Sol-gel deposition for planar components

Apr 1st, 2002

Component manufacturing

A novel sol-gel deposition process can be applied to the manufacture of silica planar lightwave circuits as an alternative to current, costly gas deposition processes.

Terahertz Photonics was founded in Scotland in 1998 to provide optoelectronic foundry services at the wafer level for the production of silica and polymer planar lightwave circuits (PLCs) as well as silica/polymer hybrid components. It claims to be the first company to perfect the use of sol-gel as a cost-effective method of depositing silica on silicon. Terahertz's Solica process is the first single-step sol-gel deposition process - a disruptive technology which could eclipse existing gas-based coating techniques.

Planar Lightwave Circuit (PLC) optical components, which are becoming increasingly central to data transmission in the optical networks, are currently processed using either chemical vapour deposition or flame hydrolysis deposition. However, these technologies involve gas phase processes that are both expensive and difficult to control.

In contrast, the Solica process reduces the cost of producing PLC components such as arrayed waveguide gratings, variable optical attenuators and couplers, a market forecast by analyst company Electronicast to be worth USD800m by 2006. Benefiting from both low capital and operational costs, the process is up to 10 times cheaper than alternative methods. It also provides a platform for a range of new devices which are both more compact and which have more features.

Director of PLC materials Dr Navin Suyal says, "We believe that Solica is a revolutionary process for the fabrication of silica and silicon planar lightwave circuits. It is capable of changing the present cost-performance curve of this sector, which will enable the next generation of metro, access and fibre-to-the-home applications."

Engineers at Terahertz identified the need to overcome the limitations of existing expensive gas techniques. Now, for the first time, the Solica process will enable the manufacture of optical-quality glass layers up to the required 10mm thickness in a single liquid-based process.

The process involves the synthesis of a sol mixture followed by a single spinning, drying and firing operation. The process uses standard tools and techniques from the semiconductor industry for both the deposition of the silica and the subsequent lithography.

As the silica is fully densified from a sol-gel at around 1150°C, it contains no organic additives present in the initial sol and does not have any impurities that could limit its performance.

The process opens up avenues for new and improved PLC components, says Terahertz. The silica layers can be doped for a refractive index of 1.45-1.50. This enables the creation of super-high-delta waveguides in which the difference in the index between core and cladding can be as high as 3%. The consequence is tighter bend radii for waveguides within a PLC and hence more compact devices, so more components can be produced from a single wafer at a higher production yield, realising lower-cost, smaller-footprint components.

Dr Suyal adds, "Versatility will be a key element. It is vital that telecom companies increase the functionality and reduce the cost of their hardware in order to reduce the cost of bandwidth to the consumer.

The sol-gel technique involves the generation of a network of Si-O-Si units suspended in a solvent through the hydrolysis and poly-condensation of metallo-organic precursors of the type Si(OR)4, where R is CH3, C2H5 etc. To control refractive index, precursors like Ge(OR)4, Ti(OR)4 or PO(OR)3 are added. Process parameters like pH, reaction temperature and dilution control the structure and density of the SiO2 units formed.

In the Solica process a sol is also synthesised by the hydrolysis and poly-condensation of the silicon precursor tetraethoxysilane. A controlled reaction then allows the synthesis of well dispersed silica units with a tailored size distribution. It is important to prevent these from agglomerating during evaporation of the solvents so they are modified by additives to control their surface properties. High-refractive-index components such as germanium, titanium and lead are added to control the refractive index of the glass layers. Phosphorous and boron are added to modify the thermal expansion and flow properties.

The sol is spin-coated on silicon substrates then dried. The film is sintered at 1050-1200°C, producing a dense and uniform optical quality oxide film exhibiting low hydroxyl content.

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