Abstract:
A method of arranging asymmetrically doped CMOS transistors in a semiconductor wafer that forms base cells within a plurality of logic standard cells in a CMOS process technology that includes conventional symmetric CMOS transistors having different threshold voltages. The asymmetrically doped CMOS transistors have a gate length exceeding 1.5 times the minimum gate length of the symmetric CMOS transistors. Regions defined by electrical junctions directly adjacent to the gate of the asymmetric transistors are formed by an implant mask exposing an area of the wafer on the source side of the transistor to receive the junction implant of the symmetric CMOS transistors with a higher threshold voltage while shielding the drain area, and a further implant mask exposing an area of the wafer on the drain side of the transistor to receive the junction implant of the symmetric CMOS transistors with a lower threshold voltage while shielding the source area.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to logic circuits constructed from asymmetrically doped complementary metal-oxide-semiconductor (CMOS) transistors. It is particularly related to, but in no way limited to, CMOS transistors used for autonomous wireless state radios and sensor actuator nodes. 
       BACKGROUND 
       [0002]    Autonomous wireless state radios and sensor/actuator systems may be characterised by having no local user interfaces and are often required to be Always-ON. Always-ON in this context is characterised by long periods of standby (more than 99% of the time) with short bursts of activity, during which, for example, a transmission is made and/or measurements are taken or actuators operated. 
         [0003]    During standby, certain system functions have to be maintained. This critically involves, for example, scheduling of the next transmission and maintaining state information, but may also include continuous sensing. 
         [0004]    Advances in low power CMOS process technology have been driven by mobile computing applications and such systems directly benefit from improved fabrication technologies. In many cases the use of advanced CMOS technology is necessary in order to support the active function of a CMOS system within the power limitations of single battery cells or energy harvesters. 
         [0005]    Advances in CMOS transistor technology have reduced the power required for both logic operations as well as the active power consumption of the analogue parts of a radio or sensor fabricated using such technology.  FIG. 1  illustrates the power consumed by a CMOS circuit in standby and transmission modes for transistors fabricated using conventional (older) and advanced (newer) CMOS transistor technology.  FIG. 1  indicates that the peak power consumption during logic operations in the ON-state is reduced in circuits fabricated using advanced CMOS transistor technology when compared to conventional CMOS fabrication technology.  FIG. 1  also shows that the static power consumption of CMOS circuits using advanced CMOS transistor fabrication technology in the standby or OFF-state is higher than in the case of CMOS circuits fabricated using conventional CMOS fabrication technology, as this is dominated by leakage power which is typically higher when using advanced CMOS transistor fabrication technology. 
         [0006]    We distinguish different operating states of the system:
       Active: Key processing blocks are powered-up and operate at high clock frequency. Typically active power consumption dominates.   Sleep: Most circuitry powered off, with a small subset of “Always-ON” circuitry clocked at low clock frequency and powered with reduced supply voltage, providing a keep-alive function and scheduling active operation. Typically static power consumption dominates over the active switching power in this case.       
 
         [0009]    Due to higher static power consumption, the average power consumption can be dominated by the leakage power consumed by circuits that remain powered during the sleep. This is seen by the higher average power consumption in  FIG. 1  for circuits fabricated using advanced CMOS technology compared to those fabricated using conventional CMOS technology. 
         [0010]    CMOS circuits for certain types of device can be split into blocks of Always-ON circuitry and blocks which are powered only during Active operation. The Always-ON logic circuitry represents a fraction of the total standard cell area, for which low leakage is the single most important requirement. These blocks conventionally use devices with a high threshold voltage (HVT), and are responsible for the higher power consumption in the sleep state as seen in  FIG. 1 . 
         [0011]    The blocks powered up during Active operation are usually optimised for speed and low dynamic power and contain higher performance devices with lower threshold voltage (LVT). 
         [0012]    An inverter is a representative of a functional building block implementing a logic function. It is widely known in the art of circuit design that an inverter comprises NMOS and PMOS transistors connected in series with tied inputs. There are two distinct leakage mechanisms at work in an inverter:
       Transistors in the OFF state (VGS=0, VDS=VDD):    The drain off-current divides into sub-threshold current to the source, bulk leakage current and gate leakage current.   Transistors in the ON state (VGS=VDD, VDS=0):    In this case leakage current is due to tunnelling currents between the channel and the gate.       
