Abstract:
The present invention provides a sidewall oxygen diffusion barrier and a method for fabricating the sidewall oxygen diffusion barrier that reduces the diffusion of oxygen into contact plugs during a CW hole reactive ion etch of a ferroelectric capacitor of an FeRAM device. In one embodiment the sidewall barrier is formed from a substrate fence. In another embodiment, the sidewall barrier is formed by etching back an oxygen barrier.

Description:
FIELD OF THE INVENTION 
   The present invention relates to reducing oxidation of contact plugs and relates more particularly to structures and processes for reducing oxygen diffusion through bottom electrode fences during CW hole reactive ion etch processing. 
   BACKGROUND OF THE INVENTION 
   In prior-art COP (capacitor on plug) devices, contact plugs are often used as vertical interconnects between metal lines in multilevel interconnect schemes During processing of a CW (contact window) hole opening using an oxide reactive ion etch (RIE), oxygen forms in the TEOS (Tetraethyl Orthosilicate) hardmask covering the capacitor. An Ir (Iridium) based barrier is often positioned between a bottom electrode (BE) and the TEOS substrate to block oxygen from causing damage when it diffuses to the plug. However, TEOS residues of the etching process (“fences”) can form during RIE processing of the bottom electrode. These fences allow the oxygen to diffuse from the TEOS hardmask to the plugs during the CW hole RIE processing. This oxygen reacts with the material of the plug, for example poly silicon or W (Tungsten), causing functional damage to the plug structure, in particular causing contact degradation. 
   One way to avoid this plug oxidization problem is to make the bottom electrode RIE process such that no oxygen-diffusion-allowing fences are formed. However, this is difficult to achieve in typical processes where the hardmask covering the bottom electrode during bottom electrode RIE processing has a steep taper angle. 
   Another way to avoid this plug oxidization problem is to remove the TEOS fences after they are formed. However, this is a difficult and complex process. 
     FIG. 1  illustrates the above problem as found in a ferroelectric capacitor device  11  of the prior art. A ferroelectric (FE) capacitor  13  includes a ferroelectric layer  15  sandwiched between a bottom electrode  17  and a top electrode  19 . The ferroelectric layer  15  can include PZT, SBT, or BLT, for example. The top electrode  19  is covered with a TEOS hardmask  21  used during patterning of the top electrode. The TEOS hardmask  21  and capacitor  13  are also encapsulated by an Al 2 O 3  oxygen barrier layer  22 . The capacitor is covered by an additional TEOS hardmask  23  used during patterning of the bottom electrode  17 . This additional TEOS hardmask  23  is encapsulated by an additional Al 2 O 3  barrier layer  24 . 
   A Ti glue-layer  25  serves to adhere the bottom electrode  17  to a TEOS substrate  27  of the FE capacitor  13 . A plug  29  (made from poly silicon, for example) passes through the device  11  to form an electrical connection between an active region (not shown) and the bottom electrode  17 . Between the Ti glue-layer  25  and the bottom electrode  17  can be barrier layers  31  of Ir (Iridium), IrO 2  (Iridium Oxide) or other materials for blocking oxygen diffusion. During the processing of the capacitor  13 , oxygen RTA or other high temperature oxygen treatments are used. These barrier layers  31  stop the oxygen introduced from these processes from damaging the plug  29 . 
   Metal fences  33  and TEOS fences  35  can be formed during the RIE processing of the bottom electrode  17 . During processing of a CW hole opening  37  using an oxide RIE process, oxygen (schematically illustrated by the dots  39 ) enters the additional TEOS hardmask  23 . As shown schematically by the dots  39 , the oxygen travels from the additional TEOS hardmask  23 , through the TEOS fence  35  and through the TEOS substrate  27  to the plug  29 , causing damage to the plug  29 . The oxygen can similarly pass to W-plugs causing damaging oxidation. 
   It would be desirable to have sidewall structures and sidewall forming processes for reducing the contact plug oxidization. 
