Patent Publication Number: US-2007105390-A1

Title: Oxygen depleted etching process

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to processing of thin film structures. More specifically, the present invention relates to plasma etching processes for etching thin film structures.  
     BACKGROUND OF THE INVENTION  
      Titanium (Ti), titanium oxide (TiO), and titanium nitride (TiN) thin films have been used as glue layers (also called adhesion layers) and barrier layers in semiconductor and microelectronic applications. Titanium (Ti) and titanium nitride (TiN) can also be used as hard mask layers for various etching steps due to their resilient etch characteristics, particularly at high etch temperatures encountered in dry etching (e.g., plasma etching) processes. Although conventional photoresist materials can be used as a mask layer, high temperature plasma etching processes can exceed a thermal budget of photoresist materials. At processing temperatures that rise above approximately 150° C., photoresist will begin to reticulate. As the processing temperature approaches approximately 180° C., photoresist will begin to burn. Consequently, the use of photoresist as a hard mask is limited to low temperature plasma etching processes where the processing temperatures is less than approximately 150° C.  
      Therefore, in high temperature plasma etching processes, hard masks made from Titanium (Ti) and titanium nitride (TiN) have been widely used, especially as a hard mask for noble metals, such as platinum (Pt), ruthenium (Ru), and iridium (Ir), for example. Titanium (Ti) and titanium nitride (TiN) have also been used as a hard mask for conductive metal oxide materials (CMO). Examples of CMO materials include perovskites such as PCMO and LNO. The inherent etch properties of noble metals and CMO require a high temperature plasma etching processes to ensure a reasonable feature profile and to minimize residue formation due to by-product re-deposition on the surface of the film being etched. It is well understood in the microelectronics art that low temperature plasma etching of noble metals and CMO materials produces a non-volatile by-product. It is also well understood in the microelectronics art that temperature (e.g., a high temperature) is a key process parameter that can be used to control the re-deposition of by-products.  
      Although high processing temperatures can operate to limit re-deposition of etch by-products, the high processing temperatures can also accelerate chemical reactions, such as oxidation of materials exposed to the etch plasma at high temperatures, for example. During a high temperature plasma etching process, thin films of Ti, TiO, or TiN can be exposed to the plasma, where chemical processes such as ionization, recombination, and dissociation are constantly occurring. Consequently, those films become oxidized and as the oxidation continues, those films become increasingly resistant to the plasma etch process. Moreover, the oxidation of those films accelerates when oxygen (O 2 ) is introduced into the plasma etch environment, either as a gas mixed in with the etch gas or in an oxygen (O 2 ) containing material (e.g., SiO 2 ). Examples include Ti oxidizing into TiO 2  and TiN oxidizing into TiON. One consequence of the oxidation process is that TiO 2  and TiON become resistant to chemical etching, physical etching (e.g., plasma etching), and ion etching (e.g., ion bombardment).  
      The oxidation process can be exacerbated by process variables such as temperature and oxygen (O 2 ) content, for example. Higher temperatures accelerate the oxidation process; whereas, increasing oxygen (O 2 ) content in the etching environment exponentially increases in the oxidation process. The TiO 2  or TiON can form a residue that covers and shields an underlying layer from the plasma etch process. Therefore, the TiO 2  or TiON can serve as a secondary mask layer that protects the underlying layer during the plasma etching in much the same manner as a hard mask. As the plasma etching proceeds through the underlying layer, a portion of the underlying layer that is covered by the secondary mask is not etched away and remains on a subsequent layer. As a result, a residue forms on the subsequent layer. Eventually, when the plasma etching process terminates, the residue can remain on a bottom most layer and that residue can result in a yield reducing defect in a device.  
      In  FIG. 1A , a conventional plasma etching process is used to etch a stack of thin film materials  100  through a hard mask  101 . Examples of materials for the hard mask  101  include silicon nitride (Si 3 N 4 ) and silicon oxide (SiO 2 ). The stack of thin film materials  100  includes a layer  103  of a titanium material, such as the aforementioned titanium (Ti), titanium oxide (TiO), or titanium nitride (TiN) thin films, for example. Below the layer  103  is a layer  105 . The layer  105  can be a noble metal, such as platinum (Pt), ruthenium (Ru), or iridium (Ir), for example. A subsequent layer  107  can also be a titanium material as described above. For example, if the layer  105  is made of platinum (Pt), the layers  103  and  107  can be an adhesion layer. A layer  109  can be a dielectric layer (e.g. SiO 2 ) and can function as an etch stop layer. A layer  121  can be a semiconductor substrate, such as a silicon (Si) wafer, for example.  
