Abstract:
A front-end method of fabricating nickel plated caps over copper bond pads used in a memory device. The method provides protection of the bond pads from an oxidizing atmosphere without exposing sensitive structures in the memory device to the copper during fabrication.

Description:
This application is a divisional of application Ser. No. 11/399,358, filed Apr. 7, 2006 now U.S. Pat. No. 7,485,948, which is a divisional of application Ser. No. 10/902,569, filed Jul. 30, 2004, now Patent No. 7,226,857. Each of the above listed applications are herein incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of semiconductor devices and, in particular, to the formation of bond pads for memory and other integrated circuit devices. 
   BACKGROUND OF THE INVENTION 
   A well known semiconductor memory component is random access memory (RAM). RAM permits repeated read and write operations on memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors, which require periodic refreshing to maintain the stored data. 
   Recently, resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical PCRAM device is disclosed in U.S. Pat. No. 6,348,365, assigned to Micron Technology Inc. and incorporated herein by reference. In typical PCRAM devices, conductive material, such as silver, is moved into and out of a chalcogenide material to alter the cell resistance. Thus, the resistance of the chalcogenide material can be programmed to stable higher resistance and lower resistance states. The programmed lower resistance state can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. 
   One aspect of fabricating PCRAM cells, which may also occur in fabrication of other integrated circuit devices, involves bond pads used for connecting a PCRAM memory device to external leads of an encapsulated integrated circuit package. Increasingly, bond pads are formed of copper, rather than traditional aluminum, due to its superior conductivity and scalability. One drawback associated with copper, however, is that it oxidizes rapidly. Thus, leaving the copper bond pads exposed to die fabrication or packaging process steps where oxygen is present will lead to corrosion of the bond pad. Exposing copper bond pads to subsequent fabrication and/or packaging processes may also cause poisoning of a PCRAM memory cell, because copper ions may migrate from the bond pads and into an underlying chalcogenide glass layer, which changes the responsiveness of the glass to accept or expel other ions used for programming the cell. This, in turn, makes the cell unable to reliably switch between high and low resistance states. Therefore, it is important in the fabrication or packaging of PCRAM cells to limit the cells&#39; copper bond pad exposure and particularly exposure to an oxygen-filled environment. Other integrated circuits using copper bond pads should also avoid exposure of the bond pad to oxidizing environments during subsequent fabrication and/or packaging steps. 
   One method for addressing this problem involves back-end processing where nickel is plated onto the copper bond pads after their fabrication. The back-end processing, however, may involve an ion mill etch step, which is a non-selective etching procedure, on the exposed copper. As copper etches at a higher rate than other materials used in fabrication, performing this etch could degrade the copper bond pad completely. 
   Accordingly, there is a need for a method of forming PCRAM cells where the PCRAM cell materials are not exposed to copper and the copper bond pads are not oxidized and do not corrode. There is also a more general need to protect copper bond pads from an oxidizing atmosphere during subsequent fabrication steps of integrated circuit devices. 
   BRIEF SUMMARY OF THE INVENTION 
   Exemplary embodiments of the invention provide a front-end method of fabricating nickel plated caps over copper bond pads used in a memory device. The method involves depositing an oxide layer over circuitry formed on a substrate, including array and periphery circuitry. Using a layer of photoresist over the oxide layer, a bond pad pattern is formed and etched in the periphery, exposing a fabricated copper bond pad. The photoresist is removed and nickel is selectively plated onto the exposed copper pad to form a cap over the copper. Following this, fabrication steps may occur which expose the in-fabrication structure to an oxidizing atmosphere without oxidizing the copper bond pads. 
   In accordance with one exemplary embodiment, the invention is used to construct bond pads for a PCRAM memory in which PCRAM cell material is deposited and formed into memory cells after the copper bonds are formed and nickel plated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-discussed and other features and advantages of the invention will be better understood from the following detailed description, which is provided in connection with the accompanying drawings, in which: 
       FIG. 1  is a cross-sectional view of an exemplary memory device constructed in accordance with the invention; 
       FIG. 2  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication; 
       FIG. 3  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication subsequent to that shown in  FIG. 2 ; 
       FIG. 4  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication subsequent to that shown in  FIG. 3 ; 
       FIG. 5  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication subsequent to that shown in  FIG. 4 ; 
       FIG. 6  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication subsequent to that shown in  FIG. 5 ; 
       FIG. 7  is a cross-sectional view of a portion of the exemplary memory device of  FIG. 1  during a stage of fabrication subsequent to that shown in  FIG. 6 ; 
       FIG. 8  illustrates a computer system having a memory element in accordance with the invention; and 
       FIG. 9  illustrates an integrated circuit package having a memory element in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description, reference is made to various specific embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made without departing from the spirit or scope of the invention. 
