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공학석사학위논문

Flexible Silicon-based Photodiode Array

for Periscopic Camera

360도 시야각 카메라를 위한

실리콘 기반의 광 다이오드 집합체

2017년 2월

서울대학교 대학원

화학생물공학부

조 이 형

Flexible Silicon-based Photodiode Array

for Periscopic Camera

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUESTMENTS FOR THE DEGREE OF MASTER IN

ENGINEERING AT THE GRADUATE SCHOOL OF

SEOUL NATIONAL UNIVERSITY

February 2017

By

Eehyung Joh

Supervisor

Dae-Hyeong Kim

i

Abstract

Flexible Silicon-based Photodiode Array

for Periscopic Camera

Eehyung Joh

School of Chemical and Biological Engineering

The Graduate School

Seoul National University

Recently, public interest in virtual reality (VR) has skyrocketed with increases

of a various type of gadgets related to VR and contents for VR. One of the gadgets

leading current VR market is a 360- degree camera that enables capturing pictures

in every direction. This type of camera usually consists of multiple cameras placed

to face several directions or complex optical lens arrangements for wide field of view.

However, those types of configurations may have difficulties in scaling-down due to

their complexity.

To resolve such issues, I developed image sensor array which covers almost

ii

full spherical surface with ultrathin single crystal silicon based photosensitive cell

array. The arrays are fabricated on the planar substrate and transferred using special

designs and transfer printing method to wrap spherical surface. Using a single

crystalline silicon which is a conventional material for optoelectronics and

electronics, photodiode array with high performances and good accessibility is

achieved. Moreover, polymeric encapsulation and deformable ultrathin design of

silicon layer enable the transferring the device on the spherically curved surface

without mechanical breakage. This fabricated device facilitates periscopic imaging

with wide field of view(300o×170 o) which covers almost all directions, paving the

way toward to advanced virtual reality systems.

Keywords: Single crystalline silicon, photodiode array, flexible electronics,

periscopic image sensor.

Student number: 2015-21032

iii

Contents

1. Introduction∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 1

2. Ultra-thin single crystalline silicon-based photosensitive cell∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 3

2.1 Structural characteristics of the photosensitive cell∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 3

2.2 Electrical characteristics of the photosensitive cell∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 6

2.3 Optoelectrical response of the 16 by 16 planar array system∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙10

3. Periscopic image sensor ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙14

3.1 Three dimensional spherical array design∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙14

3.2 Fabrication of spherical array∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙19

3.3 Flexibility test for spherical deformation∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 21

3.4 Optoelectrical response of the periscopic image sensor∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 26

4. Experimental Section∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 31

4.1 Materials for device fabrication ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 31

4.2 Fabrication of the photosensitive cell array ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 31

4.3 Fabrication of 3D spherical structure ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 33

4.4 Experimental equipment ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 33

5. Conclusion∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 34

6. References∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 35

iv

List of Schemes

1. Exploded view of stacked metal interconnection and encapsulation polymers on

a single photosensitive cell∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙5

2. An illustration of the photosensitive array before and after deformation∙∙∙∙∙∙∙∙∙∙∙∙∙17

3. The optical system of the single photosensitive cell on spherical

structure∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙18

4. Fabrication process of the deformable photosensitive cell array for spherically

curved image sensor∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙20

v

List of Figures

1. The optical image of a single photosensitive cell∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙4

2. Current-voltage response of the blocking diode in array system∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙7

3. Current-voltage response of the photodetecting diode in array system∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙8

4. Overall current-voltage response of the photosensitive cell in array system∙∙∙∙∙∙∙∙∙∙9

5. Distribution of photocurrent at complete darkness∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙11

6. Distribution of photocurrent at illumination∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙12

7. Simple pattern recognition of the 16 by 16 planar array system∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙13

8. Optical images of bending test at various bending radius∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙22

9. Flexibility test of the photosensitive cell array∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙23

10. Durability test of the flexible photosensitive cell array∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙24

11. Schematic illustration of three dimensional integration process and an optical

image of the integrated image sensor∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙25

