Vertical memory device with bit line air gap
A structure includes a three-dimensional semiconductor device including a plurality of unit device structures located over a substrate. Each of the unit device structures includes a semiconductor channel including at least a portion extending vertically along a direction perpendicular to a top surface of the substrate, and a drain region contacting a top end of the semiconductor channel. The structure also includes a combination of a plurality of contact pillars and a contiguous volume that laterally surrounds the plurality of contact pillars. The plurality of contact pillars is in contact with the drain regions, and the contiguous volume has a dielectric constant less than 3.9.
1. A monolithic three dimensional NAND string, comprising:
a plurality of semiconductor channels, at least one end portion of each semiconductor channel extending substantially perpendicular to a top surface of a substrate;
a plurality of drain regions, wherein each drain region contacts a top end of a respective one of the plurality of semiconductor channels;
a plurality of control gate electrodes extending substantially parallel to a top surface of a substrate, wherein the plurality of control gate electrodes comprises at least a first control gate electrode located in a first device level and a second control gate electrode located in a second device level located over the major surface of the substrate and below the first device level;
a plurality of interlevel insulator layers located between the plurality of control gate electrodes, such that a first interlevel insulator layer is located between the first control gate electrode and the second control gate electrode;
a blocking dielectric located in contact with the plurality of control gate electrodes;
at least one charge storage region located in contact with the blocking dielectric;
a tunnel dielectric located between the at least one charge storage region and the semiconductor channel;
a plurality of bit lines; and
a plurality of electrical contacts, wherein each physically contacts a top surface of a respective one of the plurality of drain regions and is electrically connected to a respective one of the plurality of bit lines;
wherein a cavity or a low-k dielectric layer having a dielectric constant below 3.9 is located among the plurality of electrical contacts and above a horizontal plane including the top surfaces of the drain regions;
wherein: each of the plurality of electrical contacts comprises a lower contact pillar portion which contacts the respective one of the plurality of drain regions and an upper via contact portion which contacts a respective one of the plurality of bit lines; and
the lower contact pillar portion has a larger width or diameter than the upper via contact portion; and
wherein the cavity which comprises an air gap is located between the plurality of electrical contacts.
2. The NAND string of claim 1 , wherein the lower contact pillar portion comprises a cylindrical or inverted frustum shaped pillar, and the upper via contact portion comprises a cylindrical or inverted truncated cone shaped pillar which has a smaller average diameter than the contact pillar.
3. The NAND string of claim 1 , wherein the upper via contact portion has a first lateral dimension along a first horizontal direction that is parallel to an overlying bit line, and has a second lateral dimension along a second horizontal direction that that is perpendicular to the overlying bit line, the first lateral dimension being greater than the second lateral dimension.
4. The NAND string of claim 1 , wherein a horizontal cross section of the upper via contact portion has a first eccentricity which is greater than an eccentricity of a horizontal cross section of the lower contact pillar portion.
5. The NAND string of claim 1 , wherein the low-k dielectric layer having a dielectric constant below 3.9 is located between the plurality of electrical contacts.
6. The NAND string of claim 1 , wherein each of the plurality of electrical contacts has a substantially cylindrical shape or an inverted frustum shape.