Patent Publication Number: US-9893073-B2

Title: Semiconductor nonvolatile memory element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of parent application Ser. No. 15/013,540 filed Feb. 2, 2016, now U.S. Pat. No. 9,613,970, and claims priority of Japanese Application Nos. 2015-019469 filed Feb. 3, 2015, 2015-020600 filed Feb. 4, 2015 and 2015-194571 filed Sep. 30, 2015. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor nonvolatile memory element that can change a threshold voltage through an externally applied electrical signal and a manufacturing method thereof. 
     2. Description of the Related Art 
     An electronic circuit used in electronic equipment is driven by a power supply such as a battery. When a voltage of the power supply fluctuates, malfunction of the electronic circuit and various abnormal phenomena may be caused. Thus, it is a typical approach to place a power management IC between the electronic circuit and the power supply, which is configured to regulate the voltage so that a constant voltage is output or monitoring fluctuations of the power supply, to thereby promote stable operation. In particular, in a semiconductor integrated circuit device such as a microcomputer or a CPU that is operated at increasingly lower voltages in recent years, the power management IC has been strongly required to output an accurate constant voltage and to accurately monitor the voltage value. 
     Exemplary power management ICs configured such that a constant voltage is output from a power supply to an electronic circuit include a step-down series regulator as illustrated in  FIG. 3 . 
     In this semiconductor integrated circuit device, a power supply voltage that is applied between a ground terminal  105  and a power supply terminal  106  is divided by a PMOS output element  104  and a voltage dividing circuit  103  including resistance elements  102 . The voltage divided by the resistance elements  102  is input to a minus input terminal of an error amplifier  101 , and is compared to a certain reference voltage value generated by a reference voltage circuit  100 . Depending on a result of the comparison, the error amplifier  101  controls an input voltage of the PMOS output element  104  to change a source-drain resistance of the PMOS output element  104 . As a result, an output terminal  107  has the function of outputting a constant output voltage that does not depend on the power supply voltage, but depends on the reference voltage value of the reference voltage circuit  100  and a resistance divided voltage ratio of the voltage dividing circuit  103 . The output voltage is calculated by the following Expression (1):
 
(Output voltage)=(reference voltage value)×(resistance divided voltage ratio of voltage dividing circuit)  (1)
 
     In regulating the output voltage, by changing a resistance value of the resistance element  102  in a method described below, the divided voltage ratio of the voltage dividing circuit  103  is changed to set the output voltage value at a desired value based on Expression (1). Accordingly, the voltage dividing circuit of the semiconductor integrated circuit is required to be processed/corrected for each target output voltage. 
     Further, a voltage detector as illustrated in  FIG. 4  that has the function of outputting a signal when the power supply voltage becomes a constant voltage is also one kind of the power management IC. 
     In this semiconductor integrated circuit device, the power supply voltage that is input from the power supply terminal  108  is converted to a voltage divided by the voltage dividing circuit  103  that includes the resistance elements  102 , and the converted voltage is compared to the reference voltage value of the reference voltage circuit  100  by a comparator  108 . A voltage signal corresponding to a result of the comparison is output from the output terminal  107 . With this mechanism, a voltage detector is realized that has the function of monitoring the power supply voltage and outputting, when the voltage becomes equal to or higher than, or, equal to or lower than a certain voltage, a signal for the purpose of performing appropriate processing. 
     Also in the example illustrated in  FIG. 4 , by changing the resistance value of the resistance element  102 , the divided voltage ratio of the voltage dividing circuit  103  is changed to set a desired voltage detection value based on Expression (1). Accordingly, the voltage dividing circuit of the semiconductor integrated circuit device is required to be processed/corrected for each target output voltage. 
     As a resistance element that is used for a voltage dividing circuit of a semiconductor integrated circuit device, a diffused resistor that is a monocrystalline silicon semiconductor substrate implanted with impurities having a conductivity type opposite to that of the semiconductor substrate, a resistor formed of polycrystalline silicon implanted with impurities, or the like is used. In designing the voltage dividing circuit, when a plurality of resistors are used, the resistors are set so as to have the same length, the same width, and the same resistivity. Then, the respective resistance elements are equally subjected to variations in shape in an etching process in which the shape is determined and to variations in impurity implantation. Accordingly, even if the absolute values of the resistance elements vary, resistance ratios between the resistance elements can be maintained at a constant value. 
       FIG. 5  is an illustration of a case in which the resistance elements having a certain resistance value based on the same shape and the same resistivity are used in a voltage dividing circuit. Various resistance values are realized through series connection and parallel connection of unit resistance elements  200  such as resistor groups  201  to  204  in  FIG. 5 . As described above, the unit resistance elements  200  are resistance elements having the same shape and the same resistivity, and thus, the high resistance ratios between the resistor groups each including the unit resistance element (s) having the high resistance ratio can be maintained with high accuracy. 
     Further, fuses  301  to  304  of, for example, polycrystalline silicon, are formed in parallel with the resistor groups  201  to  204 , respectively, so as to be cut by laser radiation from the outside. Depending on whether or not the fuses are cut by the laser radiation, a resistance value between a terminal  109  and a terminal  110  can be changed as necessary. Then, a voltage corresponding to a divided voltage ratio to a fixed resistor formed between the terminal  110  and a terminal  111  is output from the terminal  110 . 
     In the voltage dividing circuit as described above that has a highly accurate resistance ratio, by cutting the polycrystalline silicon fuse(s) with a laser, a desired divided voltage ratio can be obtained with high accuracy, and products having various target output voltages can be manufactured using the same semiconductor integrated circuit device. 
     A typical method of regulating an output voltage is as illustrated in  FIG. 2 . 
     First, an output voltage of a product completed in a semiconductor processing factory is measured as it is (( 1 ) in  FIG. 2 ). Then, based on a computational expression or a database prepared in advance depending on the output voltage, the polycrystalline silicon fuses formed in the voltage dividing circuit are processed with a laser to trim the output voltage (( 2 ) in  FIG. 2 ). Finally, the output voltage of the processed product is measured again to see whether or not the product is within specification as desired (( 3 ) in  FIG. 2 ). If the product is out of specification, the product is not shipped. Other than this, there is an online trimming method in which the resistors are gradually processed while the output voltage is monitored, and the processing is stopped when the output voltage reaches a desired value. The method illustrated in  FIG. 2  is called an offline trimming method in contrast with the online trimming method. 
     Next, a reference voltage circuit that is used similarly in the circuits illustrated in  FIG. 3  and  FIG. 4  is described with reference to  FIG. 6A  and  FIG. 6B . 
     A most basic related-art reference voltage circuit includes a depression type NMOS transistor  402  and an enhancement type NMOS transistor  401 . As illustrated in  FIG. 6A , each of the transistors is formed on a P-type well region  5  in a semiconductor substrate  1 , and includes a gate electrode  6 , a gate insulating film  9 , and an N-type source/drain region  12 . The transistors are different from each other in that, as an impurity region for determining a threshold voltage that is formed under the gate insulating film  9 , an N-channel impurity region  10  is formed with regard to the depression type NMOS transistor  402  while a P-channel impurity region  11  is formed with regard to the enhancement type NMOS transistor  401 . Further, each of the transistors includes a drain terminal  2  and a source terminal  3  for controlling operation thereof, and a body terminal  4  for fixing a potential of the P-type well region. 
     By connecting in series the depression type NMOS transistor  402  and the enhancement type NMOS transistor  401  between a power supply terminal  403  and a ground terminal  404  as illustrated in  FIG. 6B , outputting a constant current from the depression type NMOS transistor  402  as a current source, and inputting the current to the drain terminal  2  of the enhancement type NMOS transistor  401  as a load element, a voltage generated at the drain terminal of the enhancement type NMOS transistor  401  that is a constant voltage is output to a reference voltage output terminal  405  (see, for example, Japanese Patent Application Laid-open No. 2008-198775). 
     The constant voltage that is output from the reference voltage circuit in this case is as expressed by the following Expression (2):
 
(Reference voltage circuit constant voltage)=√( Ktd/Kte )×| Vtd|+Vte   (2),
 
where Vtd and Ktd are a threshold voltage and a transconductance, respectively, of the depression type NMOS transistor, and Vte and Kte are a threshold voltage and a transconductance, respectively, of the enhancement type NMOS transistor.
 
     In other words, variations in output voltage in Expression (1) arise from variations in parameters that determine the constant voltage that is output from the reference voltage circuit. The variations are absorbed through regulation of the resistance divided voltage ratio of the voltage dividing circuit. 
     SUMMARY OF THE INVENTION 
     Provided are a semiconductor nonvolatile memory element enabling regulation of a threshold voltage with high accuracy, the semiconductor nonvolatile memory element being capable of regulating an output voltage without employing any trimming method using laser processing in order to reduce variations in circuit characteristics of a semiconductor integrated circuit device, and a manufacturing method thereof. 
     In order to solve the problem described above, the present invention has the following configuration. 
     That is, according to one embodiment of the present invention, there is provided a semiconductor nonvolatile memory element, including: a semiconductor substrate; a well region of a first conductivity type formed in the semiconductor substrate; a heavily doped source region and a first heavily doped drain region which are formed so as to be apart from each other, and each of which contains high-concentration impurities of a second conductivity type; a first gate insulating film formed on the semiconductor substrate between the heavily doped source region and the first heavily doped drain region so as to be adjacent to the heavily doped source region; a second gate insulating film formed on the semiconductor substrate between the heavily doped source region and the first heavily doped drain region so as to be adjacent to the first heavily doped drain region; a second heavily doped drain region of the second conductivity type formed apart from the heavily doped source region, the second heavily doped drain region including a region under the second gate insulating film and being formed in a region that overlaps the first heavily doped drain region; a first lightly doped drain region of the second conductivity type formed apart from the heavily doped source region, the first lightly doped drain region including a region under the first gate insulating film and the region under the second gate insulating film and being formed in a region that overlaps the first heavily doped drain region and the second heavily doped drain region; a channel impurity region of the second conductivity type formed under the first gate insulating film between the heavily doped source region and the first lightly doped drain region; a floating gate electrode that is made of polycrystalline silicon containing high-concentration impurities and is formed on the first gate insulating film and the second gate insulating film; a third gate insulating film formed on the floating gate electrode; and a control gate electrode that is made of polycrystalline silicon containing high-concentration impurities, and is formed on the third gate insulating film, in which the well region of the first conductivity type includes the heavily doped source region, the first heavily doped drain region, the second heavily doped drain region, the first lightly doped drain region, and the channel impurity region, and is formed to a depth that is larger than depths of the heavily doped source region, the first heavily doped drain region, the second heavily doped drain region, the first lightly doped drain region, and the channel impurity region. 
