Abstract:
A method for forming aluminum lines over aluminum-filled vias in a semiconductor substrate that can compensate for some misalignment between the filled vias and the lines. By alternately depositing liner-barrier layers and aluminum layers on the substrate, different etch chemistries can be used that can anisotropically etch an aluminum layer used to form the lines without etching voids in the aluminum-filled vias.

Description:
This invention relates to improved methods for forming aluminum lines over aluminum-filled vias or plugs. More particularly, this invention relates to methods for forming aluminum lines over vias filled with aluminum that can compensate for some misalignment. 
     BACKGROUND OF THE INVENTION 
     Aluminum has been widely used to form conductive lines and vias in the manufacture of semiconductor devices. An opening is made in a dielectric layer, such as a silicon oxide layer, and filled with aluminum metal. An overlying aluminum line is then formed over the via to provide conductive lines to connect the via to other devices on the substrate. The use of aluminum metal is advantageous because it is highly conductive, and thus a via filled with aluminum has low resistance; it is easy to deposit, either by sputtering or by chemical vapor deposition (hereinafter CVD); and an aluminum layer can be etched readily to form conductive lines by plasma etching. 
     The shrinking diameter of vias and lines has also required that the via and line diameters be about the same. In the past, a line width greater than the diameter of a via (overlap) could be used, and thus accommodation to some misalignment of the patterning of the lines to the via could be readily realized. However, because circuit density has increased and feature sizes have decreased, the overlap can no longer be tolerated and reduction or elimination of line overlap (called zero overlap) has become necessary. 
     However, aluminum layers used to define aluminum lines have a thickness of about 5000 to 10,000 angstroms. When etching an aluminum layer to form aluminum lines, the rapid etch rate required for etching such a thick aluminum layer economically is difficult to control, particularly near the end of the etch step. 
     When an aluminum line is to be formed over an aluminum via, this etch rate problem is exacerbated and any exposed aluminum via due to misalignment is etched along with the line, leaving a void in the via opening. As the spacing and line width of these features is made ever smaller, i.e., for design rules of less than 0.50 micron, and particularly as design rules are reduced to 0.25 micron and less, the problem of nonalignment becomes more pronounced. A misalignment occurs because it is more difficult to precisely pattern thin, narrow aluminum lines. A result of this misalignment is that during etching to form the aluminum lines, if the line is not aligned perfectly with the aluminum via, the etchant for the aluminum layer used to form the lines etches any exposed aluminum in the via that is not covered with the line aluminum. 
     The problem addressed by the present invention can be illustrated with reference to FIGS. 1A to  1 C. 
     FIG. 1A shows a silicon oxide substrate  12  having a via therein  14  filled with aluminum. An aluminum layer  16  has been deposited over the filled via  14  and a patterned photoresist layer  18  is formed thereover. It is apparent that there is some misalignment between the via  14  and the photoresist pattern  18 . 
     The aluminum metal layer  16  is then anisotropically etched to form an aluminum line  20  beneath the photoresist line  18 , as shown in FIG.  1 B. Since the etch is difficult to control as the substrate  12  is approached, the etch removes not only the aluminum on the surface of the substrate  12  outside of the photoresist pattern, but also removes that portion of the aluminum via that is exposed by the misaligned line. FIG. 1C shows that after the photoresist  18  is removed, an opening or void  22  has been formed in the via  14  by the etchant. 
     Thus a method of etching the aluminum lines and of compensating for some misalignment between an aluminum line over an aluminum via that does not leave a void in the aluminum via would be highly desirable. 
     SUMMARY OF THE INVENTION 
     We have found a method that etches aluminum lines over aluminum vias that compensates for some misalignment in patterning the aluminum lines, but that does not leave voids in the aluminum vias. This method deposits a thin layer of aluminum by chemical vapor deposition over a barrier lined via. The thin layer of aluminum is drawn into and fills the via by heating under vacuum. Alternate aluminum and liner-barrier layers are used to form an aluminum line over the filled via, permitting the use of different etch chemistries and allowing etching of aluminum lines without etching the aluminum to form a void in the filled via. 
     In a first embodiment, a liner-barrier layer is deposited in the via. Aluminum is then deposited onto the surface of the substrate and into the opening, partially filling the opening. This aluminum layer is about 400 to 1000 angstroms thick. The aluminum is then heated under vacuum to draw the aluminum down into the opening, filling the opening. The aluminum film on the top surface of the substrate is then removed. A liner-barrier layer, which also acts as an etch stop, is then deposited on the substrate, an aluminum layer to make the aluminum lines is deposited, and a patterned photoresist layer is formed over the filled opening. The aluminum layer is then patterned by selectively anisotropically etching the aluminum layer. Any misalignment between the aluminum line and the aluminum via is compensated for by the barrier layer, which is removed using a different etch chemistry that does not etch the aluminum in the via. 
     In a second embodiment, the initial steps are the same, but the aluminum on the substrate is not removed. A liner-barrier or etch stop layer is deposited over the substrate, aluminum is deposited, and a line pattern formed beneath a patterned photoresist layer. Any misalignment is compensated for during the etch steps because the barrier layer protects the underlying aluminum. The final aluminum layer adjacent to the substrate surface is thin, and etching down to the substrate can be readily controlled by controlling the etch rate so that the etchant removes the aluminum on the surface of the substrate but does not etch the aluminum in the vias. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1A,  1 B, and  1 C are cross sectional views of a filled via and an aluminum line using a conventional method. This method produces a void in the filled via. 
