Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to applications entitled “Multi-Zone Resistance Heater,” U.S. Provisional Ser. No. 60/156,595 filed Sep. 29, 1999; and PCT/US00/25503, entitled “Multi-Zone Resistance Heater,” filed Sep. 18, 2000. Each of those applications is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a method of and a system for controlling uniformity when etching semiconductor wafers, and more particularly to a method of and a system for controlling a temperature of an electrode (e.g., an electrode of any of: silicon, anodized aluminum, carbon and silicon carbide). 
     2. Discussion of the Background 
     Plasma systems for the etching of silicon process wafers are in widespread usage. In some embodiments, such plasma systems have an electrode plate attached to the upper electrode surface for two primary purposes: (1) to present a surface of a material compatible with the process specific chemistry; and (2) to act as a source for scavenging species in the plasma For example, in some applications where an oxide film is etched from a surface of a semiconductor wafer, RF power is applied to the plasma from an upper aluminum electrode, through a silicon electrode plate. The silicon electrode plate enhances the selectivity of oxide-to-silicon etching because when the silicon electrode plate is bombarded by high energy ions from the plasma, silicon is introduced into the system. The silicon subsequently scavenges fluorine radicals and in doing so, the selectivity of the etching of oxide to the etching of silicon can be improved. 
     Known plasma etchers using silicon electrodes to achieve good selectivity of oxide-to-silicon etching have the silicon electrodes heated by the energy imparted thereto from the RF plasma However, a drawback of such etchers is that the selectivity of the oxide-to-silicon etch is very sensitive to the temperature of the silicon plate. Since the silicon plate in some reactors is heated only by the energy imparted thereto by the plasma, the heat-up time for the silicon plate to reach steady-state conditions is generally long, especially when compared to the relatively short etch time. Therefore, it takes several semiconductor wafers to be etched before the silicon electrode reaches the appropriate temperature for steady-state conditions. This results in the undesirable so-called “first wafers effect,” wherein the first wafers to be etched are unusable thereafter because the silicon electrode has not been heated to a temperature where it is fully effective. 
     Historically, a solution to the “first wafers effect” has been to run “dummy” wafers through the system to bring the electrode up to the desired operating temperature before committing device wafers to the process. However, this solution is wasteful of silicon material, time, throughput of the machine, and money. 
     Another solution has been to use heaters to heat the process wafer in the plasma chamber. For example, the stripping of photoresist by hydrogen plasma, which heats the process wafer to a temperature of between 100° C. and 225° C., is disclosed in U.S. Pat. No. 4,201,579. On the other hand, U.S. Pat. No. 5,215,619 discloses the heating of the anode surfaces (i.e., both the walls and the gas manifold) to achieve a high etching rate and in-situ self-cleaning capability. U.S. Pat. No. 5,487,786 (hereinafter “the &#39;786 patent”) discloses heating of a high frequency application electrode to reduce the formation of silicon powder contamination during the chemical vapor deposition of silicon-hydrogen (Si—H). However, the electrode heaters of the &#39;786 patent are in two parts, wherein a first part is an annular ring clamped to the edge of the front side of the electrode and the second part is physically separated from, but in close proximity to, the sides and back of the electrode. 
     Another drawback of conventional plasma etch apparatus and methods is that there is often poor thermal contact between the electrode and the upper electrode structure. Because of the poor thermal contact between the electrode and the upper electrode structure, the temperature of the electrode may not be uniform. Typically, the electrode exhibits strong temperature non-uniformities, particularly a strong radial temperature gradient, which will cause an undesirable radial etching non-uniformity resulting in non-uniform etching on the process wafers. 
     Thus, both the “first wafers effect” and the “non-uniform etching effect” cause significant undesirable problems for wafer-to-wafer uniformity and spatial uniformity of the etch rate and etch selectivity. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of and a structure for quickly heating up an electrode (preferably a consumable electrode (e.g., a silicon, anodized aluminum or silicon carbide electrode) wherein the erosion of the electrode is sensitive to temperature) prior to the first wafer to be etched, and maintaining both a (temporally) stationary and a (spatially) homogeneous electrode temperature. This is accomplished by either embedding resistive heater elements within a housing containing an upper electrode or recessing resistive heater elements in the electrode so that the resistive heater elements (e.g., formed across multiple zones) are flush with the upper flat surface of the electrode. By preheating the electrode to the appropriate operating temperature, the “first wafer effect” can be eliminated along with its corresponding non-uniform etching of a semiconductor wafer. Further, by utilizing a multi-zone type heater, the temperature non-uniformity can be minimized. 
