Multilayer SiO2-B2O3-Na2O Films on Si for Optical Applications

P1: BHR/SYD Journal of Sol-Gel Science and Technology

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Journal of Sol-Gel Science and Technology 13, 783–787 (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. °

Multilayer SiO2 -B2 O3 -Na2 O Films on Si for Optical Applications N. PELLEGRI∗ E.J.C. DAWNAY†AND E.M. YEATMAN Department of Electrical and Electronic Engineering, Imperial College, London SW7 2BT, UK.

Abstract. The fabrication of glass by the sol-gel technique has attracted considerable attention in microelectronics, optics and other fields. For applications in integrated optics on silicon substrates, sol-gel offers the possibility of a great variety of compositions and structures. For example, the porosity may allow the insertion of a dopant species, such as nano-crystals for optical non-linearity, into the host glass. Here we have investigated SiO2 -B2 O3 -Na2 O compositions, to obtain low process temperatures, and to prepare SiO2 guiding layers with a porosity control using the Vycor glass method. Films of several micron thickness were prepared in many compositions of the SiO2 -B2 O3 -Na2 O system. The process included a two-step method to introduce the water while inhibiting crystal formation, a repetitive spincoating technique in a humidity-controlled chamber, and rapid-thermal-annealing. The results show good quality in terms of homogeneity, absence of cracks, and versatility in the studied compositions. However, films prepared using rapid thermal annealing show a high residual carbon concentration. This causes strong optical absorption, and inhibits the phase separation and leaching associated with the Vycor technique. Reduction of carbon content has been investigated through adaptation of the heat treatment. Keywords: 1.

Vycor, pyrex, porosity, nanocrystals

Introduction

The possibility of manufacturing silica-on-silicon photonic components using sol-gel films and materials giving advanced functionality has been demonstrated, but progress is needed before this technology can be accepted for practical implementation [1]. The sol-gel technique offers potential advantages in that it can be used to produce films with a great variety of composition and structure [2]. Recently, a complete process for the fabrication of SiO2 /Si integrated optic devices using sol-gel has been described [3]. This overcomes the main disadvantage, the difficulty in obtaining sufficient thickness due to shrinkage stresses, by an iterative technique of spin-coating and rapid thermal annealing. The porosity in such materials may allow the insertion of a dopant species, such as nano-crystals for third-order optical non-linearity, into the host glass. ∗ Permanent

address: Laboratorio de Materiales Ceramicos, IFIR, FCEIyA, Universidad Nacional de Rosario, Av. Pellegrini 250, 2000 Rosario, Argentina. † Present address: Bookham Technology Ltd., 90 Milton Park, Abingdon, Oxfordshire OX14 4RY, UK.

Sodium borosilicate glasses constitute an interesting system for this application because of many thermal and chemical properties. In particular, low process temperatures are possible, and porous glasses of nearly pure silica may be obtained by phase seperation and leaching, i.e., the Vycor technique. Previously, sodium borosilicate monoliths and films have been prepared by the sol-gel method using several precursors and processes [4]. Here we have studied whether such materials may be adapted to the thick film fabrication methods of [3], for applications in integrated optics. 2.

Fabrication

Tetraethylorthosilicate (TEOS), triethylborate (TEB) and sodium ethoxide (NaEtox) were used as precursors, and HCl as a catalyst, with compositions and molar ratios as summarized in Table 1. First, TEOS and ethanol were mixed and stirred vigorously for 10 min at room temperature in a molar ratio TEOS/ethanol = 1 : 4. To produce a required pre-hydrolysis of TEOS before the incorporation of the TEB and NaEtox [4], 0.1 M HCl was added gradually until a water to


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Table 1.

Compositions of the solutions. Composition (mol%)

Sample

SiO2

B2 O3

Na2 O

Al2 O3

R

SBN1

70

20

10

—

1/2.15/10

SBN2

85

10

5

—

1/2.96/10

SBN3

65

26

9

—

1/1.92/10

SBN4

80

15

5

—

1/2.67/6

SBN30

62.3

28.9

8.8

—

1/1.81/6

SBN5

67.6

23.3

9.1

—

1/2.04/6

SBNA1

62.7

26.9

6.6

3.5

1/1.83/10

R: Final molar ratio of alkoxides (TEOS + TEB + NaEtox + A1IP/H2 O/EtOH).