 
         [0015]    In addition to the above,  FIG. 2  shows the variation of drain leakage current Id(off) and bulk leakage current lb(HVT) in the OFF-state of an HVT transistor exemplified by an NMOS transistor with a gate length L of 0.09 μm. The bulk OFF-state current Ib(LVT) of a corresponding LVT NMOS transistor is also shown in the figure for comparison. It is clear from  FIG. 2  that the bulk leakage current in the HVT transistors must be reduced in order to achieve a further reduction of the total device leakage current. 
         [0016]    HVT transistors fabricated using advanced CMOS fabrication technology typically use high dose pocket and extension implants with the aim of reducing short channel effects and sub-threshold leakage currents. However, the resulting highly doped, abrupt junctions cause significant junction leakage. A further reduction in the sub-threshold leakage currents can be achieved by increasing the gate length L above the minimum gate length Lmin defined for the advanced CMOS fabrication technology being used. However, the junction leakage of the drain junction must be reduced in order to allow a reduction of the overall leakage. Finally, ON-state gate leakage sets a total leakage current floor depending on the supply voltage. 
         [0017]    There therefore remains a need of further reducing the total leakage current and supply voltage in CMOS devices used in Always-ON circuits and facilitating the use of such devices in logic standard cells without adding complexities to current state of the art CMOS fabrication technology. 
       SUMMARY OF THE INVENTION 
       [0018]    This invention relates to a CMOS transistor framework to minimize static power dissipation in logic circuits. In particular, the invention describes a method of arranging asymmetrically doped CMOS transistors into building blocks for logic standard cells. A method of fabrication thereof is also detailed. 
         [0019]    There is therefore provided a method of arranging asymmetrically doped CMOS transistors in a semiconductor wafer, said method of arrangement forming base cells within a plurality of logic standard cells in a CMOS process technology comprising conventional, symmetric CMOS transistors having different threshold voltages distinguished by junction implants forming the electrical junctions directly adjacent to the gate; said asymmetrically doped CMOS transistors having a gate length exceeding 1.5 times the minimum gate length of the symmetric CMOS transistors, whereby the regions defined by the electrical junctions directly adjacent to the gate of the asymmetric transistors are formed by: (i) an implant mask from a plurality of implant masks that exposes an area of the wafer on the source side of the transistor to receive the junction implant of the symmetric CMOS transistors with higher threshold voltage while shielding the drain area, and (ii) a further implant mask from said plurality of implant masks that exposes an area of the wafer on the drain side of the transistor to receive the junction implant of the symmetric CMOS transistors with lower threshold voltage while shielding the source area; said asymmetrical transistors being arranged into base cells such that patterns for source and drain regions formed on the relevant implant masks form a regular array which is achieved by mirroring adjacent base cells such that like regions of source or drain are directly adjacent for each pair of base cells; wherein each base cell contains exactly one asymmetric NMOS transistor and one asymmetric PMOS transistor, each formed on an individual active area, such that the gate and vertical position of the active area are aligned between said NMOS and PMOS transistors and such that the edges of the relevant implant masks forming each of the electrical junctions adjacent to the gate are aligned with the centre of the gate. 
         [0020]    There is further provided base cells, wherein the area of the implant patterns that define the junction implants directly adjacent to the transistor gate exceeds 50% of the minimum area required for a viable implant in the CMOS process technology. 
         [0021]    Also provided is a plurality of base cells with identical implant patterns differing only by the width of the NMOS and PMOS transistors as well as by the configuration of the gate connection. 