12. Images of illuminated laser spot on the sensor at different incident angle in

azimuthal direction∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙27

13. Images of illuminated laser spot on the sensor at different incident angle in

altitudinal direction∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙28

14. Optoelectrical distribution of the periscopic image sensor at complete darkness

and illumination∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙29

15. Various pattern recognition of the periscopic image sensor∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙30

1

1. Introduction

In recent years, the popularity of virtual reality (VR) has been skyrocketed

due to the advance of various types of gadgets related to VR contents. The most

notable emerging technology for the current VR contents is a 360-degree video. A

360-degree video, also known as an immersive videos or a spherical video, is a video

recording where a view in every direction is recorded at the same time. During

playback the viewer has control of the viewing direction like a panorama, a form of

virtual reality. The essential device for this 360-degree video technology is an

omnidirectional camera or a collection of cameras that can capture images in all

directions. In order to meet this wide view angle, those types of devices typically

consists of multiple camera modules, especially complex optical lens arrays oriented

in many directions1-3. However, those configurations may have difficulties in

scaling-down due to their complexity.

Recently, significant advances in the fabrication of flexible electronics4-8

have been shown some compact devices that have reached fairly wide field of view.

Light-detecting arrays in those types of devices are performed on curvilinear surfaces

such as semi-cylinder10, cylinder11 and hemisphere9, 12. For instance, the array which

is shaped into a convex cylindrically curved imager achieved a 360-degree field of

view in the latitudal direction and the arthropod eye-inspired digital camera with a

2

hemispherical shape captured 160-degree view in the both longitudal and latitudal

directions. Although these miniaturized image sensors have demonstrated fairly wide

field of view, yet there must be integration of multiple devices in order to achieve

periscopic detection.

In this paper, I developed the periscopic image sensor array which can

capture images in almost every directions. The periscopic image sensor is a three

dimensional spherical shaped structure covered with the high performance ultrathin

single crystalline silicon based photosensitive cell array. Single crystalline silicon

has been used as a conventional material in optoelectronics and electronics due to its

excellent properties such as higher carrier mobility and wide light absorption

spectrum13-14. Using single crystalline silicon as a material for the image sensor, high

optoelectrical response is achieved. The photosensitive cell array is fabricated on the

silicon on insulator wafer and transferred on flexible polyimide substrate to achieve

ultrathin feature. In addition, the array is encapsulated with polymer layers.

Therefore, ultrathin structure and polymer encapsulation further reduce risks of

mechanical fractures. Finally, the array is transferred onto spherical structure printed

by the 3D printing machine. The dimensions of the photosensitive device are

determined by analysis in optical aspects for successful imaging. As a result, this

fabricated image sensor enables periscopic imaging with FoV 300o×170 o in the

azimuthal and altitudinal directions respectively. The unique features of this

periscopic image sensor facilitate a broad range of applications, including virtual

reality, surveillance system and medical devices.

3

2. Ultra-thin single crystalline silicon-based

photosensitive cell

2.1 Structural characteristics of the photosensitive cell

Figure 1 shows the design of a single photosensitive cell. This photosensitive

cell is a single-crystalline silicon based device which occupies 210×190 um2 in area

with 1.25 um thickness. The device consists of two types of p-i-n junction diodes - a

photodetecting diode and a blocking diode. A photodetecting diode is an inter-digit

patterned p-i-n junction which contains intrinsic region that absorbs light dominantly

and produces photocurrent. The blocking diode, a p-i-n junction covered with Cr/Au

metal for light cut off, prevents crosstalk between cells and current flow through the

unwanted sneak path in array system. These two diodes are electrically connected to

the other photosensitive cells respectively with 50m wide, 120nm thick

chromium/gold metal lines. Each of the device and metal interconnect layers are

encapsulated with polymer layer (polyimide PI, 1um thick). Metal connections make

contacts on n-doped silicon region through via in both of the diodes. Overall

exploded schematic view of a single photosensitive cell, electrical connections and

encapsulation layers are shown in Scheme1.

4

Figure 1. The optical image of a single photosensitive cell. The red dashed line

outlines the blocking diode region and the blue one outlines the photodetecting

diode.

5

Scheme 1. Exploded view of stacked metal interconnections and encapsulation

polymers on a single photosensitive cell.

6

2.2 Electrical characteristics of the photosensitive cell

In the array system, a blocking diode and a photodetecting diode of the

photosensitive cell show different current-voltage response when light is treated. The

IV curve of a blocking diode is shown in Figure 2. As the intrinsic region of the

diode is shielded by Cr/Au metal, optical exciton generation in the junction is

negligible regardless of light intensity. Thus, very small amount of drift current (less

than few nano amperes) flows when reverse bias (-2 to 0V) is applied in the diode.

