Lithography-free glass surface modification by self ...

Lithography-free glass surface modification by self-masking during dry etching Eric Hein Dennis Fox Henning Fouckhardt

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Lithography-free glass surface modification by self-masking during dry etching Eric Hein, Dennis Fox, and Henning Fouckhardt Kaiserslautern University of Technology, Physics Department, Integrated Optoelectronics and Microoptics Research Group, Gottlieb-Daimler-Strasse, Kaiserslautern Rheinland-Pfalz 67653, Germany [email protected] Abstract. Glass surface morphologies with defined shapes and roughness are realized by a two-step lithography-free process: deposition of an ∼10-nm-thin lithographically unstructured metallic layer onto the surface and reactive ion etching in an Ar/CF4 high-density plasma. Because of nucleation or coalescence, the metallic layer is laterally structured during its deposition. Its morphology exhibits islands with dimensions of several tens of nanometers. These metal spots cause a locally varying etch velocity of the glass substrate, which results in surface structuring. The glass surface gets increasingly rougher with further etching. The mechanism of self-masking results in the formation of surface structures with typical heights and lateral dimensions of several hundred nanometers. Several metals, such as Ag, Al, Au, Cu, In, and Ni, can be employed as the sacrificial layer in this technology. Choice of the process parameters allows for a multitude of different glass roughness morphologies with individual defined and dosed optical scattering. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3586787] Keywords: surface roughness; dry-etching; self-masking; surface scattering; borosilicate glass; electron cyclotron resonance reactive ion etching. Paper 10065SSPRR received Sep. 29, 2010; revised manuscript received Apr. 12, 2011; accepted for publication Apr. 14, 2011; published online May 5, 2011.

1 Introduction For a certain class of applications in optics and optoelectronics, glass surface roughness allows for the scattering of the incident light power in a preferred way. Because of their excellent properties, glasses are favored over polymers in applications, in which high thermal or mechanical stress exists and in which high chemical resistance, high transparency, or high electrical isolation are requested.1 Light can be directed by scattering at rough surfaces or interfaces. In this way conversion efficiency of thin-film solar cells can be enhanced by roughening of the glass superor substrate,2–4 because the scattered or reflected waves travel a second and longer absorption path in the active media. In a similar way, the light output coupling of organic light-emitting diodes can be increased.5,6 Typically lithographical processes employing exposure and etch masks are applied for surface modification. However, such processes suffer from high complexity and costs. On the other hand, mechanical surface treatments, such as sandblasting or polishing can cause defects or line textures on the surface, which induce undesired diffraction effects;3,7 furthermore, the feasible minimal structure dimensions are of several tens of micrometers.8 Hence, there is a call for alternative noncomplex technologies that enable structuring of glass substrates from the nano- to the micrometer range. It has been known for some time that self-organization during lithography-free reactive ion etching (RIE) or reactive ion-beam etching can cause surface modifications on the nano- to micrometer scale.9,10 A special type of self-organization is termed self-masking: the generation

C 2011 SPIE 1934-2608/2011/$25.00

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Hein, Fox, and Fouckhardt: Lithography-free glass surface modification by self-masking during dry-etching

of micromasks by particle clusters that stick to the substrate surface during dry etching and exhibit a lower etch rate than the substrate.11–14 These particles, originating either from the mask material of preceding etch runs, the substrate electrode, or from the glass itself, partially form hardly volatile metal-fluoride compounds with reaction products of the etch gas. These metal-fluoride spots temporarily and locally protect the glass surface from etch attack, while other surface areas suffer from high material erosion. The locally varying etch velocity causes formation of hillocks and depressions. However, this type of self-masking strongly depends on the exact conditions of the etch chamber history and pretreatment conditions and thus is not completely reproducible.15 In our still lithography-free technology, we control the process of mask generation by deposition of a thin sacrificial metal layer prior to etching. Because of the growth behavior during deposition, the layer gets structured by another type of self-organization (i.e., nucleation and coalescence)16 and the statistically distributed metallic spots remaining on the surface after sacrificial layer deposition then cause a locally and temporarily varying etch velocity during RIE in an Ar/CF4 plasma. Chemical reactions in combination with physical sputtering ensure material erosion at the glass surface. The resulting glass surfaces again exhibit distinct roughness morphologies and, hence, individual optical scattering characteristics.17,18 In principal, etch velocity may also depend on the local glass composition. Because the reactive gas can form compounds with substrate material constituents, compounds that either stick to or desorb from the surface, self-masking may be supported. However, in our technology the latter mechanisms affect the glass roughness to a lesser extent. The surface structures of our samples are neither regular in shape, size, and topography nor of completely statistical nature. Among the most remarkable structures achieved in similar processes are cones.6 By changing process parameters, various structures/morphologies and optical scattering characteristics can be realized.17,18 With our technique, “dosed” light scattering can be achieved by proper choice of surface morphologies (i.e., the fraction of diffusely scattered light power can be changed in small steps). In our contribution,17 a similar process without a sacrificial metal layer is discussed with a closer look at the optical scattering properties of the surfaces, whereas another work of ours18 deals with the details of our process with a sacrificial copper layer and different types of glass; we also show that the process is reproducible. The main aspects of this contribution are the different influences of various sacrificial metals and their layer thicknesses on the resulting glass roughness morphology. We investigate the influence of the roughness morphology of the sacrificial metal layer on the resulting glass roughness morphology. It will be shown that copper is the most promising material for the sacrificial layer (among those tested) because it is easy to process, inexpensive, allows generation of various morphologies, and exhibits the highest initial roughness.

