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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 21 — Oct. 8, 2012
  • pp: 23676–23683
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Effect of cladding layer and subsequent heat treatment on hydrogenated amorphous silicon waveguides

Shiyang Zhu, G. Q. Lo, Weihong Li, and D. L. Kwong  »View Author Affiliations


Optics Express, Vol. 20, Issue 21, pp. 23676-23683 (2012)
http://dx.doi.org/10.1364/OE.20.023676


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Abstract

Although intrinsic hydrogenated amorphous silicon (a-Si:H) wire waveguides clad with normal SiO2 layers have low propagation loss of 2.7 ± 0.1 dB/cm for transverse electric (TE) mode in the 1550-nm range, the transparency degrades when interfaced with other dielectrics (e.g., air) and/or exposed to elevated temperatures due to degradation of surface passivation in the a-Si:H waveguides. The thermal stability of a-Si:H wire waveguides with various cladding layers is systematically investigated, showing that the a-Si:H wire waveguides are stable at annealing temperature lower than ~350°C, while they degrade quickly when annealed at a higher temperature. It indicates that the thermal stability is mainly determined by the annealing temperature rather than the annealing time, which may be attributed to quick evolution of weakly bonded hydrogen in the a-Si:H waveguides. A thin Si3N4 intercladding layer between SiO2 cladding and a-Si:H waveguide core may degrade transparency due to N-H bond absorption and is of no benefit to the thermal stability, thus its overall effect on the a-Si:H waveguides is detrimental.

© 2012 OSA

1. Introduction

In this paper, a-Si:H wire waveguides with various cladding layers (i.e., SiO2, Si3N4, and air) are fabricated, and rapid thermal annealing (RTA) with different temperature and time is carried out to simulate thermal process in CMOS backend processing steps. Their effect on the optical loss of a-Si:H waveguides is revealed, and is explained by degradation of surface passivation in a-Si:H waveguides at elevated temperatures.

2. Experimental

8-inch Si wafers with 2-μm PECVD-SiO2 were used as substrates. A 220-nm a-Si:H film was deposited by PECVD in an Applied MaterialsTM parallel plate reactor using the optimized recipe as follows [10

10. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability,” Opt. Express 18(24), 25283–25291 (2010). [CrossRef] [PubMed]

]: SiH4 flow 100 sccm, N2 flow 1500 sccm, temperature 400°C, pressure 4.2 torr, and RF power 100 W. The deposition rate is ~1.58 nm/s. Wire waveguides with inverted taper structures at both input/output terminals were patterned by 248-nm deep UV lithography and dry etched down to the bottom SiO2 using a thin SiO2 layer as the hard mask. The fabrication details have been described elsewhere [10

10. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability,” Opt. Express 18(24), 25283–25291 (2010). [CrossRef] [PubMed]

]. Total 8 samples were fabricated with different bottom and upper cladding layers, as listed in Table 1

Table 1. Propagation losses of 220-nm (height) × 500-nm (width) a-Si:H wire waveguides with various cladding layers at TE mode in 1550 nm range

table-icon
View This Table
. Si3N4 and SiO2 layers (except S3 sample) are also deposited in the Applied MaterialsTM parallel plate reactor. The recipe for Si3N4 deposition is: SiH4 flow 110 sccm, NH3 flow 38 sccm, N2 flow 2500 sccm, temperature 400°C, pressure 4.2 torr, and RF power 410 W. The deposition rate is ~5 nm/s. The recipe for SiO2 deposition is: SiH4 flow 115 sccm, N2O flow 2000 sccm, temperature 400°C, pressure 4.2 torr, and RF power 270 W. The deposition rate is ~3.4 nm/s. For S3 sample, the upper cladding SiO2 is deposited at 150°C in a STS-CVD tool. Figure 1
Fig. 1 XTEM images for S4, S6, and S8 samples, the indicated thicknesses of surrounding Si3N4 are measured from the enlarged XTEM images, other samples have similar a-Si:H waveguide core profile.
shows cross sectional transmission electron microscopy (XTEM) images for S4, S6 and S8 samples. The a-Si:H waveguide core has almost rectangular profile and has no difference due to the different cladding layer. A thin Si3N4 layer covers the a-Si:H core conformally. The Si3N4 thicknesses indicated in Figs. 1(b) and 1(c) are measured from the enlarged XTEM images (not shown here). The sidewall Si3N4 is proportionally thinner than the top Si3N4. Other samples have similar cross section for a-Si:H waveguide cores, thus they are not shown here for simplicity. The surface roughness (root-mean-square (RMS) within 5-μm × 5-μm area) measured by atomic force microscope (AFM) is ~0.47 nm for the initial bottom SiO2 layer, ~1.90 nm after 220-nm a-Si film deposition, and ~1.83 nm after additional 20-nm Si3N4 deposition, as shown in Fig. 2
Fig. 2 Surface roughness measured by AFM for (a) the initial 2-μm PECVD SiO2, (b) after 220-nm a-Si film deposition; and (c) after additional 20-nm Si3N4 deposition.
. It indicates that the thin Si3N4 deposition may smooth the a-Si film slightly.

