Low temperature Electron Cyclotron Resonance plasma technique for low loss integrated optics

MIGROI~,IIC'IlIltlNE ELSEVIER

Microelectronic Engineering 53 (2000) 407-410 www.elsevier.nl/locate/ mee

L o w t e m p e r a t u r e electron cyclotron resonance p l a s m a technique for l o w loss integrated optics P. L. Pernas a, E. Ruiza, M. J. Hern~dez a, J. Garridoc, B. J. Garciaa, J. L. Castafio ~, J. M. Requejo b, J. Soils b, R. Semab, C. N. Afonsob and J. Piqueras a a Laboratorio de Microelectr6nica, Departamento de Fisica Aplicada CXII, Universidad Aut6noma de Madrid, 28049 Madrid, Spain b Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain. c E. T. S. de Inform~itica, Universidad Aut6noma de Madrid, 28049, Madrid, Spain. Electron Cyclotron Resonance (ECR) plasma deposition process implies low temperatures at high deposition rates producing uniform and mechanically stable thin films. For these reasons ECR became an attractive tool for integrated optics technology. In this work we have combined ECR with other techniques as Reactive Ion Etching (RIE) and Pulsed Laser Deposition (PLD), in order to develop a new fabrication method of channel waveguides. We report here results on alumina-based strip-loaded waveguides. Amorphous A1203 core of 0.7 ~tm were deposited over Si/SiO2 substrate by PLD technique and overcladded with a 0.5 ram SiO2-ECR film. Standard UV-photolitographic techniques and RIE were used to define a set of strips on the SiO2. These strips give the additional confinement of the light in the AI203 core. The optical losses at 633 nm were measured using an imaging technique. The relative scattered light power as function of the propagation along the channels shows a maximum optical loss of 4.5 dB/cm.

1. INTRODUCTION Rare-earth (RE) doped dielectric materials have attracted a great deal of attention because of the possibility to develop integrated optical circuits and more general photonic devices. Channel waveguides in different host materials offer the possibility that active components can all be integrated in the same substrate using photolithographic techniques [1]. Results in RE-doped waveguide active devices fabricated by different techniques suggest the importance of controlling with great precision the principal optical material parameters as emission cross-section, optimal dopant concentration and quenching, scattering losses, reabsorption losses and radiative lifetime of the transition. This knowledge enables to determine the prior suitability of each material as an active media because we can increase significantly the efficiency of the device by increasing the effective gain, tailoring the dopant concentration and refractive index profiles [2].

Erbium (Er) has shown to be an interesting dopant for these devices because its 4f transition around 1.53 ~tm coincides with the eye safe and low loss infrared window of optical fibers. Because of the relatively small emission cross sections for these Er3+ ions and its relatively long first excited state lifetime, reasonable values of total optical gain can only be reached if the signal encounters a large amount of excited ions. Therefore we can say that integrated devices require high dopant concentrations because the device length must be short. But at high concentrations the distance between Ers+ ions is very small and ion-ion interactions become important. Accurate numerical models are also needed taking to account also the excited state absorption (ESA) and up-conversion processes, because these phenomena degrade prohibitively the emission signal at 1.54 ~tm [3]. This basic limitations imposed by physics has forced to the material science in to obtain new host materials and fabrication technologies where the active ions could be introduced in a very controlled

0167-9317/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0167-9317(00)00344-0

