Patent Publication Number: US-8525144-B2

Title: Programmable via devices

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 11/874,582, filed on Oct. 18, 2007, the disclosure of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to integrated circuits, and, more particularly, to programmable via devices in integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Reconfigurable circuits have been widely used in the semiconductor industry for field programmable gate arrays (FPGAs) and for the repair of defective memory elements. Moreover, reconfigurable circuits such as FPGAs are also anticipated to play a significant role in the new Three Dimensional Integration (3DI) technology currently under development. 3DI structures typically include multilayer structures that can form a single chip combination with different functionalities. In these multilayer (and multifunctional) systems, reconfigurable circuit connections are needed to provide controllable logic functionality, memory repair, data encryption, as well as other functions. 
     A programmable via device (PVD) is an enabling technology for high-performance reconfigurable logic applications such as those required for 3DI applications. As the name would suggest, a PVD comprises vias (i.e., vertical contacts) whose resistance can be temporarily or permanently switched (i.e., programmed) between two or more resistance states by applying one or more signals to the device. Recently, the possibility of using phase change materials (PCMs) in PVDs has gained momentum as more is learned about these materials and their integration into integrated circuits. Currently, a ternary alloy of germanium (Ge), antimony (Sb), and tellurium (Te) (GST) (e.g., Ge 2 Sb 2 T 5 ) is showing the greatest promise for use in practical PCM-based PVDs, although several other materials are presently under investigation. At room temperature and at moderately elevated temperatures, for example, GST is stable in two phases, a crystalline phase, which is a moderately good conductor of electrical current, and an amorphous phase, which is largely insulating. The GST in a PVD may be converted between these phases by applying a pulse of current (“switching current pulse”) to a heating feature that is located proximate to the GST. This switching current pulse, in turn, acts to resistively heat the heating feature and, as a result, the adjacent GST. A RESET switching current pulse is designed to rapidly heat the GST above its melting point and then to rapidly quench the melted material so that its disordered arrangement of atoms is retained. In this manner, the RESET switching current pulse converts at least a fraction of the GST from a crystalline phase to an amorphous phase. In contrast, a SET switching current pulse is designed to anneal the GST at temperatures below its melting point for a time somewhat longer than the RESET pulse. Such a SET switching pulse converts at least a fraction of the GST from the amorphous phase into the crystalline phase. 
     Several designs and methods of forming PCM-based PVDs are described in, for example, U.S. Patent Publication No. 2007/0235708 A1, entitled “Programmable via structure for three dimensional integration technology,” and U.S. patent application Ser. No. 11/612,631, entitled “Programmable via structure and method of fabricating same,” which are commonly assigned herewith and are incorporated by reference herein. Nevertheless, despite their promise for use in reconfigurable circuits, there remains a need for refined designs and fabrication techniques for PCM-based PVDs. Ideally such designs and fabrication techniques will be compatible with conventional complimentary-metal-oxide-semiconductor (CMOS) processing. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention address the above-identified need by providing designs and methods of fabricating PCM-based PVDS that are compatible with conventional CMOS processing. 
     In accordance with an aspect of the invention, a device comprises a heater, a dielectric layer, a phase-change element, and a capping layer. The dielectric layer is disposed at least partially on the heater and defines an opening having a lower portion and an upper portion. The phase-change element occupies the lower portion of the opening and is in thermal contact with the heater. The capping layer overlies the phase-change element and occupies the upper portion of the opening. At least a fraction of the phase-change element is operative to change between lower and higher electrical resistance states in response to an application of an electrical signal to the heater. 
     In accordance with one of the above-identified embodiments of the invention, a programmable via device comprises a dielectric layer partially disposed on a heater. A PCM element occupies a lower portion of an opening in the dielectric layer, while a capping layer overlies the PCM element and occupies an upper portion of the opening. The PCM element is recessed into the opening in this manner by a polishing process (e.g., chemical mechanical polishing). Suitable signals applied to the heater act to cause the heater to heat the PCM element and to cause the PCM element to change between crystalline and amorphous phases. The capping layer acts as a diffusion barrier between the PCM element and overlying metallic features. 
