Abstract:
The invention relates to integrated circuit fabrication, and more particularly to an electronic device with an isolation structure made having almost no void. An exemplary method for fabricating an isolation structure, comprising: providing a substrate; forming a trench in the substrate; partially filling the trench with a first silicon oxide; exposing a surface of the first silicon oxide to a vapor mixture comprising NH3 and a fluorine-containing compound; heating the substrate to a temperature between 100° C. to 200° C.; and filling the trench with a second silicon oxide, whereby the isolation structure made has almost no void.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of U.S. Provisional Patent Application Ser. No. 61/179,107 filed on May 18, 2009 which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to integrated circuit fabrication, and more particularly to an electronic device with an isolation structure. 
       BACKGROUND 
       [0003]    Because miniaturization of elements in integrated circuit electronic devices drives the industry, the width and the pitch of active regions are increasingly becoming smaller, thus, the use of traditional local oxidation of silicon (LOCOS) isolation techniques is problematic. Shallow trench isolation (STI), because it creates relatively little of the bird&#39;s beak characteristic of LOCOS, is considered to be a more viable isolation technique. 
         [0004]    A conventional STI fabrication technique typically comprises: forming a pad oxide on an upper surface of a semiconductor substrate; forming a hardmask layer comprising nitride, such as silicon nitride, having a thickness generally greater than 600 Å, on the semiconductor substrate; forming an opening in the hardmask layer; performing anisotropic etching to form a trench in the semiconductor substrate; forming a thermal oxide liner in the trench and then filling the trench with silicon oxide as an insulating material; forming an overburden on the hardmask layer. Chemical vapor deposition (CVD) has been used extensively to deposit silicon oxide in the trench. During deposition, silicon oxide will collect on top corners of the trench, and overhangs will form at the top corners. These overhangs typically grow together faster than the trench is filled, and a void in the dielectric material filling the gap is created. 
         [0005]      FIG. 1  illustrates a partial cross-sectional view of a STI structure  19  having a void  18 . A pad oxide  12  is on a surface of a substrate  10  and a hardmask layer  14  is over the pad oxide  12 . A silicon oxide  16  having the void  18  is over the substrate  10  and a portion thereof is embedded in the substrate  10 . The void  18  is problematic in various respects. For example, any void  18  present in the trench fill can become a receptacle of polysilicon and/or metals during subsequent processing thereby increasing the likelihood of device instability and/or device failure. 
         [0006]    Accordingly, what is needed is a method for fabricating an isolation structure having no void in the silicon oxide from early stage of the isolation formation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0008]      FIG. 1  shows a partial cross-sectional view of a STI structure having a void; 
           [0009]      FIGS. 2   a - i  show schematic cross sections of a substrate processed according to an embodiment of a method for fabricating an isolation structure of the disclosure, showing various stages of fabrication, and 
           [0010]      FIG. 3  is a cross-sectional view of an electronic device having an isolation structure fabricated using the steps shown in  FIG. 2   a - i.    
       
    
    
     DETAILED DESCRIPTION 
       [0011]    It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
         [0012]      FIGS. 2   a - i  show schematic cross sections representing an isolation structure at various stages of feature formation in an embodiment of an electronic device manufacturing process. Referring to  FIG. 2   a,  a substrate  20  is provided. In one embodiment, the substrate  20  includes a silicon substrate (e.g., wafer) in crystalline structure. Other examples of the substrate  20  may include other elementary semiconductors such as germanium and diamond. Alternatively, the substrate  20  may include a compound semiconductor such as, silicon carbide, gallium arsenide, indium arsenide, or indium phosphide. The substrate  20  may include various doping configurations depending on design requirements (e.g., p-type substrate or n-type substrate). Further, the substrate  20  may include an epitaxial layer (epi layer), and/or may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure. 
