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Long period gratings (LPGs) are coated with hafnium oxide using plasma-enhanced atomic layer deposition (PEALD) to increase the sensitivity of these devices to the refractive index of the surrounding medium. PEALD allows deposition at low temperatures which reduces thermal degradation of UV-written LPGs. Depositions targeting three different coating thicknesses are investigated: 30 nm, 50 nm and 70 nm. Coating thickness measurements taken by scanning electron microscopy of the optical fibers confirm deposition of uniform coatings. The performance of the coated LPGs shows that deposition of hafnium oxide on LPGs induces two-step transition behavior of the cladding modes.
© 2016 Optical Society of America
Introduction
Over the past two decades, fiber-optic based refractive index (RI) sensors have been widely investigated for chemical [1], biological [2] and environmental applications [3]. Long period gratings have attracted particular attention due to their intrinsic sensitivity to the RI of the surrounding medium (SRI), and the possibility of producing sensors with low insertion loss and low polarization dependence [4]. The SRI sensitivity of conventional LPGs is relatively low when the devices operate in a media with low SRI, such as water (i.e. SRI of 1.33), and increases as the SRI approaches the RI of silica (i.e. RI of 1.45) [4]. However, a number of approaches have been proposed to develop LPG sensors with high sensitivity at low SRIs. These include designing LPGs that promote coupling between the core mode and a specific cladding mode near the dispersion turning point [5], etching the fiber [6], and coating the fiber [7].
Coating an LPG with a material that has a higher RI than silica promotes a phenomenon referred to as the transition mode [8]. For a fixed coating thickness of a material with a given RI, there is a value of SRI at which a lower order cladding guided mode is coupled to the coating (i.e. the coating acts as a waveguide) [7–9]. As a consequence, higher order cladding modes than that guided by the coating shift their resonance wavelength towards the original wavelength of the adjacent lower order cladding mode, causing a redistribution of the cladding modes. This leads to a significant increase of the SRI sensitivity of coated LPGs, especially in the SRI range corresponding to the transition of cladding modes to the coating (i.e. transition region) [8]. For a coating material with a given RI, it is possible to tune the transition region to low SRIs by selecting an appropriate coating thickness [7–9].
Various coating techniques and materials have been investigated to promote transition modes in LPGs. Rees et al. coated an LPG with tricosenoic acid using the Langmuir-Blodgett technique [7]. Villar et al. and Korposh et al. used electrostatic self-assembly to deposit polymers [10] and silica nanoparticles [11], respectively, on LPGs. Although these techniques allow control of coating thickness, the coatings show weak adhesion to the optical fiber. The dip-coating technique has also been investigated to coat LPGs with polystyrene [8,12]. This method enables high RI sensitivities but with poor control of coating thickness.
Advanced coating techniques have also been investigated to promote transition mode behavior in LPGs with materials that show good adhesion to optical fibers. Radio-frequency plasma-enhanced chemical vapor deposition (RF PECVD) has been used to deposit silicon nitride [13] and diamond-like carbon coatings on LPGs [14]. Thermal evaporation has been used to deposit titanium dioxide (TiO2) on LPGs [15]. Chemical vapor deposition (CVD) and thermal evaporation are directional deposition methods and, thus, must be adapted to coat the cylindrical surface of optical fibers by rotating the fiber [15] or by suspending the fiber above the substrate Table [16]. Although, significant progress has been made to achieve uniform depositions with CVD by suspending the optical fibers above the substrate Table [16], control of coating thickness at the nanometer scale remains challenging using these directional deposition methods [17]. Additionally, it has been shown that CVD can cause thermal degradation of UV-written LPGs due to the high deposition temperatures [18].
Atomic layer deposition (ALD) has become an important technique in the semiconductor industry to deposit uniform layers of metal oxides with high dielectric constants, such as aluminum oxide (Al2O3) and hafnium oxide (HfO2), in MOSFET devices [19]. Metal oxide deposition by ALD uses two gas precursors: the metal precursor is the source of the reactive metal and the oxygen precursor is the source of oxygen. The film is grown in a cyclic process comprising injection of the metal precursor, purge of the metal precursor, injection of the oxygen precursor and purge of the oxygen precursor. Inert gases, such as argon (Ar) or nitrogen (N2), are typically used to carry the gas precursors from an external reservoir to the process chamber and to purge the process chamber between injections.
Each precursor is injected into the process chamber in a sufficient amount to ensure that the gas reaches all active sites on the substrate. The purge time is selected to ensure removal of unreacted precursors and by-products from the process chamber. If both of these conditions are met, then each gas precursor is present in the process chamber individually and undergoes gas-solid reactions solely on the surface of the substrate (i.e. self-limiting surface reactions). As a result, ALD is non-directional and is, therefore, well suited for deposition of conformal coatings on complex substrates [20]. The main disadvantage of ALD is its low deposition rate compared with other deposition methods such as CVD. The deposition rates of ALD are typically less than 100 nm/hr and are strongly dependent on the deposition temperature.
