Abstract:
An asymmetrical N-channel IGFET and an asymmetrical P-channel IGFET are disclosed. One or both IGFETs include a lightly doped drain region, heavily doped source and drain regions, and an ultra-heavily doped source region. Preferably, the heavily doped source region and lightly doped drain region provide channel junctions. Forming a first asymmetrical IGFET includes forming a gate with first and second opposing sidewalls over a first active region, applying a first ion implantation to implant lightly doped source and drain regions into the first active region, applying a second ion implantation to convert substantially all of the lightly doped source region into a heavily doped source region without doping the lightly doped drain region, forming first and second spacers adjacent to the first and second sidewalls, respectively, and applying a third ion implantation to convert a portion of the heavily doped source region outside the first spacer into an ultra-heavily doped source region without doping a portion of the heavily doped source region beneath the first spacer, and to convert a portion of the lightly doped drain region outside the second spacer into a heavily doped drain region without doping a portion of the lightly doped drain region beneath the second spacer. A second asymmetrical IGFET is formed in a related manner. Advantageously, one or both IGFETs have low source-drain series resistance and reduce hot carrier effects.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 08/963,897 filed Nov. 4, 1997 and now U.S. Pat. No. 5,789,787, which is a continuation of application Ser. No. 08/821,123 filed Mar. 20, 1997 and now abandoned, which is a divisional of application Ser. No. 08/711,381 filed Sep. 3, 1996 and now U.S. Pat. No. 5,677,224. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to integrated circuit manufacturing, and more particularly to insulated-gate field-effect transistors. 
     2. Description of Related Art 
     An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel. 
     In typical IGFET processing, the source and drain are formed by introducing dopants of second conductivity type (P or N) into a semiconductor substrate of first conductivity type (N or P) using a patterned gate as a mask. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain. 
     Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide a gate which provides a mask during formation of the source and drain by ion implantation. Thereafter, a drive-in step is applied to repair crystalline damage and to drive-in and activate the implanted dopant. 
     As IGFET dimensions are reduced and the supply voltage remains constant (e.g., 3V), the electric field in the channel near the drain tends to increase. If the electric field becomes strong enough, it can give rise to so-called hot-carrier effects. For instance, hot electrons can overcome the potential energy barrier between the substrate and the gate insulator thereby causing hot carriers to become injected into the gate insulator. Trapped charge in the gate insulator due to injected hot carriers accumulates over time and can lead to a permanent change in the threshold voltage of the device. 
     A number of techniques have been utilized to reduce hot carrier effects. One such technique is a lightly doped drain (LDD). The LDD reduces hot carrier effects by reducing the maximum lateral electric field. The drain is typically formed by two ion implants. A light implant is self-aligned to the gate, and a heavy implant is self-aligned to the gate on which sidewall spacers have been formed. The spacers are typically oxides or nitrides. The purpose of the lighter first dose is to form a lightly doped region of the drain (or LDD) at the edge near the channel. The second heavier dose forms a low resistivity heavily doped region of the drain, which is subsequently merged with the lightly doped region. Since the heavily doped region is farther away from the channel than a conventional drain structure, the depth of the heavily doped region can be made somewhat greater without adversely affecting the device characteristics. The lightly doped region is not necessary for the source (unless bidirectional current is used), however lightly doped regions are typically formed for both the source and drain to avoid additional processing steps. 
     Disadvantages of LDDs include increased fabrication complexity and increased parasitic resistance due to their light doping levels. During operation, LDD parasitic resistance decreases drain current. Linear drain current (i.e., drain current in the linear or triode region) is reduced by the parasitic resistance in both the source and drain. Saturation drain current (i.e., drain current in the saturation region) is largely unaffected by the parasitic resistance of the drain but greatly reduced by the parasitic resistance of the source. Therefore, saturation drain current can be improved while reducing hot carrier effects by providing a lightly doped region only on the drain side. That is, the drain includes lightly and heavily doped regions, and the entire source is heavily doped. 
