Abstract:
A method including etching a shallow trench laterally surrounding a portion of a semiconductor substrate, the semiconductor substrate comprising a semiconductor-on-insulator SOI layer, a pad oxide layer, and a pad nitride layer, depositing a first nitride liner, a dielectric liner, and a second nitride liner in the shallow trench, wherein the dielectric liner is located between the first and the second nitride liner, and filling the shallow trench with a shallow trench fill portion.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to semiconductor structures, and more particularly to electrical isolation structures for ultra-thin semiconductor-on-insulator (UTSOI) devices and methods of manufacturing the same. 
         [0003]    2. Background of Invention 
         [0004]    Ultra-thin semiconductor-on-insulator (UTSOI) devices refer to semiconductor devices formed on an ultra-thin semiconductor-on-insulator (UTSOI) substrate. A UTSOI substrate can be employed to form various semiconductor devices that provide performance advantages through the reduced thickness of the top semiconductor-on-insulator (SOI) layer and the buried oxide layer as compared with conventional SOI substrates. 
         [0005]    While UTSOI devices, and especially UTSOI field effect transistors (FETs), are promising candidates for advanced high performance devices, several manufacturing issues need to be resolved before UTSOI devices can be manufactured with high yield. One such issue is erosion of shallow trench isolation structures that are employed to provide lateral electrical isolation between adjacent devices. Specifically, shallow trench isolation structures may experience erosion due to multiple etching steps used to recess various material layers during semiconductor fabrication. The shallow trench isolation structures may be compromised to the point where an electrical short is possible between a subsequently formed contact and a base layer of the UTSOI substrate. 
         [0006]    Thus, a method of ensuring sufficient electrical isolation between the base layer of a UTSOI substrate and the contacts despite erosion of the shallow trench isolation structures during semiconductor fabrication is needed to provide functional and reliable UTSOI devices. 
       SUMMARY 
       [0007]    According to one embodiment of the present invention, a method of forming an isolation structure is provided. The method may include etching a shallow trench laterally surrounding a portion of a semiconductor substrate, the semiconductor substrate comprising a semiconductor-on-insulator SOI layer, a pad oxide layer, and a pad nitride layer. The method may further include, depositing a first nitride liner, a dielectric liner, and a second nitride liner in the shallow trench, wherein the dielectric liner is located between the first and the second nitride liner; and filling the shallow trench with a shallow trench fill portion. 
         [0008]    According another exemplary embodiment, an isolation structure is provided. The isolation structure may include a shallow trench laterally surrounding a portion of a semiconductor substrate, the semiconductor substrate comprising a semiconductor-on-insulator SOI layer, a pad oxide layer, and a pad nitride layer. The isolation structure may further include a first nitride liner, a dielectric liner, and a second nitride liner located adjacent to a sidewall and a bottom of the shallow trench, wherein the dielectric liner is located between the first and the second nitride liner, and a shallow trench fill portion configured on top of the second nitride liner. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The following detailed description, given by way of example and not intend to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which: 
           [0010]      FIGS. 1A-1L  illustrate the steps of a method of forming an isolation structure according to one embodiment. 
           [0011]      FIG. 1A  depicts an SOI substrate used in forming the isolation structure according to an exemplary embodiment. 
           [0012]      FIG. 1B  depicts a step of forming the isolation structure where a shallow trench may be etched into the SOI substrate according to an exemplary embodiment. 
           [0013]      FIG. 1C  depicts a step of forming the isolation structure where a stack of shallow trench liners may be deposited within the shallow trench according to an exemplary embodiment. 
           [0014]      FIG. 1D  depicts a step of forming the isolation structure where a shallow trench fill may be deposited on top of the stack of shallow trench liners according to an exemplary embodiment. 
           [0015]      FIG. 1E-1J  depict intermediate steps of forming the isolation structure where the shallow trench fill material, the stack of shallow trench liners are recessed according to an exemplary embodiment. 
