Characterization of optical components using contact and non-contact interferometry techniques

-Advanced metrology for optical components *Yang Yu, #Mike Conroy, #Richard Smith

*Taylor Hobson Ltd, PO Box 36, 2 New Star Road, Leicester, LE4 9JQ, UK #Taylor Hobson China, Part A 1st floor No.460 North Flute Road, Waigaoqiao, Free Trade Zone,

200131 Shanghai, China

ABSTRACT

Advanced metrology plays an important role in the research, production and quality control of optical components. With surface finish, form error and other parameter specifications becoming more stringent, precision measurements are increasingly demanded by optics manufacturers and users. The modern metrologist now has both contact and non-contact measurement solutions available and a combination of these techniques now provides a more detailed understanding of optical components. Phase Grating Interferometry (PGI) with sub-nanometre vertical resolution and sub-micron lateral resolution can provide detailed characterization of a wide range of components including shallow and steep-sided optics. PGI is ideal for precision form measurement of a comprehensive range of lenses, moulds and other spherical or aspheric products. Because of the complex nature of these components, especially precision aspheric and asphero-diffractive optics, control of the form is vital to ensure they perform correctly. Recent hardware and software developments now make it possible to gain a better understanding and control of the form and function of this optics. Another change is the use of high speed 3D non-contact measurement of optics which is becoming more popular. Often scanning interferometric techniques such as coherence correlation interferometry (CCI) can be used to study components not suited to 2D contact analysis, including fragile surfaces and structured surfaces. Scanning interferometry can also be used to measure film thickness and uniformity of any coating present. In this paper the use of both PGI and CCI to measure optical lenses and coatings is discussed. Keywords：Metrology, interferometry, aspheric, roughness, coatings

1. INTRODUCTION

1.1 Optical lens

In order for an optical system to achieve a more complex and precise functionality, the combination of lenses made by traditional spherical optics are becoming more complex and the whole optical system becomes heavier and larger. In recent years, more and more modern optical designers have been employing aspheric and diffractive optics to reduce the number of lenses needed. One aspheric or diffractive lens can replace several conventional spherical lenses and as a result the weight, cost and space used are all reduced, achieving a more compact and better performing optical system. Diffractive lenses are normally used to correct for the “chromatic aberration” by optical designers; aspheric optics can be used to reduce or eliminate the “spherical aberration”, so as to improve the focus quality. Traditionally, a number of lenses with different dispersions and opposite errors are combined together to reduce the chromatic and spherical aberration. Among the various optical technologies, diffractive optics technology is providing new and powerful degrees of freedom for lens design and optimization of optical systems. The asphero-diffractive lens has become even more popular recently because it can significantly reduce the number of lenses in an optical system. More crucially, it can greatly minimise chromatic aberration and spherical aberration by means of the property of compensation, where its diffracted zones can be adopted to compensate for chromatic aberration arisen from the refractive properties of the lens.

6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Optical Test and Measurement Technology and Equipment, edited by Yudong Zhang, Libin Xiang, Sandy To,

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Nowadays, aspheric and diffractive lenses are amongst the most demanding of ultra-high precision form measurement applications. These lenses have a wide range of applications such as mobile phone cameras, DVD read/write heads, cameras, military night sights, missile nose cones, high power LED optics, Blu-Ray DVD optics, projectors, and medical and infra-red thermal imaging as used for rescue systems and security. Lens form is one of the most important optical design parameters to control the quality of the precision aspheric and asphero-diffractive optics to ensure they perform as required. In addition, surface roughness also affects the performance of the optical lens. Precision measurements of the form and surface roughness are therefore very important. With the rapid evolution of the optics, suitable advanced metrology tools are necessary for characterizing lenses with more complex shapes, of various sizes and made of different materials. A number of metrology tools have been employed to measure the aspheric and asphero-diffractive lenses. For instance, contact stylus profilometry and non-contact interferometry techniques. Phase Grating Interferometry (PGI) is a contact stylus profilometry, which can offer larger gauge range to resolution when compared with other tradional profilometers, such as inductive gauge and laser interferometers. Coherence Correlation Interferometry (CCI) instrument is a coherence scanning interferometer which is an advanced 3-dimensional non-contact optical metrology tool used for advanced surface characterization. The technique is fast and accurate and provides a high resolution of 3D image together with analysis that includes 3D roughness, 3D form analysis and 2D profile measurements

