Microsyst Technol (2012) 18:745–751 DOI 10.1007/s00542-012-1458-4

TECHNICAL PAPER

Realization of embedded capacitors using polymer matrix composites with barium titanate as high-k-active filler Thomas Hanemann • Benedikt Schumacher

Received: 24 January 2012 / Accepted: 15 February 2012 / Published online: 29 February 2012  Springer-Verlag 2012

Abstract Polymer matrix composites (PMC) with barium titanate as high-k-active filler have high potential in embedded capacitors within a printing circuit board (PCB) enabling high permittivity values and low loss factors. These PMCs allows for the use of established polymer processing techniques like screen printing and curing, which are compatible to the established PCB-materials and shaping processes. In this work a process chain, starting with a material optimization of the nano-sized barium titanate, dispersed in an unsaturated polyester-styrene reactive resin, and a further specific process development, will be presented. With respect to the optimization of the individual process steps the flow behaviour of the uncured composite, the polymerization process and the dielectric properties were characterized comprehensively. Using a composite with a barium titanate filler load of 74 wt% allows for a dielectric layer formation by a modified screen printing technique. After capacitor mounting and com2 posite curing an initial capacity density of 13.3 pF/mm could be achieved.

T. Hanemann (&)
B. Schumacher Karlsruhe Institute for Technology, Institute for Applied Materials, Materials Process Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany e-mail: [email protected] T. Hanemann
B. Schumacher Department of Microsystems Engineering, University of Freiburg, Georges-Koehler-Allee 102, 79110 Freiburg, Germany

1 Introduction In addition to silicon, polymers, ceramics and metals polymer matrix composites (PMC) gain more and more importance in microsystem technologies. The addition of micro- or nanosized fillers or dopants to polymers enables a physical property tailoring in combination with established polymer and ceramic shaping or replication technologies targeting new applications and devices. Of particular interest is the adjustment of the following physical properties (Hanemann and Szabo 2010): • • • • •

Optical: refractive index, transmittance, polarization, emission in the UV/Vis/NIR range Electrical: conductivity, permittivity, loss factor Flow behavior: temperature and shear rate dependent viscosity, viscoelastic properties Thermal: glass transition temperature, melting, coefficient of thermal expansion, continuous operation temperature Mechanical: stiffness, Young’s Modulus, hardness

The proceeding increase of functionalities in mobile electronic devices like mobile phones, netbooks or tablet PCs requires the development of new composite materials e.g. for the use as dielectric layer in an embedded capacitor enabling a further reduction of the necessitated printed circuit board (PCB) space. Besides the aspired enhanced dielectric properties the new systems must possess improved process stability, excellent film forming properties and a good chemical and thermomechanical compatibility with the established PCB material and processing technology. In addition a higher dielectric performance than established commercially available PMCs like the Huntsman Probelec 81 ceramic filled polymer with a permittivity value of 25 at 1 MHz must be achieved (http://www.huntsman.com, last Access 02.09. 2011).

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Mostly micron-sized barium titanate has been used as ceramic filler with extreme high permittivity values. The active component can be dispersed in different polymers like epoxides, polystyrenes and others enabling high sum permittivities. In 2010 a comprehensive review summarizes the dielectric properties of a huge number of different polymer-ceramic-composites (Sebastian and Jantunen 2010). For bulk barium titanate type ceramics the influence of the crystal phase and crystallite size on the permittivity and loss factor is of huge significance. Earlier investigations on nanosized barium titanate showed, that due to the vendor’s synthesis and processing conditions, the crystal lattice was distorted or a thermodynamically wrong phase was present (Schumacher et al. 2010; Hanemann et al. 2011a). A thermal pretreatment of the ceramic at elevated temperatures caused a significant increase of the permittivity and a reduction of the loss factor (Schumacher et al. 2010; Hanemann et al. 2011a). Quite recently a similar permittivity enhancement was demonstrated in case of nanosized strontium titanate (Hanemann et al. 2011b). In continuation of previous published work covering the enhancement of the active filler properties only, this paper focuses on the whole process chain development targeting device fabrication. For proof-of-concept purpose a simple capacitor test design was realized using a PMC as dielectric layer, consisting of a property enhanced barium titanate as active filler, an unsaturated polyester-styrene mixture as polymer matrix and screen printing as shaping method.

