Three dimensional non-volatile storage with asymmetrical vertical select devices
View Patent ↗A three-dimensional array adapted for memory elements that reversibly change a level of electrical conductance in response to a voltage difference being applied across them. Memory elements are formed across a plurality of planes positioned different distances above a semiconductor substrate. Bit lines to which the memory elements of all planes are connected are oriented vertically from the substrate and through the plurality of planes.
1. A method of fabricating a non-volatile storage apparatus, the method comprising:
adding one or more driver devices and signal lines to a substrate;
adding a select layer above the substrate and above the one or more driver devices and the signal lines, the adding of the select layer including adding select lines and adding asymmetrical vertically oriented select devices; and
adding a monolithic three dimensional array above the select layer, the monolithic three dimensional array includes word lines and vertically oriented bit lines connected to memory elements;
wherein the vertically oriented select devices are connected to the vertically oriented bit lines, the select lines and to global bit lines;
wherein the one or more driver devices are coupled to the select layer; and
wherein the one or more signal lines extend between the driver devices.
2. The method of claim 1 , wherein the adding select lines comprises:
depositing a lower oxide layer;
depositing gate material on top of the lower oxide layer;
depositing an upper oxide layer on top of the gate material; and
etching trenches in the lower oxide layer, the gate material and the upper oxide layer to create stacks.
3. The method of claim 2 , wherein the adding vertically oriented select devices comprises:
depositing thermal oxide material;
depositing a sidewall spacer;
etching the trenches;
performing a high threshold voltage enhancement mode angled implant toward a first angle such that a first side of the stacks receive the high threshold voltage enhancement mode angled implant;
performing a depletion mode angled implant angled toward a second angle such that a second side of the stacks receive the depletion mode angled implant;
filling the trenches with p− polysilicon;
performing a n+ source implant to create a n+ region at the top of the polysilicon; and
performing a thermal anneal to create a n+ region at the bottom of the polysilicon.
4. The method of claim 3 , further comprising:
adding a n+ polysilicon layer prior to depositing the lower oxide layer, the thermal anneal activates a junction between the p− polysilicon and the n+ polysilicon layer such that the p-polysilicon has its bottom end doped with n+ to form the drains of vertically oriented select devices due to diffusion of an n+ implant from the n+ polysilicon layer.
5. The method of claim 1 , wherein the adding vertically oriented select devices comprises:
adding a n+ polysilicon layer;
etching trenches on both sides of signal lines and below locations to be occupied by vertically oriented bit lines;
performing a high threshold voltage enhancement mode angled implant toward a first angle such that a first side of select line stacks receive the high threshold voltage enhancement mode angled implant;
performing a depletion mode angled implant angled toward a second angle such that a second side of the stacks receive the depletion mode angled implant;
filling the trenches with p− polysilicon;
performing a n+ source implant to create a n+ region at the top of the polysilicon; and
performing a thermal anneal to create a n+ region at the bottom of the polysilicon, the vertically oriented bit lines are added above the polysilicon after performing the thermal anneal.
6. The method of claim 1 , wherein the adding vertically oriented select devices comprises:
etching trenches on both sides of signal lines and below locations to be occupied by vertically oriented bit lines;
performing a high threshold voltage enhancement mode angled implant toward a first angle such that a first side of select line stacks receive the high threshold voltage enhancement mode angled implant;
performing a depletion mode angled implant angled toward a second angle such that a second side of the stacks receive the depletion mode angled implant; and
filling the trenches with polysilicon.