Patent Publication Number: US-7901588-B2

Title: MRAM wet etch method

Description:
RELATED APPLICATIONS 
     This application is a divisional application of U.S. Ser. No. 11/147,513, filed Jun. 8, 2005. 
    
    
     This application was sponsored by the United States Government under Contract No. MDA 972-99-C-0009, which was awarded by the Defense Advanced Research Projects Agency (DARPA); therefore, the United States Government has certain rights and privileges in the present application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the processing of magnetic thin film structures, and in particular to an etching process for selectively etching the exposed top magnetic layer, and the underlying tunnel barrier and pinned bottom magnetic layer of a magnetic random access memory (MRAM) stack. 
     BACKGROUND OF THE INVENTION 
     MRAM has tremendous potential as a nonvolatile, solid-state memory to replace flash memory and electronically erasable programmable read-only memory (EEPROM). To enhance the performance of MRAM chips, it is necessary to reduce the lithographic minimum printable size. However, ion beam etching (IBE), a.k.a. ion milling, has been essentially the only method available for creating fine patterns, e.g., submicron patterns, in magnetic thin film structures. Because of the lack of volatile compounds for ferrous metals other than carbonyl, reactive-ion etching (RIE) has not been a viable technique for patterning thin magnetic films; and a RIE process based on carbonyl chemistry has not yet been developed. Thus, a chemical etching technique for patterning magnetic thin films based on Fe, Co and Ni is attractive because of the thin film nature of MRAM magnetic films (20-50 Å z-direction) relative to the x-y dimensions of patterned magnetic tunnel junction (MTJ) elements. The MRAM structure represents a complex multilayer system which includes numerous magnetic thin film layers. A typical MRAM structure is shown in  FIG. 1 . Specifically, the thin film structure shown in  FIG. 1  comprises Si substrate  10 , SiO x  layer  12 , a 150 Å Ti layer  14 , Ni 81 Fe 19  (40 Å) layer  16 , Ir 20 Mn 80  (120 Å) layer  18 , CO 90 Fe 10  (20 Å) layer  20 , Al 2 O 3  (10 Å) layer  22 , Ni 81 Fe 19  (40 Å) layer  24  and Ti (100 Å) layer  26 . In this prior art magnetic structure, Al 2 O 3  layer  22  serves as a tunnel barrier between the top magnetic film layer, i.e., Ni 81 Fe 19  layer  24 , and the pinned bottom magnetic layer, i.e., Co 90 Fe 10  layer  20 , the antiferromagnetic layer  18  and the magnetic layer  16  which are present beneath the tunnel barrier layer  22 . Layer  26  is a passivating layer that prevents moisture, air or other contaminants from entering into the structure, while layer  14  is an adhesion layer. In the case of AP-pinned MTJs, the antiferromagnetic layer  18  can be a Ru spacer. The term “AP-pinned MTJS” is used herein to denote MTJs which contain an antiparallel-pinned (AP) layer structure, wherein the AP layer structure includes at least two pinned layers having magnetic moments that are self-pinned antiparallel to each other and the at least two pinned layers are separated by an AP coupling layer or a spacer. 
     As is well known to those skilled in the art, the magnetic films of a MRAM structure, such as illustrated in  FIG. 1 , are quite thin. Patterning of prior art MRAM structures, such as shown in  FIG. 1 , is typically carried out by first applying a mask to the MRAM structure and patterning the mask by lithography (exposure and development).  FIG. 2  shows the structure after these steps wherein reference number  28  represents the patterned mask. The pattern is transferred to the MRAM structure by RIE, IBE, or wet etching. And then the exposed Ni 81 Fe 19  (40 Å) layer  24  can be pattern-wise etched by RIE, IBE, or wet etching. In a traditional wet etching process, a standard aqueous acid solution, such as sulfuric and/or nitric acid, is employed to etch the exposed Ni 18 Fe 19  (40 Å) layer  24 . Although the acid etchants are capable of etching through the exposed top magnetic layer  24  of the structure, the acid etchants are not selective for removing just that magnetic layer  24 . Instead, when the acid etchants are employed, they also etch the underlying alumina tunnel barrier layer  22 , the pinned Co 90 Fe 10  layer  20 , and the Mn in the Ir 20 Mn 80  layer  18  of the magnetic thin film stack providing the structure shown in  FIG. 3 . 
