Failure of Copper Oxide Line Under High Current Density
Authors: Moriwaki Takeshi , Sugawara Yusuke , Takaya Ryuji , Fujisaki Kazuhiro , Sasagawa Kazuhiko 
Published: February 2020

Cu2O shows p-type semiconductor characteristics, have been investigated intensively for applications to opto-electronic devices and thin-film transistors (TFTs). Cu2O film is built by sputtering or pulsed laser deposition technique. Thermal oxidation of Cu can also create Cu2O layer, the resistivity (depending on the degree of oxidation) of the layer is controlled by temperature of heat treatment. Recently, studies on high integration such as fabrication of Cu2O nanowires were performed, but high current density due to scaling down of electronic devices induces electromigration (EM) damages. EM is a phenomenon that atom’s transport in the line by electron wind. In general, EM is one of key reasons for the failure of metal lines in electronic devices. Recently, EM failure of p-type semiconductor made of poly-crystalline silicon nanowires was studied. In this study, we conducted acceleration tests of micro-sized line of copper oxide which had p-type semiconductor characteristics to reveal electric characteristics under high current density.
Straight-shaped copper oxide line samples were created on silicon substrates. Briefly, oxidation (SiO2) layer was grown on silicon substrate by thermal treatment (1173K, overnight). Cu layer with ca. 650 nm thickness was deposited on the SiO2 layer by sputtering. Cu line was fabricated through photolithography and wet-etching process. Width and length of straight test section of the line were ca. 20 um and 460 um, respectively. The Cu line was oxidized by thermal treatment (473K, 5 min). The resistivity of the copper oxide film prepared under the same conditions was 10-5 order [Ωm]. In energy dispersive X-ray spectroscopy (EDX), the O/Cu ratio of the thermal treated Cu layer was higher than that of before thermal treated. Also, the resistance of obtained copper oxide line decreased according to increasing of ambient temperature (303 to 373 K). It was considerable that the obtained copper oxide line was semiconductor materials with main component of Cu2O.
Acceleration electric failure test was performed to the obtained electric lines. The sample was heated to 373K by using ceramic plate heater. Constant current (1.0 MA/cm2) applied to the obtained line by using constant current power supply and galvanometer. As shown in Fig (a), sample 1 with relatively high electric resistance was broken near the center of the test section at relatively short time (several tens minute). It seems that Joule heating was the key factor of failure, and the sample line broke at the center part with high temperature. By contrast, sample 2 with relatively low electric resistance broke near the anode side in test section at relative long time (several hundreds minute), as shown in Fig (b). In the laser microscope observations, the cross-sectional area of sample 2 was increased at the cathode side and decreased at the anode side. There is a possibility that the atomic transport due to EM was affected the failure at sample 2. In general, EM failure of metal line was observed at cathode side, since atomic transport occurred from cathode to anode with electron transport. It was reported that the semiconductor line made of poly-crystalline silicon nanowires was broken at the anode side. The failure at anode side was guessed that regrowth of grain was occurred due to Joule heating, they called this phenomenon thermally-assisted electromigration. It was suggested that similar EM failure occurred in sample 2.
Acceleration electric failure tests of dumbbell-shaped copper oxide lines were performed, and failure at the anode side was observed. There is a possibility that the failure is related to thermally-assisted electromigration. It is expected that more detailed mechanism of this phenomenon can be demonstrated by the failure tests using various oxidized samples.

Figure 1