Tiny lenses make cameras an easy addition to cell phones and other products.
Lenses for miniature optical systems like optical mice and cell-phone cameras have to be manufactured in high volume. Cost and size constraints dictate that the lenses do more than simply refract light. Also, these lenses must be producible in complex forms.
The industry has responded by investing in the technology to manufacture lenses at the wafer scale. The resulting products provide optical functionality far beyond the capabilities of traditional lenses while benefiting from the economic advantages of wafer-scale manufacturing techniques.
A lens is an optical device with axial symmetry that transmits and refracts light. Its material has a refractive index different from the surrounding media, and its two optical surfaces work to converge or diverge a beam. This may seem obvious, since the limits on materials and manufacturing methods only permit a very restricted range of lens shapes and optical functions to be realized.
In many regards, lenses have not changed significantly over the last 3000 years. Materials quality and manufacturing methods may have advanced, but lenses are still essentially round in plane and curved in cross-section.
One of the few exceptions in mass production is “reading stones,” commonly known as eyeglasses. While the earliest eyeglasses were round, to simplify manufacture, modern glasses now come in a variety of shapes. This is done for aesthetics, not for reasons of optical performance or cost. To look good, the shape of the glasses should match the shape of the wearer’s face.
But what if the generic description of a lens could be changed? Going into more detail, a lens is an optical device with optional symmetry that can transmit, block, filter, refract, and diffract light. It is made of multiple materials, each with different optical properties, and has multiple optical surfaces. The range of optical functions that could be achieved with such a component would pave the way for a range of innovative products. But how could such a lens be made?
Traditional lenses are manufactured from either relatively hard materials like glass or, more commonly, from a range of specially engineered polymers. Glass lenses are fabricated in a limited range of shapes or blanks and then machined to final shape by a series of grinding and polishing operations. These processes entail the removal of material from the surface by a tool. Therefore, the surface must be accessible.
The tool has a finite radius of curvature, and the type of motion necessary to conduct grinding and polishing operations limits the complexity of the shape that can be manufactured. That’s why glass lenses tend to be symmetric about the optical axis, with large radii and smooth optical surfaces. Deviation from this simple formula greatly increases cost and decreases throughput.
Mass production of lenses using engineered polymers is frequently accomplished by injection molding. A metal mold contains a hollow lens-shaped cavity that’s filled by injecting polymer in a semi-liquid form. The polymer is allowed to set or cure before opening the mold and removing the part. Injection molding is very attractive as a high-volume manufacturing process because many parts can be produced simultaneously with rapid throughput.
By injecting the polymer under high pressure, it is possible to make an exact replica of the mold cavity, including the surface finish. As a result, the released part can be used as is, without further processing. The mold surface, which is metal, has to be machined by grinding and polishing, very much like glass lenses. Because one mold can be used many times, it is economical to make more complex lens geometries, but they are still limited by the capabilities of the mold-shaping tools.
In recent years there has been a quiet revolution in lens manufacturing technology. Much of this has been driven by the solid-state imaging industry, where there is high demand for miniaturized cameras in applications like mobile phones, Web cams, digital cameras and camcorders, automotive driving aids, and surveillance systems. Typically, over one billion miniature camera modules are manufactured each year to satisfy these markets. Photonics communications systems and optical mice are two additional industries that use significant volumes of miniature lenses.
The sheer manufacturing volume and the physically small size of the lenses required for these applications has made wafer-scale lens manufacturing both technically and economically viable (Fig. 1). In its simplest form, wafer-scale lens manufacturing involves making two wafer-sized masters, one for each optical surface.
Each master contains a closely packed array of cavities. A glass plate, or wafer, is spin-coated with liquid polymer. The master is then pressed into the liquid film, which flows to fill the cavities. The polymer is cured to fix the lens shape enabling the master to be released and reused. Not only is the cycle time fast, but the key economic advantage of wafer-scale lens manufacture is that tens of thousands of lenses can be made simultaneously on a single 300-mm diameter wafer.
Because the materials and process costs are divided among the number of lenses on the wafer, the piece part expense is extremely small. However, favorable economics isn’t the principal reason for the adoption of wafer-scale lens manufacture. This process makes it possible to manufacture optical components that have functionality far beyond what polished glass or injection-molded lenses can achieve.
