METHOD FOR PREPARING A STACK OF DIELECTRIC LAYERS ON A SUBSTRATE

Abstract

A method for preparing a stack of dielectric layer on a substrate. A substrate is provided and a first layer of liquid is printed onto a surface of the substrate. A first dielectric layer is formed by solidifying the first layer of liquid and a second layer of liquid is printed onto the first dielectric layer. A second dielectric layer is formed by solidifying the second layer of liquid. The liquid includes dielectric constituents and the liquid is printed such that droplets having a volume of less than one hundred nanoliters are locally deposited per square millimeter on the surface of the substrate.

Claims

1. A method for preparing a stack of dielectric layers on a substrate, comprising: providing the substrate, printing a first layer of liquid onto a surface of the substrate, forming a first dielectric layer by solidifying the first layer of liquid, printing a second layer of liquid onto the first dielectric layer, forming a second dielectric layer by solidifying the second layer of liquid, wherein the liquid comprises dielectric constituents and the liquid is printed such that droplets having a volume of less than one hundred nanoliters are locally deposited per square millimeter on the surface of the substrate.

2. The method according to claim 1, wherein the liquid is printed onto the surface of the substrate using an inkjet printing technique.

3. The method according to claim 1, wherein the layer of liquid is printed by depositing droplets with at least one of varying droplet volumes, varying inter-droplet distances and varying overlapping of neighboring droplets along a lateral extension of the surface of the substrate, such as to form the dielectric layer with locally varying layer thicknesses with lateral dimensions (of such locally varying layer thicknesses being smaller than 1 cm, preferably being smaller than 1 mm.

4. The method according to claim 1, wherein the liquid is printed such that for each location to be covered by the liquid layer, at least two droplets are locally deposited at the location such as to superimpose each other.

5. The method according to claim 1, wherein the method comprises printing multiple layers of liquid on top of each other thereby forming a layer stack comprising multiple dielectric layers.

6. The method according to claim 5, wherein each of the multiple liquid layers is solidified before printing a subsequent liquid layer.

7. The method according to claim 1, wherein the first layer of liquid is printed using a first liquid comprising first dielectric constituents and the second layer of liquid is printed using a second liquid comprising second dielectric constituents being different from the first dielectric constituents.

8. The method according to claim 1, wherein multiple first and second layers of liquid are printed alternately on top of each other.

9. The method according to claim 7, wherein a refractive index of the first dielectric constituent differs from a refractive index of the second dielectric constituent by at least 0.1.

10. The method according to claim 7, wherein the first liquid is adapted such that the second dielectric constituents are soluble in the first liquid at most to a minor extent and/or wherein the second liquid is adapted such that the first dielectric constituents are soluble in the second liquid at most to a minor extent.

11. The method according to claim 7, wherein the first liquid comprises titanium dioxide particles as the first dielectric constituents.

12. The method according to claim 11, wherein the titanium dioxide particles are included in a matrix material which may be solidified by UV irradiation.

13. The method according to claim 7, wherein the second liquid comprises poly(methyl methacrylate) as the second dielectric constituents.

14. The method according to claim 13, wherein the poly(methyl methacrylate) is included in a solvent comprising 1,3-dimethoxybenzene.

15. The method according to claim 1, wherein the liquid layers are solidified by applying at least one of drying the liquid layer, irradiating the liquid layer using electromagnetic radiation and submitting the liquid layer to elevated temperatures of at least 40° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In the following, advantageous embodiments of the invention will be described with reference to the enclosed drawings. However, neither the drawings nor the description shall be interpreted as limiting the invention.

[0065] FIG. 1 visualizes the process step of printing a layer of liquid during a method for preparing a dielectric layer in accordance with an embodiment of the present invention.

[0066] FIG. 2 shows a cross-sectional view of a stack of dielectric layers on a substrate prepared in accordance with the method of the present invention.

[0067] FIG. 3 shows a top view onto a dielectric layer on a substrate prepared in accordance with the method of the present invention.

[0068] The figures are only schematic and not to scale. Same reference signs refer to same or similar features.

DETAILED DESCRIPTION

[0069] FIG. 1 shows a substrate 1 onto which a dielectric layer is to be prepared. FIGS. 2 and 3 show a cross-sectional view and a top view, respectively, of a dielectric layer 9 generated on top of the substrate 1.

