Abstract:
Semiconductor structures and methods for suppressing latch-up in bulk CMOS devices. The semiconductor structure comprises a shaped-modified isolation region that is formed in a trench generally between two doped wells of the substrate in which the bulk CMOS devices are fabricated. The shaped-modified isolation region may comprise a widened dielectric-filled portion of the trench, which may optionally include a nearby damage region, or a narrowed dielectric-filled portion of the trench that partitions a damage region between the two doped wells. Latch-up may also be suppressed by providing a lattice-mismatched layer between the trench base and the dielectric filler in the trench.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 11/340,737, filed on Jan. 26, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   The invention relates generally to semiconductor structures and methods and, in particular, to methods for reducing or suppressing latch-up in bulk complementary metal-oxide-semiconductor device structures and semiconductor structures fabricated by these methods. 
   BACKGROUND OF THE INVENTION 
   Complementary metal-oxide-semiconductor (CMOS) technologies integrate P- and N-channel field effect transistors (FETs) to form an integrated circuit on a single semiconductor substrate. Latch-up, which is precipitated by unwanted transistor action of parasitic bipolar transistors inherently present in bulk CMOS devices, may be a significant issue for bulk CMOS technologies. The unwanted parasitic transistor action, which has various triggers, may cause failure of bulk CMOS devices. For space-based applications, latch-up may be induced by the impingement of high energy ionizing radiation and particles (e.g., cosmic rays, neutrons, protons, alpha particles). Because the integrated circuit cannot be easily replaced in space flight systems, the chip failure may prove catastrophic. Hence, designing bulk CMOS devices with a high tolerance to latch-up is an important consideration for circuit operation in the natural space radiation environment, as well as military systems and high reliability commercial applications. 
   Bulk CMOS device designs may be adjusted to increase latch-up immunity. For example, latch-up immunity may be increased in 0.25 micron device technologies by building bulk CMOS devices on epitaxial substrates (e.g., a p-type epitaxial layer on a highly-doped p-type substrate wafer). Highly-doped substrate wafers provide excellent current sinks for currents that, if unabated, may initiate latch-up. However, epitaxial substrates are expensive to produce and may increase the design complexity of several critical circuits, such as electrostatic discharge (ESD) protective devices. 
   Guard ring diffusions represent another conventional approach for suppressing latch-up. However, guard ring diffusions are costly because they occupy a significant amount of active area silicon real estate. In addition, although guard ring diffusions collect a majority of the minority carriers in the substrate, a significant fraction may escape collection underneath the guard ring diffusion. 
   Semiconductor-on-insulator (SOI) substrates are recognized by the semiconductor community as generally free of latch-up. However, CMOS devices are expensive to fabricate on an SOI substrate, as compared to a bulk substrate. Furthermore, SOI substrates suffer from various other radiation-induced failure mechanisms aside from latch-up. Another disadvantage is that SOI devices do not generally come with a suite of ASIC books that would enable simple assembly of low-cost designs. 
   Conventional CMOS devices are susceptible to latch-up generally because of the close proximity of N-channel and P-channel devices. For example, a typical CMOS device fabricated on a p-type substrate includes a P-channel transistor fabricated in an N-well and an N-channel transistor fabricated in a P-well. The opposite conductivity N- and P-wells are separated by only a short distance and adjoin across a well junction. This densely-packed bulk CMOS structure inherently forms a parasitic lateral bipolar (PNP) structure and parasitic vertical bipolar (NPN) structure. Latch-up may occur due to regenerative feedback between these PNP and NPN structures. 
   With reference to  FIG. 1 , a portion of a standard triple-well bulk CMOS structure  30  (i.e., CMOS inverter) includes a P-channel transistor  10  formed in an N-well  12  of a substrate  11 , an N-channel transistor  14  formed in a P-well  16  of the substrate  11  that overlies a buried N-band  18 , and a shallow trench isolation (STI) region  20  separating the N-well  12  from the P-well  16 . Other STI regions  21  are distributed across the substrate  11 . The N-channel transistor  14  includes n-type diffusions representing a source  24  and a drain  25 . The P-channel transistor  10  has p-type diffusions representing a source  27  and a drain  28 . The N-well  12  is electrically coupled by a contact  19  with the standard power supply voltage (V dd ) and the P-well  16  is electrically coupled by a contact  17  to the substrate ground potential. The input of the CMOS structure  30  is connected to a gate  13  of the P-channel transistor  10  and to a gate  15  of the N-channel transistor  14 . The output of CMOS structure  30  is connected to the drain  28  of the P-channel transistor  10  and the drain  25  of the N-channel transistor  14 . The source  27  of the P-channel transistor  10  is connected to V dd  and the source  24  of the N-channel transistor  14  is coupled to ground. Guard ring diffusions  34 ,  36  encircle the CMOS structure  30 . 
   The n-type diffusions constituting the source  24  and drain  25  of the N-channel transistor  14 , the isolated P-well  16 , and the underlying N-band  18  constitute the emitter, base, and collector, respectively, of a vertical parasitic NPN structure  22 . The p-type diffusions constituting the source  27  and drain  28  of the P-channel transistor  10 , the N-well  12 , and the isolated P-well  16  constitute the emitter, base, and collector, respectively, of a lateral parasitic PNP structure  26 . Because the N-band  18  constituting the collector of the NPN structure  22  and the N-well  12  constituting the base of the PNP structure  26  are shared and the P-well  16  constitutes the base of the NPN structure  22  and also the collector of the PNP structure  26 , the parasitic NPN and PNP structures  22 ,  26  are wired to result in a positive feedback configuration. 
