Abstract:
An improved field effect transistors (FETs) and methods of manufacturing the field effect transistors (FETs) are provided. The method of manufacturing a zero capacitance random access memory cell (ZRAM) includes comprises forming a finFET on a substrate and enhancing a storage capacitance of the finFET. The enhancement can be by either adding a storage capacity to the finFET or altering a portion of the finFET after formation of a fin body of the finFET.

Description:
FIELD OF THE INVENTION 
     The invention relates to semiconductor devices and methods of manufacture and, more particularly, to improved field effect transistors (FETs) and methods of manufacturing the field effect transistors (FETs). 
     BACKGROUND 
     Zero capacitance random access memory (ZRAM) is a type of DRAM memory based on the floating body effect of silicon on insulator (SOI) process technology. In ZRAM, the floating body effect allows the memory cell to be built without adding a separate capacitor, as the floating body effect takes the place of the conventional capacitor. Although SOI technology is a relatively expensive technology compared with more traditional CMOS technology, ZRAM offers cheaper on-chip cache memory, with little or no performance degradation. 
     ZRAM can offer memory access speeds similar to a standard six-transistor SRAM cell used in cache memory; however, ZRAM uses only a single transistor, which affords higher packing densities. The small cell size reduces the size of ZRAM memory blocks and thus reduces the physical distance that data must transit to exit the memory block. 
     ZRAM with bipolar read solves problems associated with FET-mode read; however, not enough signal is available during a read without requiring unrealistically high NPN gain, or using difficult-to-control avalanche-mode read. ZRAM suffers from low capacitance in the storage element resulting in poor retention time yield, and also from Vt-scatter making it difficult to yield large arrays with adequate signal margin. Nonetheless, ZRAM offers the possibility of very dense low cost memory for logic applications. 
     Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove. 
     SUMMARY 
     In a first aspect of the invention, a method of manufacturing a zero capacitance random access memory cell (ZRAM) comprises forming a finFET on a substrate and enhancing a storage capacitance of the finFET by either adding a storage capacity to the finFET or altering a portion of the finFET after formation of a fin body of the finFET. 
     In another aspect of the invention, a method of manufacturing a zero capacitance random access memory cell (ZRAM) comprises forming a fin body on a buried oxide layer. The fin body comprises active silicon and a hard mask. The method further includes increasing a capacitance of the fin body by doping a portion of the fin body. 
     In yet another aspect of the invention, a method of manufacturing a zero capacitance random access memory cell (ZRAM) comprises forming a fin body on a buried oxide layer. The fin body comprises active silicon and a hard mask. The method further includes increasing a capacitance of the fin body by altering a work function of a gate electrode deposited on the fin body. 
     In yet another aspect of the invention, a zero capacitance random access memory cell (ZRAM) comprises: a fin body with a gate dielectric on two sides of the fin body; a gate electrode adjacent two sides of the fin body and separated from the fin body by the gate dielectric; a storage dielectric formed remote from the gate electrode; and a storage electrode adjacent two sides of the gate electrode and separated from the gate electrode by the storage dielectric. 
     In yet another aspect of the invention, a zero capacitance random access memory cell (ZRAM) comprises a fin body having a doped portion to increase storage capacitance. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention. 
         FIGS. 1-5  show structures and methods of fabricating a device in accordance with aspects of the invention; 
         FIGS. 6-11  show structures and methods of fabricating a device in accordance with other aspects of the invention; 
         FIGS. 12-15  show structures and methods of fabricating a device in accordance with other aspects of the invention; 
         FIGS. 16-18  show structures and methods of fabricating a device in accordance with aspects of the invention, starting with the device shown in  FIG. 15 ; 
         FIGS. 19-21  show structures and methods of fabricating a device in accordance with other aspects of the invention; 
         FIG. 22  shows an alternative embodiment of the invention, with an effective work function nearby the valence band in accordance with aspects of the invention; 
         FIG. 23  shows an asymmetrical device and methods of fabricating the device in accordance with other aspects of the invention; and 
         FIG. 24  shows a flow diagram of a design process used in semiconductor design, manufacture, and/or test. 
