Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Mitigation of UV-Induced Propagation Loss in PECVD Silicon Nitride Photonic Waveguides Pieter Neutens,*,† Monika Rutowska,† Willem Van Roy,† Roelof Jansen,† Federico Buja,† and Pol Van Dorpe†,‡ †
Imec, Kapeldreef 75, 3001 Leuven, Belgium KU Leuven, Department of Physics, Celestijnenlaan 200D, 3001 Leuven, Belgium
‡
S Supporting Information *
ABSTRACT: We observe a drastic increase in propagation loss at visible wavelengths in PECVD silicon nitride waveguides after exposure to ultraviolet (UV) light. Low temperature annealing or high intensity optical exposure at visible wavelengths brings the propagation loss back toward the original value before UV exposure or even lower. We postulate that these effects can be explained by the population and depopulation of defect centers in the silicon nitride. We demonstrate the importance of defect depopulation before using waveguides in any silicon nitride based visible photonic application and provide a cleaning procedure that yields reproducible, low-loss, initial waveguide conditions. KEYWORDS: silicon nitride, photonic waveguide, defect states, optical absorption, UV n the past decade, silicon nitride (SixNy, x ∼ 3, y ∼ 4) has become the waveguide material of choice for visible wavelength photonic circuits, especially for on-chip integrated biosensors. In the visible spectrum, SixNy has excellent optical properties compared to several other materials. It is transparent in the complete visible-NIR wavelength range, compatible with standard complementary metal-oxide semiconductor (CMOS) processing for low-cost mass fabrication, has a relatively high refractive index (n ∼ 1.9−2.1) and has a low-temperature sensitivity. Moreover, low-temperature, plasma-enhanced chemical vapor deposition (PECVD) SixN y allows for integration of photonic components monolithically in the back end of line of CMOS chips (cameras, detectors, FinFETs, ...).1 SixNy photonic waveguides have been used in (bio)sensing applications, demonstrating integrated fluorescence excitation and detection,2 on-chip Raman spectroscopy,3,4 and label-free refractive index sensing with resonators5,6 or interferometers.7 Almost all biosensors require clean, well-defined, surface conditions before applying a biological coating or performing an experiment on the bare waveguide material. Therefore, it is of paramount importance to understand how processing and cleaning procedures affect the waveguide material and its surface properties. In this paper, we show that deep-UV lithography and certain cleaning procedures can lead to dramatically increased propagation loss in SixNy waveguides, propose a qualitative model of the origin of the induced loss, and demonstrate an adapted cleaning recipe resulting in a surface suitable for coating combined with a repeatable, low propagation loss. In this study, we work with PECVD SixNy strip waveguides fabricated in a 200 mm CMOS pilot line. The process flow has
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© XXXX American Chemical Society
been described by Subramanian et al.8 and a schematic overview of the waveguide geometry is shown in Figure 1a. The PECVD SiO2 bottom and top cladding have a thickness of 2300 and 1000 nm, respectively. The thickness of the PECVD SixNy waveguides is 220 nm and the width of the waveguides is 340 nm in case of oxide cladding (Figure 1a, left) and 480 nm in case of air cladding (Figure 1a, right). Air-cladded waveguides are manufactured by opening the top oxide cladding above the SixNy waveguide. The air-cladded waveguide sets contain 16 different waveguides with relative length differences between 0.007 and 4.68 cm to determine the waveguide propagation loss in air using the cutback method (Supporting Information, page S5). One set for air-cladded waveguide measurements is depicted in Figure 1b. Oxidecladded waveguide sets contain four waveguides with length differences between 1 and 3 cm to assess the propagation loss of oxide cladded waveguides. All waveguides and grating couplers are designed for a wavelength of 638 nm and for TE polarization. To measure the propagation loss, we use a custom-made microscope with an automated fiber coupler (Supporting Information, page S2). All output couplers from a single set can be measured in a single camera frame, allowing us to determine both the oxide-cladded and air-cladded propagation loss for all waveguide sets on a chip. In the remainder of this paper, we will only work with two optical input powers at the grating coupler: 3 μW and 3 mW, which we name the Plow and the Phigh power settings. Considering clean, Received: January 3, 2018 Published: March 13, 2018 A
DOI: 10.1021/acsphotonics.8b00014 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
Figure 1. (a) Schematic overview of the waveguide structure. Left, an oxide cladded waveguide. Right, a waveguide after cladding removal. (b) Schematic representation of an air-cladded waveguide set and the waveguide excitation and the detection principle.