   SUMMARY OF THE INVENTION 
   The present invention provides a sidewall oxygen diffusion barrier and method for fabricating the sidewall oxygen diffusion barrier for reducing oxygen diffusion to contact plugs during CW hole reactive ion etch processing of a ferroelectric capacitor of an FeRAM device. In one embodiment the sidewall barrier is formed from a substrate fence, while in another embodiment the sidewall barrier is formed by etching back an oxygen barrier. The invention includes a hardmask used for patterning an electrode formed on a substrate. A contact plug passes through the substrate and is electrically connected to the electrode. A barrier layer is between the electrode and the plug and reduces the diffusion to the plug of oxygen introduced during capacitor processing. A sidewall oxygen diffusion barrier extends from the hardmask to the barrier layer and forms an oxygen-tight seal with the barrier layer for reducing the diffusion of oxygen from the hardmask to the plug. 
   The present invention also includes a method for fabricating the sidewall oxygen diffusion barrier. The method includes forming a substrate having a contact plug passing therethrough for electrically connecting a bottom electrode of the capacitor to an underlying active layer; depositing over the substrate the bottom electrode including a barrier layer intermediate therebetween; depositing on the bottom electrode a hardmask; etching to pattern the bottom electrode using the hardmask; forming, during the etching step, a sidewall oxygen diffusion barrier extending from the hardmask to the barrier layer and forming an oxygen-tight seal with the barrier layer for reducing the diffusion of oxygen from the hardmask to the contact plug. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     Further preferred features of the invention will now be described for the sake of example only with reference to the following figures, in which: 
       FIG. 1  shows a ferroelectric capacitor device of the prior art having fences through which oxygen diffuses. 
       FIG. 2  shows a first embodiment of the invention wherein a sidewall oxygen diffusion barrier is formed from substrate fences to reduce oxidation of the plug. 
       FIG. 3  is a flowchart showing the process for fabricating the embodiment of  FIG. 2  using HCD Nitride with the capacitor barrier, capacitor layer structures and processes of the prior art. 
       FIG. 4  illustrates the second embodiment during the etching of the bottom electrode wherein the etching is stopped before the etch frontier reaches the TEOS substrate. 
       FIG. 5  shows an oxygen stopping cover layer deposited over the outer portions including the TEOS hardmask, metal fences, and remaining underlying layers such as the barrier layers and Ti glue-layer. 
       FIG. 6  illustrates an additional anisotropic RIE processing step of the second embodiment in which the etching is continued into the TEOS substrate and the oxygen stopping cover layer is etched-back to form a sidewall oxygen diffusion barrier. 
       FIG. 7  shows the processed device with a sidewall oxygen diffusion barrier of the second embodiment for reducing oxidation of the plug. 
       FIG. 8  is a flowchart showing the etch-back process for fabricating the second embodiment of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 2  shows a first embodiment of the present invention for reducing oxidation of the plug  29 . The prior-art TEOS substrate  27  illustrated in  FIG. 1  is replaced with the substrate  41  composed of HCD Nitride (Hexachlorodisilane SiN). By using the HCD Nitride substrate  41 , the resulting fences  43  formed during patterning of the bottom electrode  17  are composed of HCD Nitride, rather than TEOS as in the prior art. The HCD Nitride has better oxygen barrier performance than TEOS, thereby reducing the diffusion of the oxygen  39  from the TEOS hard mask  23  to the plug  29 . Thus, rather than having the oxygen passing fences  35  of the prior art, the fences  43  of the present embodiment serve as sidewall oxygen diffusion barriers. These sidewall oxygen diffusion barriers extend from the TEOS hardmask  23  to the underlying oxygen barrier layers  31 , forming an oxygen-tight seal with the oxygen barrier layers  31 . 
   HCD Nitride is also superior to LP Nitride (low pressure Nitride) because it has better adhesion to metal barrier properties. HCD Nitride also shows special characteristics such as lower k and a high wet etch rate. 