     
    
    
     DETAILED DESCRIPTION 
     The invention relates to semiconductor devices and methods of manufacture and, more particularly, to improved field effect transistors (FETs) and methods of manufacturing the field effect transistors (FETs). More specifically, the present invention is directed to methods of fabricating memory elements and more specifically ZRAM (zero-capacitor RAM). The devices of the present invention have enhanced body capacitance, e.g., high capacitance from the body to other elements of the cell structure, to improve storage capacity, as well as high signal and low Vt scatter. More specifically, the present invention includes a compact high-capacitance ZRAM cell that allows a bipolar read without requiring avalanche for a large signal, as well as a FinFET structure which both increases the storage capacitance and reduces the Vt scatter while maintaining high density. The present invention provides several different devices, each advantageously providing a low-cost, high signal-to-noise ratio memory. For example, the devices of the present invention include: (i) a finFET with a storage dielectric, (ii) an inverted T-shaped inverter with a vertical fin region which is either fully or partially depleted, (iii) a segmented finFET, (iv) a finFET with an asymmetric charge distribution and (v) a finFET with an asymmetrical gate structure. 
       FIGS. 1-5  show structures and methods of fabricating a device in accordance with aspects of the invention. In particular,  FIGS. 1-5  show methods of manufacturing a semiconductor device (finFET) having a storage dielectric to enhance body capacitance. The finFET can be a ZRAM (zero-capacitor RAM) that provides a low-cost, high signal-to-noise ratio memory. In embodiments, the semiconductor device comprises, for example, a fin body with a gate dielectric on two sides of the fin body. A gate electrode is adjacent two sides of the fin body and separated from the fin body by the gate dielectric. A storage dielectric is formed remote from the gate electrode, and a storage electrode is adjacent two sides of the gate electrode and separated from the gate electrode by the storage dielectric. In embodiments, the storage electrode is electrically connected to the fin body on a top of the fin body. 
     More specifically,  FIG. 1  shows a wafer  5  in accordance with the invention. The wafer  5  comprises a substrate carrier  10 , a buried oxide layer  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known materials used for a substrate carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . 
     As shown in  FIG. 2 , the active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE). The conventional etching process includes, for example, a resist (not shown) formed on the hard mask  16  which is exposed to light to form openings. Thereafter, patterns  18  are etched into the active silicon layer  14  and hard mask  16  using a conventional etchant. The patterning exposes portions of the buried oxide layer  12 , and defines one or more fin bodies  38 . 
       FIG. 3  represents several additional fabrication processes in accordance with aspects of the invention. By way of example, a gate electric  20  is formed over the patterned structure (e.g., fin body  38  formed from the patterned active silicon layer  14  and hard mask  16 ) and exposed portions of the buried oxide layer  12 . The gate dielectric  20  can be, for example, a high-k dielectric formed by a conventional deposition process. The gate dielectric  20  can range in thickness from about 8 Å to 55 Å, although other dimensions are also contemplated by the present invention. The gate dielectric  20  can also be a thermally grown oxide, silicon oxynitride or other gate dielectric material. These same properties can be common throughout all aspects of the invention. 
     Still referring to  FIG. 3 , a gate electrode  22  is formed on the gate dielectric  20 . The gate electrode  22  may be about 5 nm to 10 nm in thickness, although other dimensions are also contemplated by the present invention. The gate electrode  22  can be, for example, TiN or TaN with Al to adjust the work function, or other metals or alloys known to be used for a gate electrode. These same properties can be common throughout all aspects of the invention. A storage dielectric  24  is formed on the gate electrode  22 . The storage dielectric  24  is a high quality dielectric material such as, for example, oxide. 
