Abstract:
A method of electrochemically filling features on a wafer surface to form a substantially planar copper layer is provided. The features to be filled includes a first feature that is an unfilled feature with the smallest width and a second feature having the next larger width after the smallest feature. The first and the second features are less than  10  micrometers in width. The method comprises applying a first cathodic current to form a first copper layer on the wafer surface. The first copper layer has a planar portion over a first feature and a non-planar portion over a second feature. After a surface of the first copper layer is treated by applying a first pulsed current, a second cathodic current is applied to form a second copper layer on the first copper layer. The second copper layer has a planar portion over both the first and second features.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Prov. No. 60/444,355 filed Jan. 30, 2003 (NT-290-P), which is incorporated herein by reference. 
     
    
     
       FIELD  
         [0002]    The present invention generally relates to semiconductor integrated circuit manufacturing, and more particularly to a method for electroplating copper on a semiconductor wafer to form copper interconnects.  
         BACKGROUND  
         [0003]    Modem integrated circuits contain millions of devices to achieve complex functions. The electrical connection between the devices in such semiconductor circuits is provided by fine wires of conductive metals known as interconnects. As integrated circuit chips have become larger and more complex the requirements placed on the interconnect systems have also increased. As a result, interconnects have evolved from a single layer of aluminum to several levels of metal interconnects extending in channels formed horizontally and vertically in the body of the chip. In the multilevel metallization scheme insulating interlayer dielectric layers separate the silicon or local interconnect lines from each other. The linkage from one layer of interconnect to another is provided by vias, which are opened in the interlayer dielectric layers and then filled by metal.  
           [0004]    Because of several advantages, copper has recently become the preferred metal of interconnect applications, replacing aluminum and its alloys. The preferred method of forming copper interconnects is the damascene process. In the damascene method, copper is generally deposited using electroplating processes onto a dielectric diffusion layer previously deposited into vias and trenches that are previously etched in the interlayer dielectric. Chemical mechanical planarization (CMP) is then used to planarize the deposited copper layer, barrier layer and even the interlayer dielectric following the copper deposition.  
           [0005]    [0005]FIG. 1 schematically shows exemplary arrays of features that are formed in the dielectric layer  10  on an exemplary semiconductor substrate. These features need to be filled by electroplated copper to form copper interconnects. As an example, a first array  11  of features may be sub-micron size trenches with widths in the range of 0.05-0.5 microns. A second array  12  of features may be trenches with widths in the range of 0.5-2 microns. A third array  13  of features may be trenches with widths in the range of 2-5 microns. Before electroplating, barrier and seed layers are deposited over the whole surface and into the arrays  11 ,  12  and  13  of the substrate as is well known in the field. Such layers will not be shown in FIG. 1 for clarification purpose.  
           [0006]    Electroplating solutions typically contain organic additives such as accelerators, suppressors and levelers in their formulations. Commonly used electrolytes are copper sulfate solutions supplied by companies such as Enthone and Shipley. The additives in the plating solutions help provide smooth deposits and bottom up growth in small features.  
           [0007]    [0007]FIG. 2 shows the typical evolution of copper thickness profiles A, B, C and D over the substrate surface depicted in FIG. 1, when electrodeposition is carried out from electrolytes containing accelerator and suppressor species. As the plating is initiated by applying a cathodic voltage to the wafer with respect to an anode, copper first starts to deposit conformally. Then bottom-up growth is initiated and the first array  11  of the smallest features is quickly filled, for example at time T 1 . Thickness profile at time T 1  is represented by profile A. As can be seen from FIG. 2, at time T 1  the larger features are still not fully filled with copper. Deposition is then continued to fill the larger features. Thickness profile B represents the copper profile at a time T 2  when the second array  12  of features is completely filled by copper. As the second array  12  is filled however, the accelerator species that are responsible for bottom-up fill of the features cause a bump or overfill  20  over the first array  11 . The reason for this phenomenon is believed to be the high accelerator concentration that stays over the first array  11 , even after the fill is complete. Similarly, thickness profile C shows that as the third array is completely filled another bump is formed, this time over the second array  12 . If plating is continued, eventually a third bump may also be formed over the third array  13  as shown in profile D.  