     Further, in order to solve the problem described above, the present invention employs the following method. 
     That is, according to one embodiment of the present invention, there is provided a method of manufacturing a semiconductor nonvolatile memory element, including: 
     a P-type well region forming step of forming, in a semiconductor substrate, a P-type well region containing P-type impurities; 
     an element isolation insulating film forming step of forming a LOCOS oxide film around the P-type well region; 
     an N-type heavily doped impurity region forming step of forming, in a region in which a drain is to be formed, an N-type heavily doped impurity region containing N-type impurities; 
     an N-type lightly doped impurity region forming step of forming a first N-type lightly doped impurity region at an N-type impurity concentration that is lower than an N-type impurity concentration of the N-type heavily doped impurity region to a depth that is larger than a depth of the N-type heavily doped impurity region; 
     a channel region forming step of forming an N-type impurity region in a region in which a channel is to be formed in the P-type well region; 
     a gate insulating film forming step of forming a second gate insulating film in the region in which the drain is to be formed so as to overlap the N-type heavily doped impurity region, and forming a first gate insulating film that is thinner than the second gate insulating film in the region in which the channel is to be formed; 
     a gate electrode forming step of forming, on the first gate insulating film and on the second gate insulating film, a polycrystalline silicon layer that contains impurities, forming a third gate insulating film on the polycrystalline silicon layer, and forming, on the third gate insulating film, a polycrystalline silicon layer that contains impurities; and 
     a source/drain forming step of forming N-type impurity regions in a region in which a source is to be formed and in the region in which the drain is to be formed, respectively. 
     According to the one embodiment of the present invention, the semiconductor nonvolatile memory element enabling regulation of a threshold voltage with an external electrical signal can be provided, and an output voltage of a semiconductor integrated circuit device can be regulated with high accuracy and with ease. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a process flow chart for illustrating a method of regulating an output voltage of a semiconductor integrated circuit device according to the present invention. 
         FIG. 2  is a process flow chart for illustrating a method of regulating an output voltage of a related-art semiconductor integrated circuit device. 
         FIG. 3  is a schematic circuit diagram of a step-down series regulator of the related-art semiconductor integrated circuit device. 
         FIG. 4  is a schematic circuit diagram of a voltage detector of the related-art semiconductor integrated circuit device. 
         FIG. 5  is an illustration of an exemplary voltage dividing circuit in which related-art resistance elements are combined. 
         FIG. 6A  is a schematic sectional view for illustrating a related-art reference voltage circuit. 
         FIG. 6B  is an illustration of an exemplary related-art reference voltage circuit. 
         FIG. 7A  is a schematic sectional view for illustrating a reference voltage circuit according to the present invention. 
         FIG. 7B  is an illustration of an exemplary reference voltage circuit according to the present invention. 
         FIG. 8  is a schematic circuit diagram of a step-down series regulator of the semiconductor integrated circuit device according to the present invention. 
         FIG. 9  is a schematic circuit diagram of a voltage detector of the semiconductor integrated circuit device according to the present invention. 
         FIG. 10  is a schematic sectional view for illustrating a semiconductor nonvolatile memory element according to a first embodiment of the present invention. 
         FIG. 11  is a schematic sectional view for illustrating a semiconductor nonvolatile memory element according to a second embodiment of the present invention. 
         FIG. 12  is a schematic sectional view for illustrating a semiconductor nonvolatile memory element according to a third embodiment of the present invention. 
         FIG. 13  is a schematic sectional view for illustrating a semiconductor nonvolatile memory element according to a fourth embodiment of the present invention. 
         FIG. 14A ,  FIG. 14B , and  FIG. 14C  are schematic sectional views for illustrating a semiconductor nonvolatile memory element according to a fifth embodiment of the present invention. 
         FIG. 15A ,  FIG. 15B , and  FIG. 15C  are schematic sectional views for illustrating a semiconductor nonvolatile memory element according to a sixth embodiment of the present invention. 
         FIG. 16A ,  FIG. 16B , and  FIG. 16C  are schematic sectional views for illustrating a semiconductor nonvolatile memory element according to a seventh embodiment of the present invention. 
         FIG. 17A ,  FIG. 17B , and  FIG. 17C  are schematic sectional views for illustrating a semiconductor nonvolatile memory element according to an eighth embodiment of the present invention. 
         FIG. 18  is an equivalent circuit diagram of capacitances of gate insulating films when seen from a drain terminal according to the present invention. 
         FIG. 19A  and  FIG. 19B  are graphs for showing electrical characteristics when the present invention is applied to a step-down series regulator. 
         FIG. 20A ,  FIG. 20B ,  FIG. 20C , and  FIG. 20D  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the first embodiment of the present invention. 
         FIG. 21E ,  FIG. 21F ,  FIG. 21G , and  FIG. 21H  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the first embodiment of the present invention, continued from  FIG. 20D . 
         FIG. 22A ,  FIG. 22B ,  FIG. 22C , and  FIG. 22D  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the second embodiment of the present invention. 
         FIG. 23A ,  FIG. 23B ,  FIG. 23C , and  FIG. 23D  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the third embodiment of the present invention. 
         FIG. 24E  and  FIG. 24F  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the third embodiment of the present invention, continued from  FIG. 23D . 
         FIG. 25A ,  FIG. 25B ,  FIG. 25C , and  FIG. 25D  are sectional views for illustrating process steps for manufacturing the semiconductor nonvolatile memory element according to the fourth embodiment of the present invention. 
         FIG. 26A  and  FIG. 26B  are sectional views for illustrating a second group of process steps for manufacturing a first gate insulating film and a second gate insulating film of the semiconductor nonvolatile memory element according to the present invention. 
         FIG. 27A ,  FIG. 27B , and  FIG. 27C  are sectional views for illustrating a third group of process steps for manufacturing the first gate insulating film and the second gate insulating film of the semiconductor nonvolatile memory element according to the present invention. 
         FIG. 28A ,  FIG. 28B , and  FIG. 28C  are sectional views for illustrating a fourth group of process steps for manufacturing the first gate insulating film and the second gate insulating film of the semiconductor nonvolatile memory element according to the present invention. 
         FIG. 29A ,  FIG. 29B ,  FIG. 29C , and  FIG. 29D  are sectional views for illustrating a fifth group of process steps for manufacturing the first gate insulating film and the second gate insulating film of the semiconductor nonvolatile memory element according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, embodiments of the present invention are described with reference to the attached drawings. 
     First, an embodiment in which the present invention is applied to the related-art semiconductor integrated circuit device described above with reference to  FIG. 3  and  FIG. 4  is described with reference to  FIG. 8  and  FIG. 9 . As illustrated in  FIG. 8  and  FIG. 9 , an input terminal for regulation  112  for inputting a voltage/current applied from the outside is added to the reference voltage circuit  100 . A specific element in the reference voltage circuit is replaced by a semiconductor nonvolatile memory element. The semiconductor nonvolatile memory element can change a threshold voltage depending on the voltage/current input to the input terminal for regulation  112  from the outside. 
     A method of regulating an output voltage is now described with reference to  FIG. 1 . 
     First, an output voltage of a product completed in a semiconductor processing factory is measured as it is (a process step of ( 1 ) in  FIG. 1 ). 
     Then, a voltage/current is applied to the semiconductor nonvolatile memory element in the reference voltage circuit via the input terminal for regulation to change the threshold voltage of the semiconductor nonvolatile memory element (a process step of ( 2 ) in  FIG. 1 ). In the semiconductor integrated circuit device as illustrated in  FIG. 8  or  FIG. 9 , when the reference voltage value that is output from the reference voltage circuit changes, in accordance with Expression (1), the output voltage is also changed in proportion thereto, and thus, the amount of the voltage/current applied to the input terminal for regulation and an amount of the output voltage are directly proportional to each other. 
     After that, the output voltage is measured. If the output voltage is out of tolerance required for the product, the process returns to the process step of ( 2 ) in  FIG. 1  to start again application of a voltage/current to the semiconductor nonvolatile memory element. In this case, a method in which the reference voltage value of the reference voltage circuit is set in advance so that an initial output voltage value is out of specification and the voltage/current applied to the semiconductor nonvolatile memory element is gradually changed in one direction, i.e., in a +direction or in a −direction to fit the product into specification is an easy way of regulation, and thus, is preferred. 
     The process step of ( 2 ) and a process step of ( 3 ) in  FIG. 1  are repeated, and, when the output voltage value is within specification, the series of processing ends (a process step of ( 4 ) in  FIG. 1 ). The process step of ( 2 ) and the process step of ( 3 ) in  FIG. 1  can be, in reality, performed not intermittently but electrically continuously, and thus, by writing a program software and exerting automatic control, it takes only a short time to fit an out-of-specification product into specification. 
     By adopting such a method, the related-art three-step process from the process step of ( 1 ) to the process step of ( 3 ) in FIG.  2  that cannot be redone can be completed in single electrical processing, which can simplify the method of regulating the output voltage and can reduce to a large extent time necessary for completing the product. Further, the method regulates the output voltage through online trimming during which the output voltage is monitored, and thus, faulty products out of specification are inhibited, and improvement in yield can be expected. 
     Further, influence of high heat (temperature coefficient and recrystallization of the resistors) such as in a related-art online trimming through processing of resistors with a laser can be eliminated, and thus, it is not necessary to fear an error in and reregulation of the output voltage, and a stable output voltage can be maintained. 