       FIG. 4  represents several additional fabrication processes in accordance with aspects of the invention. More specifically, a storage electrode  26  is formed on the storage dielectric  24 . The storage electrode  26  can be formed by a conformal deposition process to a thickness of about 5 nm to 10 nm in thickness; although other dimensions are also contemplated by the present invention. The storage electrode  26  can be, for example, TiN or other metals or alloys. 
     The storage electrode  26  can be patterned using a conventional patterning process such as, for example, RIE. After patterning, an insulating material  28  is deposited in the patterned regions, e.g., between the fin bodies  38 . The insulating layer  28  can be, for example, silicon dioxide, although other materials are also contemplated by the present invention such as, for example, low-k dielectrics such as fluorinated SiO 2 . The insulating material  28  and exposed portions of the storage electrode  26  may be planarized using, for example, a chemical mechanical polish (CMP). Openings  32  are patterned into the fin body  38 , e.g., gate structures  30 . More specifically, the openings  32  are patterned through the storage electrode  26 , storage dielectric  24 , gate electrode  22 , gate dielectric  20  and portions of the hard mask  16 , to expose, for example, the active silicon layer  14  of the fin body  38 . 
     In  FIG. 5 , an insulator layer  34  is deposited on the sidewalls of the openings  32 . The insulator layer  34  can be a high-k dielectric material, although other insulating materials are also contemplated by the present invention such as, for example, SiN. In embodiments, the insulator layer  34  may be different than the storage dielectric  24  and is preferably selected for its insulative and conformal properties. In embodiments, the insulator layer  34  is contemplated to be different than the storage dielectric  24  so that etching processes of the insulator layer  34  will not affect the storage dielectric  24  (or vice versa). A gate contact  36  is deposited in the openings  32 , contacting the active silicon layer  14  and the insulator layer  34  previously deposited on the sidewalls of the openings  32 . The gate contact  36  can be, for example, TiN, tungsten or other metals or alloys known to be used for contacts. In embodiments, the gate contact  36  can also be a doped poly. 
       FIGS. 6-11  show structures and methods of fabricating a device in accordance with other aspects of the invention.  FIGS. 6-11  can represent methods of manufacturing a finFET (ZRAM) that provides a low-cost, high signal-to-noise ratio memory. In embodiments, the finFET comprises, for example, an inverted-T-shaped semiconductor structure comprising a planar region doped to have at least a region which is electrically neutral and conducting, and a vertical fin region which is either fully or partially depleted. In embodiments, the gate structure covers the planar region and the vertical fin region. The gate structure further comprises an insulating dielectric in contact with the planar and vertical fin regions, and a conducting gate electrode in contact with the insulating dielectric. The insulating dielectric electrically insulates the semiconductor structure from the gate electrode. 
     More specifically,  FIG. 6  shows a wafer  5  comprising a substrate carrier  10 , a buried oxide  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known conventional carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . 
     As shown in  FIG. 7 , the active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE). In embodiments, the active silicon layer  14  is only partially etched to form one or more fin bodies  38  with a lower, planar portion  38   a . The lower, planar portion  38   a  can act as a planar storage area beneath the fin body, self-aligned to the wordline. 
     In  FIG. 8 , spacers  40  are formed on the lower, planar portion  38   a  formed from the active silicon layer  14 , as well as on the sides of the fin body  38 . The spacers  40  can be formed using conventional conformal deposition processes. In embodiments, the spacers  40  can be a nitride material (e.g., Si 3 N 4 ), although other materials are also contemplated by the present invention. After the formation of the spacers  40 , the exposed active silicon layer  14  (e.g., exposed portions of the planar region  38   a ) can be removed using conventional lithographic and etching processes, known to those of skill in the art. The etching process forms an inverted T fin body, with openings  42  formed therebetween (which exposes portions of the buried oxide layer  12 ). 
