Low Temperature Co-fired Ceramic Substrate Research Articles

Synthesis and characterization of single phase and low temperature co-fired cordierite glass-ceramics from perlite

Single α-cordierite glass-ceramics for low temperature co-fired ceramics (LTCCs) substrates were prepared successfully from perlite as the basic raw material. The glass-ceramics could be highly densified above 850°C and the α-cordierite being the only crystalline phase precipitated from the glass power sintered within 900–950°C. With increasing sintering temperature or sintering holding time, μ-cordierite was effectively transformed into α-cordierite, and completely transformed into α-cordierite at 900°C for 6h. The synthesized single-phase α-cordierite glass-ceramics possessed low dielectric constants (5.6–6.1), low dielectric losses (0.0019–0.02), appropriate flexural strength (112–117MPa) and suitable coefficients of thermal expansion (2.56–2.91×10−6K−1), which qualify them as a potential material for LTCC substrates application.

Thermodynamic Analysis of Physical Vapor Deposited Inorganic Thin Films on Low Temperature Cofired Ceramic

Low temperature cofired ceramic (LTCC) has been established as an excellent packaging technology for high-reliability, high-density microelectronics. The functionality and robustness of rework have been increased through the incorporation of a physical vapor deposition (PVD) thin film Ti/Cu/Pt/Au metallization. PVD metallization is suitable for radio frequency (RF) applications as well as digital systems. Adhesion of the Ti “adhesion layer” to the LTCC as-fired surface is not well understood. Although previous work has established extrinsic parameters for delamination mechanisms of thin films on LTCC substrates, there is incomplete information regarding the intrinsic (i.e., thermodynamic) parameters in the literature. This article analyzes the thermodynamic favorability of adhesion between Ti, Cr, and their oxide coatings on LTCC (assumed as amorphous silica glass and Al2O3). Computational molecular calculations are used to determine interface energy as an indication of molecular stability between pair of materials at specific temperature. The end result will expand the understanding of thin film adhesion to LTCC surfaces and assist in increasing the long-term reliability of the interface bonding on RF microelectronic layers.

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

Non-stoichiometric α-cordierite glass ceramic doped with H3BO3 and NH4H2PO4 as additives has been fabricated successfully from sandy kaolin as low temperature co-fired ceramics (LTCC) substrate materials. The sintering and crystallization behaviors of the glass–ceramics were investigated by the differential scanning calorimetry, X-ray diffraction, and field emission scanning electron microscope. In addition, various physical properties were characterized, such as dielectric properties, thermal expansion and flexural strength. The results indicated that there was only onefold α-cordierite formed from MgO–Al2O3–SiO2 glasses in the temperature range from 875 to 925 °C. The glass–ceramics could been highly densified at any experimental temperature and they showed excellent properties: low dielectric constants in the range of 5.5–7.5, low dielectric losses in the range of 0.015–0.025, low coefficients of thermal expansion between 1.22–4.32 × 10−6 K−1 and applicable flexural strength at the level from 110 to 145 MPa. All of the performance qualified cordierite glass ceramic to be used as potential LTCC substrate.

Thermodynamic Analysis of Physical Vapor Deposition (PVD) Inorganic Thin Films on Low Temperature Cofired Ceramic (LTCC)

Abstract Low temperature cofired ceramic (LTCC) has been established as an excellent packaging technology for high reliability, high density microelectronics. The functionality and robustness of rework has been increased through the incorporation of a Physical Vapor Deposition (PVD) thin film Ti/Cu/Pt/Au metallization. PVD metallization is suitable for RF (Radio Frequency) applications as well as digital systems. Adhesion of the Ti “adhesion layer” to the LTCC as-fired surface is not well understood. While past work has established extrinsic parameters for delamination mechanisms of thin films on LTCC substrates, there is incomplete information regarding the intrinsic (i.e. thermodynamic) parameters in literature. This paper analyzes the thermodynamic favorability of adhesion between Ti, Cr, and their oxides coatings on LTCC (assumed as amorphous silica glass and Al2O3). Computational molecular calculations are used to determine interface energy as an indication of molecular stability over a range of temperatures. The end result will expand the understanding of thin film adhesion to LTCC surfaces and assist in increasing the long-term reliability of the interface bonding on RF microelectronic layers.

