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Study of the impact of winding form and film thickness on thin-film inductors
Microelectronic Engineering 81 (2005) 212–216 www.elsevier.com/locate/mee
Study of the impact of winding form and film thickness on thin-film inductors Xiao-Li Tang *, H.W. Zhang, H. Su, Y. Shi, X.D. Jiang Institute of Micro-electronics and Solid-state Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China Available online 7 April 2005
Abstract Using a DC-magnetron sputtering system, we fabricated several forms of double-sided coupling thin-film inductors on a PCB (Printed Circuit Board). Due to the effects of winding forms and thickness of the film, the inductance, Q factor and resonance frequency of the inductors are different. Of the three types of inductors covering the same area, the meander inductor has the highest resonance frequency (up to 400 MHz), the circular-spiral inductor has the largest inductance (up to 1 lH/cm2) and Q factor (up to 25) at low frequency, and the hexagon-spiral inductor has moderate inductance and Q factor at an intermediate frequency. So the proper choice of winding form will depend on the specific application. 2005 Elsevier B.V. All rights reserved. Keywords: Thin film inductor; Inductance; Magnetic film; Thickness of the film
1. Introduction At present, devices in electronic system towards Ôsmall, thin and precisionÕ are following the development of the information technology. The inductor, which is one of the most important passive components, is used widely in personal computer, mobile telephone, digital camera and RF communicated circuit [1–3]. But if we cannot diminish its *
Corresponding author. Tel.: +8602883201440; fax: +8602883201810. E-mail address: [email protected] (X.-L. Tang).
size effectively, the inductor will be a bottleneck in the scaling of whole electronic system. So, the thin film inductor appears to meet this demand. Presently, research on thin film inductor is prevailing. It is commonly fabricated on silicon wafer by photolithography technique. Adopting this method, the size of inductor can be diminished and integrated with semiconductor components. But in numerous research papers, forms of the thin film inductor are mostly rectangle spiral or dual rectangle spiral [4–7]. It is known that different forms of the inductor result in different magnetic field distributing. Therefore, inductance, quality
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.03.009
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
factor and operating frequency are different. As a result, we fabricated three kinds of thin film inductors to research the impacting on the winding form, and in order to improve the unit area inductance, double-sided coupling inductors were made on a PCB.
213
Table 1 Dimensions of the inductors Form
Turns
Turn space (mil)
Turn width (mil)
Radius of the first coil (mil)
Circular-spiral Hexagonal-spiral Meander
8 5 14
10 10 10
10 10 10
45 45 –
2. Design and structure Fig. 1 shows the top and cross-section views of different forms integrated inductors, and the bottom side of the coil is reverse to the topside in order to get positive mutual inductance. A key point of this paper is to study the impact of the winding forms, so we fabricated meander, circular-spiral and hexagonal-spiral inductors. In order to simplify the process and keep inductors the same, we chose PCB technologies to make them. The conductors were set on double-sided of the PCB, and connected by via hole. Doublesided coupling inductor can improve inductance in contrast to the single sided, because L = L1 + L2 + 2M (where L is the total inductance, L1 is the inductance of top side, L2 is the inductance of bottom side, M is the mutual inductance pffiffiffiffiffiffiffiffiffi and M max ¼ L1 L2 Þ. High Q factors were also got in contrast to the inductor fabricated on the silicon wafer. So it was an effective method to get higher inductance in a small substrate. Table 1
listed dimensions of the inductors, and the all were fabricated on 10 mm · 10 mm board. In this paper, conventional amorphous CoNbZr films were used. Its intrinsic resistivity was only 140 X cm, coercive force Hc was 1.8Oe, Ms was 1.25 T, and the ferromagnetic resonance frequency was about 1 GHz [8]. Shown in Fig. 2 was the frequency profile of complex permeability for the CoNbZr film. The real part permeability exhibited high value up to 1 GHz, and the imaginary part also had low value straight to 1 GHz. So, the CoNbZr film can act as the magnetic core up to 1 GHz.
3. Measurement results and discussion According to the design, we fabricated three types of inductors. The substrates were prepared with a standard washing process before the deposition.
Fig. 1. Structure of double-sided coupling thin film inductors: (a) top view of the coils and (b) cross-sectional view.
