(Color online) Schematic of the six-transistors SRAM structure. (a) The planar view of a cell and (b) the cross section of the SRAM structure.
(Color online) Schematic diagram illustrating the interaction of tip with the sample surface during C-AFM scanning. (a) Tungsten contact is partially buried by the dielectric where the scanning tip is blocked by the edges of the dielectric. (b) Over-etched ILD results in apparent short of neighboring contacts by the tip. (c) The optimally etched ILD which is able to provide a well resolved C-AFM image.
(Color online) AFM image of the 20 nm SRAM sample when it has just been polished to the contact layer without undergoing any surface treatment. (a) The topographical image and (b) the current image. The tungsten contact is too deeply recessed (∼3.2 nm) from the dielectric surface at which the C-AFM tip could not establish a good contact with the tungsten component, resulting in a poorly resolved current image.
(Color online) Geometrical model describing the maximum allowable recessed depth beyond which the tip would physically fail to sense the current signal from the tungsten contact.
(Color online) Geometrical model describing the maximum allowable protruding height of the tungsten contacts beyond which the scanning tip would touch two adjacent tungsten contacts creating an apparent short.
(Color online) Current images of the tungsten at four different heights with reference to the dielectric surface: (a) excessive protruding height of 4 nm where tall tungsten contacts are apparently shorted together by the C-AFM tip, (b) less excessive protruding height of 3.5 nm at which current image resolution starts to improve, (c) optimum protruding height of 3 nm, and (d) optimum receding depth of ∼1.3 nm. The experiment agrees with the theoretical model that a well-resolved current image could be obtained if the tungsten contact protrudes not more than 3 nm, or recede not deeper than 1.4 nm from the dielectric surface.
(Color online) C-AFM and TEM images of the 20 nm SRAM. (a) The C-AFM image with the abnormal component (highlighted in rectangle) identified to be at the ground terminal, which is also the source terminal of the pull down transistor. (b) The cross section TEM image of the failed contacts. Ni diffusion was identified to be the cause of failure.
(Color online) Current–voltage characteristics of the source (gnd) terminal of the pull-down transistor. The good reference node shows a diode characteristic while the faulty node with diffused Ni shows a much smaller breakdown voltage.
(Color online) C-AFM and TEM images of the 20 nm SRAM. (a) The C-AFM image of the site with missing current signal (pointed by the arrow) and the abnormal component was identified to be the Vdd (or the source terminal of the pull-up transistor). (b) The TEM image of the failed contacts where a void exists between the tungsten contact interface.
(Color online) Current–voltage characteristics of the Vdd terminal of the pull-up transistor. The reference node shows a good diode characteristic while the faulty node with a void at the contact interface shows poorer conductivity due to restricted current passage when the same voltage is being applied.
(Color online) C-AFM and TEM images of the 20 nm SRAM. (a) The C-AFM image where the abnormal component (represented by the rectangle) is identified to be the word line (or the gate terminal) of the pass gate transistor. (b) The TEM image of the failed contacts where tungsten is filled into the gate (highlighted by arrow) and there is a misalignment between the gate and its neighboring terminal.
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