     A multi-zone heater embodiment of the present invention ameliorates both the first wafer effect and the non-uniform temperature effect of the electrode. The first wafer effect is ameliorated because the electrode is heated to the desired temperature prior to beginning to etch the first wafer. The non-uniform temperature is ameliorated because multi-zone heaters enable the power to each zone to be adjusted individually. Such a (multi-zone) heater can be achieved (1) using a heater integrated with the electrode or (2) using the electrode as a heater itself. 
     A first embodiment of the present invention uses at least one resistive electrode heater element to preheat the electrode(s) in a plasma or reactive ion etching system such that the temperature of the electrode(s) is stable and constant throughout the entire etching cycle. A second embodiment of the present invention uses at least one diffused region in the electrode(s) as the heating element(s) to provide the preheating and temperature controlling functions. 
     Another object of the present invention is to monitor the temperature of the electrode(s). Such an object is addressed by including temperature sensing elements in the heater/electrode system. According to one aspect of the present invention, diffused p-n junction diodes are fabricated in the electrode(s) as the temperature sensing elements. 
    
    
     
       BRIFF DESCRIPTION OF THE DRAWING FIGURES 
       A more complete appreciation of the present invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a cross-sectional view of a first embodiment of the present invention showing an upper electrode structure, wherein the upper electrode structure includes an inset plate with resistive heater elements embedded therein; 
         FIG. 1B  is a cross-sectional view taken through line  1 B— 1 B of  FIG. 1A  showing a first embodiment comprising a five-zoned pattern for the resistive heater elements embedded within the plate; 
         FIG. 1C  is a cross-sectional view taken through line  1 B— 1 B of  FIG. 1A  showing a second embodiment comprising a five-zoned concentric ring pattern for the resistive heater elements embedded within the plate; 
         FIG. 2  is a cross-sectional view of a first variation of the first embodiment of the present invention showing the details of the housing; 
         FIG. 3  is a cross-sectional view of a second variation of the first embodiment of the present invention showing the details of the housing, wherein the plate is separated from the electrode by a metal (e.g., aluminum) electrode plate; 
         FIG. 4A  is a cross-sectional view of the second embodiment of the present invention showing the housing opposite an electrode in which the resistive heater elements are recessed so as to be flush with the upper surface of the electrode; 
         FIG. 4B  is a cross-sectional view taken through line IVB—IVB of  FIG. 4A  showing the five-zoned pattern of the resistive heater elements embedded in the electrode similar to that shown in  FIG. 1B ; 
         FIG. 5  is a cross-sectional view of the second embodiment of the present invention through the electrode plate showing either P- or N-type diffused resistive heater elements recessed in the electrode so as to be flush with the upper surface of the electrode and openings in the silicon oxide coating the electrode; 
         FIG. 6  is a cross-sectional view of the second embodiment of the present invention showing the use of a bolt to connect wires passed through the RF transmission feed of the upper electrode to contact surfaces with the electrode; 
         FIG. 7  is a schematic showing a housing and electrode/semiconductor wafer within a processing chamber which is connected to a centralized computer for controlling numerous functions of the wafer etching process; and 
         FIG. 8  is a schematic showing a computer system for controlling the temperature of the electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawing figures, like reference numerals designate identical or corresponding parts throughout the several views.  FIG. 1A  illustrates a first embodiment of the present invention, in which resistive heater elements  1  are embedded in a plate  2  (e.g., made of quartz, alumina or generally an electrically insulating material that has a low coefficient of thermal expansion and is compatible with an etch process). In turn, the plate  2  is inset in and affixed to an electrode housing  3 . The electrode housing  3  is made of metal (e.g., aluminum) and it is machined with an inset to receive the plate  2 . Furthermore, the outer edge of electrode housing  3  is machined with a slight convexity in order to be flush with the convexity of the bottom surface  3 A of plate  2 . As stated, the plate  2  is machined so that a bottom surface  3 A thereof has a slight convexity (e.g., approximately 0.025 to 2 millimeters). An electrode  4  (e.g., an electrode of any of: silicon, anodized aluminum, carbon and silicon carbide) is attached to the integrated housing  3  and the plate  2  structure (e.g. by bolts at several locations about the periphery of the electrode  4 ) such that the upper surface  4 A of electrode plate  4  is pressed against the bottom surface  3 A of the plate  2 . The electrode  4  has a flat upper surface  4 A facing the convex bottom surface  3 A of the housing  3  and a slightly concave bottom surface  4 B. The convexity of the bottom surface  3 A of plate  2  and the flatness of the upper surface  4 A of electrode  4  create a spatially homogeneous mechanical pressure between the two surfaces when the electrode  4  is pressed against the plate  2  and attached to the electrode housing  3 . This resultant mechanical pressure leads to an improved physical, thermal and electrical contact between the two surfaces. 
