FIBRE-COUPLED HIGH-INDEX PECVD SILICON- OXYNITRIDE WAVEGUIDES ON SILICON

Transcription

1 FIBRE-COUPLED HIGH-INDEX PECVD SILICON- OXYNITRIDE WAVEGUIDES ON SILICON Maxim Fadel and Edgar Voges University of Dortmund, High Frequency Institute, Friedrich-Woehler Weg 4, Dortmund, Germany ABSTRACT The deposition of PECVD silicon-oxide (SiO x ) and -oxynitride (SiO x N y ) for fabrication of planar waveguides with small bending radii is investigated. The focus is on fabricating homogeneous and during annealing non-cracking core layers. Therefore, the stress behaviour and the hydrogen content are examined. It is shown that a stress-free cover layer can be achieved by adjusting the as-deposited stress and the annealing process. High numerical aperture (NA) waveguides with small bending radii are fabricated. A low-loss coupling between waveguide and fiber is achieved by using special high-na fibre. Their mode field diameter fits very well to the waveguides. Low-loss splicing to standard transmission fibres is optimized. INTRODUCTION Because of the homogeneous and wide refractive index range, silicon-oxynitride (SiON) fabricated by Plasma Enhanced Chemical Vapor Deposition (PECVD) is well-suited as a core layer for planar waveguides. Compared to fiber-matched SiON-waveguides [1], thicker core layers are required for strong guidance, which means small bending radii. It is also well-known that during PECVD-deposition hydrogen is built-in [2], and an annealing treatment is crucial [3] to achieve low-loss optical layers for the communication range at 1.5 µm. But the annealing of thicker layers could lead to cracking. Therefore the hydrogen content and the stress in the layer are examined between annealing steps to understand the crack behaviour of the layer. The principle waveguide design is already described and used by others [4]. Figure 1 shows the geometry of the fabricated waveguides with characteristic dimensions and refractive indices n (n given for λ=633 nm). LAYER DEPOSITION AND CHARACTERIZATION For layer deposition the Oxford Instruments PLASMALAB SYSTEM 100 is used. Silicon-oxide and -oxynitride are deposited using SiH 4, N 2 O and NH 3. The RF power at

2 13.56 MHz frequency is tuned between 40 and 100 W. The test layers were deposited on 4 blank silicon <100> wafers. Figure 1. Structure of the SiON- waveguide; the refractive index given for λ=633 nm After deposition of about 2 µm oxide the wafers show generally concave bending caused by the difference of the thermal expansion of Si and SiO 2 / SiON [5]. This leads to birefringence and therefore polarization dependence of the waveguide material. The stress is calculated from the bending of the 4 wafer (blank wafer thickness=525µm) which is measured with an UBM microfocus surface profilometer. The absorption of the hydrogen bonds overtone -Si-H, -N-H, -Si-O-H is located in the µm wavelength range. They are detected by a FTIR-measuring system that shows the infrared absorption between the wave numbers 400 and 4000 cm -1. The hydrogen can be driven out by annealing the layer at a high temperature. If much hydrogen is built-in, the layer cracks under tensile stress during annealing [3]. This problem is discussed in II.2. Refractive index and deposition rate The refractive index n and thickness d are examined using a Sentech 500 ellipsometer. The uniformity is better than δn=0.001 resp. δd<1 nm (d=140 nm) across 4 wafers. The refractive index measurement shows a dependence on the thickness of the measured layer (Figure 2). Therefore it is only reliable for layer thicknesses from 110 to 160 nm at 633 nm wavelength. Besides, at higher layer thicknesses (>300 nm) the ellipsometer thickness measurement is periodical and a white light reflectometer accessory (FTP 500) must be used.