     Despite being capable of etching numerous magnetic layers in a MRAM structure, the use of prior art aqueous acid solutions causes Galvanic-coupling-accelerated dissolution of the Co 90 Fe 10  (20 Å) layer  20  which is coupled to an antiferromagnetic layer in simple-pinned MTJs and to a Ru spacer in AP-pinned MTJs. The term “simple-pinned MTJs” is used herein to denote MTJs which contain a single reference layer, wherein the single reference layer has its magnetization typically pinned by exchange coupling with an antiferromagnetic layer. In MRAM, due to coupled active magnetic layers and noble metals, avoidance of Galvanically-accelerated dissolution is a major concern. 
     U.S. Pat. No. 6,426,012 describes a method to pattern the magnetic soft layer of a MRAM structure while avoiding Galvanically-enhanced etching reactions. However, the carboxylic acids employed in U.S. Pat. No. 6,426,012 are weak acids, and thus are not capable of etching through the alumina tunnel barrier  22 . 
     A desirable situation would be to selectively etch through the exposed top magnetic layer, i.e., layer  24 , as well as the underlying thin Al 2 O 3  layer  22 , and the pinned bottom magnetic layer, i.e., layer  20 , in the MRAM structure, and stop the etch process at the antiferromagnetic layer  18 , while avoiding Galvanically-enhanced etching reactions. In the case of AP-pinned MTJs, the etching process stops at the Ru spacer of the AP-pinned layer structure. Such a method would leave the antiferromagnetic layer, in the case of simple-pinned MTJs, and the Ru spacer, in the case of AP-pinned MTJs, unetched. 
     To date, applicants are unaware of any etching process which selectively etches a magnetic thin film structure so as to stop on the antiferromagnetic layer, in the case of simple-pinned MTJs, or on the Ru spacer, in the case of AP-pinned MTJs, while avoiding Galvanic corrosion or Galvanically-assisted lateral etching of the edges of exposed magnetic layers. There is thus a need for developing an etching process which is capable of selectively etching a magnetic thin film structure to provide a patterned structure wherein the pattern is not formed in the antiferromagnetic layer, in the case of simple-pinned MTJs, or in the Ru spacer, in the case of AP-pinned MTJs. Such an etching process would be beneficial since it would prevent unwanted Galvanic corrosion of the inner magnetic layers, while being able to pattern the top magnetic film layer, the tunnel barrier layer, and the pinned bottom magnetic layer of the structure. The ability to etch down to an etch stop layer such as the antiferromagnetic layer or the Ru spacer further has advantages of superior process repeatability and avoiding the difficulties associated with stopping the etch process at alumina and leaving intact the pinned magnetic layer. 
     SUMMARY OF THE INVENTION 
     The present intention is directed to a method of selectively patterning the exposed top magnetic film layer, and the underlying tunnel barrier and the pinned bottom magnetic layer of a magnetic structure, stopping on the antiferromagnetic layer, in the case of simple-pinned MTJs, or on the Ru spacer, in the case of AP-pinned MTJs, in which the various etching processes employed do not adversely damage the antiferromagnetic layer or the Ru spacer that are present beneath the pinned bottom magnetic layer. The aforementioned object is achieved by utilizing a selective etching process. Specifically, the above object is obtainable utilizing processing steps that include:
         (a) providing a magnetic structure comprising at least one pinned bottom magnetic film layer and at least one top magnetic film layer, wherein said at least one top and said at least one pinned bottom magnetic film layers are separated by a tunnel barrier layer, and said at least one top magnetic film layer has a passivating layer located thereon;   (b) forming a patterned resist atop said passivating layer, wherein a portion of said passivating layer is exposed;   (c) selectively etching said exposed portion of said passivating layer by a RIE process to expose a portion of said at least one top magnetic film layer; and   (d) selectively etching said exposed portion of said at least one top magnetic film layer, and underlying portions of said tunnel barrier and said at least one pinned bottom magnetic film layer by a wet etch process which includes an etchant solution comprising an arylsulfonic acid and an aliphatic or alicyclic amine.       

     The present intention is also directed to a method of monitoring the progress of a selective etching process and determining the etch endpoint of said selective etching process, which comprises measuring the electrochemical potential difference of a part or wafer containing a magnetic structure with respect to a reference electrode simultaneously with the selective etching process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-3  are pictorial views through cross-sections illustrating a typical prior art process of patterning a magnetic thin film structure. 
         FIGS. 4-8  are pictorial views through cross-sections illustrating the basic processing steps of the present invention which are used in patterning a magnetic thin film structure. 
         FIG. 9  is a schematic illustration of a typical wet etching experimental setup that can be employed in the present invention. 
