氮化铝共烧基板金属化及其薄膜金属化特性研究

Study of the Character of Aluminum Nitride Co-fire Metallization and Multilayer Thin Film Metalization

大规模集成电路的发展,对芯片之间的互连提出了更高的要求,高端电子系统中高密度封装技术逐渐成为发展的主流。多芯片组件(MCM)是微电子封装的高级形式,它是把裸芯片与微型元件组装在同一个高密度布线基板上,组成能够完成一定的功能的模块甚至子系统。MCM还能够实现电子系统的小型化、高密度化,是实现系统集成的重要途径,在MCM中高密度布线的多层基板技术是实现高密度封装的关键。

With the development of VLSI, there are increasing demands for IC interconnection. The high-density package technology becomes mainstream in advanced systems. Multichip module (MCM) is high-level mode in electronic package. MCM is that bare dice and microelements are assembled on a high-density interconnection (HDI) substrate. MCM can meet the demands of compact packaging and high density. It抯 a method to realize system integration. The HDI substrate is critical to MCM. Aluminum nitride (AlN) has been considered as a material for ceramic packaging in view of the recent trends in the semiconductor industry toward higher speed, power dissipation and packaging density. This is because of its natural properties. These properties include a high thermal conductivity (≥200 W/(m·K)), a high mechanical strength and a thermal expansion coefficient close to that of silicon. Other electrical characteristics of AlN, such as insulation resistance, dielectric constant and dielectric loss are very close to those of Al2O3. These characteristics are suitable for semiconductor packaging that is required to have particularly high reliability. Thus, AlN substrate is useful for multi-chip module and for MEMS as packaging material. Low sheet resistance and good matching between substrate and paste are the principal requirements for conductor paste in multi-layer cofire substrate. We need to start their sintering at same temperature and to ensure conductor paste and green-sheet ceramic bodies have the same shrinkage rate. In this sense, SiO2 is a good additive. SiO2 is used as a component in many kinds of pastes. The softening point of pure SiO2 glass is at about 1 600℃, the same temperature at which AlN green-sheet ceramic bodies start shrinkage. SiO2 can react with AlN and form sialon glass. In this paper, a tungsten paste is proposed with SiO2 glass as the only additive. CaO and Y2O3 are additives in AlN green sheet. YAlO and CaAlO complex oxides have low melting point compare to the soaking temperature of AlN sintering. YAlO and CaAlO complex oxides become liquid phase in the process of sintering. SiO2 reacts with CaO, Al2O3 and Y2O3, producing CaSiAlO and YSiO phases which are detected at the interface between conductor paste and AlN substrate. Ysialon and casialon glasses also form at the interface. If 0.45%wt SiO2 is added in the paste, the tungsten paste has low sheet resistance and matching shrinkage of AlN green-sheet bodies. At this concentration of SiO2 in the paste, AlN substrate is little porous at interface. AlN grains at interface are no different from those in the bulk. The sintering stress is decreased to the degree that cannot influence the substrate shrinkage. The AlN substrate contains nine levels with size of 50 mm×50 mm. The sheet resistance is 10 mΩ/□, substrate bending is smaller than 50 μm/50mm and adhesion strength is greater than 30 MPa. Polyimide (PI) is a thermally stable insulator and is used in multi layer thin-film interconnection (MCM-D). PI is made from precursors after heat treat. These precursors polymerize to form polyamic acid (PAA). The PAA reacts with metal film such as copper. Thus its character become worse and circuit resistance increases. Having reacted with copper, PAA becomes complex. The complex decomposes after heat treating and cause the increase in dielectric constant and reduction in dielectric breakdown strength. Otherwise Cu2O and CuO emerge from the complex, too. These defects can be overcome by a film structure of PI/Cr/Cu/Cr. Cr/Cu/Cr film has been widely used in MCM—D. Having been treated at variable temperature in atmosphere, we can get the conclusions: (1)Treated at 300℃, there are little diffusion coefficients between Cu and Cr; (2)Treated over 500℃, Cr and Cu diffuse into each other evidently. Cu film oxidizes and there are cracks in Cr film. Cr and Cu cannot form alloy and compound at any temperature.