     Further, this regulating method is electrical processing that is performed irrespective of a form of the product (wafer or package), and thus, even if the form of the product changes and the characteristics fluctuate accordingly, electrical reregulation through a terminal can be performed. For example, when an output voltage of a product in a wafer state is regulated, and then, after the product is packaged, the output voltage is changed under the influence of thermal hysteresis, resin stress, or the like and the product goes out of specification, the product in a package state can be reregulated to be fit into specification. Alternatively, the output voltage may be regulated only in a final form. By omitting measurement/processing in the wafer state, the frequency of the test can be lowered and the number of the process steps can be reduced. 
     Further, in addition to the lowered frequency of the test described above, the need for the laser trimming step is eliminated, and thus, the effect of inhibiting investment in equipment such as the measurement apparatus and the laser apparatus is also great. 
     Further, the voltage dividing circuit  103  including the resistance elements  102  illustrated in  FIG. 8  or  FIG. 9  is not required to be highly accurate. Even if the accuracy of the voltage dividing circuit  103  is low, the method according to the present invention enables the output voltage value to be fit into specification, and thus, preparing a plurality of uniform resistance elements and consideration of a pattern layout thereof as in the related art are not necessary, and the fuse elements are also not necessary. Accordingly, there is an advantage in that the chip size and a layout load are expected to be reduced. 
     Next, a reference voltage circuit for carrying out the present invention is described with reference to  FIG. 7A  and  FIG. 7B . As illustrated in  FIG. 7B , in the reference voltage circuit, the depression type NMOS transistor  402  and the enhancement type NMOS transistor  401  are connected in series between an input terminal for regulation  406  and the ground terminal  404 . A constant current is output from the depression type NMOS transistor  402  serving as a current source. A voltage generated at the drain terminal of the enhancement type NMOS transistor  401  serving as a load element is output as a constant voltage to the reference voltage output terminal  405 . 
     However, in this case, as illustrated in  FIG. 7A , the depression type NMOS transistor  402  used in the present invention has a structure in which polycrystalline silicon gate electrodes are stacked, with an upper layer being a control gate electrode  8  for controlling the voltage, and a lower layer being a floating gate electrode  7  for injecting and accumulating charge. 
     In the exemplary circuit structure illustrated in  FIG. 7B , when a voltage at the input terminal for regulation  406  rises, because a voltage between the reference voltage output terminal  405  and the ground terminal  404  is fixed at a certain value at all times, the voltage rise is borne between the input terminal for regulation  406  and the reference voltage output terminal  405 . Accordingly, as a voltage applied to the input terminal for regulation  406  rises, a drain-source voltage of the depression type NMOS transistor  402  rises. Carriers with charges, in this case, holes are injected into, via a gate insulating film, the floating gate electrode  7  at a lower potential by a method to be described below, thereby being capable of charging the floating gate electrode to a positive side. This is equivalent to a lowered threshold voltage of the depression type NMOS transistor, when seen from a control gate electrode side. Under the influence of this, the amount of current of the depression type NMOS transistor increases, and the potential of the reference voltage output terminal  405  increases accordingly. 
     When the reference voltage value of the reference voltage circuit increases, in accordance with Expression (1), the output voltage of the step-down series regulator illustrated in  FIG. 8  rises. Specifically, by controlling the voltage of the reference voltage circuit input terminal, the output voltage of the step-down series regulator can be arbitrarily changed. Note that, in this case, the input terminal for regulation  112  corresponds to the input terminal for regulation  406  illustrated in  FIG. 7A  and  FIG. 7B . 
     In this case, the threshold voltage of the semiconductor nonvolatile memory element is changed to a negative direction by voltage regulation via the input terminal for regulation, and thus, in accordance with Expression (2), the value of Vtd that is originally negative is changed further to the negative side, |Vtd| as an absolute value thereof increases, and the reference voltage that is output from the reference voltage circuit is changed to a higher direction. Then, the output voltage of the step-down series regulator is changed in a higher direction accordingly. Thus, when the output voltage of the step-down series regulator according to the present invention is designed so as to have a lower value than that of required specifications before regulation by the input terminal for regulation, through regulation of the output voltage by the input terminal for regulation, a wide range of required specifications on the output voltage can be accommodated. 
     Further, according to this method, the output voltage can be fit into a predetermined target voltage value not via a laser trimming step but only through electrical control with high accuracy. 
     A specific example is described with reference to  FIG. 19A  and  FIG. 19B . In graphs of  FIG. 19A  and  FIG. 19B , the horizontal axis denotes a voltage that is input to the input terminal for regulation  406  of the reference voltage circuit as illustrated in  FIG. 7A  and  FIG. 7B  and the vertical axis denotes a voltage that is output from the output terminal  107  of the step-down series regulator as illustrated in  FIG. 8  including the reference voltage circuit.  FIG. 19A  shows output voltage characteristics before the regulation by the input terminal for regulation, and  FIG. 19B  shows output voltage characteristics after the regulation. 
     First, before the regulation of the input, as shown in  FIG. 19A , until the input voltage reaches a point (a) (range in which the reference voltage circuit normally operates), the output voltage increases as the input voltage to the reference voltage circuit increases. When the output voltage reaches the voltage calculated by Expression (1), the output voltage remains constant until a point (b) is reached. Up to this point, the electrical characteristics are completely the same as those of the related-art step-down series regulator. 
     After that, when the point (b) is reached at which the input voltage becomes high enough for the carriers to be injected into the floating gate electrode of the semiconductor nonvolatile memory element, injection of the carriers into the semiconductor nonvolatile memory element begins. At the same time, the threshold voltage of the semiconductor nonvolatile memory element changes. Accordingly, in accordance with the amount of the injected carriers, the output voltage begins to increase again. When application of higher input voltage is stopped when a point (c) is reached at which the output voltage has a desired value, the carrier injection into the semiconductor nonvolatile memory element is stopped, and the carriers are stored in the floating gate electrode. The electrical characteristics after the above-mentioned operations end are as shown in  FIG. 19B . 
     In other words, the threshold voltage of the semiconductor nonvolatile memory element changes in accordance with the amount of the carriers injected into the semiconductor nonvolatile memory element, and thus, |Vtd| increases in accordance with Expression (2), and the constant output voltage in accordance with the reference voltage circuit constant voltage and Expression (1) is also changed to the high output voltage at the point (c). When a voltage equal to or higher than that at the point (b) is applied to the input terminal for regulation, carrier injection into the semiconductor nonvolatile memory element begins again and the output voltage begins to increase again. 
     Attention needs to be given to, first, a point that, in order to enable the output voltage regulation as shown in  FIG. 19A , the semiconductor nonvolatile memory element needs to be designed so as to have a sufficiently high drain breakdown voltage so that the element may not be broken down even when the input voltage reaches the value of the point (c). 
     A second point to which attention needs to be given is that, after the output voltage is regulated to have the desired value, a maximum value of the input voltage when the product is actually used needs to be sufficiently lower than that at the point (b) so that the output voltage may not be changed due to the carrier reinjection while the product is used. In other words, it is necessary that a semiconductor integrated circuit device to which the present invention is applied has such a product specification that an operating voltage thereof is equal to or lower than the voltage at the point (b) without fail. Accordingly, semiconductor nonvolatile memory elements having characteristics suitable for respective product specifications of semiconductor integrated circuit devices are prepared in advance. 
     Similarly, by realizing the reference voltage circuit in a voltage detection circuit illustrated in  FIG. 4  using a similar circuit, the output voltages of the respective semiconductor integrated circuit devices can be similarly controlled and set through voltage control of the input terminal for regulation connected to the reference voltage circuit. 
     Further, with regard to the reference voltage circuit, it goes without saying that the present invention can be applied to any structure of a circuit insofar as the circuit basically operates with a combination of an element serving as a current source and an element serving as a load as described above. 
     Further, the semiconductor nonvolatile memory element described herein is meant to refer to any kind of element whose threshold voltage can be shifted through carrier injection such as charge injection into the floating gate electrode as hot carrier injection, carrier injection using an FN tunnel current via a gate insulating film, or trapping of carriers in a level existing in an insulating film. 
     Next, the semiconductor nonvolatile memory element to which the present invention is applied is described in detail. 
       FIG. 10  is a sectional view for illustrating a semiconductor nonvolatile memory element according to the first embodiment of the present invention. The element illustrated in  FIG. 10  is formed in a P-type well region  5  containing boron as impurities at a concentration of from about 7×10 15 /cm 3  to about 7×10 16 /cm 3  that is formed on a semiconductor substrate  1 . A depression type NMOS transistor including N-type source/drain regions  12 , an N-type channel impurity region  10 , gate insulating films  9 ,  14 , and  15 , and gate electrodes  7  and  8  is formed in a region surrounded by a LOCOS oxide film  13  used for element isolation and having a thickness of several thousands of angstromes to 2 μm. 
     Features are that the gate electrode has a structure in which the floating gate electrode  7  and a control gate electrode  8  that are formed of polycrystalline silicon are stacked, that the control gate electrode is connected to a source terminal  3  of the depression type NMOS transistor via metal wiring or the like (not shown), and that the floating gate electrode  7  is surrounded by the first gate insulating film  9 , the second gate insulating film  14 , and the third gate insulating film  15  and is in a state of having no electrical connection. 
     Carriers such as electrons or holes are injected into the floating gate electrode  7  from a drain terminal  2  via the second gate insulating film  14 . When positive or negative carriers are injected into the floating gate electrode  7 , similarly to a case of fixed charge existing between a gate electrode and a channel region in an ordinary MOSFET, the threshold voltage of the depression type NMOS transistor changes in accordance with the injected amount. 
     Through holding the injected carriers under such structural conditions that the carriers do not escape with heat or electrical stress in normal operation, the threshold voltage of the depression type NMOS transistor can be regulated to and maintained at a desired value. 
     The threshold voltage when carriers are not injected into the floating gate electrode  7  is set to be a certain negative value in accordance with the amount of impurities in the N-type channel impurity region  10  so that the depression type NMOS transistor is in a normally on state in which current flows even when a gate-source voltage is 0 V, whenever a voltage is applied between the drain and the source. 
