may decrease from the center  210  of the interior portion  230  to the outer perimeter  220  of the interior portion  230 . The thermal mass density variation may be substantially gradual and/or linear along each radial line. Alternatively, the thermal mass density may vary in discrete steps, such as by providing multiple radial sections with different substantially uniform thermal mass densities. One of ordinary skill in the art will appreciate that various thermal mass density variation profiles may be utilized to minimize the occurrence of temperature gradients across the substrate  16 . For example, the thermal mass density may be very small in areas close to or around the supports  240 , where the substrate holder  200  actually contacts and supports the substrate  16 . 
     One way of varying the thermal mass density is by varying the mass density of the substrate holder  200 . Hence, in various different embodiments, the mass density may decrease along lines extending radially from the center  210  or location  265  of the interior portion  230  of the substrate holder  200 . The substrate holder  200  is preferably configured to have a mass density that decreases along a radius from the center  210  or location  265  to the outer annular shoulder  225 . The mass density may vary substantially gradually, linearly, and/or continuously. Alternatively, it can vary non-smoothly as described above. That is to say that the mass density may be different in different distinct regions of the substrate holder  200 . In some embodiments, the mass density may be greater near the center of the interior portion  230  of the substrate holder  200  than near the outer perimeter  220  of the interior portion  230  of the substrate holder  200 . In yet other embodiments, the mass density may be greatest at or near the center  210  or location  265  of the interior portion  230  of the substrate holder  200 , with the mass density varying as desired along the radius  270  out to the outer perimeter  220  of the interior portion  230 . For example, the mass density may be anywhere from 10% to 100% of the nominal mass density of the bulk solid material from which the interior portion of the substrate holder is formed. Therefore, in some embodiments, the mass density near the center  210  of the substrate holder  200  may be equal to the mass density of the bulk solid material. The mass density may be varied to be a fraction of the nominal mass density of the bulk solid material at various points along a radius  270  away from the center  210  as desired. 
     In some embodiments, the substrate holder  200  may comprise holes  260  each extending from the top surface  250  to a bottom surface  280  of the holder  200 . The substrate holder  200  may also have an outer annular shoulder  225  configured to extend slightly beyond an outer perimeter or edge  17  of the substrate  16 . The mass density may vary by varying a density of the holes  260  (see  FIGS. 3 and 4 ) along a radius from the location  265  to the outer annular shoulder  225 . Alternatively, the mass density can be varied by varying a size of the holes (see  FIGS. 5 and 6 ). Similarly, the mass density may be varied by varying a density or size of recesses provided in place of the holes  260  (see  FIGS. 7 and 8 ). As used herein, a hole or recess density is the number of holes or recesses per unit area of top surface  250  or bottom surface  280 . The holes or recesses may all be of an equal size, or may vary in size. 
     One way to vary the mass density of the substrate holder  200  is to vary the hole density along the radius  270  of the interior portion  230  of the substrate holder  200 . The density of the holes  260 , or the hole density, can vary substantially gradually, linearly, and/or continuously. Alternatively, multiple discrete radial sections can have different substantially uniform hole densities.  FIGS. 3 and 4  show schematic top views substrate holders  300 ,  400  having a hole density that varies along a radius  270 . The variation in the hole density, and hence the mass density, may be in the interior portion or across the entire substrate holder  300 ,  400 . In the embodiment of  FIG. 3 , the holes  260  are preferably all of an equal size, although the hole density varies across the substrate holder  300 .  FIG. 3  illustrates a substrate holder  300 , whose mass density is varied by varying the hole density gradually. As illustrated in  FIG. 3 , the hole density varies substantially continuously, and preferably substantially axisymmetrically, along the radius  270  of the substrate holder  300 .  FIG. 4  illustrates a substrate holder  400  whose mass density is varied by varying the hole density in multiple discrete central and radial/annular sections. For example, the illustrated substrate holder  400  comprises a central circular region  410  having a first substantially uniform hole density and an annular region  420  surrounding the central region  410 , the annular region  420  having a second substantially uniform hole density that is different from the first hole density. Annular region  420  may extend to the outer perimeter  220  ( FIG. 2 ) of the substrate  16  or may be surrounded by another annular region  430 , as illustrated, having a third substantially uniform hole density that is different from the first and second hole densities. In other embodiments, the substrate holder  400  may comprise a central circular region  410  having a first substantially uniform hole density and a plurality of substantially concentric annular regions  420 ,  430 , etc. surrounding the central region  410 , the annular regions  420 ,  430 , etc. each having a substantially uniform hole density that is different from the first hole density. The hole densities of the annular regions  420 ,  430 , etc. may also be different from each other. 
