Integration Concepts for the Fabrication of LTCC Structures

At the Keck Smart Materials Integration Laboratory at Penn State University, low-temperature co-fired ceramic (LTCC) material systems have been used to fabricate a number of devices for a variety of applications. This article presents an overview of the integration of the concepts and materials that we have used to achieve miniaturization and unique device function. Examples of microwave filters, metamaterial antennas, and a dielectrophoretic cell sorter will be presented, with emphasis on device modeling and design, prototype construction methods, and test results.     

Characterization of the Fabrication Process of Rolled LTCC Structures

This work focuses on the fabrication and assembly of cylindrical plasma containment tubes using DuPont's 951 low temperature co‐fired ceramics (LTCC) for use in miniature electrostatic thrusters. The tube is used to contain argon plasma, which is generated by a spiral inductively coupled plasma antenna, which is also fabricated in LTCC. The tube also interfaces with two electrically biased grids on the opposite end, which accelerate the plasma out of the tube. These interfaces are highly dependent on the dimensions and tolerances of the containment tube. The development of the fabrication process will be presented for the incorporation of the tubes and grids onto the base as a single structure. This includes constructing the antenna base, shaping the “rolled” LTCC containment tube using a jig and isostatic press, and integrating the tube and antenna base during the firing. Following the fabrication, measurements will be taken to determine tube circularity and hermeticity of the seal at the interface between the tube and the antenna base. The results will be presented and characterized to evaluate the effectiveness of the structure as well as the documentation of the development of a rolled LTCC tube structure integrated with a planar LTCC antenna base.     

LTCC Fabrication for a Leaf Spring Vertical Actuator

Ceramic laminate systems (LTCC) provide an alternative to silicon for three-dimensional actuator fabrication. The results of a LTCC vertical actuator study are presented. Based on a multi-fold leaf spring design, appearing as a series of cantilevers oriented in opposite directions, vertical motion is achieved via electrostatic actuation to compress and release the structure. Single and multi-fold proof-of-concept devices were modeled, fabricated, and tested. The device can vertically actuate 10-μm per fold under an applied voltage of 35 V. The actuation per fold may be controlled at 0.33-μm/V, over the range of 5–35 V.     

Fabrication of Ceramic Microscale Structures

Ceramics are attractive materials for engineering applications involving high temperatures and corrosive chemicals. Here, an inexpensive and reproducible ceramic microfabrication technology was used to fabricate high-density alumina structures for applications in microchemical systems. The new method is based on the gelcasting procedure, but key drying and sintering steps have been adjusted to avoid warpage and cracking of the final structures. Centimeter-scale ceramic structures with submillimeter features can be accurately replicated from an elastomeric mold. Further study of the effect of alumina particle size, D _p, on the smallest achievable and reproducible feature size showed that excellent replication of patterns can be achieved as long as the dimensions of features in the mold are greater than 30 D _p for the range of D _p from 0.3 to 3.0 μm.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

Fabrication and fluidic characterization of static micromixers made of low temperature cofired ceramic (LTCC)

Microreaction devices used for chemical synthesis must possess a high resistance against corrosive chemicals. Therefore, microreaction devices were made of glass, steel or ceramics. Photolithographic steps combined with etching processes as well as micropowder blasting or micromilling processes were applied for the formation of appropriate structures. The low temperature cofired ceramics (LTCC) technology combines easy structuring, assembling and packaging techniques with the high chemical resistance of a glass ceramic material. In contrast to the known ceramic technologies, the LTCC technology enables a fast and easy fabrication of microfluidic devices. Here, we present two micromixers made of LTCC and its fluidic characterization. Laser ablation was used for the structuring of green tapes which were layered and cofired to form the micromixers. X-type fluidic barriers were realized inside a squared meandered channel of about 160 mm length. A meandered channel mixer without X-type mixing structures was used as a reference. The pressure drop was measured for aqueous media with various viscosities and the friction factors were calculated. An exponential equation for the friction factor prediction is given. The residence time distribution was determined for both devices by pulse trace experiments and the dispersion model was used to describe the residence time distribution for low Reynolds numbers.     

具有梯度孔结构多孔陶瓷膜支撑体的制备

采用平均粒径为0.56μm的Al2O3为原料,研究了不同固相含量的Al2O3悬浮液的流变性能.在此基础上,采用含10%(质量分数,下同)Al2O3的悬浮液,通过重力沉降和真空抽吸过程,成功制备出具有梯度孔结构的片状Al2O3支撑体.对具有梯度孔结构支撑体的微观结构进行了表征,并研究了支撑体的孔隙率和渗透性能随烧成温度的...     

