Abstract:
A method of etching a wafer using resonant infrared energy and a filter to control non-uniformities during plasma etch processing. The filter includes a predetermined array or stacked arrangement of variable transmission regions that mirror the spatial etch distortions caused by the plasma etching process. By spatially attenuating the levels of IR energy that reach the wafer, the filter improves uniformity in the etching process. Filters may be designed to compensate for edge fast etching due to macro-loading, asymmetric pumping in a plasma chamber, and magnetic field cusping.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to the manufacture of silicon wafers and, more specifically, to a method of using a filter to control non-uniformities in wafer processing using resonant heating. 
     2. Description of Prior Art 
     Silicon wafers may be processed using plasma etch reactors. Layers of conducting or insulating material are deposited onto a silicon wafer and circuit features are etched into the wafer by bombarding the wafer with a reactive gas and an ion stream in near-vacuum conditions to carve out circuit features. Plasma processing has some drawbacks, however, as it often results in spatial non-uniformity at both the wafer and chip scale. 
     At the wafer scale, the design of the plasma chamber can adversely affect the gas flow profiles and the plasma itself may exacerbate etching at the wafer edge due to the high density of hot electrons and radicals. At the chip scale, loading adversely affects etch profiles that, for example, may have isolated to nested bias. As a result of these shortcomings, the full capability of plasma etching is unrealized and the overall process yields lower amounts of finished product than otherwise possible. 
     Some attempts to alleviate center/edge non-uniformity involve cooling the backside of the wafer with helium. This method does not provide selective or precise control over the etching process, however, as the temperature gradients are smoothed from the back to the front of the wafer and the same chilling substance is used for all zones. Additionally, the plasma heating generally dominates the thermal characteristics of the wafer surface, thereby reducing the impact of any helium cooling. 
     Other attempts to overcome the disadvantages of plasma processing involve the use of masks that reduce the amount of IR heating to selected areas of the wafer or multiple energy sources for differential heating. The systems are generally limited to inhibiting heating at the edge of the wafer or are extremely limited in their ability to provide high-resolution selection. 
     3. Objects and Advantages 
     It is a principal object and advantage of the present invention to improve the overall uniformity of plasma-etched wafers. 
     It is an additional object and advantage of the present invention to provide a method of reducing complex non-uniformities in a plasma-etched wafer. 
     It is a further object and advantage of the present invention to provide a system for reducing complex non-uniformities that result from multiple anomalies in the plasma etching process. 
     Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a method of improving the etching of silicon wafers by using an infrared (IR) heating device which applies resonant energy through an IR filter that spatially attenuates the strength of the radiation to compensate for spatial etch distortions. The use of resonant IR in combination with a filter for selectively controlling the location where energy is applied provides a high degree of resolution, thereby allowing for improved wafer processing. The filter comprises a predetermined array of variable transmission regions that mirror the spatial etch distortions caused by the plasma etching process. By spatially attenuating the levels of IR energy that reach the wafer, the filter improves uniformity in the etching process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a resonance plasma etching system. 
         FIG. 2  is a schematic diagram of the etching process of a chip manufactured according to the present invention. 
         FIG. 3  is a schematic diagram of a chip manufactured according to a prior art method. 
         FIG. 4  is a schematic diagram of a chip manufactured according to the present invention. 
         FIG. 5  is a schematic diagram of edge fast etching of a chip due to micro-loading. 
         FIG. 6  is a schematic diagram of the attenuation of infrared intensity to prevent edge fast etching according to the present invention. 
         FIGS. 7   a  and  7   b  are side elevation and top plan views, respectively, of a filter manufactured according to the present invention to prevent edge fast etching. 
         FIG. 8  is a schematic diagram of asymmetric pumping in a plasma etch reactor. 
         FIG. 9  is a top plan view of the asymmetric etch profile of a wafer subject to asymmetric pumping in a plasma etch reactor. 
         FIG. 10  is a top plan view of a filter manufactured according to the present invention to prevent an asymmetric etch profile. 
         FIG. 11  is a schematic diagram of magnetic filed cusping is an etch reactor having electromagnets. 
         FIG. 12  is a top plan view of the non-uniformities in the etch profile in a wafer subjected to magnetic field cusping. 
         FIG. 13  is a top plan view of a filter manufactured according to the present invention to prevent non-uniformities in the etch profile in a wafer caused by magnetic field cusping. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein like numeral refer to like parts throughout, there is seen in  FIG. 1  a plasma etch system  10  comprising a chamber  12 , a wafer chuck  14 , solenoid coils  16 , a transmissive window  20  position in the top of chamber  12 , and IR sources  22 . A wafer  24  is positioned on top of wafer chuck  14 . Chamber  12  is fitted with an inlet  26  for receiving a reactant gas supply and an exhaust port  28  for expelling reactant gas. 
     A radio frequency (RF) power supply  30  is coupled to solenoid coils  16  and to ground in order to strike and maintain a free radical plasma  32  and an RF bias power supply  34  is coupled to wafer chuck  14  and to ground in order to control forward bias (etch) power. IR sources  22  generate infrared radiation  36 , which pass through window  20  to strike the surface of wafer  24 . 
