Nanophotonic Waveguides and Photonic Crystals in Silicon-on-Insulator

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1 Nanophotonic Waveguides and Photonic Crystals in Silicon-on-Insulator Wim Bogaerts 19 April 2004 Photonics Research Group

2 nano = small photon = elementary on a scale of particle of light 1nm = 1 billionth of a meter guiding of light waves along a given path Nanophotonic Waveguides and Photonic Crystals in Silicon-on-Insulator A material consisting of a thin layer of Silicon on top of a layer of glass (isolator) Silicon Photonics Insulator (glass) Research Group Substrate (Silicion) A periodic stacking of materials

3 3 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? Foreground Nanophotonic waveguides What are the difficulties? Can we make it? What comes out?

4 4 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? = What will we use it for? Foreground Nanophotonic waveguides Answer: Telecommunication What are the difficulties? Can we make it? What comes out?

5 Telecommunication Bring information from A to B Long ago: on foot, by horse, ship,... Slow Much capacity 19th, 20th century: elektricity (telegraph, telephone,...) Fast Insufficient capacity for today s needs Now: Optical fibers (using light) Fast Large capacity Long distance intec

6 7 Telecommunication networks Cubicle on the curbside submarine cable

7 8 Telecommunication networks

8 9 Telecommunication networks

9 10 Telecommunication networks Fibre to the curbside

10 Telecommunication networks Fibre to the home intec 2004 Required: optical fibres components in between the fibres electro-optic conversion LARGE QUANTITIES AND CHEAP 11

11 Components between optical fibres must Amplify light signals Now: large cupboard Distribute light signals Restore light signals Must be smaller and cheaper intec

12 Integrated Circuits = bringing various functions together on a chip univac 1962 Nortel OPtera DT 2002 Elektronics: transistors metal wires for electrical connections between components Fotonics: switching functions intel intec 2004 waveguides to transport light between components Photonic Integrated Circuit 13

13 Photonic Integrated Circuit Waveguides on a chip Problem: taday s waveguides are too weak large bends (otherwise light misses the bend ) Few functions on a chip Large chip area Expensive components Inefficient Fabrication intec 2004 TU Delft,

14 15 Integration of multiple functions intel pentium More functions on a single circuit: Reduce size of individual functional elements Connections between elements (waveguides) must be smaller narrow waveguides sharp bends NANOphotonic Integrated Circuit TU Delft 1999

15 16 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? What is light? Foreground How can we guide light? Nanophotonic waveguides What is a good waveguide? What are the difficulties? Can we make it? What comes out?

16 17 Light = Electromagnetic Radiation Ray of light Electromagnetic wave Propagates at speed of light c Electrical oscillation E Magnetic oscillation H Oscillation frequency f f λ= c with a wavelength λ E H c wavelength λ

17 18 Electromagnetic Radiation wavelength (meter) frequency (Hertz) λ cosmis radiation γ -radiation röntgen (X-rays) ultraviolet infrared Visible light: nm wavelength (nm) violet blue green yellow orange red Radar TV - radio Light for optical fibre communication

18 19 Propagation of light In vacuum: light propagates at the speed of light c wavelength low refractive index In material: light propagates n time slower refractive index n wavelength becomes n times shorter for the same frequency wavelength high refractive index

19 20 Light at an interface Change in refractive index n low refractive index Light rays change direction Light is partially reflected Effect is more pronounced with a stronger contrast in refractive index high refractive index

20 21 Total internal reflection Van inside to outside : Very oblique rays are totally reflected low refractive index = Total internal reflection The critical angle with the surface is larger for a stronger contrast in refractive index (less oblique rays are also reflected) high refractive index

21 22 Layered (Slab) Waveguide Sandwich of material with a high refractive index between material with a low refractive index Light is guided by total internal reflection in a core of high refractive index surrounded by a cladding of low refractive index low refractive index CLADDING CORE CLADDING high refractive index low refractive index

22 Bends in waveguides Some rays can escape from the waveguide Better confinement if the contrast in refractive index is adequately large Less loss if the bends are made sufficiently wide Sharp bends possible with large refractive index contrast intec

23 24 Mode of a waveguide Thin core: Rays are an inaccurate model Light is located in a smeared-out blob in and around the waveguide core = a mode low refractive index a mode propagates as a single entity Guided modes: remain localised around the core high refractive index low refractive index

24 25 Waveguides Refractive index contrast in more directions: confine light in a core Axis of propagation Core Cladding Carrier substrate Carrier substrate

25 26 Guided modes in a waveguide Some waveguides can support multiple guided modes Mode 0 (ground mode) is the most useful best confinement: Smallest cross section most elegant distribution (no zeroes) We d like a waveguide that only supports a ground mode (= single-mode waveguide)

26 27 Single-mode waveguide For telecommunication: Waveguides should guide only a single mode: Core must be sufficiently large Optical fibres (low refractive index contrast): Core diameter ~ 10µm Larger refractive index contrast smaller core Waveguides in Silicon-on-Insulator (high index contrast): Core ~ 0.2 x 0.5µm.

