SRAM with improved program and sensing margin for scaled nanosheet devices
An integrated circuit structure includes a memory cell and multiple transistors therein. The multiple transistors are formed using channels including a stack having alternating layers of conductive semiconductor material and layers of other material that are insulative. Two or more of the multiple transistors have a same number of layers of the conductive semiconductor material in corresponding channel regions but have different numbers of active layers and inactive layers of the conductive semiconductor material. An active layer is a layer forming a channel in the channel region that is electrically coupled to S/D regions in a corresponding transistor, while a floating layer is a layer in the channel region electrically isolated from the S/D regions in the corresponding transistor. Methods for forming the integrated circuit structure are disclosed.
1 . An integrated circuit structure, comprising:
a memory cell; and
multiple transistors forming at least part of the memory cell, the multiple transistors formed using channels comprising a stack, the stack comprising alternating layers of conductive semiconductor material and layers of other material that are insulative, wherein two or more of the multiple transistors have a same number of layers of the conductive semiconductor material in corresponding channel regions but have different numbers of active layers and inactive layers of the conductive semiconductor material, where an active layer is a layer of the conductive semiconductor material forming a channel in the channel region that is electrically coupled to source and drain regions in a corresponding transistor, while a floating layer is a layer of the conductive semiconductor material in the channel region electrically isolated from the source and drain regions in the corresponding transistor.
2 . The integrated circuit structure according to claim 1 , wherein:
the integrated circuit structure comprises a substrate;
the memory cell is formed on the substrate;
the multiple transistors comprise a first transistor comprising a first source region on the substrate, a first drain region on the substrate, a first channel region extending laterally between the first source region and the first drain region, and a gate wrapping around at least part of the channel region, wherein the first transistor has at least one floating layer formed because the corresponding at least one layer of conductive semiconductor material has an insulator on first and second lateral surfaces of the at least one layer, the insulator electrically isolating the at least one floating layer from material that abuts the isolator.
3 . The integrated circuit structure according to claim 2 , wherein the multiple transistors comprise:
a second transistor comprising a second source region on the substrate, a second drain region on the substrate, a second channel region extending laterally between the second source region and the second drain region, and a gate wrapping around at least part of the second channel region, wherein all of the sheets of semiconductor material in the second channel region electrically couple to the second source region and second drain region.
4 . The integrated circuit structure according to claim 3 , wherein:
the at least one floating sheet of the first transistor is a single floating sheet; and
the multiple transistors comprise a third transistor comprising a third source region on the substrate, a third drain region on the substrate, a third channel region extending laterally between the third source region and the third drain region, and a gate wrapping around at least part of the third channel region, wherein the third transistor has multiple floating sheets formed because corresponding multiple layers of conductive semiconductor material have an insulator on first and second lateral surfaces of the multiple layers of semiconductor material, the insulator electrically isolating the at least one floating layer from material that abuts the isolator.
5 . The integrated circuit structure according to claim 4 , wherein the memory cell comprises a static random access memory cell, the first transistor is an N-type pass-gate transistor, the second transistor is a P-type pull up transistor, and the third transistor is an N-type pull down transistor.
6 . The integrated circuit structure according to claim 4 , wherein inactive sheets in the multiple transistors are farther away from the top surface of the substrate than are active sheets in the multiple transistors.
7 . The integrated circuit structure according to claim 6 , wherein the third transistor has two floating sheets, wherein the conductive semiconductor material in the two floating sheets of the third transistor have different widths between corresponding first and second lateral surfaces.
8 . The integrated circuit structure according to claim 6 , wherein source regions and drain regions for the multiple transistors have a top surface far enough away from the top surface to cover the active sheets but not far enough away cover the inactive sheets.
9 . The integrated circuit structure according to claim 2 , wherein the layers in a corresponding channel region of conductive semiconductor material are parallel to a top surface of the substrate, physically separated by the other material, and stacked vertically away from the top surface of the substrate.
10 . The integrated circuit structure according to claim 2 , wherein the insulator comprises silicon nitride, SiBCN or SiOCN.
11 . The integrated circuit structure according to claim 1 , wherein the active and inactive layers are formed from one of nanosheets or nanowires.
