Development and Optimization of Thin-Film Technology Based Micro Inductors and Transformers
Authors: Beringer Sebastian , Dinulovic Dragan , Haug Martin , Rissing Lutz , Wurz Marc Christopher 
Published: February 2020

In order to follow the current trends of electronic device development like miniaturization and increase of switching frequencies especially for magnetic passive components e.g. inductors and transformers innovative fabrication technology is expedient. In general switching frequencies in the range of 15 MHz to 50 MHz are intended. Some researcher teams focus on manufacturing micro transformer and inductors with thin-film technology in order to satisfy the market demands [1,2]. The design, the fabrication technology and the characteristics of the manufactured micro inductor and transformer were previous published. The 1st and 2nd generation were equipped with 4 contact pads on each side, primary and secondary side. The 5 µm thick soft magnetic closed oval core was deposited electrochemically. It consists of NiFe 80/20 [3] or CoFe 45/55 [4] and a multilayer coil with non-copper seed layer. The 3rd generation was equipped with 3 contact pads on each side and 10 µm [5] or 5 µm [6]thick electrochemically deposited CoFe 45/55 closed oval core with an electroplated multilayer coil with non-copper seed layers.
The 4th generation of this solenoid-core transformer is developed and presented in this work. The dies have 3 contact pads on each side, 5 µm thick electroplated CoFe 45/55 core is enclosed in pure copper coils. The impetus to further development of the devices is the electrical and magnetic performance optimization. The process flow is modified in order to eliminate the interplies of chromium and gold -which were used as seed layers for electrochemical deposition of copper on copper. The 4th generation of the micro inductor and transformer with Electronic Industries Alliance Standard 1008 footprint is based on thin-film technology namely electrochemical deposition of copper and soft magnetic alloy CoFe 45/55 , physical vapour deposition, photolithography and encapsulating with copper compatible, photosensitive polyimide. The optimization of the process flow shows lower electrical resistance of the coils in comparison to earlier generations. Whole multilayer coil structures are made of pure copper and have no delamination defects. The closed magnetic core is built of electrochemically deposited soft magnetic alloy CoFe 45/55 . In order to reduce the residual low electrical conductivity of the polyimide surface which will be brought by ion beam etching substrate cooled sputter etching is used. Another step of preparing the copper surface after wet chemical development of photosensitive polyimide is presented. The surface cleaning is necessary for successful ultrasonic wire bonding. The final embedding of the device into a QFN-package is needed for the characterization. It is fulfilled with two component epoxy which has a wide temperature working range.
The functionality of fabricated micro magnetic device was tested in high frequency DC-DC step down converter. For this test a specific integrated circuit MDCD073 a dual 7V PN half bridge with pre-drivers in 180 nm SOI technology is used. The frequency of the convertor was from 15 MHz to 30 MHz. The micro device shows the efficiency of about 80 % for output currents between 300 mA to 800 mA. The electrical resistance of the devices, with all four coils in serial connection, is 250 mΩ. The measured inductivity is above 50 nH which is stable up to very high frequencies. Yet the breakthrough voltage has to be determined as well as the impedance parameter which will be calculated from the s-parameters by matrix transformation.
[1] Xing, Xing; Sun, Nian X;Chen, Baoxing; High-Bandwidth Low-Insertion Loss Solenoid Transformers Using FeCoB Multilayers; IEE Transactions on Power Electronics 2012 Vol 28 4395-4401
[2] Feeney, Ciaran; Wang, Nigning; Mathúna, Seán Cian; Duffy, Maeve; A 20-MHz 1.8-W DC–DC Converter With Parallel Microinductors and Improved Light-Load Efficiency; IEEE Transactions on Power Electronics 2015 Vol 30 771-779
[3] Dinulovic, Dragan; Gerfer, Alexander; Opitz, Oliver; Kaiser, Matthias; Wurz, Marc C., Rissing, Lutz; Thin-Film Microtransformer for High Frequency Power Application; Joint European Magnetic Symposia 2014 Vol 75
[4] Dinulovic, Dragan; Kaiser, Matthias; Gerfer, Optiz, Oliver, Martin; Wurz, Marc C., Rissing, Lutz; Microtransformer with closed Fe-Co magnetic core for high frequency power applications; Journal of Applied Physics 2014 Vol 115 17A317-1 – 17A317-3
[5] Dinulovic, Dragan; Gerfer, Alexander; Haug, Martin; Kaiser, Matthias; Wurz, Marc C., Rissing, Lutz; Improved Microtransformer Design Utilizing Fe-Co Magnetic Core; Physics Procedia 2015 Vol 75 1252-1258
[6] Dinulovic, Dragan; Shousha, Mahmoud; Haug, Martin; Beringer, Sebastian; Wurz, Marc C.; Thin-Film Based Microtransformer Suitbale for High Switching Frequency Power Applications, in press

Figure 1