         [0013]    Still referring to  FIG. 2   a,  a pad oxide layer  22  is formed over the top surface of the substrate  20 . The pad oxide layer  22  is preferably formed of silicon oxide grown by a thermal oxidation process, having a thickness of about 80 to 150 Å. For example, the pad oxide layer  22  can be grown by the rapid thermal oxidation (RTO) process or in a conventional annealing process which includes oxygen. A hardmask layer  24 , for example a silicon nitride or silicon oxynitride layer, is formed over the pad oxide layer  22 . The hardmask layer  24  can be deposited by, for example, a CVD process, or a low pressure CVD (LPCVD) process or a diffusion process. Preferably the formed hardmask layer  24  has a thickness of about 600 to 1500 Å. 
         [0014]    Referring to  FIG. 2   b,  following formation of the hardmask layer  24 , a patterned photo-sensitive layer (not shown) is formed on the hardmask layer  24 . A reactive ion etching (RIE) may, for example, be used to anisotropically etch through the hardmask layer  24  and the pad oxide layer  22  to form an opening  26  in the hardmask layer  24   a  and the pad oxide layer  22   a,  exposing a portion of the substrate  20 . 
         [0015]    Referring to  FIG. 2   c,  following formation of the opening  26  in the hardmask layer  24   a  and the pad oxide layer  22   a,  the exposed portion of the substrate  20  is etched to form a trench  28  having a predetermined depth of between about 300 to 3000 Å in the substrate  20 . Preferably, the trench  28  is etched to have sloped trench sidewalls, preferably having an angle between about 80°˜90° with rounded top and bottom rounded corners to minimize stress. Subsequently, the patterned photo-sensitive layer is stripped after the trench  28  formation. 
         [0016]    Referring to  FIG. 2   d,  following formation of the trench  28 , a liner layer (not shown) may be formed substantially conformal over the substrate  20 , including along the walls of the trench  28 . The liner layer is a dielectric layer (e.g., an oxide layer, nitride layer, oxynitride layer or combination thereof) formed by a thermal oxidation process or CVD process. Preferably, the liner layer may have a thickness of about 30 to 200 Å. In some embodiments, the liner layer is provided for reducing damage on the surface of the trench  28  created by the opening-etch process as set forth above. In some embodiments, the liner layer is not used. 
         [0017]    Still referring to  FIG. 2   d,  following formation of the liner layer, a first silicon oxide layer  30  is formed over the liner layer, partially filling the trench  28  and the opening  26 . The first silicon oxide layer  30  has less conformal step coverage so that it can be formed thicker at a top portion of the sidewalls than a bottom portion of the sidewalls of the trench  28  and the opening  26 . In other words, the first silicon oxide layer  30  is formed on the sidewalls of the trench  28  and the opening  26  to form a constricted opening having an overhang  32  structure, leading to a shadowing effect as the first silicon oxide layer  30  is deposited within the trench  28  and the opening  26 . 
         [0018]    Preferably, the first silicon oxide layer  30  can be formed using a high-density plasma chemical vapor deposition (HDP-CVD) process. HDP-CVD forms a pure oxide than other CVD processes, and it is preferred to have a more pure oxide in contact with the substrate  20 . For example, the first silicon oxide  30  can be deposited under a low frequency power less than 5000 W, a high frequency power less than 3500 W, a pressure less than 10 mTorr and a temperature of about 500 to 1000° C., using silane and oxygen as reacting precursors. The first silicon oxide layer  30  is preferably formed to a thickness of about 300 to about 2000 angstroms. 
         [0019]    Still referring to  FIG. 2   d,  following formation of the first silicon oxide layer  30  within the trench  28  and the opening  26 , an anneal process may be performed to increase the density of the first silicon oxide layer  30 . The anneal process results in the removal of any an interface between the liner layer (not shown) and the first silicon oxide layer  30 . The anneal process can be performed, for example, in a furnace, a rapid thermal process (RTP) system or other thermal system that is adapted to provide a thermal treatment for the first silicon oxide layer  30  to obtain a desired film quality. In some embodiments, the anneal process may be performed at about 1000° C. for about 20 seconds in a RTP system in an environment containing nitrogen, an inert gas or other gas that will not substantially react with the first silicon oxide layer  30 . In some embodiments, the anneal process is not performed. 
