Engineered nanoporous and nanostructured films

ISSN:1369 7021 ? Elsevier Ltd 2009

JUNE 2009 | VOLUME 12 | NUMBER 636Engineered nanoporous and nanostructured films

Nanoporous and nanostructured films and surfaces have been exploited by nature to spectacular effect. Plant leaves use nanostructured surfaces to shed water 1, 2 as shown in Fig. 1 and the Namib desert beetle uses a similar surface to collect water from dew 3, 4. Butterflies have fashioned nanostructured surfaces to attract mates, deter predators, and provide camouflage 5, 6 (Fig. 2). Geckos 7, 8, flies, and other insects 9 use nanostructured surfaces to adhere to walls (Fig. 3), and all cell membranes can be thought of as sophisticated nanoporous films 10. The development of nanoporous or

nanostructured thin films is relatively recent and has been driven by the need for low dielectric constant materials in the semiconductor industry 11-15, the need for low refractive index materials in the photonics industry 16-20, the need for nearly blackbody absorptivity in the solar cell industry 21, the need for nanoporous membranes in the gas separations industry 22, the need for superhydrophobic or superoleophobic materials to control wetting and spreading 23, 24, and the general need for thin film catalytic and separations processes in the fuel cell 25, 26, and biotechnology 27, 28 fields. Since the range

Nanoporous and nanostructured films have become increasingly

important to the microelectronics and photonics industries. They provide a route to low dielectric constant materials that will enable future generations of powerful microprocessors. They are the only route to achieving materials with refractive indices less than 1.2, a key feature for the future development of photonic crystal devices, enhanced omni-directional reflectors, enhanced anti-reflection coatings and black-body absorbers. In addition, these films exhibit tremendous potential for separations, catalytic, biomedical and heat transfer applications. This article will review two primary techniques for manufacturing these films, evaporation induced self-assembly and oblique or glancing angle deposition, and will discuss some of the film properties critical to their use in the microelectronics and photonics industries.

Joel L. Plawsky 1, 3*, Jong Kyu Kim 2, 3, and E. Fred Schubert 2, 3

1Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY USA 121802Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY USA 121803Smart Lighting Engineering Research Center, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY USA 12180

*Email: plawsky@https://www.360docs.net/doc/5a6645934.html,

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of applications is so vast and the materials and methods used to fabricate these materials is so broad, this article will focus primarily on nanoporous and nanostructured dielectric films for potential use in the microelectronics and photonics industries.

Classification

Nanoporous and nanostructured films can be separated into two

broad classes. The first class is represented by the cellular or foam-like materials 30 where all elements of the entire structure are linked together to form a three-dimensional, continuous network. The second class of materials is the free-standing structures typified by films formed via glancing angle (GLAD) or oblique angle (OAD) deposition 31-36. In these films, each element is isolated from its neighbors leading to a brush-like or hair-like morphology. Fig. 4a -b shows a 100 nm thick xerogel film (foam) and Fig. 4c -d show OAD films 10 nm and 30 nm thick respectively. We will often refer to both types of films as being porous and having an effective porosity even though the fundamental structure of each film is different.

Fabrication methods

There have been a vast number of methods developed to produce nanoporous and nanostructured films 38-42. Either liquid-phase 37 or vapor-phase processing 39 can be used to produce foam-like or free-standing film materials. Within each phase, the structure of the

material may be random 15 or ordered 38. The former leads to amorphous materials, the latter, to more crystalline-like materials. Each film can be considered as either intrinsic or extrinsic. Intrinsic or additive structures are formed directly as the film is assembling or growing much as the skeleton of a skyscraper. Extrinsic or subtractive structures are formed Fig. 3 (a, b) Micrographs of a gecko’s foot showing the structures and porosity responsible for its climbing ability. (c–h) Carbon nanotube array forming a gecko-like tape. (Reproduced with permission of 29). Though some of the small-scale features of the gecko’s foot are reproduced by the nanotube array, the exquisite hierarchical nature of the gecko foot structure cannot yet be reproduced artificially.

(a)

(e)(b)

(f)(d)

(h)

(c)

(g)Fig. 1 Scanning electron micrographs of a rose petal (a, b) and a lotus leaf (c) showing the structures responsible for their hydrophobic properties.

