Abstract:
A semiconductor device and a method for forming the same are disclosed. The semiconductor device includes an epitaxial source/drain junction layer having an insulating film thereunder. The method comprises the step of forming a under-cut under an epitaxial source/drain junction layer so that an insulating film filling the under-cut can be formed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to transistor of semiconductor device and method for manufacturing the same, and more particularly to improved transistor of memory device and method for manufacturing the same which provides a high speed and high integrated device by preventing deterioration of the characteristics of the device resulting from increase of impurity concentration as the integration of the device is increased.  
         [0003]     2. Description of the Background Art  
         [0004]     As the integration of a semiconductor device has been increased, integration of a DRAM has increased. However, the refresh time of the DRAM is almost double in each generation because of a high speed and low power requirements.  
         [0005]     As the density of a memory device continuously increases, the concentration of impurities implanted into a substrate must be increased to minimize short channel effects, threshold voltages and leakage currents. However, the increase of the concentration of impurities results in increase of junction electric intensity, which generates short channel effects and leakage currents.  
         [0006]     Moreover, the increase of concentration increases junction capacitance, thereby reducing the operation speed of the device.  
         [0007]     As described above, in the conventional transistor of the semiconductor device and a method for manufacturing the same, the characteristics of the device is degradeddue to the increased concentration of impurities implanted into the substrate, thereby making the achievement of the high operation speed and high integration of the semiconductor device more difficult.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, it is an object of the present invention to provide a transistor of a semiconductor device and a method for forming the same wherein capacitance generated by junction leakage current and junction depletion in a source/drain junction layer is remove to achieve a high speed and high integration of the device.  
         [0009]     According to one aspect of the present invention, a transistor of a semiconductor device, comprising: an epitaxial channel region disposed on an active region of a semiconductor substrate; a stacked structure of an insulating film and an epitaxial source/drain junction layer disposed at both sides of the channel region; and a stacked structure of a gate insulating film and a gate electrode disposed on the epitaxial channel region, wherein at least a portion of the gate insulating film overlap with the source/drain junction layer is provided.  
         [0010]     According to another aspect of the invention, a method for forming a transistor of a semiconductor device., comprising the steps of: forming a stacked structure of a first epitaxial layer and a second epitaxial layer on a semiconductor substrate; forming a device isolation film of trench type defining an active region, wherein a thickness of the device isolation film is substantially the same as that of the stacked structure; implanting an impurity into the second epitaxial layer using the device isolation film as a mask; sequentially forming a thermal oxide film and a sacrificial film on the entire surface of the resulting structure; etching the sacrificial film, the thermal oxide film, and the second and first epitaxial layers using a gate electrode mask to form an opening exposing the semiconductor substrate; removing the first epitaxial layer to form an under-cut under the second epitaxial layer; forming an insulating film filling the under-cut; growing a third epitaxial layer on the semiconductor substrate exposed by the opening; removing the sacrificial film and the thermal oxide film; implanting an impurity into the third epitaxial layer to form a channel region; and forming a gate electrode on the channel region is provided.  
         [0011]     According to yet another aspect of the invention, a method for forming a transistor of a semiconductor device, comprising the steps of: forming a stacked structure of a first epitaxial layer and a second epitaxial layer on a semiconductor substrate; forming a device isolation film of trench type defining an active region; forming a dummy gate electrode on the second epitaxial layer; implanting an impurity into the second epitaxial layer using the dummy gate electrode as a mask; forming an insulating film spacer at a sidewall of the dummy gate; forming a thermal oxide film on the entire surface of the resulting structure; forming a planarized interlayer insulating film exposing the top surface of the dummy gate; etching the dummy gate and the second and first epitaxial layers therebelow to form an opening exposing the semiconductor substrate; removing the first epitaxial layer to form an under-cut under the second epitaxial layer; forming an insulating film filling the under-cut; growing a third epitaxial layer having an impurity implanted therein on the semiconductor substrate exposed by the opening; and forming a stacked structure of a gate oxide film and a gate electrode on the third epitaxial layer is provided.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention will become better understood with reference to the accompanying drawings which are given only by way of illustration and thus are not limitative of the present invention, wherein:  
         [0013]      FIG. 1  is a layout diagram illustrating an active region and a gate region formed in one field;  
         [0014]      FIGS. 2   a  to  2   f  are cross-sectional diagrams illustrating sequential steps of a method for forming a transistor of a semiconductor device in accordance with a first embodiment of the present invention; and  
         [0015]      FIGS. 3   a  to  3   i  are cross-sectional diagrams illustrating sequential steps of a method for forming a transistor of a semiconductor device in accordance with a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     A transistor of a semiconductor device and a method for forming the same in accordance with preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0017]      FIG. 1  is a layout diagram illustrating an active region and a gate electrode region  22  formed on a semiconductor substrate  11 , and  FIGS. 2   a  to  2   f  are cross-sectional diagrams illustrating sequential steps of a method for forming a transistor of a semiconductor device in accordance with a first embodiment of the present invention, taken along lines I-I of  FIG. 1 .  
