Patent Publication Number: US-10319615-B2

Title: Semiconductor bonding apparatus and related techniques

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 15/440,211, titled “Semiconductor Bonding Apparatus and Related Techniques,” filed on Feb. 23, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/299,349, titled “Simplified Apparatus and Method for Semiconductor Bonding,” filed on Feb. 24, 2016, and is related to U.S. Non-Provisional patent application Ser. No. 11/766,531, titled “Apparatus and Method for Semiconductor Bonding,” filed on Jun. 21, 2007, now issued as U.S. Pat. No. 7,948,034, dated May 24, 2011. Each of these patent applications and patents is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to an apparatus and a method for semiconductor bonding and more particularly to a simplified high-force semiconductor bonding apparatus and method. 
     BACKGROUND 
     This disclosure relates to improvements to methods and apparatus described in commonly-owned U.S. Pat. No. 7,948,034, the contents of which are incorporated herein by reference as if fully set forth herein. 
     Consumers desire ever cheaper electrical and electronic devices. A major part of the cost in producing consumer electrical and electronic devices is the cost of the semiconductor devices that provide the very features that make the electronic devices so desired by consumers. Manufacturers of the semiconductor devices thus continue to seek ways to lessen manufacturing costs of the semiconductors. A significant factor in the determination of unit cost for semiconductor devices is defects that may present themselves in a given production lot. As may be realized, loss of semiconductor devices through defects presents a fiscal loss to manufacturers that may generally be accommodated by increasing unit price. An area where defects may be introduced in fabrication of semiconductor devices is in the wafer of substrate bonding. Wafer bonding involves applying heat, force, and sometimes voltage to an aligned stack of two or more wafers in a controlled atmosphere. The goal of any wafer bonding is to produce high integrity bonds, uniformly across the entire wafer area without negatively influencing the wafer to wafer alignment. Improved bonding integrity has been achieved by generating higher interfacial pressures. For improved bonding results, the interfacial pressure can be quite high, and hence it is desired that substantial force be applied to the wafers to be bonded. For example, 100 kN on a 200-mm diameter wafer or 225 kN on a 300-mm diameter wafer. However, although enabling the bond, the high forces also cause flex and distortion of conventional bonding tools that apply the forces, resulting in poor interfacial pressure uniformity, bond quality variability, wafer shift, and post-bond bowing, and defeating the improvements sought by using high bonding forces. In conventional systems, the pressure non-uniformity approaches 50% across the bond interface. 
     Accordingly, it would be desirable to provide a bonding apparatus that could apply uniform pressure across the entire bond interface. It would also be desirable to have a simplified, less expensive apparatus than has been available to date, and such apparatus that is easier to set up and use than prior solutions. 
     SUMMARY 
     The subject matter of this application may involve, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of a single system or article. 
     One example embodiment provides a semiconductor structure bonding apparatus. The semiconductor structure bonding apparatus includes a lower block assembly including a first surface configured to receive at least one semiconductor wafer thereon. The semiconductor structure bonding apparatus also includes an upper block assembly. The upper block assembly includes a chuck configured to provide a second surface configured to be brought into contact with the first surface in application of bonding pressure to the at least one semiconductor wafer. The upper block assembly also includes a reaction plate disposed over the chuck and configured to reduce deformation of the upper block assembly. The reaction plate includes: a monolithic plate member; and a plurality of concentric grooves defined in the monolithic plate member and configured to receive a corresponding plurality of seals. The semiconductor structure bonding apparatus also includes a plurality of ports configured to deliver pressurized gas to a plurality of regions of the reaction plate. 
     In some cases, the plurality of concentric grooves of the reaction plate includes: a first groove; and a second groove concentrically exterior to the first groove. Furthermore, the plurality of ports is configured to deliver pressurized gas to the plurality of regions of the reaction plate such that in bonding 4-inch diameter semiconductor wafers, pressurized gas is applied to a first region defined between the first groove and the second groove. In some such instances: the plurality of concentric grooves of the reaction plate further includes a third groove concentrically exterior to the second groove; and the plurality of ports is further configured to deliver pressurized gas to the plurality of regions of the reaction plate such that in bonding 6-inch diameter semiconductor wafers, pressurized gas is applied to the first region and a second region defined between the second groove and the third groove. In some such instances: the plurality of concentric grooves of the reaction plate further includes a fourth groove concentrically exterior to the third groove; and the plurality of ports is further configured to deliver pressurized gas to the plurality of regions of the reaction plate such that in bonding 8-inch diameter semiconductor wafers, pressurized gas is applied to the first region, the second region, and a third region defined between the third groove and the fourth groove. 
     In some cases, the reaction plate has defined therein a cutaway portion configured to allow flexure of the reaction plate thereat. In some such instances, at least one of the plurality of seals is concentrically exterior to the cutaway portion. 
     In some cases, the upper block assembly further includes a thermal isolation plate disposed between the reaction plate and the chuck. In some such instances, the thermal isolation plate includes a plurality of wedge-shaped pieces that are physically separate from one another and configured to move with respect to one another. In some such instances, the plurality of wedge-shaped pieces is configured to be arranged in a circular manner with vertices pointing towards a common center. In some other such instances, at least one of the wedge-shaped pieces includes a plurality of raised projections extending from a surface thereof, the raised projections spaced apart from one another around the surface. In some still other such instances, at least one of the wedge-shaped pieces constitutes a monolithic element. In some still other such instances, at least one of the wedge-shaped pieces constitutes a polylithic element including: a lower plate portion; and an upper plate portion configured to be disposed over and operatively coupled with the lower plate portion such that vacuum can be maintained in a void defined between the lower plate portion and the upper plate portion. In some such cases, the lower plate portion includes a plurality of raised projections disposed on an interior surface thereof and extending toward the upper plate portion within the void defined between the lower plate portion and the upper plate portion, the raised projections spaced apart from one another around the interior surface of the lower plate portion. In some cases, the thermal isolation plate includes: a lower plate portion; and an upper plate portion configured to be disposed over and operatively coupled with the lower plate portion such that vacuum can be maintained in a void defined between the lower plate portion and the upper plate portion. In some such instances, the lower plate portion includes a plurality of raised projections disposed on an interior surface thereof and extending toward the upper plate portion within the void defined between the lower plate portion and the upper plate portion, the raised projections spaced apart from one another around the interior surface of the lower plate portion. In some other such instances, at least one of the lower plate portion and the upper plate portion constitutes a monolithic element. In some cases, the thermal isolation plate is configured to provide compliance deflection of about 50 μm or less. 