     FIGS. 2A to  2 H are cross sectional views showing method steps of one embodiment of the method of the invention that forms an aluminum line over an aluminum filled via but avoids the formation of a void in the filled via. 
     FIGS. 3A to  3 I are cross sectional views showing method steps of another embodiment of the method of the invention that forms an aluminum line over an aluminum filled via but avoids the formation of a void in the filled via. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An integrated Endura® system, available from Applied Materials, Inc of Santa Clara, CA, can be fitted with various processing chambers for sequential deposition and etch steps. For example, liner/barrier layers can be deposited by sputtering in a physical vapor deposition (PVD) chamber; CVD aluminum can be deposited in a chemical vapor deposition (CVD) chamber; and one or more Centura® DPS™ RIE metal etch chambers, also trademarks of Applied Materials, Inc, can be used for the etch steps. Equipment for spinning and patterning photoresist layers are also well known. 
     The first embodiment of the present method will be described with respect to FIGS. 2A to  2 H. 
     Referring to FIG. 2A, a dielectric substrate  22  has a via  24  formed therein. The dielectric substrate  22  is more commonly formed of silicon oxide, but fluorine-doped silicon oxide and other dielectric materials can also be used. A conformal barrier layer  26  is then deposited over the substrate  22  including the top surface  27  of the dielectric substrate  22  and along the walls  28  of the via  24 . The barrier layer  26  can be made of one or more of tantalum, tantalum nitride, titanium or titanium nitride, as examples. This barrier layer  26  is preferably formed by a plasma sputtering method that provides a conformal layer. 
     A thin layer of aluminum  30  is then deposited by CVD from an aluminum compound, such as dimethyl aluminum hydrate. This aluminum layer partially fills the via  24  and then covers the top surface  27  of the dielectric substrate  22 . Sufficient aluminum must be deposited to be able to fill the via  24 , but this aluminum layer  30  is preferably no more than 1000 angstroms thick. FIG. 2B illustrates this step. 
     In order to fill in the via  24  with aluminum, the substrate  22  is heated to a temperature of about 350-500° C. under a pressure of about 0.2 to 30 millitorr, when the aluminum is softened and drawn into the via  24  to fill the via  24 . This is shown in FIG.  2 C. The thin aluminum layer  30  remaining over the top surface  27  of the substrate  22  is etched away using a chlorine-based etch. This etch removes the aluminum metal layer  30  and part or all of the barrier layer  26  on the top surface  27  of the substrate  22 , as shown in FIG.  2 D. 
     A second liner/barrier layer  32  is deposited over the substrate  22  and the filled via  24  and a second layer of aluminum  34  is deposited thereover, as shown in FIG.  2 E. This second layer of aluminum  34  can be deposited either by CVD or by sputtering. A layer of photoresist is deposited and patterned to form a photoresist line  36 . This is shown in FIG.  2 F. In FIG. 2F the photoresist line  36  is shown somewhat misaligned with respect to the filled via  24 . An aluminum line  38  is then formed by anisotropic etching using a chlorine-based chemistry. This step is shown in FIG.  2 G. Lastly, the second liner/barrier layer  32  is etched using a fluorine-based chemistry to form a barrier line  40 . Since aluminum metal is not rapidly etched with this chemistry, no etching of aluminum occurs and no void is formed in the filled via  24 . This step is shown in FIG.  2 H. Although the aluminum line  38  is somewhat misaligned with the filled via  24 , the filled via  24  does not include any voids. 
     If desired, the substrate can be covered with a dielectric layer, such as silicon oxide (not shown) 
     FIGS. 3A to  3 I illustrates an alternative method for forming filled vias and metal lines in accordance with the invention. 
     The steps shown in FIGS. 3A to  3 C are the same as those of FIGS. 2A to  2 C. Without removing the thin aluminum layer  30 , a liner/barrier layer  42  is deposited over the filled via  24  as shown in FIG. 3D and a second layer of aluminum  44  is deposited thereover as shown in FIG.  3 E. This aluminum layer  44  can be deposited by CVD or by sputtering. A photoresist layer is deposited and patterned to form a photoresist line  46 , as shown in FIG.  3 F. This photoresist line  46  is also somewhat misaligned with the filled via  24 . The aluminum layer  44  is etched anisotropically to form an aluminum line  48  using a chlorine-based chemistry. This etch stops at the liner/barrier layer  42 , as shown in FIG.  3 G. The liner/barrier layer  42  is then anisotropically etched down to the aluminum layer  30 , forming a line  50 , as shown in FIG.  3 H. 
     The aluminum layer  30  remaining on the top surface  27  of the substrate  22  is etched away using a carefully controlled, low etch rate method and a fluorine-based chemistry to form a thin line  52 . As shown in FIG. 3I, the aluminum lines  48  and  52 , although not perfectly aligned with the filled via  24 , does not include a void. Lastly, the photoresist line  46  is removed. 
     The use of a liner barrier layer under the aluminum line permits a different etch chemistry to be used to etch the aluminum line and to etch the liner-barrier line. The latter etch can be controlled so that it does not remove aluminum from the filled via. When the aluminum remaining on the surface of the substrate is etched, since it is a very thin layer, it can be etched at a low etch rate, as by using a fluorine-based, rather than the faster etching chlorine-based etchant, to avoid etching aluminum in the filled via. 
     Although the invention has been described in terms of specific method steps, one skilled in the art can vary the nature of the materials and processing variables, as is known. Thus the invention is only to be limited by the scope of the appended claims.