2 Experimental For all investigations targeting device fabrication two barium titanates with different primary particle sizes (100, 700 nm: vendor information, Inframat Advanced Materials) where used. The measured average particle sizes (d50, 271 and 258 nm), the specific surface areas (10.4 and 2 1.7 m /g) and SEM-images have been published earlier (Hanemann et al. 2011a). For the measurement of the time, temperature and filler load dependent viscosity a barium titanate with an average particle size around 500 nm a specific surface area around 2 m2/g (vendor information, Alfa Aesar company) was used. A commercially available unsaturated polyester–styrene resin (Carl Roth GmbH) was diluted with 20 wt% of the active solvent styrene to reduce the viscosity of the base polymer matrix. 2 wt% INT-54 (E.u.P. Wuertz GmbH) were added as release agent. The barium titanates were added to the unsaturated polyester–styrene mixture using a dissolver stirrer (IKA, 30 min, 800 rpm, ambient conditions). The cold hardener methyl ethyl ketone peroxide (MEKP; Carl Roth GmbH) was used at a concentration of

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3 wt% to start the polymerization reaction. The peroxide was added subsequently after the dispersing step. With respect to test specimen or film layer fabrication the PMC was then allowed to harden at 50C for at least 1 hour. For dielectric measurements disk shaped test specimen with a diameter of 50 mm and a thickness of approx. 9 mm were cast using silicone models. The huge thickness was necessary for a better sample handling during surface polishing and applying silver conducting paint electrodes. The dielectric properties were characterized using an Agilent HP 4194A impedance analyser, equipped with the sample holder Agilent 16451B (integration and averaging of 401 data points). The simple capacitor test design was realized by using a processed printed circuit board (Bungard, Conrad Electronics) and an own constructed modified tape casting setup enabling screen printing of the PMC layer. A climate chamber (Espec SH-261) was used for the temperature dependent capacity measurements.

3 Results and discussion The whole process chain consists of several individual steps (Fig. 1). First the individual main components of the PMC, the high-k-dielectric barium titanates and the polymer matrix, were optimized with respect to highest permittivity and low loss values as well as on the polymer’s curing behaviour and thermal stability. Second the resulting PMC was characterized with respect to thin film formation via screen printing (viscosity) prior to solidification as well as dielectric properties (permittivity, loss factor) after polymerization as function of solid barium titanate load. Third a suitable simple capacitor design was selected and a demonstrator device was fabricated by screen printing. Finally the dielectric properties of the resulting capacitor device as function of temperature were measured. 3.1 PMC main component optimization 3.1.1 High-k-ceramic filler Starting in the micrometre range, decreasing grain sizes cause a pronounced increase of the barium titanate permittivity (Kinoshita and Yamaji 1976). Starting on the nanoscale, a crystallite size increase results in a permittivity increase also (Buscaglia et al. 2006). Therefore, it can be assumed that a maximum permittivity in the crystallite size range between 100 nm and 1 lm is positioned. Earlier investigations showed that PMCs containing nanosized ceramics, like barium titanate and others, delivered poor relative dielectric constants and huge loss factors (Hanemann and Schumacher 2010a). In case of

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Fig. 1 Process chain development

barium titanate it was demonstrated that these low values can be attributed to the presence of a thermodynamically unstable cubic phase instead of the expected tetragonal crystal phase. A selective thermal pretreatment e.g. at 1,000C induced a transition into the tetragonal phase enabling a significantly improved permittivity at least by a factor of two and reduced loss factors (Schumacher et al. 2010; Hanemann et al. 2011a). 3.1.2 Polymer matrix A commercially available low viscous unsaturated polyester–styrene resin (polymer content: 52 wt%, styrene: 48 wt%, Roth GmbH, FRG) was used as polymer matrix. In terms of a reliable processing parameter window the influence of the polymerization conditions (thermal initiator concentration, curing temperature, post-cure conditions) on the resulting polymer properties (glass transition, thermal stability, dielectric properties) was investigated systematically previously (Hanemann et al. 2010). As main result small deviations of the curing conditions like initial cold-hardener concentration or curing temperature do not affect the resulting dielectric properties as well as the glass transition and decomposition temperatures significantly. Hence the polymerization reaction is robust against slight process parameter variations (Hanemann et al. 2010).

