1. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 Dean P. Neikirk and Sangwook Han Microelectronics Research Center Department of Electrical and Computer Engineering The University of Texas at Austin Austin, TX 78712 USA SPIE’s Microtechnologies for the New Millennium 15-18 May 2003 Hotel Meliá Sevilla Sevilla, Spain Proceedings of SPIE Vol. #5836 : Smart Sensors, Actuators, and MEMS SESSION 16, Room: Arenal I Wed. May 18, 12.00 to 13.00: Infrared Sensors 12.20-12.35: Design of infrared wavelength-selective microbolometers using planar multimode detectors, D. P. Neikirk, S. Han, Univ. of Texas/Austin (USA) [5836-60] link to pdf of proceedings paperlink to pdf of proceedings paper. Design of Infrared Wavelength-Selective Microbolometers Using Planar Multimode Detectors

2. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 2 Fabry-Perot Microbolometer Array d mirror resistive sheet http://lep694.gsfc.nasa.gov/code693/tdw03/proceedings/docs/session_2/Ngo.pdf Conventional microbolometer infrared focal plane detectors in an ideal device, the absorber should provide total absorption of the incoming radiation and convert the electromagnetic radiation into heat to “match” an absorber to free space requires –absorber: e.g., a thin conductor with sheet resistance 377 ohms –mirror placed (odd integer)· /4 behind absorbing layer –essentially a Fabry-Perot cavity –this is sometimes referred to as “space cloth” incident thin conductor (absorber) Mirror layer Gap

3. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 3 Spectral response of Fabry-Perot microbolometers 377  mirror is it possible to build “multi-color” IR F-P microbolometer focal plane arrays? –the primary “design variable” is the distance to the mirror 3  m gap 2.5  m gap 2  m gap LWIR wavelength (microns) coupling efficiency 468101214 0.2 0.4 0.6 0.8 0 1 the bandwidth of conventional Fabry-Perot microbolometers is too wide to allow easy “color” discrimination in the LWIR wavelength band

4. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 4 Single Element in Array g w a d metal grid mirror resistive microbolometer Planar Multimode Detector Array metal grid mirror Alternative: planar multimode detectors replace standard thin film bolometer with a true antenna coupled microbolometer array –essentially a resistively loaded inductive/capacitive mesh –planar multimode detectors were extensively studied for infrared and millimeter wave detection by Rutledge and Schwarz in the late 1970’s

5. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 5 Planar Multimode Detectors support layer resistive microbolometer gridmirror single pixel η0η0 η0η0 jB c jB l GdGd Z in d grid equivalentmirror equivalent array grid period a < the shortest wavelength analysis can be performed using a modification of Eisenhart & Kahn’s waveguide post model single period a g d w

6. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 6 Spectral response of planar multimode grids Blue aNA g w d2.50 RsRs 377Ω all lengths in [micron] grid response depends on array period a, gap g, post width w, distance to mirror d, and sheet resistance R S of microbolometer material space cloth 7891011121314 wavelength [micron] 0 0.5 1 power absorption efficiency * * 377Ω 2.50 4.50 5.00 7.00 Mag 30Ω 3.61 0.96 1.00 5.33 Green 53.5Ω 3.29 3.00 0.20 6.90 Red wide range of achievable bandwidths, from broad to narrow grid a g d w

7. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 7 Methods of to achieve varied wavelength selectivity mechanical actuation for fixed grid dimensions, vary the distance between the array and the mirror –three “color” array using three different mirror distances for fixed grid dimensions, use an actuator to vary the distance between the array and the mirror

8. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 8 d=0μm d=6μm 06 24 135 d in [μm] 7 14 13 12 11 10 9 8 wavelength in [μm] power absorption efficiency d=3.29μm a=6.90μm g=0.20μm w=3.00μm R s =53.5Ω Wavelength selectivity varied by changing distance d to the mirror narrow spectral response allows greater “color” sensitivity

9. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 9 Selecting spectral response for “color” pixels 7 8 910 1112 13 14 0 1 0 1 0 1 wavelength in [micron] power absorption efficiency 7 8 910 1112 13 14 0 1 0 1 0 1 wavelength in [micron] power absorption efficiency ideal ambiguous if a design produces more than one peak in absorption then the “color” becomes ambiguous pixel 1 pixel 2 pixel 3 “ghost” peak

10. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 10 Achieving wavelength selectivity by varying d d=3.29 micron d=5.70 micron 06 24 135 d in [micron] 7 14 13 12 11 10 9 8 wavelength in [micron] power absorption efficiency a=6.90 micron g=0.20 micron w=3.00 micron R s =53.5Ω “ghost” peak: bad 7891011121314 wavelength in [micron] 0 1 power absorption efficiency Good Good

11. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 11 d=2.0micron d=2.5micron d=3.0micron R s =377Ω 7891011121314 wavelength in [micron] 1 power absorption efficiency 0 d=1.65micron d=3.75micron d=5.10micron a=5.07micron g=1.43micron w=0.74micron R s =21.0Ω 7891011121314 wavelength in [micron] 1 power absorption efficiency 0 Space cloth Optimized 3-color wavelength selectivity by varying only d optimization performed using a genetic algorithm –design goal: three minimum width spectral peaks centered at 8, 10 and 12 microns insufficient spectral selectivity

12. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 12 7891011121314 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 wavelength in [micron] power absorption efficiency Problem with variable d: complex fabrication processes requires either –fabrication process with three different sacrificial layer thicknesses or –mems actuator in both cases the fabrication would be more complicated than current micromachined microbolometer focal plane array processes –there is still some color ambiguity for the longest wavelength (12 micron) pixel due to “ghost” peak at 7 microns

13. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 13 Wavelength selectivity by varying lithographically- drawn parameters a g d w potentially simpler process if the sacrificial layer is held fixed for all pixels –sheet resistance of bolometer material also held constant (same material for all pixels) –vary ONLY the lithographically drawn features of the grid array period a, gap width g, and post width w

14. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 14 Optimized designs genetic algorithm used for optimization –design goal: three minimum width spectral peaks centered at 8, 10 and 12 microns –constraints: d (distance to mirror) is varied, but must be the same under all three pixels sheet resistance of bolometer material also held constant vary only grid period a, gap width g, and post width w w = 4.57  m wavelength in [micron] 891011121314 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 7 power absorption efficiency w = 2.80  m w = 1.30  m results from design optimization –optimum distance to mirror d = 3.14  m –optimum R S = 56.6  –all three pixels share common grid period a and gap width g a = 6.80  m g = 0.20  m –post width w is critical in determining location of absorption peak

15. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 15 absorption power absorption efficiencies Comparison: power absorption efficiencies 7 14 wavelength in [micron] 0 1 7 14 7 Fabry-Perot microbolometer variable mirror grid microbolometer grid dimensions varied

16. Microelectronics Research Center, The University of Texas at Austin Microtechnologies for the New Millennium 2005 16 IR wavelength-selective focal plane arrays planar multimode detectors exhibit widely tunable spectral response –can tune for much narrower spectral response than conventional Fabry-Perot microbolometers tuning of wavelength response can be achieved using several methods –for fixed grid dimensions distance to tuning mirror can be used multiple sacrificial layer thicknesses mechanical actuation a wavelength-selective three pixel design, each pixel using different lithographically drawn dimensions with constant mirror separation, shows excellent narrow band response through the use of planar multimode detectors color vision in the long wavelength band should be achievable