27 28 Reducing waveguide circuits in size Today s circuits: Large bend radius Reduce bend radius: increase refractive index contrast From 1.46-to-1.44 to 3.45-to-1 SILICON-ON-INSULATOR Keep only one guided mode: Reduce dimensions From 10µm to 0.5µm Silica buffer Silicon core PHOTONIC WIRES Si substrate

28 29 Waveguide circuits : Photonic wire Silica-on-Silicon Contrast: 1.46 to 1.44 Bend radius = 2cm Silicon-on-Insulator Contrast: 3.45 to 1 Bend radius = 5µm

29 30 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? Foreground What is it? Nanophotonic waveguides What can we use it for? What are the difficulties? Can we make it? What comes out?

30 A uniform material source detector 100% transmission intec 2004 wavelength 31

31 One interface source detector R 100% transmission 100%-R intec 2004 wavelength 32

32 A periodically layered structure source detector R 100% transmission a Photonic Band Gap (PBG) intec 2004 ~2a wavelength 33

33 Additional layers source detector R 100% transmission a intec 2004 ~2a wavelength Photonic Band Gap (PBG) 34

34 Higher index contrast source detector R 100% transmission a Photonic Band Gap (PBG) intec 2004 ~2a wavelength 35

35 Periodicity in more directions low refractive index contrast high refractive index contrast A A B transmission B transmission Full Photonic Band Gap B A B A intec 2004 wavelength wavelength 36

36 37 Periodicity in more directions Periodic structures for light = photonic crystals 1-D 2-D 3-D High refractive index contrast (larger than 2-to-1) needed for Full photonic band gap

37 2-D photonic crystals Pillars in air Only a photonic bandgap for light with the electric field parallel to the pillar axis (= TM-polarisation) holes in material Only a photonic bandgap for light with the electric field perpendicular to the pillar axis (= TE-polarisation) E TM H H TE E E TM H H TE E intec 2004 High refractive index contrast needed 38

38 39 A cage for light Perfect crystal with holes No light can exist there with a wavelength in the photonic band gap Defect: change holes locally Around the defect light can exist with wavelengths in the PBG The light cannot propagate away because of the photonic crystal e.g. in a line defect light has to follow the defect = a waveguide light cannot miss the bend

39 40 A waveguide in a 2-D crystal Infinitely extended 2-D crystal remove one row of holes = waveguide Light is confined by the crystal in the horizontal direction Light can spread out in the vertical direction How do we confine the light vertically

40 D crystal + slab waveguide Solution: a layered waveguide Light is confined vertically by total internal reflection or more correct: a guided mode

41 42 Photonic Crystal Slab Waveguide x y z waveguide defect photonic crystal holes slab waveguide Vertical confinement by a layered waveguide Horizontal confinement by photonic crystal: high index contrast required

$Transparent at telecom wavelengths (1550nm en 1300nm) High refractive index$ contrast in-plane: 3.45 (Silicon) to 1.0 (air holes) out-of-plane: 3.

42 Silicon-on-Insulator Why this material system? Transparent at telecom wavelengths (1550nm en 1300nm) High refractive index contrast in-plane: 3.45 (Silicon) to 1.0 (air holes) out-of-plane: 3.45 (Silicon) to 1.45 (silica) 500nm intec 2004 Layer structure: 220nm Si 1000nm SiO 2 1µm Silica Silicon 43

43 44 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? Foreground Nanophotonic waveguides What are the difficulties? Can we make it? Photonic What comes out? wires or crystals?

44 45 Nanophotonic Waveguides Photonic Crystals: In-plane: Guiding by the photonic band gap Vertical: Total internal reflection Photonic Wires: In-plane: Guiding by Total internal reflection Vertical: total internal reflection Both cases: Details : a few 100 nm Required precision: <10 nm NANOPHOTONIC waveguides

45 46 Early days of Nanophotonics Look Zog!. This will be the next breakthrough in telecommunications

46 48 Losses though out-of-plane scattering Photonic Crystal slab: Vertical confinement by layered waveguide But: No vertical confinement in the holes

47 49 Losses though out-of-plane scattering Hight vertical refractive index contrast: No radiation losses in straight sections Possible losses in bends, splitters,... vb. Silicon-on-Insulator High contrast