For a start, because the master is only filled with liquid and the optical polymer is usually UV-cured, the master itself can be made of a polymer. This means it is both cheap and very easily shaped. Because the substrate is a wafer, there are advantages to utilizing equipment from the semiconductor industry to shape the master.
One method is to create the lens shapes in photoresist using grey-scale processing to obtain 3D profiles. The photoresist is then cured and coated with a thin layer of electroless nickel to boost the durability. A polymer is cast over the nickel to provide mechanical support, and the photo resist is then removed.
Because the lens shape is lithographically derived, it is therefore no more difficult to make axially symmetric lenses than asymmetric lenses. For example, close-packed lens arrays are optically more efficient when the lenses are rounded squares, rather than circles. The fill factor of the square-base lenses is proportionately higher (Fig. 2).
Another benefit of wafer-scale manufacture is that the two optical surfaces are made using totally independent processes. This means their shapes have no physical connection and can even be different sizes. More interestingly still is that the two optical surfaces can be produced from different materials possessing different optical properties. The differences can be dramatic (e.g., refractive index) or more subtle (e.g., chromatic dispersion).
Because the two optical surfaces are fabricated on either side of a substrate—typically a glass plate—this does not mean that the substrate will be optically passive. The glass can provide a filter action, for instance, neutral density or wavelength-specific like infrared cut. The substrate could even be a nonlinear optical polymer and so provide wavelength conversion as the light passes through the structure.
The substrate is typically a flat plate so it is compatible with the spin-on process used to apply the liquid polymer film prior to the replication process. However, a spin-on process does not require the plate to be perfectly flat. Permitting some topography allows the surface of the glass plate to be patterned.
For example, applying a thin layer of metal containing a hole on the optical axis of each lens provides each optical element with an internal aperture in the structure. Likewise, a diffractive structure is physically flat as far as a spin-on coating process is concerned and can have a profound influence on the light path through the wafer-scale lens, depending on the complexity of the diffractive structure. The substrate has two surfaces available for modification, providing scope for much innovation in optical design.
Another intriguing possibility arising from wafer-scale lens manufacture results from the combination of materials and process. Because the master only comes in contact with liquid polymer, it does not have to be rigid like a mold for an injection-molded lens. Instead, the master can be compliant or slightly rubbery. This makes it possible to design optical surfaces with re-entrant features. None of the other volume manufacturing techniques have that capability.
In common with lenses manufactured using traditional processes, wafer-scale lenses can be coated to provide additional optical functionality or mechanical durability.
Wafer-scale processing makes it possible to manufacture small, simple lenses very economically in high volume. But with minimal additional manufacturing effort, it is possible to fabricate optically complex components. The challenge of wafer-scale lens technology is determining how to leverage the technical possibilities offered to produce optical designs where a multi-functional component substitutes for a single discrete lens in an optical train. Figure 3 shows an example of the type of integrated optical component that can be realized using wafer-scale lens manufacturing techniques.
Wafer-scale manufacturing ideally suits the manufacture of miniature optical lenses. The smaller the area of the part, the more economically attractive the process becomes. The wafer-scale manufacturing process entails replication of masters on flat substrates from liquid precursors. This provides great flexibility in terms of the materials for each optical surface as well as the lens shape and profile. The substrate can be optically active and provide additional functionality like filtering, stopping and diffraction. Wafer-scale manufacturing therefore makes it possible to produce multi-functional optical components in the same form factor as a single discrete lens.
Yehudit Dagan is vice president of marketing for Tessera Technology’s wafer-level camera (WLC) technology. She holds a bachelor’s degree in physics and a master’s degree in heat transfer engineering, both from Tel Aviv University, Israel, as well as a master’s degree in management from the Polytechnic University in New York. She can be reached at email@example.com.
Giles Humpston serves as director of applications for Tessera (UK). He is a metallurgist by profession and has a doctorate in alloy phase equilibria. Also, he is a cited inventor on more than 75 patents and has co-authored several textbooks on metallic joining processes. He can be reached at firstname.lastname@example.org.
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