This paper present a preparative technique for spiral thin-film transformer with ferrite magnetic core. RF magnetron sputtering is used to prepare ferrite thin film on SiO2 layer. Compatible problems of thin film with IC technology are observed by SEM. The problems are resolved through sputtering parameters modification and heating treatment addition. S-parameters are measured at 10 MHz-20 GHz. The result shows that The experience result shows, magnetic core thin-film transformer with 15:15 ratio-turn can obtain the maximal transmission efficiency 80.9% at 10 MHz-20 GHz, and the air core transformer can obtain the maximal transmission efficiency 55.4% at same frequency range. Ferrite thin film can improve the transmission efficiency evidently

Daniels, Stephen

Qin, Huibin

Zheng, Liang

DCU Online Research Access Service

A Preparative Technique for Spiral Thin-Film Transformer at GHz Frequency Band Liang Zhenga,b aNational Centre for Plasma Science and Technology Dublin City University Dublin, Ireland zhlbsbx@hotmail.com Huibin Qinb bInstitute of Electron Device & Application Hangzhou Dianzi University Hangzhou, China qhb@hdu.edu.cn   Stephen Danielsa aNational Centre for Plasma Science and Technology Dublin City University Dublin, Ireland daniels@eeng.dcu.ie  Abstract—This paper present a preparative technique for spiral thin-film transformer with ferrite magnetic core. RF magnetron sputtering is used to prepare ferrite thin film on SiO2 layer. Compatible problems of thin film with IC technology are observed by SEM. The problems are resolved through sputtering parameters modification and heating treatment addition. S-parameters are measured at 10MHz-20GHz. The result shows that The experience result shows, magnetic core thin-film transformer with 15:15 ratio-turn can obtain the maximal transmission efficiency 80.9% at 10MHz-20GHz, and the air core transformer can obtain the maximal transmission efficiency 55.4% at same frequency range. Ferrite thin film can improve the transmission efficiency evidently.  I. INTRODUCTION Thin-film transformer is an indispensable component because of its performance of signals insulation and transmission, signals synthesis and conversion. It is being found increasing use in radio frequency circuit such as oscillators, mixers, low-noise amplifiers (LNAs), baluns, matching networks and filters [1-3]. But it is not catch more attention until the 90’s of last century. Some technologies have been used to fabricate the thin-film transformer like multilayer printed technology, MEMS, superconducting thin film and so on[4-7]. However, there are still exist disadvantages, the areas of transformer fabricated by multilayer printed technology are too large (usually more than 1mm*1mm), transformers fabricated by MEMS technology are more expensive because of complicated technique, and the transmission efficiency of all these transformers must be further upgraded. In this paper, IC technology and Ni-Zn microwave ferrite is used to resolve these problems and improve the performance of transformer. II. DESIGN OF THIN-FILM TRANSFORMER 2.1. Design principle In the past, the microwave loss of common low-resistivity silicon substrate (LRS) is too high to use in RFIC. Compound semiconductor technology played a crucial role in RFIC. However, with the development of IC technology, using many new technologies appeared and improved the microwave performance of substrate, such as high-resistivity silicon substrate (HRS) technology and LRS with a thick silicon oxide interlayer technology. Now, IC technology exhibit excellent performance in radio frequency range. Adopting IC technology can not only decrease the area and cost of thin-film transformer, but also increase their consistency. An air core spiral can’t achieve big inductance in radio frequency range. Using magnetic core spiral is a way to resolve the problems above. Ni-Zn microwave ferrite is a suitable material to make thin film magnetic core. However, it is difficult to use fabricate and etching technology of magnetic material in standard IC technology. For improve the performance of transformer, it is necessary to use magnetic material, and the use of magnetic material brings the compatible problems of magnetic thin film technology with IC technology. The way to fabricate a transformer with magnetic core based on IC technology is presented below. 2.2. Structrue design Fig1 show the schematic structure of thin-film transformer. We designed stacked spiral thin-film transformers included octagonal spiral and square spiral. The primary coil and secondary coil are overlapped completely. Overlapping coils has high magnetic coupling which can transmit energy more effective. The whole structure consists of 10 layers besides the Silicon substrate. 