       FIG. 1B  is a cross-sectional view through line  1 B— 1 B of  FIG. 1A .  FIG. 1B  illustrates a preferred pattern of the resistive heater elements  1  embedded within the plate  2 . The preferred pattern of the resistive heater elements  1  includes five different zones Z 1 , Z 2 , Z 3 , Z 4 , and Z 5 . Zones Z 1 –Z 4  are outer zones and zone Z 5  is an inner zone. The boundaries of the five zones Z 1 –Z 5  are shown in  FIG. 1B  by dashed lines. Each of the resistive heater elements  1  in one of the five zones Z 1 –Z 5  is supplied with power from a separate power source (not shown). By adjusting the power to each of the zones Z 1 –Z 5 , the temperature of each zone Z 1 –Z 5  can be controlled independently. Thus, the desired temperature and temperature uniformity of the electrode  4  can be achieved. Furthermore, an RF filter or an RF choke (see  FIG. 6 ) can be employed to isolate the power supplied to the resistive heater elements  1  of each of the zones Z 1 –Z 5  from RF energy supplied to a plasma. 
     A second configuration of the heater elements  1  can take the form of a ring pattern wherein a multiplicity of concentric ring heating elements include a multiplicity of radial heating zones. For instance,  FIG. 1C  presents a concentric ring heating configuration with 5 radial heating zones, wherein the contacts for each concentric ring heating element are alternately rotated 180 degrees between adjacent rings to permit a more azimuthally symmetric design. In fact, the preferred contact rotation may be every 72 degrees. The design shown in  FIG. 1C  can permit highly resolved radial control of the electrode temperature; however, it would lack control of azimuthal temperature variations. 
     As stated above, the slight convexity of the bottom surface  3 A of the plate  2  and the electrode housing  3  permits good thermal contact between the electrode  4  and the plate  2  having the resistive heater elements  1  embedded therein. When the flat back side  2 A of the plate  2  is pressed against and attached to the inner surface  3 B of the electrode housing  3 , a good spatially homogeneous thermal contact can be achieved as a result of applied mechanical pressure. Further, when the flat upper surface  4 A of the electrode  4  is pressed against the resultant convex surface  3 A of the plate  2  and the electrode housing  3 , a good spatially homogeneous thermal contact is also achieved. The slight concavity of the bottom surface  4 B of the electrode  4  compensates for bending when the electrode  4  is pressed against the convex bottom surface  3 A of the plate  2  and the electrode housing  3 . 
       FIG. 2  shows a first variation of the first embodiment of the present invention, wherein direct current or DC power is communicated to the resistive heater elements  1 . For example, an electrical conduit  5  may be passed along side of a gas conduit  6  through a RF transmission feed  7  at the top of the electrode housing  3  as shown in  FIG. 2 . Wires  8 , as illustrated by dashed lines, may be passed through the electrical conduit  5  and also through gas baffle plates  9  located between the inner surface  3 B of the housing  3  and the flat back side  2 A of the plate  2 . The wires  8  may be connected directly, such as by soldering, to the resistive heating elements  1  embedded in the plate  2 . Furthermore, for gas distribution to the processing chamber (see  FIG. 7 ), the plate  2  has a plurality of gas orifices  10  as shown in cross-section in  FIG. 2  by dashed lines. 
