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Abstract
Broadband multilayer dielectric gratings (MDGs) with rectangular HfO2 grating profile were realized for the first time using a novel fabrication process that combines laser interference lithography, nanoimprint, atomic layer deposition and reactive ion-beam etching. The laser-induced damage initiating at the grating ridge was mitigated for two reasons. First, the rectangular grating profile exhibits the minimum electric-field intensity (EFI) enhancement inside the grating pillar compared to other trapezoidal profiles. Second, our etching process did not create nano-absorbing defects at the edge of the HfO2 grating where the peak EFI locates, which is unavoidable in traditional fabrication process. The fabricated MDGs showed a high laser induced damage threshold of 0.59J/cm2 for a Ti-sapphire laser with pulse width of 40 fs and an excellent broadband diffraction spectrum with 95% efficiency over 150 nm in TE polarization.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The invention of chirped-pulse amplification (CPA) technique is a major breakthrough for the ultra-intense ultra-short lasers [1,2]. The diffraction gratings, as the key optical components in compressors of CPA [3], should meet the requirements of high diffraction efficiency (DE) over a broad bandwidth [4] and high laser-induced damage threshold (LIDT) [5].
The metallic gratings were first used as the diffraction gratings in the CPA laser systems. Even though the diffraction efficiency of the gold gratings exceeds 95% over a wide bandwidth for the m=-1 order in a near-Littrow configuration [6], the inherent absorption losses of the gold grating materials can significantly limit the optical performance and resistance to laser damage [7]. Multilayer dielectric grating (MDG) was first proposed by Svakhin et al. [8] to increase the LIDT because of the negligible absorption losses of the dielectric materials [9,10]. The frequently-used grating materials are SiO2 and HfO2. SiO2 gratings lead to higher LIDT but limited diffraction bandwidth, therefore, HfO2 gratings are more preferred regarding to broadband high diffraction efficiency and higher tolerance to the fabrication errors [11,12]. For example, a rectangular MDG that comprises Ta2O5/SiO2 high-reflection multilayers and HfO2 grating structure could yield DE higher than 97.5% for TE polarization light over the 100 nm bandwidth to meet the requirement of bandwidth in a femtosecond pulse laser system [13].
However, the fabrication of rectangular HfO2 grating profile is difficult, which inevitably leads to the degraded LIDT and diffraction efficiency of MDGs. Laser interference lithography followed by ion-beam etching is the predominant process to make MDGs over a large area [14,15]. Unfortunately, the faceting phenomenon and the redeposition of HfO2 during the conventional ion beam etching process always results in trapezoidal grating profile with a sidewall angle as low as 70 degree [16–19]. Numerical simulation reflected that the peak EFI in the grating ridge increased with the decreasing sidewall angle [20, 21]. Meanwhile, during the etching process, the ion bombardment on the sidewall of HfO2 gratings would induce the absorption defects in the edge of grating pillars where exists strongest local electric field intensity (EFI). The joint effect of the intensification of electric field enhancement and the nano-absorbing defects in the sidewall of grating pillars decreased the LIDT of MDGs significantly [22–24]. So, it is highly desirable to explore new fabrication process that realizes rectangular HfO2 grating profile and avoids creating nano-absorbing defects at HfO2 grating ridge where the peak EFI locates, which will lead to higher LIDT and better diffraction efficiency of MDGs.
In this paper, the influence of sidewall angle on the diffraction efficiency and the EFI enhancement of MDGs was first analyzed. Then a damascene process that combines laser interference lithography, nanoimprint, atomic layer deposition and reactive ion-beam etching was proposed to manufacture the rectangular HfO2 grating profile that minimized the EFI in grating pillar. Moreover, the ion beam bombardment to the sidewall of HfO2 grating pillars was prevented during our fabrication process and no extra nano-absorbing defects were created at the region of peak EFI. Finally, we obtained a rectangular MDG with an efficiency exceeding 95% over a wide bandwidth from 714nm to 865nm and high LIDT of 0.59J/cm2 with a laser pulse of 40 fs.
