Abstract:
A method for monitoring a device fabrication process. The method includes etching into a wafer disposed inside a chamber and detecting the intensity of a portion of a light reflected from a surface of the wafer and further scattered at a scattering inside surface of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to plasma etch processes, and more particularly to an interferometric method and apparatus for monitoring a plasma etch process. 
     In the fabrication of integrated circuits, the removal of various layers or thin films of materials formed on a silicon wafer to define device patterns is commonly accomplished by means of an etching process. Etching techniques in use include wet, or chemical etching, and dry, or plasma etching. The latter technique is typically dependent upon the generation of reactive species from process gases that are impinged on the surface of the material to be etched. A chemical reaction takes place between the material and these species and the gaseous reaction product is then removed from the surface. 
     An important consideration in all etch processes is control of the extent to which the wafer is etched and determining a time, referred to as the endpoint, at which to end the process. Common methods for monitoring the etch process and determining the endpoint include spectroscopy and interferometry. Interferometric methods known in the prior art include laser interferometry and plasma emission interferometry as disclosed in U.S. Pat. No. 5,450,205 to Sawin et al. In the laser interferometric method, a laser beam I generated by laser  10  is directed through an optical window  12  and onto an area of a wafer  14  undergoing etching within a plasma chamber  16  as shown in FIG.  1 . The intensity of the reflected beam R is detected by a detector  18  and recorded as a function of time. The detector may be a bandpass filter coupled with a silicon photodiode, a spectrometer, or a CCD camera. 
     When the material being etched is relatively transparent to the incident light, such as layer A as shown in FIG. 2, and overlies a reflective surface, such as layer B, the detected light intensity goes through a series of maxima and minima. As layer A is transparent to the incident light, the incident light is both reflected from the upper surface of the layer A and is refracted through the material. At the reflective surface of layer B, the refracted light is also reflected upwardly through layer A, exiting layer A to interfere with the light reflected from the upper surface of layer A. As layer A is etched, the optical path through layer A decreases in length resulting in varying interference patterns. 
     Plasma emission interferometry also analyzes the interference of light reflected from the surface of a wafer but uses etch reactor plasma optical emission as the light source. As shown in FIG. 3, incident light I′ generated from plasma emission  20  formed within the plasma chamber  22  is reflected from the surface of a wafer  24  disposed within the chamber  22 . The reflected light R′ from the wafer  24  passes through an optical window  26  and is detected by a detector  28 . 
     A plasma chamber  30  having a top portion  32  formed of a dielectric material transmissive to radiation is shown in FIG.  4 . In the case where the dielectric material is transparent, such as fused silica, the top portion  32  can serve as an optical window  33 . As shown, a light source  34  provides an incident beam I″ which illuminates the surface of a wafer  36  through the optical window  33 . The reflected light R″ exits a plasma chamber  38  through the optical window  33  and is detected by detector  39 . Although not shown, optical emission generated by the plasma may also be detected by the detector  39  in the case where no light source  34  is used. 
     A common problem with the prior art systems shown in FIGS.  1  and  3 - 4  relates to the difficulty in maintaining the optical quality of a window exposed to the plasma. The plasma either etches the window, in which case the window loses its clarity, or the plasma deposits material onto the window, which also leads to a loss of clarity. In the case of optical window  33  shown in FIG. 4, these problems are further exacerbated by the fact that a bottom surface  31  facing the plasma  35  of the top portion  32  is typically roughened. Deposited materials adhere better to roughened surfaces than to smooth surfaces and are less likely to flake off onto the wafer being etched. As a consequence of roughening the bottom surface  31 , the optical window  33  becomes translucent rather than transparent and is not very useful for as an optical window for prior art interferometric monitoring methods. 
     A solution to maintaining the optical quality of a window is disclosed in pending application Ser. No. 09/282,519 to Ni et al. assigned to LAM Research Corporation. With reference to FIG. 5, a plasma chamber  40  includes a radiation transmissive top portion  42  having a recessed optical window  44  formed therethrough. Process gas flows into the plasma chamber  40  through an inlet  45  in communication with a prechamber  46 , the prechamber being in communication with the interior of the plasma chamber  40 . The flow of process gases prevents the plasma  47  from etching or depositing material on the optical window  44 . Interferometry is then performed conventionally using a light source  48  and detector  49 . 
     While the optical window  44  works optically well, it suffers from the disadvantage of increasing the cost of the fused silica dielectric window. In addition to the cost of machining a hole in the dielectric window to accommodate the prechamber  46 , the window is structurally weakened by the machining of the hole. As the dielectric window serves as portion of a vacuum chamber, it must be made thicker to restore the loss in structural strength. This further increases the cost of the dielectric window and reduces the effectiveness of the dielectric window in coupling the radiation to the plasma. An additional drawback of the recessed window solution disclosed is that the top center of the plasma chamber is not the optimum location for the process gas feed for all purposes. 
     It would therefore be desirable to be able to detect interferometric signals from a wafer being etched without incurring the additional costs, shifting the process, or constraining the gas injection as is required by prior art methods of keeping the optical window clean. 
     SUMMARY OF THE INVENTION 
     The present invention provides an interferometric method and apparatus for monitoring a plasma etch process that interposes a diffusing or scattering element between the wafer and the detector. The diffusing or scattering element eliminates the need for a hard-to-maintain transparent optical window located in the top wall of the chamber. It either replaces the transparent window, or allows the transparent window to be moved from a position in the top wall of the chamber to a position in the side wall of the chamber, wherein the window is less susceptible to being degraded by high-density plasma. 