      Although not shown, the hard mask  101  can be a layer of material that is deposited on the layer  103  and is subsequently patterned and etch to form the hard mask  101 . In  FIGS. 1A and 1B , an etch plasma p selectively etches the layer  103  during a plasma etch process as depicted by the dashed arrows in  FIGS. 1A and 1B . The etch materials for the plasma p can be selected so that the etch process is anisotropic and results in two discrete thin film stacks (see reference numerals  104  in  FIG. 1D ) being formed as the plasma etch proceeds as depicted by heavy dashed lines  102 . As the etch proceeds, oxygen (O 2 ) in the plasma p and/or the hard mask  101 , chemically reacts with the titanium material in the layer  103 . A chemical reaction between the etch materials in the plasma p (e.g., the O 2 ) and the titanium material in the layer  103  forms a thin layer of a titanium oxide (TiO 2 ) residue  103   r.  The residue  103   r  is resistant to the etch materials in the plasma p and serves as a secondary hard mask. Moreover, the oxidation process caused by the oxygen in the plasma p can be accelerated by heating h the stack  100  to a high temperature. For some materials, such as the aforementioned noble metals, high temperature processing is necessary for the plasma etching process to effectively etch the material with a desired profile. Although the plasma p is selective to the layer  103 , the residue  103   r  is highly resistant to the plasma p and is not dissolved by the etch materials in the plasma p. Consequently, as the plasma p etches through the layer  103 , the residue  103   r  continuously forms and propagates in a direction  103   p  along a receding surface of the layer  103  as depicted in  FIG. 1B . Eventually, the residue  103   r  is positioned over the layer  105  and partially shields a portion of the layer  105  from the plasma p.  
      In  FIG. 1C , the shielding by the secondary hard mask results in a residue  105   r  forming in the layer  105 . The residue  105   r  serves as a secondary hard mask and propagates  105   p  in the direction of the etching. Finally, in  FIG. 1D , when the etching process has ended, a residue  115   r  resides on the layer  109 . The residue  115   r  can cause defects or contamination that can reduce device yields. For example, the residue  115   r  can include materials from some or all of the layers that preceded the layer  109 . If any of the preceding layers included an electrically conductive material (e.g. platinum Pt in the layer  105 ), then the residue  115   r  can create an electrical short between adjacent thin film stacks  104 .  
      Sources for the oxygen (O 2 ) that cause the oxidation of the titanium material include the etch gasses used for the plasma p and/or the thin film materials in the stack  100 . The hard mask  101  can be an oxygen (O 2 ) containing material O 2  (e.g., silicon oxide SiO 2 ). Examples of etch gasses that the oxygen (O 2 ) can be a component of include but are not limited to argon (Ar), chlorine (Cl 2 ), boron trichloride (BCl 3 ), and fluorinated gasses (CF x ).  
      There are continuing efforts to improve etch chemistry and etch processes for plasma etching of thin film materials.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A through 1D  depict a conventional plasma etching process using a plasma containing oxygen;  
       FIG. 2  is a flow chart depicting one embodiment of an oxygen depleted plasma etching process;  
       FIG. 3  is a flow chart depicting an alternative embodiment of an oxygen depleted plasma etching process;  
       FIGS. 4A through 4C  depict a patterning and a developing of a mask layer to form an etch mask on an oxygen free hard mask layer;  
       FIGS. 4D through 4G  depict an oxygen depleted etching of an oxygen free hard mask layer to form an oxygen free hard mask;  
       FIG. 4H  depicts an oxygen free hard mask positioned on a stack of thin film materials;  
       FIGS. 4I through 4J  depict an oxygen depleted plasma etching of a stack of thin film materials;  
       FIG. 5  is a flow chart depicting yet another embodiment of an oxygen depleted plasma etching process;  
       FIG. 6A  depicts patterning a mask layer formed on an oxygen free hard mask layer that includes titanium;  
       FIG. 6B  depicts an oxygen free plasma etching of the oxygen free hard mask layer depicted in  FIG. 6A ;  
       FIG. 6C  depicts an oxygen free hard mask that includes titanium;  
       FIG. 6D  depicts an oxygen depleted etching of a stack of thin film materials;  
       FIGS. 7A and 7B  depict an embodiment of a mixed mode plasma etching process;  
       FIG. 8A  depicts a patterning of a mask layer to form an etch mask on a hard mask layer;  
       FIG. 8B  depicts a patterned mask layer for forming an etch mask on a composite hard mask layer;  
       FIG. 8C  depicts a developing of a mask layer to form an etch mask.  
       FIGS. 8D through 8F  depict an oxygen depleted plasma etching of a hard mask layer to form a hard mask;  
       FIG. 8G  depicts an oxygen depleted plasma etching of a composite hard mask layer to form a hard mask;  
       FIG. 8H  depicts an oxygen depleted plasma etching of a stack of thin film materials to a first predetermined layer in the stack;  
       FIGS. 8I through 8J  depict an oxygen containing plasma etching of a stack of thin film materials to a second predetermined layer in the stack;  
       FIGS. 8K through 8M  depict an oxygen depleted plasma etching of a stack of thin film materials to a third predetermined layer in the stack;  
       FIG. 9A  depicts a stack of thin film materials that includes a very thin layer of a dielectric material positioned between layers in the stack; and  
       FIG. 9B  depicts an oxygen depleted plasma etching of the stack of thin film materials of  FIG. 9A . 
