MOEMS – Micro-Opto-Electro-Mechanical-System EMT496

1. MOEMS – Micro-Opto-Electro-Mechanical-System EMT496

2. Micro-Optics • Micro-optics include a family of optical components such as lenses, mirrors, prism etc. • These elements are fabricated by modern micromachining such as optical lithography, e-beam writing or RIE. • One of the major strengths of micro-optics is that it allows integration of large, complex optical system into much more compact form. • Additionally, it also allows to lower the cost of mass production of micro-optical elements. • Examples of micro optics are to be found in nature ranging from simple structures to gather light for photosynthesis in leaves to compound eyes in insects.

3. On wafers up to 8 in. in diameter, intricate diffractive optical elements are possible, including this Fresnel design for shaping the beam of an excimer laser. Adapted from: V. K. Parashar et. al., Microelectronic Eng., vol. 67–68, pg. 710, (2003). Diffractive optical elements in glass. (l-r) (a) Channel gratings; (b) holographic gratings of 1 μm (350 nm/650 nm) periodicity; (c) crossed grating structure; (d) Fresnel lens with micro features; (e) Fresnel lens with nano features; (f) central part of a Fresnel lens.

4. Fresnel Lens • A type of compact lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses in 1818. • The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. • A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. • A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances. • Uses ?

5. Applications: • Fresnel lenses are used in left-hand-drive European lorries entering the UK and Republic of Ireland (and vice versa, right-hand-drive Irish and British trucks entering mainland Europe) to overcome the blind spots caused by the driver operating the lorry while sitting on the wrong side of the cab relative to the side of the road the car is on. They attach to the passenger-side window. • Aircraft carriers and naval air stations typically use Fresnel lenses in their optical landing systems. The "meatball" light aids the pilot in maintaining proper glide slope for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot's point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low. • Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, in solar forges, and in solar collectors used to heat water for domestic use

6. History • History of micro-optics is the history of the microfabrication of fine structures. • At first, the elements were simple grating structures for spectroscopy. • Then, the elements became more complex. • Fresnel zone, computer-generated holograms, diffractive optics and resonant filters are such elements. • The dispersion of light into spectral colors by a periodic structure was first reported by David Rittenhouse in 1786

7. In 1882, Henry Augustus Rowland built a ruling engine on which he produced both plane and concave grating for spectrometer applications. • The Fresnal zone plate, a diffractive element with focusing power, was hand-drawn as an amplitude mask by Lord Rayleigh (1871) and later created a phase structure by Wood (1898).

8. Rayleigh • is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. After the Rayleigh scattering the state of material remains unchanged. • The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases. • A portion of the beam of light coming from the sun scatters off molecules of gas and other small particles in the atmosphere. Here, Rayleigh scattering primarily occurs through sunlight's interaction with randomly located air molecules. • From a purely macroscopic point of view, the scattering comes from the microscopic density fluctuations which result from the random distribution of molecules in the air. • A region of higher or lower density has a slightly different refractive index from the surrounding medium, and therefore it acts like a short-lived scattering particle. It is this scattered light that gives the surrounding sky its brightness and its color.

9. Rayleigh

10. Holography • Holography was invented in 1947 by Gabor and revived in the early 1960 through the work of Leith and Upatnieks. • The introduction of computer-generated holograms (CGHs) was the direct results for the evolution of holograms. • CGHs produces graphical output from digital computer The coding of complex wavefronts to make CGHs was first demonstrated by Brown and Lohmann in 1966.

11. Holography • Holography is a technique which enables three-dimensional images to be made. It involves the use of a laser, interference, diffraction, light intensity recording and suitable illumination of the recording. • The image changes as the position and orientation of the viewing system in exactly the same way as if the object were still present, thus making the image appear three-dimensional. • Holography is a technique that enables a light field, which is generally the product of a light source scattered off objects, to be recorded and later reconstructed when the original light field is no longer present, due to the absence of the original objects. • Somewhat similar to sound recording, where sound field created by vibrating elements, is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.

12. Over 300 years later, Robert Hooke melted a glass rods to form small lenses. • Three lenses were used as microscope objective to study insect. • The term micro-optics was proposed by Uchida and Kitano (1960s). • They formed practical optical components based on gradient index fibers and lenses. • Technique for the monolothic fabrication of refractive microlens arrays was reported by Popovic (1988) and Daly (1990). • They applied lithographic patterning and thermal reflow in pr. • To improve robustness of the lenses, it was transferred into substrate by RIE.