After dicing, the chips were performed by RTA at N2 ambient with different temperature and time. In an isochronal RTA experiment, the annealing temperature ranges from 250°C to 500°C in 50°C increments while the annealing time keeps for 20 min. In an isothermal RTA experiment, the annealing time ranges from 20 s to 30 min while the annealing temperature keeps at 400°C or 500°C.

The conventional cutback method is used to extract the propagation loss. A set of 7 wire waveguides located on the same chip is measured at room temperature. The waveguide length ranges from 0.69 cm to 2.68 cm. Because the propagation loss of a-Si:H waveguides depends slightly on the waveguide dimensions and the input light, for simplicity, we fix the waveguide dimension to 220-nm (height) × 500-nm (width) and the input light to transverse electric (TE)-polarized mode in 1550 nm. The detailed measurement setup has been described elsewhere [10

10. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability,” Opt. Express 18(24), 25283–25291 (2010). [CrossRef] [PubMed]

]. Figure 3
Fig. 3 Output light power versus waveguide length for some samples measured at TE mode in 1550 nm. For samples with relatively low propagation loss, linear fitting is made for all 7 data points to extract the propagation loss (shown by solid lines). For samples with large propagation loss, the propagation loss is estimated from output powers measured on short waveguides by assuming that its coupler loss is only slightly larger than that extracted from low loss waveguides (shown by the dotted lines).
shows the row measurement results for some samples. For waveguides with relatively low optical loss (e.g., S4, S6, and S8 in Fig. 2), the measured output power exhibits good linearity with the waveguide length, thus enabling linear fitting the experimental data points to extract the propagation loss and the coupling loss between the waveguide and the fiber. To minimize the experimental error, three chips are measured for each condition. For waveguides with propagation loss ranging from ~2–15 dB/cm, the coupling loss is in the range of ~3-5 dB/facet, and there is a rough tendency that the waveguide with larger propagation loss has a slightly larger coupling loss, probably because (1) the weak output light may have relatively large measurement error due to the leakage of light besides that transports through the waveguide and/or (2) the factors (such as insufficient passivation) that degrade the transparency of a-Si:H wire waveguides may also degrade their coupling loss. For waveguides with large loss (e.g., S1 and S2 in Fig. 2), the output power through the long waveguide is too weak to be measured (i.e., smaller than the detection limit of our system of ~-55 dBm). The propagation loss is then roughly estimated from the output powers measured on the short waveguides by a simple assumption that their coupling loss is only slightly larger than that extracted from low loss waveguides. Be noted that the propagation loss estimated by this method has a large experimental error.

3. Results and discussion

Secondly, we find that a thin Si3N4 intercladding layer degrade the propagation loss as compared to that without the Si3N4 intercladding layer (i.e., S4 sample). For S5, S6, and S7 samples which have a thin S3N4 intercladding layer between upper cladding SiO2 and a-Si:H waveguide core, the propagation loss is ~5-7 dB/cm, close to that reported in Ref [8

8. R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel, and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer,” Appl. Phys. Lett. 94(14), 141108 (2009). [CrossRef]

]. for a-Si:H wire waveguides with an as-deposited Si3N4 intercladding layer. For S8 sample which has thin S3N4 intercladding layers both in upper and bottom interfaces, the propagation loss is even larger (~13.9 dB/cm). The large propagation loss of these samples cannot be attributed to the surface roughness of Si3N4 film as reported in Ref [17

17. J. Kang, Y. Atsumi, M. Oda, T. Amemiya, N. Nishiyama, and S. Arai, “Low-loss amorphous silicon multilayer waveguides vertically stacked on silicon-on-insulator substrate,” Jpn. J. Appl. Phys. 50, 120208 (2011). [CrossRef]