The deposition temperature of ALD depends on the type of process that is used to activate the gas precursors. The reactions in ALD can be thermally activated at various temperatures (i.e. typically 150 - 350 °C) and, therefore, this process is commonly referred to as thermal ALD. Process parameters such as pulse and purge times are optimized for each deposition temperature [21,22]. In thermal ALD, lower deposition temperatures require considerably longer purge times, due primarily to the slower desorption of the oxygen precursor (e.g. H2O) from the internal surfaces of the process chamber [23]. If purge times are not extended, incomplete purging of the oxygen precursor will occur and this will cause CVD-type reactions between the metal precursor and the residual oxygen precursor [23]. This can result in deposition of a non-uniform coating thickness [23].
ALD can also be performed with the aid of oxygen plasma following the metal precursor pulse, replacing the oxygen precursor used in the thermal process [24]. This process is referred to as plasma-enhanced ALD (PEALD). The plasma produces oxygen radicals that react with the metal precursor to form the metal oxide. The oxygen radicals have higher chemical reactivity than the oxygen precursors used in the thermal process. For this reason, as the deposition temperature decreases, the purge time of the oxygen precursor in thermal ALD becomes longer whereas the purge time of the oxygen in PEALD remains constant. Thus, as deposition temperature decreases, the deposition time of thermal ALD becomes longer than the deposition time of PEALD.
Deposition temperature and deposition time are important parameters when coating LPGs by ALD because, depending on the writing process, gratings can degrade at elevated temperatures and the rate of degradation increases with temperature. Thermal degradation of an LPG is manifest as a permanent decrease of the depth of the attenuation bands [18]. This decreased depth is undesirable for coated LPGs working as SRI sensors because the attenuation bands can become difficult to detect [18].
The most common process to write LPGs is by UV irradiation of hydrogen loaded germanium doped fibers. Dianov et al. showed that thermal degradation of UV-written LPGs in this type of fiber starts at temperatures between 300 °C and 400 °C [25]. The study was conducted by increasing the annealing temperature in steps of 100 °C with a waiting period of 5 minutes at each step. However, the coating of LPGs by ALD requires significantly longer exposure durations (i.e. hours) at temperatures between 150 - 350 °C to achieve coating thicknesses that promote transition mode behavior, assuming typical ALD deposition rates of less than 100 nm/hr. Studies of thermal degradation of UV-written LPGs in hydrogen loaded fibers at temperatures and over durations typically used in ALD are not available in the literature.
The development of ALD coating of LPGs to promote transition mode behavior is, relatively speaking, at an early stage. Śmietana et al. investigated the coating of LPGs, written by electric arc-discharge, with Al2O3 by thermal ALD at 150 °C [26]. Zou et al. investigated the coating of LPGs, written by CO2 laser, with Al2O3 by thermal ALD at 210 °C [27]. Śmietana et al. report a sensitivity value of 1850 nm/SRI in the SRI range between 1.3330 – 1.342 [26], and Zou et al. report a sensitivity value of 1500 nm/SRI in the SRI range between 1.3622 – 1.39 [27]. In these studies, thermal degradation is not expected because gratings written by either electric-are discharge [28] or CO2 laser [25] are stable at these ALD temperatures.
Both of Smietana and Zou provide limited coating thicknesses data based on scanning electron microscopy (SEM) measurements [26,27]. This data suggests that, as expected, thermal ALD at 210 °C results in higher coating uniformity than thermal ALD at 150 °C. However, as noted by Smietana, non-uniformity may be due to a lack of precision of the SEM measurements [26].
Smietana et al. also report UV-written LPGs coated with TiO2 by thermal ALD at 85 °C [17]. The authors selected this low deposition temperature to avoid removing the polymer layer that protects the optical fiber. Purge times and coating thickness uniformity are not reported.
PEALD is a promising alternative method for coating UV-written LPGs. As noted above, this process is non-directional and can be performed at low temperatures, thereby avoiding degradation of UV-written LPGs, while also maintaining shorter purge times than are required in thermal ALD to achieve uniform coating thickness. In this work, the first use of PEALD to coat a UV-written LPG with a metal oxide (HfO2) film is presented. HfO2 is a promising candidate for optical coatings because it has a high RI (i.e. approximately 2.0 at 1550 nm) and high transmission coefficient [29]. Additionally, HfO2 has higher density (i.e. greater than 9.0 g/cm3) compared with other materials used in ALD such as Al2O3 (i.e. between 2.5 - 3 g/cm3) and TiO2 (i.e. between 3 - 4 g/cm3). This is important because high-density films can prevent the penetration of chemical species such as water and hydrogen into the optical fiber. The presence of these species in the optical fiber leads to the formation of hydroxyl groups which may increase optical losses. Therefore, HfO2 has the potential to increase the stability of SRI sensors for long-term monitoring in harsh environments [30].
