Abstract:
A method of fabricating an integrated circuit (IC) including a first plurality of MOS transistors having a first gate dielectric having a first thickness in first regions, and a second plurality of MOS transistors having a second gate dielectric having a second thickness in second regions, wherein the first thickness&lt;the second thickness. A substrate having a semiconducting surface is provided. A pad dielectric layer having a thickness≦the second thickness is formed on the semiconductor surface including over the second regions, wherein the pad dielectric layer provides at least a portion of the second thickness for the second gate dielectric. A hard mask layer is formed on the semiconductor surface including over the second regions. A plurality of trench isolation regions are formed by etching through the pad dielectric layer and a portion of the semiconductor surface. The plurality of trench isolation regions are filled with a dielectric fill material to form trench isolation regions, and the hard mask layer is then removed. A patterned gate electrode layer is formed over the second gate dielectric, wherein said patterned gate electrode layer extends over a surface of at least one of the trench isolation regions. Fabrication of the MOS transistors in the first and second regions is then completed.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to methods of manufacturing integrated circuits (ICs) and in particular to methods of manufacturing Metal Oxide Semiconductor (MOS) field effect transistors (FETs) having trench isolation and two or more different gate dielectric thicknesses on the same IC, and ICs therefrom. 
       BACKGROUND 
       [0002]    It has become increasingly common to integrate devices with different characteristics on a single IC. The different types of devices have different operating and breakdown voltages thus requiring multiple gate dielectric layers (e.g. oxide layers) having different thicknesses to be formed. For example, the IC may have low, medium and high voltage device regions with the gate dielectric associated with the FET in the high voltage region being the thickest, the gate dielectric associated with the FETs in the low voltage device region being the thinnest, and the gate dielectric associated with the medium voltage device region being somewhere between the thinnest and the thickest. ICs having FETs formed as such are referred to as triple gate oxide (TGO) chips, and are referred to more generally as triple gate dielectric (TGD) chips. 
         [0003]    In order to form two or more different gate dielectric thicknesses in the respective device regions of an IC, several dielectric removal and deposition or growth steps are generally necessary. However, when integrated into a trench isolation process comprising flow, as described below, the dielectric removal processes generally have a detrimental impact on the trench isolation comprising structures which are commonly used for electrical isolation between adjacent FETs and between certain regions of individual FETs. 
         [0004]    As used herein, the term “trench isolation” applies for both conventional (e.g. bulk Si) substrates as well as silicon on insulator (SOI) substrates. Applied to conventional substrates, as used herein the term trench isolation includes deep trench isolation which is typically 1-5 μm deep, and shallow trench isolation which is typically &lt;1 μm deep, such as 0.3 to 0.7 μm deep. Applied to SOI substrates, as used herein, trench isolation includes the isolation regions between the active area islands. In the case of thin film SOI, the trench isolation regions like in the conventional substrate case are generally filled with a deposited dielectric, but are typically shallower that their conventional substrate counterparts, being generally &lt;0.5 μm deep, such as 0.01 to 0.3 μm deep. 
         [0005]    Following formation by a generally anisotropic etch, the resulting trench is generally first lined with a thin thermal “liner” oxide and then filled with a deposited dielectric, such as a plasma enhanced oxide. As known in the art, deposited oxides generally have a higher etch rate as compared to the thermally grown oxides which are generally used as gate dielectrics. As a result, a wet etch (e.g. dilute HF deglaze) of a gate dielectric in the non-gate areas after trench formation generally results in a significant loss in trench dielectric thickness, with a larger loss (e.g. about 1.5 to 2 times) being at the peripheral edges of the trench isolation regions which etch at a higher rate as compared to the trench dielectric away from the peripheral edges. The trench edges thus become recessed, giving rise to what are commonly referred to as divots. 