   A disturbance, such as impinging ionizing radiation, a voltage overshoot on the source  27  of the P-channel transistor  10 , or a voltage undershoot on the source  24  of the N-channel transistor  14 , may result in the onset of regenerative action. This results in negative differential resistance behavior and, eventually, latch-up of the bulk CMOS structure  30 . In latch-up, an extremely low-impedance path is formed between emitters of the vertical parasitic NPN structure  22  and the lateral parasitic PNP structure  26 , as a result of the bipolar bases being flooded with carriers. The low-impedance state may precipitate catastrophic failure of that portion of the integrated circuit. The latched state may only be exited by removal of, or drastic lowering of, the power supply voltage below the holding voltage. Unfortunately, irreversible damage to the integrated circuit may occur almost instantaneously with the onset of the disturbance so that any reaction to exit the latched state is belated. 
   What is needed, therefore, is a semiconductor structure and fabrication method for modifying standard bulk CMOS device designs that suppresses latch-up, while being cost effective to integrate into the process flow, and that overcomes the disadvantages of conventional bulk CMOS semiconductor structures and methods of manufacturing such bulk CMOS semiconductor structures. 
   SUMMARY OF THE INVENTION 
   The present invention is generally directed to semiconductor structures and methods that improve latch-up immunity or suppression in standard bulk CMOS device designs, while retaining cost effectiveness for integration into the process flow forming the P-channel and N-channel field effect transistors characteristic of bulk CMOS devices. In accordance with an embodiment of the present invention, a semiconductor structure comprises a substrate of a semiconductor material and first and second doped wells formed in the semiconductor material of the substrate. The second doped well is disposed adjacent to the first doped well. A dielectric-filled trench is defined in the substrate between the first and second doped wells. The trench has a base, first sidewalls intersecting a top surface of the substrate, and second sidewalls disposed between the base and the first sidewalls. The second sidewalls have a wider separation than the first sidewalls. 
   In accordance with another embodiment of the present invention, a semiconductor structure comprises a substrate of a semiconductor material and first and second doped wells formed in the semiconductor material of the substrate. The second doped well is disposed adjacent to the first doped well along a well junction. A dielectric-filled trench is defined in the substrate between the first and second doped wells. The trench includes a base, first sidewalls intersecting a top surface of the substrate, and second sidewalls between the base and the first sidewalls. The second sidewalls have a narrower separation than the first sidewalls. The semiconductor material of the substrate bordering the second sidewalls includes a damage region comprising non-monocrystalline semiconductor material. The base of the trench is at a greater depth than the damage region for interrupting the continuity of the non-monocrystalline semiconductor material across the well junction. 
   In accordance with another embodiment of the present invention, a semiconductor structure comprises a substrate of a first material characterized by semiconducting properties, first and second doped wells formed in the substrate, a trench defined in the substrate between the first and second doped wells, and a dielectric material filler in the trench. The second doped well is disposed adjacent to the first doped well. The trench includes a base and sidewalls intersecting a top surface of the substrate. A layer of a second material is disposed between the first material at the base of the trench and the dielectric material filler. The first and second materials have a crystal lattice constant difference sufficient to increase carrier recombination velocity. 
   In another embodiment of the present invention, a method is provided for fabricating a semiconductor structure in a substrate of semiconductor material. The method comprises forming a trench in the semiconductor material with a first sidewall and a second sidewall each disposed between a base of the trench and a top surface of the substrate. The method further comprises forming an oxygen-enriched region in the semiconductor material of the substrate bounding the first sidewall of the trench near the base and converting the oxygen-enriched region to an oxide region. 
   In yet another embodiment of the present invention, a method is provided for fabricating a semiconductor structure in a substrate of semiconductor material. The method comprises forming a trench in the semiconductor material with first sidewalls extending from a base toward a top surface of the substrate and forming a damage region comprising non-monocrystalline semiconductor material at a first depth in the substrate below the base of the first trench. The method further comprises forming a second trench registered with the first trench and having second sidewalls between the base of the first trench to a second depth greater than the first depth. The second trench partitions the damage region such that the non-monocrystalline semiconductor material is discontinuous. 
   In yet another embodiment of the present invention, a method is provided for fabricating a semiconductor structure in a substrate of semiconductor material. The method comprises forming a trench in the semiconductor material with sidewalls extending from a base toward a top surface of the substrate and forming an etch mask on the sidewalls. The method further comprises etching the trench to increase a depth of the base from the top surface using an isotropic etchant that removes the semiconductor material of the substrate bordering the trench below the etch mask to widen the sidewalls of the trench below the etch mask. 
   In another embodiment of the present invention, a method is provided for fabricating a semiconductor structure in a substrate of a first material characterized by semiconducting properties. The method comprises forming a trench in the first material with sidewalls between a base and a top surface of the substrate and forming a layer of a second material on the base of the trench that has a crystal lattice constant difference in comparison with the first material sufficient to increase carrier recombination velocity in the first material adjacent to the base. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a diagrammatic view of a portion of a substrate with a bulk CMOS device constructed in accordance with the prior art. 
       FIGS. 2-5  are diagrammatic views of a portion of a substrate at various fabrication stages of a processing method in accordance with an embodiment of the present invention 
       FIG. 5A  is a top view of the substrate portion at the fabrication stage of  FIG. 5 . 
       FIG. 6  is a diagrammatic view of the portion of the substrate at a fabrication stage subsequent to the fabrication stage of  FIG. 5 . 
       FIGS. 7-12  are diagrammatic views of a portion of a substrate at various fabrication stages of a processing method in accordance with an alternative embodiment of the present invention. 
       FIGS. 13-15  are diagrammatic views of a portion of a substrate at various fabrication stages of a processing method in accordance with an alternative embodiment of the present invention. 
       FIG. 16  is a diagrammatic view similar to  FIG. 14  depicting a portion of a substrate at a fabrication stage of a processing method in accordance with an alternative embodiment of the present invention. 
