Abstract:
The disclosed subject matter provides a Fin-FET with a thinned-down InP layer and thinning-down method thereof. In a Fin-FET, the fin structure is made of InGaAs and an InP layer is formed to cover the fin structure. The InP layer is obtained from an initial InP layer formed on the fin structure through a thinning down process including converting a surface portion of the InP layer into a Phosphor-rich layer and removing the Phosphor-rich layer. The thickness of the ultimately-formed InP layer is less than or equal to 1 nm. According to the disclosed method, the InP layer in the Fin-FET may be easily thinned down, and during the thinning-down process, contamination may be avoided.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Chinese Patent Application No. CN201510531734.7, filed on Aug. 26, 2015, the entire content of which is incorporated herein by reference. 
       FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure generally relates to the field of semiconductor fabrication technology and, more particularly, relates to a method to thin down an InP layer during the fabrication of a Fin-FET. 
       BACKGROUND 
       [0003]    With the development of the semiconductor integrated circuit (IC) technology, the critical dimension of semiconductor devices also continuously becomes smaller. For conventional metal-oxide-semiconductor (MOS) field-effect transistors (FET), such a small critical dimension may lead to the short-channel effect as well as other disadvantages. With a relatively large channel, Fin-FET may overcome the short-channel effect and, thus, has been widely used. However, for Fin-FET with a fin structure made of Si, when the critical dimension shrinks to approach 16 nm, the carrier mobility in the channel may be low due to low mobility of electrons in Si, leading to a relatively small drive current and high energy consumption for the Fin-FET. Therefore, a material with higher electron mobility may be required to replace Si for fabricating the fin structure. 
         [0004]    Certain technology is developed to use a Fin-FET with the fin structure made of III-V group elements (such as InGaAs). Because of the low energy consumption characteristics, the III-V group elements provide higher carrier mobility. However, when the critical dimension further decreases to 7 nm or even 5 nm, lower energy consumption of the Fin-FET may be required. 
         [0005]    The Fin-FET can also have the fin structure made of InGaAs and covered by an InP layer. Because the lattice mismatch between the InP layer and InGaAs is small while the electron saturation velocity is high, the Fin-FET may demonstrate characteristics of low energy consumption. However, due to the limitation of process conditions, an InP layer formed by existing methods is usually thick. A thick InP layer may not be good for heat dispersion. In addition, such a thick InP layer may also require a relatively large drive voltage. Therefore, the InP layer may need to be thinned down to approach a thickness equal to or less than 1 nm. 
         [0006]    Currently, a method for thinning down the InP layer often includes performing an ashing process on the InP layer using oxygen gas and then performing a wet etching process at room temperature to remove the ashed portion of the InP layer. An etch solution mixed by diluted sulfuric acid and water at a 1:1 ratio may be used during the wet etching process. 
         [0007]    However, the wet etching process may introduce some contaminants, which may affect the performance of the device. In addition, as the device may need to be alternatively handled in dry and wet operation environments, the fabrication process may be more complicated, the processing time may be longer, and the probability of getting the device contaminated may also increase. 
         [0008]    The disclosed fabrication methods and 3D transistors are directed to solve one or more problems set forth above and other problems in the art. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    One aspect of the present disclosure includes a method for thinning down InP layer. The method includes providing a fin structure, wherein the fin structure is made of InGaAs, forming an InP layer to cover the fin structure, converting a surface portion of the InP layer into a Phosphor-rich layer, and removing the Phosphor-rich layer. After removing the 
         [0010]    Phosphor-rich layer, the remaining portion of the InP layer becomes a thinned-down InP layer with a thickness less than or equal to 1 nm. 
         [0011]    Another aspect of the present disclosure provides a Fin-FET. The Fin-FET includes a fin structure formed on a semiconductor substrate. The fin structure is made of InGaAs. The Fin-FET further includes an InP layer formed on the fin structure with a thickness less than or equal to 1 nm. The InP layer is obtained from an initial InP layer formed on the fin structure through a thinning down process including converting a surface portion of the InP layer into a Phosphor-rich layer and removing the Phosphor-rich layer. The Fin-FET also includes a dielectric layer formed on the InP layer and a gate electrode formed on the dielectric layer. 
