A comparison between PECVD and ALD for the fabrication of slot waveguide based sensors

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1 A comparison between PECVD and ALD for the fabrication of slot waveguide based sensors Grégory Pandraud* a, Agung Purniawan b, Eduardo Margallo-Balbás c and Pasqualina M. Sarro a a Laboratory of Electronic Components, Materials and Technology, TU Delft, Feldmannweg 17, 2628CT, Delft The Netherlands b Electronic Instrumentation Laboratory, TU Delft, Mekelweg 4, 2628CD, Delft, The Netherlands c Medlumics, C/Cidro, Madrid, Spain *g.pandraud@tudelft.nl; ABSTRACT We fabricated horizontal slot waveguides using two low temperature deposition techniques ensuring the full compatibility of the processes with CMOS technology. Slots width as thin as 45 nm with smooth slot surfaces can easily be fabricated with simple photolithographic steps. Fundamental TM-like slot mode in which the E-field is greatly enhanced within slot showed a 23.9 db/cm and a 18 db/cm in a PECVD SiC/SiO 2 /SiC and a ALD TiO 2 /Al 2 O 3 /TiO 2 vertical slot waveguide, respectively. Keywords: PECVD, ALD, Silicon Carbide, TiO 2, Slot Waveguides, Sensing. INTRODUCTION Slot waveguides enable high confinement of light in low-index regions (slot material of Fig.1) of small dimensions and are very sensitive to either cover medium refractive index change or deposited receptor layer thickness increase [1]. They are therefore ideal candidate for lab on chips applications or in microfluidics when optical signals are required. Up to date, most of the reported work on the subject focused on producing the slot waveguides with very advanced lithography and on the use of Si as confining material (slab material in Fig.1). However the choice of silicon to explore sensing opportunities in water [2] is not the best because of the absorption the material and the choice of expensive production techniques does not match the price requirements of the sensor industry. Recently work has been carried out to extend the working wavelength range of slot waveguides in the visible by using, for example, TiO 2. A theoretical study [3] showed the potential of such layer. LPCVD SiN has also been used in sensors [4] but such a high temperature process is not compatible with state of the art light detector (i.e. CMOS). In this paper, we present the first results obtained with ALD TiO 2. We also compare it with another promising layer, SiC, also deposited at a relatively low temperature. By choosing Plasma Enhanced Chemical Vapor Deposited (PECVD) SiC or Atomic Layer Deposited (ALD) TiO 2, the size of the slot region becomes higher (compare to silicon) due to a weaker optical confinement [5] making the filling of the slot waveguide and the design of the fluidics for sensing applications easier. SiC has in this case a superior advantage as it is a stronger material and can easily find applications in harsh environments. SiC and ALD layers have the potential to be doped with metals to produce active layers offering the possibility of on chip amplification [6]. Further by using vertical slot waveguides (Fig.1) we increase the tolerance to dimensions deviations during fabrication and do not require the etching of the high refractive index region limiting then the scattering losses due to roughness induced by the etching. Nanophotonics IV, edited by David L. Andrews, Jean-Michel Nunzi, Andreas Ostendorf, Proc. of SPIE Vol. 8424, 84242P 2012 SPIE CCC code: X/12/$18 doi: / Proc. of SPIE Vol P-1

2 In the paper, we report a 23.9 db/cm and a 18 db/cm for the quasi-tm mode in a PECVD SiC/SiO 2 /SiC and a ALD TiO 2 /Al 2 O 3 /TiO 2 vertical slot waveguide, respectively. The roughness of the PECVD layers limit however their use to large slot waveguide. Slots as small as 45 nm can be fabricated using ALD making them attractive for highly sensitive optical biosensors [7]. SiO 2 top cladding W slab Wslot SiO 2 substrate Fig.1Schematic of an horizontal slot waveguide. PECVD SiC SLOT WAVEGUIDES The refractive index and the absorption of a 100 nm SiC film on Si are presented in Fig. 1. The index equals 2.4 over a large wavelength range. There is no absorption from 600 nm and previous work on PECVD SiC waveguides has shown reasonably low losses in the visible (5 db/cm) [8] and in the near infrared [9]. It is therefore possible to have sensors based on slot waveguides working in a very large wavelength range and specifically in the visible, as required for sensing in water. Refractive index (n) 3,1 2,9 2,7 2,5 2,3 1 0,8 0,6 0,4 0, Wavelength (nm) Absorption (k) 1) Fabrication Fig. 2. Refractive index and absorption of PECVD SiC obtained by ellipsometry. We first deposited a 2 µm thick PECVD SiO 2 layer on a standard <100> Si wafer. The oxide serves to optically isolate the waveguide from the substrate and reduce the loss due to substrate leakage. Then a 118 nm thick PECVD SiC layer is deposited using silane and methane. A 268 nm SiO 2 is then deposited to form the slot and again 118 nm SiC are deposited. Finally a 2 µm thick PECVD SiO 2 layer covers the stack to make a perfectly symmetrical structure. As we use here as confining material a layer with a lower Proc. of SPIE Vol P-2

