Patent Publication Number: US-9847422-B2

Title: Semiconductor device and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/006,522 filed on Jan. 26, 2016 which is a continuation of U.S. patent application Ser. No. 14/635,034 filed on Mar. 2, 2015, now U.S. Pat. No. 9,263,588 issued Feb. 16, 2016 which is a continuation of U.S. application Ser. No. 14/538,046 filed on Nov. 11, 2014, now U.S. Pat. No. 9,184,232 issued Nov. 10, 2015, which is a continuation of U.S. application Ser. No. 13/960,434 filed on Aug. 6, 2013, now U.S. Pat. No. 8,969,939, issued Mar. 3, 2015 which is a continuation application of U.S. patent application Ser. No. 13/553,386, filed Jul. 19, 2012, now U.S. Pat. No. 8,519,456 issued Aug. 27, 2013, which is a continuation application of U.S. patent application Ser. No. 13/079,635, filed Apr. 4, 2011, now U.S. Pat. No. 8,247,859 issued Aug. 21, 2012, which is a continuation application of U.S. patent application Ser. No. 12/906,652, filed Oct. 18, 2010, now U.S. Pat. No. 7,928,495 issued Apr. 19, 2011, which is a divisional application of U.S. patent application Ser. No. 11/931,571, filed Oct. 31, 2007, now U.S. Pat. No. 7,833,875, issued Nov. 16, 2010, which is a continuation-in-part application of U.S. patent application Ser. No. 11/149,396, filed Jun. 9, 2005, now U.S. Pat. No. 7,342,280 issued Mar. 11, 2008, which is a divisional application of U.S. patent application Ser. No. 10/446,970, filed May 28, 2003, now U.S. Pat. No. 6,913,969 issued Jul. 5, 2005, the disclosures of which are herein incorporated by reference in their entirety. 
     This application also claims the benefit of Korean Patent Application No. 10-2007-0038327, filed on Apr. 19, 2007, in the Korean Intellectual Property Office, the disclosure of which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method of fabricating the same. 
     2. Description of Related Art 
     As semiconductor device integration increases, a size of an active region on which a channel is formed is reduced. Semiconductor devices having small active regions may exhibit high leakage currents and low driving performances. For example, when a channel length is reduced, a short channel effect can occur, and when a channel width is reduced, a driving current can be decreased. 
     Accordingly, there is a need to increase a channel area in highly integrated semiconductor devices. For example, the active region can have a larger surface area by forming a protrusion in the active region with respect to an isolation layer. In the active region, sidewalls as well as an upper surface can be used as a channel, and thus, the driving performance of the semiconductor device can be increased. 
     When the sidewalls of an active region are used as a channel, an electric field can be enhanced in an edge portion of the active region. The electric field increases as the curvature radius of the edge portion of the active region decreases. A threshold voltage of a semiconductor device can be changed according to the profile of the edge portion of the active region, and the threshold voltages between semiconductor devices respectively fabricated using a single wafer or a single batch have a wide distribution range. The wide distribution range of the threshold voltages decreases the reliability of the semiconductor device. 
     Furthermore, the profile of the edge portion of the active region can affect the programming characteristics of a non-volatile memory device. When the electric field is enhanced in the edge portion of the active region, more tunneling effects of electrons or holes may occur in the edge portion of the active region, As a result, a tunneling insulating layer disposed on the edge portion of the active region deteriorates, and the non-volatile memory device has low durability and low high-temperature reliability. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, an isolation layer may be recessed from the surface of a semiconductor substrate. An active region may be defined in the semiconductor substrate by the isolation layer, the active region protruding upward with respect to the isolation layer. A curvature radius of edge portions of the active region may be in the range from about ⅓ to about ½ of the width of an upper portion of the active region. 
     According to another embodiment of the present invention, a method of fabricating a semiconductor device includes forming an isolation layer defining an active region in a semiconductor substrate. A plurality of recessing processes may be performed on the isolation layer to expose edge portions of the active region. A plurality of rounding processes may be performed to round the edge portions of the active region. 
     The rounding processes and the recessing processes may be performed alternately. 
     According to another embodiment of the present invention, at least one rounding process from among a plurality of rounding processes may include etching edge portions of the active region. 
     According to another embodiment of the present invention, at least one rounding process from among a plurality of rounding processes may include oxidizing edge portions of the active region. 
