Fabrication of Microlens Array and Its Application: A Review
Yuan et al. Chin. J. Mech. Eng.
Fabrication of Microlens Array and Its Application: A Review
Wei Yuan 0
Li‑Hua Li 0
Wing‑Bun Lee 0
ChangY‑uen Chan 0
0 Shenzhen Branch of State Key Laboratory of Ultra‐precision Machining Technology, PolyU Shenzhen Research Institute , Shenzhen 518000 , China
Microlens arrays are the key component in the next generation of 3D imaging system, for it exhibits some good optical properties such as extremely large field of view angles, low aberration and distortion, high temporal resolution and infinite depth of field. Although many fabrication methods or processes are proposed for manufacturing such precision component, however, those methods still need to be improved. In this review, those fabrication methods are categorized into direct and indirect method and compared in detail. Two main challenges in manufacturing microlens array are identified: how to obtain a microlens array with good uniformity in a large area and how to produce the microlens array on a curved surface? In order to effectively achieve control of the geometry of a microlens, indirect methods involving the use of 3D molds and replication technologies are suggested. Further development of ultraprecision machining technology is needed to reduce the surface fluctuation by considering the dynamics of machine tool in tool path planning. Finally, the challenges and opportunities of manufacturing microlens array in industry and academic research are discussed and several principle conclusions are drawn.
Microlens array; Ultraprecision machining; 3D image system; MEMS
Natural compound eyes are extensively prominent in
the biological optical systems of many diurnal insects
or deep-water crustaceans, and such eyes consist of a
mosaic of hexagonal ommatidia that work as tiny
optical units [
]. Unlike single aperture eyes, natural
compound eyes are characterized as having extremely large
field of view angles, low aberration and distortion, high
temporal resolution and infinite depth of field [
However, the compound eye image system has intrinsic
low resolution and sensitivity [
]. The image resolution is
subject to both the number and size of the ommatidia. If
the image resolution of compound eyes increases to the
same level as the human aperture eye, the radius of the
overall lens would be at least 1 meter [
Although the image resolution and sensitivity of
compound eyes are relatively low, microlens arrays, the
artificial counterpart of natural compound eyes, still have
crucial potential in a variety of applications in image
systems, under the condition that high-resolution is not
always required. For example, microlens array are more
suitable in the extremely miniaturized imaging systems
and 3D light field cameras [
]. In addition, the high
quality microlens arrays were applied in color imaging
systems, 3D image acquisition systems and fingerprint
identification systems [
Since the 1980s, the fabrication of microlens array is
realized by different methods, such as
Micro-electromechanical Systems (MEMS) based technologies [
and ultraprecision machining technologies [
However, little work has been focused on the comparison
of these method in terms of the surface finish, form error
and the efficiency of production. One of the major
challenge in the fabrication of the microlens array is the
fabrication and assembly accuracy in a large area [
the image resolution of a compound eye optical system is
increased with the number of microlens and the radius
of each microlens unit, enlarging the overall size of a
microlens array can make up the deficiency. However, to
achieve the required uniformity in a large area is very
difficult . Another challenge for microlens fabrication is
producing microlens array on a flexible layer or a curved
surface. The curved artificial compound eye is similar to
the eye of the fruit fly Drosophila, which is more
compatible and has a larger Field of View (FOV) [
curved compound eye imaging systems may have great
potential in terrestrial aerial vehicles, visual reality
systems, surveillance etc. Image detectors, such as
conventional complementary metal-oxide-semiconductor
(COMS) and charge-coupled device (CCD), are planar
and not suitable for curved image systems. Recent
] in flexible technologies enable the
formation of microlens arrays on flexible substrates which are
bent to a spherical surface. Similar to the problem in the
fabrication of planar compound eye, the requirement of
precise alignment of the photodetector and microlens is
hard to achieve.
In the light of the above, this paper aims to review the
latest research on the progress of microlens array
fabrication technologies. In Section 2, the operation principle of
the compound eye is briefly introduced to provide
background for the design of microlens array. In Section 3, the
state-of-art technologies, including the direct and
indirect methods for fabricating microlens array are reviewed
and compared. Section 4 describes the applications of
microlens array. Finally, the challenges and opportunities
of manufacturing microlens arrays in industry and
academic research are discussed and several principle
conclusions are drawn in Section 5.
