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: [email protected], 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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