         FIG. 10  is a graph plotting substrate potential (mV v. Ag/AgCl) v. time (minute) illustrating a typical Open Circuit Potential (E oc ) v. Time curve of one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention, which provides a method for selectively etching the top magnetic layer, the tunnel barrier, and the pinned bottom magnetic layer of a magnetic thin film structure, without adversely affecting the antiferromagnetic layer, in the case of simple-pinned MTJs, or the Ru spacer, in the case of AP-pinned MTJs, will now be described in more detail by referring to the drawings that accompany the present application. It is noted that the drawings of the present invention are provided for illustrative purposes and thus they are not drawn to scale. 
     Reference is first made to  FIG. 4  which includes a portion of a magnetic thin film structure that can be utilized in the present invention. Specifically, the portion of the magnetic structure shown in  FIG. 4  includes at least one pinned bottom magnetic thin film layer  52  and at least one top magnetic film layer  56  which are separated by a tunnel barrier layer  54 . The upper most surface layer of the at least one top magnetic layer  56  has a passivating layer  58  formed thereon and the passivating layer  58  includes a surface oxide  60 . In the case of simple-pinned MTJs, the at least one pinned bottom magnetic thin film layer  52  has an antiferromagnetic layer  50  formed thereunder. In the case of AP-pinned MTJs, the at least one pinned bottom magnetic thin film layer  52  has a Ru spacer  50  formed thereunder. The portion of the magnetic thin film structure shown in  FIG. 4  may further include a semiconductor substrate, a SiO x  layer, a Ti layer, another magnetic film layer, or other adhesion layers which would be located beneath the antiferromagnetic layer  50 , in the case of simple-pinned MTJs, or the Ru spacer  50 , in the case of AP-pinned MTJs. When these elements are present, the magnetic thin film structure would look similar to the one depicted in  FIG. 1 . For clarity, however, the material layers located beneath the antiferromagnetic layer  50 , in the case of simple-pinned MTJs, or the Ru spacer  50 , in the case of AP-pinned MTJs, are not shown in  FIGS. 4-8 . 
     The portion of the magnetic thin film structure illustrated in  FIG. 4  is formed utilizing conventional techniques well known in the art that are capable of forming such a structure. For example, the various magnetic and non-magnetic layers may be formed by utilizing the same or different deposition process including, for example, chemical vapor deposition (CVD), plasma-assisted CVD, plating, evaporation, sputtering, chemical solution deposition and other like deposition processes. 
     The top and pinned bottom magnetic film layers ( 56  and  52 , respectively) of the illustrated structure may be composed of the same or different magnetic layers in which each individual layer has a thickness of less than 150 Å. Preferably, each individual magnetic film layer ( 52  or  56 ) has a thickness of less than 100 Å. 
     Illustrative examples of magnetic films that can be employed in the present invention as layers  52  or  56  include, but are not limited to: Ni x Fe y , Ni x Co y Fe z  and other like magnetic films. In the above formulas, x, y, and z are any integers whose sum adds up to 100. In accordance with the present invention, the pinned bottom magnetic film layer  52  as well as the top magnetic film layer  56  may comprise only one magnetic film or they may comprise a stack of magnetic films. 
     In one embodiment of the present invention, the top magnetic film layer  56  is comprised of a Permalloy layer, i.e., a Ni x Fe y , or Co x Fe y  layer, while the bottom magnetic film layer  52  comprises a stack consisting of Ni x Fe y  and Co x Fe y . 
     The antiferromagnetic layer  50  may be composed of H x Mn y , wherein H is a noble metal and x, and y are any integers whose sum adds up to 100. 
     Illustrative examples of antiferromagnetic layers  50  that can be employed in the present invention include, but are not limited to: Ir 20 Mn 80  or Pt 50 Mn 50  and other like noble metal-containing antiferromagnetic layers. 
     In one embodiment of the present invention, any exposed edge of the magnetic structure may be coated with a conventional passivating material so as to prevent certain underlying films, notably the noble metal containing exchange bias layer from engaging in Galvanic type reactions that could inhibit the etching process. 
     The tunnel barrier layer  54  employed in the present invention includes any conventional material layer such as Al 2 O 3  which is capable of sustaining a tunneling current and which does not chemically degrade the properties of the top and pinned bottom magnetic layers ( 56  and  52 , respectively). In some instances, the tunnel barrier layer  54  may also serve as a diffusion barrier. The tunnel barrier layer  54  employed in the present invention is a thin layer which typically has a thickness of less than about 15 Å. 
     The passivating layer  58  employed in the present invention includes any metal layer such as Ti, TiN, Ta, or TaN which serves as a barrier layer preventing diffusion of moisture, air and other contaminants from contacting with the underlying magnetic layers and the tunnel barrier layer  54 . The thickness of this layer may vary, but typically the passivating layer  58  has a thickness from about 20 to about 1000 Å. 