     In  FIG. 9 , the spacers are removed and a vertical ion implantation is performed on the active silicon layer (e.g., planar portion  38   a  of the inverted T fin body). The vertical ion implantation process can use P-type dopants such as, for example, Boron, BF 2 , B 10  or other known P-type dopants. In embodiments, the hardmask  16  will protect the fin body  38  from the dopants. In embodiments, the dopants are activated by annealing at a high temperature, e.g., approximately 1000° C. for about 1 second. 
     As shown in  FIG. 10 , a dielectric material  20  can be deposited over (about) the fin body  38  and on the heavily doped planar portion  38   a  of the inverted T fin body and exposed buried oxide layer  12 . In embodiments, the dielectric material  20  can be, for example, SiO 2 , or high-k material such as, for example, HfSiO 4  or HfO 2 , amongst other high-k materials. A gate electrode  43  is deposited on the dielectric material  20 . 
       FIG. 11  shows a cross sectional view of the device of  FIG. 10 , along line A-A. In embodiments, the gate electrode  43  may be planarized and patterned to form a gate “G”. As shown in  FIG. 11 , the patterning of the gate electrode  43  can expose portions of the underlying buried oxide layer  12 . In further embodiments, an isotropic etching process can remove a portion of the doped region under the fin body  38 . 
       FIGS. 12-15  show structures and methods of fabricating a device in accordance with other aspects of the invention. In particular,  FIGS. 12-15  show methods of manufacturing a segmented finFET device with enhanced body capacitance. The segmented finFET includes a lower portion of the gated body with a heavily doped region resulting in a high body-to-gate capacitance, i.e., the heavily doped lower portion of the fin body adds storage capacity. In embodiments, the upper portion of the fin body is largely undoped to provide low Vt-scatter values. The fully depleted undoped channel minimizes Vt scatter, which is beneficial for array yield. The source/drain in the upper region of the fin body, adjacent to the low-doped regions of the body, offers low leakage to the p-doped fin body region for improved retention time and yield. In embodiments, the gate structure covers the upper and lower portions of the fin body, where a portion of the fin body is not covered by the gate structure. 
     More specifically,  FIG. 12  shows a wafer  5  comprising a substrate carrier  10 , a buried oxide  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known conventional carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . 
     As shown in  FIG. 13 , the active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE), to form one or more fin bodies  38 . In embodiments, the unprotected regions of the active silicon layer  14  are completely etched to expose the underlying buried oxide layer  12 . A Borosilicate glass  44  is deposited on the buried oxide layer  12 , surrounding the fin body  38 . The Borosilicate glass  44  is partially etched to expose an upper portion of the fin body  38 . The Borosilicate glass  44  is then subjected to an annealing process, e.g., about 500° C., which out diffuses boron into the adjacent portion (e.g., lower portion  41 ) of the fin body  38 . 
     In  FIG. 14 , the Borosilicate glass is removed by a selective etch process. A dielectric layer  20  is deposited on the fin body  38  and buried oxide layer  12 . In embodiments, the dielectric material  20  can be, for example, SiO 2 , or high-k material such as, for example, HfSiO 4  or HfO 2 , amongst other high-k materials. A gate electrode  43  is then deposited on the dielectric material  20 . 
       FIG. 15  is a cross sectional view of  FIG. 14 , along line B-B. As shown in  FIG. 15 , in embodiments, the gate electrode  43  can be planarized and patterned to form the gate “G”. In embodiments, the patterning of the gate electrode  43  can expose portions of the underlying buried oxide layer  12 . In embodiments, an upper portion of the fin body  38  is subjected to an ion implantation to form the source (S) and drain (D) regions. 
