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We present a new approach to remove monolayer graphene transferred on top of a silicon-on-insulator (SOI) photonic integrated chip. Femtosecond laser ablation is used for the first time to remove graphene from SOI waveguides, whereas oxygen plasma etching through a metal mask is employed to peel off graphene from the grating couplers attached to the waveguides. We show by means of Raman spectroscopy and atomic force microscopy that the removal of graphene is successful with minimal damage to the underlying SOI waveguides. Finally, we employ both removal techniques to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides using the cut-back method.
© 2015 Optical Society of America
1. Introduction
In recent years graphene, a two-dimensional hexagonal lattice of carbon atoms, has been shown to exhibit unique optical properties that can strongly improve the functioning of photonic devices and even of entire photonic circuits [1, 2]. When depositing graphene on chip-scale pho-tonic circuits consisting of silicon waveguides, a high-resolution patterning method to locally remove the graphene layer without affecting the underlying silicon waveguide is desired for most applications. Indeed, an accurate control of the length of the graphene strips on top of the waveguides is of high importance, since the graphene induces a high loss, especially when the graphene sheet covers the grating couplers that are used to couple light in and out of the on-chip waveguides [3].
High-resolution patterning of graphene on silicon waveguides is typically established using photolithography or electron beam lithography [4,5]. Yet there are important limitations associated with these techniques, such as minimal wafer size requirements which prohibit chip-scale processing. Most importantly, however, the subsequent steps of lithography may seriously damage the underlying graphene sheet or at least significantly alter its electrical and optical properties through chemicals-induced unintentional doping [6, 7]. A promising alternative technique for realizing high-resolution graphene patterning without employing chemicals is laser ablation [8–10 ]. The viability of femtosecond laser ablation has already been demonstrated for graphene deposited on SiO2, but ablation of graphene directly deposited on silicon has not yet been reported. Proper selection of the laser parameters is vital to avoid damage to the underlying substrate [11]. As opposed to SiO2, silicon absorbs infrared light with wavelengths shorter than 1100 nm and thus is more prone to laser-induced damage at these wavelengths. Furthermore, laser ablation of graphene on waveguide structures –regardless the material used–has to our knowledge never been investigated, but has great potential for providing the required control over the length of the graphene-covered waveguide sections in integrated optics applications.
In this paper we show that laser ablation can successfully remove graphene on a silicon waveguide without deteriorating the waveguide shape. We also find that laser ablation is not suitable for peeling off graphene from the grating couplers attached to the waveguide input and output to enable light incoupling and outcoupling. Instead, oxygen plasma etching through a mechanical mask can be employed for this purpose. As a case study, we apply both graphene removal techniques to determine the linear loss of grating-coupled graphene-covered silicon waveguides along the cut-back method.
2. Monolayer graphene transfer
Monolayer graphene was grown on high-purity polycrystalline copper foil in a commercial Black Magic system by the chemical vapor deposition (CVD) method. The pre-growth treatment of copper included annealing at 1000°C in Ar atmosphere followed by H2 etching at a pressure of 20 mbar [12]. The purpose of this step is to improve the quality and enlarge the grain size of copper as well as to ensure a reduction of oxides from the copper surface. Later, methane was introduced into the reactor with time and flow rate settings ensuring monolayer graphene growth. The as-grown graphene was covered with a 200 nm-thin layer of PMMA by spin-coating and transferred onto an SOI photonic chip by means of the electrochemical delamination method carried out in 1M aqueous solution of potassium chloride [12].
The monolayer character of the transferred graphene was confirmed by Raman spectroscopy performed using a Renishaw inVia microscope powered by a 532 nm continuous-wave (CW) frequency-doubled Nd:YAG laser. A laser power of 5 mW and a spot size of 0.4 μm (obtained by means of a 100× objective) were used. The shape, position and width of the G and 2D peaks in the Raman spectrum can serve to distinguish mono- and bilayer graphene [13–16 ]. A typical Raman spectrum of transferred graphene with indicated D, G and 2D bands is shown in Fig. 1. For that spectrum the 2D band position is 2673 cm−1 and G band position is 1582 cm−1. This is consistent with other reports on monolayer CVD graphene [6, 17, 18]. The full width at half maximum (FWHM) of the 2D band (about 38 cm−1) as well as the ratio of the peak intensities (I 2 D/IG ≈ 2) are also typical for monolayer graphene [19].