214
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216 2500
imaginary part of permeability
Real part of permeability
6000
5000
4000
3000
2000
1000
2000
1500
1000
500
0
0
1
(a)
10
100
1000
1
10
(b)
Frequency (MHz)
100
1000
Frequency (MHz)
Fig. 2. Frequency profile of complex permeability for CoNbZr film: (a) real part and (b) imaginary part.
A HP 4291B Material/Impendence Analyzer(1 M–1.8 GHz) was used to measure the inductance and Q factors of different type inductors. In order to comparison the magnetic film impacting on the inductorsÕ performance, we first measured 3000
25
2000
600
20 15 10
-1000
200
15
0
10
-200
5
-2000
L 1
5 -400
Q
-3000
0 100
10
L
0 1
10
(b)
100
1000
Frequency(MHz)
14
600
12
400
10
200
8
0
6
-200
4
Q
800
-400
L
2
Q
-600
0 1
(c)
Q
-600
Frequency(MHz)
Inductance (nH)
(a)
400
Q
Inductance(nH)
20
1000 0
25
800
Q
Inductance (nH)
the three kinds of the inductors without magnetic films, the results were shown in shown in Fig. 3. Then, the inductors with the magnetic film were fabricated. The thickness was determined with a quartz crystal thickness monitor system, and
10
100 Frequency (MHz)
1000
Fig. 3. Inductance and quality factor for the different winding form inductors without the magnetic film: (a) circular-spiral inductor; (b) hexagonal-spiral inductor and (c) meander inductor.
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
ranged from 120 to 520 nm. The inductorsÕ properties with different film thickness were shown in Fig. 4. Comparison of Figs. 3 and 4, it was clear to see the inductance and Q factors of the inductors with magnetic film were much higher than the inductors without the film. So, the devices performance can be improved by applying magnetic material.
215
As shown in Fig. 4, of the three types of inductor covering the same area, the meander inductor has the highest resonance frequency (up to 400 MHz), the circular-spiral inductor has the largest inductance (up to 1 lH/cm2) and Q factor (up to 25) at low frequency, and the hexagon-spiral inductor has moderate inductance and Q factor at an intermediate frequency. The circular-spiral,
Fig. 4. Inductance and Q value versus frequency for different thickness of: (a) circular-spiral inductor; (b) hexagonal-spiral inductor; (c) meander inductor.
216
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
hexagonal-spiral and meander thin film inductors have the best properties not with the same thickness and influences on the film thickness are obviously. Increasing the film thickness can enhance the inductance within the scope of certain, but if the thickness exceeds the certain limit, the properties of the inductor get worse. It is because the eddy current and magnetic core loss will become large when increasing the film thickness. In order to improve the unit area inductance, we found that the circular-spiral form was suitable. It can realize tight reel to enhance interturn mutual inductance, to decrease leak-inductance and to roll more turns in a fixed area. The meander inductor has lower inductance due to the sign of the mutual inductance between the neighboring conductor lines, but its resonance frequency is much higher than other types. The reason is based on its form, because resonance frequency of thin-film inductor is not only decided by the magnetic material, but also by the resonance frequency of the device itself. It is clear from Fig. 4 that the inductors are more complex and are displaying other significant effects from the stray capacitance. Frequency characteristic of the thin film inductors are mainly influenced by LC resonance. The resonance frequency of the inductor can express as: f ¼ 2pp1 ffiffiffiffi . So the inducLC tance and the stray capacitance influence the properties of the device together. In the thin film inductor, there are two chief stray capacitance: (1) the stray capacitance between the conductor lines, (2) the stray capacitance between the magnetic film and the coil layer. According to [9], we can neglect the influence of the stray capacitance between the conductor lines up to several hundreds of megahertz range. So it is not the major reason for the resonance frequency. Then, we consider another stray capacitance – the stray capacitance between the magnetic film and coil layer, which is the really element to impact the resonance frequency. Regards the magnetic film and coil layer with polyimide between them as one planar capacitor, therefore its capacitance describes as: C cm ¼
e0 er s ; d
where ÔsÕ is the area of the magnetic film and ÔdÕ is the distance between magnetic film center sheet