3 Refractive Index Refractive Index Measurement in Dependence on the Layer Thickness Annealed As-Deposited nm 230 Thickness (As-Deposited) Figure 2. It depends on the layer thickness if the ellipsometer measurement of the refractive index is reliable. By changing the NH 3 -flow a refractive index difference up to Δn 0.08 is achieved between SiON and SiO 2 as shown in Figure 3. The effective deposition rate is used to determine the thickness for the as-deposited layer taking the shrinkage during annealing into account. Eff. Deposition Rate 110 nm/min SiON: N 2 0/SiH 4 =55; RF Power=50 W. As Deposited Ref. Index (Annealed) Effective Deposition Rate (Annealed) sccm80 NH Ref. Index Figure 3. Refractive index (@ 633 nm) and effective deposition rate of the core layer in dependence on the NH 3 -flow At higher temperature Si-N bonds are generated with a subsequent increase Δn of the refractive index up to It is also noticed in Figure 3 that without NH 3 -flow the refractive index of the layer nears the index of the thermal oxide. That means there is no silicon doping in the layer to allow the building of SiO x. Therefore this parameter range has a nitrogen-like character.

4 Hydrogen bonds absorption and stress control in SiON layers Different deposition parameter ranges are used. Besides the NH 3 -flow the Total gas flow and the RF power are tuned. The FTIR-measurements show that almost all the hydrogen bonds are driven out of the layers after 3 h at 1200 C (this temperature is measured outside the sample chamber). After this treatment all layers reach a stress comparable to thermal oxide. In some samples cracks are observed already at 700 C. They result under the increasing tensile stress at this temperature. It seems that the Si-H bonds in SiON layers indicate a weak and cracking layer. Figure 4 and Figure 5 show the FTIR resp. stress measurements of the same SiON layers. Those containing Si-H bonds crack under the tensile stress at 700 C % 0.01 Si-H Content in the As-Deposited SiON Layers Non-Cracking Samples Absoption Cracking Samples cm Wavenumber Figure 4. Absorption of Si-H bonds by FTIR measurements for as-deposited SiON Layer A low Si-H content is achieved for proper deposition parameters with an effective deposition rate from 80 to 100 nm/min and an index increase up to Furthermore, optical loss measurements of 2 µm SiON layers with Δn = 0.05 and 10 µm thermal oxide have been performed at the IOMS Institute, University of Twente, Netherlands. The attenuation coefficient is determined by a prism coupling set-up with a white light source. The non-annealed samples show optical losses for more than 30 db/cm around 1510 nm (N-H bonds). After a first annealing at 1200 C for 3 h the additional attenuation at 1510 nm due to N-H bonds is still 1 db/cm (Figure 6). After 12 h annealing there are only about 0.1 db/cm additional losses left.

5 Pa Stress Tensile Compressiv Crack-Behavior of SiON Layers during Annealing; Refractive Index > 1.5 Non-Cracking Samples Cracking Samples C 1300 Temperature Figure 5. Stress trend during annealing of SiON Layers 24 db/cm Attenuation Coefficient Non annealed Annealed 3 h Annealed 9 h Annealed 12 h O-H Bonds N-H Bonds nm 1600 Wavelength Figure 6. Prism coupling measurement of optical losses for as-deposited and annealed layers (University of Twente, R. Dekker) It should be noted that the measuring set-up causes a slight base line shift which increases with the wavelength. Therefore, the absolute value of the total attenuation is uncertain within a few 0.1 db/cm. Annealing of the cover layer The PECVD SiO 2 -layers show a similar stress trend to that of the cracking SiON layers with the difference of being shifted to compressive stress. What keeps them from cracking (Figure 7).

6 10 6 Pa Stress Stess Control in Cover Layer Increased Deposition Rate Temperature Figure 7. Stress trend of the cover layer for different parameter sets On the other hand, it appears that these SiO 2 layers contain only N-H bonds that can be eliminated at already 700 C (Figure 8). C 0.04 % 0.02 N-H Existence in the Cladding Layer After Annealing at 700 C 0.00 Absorption As-Deposited cm Wavenumber Figure 8. N-H-absorption spectra of as-deposited and annealed cover layers The annealing of about 700 C does not affect the previously annealed SiON core layer. Both layers show a resulting compressive stress. A possible compensation of geometrically induced and stress-induced waveguide birefringence requires further investigation.