It is shown in this work that PEALD enables deposition of a uniform coating of a metal oxide on a UV-written LPG without causing significant thermal degradation of the LPG. The RI sensitivity of the coated LPG is investigated and it is shown that the high RI of the HfO2 coating leads to a two-step transition of the cladding modes which is explained using hybrid mode formulation (i.e. HE and EH modes). The depth of the attenuation band of the EH modes increases in the SRI range corresponding to the transition region and both HE and EH modes remain readily detectable at the transition point. This finding agrees with the numerical work conducted by Villar et al. [31] and is important because it enables the design of LPG sensors that operate close to the transition point, where the sensitivity of coated LPGs is high.
Methods
This work is performed using UV-written LPGs. Prior to deposition of the HfO2 coating, tests are conducted to determine a deposition temperature at which the rate of LPG degradation is acceptably low. LPGs and, for reference, silicon wafers are then coated with HfO2 using PEALD. Various coating thicknesses are investigated to determine the thickness range that promotes transition modes at low SRI, such as the RI of water. Coating thickness and uniformity are measured by SEM on coated optical fiber samples. As a reference, film thickness is measured by ellipsometry on a coated silicon wafer. For each deposition, the RI of the HfO2 coating is also determined by ellipsometry on a coated silicon wafer. Two LPGs coated in the same deposition are characterized to investigate the performance of PEALD for tuning the transition point of different LPGs at a given SRI. This data is compared with the response of a coated LPG modeled using the software Optigrating.
2.1 High-temperature stability of LPGs
Commercial LPGs with a period of 450 µm and a length of 20 mm (Technica S.A, Beijing) inscribed in Corning single mode fiber, SMF-28e, were used in this work. One LPG was held at 150 °C for 18 hrs and a second LPG was held at 200 °C for 6.5 hrs. The gratings were fixed to a glass slide using epoxy, to keep fiber tension relatively constant during the experiment. The wavelength shift and intensity of the attenuation band was monitored using a Micron Optics sm125 interrogator with a scan rate of 2 Hz and a resolution of 1 pm. Transmission spectra were acquired before and after high-temperature stability tests.
2.2 Deposition of HfO2 by PEALD
Depositions were performed using the Fiji F200 (Cambridge NanoTech Inc., USA) ALD system equipped with a remote inductively coupled plasma source. HfO2 was grown using TDMAHf as the metal precursor and O2 plasma as the oxidizer. The deposition temperature was 150 °C and the external reservoir of the precursor TDMAHf was maintained at 75 °C. Rationale for the selection of this deposition temperature is presented in Section 3.1.
The deposition of HfO2 comprised four steps: (1) TDMAHf pulse with Ar flow set at 60 standard cubic centimetres per minute (sccm); (2) TDMAHf purge with Ar flow set at 60 sccm; (3) plasma generated at 300 W, O2 flow set at 20 sccm, and Ar flow set at 200 sccm; (4) O2 purge with Ar flow set at 60 sccm. The pulse times for (1) and (3) are 0.25 s and 20 s, respectively. The purge times for (2) and (4) are 60 s and 10 s. These purge times are twice the duration specified by the system manufacturer for deposition on flat substrates [32].
Three depositions were conducted targeting coating thicknesses of 30 nm, 50 nm and 70 nm, referred to as Deposition 30, Deposition 50 and Deposition 70, respectively. Note that these labels correspond to nominal values, exact thickness measurements are obtained using characterization techniques described in Section 2.3.
The deposition growth rate provided by the manufacturer is approximately 0.108 nm/cycle; therefore, the number of cycles required to achieve the coating thicknesses of 30 nm, 50 nm and 70 nm are 279, 464 and 649, respectively. The time to complete each deposition was 7.7 hrs, 12.6 hrs and 17.6 hrs, respectively. The deposition times include initial waiting periods for the heaters to stabilize and for the substrates to achieve thermal equilibrium in the process chamber.
One LPG was coated with HfO2 in Deposition 30 and is labeled as LPG30. Similarly, one LPG was coated in Deposition 70, labeled as LPG70. For Deposition 50, two LPGs were coated with HfO2, labeled as LPG50 and LPG50*. Additionally, for Deposition 50, two optical fibers with no gratings were coated with HfO2 and coating thickness was determined by scanning electron microscopy (SEM). For each deposition, a 2 inch diameter silicon wafer was placed near the LPGs and film thickness and RI were measured by ellipsometry.
