The porosification of fired LTCC substrates by applying a wet chemical etching procedure

https://doi.org/10.1016/j.jeurceramsoc.2008.05.030Get rights and content

Abstract

In this study, a novel process is presented to generate a defined and homogeneous degree of porosity in fired low temperature co-fired ceramics (LTCC) substrates. For this purpose, a phosphoric-based acid is used which is a standard wet chemical etchant in the MEMS and microelectronic industry for the patterning of aluminium-based conductors and strip lines. Varying the bath temperature between 90 and 130 °C within a time frame of up to 8 h, a maximum penetration depth of 40 μm is achieved. At short etch times up to 5 h, the porosification process is reaction controlled, while at longer exposure times, diffusion-related effects dominate verified by the determination of the corresponding activation energies. In combination with morphological investigations using scanning electron microscopy and micro-X-ray diffraction techniques, it is demonstrated that the anorthite-phase crystallizing during liquid sintering in the vicinity of the Al2O3 grains shows a high dissolvability in phosphoric acid and is very important to enable its penetration into the LTCC body. This surface-near process is very attractive for the realization of selected areas on conventional LTCC substrates having modified dielectric properties, especially for high frequency applications.

Introduction

In recent years, low temperature co-fired ceramics (LTCC) have attracted much attention both as device and substrate structures. The possibility to implement vias with a low sheet resistance based on Au, Ag or Ag/Pd and the integration of passive electronic components (i.e. inductors, resistors and capacities) into the ceramic body, makes it possible to exploit the third dimension.1, 2 This enables the arrangement of electronic components in a compact way within a gas-proof body. Therefore, the components are well protected from environmental impacts when operated under harsh environmental conditions, such as high temperatures.3 LTCC are commonly based on a glass–ceramic consisting of a glass matrix in which aluminium oxide particles with a typical diameter in the range of 2–3 μm are embedded as a filler material. For metallization purposes, the thick film printing technique is the standard technology used. Due to the low sintering temperatures with peak levels in the range of 850 °C the complete assembly of filled vias and printed structures is fired in one single step with the substrate. By the liquid-phase sintering process the soft sheets are densified to form the monolithic ceramic body.

On the device and system level, many novel and sophisticated approaches are reported in literature, such as the fabrication of miniaturized actuators,4, 5 different types of sensor elements for the determination of, i.e. force, gas concentration, mass flow or temperature6, 7, 8, 9, 10 and even complete systems, especially for microfluidic applications.11 A comprehensive overview is given in Refs.12, 13 In this context, reliability in respect to mechanical strength is an important issue, especially when the LTCC needs to fulfil not only the requirements of a functional, but also of a structural material subjected to high mechanical load.14, 15

Besides these applications, LTCC is most favourably used as substrate for micromachined devices and systems operated at high frequencies typically ranging up to the microwave region. Although there are also other types of high-density, multilayer substrates available, based on organic laminates, further outstanding features of the LTCC for this field of application are the excellent thermal conductivity compared to organic materials and a coefficient of thermal expansion close to that of silicon.13 Compared to a standard organic substrate reinforced with a glass–fibre-based component, such as FR-4 or RT/Duroid, the dielectric losses are low. In contrast, the relatively high permittivity of ɛr = 7–8 is disadvantageous for some applications, such as microwave antennas directly arranged on the LTCC surface. To avoid this drawback either a combination of LTCC with a local application of a low-k organic material16 or the modification of the LTCC substrate itself is targeted. Beside the modification of the glass-matrix and the crystallization behaviour,17, 18 the generation of a defined porosity is the most commonly used approach to reduce the dielectric constant and the dielectric losses of materials.19 In Refs.20, 21 mullite or ceramic-based bubbles are dispersed in the glass-matrix modifying the dielectric properties positively, but resulting in a poor topography, especially when aiming for the application of structures on the surface made by thin film technology. In addition, the use of hollow microspheres is proposed which causes problems when these break during casting and firing and hence, reproducible dielectric properties cannot be guaranteed.22, 23 Schuler et al. modulated the material properties by punching holes in the substrate.24 The effective permittivity is determined by the volumetrically weighted median of the relative permittivity values associated with LTCC and air. In Ref.25 the capacitive coupling between a strip line and ground plane is reduced by embedding air cavities below the conductor. Although in the latter two cases, a local modification of the dielectric properties is in principal possible, an additional ceramic layer has to be arranged above the perforated substrate to support the elements resulting in a fragile overall structure.

It is the objective of this paper to report a novel process to generate locally a defined porosity in LTCC in the fired state. Up to now, a maximum penetration depth for the porosification process of about 40 μm below the substrate surface has been achieved. Phosphoric-based acid is used which is a well-established chemical product used for the patterning of aluminium-based strip lines within the fabrication process of micromachined devices. The process and hence, the degree of porosification and the corresponding penetration depth, can be controlled at a given bath concentration very easily by monitoring the etch time and the temperature of the etchant. Both parameters have a high impact on the dominating etch regime (i.e. either reaction or diffusion-controlled). Applying techniques such as scanning electron microscopy (SEM), focused ion beam (FIB) and micro-X-ray diffraction (μ-XRD), the microstructure and the phase composition of the LTCC are investigated before and upon exposure to phosphoric acid.

Section snippets

Experimental details

To study the porosifiaction process, commercially available LTCC substrates (DP 951 AX) from DuPont are used. The blank sheets were laminated at a pressure of 20 MPa and fired at a peak temperature of 850 °C for 30 min in a batch furnace. Further details of the fabrication process can be found elsewhere.26 After co-firing, a compound material is generated consisting of a glass matrix with different crystalline and chemical phases in which Al2O3 particles with a typical size in the μm-range are

Results and discussion

In Fig. 2, the porosification depth dp in fired LTCC substrates as a function of time t at different bath temperature levels Tb of the phosphoric acid is shown. A minimum value for Tb of 90 °C is required to obtain in a time frame of about 1 h a detectable porosification depth of 30 nm. As expected, dp increases when enlarging the duration for the etch attack at a given bath temperature. Increasing Tb has a similar impact on dp while keeping the parameter t fixed. At the onset of the etch attack, d

Conclusions

In this study, a novel process based on a wet chemical etchant is introduced to generate a tailored porosity in fired LTCC substrates (DP 951). Due to the use of a phosphoric acid which is well known for the patterning of aluminium thin films in MEMS or microelectronic industry a local porosification is feasible by using a photosensitive polyimide as mask material. The method is very simple to monitor, as important parameters, such as the bath temperature and the exposure time, strongly

Acknowledgements

This work was performed within the RADARAUGE project (http://www.radarauge-project.com/) financially supported by the Federal Ministry of Education and Research (BMBF) under contract number 16SV2080. This support is gratefully acknowledged. Furthermore, the authors wish to express their thanks Dr. D. Schwanke and T. Haas being with the project partner Micro Systems Engineering GmbH for providing the LTCC substrates.

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      To verify the assumption on the presence of two dominating etching regimes, Arrhenius-type diagrams of dp as a function of the reciprocal bath temperature were plotted, so that the activation energy (Ea) was determined for fixed etching times through a linear regression procedure. The calculated Ea values were used to acquire further information about the etching mechanism because, in a wet chemical etching process, Ea values of about 0.2 eV and below represent the domination of diffusion-controlled, while higher Ea values indicate the presence of reaction-controlled dissolution mechanisms [37,50]. Time-dependent evolution of Ea for etching Ferro L8 is represented in Fig. 2c and d, and it can be observed that the porosification with P50 follows the Arrhenius law over the whole temperature range up to 240 min.

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