Abstract:
A method of forming a piezoelectronic transistor (PET) device, the PET device, and a semiconductor including the PET device are described. The method includes forming a first metal layer, forming a layer of a piezoelectric (PE) element on the first metal layer, and forming a second metal layer on the PE element. The method also includes forming a well above the second metal layer, forming a piezoresistive (PR) material in the well and above the well, and forming a passivation layer and a top metal layer above the PR material at the diameter of the PR material above the well, wherein a cross sectional shape of the well, the PR material above the well, the passivation layer, and the top metal layer is a T-shaped structure. The method further includes forming a metal clamp layer as a top layer of the PET device.

Description:
[0001]    This application is a continuation of U.S. application Ser. No. 14/529,929 filed Oct. 31, 2014, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     STATEMENT OF FEDERAL SUPPORT 
       [0002]    This invention was made with Government support under contract number N66001-11-C-4109 awarded by Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The present invention relates to a piezoelectronic transistor, and more specifically, to passivation and alignment of a piezoelectronic transistor piezoresistor. 
         [0004]    Generally, a piezoelectronic transistor (PET) device is one that can be controlled to change resistivity states such that the PET device may be used as a switch or memory device, for example. Voltage applied across a piezoelectric (PE) element causes displacement of the PE element. The PET device is arranged such that the displacement of the PE element causes the desired modulation of the resistance of a piezoresistive (PR) element. In the fabrication of a PET device, the PR element poses some challenges because the material itself must be handled carefully and must be arranged such that the PET device functions as expected. 
       SUMMARY 
       [0005]    According to one embodiment of the present invention, a method of forming a piezoelectronic transistor (PET) device includes forming a first metal layer; forming a layer of a piezoelectric (PE) element on the first metal layer; forming a second metal layer on the PE element; forming a well above the second metal layer, sides of the well being lined with a passivation film; forming a piezoresistive (PR) material in the well and above the well, a diameter of the PR material above the well being greater than a diameter of the well; forming a passivation layer and a top metal layer above the PR material at the diameter of the PR material above the well, wherein a cross sectional shape of the well, the PR material above the well, the passivation layer, and the top metal layer is a T-shaped structure; and forming a metal clamp layer as a top layer of the PET device. 
         [0006]    According to another embodiment, a piezoelectronic transistor (PET) device includes a first metal layer; a layer of piezoelectric (PE) element formed on the first metal layer; a second metal layer on the PE element; piezoresistive (PR) material formed in a well above the second metal layer and above the well, sides of the well being lined with a passivation film and a diameter of the PR material formed above the well being greater than a diameter of the well; a passivation layer and a top metal layer formed above the PR material at the diameter of the PR material above the well, a cross sectional shape of the well, the PR material above the well, the passivation layer, and the top metal layer being a T-shaped structure; and a metal clamp layer as a top layer of the PET device. 
         [0007]    According to yet another embodiment, a semiconductor device includes a piezoelectronic transistor (PET) device including a first metal layer; a layer of piezoelectric (PE) element formed on the first metal layer; a second metal layer on the PE element; piezoresistive (PR) material formed in a well above the second metal layer and above the well, sides of the well being lined with a passivation film and a diameter of the PR material formed above the well being greater than a diameter of the well; a passivation layer and a top metal layer formed above the PR material at the diameter of the PR material above the well, a cross sectional shape of the well, the PR material above the well, the passivation layer, and the top metal layer being a T-shaped structure; and a metal clamp layer as a top layer of the PET device; and a voltage source configured to apply a voltage between the first metal layer and the second metal layer. 
