Vapour HF release of airgap-based UV-visible optical filters

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Procedia Engineering 120 ( 2015 ) 816 – 819

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015 doi: 10.1016/j.proeng.2015.08.674

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EUROSENSORS 2015

Vapour HF release of airgap-based UV-visible optical filters

M. Ghaderi*, N. P. Ayerden, G. de Graaf, R. F. Wolffenbuttel

Electronic Instrumentation Laboratory, Microelectronics Department, Faculty of EEMCS, Delft University of Technology, Delft, Netherlands

Abstract

The design and CMOS-compatible fabrication of airgap-based optical filters in a surface micromachining process with sacrificial release using the vapour phase is presented. An airgap-dielectric layer combination offers a higher refractive index contrast, as compared to the conventional all-dielectric layer based filters, which results in a higher peak reflectance and a wider bandwidth in a distributed Bragg reflector (DBR). Several DBRs and Fabry-Perot filters with multiple (low-n) airgap layers separating a stack of (high-n) layers of a dielectric material were designed for operation in the UV-visible spectrum. The fabrication was based on an Al2O3 and SiO2 system. The lateral design includes a set of anchor points and access holes. Finally, a vapour phase HF etching was used to remove the SiO2 layers and release the Al2O3 membranes. This method prevents the stiction of the membranes and provides a higher control and uniformity over the filter area.

Peer-review under responsibility of the organizing committee of EUROSENSORS 2015.

Keywords: Distributed Bragg reflector, airgap filters, Fabry-Perot filters, vapour HF etching, UV-visible range.

1. Introduction

Airgap-based optical filters have been implemented in various optical devices. The conventional multi-layered optical filters (OF) are made of an alternating layers of high and low index materials and the refractive index contrast sets the optical quality of the filter. In a distributed Bragg reflector for instance, both peak reflectance and bandwidth depend on the refractive index contrast between the high/low-index materials and the number of the pairs [2]. Using air as a low-index material in such applications provides a higher contrast, as compared to all-dielectric ones. Consequently, a reduced number of pairs are needed to achieve a particular optical performance. Furthermore, the negligible absorption in the air over a wide spectrum offers a higher optical quality in the resulting filters.

The OFs designed for the mid-infrared have layer infrared provides a promising airgap-based optical filter structure. In the UV-visible range, the available CMOS-compatible optical materials with negligible or low optical absorption is limited. The low refractive index contrast motivates the use of airgap-based OFs. The optical design of

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such an airgap-based filter requires layer thicknesses in the order of 100 nm. The etching selectivity and preventing the stiction subsequent to the release makes the fabrication process more challenging. The fabrication of the airgap-based DBRs using electrochemical etching has been investigated for GaN photonics [3]. However, the low etch rate of the sacrificial material limits the active area in these devices.

In reported research so far, the CMOS-compatible fabrication of large-area DBRs using the polySi and SiO2

combination was addressed. A modified sacrificial etching with a TMAH-based etchant followed by supercritical drying was applied to release the structures, while preventing stiction [4, 5]. However, these methods were found to have a limited compatibility with organic materials and a low fabrication yield. This work presents our work designs and preliminary results on the fabrication of airgap-based OFs based on vapour HF etching technique.

2. Optical design and fabrication

Alumina (Al2O3) has a high refractive index and was chosen as the optical and structural layer. The layers were

deposited using reactive RF sputtering (FHR MS150). The refractive index and absorption coefficient of the layers were measured using variable angle spectroscopic ellipsometry and the results are presented in Figure 1. Accordingly, an alumina-air layer pair provides a refractive index contrast of ሺ݊ଵଶെ ݊ଶଶሻ ʹ݊Τ ଵଶ؆ ͲǤ͸ (at 400 nm)

which gives a FWHM of more than 200 nm for a distributed Bragg reflector centred at 400 nm. Furthermore, the optical absorption in the film was below 0.002 which results in a less than 2% absorption loss in the distributed Bragg reflector. The acquired optical data was subsequently used for the design of a Fabry-Perot filter. Table 1 lists the layer configuration of a Fabry-Perot filter centred at 400 nm with an FWHM of 26.8 nm. The mirror reflectance and the transmittance of the resulting filter is shown in Figure 2.

Figure 1. The refractive index of sputtered Al2O3, measured by variable angle spectral ellipsometry (J.A. Woollam, NE, USA).