MEMS Fabrication on LTCC Substrates for RF Applications: Challenges and Perspectives

Abstract Microelectromechanical Systems (MEMS) are often used in transceiver modules, especially for telecommunication and radar applications. In this paper, we present recent progress in the development on a MEMS-on-LTCC process. We focus on the Low Temperature Co-fired Ceramic (LTCC) substrate issues and we present a successful solution for overcoming the substrate challenges through surface pre-treatment using a chemical mechanical surface polishing (CMP) process which allows us to reach the required smoothness for the fabrication of MEMS devices. We discuss various process parameters such as slurry type, rotating pad and rotation speed, and show their impact on the final surface finish. With an optimized process, the maximum roughness was decreased from more than 10μm to less than 0.5 μm over a 640 × 640 μm2 LTCC sample. Also, we present the various MEMS process steps starting with the deposition and patterning of various layers to a prototype switch highlighting the validated steps and the challenges encountered. A brief discussion of the perspectives for the integration of MEMS and LTCC technologies is also presented.

Fabrication, response and stability of miniature piezoresistive force-sensing thick-film cantilevers

Abstract Miniature ceramic cantilevers have been successfully applied to the fabrication of simple and low-cost piezoresistive thick-film force-sensing cells, using different thick-film and LTCC (low-temperature co-fired ceramic) substrates. The availability of thin substrates for some materials allows much improved sensitivity compared to classical thick-film technology, with LTCC also featuring rather low substrate elastic modulus and fine structurability. However, practical applicability may be hindered by processing difficulties, such as printing and handling very thin fired substrates, or, in the case of co-fired tapes, warpage during firing. Also, signal drift is observed with some devices. In this work, we show that most of the previously-observed signal drift in some LTCC sensors is not due to self-heating, and therefore stems from defects such as micro-cracks within the ceramic cantilevers or plastic deformations in internal conductors. In a second step, we explore manufacturability of thick-film cantilevers on very thin substrates, and show that it is possible to print a single-sided design on substrates with thickness as low as 45 μm, although a lower limit of ~100 μm, depending on substrate material, is more practical.

Pulse plated silver metallization on porosified LTCC substrates for high frequency applications

Advanced high frequency systems such as needed in modern radar applications, require high conductive metallizations as well as substrates with areas of variable permittivity. This paper presents the combination of the selective porosification technology of low temperature co-fired ceramics (LTCC) and electro pulse plated silver microstrip lines. By means of selective plating methods, line widths of 20μm can be manufactured featuring low resistivity values down to 2.33μΩcm, without detectable pore penetration. The substrate permittivity is measured facilitating a combined method of ring resonator detuning and 3D field simulations resulting in a reduction of 6.5% with a shift from approx. 7.52 to 7.03 at 66GHz due to the porosification. As often outlined in literature, the major challenge in using silver as a conductor lies in its high tendency of agglomeration and microstructural transformation especially in oxygen containing atmosphere even at low temperatures. Therefore, the effect of different temperature loads up to 500°C on the dc film resistivity is measured using the van der Pauw technique and is compared to scanning electron microscope analyses.

Surface-mountable capacitive tactile sensors with flipped CMOS-diaphragm on a flexible and stretchable bus line

This paper describes a MEMS-CMOS integrated tactile sensor for surface mounting on a flexible and stretchable bus line. The sensor is featured by the following configurations; (1) A sensing diaphragm is formed on a CMOS substrate by backside etching, and (2) the CMOS substrate is flip-bonded to a special low temperature cofired ceramic (LTCC) substrate with Au through vias. The flipped CMOS substrate and the LTCC substrate were electrically and mechanically connected using Au–Au bonding, which also formed differential capacitive gaps. The structure and fabrication process are simple compared with the existing surface-mountable MEMS-CMOS integrated sensors. Bendable and stretchable bus lines were fabricated by etching metal, which is deposited on a polyimide substrate and the outer shape is then determined by laser cutting. The bus line with a silicone protection coat is capable of stretching up to 50%. The sensors covered with silicone worked even under 10% stretching of the bus line. The tactile sensors mounted on the surface of the flexible bus line were characterized in terms of force sensitivity. Also, smart functions such as threshold and adaptation operations and the configuration of sensor parameters were demonstrated.