[0070] The substrate 1 may for example be a flexible foil or a rigid plate. For preparing the dielectric layer 9, a layer 3 of liquid is printed onto a surface 5 of the substrate 1. The liquid comprises dielectric constituents such as PMMA or titanium dioxide particles. Furthermore, the liquid comprises for example a fluid solvent and/or a fluid matrix material. Fluid properties of the liquid are adapted such that the liquid may form a printable ink. The ink is printed by propelling little droplets 7 from a print head 13 of an inkjet printer 15. The droplets 7 are ejected from the print head 13 via nozzles 17. In the example presented in the FIG. 1, the print head 13 comprises several nozzles 17. Therein, at least two of these nozzles 17 are oriented and directed such that droplets 7 ejected through these nozzles 17 are deposited on a same location 19 on the surface 5 of the substrate 1. By consecutively depositing droplets 7 on the surface 5 of the substrate 1, the layer 3 of liquid is successively generated along the surface 5.

[0071] After having prepared such layer 3 of liquid, the liquid is solidified for forming the dielectric layer 9. Various techniques may be applied for such solidification. For example, the liquid may be dried such that substantially all fluid components are evaporated. Alternatively or additionally, for example polymers in the matrix material may be solidified by irradiating UV light, thereby cross-linking and curing the polymers.

[0072] Subsequently, another layer 3 of liquid may be deposited on top of the solidified dielectric layer 9. This further layer 3 of liquid may again be solidified for forming another dielectric layer 9. Preferably, the further layer 3 of liquid comprises second dielectric constituents which differ from the first dielectric constituents comprised in the liquid used for the preceding layer 3 of liquid. By repeating such a procedure multiple times, a stack 11 of dielectric layers 9 is generated. This stack 11 comprises an alternating layer sequence of dielectric layers 9 comprising the first dielectric constituents and dielectric layers 9 comprising the second dielectric constituents. Due to the different dielectric constituents, the alternating dielectric layers 9 may be provided with a dielectric material having different refractive indices.

[0073] As shown in FIG. 2, the dielectric layers 9 generally have a substantially smaller layer thickness t as compared to the thickness of the substrate 1. Such layer thickness may be in a range of less than 1 μm, typically less than 200 nm. Due to using the inkjet printing technique, the volume of the droplets 7 and the locations 19 where these droplets 7 are deposited may be selected such that the layer thickness may be adapted with a precision of less than 20 nm, preferably less than 5 nm.

[0074] In the example shown in FIG. 2, an additional adhesion layer 21 is deposited on top of the surface 5 of the substrate 1 before printing the first layer 3 of liquid thereon. This adhesion layer 21 may increase adhesion between the material of the substrate 1 and the material of the adjacent dielectric layer 9.

[0075] As shown in the top view of FIG. 3, the layer 3 of liquid is not printed along the entire surface 5 of the substrate 1 but only at predetermined locations 19. Accordingly, the resulting dielectric layer 9 covers the surface 5 of the substrate 1 with a pattern. Therein, first partial areas 23 covered by the dielectric layer 9 are separated from neighboring second partial areas 25 also covered by the dielectric layer 9 via intermediate partial areas 27 which are not covered by any dielectric layer 9 such that, in the intermediate partial area, the surface 5 of the substrate 1 is exposed. In other words, while in the first and second partial areas 23, 25, a thickness of the dielectric layer 9 is non-zero, a thickness of the dielectric layer 9 in the intermediate partial area 27 is zero. Lateral dimensions ld of such locally varying layer thicknesses may be smaller than 1 cm or even smaller than 1 mm. Therein, the lateral dimensions ld may correspond to a width of a smallest one of the partial areas 23, 25 and the intermediate partial area 27.

[0076] It shall be noted that the terms “first” and “second” are used herein only for distinguishing respective features and shall not be interpreted as necessarily indicating an order or sequence. For example, a “first layer” does not necessarily have to be printed before a “second layer” and the “second layer” does not necessarily have to be printed on top of a “first layer”, but the order may also be inverse. Furthermore, it shall be noted that the pattern of dielectric layers 9 shown in FIG. 3 is exemplary only and the locations 19 and shapes of the partial areas 23, 25, 27 are visualized in an arbitrary manner only.

