Abstract:
A method of forming a current mirror device for an integrated circuit includes configuring a reference current source; forming a first field effect transistor (FET) in series with the reference current source, the first FET of a first conductivity type formed on a first portion of a substrate having a first crystal lattice orientation; and forming a second FET of the first conductivity type on a second portion of the substrate having a second crystal lattice orientation, with a gate terminal of the first FET coupled to a gate terminal of the second FET, and the gate terminals of the first and second FETs coupled to the reference current source; wherein the carrier mobility of the first FET formed on the first portion of the substrate is different than the carrier mobility of the second FET formed on the second portion of the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/161,337, filed Jul. 29, 2005, now U.S. Pat. No. 7,382,029, the disclosure of which is incorporated by reference herein in its entirety. 

   BACKGROUND 
   The present invention relates generally to integrated circuit devices, and, more particularly, to a method for improving integrated circuit device performance using hybrid crystal orientations. 
   Complementary metal-oxide-semiconductor (CMOS) technology is the predominant semiconductor technology used for the manufacture of ultra-large scale integrated (ULSI) circuits today. Conventional CMOS technology generally employs silicon (Si) wafers as the starting material for volume production. These silicon wafers generally have a single crystal surface that is (100) oriented; i.e., the normal to the silicon wafer surface is in the [100] direction. Hence, conventional planar metal-oxide-semiconductor field-effect transistors (MOSFETs) formed on (100) silicon wafers have a gate dielectric-channel interface plane that is in the (100) plane of the silicon substrate. 
   Electrons are known to have a relatively high mobility for a (100) Si surface orientation, whereas holes are known to have high mobility for a (110) surface orientation. More specifically, hole mobility values for (100) Si are roughly two to four times lower than the corresponding electron mobility values for this crystallographic orientation. As a result, n-type devices formed in (100) silicon have a higher carrier mobility than p-type devices (with comparable device geometry) formed in (100) silicon. Thus, in order to compensate for this discrepancy, PFET devices are typically designed with larger widths with respect to NFET devices in order to balance the PFET pull-up currents against the NFET pull-down currents and achieve uniform circuit switching. 
   On the other hand, hole mobilities for (110) Si are about twice as high than for (100) Si. Consequently, PFETs formed on a (110) Si surface will exhibit significantly higher drive currents than PFETs formed on a (100) surface. Unfortunately, electron mobilities for (110) Si surfaces are significantly degraded as compared to (100) Si surfaces. A comparison of electron and hole mobilities with respect to different crystal orientations is illustrated in  FIG. 1 . As will be noted from a speed perspective, a (110) Si surface is optimal for PFET devices because of the excellent hole mobility, while a (100) Si surface is optimal for NFET devices because of the excellent electron mobility. 
   In order to fully utilize the advantage of the carrier mobility dependence on substrate orientation, various technologies of fabricating CMOS on hybrid substrates with different crystal orientations have been developed, in which the NFETs are manufactured at (100) surface orientations within the hybrid substrates, while the PFETs are manufactured at (110) surface orientations within the hybrid substrate. Because both types of devices (NFETs and PFETs) are operating with their respective carriers at peak mobilities, the resulting CMOS has overall higher speed and transconductance (or gain) as compared with a traditional CMOS structure where both the NFET and PFET are located on the same (100) substrate. In this application, the main benefit is the optimization of device performance since the PFET devices, being located on (110) substrates, now have comparable carrier mobilities as the (100) NFET devices. 
   In other instances, however, maximization of carrier mobility is not the desired goal of certain semiconductor devices. Thus, it is also desirable to be able to fabricate semiconductor devices having, for example, offset characteristics with respect to one another, and in a manner that conserves device real estate. 
   SUMMARY 
   The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method of forming a current mirror device for an integrated circuit, including configuring a reference current source; forming a first field effect transistor (FET) in series with the reference current source, the first FET of a first conductivity type formed on a first portion of a substrate having a first crystal lattice orientation; and forming a second FET of the first conductivity type on a second portion of the substrate having a second crystal lattice orientation, with a gate terminal of the first FET coupled to a gate terminal of the second FET, and the gate terminals of the first and second FETs coupled to the reference current source; wherein the carrier mobility of the first FET formed on the first portion of the substrate with the first crystal lattice orientation is different than the carrier mobility of the second FET formed on the second portion of the substrate having with the second crystal lattice orientation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
       FIG. 1  is a pair of graphs illustrating a comparison of electron and hole mobilities with respect to different crystal orientations; 
       FIG. 2  is a schematic diagram of an existing NFET current mirror device; 
       FIG. 3(   a ) is a schematic diagram of an NFET current mirror device in which one NFET is formed on a substrate of a first crystalline orientation, and another NFET is formed on a substrate of a second crystalline orientation, in accordance with an embodiment of the invention; 
       FIG. 3(   b ) is a schematic diagram of a PFET implementation of a current mirror device in accordance with another embodiment of the invention; 
       FIG. 4  is a schematic diagram of an existing constant transconductance circuit; 
       FIG. 5  is a schematic diagram of a constant transconductance circuit in which one NFET is formed on a substrate of a first crystalline orientation, and another NFET is formed on a substrate of a second crystalline orientation, in accordance with another embodiment of the invention; 
       FIGS. 6(   a ) and  6 ( b ) illustrate an inverter pair, representing an additional application for an embodiment of the invention, in which the NFET and PFET devices of a first inverter are formed on substrates that increase respective carrier mobilities and the NFET and PFET devices of a second inverter are formed on substrates that decrease respective carrier mobilities; and 
       FIG. 7  is a schematic diagram of a pair of ring oscillator circuits, illustrating one possible use for inverter pair configured in accordance with  FIG. 6(   b ). 
   