In contrast, the photodetecting diode is a light-sensitive device. At forward bias (0 to

2V) condition, increased photocurrent is achieved in the photodiode through the

photon absorption (Figure 3). Figure 4 outlines the overall opto-electrical response

of the photosensitive cell in array system. This behavior can be described as a

combination of the current-voltage responses of the two diodes above. Large

photocurrent at forward bias and the very low reverse bias current are key features

in the IV plot. The forward bias current increases in proportion to the intensity of the

light under the influence of the photodetecting diode. The low reverse bias current

means that crosstalk between cell in the array system is effectively blocked.

7

Figure 2. Current-voltage response of the blocking diode in array system. The

reverse bias current appears to be very low as shown in the inset.

8

Figure 3. Current-voltage response of the photodetecting diode in array system.

The inset shows forward bias current at various optical intensities.

9

Figure 4. Overall current-voltage response of the photosensitive cell in array

system. The forward bias increases in proportion to the incident light power.

10

2.3 Optoelectrical responses of the 16 by 16 planar array

system

Optoelectronic responses of the 16 by 16 photosensitive cell array on a

planar substrate were obtained by a custom-made data acquisition (DAQ) system.

The current response of a unit cell according to the intensity of the light and applied

voltage were measured by contacting the anisotropic conductive film corresponding

to the unit cell coordinates in the array system. Photocurrent distribution of the planar

array system at complete darkness (off state) and illumination (on state) are shown

in Figure 5 and Figure 6 respectively. The histograms on the left of each figures

show the number of unit cells corresponding to a certain resistance. When the light

is blocked, the resistance of each unit cell was measured to be 10 times larger than

that of the case where light was irradiated. The color map on the right of each figures

show the distribution of the photo responses when these measured resistance values

are converted to current. The closer to blue, the less photocurrent, and the closer to

yellow, the higher the photocurrent flow. Comparison of these two color maps

indicates that the 16 by 16 planar array system is very sensitive to light in all parts.

In addition, not only the overall response to light but also the local light, such as

simple letter patterns, could be recognized by this planar array system. Figure 7

shows the distribution of optoelectrical response when the array system is

illuminated with S, N and U patterns.

11

Figure 5. Distribution of photocurrent at complete darkness. The histogram

(left) shows the number of unit cells corresponding to a certain resistance, and

the color map (right) represents the photocurrent level according to the

coordinates in array system.

12

Figure 6. Distribution of photocurrent at illumination.

13

Figure 7. Simple pattern recognition of the 16 by 16 planar array system.

Optoelectronics responses of the array was obtained by a custom-made DAQ

system.

14

3. Periscopic image sensor

3.1 Three dimensional spherical array design

To transform a planar array system into a spherical periscopic image sensor,

the distance between the photosensitive cells must be adjusted considering the

optical system. Scheme 2 describes an illustration of the photosensitive cell array

before (plane) and after (full sphere) deformation. The key for successful imaging in

the periscopic device is preventing overlap of light signals received by adjacent

photosensitive cells on spherical structure. To satisfy this condition, the acceptance

angle should be smaller than the angle between the nearest photosensitive cell and

the center of the sphere. The acceptance angle, Δφ, is the maximum angle at which

incident light can be detected by a photosensitive cell.

The acceptance angle of a single photosensitive cell in the spherical array

system can be determined by various geometrical and optical parameters of the

system. As shown in Scheme 3, θ1 is an angle of incident ray that reaches at the edge

of the photosensitive cell after refraction through the 2.2mm-thick

polydimethylsiloxane (PDMS) media and θ2 is the refraction angle of the ray.

Considering the geometry of the system, the refraction angle, θ2, can be calculated

by the formula

15

𝜃2 = 𝑡𝑎𝑛−1 (𝑑

2𝑓) = 5.2624𝑜

where 𝑑 is the diameter of the photodiode and 𝑓 is the focal length of the microlens.

In the current device, the diameter of the photosensitive cell and the focal length of

microlens were decided as 283.2mm and 2.2mm respectively, due to the optimized

fabrication condition. Therefore, θ2=5.2624o can be obtained by substituting these

values.

The next step to determine the acceptance angle is applying Snell’s law to

get the incident angle θ1. Snell’s law states that the ratio of the sines of the angles of

incidence and refraction is equivalent to the reciprocal of the ratio of the indices of

refraction:

sin𝜃1sin𝜃2

=𝑛𝑃𝐷𝑀𝑆

𝑛𝐴𝑖𝑟

where 𝑛𝐴𝑖𝑟 and 𝑛𝑃𝐷𝑀𝑆 are the refractive index of air and PDMS media respectively.

