Abstract:
An integrated circuit comprising one or more dielectric layers the or each dielectric layer being provided with one or more interconnects wherein the interconnect comprises metallic atoms moving from a first region of the interconnect to a second region of the interconnect when a current flows, characterized in that the interconnect comprises a donor zone in the first region of the interconnect for providing metallic atoms in order to compensate for movement of atoms from the first region and a receptor zone at the second region of the interconnect for receiving metallic atoms in order to compensate for movement of atoms to the second region.

Description:
FIELD OF THE INVENTION 
     This invention relates to improvements for reducing electromigration effect in an integrated circuit and more particularly describes a new method and apparatus for integrated circuit layout to reduce electromigration failures. 
     BACKGROUND OF THE INVENTION 
     In an integrated circuit, many different electronic devices or components such as transistors or capacitances are formed in or on a specific substrate, which generally has a planar configuration. The layer may be a metal dielectric or any other type of material appropriate to the circuit design. The electrical connections between the electronic devices located on the substrate or on a surface of a dielectric layer occurs through a network of metal interconnects or interconnections. Generally, complex integrated circuits are formed with different metal layers or levels. The network of interconnects may include intra-level connections also called metal lines which make connections in the same layer and inter-level connections called vias, which, are metal links which connect consecutive metals layers. 
     Metal lines and vias represent metallization layers also called interconnections or interconnects of an integrated circuit. Interconnections may be made of Copper, Aluminium, or any other conducting material. Most recent integrated circuits use Copper as the interconnection material as Copper has a better electrical conductivity and a better electromigration resistance than Aluminium. Integrated circuit interconnects generally comprise a diffusion barrier liner in order to separate vias of metal material from layers of dielectric material located above and below the vias. The barrier liner is made of a specific material impermeable to Copper or Aluminium for example, in order to prevent the diffusion of metal material into dielectric material. 
     A phenomenon called electromigration is a well-know problem in electronic integrated circuits. Electromigration occurs when a current flows in an integrated circuit. Electromigration relates to displacement of atoms in an interconnect such as metal lines or vias. The movement of atoms produces a void of atoms of Copper for example at one part of the interconnect and accumulation of Copper at another part of the interconnect. This movement of atoms can produce weakness in the interconnect and may ultimately lead to failure: either due to open-circuits caused by voids, or by short-circuits caused by Copper accumulation and cracking of the dielectric material. 
     When electromigration occurs in an electronic circuit, which comprises a barrier liner, two effects are visible. Firstly in the interconnect, movement of atoms leads to voids of atoms above or below the barrier liner, depending on current flow and secondly movement of atoms also leads to accumulation of atoms below or above the barrier liner. This movement of atoms in the interconnect may produce failures in the electronic circuit as above. In short, barrier liners at the via bottom stop the movements of the metal atoms. Thus, by far the most electromigration failure sites are located at the barrier liner. 
     In order to avoid the voids and accumulations of atoms, U.S. Pat. No. 6,380,075 discloses a solution using a specific process called the “punch through barrier deposition process” where the barrier liner is only made on the sidewalls of vias. The bottom of the via is free of barrier liner. Thus, when a current flows in the electronic circuit, atoms of Copper are no longer stopped and accumulated at the barrier liner. Therefore, electromigration is reduced. However the problem of accumulation of atoms at the other side of the interconnect still remains unresolved and leads to further electromigration degradation problems. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus which overcomes at least some of the problems associated with the prior art. 
     According to one aspect of the invention there is provided a method and an apparatus for integrated circuit layout as described in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a circuit layer diagram of a first electronic integrated circuit, of the prior art. 
         FIG. 2   a  is a circuit layer diagram of an electronic integrated circuit, in accordance with one embodiment of the invention, given by way of example, 
         FIG. 2   b  is a circuit layer diagram of an electronic integrated circuit, in accordance with one embodiment of the invention, given by way of example, 
         FIG. 3  is a circuit layer diagram of an electronic integrated circuit, in accordance with one embodiment of the invention, given by way of example, 
         FIG. 4  is a circuit layer diagram of an electronic integrated circuit, in accordance with one embodiment of the invention, given by way of example and 
         FIG. 5  is a flowchart of the method steps, in accordance with one embodiment of the invention, given by way of example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a layer structure  100  of an electronic integrated circuit as known in the prior art. An electronic component  110  such as a transistor or a capacitance for example is located on the bottom of the structure. Another similar electronic component  112  or a bondpad is located at the top of the structure. Layers  114  and  116  separate both components and are defined as inter-level dielectric layers. Inter-level dielectric layers  114  and  116  are made of dielectric material. Layers  114  and  116  comprise interconnects defined as metal lines  118 , 120  and vias  122 , 124 . Both metal lines and vias are made of Copper or Aluminium for example. A barrier line  126 ,  128  forms a separation between interconnects  118 ,  120 ,  122 ,  124  and inter-level dielectric layer  114  and  116 . The barrier liner  126 ,  128  is not provided at the bottom of both vias  122 ,  124 . The deposition of such a barrier liner occurs through a well-known process called “punch through deposition barrier process”. 