           [0016]      FIG. 1K  depicts a step of forming the isolation structure where a gate dielectric layer, a work function metal layer, and a gate material layer may be deposited and subsequently patterned to form a semiconductor device stack according to an exemplary embodiment. 
           [0017]      FIG. 1L  depicts the final isolation structure where the semiconductor device may be formed according to an exemplary embodiment. 
       
    
    
       [0018]    The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements. 
       DETAILED DESCRIPTION 
       [0019]    Detailed embodiments of the claimed structures and methods are disclosed herein; however, it can be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiment set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this invention to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments. 
         [0020]    Referring now to  FIGS. 1A-1L , exemplary process steps of forming an isolation structure  100  for a semiconductor device are shown. Specifically a shallow trench may be formed in an SOI substrate. Using standard processes, a stack of liners may be deposited within the shallow trench followed by the deposition of a shallow trench fill portion. A chemical mechanical polishing (CMP) technique may be used to remove excess shallow trench fill portion from atop the SOI substrate. Next the stack of liners including a first nitride liner, a dielectric liner, and a second nitride liner may be partially removed. Finally, a gate dielectric layer, a work function metal layer, and a gate conductor may be deposited. 
         [0021]    Referring now to  FIG. 1A , a silicon-on-insulator (SOI) substrate  101  is shown. The SOI substrate  101  may include a base layer  102 , a buried oxide (BOX) layer  104  formed on top of the base layer  102 , and a SOI layer  106  formed on top of the BOX layer  104 . The BOX layer  104  isolates the SOI layer  106  from the base layer  102 . In one embodiment, the SOI substrate  101  may have a pad oxide layer  108  and a pad nitride layer  110  formed on a top surface of the SOI layer  106 , where the pad nitride layer  110  may be located directly on top of the pad oxide layer  108 . The base layer  102  may be made from any of several known semiconductor materials such as, for example, a bulk silicon substrate. Other non-limiting examples include silicon, germanium, silicon-germanium alloy, silicon carbide, silicon-germanium carbide alloy, and compound (e.g. III-V and II-VI) semiconductor materials. Non-limiting examples of compound semiconductor materials include gallium arsenide, indium arsenide, and indium phosphide. Typically the base layer  102  may be about, but is not limited to, several hundred microns thick. For example, the base layer  102  may include a thickness ranging from 0.5 mm to about 1.5 mm. 
         [0022]    The BOX layer  104  may be formed from any of several dielectric materials. Non-limiting examples include, for example, oxides, nitrides and oxynitrides of silicon. The BOX layer  104  may also include oxides, nitrides and oxynitrides of elements other than silicon. In addition, the BOX layer  104  may include crystalline or non-crystalline dielectric material. Moreover, the BOX layer  104  may be formed using any of several methods. Non-limiting examples include ion implantation methods, thermal or plasma oxidation or nitridation methods, chemical vapor deposition methods and physical vapor deposition methods. The BOX layer  104  may include a thickness ranging from about 5 nm to about 200 nm. In one embodiment, the BOX layer  104  may be about 25 nm thick. 
         [0023]    The SOI layer  106  may include any of the several semiconductor materials included in the base layer  102 . In general, the base layer  102  and the SOI layer  106  may include either identical or different semiconducting materials with respect to chemical composition, dopant concentration and crystallographic orientation. In one embodiment, the base layer  102  and the SOI layer  106  include semiconducting materials that include at least different crystallographic orientations. Typically the base layer  102  or the SOI layer  106  include a {110} crystallographic orientation and the other of the base layer  102  or the SOI layer  106  includes a {100} crystallographic orientation. Typically, the SOI layer  106  includes a thickness ranging from about 5 nm to about 100 nm. Methods for making the SOI layer  106  are well known in the art. Non-limiting examples include SIMOX (Separation by Implantation of OXygen), wafer bonding, and ELTRAN® (Epitaxial Layer TRANsfer). 
