(a) Drawings of the process steps: (1) a thick silicon nitride film is deposited on a silicon substrate by LPCVD, (2) AFM local oxidation is performed by applying a voltage to the sample while the tip scans squared areas, and (3) oxide etching by dipping the sample in a HF solution. The last step can be either the transference of the pattern to the silicon substrate by means of a TMAH etching (4a) or removing the silicon nitride film by dipping the sample during in a solution (4b). In this case, a hole is observed on the silicon substrate. (b) AFM images and profiles of the results of each step in the process described in (a).
Comparison of total oxide thickness dependence on voltage for LAO of bulk silicon and silicon nitride thin film for different tip velocities. For silicon, the dependence is lineal, but for the the dependence decreases for higher oxide thickness. Silicon nitride oxidizes for lower voltages and faster tip velocities than those required for silicon.
Comparison of the consumed silicon or silicon nitride (, depth of the hole after the etching) as a function of oxide height for different tip velocities. Average slopes are 1.25 for silicon nitride and 0.62 for silicon. The ratio indicates the volume expansion factor. For silicon, a 2.6 factor is obtained (2.27 is the value for the thermal grown oxide), and 1.8 for silicon nitride (1.64 for thermal grown oxide).
Comparison of oxide thickness dependence on total injected charge (measured current/tip velocity) for silicon and silicon nitride thin film for different tip velocities. Data at different velocities follow the same trend for low oxide thicknesses, but diverge for higher. For silicon nitride, two regimes appear, indicating the existence of two different phenomena/reactions that are taking place simultaneously.
SEM images of pitch lines patterned at different LAO conditions (applied voltage and tip velocity), after dipping the sample in Sioetch and in TMAH. It can be observed that for low voltages the lines are not continuous. For fast oxidation velocities, lines are straight and resolution increases.
(a) Contact mode AFM image of the oxidized pattern, just after LAO. Nanolithography conditions were for the applied voltage and for tip velocity. (b) SEM image of a fabricated stamp, after removing the oxide and transferring the pattern into the silicon substrate. Line pitch is , and feature depth is . (c) Nanoimprinted structures in PMMA, using the stamp shown in (b). Nanoimprinting conditions were and for . Resulting structures are in height, equal to the depth of the trenches of the stamp.
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