         [0006]    The divots in the trench isolation structures are undesirable as they increase the sub-threshold leakage current of FETs in the adjacent active semiconductor regions. This effect can be particularly significant for the low voltage regions having thin gate dielectric comprising FETs. In addition, as known in the art, relatively thick gate oxides (e.g. &gt;300 Angstroms) grow non-uniformly being thicker away from the trench edge and being thinner (typically ≦75% of the thicker thickness) at or near the trench/active area edges/corners. This corner thinning results in processes targeting thick oxide to be thicker than otherwise necessary to avoid gate dielectric breakdown at the trench corner. Furthermore, sharpening at the trench corner can cause high stress levels that can result in crystal defects in the active area that can reduce circuit yield. Accordingly, it is desirable to provide a method for fabricating a multiple gate dielectric chip in which divot formation and corner sharpening at the trench corners is significantly reduced. 
       SUMMARY 
       [0007]    This Summary is provided to comply with 37 C.F.R. §1.73, presenting a summary of the invention to briefly indicate the nature and substance of the invention. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
         [0008]    Embodiments of the present invention describe methods to integrate thick gate dielectrics (e.g. 200 to 2,000 Angstroms thick silicon oxide) in trench isolation-based process flows that include multiple (i.e.  2  or more) different gate dielectric thicknesses. The thick gate dielectric is formed before trench isolation formation. This is in contrast to conventional process flows which form the thick gate dielectric (e.g. oxide) after trench isolation. In such conventional processes, the trench etch is done through a single oxide thickness across the entire wafer. Since embodiments of the present invention form the thick gate dielectric before the trenches are formed, the trench corners for the thick oxide is generally protected and the gate electrode (e.g. polysilicon) deposited early in the process flow, thus further reducing the depth of the divots, and in some cases essentially eliminating the divots at the trench corners. 
         [0009]    A first embodiment of the invention comprises a method of fabricating an IC comprising a first plurality of MOS transistors having a first gate dielectric having a first thickness in first regions of the substrate, and a second plurality of MOS transistors having a second gate dielectric having a second thickness in second regions of the substrate, wherein the first thickness&lt;the second thickness. A substrate having a semiconducting surface is provided. A pad dielectric layer having a thickness≦the second thickness is formed on the semiconductor surface including over the second regions, wherein the pad dielectric layer provides at least a portion of the second thickness for the second gate dielectric. A hard mask layer is formed on the semiconductor surface including over the second regions. A plurality of trench isolation regions are then formed by etching through the pad dielectric layer and a portion of the semiconductor surface under the trench isolation regions. The plurality of trench isolation regions are filled with a dielectric fill material to form trench isolation regions, and the hard mask layer is then removed. A patterned gate electrode layer is formed over the second gate dielectric, wherein the second gate electrode layer extends over a surface of at least one of the trench isolation regions. Fabrication of the MOS transistors in the first and second regions is then completed. 
         [0010]    ICs comprising a first plurality of MOS transistors having a first gate dielectric having a first thickness in first regions, and a second plurality of MOS transistors having a second gate dielectric having a second thickness in second regions, wherein the first thickness&lt;said second thickness are also disclosed. The second thickness is generally 200 to 2,000 Angstroms. ICs according to embodiments of the invention have significantly reduced divot formation and corner sharpening at the trench corners. For example, the second thickness over the trench isolation active area edges are generally at least 90%, and generally at least 95%, of the second thickness away (e.g. 0.3 μm away) from the trench isolation active area edges. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1-6  are cross sectional views of a multiple gate dielectric thickness comprising device at various intermediate stages of fabrication, according to a first embodiment of the invention. 
           [0012]      FIGS. 7-10  are cross sectional views of a multiple gate dielectric thickness comprising device at various intermediate stages of fabrication according to a second embodiment of the invention. 
           [0013]      FIGS. 11-12  are cross sectional views of a multiple gate dielectric thickness comprising device at various intermediate stages of fabrication according to a third embodiment of the invention. 
           [0014]      FIG. 13  is a simplified cross sectional view of a triple gate dielectric (TGD) device according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention. 
