Abstract:
To provide a simple method for evaluating reliability of a transistor, a simple test which correlates with a bias-temperature stress test (BT test) is performed instead of the BT test. Specifically, a gate current value is measured in the state where a voltage lower than the threshold voltage of an n-channel transistor whose channel region includes an oxide semiconductor is applied between a gate and a source of the transistor and a potential applied to a drain is higher than a potential applied to the gate. The evaluation of the gate current value can be simply performed compared to the case where the BT test is performed; for example, it takes short time to measure the gate current value. That is, reliability of a semiconductor device including the transistor can be easily evaluated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for evaluating reliability of a semiconductor device including a transistor. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) have attracted attention. For example, a thin film transistor which includes a semiconductor layer using an amorphous oxide (an oxide semiconductor) whose electron carrier concentration is less than 10 18 /cm 3  is disclosed in the Patent Documents 1 to 3. 
         [0005]    When a transistor is applied to various products, the transistor preferably has high reliability. From this reason, characteristics of a transistor under various environments are needed to be recognized. For example, as a test for recognizing the characteristics of a transistor, a bias-temperature stress test (hereinafter referred to as a BT test) is known. Temperature, a bias condition, and the like are considered as test conditions of a BT test (see Patent Document 4). 
       REFERENCE 
       [0000]    
       
         [Patent Document 1] Japanese Published Patent Application No. 2006-165527 
         [Patent Document 2] Japanese Published Patent Application No. 2006-165528 
         [Patent Document 3] Japanese Published Patent Application No. 2006-165529 
         [Patent Document 4] Japanese Published Patent Application No. 2001-274210 
       
