Abstract:
Low dielectric inter-metal dielectric (IMD) layers made of hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ) spin-on-glass do not have good thermal conductivity as compared to regular oxides, in addition the adhesion of HSQ or MSQ is worse than that of oxide to oxide layers Methods are disclosed and illustrated to improve the heat transfer by providing metal dummy plugs under and/or around bonding pads or between metallization layers. The arrangement and numbers of dummy plugs depends on the heat to be transferred and varies with the application. Good thermal conductivity is of particular importance because the effects of high local temperature around bonding pads during chip bonding results in thermal stress and delamination of the IMD layers. The use of bonding pads provides other benefits as well.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to methods of fabricating a semiconductor wafer, and more particularly to methods of thermal stress release by providing dummy plugs in a low-k, low thermal conductivity dielectric. 
     2. Description of the Related Art 
     Materials used for intermetal dielectrics (IMDs) in the related art comprise plasma enhanced silicon oxide (PE-OX), plasma enhanced tetraethyl orthosilicate glass (PETEOS), spin-on-glass (available from Allied Signal as #314), low-k dielectric (Dow-Corning FO X -15), and others. Along with their advantages come certain disadvantages such as the problem of moisture uptake (through etching) and release, contributing to bonding failure or poor thermal conductivity in case of the low-k dielectric. 
     FIG. 1 illustrates a typical IMD arrangement of the prior art, and having the problems discussed in the previous paragraph, showing a partial cross-section of a semiconductor wafer  10 . Reference number  11  indicates the top of the wafer, with a bonding pad  12  on top of  11 . A metal plug  13  connects the bonding pad  12  to a metal line (not shown). A PE-OX or PETEOS layer  14  is deposited on top of a SOG or low-k dielectric layer  15 . Layer  15  in turn is deposited on a liner PE-OX layer  16 . 
     In ultra-large-scale-integration (ULSI) interconnect scaling has become the performance limiting factor for new designs. Both interconnect resistance and capacitance are limiting factors to overall performance. To minimize crosstalk and RC time delay it is very important to keep the inter-metal dielectric capacitance as low as possible. This has been done through the introduction of inter-metal dielectrics (IMD&#39;s) with a low dielectric constant (low-k) such as hydrogen silsesquioxane (HSQ) and methyl silsesquioxane (MSQ). One drawback of these materials is that their thermal conductivity is not as good as that of regular oxides such as silicon oxide (SiO 2 ). The reduced thermal conductivity of these low-k insulators introduces thermal stress during chip bonding, because the local temperature around bonding pads is then especially high. This thermal stress may cause delamination of inter-metal dielectric and metallization layers. 
     One method to alleviate the poor thermal conductivity is to introduce “dummy plugs” made of various thermally conductive materials, which transfer the excessive heat around the bonding pads and distribute it to other layers of the semiconductor wafer. Another problem faced with low-k IMDs is that their adhesion to oxides is worse than that of oxide to oxide. Dummy plugs alleviate this problem by tying the IMD layer to the other layers. 
     The use of dummy pads is not new in the manufacture of semiconductor chips. There are a number of U.S. patents that describe their use and which are listed below. 
     U.S. Pat. No. 5,629,236 (Wada et al.) shows a method of converting polycrystal A 1  wiring to single crystal A 1  (connected by vias) to reduce electromigration. A dummy plug is used to supply atoms to reduce electromigration. 
     U.S. Pat. No. 5,430,325 (Sawada et al.) describes a chip having a dummy pattern. The dummy pattern reduces recognition errors. 
     U.S. Pat. No. 5,319,224 (Sakashita et al.) teaches a method for a bond pad layout. 
     It should be noted that none of the above cited examples of the related prior art address alleviating the poor thermal conductivity and adhesion problems of HSQ and MSQ. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide methods to improve thermal conductivity and to reduce the resulting thermal stress and subsequent delamination, when using low-k, low thermal conductivity silsesquioxane inter-metal dielectric layers in the manufacture of semiconductor wafers. 
     Another object of the present invention is to improve the adhesion of these low-k inter-metal dielectric layers to other types of oxide layers. 
     A further object of the present invention is to improve the removal of moisture taken up during the etching step. 
     These objects have been achieved by providing heat transferring metal dummy plugs under and/or outside bonding pads, placing dummy plugs in bonding pad areas that are unused, and by providing dummy plugs between any two metallization layers. Dummy plugs also provide increased mechanical strength by tying together the different types of layers encountered in a semiconductor wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art partial cross section of the top layers of a semiconductor wafer showing a bonding pad and via filled with a metal plug. 