         [0022]    A pair of tap cells is also provided, said pair of tap cells having the same size and type of implant opening for the implant defining the transistor junctions directly adjacent to the asymmetric transistor gates, wherein one of the pair of tap cells is used for the source side, and the other of the pair of tap cells is used for the drain side, said pair of tap cells containing a source-drain implant of opposite type to that of the junction directly adjacent to the transistor gate and forming an ohmic connection to the transistor bulk. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  illustrates the power consumption in the Active operation and Sleep-state of circuits using transistors fabricated using conventional and advanced CMOS fabrication technology, 
           [0024]      FIG. 2  charts the drain and bulk current in the OFF-state of a high threshold voltage (HVT) NMOS transistor with L=0.09 μm in an exemplary NMOS transistor; the bulk current of a corresponding low threshold voltage (LVT) transistor in the OFF-state is also shown for comparison, 
           [0025]      FIG. 3A  depicts the cross section of a standard HVT transistor known in the art, 
           [0026]      FIG. 3B  depicts the cross section of an asymmetric transistor with an LVT pocket &amp; extension implant on the drain terminal according to an embodiment of the present invention, 
           [0027]      FIG. 4  illustrates a schematic layout exemplifying base cell patterns for the fabrication of asymmetric transistors according to an embodiment of the present invention, and 
           [0028]      FIGS. 5A and 5B  illustrates an example CMOS layout for the fabrication of asymmetric transistors according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 3A  shows a thin gate-oxide transistor  10  in which the source region  20  is fabricated with a pocket implant  25 . The drain region  30  of the transistor  10  is similarly fabricated with a pocket implant  35 . Conventionally, the source and drain regions are implanted with the same masking step, and the implantation results in an extension of either region (see regions  25  and  35  in  FIG. 3A ). The resulting pocket implant and extension are coupled together for each of the source and drain regions. With reference to  FIG. 3A , an embodiment of the present invention is shown in  FIG. 3B  in which the doping of the extension and pocket implant  25  in the source region  20  is different to that of the extension and pocket implant  36  of the drain region (as indicated by the different shading for regions  35  and  36  in  FIGS. 3A and 3B ). 
         [0030]    Standard CMOS fabrication technologies use multiple pocket and extension implants to realise a desired threshold voltage VT. In the conventional transistor of  FIG. 3A , the same implant steps on the source  20  and drain  30  regions of the transistor  10  are used. 
         [0031]    For transistors powered in the Sleep mode, greater than minimum channel length can be used in CMOS logic cells to reduce leakage with small impact on the die size. This also makes it possible to place the edge of an implant mask on the centre of the gate  40 , therefore allowing for the fabrication of the transistor  10  with source  20  and drain  30  regions comprising different doping levels. Transistor  15  of  FIG. 3B  is fabricated according to an embodiment of the present invention in which the drain region  30  has a different doping to the corresponding source region  20  of the device such that the junction profile and electrostatic barrier of the drain-channel junction  38  is different to that of the source-channel junction  28 . In one embodiment of the present invention, the drain extension and pocket implant  36  has a lower doping than the corresponding source implant  25  of the device such that the junction leakage caused by band-to-band tunnelling on the drain-channel junction  38  is lower than that of the source-channel junction  28 . 
         [0032]    The drain junction  38  determines the junction leakage current, whereas the source junction  28  largely determines the transistor threshold voltage. The asymmetry illustrated in  FIG. 3B  therefore allows a CMOS device fabricated according to an embodiment of the present invention to have a highly doped and abrupt source-channel junction  28  to maintain high threshold voltage thus controlling the sub-threshold current, in combination with a soft drain junction using the pocket and extension implants  36  of a low threshold voltage (LVT) transistor to reduce junction leakage into the bulk  50 . 
         [0033]    As an alternative to LVT implants  36  on the drain side  30 , in a further embodiment, light pocket and extension implants commonly used for I/O transistors could be used. 
         [0034]    In accordance with an embodiment of the present invention, an LVT implant  36  is used at the drain terminal  30 ; the reduced junction leakage due to the LVT implant  36  at the drain region  30  allows a further reduction of the overall leakage at longer channel length than is possible with HVT transistors. 
         [0035]    Conventionally, terminal implantation of CMOS transistors is performed using ¼ of the total implant dose to implant the required area four times, each of the ¼ doses being performed after rotating the semiconductor wafer 90 degrees. As an alternative, implants using a 180 degree rotation of the wafer with respect to the implant beam (½ of the total dose implanted for each rotation) can be used instead of conventional quad-implants (90 degree rotation for each ¼ of the total implant dose). Transistors resulting from these dual-implants cannot be rotated 90 degrees on the layout, however they have lower leakage. 
         [0036]    The present invention presents a placement architecture to facilitate issues of implementing logic standard cells using asymmetrically doped CMOS transistors. 