2 Principle of Compound Eyes
In nature, compound eyes can be categorized into 2
types, e.g., apposition compound eyes and superposition
compound eyes, as shown in Figure 1. In natural
apposition compound eyes, the light through each ommatidia
is received by only one photo receptor [
]. In contrast,
every photo receptor in the superposition compound eye
is able to acquire light from several ommatidia. Therefore,
superposition eyes are much more light-sensitive, and
more suitable for deep-water crustaceans living in dim
light. However, the main drawbacks of superposition eyes
are the aberrations as the consequence of the combination
of light from different ommatidia. Therefore, the artificial
compound eyes are mainly of the apposition form.
As shown in Figure 1(a), in nature, the ommatidia of
apposition compound eyes are arranged on a curved
surface of radius Re and the receptors of diameter d are
distributed on the focal points of the ommatidia. The
geometric size of each ommatidia is denoted by the pitch
D and focal length f. The acceptance angle ϕ and
interommatidial angle Φ can be expressed as follows[
where D is the full width maximum of the Gaussian
approximation of the Airy function.
In most of artificial compound eye optical systems,
microlens array which is the counterpart of ommatidia,
is arranged on a plane (Figure 2) to fit with CCD and
CMOS. Moreover, the fabrication process of microlens
array on a plane is much simpler.
For each unit of the microlens array, the geometric size
is determined by the pitch (D), the height (h), the radius
of curvature (Ru) and contact angle (θ), shown in
Figure 3. These parameters can be measured through optical
microscopy, scanning electron microscopy as well as
contact profilometry. The quality of the microlens often is
denoted by the numerical aperture (NA), surface
roughness and array uniformity.
The numerical aperture is calculated as [
NA = n · sin α,
where n is the refractive index of the medium between
the object and the microlens. The half aperture angle α
Figure 3 Geometry parameters of microlens array
α = arccos
Ru − h .
F # =
2n · sinα
can be obtained from the height (h), the radius of
With the increase of the value of NA, the resolution
and magnification of the microlens also increases. The
contact angle θ is equal to half aperture angle α and the
F-number F# is defined as [
Surface roughness is another important parameter in
evaluating the optical performance of a lens. And it is
strongly affected by the fabrication process. Optics with
large surface roughness may suffer from scattering issues,
decreasing the efficiency of the contrast and light
]. The array uniformity is of great importance in
the imaging system, especially when the area of
microlens array is large, and light retrace is needed for further
process. The array uniformity can be described by the
standard deviation of the height and the radius of
3 Fabrication Methods
The fabrication methods for microlens arrays are
categorized into direct methods and indirect methods. The
direct method does not need to fabricate a mask or a
mold insert with concave 3D microstructures. The shape
of microlens is usually formed based on the surface
tension effect when the material is in a thermoplastic state
or liquid state resulting a super smooth surface
(arithmetic average roughness Ra less than 1 nm) [
More importantly, these methods involve simple and
cost-effective processes, which are preferred in industry.
However, it is still very difficult to control the microlens
precision because the geometry of the microlens is only
determined by the controlling parameters such as
temperature, wettability, pressure and process time. The
indirect methods need to fabricate the mold with concave
microlenses and produce the final lenses by replication
technologies, such as hot embossing, compact molding
and injection molding. Using the indirect method, the
shape of microlens array can be well-controlled but the
process is complex.
3.1 Direct Fabrication Methods
3.1.1 Thermal Reflow Method
The thermal reflow method has been used to produce
microlens arrays in the last few decades [
fabrication process is depict in Figure 4 [
]: first, the
photoresist layer is coated onto the substrate to let the
UV-light thrust through the mask which has circular
array patterns. Second, the photo-resist layer is
developed and the cylindrical isolated-islands are then
generated. Third, the isolated-islands are heated to a certain
temperature, then the cylindrical island will turn into a
spherical structure due to the effect of surface tension.