     
       SUMMARY OF THE INVENTION 
       [0010]    Note that several hours to several tens of hours are required for measurement by the above described BT test. 
         [0011]    In view of the above, an object of one embodiment of the present invention is to provide a method for evaluating reliability of a semiconductor device including a transistor by a simple method. 
         [0012]    The present inventor has found that there is a correlation between a gate current value of an n-channel transistor whose channel region includes an oxide semiconductor under a specific condition, and a result of a BT test of the transistor under the same condition. Specifically, the present inventor has found that there is a correlation between a gate current value and a result of a BT test which are in the same state where a voltage lower than the threshold voltage of the transistor is applied between a gate and a source of the transistor and a potential applied to a drain is higher than a potential applied to the gate. In other words, the present inventor has found that a gate current value in the state where the transistor is in an off state (a state where a channel region is not formed) and an electric field is generated from the drain of the transistor to the gate of the transistor correlates with a result of a BT test performed under the same state. Note that the gate current refers to a current generated in the gate. 
         [0013]    As will be described in detail, when a gate current value is measured under the above condition, a current (transient current) is generated in the gate even after the gate is electrically connected to a wiring which supplies a fixed potential. Note that the gate current value decreases with time. Here, from what the transient current results is considered. For example, when the transient current results only from a leakage from the gate to a source (a gate leakage), a gate current generated by the gate leakage should be in a steady state because a fixed potential is applied to the gate and the source under the above condition. In other words, the gate current value should be constant and does not vary with time. However, in reality, the gate current value decreases with time. That is, it can be considered that there is a factor other than the gate leakage which causes the generation of the transient current. The present inventor has considered that the transient current results from accumulation of positive charges in an oxide semiconductor or at the interface between an oxide semiconductor and a gate insulating film. In other words, under the above condition, negative charges are accumulated with time at the interface between the gate and the gate insulating film through the gate insulating film in accordance with the accumulation of positive charges with time in the oxide semiconductor or at the interface between the oxide semiconductor and the gate insulating film. Therefore, it is considered that the transient current is generated even after the gate is electrically connected to the wiring which supplies a fixed potential. 
         [0014]    On the other hand, when the BT test is performed under the above condition, deterioration such as a negative shift of a drain current (Id)-gate voltage (Vg) curve and an increase of an off state current value is generated in some cases. Note that a degree of the deterioration depends on a sample condition, a test condition, and the like. It can be considered that the deterioration is caused by the accumulation of positive charges in the oxide semiconductor or at the interface between the oxide semiconductor and the gate insulating film, and by trapping of part of the positive charges in the oxide semiconductor or at the interface between the oxide semiconductor and the gate insulating film. That is, it can be considered that when part of the accumulated positive charges are trapped in the oxide semiconductor or at the interface between the oxide semiconductor and the gate insulating film under the above condition, the negative shift of the drain current (Id)-gate voltage (Vg) curve and the increase of an off state current value occur, for example. 
         [0015]    The present inventor has considered that both of them (the transient current generated in the gate and the deterioration due to the BT test) depend on the same phenomenon (the accumulation of positive charges in the oxide semiconductor or at the interface between the oxide semiconductor and the gate insulating film). In fact, a correlation was found between these two as a result of the evaluation. 
         [0016]    The present inventor has found that there is a correlation between a gate current value and a result of a BT test which are in the same state where a voltage lower than the threshold voltage of an n-channel transistor whose channel region includes an oxide semiconductor is applied between a gate and a source of the transistor and a potential applied to a drain is higher than a potential applied to the gate. The evaluation of the gate current value can be simply performed compared to the case where the BT test is performed; for example, it takes short time to measure the gate current value. In other words, reliability of a semiconductor device including the transistor can be easily evaluated by the measurement of the gate current value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1A to 1D  are cross-sectional views each illustrating a structural example of a transistor. 
