Chapter 4 Ferromagnetic-AAO Nanocomposite for
4.3 NiFe-AAO Nanocomposite
4.3.1 Synthesis and Characterization
In comparison with the previously developed electroless deposited Ni-P-AAO nanocomposite process, the synthesis of NiFe-AAO nanocomposite requires a seed layer, Ti, under AAO layer as a conducting layer for electroplating. Therefore, two additional steps, applied voltage adjustment while Al is anodized and reversed-biasing in KCl, are applied for thinning the alumina oxide barrier layer located at the bottom of AAO nanopores to ensure electroplating stability. The detail process steps are listed as follows.
The composite synthesis begins with a Ti (200 nm)/Al (1000 nm) layer deposition by e-gun evaporator on a 4” silicon substrate grown with a layer of 0.75 μm thermal oxide as an electrical isolation layer. The AAO template is then fabricated in a 0.3M oxalic acid (H2C2O4) solution at 2°C with a bias of 40V applied on the substrate. The low temperature anodization process can reduce excessive current flow and heat evolution to make the AAO with the characteristics of small-pore films [115]. Once the film is anodized, the
voltage is reduced from 40V to 15V and then kept for 15mins for thinning the alumina oxide barrier layer. The insulting barrier layer is then further thinned down to completely anodize Al film by applying a reversed-bias voltage (~-2V) to the substrate in a 0.5M saturated KCl for 10mins [115, 116]. Finally, the AAO temple is put in a 5% H3PO4 solution at 30°C for 25mins to form uniformly distributed nanopores which are 70 nm in diameter. At final, the as-fabricated AAO temple is electroplated in a Ni-sulfate-based NiFe bath to form a layer of NiFe-AAO nanocomposite which the inductor can then be fabricated on.
Figure 4.9 shows the top view and cross sectional view of as-deposited NiFe-AAO nanocomposite. NiFe nanorods are grown within the AAO matrix.
Figure 4.10(a) shows hysteresis loop of the Ni-based-AAO nanocomposite film measured by SQUID. The relative permeability which can be calculated from the slope of M-H curve of the NiFe-AAO nanocomposite is about 20.3, 11-times larger than that of Ni-P-AAO nanocomposite. The energy dispersive spectrometer (EDS) spectrum, as shown in Figure 4.10(b), of NiFe-AAO nanocomposite verifies the composition NiFe.
4.3.2 Measurement and Discussion
Figure 4.11 shows extracted inductance and quality factor of as-fabricated inductors. The 3.5-turns inductor with a NiFe-AAO nanocomposite core averagely has 25% inductance enhancement up to 1GHz in comparison with that of the inductor without the core. Meanwhile, the maximum Q-factor decreases from 12.1 to 5.9, which is about 51% reduction. Similar inductance enhancement and Q degradation are also found in the case of the 4.5-turns
inductor. Furthermore, the self-resonance frequency of inductors with the nanocomposite core has drastically decreased from about 14GHz to about 3GHz.
In comparison with the previously developed Ni-P-AAO inductor which is about 6.5% Q-factor reduction and has almost no self-resonance frequency shift, the reduction of inductor performance using NiFe-AAO magnetic core could be attributed to the existence of a conductive 200nm Ti layer underneath the NiFe-AAO. From the point of view of self-resonance frequency, the continuous Ti layer obviously contributes extra parasitic capacitance to the inductor as a result of a lower frequency. Meanwhile, the shunt parasitic capacitance would also degrade the Q value of inductors. For a conventional on-chip spiral inductor, the Q can be depicted as the following equation [117]:
( ) ( )
feed-forward capacitance of the inductor, parasitic shunt capacitance, resistance, and signal frequency, respectively. The last term of (4.15) is a self-resonance factor with the value less than 1. The existence of Ti seed layer would increase the parasitic shunt capacitance, Cp, so as to not only limit the usable bandwidth of inductor but also decrease the Q performance. In addition, another source of the Q degradation can be attributed to the induced eddy current loss within Ti layer and magnetic loss in NiFe.In order to eliminate the effect of Ti layer, thermal annealing process at 450°C is utilized to form an amorphous TiOx layer at the interface between Ti and the SiO2[118]. Figure 4.12 shows the frequency dependence of the inductance and Q-factor of the fabricated inductors annealed for 60 and 120 mins, respectively. After the thermal anneal, the inductance of NiFe-AAO
doesn’t have a great difference. However, the self-resonance frequency has increased ~600MHz and 400MHz for the cases of the 4.5-turns and 3.5-turns inductors, respectively. On the other hand, the maximum Q values increase with the annealing time. For the case of a 4.5-turns inductor after with 120mins thermal anneal at 450°C, the maximum Q factor can be improved with a larger value than that of the inductor without the core. These results indicate that the presence of Ti layer indeed degrades the inductor performance and must be removed for practical use. Therefore, a series of micromachining process is designed and still under way to remove the Si substrate and Ti seed layer underneath NiFe-AAO layer for the reduction purpose of the eddy current loss and parasitic capacitance. After that, the high frequency magnetic property and loss mechanism including eddy current loss and hysteresis loss of NiFe-AAO can be well understood.