1.2 Optical coating

Most optical coatings are used to enhance the reflectance and transmittance properties of a substrate material. They usually consist of one or more thin layers of material, deposited on an optical component such as a lens or mirror. They fall into a number of categories such as 'anti-reflection coatings', 'high-reflection coatings', 'extreme ultraviolet coatings' and 'transparent coatings.' The optical coatings are used widely in numerous technologies and the list of applications is growing all the time. Typical applications include coated spectacles, camera lenses, LCD screens, mobile phones and astronomical telescopes. For example, most flat panel displays including LCD, OLED, and many other display technologies employ transparent conductive oxides (TCOs) to transport current. It is very important to measure the thickness of liquid crystal layers and for OLED displays the layers such as emissive, injection, buffer, and the encapsulation layer. Ever demands are leading to advances in optical coating techniques, and measuring these coatings quickly and accurately is essential in producing high quality optical products. It is essential to control both thickness and uniformity for most optical coatings in order to ensure the quality, efficiency and function of optical devices. An accurate and fast metrology tool is therefore essential. A number of metrology tools have been employed to measure optical coating thickness. These include conventional methods of spectrophotometry, ellipsometry, and physical step measurement [1-4]. Coherence Scanning Interferometry (CSI) is becoming a popular technique because of its high lateral resolution and speed. However, one of the limitations of traditional interferometry is the thickness of the coating that can be measured. Typically it needs to be larger than 1.5 µm to obtain accurate data. It is now possible to measure thicknesses down to 50 nm or less using Coherence Correlation Interferometry (CCI) together with HCF (helical complex field) techniques. Other methods have also been used to investigate the film thickness, for example wavelength scanning interferometry, prism coupler and thermal wave detection with a laser beam[5,6].

2. MEASUREMENT TECHNIQUES

2.1 Phase Grating Interferometry (PGI)[7]

The ‘Phase Grating Interferometry’ (PGI) gauge employs a cylindrical holographic grating, fitted to the end of the pivoted stylus arm, which rotates as stylus moves along the surface. The grating is illuminated using a collimated laser beam derived from a low power laser diode. Four photodiodes are used to detect the interference signal formed by super-

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imposing the shifted diffracted beams generated from the rotation of the curved grating, which reflects the stylus movement. The phase of an interference signal corresponds to the stylus position and therefore provides the information for the surface profile. Typically, this metrological gauge has a range of 12.5 mm with a resolution of up to 0.2 nm and when compared with traditional inductive gauge and laser interferometer gauge, this type of transducer offers a larger gauge range to resolution whilst giving a reduction in physical size due to the use of a laser diode as opposed to a gas laser. The measurements described in this paper were carried out on a PGI Dimension Phase Grating Interferometry instrument. The PGI Dimension is based on two technologies – Aspheric profilometry and high accuracy roundness. It is an important extension of the PGI gauge technology by means of employing a high accurate air bearing spindle to improve the measurement accuracy of the aspheric lens, asphero-diffractive lens and other different type of lenses and moulds. This precise metrology system is designed to fit within manufacturing cycle times. It can measure and analyze a wide range of shapes, with slopes up to 850, diameters up to 300 mm and sags up to 50 mm with an auto high accurate alignment of the rotate part axis and a patented proven fusion process; the alignment accuracy is less than 0.3 µm for centring and smaller than 0.0080 for levelling.

2.2 Coherence Correlation Interferometry (CCI)[8]

The CCI is a coherence scanning interferometer that uses special patented coherence correlation algorithms [8]. The CCI instrument is equipped with a high resolution digital cameral array for use as a 3D non-contact optical metrology tool optimized for advanced surface characterization. These instruments provide a true morphological representation of a surface with sub-nanometer vertical resolution and offer a range of lateral measurement areas by use of different objective lenses. The lateral resolution is mainly determined by the wavelength of light and the NA (Numerical Aperture) of the objective lens. Individual images can be stitched together to produce a verge area measurement. An important extension of the CCI’s capability is the non-destructive measurement of film thickness of transparent and semi-transparent thin films. CCI HD is a CCI instrument equipped with a 4M pixel digital camera, which also can provide different type of film measurements.

2.3 Traditional thick film analysis

When the thickness of the film is larger than ~1.5 µm (depending on refractive index and NA), the CSI interaction with the layer results in the formation of two fringes, each corresponding to a surface interface. This is shown in Fig. 1. The CCI algorithm can therefore be used to determine the thickness, by locating the positions of the two envelope maxima, assuming the refractive index is known. In addition, the surface structure of both the top surface (air/film) and the bottom surface (film/substrate) can also be obtained.[9]

2.4 Film Thickness Analysis

For thicknesses of films less than ~1.5 µm the two CSI-formed fringes overlap and appear as a single interference fringe bunch as shown in Fig. 2. Such film thicknesses cannot be accurately calculated using the thick film technique. An alternative method has to be employed. The recently introduced ‘helical complex field’ (HCF)[9] function has therefore

Fig.1 Single pixel measurement from a 7 µm thick film

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been incorporated to extract the film information. In order to obtain accurate film thickness, it is necessary to have a prior knowledge of the dispersive film index n. HCF can be used for the measurement of film thickness within the range of ~50 nm to ~5 µm.

