Integrated On-Chip Inductors with Magnetic Films Donald S. Gardner, Gerhard Schrom, Peter Hazucha, Fabrice Paillet, Tanay Karnik, Shekhar Borkar

Circuits Research, Intel Labs, Hillsboro, OR, USA Jason Saulters, Jordan Owens, Jeff Wetzel

ATDF, Inc., Austin, TX, USAAbstract

On-chip inductors with 2 levels of magnetic material were integrated into an advanced 130 nm CMOS process to ob-tain over an order of magnitude (>14×) increase in induc-tance and Q-factor, significantly greater than prior values of 2.3× for high frequency inductors. The magnetic mate-rial is shown to operate at frequencies beyond 6.4 GHz for 1 nH inductors. An amorphous CoZrTa alloy exhibiting high permeability, good high-temperature stability (>250°C), high saturation magnetization, low magne-tostriction, high resistivity, minimal hysteretic loss, and compatible with silicon technology was used in combina-tion with magnetic vias and elongated structures that take advantage of the uniaxial magnetic anisotropy.

Introduction The integration of on-chip inductors with magnetic materials into silicon process technology has been a major challenge in the move towards monolithic solutions for wireless microelec-tronics, power delivery, and EMI noise reduction. Spiral in-ductors fabricated on silicon without magnetic materials ex-hibit inductances ranging from 1-10 nH [1], but consume large amounts of area. With the addition of magnetic material, in-creased L and Q with decreased C can be achieved. Early re-search on magnetic films for inductors has focused on low-frequency materials [2] or processes with magnetic films (e.g. Ni80Fe20 permalloy) deposited after completing high-temperature (>250°C) processing or packaging [3, 4] making it impractical to manufacture. A limited increase in inductance (40-60%) has been previously demonstrated by deposing a single layer of magnetic material [5, 6] but the theoretical limit is only 100% when using a single magnetic film.

Although software simulation of inductors using two layers of magnetic material have been suggesting that very large increases in inductance are possible [5], it has proven to be difficult to achieve improvements as large as predicted with 47% [7], 65% [8], and 130% [9] being reported. Some studies have focused on obtaining high Q-factors, but in one case, the design was chosen such that the gain in inductance was only 8~23% [3, 7] or in another, 17% because of low permeabili-ties [10]. The coercivity and saturation magnetization of the magnetic material are also important because they will deter-mine the hysteretic losses and maximum current. Coercivities of 0.7 Oe [2] to 8 Oe [10] and higher have been reported in materials for inductors. Other studies are reviewed in [11].

In this work, optimized inductor structures (see Fig. 1) and a high-frequency magnetic material with low coercivity, high saturation magnetization, and high resistivity are integrated into an advanced Si process technology (see Fig. 2) to achieve up to over an order of magnitude (14.7×) increase in induc-

tance and a similar increase in the quality factor. The spirals are elongated to take advantage of the uniaxial magnetic ani-sotropy of the film. Magnetic vias are incorporated into the design to complete the path for the magnetic flux. With such improvements, the effects of eddy currents, skin effect, and proximity effect become clearly visible. Magnetic vias are instrumental in achieving these results. A magnetic via refers to the region where two magnetic layers contact each other to complete the circuit for the magnetic flux. The term “magnetic via” is used because the optimal structure for a magnetic via differs from an electrical via and may not be conductive.

Fig. 1. Microscope images of a spiral inductor (left), transmission lines (bottom) and a spiral inductor with slotted magnetic material (right).

CoZrTa

CoZrTa

Inductor

Cu (M6)

Cu (M4)

Cu (M2) Cu (M1)

Cu (M3)

Cu (M5)

Magnetic Via

Fig. 2. Cross sectional SEM image of inductors integrated on an 130 nm CMOS process with 6 metal levels. Two levels of CoZrTa magnetic material were deposited around the inductor wires using magnetic vias to complete the magnetic circuit.

Experiment Inductors and transmission lines using magnetic materials

were fabricated as part of a 130 nm CMOS process with 6-levels of metal. The amorphous Co-4.5%Ta-4.0%Zr (at.%) alloy was rendered magnetically anisotropic by application of a magnetic field during deposition, resulting in easy and hard axis orientations (see Fig. 3) and a low coercivity of 0.015 Oe that minimize hysteretic losses. A high coercivity will nega-tively impact the small-signal permeability. Zr is used to make the material amorphous and Ta is used to control mag-netostriction (coefficient = 0.2 µstrain as compared to 60 µstrain for pure Co). Figure 4 shows a low-frequency perme-ability of 875 G/Oe along the hard axis for a wide range of applied magnetic fields. The alloy exhibits a relatively high saturation magnetization of 15 kG. Figure 5 shows the mag-netic moment of CoZrTa after annealing at increasing tem-peratures up to 450°C. The magnetic moment decreases dur-ing extended anneals at 400°C and above. When the CoZrTa is annealed at 450°C, it forms crystalline Co and/or CoTax

