A self-aligned dry etching method for mechanical ...

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A self-aligned dry etching method for mechanical strain enhancement of germanium and its uniformity improvement for photonic applications Yiding Lin, Danhao Ma, Kwang Hong Lee, Jurgen Michel, Chuan Seng Tan

Yiding Lin, Danhao Ma, Kwang Hong Lee, Jurgen Michel, Chuan Seng Tan, "A self-aligned dry etching method for mechanical strain enhancement of germanium and its uniformity improvement for photonic applications," Proc. SPIE 10537, Silicon Photonics XIII, 1053704 (22 February 2018); doi: 10.1117/12.2288154 Event: SPIE OPTO, 2018, San Francisco, California, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/2/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

A self-aligned dry etching method for mechanical strain enhancement of germanium and its uniformity improvement for photonic applications Yiding Lin*,a,c, Danhao Mab, Kwang Hong Leec, Jurgen Michelb,c and Chuan Seng Tan*,a,c

a

School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798; bDept. of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Mass. Ave., Cambridge, MA 02139, USA; cLow Energy Electronic Systems IRG (LEES), Singapore-MIT Alliance for Research and Technology, 1 Create Way, Singapore 138602; Emails: [email protected]; [email protected] ABSTRACT A self-aligned dry etching method was proposed and verified theoretically to enhance the magnitude and simultaneously improve the uniformity of the tensile strain in a germanium (Ge) wave-guide (WG), with the help of tensile-stressed SiN stressor at the WG sidewalls. The SiN-strained germanium-on-insulator (GOI) WG was also experimentally demonstrated. Significant tensile strain was observed in the Ge material via micro-Raman measurements. This method could potentially facilitate a Ge photodetector with its optical detection range extended further towards longer wavelength and to be comparable with that of state-of-the-art InGaAs detectors.

Keywords: germanium-on-insulator (GOI), SiN stressor, Ge strain engineering, photodetectors, longer wavelength detection

1. INTRODUCTION In recent years, optical interconnects has been proposed as a potential alternative to address the bottleneck of conventional metal interconnects arisen from the miniaturization of transistors in the complementary-metal-oxide-semiconductor (CMOS) integrated circuits [1]. Data communication via optical interconnects could facilitate faster transmission with larger bandwidth and lower power consumption [2]. Germanium (Ge) is a promising material to realize such photonicintegrated circuits (PICs). Besides its CMOS process compatibility, Ge has a direct bandgap of ~0.8eV at Г-valley, corresponding to its optical absorption till ~1550nm within state-of-the-art tele-communication C-band. This enables key building blocks in the PICs such as photodetectors [3] and modulators [4] to be developed and monolithically integrated [5]. The ~0.2% tensile strain in the Ge film, resulting from the Ge-on-silicon (Si) epitaxy and subsequent cooling, accounts for an additional ~50nm extension of its photo-detection range into the L-band due to the strain-induced Ge bandgap shrinkage [6]. However, the responsivity of these normal-incidence detectors at L-band is still relatively low due to the roll-off of the Ge absorption coefficient. One has to design the Ge absorption layer thicker and longer, respectively, for normal-incidence and waveguide-integrated scheme detectors, to compensate for the responsivity drop, whereas the resulting increased carrier transit time and device capacitance would correspondingly limit their high-speed performance. Therefore, trade-off exists to further improve the performance of Ge-based detectors at L-band. If the mechanical tensile strain can be further enhanced in Ge, its energy bandgap would continue to decrease which extends the corresponding absorption coverage towards longer wavelength. This might potentially mitigate the performance trade-off of the Ge detectors due to the increased absorption coefficient at L-band. Applying highly-stressed silicon nitride (SiN) material on Ge is an intriguing approach since it had similarly been utilized to strain Si in CMOS fabrication to boost transistor performance [7]. A Ge photodetector was thus demonstrated without responsivity roll-off across the entire L-band [8]. Zhang et al. [9-11] also theoretically studied the extension of photo-detection range for GeSn Silicon Photonics XIII, edited by Graham T. Reed, Andrew P. Knights, Proc. of SPIE Vol. 10537, 1053704 · © 2018 SPIE · CCC code: 0277-786X/18/$18 doi: 10.1117/12.2288154 Proc. of SPIE Vol. 10537 1053704-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/2/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

