This invention relates to a substrate for mounting an optical component and an optical module which are suitably used in optical transmission, optical communication and the like.
In recent years, optical fiber communication has been put into practice in the fields of CATV and public communication. Conventionally, optical modules having a high operation speed and high reliability have been realized by the so-called coaxial or dual-inline type module construction. These optical modules have been already put into practice mainly in the so-called trunk systems.
Contrary to this, optical modules have been recently vigorously developed using a technique for highly precisely positioning and mounting optical components and an optical fiber on a silicon auxiliary substrate (auxiliary mount placed in a package, also called silicon platform) only by mechanical precision. These optical modules are targeting a practical use mainly in an area called subscriber system, and are required to be smaller, thinner and less expensive. On the other hand, it has become essential for optical modules to operate in a wider band and, accordingly, there is a demand for realization of substrate for mounting an optical components which can operate in a high-frequency region.
In the substrates for mounting an optical component, which are used for these optical modules, markers used to position optical semiconductor devices on a substrate as a base member have been frequently formed simultaneously with a V-shaped groove in which an optical fiber is mounted. For example, as shown in FIG. 11, markers 102 in the form of V-shaped grooves are formed in a substrate 101 by anisotropic etching by the same mask as a V-shaped groove 103 for mounting an optical fiber, using a photolithography technique. Accordingly, a relative positioning precision of the markers 102 and the V-shaped groove 103 is determined by the precision of a photomask, and the markers 102 and the V-shaped groove 103 are hardly displaced with respect to each other. Indicated at 104 is an electrode for mounting optical semiconductor devices.
Further, it is also known to form the so-called self-alignment marker for simultaneously fabricating a pattern for forming the V-shaped groove 103, a pattern for forming electrodes 104 for mounting optical semiconductor devices and an electrode 106 for driving the optical semiconductor devices, and a pattern for forming alignment markers 105 as shown in FIG. 12.
However, if the substrates 101 shown in FIGS. 11 and 12 are made of a silicon monocrystal, a dielectric loss increases in a high-frequency region due to a large dielectric loss tangent if electrical signals for driving the optical semiconductor devices are high-frequency signals. This causes a problem of degraded characteristics of the optical module.
Further, in the case of the V-shaped groove markers 102 shown in FIG. 11, they are generally formed by anisotropic etching. Accordingly, even if the mask configuration is circular in a plane, the selective etching causes a specific rectangular configuration, consequently restricting the configuration of the markers 102. In order to increase the mounting precision, it may be considered to combine a plurality of V-shaped groove markers formed in positions surrounding the same optical semiconductor device. However, the combination of a plurality of alignment markers makes the entire arrangement area of those markers larger. Therefore, it becomes difficult to observe each alignment marker unit by enlarging it in order to increase the mounting precision.