         [0020]    Referring to  FIG. 2   e,  after the first silicon oxide layer  30  formation process, a vapor phase etching process is used to remove the overhang  32  structure. The vapor phase etching process starts with introducing the structure of  FIG. 2   d  into a sealed reaction chamber in which the vapor phase etching process uses gas phase reactants. The etching process is self-limiting, in that amount of material removed is determined by amount of the gas phase reactants introduced into the reaction chamber. In some embodiments, the vapor phase etching process employed in the present disclosure comprises a vapor mixture  34  including at least an NH3 and a fluorine-containing compound employed as a catalyst and an etchant, respectively. The fluorine-containing compound may be a compound selected from the group consisting of HF or NF3. 
         [0021]    In one embodiment, the vapor mixture  34  comprises HF and NH3. The vapor mixture of NH3 and HF comprises a ratio of NH3 to HF between about 0.1 to 10, and preferably a ratio of 1 part NH3 to 1 part HF. In another embodiment, the vapor mixture  34  comprises NH3 and NF3. The vapor mixture of NH3 and NF3 comprises a ratio of NH3 to NF3 between about 0.5 to 5, preferably a ratio of 2 parts NH3 to 1 part NF3. 
         [0022]    The vapor phase etching process is a multiple step process. For a first step, a blanket adsorbed reactant film (not shown) of the vapor mixture  34  of fluorine-containing compound and NH3 vapor may be formed over the top surface of the first silicon oxide layer  30  in the reaction chamber. The blanket adsorbed reactant film is non-uniform due to the overhang  32  structure partially blocking the opening  26  and limiting entrance of the vapor mixture  34  of fluorine-containing compound and NH3 vapor into interior surface of the trench  28 . Because of the overhang  32 , less reaction gas reaches bottom of the trench  28 , so more of the overhang  32  reacts and less material is removed from the bottom of the trench  28 . In one embodiment, the first step using the vapor mixture  34  of NH3 and HF is performed at a pressure between 20 mTorr and 100 mTorr and at a temperature between 20° C. and 70° C. In another embodiment, the first step using the vapor mixture  34  of NH3 and NF3 is performed at a pressure between 2 Torr and 4 Torr and at a temperature between 20° C. and 70° C. 
         [0023]    For a second step, the adsorbed reactant film may react with the top surface of the first silicon oxide layer  30  in contact therewith to form a condensed and solid reaction product  36  beneath the adsorbed reactant film. In some embodiments, reaction radicals may be generated in a plasma from fluorine-containing compound and NH3 precursor gases in the reaction chamber. The reaction radicals may react with the top surface of the first silicon oxide layer  30  in contact therewith to form a condensed and solid reaction product  36 . 
         [0024]    Next, the reaction chamber may be heated to a temperature between 100° C. to 200° C. while sublimation products of the solid reaction product  36  may be pumped out from the reaction chamber. In alternative embodiments, the reaction chamber may be heated to a temperature between 100° C. to 200° C. while flowing a carrier gas over the substrate  20  to remove sublimation products of the solid reaction product  36  from the reaction chamber. The carrier gas can be any inert gas. Preferably, the carrier gas comprises N2, He, or Ar. In some embodiments, the substrate  20  is transferred into a heated chamber that is heated to a temperature between 100° C. to 200° C. while sublimation products of the solid reaction product  36  may be pumped out from the heated chamber. In alternative embodiments, the substrate  20  is transferred into a heated chamber that is heated to a temperature between 100° C. to 200° C. while flowing a carrier gas over the substrate  20  to remove sublimation products of the solid reaction product  36  from the heated chamber. The carrier gas can be any inert gas. Preferably, the inert gas includes N2, He, and Ar. 
