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:jypark@kist.re.kr; 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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