Abstract:
A device and method of fabricating the same are disclosed. In an example, a device includes a first fin Field Effect Transistors (finFET) formed on a substrate. The first finFET including a fin formed on the substrate. The device further includes a second finFET formed on the substrate. The first finFET and the second finFET share the fin and wherein the first finFET is without any low density doped (LDD) extension region in the substrate and wherein the second FinFET is associated with a first LDD extension region formed in the substrate such that a drive strength of the second finFET is greater relative to a drive strength of the first finFET.

Description:
CROSS REFERENCE 
     This application is a continuation application of U.S. application Ser. No. 13/647,091, filed Oct. 2, 2012, which is a divisional application of U.S. patent application Ser. No. 12/483,133, filed Jun. 11, 2009, now U.S. Pat. No. 8,283,231, which claims priority to European patent application number 08104375.4, filed Jun. 11, 2008, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to the selective modification of the drive strength of a finFET (Fin Field Effect Transistor). In particular the invention provides methods for forming finFETs with a selectively modified drive strength and to circuits resulting from the application of those methods to the fabrication of integrated circuits. The present invention is particularly suited to, but not limited to, use in a Static Random Access Memory (SRAM) cell. 
     CMOS (Complementary Metal Oxide Semi-conductor) technology is widely used today in integrated circuits used in, for example, microprocessors, microcontrollers, memory circuits and other digital logic circuits, as well as in a wide variety of analog circuits. CMOS technology relies on MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) to amplify or to switch signals. 
     It is desirable to be able to differentiate the drive strengths of transistors within a CMOS circuit. For example one area in which it has been found to be advantageous to differentiate the drive strengths of transistors is within a Static Random Access Memory (SRAM) cell. A typical SRAM cell circuit with 6 transistors (6-T SRAM cell) is shown in  FIG. 1 , and a typical implementation of such a cell in conventional planar CMOS technology is shown in  FIG. 2 . 
     The exemplary 6-T SRAM cell shown in  FIG. 1  comprises first and second cross-coupled inverters  2 ,  4  each inverter  2 ,  4  comprising a respective P-MOS Pull-Up (PU) transistor  6 ,  8  and a N-MOS Pull-down (PD) transistor  10 ,  12  coupled in series between the power source Vdd and ground Gnd. A first respective N-MOS Pass-gate (PG) transistor  14  is coupled between a bit-line  16  and an internal node  18  connecting the first pull-up transistor  6  and the first pull-down transistor  10 . A second respective N-MOS pass-gate transistor  20  is coupled between an inverted bit-line  22  and an internal node  24  connecting the second pull-up transistor  8  and the second pull-down transistor  12 . As will be apparent to a skilled person, corresponding inverted values are held by the internal nodes  18 ,  24 . 
     The exemplary 6-T SRAM cell stores one bit and has two stable states, in which either internal node  18  or internal node  24  are asserted, which are used to denote a 1 or 0 for the bit. Access to the exemplary 6-T SRAM cell during both a read operation and a write operation is enabled by the word line WL, which turns on the first and second pass-gate transistors  14 ,  20  thus coupling the internal nodes  18 ,  24  to the bit lines  16 ,  22  respectively. For example, during a read operation, the bit lines  16  and  22  are both pre-charged to logic 1, before asserting the word line WL to turn on first and second pass-gate transistors  14 ,  20 . Thereafter, the bit line corresponding to whichever of the internal node  18  or  24  started at logic 0 will discharge into the cell via the respective pass-gate transistor  14 ,  20  and the respective pull-down transistor  10 ,  12 . A detailed discussion of the read and write operation of a 6-T SRAM cell will be omitted since a skilled person will be familiar with the layout and operation of 6-T SRAM cell. 
     As is well known, the performance of a SRAM cell is dependent on the relative drive strengths of the transistors making up the SRAM cell. In this context, the term drive strength of a transistor is intended to mean the magnitude of the current flowing through the transistor when the transistor is switched on. 
     Conventionally designers of SRAM cells have been able to adjust all the major performance metrics of a SRAM cell, such as access time, static noise margin and write margin, by adjusting the transistor widths and lengths in planar CMOS technology to control the drive strengths of the respective transistors. Thus, for example in a SRAM cell implemented in conventional planar bulk CMOS technology the source and drain regions of the pull-down transistor can be made wider than the source and drain regions of the corresponding pass-gate transistor, thus increasing the drive strength of the pull-down transistor relative to the drive strength of the pass-gate transistor. The read time of the cell can therefore be improved. 