and conductor layer. Increasing the film thickness, the value of the ÔdÕ is increased. Therefore, the ÔCcmÕ is decreased. Although with the drop of the inductance and capacitance the resonance frequency is brought to high range, but the inductors primary properties get worse. The inductance and the stray capacitance are changed at the same time and cannot be separated. So research on the varieties of the film thickness are very important when the winding form of the inductor is chosen. 4. Conclusion We fabricated three types of double-sided coupling thin film inductor on PCB. They each have advantage with weakness. In same area, the circular-spiral inductor is suitable for using in low frequency to get large inductance, the hexagonspiral inductor is suitable for using in intermediate frequency to get moderate inductance and the meander inductor is suitable for using in high frequency to get small inductance. So the proper choice of winding form will depend on the specific application. Influence of stray capacity is also analyzed and summarized in this paper. Special attention should be paid to the capacitance between conductor and magnetic film which relating to the film thickness. References [1] M. Yamaguchi, K. Ishihara, K.I. Arai, IEEE Trans. Magn. 29 (1993) 3222. [2] Haruo Nakazawa, Masaharu Edo, Yasushi Katayama, Masakazu Gekinozu, Satoshi Sugahara, Zenchi Hayashi et al., IEEE Trans. Magn. 36 (2000) 3518. [3] Ki Hyeon Kim, Jongryoul Kim, Hee Jun Kim, Suk Hee Han, Hi Jung Kim, IEEE Trans. Magn. 38 (2002) 3162. [4] H.J. Ryu, S.H. Han, H.J. Kim, IEEE Trans. Magn. 35 (1999) 3568. [5] J.S. Song, B.K. Min, D.Y. Jeong, H.S. Kim, J.S. Heo, J. Magn. Magn. Mater. 234 (2001) 494. [6] Choong-Sik Kim, Seok Bae, Hee-Jun Kim, Seoung-Eui Nam, Hyoung-June Kim, IEEE Trans. Magn. 37 (2001) 2894. [7] K.H. Kim, D.W. Yoo, J.H. Jeong, J. Kim, S.H. Han, H.J. Kim, J. Magn. Magn. Mater. 239 (2002) 579. [8] Xiao-Li Tang, Huai-Wu Zhang, Hua Su, Xiang-Dong Jiang, J. Magn. Magn. Mater. 270 (2004) 84. [9] M. Yamaguchi, M. Matsumoto, H. Ohzeki, K.I. Arai, IEEE Trans. Magn. 27 (1991) 5274.
Study of the impact of winding form and film thickness on thin-film inductors Xiao-Li Tang *, H.W. Zhang, H. Su, Y. Shi, X.D. Jiang Institute of Micro-electronics and Solid-state Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China Available online 7 April 2005
Abstract Using a DC-magnetron sputtering system, we fabricated several forms of double-sided coupling thin-film inductors on a PCB (Printed Circuit Board). Due to the effects of winding forms and thickness of the film, the inductance, Q factor and resonance frequency of the inductors are different. Of the three types of inductors covering the same area, the meander inductor has the highest resonance frequency (up to 400 MHz), the circular-spiral inductor has the largest inductance (up to 1 lH/cm2) and Q factor (up to 25) at low frequency, and the hexagon-spiral inductor has moderate inductance and Q factor at an intermediate frequency. So the proper choice of winding form will depend on the specific application. 2005 Elsevier B.V. All rights reserved. Keywords: Thin film inductor; Inductance; Magnetic film; Thickness of the film
1. Introduction At present, devices in electronic system towards Ôsmall, thin and precisionÕ are following the development of the information technology. The inductor, which is one of the most important passive components, is used widely in personal computer, mobile telephone, digital camera and RF communicated circuit [1–3]. But if we cannot diminish its *
Corresponding author. Tel.: +8602883201440; fax: +8602883201810. E-mail address: [email protected] (X.-L. Tang).
size effectively, the inductor will be a bottleneck in the scaling of whole electronic system. So, the thin film inductor appears to meet this demand. Presently, research on thin film inductor is prevailing. It is commonly fabricated on silicon wafer by photolithography technique. Adopting this method, the size of inductor can be diminished and integrated with semiconductor components. But in numerous research papers, forms of the thin film inductor are mostly rectangle spiral or dual rectangle spiral [4–7]. It is known that different forms of the inductor result in different magnetic field distributing. Therefore, inductance, quality
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.03.009
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
factor and operating frequency are different. As a result, we fabricated three kinds of thin film inductors to research the impacting on the winding form, and in order to improve the unit area inductance, double-sided coupling inductors were made on a PCB.