     Asymmetrical IGFETs (with asymmetrically doped sources and drains) are known in the art. For instance, U.S. Pat. No. 5,424,229 entitled “Method For Manufacturing MOSFET Having An LDD Structure” by Oyamatsu discloses providing a mask with an opening over a substrate, implanting a dopant through the opening at an angle to the substrate to form a lightly doped drain region on one side without a corresponding source region on the other side, forming a gate in the opening which overlaps the lightly doped drain region, removing the mask, and implanting heavily doped source and drain regions using the gate as an implant mask. As another example, U.S. Pat. No. 5,286,664 entitled “Method For Fabricating The LDD-MOSFET” by Horiuchi discloses forming a gate, implanting lightly doped source and drain regions using the gate as an implant mask, forming a photoresist layer that covers the source side and exposes the drain side, depositing a single spacer on the drain side using liquid phase deposition (LPD) of silicon dioxide, stripping the photoresist, and implanting heavily doped source and drain regions using the gate and single spacer as an implant mask. 
     A drawback to these and other conventional asymmetrical IGFETs is that the heavily doped source and drain regions typically have identical dopant concentrations. Although the doping concentration of the heavily doped drain region may be constrained in order to reduce hot carrier effects, the doping concentration of the heavily doped source region need not be constrained in this manner. Furthermore, increasing the doping concentration of the heavily doped source region reduces the source-drain series resistance, thereby improving drive current. 
     Complementary metal-oxide semiconductor (CMOS) circuits typically include adjacent N-channel (NMOS) and P-channel (PMOS) devices. Since CMOS inverter circuits use very little power, CMOS is particularly useful in very large-scale integrated (VLSI) circuits where even small power dissipation in each transistor becomes a problem when thousands or millions of transistors are integrated on a chip. CMOS processes typically use N-well and P-well masks early in the processing sequence to define N-type and P-type active regions. CMOS processes also typically include a single masking step for forming the gates, separate masking steps for implanting lightly doped N-type source/drain regions into the P-type active region and lightly doped P-type source/drain regions into the N-type active region, formation of oxide spacers adjacent to the gates, and then separate masking steps for implanting heavily doped N-type source/drain regions into the P-type active region and heavily doped P-type source/drain regions into the N-type active region. 
     Accordingly, a need exists for improved asymmetrical N-channel and P-channel IGFETs that reduce both source-drain series resistance and hot carrier effects. 
     SUMMARY OF THE INVENTION 
     The present invention provides an asymmetrical N-channel IGFET and an asymmetrical P-channel IGFET. One or both IGFETs include a lightly doped drain region, heavily doped source and drain regions, and an ultra-heavily doped source region. Preferably, the lightly doped drain region and the heavily doped source region provide channel junctions, and the heavily doped drain region and the ultra-heavily doped source region are spaced from the channel junctions. Advantageously, one or both IGFETs have low source-drain series resistance and reduce hot carrier effects. 
     By definition, the dopant concentration of the ultra-heavily doped source region exceeds that of the heavily doped source and drain regions, and the dopant concentration of the heavily doped source and drain regions exceeds that of the lightly doped drain region. Furthermore, the heavily doped source and drain regions need not have similar dopant concentrations. 
     Preferably, both the N-channel and P-channel IGFETs include a source that consists of heavily doped and ultra-heavily doped source regions, and a drain that consists of the lightly doped and heavily doped drain regions. It is also preferred that the dopant concentration of the ultra-heavily doped source regions is in the range of 1.5 to 10 times that of the heavily doped source and drain regions, and the dopant concentration of the heavily doped source and drain regions is in the range of 10 to 100 times that of the lightly doped drain regions, and furthermore that the dopant concentration of the lightly doped drain regions is in the range of about 1×10 17  to 5×10 18  atoms/cm 3 , the dopant concentration of the heavily doped source and drain regions is in the range of about 1×10 19  to 1×10 20  atoms/cm 3 , and the dopant concentration of the ultra-heavily doped source regions is in the range of about 1.5×10 19  to 1×10 21  atoms/cm 3 . 
