The flexible inductive coils tag is fabricated using a previously developed SU-8 flexible technology [9], The flexible inductive coils of 30-turns, 70-turns and 90-turns are made of 5μm thick electroplated Cu. The coils of 30-turns, 70-turns and 90-turns are all designed in an area of 5mm x 10mm and 30μm, 15μm and 10μm in line width and 30μm, 10μm and 10μm in line-spacing, respectively.Fig. 3-1 illustrates the fabrication process of the SU-8 flexible inductive coil tags. A silicon substrate is first sputtered with 50nm thick Cr as a sacrificial layer as shown in Fig. 3-1 (a), followed by 35μm thick SU-8 spin coating as shown in Fig. 3-1 (b). After photo-patterning the SU-8 (Gersteltec Sarl GM 1060), a 50nm/100nm Ti/Cu seeding layer is deposited on the SU-8 as shown in Fig. 3-1(c) and followed by a photolithograph process using a 6μm thick AZ 4620 photoresist to patterned to define the region for coil fabrication of inductive sensor and electroplating an 5μm thick copper on that region as shown in Fig. 3-1 (d). After the first layer of 5μm thick Cu plating, another 6μm thick AZ 4620 is spun,patterned, and electroplating a 5μm thick copper for the via as shown in Fig. 3-1 (e), and sputtered with 150nm Cu seed layer for the via filling of Cu as shown in Fig. 3-1 (f). Fig. 3-1(g) shows another 6μm thick AZ 4620 is spun onto the plated structure,patterned, and plated with 5μm thick Cu to define air bridge after via filling.The AZ 4620 and seed layer are then removed using ACE, CR-7T as shows in Fig. 3-1(h). At the stage, the flexible inductive coil without a magnetic core is completed. For the case of the sensor tag with the NiFe magentci core, AZ4620 photoresist is spin-coated and patterned again to define the magnetic core region on the central area of the coil and used as a
9
Figure 3-1. illustrates the fabrication process of the SU-8 flexible current sensor.
Silicon Cr SU-8 NiFe
Ti/Cu AZ 4620 Copper
10
mold for electroplating a 4µm-thick NiFe layer as shows in Fig. 3-1(i), and AZ 4620 and Ti/Cu seed layer are then removed using ACE, CR-7T, BOE, respectively as shows in Fig. 3-1(j). At final, the SU-8 flexible inductive coil with NiFe core is released from the silicon substrate by dipping in a Cr etchant layer (H2O:HCl=100:30) for the sacrificial layer removal as shows in Fig.
3-1(k).
(a)
(b) (c)
Figure 3-2. The optical photographs of the flexible inductive coil tag fabrication (a) that before sacrificial Cr layer released and (b) released from the substrate (c) after released for inductive coil with NiFe attached to the cord
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Fig. 3-2 shows the optical photographs of the as-fabricated sensor tag before (Fig. 3-2(a)) and after (Fig. 3-2(b)) the sacrificial Cr layer release and the tag with a magnetic core after (Fig. 3-2(c)) the sacrificial Cr layer release and then attached to the power cord, respectively. Fig. 3-3 shows the SEM image on the top of the 30-turns inductive coil (Fig. 3-3(a)) which is designed with 30μm in width and 30μm line-spacing (Fig. 3-3(b)). Fig. 3-3 (b) and (c) show the inductive coil tags with the cores of NiFe array and NiFe membrane, respectively. Fig. 3-3(e) shows the NiFe array core where the size of each element is 10x10 μm2and 10μm in spacing.
(a) (b)
(c) (d)
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(e)
Figure 3-3. SEM photograph of as-fabricated on inductive coil (a)top view of 30-turns air coil (b) 30μm line width of 30-turns coil (c) 30-turns coil with NiFe array (d) 30-turns coil with NiFe membrane (e)NiFe array each side is 10μm
13
Chapter 4 Results And Discussion
According to the aforementioned analysis, the sensed signal is about several tens of µV/A which requires a signal amplifier for the electrical characterization of the flexible inductive coil. Fig. 4-1 shows an active low-pass filter circuit which is designed with 66.9dB gain resulted by the resistance ration of R1(R2) to R3. In the work, 300 and 1MΩ variable resistors are chosen for a flat gain response in a frequency range of 45 to 100 Hz which is a typical range of the household AC power signals. The low noise operational amplifier, OP-27G (Texas Instruments Inc.), is used for the gain stage. The flexible inductive coil tag is then soldered with two metal wires connected to the circuit formed on a PCB.
Figure 4-1. The active low-pass filter circuit scheme on the lower left hand side and the transfer function whose circuit gain is 66dB in spice simulation and 66.9dB in measurement respectively.
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(a)
(b)
Figure 4-2. The oscilloscope monitor shows (a) a sinusoidal wave while the sensor pastes on and (b) no signal output while the sensor takes up from the power cord with 3A, 60Hz current input. The inset of (b) shows the as-fabricated device.
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Fig. 4-2 shows the setup for the current sensing measurement. All feedthroughs are ground shielded to avoid electrical coupling and interference with the testing system. The senor tag shown in the inset of is attached to (Fig.