Fabrication of membranes and microchannels in low-temperature co-fired ceramic (LTCC) substrate using novel water-based sacrificial carbon pastes

In this work, 3D structuration of LTCC (low-temperature co-fired ceramic) for microfluidics was studied, using two novel sacrificial carbon paste compositions. These pastes are based on graphite with a water-soluble vehicle consisting of polyvinylpyrrolidone binder (PVP) dissolved in propylene glycol (PG), which is not aggressive to green LTCC material. Both examined pastes differ slightly in binder content and added plasticizer, glycerol (G) or trimethylolpropane (TMP). The thermal properties of the sacrificial carbon pastes have been examined using combined thermo-gravimetric analysis (TGA), differential thermal analysis (DTA) and differential thermo-gravimetry (DTG). The sacrificial carbon pastes have been applied to the fabrication of membranes and microchannels in LTCC substrate. A comparison of the obtained features has been made using X-ray tomography and optical profile measurements. Moreover, changes in the composition of the fired LTCC material after co-firing have been studied using X-ray photoelectron spectroscopy (XPS) and surface wettability measurements.     

High-Frequency Electromagnetic Bandgap Structures via Indirect Solid Freeform Fabrication

The indirect solid freeform fabrication process is used to fabricate low-loss complex geometry periodic dielectrics for use in the microwave range. Relevant processing details, such as mold fabrication, slurry rheology, curing, and binder burnout, are discussed. Woodpile electromagnetic bandgap structures exhibiting a −40 dB well were shown to be operational at frequencies >110 GHz. Novel designs of planar periodic dielectric resonators with Q = 965 at 33 GHz ( Q = ratio of the power stored to power lost in the resonator) were realized in alumina_. Metallo-dielectric filters with low-insertion loss were also created via ISFF; modeling of these structures using resistor-inductor-capacitor-equivalent circuit analysis was performed to calculate a Q of 645.     

Fabrication of a high-performance film based borosilicate glass/Al2O3 ceramics for LTCC application

The present work was initiated aiming at developing a high-performance film for LTCC application using a kind of polymeric dispersant in tape casting process of borosilicate-based glass/Al₂O₃ ceramics. The dispersion mechanism of the polymeric dispersant with the powders was analyzed in detail and the optimal amount of the dispersant addition was also determined through the sedimentation experiment. Its effects on the compactness, tensile strength and surface roughness of the films were analyzed. In addition, the sintering and microwave properties of the composite fired at 850 °C were also determined. For the application of tape technology, the co-firing compatibility between the composite and Ag electrodes at 850 °C was also verified. After testing and verification with different qualitative and quantitative instruments, a flexible film optimized by 1.5 wt% polymeric dispersant achieved the best overall performance.     

Printable resistors in LTCC systems

Printable resistors in LTCC technology offer some advantages in comparison with discrete components, such as the increased power handling and reliability. As the size of chip resistors decreases, also the size of printed resistors has to be reduced. For optimizing high-frequency performance the size, i.e. width in particular, has to match the width of the transmission lines. In this study the effects of different resistor dimensions and processing parameters have been analyzed. The experimental work has been made with Du Pont 951 and Ferro A6-S LTCC systems. By optimizing the processing, the resistance tolerances smaller than ±12% were achieved.     

Solid Freeform Fabrication of Thin-Walled Ceramic Structures Using an Electrohydrodynamic Jet

A printing device, which operates on an electrohydrodynamic principle, was used to deposit 100 layers of a 10 vol% zirconia suspension. Thus, a wall ∼100 μm in thickness was freeformed. The wall thickness achieved in this work is almost 30% less than that of similar structures built by ink-jet printing and therefore we demonstrate a novel print deposition based solid freeform fabrication method, which can be developed further for preparing fine ceramic structures.     

软膜紫外光固化纳米压印制备等离子体纳米孔结构的工艺优化

利用软膜紫外光固化纳米压印技术和反应离子刻蚀技术在玻璃衬底上制备了等离子体纳米孔阵列.为了实现高分辨低成本软膜紫外光固化纳米压印技术在生物传感领域的应用和产业化,必须对压印的一些具体工艺参数如压印压力进行优化.在这项工作中,压印压力最小化到10 psi,结果表明这一数值足以保证图案的精确复制以及图案转移的保真度.最后,...     

Shrinkage behavior of LTCC hetero-laminates

Shrinkage behavior of various stacking configurations consisting of three dissimilar dielectric tapes with different dielectric constants of k ～ 8.2, 14.8 and 48.2 were investigated. The shrinkage behavior has been critical as a sensitive parameter in the design of embedded microcircuit passive components. Unexpectedly low x–y shrinkages of 2.5–7% observed for the hetero-configurations are believed to be associated with the physical constraining effect that results from the dissimilar sintering route and crystallization behavior of each tape. The constraining effect found depends on the sintering temperature and the total number of each tape layer in a given hetero-structure. The tendency of slope variation in the Arrhenius plots of thermo–mechanical curves suggests that increasing the number of embedded tapes can produce a potential lower activation energy with a higher x–y shrinkage.     