         [0024]    The pad oxide layer  108  may include a silicon oxide or a silicon oxynitride. In one embodiment, the pad oxide layer  108  can be formed, for example, by thermal or plasma conversion of a top surface of the SOI layer  106  into a dielectric material such as silicon oxide or silicon oxynitride. In one embodiment, the pad oxide layer  108  can be formed by deposition of silicon oxide or silicon oxynitride by chemical vapor deposition (CVD) or atomic layer deposition (ALD). The pad oxide layer  108  may have a thickness ranging from about 1 nm to about 10 nm, although a thickness less than 1 nm and greater than 10 nm may be acceptable. In one embodiment, the pad oxide layer  108  may be about 5 nm thick. 
         [0025]    The pad nitride layer  110  may include an insulating material such as, for example, silicon nitride. The pad nitride layer  110  may be formed using conventional deposition methods, for example, low-pressure chemical vapor deposition (LPCVD). In one embodiment, the pad nitride layer  110  may have a thickness ranging from about 5 nm to about 100 nm. In one particular embodiment, the pad nitride layer  110  may be about 50 nm thick. 
         [0026]    In one embodiment, the SOI substrate  101  can be an ultra-thin semiconductor-on-insulator (UTSOI) substrate. The top SOI layer (e.g.  106 ) of a typical UTSOI substrate may also be referred to as an ultra-thin semiconductor-on-insulator (UTSOI) layer, and have a thickness ranging from about 3 nm to about 15 nm. The BOX layer (e.g.  104 ) beneath the UTSOI of a UTSOI substrate can have a thickness ranging from about 10 nm to about 50 nm. 
         [0027]    Referring now to  FIG. 1B , a cell location is identified and a mask layer  120  of a suitable masking material may be deposited on a top surface of the pad nitride layer  110  (shown in  FIG. 1A ) and patterned using a conventional photolithographic techniques. The mask layer  120  may include suitable masking materials such as, for example, photoresist or hardmask such as silicon dioxide. A shallow trench  122  is formed by etching through the pad nitride layer  110  (shown in  FIG. 1A ), the pad oxide  108  (shown in  FIG. 1A ), the SOI layer  106  (shown in  FIG. 1A ), and an upper portion  111  of the base layer  102  as illustrated by the figure. The shallow trench  122  extends vertically from the top surface of the pad nitride layer  110  (shown in  FIG. 1A ) to a depth below the interface between the base layer  102  and the BOX layer  104  (shown in  FIG. 1A ). The shallow trench  122  may be formed using, for example, an anisotropic dry etching technique, such as reactive ion etching (RIE). The shallow trench  122  laterally surrounds a vertical stack, from bottom to top, the upper portion  111  of the base layer  102 , a BOX layer region  112 , a SOI layer region  114 , a pad oxide layer region  116 , and a pad nitride layer region  118 . 
         [0028]    Referring now to  FIG. 1C , a stack of liners may be deposited on top of the pad nitride layer region  118  and within the shallow trench  122  (shown in  FIG. 1B ). A first nitride liner  124  may be deposited first, as shown in the figure. In one embodiment, the first nitride liner  124  may include silicon nitride. The first nitride liner  124  may be deposited as a contiguous layer on the entirety of the physically exposed surfaces of the base layer  102 , the BOX layer region  112 , the SOI layer region  114 , the pad oxide layer region  116 , and the pad nitride layer region  118 . The first nitride liner  124  may be deposited, for example, by chemical vapor deposition (CVD), molecular layer deposition (MLD), or a combination thereof. The first nitride liner  124  can be stoichiometric (i.e., have a composition of Si 3 N 4 ) or non-stoichiometric. The first nitride liner  124  may range in thickness from about 1 nm to about 10 nm, although a thickness less than 1 nm and greater than 10 nm may be acceptable. 
