Abstract:
A method and apparatus for processing a workpiece in a chamber by providing an asymmetric flow of process gas and processing the workpiece with the process gas. The asymmetric flow counteracts a non-uniform distribution of reactive species in the chamber. The asymmetric flow can be accomplished by introducing the process gas through a plurality of gas nozzles that communicate through a side wall of the chamber proximate a pump port while pumping gas with a pump coupled to the pump port. The inventive method can be used with a conventional processing chamber by only opening the gas nozzles closest to the pump and blocking any other gas nozzles. Alternatively, the method can be implemented in a processing chamber having gas nozzles located only proximate the pump port.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to semiconductor wafer processing systems. More specifically, the invention relates to a method and apparatus for etching dielectric films. 
     2. Description of the Background Art 
     Semiconductor wafer processing involves processes having multiple steps including deposition steps and etch steps. Many of these processes use a plasma to process the wafer. In a typical etch step, for example a plasma, generated in a process chamber, produces reactive ions, free radicals or both. These reactive species remove (etch) material from the surface of the semiconductor wafer. In the prior art etching systems, the plasma is generated from a process gas introduced to the chamber through the chamber wall by a plurality of (typically four) gas nozzles symmetrically distributed about a pedestal that supports the workpiece in the process chamber. Approximately equal amounts of gas are delivered through each of the four nozzles. A pump, connected to the chamber by a pump port located on one side of the chamber, regulates the pressure in the chamber by continuously exhausting gases. Unfortunately, byproducts of the etch process tend to collect in the vicinity of the pump port. Consequently, there are fewer reactive species near the pump port and more in other parts of the chamber. This skewing of the distribution of reactive species and byproducts in the plasma causes a non-symmetric etching of the workpiece. 
     In a prior art etching system (such as a metal etch DPS chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif.), if symmetric gas flow is utilized (i.e. process gas flows equally through all four nozzles), as in FIGS. 3 a  and  3   b,  the etch contour maps tend to tilt toward the pump, i.e., the etch contour is not symmetric about the center of the wafer. Specifically, FIG. 3 a  illustrates an etch contour map for an oxide wafer and FIG. 3 b  illustrates an etch contour map for a BCB wafer. 
     Thus, there is a need in the art for a method and apparatus for improving the uniformity of the distribution of reactive ions in a plasma process to improve the symmetry of wafer processing. 
     SUMMARY OF THE INVENTION 
     The disadvantages heretofore associated with the prior art are overcome by a method and apparatus for processing a workpiece in a chamber by providing an asymmetric flow of process gas and processing the workpiece with the process gas. The asymmetric flow counteracts a non-uniform distribution of reactive species and process byproducts in the chamber. The asymmetric flow can be accomplished by opening one or more gas nozzles located proximate the pump port and blocking one or more other gas nozzles. Consequently, the process gas flows primarily through the gas nozzles located proximate the pump port. As the process gas and process byproducts produced in the chamber are exhausted through the pump port, the process gas is replenished from the nozzles near the pump. Additional control of the non-uniformity can be achieved by providing process gas through a showerhead attached to a lid of the chamber. The method of the present invention improves the uniformity of, for example, an etch process such as a dielectric etch. 
     The invention may be embodied in an apparatus comprising a processing chamber having a plurality of asymmetrically distributed gas nozzles communicating through a wall of the chamber located proximate a pump port. 
     This invention fulfills the need for a method and apparatus that improves the symmetry of plasma processes such as dielectric etch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a horizontal cross sectional schematic view of a semiconductor wafer processing chamber as employed in the method of the present invention taken along line  1 — 1  of FIG. 2; 
     FIG. 2 depicts a vertical cross sectional schematic view of the processing chamber taken along line  2 — 2  of FIG. 1; 
     FIGS. 3 a-   3   b  depict etch contour maps for BCB and Oxide wafers using the method of the prior art; 
     FIGS. 3 c-   3   d  depict etch contour maps for BCB and Oxide wafers using the method of the present invention; 
     FIG. 4 depicts a horizontal cross sectional schematic view of a semiconductor wafer processing chamber of the present invention; and 
     FIG. 5 depicts a vertical cross sectional schematic view of a semiconductor wafer processing chamber of an alternative embodiment of the present invention. 
