High frequency behaviour and low frequency noise of LTCC resistors

XXVI International Conference of IMAPS Poland Chapter Warsaw, 25-27 September 2002

High Frequency Behaviour and Low Frequency Noise of LTCC Resistors Andrzej Dziedzic1), Karl-Heinz Drüe2), Jarosław Kita1), Andrzej Kolek3), Piotr Ptak3) 1) Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland e-mail: [email protected], 2)Ilmenau University of Technology, D-98684 Ilmenau, Germany, 3)Rzeszów University of Technology, W. Pola 2, 35-959 Rzeszów, Poland Key words: thick-film resistor, LTCC, high frequency behaviour, 1/f noise

Abstract AC electrical behaviour as well as low frequency noise belong to very important exploitation parameters of thick-film resistors. This paper presents high frequency impedance spectra as well as low-frequency noise of surface and buried LTCC resistors made from CF Du Pont resistor system, specially made for fabrication of buried resistors. The influence of conductive phase content in resistive film (represented by DC sheet resistance), the resistor position and its dimension both on complex impedance spectra in the range from 0.3 to 3000 MHz as well as on 1/f noise level were tested, analysed and correlated.

1. Introduction Low Temperature Co-Fired Ceramics (LTCC) technology is used in fabrication of high frequency and microwave circuits more and more often. Therefore the behaviour of various LTCC components should be tested in these frequency range. This paper presents and discusses the impedance spectra of LTCC resistors in the frequency range from 300 kHz to 3 GHz. The surface and buried resistors were made from CF 021 (100 ohm/sq.) and CF 041 (10 kohm/sq.) resistive ink with various area of unit square. The complex impedance spectra of tested components depend both on components resistance as well as on their parasitics. It was suggested earlier [1], that the current noise is connected with value of parasitics in resistor electrical equivalent circuit. Therefore the low-frequency noise of the same test resistors was measured in the frequency range from 0.06 to 10 Hz. Both electrical characteristics were tested and analysed in dependence of the test resistor resistance, position and dimension.

2. Test sample fabrication Special resistor test pattern with coaxial form of termination layer (diameter of inner pad – 3 mm, diameter of the outer ring – 7 mm) and four resistors (resistor length (width) – 0.3, 0.6, 0.9 or 1.2 mm, respectively; resistor aspect ratio equal to 1) connected in parallel was designed for high frequency impedance and low frequency noise measurements (Fig. 1). Table 1. DC resistance (mean value;resistive ink, resistor position and dimensio in ohms) of tested LTCC resistors Resistor Dimensions 0.3×0.3 mm2 0.6×0.6 mm2 0.9×0.9 mm2 1.2×1.2 mm2

CF 021 S 10.8 12.1 12.2 11.7

CF 021 B 48.2 45.8 53.6 50.5

CF 041 S 596 727 1116 1172

CF 041 B 3576 2814 2848 2202

Fig. 1. Design of embedded and surface LTCC resistors for high frequency measurements

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XXVI International Conference of IMAPS Poland Chapter, Warsaw 25-27.08.2002

The resistors were made on the top (surface component) or between first and second layer from the top (buried one) on/in LTCC substrates consisting of three DP 951 A2 green tapes. The resistive (CF 021 – 100 ohm/sq. or CF 041 – 10 kohm/sq.; both from Du Pont) and PdAg-based DP 6146 conductive films were screen printed through 325 mesh stainless steel and then dried at 70oC for 10 min. The lamination process was carried out in an isostatic press at 210 bar for 10 min. held at 70oC. Next, independently on resistor position, the test samples were fired under time-temperature profile recommended by tape manufacturers, i.e. they were burned out at 450oC and fired at 875oC. The sheet resistance for various resistive ink as well as resistor position and dimensions are collected in Table 1.