       FIGS. 16-18  show structures and methods of fabricating a device in accordance with aspects of the invention, starting with the structure shown in  FIG. 15 . More specifically, the semiconductor device fabricated using the methods of  FIGS. 16-18  includes selective removal of the active silicon layer  14  below the source and drain regions. As shown in  FIG. 16 , for example, a silicon dioxide layer  46  is deposited on the structure, e.g., encapsulating the fin body  38 . In embodiments, the silicon dioxide layer  46  can be a doped silicon dioxide. The silicon dioxide layer  46  can be etched back to expose an upper portion of the fin body  38 , e.g., expose the source and drain regions. 
     In  FIG. 17 , a spacer material  48  such as, for example, Si 3 N 4  is conformally deposited on the fin body  38 . The silicon dioxide is removed using a selective etching process, e.g., selective to a doped oxide. This selective etching process exposes a lower portion of the fin body  38 , e.g., the active silicon layer  14 . The exposed lower portion of the fin body  38  is then selectively etched to partially remove the active silicon layer  14  of the fin body  38  to form an undercut  50 . The spacer material  48  and gate electrode material  43  protect the central portion of the fin body  38  from the etchant. In  FIG. 18 , the undercut is filled with a dielectric material  52  such as, for example, SiO 2 , using, for example, a Chemical Vapor Deposition. 
       FIGS. 19-21  show structures and methods of fabricating a device in accordance with other aspects of the invention.  FIGS. 19-21  can represent methods of manufacturing a semiconductor device (finFET) with an asymmetric charge distribution that provides enhanced body capacitance. The finFET can be a ZRAM (zero-capacitor RAM) that provides a low-cost, high signal-to-noise ratio memory. In embodiments, the finFET has an asymmetrically accumulated body with decreased Vt-scatter (compared to doped-body designs). The finFET includes a gate structure covering the first and second sides of a fin body. The first side of the gate structure has an effective work function near the conduction band of the fin body and the second side of the gate structure has an effective work function near the valence band of the fin body, which serves to accumulate holes in the portions of the body which are adjacent to that portion of the gate structure having workfunction near the valence band. 
     More specifically,  FIG. 19  shows a wafer  5  comprising a substrate carrier  10 , a buried oxide  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known conventional carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . 
     In  FIG. 20 , the active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE), to form one or more fin bodies  38 . A dielectric layer  20  is deposited on the fin body  38  and exposed buried oxide layer  12 . The dielectric material  20  can be, for example, SiO 2 , or high-k material such as, for example, HfSiO 4  or HfO 2 , amongst other high-k materials. A metal layer  54  such as, for example, TiN is deposited on the dielectric material  20 . A layer  56  is deposited on the metal layer  54 . In embodiments, the layer  56  can be a metal such as, for example, TaN, or a metal with workfunction nearby the valence band of the fin body, such as Mg. In alternative embodiments, the layer  56  may be doped via a layer of magnesium, or, alternatively, the dielectric below layer  56  may be modified with a thin layer of aluminum oxide, which introduces a fixed charge or dipoles, and modifies the flat-band voltage, or effective workfuntion of the gate stack following a rapid thermal anneal at approximately 1000° C. Yet another alternative comprises introduction of oxygen into the exposed section of the gate, which also serves to shift the workfunction to a value nearby that of the valence band of the fin body. 
     As shown in  FIG. 21 , the layer  56  is patterned to expose a portion of the underlying layer  54  on a side  38   b  of the fin body  38 . In this patterning process, the layer  56  protects the other side  38   c  of the fin body  38 . In embodiments, a work function modifying layer  58  is deposited on the fin body  38 , i.e., on the protected portion  38   c  and unprotected portion  38   b  of the fin body  38 . In embodiments, the work function modifying layer  58  is deposited on the barrier metal layer  56 . In this embodiment, the modifying layer  58  induces an effective work function shift of the side  38   b  of the fin body  38 . In embodiments, the modifying layer may be, for example, Mg. In alternative embodiments, the work function modifying layer  58  can be a gate electrode material that is annealed at about 450° C. to about 500° C. to induce an effective work function shift of the side  38   b  of the fin body  38 . The work function modifying layer  58 , as well as the dielectric layer  20 , metal layer  54  and layer  56  can be patterned to form a gate stack “G”. As should now be understood by those of skill in the art, due to the layer  56 , the first side of the gate structure has an effective work function near the conduction band of the fin body  38  and the second side of the gate structure has an effective work function near the valence band of the fin body  38 . 