       FIG. 17  is a diagrammatic view similar to  FIG. 13  depicting a portion of a substrate at a fabrication stage of a processing method in accordance with an alternative embodiment of the present invention. 
       FIG. 18  is a diagrammatic view similar to  FIG. 3  depicting a portion of a substrate at a fabrication stage of a processing method in accordance with an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention provides an isolation region that limits the effect of the vertical parasitic NPN structure and the lateral parasitic PNP structure responsible for latch-up in triple-well bulk CMOS devices. The invention is advantageously implemented in the context of bulk CMOS devices where pairs of N-channel and P-channel field effect transistors are formed adjacent to each other in a P-well within an N-band and an N-well, respectively, and the P-well is isolated from the N-well by a shallow trench isolation (STI) region. Specifically, the latchup immunity of a standard bulk CMOS triple well structure is improved by modifying the geometry of the STI region or by selectively adding damage regions to the STI region in a manner that significantly reduces the susceptibility to latch-up. The present invention will now be described in greater detail by referring to the drawings that accompany the present application. 
   With reference to  FIG. 2 , a bulk substrate  40  of a monocrystalline semiconductor material is obtained. Substrate  40  may include a low-defect epitaxial layer for device fabrication that is grown by an epitaxial growth process, such as chemical vapor deposition (CVD) using a silicon source gas (e.g., silane). Substrate  40  may be a single crystal silicon wafer containing a relatively light concentration of a dopant providing p-type conductivity. For example, substrate  40  may be lightly doped with 5×10 15  cm −3  to 1×10 17  cm −3  of a p-type dopant, such as boron, by in situ doping during deposition of the epitaxial layer. 
   A pad structure  42  is formed on a top surface  41  of the substrate  40 . The pad structure  42  includes a first pad layer  44  and a thinner second pad layer  46  separating the first pad layer  44  from the substrate  40 . The constituent material(s) of pad layers  44 ,  46  advantageously etch selectively to the semiconductor material constituting substrate  40 . The first pad layer  44  may be a conformal layer of nitride (Si 3 N 4 ) formed by a thermal CVD process like low pressure chemical vapor deposition (LPCVD) or a plasma-assisted CVD process. The second pad layer  46  may be silicon oxide (SiO 2 ) grown by exposing substrate  40  to either a dry oxygen ambient or steam in a heated environment or deposited by a thermal CVD process. The second pad layer  46  may operate as a buffer layer to prevent any stresses in the material constituting the first pad layer  44  from causing dislocations in the semiconductor material of substrate  40 . 
   Shallow trenches  48  are formed in the semiconductor material of substrate  40  by a conventional lithography and subtractive etching process that utilizes a shallow trench pattern imparted in the pad structure  42  ( FIG. 2 ) or, optionally, in a patterned hard mask (not shown) formed on pad structure  42 . The shallow trench pattern may be created in pad structure  42  by applying a photoresist (not shown) on pad layer  44 , exposing the photoresist to a pattern of radiation to create a latent shallow trench pattern in the photoresist, and developing the latent shallow trench pattern in the exposed photoresist. An anisotropic dry etching process, such as reactive-ion etching (RIE) or plasma etching, may then be used to transfer the trench pattern from the patterned resist into the pad layers  44 ,  46 . The etching process, which may be conducted in a single etching step or multiple etching steps with different etch chemistries, removes portions of the pad structure  42  visible through the trench pattern in the patterned resist and stops vertically on the substrate  40 . After etching is concluded, residual resist is stripped from the pad structure  42  by, for example, plasma ashing or a chemical stripper. 
   The shallow trench pattern is then transferred from the patterned pad layer  44  into the underlying substrate  40  with an anisotropic dry etching process. The anisotropic dry etching process may be constituted by, for example, RIE, ion beam etching, or plasma etching using an etch chemistry (e.g., a standard silicon RIE process) that removes the constituent semiconductor material of substrate  40  selective to the materials constituting the pad layers  44 ,  46 . Each of the shallow trenches  48  defined in the semiconductor material of substrate  40  includes opposite sidewalls  50 ,  52 , which are substantially mutually parallel and oriented substantially perpendicular to the top surface  41  of substrate  40 , that extend into the substrate  40  to a bottom surface or base  54 . 
   Energetic ions, as indicated diagrammatically by singled-headed arrows  56 , are introduced by an ion implantation process into the substrate  40  to create an oxygen-enriched or oxygen implanted region  58  proximate to and just beneath the base  54  of each shallow trench  48 . The energetic ions  56 , which are generated from a source gas, are directed to impinge the top surface  41  of the substrate  40  at normal or near-normal incidence, although the invention is not so limited. The ions  56  may be implanted with the substrate  40  at or near room or ambient temperature, although the present invention is not so limited. 
   The ions  56  lose energy via scattering events with atoms and electrons in the semiconductor material constituting substrate  40  as the ions  56  penetrate the substrate  40 . The ions  56  eventually dissipate all of their initial kinetic energy and stop in the substrate  40  to produce the oxygen implanted regions  58 . The stopped ions  56  in the oxygen implanted regions  58  are characterized by a depth profile distributed about a projected range, which is measured as a perpendicular distance of the damage peak from the top surface  41 . The depth profile is characterized by a range straggle, which represents a deviation or second moment of the stopped ions  56  about the projected range. Essentially all of the implanted ions  56  come to rest in the semiconductor material of substrate  40  within a distance of three times the range straggle from the projected range. The implanted ions  56  also have a lateral straggle that causes side edges  60 ,  62  of the oxygen implanted regions  58  to extend beyond the sidewalls  50 ,  52  of each shallow trench  48 . 