     In accordance with an aspect of the invention, a method of making asymmetrical N-channel and P-channel IGFETs includes providing a semiconductor substrate with a first active region of first conductivity type and a second active region of second conductivity type adjacent to an isolation region. 
     Forming a first IGFET includes forming a first gate with first and second opposing sidewalls over the first active region, applying a first ion implantation of second conductivity type to implant first lightly doped source and drain regions into the first active region, applying a second ion implantation of second conductivity type to convert substantially all of the first lightly doped source region into a first heavily doped source region without doping the first lightly doped drain region, forming first and second spacers adjacent to the first and second sidewalls, respectively, and applying a third ion implantation of second conductivity type to convert a portion of the first heavily doped source region outside the first spacer into a first ultra-heavily doped source region without doping a portion of the first heavily doped source region beneath the first spacer, and to convert a portion of the first lightly doped drain region outside the second spacer into a first heavily doped drain region without doping a portion of the first lightly doped drain region beneath the second spacer. A first source in the first active region includes the first heavily doped and ultra-heavily doped source regions, and a first drain in the first active region includes the first lightly doped and heavily doped drain regions. 
     Forming a second IGFET includes forming a second gate with third and fourth opposing sidewalls over the second active region, applying a first ion implantation of first conductivity type to implant second light doped source and drain regions into the second active region, forming third and fourth spacers adjacent to the third and fourth sidewalls, respectively, applying a second ion implantation of first conductivity type to convert a portion of the second lightly doped source region outside the third spacer into a second heavily doped source region without doping a portion of the second lightly doped source region beneath the third spacer, and to convert a portion of the second lightly doped drain region outside the fourth spacer into a second heavily doped drain region without doping a portion of the second lightly doped drain region beneath the fourth spacer, removing at least portions of the third and fourth spacers, and applying a third ion implantation of first conductivity type to convert the second heavily doped source region into a second ultra-heavily doped source region and to convert substantially all of the second lightly doped source region into a third heavily doped source region without doping the second lightly and heavily doped drain regions. A second source in the second active region includes the third heavily doped and the second ultra-heavily doped source regions, and a second drain in the second active region includes the second lightly doped and heavily doped drain regions. 
     Preferably, the method includes forming an insulating layer over the substrate to provide first, second, third and fourth sidewall insulators adjacent to the first, second, third and fourth sidewalls, respectively, depositing a blanket layer of insulative spacer material on the insulating layer, and applying an anisotropic etch such that first, second, third and fourth insulative spacers are adjacent to the first, second, third and fourth sidewall insulators, respectively. In this manner, the first spacer includes the first sidewall insulator and the first insulative spacer, the second spacer includes the second sidewall insulator and the second insulative spacer, the third spacer includes the third sidewall insulator and the third insulative spacer, and the fourth spacer includes the fourth sidewall insulator and the fourth insulative spacer. Removing at least portions of the third and fourth spacers is accomplished by removing the insulative spacers without removing the sidewall insulators. 
     Another aspect of the method includes forming the first and second gates, forming a first photoresist layer that covers the second active region, applying the first ion implantation of second conductivity type using the first photoresist layer and the first gate as an implant mask, forming a second photoresist layer that covers the first active region, applying the first ion implantation of first conductivity type using the second photoresist layer and the second gate as an implant mask, forming the insulating layer, forming a third photoresist layer that covers the second active region and the first lightly doped drain region, applying the second ion implantation of second conductivity type using the third photoresist layer and the first sidewall insulator and a portion of the first gate as an implant mask, forming the insulative spacers, forming a fourth photoresist layer that covers the second active region, applying the third ion implantation of second conductivity type using the fourth photoresist layer and the first gate and the first and second spacers as an implant mask, forming a fifth photoresist layer that covers the first active region, applying the second ion implantation of first conductivity type using the fifth photoresist layer and the second gate and the third and fourth spacers as an implant mask, removing the insulative spacers, forming a sixth photoresist layer that covers the first active region and the second lightly and heavily doped drain regions, and applying the third ion implantation of first conductivity type using the sixth photoresist layer and the third sidewall insulator and a portion of the second gate as an implant mask. 