         [0016]    Embodiments of the invention describe methods for fabricating a IC having multiple (2 or more) different gate dielectric thicknesses in a trench isolation process flow in which divot formation and corner sharpening are both significantly reduced or eliminated, and ICs therefrom. A first embodiments of the present invention is described with the aid of  FIGS. 1-6  which are various cross-sections of a multiple gate dielectric thickness comprising chip at progressively later intermediate stage of manufacture. 
         [0017]      FIG. 1  shows a cross sectional view of multiple gate dielectric thickness comprising device  100  at a first intermediate stage of fabrication, according to an embodiment of the invention. Device  100  is shown including a substrate  105  having a semiconductor surface  106 , such as a Si or Si/Ge surface. The substrate  105  can comprise a conventional single crystal substrate or a SOI substrate. 
         [0018]      FIG. 2  shows a cross sectional view of the multiple gate dielectric thickness comprising device  100  at a later intermediate stage of fabrication, according to an embodiment of the invention, where a patterned thick oxide layer  112  which is used as a thick gate oxide layer, generally being 200 to 2,000 Angstroms thick is formed, such as, for example, 425-475 A of thermal oxide. A standard resist pattern and etch process can be used to define areas that require the thick oxide layer (e.g. where high voltage devices are needed), wherein the remainder of the thick oxide layer  112  is etched away. 
         [0019]      FIG. 3  shows a cross sectional view of the multiple gate dielectric thickness comprising device  100  at a later intermediate stage of fabrication, according to an embodiment of the invention. An intermediate dielectric thickness layer  114  which is used as a gate oxide layer having intermediate thickness, such as 50 to 200 Angstroms thick is formed, for example a 100 to 150 A thick thermal oxide. Assuming dielectric layer  114  is thermally grown, the oxide thickness of thick oxide layer  112  will increase, so that if initially 450 Angstroms thick, will grow to about 500 Angstroms. A hard mask layer  115 , such as a silicon nitride, silicon oxynitride (SiON) or silicon carbide layer  115  is then deposited on layers  112  and  114 . The hard mask layer  115  thickness can be about 1,500 to 3,000 Angstroms, such around 2,000 Angstroms. 
         [0020]      FIG. 4  shows a cross sectional view of the multiple gate dielectric thickness comprising device  100  at a later intermediate stage of fabrication, according to an embodiment of the invention, following trench isolation etch wherein trenches  140  in the substrate  105  are formed. The trench etch is generally modified to account for the thicker dielectric (e.g. thick oxide layer  112 ) in parts of device  100 , as compared to a conventional process in which the thick gate oxide layer is formed after trench processing. 
         [0021]    Although not shown, a liner oxide and liner anneal generally follow trench etch to condition the trench  140 . In embodiments of the invention, nitride pullback processing as known in the art to recess the hard mask layer (e.g. nitride layer)  115  from the edges of trench  140  occurs next, occurring before trench fill. As known in the art, nitride pullback helps protect the trench edges/corners. 
         [0022]      FIG. 5  shows a cross sectional view of the multiple gate dielectric thickness comprising device  100  at a later intermediate stage of fabrication, according to an embodiment of the invention, following trench fill with trench dielectric material  145 , followed by chemical-mechanical polish (CMP) and then removal of hard mask layer (e.g. nitride)  115 . Phosphoric acid can be used in the case of a nitride hardmask layer for removal. 
         [0023]    An optional short prefurnace clean sufficient to remove native oxide (e.g. 10-20 A oxide removal with dilute HF) then can occur. Thereafter, although not shown, in another region of the device  100  a third gate dielectric thickness, the thinnest, such as about 15 to 50 A of a dielectric can be grown or deposited and/or annealed to form thin gate dielectric regions. 