         [0029]    Next, a dielectric liner  126  may be deposited on top of the first nitride liner  124 , as shown in the  FIG. 1C . The dielectric liner  126  may include a dielectric metal oxide material, i.e., a dielectric compound including at least one metal and oxygen. The dielectric metal oxide material can optionally include nitrogen, carbon, fluorine, chlorine, or some combination thereof. In one embodiment, the dielectric metal oxide material may include silicon. For example, the dielectric metal oxide material can be a material known in the art as high-k gate dielectric materials having a dielectric constant greater than the dielectric constant of silicon nitride, i.e., 7.9. Dielectric metal oxide materials may be deposited by methods well known in the art including, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), molecular beam deposition (MBD), pulsed laser deposition (PLD), liquid source misted chemical deposition (LSMCD), atomic layer deposition (ALD), and the like. The dielectric liner  126  may be deposited as a contiguous layer on top of the first nitride liner  124 . Exemplary high-k dielectric materials include ZrO 2 , La 2 O 3 , Al 2 O 3 , TiO 2 , SrTiO 3 , LaAlO 3 , Y 2 O 3 . Other exemplary high-k materials may include HfO x N y , ZrO x N y , La 2 O x N y , Al 2 O x N y , TiO x N y , SrTiO x N y , LaAlO x N y , Y 2 O x N y , a silicate thereof, and an alloy thereof. Each value of x may independently range from 0.5 to 3 and each value of y may independently range from 0 to 2. Preferably, the dielectric liner  126  may include hafnium oxide or hafnium silicate. The dielectric liner  126  may range in thickness from about 1 nm to about 10 nm, although a thickness less than 1 nm and greater than 10 nm may be acceptable. Preferably, the dielectric liner  126  may range in thickness from about 3 nm to about 6 nm. 
         [0030]    A second nitride liner  128  may be deposited on top of the dielectric liner  126  as shown in  FIG. 1C . The second nitride liner  128  may be deposited as a contiguous layer on top of the dielectric liner  126 . In one embodiment, the second nitride liner  128  may include silicon nitride. The second nitride liner  128  may be deposited, for example, by chemical vapor deposition (CVD), molecular layer deposition (MLD), or a combination thereof. The second nitride liner  128  can be stoichiometric (i.e., have a composition of Si 3 N 4 ) or non-stoichiometric. The second nitride liner  128  may range in thickness from about 1 nm to about 10 nm, although a thickness less than 1 nm and greater than 10 nm may be acceptable. The dielectric liner  126  between the first nitride liner  124  and the second nitride liner  128  may be used to prevent a short circuit between a device subsequently formed on the SOI layer  106  and the base substrate  102 . 
         [0031]    Referring now to  FIG. 1D , a shallow trench fill layer  130  may be deposited on top of the second nitride liner  128 . The shallow trench fill layer  130  may be sequentially deposited after the deposition of the second nitride liner  128 . In one embodiment, the shallow trench fill layer  130  may include silicon oxide. The shallow trench fill layer  130  may be deposited, for example, by chemical vapor deposition (CVD). The thickness (H 1 ) of the shallow trench fill layer  130  as measured from the top surface of the pad nitride layer region  118 , may be greater than the depth (H 2 ) of the shallow trench  122 , as measured between the topmost surface of the pad nitride layer region  118  and the bottommost surface of the shallow trench  122 . The shallow trench fill layer  130  may fill the entirety of the shallow trench  122 . 
         [0032]    Referring now to  FIG. 1E , a portion of the shallow trench fill layer  130  located above the horizontal plane of the topmost surface of the second nitride liner  128  may be removed, for example, by chemical mechanical planarization (CMP), a recess etch, or a combination thereof. A remaining portion of the shallow trench fill layer  130  after planarization includes a shallow trench fill portion  132 . The shallow trench fill portion  132  contiguously and laterally surrounds the vertical stack of the upper portion of the base layer  102 , the BOX layer region  112 , the SOI layer region  114 , the pad oxide layer region  116 , and the pad nitride layer region  118 . The shallow trench fill portion  132  is laterally spaced from the vertical stack of the upper portion  111  (shown in  FIG. 1B ) of the base layer  102 , the BOX layer region  112 , the SOI layer region  114 , the pad oxide layer region  116 , and the pad nitride layer region  118  by substantially vertical portions of the first nitride liner  124 , the dielectric liner  126 , and the second nitride liner  128 . The shallow trench fill portion  132  fills the shallow trench  122  (shown in  FIG. 1B ). 
