Micro-LEDs (μLEDs) have advantages in terms of brightness, power consumption, and response speed. In addition, they can also be used as micro-sensors implanted in the body via flexible electronic skin. One of the key techniques involved in the fabrication of μLED-based devices is transfer printing. Although numerous methods have been proposed for transfer printing, improving the yield of μLED arrays is still a formidable task. In this paper, we propose a novel method for improving the yield of μLED arrays transferred by the stamping method, using an innovative design of piezoelectrically driven asymmetric micro-gripper. Traditional grippers are too large to manipulate μLEDs, and therefore two micro-sized cantilevers are added at the gripper tips. A μLED manipulation system is constructed based on the micro-gripper together with a three-dimensional positioning system. Experimental results using this system show that it can be used successfully to manipulate μLED arrays.

HIGHLIGHTS

  • A piezoelectrically driven asymmetric micro-gripper is proposed.

  • A μLED manipulation system is constructed based on the micro-gripper together with a three-dimensional positioning system.

  • A single μLED array is manipulated by the proposed micro-gripper-based system.

In recent years, Apple, Samsung, Sony, and other companies have initiated research into micro-LEDs (μLED),1,2 which represent a promising new approach in the development of display technology, particularly with regard to displays in smart devices.3,4 μLED technology is based on miniaturization and matrixing, with high-density and micro-sized LED arrays being integrated on a chip, with a pixel size generally less than 100 μm. Images of an μLED array as observed with a field emission scanning electron microscope (SWM, Zeiss Sigma 300) are shown in Figs. 1(a) and 1(b), and the corresponding surface texture obtained using white light interferometry is shown in Figs. 1(c) and 1(d).

FIG. 1.

(a) and (b) SEM images of an array of μLEDs with magnifications of 500× and 1500×, respectively. (c) and (d) Surface texture of μLED array captured by a white light interferometer.

FIG. 1.

(a) and (b) SEM images of an array of μLEDs with magnifications of 500× and 1500×, respectively. (c) and (d) Surface texture of μLED array captured by a white light interferometer.

Close modal

The main advantage of μLEDs is that each pixel cell can be individually controlled and driven. In terms of brightness, contrast ratio, power dissipation, resolution, and other parameters, they are superior to conventional LEDs while meeting standard requirement with their characteristic micrometer-scale pixel spacing. μLEDs provide brighter illumination than competing technologies, even under low-power conditions. Specifically, for a given brightness, they consume 90% less power than LCDs and 50% less than OLEDs. Accordingly, if smart phones were to use μLED displays, their battery life would be greatly improved—a critical requirement for portable devices. Furthermore, the response time of μLEDs is in the range of nanoseconds. This quick response is faster than those of LCDs (in the millisecond range) and OLED (in the microsecond range), making μLEDs the best choice for virtual reality (VR) applications of 5G technology.2 In another words, μLEDs are at the forefront as technology enablers for display applications in the 5G era.3,4

To transfer μLEDs, the most commonly used techniques are the stamp method,5–14 the roller method,15–18 the laser method,19–22 the electrostatic method,23–27 the magnetoelectric method,28 and the fluid method.29–31 Among these, the most commonly used is the stamp method (also known as transfer printing), in which the μLEDs comprising the array are picked up from the donor substrate on which the μLEDs were formed and transferred to the receiving substrate in parallel using an elastomeric stamp with a microstructured surface. Park et al.5 have proposed the use of such stamps made of polydimethylsiloxane (PDMS)-based materials. In this transfer method, the adherence and release of the μLEDs is achieved by controlling the speed of the stamp. When the stamp moves rapidly, the μLEDs adhere to the stamp owing to van der Waals forces. When the stamp moves slowly, the μLEDs detach from it and adhere instead to the receiving substrate. The Korea Institute of Machining and Materials has proposed a transfer technique based on the use of barrel seals,15–19 with which it is possible to achieve transfer speeds reaching 10 000 μLED crystals per second.

