Abstract:
A semiconductor device, a piezoelectronic transistor (PET) device, and a method of fabricating the PET device are described. The method includes forming a first stack of dielectric layers, forming a first metal layer over the first stack, forming a piezoelectric (PE) material on the first metal layer, and forming a second metal layer on the PE material. The method also includes forming a piezoresistive (PR) element on the second metal layer through a gap in a first membrane formed a distance d above the second metal layer.

Description:
DOMESTIC BENEFIT/NATIONAL STAGE INFORMATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/529,886 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 piezoelectronic transistors, and more specifically, to integrating a piezoresistive element in a piezoelectronic transistor. 
         [0004]    A piezoelectronic transistor (PET) includes a piezoelectric element (PE) that may be displaced to modulate the resistance of a piezoresistive (PR) element. The materials used to create a PET pose challenges in formation of the PET. One such challenge is instability of the PR element in air. The PR element may be comprised of samarium selenide (SmSe), samarium monosulfide (SmS), samarium telluride (SmTe), or thulium telluride (TmTe), for example. Long term stability of the PR element, particularly for thin films, currently requires passivation. The passivation may be achieved through application of an atomic layer deposition (ALD) film (e.g., hafnium dioxide (HfO 2 ) film) to the surface of the PR element, for example. 
       SUMMARY 
       [0005]    According to one embodiment of the present invention, a method of fabricating a piezoelectronic transistor (PET) device includes forming a first stack including dielectric layers; forming a first metal layer over the first stack; forming a piezoelectric (PE) material on the first metal layer; forming a second metal layer on the PE material; and forming a piezoresistive (PR) element on the second metal layer through a gap in a first membrane formed a distance d above the second metal layer. 
         [0006]    According to another embodiment, a piezoelectronic transistor (PET) device includes a first stack including dielectric layers; a first metal layer formed over the first stack; a piezoelectric (PE) material formed over the first metal layer; a second metal layer on the PE material; and a layer comprising a piezoresistive (PR) element and a passivation layer disposed on the second metal layer, the passivation layer filling a gap in a membrane to hermetically seal the PET device. 
         [0007]    According to yet another embodiment, a semiconductor device includes a piezoelectronic transistor (PET) device comprising a first stack including dielectric layers, a first metal layer formed over the first stack, a piezoelectric (PE) material formed over the first metal layer, a second metal layer on the PE material, and a layer comprising a piezoresistive (PR) element and a passivation layer disposed on the second metal layer, the passivation layer filling a gap in a membrane to hermetically seal 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  illustrates a stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0011]      FIG. 2  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0012]      FIG. 3  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0013]      FIG. 4  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0014]      FIG. 5  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0015]      FIG. 6  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0016]      FIG. 7  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0017]      FIG. 8  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0018]      FIG. 9  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0019]      FIG. 10  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0020]      FIG. 11  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0021]      FIG. 12  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0022]      FIG. 13  illustrates another stage in the fabrication of a PET device according to an embodiment of the invention; 
           [0023]      FIG. 14  is a cross sectional block diagram of the PET device according to an embodiment of the invention; 
           [0024]      FIG. 15  is a perspective top-down cross sectional view of an aspect of the PET device according to an embodiment of the invention; 
           [0025]      FIG. 16  is a perspective top-down cross sectional view of the PET device according to an embodiment of the invention; 
           [0026]      FIG. 17  shows a stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0027]      FIG. 18  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0028]      FIG. 19  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0029]      FIG. 20  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0030]      FIG. 21  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0031]      FIG. 22  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0032]      FIG. 23  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0033]      FIG. 24  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0034]      FIG. 25  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0035]      FIG. 26  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0036]      FIG. 27  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0037]      FIG. 28  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0038]      FIG. 29  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0039]      FIG. 30  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0040]      FIG. 31  shows another stage in the fabrication of a PET device according to another embodiment of the invention; 
           [0041]      FIG. 32  is a cross sectional block diagram of the PET device according to another embodiment of the invention; and 
           [0042]      FIG. 33  is a block diagram of aspects of a semiconductor device including a PET device according to embodiments of the invention 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    As noted above, the formation of a PET device involves challenges including those related to the stability of the PR element. Currently, passivation of the PR element is used to achieve long term stability. However, the application of an ALD film after deposition of, for example, SmSe as the PR element results in significant changes to the material. As a result, passivation is not performed after patterning of the PR element. Instead, a sidewall based passivation may be possible. However, this requires a highly selective etch between the PR element and the metal layer below. Embodiments of the devices and methods detailed herein relate to deposition of the PR element through an evacuated suspended membrane. As a result, sidewall free passivation and local patterning without etching the PR element in the modulated region are achieved. 