3 refractive index than Si, the SiC/ SiO 2 system has much higher tolerance in the final thickness of the layers. The confinement factor (the part of the incoming light traveling in the slot itself) computed with the deposited thicknesses (see Fig. 2) was found to be at The temperature of deposition of all the layers is 400 ºC ensuring compatibility with CMOS processes. To ensure a horizontal confinement we then patterned the whole stack with a mask containing 2.8 µm wide waveguides. The stack is then etched down to the first oxide layer by an Inductively Coupled Plasma process. Fig. 3 shows a scanning electron microscope (SEM) view of the fabricated slot waveguide. 118 nm SiC 268 nm SiO nm SiC Fig. 3 SEM view of the fabricated SiO 2 /SiC/SiO 2 vertical slot waveguide with the dimensions of deposited layers. 2) Propagation losses The SiO 2 /SiC/SiO 2 slot waveguides were characterised in terms of losses using the cutback method. The technique consists of measuring the total insertion loss caused by the waveguide for different waveguide lengths. By doing so, it is possible to isolate the contributions from the propagation in the waveguide and the total coupling losses. The characterization was done at 1.3 µm using a fiber coupled superluminescent diode (SLD). Light was sent into the waveguide and collected at the output using butt-coupled standard single-mode fibres (SMF28). The optical bench used here is depicted elsewhere [10]. The results for both the quasi-te and the quasi-tm modes are presented in Fig. 4. As expected, the quasi-tm mode gives the lowest losses (23.9 db/cm), while the quasi-te mode has higher losses (38.8 db/cm). Error bars have been added to show the consistency of the measurements. -30 Loss (db) ,4 0,6 0,8 1 1,2 Length (cm ) Fig. 4. Measured transmission losses (squares TE circles TM) Proc. of SPIE Vol P-3

4 ALD TiO2 SLOT WAVEGUIDES The refractive index and the absorption of an ALD TiO2 film on Si are presented in Fig. 5. Like the PECVD SiC used in the previous section the refractive index stays above 2.4 over a large wavelength range and the layer doesn t show any absorption in the visible. 1) Fabrication Fig. 5. Refractive index and absorption of ALD TiO 2 obtained by ellipsometry. To fabricate the slot presented in Fig. 6, a 130 nm thick ALD TiO 2 layer is deposited on 2 μm thick thermal SiO 2 on Si. A 45 nm ALD Al 2 O 3 layer forms the slot followed by again a 130 nm TiO 2 layer. Finally a 2 μm thick PECVD SiO 2 layer covers the stack to make a perfectly symmetrical structure. Like for the PECVD SiC to ensure a horizontal confinement we then patterned the whole stack with the same mask containing 2.8 μm wide lines. In the following section, we measured the optical losses of such waveguides and for that we defined by cleavage sections of different lengths. TiO 2 Al 2 O 3 Fig. 6 SEM view of the fabricated the TiO 2 /Al 2 O 3 /TiO 2 slot waveguides. W slot being 45 nm here. Proc. of SPIE Vol P-4

5 2) Propagation losses The cut back technique is here also applied using the set up briefly described in the previous section. The measurements are shown in Fig. 7 and the losses found to be 18 db/cm for the quasi-tm mode. Measurements were repeated in three sets of waveguides. Fig. 7 Measured transmission losses in a TiO 2 /Al 2 O 3 /TiO 2 slot waveguide (W slot =45 nm) for the quasi-tm mode. DISCUSSION Fig. 8 presents the electric field of the SiC/SiO 2 /SiC (quasi-tm mode) slot waveguides measured in this work. A similar field can be obtained for the ALD TiO 2 /Al 2 O 3 /TiO 2 slot waveguide. It shows that a very important part of the field is at the interface between the high and low refractive index regions. Roughness is known to play a significant role in optical losses and similarly we can also see that it does play also a key role in the measured structures. To show the effect of roughness in PECVD SiC slot waveguides we devided W slot by 2 and measured the waveguides again. A loss value larger than 40 db/cm was found for the quasi-tm mode setting the minimum slot width for the PECVD waveguide to 268 nm. Fig. 8. Field of the fundamental quasi-tm polarized slot mode at 1300 nm as calculated using [11] and the schematic cross section view of the simulated slot. Proc. of SPIE Vol P-5

