Influence of various multilayer LTCC systems on

Microelectronics International Influence of various multilayer LTCC systems on dielectric properties’ stability in GHz frequency range Tibor Rovensky, Alena Pietrikova, Igor Vehec, Martin Kmec,

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Influence of various multilayer LTCC systems on dielectric properties’ stability in GHz frequency range Tibor Rovensky, Alena Pietrikova and Igor Vehec Department of Technologies in Electronics, Technical University of Kosice, Kosice, Slovakia, and

Martin Kmec

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Institute for Information Technology, Technische Universitaet Ilmenau, Ilmenau, Germany Abstract Purpose – The purpose of this paper is to create multilayer substrate (composite) from various low temperature co-fired ceramic (LTCC) substrates by their mutual combinations and to analyse influence of these multilayer substrates on dielectric properties in GHz frequency range. Design/methodology/approach – GreenTape 951, GreenTape 9K7 and Murata LFC were used to create compound multilayer substrates that include three layers: middle layer is from Murata LFC, and both upper and bottom layers are either from GreenTape 951 or GreenTape 9K7. Shrinkage in all x-, y- and z-axes of all substrates including multilayer substrates were analysed, and influence of different shrinkage on dielectric properties was examined by microstrip ring resonators applied on all mentioned of substrates. Findings – The middle layer of Murata LFC has significant influence on shrinkage value of composites which has a good repeatability and minimalizes problems with design of multilayer LTCC devices. Impact of middle layer from Murata LFC on dielectric constant is not significant, but on the other hand Q factor (loss tangent) of these composites is increased according to inhomogeneity between single LTCC layers, especially at frequency around 6 GHz. Originality/value – The novelty of this work lies in creating multilayers systems from different types of LTCC substrates to find combination with the most suitable physical and dielectric properties for various purposes in GHz range applications. Keywords Composite, Dielectric properties, LTCC, Shrinkage, GHz frequency Paper type Research paper

1. Introduction

substrates and technology have been used for the past 15 years, so it can be considered as a new developed technology (Muller et al., 1995; Shapiro et al., 2002).

Current trends in electronics are to increase frequency used by various devices which provide faster processing of huge data’s volume. Common used materials and technologies are substituted by advanced materials and new developed technologies, e.g. printed circuit boards, are substituted by low temperature co-fired ceramic (LTCC) substrates. Devices for GHz application have been traditionally fabricated from metal and coaxial radio frequency (RF) connections are provided by connectors. In general, it leads to expensive, heavy and bulky packages (Hongwei et al., 2000; Midford et al., 1995). These metal packages are different in comparison with market demand for low cost and portable devices (Joseph and Sebastian, 2010; El-Kheshen et al., 2003). Furthermore, electronic devices for the entertainment, telecommunication and automotive industry have to handle increasing number of functions occupying as miniature space as possible. Development of complex miniaturized devices leads to using LTCC substrates as key material. The LTCC

2. Low temperature co-fired ceramic substrates Nowadays, market offers many types of LTCC substrates (Sebastian and Jantunen, 2008), and this paper deals with investigating of three various LTCC substrates (Du Pont GreenTape 951, Du Pont GreenTape 9K7 and Murata LFC) and their two mutual combinations Du Pont GreenTape 951– Murata LFC–Du Pont GreenTape 951 and Du Pont GreenTape 9K7–Murata LFC–Du Pont GreenTape 9K7. Multilayer substrates can be considered as composite, which is made from two different LTCC substrates. These LTCC substrates were chosen based on similar composition, and all of them contain glass component SiO2 which has major impact on behaviour of LTCC substrates in GHz frequency area. Dielectric constant as well as dielectric losses strictly depends on glassy phase of SiO2. 2.1 Electrical properties Du Pont GreenTape 951 (glass based on Pb-B-Si-O ⫹ Al2O3) is designed for application in GHz range, e.g. single chip

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Microelectronics International 33/3 (2016) 136 –140 © Emerald Group Publishing Limited [ISSN 1356-5362] [DOI 10.1108/MI-03-2016-0028]