         [0044]      FIGS. 1-14  illustrate stages in the fabrication of a PET device  10  according to an embodiment of the invention.  FIG. 1  illustrates a stage  100  in the fabrication of a PET device according to an embodiment of the invention. A silicon nitride (SiN) layer  135  is deposited on a stack. The stack (a dielectric stack or stack that includes dielectric layers) includes a base SiN layer  110 , a hafnium dioxide (HfO 2 ) layer  115  above the SiN layer  110 , and a metal M 0  layer  120  above the HfO 2  layer  115 . The PE material  125  is disposed on the M 0  layer  120 , and another metal M 1  layer  130  is deposited on the PE material  125 . The SiN layer  135  is deposited on the M 1  layer  130  which may be polished to achieve local flatness. The polishing of the M 1  layer  130  to achieve the required flatness may be necessary because of grain structure of the PE material  125 .  FIG. 2  illustrates another stage  200  in the fabrication of a PET device according to an embodiment of the invention. The SiN layer  135  deposited on the M 1  layer  130  is patterned using lithography and reactive ion etching (RIE) to form an opening  136 . 
         [0045]      FIG. 3  illustrates another stage  300  in the fabrication of a PET device according to an embodiment of the invention. A tungsten (W) layer  140  and platinum (Pt) layer  145  are deposited as electroplating seed layers on the SiN layer  135  and exposed M 1  layer  130 .  FIG. 4  illustrates another stage  400  in the fabrication of a PET device according to an embodiment of the invention. Nickel (Ni)  150  is electroplated through a lithographically defined resist mask on the Pt layer  145  and patterned.  FIG. 5  illustrates another stage  500  in the fabrication of a PET device according to an embodiment of the invention. The PE material  125 , M 1   130 , W  140  and Pt  145  are etched. After this the W layer  140 , Pt layer  145 , and Ni  150  are removed by a wet chemical etch. As  FIG. 5  shows, the cross section of the device shown in stage  500  after patterning the PE material  125  includes a central pillar  510  and surrounding annulus  515 . The patterning of the Ni layer  150  in  FIG. 4  is such that the Ni layer  150  edges define the width of the three pillars (including central pillar  510 ) shown in  FIG. 5 . The W layer  140  under the Pt layer  145  facilitates a wet etch that does not destroy the PE material  125 . Nickel (Ni) electroplating works well on the Pt layer  145  but benefits from the W layer  140  because the Pt layer  145  is hard to etch. The Ni  150  provides a material that does not etch quickly during the reactive ion etch needed to pattern the PE material  125 .  FIG. 6  illustrates another stage  600  in the fabrication of a PET device according to an embodiment of the invention. Silicon (Si)  155  is deposited and chemical mechanical polishing or planarization (CMP) is used to planarize the structure.  FIG. 7  illustrates another stage  700  in the fabrication of a PET device according to an embodiment of the invention. The Si  155  is recessed via RIE in the central portion of the structure in stage  600 . The mask for etching the recess in the RIE step is a lithographic photoresist. 