By rearranging this formula, the incident angle is as follows.

𝜃1 = sin−1 (𝑛𝑃𝐷𝑀𝑆

𝑛𝐴𝑖𝑟sin(𝜃2)) = 5.2698

𝑜

As a result, the acceptance angle, Δφ, can be obtained by doubling the incident angle,

θ1.

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Δφ = 2 𝜃1 =10.540𝑜

Finally, the proper distance between the adjacent photodiode on the sphere can

determined based on this acceptance angle.

𝐿 ≥ 𝑅𝛥𝜑 = 1.8419𝑚𝑚

where L is the distance between the nearest cell in array system and R is the radius

of the sphere.

17

Scheme 2. An illustration of the photosensitive cell array in planar form (left)

and the array on 3 dimensional sphere after deformation (right).

18

Scheme 3. The optical system of the single photosensitive cell on sphere. The

incident ray reaches at the edge of the cell after refraction in interface and

penetration through PDMS media.

19

3.2 Fabrication of spherical array

In order to fabricate the photosensitive array applicable to spherically curved

structure, the rigid substrate must be replaced by flexible polymer film. Plus, to avoid

fracture or mechanical breakage during the three dimensional deforming procedure,

an additional polymeric encapsulation layer is necessary. Therefore, the

photosensitive array was fabricated on a poly imide (PI) layer which was coated on

the SiO2 wafer and covered by PI again for encapsulation. This multi-layered device

subsequently transferred onto 3D printed spherical structure using water-soluble tape.

The overall fabrication process of the deformable photosensitive cell array is shown

in Scheme 4.

20

Scheme 4. Fabrication process of the deformable photosensitive cell array for

spherically curved image sensor.

21

3.3 Flexibility test for spherical deformation

The spherical array was transferred onto 0.15mm-thick PDMS/PET

substrate using water soluble tape for mechanical examination. The photosensitive

device bended with various bending radius (Figure 8). Figure 9 describes that the

optoelectrical response of the array device did not affected by bending-induced

strains owing the neutral mechanical plane design and ultra-thin structure. Moreover,

the device showed the almost same performance as the first at least 1000 bending

cycles, as shown in (Figure 10). Consequently, the photosensitive cell array could

operate suitably at deformed conditions. Figure 11 shows the integration of the

photosensitive cell array on 3D spherical structure.

22

Figure 8. Optical images of bending test at various bending radius.

23

Figure 9. Flexibility test at various bending radius. (a) IV curve of the

photosensitive cell array (b) On/off current at 1V.

24

Figure 10. Durability test of the flexible photosensitive cell array.

25

Figure 11. Schematic illustration of 3D integration process (left) and an optical

image of the integrated image sensor (right).

26

3.4 Optoelectrical response of the periscopic image sensor

The light sensitivity of the integrated spherical image sensor was measured

by varying the incident angle of the laser in two directions, altitudinal and azimuthal.

The angle of incidence were changed in 20 o and 10.6o increments respectively. When

the light is applied, the part of the image sensor that reacts varies depending on the

direction, as shown in Figure 12 and 13. Overall optoelectronic responses of the

image sensor were obtained by the DAQ system in the same way as when measuring

the planar arrays (chapter 2.3). Photocurrent distribution of the sensor at complete

darkness (off state) and illumination (on state) are shown in Figure 14. As can be

seen from this photocurrent distribution, this periscopic image sensor has 300 o FoV

in azimuthal and 170 o FoV in altitudinal direction. Additionally, the histograms in

the Figure 14 show the number of unit cells in the spherical array system

corresponding to a certain current level. As shown in Figure 15, various pattern

signal from wide incident angle could be recognized by the spherical array system.

27

Figure 12. Images of illuminated laser spot on the sensor at different incident

angle in azimuthal direction.

28

Figure 13. Images of illuminated laser spot on the sensor at different incident

angle in altitudinal direction.

29

Figure 14. Optoelectrical response of the periscopic image sensor at complete

darkness and illumination. The histograms (left) indicate the number of unit

cells in the spherical array system corresponding to a certain current level and

3D colormaps (right) show the photocurrent distribution at each circumstances.

