coatings Article
Design and Preparation of a Micro-Pyramid Structured Thin Film for Broadband Infrared Antireflection Shaobo Ge 1,† , Weiguo Liu 1, *,† , Shun Zhou 1 , Shijie Li 1 , Xueping Sun 1 , Yuetian Huang 2 , Pengfei Yang 1 , Jin Zhang 1 and Dabin Lin 1 1
2
* †
Shaanxi Province Key Laboratory of Thin Films Technology and Optical Test, Xi’an Technological University, Xi’an 710032, China;
[email protected] (S.G.);
[email protected] (S.Z.);
[email protected] (S.L.);
[email protected] (X.S.);
[email protected] (P.Y.);
[email protected] (J.Z.);
[email protected] (D.L.) Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610000, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-029-83208114 These authors contributed equally to this work.
Received: 8 April 2018; Accepted: 8 May 2018; Published: 21 May 2018
Abstract: A micro-pyramid structured thin film with a broad-band infrared antireflection property is designed and fabricated by using the single-point diamond turning (SPDT) technique and combined with nano-imprint lithography (NIL). A structure with dimensions of 10 µm pitch and 5 µm height is transferred from the copper mold to the silicon nitride optical film by using NIL and proportional inductively-coupled plasma (ICP) etching. Reflectance of the micro-optical surface is reduced below 1.0% over the infrared spectral range (800–2500 nm). A finite-difference-time-domain (FDTD) analysis indicates that this micro-structure can localize photons and enhance the absorption inside the micro-pyramid at long wavelengths. As described above, the micro-pyramid array has been integrated in an optical film successfully. Distinguishing from the traditional micro-optical components, considering the effect of refraction and diffraction, it is a valuable and flexible method to take account of the interference effect of optical film. Keywords: single-point diamond turning; nano-imprint lithography; micro-pyramid; micro-optics
1. Introduction Antireflective (AR) surfaces are of great interest in many applications ranging from military applications to coated displays, as well as being a topic that is intensely researched in optics and optoelectronics. AR layers are needed when two adjacent materials have different refractive indices, thus leading to unwanted Fresnel reflections from their interface. Coating a single layer on the surface is one common methodology for reducing reflection, however, this method is of limited functionality and usually possesses a narrow wavelength band. Wavelength bands can be broadened by applying multiple thin film layers, but it is difficult to find material combinations with appropriate refractive indices and transparencies [1]. Especially at long wavelengths from 800 to 2500 nm, covering the main solar irradiance spectrum, design and preparation of the antireflective surfaces for the broadband infrared region is a fairly far-reaching research. Solar energy is clean and nonpolluting, which can be used without limit. Photovoltaic technology is regarded as one of the most efficient light utilization technologies [2,3]. AR surfaces play an important role in photovoltaic technology. Firstly, in order to further expedite the electron transport and reduce costs, the modern solar cell requires a much thinner active layer, which means multiple thin film layers are not suitable. Secondly, light trapping Coatings 2018, 8, 192; doi:10.3390/coatings8050192
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strategies have been developed to enhance the light absorption of the limited film thickness of the active layer. In conventional silicon-based solar cells, light trapping technologies have been generally used to enhance the photoelectrical properties by pattering the silicon substrate for low reflection and reduced resistivity [4]. Alternatively, nano-patterning the surface [5,6] has the advantage of wide spectral range and broad incident angles. A variety of photonic nanostructures [7] have been extensively studied. Especially, the micro-pyramid structure has been proved to be one of the most effective anti-reflective textures [8–10]. The pyramid structures can increase the optical path length by factors as high as 40 and have been experimentally demonstrated to be one of the most efficient light-trapping structures [11,12]. Chong et al. [10] confirmed that 91% of the ideal light-ftrapping limit (the Lambertian limit) could be achieved by a skewed silicon pyramid grating placed on the front of a thin silicon solar cell. Several preparations were proposed to fabricate optical elements with a nano-structure surface, relying on direct writing, or electron or ion beam lithography. However, the pyramid structure cannot be well prepared by these above methods. Previous studies have already shown many alternative approaches [8,13,14] and nano-imprint lithography (NIL) has proved to be more appropriate for processing the large-area surface texture [15–20]. Meanwhile, wet etching of absorbing materials (such as silicon) is widely used in the fabrication of the micro-pyramid structure by combining with the NIL technique [15,21]. Based on the better AR property of the micro-pyramid, a method to fabricate an optical multifunction structure was proposed by Päivänranta et al. [22] by means of wet etching, nano-imprint lithography (NIL), and reactive ion etching (RIE). Even in this study, the processing of electron beam lithography, as the first step of their mask fabrication, is still expensive. It is