2 Methodology

2.1 Etch System and Metrology The employed etch apparatus is an electron cyclotron resonance RIE (ECR-RIE) machine, MicroSys 350 (Roth&Rau, Wuestenbrand, Germany). Besides Ar as the plasma feed gas, CF4 is used as the reactive gas. Both gas fluxes can be adjusted separately by mass flow controllers. The operating pressure is varied between 10−4 and 10−3 h Pa. The ECR-RIE system enables individual control of the plasma density and ion energy by choice of the microwave power and substrate bias, respectively. Microwave power is varied between 200 and 800 W and the negative self-bias of the substrate electrode between 240 and 600 V. A He gas back-side cooling serves as a heat sink for the substrate. Typically, an etch process lasts between 30 and 60 min. Between two etch runs, we employ a chamber cleaning plasma with O2 and Ar for 30 min to reduce chamber contamination and to guarantee the same initial conditions for each etch run. The different surface morphologies and topographies are investigated mainly using a Parker Systems XE-70 atomic force microscope (AFM), a Zeiss DSM 960 scanning electron Journal of Nanophotonics

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Hein, Fox, and Fouckhardt: Lithography-free glass surface modification by self-masking during dry-etching R microscope (SEM), or a Raith e_LiNE electron beam lithography system with a Zeiss Gemini electron beam column. Before SEM inspection of certain glass samples, an ∼5-nm thin layer of Cr is deposited onto the substrates to avoid electronic charging. The Cr coating does not noticeably influence the shape or roughness of the surface morphologies.

2.2 Glass Substrates As substrates, we use D 263TM T thin glass (Schott AG, Mainz, Germany), with a typical thickness of 550 μm. This borosilicate glass is often chosen for optoelectronic displays or for chemical applications due to its high chemical resistance.1 Besides silicon and boron oxides, the glass contains alkali- and alkali-earth metal oxides, aluminum, tin, and zinc oxides.14 Our samples usually have an area of 2.5 × 2.5 cm2 . Before etching, they are cleaned in an ExtranTM solution (Merck, Darmstadt, Germany), rinsed in an ultrasonic bath with deionized water, and dried in a nitrogen flow. After cleaning, the glass is coated with the sacrificial metal layer (process step 1). This is either performed inside a self-made magnetron sputter system or by evaporation in a Balzers Type 510 machine. Choice of the coating machine depends on the material to be deposited. Finally, the main dry etch (process step 2) is applied.

3 Materials to Induce Self-Masking

3.1 Polymers Before we decided to employ a metal layer to induce self-masking, we had been aiming for polymer seed/sacrificial layers. It is well known that reaction products of the etch gas CF4 or of other fluorocarbon gases combine with SiO2 , metals, or glass constituents during RIE or sputtering.19–21 Under certain etch conditions, this may cause formation of a polymer film or polymer spots on the substrate.22 These spots act as microetch masks during continuous ion bombardment; they prohibit further etching, resulting in surface roughening of the substrate. The formation of polymers is a random and weak process. In principle, even stronger polymer formation should be achievable by placing a piece of the polymer in question on the substrate electrode inside the plasma chamber close to the sample. Physical sputtering and redeposition of polymer fragments on the sample during RIE might result in a fissured polymer film that could serve as an etch mask. However, in this way no uniform glass roughness could be achieved in our investigations. It is assumed that the amount of sputtered polymer, which redeposited onto the glass substrates, was too small to act as micromasks on the whole sample area.