     The temperature of the plate  2  and the resistive heater elements  1  within each of the five zones Z 1 –Z 5  is regulated, (e.g., by monitoring the resistance of each of the resistive heating elements  1  and then sensing the temperature as a result of the dependence of the material resistivity). 
       FIG. 3  illustrates a second variation of the first embodiment of the present invention, wherein the plate  2  is entirely embedded within the electrode housing  3  via an aluminum electrode plate  11 . Thus, the aluminum electrode plate  11  separates the plate  2  from the electrode  4 . The gas orifices  10  through the plate  2  are aligned with corresponding smaller orifices  12  through the aluminum electrode plate  11 , which in turn are aligned with correspondingly smaller orifices  13  through the electrode  4  as shown in  FIG. 3 . This arrangement of smaller and smaller orifices  10 ,  12  and  13  allows for the passage of gas through a showerhead inject plate to the processing chamber (see  FIG. 7 ). The electrical connections of the second variation of the first embodiment of the present invention shown in  FIG. 3  are similar to the electrical connections of the first variation of the first embodiment of the present invention shown in  FIG. 2 . 
     Referring to  FIG. 4A , a second embodiment of the present invention is shown, wherein resistive heater elements  1  are recessed in the electrode  4  so that the top surface  1 A of the resistive heater elements  1  are flush with the top flat surface  4 A of the electrode  4 . The electrode housing  3  is preferably made of aluminum and has a slightly convex bottom surface  3 A. However, unlike the first embodiment shown in  FIGS. 1A ,  2 , and  3 , the electrode housing  3  of the second embodiment has no plate inset therein and attached thereto. 
       FIG. 4B  is a cross-sectional view through line IVB—IVB of  FIG. 4A . Similar to  FIG. 1B  of the first embodiment of the present invention,  FIG. 4B  illustrates a preferred pattern of the resistive heater elements  1  which is the same as that described for  FIG. 1B  above. However, a concentric ring pattern may also be employed as shown in  FIG. 1C  where greater radial control is preferred and azimuthal asymmetries are negligible. 
       FIG. 5  illustrates a partial cross-section through the electrode  4  of  FIG. 4B . The electrode  4  has diffused resistive heater elements  1  which are doped either P- or N-type. The resistive heater elements  1  are recessed in the top flat surface  4 A of the electrode  4  of the second embodiment of the present invention to function similarly to the resistive heating elements  1  embedded in the plate  2  of the first embodiment of the present invention. The recessing of the resistive heater elements  1  so as to be flush with the top surface  4 A of the electrode  4  may be done by oxide masking and diffusion. 
     The temperature of the electrode is preferably controllable. First, the electrode  4  is oxidized to form a thin (e.g., approximately 1 micrometer thick) SiO 2  layer  14 . Then, the semiconductor wafer (see  FIG. 6 ) is coated with a photo-resist using standard photo-resist techniques, such as spin coating. The pattern of the resistive heater elements  1  is then exposed using a mask having the appropriate pattern for the resistive heater elements  1 . The pattern of the resistive heater element is etched into the SiO 2  oxide layer  14  using either wet or dry etch techniques. After etching the pattern into the oxide and removing the photo-resist the appropriate impurity (from either a Group III element or a Group V element) is diffused into the electrode  4  through the openings  15  in the SiO 2  oxide layer  14 . Suitable metallic contacts may then be made to the diffused regions in order to provide the means for applying DC power to the resistive heating elements  1 . 
     The resistance of the resistive heater elements  1  may be controlled by means of controlling the time and temperature of the diffusion. Since the control of the actual temperature of the electrode  4  is important, it should be possible to form an array of p-n junction diodes at the same time as the fabrication of the resistive heating elements  1 , possibly using an extra process step. The forward voltage drop of the diodes, Vf, which is very predictably a function of temperature, could be used to monitor temperature. 
     Due to the inherent physics of silicon as a semiconductor wafer, and depending on the doping level of the electrode, this system would have an absolute upper limit of operation of 300 C. (575 K). The second embodiment of the present invention may have an operability issue because of the inherent upper limit of the operating temperature being about 300 C. as a result of the physics of using a semiconductor as a heater. However, the 300 C. upper limit is above the normally desired operating temperature and therefore, should not be a limitation. 