   The ions  56  may originate from a source selected to provide, when ionized and accelerated to impart kinetic energy, oxygen ions. The implanted species may be either charged atomic oxygen ions (O + ) or molecular ions (O +2 ). Advantageously, the peak atomic concentration for the implanted ions  56  in the oxygen implanted regions  58  may be in the range of 5×10 19  cm −3  to 5×10 21  cm −3  and, in certain embodiments, may be as low as 5×10 18  cm −3  to provide the requisite oxygen concentration. For example, a suitable dose of implanted O +  may range from 1×10 14  cm −2  to 5×10 16  cm −2  at a kinetic energy between about 10 keV and about 50 keV, although the invention is not so limited. The present invention contemplates other implant conditions, i.e., energy and dose, may be used that are capable of forming oxygen implanted regions  58  in substrate  40 . The ions  56  are implanted across the top surface  41  of the entire substrate  40 , although certain regions of substrate  40  may be optionally protected by a block mask during implantation. Ions of an oxidation rate enhancing atomic species, such as germanium (Ge), silicon (Si), or arsenic (As) for n-well applications, or boron difluoride (BF 2 ) for p-well applications, may be co-implanted with ions  56 . A block mask (not shown) of, for example, photoresist may protect a portion of the substrate  40  during the ion implantation process. 
   With reference to  FIG. 3  in which like reference numerals refer to like features in  FIG. 2  and at a subsequent fabrication stage, spacers  64 ,  66  are formed on the sidewalls  50 ,  52  of each shallow trench  48 . Spacers  64 ,  66  may be defined from a conformal layer (not shown) of a dielectric material, such as 5 nm to 15 nm of nitride deposited by a CVD process, that is anisotropically etched using a reactive ion etch (RIE) or plasma etching process. The etching process removes the material of the conformal layer (not shown) primarily from horizontal surfaces selective to (i.e., with a significantly greater etch rate than) the constituent semiconductor material of substrate  40 . The base  54  of each shallow trench  48  is exposed after the spacers  64 ,  66  are formed. 
   With reference to  FIG. 4  in which like reference numerals refer to like features in  FIG. 3  and at a subsequent fabrication stage, a majority of each of the oxygen implanted regions  58  ( FIG. 3 ) is converted by a thermal oxidation process to one of a plurality of oxide regions  68  each consisting of an oxide (e.g., silicon dioxide). The thermal oxidation process may be performed in a dry or wet oxidizing ambient atmosphere and at a temperature ranging from about 750° C. to about 1100° C. The oxidizing species from the oxidizing ambient atmosphere penetrates the substrate  40  through the exposed base  54  of each shallow trench  48  in order to reach the oxygen implanted regions  58 . Spacers  64 ,  66  protect the sidewalls  50 ,  52  of each shallow trench  48  against unwanted oxidation. The implanted oxygen in the oxygen implanted regions  58  ( FIG. 3 ) enhances the oxidation rate for the corresponding semiconductor material of substrate  40  when exposed to the oxidizing species from the oxidizing ambient atmosphere. 
   The perimeter of each oxide region  68  defines a curved boundary  69  that extends laterally or horizontally of the sidewalls  50 ,  52  of each shallow trench  48  because of the lateral oxide growth during the thermal oxidation process. The lateral extent of the concave boundary  69  of each oxide region  68  roughly coincides with the side edges  60 ,  62  of the oxygen implanted regions  58  or may be slightly narrower than the side edges  60 ,  62 . Each oxide region  68  defines a degree of undercut relative to the sidewalls  50 ,  52  and the degree of undercut increases with increasing energy of the implanted ions  56 . The lateral oxide growth defines lateral extensions  73 ,  75  of STI regions  74  ( FIG. 5 ), effectively defining sidewalls  70 ,  72  that widen each shallow trench  48  relative to sidewalls  50 ,  52 , and increases the depth of base  54  relative to the top surface  41 . The base  54  of each shallow trench  48  is effectively deepened to a greater depth by the oxidation process forming oxide regions  68  as the remaining open volume in each shallow trench  48  is filled with dielectric material in a subsequent fabrication stage. 
   A portion of each oxygen implanted region  58  ( FIG. 3 ) may remain, after the thermal oxidation process, as a damage region  71  proximate to the curved boundary  69  of the corresponding oxide region  68 . The stopping of the ions  56  ( FIG. 2 ) implanted in substrate  40  damages the constituent semiconductor material to form non-monocrystalline semiconductor material confined within damage region  71 . Energy transferred by nuclear collisions between ions  56  and target atoms in the substrate  40  displaces those target atoms from their original lattice sites and, as a consequence, permanently damages the semiconductor material of the substrate  40 . When each individual ion  56  displaces a target atom of the substrate  40  in a nuclear collision, a recoil cascade is initiated that dissipates the transferred kinetic energy by collisions with other target atoms. The recoil cascade generates additional vacancies and interstitial atoms in the lattice structure of substrate  40  dispersed among the atoms in the crystalline lattice structure remaining on regular lattice sites. The damage in the damage region  71  may comprise extended crystal lattice defects that are larger than point defects and disrupt long range order, or may render the crystalline structure amorphous. The crystalline damage in the damage region  71  coincides approximately with the depth profile of the stopped ions  56  and is stable in that the damage region  71  remains after subsequent fabrication steps. 
   With reference to  FIGS. 5 and 5A  in which like reference numerals refer to like features in  FIG. 4  and at a subsequent fabrication stage, the shallow trenches  48  are filled with amounts of an insulating or dielectric material, such as a high-density plasma (HDP) oxide or tetraethylorthosilicate (TEOS), deposited across the pad layer  44  and planarized by, for example, a CMP process. An optional high temperature process step may be used to densify the TEOS fill. The pad structure  42  is removed by a planarization process to define shallow trench isolation (STI) regions  74  in the substrate  40  having a top surface substantially co-planar or flush with the top surface  41  of substrate  40 . 