         [0008]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a cross sectional view of aspects of a PET device according to an embodiment of the invention; 
           [0011]      FIG. 2  is a top view of a PET device without the metal clamp layer according to an embodiment of the invention; 
           [0012]      FIG. 3  is a cross sectional view of the PET device shown in  FIG. 2  along A-A; 
           [0013]      FIG. 4  is a cross-sectional view of the PET device shown in  FIG. 2  along B-B; 
           [0014]      FIG. 5  is a cross sectional view of a stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0015]      FIG. 6  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0016]      FIG. 7  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0017]      FIG. 8  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0018]      FIG. 9  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0019]      FIG. 10  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0020]      FIG. 11  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0021]      FIG. 12  is a cross sectional view of another stage in the fabrication of the PET device according to an embodiment of the invention; 
           [0022]      FIG. 13  is a cross-sectional view of a stage in the formation of the PET device according to an embodiment of the invention; and 
           [0023]      FIG. 14  is a block diagram of aspects of a semiconductor device including a PET device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    As noted above, the PR element of a PET device poses challenges. The PR element may be unstable in the presence of oxygen, water vapor, or other materials typically present in the atmosphere, and should be passivated for long term stability of the PET device. The pressure placed on the PR element by displacement of the PE element must be below the yield strength of the clamp that maintains the overall dimensions of the PET device. The PR element must be aligned to add contacts and must be patterned without damaging the material at scale, but patterning the PR element following the passivation process proves problematic. Embodiments of the devices and methods detailed herein relate to achieving both the requisite passivation and alignment for the PR element of the PET device. 
         [0025]      FIG. 1  is a cross sectional view of aspects of a PET device  100  according to an embodiment of the invention. The PE element  110  ( FIG. 3 ) is not shown in  FIG. 1 . A well  115  for deposition of the PR element  200  is detailed. A bottom contact is comprised of layers of a transition metal  132  such as ruthenium (Ru), and a conductive oxygen blocking passivation layer  134  such as titanium nitride (TiN), collectively referred to as metal layer  130 . The PE element  110  would be below the Ru or transition metal  132 , and the metal layer  130  (bottom contact for the PR element  200 ) is the top electrode of the PE element  110 . The well  115  above the bottom contact is lined with a passivation layer  150  such as atomic layer deposited (ALD) hafnium oxide (HfO 2 ) or silicon dioxide (SiO 2 ). The well  115  may instead be passivated by aluminum oxide (Al 2 O 3 ), or silicon nitride (Si 3 N 4 ). The passivation layer  150  protects the PR element  200 , which may be samarium (Sm) or thulium (Tm) based monochalcogenide, for example. Exemplary PR elements  200  include samarium selenide (SmSe), samarium monosulfide (SmS), samarium telluride (SmTe), thulium telluride (TmTe), thulium monosulfide (TmS), and thulium selenide (TmSe). The well  115  diameter may be on the order of 50 nanometers (nm), for example. More than one well  115  may be formed for the deposition of the PR element  200 . The well  115  is passivated at the sidewall of the well  115  and is additionally be passivated along the edges  117  of the PR element  200  layer to the well  115 . 
         [0026]    The pressure modulated region is the well  115  in contact with the conductive passivation layer  134  (TiN). That is, the active portion of the PR element  200 , the portion that is modulated by displacement of the PE element  110  ( FIG. 3 ), is not patterned or processed. Thus, the passivated well  115  structure acts to protect the active portion of the PR element  200  (the portion of the PR element  200  that is in the well  115 . The wider portion of the PR element  200  (the portion above the well  115 ) aids in alignment of the PR element  200  with the top metal layers. These top metal layers disposed above the PR element  110  include another conductive passivation layer of TiN  205  and metal Ru  210 , the same materials as the bottom contact. The relatively large diameter (as compared with the well  115 , for example) of the stress spreading top metal layers enables alignment of additional layers to the PET device  100  structure, and keeps the stress in the structure below the yield strength of the materials used. The structure formed by the well  115  and layers  200 ,  205 ,  210  is collectively referred to as the T-shaped structure  202 . A sidewall  118  at the edge of the T-shaped structure  202  may optionally be any metal or insulating thin film that blocks oxygen (e.g., TIN). 
         [0027]      FIG. 2  is a top view of a PET device  100  without the metal clamp layer  230  ( FIG. 3 ) according to an embodiment of the invention. A metal layer  120  below the PE element  110  is shown. A spring  135  formed from a metal is also shown and is detailed below with reference to  FIG. 4 . The layer of Ru  210  is also shown. A cross sectional view across the Ru  210  is shown in  FIG. 3 , and a cross sectional view along the spring  135  is shown in  FIG. 4 . Exposed metals (e.g., the TiN  205  and Ru  210  layers) may be capped in materials that are resistant to xenon difluoride (XeF 2 ) etching or they must be resistant themselves. Exemplary caps are HfO 2  and Al 2 O 3 . In addition, TiN  205  and Ru  210  may instead be Ru/titanium aluminum nitride (TiAN)/platinum (Pt), ruthenium oxide (RuO 2 )/TiN/palladium (Pd), iridium oxide, or (IrO 2 )/Pt. 
