A novel technological approach to a nano-on-micro fabrication process: case study and perspectives

The demand of production processes for the parallel functionalization of MEMS batches with nanomaterials, compatible with planar technology of silicon, is mostly unsolved. Here we present a new method for the direct integration of nanostructured oxide layers on micromachined silicon platform batches. In order to validate the proposed technological approach, a case-study was investigated: functionalization of silicon micro- hotplates [1] with nanostructured WO3, for large-scale production of chemical microsensors. Nanostructured oxide layers were produced by supersonic cluster beam deposition (SCBD), using a pulsed microplasma cluster source (PMCS) [2]. Fig. 1 shows a scheme of the deposition apparatus. This preserves the original cluster structure, promotes a good adhesion of the resulting film, and avoids any significant damage or heating of the substrate. By combining PMCS and aerodynamic lenses, a highly collimated and intense cluster beam, suitable for hard mask patterning, can be obtained. Process cleanness and delicacy, lateral resolution, and batch uniformity are the main benefits arising from this approach. PMCS allows the production of several oxides typically used in chemoresistive sensors, such SnO2, TiO2, WO3, FeOx, MoOx, NbOx, ZnO, and PdOx. The as-deposited films have an amorphous and porous structure at the nanoscales. After annealing at 400 �C (beyond the typical operating temperatures of chemoresistive sensors), the amorphous grains rearrange into crystalline, while limited grain growth preserves the nanostructure and porosity. These general features hold for most of the listed oxides. Batches of 100 micro-hotplates arranged in a 10x10 array wafer were used as deposition substrates. The single device measures 2x2 mm2, and contains a 1x1 mm2 suspended membrane with 1.2 �m thickness. Auto-aligning micromachined silicon masks (Fig. 2) were designed and fabricated to match micro-hotplates wafer and provide patterned deposition on each single device (Fig. 3). Sensing properties of WO3 functionalized micro-hotplates were characterized respect to various oxidizing and reducing species, as shown in Fig. 4 in the case of ethanol. These measurements suggest a detection limit in the 10-100 ppb range, linearity up to several tens of ppm, and fast response and recovery times. Microsensors operate at temperatures in the range 200-300 �C spending as low as 10-20 mW of heating power. With the same approach we also functionalized micromachined platforms where chemical sensing action is coupled to a physical measurement (such as pressure, temperature or flow), obtaining a hybrid physico-chemical device. In conclusion, we showed a gas-phase deposition method, coupling micromachined hard masks and SCBD/PMCS, to pattern nanostructured oxides onto micro-hotplates wafer for batch production of chemical microsensors. The functionality of the final devices was demonstrated. Due to its generality, this method discloses new stimulating perspectives in the use of gas-phase cluster beam deposition for the direct and parallel integration of nanomaterials in MEMS, spreading from chemical microsensors, to hybrid physico-chemical sensors, to cantilevers-based array biosensors, to microfluidic systems, to microdevices requiring gas gettering parts etc.