     A typical implementation of the SRAM cell of  FIG. 1  in conventional planar CMOS technology showing the differentiation of the transistor dimensions is shown in  FIG. 2 . In  FIG. 2  the same reference numerals as the reference numerals for the corresponding features in  FIG. 1  have been used. In addition, interconnection  26  interconnects the gate of pull-up transistor  8 ; the gate of pull-down transistor  12 ; the drain of pull-up transistor  6 ; the drain of pull-down transistor  10  and the internal node side of the pass-gate transistor  14 , and thus represents the internal node  18 . An interconnection  28  interconnects the gate of pull-up transistor  6 ; the gate of pull-down transistor  10 ; the source of the pull-up transistor  8 ; the drain of pull-down transistor  12  and the internal node side of pass-gate transistor  20 , and thus represents the internal node  24 . As can be seen, the widths of the pull-down transistors  10 ,  12  are greater than the widths of the corresponding pass-gate transistors  14 ,  20 , giving the pull-down transistors  10 ,  12  greater drive strength and thus improving the performance of the SRAM cell. It is noted that the pull-up transistors  6 ,  8  are P-MOS transistors, while the other transistors are N-MOS transistors. 
     However, due to increasing variability, it is becoming increasingly difficult to manufacture a SRAM array of several Mbits in planar CMOS technology with an acceptable yield as the density of the integrated circuits is increased. In addition, as the size of the individual MOSFETS is reduced, the performance of the MOSFETS degrades, in particular due to so-called short channel effects. At the present it is expected that conventional planar MOSFETs will reach the limit of miniaturisation by 2010, concurrent with the widespread adoption of 32 nm technologies. 
     FinFETs have been suggested as potential replacements for conventional planar MOSFETS. The distinguishing characteristic of the finFET is that the conducting channel is wrapped around a thin silicon “fin” which forms the body of the device. One or more gate electrodes are formed adjacent the fin. In a dual-gate device, one gate electrode on each side of the fin is formed. In a tri-ate device, one gate electrode on each side of the fin and one gate electrode at the top of the fin are formed. These arrangements enable good gate control over the channel, resulting in improved short channel immunity and improved ratio between the channel current when the transistor is switched on and the channel current when the transistor is switched off. The structure of a typical finFET will be described with reference to  FIGS. 3-5 . 
       FIG. 3  is a plan view of a typical finFET structure.  FIG. 4  is a longitudinal cross-section through the line A-A in  FIG. 1 .  FIG. 5  is a perspective view of a typical finFET structure showing the fin and the gate structures. In  FIG. 5  the drain region and the source region of the finFET have been omitted for clarity. 
     The finFET  30  is provided with a narrow and relatively high fin  32  upstanding from the surface of the substrate  34  (seen best in  FIGS. 4 and 5 ). The fin  32  is typically formed from Silicon on a buried Oxide (BOX) layer of a Silicon (SOI) substrate  34  or on a silicon substrate and has a width W fin  and a Height H fin . One or more gate electrodes  36  having a gate length L gate  are disposed over the top and sides of the fin  32  and spaced apart from the fin  32  by a spacer  38 . Typical dimensions for the fin  32  and gate electrodes  36  for the 32 nm CMOS node would be L gate =20-50 nm; W fin =5-20 nm; H fin =20-100 nm. Adjacent each end of the fin  32  are formed two high density doped (HDD) regions forming a source HDD region  40  and a drain HDD region  42  of the finFET  30 . In addition, typically, low density doped (LDD) extension regions, forming source LDD extension region  44  and drain LDD extension region  46 , are formed within the channel extending from the source HDD region  40  and drain HDD region  42  respectively towards the gate electrode(s)  36 . 
     It is known to omit the drain LDD extension region of a finFET in order to reduce Gate Induced Drain leakage (GIDL) current within the channel of a finFET, as for example discussed in the article “GIDL (Gate-Induced Drain Leakage) and Parasitic Schottky barrier Leakage Elimination in aggressively scaled HfO 2 /TiN FinFET devices” Hoffmann et al, Electron Devices Meeting, 2005 IEDM Technical Digest. IEEE International Dec. 5, 2005, Piscataway, N.J., USA. 
     Many of the performance metrics of a finFET, for example the drive strength of the finFET, are determined by the effective channel width of a finFET which in turn depends upon the dimensions of the fin. In particular, the effective channel width of a finFET is given by 2×H fin  (for a dual gate FinFET) and 2×H fin +W fin  (for a tri-gate finFET). The person skilled in the art will be familiar with the principles of operation of a Field Effect Transistor (FET) and therefore a detailed description of the operation of the finFET will be omitted. 