   The term “substrate” used in the following description may include any supporting structure including, but not limited to, a semiconductor substrate that has an exposed substrate surface. A semiconductor substrate should be understood to include silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. When reference is made to a semiconductor substrate or wafer in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation. The substrate need not be semiconductor-based, but may be any support structure suitable for supporting an integrated circuit. 
   The term “resistance variable material” is intended to include chalcogenide glasses, and chalcogenide glasses comprising a metal, such as silver. For instance the term “resistance variable material” includes silver doped chalcogenide glasses, silver-germanium-selenide glasses, and chalcogenide glass comprising a silver selenide layer. 
   The term “resistance variable memory element” is intended to include any memory element, including programmable conductor memory elements, semi-volatile memory elements, and non-volatile memory elements which exhibit a resistance change in response to an applied voltage. 
   The term “chalcogenide glass” is intended to include glasses that comprise an element from group VIA (or group 16) of the periodic table. Group VIA elements, also referred to as chalcogens, include sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and oxygen (O). 
   The invention is now explained with reference to the figures, which illustrate exemplary embodiments and where like reference numbers indicate like features.  FIG. 1  shows array and peripheral circuitry portions of a resistance variable memory element  100  constructed in accordance with the invention. It should be understood that the portions shown are illustrative of one embodiment of the invention, and that the invention encompasses other devices that can be formed using different materials and processes than those described herein. The memory element  100  has copper bond pads  92  in the periphery which are covered with nickel plating  82 . The pads  92 , as discussed below, are constructed such that the memory cell material  69  in the array was not exposed to copper during fabrication of the device  100 . Further, and as described in more detail below, the copper bond pad  92  was not exposed to an oxygen ambient during device  100  fabrication, which could have oxidized the copper and degraded the quality of the bond pad  92 . 
   For exemplary purposes only, memory element  100  is shown with an example of the circuitry  50  that the copper bond pads  92  may be used in connection with. In the array and periphery portions of a substrate  200 , transistors  40  are formed having source/drain active regions  101  in the substrate  200 . A first insulating layer  32 , e.g., a boro-phospho-silicate glass (BPSG) layer, is formed over the transistor gatestacks. Conductive plugs  41 , which may be formed of polysilicon, are formed in the first insulating layer  32  connecting to the source drain regions  101  in the substrate  200 . A second insulating layer  34  is formed over the first insulating layer  32 , and may again comprise a BPSG layer. Conductive plugs  49  are formed in the second insulating layer  34  and are electrically connected to the conductive plugs  41  in the first insulating layer  32  which connects through some of plugs  41  to selected transistors  40 . A conductive bit line  55  is formed between the conductive plugs  49  in the second insulating layer  34 . The bit line illustrated has layers X, Y, Z formed of tungsten nitride, tungsten, and silicon nitride, respectively. A third insulating layer  36  is formed over the second insulating layer  34 , and again openings in the insulating layer are formed and filled with a conductive material to form conductive plugs  60 . Next, metallization layers having conductive traces and/or contacts  91  are formed over the third insulating layer  36  and are insulated with an interlevel dielectric (ILD) layer  38 . 
   Referring now to  FIGS. 2-7 , an exemplary method of forming the bond pads  92  for memory element  100  in accordance with the invention is now described. It should be understood that the description of materials and fabrication steps just described for circuitry  50  were illustrative only, and that other types of integrated circuitry is within the scope of the invention. Thus, for purposes of the remaining fabrication steps, the layers of the circuitry  50  are not depicted in the fabrication steps described with reference to  FIGS. 2-7 . 
   Turning to  FIG. 2 , an inter level dielectric (ILD) layer  40  is formed. In this layer  40  in the periphery, a dual damascene pattern is formed and filled with copper to create a copper connection  61  and a copper bond pad  92 . In both the array and the periphery, an oxide layer  56  and a nitride layer  57  are then deposited over the ILD layer  40 . Vias  62  are formed through layers  56 ,  57  and the ILD layer  40  and filled with a conductive material to connect with conductive areas of the circuitry  50  below (such as contacts  91  of  FIG. 1 ). The vias  62  are filled with a conductive material, such as tungsten, and the vias  62  are either dry etched or chemical mechanical polished (CMP) to planarize the top of the vias  62  even with the nitride layer  57 . Thus, at this stage, tungsten is exposed at the top of the vias  62  and the copper bond pad is covered with oxide layer  56  and nitride layer  57 . 