         [0012]    Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure. 
           [0014]      FIG. 1  illustrates a flowchart of an exemplary fabrication process consistent with disclosed embodiments; and 
           [0015]      FIGS. 2-7  illustrate schematic views of semiconductor structures corresponding to certain stages of another exemplary fabrication method consistent with disclosed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
         [0017]    As described in above background section, existing methods to thin down an InP layer formed on an InGaAs fin structure usually includes performing an ashing process on the InP layer using oxygen gas and then performing a wet etching process at room temperature to remove the ashed portion of the InP layer. During the wet etching process, an etch solution mixed by diluted sulfuric acid and water at a 1:1 ratio may be used. However, the wet etching process may introduce some contaminants, which may affect the performance of the device. In addition, as the device may need to be alternatively handled in dry and wet operation environments, the fabrication process may be more complicated, the processing time may be longer, and the probability of getting the device contaminated may also increase. 
         [0018]    Accordingly, the present disclosure provides an improved method to thin down the InP layer formed on the InGaAs fin structure.  FIG. 1  shows a flowchart of an exemplary fabrication process consistent with various disclosed embodiments.  FIGS. 2-7  show schematic views of the structures corresponding to certain stages of the exemplary fabrication method illustrated in  FIG. 1 . 
         [0019]    Referring to  FIG. 1 , at the beginning of the fabrication process, a plurality of fin structures formed on a semiconductor substrate is provided (S 101 ).  FIG. 2  shows a schematic cross-section view of the semiconductor substrate, the active region, and the two isolation structures. 
         [0020]    Referring to  FIG. 2 , a semiconductor substrate  200  is provided. A plurality of fin structures  201  made of InGaAs is formed on the semiconductor substrate  200 . 
         [0021]    The semiconductor substrate  200  may have a single-layer structure or a multiple-layer structure. When the semiconductor substrate  200  has a single-layer structure, the semiconductor substrate  200  may be made of one of Si, Ge, SiGe, SiC, GaN, etc. When the semiconductor substrate  200  has a multiple-layer structure, the semiconductor substrate  200  may include a semiconductor layer (not shown) and one or more interlayer dielectric layers (not shown) formed on the surface of the semiconductor layer. 
         [0022]    The plurality of fin structures  201  may be made of InGaAs. InGaAs is a III-V group compound and has characteristics of low energy consumption. Therefore, the carrier mobility in the fin structures  201  may be improved. 
         [0023]    The cross-section of each fin structure along a direction perpendicular to the elongation direction of the fin structure may be a rectangle, a trapezoid, or a combination of rectangle and trapezoid. In one embodiment, the cross-section of each fin structure along a direction perpendicular to the elongation direction of the fin structure is a trapezoid. The plurality of fin structures  201  may be fabricated on the semiconductor substrate  200  by any appropriate method known in the field. 
         [0024]    Further, returning to  FIG. 1 , an InP layer may be formed on the fin structures (S 103 ).  FIG. 3  shows a schematic cross-section view of the corresponding semiconductor structure. 
         [0025]    Referring to  FIG. 3 , an InP layer  203  may be formed to cove the fin structures  201 . The InP layer  203  may be formed by any appropriate method know in the field and no specific restriction is required. For example, the InP layer  203  may be formed by a physical vapor deposition method such as plasma sputtering, molecular beam epitaxy (MBE), e-beam evaporation, pulsed-laser deposition, magnetron sputtering, etc. In addition, in certain embodiments, a metal organic chemical vapor deposition (MOCVD) method may also be adopted to form the InP layer  203 . Due to the limitation of the processing conditions, the thickness of the InP layer formed by above methods may be at least several tens of nanometers. 
         [0026]    In one embodiment, the InP layer is formed by an MOCVD method and the thickness of the InP layer  203  is 30 nm. In a subsequent process, the thickness of the InP layer may be reduced in order to reach a relatively thin InP layer. 
         [0027]    Further, returning to  FIG. 1 , a surface portion of the InP layer may be converted into a Phosphor-rich layer (S 105 ).  FIG. 4  shows a schematic cross-section view of the corresponding semiconductor structure. 