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION 
     The present invention is best described in terms of a semiconductor wafer processing chamber such as that depicted in FIGS. 1 and 2. The invention is best understood by referring to FIGS. 1 and 2 simultaneously. The chamber  100  has a side wall  102 , a lid  201  (dome) and a bottom wall  203 . A pump  106  is coupled to the chamber  100  through a pump port  104  that communicates with the interior of the chamber  100 . The pump  106  regulates a pressure within the chamber  100  by pumping out gases from the interior. The invention is generally implemented in a Decoupled Plasma Source (DPS) etch chamber manufactured by Applied Materials Inc. of Santa Clara, Calif. This chamber has four process gas inlet ports that are symmetrically distributed about the sidewall  102  of the chamber  100 . 
     In accordance with the invention, process gas is asymmetrically introduced to the chamber  100 , for example, only through one or more gas nozzles  108  located proximate the pump port  104 . Gas could also be introduced through a single nozzle provided the nozzle lies in a vertical plane that intersects an axis running between a slit valve  110  and the pump port  104 . 
     Process gas flows into the chamber  100  as indicated by arrows  120 . Other gas nozzles such as nozzles  109  may be located opposite the pump port  104 , however, these nozzles  109  are closed to produce a non-uniform gas flow in accordance with the method of the present invention. Alternatively, a small flow of gas can be provided to the nozzles  109  while a larger flow of gas is provided through the nozzles  108 . Asymmetric gas flow is also possible with process gas flow provided from the top of the chamber. For example, a showerhead  202 , attached to the lid  201  of the chamber  100 , delivers process gas through a plurality of apertures  205 . To facilitate asymmetric gas distribution within the chamber, the showerhead apertures  205  are not uniformly distributed across the showerhead  202 . Such a showerhead  202  may be used in conjunction with the nozzles  108  or in lieu of the nozzles  108 . 
     The chamber  100  further includes the necessary elements for processing a wafer. For example, the chamber  100  is equipped with a wafer support  210  (depicted in FIG.  2 ), slit valve  110 , and a robot arm  112 . The wafer support  210  comprises a susceptor  212  mounted to a pedestal  214 . The pedestal  214  is typically fabricated from a metal such as aluminum. The susceptor  212  is typically fabricated from a dielectric material such as a polyimide or ceramic. Normally, a workpiece such as a semiconductor wafer  101  rests on the susceptor  212  during processing. The susceptor  212  includes components such as resistive heaters, bias electrodes or electrostatic chuck electrodes. The latter can be implemented using any number of chucking electrodes and any type of chucking electrode structure including monopolar, bipolar, tripolar, interdigitated, zonal and the like. Similarly, any number or arrangement of heaters can be used including a single heater, or two or more heaters can be used for zoned heating and the like. 
     A workpiece such as a semiconductor wafer  101  is introduced to the interior of the chamber  100  through a slit valve  110 . The robot arm  112  (shown in phantom) positions the wafer  101  on the wafer support  210 . To operate the chamber  100  by the method of the present invention, the gas nozzles  108  located proximate the pump port  104  are opened. Other gas nozzles, such as the nozzles  109  located opposite the pump port  104  are closed. A process gas (such as argon, CF 4  and/or CHF 3  for oxide etch) is introduced to the chamber through the nozzles  108  proximate the pump port  104  as indicated by the arrows  120 . For BCB etch, Cl 2  and/or O 2  are used as the process gases and, for Al etch, Cl 2 , BCl 3 , and N 2  are used as the process gases. 
     The process gas is used to process the wafer  101 . For example, reactive species (e.g., ions, free radicals, or molecules)  132  and byproducts  134  are generated from the process gas by a plasma process occurring in the chamber  100 . The reactive species  132  process the workpiece  101 . For example, in an etch process the reactive species react with the workpiece  101  in such a way as to remove material from the surface of the workpiece. The byproducts  134  tend to collect near the pump port  104  and decrease the local density of reactive species on the side of the wafer  101  proximate the pump port  104 . However, because the process gas introduced proximate the pump port  104 , there is a greater flow of process gas proximate the pump port  104 . This tends to increase the local density of reactive species  132  on the side of the wafer  101  proximate the pump port  104 . As such, the density of reactive species  132  becomes more symmetric about the center of the wafer  101 . Since the processing rate (e.g., the etch rate) depends on the density of reactive species, the processing of the wafer  101  becomes more symmetric. The showerhead  202  may provide an additional flow of process gas that can be adjusted to optimize the symmetry of wafer processing. 