[0077] In the following, possible features and advantages of an experimental implementation of the method described herein will be described with reference to a preferred embodiment. Therein, some of the features are described with a slightly different wording as compared to the preceding sections.

[0078] One-dimensional photonic crystals (1DPCs) have been widely investigated since the last century due to their fascinating capability of optical manipulation and structural coloration. A 1DPC has a structure where the refractive index (RI) is periodically distributed along one dimension in space, which can be built by alternating two materials with different RI or tuning the porosity of one material. Due to the multilayer interference effect, 1DPCs possess a photonic bandgap (PBG) analogous to the electronic bandgap in semiconductors. Electromagnetic waves at specific frequencies cannot propagate inside these media. By tailoring the stacking sequence, layer thickness, and material composition, it is possible to tune the photonic stopband to meet almost any desired characteristic, e.g. wavelength-selective filters and ultra-high reflectivity dielectric mirrors. Dielectric laser mirrors and dichroic beamsplitters are essential discrete elements of different optical systems. In many integrated optical components and systems, 1DPCs are used as distributed Bragg reflectors (DBR). Furthermore, 1DPCs enable highly sensitive optical sensors and colorful coatings for solar cells. Various materials have been used to fabricate 1DPCs, including inorganic, organic and hybrid materials. Different fabrication methods have been applied to manufacture 1DPCs: Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are widely used in industry. CVD-based dielectric mirrors are used for realizing vertical-cavity surface-emitting lasers (VCSELs). This technology, however, is very specialized and time-consuming. What's more, both methods need to be combined with lithographic patterning to obtain lateral structures. More recently, solution-based processes such as spin coating, dip coating, doctor blading, and self-assembly have emerged as attractive methods. As for spin coating, the drawbacks are the high material waste, limitations on substrate size, and the defects on thin film such as striations and comets which lead to poor stack quality. Doctor blading and self-assembly, on the other hand, can be used for large-scale production; however, it is very challenging to land with a good reproducibility on the layer thickness and good control on the thin film quality. Moreover, the lateral definition of 1DPC pixels is not possible with any of the abovementioned approaches.

[0079] A fully digitally fabricated 1DPCs prepared by inkjet printing is reported herein. Therein, a high RI contrast hybrid material pair based on printable nanoparticulate titanium dioxide (TiO.sub.2) and poly(methyl methacrylate) (PMMA) is used as inks. With ten bilayers, a maximum reflectance of around 99% was achieved, while a reflectance peak of more than 80% can be reached with five bilayers only. The central wavelength can be tuned in the spectrum from purple, through the whole visible range of the electromagnetic spectrum, into the infrared region by simply changing the layer thickness via modifying the printing parameters. Furthermore, not only small 1DPCs on rigid glass substrates were fabricated, but also large and patterned 1DPCs were successfully built on flexible foils. Inkjet printing, a simple, fast, and low-cost technique, therefore enables the fabrication of 1DPCs in various forms in different optoelectronic devices. Hence, it offers a pathway towards either up-scaling in macroscopic scale, such as a colorful patterned coating for solar cells, or pixelwise in integrated photonics applications, such as a spectrometer with high spatial and spectral resolutions.

[0080] A Bragg mirror is a device based on 1DPC. Therein, a central wavelength of the photonic bandgap generally only depends on an optical thickness of a first dielectric layer (H) having a high refractive index and a second dielectric layer (L) having a low refractive index. Hence, it can be tuned by changing the material composition and even simpler by the layer thickness. With the same constituent materials, the maximum reflectance is only influenced by the number of layers, and a higher reflectance can be obtained by stacking more layers.

[0081] In principle, the layer thickness can be controlled in an inkjet printing process by modifying the printing parameters. However, key challenges are the nanometer control of the thickness, the uniformity of the printed layers, and the choice of solvents such that intermixing of L and H layers is avoided.

