Workpiece chuck with temperature control assembly having spacers between layers providing clearance for thermoelectric modules

[0002] In semiconductor wafer processing, it is often required to process and/or test a wafer over temperature. Temperature-controlled wafer chucks have been developed to support a wafer and cycle the wafer over temperature during testing and/or processing. In a typical temperature-controlled wafer chuck, a temperature control module such as a heater and heat sink assembly is provided as a layer in the chuck. A top surface on which the wafer is mounted is provided at the top of the heater and heat sink assembly, and a base by which the chuck is mounted to the host apparatus, e.g., wafer prober, is provided beneath the heater and heat sink assembly. In some heater/heat sink assemblies, thermoelectric modules, also known as Peltier devices, are used to provide the temperature control. Peltier devices are small solid-state devices that function as heat pumps. The typical device is a sandwich formed by two ceramic plates with an array of small bismuth telluride (Bi₂Te₃) cubes in between. When a DC current is applied, heat is moved from one side of the device to the other, where it is removed with a heat sink. The cold side of the device is commonly used to cool a wafer. If the current is reversed, the device can be used as a heater by moving heat in the opposite direction.

[0003] One drawback to Peltier devices is that they are subject to mechanical stresses. For example, mechanical stresses induced by expansion and contraction of chuck layers over temperature can cause the devices to become unreliable and eventually fail. In some cases, the devices crack and are destroyed. This is particularly true in wafer chucks where the Peltier devices are rigidly attached to one or both of the upper and lower layers of the heater/heat sink assembly. As the assembly expands and contracts, the Peltier devices fail due to the mechanical stresses induced by thermal effects.

[0004] The present invention is directed to a workpiece chuck and method for supporting a workpiece such as a semiconductor wafer. The chuck includes a top layer on which the workpiece can be mounted and a temperature control assembly in thermal communication with the top layer to control temperature in the workpiece. The temperature control assembly includes an upper layer and a lower layer. At least one thermoelectric module is disposed between the upper and lower layers and at least one spacer is provided between the upper and lower layers. The spacer is sized to vertically space the upper and lower layers such that the thermoelectric module vertically floats in a space between the upper and lower layers. That is, the thermoelectric module is not mechanically or rigidly constrained within the temperature control assembly. As a result, mechanical stresses due to thermal effects are substantially reduced or eliminated.

[0005] In one embodiment, a thermally conductive medium is provided in the space between the upper and lower layers of the temperature control assembly. The thermally conductive medium thermally couples one or more thermoelectric modules to the upper and/or lower layers of the temperature control assembly. In one embodiment, the thermally conductive medium comprises a thermal heat sink grease. In another embodiment, the thermally conductive medium comprises a metallic foil. In another embodiment, the thermally conductive medium comprises a thermally conductive pad. In any of these embodiments, the thermally conductive medium is resilient such that the thermoelectric module is thermally coupled to the upper and/or lower layers while not being physically constrained between the upper and lower layers.

[0006] In one embodiment, the thermoelectric module is a Peltier device. The thermoelectric module can comprise bismuth telluride.

[0007] The upper and lower layers of the temperature control assembly can be fastened together by one or more screws. In one embodiment, the spacer is actually a plurality of washer or bushing-type spacers which include clearance holes through which the screws pass. As the screws are tightened to fasten the upper and lower layers they capture the spacers between the layers, while allowing sufficient space to prevent the thermoelectric modules from being vertically constrained. In another embodiment, the spacer is a unitary device, instead of a washer or bushing-type device as in the previously mentioned embodiment. The unitary device can be made in a star shape in which multiple spacing arms extend radially from the center of the temperature control assembly toward its outer edges.

[0008] The present invention provides numerous advantages over prior configurations. By providing sufficient space between the layers of the temperature control assembly, the thermoelectric modules are not subject to mechanical stresses due to thermal expansion and contraction effects. As a result, damage and failure of the devices is substantially reduced. Also, the spacers provide added rigidity and support in the wafer chuck, resulting in improved flatness of the top surface of the chuck, which in turn results in improved wafer processing and/or testing results.

[0009] The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

[0010] FIG. 1 is a schematic cross-sectional view of a workpiece chuck in accordance with one embodiment of the present invention.

[0011] FIG. 2 is a schematic exploded view of a portion of the workpiece chuck of FIG. 1.

[0012] FIG. 3 is a schematic detail cross-sectional view of a portion of the workpiece chuck of FIGS. 1 and 2.

[0013] FIG. 4 is a schematic cross-sectional view of a workpiece chuck in accordance with a second embodiment of the present invention.

[0014] FIG. 5 is a schematic exploded view of a portion of the workpiece chuck shown in FIG. 4.

[0015] FIG. 6 is a schematic plan view of an array of thermoelectric modules in the workpiece chuck of FIGS. 4 and 5.