Although FMR effect can not be observed in the proposed NiFe-AAO inductor due to the low self-resonance frequency, the potential of inductance enhancement using the ferromagnetic-AAO nanocomposite material has been verified again in this study. The work further validates the conjecture with the performance improvement summarized in Table 4.1. Table 4.2 summarizes the comparison of this work with the prior arts. It indicates the NiFe-AAO nanocomposite core can provide an alternative cost-effective and practical solution for the performance enhancement of on-chip inductors in terms of CMOS foundry manufacturability.
4.4 Summary
The performance enhancement scheme of on-chip spiral inductor using
ferromagnetic-AAO nanocomposite core is presented in this chapter. Two nanocomposite structures, Ni-P-AAO and NiFe-AAO, with different synthesis procedure have been successfully developed for inductance enhancement up to GHz region. By take advantage of high aspect ratio and isolated ferromagnetic nanorods structure, it can be expected that such a nanocomposite could has great potential for integrated inductor with high FMR frequency and low eddy current loss behavior. The high-frequency characteristics of on-chip spiral inductor with Ni-P-AAO nanocomposite core have been investigated. Incorporated with the nanocomposite core, the 3.5-and 4.5-turn inductors can have improved inductance up to 6.9 and 5.5 GHz, respectively. The insignificant inductance enhancement and Q degradation have been found in the inductors with the nanocomposite core, it could be further improved by incorporation of ferromagnetic metal with larger permeability. Thus, NiFe-AAO nanocomposite core has been synthesized and demonstrated its promise capability for inductance enhancement up to 1 GHz. Although the eddy current loss and the parasitic capacitance effect resulted by Ti seed layer cause the degradation of Q factor and limited usable bandwidth of the inductor, the drawbacks can be overcome by thermal annealing to form high resistive TiOx. Based on the proposed CMOS-compatible fabrication scheme, it is believed that the ferromagnetic-AAO nanocomposite can have a great potential application for future RF SOP manufacture.
Figure 4.1 A schematic of dipole moment Mr precessing about a static magnetic field, Heff. The alternating magnetic field, Hac, is applied normal to the static field.
Figure 4.2 A schematic 3-D cross section view of the on-chip spiral inductor with Ferromagnetic-AAO nanocomposite core.
(a) (b)
Figure 4.3 SEM photographs of the top views of (a) AAO template and (b) Ni nanorods
(a) (b)
Figure 4.4 M-H loops of Ni-P-AAO nanocomposite with different forms of applied magnetic fields. (a) Out-of-plane. (b) In-plane.
Figure 4.5 Fabrication process flow. (a) SiO2 and Al layer deposition. (b) Al anodized as AAO template. (c) Electroless Ni plating. (d) SiO2 and Cr/Cu seed/adhesion layer deposition. (e) First Cu plating for the coil part of the inductor. (f) Air bridge seed layer deposition and patterning. (g) Air bridge and via plating. (h) Seed layer and PR removal.
(a) (b)
Figure 4.6 SEM photographs of as-fabricated on-chip spiral inductors with the Ni-P-AAO nanocomposite (a) under whole area and (b) only in the central region.
(a) (b)
(c) (d)
Figure 4.7 Measured high-frequency characteristics of spiral inductors with Ni-P-AAO nanocomposite core. (a) Inductance and (b) Q-factor of spiral inductor with N=3.5, din =100 µm. (c) Inductance and (d) Q-factor of the spiral inductor with N=4.5, din =70 µm.
Figure 4.8 Simulated magnitude distribution of magnetic field of spiral inductor using Ansoft HFSS.
Figure 4.9 The (a) top view and (b) cross section view SEM micrographs of as-deposited NiFe-AAO magnetic nanocomposite.
Air-Bridge Metal Coil
Figure 4.10 (a) The M-H loops of Ni-P-AAO and NiFe-AAO nanocomposite with applied out-of-plane magnetic field, and (b) the EDS analysis of NiFe-AAO nanocomposite.
(a)
(b)
Figure 4.11 The measured inductance and quality factor of rectangular spiral inductors which are designed with (a) n=3.5, din=100μm and (b) n=4.5, din=70μm, with and without NiFe magnetic nanocomposite core.
Figure 4.12 The measured high-frequency characteristics of NiFe-AAO spiral inductors after annealing at 450°C for 60mins and 120mins. The Inductance of inductor with (a) n=3.5 and (b) n=4.5, and the quality factor of inductor with (c) n=3.5 and (d) n=4.5.
Table 4.1 Comparisons of performance variation between developed Ni-Based-AAO nanocomposite inductors with relative to air-core inductor.
Material Inductance enhancement
Quality factor
reduction μ
Chapter 4.2 Ni-P 3% 6.7% 1.8
Chapter 4.3 NiFe >25% No degradation 20.3
Table 4.2 Comparisons between previously developed technologies [94-97] and this work.
Ref. [94] Ref. [95] Ref. [96] Ref. [97] This Work
Magnetic Material Co85Nb12Zr3 (Fe97.6Hf2.6)90N10 Fe3O4 IrMn/CoFe NiFe-AAO Deposition Method RF Sputter
Ion milling RF Sputter Gel-sol method DC Sputter Electroplating Structure Patterned sandwich
magnetic film Laminated film Magnetite
nanorod core Mulitayers Planar nanorod Inductance
Enhancement 8.3% @1GHz 33% @1.5GHz 3.5% @3GHz 30% @1.8GHz Max. 32% @1GHz