         [0046]      FIG. 8  illustrates another stage  800  in the fabrication of a PET device according to an embodiment of the invention. A metal cantilever  160  is defined joining the M 1  layers  130  of the central pillar  510  and the annular ring ( 520 ). This metal cantilever  160  is defined by lithography in combination with physical vapor deposition of the metal and either liftoff of a resist mask or RIE through a resist mask. As  FIG. 8  indicates, the cantilever  160  will act as a flexible bridge that supplies voltage to the center PE material  125 - 1 .  FIG. 9  illustrates another stage  900  in the fabrication of a PET device according to an embodiment of the invention. Additional Si  155  is deposited as shown and planarized using CMP. The distance between the M 1  layer  130  and the top of the Si  155  defines the thickness of the subsequent PR element  170  ( FIG. 12 ).  FIG. 10  illustrates another stage  1000  in the fabrication of a PET device according to an embodiment of the invention. A thin SiN membrane  165  is deposited above the SiN layer  135  and the Si  155 .  FIG. 11  illustrates another stage  1100  in the fabrication of a PET device according to an embodiment of the invention. A small circular deposition window  166  is etched (via RIE and electron beam lithography) in the SiN membrane  165 , and the sacrificial silicon fill (Si  155 ) is removed with a xenon difluoride (XeF 2 ) etch. This releases the central pillar  510 , the SiN membrane  165  layer and the contact cantilever  160 . Generally, the circular deposition window  166  is sized to have a diameter larger than the distance between the bottom of the SiN membrane  165  layer and the top of the M 1  layer  130 . The pressure in the PR element  170 - 1  ( FIG. 12 ) (which is caused by displacement of the PE material  125  and which results in a change in resistive state) will scale with the volume of the compressed PR element  170 - 1 . Thus, thickness of the PR element  170  is minimized to achieve good piezo response. 
         [0047]      FIG. 12  illustrates another stage  1200  in the fabrication of a PET device according to an embodiment of the invention. The SiN membrane  165  defined in stages  1000 - 1100  constitutes a local shadow mask. In stage  1200 , the PR element  170  and a titanium nitride (TiN) cap  175  are deposited through this mask resulting in a local PR pillar (PR element  170 - 1  in contact with the central PE material  125 - 1  stack). This deposition also closes all of the openings used to release the Si  155  in stage  1100 . Thus at stage  1200 , the device shown is hermetically sealed and should not age. The PR element  170 - 1  does not have a sidewall that will reduce its modulation, as in prior devices. As noted above, the PR element  170  may be SmSe, for example, or may be SmS, SmTe, or TmTe. Other materials that may be used as the PR element  170  are discussed below.  FIG. 13  illustrates another stage  1300  in the fabrication of a PET device according to an embodiment of the invention. The PR element  170  and TiN cap  175  are etched back via a lithographically defined RIE. As a result, individual PET devices  10  on a wafer are electrically separated.  FIG. 14  is a cross sectional block diagram of the PET device  10  according to an embodiment of the invention. An aluminum (Al) layer  180  is added. The Al layer  180  prevents the TiN cap  175  from bending and acts as a clamp that is anchored in place despite displacement of the PE  125 - 1 . 
         [0048]      FIGS. 15 and 16  depict a perspective top-down view of aspects of the PET device  10 .  FIG. 15  is a perspective top-down cross sectional view of an aspect of the PET device  10  according to an embodiment of the invention. The PET device  10  is shown without the Al layer  180  and portions of the PR element  170 , the TiN cap  175 , and the SiN membrane  165  to expose the active PR element  170 - 1 .  FIG. 16  is a perspective top-down view of the PET device  10  according to an embodiment of the invention. 