(Composite image reproduced with permission from 2 (a,b) and photo courtesy William Thielicke)

(a)(b)(c)(a)(b)

(c)

(d)

Fig. 2 (a) Optical micrograph of Morpho Menealus. (b) – (d) Scanning

electron micrographs at increasing resolution showing the nanostructures and nanoporosity that give rise to the butterfly’s optical properties and mechanical stability. The width of a long spine in (d) is about 100 nm (Composite image reproduced with permission from 5).

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38as a solid mass containing sacrificial material that is either burned out, dissolved out, or milled out to yield a final structure. Finally, the film material can be either inorganic 15, organic 43, or a hybrid of both 41. Cleary it will be impossible to discuss all these methods in detail in this paper and so we will focus on only two versatile examples.

Evaporation induced self-assembly

One of the first processes to produce nanoporous materials is based on evaporation induced self assembly (EISA) developed by Brinker and coworkers 44. This process can produce either random or ordered (templated) structures depending upon the chemistry used. In general, the process uses sol-gel chemistry, a form of condensation polymerization that uses metal alkoxides as the monomer. The general scheme is shown in Fig. 5.

Films can be formed either by spin or dip coating and the final film thickness is controlled by a complicated interplay between the polymerization kinetics, the evaporation rate of the solvents used, and the fluid mechanics of the coating process. Though the majority of materials are formed using silicon alkoxide chemistry due to its relatively slow reaction rate, virtually any metal alkoxide, even those containing organic substitutents can be used and so hybrid materials may be prepared. Excellent reviews of the technology have been written by Volksen et al.46 and Dubois et al.47 who pioneered a number of polymeric, inorganic, and hybrid spin-on films.

Randomly oriented material structures can be produced using only the metal alkoxide and a single or more often, mixed solvent approach. The key to the mixed solvent approach is to use solvents of widely different volatility so that film thickness is controlled by the evaporation of the more volatile solvent while film porosity is controlled by the amount of the less volatile solvent. Ordered or templated materials are formed using a templating agent as shown in Fig. 5. The templating agent is a

surfactant that can be anionic, cationic, or nonionic. Block copolymer templating agents such as the Pluronic TM family of surfactants are one family of nonionic surfactants. Living cells can be incorporated into the templated structure if biological surfactants, lipids, are used 28. The advantages of the templating route are that it is more versatile, the pore size and pore size distribution can be easily controlled, pore morphology can be specified by constraining the templated phase that forms, and the materials are often more mechanically robust than porous films that are based on a random pore network. The drawbacks are that the chemistry is more complicated, the mechanics of controlling the film porosity is more complicated, and the thermodynamics of self-assembly, the kinetics of polymerization, and the transport processes underlying the formation of the film are more tightly coupled.

Vapor deposited, nanoporous films

Spin-on or dip-coated films were some of the first nanoporous materials to be investigated. However, they required water to fuel the condensation polymerization reaction. This water affected the dielectric constant of the material and was difficult to remove. Thus, the semiconductor industry sought alternatives in the form of dry or vapor deposited systems. Vapor-phase systems were preferred because they could be easily integrated into existing process lines, required no water, and yielded hybrid materials with enhanced mechanical properties. The generic family of such materials, termed SiCOH, are formed using plasma enhanced chemical vapor deposition. The process uses organo-siloxane precursors, most commonly cyclic tetrasiloxanes, and process conditions are adjusted so that the material is deposited without wholesale disruption of the ring-based structure. An excellent review of the technology is given by Grill 48. Much of the more recent work has been devoted to developing methods for producing nanoporous materials. Porous versions of the SiCOH family are produced via a

Fig. 5 Process flow for evaporation induced self-assembly (Reproduced with permission from 45).

Fig. 4 (a) Cross section of a 100 nm thick xerogel film 37. (b) Top view of xerogel film. (c) 10 nm high SiO 2 GLAD film. (d) 30 nm high SiO 2 GLAD film.

(a)(c)(b)

(d)

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sacrificial technique where a porogen is deposited along with the cyclic tetrasiloxane precursor 49, 50. The porogen is thermally unstable and removed via an annealing step at temperatures on the order of 400 °C. Work continues on chemistries that will allow for lower temperature deposition and annealing steps.