3.2 Sacrificial Layers Made of a Metal In our technology, we control the process of self-masking by deposition of a thin sacrificial metal layer prior to etching. If an ∼10-nm thin metal layer is deposited onto a substrate, metal islands separated by small fissures might appear in the layer during deposition [see e.g., Fig. 1(a)]. The metal spots—possessing lateral dimensions of several tens of nanometers—can initiate masking to some extent.23 Energy-dispersive x-ray measurements of dry-etched glass samples verify that the sacrificial metal layer is completely removed after a few minutes of etching. Removal duration depends both on the used metal and the etch parameters. Figure 1 shows the roughness morphologies of glass samples after metal layer deposition and 0, 2, and 5 min of etching, respectively (all other etch parameters are the same). It can be seen that the lateral dimensions of the surface structures in Fig. 1(b) are slightly above those of the metal islands on the unetched glass sample from Fig. 1(a). After 5 min of RIE, the dimensions are further increased above 100 nm [compare Fig. 1(c)]. This reveals that the island morphology of the sacrificial metal layer is initially transferred to the glass surface during dry etching. AFM measurements at a series of samples Journal of Nanophotonics

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Fig. 1 SEM images of (a) an unetched glass sample, (b) after 2 min of RIE, and (c) after 5 min of RIE. In a first process step, all samples had been coated with a nearly 10-nm thin sacrificial Cu layer. The sacrificial metal layer is laterally structured during deposition by nucleation and coalescence, as can be seen in (a).

etched with different duration—while keeping all other process parameters the same—show an increase of surface roughness with increasing etch duration. X-ray photoelectron spectroscopy of etched samples show no noticeable differences in surface compositions or in chemical bonds compared to an unetched sample (as a reference). Hence, in our process, self-masking is hardly supported by compounds of the etch gas and glass constituents or polymerization, but rather by the nucleation and coalescence of the sacrificial metal layer. In Figs. 2–4 the reader will find prominent etch results in the case of different sacrificial metal layer materials and for various etch conditions. Those are only examples from dozens of different morphologies seen thus far, and the authors cannot even be sure whether a totally different parameter range (etch gas, gas flow rates, operating pressure, microwave power, self-bias, substrate cooling, etc.) would give other prominent features and morphologies, not encountered thus far.

3.2.1 Sacrificial Copper Layer Roughness morphologies of a substrate can easily be initiated by a sacrificial layer made of copper.18,24 Copper (Cu) tends to lower the etch rate and especially forms the hardly volatile CuF2 or rather CuF compound in fluorine-containing plasmas, thus inducing self-masking.24,25 Optical emission spectroscopy of the plasma during the etch process of our samples indicated a decrease of emission intensity in the wavelength range from 554 to 568 nm for increasing etch durations. Spectral lines at such wavelengths can be attributed to excited Cu atoms and CuF molecules, which recombine by emission of radiation. Therefore, at the beginning of the etch runs, the glass surface may also be partially protected from material erosion by hardly volatile CuF compounds. Figure 2 shows two morphologies that emerge during dry etching under different etch conditions. Both samples had been coated with an ∼10-nm thin copper layer before etching. Figure 2(b), for example, verifies that even cones—as the most prominent quasi-benchmark— can be achieved this way. Journal of Nanophotonics

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Fig. 2 SEM images of borosilicate glass samples (a) DT203Cu and (b) DT251Cu after dry etching. Both samples had been coated with an ∼10 nm thin sacrificial layer of Cu before etching. The different glass roughness morphologies result from varied etch conditions. Etch parameters of (a) DT203Cu are: Ar flux: 4 sccm, CF4 flux: 16 sccm, pressure: 2.34 × 10−3 h Pa, microwave power: 529 W, self-bias: 400 V, etch time: 30 min; and for (b) DT251Cu: Ar flux: 2 sccm, CF4 flux: 6 sccm, pressure: 0.67 × 10−3 h Pa, microwave power: 433 W, self-bias: 340 V, etch time: 60 min.

AFM measurements of 10-nm thin Cu, Ag, or Al layer deposited onto glass substrates exhibit different roughness morphologies. The root-mean-square roughness Rq of the Cu-coated sample is nearly Rq ≈ 5 nm, while the Ag- or Al-coated samples possess lower roughness values (Rq ≈ 3 nm and Rq ≈ 1 nm, respectively). AFM inspection of the glass samples after dry etching with the same etch parameters reveals a clear correlation between metal-layer roughness and resulting glass roughness: the larger the initial roughness of the metal layer is, the larger the roughness of the glass sample is. In the case of Cu-coated substrates, the diverse glass roughness morphologies arise over a large range of parameters and are manifold. Thus, Cu is the most promising material for the sacrificial layer among those tested.