       FIG. 2   a  describes a modified structure as defined in the present invention. Metal line  118  comprises specific zones  130  and  132  on both extremities of metal line  118 . The zones  130  and  132  belong to the original metal line and are called sink zones or receptor zones. Differing from prior art and original metal line, both zones can be made of a specific material called receptor material such as a porous material different from Copper or Aluminium material. The material can be for example a porous low k such as SiCOH. The function of zones  130  and  132  called receptor zones is to absorb atoms of Copper during electromigration effect when a current flows in the electronic circuit as will be described later in further details. Ideally, the material of the interconnect is already a porous dielectric material or low k material, thus no additional process steps are required. If the integration uses a non-porous dielectric the sink zones are formed by a porous low k material commonly used in industry. The sink zones must be chosen to be large enough to allow all the Copper, which migrates during the expected life of the device, to accumulate. 
     As shown in  FIG. 2   b , an alternative modification of the original metal line  118  is to put a relatively malleable material inside zones  130  and  132  or all around the metal line  118 . In this way, the malleable material can absorb atoms of Copper by extending the length of zones  130  and  132 . Differing from the previous solution, as the zones are extendible with the malleable material, the zones do not show any accumulation of Copper atoms. Thus, zones  130  and  132  do not create any degradation in the electronic circuit. 
     Another alternative modification of the original metal line  118  is to not use any dielectric material at all, and instead use a so called airgap. In the case of airgaps, Copper atoms are free to accumulate. The distance between lines must be designed to be sufficiently large to allow accumulation without causing shorting. 
       FIG. 3  shows a modified structure as defined in the present invention. Metal line  120  comprises a specific zone  134 . This zone  134  called the source zone or donor zone may or may not already belong to the metal line  120 . This zone  134  is made of a donor material. This zone may be a bondpad made of Copper already used in the electronic circuit  100  in order to connect the circuit with an outside device. This zone  134  can also be an additional zone made of Copper. The function of the specific zone  134  is to release atoms of Copper during electromigration effect as will be described later as a result the specific zone  134  may be referred as a donor zone. 
     The process in accordance with the present invention includes a number of steps. 
     The first step is to detect the movement of atoms of Copper in interconnects during electromigration phenomenon. A specific simulation tool, for example a tool having a three dimensional Poisson solver module can carry out the detection. The aim of the detection is to locate weakness zones around areas of high current density where atoms of Copper accumulate and/or are depleted. The zones are generally located as described in  FIG. 4 . As previously mentioned zones  130  and  132  refer to receptor zones where atoms of Copper generally accumulate during electromigration effect. Therefore, the process allows creation of a sink in order to receive all the Copper atoms coming into this zone when electromigration effect occurs. As previously mentioned donor zone  134  refers to a zone where atoms are depleted. Therefore, the process allows creation of a source in order to donate all Copper atoms from the source without damaging the existing Copper material of the integrated circuit. After identifying and localizing these zones, the circuit design is altered to include space for the sink zones which are then formed during the building of the device. The simulation tool also provides information on how to fill zone  134  with Copper whilst the device is built. If donor zone  134  already refers to a zone already made of Copper material such as a bond pad for example, the simulation tool does not carry out any filling up. 
     The behaviour of an electronic circuit, as defined in the present invention, will now be described when a current flows therein. 
     When a current is applied to the electronic circuit  100 , atoms of Copper in the interconnects are free to move from one interconnect to another in another layer as there is no barrier liner at the bottom of the vias. Atoms of Copper move in the same direction as the flow of electrons i.e. in an opposite direction to the current. Therefore as indicated on  FIG. 4 , atoms of Copper migrate from metal line  120  to metal line  118 . More specifically, atoms of Copper move from zone  134  to zones  130  and  132 . As zone  134  is well provided with atoms of Copper through existing bond pads or through additional Copper material, zone  134  keeps a homogeneous surface. This means that no weakness, failures or cracks appear in zone  134  during electromigration process. Similarly, as zones  130  and  132  are provided with specific porous material or malleable material, this specific material can absorb all atoms of Copper coming from interconnect  118  or  120 . Therefore, no accumulation occurs in zones  118  and  120 . Therefore, no degradation occurs in the electronic circuit because of void formation along the interconnect or accumulation on the interconnect, at these specific zones  118  and  120 . 
     Indeed, the present invention refers to electromigration phenomenon in an integrated circuit using a punch through barrier deposition process to avoid having a barrier liner at the bottom of vias. The present invention allows compensating movement of atoms of Copper during the electromigration phenomenon, using the barrier-free via bottom process. The modification of specific zones in the interconnects allows the creation of a compensation process in order to avoid failures in the interconnect. Specific zones are provided with corresponding specific material in order to achieve two different functions: sink function and source function. The source zone  134  allows a release of atoms of Copper without damaging the interconnect. The sink zones  130 ,  132  allow reception of atoms of Copper without damaging the interconnect. Therefore, electrical conductivity of electronic circuit is maintained when electromigration effect occurs. 
     Examples of combinations, metals, different circuits, different zones, (all or some) look at all the variations. 
     It will be appreciated that the examples described above are just that and many other alternatives may exist, which fall within the scope of the present invention.