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• Abstract

SECTION I

## INTRODUCTION

THE characteristics of a laser system are strongly influenced by the response of its out-coupler (OC) components. Highly reflective narrowband mirrors are crucial for the stability of high-power laser systems. Typically, Distributed Bragg Reflectors (DBR) have been used as out-couplers, relying on constructive interference from alternating layers of high and low-index materials [1]. Alternatively, guided-mode resonance (GMR) may be exploited to create a narrowband reflectance at the desired spectral location with just a few layers of material [2]. However, conventional GMR filters place the grating as the top feature, exposing it to surface abrasion or contamination. This severely limits the applications of these designs to those which can withstand surface gratings. In order to protect the grating and reduce the variation in materials, quasi-monolithic high-contrast gratings have been introduced, encapsulating air pockets in a semiconductor substrate [3]. Other examples have embedded inverse-tone high contrast gratings to replace DBR layers in surface-emitting laser devices [4]. However, silicon-based structures have a large thermo-optic coefficient, dn/dT, and are not optimal for high power applications. Other designs for narrow linewidths have been demonstrated with monolithic low-contrast materials [5], [6]. Control over polarization sensitivity was demonstrated by implementing a hexagonal unit cell in a similar geometry based on ${\rm SiO}_{2}$ [7]. While the grating is protected, these designs leave the waveguide exposed and the index contrast is limited by the substrate material. Another method of protecting the grating is with epitaxial deposition above the structure, allowing it to planarize above the grating [8]. If the grating is coated conformally, the top of the grating may be selectively etched to expose the top of the grating ridges [9]. This approach still requires a thick layer to make sure the evanescent fields are within the cladding region. In recent work, wafer bonding techniques that encapsulate a Littrow grating composed of air holes/grooves in fused silica have been used for high reflection grating mirrors [10].

In this letter, we introduce a fabrication technique that leverages off of the conformality of Atomic Layer Deposition (ALD) to deposit films onto patterned substrates, which can then be selectively etched and bonded to form encapsulated grating structures as shown in Fig. 1. Compared to previous work, the use of low-index materials allows us to obtain a narrowband resonance while utilizing materials that can be used in high power systems. Partially filling the grating trenches adds control over the effective refractive index of the waveguide grating, therefore better controlling the resonance profile. The fabricated devices demonstrate a strong, polarization sensitive resonance while protecting the waveguide and grating from external damage.

Fig. 1. The formation and encapsulation process for an encapsulated waveguide grating is illustrated. An etched structure (a) is conformally coated using ALD deposition (b, c). The deposition atop the grating ridges is selectively etched away (d) allowing for the device to be bonded to another material (e).
SECTION II

## DESIGN

In order to demonstrate the concept, a baseline material system was selected based on fused silica for the substrate and superstrate. The interface is an etched linear grating that is subsequently backfilled with alumina $({\rm Al}_{2}{\rm O}_{3})$ and etched away as highlighted in Fig. 1. In order to optimize this configuration, Rigorous Coupled-Wave Analysis (RCWA) was used [11]. Results confirmed a strong, narrowband resonance unique to the TE polarization for a linear grating geometry, and no noticeable resonance in the orthogonal polarization. The spectral response is shown in Fig. 2. The encapsulated grating has a lateral period, depth, and duty cycle of 950 nm, 900 nm, and 32% respectively. This achieved a 100% reflection in TE polarization at 1550 nm, with ${<}{\rm 1} {\rm nm}$ full-width at half-maximum at 4.9 degrees angle of incidence.

