Basic Optics : Microlithography Optics Part 4: Polarization

Transcription

1 Electromagnetic Radiation Polarization: Linear, Circular, Elliptical Ordinary and extraordinary rays Polarization by reflection: Brewster angle Polarization by Dichroism Double refraction (Birefringence) Wave Plates: 1/4 and 1/2 wave Microlithographic applications 1

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3 Properties of light can be subdivided as: Geometrical Optics: Rectilinear propagation: Light travels in straight lines until interaction with another medium, barrier or gravitational field Finite Speed C Reflection Refraction Dispersion Wave Optics: Electromagnetic Properties Diffraction Interference Polarization Double refraction or Birefringence 3

4 We know that light has a wave nature from the phenomena of diffraction and interference, but we know nothing about the orientation of this wave motion (vibrations). EM waves oscillations are transverse (perpendicular to the direction of motion). The electric and magnetic waves are perpendicular to the direction of motion. These vibrations are confined to the wavefront! 4

5 EM Polarization 5

6 Unpolarized Light can: Electric waves will vibrate at all angles at random. Represented as the following diagram as viewed straight on: To Simplify this unpolarized light is represented by 2 planes of vibration at right angles of equal amplitude. 6

7 Polarized Light can: Electric waves will vibrate in one plane. Represented as the following diagram as viewed straight on: Linear Polarized light is represented by 1 planes of vibration either vertical or horizontal 7

8 Polarization: Unpolarized 8

9 Polarized Light: Human eye cannot see polarized light without aids. Some insects can see polarized light, which they use to navigate. Linear and circularly polarized light 9

10 Polarization: Linear 10

11 Polarization: Circular 11

12 Polarization: Linear: If two polarizers are set up in series so that their optical axes are parallel, light passes through both. However, if the axes are set up 90 degrees apart (crossed), the polarized light from the first is extinguished by the second. As the angle rotates from 0 to 90 degrees, the amount of light that is transmitted decreases. 12

13 Polarization: Linear:Wire Grid: Transmission axis is perpendicular to axis of wires. 13

14 Polarization by Dichroism ( Selective absorption of one of the two orthogonal polarization components of the incident light) : Transmission axis is perpendicular to axis of wires. Polaroid H filters: Like Polaroid sun glasses. Similar to wire grid idea. A polyvinyl alcohol ( PVA) sheet is stretched to create long chain molecules. Iodine is absorbed by these long chains forming a grid of iodine. These iodine wires absorb light vibrating parallel to the molecule chains! 14

15 Polarization: Linear Malus law 15

16 Polarization: Linear polarized light: Malus law 16

17 Ordinary and extraordinary rays: o-ray (oordinary ray): polarized parallel to plane of incidence. Also termed p-polarization. e-ray (extraordinary ray): polarized perpendicular to plane of incidence. Also termed s-polarization. 17

18 Polarization by reflection: Brewster angle ( angle of incidence): Reflected light is linearly polarized when the angle of reflection + angle of refraction = 90 o. 18

19 Birefringence Polarized light produced by Double refraction (Birefringence): This is exhibited in certain Anisotropic crystals such as quartz, mica, sapphire, and calcite. Anisotropic :Crystals have regular repetitive arrays of atoms, but the atomic forces on the electron clouds are different in different directions. This results in optical properties that are dependent upon the direction of the rays propagation in the crystal. The refractive index is dependent on the rays plane of polarization. 19

20 Birefringence Double refraction (Birefringence): Snells law deviates here as there are two refractive indices within the same material depending upon the plane of polarization. 20

21 Birefringence Birefringence: 3 conditions: 1. Rays propagating parallel to the crystal s optic axis have a constant refractive index. Here the e-rays and o-rays have the same refractive index. 2. Rays propagating perpendicular to the crystal s optic : The e-ray will travel faster than the o-ray due to it s lower refractive index in this direction. Both rays will travel in the same direction. This results in a phase shift between the two rays! 3. Rays propagating 0 to 90 o to the crystal s optic : The o-ray will be refracted according to Snells Law. The e-ray will deviate from the o-ray due to the different refractive index in this direction. The e-ray will deviate away from the optic axis and out of the plane of incidence! 21

22 Birefringence Double refraction (Birefringence) 22

23 Birefringence Birefringence measured as: n = (n e -n o ) (n e = index of refefctaion for the e-ray (n o = index of refefctaion for the o-ray if n is positive, crystal is positive uniaxial. if n is negative, crystal is negative uniaxial. 23

24 Birefringence Birefringence can also be measured as: L = phase shift difference between e-ray and o-ray. Shift measured as nm/cm 24

25 Birefringence Phase Shift 25

26 Wave Plates: 1/4 and 1/2 wave: Birefringent crystals cut to a specific thickness to achieve a desired e-ray and o-ray phase shift! 26

27 Wave Plates: 1/2 wave (180 o retarder) : e-ray and o-ray have a half wave( 180 o ) phase shift: rotates linearly polarized light 90 o. ( thickness 2n+1 multiple of λ/2) Thickness = d= (2n+1) λ/(2*(n e -n o )) 27

28 Wave Plates: 1/4 wave (90 o retarder) : e-ray and o- ray have a quarter wave( 90 o ) phase shift: changes linearly polarized light to circular polarized light. ( thickness 2n+1 multiple of λ/4) Thickness = d= (2n+1)λ/(4*(n e -n o )) 28

29 Wave Plates: 1/4 wave (90 o retarder) : Specification example 29

30 Microlithographic applications: 1. ASML alignment system: Calcite plate to split beam 2. Quartz reticles have stress induced birefringence, which can effect the SVGL scanner polarized illumination and the ASML alignment system. Can also effect PSMs! 3. Lens materials have stress induced birefringence, which can impact illumination! 30

31 reference: SVGL Micrascan III and III+ manual SVGL MS III+ scanner: Dose Control: mj/cm2 = mw/cm2*(slit widthin mm)/(stage speed mm/sec) uses Malus law to change irradiance of illumination Uses Quartz beam splitter to polarize the light to reduce absorption ( light loss) 31

32 Projection printing: SVGL Micrascan scanner ¼ Wave plates How does the light reflect off the beam splitter once an d then on the second path it is transmitted? 32

33 Projection printing: SVGL Micrascan scanner 1. Incoming light is linearly polarized (out of page) 2. The Beam splitter reflects this orientation. 3. It passes through the quarter waveplate converted to circularly polarized light 4. Reflects off aspheric mirror (MAG). 5. This circularly polarized light passes through the quarter waveplate converted to linearly polarized light rotated 90 o (parallel to page). 6. The beam splitter allows transmission of this linearly polarized light orientation

34 157nm issues Form Phil Ware Canon Intrinsic Birefringence in CaF 2 34

35 157nm issues Form Phil Ware Canon Intrinsic Birefringence in CaF 2 Birefringence dependent on crystalline orientation 35

36 157nm issues Form Phil Ware Canon Intrinsic Birefringence in CaF 2 Complex optical design! 36

37 157nm issues Form Phil Ware Canon Intrinsic Birefringence in CaF 2 Complex optical design! 37

38 157nm issues Form Phil Ware Canon Intrinsic Birefringence in CaF 2 Need to cut and polish lens on specific crystalline axis 38