Received 15 January 2016 Revised 22 March 2016 Accepted 29 April 2016

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Influence of various multilayer LTCC systems

Microelectronics International

Tibor Rovensky, Alena Pietrikova, Igor Vehec and Martin Kmec

Volume 33 · Number 3 · 2016 · 136 –140

3. Measurements of dielectric properties

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packages, RF modules and multichip modules providing stable dielectric constant and loss tangent up to tens of GHz. Du Pont GreenTape 9K7 (glass ⫹ Al2O3) is usually used for applications in GHz range requiring excellent stable and low loss properties, e.g. advanced high frequency (HF) applications, wireless and mobile communications. Murata LFC (glass based on CaO-Al2O3-SiO2-B2O3 ⫹ Al2O3) has stable dielectric properties in wide range of frequencies, and it is designed for GHz applications, e.g. RF modules and multichip modules in automotive industry. Electrical properties of used LTCC substrates are shown in Table I.

Various methods for measuring dielectric properties at single frequency are well known (e.g. split cavity resonator, cylinder resonator, etc.), but measurements in GHz area over wide frequency range are a challenging problem (Agilent, 2006; Chen et al., 2004). The method of microstrip ring resonators was used to provide numerous data points over a wide frequency range (Heinola et al., 2004). Microstrip ring resonators, shown in Figure 1, were designed by self-made software tool (Rovensky et al., 2015) and fabricated by common thick film technology. Ring resonators are tuned to primary resonance frequency at 2 GHz and were measured by probe station to minimalize errors caused by soldering connectors (Figure 2).

2.2 Physical properties – shrinkage Sintering process of glass-ceramic substrates is one of the most important steps when the final LTCC device is made from green laminate tape. During sintering process, particles are bonded together by heating, causing densification and resulting in shrinkage of the LTCC substrate (Yamaguchi, 1987). Shrinkage of all substrates was verified by us to provide as precise design of ring resonators as it is possible. Shrinkage rate is very important for designers of LTCC devices for two reasons. First, according to value of shrinkage in x- and y-axes, it is necessary to enlarge all dimensions of designed device, and second, according to value of shrinkage in z-axis, it is necessary to know exact thickness of substrate after sintering in case for matching device to 50 ⍀ characteristic impedance (microstrip, stripline, etc.). Consistency and repeatability of shrinkage rate must be the top criteria when designing any LTCC device (Alias, 2013). Shrinkage rate of used LTCC substrates is shown in Table II. Created LTCC composite which consists of different LTCC substrates has different shrinkage caused by different composition and melting point. Our measured shrinkage rates of single LTCC substrates in composite is shown in Table III.

4. Scattering parameters and dielectric properties Dielectric properties are determined from measured scattering parameters (forward transmission coefficient) which are shown in Figure 2. Resonance frequency and bandwidth at ⫺3 dB are used as input parameters for self-made software tool (Rovensky et al., 2015). Based on input parameters, equations (Heinola et al., 2004) are solved, and dielectric constant and Q factor are calculated. Measured forward transmission coefficient (S21) of ring resonators based on two different substrates is shown in Figure 2. Green curve represents S21 parameter of composite Du Pont GreenTape 9K7–Murata–Du Pont GreenTape 9K7 and blue curve depicts S21 parameter of Du Pont GreenTape 951. Both ring resonator resonate at each integer multiplies of its tuned primary resonance frequency. In these cases, they were tuned to resonate at primary resonance frequency 2 GHz. All subsequent resonance frequencies (4, 6, 8 and 10 GHz) are significant except one case. Even after multiple measurements of a few ring resonators based on Du Pont GreenTape 951, it was unable to measure resonance frequency at 10 GHz. To provide clear and simply understandable results of measured dielectric properties, graphs are divided into three partial figures. In Figure 3, dielectric constants of Du Pont GreenTape 9K7 (red curve) and composite Du Pont GreenTape 9K7–