Fig. 2. The spectral response is shown for an encapsulated grating with a keyhole pocket of air. The lateral period, duty cycle, and thickness of the grating are 950 nm, 32%, and 900 nm respectively. The superstrate and substrate are fused silica. The angle of incidence was simulated at 4.9 degrees, producing the ${\rm TE}_{0}$ resonance mode in the leaky waveguide.
SECTION III

## FABRICATION

Prior to device fabrication, 1 mm thick, 100 mm diameter UV-grade fused silica substrates were cleaned with an NMP bath and subsequent alcohol rinse. A monolayer adhesive was applied to the substrates to promote resist adhesion during processing. AZ MiR 701 photoresist was spun onto the primed substrate to a thickness of ${\sim}{\rm 1} \mu{\rm m}$. Patterning was done with a GCA I-line 5X reduction stepper tool, creating an array of devices each 25 ${\rm mm}^{2}$ in area. A baking process was performed before and after the lithographic patterning to reduce stress and aid the development process.

The wafer was immersed in AZ 300 MiF photoresist developer. A descum etch was performed before and after the transfer etch to remove remaining photoresist in the grating trenches and on the surface of the wafer. The pattern was transferred 900 nm into the substrate. The pre-etch descum, post-etch descum, and transfer etch utilized a Unaxis Versaline inductively-coupled plasma (ICP) reactive-ion etch (RIE) tool. The etched grating was conformally coated using ALD using an Oxford OpAL tool with a Trimethyl Aluminum (TMA) precursor. The TMA is drawn directly into the chamber due to its high vapor pressure, attaching itself to the surface of the wafer via chemisorbtion. Following an argon purge, oxygen plasma is used to form an atomic layer of ${\rm Al}_{2}{\rm O}_{3}$ by interacting with the TMA. A post-plasma purge completes a single ALD cycle. This method yields a deposition rate of 0.122 nm/cycle; each cycle takes ${\sim}{\rm 5} {\rm seconds}$ to complete. A 350 nm deposition of ${\rm Al}_{2}{\rm O}_{3}$ nearly fills the etched trenches.

Conformal deposition is crucial to the formation of the encapsulated waveguide. Due to the conformal nature of the deposition, the trench fills equally on all sides, prohibiting the formation of air pockets. Since the deposition mechanism is similar for each material, the fabrication of the device is independent of the materials used, given that the deposited materials and substrate can be selectively etched. With a thick enough deposition, the etched trenches will fill completely, planarizing above the grating. In this case, the deposition was shortened to partially fill the trench, in order to control the waveguide grating's effective index. Moreover, the remaining “air-rectangle” can be controlled with atomic layer precision inherent in the ALD process.

Following the deposition, an STS III-V ICP plasma etcher was used to selectively etch the top ${\rm Al}_{2}{\rm O}_{3}$ layer from the top fused silica grating ridges. The etch time was optimized to properly clear the top of the grating in preparation for bonding, causing the funneling seen in the device's cross-section. A 50 sccm gas flow of ${\rm BCl}_{3}$ is used to achieve an alumina etch rate of 40 nm/min with ICP powers of 800 W and 250 W for the coil and platen, respectively. Since the III-V etch chemistry does not attack fused silica, the optical quality polish of the fused silica wafer is preserved in preparation for bonding. Another feature of this process is the natural etch bias of the holes/grooves as compared to open areas. Since the top coating of the deposition etches faster than the material inside the grooves, a wedge (or cone in the case of circular holes) is formed to give rise to the non-ideal profile. Based on the RIE etch bias, the funnel-shaped trench will also form in fully-filled gratings. An SEM of the fabricated device is shown in Fig. 3(a). This change from the “ideal” structure in Fig. 2 will change the spectral response of the device as is shown in Section IV.

Fig. 3. (Top) Graphical illustration of etch process and (Bottom) (a) a cross-section SEM image of the fabricated device is shown prior to the bonding step. (b) The cross-section may be directly imported as a refractive index profile for accurate representation in the simulation routine. The blue, red, and yellow regions represent air, ${\rm Al}_{2}{\rm O}_{3}$, and fused silica, respectively.