     The N-type source/drain regions  12  function as source/drain terminals by lowering resistance thereof under a state in which N-type impurities such as or P of 1×10 20 /cm 3  or more are injected thereinto. The drain terminal side further includes an N-type heavily doped impurity region  17  containing N-type impurities such as or P of 5×10 18 /cm 3  or more and a first N-type lightly doped impurity region  18  containing N-type impurities such as or P of 1×10 17 /cm 3  or more and 1×10 18 /cm 3  or less between the drain terminal side and the channel impurity region  10 . 
     The first N-type lightly doped impurity region  18  plays a role in relaxing an electric field through extension of a depletion layer on the drain side when a drain voltage is applied and in realizing thereby a higher breakdown voltage. The N-type heavily doped impurity region  17  is used as a lower electrode when carriers are injected into the floating gate electrode  7 . 
     A first reason why the impurity concentration of the N-type heavily doped impurity region  17  is 5×10 18 /cm 3  or more is to prevent injection efficiency from being lowered due to a depleted surface of the N-type heavily doped impurity region  17  to which a positive voltage is applied when holes are injected as carriers into the floating gate electrode  7  and due to a relaxed electric field between the floating gate electrode  7  and the N-type heavily doped impurity region  17 . 
     A second reason is to prevent a depletion layer between the drain and a well that extends from the N-type lightly doped impurity region  18  to the drain side when, similarly, a high voltage is applied to the N-type heavily doped impurity region  17  for the purpose of injecting holes as carriers into the floating gate electrode  7  and to thereby prevent carrier injection efficiency from being lowered. 
     Incidentally, the N-type heavily doped impurity region  17  and the floating gate electrode  7  have an overlapping portion  16 . The second gate insulating film  14  that exists in the overlapping portion  16  has a thickness that is different from that of the gate insulating film  9  formed on the N-type channel impurity region  10 . In general, a gate insulating film is set to have a predetermined thickness in accordance with an operating voltage of a semiconductor integrated circuit device including a MOSFET that includes the gate insulating film from the viewpoint of reliability for a long time. However, the second gate insulating film  14  of the semiconductor nonvolatile memory element according to this embodiment adopts a thickness that is determined so as to be suitable for carrier injection into the floating gate electrode  7  and that is larger than the above-mentioned thickness determined by the reliability for a long time in accordance with the operating voltage for the purpose of preventing charge from escaping within an operating voltage range. 
     Accordingly, according to this embodiment, the second gate insulating film  14  has a thickness that is larger than that of the gate insulating film  9  formed on the N-type channel impurity region  10 . 
     Another feature of the semiconductor nonvolatile memory element according to the present invention is that regulation specialized for characteristics of a semiconductor nonvolatile memory element and semiconductor integrated circuit device including the same is performed in an analog way, and the regulation is performed through replacement of an element that is a part of a related-art semiconductor nonvolatile memory element. Accordingly, formation of a memory array for the purpose of storing information is not assumed, and a structure such as a select gate for identifying an address that is necessary when a memory array is formed is not required. 
     Next, electrical operation according to the present invention is described in detail. 
     For example, a potential of the floating gate electrode  7  changes to a negative potential when electrons having negative charge are injected thereinto. In that case, in response to the negative charge, positive charge is induced in the channel region, or, electrons in the N-type channel impurity region  10  reduce, and the threshold voltage of the N-channel type MOSFET changes to the positive side. 
     On the other hand, when holes having positive charge are injected into the floating gate electrode  7 , the potential of the floating gate electrode  7  is shifted to the positive side and an electron concentration in the N-type channel impurity region  10  increases. As a result, the threshold voltage of the N-channel type MOSFET changes to the negative side. 
     According to the present invention, due to the existence of the N-type channel impurity region  10 , the threshold voltage of the depression type NMOS transistor has a negative value under a state in which no carrier is injected, and thus, through application of a positive potential to the floating gate electrode  7  and injection of holes as carriers from the N-type heavily doped impurity region  17  side, the negative threshold voltage is changed further to the negative side, and the threshold voltage is controlled with high accuracy. 
     An ordinary semiconductor nonvolatile memory element is under binary control with threshold voltages, for example, one is set higher than 0 V and the other is set lower than 0 V. A plurality of such elements is combined to be used for holding necessary information in a digital way. The present invention is different from the related-art method in that only one element is used, information is determined in an analog way by the amount of carriers in the floating gate electrode, and the information is kept held. 
     According to the present invention, making full use of such a function of changing the threshold voltage and nonvolatility, carriers are injected into the semiconductor nonvolatile memory element in advance before shipment to a customer to regulate the threshold voltage of the semiconductor nonvolatile memory element, thereby regulating circuit characteristics of a semiconductor integrated circuit device including the semiconductor nonvolatile memory element to have a desired value. After that, no carriers are put in/out during usage by the customer, and reliability at a high level for repeated erasing and writing is not necessary for the second gate insulating film  14 . 
     According to the present invention, carriers are injected into the floating gate electrode  7  as follows. First, the amount of impurities in the N-type channel impurity region  10  is set such that, under a state in which no carriers are injected, the threshold voltage is set to be of a negative value that is higher than (on the positive side with respect to) a final target value. 
     Then, in a step of testing the semiconductor integrated circuit device after a semiconductor manufacturing process, the source potential and the control gate electrode potential are set to be the same low potential while the drain potential is changed to a higher potential on the positive side. In this state, the floating gate potential is of a value between the drain potential and the source potential/control gate electrode potential, in accordance with a capacitance ratio that is determined by thicknesses of the first gate insulating film  9 , the second gate insulating film  14 , and the third gate insulating film  15  and by sizes of the control gate electrode  8  and the floating gate electrode  7 . Through regulation of the sizes and the thicknesses described above, the value is set to be a low value close to the source potential/control gate electrode potential so that most of the drain-source voltage is applied to the second gate insulating film  14 . 
     In this depression type NMOS transistor, even if the potential of the control gate electrode  8  is 0 V, the threshold voltage is of a negative value, and thus, as the drain potential rises, a drain current flows. After the current characteristics goes out of an non-saturated region into a saturated region, the current is stabilized at a constant saturation current value that does not depend on the drain potential. A potential at an end of the depletion layer generated on the drain side in the N-type channel impurity region  10  is fixed to a low constant value (pinch-off voltage) determined by the gate-source voltage and the threshold voltage, and thus, a voltage that is a difference between the drain-source voltage and the pinch-off voltage is applied to the depletion layer generated in the first N-type lightly doped impurity region  18 . The depletion layer has an impurity concentration that is lower than the sufficiently high impurity concentration in the N-type heavily doped impurity region  17 , and thus, with regard to the potential of the N-type heavily doped impurity region  17 , the drain potential applied to the drain terminal  2  is applied as it is to the N-type heavily doped impurity region  17 , and it is easy to control a potential difference between the N-type heavily doped impurity region  17  and the control gate electrode  8 . 
     At this time, by arbitrarily setting the impurity concentration and a length in a plane direction of the first N-type lightly doped impurity region  18 , the amount of extension of the depletion layer generated in the N-type lightly doped impurity region  18  can be controlled, and an upper limit of the applied drain voltage due to avalanche breakdown can be raised. This enables setting a potential applied to the N-type heavily doped impurity region  17  to have a large value, and thus, a potential for injecting carriers can be secured even if the second gate insulating film  14  is set to have a large thickness. 
     For example, by setting the first N-type lightly doped impurity region  18  to have an impurity concentration of from 1×10 17 /cm 3  to 1×10 18 /cm 3  and setting a length from the channel region  10  to the N-type heavily doped impurity region  17  in the plane direction to be 1.5 μm or more, the drain breakdown voltage can be 20 V or higher, and a carrier injection voltage of 20 V or higher can be secured. 
     In this case, when the second gate insulating film  14  has a thickness of, for example, 400 Å, tunneling can be induced by applying a voltage of about 20 V or higher. Thus, in the drain structure described above, by setting the drain voltage to be 20 H V or higher, injection of holes having positive charge based on the tunneling is realized in the overlapping portion  16  of the N-type heavily doped impurity region  17  and the floating gate electrode  7 . On the other hand, a voltage applied to the N-type channel impurity region  10  is equal to or lower than the pinch-off voltage described above, and thus, insofar as an insulating film thickness in accordance with the pinch-off voltage is secured, tunneling is not induced in the first gate insulating film  9  between the N-type channel impurity region  10  and the floating gate electrode  7 . 
     As described above, it is preferred that the drain voltage applied in the testing step be sufficiently higher than the operating voltage of the semiconductor integrated circuit device including this semiconductor nonvolatile memory element. This can prevent carriers from being injected into the floating gate electrode  7  while a power supply voltage fluctuates within the range of the operating voltage of the semiconductor integrated circuit device, to thereby inhibit fluctuations in threshold voltage of the semiconductor nonvolatile memory element and change in circuit characteristics of the semiconductor integrated circuit device that is caused due to the fluctuations. For example, in the example described above, it is desired that the operating voltage of the semiconductor integrated circuit device be 10 V or lower. In this way, for the purpose of securing a sufficient potential difference between the operating voltage and the carrier injection voltage (in the example described above, 20 V−10 V=10 V), it is necessary to set the thickness of the second gate insulating film  14  and the conditions on the first N-type lightly doped impurity region  18 . 
     The amount of carriers to be stored in the floating gate electrode  7  can be determined through gradual increase in drain voltage as illustrated in  FIG. 19A  and  FIG. 19B . Alternately, the amount of charge of holes to be stored in the floating gate electrode  7  can also be controlled by a product of the value of the drain voltage and a time period of application thereof. As described above, the threshold voltage of the depression type NMOS transistor is shifted further to the negative side in accordance with the amount of stored charge of the holes, and thus, it is possible to apply a high constant drain voltage until a desired threshold voltage is reached, to thereby regulate, with high accuracy, the threshold voltage of the N-channel type MOSFET with the time period of application thereof while the output voltage is being monitored. 
       FIG. 11  is a sectional view for illustrating a semiconductor nonvolatile memory element according to a second embodiment of the present invention. In the semiconductor nonvolatile memory element illustrated in  FIG. 11 , in addition to the structure illustrated in  FIG. 10 , a second N-type lightly doped impurity region  19  containing As or P having an impurity concentration of from about 2×10 16 /cm 3  to about 2×10 17 /cm 3  is added under the first N-type lightly doped impurity region  18 . In the case illustrated in  FIG. 10 , depending on the conditions on the first N-type lightly doped impurity region  18 , it is easy to set the drain breakdown voltage to be as high as about 30 V. 