     According to another embodiment of the present invention, a method of fabricating a semiconductor device includes forming an isolation layer defining an active region in a semiconductor substrate. A first recessing process may be performed on the isolation layer to expose edge portions of the active region. A first rounding process may be performed to round the edge portions of the active region. A second recessing process may be performed on the isolation layer. A second rounding process may be performed to round the edge portions of the active region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a sectional view of a semiconductor device according to an embodiment of the present invention; 
         FIGS. 2 through 6  are sectional views illustrating a method of fabricating a semiconductor device according to an embodiment of the present invention; 
         FIG. 7  is a sectional view illustrating a method of fabricating a semiconductor device according to another embodiment of the present invention; 
         FIG. 8  is a sectional view illustrating a method of a semiconductor device according to another embodiment of the present invention; 
         FIG. 9  is a transmission electron microscopic (TEM) image of a sectional view of a semiconductor device fabricated according to a comparative example of the present invention; 
         FIG. 10  is a TEM image of a sectional view of a semiconductor device fabricated according to an example of the present invention; 
         FIG. 11  is a graph illustrating the distribution of threshold voltages of a semiconductor device prepared according to the comparative example of the present invention; and 
         FIG. 12  is a graph illustrating the distribution of threshold voltages of a semiconductor device prepared according to an example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to embodiments set forth herein; rather, embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements, and thus their description will be omitted. 
     A semiconductor device according to embodiments of the present invention may include memory devices and/or logic devices. 
       FIG. 1  is a sectional view of a semiconductor device according to an embodiment of the present invention. 
     Referring to  FIG. 1 , an active region  115  may be defined in a semiconductor substrate  105  by an isolation layer  110 . The semiconductor substrate  105  includes, for example, silicon (Si), germanium (Ge), or silicongermanium (SiGe). The active region  115  may be used to form an active device such as a transistor or a capacitor. The isolation layer  110  may electrically isolate the active device. The isolation layer  110  may include an insulating layer, for example, an oxide layer or nitride layer. 
     The isolation layer  110  is, for example, a shallow trench isolation (STI) layer. The isolation layer  110  may be formed by filling a trench extending to an inner portion of the semiconductor substrate  105 . The isolation layer  110  may be recessed from a surface of the semiconductor substrate  105  to a predetermined depth. As a result, edge portions E of the active region  115  may be exposed by the isolation layer  110 . The isolation layer  110  may be recessed to expose a part  120   b ′ of sidewalls  120  of the active region  115 . 
     The surface of the active region  115  exposed by the isolation layer  110  may be used as a channel that is a conductive passage for charges. A gate electrode (not shown) may cover the exposed surface of the active region  115 . The active region  115  protruding with respect to the isolation layer  110  may have a different structure from a planar-type structure, that is, a fin-type structure. Accordingly, the structure of the active region  115  may provide a greater driving current than the planar structure, and thus, the driving performance of a semiconductor device may be improved. 
     The edge portions E of the active region  115  may be rounded. Such a rounded shape may substantially prevent enhancement of an electric field generated from the gate electrode at the edge portions E of the active region  115 . As a result, threshold voltage irregularity due to irregular electron fields at the edge portions E of the active region  115  can be decreased, and reliability of a semiconductor device can be improved. 
     For example, a curvature radius R of the edge portions E of the active region  115  may be in the range from about ⅓ to about ½ of the width W of an upper portion of the active region  115 . When the curvature radius R is smaller than about ⅓ of the width W, an electric field enhancement decrease effect is small, and thus, threshold voltages may be irregular. When the curvature radius R is about ½ of the width W, the upper portion of the active region  115  is rounded and has a curvature radius, and a high electric field enhancement decrease effect can be obtained. When the curvature radius R is greater than about ½ of the width W, the upper portion of the active region  115  may have a sharp pointed part, and thus, an electric field enhancement may occur. 
     When the semiconductor device according to an embodiment of the present invention is a non-volatile memory device, an electric field enhancement decrease at the edge portions E of the active region  115  may contribute to high reliability of a tunneling insulating layer (not shown) of the non-volatile memory device, where a local electric field enhancement may cause tunneling of charges in a portion of the tunneling insulating layer on the active region  115 . In addition, the active region  115  has a larger surface area and a charge storage layer formed on the active region  115  may also have a larger area. For the active region  115  having a larger surface area, the charge storage layer may store more charges, and reliability of multi-bit operation using a local charge trap can be improved. 
       FIGS. 2 through 6  are sectional views illustrating a method of fabricating a semiconductor device according to an embodiment of the present invention. 
