Abstract:
Methods and apparatuses relate to implanting a surface of a semiconductor substrate with dopants, making undoped semiconductor material directly on the surface implanted with the dopants, and making a transistor with a transistor channel in the undoped semiconductor material, such that the transistor channel of the transistor remains undoped throughout manufacture of the integrated circuit.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The present technology relates to methods and apparatuses for channel stop doping, which can decrease the effect of parasitic transistors and improve the current of small dimension transistors. 
         [0003]    2. Description of Related Art 
         [0004]    Small dimension transistors such as finFETs and planar transistors are particularly sensitive to changes in the density of dopants in the transistor channel. Accordingly, a fabrication goal is to decrease or eliminate dopants in the transistor channel. 
         [0005]    One approach is to form shallow trench isolation (STI) oxide regions, and then perform high implant energy implants through the semiconductor surface to form the n-wells and p-wells. However, such an approach has the side effect of undesirable channel doping in the surface of the semiconductor, which runs counter to the original goal of decreasing or eliminating dopants in the transistor channel. Such an approach also fails to have abrupt doping junctions between the lightly doped or undoped channel, and the moderately to heavily doped semiconductor material underlying the channel. 
       SUMMARY 
       [0006]    Various embodiments are directed to channel stop doping. 
         [0007]    One aspect of the technology is a method of performing channel stop doping in an integrated circuit, comprising:
       implanting a surface of a semiconductor substrate with dopants;   making undoped semiconductor material directly on the surface implanted with the dopants; and   making a transistor with a transistor channel in the undoped semiconductor material, such that the transistor channel of the transistor remains undoped throughout manufacture of the integrated circuit.       
 
         [0011]    Another aspect of the technology is a nontransitory computer readable medium with computer executable instructions determining a layout of an integrated circuit design. The computer executable instructions include:
       first computer executable instructions defining first layout locations for implanting a surface of a semiconductor substrate with dopants; and   second computer executable instructions defining second layout locations making a transistor with a transistor channel in the undoped semiconductor material, the undoped semiconductor material directly on the surface implanted with the dopants, such that the transistor channel of the transistor remains undoped throughout manufacture of the integrated circuit.       
 
         [0014]    Another aspect of the technology is a set of lithographic masks determining a layout of an integrated circuit design, including:
       a first lithographic mask defining first layout locations for implanting a surface of a semiconductor substrate with dopants; and   a second lithographic mask defining second layout locations making a transistor with a transistor channel in the undoped semiconductor material, the undoped semiconductor material directly on the surface implanted with the dopants, such that the transistor channel of the transistor remains undoped throughout manufacture of the integrated circuit.       
 
         [0017]    Another aspect of the technology is a computer system for laying out an integrated circuit design. The computer system comprises a processor and a memory, and generates a layout of the integrated circuit design that defines a plurality of masks. The computer system is configured to perform computer executable instructions comprising:
       first computer executable instructions defining first layout locations for implanting a surface of a semiconductor substrate with dopants; and   second computer executable instructions defining second layout locations making a transistor with a transistor channel in the undoped semiconductor material, the undoped semiconductor material directly on the surface implanted with the dopants, such that the transistor channel of the transistor remains undoped throughout manufacture of the integrated circuit.       
 
         [0020]    The various aspects of the technology have various embodiments. 
         [0021]    In some embodiments, it is unnecessary to define layout locations for making the undoped semiconductor material directly on the surface implanted with the dopants. If both the NMOS and the PMOS channels are built with the same undoped semiconductor, such as Si, then the undoped semiconductor material can be grown on the entire chip, such that no mask is necessary. 
         [0022]    In other embodiments, third layout locations are defined for making undoped semiconductor material directly on the surface implanted with the dopants. Different undoped semiconductors can be grown for the NMOS and the PMOS transistors, such as InGaAs and Ge. Such locations can be defined via computer executable instructions or a lithographic mask, where the computer executable instructions can be on a nontransitory computer readable medium, or on a computer system configured to perform the instructions. 
         [0023]    In some embodiments, the transistor is a finFET or a planar transistor. 
         [0024]    In some embodiments, the implanted dopants provide latchup protection of the transistor against other structures. 
         [0025]    In some embodiments, the transistor is one of a plurality of transistors with the transistor channel in the undoped semiconductor material. The transistor channels of plurality of transistors share the same number of dopants. 
