EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie Lecture 22 Optical MEMS (4) Agenda: Refractive Optical Elements Microlenses GRIN Lenses Microprisms Reference: S. Sinzinger and J. Jahns, Chapter 5 in Microoptics, Wiley-VCH, 2003 EEL6935 Advanced MEMS 2005 H. Xie 4/4/2005 1 Optical Functions and Their Implementation EEL6935 Advanced MEMS 2005 H. Xie 2
Classification of Refractive Optical Elements EEL6935 Advanced MEMS 2005 H. Xie 3 Refractive Micro-Optics Surface Profile Microlenses Melted photoresist lenses reflow lenses Mass transport Volume change Lithographically initiated volume growth Dispensed or droplet microlenses Direct writing Grey-scale lithography Gradient-index (GRIN) Optics GRIN rod lenses Planar GRIN lenses Microprisms EEL6935 Advanced MEMS 2005 H. Xie 4
1. Surface Profile Microlenses 1.1 Melted Photoresist Lenses Reflow Lenses Fig 5.1 EEL6935 Advanced MEMS 2005 H. Xie 5 1.1 Photoresist Reflow Microlenses Focal length f r c = n 1 r c : radius of curvature of the spherical lens n: refractive index of the lens material. n~1.4-1.6 for most polymers. Photoresist volumes before and after photoresist reflow: V cyl 2 D = π t 2 ( /3) 2 Vsph = π h rc h EEL6935 Advanced MEMS 2005 H. Xie 6
1.1 Photoresist Reflow Microlenses D h ( ) 2 2 D 2 c rc h + = r 2 r c -h 2 h + D 2 /4 r c rc = 2h We assume that the photoresist volume does not change during fabrication, i.e., V cyl = V sph.. Thus, the thickness is given by 2 h 4 h t = 1+ 2 3 D EEL6935 Advanced MEMS 2005 H. Xie 7 1.1 Photoresist Reflow Microlenses Figure 5.2 EEL6935 Advanced MEMS 2005 H. Xie 8
1.1 Photoresist Reflow Microlenses Figure 5.3 Surface Profile Surface tension Surface energy: photoresist-air interface; photoresist-substrate interface Gravitational energy Energy balance before and after reflow Substrate material Surface treatment Surface roughness Facing up or down Processing temperature Issue: outer and inner parts reaches to melting temperature at different times. EEL6935 Advanced MEMS 2005 H. Xie 9 Preshaping 1.1 Photoresist Reflow Microlenses Long focal length Aspheric profile Melting temperature, processing time Local heating of just the surface EEL6935 Advanced MEMS 2005 H. Xie 10
Pattern Transfer 1.1 Photoresist Reflow Microlenses Photoresist Si Photoresist Si Si Lenses made of substrate such as silicon, fused silica, GaAs, InP Anisotropic RIE etching needed Equal etching rate for photoresist and substrate EEL6935 Advanced MEMS 2005 H. Xie 11 1.2 Microlens Formed by Volume Change Photosensitive glass (e.g., Fotoform by Corning) Photocolouration: color change under intense UV illumination After UV exposure, heated to near melting temperature Regional crystallization shrinkage local swelling spherical lenses Typical lens diameters: 400-800µm Typical numerical aperture: 0.11-0.19 EEL6935 Advanced MEMS 2005 H. Xie 12
1.3 Microlens Formed by Volume Growth PMMA (polymethyl methacrylate) High energy radiation (e.g., UV laser, x-ray, electron or proton beams) breaks polymer chains reduce molecular weight reduced stability Exposed to monomer vapor, monomer molecules diffuse into PMMA. The smaller the molecular weight, the more the monomer diffusion Different swellings at different regions microlenses UV curing EEL6935 Advanced MEMS 2005 H. Xie 13 1.4 Dispensed or Droplet Microlenses EEL6935 Advanced MEMS 2005 H. Xie 14
1.5 Microlens Examples -1 AZ4620 photoresist (n=1.62) 200ºC for 20 min Diameter 300µm Focal length: 670 µm C. King, L. Lin and M. Wu, IEEE Photonics Technology Letters, 1996 EEL6935 Advanced MEMS 2005 H. Xie 15 1.5 Microlens Examples -2 Ring-shape holder UV curable polymer droplet Manually dispense droplets using micromanipulator Surface tension Biconvex lens Diameter 400µm Height: 84 µm NA: 0.39 Kwon and Lee, MEMS 2002 EEL6935 Advanced MEMS 2005 H. Xie 16
1.5 Microlens Examples -3 Scratch-drive actuators (SDAs) 2-D scanning For 1.55µm wavelength AZ4620: 11 µm thick Hotplate: 150ºC for 1min Diameter 270µm Focal length: 670 µm Toshiyoshi et al, J. Lightwave Technology, 2003 EEL6935 Advanced MEMS 2005 H. Xie 17 1.5 Microlens Examples -4 T.K. Shin et al, IEEE Photonics Technology Letters, 2004 EEL6935 Advanced MEMS 2005 H. Xie 18
1.5 Microlens Examples -5 2µm-thick transparent nitride lens holder with 20µm-deep circular well Polymer jet printing Lens diameter: 800µm Focal length: 2-7mm Choo and Muller (UC-Berkeley), Hilton Head Workshop 2004 EEL6935 Advanced MEMS 2005 H. Xie 19 1.5 Microlens Examples -5 Choo and Muller (UC-Berkeley), Hilton Head Workshop 2004 EEL6935 Advanced MEMS 2005 H. Xie 20
1.5 Microlens Examples -6 Maximum displacement of 280 µm achieved Actuation Voltage: <10V A. Jain and H. Xie, MEMS 2005 EEL6935 Advanced MEMS 2005 H. Xie 21 2. GRIN Microlenses Graded-Index (GRIN) Fiber EEL6935 Advanced MEMS 2005 H. Xie 22
2. GRIN Microlenses GRIN Rod Lenses or GRIN Fiber Lenses Selfoc TM One cycle Input angle may not equal to output angle which depends on the length. At half or full cycle, the input and output angles are the same, or focused At ¼ or ¾ cycle, the output light rays are parallel, or collimated Pitch: The fraction of a full sinusoidal cycle that light goes through before leaving the fiber. For example, a 0.25-pitch lens collimates the input light. EEL6935 Advanced MEMS 2005 H. Xie 23 Planar GRIN Microlenses PML TM 2. GRIN Microlenses Ion-exchange process: Thermal or field-assisted (electromigration) Index change is proportional to the percentage of exchanged ions The concentration of exchanged ions changes gradually according to the diffusion process Thermal Ion-exchange process Index change is proportional to the percentage of exchanged ions The concentration of exchanged ions changes gradually according to the diffusion process EEL6935 Advanced MEMS 2005 H. Xie 24
3. Microprisms Challenge: Linear slope with sharp edge Fabrication Techniques: Deep synchrotron or proton lithography Analog lithography Reflow and Mass-transport techniques Fabricated using analog lithography by E.B. Kley and F. Thoma EEL6935 Advanced MEMS 2005 H. Xie 25