     Another way to vary the mass density of the substrate holder  200  is to vary the size of the holes  260  along the radius  270 . The hole size can vary substantially gradually, linearly, and/or continuously. Alternatively, multiple discrete radial/annular sections can have differently sized holes.  FIGS. 5 and 6  show schematic top views of substrate holders  500 ,  600  having a hole size that varies along a radius  270 . The variation in the hole size, and hence the mass density of the substrate holder, may be in the interior portion or across the entire substrate holder  500 ,  600 .  FIG. 5  illustrates a substrate holder  500 , where the mass density is varied by varying the hole size gradually or smoothly. As illustrated in  FIG. 5 , the hole size varies substantially gradually and continuously along the radius  270  of the substrate holder  500 .  FIG. 6  illustrates a substrate holder  600 , where the mass density is varied by varying the hole size in a central section and one or more annular sections surrounding the central section. For example, the illustrated substrate holder  600  comprises a central circular region  610  where the holes  260  have a substantially uniform first hole size and an annular region  620  surrounding the central region  610 , the annular region  620  having holes with a substantially uniform second hole size that is different from the first hole size. Annular region  620  may extend to the outer perimeter  220  ( FIG. 2 ) of the substrate  16  or may be surrounded by another annular region  630  having a third substantially uniform hole size that is different from the first and second hole sizes. In other embodiments, the substrate holder  600  may comprise a central circular region  610  where the holes  260  have a substantially uniform first hole size and a plurality of substantially concentric annular regions  620 ,  630 , etc. surrounding the central region  610 , the annular regions  620 ,  630 , etc. each having holes  260  with a substantially uniform hole size that is different from the first hole size. The hole sizes of the annular regions  620 ,  630 , etc. may also be different from each other. 
     The holes  260  of  FIGS. 3-6  may be of a suitable size for the purposes described herein. While the size of the holes  260  is shown quite large (as in  FIGS. 5 and 6 ), it will be understood that this is done for the purpose of illustration, and that the holes may be smaller or even larger than depicted, as required. In some embodiments, each of the holes  260  in the interior portion  230  of the substrate holder have only one upper end at the upper surface  250  of the substrate holder and only one lower end at the lower surface  280  of the substrate holder, wherein none of the holes  260  are connected to any others of the holes  260 . 
     The holes  260  of  FIGS. 3-6  may help to prevent autodoping, as discussed previously. Sometimes the holes can result in the direct impingement of relatively focused, high velocity flows of purge gas onto the substrate backside. These focused, high velocity flows of purge gas onto the substrate backside can cause localized cooling or “cold spots” in the substrate, which adversely affect the uniformity of deposited materials on the substrate. Hence, an alternative approach to preventing autodoping is to form the substrate holder  200  from a porous material that allows diffused dopant atoms to flow downwardly through the substrate holder  200 , without providing through holes  260 . Hence, in an embodiment, the substrate holder  200 , or the interior portion  230  of the substrate holder  200 , may comprise a porous material, such as a material that is sponge-like yet rigid, having a porosity of greater than 10%, or within 10-40%. 
     In embodiments where the substrate holder  200  ( FIG. 2 ) comprises a porous material, it may be desirable to vary the thermal mass density along the radius  270  of the substrate holder  200 . Hence, in various embodiments, the substrate holder  200  has a mass density that varies, preferably axisymmetrically, along a radius  270  from the center  210  or substrate center alignment point  265  of the substrate holder, wherein the substrate holder  200  is formed of a material having a porosity preferably between about 10%-40% and configured to allow gas flow therethrough. In some embodiments, the mass density of the porous material may be varied by varying the porosity of the material across the substrate holder. In such embodiments, the through holes  260  are preferably omitted. 
