Low Temperature Co-fired Ceramic Substrate Research Articles

Packaging for RF MEMS devices using LTCC substrate and BCB adhesive layer

In this paper, a packaging method utilizing an LTCC (low temperature co-fired ceramic) substrate and a BCB (benzocyclobutene) adhesive layer has been developed for RF MEMS devices, and the RF performance and characteristic parameters of the package have been evaluated. LTCC substrates have good RF characteristics in high-frequency applications, and via feedthroughs can be easily incorporated during the manufacturing process. In this paper, an LTCC substrate is used as a capping wafer to reduce the complex processes for vertical interconnections. A layer of BCB, in the form of sealing rims, is used as an adhesive to bond the MEMS substrate with the LTCC cap due to the excellent properties of BCB as a packaging material. A CPW (coplanar waveguide) line has been fabricated on a quartz substrate and packaged to demonstrate the performance of the proposed packaging method. After forming the CPW lines, a 28 µm thick BCB layer is patterned by double-coating photolithography for an adhesive bonding. On the backside of the LTCC cap, a 150 µm deep cavity is formed to improve the RF characteristics. The CPW and the contact pad are connected electrically through the silver via-post in the LTCC substrate by screen-printed silver epoxy. The RF characteristics of the CPW line have been measured before and after packaging. The insertion loss of a bare CPW is 0.047 dB at 2 GHz and 0.092 dB at 20 GHz. After packaging, the insertion loss of the packaged CPW is 0.091 dB at 2 GHz and 0.312 dB at 20 GHz. A leak test has been performed using both IPA (isopropyl alcohol) soaking and the He leak tester. Most of the samples show no leakage for the IPA test, and a measured leak rate of 10−8 atm cc s−1 for the He leak test. In addition, the shear strength of the package was measured to be 25–35 MPa. From the experimental results, we showed the feasibility of a low-loss RF MEMS package from dc to 20 GHz with acceptable package performances.

Structural and electrical properties of perovskite ruthenate-based lead-free thick film resistors on alumina and LTCC

CaRuO3 perovskite-based lead-free thick-film resistors (TFRs) were prepared on 96%-alumina and Low Temperature Co-fired Ceramic (LTCC) substrates. The microstructure evolution, possible interactions, and electrical properties of resistors were investigated. The hot and cold TCR values of all the resistors were measured in the temperature range (+20 to +120 ∘C) and (+20 to −120 ∘C), respectively. TFRs with 25% by vol. of CaRuO3 on alumina exhibit a sheet resistance Rs = 5kΩ/sq. with hot and cold TCR of 225 and 470 ppm/∘C respectively, whereas the same composition gives 1.2 kΩ/sq., 16.5 kΩ/sq. and 0.7 kΩ/sq. for co-fired, post-fired resistors on LTCC and buried resistors, respectively. The hot (HTCR) and cold (CTCR) values were evaluated; HTCR = 190 ppm/∘C and CTCR = 314 ppm/∘C were found for co-fired structures; HTCR = 216 ppm/∘C and CTCR = 205 ppm/∘C for post-fired samples and HTCR = 520 ppm/∘C and CTCR = 350 ppm/∘C for buried in LTCC structures.

Laser profilometer module based on a low-temperature cofired ceramic substrate

We realized a laser profilometer module using low temperature cofired ceramics technology. The device consists of a vertical-cavity surface-emitting laser as the light source and a complementary metal oxide semiconductor image sensor as the detector. The laser transmitter produces a thin light stripe on the measurable object, and the receiver calculates the distance profile using triangulation. Because the design of optoelectronic modules, such as the laser profilometer, is usually carried out using specialized software, its electronic compatibility is very important. We developed a data transmission network using commercial optical, electrical, and mechanical design software, which enabled us to electronically transfer data between the designers. The module electronics were realized with multilayer ceramics technology that eases component assembly by providing precision alignment features in the substrate. The housing was manufactured from aluminum using electronic data transfer from the mechanical design software to the five-axis milling workstation. Target distance profiles were obtained from 100 points with an accuracy varying from 0.1 mm at a 5-cm distance to 2 cm at 1.5 m. The module has potential for distance measurement in portable devices where small size, light weight, and low power consumption are important.

