Abstract:
The present invention relates to a capacitor in a semiconductor device and a method for fabricating the same. The capacitor fabrication method includes the steps of: forming a lower electrode by using a thin film of (Ba,Sr)RuO 3  (BSR) on a substrate provided with various device elements; forming a dielectric layer on the lower electrode by using a thin film of barium strontium titanate (BST); and forming an upper electrode on the dielectric layer.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for fabricating a semiconductor device; and, more particularly, to a method for fabricating a capacitor in a semiconductor device by using barium strontium titanate (BST) as a dielectric layer.  
       DESCRIPTION OF RELATED ARTS  
       [0002]     In a semiconductor memory device, particularly in a dynamic random access memory (DRAM) device, a large scale of integration of semiconductor devices has been led to a significant decrease in an area of a memory cell which is a basic unit for storing information.  
         [0003]     The decrease in the memory cell area has further led to a decrease in a cell capacitor area which causes a sensing margin and a sensing speed to decrease and results in a weak tolerance to a soft error caused by a-particles. Therefore, it is necessary to develop a method for securing a sufficient capacitance within a limited cell area.  
         [0004]     The capacitor capacitance (C) is defined as follows. 
 
 C=ε*As/d   Eq. 1 
 
         [0005]     Herein, ε, As and d are a dielectric constant, an effective surface area and a distance between electrodes, respectively. Therefore, to increase the capacitor capacitance, an effective surface area of an electrode or a dielectric constant is increased, or a thickness of a dielectric layer is decreased.  
         [0006]     Among the above mentioned methods, the method of increasing the effective surface area of the electrode has been primarily considered. The increase in the effective surface area of the electrode within a limited area is achieved by forming the capacitor in three dimensions such as a concave structure, a cylinder structure, a multi-pin structure and the like. However, this method of increasing the effective surface area of the electrode is limited as semiconductor devices have been highly integrated.  
         [0007]     As another approach, decreasing a distance between electrodes is considered. To minimize the distance between the electrodes, it is proposed to decrease a thickness of a dielectric layer. However, there is a problem in that leakage currents increase as the thickness of the dielectric layer increases. Therefore, a current focus of increasing the capacitor capacitance is to increase a dielectric constant of the dielectric layer.  
         [0008]     Typically, a capacitor is mainly formed in a nitride/oxide (NO) structure by using dielectric materials such as silicon oxide and silicon nitride. Recently, high dielectric materials such as HfO 2 , Ta 2 O 5 , (Ba,Sr)TiO 3  (BST) and so on or ferroelectric materials such as (Pb,Zr)TiO 3 (PZT), (Pb,La) (Zr,Ti)O 3 (PLZT) and so on are used for a dielectric layer of the capacitor.  
         [0009]     When a high dielectric capacitor or a ferroelectric capacitor is formed based on the above high dielectric material or ferroelectric material, it is required to control other materials adjacent to the employed dielectric material and related processes for the purpose of obtaining an intended dielectric characteristic of the high dielectric material or the ferroelectric material.  
         [0010]     Generally, noble metals or noble-metal containing compounds are used for forming lower electrodes and upper electrodes of high dielectric capacitors or ferroelectric capacitors. Examples of such lower electrode and upper electrode materials are platinum (Pt), iridium (Ir), ruthenium (Ru), ruthenium oxide (RuO 2 ) and iridium oxide (IrO 2 ).  
         [0011]      FIGS. 1A and 1B  are cross-sectional views showing a conventional capacitor in a semiconductor device.  
         [0012]     Referring to  FIG. 1A , an inter-layer insulation layer  12  is formed on a substrate  10  provided with an active region  11 . Although not illustrated, the inter-layer insulation layer  12  is etched to form a contact hole exposing the active region  11  of the substrate  10 . Then, the contact plug  13  is filled with a conductive material. Herein, polysilicon is used as the conductive material for the contact plug  13 .  
         [0013]     Next, a lower electrode  14  and a dielectric layer  15  are sequentially formed on the contact plug  13  and a portion of the inter-layer insulation layer  12 .  
         [0014]     Referring to  FIG. 1B , an upper electrode  16  is formed on the dielectric layer  15 . At this time, if (Ba,Sr)TiO 3  (BST) is used as the dielectric material, the upper electrode  16  and the lower electrode  14  are made of a noble metal such as Ir, Ru or the like.  
