Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-04-22
2001-05-08
Mulpuri, Savitri (Department: 2812)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S044000
Reexamination Certificate
active
06228670
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a semiconductor optical waveguide array and an array-structured semiconductor optical device, and in particular, a method of manufacturing a semiconductor optical waveguide array and an array-structured semiconductor optical device in which the array has a uniform optical device characteristic and the array is considerably minimized in size when compared with that of conventional technology.
Description of the Prior Art
For the practical use of an optical communication system employing a wavelength-division multiplexing (WDM), it is essential to develop a technology to supply such key devices at a low cost as an light source with multiple wavelengths and a wavelength selective light source. Particularly, the wavelength selective light source, which enables system users to select an arbitrary oscillation wavelength, is one of the light sources especially suitable for practical uses of the system because it is quite easy to operate.
To manufacture a wavelength selective light source, there has been considered a technology in which a plurality of optical sources having mutually different wavelengths are integrally fabricated in one chip in a monolithic form. As an example of such devices, a different wavelength semiconductor laser array has been developed.
In pages 824 and 825 of the “IEE Electronics Letters”, Vol. 28, No. 9 (to be referred to as Literature 1 hereinafter), one of the representative methods of fabricating a semiconductor laser array with different wavelengths has been described.
In the method, an active layer such as a multi-quantum well (MQW) and an optical waveguide layer or the like are grown on an InP wafer by an ordinary process such as metallo-organic vapor phase epitaxy (MOVPE), and then diffraction gratings having mutually different periods are fabricated on the optical waveguide layer by electron beam exposure or the like.
Thereafter, a current block structure is formed by embedding resistors having high resistance and constructing semiconductor lasers of the laser array are processed respectively and electrically separated from each other by dry etching and the like so as to complete a different wavelength semiconductor laser array.
In the laser array, by using different pitch diffraction gratings, the respective semiconductor lasers of the array can be oscillated with mutually different wavelengths, i.e., Bragg wavelengths of the respective diffraction gratings.
However, a gain peak wavelength of the MQW active layer is fixed in the array manufactured in this method. Therefor, the Bragg wavelength of the semiconductor laser determined by the diffraction grating pitch varies. Hence there appears a shift or difference relative to the gain peak wavelength, there arises a problem of deterioration of laser characteristics such as an optical output power at oscillation and threshold current of the laser. This leads to disadvantages as follows. Namely, it is impossible to obtain the desired favorable laser characteristics from all the semiconductor lasers composing the different wavelength semiconductor laser array.
The above problem can be removed by a method of manufacturing a different-wavelength semiconductor laser array described in Japanese Patent Laid-Open Application No. 8-153928 (to be referred to as Literature 2 hereinafter). In the method, a selective growth is employed to fabricate an MQW active layer forming the gain of the semiconductor laser.
In accordance with this method, the gain peak wavelength of the MQW active layer can continuously follow an oscillation wavelength of each semiconductor laser determined by a diffraction grating. It is performed to obtain the desired steady and preferable laser characteristics such as low threshold current and high output power from all semiconductor lasers composing different-wavelength oscillation laser array.
As described above, in order to implement a different wavelength semiconductor laser array having favorable characteristics. There are essentially required 1) a technology to fabricate a different pitch diffraction grating to change the oscillation wavelength and 2) a technology to make the gain peak wavelength of the MQW active layer continuously follow any change in each Bragg wavelength of the different pitch diffraction grating.
From this viewpoint, the technology disclosed of Literature 2, namely, a manufacturing method in which 1) the electron beam exposing technology and 2) the selective growing technology are combined with each other can be regarded as a technology to satisfy two necessary conditions for the fabrication of a high-performance different wavelength semiconductor laser array.
However, the array-structured semiconductor optical devices such as the different wavelength semiconductor laser array fabricated in the method of Literature 2 are attended with a fatal problem when the devices are required to be supplying at a low cost. That is, the array-structured semiconductor optical device is increased in size.
When the manufacturing method of Literature 2 is employed to fabricate a different wavelength semiconductor laser array, the semiconductor laser devices of the array are separated by a distance of about 250 micrometers (&mgr;m). As a result, when a different wavelength semiconductor laser array including 8 lasers each having a cavity length of about 300 &mgr;m for eight wavelengths is manufactured, the overall size of the array device becomes about 2 millimeters (mm)×300 &mgr;m.
This is because that the selective growth technology described in Literature 2 requires an about 40 &mgr;m wide dielectric mask between the optical waveguides structuring the array. Namely, to prevent influence from taking place between the dielectric masks of the adjacent optical waveguides of the array, an array interval of at least about 200 &mgr;m is required. In consequence, in the method of manufacturing a different wavelength semiconductor laser array of Literature 2, it is inherently impossible to minimize the array interval.
Consequently, the yield of array devices per wafer is about ⅛ or less that of semiconductor laser devices per wafer. This greatly increases the manufacturing cost and hence results in a serious problem in practical applications of the array-structured semiconductor device such as a different wavelength semiconductor array.
The conventional methods of fabricating a different wavelength semiconductor laser array described in Literature 1 and 2 are therefore attended with problems as follows. 1) It is impossible to obtain desired excellent and uniform characteristics for all semiconductor lasers of the array. Moreover, 2) the device size of the array becomes greater, the wafer yield per wafer is lowered, and the device cost is conspicuously soared.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention, which has been devised in consideration of the problems above, to provide a method of manufacturing a semiconductor optical waveguide array capable of minimizing the array size and a array-structured semiconductor optical device including a semiconductor optical waveguide array in a fine structure in which desired excellent and uniform characteristics can be obtained.
The present inventor determined widths of dielectric thin films through various experiments including Experimental Examples 1 to 3, which will be described later, to minimize the array size. Additionally, it has been found that uniform characteristics can be obtained in the optical devices by changing the composition and layer thickness between adjacent optical waveguides of the array, which results in the present invention.
To achieve the above-described object in accordance with the present invention, there is provided a method of manufacturing a semiconductor optical waveguide array including a plurality of optical waveguides in an array structure in stripe-shaped growth regions enclosed by dielectric thin films on a substrate, the optical waveguide being fabricated through a selective cryst
Mulpuri Savitri
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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