     In some cases, at least one of the plurality of concentric grooves is configured to receive at least one of the corresponding plurality of seals having a 3-mm diameter. In some cases, the plurality of concentric grooves is configured to receive the corresponding plurality of seals such that sealing is maintained at pressures up to about 37 bar. In some cases, the semiconductor structure bonding apparatus further includes a water-cooled flange disposed between the chuck and the reaction plate. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been selected principally for readability and instructional purposes and not to limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the figures, wherein like numerals represent like parts throughout the several views; 
         FIG. 1  is a schematic diagram of a prior art wafer bonding system; 
         FIG. 2A  is a finite element analysis result displaying the displacement along the bond interface for the prior art wafer bonding system of  FIG. 1 ; 
         FIG. 2B  is a finite element analysis result displaying the Von Mises stress along the bond interface for the prior art wafer bonding system of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a wafer bonding system; 
         FIG. 4A  is a finite element analysis result displaying the displacement along the bond interface for the wafer bonding system of  FIG. 3 ; 
         FIG. 4B  is a finite element analysis result displaying the Von Mises stress along the bond interface for the wafer bonding system of  FIG. 3 ; 
         FIG. 5  is a schematic diagram of another embodiment of a wafer bonding system; 
         FIG. 6  is a schematic cross-sectional diagram of a wafer bonding apparatus; 
         FIG. 7  is a perspective view of a wafer bonding apparatus; 
         FIG. 8  is a cross-sectional view of the wafer bonding apparatus of  FIG. 7 ; 
         FIG. 9A  is a cross-sectional view of the wafer bonding apparatus of  FIG. 8  including a wafer transport fixture; 
         FIG. 9B  is a detailed cross-sectional view of a portion of the upper block assembly of  FIG. 9A ; 
         FIG. 10  is a cross-sectional view of the wafer bonding apparatus of  FIG. 9  with the wafers being in contact with the top and bottom block assemblies (proximity position); 
         FIG. 11  is a detailed cross-sectional view of the wafer bonding apparatus of  FIG. 10 ; 
         FIG. 12  is a cross-sectional view of the wafer bonding apparatus of  FIG. 8  including cross-sectional views of the top and bottom assemblies; 
         FIG. 13  is a detailed cross-sectional view of one embodiment of the alignment system in the wafer bonding apparatus of  FIG. 8 ; 
         FIG. 14A  is a detailed cross-sectional view of the thermal isolation layer in the bonding apparatus of  FIG. 8 ; 
         FIG. 14B  is a schematic cross-sectional diagram of area A of  FIG. 14A ; 
         FIG. 15  is a detailed cross-sectional view of a portion of the upper block assembly of  FIG. 8 ; 
         FIG. 16  is a perspective view of the wafer carrier fixture and the wafer loading system; 
         FIG. 17A  is a top perspective view of the wafer carrier fixture; 
         FIG. 17B  is a detailed view of the wafer spacers and clamping system in the wafer carrier fixture of  FIG. 17A ; 
         FIG. 18  is a schematic diagram of the wafer heater system; 
         FIG. 19  is an exploded view of the wafer heater and thermal isolation systems; and 
         FIG. 20  is a cross-sectional view of another embodiment of the wafer bonding system. 
         FIG. 21  is a cross-sectional view of a leveling mechanism. 
         FIG. 22  is a perspective view of the leveling mechanism of  FIG. 21 . 
         FIG. 23  is cross-sectional view of a gimbal mount for use with the leveling mechanism. 
         FIG. 24  is a cross-sectional view of a reaction plate and associated components. 
         FIG. 25  is a cutaway perspective view of the reaction plate of  FIG. 24 . 
         FIG. 26  is a perspective view of a thermal isolation plate made up of wedge-shaped sections. 
         FIG. 27  is a perspective view of a two-portion thermal isolation plate. 
     
    
    
     These and other features of the present embodiments will be understood better by reading the following detailed description, taken together with the figures herein described. In the drawings, each identical or nearly identical component that is Illustrated in various figures may be represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. Furthermore, as will be appreciated in light of this disclosure, the accompanying drawings are not intended to be drawn to scale or to limit the described embodiments to the specific configurations shown. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , in a prior art wafer bonding system  300 , a first wafer  310  having a bond layer  312  on a first surface  310   a  is brought into contact with a second wafer  320  having a bond layer  322  on a first surface  320   a , so that the two bond layers  312  and  322  are opposite to each other. The wafer bonding process involves compressing the two wafers together by applying a force  350  on a second surface  310   b  of the first wafer  310 . Force  350  is usually applied to the center of the wafer stack  302  with a piston-type mechanism, as shown in  FIG. 1 . In other embodiments, force  350  may be applied in the periphery of the wafer stack  302  or a second force may be applied simultaneously with the force  350  on the second surface  320   b  of the second wafer  320 . Finite element analysis (FEA) of the displacement along the bond interface  305  is shown in  FIG. 2A . We observe the formation of a “hot pressure spot” directly underneath the central area  301  where the force  350  is applied. A first spherical area  302  directly underneath the central area  301  has a displacement of the order of 30μ. Directly under area  302  is another spherical area  303  where the displacement is of the order of 2-3μ and directly under area  303  is area  304  where the displacement is in the range of 1 μm. The spherical front of the “hot pressure spot” propagates down to the bond interface  305  and causes the central region  306  to be more bowed than the edge regions  307 . As we mentioned above, the pressure non-uniformity across the bond interface can reach up to 50%. The Von Mises stresses of the FEA are shown in  FIG. 2B . We observe again a spherical stress front propagating down to the bond interface  305 , where it causes stress variations between the central  306  and periphery regions  307 . Areas  308 ,  309 , and  311  have stresses of the order of 100 Factor of Safety (FOS), 50 FOS, and 10 FOS, respectively. 
