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Fabrication Method of 3D Feed Horn Shape MEMS Antenna Array
Using MRPBI System and Application for Microbolometer
Jong-yeon Park*,** , Kun-tae Kim*, Sung Moon*, Jong-oh Park*,Myung-Hwan Oh*, James jungho Pak**
*Microsystem Research Center, Korea Institute of Science and Technology P.O.BOX 131,Cheongryang, Seoul, 130-650, Korea
**Dept. of Electrical Engineering, Korea University, Seoul, Korea
ABSTRACT
A 3D Feed horn shape MEMS antenna has some attractive features for array application, which can be used to improve
microbolometer performance. Since MEMS technology have been faced many difficulties to fabrication of 3D feed
horn shape MEMS antenna array itself. The purpose of this paper is to propose a new fabrication method to realize a 3D
feed horn shape MEMS antenna array using a MRPBI(Mirror Reflected Parallel Beam Illuminator) system with an
ultra-slow-rotated and inclined x-y-z stage. A high-aspect-ratio 300 µm sidewalls had been fabricated using SU-8
negative photo resist. It can be demonstrated to feasibility of realize 3D feed horn shape MEMS antenna array
fabrication. In order to study the effect of this novel technique, the 3D feed horn shape MEMS antenna array had been
simulated with HFSS(High Frequency Structure Simulator) tools and then compared with traditional 3D theoretical
antenna models. As a result, it seems possible to use a 3D feed horn shape MEMS antenna at the tera hertz band to
improve microbolometer performance and optical MEMS device fabrication.
Keywords: MRPBI, 3D MEMS Antenna, Microbolometer, HARS, 3D UV-Lithography, IR image sensor
1. INTRODUCTION
Recent advances in MEMS (Micro Electro Mechanical Systems) industries have given rise to the advent of various
MEMS fabrication techniques to fabricate microstructures made from various materials. There is also a need for
fabricating complicated 3-Dimensional micro structures with high aspect ratios, such as application for enhanced
Microbolometer coupled with a 3D MEMS antenna array, enhanced optical efficiency of TFT-LCD and display devices
* Correspondence: E-mail:[email protected]; Phone:+82-2-958-6809,6758; Fax:+82-2-958-6910,6909
application. Although the 3D feed horn MEMS Antenna structure has many advantages, could not be carried out due to
the difficulty of fabrication using conventional UV (Ultra-violet) lithography techniques. In this paper, a novel method
to realize 3D feed horn MEMS Antenna array using MRPBI (Mirror Reflected Parallel Beam Illuminator) System is
presented.
54㎛54㎛54㎛54㎛
22㎛22㎛22㎛22㎛
2.5㎛2.5㎛2.5㎛2.5㎛
Absorption layerAbsorption layerAbsorption layerAbsorption layer Supporting legSupporting legSupporting legSupporting legSubstrateSubstrateSubstrateSubstrate
AntennaAntennaAntennaAntenna
Figure 1: Schematic drawing of 3D feed horn MEMS antenna coupled with Microbolometer.
2. CONCEPTS AND IMPLEMENTATION OF MRPBI SYSTEM
The most difficult problem in HARS(High Aspect Ratio Structure) and 3D feed horn shape MEMS antenna array
fabrication is how to exposure parallel beam using UV lithography apparatus. We know need to make parallel beam
over 6meter UV light propagation ray path for 4 inch exposure area using CODE V optical simulator but usual
laboratory height is smaller than 6 meter. Therefore, higher height apparatus is difficult to set up at usual laboratory.
UV cold mirror
Lamp housing
Rear reflector
Optical boardMotor, gear, sensor
Sample stage
Shutter, filter
UV cold mirror
Lamp housing
Rear reflector
Optical boardMotor, gear, sensor
Sample stage
Shutter, filter
Figure 2: Schematic drawing of MRPBI System
So we tried to solve the problem using UV light long propagation ray by several UV cold mirror reflection method.
These results can be supported by fig.2 Schematic drawing of MRPBI System and fig.3 Configuration of MRPBI
System.
Figure 3: Configuration of MRPBI System
Conventional UV-lithography apparatus have been exposed on the planar stage but MRPBI System exposure method is
with a difference. For fabrication of more high aspect ratio 3D structure array, stage is x-y-z direction tilted, 360°
automatic control rotated and exposure simultaneously. In consideration of more parallel UV light, MRPBI system
exposure area is smaller than conventional UV-lithography apparatus. Mask coupled with a wafer can be employed 2-
way vacuum fixed and it can be controlled to selection of hard contact, soft contact, and several contact conditions
respectively using 2- way fixed vacuum system. Fig.4 show to inside photography of MRPBI system. Its rotational
axis can be controlled two ways that first is manual controlled and second is computer controlled by RS-232C
communication port. So we can change experiment parameter: rotation time and exposure time. MRPBI�s wavelength is
365nm by 1kw SHP Hg lamp and radiation intensity can be changed to maximum 10mw/Cm2 .