         [0024]      FIG. 6  shows a cross sectional view of the multiple gate dielectric thickness comprising device  100  at a later intermediate stage of fabrication, according to an embodiment of the invention, following gate electrode deposition, such as polysilicon deposition, and patterning and etching. In the embodiment described herein, polysilicon layer  160  is a dedicated poly level for the thick gate dielectric devices, with other devices, such as core and I/O devices which receive thinner gate dielectrics receiving their gate electrode (e.g. polysilicon) in a subsequent step. Being over the trench corner, polysilicon layer  160  protects the trench corner  164  against divoting for the balance of the processing of device  100 , such as from deglazes. Fabrication of device  100  is then completed generally according to standard manufacturing procedures, such as the remainder of the front end of the line (FEOL), and the back end of the line (BEOL). 
         [0025]    A second embodiment of the present invention is described with the aid of  FIGS. 7-10  which are various cross-sections of a multiple gate dielectric thickness comprising device  200  at several intermediate stages of manufacture.  FIG. 7  is a cross section depiction of a multiple gate dielectric thickness comprising device  200  after a thick oxide layer  112  of about 200 to 2,000 Angstroms (e.g. about 500 A) is grown on substrate  105 , wherein the thick gate dielectric device areas are masked with a patterned masking layer  171  (e.g. with photoresist), and areas other than the thick gate dielectric device areas etched (e.g. deglazed) to a lower thickness shown as regions  144 , such as to a thickness of 50 to 200 Angstroms. Following removal of masking layer  171 , hard mask layer  115  is deposited, and patterned using masking layer  181 , and trenches  140  are formed using the pattern to result in the intermediate structure of device  200  shown in  FIG. 8 . 
         [0026]    The trench etch process is modified slightly to permit etching through the full thickness of thick oxide layer  112 . Following removal of masking material  181  and trench filling with trench dielectric material  145 , hard mask layer  115  is removed, and a dedicated polysilicon layer  160  is patterned and etched to result in the structure shown in  FIG. 9 . The thick gate dielectric device areas are patterned generally with a resist, oxide layer  144  is removed, and a gate oxide  185  for certain intermediate voltage devices, such as I/O devices is grown, such as 50 to 200 Angstroms thick. Subsequent steps parallel those above relative to device  100  including formation of a third gate dielectric thickness, the thinnest, such as about 15 to 50 A of a dielectric can be grown or deposited and/or annealed to form a thin gate dielectric, followed by gate electrode formation for these devices  170  (e.g. polysilicon) to result in the structure shown in  FIG. 10 . 
         [0027]    A third embodiment of the present invention is described with the aid of  FIGS. 11-12  which are various cross-sections of a multiple gate dielectric thickness comprising chip  300  at several intermediate stages of manufacture.  FIG. 11  is a cross section depiction of a multiple gate dielectric thickness comprising device  300  showing device  300  after formation of trenches  140  using a patterned stack comprising resist  171 , on hard mask  115  on a grown or deposited pad oxide  112  which is used for the trench isolation processing and as the gate dielectric for the high voltage devices.  FIG. 12  is a cross section depiction of the multiple gate dielectric thickness comprising device  300  after trench formation, hard mask layer  115  strip, and dedicated gate electrode layer  160  deposition, patterning and etching, and the dielectric (e.g. oxide) removed from the etched regions. Subsequent steps parallel those above relative to devices  100  and  200  including formation of a third gate dielectric thickness, the thinnest, such as about 15 to 50 A of a dielectric can be grown or deposited and/or annealed to form thin gate dielectric, followed by gate electrode formation for these devices. 
         [0028]    As another processing alternative, the standard polysilicon deposition can be used to cover both the core, IO and HV-GOX layers. This alternative comprises use of a single polysilicon layer that is deposited after all the gate oxides are formed. 