   With reference to  FIG. 6  in which like reference numerals refer to like features in  FIG. 5  and at a subsequent fabrication stage, standard bulk CMOS processing follows, which includes formation of a triple-well structure consisting of an N-well  76 , a P-well  78 , and a deep buried N-well or N-band  80  in the substrate  40 . The buried N-band  80  supplies electrical isolation for the P-well  78 . This triple-well construction permits the optimization of bias potentials for both N- and P-wells  76 ,  78 . The P-well  78  is arranged between the N-band  80  and the top surface  41 . 
   The N-well  76 , as well as other N-wells (not shown) dispersed across the substrate  40 , are likewise formed by patterning a mask layer (not shown) applied on the top surface  41  with techniques known in the art, and implanting an appropriate n-conductivity type impurity into the substrate  40  in unmasked regions. The N-band  80 , as well as other N-bands (not shown) dispersed across the substrate  40 , are formed by patterning another mask layer (not shown), such as a photoresist, applied on top surface  41  and implanting an appropriate n-conductivity type impurity into the substrate  40  in this set of unmasked regions. The P-well  78 , as well as other P-wells (not shown) dispersed across the substrate  40 , are likewise formed by patterning another mask layer (not shown) applied on top surface  41  and implanting an appropriate p-conductivity type impurity into the substrate  40  in this set of unmasked regions. Typically, the P-well  78  is formed by counterdoping the N-band  80  and has an opposite conductivity type from the N-well  76  and N-band  80 . Generally, the dopant concentration in the N-well  76  ranges from about 5.0×10 17  cm −3  to about 7.0×10 18  cm −3 , the dopant concentration in the P-well  78  ranges from about 5.0×10 17  cm −3  to about 7.0×10 18  cm −3 , and the dopant concentration in the N-band  80  ranges from about 5.0×10 17  cm −3  to about 7.0×10 18  cm −3 . A thermal anneal may be required to electrically activate the implanted impurities operating as the p-type and n-type dopants. 
   An N-channel transistor  82  is built using the P-well  78 , and a P-channel transistor  84  is built using the N-well  78  to define a bulk CMOS device. The N-channel transistor  82  includes n-type diffusions in the semiconductor material of substrate  40  representing a source region  86  and a drain region  88  that flank opposite sides of a channel region in the semiconductor material of substrate  40 , a gate electrode  90  overlying the channel region, and a gate dielectric  92  electrically isolating the gate electrode  90  from the substrate  40 . The P-channel transistor  84  includes p-type diffusions in the semiconductor material of substrate  40  representing a source region  94  and a drain region  96  that flank opposite sides of a channel region in the semiconductor material of substrate  40 , a gate electrode  98  overlying the channel region, and a gate dielectric  100  electrically isolating the gate electrode  98  from the substrate  40 . Other structures, such as sidewall spacers (not shown), may be included in the construction of the N-channel transistor  82  and the P-channel transistor  84 . 
   The conductor used to form the gate electrodes  90 ,  98  may be, for example, polysilicon, silicide, metal, or any other appropriate material deposited by a CVD process, etc. The source and drain regions  86 ,  88  and the source and drain regions  94 ,  96  may be formed in the semiconductor material of substrate  40  by ion implantation of suitable dopant species having an appropriate conductivity type. The gate dielectrics  92 ,  100  may comprise any suitable dielectric or insulating material like silicon dioxide, silicon oxynitride, a high-k dielectric, or combinations of these dielectrics. The dielectric material constituting dielectrics  92 ,  100  may be between about 1 nm and about 10 nm thick, and may be formed by thermal reaction of the semiconductor material of the substrate  40  with a reactant, a CVD process, a physical vapor deposition (PVD) technique, or a combination thereof. 
   Processing continues to complete the semiconductor structure, including forming electrical contacts to the gate electrodes  90 ,  98 , source region  86 , drain region  88 , source region  94 , and drain region  96 . The contacts may be formed using any suitable technique, such as a damascene process in which an insulator is deposited and patterned to open vias, and then the vias are filled with a suitable conductive material, as understood by a person having ordinary skill in the art. The N-channel and P-channel transistors  82 ,  84  are coupled using the contacts with other devices on substrate  40  and peripheral devices with a multilevel interconnect structure consisting of conductive wiring and interlevel dielectrics (not shown). A contact  102  is also formed in substrate  40  that is electrically coupled with the N-well  76  for supplying the standard power supply voltage (V dd ) to the N-well  76 . Another contact  104  is formed in substrate  40  for coupling the P-well  78  with the substrate ground potential. 
   In accordance with the principles of the invention, the lateral extensions  73 ,  75  of the bottom portion of the STI regions  74  increase the base width or P-well path for the parasitic NPN structure  22  ( FIG. 1 ) and the base width or N-well path for the PNP structure  26  ( FIG. 1 ). As a consequence, holes traversing the N-well  76  to the P-well  78 , which constitutes the collector of the PNP structure  26 , and electrons traversing the P-well  78  to the N-well  76 , which constitutes the collector of the NPN structure  22 , must flow around the lateral extensions  73 ,  75 , which defines an inverted-T structure. A bump in the electric potential forms at the lateral extensions  73 ,  75 , due to the concavity of the silicon surface about curved boundary  69  and bounding the side edges  70 ,  72  of the lateral extensions  73 ,  75 . This potential bump impedes the flow of minority carriers and results in reduced beta for both parasitic NPN and PNP structures  22 ,  26 . The damage regions  71  in the semiconductor material of the substrate  40 , if present, are believed to reduce the minority carrier lifetimes and to contribute to the reduction of the bipolar gain of the parasitic NPN and PNP structures  22 ,  26 . 