   DETAILED DESCRIPTION 
   Disclosed herein is a method and apparatus for improving integrated circuit device performance using hybrid crystal orientations. For certain integrated circuit applications, there is a benefit to having NFET (or PFET) pairs manufactured on hybrid substrates having different orientations. For example, an NFET (or PFET) pair formed with the same device geometry, but on substrates of different orientations, will primarily demonstrate the same electrical performance except for performance related to carrier mobility. Advantageously, such NFET (or PFET) pairs may be used in applications such as current/voltage generators, for example, or used to generate a repeatable offset in device characteristics, rather than for maximizing speed as previously used. In previous applications where a repeatable offset is designed, the existing approaches included, for example, varying the dimensions, operating conditions and/or process characteristics of the devices. The present invention embodiments introduce a new approach in producing repeatable offsets for device pairs. 
   Briefly stated, the present invention embodiments provide for a pair of MOS devices of the same conductivity and geometry (e.g., an NFET pair or PFET pair), in which one of the devices is formed on a substrate of a first crystal orientation and the other device is formed in a substrate of a different crystal orientation. This pair of MOS devices may in turn be used to improve the matching performance of elements included within, for example, a current mirror, a constant-gm biasing circuit and/or any other device in which highly repeatable offset characteristics are desired. 
   As indicated above, various techniques have been developed for forming a single, hybrid substrate having multiple crystal orientations. For example, in U.S. Patent Application Publication 2004/0151917, incorporated herein by reference in its entirety, a hybrid silicon structure is provided by physically bonding a first substrate of a (100) crystallographic orientation with a second substrate of a (110) crystallographic orientation. Another possible approach to forming a pair of devices in different crystal orientations is to orient the two devices at 45-degree angles with respect to one another, as is described in M. Shima, et al., “&lt;100&gt; Channel Strained-SiGe p-MOSFET with Enhanced Hole Mobility and Lower Parasitic Resistance”, IEEE 2002 Symposium on VLSI Technology Digest of Technical Papers, pp. 94-95, incorporated herein by reference in its entirety. Regardless of how the different crystal orientations are realized for each device pair, the present approach invention exploits the conventional wisdom of using identical device layout in order to achieve optional matching characteristics, but further utilizes the difference in carrier mobility to achieve desired controlled offsets in circuit behavior. 
   Referring once again to  FIG. 1 , it will be seen from the electron mobility curves therein that the electron mobility varies from about 100 cm 2 /V·S on a (110) Si substrate to about 260 cm 2 /V·S on a (100) Si substrate when the inversion charge density is about 9×10 12  cm −2 . If a bias voltage V GS  is applied to a particular NFET device, then the current therethrough will be given by the expression: 
   
