InP wafers ready for high volume optical applications

InP wafers ready for high volume optical applications
Several recent technology developments have allowed indium phosphide (InP)-based circuit and optical component fabrication to scale to higher volumes and lower cost. The availability of 4-inch InP substrates with acceptable defect densities and good flatness is one of the key improvements driving volume manufacturing of InP-based components for long-wavelength communications systems.

Two principal developments in wafer fabrication have yielded those improvements. First has been the ability to control the tendency of InP to twin during crystal growth, allowing the growth of relatively large, 4-inch-diameter single-crystal boules. The second has been the development of polishing techniques capable of producing submicron local thickness induced reading (LTIR). That measure of a wafer's flatness directly affects the ability to print submicron geometries over large field sizes.

Advancements in epitaxial growth of InP and lattice-matched compounds is also driving capability. Phosphorus-containing compounds can now be grown by molecular beam epitaxy (MBE) in volume in multiple-wafer machines and are available in quantity from several epi service suppliers. MBE allows either beryllium or carbon to be used as a p-type dopant, a choice not available by metal-organic chemical vapor deposition (MOCVD). In the past, phosphorus-containing compounds have not been popular with MBE growers, because of the maintenance headaches created by phosphorus deposition on the interior of the MBE growth chamber. Those problems have been overcome.

Group V-graded structures are more difficult to grow by MBE than MOCVD, making variable-width superlattices more common in MBE-grown layer stacks. MOCVD offers a competing growth process more widely used in the deposition of epi layers used in optical components. For InP-based transistor structures grown by MOCVD, carbon is the only real choice of p-type dopant. Problems with hydrogen passivation both during epi layer growth and during subsequent device processing once limited the peak doping in the base to low, 1E19 levels, limiting the high-frequency gain.

Advancements in carbon doping and the ability to control carbon passivation during the growth process have made MOCVD a good choice for the growth of HBTs as well. The ability to grade the Group V elements smoothly throughout the epi layer stack makes MOCVD an attractive choice, particularly in the growth of double heterostructures and more complicated optical structures.

Another attractive feature of MOCVD is the ability to do selective growth and regrowth. That ability makes it possible to perform semiconductor junction passivation by overgrowth and two-dimensional refractive index grading — very desirable features in the fabrication of both electronic and optical components.

Device fabrication is still most often done by creating mesa structures using wet-etching techniques which provide high selectivity between the epi layers. The ongoing development of reactive ion etching (RIE)-based dry etches will be critical in the fabrication of smaller device sizes required to reduce power, increase performance and increase levels of integration. Those same dry etching techniques are critical to the manufacture of optical components and integrated optics since they allow the fabrication of waveguides with square cross sections, an important feature needed to minimize polarization distortion.

Ultimately, spacer-based processes will replace most of the wet-etched and shadow-masked processes currently used in transistor fabrication. The spacer-based processes allow for tighter control and much smaller self-aligned overlaps, resulting in superior device performance. Lift-off techniques ultimately need to be replaced by etching and plating processes similar to the dual-damascene copper processes used by the silicon fabrication industry.

Advanced processing and interconnect technologies now allow the monolithic integration of optical components, such as photodetectors and waveguides, with electronic circuits, such as amplifiers. The critical interfaces then become well-controlled by wafer-processing techniques. The development of optoelectronic products that leverage those capabilities will ultimately ramp the volume of InP wafers.

The critical technical obstacles in wafer supply, epi growth and processing have been solved. The focus is now centered on the development of advanced optoelectronic products that take advantage of these capabilities.

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