     Referring to  FIG. 2 , an isolation layer  110  may be formed on a semiconductor substrate  105  to define an active region  115 . For example, a trench (not shown) is formed in the semiconductor substrate  105 , and filled with an insulating layer. For example, the insulating layer may include an oxide layer or a nitride layer. The insulating layer may be planarized using an etch-back method or a chemical mechanical polishing (CMP) method. The planarizing process can be performed using a protective layer (not shown) disposed on the active region  115  as a stop point, e.g., an etch stop. At this time, the isolation layer  110  may surround the sidewalls  120  of the active region  115  and may protrude from the surface of the semiconductor substrate  105  to a predetermined height. 
     Referring to  FIG. 3 , a first recessing process is performed to recess the isolation layer  110  so that edge portions E of the active region  115  are exposed by the isolation layer  110 . For example, the isolation layer  110  can be recessed to expose a first portion  120   a  of the sidewalls  120 . The height of the first portion  120   a  can be adjusted according to a number of rounding processes and a rounding efficiency. 
     For example, the first recessing process of the isolation layer  110  can be performed using a wet etching method or a dry etching method. When the isolation layer  110  is an oxide layer, the wet etching may be performed using a HF solution. 
     Referring to  FIG. 4 , a first rounding process is preformed to round the edge portions E of the active region  115 . For example, the first rounding process may be performed by partially etch the active region  115 . The edge portions E of the active region  115  can be rounded since the edge portions E of the active region  115  having a wide surface area are more quickly etched than other portions of the active region  115 . The width of a first portion  120   a ′ may be smaller than the width of a first portion  120   a  (see  FIG. 3 ) due to the first rounding process. 
     For example, the active region  115  can be isotropically and/or anisotropically etched. The isotropic etching may be performed using a wet etching method or a chemical dry etching (CDE) method, For example, the wet etching method may use a mixture (SCI) solution of NH 4 OH, H 2 O 2 , and H 2 O. The anisotropic etching can be performed using a plasma dry etching method. The rounding process can be performed using the anisotropic etching according to the shape of the active region  116  and the concentration of radicals in plasma, 
     Referring to  FIG. 5 , a second recessing process is performed on the isolation layer  110 . For example, the isolation layer  110  can be recessed to expose a second portion  120   b  of the sidewalls  120  of the active region  115 . The height of the second portion  120   b  may be larger than the height of the first portion  120   a . The second recessing process may be performed on the isolation layer  110  using, for example, a wet etching method or a dry etching method, 
     Referring to  FIG. 6 , a second rounding process is performed to round the edge portions E of the active region  115 . For example, the second rounding process can be performed by partially etching the active region  115 . The edge portions E of the active region  115  can be rounded since the edge portions E of the active region  115  having a wide surface area are more quickly etched than other portions of the active region  116 . The width of a second portion  120   b ′ may be smaller than the width of the first portion  120   a ′ (see  FIG. 4 ) due to the second rounding process. For example, the active region  116  can be isotropically and/or anisotropically etched, as described with reference to the first rounding process. 
     The isolation layer  110  is gradually recessed through the first and second rounding processes. The first portion  120   a  of the sidewalls  120  of the active region  115  is exposed, and then the second portion  120   b  of the sidewalls  120  of the active region  115  is exposed. Accordingly, the first portion  120   a  exposed through the first recessing process may be etched twice through first and second rounding processes, and a newly exposed portion of the sidewalls  120  of the active region  115  through the second recessing process may be etched once. As a result, the width of the active region  115  may be increased in a direction toward the isolation layer  110 . Accordingly, in the rounding processes, a decrease in the surface area of the active region  115  due to a decrease in the width of the active region  115  can be substantially prevented. 
     Through the first and second rounding processes, the edge portions E can be sufficiently rounded. For example, a curvature radius R of the edge portions E of the active region  115  may be in the range from about ⅓ to about ½ of a width of an upper portion of the active region  115 . Accordingly, through first and second recessing processes and first and second rounding processes, the edge portions E of the active region  115  are sufficiently rounded and a decrease in the width and surface area of the active region  115  can be substantially prevented. 
     Subsequently, a semiconductor device can be completely fabricated using a method of fabricating a semiconductor device known to those of ordinary skill in the art. 
       FIG. 7  is a sectional view illustrating a method of a semiconductor device according to another embodiment of the present invention. The method of fabricating a semiconductor device is the same as the method according to the previous embodiment described with reference to  FIGS. 2 through 6 , except that the first rounding process is modified.  FIG. 7  shows a sectional view of a semiconductor device fabricated by modifying the first rounding process described with reference to  FIG. 4 . The description of elements in the  FIG. 7  similar to the elements in  FIGS. 2 through 6  will not be repeated. 