One-Step Mask-Based Diffraction Lithography for the Fabrication of 3D Suspended Structures

We propose a novel one-step exposure method for fabricating three-dimensional (3D) suspended structures, utilizing the diffraction of mask patterns with small line width. An optical model of the exposure process is built, and the 3D light intensity distribution in the photoresist is calculated based on Fresnel-Kirchhoff diffraction formulation. Several 3D suspended photoresist structures have been achieved, such as beams, meshes, word patterns, and multilayer structures. After the pyrolysis of SU-8 structures, suspended and free-standing 3D carbon structures are further obtained, which show great potential in the application of transparent electrode, semitransparent solar cells, and energy storage devices.     

微流控芯片注射成型过程的数值模拟研究

以生物测试上广泛使用的微流控芯片为研究对象,研究使用聚合物微型注塑方法进行类似产品大规模、批量化生产的可能性。在建立微流控芯片结构模型的基础上,运用聚合物成型分析软件Moldllow对其在不同工艺参数下的成型过程进行了系统研究。结果表明,熔体温度的改变对充填时间的影响甚微,充填时间随着注射速度的增加而明显缩短,注射压力随熔体温度的增加而减小,随注射速度的增加而增加。增加熔体温度和注射速度可以降低翘曲变形。     

Fabrication of Ceramics with Complex Porous Structures by the Impregnate–Freeze-Casting Process

The ceramics with complex porous structures were fabricated by a combination of impregnating and freeze-casting process. The polyurethane sponge was impregnated in the mold with 20 vol% of aqueous alumina slurry, and then the bottom of the cast body was kept at a constant cooling rate of 6°C/min to induce unidirectional solidification. After drying and sintering of the green part, porous ceramic with obvious lamellar architectures was prepared. The lamellae thickness and interlayer distance were as large as ∼9 and ∼15 μm, respectively. The large pores, which resulted from the burn-up of sponge struts were homogeneously distributed in the sample. The use of the porous template introduced some local interruption of the lamellar structures. However, high compression strength for the porous ceramic can still be obtained.     

Zero Shrinkage of LTCC by Self-Constrained Sintering

Low shrinkage in x and y direction and low tolerances of shrinkage are an indispensable precondition for high-density component configuration. Therefore, zero shrinkage sintering technologies as pressure-assisted sintering and sacrificial tapes have been introduced in the low-temperature co-fired ceramics (LTCC) production by different manufacturers. Disadvantages of these methods are high costs of sintering equipment and an additional process step to remove the sacrificial tapes. In this article, newly developed self-constrained sintering methods are presented. The new technology, HeraLock^®, delivers LTCC modules with a sintering shrinkage in x and y direction of less than 0.2% and with a shrinkage tolerance of ±0.02% without sacrificial layers and external pressure. Each tape is self-constrained by integration of a layer showing no shrinkage in the sintering temperature range of the LTCC. Large area metallization, integration of channels, cavities and passive electronic components are possible without waviness and camber. Self-constrained laminates are an alternative way to produce zero shrinkage LTCC. They consist of tapes sintering at different temperature intervals. Precondition for a successful production of a self-constrained LTCC laminate is the development of well-adapted material and tapes, respectively. This task is very challenging, because sintering range, high-temperature reactivity and thermal expansion coefficient have to be matched and each tape has to fulfill specific functions in the final component, which requires the tailoring of many properties as permittivity, dielectric loss, mechanical strength, and roughness. A self-constrained laminate is introduced in this article. It consists of inner tapes sintering at especially low-temperature range between 650°C and 720°C and outer tapes with an as-fired surface suitable for thin-film processes.     

Fabrication of Transparent Spinel Honeycomb Structures by Methyl Cellulose–Based Thermal Gelation Processing

Transparent MgAl₂O₄ spinel of flat as well as honeycomb structure was fabricated by employing thermally induced gel casting of the slurry with 56 wt% solid loading containing 0.2 wt% of methylcellulose. The green specimens were pressureless sintered to 98%–99% of theoretical density with no open porosity at an optimum temperature of 1700°C. Final densification by hot isostatic pressing of both the specimens at the optimum temperature of 1800°C and 195 MPa pressure enabled further elimination of residual porosities and full densification resulting in theoretical density of 3.58 g/cc. The design of the honeycomb was such that it exhibited a surface area to volume ratio of 0.65 cm²/cm³ and a relative density of 0.69. The hardness of the honeycomb specimens has been found to be 13 GPa, which is at par with the solid specimens processed under identical conditions. Solid specimens of around 4 mm thickness exhibited a transmission of >80% in the visible (0.4–0.8 μm) region. Specimens were also tested according to ASTM procedures and have shown a flexural strength (σ_f) of 195 MPa and plane‐strain fracture toughness (K_Ic) of 1.87 MPa·m^1/2 as reported in this study.