1
26
4
3
5
7
1
26
4
3
5
7
1. Lam p M odule
2. M otorized stage
3. U .V cold M irror
4. Cooling line
5. External housing
6. O ptical board
7. Control box
1. Lam p M odule
2. M otorized stage
3. U .V cold M irror
4. Cooling line
5. External housing
6. O ptical board
7. Control box
LAMP HOUSING
REARREFLECTOR
SHUTTER & FILTER
UV COLD MIRROR
SAMPLE STAGE
AIR OUT
MOTOR, GEAR
& SENSOR
MIRROR MIRROR
HOUSING(AL PROFILE)
2t STEEL
OPTICAL BOARD
Figure 4: Inside photograph of MRPBI System
3. OPTIMAL DESIGN OF 3D MEMS ANTENNA USING HIGH FREQUENCY STRUCTURE SIMULATOR
The incident thermal, or blackbody, radiation emitted by all objects of a given physical temperature is a maximum at the
8-12 µm wavelength range. So we considered 3D antenna dimensions in relation to the development of a new form of
IR imaging array that uses 3D feed horn MEMS antenna technology to couple the incident thermal radiation into an
individual array element or pixel. Fig.8 shows optimal design parameter and incident light radiation pattern of 3D feed
horn MEMS antenna using HFSS(High Frequency Structure Simulator). Following equations (1),(2),(3) are relative to
conical feed horn antenna and equation (4),(5),(6) are relative to feed horn antenna optimal design and simulation.
( ) )(log104log10)(2
102
210 sLCadBD apc −
=
=
λπ
λπε
(1)
)79.1725.2671.18.0()(log10)( 32
10 ssssL ap −+−≅−= ε (2)
lds m
λ8
2
= (3)
a : Radius of horn at the aperture
L(s): directivity loss of aperture efficiency
C : aperture circumference
s : maximum phase deviation
θθ×.=
λL
cos1cos30
− (4)
(5)
θsin2×mdL= (6)
L: Feed horn length
Dm: Feed horn diameter
Following Fig 5, fig.6 and fig.7 are to 3D Cylinder MEMS antenna simulation, 3D Conical horn MEMS antenna
simulation and 3D feed horn MEMS antenna simulation respectively. Fig 5,fig.6 and fig.7 are scattered radiation pattern.
This result indicated the side lobe by miss tuning 3D MEMS antenna modeling.
22222222
100100100100
22222222
100100100100
Figure 5: 3D Cylinder MEMS antenna simulation using HFSS(High Frequency Structure Simulator)
11.511.511.511.511.511.511.511.5 22222222
100100100100
100100100100
62.862.862.862.8
Figure 6: 3D Conical horn MEMS antenna simulation using HFSS(High Frequency Structure Simulator)
θθ×=
λdm
cos-1cossin6.0 ×θ
100100100100
22222222
°11.511.511.511.5
62.862.862.862.8
100100100100
22222222
°11.511.511.511.511.511.511.511.5
62.862.862.862.8
Figure 7: 3D Feed horn MEMS antenna simulation using HFSS(High Frequency Structure Simulator)
Fig.8 shows optimal design of 3D feed horn MEMS antenna using HFSS(High Frequency Structure Simulator). The
result indicated that directivity is 20.42 dB and gain is 20.77 dB. Feed horn angle parameter is 11.5° and feed horn
diameter is 22µm.
44444444
22222222
11.511.511.511.5°
40404040
Figure 8: Optimal design of 3D feed horn MEMS antenna using HFSS(High Frequency Structure Simulator)
Table. 1 Simulation numerical value of 3D feed horn MEMS antenna design using HFSS
(A unit Diameter & Length: µm)
Table 1 is indicated that 3D feed horn MEMS antenna simulation numerical value using HFSS. Fig.9 shows comparison
of 3D MEMS antenna directivity gain. Consequently, directivity gain of feed horn shape is highest about 22dB.