compounds as can be seen in the X-ray diffraction measure-ments in Fig. 6. The result is an increase in the coercivity re-sulting in large hysteretic losses and a significant drop in the small-signal permeability (see Fig. 4). This crystallization can easily occur with cumulative exposure to high temperatures as shown in the TEM images of Fig. 7, so a key to integration is to maintain good control over the high temperature steps.

The CoZrTa has a high resistivity of 99 µΩ-cm that will re-duce eddy currents and a ferromagnetic resonance of 1.4 GHz, as measured on an unpatterned film (see Fig. 8). Pat-terning the film increases this frequency [5]. As the thickness increases, the real component of the permeability versus fre-quency decreases and the imaginary component becomes large.

Results and Discussion The uniaxial magnetic anisotropy of CoZrTa is ideal for

transmission lines. Measurements of inductance versus fre-quency of the transmission lines using two levels of 1.5µm CoZrTa show an inductance increase of up to 14.7× with the largest improvement occurring at 200 MHz for the narrowest 4 µm wide lines (see Fig. 9). This is a significant increase for

Fig. 4. Permeability versus applied magnetic field of amorphous and crystalline CoZrTa. The permeability decreases when the magnetic material saturates. The crystallized film was annealed at 450°C for 30 min. The small-signal permeability drops to ~25 because the coercivity becomes large.

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Co (101)Co11Ta4 (008)

Co0.745Ta0.255 (107)

Fig. 6. X-ray Diffraction analysis of CoZrTa as-deposited and after an-nealing for 30 minutes. The amorphous material can form crystalline Co and CoTax compounds upon annealing.

Fig. 5. Magnetic moment versus temperature of a CoZrTa sample during thermal cycles held for 30 minutes at the noted temperatures. The moment starts to decay when the material crystallizes. This would cause a significant inductance drop.

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Fig. 3. B-H magnetic hysteresis loops or magnetization curves of the hard and easy axis of magnetization for Co-ZrTa. The slope corresponds to the permeability. Insert shows that the coercivity is only 0.015 Oe resulting in minimal hysteretic losses when used to build inductors.

an on-chip inductor, higher than any published value known to the authors. Well designed magnetic vias on each side of the lines are instrumental in achieving these results in that they create a complete loop for the magnetic flux. The quality fac-tor increases by a similar factor of 13.7× for the 8 µm lines.

The inductance however drops at frequencies >200 MHz because eddy currents circulate in the magnetic film generat-ing resistive losses. For applications requiring higher frequen-cies, laminations and slots can be used to attenuate the eddy currents and extend the inductor usable frequency range. Laminations were fabricated by alternately depositing layers

of CoZrTa followed by exposure to an oxidizing ambient to form Co oxide that can be easily etched. In measurements of inductors using a 0.5 µm magnetic film composed of five 0.1 µm laminations, the rolloff frequency increased from 0.3 GHz to 0.8 GHz (see Fig. 10).

The quality factor of the inductors is improved by using magnetic films, but eddy currents limit the high frequency (GHz) performance. The quality factor of the inductors is also limited here from using thin aluminum for the conductor. The maximum quality factor for transmission lines using 0.5 µm thick CoZrTa is 4 at 100 MHz, significantly higher than what could be achieved with aluminum spiral inductors at that fre-quency (see Fig. 11). Analytical modeling shows that thinner magnetic films can be used to increase the quality factor be-cause of lower eddy currents (see Fig. 12), but inductance decreases. Slotting the magnetic film as in Fig. 1 helped to reduce the eddy currents, but the inductance drops because of less magnetic material and smaller magnetic vias. Combining laminations [12], patterned slots, and copper for the conduc-tor can help to improve the quality factor at high frequencies.

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Fig. 9. Inductance measurements of 940 µm long transmission lines with various wire widths with 2 levels of 1.5 µm thick CoZrTa ver-sus with no magnetic film and an aluminum ground plane. The in-ductance is increased by up to 14.7× using magnetic films with mag-netic vias.

