Characterization of Dielectric LTCC Tapes - IEEE Xplore

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Abstract: Low Temperature Co-Fired Ceramic (LTCC) technology is one of the most ... characterization of two dielectric LTCC tapes (Heraeus AHT 01-005 and ...
Characterization of Dielectric LTCC Tapes G. Mišković1), S. Toškov1), A. Marić2), N. Blaž2), G. Radosavljević1) 1)

Department of Applied Electronic Materials, Institute of Sensor and Actuator Systems, Vienna University of Technology, Vienna, Austria 2) Department of Power, Electronics and Communication, Faculty of Technical Sciences, University of Novi Sad, Serbia [email protected]

Abstract: Low Temperature Co-Fired Ceramic (LTCC) technology is one of the most widely used technologies for producing reliable electronic components, modules, and sensors, capable for working in high temperature applications and applications requiring low dielectric losses. In this paper characterization of two dielectric LTCC tapes (Heraeus AHT 01-005 and Heraeus AHT 08-047) is presented. The virtual relative permittivity of LTCC tapes as well as its dependence on temperature is presented in the paper. An investigation on weight loss before and after tape firing, material shrinkage, and surface roughness for both LTCC tapes is given. Material reliability in water environment is also investigated and reported. For this purpose, an interdigital capacitor (IDC) was embedded between four, six, and eight layers of LTCC tape, in which the IDC is buried in the middle of the structure. Sensing structures were placed in tap water and the capacitance of the buried IDC was measured over a period of 200 days. The most of the IDC structures buried inside were unaffected by water which is surrounding the structures, indicating their long-term stability in water environment. 1. INTRODUCTION For every research, knowing material properties and characteristic is important in order to perform scientific work and investigations as best as possible. The history of LTCC technology actually dates back to early 1980s, when it was first developed by Hughes and DuPont for military systems. Commercialization of the technology, which was accelerated after the cooperation of LTCC tape producers such as DuPont, Heraeus and Ferro with packaging companies in the late 80s, broadened the application areas to avionics and automotive industries. Today Low Temperature Co-Fired Ceramic (LTCC) technology is one of the most widely used technologies for producing reliable electronic components, modules, sensors etc. Although LTCC technology is cited as the most promising packaging concept for realization of highly functional, reliable and low-cost electronic devices [1], it is evident that progress can only be achieved by using advanced materials systems. However, frequently the materials’ data, in particular the ones of LTCC tapes are insufficient to design a product and predict its properties enough accurate. For this reason, 978-1-4799-0036-7/13/$31.00 ©2013 IEEE

the materials’ properties must be experimentally characterized and specified precisely for the intended application in advance. There are a large number of investigations focused on characterization of LTCC tapes [2-11]. Some of them are focused on ferrite LTTC tapes, other are oriented on dielectric LTCC tapes. There are investigations on electrical properties [2-4], magnetic properties [5-8], mechanical and microwave properties [8-11]. In this paper electrical, thermal and physical properties are investigated. In a very simple “straightforward” approach the “virtual” electrical permittivity as function of temperature is measured up to 850 ºC using a plate capacitor. Moreover the surface roughness parameters are obtained by measurements with an optical profilometer, and the weight loss is investigated using a precision scale. Additionally, the long-term stability of an LTCC-test device with embedded IDC was investigated under the conditions of being permanently immersed in water [12-14]. All LTCC tapes have been shaped by laser (Nd:YAG, Rofin RSM 100 D II TEM00) in order to

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obtain high precision. IDC structures were printed on green tapes using a DeHaart SPSA-10 semi-automatic screen printer. Commercially available conductive paste Heraeus TC 7306 A was used for the conductor structure. Tapes have been stacked together in a press mold and then the multi-layered samples were laminated. Lamination was performed in a uniaxial press (Carver 3895CE Auto Four/30) under a pressure of 8.5 MPa for 3 minutes at a temperature of 75 ºC. All samples were placed between two Ceramtec A (Ceramtec GmbH) alumina tapes to reduce warping, and then unconstrained co-fired in a belt furnace (BTU Systems) at a peak temperature of 850 ºC in a total firing cycle of approximately one hour and fifty minutes.

