End-of-life reliability for non-volatile memory cells

1. A memory chip comprising:

a memory array comprising a multiplicity of memory cells connected in rows by word lines and in columns by bit lines; and

a two-dimensional sensing system to move a read point two-dimensionally within a two-dimensional current and voltage read space as said two-dimensional read space shrinks and shifts over the life of said chip, said sensing system comprising a word line control unit to maintain a predefined fixed DC current level for reference ones of said cells during sensing at all points in said life of said chip,

and wherein said word line control unit comprises a feedback unit to generate a word line voltage source for said word lines as a function of the difference of a fixed DC voltage and a changing average reference voltage from said reference cells.

2. The memory chip according to claim 1 and wherein said feedback unit comprises an integrator to integrate the output of said word line voltage source.

3. A method for operating a memory chip, the method comprising:

two-dimensionally moving a read point of a memory array within a two-dimensional current and voltage read space as said two-dimensional read space shrinks and shifts over the life of said chip,

wherein said two-dimensional read space is comprised within a space defined by lowest reaction lines within said two-dimensional read space for said memory cells when erased and by highest reaction lines when programmed,

wherein said moving comprises:

maintaining a predefined fixed DC current level for reference ones of said cells;

generating a word line voltage for word lines of said memory as a function of the difference of a fixed DC voltage and a changing average reference voltage from said reference cells, and

sensing an average current of said reference cells.

4. The method according to claim 3 and wherein said generating comprises integrating the output of said word line voltage.