     An exemplary implementation of the SRAM cell of  FIG. 1  using finFETs is shown in  FIG. 6 . In  FIG. 6  reference numerals corresponding to reference numerals used in  FIG. 2  are used for the corresponding features. As can be seen from a consideration of  FIG. 6 , in this exemplary silicon layout implementation, the finFET pass-gate transistor  114  shares a common fin with the finFET pull-down transistor  110  and the finFET pass-gate transistor  120  shares a common fin with the finFET pull-down transistor  112 . As a result, the drive strengths of the transistors in the SRAM cell implementation shown in  FIG. 6 , in particular the pass-gate transistors  114 ,  120  and the pull-down transistors  110 ,  112  cannot easily be altered independently. 
     Previously several different methods have been suggested to increase the drive strength of a finFET. In a first method disclosed for example in U.S. Pat. No. 6,706,571 an additional fin parallel to the fin common with the pass-gate transistor is provided for each pull-down transistor in a SRAM cell. However, the addition of the extra fin results in an undesirable increase in area used by the SRAM cell. In a second method disclosed for example in H. Kawasaki et al “Embedded Bulk FinFET SRAM cell Technology with planar FET peripheral circuit for hp32 nm node and beyond”, Symposium on VLSI Technology digest of Technical papers, 2006, the fin height of adjacent finFETs in a cell built on bulk Si is adjusted to alter the drive strength of the finFETs. However, it is difficult to apply this technique to the mass fabrication of integrated circuits and especially difficult to apply this technique to finFETs built on a SOI substrate. In a third method proposed in Z. Guo et al., “FinFET-based SRAM Design”, Proceedings of the 2005 International Symposium on Low Power Electronics and Design (ISPLED&#39;05), pp 2-7, 2005, the pull-down transistors are rotated by 45 degrees so that the channels are formed in the [100] plane, resulting in an improvement in the Signal Noise Margin (SNM) owing to the improved electron mobility in the [100] plane compared with the conventional [ 110 ]. However, again this technique is complex and it is difficult to apply this technique to the mass fabrication of integrated circuits. 
     The present inventors have realised it would be desirable to be able to selectively modify the drive strength of a finFET in a CMOS circuit or to differentiate the drive strengths of two or more finFETs in a CMOS circuit. 
     SUMMARY 
     In accordance with one aspect of the present invention, there is provided a method of fabricating a fin Field Effect Transistor (finFET), the finFET being formed on a substrate and comprising a fin formed on the substrate, a first high density doped (HDD) region and a second high density doped (HDD) region formed in the substrate at longitudinal ends of the fin and at least one gate region formed adjacent the fin, the method comprising the steps of: determining whether the finFET should have a maximum drive strength or a relatively lower drive strength; and fabricating the finFET with a first low density doped (LDD) extension region in the substrate extending from the first high density doped (HDD) region toward the gate region and a second low density doped (LDD) extension region in the substrate extending from the second high density doped (HDD) region toward the gate region where a maximum drive strength is determined; and fabricating the finFET without one or both of said LDD extension regions in the substrate when a relatively lower drive strength is determined. 
     In accordance with another aspect of the invention, there is provided a method of forming a circuit comprising at least a first fin Field Effect Transistor (finFET) and a second finFET, the at least first and second finFETs being formed on a substrate and comprising a fin formed on the substrate, a first high density doped (HDD) region and a second high density doped (HDD) region formed in the substrate at longitudinal ends of the fin and at least one gate region formed adjacent the fin, where the drive strength of the first finFET is selected to be higher than the drive strength of the second finFET, by forming the first finFET with at least one of a first low density doped (LDD) extension region in the substrate extending from the first HDD region toward the gate region and a second low density doped (LDD) extension region in the substrate extending from the second HDD region toward the gate region; and the drive strength of the second finFET is selected to be lower than the drive strength of the first finFET by forming the second finFET with no LDD extension regions in the substrate or with only one of said first LDD extension region or said second LDD extension region. 
     In accordance with a further aspect of the invention there is provided a circuit comprising at least a first and second fin Field Effect Transistors (finFET), the at least first and second finFETs being formed on a substrate and comprising a fin formed on the substrate, a first high density doped (HDD) region and a second high density doped (HDD) region formed in the substrate at longitudinal ends of the fin and at least one gate region formed adjacent the fin, wherein the first finFET has at least one of a first low density doped (LDD) extension region in the substrate extending from the first HDD region toward the gate region and a second low density doped (LDD) extension region in the substrate extending from the second HDD region toward the gate region; and the second finFET has no LDD extension regions in the substrate or has only one of said first LDD extension region or said second LDD extension region such that the drive strength of the second finFET is reduced relative to the drive strength of the first finFET. 
     The first FinFET may be selected or determined to have maximum drive strength and be formed with a first LDD extension region and a second LDD extension region in the substrate. The first LDD extension region may be formed in the source region of the finFET and the second LDD extension region may be formed in the drain region of the finFET. 