13. Refractive and Diffractive • The direction of light propagation can be changed at the boundary of two media having different densities. This property is called refraction, and is illustrated in the following figure for the boundary between air and water. • The apparent and actual positions of the fish differ because the direction of light propagation has been changed as light passes from the more dense water into the less dense air.

14. Refractive and Diffractive • Because light is a wave, it has the capability to "bend around corners“. • Diffractive effects occur generally when a part of a light wave is cut off by an obstruction (slit), causing a diffraction pattern to form . • In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. • Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves. • Richard Feynman wrote: No-one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them.

15. Refractive and Diffractive • Basic different between refractive and diffractive structure shows in Fig. 1. • For prism, the light is deflected by varying the optical path length d(x) ∆n, where: • d(x) is the thickness profile • ∆nis the refractive index difference. • In air, ∆n = n – 1. An incident plane wave is deflected through the angle θ. • For thin prism, θ is: θ ≈ ∆n α where α is the prism angle as shown in Fig. 1.

16. The diffractive blazed grating profile can be obtained by wrapping the depth profile d(x) to an interval between 0 and λ / ∆n (Fig. 1(b)). • The depth difference λ / ∆n corresponds to a phase change of 2π. • The blazed grating has period ᴧ and each period has the same slope as the original prism. • The binary optics grating is then obtained by replacing the prism shape of each period by a binary optics profile with depths 0 and λ / (2 ∆n) as shown in Fig. 1(c). • The sine of the deflection angle θ varies linearly with the wavelength λ and inversely with the grating period ᴧ:

17. sin θ = m (λ / ᴧ) where m is the diffraction order. • In ideal case, the blazed grating generates only one diffraction order (m = 1), whereas the binary grating generates several orders. • Binary optics grating is used for applications where several diffraction orders are desired such as fan-out elements or far-field beam shaping.

18. Figure 1: Optical elements: (a) prism; (b) blazed grating; (c) binary optics grating

19. Photonic Crystal • What is photonic crystal? • Photonic crystals are periodic optical nanostructures that affect the motion of photons in much the same way that ionic lattices affect electrons in solids. • Photonic crystals occur in nature in the form of structural coloration. • Photonic crystals can be fabricated for one, two, or three dimensions. • 1D photonic crystals can be made of layers deposited or stuck together; 2D ones can be made by drilling holes in a suitable substrate. 3D ones by, for example, stacking spheres in a matrix and dissolving the spheres

20. 1D Photonic Crystal • In a 1D photonic crystal, layers of different dielectric constant may be deposited or adhered together to form a band gap in a single direction. • E.g.: A Bragg grating. • Can be either isotropic or anisotropic • Uses as an optical switch. • Recently, a graphene-based Bragg grating has been fabricated and demonstrated its capability for excitation of surface electromagnetic waves in the periodic structure by using 633 nm He-Ne laser as the light source. • A novel type of 1D graphene-dielectric photonic crystal has also been proposed. This structure can act as a far-IR filter and also is capable of supporting low-loss surface plasmons for waveguide and sensing applications.

21. 2D Photonic Crystal • In 2D, holes may be drilled in a substrate that is transparent to the wavelength of radiation that the bandgap is designed to block. • Triangular and square lattices of holes have been successfully employed. • The Holey fiber or photonic crystal fiber can be made by taking cylindrical rods of glass in hexagonal lattice, and then heating and stretching them, the triangle-like airgaps between the glass rods become the holes that confine the modes.

22. Photonic Crystal • If the incident light propagates within the periodic structure and is reflected back and forth, then the element become selective, such as the Bragg grating shown in Fig. 2 (1D crystal). • That structure which are periodic in 1, 2, and 3D are known a photonic crystal. • The goal was to control spontaneous emission. • The mechanism of operation is familiar from solid-state physic. • The electron wave function is modulated by coherent scattering by the periodic potential of the crystalline lattice.

23. The presence of big electronic bandgap has consequences for the electrical and thermodynamic properties of solids. • Similarly, when periodicity is introduced in the dielectric constant of a medium, a photonic bandgap can open. • When that happens, emt waves of certain frequencies are forbidden to propagate through the structured medium. • This principle has been applied to suppress spontaneous emission.