However, differences between the roughness of the stamp surface and that of the donor substrate will lead to a large mismatch error, as shown in Fig. 2. Local temperature inhomogeneities could also contribute to mismatch error. The thermal coefficient of PDMS is about 340 ppm/°C, while the thermal coefficient of the donor substrate (Si) is about 5–9 ppm/°C. There would be a mismatch error of 33 μm between stamp and μLED array for a stamp of side length 100 mm if the ambient temperature changed by 1 °C. Such errors contribute to the low yield rate of stamp transfer printing. In view of this low yield rate, we propose here a micro-gripper32 to manipulate μLED arrays and thereby improve the yield of micro-stamp transfer.

FIG. 2.

Mismatch error associated with the stamp method.

FIG. 2.

Mismatch error associated with the stamp method.

Close modal

The micro-gripper is aimed at fixing mismatch errors associated with the stamp method in mass production. It should be possible for a μLED to be picked up and released with highly accurate guidance of the positioning system. However, it is hard for traditional methods to obtain high accuracy in positioning the μLED and the receiving substrate. In a previous study,33 we proposed a highly precise method for depth measurements using a 3D microscope system that can be used to determine the positions of both the μLED and the receiving substrate. Owing to the small size of μLED crystals, it is necessary to employ specially designed mechanical grippers to handle them. Mechanical clamping has an advantage over other clamping methods in that it can be used in conjunction with a microscope camera for determining the position of the μLED. Figure 3(a) shows a piezoelectrically driven asymmetric micro-gripper.34–36 However, on its own, such a gripper is too large to manipulate μLEDs. Therefore, to meet the needs of μLED crystal clamping and transfer, two micro-sized cantilevers, shown in Fig. 3(b), are added at the tips of the gripper as shown in Fig. 3(c).

FIG. 3.

Schematic of a gripper-based μLED manipulation system.

FIG. 3.

Schematic of a gripper-based μLED manipulation system.

Close modal

μLEDs have already been cleaved from their donor substrate before they are transferred. The micro-gripper only needs to overcome the adhesion between μLED and substrate, which is about 2 μN. The micro-gripper can be used to carry out micromanipulations such as clamping, transfer, and release. Previous work on micro-gripper has been reviewed in Refs. 37 and 38.

The structure of the proposed gripper based on the asymmetric clamping mode is shown in Fig. 4(a). The clamping arm on one side is attached rigidly to the frame of the gripper. The clamping arm on the other side is connected to the lever mechanism via the output end of the gripper. This structure ensures stability during clamping and avoids instability caused by manufacturing error. At the same time, it makes operation of the gripper easier. In practical operation, only a piezoelectric drive is needed to control the displacement of the clamping arm to grip the object.

FIG. 4.

Micro-gripper: (a) diagram of gripper; (b) photograph of gripper; (c) finite element simulation model of gripper for an input displacement of 2 μm.

FIG. 4.

Micro-gripper: (a) diagram of gripper; (b) photograph of gripper; (c) finite element simulation model of gripper for an input displacement of 2 μm.

Close modal
The movable clamping arm of the micro-gripper moves in a translational mode. The two clamping arms are always in a parallel alignment and the Y displacement is approximately zero. This ensures that when the gripper clamps an object, even if there is interference by external forces, the clamped object will not slip off. In addition, this gripper uses a parallelogram mechanism as an amplifying mechanism for the final stage. When the angle of rotation of the parallelogram mechanism is small, the displacement generated by the system in the horizontal direction is much greater than that generated in the vertical direction. The relationship between the vertical displacement y and the angle of rotation θ is as follows:
y=l(1cosθ),
(1)
where l is the length of the moving part of the parallelogram mechanism. It can be seen from Eq. (1) that when the angle of rotation is extremely small, and so cos θ is close to 1, the vertical displacement is close to zero. The parallelogram mechanism is connected directly to the output end of the micro-gripper, which ensures parallel movement of the clamping end of the micro-gripper.

This micro-gripper is driven by stacked of piezoelectric ceramics where the output displacement of the gripper depends on the input displacement of the piezoelectric ceramics. We simulated the displacement amplification ratio of different depths by finite element method as shown in Fig. 4(c).