3. High-frequency behaviour All high-frequency measurements were done using HP 8753C network analyzer in the frequency range from 300 kHz to 3 GHz. The test samples were connected to one port of the network analyze3r by a N-type cable and air-filled line of about 35 mm length. The end of this line was adhered to the test substrates [2,3]. Then the reflection coefficient was measured and the complex impedance of the resistor under test was determined. The results shown in Figs. 2 and 3 were received after calibration procedure. This means, that in the first stage the impedance spectra were measured for test samples from Fig. 1. The same measurements were repeated for the samples containing only conductive layers inside LTCC structures in the second stage and the impedance of an empty termination was subtracted from the measured in the first stage.

15

0.3 0.6 0.9 1.2

14 13 12

1

6

10

7

8

10 10 f [Hz]

5

50 48

0.3 0.6 0.9 1.2

X [ohm]

52

46 44 42 5 10

10

6

7

10 10 f [Hz]

8

10

9

10

8 CF021, buried 0.3 0.6 6 0.9 1.2 4

CF021, buried 54

10

7

8

10 10 f [Hz]

10

9

11

12

13 14 R [ohm]

15

16

CF021, buried 0.3 0.6 0.9 1.2

6 4

2 0

2 0

-2 6

1 0

10

9

10

0.3 0.6 0.9 1.2

2

0

10 5 10

CF021, surface

3

0.3 0.6 0.9 1.2

2

11

R [ohm]

CF021, surface

X [ohm]

R [ohm]

3

X [ohm]

CF021, surface

X [ohm]

16

-2 7

8

10

10 f [Hz]

9

10

42

44

46

48 50 R [ohm]

52

54

Fig. 2. Bode and Nyquist plots of buried and surface CF 021 LTCC resistors with various unit square

CF 021 ink serve for fabrication of low ohmic resistors whereas CF 041 for medium ohmic ones. Therefore all CF 041 (Fig. 3) resistors exhibit capacitive character (X = Im Z< 0) whereas the inductive character prevails in CF 021 resistors (Fig. 2). But pure inductive behaviour is characteristic only for CF 021 surface resistors with 0.3 or 0.6 mm resistor length. The impedance spectra of other low ohmic components suggest the presence of small capacitor in their electrical equivalent circuit. This is more visible for resistors with larger unit square because they have very small reactive parts. Reactive (inductive or capacitive) character starts to manifest itself above 108 Hz for low ohmic resistors or above 107 Hz for CF 041 resistors. One should mention that similar behaviour is observed for other thick-film resistors – polymer [4,5,6], cermet [1,2,7,8] or LTCC [2,3,9] ones.

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-200

0.3 0.6 0.9 1.2

400 200 5

10

4000

6

10

-300

0.3 0.6 0.9 1.2

-400 -500

7

8

10 10 f [Hz]

5

10

9

10

CF041, buried

400

7

8

10 10 f [Hz]

9

CF041, buried

-400 0.3 0.6 0.9 1.2

-800

-1200 5

10

6

7

8

10 10 f [Hz]

9

10

10

-400

10

X [ohm]

X [ohm]

R [ohm]

0 5 10

0.3 0.6 0.9 1.2

-200

-600

6

10

0

2000

0.3 0.6 0.9 1.2

0

X [ohm]

-100

800

X [ohm]

1000 R [ohm]

0

600

CF041, surface

CF041, surface

CF041, surface 1200

250

500

750 R [ohm]

1000

1250

400 CF041, buried 0.3 0.6 0 0.9 1.2 -400 -800

-1200

6

7

10

8

10 10 f [Hz]

9

0

10

2000 R [ohm]

4000

Fig. 3. Bode and Nyquist plots of buried and surface CF 041 LTCC resistors with various unit square

4. Low-frequency noise Low frequency noise measurement was performed with AC technique proposed by Scofield [10,11], where bridge with measured resistor was supplied through ballast resistor by a single frequency alternating current I = I0cosω0t. It is important to note, that such AC technique allows noise measurements for frequencies down to mHz and can be performed at lower voltages in comparison with a standard DC technique (see eg. [9,12,13]). The excitation frequency used in measurements was f0 = 325 Hz. This frequency was chosen in order to make peaks arising from mixing carrier frequency and harmonics of 50 Hz (line frequency) as low as possible. The typical resistance fluctuations received in AC Scofield method are shown in Fig. 4 where the lined area indicates the excess noise. Data obtained in this way were arranged as log versus logV plots at some fixed f (Fig. 5 – CF 021 resistors, Fig. 7 - CF 041 resistors). As is visible in the log-log coordinates the data follow line with slope α not always equal to 2. The results of α are given in Table 2. They differ from 2 especially for low ohmic CF 021 resistors. However this could be a result of measurement accuracy.

