Abstract:
A BEOL manufacturing process for forming a via process between two metal lines on a semiconductor wafer comprises depositing a portion of a first metal adhesion layer within a patterned via hole, followed by a cooling step. The cooling step is then followed by formation of the remainder of the first metal adhesion layer and formation of a second metal adhesion layer within the patterned via hole. This process of forming the remaining portion of the first metal adhesion layer can be referred to as a wafer load, unload, load (LUL) process. By using a LUL process, thermal history is minimized, which reduces Al extrusion at the via interfaces.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation-in-part application of U.S. patent application Ser. No. 11/562,834 filed on Nov. 22, 2006 including the specification, claims, drawings and summary. The disclosure of the above patent application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to semiconductor fabrication, and more particularly to back end of line (BEOL) techniques that reduce manufacturing errors related to via metal melt extrusion 
         [0004]    2. Background of the Invention 
         [0005]    Semiconductor wafers are manufactured in accordance with a process flow. The process flow comprises all of the different processing steps, such as etching and photolithography, involved in the process of manufacturing semiconductor wafers. A typical process flow can consist of 300-400 steps, wherein each of these steps contributes to the final circuit structures formed within a single chip on the semiconductor wafer. A typical process flow is divided into two main sub-processes. The first of these main sub-processes can be termed the front end of line (FEOL) process, and the second of these main sub-processes can be termed the back end of line (BEOL) process. 
         [0006]    The FEOL process typically starts from the laser marking of wafer lots and continues through the formation of Shallow Trench Isolation (STI), implantation of P and N wells, etching of poly, and followed by implantation of various regions such as the drain and source regions of a transistor structure. 
         [0007]    The BEOL process can comprise the formation of metal lines and via contacts between metal lines in different layers of the wafer. Often, there are two, or more metal layers comprising metal interconnection lines. Vias run between two metal layers. The BEOL process is a process whereby devices in the FEOL layers are interconnected with other circuits forming the chip and to the outside world. 
         [0008]    Metal layers are typically formed via a Physical Vapor Deposition (PVD) process. A typical PVD process comprises the deposition of metal layers as followed. These metal layers typically comprise Ti, TiN, Al or AlCu, and Ti and TiN. First, for example, Ti layer and TiN layer are deposited followed by the deposition of an Al layer, and then the deposition of Ti layer and TiN layer. 
         [0009]    Vias are then patterned in the lower metal layers and further metal layers are formed over the lower metal layers. Conventionally, a via is formed by first forming metal adhesion layers within the patterned via hole and then forming a tungsten (W) plug inside of the metal adhesion layers. The metal adhesion layers often comprise a Titanium (Ti) metal layer formed within the patterned via hole and a Titanium Nitride (TiN) layer formed with the Ti Layer. The Ti Layer is often formed in a first deposition chamber (CH  1 ) and the TiN layer is often formed in a second deposition chamber (CH  2 ). The Ti adhesion layer can be formed using PVD, or more specifically Ionized Metal Plasma (IMP) PVD. The TiN adhesion layer can be formed using Metal Organic Chemical Vapor Deposition (MOCVD) 
         [0010]    It will be understood that the metal adhesion layer formation can subject the wafer to high heat. For example, in a conventional process, the temperature in CH  1  can be as high as 200° C. and rising throughout the process. The temperature in CH  2  can be as high as 450° C. Unfortunately, the Al in the metal layers can melt at temperatures around 600° C. The back-to-back heating processes can actually cause the wafer temperature to exceed the Al melting point, which cause the Al to extrude into the via. This can increase the via resistance and lead to device performance problems and even failures. 
         [0011]    One solution to this problem is to reduce the heat cycle times associated with deposition of the Ti layers by thinning the Ti layers in adhesion layer; however, such solutions are not ideal since reducing the heat cycling time by reducing the thickness of the Ti layers will reduce reliability as well. 