     With reference to  FIGS. 2 ,  7 , and  8 , in embodiments where the substrate holder  200  comprises a porous material, the substrate holder  200  may comprise recesses  290  defining thinned portions to provide for easier diffusion of gas through the porous material. Such recesses  290  may be provided as an alternative to the holes  260  described above. Recesses  290  may be formed in the interior portion  230  of the substrate holder  200 , or alternatively throughout the substrate holder  200 . Porous material substrate holders with recesses for defining thinned portions are more fully described in U.S. patent application Ser. No. 12/116,114, filed May 6, 2008. As discussed above with respect to holes  260 , the mass density of the interior portion  230  of the substrate holder can be varied by varying the density and/or size of the recesses  290 . The density and/or size of the recesses  290  may vary substantially gradually, linearly, smoothly, and/or continuously along the radius of the substrate holder  200 . In other embodiments, the mass density of the porous material may vary discontinuously, such as by discontinuously varying the density and/or size of recesses  290 . 
     As used herein, a “porous material” refers to a material that is inherently porous and gas-permeable. Thus a substrate holder formed of a “porous material” is gas-permeable regardless of the presence or non-presence of the man-made holes  260  formed within the substrate holder  200 . In one embodiment, the porosity of the porous material is between about 10-40%. In another embodiment, the porosity of the porous material is between about 20-30%. Such porosity of the substrate holder  200  allows sufficient flow therethrough of gas in thinned portions formed by recesses or cut-outs  290  in the upper surface  250  or lower surface  280 . Such gas flow prevents or reduces backside deposition and autodoping, as described above. According to an embodiment, the porous material is a composite silicon carbide material, such as one available from XyCarb Ceramics/Schunck Semiconductor of The Netherlands. In an embodiment, the porous material has a density in a range of about 0.5-1.5 g/cm 3 , such as about 1.0 g/cm 3 . In some embodiments, the mass density varies along a radius from the center  210  or substrate center alignment point  265  of the interior portion  230  to the outer perimeter of the substrate holder. 
       FIGS. 7 and 8  respectively show embodiments of substrate holders  700  and  800  formed of a porous material. The interior portion of the substrate holder  700 ,  800  includes a plurality of recesses or cut-outs  290  to produce thinned portions of the substrate holder  700 ,  800  for facilitating fluid flow through the substrate holder. It will be understood that a recess  290  is a cut-out, hole, or opening that does not extend completely through the substrate holder  700 ,  800 . It is understood that although the recesses or cut-outs  290  are illustrated as being formed on the lower surface  710 , the recesses  290  may also be formed on the upper surface  250  of the substrate holder as well, or even on both the upper and lower surfaces. The recesses  290  interior portion may have various shapes and sizes. 
     It will be understood that in embodiments using a porous material, the thinned portions defined by recesses  290  allow a sufficient amount of gas, such as cleaning gas, purge gas, etc., to flow though the substrate holder  700 ,  800  to reduce or prevent backside deposition as well as autodoping. The skilled artisan will also readily appreciate that recesses  290 , in combination with the porous material, allow gas flow through the substrate holder  700 ,  800 , but do not allow direct gas flow on the backside of the substrate. As discussed above, direct impingement of relatively focused, high velocity flows onto the substrate backside can cause localized cooling or “cold spots” in the substrate, which adversely affect the uniformity of deposited materials on the substrate. Furthermore, the skilled artisan will appreciate that a substrate holder formed of the porous material has less thermal mass than a conventional substrate holder formed of a non-porous material, thereby increasing throughput as well as slip performance. Radial variation of the mass density of the substrate holder  700 ,  800  may further reduce the possibility of temperature gradients, thereby enhancing slip performance even further. 
       FIGS. 7 and 8  depict schematic cross-sectional views of a substrate holder wherein the mass density is varied across the substrate holder  700 ,  800  by varying the density or size of the recesses  290 . The substrate holder  700 ,  800  may comprise a porous material, although it is understood that in other embodiments, substrate holder  700 ,  800  may be formed of a non-porous material. For example, the substrate holder may not be designed for sweep gas flow, in which case the recesses  290  do not convey gas through the holder. The variation in the recess  290  density or size may be in the interior portion or across the entire substrate holder  700 ,  800 .  FIG. 7  illustrates a substrate holder  700  in which the mass density is varied by varying the density of the recesses, i.e., the number of recesses  290  per unit area of surface  250  or surface  280 . As explained with reference to  FIGS. 3 and 4 , the hole  260  density may be varied gradually, linearly, continuously, or non-smoothly by providing a central section and one or more surrounding annular sections with different substantially uniform hole  260  densities. Similarly, the recess  290  density may be varied in ways similar to those described above. Hence, in the embodiment illustrated in  FIG. 7 , the mass density is varied by gradually varying the density of the recesses  290  along the radius  270  of the substrate holder  700 . In other embodiments, the substrate holder  700  may comprise a central circular region where the recesses  290  have a substantially uniform first recess density and an annular region surrounding the central region, the annular region having a substantially uniform second recess density that is different from the first recess density (as illustrated in  FIG. 4  with respect to the holes  260 ). In yet other embodiments, the substrate holder  700  may comprise a central circular region with a substantially uniform first recess density and a plurality of concentric annular regions surrounding the central region, the annular regions each having a substantially uniform recess density that is different from the first recess density. The recess densities of the annular regions may also be different from each other. The recess  290  density is preferably axisymmetric. 
