Wafer carrier with temperature distribution control

Wafer carrier arranged to hold a plurality wafers and to inject a fill gas into gaps between the wafers and the wafer carrier for enhanced heat transfer and to promote uniform temperature of the wafers. The apparatus is arranged to vary the composition, flow rate, or both of the fill gas so as to counteract undesired patterns of temperature non-uniformity of the wafers. In various embodiments, the wafer carrier utilizes at least one plenum structure contained within the wafer carrier to source a plurality of weep holes for passing a fill gas into the wafer retention pockets of the wafer carrier. The plenum(s) promote the uniformity of the flow, thus providing efficient heat transfer and enhanced uniformity of wafer temperatures.

FIELD OF THE INVENTION

The invention relates generally to semiconductor fabrication technology and, more particularly, to chemical vapor deposition (CVD) processing and associated apparatus for reducing temperature non-uniformities on semiconductor wafer surfaces.

BACKGROUND OF THE INVENTION

Many semiconductor devices are formed by processes performed on a substrate. For example, in the fabrication of light-emitting diodes (LEDs) and other high-performance devices such as laser diodes, optical detectors, and field effect transistors, a chemical vapor deposition (CVD) process is typically used to grow a thin film stack structure using materials such as gallium nitride over a sapphire or silicon substrate. The substrate typically is slab of a crystalline material, commonly referred to as a “wafer,” typically in the form of a disc.

One common process is epitaxial growth. For example, devices formed from compound semiconductors such as III-V semiconductors typically are formed by growing successive layers of the compound semiconductor using metal organic chemical vapor deposition or “MOCVD.” In this process, the wafers are exposed to a combination of gases, typically including a metal organic compound as a source of a group III metal, and also including a source of a group V element which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Typically, the metal organic compound and group V source are combined with a carrier gas which does not participate appreciably in the reaction as, for example, nitrogen. One example of a III-V semiconductor is gallium nitride, which can be formed by reaction of an organo gallium compound and ammonia on a substrate having a suitable crystal lattice spacing, as for example, a sapphire wafer. Typically, the wafer is maintained at a temperature on the order of 1000-1100° C. during deposition of gallium nitride and related compounds.

Composite devices can be fabricated by depositing numerous layers in succession on the surface of the wafer under slightly different reaction conditions, as for example, additions of other group III or group V elements to vary the crystal structure and band gap of the semiconductor. For example, in a gallium nitride based semiconductor, indium, aluminum or both can be used in varying proportion to vary the band gap of the semiconductor. Also, p-type or n-type dopants can be added to control the conductivity of each layer. After all of the semiconductor layers have been formed and, typically, after appropriate electric contacts have been applied, the wafer is cut into individual devices. Devices such as light-emitting diodes (“LEDs”), lasers, and other optoelectronic devices can be fabricated in this way.

The epitaxial growth process can be carried out in a CVD tool that includes a process or reaction chamber, which provides a sealed environment that allows infused gases to be deposited upon the substrate to grow the thin film layers.

One type of CVD tool which has been widely accepted in the industry uses a device commonly referred to as a “wafer carrier.” Wafer carriers typically comprise a large disc with numerous wafer retaining regions or “pockets,” each pocket adapted to hold one wafer. The wafer retaining pockets are typically comprise a recess or cavity formed on the top surface dimensioned to receive a wafer, and are characterized as having an upwardly-facing floor and a radially inward-facing perimeter wall.

Typically, the wafers are supported by protrusions that extend from the floor and the perimeter wall the wafer retaining pocket to suspend the wafer above the surface of the floor of the wafer retaining pocket. The wafer can bow downward (concave) in different process layers. Suspension above the pocket floor at a predetermined distance prevents wafer from unevenly sitting bottomed out in the pocket due to this shape, which can cause wafer toss and large temperature non-uniformities. Maintaining a certain distance between the wafer and the pocket floor also reduces the wafer temperature non-uniformity that results from an uneven wafer-pocket gap due to the bowing of the wafer. The suspension of the wafer also promotes uniformity of the temperature of the wafer by eliminating random contact points that would otherwise result between the wafer and the pocket floor. Moreover, certain wafer carriers implement pocket floors that are appropriately bowed or shaped to help with uniformity at certain process layers. If the wafer rests on the floor directly, layers that the floors were not designed for can incur large non-uniformities due to bottoming out.

The wafer carrier is supported on a spindle within the reaction chamber so that the top surface of the wafer carrier with exposed surfaces of the wafers is oriented to face upwards, toward a gas distribution element of the CVD tool. While the spindle is rotated, the gas is directed downwardly onto the top surface of the wafer carrier and flows across the top surface toward the periphery of the wafer carrier. The wafer carrier is maintained at the desired elevated temperature by heating elements, typically electrical resistive heating elements disposed below the bottom surface of the wafer carrier. These heating elements are maintained at a temperature above the desired temperature of the wafer surfaces, whereas the gas distribution element typically is maintained at a temperature well below the desired reaction temperature so as to prevent premature reaction of the gases. Therefore, heat is transferred from the heating elements to the bottom surface of the wafer carrier and flows upwardly through the wafer carrier to the individual wafers.

