Self-assembled nanostructures via nanosphere lithography and their application to gas sensors

Abstract

In this master’s thesis, we explored the potential gas sensors application of self-assembled nanomesh structure via polystyrene nanosphere lithography (NSL). The PS-NSL method was explored on both rigid silicon substrate. Oxygen ($O_2$) plasma treatment was used to increase the surface wettability of substrate to make a larger coverage and more uniform coating of PS beads. $O_2$ RIE can effectively etch PS bead controlling the size of PS beads while making gaps among individual beads. Those shrunk beads serve as the shadow mask while depositing the desired materials. After the removal of PS beads through mild DI sonication bath, a honeycomb structure with minimum dimension lower than 100nm was achieved. Two types of device were fabricated to investigate the hydrogen sensing performance- Pd-decorated silicon honeycomb structure and Pd honeycomb structure. Silicon thin film transistor with palladium (Pd) decorated was fabricated through the conventional complementary metal-oxide-semiconductor (CMOS) process. Buffered oxide etchant (BOE) was used to clean the silicon surface before Pd deposition via electron beam evaporation. A clear raise of electrical resistance was observed after relative long time (3min~20min) BOE treatment. Meanwhile that caused an dramatic improvement of hydrogen sensing performance after decoration of Pd. Hydrogen sensitivity of the sensors increased from 0min, 3min to 5min BOE treatment, and when BOE etching time was over 5min the sensors showed similar sensitivity. Sensors with 3min and 5min BOE treatment showed faster response compared with other samples, while sensors with 5min and 10min BOE treatment showed quicker recovery after exposure to hydrogen gas. Overall, device after 5min BOE treatment was found to have the best complex performance- high sensitivity, fast response and quick recovery. Based on the thin film device structure, nanomesh pattern was applied at the channel, making the thin film a porous-honeycomb-like structure. The minimum feature among the silicon honeycomb-like nanomesh structure obtained was 40 to 80nm. As a result, the silicon honeycomb nanostructure was fabricated with minimum dimension of 80 nm. The small dimension enlarged the Pd-gating-effect and thus improved the sensitivity. By using nanosphere lithography, high-resolution lithography process was avoided and cost was reduced. 2 mins BOE treatment made the Si nanomesh surface rougher and induced a clear increase of the sensitivity. Additionally, the sensor with Pd-decorated BOE-treated Si nanomesh channel structure shows excellent selectivity among the common interfering gases and low gas sensing degradation with up to 80% RH environment condition. Compared with other conventional Si-based hydrogen sensors, the honeycomb hydrogen sensors was fabricated through a low cost process and showed better performance. And those two simple strategies of NSL nanopattern and BOE treated rough surface for sensitivity enhanment, shown in this paper are applicable to other Si-based chemical- and bio-sensors.