2. ELECTRICAL PERMITTIVITY One of the most important parameter of dielectric material is their electrical permittivity. Unfortunately, information regarding permittivity of Heraeus AHT 01-005 and Heraeus AHT 08-047, is not provided in data sheets. In order to obtain a first estimation of the relative electrical permittivity, of the two mentioned types of tapes, samples with simple plate capacitor were fabricated.

were placed as close as possible to the samples. Using the LCR meter the impedance of the samples (including their silver leading wires) are measured point wise at each temperature value after reaching equilibrium and interpreted in accordance to the equivalent circuit as depicted in Fig 5a. From capacitance value Cm using the formula C = εoεr(A/d) “the virtual“ relative electrical permittivity of dielectric (εr) was calculated, where ε0 is relative permittivity of vacuum, A is area of electrode plate and d is a distance between two electrode plates. In Fig. 3, the permittivity for both tapes is presented as functions of temperature. The reason for increasing “virtual” relative electrical permittivity with temperature can be explained with the equivalent electrical model of the LCR meter (Fig. 2a). In reality, there is a slightly different electrical model, Fig. 2b.

In order to obtain the virtual relative permittivity, the capacitance method was used. Simple plate capacitors were realized, where LTCC tapes are used as a dielectric material. Fig. 2. Equivalent electrical models: a) Theoretical, b) In reality.

The LCR meter is actually measuring impedance (Z) between the nodes A and B and from Z it is extracting the capacitance (Cm). Resistivity Rm and Rc can be neglected in calculation, since their value is very big. As it can be observed from Fig. 2 in reality there is resistance (Rs) from the contact wires which obviously contributes to the overall Z. For Heraeus AHT 01-005 samples, the following parameters are valid: A = 95 ± 5 mm2, d = 190 ± 10 μm, virtual εr = 9 ± 10 %, at 50 kHz was obtained at room temperature. Fig. 1. Capacitance vs. temperature of Heraeus AHT 01-005 and AHT 08-047.

The measurement set-up consists of a box furnace, high frequency LCR meter (Wayne Kerr 6500P) and a thermometer connected to a PC. Thermocouple probes

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For the Heraeus AHT 08-047 samples (A = 86.5 ± 3 mm2, d = 325 ± 10 μm, at 50 kHz) at room temperature a virtual εr = 7.9 ± 10 % was obtained. The temperature dependences of virtual relative electrical permittivity of both tapes (materials) are presented in Fig. 1.

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From Fig. 1 it is evident that for both LTCC tapes virtual permittivity changing with increasing temperature. It can be also observed that up to around 500 ºC, virtual relative permittivity is slightly changed, but there is enormous increase above of the virtual εr.

Table 1. Shrinkage estimation for Heraeus AHT 01-005 and Heraeus AHT 08-047 by x, y and z axis. Shrinkage

3. PHYSICAL CHARACTERIZATIONS For investigation of the tape shrinkage during sintering, weight loss and surface roughness of Heraeus AHT 01-005 and Heraeus AHT 08-047 LTCC tapes, samples have been realized. Unfired tapes have been laser machined in order to obtain stripes with precise dimensions. Afterwards the samples were fired. The appearance of fired and unfired sample blocks is given in Fig. 3.

Heraeus AHT 01-005 Heraeus AHT 08-047

x in %

24.34

21.50

y in %

23.76

21.26

z in %

12.50

25.00

For the investigation regarding weight loss, the weight of samples was measured before and after firing. The main reason for weight loss is the evaporation of organic compounds. The estimation of weight loss is presented in Table 2. Table 2. Weight loss estimation for Heraeus AHT 01-005 and Heraeus AHT 08-047 tapes. Heraeus AHT 01-005 Heraeus AHT 08-047 Weight Loss

14.30 %

15.47 %

From Table 2, it can be noticed that the Heraeus AHT 08-047 LTCC tape has more weight loss than Heraeus AHT 01-005. a) Fired (left) and unfired (right) samples.

3.2. Material Surface Roughness

b) Dimensions of stripes for unfired samples.

Fig. 3. Fired and unfired sample blocks and their dimensions.

3.1. Shrinkage and Weight Loss As can be seen from Fig. 3, the fired samples are smaller than the unfired ones. During the firing of the LTCC tape, sintering occurs and due to that shrinkage and weight loss of tapes are present and obvious. The relevant dimensions, width and length of the sample as well as width and length of the channels, were measured before and after firing for all samples. Using average measurement values, the relative shrinkage is estimated. The estimated shrinkage for both tapes and it is presented in Table 1, from which can be observed that the shrinkage of the tapes in x and y direction is approximately the same, which was expected. It can also be observed that the Heraeus AHT 01-005 LTCC tape is shrinking more plane direction, but less in perpendicular direction than the Heraeus AHT 08-047.

