基于纳米颗粒热效应的飞秒激光高效直写金属铜微结构

Femtosecond Laser Direct Writing of Copper Microstructures with High Efficiency via Thermal Effect of Nanoparticles

在Cu(NO_(3))_(2)前驱体溶液中添加硅纳米颗粒,采用飞秒激光在透明基底表面成功直写了导电金属铜微结构。前驱体溶液中的硅颗粒作为吸光粒子吸收激光能量后对溶液进行加热,使Cu^(2+)还原为金属铜并沉积在基底表面。结果表明:当激光光强为5.32×10^(9)~8.51×10^(9)W·cm^(-2)、扫描速度为100~500mm·s^(-1)时,微结构主要由铜、Cu_(2)O及微量硅组成,铜含量及微结构的导电性随着光强的增加或扫描速度的降低而逐渐增加;在光强为5.32×10^(9)W·cm^(-2)、扫描速度为100mm·s^(-1)的条件下,铜微结构的方阻为0.28Ω·sq^(-1),电阻率为4.67×10^(-6)Ω·m。与已有的飞秒激光直写铜微结构的技术相比,这种方法使激光光强降低了2个数量级,直写效率提高了1~3个数量级。

Objective The precise conductive Cu micropatterns have been used in a variety of electronic devices.Compared to other traditional fabrication methods,laser direct writing is more efficient and reliable.The femtosecond laser direct writing technique,in particular,is used to construct highly conductive Cu microstructures.Femtosecond laser with ultrashort pulse duration can precisely control the heat input resulting in the reduction of Cu^(2+)in the laser irradiation zone without the damage of substrate.However,the intensity is as high as 10^(11) W·cm^(-2) and the scanning speed is generally lower than 10mm·s^(-1) to achieve the necessary reduction temperature.Si nanoparticles were added to Cu^(2+) solution in this study,acting as photon-absorbing nanoparticles due to their narrow band-gap.The photon-absorbing nanoparticles reduced the volume of the reduction zone by decreasing the penetration depth.The temperature of the reduction zone was rising,resulting in more efficient and less expensive direct writing.As a result,the conductive Cu microstructures were deposited on the substrate with the intensity from 5.32×10^(9)to 8.51×10^(9) W·cm^(-2)and the scanning speed from 100to 500mm·s^(-1).The intensity was two orders of magnitude lower,and the direct writing efficiency was three orders of magnitude higher,compared to previously reported work.The impacts of scanning speed and intensity on the morphology,chemical composition,and conductivity of Cu microstructures were investigated.The lowest sheet resistance was 0.28Ω·sq^(-1) and the lowest electrical resistivity was 4.67×10^(-6)Ω·m at the intensity of 5.32×10^(9) W·cm^(-2)with a scanning speed of 100mm·s^(-1),respectively.Methods The solvent was prepared by mixing 6 mL of ethylene glycol and 3 mL of deionized water.4g of Cu(NO_(3))_(2)·3H_(2)O was added to the solvent with ultrasonication for at least 30 min to thoroughly dissolveCu(NO_(3))_(2)·3H_(2)O .For 2minutes,the liquid was heated to 170℃.The solvent received 100mg of Si nanoparticles.To obtain the suspension liquid,the mixed solution was ultrasonically homogenized for 1h.Glass was used as a substrate that was adhered to the suspension liquid’s surface.The laser beam scanning was controlled by a femtosecond laser equipped with a galvanometer system.After the femtosecond laser irradiation,the conductive Cu microstructure was formed on the backside of the substrate.Then,the morphologies of the Cu microstructures were characterized by optical microscopy and field emission scanning electron microscopy.The composition of the Cu microstructures was verified using X-ray diffraction.The thickness of the microstructures was measured and the three-dimensional topography of the microstructures was depicted using a surface profiler.Cu microstructures’electrical properties were measured using a source meter based on the four-point probe method.Results and Discussions The continuity of laser-fabricated microstructures and the proportion of Cu increased with the increasing intensity(Fig.2 and Fig.3).The intensity was two orders of magnitude lower than that in previous experiments.The addition of photon-absorbing Si nanoparticles to the suspension liquid resulted in a decrease in laser penetration depth in solution,raising the temperature of the laser-induced reduction zone(Fig.4).The more metallic Cu was obtained.The continuity of microstructures and the proportion of Cu also increased with the decreasing scanning speed(Fig.5and Fig.6).The direct writing efficiency was one to three orders of magnitude higher than that in previous work(Table 1).The sheet resistance and electrical resistivity of as-fabricated Cu microstructures tended to decrease with increasing intensity or decreasing scanning speed(Fig.7).The Cu microstructure obtained at5.32×10^(9) W·cm^(-2)intensity and 100mm·s^(-1) scanning speed exhibited the lowest sheet resistance of 0.28Ω·sq^(-1).Moreover,as a result of the reduction reaction threshold,the microstructure’s line width was narrower than the laser spot’s diameter.As a result,the heat input to the irradiation zone was precisely controlled,limiting the reduction zone area and resulting in finer line width formation(Fig.9).Conclusions In this study,highly conductive Cu microstructures were formed on aglass substrate using femtosecond laser direct writing.As photon-absorbing nanoparticles,Si nanoparticles were added to the precursor solution.With the intensity ranging from 5.32×10^(9) W·cm^(-2)to 8.51×10^(9) W·cm^(-2)and the scanning speed ranging from 100mm·s^(-1) to 500mm·s^(-1),the Cu microstructures were formed on substrates.Metallic copper,Cu2O,and minor Si were found in the copper microstructures.The results show that the continuity of the microstructure,the proportion of Cu,and the conductivity of the microstructures all increased with increasing intensity or decreasing scanning speed.At the scanning speed of 100mm·s^(-1),the lowest sheet resistance of 0.28Ω·sq^(-1) and the lowest electrical resistivity of 4.67×10^(-6)Ω·m were obtained.The intensity was two orders of magnitude lower than that in previous work,and the direct writing efficiency was one to three orders of magnitude higher than that in previous work.Moreover,the line width of the microstructure was significantly smaller than the diameter of the laser spot.