       FIG. 22  shows an alternative embodiment of the invention, with an effective work function nearby the valence band. More specifically, a beginning wafer  5  comprises a substrate carrier  10 , a buried oxide  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known conventional carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . The active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE), to form one or more fin bodies  38 . A dielectric layer  20  is deposited on the fin bodies  38  and exposed buried oxide layer  12 . The dielectric material  20  can be, for example, SiO 2 , or high-k material such as, for example, HfSiO 4  or HfO 2 , amongst other high-k materials. A dipole forming layer such as Al 2 O 3  is formed on an unprotected side  38   b  of the gate structure  39  and diffused through the gate electrode metal into the dielectric layer  20 . The charge in the dielectric  20  sets (shifts) the effective work function to near the valance band. The other side  38   c  of the fin body  38  (gate) has an effective work function near the conduction band. 
       FIG. 23  shows an asymmetrical device and methods of fabricating the device in accordance with aspects of the invention. The device of  FIG. 23  can represent a finFET, e.g., ZRAM that employs an asymmetric charge distribution. In embodiments, the finFET includes a heavily doped portion which provides large storage capacitance for good retention time. Also, the source/drain in the side of the fin body, opposite that of a highly doped body region, offers low leakage to the p-doped fin region for improved retention time and yield. 
     In  FIG. 23 , a wafer  5  comprises a substrate carrier  10 , a buried oxide  12  and an active silicon layer  14 . The substrate carrier  10  can be, for example, silicon or any other known conventional carrier. A hard mask  16  is formed on the active silicon layer  14 . The hard mask  16  can be, for example, SiO 2  or Si 3 N 4 . The active silicon layer  14  and hard mask  16  are patterned using a conventional etching process such as, for example, reactive ion etching (RIE), to form one or more fin bodies  38 . A P-type dopant, e.g., boron, is implanted into an unprotected side of the fin body  38  to form a P+ doped region  60 . The P+ doped region  60  provides a charge storage region. The other side  38   c  of the fin body is nearly identical to intrinsic silicon. A dielectric layer  20  is deposited on the fin body  38  and exposed buried oxide layer  12 . The dielectric material  20  can be, for example, SiO 2 , or high-k material such as, for example, HfSiO 4  or HfO 2 , amongst other high-k materials. A layer (gate electrode)  43  such as, for example, TiN is deposited on the dielectric material  20 . 
     DESIGN STRUCTURE 
       FIG. 24  illustrates multiple design structures including an input design structure  920  that is preferably processed by a design process  910 . Design structure  920  may be a logical simulation design structure generated and processed by design process  910  to produce a logically equivalent functional representation of a hardware device. Design structure  920  may also or alternatively comprise data and/or program instructions that when processed by design process  910 , generate a functional representation of the physical structure of a hardware device. Whether representing functional and/or structural design features, design structure  920  may be generated using electronic computer-aided design (ECAD) such as implemented by a core developer/designer. When encoded on a machine-readable data transmission, gate array, or storage medium, design structure  920  may be accessed and processed by one or more hardware and/or software modules within design process  910  to simulate or otherwise functionally represent an electronic component, circuit, electronic or logic module, apparatus, device, or system such as those shown in  FIGS. 1-23 . As such, design structure  920  may comprise files or other data structures including human and/or machine-readable source code, compiled structures, and computer-executable code structures that when processed by a design or simulation data processing system, functionally simulate or otherwise represent circuits or other levels of hardware logic design. Such data structures may include hardware-description language (HDL) design entities or other data structures conforming to and/or compatible with lower-level HDL design languages such as Verilog and VHDL, and/or higher level design languages such as C or C++. 