         [0026]    Other aspects are directed to a computer readable medium storing computer instructions to perform a method of designing an integrated circuit, the method for use by a computer system having a processor and memory. The computer instructions are executable by the computer system to design the integrated circuit as described herein. 
         [0027]    Other aspects are directed to a computer system designing an integrated circuit, comprising a processor and memory, configured to design an integrated circuit as described herein. 
         [0028]    Other aspects are directed to a method of operating an integrated circuit as described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  shows a simplified representation of an illustrative integrated circuit design flow incorporating features of the technology. 
           [0030]      FIG. 2A  is a simplified block diagram of a computer system that can be used to implement software incorporating aspects of the technology. 
           [0031]      FIG. 2B  is a memory that can be used to implement aspects of the technology. 
           [0032]      FIG. 2C  is an integrated circuit created with the described technology. 
           [0033]      FIG. 3A  shows mid-energy implants of p-type dopants such as boron into the silicon substrate. 
           [0034]      FIG. 3B  is a doping graph that shows dopant implant depth versus dopant concentration with respect to  FIG. 3A . 
           [0035]      FIG. 4A  shows mid-energy implants of n-type dopants such as phosphorus into the silicon substrate. 
           [0036]      FIG. 4B  is a doping graph that shows dopant implant depth versus dopant concentration with respect to  FIG. 4A . 
           [0037]      FIG. 5A  shows the p-well and n-well formed from annealing and implantation of dopants. 
           [0038]      FIG. 5B  is a doping graph that shows the dopant implant depth versus dopant concentration for the p-well with respect to  FIG. 5A . 
           [0039]      FIG. 5C  is a doping graph that shows the dopant implant depth versus dopant concentration for the n-well with respect to  FIG. 5A . 
           [0040]      FIG. 6A  shows epitaxial silicon above the p-well and n-well formed from annealing and implantation of dopants. 
           [0041]      FIG. 6B  is a doping graph that shows the dopant implant depth versus dopant concentration for the p-well with respect to  FIG. 6A . 
           [0042]      FIG. 6C  is a doping graph that shows the dopant implant depth versus dopant concentration for the n-well with respect to  FIG. 6A . 
           [0043]      FIG. 7A  shows the formation of shallow trench isolation (STI) regions in between the p-well and the n-well. 
           [0044]      FIG. 7B  is a doping graph that shows the dopant implant depth versus dopant concentration for the p-well with respect to  FIG. 7A . 
           [0045]      FIG. 7C  is a doping graph that shows the dopant implant depth versus dopant concentration for the n-well with respect to  FIG. 7A . 
           [0046]      FIG. 8  is a cross-section of finFET transistors, where the fin structures of the finFET transistors have been made according to the process flow of  FIGS. 1-5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIG. 1  is a simplified representation of an integrated circuit design flow. As with all flowcharts herein, it will be appreciated that many of the steps of  FIG. 1  can be combined, performed in parallel or performed in a different sequence without affecting the functions achieved. In some cases a rearrangement of steps will achieve the same results only if certain other changes are made as well, and in other cases a rearrangement of steps will achieve the same results only if certain conditions are satisfied. 
         [0048]    At a high level, the process of  FIG. 1  starts with the product idea (block  100 ) and is realized in an EDA (Electronic Design Automation) software design process (block  110 ). When the design is finalized, the fabrication process (block  150 ) and packaging and assembly processes (block  160 ) occur, ultimately resulting in finished integrated circuit chips (result  170 ). 
         [0049]    The EDA software design process (block  110 ) is actually composed of a number of steps  112 - 130 , shown in linear fashion for simplicity. In an actual integrated circuit design process, the particular design might have to go back through steps until certain tests are passed. Similarly, in any actual design process, these steps may occur in different orders and combinations. This description is therefore provided by way of context and general explanation rather than as a specific, or recommended, design flow for a particular integrated circuit. 
         [0050]    A brief description of the component steps of the EDA software design process (block  110 ) will now be provided. The following stages reflect the improved device design using channel stop doping. 
         [0051]    System design (block  112 ): The designers describe the functionality that they want to implement, they can perform what-if planning to refine functionality, check costs, etc. Hardware-software architecture selection can occur at this stage. Example EDA software products that have been available from Synopsys, Inc. that could be used at this step include Model Architect, Saber, System Studio, and DesignWare® products. 