under SiN liner stressor. In addition, SiN stressor has been applied at the top [12] and all-around the Ge micro-structures [13] to minimize its L-Г valley energy gap in pursuit for an on-chip laser. However, the reported strain distribution profiles in Ge [9, 14] were not uniform, with reduced-tensile and even compressive strain observed close to the substrate and near the edges of the structure, respectively. This may still lead to early roll-off of the detector responsivity and degrade its performance uniformity. The strain non-uniformity is due to the presence of SiN stressor only at the top or sides of the Ge micro-structure, which leads to hindered elastic deformation of Ge near the bottom constrained by the substrate. Although the all-around stressor provides a more uniform strain distribution, thin pedestal structure under Ge is necessary which poses new challenges for CMOS integration. In this work, tensile-stressed SiN stressor is applied at Ge wave-guide (WG) sidewalls after a process named self-aligned dry etching (SADE) which forms trenches in the substrate material (SiO2 in this work) aligning with the WG sidewalls. The stressor could, in this case, not only tensile-strain the Ge material, but also pull the substrate material underneath via the SiN filled in the trenches. Enhanced Ge mechanical tensile strain as well as improved strain uniformity was thus established by this simple and completely CMOS-compatible method. This would potentially facilitate a Ge photodetector with even longer wavelength coverage and mitigated responsivity-speed trade-off.

2. FINITE ELEMENT METHOD (FEM) STRAIN MODELLING AND ABSORPTION EDGE CALCULATION A Ge-on-insulator (GOI) platform was used in this work for the FEM modelling as well as the subsequent experimental demonstration. The insulator layer refers to SiO2. The epitaxy and layer transfer technique for the GOI fabrication was discussed elsewhere [15]. The FEM modelling followed the GOI fabrication process consisting of Ge-on-Si epitaxy at 600oC and post-bonding annealing at 300oC. The thickness (along z-direction in Figure 2) of the Ge film and SiO2 were 200 and 700nm, respectively, with Si as the substrate at the bottom. Tensile strain of ~0.15% in the Ge film from the model matches with the X-ray diffractometer (XRD) measurement result [16]. A new solid mechanics module was then built to shape the Ge film into WG (length along , namely y-direction in Figure 2) without temperature change. The length of the WG (100µm) is much longer than its width (500nm along x-direction in Figure 2). A SiN layer with the same thickness as the GOI WG was then placed at both sidewalls of the GOI WG as stressor. A typical tensile stress value of 1GPa [17] was used for the stressor. The Young’s modulus used for Ge and SiN are 103 and 200GPa, respectively. The plane of symmetry penetrates the model through the y-z plane across the centre of the WG. For the model with SADE approach, all the parameter settings are identical except that the WG sidewalls extend 200nm deeper into the bottom SiO2 layer, with the SiN stressor filled all along till the top plane of the GOI WG. Cross-sectional schematics of the FEM models without and with the SADE method were shown in Figure 1 (a) and (b), respectively.

SiN (tensile)

(a)

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SiN (tensile)

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SiN (tensile) S102

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Figure 1. Cross-sectional (on the x-z plane in Figure 2) schematics of the FEM models (a) without and (b) with the SADE method.

Figure 2 (a) and (b) show the strain profiles in the GOI WG along y- and x-direction, respectively, before the SiN stressor is applied. The strain along y-direction remains the same as that in the Ge film (~0.15% tensile), while the strain along the

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x-direction relaxes (~-0.02% to ~0.02%). The reason to the x-direction strain relaxation might be due to the reduced constraint from the insulator substrate along the x-direction, compared to that along y-direction, since the length of the WG along y-direction is much longer than that along x-direction. Uniaxial strain along y- direction

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Figure 2. FEM strain profiles of a Ge WG-on-SiO2-insulator with 500nm width (x-direction) and 200nm height (z-direction) along (a) y-direction and (b) x-direction, before the SiN stressor is applied. The negative strain values refer to compressive strain while the positive ones refer to tensile strain.