2.3 Characterization of coatings
The thickness and RI of the coatings applied to the silicon wafers were determined by ellipsometry (Uvisel, Horiba Scientific, US). The ellipsometer acquires spectral data over the wavelength range between 245 - 2100 nm to which the Cauchy dispersion model is fit [33]. Thickness is a direct output of this model and RI is determined as a function of wavelength n(λ) using Eq. (1),
where A, B and C are optical coefficients that are also direct outputs of the Cauchy dispersion model.For each silicon wafer, the optical coefficients were obtained at the center of the wafer by a single measurement. Film thickness was measured at five locations, one point at the center of the wafer and four points located at the corners of a 15 mm square, centered on the first location.
Coating thickness was measured on two optical fibers, referred to as fiber 1 and fiber 2, both of which were coated in Deposition 50. Each fiber was cleaved into three sections labeled as sections A, B and C. For each section, a total of 40 film thickness measurements were taken from 8 SEM images using the method described in [3].
2.4 LPG characterization after deposition
After deposition, LPGs were placed in custom fixtures that allow immersion of the grating in glycerin solutions while maintain constant tension (see Fig. 1). The RI of the solutions was measured with a commercial Abbe refractometer (Leica, Mark II plus, Canada) at 589 nm with a resolution of 1 × 10−4 RI units.
The transmission spectrum of LPGs was obtained using a white light source (Yokogawa, AQ4305) and an optical spectrum analyzer (Yokogawa, AQ 6331) providing a wavelength range between 1200 - 1700 nm with a resolution of 0.5 nm. To acquire spectra with higher resolution, the interrogator sm125 was used.
2.5 Modeling
Modeling was carried out using the software Optigrating (Optiwave, Canada). The linear polarized modes (LP0m) with m≥1 are calculated based on the assumption that the difference between the RI of the core and cladding is small (i.e. weak guiding fibers).
The fiber parameters of the Corning SMF-28e used in this work are not fully disclosed by the manufacturer. The diameter of the core is defined as the mode-field diameter, the cladding diameter is given with an accuracy of ± 7 µm, and the value of the core and cladding refractive indices vary with dopant concentration. Therefore, the refractive index of the core iss slightly adjusted to achieve a modeled spectrum that was equivalent to the measured spectrum of the physical grating. The core and cladding radii used in the model are 4.5 µm and 62.5 µm, respectively. The core and cladding refractive indices are 1.4683 and 1.4628, respectively. The grating period and length are as specified by the LPG manufacturer for the physical grating (see Section 2.1) and index modulation was assumed to be 3.8 × 10−4.
The model was used to simulate transition mode behavior of an LPG with a 60 nm coating with an RI of 1.932. The thickness and RI of the coating for the model were obtained by SEM and ellipsometry presented in Section 3.2, respectively.
Results
3.1 High-temperature stability of LPGs
The spectra of two LPGs in air at room temperature, before and after high-temperature stability tests, are shown in Fig. 2. For the LPG tested at 150 °C, the resonance wavelength decreases by 10.2 nm. The depth of the attenuation band increases from 25 dB before the test to 27 dB after the test, corresponding to coupling efficiencies between core-cladding modes of 99.7% and 99.8%, respectively. The shape of the attenuation band after the test remains similar to the original spectrum. The full width at half maximum (FWHM) decreases from 2.0 nm before the test to 1.5 nm after the test.
For the LPG tested at 200 °C, the resonance wavelength decreases by 21 nm and the depth of the attenuation band decreases by 25 dB. The coupling efficiency between core-cladding modes decreases from 99.98% to 89%. Most notably, the shape of the attenuation band changes significantly and the FWHM increases from 0.90 nm to 13 nm.
It can be concluded from these results that long-term (i.e. 6.5 hours) exposure to temperatures at or above 200 °C will lead to significant thermal degradation of UV-written LPGs. This exposure time is on the order of magnitude required to increase the RI sensitivity of LPGs coated with HfO2 by thermal ALD.
Based on results presented in Fig. 2, thermal degradation of UV-written LPGs at 150 °C over 18 hrs is relatively low. This exposure time is longer than the deposition time required to coat optical fibers with HfO2 by PEALD at the targeted coating thicknesses tested in this work (i.e. between 30 - 70 nm). Therefore, the temperature of 150 °C was selected as the deposition temperature to coat LPGs with HfO2 by PEALD.
3.2 Characterization of films
Ellipsometry measurements of thickness and RI for films deposited on flat silicon wafers are shown in Table 1. Non-uniformity was calculated as the difference between extreme values normalized by twice the mean, based on five measurement points.
Table 1. Thickness and RI of HfO2 for different depositions measured on silicon wafers by ellipsometry
For each deposition, n(λ) was calculated using Eq. (1), and the RI value at 1550 nm is shown in Table 1. The data shows that the RI of the film increases as the film thickness increases. The increase of RI with film thickness on silicon wafers has also been observed in the deposition of silicon nitride and diamond-like carbon by RF PECVD [34], as well as in the deposition of TiO2 by thermal ALD [17]. However, there are also studies that report the opposite trend, for instance, the deposition of Al2O3 on silicon wafers by thermal ALD [26]. Where the RI increases with film thickness, this phenomenon is attributed to an increase in film density and/or by the stress that the substrate induces in the film [17,34].