3.2.2 Sacrificial Silver Layer Evaporated or sputtered silver (Ag) often shows fissures like those in Fig. 3(a). The typical irregular distance of the fissures/grooves is on the order of 100 nm. One of the various glass roughness morphologies that is achieved after 45 min of dry etching shows cones with somewhat eroded tops [see Fig. 3(b)]. The cones have typical lateral dimensions between 0.5 and 1 μm. Again, there is a connection between the two morphologies: the metal spots act as a statistically distributed etch mask that causes an initial roughening of the glass surface. After removal of the sacrificial layer during the etch process, the glass roughness is getting coarser by continuing dry etching. The diameters and distances of the optical scatterers (here, eroded cones) can be

Fig. 3 SEM images of (a) a 10-nm thin Ag layer deposited on top of a smooth borosilicate glass sample before (DT157Ag) and (b) another sample (DT133Ag) after dry etching. The black structures (fissures) in the Ag layer (a) are much smaller and less far apart than those of the glass surface after etching as shown in (b). Here, the structures have typical sizes and distances between 0.5 and 1 μm. Note, the white bar refers to 100 nm in (a) and to 1 μm in (b), respectively. Journal of Nanophotonics

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tuned by changing the etch parameters, such as self-bias and gas flow, although the fissure parameters had remained the same. Initially, formed scatterers can even be “grinded down” to smooth morphologies by choice of higher self-bias.

3.2.3 Sacrificial Gold or Indium Layer Etch runs with a sacrificial layer of indium (In) can result in glass surfaces structured with dense-lying spiky conelike single scatterers, as illustrated in Fig. 4(a). In comparison to the etch result of sample DT251Cu in Fig. 2(b) (a sample that was initially covered with a seed layer of Cu and etched for 60 min), the cones of DT244In from Fig. 4(a) are clearly smaller for an etch duration of 30 min (all other etch parameters the same for both samples). Dry etching of a glass sample initially covered with a gold (Au) layer results in features on the order of 1.5-μm characteristic size, like those shown in Fig. 4(b). By changing the etch parameters (i.e., gas flux, self-bias, and microwave power), self-bias and microwave power, the ratio between the rough and smooth areas on the glass surface can be tuned. After a series of etch runs with various sets of etch parameters, no other morphologies could be observed for samples with an indium or gold sacrificial layer.

3.2.4 Other Sacrificial Layer Materials Using a sacrificial layer of aluminum (Al) or nickel (Ni), glass roughness can only be observed after dry etching for a small range of parameters. However, the results cannot be achieved homogeneously over the whole sample surface area in all cases. Partially, there are large smooth (unetched) areas between rough spots. Glass samples initially covered by titanium (Ti) or chromium (Cr) layers cannot be etched reproducibly nor can regular roughness be achieved within the chosen parameter ranges. On the one hand, this observation can be attributed to the fact that these metal layers are not laterally structured after deposition, while for glass roughening, the sacrificial metal layer must be inhomogeneous to cause a locally and temporarily etch rate. On the other hand, the diverse metals interact in an individual manner with the etch gas and thus can act as etch mask spots for a longer or shorter duration. For glass surface structuring, the deposition of a sacrificial Ag or Cu layer is found to be most promising because such materials are easy to process and enable the realization of a large variety of different glass roughness morphologies by subsequent RIE. Compared to Ag, a sacrificial Cu layer is less expensive and thus preferred in our technology.

Fig. 4 SEM images of borosilicate glass samples (a) DT244In with indium sacrificial layer and (b) DT193Au with gold sacrificial layer, both after dry etching. Both samples had been coated with an ∼10-nm thin sacrificial layer. Note, the white bar refers to 1 μm in (a) and to 2 μm in (b), respectively. Journal of Nanophotonics

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4 Results

4.1 Variation of Morphologies To achieve different glass roughness morphologies with the process described in Sec. 3 (i.e., deposition of a sacrificial metal layer and subsequent dry etching), the following parameters are varied: the thickness of the sacrificial layer and the etch parameters, such as gas flow, total gas pressure, microwave power, and self-bias.