    
    
      Although the previous Drawings depict various examples of the invention, the invention is not limited by the depicted examples. Furthermore, the depictions are not necessarily to scale.  
     DETAILED DESCRIPTION  
      In the following detailed description and in the several figures of the drawings, like elements are identified with like reference numerals.  
      As shown in the drawings for purpose of illustration, the present invention is embodied in a method of oxygen depleted etching of thin films at a high temperature. In a first embodiment, the method includes forming a mask layer on an oxygen free hard mask layer, patterning the mask layer, developing the mask layer to form an etch mask on the oxygen free hard mask layer, etching the oxygen free hard mask layer in an oxygen free etch plasma to form an oxygen free hard mask, optionally removing the etch mask, etching a stack of thin film materials patterned by the oxygen free hard mask in an oxygen free etch plasma at a high temperature, and terminating the etching at a predetermined layer in the stack of thin film materials.  
      In a second embodiment, the method includes forming a mask layer on a hard mask layer that is not oxygen free, patterning the mask layer, developing the mask layer to form an etch mask on the hard mask layer, etching the hard mask layer in a substantially oxygen free etch plasma to form a hard mask, optionally removing the etch mask, etching a stack of thin film materials patterned by the hard mask in a substantially oxygen free etch plasma at a high temperature, and terminating the etching at a predetermined layer in the stack of thin film materials.  
      In a third embodiment, the method includes forming a mask layer on an oxygen free titanium hard mask layer, patterning the mask layer, developing the mask layer to form an etch mask on the oxygen free titanium hard mask layer, etching the oxygen free titanium hard mask layer in an oxygen free etch plasma to form an oxygen free titanium hard mask, etching a stack of thin film materials patterned by the oxygen free titanium hard mask in an oxygen free etch plasma at a high temperature, and terminating the etching at a predetermined layer in the stack of thin film materials.  
      In a fourth embodiment, a mixed mode method includes forming a mask layer on a hard mask layer, patterning the mask layer, developing the mask layer to form an etch mask on the hard mask layer, etching the hard mask layer in a first oxygen free etch plasma to form a hard mask, etching a stack of thin film materials patterned by the hard mask in the first oxygen free etch plasma at a high temperature, terminating the etching at a first predetermined layer in the stack of thin film materials, continuing to etch the stack in a second oxygen containing etch plasma at a high temperature, terminating the etching at a second predetermined layer in the stack, continuing to etch the stack in a third oxygen free etch plasma at a high temperature, and terminating the etching at a third predetermined layer in the stack.  
      An Oxygen Free Hard Mask  
      Referring now to  FIG. 2  and  FIGS. 4A through 4C , a method  200  of oxygen depleted etching includes, at a stage  203 , forming a mask layer  407  on an oxygen (O 2 ) free hard mask layer  409 . Preferably, the hard mask layer  409  is made from an electrically non-conductive material. At a stage  205 , the mask layer  407  is patterned. At a stage  207 , the mask layer  407  is developed to form an etch mask  407 M on the hard mask layer  409 . The forming, patterning, and developing of the mask layer  407  can be accomplished using processes that are well understood in the microelectronics art. For example, the mask layer  407  can be a photoresist material that is spin deposited on a surface of the hard mask layer  409 . Photolithography can be used to expose a pattern  407 P in the mask layer  407  using light L (see  FIG. 4A ). Subsequently, the mask layer  407  can be developed D using a wet or a dry etching process to form the etch mask  407 M (see  FIG. 4C ). If dry etching (e.g. plasma etching) is used to develop the mask layer  407 , then a temperature in the plasma environment should be within an acceptable range of temperatures for the material selected for the mask layer  407 . Therefore, if the mask layer  407  is a photoresist material, then the temperature should be adjusted to an appropriate temperature range. Typically, a temperature of approximately 150° C. or less is suitable for plasma etching of photoresists. More preferably, the temperature is approximately 100° C. or less for photoresist materials. For example, the plasma etching can occur at approximately room temperature (e.g., about 25° C.).  
      Turning now to  FIGS. 4D through 4G , after the etch mask  407 M is formed, at a stage  209 , the hard mask layer  409  is etched in an oxygen (O 2 ) free etch plasma P to form a hard mask  409 M. As the plasma etch proceeds, a surface  409 S of the mask layer  409  that is not covered by the etch mask  407 M, recedes  409 R in a direction towards an underlying layer  411 . The plasma etching can be halted when a surface  411 S of the underlying layer  411  is exposed. Endpoint detection techniques that are well understood by those skilled in the microelectronics art can be used to determine when to terminate the etching at the stage  209 . Determining the endpoint for terminating the etching at the stage  209  can include but is not limited to techniques such as etch time, spectral analysis of a light spectra emitted by the oxygen free plasma P, the use of a material suitable as an etch stop layer, and chemical analysis of the plasma P to detect one or more constituent compounds in the oxygen free plasma P that are indicative of having reached the endpoint, for example.  