Growth per cycle (GPC) for the ALD process is calculated by a linear regression of the experimental film thickness data, expressed as a function of number of deposition cycles. Based on the three data points obtained in this work, the calculated GPC is 0.108 nm/cycle. This value is in agreement with the expected GPC provided by the ALD manufacturer.
The results of the SEM-based coating thickness measurements taken on fiber 1 and fiber 2 after Deposition 50 are shown in Fig. 3. For each section, the standard deviation was calculated from a total of 40 thickness measurements taken from 8 SEM images (see Section 2.3).
Fig. 3 Coating thickness measured by scanning electron microscopy on fibers coated by Deposition 50.
The relative standard deviation presented in Fig. 3 for Section A, B, C of Fiber 1 is 6.7%, 9.5%, and 6.4%, respectively, and for Section A, B, C of Fiber 2 is 4.4%, 5.3%, and 4.5%, respectively.
The coating thickness and standard deviation calculated from measurements taken on the three fiber sections of fiber 1 and fiber 2 is 59.7 ± 5 nm, and 59.5 ± 3 nm, respectively.
3.3 LPG characterization after deposition
Figure 4 shows the transmission spectra of LPG30, LPG50 and LPG70 in air, before (i.e. bare LPGs) and after deposition. The wavelength shifts after deposition for LPG30, LPG50 and LPG70 are 9.9 nm, 11.5 nm, and 11.5 nm, respectively. The depth of the attenuation band of LPG30 decreases by 6.3 dB and increases by 6.0 dB and 4.0 dB for LPG50 and LPG70, respectively. The coupling efficiency between core-cladding modes for all coated LPGs is higher than 99.0%. Additionally, for all LPGs investigated in this work, the shape of the attenuation band is preserved after the depositions. As expected, degradation of the transmission spectrum of the LPGs after deposition at 150 °C is acceptably low.
The wavelength shifts and the changes in the depth of the attenuation bands shown in Fig. 4 are believed to result both from exposure to elevated temperatures, as discussed earlier, and from the presence of the coating. The deposition of a coating material with an RI higher than silica is also expected to affect the transmission spectra of the LPGs [35]. The coating increases the effective RI of the cladding modes which leads to a decrease of the resonance wavelength. The effect of the coating on the depth of the attenuation bands is less clear. Results in the literature show that the coating of LPGs can lead both to an increase [8] and to a decrease in the depth of the attenuation band [12,35].
Figure 5 shows the characterization curves for LPG30, LPG50 and LPG70 based on immersion in solutions having a range of RI values. The characterization of a bare LPG is also depicted in Fig. 5. To help data visualization, the experimental data of the bare LPG was offset to the value of the coated LPGs, where the SRI is 1.0.
The modes depicted in Fig. 5 are hybrid modes (i.e. HE1,j and EH1,j). In the absence of a coating, linearly polarized (LP) mode approximation (i.e. LP0,j modes) is typically used, based on the assumption that the contrast between the RI of the core and cladding is small (i.e. weak guiding fibers). However, when LPGs are coated with a material of high RI, the contrast between the RI of the cladding and the coating becomes large and hybrid mode formulation is adopted [31,36,37]. The notation used to label the modes is based on the transmission spectra of the LPGs in air. The first attenuation band centered at the lowest wavelength is labeled as the HE1,2 mode. The HE1,2 mode is equivalent to the LP0,2 mode described by the LP mode approximation. The attenuation band of the HE1,2 mode is centered at a wavelength which is approximately equal to the center wavelength of the modeled LP0,2 mode. The second attenuation band with significant depth is labeled as the HE1,4 mode which is equivalent to the LP0,3 mode, and so on. Between each HE mode, an attenuation band with significantly reduced depth is labeled as EH mode. For example, the EH1,7 mode is located between the HE1,6 and the HE1,8 modes. The depth of the attenuation bands of the EH1,j modes increases with the increase of SRI, as described later.
Figure 5 shows a phenomenon referred to as the two-step transition [31]. As the SRI increases, the resonance wavelength of the HE1,8 mode of LPG70 decreases until it approaches the original wavelength of the EH1,7 mode at an SRI of 1.365 (see end of 1st step transition in Fig. 5), at which point the data plateaus. As SRI increases above 1.384, the resonance wavelength of HE1,8 mode of LPG70 continues to decrease until it approaches the original wavelength of the HE1,6 mode.
The two-step transition is a behavior of transition modes that can be explained using the hybrid mode formulation. As the SRI increases, cladding modes (e.g. HE) higher than that guided by the coating (e.g. HE1,j) shift their resonance wavelength towards EH1,j-1 mode (i.e. first-step transition), followed by another transition of HE1,j towards HE1,j-2 mode (i.e. second-step transition). The same is true for EH modes, i.e. there is a first-step transition of EH1,j to HE1,j-1 mode, followed by a second-step transition of EH1,j to EH1,j-2. This phenomenon has been theoretically investigated by Villar et al. [31] and experimentally observed by Tatam et al. [36].