           [0018]      FIG. 2  is a graph showing variations in gate current values with time. 
           [0019]      FIG. 3  is a graph showing variations in gate current values with time. 
           [0020]      FIG. 4  is a graph showing drain current (Id)-gate voltage (Vg) characteristics of transistors. 
           [0021]      FIG. 5  is a graph showing drain current (Id)-gate voltage (Vg) characteristics of transistors. 
           [0022]      FIG. 6  is a graph showing evaluation of a relation between gate current values and deterioration due to a BT test. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    First, structural examples of a transistor to be a sample will be described with reference to  FIGS. 1A to 1D . 
         [0024]      FIGS. 1A to 1D  are cross-sectional views each illustrating the structural examples of the transistor. A transistor  211  illustrated in  FIG. 1A  includes a gate layer  221  provided over a substrate  220  having an insulating surface, a gate insulating film  222  provided over the gate layer  221 , an oxide semiconductor layer  223  provided over the gate insulating film  222 , and a source layer  224   a  and a drain layer  224   b  provided over the oxide semiconductor layer  223 . 
         [0025]    As described above, the transistor  211  illustrated in  FIG. 1A  includes the oxide semiconductor layer  223  as a semiconductor layer where a channel region is formed. As an oxide semiconductor used for the oxide semiconductor layer  223 , any of the following can be used: an In—Sn—Ga—Zn—O-based oxide semiconductor which is a four-component metal oxide; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and a Sn—Al—Zn—O-based oxide semiconductor which are three-component metal oxides; an In—Ga—O-based oxide semiconductor, an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, and an In—Mg—O-based oxide semiconductor which are two-component metal oxides; and an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor which are single-component metal oxides. Further, SiO 2  may be contained in the above oxide semiconductors. Here, for example, the In—Ga—Zn—O-based oxide semiconductor means an oxide containing at least In, Ga, and Zn, and the composition ratio of the elements is not particularly limited. The In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn. 
         [0026]    For the oxide semiconductor layer  223 , a thin film represented by the chemical formula, InMO 3 (ZnO) m  (m&gt;0), can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. 
         [0027]    In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In 2 O 3 :ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In 2 O 3 :ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=1.5:1 to 15:1 in an atomic ratio (In 2 O 3 :ZnO=3:4 to 15:2 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z&gt;1.5X+Y is satisfied. 
         [0028]    As the substrate  220  having an insulating surface, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like can be used, for example. Note that a substrate formed of an insulator such as a ceramic substrate, a quartz substrate, or a sapphire substrate may be used for the substrate  220 , instead of the glass substrate. Alternatively, crystallized glass or the like can be used. 
         [0029]    In the transistor  211 , an insulating film serving as a base film may be provided between the substrate  220  and the gate layer  221 . The base film has a function of preventing diffusion of an impurity element from the substrate, and can be formed to have a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. 
         [0030]    The gate layer  221  can be formed to have a single-layer structure or a stacked-layer structure using a metal material such as molybdenum (Mo), titanium (Ti), chromium (Cr), tantalum (Ta), tungsten (W), aluminum (Al), copper (Cu), neodymium (Nd), or scandium (Sc), or an alloy material including any of these as a main component. 
         [0031]    The gate insulating film  222  can be formed by a plasma CVD method, a sputtering method, or the like as a single-layer structure or a stacked-layer structure of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, an aluminum oxide layer, an aluminum nitride layer, an aluminum oxynitride layer, an aluminum nitride oxide layer, or a hafnium oxide layer. For example, by a plasma CVD method, a silicon nitride layer (SiN y  (y &gt;0)) with a thickness of 50 nm or more and 200 nm or less is formed as a first gate insulating film, and a silicon oxide layer (SiO x  (x &gt;0)) with a thickness of 5 nm or more and 300 nm or less can be formed as a second gate insulating film over the first gate insulating film. 