         [0018]     Referring to  FIG. 2   a , an first epitaxial layer  13  and a conductive second epitaxial layer  15  are sequentially formed on the semiconductor substrate  11  consisting of silicon.  
         [0019]     Preferably, the first epitaxial layer  13  is an epitaxial SiGe layer formed under an atmosphere of a mixture gas of a gas from the group consisting of GeH 4 , SiH 4 , SiH 2 Cl 2  and combinations thereof, HCl and H 2 , and has a thickness ranging from 50 to 1000 Å, and the second epitaxial layer  15  is an epitaxial Si layer formed under an atmosphere of a gas from the group consisting of SiH 4 , SiH 2 Cl 2  and combinations thereof, HCl and H 2 , and has a thickness ranging from 50 to 1000 Å.  
         [0020]     A pad oxide film (not shown) and a nitride film (not shown) are sequentially formed on the entire surface of the resulting structure.  
         [0021]     Thereafter, the nitride film, the oxide film, the second epitaxial layer  15 , the first epitaxial layer  13  and a predetermined depth of semiconductor substrate  11  are etched via a photoetching process using a device isolation mask (not shown) to form a trench. A device isolation film  17  is formed by filling the trench to define an active region.  
         [0022]     Next, the second epitaxial layer  15  is subjected to an implant process using the device isolation film  17  as a mask. The implant process is preferably performed using As having a concentration ranging from 1.0 E12 to 5.0 E13 atoms/cm 2  with an energy ranging from 10 to 100 KeV. Other conventional impurities may be used.  
         [0023]     A thermal oxide film  19  is formed on the entire surface of the resulting structure. The thermal oxide film preferably has a thickness ranging from 10 to 200 Å. A sacrificial film  21  is then formed on the thermal oxide film  19 . Preferably, the sacrificial film  21  may be an oxide film, nitride film or polysilicon film having a thickness ranging from 500 to 3000 Å and used as a mask.  
         [0024]     Referring to  FIG. 2   b , a gate electrode region  22  exposing the semiconductor substrate  11  is formed by sequentially etching the sacrificial film  21 , the thermal oxide film  19 , the second epitaxial layer  15  and the first epitaxial layer  13  via a photoetching process using a mask exposing an region corresponding to the gate electrode region  22  of  FIG. 1 .  
         [0025]     Referring to  FIG. 2   c , the first epitaxial layer  13  exposed by a lower sidewall of the gate electrode region  22  is removed to form an under-cut  23  under the second epitaxial layer  15 . Preferably, the process for removing the exposed portion of the first epitaxial layer  13  is a wet etching process or an isotropic dry etching process utilizing etching selectivity differences among adjacent layers and the first epitaxial layer  13 .  
         [0026]     Specifically, the wet etching process is preferably performed using a solution containing H 2 O, H 2 O 2  and NH 4 OH having a ratio of 5:1:1 at a temperature ranging from 70 to 80° C. (ref. F. S. Johnson et al. journal of electronic materials, vol 21, p. 805-810, 1992). The isotropic dry etching process is preferably a plasma etching process using HBr, O 2  and Cl 2 , and more preferably performed using microwaves to improve isotropic etching properties. In addition, the plasma etching process may be performed using SF 6 .  
         [0027]     Now referring to  FIG. 2   d , an insulating film  25  for filling the under-cut  23  is formed on the entire surface of the resulting structure. Preferably, the insulating film  25  is an oxide film or nitride film.  
         [0028]     The insulating film  25  is preferably formed via thermal oxidation, chemical vapor deposition (CVD) or atomic layer deposition (ALD).  
         [0029]     Preferably, the CVD process is performed using SiH 4  and N 2 O under a pressure of 50 Torr and at a temperature ranging from 50 to 800° C. The thermal oxidation process is a dry or wet process performed at a temperature of 700 to 1100° C.  
         [0030]     Referring to  FIG. 2   e , a wet etching process is performed so that only the portion of the insulating film  25  in the under-cut  23  remains. The wet etching process is preferably performed using a HF group etching solution.  