The two-step transition is less pronounced for LPGs with thinner coatings. For instance, the plateau between the first and second steps of the transition of the HE1,8 mode of LPG30 is not apparent in Fig. 5.
Figure 6 shows the change of wavelength and intensity of the transmission spectrum of LPG 70 as a function of SRI. It can be observed that, as the SRI increases, the intensity of the HE1,8 mode decreases until the resonance wavelength of this mode approaches the original wavelength of the EH1,7 mode. At this point the intensity of the HE1,8 is approximately 0 dB which is approximately the intensity of the EH1,7 mode at an SRI of 1.0. As the SRI increases above 1.4100, the intensity of the HE1,8 mode increases until the resonance wavelength of this mode approaches the original wavelength of the HE1,6 mode. At this point, the intensity of the HE1,8 mode is −15 dB which is similar to the intensity of HE1,6 mode at an SRI of 1.0. The intensity of the EH1,7 mode undergoes an opposite trend. As the SRI increases, the intensity of the EH1,7 mode increases and reaches a maximum when the wavelength of this mode approaches the wavelength of the HE1,6 mode. After this, the intensity of the EH1,7 mode decreases until reaching approximately 0 dB. This behavior is discussed further in Section 4.
Figure 7 shows the spectrum of the LPG70 at different SRIs for additional visualization of the behavior shown in Fig. 6. In this figure, the HE1,6 mode is represented by letters A and B, the EH1,7 mode is represented by letters B’ to G’, and the HE1,8 mode is represented by G” and H”. Each letter is associated with a different SRI, for example, B and B’ correspond to the spectrum of LPG70 when surrounded by water (SRI of 1.3327).
Fig. 7 Spectrum of LPG70 at different SRIs. The information is organized by (label, mode, SRI) as following: (A, HE1,6, air), (B, HE1,6, 0%), (B’, EH1,7, 0%), (C’, EH1,7, 4%), (D’, EH1,7, 8%), (E’, EH1,7, 30%), (F’, EH1,7, 50%), (G’, EH1,7, 60%), (G”, HE1,8, 60%), and (H”, HE1,8, 70%).
Figure 7 shows that the depth of the attenuation band corresponding to the HE1,6 mode is significantly reduced when the SRI increases from 1.0 (see label A) to 1.3327 (see label B). This occurs because the HE1,6 mode is operating in the transition region at the SRI of 1.3327 and the coupling coefficient of the HE1,6 mode decreases in this region (see Section 4) .
Similarly, the HE1,8 mode is operating in the second transition region at the SRI of 1.4204 (see label G”). When the SRI increases from 1.4204 to 1.4348, the HE1,8 mode shifts away of the transition region and the depth of the attenuation band increases (see label G” and H”).
The depth of the attenuation band of the EH1,7 mode in Fig. 7 shows a different behavior with the variation of the SRI in the transition region than the HE1,6 and HE1,8 modes. Between an SRI of 1.3327 (see label B’) and 1.3448 (see label D’) the depth of the attenuation band of EH1,7 mode increases. In this SRI range, this mode is operating in the transition region. The depth of the attenuation band reaches a maximum value at approximately 1533 nm (see label E’). The region of maximum depth corresponds to the end of the 1st step transition of the EH1,7 mode of LPG70. After this, the depth of the EH1,7 mode decreases with further increase of SRI (see trend E’ - G’).
Experimental data presented in Fig. 5 is fitted by the Cumulative Lorentz equation, expressed as following [38]:
For each mode, λ0 is the original resonance wavelength, Smax is the maximum sensitivity, ∆SRI represents the bandwidth at half maximum of the curve fits depicted in Fig. 8 and SRItp is the SRI of the transition point. For LPG30, only one curve fit was calculated for each mode because the characterization curves presented in Fig. 5 only show one-step transition. For LPG50 and LPG70, one curve fit was calculated for the HE1,6 and two curve fits were calculated for each EH1,7 and HE1,8 modes due to the two-step transition behavior of these modes.
Fig. 8 Sensitivity curves of (a) LPG30, (b) LPG50, and (c) LPG70. Peaks correspond to transitions identified in Fig. 5.
Sensitivity data for coated LPGs are calculated as the absolute value of the first derivative of the Cumulative Lorentz fits. Sensitivity curves of LPG30, LPG50 and LPG70 are depicted in Figs. 8(a)-8(c), respectively.