408

P.L. Pernas et al. / Microelectronic Engineering 53 (2000) 4 0 7 - 4 1 0

way without increasing substrate damage. Furthermore, the fabrication technique must be flexible and compatible with standard silicon process techniques to transfer properly the pattern of waveguide structures as couplers, splitters, amplifiers and lasers. In this paper we report a new fabrication method of strip-loaded waveguides based on amorphous A1203 and SiO2 films deposited by Pulsed Laser Deposition (PLD) and Electron Cyclotron Resonance (ECR) plasma. In these devices ECRSlOE strips give to the PLD-A1203 plane waveguide the additional lateral optical confinement [4]. 2. EXPERIMENTAL RESULTS AND DISCUSSION PLD is a recently, simple and low cost stoichiometric technology that produces high density films with good adhesion and it has been successfully applied to produce a variety of glasses [5-7]. A1203 material is an interesting host for Er because its relative high refractive index respect to SiO2 cladding allows for high confinement of the optical mode. Furthermore, the similarity in valence between A1203 and ErO3 allows for high concentrations in the crystal structure [8]. With PLD technique we can control the incorporation of Er ions during growth by alternative ablation of Er and A1203 targets in a single step process. The final Er concentration profile of the A1203 film depends on the number of pulses at the Er target and this is very convenient for active a waveguide device [9-10]. Considering that the minimization of the temperature in any waveguide fabrication method is important to reduce propagation losses, Low Pressure Chemical Vapor Deposition (LPCVD) SiO2 films, that implies temperatures of about 700°C are discarded. Alternatively, ECR plasma deposition technique have attained a high degree of reliability for passivation coatings and isolation layers in VLSI technology, and implies low temperatures at high deposition rates producing uniform and mechanically stable thin films. The resulting surface damage or structural changes produced in the fabrication of the waveguide are in straightforward relation with the surface scattering losses and they are generally the dominant loss mechanism in dielectric waveguides because light waves interact strongly with the surface and could be easily scattered by defects. During the deposition with

ECR technique the sample is maintained at temperatures below 300°C in a plasma that is characterized by the low energy of the ions (between 10 to 20 eV) resulting in the absence of surface damage [11-12]. The initial stage of the waveguide fabrication starts with the careful cleaning of the Si wafers; followed by the ECR plasma deposition of the SiO2 cladding from 5% Ar diluted Sill4 and 0 2 precursor gases. Considering the high index difference between the two materials, nsi02 =1.46 and nA/2o3 = 1.64, a cladding thickness ti = 0.5 ktm has

been proved adequate to avoid wave propagation into the Si wafer. The typical deposition time for this thickness is about 10 min using a microwave power of 1 kW. Laser ablation is carried out by focussing an ArF laser (~ =12 ns, ~=193 nm) at 5 Hz onto the targets with an energy density of 1-3 J/cm2. The experiment is carried out in a vacuum chamber (base pressure of 4x10-7 Ton') or in oxygen up to pressures of 10"1 Torr. An He-Ne laser beam (633 nm) is used to record in real time the evolution of refle,;tivity of the film as it grows, which allows mollitoring the sample thickness and deposition rate. We have deposited a thickness t2= 0.7 ~tm of PLD-A1203 [10].

ECRSlOz

n3= 1"46

~

It3 .0.6~=m

ECRSiO 2

'::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: = 146 ;':'::':':+:':':'; :':+:':+:+:'::':::+: :+: ~ tl =06~m • //////////////////s

Sl

Figure 1. Schematic cross section of the waveguide structure. The lateral confinement was achieved with the so-called strip-loaded method that consists in depositing an over-cladding film of ECR-SiOv The strips of this material change the effective refractive index inside the core of A1203 producing a convenient lateral optical confinement. Previous finite difference calculations where performed in order to obtain monomode waveguides at 1.5 lam, with a minimum thickness. From these results t3= 0.5 p.m has been proved adequate. Figure 1 shows

409

P.L. Pernas et al. / Microelectronic Engineering 53 (2000) 407-410

the schematic cross section of the waveguide fabrication method. To define the strips on this final cladding, standard UV-photolithographic techniques were used. The pattern was obtained from an e-beam processed Cr203 quartz mask consisting in a single block of six lines of different widths, w = 20, 10, 6, 4, 2 and 1 ~tm with a pitch of 50 ~tm. This block is periodically repeated across the mask with a separation of 100 lam. A positive photoresist deposited and spun at 6000 rpm with pre-bake of 15 min at 90°C were performed. The exposure time of the photoresist was approximately 30 s of UV mercury lamp. After that, a post-bake of 15 min is performed in order to eliminate the effect of standing waves in the photoresist film. The development time for the activated photoresist is around 10 s. SiO2 is finally etched by Reactive Ion Etching (RIE), using CHF3 and SF6 gases, with a RF power of 80 W. The time required to etch the 0.5 ~tm of SiO2 film is around 10 min. Figure 2 shows a SEM image of the set of six waveguides at the cleaved edge of the Si-wafer.

InGaAs detector. The 514.5 nm line of an Ar ÷ laser was used as excitation source. Figure 3 shows the PL spectrum of the waveguide film. This spectrum is broad, typical of a glass matrix and shows a peak at 1.53 p.m, confirming the presence of optical active Er ions [10].

