Patent Publication Number: US-2007114581-A1

Title: Transistor of semiconductor device and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a transistor of a semiconductor device and a method for manufacturing the same, and more particularly, to a transistor capable of improving the reliability of the device, and a method for manufacturing the same.  
      2. Description of the Related Art  
      In general, as the integration degree of a semiconductor device increases, there is a demand for transistors smaller in size. However, there is a limitation to the shallowness of a junction depth of a source/drain terminal.  
      This is the reason why a short channel effect occurs. Here, as a length of a channel is reduced from a long channel of a related art to a short channel equal to or less than 0.5 μm, a source/drain depletion region penetrates into a channel to reduce a length of a valid channel and a threshold voltage. In the event, the short channel effect indicating a function of a gate control in a MOS transistor occurs.  
      So as to suppress such a short channel effect, a thickness of a gate insulating layer should be reduced. Further, the channel between the source and drain terminals, a maximum width of a depletion region under a gate, as well as a concentration of impurity ions in a semiconductor substrate should be reduced.  
      However, first of all, it is important to form a shallow junction. To do this, there have continuously been attempts to embody a shallow junction by a subsequent thermal treatment processes using ion implantation equipment in a manufacturing process of a semiconductor device.  
      Further, a MOS transistor may typically include a light doped drain (referred to as ‘LDD’ hereinafter) structure.  
      On the other hand, a MOS transistor mainly used in a semiconductor memory device such as DRAM typically is a planar type transistor, in which a gate insulating layer is formed at a surface of a silicon substrate, and a conductive layer pattern is formed on the gate insulating layer.  
      However, as semiconductor devices are highly integrated, a line width of a gate pattern, and a length and a width of a channel are reduced, undesirable effects such as a short channel effect or a shallow channel effect may increase in the operation of the transistor.  
      Further, a drive current in the MOS transistor flows through a substrate channel formed under a gate electrode in each cell. When a size of a device is reduced according to higher integration levels of the semiconductor device, the drive current flows through a limited depth and width, which is adjacent to a gate electrode. Accordingly, an amount of the drive current may be limited, thereby limiting operation characteristics of the transistor.  
      In order to solve a short channel effect and a drive current limit problem in the MOS transistor, a pin type MOS transistor has been suggested, which increases a drive current by increasing a contact area of a substrate and a gate electrode in a shallow junction structure.  
      Hereinafter, a transistor of a semiconductor device according to the related art will be explained with reference to accompanying drawings.  
       FIG. 1  is a perspective view showing a pin type MOS transistor according to the related art.  FIG. 2  is a cross-sectional view of the pin type MOS transistor taken along line I-I of  FIG. 1 .  
      As shown in  FIGS. 1 and 2 , the pin type MOS transistor according to the related art includes a device isolation layer  101 , an active portion  105 , a gate electrode  106 , a gate insulating layer  130 , and source/drain terminals (not shown). The device isolation layer  101  is formed in a device isolation region of the semiconductor device  100 . The active portion  105  protrudes upwards from an upper side of the device isolation layer  101 . The active portion  105  has a line shape upon viewing the active portion in a layout or a cross-section. The gate electrode  106  is arranged perpendicular to the active portion  105 . The gate insulating layer  130  insulates the active portion  105  from the gate electrode  106 . The source/drain terminals are formed in the active portion  105  at opposite sides of the gate electrode  106 .  
      Here, the gate electrode  106  passes over and encloses two side portions of a protruded active portion  105  and an upper surface connected thereto. Accordingly, the pin type MOS transistor has a gate length greater than that of a general (planar) MOS transistor by about two times the protruded height of the active portion  105 , so that an amount of a drive current may be increased.  
      However, a transistor according to the related art has following problems.  
      In the MOS transistor shown in  FIG. 5 , because the upper surface and sides of the protruded active portion  105  are substantially arranged perpendicular to each other, an edge part A of the active portion  105  makes a right angle. This may cause damage to the gate insulating layer  101  in the edge part A of the active portion  105 .  