         [0031]     As shown in  FIG. 2   f , a third epitaxial layer  27  is grown on the semiconductor substrate  11  exposed by the gate electrode region  22 .  
         [0032]     Preferably, the third epitaxial layer  27  is an epitaxial Si layer having a thickness ranging from 100 to 2000 Å formed under an atmosphere of a mixture gas of a gas from the group consisting of SiH 4 , SiH 2 Cl 2  and combinations thereof, HCl and H 2 .  
         [0033]     The sacrificial oxide film  21  and the thermal oxide film  19  are removed via a wet etching process. Preferably, the wet etching process is performed by utilizing the etching selectivity difference among adjacent other layers and the sacrificial oxide film  21 .  
         [0034]     A channel region is formed on the third epitaxial layer  27  by performing a channel implant process and a punch stop implant process.  
         [0035]     A gate electrode having a stacked structure of a gate oxide film  29 , a conductive layer for gate electrode  31  and a hard mask layer  33  is formed via a photoetching process using a gate electrode mask. Conventional conductive materials may be used as the conductive layer for gate electrode  31 . The hard mask layer  33  is preferably a nitride film or an oxide film.  
         [0036]     The formation of the channel region and removal of the sacrificial oxide film  21  and the thermal oxide film  19  may be performed prior to the formation of the gate electrode.  
         [0037]     In the transistor of the semiconductor device formed according to the method of  FIGS. 2   a  to  2   f , the conductive third epitaxial layer  27  formed in the active region of the semiconductor substrate  11  corresponds to the channel region of the transistor. The stacked structure of the insulating film  25  and the conductive second epitaxial layer  15  is formed at both sides of the third epitaxial layer  27 , and the gate insulating film  29  and the gate electrode  31  are formed on the third epitaxial layer  27 . The second epitaxial layer  15  corresponds to a source/drain junction layer, and at least a portion of the gate insulating film  29  overlap with the second epitaxial layer  15 .  
         [0038]      FIGS. 3   a  to  3   i  are cross-sectional diagrams illustrating sequential steps of a method for forming a transistor of a semiconductor device in accordance with a second embodiment of the present invention, taken along lines I-I of  FIG. 1 .  
         [0039]     Referring to  FIG. 3   a , an first epitaxial layer  43  and a conductive second epitaxial layer  45  are sequentially formed on the semiconductor substrate  41  comprised of silicon. Preferably, the first epitaxial layer  43  is an epitaxial SiGe layer having a thickness ranging from 50 to 1000 Å formed under an atmosphere of a mixture gas of a gas from the group consisting of GeH 4 , SiH 4 , SiH 2 Cl 2  and combinations thereof, HCl and H 2 , and the second epitaxial layer.  45  is an epitaxial Si layer having a thickness ranging from 50 to 1000 Å formed under an atmosphere of a mixture gas of a gas from the group consisting of SiH 4 , SiH 2 Cl 2  and combinations thereof, HCl and H 2 .  
         [0040]     A pad oxide film (not shown) and a nitride film (not shown) are sequentially formed on the entire surface of the resulting structure.  
         [0041]     Thereafter, the nitride film, the oxide film, the second epitaxial layer  45 , the first epitaxial layer  43  and a predetermined depth of semiconductor substrate  41  are etched via a photoetching process using a device isolation mask (not shown) to form a trench. A device isolation film  47  is formed by filling the trench to define an active region.  
         [0042]     Referring to  FIG. 3   b , a polysilicon layer (not shown) is formed on the entire surface of the resulting structure., and then etched via a photoetching process using a gate electrode mask to form a dummy gate electrode  49  in a predetermined region where a gate electrode region is to be formed. The thickness of the polysilicon layer is substantially the same as that of the gate electrode formed in a subsequent process, for example, 500 to 3000 Å in thickness. An oxide film or nitride film may be used other than the polysilicon layer.  
         [0043]     Thereafter, the second epitaxial layer  45  is subjected to an implant process using the dummy gate electrode  49  as a mask. The implant process is preferably performed using As having a concentration ranging from 1.0 E12 to 5.0 E13 atoms/cm 2  with an energy ranging from 10 to 100 KeV. Other conventional impurities may be used.  
         [0044]     The implant process may also be performed prior to the formation of the polysilicon layer using the device isolation film  47  as a mask.  