           [0008]    Non-flat profile depicted as profile D in FIG. 2 presents challenges for the CMP process. During CMP, surface of dense arrays that are overfilled need to be cleared off copper to avoid shorting between the features. This requires over-polishing, which in turn causes dishing and erosion defects as well known in the art. It should be noted that the bumps may have a copper thickness of more than 2000 Å (for example 2000-6000 Å) compared to the region of the wafer where there are isolated large features. A flat copper thickness profile is therefore preferred for the best results after the CMP step of the interconnect fabrication process. In some prior art applications, levelers may be used to minimize bump formation over the dense arrays. However, levelers cannot completely eliminate the bump formation problems.  
           [0009]    Defect-free filling of the small features is another requirement for interconnect fabrication. Copper deposited into the vias and trenches needs to be free of voids, seams and other defects to avoid high resistance and reliability problems. Plating solutions with bottom-up filling capability are formulated to minimize such defects. In addition to the formulation of the bath, plating waveform which is the voltage/current applied to the wafer is also an important factor in minimizing or eliminating fill-related defects. For example, in the prior art, applying reverse pulses (anodic pulses) to the wafer during early stages of the plating period is shown to improve filling properties of the smallest features. Example of such prior art processes may be seen in U.S. Pat. Nos. 5,972,192, 6,297,155 and 6,303,014. Generally, reverse pulses used in the prior art are applied in short time durations in millisecond level. Further, prior art reverse pulses are either applied during the gap file period or throughout the plating period.  
         SUMMARY  
         [0010]    A method is developed to control the planarity of the copper or copper alloy deposition over a semiconductor substrate with features of various sizes and densities. In this method, the plating potential is reversed multiple times with different durations and waveforms to prevent excessive deposition of copper over small features. In the absence of potential reversal, the growth rate of film over small features is very fast due to the accumulation of accelerator species at high concentrations following the bottom-up fill. Reversing the plating potential at carefully selected intervals reduces the growth rate over such features. In the present method, each potential reversal period is aimed at prevention of bump formation at a specific feature size and is applied when the bottom-up fill is nearly completed at that feature size.  
           [0011]    In one aspect of the present invention, a method of electrochemically filling cavities on a wafer surface to form a substantially planar conductive layer is provided. During the process, initially, a first cathodic current is applied to form a first conductive layer on the wafer surface. The first conductive layer includes a planar portion over a first cavity and a non-planar portion over a second cavity. The first cavity is an unfilled cavity with the smallest width and the second cavity has the next larger width after the smallest cavity. The first and the second cavities are less than 10 micrometers in width. In the next step of the process, surface of the first conductive layer is treated by applying a first pulsed current. In the following step, a second cathodic current is applied to form a second conductive layer on the first conductive layer. The second conductive layer has a planar portion over both the first and second cavities. The steps of treating and applying is repeated until all of the cavities are filled.  
           [0012]    These and other features and advantages of the present invention will be described below with reference to the associated drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic illustration of arrays of features that are formed on a semiconductor substrate;  
         [0014]    [0014]FIG. 2 is a schematic illustration of stages of prior art copper deposition over the substrate shown in FIG. 1;  
         [0015]    [0015]FIG. 3 is a schematic illustration of a substrate having arrays of features that are filled with the process of the present invention; and  
         [0016]    [0016]FIG. 4 is a graph showing wave-forms used during the various stages of the electrochemical deposition process of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The microscopic and macroscopic uniformity of copper film after electrodeposition step is critical for the performance of CMP to efficiently polish copper from the whole wafer without excessive overpolish. The present invention is aimed at controlling the growth rate of copper deposition over the high density feature areas by applying a series of reversed potential pulse sequences to eliminate or reduce microscopic thickness non-uniformities over the dense array of small features. By selecting an optimized reverse potential pulse sequence, bump formation, which is defined in the background section, could be eliminated or minimized over the features narrower than about 10 microns, preferably narrower than 5 microns and therefore the micro-scale planarity of the film could be highly improved.  