         [0015]     Among various high dielectric materials, the above mentioned thin BST layer with the Perovskite structure has a high dielectric constant and a good insulation characteristic. Also, a dielectric characteristic of the thin BST layer is minimally lost or dispersed even under a high frequency. Furthermore, since the thin BST layer exists as a paraelectric phase, there is not a fatigue or degradation problem. As a result of this effect, the thin BST layer can be applied to a highly dense and integrated capacitor requiring a high dielectric constant and low leakage current.  
         [0016]     However, the dielectric constant and leakage current characteristics vary greatly depending on a process for fabricating a BST layer and a type of a capacitor material in contact with the thin BST layer. Also, even characteristics of the compounded BST layer vary differently depending on a composition ratio of the BST layer.  
         [0017]     Since a noble metal such as iridium is used as an electrode material and the above mentioned BST layer is used as a dielectric material, it is difficult to form the thin BST layer stably on the electrode made of the noble metal due to irrelevant material characteristics between the BST and the noble metal.  
       SUMMARY OF THE INVENTION  
       [0018]     It is, therefore, an object of the present invention to provide a method for fabricating a capacitor through a simplified process by using a material with a similar composition to that of a thin BST film used as a dielectric material of the capacitor.  
         [0019]     In accordance with an aspect of the present invention, there is provided a method for fabricating a capacitor in a semiconductor device, including the steps of: forming a lower electrode by using a thin film of (Ba,Sr)RuO 3  (BSR) on a substrate provided with various device elements; forming a dielectric layer on the lower electrode by using a thin film of barium strontium titanate (BST); and forming an upper electrode on the dielectric layer.  
         [0020]     In accordance with another aspect of the present invention, there is provided a capacitor in a semiconductor device, including: a thin BSR layer for use in a lower electrode formed on a substrate; a thin BST layer for use in a dielectric layer formed on the thin BSR layer; and an upper electrode formed on the thin BST layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects and features of the present invention will become better understood with respect to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0022]      FIGS. 1A and 1B  are cross-sectional views showing a conventional method for fabricating a capacitor; and  
         [0023]      FIGS. 2A  to  2 D are cross-sectional views showing a method for fabricating a semiconductor device in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     A capacitor in a semiconductor device and a method for fabricating the same in accordance with a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.  
         [0025]      FIGS. 2A  to  2 D are cross-sectional views illustrating a method for fabricating a capacitor in a semiconductor device in accordance with a preferred embodiment of the present invention.  
         [0026]     Referring to  FIG. 2A , an inter-layer insulation layer  32  is formed on a substrate  30  provided with an active region  31 . Then, although not illustrated, a contact hole that exposes the active region  31  of the substrate  30  by passing through the inter-layer insulation layer  32  is formed.  
         [0027]     The inter-layer insulation layer  32  is formed by using an oxide-based material such as undoped silicate glass (USG), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG) or high density plasma (HDP) oxide, spin on glass (SOG), or tetraethylorthosilicate (TEOS). Also, the inter-layer insulation layer  32  can be made of a thermal oxide layer that is formed by oxidizing a silicon substrate at a high temperature ranging from approximately 600° C. to approximately 1,100° C. at a furnace.  
         [0028]     Next, a conductive material is filled into the contact hole, thereby forming a contact plug  33 . Herein, the contact plug  33  is frequently made of polysilicon.  
         [0029]     A titanium silicide layer  34  is formed in a manner to make a contact with the contact plug  33 . The titanium silicide layer  34  acts as a diffusion barrier layer for preventing an increase of a contact resistance between the contact plug  33  and a subsequent lower electrode. Also, when an electrode is made of an oxide-based material, especially, (Ba,Sr)RuO 3  (BSR) and is formed on the contact plug  33  made of polysilicon, oxygen from the thin BSR layer and polysilicon from the contact plug  33  react with each other, producing a compound with a bad electric characteristic. Thus, the titanium silicide layer  34  serves another role in preventing generation of such compound. That is, the titanium silicide layer  34  blocks oxygen from getting into the contact plug  33 .  