     These and other aspects, features and advantages of the invention will be further described and more readily apparent from a review of the detailed description of the preferred embodiments which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the preferred embodiments can best be understood when read in conjunction with the following drawings, in which: 
     FIGS. 1A-1U show cross-sectional views of successive process steps for making an asymmetrical N-channel IGFET and an asymmetrical P-channel IGFET in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, depicted elements are not necessarily drawn to scale and like or similar elements may be designated by the same reference numeral throughout the several views. 
     In FIG. 1A, silicon substrate  102  suitable for integrated circuit manufacture includes P− type epitaxial layer  104 . Preferably, epitaxial layer  104  is disposed on a P+ base layer (not shown). Substrate  102  also includes P− active region  106  and N− active region  108  adjacent to a planar top surface. Active region  106  has a boron background concentration on the order of 1×10 16  atoms/cm 3 , a &lt;100&gt; orientation and a resistivity of 12 ohm-cm, and active region  108  has an arsenic background concentration on the order of 1×10 16  atoms/cm 3 , a &lt;100&gt; orientation and a resistivity of 12 ohm-cm. Trench oxide  110 , composed of silicon dioxide (SiO 2 ), provides dielectric isolation between active regions  106  and  108 . Gate oxide  112 , composed of silicon dioxide, is formed on the top surface of substrate  102  using tube growth at a temperature of 700 to 1000° C. in an O 2  containing ambient. Gate oxide  112  has a thickness of 50 angstroms. Thereafter, a blanket layer of undoped polysilicon  114  is deposited by low pressure chemical vapor deposition (LPCVD) on the top surface of gate oxide  112 . Polysilicon  114  has a thickness of 2000 angstroms. If desired, polysilicon  114  can be doped in situ as deposition occurs, or doped before a subsequent etch step by implanting arsenic with a dosage in the range of 1×10 15  to 5×10 15  atoms/cm 2  and an energy in the range of 2 to 80 kiloelectron-volts. However, it is generally preferred that polysilicon  114  be doped during an implantation step following a subsequent etch step. 
     In FIG. 1B, photoresist  116  is deposited as a continuous layer on polysilicon  114  and selectively irradiated using a photolithographic system, such as a step and repeat optical projection system, in which I-line ultraviolet light from a mercury-vapor lamp is projected through a first reticle. Thereafter, photoresist  116  is developed and the irradiated portions are removed to provide openings in photoresist  116 . The openings expose portions of polysilicon  114 , thereby defining first and second gates. 
     In FIG. 1C, an anisotropic etch is applied that removes the exposed portions of polysilicon  114  and the underlying portions of gate oxide  112 . Preferably, a first dry etch is applied that is highly selective of polysilicon, and a second dry etch is applied that is highly selective of silicon dioxide, using photoresist  116  as an etch mask. After etching occurs, the remaining portions of polysilicon  114  and gate oxide  112  above active region  106  provide polysilicon gate  120  with opposing vertical sidewalls  122  and  124  on gate oxide  126 , and polysilicon gate  130  with opposing vertical sidewalls  132  and  134  on gate oxide  136 . Polysilicon gate  120  has a length (between sidewalls  122  and  124 ) of 3500 angstroms, and polysilicon gate  130  has a length (between sidewalls  132  and  134 ) of 3500 angstroms. 
     In FIG. 1D, photoresist  116  is stripped, photoresist  138  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and a second reticle, and the irradiated portions are removed to provide an opening in photoresist  138 . The opening is above active region  106 , and photoresist  138  covers active region  108 . 
