Scalable transparent metal mesh heater on flexible substrate prepared using reverse-offset printed seed layer

We demonstrate a manufacturing process for a transparent metal mesh heater using reverse offset printing (ROP), metal lift-off and Ni electroplating. All used methods are scalable and compatible with high-throughput roll-to-roll (R2R) fabrication. First, a Cu-mesh seed layer is produced by ROP printing of a ∼ 70 nm thick semi-dry poly 4-vinylphenol (PVPh) ink on polyethylene terephthalate (PET) substrate and followed by metal evaporation and a lift-off process using the patterned PVPh as a mask. After this, the thickness of the relatively thin Cu-mesh (40 nm) is increased using a Ni-electroplating process to achieve the desired resistivity (< 3 Ω/□) and metal thickness (1–5 μm) of the heater. Optical transparency of the metal heater is achieved by suitable design and a low linewidth (2 μm) of the ROP/lift-off patterned seed Cu-mesh. The performance of the transparent heater is evaluated e.g. by using a figure of merit (FoM) value and compared against indium tin oxide (ITO) based heaters on PET substrate with a sheet resistance of 50 Ω/□. The FoM value is ∼400 for 4.5 cm × 4.5 cm size heaters, compared to ∼50 for the reference ITO. Four different sized heaters are fabricated and tested with a constant voltage until the temperature of the heater is saturated, yielding >260 °C∙cm²/W thermal resistance, thus much higher than 68 °C∙cm²/W obtained for the ITO reference. The measured optical transparency of ∼74 % of the metal heater is close to the calculated transparency of 77 %. The transparency of the metal mesh is impacted by the relatively high ∼15 % haze, which is probably due to the high surface roughness of Ni. For ITO reference, the transparency and haze are ∼85 % and 1 %, respectively. The operation of the metal mesh heater is demonstrated in a defogging test where water vapor was removed from poly(methyl methacrylate) (PMMA) surface within 10 s of activating the heater. The heater uses relative low voltage (2.5 V) resulting in a surface temperature ∼ 50 °C. The ROP lift-off process produces superior quality of the Cu-seed layer at low-temperature, enables high transparency, allows the use of complex designs and a variety of substrates. The results indicate that the proposed metal mesh heater is a good candidate for scalable, high-volume manufacturing.