6 The ALD TiO 2 /Al 2 O 3 /TiO 2 based slot waveguides can be used with slot widths down to 45 nm (see previous section) with acceptable losses. Such thickness compares to the ones commonly used in biosensing [7] applications However the absorption efficiency of bio materials is expected to be lower in such thin slot due to capillary filling effect [12]. To avoid such limitations the ALD slot waveguides presented her can be efficiently used in cantilever based sensors where (the field being concentrated in the 45 nm slot) a high sensitivity is expected [13]. CONCLUSIONS We have shown that the choice of material to fabricate slot waveguides could be extended to PECVD and ALD layers. In the work we focused on SiC and TiO 2 that have high refractive indexes and we found propagation losses of 23.9 db/cm and 18 db/cm for the quasi-tm mode in a PECVD SiC/SiO 2 /SiC and a ALD TiO 2 /Al 2 O 3 /TiO 2 vertical slot waveguide, respectively. Such values are in agreement with reported losses using SiN (20 db/cm) [7] making the structures investigated in this work applicable for biosensors. However if capillary filling is employed SiC should be preferred as it allows much larger slot width while the extremely concentrated field in atio 2 based slot can find application in cantilever based sensor or in ring resonators. ACKNOWLEDGMENT The authors would like to acknowledge useful discussions with C. de Boer of the DIMES Technology Center. They are also grateful to A. Barbosa Neira for simulating the different slot waveguides. REFERENCES [1] F. Dell Olio, V. M. N. Passaro, Optical sensing by optimized silicon slot waveguides, Opt. Express, 15, (2007). [2] A. H. J. Yang, S. D. Moore, B. S. Schmidt, M. Klug, M. Lipson and David Erickson, Optical manipulation of nanoparticles and biomolecules in sub-wavelength slot waveguides, Nature, 457, 71-75, (2009). [3] G. Testa and R. Bernini, Slot and layer-slot waveguide in the visible spectrum, J. of Light. Technol., 29, (2011). [4] C. A. Barrios, K. B. Gylfason, B. Sánchez, A. Griol, H. Sohlström, M. Holgado,and R. Casquel, Slotwaveguide biochemical sensor Opt. Letters, 32, , (2007). [5] C. A. Barrios, B. Sanchez, K.B. Gylfason, A. Griol, H. Sohlstrom, M. Holgado, R. Gasquel, Demonstration of slot-waveguide structures on silicon nitride/silicon oxide platform, Opt. Express, 15, , (2007). [6] K. Wörhoff, J.D.B. Bradley, F. Ay, D. Geskus, T.P. Blauwendraat, and M. Pollnau, Reliable low-cost fabrication of low-loss Al 2 O 3 :Er 3+ waveguides with 5.4-dB optical gain IEEE J. Quantum Electron., 45, , (2009). [7] S. Lee, S. C. Eom, J. S. Chang, C. Huh, G. Y. Sung, and J. H. Shin, "Label-free optical biosensing using a horizontal air-slot SiNx microdisk resonator", Opt. Express 18, 20638, (2010). [8] G. Pandraud, P.M. Sarro and P.J. French, Fabrication and characteristics of a PECVD SiC evanescent wave optical sensor, Sens. and Act. A 142, 61-66, (2008). [9] G. Pandraud, E. Margallo-Balbas, Chung-Kai Yang and P.J. French, Experimental Characterization of Roughness Induced Scattering Losses in PECVD SiC Waveguides, J. Light. Technol. 29, , (2011). [10] G. Pandraud, P.M. Sarro and P.J. French, PDL free plasma enhanced chemical vapor deposition SiC optical waveguides and devices, Opt. Com. 269, , (2007). [11] S. G. Johnson and J. D. Joannopoulos, "Block-iterative frequency-domain methods for Maxwell's equations in a planewave basis", Opt. Express 8, , (2001). Proc. of SPIE Vol P-6

7 [12] A.V. Pesse, G.R. Warrierand V.K Dhir An experimental study of gas entrapment process in closedend microchannels, Int. J. Heat Mass Transfer 48, (2005). [13] A. Purniawan, P.J. French, G. Pandraud, Y. Huang and P.M. Sarro, All ALD TiO2-Al2O3-TiO2 horizontal slot waveguides for optical sensing, Proc. IEEE Sensors, , (2011) Proc. of SPIE Vol P-7