     These and other features and advantages of the present invention will become apparent from the following detailed description which is to be read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a sectional view of a programmable via device in accordance with an illustrative embodiment of the invention. 
         FIG. 2  shows a flow chart of an illustrative method of forming the  FIG. 1  programmable via device. 
         FIGS. 3A-3J  show sectional views of the  FIG. 1  programmable via device in various stages of formation using the  FIG. 2  method. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention will be illustrated herein in conjunction with an exemplary PVD for use in integrated circuits and methods for forming such a PVD. Such a device may be used in a range of applications including, but not limited to, reconfigurable circuits. It should be understood, however, that the invention is not limited to the particular materials, features, and processing steps shown and described herein. Modifications to the illustrative embodiments will be apparent to those skilled in the art. 
     Particularly with respect to processing steps, it is emphasized that the descriptions provided herein are not intended to encompass all of the processing steps that may be required to successfully form a functional device in an integrated circuit. Rather, certain processing steps which are conventionally used in forming integrated circuit devices, such as, for example, wet cleaning and annealing steps, are purposefully not described herein for economy of description. However one skilled in the art will readily recognize those processing steps omitted from this generalized description. Moreover, details of conventional semiconductor processing steps described herein will only be described generally since the details of these conventional processes will be known to one skilled in the art and since there are commercially available semiconductor processing tools for implementing these processing steps. Details of the processing steps used to fabricate semiconductor devices may be found in a number of publications, for example, S. Wolf and R. N. Tauber,  Silicon Processing for the VLSI Era, Volume  1, Lattice Press, 1986; and S. Wolf,  Silicon Processing for the VLSI Era, Volume  4:  Deep Submicron Process Technology , Lattice Press, 2002, both of which are incorporated herein by reference. 
     The term “phase change material” (PCM) as used herein is intended to encompass any material displaying more than one programmable electrical resistance state for use in integrated circuits. PCMs comprise, for example, various chalcogenides and transition metal oxides and include, but are not limited to, doped or undoped Ge 2 Sb 2 Te 5 , GeSb, GeSb 4 , SbTe, , SrTiO 3 , BaTiO 3 , (Sr,Ba)TiO 3 , SrZrO 3 , Ca 2 Nb 2 O 7 , (Pr,Ca)MnO 3 , Ta 2 O 5 , NiO x  and TiO x , as well as other suitable materials. 
     It should also be understood that the various layers and/or regions shown in the accompanying figures are not drawn to scale, and that one or more layers and/or regions of a type commonly used in integrated circuits may not be explicitly shown in a given figure. This does not imply that the layers and/or regions not explicitly shown are omitted from the actual integrated circuit. 
       FIG. 1  shows a PVD  100  in accordance with an illustrative embodiment of the invention. This illustrative PVD comprises a substrate  105 , a lower dielectric layer  110 , a heater  115 , and an upper dielectric layer  120 . The upper dielectric layer comprises a first opening  125  and two second openings  130 . A PCM element  135  and a capping layer  140  occupy the first opening, while metal vias  145  occupy the second openings. Metal lines  150  are disposed on the metal vias and on the capping layer. 
     In the illustrative PVD  100 , the substrate preferably comprises silicon or some other suitable semiconductor material, while the lower dielectric layer  110  and upper dielectric layer  120  preferably comprise silicon oxide (e.g., SiO x ) or some other suitable dielectric material such as silicon nitride (e.g., Si x N y ); silicon carbonitride (e.g., SiC x N y ); silicon-carbon hydroxide (e.g., SiCOH); or silicon oxynitride (e.g., SiO x N y ). Moreover, the heater  115  preferably comprises tantalum silicon nitride (e.g., Ta x Si y N z ) or some other suitable conductive material with a relatively high resistivity (e.g., from about 100 ohm-cm to about 1,000 ohm-cm) such as, but not limited to, tantalum nitride (e.g., TaN), chromium oxide (e.g., CrO x ), or ruthenium oxide (e.g., RuO x ). The PCM element  135  preferably comprises GST or some other suitable PCM (further described above), while the metal vias  145  and metal lines  150  preferably comprise a conductive metallic material such as tungsten, copper, aluminum, or alloys thereof. 