also difficult to improve the depth-to-width ratio because of the strong crystallographic orientations of silicon, which means the tunable size of the pyramid structure cannot be achieved. These above factors are restrictions on mask design and fabrication processing, such as pattern transfer. Additionally, for widely-used AR applications, easy fabrication flow with less cost or larger size still cannot be achieved by these methods. Previously, the single-point diamond turning (SPDT) technique have been proved to fabricate micro-pyramids successfully with less cost [23–25]. Distinguished from the traditional optical and electric processes, ultra-precision diamond machining improves the surface quality with high flexibility even in fabricating more complex three-dimensional structures. As we can fabricate the microstructure with the NIL technique in any optical material which cannot be cut by SPDT, such as fused quartz, the fabrication on thin films can be realized with reliability. Thin films have the property of excellent hardness over other optical materials, especially some specific antireflective thin film materials, like SiNx , which can act as a passivation layer [26] and be more effective with respect to antireflection to broadband wavelengths. Moreover, benefiting from the interference effect of thin films, and the light-trapping structures of the pyramid, it is worth investigating microstructures on the optical thin film. Compared to previous photolithography studies, a larger sample size with lower-cost and higher depth-to-width ratio, which can be improved by a better diamond tool design, has been realized by our fabrication process. With this, we can generate an angle-tunable pyramid structure which cannot be realized by wet processing. In previous studies, the prepared pyramid array structure was directly transferred to the polymer as an antireflective layer by nano-imprinting [27,28]. However, the polymer coating is not resistant to high temperatures, acid corrosion, and has poor adhesion. In conclusion, design and preparation of the micro-pyramid structures on the dielectric thin film has proved to be extremely valuable. In this study, a pyramid structure can be generated using the SPDT technique. With the help of the NIL technique, a micro-pyramid array structure is obtained in an optical thin film which is usually used as an antireflective material. In order to realize an optimal AR property, the structure on the surface of the thin film is pyramidal and the antireflection material is a silicon nitride film. The influence factors on antireflection are analyzed with the simulation analyses of finite-difference-time-domain (FDTD).
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Distinguishing from the traditional micro-optical components, considering the effect of refraction and diffraction, it isofa valuable andfilm. flexible method to take into account the thin interference of the interference effect the optical Here, a micro-pyramid structured film foreffect broadband optical Here, a micro-pyramid structured film forThe broadband infrared antireflection has been infraredfilm. antireflection has been designed and thin generated. optimized pyramid form is simulated designed and generated. optimized form is Then, simulated by the effective array medium theory by the effective medium The theory and thepyramid FDTD method. the micro-pyramid mold was and the FDTD method. Then, the micro-pyramid array mold was obtained using the SPDT technique. obtained using the SPDT technique. The mask layer is fabricated with a process where NIL is used The layer is fabricated with a process where NIL is used from as an the intermediate step as anmask intermediate step in transferring the pyramid structure metal mold to in thetransferring NIL resist. the pyramid structure from the metal mold to the NIL resist. Finally, our micro-pyramid structure is Finally, our micro-pyramid structure is transferred into the silicon nitride layer using proportional transferred into the silicon nitride using proportional inductively-coupled plasma (ICP) etching. inductively-coupled plasma (ICP)layer etching. 2. 2. Materials Materials and and Methods Methods 2.1. Modeling Methods 2.1. Modeling Methods The effect of anti-reflection comes from two aspects: First, the effective refractive index of silicon The effect of anti-reflection comes from two aspects: First, the effective refractive index of silicon nitride layer is decreased by the micro-pyramid, which acts as a single-layer anti-reflection film. nitride layer is decreased by the micro-pyramid, which acts as a single-layer anti-reflection film. Second, the localized absorption enhancement is brought by the pyramid structure. Second, the localized absorption enhancement is brought by the pyramid structure. When antireflection properties are realized by patterning the surface with a pyramidal structure, When antireflection properties are realized by patterning the surface with a pyramidal structure, the incoming light perceives this modified region as a thin layer with a certain refractive index, known the incoming light perceives this modified region as a thin layer with a certain refractive index, as the effective refractive index. In this case, the equivalent refractive index gradually increases from known as the effective refractive index. In this case, the equivalent refractive index gradually the refractive index of air to the refractive index of the absorbing