     The second FinFET may be selected or determined to have a first drive strength lower than the maximum drive strength and be formed with only a second LDD extension region in the substrate. Alternatively, the second FinFET may be selected or determined to have a second drive strength lower than the maximum drive strength and be formed with a first LDD extension region in the substrate. Further, the second FinFET may be selected or determined to have a minimum drive strength and be formed with no LDD extension regions in the substrate. 
     In this context it will be understood that the terms “minimum drive strength” and “maximum drive strength”, and the “first drive strength” and “second drive strength” intermediate the maximum and minimum drive strengths, are intended to refer to the variation in drive strength of a finFET achievable in accordance with the omission of the source and/or drain LDD extensions in a finFET. It is not necessary that absolute values of the drive strength of the finFET are known: instead it is intended that the drive strength of at least one finFET can be selectively modified relative to either the maximum available drive strength of a finFET having the same dimensions, or relative to another finFET in the circuit. 
     Advantageously the invention may be applied to finFETs sharing a common fin, such as the pass-gate and pull-down finFETs in a SRAM. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic drawing showing a typical SRAM cell circuit with 6 transistors (6-T SRAM cell); 
         FIG. 2  illustrates a typical implementation of the circuit shown in  FIG. 1  in conventional planar CMOS technology; 
         FIG. 3  is a plan view of a typical finFET structure; 
         FIG. 4  is a longitudinal cross-section through the line A-A in  FIG. 3 ; 
         FIG. 5  is a partial perspective view of the typical finFET structure shown in  FIGS. 3 and 4 ; 
         FIG. 6  illustrates an exemplary implementation of the circuit of  FIG. 1  using finFETS; 
         FIG. 7  is a plan view of a finFET structure in accordance with one embodiment of the invention; 
         FIG. 8  is a longitudinal cross-section through the line A-A in  FIG. 7 ; 
         FIGS. 9   a ) to  9   c ) show the variation of drive strength of a finFET; 
         FIG. 10  compares simulated butterfly curves of a prior art SRAM cell and a SRAM cell implementing the present invention; 
         FIG. 11  shows masks showing doping layer areas up to the first metal layer (M1) overlaid on a typical 6-T SRAM layout as shown in  FIG. 6 ; 
         FIG. 12  illustrates the essential steps in a typical FinFET CMOS process flow using the masks shown in  FIG. 11   
         FIG. 13  shows masks showing doping layer areas up to the first metal layer (M1) overlaid on a typical 6-T SRAM layout as shown in  FIG. 6 ; 
         FIG. 14  illustrates the essential steps in a typical FinFET CMOS process flow using the masks shown in  FIG. 13   
         FIG. 15  shows masks showing doping layer areas up to the first metal layer (M1) overlaid on a typical 6-T SRAM layout as shown in  FIG. 6 ; 
         FIG. 16  illustrates the essential steps in a typical FinFET CMOS process flow using the masks shown in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the invention the drive strength of a FinFET transistor can be selectively modified, and in particular can be selectively reduced, by omitting the LDD extension region formation in the source and/or in the drain of the finFET. One application of this approach is to enable differentiation of the drive strengths of transistors in an integrated circuit by applying the technique to some, but not all, of the transistors in the integrated circuit. In particular in a SRAM cell formed from finFET transistors the application of the technique to the pass-gate transistors, which leads to a reduction of the drive strength of the pass-gate transistors relative to the drive strength of the pull-up and pull-down transistors, results in improved SRAM cell performance. 
     The invention will now be explained with reference to  FIG. 7 , which shows a plan view of a finFET structure in accordance with one embodiment of the invention, and  FIG. 8 , which is a longitudinal cross-section through the line A-A in  FIG. 7 . Corresponding reference numerals have been used for features corresponding to  FIGS. 3 and 4 . 
     In this embodiment of the invention adjacent each end of the fin are formed two high density doped (HDD) regions forming a source HDD region  140  and a drain HDD region  144  of the finFET. The source LDD extension regions  132  have been formed in the channel, extending from the source HDD region  140  towards the gate electrode(s)  136  but drain LDD extension regions have not been formed. 
     As indicated above, in embodiments of the invention the drain LDD extension regions or the source LDD extension regions or both may be omitted, depending upon the desired extent of the reduction in the drive strength of the transistor required. Typical reductions in the drive strength that may be obtained in an NMOS finFET by omitting the drain LDD extension region, the source LDD extension region and both the source LDD extension region and the drain LDD extension region are shown in  FIGS. 9   a  to  9   c  respectively. 
       FIGS. 9   a  to  9   c  show a plot of the drive strength of a single fin finFET, i.e. the current flowing through the fin of the finFET, against the gate  36  voltage applied to the finFET for a drain to source voltage equal to 1 Volt. In each of  FIGS. 9   a  to  9   c  the line  50  shows the drive strength of a first finFET having both a source LDD extension region and a drain LDD extension, for example the finFET  30  shown in  FIG. 3 . Therefore line  50  shows the maximum drive strength that can be obtained without changing the dimensions of the fin. 
