An LTCC Design Technique Based on FDTD EM Simulations

Zulkifli Ambak, Rosidah Alias, Azmi Ibrahim, Sabrina Mohd Shapee, Samsiah Ahmad, Mohamed Razman Yahya and Abdul Fatah Awang Mat

Microelectronics and Nano Technology Program, TM Research & Development Sdn. Bhd.,

Idea Tower I & II, UPM-MTDC Lebuh Silikon, 43400 Serdang, Selangor,Malaysia. zulkifliambak@tmrnd.com.my , rosidah@rndtm.net.my , iazmi@tmrnd.com.my , sabrina@tmrnd.com.my, samsiah@tmrnd.com.my, razman@tmrnd.com.my,drfatah@tmrnd.com.my

Abstract: This paper described the Low Technology Co-fired Ceramic (LTCC) design methodology at TMRND based on FDTD EM simulations that are required when designing the RF/microwave circuit. In this paper, 3D EM analysis and optimization with Finite Different Time Domain (FDTD) software Empire XcCel TM from IMST was applied to achieve accurate modeling of the RF/microwave circuit using LTCC technology. A multilayer spiral inductor was selected for discussion to show this LTCC design technique. The main elements of the LTCC design technique relies on three important guidelines; use accurate component models, link the circuit schematic to the layout to reduce errors and finally link the layout to an EM simulator to detect coupling problems. Keywords: Low Temperature co-fired ceramics (LTCC), Finite Different Time Domain (FDTD), Electro-Magnetic(EM) simulations.

1. Introduction LTCC (low temperature co-fired ceramic) stands for a ceramic substrate system which is applicable in electronic circuits. LTCC is a multilayer platform technology that can be used in fabricating components, modules and packages for the millimeter-wave frequencies. Main benefits of the LTCC are high packaging density, dielectric constant ,high thermal conductivity , reliability and stability. The technology steps for fabrication a LTCC circuit which developed in TMRND is shown in Fig 1.0. Meanwhile, Figure 2.0 shows the LTCC process is started with punched the via holes into the un-fired or “green” glass ceramic tapes. Then, vias are filled, and conductors are printed on each tape separately. The different tape layers are aligned, laminated and sintered together with conductors. Finally, the substrate is dices. 1-4244-1435-0/07/$25.00©2007 IEEE

The temperature of sintering temperature is below 900oC for LTCC glass ceramic. The metal with high conductivity, such as silver, gold or copper can be used as the conductor materials, which results in the low conductor loss. During the firing, the ceramic sheets shrink of the ceramic tape. The shrinking has to be taken into account in the LTCC process. Very accurate values of shrinkage of glass/ceramic sheet and metallization are quite difficult to predict because of the differences between the material loss.[1]. In this presentation the fundamentals of LTCC design technique based on FDTD EM simulation will be explained and examples of multilayer inductor will be simulated and discussed. The EM simulations were performed on Empire XcCel TM [2].

Figure 1.0: Development of Multilayer LTCC at TMRND

Figure 2.0 : Process flow for fabrication a LTCC circuit [1]

2. FDTD theory and analysis

Most of problems encountered in analyzing and designing high frequency elements are due to the unknown electromagnetic field behavior called “parasitic” or “coupling effect”. The reason for the inability to predict the electromagnetic field behavior is that Maxwell’s equation can not be solved analytically for any practical structure. One method to solve Maxwell’s equations with very few approximations is the Finite Different Time Domain method (FDTD). On the other hand, the FDTD technique is a time domain simulation technique and therefore very good suited for simulation the LTCC Multilayer circuit like passive component. Within one time domain simulation, the whole frequency band can be covered. This technique is very memory efficient and allows the calculations of large scale problems. The aim of the FDTD method is to solve Maxwell’s equations and its associated material relations for a given structure and boundary conditions [2].Maxwell equations can be reformulated as hyperbolic partial differential equations is given as:

where E, H, ε ,μ and σ are the electric field, magnetic field, permittivity, permeability and conductivity of the material, respectively. The FDTD method employs an efficient time stepping algorithm, known as the Yee’s cell[3] as shown in Figure 3.0.The Yee cell (see the left side of figure 3) defines the location of the representative field samples for the electric on the FDTD grid. A new equivalent circuit for the FDTD technique (see the right side of Figure 3.0), which is based on the definition from the Yee cell, is derived in [3].

