Infineon enables third dimension of chip integration; develops SOLID stacking technology to connect multiple chips for "system-in-package" electronics
Chip stacking, or 3D integration, is an alternate method for reaching high levels of integration while improving the efficiency and reliability of advanced electronic system designs. Several methods are currently used to create close connections between chips that work together to form a system. Compared to the currently used gluing or wire bonding techniques for stacking chips, Infineons new SOLID technology significantly reduces the size of the electrical contacts in the package, leading to greater operating efficiency and contributing to overall cost savings.
Infineon is working to bring together two complementary technologies in chip design and fabrication, said Dr. Soenke Mehrgardt, Chief Technology Officer of Infineon Technologies AG. The system-on-chip integrates many different functions on a single chip die, while the system-in-package connects multiple chips in one compact solution. By developing the capability to use both of these approaches, Infineon is well-positioned to provide our customers with optimal solutions across a wide range of cost and performance parameters.
There are many cases in which on-chip integration is either physically impossible or not economically viable, such as when dissimilar chip processing technologies or circuit architectures are used to obtain optimal performance of different components. The SOLID technology developed by Infineon solves the problem of connecting these dissimilar chips, providing fast chip-to-chip communication and very tight integration. Stacking technology has the potential to combine mixed technologies (logic, memory functions, sensors, bipolar processes, CMOS, different generations and wafer sizes, etc.) and optimise performance, while the use of standard processes for the individual chips also reduce costs.
The first prototype produced by Infineon based on this 3D integration technology is a chip card controller. In physical space and power-constrained systems such as smart cards, wireless communications and many types of portable devices, connecting highly integrated single chip systems onto a circuit board may not be either technically or economically feasible. Complex ICs are housed in chip packages that sometimes have over a thousand pins or balls, which are the electrical connections needed to operate the chip. These have to be connected to other components on multilayer boards, in a layout process that is very work-intensive. Additionally, with frequencies at the GHz level, electrical signal integrity is often weakened by the parasitic effects caused by the relatively long tracks and large solder contacts. In many applications, the very short connections and small contact pitch of 3D chip technology provide a wide range of benefits such as reduced board space, reduced signal interference, lower costs, reduced power consumption, increased speed and a simpler board layout.
SOLID (solid-liquid interdiffusion) is a new soldering method in which separated chips are fitted together in a stacked structure and permanently bonded in a soldering process. To create a microcontroller and memory system-in-package, for example, one of the two chips would be turned over (flip-chip) to create a face-to-face arrangement with the other. A thin copper layer is applied to the face of each chip for metallisation. The copper metallisation layer contains small inside contact pads (10 µm) to connect the chips, and bond pads on the edges of the bottom chip to connect the combined chips via wire bonding to the package. The copper layer can also be used to integrate jumpers and other passive components such as coils, or strip or delay lines. After applying the copper metallisation, a very thin layer (3 µm) of tin solder is applied between the two chips. The two chips are then soldered with a low temperature process at 270 °C to 300 °C and at a pressure of 3 bars, and are thus permanently attached to each other. Although the process temperature of less than 300 °C is relatively low, the created intermetallic phase has a high melting point and can withstand even very high temperatures (above 600 °C).
With SOLID the chips are electronically connected via the inside pads on the copper layer. Compared with the balls of BGA packages (150 µm) or the bond pads in traditional stacking methods (100 µm) these have a considerably smaller contact pitch (20 µm). Their very low level of parasitic inductivity and capacity allows high signal speeds while retaining high signal integrity.
In specific applications such as chip cards, the vertical chip combination cannot exceed a certain height. For this reason Infineon has reduced the wafer thickness for the face-to-face method from 120 µm to 60 µm. The chip sandwich is fitted into a standard chip housing. This method allows a saving of fifty percent on material and costs for the housing. Initially Infineon will be using SOLID for two chips or wafers on top of each other, although in principle it is possible to have several layers.
The first product to be manufactured based on SOLID technology will be a chip card controller, planned for next year. The use of 3D integration will give this controller clear advantages, including a considerably higher memory capacity than traditional products. The SOLID method is also an interesting alternative for mobile communications applications. Many other embedded control applications, which typically require very high reliability components in compact and power efficient system configuration, are potential markets for this innovative chip stacking method.
Infineon Technologies AG, Munich, Germany, offers semiconductor and system solutions for applications in the wired and wireless communications markets, for security systems and smartcards, for the automotive and industrial sectors, as well as memory products. With a global presence, Infineon operates in the US from San Jose, CA, in the Asia-Pacific region from Singapore and in Japan from Tokyo. In the fiscal year 2001 (ending September), the company achieved sales of Euro 5.67 billion with about 33,800 employees worldwide. Infineon is listed on the DAX index of the Frankfurt Stock Exchange and on the New York Sock Exchange (ticker symbol: IFX). Further information is available at www.infineon.com.
Infineon enables third dimension of chip integration; develops SOLID stacking technology to connect multiple chips for "system-in-package" electronicsPress Picture
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