     The material for the capping layer  140  is preferably chosen so that the capping layer acts as diffusion barrier between the PCM element  135  and the overlying metal line  150 . The capping layer may, for example, comprise a layer of titanium nitride (e.g., TiN) deposited on a thin layer (e.g., less than about one nanometer) of titanium or a layer of tantalum nitride (e.g., TaN) deposited on a thin layer of tantalum. The purely metallic sublayers typically act as adhesion layers for the metal nitride sublayers. The metal nitride sublayers typically act as the diffusion barriers. 
     In all cases, however, it should be noted that the above-described choices of materials for the elements in the PVD  100  are merely illustrative and that other suitable materials may be utilized as substitutes for the ones explicitly presented herein. Embodiments comprising such alternative materials will be apparent to one skilled in the art and may still fall within the scope of the invention. 
     The PVD  100  is programmable between two resistance states by applying a switching current pulse between the metal lines  150  contacting the metal vias  145 . Such a signal application causes the switching current pulse to pass through the heater  115  where it is partially converted into heat by resistive (i.e., ohmic) heating. This, in turn, causes the adjacent PCM element  135  to be heated by conduction. A RESET switching current pulse is applied to the PVD in order to rapidly melt and quench the PCM element and to thereby convert at least a fraction of the PCM from its moderately conductive crystalline phase to its insulating amorphous phase. A RESET pulse may, as an example, ramp up to peak current in about ten nanoseconds (ns), stay at peak current for 50 ns, and ramp down in two ns. A SET switching current pulse is, in contrast, applied to the PVD in order to anneal the PCM element below its melting point and to convert at least a fraction of the PCM from its amorphous phase into its crystalline phase. A SET switching pulse may, for example, ramp up to peak current in about ten ns, stay at peak current for about 1,000 ns, and ramp down in about 200 ns. In this manner, the PVD may be cycled between resistance states. 
     After being so programmed, any signals applied to the PVD  100  that must travel through the PCM element  135  will be affected by the PVD&#39;s programmed state. The resistance of a PVD after a suitable SET switching current pulse may be more than three orders of magnitude higher than the resistance after a suitable RESET switching current pulse. 
       FIG. 2  shows a flow chart of an illustrative method  200  in accordance with aspects of the invention for forming the PVD  100 . Moreover,  FIGS. 3A-3J  show sectional views of the PVD in various stages of formation using this illustrative method. 
     In step  205  of the method  200 , a dielectric layer is grown or deposited on the substrate  105  to form the lower dielectric layer  110 . If the lower dielectric layer comprises silicon oxide, for example, it may be thermally grown on a silicon substrate. Next, in step  210 , a layer of heater material  115 ′ for the heater  115  is deposited on the lower dielectric layer, as shown in  FIG. 3A . In step  215 , the layer of heater material is patterned such that it forms the discrete heater, as shown in  FIG. 3B . 
     In step  220 , a layer of dielectric material is deposited on the film stack to form the upper dielectric layer  120 , as shown in  FIG. 3C . Subsequently, in step  225 , the first opening  125  is patterned into the upper dielectric layer. The first opening lands on the heater  115 , as shown in  FIG. 3D . 
     In step  230 , a layer of PCM  135 ′ for the PCM element  135  is conformally deposited onto the uppermost surface of the upper dielectric layer  120  and into the first opening  125 . The layer of PCM fills the entire first opening, as shown in  FIG. 3E . 
     In step  235 , the layer of PCM  135 ′ is polished such that it is entirely removed from the uppermost surface of the upper dielectric layer  120 . The uppermost surface is also polished somewhat by this process. Moreover, in accordance with aspects of the invention, the polishing process is allowed to recess the PCM into the first opening  125  such that the PCM element  135  is defined, as shown in  FIG. 3F . One will note that, as a result of this polishing process, the PCM element only occupies a lower portion of the first opening. In other words, the PCM is purposefully removed from the upper portion of the first opening. 
     The just-described polishing process may comprise chemical-mechanical polishing (CMP). This CMP process is designed to take advantage of the softness of the PCM in relation to upper dielectric layer  120 , effectively allowing the PCM element  135  to be recessed into the first opening  125 . Such a CMP process will be familiar to one skilled in the art. 
