Abstract:
High performance digital transistors ( 140 ) and analog transistors ( 144 ) are formed at the same time. The digital transistors ( 140 ) include pocket regions ( 134 ) for optimum performance. These pocket regions ( 134 ) are partially or completely suppressed from at least the drain side of the analog transistors ( 144 ) to provide a flat channel doping profile on the drain side. The flat channel doping profile provides high early voltage and higher gain. The suppression is accomplished by using the HVLDD implants for the analog transistors ( 144 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 USC § 119(e)(1) of provisional application numbers 60/138,728 filed Jun. 11, 1999. 
     The following co-pending patent application is related and hereby incorporated by reference: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Serial No. 
                 Filing Date 
                 Inventors 
               
               
                   
                   
               
             
             
               
                   
                 (TI-29254) 
                 06/11/99 
                 Efland et al. 
               
               
                   
                   
               
             
          
         
       
     
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to the field of semiconductor devices and more specifically to fabricating semiconductor devices having both analog and digital transistors. 
     BACKGROUND OF THE INVENTION 
     As digital CMOS technology extended into the deep submicron range (e.g., 0.35 μm and below), a device feature was developed to enable a much shorter channel length. This particular feature is a pocket implant. As shown in FIG. 1, pocket implants are used to provide a pocket region  16  of heavier doping of the same conductivity type as the channel/body  18  of the CMOS transistor  20 . In some approaches, multiple pocket implants are performed. Pocket regions  16  are extend further under the gate than drain extension regions  24 . The drain  12  and source  14  regions are generally of opposite conductivity type with respect to the channel/body  18 . 
     The pocket implant energy and the anneal temperatures and times are designed such that the final dopant distribution serves to increase the channel doping concentration. The increased channel doping concentration reduces the depletion regions arising due to the electrostatic potential differences between source and the substrate and, more importantly, between the drain and the substrate. Such reduction in the depletion regions is known to result in improved short-channel characteristics, in particular, it leads to a reduced drain-induced barrier lowering or DIBL effect. Reduction in DIBL is critical to the ability to scale the MOSFET to smaller channel lengths without a concurrently sharp increase in subthreshold leakage. The key feature that has led to the use of pocket implants to increase the channel doping concentration in preference to the more conventional threshold voltage adjust implants is the fact that pocket implants are self-aligned to the gate. This means that the implant is largely blocked by the gate material so that the increased channel doping density due to the implant occurs only in the vicinity of the source and drain regions. One consequence of this is that the average doping density in the channel is low at long channel lengths and increases as the channel length is reduced. This is a desirable feature because the higher doping helps suppress the undesirable DIBL effect which is worse at smaller channel lengths, while at the longer lengths where such a high doping density is not needed the lower doping density results in higher inversion charge when the transistor gate is biased to turn on and conduct current. Higher inversion charge leads to a correspondingly higher drive current. Thus, over a statistical distribution of channel lengths one is able to obtain a higher drive current while maintaining a low off current. 
     High drive current increases the switching speed of digital logic gates, especially when the gates are required to drive large loads such as a long metal line. Low off current leads to a low standby power for CMOS logic circuits. Thus, MOSFETs designed with pocket implants are very attractive for high performance CMOS digital logic circuits. However, that is not true for many of the CMOS analog circuits. For example, the dc voltage gain of a CMOS differential amplifier is high for high values of gm/gds, where gm is the MOSFET&#39;s small signal transconductance and gds is the MOSFET&#39;s small-signal output conductance. Also, the standby power depends on the dc bias current, where the bias current value is designed for high gm/gds. 