       FIG. 8  illustrates a substrate holder  800 , where the mass density is varied by varying the recess  290  size. As illustrated in  FIG. 8 , the mass density may be varied by varying the size of the recesses  290  gradually, linearly, and/or continuously along the radius  270  of the substrate holder. In other embodiments, the substrate holder  800  may comprise a central circular region where the recesses  290  have a substantially uniform first size and an annular region surrounding the central region, the annular region having recesses  290  with a substantially uniform second recess size that is different from the first recess size. In yet other embodiments, the substrate holder  800  may comprise a central circular region with a substantially uniform first recess size and a plurality of concentric annular regions surrounding the central region, the annular regions each having recesses  290  with a substantially uniform recess size that is different from the first recess size. The recess  290  sizes of the annular regions may also be different from each other. 
     The arrangement of holes  260  or recesses  290  may be axisymmetric with respect to the center axis of the substrate holder  200 . Any suitable number of holes  260  or recesses  290  may be provided. It will be understood that there are a great variety of possible arrangements of the holes  260  or recesses  290 , and that the illustrated arrangements are merely possibilities. In some embodiments, about 20-80% of an upper  250  or lower  280  surface of the substrate holder  200  has such holes  260  or recesses  290 . 
     The holes  260  and recesses  290  can have cross-sections of various shapes. In practice, it is relatively easier to produce holes and recesses with circular cross-sections, by conventional drilling. Hence the holes  260  and recesses  290  may have diameters ranging from 0.1 mm to 5 mm. In certain embodiments with recesses  290 , where the recesses  290  define thinned portions of the substrate holder, each of the thinned portions is no thicker than 90% of the total substrate holder thickness at that location, i.e., less than 90% of the thickness of the substrate holder immediately surrounding the recess defining the thinned portion. 
     In various other embodiments, the holes  260  or recesses  290  may radially vary in density or size linearly from the center  210  or location  265  of the substrate holder. In other words, the size or density varies linearly with displacement from the center  210  or substrate center alignment point  265 . The diameter of the holes  260  or recesses  290  can be determined, in part, based upon empirical haze and resistivity results, as well as, for example, the desired flow rate of the gas passing through the interior portion  230 . Additionally, the holes  260  or recesses  290  can be similar to or different than one another, as desired. 
     The skilled artisan will appreciate that various arrangements of the holes  260  or recesses  290  in the substrate holder are possible and are preferably optimized for strength as well as process control, for example, reducing haze/halo problem, resistivity, slip, nanotopography, etc. Furthermore, the spider  22  (see  FIG. 1 ) can be hollow and can convey sweep gas into the holes  260  or recesses  290  of the embodiments described above. 
     The skilled artisan will also recognize that a substrate holder  200  with varying thermal mass density may be used to tailor effective thermal coupling, and the resultant substrate thermal profile, to compensate for non-uniformities other than a concavity in the top surface  250  of the substrate holder  200 . The non-uniformities may result from masses or structures within a chamber or from process effects. For example, the variation in mass density of the substrate holder  200  could be made to be non-axisymmetric in order to non-axisymmetrically tailor the thermal coupling between the substrate holder  200  and the substrate  16  in order to achieve a nominally uniform temperature profile on a non-rotated wafer. This may be desirable, for example, to compensate for a system-specific non-rotated thermal “signature.” 
     Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various features and steps discussed above, as well as other known equivalents for each such feature or step, can be mixed and matched by one of ordinary skill in this art to perform methods in accordance with principles described herein. 
     Although this invention has been disclosed in the context of certain embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modification thereof Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.