Indian Journal of Engineering & Materials Sciences Vol. 12, February 2005, pp. 12-16

Silica-on-silicon based 1×N optical splitter: Design, fabrication and characterization

Aji Baby, C Dhanavantri, J P Pachauri, S Johri, Pawan Kumar & B R Singh* Optoelectronics Devices Group

Central Electronics Engineering Research Institute, Pilani, 333 031, India

Received 8 December 2003; accepted 10 December 2004

The present paper deals with the design, fabrication technology and performance of silica-on-silicon based 1×4 and 1×8 optical power splitter. The basic waveguide structure with S-bends and Y-junctions have been used for the splitter design using BPM software. The optimized process parameters related to choice of metal masking layer, chemistry used for deep dry etching of core layer, dependence of etch process parameters on anisotropy and sidewall smoothness, dicing and fibre alignment are presented. An indigenously built ECR/RIE system especially for deep dry etching of silica layer is also described. The results on splitting insertion loss and uniformity maximum across the channel measured on 1×4 and 1×8 splitters are presented.

IPC Code: H 01 R 31/02

The ever increasing demand of multimedia communications and other digital communication is driving the transition from electronic to optical networks, however, the success of optical networking would heavily depend on the availability of new family of active as well as passive optical components. The major trends in optical networks are rapid capacity expansion and networking which are more adoptable. But the success of optical networking would heavily depend on the availability of new family of optical active as well as passive components. Most of these passive components, till date, have been realised in bulk-optic configuration using micro lenses and prisms and in fibre optic configuration using fused fibre couplers. These bulk-optic and fibre optic approaches have some limitations in terms of productivity, device stability and suitability for integration. A possible approach to overcome this problem is to introduce channel waveguide technologies to form integrated-optic components. A variety of optical materials like glasses, lithium-niobates and III-V semiconductors have been used to realize these components but the dream of integrating every functional devices on a planar substrate is far from realization1.

Due to their inherent superiority like low insertion loss, possibility of hybrid integration, reproducibility and long term reliability, silica based planar lightwave

circuits (PLCs) have lately attracted considerable attention for the development of wavelength division multiplexing(WDM) components. These components are finding application in optical branching, switching and filtering application in WDM optical networks and offer many attractive applications for passive components like splitters, couplers in optical access networks and PLC switches and filters in DWDM networks2,4. Future developments are now focused to hybrid integration, in which the PLCs based on silica-on-silicon technology is considered to be the most promising candidate for integration platform because silica based waveguide provides high performance planar lightwave circuits and silicon has excellent mechanical and thermal properties which are suitable for optical bench5,7.

This paper presents the results of our investigation related to design, fabrication and characterization of silica-on-silicon based 1×N optical power splitter. Our efforts on developing an indigenous deep dry etching system for this work is also included. Design

Design of integrated optic (IO) components involves the numerical analysis of geometry and light propagation through it. Various methods like effective index method (EIM), beam propagation method (BPM), finite element method (FEM) and method of lines, can be used for the modelling of different geometries of the integrated optic devices8,12. A

_____________________ *For correspondence (E-mail:brs@ceeri.ernet.in)

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“WDM Phasor” Software from M/s Optiwave Corporation, Canada which has inbuilt 2D BPM was extensively used in designing splitter. As an example, a typical simulation result for 1×4 splitter along with the input parameters is shown in Fig. 1 and Table 1 respectively. The wavelength for operation has been chosen as 1550.52 nm. Technology

The fabrication steps along with its cross-sectional view for developing silica on silicon single mode

buried waveguide as well the starting material structure used for this investigation is shown in Fig. 2. A two layer structure consisting of 30 µm thick bottom silica cladding layer 7µm thick doped silica layer deposited using flame hydrolysis deposition (FHD) technique was used as starting material, procured from M/s Piri (Photonic Integration Research Inc.), USA. The core layer was defined using conventional photolithography in combination with metal etch masking layer and deep dry etching. Undoped silica film of 5 µm thickness deposited in-house using plasma enhanced chemical vapour deposition (PECVD).

It can be seen from the above that the most important processing step involved in developing IO components are material deposition and deep etching (7-8 µm) of silica core layer while maintaining the sidewall geometry and smoothness. An ECR/RIE

Fig. 1⎯Simulation result of 1×4 splitter [

Table1⎯Design parameters for 1×8 splitter

Material / Device Parameters (Input Design Parameters)

Values

Wavelength for operation 1550.52nmNclad 1.445Ncore 1.449N 0.3%D=250 μm ( center to center) 250 µm

Simulated Results S band length for d (500) 8220 µmTapper angle 0.5оTapper length 450 µmWidth 8.0 µmThickness 8.0 µmDevice length 21525µmDevice width 1750µmRadi of curvature 34.0 mmMax Insertion loss ≤ 10.5 dB

Fig. 2⎯Process flow chart

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system was indigenously developed under this programme for deep dry etching of silica core layer (Fig. 3). The system consists of a source region with gas inlet, a cylindrical process chamber and a pumping system. ECR is capable of delivering 0-800 W power at 2.48 GHz through a quartz window. The magnetic flux density required for resonance (87.5 mT) is generated by water cooled electromagnet coils. The RF power at 13.56 MHz can be applied to the substrate holder through a matching network with a provision for measuring the dc substrate bias. It has Turbo pump backed by Rotary for vacuum, four MFCs for different gas controls and two vacuum gauges for vacuum measurements. A especially designed substrate holder with RF feed through and water cooling has been incorporated in order to avoid

excessive heating during etching process. The safety interlocks among various sub-modules and systems, status displays, chambers, fixtures and all necessary utility services have been incorporated in this system.