         [0029]      FIG. 13  is a simplified cross sectional view of a TGD device  1300  according to an embodiment of the invention. TGD chip  1300  includes a p-substrate  185  having semiconductor regions of the first kind  150 , semiconductor regions of the second kind  160 , and semiconductor regions of the third kind  170  formed therein. Semiconductor regions of the first kind  150  are generally low voltage regions over which devices with thin gate dielectrics  116  are formed, semiconductor regions of the second kind  160  are intermediate voltage regions over which devices with gate dielectrics of intermediate thickness  114  are formed and semiconductor regions of the third kind  170  are high voltage regions over which devices with thick gate dielectrics  112  are formed. Correspondingly, there is a low gate breakdown voltage for MOSFETs in semiconductor regions of the first kind, an intermediate gate breakdown voltage for MOSFETs in semiconductor regions of the second kind and a high gate breakdown voltage for MOSFETs in semiconductor regions of the third kind. Gate electrodes are not shown. 
         [0030]    In the semiconductor regions of the first kind  150  there is an n-well  10  and a p-well  12 , which are formed in a deep n-well  14  that is formed in substrate  185 . An isolation region,  18 , which is generally an STI region, separates the n-well  10  from the p-well  12 . It can be seen the edge of the isolation region  18  does not have any noticeable divot, whether in regions of the first kind  150 , regions of the second kind  160 , or regions of the third kind  170 . ICs according to embodiments of the invention have significantly reduced divot formation and corner sharpening at the trench corners. For example, the thickness of dielectric layer  112  in regions of the third kind  170  over the trench isolation active area edges are generally at least 90%, and generally at least 95%, of the thickness of dielectric layer  112  a distance of 0.3 μm away from the trench isolation active area edges. 
         [0031]      FIG. 13  depicts examples of what could constitute semiconductor regions of the first kind, which could contain any number of n-type or p-type semiconductor regions. The semiconductor regions can be any kind of semiconductor region and not necessarily wells. The semiconductor regions need not be formed in a deep n-well that is formed in a p-substrate, but could for example be formed in a deep p-well that is formed in an n-substrate. 
         [0032]    Semiconductor regions of the second kind  160  includes an n-well  22  and a p-well  24 , which are formed in a deep n-well  26  that is formed in a p-substrate  185 . An isolation region,  18 , which is generally an STI region, separates the n-well  22  from the p-well  24 .  FIG. 13  depicts examples of what could constitute semiconductor regions of the second kind, which could contain any number of n-type or p-type semiconductor regions. The semiconductor regions can be any kind of semiconductor region and not necessarily wells. The semiconductor regions need not be formed in a deep n-well that is formed in a p-substrate, but could for example be formed in a deep p-well that is formed in an n-substrate. 
         [0033]    In the same way, semiconductor regions of the third kind  170  includes an n-well  28  and a p-well  30 , which are formed in substrate  185 . An isolation region,  18 , which is generally an STI region, separates the n-well  28  from the p-well  30 .  FIG. 13  depicts examples of what could constitute semiconductor regions of the third kind, which could contain any number of n-type or p-type semiconductor regions. The semiconductor regions can be any kind of semiconductor region and not necessarily wells and they could be formed in an n-substrate. 
         [0034]    Isolation regions  20  are shown in  FIG. 13  separating regions of different kinds  150 ,  160  and  170 . These isolation regions can be trench isolation regions that are disposed over field implant regions  32 , which can be added as known in the art to enhance the isolation. 
         [0035]    Embodiments of the invention can be integrated into a variety of process flows to form a variety of devices and related products that generally require high voltage MOS devices, which provide gate to body breakdown voltages of ≧10 volts. For example, the high voltage devices can be embodied as conventional CMOS devices, DEMOS or LDMOS. Exemplary devices include power management devices, display drivers and medical applications (e.g. defibrillators). 
         [0036]    The semiconductor substrates may include various elements therein and/or layers thereon. These can include barrier layers, other dielectric layers, device structures, active elements and passive elements including source regions, drain regions, bit lines, bases, emitters, collectors, conductive lines, conductive vias, etc. Moreover, the invention can be used in a variety of processes including bipolar, CMOS, BiCMOS and MEMS. 
         [0037]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents. 
         [0038]    Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. 
         [0039]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
         [0040]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0041]    The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the following claims.