The importance of maintaining uniform conditions at all points on the top surfaces of the various wafers during the CVD process has long been recognized. Minor variations in composition of the reactive gases and in the temperature of the wafer surfaces cause undesired variations in the properties of the resulting semiconductor device. For example, if a gallium and indium nitride layer is deposited, variations in wafer surface temperature will cause variations in the composition and band gap of the deposited layer. Because indium has a relatively high vapor pressure, the deposited layer will have a lower proportion of indium and a greater band gap in those regions of the wafer where the surface temperature is higher. If the deposited layer is an active, light-emitting layer of an LED structure, the emission wavelength of the LEDs formed from the wafer will also vary. Thus, considerable effort has been devoted in the art heretofore towards maintaining uniform conditions.

While considerable effort has been devoted to design an optimization of such systems, further improvement would be desirable. In particular, it is desirable to provide better uniformity of temperature across the surface of each wafer.

SUMMARY OF THE INVENTION

Various embodiments of the invention provide a uniform distribution of inert, high conductivity gas, referred to herein as a “fill gas,” to a plurality of wafer pockets in a wafer carrier. The uniform distribution of fill gas to the pockets promotes the uniform diffusion of heat transferred between the pocket floors and the wafers to provide efficient heat transfer and enhanced uniformity of temperatures, both across the surface of a given wafer as well as between the plurality of wafers. The fill gas delivered in various embodiments of the invention can have a high conductivity relative to the process gases that would otherwise occupy the interstitial space between the pocket floor and the wafer. The higher conductivity fill gas enhances the heat transfer therebewteen and reduces sensitivity to variations in the thermal conductive coupling.

United States Patent Application Publication No. 2011/0206843 to Gurary et al. (Gurary), assigned to the owner of the instant application, discloses a system that delivers fill gas to a plurality of wafer retaining pockets of a wafer carrier. Each wafer retaining pocket is in fluid communication with a respective conduit that is sourced through the spindle of the CVD tool, and terminates at the face of the pocket floor to define a single exit aperture, preferably at the center of the wafer retaining pocket. It has been found that such an arrangement can cause localized heating or cooling of the wafer at locations immediately above the exit aperture. Localized heating or cooling adversely affects the temperature profile of exposed face of the wafer, and can also cause the wafer to bow slightly due to the presence of thermal gradients.

Various embodiments of the present invention mitigate the effect of localized heating or cooling by delivering the fill gas through a plurality of weep orifices that pass through the floor of the retainer pocket. The “weep orifices” are so named because the flow therethrough is slow enough so as to avoid or reduce convection heat transfer between the wafer and the fill gas. In one embodiment, the weep orifices are distributed in a uniform, matrixical arrangement to promote uniform distribution of the fill gas within the wafer retaining pocket. In order to source the plurality of weep orifices among the plurality of wafer retaining pockets, various plenum configurations and distribution manifold arrangements are implemented to prevent those weep orifices that are closer to the source of the fill gas from starving those weep orifices which are located further from the source of the fill gas.

Structurally, various embodiments of the invention include a wafer carrier for use in a system for growing epitaxial layers on a plurality of wafers by chemical vapor deposition. The wafer carrier can include a body portion formed symmetrically about a central axis, the body portion including generally planar top and bottom surfaces that are substantially perpendicular to the central axis. A plurality of wafer retention pockets can be recessed relative to the top surface of the body portion, each of the wafer retention pockets including a floor portion having an upper surface generally parallel to the top surface of the body portion. A center receptacle can be recessed relative to the bottom surface of the body portion, the center receptacle being concentric with the central axis. In one embodiment, a plurality of flow passages extending radially outward from and are in fluid communication with the center receptacle. At least one plenum chamber can be defined within the body portion, the at least one plenum chamber being in fluid communication with at least one of the plurality of flow passages. In certain embodiments, a plurality of manifold passages are in fluid communication with the at least one plenum chamber, the manifold passages having an orientation that is substantially parallel with the top surface of the body portion of the wafer carrier and extending underneath the surface of the floor portion of one of the plurality of wafer retention pockets. A plurality of weep orifices are formed in the floor portion of the one of the plurality of wafer retention pockets, each of the weep orifices being in fluid communication with one of the plurality of manifold passages and passing through the upper surface of the floor portion of the wafer retention pocket. The weep orifices can be oriented substantially parallel with the central axis. The plurality of manifold passages can be parallel to each other.

In one embodiment, the plenum chamber or chambers can be characterized as having a first hydraulic diameter and each of the manifold passages have a second hydraulic diameter, the first hydraulic diameter being about 2 to about 100 times the second hydraulic diameter. The manifold passages can have a second hydraulic diameter and each of the weep orifices have a third hydraulic diameter, the second hydraulic diameter being the same to about 50 times the third hydraulic diameter. Each of the wafer retention pockets can be at least partially surrounded by a thermal isolation slot, the thermal isolation slot being defined in and recessed from the bottom surface of the wafer carrier.

