3.3 Results and Discussions
3.3.4 Electrical characterisation of AgNP printed micro-resistors
This section discusses the I-V characterisation for the above-printed micro-resistors. DC probe station of Lakeshore cryotronics (Model: CPX) and EverBeing (Model: BD6) was used along with Keithley 4200A-SCS Parametric Analyzer [61] to probe the printed resistors. Figure 3.9(a) shows the I-V characteristics of the spillover AgNP layer over the glass, SiO2/Si and polyethylene terephthalate TH-2495_146102016
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-0.10 -0.05 0.00 0.05 0.10 -0.02
-0.01 0.00 0.01 0.02
Current(A)
Voltage (V)
Glass
SiO 2
/Si
PET
(a)
-0.10 -0.05 0.00 0.05 0.10 -0.0020
-0.0015 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015
Current(A)
Voltage (V)
SOP R1
SOP R2
SOP R3
(b)
Fig. 3.9: (a) I-V characteristics of AgNP spill layer over different substrates (b) I-V characteristics of repre- sentative SOP-PMRs after annealing
−0.4−2 −0.2 0 0.2 0.4
0 2 4x 10−6
Voltage (V)
Current (A)
−0.4 −0.2 0 0.2 0.4
−0.1
−0.05 0 0.05 0.1
Voltage (V)
Current (A)
0 5 10 15 20
−1 0 1 2 3x 10−8
Voltage (V)
Current (A)
0 5 10 15 20
−1 0 1 2 3x 10−6
Voltage (V)
Current (A)
(a)
(d) (b)
(c)
Fig. 3.10: I-V characteristics of drop-casted layer of AgNP ink on glass (a) before annealing (b) after annealing.
I-V characteristics of SOP-PMR on glass (c) before annealing (d) after annealing
(PET). The I-V curve of AgNP spill layer on glass has been used as a reference to compare the I- V characteristics of the SOP, DIPSOP and SDP based PMRs on the glass. It has been measured that AgNP spill layer on glass has R of 4.4 Ω. The reason behind such low R and higher current of the AgNP spill layer is its higher thickness and surface uniformity. Figure 3.9(b) shows the I-V characteristics of SOP-PMRs on the glass substrate. It is observed that current varies in the range of -1.67 mA to 0.573 mA for an applied voltage linear sweep of ± 0.1 V while the measured R, varies in range of 94 Ωto 224Ω. Figure 3.11(a) shows the I-V plot for SOP R4 for different levels of probe
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−0.5 0 0.5
−2 0 2 4 6 8 10x 10−11
Voltage (V)
Current (A)
0 500 1000 1500
0 20 40 60 80 100 120 140 160 180 200
Thickness (nm)
Average electrical resistance, R ( Ω)
Probe tip at starting level Probe tip at first deep level Probe tip at second deep level Probe tip at third deep level (TSE)
(a) (b)
Fig. 3.11: (a) I-V characteristics of AgNP SOP R4 on glass surface at different levels of dc probe tip adjustment indicating a bad contact (b) Plot of measured average electrical resistance of PMRs against their thickness
−0.1 −0.05 0 0.05 0.1
−1
−0.8
−0.6
−0.4
−0.2 0 0.2 0.4 0.6 0.8 1
x 10−3
Voltage (V)
Current (A)
−0.1 −0.05 0 0.05 0.1
−5
−4
−3
−2
−1 0 1 2 3 4 5x 10−6
Voltage (V)
Current (A)
DIPSOP R1
DIPSOP R2 DIPSOP R3
(a) (b)
Fig. 3.12: I-V characteristics DIPSOP-PMRS after annealing
tip adjustment. It is observed that as the probe tip is forced more deep into the device, current initially increases but as the tip is forced further deep into the device, TSE occurs (Figure 3.13(a)) and device starts exhibiting non-conducting behaviour. Therefore, proper tip adjustment is also a crucial parameter to achieve the desired characteristics for the electrical characterization of a bare printed resistor. However, Figure 3.11(a) shows that current varies non-linearly with the applied voltage and gets saturated after a certain voltage in the first three cases which are free from TSE. The probable reason for such non-linear behaviour seems to be the non-uniform surface of the glass substrate and TH-2495_146102016
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Fig. 3.13: (a) Optical image showing TSE effect seen in SOP R4 on glass in third level of probing (b) Optical image showing ‘IITG’ printed using SOP technique over glass substrate
PMR. Figure 3.13(b) shows the printing of ‘IITG’ using SOP technique over coverslip gass. The inset- region in Figure 3.13(b) has been used for study of surface uniformity of SOP-printed stucture after annealing. Figure 3.11(b) shows a combined plot of measured R of the PMRs against their thickness.