Table I Electrical properties of used LTCC substrates Dielectric material Fired thickness, (␮m) Dielectric constant, (10 GHz)a Loss tangent, (10 GHz)a

GT 951 PX GT 9K7 PX Murata LFC 0.22 7.8 ⫾ 0.2 0.0140

0.224 7.1 ⫾ 0.2 0.0010

0.226 7.55 ⫾ 0.2 0.0045

Note: a Split cavity measurement method Table II Physical (shrinkage) properties of used LTCC substrates Dielectric material

GT 951 PX

GT 9K7 PX

Murata LFC

GT 951 PX – Murata LFC – GT 951 PX

GT 9K7 PX – Murata LFC – GT 9K7 PX

X, Y, shrinkage, (%)a Z, shrinkage, (%)a

12.7 ⫾ 0.3 15 ⫾ 0.5

9.1 ⫾ 0.3 11.8 ⫾ 0.5

17 ⫾ 0.3 13.6 ⫾ 0.5

17.1 ⫾ 0.3 7.9 ⫾ 0.3

1.6 ⫾ 0.2 21.8 ⫾ 0.3

Notes: a Isostatic lamination; 3,000 psi; 70°C; 10 minS Table III Shrinkage rate of single LTCC substrates in composite Composite Dielectric material

GT 951 PX – Murata LFC – GT 951 PX GT 951 PX Murata LFC

GT 9K7 PX – Murata LFC – GT 9K7 PX GT 9K7 PX Murata LFC

X, Y, shrinkage, (%)a Z, shrinkage, (%)a

17.1 ⫾ 0.3 6.3 ⫾ 0.3

1.6 ⫾ 0.2 18.1 ⫾ 0.3

17.1 ⫾ 0.3 10.7 ⫾ 0.3

Notes: a Isostatic lamination; 3,000 psi; 70°C; 10 min

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1.6 ⫾ 0.2 29.2 ⫾ 0.3

Influence of various multilayer LTCC systems

Microelectronics International

Tibor Rovensky, Alena Pietrikova, Igor Vehec and Martin Kmec

Volume 33 · Number 3 · 2016 · 136 –140

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Figure 1 (a) Fabricated ring resonators using all LTCC substrates; (b) Measuring by probe heads (up), view via microscope’s ocular (down)

Figure 2 Measured forward transmission coefficient of ring resonator

In Figure 4, dielectric constants of Du Pont GreenTape 951 (orange curve), Murata LFC (blue curve) and composite Du Pont GreenTape 951–Murata LFC–Du Pont GreenTape 951 (purple curve) are shown. Dielectric constant of Du Pont GreenTape 951 is stable up to 8 GHz (measurement at 10 GHz was unsuccessful). Manufacturer states value 7.55 of Murata’s LFC dielectric constant at 1 GHz, and our measurements show value 7.77 which is not in tolerance (⫾0.2), and at 10 GHz, it is even more (7.94). Dielectric constant of Murata LFC and composite Du Pont GreenTape 951–Murata LFC–Du Pont GreenTape 951 have a tendency to rise with increasing frequency. Divergences between the lowest and the highest value of dielectric constant over frequency up to 10 GHz is 0.23 for Murata LFC and 0.25 for composite. Middle layer of Murata LFC between Du Pont GreenTape 951 has no major impact on dielectric constant of composite due to almost the same value of dielectric constant. In Figure 5, the Q factor of all measured substrates is shown. Q factor of LTCC one type’s substrates is rising up to 4.5 GHz, and after reaching this frequency, it starts slightly falling down (Du Pont GreenTape 951 from 93 to 88, Du Pont GreenTape 9K7 from 110 to 99 and Murata LFC from 110 to 96). Q factor of composite is rising up to 4 GHz and then drops at 6 GHz; it again rises at 8 GHz and drops at 10 GHz. Q factor’s range of composite Du Pont GreenTape 951–Murata LFC – Du Pont GreenTape 951 is from 60 to

Murata LFC–Du Pont GreenTape 9K7 (green curve) are shown. Dielectric constant of both substrates have tendency to rise with increasing frequency. Divergences between the lowest and the highest value of dielectric constant over frequency up to 10 GHz is 0.15 for Du Pont GreenTape 9K7 and 0.13 for composite, and both are in tolerance ⫾0.2. Middle layer of Murata LFC between Du Pont GreenTape 9K7 has no major impact on dielectric constant of composite despite that dielectric constant stated by manufacturer of Murata LFC is 7.55 (value measured by us is 7.7) and Du Pont GreenTape 9K7 is 7.1.

