Low Temperature Cofired Ceramic (LTCC) technology is an eye-catching integrated component technology developed in recent years. It represents the development direction of miniaturization, high frequency, integration and low cost of electronic components. Has become the mainstream implementation of passive integration.
LTCC is a new material technology developed by Hughes in 1982. It uses thick film materials and fires electrode materials, substrates, and electronic devices at one time according to a preset structure. It can be used to achieve high integration, High-performance electronic packaging technology port. The development of LTCC technology integrates electronic components including passive components and active components and circuits into a multi-layer structure to achieve integration, so that the volume utilization rate of electronic products can be improved.
The LTCC substrate has the advantages of high-frequency characteristics, thermal stability, and integration of passive components: (1) It has excellent high-frequency, high-Q characteristics and high-speed transmission characteristics; (2) It has good temperature characteristics and can adapt to large currents and resistance High-temperature characteristics are required; (3) It is easy to realize multi-functionality and increase assembly density, high reliability, high temperature resistance, high humidity, shock and vibration, and can be used in harsh environments. Therefore, LTCC is considered to be the most promising technology for future integration of components and substrate materials for high-frequency applications.
Material system
From the perspective of the preparation process and application of LTCC, LTCC's requirements for material properties are: (1) The dielectric constant εr is serialized in the range of 2 to 20000 to be suitable for different operating frequencies; (2) Low dielectric loss is Ensure the high frequency characteristics of the device; (3) The sintering temperature is less than 900C to facilitate co-firing with conductive materials such as Ag and Cu; (4) Good thermal stability, the thermal expansion coefficient (TEC) can be adjusted to be close to the TEC of the chip , The temperature coefficient of resonance frequency τf is as small as possible; (5) High thermal conductivity to improve power and packaging density; (6) High physical and chemical stability, powder is conducive to slurry configuration and casting molding, co-firing characteristics match, local There are as few defects as possible.
Ceramic material classification
There are currently three types of low-temperature co-fired ceramic materials: glass-ceramics, glass + ceramic composite systems and amorphous glass systems.
Glass-ceramic system: Glass-ceramic is a composite body made of a large number of tiny crystals and a small amount of residual glass phase made by controlled crystallization of a certain composition of glass. It has the characteristics of easy adjustment of formula, simple process and superior performance, such as low dielectric loss, suitable for making devices with operating frequency of 20 ~ 30GHz, and cordierite, wollastonite and spodumene are the most widely used. According to the basic glass composition, glass-ceramics can generally be divided into five categories: silicate system, aluminosilicate system, borosilicate system, borate system and phosphate system. Glass-ceramics use silicate glass-ceramic materials, add one or more oxides, such as ZrO2, ZnO, SnO2, sintering temperature is 850-1050 ℃, dielectric constant and thermal expansion coefficient are small.
Glass + ceramic composite system: This is currently the most commonly used LTCC material. Adding a low-melting glass phase to the ceramic will soften the glass and decrease the viscosity during sintering, thereby reducing the sintering temperature. The glass is mainly a variety of crystallized glass, and the ceramic filling phase is mainly Al2O3, SiO2, cordierite, mullite, etc. The sintering temperature is about 900°C, the process is simple and flexible, and the sintering characteristics and physical properties of the composite material are easily controlled and adjusted. The dielectric constant and temperature coefficient are small, the resistivity is high, and the chemical stability is good.
Amorphous glass series: The oxides that form the glass are thoroughly mixed, calcined at 800 ~ 950°C, then ball milled through a sieve, and shaped and sintered according to the ceramic process to form a dense ceramic substrate. The process of this system is simple and the composition is easy to control, but the overall performance of the ceramic substrate is not ideal, such as low mechanical strength and large dielectric loss, which is rarely used at present.
In addition to forming glass or ceramic matrix materials, some additives are usually added to the system, such as crystal nucleating agents, sintering aids, etc., to improve the crystallization ability, sintering performance, and electrical properties of the system.
Application of LTCC material
LTCC materials play a key role in circuit performance, mainly dielectric loss, dielectric constant, insulation resistance and dielectric strength. For transmitting and receiving signals, low loss is needed, and low dielectric constant is also important for high-speed signal processing. At the same time, high insulation resistance and dielectric strength are also required. These characteristics are closely related to chemical composition, technology and conductive materials.
According to the use of LTCC ceramic materials, it is mainly reflected in the following two aspects.
1. LTCC substrate and packaging materials
Traditional substrate materials (Al2O3, SiC, etc.) and high temperature sintered ceramics (HTCC) not only have a high sintering temperature (> 1500°C), but also can only be co-fired with metals with high melting point and high resistance (Mo, W, etc.), which is not conducive to reduction Cost of production. For this reason, a new low-temperature co-fired ceramic (LTCC) technology has been developed. The low sintering temperature can make good metal conductors (Cu, Ag, etc.) co-fired with the ceramic casting wafer, improving the conductivity of thick film circuits and reducing costs.
Due to the development of large-scale integrated circuits, the improvement of IC chip integration, speed, and power requires improved heat dissipation conditions, increased number of I/Os, less interconnection size, reduced signal loss, reduced device volume and reduced packaging. Cost, which requires that the substrate material must have high thermal conductivity, low dielectric constant and loss; multilayer ceramic low-temperature co-fired substrates have simple equipment, low cost, good thermal expansion coefficient matching between ceramic components and chip materials, and easy metal wiring. The advantages are widely used. In fact, the multilayer ceramic co-fired substrate is an application of LTCC.
2. LTCC electronic component materials
In the field of communication technology, the miniaturization and light weight of microwave devices have received increasing attention. In order to reduce the size of microwave devices and meet the miniaturization requirements of communication systems, the multilayer structure chip LC ceramic filter based on LTCC and other structural microwave filters greatly reduce the size of the filter, which is the miniaturization of communication equipment. And portability has laid a good foundation.
To manufacture chip filters with LTCC technology, ceramic materials should meet the following requirements: (1) The sintering temperature should be lower than 950°C; (2) The dielectric constant and dielectric loss are appropriate. Generally, the larger the Q value, the better; (3) The temperature coefficient of the resonance frequency should be small; (4) There is no interface reaction between ceramics and internal electrode materials, and the diffusion is small, and the co-firing should be matched with each other; (5) The powder characteristics should be conducive to slurry preparation and casting molding Wait.
In order to meet the requirements of LTCC microwave devices, many low-fired ceramic systems have been widely developed and utilized, such as MgTiO3-CaTiO3 system, (Zr, Sn)TiO3-BaO-TiO2 system, BaO-Ln2O3-TiO2 system, Bi2O3-ZnO-Nb2O5 system , BiNbO4 system, composite perovskite structure and tungsten bronze structure material system, etc.
Although some progress has been made in the research of microwave dielectric materials, the selection of additives, sintering principles and kinetics, phase composition, microstructure and the influence mechanism of dielectric properties are still lacking in theory. For this reason, material research, device design and production preparation should be unified, which is particularly important for domestic electronic component companies and research institutions.