Fig. 1 Raman spectrum measured after graphene transfer on the SOI waveguides, showing the monolayer nature of the transferred graphene.
3. Femtosecond laser ablation
Graphene ablation was investigated using an Yb-doped fiber laser (Satsuma, Amplitude Systems, 5 W, 1030 nm, 400 fs). During the ablation experiments, the pulse repetition rate was fixed to 10 kHz, whereas the laser pulse energy and the amount of shots (i.e. the laser scanning speed) were varied. An objective lens with a focal length of 60 mm and a numerical aperture (NA) of 0.2 was used to focus the laser beam and all ablation experiments were carried out in air at room temperature. The beam waist of the laser beam was determined to be w 0 = 9.9 μm [20].
We started with investigating laser ablation of a monolayer graphene sheet (with the PMMA layer removed) on an SiO2 substrate. The graph in Fig. 2 shows the squared diameter of the ablated areas (D2) as a function of the peak fluence for single shots. From this graph, the ablation threshold was determined to be 0.076 J/cm2. This ablation threshold is higher than the value found by Zhang et al. [21], but we also worked at a longer wavelength, namely 1030 nm versus 800 nm. This seems to be in line with earlier wavelength dependence studies [22]. The smallest measured diameter of the ablated spot was 8 μm, which in combination with high-precision alignment stages enables patterning of graphene with micron-scale resolution.
Fig. 2 Experimental measurement of the squared diameter of the ablated areas (D2) as a function of the peak fluence to determine the ablation threshold, which is the intersection of the fitted curve with the X-axis. The smallest measured diameter of the ablated spot was 8 μm for graphene on SiO2, 5 μm for graphene on SOI, and 4 μm for bare SOI. A Nikon Optiphot 200 high-resolution, high-magnification optical microscope was used to determine D2.
Next, we investigated the ablation of graphene deposited on an SOI substrate with a 220nm-thick top silicon layer, and we found that the ablation threshold for graphene removal, i.e. 0.147 J/cm2 (Fig. 2), was a factor 2 higher for graphene on silicon than for graphene on SiO2 (0.076 J/cm2), while the damage threshold for bare SOI was 0.546 J/cm2 (Fig. 2). The smallest measured diameter of the ablated spot was 5 μm for graphene on SOI and 4 μm for bare SOI (Fig. 2). Having determined the threshold for ablation of graphene on a blank SOI sample, we proceeded to the local removal of graphene on a waveguide-containing SOI photonic chip featuring a 220 nm-thick top silicon layer. Notice that since the absorption depth of silicon at 1030 nm is much greater than 10 μm, we expected to induce less damage to the silicon at 1030 nm as compared to shorter wavelengths, especially since the silicon layer is only 220 nm thick. The chip contained 450 nm-wide waveguides fabricated in the 220 nm-thick silicon top layer, which are the standard waveguide dimensions for operation at the telecom wavelength of 1550 nm. The waveguides’ grating couplers were also designed for 1550 nm. We found a fluence of 0.16 J/cm2 and a laser scanning speed of 70 mm/s to be the optimal parameters for continuous removal of graphene from the silicon waveguides. The threshold for graphene ablation on the SOI waveguides (i.e. 0.137 J/cm2) was found to be slightly lower than the threshold for graphene ablation on bulk SOI (i.e. 0.147 J/cm2). As a result, the diameter of the ablated spot on the waveguides at our working point of 0.16 J/cm2 is slightly larger (i.e. 7 μm) than the corresponding spot diameter found for ablation of graphene on bulk SOI (i.e. 5 μm). A scanning speed of 70 mm/s for a free-running laser with a repetition rate of 10 kHz corresponds to ablation in the single pulse regime when taking into account this 7 μm ablation spot diameter for graphene on SOI waveguides.