      At a stage  210 , the etch mask  407 M may optionally be removed from the hard mask  409 M. The decision to remove the etch mask  407 M can be based on the material used for the etch mask  407 M and its ability to withstand high temperatures in a subsequent plasma etching process at a stage  213 . If the etch mask  407 M is made from a photoresist material or some other material that cannot withstand high temperature processing, then at a stage  211 , the etch mask  407 M can be removed from the hard mask  409 M. For example, if the etch mask  407 M is a photoresist material, then an ashing or stripping process can be used to remove the etch mask  407 M. If ashing is used, it is preferable that the ashing process is also oxygen free because the underlying layer  411  can be made from a material that includes titanium or an alloy of titanium that can be oxidized if exposed to oxygen. On the other hand, if the etch mask  407 M can withstand high temperature processing, then the etch mask  407 M need not be removed and the stage  211  can be skipped. However, one skilled in the art will appreciate that the etch mask  407 M may need to be removed to achieve some other process related goal. Therefore, the stage  211  may be implemented to achieve that processing goal.  
      In  FIG. 4G , the oxygen free hard mask  409 M is positioned on the surface  411 S of the underlying layer  411 . The underlying layer  411  is one of a plurality of thin film layers in a stack  400  of thin film materials. The hard mask  409 M will be used to etch through the plurality of thin film layers in the stack  400  that are positioned below the hard mask  409 M as depicted by heavy dashed lines  425 L in  FIG. 4H . The hard mask  409 M opening process using an oxygen free etch plasma P is critical in preventing a formation of a highly etch resistant secondary mask layer proximate the surface  411 S of the underlying layer  411 . By eliminating oxygen (O 2 ) from the material for the hard mask layer  409  and from the plasma P, titanium (Ti) or an alloy of titanium in the layer  411  will not form oxides of titanium (e.g., TION or TiO 2 ) on the surface  411 S that will serve as the highly etch resistant secondary mask layer.  
      In  FIG. 4I , at a stage  213 , exposed layers in the stack  400  are plasma etched through the openings in the hard mask  409 M using an oxygen (O 2 ) free etch plasma P at a high temperature H. The layer  411  is the first layer to be etched, followed by the layers positioned below it. For the same reasons stated above, the oxygen (O 2 ) free etch plasma P prevents by-product re-deposition of oxides of titanium on exposed etch surfaces so that a secondary hard mask layer is not formed. Therefore, a portion  411 F of the surface  411 S of the layer  411  is free of oxides of titanium that can mask subsequent layers in the stack  400  from being etched by the plasma P.  
      In  FIG. 4J , at a stage  215 , the etching terminates at a predetermined layer in the stack  400 . In  FIG. 4J , the etching terminates at a surface  419 S of a layer  419  in the stack  400 . The termination at the stage  215  can be controlled by process parameters such as time or a selection of an appropriate endpoint indicator, for example. As one example, the etching can run for a predetermined period of time and can be halted at the stage  215 . As another example, the plasma P can be monitored by a sensor and an output from the sensor can be coupled with a computer or process controller to determine that an endpoint for the stage  215  has been reached. The sensor can analyze the gasses in the plasma P or a light spectra of the plasma P to detect a condition indicative of the endpoint that triggers termination at the stage  215 .  
      On the other hand, the layer  419  can be made from a material that serves as an etch stop layer that is resistant to the etch plasma P. For example, the layer  419  can be made from a dielectric material, such as silicon nitride (Si 3 N 4 ) or silicon oxide (SiO 2 ). After the stage  215 , the layers in the stack  400  form discrete columns of thin film materials  425 C. Each discrete column  425 C can represent an active device, such as a resistive state memory device, for example. The elimination of by-product re-deposition of oxides of titanium during the plasma etching at stages  209  and  213  can prevent or substantially eliminate secondary mask layer propagation downward in the stack  400  as the etching proceeds so that masking effects of a secondary mask layer does not result in a residue forming on the surface  419 S.  
      One possible consequence of not preventing secondary mask layer formation is that electrically conductive residue can form and create defects or electrical shorts. For example, if an electrically conductive residue is present on the surface  419 S, then a conductive path between sidewall surfaces  417 E of a layer  417  can be formed by the residue, creating a short circuit between the adjacent discrete columns  425 C. If the adjacent columns  425 C define active electrical devices, then those devices can be rendered inoperative due to the short circuit path electrically coupling the devices to each other.  
      The layers of thin film materials in the stack  400  will be application dependent and the stack  400  can include more layers or fewer layers than depicted in  FIG. 4A . TABLE 1 below lists examples of materials that can be used for the layers in the stack  400 . In the examples listed in TABLE 1, the layer  411  can be a titanium (Ti) glue or adhesion layer between the hard mask layer  409  and a noble metal layer  413 . The stack  400  can be fabricated using thin film deposition processes to build the layers up from a substrate layer  421 .  