     In FIG. 1E, lightly doped source and drain regions  140  and  142  are implanted into active region  106  by subjecting the structure to ion implantation of phosphorus, indicated by arrows  144 , at a dose in the range of 1×10 13  to 5×10 14  atoms/cm 2  and an energy in the range of 2 to 35 kiloelectron-volts. Polysilicon gate  120  and photoresist  138  provide an implant mask for the underlying portion of substrate  102 . As a result, lightly doped source and drain regions  140  and  142  are substantially aligned with sidewalls  122  and  124 , respectively. Lightly doped source and drain regions  140  and  142  are doped N− with a phosphorus concentration in the range of about 1×10 17  to 5×10 18  atoms/cm 3 . 
     In FIG. 1F, photoresist  138  is stripped, photoresist  146  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and a third reticle, and the irradiated portions are removed to provide an opening in photoresist  146 . The opening is above active region  108 , and photoresist  146  covers active region  106 . 
     In FIG. 1G, lightly doped source and drain regions  150  and  152  are implanted into active region  108  by subjecting the structure to ion implantation of boron difluoride (BF 2 ), indicated by arrows  154 , at a dose in the range of 1×10 13  to 5×10 14  atoms/cm 2  and an energy in the range of 2 to 35 kiloelectron-volts. Polysilicon gate  130  and photoresist  146  provide an implant mask for the underlying portion of substrate  102 . As a result, lightly doped source and drain regions  150  and  152  are substantially aligned with sidewalls  132  and  134 , respectively. Lightly doped source and drain regions  150  and  152  are doped P− with a boron concentration in the range of about 1×10 17  to 5×10 18  atoms/cm 3 . 
     In FIG. 1H, photoresist  146  is stripped, and oxide layer  160  is deposited over substrate  102  using a low temperature deposition process. Oxide layer  160  has a thickness in the range of 100 to 500 angstroms. Oxide layer  160  includes sidewall oxide  162  adjacent to sidewall  122 , sidewall oxide  164  adjacent to sidewall  124 , sidewall oxide  166  adjacent to sidewall  132 , and sidewall oxide  168  adjacent to sidewall  134 . 
     In FIG. 1I, photoresist  170  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and a fourth reticle, and the irradiated portions are removed to provide an opening in photoresist  170 . The opening is above lightly doped source region  140 , sidewall oxide  162 , and a first portion of polysilicon gate  120  adjacent to sidewall  122 . Photoresist  170  covers lightly doped drain region  142 , sidewall oxide  164 , a second portion of polysilicon gate  120  adjacent to sidewall  124 , and active region  108 . 
     In FIG. 1J, substantially all of lightly doped source region  140  is converted into heavily doped source region  172  by subjecting the structure to ion implantation of arsenic, indicated by arrows  174 , at a dose of 4.5×10 15  atoms/cm 2  and an energy in the range of 10 to 80 kiloelectron-volts. Photoresist  170  and sidewall oxide  162  and the first portion of polysilicon gate  120  (outside photoresist  170 ) provide an implant mask for the underlying portion of substrate  102 . As a result, heavily doped source region  172  is substantially aligned with sidewall oxide  162  on the side opposite polysilicon gate  120 , and lightly doped drain region  142  is essentially unaffected. Heavily doped source region  172  is doped N+ with an arsenic concentration in the range of about 1×10 19  to 1×10 20  atoms/cm 3 . Preferably, the dopant concentration of heavily doped source region  172  is at least 10 times that of lightly doped drain region  142 . As is seen, a very small portion of lightly doped source region  140  remains beneath sidewall oxide  162 . 
     In FIG. 1K, photoresist  170  is stripped, and a tube anneal on the order of 850° C. for 60 minutes is applied to remove crystalline damage and to drive-in and activate the implanted dopants. As a result, heavily doped source region  172  diffuses into and essentially eliminates lightly doped source region  140 . Furthermore, heavily doped source region  172  and lightly doped drain region  142  diffuse slightly beneath sidewalls  122  and  124 , respectively, and lightly doped source region  150  and lightly doped drain region  152  diffuse slightly beneath sidewalls  132  and  134 , respectively. 