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In order to investigate material surface roughness before and after firing, 2-D surface roughness measurements were performed, using an FRTmicroprof, FRT CWL II sensor. The measurement length ln and respective cutoff wavelength λc (Butterworthfilter) were chosen according to EN ISO 3274 and EN ISO 13565-1 (ln = 4 mm, λc = 0.8 mm) standards [cite standard]. The average values of the measurements are presented in Table 3. Table 3. Surface roughness estimation for Heraeus AHT 01-005 and Heraeus AHT 08-047 tapes. Surface parameter Ra in µm Green

Fired

Heraeus AHT 08-047

0.34

0.38

Heraeus AHT 01-005

0.38

0.45

From Table 3 it can be concluded that surface roughness increases after firing. For some application, such as resistor or heater realization, increased surface roughness can present a problem.

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4. MATERIAL RELIABILITY IN WATER ENVIRONMENT An interdigital capacitor was embedded between four, six, and eight layers of LTCC tape, in which the IDC is buried in the middle of the structure. All structures were stored in tap water and the capacitance of the buried IDC was measured over a period of 200 days in order to investigate the material’s reliability in water environment. 4.1. Model and Fabrication

a)

The geometry of the IDC layout is shown in Fig. 4, while corresponding geometrical parameters and their values are summarized in Table 4.

Exploded view

b) Fabricated sample Fig. 5. LTCC sensing samples with buried IDC. Fig. 4. The layout of IDC with parameters.

4.2. Measurement Results

Table 4. Geometrical parameters of IDC.

All measurements were performed with High Frequency LCR meter Wayne Kerr 6500P. Capacitance of sensing structures was measured at room temperature at 50 kHz.

Parameter

Value

Finger width w [μm]

250

Spacing s [μm]

250

Length of fingers l [mm]

13

Thickness of conductor t [μm]

12

Thickness of substrate h [μm]

see text

Total length dx [mm]

24.75

Total width dy [mm]

14

Number of fingers N

50

The total thickness of the IDC samples after sintering for Heraeus AHT 01-005 tape for two, six and eight layers were 400 µm, 680 µm and 800 µm, respectively. The thickness of Heraeus AHT 08-047 tape stacks with four, six and eight layers are 320 µm, 500 µm and 640 µm, respectively. Fabricated sample as well as 3D exploded model of sensing LTCC structure is given in Fig. 5.

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Fig. 6. The influence of the tap water on the capacitance of the buried IDCs for Heraeus AHT01-005 for different number of covering layers.

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The measurement results for capacitance value of both LTCC tapes are shown in Fig. 6 and Fig. 7. Structures which are made of Heraeus AHT 01-005 show excellent long-term stability. All three samples have unswerving capacitance values. On the other side structures made of Heraeus AHT 08-047 with six and eight layers also show good stability, while the structure with four layers showed a lack of stability.

ACKNOWLEDGMENT This work is done within the scope of the FP7 SENSEIVER project (Low-cost and energy-efficient LTCC sensor/IR-UWB transceiver solutions for sustainable healthy environment project references 289481).

REFERENCES

Fig. 7. The influence of the tap water on the capacitance of the buried IDCs for Heraeus AHT08-047 for different number of covering layers.

5. CONCLUSION In this paper characterization of two dielectric LTCC tapes (Heraeus AHT 01-005 and Heraeus AHT 08-8047) is presented. The virtual relative electrical permittivity of LTCC tapes in the dependence on temperature is shown. Further research work is needed for a more accurate analysis of the temperature dependence of the ceramic materials particularly in the higher temperature range. Weight and material shrinkage are present after firing of the LTCC tapes. Surface roughness increased after firing for both LTCC tapes. The influence of the tap water on the capacitance of the buried IDCs was measured. It is evident that most of the IDC structures buried inside were unaffected (± 2 %) by water which is surrounding the structures, indicating their reliability in water environment, except the structure with four layers of Heraeus AHT08-047 where a significant increase of capacitance is observed (± 10 %). Plans for future investigations are to continue with measurements, perform simulations for presented structures and further material characterization. 978-1-4799-0036-7/13/$31.00 ©2013 IEEE