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Figure 15. Various pattern recognition of the periscopic image sensor

31

4. Experimental section

4.1 Materials for device fabrication

1.25mm (100) top silicon p-type silicon-on-insulator (SOI) from Soitec

(France) was used for the photosensitive cell array. Spin-on-diffusant (SOD) P509

and B153 from Filmtronics (USA) were used for n-doping and p-doping respectively.

SiO2 wafer was purchased from 4Science (Korea) and made from LG Siltron (Korea).

Thermal evaporation source of Cr (adhesion layer) and Au were purchased from

Taewon Scientific Co., LTd(Korea). Poly (pyromellitic dianhydride-co-4m4’-

oxydianiline)amic acid solution (polyimide, PI, electronic grade, Sigma-Aldrich),

polydimethylsiloxane base and curing agent (PDMS, sylgard 184, Dow corning),

positive photoresist S1805, AZ5214, AZ4620 (AZ electronics Materials) were used

for the device fabrication.

4.2 Fabrication of the photosensitive cell array

The first step to fabricate the photosensitive cell array is n-type doping of

silicon-on-insulator (SOI) wafer with a phosphorous spin-on-diffusant (SOD). For

doping mask, 4000Å of SiO2 layer was deposited on pre-cleaned 1.25mm SOI wafer

using plasma enhanced chemical vapor deposition (PECVD). Photoresist (PR;

32

AZ5214) was spin-coated (3000rpm, 30 s) on the hexamethyldisilazane (HMDS)

pretreated SiO2 layer and patterned with UV optical lithography through chrome

mask. Patterned PR was developed by aqueous base developer (AZ 300 MF). The

PR developed SiO2 layer was subsequently etched with buffered oxide etcher (BOE;

6:1). PR was removed by acetone rinsing, followed by piranha treatment for

3minutes. Phosphorus SOD was coated on the etched SiO2 layer and annealed at

200 °C for 15 minutes. Then, the annealed wafer was put into 975°C furnace for 20

minutes to diffuse the dopant. Finally, the n-doped SOI wafer was treated with HF

and piranha solution alternately for removal of the SiO2 doping mask. Similar to

above procedure, the p-type doping process was carried out using a boron SOD

instead of the phosphorous SOD.

After the doping process, the doped n-p-n diode array were transfer printed

onto polyimide (PI) film coated on a silicon oxide (SiO2) wafer using

polydimethylsiloxane (PDMS) elastomeric stamp. Using reactive ion etching (RIE

(SF6 plasma)) with photolithography, the photosensitive cell array were additionally

patterned for silicon isolation. Then, PI layer was spin-coated and cured at 250 °C

for 1 hour, followed by patterning using RIE (O2 plasma) for via. Thermal

evaporation for metallization (Au/Cr, 100 nm/8 nm), photolithography and wet-

etching steps defined the interconnected metal lines. Subsequently, the second PI via

layer and second Au/Cr metal connections were deposited with aforementioned

method. Lastly, top PI layer was spin coated and the entire layer was patterned by

33

RIE (O2 plasma). The whole device was then transferred onto 3D printed structure

using a thermal-release tape, followed by removing the tape through immersion in

DI water.

4.3 Fabrication of the 3D sphere structure

The spherical structure for periscopic image sensor is fabricated by 3D

printing called PolyJet. This 3D printing works similarly to inkjet printing, but

instead of jetting drops of ink onto paper, the 3D Printers jet layers of curable liquid

photopolymer onto a build tray. This process consists of three steps: Pre-processing,

production and support removal. First, build-preparation software automatically

calculates the placement of photopolymers and support material from a 3D CAD file.

Subsequently the 3D printer jets and instantly UV-cures tiny droplets of liquid

photopolymer. Fine layers accumulate on the build tray to create one or several

precise 3D models or parts. Finally, support material is removed by hand with water.

4.4 Experimental equipment

Electrical properties of the photosensitive cell array were measured with

semiconductor device parameter analyzer B1500A (Agilent, USA). Optoelectronics

responses of the 16 X 16 planar array and the periscopic image sensor were operated

by a custom-made DAQ system.