     
       
         
           
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   for the biasing condition of V DS &gt;V GS −V TH . Given the same bias voltage, an NFET formed on a (100) oriented substrate will pass about 2.6 times more current than an NFET with identical geometry but formed on a (110) oriented substrate. In the present disclosure, such a pair is an example of what will be also referred to hereinafter as a Hybrid Orientation Substrate Technique (HOST) device pair. The specific properties of a HOST pair (i.e., a first device of a given conductivity formed on a substrate of a first crystalline orientation and a second device of the same conductivity formed on a substrate of a second crystalline orientation) make such a pair useful in various exemplary circuit designs presented below. 
   For example, a current mirror is a circuit device used to produce an output current that has a fixed relationship or ratio to a reference current. Current mirrors are one of the most popular circuit blocks used in integrated circuits. In a typical application, the fixed relationship of the generated output current is such that its value is usually either a multiple or a fraction of the value of reference current. A conventional NFET implementation of a current mirror  200  is illustrated in the schematic diagram of  FIG. 2 . Since both NFETs N 1  and N 2  are biased at the same gate voltage (V gs ), the ratio of the output current to the reference current corresponds to the ratio of the device width of N 2  to the device width of N 1  (i.e., I out /I ref =W N2 /W N1 ), assuming that the device length of the N 1  and N 2  are the same. If the circuit design calls for a larger output current with respect to the reference current, then the current mirror circuit  200  is configured such that the effective device width of N 2  is larger than the effective device width of N 1 . Conversely, if a smaller magnitude of output current is desired with respect to the reference current, then N 1  will have a larger effective device width than N 2 . Regardless of whether I out  is larger or smaller than I ref , an integral multiple relationship between I ref  and I out  is typically used in the design. 
   For example, if it is desired to configure the current mirror  200  such that the value of I out  is K times the value of I ref  (with K being an integer), then the device depicted by N 2  in  FIG. 2  may actually be implemented by K individual NFETs connected in parallel with one another, with each of the K NFETs having the same geometry as NFET N 1 . Such a configuration is commonly configured through “multi-finger” devices, and generally works well so long as N 1  and each of the individual NFETs of N 2  have the same electrical performance. As a practical matter, however, a multi-finger device (such as might be embodied by N 2 ) may demonstrate different electrical performance with respect to the single finger device (N 1 ) as a result of different device stresses during the formation thereof. 
   Accordingly, a HOST pair as set forth in the present disclosure provides a novel way of implementing such a current mirror, because the variation in carrier mobility due to forming like conductivity devices on differently oriented substrate provides another design variable, in addition to device number and/or device size.  FIG. 3(   a ) is a schematic of a NFET type current mirror  300  configured in accordance with an embodiment of the invention. As is illustrated, the N 1  is formed on a portion of a substrate having a (110) orientation, while N 2  is formed on another portion of the substrate having a (100) orientation. Where the NFET pair is constructed with the same device geometry, the ratio of the output current to the reference current is dictated by the ratio of the carrier mobility (electron in this example) of N 2  to the carrier mobility of N 1 ; that is, I out /I ref =μ N2 /μ N1 . 
   Thus, for equal device geometries, the value of I out  will be roughly 2.6 times that of I ref  since N 1  is formed on the (110) orientation Si substrate, which has a lower electron mobility than NFET N 2  formed on the (100) orientation. It will be appreciated that where this specific current offset (due to just the difference in substrate mobilities) is a desired amount, then the need for creating the NFET pair at different geometries or adding additional devices in parallel is obviated. However, it is also contemplated that the HOST technique disclosed herein could also be combined with existing methods of establishing the ratio of I out /I ref , if other ratios are so desired. 
     FIG. 3(   b ) is a PFET embodiment  302  of the current mirror shown in  FIG. 3(   a ), demonstrating the applicability of p-type devices to the HOST technique as well. In this example, I out  is also intended to be greater than I ref . Thus, for this implementation, it is noted that P 2  is formed on the (110) orientation substrate and P 1  is formed on the (100) orientation substrate since the hole mobility is larger for the (110) orientation. 
   Another example of a practical circuit to which the HOST pairs are applicable is a constant transconductance (g m ) circuit employing FET technology. An example of a conventional transconductance circuit  400  is illustrated in the schematic diagram of  FIG. 4 . The transconductance circuit  400  includes a current mirror portion comprising PFETs TP 6  and TP 15  configured for equalizing the currents I 1 , I 2 , flowing therethrough. The gate of PFET TP 7  is also coupled to the gates of TP 6  and TP 15  to provide a mirrored output of I 1  and I 2  at I out . In addition, the current mirror portion provides NFETs TN 6  and TN 7  for adjusting the drain voltage thereof. As a result of the resistor R B , the group of NFETs represented by M 2  on the left side of the transconductance circuit  400  are configured to source the same amount of current as the single NFET M 1  on the right side of the circuit  400 . Where the M 2  devices are of equal channel length with respect to the M 1  device, then effective size of the M 2  channel width is N times the effective channel width of M 1 , with N representing the number of parallel NFETs used for M 2 . The device characteristics are given by:
 