Vapor deposited, nanostructured films

The other process to produce nanoporous films is based on oblique-angle (OAD) or glancing angle deposition (GLAD). The technique was pioneered in the 1950’s 51 and has been used to grow porous, sculptured thin films 52, 53. In OAD, the substrate is tilted at a steep deposition angle, defined as the angle between the normal to the substrate surface and incident vapor flux. A random growth fluctuation on the substrate produces small islands of deposited material that form shadowed regions 31, 54, 55 behind which the incident vapor flux cannot reach (see Fig. 6a ). Subsequent vapor deposition occurs preferentially on the non-shadowed regions, creating an oriented, rod-like structure with high porosity, as shown in Fig. 6b . As the vapor incident angle approaches 90° the shadowed area increases and a more porous film is produced. Fig. 6c -e show cross-sectional scanning-electron micrographs of nanostructured SiO 2, ITO, and TiO 2 films respectively. The OAD results in tilted, nanorod structures whose angle relative to the normal to the sample surface is 56:θt = θ ? sin ?1

[

1 ? cos θ

]

(1)2

There are several advantages to the OAD method. Since the film is deposited by evaporation (electron-beam or thermal) or sputtering, the film thickness can be precisely controlled by adjusting the power

and deposition time. The thickness can also be easily measured in-situ via a quartz crystal microbalance. By comparison, the thickness of spin-on films depends upon the spin speed, the spin time and the fluid rheology, all of which interact with one another in a nonlinear way. While film thickness can be measured in-situ, it is a much more complicated task. The advantages of OAD are a very important feature for optical components whose layer thicknesses usually must be less than the wavelength of light. Finally, the refractive index of OAD films can be tuned to any desired value between its bulk, dense value and a value close to the index of air, just by controlling the deposition angle 57. Spin-on films require an adjustment in their composition to affect the same control.

The refractive index of nanoporous or nanostructured films can be related to the density or volume fraction of the constituent materials using effective medium theories 11, 15. For either spin-on, dip-coat, or OAD materials, the pores or the gaps between the nanorods are much smaller than the wavelength of visible light. Mie and Rayleigh scattering can therefore be neglected and the layer can be treated as a quasi-homogeneous film with a uniform refractive index 58.

Mechanical properties

The mechanical properties of nanostructured films depend upon the film composition, structure and processing history. An introduction to the mechanical properties of low-κ materials, especially nanoporous,

low-κ dielectrics, is given in the text Interlayer Dielectrics for Semiconductor Technologies 11.

The elastic modulus is widely used to assess the mechanical strength of nanostructured materials though for discrete element films, the spring constant is more important since it defines how much the films

Fig. 6 (a) and (b): Illustration of deposition kinetics of oblique-angle deposition including the shadowed regions that result in the growth of an array of directed nano-rods. Cross-sectional scanning electron micrographs of (c) SiO 2, (d) ITO, and (e) TiO 2 nanorods deposited by oblique angle deposition.

(a)(c)(e)

(d)(b)

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can be compressed or deflected.While properties such as fracture strength, flexural strength, and fracture toughness are important, if the elastic modulus cannot meet a minimum standard, the material cannot be used outside a laboratory setting.

Models for generic nanocomposite/ nanostructured materials

Several classes of models have been introduced to relate the mechanical properties of nanoporous/nanostructured materials to their density

or porosity. Some of these models are simple enough to be used for screening and rough predictions. Two of the most popular approaches will be discussed briefly below.

Exact/fundamental models

Micro-mechanics59models have been used to predict what happens when a single inclusion or low volume of inclusions is incorporated into a matrix.Work has been done to extrapolate and extend

these results to higher volume fractions of spherical or ellipsoidal pores60. Theoretical bounds61exist to predict the elastic properties

of composites, but since porous materials have no lower bound

to their elastic properties, these lose their predictive power as porosity increases.

Similar models have been developed for the nanostructures associated with OAD films62-64.Most of the work focuses on single structures but there are some reports that look at a forest of such materials65.In a generic slanted rod, the deflection, δ, associated with an applied force F can be related to Young’s modulus, E, the rod angle, θ, and the moment of inertia for the column, I:

δ = F(L3 cos2θ)or k = F=(3EI)(2) 3EIδL3 cos2θ

where the moment of inertia depends on the shape of the column cross section.

