Abstract:
A hard mask stack for etching a magnetic tunneling junction (MTJ) structure is described. An electrode layer is deposited on a stack of MTJ layers on a bottom electrode. A photoresist mask is formed on the electrode layer. The electrode layer is etched away where it is not covered by the photoresist mask to form a metal hard mask. The metal hard mask is passivated during or after etching to form a smooth hard mask profile. Thereafter, the photoresist mask is removed and the MTJ structure is etched using the metal hard mask wherein the metal hard mask remaining acts as a top electrode. The resulting MTJ device has smooth sidewalls and uniform device shape.

Description:
TECHNICAL FIELD 
     This application relates to the general field of magnetic tunneling junctions (MTJ) and, more particularly, to etching methods for forming MTJ structures. 
     BACKGROUND 
     Tantalum (Ta) is one of the best hard masks for MTJ reactive ion etching (RIE) due to a very low etching rate using Methanol and high etching selectivity to MTJ materials. Usually, a Ta hard mask is patterned by a dielectric hard mask, for example silicon dioxide (SiO 2 ) or silicon nitride (SiN x ), and the dielectric hard mask is patterned by photoresist (PR). This is because there will be a fencing issue if PR is used directly to pattern Ta. However, SiO 2  etching by high SiO 2 /PR selectivity chemical etching, e.g. C 4 F 8 , will generate severe striations around the SiO 2  sidewall which will transfer to the Ta and even to the MTJ sidewall and lead to rough sidewalls and poor uniformity within the chip. Etching SiO 2  by CF 4  usually has much less sidewall striation; however, the poor SiO 2 /PR selectivity will limit the thickness of SiO 2  and also Ta that can be etched with the same thickness PR. 
     Several patents show the use of various hard masks in etching MTJ stacks. These include U.S. Pat. No. 8,722,543 (Belen et al), U.S. Pat. No. 8,450,119 (Torng et al), U.S. Pat. No. 7,593,193 (Inomata et al), and U.S. Pat. No. 7,001,783 (Costrini et al). Other patents show passivating processes: U.S. Pat. No. 7,169,654 (Zia et al), U.S. Pat. No. 6,395,621 (Mizushima et al), U.S. Pat. No. 8,716,072 (Bangsaruntip et al), and U.S. Pat. No. 7,471,445 (Pan). The process of the present disclosure is not taught or suggested by any of these references. 
     SUMMARY 
     It is an object of the present disclosure to provide a hard mask and an etching scheme to improve etching profile and etching uniformity in forming MTJ structures. 
     Yet another object of the present disclosure is to provide a hard mask and etching scheme for MTJ structures wherein a thick Ta layer remains as a top electrode without increasing the thickness of a photoresist layer. 
     In accordance with the objectives of the present disclosure, a method for etching a magnetic tunneling junction (MTJ) structure is achieved. A stack of MTJ layers is provided on a bottom electrode. An electrode layer is deposited on the stack of MTJ layers. A photoresist mask is formed on the electrode layer. The electrode layer is etched away where it is not covered by the photoresist mask to form a metal hard mask. The metal hard mask is passivated during or after etching to form a smooth hard mask profile. Thereafter, the photoresist mask is removed and the MTJ structure is etched using the metal hard mask wherein the metal hard mask remaining acts as a top electrode. The resulting MTJ device has smooth sidewalls and uniform device shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings forming a material part of this description, there is shown: 
         FIGS. 1 through 4  illustrate in cross-sectional representation steps in a preferred embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides a method of etching the whole MTJ structure to make the electric isolation between MTJ junctions. This method can improve etching uniformity and etching profile within wafers and within chips. For example, a STT-RAM (spin transfer torque random access memory) can be manufactured based on a Ta (or TaN, Ti, TiN, W, etc.)/PR etching mask scheme. Usually when PR is used to directly pattern a metal hard mask layer, fencing occurs. The fencing issue can be solved by a passivation step, which is to purge O 2 , H 2 O vapor, or air into the chamber or to treat the wafer by low power O 2  plasma after the metal hard mask opening or in between metal hard mask etching steps but before the PR strip, or to expose the wafer to air, or to water-rinse the wafer after metal etching This passivation step can generate a more uniform etching profile with smoother sidewalls. This method can also generate thick Ta remaining as the top electrode without increasing the thickness of the PR. 
     One previous method of etching a MTJ stack used a hard mask stack of Ta/SiO 2 /PR, in which Ta was also used for the top electrode of the MTJ. The SiO 2  (or similar dielectric layer) was defined by PR and etched by CF 4 . The Ta (or similar metal layer) was defined by SiO 2  and etched by CF 4 . Then, the Ta layer was used as the MTJ etching mask and as the top electrode after the MTJ etch. 
     Etch rate and selectivity are listed in Table I and Table II. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Etch Rate of Different MTJ Etch Mask Materials. 
               