3. MEASUREMENTS AND RESULTS

3.1 Measurement results of optical lenses using Phase Grating Interferometry (PGI) and Coherence Correlation Interferometry (CCI)

3.1.1 Measurement of asphero-diffractive lens using PGI Dimension

A large and shallow asphero-diffractive concave lens with a 75 mm diameter and sag of 0.5 mm was tested using PGI Dimension. The ratio of the sag against the diameter is less than 0.01, making the centring and levelling of the measurement very challenging for most of the instruments. The PGI employs a patented feature of advanced centring and levelling, which utilizes the principal of symmetry to align the part to real aspheric axis, and makes the precise measurement of a large and shallow lens possible. The measurement process and the results are shown as below.

Fig.2 A single pixel fringe envelope from an actual interferometry measurement of Ta2O5 thin film (270 nm)

a). Profile of asphero-diffractive lens showing the location of diffractive zones

Asphero- Diffractive lens – analysis process

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3.1.2. Tests of process improvement after PGI Dimension measurement

An asphero-diffractive convex lens with a 60 mm diameter and a 3 µm step height was measured before and after the compensation of the lens form during the manufacturing process using PGI Dimension. The test results are shown as in Figure 4.

b). Diffractive zones: Aspheric form removed

c). Asphero-diffractive residual form error

Figure 3. Measurement of a large and shallow Asphero- diffractive concave lens using PGI Dimension

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The tests indicate that PGI Dimension allows manufacturers to precisely control key parameters such as underlying form error, step height and zone position during and after the manufacturing process, in order to ensure good quality of the optical lens so that the lens can perform correctly to cut the production costs.

3.1.3 Miniature lens measurement using CCI

Small aspheric lenses are often made of glass or plastic. They are mostly made by moulding. These lenses are generally used in inexpensive consumer products such as cameras, camera phones and CD players due to their low cost and good performance. They are also commonly used for laser diode collimation, for coupling light into and out of optical fibers due to their capability of correcting the “spherical aberration”. A 0.4 mm diameter aspheric concave glass lens was tested using CCI instrument. CCI was selected to measure this part because it may be too difficult for PGI Dimension to make an accurate measurement regarding the fixture, centring and levelling. The advantages of CCI technique are that it is non-contact and therefore non-destructive, and there are no strict requirements on the fixture. Particularly for these small sizes of optical components, CCI provides rapid and accurate 3D morphological measurements. It can provide both 3D roughness results and form error in one measurement to completely suit the requirements from the optical manufacturers.

Length = 49 mm Pt = 3.97 µm Scale = 10 µm

0 5 10 15 20 25 30 35 40 45 mm

µm

-5-4-3-2-101234

a). Pre-compensation with aspheric form removed b). Post-compensation with aspheric form removedLength = 49.8 mm Pt = 3.06 µm Scale = 5 µm

0 5 10 15 20 25 30 35 40 45 mm

µm

-2.5-2

-1.5-1

-0.50

0.51

1.52

Test of process improvement on an Asphero-Diffractive lens

8x improvement

- 60mm convex radius - K= -1 - Diffractive coefficient -4e-5 - 3um Step Size

1: Pre-compensation 2: Post-compensation

c). Residual error comparison with Asphero-Diffractive form removed

1

2

Pt1 = 1.10 µm Pt2 = 0.14 µm

Design data of the axisymmetric diffractive lens

Figure 4. Test of process improvement after the measurement using PGI Dimension on an Asphero-Diffractive convex lens

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Note: Only 15 seconds was taken for the measurement

3.2 Tests of optical coating using Coherence Correlation Interferometry (CCI)

3.2.1 Testing of the HCF approach

This work was carried out with the measurements of SiO2 films on Si substrate. 5 samples ranging in thickness from ~50 nm to ~2000 nm were tested using CCI HD (HCF). The results are shown in Tab.1.