   With reference to  FIG. 7  in which like reference numerals refer to like features in  FIG. 2  and in accordance with an alternative embodiment of the present invention that does not rely on an ion implantation process, the anisotropic dry etching process transferring the trenches  48  from the patterned pad layer  44  into the underlying substrate  40  is halted at an intermediate base  106  that is shallower than base  54  ( FIG. 2 ). A conformal layer  108  of a dielectric material, such as 5 nm to 15 nm of silicon nitride deposited by a CVD process, is formed on the pad layer  44  and the sidewalls  50 ,  52  and intermediate base  106  of trenches  48 . 
   With reference to  FIG. 8  in which like reference numerals refer to like features in  FIG. 7  and at a subsequent fabrication stage, the conformal layer  108  is anisotropically etched using, for example, an RIE or plasma etching process that removes the material constituting the conformal layer primarily from horizontal surfaces selective to (i.e., with a significantly greater etch rate than) the constituent semiconductor material of substrate  40 . Un-removed portions of the conformal layer  108  define spacers  110 ,  112  on the sidewalls  50 ,  52  of each shallow trench  48 . 
   Using the pad structure  42  and spacers  110 ,  112  as a mask, an anisotropic etching process is used to deepen the shallow trenches  48 , which defines base  54 . Respective surfaces  50   a ,  52   a  of the semiconductor material of substrate  40  are exposed between base  54  and the spacers  110 ,  112 , as is the surface along base  54 . The depth difference between base  54  and intermediate base  106 , which is determined based upon depths measured as a perpendicular distance relative to surface  41 , may be, for example, about 0.1 μm. The depth difference also defines the vertical height of the surfaces  50   a ,  52   a  across which the semiconductor material of substrate  40  borders the shallow trench  48  and, hence, is unmasked by spacers  110 ,  112 . The absolute depths to which the shallow trenches  48  are etched may vary according to device design. 
   With reference to  FIG. 9  in which like reference numerals refer to like features in  FIG. 8  and at a subsequent fabrication stage, an isotropic etching process is used to etch the semiconductor material of substrate  40  bordering shallow trenches  48  exposed across the surfaces  50   a ,  52   a  and base  54 . The isotropic etching process, which may be conducted in a single etching step or multiple steps with different etch chemistries, selectively removes the semiconductor material of substrate  40  vertically to slightly deepen base  54 . The isotropic etching process also removes the semiconductor material of substrate  40  laterally across the surfaces  50   a ,  52   a  to define sidewalls  114 ,  116  that have a wider separation than sidewalls  50 ,  52 . During the isotropic etching process, the spacers  110 ,  112  mask and protect sidewalls  50 ,  52  above surfaces  50   a ,  52   a  against removal. For example, the etching process may rely on an isotropic silicon etchant such as a wet or dry hydrofluoric acid etchant. 
   With reference to  FIG. 10  in which like reference numerals refer to like features in  FIG. 9  and at a subsequent fabrication stage, spacers  110 ,  112  are stripped from the sidewalls  50 ,  52  of each shallow trench  48  using an appropriate etching process. A liner  118  is formed on the sidewalls  50 ,  52 , sidewalls  114 ,  116 , and base  54 , as well as on the pad layer  44 . The liner  118  may be, for example, silicon oxide grown by exposing the unmasked semiconductor material of substrate  40  to either a dry oxygen ambient or steam in a heated environment. Another optional liner (not shown) of, for example, silicon nitride may be applied as a diffusion barrier to prevent impurity migration from the trench fill material into the semiconductor material of substrate  40  bordering the shallow trenches  48 . The liner  118  also operates to repair any etch damage incurred by the sidewalls  50 ,  52 , sidewalls  114 ,  116 , and base  54  of each shallow trench  48 . 
   With reference to  FIG. 11  in which like reference numerals refer to like features in  FIG. 10  and at a subsequent fabrication stage, the shallow trenches  48  are filled with an insulating or dielectric material, such as HDP oxide or TEOS, deposited across the pad layer  44  and planarized by, for example, a CMP process. The pad structure  42  and excess liner  118  on the pad layer  44  are removed and planarized to define the STI regions  74  in the substrate  40  by a planarization process that makes the top surface of the STI regions  74  substantially co-planar or flush with the top surface  41  of substrate  40 . Portions of the dielectric material fill the concavities bounded by sidewalls  114 ,  116  to form the lateral extensions  73 ,  75  at the bottom of the STI regions  74 . 
   With reference to  FIG. 12  in which like reference numerals refer to like features in  FIG. 11  and at a subsequent fabrication stage, standard bulk CMOS processing follows as described above with regard to  FIG. 6  to form the N- and P-wells  76 ,  78 , the N-band  80 , the N-channel transistor  82 , the P-channel transistor  84 , and contacts  102 ,  104  in the substrate  40 . A person having ordinary skill in the art will appreciate that this embodiment of the present invention may be advantageously implemented in a dual-well CMOS structure that lacks the N-band  80 . 
   With reference to  FIG. 13  in which like reference numerals refer to like features in  FIG. 2  and in accordance with an alternative embodiment of the present invention that minimizes damage across a well junction  142  ( FIG. 15 ) between the subsequently-formed N- and P-wells  138 ,  140  ( FIG. 15 ) in a dual-well structure, a crystal damaging species is ion implanted into the base  54  of the shallow trenches  48  before the STI regions  74  ( FIG. 15 ) are defined by filling the shallow trenches  48  with dielectric material. Before implantation, the shallow trenches  48  are lined with a liner  121  consisting of one or more individual layers (not shown) of suitable materials, such as 1 nm to 3 nm of thermally grown silicon oxide covered by 4 nm to 20 nm of silicon nitride deposited conformally by a CVD process. 