                   TABLE 1                       Layer   Example Materials                  407   An O 2  Free Etch Mask Material (e.g., photoresist)       409   An O 2  Free Hard Mask Material including an O 2  Free Dielectric           Material       411   titanium (Ti) or an alloy of titanium: (e.g., Ti, TiN, or TiO)       413   A noble metal or an alloy of a noble metal:           (e.g., platinum (Pt), ruthenium (Ru), or iridium (Ir))       415   A Conductive Metal Oxide (CMO) (e.g., a perovskite, PCMO, or           LNO)       417   A noble metal or an alloy of a noble metal:           (e.g., platinum (Pt), ruthenium (Ru), or iridium (Ir))       419   An Electrically Nonconductive Layer:           (e.g. a dielectric layer, Si 3 N 4 , or SiO 2 )       421   A Substrate: (e.g., a semiconductor, silicon (Si),           single crystal Si, or a Si wafer)                  
 
      Preferably, a total thickness T of the layers  413 ,  415 , and  417  is less than approximately 1500 Å (see  FIG. 4A ). For example, a thickness t 1 , t 2 , and t 3  of the layers  413 ,  415 , and  417  respectively, can be approximately 500 Å or less so that the total thickness T of the three layers is less than approximately 1500 Å. The aforementioned residue formation due to by-product re-deposition is more common when a thickness of the thin film (e.g., a noble metal) is approximately 500 Å or less. On the other hand, when the total thickness T of the thin film layer is on the order of thousands of angstroms, then there is a higher probability of the secondary mask layer (e.g., TiO 2  or TiON) being cleared away during the etching of the thin film layer (e.g., a layer of Pt). Therefore, in some applications, it may not be useful to use the oxygen depleted etch process for layer thicknesses on the order of thousands of angstroms (e.g., T&gt;1500 Å).  
      An Oxygen Containing Hard Mask  
      Referring now to  FIG. 3  and  FIGS. 4A through 4C , a method  300  of oxygen depleted etching includes, at a stage  303 , forming the mask layer  407  on the hard mask layer  409  in the stack  400  of thin film layers. However, unlike the method  200  as described above, in the method  300 , the hard mask layer  409  is not oxygen (O 2 ) free. Instead, the hard mask layer  409  is an oxygen (O 2 ) containing material. For example, the hard mask layer  409  can be an electrically nonconductive material such as silicon oxide (SiO 2 ). Alternatively, the hard mask layer  409  can be an electrically conductive material such as titanium oxide (TiO), for example. At a stage  305 , the mask layer  407  is patterned  407 P. At a stage  307 , the mask layer  407  is developed to form the etch mask  407 M (see  FIGS. 4B and 4C ). At a stage  309 , the hard mask layer  409  is etched in a substantially oxygen (O 2 ) free etch plasma P to form a hard mask  409 M (see  FIGS. 4D-4G ). Optionally, at a stage  310 , the etch mask  407 M can be removed at a stage  311  as was described above or the method  300  can continue at a stage  313 .  
      At the stage  313 , exposed layers in the stack  400  are plasma etched through the openings in the oxygen containing hard mask  409 M using a substantially oxygen (O 2 ) free etch plasma P at a high temperature H. Although the etch plasma P at the stages  309  and  313  is initially oxygen free because oxygen (O 2 ) is not intentionally included in the etch gasses that form the plasma P, chemical processes caused by the plasma P reacting with the hard mask  409 M can liberate some of the oxygen (O 2 ) from the hard mask  409 M. Therefore, during the stages  309  and  313 , some of the oxygen (O 2 ) in the hard mask  409 M can be introduced into the plasma P. As a result, the plasma P is not totally free of oxygen (O 2 ). However, the amount of oxygen (O 2 ) introduced into the plasma P is substantially lower than the case where oxygen (O 2 ) is intentionally introduced into the plasma P as one of the etch gasses. Therefore, any residual oxygen (O 2 ) remaining in the plasma P during the etching of the stack  400  results in a substantially oxygen free etch plasma P. At a stage  315 , the etching can be terminated at a predetermined layer in the stack  400  as was described above.  
      Oxygen Free Titanium Hard Mask  
      Turning to  FIG. 5  and  FIGS. 6A through 6D , a method  500  of oxygen depleted etching includes, at a stage  503 , forming a mask layer  605  on an oxygen free titanium hard mask layer  623 . The oxygen free titanium hard mask layer  623  can include other materials or compounds and need not be a titanium (Ti) only layer. At a stage  505 , the mask layer  605  is patterned  605 P. At a stage  507 , the mask layer  605  is developed to form an etch mask  605 M. At a stage  509 , the oxygen free titanium hard mask layer  623  is etched in an oxygen free etch plasma P to form an oxygen free titanium hard mask  623 M. Optionally, at a stage  510 , the etch mask  605 M can be removed at a stage  511  as was described above or the processing can continue at a stage  513 . At the stage  513 , exposed layers in a stack  600  of thin film materials are etched through the openings in the oxygen free titanium hard mask  623 M using an oxygen free etch plasma P at a high temperature H. At a stage  515 , the etching can be terminated at a predetermined layer in the stack  600  as was described above.  
