Preparation Of [Cu/CoCu]_n Multilayer Films And Its Application In Microwave Transmission Lines

With the continuous miniaturization of on-chip integrated devices,the use of patterned thin films as interconnect lines has become the mainstream of current integrated RF passive devices.However,at high frequency,the eddy current loss in the interconnect has a negative effect on the application of the device.At the same time,the cost of the prepared film is higher.In this paper,a new type of [Cu/CoCu]_n multilayer transmission line was fabricated by alternating electrochemical deposition of nonmagnetic metal layer and ferromagnetic metal layer.The relative permeability of the ferromagnetic metal layer is negative when it beyond the ferromagnetic resonance frequency,it can compensate the positive permeability of the nonmagnetic metal layers,leading to near zero effective permeability and a very deep skin depth,effectively suppress high-frequency eddy current loss.In this paper,the composition,surface morphology and magnetic properties of CoCu alloy films were studied.Change the cathodic potential in the range of-2.1V to-1.1V,found that the deposition potential in-1.3V,keep the pH value of the solution is4,the content of cobalt films deposited in about 0.526,mainly at the crystal structure of the films is face centered cubic structure.Secondly,the surface of the CoCu alloy film can be more compact and uniform,and the gap between grains is smaller and the film quality is higher upon adding sodium citrate.Secondly,the soft magnetic properties of the deposited CoCu alloy films were controlled by changing the ratio of cobalt sulfate and copper sulfate.When the deposition potential was-1.3 V,the saturation magnetization of the CoCu alloy films was maintained at 0.8 T,and the coercivity was maintained at about 10 Oe,it shows that the CoCu alloy films has very good soft magnetic properties.In addition,in order to make the CoCu alloy thin film work in the high frequency region,it is necessary to induce uniaxial anisotropy under the condition of lower saturation magnetization.In the electrodeposition process,we applied the parallel magnetic field of 800 Oe in the plating bath on both sides as the induced magnetic field,a detailed study of the cathodic potential,the effects of the deposition potential,the concentration of solution,the concentration of boric acid and the concentration of sodium citrate on the anisotropy of the films were studied in detail.The results showed that when theconcentration of sodium citrate was 0.0453 mol/L,the solution pH was 4,and the deposition potential was-1.3V,the uniaxial anisotropy CoCu magnetic film can be obtained,and the anisotropy field is about 50 Oe,which is in line with the requirements of the device design.At this time,the relative permeability of the film is negative in the frequency band above 3 GHz,it is helpful to realize the control of the performance of multilayer structure.Finally,we studied the effect of changing the number of alternating deposition cycles and the thickness ratio of two metal layers on the inhibition of skin effect.The microwave transmission line of [Cu/CoCu]_n multilayer film was prepared by changing the cathodic deposition potential in the same bath.The S parameters were measured by microwave probe and vector network analyzer,after conversion into the transmission resistance,it is observed that the transmission resistance in the 10 GHz band has decreased significantly.Compared with pure copper transmission line,the microwave transmission line of [Cu/CoCu]_n multilayer film has the effect of improving the skin depth and suppressing the eddy current loss.