Enhanced performance of microbolometer

using coupled feed horn antenna

Kuntae Kim*,a, Jong-Yeon Park*, Ho-Kwan Kang*, Jong-oh Park*, Sung Moon*, Jung-ho Parka * Korea Institute of Science and Technology, Seoul, Korea

a Korea University, Seoul, Korea

ABSTRACT

In the paper, we improved the performance of the microbolometer using coupled feed horn antenna. The response time

of the device was improved by reducing thermal time constant as the area of the absorption layer was reduced. We

designed the shape of an absorption layer as circular structure in order to reduce the coupling loss between the antenna

and the bolometer. A supporting leg for thermal isolation also has circular structure and its length increased up to 82㎛,

it reduced the thermal conductance to 4.65× 10-8[W/K]. The directivity of the designed antenna has 20.8dB. So the

detectivity of the bolometer was improved to 2.37× 10-9 [ WHzcm / ] as the noise characteristics of the bolometer was

enhanced by coupling feed horn antenna. The fabrication of the bolometer are carried out by a surface micromachining

method that uses a polyimide as a sacrificial layer. The absorption layer material of the bolometer is VOx and its TCR

value has above 2%/K. The 3-D feed horn antenna structure can be constructed by using a PMER negative photoresist.

The antenna and the bolometer can be bonded by Au-Au flip chip bonding method.

Keywords: microbolometer, feed horn antenna, antenna coupled, detectivity

1. INTRODUCTION Recently, the demand for inexpensive and uncooled infrared detectors has grown for both civilian and military

application. In this respect, thermal detector has advantage over photon detector.1. Thermal detector can be divided into

three types: 1)bolometer, 2)thermopile, 3)pyroelectric detector.2. The bolometer is most widely researching now because

it doesn�t need the chopper and it can be fabricated monolithically.3,4,5,6,7,8. The bolometer converts absorbed IR

radiation into heat, which in turn changes the resistance of the absorption layer. The bolometer can be modeled as an

IR-sensitive element of thermal mass C linked via a thermal conductance G to a substrate serving as the heat sink. The

performance of the bolometer is characterized by certain figures of merit such as temperature coefficient of resistance

*Contact: E-mail: korion@kist.re.kr, phone:82-2-958-6809

(TCR), responsivity R and detectivity D*. The TCR is an measure of how rapidly the electrical resistance of a material

responds to a change in temperature, the responsivity is the output signal voltage per unit incident infrared power and

the detectivity D* is a figure of merit that normalizes the performance of the bolometer with respect to the size and

noise signal.

To date, bolometer array structures have been fabricated as open structure which has little directionality and, as a

consequence, are sensitivity to radiation incident from all angles. The result of this is a degradation of the signal to

noise ratio of each array pixel element and a corresponding decrease in the overall detectivity of the bolometer array. A

method of enhancing individual pixel directionality is therefore desirable and can be achieved by the use of feed horn

antenna.9. The feed horn would provide efficient coupling between focal plane image and the bolometer array and

would significantly reduce the amount of radiation incident on a given pixel element. And it can lead to significant

improvement in detectivity of bolometer. Besides of this effect, the size of the bolometer is reduced, so it can lead to

reducing the total power consumption and it can be applied to the fast image detection system. Figure 1 is schematic

view of the proposed feed horn antenna coupled bolometer. Figure 2 shows the advantage of the antenna coupled

bolometer by reducing the signal to noise ratio.

Figure 1: Schematic view of feed horn antenna coupled bolometer

Figure 2: Advantage of the antenna coupled bolometer

2. DESIGNS

We designed the bolometer and the antenna focusing on enhancement of the detectivity. First, we designed optimal size

of the antenna that has most excellent directivity in 10㎛ wavelength. The directivity of antenna expressed as 10.

( ) )(log104log10)(2

102

210 sLCadBD apc −

=

=

λπ

λπε (1)

)79.1725.2671.18.0()(log10)( 3210 ssssL ap −+−≅−= ε ,

lds m

λ8

2

=

where a is radius of horn at the aperture , L(s) is directivity loss for aperture efficiency, and C is aperture circumference

and s is maximum phase deviation. Figure 3 shows designed feed horn antenna structure and simulation result of the

antenna�s directivity. Directivity of the designed feed horn antenna has 20.8dB where the radius of horn is 54㎛. Next,

we determined the width of an absorption layer and the length of a thermal isolation leg for most excellent detectivity.

Absorption layer width of the bolometer was matched to the diameter of the antenna in order to reduce the coupling

loss between the bolometer and the antenna. The thermal conductance G and heat capacitance C of the bolometer has

4.65× 10-8[W/K] and 9.31×10-10[J/K] respectively from the following equations. 11.

cVC ρ= , where ρ : density, c:heat capacity, V:volume for absorption layer

lWdKG = , where K:thermal conductance, W:width, d:thickness, l:length for thermal isolation leg

The detectivity D* was calculated using the above equations and following equation (2) and it�s value has 2.37×

109[ WHzcm / ]

2221*

Vn

fAd

GRID

∆

+=

τωηα (2)

Where α (TCR):0.02K-1, R(bolometer resistance):100K, η (absoption ratio):0.9, I(bias current):5㎂, τ (thermal time

constant):0.02s, ω =2π ×30Hz, Ad(absoption layer area):π ×11㎛ 2 , 92 107.404 −×=≅∆ kTRfVn .

