Abstract:
A plasma CVD apparatus comprises a reaction container for allowing a reaction for forming a thin film on a semiconductor wafer to be performed, a bias electrode which applies a high frequency bias for sputtering to the semiconductor wafer, a nozzle which supplies SiH 4  gas including at least hydrogen into the reaction container, and a control circuit which on/off-controls the high frequency bias through a switch and which on/off-controls the supply of SiH 4  gas through a flow rate controller based on an opposite control logic to a high frequency bias control logic.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a plasma CVD apparatus which can suppress the deterioration of hydrogen reduction during film formation.  
         BACKGROUND OF THE INVENTION  
         [0002]    A plasma CVD apparatus which forms a thin film such as an insulating film on a semiconductor wafer using plasma vapor phase excitation has been conventionally used in semiconductor device manufacturing process. This plasma CVD apparatus supplies material gas which consists of elements constituting a thin film onto the semiconductor wafer and forms a desired thin film by a vapor phase or a chemical reaction on the surface of the semiconductor wafer. Plasma discharge is used to excite gas molecules.  
           [0003]    [0003]FIG. 4 shows the configuration of a conventional plasma CVD device. In FIG. 4, a reaction container  10  is a container which has an evacuated interior and which allows an insulating film to be formed on a semiconductor wafer  19  having a diameter of  12  inches. A nozzle  11  which emits Ar gas, a nozzle  12  which emits O 2  gas and a nozzle  13  which emits SiH 4  gas which serves as the material gas explained above are provided on the inner side face of the reaction container  10 .  
           [0004]    An RF electrode  14  is provided on the upper section of the reaction container  10  and connected to a high frequency power supply  15 . This RF electrode  14  generates a high frequency electric field to deposit SiO X  on the semiconductor wafer  19 . As shown in FIG. 5A, during the vapor deposition, an insulating film  19   b  is formed to cover wirings  19   a  formed on the semiconductor wafer  19 . At this moment, however, the insulating film  19   b  does not completely reach gaps between the wirings  19   a . In FIG. 5A, an RF input is a high frequency input from the RF electrode  14 . In addition, the RF power of the RF electrode  14  is set at, for example, 3 kW.  
           [0005]    A support base  16  is provided in the reaction container  10  and supports the semiconductor wafer  19  by an electrostatic force. A bias electrode  17  is embedded in the support base  16  so as to be opposed to the RF electrode  14  and is connected to a high frequency power supply  18 .  
           [0006]    The bias electrode  17  applies a bias so as to draw ionized Ar +  into the semiconductor wafer  19 . The ionized Ar +  etches the insulating film  19   b  deposited on the upper corner sections of the wirings  19   a . In this instance, therefore, the upper sections of the gaps between the wirings  19   a  are always opened, making it possible to evaporate the insulating film  19   b  compactly into the gaps between the wirings  19   a . In FIG. 5B, an LF input is a bias input from the bias electrode  17 . The bias power of the bias electrode  17  is, for example, 1 kW.  
           [0007]    According to the configuration explained above, the Ar gas, the O 2  gas and the SiH 4  gas are constantly emitted from the nozzles  11 ,  12  and  13  into the reaction container  10 , respectively, as can be seen from “B”, “C” and “E” shown in FIG. 6. Likewise, the high frequency power supplies  15  and  18  are constantly connected to the RF electrode  14  and the bias electrode  17 , respectively. That is, as can be seen from “A” and “D” shown in FIG. 6, the RF electrode  14  and the bias electrode  17  are kept in an RF input (high frequency input) state and an LF input (bias input) state, respectively. Therefore, vapor deposition due to the RF input and sputtering due to the LF input are simultaneously carried out in the reaction container  10 .  
           [0008]    In other words, as shown in FIG. 5B, the insulating film  19   b  which consists of SiH 4  is evaporated on the surface of the semiconductor wafer  19  and sputtering is carried out so that Ar +  is drawn into the semiconductor wafer  19  side. As a result of this sputtering, the excess insulating film  19   b  is scraped off and the insulating film  19   b  spreads into the gaps between the wirings  19   a .  