         [0018]    [0018]FIG. 3 illustrates a substrate  100  having a copper layer  102  deposited in accordance with the principles of the present invention. The substrate  100  is a semiconductor substrate, preferably silicon, comprising an insulating layer  104  such as a dielectric layer, for example SiO 2 , on top of it. The dielectric layer  104  is previously patterned and processed with known semiconductor process technologies to form exemplary first, second and third feature or cavity arrays  106 ,  108  and  110 . Accordingly, the first, second and third feature arrays are comprised of small width (about 0.05-0.5 microns) or small, medium width (about 0.5-2 microns) or medium and large width (2-5 microns) or large trenches  107 ,  109  and  111 , respectively. The trenches  107 ,  109  and  111  and surface  112  of the dielectric layer  104  are lined with a barrier layer  114 , for example Ta or TaN or both, and a copper seed layer  116  on top of the barrier layer. It is understood that the substrate  100 , the layers on top it and the way they are processed are the same as the substrate shown in FIG. 1. However, in order to describe the present invention in detail and more clearly, different reference numerals are used.  
         [0019]    The electrodeposition process of the present invention will be described in connection with FIGS. 3 and 4. The graph  200  in FIG. 4 shows an exemplary electric field waveform used in the present invention. Referring to FIG. 3, the electrochemical deposition of the copper layer  102  may be described using a multi-step selective filling process that fills each feature group in predetermined time intervals. After filling of the features that are approximately less than 5-10 microns in width with the process of the present invention, the process may proceed with a selected conventional electroplating process to fill the features having more than 10 microns feature widths. Of course, although the exemplary process of the present invention has three process steps, depending on the number of feature array groups of interest, the process may have multiple process steps to fill features grouped according to their sizes.  
         [0020]    The electrodeposition is carried out from a copper ion containing plating or electrolyte solution having accelerator and suppressor species. The electrolyte solution may not include levelers, although their inclusion does not affect the process of the present invention. During the process, either the substrate is immersed in the electrolyte, or the seed layer lined surface of the substrate is contacted with the electrolyte. An electrode is also immersed in the electrolyte, and the substrate to be plated and the electrode are connected to a power supply that is able to apply a voltage, or able to reverse or pulsate the voltage, between the electrode and the substrate. During the electroplating process, the electrode functions as an anode while the conductive substrate surface becomes cathode.  
         [0021]    Accordingly, in a first exemplary process step, a first copper layer  118  is deposited over the substrate to completely fill the small trenches  107  in the array  106 . As shown in FIGS. 3 and 4, the plating is initiated by applying a cathodic voltage to the substrate with respect to the electrode (not shown). During the process, copper first starts to deposit conformally, and then bottom-up growth is initiated and the small trenches  107  are quickly filled by the time t 1 . As shown in FIG. 4, this stage of the process comprises a first cathodic waveform  202 . The waveform may be a rectangular DC wave form as shown in FIG. 4, however, any waveform, DC or AC or varying may be used to fill the features. It should be understood that the prior art defect-free filling of the small features that is mentioned in the background section is carried out before the time t 1 . Therefore, the prior art does not address the bump formation problem, which occurs after the time t 1 .  
         [0022]    Time t 1  is a predetermined filling time and depends on the width of the trench. Once the small trenches  107  are filled with the layer  118 , a first set of anodic pulses  204  are applied until a predetermined time t 1 ′. The set of pulses may comprise at least one pulse of 0.5 to 5 seconds in duration, preferably 1 to 2 seconds. The application of pulsed waveform  204  prevents bump formation over the layer  118  above the array  106  when the deposition is continued. As mentioned before, in the prior art applications, the accelerator species that are responsible for bottom-up fill of the features can cause a bump or overfill over the small trenches, as the deposition progresses after the filling of the smaller features. In the present invention, use of pulsed wave forms advantageously reduce accelerator concentrations over the deposited layer and hence inhibit bump formation, when the deposition process continues. Although the pulses shown in FIG. 4 are preferably completely anodic, it is also possible to have cathodic components of these pulses.  
