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  4. No.35 January,2006
Fujikura Technical Review

No.35 January,2006

Recently, there has been a remarkable progress in the compactness and multifunctionality of a mobile phone. A radio-frequency equipment such as the mobile phone has a radio-frequency integrated circuit to process radio waves received generally by an antenna, and the integrated circuit is required to have space-saving and high-performance features. In order to meet these requirements, we have developed high-performance on-chip inductors employing Wafer Level Packaging (WLP) technology. Currently, an on-chip inductor based on Silicon CMOS process is widely used in radio-frequency integrated circuit. However, it limits the performance of the circuit because of the loss caused by the resistivity of Al routing and by magnetic coupling between the inductor and the underneath Si substrate. We successfully reduced the routing loss by introducing electroplated Cu multilayer; moreover, we also reduced the loss originating from magnetic coupling by introducing thick and multiple resin layers. Both layers are based on WLP technology, which is already established as one of the high-density packaging technologies. Four pictures show different types of on-chip inductors based on WLP technology, namely, square spiral, circular spiral, solenoid, and meanda, from the top. Each has different characteristics because of different wiring length and magnetic-field distribution. Common patterns at both sides of inductors are ground patterns put for measurement. We have already been producing the WLP as a foundry and are to provide the on-chip inductor as one of our product lines, while we are developing new applications of the WLP technology such as an antenna for RFID tags as well.


White Light-emitting Diode Lamps for Lighting Applications

White Light-emitting diode (LED) lamps are expected to take the place of conventional incandescent and fluorescent lamps for general lighting applications in the near future. For general illuminations, there are various demands such as high luminous efficacy, high color rendering properties, various correlated color temperatures and so on, corresponding to installation locations and applications. We have developed two types of white LED lamps, which satisfy each demand using novel oxynitride/nitride phosphors. One is a high-efficacy-type warm-white LED lamp and the other is a high-color-rendering-type white LED lamp with various correlated color temperatures. In this paper, excellent optical properties of these white LED lamps are reported. 

Arc-fusion Splicing Techniques for Holey Fibers

We report experimental results of fusion splicing for a hole-assisted type holey fiber (HA-HF) and a pure silica holey fiber (PS-HF). The splice losses between the standard single-mode fiber (S-SMF) and the holey fibers have been found to be reduced by employing the special functions provided by a Fujikura fusion splicer, FSM-40F. In the case of fusion splicing between HA-HF and Conventional single-mode fiber (C-SMF), we have achieved a splice loss of 0.05 dB in average by relaxing the longitudinal holes' tapers at the spliced point using sweep discharge. We have studied the connectorization for the HA-HF using this technique. In the case of fusion splicing between PS-HF and C-SMF, we have achieved a splice loss of 0.20 dB and a tensile strength of 0.12 GPa, in average, employing an intermitting discharge after short time discharge. 

Small Fusion Splicer and Fiber Cleaver

The installation of Japanese fiber to the premises (FTTP) networks often includes mechanical splices and optical fiber connectors to connect the drop cable to premises or to connect an indoor cable within a premises. However, fusion splices are still the preferred method of connecting fiber at locations away from the premises. These remote locations include a variety of splicing environments such as aerial splicing that demands equipment portability and versatility. For these reasons, an increasing number of customers require an easy to use, smaller, and lighter splicer with a full set of features. To meet these customer requirements, the highest priority when we developed our new fusion splicer and cleaver was that it would be the smallest, lightest, and most versatile in the industry. 

New Optical Fiber Identifier

In the construction or maintenance work for fiber to the premise (FTTP) networks, an optical fiber identification function and optical power measurement are both necessary. As working space for the optical fiber identification is limited, e.g. in a cable closure, the optical fiber identifier is required to have easy operation and portability. The light source to be used with the optical fiber identifier also needs various wavelengths in order to adapt to different networks. To meet these requirements of the customers, we have developed a new optical fiber identifier and a light source to work with the identifier. 

SZ-slotted Core Cable with Two-step Splittable Ribbons

To construct fiber to the home (FTTH) network economically, it is important to use the fiber efficiently. Ribbon-type optical cables are widely used in Japan. But nowadays, the splittable ribbon, which can be easily separated into individual fibers at any point where the user operates, is getting a lot of attention. On the other hand, considering the splicing with the ribbon in an existing cable, the ribbon structure will be preferred to the individual fiber structure. We have developed the two-step splittable ribbon, which can be divided into 2-fiber or 4-fiber ribbon first, and then eventually separated into individual fibers. This two-step splittable ribbon can be applicable to the SZ-slotted core cable, which are often used for mid-span branching situation. We have developed the 100-fiber and 640-fiber SZ-slotted core cable using the two-step splittable ribbons, and these cables showed good performance. 