         [0037]    In a device layout using cell libraries based on standard CMOS design rules for active circuitry, the standard-cell area providing the keep-alive functionality in the Sleep mode, using the asymmetric transistors  15  of the present invention, can be clearly recognised as having a gate pitch that is significantly larger than the areas using a high-density standard cell library, yet being significantly more compact than areas using cell libraries based around the design rules of standard I/O transistors. This keep-alive circuitry is further characterised by having fully aligned and regular edges of the gates and active areas in the channel direction as well as by having always exactly one gate per active area, rather than sharing one active area for multiple transistors. 
         [0038]    An integrated circuit die may contain a majority of symmetrical CMOS transistors in standard cells with L&lt;1.5×Lmin in the chip area powered in the active mode, and a fraction, typically less than ¼ of the total gate count, will comprise asymmetrically doped transistors to provide the keep-alive functionality in the Sleep mode. 
         [0039]    The asymmetrically doped transistors of the present invention are fabricated using the same CMOS technology used to fabricate the symmetrical transistors. This allows the Always-ON and non-Always-ON transistors to be fabricated together and on the same substrate, thereby requiring no separate fabrication steps. In order to achieve this, a unique CMOS layout is required, as will be detailed below. 
         [0040]    A CMOS layout  400  for the fabrication of asymmetric transistors according to an embodiment of the present invention is shown in  FIG. 4 . The example CMOS layout  400  in  FIG. 4  shows four asymmetric NMOS transistors TN 1 , TN 2 , TN 3  and TN 4 , and four asymmetric PMOS transistors TP 1 , TP 2 , TP 3  and TP 4 . However any number of such transistors, and, preferably, an equal number of transistors in which there are as many asymmetric NMOS transistors as there are asymmetric PMOS transistors, may be used. 
         [0041]    In one embodiment of the present invention, asymmetric NMOS and PMOS transistors are abutted together in a CMOS layout  400  as shown in  FIG. 4 . In the CMOS layout  400  of  FIG. 4 , regions  411 ,  422 ,  431  and  442  are the source regions of the asymmetric NMOS transistors  410 ,  420 ,  430  and  440 , respectively, and regions  412 ,  421 ,  432  and  441  are the drain regions of the asymmetric NMOS transistors  410 ,  420 ,  430  and  440 , respectively. Regions  413 ,  424 ,  433  and  444  are the source regions of the asymmetric PMOS transistors  415 ,  425 ,  435  and  445 , respectively, and regions  414 ,  423 ,  434  and  443  are the drain regions of the asymmetric PMOS transistors  415 ,  425 ,  435  and  445 , respectively. 
         [0042]      FIG. 4  also shows four example base patterns  405 ,  406 ,  407  and  408 . The construction of all base patterns is based on a regular pattern for the pocket and extension implants  410 ,  415 ,  420 ,  425 ,  430 ,  435 ,  440  and  445 , which comply with the normal design rules for minimum edge-length and minimum area required for viable implants in the CMOS technology. The minimum allowed edge length for the implant opening defines the transistor pitch. Each base cell contains one NMOS and one PMOS transistor, whereby the each transistor has an individual active Silicon area and all edges of gate and active area are aligned in the channel direction of the transistor. Each implant area  464  to  466  is shared between the source areas of two adjacent base cells; each implant area  460  to  463  is shared between the drain areas of two adjacent base cells. This implies a pattern of even and odd base patterns where all odd base patterns are mirrored along a vertical axis. In an alternative embodiment of the present invention, implant areas  464  to  466  is shared for the drain areas of two adjacent base cells while each implant area  460  to  463  is shared between the source areas of two adjacent base cells. 
         [0043]    The four base cell patterns shown vary only in the number and location of gate connections as well as in the gate widths of the NMOS and PMOS transistors within the cell. Base cell  405  has separate gate connections for each of TN 1  and TP 1 . Accordingly, contacts to the each gate are positioned a minimum allowed (vertical) distance away from the active regions of the device. In base cell  405 , gate connections are made to the top of the NMOS transistor and to the bottom of the PMOS transistor. A single shared gate electrode with a centrally located gate contact is adopted in base cell  406 , connecting the gates of transistors TN 2  and TP 2 . Base cells  407  and  408  have separate gate connections for each of transistors TN 3 , TP 3 , TN 4  and TP 4 , the contacts of each being located at different positions. It should be noted that transistors TN 1  to TN 4  and TP 1  to TP 4  have different gate widths (identified by the vertical length of overlap of the respective gate electrodes and the width of the active silicon area; this has been identified by dimension  401  in  FIG. 4 ). It will be recognised that the above described variations in base cell patterns fall within the scope of the present invention. 