Figure 2. Propagation loss measurement on oxide cladded (full markers) and air cladded (open markers) SixNy waveguides before and after cleaning. Measurements at low and high optical input power are indicated by, respectively, a white and gray background. (a) Effect of O2-plasma cleaning on the propagation loss. (b) Effect of UV-ozone cleaning on the propagation loss.
O2, 50 mTorr, 30 min), we observe an increase from 2.1 to 4.2 dB/cm for air-cladded waveguides and from 1.2 to 1.7 dB/cm for oxide-cladded waveguides. This effect is even larger for UVozone cleaning (Jelight144AX UV ozone cleaner, 15 min), where we observe an increase from 3.2 to 12.5 dB/cm for air cladding and from 1.2 to 7.9 dB/cm for oxide cladding. Two factors that the two cleaning procedures have in common that could cause the change in propagation loss are the presence of atomic oxygen and UV irradiation. The lamp in a UV-ozone cleaner has two main wavelength peaks in the UV spectrum: 184 and 253 nm. Cleaning plasmas are also known to emit light in the UV spectrum down to wavelengths of 150 nm, depending on the gases present in the chamber and the cleaning recipe parameters.9 In order to verify that this effect is due to the UV exposure and not created by the presence of atomic oxygen, the test was repeated in a cross-linker chamber that only has the 253 nm peak preventing the formation of ozone, and exactly the same effect was observed. From these experimental observations, we conclude that UV irradiation can have a large influence on the SixNy waveguide properties. These initial measurements suggest that the origin of the observed effect can be found in the existence of optically active defect states in the SixNy layer. For a long time, it has been wellknown that chemical vapor deposition (CVD) SixNy contains a high defect density (1017−1020 cm−3). Since the early 1970s, these point defects have been exploited to store charges in silicon-oxide-nitride-oxide-silicon (SONOS), metal-nitrideoxide-silicon (MNOS), and other SixNy-based memory devices.10−15 In SONOS devices, the memory cell is for example formed by inserting a thin SixNy layer in the gate oxide of a standard polysilicon metal oxide semiconductor field effect transistor. Programming and erasing the memory cell relies on Fowler−Nordheim tunneling through the thin oxide barrier. The theory and the properties of the SixNy trap states used in nitride-based devices have been described and extensively
low-loss routing waveguides and taking into account the grating coupler loss and all splitter levels, this translates to a power of 1.4 nW and 1.4 μW per measurement waveguide, respectively. A full description of the chip layout and the resulting powers and power differences can be found on pages S3 and S4 of the Supporting Information. As fabricated, the waveguide surface is not sufficiently clean for surface functionalization or to perform biosensing experiments. After processing, the chip surface contains contaminants from dicing tape and photoresist and developer residues. Before cleaning, a contact angle for water larger than 30° is measured on blanket SixNy films. In general, UV-ozone cleaning is performed as standard clean before applying a biological coating. This reduces the contact angle below 5° and strongly reduces any surface contamination. Another frequently applied cleaning method to remove organic residues is the use of an oxygen plasma. The measured propagation loss before and after oxygen plasma and UV-ozone cleaning is shown in Figure 2a and b, respectively. For all measurements presented in Figures 2 and 3, the propagation loss was measured for four air-cladded waveguide sets (open markers) and one oxide-cladded waveguide set (filled markers). For the measurements in Figure 2a,b, the propagation loss was measured using Plow (white area) and Phigh (gray area) before cleaning. At the 15 min time point, the samples were taken out of the measurement setup and the respective clean was performed, indicated by the break in the x-axis. Right after the clean, the samples were reinstalled on the measurement setup and the propagation loss was extracted at low optical power Plow to avoid inflicting possible changes induced by high-power optical waveguide illumination. After both cleaning procedures, we observe a drastic propagation loss increase of the air-cladded waveguides and, interestingly, also of the oxide-cladded waveguides. After oxygen plasma cleaning (100 W, 2 sccm B
DOI: 10.1021/acsphotonics.8b00014 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
Figure 3. Optical and thermal depopulation. Measurements at low and high optical input power are indicated by, respectively, a white and gray background. (a) Thermal defect depopulation after O2-plasma cleaning. (b) Thermal defect depopulation after UV-ozone cleaning. (c) Optical defect depopulation after O2-plasma cleaning. (d) Optical defect depopulation after UV-ozone cleaning.