In this method, an optical separator is not needed and
the microlens array is fabricated by the common MEMS
technique. However, the contact angle is hard to control
because it is only affected by the wettability of both the
material on the substrate and the surrounded air around
the microlens, rather than the size of the isolated-islands.
3.1.2 Microplastic Embossing Method
The microplastic embossing method is a low-cost and
highly efficient technique, which was developed to the
fabrication of plastic microlens [
partial-filling technology  based on the microplastic
embossing process does not require a mold insert with
the desired microlens geometry, and thereby alleviates
the surface defects induced by contact between mold
insert and injected materials. Based on this method, a
silicon mold insert of circular openings with a few
hundred micrometers is first fabricated by a deep reactive
ion etching process. Then a polymer substrate is placed
between the heating plates and the silicon mold insert,
as shown in Figure 5. The microlenses are constructed by
applying an external pressure at an elevated temperature
above the glass transition point for a given time. Finally,
the newly formed microlens is cooled down by
decreasing the temperature as slowly as possible to reduce the
thermal stress and attendant replication errors. The
temperature-dependent viscosity and surface tension are
the major factors in the fabrication process, determining
both the height and radius of the microlens. The applied
pressure has a linear relationship with the height of the
microlens but has little effect on the radius. The
processing temperature is capable of affecting both the radius
and height in a complicated relationship. The
microplastic embossing method is considered as a one-step
molding process, and is not influenced by the quality of the
mold surface [
3.1.3 Microdroplet Jetting Method
Microdroplet jetting, also known as ink jet printing,
is another direct method for fabricating microlenses
]. Figure 6 shows a schematic diagram of the
working principle of this method [
]. The droplets of a
UV-polymerizable liquid are ejected from a nozzle to a
substrate. When the droplets reach this substrate, they
are exposed to UV light and converted to a solid state
with a super smooth shape. In order to obtain a
microlens with large a NA (more than 0.4), the substrate is
treated by Nano texturing and fluorodecyltrichlorosilane
(PFTS) or C4F8 coating [
]. This method is suitable
for fabricating microlens arrays in a large area rapidly at
room temperature. However, it is hard to control the
consistency and feature size of the microlens.
3.2 Indirect Method
In contrast with direct methods for the fabrication of
compound eye microlens arrays, indirect methods need
a mold with a concave spherical microlens. The final
microlens array is produced by replication technologies
such as injection molding, hot embossing or UV
molding. The key to the indirect methods is how to generate
concave microlenses with precise geometry. The
potential technologies are divided into two categories, i.e.,
MEMS based technologies and ultraprecision machining
3.2.1 MEMS Based Methods
The standard MEMS methods utilizes
photolithography to generate patterns on the mask layer and
chemical reactions to etching the curvature of microlens onto
the substrate [
]. Albero et al. [
] proposed a
novel microlens wafer-scale fabrication method based
on isotropic wet etching technology, shown in Figure 7.
First, the protection layers of SiO2, Si3N4 and NiCr are
coated on the substrate. Then the patterns are generated
though photolithography, reactive ion etching (RIE) and
the HF solution step by step. After that, the wafers are
immersed in the isotropic etch solution to generate the
concave microlenses. Finally, the mold is finished after
the removal of the mask layer. A wide range of lens
geometries and lens arrays with good surface smoothness, high
uniformity and repeatability can be achieved using on
this method. However, this approach requires expensive
equipment and complicated procedures to produce the
mask on the wafer. Besides, the control precision of the
mask fabrication technology must be improved to meet
the requirements in the miniaturization of lenses.