     Unfortunately, it turns out that the high doping density localized near the drain region of MOSFETs, so typical of transistors with pocket implants, also makes it difficult to achieve a low gds. The self-aligned pocket implants naturally lead to a laterally nonuniform channel doping profile with the doping density low at the center and increasing to a pocket of high doping near the drain, as shown in FIG.  2 . Such a profile causes an increasing potential barrier for the charge carriers (electrons for nMOS and holes for PMOS) to flow from the center to the drain. Being coincident with the pocket of high channel doping density in the vicinity of the drain, the magnitude of potential barrier is relatively easily modulated by the voltage Vds applied to the drain electrode. Increasing the magnitude of Vds (Normally, Vds &gt; 0  for nMOS and Vds &lt; 0  for pMOS) lowers the barrier and vice versa. Consequently, the MOSFET output characteristic, which is the drain current Ids vs Vds at a constant gate bias of Vgs, fails to saturate as well as it otherwise should. Thus, generally, pocket implanted MOSFETs exhibit a relatively high slope (high gds) in the saturation region compared to conventional MOSFETs with a laterally uniform channel, as shown in FIG.  3 . Furthermore, since the pocket and the accompanying potential barrier is present at long and short lengths, the rapid improvement (i.e., reduction) in gds with increasing channel length, commonly seen for conventional MOSFETs, is typically absent for MOSFETs with pocket implants. Thus, with pocket implanted devices one does not even have the option of increasing the gate length to achieve the high gm/gds needed for acceptable analog performance. 
     There is a need in modern technologies to be able to build advanced circuitry of both a digital and analog nature on the same integrated circuit. Therefore, there is a need for a method of producing transistors that are more analog friendly along with digital transistors in the same monolithic solution. 
     SUMMARY OF THE INVENTION 
     The invention uses conventional mask steps in a process flow with high and low voltage transistors to provide at least a more laterally uniform channel profile near the drain end of a second type of low voltage transistor (e.g., analog) relative to the corresponding profile in a first type of low voltage transistor (e.g., core digital transistors). Drain extension implants for the high voltage transistors are simultaneously performed in the high voltage transistor regions and the second type of low voltage transistor regions. Thus, the second type of low voltage transistors exhibit output characteristics (drain current I d  vs drain voltage V ds  at constant gate and body potentials) where the drain currents saturate to low values of small signal output conductance g ds  (g ds ≡∂I d /∂V ds ). In contrast, the first type of low voltage digital transistors retains the highly nonuniform pocket profiles and exhibit output characteristics with higher values of gds in the saturation region. 
     These and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional diagram of a prior art MOS transistor with pocket implants; 
     FIG. 2 is a graph of doping concentration across the channel of the MOS transistor with pockets and without pockets; 
     FIG. 3 is a graph of potential barrier for a MOS transistor with and without pockets; 
     FIG. 4 is a graph of barrier height, V ds  for a MOS transistor with pockets; 
     FIG. 5 is a graph of I ds -V ds  and g ds -V ds  characteristics for a MOS transistor with pockets and without pockets; 
     FIG. 6 is a graph of Early Voltage versus gate length for a MOS transistor with and without pockets; 
     FIGS. 7A-7C are cross-sectional diagrams of digital and analog transistors according to the first embodiment of the invention at various stages of fabrication; 
     FIGS. 8A and 8B are top view diagrams of the digital and analog transistors according to the first embodiment of the invention; 
     FIGS. 9A-9B are cross-sectional diagrams of digital and analog transistors according to the second embodiment of the invention at various stages of fabrication; 
     FIGS. 10A and 10B are top view diagrams of the digital and analog transistors according to the second embodiment of the invention; 
     FIGS. 11A-11B are cross-sectional diagrams of digital and analog transistors according to the third embodiment of the invention at various stages of fabrication; 
     FIGS. 12A and 12B are top view diagrams of the digital and analog transistors according to the third embodiment of the invention; 
     FIG. 13 is a cross-sectional diagram of digital and analog transistors according to the fourth embodiment of the invention; 
     FIG. 14 is a top view diagram of the digital and analog transistors according to the fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention is a method for partially or completely suppressing the pocket implant in selective circuit elements with no additional mask steps in a CMOS process with separate masking steps for the drain extension implants in low voltage and high voltage transistors. The invention is particularly applicable to form analog devices with partially or completely suppressed pockets. It will be apparent to those of ordinary skill in the art that the benefits of the invention may be applied to other processes and devices. 
     The conventional pocket regions are undesirable for analog devices because the pocket region gives an output characteristic that is undesirable. The pocket of higher channel doping near the drain region gives rise to a potential barrier that is easily modulated by the drain voltage (V ds ) Consequently, the output conductance in saturation is high and does not reduce with increasing gate length as significantly as it does for MOSFETs without pocket regions. 