To simulate the output of the gripper, the input displacements are set at positions on the piezoelectric ceramic of 2 μm, 4 μm, 6 μm, and 8 μm. The X and Y displacements of the movable clamping arm of the micro-gripper under different inputs are recorded. The results show that the displacement in the Y direction is much less than that in the X direction. The displacement amplification is the ratio of output displacement to input displacement and for this gripper is about 10.2. The data from the simulation are shown in Table I.

TABLE I.

Displacement amplification of gripper according to simulation.

Input displacement (μm)X (μm)Y (μm)Y/XDisplacement amplification ratio
20.438 0.183 0.008 95 10.219 
40.876 0.367 0.008 99 10.219 
61.313 0.551 0.008 97 10.219 
81.751 0.734 0.008 98 10.219 
10 102.200 0.918 0.008 98 10.220 
Input displacement (μm)X (μm)Y (μm)Y/XDisplacement amplification ratio
20.438 0.183 0.008 95 10.219 
40.876 0.367 0.008 99 10.219 
61.313 0.551 0.008 97 10.219 
81.751 0.734 0.008 98 10.219 
10 102.200 0.918 0.008 98 10.220 

The selection of the size of the cantilever beam is extremely important. The stress introduced by the cantilever when the μLED crystal is clamped needs to be assessed to ensure that the clamping end will not damage the μLED during the clamping process. According to Euler’s formula, the thickness of the cantilever has the greatest influence on the magnitude of the stress introduced by the micro-gripper. Therefore, the finite element method is used to simulate the stress conditions of cantilevers of different thickness when they hold and transfer μLED crystals.

Considering the size of the μLED crystals, the length of the cantilever is set at 100 μm and the width at 30 μm. Cantilevers with thicknesses of 0.5 μm, 1.0 μm, and 1.5 μm are selected. The maximum stress at the movable end when clamping is simulated under different input displacements at the cantilever tip. Thereby, the working ranges of cantilevers with different thicknesses are determined. The simulation results are shown in Fig. 5.

FIG. 5.

Simulated maximum stress at the tip of the gripper vs deflection.

FIG. 5.

Simulated maximum stress at the tip of the gripper vs deflection.

Close modal

Both the cantilever and μLED are fabricated from silicon, which has a yield strength of 503 MPa. To prevent damage to the μLED and cantilever, the maximum clamping force is set at one-tenth of this yield strength, i.e., 50.3 MPa. The simulation results show that for cantilevers of thicknesses 0.5 μm and 1 μm, the maximum stress at the tip remains below the yield strength throughout the entire working range of 10 μm. Thus, even in extreme cases, neither the clamping end of the micro-gripper nor the μLED will be damaged by excessive stress during the manipulation procedure. The proposed method is robust enough to be used repeatedly.

For a cantilever of thickness of 1.5 μm, when the input displacement is 8 μm, the maximum stress will exceed the yield strength and thus cause damage. Although this cantilever would be damaged in the extreme case where the μLED crystal was immovable, irreversible damage would only occur when the input displacement reached 7.5 μm. By then, the micro-gripper would have covered three-quarters of the working range. Also, in actual operation, the driving voltage applied to the piezoelectric ceramic end of the micro-gripper increases slowly from low to high. Therefore, it can be considered that a cantilever of thickness of 1.5 μm performs well under extreme conditions and will not cause any damage. Thus, the above analysis indicates that the ideal thickness of the cantilever of the micro-gripper should be 1.5 μm.

To investigate the performance characteristics of the micro-gripper, the voltages at both ends of the piezoelectric ceramic are applied from 0 to 120 V in steps of 10 V. The input voltage is converted into the corresponding input displacement of the piezoelectric ceramic. The relationship between the voltages and the displacement has been calibrated by the manufacturer. At the same time, the output displacement of the micro-gripper is measured by a laser interferometer. The ratio of the output displacement of the clamping arm to the input displacement of the piezoelectric ceramic is the amplification factor of the gripper. The data are shown in Table II.