Figure 4 shows the final designed antenna coupled bolometer.

44 44 44 44 ㎛㎛㎛㎛

22222222 ㎛㎛㎛㎛

54545454㎛㎛㎛㎛

Figure 3: Designed antenna structure and directivity simulation

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 4: Designed antenna coupled bolometer

3. FABRICATIONS

3.1 Antenna fabrication

Antenna fabrication can be carried out like figure 5. Firstly, deposit Cr/Au seed layer. Secondly, tilted and rotated

illumination on PMER negative photoresist. After developed the photoresist, the horn shape mold are formed. Thirdly,

electroplate the Ni on the seed layer using the PMER mold. After chemical mechanical polishing(CMP) process,

removing the PR mold and seed layer, then the antenna is constructed. The key technology to make the horn shape

mold is tilted and rotated illumination using a Mirror Reflected Parallel Beam Illuminator(MRPBI) which is invented

for parallel beam illumination. Figure 6 is MRPBI system and Figure 7 is PMER mold which was made by using the

MRPBI system.

1. seed layer deposition1. seed layer deposition1. seed layer deposition1. seed layer deposition

2. Tilted and rotation illumination2. Tilted and rotation illumination2. Tilted and rotation illumination2. Tilted and rotation illumination

3.Electroplating3.Electroplating3.Electroplating3.Electroplating

5. PR removing5. PR removing5. PR removing5. PR removing

6. Seed layer removing6. Seed layer removing6. Seed layer removing6. Seed layer removing

4. CMP4. CMP4. CMP4. CMP

Figure 5: Antenna fabrication process

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 6: MRPBI system

Figure 7: PMER mold made by MRPBI

3.2 Bolometer fabrication

The bolometer can be made by surface micromachining method. Figure 8 shows the bolometer fabrication process.

Firstly, polyimide was used as a sacrificial layer. Polyimide was cured on high temperature oven and had 2.5㎛

thickness, it is etched by RIE using AZ9260 thick PR which plays a role as an etch mask. The polyimide has to

rounded side-wall for post SiNx, metal deposition and patterning. Figure 9 shows SEM image of the rounded side-wall

of polyimide and 2 × 4patterned polyimide array. Secondly, a SiNx was used as a thermal isolation supporting leg.

SiNx was deposited using the PECVD and it was patterned using the RIE. Thirdly, a VOx was used as a absorption

layer and it�s TCR has above 2%. Figure 10 is optical image of a patterned SiNx and VOx. Fourthly, Cr/Au layer for

contact layer was deposited using the evporator and it was patterned using the RIE. Finally, ashing the sacrificial layer

using the microwave plasma asher. Figure 11 is a single pixel mask layout and patterned bolometer. Figure 12 shows 2

× 4 patterned bolometer array. After fabricated the antenna and the bolometer respectively, they are bonded by Au-Au

flip chip bonding like figure 13.

Mask 1Mask 1Mask 1Mask 1

1. Polyimide patterning1. Polyimide patterning1. Polyimide patterning1. Polyimide patterning

Mask 2Mask 2Mask 2Mask 2

2. Si3N4 etching2. Si3N4 etching2. Si3N4 etching2. Si3N4 etching

3. VOx etching3. VOx etching3. VOx etching3. VOx etching

Mask 3Mask 3Mask 3Mask 3

Mask 4Mask 4Mask 4Mask 4

4.Cr/Au etching4.Cr/Au etching4.Cr/Au etching4.Cr/Au etching

6. Sacrifitial layer removing6. Sacrifitial layer removing6. Sacrifitial layer removing6. Sacrifitial layer removing

Mask 2Mask 2Mask 2Mask 2

5. Si3N4 etching(upper layer)5. Si3N4 etching(upper layer)5. Si3N4 etching(upper layer)5. Si3N4 etching(upper layer)

Figure 8: Bolometer fabrication process

(a) (b)

Figure 9: Patterned polyimide (a)rounded side-wall of polyimide, (b) 2× 4 patterned bolometer array

(a) (b)

Figure 10: Patterned SiNx and VOx (a) single pixel , (b) 2× 4 array

Figure 11: Mask layout and patterned bolometer

Figure 12: 2 × 4 bolometer array

54㎛

22㎛

15㎛

1㎛

3㎛

5㎛AuAuAuAu

Figure 13: Bonding of the antenna and the bolometer

4. CONCLUSIONS

We improved detectivity of the bolometer using the couped feed horn antenna which increases signal to noise ratio.

Optimal size of the antenna and the bolometer was designed in order to enhance the detectivity of the bolometer.

Antenna simulation was carried out by HFSS simulation tool. The directivity of designed antenna had 20.8dB and the

detectivity of the device had 2.37× 10-9 [ WHzcm / ]. To make the horn shape antenna, we invented MRPBI system

which can illuminate parallel beam and it�s stage can be tilted and rotated. We convinced that the bolometer can be

fabricated by surface micromachining method. We also confirmed that the antenna and the bolometer can be fabricated

respectively and they can be bonded each other by Au-Au flip chip bonding.

ACKNOWLEDGMENTS

This work has been supported by the 21C frontier project and Intelligent microsystem program

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