Tunable Fabry-Pérot Filter for Optical Glucose Monitoring

Xin Zhao

1,2

and Schmidt J. Dominik

College of Physical Science and Technology, Sichuan University, 29 Wangjiang Road, Chengdu, Sichuan, P.R. China,

Department of Electrical Engineering, International Technological University,

355W San Fernando St, San Jose, CA, U.S.A.

Keywords: Micro-electro-mechanical Systems, Long-Wave Infrared, Tunable Fabry-Pérot Filter, Glucose Sensor.

Abstract: This paper presents a Tunable MEMS Fabry-Pérot (FP) filter for the application of optical glucose sensor in

Long-Wave Infrared (LWIR) range. The structure design, simulation, fabrication process and testing system

are reported. The spectral wavelength tuning range from 10.9 μm to 12.19 μm was designed for the

transmission peak which is related to the glucose concentration. A tuning distance between top and bottom

reflector of 2.25µm has been achieved, while maintaining a relatively low tuning voltage of 20V. The

Tunable FP filter is the core part of glucose sensor and has other applications in Bio-sensor, hyperspectral

imaging and tunable lasers, etc.

1 INTRODUCTION

Blood Glucose monitoring technologies have been

used in the treatment of diabetes for three decades.

The invasive methods are based on microneedle,

impedance spectroscopy, microdialysis and

subcutaneous sensor, etc. The developing non-

invasive sensors use infrared absorption, optical

coherence tomography, Raman scattering, and

polarimetry as measurement technologies.

Recently, infrared spectroscopy has shown the

potential for an analytical method in non-invasive

glucose sensing based on the optical characteristic of

the glucose concentration. On the other hand, with

the development of micromachining technologies,

Micro-electro-mechanical Systems (MEMS)

emerges as the choice for the fabrication of Infrared

(IR) instrument. Compared with conventional IR

analysis systems, the MEMS IR sensors offer lower

cost, lower power consumption and more portability

due to their small size.

As the key part of optical non-invasive glucose

sensor, the tunable MEMS FP filter was originally

developed by several different groups. Noro’s group

reported the 3-5 μm Mid-wave Infrared (MWIR)

filter for the gas sensing. Stupar’s group presented

the first LWIR tunable filter range from 8 μm to 11

μm. The dual-band FP filter with two reflectors was

reported by Neumann’s group. The spectral ranges

are from 4 μm to 5 μm and from 8 μm to 10.5 μm.

In this paper, a tunable MEMS FP filter for the

LWIR range is presented. The proposed bulk

micromaching technology is used to form the

reflector carriers. The tunable FP filter could be

combined with a detector to test the central

wavelength shift of transmission peak which can

read out the change of glucose concentration in the

human body.

2 DESIGN OF TUNABLE FP

FILTER

2.1 MEMS Design

Figure 1(a) shows the structure of the tunable FP

filter, which consists of two 2.1× 2.1 mm² reflectors,

two 3.6 × 3.6 mm² reflector carriers, and four

springs. The bottom reflector carrier is fixed and the

other suspended by springs. The reflectors are

surrounded by a metal electrode which uses an

electrostatic force between the two reflector carriers

to change the distance between the reflectors as

shown in Fig. 1(b). The L shaped springs are applied

on the structure design to optimize the the flatness

across the central area of the top reflector.

171

Zhao X. and J. Dominik S..

Tunable Fabry-Pérot Filter for Optical Glucose Monitoring.

DOI: 10.5220/0004871101710175

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2014), pages 171-175

ISBN: 978-989-758-013-0

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

(a)

(b)

Figure 1(a): Schematic of the tunable FP filter, (b) cross-

section of the tunable FP filter.

2.2 Optical System

The electromagnetic waves enter the optical cavity

through the top reflector and are reflected by both

the top and bottom reflector many times. The output

transmittance spectrum T(λ) of the FP filter is

described by the Airy-Function





T



1



1R





sin



δ



(1)

with the optical phase











2πndcos

ϕ

(2)

and the peak transmittance





1



1





(3)

Where λ is the wavelength of the electromagnetic, R

is the reflectance, n is the refractive index of the

material between the reflectors, d is the distance

between the top and bottom reflector, θ is the angle

of incidence, ϕ is phase shift on reflection, and A is

the absorption as shown in Fig.2.

Figure 2: Schematic of Fabry-Pérot filter.

The reflectors are distributed Bragg reflector

(DBR) mirrors which contain Ge/Al2O3/Ge layers.

The thickness of the Ge layers and Al

2O3 layer are

850 nm and 629 nm, respectively. The electrical

field distribution of DBR is related to the peak value

of the transmission. The simulation result of the 3D

electrical field distribution of the DBR is shown in

Fig. 3. The optical properties simulation is

performed by software COMSOL by using finite

element methods.

Figure 3: The 3D electrical field distribution.

The central wavelength (CWL) of the output

transmission peak is related to the distance between

the top and bottom reflector. As shown in Fig. 4,

The CWL is 11.56 μm, when the distance between

the top and bottom reflector is 6.17 μm.

Figure 4: The simulated transmission of the tunable FP

filter.

R d

108.75

11.25

12.5 13.75

0.25

0.75

Wavelength (μ

)

The transmission of tunable FP Filter

0.5

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2.3 Electromechanical System

The metal layers in top and bottom reflector carriers

consist of capacitor structure. The electrostatic force

of the capacitor results in the change of the distance

between the top and bottom reflector when the

voltage is applied on the planes of capacitor.