     However, with regard to the depletion layer in the first N-type lightly doped impurity region  18  and in the P-type well region  5  thereunder, extension of the depletion layer to the first N-type lightly doped impurity region  18  side is limited, and it is difficult to set the breakdown voltage to be higher than 30 V. Accordingly, by adding the second N-type lightly doped impurity region  19  as illustrated in  FIG. 11  to secure extension of the depletion layer corresponding to a diffusion depth thereof, a drain breakdown voltage that is higher than 30 V can be attained. This is effective in accommodating a semiconductor integrated circuit device having a higher operating voltage and in securing a larger margin between the operating voltage and a tunneling voltage. 
       FIG. 12  is a sectional view for illustrating a semiconductor nonvolatile memory element according to a third embodiment of the present invention. In the semiconductor nonvolatile memory element illustrated in  FIG. 12 , an oxide film  13  having a thickness that is larger than those of the first gate insulating film  9  and the second gate insulating film  14  is formed between the floating gate electrode  7  and the first N-type lightly doped impurity region  18 . Such a structure can relax a high electric field between the floating gate electrode  7  having a low potential and the first N-type lightly doped impurity region  18  that is induced when the drain voltage is increased, and the drain breakdown voltage can be set to be as high as about 60 V. 
     The thick oxide film  13  can have an arbitrary thickness depending on the extent of the necessary relaxation of the electric field. When the drain breakdown voltage is higher than 30 V, it is preferred that the thickness be 1,000 Å or more. Further, by forming the thick oxide film  13  simultaneously with a LOCOS oxide film in the element isolation region, increase in the number of process steps can be prevented. 
       FIG. 13  is a sectional view for illustrating a semiconductor nonvolatile memory element according to a fourth embodiment of the present invention. In the semiconductor nonvolatile memory element illustrated in  FIG. 13 , the second N-type lightly doped impurity region  19  illustrated in  FIG. 12  is extended to a direction to the source terminal  3  side to an extent so as to overlap the N-type channel impurity region  10 . In addition, a P-type lightly doped impurity region  20  having an impurity concentration that is higher than that of the second N-type lightly doped impurity region  19  is formed so as to surround the source terminal  3 . 
     The P-type lightly doped impurity region  20  is formed in a region that is close to the first gate insulating film  9  and that does not exceed the thick oxide film  13 , and contains B or BF 2  having an impurity concentration of from about 2×10 16 /cm 3  to about 2×10 17 /cm 3 , which is higher than that of the second N-type lightly doped impurity region  19 . The structure in which the P-type lightly doped impurity region  20  has an impurity concentration that is higher than that of the second N-type lightly doped impurity region  19  enables further extension to the drain side of the depletion layer on the channel side and on the drain side that is generated when the drain voltage is increased, which is effective when a drain breakdown voltage of 60 V or higher is required to be obtained. 
     In the first to fourth embodiments described above, as illustrated in  FIG. 10  to  FIG. 13 , the floating gate electrode  7  and the control gate electrode  8  are formed by stacked polycrystalline silicon layers. Such a method using two polycrystalline silicon layers can inhibit increase in area of the gate electrode to facilitate lower costs, but the number of process steps increases and the processing is complicated. This leads to, for example, difficult selection of conditions on dry etching when the floating gate electrode  7 , the control gate electrode  8 , and the third gate insulating film  15  therebetween are collectively processed and deteriorated flatness due to etching resistance of a resist to be a mask, a stringer of polycrystalline silicon formed at a portion having a level difference, or a high-aspect-ratio gate electrode structure. 
       FIG. 14A  to  FIG. 17C  are illustrations of methods of realizing a semiconductor nonvolatile memory element using only one polycrystalline silicon layer for the purpose of overcoming the complication, and correspond to the structures illustrated in  FIG. 10  to  FIG. 13 , respectively. 
     First,  FIG. 14A ,  FIG. 14B , and  FIG. 14C  are illustrations of a fifth embodiment of the present invention in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 10  is changed to a single-layer structure. 
       FIG. 14A  is a plan view, and  FIG. 14B  and  FIG. 14C  are sectional views taken along the lines A-A′ and B-B′, respectively, of  FIG. 14A . The polycrystalline silicon structure including two stacked layers illustrated in  FIG. 10  is changed to a single-layer structure including only the floating gate electrode  7 . As illustrated in  FIG. 14B , the control gate electrode  8  and the third gate insulating film  15  are not formed on the floating gate electrode  7 . Instead, as illustrated in  FIG. 14A , the floating gate electrode  7  extends beyond the channel region so as to overlap the control gate electrode  8  that uses an impurity diffusion region in the semiconductor substrate  1 . The control gate electrode  8  that uses the impurity diffusion region in the semiconductor substrate  1  includes a potential output portion  6 . The control gate electrode  8  may use the impurities and a structure and a process step of, for example, the N-type heavily doped impurity region  17  in common, and the N-type high-concentration impurities in the source/drain regions  12  may also be used in the potential output portion  6 . 
     Further, the third gate insulating film  15  between the floating gate electrode  7  and the control gate electrode  8  as used in the structure illustrated in  FIG. 10  is formed of an oxide film on a surface of the semiconductor substrate  1  that is formed between the floating gate electrode  7  and the control gate electrode  8  serving as the impurity diffusion region in the semiconductor substrate  1 . In this case, an oxide film formed simultaneously with the first gate insulating film  9  formed outside the channel region  10  is used. 
     The structure illustrated in  FIG. 14A ,  FIG. 14B , and  FIG. 14C  requires two occupation areas for the control gate electrode  8  and the floating gate electrode  7  in the semiconductor integrated circuit device, and thus, the occupation area in the chip increases and cost also increases accordingly. However, the present invention is not applied to a memory array in which a large number of semiconductor nonvolatile memory elements are arrayed in a semiconductor integrated circuit device. Thus, the increase in occupied area is not to a large extent, and the increase in costs is almost no problem in the semiconductor integrated circuit device. On the other hand, as described above, the present invention has a strong point that, through elimination of the complication and the difficulty in manufacturing process, the effects of stabilized quality and a reduced number of process steps can be enjoyed. 
     Further, in the structure illustrated in  FIG. 14A ,  FIG. 14B , and  FIG. 14C , when a circuit is adopted in which the gate potential and the source potential, and the potential of the P-type well region that are used are a common low potential realized through connection using metal wiring or the like, the impurities in the control gate electrode  8  illustrated in  FIG. 14C  may be P-type high-concentration impurities, and further, the control gate electrode  8  may be the P-type well region  5  as it is. 
     The reason is that the semiconductor nonvolatile memory element according to the present invention is of a normally on type in which, due to the existence of the N-type channel impurity region  10 , a current flows in accordance with the drain voltage even when the gate-source voltage is 0 V. Accordingly, when the P-type well region  5  is connected to the source terminal  3  using metal wiring or the like (not shown) located somewhere, the potential relationship is the same even if the impurities diffused in the control gate electrode  8  are of the P type. 
       FIG. 15A ,  FIG. 15B , and  FIG. 15C  are illustrations of a sixth embodiment of the present invention in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 11  is changed to a single-layer structure, which has an effect that is similar to that of the fifth embodiment described with reference to  FIG. 14A ,  FIG. 14B , and  FIG. 14C .  FIG. 15A  is a plan view, and  FIG. 15B  and  FIG. 15C  are sectional views taken along the lines A-A′ and B-B′, respectively of  FIG. 15A . 
     Further,  FIG. 16A ,  FIG. 16B , and  FIG. 16C  are illustrations of a seventh embodiment of the present invention in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 12  is changed to a single-layer structure, which has an effect that is similar to that of the fifth embodiment described with reference to  FIG. 14A ,  FIG. 14B , and  FIG. 14C .  FIG. 16A  is a plan view, and  FIG. 16B  and  FIG. 16C  are sectional views taken along the lines A-A′ and B-B′, respectively of  FIG. 16A . 
     Further,  FIG. 17A ,  FIG. 17B , and  FIG. 17C  are illustrations of an eighth embodiment of the present invention in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 13  is changed to a single-layer structure, which has an effect that is similar to that of the fifth embodiment described with reference to  FIG. 14A ,  FIG. 14B , and  FIG. 14C .  FIG. 17A  is a plan view, and  FIG. 17B  and  FIG. 17C  are sectional views taken along the lines A-A′ and B-B′, respectively of  FIG. 17A . 
     Next, the three kinds of gate insulating films used in the present invention are described. 
     First, it is desired that the second gate insulating film  14  used when carriers are injected into the floating gate electrode  7  using tunneling according to the present invention be a silicon oxide film formed by thermal oxidation that is excellent in film thickness controllability and film quality stability. Further, carriers are injected into the floating gate electrode  7  only once or several times in the testing step after the semiconductor manufacturing process ends, and thus, special conditions on the film formation and additional processing for the purpose of obtaining high resistance to repeated erasing and writing, are not necessary. Meanwhile, the second gate insulating film  14  is set to have a thickness that is large enough that a desired tunneling current value is obtained when a drain voltage is applied that is sufficiently higher than the operating voltage applied to the semiconductor integrated circuit device in the testing step after the semiconductor manufacturing process ends. 
     On the other hand, it is desired that the first gate insulating film  9  and the third gate insulating film  15  have a large capacitance value. This is for the purpose of, when the drain voltage is applied to inject holes into the floating gate electrode  7  in the testing step, sufficiently lowering the potential of the floating gate electrode  7  that is determined by capacitive coupling in order to efficiently apply a voltage to the second gate insulating film  14 . 