      More particularly, an electric field may be concentrated in a part corresponding to the edge A of the active portion  105  in the gate insulating layer  101 .  
     SUMMARY OF THE INVENTION  
      Accordingly, the present invention is directed to a transistor of a semiconductor device and a method for manufacturing the same that substantially obviates one or more problems due to limitations and disadvantages of the related art.  
      An object of the present invention is to provide a transistor that improves device reliability by preventing or reducing a likelihood of damage to a gate insulation layer.  
      Another object of the present invention is to provide a method for manufacturing the transistor.  
      Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure(s) particularly pointed out in the written description and claims hereof as well as the appended drawings.  
      To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided transistor comprising: an active portion having a first height from a surface of a semiconductor substrate, and having a line-shaped cross-section; a device isolation layer having a round portion at a second height lower than the first height; a gate insulating layer on the active portion; a gate electrode on the gate insulating layer, intersecting the active portion; and source/drain terminals in the active region on opposite sides of the gate electrode.  
      In another aspect of the present invention, there is provided a method for manufacturing a transistor comprising: forming first and second insulating layer patterns defining an active region of a semiconductor substrate; selectively removing portions of the substrate in device isolation regions using the first and second insulating layer patterns as a mask to form a trench and an active portion having a first height; forming a device isolation layer having a second height lower than the first height in the trench, the device isolation layer including a round portion at a sidewall of the active portion; forming a gate electrode structure intersecting the active region; and forming source/drain terminals at opposite sides of the gate electrode structure in the active portion.  
      It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle(s) of the invention. In the drawings:  
       FIG. 1  is a perspective view showing a pin type MOS transistor according to the related art;  
       FIG. 2  is a cross-sectional view of the pin type MOS transistor taken along line I-I of  FIG. 1 ;  
       FIG. 3  is a perspective view showing a pin type MOS transistor according to an embodiment of the present invention;  
       FIG. 4  is a cross-sectional view of the pin type MOS transistor taken along line II-II of  FIG. 3 ;  
       FIG. 5  is a cross-sectional view of the pin type MOS transistor taken along line III-III of  FIG. 3 ; and  
       FIGS. 6A through 6J  are cross-sectional views of a pin type MOS transistor for describing a method for manufacturing a transistor according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Reference will now be made in detail to the preferred embodiments of the present invention, examples of 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.  
      Hereinafter, a transistor of a semiconductor device and a method for manufacturing the same according to an embodiment of the present invention will be described with reference to the accompanying drawings.  
       FIG. 3  is a perspective view showing a pin type MOS transistor according to an embodiment of the present invention.  FIG. 4  is a cross-sectional view of the pin type MOS transistor taken along line II-II of  FIG. 3 .  FIG. 5  is a cross-sectional view of the pin type MOS transistor taken along line III-III of  FIG. 3 .  
      As shown in  FIGS. 3 through 5 , the pin type MOS transistor according to the embodiment of the present invention includes a device isolation layer  306  with a round portion  306   d , an active portion  305 , a gate insulating layer  308 , a gate electrode  309 , a sidewall spacer  311 , an LDD  310 , and source/drain terminals  312 .  
      The active portion  305  protrudes to a first height from a surface of the semiconductor substrate  301 . The active portion  305  may have a long line shape upon viewing it on a plane (e.g., by viewing a two-dimensional cross-section, such as the view of  FIG. 5 , or as viewed in a top-down, layout view).  
      On the other hand, active portion  305  may have a trapezoid shape in a cross-sectional view. More particularly, the active portion  305  has a shape in which a sectional area or dimension (e.g., a width) becomes gradually smaller as it goes from the semiconductor substrate  301  toward the uppermost surface of the active portion  305 . Accordingly, the active portion  305  may form an obtuse angle (e.g., from 75° to 88°, from 80° to 85°, or any range of values therein) with respect to the semiconductor substrate  301 .  
      On the other hand, an upper edge of the active portion  305  may be rounded. Rounding the upper edge of the active portion  305  may prevent or reduce an electric field from being concentrated in a corresponding portion of gate insulating layer  308  as will be described later.  