4.1.1 Metal Layer Thickness The thickness of the metal layer has nearly no influence on the scatterers’ shapes, but does have influence on the height and surface density of the scatterers. For a series of etch results with various silver seed layer thicknesses, it is observed that the scatterers’ shapes are very much alike. But for initial layer thicknesses clearly greater than 100 nm, no prominent roughness is observable. Thinner seed layers are better suited for the purpose of introducing statistically distributed etch mask spots. The desired inhomogeneities in the sacrificial layer thickness are best for thin seed layers of ∼10-nm thickness. For thick seed layers, a dense metal film is formed. Moreover, we typically used a fixed etch duration of 30 min. With too thick a seed layer (more than ∼100 nm) most of the fixed etch duration of 30 min will be used to sputter the metal film. The exposure time of the glass to incident ions is considerably reduced, and no remarkable surface patterning occurs. With too thin a layer, there are not enough metal spots that could induce a locally strongly varying etch velocity of the substrate.

4.1.2 Etch Parameters The microwave source of the RIE apparatus ionizes the Ar/CF4 gas mixture and keeps up a high-density plasma of up to 1013 ions cm−3 .16 With too high or too low a microwave power, no prominent roughness is observed; thus, a medium value of ∼530 W must be chosen to obtain a maximum of surface roughness.18 A variation of self-bias exhibits a similar characteristic: a maximum of roughness is found for 400 V. Applying higher self-biases typically results in less rough surfaces in terms of root-mean-square roughness Rq ; the structures (if any) are eroded by the etch process. The same result is observed for lower or no self-bias because, in this case, the kinetic energy of the plasma ions is too small to erode the glass surface. For a series of samples with a 10-nm sacrificial Cu layer, the Ar/CF4 ratio was varied, while all other etch parameters were kept constant. With higher Ar flow, the glass surface exhibits large smooth areas between isolated scatterers. Lowering the Ar flow, the surface shows finer features on a 50-nm scale on top of the structures [see also Fig. 4(b)]. It must be kept in mind that the values for the optimum layer thickness and the optimum etch parameters are related to each other and also machine dependent. Another RIE system might have other optimum etch parameters and conditions. Table 1 shows the process parameters of a selection of samples. The last two characters of each sample’s denomination indicate the material that was used for the sacrificial layer. Some roughness morphologies are presented in Sec. 3. The etched glass samples are characterized by their root-mean-square roughness Rq and the averaged correlation length x1/e of the twodimensional surface profile. The latter quantity equals a mean lateral structure size (radius) of the statistically distributed single scatterers. Both Rq and x1/e are obtained by evaluation of AFM measurements (20 × 20 μm2 scan areas with 512 × 512 data points). Figure 5 verifies that—for our samples and our etch process—in first approximation, the correlation length is proportional to Rq . This means that the single scatterers are either large and broad or small and narrow. This behavior is indicated by the dashed line, which illustrates a linear fit to the measured data. Journal of Nanophotonics

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Table 1 Process parameters of a selection of samples. The two last characters of the sample denomination indicate the material that was used as sacrificial metal layer before dry etching (each 10 nm thin).

Sample DT178Cu DT254Cu DT201Cu DT204Cu DT144Ag DT160Al DT176Cu DT244In DT193Au DT145Ag DT142Ag DT251Cu

Ar flux (sccm)

CF4 flux (sccm)

Pressure (10−3 h Pa)

Etch duration (min)

Microwave power (W)

Self-bias (V)

4 2 4 4 4 4 4 2 4 4 4 2

16 6 16 16 12 8 8 6 12 12 12 6

2.46 0.67 2.29 2.34 1.68 1.91 1.24 0.67 2.34 1.82 1.65 0.67

30 30 45 30 15 60 30 30 45 60 60 60

529 433 528 529 716 716 529 433 528 715 715 433

250 240 400 400 280 340 400 340 240 350 280 340

We found that our technology is reproducible in the sense that the roughness morphologies of identically processed samples show the same shapes of single scatterers. Also, surface roughness Rq of these samples coincides within a deviation of 8%.