     An equivalent capacitive coupling circuit of the drain terminal  2  to which a high potential is applied, the control gate electrode  8  and the P-type well region  5  that are set to have a low potential, and the body terminal  4  at the same potential as that of the P-type well region  5  is as illustrated in  FIG. 18 . As can be seen from  FIG. 18 , by setting the first gate insulating film  9  and the third gate insulating film  15  to have a large capacitance and setting the ratio thereof to the capacitance of the second gate insulating film  14  having a small capacitance and having a large thickness to be large, the potential of the floating gate electrode  7  at an intermediate potential of the capacitive coupling is lowered. In this manner, a high voltage can be applied to the second gate insulating film  14  formed between the drain terminal  2  and the floating gate electrode  7 , and thus, the tunneling can be promoted. 
     Accordingly, the first gate insulating film  9  and the third gate insulating film  15  are required to have a large electrode size when seen in a plan view. The reason is that the capacitance value can be large. It is sufficient that the first gate insulating film  9  and the third gate insulating film  15  have an area that is ten times or more as large as the area of the second gate insulating film  14  when seen in a plan view. 
     Further, for the purpose of having a large capacitance value, it is desired that the first gate insulating film  9  and the third gate insulating film  15  be as thin as possible. The floating gate electrode  7 , the control gate electrode  8 , and the P-type well region  5  are fixed to the same low potential in terms of the circuit, and thus, the insulating film thickness is not limited by the operating voltage of the semiconductor integrated circuit device. Accordingly, in the case of a thermal oxide film, taking into consideration leakage of carriers in the floating gate electrode  7  due to a high temperature environment, it is desired that the film thickness be from about 100 Å to about 200 Å. 
     Further, from the viewpoint of a large capacitance, it is desired that the first gate insulating film  9  and the third gate insulating film  15  have a relative permittivity that is as high as possible, which can be realized by using SiON, SiN, HfO 2 , or the like rather than a silicon thermal oxide film. Characteristics such as a threshold voltage of films other than a silicon thermal oxide film generally fluctuate to a large extent due to instability of characteristics at a film interface. However, according to the present invention, even if there are variations in characteristics, the variations are adjusted finally by the regulation at the testing step, and thus, no problem arises. 
     Further, adopting the first gate insulating film  9  and the third gate insulating film  15  that are thin or that have a high relative permittivity has a strong point that the size thereof can be reduced and the costs can be reduced accordingly. Further, this method leads to increase in C as a capacitance value per unit area in the following general Expression (3), and thus, this method also has a strong point that the amount of fluctuations in threshold voltage can be lowered when Q reduces due to leakage of carriers in the floating gate electrode  7  or the like:
 
 V=Q/C   (3)
 
     As described above, by adopting the semiconductor nonvolatile memory element according to the present invention, the threshold voltage can be regulated with ease, the stability is high for a long time, and variations in circuit characteristics due to variations in element characteristics can be absorbed in electrical regulation in the testing step, and thus, a highly accurate semiconductor integrated circuit device can be provided. 
     Next, a method of manufacturing the semiconductor nonvolatile memory element having the structure according to the first embodiment illustrated in  FIG. 10  is described with reference to  FIG. 20A  to  FIG. 20D  and  FIG. 21E  to  FIG. 21H  for illustrating process steps thereof. 
     First, the semiconductor substrate  1  of the P type or of the N type is prepared. After P-type impurities such as B or BF 2  are injected by ion injection into a region in which the semiconductor nonvolatile memory element is to be formed, thermal diffusion is performed to form the P-type well region  5  ( FIG. 20A ). 
     The polarity of the semiconductor substrate  1  is selected in accordance with a demand for the semiconductor integrated circuit device including the semiconductor nonvolatile memory element according to the present invention. Specifically, if the P-type well region  5  is required to be electrically isolated so that the potential of the P-type well region  5  is prevented from being the lowest potential on the semiconductor integrated circuit device, it is desired that the N-type semiconductor substrate  1  be prepared. However, if the P-type well region  5  has the lowest potential on the semiconductor integrated circuit device, a P-type semiconductor substrate that is less expensive can be used. 
     The amount of impurities to be injected and conditions on the thermal diffusion are selected so that the P-type well region  5  has an impurity concentration of from 7×10 15 /cm 3  to 7×10 16 /cm 3  and has a depth of from 6 μm to 10 μm. More specifically, the above-mentioned values are realized by an area density of impurity injection of 1×10 12 /cm 2  to 1×10 13 /cm 2  and by performing the thermal diffusion through heat treatment at from 1,100° C. to 1,200° C. for several hours to ten-odd hours. 
     Next, for the purpose of electrically isolating elements from one another, LOCOS or the like is used to form the element isolation region  13  as a silicon oxide film around the P-type well region  5 , and at the same time, a semiconductor nonvolatile memory element region surrounded by the element isolation region  13  is defined ( FIG. 20B ). 
     Then, N-type impurities of As or P are injected by ion injection into a region to be the drain region of the semiconductor nonvolatile memory element, to thereby form the N-type heavily doped impurity region  17  and the first N-type lightly doped impurity region  18  ( FIG. 20C ). 
     The N-type heavily doped impurity region  17  serves as the lower electrode for injecting, using tunneling, carriers such as electrons or holes into the floating gate electrode  7  thereabove via the silicon oxide film in a later stage. In order to inhibit depletion on the surface of the N-type heavily doped impurity region  17  when a high voltage on the positive side is applied in the carrier injection, it is desired that the N-type heavily doped impurity region  17  be formed through injection of As at an impurity concentration of 5×10 18 /cm 3  or more. The injecting energy in this case is to the extent that enables the impurities to pass through the oxide film on a surface of the semiconductor substrate  1 , and is, for example, about 100 keV. 
     Further, for the purpose of obtaining a drain breakdown voltage of a desired value or more, it is desired that the first N-type lightly doped impurity region  18  be formed by injecting P at an impurity concentration of 1×10 17 /cm 3  or more and 1×10 18 /cm 3  or less. Further, by setting the injecting energy to be 90 keV or more, the impurities can be diffused to a depth that is larger than that in the N-type heavily doped impurity region  17 , and a PN junction breakdown voltage with the P-type well region  5  below the N-type heavily doped impurity region  17  can be set to be high. 
     Next, for the purpose of manufacturing the semiconductor nonvolatile memory element as a normally on type depression type MOSFET, N-type impurities of As or P are injected by ion injection into a region in which the channel is to be formed so that the threshold voltage may have a desired negative value, to thereby form the N-type channel impurity region  10  ( FIG. 20D ). 
     Then, by thermal oxidation, CVD, or the like, the first gate insulating film  9  having a thickness of from about 100 Å to about 200 Å is formed in the region in which the channel is to be formed, and the second gate insulating film  14  having a thickness of about several hundred angstroms that is larger than that of the first gate insulating film  9  is formed in a region in which the drain is to be formed ( FIG. 21E ). 
     The two gate insulating films having different thicknesses are realized as follows. First, the thicker second gate insulating film  14  is formed over an entire surface of the element region as a silicon oxide film by thermal oxidation, then, the second gate insulating film  14  in regions other than the region in which the drain is to be formed is removed by photolithography and etching using HF or the like. After that, the first gate insulating film  9  is formed as a silicon oxide film by thermal oxidation. 
     In this method, the second gate insulating film  14  is subjected to thermal oxidation when the first gate insulating film  9  is formed, and the silicon oxide film forming the second gate insulating film  14  is grown again. However, the second gate insulating film  14  already has a large thickness, and thus, when the first gate insulating film  9  serving as a thin gate insulating film undergoes the thermal oxidation to be formed, the speed of oxygen reaching silicon is slow, and the film thickness increases at a very low speed and the amount of the growth is small. Accordingly, the thickness of the second gate insulating film  14  after the thermal oxidation twice is dominantly affected by the first thermal oxidation, and the thickness can be easily predicted. 
     Then, a polycrystalline silicon layer is formed on the first gate insulating film  9  and the second gate insulating film  14 , and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more, and then, photolithography and dry etching are performed to form the floating gate electrode  7  of the semiconductor nonvolatile memory element. At this time, the floating gate electrode  7  and the second gate insulating film  14  has an overlapping portion for the purpose of carrier injection using tunneling ( FIG. 21F ). 
     Then, an insulating film is formed by thermal oxidation, CVD, or the like on the floating gate electrode  7  of the semiconductor nonvolatile memory element for the purpose of forming the third gate insulating film  15 . After that, a polycrystalline silicon layer is formed in succession, and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more, and then photolithography and dry etching are used to form the control gate electrode  8  by patterning ( FIG. 21G ). 
     In this case, the floating gate electrode  7  and the control gate electrode  8  may be collectively formed by performing photolithography and dry etching once. Specifically, a polycrystalline silicon layer is formed on the first gate insulating film  9  and the second gate insulating film  14 , and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more. Then, the third gate insulating film  15  is formed in succession by thermal oxidation, CVD, or the like. Further, a polycrystalline silicon layer is formed in succession, and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more. After that, the control gate electrode  8  and the floating gate electrode  7  are collectively formed by patterning using photolithography and dry etching. 
     Then, in order to form the source/drain regions  12  of the semiconductor nonvolatile memory element, N-type impurities of As or P are injected by ion injection so that the concentration is 1×10 20 /cm 3  or more ( FIG. 21H ). 
     The above description is made with reference to  FIG. 20A  to  FIG. 20D  and  FIG. 21E  to  FIG. 21H  for illustrating the process steps. 
     Then, an oxide film serving as an insulating film (not shown) is formed on an entire surface. After a contact hole is formed in a predetermined location, for the purpose of giving the gate potential, the source potential, the drain potential, and a body potential of the semiconductor nonvolatile memory element, metal wiring is formed by sputtering and patterning a metal film. 
     Note that, when the structure of the fifth embodiment described with reference to  FIG. 14A ,  FIG. 14B , and  FIG. 14C  in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 10  is changed to the single-layer structure is manufactured, the process steps up to the step of forming the floating gate electrode  7  on the first gate insulating film  9  and the second gate insulating film  14  illustrated in  FIG. 21F  are common to the manufacturing method described above. After this, the source/drain regions  12  of the semiconductor nonvolatile memory element are formed. In this way, the structure of the fifth embodiment can be similarly manufactured. The control gate electrode  8  can be formed using, for example, the impurities and the structure and the process step of the N-type heavily doped impurity region  17  in common. 