         [0028]      FIG. 3  is a cross sectional view of the PET device  100  shown in  FIG. 2  along A-A.  FIG. 3  shows the PE element  110  between the metal layer  120  and the metal layer  130 . Application of a voltage between the two metal layers  120 ,  130  causes displacement of the PE element  110  which in turn causes modulation of the PR element  200 . The T-shaped structure  202  including the well  115  is shown above the metal layer  130 . Above the T-shaped structure  202 , a metal clamp layer  230  (a portion of the metal clamp layer  230 - 2  associated with the T-shaped structure  202  is shown) maintains the height of the PET device  100  despite movement of the PE element  110 . That is, the displacement of the PE element  110  coupled with the rigidity of the metal clamp layer  230  causes the compression of the PR element  200  resulting in modulation of its conductivity. As indicated in the figure, SiO 2    300  surrounds the active elements of the PET device  100 . 
         [0029]      FIG. 4  is a cross-sectional view of the PET device  100  shown in  FIG. 2  along B-B. The spring  135  is shown above the stack of the metal layer  120 , PE element  110 , and metal layer  130 . A portion metal clamp layer  230 - 1  is shown above the spring  135 . As shown in  FIG. 13 , the metal clamp layer  230  (which becomes divided into portions  230 - 1  and  230 - 2  shown in  FIGS. 3 and 4 ) seals the vias  250  and provides electrical contact to the T-shaped structure  202  and the spring  135 . The portions of the metal clamp layer  230 - 1  and  230 - 2  are anchored in place by their adhesion to the insulating portion of the SiO2  300 . The clamp is constituted by the oxide layer SiO2  300  and the metal clamp layer  230 - 1 ,  230 - 2 . Thus, the purpose of the spring  135  is to ensure the force created by the PE element  110  displacement acts primarily on the PR element  200  in the well  115  (part of the T-shaped structure  202 ) while creating a conductive metal contact to metal layer  130 . 
         [0030]      FIGS. 5 to 12  illustrate stages in the formation of the PET device  100  according to an embodiment of the invention.  FIG. 5  is a cross sectional view of a stage  500  in the fabrication of the PET device  100  according to an embodiment of the invention. In the stage  500  shown in  FIG. 5 , the metal layer  120 , the PE element  110 , and the metal layer  130  are stacked. Specifically, to form the metal layer  120  according to one embodiment, silicon oxide (SiO 2 ) is grown, titanium (Ti) is deposited and oxidized, and platinum (Pt) is deposited (each of these steps is not illustrated). To form the metal layer  130 , Ru ( 132 )/TiN ( 134 ) is deposited.  FIG. 6  is a cross sectional view of another stage  600  in the fabrication of the PET device  100  according to an embodiment of the invention. Silicon (Si)  140  is deposited on the metal layer  130  and etched via RIE (in combination with photolithography) such that a portion of the metal layer  130  remains covered with the Si  140 . The Si  140  is a sacrificial layer that facilitates the formation of the spring  135 . Ru/TiN is deposited such that a portion is on the Si  140  and another portion is on the metal layer  130  and is etched via a lithographically defined RIE to define the spring  135 . 