     In FIG. 1L, a blanket layer of silicon nitride (Si 3 N 4 ) with a thickness of 2500 angstroms is conformally deposited over the exposed surfaces by plasma enhanced chemical vapor deposition (PECVD) at a temperature in the range of 300 to 800° C. Thereafter, the structure is subjected to an anisotropic etch, such as a reactive ion etch, that is highly selective of silicon nitride with respect to silicon dioxide. The anisotropic etch forms nitride spacers  176 ,  178 ,  180  and  182  adjacent to sidewall oxides  162 ,  164 ,  166  and  168 , respectively. Nitride spacers  176 ,  178 ,  180  and  182  each extend 1200 angstroms across substrate  102 . Moreover, sidewall oxide  162  and nitride spacer  176  collectively form a source-side spacer for the active region  106 , sidewall oxide  164  and nitride spacer  178  collectively form a drain-side spacer for active region  106 , sidewall oxide  166  and nitride spacer  180  collectively form a source-side spacer for active region  108 , and sidewall oxide  168  and nitride spacer  182  collectively form a drain-side spacer for active region  108 . 
     In FIG. 1M, photoresist  184  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and the second reticle, and the irradiated portions are removed to provide an opening in photoresist  184 . The opening is above active region  106 , and photoresist  184  covers active region  108 . 
     In FIG. 1N, a portion of heavily doped source region  172  outside oxide sidewall  162  and nitride spacer  176  is converted into ultra-heavily doped source region  186 , and a portion of lightly doped drain region  142  outside oxide sidewall  164  and nitride spacer  178  is converted into heavily doped drain region  188  by subjecting the structure to ion implantation of arsenic, indicated by arrows  190 , at a dose in the range of 2×10 15  to 3×10 15  atoms/cm 2  and an energy in the range of 20 to 80 kiloelectron-volts. Polysilicon gate  120 , sidewall oxides  162  and  164 , nitride spacers  176  and  178 , and photoresist  184  provide an implant mask for the underlying portion of substrate  102 . As a result, ultra-heavily doped source region  186  is substantially aligned with nitride spacer  176  on the side opposite sidewall oxide  162 , and heavily doped drain region  188  is substantially aligned with nitride spacer  178  on the side opposite sidewall oxide  164 . Furthermore, the portion of heavily doped source region  172  beneath sidewall oxide  162  and nitride spacer  176  and the portion of lightly doped drain region  142  beneath sidewall oxide  164  and nitride spacer  178  are essentially unaffected. Ultra-heavily doped source region  186  is doped N++ with an arsenic concentration in the range of about 1.5×10 19  to 1×10 21  atoms/cm 3 , and heavily doped drain region  188  is doped N+ with an arsenic concentration in the range of about 1×10 19  to 1×10 20  atoms/cm 3 . Preferably, the dopant concentration of ultra-heavily doped source region  186  is at least 1.5 times that of heavily doped source region  172  and heavily doped drain region  188 . 
     In FIG. 1O, photoresist  184  is stripped, and a rapid thermal anneal on the order of 900 to 1050° C. for 10 to 30 seconds is applied to remove crystalline damage and to drive-in and activate the implanted arsenic from the previous two ion implantations. As a result, heavily doped source region  172  and ultra-heavily doped source region  186  merge to form a source, and lightly doped drain region  142  and heavily doped drain region  188  merge to form a drain for an NMOS device controlled by polysilicon gate  120 . Heavily doped source region  172  provides a first channel junction  190  that is substantially aligned with sidewall  122 , and lightly doped drain region  142  provides a second channel junction  192  that is substantially aligned with sidewall  124 . In addition, ultra-heavily doped source region  186  and heavily doped drain region  188  are spaced from channel junctions  190  and  192 . 
     In FIG. 1P, photoresist  194  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and the third reticle, and the irradiated portions are removed to provide an opening in photoresist  194 . The opening is above active region  108 , and photoresist  194  covers active region  106 . 