Flip-chip Bonded SiC Power Devices on a Low Temperature Co-fired Ceramic (LTCC) Substrate for Next Generation Power Modules

Abstract This paper explores the design and performance benefits of an LTCC-based power module using SiC power devices. The goal of the design is to achieve high power density with an improved level of reliability as compared to the state-of-the-art, especially at elevated operating temperatures. This will enable a more complete leveraging of the benefits of SiC semiconductor technology. The reliability of existing power modules under high thermo-mechanical stress is adversely affected by the presence of wire bonds and by delamination at the die attachment interface between the die and substrate. As power devices are driven at higher frequencies, wire bonds will inhibit performance by introducing ringing and large overshoots due to the parasitic inductances they introduce in the critical switching loops in the circuit. A flip-chip bonding process for bonding the power devices has been investigated in this paper as an alternative to wire-bonding. It was found that flip-chip interconnects not only improved the switching characteristics of the device, but also reduced thermo-mechanical stresses on the bonding interface.

The Direct plating copper (DPC) ceramic material on Al2O3/AlN or LTCC (Low-temperature co-fired ceramic) substrates

RESEARCH BACKGROUND: The now used ceramic substrate or sub-mounted are normally based on Ag-printed, direct bonding copper (DBC) ceramic or LTCC (Low temperature co-fired ceramic)/HTCC (High temperature co-fired ceramic) technology.Due to the limit of the screen-printed process, the resolution and conducting material thickness the Ag-printed, LTCC and HTCC substrate are poor. The poor resolutions make these materials difficult to use in high density and flip-chip device design. And the related thinner conducting material (normally <20um) limits the power rating of the design.DBC is now widely applied in power circuit design, however, duo to the copper lamination process requirement, more than 300um in thickness of copper layer is needed. Any lower copper thickness design should have an extra costly grind to reach. Also, the DBC material is difficult to provide to the multilayer trace design. OUR GOAL: We want to provide a solution with multilayer ceramic substrate for high power and high device density applications. Besides, the material properties, the adhesion of the metal/ceramic also be considered. Following are the material characteristics required for the development:A low electrical resistance material: Copper.A thick trace material thickness of more than 3 oz.A high thermal conductivity and stability ceramics with via-holes for TSV plating (Drilled Al2O3/AlN substrate ) or non- shrinking LTCC materialHigh metal trace resolution whose line width and space could be only 50 umWell metal/ceramic adhesion uniformity and strength is required: The voids between metal/ceramic < 1%; The adhesion strength> 2 kg/2*2mm2. METHODS & RESULTS: Metal trace plating: For high resolution and lower material electrical resistance request of the trace metal, we introduce electrical casting direct-plating copper (DPC) technology. The first copper is sputtered on the ceramic substrate using Ti as combined/buffer layer between copper and ceramic to provide good adhesion strength and stability. The second copper is made by electrical casting process to increase its thickness to 3 to 5 oz. (100~150um). The key technology of the metal trace plating is the material control of the sputter layers and the second copper layer stress release during plating. Multilayer Ceramic substrates: For double layers design, we use sintered Al2O3 or AlN substrates with electrical conducting via-holes design. The via-holes are made by laser drilling. And the conducting of the front and back side is connected by the following plating process. The key technology of this process is the stability of the via-holes. We have to make sure the via-holes cleaning, impurity removing and material variation during high temperature laser drilled is well controlled. For the more than three layers design, the non-shrinking LTCC is used. The dimension mismatch of the non-shrinking LTCC can controlled less than 100um., much better than that of normal LTCC/HTCC. By the correction of the following DPC process, the tolerance of the metal trace can be controlled < 30 um. The key technology of this process is the non-shrinking LTCC technology and the adhesion of the DPC metal on LTCC material.