         [0044]    Base cell  405  consists of an NMOS transistor  410  and a PMOS transistor  415  with the like terminals vertically aligned where the source region  411  of the NMOS transistor  410  is in line with the source region  413  of the PMOS transistor  415 . In the CMOS layout  400 , base cells are laterally mirrored such that the interior source and drain regions abut each other. 
         [0045]    Region  460  is configured such that the edge of the photoresist mask vertically falls on the centre of gates  471  and  472 . To ensure that the implant area  460  is not obstructed by photoresist a minimum overlap of the gate is required, typically requiring a gate length of at least  1 . 5  x Lmin. Similarly, the edges of the photoresist mask defining regions  461  to  463  fall on the centre of gates  474  and  476 ,  470  and  472 , and  473  and  475 , respectively, thereby ensuring that the photoresist can be fully removed between these gates to enable the implantation. This results in the source and drain regions of each of the asymmetric NMOS or PMOS transistors having different junctions. In one embodiment of the present invention, the drain region has a lower doping than the source region in each asymmetric transistor. 
         [0046]    In the above described embodiments, the mirroring of the base cells in the CMOS layout  400  complies with minimum implant area rules such that the area between regions  460  to  463 , for example, remains viably implanted to realise regions of higher doping; this is achieved by ensuring that the width of the implant mask openings defining regions  460  to  463  complies with the minimum width for pocket and extension implantation defined by the CMOS technology used for fabrication of the symmetrical transistors. It can be seen that the advantage of this asymmetric CMOS transistor placement methodology is that the loss of gate density forced by the requirements of the different implantation of source and drain is minimised and a regular placement is adopted, which minimises the variation in the layout environment of each transistor and the number of viable base cells. The limited number of fixed base cells allows these to be better characterised and modelled by precise fixed point models rather than fully scalable models including extensions to account for some of the variations caused by the local environment. 
         [0047]    Further, in the embodiments described above, within each base cell, no ohmic contact is made to the respective n-wells and p-wells in which the respective NMOS and PMOS transistors are located. This ensures a higher packing density of cells giving a higher density of gate electrodes per unit area. In an embodiment of the present invention, dedicated tap-cells  450  and  455  are inserted at the end of a row of asymmetric transistors, as seen in  FIG. 4 . These tap cells are defined to receive either the source or the drain implants. 
         [0048]    In a further embodiment of the present invention, when tap-cells comply with area rules of the CMOS technology used for fabrication, they can be used at the edges of logic cells, as shown in the CMOS layout in  FIGS. 5A and 5B . In this manner, and in accordance with an embodiment of the present invention, the height of the base patterns can be aligned to conventional standard cells, such as a  9  track cell height, and can be butted to conventional standard cells without the need for border dummy cells. 
         [0049]    In one example based on 40 nm CMOS technology, according to layout design rules, the minimum implant width is 0.18 μm and the minimum diffusion extension in the lateral (source/drain) direction is 0.14 μm. This gives a minimum lateral gate extension of 0.08 μm (because Lmin=2×(0.18 μm−0.14 μm)=0.08 μm). For a base cell height of 1.26 μm in which the NMOS transistor height is 0.58 μm and the PMOS transistor height is 0.68 μm, and a base cell width of 0.4 μm, the unit implant area for an NMOS asymmetric transistor is 0.232 μm 2  while that for a PMOS asymmetric transistor is 0.272 μm 2 . 
         [0050]    The increased gate area and reduced net channel doping reduces the threshold voltage variability of the resulting asymmetric transistor, and therefore allows these transistors to operate at less gate-overdrive, i.e. reduced supply voltage. This allows the supply voltage to be reduced during sleep mode in which only the keep-alive circuitry is powered and active. 
         [0051]    Preferably the transistors of the present invention are fabricated with existing pocket and extension implants used for different types of symmetric core logic, SRAM or I/O transistors in the CMOS process technology. 
         [0052]    It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.