24. If light of a certain frequency is forbidden to propagate through the medium surrounding an atom, this atom will not be able to radiate photons of the corresponding energy. • Thus, certain direct transitions between energy levels will be forbidden. • Fully 3D periodic structure is required in order to get complete photonic bandgap.

25. 2D photonic crystal was used in optical waveguides or sensor. • In 2D, the light is confined in the third dimension by total internal reflection. • Similarly, we can consider a Bragg grating within a fibre as a 1D photonic crystal. • There is strong interest in 2D crystal because they can be realized with lithography technology, enabling a large variety of shapes.

26. Figure 2: Various type of photonic crystal

27. Resonant Filter • Periodic surface relief structure can also be combined with waveguide structures as in the case of resonant grating filters. • The basic concept shown in Fig. 3. • Resonant grating filters use guided-mode resonance effects in waveguide grating, yielding sharp intensity variations of the observable propagating waves. • This resonance results from an evanescent diffracted waves that is parametrically near to a corresponding leaky mode of the waveguide.

28. Resonant grating filters offer the advantage of obtaining a sharp linewidth response with a small number of layers, compared to standard thin film technique. Figure 3: Resonant grating filter with a grating on top of a waveguide.

29. Binary Optics • What is binary optics? • Binary optics is a surface-relief optics technology based on VLSI fabrication techniques (primarily photolithography and etching). • The ‘‘binary’’ in the name referring to the binary coding scheme used in creating the photolithographic masks . • The technology allows the creation of new , unconventional optical elements and provides greater design freedom and new materials choices for conventional elements. • Binary optic fabrication is a well-known technique for fabricating multiple level diffractive structures.

30. Binary Optics:Fabrication • The basic structure to fabricate binary optics is to pattern a photoresist by lithography methods (E-beam, laser-beam writing, optical lithography). • Then the structure is transfer into substrate by RIE or replicate it at low cost in plastic or epoxy. • Several techniques were existed. • The oldest and simplest is contact printing, where the photomask is brought into contact with the pr on the wafer for exposure. • Slightly more complicated technique is called proximity printing.

31. The idea is to keep the photomask in close proximity to the wafer without actual contact. • This can avoid contamination and damage. • But diffraction effects reduce the maximum resolution of the process. • Modern technology system are based on use of a projection system to image the mask on the wafer. • Wafer stepper is a typical lithography system used in microchip fabrication. • The basic idea is to image only small region of the mask onto the wafer at a time, and step over the surface to pattern the entire wafer.

32. Structure of blazed-binary optics diffractive elements Schematic illustration of the fabrication of diffractive optical elements

33. A stepper is a device used in lithography step that is similar in operation to a slide projector or a photographic enlarger. • It can creates millions of microscopic circuit elements on the surface of tiny chips of silicon.

34. Multilevel Optics • Multilevel structures are an extension of the binary optics technology. • Using several masks, multilevel profiles can be fabricated in order to approach continuous surface-relief profiles. • The basic fabrication steps were shown in Figure 4. • The first mask creates a two-step profile and each succeeding mask doubles the number of phase levels. • The mask with smallest feature is usually replicated first. • The major difficulty is the relative alignment of the different masks and the minimum feature size, which limits the maximum number of steps.

35. Fabrication step for a multilevel phase profile

36. Technology for surface profile • Because of vast development of lithography techniques, the fabrication of binary optics and multilevel surface profiles are become simple and available for optical applications. • Two of most characteristic requirements for optical elements are the smoothness and the profile accuracy of their surfaces. • These can be fulfilled for specific surface profiles by exploiting the physical effect of surface tension. • The physic behind it is the force between atoms and molecules. • If the surface tension shapes a surface (e.g., in the case of fluid material), the resulting profile will be a very smooth, minimal surface formed under the influence of the boundary conditions (wetting angle).

37. Lens melting and reflow • The easiest way to form spherical microlens is resist melting technique. • Firstly, a cylindrical resist structure has to be realized on the basis of normal photolithography. • Followed by a well-controlled baking step to melt the resist. • During this melting, the resist forms a minimal surface based on the static boundary conditions, which are resist footprint, wetting angle, resist volume. • This technique is very popular for microlens fabrication because spherical and cylindrical profiles are the minimal surfaces above circular and rectangular footprints. • For spherical lens fabrication the design of the resist thickness is simple.