         [0052]    Logic design and functional verification (block  114 ): At this stage, high level description language (HDL) code, such as VHDL or Verilog code, is written for modules in the system and the design is checked for functional accuracy. More specifically, the design is checked to ensure that it produces the correct outputs in response to particular input stimuli. Example EDA software products that have been available from Synopsys, Inc. that can be used at this step include VCS, VERA, DesignWare®, Magellan, Formality, ESP and LEDA products. 
         [0053]    Synthesis and design for test (block  116 ): Here, the VHDL/Verilog code is translated to a netlist. The netlist can be optimized for the target technology. Additionally, the design and implementation of tests to permit checking of the finished chip occur. Example EDA software products that have been available from Synopsys, Inc. that could be used at this step include Design Compiler®, Physical Compiler, Test Compiler, Power Compiler, FPGA Compiler, TetraMAX, and DesignWare® products. 
         [0054]    Netlist verification (block  118 ): At this step, the netlist is checked for compliance with timing constraints and for correspondence with the VHDL/Verilog source code. Example EDA software products that have been available from Synopsys, Inc. that can be used at this step include Formality, PrimeTime, and VCS products. 
         [0055]    Design planning (block  120 ): Here, an overall floor plan for the chip is constructed and analyzed for timing and top-level routing. Example EDA software products that have been available from Synopsys, Inc. that can be used at this step include Astro and IC Compiler products. 
         [0056]    Physical implementation (block  122 ): The placement (positioning of circuit elements) and routing (connection of the same) occurs at this step. Example EDA software products that have been available from Synopsys, Inc. that can be used at this step include AstroRail, Primetime, and Star RC/XT products. 
         [0057]    Analysis and extraction (block  124 ): At this step, the circuit function is verified at a transistor level; this in turn permits what-if refinement. Example EDA software products that have been available from Synopsys, Inc. that can be used at this stage include Custom Designer, AstroRail, PrimeRail, Primetime, and Star RC/XT products. 
         [0058]    Physical verification (block  126 ): At this stage various checking functions are performed to ensure correctness for: manufacturing, electrical issues, lithographic issues, and circuitry. Example EDA software products that have been available from Synopsys, Inc. that can used at this stage include the Hercules product. 
         [0059]    Tape-out (block  127 ): This stage provides the “tape-out” data for production of masks for lithographic use to produce finished chips. Example EDA software products that have been available from Synopsys, Inc. that can be used at this stage include the CATS(R) family of products. 
         [0060]    Resolution enhancement (block  128 ): This stage involves geometric manipulations of the layout to improve manufacturability of the design. Example EDA software products that have been available from Synopsys, Inc. that can be used at this stage include Proteus/Progen, ProteusAF, and PSMGen products. 
         [0061]    Mask preparation (block  130 ): This stage includes both mask data preparation and the writing of the masks themselves. Example EDA software products that have been available from Synopsys, Inc. that can be used at this stage include CATS(R) family of products. 
         [0062]    The integrated circuit manufacturing flow includes a parallel flow, as follows: 
         [0063]    (1) Develop individual process steps for manufacturing the integrated circuit. This can be modeled with the Synopsys tools “Sentaurus Process”, “Sentaurus Topography”, and “Sentaurus Lithography”. The input information here is the process conditions like temperature, reactor ambient, implant energy, etc. The output information is the change in geometry or doping profiles or stress distribution. 
         [0064]    (2) Integrate the individual process steps into the complete process flow. This can be modeled with the Synopsys tool “Sentaurus Process”. The input information here is the collection of the process steps in the appropriate sequence. The output is the geometry, the doping profiles, and the stress distribution for the transistors and the space in between the transistors. 
         [0065]    (3) Analyze performance of the transistor manufactured with this process flow. This can be done with the Synopsys tool “Sentaurus Device”. The input information here is the output of step (3) and the biases applied to transistor terminals. The output information is the currents and capacitances for each bias combination. 
         [0066]    (4) If necessary, modify the process steps and the process flow to achieve the desired transistor performance. This can be done iteratively by using the Synopsys tools mentioned above. 
         [0067]    Once the process flow is ready, it can be used for manufacturing multiple circuit designs coming from different fabless companies. The EDA flow can be used by such fabless companies. The parallel flow described here is used at a foundry to develop a process flow that can be used to manufacture designs coming from their fabless customers. A combination of the process flow and the masks are used to manufacture any particular circuit. If the integrated circuit is manufactured at an IDM (integrated device manufacturer) company instead of the combination of a fabless company and a foundry, then both parallel flows described above are done at the same IDM company. 