studied experimentally.16−20 Next to charging by Fowler− Nordheim tunneling through the oxide barrier, several other ways exist to change the electronic state of the defects. Electrons in the dielectric can be excited by X-ray irradiation, electron or ion irradiation, ultraviolet (UV) light, and mechanical treatment. In 1988, Krick et al. reported on the nature of the dominant deep trap in amorphous SixNy. They experimentally demonstrated that the main deep trap in SixNy consists of a silicon dangling-bond defect, called the K center.21,22 In the neutral configuration (K0) the K center is amphoteric, so it can capture either electrons (K−) or holes (K+). By using the etchback method, it was shown that these defect states are distributed uniformly across the layer. Electron spin resonance measurements on different PECVD and LPCVD SixNy layers revealed that optical irradiation with an energy larger than 4.1 eV can generate K0 centers in the nitride layer. Annealing the layer for 1 h at 250 °C was found to annihilate the K0 defect state. The observed reversible switching between its neutral state and its charged state suggested that the optical generation of K0 defect states in SixNy involved no structural rearrangement, but only a change in charge and spin of the defect state.21−24 Later, it was also shown that the Si−H defect (substitution of a N atom with one H, leading to a single Si−H bond and a weak Si−Si covalent bond) also plays a large role in the trapping properties of SixNy layers.25 In this paper, we postulate that UV light with an energy larger than the SixNy bandgap (∼5 eV) can indeed generate K0 centers in the SixNy waveguide material as was demonstrated by Krick et al.21−24 Furthermore, we propose the hypothesis that these defect states are optically active and can absorb incident radiation with energies far below the SixNy bandgap energy. Optical absorption from the generated defect centers thereby explains the observed increase in SixNy waveguide propagation
loss after UV exposure by plasma cleaning (Figure 2a) or UVozone cleaning (Figure 2b). We will confirm that activation by thermal annealing and consecutive depopulation of the defect centers yields the expected reduction in propagation loss down to the initial value. Furthermore, we will also establish that light irradiation with sub-bandgap energies can also activate the defect states and cause a reduction in the propagation loss, similar to the annealing process. We present a qualitative explanation for the observed effects based on the known properties of the electronic configuration of SixNy. From research on SixNy memory cells, it is well-known that the defect state energy is located 1.4−2.3 eV from the mobility band. In thermal equilibrium, all lower energy defect levels are completely occupied. Therefore, a photon with an energy far below the SixNy bandgap cannot excite an electron from the valence band into the completely occupied low energy defect states. Neither has it sufficient energy to excite an electron from the highest filled defect state into the conduction band. Therefore, at thermal equilibrium, no optical absorption will take place for light energies far below the SixNy bandgap. By means of a UV exposure with energies comparable to the SixNy bandgap, electrons from both the defect states as from the SixNy valence band can be directly excited into the conduction band where they become mobile and can be trapped by any accessible defect center. This process induces single or double occupied states in the shallow energy levels, allowing optical excitation of electrons with light energies down to ∼1.4 eV (depending on the trap state energy and the width of the defect energy band). Rendering the SixNy back to a nonabsorbing material requires activation of the trapped electrons or holes so they can move into the mobility band and be retrapped by another defect. This continues until all accessible electrons fall down to an energy level that cannot be excited with the working wavelength. This process of activation C
DOI: 10.1021/acsphotonics.8b00014 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
Figure 4. (a) Propagation loss in air as a function of annealing time after UV-ozone cleaning. (b) Waveguide propagation loss in air after UV-ozone cleaning and thermal annealing for 65 min at different temperatures. For each temperature point on the x-axis, the sample received a UV-ozone clean to bring it back to the same high-loss initial condition before annealing at different temperatures. The first points at T = 20 °C equals the initial waveguide loss as fabricated.
description of the chip layout and the resulting power level differences at the measurement waveguides causing the different depopulation rates between sets can be found on pages S3−S4 of the Supporting Information. We found that a high temperature thermal anneal after UVozone cleaning yields the lowest and most reproducible propagation loss value, combined with a very clean surface for biological coating or experiments, confirmed by standard contact angle measurements and FTIR spectroscopy. As fabricated, the standard deviation over eight different aircladded waveguide sets on a single chip is 0.93 dB/cm, while after UV-ozone cleaning followed by thermal defect depopulation this is reduced to