     More particularly, the present invention is embodied in a plasma chamber having a top wall formed of fused silica, the top wall having a top surface and a bottom surface facing the interior of the plasma chamber. In a first embodiment of the invention, light generated by plasma emission is reflected from the wafer, is scattered at the bottom surface of the top wall, and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer. 
     In another embodiment of the invention, a light source is provided for illuminating the wafer. Light from the light source passes through the top surface of the top wall and is diffused at the bottom surface of the top wall. The diffused light from the light source illuminates the wafer and is reflected from the wafer. The light reflected from the wafer illuminates the bottom surface of the top wall, is scattered by that surface and is transmitted through the top surface of the top wall. Detecting apparatus comprising a lens, optical fiber and a detector detect the light opposite the top wall from the wafer. 
     In another embodiment of the invention, a screen is disposed inside the plasma chamber. Light from plasma emission is reflected from the wafer, scattered from the screen and detected through a viewing window by a detecting apparatus. 
     In another embodiment of the invention, a screen is disposed inside the plasma chamber and a light source illuminates the wafer through a window. Light reflected from the wafer is scattered by the screen and is detected through a viewing window. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following description of the invention and a study of the several figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, with like reference numerals designating like elements. 
     FIG. 1 is a schematic view of an interferometric apparatus of the prior art. 
     FIG. 2 is a graphical representation showing light both reflected and refracted to produce interference maxima and minima. 
     FIG. 3 is a schematic view of another interferometric apparatus of the prior art. 
     FIG. 4 is a schematic view of yet another interferometric apparatus of the prior art. 
     FIG. 5 is a schematic view of another interferometric apparatus of the prior art. 
     FIG. 6 is a schematic view of a first embodiment of the invention. 
     FIG. 7 is a graph showing interference patterns detected in accordance with the present invention. 
     FIG. 8 is a schematic view of a second embodiment of the invention. 
     FIG. 9 is a schematic view of a third embodiment of the invention. 
     FIG. 10 is a schematic view of a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with a first embodiment of the invention, a plasma chamber  50  comprises a top wall  52  formed of fused silica as shown in FIG.  6 . Top wall  52  includes a roughened surface  54  which faces the interior of plasma chamber  50  and a top surface  53 . The roughened surface  54  is provided to prevent material deposited on the roughened surface  54  from flaking off onto a wafer  55 . A detecting apparatus, generally designated  60 , is disposed outside of plasma chamber  50  and includes a lens  62  and a detector  64 . Light generated by plasma emission is reflected from the wafer  55 , scattered at the roughened surface  54  of the top wall  52  and transmitted through the top surface  53  of the top wall  52 . The transmitted light is then detected by detecting apparatus  60 . In this manner interference patterns such as those shown in FIG. 7 are detectable for use in monitoring the etch process. 
     During the plasma etch process, the roughened surface  54  undergoes localized etching and material deposition due to the positioning of a radiation inducing coil (not shown). The detecting apparatus  60  is preferably positioned at a location in such manner that the detected light is being transmitted through the location of the roughened surface  54  being etched. In this configuration, the detected light does not have to traverse material deposits which may be opaque. The effect of plasma etching on the roughened surface is typically to further roughen the surface. The additional roughening does not significantly alter the optical diffusing properties of the surface. 
     A second embodiment of the invention is shown in FIG. 8. A plasma chamber  70  comprises a top wall  72  formed of fused silica. The top wall  72  includes a roughened surface  74  which faces the interior of plasma chamber  70  and a top surface  73 . The roughened surface  74  is provided to prevent material deposited on the roughened surface  74  from flaking off onto a wafer  75 . A detecting apparatus, generally designated  80 , is disposed outside of plasma chamber  70  and includes a lens  82  and a detector  84 . Incident light provided by a light source  76  is scattered by the top wall  72 , reflected from the wafer  55 , scattered at the roughened surface  74  of the top wall  72 , and transmitted through the top surface  73  of the top wall  72 . The transmitted light is then detected by detecting apparatus  80 . In this manner interference patterns are detectable for use in monitoring the etch process. 
     A third embodiment of the invention is shown in FIG. 9. A scattering screen  90 , preferably a ceramic screen, is disposed within a plasma chamber  92 . Incident light provided by a light source  94  is reflected from a wafer  95 , scattered and reflected from the screen  90  and detected by a detecting apparatus generally designated  100  through a viewing window  96 . In this manner interference patterns are detectable for use in monitoring the etch process. 
     A fourth embodiment of the invention is shown in FIG. 10. A scattering screen  110 , preferably a ceramic screen, is disposed within a plasma chamber  112 . Light generated by plasma emission is reflected from the wafer  115 , scattered and reflected from the screen  110  and detected by a detecting apparatus generally designated  120  through a viewing window  116 . Window  116  is preferably located in the side wall of the chamber away from the region of highest plasma density near the top wall of the chamber. At this location, plasma etching of the window is greatly reduced and deposition of material onto the window can be controlled by standard methods such as heating the window. In this manner interference patterns are detectable for use in monitoring the etch process. 
     Although only a few embodiments of the present invention have been described in detail herein, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. In particular, while the detecting apparatus has been described as comprising a lens and a detector, the use of any optical detecting apparatus is within the scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.