     As indicated above, the passivating layer  58  contains a surface oxide  60  which is located on the upper surface of the passivating layer  58  when the structure is exposed to air. 
     Next, and as illustrated in  FIG. 5 , patterned resist  62  is formed on the surface oxide  60  present on the passivating layer  58  as shown in  FIG. 4  utilizing conventional lithography. Specifically, the lithography process includes: (i) applying a conventional resist to the surface oxide  60  of the passivating layer  58  via a conventional deposition process such as spin-on coating, dip coating, CVD, plasma-assisted CVD, evaporation and chemical solution deposition; (ii) exposing the resist to radiation to form a desired pattern therein; and (III) developing the desired pattern using a conventional developer solution to expose a portion of the surface oxide  60  on the passivating layer  58 . 
     Following the formation of the patterned resist, the inventive etching process, which will be described in more detail below, is performed. First, the exposed surface oxide  60 , not protected with the patterned resist  62 , is removed by utilizing a reactive-ion etching (RIE) process providing the structure shown  FIG. 6 . The RIE process may include CF 4 /Ar or CBrF 3 /SF 6  as etchant gases. Any suitable RIE process may be used, such as the process disclosed in U.S. Pat. No. 6,426,012; the entire content of which is incorporated herein by reference. Those skilled in the art can ascertain the suitable conditions without undue experiments. As is illustrated, this etching step exposes a portion of the passivating layer  58 . 
     In one embodiment of the present invention, an optional oxygen ashing process is performed prior to removing the exposed surface oxide  60 . The optional oxygen ashing process includes the use of oxygen ashing conditions well known to those skilled in the art that are capable of removing any etched resist residue from the exposed surface of the surface oxide  60  of the passivating layer  58 . In another embodiment of the present invention, a conventional oxygen ashing process which is capable of removing any fluorine-related resist residue may be performed after conducting the above described RIE steps. 
     Next, the exposed portion of the passivating layer  58  that does not include any inert surface oxide layer is subjected to a suitable etching process that includes the use of RIE process. Any suitable etching process may be used, such as the etching process disclosed in U.S. Pat. No. 6,426,012. Those ordinarily skilled in the art can ascertain the suitable conditions without undue experiments. In this step of the present invention, the passivating layer  58  is selectively patterned so as to expose a portion of the at least one top magnetic film layer  56 , See  FIG. 7 . 
     Alternatively, and when a Ta (or TaN) passivating layer  58  is employed, the above wet etching step may be replaced with a SF 6  RIE plasma etching process. The use of SF 6  RIE plasma etching to remove the Ta (or TaN) passivating layer  58  also modifies the surface of the exposed magnetic layer  56  to include sulfur. Alternatively, another RIE etchant gas which removes Ta (or TaN) could be used first and thereafter the etched surface is treated with a SF 6  gas. 
     The presence of sulfur on a magnetic thin film layer  56  is advantageous since sulfur-containing magnetic film layers etch at a much faster rate than magnetic film layers which does not include sulfur fragments. 
       FIG. 8  shows the next step of the present invention in which exposed top magnetic layer  58 , as well as the underlying tunnel barrier  54 , and the bottom magnetic layer  52 , are selectively etched utilizing an etching process that is capable of stopping on the antiferromagnetic layer  50 , in the case of simple-pinned MTJs, or on the Ru spacer  50 , in the case of AP-pinned MTJs. In this step of the present invention, the exposed top magnetic film layer  56 , the underlying tunnel barrier  54 , and the bottom magnetic layer  52 , are selectively etched utilizing the inventive etchant solution that comprises an arylsulfonic acid or a salt thereof and an aliphatic or alicyclic amine. By “an arylsulfonic acid or a salt thereof”, it is meant a sulfonic acid derivative that is substituted with an aryl group or any metal salt of such an arylsulfonic acid. By “an aliphatic amine” it is meant an amine derivative that is substituted with a hydrocarbon compound having an open-chain structure. By “an alicyclic amine” it is meant an amine derivative that is substituted with a hydrocarbon compound that contains a ring, but is not aromatic. In a preferred embodiment of the present invention, the salt thereof is an alkali metal salt of the arylsulfonic acid. 
     The arylsulfonic acids suitable for the present invention include nitrobenzene sulfonic acids having one of the following structures: 
                         
wherein R1, R2, R3, and R4 are the same or different, and are independently hydrogen, C 1 -C 3  alkyl, halogen, amino, or hydroxyl group.