     Subsequently, in step  240  of the method  200 , a layer of capping material  140 ′ for the capping layer  140  is conformally deposited on the uppermost surface of the upper dielectric layer  120  and into the upper portion of the first opening  125  so that it overlies the PCM element  135 . In step  245 , this layer of capping material is polished so that it is removed from the uppermost surface of the dielectric layer. This leaves the discrete capping layer in the upper portion of the first opening, as shown in  FIG. 3H . As indicated in the figure, the capping layer has an uppermost surface that is substantially coplanar with the uppermost surface of the upper dielectric layer. 
     In step  250 , the second openings  130  are patterned into the upper dielectric layer  120 . The second openings also land on the heater  115 , as shown in  FIG. 3I . In step  255 , a first layer of metallic material for the metal vias  145  is conformally deposited on the uppermost surface of the upper dielectric layer, onto the capping layer  140 , and into the second openings. In step  260 , this metallic material is then be polished such that it only remains in the second openings, thereby forming the metal vias  145 , as shown in  FIG. 3J . Subsequently, in step  265 , a second layer of metallic material is deposited on the film stack. In step  270 , this second layer of metallic material is patterned to form the metal lines  150 . After completing these processing steps, the PVD  100  is formed as shown in  FIG. 1 . 
     Advantageously, the PVD  100  and the method  200  are compatible with conventional CMOS processing. That is, they do not require processing steps that are not conventionally used in modern CMOS processing. What is more, methods in accordance with aspects of the invention may have several advantages when compared to other processing techniques for PCM-based PVDs. As described above with reference to steps  235 - 245 , for example, the capping layer  140  in the method  200  is formed by the steps of: recessing the PCM element  135  by polishing such that it only occupies the lower portion of the first opening  125  in the upper dielectric layer  120 , depositing a layer of capping material  140 ′ on the film stack, and then polishing the capping material such that it only remains in the upper portion of the first opening. As a result, the processing sequence used to form the capping layer does not use a purpose-specific photolithography step and is likely to be significantly less expensive than a processing sequence that forms the capping layer by depositing the layer of capping material and then using photolithography and anisotropic etching to pattern the layer of capping material. Steps comprising photolithography are typically among the most expensive steps in a given semiconductor processing scheme. 
     Moreover, the ability to form the metal vias  145  by depositing a layer of metallic material into the second openings  130  and polishing the metallic material, as described above with reference to steps  255  and  260 , is facilitated by the fact that the capping layer  140  does not protrude above the uppermost surface of the upper dielectric layer  120 . If the capping layer were to protrude in this manner, it would be difficult to remove the metallic material completely from the uppermost surface of the upper dielectric layer by polishing since the protruding capping layer would interfere with this polishing or be damaged by this processing step. 
     The method  200  also allows the metal vias  145  to be formed after forming the PCM element  135  and the capping layer  140 . This sequence mitigates cross-contamination issues. It is widely recognized, for example, that the presence of certain metallic materials for the metal vias such as copper during the deposition of GST may adversely affect the physical properties of the GST. With the fabrication sequence provided herein, the PCM element is already formed and encapsulated before any metallic material for the metal vias is deposited. 
     Devices and fabrication methods in accordance with aspects of the invention may be utilized to form an integrated circuit. The integrated circuit design is created in a graphical computer programming language, and is stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate integrated circuits or photolithographic masks used to fabricate integrated circuits, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the integrated circuit design in question that are formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     The resulting integrated circuits may be distributed by the fabricator in raw wafer form (i.e., as a single wafer that has multiple unpackaged integrated circuits), as a bare die, or in packaged form. In the latter case, the integrated circuit is mounted in a single integrated circuit package (e.g., plastic carrier with leads that are affixed to a motherboard or other higher level carrier) or in a multi-integrated circuit package (e.g., ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the integrated circuit is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either an intermediate product (e.g., motherboard) or an end product. The end product may be any product that includes integrated circuits, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying figures, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made to these embodiments by one skilled in the art without departing from the scope of the appended claims.