# A Wideband Termination Based on Laser-Scribed Lossy Microstrip Line Structures
## 1 Introduction  
Terminations are single-port devices with high absorption and minimized reflection of the input signal, which provide a variety of applications such as attenuation, isolation, measurement calibration, power divider, and multi ports network for measurement instruments and microwave systems. Conventionally, a planar termination commonly uses a chip resistor or a sheet resistor in series with a short end [1, 2, 3, 4, 5]. However, this configuration shows a low-pass response due to the series parasitic inductance of the short end. The signals higher than the cutoff frequency are blocked. On the other hand, open-end configurations consist of lossy transmission lines to gradually attenuate the microwave power [6, 7, 8]. The response demonstrates a high-pass characteristic, which is fit for high-frequency broadband networks. The core to construct a planar open-end termination is to design and optimize lossy transmission structures with broadband absorption and impedance match. Various printing techniques have been introduced to the manufacture of lossy films with desired resistance. For example, [PERSON] et.al. reported inkjet-printed microstrip line terminations with high-pass response characteristics [1]. The sheet resistance of the thin-film conductors was optimized at around a low value of 20 \(\Omega/\square\). The aerosol-jet printed wideband matched load up to 67 GHz was also realized by [PERSON] et al. [9]. The material was sintered with a low resistivity of 10\({}^{4}\) S/m. These works demonstrated simple, low-cost routes to fabricate wideband terminations to high frequencies. The laser direct writing (LDW) technique has become a promising alternative for the fabrication of electronic devices due to rapid prototyping, maskless patterning, and simplified fabrication [10; 11]. A variety of demonstrations have been reported including laser-scripted conductive electrodes [12], supercapacitors [13; 14; 15], and sensors [16; 17]. Recently, this technique has been extended to the field of microwave devices [18; 19]. Compared with protocols such as inkjet printing and aerosol-jet printing, which require inks with small particle sizes, LDW demonstrates good material compatibility that can in-situ modify the structures and materials on substrates with a precision down to a micro-size or even sub-micrometer scale. The localized reactions make it less possible to damage the surroundings. Moreover, due to the controllable laser characteristics, LDW provides a versatile tool to construct conductors with a tunable conductivity ranging from 10 S/m to above \(10^{6}\) S/m for microwave devices. The resistive nature of the laser-scripted conductors enables manipulation of the microwave signal via both absorption and reflection of the signal due to ohmic losses and impendence mismatch, respectively [20]. This also provides ideal resistive film for terminations. In this paper, we demonstrate a laser-scribed wideband open-end termination, which relies on conductor loss of the microstrip (MS) line structure to obtain effective absorption. The proposed design consists of a resistive film overlapped on a strip conductor, providing an enlarged sheet-resistance range of the resistive film to reduce the fabrication difficulties and give more choices of materials. A tapered-shaped resistive film is used to enable small gradual changes in impedance and obtain minimized reflection. Furthermore, the use of the termination as an attenuator is demonstrated in the two-port configuration. Prototypes are experimentally demonstrated utilizing laser-scribed reduced graphene oxide (rGO) film. The merits include the lower cost of graphene oxide compared with silver and chemical-vapor-deposited graphene, the adaptive sheet-resistances of rGO films that match design, the simplified process of combining the reduction in graphene oxide and pattering into a single step via LDW, and high accurate modification. The measurement results are shown to evaluate the performance of the termination from 1 GHz to 30 GHz.