     Referring to  FIG. 3 , in a wafer bonding system  400 , a first wafer  410  having a first surface  410   a  is brought into contact with a second wafer  420  having a first surface  420   a , so that the two surfaces  410   a ,  420   a  are opposite to each other. The wafer bonding process involves compressing the two wafers together by applying a “force column”  450  on a second surface  410   b  of the first wafer  410 . Force column  450  includes a plurality of forces arranged in a column having a base dimensioned to cover the entire second surface  410   b  of the first semiconductor wafer  410  and is configured to apply a uniform pressure to the entire second surface  410   b  of the first wafer  410  and to transfer a uniform pressure to the bond interface  405  of the wafer stack  302 . In other embodiments, a second force column  460  may be applied simultaneously with force column  450  on the second surface  420   b  of the second wafer  420 , as shown in  FIG. 5 . In one example, force column  450  is a pressurized gas column and applies forces of the order of 100 kN on a 200-mm wafer, which generates a pressure of approximately 32,000 mbar. Finite element analysis of the displacement and of the Von Mises stresses along the bond interface  405  are shown in  FIG. 4A  and  FIG. 4B , respectively. We observe layers  401 ,  402 , and  403  with uniform displacement and a uniform stress region  404  with no variations between the central  406  and periphery regions  407  of the bond interface  405 . In some embodiments, surfaces  410   a ,  420   a  have bond layers  412 ,  422 , respectively, configured to promote a specific type of bonding between the two wafer surfaces  410   a ,  420   a . Bond layers  412 ,  422  may be grid structures, metal, glass, semiconductor structures, insulators, integrated devices, adhesives, or any other bond-promoting material or structure. The system is designed to perform any desirable substrate bond process including anodic, eutectic, adhesive, fusion, glass fit, and thermocompression bond processes for wafer-to-wafer bonding. Accordingly, the system has suitable controls for controlling the bonding operation parameters, including substrate temperature, bond pressure, and chamber atmosphere, among others. In other embodiments, system  400  is used to bond any type of semiconductor structures or materials including flat-panel structures, integrated circuit devices, 3D integration of microelectronics, and packaging of Micro-Electro-Mechanical-Systems (MEMS), among others. 
     Referring to  FIGS. 6-14 , bond apparatus  10  operates generally as a clamp. The apparatus  10  has opposing clamping blocks in this embodiment, an upper block assembly  20  and opposing lower block assembly  22 . The lower block  22  assembly has a chuck  21  for holding or otherwise receiving one or more wafers thereon. One or more stacks  430  of one or more wafers  410 ,  420 , shown in  FIG. 3 , are positioned on the wafer chuck  21  of the apparatus  10 . Lower block assembly  22  is supported by the bottom plate  56 , and upper block assembly  20  is supported by the top plate  53 . Bottom plate  56  and top plate  53  are movably connected to posts  42 . In this embodiment, the lower block assembly  22  and the bottom plate  56  move upward along the Z-direction to bring the wafer(s)/stack(s) substantially to or near contact with bearing surfaces  23 S of the upper block assembly  20 . When this proximity position is reached, the positions of the bottom plate  56 , top plate  53 , and upper block assembly  20  are fixed, and the lower block assembly  22  is moved upward along the direction of arrow P 1  toward the upper block assembly  20  to apply a desired high bonding pressure on the wafer stack  430 . In one example, the desired bonding pressure is 100 kN on a 200-mm wafer stack or 225 kN on a 300-mm wafer stack. In alternate embodiments, the upper block  20  or both the upper block  20  and the lower block  22  are moved together to apply the desired high bonding pressure on the wafer stack(s)  430  and effectuate bonding between the interfacing wafer surfaces  410   a ,  420   a . The upper block assembly  20  and lower block assembly  22  deliver the high bonding pressure substantially uniformly (i.e., without significant pressure variance) across the area of the wafer bond interface  405  and without inducing substantially any shearing stress at the interface (e.g., substantially zero shearing stress at the bond interface of the wafers), as will be described in greater detail below. The load distribution within the upper block assembly  20  and lower block assembly  22 , resulting in the aforementioned bonding pressure, is a substantially true column loading in respective load bearing members, substantially eliminating load eccentricities and bending moments causing flexure in the upper and lower block assemblies  20 ,  22 , as well as other portions of the apparatus  10 . Loading uniformity and repeatability are provided by a structural skeleton  16  of the apparatus  10  that substantially bypasses the chamber housing  12  as load bearing member of the apparatus  10 . Loading uniformity on the bond interface  405  is further established by the apparatus  10 , with a leveling system  82  that maintains the wafer bearing surfaces  23 S,  21 S of the upper block  20  and lower block  22  assemblies, respectively, substantially level or parallel with each other, and ensures that bonding forces are applied by the lower block and upper block assemblies  22 ,  20  substantially normal to the bonding interface  405  of the wafer stack  430 . Also, as will be described further below, the upper block  20  and lower block  22  assemblies in the exemplary embodiments include heaters  30 ,  32 , respectively, (or thermal cyclers for the thermal cycling of the wafer contact surfaces  23 S,  21 S) that are thermally isolated from the apparatus structure by load-bearing, vacuum isolation systems  70 ,  72 , respectively. The load-bearing, vacuum isolation systems  70 ,  72  provide optimal thermal isolation performance while eliminating undesired thermal leaks and reducing thermal mass (and, hence, inertia) of the thermally cycled portion (with commensurate fast cycle time performance), and nevertheless are capable of supporting desired loads (e.g., bonding pressure loads in the exemplary embodiment). In some embodiments, heaters  30 ,  32  may have more than one heating zone. Referring to  FIG. 18 , heater  32  includes a first heating zone  32 B configured to heat the center region of the wafer and a second heating zone  32 A configured to heat the periphery of the wafer. Heating zone  32 A is controlled independently from heating zone  32 B in order to achieve thermal uniformity throughout the entire bond interface  405  and to mitigate thermal losses at the edges of the wafer stack. 
     The apparatus  10  is capable of bonding wafers or substrates  410 ,  420  of any suitable type and size. For example, the substrates  410 ,  420  may be 100 mm, 200 mm, or 300 mm diameter semiconductor substrates. In the embodiment shown in  FIG. 3 , the wafers  410 ,  420  are substantially similar to each other. In alternate embodiments, the stack  430  may comprise different types or different size wafers. Stack  430  is shown in  FIG. 3  as having two wafers  410 ,  420  for example purposes. As may be realized, stack  430  may include any desired number of wafers being bonded together. The bonded surfaces  410   a ,  410   b  may include bond layers  412  and  422 , respectively, and the bond layer  412 ,  422  may be metal, grid structures, semiconductor structures, insulators, adhesives, or glass, among others. 