 
     Illustrative examples of arylsulfonic acids and salts thereof that can be employed in the present invention include, but are not limited to: 3-nitrobenzene sulfonic acid, a.k.a. m-nitrobenzene sulfonic acid; 4-chloro-3-nitrobenzene sulfonic acid; sodium 3-nitrobenzene sulfonate; 4-nitrobenzene sulfonic acid; 2-methyl-5-nitrobenzene sulfonic acid; 2-amino-4-nitrophenol-6-sulfonic acid; 2-nitrobenzene sulfonic acid; 2-chloro-5-nitrobenzene sulfonic acid; 3-amino-4-hydroxy-5-nitrobenzene sulfonic acid; and 3-amino-2-hydroxy-5-nitrobenzene sulfonic acid. In a preferred embodiment of the present invention, the arylsulfonic acid is m-nitrobenzene sulfonic acid. In one embodiment of the inventive etchant solution, the arylsulfonic acid is m-nitrobenzenesulfonic acid (NBSA). 
     The aliphatic or alicyclic amines suitable for the present invention include primary, secondary, and tertiary amines. Illustrative examples of aliphatic or alicyclic amines that can be employed in the present invention include, but are not limited to: propylamine, ethylenediamine, diethylenetriamine, triethylenetetramine and other analogous amines. It is understood to one skilled in the art that triethylenetetramine can be linear, branched, cyclic, or a mixture thereof. In one embodiment of the inventive etchant solution, the aliphatic or alicyclic amine is ethylenediamine. 
     The inventive etchant solution may further comprise dissolved O 2 . The dissolved O 2  is at an equilibrium concentration due to air atmosphere. The inventive etchant solution includes a mole ratio of the arylsulfonic acid to the aliphatic or alicyclic amine from about 1:3 to about 1:20, with a mole ratio of the arylsulfonic acid to the aliphatic or alicyclic amine from about 1:6 to about 1:10 being more preferred. The etchant solution typically comprises from about 0.1 to m parts by weight of an arylsulfonic acid per 100 parts by weight water where m is limited by the solubility of the acid. Typically, the pH of the inventive etchant solution is from about 5 to about 10, with the pH of about 6.5 to about 8.5 more preferred. In one embodiment of the present invention, the pH of the inventive etchant solution is about 7.5. 
     This etching step is typically carried out at temperature of about 40° to about 50° C. for a period of time from about 3 to about 12 minutes, depending on the source and type of parts, the condition of the free top magnetic layer  56 , and the thickness of the tunnel barrier layer  54 . 
     The progress of a selective etching process, such as the above etch step, may be monitored by measuring the electrochemical potential difference of a part or wafer containing a magnetic structure with respect to a reference electrode simultaneously with the selective etching process.  FIG. 9  shows a typical experimental setup of the inventive wet etching process. A typical reference electrode is Ag/AgCl electrode. In one embodiment, the etch tank is a thermostatted Jacketed polymer tank for 200 mm substrates. In another embodiment, the etch tank is a thermostatted jacketed Pyrex tank with magnetic stirring for preparing conducting atomic force microscopy (CAFM) parts. The etchant solution is preferably pre-filtered using 0.2 μm filter. 
     The etch endpoint of the selective etching process, such as the above etch step, may be determined by measuring the electrochemical potential difference of a part or wafer containing a magnetic structure with respect to a reference electrode simultaneously with the selective etching process.  FIG. 10  shows the curve of substrate potential v. etch time for one embodiment of simple pinned MTJs etched in NBSA/ethylenediamine solution at 45° C. The curve exhibits a steady decrease in substrate potential when the Ni 81 Fe 19  top layer is dissolved, and reaches the bottom when the AlO x  layer is dissolved. While dissolution of Co x Fe y  layer is underway, the curve exhibits a rapid rise in substrate potential. When Pt 50 Mn 50  layer is exposed, the oblique curve becomes a plateau in substrate potential. The characteristic of exposed Pt 50 Mn 50  is the etch endpoint of this etching process. 
     The application of the novel electrochemical monitoring technique for determining etch endpoint enables the inventive wet etch method can be used for preparing samples for routine CAFM screening tests, and the ability of the inventive wet etch method to etch past the tunnel barrier and minimize or eliminate Galvanic corrosion effects are consistently demonstrated. 
     Following the third etching step of the present invention, the patterned structure may be rinsed with a suitable inert solvent or supercritical fluid and the patterned resist may then be stripped utilizing any well known stripping process. The patterned magnetic structure may then be subjected to conventional MRAM processing techniques also well known to those skilled in the art. 
     While this invention has been particularly shown and described with respect to preferred embodiments thereof it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but all within the scope of the appended claims.