material (silicon nitride, for example). increases from the refractive index of air to the refractive index of the absorbing material (silicon The pyramid structure causes the diminished effective refractive index of silicon nitride, acting as nitride, for example). The pyramid structure causes the diminished effective refractive index of a single-layer antireflection film. Here, three-dimensional vector diffraction theory was used to silicon nitride, acting as a single-layer antireflection film. Here, three-dimensional vector diffraction analyze the influence of the equivalent refractive index on the anti-reflection characteristics [29,30]. theory was used to analyze the influence of the equivalent refractive index on the anti-reflection The equivalent refractive index can be determined from the grating equation: characteristics [29,30]. The equivalent refractive index can be determined from the grating equation: 2 2 22 2 sin22 θθm,n = = ((nni sin sin θ θin cos φφ + λm/d θθ φφ++λn/d (1) x ) ) ++ nn yd i sin in sin (1) nn121sin cos + λ m d sin sin λ n m ,n i in x i in y
(
)
where where nn11 is is the the refractive refractive index index on on the the incident incident and and reflection reflection side, side, nnii is is the the refractive refractive index index of of the the grating layer, and λ is the wavelength assuming as the interested waveband from 800 to 2500 nm. grating layer, and λ is the wavelength assuming as the interested waveband from 800 to 2500 nm. θθm,n , θ in,, and and ϕ φ are are the the propagation propagation angle, angle, the the angle angle of of incidence incidence and and the the conical conical angle, angle, respectively. respectively. m,n, θin Furthermore, m and and nnare arethe thediffraction diffractionorders ordersininthe the x and y directions, respectively, is Furthermore, m x and y directions, respectively, andand λ isλthe the wavelength. The d = d = d are the micro-pyramid period, and the h is the pyramid height. x y wavelength. The dx = dy = d are the micro-pyramid period, and the h is the pyramid height. Additionally, Additionally, the the effective effective refractive refractive index index of of the the grating grating layer layer is is decreased decreased by by the the micro-pyramid, micro-pyramid, which acts as a single-layer anti-reflection film. Here, the micro-pyramid structure is which acts as a single-layer anti-reflection film. Here, the micro-pyramid structure is divided divided into into N N layers as shown in Figure 1a. The n can be calculated by the effective index theory taking into account layers as shown in Figure 1a. The nii can be calculated by the effective index theory taking into account traditional traditional absorbent absorbent materials, materials, as as shown shown in in Figure Figure 1b. 1b.
(a)
(b) Figure 1. 1. The The modeling modeling methods methods (a) (a) and and nni (b). (b). Figure i
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With With the the simulation simulation method method of of FDTD, FDTD, the the appearance appearance of of the the reflection reflection suppression suppression with with our our With the simulation method of FDTD, the appearance of the reflection suppression with our micro-pyramid array, which enhance the absorption at long wavelengths from 800 to 2500 nm covered micro-pyramid array, which enhance the absorption at long wavelengths from 800 to 2500 nm micro-pyramid array, enhance thebeabsorption at long wavelengths from 800 to nm the main the solar irradiance spectrum, would observed. Meanwhile, this micro-structure can2500 localize covered main solar which irradiance spectrum, would be observed. Meanwhile, this micro-structure can covered the main solar irradiance spectrum, would be observed. Meanwhile, this micro-structure can photons and realize the broad-spectrum absorption enhancement inside theinside micro-pyramid. Figure 2 localize photons and realize the broad-spectrum absorption enhancement the micro-pyramid. localize photons and realize the broad-spectrum absorption enhancement inside the micro-pyramid. gives evidence of this light trapping effect by intensity distribution of light field with incoming wave Figure 2 gives evidence of this light trapping effect by intensity distribution of light field with Figure 2 gives evidence this light trapping effect by intensity distribution of light field with of (a) 800, (b) 1500, (c) of 2100 nm, respectively. with theComparing absorption with inside theabsorption substrate, incoming wave of and (a) 800, (b) 1500, and (c) 2100Comparing nm, respectively. the incoming of (a)the 800, (b) 1500, inside and (c) 2100 nm,torespectively. Comparing with the absorption inside our micro-pyramids isour enhanced almost tripling whento thealmost dx = dthe =absorption d = 10when µm, inside the wave substrate, absorption micro-pyramids is enhanced ytripling inside the substrate, the absorption inside our micro-pyramids is enhanced to almost tripling when and 5 dµm. Asμm, performed FDTD method, it shows that this micro-structure canthat localize the dthe x =hd= y = = 10 and thewith h = 5the μm. As performed with the FDTD method, it shows this the dx = dand y = d = 10 the h = 5inside μm.enhance As with theinside FDTDthe method, it shows that this photons enhance theand absorption theperformed micro-pyramid. micro-structure canμm, localize photons and the absorption micro-pyramid. micro-structure can localize photons and enhance the absorption inside the micro-pyramid.
(a) (a)
(b) (b)
(c) (c)
Figure 2. Photon absorption profile at cross section at (a) 800 nm; (b) 1500 nm; and (c) 2100 nm Figure 2. Photon absorption profile at cross section at (a) 800 nm; (b) 1500 nm; and (c) 2100 Figure 2. Photon absorption profile at cross section at (a) 800 nm; (b) 1500 nm; and (c) 2100 nm wavelength. nm wavelength. wavelength.
2.2. Fabrication of the Micro-Pyramid Structured Thin Film 2.2. 2.2. Fabrication Fabrication of of the the Micro-Pyramid Micro-Pyramid Structured Structured Thin Thin Film Film There are four main steps of the fabrication process of micro-pyramid structured thin film for There are four four main steps stepsofofthe thefabrication fabrication process of micro-pyramid structured There are structured thinthin filmfilm for broadband infraredmain antireflection, as shown in process Figure of 3. micro-pyramid Firstly, a pyramid grid was precision for broadband infrared antireflection, as shown in Figure 3. Firstly, a pyramid grid was precision broadband infrared antireflection, as shown in Figure 3. Firstly, a pyramid grid was precision machining on a copper mold by the SPDT technique (Figure 3a). Then, the micro-pyramid array was machining on aa copper mold SPDT technique (Figure 3a). Then, the array machining copperresist mold by by the theNIL SPDT technique 3a).final Then, the micro-pyramid micro-pyramid array was was printed in on nanoprint with (Figure 3b,c)(Figure and the micro-pyramid structures was printed in nanoprint resist with NIL (Figure 3b,c) and the final micro-pyramid structures was patterned printed in nanoprint resist with NIL (Figure 3b,c) and the final micro-pyramid structures was patterned in SiNx thin film by proportional inductively-coupled plasma etching (ICP) (Figure 3d). in SiNx thin proportional inductively-coupled plasma etching (ICP) (Figure 3d).(Figure 3d). patterned infilm SiNxby thin film by proportional inductively-coupled plasma etching (ICP)
Figure 3. Preparation of micro-pyramid structured thin film for broadband infrared antireflection. Figure 3. Preparation of micro-pyramid structured film mold for broadband infrared antireflection. (a) Firstly, the micro-pyramid array is generated on athin copper by the SPDT technique; (b) Next, Figure 3. Preparation of micro-pyramid structured thin film for broadband infrared antireflection. (a) Firstly, the micro-pyramid array is generated on a copper mold by the SPDT technique; (b) Next, a soft composite PDMS (Polydimethylsiloxane) was demolded the copper master (a) Firstly, the micro-pyramid array is generatedstamp on a copper mold by from the SPDT technique; (b)stamp; Next, a soft composite PDMS (Polydimethylsiloxane) stamp was demolded from the copper master stamp; Then, a nano-imprint lithography resist is spun ontowas a silicon nitride layer the micro-pyramid a(c) soft composite PDMS (Polydimethylsiloxane) stamp demolded from theand copper master stamp; (c) Then, a nano-imprint lithography resist is spun onto a silicon nitride layer and the micro-pyramid array is copied into resist by using the PDMS stamp; (d) The pattern is transferred to silicon nitride (c) Then, a nano-imprint lithography resist is spun onto a silicon nitride layer and thethe micro-pyramid array is copied into resist by using the PDMS stamp; (d) The pattern is transferred to the silicon nitride thin film by proportional inductively-coupled plasma array is copied into resist by using the PDMS stamp; (d)etching. The pattern is transferred to the silicon nitride thin film by proportional inductively-coupled plasma etching. thin film by proportional inductively-coupled plasma etching.
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Ultra-precision machining [31] is adopted to precisely fabricate the micro-optical threeUltra-precision machining [31] practice is adopted to precisely fabricateisthe micro-optical dimensional structures. Theory and indicates that fly-cutting extremely suitablethreefor dimensional structures. Theory and practice indicates that fly-cutting is extremely suitable for machining repeated complex structures as prism arrays and pyramid arrays with high surface quality machining repeated complex structures prism arrays with high surface quality and flexibility. In diamond fly-cutting,asthe tool doesand notpyramid remain arrays in constant contact with the and flexibility. fly-cutting, the tool does remain to in the constant contactmilling. with theAworkpiece, workpiece, it is In andiamond interrupted cutting process, whichnot is similar single-edge Cu roller it is an interrupted cutting process, the single-edge A aCu roller workpiece workpiece of 200 mm in diameter is which chosenistosimilar fix on to a workpiece holdermilling. which is vacuum chuck on of 200 mm Bintable, diameter is chosen to fix onisa workpiece holder which isC. a vacuum on thethe rotary the rotary while diamond tool mounted on the spindle During chuck fly-cutting, toolB table, while diamond tool is mounted on the spindle C. During fly-cutting, the tool generally rotates generally rotates and the workpiece remains fixed. An extremely sharpened V-type diamond tool is and the workpiece remainsFigure fixed. 4An extremely sharpened V-type diamond toolmachine is used tool to make used to make microgrooves. shows the structural layout of ultra-precision and microgrooves. Figure 4In shows structural layout of ultra-precision machine tool and intersecting on-machine on-machine fly-cutting. orderthe to fabricate the micro-pyramid structure, two V-grooves ◦ should fly-cutting. to fabricate the micro-pyramid structure, two V-grooves intersecting at 90° shouldInbeorder prepared. The experimental conditions of fly-cutting is shown in Table 1.atA90pyramid be prepared. experimental fly-cutting is shown in been Table generated 1. A pyramid structure with structure withThe dimension of 10conditions μm pitchofand 5 μm height has using the SPDT dimension(Figure of 10 µm pitch and 5 µm height has been generated using the SPDT technique (Figure 5a–c). technique 5a–c).
Figure Figure4.4.Structural Structurallayout layoutofofultra-precision ultra-precisionmachine machinetool tooland andon-machine on-machinefly-cutting. fly-cutting. Table Table1.1.The Theexperimental experimentalconditions conditionsof offly-cutting. fly-cutting. Tool Included Angle Workpiece Tool Included Angle (°) Workpiece (◦ ) Copper alloy 90 Copper alloy 90
Depth of Cut Depth of Cut (μm) (µm) 5 5
Feed Rate Spindle Rotational Speed Feed Rate Spindle Rotational Speed (μm/s) (rpm) (µm/s) (rpm) 500 3000 500 3000
A 5 μm thick SiNx thin film was deposited on the standard -oriented silicon wafer because of theA55μm 10xμm micro-pyramid mold chosen for fabrication of wafer antireflective µmheight, thick SiN thinpitch film was deposited onarray the standard -oriented silicon because (AR) surfaces. Among SiNx layer deposition methods, plasma-enhanced vapor of the 5 µm height, 10the µmcurrent pitch micro-pyramid array mold chosen for fabrication chemical of antireflective deposition (PECVD) is the onecurrent of the best, the low temperature of the process (usually around (AR) surfaces. Among SiNx due layertodeposition methods, plasma-enhanced chemical vapor 300 °C) and more uniform and less stress variation through wafers. The low deposition temperature deposition (PECVD) is one of the best, due to the low temperature of the process (usually around ◦ C) and of PECVD can alleviate theand effects thermal expansion mismatch, produces stress between 300 more uniform lessof stress variation through wafers. which The low deposition temperature the substrate and coating leading to problem of adhesion. Here, a plasma-enhanced chemical vapor of PECVD can alleviate the effects of thermal expansion mismatch, which produces stress between deposition (PECVD) system (SAMCO PD-220, SAMCO, Tokyo, Japan) had been used to investigate the substrate and coating leading to problem of adhesion. Here, a plasma-enhanced chemical vapor some important parameters, as pressure, andTokyo, gas flow rates. Additionally, deposition (PECVD) system such (SAMCO PD-220,power, SAMCO, Japan) had been used tointroducing investigate this optimized process, a 5.49 μm thick low stress layer was generated, which would be usedthis to some important parameters, such as pressure, power, and gas flow rates. Additionally, introducing patterned array as presented the was Figure 5d. optimizedmicro-pyramid process, a 5.49 µm thick low stressin layer generated, which would be used to patterned Next, the patterned mold coated5d. with anti-adhesive layer and a soft PDMS layer was micro-pyramid array as copper presented in was the Figure spun-coated above pyramid texture, andwas finally covered a hard, thicklayer PDMS layer. Then thislayer soft Next, the patterned copper mold coated with by anti-adhesive and a soft PDMS composite stamp was demolded theand copper master stamp coated anti-sticking layer was spun-coated above pyramidfrom texture, finally covered by and a hard, thickwith PDMS layer. Then this lowering the surface energy prior to the hot embossing. For the nanoimprint step, a mr-I 7030 R soft composite stamp was demolded from the copper master stamp and coated with anti-sticking (Micro resist Technology, Berlin, Germany) resist layer was spun onto the SiN x film sample. layer lowering the surface energy prior to the hot embossing. For the nanoimprint step, a mr-I 7030 R A hot nanoimprint lithography technique wasresist usedlayer to print micro-pyramid the resist at (Micro resist Technology, Berlin, Germany) wasthe spun onto the SiNstamp sample. A hot x film to ◦ 135 °C under 40 bars for 5 min (Figure 5e,f). Due to the low stiffness of the composite mold and nanoimprint lithography technique was used to print the micro-pyramid stamp to the resist at 135 C micrometer scale the pyramid shape, the would be printed without deforming in the NIL under 40 bars forof 5 min (Figure 5e,f). Due tostamp the low stiffness of the composite mold and micrometer process. scale of the pyramid shape, the stamp would be printed without deforming in the NIL process. After cooling the sample, the pyramid structure was obtained in the resist material. Then, this texture was generated on the SiNx layer by using a proportional inductively-coupled plasma (ICP) etching process with sulfur hexafluoride (SF6).
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After cooling the sample, the pyramid structure was obtained in the resist material. Then, this texture was8,generated the SiNx layer by using a proportional inductively-coupled plasma (ICP) Coatings 2018, x FOR PEERon REVIEW 6 of 9 etching process with sulfur hexafluoride (SF6 ). The morphology of the pyramid-SiNx structure structure were observed by scanning electron microscope (SEM) shown in Figure Figure 5g,h. 5g,h.In Inaddition, addition,atomic atomicforce forcemicroscopy microscopy(AFM, (AFM,tapping tapping mode, MultiMode mode, MultiMode 8, 8, Bruker, Madison, USA) could estimate the repeatable uniformity the surface over the Bruker, Madison, WI,WI, USA) could estimate the repeatable uniformity of the of surface over the micromicro-pyramid texture, as shown in Figure 5i.reflectance The reflectance thex SiN without pyramid texture, as shown in Figure 5i. The of theofSiN thinx thin film film withwith and and without the the micro-pyramid structures obtained using an optical spectrometer (PekinElmer Lambda micro-pyramid structures was was obtained using an optical spectrometer (PekinElmer Lambda 950 950 UV/VIS/NIR spectrophotometer with an integrating sphere, 150 PerkinElmer, mm, PerkinElmer, Inc., Boston, UV/VIS/NIR spectrophotometer with an integrating sphere, 150 mm, Inc., Boston, MA, MA, USA) over the wavelength range of 800–2500 nm. USA) over the wavelength range of 800–2500 nm. 3. Results and and Discussion Discussion 3. Results As above, aa 55 µm As described described above, μm height height and and aa 10 10 µm μm pitch pitch micro-pyramid micro-pyramid array array was was generated generated in in copper with the SPDT process (Figure 5a–c). A 5.49 µm thick SiN layer with low stress is by x x layer with low stress produced copper with the SPDT process (Figure 5a–c). A 5.49 μm thick SiN is produced the PECVD method (Figure 5d), 5d), the micro-pyramid texture was transferred to theto nano-imprint resist by the PECVD method (Figure the micro-pyramid texture was transferred the nano-imprint by using the NIL process (Figure 5e,f), and the final texture was fabricated in the silicon nitride layer resist by using the NIL process (Figure 5e,f), and the final texture was fabricated in the silicon nitride by using proportional ICP etching (Figure 5g–i). 5g–i). layer by using proportional ICP etching (Figure
Figure 5. SEM images and AFM images of the samples with the micro-pyramids texture. (a–c) An Figure 5. SEM images and AFM images of the samples with the micro-pyramids texture. (a–c) An array array of micro-pyramids is obtained with the SPDT process; (d) a 5.49 μm thick SiNx layer with low of micro-pyramids is obtained with the SPDT process; (d) a 5.49 µm thick SiNx layer with low stress stress is produced by the PECVD method; (e,f) a micro-pyramid grid in the NIL resist is obtained and is produced by the PECVD method; (e,f) a micro-pyramid grid in the NIL resist is obtained and film is is generated. generated. (g–i) aa final final micro-pyramid micro-pyramid texture texture in in the the SiN SiNx film (g–i) x
According to the simulation results, a pyramid structure with a dimension of 10 μm pitch and According to the simulation a pyramid structurefor with dimension of 10 µm pitch and 5 μm height represented excellentresults, antireflective performance theamain solar irradiance spectrum 5(800 µmtoheight represented excellent antireflective performance the main solar irradiance spectrum 2500 nm). The effect of anti-reflection comes from the for effective refractive index of the silicon (800 to 2500 nm). The effect of anti-reflection comes from the effective refractive index of the silicon nitride layer with the micro-pyramid array and the localized absorption enhancement by the pyramid nitride layer withisthe micro-pyramid array and the localized absorption enhancement by the pyramid structure, which shown by the FDTD analysis. structure, is shown by the FDTD analysis. In thewhich process of fabrication, the pyramid array on the copper mold was generated by the SPDT technique. The area of the micro-structured surface is a circle 12.5 mm in diameter, or a square area 30 mm long and 25 mm wide. It only takes a few minutes to finish the fly-cutting to obtain the pyramid array with a feed rate 500 μm/s. In fact, the size area of the micro-pyramid surface could be generated as large as 300 mm in diameter using the SPDT method. It would take only 20 min to fabricate the pyramid array on the whole surface. There are almost no defects during the fly-cutting,
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In the process of fabrication, the pyramid array on the copper mold was generated by the SPDT technique. The area of the micro-structured surface is a circle 12.5 mm in diameter, or a square area 30 mm long and 25 mm wide. It only takes a few minutes to finish the fly-cutting to obtain the pyramid array with a feed rate 500 µm/s. In fact, the size area of the micro-pyramid surface could be generated as large2018, as 300 mmPEER in diameter Coatings 8, x FOR REVIEW using the SPDT method. It would take only 20 min to fabricate 7 the of 9 pyramid array on the whole surface. There are almost no defects during the fly-cutting, as shown in Figure 5a–c. pinnacle of the pyramid extremely and the sharp, repeatability of repeatability the pyramid as shown in The Figure 5a–c. The pinnacle ofunit the is pyramid unitsharp, is extremely and the shape high. Compared with the traditional preparation method, the newly-developed of the ispyramid shape is high. Compared with the traditional preparation method, themethod newlyhas the advantages highthe yield, low cost, large areas, and ease industrialization. Additionally, developed method ofhas advantages of high yield, low ofcost, large areas, and ease of the roughness of the surface is pretty good. This a large area ofgood. the structured surface can be industrialization. Additionally, the roughness of means the surface is pretty This means a large area directly generatedsurface on the can plastic film andgenerated thin metalon film. of the structured be directly the plastic film and thin metal film. As we know by now, now, using using the the NIL NIL process process can can generate generate the the flexible flexible structured structured surface surface after fabricating the metal mold. In Figure 5e,f, the micro-pyramid array array is is replicated replicated well well in in aa large large area. The mold. The structures structures on on the the NIL resist maintain good consistency as the pyramid array on the copper mold. In order to enhance the localized absorption, the micro-pyramid array should be transferred to order to enhance the localized absorption, the micro-pyramid array should be transferredthe to silicon nitride thin thin film.film. It turned out toout be to an be effective method to reduce the stress during the silicon nitride It turned an effective method to reduce theconcentration stress concentration the deposition of the SiNofx layer using PECVD. SiNx coating isx5.49 µm thick and retains during the deposition the SiN x layer usingThe PECVD. The SiN coating is 5.49 μm thickthe andsuperior retains characteristics of low stress and good compactness, as shown in Figure 5d. The micro-pyramids of the the superior characteristics of low stress and good compactness, as shown in Figure 5d. The microSiN generated ICP etching were 4 µm in height, to corresponds a selectivity of pyramids of the SiNx thinby film generated by ICP etching were 4which μm incorresponds height, which to x thin film 1.25 betweenof the nanoimprint resist and the SiN coating. Additionally, the side walls of the pyramid a selectivity 1.25 between the nanoimprint resist and the SiN x coating. Additionally, the side walls x are quitepyramid smooth after the ICP process. Thethe fabrication of the micro-pyramid-structured dielectric thin of the are quite smooth after ICP process. The fabrication of the micro-pyramidfilm is quitedielectric successful in film Figure 5g–i. successful in Figure 5g–i. structured thin is quite During the entire fabrication process, a small gap between the micro-pyramid units is formed. formed. Obtaining Obtaining the the texture texture by by SPDT, SPDT, the the flat flat area area is is nearly nearly tens of nanometers nanometers wide, wide, which has no influence on the pyramid height of 5 µm μm in the the nano-imprint nano-imprint resist before ICP. ICP. The The proportional proportional ICP ICP process process led to the increase of the gap width to more than one hundred nanometers because of the nonlinear the gap width to more than one hundred nanometers nonlinear feature feature of of the the resist. resist. In Figure 5.55.5 µmμm thick SiNSiN andand thethe transferred patterns are Figure 6,6,the thereflectance reflectancecurves curvesfor forthe the thick x layer transferred patterns x layer measured whenwhen the incident light islight unpolarized. In the condition of verticalof incidence, reflectance are measured the incident is unpolarized. In the condition vertical the incidence, the is reduced below 1.0% below through thethrough wavelengths concerned. concerned. reflectance is reduced 1.0% the wavelengths
Figure curves for for 5.49 5.49 µm μm thick thick SiN SiNxx layers micro-pyramid array. array. Figure 6. 6. Reflectance Reflectance curves layers and and the the measured measured micro-pyramid
The effect of anti-reflection comes from two aspects: First, the effective refractive index of silicon The effect of anti-reflection comes from two aspects: First, the effective refractive index of silicon nitride layer is decreased by the micro pyramid, which acts as a single-layer anti-reflection film. nitride layer is decreased by the micro pyramid, which acts as a single-layer anti-reflection film. Second, Second, the localized absorption enhancement is brought by the pyramid structure. the localized absorption enhancement is brought by the pyramid structure. Owing to the excitation of guided resonances, light-trapping can be achieved. The micro-pyramid Owing to the excitation of guided resonances, light-trapping can be achieved. The micro-pyramid texture can reduce the front surface reflection and scatter light efficiency into oblique angles. It was texture can reduce the front surface reflection and scatter light efficiency into oblique angles. It was generated by a combination of the properties of the relatively low surface area enhancement of about 70% and the gradient refractive index tapered profile. The interface effects in the coating ensure the wave reflected from the antireflection coating top surface to be out-of-phase with that reflected from the semiconductor surface. We consider the interference effect of the reflected light from the interface and how the reflections break there as well.
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generated by a combination of the properties of the relatively low surface area enhancement of about 70% and the gradient refractive index tapered profile. The interface effects in the coating ensure the wave reflected from the antireflection coating top surface to be out-of-phase with that reflected from the semiconductor surface. We consider the interference effect of the reflected light from the interface and how the reflections break there as well. Furthermore, based on the absorbing material, silicon nitride, the path length of the light would be increasing, by coupling the light into higher diffraction orders. The refraction of light into large angles allows the light to be reflected multiple times, and thereby improves light absorption. It means tapered micro-structures can greatly improve the light absorption over a wide spectral range because of the suppression of reflections and the light-trapping in the long wavelength range. As performed with the FDTD method, it shows that this micro-structure can localize photons and enhance the absorption inside the micro-pyramid. To summarize, the two influences, a single silicon nitride layer as an anti-reflection film and the micro-pyramid structure analyzed by FDTD, are superimposed together to achieve the final anti-reflection effect. These results show that our structure can realize anti-reflectance efficiently, and this fabrication process is effective with less cost and greater flexibility. 4. Conclusions In summary, a micro-optical pattern-based optical film, which has a rectangular pyramid shape and a 10 µm pattern size, was designed and fabricated. The patterns were fabricated using the SPDT technique, a hot embossing and ICP etching process. Compared to the wet-patterning approach, we can generate an angle-tunable pyramid structure. Micro-pyramid structures in the silicon nitride thin film are resistant to high temperatures, acid corrosion, and have better adhesion compared to polymer coatings. An optical property of reflectivity, less than 1.0%, was achieved by our approach. Thus, proving that large-scale pyramid structures are effective for antireflection in broadband wavelengths. Moreover, the performance of antireflection is analyzed by simulating the micro-pyramid structures by the FDTD method. This low reflectance is associated to light trapping and multiple reflections in the very sharp pyramid shape manufactured within the silicon nitride layer. Our results show that the interference effect of thin films and the pyramid structures are beneficial to the light-trapping effect. Author Contributions: S.G. and W.L. conceived and designed the experiments; S.G., S.Z., S.L., and Y.H. performed the experiments; S.G., W.L., X.S., and P.Y. analyzed the data; J.Z. and D.L. contributed analysis tools; and S.G. and W.L. wrote the paper. Funding: This research was supported by the National Basic Research Project (JCKY2016208A002), Advanced Manufacturing Project (41423020111), the National Natural Science Foundation of China (no. 61505157), the National Natural Science Foundation of China (no. 51502232), Scientific Research Plan Projects of Shaanxi Education Department (16JK1363), and the National R and D Program of China (2017YFA0207400), the principal fund of Xi’an Technological University (no. 302021501). Conflicts of Interest: The authors declare no conflict of interest.
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