TEOS molar ratio of 2 was attained. The solution was placed in the refluxing bath immediately after mixing, and the temperature of the bath increased to 70◦ C in 15 min, while stirring, and kept there for 2 hours. Meanwhile, a solution of TEB and ethanol was prepared and stirred under nitrogen for 30 min. This solution was then added, dropwise, to the TEOS solution under N2 and refluxed for 90 min. The temperature was decreased to room temperature and the nitrogen was removed. A solution of NaEtox, acetic acid and ethanol was prepared and stirred for 30 min; and further added, dropwise, to the TEOS-TEB solution and stirred at room temperature for 1 hour. Then, more water was added with a suitable quantity of ethanol, to yield the desired concentration or film thickness, stirred for 30 min and used for the films’ preparation. All the chemicals were obtained from Aldrich. The sols were dispensed on p-type, 3 in. diameter silicon wafers, through a 0.1 µm filter (PTFE Whatman), and

thereafter the substrate was spun at 2000 or 3000 rpm. for 30 s. For monolayer films the coated substrates were baked in air, at temperatures in the range 100◦ C to 1000◦ C, for 30 min, or in the rapid thermal annealer (RTA) in oxygen at the desired temperature for 10 s. Thick layers were built up by repetition of spin-coating, annealing for 10 s in the RTA at 650◦ C, and cooling by convection [3]. Due to the reactivity of the film with atmospheric humidity, all coatings were made in an atmosphere-controlled chamber, with a relative humidity below 20% [4, 5]. The thickness and refractive index of the samples were measured using a Rudolph AutoEl III ellipsometer, with an operating wavelength ˚ in of 633 nm, and precisions of about 0.002 and 3 A index and thickness, respectively. For microporous films, the measured index is strongly dependent on relative humidity, because of condensation of water in the pores. By measuring the dependence of index on humidity, information about porosity can be obtained [6]. The quality of the films was determined by visual examination and with an optical microscope. To analyze the internal structure and final composition of the films, Infrared (FTIR), Auger and energy dispersion (EDS) spectroscopy were used. 3.

Results and Discussions

Water content and the way in which it is introduced are key parameters in obtaining optical quality films. Crystal formation is observed when all the water is introduced with the TEOS solution, and these crystals grow with heat treatment. Their presence was detected by optical microscopy (Fig. 1) and confirmed by X-Ray

Figure 1. Optical microscope photographs of single-layer coatings on silicon, processed at 1000◦ C for 30 min, of compositions: (a) SBN3 and (b) SBNA1.


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Figure 2. Film refractive index vs. annealing temperature, measured in dry N2 and 100% RH atmospheres, for single layer films on silicon: (a) sample SBN3 annealed in conventional furnace for 30 min and (b) sample SBN4 annealed in RTA for 10 s. Lines are for ease of viewing only.

Diffraction. To avoid this devitrification, the two-step hydrolysis method described above was found to be effective. Another approach is to incorporate Al2 O3 into the glass [7]. We selected aluminum isoproposide in solution with ethanol as the precursor, introduced in the final step of the solution preparation. In Fig. 1 the absence of crystals on an Al2 O3 co-doped sample is shown, in relation with the non-codoped sample. The SiO2 -B2 O3 -Na2 O films obtained by these two methods

showed good quality, in terms of absence of cracks, homogeneity, thickness and versatility in the studied compositions. Using both techniques described above, film thicknesses of several microns were obtained. To determine the optimum heat treatment, evolution of the thickness and the refractive index with annealing temperature was studied for monolayer samples [8]. Figure 2 displays the evolution of refractive index for SBN3


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Figure 3.

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Depth profile through SBN5 sample on silicon vs. electron etching time by the Auger technique.

and SBN4 compositions. Around 600◦ C complete densification of the films is achieved, as compared to >1000◦ C for pure SiO2 films. This reduction in softening temperature greatly eases preparation of waveguides. The second goal was to prepare guiding layers with a porosity control based on Vycor type glasses. We performed a set of experiments to leach the samples of compositions SBN30 and SBN5. Several heat treatments (from 0 to 48 hours) at different temperatures (room temperature, 550, 600 and 650◦ C) were performed to induce phase separation. Then, leaching was attempted with HCl 0.1, 1 and 3 N, for seconds up to several hours, with a further rinse and drying to remove the alkali and boric oxide rich phase [7, 9]. However, refractive index measurements showed that phase separation did not occur. To obtain information about the internal structure of the films, before and after the leaching treatment, we measured FTIR spectra on films of composition SBN30 before and after the heat treatment to induce phase separation, and after the leaching with HCl. The spectra were qualitatively similar. Peaks corresponding to Si O Si vibrational bonds at 1050, and to O Si O at 800 and 470 cm−1 were detected, as were peaks for Si OH vibrations at 960 and 1150 cm−1 . All three spectra show a peak which corresponds to the B O B bonds at 1380 cm−1 , but there is no signal for the B OH