       SUMMARY 
       [0012]    A BEOL via adhesion manufacturing process comprises forming a first portion of a metal adhesion layer, such as a Ti layer, within a patterned via and then removing the wafer from the chamber and placing it in a cooling chamber to undergo a cooling process. The wafer can then be placed back into the chamber for formation of the remainder of the metal adhesion layer. The wafer can then be placed into another chamber for formation of a second metal adhesion layer, such as a TiN layer, within the patterned via. The process can be referred to as a wafer Load-Unload-Load (LUL) process. By using the LUL process the thermal processing of the wafer is minimized, which reduces Al extrusion at the via interfaces. 
         [0013]    These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.” 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a diagram illustrating an exemplary semiconductor circuit that can comprise a single chip on a semiconductor wafer; 
           [0016]      FIG. 2  is a diagram illustrating a portion of via within the circuit of  FIG. 1  in more detail; and 
           [0017]      FIG. 3  is a Transmission Electron Microscope (TEM) image illustrating Al extrusion that can result from a conventional via formation process; and 
           [0018]      FIG. 4  is a flowchart illustrating an example method for depositing metal adhesion layers during via formation in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is a diagram illustrating an example semiconductor circuit  100  that can comprise a single chip on a semiconductor wafer. In the example of  FIG. 1 , circuit  100  is a memory circuit comprising a main memory portion  120  and a periphery portion  122 . Main memory portion  120  can comprise the structures that form memory circuit  100 , while periphery portion  122  can comprise the interconnects and control circuitry required to interfaced the main memory portion  120  with the control circuits and interconnects required to control and access main memory portion  120 . 
         [0020]    It will be understood that while the systems and methods described below will be described in relation to a memory circuit such as circuit  100 , the invention is in no way restricted to memory circuits. Rather, it will be clear that the systems and methods described herein can be applied to any BEOL process regardless of the type of circuit involved. Further, while there are two metal layers  108  and  110  illustrated in the example of  FIG. 1 , it will be understood that the systems and methods described below can be extended to circuits with fewer or more metal layers, and that two metal layers are shown by way of example only. 
         [0021]    Circuit  100  comprises several layers constructed using various well-known semiconductor processing techniques. For example, circuit  100  can comprise device layer  124 , which can be constructed using FEOL techniques, and an interconnect layer  126 , which can be constructed using BEOL techniques. 
         [0022]    Device layer  124 , can comprise several sub-layers. These sub-layers can include a well  102 , which can comprise a silicon well and various regions, i.e., drain and source regions, implanted therein. The semiconductor well and implanted regions can be formed using well-known semiconductor techniques. A word/bit line layer  104  can then be formed on top of well layer  102 . Word/bit line layer  104  can comprise various interconnect lines such as word lines and bit lines formed using well-known semiconductor techniques. A storage layer  106  can then be formed on top of word/bit line layer  104 . 
         [0023]    Interconnect layer  126  can comprise several metal layers, of which metal layer  1   108  and metal layer  2   110  are illustrated by way of example. Metal layers  108  and  110  can comprise metal contacts such as metal contacts  128  and  130  as well as interconnecting vias, such as vias  112 ,  114  and  116 , configured to connect metal contacts  130  and  128  as shown. In addition, vias such as vias  118 ,  120 ,  121 ,  122 , and  124 , can also be included to connect metal contacts  128  to various layers within device layer  124  as illustrated. 
         [0024]      FIG. 2  is a diagram illustrating a portion of interconnect layer  126  in more detail.  FIG. 2  illustrates a portion of interconnect layer  126  surrounding via  112  in metal layer  1   108 . Thus, via  112  is bounded above and below by metal contacts  130  and  128 , respectively. As can be seen, a metal contact, such as metal contact  128 , can comprise a plurality of metal layers. These layers can include Ti layers  206 , TiN layers  210 , and Al layer  208 . 
         [0025]    It will be understood that while the example of  FIG. 2  metal layer  128  includes Ti layers  206 , TiN layers  210 , and an Al layer  208 , other layers can also be incorporated in addition, or in place of the layers illustrated in  FIG. 2 . For example, Al layer  208  can be replaced by an AlCu layer. 