4.2 Optical Scattering Properties Because of their different surface roughness morphologies, the etched glass samples show individual optical scattering characteristics. All optical measurements have been performed by Brinkmann’s group at the University of Applied Sciences Darmstadt, Germany. Because measurements at various wavelengths (473, 532, and 660 nm) showed no strong wavelength dependence within the measurement uncertainty, most of the measurements reported here have been carried out at only one vacuum wavelength (i.e., 473 nm). (Only effects that are related to the fact that the ratio Rq /λ is dependent on the vacuum wavelength λ are observed.) Optical scattering inspections are performed with two measurement setups: an integrating sphere for

Fig. 5 Interdependence between surface roughness Rq to mean lateral size (i.e., radius) of the individual structures x1/e . The latter quantity is the averaged two-dimensional surface height profile correlation length. In the first approximation x1/e shows a linear relationship to Rq as indicated by the dashed line. Journal of Nanophotonics

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Fig. 6 Outline of the main part of the optical measurement setups. The quantity Ts is the specular transmission.

determination of the total transmitted and reflected power (T and R, respectively) and a one-axis goniometer for angle-resolved measurements of the scattered light power. The goniometer setup enables scatter measurements in one plane of a hemisphere either in transmission or reflection. As the angle of incidence was to be varied over a wide range, a glass hemisphere has been fitted to the glass sample at its smooth unstructured face with a layer of an index-matching liquid (see Fig. 6). The hemisphere allows for an increase of the angle of incidence up to nearly 90 deg because strong reflection is avoided. All measurements are performed with the light incident from the smooth back side of the glass samples. The background of the specular (nonscattered) peaks Ts and Rs is represented by the diffusely scattered transmitted or reflected relative power Td and Rd , respectively. All kinds of measurement losses are summarized in quantity A. Table 2 shows optical scattering results of a selection of samples. Both the material of the sacrificial metal layer as well as the etch parameters were varied, as specified in Table 1. Because of the fact that local refraction and total internal reflection at each single optical scatterer adds up to the total scattering, the ratio of diffuse to specular power is and must be different in transmission and reflection.17 Diffuse transmission Td ranges from 10 to 78% in small steps; the technological process described thus far enables realization of virtually any ratio of the diffuse and specular light power portion in transmission by variation of the process parameters Table 2 Optical scattering properties of various samples at a vacuum wavelength of 473 nm and normal incidence: diffuse and specular transmission (Td , Ts ) and reflection (Rd , Rs ). In the evaluation, the loss A incorporates all effects that reduce the signal. Fresnel reflections at the first air/glass hemisphere interface are already subtracted from total power. Because, in general, reflection is weak and, thus, harder to measure accurately, Rs is given together with A. It is obvious that diffuse transmission can be tuned over a wide range.

Sample DT178Cu DT254Cu DT201Cu DT204Cu DT144Ag DT160Al DT176Cu DT244In DT193Au DT145Ag DT142Ag DT251Cu

Diffuse transmission Td (%)

Specular transmission Ts (%)

Diffuse reflection Rd (%)

Specular reflection Rs + measurement losses A (%)

10 14 25 30 31 47 51 52 61 68 77 78

85 81 70 62 56 48 36 36 24 24 10 4

4 5 4 7 5 5 12 12 13 9 13 15

1 0 1 1 8 0 1 0 1 0 1 2


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Fig. 7 Interdependence between the root-mean-square roughness Rq to TIS in transmission TIST (λ = 473 nm, normal incidence); the solid curve illustrates the relationship between Rq and TISLit given in Eq. (1).

(i.e., there is an opportunity for “dosed” optical scattering with a reliable and reproducible process). Another important advantage of our process is that no line texture is created that would result in diffraction. The ratio Td /T = Td /(Ts +Td ) is called total integrated scatter (TIST ) (here, in transmission) and can be related to the morphological surface quantity Rq , i.e., Rq 2 TISLit = 1 − exp − 2π (n1 − n2 ) , (1) λ according to the literature.7,26 Here the parameters n1 and n2 are the refractive indices of the glass (n1 = 1,5305)1 and air (n2 = 1), respectively, and λ denotes the wavelength of the incident light, in case of our measurement wavelength of λ = 473 nm. The relationship from literature (illustrated by the solid curve in Fig. 7) is valid for a normalized wavelength λnorm =

λ 2π (n1 − n2 )

≈ Rq

(2)

on the order of the root-mean-square roughness Rq or slightly above. For most of our samples, Rq is on the order of λnorm ≈ 140 nm; thus, there is good agreement with Eq. (1). However, some of the measured TIST values are larger than expected, indicating that subwavelength features in the morphology have some impact on the optical scattering. Obviously, the larger the roughness as well as the scatterers are, the stronger the diffuse transmission is. These results are characteristic for the samples realized with our technological process. Further optical scattering results of our samples are discussed in Ref. 17.