Experimental Evaluation of Arrayed Microcolumns with Monolithic Structure

A 2×2 arrayed microcolumn structure based on a monolithic deflector was evaluated experimentally. The microcolumns of this structure consist of electron emission tungsten tips, silicon membrane microlenses, double silicon deflectors, and Cu grid samples. The assembly of the microlens and column component was performed on a custom-built alignment unit consisting of an optical microscope, an alignment jig, and a motor-driven micromanipulator. The alignment of the tungsten tip and microlens hole is performed under direct observation with two perpendicularly aligned microscopes. Microlenses and deflectors (silicon octupoles) are mounted on a low temperature co-fired ceramic (LTCC) substrate. The three-dimensional interconnection structure of the LTCC substrate provides common wiring for each double octupole for synchronous operation. Each deflector was operated with same voltage sources. The 2×2 arrayed microcolumns were tested in three modes; single column, independent, and parallel mode, and the properties of these modes were compared.

Capacitive pressure sensor in post-processing on LTCC substrates

A capacitive pressure sensor was realized by means of a post-processing step on a low temperature co-fired ceramics (LTCC) substrate. The new sensor fabrication technology allows for integration of the sensor with interface circuitry and possibly also wireless transmission circuits on LTCC substrates to realize a truly autonomous sensor unit. A special feature of this sensor technology is the flush surface. The article describes the design considerations, and compares experimental data to the theoretical design. Special point of attention is the long-term behaviour of the soldering joint. Various design variants have been evaluated considering reproducibility and creep behaviour.

Effect of substrate properties and thermal annealing on the resistivity of molybdenum thin films

In this study, the influence of substrate properties (e.g. roughness characteristics and chemical composition) on the electrical resistivity of evaporated molybdenum thin films is investigated as a function of varying parameters, such as film thickness (25–115 nm) and post-deposition annealing with temperatures up to T PDA = 900 °C. A thermally oxidized silicon wafer with very low surface roughness was used as one substrate type. In contrast, a low temperature co-fired ceramics substrate with a glass encapsulant printed in thick film technology is the representative for rough surface morphology. The electrical resistivity follows the prediction of the size effect up to T PDA = 600 °C independent of substrate nature. On the silicon-based substrate, the thickness-independent portion of the film resistivity ρ g in the “as deposited” state is about 29 times higher than the corresponding bulk value for a mono-crystalline sample. Thin films of this refractory metal on the SiO 2/Si substrate exhibit an average grain size of 4.9 nm and a negative temperature coefficient of resistivity (TCR). On the glass/ceramic-based substrate, however, ρ g is half the value as compared to that obtained on the SiO 2/Si substrate and the TCR is positive.

An adaptive impedance tuning CMOS circuit for ISM 2.4-GHz band

The difficulties encountered in matching an antenna to its optimal impedance are reduced with an adaptive 0.35-/spl mu/m CMOS circuit based on several switched shunt capacitors arranged in capacitor banks and on a few external series inductors. As high-quality inductors are difficult to obtain in CMOS, the inductors are placed either in an low-temperature cofired ceramic (LTCC) substrate or is a lumped component outside the core circuit. The circuits, presented here through a range of simulations, are optimized to function within the ISM 2.4-GHz band, but the general approach employed to improve matching can be used for other frequency bands as well. The circuits discussed provide a VSWR/spl les/2 match for every impedance with VSWR/spl les/5. There is a 1-dB power loss for a perfect 50 /spl Omega//spl rarr/50 /spl Omega/ transformation, a break-even point at VSWR=1.5, and a 3-dB increase in delivered power for VSWR= 4.3.

Evaluation of compatibility of thick-film PTC thermistors and LTCC structures

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film thermistors with high positive temperature coefficients of resistivity, i.e. PTC 5093 (Du Pont) fired either on “green” LTCC (low temperature cofired ceramics) substrates or buried within LTCC structures, were evaluated. The active phase (ruthenium oxide) in the PTC thermistors is not present in the dried films but is formed during firing due to the decomposition of the ruthenate phase. Because of interactions between a glassy LTCC material and thermistors electrical characteristic, i.e. sheet resistivities and noise indices of thermistors fired on the surface LTCC substrates, changed from values obtained on alumina substrates. The differences in the measured electrical parameters were attributed to the interactions between the thermistor layers and the glassy LTCC substrates. The inter-diffusion of oxides, mainly PbO and Al 2O 3, was confirmed by microanalysis. In the case of buried thermistors, presumably due to the incompatibility of both materials, the structure de-laminated during firing. Cracks between the buried PTC films and the LTCC substrates as well as cracks in PTC films resulted in high sheet resistivities and high noise indices.