         [0045]     Referring to  FIG. 3   c , an insulating film spacer  51 , preferably a nitride film or an oxide film is formed at sidewalls of the dummy gate electrode  49 . A thermal oxide film  53  is then formed on the entire surface of the resulting structure.  
         [0046]     Now referring to  FIG. 3   d , an interlayer insulating film  55  is formed on the entire surface of the resulting structure, and then planarized to expose the top surface of the dummy gate electrode  49 . Preferably, the planarization process is a CMP process.  
         [0047]     Referring to  FIG. 3   e , the exposed dummy gate electrode  49  is first removed, and the second epitaxial layer  45  and the first epitaxial layer  43  are etched using the interlayer insulating film  55  and the insulating film spacer  51  as a mask to form a gate electrode region  57  exposing the semiconductor substrate  41 .  
         [0048]     Referring to  FIG. 3   f , the first epitaxial layer  43  exposed by a lower sidewall of the gate electrode region  57  is removed to form an under-cut  59  under the second epitaxial layer  45 . Preferably, the process for removing the exposed portion of the first epitaxial layer  43  is a wet etching process or an isotropic dry etching process utilizing etching selectivity differences among adjacent and the first signal-crystalline layer  43 .  
         [0049]     Specifically, the wet etching process is preferably performed using a solution containing H 2 O, H 2 O 2 , NH 4 OH having a ratio of 5:1:1 at a temperature ranging from 70 to 80° C. (ref. F. S. Johnson et al. journal of electronic materials, vol 21, p. 805-810, 1992). The isotropic dry etching process is preferably a plasma etching process using HBr, O 2  and Cl 2 , and more preferably performed using microwaves to improve isotropic etching properties. In addition, the plasma etching process may be performed using SF 6 .  
         [0050]     Referring to  FIG. 3   g , an insulating film  61  for filling the under-cut  59  is formed on the entire surface of the resulting structure. Preferably, the insulating film  61  is an oxide film or a nitride film.  
         [0051]     The insulating film  61  is preferably formed via thermal oxidation, chemical vapor deposition (CVD) or atomic layer deposition (ALD).  
         [0052]     Preferably, the CVD process is performed using SiH 4  and N 2 O under a pressure of 50 Torr and at a temperature ranging from 50 to 800° C. The thermal oxidation process is a dry or wet process performed at a temperature ranging from 700 to 1100° C.  
         [0053]     Referring to  FIG. 3   h , a wet etching process is performed so that only the portion of the insulating film  61  in the under-cut  59  remains. The wet etching process is preferably performed using a HF group etching solution.  
         [0054]     Referring to  FIG. 3   i , a third epitaxial layer  63 , which is preferably an epitaxial Si layer having a thickness ranging from 100 to 2000 Å, is grown on the semiconductor substrate  41  exposed by the gate electrode region  57 .  
         [0055]     A channel region is formed on the third epitaxial layer  63  by performing a channel implant process and a punch stop implant process.  
         [0056]     A gate oxide film  65  is grown on the third epitaxial layer  63 , and a conductive layer for gate electrode  67  is formed on the gate oxide film  65 , and then planarized using the interlayer insulating film  55  as an etch barrier to form a gate electrode.  
         [0057]     The gate electrode may include a stacked structure of a conductive layer and a hard mask layer.  
         [0058]     As discussed earlier, in accordance with the present invention, the transistor of the semiconductor device and the method for forming the same have the following advantages:  
         [0059]     1. Generation of junction leakage current is prevented.  
         [0060]     2. Removal of capacitance generated by junction depletion provides improved operation speed of the device.  
         [0061]     3. Improved short channel effects/drain induced barrier lowering characteristics due to the decrease in junction depth provide decrease of a critical dimension of the gate electrode and less threshold voltage reduction.  
         [0062]     4. The epitaxial Si layer consisting the source/drain junction region represses depletion in a bulk-wise direction to improve punch-through characteristics and allows a reduction of the dose of punch stop implant to improve refresh characteristics of the DRAM.  
         [0063]     5. The improved punch-through characteristics allow reduction of channel threshold voltage control implant dose, thereby improving swing phenomenon and leakage current characteristics in an off state.  
         [0064]     6. An increase in junction breakdown voltage allows a high-speed operation of the device which uses a high driving voltage.  
         [0065]     7. A remarkable decrease in leakage current between the devices allows reduction of the depth and width of the device isolation film and achievement of high integration.  
         [0066]     8. The epitaxial channel region and the source/drain junction region provide an improved interface characteristics of the semiconductor substrate.  
         [0067]     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are therefore intended to be embraced by the appended claims.