         [0049]      FIGS. 17-32  illustrate stages in the fabrication of a PET device  20  according to another embodiment of the invention. The stages shown in  FIGS. 17-32  begin with the stage  100  shown in  FIG. 1 . While the processes illustrated in several of the figures according to this alternate embodiment are the same as for the embodiment illustrated in  FIGS. 1-14  (and labels are reused for many of the layers), the deposition window  245  ( FIG. 29 ) for the PR  170  ( FIG. 30 ) is different than the deposition window  166  ( FIG. 11 ).  FIG. 17  shows a stage  1700  in the fabrication of a PET device  20  according to another embodiment of the invention. The SiN layer  135  of stage  100  is etched and a disk of Si  210  and SiN membrane  220  (or atomic layer deposition (ALD) film) is created on the M 1  layer  130 .  FIG. 18  shows another stage  1800  in the fabrication of a PET device  20  according to another embodiment of the invention. A W layer  140  and Pt layer  145  are deposited as electroplating seed layers.  FIG. 19  shows another stage  1900  in the fabrication of a PET device  20  according to another embodiment of the invention. Ni  150  is electroplated on the Pt layer  145  inside areas not masked by a photoresist.  FIG. 20  shows another stage  2000  in the fabrication of a PET device according to another embodiment of the invention. The PE material  125 , M 1   130 , W  140  and Pt  145  are etched. After this the W layer  140 , Pt layer  145 , and Ni  150  are removed by a wet chemical etch. The creates the central pillar  2010  that includes the PE material  125 - 1  that will be displaced based on a voltage application and annular ring  2015 .  FIG. 21  shows another stage  2100  in the fabrication of a PET device  20  according to another embodiment of the invention. Si  155  is deposited and CMP is used to planarize the structure. 
         [0050]      FIG. 22  shows another stage  2200  in the fabrication of a PET device  20  according to another embodiment of the invention. The Si  155  is recessed via RIE in the central portion of the structure shown in stage  2100 .  FIG. 23  shows another stage  2300  in the fabrication of a PET device  20  according to another embodiment of the invention. A metal cantilever  160  is defined joining the M 1  layers  130  of the central metal pillar  2010  and the annular ring  2015 . This metal cantilever  160  is defined by lithography in combination with physical vapor deposition of the metal and either liftoff of a resist mask or reactive ion etching through a resist mask.  FIG. 24  shows another stage  2400  in the fabrication of a PET device  20  according to another embodiment of the invention. A small hole  230  is etched in the SiN membrane  220  via RIE combined with electron beam lithography. This hole  230  forms an integral part of the deposition window for the PR layer ( 170 ,  FIG. 30 ) later in the process. This process (formation of the hole  230 ) differentiates the current embodiment from the embodiment discussed with reference to  FIGS. 1-14  because the gap between the SiN membrane  220  and the M 1  layer  130  surface is defined by the thickness of a thin film (Si  210 ) rather than via CMP. While electron beam lithography is used to etch the hole  230 , the other RIE steps according to the current embodiment use resist and photolithography.  FIG. 25  shows another stage  2500  in the fabrication of a PET device  20  according to another embodiment of the invention. Si  155  is deposited and planarized using CMP.  FIG. 26  shows another stage  2600  in the fabrication of a PET device  20  according to another embodiment of the invention. The contact annulus  235  is etched via RIE to the lower membrane (SiN  220 ).  FIG. 27  shows another stage  2700  in the fabrication of a PET device  20  according to another embodiment of the invention. Another SiN membrane  240  is deposited over the SiN layer  135  and in the annulus  235  above the SiN membrane  220 .  FIG. 28  shows another stage  2800  in the fabrication of a PET device  20  according to another embodiment of the invention. A window  245  is etched via a lithographically defined RIE in the SiN membrane  240 . The window  245  combined with the hole  230  form the composite PR deposition window for the current embodiment.  FIG. 29  shows another stage  2900  in the fabrication of a PET device  20  according to another embodiment of the invention. A dry release etch process using xenon difluoride (XeF 2 ) is used to remove the Si  210  and Si  155 . 