Figure 5. A 0.4 mm diameter glass aspheric concave lens was measured using CCI

b). Extracted profile from 900 - 2700

µm

0

0.025

0.05

0.075

0.1

0.125

0.15

0.175

0.2

0.225

0.25

0.275

0.3

0.325

0.35

0.375

0.4

0.425Sq 0.077109 µm Sa 0.057006 µm St 0.42984 µm

a). Aspheric form removed 3D topography

Length = 0.40378 mm Pt = 232.08 nm Scale = 600.00 nm

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 mm

nm

-300

-200

-100

0

100

200

300

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Tab.1 Correlation of HCF technique with other common techniques

Sample VLSI sample set Other samples

Film/substrate SiO2/Si SiO2/Si SiO2/Si SiO2/Si SiO2/Si Film thickness )(nm) (ellipsometry)(NIST) spectrophotometry

47.0 191.3 1049.1 294.8 2077.7

Film thickness (HCF) (nm) 47.0 191.2 1055.7 297.4 2080.4

% Difference 0.01 0.03 0.63 0.89 0.13 The results from the above table show good correlation between the CCI/HCF approach and ellipsometry/spectrophotometry for thicknesses of SiO2 films on Si substrate ranging from ~50 nm to ~2 µm.

3.2.2 Comparison test between the HCF approach and physical step measurement

In this work, a PTB standard which contains a 100 µm strip with SiO2 coating on Si substrate has been measured by using CCI HD and Nanostep. The test results are given in Tab.2.

Tab.2 Comparison of results between film thickness analysis (HCF) and physical step measurement technique

Nanostep (nm) CCI HD (nm) Difference (nm) Accuracy

290.6 292.7 2.1 0.7%

Repeatability (σ) 0.6

Note: Standard deviation was calculated by means of 20 measurements The results show that good agreement was obtained between the HCF technique and physical step measurement.

3.2.3 Tests of optical coating using Coherence Correlation Interferometry (CCI)

Tantalum pentoxide (Ta2O5) was selected for the tests as it is a typical surface coating material used in the optical industry. It has the required properties of high-index and low-absorption for optical coatings, usable in the near-UV (350 nm) to IR (~8 µm) wide regions. It can sometimes be with silicon dioxide (n = 1.48) for UV laser applications. It is also suitable for hard, scratch-resistant and adherent coatings.

A sample was prepared of BK7 glass partly coated with Ta2O5 (nominal thickness 270 nm). This sample was chosen because it would be expected that the standard CCI step height measurements should give an accurate measurement of the film thickness.

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A series of film thickness measurements (HCF) were made on the Ta2O5 coating where close to the intersection line between the coating and the glass, in order to compare with a standard step height measurement. As expected, the results of film thickness measurement (HCF) well agree with the step height measurement.

The results show that the HCF technique is ideal for measuring optical film thickness where no substrate exposed, and is an ideal way to measure the film thickness uniformity across a coated surface.

4. CONCLUSIONS

PGI dimension provides accurate 3D form measurements for a wide range of optical lenses and moulds, with different steepnesses and sizes. The 3D non-contact CCI instruments provide rapid and precise topographical measurements. This technique is particularly suitable for the measurements of some optical lenses or moulds with miniature sizes, shallow shapes and made of malleable materials. PGI dimension plus CCI instruments can therefore cover most requirements of the surface characterization for all the optical industries. The development of the helical complex field (HCF) function together with Coherence Correlation Interferometry provides us with the ideal metrology tool to perform fast and accurate measurements of coated optical surfaces.

5. ACKNOWLEDGMENTS

The authors would like to express thanks to Erik Stover and Danny Mansfield at Taylor Hobson for their support with measurements and valuable discussions.

REFERENCES

1. Coates., U.S. Patent 4308586 (29 Dec, 1981) 2. Opsal et al., U.S. Patent 6753962B (22 Jun, 2004) 3. Débora Gonçalves* and Eugene A. Irene , Fundamentals and Applications of Spectroscopic Ellipsometry, Quim.

Nova, Vol. 25, No. 5, 794-800, 2002

-50 0

50 100 150 200 250 300

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Lateral dimensions (mm)

Z height (nm)

CCI Step Height Measurements Film Thickness Measurement

Fig. 6 Graph comparing standard CCI measurement data with thin film measurement of Ta2O5 thin film coated on BK7 glass

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4. Andrew R. Hind PhDa and Lisette Chomette, The Determination of Thin Film Thickness using Reflectance Spectroscopy, Varian UV At Work No. 090

5. Rosencwaig et al., U.S. Patent 45225510 (11 Jun, 1985) 6. R.Ulrich and R.Torge, Measurement of Thin Film Parameters with a Prism Coupler, Appl. Opt. 12(12), 2901-

2908(December 1973). 7. Ian K. Buehring and Daniel Mansfield, U.S. Patent 5517307 (14 May, 1996) 8. A.Bankhead et al, Interferometric Surface Profiling, GB2390676, 2004 9. Mansfield D, Thin Film Extraction from Scanning White Light Interferometry, Proc. of the Twenty First Annual

ASPE Meeting, Oct 2006

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