], but be preferably attributed to N-H bond absorption. It has been reported that the N-H concentration in PECVD Si3N4 can be reduced by optimization of Si3N4 deposition parameters [18

18. S. C. Mao, S. H. Tao, Y. L. Xu, X. W. Sun, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Low propagation loss SiN optical waveguide prepared by optimal low-hydrogen module,” Opt. Express 16(25), 20809–20816 (2008). [CrossRef] [PubMed]

] or using an in situ N2/Ar plasma treatment [8

8. R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel, and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer,” Appl. Phys. Lett. 94(14), 141108 (2009). [CrossRef]

]. It indicates that the thin Si3N4 intercladding layer has two completing effects on the optical loss: increasing the loss due to N-H absorption and decreasing the loss due to its mediate refractive index between a-Si:H and SiO2 (which can reduce the waveguide roughness induced optical loss). For samples reported in Ref [8

8. R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel, and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer,” Appl. Phys. Lett. 94(14), 141108 (2009). [CrossRef]

], the N-H absorption is reduced by the in situ N2/Ar plasma treatment and the reference a-Si:H waveguides have large loss probably due to large sidewall roughness, thus the overall effect of the thin Si3N4 intercladding layer is beneficial. In contrast, for our samples, the reference a-Si:H waveguide (i.e., S4) already has a very low propagation loss (it implies that the waveguide sidewall of our samples is quite smooth) and the thin Si3N4 film is as-deposited, thus the overall effect of the thin Si3N4 intercladding layer is detrimental. S7 sample has similar optical loss as S6 sample (within the measurement error) and is only slightly larger than S5 sample. The very weak dependence of optical loss on the intercladding Si3N4 thickness may also be attributed to the above two completing effects.

Figure 4
Fig. 4 Propagation loss versus annealing temperature for various a-Si:H wire waveguide listed in Table 1 after isochronal RTA in N2 ambient for 20 min.
plots propagation losses for various a-Si:H wire waveguides after isochronal RTA in N2 ambient for 20 min. The annealing temperature ranges from 250°C to 550°C. The data for S1 and S2 samples are not included because their propagation losses are too large to be accurately measured. We can see in Fig. 4 that all samples exhibits similar thermal behavior: the propagation loss keeps almost unchanged after annealing at temperature <350°C, degrades substantially at 400°C, and becomes very larger after annealing at temperature >450°C. It indicates that the annealing temperature is a key factor that determines the thermal stability of a-Si:H waveguides.

Figure 5
Fig. 5 Propagation loss versus annealing time for various a-Si:H wire waveguide listed in Table 1 after isothermal RTA in N2 ambient at 400°C.
plots propagation losses for various a-Si:H wire waveguides after isothermal RTA at 400°C. The annealing time ranges from 20 s to 30 min. All samples exhibit similar lifetime behavior: the propagation loss increases substantially after RTA for 20 s, then keeps almost constant over a long annealing time, and then increases slowly with annealing time further increasing. For 500°C RTA, the propagation loss becomes very large (>20 dB/cm) after annealing for 20 s for all a-Si:H wire waveguides, as shown in Fig. 6
Fig. 6 Propagation losses for various a-Si:H waveguides listed in Table 1 after RTA in N2 ambient at 500°C for 20 sec (shown as solid circles), their initial propagation losses are also shown (open squares) for comparison.
. It indicates that the degradation of a-Si waveguides occurs within a very short period of time during annealing, namely, the annealing time plays a minor role to the thermal stability of a-Si:H waveguides.

4. Conclusion

Acknowledgments

This work was supported by Singapore A*STAR Infuse Exploratory Grant I02-0331-12.

References and links

1.

A. Harke, M. Krause, and J. Mueller, “Low-loss single mode amorphous silicon waveguides,” Electron. Lett. 41(25), 1377–1379 (2005). [CrossRef]

2.

R. A. Street, Hydrogenated Amorphous Silicon (Cambridge University Press, 1991).

3.

K. Narayanan, A. W. Elshaari, and S. F. Preble, “Broadband all-optical modulation in hydrogenated-amorphous silicon waveguides,” Opt. Express 18(10), 9809–9814 (2010). [CrossRef] [PubMed]

4.

Y. Shoji, T. Ogasawara, T. Kamei, Y. Sakakibara, S. Suda, K. Kintaka, H. Kawashima, M. Okano, T. Hasama, H. Ishikawa, and M. Mori, “Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide,” Opt. Express 18(6), 5668–5673 (2010). [CrossRef] [PubMed]

5.