213
Table 1 Dimensions of the inductors Form
Turns
Turn space (mil)
Turn width (mil)
Radius of the first coil (mil)
Circular-spiral Hexagonal-spiral Meander
8 5 14
10 10 10
10 10 10
45 45 –
2. Design and structure Fig. 1 shows the top and cross-section views of different forms integrated inductors, and the bottom side of the coil is reverse to the topside in order to get positive mutual inductance. A key point of this paper is to study the impact of the winding forms, so we fabricated meander, circular-spiral and hexagonal-spiral inductors. In order to simplify the process and keep inductors the same, we chose PCB technologies to make them. The conductors were set on double-sided of the PCB, and connected by via hole. Doublesided coupling inductor can improve inductance in contrast to the single sided, because L = L1 + L2 + 2M (where L is the total inductance, L1 is the inductance of top side, L2 is the inductance of bottom side, M is the mutual inductance pffiffiffiffiffiffiffiffiffi and M max ¼ L1 L2 Þ. High Q factors were also got in contrast to the inductor fabricated on the silicon wafer. So it was an effective method to get higher inductance in a small substrate. Table 1
listed dimensions of the inductors, and the all were fabricated on 10 mm · 10 mm board. In this paper, conventional amorphous CoNbZr films were used. Its intrinsic resistivity was only 140 X cm, coercive force Hc was 1.8Oe, Ms was 1.25 T, and the ferromagnetic resonance frequency was about 1 GHz [8]. Shown in Fig. 2 was the frequency profile of complex permeability for the CoNbZr film. The real part permeability exhibited high value up to 1 GHz, and the imaginary part also had low value straight to 1 GHz. So, the CoNbZr film can act as the magnetic core up to 1 GHz.
3. Measurement results and discussion According to the design, we fabricated three types of inductors. The substrates were prepared with a standard washing process before the deposition.
Fig. 1. Structure of double-sided coupling thin film inductors: (a) top view of the coils and (b) cross-sectional view.
214
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216 2500
imaginary part of permeability
Real part of permeability
6000
5000
4000
3000
2000
1000
2000
1500
1000
500
0
0
1
(a)
10
100
1000
1
10
(b)
Frequency (MHz)
100
1000
Frequency (MHz)
Fig. 2. Frequency profile of complex permeability for CoNbZr film: (a) real part and (b) imaginary part.
A HP 4291B Material/Impendence Analyzer(1 M–1.8 GHz) was used to measure the inductance and Q factors of different type inductors. In order to comparison the magnetic film impacting on the inductorsÕ performance, we first measured 3000
25
2000
600
20 15 10
-1000
200
15
0
10
-200
5
-2000
L 1
5 -400
Q
-3000
0 100
10
L
0 1
10
(b)
100
1000
Frequency(MHz)
14
600
12
400
10
200
8
0
6
-200
4
Q
800
-400
L
2
Q
-600
0 1
(c)
Q
-600
Frequency(MHz)
Inductance (nH)
(a)
400
Q
Inductance(nH)
20
1000 0
25
800
Q
Inductance (nH)
the three kinds of the inductors without magnetic films, the results were shown in shown in Fig. 3. Then, the inductors with the magnetic film were fabricated. The thickness was determined with a quartz crystal thickness monitor system, and
10
100 Frequency (MHz)
1000
Fig. 3. Inductance and quality factor for the different winding form inductors without the magnetic film: (a) circular-spiral inductor; (b) hexagonal-spiral inductor and (c) meander inductor.
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
ranged from 120 to 520 nm. The inductorsÕ properties with different film thickness were shown in Fig. 4. Comparison of Figs. 3 and 4, it was clear to see the inductance and Q factors of the inductors with magnetic film were much higher than the inductors without the film. So, the devices performance can be improved by applying magnetic material.
215
As shown in Fig. 4, of the three types of inductor covering the same area, the meander inductor has the highest resonance frequency (up to 400 MHz), the circular-spiral inductor has the largest inductance (up to 1 lH/cm2) and Q factor (up to 25) at low frequency, and the hexagon-spiral inductor has moderate inductance and Q factor at an intermediate frequency. The circular-spiral,
Fig. 4. Inductance and Q value versus frequency for different thickness of: (a) circular-spiral inductor; (b) hexagonal-spiral inductor; (c) meander inductor.