Design and Analysis of LTCC-Integrated Planar Microstrip-to-Waveguide Transition at 300 GHz

A 300-GHz planar microstrip-to-waveguide transition in low-temperature co-fired ceramic (LTCC) is numerically and experimentally analyzed. A via fence and air holes are used to form a three-dimensional transition structure and a vertical hollow waveguide inside the multilayer LTCC substrate. The transition consists of a coplanar-waveguide-fed slot radiator backed by a short-wall of the waveguide and an open-circuited microstrip resonator. On the basis of the dual operation mechanism, the design guideline for the complex 3D structure is described using full wave analysis. The overall design is experimentally verified by a back-to-back transition which exhibits insertion loss of 4 dB at 300 GHz and 36-GHz bandwidth with better than 10-dB return loss. To evaluate the loss of a single transition, we carried out a loss component analysis by evaluating different lengths of microstrip line and hollow waveguide. The estimated loss for a single transition is 1 dB at 300 GHz. The planar transition without a metal back-short significantly reduces the size of terahertz packages and eliminates the need for additional components for hermetic sealing. The compact transition is easy to integrate in a low-cost LTCC package with an MMIC chip.

High-Gain L-Probe Excited Substrate Integrated Cavity Antenna Array with LTCC-Based Gap Waveguide Feeding Network for W-Band Application

The W-band high-gain L-probe excited substrate integrated cavity (SIC) antenna and its arrays on a low-temperature cofired ceramic (LTCC) substrate are presented in this paper. To achieve a high gain, the SIC antenna is excited using L-probe feeding techniques. The SIC of the antenna servers as main radiator and the L-probe is involved in the radiation. A novel low-loss LTCC-based ridge gap waveguide (GWG) is also designed in this work. This GWG forms a compact hybrid two-layer low-loss feeding network with a substrate integrated waveguide (SIW) feeding network for an $8 \times 8$ antenna array. A high-gain antenna array is achieved by combining the high-gain antenna element and low-loss hybrid feeding network. In addition, a reference antenna array with a feeding network composed solely of SIW is designed for comparison. With the reconfiguration of the radiating element, feeding network, and transition, the proposed antenna array shows a wide measured impedance bandwidth of 14.9% from 87 to 101 GHz for $\vert \text{S}11\vert and a gain of up to 23.8 dBi. The measured aperture efficiency of the proposed array is 42.3%, which is 8% higher than that of the reference array.

Influence of B2O3–SiO2–ZnO–BaO glass ratio and sintering temperature on the microstructure and property of copper thick film for low temperature co-fired ceramic

In this study, B2O3–SiO2–ZnO–BaO (BSZB) glass-coated copper particles were prepared by sol–gel process for use in copper pastes. The pastes with different ratio of BSZB glass were sintered at temperatures ranging from 850 to 910 °C with low temperature co-fired ceramic (LTCC) substrate. When pastes prepared from 1 wt% glass-coated copper powder were sintered at 910 °C, both a dense copper film and a dense LTCC structure were obtained. At this condition, the film had a thickness of approximately 15 μm and a sheet resistance of 1.9 mΩ/□, and a good bonding behavior between copper film and LTCC substrate was also observed.

Development and characterization of miniaturized LTCC pH sensors with RuO2 based sensing electrodes

This work describes the development of a low cost miniaturized solid state pH sensor using LTCC (low temperature cofired ceramic) technology. Thick film sensing electrodes screen printed on LTCC substrates were made of RuO2 and RuO2-Ta2O5-glass based pastes. The Ag/AgCl/KCl reference electrode for the potentiometric sensor was deposited by screen printing on the same LTCC substrate. The sensing performance and mechanism were investigated by using the potentiometric method and electrochemical impedance spectroscopy (EIS). The fabricated potentiometric sensors show close to the Nernstian behavior with the sensitivity being 56mV/pH for RuO2 layer and 58mV/pH for RuO2-Ta2O5-glass layer in a wide pH range (2–11). The sensors exhibit very fast response and excellent stability. A wireless system for online solution monitoring operating together with the fabricated sensor was successfully tested. Good performance of the prepared LTCC sensors suggests their applicability in the next generation of solid state pH sensors destined for online solution monitoring.

