Abstract:
A device comprising a diblock copolymer mask for fabricating a magnetoresistive random access memory (MRAM) includes a magnetic layer; a mask formed on the magnetic layer; a template formed on the mask; and the diblock copolymer mask, the diblock copolymer mask comprising a first plurality of uniform shapes formed on and registered to the template.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 12/612,258, filed on Nov. 4, 2009. 
    
    
     FIELD 
     This disclosure relates generally to the field of magnetoresistive random access memory. 
     DESCRIPTION OF RELATED ART 
     Magnetoresistive random access memory (MRAM) is a type of non-volatile memory that stores data bits in cells comprising magnetic storage elements that may comprise magnetic tunnel junctions (MTJs). An individual MTJ includes two magnetic regions, each with internal magnetization set in a particular direction, separated by an insulating layer, or tunnel barrier. One of the magnetic regions is a pinned reference layer magnet with magnetization set to a particular unchanging polarity; the other region may change its magnetization polarity to due to external magnetic fields or magnetically-polarized electron flow. The state of the magnetization in the second region is set either parallel or antiparallel to the first region&#39;s magnetization. The two different states result in different resistance values that may be measured across the MTJ tunnel barrier, allowing the MTJ to store data. An MRAM may comprise spin momentum transfer (SMT) MRAM, which uses spin-polarized electrons to perform write operations. When electrons flowing from one magnetic region across a tunnel barrier to a second region change their spin, the electron exerts a torque on the magnetization of the second region. For sufficiently large polarized electron currents, this torque may be large enough to cause the second magnetic region&#39;s magnetization to change direction. The necessary electron current required to change the direction increases as the volume of the second magnetic region increases. Therefore, to reduce power requirements for operation of SMT MRAM, it is necessary to reduce the volume of the second magnetic region as much as possible without making the second region thermally unstable. 
     To fabricate a useful SMT MRAM array, the MTJ devices must be uniform in size, with a diameter of less than about 100 nanometers (nm). MTJs larger than 100 nm in diameter require relatively high current to switch state, leading to higher power consumption and a need for less-dense arrays. Inadequate uniformity of MTJ size in the array may lead to an excessive spread of device writing and reading conditions, making operation of large SMT MRAM arrays impractical. The MTJ devices may be formed using a lithographic pattern that is transferred to a mask that is located on top of a series of magnetic layers. The patterned mask is used to define the individual MTJ shapes as an etch process removes magnetic layer material from regions that are unmasked. However, uniform lithography at sub-100 nm dimensions is difficult, particularly for isolated pillar-like structures such as MTJs. Non-uniformity in the lithography may lead to undesirable non-uniformity of MTJ shapes and sizes, which may result in an inoperable SMT MRAM array. 
     SUMMARY 
     In one aspect, a device comprising a diblock copolymer mask for fabricating a magnetoresistive random access memory (MRAM) includes a magnetic layer; a mask formed on the magnetic layer; a template formed on the mask; and the diblock copolymer mask, the diblock copolymer mask comprising a first plurality of uniform shapes formed on and registered to the template. 
     Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of a method for a diblock copolymer mask for MRAM device formation. 
         FIG. 2  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after formation of a mask on the magnetic layer. 
         FIG. 3  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after formation of a template to be used to register the diblock copolymer features on the mask. 
         FIG. 4A  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after application of diblock copolymer to the diblock copolymer template, and subsequent patterning of the diblock copolymer such that only one component of the copolymer remains, as cylindrical pillars. 
         FIG. 4B  illustrates a top view of an embodiment of a process for a diblock copolymer mask for MRAM device formation after application and patterning of cylindrical diblock copolymer on the template on the mask. 
         FIG. 5  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after patterning the mask and stripping the diblock copolymer. 
         FIG. 6  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after patterning the magnetic layers and tunnel barrier. 
         FIG. 7  illustrates a cross section of an embodiment of a process for a diblock copolymer mask for MRAM device formation after formation of insulation and a top contact wiring layer. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of systems and methods for a template-registered diblock copolymer mask for MRAM device formation are provided, with exemplary embodiments being discussed below in detail. Diblock copolymers may form uniform features having dimensions as small as about 10 nm, and such features are suitable for masking etch processes commonly used in semiconductor device fabrication. Use of self-assembling block copolymers to form a uniform grid of cylinders perpendicular to a mask surface allows for creation of a masking structure having uniformity of shape and size on the order of about 1 nm. By transferring the diblock mask structure to an underlying series of magnetic films, sub-100 nm devices may be created with shape and size uniformity needed to fabricate a large SMT device array. 
     To connect the SMT devices properly to submicron wiring features below the devices, it is critical to align the diblock features with the wiring layers beneath the magnetic device layers. This can be done by employing a topographical or chemical/material composition template to the wafer surface prior to diblock curing. The template may be patterned with conventional lithography as it can be substantially larger than individual diblock features. The smaller diblock features may register to the edges of the template to create a low-energy state wherein diblock features fill the template in a well-ordered pattern with predictable spacing and location. 
       FIG. 1  illustrates an embodiment of a method  100  for MRAM device formation using a template-registered diblock copolymer mask. Blocks  101 - 106  of  FIG. 1  are illustrated by  FIGS. 2-7 , respectively. In block  101 , mask  206  is formed on magnetic layers  205 , as shown in cross section  200  of  FIG. 2 . Magnetic layers  205  are separated from magnetic layers  203  by insulating tunnel barrier  204 . Base contacts  202   a - d  are located in wiring layer  201 . Base contacts  202   a - d  are shown for illustrative purposes only; wiring layer  201  may comprise any appropriate number of contacts. Mask  206  may comprise any conductive etchable material having a relatively low electrical resistance, for example, tantalum (Ta), tantalum nitride (TaN), titanium nitride (TiN), or any other appropriate conductive mask material. In some embodiments, mask  206  may comprise an optional top layer of intermediate mask, comprising a material including but not limited to silicon (Si), silicon oxide (Si0 2 ), or silicon nitride (SiN). 