vibration bond [10–12]. There is no clear detection of organic group peaks (C C, C O, etc.). Clearly the internal structure is not affected by the two treatments, so the boron rich phase is not eliminated. Finally, to confirm the existence of a phase separation inhibitor we performed compositional analysis by Auger spectroscopy in several thick samples. The results are semi-quantitative, and the relative elemental compositions should be taken as indicative only. Figure 3 shows the depth profile through an SBN5 sample (2 µm thickness) on silicon as a function of electron etching time by the Auger technique. The maximum etch time (60 min) corresponds to an etch depth of about 1 µm. The remarkable point is the large quantity of carbon in all the studied samples. This appears to decrease with depth, with a corresponding increase in Si and O. A likely cause of this variation is that the etched depth corresponds to about 10 deposited layers, and the lower layers, having undergone more annealing steps, have lost more carbon (some loss of boron with annealing is also suggested). Sodium content appears to drop rapidly with depth, but this we believe is an artefact, caused by charge induced migration of the highly mobile Na+ ions in reponse to the Auger beam itself. We infer that the presence of carbon is responsible for the phase separation inhibition, and for the high optical absorption found in thick samples, which makes


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Multilayer SiO2 -B2 O3 -Na2 O Films

the films unsuitable for waveguides. To eliminate the presence of carbon, several heat treatment experiments have been made. Preliminary results show that a reduction of carbon content, detected by optical quality observations, was obtained using an adapted heat treatment, in which the rate of temperature increase was reduced from 15 to 5◦ C/s. 4.

Conclusions

This study has shown that films of several microns thickness, in many compositions of the SiO2 -B2 O3 Na2 O system, are obtained presenting good quality in terms of homogeneity and absence of cracks. These compositions have the advantage of lowering the necessary process temperatures for the fabrication of planar waveguides. The process included a two-step method to introduce the water while inhibiting crystal formation, a repetitive spin-coating technique in a chamber to control the humidity and rapid-thermalannealing. The films, however, showed a high carbon concentration which causes strong optical absorption, and inhibits the phase separation and the leaching process associated with the Vycor technique. We believe that the reduction of softening temperature to levels approaching those of organic decomposition results in organic ligands being trapped within a fully dense matrix, and therefore decomposing without sufficient diffusion of oxygen into the matrix from the annealing atmosphere. Therefore, rapid thermal processing becomes

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more difficult to implement in such glass compositions. Acknowledgments N.P. is grateful for support from Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina. Support from the European Commission (ACTS project AC047: CAPITAL) is also acknowledged. References 1. E.M. Yeatman and E.J.C. Dawnay, J. Sol-Gel Sci. Tech. 8, 1007 (1997). 2. J.D. Mackenzie and Y.H. Kao, in Glass Integrated Optics and Optical Fiber Devices, SPIE CR53, 83 (1994). 3. R.R.A. Syms and A.S. Holmes, J. Non-Cryst. Solids 170, 223 (1994). 4. N. Pellegri, O. de Sanctis, and A. Duran, J. Sol-Gel Sci. Tech. 2, 519 (1994). 5. N. Tohge and T. Minami, J. Non-Cryst. Solids 112, 432 (1989). 6. E.M. Yeatman, Mino Green, E.J.C. Dawnay, M.A. Fardad, and F. Horowitz, J. Sol-Gel Sci. Tech. 2, 711 (1994). 7. Glass Science and Technology Vol. 1: Glass Forming Systems, edited by D.R. Ulhmann and N.J. Kreidl (Academic Press, NY 1983), p. 179. 8. M.A. Fardad, Ph.D. Thesis, Imperial College, University of London, 1995. 9. J. Rincon and A. Duran, Separacion de Fases en Vidrios (Sociedad Espanola de Ceramica y Vidrio, 1982). 10. D. Niznansky and J.L. Rechspringer, J. Non-Cryst. Solids 180, 191 (1995). 11. R. Almeida, International J. Optoelec. 9, 135 (1994). 12. J. Brinker and D. Haaland, J. Am. Ceram. Soc. 66, 758 (1983).