Lateral temperature equalizing system for large area surfaces during processing
View Patent ↗In many processes used in fabricating semiconductors the wafer is seated on the top surface of a pedestal and heated in a high energy process step, such as plasma etching. The pedestal, chuck or platen may be cooling but the wafer gradually heats until the process can no longer continue. Where large, e.g. 300 mm diameter, wafers are being processed the temperature level across the wafer is difficult to maintain substantially constant. In this system and method the lateral temperature distribution is equalized by a heat sink structure in a chamber immediately under the wafer support on top of the pedestal. A number of spatially distributed wicking posts extend downwardly from a layer of wicking material across the top of the chamber, into a pool of a vaporizable liquid. At hot spots, vaporized liquid is generated and transported to adjacent condensation posts extending up from the liquid. The system thus passively extracts heat to equalize temperatures while recirculating liquid and assuring adequate supply. The free volume above and within the liquid, and the short distances between posts, assure adequate heat transfer rates.
1. A wafer supporting pedestal for maintaining the temperature across a semiconductor wafer substantially uniform as the wafer is heated on its upper side and the pedestal is cooled beneath the wafer, comprising:
a pedestal body having a top surface and a pedestal compartment adjacent the underside of the top surface, the top surface supporting the wafer thereon, the compartment defining a shallow open volume below the top surface and including a sink having vaporizable liquid in the open volume;
a surface of wicking material disposed on the underside of the top surface of the compartment, and including a plurality of distributed first wicking elements extending into vaporizable liquid, but not contacting the bottom of the compartment;
a condensing surface comprising a second plurality of wicking elements protruding up from the bottom of the volume compartment to above the liquid level, but without contacting the top surface of the compartment, and
the lateral distances between the downwardly extending and the upwardly protruding wicking elements each being respectively selected with respect to the transport distance and pressure of the liquid undergoing wicking action to minimize wicking transport distances.
2. An apparatus as set forth in claim 1 above, wherein the pedestal is configured to receive a semiconductor wafer of approximately 300 mm in diameter on the top surface, and wherein the liquid is a vaporizable liquid from the class including ammonia, H 2 O and alcohol.
3. An apparatus as set forth in claim 1 above, wherein the pedestal includes a gas cooling system interjecting gas between the top surface of the pedestal and the adjacent under surface of the wafer, and wherein the pedestal includes a cooling system in operative relation to the pedestal body below the compartment.
4. An apparatus as set forth in claim 3 above, wherein the upper surface of the compartment is covered with a wicking material and a plurality of depending wicking posts extending from the upper surface of the compartment into the liquid, and wherein the protruding elements also comprise condensing posts of wicking material interdigitized among the wicking posts.
5. An apparatus as set forth in claim 4 above, wherein the apparatus includes strength posts extending from the bottom of the housing to the top surface of the pedestal for withstanding interior pressures.
6. A structure providing a lateral heat distribution function for maintaining the temperature of a semiconductor wafer substantially uniform in the lateral direction during a heating process, comprising:
a chamber having a roof and a floor for providing a substantially compact temperature equalizer for contact with a wafer supporting surface disposed on top of the roof that is in heat conductive engagement across the area of a semiconductor wafer;
a plurality of first modular elements having heads with hexagonal outlines nested together to form a wall covering the upper side inside of the roof of the chamber, each hexagonal element having a depending post, and the hexagonal elements and posts each being of a wicking material;
a plurality of second elements, each having a hexagonal head and of like configuration so that they fit together to form a planar base structure covering the floor of the chamber, each of the plurality of second hexagonal elements including an upstanding condensing post, the elements and posts being of wicking material, and nested together to provide a wall covering the floor of the chamber;
a locking element having a substantially planar body for sandwiching between the plurality of the first and the second elements, which nest together into an interdigitated structure, the locking element having a plurality of openings for receiving the upstanding condensing posts of the lower structure and a plurality of spring elements for engaging the tips of the depending posts to urge the plurality of first modular elements into engagement with the upper surface of the chamber, the spring elements each holding a different one of the upper hexagonal heads out of contact with the adjacent condensing posts and the hexagonal heads of the second elements, and
a vaporizable liquid in the chamber to a level wetting both the depending posts and the upstanding posts.