To overcome this problem, direct writing technologies
have been developed. For example, femtosecond laser
wet etching was developed in which the patterns are
directly generated on the wafer by laser processing. Then,
in the wet etching process, the concave microlens are
formed as the induced region has a higher selective
etching rate than the other parts. Other examples of direct
writing techniques includes focused ion beam writing
] and electron beams writing [
Direct laser writing can be used to produce patterns on
a spherical substrate. Curved artificial compound eyes
(CACEs) are realized by this method [
]. However, the
optical system is hard to be miniaturized because it needs
overlapping of the concave and the convex bulk lenses
(the radius of the spherical lenses is 40 mm). Another
approach to fabricate the CACEs is by transforming the
2D microlens-pattern films into a 3D shape by the
negative pressure deformation process [
the uniformity of the microlenses need to be improved
because in the negative pressure deformation process,
some microlens at the marginal areas may be damaged
3.2.2 Ultraprecision Machining Methods
Ultraprecision machining technologies, such as diamond
micro-milling and single point diamond turning (SPDT)
are effective methods to fabricate microstructures and
nanostructures with good uniformity in a large area [
]. Such ultraprecision technologies are
integrated into the process chain for mass fabrication of
microlens arrays. Ball-ending milling usually utilizes
half-arc single crystal diamond tools to removal metallic
]. Metallic materials such as OFHC-Cu,
AlMg3 and NiP can be processed. The achieved surface
roughness (Ra) is as low as 5 nm. However, this method
needs to machine the microlenses one by one, which
severely extends the operation time and increases the
cost. To our knowledge, the bottom of the each concave
microlens is affected by the alignment error (around 1
μm) between the vertex of the cutting edge and the
spindle axis. With regard to this, the single point diamond
turning (SPDT) method is performed to produce
microlens arrays with high quality by a slow slide servo [
] or a fast tool servo [
21, 59, 60
The procedures of single point diamond turning of
microlens arrays on a flat surface were described by
Zhang et al. [
]. A design model is first generated based
on the ideal structures of microlens array. Then the ideal
tool path is calculated based on the design model and
cutting parameters such as spindle speed (S) and feedrate
(F) based on the cylindrical coordinate method [
After that, the final tool path is modified by considering
alignment error, tool radius error and squareness error.
Two translation axes (X and Z) and a rotational
spindle axis (C) are precisely controlled to generate the 3D
structures on the cylindrical end face of the workpiece.
The form errors of the microlens array influence the
machining errors. However, the surface fluctuation
(Figure 8) is probably noticeable on the cutting parts around
the inflection points of the path, which reduces the
optical performance severely. The surface fluctuation appears
due to the limit of the machine dynamic response. With
regard to this, a new fabrication method is proposed to
cut the microlens on a metal sheet mounted on the side
face of a cylindrical basement, as shown in Figure 9.
After the turning process, the metal sheet is striped off
and inserted in the injection mold. Then the microlenses
are produced by microinjection molding process. Based
on this method, the surface fluctuation is reduced and a
smoother surface is obtained, with the surface roughness
(Ra) less than 10 nm and the PV-value less than 0.150
µm . One of the drawbacks of this method is that,
in order to avoid the interference between the clearance
face of diamond tool and the machined concave
microlens, a small F number (less than 4) is difficult to achieve.
It is expected that the surface finish will be further
improved by adaptive tool path planning, considering the
dynamic response of the machine tool. Some
improvement has been achieved by developing the quasi-elliptical
tool servo (QETS) technique when producing a single
microlens on the mold [
Ultraprecision machining was also applied to fabricate
3D compound eye lens arrays [
]. It was reported
that 601 individual compound eye microlenses (aperture
of 0.58 mm) and the related microprisms were produced
in a 20 mm diameter area, providing a large light
deviation angle of 18.43° and maximal FOV of 180°, if the entire
hemispherical surface is fabricated with microlenses. The
microprism array and microlens array were precisely
fabricated on a curved and a flat surface respectively, with
a combination of single point diamond turning, diamond
broaching and micromilling processes . However, the
intensity of the microlens on the hemispherical surface is
low, therefore the measured FOV is much smaller than
the maximum theoretical value.
Microlens arrays can be integrated into a light field
camera to achieve the function of “take photo and then focus”
] and obtain an image with both large FOV and
aperture. Figure 10 shows the current light field of
cameras, including the camera arrays developed by Stanford
]; Integrated lens array designed by the Adobe
]; light field camera and microscope
developed by Lytro [
]; 3D light field camera produced by
Raytrix  and compound eye camera module
developed by the Toshiba company [
]. Compared with the
camera array based devices, the microlens based devices
have more potential in the market because of their light
and compact structure. Such microlens array is mounted
between the main lens and the light sensor, changing the
light path (Figure 11). Figure 12 shows the experimental
setup and the image acquired by the camera, having a
compound eye microlens array fabricated by single point
diamond turning and injection molding. Figure 12(b)
shows the images processed by the digital refocus
algorithm. To our knowledge, although 3D images can be
obtained, however, the resolution of image obtained from
current light field camera (less than 1200 × 900 pixels) is
much lower than the single-lens reflex camera.