     In block  102 , a diblock copolymer template  301  is formed on top of mask  206 , as shown in cross section  300  of  FIG. 3 . Diblock copolymer template  301  may be formed through various means, including: etching a recess or mesa into mask  206  (topographical template); modifying the surface of mask  206 , e.g. with a reactive plasma exposed to only a certain region of the mask  206 , or through physical adjustment of surface roughness, by ion beam exposure or smoothing polish applied only to desired template regions (surface modification template); creation of a multi-film mask  206  through a process such as deposition through a shadow-mask or Damascene etch, fill, and polish, wherein the surface is relatively flat, but different materials are exposed on the surface in desired regions (multi-material template); or combinations of any of the above processes. The template size and shape may be chosen to restrict a well-defined array of diblock features to fit within the template  301 . Diblock copolymer template  301  may be aligned with base contacts  202   a - d.    
     In block  103 , a diblock copolymer is applied to the diblock copolymer template  301 , and the applied diblock copolymer is cured, as is shown in cross section  400   a  of  FIG. 4A  and top view  400   b  of  FIG. 4B . The diblock copolymer comprises a 2-part polymer, i.e., 2-part chains with heads and tails that either attract or repulse one another. The chains allow the diblock copolymer to self-assemble into uniform shapes. For proper composition of the diblock material, the applied diblock copolymer forms copolymer cylinders  401   a - g  on topographical diblock copolymer template recess  301 . The diblock composition is tuned to generate features with desired dimensions, and spacing between features such that the features will register to the template edge in a well-defined, predictable fashion as the diblock cures and the system seeks to minimize the energy of the resultant state. For example, the diblock features may form preferentially on a certain type of surface (adjustable with surface-modification templating or material templating), or the diblock features may seek to avoid locating atop a topographical edge (topographical templating). Copolymer cylinders  401   a - g  may be aligned with base contacts  202   a - d , through intermediate alignment of the template with the base contacts  202   a - d . Copolymer cylinders  401   a - g  may be less than 100 nm in diameter, and in some embodiments may be about 50 nanometers (nm) in diameter. Copolymer cylinders  401   a - g  are shown for illustrative purposes only; any appropriate configuration and number of copolymer shapes may be formed. Note the lithographically-patterned template (a rectangle in this example) is substantially larger than the dimensions of the self-assembled diblock pillars. The self-assembled features offer improved uniformity of shapes at very small dimensions, whereas the larger lithographic-defined template is straightforward to create and align with standard lithography tools. By using the self-assembly in conjunction with the lithographic template, the process offers highly uniform shapes well-aligned to wiring layers underneath. 
     In block  104 , mask  206  is patterned to form cylinders  501   a - g , and copolymer cylinders  401   a - g  are removed, as is shown in cross section  500  of  FIG. 5 . Reactive ion etching may be used to pattern mask  206  and form mask cylinders  501   a - g . Oxygen plasma etching may be used to remove copolymer cylinders  401   a - g . Mask cylinders  501   a - g  may be less than 100 nm in diameter, and in some embodiments may be about 50 nanometers (nm) in diameter. The dimensions of mask cylinders  501   a - g  are commensurate with the dimensions of copolymer cylinders  401   a - g . In embodiments where mask  206  comprises a top layer of intermediate mask, the intermediate mask is etched first, then the conductive mask is etched, as copolymer cylinders  401   a - g  may not withstand the etch required to pattern the conductive mask. The intermediate mask is then removed. 
     In block  105 , magnetic layers  205 , tunnel barrier  204 , and magnetic layers  203  are etched to form a plurality of MTJs  604   a - g , as is shown in cross section  600  of  FIG. 6 . Mask cylinders  501   a - g  act as a mask during etching of the magnetic layers  203  and  205  and tunnel barrier  204 . Magnetic layers  601   a , tunnel barrier  602   a , and magnetic layers  603   a  taken together form MTJ  604   a ; MTJs  604   b - g  similarly comprise respective magnetic layers  601   b - g  and magnetic layers  603   b - g  separated by tunnel barriers  602   b - g . The plurality of MTJs  604   a - g  are uniform in size, and isolated from each other. MTJs  604   a - g  may be less than 100 nm in diameter, and in some embodiments may be about 50 nanometers (nm) in diameter. MTJs  604   a - g  may contact base contacts  202   a - d  located in wiring layer  201 . MTJs  604   a - g  are shown for illustrative purposes only; any appropriate configuration and number of MTJs may be formed. For illustrative purposes,  FIG. 6  shows several MTJs which do not contact underlying wiring layer conductors  202   a - d . This shows how one can handle diblock copolymer compositions that yield relatively high density features, but memory element cell size is larger than the period of the diblock features. By disconnecting one end of the unwanted MTJs, they will not be operable in the array, and will serve only as dummy fill devices. 
     In block  106 , insulation  702  and top contact wiring layer  701  are formed, resulting in MRAM array  700 , as is shown in  FIG. 7 . The MTJs  604   a - g  and mask cylinders  501   a - g  bridge between wiring layers  201  and  701 . The material comprising mask cylinders  501   a - g  has a relatively low resistance compared to MTJs  604   a - g , and does not interfere with the operation of MTJs  604   a - g . MRAM  700  may comprise an SMT MRAM. 
     The technical effects and benefits of exemplary embodiments include formation of small, uniform MTJ devices aligned to underlying wiring contacts for use in an MRAM. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.