           [0009]    The conventional plasma CVD apparatus draws Ar +  into the semiconductor wafer  19  by applying a bias thereto from the bias electrode  17  shown in FIG. 4. However, when the bias is applied, hydrogen existing in the reaction container  10  is also drawn into the semiconductor wafer  19 . FIG. 7A is a view which shows a relationship between the mass number of an element and current (drawn-in quantity) when bias is OFF. FIG. 7B is a view which shows a relationship between the mass number of an element and current (drawn-in quantity) when bias is ON. The mass number of an element= 2  corresponds to that of hydrogen molecules (H 2 )The quantity of hydrogen which is drawn into the semiconductor wafer  19  rapidly increases when bias is OFF and ON. In this instance, hydrogen reduction is deteriorated in the semiconductor wafer  19 , which adversely influences device characteristic. If the semiconductor wafer  19  is made of a ferroelectric material, in particular, the P(polarization)-V(applied voltage) characteristic of the semiconductor wafer  19  (semiconductor device) deteriorates as shown in FIG. 8. That is, before film formation, the P-V characteristic has an ordered before-film-formation hysteresis loop  30 . After film formation, the P-V characteristic has a disordered after-film-formation hysteresis loop  31 .  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a plasma CVD apparatus which can suppress the deterioration of hydrogen reduction in a semiconductor wafer during film formation.  
           [0011]    The plasma CVD apparatus according to this invention comprises a reaction container for allowing a reaction for forming a thin film on a semiconductor wafer to be performed, a high frequency bias unit which applies a high frequency bias for sputtering to the semiconductor wafer, and a high frequency bias control unit which on/off-controls the high frequency bias. The plasma CVD apparatus also comprises a gas supply unit which supplies gas containing at least hydrogen to the reaction container, and a gas supply control unit which on/off-controls supply of the gas based on an opposite control logic to a control logic of the high frequency bias control unit.  
           [0012]    Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a view which shows the configuration of one embodiment of the invention according to the present invention,  
         [0014]    [0014]FIG. 2A to FIG. 2D are timing charts which explain various gas outputs, an RF input and an LF input in the embodiment of the invention,  
         [0015]    [0015]FIG. 3 is a view which shows P-V characteristic in the embodiment of the invention,  
         [0016]    [0016]FIG. 4 is a view which shows the configuration of a conventional plasma CVD apparatus,  
         [0017]    [0017]FIG. 5A and FIG. 5B are views which explain film formation in the conventional plasma CVD apparatus,  
         [0018]    [0018]FIG. 6 is a timing chart which explains various gas outputs, an RF input, and an LF input in the conventional plasma CVD apparatus,  
         [0019]    [0019]FIG. 7A and FIG. 7B are views which explain the problems of the conventional plasma CVD apparatus, and  
         [0020]    [0020]FIG. 8 is a view which shows the P-V characteristic of the conventional plasma CVD apparatus. 
     
    
     DETAILED DESCRIPTION  
       [0021]    An embodiment of the plasma CVD apparatus according to the present invention will be explained hereinafter in detail with reference to the drawings.  
         [0022]    [0022]FIG. 1 shows the configuration of one embodiment of the apparatus according to the present invention. In FIG. 1, sections corresponding to those shown in FIG. 4 are denoted by the same reference symbols, respectively. In FIG. 1, a flow rate controller  100 , a flow rate controller  101 , a switch  102  and a control circuit  103  are newly provided.  
         [0023]    The flow rate controller  100  turns on and off the flow rate of O 2  gas emitted from a nozzle  12  based on an O 2  gas flow rate control signal S 1  (see FIG. 2B) reversed from the control circuit  103 . The O 2  gas flow rate control signal S 1  shown in FIG. 2B is a signal which is repeatedly turned on and off at predetermined time intervals.  
         [0024]    The flow rate controller  101  turns on and off the flow rate of SiH 4  gas emitted from a nozzle  13  based on an SiH 4  gas flow rate control signal S 2  (see FIG. 2C) output from the control circuit  103 . The SiH 4  gas flow rate control signal S 2  shown in FIG. 2C is a signal which is synchronized with the O 2  gas flow rate control signal S 1  and which is repeatedly turned on and off at predetermined time intervals.  