         [0051]      FIG. 30  shows another stage  3000  in the fabrication of a PET device  20  according to another embodiment of the invention. The deposition window (window  245  and hole  230 ) formed in the stages preceding stage  3000  constitutes a local shadow mask. In stage  3000  the PR element  170  and a titanium nitride (TiN) cap  175  are deposited through this mask resulting in a local PR element  170 - 1  pillar in contact with the central PE material  125 - 1  stack  2010 . This deposition also closes all of the openings used to release the silicon layer (Si  210  and Si  155 ) in stage  2900 . Thus, the device shown at stage  3000  is hermetically sealed and should not age.  FIG. 31  shows another stage  3100  in the fabrication of a PET device  20  according to another embodiment of the invention. The PR element  170  and TiN cap  175  are etched back via a lithographically defined RIE.  FIG. 32  is a cross sectional block diagram of the PET device  20  according to another embodiment of the invention. An aluminum (Al) layer  180  is added to the structure as discussed above. The Al layer  180  acts as a clamp that is anchored in place despite displacement of the central portion PE material  125 - 1 . A comparison of  FIG. 14  with  FIG. 32  indicates the difference between the two exemplary embodiments with regard to the deposition of the PR element  170 - 1  above the M 1  layer  130 . In the embodiment associated with  FIG. 32 , the SiN deposition window  245  may be closer to the M 1  layer  130  than in the embodiment associated with  FIG. 14 . This is because of two factors. First, the thickness between the window  245  and the M 1  layer  130  surface is defined by the thickness of a thin film (Si  210 ) rather than CMP. Second, the SiN membrane  220  close to the M 1  layer  130  does not need to be as wide as the entire device, thus it may be made thinner without risking buckling. As a result, the thickness of the PR element  170  may be less for the PET device  20  than for the PET device  10 . 
         [0052]    The SiN membrane  240  must be under tensile stress to not buckle and touch the M 1  layer  130 . The SiN membrane  240  must also be far enough away from the M 1  layer  130  such that Van der Waal forces do not pull the SiN membrane  240  in. The release etch discussed with reference to  FIG. 29  is done as a gas phase process to prevent pull-in of the SiN membrane  240  due to strong wetting forces. The SiN membrane  240  must be kept dry until the PR element  170  is deposited. Thus, an in-situ XeF 2  release inside the same vacuum system that deposits the PR layer  170  and capping layer  175  may be most effective. The distance between the M 1  layer  130  and the SiN membrane  240  should generally be less than the diameter of the SiN membrane  240  opening (window  245 ). Volume of the PR element  170  may be scaled down to reach higher pressures. There is no lower bound on thickness of the PR element  170  and, thus, on the volume. The speed of the PET device  10 ,  20  depends on total stack height. Thus, to achieve maximum speed of the PET device  10 ,  20 , distance between the SiN membrane  240  and the M 1  layer  130  should be minimized. The thickness of the PR element  170  should be less than the thickness of the window  245  to prevent clamping part of the PR element  170  by the SiN membrane  240  which is made rigid by the subsequent metal deposition steps. The hermetically sealed device (PET device  10 ,  20 ) will not become oxidized because the PE material  125  typically has very low partial pressures of oxygen in vacuum. 
         [0053]    In alternatives to both of the exemplary embodiments discussed above, the TiN cap  175  may instead be comprised of a stack of metals such as, for example, Pt/TiN/Pt, titanium aluminum nitride (TiAlN) and TiN, Pt/TiN, and palladium (Pd) and TiN. The M 1  layer  130  may be a stack of metals that are resistant to XeF 2  and with a bottom metal of the stack that does not reduce the PE material  125 . For example, the M 1  layer  130  may be comprised of ruthenium (Ru)/TiAlN/Pt, ruthenium oxide (RuO 2 )/TiN/Pd, or iridium oxide (IrO 2 )/Pt. In alternate embodiments, the SiN membrane  240  may be comprised of ALD films such as HfO 2  or aluminum oxide (Al 2 O 3 ). In addition to the above-discussed exemplary materials that may be used for the PR element  170 , the PR element  170  may be comprised of thulium monosulfide (TmS), or thulium selenide (TmSe). 
         [0054]      FIG. 33  is a block diagram of aspects of a semiconductor device  30  including a PET device  10 ,  20  according to embodiments of the invention. A voltage source  15  is used to apply a voltage across the M 0  metal layer  120  and the M 1  metal layer  130  to modulate the PE element  125 . This modulation results in switching the PR element  170  between a low and high resistive state. When the PET device  10 ,  20  is used as a switch element, read and write components  17  may be coupled to the PET device  10 ,  20 . The PET device  10 ,  20  may be used as a memory in another context as part of semiconductor device  30 , as well. 
         [0055]    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. 
         [0056]    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. 
         [0057]    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. 
         [0058]    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.