[0082] Here, TiO.sub.2 and PMMA were chosen as the constituent materials due to their widespread usage in optical applications and their high contrast in RI. The fabrication process of 1DPCs was completed by alternately printing the PMMA and Ti.sub.02 layers on each other, as illustrated in FIG. 2. In order to be applied as suitable inks for the inkjet printing process, TiO.sub.2 nanoparticle dispersion with a UV curable organic matrix and PMMA ink were developed. At room temperature, PMMA showed a very low solubility in the solvent used for TiO.sub.2 ink. Therefore, it ensures that the previously printed PMMA layer would not be dissolved during the next TiO.sub.2 printing step. The RI of both materials were measured by ellipsometry. The RI of a printed TiO.sub.2 layer is 2.08 at 380 nm and 1.87 at 780 nm, while at these wavelengths, the RI of a PMMA layer is 1.49 and 1.48, respectively. This gives a high RI contrast ranging from 0.59 to 0.39 in the visible light range and renders these materials suitable to be used as high index and low index layers in the Bragg stack. In order to have different spectra with central wavelength over the whole visible light range, the thickness of each layer was adjusted by changing the printing resolution, i.e. the deposition volume per unit area. During printing, at least ten nozzles were used for jetting, and the quality factor was chosen as 2, which means one vertical line will be printed by two nozzles. By these means, the variation in individual droplet volume was minimized in each printing round. After printing the TiO2 layer, it was first dried at ambient temperature for 2 min, prebaked at 100° C. for 5 min, and then followed by a UV curing step, which is necessary to harden the film. At the same time, the UV—curable polymer matrix (PM) helped to enhance the stability of the TiO.sub.2layer without the need for a high sintering temperature. After the UV exposure, the TiO.sub.2 film was further hard-baked at 100° C. for 10 min. The UV curing step is an essential process, and an intermixing of the subsequent PMMA layer with TiO.sub.2 layer was observed when the UV radiation dose was not sufficient during exposure. As for the PMMA layer, it was first dried in a vacuum chamber at 10 mbar for 2 min and then heated at 50° C. for 5 min to eliminate the solvent residuals.

[0083] In order to realize a highly reflective mirror, a larger number of bilayers has to be deposited on top of each other. For achieving a reflectance of more than 98%, ten bilayers are needed by calculations. FIG. 2 shows a cross-sectional view of an exemplary 1DPC with several bilayers, the view substantially representing a cross-sectional scanning electron microscopy (SEM) of an actual printed 1DPC with ten bilayers. The bottom PMMA layer was intentionally designed to have a relatively larger thickness to ensure a closed PMMA film on the substrate. Meanwhile, this has little impact on the final optical property of the 1DPC due to its similar RI as the substrate. From the multilayer structure, it can be clearly seen that the thickness of each layer is uniform over a large area. Furthermore, the repeatability of the printing process is evident from the constant layer thickness of the same material. The inkjet printing may result in a closed Ti02 film with densely packed nanoparticles embedded in a polymer matrix. Measured reflectance and transmittance spectra of fabricated 10-bilayer 1DPCs show their excellent reflection/dichroic behavior with different central wavelengths. A maximum reflectance of about 99% was achieved, while the highest reflectance in the other 1DPCs reached around 98%.

[0084] The good complementary between the reflectance and transmittance spectra validates the peak values and also indicates that there is neither absorption nor optical scattering in the materials in the visible light range. The bandwidth of the PBG becomes larger with increasing central wavelength, as predicted by mathematical calculation. In addition, the 1DPC with a central wavelength at 416 nm shows a significant drop in transmittance near/in the ultraviolet light range due to the light absorption in both constituent materials and the glass substrate.

[0085] The prepared inkjet-printed 1DPCs can not only be used as high reflectivity mirrors but also as dichroic beamsplitters.

[0086] After the inks have been developed, the inkjet printing process can be transferred to a large variety of substrates. Only the printing parameters for the first PMMA layer need to be adjusted according to the wettability of the substrate. Therefore, 1DPCs can be printed not only on small and rigid substrates but also in specific patterns on large and flexible substrates. Hence, 1DPCs were also printed on 12×12 cm.sup.2 polyester (PET) foils. The deformability of the printed 1DPCs, therefore, allows for their applications on curved surfaces with a high degree of freedom. The excellent color homogeneity over a large area and the high reflectance give the inkjet-printed 1DPCs a massive potential for a large variety of applications.