The at least one plenum chamber can be a single plenum chamber is centrally located within the body portion and symmetric about the central axis, wherein the single plenum chamber is defined by a plenum cavity formed in a central region of the body portion and a closure portion disposed within the plenum cavity, the plenum cavity being in fluid communication with the plurality of manifold passages. In one embodiment, the cavity is recessed from the bottom surface of the body portion. In another embodiment, the single plenum chamber is defined by a two-piece plenum structure, the two-piece plenum structure being disposed within the body portion of the wafer carrier.

Alternatively, the wafer carrier can include a plurality of distributed plenum chambers, each being associated with a respective one of the wafer carrier pockets. Each plenum chamber can surround the wafer pocket, the plenum chamber providing thermal isolation of the wafer pocket from a body portion of the wafer carrier.

In another embodiment of the invention, a method for controlling the temperature distribution in a wafer carrier includes providing a wafer carrier that defines a central axis and includes a wafer pocket, the wafer pocket being in fluid communication with a plurality of weep orifices, each of the plurality of weep orifices being in fluid communication with a plenum chamber, the plenum chamber being contained within the wafer carrier and in fluid communication with a center receptacle, the center receptacle being concentric about the central axis; and charging the plenum chamber with a fill gas via the center receptacle, thereby causing the fill gas to enter the wafer pocket via the plurality of weep orifices.

DETAILED DESCRIPTION

Referring toFIGS. 1 and 2, a schematic of a chemical vapor deposition (CVD) apparatus11is depicted an embodiment of the invention. The CVD apparatus11includes a reaction chamber10having a gas distribution element12arranged at a top end13of the reaction chamber10. In certain embodiments, the gas distribution element12is connected to sources14a,14b,14cfor supplying process gases to be used in the wafer treatment process, such as a carrier gas and reactant gases such as a metalorganic compound and a source of a group V metal. The gas distribution element12is arranged to receive the various gases and direct a flow of process gasses generally in the downward direction. The gas distribution element12can also be connected to a coolant system16arranged to circulate a liquid through the gas distribution element12so as to maintain the temperature of the element at a desired temperature during operation. A similar coolant arrangement (not depicted) can be provided for cooling walls17of reaction chamber10. The reaction chamber10can also equipped with an exhaust system18arranged to remove spent gases from the interior of the chamber through ports (not depicted) at or near the bottom of the chamber so as to permit continuous flow of gas in the downward direction from the gas distribution element12.

In the depicted embodiment, a spindle20is arranged within the chamber so that a rotational axis22of the spindle20extends in the upward and downward directions. The spindle20can be mounted to the reaction chamber10by a conventional rotary pass-through device25that incorporates bearings and seals (not depicted) for rotation of the spindle20about the rotational axis22, while maintaining a seal between the spindle20and the reaction chamber10. The spindle20has a fitting portion24at a top end23, the fitting portion24being adapted to releasably engage a wafer carrier. In the depicted embodiment, the fitting portion24is a generally frustoconical, tapering toward the top end23of the spindle20. The spindle20can be connected to a rotary drive mechanism26such as an electric motor drive, which is arranged to rotate the spindle about axis22. In some embodiments, the spindle20has an internal gas passageway28terminating at an opening30at the top end23of the spindle20and within fitting portion24(FIG. 2).

In one embodiment, the spindle20has a gas entry port38in fluid communication with the gas passageway28, the gas entry port38being remote from the upper end of the spindle and being below the chamber10and the pass-through25. A rotary connection assembly40can define an annular space42surrounding the gas entry port38, so that the port38remains in communication with annular space42during rotation of the spindle20. The rotary connection assembly40can include an inlet44communicating with the annular space42and hence with the gas entry port38and gas passageway28of the spindle. The rotary connection assembly40can also include conventional seals46. Although the seals46are diagrammatically depicted as O-rings, the seals may be of any type available to the artisan, such as a lip seal or packing gland.

In various embodiments, the inlet44of the rotary connection assembly40is connected to fill gas supply sources48aand48b(FIG. 1), arranged to supply different gas components having different thermal conductivities. Flow control elements50aand50bare provided in the depicted embodiment to regulate the flow of gas from each of sources48aand48bindependently. The flow control elements50a,50bcan be connected to signal outputs of a control system52. In one embodiment, the flow control elements50aand50bare arranged for adjustment during operation in response to control signals applied by the control system52. For example, flow control elements50aand50bcan be conventional electrically-controllable valves or mass flow controllers provided in the connections between the rotary connection assembly40and the sources48aand48b. However, any other arrangement for regulation of flow from a gas source in response to a control signal can be utilized.

The spindle20can also be provided with internal coolant passages54and56(FIG. 2) extending generally in the axial direction of the spindle20within gas passageway28. The internal coolant passages54and56can be connected to a coolant source60via a second rotary connection assembly58(FIG. 1). A fluid coolant can be circulated in a loop through the coolant passages54and56, originating and returning to the coolant source60. The coolant source60can include conventional devices for regulating the flow and maintaining the temperature of the circulating coolant.