 I   2 =( V   gs1   −V   gs2 )/ R   B  
 
   
     
       
         
           
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   Thus, the transconductance of the circuit will be: 
   
     
       
         
           
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   It can be seen that g m  is a value independent with respect to temperature, supply voltage and MOS device parameters, which is a very desirable feature for the circuit design. 
   As is the case with a current mirror circuit, the present HOST technique can be used in such a constant transconductance circuit. Instead of having N number of NFET devices in parallel on one side of the circuit, the embodiment of  FIG. 5  illustrates a constant g m  circuit  500  built using a single pair of NFET devices. In this instance, the M 1  NFET coupled to R B  is formed on the (100) substrate having the higher carrier mobility, while the M 2  NFET device on the opposite side is formed on the (110) substrate having the lower carrier mobility. A similar circuit analysis reveals that the current I 2  is given by: 
   
     
       
         
           
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   with the transconductance given by 
   
     
       
         
           
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   wherein μ n1  and μ n2  are the mobility of the electrons on the (100) substrate and (110) substrate respectively. As should be appreciated, a PFET version of the constant transconductance circuit using the HOST technique could also be implemented. 
   Still another practical application of a HOST device pair is in conjunction with applications where a precision fixed delay is desired. Since HOST pairs provide a fixed current offset when biased at the same voltage, and since the offset is defined by the mobility of the FET, the repeatability of this offset is excellent. For example, in a D flip-flop, inverter or a buffer, a signal delay therethrough is proportional to the biasing current; thus, a fixed signal delay can be generated when a HOST pair is used in such devices.  FIG. 6(   a ) schematically represents an inverter pair, in which the upper inverter is intended to pass a signal quicker than the lower inverter, thereby generating a fixed delay with respect to Out  1  and Out  2 . A HOST based realization of this inverter pair, in accordance with still a further embodiment of the invention, is shown in  FIG. 6(   b ). As is shown, both the PFET device and NFET device of the first inverter are formed on respective substrates that enhance carrier mobility for quickest response time (i.e., the PFET on (110) silicon and the NFET on (100) silicon). In contrast, the PFET and NFET device of the second inverter are formed on respective substrates that reduce carrier mobility for a slower response time with respect to the first inverter (i.e., the PFET on (100) silicon and the NFET on (110) silicon). 
   Thus configured, an inverter pair according to the present hybrid substrate approach can further be used to create additional circuit functions, such as a pair of ring oscillators with a fixed frequency difference as shown in  FIG. 7 . The oscillators  702 ,  704 , are formed through a chain of individual inverter elements. When used in a jitter test system for example, the ring oscillators are conventionally designed to provide two different frequencies by either biasing the inverters of each ring oscillator at different voltages or by using different device sizes. However, by using HOST devices to build the two ring oscillators  702 ,  704 , the same biasing voltage can be used for each inverter along with FETs of like geometry. Again, the fixed offset is achieved by increasing the carrier mobility of an FET of a given conductivity and decreasing the carrier mobility of the same conductivity device in the other inverter. This architecture significantly improves the matching performance and, since the same biasing voltage is applied, the biasing circuit is much simpler to design. 
   While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.