12 '-1

8

_z, t-

t,'.n

4

.T= ,-I

0 1,4

1,5

1,6

1,7

Wavelength (lam) Figure 3. Room temperature PL spectrum of an

A1203plane waveguide doped with Er dvring PLD.

~. 3x10 4

• 0 Q-2x10

7= |

Experimentaldata

I

Exponential fitting

4

0

(0 l x 1 0 ' . I1) o.oo

n-

. . .

I

o.os

. . . .

I

o.lo

. . . .

I

. . . .

o.15

020

Propagation length (cm)

Figure 2. Cleaved edge of the SiO2 strips and A1203 substrate showing the set of six channels waveguides of 20, 10, 6, 4, 2 and 1 microns.

Figure 4. Relative scattered light power as function of propagation in the waveguide.

The characterization of the A1203 plane waveguides has been performed at 633 nm by using dark-mode method [4]. From these measurements the thickness and the refractive index were obtained. Room temperature photoluminescence measurements of the E r 4113/2-~4115/2transition (first excited state) were performed on doped plane waveguides using a single monochromator and an

It is worth mentioning that these waveguides were successfully end-fire coupled with an He-Ne laser (~.=633nm) and microscope objectives at both sides• The optical losses at 633 nm were measured using an imaging technique. The relative scattered light power as function of the propagation in the channel is presented in figure 4. From this data a maximum optical loss of 4.5 dB/cm has been determined. Future work includes identifying the

410

P.L. Pernas et al. / Microelectronic Engineering 53 (2000) 407-410

relation between scattering losses and the width w of the stripe in the near infrared region. Lower scattering losses are expected in the thinnest waveguides at 3.=1.53 lam. 3. CONCLUSIONS We have demonstrated a new fabrication method of strip-loaded waveguides on A1203 and SiO2 materials by a low temperature process adapted from microelectronic technology. The obtained waveguides combine the advantages of Pulsed Laser Deposition technique to growth RE doped A1203 thin films and Electron Cyclotron Resonance plasma Silicon Dioxide films. The SiO2 film is deposited at very low temperature in short times and is easily etched by RIE technique. SEM images were used for waveguide characterization and low surface damage was observed. The high refractive index difference between core and cladding allows for high confinement of the optical mode. Low linear losses were measured along the waveguides and the relative scattered light power as function of the propagation along the channel shows an optical loss of 4.5 dB/cm. All these properties, as well as the flexibility in the pattern transfer, make this a promising fabrication technique for active integrated optics. Further studies are in progress to improve the performance of these waveguides.

Acknowledgement This work was partially supported by CICyT projects TIC96-0467 and MAT98-0823-C0303.

REFERENCES 1. A. Grisard, E. Lallier, G. Garry and P. Aubert, IEEE J. Quantum Electron. 33 No.10 (1997) 1627. 2. F. Di Pasquale, M. Zoboli, M. Federighi and I. Massarek, IEEE J. Quantum Electron. 30 No.5 (1994) 1277. 3. E. Snoeks, G. N. van den Hoven and A. Polman, IEEE J. Quantum Electron. 32 No.9 (1996) 1680. 4. T. Tamir (eds.), Integrated Optics, Topics in Applied Physics Vol.7, Springer-Verlag Berlin Heidelberg New York, 1982. 5. K.E. Youden, T. Grevatt, R. W. Eason, H. N. Rutt, R. S. Deol, and Wylangowski, Appl. Phys. Lett. 63 (1993) 1601. 6. S. Ohtsuka, T. Koyama, K. Tsunetomo, H. Nagata, and S. Tanaka, Appl. Phys. Lett. 61 (1992) 2953. 7. C. N. Afonso, in: F. Agull6-L6pez (eds.), Insulating Materials for Optoelectronics, World Scientific, Singapore, 1995. 8. A. Polman, J. Appl. Phys. 82 (1997) 1. 9. R. Serna and C. N. Afonso, Appl. Phys. Lett. 69 (1996) 1541. 10. R. Serna, C. N. Afonso, J. M. Ballesteros, A. Zschocke, Appl. Surf. Sci. 109/110 (1997) 524. 11. M. J. Hern~lndez,J. Garrido, and J. Piqueras, J. Vac. Sci. Technol. B 12 (1994) 581. 12. M. J. Hermindez, J. Garrido, J. Martinez and J. Piqueras, Semicond. Sci. Technol. 11 (1996) 422.