For LPG30, Fig. 8(a) shows that the transition point (i.e. maximum sensitivity) of HE1,6, EH1,7 and HE1,8 modes occurs at SRIs of 1.4340, 1.4430, and 1.4430, respectively. For LPG 50, Fig. 8(b) shows that first-step transition of modes EH1,7 and HE1,8 is located at lower SRI, relative to second-step transitions. Figures 8(b) and 8(c) show that with the increase of coating thickness, the separation between the curves of first-step transition and second-step transition increases. For instance, for the HE1,8 mode of LPG50, the first and second step transition occurs at the SRI of 1.4100 and 1.4371, respectively, whereas for the same mode of LPG70, the first and second step transition occurs at the SRI of 1.3355 and 1.4222, respectively.
Figure 9 shows the characterization curves of LPG50 and LPG50*. Additionally, the figure shows the modeling of an LPG with a coating thickness of 60 nm and an RI of 1.932. The value of coating thickness was determined from thickness measurements taken on fiber 1 and fiber 2. The RI was measured by ellipsometry on the silicon wafer of Deposition 50 (see Section 3.2). The dispersion of the RI of HfO2 was calculated in the wavelength range between 1450 - 1700 nm using Eq. (1). The maximum RI variation is only 0.002 RI units. Therefore, the approximation of a constant RI value for a coating thickness of 60 nm is reasonable due to the low dispersion of the coating in the wavelength range investigated in this work. For comparison between the model and experimental results, data for LPG 50 and LPG50* were offset to the value of the LP04 mode of the modeled LPG, where SRI is equal to 1.0.
Figure 9 shows that the behavior of LPG50 is identical to LPG50* for all the modes monitored in this work. For the modeled LPG, the LP04 mode follows the same trend of the experimental LPGs; however, the LP04 mode undergoes a wider wavelength shift relative to the experimental data. Figure 9 also shows that the LP05 mode agrees with the HE1,8 mode at the first-step transition. Model and experimental results differ for higher SRIs than that corresponding to end of first-step transition.
Figure 10 shows the sensitivity curves of the HE1,6 mode for LPG50 and LPG50*, and the LP04 mode for the modeled LPG. The values of sensitivity predicted by the model are higher than the experimental results. The transition points of LPG50, LPG50*, and LP04 mode occur at the SRI of 1.4000.
Discussion
In this work, the first use of PEALD to coat UV-written LPGs is reported. This method allows deposition of conformal and uniform coating thicknesses on optical fibers. The measurements of coating thickness taken by SEM show low variability for two optical fibers coated in the same deposition. The mean coating thickness and standard deviation of fiber 1 and fiber 2 coated in Deposition 50 are 59.7 ± 5 nm, and 59.5 ± 3 nm, respectively.
As a result of this uniform deposition, two LPGs coated in the same deposition process exhibit identical behavior as a function of SRI. Both sensors show the transition point at the same SRI (i.e. 1.4000). This result suggests that multiple LPGs can be coated simultaneously with highly uniform coating thickness and with consistent thickness from fiber to fiber. This finding suggests that PEALD will allow scaling the production of sensors and thus reduce the deposition cost per sensor.
Film thickness measured on the silicon wafer coated by deposition 50 is lower than coating thickness measured by SEM on fiber 1 and 2 (see Table 1). This difference may be due to the initial growth of HfO2 on each substrate. In an ideal ALD process, GPC is constant during the deposition. However, in practice, GPC at the start of the deposition may differ due to the initial growth of HfO2 on a substrate, which is referred to as nucleation [39]. Nucleation depends on the concentration of reactive sites available on a substrate. In the case of HfO2, the reactive sites are hydroxyl groups (OH). Therefore, the amount of HfO2 that is deposited during the initial cycles is proportional to the concentration of OH groups on the surface of the substrate [39]. Optical fibers used in this work are comprised primarily of silicon dioxide which provides more sites to which the OH groups can bond than is the case for the wafers, which are made of pure silicon. Therefore, optical fibers provide a higher concentration of nucleation sites and initial GPC is expected to be higher than on silicon wafers. Once the substrate is fully covered with HfO2, GPC is expected to be relatively constant, regardless of the substrate material because the growth takes place on the HfO2 film [39].
The major advantage of PEALD over thermal ALD is that the deposition temperature can be reduced, so as to avoid thermal degradation of the grating, without a significant increase in the purge times. The cycle durations specified by the ALD manufacturer for PEALD and thermal ALD at 150 °C are 55.3 s and 60.3 s, respectively. Considering a target coating thickness of 70 nm and assuming purge times with twice the duration specified by the ALD manufacturer, the times to complete each deposition for PEALD and thermal ALD are 16. 2 hrs and 21.7 hrs, respectively, excluding time for thermal stabilization. The difference between deposition times of PEALD and thermal ALD becomes even more significant at lower deposition temperatures. For example, the recommended cycle durations for PEALD and thermal ALD at 90 °C are 145.3 s and 240.3 s, respectively. These cycle times correspond to deposition times of 43.8 hrs for PEALD and 86.5 hrs for thermal ALD to achieve a coating thickness of 70 nm, assuming purge times with twice the duration specified by the ALD manufacturer. Depositions at temperatures in the range of 85 °C may be needed in applications that require retention of the polymer coating that protects the optical fiber [17].