     FIG. 3  is a flowchart showing the process for using HCD Nitride with the capacitor barrier, capacitor layer structures and processes of the prior art. In the flowchart, references to O2 mean that the RTA process in performed in Oxygen, Also, the thickness are provided in unites of Angstroms (A). As a final step illustrated in  FIG. 3 , a peeling test was performed. Scanning Electron Microscope examination revealed that the resulting structure incorporating the HCD Nitride layer passed the peeling test. 
     FIGS. 4–6  show the device of the second embodiment of the present invention during the various processing steps resulting in the processed device of  FIG. 7 . The flowchart of  FIG. 8  shows the method steps for fabricating the second embodiment. The flowchart of  FIG. 8  starts with the step  101  of RIE patterning of the bottom electrode  17 . Like the first embodiment of  FIG. 2 , the second embodiment results in a sidewall oxygen diffusion barrier  47  extending from the TEOS hardmask  23  to the underlying oxygen barrier layers  31 , forming an oxygen-tight seal with the oxygen barrier layers  31  (see  FIG. 7 ). 
   In the prior art of  FIG. 1  and the first embodiment of  FIG. 2 , the initial etching of the bottom electrode continues all the way into the substrates  27  and  41 , respectively, of the capacitors. This results in the fences  35 ,  43  formed from the respective TEOS and HCD Nitride materials. In the second embodiment, the RIE processing of the bottom electrode can etch into the underlying oxygen barrier layers  31  or Ti glue-layer  25 , but the etch is stopped before the etch frontier reaches the TEOS layer  27  as illustrated in  FIG. 4  and step  103  of  FIG. 8 . The metal fences  33  are formed during the bottom electrode etching. 
   Next, as shown in  FIG. 5  and step  105  of  FIG. 8 , an oxygen stopping cover layer  45  is deposited over the outer portions including the TEOS hardmask  23 , metal fences  33  and remaining underlying layers such as the barrier layers  31  and Ti glue-layer  25 . The oxygen stopping cover layer  45  can be composed of a Nitride, such as HCD Nitride, or Al 2 O 3 , for example. The cover layer  45  can be deposited using PVD (physical vapor deposition), CVD (chemical vapor deposition) or ALD (atomic layer deposition) processes, for example. 
   As illustrated by step  107  of  FIG. 8  and by  FIG. 6 , an additional RIE processing step (or continuation of the previous RIE processing step) is performed in which the etch is continued into the TEOS substrate  27 . The RIE process is anisotropic, etching away more horizontally oriented portions of the oxygen stopping cover layer  45  while leaving more vertically oriented portions of the oxygen stopping cover layer  45  clinging to the TEOS hardmask  23 , metal fences  33 , or underlying layers The remaining more vertically oriented portions of the oxygen stopping cover layer become the oxygen barrier fence or sidewall oxygen diffusion barrier  47 . In other words, the cover layer  45  is etched back to leave the sidewall oxygen diffusion barrier  47 . 
   Finally, an additional Al 2 O 3  barrier layer  24  is deposited around the TEOS hardmask  23  and sidewall barrier  47  at step  109  of  FIG. 8 .  FIG. 8  shows a device of the present invention after processing. Once again, during processing of the CW hole opening  37  using an oxide RIE process, oxygen  39  enters the additional TEOS hardmask  23 . The sidewall barrier  47  forms an oxygen-tight seal with the barrier layer  24  as well as with the oxygen barrier layers  31  to form a continuous encapsulation barrier for reducing the oxygen  39  passing from the hardmask  23  to the plug  29 . 
     FIG. 7  differs slightly from  FIG. 6  in that it shows additional TEOS fences  35  formed during the RIE processing of the cover layer  45 . Even with these fences  35 , due to the sidewall barrier  47  with covering barrier layer  24 , oxygen  39  is still substantially prevented from passing from the hardmask  23  to the plug  29 . 
   Other materials and method steps can be added or substituted for those above. Thus, although the invention has been described above using particular embodiments, many variations are possible within the scope of the claims, as will be clear to a skilled reader.