Figure 3.0 : Arrangement of field components in a Yee cell and FDTD equivalent circuit(right side)

3. LTCC design concepts and tools

The limited availability of design libraries and reliable multilayer models has impeded the ability of hardware developers to take full advantages of the miniaturization potential of LTCC technology. The availability of commercial Electromagnetic (EM) simulation packages and powerful computer processing is easing the difficulty in designing multilayer structures. However, simulating “Electromagnetically”, relatively complex multilayer structure can be very time consuming, and although EM packages offer an optimization function the simulation time maybe excessive.[4]. Therefore, the simulation program Empire XcCel from IMST has been used. This is based on the Finite Different Time Domain (FDTF) method and allows the efficient simulation of large multilayer structure. One of the main strength of LTCC design based on FDTD is its unique runtime compilation and able to extensively exploit modern processor architectures which leads to a performance up to 850million cells per seconds (Mcell/s) [5]. An Empire XcCel TM has been selected since this tool based on the FDTD method has proven to be very accurate. It includes specially modified algorithms and methods for efficient utilization of multiple floating points and caches. The software’s newest graphical user interface, known as Ganymede, provides tools for the construction and modification of the objects that are being analyzed.[2]. This package utilizes the Finite Different Time Domain (FDTD) which has become a standard for RF component design. Its applicability, ranges from analyzing planar structures, interconnects, and multi-port packaged to waveguides, antennas and EMC problems. The table 1.0 as below show that different between Empire(FDTD) and Momentum(Method of Moment).Every software has it is own advantages and disadvantages. Empire(FDTD) Momentum(MoM) 1) Finite substrates 1) infinity substrate 2) Capable of simulating the exact metallization

2) Assumes zero metallization thickness

3) Easy structure set up

3) Need to define each structure and substrate characteristic.

3) Less amount of memory (40,000KB)

3) Consumed large amount of memory (225,380KB)

Table 1.0 : Comparison Empire(FDTD) and Momentum (MoM).

4.Experimental Design of embedded multilayer unductor

Inductors are one of the critical components for RF designs because of their application in several functionalities such as biasing, matching, filtering and feedback circuits. For that reason, we select the multilayer inductor in order to establish the LTCC design technique based on FDTD EM simulation as shown in Figure 4.0.

Figure 4.0 : 3D View of an Multilayer Spiral Inductor in LTCC multilayer technology by using Empire XcCel TM

The LTCC design aspects of microwave applications can be obtained from standard literature like [6].An important part of the design cycle is the extraction of accurate component models for use in the design. An equivalent circuit models are limited to the first self-resonant frequency.

5. Result and discussion Figure 5.0 shows an example of the simulated result for multilayer inductor. The desired inductance of the spiral inductor was found with a simple circuit optimization. The optimization goal was to achieve L=imag{Zin/2πf).From the simulated result, the inductor value, ZL is 82.41nH at 400MHz & ZQ is 73 at frequency 400MHz.

0

2e-008

4e-008

6e-008

8e-008

1e-007

1.2e-007

1.4e-007

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Impedance in Ohms

frequency in MHz

1

1: 82.417e-9@399.500

Re(./ sub-1/ ZL)

Im(./ sub-1/ZL)

Figure 5.0(a) : Simulation result ZL

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0 50 100 150 200 250 300 350 400 450 500

Impedance in Ohms

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Re(./ sub-1/ZQ)

Im(./ sub-1/ ZQ)

Figure 5.0(b) : Simulation result ZQ

6.Conclusions

The aim of the modeling and EM simulation is to reduce the number of prototype sample iteration with the eventual goal of a right first time design methodology. As conclusion, the development of LTCC circuit can be significantly accelerated with the help of electromagnetic analyses. An efficient design flow can be obtained when an appropriate electromagnetic analysis method is used. In this paper, it has been presented how electromagnetic analysis with FDTD 3D Empire XcCelTM was used to achieve the design goal with LTCC technology.

Acknowledgement The author would like to thank TM for financing this research work under project R05-0606-0.

References

1)Paula Lucchini, Nathan Roberts, Marcos Vargas,Dr Mikaya Lumori, “ Full wave analysis and characterization of via grounding techniques used to decrease electromagnetic coupling between striplines,”University of san diego,2005. 2 )IMST GmbH: User and Reference Manual for the 3D EM Time Domain Simulator Empire,http://www.empire.de,2007. 3)K.S.Yee, “Numerical solution of initial boundary value problems involving Maxwell’s equation in isotropic media,”IEEE Trans. Antennas Propagat.,vol AP-14,pp302-307,May 1966. 4)Sarmad Al-Taei,David Haigh,George Passiopoulos, “Multilayer Ceramic Integrated Circuits(MCICs) Technology and passive circuit design,”. 5)Nancy Friedrich, “EM Simulators reveal Contents of Crystal Ball”,August 2005,Penton Media Inc. 6)H.Jantunen,T.Kangasvieri,J.Vahakangas, S.Leppavuori,“ Design aspects of microwave components with LTCC technique”, Journal of the European Ceramics Society, pp 2541-2548,2003