It is verified that R decreases with increasing the print-thickness. The resistivity of PMRs on glass having R inΩ-range is computed using standard formula,ρ= RWt/L, where R is the average electrical resistance, W is the channel width, L is the channel length and t is the print-thickness of PMR. The order of the resistivity is same for all the PMRs which is10−5 Ω-m, with variation in magnitude from 1.22 to 5.4. This variation is observed probably due to higher surface roughness of the PMRs which results into the surface scattering of the charge carriers and subsequent variation in resistivity [75, 76].
Figure 3.12 shows the I-V characteristics of representative DIPSOP-PMRs. The measured R varies in range of 120Ωand 194Ωwith a current variation of±0.2 mA to ±1.2 mA for an applied voltage sweep of ± 0.1 V in DIPSOP R1 and DIPSOP R2. However, some PMRs such as DIPSOP R3 have comparatively higher R of 24 kΩwith a current variation of±0.004 mA for the same applied voltage range. The probable reason can be its thinner channel region and higher surface roughness. The SDP based printing technique is optimised and the ink is properly distributed over the two contact pads and the channel by the SPT micro-cantilever to achieve uniform thickness throughout the device.
Figure 3.14 shows the electrical response of SDP R1. The current varies from ±6 mA (with an R of 16 Ω) for a voltage linear sweep from ± 0.1 V. This result establishes SDP technique as a potential technique to print micro-structures of required dimensions and desired functionality. Moreover, the linear variation of current with applied voltage in the I-V plots of each printing technique validates our
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−0.1 −0.08 −0.06 −0.04 −0.02 0 0.02 0.04 0.06 0.08 0.1
−8
−6
−4
−2 0 2 4 6 8x 10−3
Voltage (V)
Current (A)
SDP R1
Fig. 3.14: I-V characteristics of SDP R1 on SiO2/Si
Table 3.3: Table for comparison between standard SMD chip resistors and MCP based PMRs Make and Model Resistance Length Width Thickness Vishay CRCW e3 Series, Model: 1206 16Ω L∼3.2 mm W∼1.6 mm t ∼0.55 mm
SDP R4 16Ω L∼ 500 µm W∼ 200µm t ∼700 nm WALSIN WR06 Series, Model: 0603 93.1Ω L∼1.6 mm W∼0.8 mm t ∼0.45 mm SOP R1 94Ω L∼ 475 µm W∼ 200µm t ∼360 nm TE CONNECTIVITY CPF-A Series, Model: 0603 120Ω L∼1.55 mm W∼0.8 mm t ∼0.45 mm DIPSOP R2 120Ω L∼ 300 µm W∼ 120µm t ∼350 nm PANASONIC ERA6A Series, Model: 0805 133Ω L∼2 mm W∼1.25 mm t∼0.5 mm
SOP R2 134Ω L∼ 450 µm W∼ 200µm t ∼353 nm MULTICOMP MCT05 Series, Model: 0805 196Ω L∼2 mm W∼1.25 mm t ∼0.55 mm DIPSOP R1 194Ω L∼ 190 µm W∼ 125µm t ∼300 nm MultiComp MCMR Series, Model: 0402 226Ω L∼1 mm W∼0.5 mm t ∼0.35 mm SOP R3 224Ω L∼ 460 µm W∼ 200µm t ∼386 nm
* L,W and t are overall device Length, Width and Thickness
claim to consider the printed structures as micro-resistors. When I-V measurements were performed after 1 year, a shift of few ohms was observed in R of PMRs which states that MCP based PMRs are electrically stable even after 1 year. A summary of fabricated and measured PMRs is presented in Table 7.2 where spill layer data has been presented as a reference for comparison. Since the printed contact pads are not sufficiently thick, the level of probing varies from one PMR to other PMR.
Therefore, R varies somewhat inconsistently with the product of W/L ratio and t from one PMR to other as seen in Table 7.2.
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Fig. 3.15: AFM images for different printing methods and printed layers