1E-16

2

SV , V /Hz

1E-16

A -14,87 B 1,95

, V

2

A -14,89 B 2,24

1E-17

1E-17

1E-18

0.1

A -16,06 B 2,16 A -16,39 B 1,66 0,1

1 f, Hz

1

V, Vrms

Fig. 4. Typical resistance fluctuation spectra in AC Scofield method (data for CF 021S-R4 resistor)

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Fig. 5. Average noise intensity for CF 021 LTCC resistors versus the voltage V biasing the specimens (full symbol – buried resistor, open symbol – surface one; olive colour – 0.3×0.3 mm2 structure, black one – 1.2×1.2 mm2 one)


This is well known that in the case of thick-film resistors the relative power spectral density S = SV/V2 should be inversely proportional to the resistor volume Ω. Thus, the noise intensity C = fSΩ appears as the most universal quantity that should be used to characterize the noisy properties of the material from which the resistors is fabricated [14,15]. It was impossible to determine resistor volume, especially for buried one. But we believe that the products S⋅ (ΩR1/ΩR4) or S⋅ (ΩR1/ΩR4)⋅ (R1/R4), which values are given in Table 2, make possible comparison of noise intensity of investigated resistive films. In general the resistance of CF 021 resistors is almost two order smaller than for CF 041 ones. But in the same case the relative noise intensity S of adequate components exceeds 2.5÷3 orders of magnitude. Moreover we have almost the same value of product S⋅ (ΩR1/ΩR4)⋅ (R1/R4) for CF 041 surface resistors independently on their area. This value is approximately 2 times larger than for buried resistors although their resistance is larger than surface components. In the case of CF 041 resistive ink we have very similar value of S⋅ (ΩR1/ΩR4)⋅ (R1/R4) for CF 041BR1 and CF 041S-R1 resistors. The same situation appears for SF 041S-R4 and CF 041B-R4 resistors in spite of significant difference in their resistance (about 4÷6 times). The same these measurements confirm results presented in [13]. Buried resistors had larger sheet resistance than surface ones but similar (or even smaller) low frequency noise level. This leads us to conclusion that the change of resistance connected with resistor position is rather a result of geometrical than microstructural changes. 1E-13

1E-14 SV V2/Hz 1E-15

, V

2

1E-14

1E-16

1E-15 0,1

1

10

f, Hz

0,1

V, Vrms

Fig. 7. Average noise intensity for CF 041

2

LTCC resistors versus the voltage V biasing the Fig. 6. Spectra of “excess” noise for 1.2×1.2 mm CF 041 surface resistor. Successive lines from up to specimens (full symbol – buried resistor, open symbol – down are for decreasing bias voltage, V surface one; olive colour – 0.3×0.3 mm2 structure, black one – 1.2×1.2 mm2 one) Table 2. Low frequency noise properties of CF 021 and CF 041 LTCC resistors Dimen- R [ohm] R1/R4 ΩR1/ΩR4 2) α3) S = SV/V2 S)⋅ (ΩR1/ΩR4) S⋅ (ΩR1/ΩR4)⋅ 1) sion [1/Hz] (R1/R4) CF021 S R4 9.4 1.03 1 2.24 1.29E-15 1.29E-15 1.33E-15 CF021 S R1 9.7 1.00 16 2.16 8.71E-17 1.39E-15 1.39E-15 CF021 B R4 52.0 0.47 1 1.95 1.35E-15 1.35E-15 6.37E-16 CF021 B R1 24.5 1.00 16 1.66 4.09E-17 6.55E-16 6.55E-16 CF041 S R4 550 1.36 1 2.06 3.33E-13 3.33E-13 4.53E-13 CF041 S R1 747 1.00 16 1.95 7.55E-14 1.21E-12 1.21E-12 CF041 B R4 3112 0.86 1 2.02 6.83E-13 6.83E-13 5.88E-13 CF041 B R1 2679 1.00 16 1.93 7.88E-14 1.26E-12 1.26E-12 1) R1 - 1.2×1.2 mm2, R4 - 0.3×0.3 mm2; 2) Ω - volume of resistor; 3) α - index in relation SV ~ Vα Mark