With respect to processing the addition of the coldhardener MEKP to the low viscous polymer matrix should allow a stable process window with low viscosity values prior to the initiation of the polymerization reaction. The latter one is, due to chain growth, accompanied by a pronounced viscosity increase preventing thin film preparation by tape casting or screen printing. The polymerisation reaction rate can be influenced by temperature following the well-established van’t Hoff’s rule, which assumes a reaction rate increase by a factor of 2–3 during a temperature increase of 10 K (Q10 temperature coefficient (http://en.wikipedia.org/wiki/Q10_(temperature_coefficient), access 22.12.2011). Figure 2 shows the viscosity increase of the polymer matrix after MEKP addition with time and at different temperatures. Two main aspects can be depicted: First the influence of the measuring temperature on the polymer matrix prior (t \ 100 s) to the curing process can be seen. A temperature increase of 30C lowers the viscosity almost by a factor of five. Second after the initial onset time a significant viscosity increase over a few decades within minutes, especially at elevated temperatures, can be observed. Following these curing behaviour it is strongly recommended to keep the storage and processing temperature prior to curing below 30C retaining sufficient processing time (here 10 min) for shaping.

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Fig. 2 Time and temperature dependent viscosity (pure polymer matrix) increase during polymerization

3.2 PMC characterization It is known from ceramic slurry processing that bimodal ceramic filler mixtures enable higher filler loadings in a liquid polymer matrix retaining moderate viscosities. The barium titanate fillers were added to the low viscous reactive resin matrix under ambient conditions applying a small sized dissolver stirrer. With respect to shaping it was shown earlier, that a composite containing up to 78 wt% of bimodal barium titanates (composition: 70% Inframat 700, 30% Inframat 100) possesses acceptable viscosity values below 10 Pa s and a pronounced pseudoplastic flow behavior (Hanemann et al. 2011a). With respect to shaping the knowledge of the influence of the solid load in the PMC on the curing behavior is crucial. Figure 3 shows the change of the viscosity with time (after MEKP addition) and barium titanate load (up to 55 wt%) at 25C. For comparison the data for the pure polymer matrix is included also. Due to experimental reasons (gap emptying at these high shear rates), the viscosity measurements at higher solid loadings were omitted. Again two main aspects can be seen: First the addition of barium titanate causes a pronounced PMC viscosity increase almost by a factor of 10 prior to curing. Second the addition of the filler attenuates the viscosity rise during the preceding curing reaction. This can be explained by a hindered diffusion of the reactive centers and molecules during chain growth due to the presence of inactive filler particles. The prolongation of the onset time by a factor of five at higher barium titanate contents extends the processing window for the shaping step. Figure 4 shows the permittivity change of cured test specimen with temperature and bimodal solid loading. Under ambient conditions, a relative permittivity value of 50 (at 78 wt% (40 vol%) bimodal barium titanate) can be

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Fig. 3 Time and solid load dependent viscosity (PMC) increase at 25C during polymerization

achieved, which is twice of the commercially available Huntsman product. The given permittivity value of 50 (at 1 kHz) is significantly higher than for a polyimide-BaTiO3composite (27 at 1 kHz, 40 vol%) described in (Sebastian and Jantunen 2010) or an epoxy-BaTiO3-mixture (*25 at 1 MHz, 40 vol%), published by (Imanaki et al. 2011). The latter system enables the fabrication of composites with a filler load up to 70 vol% and a resulting permittivity value of 120 (1 MHz). The pronounced permittivity increase passing 50C can be attributed to the increasing polymer matrix flexibility approaching the glass transition temperature around 86C. This effect was observed also for polyester-styrene based PMCs containing different barium and strontium titanates (Schumacher et al. 2010; Hanemann et al. 2011b).

