Fabrication of spin-coat optical waveguides for optically interconnected LSI and influence if fabrication process on lower layer MOS capacitors

Abstract

Next, in order to examine the influence of the FPI waveguide fabrication process on lower devices, MOS capacitors were made and FPI waveguides were formed on it as shown in Fig.1 (b). After the SiO2 film of 7 nm was formed on the LOCOS substrate by dry oxidation at 850°C, the poly-Si gate (area 100×100 μm), the phosphosilicate glass layer and the Al electrode (area 150×150 μm) were formed and the MOS capacitor fabrication was finished. Then FPI waveguides were fabricated after SiO2 layer by Atmospheric Pressure CVD, APCVD or SOG layer was formed on the MOS capacitor. Finally the contact hole for the measurement was formed into the SiO2 or SOG layer. Introduction Optical interconnection was proposed for high-speed and high-performance of LSI [1], and then the optical waveguide is necessary to be integrated on LSI without any significant damage on the lower layer devices. Spin-coat heat-resistant polyimide resin is a suitable material for optical waveguide on LSI because it is easily fabricated at low temperature. Then the fluorinated polyimide, FPI, which was developed for long-distance optical-communication devices [2], is adopted for material of waveguide on Si chips, and the fabrication process of low-loss and miniaturized waveguide on Si substrates is studied. Result and Discussion It is an important interest that whether the fabrication process of FPI waveguide damages the lower layer devices. Then, the FPI waveguide is made on MOS devices, and influence of the FPI waveguide fabrication process on the MOS devices is investigated for the first time in this work. Figure 2 shows waveguide-width dependence of propagation loss for the fabricated FPI waveguide. The He-Ne laser (wavelength 633 nm) was used for the source of light. It is found that propagation loss becomes large when the width of core becomes small. Figure 3 (a) shows SEM image of FPI waveguide and the roughness can be seen on the side wall. As shown in Fig.3 (b), the size of the waveguide becomes smaller then the frequency to which light reflects on boundary of the core and cladding layer increases. Therefore, the propagation loss originated mainly from the boundary of the core and cladding layer and is caused by surface roughness of side wall of FPI. On the other hand, when SOG is used for clad, the propagation loss is larger than the thermal oxide film. It is thought that the propagation loss is caused by the surface roughness of SOG. Although the propagation loss of SOG is somewhat large, it is possible to use it for optical interconnection in the chip. Waveguides Fabrication Firstly an optical waveguide is fabricated as shown in Fig. 1 (a) to investigate the propagation characteristics for the FPI waveguide on Si substrates. The core layer is made of FPI (refractive index 1.55), bottom clad layer is made of thermal oxide film (refractive index 1.46) or Spin-On-Glass, SOG (refractive index 1.38) and side and upper clad layer are made of air. The thickness of the thermal oxide film is set to 2.2 μm, SOG to 1.5 μm and FPI to 1 or 1.5 μm. After spin-coating FPI film, Al hard mask (200 nm) was deposited on FPI. By means of resist masks patterned by the electron beam lithography, Al was etched using inductively coupled plasma etcher (etching gases: BCl3 + Cl2). Then FPI film was shaped to waveguide by reactive ion etching with the O2 plasma, and the FPI waveguide fabrication was completed by removing Al mask by wet etching.