48 Bends: not that simple In s simple bend: Out-of-plane scattering Backreflection Solution: Optimise the bend geometry (heavy number crunching) intec

49 Nanophotonic Waveguides Photonic crystals: Many possibilities Hard to design Losses Photonic Wires: Simple Less loss (given good fabrication technology) Use for compact functional elements Use for waveguides (connections between elements) intec 2004 Good fabrication technology needed 53

50 The troubles of nanophotonics Nanophotonic components Hard to measure Hard to model index contrast: 3.5 to 1 fine details: 150nm - 1µm Hard to Make high resolution precision: <10nm What goes on inside the structure? Hard to Match how to get the light in and out Hard to Design many parameters fabrication tolerances intec

51 The troubles of nanophotonics Nanophotonic components Hard to measure Hard to model index contrast: 3.5 to 1 fine details: 150nm - 1µm Hard to Make high resolution precision: <10nm What goes on inside the structure? Hard to Match how to get the light in and out Hard to Design many parameters fabrication tolerances intec

52 56 Overview of the presentation Background What s the use? How does a waveguide work? What s a photonic crystal? Foreground Which techniques are there? Nanophotonic waveguides What do we use? What are the difficulties? What are the difficulties? Can we make it? What comes out? You bet!

53 57 Litho-graphy = Stone-writing So you want to make these structure really accurately?

54 58 Lithographic techniques Goal: Imprint a pattern into the Silicon Solution Imprint the pattern in a sensitive polymer (resist) = lithography Transfer the pattern into the Silicon = etching Remove the resist Resist Silicon? Glas Si-substrate

55 59 Lithographic Techniques Optical lithography: Pattern of a mask is projected into the resist Electron beam Pattern is written directly into the resist light source elektron gun mask with pattern SLOW guidance of the electron beam lens photosensitive resist electronsensitive resist

56 Optical Lithography Size of smallest patternis determined by the wavelength of the projection light source Shorter wavelength narrower lines, smaller holes light source Deep UV (excimer lasers) 193nm 248nm 200 Near UV 365nm 300 mask with pattern lens Visible light: nm violet blue green yellow orange photoresist red 700 wavelength (nm) intec

57 61 Fabrication in Silicon-on-Insulator Facilities of IMEC (Leuven) Research Center for Microelectronics Use of advanced technologies for the fabrication of CMOS chips : Deep-UV lithography at 248nm and 193nm. Electronic Chips = Based on Silicon COMPATIBLE PROCESSES We use: Silicon-on-Insulator

58 62 Step 1: A bare SOI wafer Layer Structure 220nm Silicon 1000nm Silica buffer 220nm 1000nm Silicon Silica Silicon substrate

59 63 Step 2: Apply Photoresist Photoresist: applied by spinning Shipley UV3 650nm thick layer photosensitive resist

60 64 Step 3: Baking of the photoresist photosensitive resist

61 65 Step 4: Antireflective coating AR coating to avoid reflections at the air-photoresist interface AR-coating

62 66 Step 5: Illumation Deep UV Lithography Wavelegth = 248nm NA = 0.63 Dose = mj/cm 2 Reduction factor = 4X

63 67 Step 6: Post-exposure bake

64 68 Step 7: Development of the resist Unexposed areas become solid Exposed areas are dissolved

65 69 Step 8: Resist hardening The photoresist is exposed to a plasma which partially etched the photoresist

66 70 Step 9: Silicon etch A plasma etched the Silicon where it is not protected by the layer of photoresist

67 71 Step 10: Strip the resist The residue of the photoresist is removed

68 72 A fistful of photonic crystals 8 SOI wafer: Structures are repeated many times µm cm mm

69 73 Problem: Proximity effects Problem: Holes near edges differ from holes in the bulk (while they should be identical!) photoresist pattern hole in the bulk = 420nm Hole on the edge = 380nm 1um Hole on the corner = 350nm

70 74 Solution: Proximity corrections The patterns on the mask are altered in such a way that they are imaged correctly in the photoresist. Corrections should be known in advance Calculate (difficult) Measure empirically Desired patroon Resulting Photoresist Corrected pattern

71 76 What can we make? Spot-size Converters Photonic Crystals Gratings Silicon Art Photonic Wires

72 77 Overview of this presentation Background What s the use? How does a waveguide work? What s a photonic crystal? How do we measure? Foreground How good are our waveguides? Nanophotonic waveguides What are the difficulties? Can we make it? What comes out?

73 78 Measurements So you re convinced that there should be some light coming through?

