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1. (US20180108395) Semiconductor storage device and method for writing of the same
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CROSS REFERENCE TO RELATED APPLICATIONS

      This application is a U.S. national stage of International Patent Application No. PCT/JP2016/072463 filed on Aug. 1, 2016 and is based on Japanese Patent Application No. 2015-155081 filed on Aug. 5, 2015, the disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

      The present disclosure relates to a semiconductor storage device and a method for writing of the same.

BACKGROUND ART

      For example, in a conventional writing to a p-channel flash memory cell, an application voltage to the memory cell had a fixed value.

PRIOR ART LITERATURE

Patent Literature

      Patent Literature 1 JP2004-55004A

Non-Patent Literature

      Non-patent Literature 1: Joe Brewer, Manzur Gill, “Nonvolatile Memory Technologies with Emphasis on Flash: A Comprehensive Guide to Understanding and Using Flash Memory Devices”, Wiley-IEEE Press

SUMMARY OF INVENTION

      Suitable writing conditions are likely to be different between respective memory cells depending on variations in processing, the temperature, and states of writing. Thus, if the writing is executed under the same condition to all of the memory cells, it is not necessarily true that the writing is efficiently conducted to all of the memory cells. In such a case, since the state of writing is not uniform, there is a possibility that variations in thresholds of the memory cells to which data have been written will arise.
      It is an object of the present disclosure to provide a semiconductor storage device and a method for writing of the semiconductor storage device, which are capable of improving the efficiency of writing, and the uniformity of the writing.
      According to an aspect of the present disclosure, a semiconductor storage device includes a memory cell having a control electrode, a floating electrode, a source and a drain. The semiconductor storage device further includes a switch that connects the control electrode and the drain, a source driver that is connected to the source and applies a predetermined voltage to the source, a current source that is configured to be connected to the source and to supply a predetermined current to the source, and a drain driver that is connected to the drain and applies a predetermined voltage to the drain. The semiconductor storage device still includes a voltage measurement circuit that measures a voltage occurring between the control electrode and the source, and a control electrode driver that applies a predetermined voltage to the control electrode. In writing to the memory cell, the voltage measurement circuit measures a voltage occurring between the control electrode and the source when the predetermined current flows from the current source to the memory cell in a state where the switch is turned on. The control electrode driver applies a voltage controlled based on the voltage measures to the control electrode.
      According to a second aspect of the present disclosure, a method for writing of the semiconductor storage device according to the first aspect described above includes: in writing the memory cell, turning on the switch, and subsequently conducting the predetermined current from the current source to the memory cell; measuring the voltage occurring between the control electrode and the source by the voltage measurement circuit when the predetermined current flows between the source and the drain by the voltage measurement circuit, and subsequently applying a voltage controlled based on the voltage measured to the control electrode by the control electrode driver.
      According to a third aspect of the present disclosure, a method for writing of the semiconductor storage device according to the first aspect described above includes: in writing the memory cell, turning on the switch and causing the predetermined current to flow from the current source to the memory cell; measuring a first voltage generated between the control electrode and the source by the voltage measurement circuit when the predetermined current flows between the source and the drain; executing a first writing by applying a voltage that is controlled based on the first voltage to the control electrode from the control electrode driver: turning on the switch and causing the predetermined current to flow from the current source to the memory cell; measuring a second voltage generated between the control electrode and the source by the voltage measurement circuit when the predetermined current flows between the source and the drain; and executing a second writing by applying a voltage that is controlled based on the second voltage to the control electrode from the control electrode driver.
      According to a fourth aspect of the present disclosure, a method for writing of the semiconductor storage device according to the first aspect described above includes: in writing the memory cell, turning on the switch and causing the predetermined current to flow from the current source to the memory cell; measuring a voltage generated between the control electrode and the source by the voltage measurement circuit when the predetermined current flows between the source and the drain; and executing a writing by applying a voltage that is controlled based on the voltage measured to the control electrode from the control electrode driver. The turning on the switch, the causing the predetermined current to flow, the measuring the voltage, and the executing the writing are repeated until the voltage reaches a predetermined value.
      According to a fifth aspect of the present disclosure, a semiconductor storage device including: a reference memory cell that at least includes a first control electrode, a first floating electrode, a first source, and a first drain; a storage memory cell that at least includes a second control electrode, a second floating electrode, a second source, and a second drain; a first source driver that is connected to the first source and is configured to apply a predetermined voltage to the first source; a current source that is configured to be connected to the first source and the second source and to supply a predetermined current to the first source or the second source; a first drain driver that is connected to the first drain and is configured to apply a predetermined voltage to the first drain; a switch that is configured to connect the first control electrode and the first drain; a voltage measurement circuit that is configured to measure a voltage generated between the first control electrode and the first source; a first control electrode driver that is configured to apply a predetermined voltage to the first control electrode; a second source driver that is connected to the second source and is configured to apply a predetermined voltage to the second source; a second drain driver that is connected to the second drain and is configured to apply a predetermined voltage to the second drain; and a second control electrode driver that is configured to apply a predetermined voltage to the second control electrode. The voltage measurement circuit measures a voltage generated between the first control electrode and the first source when the switch is in an on state and the predetermined current flows from the current source to the memory cell. The second source driver, the second drain driver, the second control electrode driver generate voltages that are controlled based on the voltage that is generated between the first control electrode and the first source electrode and is measured by the voltage measurement circuit.
      According to a sixth aspect of the present disclosure, a method for writing of the semiconductor storage device according to the fifth aspect includes: measuring a voltage generated between the first control electrode and the first source when the switch is in an on state and the predetermined current flow from the current source to the reference memory cell; and controlling voltages to be generated from the second source driver, the second drain driver and the second control electrode driver based on the voltage that is generated between the first control electrode and the first source and is measured by the voltage measurement circuit.
      According to the structures and methods described hereinabove, the writing can be executed to the memory cell of the semiconductor storage device using the optimum condition, resulting in improvement of the writing efficiency and improvement of the uniformity of the writing.

BRIEF DESCRIPTION OF DRAWINGS

      The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings, in which:
       FIG. 1 is a block diagram illustrating a schematic structure of a semiconductor storage device according to a first embodiment;
       FIG. 2 is a flowchart illustrating an outline of a writing processing of the semiconductor storage device according to the first embodiment;
       FIG. 3 is a flowchart illustrating an outline of a writing processing of a semiconductor storage device according to a second embodiment;
       FIG. 4 is a graph illustrating a change in threshold of the semiconductor storage device according to the second embodiment;
       FIG. 5 is a block diagram illustrating an example of a structure of a semiconductor storage device according to a third embodiment;
       FIG. 6 is a flowchart illustrating an outline of a writing processing of the semiconductor storage device according to the third embodiment;
       FIG. 7 is a block diagram illustrating an example of a structure of a semiconductor storage device according to a fourth embodiment;
       FIG. 8 is a flowchart illustrating an outline of a writing processing of the semiconductor storage device according to the fourth embodiment;
       FIG. 9 is a block diagram illustrating an example of a structure of a semiconductor storage device according to a fifth embodiment; and
       FIG. 10 is a flowchart illustrating an outline of a writing processing of the semiconductor storage device according to the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

      Hereinafter, embodiments in which the present disclosure is implemented will be described with reference to the drawings. In the embodiments, the same elements will be designated with the same reference numbers, and a description thereof will be omitted.

First Embodiment

      As shown in FIG. 1, a semiconductor storage device 10 according to the present embodiment is, for example, a p-channel flash memory, and has a memory cell 121. The memory cell 121 has a control electrode 122, a floating electrode 123, a source 124 and a drain 125. Writing to the memory cell 121 is implemented as electrons are injected to the floating electrode 123.
      The semiconductor storage device 10 includes a source driver 14 and a drain driver 16. The source driver 14 is connected to the source 124, and applies a predetermined voltage to the source 124. The drain driver 16 is connected to the drain 125, and applies a predetermined voltage to the drain 125. The semiconductor storage device 10 includes a control electrode driver 18. The control electrode driver 18 is connected to the control electrode 122, and applies a predetermined voltage to the control electrode 122.
      A switch 30 is provided between the control electrode 122 and the drain 125, and is configured to switch on and off the connection between the control electrode 122 and the drain 125. The semiconductor storage device 10 includes a current source 20 connecting the source 124 and the drain 125, and a voltage measurement circuit 22 that is configured to measure a voltage between the source 124 and the drain 125. A switch 32 is provided between the source 124 and the current source 20 and voltage measurement circuit 22, and is configured to switch on and off the connection between the source 124 and the current source 20 and voltage measurement circuit 22. A switch 33 is provided between the drain 125 and the current source 20 and voltage measurement circuit 22, and is configured to switch on and off the connection between the drain 125 and the current source 20 and voltage measurement circuit 22.
      The voltage measurement circuit 22 can measure a voltage between the control electrode 122 and the source 124 when the switches 30, 32, 33 are in on states (i.e., connected).
      As a procedure of a writing processing of the semiconductor storage device 10 of the present embodiment, as shown in FIG. 2, firstly, the switches 30, 32 and 33 are turned on (A 1). Next, the control electrode driver 18 applies a voltage to the control electrode 122 until a predetermined current flows to the memory cell 121 from the current source 20 (A 2). That is, a voltage is applied between the control electrode 122 and the source 124. In this case, the value of the predetermined current is very small, and is for example defined as 1 μA. These steps are for measuring a threshold of the memory cell 121. Next, a potential difference (i.e., voltage, voltage difference) between the source 124 and the drain 125 is measured by the voltage measurement circuit 22 (A 3). Since the control electrode 122 and the drain 125 are in a connected state, the potential difference (voltage) is a threshold voltage (Vth) of the memory cell 121.
      Next, based on the potential difference measured, that is, based on the threshold voltage of the memory cell 121, a writing control electrode voltage to be applied to the control electrode 122, a writing source voltage to be applied to the source 124, a writing drain voltage to be applied to the drain 125 are controlled (A 4). The writing control electrode voltage to be applied to the control electrode 122 is controlled by the control electrode driver 18. The writing source voltage to be applied to the source 124 is controlled by the source driver 14. The writing drain voltage to be applied to the drain 125 is controlled by the drain driver 16.
      These voltages are controlled based on the threshold voltage (Vth) of the memory cell 121 obtained in a step A 3 so that an injection efficiency of electrons to the floating electrode 123 of the memory cell 121 is optimum. For example, in a case where the memory cell 121 is a p-channel memory cell, these voltages are controlled so that a difference between the voltage applied to the control electrode 122 and the voltage applied to the source 124 in the writing has a value approximate to the threshold voltage. In this case, the injection efficiency of electrons to the floating electrode 123 improves, and thus the writing efficiency improves.
      Subsequently, the voltages controlled as described above are applied to the memory cell 121 respectively from the control electrode driver 18, the source driver 14 and the drain driver 16, to thereby execute the writing processing to the memory cell (A 5).
      The threshold voltage of the memory cell 121 is monitored by the steps A 1 to A 3 described above. Based on the threshold voltage monitored, the voltages applied to the control electrode 122, the source 124 and the drain 125 of the memory cell 121 are controlled by the steps A 4 and A 5, so the writing processing is executed under a condition where the injection efficiency of electrons to the floating electrode 123 is optimum.
      As described above, according to the semiconductor storage device 10 and the method for writing of the semiconductor storage device 10 of the present embodiment, the threshold voltage of each of memory cells 121 of the semiconductor storage device 10 is measured, and the writing voltages to the respective memory cells 121 can be controlled based on the threshold voltages measured.
      Accordingly, the injection efficiency of electrons to the floating electrode 123 improves, and the writing efficiency of the memory cell 121 improves. Therefore, the total writing time can be shortened in the semiconductor storage device 10 as a whole. Further, variations in writing can be reduced, and thus the writing can be uniformly executed.

Second Embodiment

      Next, a second embodiment will be described. Hereinafter, only the differences from the first embodiment will be described. The structure of a semiconductor storage device 10 of the second embodiment is the same as the structure of the first embodiment.
      As shown in FIG. 3, in a writing processing of the second embodiment, firstly, the switches 30, 32 and 33 are turned on (B 1). Next, a predetermined current is caused to flow to the memory cell 121 from the current source 20 (B 2). That is, a voltage is applied between the control electrode 122 connected to the drain 125 and the source 124 so that the predetermined current flows in the memory cell 121. Next, a first potential difference (La, voltage, voltage difference) generated between the source 124 and the drain 125 is measured by the voltage measurement circuit 22 (B 3). Since the control electrode 122 and the drain 125 are in the connected state, the first potential difference (voltage) is a threshold voltage of the memory cell 121 before writing. Hereinafter, the first potential difference will be referred to as a first threshold voltage (Vth 1).
      Next, based on the first potential difference, that is, based on the first threshold voltage of the memory cell 121, a writing control electrode voltage to be applied to the control electrode 122, a writing source voltage to be applied to the source 124, and a writing drain electrode voltage to be applied to the drain 125 are controlled (B 4). These voltages are controlled based on the first threshold voltage (Vth 1) obtained by the step B 3 so that the injection efficiency of electrons to the floating electrode 123 of the memory cell 121 at the first threshold voltage is optimum.
      Next, the switches 30, 32 and 33 are turned off, and then the voltages controlled are applied to the memory cell 121 respectively from the control electrode driver 18, the source driver 14 and the drain driver 16, to thereby execute the writing to the memory cell 121. In this case, the writing is not completed, but is conducted partway (B 5). For example, assumed that the writing time in the first embodiment is 200 μs, and the writing time in this case is, for example, 100 μs.
      The steps B 1 to B 5 described hereinabove are categorized into a first step-group, and the writing executed by the first step-group is referred to as a first writing.
      Next, the switches 30, 32 and 33 are turned on (B 6). Next, a predetermined current is caused to flow to the memory cell 121 from the current source 20 (B 7). Subsequently, a second potential difference (i.e., voltage, voltage difference) between the source 124 and the drain 125 is measured by the voltage measurement circuit 22 (B 8). Since the control electrode 122 and the drain 125 are in the connected state, the second potential difference (voltage) is a threshold voltage of the memory cell 121 in the state where the writing has been executed by the first writing. Hereinafter, the second potential difference will be referred to as a second threshold voltage (Vth 2).
      Next, based on the second potential difference measured, that is, based on the second threshold voltage of the memory cell 121, the writing control electrode voltage applied to the control electrode 122, the writing source voltage applied to the source 124, and the writing drain voltage applied to the drain 125 are controlled (B 9). These voltages are controlled based on the second threshold voltage (Vth 2) of the memory cell obtained by the step B 8 so that the injection efficiency of the floating electrode 123 of the memory cell 121 at the second threshold voltage is optimum.
      Next, after the switches 30, 32 and 33 are turned off, the voltages controlled above are applied to the memory cell 121 respectively from the control electrode driver 18, the source driver 14 and the drain driver 16, to thereby execute the writing to the memory cell 121 ( 310). Thereafter, this processing is repeated until the threshold voltage of the memory cell 121 reaches a target value.
      The steps B 6 to B 10 described hereinabove are categorized into a second step-group, and the writing executed by the second step-group will be referred to as a second writing.
       FIG. 4 is a graph illustrating the threshold voltage that varies in accordance with the progression of the writing by the first writing of the first step-group and the second writing of the second step-group. It is appreciated that the threshold voltage converges to a predetermined value as the steps are repeated.
      In the case where the memory cell 121 is a p-channel memory cell, as described hereinabove, the writing efficiency is optimum when the application voltage to the control electrode 122 is approximate to the threshold voltage. However, the threshold voltage varies constantly during the writing. The threshold voltage at the time of beginning the writing and the threshold voltage while after the writing has begun are different. Therefore, the writing voltage that has been applied as an optimum voltage at the time of beginning the writing is not optimum at a time while after the writing has begun.
      In the second embodiment, the writing is executed to a halfway of a target threshold by the first writing processing, and the second threshold voltage during the writing is measured by the second step group. Based on the second threshold voltage measured, the writing voltage of the second writing processing is controlled. Therefore, the writing voltage can be kept at an optimum value from the beginning of the writing to the memory cell 121 to the end of the writing. Accordingly, the writing efficiency can be improved, and unevenness of the writing can be reduced, and the writing can be uniform.
      Note that it has been described that the writing can be categorized into the first step group and the second step group. However, the writing is not limited to this example, and but may be categorized into further groups by adding a third step group (third writing) and a fourth step group (fourth writing).

Third Embodiment

      Next, a third embodiment will be described. Hereinafter, only the differences from the first embodiment will be described. As shown in FIG. 5, the third embodiment is different from the first embodiment as having a reference memory cell 121 a and a storage memory cell 131.
      As shown in FIG. 5, a semiconductor storage device 10 has a reference memory cell 121 a and a storage memory cell 131. The semiconductor storage device 10 includes a source driver 141 that is connected to a source 124 a of the reference memory cell 121 a, and applies a predetermined voltage to the source 124 a. The semiconductor storage device 10 includes a drain driver 161 that is connected to a drain 125 a, and applies a predetermined voltage to the drain 125 a. Also, the semiconductor storage device 10 includes a control electrode driver 181 that is connected to a control electrode 122 a, and applies a predetermined voltage to the control electrode 122 a.
      A switch 30 is provided between the control electrode 122 a and the drain 125 a, and is configured to switch on and off the connection between the control electrode 122 a and the drain 125 a. The semiconductor storage device 10 includes a current source 20 that connects the source 124 a and the drain 125 a, and a voltage measurement circuit 22 that is configured to measure a voltage between the source 124 a and the drain 125 a. A switch 32 is provided between the source 124 a and the current source 20 and voltage measurement circuit 22. The switch 32 is configured to witch on and off the connection between the source 124 a and the current source 20 and voltage measurement circuit 22.
      When the switches 30, 32 and 33 are in the on states (i.e., connected), the voltage measurement circuit 22 measures the voltage between the source 124 a and the drain 125 a. The voltage measured is a threshold voltage of the reference memory cell 121 a, as will be described hereinafter.
      The semiconductor storage device 10 includes a source 134 of the storage memory cell 131, and a source driver 142 that applies a predetermined voltage to the source 134. The semiconductor storage device 10 also includes a drain driver 162 that is connected to a drain 135, and applies a predetermined voltage to the drain 135. The semiconductor storage device 10 further includes a control electrode driver 182 that is connected to a control electrode 132, and applies a predetermined voltage to the control electrode 132.
      The threshold voltage of the reference memory cell 121 a measured by the voltage measurement circuit 22 is transmitted to the source driver 142, the drain driver 162 and the control electrode driver 182. The source driver 142, the drain driver 162 and the control electrode driver 182 respectively control the voltages to be applied to the control electrode 132, the source 134 and the drain 135 based on the threshold voltage.
      In a writing processing of the third embodiment, as shown in FIG. 6, firstly, the switches 30, 32 and 33 are turned on (C 1). Next, a predetermined current is caused from the current source 20 to the reference memory cell 121 a (C 2). That is, a voltage is applied between the control electrode 122 a connected to the drain 125 a and the source 124 a. Subsequently, a potential difference (i.e., voltage, potential difference) between the source 124 a and the drain 125 a is measured by the voltage measurement circuit 22 (C 3). In this case, the value of the predetermined current is very small, and is for example, defined as 1 μA. Since the control electrode 122 a and the drain 125 a are in the connected state, the threshold voltage (Vth) of the reference memory cell 121 a can be measured by measuring the potential difference (voltage) between the source 124 a and the drain 125 a.
      Next, based on the potential difference described above, that is, based on the threshold voltage of the reference memory cell 121 a, a writing control voltage to be applied to the control electrode 132 of the storage memory cell 131, a writing source voltage to be applied to the source 134, and a writing drain voltage to be applied to the drain 135 are controlled (C 4). These voltages are controlled based on the threshold voltage (Vth) of the memory cell obtained by the step C 3 so that the injection efficiency of electrons to a floating electrode 133 of the storage memory cell 131 at the threshold voltage is optimum.
      Next, the voltages controlled are applied to the storage memory cell 131 respectively from the control electrode driver 182, the source driver 142 and the drain driver 162, to thereby execute the writing to the storage memory cell 131 until the threshold voltage reaches a target threshold voltage (C 5).
      According to the semiconductor storage device 10 and the method for writing of the semiconductor storage device 10 of the third embodiment, in addition to the advantageous effects similar to the first embodiment, the following effects can be achieved. That is, since the reference memory cell 121 a and the storage memory cell 131 are separately provided, the writing to the storage memory cell 131 can be started at the same time as measuring the threshold voltage of the reference memory cell 121 a. Therefore, the total writing time can be shortened.

Fourth Embodiment

      Next, a fourth embodiment will be described. In the fourth embodiment, the reference number “ 121 a” denotes a first reference memory cell (first reference flash memory) 121 a. The fourth embodiment is different from the third embodiment on a point that the first reference memory cell 121 a and a second reference memory cell 121 b are provided. As shown in FIG. 7, a semiconductor storage device 10 includes the first reference memory cell 121 a and the second reference memory cell 121 b. The first reference memory cell 121 a includes a control electrode 122 a, and a floating electrode 123 a. The second memory cell 121 b includes a control electrode 122 b and a floating electrode 123 b. The first reference memory cell 121 a and the second reference memory cell 121 b are connected symmetric with respect to a current source 20 and a switch 30.
      A source 124 a of the first reference memory cell 121 a is connected to the current source 20 and one of terminals of the voltage measurement circuit 22 through a switch 32 a. A drain 125 a of the first reference memory cell 121 a is connected to the current source 20 and the other one of the terminals of the voltage measurement circuit 22 through a switch 33 a. Assumed that the first reference memory cell 121 a is in a state of before the writing, and a threshold voltage of the first reference memory cell 121 a is adjusted to be at a level of before the writing.
      A source driver 141 a is connected to the source 124 a of the first reference memory cell 121 a. A drain driver 161 a is connected to the drain 125 a. A control electrode driver 181 a is connected to the control electrode 122 a, A switch 30 is provided between the control electrode 122 a and the drain 125 a, and is configured to switch on and off the connection between the control electrode 122 a and the drain 125 a. The switch 32 a and the switch 33 a are respectively provided between the source 124 a and the current source 20 and voltage measurement circuit 22, and between the drain 125 a and the current source 20 and voltage measurement circuit 22, and are configured to switch on and off the connections therebetween.
      The second reference memory cell 121 b has similar structures to those of the first reference memory cell 121 a. The second reference memory cell 121 b and the first reference memory cell 121 a are symmetrically connected with respect to the current source 20 and the voltage measurement circuit 22. The structures of the second reference memory cell (second reference flash memory) 121 b and the structures around the second reference memory cell 121 b can be understood by replacing the suffix “a” of the reference number of each structure in association with the first reference cell 121 a with “b”, and thus the description of the structures of the second reference memory cell 121 b and the structures around the second reference memory cell 121 b will be omitted. Assumed that the second reference memory cell 121 b is in a state during the writing. For example, the second reference memory cell 121 b is adjusted to have a threshold voltage of approximately 70% of the target threshold voltage.
      The voltage measurement circuit 22 measures the threshold voltages of the first reference memory cell 121 a and the second reference memory cell 121 b, and outputs the measurement results to the source driver 142, the control driver 182 and the drain driver 162. The source driver 142, the control electrode driver 182 and the drain driver 162 control the writing to the storage memory cell 131 based on the threshold voltages received.
      In a writing processing of the fourth embodiment, as shown in FIG. 8, firstly, the switch 30 a, 32 a, and 33 a are turned on, and the switches 30 b, 32 b and 33 b are turned off (D 1). Next, a predetermined current is caused to flow to the first reference memory cell 121 a from the current source 20 (D 2). That is, a voltage is applied between the control electrode 122 a connected to the drain 125 a and the source 124 a. Next, a first voltage between the source 124 a and the drain 125 a is measured by the voltage measurement circuit 22 (D 3). In this case, the predetermined current has a very small current value, which is for example defined as 1 μA. Since the control electrode 122 a and the drain 125 a are in a connected state, the first voltage is the threshold voltage of the first reference memory cell 121 a. Hereinafter, the first voltage will be referred to as a first threshold voltage (Vth 1).
      Next, based on the first voltage described hereinabove, that is, based on the first threshold voltage of the first reference memory cell 121 a, a writing control electrode voltage to be applied to the control electrode 132 of the storage memory cell 131, a writing source voltage to be applied to the source 134, and a writing drain voltage to be applied to the drain 135 are controlled (D 4). These voltages are controlled based on the first threshold voltage (Vth 1) obtained by the step D 3 so that the injection efficiency of electrons to the floating electrode 133 of the storage memory cell 131 at the first threshold voltage is optimum.
      Next, the voltages controlled are applied to the storage memory cell 131 respectively from the control electrode driver 182, the source driver 142, and the drain driver 162, thereby to execute the writing to the storage memory cell 131. In this case, however, the writing is not completed, but is executed until the threshold voltage of the storage memory cell 131 reaches a second threshold voltage, which will be described hereinafter (D 5). The second threshold voltage is, for example, defined as approximately 70% of the target threshold voltage.
      The steps D 1 to D 5 described hereinabove are categorized into a first step group, and the writing executed by the first step group is referred to as the first writing.
      Next, the switches 30 a, 32 a and 33 a are turned off, and the switches 30 b, 32 b and 33 b are turned on (D 6). As such, the current source 20 and the voltage measurement circuit 22 are connected to the second reference memory cell 121 b.
      Next, a predetermined current is caused to flow to the second memory cell 121 b from the current source 20 (D 7). Subsequently, a second potential difference (i.e., voltage, potential difference) between the source 124 b and the drain 125 b is measured by the voltage measurement circuit 22 (D 8). In this case, the predetermined current has a very small current value, which is, for example, defined as 1 μA. Since the control electrode 122 b and the drain 125 b are in the connected state, the second potential difference (voltage) is the threshold voltage of the second reference memory cell 121 b. Hereinafter, the second potential difference will be referred to as a second threshold voltage (Vth 2).
      Next, based on the second potential difference, that is, based on the second threshold voltage of the second reference memory cell 121 b, a writing control electrode voltage to be applied to the control electrode 132 of the storage memory cell 131, a writing source voltage to be applied to the source 134, and a writing drain voltage to be applied to the drain 135 are controlled (D 9). These voltages are controlled based on the second threshold voltage (Vth 2) obtained by the step D 8 so that the injection efficiency of electrons to the floating electrode 133 of the storage memory cell 131 at the second threshold voltage is optimum.
      Next, the voltages controlled are applied to the storage memory cell 131 respectively from the control electrode driver 182, the source driver 142, and the drain driver 162, thereby to execute the writing to the storage memory cell 131 (D 10). In this case, the writing to the storage memory cell 131 is executed until the threshold voltage reaches a target threshold voltage.
      The steps D 6 to D 10 described hereinabove are categorized into a second step group, and the writing executed by the second step group is referred to as the second writing.
      According to the fourth embodiment, the similar advantageous effects to those of the first embodiment, the second embodiment and the third embodiment will be achieved. In the fourth embodiment, further, the first reference memory cell 121 a and the second reference memory cell 121 b are provided separately from the storage memory cell 131. The writing processing to the storage memory cell 131 is firstly controlled based on the first threshold voltage of the first reference memory cell 121 a monitored, and is subsequently switched to control based on the second threshold voltage of the second reference memory cell 121 b. In this way, the writing to the storage memory cell 131 is executed. Accordingly, the first threshold voltage and the second threshold voltage are further accurately monitored, and thus the writing can be efficiently executed. Also, the writing can be uniform.
      Note that, in the fourth embodiment, the writing to the storage memory cell 131 is divided into the first writing executed by the first step group and the second writing executed by the second step group, as exemplarily described hereinabove. However, the writing to the storage memory cell 131 is not limited to the example. As another example, a plurality of reference memory cells may be provided to have a plurality of threshold voltages that are leveled from the threshold voltage before the writing to the threshold voltage of the timing of the completion of the writing. The writing to the storage memory cell 131 may be repeated until the threshold voltage of the storage memory cell 131 reaches the target threshold voltage while monitoring the threshold voltage of each of the plurality of memory cells and switching the writing voltages. That is, in addition to the first and second reference memory cells, a third reference memory cell and a fourth reference memory cell are provided, for example. The threshold voltages of these reference memory cells are adjusted so that the these reference memory cells have threshold voltages, which are different subsequently from the threshold voltage before the writing to the threshold voltage at the timing of the completion of the writing. Further, the writing to the storage memory cell 131 is repeated, as in a manner of a first writing, a second writing, a third writing and a fourth writing, until the threshold voltage of the storage memory cell 131 reaches a target threshold voltage while monitoring the threshold voltages of the reference memory cells and switching the writing voltages.

Fifth Embodiment

      Next, a fifth embodiment will be described. In the fifth embodiment, a comparator 40 connected to a threshold reference memory cell 121 c and voltage measurement circuits 22 c and 22 b is provided. The comparator 40 compares a threshold of a storage memory cell 131 after writing and a threshold of the threshold reference memory cell 121 a, to thereby verify the threshold of the storage memory cell 131 after the writing.
      As shown in FIG. 9, a semiconductor storage device 10 includes the threshold reference memory cell 121 c, the storage memory cell 131, and the comparator 40. The structure of the threshold reference memory cell 121 c and the structures around the threshold reference memory cell 121 c are similar to those of the first reference memory cell 121 a of the third embodiment shown in FIG. 5, and can be understood by replacing suffix of the reference number with “c”. Thus, descriptions of portions that are substantially the same will be omitted.
      The threshold reference memory cell 121 c has the voltage measurement circuit 22 c. The voltage measurement circuit 22 c is connected to the comparator 40. The measurement result of the voltage measurement circuit 22 c is provided to the comparator 40. The threshold reference memory cell 121 c is written in advance, and has the threshold after the writing.
      A source 134 of the storage memory cell 131 is connected to a current source 20 b and one of the terminals of the voltage measurement circuit 22 b through a switch 32 b. A drain 135 of the storage memory cell 131 is connected to the current source 20 and the other one of the terminals of the voltage measurement circuit 22 b through a switch 33 b. The voltage measurement circuit 22 b is connected to the comparator 40, and provides the measurement result to the comparator 40.
      The comparator 40 compares the measurement value received from the voltage measurement circuit 22 and the measurement value received from the voltage measurement circuit 22 b, that is, compares the threshold of the threshold reference memory cell 121 c and the threshold of the storage memory cell 131.
       FIG. 10 shows a writing processing of the fifth embodiment. The writing to the storage memory cell 131 may employ any of the writings of the first to fourth embodiments. For example, after the writing to the storage memory cell 131 is executed by the method of the third embodiment including the steps C 1 to C 5, the switches 30 c, 32 c and 33 c of the threshold reference memory cell 121 c are turned on (E 1). Next, a predetermined current is caused to flow to the threshold reference memory cell 121 c from the current source 20 c (E 2). Subsequently, a voltage generated between the source 124 c and the drain 125 c is measured by the voltage measurement circuit 22 c (E 3). Since the control electrode 122 c and the drain 125 c are in the connected state, the voltage between the source 124 c and the drain 125 c is the threshold voltage (Vth) of the threshold reference memory cell 121 c.
      Next, the switches 30 b, 32 b and 33 b of the storage memory cell 131 are turned on (E 4). Next, the control electrode driver 182 applies a voltage to the control electrode 132 until a predetermined current flows from the current source 20 b to the storage memory cell 131 (E 5). That is, the voltage is applied between the control electrode 132 and the source 134. Subsequently, a potential difference (i.e., voltage, potential difference) between the source 134 and the drain 135 is measured by the voltage measurement circuit 22 b (E 6). Since the control electrode 132 and the drain 135 are in the connected state, the potential difference is the threshold voltage (Vth) of the storage memory cell 131. Note that the steps of E 1 to E 3 and the steps of E 4 to E 6 may be conducted in reversed order.
      Next, the comparator 40 compares the threshold of the threshold reference memory cell 121 c and the threshold of the storage memory cell 131 (E 7), and the threshold written in the storage memory cell 131 is verified (E 8).
      As a result of the verification, if the threshold of the storage memory cell 131 has not reached a target threshold, it is determined that additional writing is necessary (E 9: Yes), and the additional writing is executed (E 10). The additional writing is repeated until the threshold of the storage memory cell 131 reaches the target value. The additional writing can be executed by any writing method of the first to fourth embodiments. As a result of the verification, if the threshold of the storage memory cell 131 has reached the target threshold (E 9: No), the writing processing is ended (E 11).
      According to the fifth embodiment, the state of writing to the storage memory cell 131 can be known by the verification. Therefore, the additional writing can be executed, as required. As such, the writing can be further accurately executed, contributing to evenness of the writing.
      While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.
      For example, the writing to the storage memory cell 131 may be executed by writing methods of the first to fourth embodiments in any combination.

DESCRIPTION OF REFERENCE NUMBERS

10: Semiconductor storage device,

121: Memory cell

121a: First reference memory cell (Reference memory cell)

121b: Second reference memory cell (Reference memory cell)

121c: Threshold reference memory cell

122: Control electrode

122a: Control electrode (First control electrode)

122c: Control electrode (Third control electrode)

123: Floating electrode

123a: Floating electrode (First floating electrode)

123c: Third floating electrode

124: Source

124a: Source (First source)

124c: Source (Third source)

125: Drain

125a: Drain (First drain)

125c: Drain (Third drain)

131: Storage memory cell

132: Control electrode (Second control electrode)

133: Floating electrode (Second floating electrode)

134: Source (Second source)

135: Drain (Second drain)

14: Source driver

141: Source driver (First source driver)

142: Source driver (Second source driver)

16: Drain driver

161: Drain driver (First drain driver)

162: Drain driver (Second drain driver)

18: Control electrode driver

181: Control electrode driver (First control electrode driver)

182: Control electrode driver (Second control electrode driver)

20: Current source

22: Voltage measurement circuit

22b: Voltage measurement circuit (Second voltage measurement circuit)

22c: Voltage measurement circuit (Third voltage measurement circuit)

30: Switch

30b: Switch (Second switch)

30c: Switch (Third switch)

40: Comparator

10: Semiconductor storage device,

121: Memory cell

121a: First reference memory cell (Reference memory cell)

121b: Second reference memory cell (Reference memory cell)

121c: Threshold reference memory cell

122: Control electrode

122a: Control electrode (First control electrode)

122c: Control electrode (Third control electrode)

123: Floating electrode

123a: Floating electrode (First floating electrode)

123c: Third floating electrode

124: Source

124a: Source (First source)

124c: Source (Third source)

125: Drain

125a: Drain (First drain)

125c: Drain (Third drain)

131: Storage memory cell

132: Control electrode (Second control electrode)

133: Floating electrode (Second floating electrode)

134: Source (Second source)

135: Drain (Second drain)

14: Source driver

141: Source driver (First source driver)

142: Source driver (Second source driver)

16: Drain driver

161: Drain driver (First drain driver)

162: Drain driver (Second drain driver)

18: Control electrode driver

181: Control electrode driver (First control electrode driver)

182: Control electrode driver (Second control electrode driver)

20: Current source

22: Voltage measurement circuit

22b: Voltage measurement circuit (Second voltage measurement circuit)

22c: Voltage measurement circuit (Third voltage measurement circuit)

30: Switch

30b: Switch (Second switch)

30c: Switch (Third switch)

40: Comparator