Nano Hotplate Fabrication for Metal Oxide-Based Gas Sensors by Combining Electron Beam and Focused Ion Beam Lithography

Abstract

Metal oxide semiconductor (MOS) gas sensors are widely used for gas detection. Typically, the hotplate element is the key component in MOS gas sensors which provide a proper and tunable operation temperature. However, the low power efficiency of the standard hotplates greatly limits the portable application of MOS gas sensors. The miniaturization of the hotplate geometry is one of the most effective methods used to reduce its power consumption. In this work, a new method is presented, combining electron beam lithography (EBL) and focused ion beam (FIB) technologies to obtain low power consumption. EBL is used to define the low-resolution section of the electrode, and FIB technology is utilized to pattern the high-resolution part. Different Au⁺⁺ ion fluences in FIBs are tested in different milling strategies. The resulting devices are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS). Furthermore, the electrical resistance of the hotplate is measured at different voltages, and the operational temperature is calculated based on the Pt temperature coefficient of resistance value. In addition, the thermal heater and electrical stability is studied at different temperatures for 110 h. Finally, the implementation of the fabricated hotplate in ZnO gas sensors is investigated using ethanol at 250 °C.

Keywords: electron beam lithography; focused ion beam; gas sensors; ion beam lithography; nano heaters; power consumption.

Figure 1

Layout design of the nano-heater structure. (a) Layout of electrodes and contacts for EBL patterning; (b) Layout of the heating element for FIB patterning.

Figure 2

Schematic diagram of the fabrication process: (a) ONO stack layer preparation; (b) EBL exposure and development; (c) Metallic layer deposition (Pt and Ti); (d) Metal lift-off; (e) Electrode calcination and metallization.

Figure 3

The FIB milling process: (a) Substrate after EBL patterning and metal layer deposition; (b) Fine FIB milling of the heating circuit part; (c) Electrode calcination and metallization.

Figure 4

Upper, optical microscope image of a full device defined by EBL. Lower, SEM images of electrode circuits patterned by EBL at 30 keV with different electron doses of (a) 750 μC/cm²; (b) 800 μC/cm² and (c) 850 μC/cm².

Figure 5

Silicon and gold ion doses versus ion energies for a 100 nm Pt film milling.

Figure 6

Schematic diagram of the FIB milling process.

Figure 7

AFM images of squares milled on Pt film by Au⁺⁺ ion beam at fluences of 60, 75 and 90 kμC/cm² in (a) a 1-loop mode and (b) a 10-loop mode at a 70 keV energy.

Figure 8

AFM results of 70 keV Au⁺⁺ ion beam milling on Pt film with different doses and loops: (a) Milling depths; (b) Average roughness.

Figure 9

Au element distribution and Si ion intensities after Au⁺⁺ milling by FIB on samples with different doses at (a) 1 loop and (b) 10 loops.

Figure 10

(a) Results of the FIB Au ion milling of the NHP by applying 60, 75 and 90 kμC/cm² Au⁺⁺ doses at 35 kV (left to right) in a 10-loop; (b) The corresponding electrode circuit magnified images at different doses.

Figure 11

(a) The relationship between input voltage with working temperature and power consumption; (b) The stability of the NHP at different input voltages over 110 h.

Figure 12

The response of the ZnO sensing film deposited on the NHP (as shown in the inset) at 5 V and 250 °C towards the target gas of ethanol at concentrations of 5 ppm, 10 ppm and 20 ppm.