74 79 End-fire measurements Optical fibre with lens-shaped facet The photonic circuit Translation stage Objective lens to capture the light

lens for outcoupling monitor optical vezel IR camera suspended tafel rail along the

75 Measurement Setup tunable laser Interface to PC microscope measurement and control software polarisation control lesed fibre power meter aperture polariser objective lens for outcoupling monitor optical vezel IR camera suspended tafel rail along the optical axis x y z intec 2004 translation stages 80

76 81 Measuring waveguide losses Measured optical power [Watt] wavelength [nm] Measure the transmission Measure transmission as a function of wavelength Measure transmission for various waveguide lengths Transmission drops for longer waveguides Lengte L

77 82 Measuring waveguide losses measured optical power [Watt] measured optical power [Watt] wavelength [nm] Length L [mm] Length L

78 83 Waveguide losses Express power in db with respect to input power -10dB = 10x drop in power -20dB = 100x drop in power Waveguide losses in db/mm: Measured values are on a straight line Slope of the line: waveguide loss in db/mm measured optical power [Watt] measured optical power [db] Length L [mm] Length L [mm]

79 Photonic Crystal Waveguide Measure Transmission as a function of wavlength for various waveguide length (25, 50, 100, 200, 500, 1000µm) L 1.5% Transmission 1.0% 0.5% L=25µm L=250µm 0.0% intec wavelength [nm] L=1mm 84

80 -5 Photonic Crystal Waveguide Behaviour is strongly wavelength dependent For some wavelength ranges there is a fully guided mode For some wavelength ranges the mode is not fully guided For some wavelength ranges there no guided mode at all Waveguide loss [db/mm] TE-polarisation Wavelength[nm] intec 2004 Guided mode in the photonic band gap 7.5 db/mm Period: 500nm Holes: 320nm Silicon-etch measurement for 85

81 86 Photonic Crystal Waveguides Our best result: 7.5 db/mm Competition: 2.4 db/mm McNab et al., LEOS Topicals, Vancouver 2003

82 Losses in photonic wires Photonic wires: Loss less wavelength dependent Lower loss than photonic crystals width Silicon 220nm silica 1µm Si substrate 0-1 width = 450nm 0.74 db/mm Transmission [db] slope wavguide loss Length of the photonic wire [mm] intec

83 Photonic Wires width Silicon width 220nm width silica silica 1µm silica Si substrate Si substrate Si substrate Loss [db/mm] Width 400nm 440nm 450nm 500nm Waveguide loss 3.4 db/mm 0.96 db/mm 0.74 db/mm 0.24 db/mm intec Width [nm] 88

Waveguide Losses Our best results: 7.5 db/mm The competition: 2.4 db/mm McNab et al., LEOS Topicals, Vancouver 2003 0.

84 Waveguide Losses Our best results: 7.5 db/mm The competition: 2.4 db/mm McNab et al., LEOS Topicals, Vancouver db/mm 0.35 db/mm Sorry, no picture intec 2004 McNab et al. Opt. expr. 11(22) p

85 Summarised Wanted: Cheaper components for optical fibre communications Compact waveguide circuits Photonic wires: perfect for connections Photonic Crystals: more suitable for compact functional elements Fabrication process e-beam lithography: fine details, but too slow Deep-UV lithography high resolution large throughput commercially proven in CMOS industry intec

http://photonics.intec.ugent.be 91 What have we done?

86 91 What have we done? Study of losses Out-of-plane scattering Scattering at roughness Fabrication with deep-uv lithography Optimised the fabrication process Characterisation and study of components 500nm Silicon silica 220nm Measurement results Photonic Wire: 0.24dB/mm Photonic Crystal Waveguide: 7.5dB/mm

Flemish institute for the advancement of Scientific- Technological research

87 92 Many thanks to the IST-PICCO project the IAP-PHOTON network The Flemish institute for the advancement of Scientific- Technological research in the Industry (IWT) Roel and the complete photonics-group The people in IMEC-Leuven involved in the fabrication of my designs

http://photonics.intec.ugent.be 93 Any questions?

88 93 Any questions?... I must admit that photonic crystals made quite an impression

89 Some meaningless statistics ± holes etched = 0.4 Terahole = 3000 holes per second = 10 holes on each millimeter around the equator = 12 years of e-beam writing ± 30 km straight waveguides = db loss intec 2004 roughly equivalent to the combined propagation loss of all submarine cable combined 94

Section 2: Lithography. Jaeger Chapter 2 Litho Reader. The lithographic process

Section 2: Lithography. Jaeger Chapter 2 Litho Reader. The lithographic process Section 2: Lithography Jaeger Chapter 2 Litho Reader The lithographic process Photolithographic Process (a) (b) (c) (d) (e) (f) (g) Substrate covered with silicon dioxide barrier layer Positive photoresist