Exemplary Substrate Preparation

[0087] A 2.5×2.5 cm.sup.2 glass substrates (soda lime glass) and 12×12 cm.sup.2 PET foils (Puetz Folien) were cleaned in an ultrasonic bath in deionized water, acetone, and isopropanol for 10 min each. Then the substrates were treated by oxygen plasma in a plasma chamber (PlasmaFlecto 30, Plasma technology) with a power of 100 W for 10 min.

Exemplary Ink Preparation and Inkjet Printing

[0088] PMMA with a molecular weight of 65000 Da (PSS-polymer) was dissolved in 1,3-dimethoxybenzene (≥98%, Sigma-Aldrich) to achieve a concentration of 40 mg/ml, and 10% hexylbenzene (97%, Sigma-Aldrich) was added to mitigate the coffee ring effect. Ti02 ink was prepared by diluting the TiO.sub.2 nanoparticle dispersion (RF-IO-UV, Avantamar) with ethylene glycol monopropyl ether (99.4%, Sigma-Aldrich) to reach a final concentration of 3.8 wt. %. The concentrations of the inks were determined to reach the target thickness range with suitable printing parameters. Before printing, both inks were placed in an ultrasonic bath for 5 min and then filtered using PTFE filters with a pore size of 0.2 μm. The inkjet printer (PixDro LP50) was equipped with 10 pL cartridges (Fujifilm Dimatix). During printing, at least 10 nozzles were used for jetting. The substrate temperature was set at 24° C. The print head temperature was set at 27° C. and 28° C. for TiO.sub.2 and PMMA ink, respectively. The waveforms were custom-made for up to 2.5 kHz jetting frequency. Both waveforms were simple single peak waveforms with a maximum voltage of 22 V, because the inks have been developed to be in the most suitable range for inkjet printing. First, a PMMA layer was printed directly on the substrate, and then the layer was vacuum dried at 10 mbar for 2 min and then placed on a hotplate at 50° C. for 5 min. TiO.sub.2 layer was subsequently printed on top of the PMMA layer, and this layer was first dried at ambient temperature for 2 min, and then pre-baked on a hotplate at 100° C. for 5 min, UV-cured for 10 min with a UV-LED light source (GC 77, Hamamatsu), and subsequently post-baked at 100° C. for 10 min. The following PMMA and TiO.sub.2 layers were alternately printed and processed in the same way. The thickness of each layer was controlled by changing the printing resolution, which determines how much volume is finally deposited on a unit area. The printing resolutions were in the range of 550 to 900 dpi for Ti02 ink and 500 to 700 dpi for PMMA ink. The whole fabrication process was completed in a cleanroom with the environment temperature at 21-22° C. and humidity at 40-50%.

[0089] In conclusion, the first fully digitally manufactured 1DPCs by multilayer inkjet printing has been demonstrated. The central wavelength can be tuned from the purple to infrared spectral range, i.e. from 416 nm to 808 nm. The central wavelength was tuned by changing the printed layer thickness, and the maximum reflectance was adjusted by controlling the number of bilayers. The reflectance peak reached up to around 99% when ten bilayers were printed. Without the need for a high sintering temperature, 1DPCs were successfully printed on glass substrates as well as on flexible PET foils in designed patterns. The printed 1DPCs showed a good color homogeneity and an overall high quality in optical property, making inkjet printing a highly competitive candidate for the large-scale fabrication of high-quality 1DPCs. Inkjet printing offers a fast, simple, material-saving, and low-cost fabrication route. Inkjet-printed 1DPCs, large or small, patterned or unpatterned, can be used in numerous different fields, as functional and decorative optical components. Vast applications may be foreseen, ranging from additive manufacturing of integrated photonic systems (e.g., in sensing systems) to large-area applications such as aesthetically appealing photovoltaics.

[0090] Finally, it should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

LIST OF REFERENCE SIGNS

[0091] 1 substrate [0092] 3 layer of liquid [0093] 5 surface of the substrate [0094] 7 droplet [0095] 9 dielectric layer [0096] 11 stack [0097] 13 print head [0098] 15 inkjet printer [0099] 17 nozzle [0100] 19 location [0101] 21 adhesion layer [0102] 23 first partial area [0103] 25 second partial area [0104] 27 intermediate partial area [0105] t layer thickness [0106] ld lateral dimension