2. Design and analysis
The MDG is composed of a rectangular surface-relief grating, a matching layer, a multilayer dielectric high-reflection coating and the substrate, as illustrated in Fig. 1 [11]. The HfO2 (ng = 2.01 at 800nm) is chosen as the grating material, while SiO2 match layer and the Ta2O5/SiO2 high reflector helps to obtain high efficiency over a broad wavelength range [13]. The coating structure of Ta2O5/SiO2 high reflector is (HL)^9H, where H and L denote the high-index (Ta2O5, nH = 2.17 at 800nm) layer and the low-index (SiO2, nL = 1.47 at 800nm) layer, respectively. The optical thicknesses of the H and L layers are quarter-reference wavelength. The incident angle is 57° and the period Λ is chosen as 510nm to avoid the guide-mode resonance (GMR) inhibition in the MDGs. Rigorous coupled-wave analysis (RCWA) [25] was employed to calculate the DE and the EFI distribution in MDGs.
Fig. 1. Schematic representation of an MDG that is composed of the rectangular surface-relief HfO2 grating, a SiO2 matching layer, a Ta2O5/SiO2 multilayer dielectric high-reflection mirror and the quartz substrate.
A merit function considering both -1st broadband diffraction efficiency centered at 800 nm and minimizing EFI enhancement is chosen:
The optimized four parameters {h, t, f, λ} of the MDG are given in Table 1. The corresponding spectrum is shown in Fig. 2(a1). The −1st order diffraction efficiency for TE polarization of the designed MDG exceeds 95% in a 150-nm-wide wavelength range (from 727 nm to 877 nm) and the maximum value is 99% at a wavelength of 757 nm. Moreover, as shown in Fig. 2(a2), the peak EFI in the grating ridge is 2.125 at the wavelength of 800 nm, which exists in the sidewall of HfO2 grating at the opposite side of the incoming wave. For comparison, the maximum of the EFI in multilayer dielectric high-reflection coating is 0.893, which is much smaller than the peak EFI in the grating ridge.
Fig. 2. The diffraction spectra and peak EFI locations in rectangular and trapezoidal gratings at the wavelength of 800nm.
Table 1. Optimization results of rectangular grating parameters
In the conventional fabrication process, the profiles of HfO2 gratings are deteriorated to trapezoidal form, which will affect the diffraction efficiency and the near field EFI enhancement of MDGs. It has been reported that the sidewall angle that is defined as the corner between the sidewall of the gratings and the coating plane deviated from 90° to 70° [18]. Previous studies have revealed that the maximum EFI in the grating ridge monotonously increases with the decreasing sidewall angle [21]. Here, we analyzed the diffraction efficiency and the EFI enhancement of two trapezoidal HfO2 gratings with sidewall angle of 70°. The first trapezoidal HfO2 grating kept the bottom width as 153 nm, which was corresponded to a duty cycle f as 0.3. The shrunken grating pillars lead to a deteriorated diffraction performance in the region of longer wavelength, as shown in Fig. 2(b1). What is worse, the maximum EFI in the grating ridge increases to 3.829, as shown in Fig. 2(b2). The maximum EFI is more than 80% higher than that of the rectangular grating profile, where the increment is at the similar level as previously reported [18]. This will decrease the LIDT greatly because the damage threshold is inversely proportional to the maximum EFI in grating [5]. We also considered that the trapezoidal HfO2 grating maintain the top width as 153 nm. The enlarged grating pillars also resulted in a degraded diffraction performance but in the region of shorter wavelength, as shown in Fig. 2(c1). What is common is that the maximum EFI in the grating ridge also increases to 3.040, as shown in Fig. 2(c2). Therefore, much efforts are worth to be devoted to control the HfO2 grating to be rectangular profile. It is worth to note that the rectangular grating profile still give the minimum EFI enhancement in the grating ridge even the trapezoidal profile was used in the optimization process.