         [0033]    Referring now to  FIG. 1F , a portion of the second nitride liner  128  may be removed from above the dielectric liner  126 . In one embodiment, the portion of the second nitride liner  128  may be removed, for example, by a wet etching technique using hot phosphoric acid or by a dry etching technique. The shallow trench portion  132  may not be affected during the removal of the portion of the second nitride liner  128  because the etching technique used can be very selective to oxide, or the material of the shallow trench fill portion  132 . 
         [0034]    In an additional process step the shallow trench fill portion  132  may be recessed with an etching technique using different etching chemistries than that used for the removal of the portion of the second nitride liner  128 . In one embodiment, the shallow trench fill portion  132  may be recessed, for example, by a wet etching technique or by a dry etching technique. 
         [0035]    Referring now to  FIG. 1G , a portion of the dielectric liner  126  located directly above the first nitride liner  124  may be removed. In one embodiment, the portion of the dielectric liner  126  may be removed, for example, by a wet etching technique or by a dry etching technique. The chemistry for etching the dielectric liner  126  depends on the composition of the dielectric liner  126 . Any chemistry for etching the dielectric liner  126  as known in the art can be used. For example, a chlorine based dry etching technique may be used to remove a hafnium oxide dielectric liner. Depending on the etching technique used the shallow trench fill portion  132  may be further recessed during the removal of the dielectric liner  126 . 
         [0036]    Referring now to  FIG. 1H , a portion of the first nitride liner  124  located directly above the pad nitride layer region  118  may be removed. In one embodiment, the portion of the first nitride liner  124  may be removed, for example, by a wet etching technique using hot phosphoric acid or by a dry etching technique. 
         [0037]    With continued reference to  FIG. 1H , the pad nitride layer region  118  may be removed. The pad nitride layer region  118  may be removed for example, by a wet etching technique or a dry etching technique. In one embodiment, the pad nitride layer region  118  may include silicon nitride, and a wet etching technique using hot phosphoric acid may be used to remove the pad nitride layer region  118 . After removal of the portion of the first nitride liner  124 , the portion of the dielectric liner  126 , the portion of the second nitride liner  128 , and the pad nitride layer region  118  a small portion  134  of the dielectric liner  126  may remain extending above the top surface of the pad oxide layer region  116 , as shown in  FIG. 1H . 
         [0038]    Referring now to  FIG. 1I , the small portion  134  (shown in  FIG. 1H ) of the dielectric liner  126  may be recessed to a level equal with a top surface of the first nitride liner  124 . In one embodiment, the small portion  134  (shown in  FIG. 1H ) of the dielectric liner  126  may be recessed, for example, by using a wet etching technique having a chemistry capable of recessing the small portion  134  (shown in  FIG. 1H ) of the dielectric liner  126  selective to the first nitride liner  124 , the second nitride liner  128 , and the pad oxide layer region  116 . In one embodiment, the etch chemistry may be selective to silicon oxide. In one embodiment, a chlorine based dry etching technique may be used to recess the small portion  134  (shown in  FIG. 1H ) of the dielectric liner  126 . Because the chlorine based etching technique is relatively selective to oxide the shallow trench fill portion  132  may not be further recessed during the removal of the small portion  134  of the dielectric liner  126 , however, some recess is acceptable. 
         [0039]    In one embodiment, the top portion of the shallow trench fill portion  132  may be recessed during the removal of the pad oxide layer region  116  or in a different recess etching step so that the top surface of the shallow trench fill portion  132  becomes substantially coplanar with the top surface of the first nitride liner  124 , the dielectric liner  126 , and the second nitride liner  128 . As used herein, a first surface is substantially coplanar with a second surface if the difference in height between the first surface and the second surface is limited by inherent limitations of processing techniques intended to make the first and second surfaces coplanar. It should be noted that the difference in height depicted in the figures may be exaggerated and is merely a pictorial representation. 