Various embodiments of the invention include heating elements70mounted within the reaction chamber10which surrounds the spindle20below the fitting portion24. The reaction chamber typically includes an entry opening72leading to an antechamber76, and a door74for closing and opening the entry opening. The door74is depicted schematically inFIG. 1as movable between the closed position (solid lines) and an open position74′ (broken lines). In the closed position, the door74isolates the interior of the reaction chamber10from antechamber76. The door74can equipped with control and actuation devices for movement between the open position and closed positions. In practice, the door74can comprise a shutter movable in the upward and downward directions as disclosed, for example, in U.S. Pat. No. 7,276,124. The CVD apparatus11can further include a loading mechanism (not depicted) for moving a wafer carrier from the antechamber76into the reaction chamber10, engaging the wafer carrier with the spindle20in the operative condition, and for moving a wafer carrier off of the spindle20and into the antechamber.

A wafer carrier80is mounted to the spindle20in an operative position. The wafer carrier80includes a body82which can be substantially circular about a central axis84. The body82is characterized as having a generally planar top surface88, a generally planar bottom surface90, and an outer peripheral edge91. The top and bottom surfaces88and90can extend generally parallel to each other and can be generally perpendicular to the central axis84of the body82. The body82also has a plurality of wafer retention pockets92, each adapted to hold a corresponding one of a plurality of wafers94.

Various configurations of the wafer carriers80of the present invention are described below.

Referring toFIGS. 3-10, a two-piece wafer carrier80acomprising a body portion82aand a closure portion98is depicted in an embodiment of the invention. The body portion82adefines a plurality of wafer retention pockets92arecessed from a top surface88a. The body portion82afurther comprises a plenum cavity96recessed from a bottom surface90a. The closure portion98is disposed within the plenum cavity96to define a plenum chamber100within the two-piece wafer carrier80a.

Each wafer retention pocket92aincludes a floor portion102and a peripheral wall104. The floor portion102is characterized as having a thickness106and an upper surface108facing in the upward direction. A plurality of protrusions112extend in the upward direction from the upper surface108and radially inward from the peripheral wall104. The protrusions112can be configured and distributed about the periphery of the wafer retention pocket92aas described in U.S. patent application Ser. No. 13/450,062, filed on Apr. 18, 2012 and owned by the owner of the present application.

The floor portion102of a given wafer retention pocket92aincludes a plurality of manifold passages114that extend into the thickness106of the floor portion102substantially parallel to the upper surface108. A plurality of weep orifices116extend in the upward direction from each of the manifold passages114, passing through the upper surface108of the floor portion102, thereby establishing fluid communication between the wafer retention pocket92aand the respective manifold passage114.

Flow passageways, such as the weep orifices116, manifold passages114and plenum chambers (discussed below) can be characterized as having hydraulic diameters DH, defined as
DH=4·A/PEq. (1)
where A is the cross-sectional area of the flow passageway and P is the wetted perimeter of the flow passageway. The hydraulic diameters of the weep orifices116and manifold passages114are denoted by numerical references118and120, respectively (FIG. 7). The hydraulic diameters118of the weep orifices116can be of substantially smaller dimension than the hydraulic diameters120of the manifold passages114. In a non-limiting example, the weep orifices116have a diameter on the order of 0.6 mm (0.025 inches), whereas the manifold passages114have a diameter on the order of 1.5 mm (0.06 inches). In various embodiments, the ratio of the hydraulic diameter120of the manifold passage114to the hydraulic diameter118of the weep orifices116is in the range of about 1 to about 50.

The manifold passages114can be substantially parallel with respect to each other and spaced apart at uniform intervals. Likewise, the weep orifices116can be uniformly spaced along the respective manifold passage114. In such an arrangement, the weep orifices116define a matrixical arrangement on the upper surface108of the floor portion102. In the depicted embodiment, the manifold passages114and the weep orifices116of each of the manifold passages114are spaced at a substantially equal dimension, thereby defining a square matrixical arrangement.

Alternatively, the weep orifices116can be in other than a matrixical arrangement or otherwise non-uniformly spaced (FIGS. 16A through 16C). For example, the weep orifices116can be arranged in a series of concentric circular patterns in between certain limiting radii on the pocket. Alternatively, they can be of different densities in different sections of the pocket.

The plenum cavity96of the two-piece wafer carrier80ais recessed into the body portion82afrom a bottom surface90a. The plenum cavity96is characterized as having a perimeter wall124and a ceiling portion126, the ceiling portion126having a lower surface128. The body portion82acan further define a center receptacle130concentric with the central axis84and extending upward from the lower surface128of the ceiling portion126. In the depicted embodiment, the center receptacle is blind (i.e., does not pass through the top surface88aof the body portion82a). A plurality of threaded holes132(FIG. 5) can be formed on the ceiling portion126, the threaded holes132being configured to receive fasteners134(FIG. 3). In one embodiment, the threaded holes132are blind so as not to disturb the upper surface88aof the body portion82a. The perimeter wall124defines exposed edges136of the floor portions102of the wafer retention pockets92a. In one embodiment, each of the manifold passages114of the various wafer retention pockets92aare oriented to pass through the exposed edges136of the respective floor portion102, thereby establishing fluid communication between the plenum cavity96and the wafer retention pockets92avia the manifold passages114and weep orifices116.

