Surface and Wavefront Interference Testing

Robert Smythe

Interference testing continues to expand in support of fields that are confronting increasingly stringent manufacturing tolerances, including the automotive, data storage, semiconductor and optical manufacturing industries. The performance, flexibility and adaptability of modern systems that are used in interference testing make them uniquely suited to meet these challenges.

Interferometric analysis and signal processing continue to evolve, as advancements in other technologies make new techniques possible. For example, many applications require specific wavelengths to enable measurement. Now, laser and incandescent light sources are available to meet these needs, and continued improvements in data processing are encouraging new approaches to measurement.

Automotive applications

Pump valve sealing surfaces and fuel injectors play key roles in meeting the strict regulatory requirements for fuel efficiency and emissions in the automotive industry. Sealing surfaces often have tolerances beyond the capabilities of traditional tactile gauges, but optical surface interferometry provides both the gauge repeatability performance and critical full-surface maps to effectively quantify these surfaces.

In this bore surface of an automotive fuel injector measured on a Zygo NewView 5000 interferometric profiler, the crosshatch pattern, roughness, microburr and form indicate critical part functions related to fit, performance and failure uncertainty. Interferometry enables simultaneous measurement of these functions in less than 10 seconds.

Because some precision-machined surfaces are not specular in the visible range, interferometers that operate at infrared wavelengths (10 to 12 μm) have been developed to measure them.

With the continued push to develop increasingly durable, efficient and high-performance engines, the use of interferometry will continue to grow in the manufacture of automotive components.

Data storage

The advent of audio and video applications for PCs has created a need for unprecedented levels of affordable storage. Manufacturers have continuously refined their designs and manufacturing technologies, resulting in subsequent generations of drives with geometrically increased capacity. Both the optical and magnetic data storage industries depend upon interferometry to develop next-generation products and to maintain quality control.

Optical data storage in the form of CDs and DVDs requires high-volume manufacturing of diffraction-limited aspheric optical components and assemblies. Interferometry has been key in the development of these formats, and it will continue to be an enabling technology for future standards. With magnetic hard drives, capacity is directly related to precise control of the gap or “flying height” of the head-disc interface. To precisely control the parameters affecting flying height, manufacturers use interferometry during development and manufacturing to achieve the nanometer tolerances of the magnetic head’s air-bearing surfaces.

Semiconductor manufacturing

Optical lithography tools are at the core of the semiconductor manufacturing process, and interferometry is essential to the development and fabrication of the lenses used in these tools. For example, homogeneity of lens blanks must be measured to less than 0.01 ppm. Low-noise interferometers and data-acquisition techniques are required to meet this level of performance.

Final lens assembly, alignment and testing are performed at the operating wavelength of the lithography lens. Operating wavelengths are currently in the deep-UV range, starting at 248 nm and going down to 157 nm. At these wavelengths, interferometers measure through lens wavefront performance to meet the nanometer-level accuracy that is required.

The next-generation lithography tools will operate in the extreme-UV at 13.4 nm, and interferometers have already been developed to measure beta optics for these critical optical systems.

Optics manufacturing

Optics manufacturing is driven by many industries and applications — from aerospace to digital cameras, to laser fusion research, to medical instrumentation — and each application has unique metrology needs. Some require in-process inspection for quality control on mass-production lines, while others may use customized setups for low-volume production or product development. Interferometry has both the flexibility and the precision to accommodate these divergent needs.

Several factors contribute to this flexibility. Modern laser systems have greatly expanded the wavelengths at which interferometers can operate. Systems can be designed with apertures ranging from 5 to more than 800 mm. Custom fixturing combined with stand-alone workstations increase throughput, minimize footprint and improve measurement accuracy. Innovations in analysis software expand the type and reliability of measurement data, while built-in statistical process control functions and real-time feedback help to regulate production processes.

Interferometry’s future

Interferometry will continue to be a fruitful and dynamic area of invention and product development. Currently, the main driving force is continued improvement of production tolerances in several industries, which necessitates new metrology solutions, many of which will require new interferometer designs rather than reapplication of old techniques. Interferometry has the inherent ability to measure an expanding set of applications into the foreseeable future.

Meet the author

Robert Smythe is vice president of marketing and sales at Zygo Corp. in Middlefield, Conn. For the past 20 years, he has been active in the design, development and marketing of interferometer systems for the optical, semiconductor and automotive markets.