         [0068]    There is also a bridge between these tools and the EDA tools. The bridge is a Synopsys tool “Seismos” that applies compact proximity models for particular circuit design and layout to obtain netlist with instance parameters for each individual transistor in the circuit as a function of its neighborhood and stress, including material conversion stress. This netlist is used in the analysis step. 
         [0069]      FIG. 2A  is a simplified block diagram of a computer system  210  suitable for use with embodiments of the technology. Computer system  210  typically includes at least one processor  214  which communicates with a number of peripheral devices via bus subsystem  212 . These peripheral devices may include a storage subsystem  224 , comprising a memory subsystem  226  and a file storage subsystem  228 , user interface input devices  222 , user interface output devices  220 , and a network interface subsystem  216 . The input and output devices allow user interaction with computer system  210 . Network interface subsystem  216  provides an interface to outside networks, including an interface to communication network  218 , and is coupled via communication network  218  to corresponding interface devices in other computer systems. Communication network  218  may comprise many interconnected computer systems and communication links. These communication links may be wireline links, optical links, wireless links, or any other mechanisms for communication of information. While in one embodiment, communication network  218  is the Internet, communication network  218  may be any suitable computer network. 
         [0070]    User interface input devices  222  may include a keyboard, pointing devices such as a mouse, trackball, touchpad, or graphics tablet, a scanner, a touchscreen incorporated into the display, audio input devices such as voice recognition systems, microphones, and other types of input devices. In general, use of the term “input device” is intended to include all possible types of devices and ways to input information into computer system  210  or onto communication network  218 . 
         [0071]    User interface output devices  220  may include a display subsystem, a printer, a fax machine, or non-visual displays such as audio output devices. The display subsystem may include a cathode ray tube (CRT), a flat-panel device such as a liquid crystal display (LCD), a projection device, or some other mechanism for creating a visible image. The display subsystem may also provide non-visual display such as via audio output devices. In general, use of the term “output device” is intended to include all possible types of devices and ways to output information from computer system  210  to the user or to another machine or computer system. 
         [0072]    Storage subsystem  224  stores the basic programming and data constructs that provide the functionality of some or all of the EDA tools described herein, including cell library devices having channel stop doping and tools applied for development of a layout using the library and corresponding manufacturing. These software modules are generally executed by processor  214 . 
         [0073]    Memory subsystem  226  typically includes a number of memories including a main random access memory (RAM)  230  for storage of instructions and data during program execution and a read only memory (ROM)  232  in which fixed instructions are stored. File storage subsystem  228  provides persistent storage for program and data files, and may include a hard disk drive, a floppy disk drive along with associated removable media, a CD-ROM drive, an optical drive, or removable media cartridges. The databases and modules implementing the functionality of certain embodiments may be stored by file storage subsystem  228 . 
         [0074]    Bus subsystem  212  provides a mechanism for letting the various components and subsystems of computer system  210  communicate with each other as intended. Although bus subsystem  212  is shown schematically as a single bus, alternative embodiments of the bus subsystem may use multiple busses. 
         [0075]    Computer system  210  itself can be of varying types including a personal computer, a portable computer, a workstation, a computer terminal, a network computer, a television, a mainframe, or any other data processing system or user device. Due to the ever-changing nature of computers and networks, the description of computer system  210  depicted in  FIG. 2A  is intended only as a specific example for purposes of illustrating the preferred embodiments. Many other configurations of computer system  210  are possible having more or less components than the computer system depicted in  FIG. 2A . 
         [0076]      FIG. 2B  shows a memory  240  such as a non-transitory, computer readable data storage medium associated with file storage subsystem  228 , and/or with network interface subsystem  216  can include a data structure specifying a circuit design that includes cell library devices having channel stop doping. The memory  240  can be a hard disk, a floppy disk, a CD-ROM, an optical medium, removable media cartridge, or other medium that stores computer readable data in a volatile or non-volatile form. The memory  240  is shown storing a circuit design  280  including, for example, an HDL description of a circuit design that includes one or more cell library devices having channel stop doping.  FIG. 2C  is a block representing an integrated circuit  290  created with the described technology that includes one or more cell library devices having channel stop doping. 
         [0077]      FIG. 3A  shows mid-energy implants of p-type dopants such as boron into the silicon substrate  301 . A photolithographically defined pattern of photoresist  303  determines ion implantation windows. The final resting position of the implants is shown as slightly impinging from the volume under the open windows, to the volume covered by the photoresist. 