I rectangular =

bh3

circular

πr4

o(3) 124

If the rods are subject to mechanical vibration, their fundamental resonant frequency can be related to Young’s modulus, the column’s density, cross-sectional shape, and length, L.

f square = (1.875)2?b2 E f circular = (1.875)2?r2o E(4) 48π2 ρL464π2 ρL4

Cellular models

Cellular models of nanoporous materials view the composite system as if it were a foam, built of thin walls framing the surfaces of a hollow cell as shown in Fig. 7. The whole solid is assumed to behave in a linearly-elastic way and the deflection of the structure is proportional to the applied force, F. Young’s modulus for the foam can be related directly to the relative density,

= C(ρ)m(5) E

dense

where C is constant of order 1 and m is another constant that should equal 2 for a perfect foam structure.

Modulus measurements

The modulus of a thin film is most often measured by nanoindentation66, 67 but other techniques including beam bending, surface acoustic wave, and Brillouin scattering are used. Discrepancies between the techniques are common68 and published data over a full range of porosity or density is rare, especially so for OAD films.

The modulus or spring constant of OAD films is more difficult to assess due to the range of materials deposited, the number of different rod morphologies used, and the fact that unlike the foam materials, there appears to be no systematic study of modulus as a function

of bulk density for any one type of material or morphology. Table 1 summarizes some of the results that can be found in the literature.

Table 1 Spring Constant Data for Some Nanostructured Films.

Fig. 7 Unit cell for an open cell foam showing what happens during

loading(Reproduced with permission from30 ?Cambridge University Press).

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Since the OAD films are composed of individual elements with a defined structure, a collection of structures can show anisotropic deformation as in the Ta 2O 5 films. The growth rate is also important since the faster the films grow, the more defects are introduced within the columns. Fig. 8 shows this explicitly for the diamond-like carbon films 63.There is much more data for Young’s modulus of nanoporous, silica-based, and hybrid films formed using sol-gel and EISA techniques. Fig. 9 shows most materials are relatively weak and that m (equation 5) varies between 2 and 4 regardless of whether the materials are deposited via a liquid-phase or vapor-phase process. Vapor-phase deposition of organosiloxanes generally leads to higher values of elastic modulus for a given dielectric constant, but not higher than the sintered, pure inorganic films. Recently a class of materials with m < 2 have been developed 70.

These materials achieve a lower value for m because the assumption that the individual struts making up the foam have elastic properties similar to bulk, dense materials was incorrect. The modulus for the struts in these materials, because they were formed by the liquid-phase, EISA technique, is larger than for bulk silica. As the film density decreases, the modulus of the struts increases, leading to the observed behavior. The modulus of sintered xerogels and templated silica materials are relatively similar 70, 71 and this supports the hypothesis that all these materials have continuous solid matrices made up of nearly pure SiO 2. Elastic modulus values for unsintered xerogel films are comparable to corresponding methyl-silsesquioxane 72-74 films. Fig. 9 shows that families of porous materials can be identified based on their composition and the defect density inherent in their processing history.

Fig. 8 Dependence of Young’s modulus on the growth rate of diamond-like carbon nanopillars (Redrawn with permission from 63 ).

Fig. 9. Elastic modulus for a number of popular low-κ silica-based materials. Solid lines are power-law fits for xerogel films made using different processing techniques (Updated with permission from 75).

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Thermal properties

Of the three modes of heat transfer, conduction, convection, and radiation, the dominant mode in most microelectronic or photonic applications is conduction. The other modes of transport are important in larger-scale packaging configurations and when trying to maximize heat rejection. The thermal conductivity, λ, of nanporous and nanostructured materials must include contributions from conduction through the solid

matrix, λ

s , conduction through the gas space, λ

, and in some cases,

radiation through the solid matrix and voids λ

76 (equation 6).

λt = λ

+ λ

(6) Most solid matrices of nanoporous and nanostructured materials

are approximated as disordered, or amorphous, dielectric solids.Heat transfer in these materials is treated in terms of the propagation of phonons that arise due to vibrations of the molecules making up the solid matrix. At temperatures above 50 K, phonon motion loses all coherence and heat transfer through disordered dielectric films is a diffusion-like process.Borrowing the form from kinetic theory, we can write the thermal diffusivity (α) of such materials as:

α(T) = k(T) ? = (1/3) νl(T)(6)ρ C

where the macroscopic density ρ, specific heat C

of the material, the transport velocity, ν, of the lattice waves (or phonons) and the phonon mean free path, l, are the dominant factors.For amorphous disordered solids like glasses, the mean free path is almost constant at room temperature77 and is limited to several interatomic spacings.

