Boundary Word Line Search and Open Block Read Methods with Reduced Read Disturb

A system, comprising:

• an array of non-volatile memory cells having a plurality of blocks in which memory cells of a block are formed along a plurality of word lines, including a first word line, and in which the word lines of a the block are written sequentially from a first end to a second end of the block;
• and
• a controller for the array of non-volatile memory cells, the controller performing a write operation on the first word line of the block and performing a post-write read operation for the first word line
• in response to a read command indicating unwritten word lines of the block, the post-write read operation including:
• applying a first sensing voltage along the first word line;
• applying a first non-selected word line read voltage along word lines between the first word line and the first end of the block;
• and
• applying a second non-selected word line read voltage along one or more of the indicated unwritten word lines between the first word line and the second end of the block, wherein the second non-selected word line read voltage is less than the first non-selected word line read voltage.

An apparatus comprising:

• a controller for a semiconductor memory device, the controller configured to:
• locate a last written word line in a partially written block of the semiconductor memory device by biasing word lines of different zones of the block to different read voltages and determining whether a number of non-conducting strings of the block satisfy a criterion;
• apply a first non-selected word line read voltage to one or more word lines between a first end of the partially written block and the last written word line in a read operation for the partially written block;
• and
• apply a second non-selected word line read voltage to one or more word lines between the last written word line and a second end of the partially written block in the read operation, the second non-selected word line read voltage being less than the first non-selected word line read voltage.