To show that the local removal of graphene was successful, we measured the Raman spectrum at several positions along the waveguide that was ablated and at positions that were not ablated. The Raman spectra were obtained using a Horiba Scientific LabRam HR with a 532 nm CW frequency-doubled Nd:YAG laser and 0.25 mW laser power (notice that the Raman spectra for the ablation experiments were obtained with a different Raman microscope than the one used after graphene transfer and the plasma etching experiments because we measured the spectra directly after the ablation process, which was carried out at a different location than the plasma etching). The spot size was 0.7 μm, obtained with a 100× objective (NA of 0.9). Figure 3 shows a typical result of these measurements. Whereas in the blue curve the characteristic peaks clearly show the presence of monolayer graphene, the red curve shows that there is no graphene remaining on top of the waveguide after laser ablation. In addition, atomic force microscopy (AFM) was used to check the geometrical shape and surface roughness of the SOI waveguides after laser ablation, to identify potential damage to the waveguide during ablation. A high aspect ratio tip was used in tapping mode (256 × 256 data points, 1 Hz scan rate) to acquire the AFM images in Figs. 4 and 5, showing the surface profile and cross-section of an SOI waveguide before and after ablation respectively. The root mean square (RMS) surface roughness Rq of a graphene-covered SOI waveguide, measured in an area of 400 nm × 400 nm, was 0.57 nm (Fig. 4). After ablation, we measured an RMS surface roughness of Rq = 2.12 nm (Fig. 5), which means that the ablation-induced increase in surface roughness equals 1.55 nm. We found that the parameter window for graphene removal with such small damage to the underlying waveguide is relatively small: the lower limit is determined by the ablation threshold for graphene on SOI waveguides (i.e. 0.137 J/cm2), whereas the upper limit is found to be 0.16 J/cm2 since the ablation-induced surface roughness increases rapidly for ablation fluences larger than this value.
Fig. 3 Raman spectra measured in the ablated and non-ablated regions of a graphene-covered SOI waveguide.
Fig. 4 AFM measurement showing the waveguide profile before laser ablation (left). The right part shows a cross-section of the waveguide along the blue line in the left AFM image (bottom) and a zoom on the 400 nm × 400 nm area indicated by the dashed square in the left AFM image (top). The measured surface roughness is 0.57 nm.
Fig. 5 AFM measurement showing the waveguide profile after laser ablation (left). The entire area shown in this measurement has been ablated. The right part of the figure shows a cross-section of the ablated waveguide along the blue line in the left AFM image (bottom) and a zoom on the 400 nm × 400 nm area indicated by the dashed square in the left AFM image (top). The measured surface roughness is 2.12 nm.
We also observed that whereas for the waveguides the removal of graphene with laser ablation was successful, it was not possible to achieve the same result on the grating couplers. We believe that resonant reflections of the laser ablation beam on the gratings results in damage to the gratings even at very low ablation powers. For the tapered SOI sections between the waveguides and the grating couplers, we found that the level of adhesion of the graphene layer depends on the local width of the tapers, so that removing the graphene along the tapers would require a gradual variation of the laser ablation settings along the taper. To avoid this and to solve the issue of graphene removal from the grating couplers, we used O2 plasma etching [23,24] to peel off the graphene from the grating couplers and the tapers.
4. Oxygen plasma etching
As opposed to laser ablation, the plasma etching step can be performed before the PMMA layer used for the graphene transfer is washed off of the graphene sheet. The O2 plasma etching was carried out in a 13.56 MHz chemically reactive plasma etcher. The oxygen flow was set to 5 sccm and the pressure reached 0.3 mbar. At 50 W of applied RF power the etching continued for 30 min. A mechanical mask was designed and laser-cut in a 200 μm-thin steel plate to define the openings to be etched. Taking into account that the minimal attainable size of the openings is about 100 μm, this approach of O2 plasma etching with a mechanical mask is particularly suited for the removal of graphene on larger areas such as in this case grating couplers and waveguide tapers. The alignment of the mask with respect to the chip proved a challenging task and was carried out with the help of custom-adapted pick-and-place equipment which allowed us to achieve an alignment accuracy of 10 μm. The patterning resolution for graphene removal with plasma etching is thus much lower than with laser ablation. Figure 6 shows the SOI chip after the mask alignment has been carried out and openings have been etched in the graphene/PMMA layer. The brown area indicates where the graphene/PMMA layer was still present and the dark blue area where it was etched away. The selectively etched graphene/PMMA layer is afterwards rinsed in an acetone/ethanol bath to strip the PMMA layer and reveal the underlying graphene. Figure 6 also shows a zoom of three grating couplers situated within one of the etched openings.