     In  FIG. 9   a  the line  52  shows the drive strength of a finFET in accordance with an exemplary embodiment having a source LDD extension region formed therein but no drain LDD extension region formed therein, for example the finFET  130  according to the exemplary embodiment shown in  FIGS. 7 and 8 . As can be seen, the drive current of the finFET in this embodiment shown by line  52  is reduced with respect to the maximum drive strength shown by line  50  by up to 27%. 
     In  FIG. 9   b  the line  54  shows the drive strength of a finFET in accordance with an exemplary embodiment having a drain LDD extension region formed therein but no source LDD extension region formed therein (not shown). As can be seen, the drive current of the finFET in this embodiment shown by line  54  is reduced with respect to the maximum drive strength shown by line  50  by up to 47%. 
     In  FIG. 9   c  the line  56  shows the drive strength of a finFET in accordance with an exemplary embodiment having no source LDD extension region or drain LDD extension region formed therein (not shown). As can be seen, the drive current of the finFET in this embodiment shown by line  56  is reduced with respect to the maximum drive strength shown by line  50  by up to 63%. Line  56  shows the minimum drive strength that can be obtained without changing the dimensions of the fin. 
     As will be appreciated by a skilled person, the invention may be applied to any situation in which it is desired to selectively modify the drive current of a finFET transistor. Thus it can be determined during a design process whether a finFET should have a maximum drive strength or should have a relatively lower drive strength. The finFET can then be designed, and then fabricated, with or without an LDD extension region for the source and/or the drain, according to the determined relative drive strength. 
     The invention is particularly suitable where a differentiation between the drive currents of two or more finFET transistors within a circuit is required. In this case, the drive strength of a first finFET of a circuit can be selected to be higher that a second finFET of the circuit, and the second finFET can be formed having fewer LDD extension regions than the first finFET, and thus having a lower drive strength. 
     One situation in which the invention can be applied is in a SRAM cell, and therefore embodiments of the invention will now be described with reference to a SRAM cell. However a skilled person will appreciate that the invention is not limited to application in a SRAM cell. 
     In the exemplary embodiment of the invention applied to a SRAM cell, the drive strength of the pass-gate transistors are modified by omitting source LDD extension regions or drain LDD extension regions, in order to reduce the drive strength of the pass-gate transistor. It should be noted that in the pass-gate transistor of an SRAM, the source and drain change role depending on the state of the cell (read/write/standby), therefore we will refer to the bit line side LDD extension regions or the internal node side LDD extension regions for the pass-gate transistors instead of source and drain LDD extension regions. However, the skilled person will understand that the principles of omitting one or both of the LDD extension regions on the finFET may be applied equally to this situation. 
     The static noise margin is given by the drive strength ratio of the pass-gate and the pull-down transistors. In the exemplary embodiment of the invention applied to a SRAM cell, the modification of the drive strength of the pass-gate transistors by omitting the bit line side LDD extension regions or the internal node side LDD extension regions as described above and as shown in  FIGS. 9   a ) to  9   c ) will improve the static noise margin of the cell. 
     The static noise margin can be illustrated by means of a SRAM cell butterfly curve.  FIG. 10  shows simulated butterfly curves of a first cell with internal node LDD extension regions in the pass-gate transistors (line  62 ) and a second cell without internal node LDD extension regions in the pass-gate transistors (line  64 ). Each cell has the same pull-up, pull-down and pass-gate transistor dimensions (L gate 50 nm; W fin =10 nm, H fin =60 nm). V dd  is equal to 1. 
     The Static Noise Margin can be represented visually by the size of the biggest square that can be inserted in the “eyes” of the butterfly curve. As is clear from  FIG. 10 , the static noise margin of the second cell, represented by square  66 , is larger than the static noise margin of the first cell, represented by square  68 . 
     Thus it will be apparent to a skilled person that if the drive strength of the pass-gate transistor is designed to be weaker than the drive strength of the pull-down transistor by application of the principles of the invention, the read operation of a SRAM cell will be easier and the static noise margin will increase. A skilled person can select whether or not bit line side LDD extension regions and/or the internal node side LDD extension regions for the pass-gate transistors are implanted in order to achieve the desired drive strength decrease required for the pass gate transistor. In this context it should be noted that the drive strength of the pass-gate transistor should not be made too weak. In particular, a weak pass-gate finFET drive strength will also result firstly in a degraded write margin, since the write margin is reduced if the pass gate transistor drive strength decreases with respect to the pull-up transistor, and secondly in reduced read current I read , since the read current I read  is improved if the drive strength of both pass-gate transistor and pull-down transistor is increased. 