[1] R.C. Buchanan: “Ceramic Materials for Electronics”, 2nd edition, Marcel Dekker, Chapter 9: Multilayer Ceramic Technology, 1991, pp. 488-526. [2] N. Blaž, G. Radosavljević, Lj. Živanov, W. Smetana: “Characterisation of Dielectric LTCC Tapes Using the Capacitance Method”, International Semiconductor Conference, ISBN: 978-1-4244-4413-7, 2009, pp. 447-450. [3] Vadiveloo, P. L., Wai, Lai Lai, Fan, Wei, Lu, C. W.:” Processing and Electrical Characterization of CoSintered Composite Glass Ceramics”, 8th Electronics Packaging Technology Conference, ISBN: 1-42440665-X, 2006, pp. 655 - 658. [4] Alias Rosidah, Ambak Zulkifli, Yusoff Mohd Zulfadli Mohamed, Shapee Sabrina Mohd, Saad, Muhammad Redzuan, Yusoff Ashaari, Mahmood Che Seman, Al Rashid Megat Ahmad Megat Harun: “Characterization of Alumina-based LTCC Composite Materials: Thermal and Electrical Properties”, 12th Electronics Packaging Technology Conference (EPTC), ISBN: 978-1-4244-8561-1, 2010, pp. 269 - 272. [5] Su Yipeng P., Li Qiang, Lee Fred C Y: “Magnetic Characterization of Low Temperature Co-fired Ceramic (LTCC) Ferrite Materials for High Frequency Power Converters”, Energy Conversion Congress and Exposition (ECCE), ISBN: 978-1-4577-0542-7, 2011, pp. 2133 - 2138. [6] Bray Joey R., Kautio Kari T., Roy Langis: “Characterization of an Experimental Ferrite LTCC Tape System for Microwave and Millimeter-Wave Applications”, IEEE Transactions on Advanced Packaging, ISSN: 1521-3323, 2004, pp. 558 - 565. [7] Marić, A. M., Blaž N.; Radosavljević, G. J.; Atassi I.; Živanov L.; Smetana W.: “Complex Permeability Dependence of Commercial LTCC Ferritic Tape on Ambient Temperature”, 35th International Spring Seminar on Electronics Technology (ISSE), ISBN: 978-1-4673-2239-3, 2012, pp. 65 - 69. [8] Bray J., Hojjat N., Elasoued R.A., Baillargeat D.:” Microwave and Magnetostatic Characterization of Ferrite LTCC for Tunable and Reconfigurable Sip Applications”, IEEE International Microwave Symposium, ISBN: 1-4244-0688-9, 2007, pp. 691-694. [9] Sunappan Vasudivan, Periannan, Arnlvanan, Meng Chua Kai, Khuen Wong Chee: “Process Issues and Characterization of LTCC Substrates”, 54th Electronic Components and Technology Conference Proceedings Vol. 2, ISBN: 0-7803-8365-6, 2004, pp. 1933 - 1937.

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[10] Unger Michael, Smetana Walter, Ehrenzweig Wolfgang: „Mechanical Characteristics of LTCC (Low Temperature Cofired Ceramics) -Tapes for Mechanical Application“, 30th International Spring Seminar on Electronics Technology, ISBN: 987-14244-1218-1, 2007, pp. 59 - 64. [11] Asudevan, S., Shaikh Aziz S.: “Microwave Characterization of Low Temperature Cofired Ceramic System”, 3rd International Symposium on Advanced Packaging Materials Proceedings, ISBN: 07803-3818-9, 1997, pp. 152 - 157. [12] G. Mišković, S. Toškov, A. Marić, N. Blaž, G. Radosavljević: “Behavior of Heraeus CT 703 Tape in Water Environment”, 7th International Conference

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and Exposition on Electrical and Power Engineering – EPE, Iasi, Romania, ISBN: 978-1-4373-5001-3, October 25-27, 2012. pp. 137-140. [13] G. Mišković, S.Toškov, A. Marić, N. Blaž, G. Radosavljević:” Characterization of LTCC Tapes in Water Presence”, Advanced Materials Research Vol. 685 Trans Tech Publications, ISBN: 978-3-03785676-5, 2013 pp. 307-311. [14] S. Toskov, G. Miskovic, A. Maric, B. Nelu, G. Radosavljevic, ”The Effect of Water Presence on Properties of LTCC Dielectric Tapes,” 48th International Conference on Microelectronics, Devices and Materials, ISBN 978-961-92933-2-4, September, 2012, pp.285-290.

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