      Device isolation layers  306  are arranged at or in device isolation regions, which are formed at both (opposite, or surrounding all) sides of the active portion  305 . In embodiments of the present invention, each of the device isolation layers  306  has a second height lower than the first height of the active layer  305 . A rounded portion  306   d  is formed where the device isolation layer  306  contacts a sloped side of the active portion  305 . The device isolation layer  306   a  and/or the round portion  306   d  functions to prevent or reduce a contact part between the device isolation layer  306  and the active portion  305  from being excessively etched during a subsequent process.  
      The gate insulating layer  308  covers at least the part of the active portion  305  under the gate electrode  309 . Optionally, the gate insulating layer  308  covers the device isolation layer  306 , at least under the extended portions of the gate electrode  309 . In these embodiments, a sloped side of the active portion  305  and a round portion of the device isolation layer  306  prevent or reduce undesirable concentration(s) of an electric in the gate insulating layer  308 , thereby preventing or reducing the likelihood of dielectric breakdown of gate insulating layer  308 , and suppressing damage to the gate insulating layer  308 .  
      In some embodiments, the gate insulating layer  308  may comprise a hafnium silicon oxynitride (Hf—Si—O—N) or a hafnium silicon oxide (Hf—Si—O).  
      The gate electrode  309  is arranged on the gate insulating layer  308  substantially perpendicular to the active portion  305 . In this embodiment, the gate electrode  309  may have a line shape (e.g., a line-shaped cross-section, or a general line shape when a user views a layout thereof).  
      A sidewall spacer  309  is arranged at both sides of the gate electrode  308 . The sidewall spacer  309  is used to protect the LDD  310  in the active portion  305  after formation thereof. The LDD  310  is formed at a lower portion of the sidewall spacer  309 . The LDD  310  arranged at the lower portion of the sidewall spacer  309  is combined with source/drain terminals  312  of the active portion  305 .  
      In various embodiments of the present invention, the active portion  305  protruding from the semiconductor substrate  301  may have a trapezoidal shape, optionally with rounded upper corners, and the device isolation layer  306  may have a round portion  306   d  where it contacts with a sloped sidewall of the active portion  305 , thereby preventing or reducing the likelihood of damage to the gate insulating layer  308 .  
       FIGS. 6A through 6J  are cross-sectional views of a pin type MOS transistor for describing a method for manufacturing a transistor according to an embodiment of the present invention.  
      Firstly, as shown in  FIG. 6A , a first insulating layer  302  and a second insulating layer  303  are sequentially formed on the semiconductor substrate  301 .  
      The first insulating layer  302  may include an oxide layer (e.g., comprising silicon dioxide), which may have a thickness ranging from 20 to 100 Å and be formed by conventional wet or dry thermal oxidation (e.g., of silicon) or conventional chemical vapor deposition (CVD) (e.g., from precursor gases such as a silane or TEOS and dioxygen) with optional annealing for densification. Further, the second insulating layer  303  can include a nitride layer, which has a thickness ranging from 500 to 1,500 Å and be formed by conventional thermal nitridation (e.g., of silicon dioxide) or conventional chemical vapor deposition (e.g., from precursor gases such as a silane and dinitrogen or ammonia).  
      Although this embodiment of the present invention has been described in that the first insulating layer  302  and the second insulating layer  303  are sequentially formed, the present invention is not limited thereto. That is, a single insulating layer (e.g., comprising silicon nitride or a silicon oxynitride) can be formed for a mask.  
      After a photo resist layer is coated on the second insulating layer  303 , the photo resist layer is patterned by a conventional photolithography process including exposure and developing processes to form a first photo resist pattern  304 .  
      The first photo resist pattern  304  defines an active region where an active portion  305  to be described later will be formed. The removed part of the first photo resist pattern  304  defines a device isolation region, where the device isolation layer  306  to be explained below will be formed.  