     As shown in FIG. 2, the pocket implants for a nMOS device create regions where the dopant concentration increases to a peak  30  at the source and drain ends of the channel. Such nonuniform profiles with regions of high dopant concentration do not occur without pockets. Without pockets, the channel dopant profile is relatively flat in the lateral direction. The peaks  30  result in potential barriers for electrons at both the source and drain ends at V gs =0 and V ds  biased in saturation, as shown in FIG.  3 . The potential barrier  32  at the drain end is not present without pockets. For the pocket devices, the barrier  32  at the drain end persists even for V gs &gt;V T , (V T  is the MOSFET threshold voltage). Because of its proximity to the drain, the height of the barrier  32  is actually modulated with drain voltage as shown in FIG.  4 . This modulation is reflected by a corresponding modulation of I d  in saturation. On the other hand, the barrier at the source end, being further from the drain, is relatively unaffected by V ds . Without pockets, relatively constant saturation I d , i.e., a low g ds , is obtained. 
     In terms of electrical characterization of the device, digital performance is relatively insensitive to the flatness of the I-V characteristic. Rather, pocket implants are preferred for digital logic. Pocket implants reduce sensitivity of the drive currents I drive  and the off currents l off  to statistical variations in gate length, which makes it possible to achieve a much higher drive current at the nominal gate length to enable quick charging. However, for analog functions this slope needs to be very flat as shown in FIG. 5 (without pockets). Horizontal is ideal. If a line is drawn tangential to this saturated characteristic and extrapolated back to the Vds axis, the voltage value at which the axis is crossed is the “Early voltage” (Va). Va is a parameter that characterizes the output resistance, Rd, or the output conductance, gds, of a MOSFET. A large Va means a large Rd or small gds. Obviously, when the saturated characteristic is flat, then the Va is very high. In fact, a perfectly flat curve would relate to zero slope and Va=infinity. Generally, a slope resulting in a Va of 50V to 100V is usual and a good value for building analog circuitry. However, the Va associated with the with pockets curve of FIG. 5 is significantly less for the same saturation current than the 50V to 100V needed for analog functions. FIG. 6 is a graph of Va versus gate length. It shows that the early voltage, Va, is significantly greater without pockets versus with pockets for a given gate length. Thus, a transistor with at least partially suppressed pocket regions is desired for analog transistors. 
     In a conventional CMOS process, the drain extension or lightly doped drain (LDD) and the pocket implants are performed using the same mask for the low voltage transistors. One approach to suppressing the pocket in analog devices is to add a masking level for the pocket implant. The low voltage LDD (LVLDD) mask would be used to implant LVLDD regions in both analog and digital transistors, but the pocket mask would block the analog transistors and only implant in the digital transistors. The problem with this approach is the cost of adding masking levels. 
     The embodiments of the invention are applied to a CMOS process having both low voltage transistors and high voltage transistors. The terms low voltage and high voltage are relative to each other and refer to the operating voltage of the transistors. Typically, core transistors have a lower operating voltage (e.g., 1.8 V) while I/O transistors have a higher operating voltage (e.g., 3.3 V). A semiconductor substrate  100  is processed through gate  126  formation, as shown in FIG.  7 A. This includes the formation of isolation structures, such as STI  106 , threshold adjust implants, low voltage gate dielectric  122 , and a thicker high voltage gate dielectric  120 . FIG. 7A shows three transistor areas: a first low voltage (digital) transistor area  140 , a high voltage (I/O) transistor area  142 , and a second low voltage (analog) transistor area  144 . 
     In the first embodiment, the LVLDD mask is modified to form a LVLDD pattern  102  to expose only the first low voltage transistor areas  140 , as shown in FIGS. 7B (cross-section) and  8 A (top view). The high voltage (typically I/O) transistor areas  142  and the second low voltage (analog) transistor areas  144   90  are masked. The LVLDD mask  102  is then used for both the LDD  132  implant and the pocket  134  implant for the first low voltage transistor areas  140 . The LDD  132  implant uses a dopant of opposite conductivity to the dopant used for the pocket  134  implant. As an example, arsenic may be used for the LDD implant and BF 2  for the pocket implant for an NMOS transistor. The LVLDD mask  102  is then removed. 
     The LDD regions  148  for the high voltage transistors and the second type of low voltage transistors are formed simultaneously. For example, a HVLDD (high voltage lightly doped drain) mask creates a pattern  150  that exposes both the high voltage transistor areas  142  and the second low voltage transistor areas  144 , as shown in FIGS. 7C and 8B. Pattern  150  is then used to form a HVLDD region  148  but not a pocket region. The pocket is completely suppressed for the analog transistors. The HVLDD region  148  may, for example, be formed using arsenic and phosphorous for n-type or boron for p-type. 
     It should be noted that the order for forming core and I/O transistor regions may be reversed from that shown without departing from the invention. 