Fig. 4 Change of the PMC’s permittivity with bimodal solid load and temperature

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Table 1 Coefficient of thermal expansion (CTE) of used materials Material

CTE (106 (1/K)

Reference

Copper

16.5

a

FR4 (Epoxide, reinforced with glass fibers, PCB)

12–17 (x,y)

b

BaTiO3 (sintered)

10.1

70 Trithaveesak (2004)

a

https://secure.wikimedia.org/wikipedia/de/wiki/Ausdehnungsko effizient, access 30.12.2011

b

http://www.leiton.de/technology-rigid-printed-circuits.html, access 10.01.2012

avoided. Figure 5 shows the dependence of the PMC’s CTE with solid load. It can be seen, that with increasing barium titanate content the large initial CTE of the polymer matrix can be reduced, but even at high solid loadings (30 vol%: *120 9 10-6 1/K) a CTE mismatch by a factor of 10 is still given. The measured value is in good agreement with a linear mixing model applying the individual CTEs of the PMCs components and the volume load of the ceramic filler (Sebastian and Jantunen 2010). Considering e.g. a mixture of ceria, dispersed in high density polyethylene (HDPE), a CTE around 130 9 10-6 (1/K) for a PMC with 30 vol% filler was described.

3.3 Capacitor layout and fabrication

Fig. 5 Change of the PMC’s CTE with barium titanate load

With respect to device fabrication and device stability a mismatch of the thermal expansion coefficients (Table 1) of the PCB, the electrodes and the dielectric layer has to be

Figure 6, left shows the design of a simple capacitor for the dielectric performance evaluation of newly developed polyester-styrene bimodal barium titanate composites. A screen printing approach was used to place the composite on the bottom electrodes (Fig. 6, middle). The copper contact stripes were protected using thin sticky tape. Subsequently, after casting the upper set of electrodes were positioned on the laminate and fixed. The demonstrator was then cured at 50C for several hours in a sample holder applying external pressure to avoid delamination resulting from polymerization shrinkage. The polymer acts also as glue in the sandwich capacitor. Finally an active film thickness around 40 lm could be achieved (Fig. 6, right). A similar approach was selected by (Das et al. 2008) applying an epoxy-BaTiO3 PMC. The use of a solvent, which was evaporated later, enables the realization of higher solid loading up to 70 vol% and thinner films down to 2 lm (Das et al. 2008).

Fig. 6 Left capacitor test design. Middle schematic drawing of the screen printing process. Right capacitor cross section

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phenomena can be prevented, a stable permittivity under cycling conditions can be achieved (Das et al. 2008). 4 Conclusion and outlook

Fig. 7 Capacity density change under temperature cycling conditions

A complete process chain for the realization of simple embedded capacitors has been developed. Starting with the improvement of the dielectric properties of commercially available barium titanate, followed by the development of highly filled PMCs applying bimodal filler systems in combination with a modified screen printing technique for thin layer formation, acceptable dielectric properties could be achieved. The comprehensive dielectric characterization employing thermal cycling showed the principal applicability of the chosen concept and methods. Further improvement of the capacitor fabrication e.g. reduction of the active layer thickness and a reduction of the CTE mismatch between electrodes, PCB and PMC should improve the dielectric performance and especially the longtime stability under cycling conditions. References

Fig. 8 Dielectric loss change under temperature cycling conditions

3.4 Device characterization Figure 7 shows the capacity density change with operating temperature variation. The thermal cycling between -60 and 80C caused pronounced capacity decay. This might be attributed to delamination of the active layer from the electrodes due to a mismatch of the coefficients of thermal expansion of the active layer and the electrodes or the board material (see Sect. 3.2). The dielectric loss remained almost constant at constant temperature and increasing number of cycles (Fig. 8). At temperatures close to the PMC’s glass transition temperature (*86C), a pronounced dielectric loss increase according to enhanced polymer chain mobility could be observed. The initial permittivity density value of the pristine capacitor of 2 13.3 pF/mm dropped with proceeding cycling, the loss factor remained almost constant. It can be assumed, that defect generation (delamination) is responsible for the permittivity decay. If a delamination and other aging

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