     Design process  910  preferably employs and incorporates hardware and/or software modules for synthesizing, translating, or otherwise processing a design/simulation functional equivalent of the components, circuits, devices, or logic structures shown in  FIGS. 1-23  to generate a netlist  980  which may contain design structures such as design structure  920 . Netlist  980  may comprise, for example, compiled or otherwise processed data structures representing a list of wires, discrete components, logic gates, control circuits, I/O devices, models, etc. that describes the connections to other elements and circuits in an integrated circuit design. Netlist  980  may be synthesized using an iterative process in which netlist  980  is resynthesized one or more times depending on design specifications and design attributes for the device. As with other design structure types described herein, netlist  980  may be recorded on a machine-readable data storage medium or programmed into a programmable gate array. The medium may be a non-volatile storage medium such as a magnetic or optical disk drive, a programmable gate array, a compact flash, or other flash memory. Additionally, or in the alternative, the medium may be a system or cache memory, buffer space, or electrically or optically conductive devices and materials on which data packets may be transmitted and intermediately stored via the Internet, or other networking suitable means. 
     Design process  910  may include hardware and software modules for processing a variety of input data structure types including netlist  980 . Such data structure types may reside, for example, within library elements  930  and include a set of commonly used elements, circuits, and devices, including models, layouts, and symbolic representations, for a given manufacturing technology (e.g., different technology nodes, 32 nm, 45 nm, 90 nm, etc.). The data structure types may further include design specifications  940 , characterization data  950 , verification data  960 , design rules  970 , and test data files  985  which may include input test patterns, output test results, and other testing information. Design process  910  may further include, for example, standard mechanical design processes such as stress analysis, thermal analysis, mechanical event simulation, process simulation for operations such as casting, molding, and die press forming, etc. One of ordinary skill in the art of mechanical design can appreciate the extent of possible mechanical design tools and applications used in design process  910  without deviating from the scope and spirit of the invention. Design process  910  may also include modules for performing standard circuit design processes such as timing analysis, verification, design rule checking, place and route operations, etc. 
     Design process  910  employs and incorporates logic and physical design tools such as HDL compilers and simulation model build tools to process design structure  920  together with some or all of the depicted supporting data structures along with any additional mechanical design or data (if applicable), to generate a second design structure  990 . Design structure  990  resides on a storage medium or programmable gate array in a data format used for the exchange of data of mechanical devices and structures (e.g. information stored in an IGES, DXF, Parasolid XT, JT, DRG, or any other suitable format for storing or rendering such mechanical design structures). Similar to design structure  920 , design structure  990  preferably comprises one or more files, data structures, or other computer-encoded data or instructions that reside on transmission or data storage media and that when processed by an ECAD system generate a logically or otherwise functionally equivalent form of one or more of the embodiments of the invention shown in  FIGS. 1-23 . In one embodiment, design structure  990  may comprise a compiled, executable HDL simulation model that functionally simulates the devices shown in  FIGS. 1-23 . 
     Design structure  990  may also employ a data format used for the exchange of layout data of integrated circuits and/or symbolic data format (e.g. information stored in a GDSII (GDS2), GL1, OASIS, map files, or any other suitable format for storing such design data structures). Design structure  990  may comprise information such as, for example, symbolic data, map files, test data files, design content files, manufacturing data, layout design attributes, wires, levels of metal, vias, shapes, data for routing through the manufacturing line, and any other data required by a manufacturer or other designer/developer to produce a device or structure as described above and shown in  FIGS. 1-23 . Design structure  990  may then proceed to a stage  995  where, for example, design structure  990 : proceeds to tape-out, is released to manufacturing, is released to a mask house, is sent to another design house, is sent back to the customer, etc. 
     The methods as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements, if any, 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 embodiments were 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.