     Next, a method of manufacturing the semiconductor nonvolatile memory element having the structure according to the second embodiment illustrated in  FIG. 11  is described with reference to  FIG. 22A  to  FIG. 22D  for illustrating process steps thereof. The second embodiment is different from the first embodiment in that the second N-type lightly doped impurity region  19  illustrated in  FIG. 11  is additionally formed, and in  FIG. 22A  to  FIG. 22D , process steps in a latter part are simplified accordingly. 
     First, the semiconductor substrate  1  of the P type or of the N type is prepared. The P-type well region  5  is formed in a region in which the semiconductor nonvolatile memory element is to be formed, and further, the second N-type lightly doped impurity region  19  is formed therein ( FIG. 22A ). 
     The amount of the impurities to be injected and conditions on the thermal diffusion of P-type impurities of B or BF 2  are selected so that the P-type well region  5  has an impurity concentration of from 7×10 15 /cm 3  to 7×10 16 /cm 3  and has a depth of from 6 μm to 10 μm. More specifically, the above-mentioned value is realized by an area density of impurity injection of from 1×10 12 /cm 2  to 1×10 13 /cm 2  and by performing the thermal diffusion through heat treatment at from 1,100° C. to 1,200° C. for several hours to ten-odd hours. 
     Further, the amount of impurities to be injected and conditions on the thermal diffusion of N-type impurities of P or As are selected so that the second N-type lightly doped impurity region  19  has an impurity concentration of from 2×10 16 /cm 3  to 2×10 17 /cm 3  and has a depth of from 3 μm to 6 μm. The thermal diffusion may be performed when the heat treatment is performed in forming the P-type well region  5 , or, may be performed additionally thereafter. 
     Then, for the purpose of electrically isolating elements from one another, LOCOS or the like is used to form a silicon oxide film as the element isolation region  13 , and at the same time, the semiconductor nonvolatile memory element region surrounded by the element isolation region  13  is defined (not shown). 
     Then, N-type impurities of As or P are injected by ion injection into a region to be the drain region of the semiconductor nonvolatile memory element to form the N-type heavily doped impurity region  17  and the first N-type lightly doped impurity region  18  ( FIG. 22B ). 
     It is desired that the N-type heavily doped impurity region  17  be formed by injecting As at an impurity concentration of 5×10 18 /cm 3  or more, and the injecting energy be about 100 keV so that the impurities can pass through the oxide film on the surface of the semiconductor substrate. 
     Further, it is desired that the first N-type lightly doped impurity region  18  be formed by injecting P at an impurity concentration of 1×10 17 /cm 3  or more and 1×10 18 /cm 3  or less. By setting the injecting energy to be 90 keV or more, the impurities can be diffused to a depth that is larger than that in the N-type heavily doped impurity region  17 , and a PN junction breakdown voltage with the P-type well region  5  below the N-type heavily doped impurity region  17  can be set to be high. 
     After that, the N-type channel impurity region  10  is formed as described above with reference to  FIG. 20D , and the first gate insulating film  9  and the second gate insulating film  14  are formed as described above with reference to  FIG. 21E . 
     Then,  FIG. 22A  to  FIG. 22D  are referred to again. A polycrystalline silicon layer is formed on the first gate insulating film  9  and the second gate insulating film  14 , and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more. Then, photolithography and dry etching are performed to form the floating gate electrode  7  of the semiconductor nonvolatile memory element ( FIG. 22C ). 
     Then, the third gate insulating film  15  and the floating gate electrode  7  are formed as described with reference to  FIG. 21G . 
     Then, in order to form the source/drain regions  12  of the semiconductor nonvolatile memory element, N-type impurities of As or P are injected by ion injection so that the concentration is 1×10 20 /cm 3  or more ( FIG. 22D ). 
     Then, an oxide film serving as an insulating film (not shown) is formed on an entire surface. After a contact hole is formed in a predetermined location, for the purpose of giving the gate potential, the source potential, the drain potential, and the body potential of the semiconductor nonvolatile memory element, metal wiring is formed by sputtering and patterning a metal film. 
     Note that, when the structure of the sixth embodiment described with reference to  FIG. 15A ,  FIG. 15B , and  FIG. 15C  in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 11  is changed to the single-layer structure is manufactured, the process steps up to the step of forming the floating gate electrode  7  on the first gate insulating film  9  and the second gate insulating film  14  illustrated in  FIG. 22C  are common to the manufacturing method described above. After this, the source/drain regions  12  of the semiconductor nonvolatile memory element are formed. In this way, the structure of the sixth embodiment can be similarly manufactured. The control gate electrode  8  can be formed using, for example, the impurities and the structure and the process step of the N-type heavily doped impurity region  17  in common. 
     Next, a method of manufacturing the semiconductor nonvolatile memory element having the structure according to the third embodiment illustrated in  FIG. 12  is described with reference to  FIG. 23A  to  FIG. 23D , and  FIG. 24E  and  FIG. 24F  for illustrating process steps thereof. 
     First, the semiconductor substrate  1  of the P type or of the N type is prepared. The P-type well region  5  is formed in a region in which the semiconductor nonvolatile memory element is to be formed, and further, the second N-type lightly doped impurity region  19  and the first N-type lightly doped impurity region  18  are formed therein. The P-type well region  5  and the second N-type lightly doped impurity region  19  are formed as described in the manufacturing method for the second embodiment. The first N-type lightly doped impurity region  18  is formed to have the impurity concentration of the N-type impurities of As or P that is adjusted to be from 1×10 17 /cm 3  to 1×10 18 /cm 3 . The first N-type lightly doped impurity region  18  is formed in advance so as to underlie the thick oxide film  13  to be formed adjacent to the N-type channel impurity region  10  ( FIG. 23A ). 
     Then, for the purpose of electrically isolating elements from one another, LOCOS is used to form the element isolation region  13 , and then, the thick oxide film  13  is formed on the first N-type lightly doped impurity region  18 . It is preferred that the thick oxide film  13  formed on the first N-type lightly doped impurity region  18  have a thickness of 1,000 Å or more. As described with reference to  FIG. 12 , the thick oxide film  13  may be formed simultaneously with the LOCOS oxide film  13  in the element isolation region to prevent increase in the number of process steps ( FIG. 23B ). 
     Then, N-type impurities of As or P are injected by ion injection into a region to be the drain region of the semiconductor nonvolatile memory element to form the N-type heavily doped impurity region  17 . Next, for the purpose of manufacturing the semiconductor nonvolatile memory element as a normally on type depression type MOSFET, N-type impurities of As or P are injected by ion injection into a region in which the channel is to be formed to form the N-type channel impurity region  10  ( FIG. 23C ). 
     Then, by thermal oxidation, CVD, or the like, the second gate insulating film  14  having a thickness that is larger than that of the first gate insulating film  9  is formed in a part of the region in which the drain is to be formed on the N-type heavily doped impurity region  17  so as to be in contact with the LOCOS oxide film  13  formed prior thereto. Then, the first gate insulating film  9  is formed on the region in which the channel is to be formed. The two gate insulating films having different thicknesses are realized as described with reference to  FIG. 21E . First, the thicker second gate insulating film  14  is formed. Then, the second gate insulating film  14  in regions other than the region in which the drain is to be formed is removed by photolithography and etching using HF or the like, and then, the first gate insulating film  9  is formed ( FIG. 23D ). 
     Then, a polycrystalline silicon layer is formed on the first gate insulating film  9  and the second gate insulating film  14 , and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more. Then, photolithography and dry etching are performed to form the floating gate electrode  7  of the semiconductor nonvolatile memory element. At this time, the floating gate electrode  7  and the second gate insulating film  14  has an overlapping portion for the purpose of carrier injection using tunneling ( FIG. 24E ). 
     Then, the third gate insulating film  15  and the control gate electrode  8  are formed as described with reference to  FIG. 21G  (not shown). 
     Then, in order to form the source/drain regions  12  of the semiconductor nonvolatile memory element, N-type impurities of As or P are injected by ion injection so that the concentration is 1×10 20 /cm 3  or more ( FIG. 24F ). 
     The above description is made with reference to  FIG. 23A  to  FIG. 23D , and  FIG. 24E  and  FIG. 24F  for illustrating the process steps. 
     Then, an oxide film serving as an insulating film (not shown) is formed on an entire surface. After a contact hole is formed in a predetermined location, for the purpose of giving the gate potential, the source potential, the drain potential, and the body potential of the semiconductor nonvolatile memory element, metal wiring is formed by sputtering and patterning a metal film. 
     Note that, when the structure of the seventh embodiment described with reference to  FIG. 16A ,  FIG. 16B , and  FIG. 16C  in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 12  is changed to the single-layer structure is manufactured, the process steps up to the step of forming the floating gate electrode  7  on the first gate insulating film  9  and the second gate insulating film  14  illustrated in  FIG. 24E  are common to the manufacturing method described above. After this, the source/drain regions  12  of the semiconductor nonvolatile memory element are formed. In this way, the structure of the seventh embodiment can be similarly manufactured. The control gate electrode  8  cap be formed using, for example, the impurities and the structure and the process step of the N-type heavily doped impurity region  17  in common. 
     Next, a method of manufacturing the semiconductor nonvolatile memory element having the structure according to the fourth embodiment illustrated in  FIG. 13  is described with reference to  FIG. 25A  to  FIG. 25D  for illustrating process steps thereof. 
     First, the semiconductor substrate  1  of the P type or of the N type is prepared. The P-type lightly doped impurity region  20  and the second N-type lightly doped impurity region  19  are formed in a region in which the semiconductor nonvolatile memory element is to be formed so as to partially overlap each other. The amount of impurities to be injected and conditions on the thermal diffusion of N-type impurities of P or As are selected so that the second N-type lightly doped impurity region  19  has an impurity concentration of from 2×10 16 /cm 3  to 2×10 17 /cm 3  and has a depth of from 3 μm to 6 μm. The P-type lightly doped impurity region  20  of B or BF 2  has, as described with reference to  FIG. 13 , an impurity concentration of from about 2×10 16 /cm 3  to about 2×10 17 /cm 3 , which is the impurity concentration higher than that of the second N-type lightly doped impurity region  19 , thereby seeking improvement in the drain breakdown voltage ( FIG. 25A ). 