     Still referring to  FIGS. 6-14 , and in greater detail, bond apparatus  10  includes a chamber  12 . The chamber  12  is closed or otherwise configured to have a controlled atmosphere, such as an inert gas, or is held in vacuum conditions with a turbo pump system  161 , shown in  FIG. 7 . In alternate embodiments, the apparatus may not include a chamber. As seen in  FIG. 7 , the chamber  12  includes an access port  14 . The access port  14  is sized to allow placement and removal of a carrier fixture  24  into the chamber  12 , shown in  FIG. 9 . In some embodiments, a pre-load chamber  15  communicates with chamber  12  through port  14 , as shown in  FIG. 7 . Port  14  has a door (not shown) for closing the port if desired. For loading the wafer stack into the evacuated chamber  12 , first, the port door is closed, and the carrier fixture  24  with the pre-aligned wafers  410 ,  420  is placed in the pre-load chamber  15 . Next, the pre-load chamber  15  is evacuated, and then the port door is opened, and the carrier fixture  24  with the pre-aligned wafers  410 ,  420  is placed in the chamber  12 . The port door is then closed again. For the removal of the bonded wafers, the pre-load chamber  15  is evacuated, and then the port door is opened, and the carrier fixture  24  with the bonded wafers  410 ,  420  is removed from the chamber  12 , and the port door is closed again. Carrier fixture  24  holds the previously aligned wafer stack  430 . A transport device  480 , such as a transport arm or slide, that is automated or otherwise manually operated, is used to move the carrier fixture  24  into and out of the chamber  12 , as shown in  FIG. 16 . In one embodiment, shown in  FIG. 17A , carrier fixture  24  is a circular shaped ring  280  and includes three spacer and clamp assemblies  282   a ,  282   b ,  282   c  arranged symmetrically at the periphery of the circular ring at about 120° apart. Each spacer and clamp assembly  282   a ,  282   b ,  282   c  includes a spacer  284  and a clamp  286 . Spacer  284  is configured to set the first and second wafers  410 ,  420 , at a predetermined distance. Spacers with different thicknesses may be selected for setting different spacings between the two wafers. Once the spacers are inserted between the wafers, the clamp is clamped down to lock the position of the two wafers. Each spacer  284  and each clamp  286  is independently activated by linear actuators  283  and  285 , respectively. For the bonding process, the aligned wafers  410 ,  420  are placed in the carrier fixture  24  and are spaced apart with spacers  284  and then damped down with clamps  286 . The fixture with the clamped wafers is inserted in the bonding chamber  12 , and then each damp is undamped one at a time, the spacer is removed, and then clamped again. Once all spacers are removed, the wafers are clamped again, and the two wafers are staked together with a pneumatically controlled center pin  290 , and then the force column  460  is applied to facilitate the bonding process. The wafers are staked together with a force that is automatically or manually adjustable. 
     As shown in  FIG. 8 , at least one of the upper block  20  and/or the lower block  22  is movably held in the chamber  12 . In the embodiment shown in  FIG. 8 , the upper block  20  and opposing lower block  22  are depicted in a vertical clamping configuration. In alternate embodiments, the opposing upper block  20  and lower block  22  are arranged in any other desired clamping orientation including horizontal clamping configuration. In the exemplary embodiment, the upper block assembly  20  is fixed, and the lower block assembly  22  is movable along the direction indicated by arrow P 1 , shown in  FIG. 6 . The lower block assembly  22  is also moved as a unit together with the bottom support plate  56  along the Z-direction (shown in  FIG. 6 ) by a suitable drive  100 , referred hereto as a z-drive  100 . In the exemplary embodiment, the lower block  22  has a movable portion  22 M, capable of being moved in the direction indicated by arrow P 1 , independent of the z-drive  100 , by a suitable actuator  52 , as will be described below. In the exemplary embodiments, z-drive  100  provides gross motion to the lower block assembly  22  together with support plate  56 , and actuator  52  moves the movable portion  22 M of the lower block  22  assembly for bonding. In alternate embodiments, the z-drive  100  moves the upper block assembly  20  downward in a direction opposite to the indicated Z-direction. The upper block  20  and lower block  22  have corresponding seating surfaces  23 S,  21 S. The upper and lower block assemblies  20 ,  22  and seating surfaces  23 S,  21 S are sized as desired to generate suitable bonding pressure on the wafer stack. As noted before and will be described below, the seating surfaces  23 S,  21 S have heat control (i.e., are capable of being heated and/or cooled). The heat control is provided by any suitable thermal controller. In one example, seating surfaces  21 S,  23 S are made from a suitably hard material, such as silicon carbide (SiC). 
     Referring now also to  FIG. 7  and  FIG. 8 , the chamber  12  generally comprises a casing or shell  16  that is substantially closed to allow isolation of the chamber interior from outside. In the exemplary embodiment shown, the casing  16  is generally annular, though in alternate embodiments, the casing may have any desired shape. The chamber casing  16  is supported from a desired base or foundation structure  18  by a skeletal or support frame  40 . The base structure  18  is of any desired type and shape and is shown as a substantially flat plate  18  located below the chamber  12 , for example purposes. The base structure  18  is substantially rigid and, in alternate embodiments, may have any desired size, shape, and location relative to the chamber. The skeletal frame  40  of the apparatus  10  has substantially rigid members that are attached to the casing  16  and joined to the base structure  18  to carry the casing  16 . The skeletal structure  40  is also attached to the upper block and lower block assemblies  20 ,  22  of the apparatus  10  so that reaction on the upper block and lower block assemblies  20 ,  22  during the application of the bonding forces is distributed to the skeletal frame  40  and not the chamber casing  16 . In the exemplary embodiment, the skeletal frame  40  is substantially an exoskeletal frame located outside the chamber  16 . In alternate embodiments, the skeletal frame  40  may be an endoskeletal frame located within the chamber, if desired. In the exemplary embodiment, the skeletal frame  40  comprises substantially rigid post  42  (three are shown for example purposes, though any desired number may be used). Posts  42  are anchored at one end to the base structure  18 . The posts  42  are distributed substantially equally around the casing  16 . Size and shape of the posts  42  is selected as desired for desired rigidity. The skeletal frame  40  also may include a top attachment plate  46 . As seen best in  FIG. 7 , the attachment plate  46  is attached to the casing  16  by any desired attachment means, such as welding, brazing, or mechanical fasteners. In alternate embodiments, the casing  16  and attachment plate  46  may be formed as a unitary member. The attachment plate  46  is a substantially rigid member. The stiffness of the plate  46 , at least in response to reaction loads imparted thereon by the bonding press, is generally commensurate with the stiffness of the rest of the skeletal frame  40 , including posts  42 . In alternate embodiments, the attachment plate  46 , attaching the casing  16  and other bonding press components inside the chamber  12  to the skeletal frame, may have any other desired shape. As seen best in  FIG. 8 , the posts  42  are attached at another end to the attachment plate  46 . The connection  44  between each post  42  and attachment plate  46  may be bi-directional, capable of supporting axial loads, along the axis of posts  42 , both towards and away from the base plate  18 . The connection  44  of each post is adjustable (both up and down along the axis of the posts) to ensure substantially uniform loading of each post  42  under both static loads from the chamber and apparatus components, and static and dynamic loads during bonding press. In the exemplary embodiment, the connection  44  is generally symmetrical on opposite sides of the interface with the attachment plate  46 . The connection  44  may include engagement members  44 E (for example, threaded arms) that engage the post  42  (e.g., by positive engagement surface or clamping) and have a bearing surface for bearing loads from the attachment plate. The connection  44  may include bearing elements to ensure uniform load distribution from the attachment plate onto the bearing surfaces of the engagement members  44 E. In alternate embodiments, the connection between the posts  42  of the skeletal frame  40  and attachment plate  46  carrying the chamber casing and bonding press may have any suitable configuration. In the exemplary embodiment, the connection  44  may be preloaded (e.g., by torqueing engagement members  44 E) in order to eliminate undesired displacements of the posts  42  during bonding operation. 