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107 108 109 1010

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4 µm (CZT) 8 µm (CZT) 16 µm (CZT) 24 µm (CZT) 32 µm (CZT) 4 µm (Al) 8 µm (Al)

Fig. 10. Inductance measurements of 940 µm long transmission lines with 2 levels of 0.5 µm thick laminated CoZrTa versus with Al ground planes. Five laminations 0.1 µm thick each are used. The inductance is constant at frequencies to 0.8 Ghz because of lower eddy currents from laminated magnetic material.

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Fig. 7. TEM cross sectional image and diffraction patterns of amor-phous (left) and crystalline (right) CoZrTa. The CoZrTa crystal-lized after a high-temperature SiO2 deposition.

Fig. 8. Real and imaginary components of the complex permeability versus film thickness and frequency. The imaginary component becomes large with increasing thickness.

Spiral inductors can be elongated to take advantage of the uniaxial magnetic anisotropy of the film (see Fig. 1) and in-crease inductance. One level of magnetic material however show increases of only up to 36% (see Fig. 13), but high cutoff frequencies up to 6.4 GHz and beyond. The improvement is limited in part by the fact that only a fraction of the spiral is sitting over the magnetic film. Two magnetic films with mag-netic vias result in much larger increases in inductance of up to 6× with the magnetic material effective at frequencies up to 2.3 GHz to 6 GHz as shown in Fig. 14. At 1 GHz, the skin depth in cobalt is 0.5 µm, less than the 1.5 µm thickness of the magnetic material, so the field will not penetrate equally throughout the thickness of the magnetic material and the skin effect reduces the effective permeability of the medium.

Conclusion Magnetic films can be used to obtain significant increases in

inductance when using magnetic vias and elongated structures. CoZrTa is a magnetic material that combines high permeability, good thermal stability, low coercivity, high saturation magneti-zation, high resistivity, low magnetostriction, good high fre-quency properties, and is silicon process compatible. Depend-ing on the application, laminations, patterned slots, and high resistivity may be needed to control eddy currents.

References

[1] S.S. Mohan, M Hershenson, S.P. Boyd, T.H. Lee, IEEE J. of Solid State Circuits, v. 34, no. 10, pp. 1419-1424, Oct. 1999.

[2] J.Y. Park, S.H. Han, M.G. Allen, IEEE Trans. Magn. v.35, pp.4291-4300, 1999.

[3] M Yamaguchi, et al., J. Appl. Phy., v. 85(11), pp. 7919-7922, 1999. [4] Y. Zhuang, M. Vroubel, B.Rejaei, J.Burghartz., IEDM pp.475-478, 2002 [5] D.S. Gardner, A.M. Crawford, S. Wang, IEEE Intl Interconnect Tech.

Conf., pp. 101-103, 2001. [6] J. Salvia, J.A. Bain, C. Patrick Yue, IEEE IEDM, pp. 943 – 946, 2005. [7] M. Yamaguchi, M. Baba, K.I. Arai, T. Microwave Theory, vol. 49(12),

pp. 2331-2335, 2001. [8] J. Michel, Y. Lamy, A. Royet, B. Viala, Proc. Intl. Magn., p. 57, 2006. [9] A. Crawford, D.S. Gardner, S. Wang, IEEE T. Mag., v.40(4), pp. 2017-

2019, 2004. [10] C. Yang, et. al., Sensors and Actuators A 130-131, pp. 365-370, 2006. [11] V. Korenivski, J. Magnetism Mag. Materials, v. 215, pp 800-806, 2000. [12] W.P. Jayasekara, J.A. Bain, M.H. Kryder, IEEE T. Mag., v.34(4), pp.

1438-1440, 1998. Acknowledgments

We would like to thank Anupama Bowonder, Ken Sotoodeh, Brit-ton Birmingham, Tawfeeq Alzaben , Gabriel Gebara, ATDF staff, Mark McDougal, Michael Ricks, and the Mattec staff.

Fig. 13. Inductance measurements of spiral inductors with and with-out a single layer of 0.5 µm thick CoZrTa magnetic material. The inductance increases by only 36%, but up to frequencies of 6.4 GHz and beyond.

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Single 0.5 µm CoZrTa

Fig. 14. Inductance measurements of spiral inductors with and without two layers of 1.5 µm CoZrTa. The inductance increase isbetween 5× to 6× using 4 to 8 turns and magnetic vias in the center and edges of elongated spirals.

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Two 1.5 µm CoZrTa

Fig. 11. Quality factor of 940 µm long transmission lines fabricated with 1 µm thick aluminum and 2 levels of 0.5 µm thick CoZrTa or with an aluminum ground plane. The quality factor is improved by 6.5× using 0.5 µm thick CoZrTa.

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Fig. 12. Plots calculated using an analytical model of the quality factor versus the film thickness of CoZrTa.