3. Fabrication process of multilayer dielectric gratings
The multilayer dielectric high reflection coating consisted of SiO2 and Ta2O5 was deposited on fused silica substrates using ion-beam assisted deposition in an optorun OTFC- 1300 vacuum coating system. The deposition rates of SiO2 and Ta2O5 were 8.0 and 3.0 Å/s, respectively. The detailed deposition process can be found in Ref. [27].
To overcome the difficulties of fabricating rectangular gratings for MDGs, a damascene process [28] that combines laser interference lithography (LIL), nanoimprint lithography (NIL), atomic layer deposition (ALD) and reactive ion-beam etching (RIE) was utilized to manufacture the rectangular HfO2 grating profile, as illustrated in Fig. 3.
Fig. 3. Schematic representation of the new damascene process combining laser interference lithography, nanoimprint, atomic layer deposition and ion beam etching.
Firstly, the large area inverse resist structure with vertical sidewall was patterned by ultraviolet-nanoimprint lithography (UV- NIL) using soft polydimethylsiloxane (PDMS) [29]. As showed in Fig. 3 (a1-a3), the silicon master template was fabricated via laser interference lithography (LIL), lift-off process for Cr mask and reactive ion etch (RIE), which is capable of fabricating rectangular Si grating with large duty cycle on large areas under the optimum etching condition [30,31]. The Si grating template with 502nm in period, 0.73 in duty cycle and nearly vertical sidewall was achieved. The height of the Si grating master stamp is 283nm, which is larger than the height of the target HfO2 grating. A PDMS stamp was used as a soft mold that is against Si master template to improve the imprinting pattern qualities in UV- NIL step, as illustrated in Fig. 3 (a4). Then, the PDMS mold was imprinted into the resist (PMMA) spin-coated on the multilayer dielectric mirrors through UV- NIL to obtain the resist grating structure that is inverse to the target HfO2 grating structure, as illustrated in Fig. 3(a5-a6). The UV-NIL process was performed on a NanoImprinter at 65◦C and 150 kPa. After cooling and separation of the soft stamp from the substrate, the residual PMMA was removed by O2 plasma in the trench between the two grating pillars. Fig. 4(a) presents the imprinted PMMA gratings with a period of 502nm, a grating width of 365nm, which is almost the same structure as Si master template.
The HfO2 film was deposited by ALD technique as shown in Fig. 3 (a7), Tetrakis (dimethylamino) Hafnium (TDMAH) and H2O were chosen as Hf precursor and oxidant, respectively, that could maintain chemical reactions to grow the HfO2 film at a low deposition temperature of 95 °C [32]. This deposition temperature is lower than the glass transition temperature of PMMA to preserve the physical integrity of the resist grating. During the ALD process, the pulsing time of TDMAH and H2O, purging time of N2 was set as 400ms, 60ms and 60s respectively, to ensure the complete reaction and full removal of the excessive precursors and reaction byproducts. The deposition rate is about 0.12 nm/cycle. The refractive index n of HfO2 films is 2.01 at 800nm under the deposition condition in our ALD equipment. The HfO2 film measured by X-ray photoelectron spectroscopy have a O:Hf ratio of ∼2. This result shows that the ALD produce a stoichiometric HfO2 coating, which is in agreement with the study in Ref. [33]. Because the deposited HfO2 film fills the trenches from both sides due to the conformality of ALD process, the required total film thickness is larger than half of the width of trenches between the resist grating pillars. Therefore, we deposited HfO2 film with thickness of 96 nm via 800 deposition cycles to ensure that HfO2 had filled in all trenches sufficiently.