     The HVLDD implants are designed to form a graded junction to reduce the maximum lateral electric field in order to meet reliability specifications, in particular, channel hot carrier (CHC) lifetime spec. On the other hand, the lower voltage used in the core allows the LVLDD implants to be designed for an abrupt junction. The HVLDD implants may differ from the LVLDD implants in several ways. Two such differences that are relevant for this invention are: (i) the HVLDD profiles extend deeper in both vertical and lateral directions to form graded junctions and (ii) the pocket implants used in the LVLDD implants are not used in conjunction with the HVLDD implants. Thus, the masking scheme of the first embodiment (referred to as the analog symmetric) has the advantage of achieving a much lower gds in the saturation region of operation. 
     In a second embodiment of the invention, the LVLDD mask is modified to form a LVLDD pattern  160  to expose the first low voltage (digital) transistor areas  140  and a source side of the second low voltage (analog) transistor areas  144 , as shown in FIG. 9A and 10A. The high voltage (typically I/O) transistor areas  142  and the drain side of the second low voltage (analog) transistor areas  144  are masked. The LVLDD mask is then used for both the LDD  132  implant and the pocket implant  134  for the low voltage digital transistor areas  140 . The HVLDD (high voltage lightly doped drain) mask creates a pattern  170  that exposes both the high voltage transistor areas  142  and the drain side of the analog transistor areas  144 , as shown in FIG. 9B and 10B. Pattern  170  is then used to form a HVLDD region  148  but not a pocket region. The pocket is completely suppressed form the drain side of the analog transistor areas  144 . However, a pocket  134  is formed on the source side of the analog transistor areas  144 . 
     The source side only pocket (analog asymmetric) has two important advantages. First, similar to the analog symmetric, it achieves improved analog performance (higher Va) over the digital devices by allowing a flat channel profile on the drain side of the transistor as it suppresses the drain side pocket. Second, the source side only pocket suppresses DIBL at shorter channel lengths by supplying a higher doped region on the source side to stand off depletion spreading from the drain end to lower the barrier at the source end. This leads to the analog asymmetric devices to have higher Va compared to the analog symmetric devices at the shorter gate lengths. 
     The source side only pocket for high performance analog CMOS is achievable and compatible with building state of art deep submicron digital CMOS in the same process at the same time on the same silicon substrate using the same wafer fabrication steps. 
     In a third embodiment of the invention the pocket regions for the analog transistors are partially suppressed. The LVLDD mask is used to form a pattern  202  to expose the low voltage digital transistor areas  140  and the low voltage analog transistor areas  144 , as shown in FIGS. 11A and 12A. The high voltage (typically I/O) transistor areas  142  are masked. The LVLDD mask is then used for both the LDD  132  implant and the pocket  134  implant for the low voltage digital transistor areas  140  and the LDD  208  and pocket  206  in the analog transistor areas  144 . The HVLDD (high voltage lightly doped drain) mask creates a pattern  204  that exposes both the high voltage transistor areas  142  and the analog transistor areas  144 , as shown in FIGS.11 B and  12 B. Pattern  204  is then used to form a HVLDD region  148  in the high voltage digital transistor areas and to counterdope the pocket regions  206  in the analog transistor areas  144 . The pocket regions  206  are partially suppressed for the analog transistors. 
     In a fourth embodiment of the invention the pocket regions for the analog transistors are partially suppressed on the drain side only. In this embodiment, the LVLDD mask is not modified from the conventional process. The LVLDD mask is used to form a pattern  202  to expose the low voltage digital transistor areas  140  and the low voltage analog transistor areas  144 , as shown in previous FIGS. 11A and 12A. The high voltage (typically I/O) transistor areas  142  are masked. The LVLDD mask is then used for both the LDD  132  implant and the pocket  134  implant for the low voltage digital transistor areas  140  and the LDDs  208  and pockets  206  in the analog transistor areas  144 . The HVLDD (high voltage lightly doped drain) mask creates a pattern  214  that exposes both the high voltage transistor areas  142  and the drain side of the analog transistor areas  144 , as shown in FIGS. 13 and 14. Pattern  214  is then used to form a HVLDD region  148  in the high voltage digital transistor areas and to counterdope the pocket region  206  creating pocket region  207  in the drain side of the analog transistor areas  144 . The pocket region  207  is partially suppressed for the analog transistors. 
     One advantage of retaining the LVLDD implant on either the drain or source side is to lower the parasitic series resistance. The lower series resistance is a result of the typically higher number of donor type dopants for the nMOS and acceptor type dopants for the pMOS implanted during the LVLDD implants. 
     After forming the LVLDD and HVLDD regions according to any of the above embodiments, conventional processing may be used. For example, sidewall spacers may be formed followed by the n-type and p-type source/drain regions. Fabrication may then continue with a standard metal backend process. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.