     The chamber can be any type of process chamber suitable for performing plasma enhanced wafer processing such as etch, physical vapor deposition (PVD), chemical vapor deposition (CVD), and the like. The chamber  100  is, for example, an etch chamber such as the DPS chamber manufactured by Applied Materials of Santa Clara, Calif. A coil  220  connected to a first radio frequency (RF) power supply  222  supplies RF energy to inductively ignite and maintain the plasma  230  within the chamber  100 . A second RF power supply  224  is connected to the pedestal  214  which acts as a cathode. Alternately, RF power can be supplied to an RF bias electrode (not shown) within the susceptor  212 . RF voltage supplied by the second RF supply  224  to the cathode controls a bias applied to the workpiece. The bias produces an electric field  232  proximate a surface of the workpiece that is to be etched. Reactive ions  132  from the plasma  230  are accelerated toward the workpiece by the electric field and preferentially etch the workpiece in the direction of the electric field. An asymmetric flow of process gas, e.g., from the gas nozzles  108  and/or the showerhead  202 , controls the etch symmetry of the workpiece as described above. 
     The improved symmetry of wafer processing with a symmetric gas flow is illustrated for a dielectric etch process in FIGS. 3 c-   3   d.  FIG. 3 c  depicts etch contours for a BCB wafer while FIG. 3 d  depicts etch contours for an oxide wafer (e.g., SiO 2 ). 
     When the process gas is introduced only through the two nozzles  108  proximate the pump port  104  the asymmetry (tilting) of the etch contours seen in FIGS. 3 c  and  3   d  is less pronounced. In particular, the BCB wafer etch contour map shown in FIG. 3 c  exhibits contours that are less skewed, i.e., more symmetric about the wafer center, than depicted in FIG. 3 a.  The oxide wafer etch contour map shown in FIG. 3 d  exhibits a large central contour indicative of a greatly improved etch uniformity. 
     These results of the use of asymmetric process gas flow in a DPS etch chamber suggest a new configurations for the semiconductor wafer processing chambers depicted in FIGS. 4 and 5. Each chamber  400 ,  500  has a side wall  402 ,  502  a pump port  404 ,  504  that communicates with the interior of the chamber  400 ,  500 . Pumps  406 ,  506  are coupled to the chambers  400 ,  500  through the pump ports  404 ,  504 . Process gases are asymmetrically introduced to the chamber through one or more gas nozzles  408 ,  508  located mostly proximate the pump ports  404 ,  504 . The process gases flow into the chambers  400 ,  500  as indicated by arrows  420 ,  520 . The nozzles  408  are, for example, distributed parallel to the plane of the wafer  401 , as depicted in FIG.  4 . Alternatively, the nozzles  508  are vertically disposed above and below the pump port  500  within a plane that is perpendicular to the plane of the wafer  507  as depicted in FIG.  5 . Other configurations of the nozzles that lead to asymmetric gas flow are also possible. For example, the tuft chambers  400 ,  500  may be provided with gas nozzles distributed both parallel to and perpendicular to the plane of the wafer, i.e., the nozzles are distributed about the pump port. Alternatively, the chambers  400 ,  500  may be provided with nozzles at any angle with respect to the plane of the wafer. 
     Gas nozzles  409 ,  509  may also be provided opposite the pump ports  404 ,  504  to further control process gas flow. The nozzles  409 ,  509  are, for example distributed symmetrically with respect to a pump port-slit valve axis  403 ,  503 . A lesser flow of gas of provided through the nozzles  409 ,  509  opposite the pump ports  404 ,  504  than through the nozzles  408 ,  508  located proximate the pump ports. Asymmetric gas flow may be further controlled with process gas flow provided through a shower head  516  having a plurality of orifices  505 , depicted in FIG. 5 attached to a lid  501  of the chamber  500 . 
     The chambers  400 ,  500  are, by way of example, etch chambers that further include the necessary elements for processing a wafer  401 ,  501 . For example, the chamber  500  is equipped with wafer support  511  (depicted in FIG.  2 ), slit valve  510 , and robot arm  512 . The wafer support  511  comprises a susceptor  513  mounted to a pedestal  514 . A coil  521  connected to a RF source  522  inductively supplies RF energy to ignite and maintain a plasma  530  within the chamber  500  to process the wafer  501 . 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.