         [0028]    Referring to  FIG. 4 , a surface portion of the InP layer  203  (referring to  FIG. 3 ) formed on the fin structure  201  may be converted into a Phosphor-rich layer  205 . The Phosphor-rich layer  205  may be formed by a dry etching process. The dry etching process may use an etch gas including Cl 2  and Ar. During the dry etching process, a portion of the indium atoms near the surface of the InP layer  203  (referring to  FIG. 3 ) may be removed. Thus, with a major ingredient of Phosphor, the remained surface portion becomes the Phosphor-rich layer  205 . The thickness of the Phosphor-rich layer  205  after the dry etching process may be less than 1 nm. The portion of the InP layer  203  (referring to  FIG. 3 ) not affected by the dry etching process, i.e., the ultimately-formed InP layer after the thinning-down process, is labeled as  207 . 
         [0029]    In one embodiment, the dry etching process to convert the surface portion of the InP layer  203  may be a chemical dry etching process. The process parameters for the chemical dry etching process may include a process temperature in a range of −10 to 200° C., a process pressure in a range of 5 to 500 mTorr, a power in a range of 100 to 1000 W, a flow rate of Cl 2  in a range of 10 to 200 sccm, and a flow rate of Ar in a range of 10 to 500 sccm. The process conditions for such a chemical dry etching process may be mild so that damages to the surface of the unconverted InP layer, i.e., the ultimately-formed InP layer  207 , during the thinning-down process may be reduced, which further ensures that the performance of the Fin-FET is not affected. 
         [0030]    Further, returning to  FIG. 1 , the Phosphor-rich layer may be removed (S 107 ).  FIG. 5  shows a schematic cross-section view of the corresponding semiconductor structure. 
         [0031]    Referring to  FIG. 5 , the Phosphor-rich layer  205  is removed. The Phosphor-rich layer  205  may be removed by a dry etching process. The dry etching process may use HBr as an etch gas. During the dry etching process, the Phosphor-rich layer  205  is gradually removed and only the unconverted portion of the InP layer, i.e., the ultimately-formed InP layer  207 , remains. As such, the goal to thin down the InP layer  203  may be reached. 
         [0032]    In one embodiment, the dry etching process to remove the Phosphor-rich layer  205  is a chemical dry etching process. The process parameters for the chemical dry etching process may include a process pressure in a range of 5 to 500 mTorr, a power in a range of 100 to 1000 W, and a flow rate of HBr in a range of 50 to 500 sccm. The process conditions for such a chemical dry etching process may be mild so that damages to the surface of the unconverted InP layer, i.e. the ultimately-formed InP layer  207 , during the thinning-down process may be reduced, which further ensures that the performance of the Fin-FET is not affected. 
         [0033]    In other embodiments, the chemical dry etching process to form the Phosphor-rich layer  205  using Cl 2  and Ar as an etch gas and the subsequent chemical dry etching process to remove the Phosphor-rich layer  205  using HBr as an etch gas may be performed multiple times in order to obtain a desired thickness for the ultimately-formed InP layer  207 . During the dry etching process to form the Phosphor-rich layer  205 , the process temperature and the flow rate of 
         [0034]    Ar may directly affect the etching rate. Specifically, with a higher process temperature, the etching rate may be higher; while with a higher flow rate of Ar, the etching rate may also be higher. In addition, during each HBr etching process, the thickness of the removed Phosphor-rich layer  205  may be less than 1 nm. 
         [0035]    The thickness of the ultimately-formed InP layer  207  after being thinned down may not be greater than 1 nm. When the thickness of the InP layer is equal to or less than 1 nm, the heat dissipation effect may be desired while the required drive voltage is also relatively small. 
         [0036]    The fabrication method may further include forming a dielectric layer on the surface of each fin structure and a gate electrode on the dielectric layer to complete Fin-FET structures.  FIG. 6  shows a schematic cross-section view of the corresponding semiconductor structure. 
         [0037]    Referring to  FIG. 6 , a gate structure may be formed on the thinned-down InP layer  207 . The gate structure may across the whole length of the corresponding fin structure  201 . Specifically, the gate structure includes a dielectric layer  209  formed on the thinned-down InP layer  207  and a gate electrode  211  formed on the dielectric layer  209 . The gate structures may be formed by any appropriate method know in the field. 