     As indicated above, the present invention is not limited to use in finFET SRAM cells, and may be applied to a wide variety of circuits using finFETs. 
     An exemplary finFET CMOS process flow will now be described with reference to  FIGS. 11 and 12 . 
       FIG. 11  shows masks showing doping layer areas up to the first metal layer (M1) overlaid on a typical 6-T SRAM layout as shown in  FIG. 6 , and  FIG. 12  illustrates the essential steps in a typical finFET CMOS process flow using the masks shown in  FIG. 11 . 
     The person skilled in the art will appreciate that additional or alternative steps can also be used to effect fabrication of a CMOS circuit and such additional or alternative steps have accordingly not been discussed in detail. In particular, well and halo (pocket) implantation steps are not present in the illustrated process flow, nor are intermediate thermal anneal steps and epitaxial growth steps after spacer formation to form a raised source or drain. Typically fins are undoped so that well implantations are not necessary: however they can be added as additional mask layers. 
       FIG. 11  shows the mask layers used in the CMOS process flow shown in  FIG. 12 . Mask layer ACTIVE 70, defines the fins; mask layer POLY 72, defines the gate stack; mask layer NPLUS 74 defines the areas to be N-doped; mask layer PPLUS 76 defines the areas to be P-doped; mask layer METAL 78 defines areas of metal interconnect; and mask layer CONTACT 80 defines contact areas. It should be noted that the mask layers ACTIVE 70, POLY 72, METAL 78 and CONTACT 80 shown in  FIG. 11  are positive masks, i.e. the features remain where the mask is drawn. In contrast the NPLUS 74 and PPLUS 76 masks are negative, i.e. photo resist is removed where the mask is drawn such that underlying areas are open for the formation of doped layers, for example by ion implantation. 
     In the exemplary finFET CMOS process flow  1000  shown in  FIG. 12 , firstly a SOI silicon substrate is taken  1002 , and the fins are formed  1004  using the ACTIVE mask 70. The fins are typically formed from silicon. Thereafter the associated gate stacks are formed  1006  on the substrate using the POLY mask 72, and a final gate etch step carried out  1008 . 
     Then the LDD extensions are formed. Firstly the NPLUS mask, as shown in  FIG. 11 , is used in step  1010  to apply photo resist patterned to form the N-doped LDD extension regions, and then the N-LDD implant step  1012  is carried out. The N-LDD implant step  1012  may use, for example, Phosphorous (P) or arsenic (A) dopants. The photo resist is removed  1014 . Next the PPLUS mask, as shown in  FIG. 11 , is used in step  1016  to apply photo resist patterned to form the P-doped LDD extension regions, and then P-LDD implant step is carried out  1018 . The P-LDD implant step may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Then again the photo resist is removed  1020 . 
     After formation of a spacer  1022 , the HDD extension regions are formed. Firstly the NPLUS mask, as shown in  FIG. 11 , is used in step  1024  to apply photo resist patterned to form the N-doped HDD regions. Then the N-HDD implant step  1026 , which may be for example use Phosphorous (P) or arsenic (A), is carried out. The photo resist is removed, in step  1028 . Then the PPLUS mask, as shown in  FIG. 11 , is used to apply photo resist patterned to form the P-doped HDD regions in step  1030 . Then the P-HDD implant step  1032  is carried out. The N-HDD implant step  1032  may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Once again the photo resist is removed in step  1034 . A thermal anneal process in step  1036  completes the step of forming the LDD and HDD areas. Finally a salicidation step  1038  is carried out before the end of the process. 
     Modifications of the finFET process flow and/or process masks to fabricate a SRAM cell in accordance with exemplary embodiments of the invention will now be described with reference to  FIGS. 13-16 . 
     In a first embodiment illustrated with reference to  FIGS. 13 and 14  a finFET transistor having asymmetric LDD extension regions, as described above with reference to  FIGS. 7 and 8  is formed as a pass-gate transistor in a SRAM cell. As illustrated, the pass-gate transistors only have a LDD extension region on one side of the gate, in this case only the internal node side LDD extension regions are present and the bit line side LDD extension regions are omitted. 
       FIG. 13  shows masks showing doping layer areas up to the first metal layer (M1) overlaid on a typical 6-T SRAM layout. Reference numerals corresponding to reference numerals in  FIGS. 7 and 8  have been added to  FIG. 13 .  FIG. 14  illustrates the essential steps in a FinFET CMOS process flow  2000  using the masks shown in  FIG. 13 . 
     Again, the person skilled in the art will appreciate that additional or alternative steps can also be used to effect fabrication of a CMOS circuit and such additional or alternative steps have accordingly not been discussed in detail. In particular, well and halo (pocket) implantation steps are not present in the illustrated process flow, nor are intermediate thermal anneal steps and epitaxial growth steps after spacer formation to form a raised source or drain. Typically fins are undoped so that well implantations are not necessary: however they can be added as additional mask layers. 