         [0030]     Referring to  FIG. 2B , a first thin BSR layer  35 A for forming a lower electrode is formed on the titanium silicide layer  34 . The thin BSR layer is an oxide-based layer where bromine (Br), strontium (Sr) and ruthenium (Ru) are compounded together, and since the thin BSR layer has a similar composition to that of a thin BST layer which will be formed above the thin BSR layer, it is easier to set up a reliable process compared with the case of using other electrode materials.  
         [0031]     Meanwhile, an area of an individual unit cell decreases as semiconductor memory devices have been highly integrated, and thus, even if a BST thin film with a high dielectric constant is used as a dielectric material, it is mandated to decrease a thickness of the thin BST layer in more extents.  
         [0032]     More particularly, the thin BST layer is typically formed in a thickness of approximately 500 Å. However, because of the large scale of integration, the thickness of the thin BST layer should be decreased to approximately 200 Å to approximately 300 Å. If the thickness of the thin BST layer is decreased to this mentioned level, it is difficult to fabricate a capacitor in a planar structure. As a result, the capacitor is rather formed in a stack structure.  
         [0033]     A step coverage characteristic is an important factor in the stack structure of the capacitor. For this reason, instead of employing a sputtering deposition method for forming a lower electrode, a metal organic chemical vapor deposition (MOCVD) method is employed. Especially, a low temperature MOCVD method is employed to obtain a good step coverage characteristic. However, the low temperature MOCVD method is limited because of several disadvantages such as low reliability, non-uniformity in a wafer and a non-decomposed substance material.  
         [0034]     Therefore, although it is possible to use a typical tetramethylheptanedionato (TMHD)-based substance material for forming a lower electrode, there are problems of using the above substance material such as decomposition of the TMHD-based substance material and a vapor pressure decrease of the TMHD-based substance material caused by a delayed time due to oligomerization and the like.  
         [0035]     Hence, the first BSR layer  35 A is formed by using a single cocktail source obtained preferably by diluting methoxyethoxytetramethylheptanedionato barium (Ba(METHD) 2 ) which is a thermally stable organic metal substance, methoxyethoxytetramethylheptanedionato strontium (Sr(METHD) 2 ), methoxyethoxytetramethylheptanedionato ruthenium (Ru(METHD) 3 ) with an organic solvent of butylacetate (C 6 H 12 O 2 ). At this time, the first thin BSR layer  35 A is formed in a thickness of approximately 50 Å using the above mentioned single cocktail source.  
         [0036]     When the fist thin BSR layer  35 A is formed by employing the MOCVD method, there still remain organic impurities like carbons or hydrogens because of the use of a carbon or hydrogen containing metal organic precursor. These impurities result in a decrease in the conductivity of the thin BSR layer  35 A and an increase in leakage currents caused by degradation of material characteristics of the first thin BSR layer  35 A.  
         [0037]     Therefore, after the first thin BSR layer  35 A of which thickness is below approximately 50 Å is formed, the single cocktail source supply is stopped and a temperature is raised up to approximately 700° C. Afterwards, oxygen and nitrogen are supplied in-situ. At this time, if the temperature is greater than approximately 800° C., oxygen react with atoms of the first thin BSR layer  35 A inducing a reduction reaction. For this reason, the temperature should be maintained to be in a range from approximately 500° C. to approximately 800° C. but not be greater than approximately 800° C.  
         [0038]     This oxygen thermal treatment is particularly applied to effectively remove organic substances contained within the first thin BSR layer  35 A, while the nitrogen thermal treatment is applied to improve crystallization of the first thin BSR layer  35 A so as to increase the conductivity of the first thin BSR layer  35 A. Also, the oxygen thermal treatment should be carried out for a short period, for instance, within five minutes, to remove the organic impurities of the first thin BSR layer  35 A and to prevent the titanium silicide layer  34  which is a diffusion barrier layer against oxygen from oxidizing.  
         [0039]     Herein, the nitrogen thermal treatment needs to be performed after the oxygen thermal treatment. The reason for this order of the thermal treatments is because if the nitrogen thermal treatment is performed prior to the oxygen thermal treatment, nitrogen stimulates crystallization of the organic impurities with the organic materials of the thin BSR layer  35 A, thereby impairing a complete removal of the organic materials during the oxygen thermal treatment.  
