Laser reactive deposition produces glasses for PLCs

Xiangxin Bi, Michael Bryan, Eric Euvrard and Tim Jenks

Monolithic materials integration is critical to developing planar lightwave circuits, and standard deposition technologies are being challenged to meet these needs. The authors argue that laser reactive deposition can produce complex glass compositions and integrate different compositions on a substrate.

In planar lightwave circuit (PLC) technology, components such as waveguides, amplifiers, lasers, gratings, splitters, couplers, switches, and detectors are integrated onto a single planar platform. By integrating these optical devices, the final integrated component will have more functionality, reduced size, enhanced performance and reduced packaging costs.

Major building blocks for these PLCs include gain blocks such as erbium-doped waveguide amplifiers, functional blocks such as germanium-doped ultraviolet (UV) sensitive glass, and waveguides based on a silica-on-silicon structure. For example, UV-sensitive media and gain media can be co-deposited on a planar substrate to create waveguide laser chips.

CREATING GAIN MEDIUM
A complex glass composition is needed to produce commercially viable gain media.^{1, 2} High erbium concentrations of up to 10²⁰ ions/cm³ are needed to achieve reasonable amplification (3 to 4 dB/cm). This concentration allows erbium-doped planar gain blocks yielding about 15 dB gain to be fabricated on silicon wafers, which is ideal for metropolitan WDM applications.

Aluminum infusion into the glass matrix is found to better accommodate such a high erbium content. For optical amplification bands other than those centered around 1550 nm, rare earth elements such as praseodymium (around 1310 nm) and thulium (around 1490 nm) may be doped into the glass. A corresponding glass host must be selected for each of these elemental dopants. Otherwise, significant dopant clustering or crystal precipitation may occur, leading to a major degradation of the gain block's optical quality.

The glass composition becomes even more complex when one attempts to minimize the stress in the glass layers. In this case, it is common to add alkali dopants such as sodium into the glass matrix to modify the bonding between oxygen and silicon. In another optical building block, germanium-doped UV-sensitive glass is used to fabricate devices such as filters, multiplers and demultiplexers, and Bragg gratings. To produce the required photosensitivity, hydrogen or boron is introduced into the phosphate silica glass matrix. When fabricating a waveguide laser using a planar Bragg grating as a mirror and an erbium-doped waveguide amplifier as a gain block, the glass media must be both UV sensitive and amplifying, which can be accomplished by doping tin into a phosphate glass.³

If UV sensitive and amplifying materials are partitioned in different regions of the substrate, then appropriate compositions must be selected for both materials to match mechanically and optically at the interface. Furthermore, to increase the pump energy absorption cross section, ytterbium may be co-doped with erbium. Finally, basic passive components such as waveguides and arrayed waveguide gratings use phosphorus- and/or boron-doped glasses. To integrate these with amplifying and UV-sensitive media, it is critical to precisely engineer the composition of materials for each component. A materials deposition technology with an ability to produce a wide range of compositions on a single substrate is required.

DEPOSITION PROCESSES
Thin-film deposition technologies include ion sputtering, laser ablation, sol-gel, thermal evaporation, electron beam evaporation, and molecular beam epitaxy (MBE). These techniques have been designed, developed and optimized for submicron electronic thin-film depositions. For thick glass films used for silica-on-silicon PLC structures, these techniques require long process times.

Two common commercial planar glass deposition technologies are chemical vapor deposition (CVD) and flame hydrolysis deposition (FHD), the latter having been developed mainly for optical-fiber manufacturing.⁴ Both use high-purity chemicals such as tetracholorosilane (SiCl₄), germanium tetrachloride (GeCl₄), and phosphorus oxychloride (POCl₃) as precursors. These precursors react on a heated substrate to form dense glass films in the case of CVD, or react in a fuel and oxygen flame to form glass particles that are deposited onto a substrate in the case of FHD.

In both cases, a standard process step of thermal treatment produces final glasses with high-quality optical properties and low propagation loss. Chemical-vapor-deposition films typically need an annealing process of approximately 600°C to 800°C, whereas films produced by FHD need a consolidation process around 1000°C to 1300°C. Despite their success in producing passive components, neither FHD nor CVD can readily produce glasses having complex compositions.

LASER REACTIVE DEPOSITION
To produce thick-glass films with complex compositions, the deposition technology must be able to use a wide range of chemical precursors (vapors, gases, and aerosols), as well as to generate, mix, deposit, and consolidate all required elemental species onto a planar substrate. In addition, these materials must be uniformly distributed throughout the entire planar substrate. Laser reactive deposition (LRD) is a materials synthesis and deposition technology that can accomplish this and produce integrated optical materials media on planar substrates.

In the LRD process, a sheet laser beam, delivered by an optical assembly, intercepts a reactant materials stream delivered by gas, vapor, and aerosol injection (see Fig. 1). The reaction zone is defined by the overlapping regions between the laser beam and the reactant materials stream. The solid nanoscale particles are formed from rapid heating and quenching produced by the laser-driven reaction.

Planar-glass fabrication benefits greatly from using the LRD process (see Table 1). Typically, a flat-nozzle assembly is used to deliver precursors in a rectangular sheet stream. The beam is oriented perpendicularly to the precursor stream to achieve the maximum heating and quenching rate (see Fig. 2). Wafer after wafer is transported through a production chamber and over the laser-reaction zone for the deposition of high-quality optical glass materials (see Fig. 3).

COMPARING PROCESSES
The characteristics of LRD compare well with CVD and FHD with regard to their ability to synthesize complex compositions for the fabrication of glass at a commercial scale (see Table 2). Laser reactive deposition can use a wide range of precursor chemicals to provide precise control of elemental species in the final glass composition. This process is enabled by the use of a high-power industrial carbon dioxide laser. Deposition rates of 10 µm/min. are standard. This rate is 50 to 100 times faster than typical rates of 0.1 to 0.2 µm /min achieved by plasma enhanced CVD (PECVD) process.

Xiangxin Bi is founder, vice president of research, and chief scientist; Michael Bryan is vice president of business development; Eric Euvrard is director of marketing and product management; and Tim Jenks is president and CEO at NEO Photonics, 49040 Milmont Drive, Fremont, CA 94538-7301l. Michael Bryan can be reached at mbryan@neophotonics.com.

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