         [0032]    As a conductive film used for the source layer  224   a  and the drain layer  224   b , for example, an element selected from aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy containing any of these elements as a component, an alloy film combining any of these elements, or the like can be used. Alternatively, a structure may be employed in which a high-melting-point metal layer of titanium (Ti), molybdenum (Mo), tungsten (W), or the like is stacked over and/or below a metal layer of aluminum (Al), copper (Cu), or the like. In addition, heat resistance can be improved by using an aluminum (Al) material to which an element (such as Si, Nd, or Sc) for preventing generation of a hillock or a whisker in an aluminum (Al) film is added. 
         [0033]    Further alternatively, the conductive film to be the source layer  224   a  and the drain layer  224   b  (including a wiring layer formed in the same layer as the source layer  224   a  and the drain layer  224   b ) may be formed of a conductive metal oxide. As the conductive metal oxide, indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), indium oxide-tin oxide alloy (In 2 O 3 —SnO 2 ; abbreviated to ITO), indium oxide-zinc oxide alloy (In 2 O 3 −ZnO), or any of these metal oxide materials containing silicon oxide can be used. 
         [0034]    The transistors disclosed in this specification are not limited to the structure illustrated in  FIG. 1A . For example, structures illustrated in  FIGS. 1B to 1D  can be applied to these transistors. 
         [0035]    A transistor  510  illustrated in  FIG. 1B  is one of bottom-gate transistors referred to as a channel-protective (also referred to as a channel-stop) transistor. 
         [0036]    The transistor  510  includes the gate layer  221 , the gate insulating film  222 , the oxide semiconductor layer  223 , an insulating layer  511  functioning as a channel protective layer that covers a channel formation region of the oxide semiconductor layer  223 , the source layer  224   a , and the drain layer  224   b , which are formed over the substrate  220  having an insulating surface. 
         [0037]    As the insulating layer  511 , an insulator such as silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be used. A stacked-layer structure of these materials can also be used. 
         [0038]    A transistor  520  illustrated in  FIG. 1C  is a bottom-gate transistor. The transistor  520  includes the gate layer  221 , the gate insulating film  222 , the source layer  224   a , the drain layer  224   b , and the oxide semiconductor layer  223 , which are formed over the substrate  220  having an insulating surface. 
         [0039]    In the transistor  520 , the gate insulating film  222  is provided on and in contact with the substrate  220  and the gate layer  221 , and the source layer  224   a  and the drain layer  224   b  are provided on and in contact with the gate insulating film  222 . The oxide semiconductor layer  223  is provided over the gate insulating film  222 , the source layer  224   a , and the drain layer  224   b.    
         [0040]    A transistor  530  illustrated in  FIG. 1D  is one of top-gate transistors. The transistor  530  includes an insulating layer  531 , the oxide semiconductor layer  223 , the source layer  224   a , the drain layer  224   b , the gate insulating film  222 , and the gate layer  221 , which are formed over the substrate  220  having an insulating surface. A wiring layer  532   a  and a wiring layer  532   b  are provided in contact with and electrically connected to the source layer  224   a  and the drain layer  224   b  respectively. 
         [0041]    As the insulating layer  531 , an insulator such as silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be used. A stacked-layer structure of these materials can also be used. 
         [0042]    As the wiring layers  532   a  and  532   b , an element selected from aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); an alloy containing any of these elements; or a nitride containing any of these elements can be applied. A stacked-layer structure of these materials can also be used. 
       Example 1 
       [0043]    In this example, measurement results of variations in gate current values with time and results of a BT test are shown. The states of these results, which are the same, are as follows: a voltage lower than the threshold voltage of a transistor whose channel formation region includes an oxide semiconductor is applied between a gate and a source of the transistor; and a potential applied to a drain is higher than a potential applied to the gate. Further, in this example, the measurement of gate current values and the BT test were performed under five temperature conditions on two kinds of transistors having gate insulating films with different thicknesses. In other words, 20 samples were prepared for the measurement. Specific conditions of the measurement in this example are shown in Table 1. In Table 1, Samples 1 to 10 were used for the measurement of variations in the gate current values with time and Samples 11 to 20 were used for the BT test. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Thickness of 
                 Measurement 
               