A Study of Impedance Control in Flexible Printed Circuit (FPC)

Ordinary, requirements for printed circuits are only to be connected correctly and to have enough electrical current capacity. If signals used inside of equipments are very low frequency, impedance control of circuit is not needed. Impedance control is needed only for long communication cables between equipments. When electrical circuits with high frequency signal are operated, impedance control is needed for not only communication cables but also printed circuits. Recently it is not rare that the high frequency signal (several hundreds MHz to several GHz) is used inside of equipment. In this case, the circuit should be considered as distributed constant circuit even if the circuit is very short. Therefore impedance control of circuit is needed on Printed Circuit Board. Based on the background described above, we have tried to control characteristic impedance of Flexible Printed Circuit(FPC) . 

The Future Trend and Characteristics of Flexible Printed Circuit (FPC)

With household digital appliances becoming smaller, thinner, and more functional, it is thought that flexible printed circuit (FPC) will be used increasingly and there will be a greater demand for lighter, thinner, shorter, smaller, more flexible ones as well as ones with higher density. Furthermore, it is being increasingly required that consideration be given to environmental problems. Therefore, developments of FPC that are applicable for fine circuits and are multi-layered, soft/flexible, heat-resistant, having a low dielectric constant and are environment friendly is advancing.  

Applications of Highly Conductive Silver Paste

A membrane (MB) circuit board is produced by screen printing of a silver paste on a PET substrate. Compared with flexible print circuit (FPC) fabricated by the subtracting method, the advantage of MB circuit board lies in its low cost due to the simple processes. We successfully developed a highly conductive silver paste and the MB using the paste (highly conductive MB). In this paper, not only the applications and their performance but also new uses for the paste are introduced. 

Development of the "VOC-free Eco-Light"

Fujikura has developed "Eco-Light" products as a flame-resistant halogen-free recyclable material that can be separated from PVC and PE using specific gravity separation. Eco-Light has received excellent reviews as being superior in terms of its insulative and flame-resistant properties, mechanical characteristics, weather resistance, color tone, and ease in processing through extrusion and is used broadly in "green buildings" for governmental and other public offices, where the material is used as an insulator and a sheathing material for common electric cables. While PVC cables are often used in clean rooms for semiconductor manufacturing, the plasticizing agents have been found to become volatile and to outgas (placing a load on the surrounding environment), causing defects in the semiconductor products. Hence, there is a need for a cable material that not only is safe and environmentally friendly, but also has no effect on the local environment. Fujikura has developed and released to market a "VOC-free Eco-Light" product that has, in addition to the features of the earlier Eco-Light products, a total outgassing of no more than 1 ppm. 

Dielectric Properties of Flame-retardant Insulation Material in the GHz Frequency Range

The high-speed cable used with storage area network requires good dielectric properties in the GHz frequency range and high flame retardancy as a countermeasure in a disaster. First, we investigated the dielectric properties of polyethylene for insulation material and developed the polyethylene that has high performance in the GHz frequency range. Next, we investigated the fire resistance and dielectric properties of flame retardant insulation material and prepared the index of flame retardancy by using an original method. Applying these data, we developed an insulation material, which could be used for InfiniBandTM

RF Characteristics of Silicon Through-hole Interconnections

Three-dimensional (3D) stacking or system in package (SiP) is proposed for high-density packaging of electronic devices. Fujikura has been developing a new wafer level packaging technology based on silicon through-hole interconnections for 3D stacking and SiP. In this paper, RF characteristics of the silicon through-hole interconnections, especially focused on thickness of the insulation layer on the surface of the through-hole and resistivity of silicon substrate, are examined. As a result, it was confirmed that both the thickness of the insulation layer and the resistivity of the substrate affected the RF characteristics. Also, the higher resistivity and the smaller stray capacitance of the SiO2 film were important for realizing higher frequency transmission characteristics of the silicon through-hole interconnections. 

On-Chip Passive Devices Embedded in Wafer-Level Package

On-chip high-Q spiral and solenoid inductors on Si substrate embedded in WLP have been fabricated. These inductors consisted of a thick Cu electroplated rerouting to reduce electrical resistance and a thick resin layer to separate the inductors typically 20 µm from Si substrate. An inductance L of 5.0 and 4.9 nH with a quality factor Q of 28.4 and 42.9 were obtained for a 3.5 turn rectangle spiral inductor at 2 GHz on the Si substrate, which had a resistivity of 4Ωcm and 1 kΩcm, respectively. In addition, the measured results of Q, L and fres corresponded well with the simulated values by HFSS and Sonnet. This technology realizes embedded high quality inductors in WLP.