Fig. 6 Microscope image of the photonic chip on which a PMMA/graphene layer has been transferred, which is then selectively O2 plasma etched to create openings in the layer on top of the grating couplers and tapered sections of the SOI waveguides. The dashed rectangle refers to the area of the chip which was used for the cut-back measurements (see Section 5). Left: zoom on three grating couplers in an etched opening, and the transition to a graphene-covered region (i.e. the slightly darker stripe that runs vertically across the figure).
Figure 7 shows a typical Raman spectrum of graphene outside the O2 plasma etching region, showing that the remaining graphene monolayer is of good quality. In the same figure, a typical Raman spectrum taken within the plasma etched area is shown which exhibits no graphene-related peaks and shows no traces of carbon residue. This clearly shows that the O2 plasma completely removes the graphene layer in the selectively etched areas. The Raman spectra were obtained with the same Renishaw inVia Raman microscope referred to earlier. AFM measurements of the gratings (Fig. 8) confirmed that while a higher surface roughness is introduced, the grating profile shape is not significantly degraded due to O2 plasma etching (as opposed to laser ablation). From Fig. 9 it is clear that the plasma etching process also induces an increase in RMS surface roughness on the tapered sections of the waveguides to Rq =3.16 nm for an area of 400 nm × 400 nm (i.e. an increase of 2.5 nm compared to the surface roughness measured before plasma etching). This could be improved by removing the PMMA layer prior to the plasma etching to shorten the plasma etching duration.
Fig. 7 Raman spectra measured in a graphene-covered region (blue) and in the O2 plasma etched grating coupler area (red).
Fig. 8 AFM measurement showing the grating coupler profile before (a) and after (b) plasma etching. The bottom part shows a cross-section of the grating along the line in the top AFM image.
Fig. 9 AFM measurement showing the tapered waveguide section profile before (a) and after (b) plasma etching. The bottom part shows a cross-section of the grating along the line in the top AFM image. The measured surface roughness over an area of 400 nm × 400 nm was measured to increase from 0.66 nm before plasma etching to 3.16 nm after plasma etching.
5. Measurement of linear loss of graphene-covered waveguides using cut-back method
To showcase its practical use for optical experiments, we also employed our combined laser ablation and plasma etching technique for determining the optical transmission properties of graphene-covered silicon waveguides. More specifically, we have plasma etched and laser ablated the gratings and waveguides according to the pattern shown in Fig. 10. This way, the length of the graphene-covered waveguide is gradually increased from 0 μm (i.e. the graphene is removed over the full waveguide length) to 300 μm such that we can determine the linear propagation losses by means of the so-called cut-back method [25].
Fig. 10 Schematic lay-out of the laser ablation and O2 plasma etching regions on an array of identical graphene-covered waveguides (indicated by the dashed rectangle in the right part of Fig. 6), to be used for cut-back measurements.
The experimental setup is schematically illustrated in Fig. 11. To couple light between single-mode fibers and the silicon waveguides, we positioned the fibers above the grating couplers at an angle of 9° with respect to the vertical axis for optimal coupling efficiency. To be able to monitor how much power we couple into the chip, we split off 1% of the light coming from the laser source. A variable optical attenuator (VOA) is used to accurately control the power to be coupled into the chip, and just prior to entering the chip, the light passes through a fiber polarization controller (PC) to align the polarization to the desired Transverse-Electric mode of the SOI waveguides. As a reference, we first measured the grating coupler efficiency before graphene transfer on the chip. The source employed is a continuous-wave wavelength-division multiplexing (WDM) telecom laser (emitting at the wavelength of 1550 nm for which the grating couplers were designed) connected to an Erbium-doped fiber amplifier (EDFA) set to 1mW output power. The total insertion loss, including the grating coupler loss at input and output as well as the waveguide propagation loss over 800 μm (the latter comprises a 400 μm-long waveguide section and two 200 μm-long tapered sections) was found to be 13.5 dB. Since the propagation loss of the waveguides is of the order of 3 dB/cm [25], the insertion loss per grating is estimated to be 6.6 dB when using flat-cleaved fibers to couple light in and out of the SOI waveguides.