               
                   
                 Gate Insulating film 
                 Temperature 
               
               
                   
                 (nm) 
                 (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Samples 1, 11 
                 30 
                 25 
               
               
                   
                 Samples 2, 12 
                 30 
                 40 
               
               
                   
                 Samples 3, 13 
                 30 
                 85 
               
               
                   
                 Samples 4, 14 
                 30 
                 125 
               
               
                   
                 Samples 5, 15 
                 30 
                 150 
               
               
                   
                 Samples 6, 16 
                 100 
                 25 
               
               
                   
                 Samples 7, 17 
                 100 
                 40 
               
               
                   
                 Samples 8, 18 
                 100 
                 85 
               
               
                   
                 Samples 9, 19 
                 100 
                 125 
               
               
                   
                 Samples 10, 20 
                 100 
                 150 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    Note that transistors having the structure illustrated in  FIG. 1A  are used for the measurement in this example. In addition, channel length L and channel width W of each of the Samples 1 to 20 are 10 μm and 1 m respectively. Manufacturing steps of the transistors are described below in detail. 
         [0045]    First, a tungsten film with a thickness of 100 nm was formed over a glass substrate by a sputtering method. Then, a gate layer with a thickness of 100 nm was formed by a photolithography method and an etching method. 
         [0046]    Next, a gate insulating film was formed over the substrate and the gate layer by a plasma CVD method. Note that as shown in Table 1, the thicknesses of the gate insulating films of the Samples 1 to 5 and 11 to 15 are 30 nm respectively and the Samples 6 to 10 and 16 to 20 are 100 nm respectively. 
         [0047]    Next, an oxide semiconductor film with a thickness of 20 nm was formed over the gate insulating film by a sputtering method. Here, a target used for formation of the oxide semiconductor film by a sputtering method is, for example, an oxide target containing In 2 O 3 , Ga 2 O 3 , and ZnO at a composition ratio of 1:1:1 [mol], so that an In—Ga—Zn—O film was formed. Film formation was performed while heating the substrate at 200° C. As the other conditions of the film formation, the pressure was 0.6 Pa, the direct current (DC) power supply was 5 kW, and the atmosphere was a mixed atmosphere of oxygen and argon (the gas flow ratio of oxygen and argon was 1:1). 
         [0048]    Subsequently, the oxide semiconductor film was subjected to heat treatment at 650° C. in a nitrogen atmosphere for 6 minutes, using a gas rapid thermal anneal (GRTA) apparatus. 
         [0049]    Next, an oxide semiconductor layer with a thickness of 20 nm was formed by a photolithography method and an etching method. 
         [0050]    Then, a titanium film with a thickness of 100 nm, an aluminum film with a thickness of 200 nm, and a titanium film with a thickness of 100 nm were stacked and formed over the gate insulating film and the oxide semiconductor layer by a sputtering method. A source layer and a drain layer which were formed of a stacked layer of a titanium film, an aluminum film, and a titanium film and had a thickness of 400 nm were formed by a photolithography method and an etching method. 
         [0051]    Through the above steps, the transistors were completed. Note that an interlayer insulating film was further formed over each of the transistors, which were used in the measurement of this example. Specifically, a silicon oxide film with a thickness of 300 nm was formed over each of the transistors by a sputtering method, following the above steps. Moreover, an acrylic film with a thickness of 1.5 μm was formed over the silicon oxide film by a spin coating method. 
         [0052]    The measurement results of this example are shown below. In this example, the measurement of variations in the gate current values with time was performed in the state where a potential of −2.0 V was applied to the gate of each of the transistors and a potential of 0 V was applied to the source and the drain of each of the transistors. In addition, drain current (Id)-gate voltage (Vg) characteristics were evaluated in the same state after the BT test was performed for 600 seconds. 
         [0053]      FIG. 2  is a graph showing variations in the gate current values of the Samples 1 to 5 with time.  FIG. 3  is a graph showing variations in the gate current values of the Samples 6 to 10 with time. The graphs in  FIG. 2  and  FIG. 3  show that the gate current values are increased in accordance with an increase in the measurement temperature. Further, the gate current values of each of the transistors including the gate insulating film with a thickness of 100 nm is greater than the gate current values of each of the transistors including the gate insulating film with a thickness of 30 nm. 
         [0054]      FIG. 4  is a graph showing drain current (Id)-gate voltage (Vg) characteristics of the Samples 11 to 15 after the BT test was performed.  FIG. 5  is a graph showing drain current (Id)-gate voltage (Vg) characteristics of the Samples 16 to 20 after the BT test was performed. The graphs in  FIG. 4  and  FIG. 5  show that deterioration due to the BT test (a negative shift of a drain current (Id)-gate voltage (Vg) curve and an increase in the off current) is more remarkable in high temperature tests. Further, deterioration, due to the BT test, of each of the transistors including the gate insulating film with a thickness of 100 nm is more remarkable than that of each of the transistors including the gate insulating film with a thickness of 30 nm. 
         [0055]      FIG. 6  is a graph showing evaluation of a relation between the gate current values and the deterioration due to the BT test, based on the above described measurement results. Specifically, in  FIG. 6 , the horizontal axis indicates the gate current values of each of the Samples 1 to 10 at 10 seconds after each of the measurements is started under the above conditions. The vertical axis indicates the rising voltage of the drain current (Id)-gate voltage (Vg) curve after the BT test was performed on the samples under the conditions where thickness of the gate insulating film and measurement temperature are the same as that of the above conditions. Note that a “rising voltage” in this specification refers to a voltage value when a current value is increased by an order of magnitude from a reference value. Here, the reference value is a current value of a transistor in an off state.  FIG. 6  shows that the value of the rising voltage is reduced in accordance with an increase of the gate current values. In other words, it is found that a degree of deterioration due to the BT test can be observed by the gate current values. 
         [0056]    This application is based on Japanese Patent Application serial no. 2010-111158 filed with Japan Patent Office on May 13, 2010, the entire contents of which are hereby incorporated by reference.