The use of laser light for precise illumination has been limited to high-end applications like optical lithography or small niche markets like measurement technologies. Now that such industries as automotive and consumer electronics are developing and ramping up the production of LIDAR and 3D sensors, laser illumination is scaling in a new direction. For imaging applications, optics made of polymers are already the first choice for such devices as smart cameras. But in order to deliver glass micro-optics with better performance and long-term stability, the cost structure of injection molded polymer optics needed to be addressed.
The limited functionality of optical polymer materials means limited design and production opportunities for optical devices. This is particularly disadvantageous for optical devices that place a high requirement on stability and performance. This means missed opportunities for the use of optical devices in safety-related applications like LIDAR and 3D ID. In particular, well known degradation mechanisms like gloss, haze, birefringence, and ultra-violet/visible (UV/VIS) light absorption and transmission decline can limit the use of polymer-based optics in applications in harsh environments like autonomous transportation or precise optical control of industrial and consumer devices.
Similarly, in naturally illuminated objects for photography, the degradation of laser illumination reduces the resolution and functionality of the devices. Such degradation mechanisms, in combination with high flux pulsed diode laser sources, can limit the performance and lifetime of devices with safety relevant features. To address these issues, a new production technology for cylindrical lens shapes has been designed to overcome the limiting factors of cost reduction for optical components made of glass and can deliver precise polished optics at polymer cost levels.
Wafer-Based Optics Production Technology
Beam shaping, the art of controlling laser light down to the single photon, has enabled the rise of the optics market to its current near trillion-dollar level. Previously used in industrial applications for laser cutting or welding, beam shaping has found its way into the consumer electronics market. Beam shaping was initially for volume production of laser diodes for CD/DVD & Blue Ray players. It is now evolving into high-end micro-optics for smartphones, enabling face recognition, gesture control, and bright crisp images in low-light environments. In the automotive industry, beam shaping is not only used in spotlights; state-of-the-art heads-up displays and LIDAR improve driver vision and safety, opening the possibility for future autonomous cars.
To enable such beam shaping applications, the required micro-optics have to be fabricated with high precision and accuracy. The optical characteristics and long-term stability are key criteria when selecting glass for a wide range of optical functions. Yet smart consumer applications and autonomous transportation are mainly driven by cost; as a result, polymer optics are currently the first choice for applications with volumes of tens- to hundreds-of-millions of pieces.
In combination with diode laser sources, polymer optics can be used only for low power or low value applications due to UV- and high-power-degradation. For applications that require a few Watts CW (continuous wave) or 100 Watts QCW (quasi continuous wave) and higher, the safe and reliable operation of glass optics, especially in a harsh environment, is the best choice. Besides long-term stability, glass, in comparison to polymer-based optics, offers a smaller thermal expansion coefficient, far higher refractive indices, better transmission in both wavelength range and intensity, and is resistant to environmental influences.
Until recently, volume producibility and price have led product designers to choose polymer optics. Now, the development of improved non-sequential cold processing and polishing technologies for acylindrical lenses on glass wafers have reduced cost per processed mm2 down to polymer optics level. LIMO, for example, has increased the wafer size to 300 mm x 300 mm (~12 inch) of simultaneously produced micro-optics, in combination with higher grinding and polishing rates. This has led to reduced cycle times, resulting in volume production levels at low cost while maintaining high quality.
12" Wafer Level Optics Made of Glass
The process starts with a polished glass wafer. A grinding process is used to structure the surface, as shown in Figure 1. Five sizes encompass the generations that have increased over the years from 35 mm to 300 mm edge length. The surface shape is only limited by the shape of the tool, and is, thus, freeform in this direction. After structuring one side, the other side can be processed with an arbitrary shape, either parallel to the front surface or perpendicular to it. Structured area scales quadratically with edge length, while processing time only increases slightly, so that with each generation the production cost per mm2 is reduced. The latest generation has an effective processed wafer area of 90,000 mm2. Using current stealth dicing techniques, this would yield more than a million pieces of high-end 1 mm2 micro-optics with only 12 of these wafers.
Structuring both sides with an arbitrary shape enables a wide range of possible combinations, from fast- and slow-axis-collimators (FACs/SACs) for single emitter diodes or LIDAR applications, to homogenizers for lithography, to beam transformation systems (BTS). The anamorphic shaping (individual control of x- and y-beam dimension and intensity) of laser light into all kinds of rectangular, square, or line shaped beams opens a wide range of applications with laser light that had limited functionality when using only round or slightly elliptic beam shapes.
There are several methods available to produce these micro-optics. If we focus on glass, the main ones are LIMO’s mechanical wafer structuring, as shown in Figure 3, and glass molding. Both yield decent quality, but they need to be compared for design freedom, production speed, and resulting cost. Molds potentially have 2D free form, which leads to more design freedom. This advantage is diminished in edge emitters, which are the main laser source in current pumping applications, as well as many state-of-the-art LIDAR approaches, one of the main future volume markets for micro-optics. Edge emitters are asymmetric emitters, which exclude using rotational symmetric lenses due to the necessity to design both axes with different effective focal lengths, favoring acylindrical shapes.
New structuring capabilities need less than 4h for the full front-side 300 mm × 300 mm wafer, resulting in ~20,000 mm2/h, using only one set of tools. This minimizes NRE costs, compared to 7-10 sets needed in optimized mold transfer machines. This structuring time is nearly independent of the material choice and allows processing of special high index glasses, as well as a variety of hard materials, such as silicon, germanium, fused silica, or calcium fluoride. Fused silica, especially, can be troublesome for molding due to its high transition temperature of Tg~1,400°C.
In the wafer front-end production flow, an iterative improvement loop of surface profilometry and optical tests, to execute target-performance comparison and variance analysis, have been implemented. The advantage is the ability to restructure already processed wafers, in case a wafer does not meet the highest quality standards. This makes it possible to keep quality at a constant high level with maximum yield.
Structured glass wafers can easily be cleaned, shipped, and coated. Automated dicing, inspection, and packaging offer reliable, reproducible, and reasonably priced back-end processes, targeting polymer cost levels.
The ability to scale the production process of micro cylindrical lenses onto 12-inch glass wafer enables a completely new cost structure and redefines the use of cylindrical lenses made of glass in consumer and mass production applications. Now, all performance related parameters of glass lenses become available on a polymer optics price level. Wafer-based production of glass micro-optics has led to a cost structure that makes the mass production of a variety of laser illumination devices possible, such as 3D ID and LIDAR sensors. Glass micro-optics are suitable for the design of safety related laser illumination. In combination with shortest ramp-up time, the production of several million cylindrical glass lenses is now possible with LIMO’s new 12-inch production technology.