     FIG. 2 a  is a top view of a part of a semiconductor wafer showing a bonding pad and dummy plugs of the present invention. 
     FIG. 2 b  is a cross-section of FIG. 2 a.    
     FIG. 3 a  is a top view of a part of a semiconductor wafer showing a bonding pad surrounded by dummy plugs of the present invention. 
     FIG. 3 b  is a cross-section of FIG. 3 a.    
     FIG. 4 a  is a top view of a part of a semiconductor wafer showing dummy plugs in an unused bonding pad area of the present invention. 
     FIG. 4 b  is a cross-section of FIG. 4 a.    
     FIG. 5 a  is a top view of a part of a semiconductor wafer showing another arrangement of dummy plugs of the present invention. 
     FIG. 5 b  is a cross-section of FIG. 5 a  showing dummy plugs between two metallization layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The methods of the present invention of providing metal dummy plugs offer a number of features which advance the art of semiconductor wafer manufacture by promoting better heat dissipation, improved adhesion of low-k inter-metal dielectric (IMD) layers, reducing thermal stress delamination, and increased moisture release. Heat transfer away from the bonding pad to a lower metallization layer occurs through the metal of the dummy plug. As such, the number and arrangement of these plugs is of significance since the heat transfer depends on these parameters. Dummy plugs can be used either underneath and/or around bonding pads as well as in unused bonding pad areas. Dummy plugs can be implemented between the semiconductor wafer top surface and any metal level, not just the last metal layer i.e. the top most metal layer, and they can be used between metal layers. 
     The various contemplated methods illustrated in FIGS. 2 a  to  5   b  and disclosed in the subsequent description all have in common a semiconductor wafer which may be patterned with transistors, diodes, capacitors, resistors or similar devices or which may not contain such devices. On top of the semiconductor wafer is a sandwich of IMD layers and metallization layers, followed by a final IMD layer to seal off the top surface. FIGS. 2 a  to  5   b  are not to scale and are only intended as examples of the methods of the invention and thus are not intended to limit the scope of the invention. 
     The detailed method which creates this sandwich structure, comprises: 
     depositing at least one set of an alternating inter-metal dielectric layer and metallization layer on top of the semiconductor wafer, 
     providing each metallization layer with a plurality of metal lines, 
     depositing a final inter-metal dielectric layer on top of a final metallization layer. 
     The dielectric layer may consist of one or more dielectric depositions depending on the requirements of the process. 
     The First Embodiment 
     The method of the first embodiment, as illustrated in FIG. 2 a  and FIG. 2 b,  shows a section of a semiconductor wafer  20  with top surface  21  and a bonding pad  22  on top surface  21 . The number of bonding pads can vary, but there is at least one bonding pad  22  on top of the final IMD layer  26 . A plurality of dummy plugs  23  is provided underneath the bonding pad  22 . The arrangement and number of dummy plugs is for illustrative purposes only and is not intended to limit the scope of the invention but is determined by the heat transfer requirements. The bonding pad  22  is connected using at least one metal via plug  24 , and typically more than one, to a metal line  29  underneath. FIG. 2 b  is a cross-section of FIG. 2 a  and shows bonding pad  22  placed on top surface  21 . Below are IMD layer  26  and metallization layer  27 , showing metal lines  28  and  29 . The cross-section reveals three of the dummy plugs  23  connecting bonding pad  22  with metallization layer  27  but none of the dummy plugs touching metal lines  28  or  29 . Only three metal lines and cross sections thereof are shown for the sake of clarity though typically many more metal lines are used. 
     The Second Embodiment 
     The method of the second embodiment, as illustrated in FIG. 3 a  and FIG. 3 b,  shows a section of a semiconductor wafer  30  with top surface  21  and a bonding pad  22  on top surface  21 . The number of bonding pads can vary, but there is at least one bonding pad  22  on top of the final IMD layer  26 . A plurality of dummy plugs  23  is provided outside of bonding pad  22 . The arrangement and number of dummy plugs is for illustrative purposes only and is not intended to limit the scope of the invention but is determined by the heat transfer requirements. The bonding pad  22  is connected using at least one metal via plug  24 , but using typically more than one, to a metal line  29  underneath. FIG. 3 b  is a cross-section of FIG. 3 a  and shows bonding pad  22  placed on top surface  21 . Below are IMD layer  26  and metallization layer  27 , showing metal lines  28  and  29 . The cross-section reveals six of the dummy plugs  23  penetrating IMD layer  26  and terminating with metallization layer  27 . Note that dummy plugs  23  do not contact metal lines  28  or  29 . Only three metal lines and cross sections thereof are shown for the sake of clarity though typically many more metal lines are used. 
