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High-Precision Wafer-Level Optics Fabrication and Integration

Photonics Spectra
Dec 2010
Michael Kast, EV Group

The evolution of micro-optical components for camera modules is today being strongly driven by the mobile phone industry, while manufacturers of conventional camera modules – which are assembled from a multitude of individual parts – are suffering from continuously decreasing profit margins. Wafer-level fabrication and integration of micro-optical components is a promising alternative that opens up a highly competitive route for camera module manufacturing by using well-established microstructure technologies. The basic idea is to use imprinting techniques to fabricate individual microlenses at the wafer level; these are then continuously stacked by UV bonding until the ultimate optical element is fully assembled. Finally, these elements are bonded onto the image sensor at the wafer level, or a known good die approach is used.


Elements of a typical wafer-level camera module include a CMOS image sensor, polymeric lenses molded onto glass carriers by UV imprint lithography, spacers and aperture layers, as shown in this exploded view. Images courtesy of EV Group.


Key advantages of wafer-level optics fabrication and integration include the reduced form factor of the resulting wafer-level cameras and the highly accurate assembly based on state-of-the-art imprint lithography and bond-alignment technologies. Due to smaller form factors and lower cost-to-performance ratios, wafer-level cameras have already begun replacing conventional barrel-type camera modules. Key players, including Heptagon, Anteryon, Tessera, Aptina, Nemotek, Himax and Visera, have successfully demonstrated the first wafer-level camera-based modules. Market forecasts predict continually increasing demand, with wafer-level cameras breaking 1 billion units per year within the next five to seven years. This trend will be accompanied by the evolution of wafer-level cameras toward higher pixel numbers, generating a need for more complex optical systems and, consequently, tighter manufacturing tolerances.


The EVG IQ Aligner is suitable for high-precision wafer-level optics fabrication and integration of wafer sizes up of 300 mm.


Wafer-level optics fabrication

The starting point of each wafer-level optics manufacturing process flow is master stamp fabrication. Wafer-level master stamps, which are based on the design of the micro-optical component, must be fabricated for each individual microlens contributing to the optical performance of the overall lens stack. The most promising technique for master stamp fabrication is step-and-repeat ultraviolet imprint lithography. The basic idea is to repeatedly replicate individual microlens molds from a single lens master onto a master substrate, using a high-precision alignment stage as a guide. Current step-and-repeat ultraviolet imprint lithography technologies enable lens position accuracies of sub-100 nm, indispensable to meeting future tolerance requirements of wafer-level camera modules. Besides producing high uniformity of lens shape across the resulting wafer-level master stamp, step-and-repeat ultraviolet imprint lithography accelerates materials evaluation and process optimization for all relevant imprint processes due to its versatility in varying all relevant process parameters during one individual process run.


The EVG 770 NIL Stepper model for step-and-repeat master stamp fabrication features positioning accuracies in the sub-100-nm range.


In the past 10 years, wafer-level UV molding of micro-optical components has become the first high-volume manufacturing application of UV imprint lithography since its introduction in 1995. This technique replicates microstructures from a patterned stamp into an optical prepolymer using a combination of controlled imprinting and UV-initiated polymer cross-linking. Polymeric working stamps are commonly favored as stamp material, as they significantly reduce UV molding processes’ total cost of ownership. Moreover, release properties of working stamps can be tuned to enable the defectless, repetitive demolding of lens wafers that is essential for high-volume wafer-level optics manufacturing.

The most established wafer-level optics fabrication technique is sequential double-sided microlens molding using puddle dispense. In this approach, the optical prepolymer is dispensed as a single puddle onto the center of the wafer via an integrated dispense system. Following overlay alignment of stamp and substrate, the puddle is squeezed, which enables continuous filling of all lens cavities from the center to the perimeter of the working stamp. Because of the nature of the imprint process, a thin residual layer remains after UV imprint lithography, predetermined by the final imprint gap. The key advantage of this approach is the ability to perform all necessary process steps – including wedge error compensation, material dispense, overlay alignment, controlled imprint, UV exposure and demolding – on one dedicated equipment platform.


This cross section represents a fully assembled wafer-level camera module. Polymeric lens elements (blue) molded onto glass carriers by UV imprint lithography as well as spacer elements (yellow) constitute the micro-optics stack sitting on top of the image sensor (red). The opaque housing (black) blocks unwanted interfering stray light, which otherwise deteriorates the sensor response. The ball grid array at the bottom of the module indicates the electrical contacts of the image sensor.