The electrical force F of the capacitor structure can

be expressed by







2

(4)

Where C is the capacitance, V is the voltage between

the two planes. The bottom of the plane is fixed,

therefore, the pressure P on the top plane is







2

(5)

Where S is the area of the capacitor plane. The

electromechanical simulation is performed by the

software COMSOL. The C is 11.1763 pf, when the

voltage is 20 V, S is 8.55 mm



and d is 6.8 µm. The

displacement of the top reflector is 2.25 µm as

shown in the Fig. 5, when the voltage on the top

plane is 20 V. Because the bottom reflector is fixed,

the displacement of the top reflector is equal to the

change of distance between the top and bottom

reflector.

When the voltage changes from 0 V to 20 V, the

distance between the top and bottom reflector plates

increases from 0 µm to 2.25 µm as shown in the Fig.

6. Due to the pull-in effect, the tuning distance

between the two reflectors should be less than 2.27

µm which is one third of the initial distance 6.8 µm.

In order to get the stable tuning range of the

transmission, the input voltage should be less than

20V.

Figure 5: The displacement of the top reflector.

Figure 6: The change of distance between the top and

bottom reflector due to the applied voltage.

The change of distance between the top and bottom

reflector results in the transmission peak shift, when

the voltage is different as shown in Fig. 7. When the

voltages are 0 V, 14 V and 20V, the central

wavelength of the transmission peaks are 12.19 µm,

11.56 µm, and 10.9 µm, respectively. As shown in

the Fig. 8, when the voltage changes from 0V to 20

V, the transmission peak shift increases from 0 µm

to 1.29 µm.

Figure 7: The transmission peak of the tunable FP filter.

15 20

0.75

1.5

2.25

The voltage applied on the capacitor (V)

The change of distance between the

reflectors

(

)

10 8.75 11.25

12.5 13.75

0.25

0.75

Wavelength (μm)

The transmission of tunable FP Filter

0.5

14V

20V

TunableFabry-PérotFilterforOpticalGlucoseMonitoring

173

Figure 8: The transmission peak shift due to the applied

voltage on the capacitor.

2.4 Fabrication

The proposed fabrication of the MEMS Fabry-Pérot

filter is based on a bulk micromachining approach.

Two structured wafers are bonded together by an

intermediate SU8-layer to form the top and bottom

reflector. The optical area in the center of the chip is

surrounded by control electrodes.

The silicon wafer is 4 inch in diameter and 525

µm thick. In order to achieve good transmittivity, the

wafer is polished to reduce the thickness to 300 µm.

The fabrication starts with deposition of silicon

dioxide which is used as the block layer of wet

etching of silicon as shown in Fig. 9(a).

The silicon dioxide layer is under etching by

buffered oxide etch (BOE) to form the window for

silicon etching as shown in Fig. 9(b).

The silicon is

selectively etched by Tetramethylammoniµm

hydroxide (TMAH) from backside as shown in Fig.

9(c).

In the next step, the other silicon dioxide is

etched by BOE as shown in Fig. 9(d), and the

springs of the top movable reflector carrier is formed

by etching the silicon material as shown in Fig. 9(e).

The proposed DBR structure of the reflector is

formed by depositing Ge/Al

2O3/Ge layers as shown

in Fig. 9(f). The thickness of the Ge layers of is 850

nm and the thickness of the Al2O3 layer is 629 nm.

Germanium Evaporation is realized by Innotec

ES26C E-Gun Evaporator.

Al2O3 deposition is

fabricated by the plasma-enabled atomic layer

deposition (ALD) system.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 9(a)-(h): The fabrication process flow of the

tunable FP filter.

The top electrode is formed by using physical

vapour deposition Sputter System to sputter Au in

the front side of substrate as shown in Fig. 9(g). The

thickness of the Au layer is 0.5 µm.

The fabrication of the bottom reflector carrier is

similar to the top reflector carrier except for the

etching process to form the backside open and

springs. Both the wafers with the movable top and

the bottom fixed reflector carriers respectively are

connected with a SU-8 interface layer as shown in

Fig. 9(h).

The proposed layout of the tunable FP filter is

shown in Fig. 10. Ledit software is used to draw the

layout. The width of the reflector is 2.1mm. The

width of the spring is 0.75 mm and the movable area

of the top carrier is 3.6 × 3.6 mm. The area of the

metal pad is 1.2 × 1.2 mm.

5 0

15 20

0.65

0.975

1.3

The voltage applied on the capacitor (V)

The transmission peak shift (µm)

0.325

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174

Figure 10: The layout of the tunable FP filter.

2.5 Testing System

In order to control the voltage between the top and

bottom capacitor planes, a Korad voltage supply

KA6003P can be used as reference. The voltage

supply is a programmable single-channel, constant-

voltage power supply with low noise, high reliability

and high accuracy.

The glucose sensor system consists of a voltage

supply, tunable FP filter, detector, testing circuit,

and PC as shown in Fig. 11. The reference detector

is a thermopile detector MLX90614 from Melexis.

The MLX90614 is an Infrared thermometer for non-

contact temperature measurements. Both the IR

sensitive thermopile detector chip and the signal

conditioning ASIC are integrated in the same TO-39

can.

Evaluation board EVB90614 from Melexis is

used as the testing circuit which includes an easy

interface between the MLX90614 infrared

thermometer in TO-can and a PC.

Figure 11: The schematic of glucose sensor system.

3 CONCLUSIONS

In summary, a tunable MEMS Fabry-Pérot filter for

the Long-Wave Infrared range is proposed and

simulated. By applying a voltage ranging from 0 V

to 20 V, the tuning range of output transmission

peak shift from 10.9 µm to 12.19 µm is obtained.

The tunable FP filter is the key part of glucose

sensor and has other applications in hyperspectral

imaging, spectrometers, telecommunication and

light sources, etc.

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Tunable

FP filter

detector

Testing

circuit

Voltage

supply

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