         [0025]    The switch  102  is interposed between the bias electrode  17  and the high frequency power supply  18 , and is controlled to be turned on and off based on a bias control signal S 3  (see FIG. 2D) output from the control circuit  103 . The bias control signal S 3  shown in FIG. 2D is a signal which has a reversed relationship with respect to the O 2  gas flow rate control signal S 1  (see FIG. 2B) and the SiH 4  gas flow rate control signal S 2  (see FIG. 2C). The control circuit  103  outputs the O 2  gas flow rate control signal S 1 , the SiH 4  gas flow rate control signal S 2 , and the bias control signal S 3  to thereby conduct flow rate control and bias control.  
         [0026]    As the line “B” in FIG. 2A indicates that Ar gas is constantly emitted from the nozzle  11  into the reaction container  10 . Similarly, the line “A” in FIG. 2A indicates that the high frequency power supply  15  is constantly connected to the RF electrode  14 .  
         [0027]    Between time t o  and time t 1  (e.g., for 20 sec) shown in FIG. 2B to FIG. 2C, the O 2  gas flow rate control signal S 1  and the SiH 4  gas flow rate control signal S 2  are set ON. In this instance, therefore, the O 2  gas and the SiH 4  gas are emitted from the nozzles  12  and  13  into the reaction container  10 , respectively. Asaresult, an insulating film which consists of SiH 4  is evaporated on the surface of the semiconductor wafer  19 .  
         [0028]    On the other hand, between time t 0  and time t 1  shown in FIG. 2D, the bias control signal S 3  is set OFF. In this instance, since the bias from the bias electrode  17  is set OFF, sputtering is not carried out.  
         [0029]    Between time t 1  and time t 2 , the O 2  gas flow rate control signal S 1  and the SiH 4  gas flow rate control signal S 2  are changed from ON to OFF. Therefore, the emission of O 2  gas and SiH 4  gas from the nozzles  12  and  13  is stopped. In this instance, therefore, no insulating film is evaporated on the semiconductor wafer  19 .  
         [0030]    On the other hand, between time t 1 , and time t 2 , the bias control signal S 3  is changed from OFF to ON. In this instance, therefore, the bias from the bias electrode  17  is set ON and Ar +  is drawn into the semiconductor wafer  19 , i.e., sputtering is carried out. In this instance, SiH 4  and the like including hydrogen are not supplied to the reaction container  10 , which suppresses unnecessary hydrogen from being drawn into the semiconductor wafer  19 . Thereafter, the vapor deposition and the sputtering are alternately repeated at predetermined time intervals.  
         [0031]    [0031]FIG. 3 shows the P-V characteristic of the semiconductor wafer  19  in one embodiment of the invention. As can be seen from FIG. 3, before and after film formation, an ordered before-film-formation hysteresis loop  200  and an ordered after-film-formation hysteresis loop  201  are formed, respectively. This represents that the quantity of hydrogen drawn into the semiconductor wafer  19  rapidly decreases in one embodiment of the invention.  
         [0032]    According to one embodiment of the invention, sputtering using the bias is carried out while the supply of SiH 4  gas including hydrogen is stopped. Therefore, rate of drawing originally unnecessary hydrogen into the semiconductor wafer  19  sharply decreases, making it possible to suppress the deterioration of hydrogen reduction in the semiconductor wafer  19  during the film formation.  
         [0033]    Note that it is possible that the control circuit  103  adjusts the switching cycle and the duty ratio between the O 2  gas flow rate control signal S 1 , the SiH 4  gas flow rate control signal S 2 , and the bias control signal S 3 . In this instance, it is possible to minutely control vapor deposition time and sputtering time in accordance with the state of the semiconductor wafer. Further, in one embodiment of the invention, the instance in which the emission of O 2  gas is on/off controlled has been explained. However, since hydrogen is not contained in the O 2  gas in its ideal form, the O 2  gas may be continuously emitted.  
         [0034]    According to the present invention, sputtering using the high frequency bias is carried out while the supply of gas including hydrogen is stopped. Therefore, the rate of drawing originally unnecessary hydrogen into the semiconductor wafer sharply decreases, making it possible to suppress the deterioration of hydrogen reduction in the semiconductor wafer during the film formation.  
         [0035]    Moreover, the switching cycle and the duty ratio between the high frequency bias control signal and the gas supply control signal are adjusted. It is, therefore, possible to minutely control vapor deposition time and sputtering time in accordance with the state of the semiconductor wafer.  
         [0036]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.