The closure portion98of the two-piece wafer carrier80aincludes an upper face142, a lower face144and a periphery146. The periphery146is shaped to the contour of the perimeter wall124of the plenum cavity96, such that the closure portion98, when disposed in the plenum cavity96, provides a close tolerance fit with the perimeter wall124of the plenum cavity96. In one embodiment, the closure portion98can include a thin lip running along the periphery146that mates with recessed shoulders formed on the exposed edges136of the floor portions102of the wafer retention pockets92a. The lip and the shoulders can be dimensioned to overlap when fitted together, providing a mating fit.

In the depicted embodiment, the closure portion98includes a hub portion152and a plurality of spacer portions154, all extending upward from the upper face142of the closure portion98. The hub portion152includes an exterior surface156and an interior surface158defining an aperture162that passes through a top face164of the exterior surface156. The exterior surface156can be dimensioned for a close tolerance fit with the center receptacle130of the body portion82a. In one embodiment, an outer radial face163of the exterior surface156of the hub portion152, when fully engaged with the fitting portion24, extends only partway into the length of the center receptacle130, thereby defining a gap165between the top face164of the hub portion152and the center receptacle130. The interior surface158of the hub portion52can be configured to mate with the fitting portion24of the spindle20(e.g., complementary to the frustoconical or other shape of the fitting portion24).

The spacer portions154are characterized as having substantially equal thicknesses166(FIG. 4) and each defining a raised face168, the raised faces168lying substantially on a common plane. In one embodiment, the spacer portions154are of equal shape, and extend radially outward from and are uniformly distributed about the hub portion152. The distribution of the spacer portions154define plurality of channels172between the spacer portions154, the channels172extending radially outward from the hub portion152to a peripheral portion175of the closure portion98that extends radially beyond the spacer portions154.

A plurality of slots174can be formed on the outer radial face163of the exterior surface156of the hub portion152, the slots174extending axially (i.e., parallel to the central axis84) and each being adjacent a radially proximal end of a respective one of the channels172. The closure portion98can further include a plurality of through holes178arranged to align with the threaded holes132of the body portion for insertion of the fasteners134therethrough.

In one embodiment, the floor portions102are partially surrounded by thermal isolation slots182and184that extend in the upward direction from the bottom surface90aand the ceiling portion126, respectively. The thermal isolation slots182,184extend into the body portion82abut need not pass through the top surface88aof the body portion, thereby leaving the top surface88astructurally undisturbed.

In fabrication, the body portion82aof the two-piece wafer carrier80acan be initially formed as a blank circular disk having the parallel top and bottom surfaces88aand90aand an outer peripheral edge91a. A plurality of reference axes186are designated that extend through and perpendicular to the central axis84, and which can be uniformly distributed about the central axis84. The axial location of the reference axes186(i.e., the location along the central axis84where the reference axes186intersect) corresponds to the axial location where the manifold passages114are to pass through the floor portions102of the wafer retention pockets92a. Each of the wafer retention pockets92ais designated to be laterally centered about a corresponding one of the reference axes186. In the depicted embodiment, for each wafer retention pocket92a, the manifold passages114passing therethrough includes a designated central manifold passage114′ (FIG. 9) that is formed along the respective reference axis186with other manifold passages114for the corresponding wafer retention pocket92abeing formed parallel to and at the same axial location as the respective reference axis186.

The manifold passages114can be bored from the outer peripheral edge91a, extending toward the center of the body portion82a. The length any one of the respective manifold passages114is typically long enough to pass through the location of where the perimeter wall124of the plenum cavity96is or will be formed. In one embodiment, each of the manifold passages114include an enlarged diameter portion188proximate the outer peripheral edge91a. For those manifold passages114that lie along an axis that passes through a thermal isolation slot182, the enlarged diameter portion188of a given manifold passage114extends far enough into the body portion82ato entirely pass through the location where the thermal isolation slots182are or will be formed. Accordingly, the depth (distance into the body from the outer peripheral edge91a) of the enlarged diameter portions188varies with the location of the respective manifold passage114with respect to the closest reference axis186. After forming the enlarged diameter portions188, plugs192can be inserted into the enlarged diameter portions188that extend substantially the entire length of the respective enlarged diameter portion188. The plugs prevent fill gas from exiting the ends of the manifold passages114in operation.

After the manifold passages114have been formed and the plugs192inserted, the weep orifices116and the wafer retention pockets92aare formed from the top surface88aof the body portion82a, the weep orifices116extending into manifold passages114. The plenum cavity96, center receptacle130, threaded holes132and thermal isolation slots182and184are formed from the bottom surface90a.

Other than forming the various aspects of the body portion82aafter formation and plugging of the manifold passages114, there is no particular sequence to their formation. Likewise, the features and aspects of the closure portion98can be formed using standard machine practices, again without particular sequence.

In assembly, the closure portion98is aligned with and disposed within the complementary-shaped plenum cavity96so that the raised faces168of the spacer portions154register against the ceiling portion126of the plenum cavity96, and secured in place with the fasteners134. The hub portion152is thereby disposed within the center receptacle130of the body portion82ato define the gap165, the gap165being in fluid communication with the gas passageway28of the spindle20. The slots174formed on the outer radial face163of the hub portion152cooperate with the interior surface of the center receptacle130and the channels172and the peripheral portion175of the upper face142of the closure portion98cooperate with the ceiling portion126of the plenum cavity96to define a plurality of flow passages194that extend from the gap165to the plenum chamber100.