     Referring to  FIG. 7 , a first rounding process is performed to oxidize a surface of the active region  115  exposed by the isolation layer  110  to form a sacrificial layer  123 . The width of the first portion  120   a ″ of the sidewalls  120  may be smaller than the width of the edge portion  120   a  (see  FIG. 3 ) due to the first rounding process. Portions of the sacrificial layer  123  formed in edge portions E of the active region  115  to which oxygen is supplied may be thick, and thus, the edge portions E of the active region  115  can be rounded by removal of the sacrificial layer  123 . 
       FIG. 8  is a sectional view illustrating a method of a semiconductor device according to another embodiment of the present invention. The method of fabricating a semiconductor device is the same as the method described with reference to  FIGS. 2 through 6 , except that the second rounding process is modified. That is,  FIG. 8  shows a sectional view of a semiconductor device fabricated by modifying the second rounding process described with reference to  FIG. 6 . Accordingly, the description of elements similar to the elements described with reference to  FIGS. 2 through 6  will not be repeated. 
     Referring to  FIG. 8 , the second rounding process is performed to oxidize a surface of an active region  115  exposed by an isolation layer  110 , thereby forming a sacrificial layer  133 . As a result, the width of the second portion  120   b ″ of the sidewalls  120  may be smaller than the width of the first portion  120   a ′ (see  FIG. 4 ) of the sidewalls due to the second rounding process. Portions of the sacrificial layer  133  formed in edge portions E of the active region  115  to which oxygen is supplied may be thick, and thus, the edge portions E of the active region  115  can be rounded by removal of the sacrificial layer  131   
     In the method of fabricating a semiconductor device described with reference to  FIGS. 2 through 6 , the first rounding process described with reference to  FIG. 4  and the second rounding process described with reference to  FIG. 6  may be modified to the first rounding process described with reference to  FIG. 7  and the second rounding process described with reference to  FIG. 8 , respectively. 
     In previous embodiments of the present invention, two recessing processes and two rounding processes are performed. However, the number of the recessing and rounding processes are not limited thereto. For example, a plurality of recessing processes and a plurality of rounding processes can be performed alternately. A plurality of recessing processes may be understood with reference to the first and second recessing processes described above. A plurality of rounding processes may be understood with reference to the first and second rounding processes described above. The number of recessing and rounding processes may be limited in consideration of the manufacturing costs. 
       FIG. 9  is a transmission electron microscopic (TEM) image of a sectional view of a semiconductor device fabricated according to a comparative example of the present invention, and  FIG. 10  is a TEM image of a sectional view of a semiconductor device fabricated according to an exemplary embodiment of the present invention. In forming the device depicted in  FIG. 10 , two rounding processes were performed to fabricate a semiconductor device; whereas, in the comparative example, only one rounding process was performed to fabricate a semiconductor device. In  FIGS. 9 and 10 , the semiconductor device was a NAND-type non-volatile memory device. 
     Referring to  FIG. 9 , in the comparative example, edge portions E 1  of an active region  115   a  are not rounded. However, referring to  FIG. 10 , in the example, edge portions E 2  of an active region  115   b  are rounded to have a curvature radius. 
       FIG. 11  is a graph illustrating the distribution of threshold voltages of a semiconductor device fabricated according to the comparative example of  FIG. 9 , and  FIG. 12  is a graph illustrating the distribution of threshold voltages of a semiconductor device fabricated according to the example of  FIG. 10 . With respect to an NAND structure, the distribution of threshold voltages was obtained by measuring the threshold voltage of memory transistors except for two memory transistors located in both ends of the NAND structure. 
     Referring to  FIG. 11 , the memory device fabricated according to the comparative example has the threshold voltage distribution of about 3.1 V. Referring to  FIG. 12 , the memory device fabricated according to the example of  FIG. 10  has the threshold voltage distribution of about 2.5. Accordingly, the memory device fabricated according to the example of  FIG. 10  has narrower threshold voltage distribution than the memory device fabricated according to the comparative example of  FIG. 9 . Such an improvement on the threshold voltage distribution in the example of  FIG. 10  may result from a uniform electric field distribution due to high rounding effects. 
     A semiconductor device according to embodiments the present invention has a high driving performance due to a large active region, and high reliability due to low electric field enhancement in edge portions of the active region. 
     A semiconductor device according to embodiments of the present invention can be a non-volatile memory device with a tunneling insulating layer having high durability and high-temperature reliability. 
     According to a method of fabricating a semiconductor device, the surface of an active region can be efficiently widened and high rounding effects can be obtained, by repeatedly using a recessing process and a rounding process. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention.