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Abstract
In order to enhance the reliability and performance of space-based Lidar systems, it is desirable to increase the damage resistance of ultraviolet antireflective coatings. For laser pulses with nanosecond pulse duration, laser-induced damage is known to be triggered by nano-sized defects embedded in the optical coating. In this work, we demonstrate the mitigation of damage precursors during the manufacturing of ion-beam sputtered (IBS) coatings using two approaches: ion bombardment with a secondary ion source and laser irradiation with a nanosecond-pulsed laser. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in large-area raster scans.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Space-based Lidar systems – e.g. ESA’s Earth explorer missions Aeolus and EarthCARE – play an important role for monitoring the atmosphere in times of climate change. The ultraviolet laser systems developed and space-qualified for these missions can also be used to study other planets in our solar system. The MOMA laser for example will be used to examine rock samples for evidence of past life as part of the ExoMars mission [1]. In many cases, the limited lifetime of the installed optics contributes a major part to the risk of failure in spaceborne laser instruments. In this context especially, nano-sized damage precursors in antireflection coatings on the optics are considered as a major obstacle. Since these damage precursors cannot be detected by means of optical microscopy, only very time consuming and cost intensive procedures are available to qualify a space optic for being used at a specific fluence. The laser components of the Aladin instrument as part of the Aeolus mission had to be selected using raster scan tests [2].
Great progress has already been made in improving high power laser optics with respect to defect induced damage mechanisms. For example, lasers are already used in many applications to remove submicron dust particles from optical surfaces such as silicon wafers [3]. In the research activities of the National Ignition Facility, laser irradiation serves as a finishing technology for the polishing process of fused silica substrates [4]. The pretreated optics at 355 nm achieved a reduction of the defects of up to 120 times depending on the manufacturing process. An irradiation of the substrate surface in case of an antireflection coating for 1064 nm deposited by ion beam sputtering increased the laser-induced damage threshold by 40% [5]. Plasma etching was also used to improve the laser-induced damage resistance of the optical coatings. For example, this pretreatment technique resulted in a clear success within the thin film damage competition of the 2008 Boulder Damage Symposium for high reflecting mirrors at 1064 nm [6]. The use of a second ion beam in optical coating manufacturing has been investigated previously for example with improvements in the laser-induced damage resistance [7–10]. The ability to set oblique angles of the ion incidence and consider defect-induced damages in ultraviolet antireflection coatings sets this work apart.
We focus on improving the laser-induced damage stability of ultraviolet antireflection coatings (at 355 nm and 266 nm) motivated by their application in current space missions. In one approach, ion etching via argon bombardment with a secondary ion source is applied to prevent damage precursors from being buried during the deposition of the optical coating system by ion beam sputtering. As an alternative method, we investigate laser irradiation of the sample surface inside the coating system by ultraviolet laser radiation. The mitigation effects are measured by identifying the number of damages caused in a subsequent raster scan test. Furthermore, ramped raster scan tests are used to evaluate the damage resistance of laser optics at different fluences. Improvements of the laser-induced damage threshold are finally transferred from antireflection coatings for 355 nm to a coating for 266 nm. The mitigation schemes are discussed in the context of our current theoretical understanding of laser induced damage mechanisms.
2. Laser-induced damage mechanism and prevention strategy
Damage precursors can originate from grown structural defects or particles in the deposition process [11]. Additionally, precursors can be located in the substrate surface near to the coating system. These imperfections usually have a size of well below 100 nm and are not detectable using dark field microscopy. The laser irradiation of imperfections induces a local plasma with temperatures of several 1000 Kelvin [12]. The mechanical stress inside the coating finally leads to damage initiation by bursting off the overlaying material. The laser-induced damage with diameters of several micrometers and more can be detected by using conventional microscopy methods. The ion based mitigation scheme as well as the laser based mitigation scheme will be used to remove these damage precursors during the deposition process.
Antireflection coatings with a residual reflection of 0.2% under perpendicular incidence are specified. The test objects will be realized with four layers of aluminum oxide and silicon dioxide. Since the coatings are antireflective, the laser radiation reaches all damage precursors down to the substrate surface. For this reason, an ultraviolet fused silica glass with super polished surfaces is used as substrate material.
The thin films were produced by ion beam sputtering to counter the vacuum conditions of a space mission with a dense structure of the high-energy deposition technique [13]. A cryogenic pump in the bottom of the chamber can evacuate the system to a base pressure of a few 1 × 10−7 mbar. An ion beam source with neutralizer is used to sputter the coating material in reactive oxygen atmosphere. The targets with a purity of over 99.999% were moved by a rotating holder assembly. The sputter process with reactive gas pressures of a few 1 × 10−4 mbar generates a substrate temperature of about 50° C. Each layer of the antireflection coating is terminated by optical broadband monitoring as part of a fully automatic system control.
2.1 Ion based mitigation scheme
The first approach to reduce damage precursors in the antireflection coating includes a secondary ion bombardment. The potential damage sites of the layer system should be mitigated by resputtering of the growing layer material. An assist source in the rear left corner of the deposition system is targeting the deposited material on the substrate surface (see Fig. 1). The radio frequency pumped ion source extracts argon atoms using a three-grid system made of titanium. In addition to the ion current and the ion energy, the angle of incidence can be adjusted using a customized mounting system. Within the applied irradiation processes, the ion beam of the assist source has a variance of less than 10% with respect to the energy distribution. The ion current density for an ion current of 200 mA is in the range of a few 1 mA/cm2 due to the collimated propagation with a beam diameter of about 12cm. The lateral momentum of the impinging particles is supposed to distinguish the mitigation scheme from the dual ion beam sputtering process. These conditions allow the damage precursors to be sputtered away instead of being incorporated into the material by vertical impact. The secondary ion bombardment can be used for substrate preparation as well as during layer growth to optimize the ultraviolet performance.
2.2 Laser based mitigation scheme
The second approach applies laser radiation to remove the damage precursors in the antireflection coatings. A high power laser beam at the application wavelength is used to trigger the imperfections of the growing layer system, while they are still located on the surface. This interaction can remove the precursors before they are imbedded within the film matrix. Thus, a higher laser induced damage threshold is expected. The mitigation cannot cause functional damage due to the lack of sufficient mechanical stress in the coating material. Surface irregularities of the optical component are compensated by the deposition of further coating material. The laser irradiation can - similar to the ion based mitigation approach - address damage precursors on the substrate surface as well as in the coating material.
The pulsed laser radiation reaches the optical component through a window in the bottom of the deposition system. A galvanometer scanner outside of the vacuum chamber scans the deposited material line-by-line within a radius of 18 mm. The number of laser pulses can be adjusted via the scanning speed taking into account the effective beam diameter of 200 µm. A laser system generates pulses with a duration of 8.3 ns and a fluence of up to 14 J/cm2. The laser treatment was limited to the interfaces of the multilayer coating because of the duration of the raster scans.
3. Results and discussion
The power handling stability of an antireflective coating from the same coating chamber without installations was determined to identify a reference for the precursor mitigation. The ion beam sputtered samples consists of four layers of aluminum oxide and silicon dioxide with a total thickness of about 200 nm and achieve a residual reflectance of less than 0.2% at 355 nm in spectrophotometry measurements under normal incidence. The reproducibility of the reference system has been verified by performing coating runs and characterization over a period of several years. The laser-induced damage threshold of the component was measured by an S-on-1 test in accordance with EN ISO 21254-2. In this case, the sample was irradiated on 128 test sites with a spot size of 300 µm using a fluence of up to 25 J/cm2. The ultraviolet irradiation generated functional damages starting at a fluence of 12 J/cm2 as well as micro pits at a fluence of 8 J/cm2 and above. The established process thus offers a realistic reference for the reduction of damage precursors.
In the course of the study, the damage density of different process configurations was measured to optimize the mitigation schemes. A raster scan test with a test area of 12 × 5 mm2 was performed on the antireflection coatings for 355 nm. The laser light (third harmonic of a pulsed Nd:YAG laser, Innolas DPSS500 operated at 100 Hz repetition rate) with a pulse duration of 8.3 ns and a spot size of about 200 µm was applied line-by-line with a vertical displacement of 50 µm using a bidirectional translation stage. A number of 10 pulses per sample position with a fluence of 25 J/cm2 was provided. The irradiation of previously damaged sites was excluded to prevent effects from distributed debris.
Before and after the raster scan tests dark field and Nomarski microscopy was performed to identify laser induced damage sites. The irradiated sample area was mapped in individual sections by the dark field microscope (Olympus BX61 differential interference contrast and dark field microscope) using a 200x magnification. Measurements were performed in reflection mode with a halogen light source and an Olympus UC90 and XM10 camera for Nomarski and dark field mode, respectively. A computer based brightness evaluation screened the dark field micrographs for scattering objects in the antireflection coatings (see Fig. 2). Dark field images were measured with the same settings (e.g. illumination time) to ensure repeatability of the brightness distribution. For the comparison of identical positions in the micrographs, dark field images taken before and after irradiation were aligned. This was achieved via a coordinate transformation that rotates and translates the images to match the pixel coordinates of markers written to just below the surface of each optical substrate via laser beam writing. In order to detect laser-induced damages, an algorithm is used to look for connected pixels (connected horizontally, vertically and diagonally; meaning that each pixel has 8 neighbors) with an intensity above a specific brightness threshold. The developed software then counts the number and evaluates the size of “defects” by evaluating dark field micrographs taken before the raster scan and the size and number of “defects and damages” by evaluating micrographs taken after the raster scan. Additionally, it is possible to look for “damages without a detectable precursor”. This was achieved by masking areas around pre-irradiation coating defects to concentrate on laser-induced damage sites (see Fig. 3). The overall result of the raster scan tests is a density of laser-induced damage sites considering the absence of the masked areas. For simplicity, in the following we will mostly concentrate our discussion of the results on the metric of the “damages without a detectable precursor”. However, data of the laser based mitigation scheme derived without using the masking algorithm is given as supporting information in the appendix. It should be noted that depending on the analytical settings the generated data in a dark field illumination might not exactly reflect the actual damage dimensions.
Fig. 3. Example for the masking of previous objects when evaluating the darkfield images. The left micrograph shows polishing residues and a defect in the antireflective coating. These imperfections are masked in the right image to concentrate on damages of the raster scan test.
3.1 Adjustment of the mitigation approaches
The ion based mitigation scheme was initially used on the super polished surfaces of the fused silica substrates for a duration of 20 minutes. A slightly reduced damage density using an incidence angle of the argon ions of 70° compared to 45° and 30° was achieved (see Fig. 4). The ion current density for the grazing incidence is about 0.5 mA/cm2 with respect to the substrate plane. The reduced damage density may result from the polishing process or cleaning residues that were removed by the ion bombardment. The oblique incidence cause these defects to be sputtered away instead of being incorporated into the substrate by a perpendicular ion bombardment.
Fig. 4. Masked damage density of ultraviolet antireflection coatings with substrate pretreatment as a function of the ion incidence angle. During the variation of the ion incidence angle, an ion energy of 200 eV and an ion current of 200 mA was used to ensure comparability. The damage density indicates the number of detected laser induced damages after the raster scan test using the masking algorithm. The size classes subdivide the damages based on the detected area in the dark field micrograph starting at a diameter of 4.6µm2.
The second step includes continuous ion bombardment during the deposition process in addition to the substrate pretreatment. The optimization of the simultaneous mitigation is based on the damage densities generated in the raster scan tests (see Fig. 5). A variation of the operation parameters of the assist source was applied with a midpoint at an ion current of 200 mA and an ion energy of 200 eV.
Fig. 5. Masked damage density of ultraviolet antireflection coatings with continuous ion bombardment as a function of the ion source parameters. During the variation of the ion current, an ion energy of 200 eV was used and during the variation of the ion energy, an ion current of 200 mA was used to ensure comparability.
The reference for the damage density of the mitigation schemes is defined by the initial antireflection coating (see Sect. 2). The detection of laser-induced damages was limited to equivalent size of more than about 5 µm2 to achieve a good reproducibility. A decreased damage density could be observed with increasing beam current of the ion bombardment. The larger number of impinging ions could enable a more effective resputtering of weakly bounded material in the growing layer. The reduced damage density could also be observed for an increased energy. Several configurations even prevented the generation of laser-induced damages above an effective area of about 10 µm2 completely. The damage precursors could be removed more effectively due to the stronger momentum transfer of the secondary ion bombardment.
The laser based mitigation scheme was also optimized based on the damage densities of the raster scan tests. The influence on the damage resistance was investigated as a function of the fluence as well as the number of pulses. No significant improvements could be achieved by a substrate pretreatment due to the lack of imperfections on the super polished surfaces. The second step of the laser based mitigation scheme involves an additional irradiation after each deposited layer. The damage density of the antireflection coatings was evaluated as a function of the applied laser parameters (see Fig. 6). The standard configuration for the laser based mitigation scheme includes a fluence of 10 J/cm2 and a number of 10 pulses per site.
Fig. 6. Masked damage density of ultraviolet antireflection coatings with laser irradiation after each layer as a function of the laser beam parameters. During the variation of the fluence, a number of 10 pulses was used and during the variation of the number of pulses, a fluence of 10 J/cm2 was used to ensure comparability.
The precursor mitigation after each layer showed a significant improvement compared to a laser conditioning of the substrate. The damage density of the antireflection coatings could be minimized with increased fluence of the laser irradiation. The increase in the fluence of the laser irradiation triggers precursors with a higher damage threshold. Remaining damage precursors that only trigger at higher fluences could be defused by a conditioning effect of the ultraviolet laser irradiation. Precursors that are stubborn and require additional shots to be removed are addressed by the increased number of pulses during the laser irradiation. As a result, twice the number of pulses during the laser irradiation also showed a reduced damage density in the antireflection coatings. The damage precursors are removed by the laser-conditioning rather than made detectable by dark field microscopy and then ignored by the masking step in the raster scan test. This conclusion is confirmed by the treated samples, which show a significantly lower density of objects after the raster scan test even without the masking algorithm (see Fig. 10). In addition to the reference process, the reproducibility was confirmed on the most successful configurations by fabrication in several coating runs.
3.2 Ramped raster scans of improved optics
Ramped raster scan tests were performed to investigate the damage threshold of the improved antireflection coatings [14,15]. The reference deposition process will be compared with the ion based mitigation scheme taking a beam current of 200 mA and an ion energy of 250 eV. Samples of the laser based mitigation scheme taking a fluence of 10 J/cm2 and a number of 20 pulses are subjected to the ramped raster scan test as well. The ramped raster scan tests with a spot size in the range of 250 µm were performed in several stages up to a maximum fluence of 30 J/cm2 at a number of 10 pulses. Dark field micrographs within an area of 10 × 3 mm2 were analyzed for laser induced damage sites using the mentioned detection software (see Fig. 7). In contrast to the optimization phase, all of the scattering objects in the ramped raster scan tests were considered after the laser irradiation.
Fig. 7. Number of objects on the reference and improved optics after each stage of the ramped raster scan test.
The darkfield microscopy before the raster scan tests reveals coating defects and cleaning residues of the optical thin films. The laser irradiation of the first raster scans reduced the number of scattering objects due to a cleaning effect [16]. Laser-induced damage appeared first in the reference coating at a fluence of 20 J/cm2. The antireflection coating with laser based mitigation scheme indicates an increase of the laser-induced damage threshold to a fluence of 30 J/cm2. In case of the ion etched thin film, the number of detected objects did not increase at all, but was slightly higher from the beginning. Compared to the adjustment of the mitigation approaches, the standard optic showed less laser-induced damage sites at 25 J/cm2. The improved damage resistance is attributed to the successive increase of the fluence in the ramped raster scan test. This means that the previous raster scan tests resulted in a laser conditioning of the ultraviolet antireflective coating [17].
Micrographs were taken during the ramped raster scan tests to observe the development of laser-induced damage sites as a function of the fluence. For this purpose the irradiated area of the antireflection coating was mapped after each raster scan test using a Nomarski microscope (see Fig. 8). This example belongs to a laser optic manufactured via laser irradiation. Two micro-pits (without a detected precursor) were identified after performing the second raster scan test with a fluence of 10 J/cm2. Previous investigations revealed that micro pits show a stable size over a large fluence range and no growth after repeated irradiation [18]. The micro-pits expand after the raster scan with a fluence of 30 J/cm2 up to an equivalent diameter of about 10 µm (dashed circle in Fig. 8). Furthermore, additional micro-pits are formed (dash-dotted circle in Fig. 8).
Fig. 8. Development of a laser-induced damage site in a ramped raster scan. This example belongs to a laser optic manufactured with laser irradiation.
3.3 Transfer of the ion based mitigation scheme to 266 nm
The improvement of the laser-induced damage threshold was then transferred to antireflection coatings for 266 nm. High power laser optics at shorter wavelengths will enable new measurement accuracies for future space missions. The antireflection coatings were produced using the ion based mitigation scheme due to the highest damage resistance in the ramped raster scan tests. Furthermore, the ion etching offers significantly easier process control and scaling from single optics to whole batches compared to the laser irradiation. The antireflection coatings consists of four layers of aluminum oxide and silicon dioxide with a total thickness of about 160 nm and achieve a residual reflectance of less than 0.2% at 266 nm under normal incidence. Separate raster scan tests with a number of 10 pulses resulting from a repetition rate of 100 Hz and a vertical displacement of about 30 µm were analyzed by microscopy to evaluate the precursor mitigation. The mapping was performed with an AxioImager.M2 microscope from Zeiss using both dark field mode and differential interference contrast with reflected light configuration. The antireflection coatings were irradiated at the application wavelength of 266 nm using a diode pumped solid-state laser source with a pulse duration of 4.5 ns and a spot size of 130 µm. A comparison between the ion based mitigation scheme and the reference deposition process was made to identify differences in the power resistance. The reference system shows a laser-induced damage threshold of 10 to 12 J/cm2, which is a comparable high value for antireflective coatings. The applied ion treatment was able to improve the laser resistance considering a damage threshold of above 14 J/cm2 in the raster scan tests. The optical thin films did not reveal micro pits as observed for the laser irradiation at 355 nm but a full surface damage (see Fig. 9). This result indicates that the laser resistance of the coating material is the dominant factor for the power capability. The increased damage threshold shows that the intrinsic damage resistance was improved by the oblique ion bombardment. This improvement may be related to a modification of defect states near the band gap of aluminum oxide caused by the oblique ion bombardment.
Fig. 9. Darkfield micrograph of a full surface damage with a width of 3 mm after a raster scan test of the improved optic for 266 nm applying a fluence of 16 J/cm2.
3.4 Conclusion
Damage precursors with a size below the detection limit of darkfield microscopy pose a high risk for the damage resistance of for example space borne lidar systems. The precursor mitigation successfully reduced the number of laser-induced damages generated in the raster scan tests with a fluence of 25 J/cm2. The improvement in laser-induced damage resistance could be achieved with both, the angled ion bombardment and the ultraviolet laser irradiation. Both mitigation approaches revealed the biggest effects when applied during the deposition process of the antireflection coatings. This leads to the conclusion that the damage resistance at the wavelength 355 nm is determined by defects in the coating material in contrast to the substrate surface. The resistance of the fused silica glass substrates with regard to subsurface damage is probably due to the high quality surface polishing.
It has been demonstrated that the damage precursors in the ion based mitigation scheme can be eliminated by resputtering using the argon bombardment. The difference to the dual ion beam sputtering process is the lateral momentum due to the oblique incidence of the impinging ions. The damage precursors in the laser based mitigation approach were eliminated during the deposition process without bursting the entire layer system.
The ramped raster scan tests on the antireflection coatings revealed a growth of damage sites at certain fluences. It has been shown that the mitigation schemes shifted the laser-induced damage threshold to higher fluences. The improved power stability of the antireflection coatings for 266 nm indicates that the ion bombardment may have an influence although not based on the defect induced damage mechanism. The developed mitigation schemes are expected to lead to a higher reliability of the optical thin films in the next generation of laser based space missions.
Appendix
Fig. 10. Density of objects before and after the raster scan test of ultraviolet antireflection coatings with laser irradiation after each layer as a function of the laser beam parameters.
The improvement in the damage resistance of the ultraviolet antireflection coatings with laser based mitigation scheme can also be seen in the density of objects detected by dark field microscopy before and after the raster scan tests without using the masking algorithm (see Fig. 10).
Funding
European Space Agency.
Acknowledgments
The authors would like to thank Ana Baselga Mateo (ESA-ESTEC), Clemens Heese (ESA-ESTEC) and Helmut Schröder (formerly DLR) for support and scientific advice during the initiation and execution of this project. Furthermore, we would like to thank Gabriele Taube and Franz Hadinger (DLR) as well as Heinrich Mädebach and Ina Loth (LZH) for their precise and rigorous experimental work.
Disclosures
The authors declare no conflicts of interest.
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