         [0038]    In certain other embodiments, the Phosphor-rich layer  205  may be removed by a method different from the HBr chemical dry etching process described above. For example, the Phosphor-rich layer  205  may be removed by an oxygen treatment process followed by a H 2  dry etching process. Specifically, the Phosphor-rich layer  205  may be treated by O 2  and then the O 2 -treated Phosphor-rich layer may be removed by a H 2  dry etching process.  FIG. 7  shows a schematic cross-section view of the corresponding semiconductor structure after the oxygen treatment process. 
         [0039]    Referring to  FIG. 7 , an O 2 -treated Phosphor-rich layer  205 ′ is formed by using O 2  to treat the Phosphor-rich layer  205 . A hydrogen dry etching process is then preformed to remove the O 2 -treated Phosphor-rich layer  205 ′ and obtain a thinned-down InP layer. As such, the goal to thin down the InP layer  203  may be reached. 
         [0040]    The oxygen treatment and the hydrogen dry etching are both chemical dry etching process. The process parameters for the oxygen treatment process may include a process pressure in a range of 5 to 500 mTorr, a power in a range of 100 to 1000 W, and a flow rate of O 2  in a range of 10 to 200 sccm. The process parameters for the chemical dry etching process to remove the O 2 -treated Phosphor-rich layer  205 ′ may include a process pressure in a range of 5 to 500 mTorr, a power in a range of 100 to 1000 W, and a flow rate of H 2  in a range of 50 to 500 sccm. The process conditions for chemical dry etching processes may be mild so that damages to the surface of the unconverted InP layer, i.e. the ultimately-formed InP layer  207 , during the thinning-down process may be reduced, which further ensures that the performance of the Fin-FET is not affected. 
         [0041]    In certain other embodiments, the process to form a Phosphor-rich layer and the subsequent process to remove the Phosphor-rich layer may need to be repeated n times with n no less than 2. When Ar is used to form a Phosphor-rich layer, the process temperature and the flow rate of Ar may directly affect the etching rate. Specifically, with a higher process temperature, the etching rate may be higher; while with a higher flow rate of Ar, the etching rate may also be higher. In addition, during each oxygen treatment process and the following H 2  dry etching process, the thickness of the removed Phosphor-rich layer  205  may be less than 1 nm. 
         [0042]    Therefore, the etched thickness may be controlled by adjusting the process temperature, the Ar flow rate, and the etching time. When the Phosphor-rich layer formed each time is thinner, the process to form and remove Phosphor-rich layer may need to be repeated more times; in the meantime, the thickness of the ultimately thinned-down InP layer may also be more precisely controlled and damage to the surface of the thinned-down InP layer may also be reduced. In one embodiment, the thickness of the Phosphor-rich layer formed each time is smaller than 1 nm. 
         [0043]    According to the disclosed methods, a surface portion of the InP layer formed on the fin structure may be converted into a Phosphor-rich layer in order to reduce the thickness of the InP layer. The Phosphor-rich layer may be easily removed, and during the removal process, contaminations may not be induced. 
         [0044]    Further, the Phosphor-rich layer may be removed by a dry etching process. During the etching process, contaminations may be less. In addition, using a dry etching process to remove the Phosphor-rich layer may also reduce alternation of dry and wet operation environments for the device. Therefore, the processing time may be reduced, and the probability of getting the device contaminated may also decrease. 
         [0045]    Further, according to the disclosed methods, the dry etching process to form or remove the Phosphor-rich layer is a chemical dry etching process. For such a chemical dry etching process, the process conditions may be mild so that damages to the devices may be reduced. 
         [0046]    Further, according to the disclosed methods, the process to form Phosphor-rich layer and the process to remove Phosphor-rich layer may be repeated multiple times. Therefore, controllability of process engineering may be increased, which further ensures the performance of the device. 
         [0047]    The above detailed descriptions only illustrate certain exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention. Those skilled in the art can understand the specification as whole and technical features in the various embodiments can be combined into other embodiments understandable to those persons of ordinary skill in the art. Any equivalent or modification thereof, without departing from the spirit and principle of the present invention, falls within the true scope of the present invention.