Potentiometric RuO2–Ta2O5 pH sensors fabricated using thick film and LTCC technologies

The paper reports on the preparation, properties and application of potentiometric pH sensors with thick film RuO2–Ta2O5 sensing electrode and Ag/AgCl/KCl reference electrode screen printed on an alumina substrate. Furthermore, it presents fabrication procedure and characterization of a new miniaturized pH sensor on LTCC (low temperature cofired ceramics) substrate, destined for wireless monitoring. The crystal structure, phase and elemental composition, and microstructure of the films were investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Potentiometric characterization was performed in a wide pH range of 2–12 for different storage conditions and pH loops. The advantages of the proposed thick film pH sensors are: (a) low cost and easy fabrication, (b) excellent sensitivity close to the Nernstian response (56mV/pH) in the wide pH range, (c) fast response, (d) long lifetime, (e) good reproducibility, (f) low hysteresis and drift effects, and (g) low cross-sensitivity towards Li+, Na+ and K+ as interfering ions. The applicability of the sensors for pH measurement of river, tap and distilled water, and some drinks was also tested.

Bonding of zero-shrink LTCC with alumina ceramics

Purpose – The purpose of this paper is to focus on a description of reliable bonding technique of zero-shrink low-temperature co-fired ceramic (LTCC) and alumina ceramics. LTCC is widely used for manufacturing electrical systems in 3D configuration. LTCC substrates were so far bonded with alumina ceramics using additional adhesive layers with subsequent firing or curing cycle. With the advent of the zero-shrink LTCC substrates, it is now possible to bond unfired substrates with other fired substrates, for example fired LTCC or alumina substrates. Alumina substrate in combination with LTCC brings advantages of good thermal conductivity for usage in heating elements or packaging. Design/methodology/approach – The test structure contains a thick-film pattern for verification of the compatibility of the bonding process. We have used two methods for bonding the substrates: cold chemical lamination (CCL) and thermo compression method, using a dielectric thick-film paste as the adhesive. Optical microscopy, scanning electron microscopy and electric testing of the screen-printed patterns were used for verification of the bonding quality. Findings – The thermo-compression method gave poor results in comparison with the CCL method. The best quality of lamination was achieved at room temperature combined with low pressure for both types of bonding materials. In addition, a possibility of using this bonding method for sensor fabrication was investigated. The ceramic pressure sensor samples with a cavity were created. Originality/value – The possibility of bonding two different ceramic materials was investigated. A new approach to ceramic bonding showed promising results with possible use in sensors.

Integration of Silver Heat Spreaders in LTCC utilizing Thick Silver Tape in the Co-fire Process