     As seen best in  FIG. 8  and as noted before, in the exemplary embodiment, the upper block assembly  20  and the lower block assembly  22  are attached to the skeletal frame  40 . The upper block assembly  20  is attached to skeletal frame  40  by a span support structure  53 , as will be described further below. The static and dynamic loads, including bonding press loads, from the upper block assembly  20  are carried substantially entirely by the span structure  53  and distributed by the span structure  53  via attachment plate  46  to the posts  42 . The lower block assembly  22  is attached to the posts  42  via a seat structure  56 . In the exemplary embodiment shown, seat structure  56  generally has a span  56 S and a block support seat  56 T. In alternate embodiments, the seat structure supporting the lower block may have any other desired configuration. In the exemplary embodiment, the span structure  56 S is shown as a plate, for example, but may have any other desired form, and is attached to the posts  42  by linear slides  43 . Thus, in the exemplary embodiment, the seat structure  56  and, hence, the lower block assembly  22 , is capable of movement in the direction indicated by arrow z (z-direction). The posts  42  may serve as guides for z-movement of the lower block. In the exemplary embodiment shown in  FIG. 8 , the z-drive  100 , which may be any suitable drive (e.g., electric linear drive, pneumatic drive, or hydraulic drive, to name a few), is connected to the span structure  56 S and capable of moving the seat structure  56  and lower block assembly  22  in the z-direction as a unit. The z-drive  100  may be attached to the base structure  18 . As seen in  FIG. 8 , the support seat  56 T is connected to the lower block assembly  22 . In the exemplary embodiment, the support seat  56 T extends generally into the casing  16 . A bellows seal  16 S, between the casing  16  (in the example, shown attached to a closure plate  16 P of the casing) and the support seat  56 T, isolates the chamber interior and accommodates the z-motion of the seat structure  56  and lower block assembly  22 . The seat structure  56  shown in  FIG. 8  is merely exemplary, and in alternate embodiments, the structure may have any desired configuration. In the exemplary embodiment, the seat structure  56  has a seat surface  58  that engages the bottom of the lower block assembly  22 . 
     As seen best in  FIG. 8 , the lower block assembly generally includes a chuck  21 , with wafer support surface  21 S, a heater (or thermal cycler)  32 , and flange  36 . The heater  32  is supported by flange  36 . The heater  32  is thermally isolated from the flange  36  by a load-bearing, vacuum isolation system  72 , described further below. The flange  36  is maintained at a desired steady-state temperature by a thermal regulator (e.g., a water cooling system). The chuck  21  is connected to the heater  32  so that the wafer support surface  21 S and, hence, the wafer seated thereon is heated by the heater  32 . The chuck  21 , heater  32 , and flange  36  form the movable section  22 M of the block assembly  22 . Movable section  22 M is movable in direction P 1  relative to a base section  22 B of the block assembly  22 , shown in  FIG. 9 . In the exemplary embodiment, the block assembly  22  includes an actuator  52  that actuates the movable portion  22 M, independent of the z-drive motion, and generates a force column substantially uniformly distributed across the seating surface  21 S of the block assembly  22 . In the exemplary embodiment, the actuator  52  is driven by a pressurized gas, though in alternate embodiments, the actuator may be driven by hydraulic or magnetic means capable of generating a substantially uniformly distributed force column across the wafer seating surface. In the exemplary embodiment shown in  FIG. 8 , the actuator  52  has a movable plate member  54  and a base or reaction member  55 . In this embodiment, the base member  55  is fixedly seated against surface  58  of seat structure  56 . Bellows seals  52 B join the plate  54  and base members  55  of the actuator  52  and isolate the actuator from the chamber interior, as shown in  FIG. 13 . As may be realized, a desired gas (e.g., clean air or inert gas, such as nitrogen, N 2 ) is introduced between plate  54  and base members  55  for actuation. The pressure of the gas is controlled to achieve the desired high pressures (e.g., about 100 kN on 200-mm wafers; about 225 kN on 300-mm wafers) for bonding the wafer stack. The plate member  54  in the exemplary embodiment has a pressure face surface  54 F that is substantially similar (e.g., in shape and size) and aligned parallel to the wafer support surface  21 S of the chuck  21  so as to provide a substantially uniform column for loading between plate face  54 F and wafer support surface that is substantially normal to the plane of the water support surface. The bonding pressure is monitored with the pressure gauges  295 , shown in  FIG. 13 . In some embodiments, the size of the pressure face surface  54 F is adjusted via a manual or an automated mechanism in order to accommodate different size wafers. As may be realized, orthogonality of the loading by the actuator on the wafer support surface may be readily achieved by controlling planarity and degree of parallelism of the plate pressure face and wafer support surface. 