     In FIG. 1Q, a portion of lightly doped source region  150  outside sidewall  166  and nitride spacer  180  is converted into heavily doped source region  196 , and a portion of lightly doped drain region  152  outside oxide sidewall  168  and nitride spacer  182  is converted into heavily doped drain region  198  by subjecting the structure to ion implantation of boron difluoride, indicated by arrows  200 , at a dose in the range of 2×10 15  to 3×10 15  atoms/cm 2  and an energy in the range of 20 to 80 kiloelectron-volts. Polysilicon gate  130 , sidewall oxides  166  and  168 , nitride spacers  180  and  182 , and photoresist  194  provide an implant mask for the underlying portion of substrate  102 . As a result, heavily doped source region  196  is substantially aligned with nitride spacer  180  on the side opposite sidewall oxide  166 , and heavily doped drain region  198  is substantially aligned with nitride spacer  182  on the side opposite sidewall oxide  168 . Furthermore, the portion of lightly doped source region  150  beneath sidewall oxide  166  and nitride spacer  180  and the portion of lightly doped drain region  152  beneath sidewall oxide  168  and nitride spacer  182  are essentially unaffected. Heavily doped source region  196  is doped P+ with a boron concentration in the range of about 1×10 19  to 1×10 20  atoms/cm 3 , and heavily doped drain region  198  is doped P+ with a boron concentration in the range of about 1×10 19  to 1×10 20  atoms/cm 3 . Preferably, the dopant concentration of heavily doped source and drain regions  196  and  198  is at least 10 times that of lightly doped source and drain regions  150  and  152 . At this point, the doping in active region  108  continues to be symmetrical. 
     In FIG. 1R, photoresist  194  is stripped, and nitride spacers  176 ,  178 ,  180  and  182  are removed by applying a dry etch that is highly selective of silicon nitride with respect to silicon dioxide. Accordingly, oxide layer  160  is substantially unaffected by the etch, and oxide layer  160  provides an etch mask for polysilicon gates  120  and  130  and substrate  102 . 
     In FIG. 1S, photoresist  202  is deposited as a continuous layer over substrate  102  and selectively irradiated using the photolithographic system and a fifth reticle, and the irradiated portions are removed to provide an opening in photoresist  202 . The opening is above heavily doped source region  196 , lightly doped source region  150 , sidewall oxide  166 , and a first portion of polysilicon gate  130  adjacent to sidewall  132 . Photoresist  202  covers lightly doped drain region  152 , heavily doped drain region  198 , sidewall oxide  168 , a second portion of polysilicon gate  130  adjacent to sidewall  134 , and active region  106 . 
     In FIG. 1T, substantially all of the remaining lightly doped source region  150  is converted into heavily doped source region  204 , and heavily doped source region  196  is converted into ultra-heavily doped source region  206  by subjecting the structure to ion implantation of boron difluoride, indicated by arrows  208 , at a dose of 4.5×10 15  atoms/cm and an energy in the range of 10 to 80 kiloelectron-volts. Photoresist  202  and sidewall oxide  166  and the first portion of polysilicon gate  130  (outside photoresist  202 ) provide an implant mask for the underlying portion of substrate  102 . As a result, heavily doped source region  204  is substantially aligned with sidewall oxide  166  on the side opposite polysilicon gate  130 , and lightly and heavily doped drain regions  152  and  198  are essentially unaffected. Heavily doped source region  204  is doped P+ with a boron concentration in the range of about 1×10 19  to 1×10 20  atoms/cm 3 , and ultra-heavily doped source region  206  is doped P++ with a boron concentration in the range of about 1.5×10 19  to 1×10 21  atoms/cm 3 . Preferably, the dopant concentration of ultra-heavily doped source region  206  is at least 1.5 times that of heavily doped source and drain regions  204  and  198 . As is seen, a very small portion of lightly doped source region  150  remains beneath sidewall oxide  166  and polysilicon gate  130 . 