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Acknowledgements This work was supported by the Polish State Committee for Scientific Research, Grant no 8T11B 055 19

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

M. Hrovat, J. Holc, Z. Samardzija, D. Belavič; Characterisation of “equilibrated” thick-film Resistors, Proc. 34th MIDEM Conf., Roganska Slatina (Slovenia), 1998, p.311-316 K.-H. Drűe, H. Thust; RF-behavior of printed resistors in the frequency range up to 6 GHz, Proc. Int. Symp. On Microelectronics (ISHM-US), 1996, p.66-70 K.-H. Drűe, H. Thust, E.K. Polzer; Wide band characterisation of trimmed and untrimmed buried resistors in LTCC, Proc. 13th Eur. Microelectronics and packaging Conf., Strasbourg 2001, p.73-77 S. Jussila, H. Stubb, S. Pienimaa; Impedance spectroscopy of the polymer thick film resistor/conductor interface, MRS Symp. Proc., vol.203 (1991), p.41-46 K. Nitsch, A. Dziedzic, H. Czarczyńska, B.W. Licznerski; Impedance spectroscopy of carbon blackfilled polyesterimide resistors, Int. J. Electronics, vol.81 (1994), p.981-984 R. Modi, H.D. Wu, R.C.Y. Auyeung, C.M. Gilmore, D.B. Chrisey; Direct writing of polymer thick film resistors using a novel laser transfer technique, J. Mater. Res., vol.16 (2001), p.3214-3222 B.K. Koo, K.S. No, H.G. Kim; AC complex impedance investigation for RuO2-glass composites, IEEE Trans. On Comp., Hybr., and Manuf. Technol., vol.14 (1991), p.580-584 M.A. Jupina, C. El-Beyrouty, D.I. Amey, A.T. Walker; Characterization of thick film resistors up to 18 GHz for wireless and RF circuit applications, Proc. Int. Symp. On Microelectronics (IMAPS-US), 1999, p.94-99 A. Dziedzic, K. Nitsch, A. Kolek; Effect of environmental exposures on noise and AC properties of polymer thick-film resistors, Proc. 20th ISHM-Poland Chapter Conf., 1997, p.117-120 J.H. Scofield; AC method for measuring low-frequency resistance fluctuation spectra, Rev. Sci. Instrum., vol.58 (1987), p.985-993 P. Ptak, A. Kolek; Comparison of DC and AC techniques of low frequency noise measurements, Proc. 25th IMAPS-Poland Chapter Conf., 2001, p.175-178 A. Dziedzic, L.J. Golonka, A. Kolek, P. Mach, K. Nitsch; DC and AC electrical properties and longterm stability of LTCC resistors, Proc. 24th Int. Spring Seminar on Electronics Technology, ISSE 2001, Calimanesti-Caciulata (Romania), p.137-141 P. Ptak, A. Kolek, A. Dziedzic; Low frequency noise in LTCC resistors, Proc. 25th Int. Spring seminar on Electronics Technology, Prague 2002, p.142-146 A. Dziedzic, A. Kolek; 1/f noise in polymer thick-film resistors, J. Phys. D: Appl. Phys., vol.31 (1998), p.2091-2097 A. Kolek, A. Dziedzic; Low-frequency noise of polymer thick-film resistors: analysis of volume and contacts effects, Proc. 22nd Int. Conf. on Microelectronics, MIEL 2001, Nis (Yugoslavia), p.531-534

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