         [0025]    This reaction proceeds until solid reaction product  36  is removed; and continues until less thickness of the interior surface of the trench  28  is removed. Accordingly, at the end of the vapor phase etching process  34  shown in  FIG. 2   f,  a substantial amount of the first silicon oxide layer  30   a  is remained, resulting in reduced aspect ratio of the opening  26  and the trench  28 . In some embodiments, the vapor phase etching process  34  may fully etch the overhang  32  structure, exposing sidewall surfaces of the hardmask layer  24   a,  the pad oxide layer  22   a  and the silicon substrate  20  (not shown). However, it is preferable not to etch through the hardmask layer  24   a  by the vapor phase etching process  34 . The attacked hardmask layer  24   a  may not serve as a stop layer in subsequent processes thereby increasing the likelihood of active area damage. In one embodiment, a ratio of removal rates by the vapor mixture  34  of the first silicon oxide  30  and the hardmask layer  24   a  is greater than 10. In other words, the first silicon oxide layer  30  removal rate is greater than 10 times of the removal rate of the hardmask layer  24   a.  Furthermore, the silicon substrate  20  is preferably not attacked by the vapor phase etching process  34 . The attacked silicon substrate  20  will act as a source of crystal defects in subsequent processes thereby increasing the likelihood of electrical leakage. In one embodiment, a ratio of removal rates by the vapor mixture  34  of the first silicon oxide  30  and the silicon substrate  20  is greater than 30. In other words, the first silicon oxide layer  30  removal rate is greater than 30 times of the removal rate of the silicon substrate  20 . Furthermore, repeated deposition/etch sequence may be required as more reduced aspect ratio of the opening  26  and the trench  28  is needed. 
         [0026]    Referring to  FIG. 2   g,  following formation of the reduced aspect ratio of the opening  26  and the trench  28 . A second silicon oxide layer  30   b  is formed over the first silicon oxide layer  30   a  to a sufficient thickness to form a void-free silicon oxide layers  30   a  and  30   b  within the opening  26  and the trench  28 . For example, the second silicon oxide layer  30   b  is preferably deposited to a thickness of 4000 to 8000 Å. In one embodiment, the second silicon oxide layer  30   b  can be formed by a CVD process, such as HDP CVD process or sub-atmospheric CVD (SACVD) process. For example, the second silicon oxide layer  30   b  comprises a HDP-CVD oxide layer. Other deposition can be used because the second oxide layer  30   b  can be less pure than the first oxide layer  30 . The second silicon oxide layer  30   b  can be deposited under a low frequency power less than 5000 W, a high frequency power less than 3500 W, a pressure less than 10 mTorr and a temperature of about 500 to 1000° C., using silane and oxygen as reacting precursors. For another example, the second silicon oxide layer  30   b  comprises a sub-atmospheric undoped-silicon glass (SAUSG) layer. The second silicon oxide layer  30   b  can be deposited under a pressure of about 500 to 700 Torr and a temperature of about 500 to 600° C., using tetraethoxysilane (TEOS) and O 3  as reacting precursors. In other embodiment, the second silicon oxide layer  30   b  can be formed by a spin-on-dielectric (SOD) process, for example, the first silicon oxide layer  30   a  is spin coated with a material comprising the second silicon oxide layer  30   b,  such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ). The spin-coated material is baked at a temperature of 150 to 300° C., and then cured at 400 to 450° C. in a furnace or a hot-plate bake tool to form the second silicon oxide layer  30   b.    
         [0027]    Still referring to  FIG. 2   g,  following formation of the second silicon oxide layer  30   b  within the trench  28  and the opening  26 , an anneal process is performed to increase the density of the void-free silicon oxide layers  30   a  and  30   b.  This results in an interface between the first silicon oxide layer  30   a  and the second silicon oxide layer  30   b  that will disappear after the anneal process. The anneal process can be performed, for example, in a furnace, a rapid thermal process (RTP) system or other thermal system that is adapted to provide a thermal treatment for the void-free silicon oxide layers  30   a  and  30   b  to obtain a desired film quality. In some embodiments, the anneal process may be performed at about 1000° C. for about 20 seconds in a RTP system in an environment containing nitrogen, an inert gas or other gas that will not substantially react with the void-free silicon oxide layers  30   a  and  30   b.    