An alternative UV molding technique is UV imprint lithography using ink-jet dispense, in which the lens cavities on the working stamp are directly filled with optical prepolymer by using ink-jet dispensers featuring pattern recognition and autoalignment. During the subsequent imprint process, all individual lenses are molded simultaneously as the stamp is touching the substrate at the interstitial areas. This approach avoids creation of a residual layer, which significantly cuts down on materials costs. And while ink-jet dispensers exhibit a much lower throughput compared with puddle dispense, this is partly offset by the UV imprint lithography systems’ shorter imprint times.


Shown is a cutaway of a fully assembled micro-optics stack bonded onto the image sensor. Double-sided lens wafers and spacer wafers are repeatedly stacked by aligned UV bonding until the total micro-optics stack is fully assembled.


In the final step, all individual lens wafers are stacked by aligned UV bonding to form the ultimate micro-optics stack. The distance between adjacent microlenses within the stack is adjusted using dedicated spacer wafers.

Image quality

The image quality of a wafer-level camera module strongly depends on the manufacturing tolerances of the micro-optics stack sitting on top of the CMOS image sensor. Key parameters include:

• Lens-to-substrate alignment
• Lens-to-lens alignment
• Lens stacking alignment
• Optics axis tilt
• Lens shape fidelity
• Thickness uniformity of bond interfaces

Each parameter must be addressed in the manufacturing process to achieve the micro-optical system’s desired optical performance. In terms of alignment, UV microlens molding and stacking processes have to be considered as thick resist applications that necessitate dedicated proximity alignment procedures realized in state-of-the-art UV imprint lithography equipment.

Also essential for high-precision wafer-level optics fabrication are wedge error compensation routines, which literally compensate for any wedge errors induced during UV molding or stacking processes by setting the wafer parallel to the stamp. The imprint result strongly depends on the curing behavior of the optical prepolymer. Significant shrinkage during UV exposure causes detachment of the polymer from the stamp surface, which creates defects in the lens surfaces. This effect can be eliminated by precisely controlling the imprint force that enables microlens cavities to be constantly filled throughout polymer cross-linking. Finally, the optical performance of the micro-optics stack depends on the uniformity of the bond interface. This can be addressed by choosing a dispense technique that homogeneously deposits the UV-curable adhesive.

At present, the camera module assembly generally uses well-established semiconductor technologies to transition from discrete assembly to wafer-level integration. Despite the significant reduction in camera module size made possible by wafer-level optics technologies, the mobile phone industry demands thinner camera modules and, thus, further miniaturization. In this context, the limiting factor inside a wafer-level camera is the total sum of all constituting glass wafers, including individual lens substrates and spacer wafers. A promising solution to overcome this limitation is UV molding without a substrate. In this approach, the optical prepolymer is molded between two stamps, similar to a Belgian-style waffle maker. Omitting the glass carrier wafer adds more flexibility in the optical design, resulting in thinner lens wafers and a total optical stack height that is significantly lower.

Wafer-level optics fabrication technologies are being considered for other static optics applications, such as autostereoscopic 3-D displays and high-brightness LEDs. Besides static features, dynamic ones such as adaptive optics for autofocus are currently candidates for wafer-level manufacturing. The advantages of this approach are similar to those for static wafer-level optics. Form factors as well as manufacturing costs can be significantly reduced.

All above-mentioned technologies are considered to contribute to the evolution of wafer-level optics as potential successors to conventional barrel-type lens assemblies. As a consequence, equipment suppliers must be flexible, adapting their dedicated equipment to the integration of highly customized processes related to any of the above-mentioned concepts, and at a reasonable cost of ownership.

Meet the author

Michael Kast is product manager at EV Group’s headquarters in St. Florian am Inn, Austria; e-mail: m.kast@evgroup.com.



GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
optical
Pertaining to optics and the phenomena of light.
optical system
A group of lenses, or any combination of lenses, mirrors and prisms,  so constructed as to refract or reflect light to perform some definite optical function. 
polymer
A material whose molecular structure consists of long chains made up by the repetition of many (usually thousands) of similar groups of atoms.
wafer
A cross-sectional slice cut from an ingot of either single-crystal, fused, polycrystalline or amorphous material that has refined surfaces either lapped or polished. Wafers are used either as substrates for electronic device manufacturing or as optics. Typically, they are made of silicon, quartz, gallium arsenide or indium phosphide.
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