38. Example of lens melting technique. (a) Microlens fabrication step. (b) SEM picture of fabricated resist lenses.

39. Another technique is the reflow technique. • Instead of thermal melting, a reflow with a dissolution atmosphere (e.g., acetone) is used to form the lens. • Because of the very low wetting angle, the resist cylinder may flow away. • The flow can be stopped, for instance, by the use of a pedestal, which limits the final lens diameter. • Other methods are also suitable for stopping the flow of the resist. • One such method is a plasma surface treatment after the fabrication of the resist cylinder. • This procedure stops the wetting of the substrate during the reflow.

40. Schematic illustration of the reflow technique.

41. Example of melting technique for lens fabrication: • Resist: AZ 4562 • Spin coating: 300rpm – resist thickness 7um. • Prebake: 80°C, 1hr. • Photolithography: resist cylinder • Melting: Hotplate, 150°C for 5 min. • Unfortunately, pr is not suitable material for most applications in optics. • Only suitable for further replication or transfer process.

42. Glass-melting • Apart from resist melting and reflow technique, a glass-melting technology was sued for microlens fabrication. • Basic idea, use of glass layer with lower melting temp. than the substrate. • E.g., BPSG made by flame hydrolysis deposition (FHD) on a quartz substrate for instance. • BPSG, is a type of silicate glass that includes additives of both boron and phosphorus. • FHD is related to CVD but it happens at atmospheric pressure.

43. Con’t • The reaction of SiCl4, BCl3 and PCl3 with hot hydrogen-oxygen flame produces small doped SiO2 cluster of about 50 nm size. • They condense on the top of a thermally stabilized substrate, either Si wafer or silica, forming a loosely bound layer. • For micro-optics use, a subsequent sinter at temp. between the melting point of the doped cluster and the substrate material has to perform. • The goal of entire process is a stress-adapted layer of boron and phosphorus-doped quartz glass with known thickness, optical parameters and melting properties.

44. FHD for BPSG layer. Schematic of glass-melting technique for lens fabrication (above). SEM images of cylinders and lenses made by BPSG on quartz substrate (below).

45. Ink-jet lens printing • A serial working fabrication technique with highly flexible is ink-jet printing. • This method is fully automated, data driven process that allows one to write micro-optics directly onto optical substrate and components. • Optical substrate and components e.g., diode laser, optical fibres and waveguides. • Two classes of ink-jet printing: continuous and drop-on-demand. • For drop-on-demand technique, use piezoelectric force.

46. Con’t • Drop-on-demand: • A piezoelectric nozzle is connected to a reservoir with a fluid polymer as ink. • The reservoir and the print head can be heated to decrease the ink viscosity. • The piezotransducer is connected to a computer-controlled high-frequency generator. • Triggered by the signal, the nozzle is squezzed and generates a drop. • Typical diamter of the droplets is 20-60 um. • Material that are well-suited for the ink-jet lens fabrication are UV-curing, 100% solid solutions of optical polymers. • This method produce microlens with diameter between 20 um and 5mm.

47. Analog Lithography • Technologies that use surface tension are limited to minimal surfaces. • It can only fabricate spherical and cylindrical lenses. • Analog lithography was used to overcome this advantage. • The developed resist depth is control by dose of radiation. • Use e-beam and lase beam writing. Basic idea of analog lithography by controlling the develop resist depth.

48. E-beam writing • The basic components are: • (1) The electron optical column, which contains the electron gun, the electron optics, the deflection system and signal detection system (for backscattered electrons. • (2) the xy stage with substrate carrier. The xy stage, which is equipped with laser interferometers and high-precision controlled motor drivers, is one of the elements responsible for the lateral accuracy. • The whole electron lithographic exposure system is in vacuum.

49. E-beam writer. Laser-beam writer.

50. Laser-beam • This writer is equipped with the components for HeCd laser resist exposure (λ = 445 nm), a beam shutter, projection optics and xy table controlled by laser interferometer and computer controlling the whole writing procedure. • Because of direct laser linear relationship between the dose profile and developed resist profile, the final relief quality and its deviation from ideal relief are strongly determine by the precision of xy table motion.