   Next, referring to  FIG. 3 , an oxide layer  63  is formed over the tops of the vias  62  and the nitride layer  57 . The oxide layer  63  is preferably thin, approximately 100 to about 500 Angstroms thick over both the array and the periphery. A layer of photoresist  64  is formed over the oxide layer  63 . As shown in  FIG. 3 , a bond pad pattern is formed over pad  92  by patterning and developing the photoresist  64 , and as shown in  FIG. 4 , the opening is used to etch oxide layer  63 , nitride layer  57 , and oxide layer  56  down to the bond pad  92 . After etching, the photoresist  64  is stripped from the wafer. 
   At this stage in fabrication, in the area of the periphery where the bond pad is patterned, the exposed copper  92  will oxidize slightly, however, so long as the this step is not prolonged, the oxidation will enable the next formation step. As shown in  FIG. 5 , nickel is plated selectively onto the copper bond pad  92 , forming a nickel cap  82 . The nickel plating may be accomplished by an electroless nickel bath. For example, without limiting the plating chemistry that may be utilized for this invention, the copper bond pad  92  is exposed to a plating nickel bath having a pH value of approximately 8. The nickel bath may comprise a nickel salt and a reducing agent as well as a stabilizing agent. The temperature of the bath may be approximately 80 degrees Celsius or less, depending on the rate of deposition desired. A lower temperature improves the uniformity of deposition while a higher temperature increases the plating rate. The nickel cap may be approximately 4000 Angstroms thick. Post-plating, the remaining oxide layer  63  is wet etched off, leaving the tungsten vias  62  exposed. 
   Memory cell formation and patterning can now occur. As shown in  FIG. 6 , cell material  69  is deposited on the array. The cell material  69  may include resistance variable cell material, like the materials necessary for construction of PCRAM memory cells constructed according to the teachings of U.S. Pub. Appl. Nos. 2003/0155589 and 2003/0045054, each assigned to Micron Technology Inc. Appropriate PCRAM cell materials include layers of germanium selenide, chalcogenide glass, and silver-containing layers creating a resistance variable memory device  100 . Finally, a top electrode  70  is deposited over the cell material  69  as shown in  FIG. 7 . The top electrode  70  contacts the cell  69  and the periphery vias  62 . The electrode  70  can be patterned as desired. For example, the electrode  70  layer may be blanket deposited over the array; or alternatively, an electrode  70  may be deposited in a pre-determined pattern, such as in stripes over the array. In the case of PCRAM cells, the top electrode  70  should be a conductive material, such as tungsten or tantalum, but preferably not containing silver. Also, the top electrode  70  may comprise more than one layer of conductive material if desired. 
   At this stage, the memory element  100  is essentially complete. The memory cells are defined by the areas of layer  69  located between the conductive plugs  62  and the electrode  70 . Other fabrication steps to insulate the electrode  70  using techniques known in the art, are now performed to complete fabrication. 
     FIG. 9  illustrates that the memory element  100  is subsequently used to form an integrated circuit package  201  for a memory circuit  1248  ( FIG. 8 ). The memory device  100  is physically mounted on a mounting substrate  202  using a suitable attachment material. Bond wires  203  are used to provide electrical connection between the integrated chip bond pads  92  and the mounting substrate bond pads  204  and/or lead wires which connect the die  100  to circuitry external of package  201 . 
   The embodiments described above refer to the formation of a memory device  100  structure in accordance with the invention. It must be understood, however, that the invention contemplates the formation of other integrated circuit elements, and the invention is not limited to the embodiments described above. Moreover, although described as a single memory device  100 , the device  100  can be fabricated as a part of a memory array and operated with memory element access circuits. 
     FIG. 8  is a block diagram of a processor-based system  1200 , which includes a memory circuit  1248 , for example a PCRAM circuit employing non-volatile memory devices  100  fabricated in accordance with the invention. The processor system  1200 , such as a computer system, generally comprises a central processing unit (CPU)  1244 , such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device  1246  over a bus  1252 . The memory  1248  communicates with the system over bus  1252  typically through a memory controller. 
   In the case of a computer system, the processor system may include peripheral devices such as a floppy disk drive  1254  and a compact disc (CD) ROM drive  1256 , which also communicate with CPU  1244  over the bus  1252 . Memory  1248  is preferably constructed as an integrated circuit, which includes one or more resistance variable memory elements  100 . If desired, the memory  1248  may be combined with the processor, for example CPU  1244 , in a single integrated circuit. 
   The above description and drawings are only to be considered illustrative of exemplary embodiments which achieve the features and advantages of the invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.