Demonstration of Resistive Pt Temperature Sensors Directly Printed Using Novel Atomic-Layer 3D Printer

Abstract

This case study demonstrates resistive Pt temperature sensors fabricated by a novel Atomic Layer 3D-printer, a revolutionary rapid prototyping tool utilizing a combination of atomic layer deposition (ALD), microfluidics and high precision 3D printing [1], first time introduced at AVS ALD 2020 [2], and also discussed in detail in the ECS 2021 G01 symposium [3]. The printed sensors are characterized and compared to a thin film sensor made using conventional (physical vapor deposition, lithography) methods and also to a Pt100 standard.ALD works like a chemical vapor deposition (CVD) but the precursors react with the substrate one at a time, in a sequential, self-limiting manner, offering thickness control on the level of atomic layers. Combining ALD and 3D printing, the Atomic Layer 3D-printer allows for deposition of thin films with controllable thickness in the sub-nm range in a pre-programmed X-Y pattern, with no need for lithography or additional patterning, on a wide variety of substrates.The printer nozzle moves relative to the substrate in a highly controlled manner (in this case ~2 mm/s). The nozzle is a miniature spatial ALD system where the precursor (MeCpPtMe3 in this case) flows out of the center of the nozzle, surrounded by a concentric rings of vacuum and a reactant gas, in this case ozone (O3). The result is an area-selective ALD, where each nozzle pass over the substrate equals to one ALD cycle, with a typical deposition rate of ~0.9 nm/cycle.In this study, resistive Pt temperature sensors were fabricated on the SiO2/Si substrates by printing >2 mm long Pt-wires of different thicknesses (approx. range 10-40 nm thick) using 100-500 ALD cycles at 200, 225 and 250 °C, where the width of the wires (~400 µm) was defined by the used nozzle geometry (Fig. 1 shows the fabrication flow).Morphology studies of the Pt surface by scanning electron microscopy (Fig. 2) revealed that the film growth in the initial stages follows island-like nucleation, eventually becoming progressively denser with increasing number of ALD cycles. The Pt wire deposited using 200 cycles (nozzle passes) consist of interconnected network of Pt grains, while the one made using 400 cycles (~ 30 nm thick) consists of a continuous polycrystalline Pt film. This growth mechanism is typical for this thermal ALD (Pt+O3) process [4].Sensors were annealed at 600 °C in N2 to ensure temperature stability, and equipped with Au contact pads 2 mm apart (made by e-beam PVD through a shadow mask) for electrical characterization. The resistance was measured in the 25-400 °C range (selected characteristics shown in Fig. 3a), as well as in cryogenic temperatures down to 3 K (not shown here).Interestingly, higher temperature sensitivity S (Fig. 3b) was found for samples made by 200 ALD cycles, with a morphology of a network of interconnected Pt grains.The comparison of temperature coefficients of resistivity α (normalized sensitivity, Fig. 3c) clearly shown that the printed ALD Pt demonstrate better temperature sensing characteristics than the conventional 30 nm e-beam PVD Pt thin film, and is comparable also to the Pt100 standard.We will also discuss sensors printed on corrugated surfaces, i.e. black Si and Si gratings.The resistive Pt temperature sensors demonstrated in this study fabricated by a novel Atomic Layer 3D-printer are suitable for rapid printing in small-footprint areas where temperature sensing is required. Targeted, low temperature deposition with no need for lithographic masks or additional patterning paves the way for myriads of applications, e.g. on a chip, or in a battery. The characteristics of these sensors are shown to be comparable to a thin film sensor fabricated using conventional processing, as well as a Pt100 standard, thanks to high film quality delivered by the ALD process.We acknowledge the support of the H2020-EU ATOPLOT project (grant ID: 950785).