[0016] FIG. 1 is a schematic cross-sectional view of a workpiece chuck 10 in accordance with an embodiment of the invention. FIG. 2 is a schematic exploded view of the chuck 10 shown in FIG. 1. FIG. 3 is a schematic cross-sectional detailed view of a portion of the workpiece chuck 10 shown in FIGS. 1 and 2. Referring to FIGS. 1 through 3, the chuck 10 includes a top surface or vacuum plate 12 on which a workpiece such as a semiconductor wafer can be placed during processing. The top vacuum plate 12 includes concentric channels for distributing vacuum throughout the top surface to hold the wafer on the top surface. The chuck 10 also includes a heat sink or temperature control apparatus 14 fastened beneath the top vacuum 12 by one or more fastener stacks which include screws 26. Air/fluid coolant inlet and exit nozzles 16 are provided for circulating coolant fluid through the heat sink assembly 14. A vacuum inlet 18 is provided for providing vacuum to the chuck 10 to hold the wafer to the top surface 12. A mounting base 20 can be used to mount the chuck 10 to the host machine that is performing the wafer processing such as for example, a wafer prober machine.

[0017] An array of thermoelectric or Peltier modules 24 is provided in the space between the top plate 12 and the heat sink 14. The Peltier modules 24 can be of the type sold by Melcor of Trenton, N.J. The modules 24 are electrically connected to a printed circuit board 25 which is positioned between the top vacuum plate 12 and the heat sink assembly 14. The vertical spacing between the top plate 12 and the heat sink 14 is controlled by the size of the spacers 28. The spacers 28 are formed of a ceramic or similar non-conductive, low thermal expansion material. The vertical dimension of the spacers 28 is selected such that when the top plate 12 and the heat sink assembly 14 are fastened together by screws 26, the space between the top plate 12 and the heat sink assembly 14 is such that the Peltier modules 24 are free to move laterally. That is, the top plate 12 and heat sink assembly 14 do not clamp or mechanically constrain the Peltier modules 24 and the vertical direction. That is, the controlled dimension is such that the Peltier array is allowed to “float” during temperature transitions, thus effectively limiting mechanical stresses across the Peltier structure. This approach dramatically extends the life expectancy of the Peltier modules 24. A power/sensor cable 22 provides electrical power to the Peltier modules 24 and temperature and other sensing capabilities in the chuck 10.

[0018] The array of Peltier modules 24 is thermally coupled with the top vacuum plate 12 and the heat sink assembly 14 through a thermally conductive medium on the module surfaces. The thermally conductive transfer medium 27 is applied in the space between the top and bottom surfaces of the Peltier modules 24 and the top plate 12 and the heat sink assembly 14, respectively. The medium can be a thermal heat sink grease, a metallic foil, a conductive pad, or similar medium. The medium is mechanically resilient such that the Peltier modules 24 do not come under stress under thermal expansion and contraction effects due to temperature cycling.

[0019] In addition to the benefits realized by allowing the Peltier modules 24 to float in the space between the top plate 12 and the heat sink assembly 14, the spacer configuration provides a thermally stable chuck structure. The change in flatness of the top surface over a typical testing temperature range is limited to the initial ambient flatness. The rigidity also lowers the deflection caused by high wafer probing forces during test contact.

[0020] Referring to FIG. 3, the top vacuum plate 12 and the heat sink assembly 14 are held together by threaded screws 26. The screws thread into threaded holes 31 in the top vacuum plate 12. A nut or head captures washers 29 in a recess or counterbore formed in the bottom of the heat sink assembly 14.

[0021] FIG. 4 is a schematic cross-sectional view of a workpiece chuck 100 in accordance with a second embodiment of the present invention. FIG. 5 is a schematic exploded view of the chuck 100 of FIG. 4. FIG. 6 is a schematic plan view illustrating a Peltier module array and spacer configuration used in the chuck of FIGS. 4 and 5. Description of elements which are the same as those of the first embodiment will be omitted. Referring to FIGS. 4 through 6, the top vacuum plate 12 and heat sink assembly 14 are separated by a unitary spacer 128, instead of the multiple spacers used in the previously described embodiment. The unitary “star” spacer 128 is made of a ceramic or similar non-conductive, low thermal expansion material. The unitary spacer 128 includes a plurality of radial arms which extend between the Peltier modules 24 from the center of the spacer 128 toward the edges of the chuck 100. Once again, as in the previous embodiment, the thickness of the spacer 128 defines the space between the top surface plate 12 and the heat sink assembly 14. The thickness is selected to allow the Peltier modules 24 to float during temperature transitions to reduce mechanical stress, thus increasing life and reliability of the modules 24. The one-piece unitary spacer 128 has thin vertical sections and includes cutouts to minimize thermal conductivity between the top vacuum plate 12 and the heat sink assembly 14 and also to provide electrical interconnection clearance. The configuration also includes pin inserts 102 on the heat sink assembly 14 to prevent excessive position shift of the modules 24 during temperature transitions.

[0022] While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims.