Nonvolatile semiconductor memory device

A nonvolatile semiconductor memory device, comprising:

• a plurality of memory cells, each of which has a floating gate and a threshold voltage level that depends on a quantity of charge accumulated at said floating gate;
• word lines connected to said memory cells at respective control gates thereof;
• a bit line to which source and drain paths of said memory cells are connected;
• a sense amplifier that is connected to said bit line;
• a data latch circuit that is connected to said bit line and holds data to be programmed into said memory cells;
• and
• a word line voltage driver that generates a word line voltage that is applied to selected word lines connected to memory cells for which programming or verification is carried out;
• wherein said threshold voltage level of each of said memory cells is set in one of first, second, third, and fourth states of threshold voltage, depending on the data held by said data latch circuit, said second state being placed between said first state and said third states and said third state being placed between said second state and said fourth state;
• said word line voltage driver generates a first word line voltage on said selected word lines, selects memory cells to enter said fourth state, depending on the data held by said data latch circuit, and allows said sense amplifier to verify whether the memory cells selected to enter said fourth state have been programmed, according to energized or de-energized states thereof;
• programming is executed for insufficiently programmed memory cells among the memory cells selected to enter said fourth state;
• after completion of programming into the memory cells selected to enter said fourth state, said word line voltage driver generates a second word line voltage on said selected word lines, selects memory cells to enter said second state, depending on the data held by said data latch circuit, and allows said sense amplifier to verify whether the memory cells selected to enter said second state have been programmed, according to energized or de-energized states thereof;
• said word line voltage driver changes the voltage applied to the selected word lines substantially stepwise from said second word line voltage to a third word line voltage, selects memory cells to enter said third state, depending on the data held by said data latch circuit, and allows said sense amplifier to verify whether the memory cells selected to enter said third state have been programmed, according to energized or de-energized states thereof;
• and
• said third word line voltage is higher than said second word line voltage and said first word line voltage is higher than said second word line voltage and said third word line voltage.

A nonvolatile semiconductor memory device, comprising:

• a plurality of memory cells, each of which has a floating gate and a threshold voltage level that depends on a quantity of charge accumulated at said floating gate;
• word lines connected to said memory cells at respective control gates thereof;
• and
• a bit line to which source and drain paths of said memory cells are connected;
• wherein said threshold level of each of said memory cells is to be set in one of first, second, third, and fourth states of threshold voltage, said second state being placed between said first state and said third state, and said third state being placed between said second state and said fourth state;
• first programming is executed to set the threshold levels of selected memory cells among at least one of said second, third, and fourth states, depending on multilevel data to be programmed into the selected memory cells;
• verification is carried out to verify whether said memory cells for which said first programming has been executed have been programmed to the corresponding threshold voltage states and to distinguish insufficiently programmed memory cells;
• and
• second programming is executed to set the threshold levels of insufficiently programmed memory cells according to the multilevel data to be programmed therein.