Active solid-state devices (e.g. – transistors – solid-state diode – Alignment marks
Reexamination Certificate
1995-09-05
2002-09-24
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Alignment marks
C257S466000, C385S088000
Reexamination Certificate
active
06455944
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical assembly fabricated by mounting an optical device chip on a substrate, and in particular to an optical assembly of a passive alignment scheme suitable for reduction of the fabrication cost and a method for fabricating the same.
Reduction of fabrication cost of the optical device chip and the substrate is required of optical assemblies. In recent years, a passive alignment scheme (in which the optical device chip is not activated) is being studied vigorously instead of the conventional active alignment scheme (in which alignment is performed while the optical device chip is being activated).
Passive alignment schemes are divided broadly into two categories: index alignment scheme and self alignment scheme. In the index alignment scheme, alignment is performed by conducting image recognition on marks provided on the optical device chip and the substrate. In the self alignment scheme, alignment in the horizontal direction of the substrate is automatically performed by the surface tension of a solder bump which connects the optical device chip to the substrate. In order to reduce the dispersion of height in a direction perpendicular to the substrate, however, a measure such as control of the bump volume with high accuracy or formation of a standoff for positioning on the substrate is needed. In the existing circumstances, therefore, the index alignment scheme is considered to be more advantageous than the self alignment scheme.
As for the index alignment scheme, one described in Proceedings of 43rd Electronic Components and Technology Conference, pp. 808-817, 1993 and one described in 1994 Spring National Convention Record of the Institute of Electronics, Information, and Communication Engineers (IEICE), presentation No. C-291, for example, are known.
The optical assembly of the former-cited paper includes a laser chip of ridge waveguide type, a fiber carrier substrate, and an alumina substrate. Separately from them, an alignment plate is used. The chip, carrier substrate, and plate have an index having a cross-shaped ridge, an index having a cross-shaped hollow on a silicon nitride film, and an index having a cross-shaped opening in a chromic film, respectively. The chip and the carrier substrate are aligned by effecting image recognition on cross-shaped patterns while applying illumination transmitted through the plate. Thereafter, the chip and the carrier substrate are subjected to vacuum adsorption and soldered to the alumina substrate.
The optical assembly described in the latter-cited paper includes a laser chip and a silicon substrate. The substrate has a V-shaped groove for placing a fiber thereon. The chip and the substrate have an index formed by patterning an electrode circularly and an index formed by removing the electrode circularly, respectively. The chip is aligned by performing image recognition on the circular pattern while applying illumination transmitted through the substrate and the chip. The chip is soldered onto the substrate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical assembly including an optical device chip and a substrate each provided with indexes so as to allow image detection with high accuracy and formation with high accuracy.
Another object of the present invention is to provide a method for fabricating an optical assembly including an optical device chip and a substrate each provided with indexes so as to allow image detection with high accuracy and formation with high accuracy.
As a matter of course, improvement of alignment accuracy of the optical device chip and the substrate is important for optical assemblies. Typically the alignment accuracy is required to be equal to or less than the spot size of a light emission portion or a light receiving portion of the optical device chip or equal to or less than the spot size of an optical waveguide formed or mounted on the substrate. To be concrete, accuracy equal to or less than 1 &mgr;m, for example, is needed. In the index alignment scheme, high accuracy formation of indexes with respect to the optical axis of the chip or the substrate and high-accuracy image recognition of these indexes are especially primary subjects. For performing high-accuracy image recognition, it is necessary to form, on the chip or the substrate, indexes for providing high-contrast images.
In the index of the laser chip described in the aforementioned paper, a silicon nitride film and a gold reflecting film are deposited on the top of the cross-shaped ridge surface and the peripheral around the ridge surface in order to improve the contrast between the top surface and the peripheral surface. An optical interference effect is used by controlling the thickness of the silicon nitride film so as to attain a thickness of 110 nm and by illuminating with monochromatic light. In the index of a fiber carrier substrate, the silicon nitride film is controlled to have a thickness of 120 nm in order to obtain the contrast between the bottom face of a cross-shaped hollow on the silicon nitride film and the top face.
Thus in the case where the top face of the ridge and the bottom face of the hollow which are parallel to the surface of the chip and the substrate are used, it is not easy to obtain high contrast images because they are parallel to each other. Considering dispersion in film thickness and dispersion in illumination of microscope optical system at the time of mass production, it is conjectured that variation in contrast, i.e., variation in alignment accuracy is caused. In the technique of the former-cited paper, therefore, the consideration given to the fabrication yield of optical assemblies is not sufficient.
In the latter-cited paper, metal electrodes of the laser chip and the silicon substrate are used as indexes. Patterning of the index of the chip is performed separately from the photolithography process for forming a laser active layer. Patterning of the index of the substrate is performed separately from the photolithography process for forming a V-shaped fiber groove.
In the case where a metal pattern is used as the index, there is a great difference in transmissivity or reflectivity between the material forming the chip or the substrate and the metal and hence a high-contrast image is obtained easily. However, patterning of the active layer or the V-shaped groove and patterning of metal are conducted separately. Because of the alignment error of the photomask, therefore, misalignment of the index of the active layer or the V-shaped groove is caused. In the technique of the latter-cited paper, the consideration given to the alignment accuracy of optical assemblies is insufficient.
As heretofore described, any optical assembly using conventional techniques cannot satisfy requirements of both high accuracy and simplicity for index alignment. Thus the present invention aims at realizing indexes which can be formed with high accuracy in an optical assembly and allows easy detection of a high-contrast image.
First, the present invention provides an optical assembly including an optical device chip and a substrate and having index alignment means which makes possible a reconciliation of high-accuracy formation and high-accuracy image detection;
Secondly, the present invention provides more concretely an index shape associated with the alignment means and suited for high-accuracy formation;
Thirdly, the present invention provides a basic index shape suited for formation and image measurement;
Fourthly, the present invention provides an index shape captured as an image;
Fifthly, the present invention provides index forming means suited for the case where the chip or the substrate includes a crystal;
Sixthly, the present invention provides index forming means more suited for the chip or the substrate;
Seventhly, the present invention provides means for configuring indexes suited for high-accuracy image detection;
Eighthly, the present invention provides means for forming indexes with high accuracy;
Ninthly, the pre
Aoki Masahiro
Kato Takeshi
Kikuchi Satoru
Oshida Yoshitada
Teraoka Tatsuo
Kenyon & Kenyon
Wilson Allan R.
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