      One advantage to the oxygen free titanium hard mask  623 M is that it can serve as both a hard mask and an adhesion layer or glue layer for an underlying noble metal layer  613 . In some applications, the oxygen free titanium hard mask  623 M can be used instead of a dedicated adhesion/glue layer, such as the layer  411  in  FIG. 4A , for example. The layers of thin film materials in the stack  600  will be application dependent and the stack  600  can include more layers or fewer layers than depicted in  FIG. 6A . However, TABLE  2  below lists examples of materials that can be used for the layers in the stack  600 .  
                   TABLE 2                       Layer   Example Materials                  605   An O 2  Free Etch Mask Material (e.g., photoresist)       623   titanium (Ti) or an Alloy of titanium (e.g., TiN)       613   A noble metal or an alloy of a noble metal           (e.g., platinum (Pt), ruthenium (Ru), or iridium (Ir))       615   A Conductive Metal Oxide (CMO) (e.g., a perovskite, PCMO, or           LNO)       617   A noble metal or an alloy of a noble metal           (e.g., platinum (Pt), ruthenium (Ru), or iridium (Ir))       619   An Electrically Nonconductive Layer           (e.g., a dielectric material, Si 3 N 4 , or SiO 2 )       621   A Substrate (e.g., a semiconductor material,           silicon (Si), single crystal Si, or a Si wafer)                  
 
      Mix Mode Oxygen Depleted Plasma Etching  
      In some applications it may be desirable to use an etch chemistry that varies over the course of a plasma etching of a stack of thin film materials. Depending on the number of layers in the stack and the particular etching requirements for one or more layers in the stack, the etch gasses may be switched between an oxygen depleted etch gas (i.e., no O 2  is mixed with the etch gas) and oxygen containing etch gas (i.e., O 2  is intentionally added to the etch gas). Therefore, one or more layers in the stack may require etching with an oxygen depleted plasma and one or more layers in the stack may require etching with an oxygen containing plasma. Accordingly, a mixed mode etching process includes switching one or more times between oxygen depleted plasma etching and oxygen containing plasma etching. Each etching mode can be continued until a predetermined layer in the stack is reached. Upon reaching the predetermined layer, the etching mode may be switched from oxygen depleted to oxygen containing or vice-versa, or the plasma etching process can terminate at the appropriate layer in the stack or upon an endpoint condition. The composition of the etch gas can also be changed depending on the etch mode (i.e., oxygen depleted or oxygen containing). For the oxygen depleted plasma etching the etch gas can include Ar and Cl 2 ; whereas, for the oxygen containing plasma etching the etch gas can include Cl 2  and O 2 , for example.  
      Reference is now made to  FIGS. 7A and 7B  and  FIGS. 8A through 8M , where a method  700  for mixed mode plasma etching of a stack  800  of thin film materials includes at a stage  703 , forming a mask layer  805  on a hard mask layer  823 . The mask layer  805  is patterned at a stage  705  as depicted by dashed lines  805 P, followed by developing D the mask layer  805  at a stage  707  (see  FIG. 8C ) to form an etch mask  805 M (see  FIG. 8D ). The hard mask layer  823  can be a single layer of material as depicted in  FIG. 8A  (e.g., TiN or TiO 2 ) or the hard mask layer  823  can be a composite hard mask layer made from two or more layers of thin film materials that are suitable for use as a hard mask.  FIG. 8B  depicts a hard mask layer  823  that includes two layers  823   a  and  823   b.  In either case, the etch mask  805 M will be used to etch through the single layer or the composite layer to form a hard mask. Examples of materials for the layer  823   a  include but are not limited to SiO 2  and SiN 3  and materials for the layer  823   b  include but are not limited to TiN and TiO 2 .  
