Abstract:
An LDMOS transistor includes a gate including a conductive material over an insulator material, a source including a first impurity region and a second impurity region, a third impurity region, and a drain including a fourth impurity region and a fifth impurity region. The first impurity region is of a first type, and the second impurity region is of an opposite second type. The third impurity region extends from the source region under the gate and is of the first type. The fourth impurity region is of the second type, the fifth impurity region is of the second type, and the fourth impurity region impinges the third impurity region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims the benefit of priority under 35 U.S.C. Section 120 of U.S. application Ser. No. 11/488,378, filed Jul. 17, 2006, now issued as U.S. Pat. No. 7,868,378, which claims priority to U.S. Application Ser. No. 60/700,395, filed on Jul. 18, 2005. The disclosure of each prior application is considered part of and is incorporated by reference in the disclosure of this application. 
    
    
     TECHNICAL FIELD 
     The following disclosure relates to semiconductor devices, and more particularly to transistors, such as lateral double-diffused MOSFET (LDMOS) transistors. 
     BACKGROUND 
     Voltage regulators, such as DC to DC converters, are used to provide stable voltage sources for electronic systems. Efficient DC to DC converters are particularly needed for battery management in low power devices, such as laptop notebooks and cellular phones. Switching voltage regulators (or simply “switching regulators”) are known to be an efficient type of DC to DC converter. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency input voltage to generate the output DC voltage. Specifically, the switching regulator includes a switch for alternately coupling and decoupling an input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. A controller, such as a pulse width modulator or a pulse frequency modulator, controls the switch to maintain a substantially constant output DC voltage. 
     LDMOS transistors are commonly used in switching regulators as a result of their performance in terms of a tradeoff between their specific on-resistance (R dson ) and drain-to-source breakdown voltage (BV d     —     s ). Conventional LDMOS transistors are typically fabricated having optimized device performance characteristics through a complex process, such as a Bipolar-CMOS (BiCMOS) process or a Bipolar-CMOS-DMOS (BCD) process, that includes one or more process steps that are not compatible with sub-micron CMOS processes typically used by foundries specializing in production of large volumes of digital CMOS devices (e.g, 0.5 μm DRAM production technologies), as described in greater detail below. As a result, conventional LDMOS transistors are, therefore, not typically fabricated at such foundries. 
     SUMMARY 
     In one aspect, the invention is directed to method of fabricating a transistor having a source, drain, and a gate on a substrate. The method includes implanting a first impurity region, forming a gate insulator between a source region and a drain region of the transistor, covering the gate insulator with a conductive material, and implanting, into the drain region of the transistor, a second impurity region. The first impurity region has a first volume and a first surface area and is of a first type, the gate insulator covers a portion of the first surface area, and the second impurity region has a second volume and a second surface area and is of an opposite second impurity type, the second volume impinging the first volume. 
     In another aspect, the invention is directed to a transistor. The transistor includes a gate including a conductive material over an insulator material, a source including a first impurity region and a second impurity region, a third impurity region, and a drain including a fourth impurity region and a fifth impurity region. The first impurity region is of a first type, and the second impurity region is of an opposite second type. The third impurity region extends from the source region under the gate and is of the first type. The fourth impurity region is of the second type, the fifth impurity region is of the second type, and the fourth impurity region impinges the third impurity region. 
     In another aspect, the invention is directed to a transistor. The transistor includes a gate with a conductive material over an insulator material, a source including a first impurity region and a second impurity region, a third impurity region, a drain including a fourth impurity region and a fifth impurity region, and a resurf impurity region. The first impurity region is of a first type, the second impurity region is of an opposite second type, the third impurity region is of the first type, the fourth impurity region is of the second type, the fifth impurity region is of the second type, and the resurf impurity region is of the first type. The third impurity region extends from the source region under the gate, and the resurf impurity region extends laterally beneath a portion of the fourth impurity region. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic cross-sectional view of an LDMOS transistor. 
         FIG. 1B  is a schematic cross-sectional view of another implementation of an LDMOS transistor. 
         FIG. 2  is a flow diagram of a process for manufacturing an LDMOS transistor. 
         FIGS. 3A-3G  illustrate a process for manufacturing an LDMOS transistor. 
         FIG. 4  is a flow diagram of another implementation of a process for manufacturing an LDMOS transistor. 
         FIG. 5  is a schematic cross-sectional view of another implementation of an LDMOS transistor. 