F. G. Della Corte, S. Rao, G. Coppola, and C. Summonte, “Electro-optical modulation at 1550 nm in an as-deposited hydrogenated amorphous silicon p-i-n waveguiding device,” Opt. Express 19(4), 2941–2951 (2011). [CrossRef] [PubMed]

6.

S. Rao, G. Coppola, M. A. Gioffrè, and F. G. Della Corte, “A 2.5 ns switching time Mach-Zehnder modulator in as-deposited a-Si:H,” Opt. Express 20(9), 9351–9356 (2012). [CrossRef] [PubMed]

7.

A. Biberman, K. Preston, G. Hendry, N. Sherwood-Droz, J. Chan, and K. Bergman, “Photonic network-on-chip architectures using multilayer deposited silicon materials for high-performance chip multiprocessors,” ACM J. on Emerging Technologies in Computing Systems 7(2), DOI 10.1145 (2011).

8.

R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel, and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer,” Appl. Phys. Lett. 94(14), 141108 (2009). [CrossRef]

9.

R. Sun, J. Cheng, J. Michel, and L. Kimerling, “Transparent amorphous silicon channel waveguides and high-Q resonators using a damascene process,” Opt. Lett. 34(15), 2378–2380 (2009). [CrossRef] [PubMed]

10.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability,” Opt. Express 18(24), 25283–25291 (2010). [CrossRef] [PubMed]

11.

P. K. Lim, W. K. Tam, L. F. Yeung, and F. M. Lam, “Effect of hydrogen on dangling bond in a-Si thin film,” J. of Phys.: Conference Series 61, 708–712 (2007). [CrossRef]

12.

T. A. Li, F. W. Chen, A. Cuevas, and J. E. Cotter, “Thermal stability of microwave PECVD hydrogenated amorphous silicon as surface passivation for n-type heterojunction solar cells,” European Photovoltaic Solar Energy Conference 2007, ed. Conference Program Committee, WIP-Renewable Energies, Germany, 1326–1331 (2007).

13.

C. J. Arendse, D. Knoesen, and D. T. Britton, “Thermal stability of hot-wire deposited amorphous silicon,” Thin Solid Films 501(1-2), 92–94 (2006). [CrossRef]

14.

S. K. Selvaraja, E. Sleeckx, M. Schaekers, W. Bogaerts, D. V. Thourhout, P. Dumon, and R. Baets, “Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry,” Opt. Commun. 282(9), 1767–1770 (2009). [CrossRef]

15.

S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO₂-Si-SiO₂-Cu nanoplasmonic waveguides,” Opt. Express 20(6), 5867–5881 (2012). [CrossRef] [PubMed]

16.

J. Song, Y. Z. Li, X. Zhou, and X. Li, “A highly sensitive optical sensor design by integrating a circular-hole defect with an etched diffraction grating spectrometer on an amorphous-silicon photonic chip,” IEEE Photon. J. 4(2), 317–326 (2012). [CrossRef]

17.

J. Kang, Y. Atsumi, M. Oda, T. Amemiya, N. Nishiyama, and S. Arai, “Low-loss amorphous silicon multilayer waveguides vertically stacked on silicon-on-insulator substrate,” Jpn. J. Appl. Phys. 50, 120208 (2011). [CrossRef]

18.

S. C. Mao, S. H. Tao, Y. L. Xu, X. W. Sun, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Low propagation loss SiN optical waveguide prepared by optimal low-hydrogen module,” Opt. Express 16(25), 20809–20816 (2008). [CrossRef] [PubMed]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(230.3990) Optical devices : Micro-optical devices
(230.7370) Optical devices : Waveguides

ToC Category:
Integrated Optics

History
Original Manuscript: July 19, 2012
Revised Manuscript: August 27, 2012
Manuscript Accepted: September 7, 2012
Published: October 1, 2012

Citation
Shiyang Zhu, G. Q. Lo, Weihong Li, and D. L. Kwong, "Effect of cladding layer and subsequent heat treatment on hydrogenated amorphous silicon waveguides," Opt. Express 20, 23676-23683 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-21-23676