Then the overcoated HfO2 film that on the top surface of the resist was removed by RIE in a mixture of CHF3 and Ar. The CHF3 and Ar gas flow rates, working pressure, RF power, and DC bias voltage were maintained at 40 and 20 sccm, 20mTorr, 150W, and −420 V, respectively. The etch rate of HfO2 was about 4nm/min under this condition, and the etching time was carefully controlled to obtain the height of the target HfO2 grating. Finally, the remaining resist was removed by O2 plasma and the rectangular HfO2 grating structures were fabricated.
It should be emphasized here that our fabrication process can not only manufacture the rectangular grating profile, but also avoid the ion beam bombardment to the sidewall of HfO2 grating pillars and no extra nano-absorbing defects are created at the region of peak EFI compared to the conventional process. The joint effect is supposed to increase the LIDT of MDGs significantly.
4. Results and discussions
The fabricated MDG was characterized using scanning electron microscopy (SEM), and Fig. 4(b-d) displays top and cross-sectional images of the fabricated grating. As seen in cross-sectional view in the SEM image, the experimental parameters for the HfO2 grating are estimated with a period of 502nm, a width of 137 nm and a depth of 185 nm. The grating has an inverted trapezoidal profile with the sidewall angle of the pillar is about 92.5°, which is very close to rectangular profile. It is fair to say that this result shows a good correspondence to the designed grating configuration. It is the first time to realize MDGs with rectangular HfO2 grating profile using this novel fabrication process, which breaks through the limitations of conventional etching process that always results in trapezoidal grating profile of the fabricated HfO2 MDGs [17,18].
Fig. 4. (a) SEM cross-sectional view of the inverse PMMA grating patterned by nanoimprint, (b-d) SEM top and cross-sectional views of the fabricated MDG, (e) The EFI simulation for the fabricated MDG at the wavelength of 800 nm, (f) The simulation and measurement results of diffraction spectra for the fabricated MDG sample.
Then we performed a RCWA simulation with the measured structure parameters on the actual fabricated sample. The maximum EFI is still in the ridge of HfO2 grating at the opposite side of the incoming wave, and the value is 2.244, which is only 5.6% higher compared to the rectangular grating profile, as shown in Fig. 4(e). It indicates that the LIDT will not change significantly with the fabricated grating profile.
The diffraction spectrum of the MDG was tested by angle-resolved spectrum system based on Fourier transform. The measurement was carried in a darkroom to improve the signal-to-noise ratio. The −1st order DE over the wavelength ranges at an incidence angle of 57° for TE polarization by simulation and measurement of the fabricated MDG sample are shown in Fig. 4(f). The measurement result for −1st order DE of the MDG fabricated in this work exceeds 95% in a 151nm-wide-wavelength range (from 714nm to 865nm) and the highest value reaches up to 99.2%, which corresponds well with the simulation result of the fabricated MDG. Compared with the similar work, the -1st order DE of fabricated trapeziform HfO2 MDG for femtosecond pulse compressor exceeds 90% over 128 nm width [18]. To the best of our knowledge, our fabricated MDG possesses the broadest bandwidth with high diffraction performance.
A Ti-sapphire laser system operating at 800 nm with the P-polarized pulse was used to measure the laser damage threshold of the MDG. The pulse width is 40 fs and the beam profile is nearly a Gaussian profile with a diameter of 60 µm at 1/e2;. The system could be run at a repetition rate of approximately 1 kHz. A half-wave plate was used to vary the pulse from P-polarized to S-polarized. The damage test facility was similar to the experiment setup in Ref. [18]. Testing is carried out under air environment at an incidence of 57° in TE polarization. The laser damage tests on the MDG were performed in 1-on-1 mode. During the test, every damage test site was exposed to one laser pulse, and twenty sites were irradiated by the same laser fluence. The laser fluence was gradually increased and the damage was determined by a Nomarski microscope with 200× magnification. The damage threshold definition is according to ISO11254 [34]. Here, the relative error of the damage threshold by measurement is assessed closer to 3% due to the laser energy measurement uncertainty and facula effective area measurement uncertainty. As illustrated in the fitting line for the damage probability with respect to the laser fluence in Fig. 5, the measured LIDT of MDG is 0.59 ± 0.02 J/cm2 for a pulse width of 40 fs at 57° in TE polarization. Compared with the LIDT of trapeziform HfO2 MDG manufactured by traditional etching process in Ref. [18], the LIDT of our fabricated rectangular MDG shows an 11% increment.