         FIG. 6  is a schematic cross-sectional view of another implementation of an LDMOS transistor. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1A  shows a schematic cross-sectional view of an LDMOS transistor  100 . This LDMOS transistor  100  can be a switch in a switched-mode power supply voltage regulator operable to convert an input DC voltage into a high frequency voltage. 
     The LDMOS transistor  100  can be fabricated on a high voltage n-type well (HV n-well)  103  implanted in a p-type substrate  102 . An HV n-well implant is typically a deep implant and is generally more lightly doped relative to a CMOS n-well. HV n-well  103  can have a retrograded vertical doping profile. 
     The LDMOS transistor  100  includes a drain region  104 , a source region  106 , and a gate  108 . The gate  108  includes a gate conductor layer  108   b  and a gate oxide  108   a . The gate can also include an oxide spacer formed around the gate conductor layer  108   b  and gate oxide  108   a . The drain region  104  includes an n-doped n+ region  110  and an n-doped drain (NDD)  112 . Although illustrated as spaced from the gate oxide  108   a , the n+ region  110  can be self-aligned to the gate (e.g., so that the edge of the n+ region  120  is aligned with the outer edge of the oxide spacer). The source region  106  includes an n-doped n+ region  114  and a p-doped p+ region  116 . The n+ region  114  of the source  106  can include an N-LDD implanted after creation of the gate oxide but before formation of oxide spacer, and an n+ implanted after formation of the oxide spacer. In one implementation, the n+ region  114  of the source  106  includes an N-LDD but the n+ region  110  of the drain  104  does not include an N-LDD. 
     A p-doped P-body  118 , at least a portion of which can be considered part of the source region  106 , extends beneath the gate  108  and abuts the NDD  112 . A portion of the n+ region  114  can extend partially beneath the gate  108 . The interface between the P-body  118  and the NDD  112  can be aligned with the drain-side edge of the gate  108 . Alternatively, as shown in  FIG. 1B , the interface between the P-body  118  and the NDD  112  can be positioned beneath the gate  108 . In general, placement of the interface at the drain-side edge of the gate can be useful for high-frequency applications, whereas placement of the interface nearer to the source-side edge of the gate  108  can be useful for high-power applications. 
     The HV n-well  103 , the NDD  112 , and the n+ region  110  in drain region  104  are volumes composed of doped material generated by discrete implant steps. Both the NDD  112  and the HV n-well  103  are generated with implant steps which have a lower concentration of impurities than the implant steps which generate the n+ regions  110 ,  114 . Of course, portions at which these volumes overlap have a higher doping concentration than the individual volumes separately. A portion  120  that contains the overlapping volumes of the n+ region  110 , the NDD  112 , and the HV n-well  103  has the highest doping concentration of all the overlapping volume portions. A portion  122  that contains the overlapping volumes of the NDD  112  and the HV n-well  103 , but not the n+ region  110 , has a lower doping concentration than portion  120 . A portion  124  that only includes the HV n-well  103  has a lower doping concentration than either portions  120  or  122  because it does not include multiple overlapping doped volumes. Likewise, the n+ region  114 , the p+ region  116 , and the P-body  118  in source region  106  are volumes ( 126 ,  128 , and  130 , respectively) composed of doped material. 
       FIG. 2  illustrates a process  200  of fabricating a semiconductor device, including an LDMOS transistor. Conventional CMOS transistors can also be fabricated through process  200 . 
     The process  100  begins with forming a substrate (step  202 ). The substrate can be a p type substrate or an n type substrate. Referring to the example of  FIG. 3A , a semiconductor layer consisting of a p-type substrate  102  is formed. As shown in  FIG. 3B , an HV n-well  103  for the LDMOS transistor is implanted into the substrate (step  204 ). In addition, an n-well for a the PMOS transistor with floating operation capability, or NMOS transistor with floating operation capability can be implanted. Optionally, unillustrated CMOS n-wells for conventional PMOS transistors and unillustrated CMOS p-wells for conventional NMOS transistors can be implanted into the substrate (step  206 ). A non self-aligned P-body  118  for the drain region of the LDMOS transistor is implanted (step  208 ). As shown in  FIG. 3C , the P-body  118  is implanted into the HV n well  103 . During step  206 , a P-body can also be implanted for the NMOS transistor with floating operation capability. 