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References

  1. A. Harke, M. Krause, and J. Mueller, “Low-loss single mode amorphous silicon waveguides,” Electron. Lett.41(25), 1377–1379 (2005). [CrossRef]
  2. R. A. Street, Hydrogenated Amorphous Silicon (Cambridge University Press, 1991).
  3. K. Narayanan, A. W. Elshaari, and S. F. Preble, “Broadband all-optical modulation in hydrogenated-amorphous silicon waveguides,” Opt. Express18(10), 9809–9814 (2010). [CrossRef] [PubMed]
  4. Y. Shoji, T. Ogasawara, T. Kamei, Y. Sakakibara, S. Suda, K. Kintaka, H. Kawashima, M. Okano, T. Hasama, H. Ishikawa, and M. Mori, “Ultrafast nonlinear effects in hydrogenated amorphous silicon wire waveguide,” Opt. Express18(6), 5668–5673 (2010). [CrossRef] [PubMed]
  5. F. G. Della Corte, S. Rao, G. Coppola, and C. Summonte, “Electro-optical modulation at 1550 nm in an as-deposited hydrogenated amorphous silicon p-i-n waveguiding device,” Opt. Express19(4), 2941–2951 (2011). [CrossRef] [PubMed]
  6. S. Rao, G. Coppola, M. A. Gioffrè, and F. G. Della Corte, “A 2.5 ns switching time Mach-Zehnder modulator in as-deposited a-Si:H,” Opt. Express20(9), 9351–9356 (2012). [CrossRef] [PubMed]
  7. A. Biberman, K. Preston, G. Hendry, N. Sherwood-Droz, J. Chan, and K. Bergman, “Photonic network-on-chip architectures using multilayer deposited silicon materials for high-performance chip multiprocessors,” ACM J. on Emerging Technologies in Computing Systems 7(2), DOI 10.1145 (2011).
  8. R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel, and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer,” Appl. Phys. Lett.94(14), 141108 (2009). [CrossRef]
  9. R. Sun, J. Cheng, J. Michel, and L. Kimerling, “Transparent amorphous silicon channel waveguides and high-Q resonators using a damascene process,” Opt. Lett.34(15), 2378–2380 (2009). [CrossRef] [PubMed]
  10. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Low-loss amorphous silicon wire waveguide for integrated photonics: effect of fabrication process and the thermal stability,” Opt. Express18(24), 25283–25291 (2010). [CrossRef] [PubMed]
  11. P. K. Lim, W. K. Tam, L. F. Yeung, and F. M. Lam, “Effect of hydrogen on dangling bond in a-Si thin film,” J. of Phys.: Conference Series61, 708–712 (2007). [CrossRef]
  12. T. A. Li, F. W. Chen, A. Cuevas, and J. E. Cotter, “Thermal stability of microwave PECVD hydrogenated amorphous silicon as surface passivation for n-type heterojunction solar cells,” European Photovoltaic Solar Energy Conference 2007, ed. Conference Program Committee, WIP-Renewable Energies, Germany, 1326–1331 (2007).
  13. C. J. Arendse, D. Knoesen, and D. T. Britton, “Thermal stability of hot-wire deposited amorphous silicon,” Thin Solid Films501(1-2), 92–94 (2006). [CrossRef]
  14. S. K. Selvaraja, E. Sleeckx, M. Schaekers, W. Bogaerts, D. V. Thourhout, P. Dumon, and R. Baets, “Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry,” Opt. Commun.282(9), 1767–1770 (2009). [CrossRef]
  15. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, “Components for silicon plasmonic nanocircuits based on horizontal Cu-SiO₂-Si-SiO₂-Cu nanoplasmonic waveguides,” Opt. Express20(6), 5867–5881 (2012). [CrossRef] [PubMed]
  16. J. Song, Y. Z. Li, X. Zhou, and X. Li, “A highly sensitive optical sensor design by integrating a circular-hole defect with an etched diffraction grating spectrometer on an amorphous-silicon photonic chip,” IEEE Photon. J.4(2), 317–326 (2012). [CrossRef]
  17. J. Kang, Y. Atsumi, M. Oda, T. Amemiya, N. Nishiyama, and S. Arai, “Low-loss amorphous silicon multilayer waveguides vertically stacked on silicon-on-insulator substrate,” Jpn. J. Appl. Phys.50, 120208 (2011). [CrossRef]
  18. S. C. Mao, S. H. Tao, Y. L. Xu, X. W. Sun, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Low propagation loss SiN optical waveguide prepared by optimal low-hydrogen module,” Opt. Express16(25), 20809–20816 (2008). [CrossRef] [PubMed]

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