      Turning now to  FIGS. 8D through 8E , at a stage  709 , the hard mask layer  823  is etched in a first etch plasma P 1 {−O 2 } to form a hard mask. The etch gasses for the first etch plasma P 1 {−O 2 } does not contain oxygen (O 2 ). If the hard mask layer  823  is made from an oxygen free material, then the first etch plasma P 1 {−O 2 } is an oxygen free etch plasma because the material for the mask layer does not contribute oxygen to the plasma etch environment. The first etch plasma P 1 {−O 2 } etches the hard mask layer  823  through the etch mask  805 M and a surface  823 R of the hard mask layer  823  recedes in a direction towards an underlying layer  813 .  FIG. 8F  depicts a hard mask  823  formed over the layer  813 .  FIG. 8G  depicts an alternate scenario where the hard mask layer  823  comprises a composite layer (i.e., made from two or more layers of different materials) and the first etch plasma P 1 {−O 2 } results in a formation of a hard mask ( 823   a,    823   b ) formed over the layer  813 . Hereinafter, the hard mask will be denoted as  823 M regardless of whether it is formed from a single layer ( FIG. 8F ) or multiple layers ( FIG. 8G ). As was described above, at a stage  710 , the etch mask  805 M may optionally be removed at a stage  711  or the first etch plasma P 1 {−O 2 } can continue at a stage  713 . In  FIG. 7A  and  FIGS. 8A and 8B , if the hard mask layer ( 823  or  823   a  and  823   b ) are oxygen containing layers (e.g., TiO 2 ), then the fist etch plasma P 1 {−O 2 } is a substantially oxygen free etch plasma. Therefore, at the stage  709 , formation of the hard mask  823 M by the first etch plasma P 1 {−O 2 } will be result in an oxygen free etch plasma or a substantially oxygen free etch plasma, depending on the composition of the mask layer  823 .  
      In  FIGS. 8H and 8I , at the stage  713 , the first etch plasma P 1 {−O 2 } etches at a high temperature H, the stack  800  of thin film materials that are patterned by the hard mask  823 M. At a stage  715 , the etching is terminated at a first predetermined layer in the stack (e.g., a layer  817 ).  
      Turning now to  FIGS. 7B and 8J , the method  700  continues at a stage  717  where the stack  800  of thin film materials is etched in a second etch plasma P 2 {+O 2 } at a high temperature H. At the stage  717 , oxygen (O 2 ) is added to the etch gasses for the second etch plasma P 2 {+O 2 } such that the plasma is an oxygen containing plasma. At a stage  719 , the etching is terminated at a second predetermined layer in the stack  800  (e.g., a layer  819 ). In  FIG. 8K , at a stage  721 , etching of the stack  800  continues with a third etch plasma P 3 {−O 2 } at a high temperature H and at a stage  723  the etching is terminated at a third predetermined layer in the stack  800  (e.g., a layer  821 ). At the stage  721 , oxygen (O 2 ) is not added to the etch gasses for the third etch plasma P 3 {−O 2 } such that the plasma is an oxygen free etch plasma. The high temperature H need not be the same for the stages  713 ,  717 , and  721 .  
      The stacks  400 ,  600 , and  800  that were described above can include a wide variety of layered thin film materials. One of the layers can be a very thin layer of a dielectric material. In  FIG. 9A , a layer  914  is sandwiched between layers  913  and  915 . Preferably, the layer  914  has a thickness t B  that is approximately 30 Å or less. For example, the layer  914  can be a tunnel barrier layer, the layer  915  can be a CMO layer (e.g., a manganite, a perovskite, PCMO, or LNO) and the layer  913  can be a layer of an electrically conductive material such as a noble metal or an alloy of a noble metal (e.g., Pt, Ru, or Ir). A layer  917  can also be a layer of an electrically conductive material such as a noble metal or an alloy of a noble metal (e.g., Pt, Ru, or Ir). Collectively, the layers  913 ,  914 ,  915 , and  917  can be a memory element  910  that stores data as a plurality of conductivity profiles. Although not depicted in  FIGS. 9A and 9B , other layers in the stack  900  can include a plurality of thin film materials that form a metal-insulator-metal structure (e.g., a non-ohmic device) that is electrically in series with the memory element  910  and operative to impart a non-linear I-V characteristic so that the memory element  910  operates within a preferred range of voltages and currents for read and write operations to the memory element  910 . U.S. patent application Ser. No. 11/095,026, filed Mar. 30, 2005, and titled “Memory Using Mixed Valence Conductive Oxides,” hereby incorporated by reference in its entirety and for all purposes, describes non-volatile memory cells that can be arranged in a cross-point array. The application describes a two terminal memory element that changes conductivity when exposed to an appropriate voltage drop across the two terminals. The memory element includes an electrolytic tunnel barrier and a mixed valence conductive oxide. A voltage drop across the electrolytic tunnel barrier causes an electrical field within the mixed valence conductive oxide that is strong enough to move oxygen ions out of the mixed valence conductive oxide and into the electrolytic tunnel barrier. When certain mixed valence conductive oxides (e.g., praseodymium-calcium-manganese-oxygen perovskites—PCMO and lanthanum-nickel-oxygen perovskites—LNO) change valence, their conductivity changes. Additionally, oxygen accumulation in certain electrolytic tunnel barriers (e.g., yttrium stabilized zirconia—YSZ) can also change conductivity. If a portion of the mixed valence conductive oxide near the electrolytic tunnel barrier becomes less conductive, the tunnel barrier width effectively increases. If the electrolytic tunnel barrier becomes less conductive, the tunnel barrier height effectively increases. Both mechanisms are reversible if the excess oxygen from the electrolytic tunnel barrier flows back into the mixed valence conductive oxide. A memory can be designed to exploit tunnel barrier height modification, tunnel barrier width modification, or both.  