         [0031]      FIG. 7  is a cross sectional view of another stage  700  in the fabrication of the PET device  100  according to an embodiment of the invention.  FIG. 7  shows a portion of the PET device  100  where the well  115  will be formed and where the spring  135  is not formed. Si  145  is deposited over the metal layer  130  and HfO 2 /SiO 2    150  is deposited on the Si  145 .  FIG. 8  is a cross sectional view of another stage  800  in the fabrication of the PET device  100  according to an embodiment of the invention.  FIG. 8  shows the portion of the PET device  100  that includes the spring  135 . In this portion, Si  145  and HfO 2 /SiO 2    150  are deposited over the spring  135 .  FIG. 9  is a cross sectional view of another stage  900  in the fabrication of the PET device  100  according to an embodiment of the invention.  FIGS. 9-12  show the portion of the PET device  100  shown in  FIG. 7  (spring is not shown). Electron-beam lithography is used to define the well  115  in combination with RIE or ion milling. As noted above, more than one well  115  may be formed in this manner. The well  115  is etched to the depth of the Si  145  layer via RIE or ion milling to the TiN ( 134 ) layer of the metal layer  130 . Any remaining resist is then stripped. A coating of HfO 2  is added to the sides of the well  115 . This sidewall coating is created by applying the HfO 2    150  on all surfaces and then using a blanket highly anisotropic RIE or ion milling step to remove the material from the bottom of the well. Following the coating process, germanium (Ge)  160  is deposited. Lithographically patterned Ni with a Ti adhesion layer  170  is then deposited on top of the Ge  160  layer. Layers  145  through  170  are indicated as  180 , a shown in  FIG. 9 . 
         [0032]      FIG. 10  is a cross sectional view of another stage  1000  in the fabrication of the PET device  100  according to an embodiment of the invention. The PE element  110  is etched to the metal layer  120  via RIE utilizing the Ni of layer  170  as a hardmask. The metal layer  120  is patterned via RIE in combination with photolithography. After metal layer  120  is patterned the Ti/Ni  170  layer is removed by dissolving Ge  160  in hydrogen peroxide (H 2 O 2 ).  FIG. 11  is a cross sectional view of another stage  1100  in the fabrication of the PET device  100  according to an embodiment of the invention. Ge  195  is deposited and a window is opened in it using hydrogen peroxide (H 2 O 2 ) solution. This window is defined lithographically and is aligned to the area containing the well  115 . The PR element  200  and TiN  205  are deposited with a Ru  210  cap, and the layers  200 ,  205 ,  210  are etched to the Si  145  layer via RIE with a photoresist  220  above the Ru  210  such that the structure  1200  shown in  FIG. 12  is achieved.  FIG. 12  is a cross sectional view of another stage  1200  in the fabrication of the PET device  100  according to an embodiment of the invention. A layer of Ge or Si layer  240  is deposited, and SiO 2    300  is then deposited conformally around the stage  1200  structure to a thickness greater than the total height of the PE element  110  and T-shaped structure  202 . Chemical mechanical planarization (CMP) is used to polish the film stack flat resulting in a flat layer of SiO 2    300  with the stage  1200  structure embedded below the surface. 
         [0033]      FIG. 13  is a cross-sectional view of a stage  1300  in the formation of the PET device  100  according to an embodiment of the invention.  FIG. 13  shows both the spring  135  and the T-shaped structure  202  formed on the metal layer  130 . After stage  1200  ( FIG. 12 ), photolithography is used to define vias  250  in the SiO 2    300  to provide electrical access to the spring  135  and the T-shaped structure  202 . The vias  250  are etched to the Ge or Si layer  240  using RIE. The mask is photoresist. Then XeF2 is applied, which removes the Si  145 , Ge  195 , and Ge or Si layer  240 , leaving air. Metal (the metal clamp layer  230 ) is deposited (see e.g.,  FIGS. 3 and 4 ) into the vias  250  via evaporation without any mask. Deposition of the metal clamp layer  230  hermetically seals the PET device  100  and provides electrical contact to the top of the T-shaped structure  202  (and, thus, to one side of the PR element  200 ) and to the spring layer  135 . In turn the spring provides an electrical connection to layer  130  shared by the PR element  200  and PE element  110 . Access to metal layer  120  is provide for outside of the clamping structure. One final lithography step is done to provide a resist mask to etch metal layer  230  into the two parts  230 - 1  and  230 - 2  that individually contact the spring  135  and top film of the T structure  210 . 
         [0034]      FIG. 14  is a block diagram of aspects of a semiconductor device  10  including a PET device  100  according to an embodiment of the invention. A voltage source  15  is used to apply a voltage across the metal layer  120  and the metal layer  130  to modulate the PE element  110 . This modulation results in switching the resistive state of the PR element  200  between low and high. When the PET device  100  is used as a switch element, read and write components  20  may be coupled to the PET device  100 . The PET device  100  may be used as a memory in another context as part of semiconductor device  10 , as well. 
         [0035]    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 more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0036]    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 
         [0037]    The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0038]    While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.