      As shown in  FIG. 6B , the first insulating layer  302  and the second insulating layer  303  are patterned using the first photo resist pattern  304  as an etch mask to form a first insulating layer pattern  302   a  and a second insulating layer pattern  303   a.    
      After a formation of the first insulating layer pattern  302   a  and the second insulating layer pattern  303   a , the first photo resist pattern  304  is removed from the semiconductor substrate  301 .  
      The semiconductor substrate  301  is patterned (e.g., by dry etching using a conventional etch chemistry for etching silicon, which may comprise one or more fluorocarbon, hydrofluorocarbon, halogen and/or hydrogen halide etchants that are typically in the gas phase at ambient temperatures), using the first insulating layer pattern  302   a  and the second insulating layer pattern  303   a  to form a trench having a predetermined depth at a device isolation region of the semiconductor substrate  301 , and to form an active portion  305  at an active region. In this embodiment, the active portion  305  has a line shape as described herein (e.g., in a planar or cross-sectional view).  
      In this embodiment of the present invention, after the first photo resist pattern  304  has been removed, the semiconductor substrate  301  is patterned using the first insulating layer pattern  302   a  and the second insulating layer pattern  303   a  as an etch mask to form the trench. However, the trench can also be formed using the photo resist pattern  304  as the etch mask without prior removal of the photo resist pattern  304 .  
      After the trench and the active portion  305  have been formed, a third insulating layer is formed on/over an entire surface of the semiconductor substrate  301  (e.g., by a blanket deposition technique such as CVD). Generally, the third insulating layer comprises a silicon oxide (e.g., as described herein). However, prior to blanket deposition of the third insulating layer, a thin liner oxide (not shown) may be formed along the surface of the trench by wet or dry thermal oxidation.  
      Next, the third insulating layer is polished or planarized by a chemical mechanical polishing process using an upper surface of the second insulating layer pattern  303   a  as an end point to form a first preliminary device isolation layer  306   a  inside the trench.  
      As shown in  FIG. 6C , a photo resist layer is again formed at an upper surface of the first preliminary device isolation layer  306   a . Then, the photo resist layer is patterned using a photolithography process to form a second photo resist pattern  304 ′, which covers a part of the first preliminary device isolation layer  306   a  as well as first and second insulating layer patterns  302   a  and  303   a.    
      As shown in  FIG. 6D , parts of the first preliminary device isolation layer  306   a  are dry-etched using the second photo resist pattern  304  as an etch mask to form a second preliminary device isolation layer  306   b  having a stepped portion  306   c  inside the trench (or over a sidewall boundary portion of the trench, if the trench is considered to be defined by a region of the semiconductor substrate removed during the etching step described above with regard to  FIG. 6B ).  
      As shown in  FIG. 6E , after the second preliminary device isolation layer  306   b  having the stepped portion  306   c  has been formed, the second photo resist pattern  304  is removed.  
      Subsequently, the first insulating layer pattern  302   a  and the second insulating layer pattern  303   a  are removed from the active portion  305  by one or more wet etch processes. When the first insulating layer pattern  302   a  and the second insulating layer pattern  303   a  are removed from the active portion  305 , the second preliminary device isolation layer  306   b  having a stepped portion  306   c  is also etched to form a device isolation layer  306  inside the trench.  
      As the second preliminary device isolation layer  306   b  having the stepped portion  306   c  is etched (e.g., using dilute aqueous hydrofluoric acid, where the ratio by volume of deionized water to concentrated aqueous HF may be from about 1:1 to about 10:1), a round portion  306   d  is formed at a part of the device isolation layer  306  corresponding to the stepped portion  306   c . The round portion  306   c  prevents an excessive etch in a contact part between the active portion  305  and the device isolation layers  306 .  
      In the embodiment of the present invention, the second insulating layer pattern  303   a  is removed using an aqueous phosphoric acid or phosphate solution. When (or after) the first insulation layer pattern  302   a  is removed, the second preliminary isolation layer  306   b  may be removed to a predetermined thickness.  