A 60-GHz multilayer parasitic microstrip array antenna on LTCC substrate for system-on-package

This letter describes a compact and high-gain multilayer parasitic microstrip array antenna (MPMAA). The design and performance of the proposed array antenna are presented. The antenna employs three layers with a 2/spl times/2 parasitic array on each layer. The developed prototype MPMAA employs a multilayer low-temperature co-fired ceramic (LTCC) substrate that is well suited to the assembly of MMIC chips. The fabricated MPMAA achieves a 7.17 dBi absolute gain at 60 GHz including the loss derived from the feeding parts and RF probe to measure its antenna performance. The spacing of the top layer of the parasitic array constructed by 2/spl times/2 elements has a free-space wavelength of 0.36 and the chip size is 10 mm/sup 2/. The fabricated MPMAA achieves both compact and high directional gain and satisfies the requirements for a millimeter-wave system-on-package (SOP).

LTCC TECHNOLOGY FOR PHOTONIC AND MILLIMETER WAVE MODULE INTEGRATION

In the module integration process, lower level packaging elements such as devices, sub-mounts and components are combined as a functional block. Embedded sub-system consists of modules whose operations are controlled by a specific algorithm. Typically, the control algorithm is realized in a form of system software in a processor unit, which controls individual module operations and combines separate module operations as a system operation. In the creation of a cost-efficient embedded system it is essential that reliable, reproducible and cost-efficient building blocks of systems, modules, can be manufactured. We have studied Low Temperature Co-fired Ceramics (LTCC) substrate technology and its possibilities to produce reliable, reproducible and cost-efficient photonic and millimeter wave modules. In this paper we introduce our contribution to the development of LTCC photonic and millimeter wave modules. Most of the papers written on LTCC packaging are in the field of RF and millimeter wave modules and are shortly reviewed in this paper. Photonic modules utilizing LTCC substrates are almost missing in the literature. Several photonic modules based on the use of LTCC substrates developed by our group are introduced in this paper, including hermetic fiber pigtailed power laser, wavelength tunable laser and 10 GBit/s/ch optical interconnects board for photonic applications. In addition, front-end module for 24 GHz transceiver for millimeter wave applications is introduced.

Study on an injection quantity sensor. II: Evaluation of the sensing element

For further optimization of the combustion process, the information about the actually injected fuel quantity is desirable, especially in diesel engines equipped with direct injection technology. A miniaturized hot-film anemometer with a titanium/platinum metallization on a low-temperature co-fired ceramics substrate was developed and integrated into a Common Rail injection nozzle. The micro-flow sensor proved its high performance and its capability over the complete fuel quantity map of a high pressure hydraulic injection system where drive pulses for the operation of the injector range between 0.3 ms and 1.5 ms at injection pressures up to 135 MPa (1350 bar). In correspondence with measurements of an injection amount indicator integrated into the hydraulic test bench, the injected fuel quantity and the opening behaviour of the orifice were derived from the sensor signals as a function of injection parameters. Assuming a power law dependence on the fluid velocity for the flow-sensitive portion of the heat-transfer coefficient, a value of 0.5 for the exponent was determined experimentally, in excellent agreement with theoretical predictions.

LTCC technology for cost‐effective packaging of photonic modules

PurposeThe aim of the research was to evaluate the concept that utilizes structured planar substrates based on low temperature co‐fired ceramics (LTCC) as a precision platform for the passive alignment of a multimode fiber and wide‐stripe diode laser.Design/methodology/approachPresents the manufacturing process for realisation of 3D precision structures, heat dissipation structures and a cooling channel into the LTCC substrate. The developed methodology for 3D modelling and simulation of the system was used to optimize structures, materials and components in order to achieve optimal performance for the final product and still maintain reasonably low fabrication costs. The simulated optical coupling efficiency and alignment tolerances were verified by prototype realization and characterization.FindingsThe achieved passive alignment accuracy allows high coupling efficiency realisations of multimode fiber pigtailed laser modules and is suitable for mass production.Research limitations/implicationsProvides guidance in the design of LTCC precision platforms for passive alignment and presents a hybrid simulation method for photonics module concept analysis.Practical implicationsThe three‐dimensional shape of the laminated and fired ceramic substrate provides the necessary alignment structures including holes, grooves and cavities for the laser to fiber coupling. Thick‐film printing and via punching can be incorporated in order to integrate electronic assemblies directly into the opto‐mechanical platform.Originality/valueIntroduces the LTCC 3D precision structures for photonics modules enabling passive alignment of multimode fiber pigtailed laser with high efficiency optical coupling. Demonstrates the hybrid simulation methodology for concept analysis.