         [0040]     Although the MOCVD method is employed for forming the first thin BSR layer  35 A in this preferred embodiment, the first thin BSR layer  35 A can be still formed by employing other apparatuses such as a sputter, a vaporizer and the like. Also, although this preferred embodiment exemplifies the case of performing the oxygen thermal treatment and the nitrogen thermal treatment in-situ, these oxygen and nitrogen thermal treatments can be performed separately ex-situ.  
         [0041]     Referring to  FIG. 2C , a second thin BSR layer  35 B for forming the lower electrode is formed on the first thin BSR layer  35 A. As shown, the thin BSR layer  35  is formed in two layers because it is not possible to form a thin epitaxial layer owing to the fact that the thin BSR layer  35  has a different lattice orientation from that of the titanium silicide layer  34 . Thus, the first thin BSR layer  35 A is formed as a seed layer and a thermal treatment is applied to the first thin BSR layer  35 A to improve crystallinity of the first thin BSR layer  35 A. Afterwards, the second thin BSR layer  35 B is formed.  
         [0042]     As mentioned above, the first thin BSR layer  35 A and the second thin BSR layer  35 B are formed through the MOCVD method. The source material for forming the BSR thin layer  35  passes through a vaporizer, thereby producing precursors which are, in turn, decomposed under an appropriate condition. As a result of the decomposition, Ba, Sr and Ru are deposited on a substrate structure, forming the thin BSR layer  35 . Herein, as described above, the thermal treatment processes are performed after the formation of the first thin BSR layer  35 A to improve crystallinity of a thin dielectric layer and enable an epitaxial growth of the second thin BSR layer  35 B.  
         [0043]     Also, the thin BSR layer  35  has a thickness ranging from approximately 200 Å to approximately 300 Å. Also, a deposition temperature is approximately 500° C., and a pressure is maintained approximately 2 torr. For an oxidizing agent, oxygen (O 2 ) gas is used, and an amount of the O 2  gas ranges from approximately 100 sccm to approximately 700 sccm. A composition ratio of Ba to Sr to Ru of the thin BSR layer  35  is set to be approximately 0.5 to approximately 0.5 to approximately 1.  
         [0044]     After the formation of the thin BSR layer  35 , a rapid thermal process for improving properties of the thin BSR layer  35  is carried out.  
         [0045]     Referring to  FIG. 2D , a thin BST dielectric layer  36  is formed on the thin BSR layer  35 . The thin BST dielectric layer  36  can be grown epitaxially since the thin BST dielectric layer  36  is formed on top of the lower electrode made of the thin BSR layer  35 . Afterwards, an upper electrode  37  is formed on the thin BST dielectric layer  36 .  
         [0046]     Herein, the upper electrode  37  is made of the same thin BSR film used for forming the lower electrode or another metal-based material selected singly or in combination from a group consisting of titanium nitride (TiN), tantalum nitride (TaN), tungsten (W), platinum (Pt), iridium (Ir), iridium oxide (IrO 2 ), ruthenium (Ru), ruthenium oxide (RuO 2 ), tungsten nitride (WN), and strontium ruthenate (SrRuO 3 ). Also, if the thin BSR film is used for forming the upper electrode  37 , the same process recipe used for forming the lower electrode is applied to form the upper electrode  37 .  
         [0047]     In accordance with the present invention, when the dielectric layer of a capacitor is made of BST, a thin film of BSR is used for forming the lower electrode, and thus, it is possible to form a stable capacitor by forming the thin dielectric layer on the lower electrode with a similar lattice structure.  
         [0048]     Also, on the basis of the present invention, the lower electrode is formed in two thin BSR layers. That is, the first thin BSR layer is formed as a seed layer, and then the oxygen thermal treatment and the nitrogen thermal treatment are carried out to remove organic impurities from the first thin BSR layer. Afterwards, the second thin BSR layer is formed on the seed layer, i.e., the first thin BSR layer. These sequential processes improve characteristics of the thin BSR layer such as leakage current, thereby enabling formation of a reliable capacitor.  
         [0049]     The present application contains subject matter related to the Korean patent application No. KR  2004 - 0060447 , filed in the Korean Patent Office on Jul. 30, 2004, the entire contents of which being incorporated herein by reference.  
         [0050]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.