The thermal conductivity of a material is process dependent

since process history influences the number of defects in the

material and hence the scattering of phonons. The process becomes more complicated for porous or structured materials since phonon scattering can be due to defects in the matrix but also from very small pores and from the abrupt density change that occurs at a pore’s or nanostructure’s surface.Effective medium theories, such as that of Landauer78generally yield good estimates of the thermal conductivity.

Thermal conductivity measurements

The measurement of thermal conductivity for very thin films has,

until recently, been difficult to perform and the data, unreliable. Two reliable and robust techniques that have found wide usage are the 3ω technique79 and photothermal80 deflection. A schematic of a 3ω system is shown in Fig. 10. The photothermal deflection technique is based on the periodic heating of a sample by a modulated laser pump beam. The absorption of the pump laser beam causes a local-temperature rise, which in turn leads to a local surface deformation, that is detected by the probe beam and related to the thermal conductivity.

Fig. 11 displays a comparison of the thermal conductivity for

a number of nanoporous and nanostructured materials. Thermal conductivity data is most common for nanoporous materials and virtually nonexistent for OAD films.There are several problems in obtaining data for OAD films.Since they are composed of discrete structures, it is difficult to use the 3ω technique.Though adjusting the deposition angle allows for the production of a capping layer31, such

a layer is thick and so dominates the thermal response. Thus, most of the work on OAD films has been theoretical82, 83and the application

to thermal barrier coatings.Most of the work on porous films is for semiconductor dielectrics where high thermal conductivity is required. The highest values of thermal conductivity in Fig. 11 are for the sintered porous xerogel film12. The sintering process reduces matrix and pore induced phonon scattering by healing the microstructure84, reducing residual organic content, and causing the pore size distribution and pore shape distribution to become narrower85. Polymeric materials have inherently lower thermal conductivity than inorganic materials due to the flexibility of their chains and the corresponding low speed

of sound in the material. Crystalline polymeric materials have higher conductivities than amorphous materials.Making either polymers

or inorganic solids porous, reduces their thermal conductivity due

to increased phonon scattering. The hybrid organo-silicates (or silsesquioxanes) have conductivities similar to xerogel and polymeric Fig. 10 Schematic diagram of the 3ω technique. Current is applied at a frequency, ω, and the voltage signal is read at a frequency 3ω.

Fig. 11 A comparison of the thermal conductivity for several nanoporous/ nanostructured materials. (Updated with permission from81)

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materials. Additional phonon scattering occurs in hybrid materials due to the abrupt density changes. Finally, one example of a GLAD film is shown. This is an yttria-stabilized zirconia film used as a barrier coating. It has an overall porosity of 56% and for a material with a high dielectric constant, exhibits a low thermal conductivity.

Optical properties

Modern integrated optical structures require access to materials covering a wide range of refractive indices and dense materials with very low refractive indices (n < 1.39) do not exist. MgF 2, CaF 2, and SiO 2 are dense materials with refractive indices among the lowest available but their refractive indices, n MgF 2

= 1.39, n CaF 2

= 1.44, n SiO 2

= 1.46, are much higher than that of air. An optical, thin film material with a refractive index close to that of air could enhance the performance of many photonics applications, such as broad-band anti-reflection coatings 86, 87, omni-directional reflectors 20, 88, distributed Bragg reflectors (DBRs)89, 90, optical micro-resonators 91, 92, light-emitting diodes (LEDs)93, and optical inter-connects 16. Films fabricated by evaporation-induced self-assembly or oblique-angle deposition have refractive index values which can bridge the gap between conventional solid materials and air. Using these techniques, thin SiO 2 films with refractive index values as low as