   Local crystalline damage regions  124 , which include semiconductor material of substrate  40  that has been converted to a non-monocrystalline state and includes point and extended defects, are formed by introducing energetic ions, as indicated diagrammatically by singled-headed arrows  122 , by an ion implantation process into the substrate  40 . The energetic ions  122 , which are generated from a source gas, are directed to impinge the substrate  40  at normal or near-normal incidence. The ions  122  may originate from a source gas selected to provide, when ionized and accelerated to impart kinetic energy, neutral impurities in silicon like nitrogen (N), oxygen (O), carbon (C), gold (Au), platinum (Pt), germanium (Ge), and silicon (Si), and other suitable elements capable of inducing lattice damage. The ions  122  may be implanted with the substrate  40  at or near room or ambient temperature, although the present invention is not so limited. The pad structure  42  masks underlying regions of the substrate  40  against receiving an ion dose during the ion implantation process such that only damage regions  124  of the substrate  40  are implanted with a significant dose of ions  122 . 
   The trajectories of the ions  122  penetrate the substrate  40  across base  54  of at least the shallow trench  48  that, after subsequent fabrication stages, intersects the well interface  142  ( FIG. 15 ), as well as optionally other trenches  48 . The ions  122  lose energy via scattering events with atoms and electrons in the semiconductor material constituting substrate  40 . Kinetic energy lost in nuclear collisions displaces target atoms of the substrate  40  from their original lattice sites and permanently damages the substrate  40 . When each individual ion  122  collides with a target atom of the substrate  40 , a recoil cascade is initiated that dissipates the transferred kinetic energy by collisions with other target atoms. The recoil cascade generates vacancies and interstitial atoms in the lattice structure of substrate  40  among the atoms in the lattice structure remaining on regular lattice sites. 
   The ions  122  eventually lose all of their initial kinetic energy and stop in the substrate  40  to produce one of the damage regions  124  of non-monocrystalline semiconductor material near the base  54  of each shallow trench  48 . The crystalline damage in the damage regions  124  coincides approximately with the depth profile of the stopped ions  122 . Similar to the stopped ions  122 , each damage region  124  is characterized by a depth profile distributed about a projected range, which is measured as a perpendicular distance of the damage peak from the top surface  41 , and having a range straggle. Essentially all of the implanted ions  122  come to rest within a distance of three times the range straggle from the projected range, which implies that the damage has a similar distribution. After the ion implantation is concluded, uncombined vacancies and interstitial atoms remain and are distributed across the thickness of the damage regions  124 , as well as extended defects. The depth profile of the implanted ions  122  and damage also has a characteristic lateral straggle such that ions  122  and damage extend laterally of the sidewalls  50 ,  52 , as indicated generally by boundary  126 . 
   The ion dose is preferably selected such that the peak atomic concentration of the implanted ions  122  in each damage region  124  exceeds the solid solubility of the impurity in the constituent material of the substrate  40 . By exceeding the solid solubility, subsequent heated process steps do not anneal the crystalline defects in the damage regions  124 . Advantageously, the peak atomic concentration for the implanted ions  122  in each damage region  124  may be in the range of 5×10 19  cm −3  to 5×10 21  cm −3  and, in certain embodiments, may be as low as 5×10 18  cm −3  to provide the requisite crystalline damage. For example, a suitable implanted ion dose may range from 1×10 14  cm −2  to 5×10 16  cm −2  at a kinetic energy between about 10 keV and about 50 keV, although the invention is not so limited. The present invention contemplates other implant conditions, i.e., energy and dose, that are capable of forming the damage regions  124  in substrate  40 . The ions  122  are implanted across the top surface  41  of the entire substrate  40 , although certain regions of substrate  40  may be optionally protected by a block mask (not shown) during implantation. 
   With reference to  FIG. 14  in which like reference numerals refer to like features in  FIG. 13  and at a subsequent fabrication stage, a pattern of deep trenches  128  is formed in the substrate  40  by a conventional lithography and subtractive etching process. To that end, a photoresist  130  is applied on pad layer  44  and exposed to a pattern of radiation that, after developing, creates a deep trench pattern. An anisotropic dry etching process, such as reactive-ion etching (RIE) or plasma etching, may then be used to transfer each deep trench  128  from the deep trench pattern in the patterned photoresist  130  into the substrate  40 . The deep trench pattern in the photoresist  130  is tailored such that each deep trench  128  is registered with a corresponding one of the shallow trenches  48  overlying the future location of the well junction  142  ( FIG. 15 ). 
   The damage region  124  ( FIG. 13 ) coinciding with the shallow trench  48  associated with deep trench  128  is partially removed by the anisotropic dry etching process forming the deep trench  128 . As a result, sidewalls  127 ,  129  of the deep trench  128  are each flanked by a corresponding one of a pair of damage regions  132 ,  134 . The deep trench  128  separates damage regions  132 ,  134  so that the crystalline damage in the semiconductor material of substrate  40  (i.e., the non-monocrystalline semiconductor material) is discontinuous and interrupted across the well junction  142  ( FIG. 15 ). Specifically, a bottom or base  131  of the deep trench  128  is at a greater depth than the damage regions  132 ,  134 . The sidewalls  127 ,  129  are narrower than the sidewalls  50 ,  52  of the corresponding shallow trench  48  and the base  131  is at a greater depth, measured perpendicular to surface  41 , than base  54  of the corresponding shallow trench  48 . 
