Glass

SPECIAL SECTION/GLASS: Non-Contact Glass Testing

February 1, 2010
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Sophisticated but easy-to-use optical interferometers provide measurements of optical surfaces for large-scale form down to microscopic surface roughness.



Many of the devices used to make our daily lives easier, more fun, more convenient and more secure are driven by glass manufacturing technologies. Applications as far-ranging as DVD and Blu-ray disc players to display systems and military target acquisition systems all have glass optics at their core. The techniques used to shape and condition these glass surfaces are also used to meet similar surface requirements in other materials, such as ceramics and metals.

No matter what the material, various functions of these components are driven by the surface’s various measurable characteristics. Using a multitude of technologies, parameters such as surface form to surface roughness can be easily tested.

Transforming a bulk piece of glass, ceramic or metal into a precision element involves a number of manufacturing operations, ranging from simple grinding to precision polishing and surface texturing. Each step of the process offers the opportunity for inspection and quality control. For some of these measurements and processes, a tactile technique has traditionally been used. With the development of sophisticated but easy-to-use optical interferometers, measuring these optical surfaces for large-scale form down to microscopic surface roughness in a non-contact manner is a reality.

Figure 1. A precision-polished optical flat.

Why Non-Contact?

Physicists have long known that the act of observation affects that which is being observed. When it comes to tactile metrology techniques, this is a very real phenomenon. While relatively simple to perform and easy to understand, tactile measurements rely on actually contacting a work surface with a type of metrology probe.

For large-scale measurements with a coordinate measurement machine (CMM), small point contacts might be made on the surface. For roughness or profile measurements made with a tactile profilometer or atomic force microscope (AFM), the gauge is dragged along the surface. When these techniques are employed, the act of measuring the surface can cause changes or even damage the surface being measured. With non-contact metrology techniques, this uncertainty factor is removed. In many cases, the data density and speed of a non-contact measurement is actually faster than a method that requires contact with the surface.

Laser interferometers* provide an ideal way to measure the surface of a glass sample for large-scale form measurements. For decades, laser interferometers have been used to measure flats and spherical optics. Recent advances in automation techniques and computerized analyses have enabled laser interferometry to characterize even some of the most complex aspheric surfaces that are critical to so many of today’s compact and lightweight  optical systems.

While the application of the laser interferometer is rather well-known and entrenched in optical manufacturing for measuring low-frequency surface shapes, a complementary technology using white light is exceptionally powerful for characterizing mid- and high-frequency waviness and roughness. Known as scanning white light interferometry (SWLI), this technique is at the core of non-contact 3-D optical profilers.**

SWLI tools typically possess a higher lateral resolution and a smaller field of view that results from the higher magnification of the microscope objectives that are used. This allows them to measure smaller samples and smaller surface structures that might not be seen by a large-aperture laser interferometer.

SWLI instruments also differ from laser interferometers in their illumination; SWLI instruments use broadband white light, as opposed to monochromatic laser illumination. This white light has a broad wavelength range that enables the optical profiler to measure discontinuous surfaces and a large vertical range, which would confound the laser interferometer. When used together, though, the laser interferometer and the 3-D optical profiler can provide a complete non-contact metrology suite that addresses the entire range of surface structures that may be present on an optical surface.

*Examples include ZYGO's GPITM and VeriFireTM.
**ZYGO NewViewTM family of optical profilers.

Figure 2. A ground optical surface showing a 2 µm peak-valley surface figure measured over a 100 mm field of view using grazing incidence interferometry.

Grinding

One of the first steps in turning raw glass into a finished optical element is grinding the surface into a nominal shape. These surfaces are typically very rough and diffuse with large surface deviations, making them more difficult to measure in a typical laser interferometer set-up.

Solutions for form metrology of rough surfaces include the use of a longer source wavelength and/or arranging the test cavity in a grazing incidence configuration, as shown in Figure 2. The SWLI instrument, with its broadband light source, can be used to make quick inspections on these surfaces to measure roughness and localized form.

Another application at this stage of the manufacturing process can be to inspect for the presence of tooling marks from saws or cutting discs. These tooling marks can impact the duration and quality of future finishing operations, so minimizing them at the start of the process can be critical.

Figure 3. A ground glass surface under high magnification. This surface has a peak-valley departure of 13 µm and a surface roughness of 1.3 µm Ra.

Polishing

Once the base shape is formed through grinding, the optical element typically proceeds through polishing operations. It is at this stage that the form of the optic is easily measurable at a visible wavelength and normal incidence by the laser interferometer. The optical profiler adds great value at the polishing stage as well, since the surface texture of the finished optic can have a dramatic effect on the efficiency and light scattering characteristics of the surface.

An optical element that is designed as a mirror in the infrared will not require a surface nearly as smooth as a mirror designed for visible or ultraviolet illumination. Using the optical profiler to characterize the surface texture of the optic during the polishing stage can save valuable manufacturing time and money by ensuring that work is only spent improving the surface quality of an optic that requires it.

Surface textures on the order of 1 nm Ra are routine for SWLI optical profilers, and the wide range of measurement capabilities of the optical profiler makes it equally valuable for both rough and super-polished optics. Averaging multiple discrete measurements enables SWLI profiler results of better than 0.05 nm Ra (average roughness) on optical surfaces.

Figure 4. A 0.5 x 0.7 mm area on the flat shown in Figure 1. Notice the small polishing sleeks that appear as raised defects in the surface map. The surface roughness is measured to be 0.05 nm.

Coatings

After optical shaping and polishing, the specific application of an optic might require that it be coated with one or more layers of transparent optical films. These transparent optical films can complicate traditional approaches to white light interferometry, since each of the layers produces its own interference signal. The multiple interference signals can overlap and make it challenging to measure the surface.

Recently, a new software module† was developed that enables the SWLI measurement of the top layer of optical films that are thicker than 400 nm and the topography and thickness of films that are more than 1000 nm thick. By enabling the measurement of a coated optical surface, manufacturers can finally observe the final manufactured optical surface in a completely non-contact way.

Fioms Analysis software module for the NewView, developed by ZYGO.

Measurement Environment

The reduction of environmental disturbances is strongly recommended to achieve the lowest measurement uncertainty (highest accuracy) in any interferometric application. Temperature, vibration and acoustic noise can all affect the uncertainty of an interferometric measurement.

In some applications, however, it is not possible to operate in an optimum environment. For mild vibrations, software†† can be used to eliminate vibration-induced “fringe ripple.” In the case of more-challenging environments, dynamic acquisition technology enables accurate form measurements in the presence of extreme vibrations.+++

††Vibration Correction software can be used on both the NewView and VeriFire phase measuring systems.
+++ DynaPhase dynamic acquisition software enables metrology in the presence of extreme vibrations with the VeriFire instruments.

Complete Solution

As applications for optical components continue to expand, metrology demands on optical manufacturers are surely going to increase. Optical interferometry-using both laser-based, large-aperture laser interferometers and smaller-aperture scanning white light optical profilers-offers manufacturers a complete non-contact solution for characterizing optical surfaces at all stages of manufacture. In addition, technological advances in software and automation have increased the applicability of interferometry from basic and simple surfaces to complex aspheres and coated optics.

For additional information regarding glass testing, contact Zygo Corp. at Laurel Brook Rd., Middlefield, CT  06455-0448; (860) 347-8506 or (800) 994-6669; fax (860) 347-8372; e-mail inquire@zygo.com; or visit www.zygo.com.

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