Measuring Precision Optics with Combined Measurement Techniques

Table of Contents

Introduction
Overview of Optical Profiling Systems
Complex Optical Systems
Measurement Using Multiple Tools
Conclusion

Introduction

Microtechnology, materials science and photonics are just some of the technological fields that play a vital role in mainstream manufacturing. Complex MEMS structures, for instance, are widely employed in consumer electronics, and increasingly complex multi-layer structures are used to improve photovoltaic solar cell efficiency. Similarly, precision optics can now be rapidly replicated as a complex aspheric, rather than being ground separately.

Microstructures have been widely used in various high-tech applications in order to obtain specific characteristics, from diffraction effects to induce specific optical performance in optics, surface enhancement effects for enhancing wear characteristics or passivity, or microfluidic anomalies for processing small quantities of liquid materials. Moreover, application-specific micro- or nanotexturing is gaining popularity in LED/OLED, biophotonik, medical and other technology fields.

With the increase in the complexity of these structures, and therefore in the demands on manufacturing repeatability, the tools required to gauge their designed performance require great versatility.

Overview of Optical Profiling Systems

The demand for powerful, rapid, and manufacturing-friendly optical profiling systems, both for quality assessment in the production phase and as a useful tool in the design stage, continues to grow significantly. These systems should be capable of measuring smooth, rough and stepped surface topologies and textures for different materials and complex structures.

Although this task can already be accomplished with several commercially available technologies, only some of the profiling systems use complementary optical techniques to improve a single non-contact measurement system’s flexibility. Sensofar Metrology of Terrassa/Barcelona, Spain, led the integration of various optical tools, combining optical/confocal microscopy with corresponding interferometric techniques, and more recently with focus variation, thereby improving the versatility of their optical profiling systems.

Phase shift interferometry is a widely used optical profiling technology with sub-nm vertical resolution even at low optical magnification. It is suitable for smooth continuous surfaces, but unfit for steep slopes and rough surfaces. It can exhibit ambiguity at surface discontinuities.

White light interferometry can be used on moderately rough surfaces, while a confocal measurement method with a high NA objective can measure both steep slopes and rough surfaces over a wide depth range.

Modern high-end profiling systems use at least one the above techniques so as to ensure high flexibility for measuring a range of surface forms and structures as shown in Figure 1.

Figure 1. An extended topography of an LED array

Complex Optical Systems

Application-specific lenses are beginning to become widely used in scientific and consumer applications. Microlenses, for instance, are an essential component of portable microprojector designs. Fresnel lenses are capable of directing light from high-power white light LEDs, allowing for a compact design, low power consumption and high light throughput.

In biomedical applications, special optics can enhance imaging in endoscopes and the “lab on chip devices” used for comparing tissue samples. Other examples include optical couplers for VCSELs in telecommunications, and microlenses that concentrate light into a microscopic waveguide. All these applications use unique optical designs to meet the application-specific requirements. The production number for each application may vary from the hundreds into the tens of thousands.

In this case of complex aspherical optics, deliberately straying from a true spherical shape enables spherical aberration of a singlet lens to be reduced and/or specific optical performance can be achieved. In addition, integration of diffractive effects into one lens surface can correct the overall chromatic dispersion, thereby ensuring excellent overall color performance.

With the help of an optical polymer coating in a replication process, standard spherical lenses can be economically ‘aspherized’. Small or large numbers of lenses can be pressed using a mold, as shown in Figure 2. Complex, non-spherical optical designs, and some diffractive structures, can now be diamond-turned into a mold. Although these molded lenses cannot provide absolute high-end optical performance, they can achieve a level of advancement that is ususally only possible through the use of doublet or more complex lens systems. Free-form lenses are even more difficult to engineer. They can either be diamond-turned directly or manufactured through a mold process.

Figure 2. A molded high-precision lens and the measured profile

One issue related to designing and quality control of such optics is that the uniqueness of most non-rotationally symmetric designs limits the usefulness of interferometric measurement methods that use standard reference surfaces. This problem deepens with increasing design complexity, such as for surface discontinuities and/or a degree surface slope that cannot be measured easily.

Precise measurement of surface roughness of a molded lens, as shown in Figure 3, is used to determine whether the optical scatter is acceptable. In addition, transmission can be enhanced in specific spectral regions by coating the optical lenses, and hence it is key that the quality of any thin film structures is determined.

Figure 3. Surface finish on a high-precision lens – remnant surface structure is clearly visible

Measurement Using Multiple Tools

A combination of non-contact technologies can be used for an in-depth characterization of a lens or lens array and for high-precision optical surfaces. When so equipped, optical profiling systems can deliver complementary assessment of the surface properties and profile.

High-end systems, such as Sensofar’s “S neox”, deliver sub-nanometer (nm) resolution on all axes. They include the following features:

  • Phase-shift and white light vertical scanning interferometry provides sub-nm vertical resolution over smooth surfaces, or less accurate but more robust profile measurements for rougher surfaces respectively.
  • Confocal metrology provides robust, high vertical and lateral resolution on steep and complex surfaces, where modern micro-imaging techniques are used to enhance the acquisition process
  • White light interferometry (WLI) and spectral reflectometry provide information on the structure of thin films, with overall thickness ranging from sub-nm to 10µm
  • Twin CCDs are used video and metrology tasks
  • Four LEDs – red (630 nm), green (530 nm), blue (460 nm) and white

The above features can be combined differently so as to meet specific applications. For instance, a confocal microscopy technique can be adapted to quickly evaluate the basic form of rotationally symmetric lenses. In confocal microscopy, the light that does not emanate from the exact focal plane of the microscope is prevented from hitting a detector. Utilization of structured illumination allows the surface profile of an optic to be mapped out at a lateral and vertical resolution of 1 nm and 0.5 nm, respectively, through an algorithm that focuses a confocal microscope on the surface of the optic and tracks the necessary movement with a linear axis system.

Sensofar’s Confocal Tracking is the key technology in the “PLu apex” system, as shown in Figure 4. Complementary “SensoTRACK” software features a user-friendly interface to simplify sample measurement, and a basic set of tools to display and analyze data. Form assessment can be carried out using a powerful aspheric curve analysis module.

Figure 4. Sensofar’s PLu apex measurement system for aspheres, utilizing “Confocal Tracking” (right)

A full suite of metrology tools within a single system, such as in Sensofar's “S neox”, can be used for complex surface topologies, including microlens arrays for MEMS/MOEMS systems shown in Figure 5 or in photovoltaic solar cell structures. The combination can prove advantageous at the quality control stage.

Figure 5. Thorough examination of MEMs structures usually requires several complementary optical techniques

Conclusion

Micro-optical measurement technology meets two fundamental requirements of metrological applications: a combination of non- destructive measurement and high accuracy. The integration of thin film metrology capability, microdisplay technology and powerful and flexible analysis algorithms allow Sensofar’s systems to stand unique in this field of technology. In addition, the compact design of the sensor head allows for the possibility of many different configurations in the system. It possible to set-up the system for R&D and quality inspection laboratories, and to allow complete and sophisticated arrangements for on-line process control.

This information has been sourced, reviewed and adapted from materials provided by Sensofar.

For more information on this source, please visit Sensofar.

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