Figure 3 (a) and (b) show the FEM strain profiles in the GOI WG without and with the SADE method used, respectively, by applying the 1GPa tensile SiN stressor at the WG sidewalls. It can be observed from the strain distribution profiles that the GOI WG with SADE method exhibits a higher and more uniform colour code, compared to the WG without SADE use. This implies enhancement in the magnitude and uniformity of the strain. As can be recognized from the colour code in the SiO2 near the GOI WG in Figure 3 (b), the tensile stress in the SiN stressor at both sidewalls pulls the SiO2 oppositely along the x-direction, which leads to the enhancement of tensile strain in the Ge close to the bottom of the WG interfacing with the SiO2. The strain uniformity is thus improved simultaneously. The two insets show the corresponding strain profiles in the Ge WG along the y-direction. Negligible change in the strain magnitude was found after introducing the SiN stressor and the SADE method, compared to Figure 2 (a). Uniaxial strain along x- direction 0.7%

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Figure 3. FEM strain profiles of the GOI WG along the x-direction (a) without and (b) with the SADE method used. The insets show the corresponding strain profiles in the GOI WG along the y-direction.

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Figure 4. (a) FEM strain (along the x-direction) profile in the GOI WG from A to B as indicated in Figure 3; (b) calculated absorption edge of the GOI WG with respect to its strain along the x-direction.

To better reveal the enhancement in tensile strain as well as its uniformity improvement, the strains along x-direction in the Ge WG, from point A to B as indicated in Figure 3, were extracted from the FEM model and presented in Figure 4 (a). The plot clearly shows significant strain enhancement through the entire GOI WG structure by applying the SADE method, with ~90% increase at the region adjacent to the SiO2 layer. In addition, the standard deviation of the strain reduces from ~0.06% to ~0.02%, indicating a ~67% improvement on the strain uniformity. Figure 4 (b) shows the estimated Ge optical absorption edge with respect to the material tensile strain along the x-direction. The absorption edge, determined between the conduction band Г-valley minimum and the split heavy hole (Г-HH) as well as the light hole (Г-LH) maximum at valence band, were calculated using the deformation potential theory [18], considering the strain values in the GOI WG. The ~0.17% tensile strain along the y-direction was also considered. The shaded areas in Figure 4 (b) corresponds to the range of the strain in the GOI WGs with and without the SADE approach in Figure 4 (a). An additional ~50nm extension was observed for the Ge absorption edge with the SADE method used, compared with that without the SADE. The absorption edge was thus extended towards ~1611 and ~1707 nm for the Г-HH and Г-LH band, respectively. The entire L- and U- band would be covered by the Г-LH absorption in this case, further widening the communication bandwidth of the Ge photodetector. Meanwhile, the wavelength span for the absorption edge at Г-LH decreases from ~25 to ~10nm due to the improved strain uniformity, which would benefit to further prevent the early roll-off of the detector responsivity. Therefore, the absorption spectrum of this Ge detector would potentially overlap with that of state-of-the-art InGaAs-based photodetectors and provide a viable solution for a monolithically-integrated CMOS-compatible photodetector for optical detection beyond 1.7µm.

3. EXPERIMENTAL In practical fabrication, the structure with the SADE method was realized by continuing the dry etching into the bottom SiO2 layer after the GOI WG etching, utilizing the same hard mask in patterning the GOI WG. The corresponding detailed process flow for the SADE structure is shown in Figure 5. Ge WGs with width of 1μm were firstly patterned on a GOI platform using e-beam lithography (EBL). The thickness of the Ge film was 100nm. Chlorine (Cl2)-based reactive ion etching (RIE) was then performed to form the Ge strip WG. By immediately switching to the CF4-based gases after the Ge etching, the SiO2 layer underneath was etched by ~500nm using the same hard mask in protecting the GOI WG. Afterwards, tensile-stressed SiN was deposited by plasma-enhanced chemical vapour deposition (PECVD) with ~580MPa biaxial tensile stress calculated from an identical deposition on a 6-inch Si wafer using Stoney’s equation [19]. A second EBL patterning and RIE were followed to remove the SiN on the top of the GOI WG since the tensile-stressed SiN would induce compressive strain to the WG underneath. Alternatively, chemical-mechanical polishing (CMP) could be used to remove the top SiN and planarize the structure. The fabricated structures were viewed under an optical microscopy and a Field Emission Scanning Electron Microscopy (FESEM), respectively. The strain in the Ge WG were identified via LO-mode micro-Raman technique at laser wavelength of 532nm.