Thus, the plenum chamber100of the two-piece wafer carrier80ais bounded by the ceiling portion126and perimeter wall124of the plenum cavity96and peripheral portion175of the upper face142of the closure portion98, with the plurality of flow passages194leading thereto. The plenum chamber100is thus continuous in the tangential direction and has a height substantially the same as the thickness166of the spacer portions154.

After the closure portion98is secured to the body portion82a, the exterior surfaces of the assembled two-piece wafer carrier80acan be coated with a material such as silicon carbide using a CVD process to resist the chemicals of the intended environment of operation.

The treated surface exterior surface can have a thermal expansion coefficient that is different from that of the base material (e.g., SiC has a different thermal expansion coefficient than graphite). The difference in thermal expansion can cause the components to bow at temperatures that depart from the treatment temperature, particularly the peripheral portion175of the closure portion98because of the thinness of the material. Accordingly, in one embodiment, all exposed surfaces of the closure portion98(the upper face142, spacer portions154and exterior surface156of the hub portion152) can be coated prior to assembly.

Functionally, with both sides coated evenly, the differential expansion and contraction between the sides reduces bending due to differences in the coefficient of thermal expansion at temperatures different from the treatment temperature. The exterior surface144can also be masked during coating of the rest of the two-piece assembly. In this manner, after cool down following the treatment and coating of the entire assembly, the relatively thin peripheral portion of175of the closure portion98experiences the same thermal contraction on both sides, largely cancelling the bowing effect that would otherwise result from having a treated surface on only one side.

In operation, the wafers94are loaded into the two-piece wafer carrier80a, and the wafer carrier80acoupled to the fitting portion24of the spindle20. A fill gas196from one or both of the gas sources48aand48bis caused to flow into the gas passageway28via the rotary connection assembly40, wherefrom the fill gas196passes through the gap165and flow passages194and into the plenum chamber100. From the plenum chamber100, the flow of fill gas196pressurizes the plurality of manifold passages114so that the fill gas196flows through the plurality of weep orifices116to fill the voids between the upper surfaces108of the floor portions102of the wafer retention pockets92and the plurality of wafers94. The fill gas196exits the two-piece wafer carrier80aaround the perimeters of the wafers94.

The heater elements70are energized and transfer heat to the bottom surface90aof the two-piece wafer carrier80a, primarily by radiative coupling. The heat transferred to the bottom surface90aof the two-piece wafer carrier80aflows upwardly through the body portion82a, primarily by conduction through the solid portions and gases, contact conductance across the contacting surfaces (e.g., between the spacer portions154and the ceiling portion126) radiation across the internal spaces (e.g., the plenum chamber100and flow passages194) and by convection, conduction and advection to the fill gas196. Heat is radiated from the top surface88aof the two-piece wafer carrier80aand from the top surfaces of the wafers94to the lower temperature components that surround the two-piece wafer carrier80a(e.g., the walls of the reaction chamber10), and by advection to the process gas that flows out of the gas inlet element12and over the top surface88aof the two-piece wafer carrier80aand the top surfaces of the wafers94. The temperature at the top surfaces of the wafers94represents a balance between the heat transfer to the wafers94(primarily between the floor portions102of the wafer retention pockets92a) the heat transfer away from the top surface of the wafer94.

Functionally, the relative cross-sectional flow areas of the plenum chamber100, manifold passages114and weep orifices116are such that the uniformity of the flow exiting the weep orifices116is enhanced. That is, the effective cross-sectional flow area of the plenum chamber100is substantially greater than the combined cross-sectional flow area of the manifold passages114. Accordingly, the uniformity of a pressure distribution throughout the plenum chamber100is promoted throughout for better sourcing of the manifold passages114with fill the gas196. Likewise, the cross-sectional flow area of a given weep orifice116is substantially less than the cross-sectional flow area of the manifold passage114, so that manifold passage114does not experience a substantial pressure difference along its length; thus, the weep orifices116proximate the inlet to the manifold passage114do not starve the weep orifices116distal to the inlet.

Because of the convective and advective heat transfer between to the fill gas196and the floor portions102of the wafer retention pockets92, the operating temperature of the floor portions102can be substantially different than the temperatures of the portions of the two-piece wafer carrier80aadjacent thereto. The thermal isolation slots182,184act to inhibit heat transfer between the floor portions102and the adjacent areas, thus promoting the uniformity of the temperature of the wafer retention pockets92. The design and influence of the thermal isolation slots is further described at U.S. patent application Ser. No. 13/618,799, filed on Aug. 29, 2012 and owned by the assignee of the instant application.

The flow rate of the fill gas196is preferably low enough so that the back pressuring does not cause the wafers94to lift off of the protrusions112. The low flow rate also mitigates convective coupling on the backside of the wafers94, so that the heat transfer between the floor portions102and the wafers94is dominated by conduction through the fill gas196. By way of non-limiting example, for a system processing wafers of 4-inch (100 mm) diameter, with pockets slightly larger than 100 mm in diameter, the flow rate of the fill gas196into each pocket is typically less than about 100 standard cubic centimeters per minute (cc/min). More generally, the flow rate into each pocket can range from about 20 cc/min to about 1000 cc/min.