         [0078]    The doping graph  302  of  FIG. 3B  shows dopant implant depth versus dopant concentration. The implants are “mid-energy” in that neither ultra low energies like below 1 keV nor super high energies like above 500 keV are required. The doping concentration peak of over 10 19  cm −3  is below the surface of the silicon substrate  401 , but shallower than 100 nm from the surface of the silicon substrate  301 . The target dopant peak concentrations are in the range from 10 17  cm −3  to 10 21  cm −3 . The target dopant peak depths are from zero to 500 nm from the surface. 
         [0079]      FIG. 4A  shows mid-energy implants of n-type dopants such as phosphorus into the silicon substrate  401 . A photolithographically defined pattern of photoresist  403  determines ion implantation windows. The final resting position of the implants is shown as slightly impinging from the volume under the open windows, to the volume covered by the photoresist. 
         [0080]    The doping graph  402  of  FIG. 4B  shows dopant implant depth versus dopant concentration. The implants are “mid-energy” in that neither ultra low energies like below 1 keV nor super high energies like above 500 keV are required. The doping concentration peak of over 10 19  cm −3  is below the surface of the silicon substrate  401 , but shallower than 100 nm from the surface of the silicon substrate  401 . The target dopant peak concentrations are in the range from 10 17  cm −3  to 10 21  cm −3 . The target dopant peak depths are from zero to 500 nm from the surface. 
         [0081]      FIG. 5A  shows the p-well formed from the implantation of p-type dopants, and the n-well formed after the implantation of n-type dopants, after annealing to activate the implants and to sharpen the junctions. The doping graphs  502  of  FIGS. 5B and 503  of  FIG. 5C  show the dopant implant depth versus dopant concentration, for the p-well and the n-well respectively. The doping concentration peak of over 10 19  cm −3  has decreased to less than 10 19  cm −3 , and moved to the surface of the silicon substrate  501 . The target dopant peak concentrations are in the range from 10 17  cm −3  to 10 21  cm −3 . The target dopant peak depth is zero although it can be conceivably located within several tens or hundreds of nanometers from the surface.  FIG. 6A  shows the growth of epitaxial silicon on the surface of both the p-well and the n-well. The doping graphs  602  of  FIGS. 6B and 603  of  FIG. 6C  show the dopant implant depth versus dopant concentration, for the p-well and the n-well respectively. The preferred thickness range is determined by the height of the fin channel, which is typically 5 nm to 40 nm tall. The doping concentration peak was positioned at the surface of the silicon substrate after the prior step. Accordingly, following the growth of epitaxial silicon, there is an abrupt junction in the doping concentration between the epitaxial layer and the p-well and n-well. At the interface, the doping concentration changes by a decade over a depth of less than 3 nm. The doping concentration profiles from the previous step have shifted downward by a distance corresponding to the thickness of the epitaxial silicon. 
         [0082]      FIG. 7A  shows the formation of shallow trench isolation (STI) regions in between the p-well and the n-well. The resulting epitaxial surface is used to form the transistor channels in finFETs or planar transistors. The doping graphs  702  of  FIGS. 7B and 703  of  FIG. 7C  show the dopant implant depth versus dopant concentration, for the p-well and the n-well respectively. 
         [0083]      FIG. 8  is a simplified illustration showing finFET structures in a typical bulk substrate configuration. In  FIG. 8 , a plurality of fins  811 ,  812 ,  813  protrude from a bulk semiconductor body  810 , sometimes referred to as body-tied fins. In addition, the individual fins are separated by shallow trench isolation structures  816 ,  817 . A gate dielectric layer  815  overlies the fins  811 ,  812 ,  813 . A gate conductor  818  extends across the fins and over the gate dielectric layer  815 . 
         [0084]    For the embodiment of  FIG. 8 , on either side of the gate conductor  818 , source and drain regions (not shown) are implemented in the fins. The FET transistors that result have source, channel and drain regions in the fins, and a gate overlying the fins. Such transistors are often called multi-gate transistors, because the gate conductor overlies two sides of the fins, and as a result increases the effective width of the channel. The fins used to implement the finFET transistors can be quite narrow. For example, fins having widths on the order of 20 nm or less can be utilized. As a result of the multi-gate gate structure and the narrow widths of the fins, finFET transistors have excellent performance characteristics and small layout areas. 
         [0085]    While the present technology is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.