The integration density in semiconductor devices is significantly increased in the last years. This trend is already described by Moore's law what forecasts a doubling of the integration density every two years. This evolution makes greater demands on the substrate technology which is used for the first level interconnect between the semiconductor and the device package. Higher pattern resolution is required to connect more functions on a smaller chip. Also the thermal performance of the substrate is a crucial issue. The increased integration density leads to an increased power density, what means that more heat has to dissipate on a smaller area. Thus, substrates with a high thermal conductivity (e. g. direct bonded copper (DBC)) are utilized which spread the heat over a large area. However, the reduced pattern resolution caused by thick metal layers is disadvantageous for this substrate technology. Alternatively, low temperature co-fired ceramic (LTCC) can be used. This multilayer technology provides a high pattern resolution in combination with a high integration grade. The poor thermal conductivity of LTCC (3 … 5 W*m−1*K−1) requires thermal vias made of silver paste which are placed between the power chip and the heat sink and reduce the thermal resistance of the substrate. The via-pitch and diameter is limited by the LTCC technology, what allows a maximum filling grade of approx. 20 to 25 %. Alternatively, an opening in the ceramic is created, to bond the chip directly to the heat sink. This leads to technological challenges like the CTE mismatch between the chip and the heat sink material. Expensive materials like copper molybdenum composites with matched CTE have to be used. In the presented investigation, a thick silver tape is used to form a thick silver heat spreader through the LTCC substrate. An opening is structured by laser cutting in the LTCC tape and filled with a laser cut silver tape. After lamination, the substrate is fired using a constraint sintering process. The bond strength of the silver to LTCC interface is approx. 5.6 MPa. The thermal resistance of the silver structure is measured by a thermal test chip (Delphi PST1, 2.5 mm × 2.5 mm) glued with a high thermal conducting epoxy to the silver structure. The chip contains a resistor and diodes to generate heat and to determine the junction temperature respectively. The backside of the test structure is temperature stabilized by a temperature controlled heat sink. The resulting thermal resistance is in the range of 1.1 K/W to 1.5 K/W depending on the length of silver structure (5 mm to 7 mm). Advantages of the presented heat spreader are the low thermal resistance and the good embedding capability in the co-fire LTCC process.

Tunable metamaterial dual-band terahertz absorber

Abstract We report a design of a temperature controlled tunable dual band terahertz absorber. The compact single unit cell consists of two nested closed square ring resonators and a layer metallic separated by a substrate strontium titanate (STO) dielectric layer. It is found that the absorber has two distinctive absorption peaks at frequencies 0.096 THz and 0.137 THz, whose peaks are attained 97% and 75%. Cooling the absorber from 400 K to 250 K causes about 25% and 27% shift compared to the resonance frequency of room temperature, when we cooling the temperature to 150 K, we could attained both the two tunabilities exceeding 53%. The frequency tunability is owing to the variation of the dielectric constant of the low-temperature co-fired ceramic (LTCC) substrate. The mechanism of the dual band absorber is attributed to the overlapping of dual resonance frequencies, and could be demonstrated by the distributions of the electric field. The method opens up avenues for designing tunable terahertz devices in detection, imaging, and stealth technology.

Gas-Sensing Performance of M-Doped CuO-Based Thin Films Working at Different Temperatures upon Exposure to Propane.

Cupric oxide (CuO) thin films are promising materials in gas sensor applications. The CuO-based gas sensors behaved as p-type semiconductors and can be used as part of an e-nose or smart sensor array for breath analysis. The authors present the investigation results on M-doped CuO-based (M = Ag, Au, Cr, Pd, Pt, Sb, Si) sensors working at various temperatures upon exposure to a low concentration of C3H8, which can be found in exhaled human breath, and it can be considered as a one of the biomarkers of several diseases. The films have been deposited in magnetron sputtering technology on low temperature cofired ceramics substrates. The results of the gas sensors’ response are also presented and discussed. The Cr:CuO-based structure, annealed at 400 °C for 4 h in air, showed the highest sensor response, of the order of 2.7 at an operation temperature of 250 °C. The response and recovery time(s) were 10 s and 24 s, respectively. The results show that the addition of M-dopants in the cupric oxide films effectively act as catalysts in propane sensors and improve the gas sensing properties. The films’ phase composition, microstructure and surface topography have been assessed by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) methods.

Integration of a 140 GHz Packaged LTCC Grid Array Antenna With an InP Detector

Integration of a 140-GHz packaged low-temperature cofired ceramic (LTCC) antenna with a power detector is demonstrated under the concept of antenna-in-package. The detector is designed on an indium phosphide (InP) process. A grid array antenna in LTCC is designed to provide package for the detector. Coplanar ground–signal–ground (GSG) bond wires are used to connect the detector and the antenna. Parallel plate mode is observed in the InP substrate and absorbed by the LTCC substrate. The comparison between the measured responsivity of the power detector with and without the antenna indicates the acceptable insertion loss of the coplanar GSG bond wires transition. It shows a feasible solution for the D-band front-end integration and packaging.