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Ge SiOz

Si (a)

SIN (tensile)

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SIN (tensile)

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(b)

Figure 5. (a) Schematics of the fabrication process for a GOI WG strained by SiN stressor employing the SADE method; (b) a schematic of the SiN-strained GOI WG without using the SADE method, as a comparison.

Figure 6 (a) and (b) show an optical microscopy and FESEM image, respectively, of the fabricated SiN-strained GOI WG with the SADE method used. The WG highlighted with the red rectangle in Figure 6 (a) corresponds to the area within the FESEM image in Figure 6 (b). The dented region in Figure 6 (b) is the top plane of the GOI WG with SiN removed, while the WG sidewalls were fully covered by SiN. Figure 7 summarizes the LO-mode micro-Raman measurement results for the fabricated SiN-strained GOI WGs. Laser wavelength at 532nm was used. A significant left-shift of the peak phonon wavenumber was observed for both the WG with and without the SADE method applied. The micro-Raman peaks for the GOI WG and bulk Ge match with these simulated strain values, using the stress-induced LO phonon splitting relation [20] as discussed in Ref. [21]. The ~2cm-1 left-shift indicates a ~0.9% tensile strain along the x-direction, using the same relation. The strain of ~0.17% was assumed along the y-direction during the calculation, according to the FEM models. The corresponding Ge absorption edge could thus be extended towards ~1740nm from Figure 4 (b), which is red-shifted by ~150nm compared with that of intrinsically-strained Ge material from Ge-on-Si epitaxy. Further verification of the absorption edge is needed by developing the structure into a photodetector. However, since the 532nm laser could penetrate only ~9nm into Ge [22], only the strain information at the top of the GOI WG was collected. This may explain the reason why there is negligible discrepancy on the peak phonon wavenumbers for the WG with and without the SADE method, since the minimum difference on the strain was also similarly achieved at the top of the WGs in Figure 4 (a). Therefore, a longer wavelength laser is needed to gather a more complete strain distribution profile deeper inside the Ge along the zdirection. According to Ref. [22], a 785nm laser could penetrate ~89nm into Ge, which is suitable to reveal the strain information entirely through the WG in this work. Further investigation is thus required to verify the effect of SADE method on the strain in Ge WG.

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Z N

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Figure 6. (a) Optical microscopy and (b) FESEM images of SiN-strained GOI WG.

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300.79

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Wavenumber (cm-1) Figure 7. Micro-Raman results for the SiN-strained GOI WGs using 532nm laser. The respective peak LO phonon wavenumbers were indicated. The results for bulk Ge and the GOI WG without SiN stressor were also included as comparison.

4. SUMMARY In this study, a self-aligned dry etching method was introduced to enhance the magnitude and simultaneously improve the uniformity of the tensile strain in a Ge WG, with the help of the tensile-stressed SiN stressor at the WG sidewalls. From the FEM modelling, the SADE method could theoretically provide a ~90% increase and ~67% reduction on the magnitude and standard deviation of the tensile strain, respectively, in a GOI WG with 500nm width and 200nm height, along with the SiN stressor of 1GPa tensile stress. The enhanced strain could correspondingly extend the Ge absorption edge towards ~1707nm from the deformation potential theory calculation, which covers the entire L- and U- tele-communication bands. Furthermore, a GOI WG with 1µm width and 100nm height was experimentally fabricated, with ~580MPa tensile stress in the SiN stressor. Significant tensile strain was demonstrated in both of the WGs with and without SADE method used. However, longer wavelength laser is needed to further investigate the strain information entirely through the WG to analyse the effect of the SADE method. Still, the SADE method could potentially facilitate a Ge photodetector with furtherextended optical detection comparable with that of InGaAs detectors. Besides, other Ge-based active optoelectronic components such as wavelength-shifted electro-absorption modulators and low-threshold lasers could also be realized by this approach for establishing a complete photonic-integrated circuit. Acknowledgments: This research was supported by the National Research Foundation Singapore through the SingaporeMIT Alliance for Research and Technology's Low Energy Electronic Systems (LEES) IRG, NRF-CRP12-2013-04, and SMART Fellowship. The authors would also like to thank the support from Nanyang Nanofabrication Centre (N2FC) and Ms. Zhou Jin on the GOI WG fabrication and EBL process, respectively.

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