12 . A method, comprising:
forming a memory cell on a substrate; and
forming multiple transistors that form at least part of the memory cell, the forming the multiple transistors comprising:
forming and patterning a stack, the stack comprising alternating layers of conductive semiconductor material and layers of other material that are insulative;
forming channel regions comprising the stack;
forming source and drain regions on opposing sides of the channel regions,
wherein two or more of the multiple transistors have a same number of layers of the conductive semiconductor material in corresponding channel regions but have different numbers of active layers and inactive layers of the conductive semiconductor material, where an active layer is a layer of the conductive semiconductor material forming a channel in the channel region that is electrically coupled to source and drain regions in a corresponding transistor, while a floating layer is a layer of the conductive semiconductor material in the channel region electrically isolated from the source and drain regions in the corresponding transistor.
13 . The method according to claim 12 , wherein:
forming the multiple transistors comprises forming a first transistor comprising a first source region formed on the substrate, a first drain region formed on the substrate, a first channel region extending laterally between the first source region and the first drain region, and forming the first transistor comprises:
forming a gate wrapping around at least part of the channel region; and
prior to forming the first source region and first drain region and while forming the first channel region, forming at least one floating layer comprising performing an optical planarization layer coating and lithography to cover sides of fins comprising the stack, performing an optical planarization layer recess to cover active layers but leave the at least one floating layer uncovered by the optical planarization layer, performing a dent of a layer of the semiconductor material in the at least one floating layer, and forming a spacer deposition of insulator to cover sidewalls of the fins including first and second lateral surfaces of the dented layer; and
forming the first source region and second source region having sides contacting sides of the fins including sides of the active layers.
14 . The method according to claim 13 , wherein forming the multiple transistors comprises forming a second transistor comprising a second source region on the substrate, a second drain region on the substrate, a second channel region extending laterally between the second source region and the second drain region, and a gate wrapping around at least part of the second channel region, wherein all of the sheets of semiconductor material in the second channel region electrically couple to the second source region and second drain region.
15 . The method according to claim 14 , wherein:
the at least one floating sheet of the first transistor is a single floating sheet; and
forming the multiple transistors comprises forming a third transistor comprising a third source region on the substrate, a third drain region on the substrate, and a third channel region extending laterally between the third source region and the third drain region, the forming the third transistor comprising:
forming a gate wrapping around at least part of the third channel region; and
prior to forming the first source region and third drain region and while forming the third channel region, forming two floating layers comprising performing a first lithographic process comprising an optical planarization layer coating and lithography to cover sides of fins comprising the stack, performing an optical planarization layer recess to cover active layers but leave a first floating layer uncovered by the optical planarization layer, performing a dent of the layer of the semiconductor material in the first floating layer, performing a second lithographic process comprising an optical planarization layer coating and lithography to cover sides of fins comprising the stack, performing an optical planarization layer recess to cover active sheets but leave the first floating layer and a second floating layer uncovered by the optical planarization layer, performing a dent of the layers of the semiconductor material in the first and second floating layers, and forming a spacer deposition of insulator to cover sidewalls of the fins including first and second lateral surfaces the dented sheets; and
forming the third source region and third source region having sides contacting sides of the fins including sides of the active layers.
16 . The method according to claim 15 , wherein a lithography is performed with a mask having open areas for the first transistor and second transistor and having closed areas elsewhere to form the single floating layer of the first transistor and the dent of the first floating layer of the third transistor, and wherein another lithography is performed with a mask having open areas for the second transistor and closed areas elsewhere to form the dent of the first and second floating layers of the third transistor.
17 . The method according to claim 15 , wherein the memory cell comprises a static random access memory cell, wherein forming the first transistor forms an N-type pass-gate transistor, wherein forming the second transistor forms a P-type pull up transistor, and wherein forming the third transistor forms an N-type pull down transistor.
18 . The method according to claim 16 , wherein source regions and drain regions for the multiple transistors have a top surface far enough away from the top surface to cover the active sheets but not far enough away cover the floating sheets.
19 . The method according to claim 15 , wherein the memory cell comprises a static random access memory cell having first, second, and third transistors, wherein forming the first transistor forms an N-type pass-gate transistor having a single floating layer, wherein forming the second transistor forms a P-type pull up transistor having two floating layers, and wherein forming the third transistor forms an N-type pull down transistor with no floating layers and all active layers, and wherein the forming the first, second, and third transistors is performed using lithography with a mask having open areas for the first transistor and second transistor and having closed areas elsewhere to form in part the single floating layer of the first transistor and form in part the first floating layer of the third transistor, and wherein another lithography is performed with a mask having open areas for the second transistor and closed areas elsewhere to form in part the first and second floating layers of the third transistor.
20 . The method according to claim 12 , wherein the active and inactive layers are formed from one of nanosheets or nanowires.