10a)/detached from (Fig. 10b) a 2C power cord loaded with the current source (ELGAR CW801M) with a 1A, 60Hz current output. When the tag is detached, the disappearance of the sensed signal in the oscilloscope validates the sensing mechanism of magnetic flux coupling and the feasibility of the flexible inductive coil tag for current sensing. Fig. 4-3 shows the measured voltage in the coils versus the input current in the cord. The data has been converted into the intrinsic signals without amplification. For 30/70/90-turns inductive coil tags, a linear relation of the induced voltage with the input current can be accomplished with the sensitivity of 23.2/55.5/68.5µV/A, respectively. While the as-fabricated tag is applied for detecting the 60Hz electrical current inputs in a 2C power cord in the ampere regime, it has been found that, instead of 26.9/63.6/80.7µV/A derived by the theory model, the average coupling efficiency of the 30/70/90-turns inductive coil tags is only 86/87/85% of the ideal values, respectively. Meanwhile, it is found that the induced voltage of the 30-turns inductive coils both with NiFe array and membrane cores can have the larger but similar sensitivity which is 26.3/25.6/µV/A, respectively. Since the total volume of the magnetic cores are quite different, the sensitivity difference between the coil with and without the magnetic core might not be caused by the magnetic flux enhancement. It could be attributed to poor attachment of the sensor tag to the power cord. Fig. 4-4 shows the measurement versus the theoretical calculation of the 30-turns inductive coil with different gap distances between the power cord and the coil. While the
16
gap is changed from the ideal value, i.e. 1.785mm, to 1.825mm and 1.815mm, the calculated data can well fit with the measurement results of the tags with the NiFe membrane and array cores, respectively. The ideal case is defined as the total thickness of the SU-8 layer and the coated insulator of the power cord.
Although NiFe exhibits a large permeability μr
, it should be taken into account
that the resultant permeability of the core, μc, can be much lower than the material permeability due to the demagnetizing field effect which can be resolved by increase the aspect ratio of magnetic material [8]. In this work, the aspect ratios of the NiFe array and membrane cores are both less than 1, so the enhancement can be limited by the effect.Figure 4-3. The simulation and theoretical calculation of the induced voltage versus input-current for the inductive tag .
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(a)
(b)
18
(c)
Figure 4-4. The theory determined of induced voltage versus calculated values with different gap distances (a)air coil (b)NiFe membrane coil (c)NiFe array coil
In fact, it can be further verified by the magnetic hysteresis measurement.
Fig. 4-5 shows the superconducting quantum interference device (SQUID) measurements of the NiFe membrane with the fields whose directions are out of plane and in plane, respectively. From the slope of the M-H curve, the relative permeability can be calculated as follows:
0
1 4
M H
r π
µ = + (7)
where M is magnetization and H0 is the applied magnetic field. The relative
19
permeability in our case is only about 1.4 which is quite small for the out-of-plane field. Negligible enhancement can be expected.
Figure 4-5. Hysteresis loop of NiFe with applied magnetic field
The sensitivity, in fact, decreases with the gap resulting in lower magnetic flux coupling. Although the flexible inductive coil tag is aimed to take the advantage of a good proximity effect on the current sensing, the connecting wires soldered to the tag would make the coil structure difficult in closely attaching to the power cord by manual control. Owing to the process characteristic of the flexible tag which is fully compatible with the previously developed SU-8 flexible technology, it is our belief that the sensitivity reduction problem can be further resolved by integrating the coil tag with a CMOS readout circuit chip with wireless data transmission function as a result of easy
20
implementation and good proximity.
TABLE.4-1
Sensor type AMR Piezoelectric Sensor size 15.6*17.8*2mm 1*0.2mm Circuit gain 80dB 40dB
Sensitivity* 80μV/A 0.87mV/A@60Hz
DC capable Yes No
*The results derived from the amplified data.
TABLE4-1 THE LIST OF CURRENT SENSORS
Schulz et al.
(2010) [12]
This work
Sensor type Rogowski coil Faraday induction Sensor size 2.5*4.5*0.45mm 10*5mm Circuit gain 100dB 66.9dB
Sensitivity* 116nV/A-Hz 80μV/A@60Hz
DC capable No No
*The results derived from the amplified data.
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The flexible sensor tag is compared with the state-of-the-art current sensors as shown in Table 4-1[10–12]. Basically, all of these sensors require amplified circuits, but the circuit specification of the presented one is not the most stringent in comparison with the others. Meanwhile, the coil size is much smaller than the wavelength of the background 60Hz EM wave (~5000 km) as a result of low antenna gain. The signal coupled from the environment EM field would be very extremely small.
In addition, the voltage induction of the presented air-coil sensor is irrelevant to any material properties. The sensor can be important because in contrary to other magnetic sensors, e.g. piezoelectric current sensor with a permanent magnet mounted, Hall sensor, it is not sensitive to DC component of magnetic field and environmental temperature. Thus, good proximity and stability indicate that the inductively sensing mechanism is feasible and the sensor can be prevailed for the applications of household electricity monitoring systems.
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Chapter 5 Conclusion
A flexible inductive coil tag is demonstrated with the potential for the application to low-cost, reliable and pervasive DR electricity monitoring systems for residential power management. A physical model has been derived and verified for the coil design. According to the model, the sensitivity can be improved and determined immediately by reducing the metal width and increase the coil number simultaneously. Although the induced voltage will be saturated with the coil turns which is caused by less magnetic flux contribution from the inner coils closely to the central region, it can be resolved by incorporating a high aspect ratio ferromagnetic core since the sensitivity of the coil with magnetic material depends mostly on the dimensions and geometry of the core. Further experimental validation is required.
23
Reference
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Inductive Coil Tag for Household Two-Wire Current Sensing Applications,” submitted to IEEE sensors Journal
25
Vita
姓 名：余松築 (Sung-Chu Yu)
出生日期：中華民國七十六年一月二十一日 出 生 地：中壢市
E - mail：
[email protected]
學 歷：國立內壢高級中學 (2002.9～2005.6)
(National Neili Senior High School)
國立中正大學物理學系 (2005.9～2009.6)
(Department of Physics, National Chung Cheng University)
國立交通大學電子工程所碩士班 (2009.9～2011.8)
(Department of Electronics Engineering & Institute of Electronics, Nation Chiao Tung University)