     As seen in  FIG. 9 , the lower block  22  in the exemplary embodiment also includes a leveling system  82  for leveling the wafer support surface  21 S of the lower block assembly  22  with the wafer seating surface  23 S of the upper block assembly  20 . In the exemplary embodiment, the plate member  54  and, hence, the movable section  22 M of the lower block assembly  22  rides on a layer of gas with respect to the base  55  and is positionally decoupled from the base  55 , except as controlled by the leveling system  82 . In the exemplary embodiment, the leveling system  82  includes a linear guide portion  84  and a rotational guide or gimbal portion  86 , shown in  FIG. 12 . The linear guide portion  84  guides the movement of the movable block section  22 M so that the wafer support surface  21 S travel is substantially axial in the direction indicated by arrow P 1  (without any lateral translation). The rotational guide portion  86  guides the movement of the movable portion  22 M so that the wafer support surface  21 S may rotate and/or tilt around a center point  85  (shown in  FIG. 10 ) corresponding to the center of the wafer bond interface  405  without translation. The leveling system  82  may be autonomous/automatic or may be manually operated if desired. In the exemplary embodiment, the linear guide portion  84  includes a guide rod  84 R that is movably supported in a linear bearing assembly  84 B, shown in  FIG. 13 . The guide rod  84 R is connected to the plate member  54  as shown in  FIG. 13 . In alternate embodiments, the linear guide portion  84  may have any other desired configuration. As seen in  FIG. 13 , in the exemplary embodiment, the linear bearing assembly  84 B is mated to gimbal  86 , defined by a hemispherical bearing assembly. The hemispherical bearing surface radius extends from the bond interface center  85 . The gimbal  86  may be attached to the support seat  56 T. In alternate embodiments, the gimbal portion may have any other desired configuration. In still other alternate embodiments, the linear guide and gimbal portions may be mated in any other desired arrangement. As seen in  FIG. 13 , the leveling system  82  is positioned so that the linear guide portion  84  and gimbal portion  86  are not loaded by the actuator  52  or any other portion of the lower block assembly during bonding operations. In the exemplary embodiment, the gimbal portion  86  is preloaded in order to lock and unlock the bearing surface. Preload may be accomplished by any desired preload system type, such as, for example, pneumatic or hydraulic pressure or mechanical or electromechanical pressure applied against the bearing surface. The preload system may be controllable with a suitable controller (not shown) or may be set to a desired lock limit. The leveling system  82  enables dynamically leveling of the lower block assembly to the upper block assembly. This eliminates the over-constrained condition that occurs when the top and bottom assemblies are not parallel or if the wafer stack is wedge-shaped. The bearing itself does not bear the bond load, and the center of rotation is at the wafer plane so that any rotation that occurs will not impart wafer shift. 
     Referring to  FIG. 20 , in another embodiment, the leveling system  82  is positioned so as to carry the load of the actuator  52  and bears the bond load. The gimbal portion  86  is positioned below the fixed plate  55  and supports the fixed plate  55 , the movable plate  54 , and the above lying flange  36 , thermal isolation system  72 , heater  32 , chuck  21 , and wafers (not shown). In this embodiment, the size of the base of the applied force column is adjusted to accommodate wafers of various sizes. Fixed plate  55  is sealed against the movable plate  54  at the edges with bellows seal  52 B and at selectable intermediary locations with piston or zone seals  52 Z 1  and  52 Z 2 . The sealing locations of bellows seals  52 B and intermediary zone seals  52 Z 1 ,  52 Z 2  are selected based on the size of the wafer stack that needs to be bonded and determine the base area of the applied force column. Pressurized gas fills the sealed region between the selected seals. In one example, the location of bellows seals  52 B at the edges is selected for bonding 8 inch wafers, zone seal  52 Z 1  for bonding 6 inch wafers, and zone seal  52 Z 2  for bonding 4 inch wafers. 
     Referring now also to  FIG. 14 , as noted before, the lower block assembly has a thermal isolation system  72 , thermally isolating the heater  32  from the mating portion of the block assembly supporting the heater. As also noted before, in the exemplary embodiment, the thermal isolation system is a load-bearing vacuum isolation system. As seen in  FIG. 13 , the isolation system  72  is positioned across the loading path from the actuator  52  to the wafer support surface  21 S. Hence, the thermal isolation system  72  supports the bonding pressure loads. As seen in  FIG. 14 , the system  72  generally comprises a load-bearing vacuum layer confined between a plate  78  and a diaphragm  76 . Diaphragm  76  is connected to plate  78  via bellows  74  outside of the load-bearing region. The diaphragm  76  may be made of any suitable material, such as INCONEL™, and may be connected in any suitable manner, such as, for example, by welding, to the open end of the bellows  74 . As seen in  FIG. 14 , the bellows  74  are located outside the load-bearing portion of the block assembly, and the diaphragm  76  is positioned in the load-bearing portion. The diaphragm  76  is supported by plate  78  which comprises material having a low coefficient of thermal expansion (CTE). In one example, plate  78  is made of ZERODUR® glass-ceramic, manufactured by Schott AG. Plate  78  has a surface  78 S formed to minimize the contact area with the diaphragm  76  yet has sufficient strength to bear the compressive loads during bonding, shown in  FIG. 15 . This structure  72  is continually evacuated to minimize heat transfer. As noted above, surface  78 S is formed, for example, by machining or any other suitable forming process to minimize the contact area with the diaphragm and, hence, provide limited and poor thermal contact area between the diaphragm  76  and the low-CTE material layer  78 . As may be realized, the low-CTE material of layer  78  also may have a poor thermal conduction coefficient. In the exemplary embodiment shown in  FIG. 15 , the contact surface  78 S has raised projections that contact the diaphragm  76 . The projections are shown schematically in  FIG. 15  and may have any suitable shape. For example, the projections may have a cross-section that tapers in to contact the diaphragm. The number and size of projections may be as desired to achieve desired load capacity and thermal conduction properties across the diaphragm/low-CTE material layer interface. As may be realized, the thermal break provided by the isolation system  72  allows for rapid thermal cycling of the heater  32 , chuck  21 , and wafer stack  430 . 