New approaches for designing microstrip filters utilizing mixed dielectrics

A strategy is developed for designing capacitively loaded microstrip filters on low-temperature co-fired ceramic (LTCC) substrates with inclusions or superstrate layers of higher permittivity dielectrics. Finite-difference time-domain simulations of the field distribution at resonant frequencies are used to determine the optimal locations and size of capacitive loads. It is demonstrated that strategic capacitive load placement enables altering the center and attenuation pole frequencies, the shape and width of the passband, and input impedance of the filter by modification of selected resonant modes. Capacitive loading with higher permittivity dielectrics is shown to be very efficient in decreasing dimensions of microstrip filters with low-permittivity substrates. The designs of novel compact resonators and filters have been developed and the prototypes fabricated by using LTCC technology. The results of prototype measurements agree with the simulation results, which validates the proposed approach

Thick-film temperature sensors on alumina and LTCC substrates

The electrical and microstructural characteristics of 1 kΩ/sq. thick-film thermistors with high positive or negative temperature coefficients of resistivity, i.e., PTC 5093 (Du Pont) and NTC 4993 (EMCA Remex), fired on alumina or co-fired on “green” low-temperature co-fired ceramic (LTCC) substrates, are compared. The active phase in both materials (the ruthenium oxide and the semiconducting spinel in the PTC and NTC thermistors, respectively) is not present in the dried films but is formed during firing. The differences in the measured electrical parameters (sheet resistivities, temperature dependence of resistivities and noise) of the thermistors, fired on alumina or on LTCC were attributed to the interactions between the thermistor layers and the glassy LTCC substrates. The inter-diffusion of oxides, mainly PbO and SiO 2, was confirmed by microanalysis.

Design of an LTCC Tri-Band Transceiver Module for GPRS Mobile Applications

This paper presents the design and results of a low-temperature co-fired ceramic (LTCC) module for tri-band general packet radio service (GPRS) mobile applications. The proposed transceiver module integrates the tri-band (EGSM900/DCS1800/PCS1900) antenna switch, the surface-acoustic-wave filters for band selection, and a fully integrated tri-band transceiver integrated circuit into one LTCC package. By taking advantages of the three-dimensional layout capability of the LTCC substrate, the embedded circuit elements can be arranged vertically for size reduction. In addition, the utilization of mutual coupling leads to a novel design of miniature lumped-element realization of the transmission line. By combining these miniature passive component designs, the proposed module features a compact size of 13.8 mm /spl times/ 8.1 mm, with a packaged height of 1.8 mm. The module is compliant with the European Telecommunications Standards Institute regulations for GPRS mobile applications and is currently in use in commercially available mobile phones.

Studies on three-dimensional moulding, bonding and assembling of low-temperature-cofired ceramics for MEMS and MST applications

This paper broadly illustrates the preparation of three-dimensional shapes in low-temperature-cofired ceramics (LTCC). Test structures are fabricated by moulding single-layer green tapes into cylindrical form of different dimensions in order to investigate the penetration of cracks and the limiting value at which they appear. The factors responsible for the propagation of cracks have been analysed. Another important aspect which has been exemplified is the bonding of processed LTCC modules to metal parts with a dissimilar coefficient of thermal expansion. This is important from the point of view of attachment of LTCC-based micro-electromechanical systems (MEMS) or micro-system technology (MST) devices to a functional mechanical element. A methodology has been discussed for a smooth transition in the coefficient of thermal expansion of ceramic and metallic part in order to minimise stresses and avoid micro-cracks in the interconnections. An assembly of different post-fired modules by a fastening mechanism of individual modules for realizing a complex system are also described. Based on the above studies, a preliminary design to fabricate, shape, bond and post-process the LTCC substrate for the realization of a complex system together with the crucial processing issues is presented. Efforts have been made to implement this design and process steps on a real-time ball-bearing to prove the suitability of LTCC material for MEMS, MST and other complex systems.