Figure 3 Dielectric constant of Du Pont GreenTape 9K7 and composite Du Pont GreenTape 9K7–Murata LFC–Du Pont GreenTape 9K7

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Tibor Rovensky, Alena Pietrikova, Igor Vehec and Martin Kmec

Volume 33 · Number 3 · 2016 · 136 –140

Figure 4 Dielectric constant of Murata LFC, Du Pont GreenTape 951 and composite Du Pont GreenTape 951 – Murata LFC – Du Pont GreenTape 915

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Figure 5 Q factor of all measured LTCC substrates

constant of both substrates, which create composite are almost the same (Murata LFC 7.77 ⫾ 0.2, Du Pont GreenTape 951 7.8 ⫾ 0.2); therefore, no important change is reasonable. However, in the second case, when dielectric constant is different (Murata LFC 7.77 ⫾ 0.2, Du Pont GreenTape 951 7.1 ⫾ 0.2), any changes of composites’ dielectric constant were assumed, but they were not proved. For this reason, we can conclude that major impact on dielectric constant has a top layer of composite, where microstrip ring resonator was applied. Q factor of composites is influenced by inhomogeneity of the substrates. For that reason, during signal’s transition between single layers, dielectric losses are increasing, and they influence the Q factor of measured LTCC substrates. The impact of inhomogeneity is the most significant at frequency around 6 GHz.

124, and that of Du Pont GreenTape 9K7 – Murata LFC – Du Pont GreenTape 9K7 is from 119 to 42. Shape of both the Q factor curves’ is similar to sinus waveform, but the amplitudes are changing.

5. Conclusion This work describes various multilayer LTCC substrates from the view of dielectric properties in GHz frequency range and analyses dielectric constant and dielectric losses by ring resonators microstrip measurement method. Ring resonators from five different types of substrates were designed, fabricated and measured to obtain dielectric properties of these substrates in the area up to 10 GHz. By inserting Murata LFC between Du Pont GreenTape 9K7 or Du Pont GreenTape 951, new composites were originating. The middle layer of Murata LFC has significant influence on shrinkage value of composites. Du Pont GreenTape 9K7 has shrinkage of 9.1 ⫾ 0.3 per cent in x- and y-axes, and a 11.8 ⫾ 0.5 in z-axis, but in combination with Murata LFC, it has changed to 1.6 ⫾ 0.2 per cent in x- and y-axes, and to 21.8 ⫾ 0.5 in z-axis. Du Pont GreenTape 951 has shrinkage of 12.7 ⫾ 0.3 per cent in x- and y-axes, and a 15 ⫾ 0.5 in z-axis, but in combination with Murata LFC, it has changed to 17.1 ⫾ 0.3 per cent in x- and y-axes, and to 7.9 ⫾ 0.5 in z-axis. Murata LFC raises shrinkage in z-axis and reduces shrinkage in x- and y-axes in combination with Du Pont GreenTape 9K7, but on the other hand, in combination with Du Pont GreenTape 951 it reduces shrinkage in z-axis and raises shrinkage in x- and y-axes. Middle layer of Murata LFC has no significant impact on dielectric constant of both composites. In one case, dielectric

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Influence of various multilayer LTCC systems

Microelectronics International

Tibor Rovensky, Alena Pietrikova, Igor Vehec and Martin Kmec

Volume 33 · Number 3 · 2016 · 136 –140

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Corresponding author Tibor Rovensky can be contacted at: tibor.rovensky@ tuke.sk

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