     Then, the first N-type lightly doped impurity region  18  is formed in a region in which the drain is to be formed to have the impurity concentration of N-type impurities of As or P that is adjusted to be from 1×10 17 /cm 3  to 1×10 18 /cm 3  (not shown). 
     Then, for the purpose of electrically isolating elements from one another, LOCOS is used to form the element isolation region  13 , and then, the thick oxide film  13  is formed on the first N-type lightly doped impurity region  18 . It is preferred that the thick oxide film  13  formed on the first N-type lightly doped impurity region  18  have a thickness of 1,000 Å or more. As described with reference to  FIG. 12 , the thick oxide film  13  may be formed simultaneously with the LOCOS oxide film  13  in the element isolation region to prevent increase in the number of process steps ( FIG. 25B ). 
     Then, the N-type channel impurity region  10 , the first gate insulating film  9 , and the second gate insulating film  14  are formed (not shown). 
     Then, a polycrystalline silicon layer is formed on the first gate insulating film  9  and the second gate insulating film  14 , and impurities at a high concentration are injected by ion injection or thermal diffusion so that the concentration is 1×10 19 /cm 3  or more. Then, photolithography and dry etching are performed to form the floating gate electrode  7  of the semiconductor nonvolatile memory element ( FIG. 25C ). 
     Then, the third gate insulating film  15  and the control gate electrode  8  are formed (not shown). 
     Then, in order to form the source/drain regions  12  of the semiconductor nonvolatile memory element, N-type impurities of As or P are injected by ion injection so that the concentration is 1×10 20 /cm 3  or more ( FIG. 25D ). 
     Then, an oxide film serving as an insulating film (not shown) is formed on an entire surface. After a contact hole is formed in a predetermined location, for the purpose of giving the gate potential, the source potential, the drain potential, and the body potential of the semiconductor nonvolatile memory element, metal wiring is formed by sputtering and patterning a metal film. 
     Note that, when the structure of the eighth embodiment described with reference to  FIG. 17A ,  FIG. 17B , and  FIG. 17C  in which the two-layer gate electrode structure of polycrystalline silicon illustrated in  FIG. 13  is changed to the single-layer structure is manufactured, the process steps up to the step of forming the floating gate electrode  7  on the first gate insulating film  9  and the second gate insulating film  14  illustrated in  FIG. 25C  are common to the manufacturing method described above. After this, the source/drain regions  12  of the semiconductor nonvolatile memory element are formed. In this way, the structure of the eighth embodiment can be similarly manufactured. The control gate electrode  8  can be formed using, for example, the impurities and the structure and the process step of the N-type heavily doped impurity region  17  in common. 
     By the way, in the manufacturing methods of the first to fourth embodiments described above, the first insulating film  9  and the second insulating film  14  are formed in the same way. This is referred to as a first manufacturing method of forming the gate insulating films. Instead of the first method, the following second to fifth methods can be adopted for the purpose of improving the performance, reducing costs, or the like. 
     The second method is different from the first method in that the three steps of forming a thick oxide film, partly removing the oxide film, and forming a thin oxide film are not used, but the first gate insulating film  9  and the second gate insulating film  14  having different thicknesses are simultaneously formed by performing thermal oxidation once. 
     Specifically, as illustrated in  FIG. 26A , first, the N-type heavily doped impurity region  17  is formed at a high impurity concentration of 1×10 19 /cm 3  or more using As. 
     Then, a gate insulating film is formed by wet oxidation using water vapor, or pyrogenic oxidation in which oxygen gas and hydrogen gas are introduced into a furnace to be reacted with each other. The gate insulating film has a large thickness only on the N-type heavily doped impurity region  17  due to an enhanced oxidation effect and has a small thickness in a remaining region, and thus, a shape illustrated in  FIG. 26B  is obtained. 
     In this case, for example, when the first gate insulating film  9  has a thickness of 150 Å, the second gate insulating film  14  can have a thickness of about 300 Å. As the extent of a lattice defect in the semiconductor substrate  1  becomes larger, the effect of the enhanced oxidation becomes more remarkable due to entrance of oxygen and reaction. Thus, regardless of whether the impurities are of the N type or of the P type, when the impurity concentration in the semiconductor substrate  1  is high, the effect of the enhanced oxidation can be obtained in accordance with the extent of the lattice defect. However, when the film is used as a gate insulating film in particular, an oxide film that is grown on an impurity region of the N type is desired. Accordingly, this method can be said to be effective in an N-channel type semiconductor nonvolatile memory element. In this case, P-type impurities are not preferred because the P-type impurities enter the oxide film during the thermal oxidation to considerably lower the quality of the oxide film. 
     The method described above has effects that, by reducing the three steps to one step, costs can be reduced and time taken for the step can be shortened. 
     Next, the third method of forming the first gate insulating film  9  and the second gate insulating film  14  is described with reference to  FIG. 27A  to  FIG. 27C . 
     In the third method, first, a polycrystalline silicon layer  21 A having a thickness of from 100 Å to 400 Å is formed in advance on an entire surface ( FIG. 27A ). 
     Then, the polycrystalline silicon layer  21 A in regions other than a region in which the second gate insulating film  14  is to be formed is removed by photolithography and etching, and a polycrystalline silicon layer  21 B is left ( FIG. 27B ). 
     Then, the resultant is subjected to thermal oxidation for forming the first gate insulating film  9 , and a silicon oxide film ( 9  and  14 ) is formed on the semiconductor substrate  1 . At that time, by setting the second gate insulating film  14  to have a thickness so that the polycrystalline silicon layer  21 B is completely oxidized by the thermal oxidation for forming the first gate insulating film  9 , the second gate insulating film  14  may be formed of the oxide film formed by oxidizing polycrystalline silicon. In this case, polycrystalline silicon is used because, due to a lattice defect included therein, the oxidation speed can be 1.5 to 2 times as high as that of ordinary monocrystalline silicon ( FIG. 27C ). 
     The third method is different from the first method in that the need for long-time high-temperature heat treatment for forming the thick second gate insulating film  14  is eliminated, and thus, has effects that variations in impurities at a relatively low concentration in the N-type channel impurity region  10 , the first N-type lightly doped impurity region  18 , or the second N-type lightly doped impurity region  19  due to heat treatment at high temperature are inhibited and higher accuracy of element characteristics is promoted. 
     Next, the fourth method is described with reference to  FIG. 28A  to  FIG. 28C . First, an underlayer silicon oxide film having a thickness of from 10 Å to 100 Å is formed in advance on an entire surface by thermal oxidation, and then, a SiN layer  22  having a thickness of from 100 Å to 200 Å is formed on an entire upper surface of the underlayer silicon oxide film by LPCVD or the like ( FIG. 28A ). 
     Then, the SiN layer  22  in regions other than a region in which the first gate insulating film  9  is to be formed is removed by photolithography ( FIG. 28B ). 
     Then, on the resultant silicon oxide film having a thickness of several hundred angstroms for forming the second gate insulating film  14  is formed by thermal oxidation. At that time, the first gate insulating film  9  is covered with SiN having a low reactivity, and thus, the oxide film is scarcely grown thereon. Accordingly, the first gate insulating film  9  can be formed as a stacked film including the silicon oxide film having a thickness of several ten angstromes and the SiN film having a thickness of from 100 Å to 200 Å, and the second gate insulating film  14  can be formed as a silicon film having a thickness of several hundred angstromes ( FIG. 28C ). 
     The fourth method has strong points that the first gate insulating film  9  can have a large capacitance, the size of the gate electrode  8  can be reduced and costs can be reduced accordingly, and the amount of variations in threshold voltage can be small even when the charge Q is reduced due to leakage of carriers in the floating gate electrode  7 , or the like. 
     Next, the fifth method is described with reference to  FIG. 29A  to  FIG. 29D . First, similarly to the case of the first method, the second gate insulating film  14  having a thickness of from 100 Å to 1,000 Å is formed on an entire surface as a silicon oxide film by thermal oxidation ( FIG. 29A ). 
     Then, similarly to the case of the first method, the second gate insulating film  14  in a region in which the channel is to be formed is removed by photolithography and etching ( FIG. 29B ). 
     Then, the first gate insulating film  9  is formed by thermal oxidation. According to this method, the thickness of the first gate insulating film  9  is smaller than that in the first method, and is from 30 Å to 100 Å ( FIG. 29C ). 
     Then, thermal nitridation is performed in an atmosphere of ammonia at a temperature that is equal to or higher than 1,000° C. Then, nitrogen is diffused to an interface with the semiconductor substrate  1  under the first gate insulating film  9  to react with the semiconductor substrate  1 , and a SiN layer having a thickness of from about 1 Å to about 20 Å is formed. On the other hand, the second gate insulating film  14  has a sufficiently large thickness, and thus, the amount of nitrogen that is diffused to reach the interface with the semiconductor substrate  1  is very small. Thus, the extent of insulation of the formed SiN layer is not large enough to prevent tunneling of the carriers ( FIG. 29D ). 
     The silicon oxide film that forms the first gate insulating film  9  according to the fifth method has a small thickness of 100 Å or less, and thus, the carriers may dissipate in the floating gate electrode  7  due to a leakage current at high temperature. However, the SiN layer under the oxide film ensures a large extent of insulation, and thus, the leakage is inhibited, and at the same time, a large capacitance of the first gate insulating film  9  is realized as well. 
     The SiN layer is formed in the fourth method as well. However, when CVD is used as in the fourth method, controllability of a film thickness of 100 Å or less is not satisfactory, and there is a problem in that the element characteristics vary. Thermal nitridation as in the fifth method can form a thinner SiN layer with stability, which is effective for more highly accurate element characteristics. 
     The present invention is applicable to products other than a step-down series regulator and a voltage detector described above. Through adoption of a memory terminal in which the threshold voltage can be changed with an electrical signal that is input from an input terminal for regulation, an output voltage can be changed with an input electrical signal in various semiconductor integrated circuit devices including a reference voltage circuit. It goes without saying that the present invention is applicable to products other than a power management IC.