         [0026]    Via  112  can then comprise metal adhesion layers  214  and  216  and W plug  212 . For example, as explained above, metal adhesion layer  216  can comprise a TiN layer grown using MOCVD, while metal adhesion layer  214  can comprise a Ti layer grown using IMP PVD. If a conventional process is used to form layers  214  and  216 , however, then extrusion of Al layer  208  can result in via interface, and this phenomena is more obvious in the unlanding vias than the landing vias. This is illustrated in  FIG. 3 .  FIG. 3  is a TEM that illustrates Al extrusion at the lower end  304  of via  112 . In the TEM of  FIG. 3 , the top portion  302  of via  112  is unaffected; however, the extrusion of portion  304  can increase resistance and lower reliability of the connection made via vias  112 . 
         [0027]    The extrusion phenomenon illustrated in  FIG. 3  occurs, because the temperature cycle, i.e., the temperature and time, required to effect the deposition of metal adhesion layer  214  and  216  can actually cause Al layer  208  to melt. 
         [0028]      FIG. 4  is a diagram illustrating a process for forming a via, such as via  112 , in accordance with one embodiment of the systems and methods described herein. First in step  402 , a first metal layer (metal layer  1 ) can be formed. The metal layer can actual comprise a plurality of metal layers as illustrated for layer  128  in  FIG. 2 . Thus, step  402  can comprise the formation of a Ti layer, a TiN layer, an Al layer, a second Ti layer, and a second TiN layer. The metal layers can, for example, be formed via PVD, or more specifically IMP PVD. 
         [0029]    In step  404  a via, such as via  112 , can be patterned and in step  406  the wafer can be degassed. In step  408 , the wafer can be placed into a pre-cleaning chamber (PC II) and in step  410  the wafer can be removed from the PC II and placed into a cooling chamber (CH A). 
         [0030]    In step  412 , the wafer can be removed from CH A and placed into a deposition chamber (CH  1 ) in which a metal adhesion layer, e.g., Ti layer  214 , can be formed in the patterned via hole; however, only a portion of the metal layer is formed. The wafer is then removed form CH  1  and placed back into a cooling chamber or returned to the load lock, where the wafer is cooled again in step  414 . Since Ti can adsorb oxygen, there is no native oxide issue when the wafer is exposed to the atmosphere. Then, in step  416 , the wafer is placed back into CH  1  and the remainder of the adhesion layer is formed. This is the LUL process, which can reduce the overall thermal stressing of the wafer and prevent Al extrusion. 
         [0031]    The Ti Layer can be formed using IMP PVD. For example, a Ti layer of 400 Å can be formed by first forming a 200 Å layer and then forming another 200 Å layer of Ti. Accordingly, the Ti layer thickness can be maintained at a thickness of 400 Å without over stressing the wafer. Each CH  1  step can last for approximately between 100 and 300° C., preferably between 10 and 50 seconds at 200° C., and more preferably at 49 seconds. It will be understood, how ever, that the dimensions and temperatures provided are by way of example only and will depend on the requirements of specific implementation. 
         [0032]    In step  418 , the wafer can then be placed in another deposition chamber (CH  2  or CH  3 ) for formation of a second metal adhesion layer, e.g., a TiN layer  216 . The second metal adhesion layer can, e.g., be formed using MOCVD. The CH  2  or CH  3  processing time can be at a temperature of approximately between 300 and 450° C., preferably between 50 and 200 seconds at 450° C., and more preferably at between 100 and 177 seconds. 
         [0033]    In step  420 , the wafer can be removed and placed in a second cooling chamber (CH B). A W plug can then be formed in the patterned via hole in step  422 . The W plug can then be polished, e.g., using CMP, in step  424 , and the second metal layer can be formed in step  426 . Again, the second metal layer can actual comprise a plurality of metal layers as illustrated for layer  130  in  FIG. 2 . Thus, step  402  can comprise the formation of a Ti layer, a TiN layer, an Al layer, a second Ti layer, and a second TiN layer. The metal layers can, for example, be formed via PVD, or more specifically IMP PVD. 
         [0034]    Thus, by implementing the process of  FIG. 4 , Al extrusion can be avoided, while still maintaining Ti adhesion layer thickness, which can improve device reliability and lower failure rates. 
         [0035]    While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.