Fig. 11 Schematic of the experimental setup to measure the linear loss properties of graphene-covered waveguides (WDM: wavelength-division multiplexing; EDFA: Erbium-doped fiber amplifier; VOA: variable optical attenuator; PC: polarization controller).
Then we performed the cut-back experiments to measure the contribution of graphene to the loss of grating-coupled graphene-covered SOI waveguides. Figure 12 shows the total insertion loss (i.e. the waveguide loss and the coupling loss at both grating couplers) versus the length of the graphene strip in-between the ablated zones. The leftmost data point in Fig. 12 shows a total insertion loss of 18.3 dB for a waveguide which was ablated over its full length. This is about 4.8 dB higher than the insertion loss of 13.5 dB measured before the graphene transfer and patterning through laser ablation and plasma etching. This difference is due to the plasma etching-induced increase in surface roughness of the grating couplers and the tapered waveguide sections (which can be observed in Figs. 8 and 9), and the ablation-induced increase in surface roughness of the silicon waveguide (which can be observed in Fig. 5).
When applying a linear regression on all datapoints in Fig. 12, we find that the contribution of graphene to the propagation loss for the graphene-covered waveguides is about 13.2 dB per 100 μm of graphene, with a very good R2 value of 0.9925 for the fit. This loss of 0.132 dB/μm is in line with other experimental results reported by Li et al. [26]. The fact that the graphene-induced loss is so high clearly underlines the necessity of local patterning of the graphene transferred on top of SOI waveguides when targeting on-chip photonic applications. Li et al. also showed that the linear loss caused by the graphene layer on top of a waveguide strongly depends on the waveguide geometry [26]. This is because for different geometries the electric field strength in the graphene cover layer, and as such the interaction between the graphene and the waveguide mode, is different.
6. Conclusions
In conclusion, we have demonstrated the successful removal of a monolayer of graphene on SOI waveguides through femtosecond laser ablation at 1030 nm. Because laser ablation of the grating couplers was not successful, we employed O2 plasma etching through a metal mask to remove graphene at these locations, as well as on the tapered waveguide sections. Atomic force microscopy revealed that laser ablation induced an increase in surface roughness of the underlying waveguides of 1.55 nm, whereas O2 plasma etching induced an increase in surface roughness of the tapered waveguide sections of 2.5 nm. This resulted in a 4.8 dB insertion loss penalty for a 400 μm-long ablated waveguide section and O2 plasma etched gratings and tapers. Both the laser ablation and plasma etching techniques offer the important advantage that they do not involve additional chemical treatment of the graphene sheet. Laser ablation has the supplementary advantage that it allows patterning of the graphene with a much higher resolution than plasma etching. Finally, we employed both techniques for determining the optical transmission of graphene-covered SOI waveguides along the cut-back method. For a standard waveguide width of 450 nm and height of 220 nm we found the contribution of graphene to the optical loss to be 0.132 dB/μm. This high loss value per unit distance clearly shows that to apply graphene in photonic chip applications, accurate control of the overall loss through localized patterning of the graphene is essential.
Acknowledgments
This work was supported by the EU-FET GRAPHENICS (grant agreement no. 618086), BELSPO-IAP, Methusalem, ERC-FP7/2007-2013 grant 336940 and by the EU-FP7 Graphene Flagship (grant agreement no. 604391). The work of J. Van Erps was supported by the Research Foundation Flanders (FWO-Vlaanderen) under a postdoctoral research fellowship.
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