The experimental results show that the displacement amplification ratio of the micro-gripper is about 7.5.

TABLE II.

Displacement amplification ratio of micro-gripper.

Input voltage (V)Input displacement (μm)Output displacement (μm)
20 2.67 21.31 
40 5.33 42.78 
60 8.00 59.38 
80 10.67 80.23 
100 13.33 94.86 
120 16.0 112.63 
Input voltage (V)Input displacement (μm)Output displacement (μm)
20 2.67 21.31 
40 5.33 42.78 
60 8.00 59.38 
80 10.67 80.23 
100 13.33 94.86 
120 16.0 112.63 

The setup of the clamping experiment is shown in Fig. 6(a) and consists of three parts: the micro-gripper, a 3D microscope system for depth measurement, and an adjustable displacement table.

FIG. 6.

Experimental manipulation of a μLED by the micro-gripper. (a) Experimental setup. (b) Manipulation of a single μLED by the micro-gripper.

FIG. 6.

Experimental manipulation of a μLED by the micro-gripper. (a) Experimental setup. (b) Manipulation of a single μLED by the micro-gripper.

Close modal

The procedure for manipulation of a single μLED using the micro-gripper consists of the following steps.

  • Step 1. Depth measurements are performed using the 3D microscope system, and the images obtained while recording the clamping process are saved in real time. The accuracy of positioning can reach 1 μm. The advantages of the micro-gripper is that it can be used in conjunction with visual methods to achieve high-precision position control.

  • Step 2. After positioning has been performed using the visual system, the micro-gripper performs its clamping function and manipulates the μLED under control by the piezoelectric system.

  • Step 3. The displacement table and the visual positioning system are used together to transfer the micro-gripper to its target position.

  • Step 4. The micro-gripper releases the μLED under control by the piezoelectric system.

To investigate the performance of the micromanipulation system, the experimental setup shown in Fig. 6(a) was used to manipulate a μLED of length of 40 μm, width 20 μm, and thickness 4 μm as shown in Fig. 6(b). It can be seen that the micro-gripper was able to achieve a stable clamping state. Meanwhile, it was found that the difference in Y position of the two clamping arms before and after clamping was very small, thus enabling parallel clamping. It should be noted that the micro-gripper used in this study was designed with one clamping arm fixed and the other movable. The fixed clamping arm was used as the position reference in the experiment, which greatly reduced the positioning time and improved the experimental efficiency.

Current μLED handling and transfer methods are limited by their low yield. In this paper, a micro-gripper has been designed to manipulate and transfer μLED arrays with improved yield. The next step should be to investigate the application of this micro-gripper on an industrial scale, for example by developing a new method for repairing μLED arrays taking advantage of the inherent low speed of the micro-gripper-based manipulation system.