         [0028]      FIG. 2   h  shows the substrate  20  of  FIG. 2   g  after a planarization process, such as a chemical mechanical polishing (CMP) process, is performed to remove portions of the void-free silicon oxide layers  30   a  and  30   b  above the hardmask layer  24   a  to expose the hardmask layer  24   a,  thereby leaving a void-free silicon oxide layer  30   c  respectively filling the trench  28  and the openings  26 . The hardmask layer  24   a  also serves as a stop layer for stopping the planarization process on the hardmask layer  24   a.  In some embodiments, a top surface of the void-free silicon oxide layer  30   c  is coplanar with, or substantially coplanar with, the hardmask layer  24   a.    
         [0029]    Referring to  FIG. 2   i,  after the planarization process, the hardmask layer  24   a  is removed by a wet chemical etching process, for example, by dipping the substrate  20  in hot phosphoric acid (H 3 PO 4 ), exposing a top surface of the pad oxide layer  22   a.  Because the wet chemical etching process has higher etch selectivity for nitride than to oxide, the etch process removes the hardmask layer  24   a  faster than the void-free silicon oxide layer  30   c.  Accordingly, the remaining void-free silicon oxide layer  30   c  extends over a top surface of the pad oxide layer  22   a.    
         [0030]    Still referring to  FIG. 2   i,  subsequent to the hardmask layer removal process, the pad oxide layer  22   a  is removed by a vapor phase etching process or a wet etching process, for example, by dipping the substrate  20  in hydrofluoric (HF), exposing the top surface of the substrate  20 . Since the wet chemical etching process has almost no selectivity for the pad oxide layer  22   a  and the void-free silicon oxide layer  30   c,  the void-free silicon oxide layer  30   c  may lose almost the same thickness as the pad oxide layer  22   a  does. Accordingly, at the end of the wet etching process, a silicon oxide layer  30   d  made has almost no void and serves as an isolation structure  38  between electronic devices. The isolation structure  38  still partially protrudes over a top surface of the substrate  20 . Accordingly, the above method of fabricating an isolation structure produces a void-free silicon oxide layer  30   d.    
         [0031]    Referring to  FIG. 3 , an electronic device such as a metal-oxide-semiconductor (MOS) transistor  400  can be formed over a portion of the substrate  20  adjacent to the isolation structure  38 . Fabrication of the MOS transistor  400  is well known to those skilled in the art and is thus not described here, for brevity. The MOS transistor  400  now includes source/drain regions  402  formed in a portion of the substrate  20 , a gate stack comprised of a gate dielectric layer  404  and a gate electrode  406  sequentially formed over the substrate  20 , and spacers  408  respectively formed on both sidewalls of the gate stack. In some embodiment, the electronic device comprises a gate with a gate length less than 32 nm. 
         [0032]    In some embodiments, the gate dielectric layer  404  may comprise silicon oxide, silicon oxynitride, a high-k dielectric layer or combinations thereof. The high-k dielectric layer may comprise hafnium oxide (HfO 2 ), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), metal oxides, metal nitrides, metal silicates, transition metal-oxides, transition metal-nitrides, transition metal-silicates, oxynitrides of metals, metal aluminates, zirconium silicate, zirconium aluminate, silicon nitride, silicon oxynitride, zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO 2 —Al 2 O 3 ) alloy, other suitable high-k dielectric materials, and/or combinations thereof. The gate dielectric layer  404  may further comprise an interfacial layer to reduce damage between the gate dielectric layer  404  and the substrate  20 . The interfacial layer may comprise silicon oxide. 
         [0033]    In some embodiments, the gate electrode  406  may comprise a polysilicon gate and/or a metal gate. The metal gate may comprise one or more layers including Ti, TiN, TaN, Ta, TaC, TaSiN, W, WN, MoN, MoON, RuO 2 , and/or other suitable materials. The metal gate may comprise one or more layers formed by PVD, CVD, ALD, plating, and/or other suitable processes. The metal gate may be formed by a gate-first or a gate-last metal gate fabrication process. 
         [0034]    While the preferred embodiments have been described by way of example it is to be understood that the scope of invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the disclosure should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. The disclosure can be used to form or fabricate an isolation structure with a void-free silicon oxide layer. In this way, an isolation structure or region is formed with a void-free silicon oxide layer.