     In FIG. 1U, photoresist  202  is stripped, and a rapid thermal anneal on the order of 900 to 1050° C. for 10 to 30 seconds is applied to remove crystalline damage and to drive-in and activate the implanted boron from the previous two ion implantations. As a result, heavily doped source region  204  diffuses into and essentially eliminates what remains of lightly doped source region  150 . In addition, heavily doped source region  204  and ultra-heavily doped source region  206  merge to form a source, and lightly doped drain region  152  and heavily doped drain region  198  merge to form a drain for an PMOS device controlled by polysilicon gate  130 . Heavily doped source region  204  provides a first channel junction  210  that is substantially aligned with sidewall  132 , and lightly doped drain region  152  provides a second channel junction  212  that is substantially aligned with sidewall  134 . In addition, ultra-heavily doped source region  206  and heavily doped drain region  188  are spaced from channel junctions  210  and  212 . 
     Further processing steps in the fabrication of IGFETs typically include forming salicide contacts on the gates, sources and drains, forming a thick oxide layer over the active regions, forming contact windows in the oxide layer to expose the salicide contacts, forming interconnect metallization in the contact windows, and forming a passivation layer over the interconnect metallization. In addition, earlier or subsequent high-temperature process steps can be used to supplement or replace the desired anneal, activation, and drive-in functions. These further processing steps are conventional and need not be repeated herein. Likewise the principal processing steps disclosed herein may be combined with other steps apparent to those skilled in the art. 
     The present invention includes numerous variations to the embodiment described above. For instance, the gate insulators can remain outside the gates during the ion implantations. The sources may include very small lightly doped source regions adjacent to the channel junctions as long as the lightly doped source regions, if any, are far smaller than the lightly doped drain regions. The nitride spacers can be replaced by other materials, such as polysilicon, that can be selectively etched without removing the sidewall oxides. The spacers may include several layers of sequentially grown or deposited materials, of which only one layer need be subjected to the anisotropic etch. Alternatively, the sidewall insulators (e.g., sidewall oxide  162 ) can be omitted and the spacers can include a single layer of material such as silicon dioxide. The sidewall insulators can be formed at various stages between forming the gates and forming the insulative spacer (e.g., nitride spacer  176 ). For instance, the sidewall insulators can be formed before implanting any of the lightly doped regions, or the sidewall insulators can be formed after implanting the lightly doped regions and the first heavily doped source region (e.g., region  172 ). In these instances, implanting the first heavily doped source region converts the entire first lightly doped source region (e.g., region  140 ) into a heavily doped region. The gates can be various conductors, and the gate insulators can be various dielectrics. The device conductivities can be reversed. Suitable N-type dopants include arsenic, phosphorus and combinations thereof; suitable P-type dopants include boron, boron species (such as boron difluoride) and combinations thereof. 
     Further details regarding asymmetrical IGFETs are disclosed in U.S. application Ser. No. 08/711,383 filed concurrently herewith, entitled “Asymmetrical Transistor With Lightly Doped Drain Region, Heavily Doped Source and Drain Regions, and Ultra-Heavily Doped Source Region” by Gardner et al.; U.S. application Ser. No. 08/711,382 filed concurrently herewith, entitled “Asymmetrical Transistor With Lightly and Heavily Doped Drain Regions and Ultra-Heavily Doped Source Region” by Kadosh et al.; and U.S. application Ser. No. 08/711,957 filed concurrently herewith, entitled “Asymmetrical N-channel and Symmetrical P-Channel Devices” by Gardner et al.; the disclosure of which are incorporated herein by reference. 
     The invention is particularly well-suited for fabricating N-channel MOSFETs, P-channel MOSFETs, and other types of IGFETs, as well as CMOS structures such as inverter circuits, particularly for high-performance microprocessors where high circuit density is essential. Although only a single pair of N-channel and P-channel devices has been shown for purposes of illustration, it is understood that in actual practice, many devices are fabricated on a single semiconductor wafer as widely practiced in the art. Accordingly, the invention is well-suited for use in an integrated circuit chip, as well as an electronic system including a microprocessor, a memory and a system bus. 
     Those skilled in the art will readily implement the steps necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only and can be varied to achieve the desired structure as well as modifications which are within the scope of the invention. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.