In view of the large number of parameters like RF/microwave power, pressure, gas composition and masking layer which have to be optimized before ideal etch conditions are achieved, initial effort was to keep RF power/microwave power constant to 150 W/600 W and work with the pressure range from 50 mTorr to 1 mTorr and different gas compositions. Different gases like CHF3 and C2F6 mixed with O2 and argon were used for etch rate optimization. The optimized process parameter is shown in Table 2.

E-beam evaporated CrTi and NiCr films were tried as etch masking layers. In view of better edge sharpness, reproducibility of the process, and ease in after dry etch removal, NiCr films were subsequently used as etch masking layer. Conventional photolithography and chemical etching was employed for patterning. Deep dry etching of core layer was performed in ECR/RIE system with the process parameters described earlier.

The SEM photograph of optical waveguide etched using above parameters is shown in Fig. 4. The sharp

Fig. 3⎯ECR/RIE system developed indigenously

Fig 4 ⎯ SEM micrograph of etched silica wafers

Table 2 ⎯ Optimized RIE process parameters

Parameters Values RF power 150 W Microwave power 200 W Pressure 50m Torr Gas composition Ar 20 SCCM O2 20 SCCM CHF3 15 S CCM

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side walls with desired dimensional control suited for optical wave guide applications can be seen from the figure.

After removal of NiCr films, the devices are diced with (Disco, model DAD321, Japan) dicing saw and polished using a inhouse developed jig. The angled dicing and nm level of polishing is required for these devices in order to get state-of-the-art performance. Although operational devices with functional behaviour have been realized, these two steps need sophisticated jigs/chemicals to achieve the desired results and are still under development.

A manual fibre alignment system (New port, U.S.A., VH Vibration Isolation Workstation) having twin precision six axis alignment tables (incorporating embedded precision X, Y, Z, linear and angular displacement manipulators as well as piezo micro actuators) was used for device characterization. A separate video microscope with 50-300× zoom facility has also been incorporated with the system to further enhance the field of view and initial adjustments (Fig. 5). A He-Ne laser (600-630 nm), model JDS1135, having 5-10 mW output power has been used for visible alignment. For launching of the optical power in 1×4, 1×8 splitter, a FC/PC connectorised optical fibre at one end and fibre fixed in V groove etched silicon on the other end, was used. While on the other end of the splitter 8 optical fibres with FC/PC connectorised ends and all the eight fibres embedded in eight silicon V

grooves etched 250 μm apart was used for detecting the splitted optical power.

Results and Discussion Figs. 6a and 6b show the fabricated 1 × 4 and 1 × 8

splitters. The successful operation of 1×4 and 1×8 splitters could be seen from the four/eight equispaced spots observed at the waveguide output on a video terminal in Figs 7a and Fig 7b respectively. Ideally the output optical spots should be elliptical in shape but it can be seen that some of the spots are blurred and deformed. It is perhaps due to the non ideal edge polishing and/or damage created due to chipping of silica at the waveguide edges.

Various optical parameters at 1550 nm like insertion loss, uniformity were then measured using IR source and V-grooved optical fibre at the output. The splitting insertion loss of <27db for 1×4 splitter and < 32db in the case of 1×8 was observed11,15. Uniformity (maximum) in the range of 4-5 db was noted in both the devices (Table 3). It can be seen from the measured values that both these parameters are higher than the simulated values. Further optimization of material and processing steps in general and edge polishing and fibre alignment would be required in order to achieve the state-of-the-result.

Conclusions The design, fabrication technology and performance

of silica-on- silicon based 1×4 and 1×8 optical splitter

Table 3 ⎯ Optical parameters for 1×4 and 1×8 splitter (simulated versus measured)

Parameters 1×4 Splitter

simulated 1×4 Splitter measured

1×8 Splitter simulated

1×8 Splitter measured

Splitting insertion loss < 10 dB < 27 dB < 15 dB < 32 dB Uniformity maximum 1.5 dB 4 dB 2 dB 5 dB

Fig. 5 ⎯ Fibre alignment and optical characterization set-up

Fig. 6 ⎯ Micrograph of fabricated 1×4 and 1×8 splitter

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suited for optical network applications have been presented The critical issues related to device design and fabrication technology have been addressed. A brief description of an indigenously developed ECR/RIE system for deep dry etching of silica core layer is also included. The performance of optical splitters realized has been presented. The design, fabrication technology and performance of optical splitters.

Acknowledgement The authors would like to thank Miss Kiran for her

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Fig. 7⎯Micrograph of 1×4 and 1×8 splitter output light spots