     Referring again to  FIG. 8 , in the exemplary embodiment, the upper block assembly  20  is generally similar to the lower block assembly  22 , described before. In the exemplary embodiment, the upper block provides the control level surface for stack bonding, and the leveling system  82  operates to level the wafer support surface  21 S of the lower block assembly to the wafer support surface  23 S of the upper block assembly, as previously described. In alternate embodiments, the upper block assembly  20  may have an integral leveling system. In this embodiment, upper block assembly  20  is not movable. In other embodiments, similar to block assembly  22 , the upper block assembly  20  may have a movable portion  20 M, with chuck  23 , heater  30 , and support flange  34  (similar to heater  32  and flange  36  of the lower block) that is actuated in the direction indicated by arrow P 1  by actuator  50 . As seen in  FIG. 8 , in the exemplary embodiment, a bad-bearing vacuum thermal isolation system  70 , similar to previously described system  72 , defines a thermal break between heater  30  and flange  34 , The actuator  50  in the alternate embodiment also may be similar to actuator  52 . The actuator  50  may have a plate member  57  and reaction or base member  55  joined to the plate member by bellows seals  53 B, as shown in  FIG. 16 . In the exemplary embodiment, the bellows seals  53 B are configured to support the movable portion  20 M from the base member  55  under static conditions. Preload blocks  59  may be provided for preloading the bellows  53 B during static conditions, in order to provide improved control of plate member displacement during actuator operation (e.g., preload blocks counter spring forces in bellows due to weight of movable portion of upper block assembly). As seen in  FIG. 8 , in the exemplary embodiment, the base member  51  of the actuator is connected to and supported from span member  53  by connection section  102 . Connection section  102  is substantially rigid in axis z, in order to transfer z-loads between the base member  51  and the span member  53  without any substantial elongation. During the bonding process, the connection section  102  behaves as a pinned connection and, hence, is unable to transfer bonding moments. In the exemplary embodiment shown in  FIG. 8 , the connection section  102  includes an annular shell or wall  102   w  joined at one end  103  to the base member  55 . The wall  102   w  has a flange  106  that extends between wall  102   w  and span member  53  and joins the wall  102   w  to the span member  53 . Flange  106  may be formed integral to the wall  102   w  or to the span member  53 . The flange thickness is similar to the thickness of the span member at the interface between flange  106  and span member  53 . If integrally formed with the span member  53 , the flange  106  is joined in any desired manner (e.g., by welding) to the wall  102   w  and vice-versa. The flange  106  serves to offset the wall  102   w  from the span member  53  and, hence, reduces the flexural stiffness of the wall  102   w  to span member  53  joint and renders the wall  102   w  substantially unable to transfer bonding loads between actuator base member  51  and span member  53 . As may be realized, this allows the base member  51  to remain substantially flat when the actuator is pressurized for bonding the wafer stack in the chamber. 
     Although the aforementioned embodiments provide substantial improvement over known mechanisms, some aspects of them are expensive or complex to configure and use. Thus, various improvements are described below. 
     Referring again to  FIG. 6 , it is desirable that upper block assembly  20  be level with respect to lower block assembly  22  so that even pressure is applied across the entirety of surfaces  23 S and  21 S when they are brought together. As described previously with respect to  FIG. 8 , some z-axis adjustment is possible at connection  44  of each post  42  by adjustment of corresponding engagement members  44 E. In an embodiment using three such posts  42 , such two-dimensional leveling can be obtained. However, in practice, it is found that configuration using this mechanism is time-consuming and somewhat difficult. As an improvement, and referring now to  FIG. 21 , in one embodiment, in place of each of two of the three posts  42 , a threaded post  2101  is engaged with a differentially threaded adjustment collar  2102 , which is surrounded by leveling sleeve  2110 . To ensure threads of adjustment collar  2102  are properly seated before being placed under load so that adjustment does not change under load, preload springs  2105  (e.g., preload washers) are engaged between attachment plate  46  and post  2101 , with upper spacer stop  2103  and hex screw  2104  placed as shown to provide a range limit stop. A clamp  2106  is configured to provide radial clamping force to minimize z-axis movement when it is tightened after leveling adjustment is complete. Leveling sleeve  2110  includes a shoulder  2111  allowing hex shoulder screw  2112  to prevent rotation (or other movement) of leveling sleeve  2110  during adjustment. In one embodiment, springs  2105  are Belleville-style coned disc washers stacked as shown (e.g., within a generally cone-shaped seat feature defined in attachment plate  46 ) to provide the desired preload capacity and range of adjustment, which, in some embodiments, is approximately ±2 mm with a preload force of at least 5 kN or greater (e.g., about 10 kN or greater, about 15 kN or greater, and so forth). In one embodiment, threaded adjustment collar  2102  is differentially threaded in the manner of a micrometer drive system, with outside threads of 2 mm pitch and inside threads of 1.5 mm pitch, yielding an effective pitch resolution of about 1.0 mm or less (e.g., about 0.75 mm or less, about 0.5 mm or less, and so forth). In some embodiments, the pitch of the inside threads may be about 0.5 mm less than the pitch of the outside threads. 
       FIG. 22  is a perspective view illustrating the components discussed in  FIG. 21 . In the illustrated embodiment, leveling sleeve  2110  includes an integral clamp  2106  formed by cutting away a portion of leveling sleeve  2111 . In addition, vernier scale markings  2202  are etched into a portion of leveling sleeve  2110 , and vernier scale markings  2201  are also etched onto a portion of threaded adjustment collar  2102 , thus permitting user adjustment in a simpler manner than would be possible without such markings. In one embodiment, the vernier scale markings  2201  and  2202  provide on the order of one micron (1 μm) of measurement resolution. 
     As previously noted, in one embodiment, two of three posts  42  are replaced with the configurations of components shown in  FIGS. 21 and 22 . In this embodiment, the replacement for the third post  42  is a non-adjustable gimbal attachment. Having two of three posts adjustable provides the leveling capability sought, and there is no need for a third adjustment point. Referring now to  FIG. 23 , a third threaded post  2101 , rather than being fitted with the adjustment components shown in  FIGS. 21 and 22 , includes a load cell  2301  with corresponding cap  2302  and gimbal bushing  2303 . In one embodiment, cap  2302  is tightened using a cap screw (not shown) and springs  2105  in the same manner as discussed in connection with  FIG. 21 . Load cell  2301 , in one embodiment a model FD0180-N510-1379-M09 (available from ATP Messtechnik GmbH of Ettenheim, Germany) is used for load cell  2301 . An advantage of this gimbaled configuration is that load cell  2301  not only forms part of the gimbal, but is also usable as a sensor for the applied force column as previously discussed. Because of equal spacing and symmetrical arrangement of posts  42  (specifically, the posts  2101  discussed with respect to  FIGS. 21 and 23 ), the overall force applied is simply three times the force indicated by load cell  2301 . In accordance with some embodiments, a load cell  2301  may be utilized with any one or combination of the posts provided. Thus, multiple load cells  2301  may be employed in one or more locations. 