     As in the first embodiment, electrical connection of the DC power supplies to the resistive heater elements  1  are provided. Wires (not shown) are passed through the RF transmission feed  7  of the upper electrode housing  3  to contacts  16  located adjacent to the surface in contact with the electrode  4  as shown in  FIG. 6 . The contacts  16  and wires (not shown) are insulated from the surrounding conducting structure via insulation  17 . A bolt  18  connecting the electrode  4  to the electrode housing  3  may force contact between the aligned contacts  16  in the electrode housing  3  and electrode  4 . 
     In any of the above-described embodiments, the power delivered to individual zones Z 1 –Z 5  may be independently controlled by commands accepted from a centralized computer  100 , as shown in  FIGS. 7 and 8 . Referring to  FIG. 7 , the computer  100  can be employed to control other functions and can communicate with a control processor  20 . 
     In turn, the control processor  20  commands the DC power level output from the DC power supply  21 . In one embodiment of the present invention, an RF choke or filter  22  is inserted between the DC power supply  21  and the resistive heater elements  1  so as to isolate the DC power supply  21  from the applied RF signals supplied from the RF power supply  23 . Moreover, in yet another embodiment, the match network  24  enables proper RF power matching when generating the plasma. The gas box  25 , along with the pump  26  regulate gas flow and pressure within the processing chamber  27 . 
       FIG. 8  illustrates a computer system for communicating with a control processor  20  to command the DC power level output from the DC power supply  21  to the resistive heating elements  1  which are embedded within the plate  2  of the first embodiment of the present invention or recessed in the electrode  4  of the second embodiment of the present invention. The computer system includes a computer  100  to implement the method of the present invention. The computer  100  includes a computer housing  102  which houses the motherboard  104 . The motherboard  104  contains a central processing unit (hereinafter “CPU”)  106 , a memory  108  (e.g., DRAM, ROM, EPROM, EEPROM, SRAM, SDRAM, and Flash RAM), and other optional special purpose logic devices (e.g., ASICs) or configurable logic devices (e.g., GAL and reprogrammable FPGA). The memory  108  stores information for the temperature of the resistive heater elements  1 , etc. Preferably, the memory  108  stores information even when the upper electrode housing  3  and the electrode  4  are turned off and not in use. 
     The computer  100  also includes plural input devices (e.g., a keyboard  122  and mouse  124 ) and a display card  110  for controlling the monitor  120 . In addition, the computer system  100  further includes a floppy disk drive  114 ; other removable media devices (e.g. compact disc  119 , tape, and removable magneto-optical media (not shown)); and a hard disk  112 , or other fixed, high density media drives, connected using an appropriate device bus (e.g., a SCSI bus, an Enhanced IDE bus, or an Ultra DMA bus). Also connected to the same device bus or another device bus, the computer  100  may additionally include a compact disc reader  118 , a compact disc reader/writer (not shown) or a compact disc jukebox (not shown). Although the compact disc  119  is shown in a CD caddy, the compact disc  119  can be inserted directly into CD-ROM drives which do not require caddies. In addition, a printer (not shown) also provides printed listings of the electrode temperature. 
     As stated above, the computer system includes at least one computer readable medium. Examples of computer readable media are compact discs  119 , hard disks  112 , floppy discs, tape, magneto-optical discs, PROMs, (EPROM, EEPROM, Flash EPROM), DRAM, SRAM, SDRAM, etc. Stored on any one or on a combination of computer readable media, the present invention includes software for controlling both the hardware of the computer  100  and for enabling the computer  100  to interact with a human user. Such software may include, but is not limited to, device drivers, operating systems, and user applications, such as development tools. Such computer readable media further includes the computer program product of the present invention for the method of controlling electrode temperature. The computer code devices of the present invention can be any interpreted or executable code mechanism, including but not limit to, scripts, interpreters, dynamic link libraries, Java classes, and complete executable programs. 
     The first and second embodiments of the present invention and several variations thereof have been described for multi-zone heaters for a single-electrode upper structure. However, the concept is easily extended to a segmented upper electrode, wherein the pattern of resistive heater elements is either segmented per each sub-electrode, embedded within a single electrode plate, or embedded within each sub-electrode silicon plate. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore understood that, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.

Technology Category: 5