     The Third Embodiment 
     The method of the third embodiment, as illustrated in FIG. 4 a  and FIG. 4 b,  shows a section of a semiconductor wafer  40  with top surface  21  and provided with a plurality of dummy plugs  23 , of which nine are shown, in an area  22   a  reserved for a bonding pad but not used. The arrangement and number of dummy plugs is for illustrative purposes only and is not intended to limit the scope of the invention but is determined by the heat transfer requirements. FIG. 4 b  is a cross-section of FIG. 4 a.  The cross-section reveals three of the dummy plugs  23  penetrating IMD layer  26  and terminating at metallization layer  27 . Layer  27  is the metallization layer with embedded metal lines  28  and  29 . Note that dummy plugs  23  do not contact either metal line  28  or  29 . Only three metal lines and cross sections thereof are shown for the sake of clarity though typically many more metal lines are used. 
     The Fourth Embodiment 
     The method of the fourth embodiment, as illustrated in FIG. 5 a  and FIG. 5 b,  shows a section of a semiconductor wafer  50  with top surface  21  and provided with a plurality of dummy plugs  23 . The arrangement and number of dummy plugs  23  is for illustrative purposes only and is not intended to limit the scope of the invention but is determined by the heat transfer requirements. FIG. 5 b  is a cross-section of FIG. 5 a.  The cross-section shows a final, or top, IMD layer  26  with a top surface  21 , followed by metallization layer  27 . After metallization layer  27  follow IMD layer  26   a  and metallization layer  27   a.  The cross-section additionally reveals five of the plurality of dummy plugs  23  penetrating IMD layer  26   a  and terminating at metallization layer  27   a.  The remainder of dummy plugs  23  are similarly disposed between metallization layer  27  and metallization layer  27   a  and penetrating IMD layer  26   a.  Metallization layers  27  and  27   a  have embedded metal lines  28  and  28   a,  respectively, where one metal line  28   a  is hidden from view in FIG. 5 a  by the overlying metal line  28 . Note that none of dummy plugs  23  contacts metal lines  28  or  28   a.  Only two each of metal lines  28  and  28   a  and cross sections thereof are shown for the sake of clarity though typically many more metal lines are used. 
     FIG. 5 b  illustrates the method of dummy plugs  23  penetrating an inter-metal dielectric layer  26   a  placed between two successive metallization layers  27  and  27   a.  In this method, dummy plugs  23  may further penetrate the lower of the two metallization layers  27   a  to a subsequent metallization layer (not shown). 
     In any of the four embodiments each or any of the inter-metal dielectric layers  26  may consist of more than one inter-metal dielectric deposition. It is important to note that in all of the above recited embodiments, dummy plugs  23  penetrate either IMD layer  26  or  26   a  to allow for the transfer of heat. 
     The following features and characteristics of the present invention apply to all four of the above recited embodiments: 
     The low-k IMD layer is preferably selected from the group comprising hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), hydrio polysilsesquioxane (H-PSSQ), methyl polysilsesquioxane (M-PSSQ), phenyl polysilsesquioxane (P-PSSQ), FLARE from Allied Signal, Microwave Materials, SILK from Dow Chemical, Xerogel, Nanoglass, or PAE-2, etc. The dielectric constant of above listed low-k dielectrics is typically 3, but ranges from 1 to 4. 
     Additionally the inter-metal dielectric layer may be further selected from the group comprising plasma enhanced silicon oxide, low-k spin-on-glass, tetraethyl orthosilicate glass, silicon nitride, phosphorus doped SiO 2 , F—SiO 2 , SiON, FSG, PSG, HDP-USG, HDP-SiO 2 , SACVD, or O 3 -TEOS, having a dielectric constant of typically 4, but ranging from 1 to 7. The dummy plug  23  is preferably made of material selected from the group comprising tungsten, Aluminum, Copper, Gold, Platinum, TaN, Ti, TiN, or Tn. 
     Providing dummy plugs according to the disclosed methods offers the advantages of releasing moisture that was taken up during etching, where the number of dummy plugs employed is also related to the amount of moisture released. Dummy plugs promote increased heat dissipation, particularly during chip bonding when the local temperature around the bonding pad is especially high and when thermal conductivity of the inter-metal dielectric layer is poor; the resulting reduction in thermal stress reduces potential delamination of the IMD layers to other oxide layers. Lastly dummy plugs make for a mechanically stronger wafer. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.