15151515
16161616
17171717
18181818
19191919
20202020
21212121
22222222
Cylin d er Con icalHorn
F eed Horn
Directiv ity (dB)Directiv ity (dB)Directiv ity (dB)Directiv ity (dB)
15151515
16161616
17171717
18181818
19191919
20202020
21212121
22222222
Cylin d er Con icalHorn
F eed Horn
Directiv ity (dB)Directiv ity (dB)Directiv ity (dB)Directiv ity (dB)
Figure 9: Comparison of 3D MEMS antenna directivity
120120120120 115115115115 110110110110 105105105105 100100100100 Diameter
461.27 422.15 384.69 314.79 Length
12.31 9.51 10.05 13.50 13.82 Gain(dB)
14.43 12 17.56 19.40 18.76 16.83 Gain(dB)
282.35 251.58 222.48 195.05 169.35 145.23 Length
95959595 90909090 85858585 80808080 75757575 70707070 Diameter
314.79
4. FABRICATION PROCESS OF MICROBOLOMETER COUPLED WITH 3D FEED HORN MEMS ANTENNA ARRAY USING MPRBI SYSTEM
2. 2. 2. 2. Exposure with Exposure with Exposure with Exposure with Tilt+Rotation, DevelopTilt+Rotation, DevelopTilt+Rotation, DevelopTilt+Rotation, Develop
1. Seed Layer Deposition
3. Electroplating
4. 4. 4. 4. CMP ProcessCMP ProcessCMP ProcessCMP Process
5. PR remove
6. 6. 6. 6. Seed Layer RemoveSeed Layer RemoveSeed Layer RemoveSeed Layer Remove
7. 7. 7. 7. Antenna SeparationAntenna SeparationAntenna SeparationAntenna Separation
2. 2. 2. 2. Exposure with Exposure with Exposure with Exposure with Tilt+Rotation, DevelopTilt+Rotation, DevelopTilt+Rotation, DevelopTilt+Rotation, Develop
1. Seed Layer Deposition
3. Electroplating
4. 4. 4. 4. CMP ProcessCMP ProcessCMP ProcessCMP Process
5. PR remove
6. 6. 6. 6. Seed Layer RemoveSeed Layer RemoveSeed Layer RemoveSeed Layer Remove
7. 7. 7. 7. Antenna SeparationAntenna SeparationAntenna SeparationAntenna Separation
1. Seed Layer Deposition
3. Electroplating
4. 4. 4. 4. CMP ProcessCMP ProcessCMP ProcessCMP Process
5. PR remove
6. 6. 6. 6. Seed Layer RemoveSeed Layer RemoveSeed Layer RemoveSeed Layer Remove
7. 7. 7. 7. Antenna SeparationAntenna SeparationAntenna SeparationAntenna Separation
Figure 10: Process sequence for micro fabrication of 3D feed horn MEMS antenna
Fig.10 is illustrated fabrication main process procedure of 3D feed horn MEMS antenna array. Process procedure:
1.Seed layer deposition, 2.Feed horn antenna mold fabrication using MRPBI system, 3.Electroplating process, 4.
CMP(Chemical mechanical polishing) Process; Planarization, 5. Photoresist remove, 6.Seed layer remove, 7.Antenna
separation.
5. EXPERIMENTS & RESULT
Figure 11: SEM image and schematic drawing of negative photoresist PMER lithography experiment
using conventional lithography apparatus
Thick Photoresist
Figure 12: SEM images of vertical sidewall array using negative photoresist PMER by MRPBI system
Figure 13: SEM images of inclined sidewall array using negative photoresist SU-8 by MRPBI system
Fig.11 shows that negative photoresist PMER experiment result using conventional UV-lithography apparatus. In
Fig.12 we can see over 100µm vertical sidewall structure array using negative photoresist PMER. The condition are
2step spin coating: (500rpm/s, 600rpm/s), stabilization: 5min, prebake: 110!C/25min, PEB(Post Exposure Bake):
100!C/15min and exposure time is 1800sec. When expoure time is over 2400sec that happen to serious hard reflection.
Following fig.14 is show that look like teapot shape array SEM images (a),(b) and 2D microscope picture (c). In this
case, experiment parameters are 20û inclined stage, y-axis direction 360º rotated and exposure time is 1800sec. The
exposure time is the most important factor in this experiment.
(a) (b) (c)
Figure 14: SEM images (a),(b) and 2D microscope picture(c) of teapot shaped array
using negative photoresist PMER by MRPBI system
6. CONCLUSION
Novel techniques and methods have been described to fabrication of 3D feed horn MEMS antenna using a new UV
lithography apparatus called MRPBI(Mirror Reflected Parallel Beam Illuminator) system in this paper. And proposed to
optimal 3D MEMS antenna design for coupled with Microbolometer using HFSS(high frequency structure simulator).
ACKNOWLEDGMENTS
The authors wish to acknowledge that this paper is a result of the research accomplished with the financial support of
the Intelligent Microsystem Center, Seoul, Korea, which is carrying out one of the 21st century's New Frontier R&D
Projects sponsored by the Korea Ministry Of Science & Technology.
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