This research received financial support from the Scientific Research Program of the Tianjin Education Commission (No. 2019ZD08).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
Zheng
Y
,
Song
L
,
Huang
JX
, et al
Detection of the three-dimensional trajectory of an object based on a curved bionic compound eye
.
Opt Lett
2019
;
44
(
17
):
4143
4146
.
2.
Jeon
CW
,
Choi
HW
,
Dawson
MD
.
Fabrication of matrix-addressable InGaN-based microdisplays of high array density
.
IEEE Photonics Technol Lett
2003
;
15
:
1516
1518
.
3.
Liu
Z
,
Wong
KM
,
Chong
WC
,
Lau
KM
.
P‐34: Active matrix programmable monolithic light emitting diodes on silicon (LEDoS) displays
.
SID Symp Dig Tech Papers
2011
;
42
:
1215
1218
.
4.
Kamarei
ZB
.
Analysis for science librarians of the 2014 Nobel Prize in physics: Invention of efficient blue-light-emitting diodes
.
Sci Technol Libr
2015
;
34
:
19
31
.
5.
Park
SI
,
Xiong
Y
,
Kim
RH
, et al
Printed assemblies of inorganic light-emitting diodes for deformable and semitransparent displays
.
Science
2009
;
325
:
977
981
.
6.
Cok
RS
,
Meitl
M
,
Rotzoll
R
, et al
Inorganic light-emitting diode displays using micro-transfer printing
.
J Soc Inf Disp
2017
;
25
:
589
609
.
7.
Kim
TI
,
Jung
YH
,
Song
J
, et al
High-efficiency, microscale GaN light-emitting diodes and their thermal properties on unusual substrates
.
Small
2012
;
8
:
1643
1649
.
8.
Carlson
A
,
Bowen
AM
,
Huang
Y
, et al
Transfer printing techniques for materials assembly and micro/nanodevice fabrication
.
Adv Mater
2012
;
24
:
5284
5318
.
9.
Kim
S
,
Wu
J
,
Carlson
A
, et al
Microstructured elastomeric surfaces with reversible adhesion and examples of their use in deterministic assembly by transfer printing
.
Proc Natl Acad Sci U S A
2010
;
107
:
17095
17100
.
10.
Hu
HH
,
Chang
KKC
,
Bibl
A
.
Micro device with stabilization post
: U.S. Patent 9209348.
12/08/2015
.
11.
Trindade
AJ
,
Guilhabert
B
,
Xie
EY
, et al
Heterogeneous integration of gallium nitride light-emitting diodes on diamond and silica by transfer printing
.
Opt Express
2015
;
23
:
9329
9338
.
12.
Meitl
MA
,
Zhu
ZT
,
Kumar
V
, et al
Transfer printing by kinetic control of adhesion to an elastomeric stamp
.
Nat Mater
2006
;
5
:
33
38
.
13.
Feng
X
,
Meitl
MA
,
Bowen
AM
, et al
Competing fracture in kinetically controlled transfer printing
.
Langmuir
2007
;
23
:
12555
12560
.
14.
Huang
YY
,
Zhou
W
,
Hsia
KJ
, et al
Stamp collapse in soft lithography
.
Langmuir
2005
;
21
:
8058
8068
.
15.
Bae
S
,
Kim
H
,
Lee
Y
, et al
Roll-to-roll production of 30-inch graphene films for transparent electrodes
.
Nat Nanotechnol
2010
;
5
:
574
.
16.
Lee
MH
,
Lim
N
,
Ruebusch
DJ
, et al
Roll-to-roll anodization and etching of aluminum foils for high-throughput surface nanotexturing
.
Nano Lett
2011
;
11
:
3425
3430
.
17.
Tavares
L
,
Hansen
JK
,
Rubahn
HG
.
Efficient roll-on transfer technique for well-aligned organic nanofibers
.
Small
2011
;
7
:
2460
2463
.
18.
Yang
Y
,
Hwang
Y
,
Cho
HA
, et al
Arrays of silicon micro/nanostructures formed in suspended configurations for deterministic assembly using flat and roller-type stamps
.
Small
2011
;
7
:
484
491
.
19.
Marinov
V
,
Swenson
O
,
Miller
R
, et al
Laser-enabled advanced packaging of ultrathin bare dice in flexible substrates
.
IEEE Trans Compon, Packag Manuf Technol
2011
;
2
:
569
577
.
20.
Miller
R
,
Marinov
V
,
Swenson
O
, et al
Noncontact selective laser-assisted placement of thinned semiconductor dice
.
IEEE Trans Compon, Packag Manuf Technol
2012
;
2
:
971
978
.
21.
Tomoda
K
.
Method of transferring a device AMD method of manufacturing a display apparatus
: U.S. Patent 12/647826.
07/29/2010
.
22.
Bibl
A
,
Higginson
JA
,
Law
HFS
,
Hu
HH
.
Method of transferring a micro device
: U.S. Patent 8333860.
12/18/2012
.
23.