1) Layer 1 is SiO2. It supplies the whole device and isolates Si substrate. Thermal oxidation is used to fabricate This paper is supported by National Nature Science Foundation of China (No.60601022, NO.60671024) and Natural Science Foundation of Zhejiang Province (No. Y107205 and No. Y407133). 978-1-4244-3732-0/09/$25.00 ©2009 IEEE256Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on July 27,2010 at 09:58:20 UTC from IEEE Xplore.  Restrictions apply. SiO2 .The thickness of SiO2 layer is 1μm, thick oxide can reduce the parasitical capacitance[8]. 2) Layer 2 is Si3N4. PECVD is used to fabricate. The thickness of Si3N4 layer is 120 nm. The electrical resistivity of Si3N4 is 1015Ω•cm, substrate with highly electrical resistivity can reduce substrate loss. Furthermore, Si3N4 has low-fluidity and difficult to corrupt, these characteristics can improve the compatibility of subsequent etching process[9,10]. 3) Layer 3 is Metal1. Metal1 is lead wire using aluminum. DC magnetron sputtering is used to fabricate Al. Two lead terminals of primary spiral are prepared. The thickness of Al layer is 0.7 μm. 4) Layer 4 is SiO2. SiO2 with 1μm thickness isolate lead wire and primary spiral and fabricated by PECVD. 5) Layer 5 is primary spiral. DC magnetron sputtering is used to fabricate Al with 1μm thick. 6) Layer 6 is SiO2 or ferrite thin film. Air core thin-film transformer use SiO2 and magnetic core thin-film transformer use ferrite thin film. Ferrite is highly resistant material, so the thin film can be used as magnetic core and insulating layer. It brings the structure simpler. RF magnetron sputtering is used to prepare ferrite thin film or PECVD is used to prepare SiO2. The thickness is 1μm. 7) Layer 7 is second spiral. DC magnetron sputtering is used to fabricate Al with 1μm thick. 8) Layer 8 is insulation layer. PECVD is used to fabricate SiO2 layer with 1.2 μm thickness.  9) Layer 9 Metal4 is also lead wire using aluminum. DC magnetron sputtering is used to fabricate Al. Two lead terminals of primary spiral are prepared. The thickness of Al layer is 1.2 μm. 10) Layer 10 is passivation layer. PECVD is used to fabricate SiO2 layer with 1.2 μm thickness.   Figure 1.  Structure of thin-film transformer  2.3. Technics design As functional layer and insulating layer, the compatible problem of ferrite thin film with IC technology is a difficulty of thin-film transformer preparation. Ferrite is not a standard material in IC process technology. We met some problems in preparation progress. 2.3.1 Preparation of ferrite thin film There are some ways to fabricate ferrite thin film like PLD, sol-gel, but sputtering is in common usage on IC technology. So RF magnetic sputtering is used to prepare ferrite thin film. We adopted modified NiZn ferrite material to fabricate magnetic thin film through RF magnetron sputtering Alliance Concept DP650. The sputtering gas is high-purity argon atmosphere 99.999%, sputtering pressure is 8μbar, background vacuum is 4.810*10-6mbar, space between target and substrate is 100 mm, RF forward power is 1262 WRF, back forward is 4WRF, flow rate of Argon is 57 sccm, the deposition rate is 0.648μm/h. It is found that the surface of ferrite film appears crack due to high stress accumulated when the thickness of film has rise to 1μm. The crack of film surface can be found in SEM image magnified 2000 times, as showed in Fig.2 (a). This question is solved through sputtering parameters modification and deposition rate decrease. The sputtering pressure is increased to 11μbar; flow rate of Argon is increased to 60 sccm, then the deposition rate is decreased to 0.436μm/h. The SEM image magnified 10000 times is showed in Fig.2 (b). The surface of thin film is smooth and has better quality.  (a) 257Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on July 27,2010 at 09:58:20 UTC from IEEE Xplore.  Restrictions apply.  (b) Figure 2.  SEM image of ferrite thin-film (a) SEM image of ferrite thin-film with cracks magnified 2500 times. (b) SEM image of ferrites thin-film with higher quality magnified 10000 times. 2.3.2 Cohesiveness problem The ferrite film contact SiO2 and Al according to the structure of thin-film transformer. Ferrite thin film has poor cohesiveness to SiO2 layer or Al film. In subsequent processing, ferrite film will crack and desquamate after etching and cleaning, as showed in Fig.3.  (a)  (b) Figure 3.  