   With reference to  FIG. 15  in which like reference numerals refer to like features in  FIG. 14  and at a subsequent fabrication stage, residual photoresist  130  ( FIG. 14 ) is stripped by, for example, plasma ashing or a chemical stripper after the deep trenches  128  are etched. The shallow trenches  48  are filled with amounts of an insulating or dielectric material, such as HDP oxide or TEOS, deposited across the pad layer  44  and planarized by, for example, a CMP process. The pad structure  42  is removed and planarized to define the STI regions  74  in the substrate  40  by a planarization process that makes the top surface of the STI regions  74  substantially co-planar or flush with the top surface  41  of substrate  40 . Portions of the dielectric material also fill the deep trench  128  to define a pigtail or extension  136  that separates the damage regions  132 ,  134 . Standard bulk CMOS processing follows as described above with regard to  FIG. 6  to form N- and P-wells  138 ,  140 , similar to N- and P-wells  76 ,  78  ( FIG. 6 ), the N-channel transistor  82 , the P-channel transistor  84 , and contacts  102 ,  104  in the substrate  40 . A person having ordinary skill in the art will appreciate that this embodiment of the present invention may be advantageously implemented in a triple-well CMOS structure that includes an N-band (not shown) similar to N-band  80  ( FIG. 6 ). 
   The selectively introduced lattice damage reduces the current gains of the parasitic NPN and PNP structures  22 ,  26  ( FIG. 1 ) without degrading well leakage of the N- and P-wells  138 ,  140 . Ordinarily, crystal damage across well junction  142  between the N- and P-wells  138 ,  140  causes a depletion region in a dual-well bulk CMOS technology, which increases the well leakage currents. In accordance with the present invention, the relatively narrow deep trench  128  is aligned relative to the corresponding shallow trench  48  to intersect the well junction  142  between the N-well  138  and P-well  140 , thus removing the damaged semiconductor material across the well junction  142 . Thus, the damage in damage regions  132 ,  134  exists only within the portion of the N- and P-wells  138 ,  140  that constitutes the base of the parasitic NPN and PNP structures  22 ,  26  ( FIG. 1 ). The crystal damage in the base regions shortens the minority carrier lifetime of the carriers emitted by the emitters and, thereby, reduces the bipolar gain to the point where latch-up is not sustained. Because the damage is located away from the well junction  142 , well leakage is not degraded. 
   With reference to  FIG. 16  in which like reference numerals refer to like features in  FIG. 14  and in accordance with an alternative embodiment of the present invention, ions  152 , similar or identical to ions  122  ( FIG. 13 ), may be directed into the sidewalls  127 ,  129  of the deep trench  128 . An implantation mask  154  of, for example, HDP oxide is applied to self align the impinging ions  152  to the sidewalls  127 ,  129  and to prevent impinging ions  152  from entering the semiconductor material of the substrate  40  near the base  131  of the deep trench  128 . A portion of the implantation mask  154  masks the base  131  of the deep trench  128 , which prevents damage to the well junction  142 . The implanted ions  152  form damage regions  156 ,  158  in the semiconductor material of substrate  40  that are similar or identical to damage regions  132 ,  134  ( FIG. 14 ). These damage regions  156 ,  158  of non-monocrystalline semiconductor material may be used in conjunction with damage regions  132 ,  134  for suppressing latch-up. Processing continues as shown in  FIG. 15  to complete semiconductor structure. 
   With reference to  FIG. 17  in which like reference numerals refer to like features in  FIG. 13  and in accordance with an alternative embodiment of the present invention, a high defect region may be produced near the base  54  of the shallow trenches  48  without ion implantation. To that end, protective spacers  144 ,  146  of an insulating material, such as silicon oxide or silicon nitride, are formed on the sidewalls  50 ,  52  of at least the shallow trench  48  that, after subsequent fabrication stages, intersects the well junction  142 . 
   A layer  148  of a semiconductor material, such as SiGe, having a lattice mismatch with the semiconductor material of the substrate  40  is then deposited or grown at the bottom of the shallow trench  48 . The protective spacers  144 ,  146  guard the sidewalls  50 ,  52  against the formation of an extraneous layer (not shown) of the material constituting layer  148  on sidewalls  50 ,  52 . The lattice mismatch or crystal lattice constant difference between the materials in layer  148  and substrate  40  results in a region  150  of high carrier recombination velocity in the substrate  40  beneath the shallow trench  48 . Region  150  is characterized by a high recombination velocity and getters or attracts carriers in transit to the collectors of the parasitic NPN and PNP structures  22 ,  26  ( FIG. 1 ). Processing continues as shown in  FIG. 14  to complete the semiconductor structure ( FIG. 15 ). 
   With reference to  FIG. 18  in which like reference numerals refer to like features in  FIG. 3  and in accordance with an alternative embodiment of the present invention, the oxygen implanted regions  58  ( FIG. 3 ) may be removed with an appropriate isotropic etching process to leave open cavities or voids  59 . The open voids  59 , which communicate with a corresponding one of the shallow trenches  48 , are each filled with dielectric material when the shallow trenches  48  are filled. The dielectric-filled open voids  59  define the lateral extensions  73 ,  75  ( FIG. 4 ). Processing continues as shown in  FIG. 4  to complete the semiconductor structure. 
   References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term “horizontal” as used herein is defined as a plane parallel to the top surface  41  of substrate  40 , regardless of its actual spatial orientation. The term “vertical” refers to a direction perpendicular to the horizontal, as just defined. Terms, such as “on”, “above”, “below”, “side” (as in “sidewall”), “higher”, “lower”, “over”, “beneath” and “under”, are defined with respect to the horizontal plane. It is understood that various other frames of reference may be employed for describing the present invention without departing from the spirit and scope of the present invention. 
   The fabrication of the semiconductor structure herein has been described by a specific order of fabrication stages and steps. However, it is understood that the order may differ from that described. For example, the order of two or more fabrication steps may be switched relative to the order shown. Moreover, two or more fabrication steps may be conducted either concurrently or with partial concurrence. In addition, various fabrication steps may be omitted and other fabrication steps may be added. It is understood that all such variations are within the scope of the present invention. It is also understood that features of the present invention are not necessarily shown to scale in the drawings. 
   While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.