1. Technical Field
The present invention is directed to a process for making a tapered waveguide interconnect for optical devices.
2. Art Background
In integrated circuits (ICs), semiconductor devices are integrated by forming metal wires in the semiconductor chip. The metal wires provide the desired electrical interconnection between the semiconductor devices. Metal wires, however, cannot be used to similarly integrate opto-electronic devices, because the operating frequency of opto-electronic devices is much higher. A waveguide structure is therefore required to integrate opto-electronic devices. Unlike the wire used to connect semiconductor devices, the waveguide interconnect must have a certain thickness, bandgap, and strain profile to provide the desired interconnection. Consequently, the process for making such waveguide interconnects must be able to provide waveguides that meet the specifications.
One current technique that is used to fabricate these interconnect waveguides is referred to Selective Area Growth (SAG). This process is described in Gibbon, M., et al., "Selective-area low-pressure MOCVD of GaInAsP and related materials on planar InP substrates," Semicond. Sci. Technol., Vol. 8, pp. 998-1010 (1993) (Gibbon et al.), which is hereby incorporated by reference. Referring to the schematic cross-section in FIG. 1, in the SAG process, pads 12 that define the desired configuration of the waveguide are formed on a semiconductor wafer 10. In Gibbon et al. the substrate is an Indium-Phosphide (InP) substrate and the pads are a dielectric material such as silicon dioxide (SiO.sub.2) 12. Silicon dioxide is referred to as oxide hereinafter. Vapor phase epitaxy, e.g., Metal Organic Chemical Vapor Deposition (MOCVD), is then used to deposit the waveguide material 14, typically a semiconductor material such as Gallium-Indium-Arsenide-Phosphate (GaInAsP). The semiconductor material 14 does not grow on the portions of the substrate 10 that are covered by the oxide pads 12.
As noted in Gibbon et al., the composition and thickness of the waveguide material 14 changes in proximity to the pads. Specifically, the growth of the waveguide material is enhanced adjacent to the oxide pads 12. This growth enhancement effect of the oxide pads 12 on the growth of the waveguide material must be considered in placing the oxide pads on a substrate to define the substrate region on which the waveguide will be formed by SAG. The growth enhancement varies with both the dimensions of the oxide pads 12 and the distance from the oxide pads. Thus, designing an oxide mask for SAG is not simply a matter of determining the desired waveguide dimensions and forming oxide mask that defines a space corresponding to the desired waveguide dimensions. Jones, A. M., et al., "Integrated optoelectronic device by selective-area epitaxy," SPIE, Vol. 2918, pp. 146-154 (1996), which is incorporated by reference herein, note the problem associated with using SAG to form a waveguide.
One example of a device that is formed using SAG is an expanded beam laser. An example of an expanded beam laser 30 is illustrated in FIG. 2A. As illustrated in FIG. 2A, an expanded beam laser 30 has a first section 31 which transfers the mode of the laser beam to the underlying waveguide 32. The underlying waveguide is slowly tapered in a second section 33 to expand the mode of the laser.
Another example of a device formed using SAG is an electroabsorption modulated laser (EML). In the EML device a laser is optically integrated with a modulator. An example of an EML structure is illustrated in FIG. 2B. The device also has a first section 31 (the gain section) and a second section 34 (the modulator section) and a tapered section 33. However, unlike the device illustrated in FIG. 2A, all of these sections make up the waveguide and all of the layers 42, 43 and 44 are tapered. Such a device is described in Thrush, E. J., et al., "Selective and non-planar epitaxy of InP, GaInAs and GaInAsP using low pressure MOCVD," Journal of Crystal Growth, Vol. 124, pp. 249-254 (1992), which is hereby incorporated by reference. In such devices, the taper section 33 is formed by SAG. The taper must be carefully controlled in order to obtain the desired mode expansion (for devices of the type shown in FIG. 2A), or to minimize power loss while retaining desirable modulator properties (for devices of the type shown in FIG. 2B).
As noted in Jones et al., the dimensions of the dielectric pads are used to control the thickness and the composition of the waveguide material formed on the substrate. Jones et al. models the epitaxial MOCVD deposition process and uses the model to determine the dimensions of the oxide pads and the distance between those pads to provide a waveguide with a desired thickness.
However, the Jones et al. model is a two dimensional model that cannot be used to control the profile of the waveguide. As used herein, the profile of the waveguide is the taper of the waveguide as it transitions from a first thickness (e.g. the thicker laser section) to a second thickness (e.g. the thinner mode expander section). Currently, there is no process for determining a dielectric mask configuration that will provide a waveguide with a desired profile formed by an MOCVD process. Trial and error is used to determine the mask configuration that provides the desired profile. Because the profile of the waveguide must be configured precisely in order to use the waveguide to monolithically integrate an opto-electronic device (e.g. a laser) and a fiber or waveguide, it can take many iterations over a long period of time to design a mask for growing a waveguide with a desired taper on a substrate with an MOCVD process. Accordingly, a process for designing a dielectric mask for use in fabricating a waveguide with a desired profile is desired.