Bibl
A
,
Higginson
JA
,
Law
HFS
,
Hu
HH
.
Micro-LED transfer head heater assembly and method of transferring a micro-LED
: U.S. Patent 8349116.
01/08/2013
.
24.
Bibl
A
,
Golda
D
.
Micro pick up array with compliant contact
: U.S. Patent 9136161.
09/15/2015
.
25.
Hu
HH
,
Bibl
A
.
Stabilization structure including sacrificial release layer and staging cavity
: U.S. Patent 9166114.
10/20/2015
.
26.
Sakariya
KV
,
Bibl
A
,
Hu
HH
.
Active matrix display panel with ground tie lines
: U.S. Patent 9559142.
01/31/2017
.
27.
Bibl
A
,
Golda
D
.
Compliant micro device transfer head with integrated electrode leads
: U.S. Patent 8791530.
07/29/2014
.
28.
Chen
LY
,
Lee
HW
.
Method for transferring semiconductor structure
: U.S. Patent 9722134.
08/01/2017
.
29.
Jacobs
HO
.
Method of self-assembly on a surface
: U.S. Patent 7774929.
08/17/2010
.
30.
Jacobsen
JJ
,
Gengel
GW
,
Craig
GSW
.
Methods for fabricating a multiple modular assembly
: U.S. Patent 6316278.
11/13/2001
.
31.
Schuele
PJ
,
Sasaki
K
,
Ulmer
K
,
Lee
JJ
.
Display with surface mount emissive elements
: U.S. Patent 15/410001.
05/11/2017
.
32.
Liang
CM
,
Wang
FJ
,
Shi
BC
, et al
Design and control of a novel asymmetrical piezoelectric actuated microgripper for micromanipulation
.
Sens Actuators A
2018
;
269
:
227
237
.
33.
Bai
J
,
Niu
PJ
,
Cao
SN
.
Highly precise measurement of depth for μLED based on single camera
.
Measurement
2022
;
189
:
110439
.
34.
Iwatsuki
N
,
Kosaki
T
.
Large deformation analysis and synthesis of elastic closed-loop mechanism made of a certain spring wire described by free curves
.
Chin J Mech Eng
2015
;
28
(
4
):
756
762
.
35.
Chen
Z
.
Design and experimental study of compliant structural force cell based on digital image correlation
.
J Mech Eng
2013
;
49
(
9
):
12
.
36.
Chu
JK
,
Hao
XC
,
Wang
LD
.
Design and fabrication of electrothermal nickel microgripper
.
Chin J Mech Eng
2007
;
43
(
5
):
116
121
.
37.
Sun
XT
,
Chen
WH
,
Tian
YL
, et al
A novel flexure-based microgripper with double amplification mechanisms for micro/nano manipulation
.
Rev Sci Instrum
2013
;
84
(
8
):
085002
.
38.
Zheng
YL
,
Song
L
,
Hu
G
, et al
Improving environmental noise suppression for micronewton force sensing based on electrostatic by injecting air damping
.
Rev Sci Instrum
2014
;
85
(
5
):
055002
.

Jie Bai, born in 1986, is currently a Ph.D. candidate at School of Mechanical Engineering, Tiangong University, China. She received her bachelor degree from Ningxia University, China, in 2019. Her current research focuses on micro LED transfer and application.

Pingjuan Niu, born in 1973, is currently a professor at Tiangong University, China. She received her Ph.D. degree from Tianjin University, China, in 2002. Her current research interests include new semiconductor optoelectronic devices and integration, power electronic device technology, intelligent sensors, and system integration.

Erdan Gu, born in 1956, is currently a professor at the University of Strathclyde, United Kingdom. He received his Ph.D. degree from the University of Aberdeen, United Kingdom, in 1992. He is a Distinguished Professor of Changjiang Scholars. His current research interests include micro/nanophotonics and optoelectronic materials and devices.

Jianming Li is an engineer at the Tianjin Institute of Aerospace Mechanical and Electrical Equipment. His research interests include microgravity and hypogravity environmental simulation technology.

Clarence-Augustine-TH Tee is currently a professor at Zhejiang Normal University. He received his Ph.D. degree from the University of Cambridge, United Kingdom, in 2000. His current research interests are cross-disciplinary and involve deep learning, nanotechnology/nanoscience, and photonics (including plasmonics/surface optics).