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Full-wafer double-sided monolithic lens fabrication

By Vijay Ramya Kolli, Margarete Zoberbier, Eleonora Storace, Marc Hennemeyer [SUSS MicroTec Lithography GmbH]
Markus Brehm, Patrick Heißler [DELO Industrial Adhesives]
W afer-level fabrications high throughput. Monolithic lens arrays (provided that appropriate materials are

a nd i nteg rat ion a re
w e l l - e s t a b l i sh e d are a key component in next-generation used). The main disadvantage of POG
lenses is that they consist of more than one
flashlights for mobile phones, high pixel
technologies to deliver reliable and count wafer-level cameras, efficient material (polymer and glass). Effects like
repeatable manufacturing processes for coupling of light from a vertical-cavity internal interfaces, coefficient of thermal
low-cost consumer products. These surface-emitting laser (VCSEL) into an expansion (CTE) mismatch, etc., can lead
technologies also enable fabricating high- optical fiber [2], and integration of light- to reliability issues.
quality microoptical elements and packing emitting diode (LED) arrays, and many Monolithic UV imprint lithography can
thousands of sub-optical components other interesting applications. combine the best of both worlds. Because
in parallel. In addition, wafer-level When talking about microoptical it is monolithic, it avoids all the trouble
microoptics is an essential technology elements produced in large quantities at of material compatibility and at the same
that plays a significant role for many low cost, two competing processes can be time profits from a highly reliable material.
applications in semiconductor industry, considered: injection molding, and polymer While monolithic lenses offer significant
such as wafer-level cameras, 3D imaging on glass (POG) UV replication. While benefits, there are also challenges, which
and sensing, 3D integration, through- injection molding is a standard process must be solved in order to enable mass-
silicon vias (TSVs), light detection for fabrication of miniaturized optics, production of high-quality lenses at
and ranging (LiDAR) technology, and such optical elements typically cannot low cost. The main challenges are: 1)
different modules for complementary withstand high temperatures. The reason obtaining arrays with high residual layer
metal oxide semiconductor (CMOS) is inherent in the process: during injection thickness (RLT) uniformity over large
technology. Wafer-level microoptics can molding, a plastic solid material is molten, areas; 2) low aberration and distortion
also be adapted with other wafer-level transferred to a mold cavity and solidifies of the lens geometry upon thermal
technologies, microelectromechanical by cooling down [3]. Unfortunately, the stress; 3) precise lens-to-lens alignment;
systems (MEMS), and CMOS components. same thing happens when the optical and 4) manufacturing of larger sets of
The wafer-level camera industry is element is heated to high temperatures in interconnected lens arrays. In this paper
considered to be a growth powerhouse, as subsequent processing (e.g., when using we will demonstrate how the combination
it provides a promising alternative to the standard reflow soldering with 260°C peak of SUSS U V-imprint lithography
conventional barrel-type cameras along temperature): the lens would just melt equipment (SMILE) in combination with
with smaller form factor and low cost of and lose its shape. This means that optics a matching UV-curing epoxy imprint
ownership. The camera module demands can be produced in large quantities and material from DELO overcomes these
accurate assembly of various microoptical low cost, but the downstream processing challenges and allows the imprinting
elements to deliver the best possible image requires additional process steps (e.g., of interconnected and double-sided
quality [1]. Wafer-level packaging (WLP) low-temperature soldering, or mechanical monolithic lens arrays in a single step,
provides a cost-effective solution for multi- clamping) increasing both complexity while attaining high throughput, desirable
level microlens arrays integration and of the process, as well as costs. Another quality, and low cost of ownership.
miniaturized devices that are connected disadvantage of injection molding is that
with higher input/output (I/O) density. limited alignment accuracy from the top UV-imprint lithography
Wafer-level monolithic lens array to bottom of the mold results in limited The SMILE imprint technology
fabrication has emerged over the past optical performance. POG UV replication facilitates reaching the targeted epoxy
decade as a constantly growing market can address both of these problems. thickness and a very low total thickness
because it eliminates the requirement Dedicated imprint machines allow for the variation (TTV) by means of the SUSS
of using glass substrates, thereby highest alignment accuracy. At the same active wedge error compensation (WEC)
increasing the scalability and material time, the mechanism for the transition technique. This method allows for a
compatibility of optical devices. In this from liquid to solid is completely changed. constant real-time control over the relative
paper, we discuss the capability of SUSS The liquid-solid transformation is no positions of the imprint stamps/substrates
ultraviolet (UV)-imprint lithography longer a reversible melting-solidifying with a micrometric precision and force
together with an optically-tailored, UV- process, but a chemical crosslinking detection. A low TTV is not only critical
curable epoxy resin from DELO to mediates the transformation. Because for attaining high RLT uniformity, but
manufacture double-sided monolithic this reaction is not reversible, the material also for precisely aligning and producing
wafer-level-optics. Such wafer-level would stay solid and maintain its shape interconnected lens structures. During
optics enable low cost of ownership and even when heated to high temperatures the process of reaching the final gap, the

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