         [0023]    After the application of pulsed waveform  204 , in a second step of the process, a first leveled deposition layer  119  is initially formed on the copper layer  118 . In this step, a second cathodic wave form  206  is applied to initiate deposition of the layer  119  over the first layer  118  which is treated with pulsed wave  204  to assure flatness of the subsequently deposited layer  119 . As shown in FIG. 4, the second cathodic waveform  206  is applied between the time t 1 ′ and the time t 2 , and the first leveled deposition layer  119  is formed between time t 1 ′ and t 1 ″. As can be seen from FIG. 3, at time t 1 ″, although the small trenches are filled and successfully covered with the first leveled deposition layer  119 , which is bump free and flat, the medium and larger trenches are still not fully filled with copper. Deposition is then continued with the waveform  206  to fill the medium trenches  109  with a second copper layer  120 .  
         [0024]    During the deposition of the second copper layer  120  on the layer  119 , between time t 1   41  and t 2 , copper starts filling the remaining unfilled upper portions of the medium trenches and large trenches, and by the time t 2 , the medium trenches  109  are completely filled. Once the medium trenches  109  are filled, the second step of the process is continued by applying a second set of anodic pulses  208  to treat the second copper layer  120  until a predetermined time t 2 ′. As in the previous step, the application of pulsed waveform  208  prevents bump formation on the layer  120  over the array  108  when the plating continues with deposition of a new copper layer on top of the layer  120 .  
         [0025]    After the application of pulsed wave form  208 , in a third step of the process, a second leveled deposition layer  121  is formed over the second layer  120 , as a third cathodic wave form  210  is applied to initiate copper deposition over the second layer  120  which is pulsed wave treated. As shown in FIG. 4, plating process comprises application of a third cathodic waveform  210  between the time t 2 ′ and the time t 3 . In particular, the second leveled deposition layer  121  is formed between time t 2 ′ and t 2 ″ over the pulsed wave treated copper layer  120 . As can be seen from FIGS. 3 and 4, at time t 2 ″, although the second leveled deposition layer  121 , which is flat, is formed over the small and medium arrays  106  and  108 , the large trenches  111  in large array  110  are still not fully filled with copper. The third process step is then continued with deposition of a third copper layer  122  over the second leveled layer  121  to completely fill the large trenches  111 . As shown in FIG. 4, this stage of the process is performed within the third cathodic waveform  210  between the time t 2 ″ and the time t 3 .  
         [0026]    During the deposition of the third copper layer  122 , between time t 2 ″ and t 3 , copper starts filling the remaining unfilled upper portions of the large trenches and by the time t 3 , the large trenches  111  are completely filled. Once the large trenches  111  are filled, the third step of the process is continued by applying a third set of anodic pulses  212  until a predetermined time t 3 ′. As in the previous steps, the application of pulsed waveform  212  prevents bump formation on the layer  122  when the plating continues with deposition of a new copper layer on top of the layer  122 .  
         [0027]    Up to this point of the process of the present invention fills the features that are approximately less than 10 microns in width. The process proceeds with the conventional electroplating process to fill the features having more than 10 microns feature widths so that after the application of pulsed waves, a third leveled deposition layer  123  over the layer  122  may be formed to continue deposition process. The third leveled deposition layer  123  may be formed by the time t 3 ″ using for example a fourth cathodic waveform  214 . In all above steps of the present invention, time spent to fill feature arrays  106 ,  108  and  110  using forward cathodic waveform depends on the size of the features. Further, it is understood that although the waveforms  202 ,  206  and  210  are DC waveforms with equal magnitude, this is not necessary to perform the process of the present invention. The process can be performed using any DC or AC waveform having different magnitudes. As a result of electroplating process of the present invention, the copper layer  102  is formed over the feature arrays  106 ,  108  and  110 , which have approximately less than 10 microns width or preferably less than 5 microns width, in a planar manner without bumps.  
         [0028]    Although various preferred embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.