     As described above in connection with  FIGS. 6, 8, and 11 , for example, a lower block assembly  22  is configured to be brought into contact with an upper block assembly  20 , and in the specific embodiment of  FIGS. 8 and 11 , associated heater and thermal isolation components  30 ,  32 ,  34 ,  36  are all brought together via force columns  450 ,  460 . In practice, it is found that thermal differences and significant pressures can cause doming of certain components (e.g., upper and lower block assemblies  20 ,  22  and those components that may be sufficiently pressed against those assemblies to assume their shapes). Although such deformation may be exceedingly small (e.g., in a range of 25-450 μm, in some cases), this can still lead to uneven wafer bonding. Referring now to  FIG. 24 , in order to minimize such deformation, in one embodiment, a reaction plate  2401  is employed to minimize such unwanted deformations. In one embodiment, reaction plate  2401  is a monolithic machined piece of AISI 1045 steel with no complex moving parts. Reaction plate  2401  is fastened by screws to base member  51 , previously described. A set of seals  2402 - 2407  (with corresponding cross-sections on the right side of  FIG. 24  not numbered), in one embodiment O-rings, are placed in corresponding grooves in base member  51  and reaction plate  2401 . Seals  2406  and  2407  maintain pressure conditions in the process chamber, which is the region outside the space defined by seal  2407  and inside the space defined by seal  2406 . The process chamber environment is a deep vacuum in some embodiments, but may also involve higher pressures (e.g., up to 2 atmospheres in one embodiment). Seals  2402  and  2405  define inner and outer boundaries, respectfully, for introduction of pressurized gas (in the manner previously described) between base member  51  and reaction plate  2401 , via corresponding ports (not shown). The spaces between seals  2402  and  2406 , as well as between seals  2405  and  2407 , are maintained at nominal atmospheric pressure via vents  2408 . A portion of the material forming reaction plate  2401  is cut away to form a flexure  2409 . Seals  2402 - 2407  act as dynamic seals, expanding or compressing with the relative movement of corresponding portions of base member  51  with respect to reaction plate  2401 , thereby obviating the need for more complex structures, such as pistons, that would otherwise maintain a seal with such movement. In one embodiment, seals  2402 - 2405  are implemented using 3 mm diameter O-rings, which are found to maintain good sealing even at pressures on the order of 37 bar (e.g., approximately 537 psi). 
     Seals  2403 - 2405  are configured to permit pressurization appropriate for the three different standard sizes of wafers. For 4-inch diameter wafers, pressurized gas is applied via appropriate ports to the region between seals  2402  and  2403 ; for 6-inch diameter wafers, the zone between seals  2403  and  2404  is also pressurized. For 8-inch diameter wafers, the zone between seals  2404  and  2405  is pressurized as well. Note that seal  2407  is smaller than the other seals because it is outside the flexure  2409  in an area not subject to significant deformation. 
     In practice, it is found that any unwanted variation in bonding force and corresponding substrate chuck deformation due to doming of components such as base member  51  in the embodiments described (e.g., in  FIG. 15 ) is dramatically reduced by use of reaction plate  2401  pressurized in the manner described. 
     Also, illustrated in  FIG. 24  are various elements between reaction plate  2401  and chuck  23  (previously described), in one embodiment. As shown, these include a water-cooled flange assembly including cap  2410  and support flange  34 , as previously described, and heating and thermal isolation components including isolation plate  2470  and previously described heater  32  with heat shields  2478 , as further detailed below.  FIG. 25  is a cutaway perspective drawing of reaction plate  2401  as well as many of the associated components illustrated in  FIG. 24 . 
     Isolation plate  2470  is found to exhibit improved durability and service life, as well as being less complex, as compared with the thermal isolation systems previously described in connection with, for example,  FIGS. 14 and 19 , even under the temperature and pressure extremes of repeated use. In one embodiment, isolation plate  2470  is formed of low-CTE material as previously described, with a set of stacked heat shields  2478  placed between the isolation plate  2470  and heater  32  to provide enhanced thermal isolation. Referring now to  FIG. 26 , rather than using the single low-CTE plate  78  and diaphragm  76  system of  FIG. 19 , in one embodiment, the isolation plate  2470  is formed of a number of pie-shaped wedges  2671  of low-CTE material. It is found that a benefit of using such wedges  2671  rather than a single piece of low-CTE material is the minimization of detrimental effects from thermal expansion/contraction. Such expansion/contraction of a single such piece, when it is in contact with another piece having a different coefficient of thermal expansion (e.g., diaphragm  76 , shown in  FIG. 19 ) may result in abrasion of either the low-CTE material, the other piece, or both. Since the pie wedges  2671  are independent, each can move slightly with respect to another, and thus abrasion from movement due to thermal expansion/contraction is minimized or otherwise reduced. In some instances, wedges  2671  may be configured to be arranged in a generally circular manner, with vertices pointing towards a common center, though other arrangements may be provided in accordance with other embodiments. Furthermore, the use of wedges  2671  permits isolation plate  2470  to provide a small amount of desirable compliance deflection in the range of about 50 μm or less (e.g., about 30 μm or less, about 10 μm or less, about 5 μm or less, and so forth) and tolerate such deflection without cracking, as low-CTE materials typically are not particularly forgiving in these aspects. 
     In one embodiment, further improvement over diaphragm/vacuum systems such as those of  FIG. 19  is achieved by use of stacked foil heat shields  2478  rather than relying on a vacuum (and, accordingly, relying on diaphragm  76  remaining sound). In one embodiment, the heat shields include two metal foil layers, each of which has cutout holes to match the pattern of raised projections of isolation plate  2470 . In one embodiment, head shields  2478  are constructed of stainless steel; in an alternate embodiment, a low-CTE alloy such as Invar is used. In practice, it is found that performance similar to the vacuum-based system of  FIG. 19  is achieved, without need for establishing a vacuum at all, with use of isolation plate  2470  and foil heat shields  2478 . 
     Referring now to  FIG. 27 , in still another embodiment, isolation plate  2470  is formed of two pieces low-CTE material, upper portion  2771  and lower portion  2772 . In one variation on this embodiment, heat shields  2478  are not even required, particularly if a slight vacuum is maintained in the voids between upper portion  2771  and lower portion  2772 . In this manner, the thermal isolation benefits of a partial vacuum are obtained without the need for a diaphragm that may suffer from wear over time. Combinations of these features also may prove to provide improved performance and wear characteristics, in certain applications. For instance, each of the wedges  2671  of  FIG. 26  can be constructed of upper and lower portions  2771 ,  2772  as shown in  FIG. 27 . 
     The foregoing description of example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise forms disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the present disclosure be limited not by this detailed description, but rather by the claims appended hereto. Future-filed applications claiming priority to this application may claim the disclosed subject matter in a different manner and generally may include any set of one or more limitations as variously disclosed or otherwise demonstrated herein.