     As seen in  FIG. 2 , etching occurs when wafer  24 , comprising a substrate  38  and an insulator  40  that has been coated with an etchable conductor layer  42  and a masking layer  44 , is exposed to free radical plasma  32 . Free radical plasma  32  chemically interacts with the surface of wafer  24  to form a secondary compound  46  that, in the presence of heat, will evaporate. As exposure to free radical plasma  32  and evaporation of secondary compound  46  continues, a trench  48  will form in etchable conductor layer  42 . 
     Due to the partial pressure of secondary compound  46 , evaporation may not occur at a temperature that is low enough to prevent damage to other components of wafer  24 . As further seen in  FIG. 2 , selected wavelengths of infrared radiation  36  are applied to wafer  34  in combination with plasma  32  to lower the temperature at which etching will occur. Referring to  FIG. 1 , the wavelength of infrared radiation  36  is controlled by the use of a wavelength filter  52  that filters out undesirable wavelengths while allowing select wavelengths to pass through to window  20 . The particular wavelength of infrared radiation  36  is selected so that it will couple with and excite the vibrational state of the secondary compound formed by the interaction of the surface of wafer  24  and the free radical plasma  32  used in the etching process, thereby selectively heating only those areas of wafer  24  to be etched. 
     For example, plasma etching of a copper-coated wafer  24  in the presence of chlorine gas results in the formation of a layer of copper chloride (CuCl 2 ) in the non-masked areas of wafer  24 . Due to the partial pressure of CuCl 2 , the surface of wafer  24  will be passivated at temperatures below 600 degrees F. and no etching will occur. Radiating with infrared radiation  36  at a resonance wavelength will effectively lower the temperature at which the layer of CuCl 2  formed on the area of wafer  24  will evaporate to form the etching. By contrast, the surrounding areas of wafer  36  that are masked to prevent the formation of CuCl 2  will be heated to a lesser degree as selected wavelength infrared radiation  30  will not induce resonance in those regions. 
     Referring to  FIG. 3 , a wafer  60  processed by prior art, non-resonant infrared radiation  62  forms non-discrete heating zones  64 . Referring to  FIG. 4 , a wafer  70  processed with resonant infrared radiation  72  will, however, form heating zones  74  having a finer resolution. By creating zones  74  having finer resolution, resonant radiation  72  allow for more exact etching and an increased density of circuits in wafer  70 , thereby improving both the quality and overall performance of the etching process. 
     Referring to  FIG. 1 , plasma etch system  10  further includes a filter or mask  50  which spatially attenuates the strength of infrared radiation  30  to compensate for spatial etch distortions, i.e., non-uniformities in the amount of etching that occurs in various regions of a wafer  24 , thereby allowing for improved wafer processing. Filter  50  for spatially attenuating infrared radiation  30  may be separate from wavelength filter  52 , or the functions of both filters  50  and  52  may be combined into a single filter that selects for the resonant frequency and spatially attenuates to remove non-uniformities. 
     Referring to  FIG. 5 , unfiltered etching of wafer  24  may result in edge fast etching  76  due to macro-loading. Referring to  FIGS. 6 and 7 , filter  50  having spatial variations in transmission which mirror or are complementary to the non-uniformities will attenuate the infrared intensity at the edge of wafer  24  to compensate for macro-loading and allow for uniform etching. To prevent edge fast etching, filter  50  has a central region  80  having high transmittance and a peripheral region  82  having low transmittance to slow the etching of the edge of wafer  24 . 
     Referring to  FIG. 8 , asymmetric pumping of plasma  32  in chamber  12  results in the formation of non-uniformities in wafer  24 . When port  28  is positioned on one side of chamber  12 , the non-uniform flow  84  of reactant gas will lead to areas of non-uniform etching. With reference to  FIG. 9 , a non-uniform etch profile  86  is formed on wafer  24  when subjected to asymmetric pumping. Referring to  FIG. 10 , filter  50  may be designed with a series of eccentric regions  88  having gradually decreased transmittance to compensate for etch profile  86  and spatially attenuate the etching of wafer  24  to smooth the non-uniformities. 
     Referring to  FIG. 11 , magnetic field cusping of chamber  12  may also cause non-uniformities in the plasma etching of wafer  24 . During etching, magnetic lines of force from electromagnets  90  positioned around chamber  12  cause “cusp” regions that affect etch uniformity. Referring to  FIG. 12 , non-uniformities  92  are formed in wafer  24  when subjected to magnetic field cusping. Referring to  FIG. 13 , filter  50  may be designed to include complementary regions of variable transmission  94  that mirror non-uniformities  92  and improve uniformity in the etching of wafer  24 . 
     Filter  50  may comprise standard linear variable metallic neutral density filters that are modified to have transmission patterns according to the present invention. The appropriate regions of variable transmission may be created in filter  50  by attenuating the intensity of the incident (IR) beam with metallic coatings. For example, an optical quality glass filter having aluminum coating that is protected by an overcoat may used to attenuate infrared intensity, although other coating materials could also be used. The spatial variations in the attenuating power of filter  50  can be achieved by varying the thickness of the film coating in the appropriate regions of filter  50  to mirror and attenuate the undesirable regions of non-uniformity.