Referring toFIGS. 11 and 12, a two-piece wafer carrier80bhaving interior pockets202and exterior pockets204, the bottom views of which are is depicted in an embodiment of the invention. The two-piece wafer carrier80bincludes a body portion82bhaving a bottom surface90band defining a plenum cavity206. A closure portion208is disposed over and within the plenum cavity206. The plenum cavity206is formed to define the radially inward-facing perimeters of the floor portions of the exterior pockets204, and to define the perimeters floor portions of the interior pockets202. The closure portion208includes many of the same aspects as the closure portion98of the two-piece wafer carrier80aofFIGS. 3-10, including a hub portion212, spacer portions214that define channels216and a contoured periphery218. The closure portion208can also include peripheral spacer portions222that help maintain chamber spacing near the contoured periphery218. The closure portion208further includes openings220to accommodate the interior pockets202.

The fabrication process for forming manifold passages224is similar to that described for the two-piece wafer carrier80a. The manifold passages224of the interior pockets202are formed extending the passages formed in an adjacent exterior pocket204, as depicted inFIG. 12. Note that designated central manifold passages224′ (depicted with bold lines inFIG. 12) that pass through both an interior and an exterior pocket202and204are not aligned to pass through the central axis of the two-piece wafer carrier80b, but instead is aligned to pass through the full diameter of both the respective interior and exterior pockets202and204.

Referring toFIGS. 13 and 14, a three-piece wafer carrier80cis depicted in an embodiment of the invention. The three-piece wafer carrier80cincludes an outer ring or body portion82cwith a bottom surface90cand having wafer retention pockets92cand a two-piece plenum structure242. The outer ring portion82cincludes many features that are similar to the two-piece wafer carrier80aand which are identified with like-numbered numerical references. Instead of a plenum cavity, the outer ring portion82cdefines an interior opening244bounded by a radially inward-facing perimeter246. The manifold passages114pass through the radially inward-facing perimeter246to establish fluid communication with the interior opening244.

The two-piece plenum structure242includes a top portion252and a bottom portion254that cooperate to define a plenum chamber therebetween. The top portion252includes a plurality of interior wafer retention pockets256that have the same features as the wafer retention pockets92c. The top portion252further includes a contoured periphery258that substantially conforms to radially inward-facing peripheries262of the wafer retention pockets92c. The contoured periphery258extends radially outward from the inward-facing perimeter246of the outer ring portion82cto define a flange264on the top portion252. The outer ring portion82cincludes a contoured recess266from the top surface88aand proximate the inward-facing perimeter246, the contoured recess266conforming to the shape and thickness of the flange264.

The bottom portion254of the two-piece plenum structure242has generally the same features as the closure portion208of the two-piece wafer carrier80b, the likenesses of which are similarly labeled inFIG. 13. One difference is that the outer periphery268of bottom portion254is not contoured to the shape of the floor portions102of the wafer retention pockets92c, but rather for a close tolerance fit with the inward-facing perimeter246.

In fabrication, both the top portion252and the bottom portion254of the two-piece plenum structure242can be treated on the interior surfaces in the same manner as the exterior surfaces to prevent bowing due the thermal expansion differences, as discussed above in relation to the closure portion98of the two-piece wafer carrier80a. The manifold passages114for both the outer ring82cand the top portion252of the two-piece plenum structure242can be formed using methods previously described.

In assembly, the top portion252of the two-piece plenum structure242is aligned with and disposed within the contoured recess266of the outer ring portion82c. The top portion252can be affixed to the outer ring portion82cusing fasteners (not depicted) that pass through the flange264. The bottom portion254is affixed to the top portion252to define a plenum chamber (not depicted) therebetween, the plenum chamber also being bounded by the inward-facing perimeter246of the outer ring portion82c. It is noted that, for the embodiment of the three-piece wafer carrier80cas depicted, there is no fixed order for the assembly. That is, the top and bottom portions252and254of the two-piece plenum structure242can be affixed to each other before or after the top portion252is joined to the outer ring82cportion.

In operation, the three-piece wafer carrier80coperates by the same principles as the two-piece wafer carriers80aand80b. The fill gas196flows over the hub portion212and into the channels216to flood the plenum chamber. The manifold passages114of the wafer retention pockets92cand256are thereby sourced with the fill gas196, which in turn sources the weep orifices116.

It is noted that the three-piece wafer carrier80cis depicted without thermal isolation slots (e.g., numerical references182and184of the two-piece wafer carriers80aand80b). It is recognized that thermal isolation slots can also be implemented into the three-piece wafer carrier80c.

Referring toFIGS. 15 through 18, a multiple plenum wafer carrier80dis depicted in an embodiment of the invention. The multiple plenum wafer carrier80dincludes a body portion82dthat defines a plurality of wafer retention pockets282, one each disposed within a respective through aperture284formed on the body portion82d. Each through aperture284is characterized as having an inner radial perimeter285. The multiple plenum wafer carrier80dincludes many of the same aspects as the wafer carriers80a,80band80cwhich are identified with like-numbered numerical references.