5 Conclusions Surface patterning of borosilicate glasses is induced by self-organizational micromask generation during ECR-RIE. For this purpose, a sacrificial metal layer (not lithographically structured) is deposited onto the substrates before etching. The metal layer is laterally structured during deposition by self-organization (i.e., nucleation and coalescence), thus enforcing a locally varying etch velocity of the glass substrate. Proper choice of the metallic layer, its thickness, and tuning of the etch parameters enables realization of a variety of glass roughness morphologies. Journal of Nanophotonics

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An ∼10-nm-thick copper layer is most suitable as a sacrificial layer in this technology. Diverse shapes, such as cones, hemispheres, pits, or trenches of submicron to micron dimensions, are observed after dry etching of the glass. The multitude of different generated morphologies exhibits a wide range of possible optical scattering properties.

Acknowledgments The work has been financially supported by the Foundation Rheinland-Pfalz (RhinelandPalatinate) for Innovation under Contract No. 836 and by the German Federal Ministry of Education and Research (BMBF) within the funding program Optical Technologies under Contract No. 13N9451. The authors also acknowledge support by Schott AG, Mainz, Germany, both by valuable discussions and the provision of glass samples. The authors also thank Matthias Brinkmann, Malte Hagemann, and Willi Juschtschenko for the scattering measurements in Brinkmann’s group at the University of Applied Sciences Darmstadt, Germany, and, we are grateful to Sandra Wolff, Bert Lägl, and Christian Dautermann of the Nano+Bio Center at Kaiserslautern University of Technology for some technical assistance.

References 1. Advanced Materials Schott AG, “Specification Physical and chemical properties D 263 T – Thin Glass,” http://www.schott.com/special_applications/german/download/d263t.pdf. 2. Ch. Gandon, Ch. Marzolin, B. Rogier, and E. Royer, “Transparent Textured Substrate and Methods for Obtaining Same,” United States Patent Application Publication No.: US 2004/0067339 A1 (2004). 3. A. G. Aberle, P. I. Widenborg, and N. Chuangsuwanich, “Glass Texturing,” Patent Cooperation Treaty, International Publication Number WO 2004/089841 A1 (2004). 4. R. Buffaron, L. Escoubas, J. J. Simon, Ph. Toricho, F. Flory, G. Berginc, and Ph. Masclet, “Enhanced antireflecting properties of microstructured top-flat pyramids,” Opt. Express 16(23), 19304–19309 (2008). 5. S. Reineke, F. Lindner, G. Schwarz, N. Seidler, K. Walzer, B. Luessem, and K. Leo, “White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency,” Nature Lett. 459, 234–239 (2009). 6. H. Fouckhardt, I. Steingoetter, M. Brinkmann, M. Hagemann, H. Zarschizky, and L. Zschiedrich, “nm- and μm-scale surface roughness on glass with specific optical scattering characteristics on demand,” Adv. OptoElectron. 27316 (2007). 7. J. M. Bennett and L. Mattsson, Introduction to Surface Roughness and Scattering, pp. 30 and 65, second edition, Optical Society of America, Washington, D.C. (1999). 8. J. A. Plaza, M. J. Lopez, A. Moreno, M. Duch, and C. Cané, “Definition of High Aspect Ratio Glass Columns,” Sens. Actuators A 105(3), 305–310 (2003). ´ 9. M. Navez, C. Sella, and D. Chaperot, “Microscopie e´ lectronique-Etude de l’attaque du verre par bombardement ionique,” Comptes Rendus de l’Académie des Sciences 254, 240–242 (1962). 10. U. Valbusa, C. Boragno, and F. Buatier de Mongeot, “Nanostructuring surfaces by ion sputtering,” J. Phy. Cond. Matter 14(35), 8153–8175 (2002). 11. E. E. Metwalli and C. G. Pantano, “Reactive ion etching of glasses: Composition dependence,” Nucl. Instrum. Methods Phys. Res. B 207(1), 21–27 (2003). 12. D. Y. Choi, J. H. Lee, D. S. Kim, and S. T. Jung, “Formation of induced surface damage in silica glass etching for optical waveguides,” J. Appl. Phys. 95(12), 8400–8407 (2004). 13. G. K. Wehner and D. J. Hajicek, “Cone Formation on Metal Targets during Sputtering,” J. Appl. Phys. 42(3), 1145–1149 (1971). 14. L. Lallement, C. Gosse, C. Cardinaud, M.-C. Peignon-Fernandez, and A. Rhallabi, “Etching studies of silica glasses in SF6 /Ar inductively coupled plasmas: Implication for microfluidic devices fabrication,” J. Vac. Sci. Technol. A 28(2), 277–286 (2010). Journal of Nanophotonics