     The gate oxide for each of the LDMOS transistor is formed (step  210 ). The gate oxide for other components, such as the PMOS transistor with floating operation capability, and the NMOS transistor with floating operation capability, and the conventional CMOS transistors can also be formed. The gate oxide for the LDMOS transistor can be formed at the same time as a gate oxide of the conventional CMOS transistors. The LDMOS transistor can, therefore, have a similar threshold voltage and gate oxide thickness and as the conventional CMOS transistors, and can be driven directly by conventional CMOS logic circuits. Alternatively, the gate oxide of the LDMOS transistor can formed at a different time than the gate oxide of the conventional CMOS transistors to allow the LDMOS transistor to be implemented with a dedicated thick gate oxide. When implemented with a thick gate oxide, the LDMOS transistor allows for higher gate drive in applications where a lower voltage power supply may not be readily available. This flexibility allows for optimization of the LDMOS transistor depending on specific requirements of a power delivery application, such as efficiency targets at a particular frequency of operation. 
     Referring to the example of  FIG. 3D , the LDMOS gate oxide  108   a  is formed on a surface  302  of the substrate such that drain-side edge of the gate is aligned with an inner edge  304  of the P-body  118 , or such that the gate overlies the inner edge  304  of the P-body  118 . Exact alignment is not required, as the final position of the interface between the P-body and NDD will be determined by the NDD implant step. A polysilicon layer is deposited over the gate oxide (step  210 ). As shown in  FIG. 3E , a polysilicon layer  108   a  is deposited over the LDMOS gate oxide  108   b . A polysilicon layer can also be deposited over the conventional PMOS and NMOS gates. 
     A shallow drain is implanted and diffused into the drain of the LDMOS transistor (step  114 ). The shallow drain can be implanted after the LDMOS gate is formed so that the shallow drain is self aligned with respect to the LDMOS gate. The shallow drain can be implanted through a LAT implant or a normal angle tilt implant. In the example of  FIG. 3F , the shallow drain is the n-doped drain NDD  112 . 
     The n-doped drain NDD  112  is implanted such that the NDD abuts the P-body  118 . In addition, by controlling the diffusion process, the distance  307  by which the NDD extends under the gate  108  can be controlled. Thus, the position of the interface between the NDD and the P-body can be controlled in an aligned fashion relative to the drain-side edge of the gate  108 . The spacing  307  can be sized such that that the NDD  112  implant extends a predetermined distance under the LDMOS gate. The doping concentration of NDD is can be greater than the P-body so that the NDD implant extends into the P-body to define the channel. 
     The n+ regions and p+ regions of the LDMOS transistor, the PMOS transistor with floating operation capability, and the NMOS transistor with floating operation capability, and the conventional CMOS transistors, are implanted (step  216 ). A p+ region  116  is implanted at the source of the LDMOS transistor. The LDMOS transistor also include an n+ region  110  implanted at the drain and an n+ region  114  implanted at the source. 
     The process  200  provides several potential advantages. First, the P-body of the LDMOS transistor is implanted and diffused prior to formation of the gate oxide of the conventional CMOS transistors. The thermal cycle associated with the P-body implant therefore does not substantially affect the fixed thermal budget associated with sub-micron CMOS process steps (e.g., process step  206 ). Second, the placement of the interface between the P-body and the NDD can be tightly controlled due to the self-alignment of the NDD relative to the gate. 
     Referring to  FIG. 4 , although the process described above forms the gate  108  after the P-body  118  is implanted, it is also possible for the P-body  118  to be implanted after formation of the gate  108 , so that the P-body is self-aligned relative to the gate. In this case, the CMOS gates can be formed after the P-body implant. Alternatively, if the thermal budget permits, the CMOS gates can be formed at the same time as the LDMOS gate. 
     The NDD  112  can be shallower than the P-body  118 . Referring to  FIG. 5 , in another implementation, during the NDD implant step, the NDD  112 ′ is driven beneath the gate  108  and into the P-body  118 ′ such that the P-body has a portion  502  that extends laterally beneath the NDD  112 ′. This portion  502  can provide an implanted resurf region that reduces the peak surface electric field, particularly near the drain-side edge of the gate. 
     Referring to  FIG. 6 , in another implementation, the a p-resurf implant is performed to produce a p-resurf region  602  that extends below NDD  112 . The p-resurf region the can extend over just the source and gate as illustrated, or it can extend across the entire n-well. In addition, the p-resurf region can be spaced from the P-body and NDD, or in contact with one or both of the P-body and NDD. This portion p-resurf region  602  can reduce the peak surface electric field, particularly near the drain-side edge of the gate. 
     Reduction of the peak surface electric field can reduce hot carrier degradation, thus permitting the devices to be scaled smaller while maintaining device lifetime. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.