RF sputtered SiO2 layers were selected as the sacrificial layers and a layer stack according to the Table 1 was

deposited. Both SiO2 and alumina were alternatingly deposited without breaking the vacuum, respectively as the

sacrificial and structural materials. An array of access holes was patterned and etched through the layer stack. Depositing a polysilicon layer with LPCVD (at 575° C) filled the holes. The polysilicon was subsequently removed from the active area, leaving a pin-shape structure into the holes that mechanically support the membranes after the release. The pin-shape anchors can support multiple membrane layers. Another set of access holes were patterned and etched and was used for the sacrificial etching. Figure 3 shows the fabrication process used to define the structures and access holes for the sacrificial release. HF vapour etching offers a high selectivity between etching SiO2 and alumina resulting a promising optical device. The wafers were etched in a vapour etching system (uEtch,

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Figure 2. a. Expected reflectance of a DBR with two airgap-Al2O3 pairs on glass substrate and b. Expected transmittance of a Fabry-Perot using two similar DBRs. The Fabry-Perot cavity width is 200 nm. The simulations were carried out using TFCalc, (Software Spectra, OR, USA). Table 1. Layer listing for a Fabry-Perot filter centred at 400 nm. A 200 nm air layer acts as the resonator layer between two airgap based DBRs.

# Layer Thickness [nm] - Glass substrate - 1 D BR # 1 Airgap 100 2 Al2O3 62 3 Airgap 100 4 Al2O3 62 5 Airgap 200 6 D BR # 2 Al2O3 62 7 Airgap 100 8 Al2O3 62

Figure 3. The fabrication process: (a) Layer deposition, (b) masking and pin-hole opening, (c) polySi deposition (Anchor pins), (d) masking and access hole opening, and (e) sacrificial etching. PolySi is opaque in UV-visible and as a very low etch rate in vapour HF and was used as the anchoring material.

3. Preliminary results

The vapour phase etching of the SiO2 layers were performed and the each rate was measured to be about

200 nm/min, while no etching of the alumina layers were observed. Several samples were fabricated according to the optical design and the alumina layers were sacrificially released. Figure 4 shows the resulted structures after 1

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hour of sacrificial etching. While the etch rate for the first few microns was as was expected, it appeared that the etch rate decreased significantly afterwards. The very thin gap between the membrane (100 nm) and possible residual stress-induced buckling is suspected to contribute to this effect.

Figure 4. SEM image of a topside of the device. The structure is an array of hexagon-shape membrane cells anchored at the center. The enlarged image of the access hole is shown in the right.

4. Conclusion

This work presented the fabrication of airgap-based optical filters using vapour phase etching of the sacrificial SiO2 layer. The etch rates of SiO2 and Al2O3 in the vapour HF results in the adequate selectivity for this application.

Furthermore, Al2O3 has a relatively high refractive index and a low absorption in the UV-visible spectrum. The

typical thickness of the airgap layer in UV-Visible optical designs is in the order of 100 nm in which the vapour HF etching provides the possibility of achieving stiction free membranes. The preliminary results show that, despite the high etch selectivity to the alumina layers, the fabrication process must be optimized to achieve the large area openings needed for the MEMS large area OF design. Further studies are aiming at comparing and optimizing the sacrificial etching of the membranes and on the optical characterization of the filters.

Acknowledgement

This work has been supported by the Dutch Technology Foundation STW under Grant DEL.11476. The process was carried out in the Dimes technology center Delft.

References

[1] M. Tuohiniemi and M. Blomberg, "Surface-micromachined silicon air-gap Bragg reflector for thermal infrared," Journal of Micromechanics and Microengineering, vol. 21, p. 075014, 2011.

[2] H. A. Macleod, "Multilayer High-Reflectance Coatings," in Thin-Film Optical Filters, Fourth Edition ed: CRC Press, 2010, pp. 209-239. [3] W. Tao, "MOCVD Growth of Nitride DBRs for Optoelectronics," in Handbook of Optical Microcavities, ed: Pan Stanford, 2014, pp.

157-210.

[4] M. Ghaderi and R. F. Wolffenbuttel, "Design and fabrication of multiple airgap-based visible filters," in SPIE Photonics Europe, 2014, pp. 913005-913005-8.

[5] M. Ghaderi, N. P. Ayerden, G. de Graaf, and R. F. Wolffenbuttel, "Surface-micromachined Bragg Reflectors Based on Multiple Airgap/SiO2 Layers for CMOS-compatible Fabry-perot Filters in the UV-visible Spectral Range," Procedia Engineering, vol. 87, pp. 1533-1536, // 2014.