      In addition, after the second insulating layer pattern  303   a  and the first insulating layer pattern  302   a  have been removed (or after the first insulating layer pattern  302   a  is removed and during wet etching of the second insulating layer pattern  303   a ), the second preliminary device isolation layer  306   b  may be removed to a predetermined thickness from the surface (or for a predetermined period of time) through a separate etch, namely, an etch back process to protrude an active region  305 .  
      As shown in  FIG. 6F , an oxidation process (e.g., conventional wet or dry thermal oxidation) is performed on the semiconductor substrate  301  or the active portion  305  to form a fourth insulating layer  307 . Alternatively, fourth insulating layer  307  may be formed by CVD (e.g., of silicon oxide) and optional annealing, as described above for the first insulating layer  302 . The fourth insulating layer  307  on the upper surface of the protruded active portion  305  may be a sacrificial layer.  
      Here, reference symbol R represents that an edge part of the active portion  305  is rounded by an oxidizing process of the semiconductor substrate  301 . The fourth insulating layer  307  preferably has a thickness ranging from approximately 50 Å to approximately 300 Å.  
      As shown in  FIG. 6G , after forming rounded corners R on the active portion  305 , the fourth insulating layer  307  may be removed using a wet etch process, as described above for silicon oxide.  
      As shown in  FIG. 6H , impurity ions for forming a well and/or adjusting a threshold voltage are implanted in an entire surface of the semiconductor substrate  301  using a conventional ion implantation method. Alternatively, one or both ion implantations may be performed prior to removing the fourth insulating layer  307 , to reduce possible damage to the active portion  305  during the ion implantation process.  
      As shown in  FIG. 6I , a gate insulating layer  308  is formed at an entire surface of the semiconductor substrate  301 . The gate insulating layer  308  may be formed using a Chemical Vapor Deposition (CVD), a Physical Vapor Deposition (PVD), or an Atomic Layer Deposition (ALD) method. Further, the gate insulating layer  308  may comprise a hafnium silicon oxynitride (Hf—Si—O—N) or a hafnium silicon oxide (Hf—Si—O).  
      After a formation of the gate insulating layer  308 , a conductive layer for a gate electrode is formed on the gate insulating layer  308 . The conductive layer may comprise TiN, Ti/TiN, W x N y , or polysilicon.  
      Thereafter, the conductive layer and the gate insulating layer  308  are patterned and/or selectively removed through a photolithography process to form a gate electrode  309  at an upper surface of the protruded active portion  305 . The gate electrode  309  may cross over, or intersect the active portion  305  at substantially a right angle.  
      Next, a low density of n- or p-type impurities are implanted in an entire surface of the semiconductor substrate  301  using the gate electrode  309  as an ion implantation mask to from a lightly doped drain (LDD)  310  at a surface of the active portion  305  of opposite sides of the gate electrode  309 .  
      As shown in  FIG. 6J , after a fifth insulating layer (comprising an oxide layer, a nitride layer, or a multi-layered combination thereof such as an oxide-nitride bilayer or an oxide-nitride-oxide trilayer) has been formed on the entire surface of the semiconductor substrate  301 , an etch back process is performed to form sidewall spacers  311  at both sides of the gate electrode  309 . Here, the fifth insulating layer can be formed by blanket deposition of a nitride layer or an oxide layer then a nitride layer by, e.g., CVD.  
      Thereafter, a high density of n- or p-type impurities are implanted in the entire surface of the semiconductor substrate  301  using the gate electrode  309  and the sidewall spacers  311  as an ion implantation mask to from source/drain terminals  312  (combined or in electrical contact with LDD  310 ) in the active portion  305 .  
      It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.  
      As is seen from the forgoing description, in the transistor of a semiconductor device and a method for manufacturing the same, because edge parts of a protruded active region are rounded, dielectric breakdown and/or a disconnection of the gate insulating layer along the edge parts may be prevented to enhance the reliability of a device.  
      In addition, according to the present invention, since a rounded portion is formed at an interface of an active portion and a device isolation layer along the sidewall of the active portion, it may prevent the active portion and a device isolation layer part contacting with the active portion from being damaged.