     The LDD extension regions are typically formed by ion implantation of impurities, either vertically or under a tilt angle. Alternatively doping techniques such as plasma doping (PLAD) can also be applied. Also co-implantations with non-dopant impurities, e.g. Germanium (Ge) or Fluorine (F) or Hydrogen (H) or helium (He), may be made. 
       FIG. 13  shows the mask layers used in the CMOS process flow shown in  FIG. 14 . Mask layer ACTIVE 70, defines the fins; mask layer POLY 72, defines the gate stack; mask layer METAL 78 defines areas of metal interconnect; and mask layer CONTACT 80 defines contact areas correspond with the same mask layers described above with reference to  FIG. 11 , and have therefore been given the same reference numerals. New mask layer NLDD 82 defines the areas to be N-doped; and new mask layer PLDD 84 defines the areas to be P-doped. 
     Again it should be noted that the mask layers ACTIVE 70, POLY 72, METAL 78 and CONTACT 80 shown in  FIG. 13  are positive masks, i.e. the features remain where the mask is drawn. In contrast the NLDD 82 and PLDD 84 masks are negative, i.e. photo resist is removed where the mask is drawn such that underlying areas are open for the formation of doped layers, for example by ion implantation. 
     In the finFET CMOS process flow  2000  in accordance with the embodiment of the invention shown in  FIG. 14 , firstly a SOI silicon substrate is taken  2002 , and the fins are formed  2004  using the ACTIVE mask 70. The fins are typically formed from silicon. Thereafter the associated gate stacks are formed  2006  on the substrate using the POLY mask 72, and a final gate etch step carried out  2008 . 
     Then the LDD extension regions are formed. Firstly the NLDD mask 82, as shown in  FIG. 13 , is used in step  2010  to apply photo resist patterned to form the N-doped LDD extension regions, and then the N-LDD implant step  2012  is carried out. The N-LDD implant step  2012  may use, for example, Phosphorous (P) or arsenic (A) dopants. The photo resist is removed in step  2014 . Next the PLDD mask 84, as shown in  FIG. 13 , is used in step  2016  to apply photo resist patterned to form the P-doped LDD extension regions, and then P-LDD implant step is carried out  2018 . The P-LDD implant step may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Then again the photo resist is removed  2020 . 
     After formation of a spacer  2022 , the HDD regions are formed. Firstly the NPLUS mask 74, as shown in  FIG. 11 , is used in step  2024  to apply photo resist patterned to form the N-doped HDD regions. Then the N-HDD implant step  2026 , which may be for example use Phosphorous (P) or arsenic (A), is carried out. The photo resist is removed, in step  2028 . Then the PPLUS mask 76, as shown in  FIG. 11 , is used to apply photo resist patterned to form the P-doped HDD regions in step  2030 . Then the P-HDD implant step  2032  is carried out. The N-HDD implant step  2032  may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Once again the photo resist is removed in step  2034 . A thermal anneal process in step  2036  completes the step of forming the LDD and HDD areas. Finally a salicidation step  2038  is carried out before the end of the process. 
     As will be apparent from the above description, in the embodiment of the invention described with reference to  FIGS. 13 and 14  two additional masks NLDD 82 and PLDD 84 are needed in addition to the masks shown in  FIG. 11 , but no additional process steps are required. The new masks NLDD 82 and PLDD 84 do not contain small features, and are therefore relatively inexpensive. 
     In a second embodiment illustrated with reference to  FIGS. 15 and 16  a finFET transistor having asymmetric LDD extension regions, as described above with reference to  FIGS. 7 and 8 , is formed as a pass-gate transistor in a SRAM cell. As illustrated the pass-gate transistors only have a LDD extension on one side of the gate, in this case only the internal node side LDD extensions are present and the bit line side LDD extensions are omitted. 
       FIG. 15  shows a hard mask to be used in addition to the masks shown in  FIG. 11  in the FinFET CMOS process flow  3000  shown in  FIG. 16 . Reference numerals corresponding to reference numerals in  FIGS. 7 and 8  have been added to  FIG. 15 . 
     Again, the person skilled in the art will appreciate that additional or alternative steps can also be used to effect fabrication of a CMOS circuit and such additional or alternative steps have accordingly not been discussed in detail. In particular, well and halo (pocket) implantation steps are not present in the illustrated process flow, nor are intermediate thermal anneal steps and epitaxial growth steps after spacer formation to form a raised source or drain. Typically fins are undoped so that well implantations are not necessary: however they can be added as additional mask layers. 
