# Microheater Topology for Advanced Gas Sensor Applications with Carbyne-Enriched Nanomaterials
## 2 Materials and Methods  
Lithium niobate (LiNbO\({}_{3}\)) substrates were used to construct the SAW structures. This crystal is distinguished by the strong temperature stability of the coefficient of electromechanical coupling from 20 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, and the changes in this coefficient, in this case, are 1.8% [17; 18]. The substrates were cleaned in an ultrasonic bath with a solution consisting of isopropyl alcohol, acetone, and deionized water in a ratio of 1:1:3. Palladium thin film with a thickness of 280 nm was deposited via vacuum sputtering at an argon pressure of 2 \(\times\) 10\({}^{-2}\) Torr and sputtering voltage of 1.5 kV. Photolithographic patterning was conducted with a positive photoresist ma-P1215 (Micro resist technology) and wet chemical etching in heated mixture of nitric acid and hydrochloric acid (in a mixing ratio of 1:3). For the microheaters, reverse photolithography (lift-off) process was applied to avoid second chemical etching and reduce the quality of interdigitated (IDT) fingers of the SAW topology. A platinum film with a thickness of 280 nm was vacuum-sputtered on the backside of the substrates at 2.10\({}^{-2}\) Torr, and a sputtering voltage of 0.9 kV was grown over the photoresistive topologies, determined by three different geometrical parameters of the meander heaters (Figure 1a-c). The heaters were designed in COMSOL Multiphysics 5.4. (details in Supplementary Material--Figure S1a-d) with an appropriate width (in the range 300-450 \(\upmu\)m for the different topologies), length (1.2 mm for all samples), thickness (280 nm for all samples), and number of wires (1, 3 and 5), in order to provide maximal heating temperature of 80 \({}^{\circ}\)C, taking into account that the adsorbed organic analytes are volatile and greater temperature is not necessary. Regarding the selection of the microheater contour shape, the following considerations were taken into account: for a spiral microheater, it is known from previous research that this configuration has a constant heating power per active area, which induces a hot spot in the heater center [19]. A spiral configuration, specifically the square one, consists of elongated straight tracks with numerous 90\({}^{\circ}\) turns. Previous studies have effectively showcased the capability to adjust the heating power within the active area by modifying the linewidth or the spacing or adjusting both the linewidth and spacing simultaneously [20], which is a complex approach. It has been proven that the spiral layout is only suitable for circular or square substrates. In contrast, for a rectangular substrate, a serpentine (or meander) heater is more compatible with the substrate's profile. Even for a square substrate, the modified serpentine heater can achieve a larger isothermal region than an optimized spiral heater [21]. Spiral microheaters can lead to greater lateral heat dispersion beyond the sensing area due to their radial nature. This heat diffusion could potentially interfere with the accuracy and efficiency of the sensing mechanism, leading to diminished sensor performance [22]. Managing and minimizing radial heat distribution is therefore crucial for maintaining the optimal performance and longevity of SAW gas sensor systems. Afterwards, carbyne-enriched gas sensing film was deposited on the microheaters and the IDT electrodes via pulsed-plasma deposition through ion-assisted stimulation with specific energy levels. The distance between the target and the substrates was 1 m; the number of carbon plasma pulses was 3000; the voltage for an arc discharge ignited between the main discharge cathode holding the source of carbon and the main discharge anode holding the substrate was 300 V; the charge of the main capacitor that formed pulses of carbon plasma was 2000 uf at 5 Hz; and the Ar-ion plasma power was 150 W. The thickness of the carbyne-enriched layer was 150 nm.