5 Conclusions and Outlook
In this paper, the most important fabrication
technologies for microlens arrays are reviewed and compared.
By reviewing the advantages and disadvantages of those
technologies, the following conclusions are drawn.
(1) The direct fabrication technologies, including the
thermal reflow method, microplastic embossing and
microdroplet jetting method, are simple and low-cost
processes which are suitable for mass industrial
production, but it is very difficult to control the accuracy
of the microlenses shape based on the direct
(2) Compared with MEMS based technologies,
ultraprecision machining is more suitable in terms of
producing microlens array on a mold with good
uniformity in a large area. The shape of microlens can be
well controlled by ultraprecision machining. Further
improvement is needed to reduce the surface
fluctuation and surface roughness by optimizing the tool
path considering the dynamics of machine tools.
(3) Both the MEMS based technologies and
ultraprecision machining are able to fabricate curved
compound eye microlenses, but the production quality
needs to improve.
(4) 3D imaging systems inserted with microlens array
can be used to capture the light field information,
but the spatial resolution is much lower compared
with that of photos captured by 2D camera. These 3D
imaging systems may be applied in the situation that
the high spatial resolution is not required.
WY carried out the studies in the reviews of the principles of compound eye
and the fabrication methods. He wrote the draft. LL investigated the applica‑
tions of microlens array and she also contributed to the review of indirect
fabrication method including molding process. WL and CC shared many fun‑
damental ideas in the ultraprecision machining technologies, thermal reflow
method and UV molding technology. CC conducted proof reading and made
some critical revisions. All authors read and approved the final manuscript.
1 State Key Laboratory of Ultra‑precision Machining Technology, Partner Labo ‑
ratory in the Hong Kong Polytechnic University, Hong Kong 999077, China.
2 Shenzhen Branch of State Key Laboratory of Ultra‑precision Machining
Technology, PolyU Shenzhen Research Institute, Shenzhen 518000, China.
Wei Yuan born in 1990, is currently a PhD candidate at State Key Laboratory
of Ultra-precision Machining Technology, the Hong Kong Polytechnic University,
China. He received his bachelor degree from Hefei University of Technology,
China and master degree from The Chinese University of Hong Kong, China. His
interests include metal cutting theory, ultraprecision machining processing
and robotics. Li‑Hua Li born in 1981, is current a research assistant at State Key
Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic
University, China. She earned her PhD degree in measurement science and
technology from Tsinghua University, China, in 2012. Dr. Li’s research focuses
on the study of design theory of optical elements, fabrication and measure‑
ment technology of optics. Wing‑Bun Lee born in 1951, is currently the Head
of State Key Laboratory of Ultra-precision Machining Technology, the Hong Kong
Polytechnic University, China. His teaching and research interests include
advanced manufacturing technology, materials processing, ultra‑precision
machining, manufacturing strategy and knowledge management systems.
Chang‑ Yuen Chan born in 1965, is current the project manager at State Key
Laboratory of Ultra-precision Machining Technology, the Hong Kong Polytechnic
University, China. He earned his PhD degree in mechanical engineering from
Hong Kong University, China, in 1995. Dr. Chan’s research focuses on the study
of ultraprecision machining technology and 3D imaging processing.
Supported by Shenzhen Science, Technology and Innovation Commission of
China (Grant No. JCYJ20150630115257902), the Research Grants Council of the
Hong Kong Special Administrative Region of China (Grant No. ITS/339/13FX),
and Research Committee of The Hong Kong Polytechnic University, China
(Grant No. RUK0).
The authors declare that they have no competing interests.
Ethics approval and consent to participate
Springer Nature remains neutral with regard to jurisdictional claims in pub‑
lished maps and institutional affiliations.
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