SEM image of sample (a) coil (b) Pad Heating treatment is used to improve the cohesiveness of ferrite film. The experiment results shows that the stress of ferrite film can be decreased and the cohesiveness can be increased after the film having been heated for 30 minutes at 300℃ and cooled to room temperature for 25 hours. The SEM image of a sample with heat treatment is showed in Fig.4.   Figure 4.  SEM image of sample with heat treatment 2.4. Fabrication sequences The modified fabrication process is showed in fig.5. After modified the fabrication process, we can get a transformer sample with better quality.  Figure 5.  Fabrication sequences of the thin-film transformer III. MEASUREMENT RESULTS AND DISCUSSIONS Two kinds of thin-film transformers are measured: air core thin-film transformer and magnetic core thin-film transformer. The turn numbers of two spirals are all 15, the spiral width line is 12μm, the spiral distance line is 3μm, the thickness of Al layer is 1μm, and the spiral area are 1*1mm2, Fig.6 showed the surface topography of the transformer. The transmission characteristics are measured at 10MHz-20GHz 258Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on July 27,2010 at 09:58:20 UTC from IEEE Xplore.  Restrictions apply. by vector network analyser Agilent PNA E8363B and probe Cascade Microtech ACP GSG. The measured results are showed in Fig.7.  As shows in Fig.7, the transmission efficiency of air core thin-film transformer more than 50% at 4.3GHz-14.2GHz, and the maximum is 55.4% at 11.2GHz; the transmission efficiency of magnetic core thin-film transformer more than 50% at 0.5GHz-20GHz, and the maximum reached 80.9% at 9.25GHz. It means that the whole transmission efficiency of thin-film transformer gets largely improved by ferrite thin film, and it results in that the bandwidth of magnetic core thin-film transformer is larger than air core thin-film transformer.  Figure 6.  SEM image of transformer with 15:15 turn ratio    (a)  (b) Figure 7.  15:15 transformer’s S(2,1) curve (a)air core thin-film transformer (b)magnetic core thin-film transformer IV. CONCLUSIONS  A thin-film transformer using ferrite film based on Si IC technology was presented. The compatible problems of thin film with IC technology are resolved, the structure and technological design of thin-film transformers are proposed, and performance of the transformers are tested. The experience result shows, magnetic core thin-film transformer with 15:15 ratio-turn can obtain the maximal transmission efficiency 80.9% at 10MHz-20GHz, and the air core transformer can obtain the maximal transmission efficiency 55.4% at same frequency range. Ferrite thin film can improve the transmission efficiency evidently. The thin-film transformer has advantages like small in size, operating at high frequency, suitable for mass production, and so on. The preparative technique can be used to fabricate miniature ferrite device and the transformer is expected to apply on RF circuit. REFERENCES [1] Niranjan A, Talwalkar C, Patrick Yue.  “Analysis and Synthesis of On-Chip Spiral Inductors”. IEEE Transactions on Electron Devices. vol.33, pp. 2002-2027,Feb. 2005. [2] Y.Z.Xiong. “On-chip transformer-based feedback CMOS power oscillator”. Electronics Letters. vol.41, pp.2232-2239, Feb. 2005. [3] KyuchulChong and Ya-Hong Xie. “High-Performance On-Chip Transformers”. IEEEE Lectron Device Letters. vol. 26, pp. 557-559, Aug. 2005. [4] Yamaguchi K, Sugawara E, Nakajima O. “Load characteristics of a spiral coil type thin film microtransformer”. IEEE Transactions on Magnetics.vol. 29, pp. 3207-3209. Nov.1993. [5] Hiroaki Tsujimoto, Ieyasu O. “High frequency transmission characteristic of co-planar film transformer fabricated on flexible polyamide film”. IEEE transactions on magnetics, vol. 31, pp. 4232-4234. Nov. 1995. [6] Terence O’Donnell, Ningning Wang, Magali Brunet, Saibal Roy, et al. “Thin film micro-transformers for future power conversion”. APEC ’04. Nineteenth Annual IEEE, vol. 2, pp. 939-944. 2004. [7] A H Miklich, J X Przybysz, T J Smith. “Superconducting thin-film transformers at microwave frequencies”. IEEE transactions on applied superconductivity, vol.9, pp. 3062-3065. Jun. 1999. [8] Choong-Mo Nam, Young-Se Kwon. “High-performance planar inductor on thick oxidized porous silicon (OPS) substrate”. IEEE Microwave and guided wave letters, vol. 7, pp. 236-238. Aug.1997. [9] Kirk B Ashby, Ico A Koullias, William C, et al. “High Q inductors for wireless applications in a complementary silicon bipolar process”. IEEE Joural of solid-state circuits, vol. 31,pp. 4-9. Jan. 1996. 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A preparative technique for spiral thin-film transformer at GHz frequency band

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