             
          
           
               
                   
                   
                 Etch Rate (nm/min) in 
                 Etch Rate (nm/min) in 
               
               
                   
                 Material 
                 CF 4   
                 CH 3 OH 
               
               
                   
                   
               
             
          
           
               
                   
                 PR 
                 203.40 
                 NA 
               
               
                   
                 Ta 
                 83.4 
                 4.10 
               
               
                   
                 SiO2 
                 126.00 
                 26.70 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Etching Selectivity in CF 4  of Different MTJ Etch mask Materials 
               
             
          
           
               
                   
                   
                 Etch 
               
               
                   
                 Materials 
                 Selectivity 
               
               
                   
                   
               
               
                   
                 SiO 2 /PR 
                 0.62 
               
               
                   
                 Ta/SiO 2   
                 0.66 
               
               
                   
                 Ta/PR 
                 0.41 
               
               
                   
                   
               
             
          
         
       
     
     The old MTJ etch using Ta/SiO 2 /PR masks has more limitations to generating good uniformity and smooth sidewalls. One reason is the necessity to transfer the pattern twice. Although the selectivity of Ta/SiO 2 /PR using CF 4  is comparable to Ta/PR using CF 4 , as shown in Table II, a SiO 2  protection layer (˜150-200 A) was required before PR strip to obtain a better profile, which limited Ta thickness although thicker Ta is always desired. PR strip also will consume SiO 2 , which will limit the Ta thickness even further. However, CF 4  can generate a relatively smoother sidewall than other high selectivity etch chemistries. 
     The new MTJ etching mask of the present disclosure will be described in more detail with reference to the drawing  FIGS. 1-4 . Referring now more particularly to  FIG. 1 , there is shown a bottom electrode layer  10 . On the bottom electrode have been formed a stack of layers  14  that will be patterned to form a MTJ structure. A layer  16  of Ta, or alternately tantalum nitride (TaN), titanium (Ti), titanium nitride (TiN), tungsten (W), or the like, is deposited on the MTJ stack  14 . This Ta layer, or electrode layer,  16  should have a thickness of between about 380 and 1000 Angstroms. A photoresist layer is deposited to a thickness of between about 1000 and 4700 Angstroms and patterned to form photoresist mask  20 . 
     Since the PR layer is coated on Ta instead of transparent SiO 2  or SiN X , a better profile PR mask  20  can be obtained. 
     Now, referring to  FIG. 2 , the Ta or electrode layer mask  17  is defined by the PR mask  20 , using reactive ion etching (RIE) with Fluorine or Chlorine based chemicals. 
     One more step is added after Ta etching, but before stripping the PR mask. A passivation step is performed by flowing O 2 , H 2 O vapor, or air into the etching chamber or by low power O 2  plasma after Ta etching. Alternatively, the wafer can be exposed to the ambient air or be water rinsed after etching. This step is critical to form smooth sidewalls and a uniform shape of the device. 
     The passivation step after Ta-etching is critical for the Ta (or TaN, Ti, TiN, W, etc.)/PR etching mask. Without passivation or with insufficient passivation, the shape of the MTJ device will be impacted due to the fencing issue, which will lead to rough device sidewalls and a non-uniform shape of the device. The current tool has limitations on H 2 O vapor and air flow and O 2  flow rate and pressure, so we can use the alternative method for passivation, which is low power O 2  plasma treatment or exposure of the wafer to the ambient air or water rinsing the wafer. The exposure time has been found to be not very critical to the smoothness of the sidewall. For example, preferred flow rates for H 2 O vapor or O 2  or air are between about 500 and 3000 sccm, at a pressure of between about 1 and 3 Torr. The preferred O 2  plasma treatment is low power (source power&lt;100 W, and bias power=0). 
     As another alternative, the passivation may be performed during the Ta etching step instead of after the Ta etching by flowing H 2 O vapor or O 2  or air into the etching chamber along with the etching gases. For example, preferred flow rates for H 2 O vapor or O 2  or air are between about 500 and 1000 sccm, at a pressure of between about 1 and 3 Torr. 
     Sometimes, if the electrode layer  18  is thicker than 380 Angstroms, the metal etch needs to be separated into two or more steps, and the passivation is required between every two steps. 
     The critical passivation step before PR removal provides a very uniform electrode mask  18  with smooth sidewalls. Now, the PR mask  20  is removed using conventional stripping methods, leaving Ta or electrode mask  18  on the MTJ layer stack, as shown in  FIG. 3 . 
     The passivated mask  18  then is used as the MTJ etching mask to etch the MTJ device  15 , by either RIE or ion beam etching (IBE) as shown in  FIG. 4 . The MTJ is etched using CH 3 OH based chemicals or CO and NH 3 -based chemicals. For example, the etchant can be CH 3 OH or CH 3 OH with other gases such as Ar, O 2 , H 2 , N 2 , or the like. 
     The remaining mask  18  is used as the top electrode after the MTJ etch. It should be noted that a thick Ta layer is required for IBE due to poor selectivity of IBE. The method of the present disclosure can be also beneficial because this new method can etch thicker Ta with the same thickness of PR. 
     The new MTJ etch process of the present disclosure using Ta (or TaN, Ti, TiN, W, etc.)/PR etching mask with added passivation step can give us a better PR profile because:
         the PR is coated on metal instead of on transparent SiO 2  or SiN x      there are fewer pattern transfer steps   the SiO 2  or SiN x  striation effect is avoided   the hard mask has a smooth profile resulting in a better etching profile and better uniformity of the MTJ device within the wafer and within the chip   a thicker Ta layer can be etched using the same thickness of PR, which will provide a larger CMP process window, and leave enough thickness of Ta as the top electrode of the MTJ.       

     The present disclosure provides an improved process for MTJ etching by using a simple Ta/PR etching mask stack and by adding a passivation step during or after metal etching and prior to PR stripping. A better photoresist and etch profile and better uniformity within the chip and across the wafer are obtained. The passivation step by O 2 , O 2  plasma, H 2 O vapor, H 2 O, or air is critical to obtain smooth sidewalls and uniform devices. This simple mask stack also can be beneficial to etch a thicker top electrode (Ta). 
     Although the preferred embodiment of the present disclosure has been illustrated, and that form has been described in detail, it will be readily understood by those skilled in the art that various modifications may be made therein without departing from the spirit of the disclosure or from the scope of the appended claims.