In place of a conventional wafer bonder, oxygen plasma was used to activate the bonding surfaces [10], [12]. Surface activation was performed on the ALD tool for 10 minutes; platen temperature, RIE power, and ${\rm O}_{2}$ flow rates were 300°C, 200 W, and 50 sccm, respectively. In order to create a hydrophilic surface, improve contact bond strength and reduce particle contamination, the initialized surfaces were rinsed with deionized water and dried by blowing the surface with nitrogen. After the activated surfaces were brought into contact, the wafers were placed under a weight for 2 hours; bond strength was not tested, since the interest is in the functionality of the device. Fig. 4 illustrates the bonded devices.

Fig. 4. A photograph of the final resonant devices, bonded to the second 0.5 mm thick substrate. The top of the device wafer was diced for the cross section shown in Fig. 3(a).
SECTION IV

## EXPERIMENT & ANALYSIS

Device testing was performed using an amplified signal from an Agilent laser source, tunable from 1525 nm to 1610 nm; in this experiment, increments of 0.5 nm were used near the resonance region. The incident beam has a ${\rm 1/e}^{2}$ diameter of 700 $\mu{\rm m}$. The device was mounted on rotational and translation stages allowing for accurate alignment of the resonant device. Reflected and transmitted power measurements are simultaneously monitored for optimal alignment and proper normalization of the signal. Normalization is necessary due to the variation in output power with respect to wavelength. The incident beam is highly linearly polarized in the TE plane of the wafer, with the electric field parallel to the grating grooves. The spectral response of the device after bonding is shown in Fig. 5.

Fig. 5. Spectral reflectance is shown for the fabricated device (Fig. 3(a)) and the simulated device (Fig. 3(b)). Experimental and simulated data are represented by the solid and dashed curves, respectively, and are represented by Fig. 3(a) and (b). The TE response is shown in red, and the TM response is shown in blue.

The final device demonstrated a strong resonance unique to the TE polarization, with a linewidth of 5 nm with 84.6% reflectivity, tilted at 7.8 degrees angle of incidence (in air). No significant peak was seen in the orthogonal polarization. The simulated FWHM was found to be 3 nm. At 1550 nm, the device exhibited a strong extinction ratio for $({\rm R}_{\rm TE}/{\rm R}_{\rm TM})$. The fringes are due to an etalon effect between the top of the grating and the top of the superstrate. Due to undersampling away from the resonance wavelength, the interference fringes are misrepresentative of the etalon length. Finer sampling from SEM-based simulations showed fringes with a free-spectral range of ${\sim}{\rm 2} {\rm nm}$. This leads to an etalon thickness close to that of the bonded wafer. Based on data from the device cross-section (Fig. 3(a)), a modified refractive index profile was modeled. The experiment and simulation results are shown in Fig. 5. The peak of the TE resonance agrees well with that of the modified simulation. Differences of the peak amplitudes between experiment and theory are the result of fabrication tolerances, optical alignment errors and the interference fringes overlapping with the reflected signal.

SECTION V

## CONCLUSION

We have introduced a novel fabrication process for encapsulated resonant structures using the conformal deposition with selective etching to maintain the interface for subsequent wafer bonding. This approach opens the door for incorporating various metal oxides into bonded structures. Moreover, the lateral deposition of ALD eliminates the need to deposit thick films with subsequent deep etches for high aspect ratio structures and enables one to design and fabricate structures based on homogeneous films.

The device presented herein was a polarization selective device in fused silica with a fully encapsulated resonant waveguide grating. The encapsulation protects the waveguide and grating while still providing a strong resonance at non-normal angle of incidence. The response of the device may also be tailored by changing the etched grating parameters, i.e., morphology, as well as altering the deposition parameters.

## Footnotes

This work was supported in part by the HEL-JTO agency through the AFOSR under grant FA9550-10-1-0543.

The authors are with the Holcombe Department of Electrical and Computer Engineering, Clemson University, Clemson, SC 29634-0915 USA, and also with COMSET, Clemson, SC 29634 USA (e-mail: apung@g.clemson.edu; scarl@clemson.edu; iraghu@g.clemson.edu; ejohns8@clemson.edu).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

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