216
X.-L. Tang et al. / Microelectronic Engineering 81 (2005) 212–216
hexagonal-spiral and meander thin film inductors have the best properties not with the same thickness and influences on the film thickness are obviously. Increasing the film thickness can enhance the inductance within the scope of certain, but if the thickness exceeds the certain limit, the properties of the inductor get worse. It is because the eddy current and magnetic core loss will become large when increasing the film thickness. In order to improve the unit area inductance, we found that the circular-spiral form was suitable. It can realize tight reel to enhance interturn mutual inductance, to decrease leak-inductance and to roll more turns in a fixed area. The meander inductor has lower inductance due to the sign of the mutual inductance between the neighboring conductor lines, but its resonance frequency is much higher than other types. The reason is based on its form, because resonance frequency of thin-film inductor is not only decided by the magnetic material, but also by the resonance frequency of the device itself. It is clear from Fig. 4 that the inductors are more complex and are displaying other significant effects from the stray capacitance. Frequency characteristic of the thin film inductors are mainly influenced by LC resonance. The resonance frequency of the inductor can express as: f ¼ 2pp1 ffiffiffiffi . So the inducLC tance and the stray capacitance influence the properties of the device together. In the thin film inductor, there are two chief stray capacitance: (1) the stray capacitance between the conductor lines, (2) the stray capacitance between the magnetic film and the coil layer. According to [9], we can neglect the influence of the stray capacitance between the conductor lines up to several hundreds of megahertz range. So it is not the major reason for the resonance frequency. Then, we consider another stray capacitance – the stray capacitance between the magnetic film and coil layer, which is the really element to impact the resonance frequency. Regards the magnetic film and coil layer with polyimide between them as one planar capacitor, therefore its capacitance describes as: C cm ¼
e0 er s ; d
where ÔsÕ is the area of the magnetic film and ÔdÕ is the distance between magnetic film center sheet
and conductor layer. Increasing the film thickness, the value of the ÔdÕ is increased. Therefore, the ÔCcmÕ is decreased. Although with the drop of the inductance and capacitance the resonance frequency is brought to high range, but the inductors primary properties get worse. The inductance and the stray capacitance are changed at the same time and cannot be separated. So research on the varieties of the film thickness are very important when the winding form of the inductor is chosen. 4. Conclusion We fabricated three types of double-sided coupling thin film inductor on PCB. They each have advantage with weakness. In same area, the circular-spiral inductor is suitable for using in low frequency to get large inductance, the hexagonspiral inductor is suitable for using in intermediate frequency to get moderate inductance and the meander inductor is suitable for using in high frequency to get small inductance. So the proper choice of winding form will depend on the specific application. Influence of stray capacity is also analyzed and summarized in this paper. Special attention should be paid to the capacitance between conductor and magnetic film which relating to the film thickness. References [1] M. Yamaguchi, K. Ishihara, K.I. Arai, IEEE Trans. Magn. 29 (1993) 3222. [2] Haruo Nakazawa, Masaharu Edo, Yasushi Katayama, Masakazu Gekinozu, Satoshi Sugahara, Zenchi Hayashi et al., IEEE Trans. Magn. 36 (2000) 3518. [3] Ki Hyeon Kim, Jongryoul Kim, Hee Jun Kim, Suk Hee Han, Hi Jung Kim, IEEE Trans. Magn. 38 (2002) 3162. [4] H.J. Ryu, S.H. Han, H.J. Kim, IEEE Trans. Magn. 35 (1999) 3568. [5] J.S. Song, B.K. Min, D.Y. Jeong, H.S. Kim, J.S. Heo, J. Magn. Magn. Mater. 234 (2001) 494. [6] Choong-Sik Kim, Seok Bae, Hee-Jun Kim, Seoung-Eui Nam, Hyoung-June Kim, IEEE Trans. Magn. 37 (2001) 2894. [7] K.H. Kim, D.W. Yoo, J.H. Jeong, J. Kim, S.H. Han, H.J. Kim, J. Magn. Magn. Mater. 239 (2002) 579. [8] Xiao-Li Tang, Huai-Wu Zhang, Hua Su, Xiang-Dong Jiang, J. Magn. Magn. Mater. 270 (2004) 84. [9] M. Yamaguchi, M. Matsumoto, H. Ohzeki, K.I. Arai, IEEE Trans. Magn. 27 (1991) 5274.