1.05 were achieved 94. Conductive films of indium-tin-oxide(ITO) have been produced with a refractive index as low as n = 1.1793. While EISA and OAD techniques can produce tunable refractive index films, OAD has the advantages of being able to tune refractive index in-situ and to change materials and deposit multilayer films without removing the substrate from the deposition chamber. Fig. 12a -b show the refractive index at 460 nm and calculated porosity of SiO 2 and ITO films, respectively, for several incident angles. The porosity is calculated from the refractive index using effective medium theory. The refractive index decreases with increasing deposition angle so that we can freely tune the refractive index of an SiO 2 film in the range 1.05 ~ 1.46. Similarly, the refractive index of an ITO thin film can be precisely tuned in the range

2.11 (dense ITO) ~ 1.17. By employing different base materials such as titania (TiO 2) and silica (SiO 2), a wide range of refractive indices, from 2.68 to 1.05, can be attained. Further improvement is possible using polymeric materials, such as poly(p-xylylene)27.

The OAD system has another advantage. By rotating the substrate, nanosprings can be formed instead of nanorods 95. This results in the fabrication of chiral films whose optical rotation can be controlled as well as its refractive index. Such control leads to the emission of

Fig. 12 Refractive indices and porosity of (a) low-n SiO 2 and (b) low-n ITO nanostructured films as a function of deposition angle.

Fig. 13 (a) Cross section scanning-electron micrograph of three-period DBR structure on a silicon substrate. (b) Optical micrograph of one-, two-, and three-period DBR samples. The two- and three-period DBRs have a bluish color, which indicates higher reflectivity in that wavelength range. (Redrawn with permission from 97)

(a)

(b)

(a)(b)

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44circularly polarized light from luminescent materials. The polarization is opposite the handedness of the film 96.

Summary and future directions

Nanoporous films will become increasingly important in future microelectronic and photonic devices. They provide the heart of low-k dielectrics critical to the future success of the semiconductor industry yet the vast majority of potential applications have yet to be explored. The ability to alter the chemistry of the films, to incorporate or tether dye molecules, nanoparticles, viral particles, or living cells into the matrix of these materials offers up a whole host of potential sensing and biomedical applications. Several promising commercial applications are shown in Fig. 13–16.

Fig. 13a -b show images of a DBR made solely from ITO. This allowed a conductive DBR to be formed specifically for 460 nm operation 97. A DBR with three periods exhibited a reflectivity of 72.7%, in excellent agreement with theory.

Fig. 14a -b show results from the formation of a six-layer, graded-index, antireflection coating (AR) made entirely of ITO 93. GaInN LEDs

with the graded-index ITO anti-reflection contact achieved a light-extraction efficiency enhancement of 45% compared to LEDs without the AR coating and 24.3% over LEDs with a conventional ITO contact.Recently, an optimized graded-index AR coating with broadband and omni-directional characteristics was designed by a genetic algorithm 98. The optimized 3-layer graded-index AR coating for solar cell applications consisted of a dense TiO 2 and two nano-porous SiO 2 layers fabricated using OAD, as shown in Fig. 15. The normal incidence reflectance of the coating averaged between 400 and 700 nm was 3.9%, which is 37% lower than that of a conventional single-layer Si 3N 4 coating. Furthermore, the measured reflection over the 410 – 740 nm wavelength range and 40 – 80° incident angle range was reduced by 73% in comparison with the single-layer Si 3N 4 coating

ACKNOWLEDGEMENTS

The work presented here was supported by the U.S. National Science Foundation through its Smart Lighting Engineering Research Center

program and also by the Semiconductor Research Corporation through its Center for Advanced Interconnect Science and Technology.

Fig. 14 (a) Cross-sectional SEM image of graded-index ITO AR coating with modified-quintic refractive-index profile. The graded-index ITO AR coating consists of 6 ITO layers with pre-determined refractive indices for optimum AR characteristics. (b) Average light-output power of top-emitting GaInN LED chips with dense ITO coating and with graded-index ITO AR contact. (Redrawn with permission from 93)

Fig. 15 (a) Scanning electron micrograph with a tilt angle of 44° of 3-layer GRIN AR coating. (b) Comparison of the quarter-wave AR coating versus the 3-layer GRIN AR coating over angle of incidence 40 – 80°, and wavelength range 410 – 740 nm. (c) Photograph of Si substrate without an AR coating, with conventional Si 3N 3 AR coating, and 3-layer GRIN AR coating, reflecting letters from a LCD monitor. (Redrawn with permission from 98)

(a)(b)

(b)

(a)

(c)

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