     The LDD extension regions are typically formed by ion implantation of impurities, either vertically or under a tilt angle. Alternatively doping techniques such as plasma doping (PLAD) can also be applied. Also co-implantations with non-dopant impurities, e.g. Germanium (Ge) or Fluorine (F) or Hydrogen (H) or helium (He), may be made. 
       FIG. 15  shows a blocking hard mask LDD 86. The LDD mask 86 is a negative mask, so that where the mask is drawn the hard mask will be removed and the LDD extension regions formed. In addition masks shown in  FIG. 11  are also used in the CMOS process flow shown in  FIG. 1 . Process steps corresponding to the CMOS process described above with reference to  FIG. 12  or  14  will be given corresponding reference numerals. 
     In the finFET CMOS process flow  3000  in accordance with the embodiment of the invention shown in  FIG. 16 , firstly a SOI silicon substrate is taken  3002 , and the fins are formed  3004  using the ACTIVE mask 70. The fins are typically formed from silicon. Thereafter the associated gate stacks are formed  3006  on the substrate using the POLY mask 72, and a final gate etch step carried out  3008 . 
     In this CMOS process flow, firstly a hard mask layer is applied in step  3050 . The hard mask may be, for example, Silicon Nitride Si 3 N 4  or a TEOS layer or stack of layers. Next the LDD mask 86 is used in step  3052  to apply photo resist, and then the hard mask layer is etched in step  3054 . Thereafter the photo resist is removed in step  3056 . These steps result in a covering of hard mask preventing formation of LDD extension regions on the bit line side of the pass-gate transistors. 
     Then the LDD extension regions are formed. Firstly the NPLUS mask 74 is used in step  3010  to apply photo resist patterned to form the N-doped LDD extension regions, and then the N-LDD implant step  3012  is carried out. The N-LDD implant step  3012  may use, for example, Phosphorous (P) or arsenic (A) dopants. The photo resist is removed in step  3014 . Next the PPLUS mask 76 is used in step  3016  to apply photo resist patterned to form the P-doped LDD extension regions, and then P-LDD implant step is carried out  3018 . The P-LDD implant step may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Then again the photo resist is removed in step  3020 , and the hard mask removed in step  3058 . 
     After formation of a spacer  3022 , the HDD regions are formed. Firstly the NPLUS mask 74, as shown in  FIG. 11 , is used in step  3024  to apply photo resist patterned to form the N-doped HDD regions. Then the N-HDD implant step  3026 , which may be for example use Phosphorous (P) or arsenic (A), is carried out. The photo resist is removed, in step  3028 . Then the PPLUS mask 76, as shown in  FIG. 11 , is used to apply photo resist patterned to form the P-doped HDD regions in step  3030 . Then the P-HDD implant step  3032  is carried out. The N-HDD implant step  3032  may use, for example, Boron (B) or Boron Fluorine (BF 2 ). Once again the photo resist is removed in step  3034 . A thermal anneal process in step  3036  completes the step of forming the LDD and HDD areas. Finally a salicidation step  3038  is carried out before the end of the process. 
     As will be apparent from the above description, in the embodiment of the invention described with reference to  FIGS. 15 and 16  only one additional mask namely the LDD mask 86 is needed in addition to the masks shown in  FIG. 11 , but additional process steps  3050 - 3058  are required. The new LDD mask 86 does not contain small features, and is therefore relatively inexpensive. 
     Thus in the embodiments of the invention described with reference to  FIGS. 13-16  the LDD extension region is formed in the pass gate transistors only on the side in contact with the internal node. Clearly, in a first alternative the LDD extension region of the pass gate transistors on the side in contact with the bit line may be formed and the LDD extension region on the side in contact with the internal node may be omitted or in a second alternative both the LDD extension regions of the pass gate transistors on the side in contact with the bit line and on the side in contact with the internal node, may be omitted using suitable masks, depending on the required reduction in the drive strength of the transistor. 
     The skilled person will appreciate that the process flows described herein and the illustrated masks are illustrative only and changes may be made thereto without departing from the scope of the invention. In addition other methods of forming a finFET transistor without one or more LDD extensions are intended to be included within the scope of the invention. 
     Thus there is provided a method for the selective modification of the drive strength of a FinFET transistor, and in particular a method in which the drive strength of a finFET transistor can be selectively reduced, by omitting an LDD extension region formation in the source and/or in the drain of the FinFET. 
     One application of this method is to enable differentiation of the drive strengths of transistors in an integrated circuit by applying the technique to some, but not all, of the transistors in the integrated circuit. In particular in a SRAM cell formed from FinFET transistors the application of the technique to the pass-gate transistors, which leads to a reduction of the drive strength of the pass-gate transistors relative to the drive strength of the pull-up and pull-down transistors, results in improved SRAM cell performance.