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15. D. A. Zeze, D. C. Cox, B. L. Weiss, and S. R. P. Silva, “Lithography-free high aspect ratio submicron quartz columns by reactive ion etching,” Appl. Phys. Lett. 84(8), 1362–1364 (2004). 16. G. Franz, Niederdruckplasmen und Mikrostrukturiertechnik, 3rd ed., pp. 189, 273; SpringerVerlag, Berlin (2004). 17. H. Fouckhardt, E. Hein, D. Fox, and M. Jaax, “Multitude of glass surface roughness morphologies as a tool box for dosed optical scattering,” Appl. Opt. 49(8), 1364–1372 (2010). 18. E. Hein, D. Fox, and H. Fouckhardt, “Self-masking controlled by metallic seed layer during glass dry-etching for optically scattering surfaces,” J. Appl. Phys. 107(3), 033301 (2010). 19. J. W. Coburn and E. Kay, “Some chemical aspects of the fluorocarbon plasma etching of silicon and its compounds,” IBM J. Res. Dev. 23(1), 33–41 (1979). 20. E. Kay and A. Dilks, “Metal-containing plasma polymerised fluorocarbon films—their synthesis, structure, and polymerisation mechanism,” J. Vac. Sci. Technol. 16(2), 428–430 (1979). 21. Y. Ozaki and K. Hirata, “Columnar etching residue generation in reactive sputter etching of SiO2 and PSG,” J. Vac. Sci. Technol. 21(1), 61–65 (1982). 22. B. K. Smith, J. J. Sniegowski, G. LaVigne, and C. Brown, “Thin Teflon-like films for eliminating adhesion in released polysilicon microstructures,” Sens. Act. A 70(1–2), 159–163 (1998). 23. F. Frost, B. Ziberi, A. Schindler, and B. Rauschenbach, “Surface engineering with ion beams: from self-organized nanostructures to ultra-smooth surfaces,” Appl. Phys. A 91(4), 551–559 (2008). 24. J. L. Vossen, “Inhibition of chemical sputtering of organics and C by trace amounts of Cu surface contamination,” J. Appl. Phys. 47(2), 544–546 (1976). 25. A. J. van Roosmalen, “Review: dry etching of silicon oxide,” Vacuum 34(3-4), 429–436 (1984). 26. C. K. Carniglia, “Scalar scattering theory for multilayer optical coatings,” Opt. Eng. 18(2), 104–115 (1979).

Eric Hein studied physics at Kaiserslautern University of Technology, Kaiserslautern, Germany from 2002 to 2008. In 2008, he received the diploma in physics at Kaiserslautern University of Technology. Since 2008, he has been working as a PhD student in the university’s Integrated Optoelectronics and Microoptics Research Group in the Physics Department.

Dennis Fox studied physics at Kaiserslautern University of Technology, Kaiserslautern, Germany from 2002 to 2008. In 2008, he received the diploma in physics at Kaiserslautern University of Technology. Since 2010, he has been a teacher of mathematics and physics at a German high school.


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Henning Fouckhardt studied physics at the University of Goettingen, Germany, and for one year he studied computer sciences at the University of California San Diego (UCSD). He received his MS and PhD in physics, from the University of Goettingen in 1984 and 1987, respectively. In 1985, he joined the Institute for High Frequency Technology in the Electrical Engineering Department of the Technical University of Braunschweig, Germany, as a research and teaching assistant, where he was mainly concerned with optical time division switches (modulators) in semiconductors. From July 1988 to July 1989, he was a postdoctoral fellow with Bellcore, Red Bank, New Jersey, where he investigated nonlinear optical space division switches in glasses. In 1989, he joined Hewlett-Packard’s Analytical Division in Waldbronn, Germany, as a member of the technical staff in research and development and was concerned with optical detectors for capillary electrophoresis systems. In 1991, he became associate professor with tenure at the Institute for High Frequency Technology in the Electrical Engineering Department of the Technical University of Braunschweig, Germany, heading the “Optoelectronics” research group. In 1996, he became a full professor with tenure in the Physics Department of Kaiserslautern University of Technology, Germany, heading the “Integrated Optoelectronics and Microoptics” section. His current research interests focus on integrated optical systems, such as transverse mode selectors for broad-area lasers; microfluidic devices for active optics; and antimonide-based semiconductor lasers, partially with GaAsSb quantum dots. He is a member of the Optical Society of America.


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