Fig. 5. Damage threshold data with curve fits for the MDG irradiated with the 40fs pulse at the wavelength of 800nm.
The damage morphologies of the MDG were observed by SEM as shown in Fig. 6, and it is obvious to see that the grating pillars are split and destroyed under the irradiation of laser with the fluence beyond the LIDT. From the overview of the main damage site, the grating pillars damaged more seriously in the center of laser irradiation spot that shown in the right of Fig. 6(a). In the enlarged figure of the damage morphology in the edge of the damage area, the small cracks first appear on the opposite side of the incident beam, it clearly verifies that damages initiate at the edge of the grating pillars where the maximum EFI exists. Then the part of the grating pillars split in higher laser fluence, making the structure tend to be smaller, as shown in the damage morphology of the grating pillars near the center of the laser irradiation. Finally, the full grating pillars are delaminated from the HR coating. This phenomenon demonstrates the importance of reducing EFI enhancement to improve LIDT of the MDG.
Fig. 6. Typical damage morphology of the MDG (The direction of the incident beam is from right to left). (a) Overview of damage morphology (b) Detailed damage morphology in the edge of the damage area
It should be point out that although our reactive ion beam etching process does not create nano-absorbing defects at the edge of the HfO2 grating where the peak EFI locates, there is probably still some PMMA residues in the ridge of the grating structure, which may decrease the LIDT of the MDG significantly [35]. More detailed study and optimized cleaning process will be conducted to further improve the LIDT.
Furthermore, it is worth to mention that the inverted grating profiles is mainly due to the etching process of Si grating, and it results to a little trapezoidal in the top part of the PMMA grating as shown in Fig. 4(a). However, there is no obstacle to fabricate the perfect rectangular HfO2 grating profile after iterative optimization. Additionally, one can note that the width of the fabricated grating is about 137nm, that is much smaller than the design parameter, and it proved that the grating with a smaller duty cycle f can be produced with our fabrication process. Since the maximum EFI will be even lower with small duty cycle, the LIDT can be further improved.
5. Conclusions
We designed the MDG with rectangular HfO2 grating profile which possessed good broadband diffraction performance and lower EFI enhancement, and analyzed the influence of sidewall angle on the diffraction efficiency and the EFI enhancement of MDGs. Then a damascene process that combines laser interference lithography, nanoimprint, atomic layer deposition and reactive ion-beam etching was proposed to manufacture the rectangular HfO2 grating profile. Such a fabrication process avoided the ion beam bombardment to the sidewall of HfO2 grating pillars and did not create extra nano-absorbing defects at the region of peak EFI. The actual structure of fabricated MDG had a nearly rectangular grating profile, showed the diffraction efficiency exceeds 95% in a 151-nm-wide wavelength range (from 714nm to 865nm) and the high LIDT of 0.59J/cm2 with a laser pulse of 40 fs. Our study provides a novel approach to fabricate the rectangular grating profile, which can be used to improve the LIDT and spectral performance of MDGs and other nanostructure devices.
Funding
National Natural Science Foundation of China (61621001, 61675156, 61925504, 61975155); National Key Research and Development Program of China (2016YFA0200900); Major projects of Science and Technology Commission of Shanghai (17JC1400800); “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education (17SG22); Shanghai Municipal Education Commission (2017-01-07-00-07-E00063).
Acknowledgement
Part of the sample fabrication was conducted at Fudan Nano-fabrication Lab.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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