         [0040]    Referring now to  FIG. 1J , the pad oxide layer region  116  (shown in  FIG. 1I ) may be removed. In one embodiment, a wet etching technique using hydrofluoric acid may be used to remove the pad oxide layer region  116 . The second nitride liner  128  may further be recessed below the top surface of the shallow trench fill portion  132  during the removal of the pad oxide layer region  116 . After removing the pad oxide layer region  116  the top surfaces of the first nitride liner  124 , the dielectric liner  126 , and the second nitride liner  128  may be substantially flush with one another, and below the top surface of the shallow trench fill portion  132 . 
         [0041]    Referring now to  FIG. 1K , a semiconductor device may be formed on top of the isolation structure  100 . The semiconductor device can include, for example, a field effect transistor, a junction transistor, a diode, a resistor, a capacitor, an inductor, an optical device, or any other semiconductor device known in the art. In one embodiment, the semiconductor device may include a field effect transistor. In such embodiments, a gate dielectric layer  136  may be deposited by a conformal deposition process such as chemical vapor deposition (CVD) or atomic layer deposition (ALD). The gate dielectric layer  136  may be contiguously deposited on top of the shallow trench fill portion  132 , the first nitride liner  124 , the dielectric liner  126 , the second nitride liner  128 , and the SOI layer region  114 . The gate dielectric layer  136  may include any gate dielectric material known in the art including, but not limited to, silicon-oxide-based gate dielectric materials and dielectric metal oxide materials. If a dielectric metal oxide material is used as the entirety of, or as a part of, the gate dielectric layer  136 , the dielectric metal oxide material within the gate dielectric layer  136  may have the same composition as, or a different composition from, the dielectric metal oxide material of the dielectric liner  126 . Further, the dielectric metal oxide material within the gate dielectric layer  136  may have the same thickness as, or a different thickness from, the dielectric metal oxide material of the dielectric liner  126 . 
         [0042]    With continued reference to  FIG. 1K , a work function metal layer  138  and a gate material layer  140  may be conformably deposited on top of the gate dielectric layer  136 . The work function metal layer  138  and the gate material layer  140  may be subsequently patterned to form the semiconductor device gate stack. In one embodiment, the work function metal layer  138  may include, for example, TiN, Ta, or TaC and the gate material layer  140  may include, for example, polysilicon, tungsten, or aluminum. 
         [0043]    Referring to  FIG. 1L , the gate stack, pattered using conventional photolithography techniques, is shown. The gate stack may include a gate oxide  142  (made from the gate dielectric layer  136 ), a work function metal  144  (made form the work function metal layer  138 ), and a gate conductor  146  (made from the gate material layer  140 ). A pair of dielectric spacers  148  may be formed using conventional photolithography techniques on opposite sides of the gate stack as shown in the figure. A pair of raised source/drain regions  150  may be formed by selective epitaxial Si growth. The pair of raised source/drain regions  150  may be either n-doped or p-doped. Typically, n-doped source/drain regions are used for forming p-channel field effect transistors (p-FETs), and p-doped source/drain regions are used for forming n-channel field effect transistors (n-FETs). However, the source/drain regions of one device on a semiconductor substrate may be n-doped while the source/drain regions of another device on the same semiconductor substrate may be p-doped. 
         [0044]    An inter-layer dielectric (ILD) layer  152  may be deposited on top of the isolation structure  100  using conventional deposition techniques known in the art. One or more contact via holes may be etched through the ILD  152  and then filled with a conductive material to form a device contact  154 . The device contact  154  may be use to make electrical connections to the semiconductor device, and more specifically the gate conductor  146 , and the pair of source/drain regions  150 . 
         [0045]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable other of ordinary skill in the art to understand the embodiments disclosed herein.