7 WAVEGUIDE FABRICATION AND COUPLING For the fabrication of the waveguides 4 Si wafers with 10 µm thermal oxide are used. The core layer of 2 µm is deposited and annealed. A negative resist is structured by UV photolithography and used as a mask for an anisotropic Reactive Ion Etching (RIE) step with CHF 3. The resulting ridge structure of the core layer is shown in Figure 9. Figure 9. SEM photo of waveguide ridge structures Besides the stability and reproducibility of the photolithography, the RIE process is also critical. The observed side wall roughness (Figure 10) still may cause additional optical losses. Waveguide Ridge Side Wall Roughness Figure 10. SEM photo of waveguide side wall After removing the resist the SiO 2 cover layer of about 6 µm thickness is deposited. The facets of the waveguides are prepared with a special polishing saw disc. Figure 11 shows such a facet. This technique demands a low feed rate during sawing and no vibration in the system. Otherwise the facets or the saw will break. The remaining facet

8 roughness is not critical, because index-matched UV-curing adhesive or index-matched fluids are used for fibre coupling. Waveguide Core Silicon Figure 11. A waveguide facet after sawing A butt-coupling technique with index-matched fluid is used to measure the transmitted optical power. Due to the mismatch between the Mode Field Diameters (MFD) of the waveguides and the Standard Single Mode Fibre (SMF28), the coupling losses exceed 3 db per facet. This problem is solved by using an Ultra High Numerical Aperture Fibre (UHNA-fibre; from Nufern) with an MFD around 4 µm. Figure 12 shows a coupling scan measurement of a UHNA-fibre to another one (blue) and to a waveguide (red). Ultra-High-NA Fibre and Waveguide Intensity Scan 6 µm 5 4 Waveguide 4.5 µm -8.30dB Height µm dB Ultra-High-NA Fibre µm 6 Width Figure 12. Coupling scan of a UHNA-fibre to another one (blue) and to a waveguide (red)

9 The comparison indicates a low loss coupling between fibre and waveguide. Another feature of this UHNA-fibre is the possibility of low-loss splicing to SMF28. By using special splice parameters and causing thermal diffusion of the core doping material during splicing the small MFD is extended to fit that of the SMF28. The losses are optimized below 0.2 db per splice. The fibre-coupled transmission loss for waveguide with 5 cm length is below 1 db. This indicates waveguide attenuation below 0.2 db/cm. There are also no significant losses for bending radii beyond 1 mm. CONCLUSION AND OUTLOOK PECVD silicon-oxide (SiO 2 ) and -oxynitride (SiON) layers are characterized for the fabrication of optical waveguides with small bending radii. Deposition parameters are found for non-cracking layers with the required thickness. The stress in the cover layer can be minimized. Waveguide ridge structures are fabricated. The waveguide attenuation is below 0.2 db/cm. The side wall roughness can be reduced by improving the photolithography and the RIE process. The MFD of the waveguides fits well that of Ultra High NA-fibres. These are spliced to standard transmission fibres. REFERENCES [1] M. Hoffmann, P. Kopka, E. Voges, Low-loss fiber-matched low-temperature PECVD waveguides with small core dimensions for optical communication systems, IEEE Photonics Techn. Letters, vol. 9, pp , 1996 [2] Lanford, W., Rand, M.: The hydrogen content of plasma-deposited silicon nitride, J.Appl-Phys. vol. 49, pp , 1978 [3] Denisse, C., Annealing of plasma silicon oxynitride films, J. Appl. Phys. vol. 60, pp ,1986 [4] R.Germann, H.W.M. Salemink, R.Beyeler, G. L. Bona, F. Horst, "Silicon Oxynitride layers for optical waveguide application", J. Electrochem. Soc., vol. 147, pp , 2000 [5] S. M. Hu, Stress-related problems in silicon technology, J. Appl. Phys. vol. 70(6), pp. R53-R80, 1991

10 KEY WORDS Annealing Treatment, 1 Small Bending Radii, 1 Plasma Enhanced Chemical Vapor Deposition, 2 Silicon Oxynitride, 3