Multiplexed read-out electronics implemented on LTCC substrate for PbS array

Infrared detector arrays are very useful for spectroscopy to realize grating spectrometers with the ability for parallel signal detection. The signals from the detector pixels are usually very weak, and they have to be amplified. Because of the large number of pixels, it is often necessary to multiplex the signals to reduce the number of signal lines out from the package. This work presents the preamplifier and multiplexer modules built on low temperature co-fired ceramic (LTCC) substrate. The LTCC material is suitable for hermetic packaging, which is often needed for PbS detectors in process applications. The capacitive cross coupling of the modules is measured from the modules. The modules are implemented into a package next to the PbS array and the nonlinearity and normalized detectivity of the system are measured. The measurements show that special concern is needed in layout design to avoid capacitive cross coupling. The measurements also show that multiplexing does not deteriorate the good detectivity of the PbS detector.

Fabrication, characterization and sintering of glass-ceramics for low-temperature co-fired ceramic substrates

Cordierite-based glass-ceramics with non-stoichiometric composition doped with rare earth oxide (CeO2) and heavy metal oxide (Bi2O3) respectively were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including compact density and apparent porosity, were examined to evaluate the sintering behavior of cordierite-based glass-ceramics. Results showed both that the additives heavy metal oxide and rare earth oxide promoted the sintering and lowered the phase temperature from μ- to α-cordierite as well as affecting the dielectric properties of sintered glass-ceramics. The complete-densification temperature for samples was as low as 900 °C. This material has a low dielectric constant (≈5.3), a low dielectric loss (≤0.2%) and a low thermal expansion coefficient (2.8–3.52×10−6 K−1), and can be co-fired with high conductivity metals such as Au, Ag, Cu, Ag/Pd paste at low temperature (below 950 °C), which makes it a promising material for low-temperature co-fired ceramic substrates.

Thermal Assessment of RF-Integrated LTCC Front End Modules

The thermal performance of front end module (FEM) incorporating low-temperature cofired ceramic (LTCC) substrate is investigated. An infrared microscope system was used to measure device surface temperature with both RF and dc power. Various duty cycles (25% to 100%) were applied to characterize the thermal performance of the device for different power densities. The maximum junction temperature occurs at the second stage: under dc powering at 100% duty cycle, it reaches 134/spl deg/C. The comparison between numerical and experimental data indicates good agreement, with less than 10% difference in the peak temperature values. When replacing the common 2-layer organic substrate with a 14-layer LTCC substrate and silver paste metallization, the peak junction temperature reaches 130.1/spl deg/C, /spl sim/51% higher compared to the baseline case using a 2-layer organic substrate with copper metallization. However, by increasing the silver paste thermal conductivity from 90 to 350 W/mK, a significant drop in peak temperatures occurs, indicating the impact on the module's overall thermal performance. For the case with 350 W/mK, the peak temperature reaches 82.9/spl deg/C, almost 3% lower than the baseline case with 2-layer organic substrate. The top metal layer thickness (10 versus 30 /spl mu/s) contributed only to 5%-8% change in the peak junction temperature. An improved FEM design incorporates a higher thermal conductivity silver paste material (300 W/mK) and a new thermal via array structure (25 vias, 150 /spl mu/s in diameter each) in the LTCC substrate. The module junction temperature reaches 96/spl deg/C (based on 25/spl deg/C reference temperature and 100% duty cycle), corresponding to a junction-to-substrate (R/sub js/) thermal resistance of 14/spl deg/C/W. Further study reveals that 20% voiding placed at the die center has no impact on FEM thermal performance, while the voiding placed at the die corner (under the heat dissipating stages) increases the stage peak temperature by more than 40/spl deg/C. The last part of this paper focuses on several designs to reduce the manufacturing and assembly costs, while maintaining reasonable FEM thermal performance. Analyses suggest that two particular designs provide the optimal thermal performance when reducing the thermal via number/costs by almost 40%.

Compact integrated combline band‐pass filter with interdigital capacitive loads using LTCC technology

AbstractA compact embedded tapped‐line combline filter with interdigital capacitors using low‐temperature co‐fired ceramic (LTCC) technology for wireless application is proposed. The interdigital‐type capacitors have been loaded to reduce the geometrical length while extending the electrical length. The fabricated filter has a relatively small size, 2.7 × 2.03 mm, and good characteristics at the 5‐GHz range. Its small size and relatively simple structure make it a good candidate as an integrated filter for various LTCC substrates. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 42: 377–379, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20309