Each wafer retention pocket282includes an outer radial perimeter286upon which a tangential channel288is formed. Each tangential channel288is in fluid communication with the manifold passages114of the respective wafer retention pocket282. Each wafer retention pocket282can further include a flange or enlarged radius portion292formed on the outer radial perimeter286that mates within a recessed shoulder294formed on the respective through aperture284.

Each through aperture284can be placed in fluid communication with the gap165of the center receptacle130via a respective flow passage296. In one embodiment, the flow passage296comprises a bored passageway297. In one embodiment, each flow passage296includes an axially extending portion296athat extends substantially parallel to the central axis84and a radially extending portion296bthat extends substantially normal to the central axis84. During fabrication, the axially extending portions296acan be formed as blind holes that extend from the top surface88d(FIG. 18). A cap298can be placed over the axially extending portions296ato define an upper boundary of the gap165and placing the gap165in fluid communication with the flow passages296. The cap298can also enable a smooth upper surface88dafter any coating process is implemented on the multiple plenum wafer carrier80d.

In reference toFIGS. 16A through 16C, alternative configurations of wafer retention pockets282athrough282c, respectively, are depicted in embodiments of the invention. The wafer retention pockets282athrough282cpresent weep orifices16that establish different patterns. Wafer retention pocket282a(FIG. 16A) depicts the weep holes116as falling generally between a first radius R1and a second radius R2, thus providing entry of the fill gas over a substantially annular region of the upper surface108. Wafer retention pocket282b(FIG. 16B) depicts the weep holes116as falling generally outside a first radius R1, thus providing entry of the fill gas over a substantially outer annular region of the upper surface108. Wafer retention pocket282c(FIG. 16C) depicts the weep holes116as falling generally within a rectangular pattern that is substantially centered on the upper surface108.

While the various arrangements of the weep holes116are depicted and described in the context of the multiple plenum wafer carrier80d, it is understood that differing arrangements of the weep hole pattern can be applied to any of the wafer carriers80described and depicted herein.

In assembly, each wafer retention pocket282is disposed in a respective one of the through apertures284so that the flange portion292rests on the recessed shoulder294of the respective through aperture. The tangential channel288cooperates with the inner radial perimeter285of the respective through aperture284to define a local plenum chamber299that surrounds the respective wafer retention pocket282. In one embodiment, the manifold passages114are oriented substantially perpendicular to the radially extending portions296b. The cap298is placed over the axially extending portions296a.

The operational concept of the multiple plenum wafer carrier80dis to source each wafer retention pocket282with the separate, local plenum chamber299, best seen inFIG. 18. The fill gas196enters the gap165, courses through the flow passage296and enters the local plenum chamber299. The local plenum chamber299has a much larger hydraulic diameter than the manifold passages114, and therefore substantially less resistance to flow than the manifold passages114. The substantially lower resistance to flow enables the plenum chamber299to charge to a substantially uniform pressure, thus sourcing each of the manifold passages114in a substantially uniform manner. The fill gas196enters the manifold passages114to source the weep orifices116, as in the other embodiments.

Orienting the manifold passages114perpendicular to the radially extending portions296of the respective flow passage296prevents impinging any of the manifold passages114directly, thus aiding in spreading the flow of the fill gas196as it enters the plenum chamber299and enhancing the equal pressure sourcing of the manifold passages114. Also in the depicted embodiment ofFIGS. 15-18, the manifold passages114pass entirely through the floor portions103of the wafer retention pockets282, and are sourced from both ends, further reducing pressure gradients along the length of the manifold passages114.

The local plenum chamber299surrounds the respective wafer retention pocket282with fill gas196that flows tangentially around the wafer retention pocket282. The tangentially flowing fill gas196can act to thermally isolate the wafer retention pocket282from the body portion82d, thus enhancing the temperature uniformity of the resident wafer. Accordingly, while thermal isolation slots182,184of the two-piece wafer carriers80aand80bcan be implemented into the multiple plenum wafer carrier80d, the local plenum chamber299can provide the desired thermal isolation without need for thermal isolation slots.

The bodies82of the various wafer carriers80are preferably formed from materials that do not contaminate the reaction chamber10and can withstand the temperatures of operation. Typical materials include graphite, silicon carbide, or other refractory materials.

The following references, referred to above, are hereby incorporated by reference herein in their entirety except for the claims and express definitions included therein: U.S. Patent Application Publication No. 2011/0206843; U.S. Pat. No. 7,276,124; U.S. patent application Ser. No. 13/450,062; U.S. patent application Ser. No. 13/618,799.

References to “top,” “bottom,” “upper” and “lower” are used for ease of description, and do not necessarily refer to gravitational frame of reference. Rather, these terms refer to directions relative to and between the gas distribution element12and the wafer carrier80. That is, the top end13of the reaction chamber10is typically, but not necessarily, disposed at the “top” of the chamber in the normal gravitational frame of reference. The “downward direction” as used herein refers to the direction away from the gas distribution element12toward the wafer carrier80, with the “upward direction” being the direction opposite the downward direction, regardless of whether these directions are aligned with the gravitational vector. Similarly, “top” or “upper” surfaces describe surfaces that face in the upward direction, and “bottom” or “lower” surfaces describe surfaces that face in the downward direction as defined.