In this work, the use of PEALD was demonstrated to coat UV-written LPGs by selecting a deposition temperature that avoids significant thermal degradation (i.e. 150 °C). The spectra of LPGs before and after deposition show that the depth of the attenuation bands is not significantly reduced. This is important because it improves the detection of the attenuation bands, especially at the transition region where there is an intrinsic reduction of the depth of the attenuation bands.
Coating of LPGs with HfO2 promotes a two-step transition behavior, due to the high RI of HfO2, which can be explained using the hybrid mode formulation. The depth of the attenuation band of an EH mode increases within the transition region, whereas this depth decreases for an HE mode.
The change of the depth of the attenuation bands in two-step transition behavior has been numerically investigated by Villar et al. [31]. Similar to the wavelength shift of the HE1,j mode towards the EH1,j-1 mode at the first-step transition, the coupling coefficient of the HE1,j mode also approaches the coupling coefficient of the EH1,j-1 mode [31]. At the second-step transition, the coupling coefficient of the HE1,j mode approaches the coupling coefficient of the HE1,j-2 mode. This behavior is also observed for the transition of the coupling coefficient in the EH modes. The coupling coefficient of the HE modes is higher than that of the EH modes. This leads to a decrease in the depth of the attenuation bands when the HE modes approach the resonance wavelength of the EH modes, and an increase in the depth of the attenuation bands when the EH modes approach the resonance wavelength of the HE modes [31]. This behavior is illustrated in Fig. 6.
There is an important difference in the intensity of the attenuation bands in two-step transitions relative to the intensity of the attenuation bands in one-step transitions. In the latter case, the region of reduced intensity coincides with the region of higher sensitivity (i.e. transition point). In the former case, the region of the reduced intensity coincides with a region of lower sensitivity (see Fig. 6).
In this work, it is shown experimentally that the attenuation band of the EH1,7 mode of LPG70 reaches a significant depth (i.e. higher than 10 dB) when operating within the transition region (see Fig. 7). This is important because the EH modes can be tuned to operate closer to the transition point, relative to LP modes, with amplitudes that are readily detectable by commercial optical interrogators.
The transition region of LPG70 is located at low SRIs which leads to enhanced sensitivity of this grating at the SRI of water. The sensitivities of the HE1,6, EH1,7 and HE1,8 modes are 525 nm/SRI, 1050 nm/SRI and 1820 nm/SRI, respectively, at an SRI of 1.3327. The sensitivity of the HE1,6 mode of a bare LPG in the vicinity of water is 12 nm/SRI. These results show that the deposition of HfO2 on an LPG with a coating thickness of 70 nm leads to an increase in sensitivity of 43 times at an SRI in the vicinity of water. Note that the sensitivity of a coated LPG is highly dependent on the cladding mode. Therefore, the sensitivity of LPGs coated by PEALD can be further improved by tuning the grating parameters to promote cladding modes of higher order, or by etching the fiber to combine the transition mode behavior with the dispersion turning point [17,40,41].
The LP modes of a coated LPG are modeled using Optigrating software and compared with data of the HE modes that were obtained experimentally. Although this approach is not suitable to model the response of both hybrid modes (HE and EH modes), we show that it can be used to estimate the SRI at which the transition point of the HE mode occurs.
Conclusions
PEALD is investigated for coating of LPGs with HfO2. This method allows lower deposition temperatures relative to thermal ALD to enable coating of UV-written LPGs without thermal degradation.
Depositions were performed at 150 °C and the process parameters, pulse time and purge time, were selected to ensure only ALD-type reactions. As a result, excellent film uniformity was achieved on silicon wafers measured by ellipsometry (i.e. non-uniformity between 0.8% - 0.9%). Consistent wavelength shifts, as a function of SRI, were also achieved on LPGs coated by the same deposition. The transition point of both LPG50 and LPG50* occurs at 1.4000. The maximum sensitivity of LPG50 and LPG50* is 1545 nm/SRI and 1445 nm/SRI, respectively.
Coating of LPGs with HfO2 promotes two-step transitions due to the high RI of the coating material. These transitions can be explained using hybrid mode formulation. The attenuation bands of the HE and EH modes are readily detectable at the transition point. The results obtained in this work suggest that two-step transition behavior can be used to monitor SRI changes while operating closer to the transition point compared to one-step transitions.
Acknowledgments
The authors gratefully acknowledge funding provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), Carbon Management Canada (CMC), and the Korean Carbon Capture and Sequestration R and D Center (KCRC). This work made use of the shared facilities of 4D LABS, which are supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversification Canada, and Simon Fraser University.
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