      Both the electrolytic tunnel barrier and the mixed valence conductive oxide do not need to operate in a silicon substrate, and, therefore, can be fabricated above circuitry fabricated in the substrate and being used for other purposes (such as selection circuitry). Additionally, two-terminal memory elements can be arranged in a cross-point array such that one terminal is electrically coupled with an x-direction line and the other terminal is electrically coupled with a y-direction line. A stacked cross-point array consists of multiple cross-point arrays stacked upon one another, sometimes sharing x-direction and y-direction lines between layers, and sometimes having isolated lines. Both single-layer cross-point arrays and stacked cross-point arrays may be arranged as third dimension memories fabricated above a substrate including circuitry that allows data access to/from the third dimension memories.  
      In  FIG. 9B , the methods  200 ,  300 ,  500 , and  700  can be used to etch the layers of thin film materials in the stack  900  through a hard mask  925  to form columns  925   c  that define discrete memory devices. Although not depicted in  FIG. 9B , one skilled in the art will appreciate that the space between the columns  925   c  can be filled in with a dielectric material that electrically isolates the columns  925   c  from one another. Depending on the materials selected for the thin film layers, a plasma P used for etching one or more layers in the stack  900  can be the oxygen free etch plasma, the substantially oxygen free etch plasma, the oxygen containing etch plasma, or some combination thereof (e.g., mixed mode plasma etching).  
      Etch Gasses &amp; High Temperatures  
      Suitable etch gasses for the plasma P will be application dependent. However, except as described above in reference to mixed mode plasma etching, oxygen (O 2 ) should not be one of the gasses that is included with the etch gasses for the plasma P. Examples of gasses that can be used to form the oxygen depleted plasma P or substantially oxygen depleted plasma P include but are not limited to argon (Ar), chlorine (Cl 2 ), boron trichloride (BCl 3 ), and fluorinated gasses (CF x ). On the other hand, for the aforementioned mixed mode plasma etching, the oxygen containing plasma P{+O 2 } can comprise an etch gas including but not limited to Cl 2 +O 2  and the oxygen depleted plasma P{−O 2 } can comprise an etch gas including but not limited to Ar+Cl 2 . A range of vacuum conditions for the plasma etching will be application dependent. For example, an approximate range of vacuum levels for the plasma etching will be from about 1 millitorr to about 250 millitorr.  
      For those stages of the methods  200 ,  300 ,  500 ,  700  that require heating at the high temperature H, the actual temperature selected will be application dependent and the high temperature H need not be the same at each stage. As was previously discussed, materials that are amendable or designed for low temperature processing (e.g. below approximately 200° C.) should not be subjected to the high temperature H. For photoresist materials, processing below approximately 150° C. may be necessary to prevent burning. More preferably, to ensure that no burning occurs, the processing temperature for photoresist materials should be below approximately 100° C. On the other hand, the materials in the stacks ( 400 ,  600 ,  800 , and  900 ) often require high temperatures during plasma etching for several reasons including their etch characteristics, to obtain a reasonable etch profile, and to prevent or reduce by-product re-deposition, for example. Accordingly, the oxygen free etch plasma P at a high temperature H or the substantially oxygen free etch plasma P at a high temperature H will typically occur at a temperature above 200° C. For example, a high temperature H between about 350° C. and about 550° C. is suitable for some materials such as noble metals (e.g., Pt, Ir, and Ru), CMO (e.g., perovskites, LNO, and PCMO), dielectric materials (e.g., SiO 2  and SiN 3 ), and titanium and its alloys (e.g., TiN and TiO 2 ). In some applications, it may be desirable to use the high temperature H between about 350° C. and about 550° C. for some or all of the stages of the methods described above, especially if materials that cannot withstand high temperature processing are not present in the stacks of thin film materials (e.g., photoresist).  
      The methods  200 ,  300 ,  500 , and  700  can be implemented in a program fixed in a computer readable media operative to run on a computer or a process controller, for example. The term computer readable media includes a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Common forms of computer readable media includes but is not limited to floppy disk, flexible disk, hard disk, magnetic tape, any other magnetic medium, CD-ROM, DVD-ROM, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, RAM, PROM, EPROM, FLASH memory, any other memory chip or cartridge, carrier wave, or any other medium from which a computer or process controller can read. Furthermore, the term computer readable media refers to any media that participates in providing instructions to a computer or process controller for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, the aforementioned optical or magnetic disks. Transmission media includes coaxial cables, copper wire, and fiber optics. Transmission media can also take the form of acoustic waves, carrier waves, or light waves, such as those generated during radio wave and infrared data communications. In general, the steps of disclosed processes may be performed in an arbitrary order, unless otherwise provided in the claims.  
      Although several embodiments of an apparatus and a method of the present invention have been disclosed and illustrated herein, the invention is not limited to the specific forms or arrangements of parts so described and illustrated. The invention is only limited by the claims.