34

5. Conclusion

In this paper, the single crystalline silicon-based periscopic image sensor is

shown. This device is three dimensional sphere-shaped sensor which has a 300o ×

170o field of view. Single crystalline silicon-based photosensitive cell array was

fabricated considering optical aspects analysis. Ultrathin design and polymeric

encapsulation enabled the deformation and transfer process of the array on the

spherically curved structure without a mechanical breakage. As a result, this

fabricated image sensor achieved periscopic imaging with FoV 300o×170o. High

optoelectrical response and successful pattern recognition by the device are

performed. This periscopic image sensor is able to capture images in almost all

direction, facilitating a broad range of applications, including virtual reality,

surveillance system and medical devices.

35

6. References

1. Brady, D. J. et al. Multiscale gigapixel photography. Nature 486, 386-389

(2012)

2. Yoon, J. et al. Heterogeneously integrated optoelectronic devices enable by

micro-transfer printing. Adv. Opt. Mater. 3, 1313-1335 (2015)

3. Rim, S.-B. et al. The optical advantages of curved focal plane arrays. Opt.

Express 16, 4965 (2008)

4. Kim, D.-H. et al. Epidermal electronics. Science 333, 838-843, (2011).

5. Sekitani, T. et al. Organic nonvolatile memory transistors for flexible

sensor arrays. Science 326, 1516-1519 (2009).

6. Son, D. et al. Multifunctional wearable devices for diagnosis and therapy

of movement disorders. Nature Nanotech. 9, 397-404 (2014).

7. Ying, M. et al. Silicon nanomembranes for fingertip electronics.

Nanotechnology 23, 344004 (2012).

8. Wong, W. et al. Flexible electronics: materials and applications. (2009)

9. Ko, H. C. et al. A hemispherical electronic eye camera based on

compressible silicon optoelectronics. Nature 454, 748-753, (2008)

10. Floreano, D. et al. Miniature curved artificial compound eyes. PNAS 110,

9267–9272 (2013)

11. Fan, D. et al. Flexible thin-film InGaAs photodiode focal plane array. ACS

photon. 3, 670-676, (2016)

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12. Song, Y. M. et al. Digital cameras with designs inspired by the arthropod

eye. Nature 497, 95-99, (2013)

13. Green, M. A. et al. Opitical properties of intrinsic silicon at 300K. Progress

in Photovoltaics 3, 182-192 (1995)

14. Arora, N. D. et al. Electron and hole mobility in silicon as a function of

concentration and temperature IEEE 2, 292-295 (1982)

37

요약 (국문초록)

360도 시야각 카메라를 위한 실리콘 기반의

광 다이오드 집합체

서울대학교 공과대학원

화학생물공학부

조이형

최근 들어 가상현실에 대한 대중의 관심이 증가함에 따라 가상현실

콘텐트를 위한 다양한 형태의 전자 기기들이 활발하게 개발되고 있다. 현재

의 가상현실 콘텐트 시장을 선도하는 기기 중 하나는 모든 방향의 이미지를

담을 수 있는 360도 카메라다. 현재 상용화 된 360도 시야각을 갖는 전자기

기는 일반적으로 여러 개의 카메라가 다양한 방향을 향하도록 배치된 형태

로, 매우 복잡한 광학 렌즈 배열로 구성되어있다. 이러한 구성 방식은 그 구

조적 복잡성으로 인해 기기의 규모를 축소하는 과정에서 많은 어려움이 존

재한다.

본 논문에서는 이러한 문제를 해결하기 위해 매우 얇은 단결정 실리

콘 기반의 광 다이오드 집합체를 3차원 구 표면 위에 배열시켜 모든 방향의

38

시야각을 갖는 소형 이미지 센서를 개발했다. 오래 전부터 광전자공학 및 전

자공학에서 전통적인 재료로 쓰여온 단결정 실리콘을 사용해 높은 빛 감광

성을 갖는 광 다이오드 집합체를 제작했다. 다이오드 집합체를 고분자 층으

로 보호함과 동시에 변형 가능한 매우 얇은 디자인으로 제작한 후 전사기법

을 통해 기계적 파손 없이 3차원 곡면 상에 성공적으로 배열시켰다. 제작된

실리콘 기반의 소형 이미지 센서는 300도 × 170도의 시야각을 갖는다. 이

러한 이미지 센서는 모든 방향의 이미지를 획득하는 고유한 장점을 통해 가

상현실뿐만 아니라 군용 감지기 및 의료기기 분야에 폭넓게 응용될 수 있다.

주요어: 단결정 실리콘, 광 다이오드 집합체, 플렉서블 일렉트로닉스, 광 시야

각 카메라.

학번: 2015-21032

39