White light interferometry for surface profiling with a colour CCD

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Abstract

In laser based interferometry, the unambiguous measurement range is limited to half a wavelength. Multiple wavelength or white light interferometer is used to overcome this difficulty. In this paper a white light interferometer with a colour CCD camera is discussed. We access interference intensity information from the three channels of the colour CCD simulating three-wavelength measurement. This makes the data acquisition as simple as in single wavelength interferometry. The unambiguous measurement range however gets limited by the coherence length of the CCD. The usefulness of the proposed method is demonstrated on a micro-sample.

Highlights

► A white light interferometer system using a single-chip colour CCD for 3-D surface profiling on micro-samples is demonstrated. ► The proposed method makes the data acquisition and phase evaluation procedures as simple as in case of single wavelength method. ► The use of colour CCD camera in white light interferometry makes the measurement faster, simple. ► And cost effective, which is very much useful for industrial applications.

Introduction

Laser interferometry using phase shifting technique is a widely used tool for precision surface metrology [1], [2]. However, the unambiguous measurement range for the laser is limited to half a wavelength. The unambiguous range can be increased using the more wavelengths [3], [4], [5], [6], [7], [8]. Lasers have been used for this purpose. White light interferometry (WLI) is also a state-of-the-art technique for measuring discontinuous surface profiles [9], [10]. WLI combining with advanced CCD cameras, computers, image processing cards, and software packages has given an extremely powerful measurement tools. Extremely short coherence length of the white light source results in high contrast fringe occurs only when the optical path difference (OPD) is close to zero. The 3-D plot of the axial positions of the zero OPD along the optical axis represents the surface profile of the object under test. Compared to single wavelength phase shifting interferometry, the white light interferometry is rather slow, because the number of frames to be recorded and evaluated is rather large. The spectrally resolved white light interferometry (SRWLI) [11], [12], on the other hand gives only a line profile of the object, although the requirement on number of frames is similar to the single wavelength phase shifting interferometry.

In this paper, we describe a single shot white light microscopic interferometer with a colour CCD camera. For recording of interferograms a three-chip or a single-chip colour CCD cameras can be used. Maximum resolution will be achieved using 3-chip colour CCD. In this investigation, however, a single chip colour CCD camera is used and the data from the three channels is used as three wavelengths source. We believe that such experiment has not been done before. Typically five to six frames are required to use phase shifting algorithm for phase calculation. Thus the data acquisition is as simple as in single wavelength case. The stored phase shifted interference data from RGB (red–green–blue channels) is then separated into its components. The phase at individual wavelength is calculated using phase shifting algorithms. The combination of white light interferometry with colour CCD camera makes the measurement faster, simple, and cost effective. There is a practical issue in implementing this approach with a phase-shifting technique. The single chip-CCD has three separate spectral bands, R, G, B centred at Red (λ1=620 nm), Green (λ2=540 nm), and Blue (λ3=460 nm) wavelengths, respectively. The phase shifts are usually produced by using a piezoelectric transducer (PZT) to change the length of one of the optical paths. The phase step produced at single wavelength can be set precisely at 90°, but the same motion of the PZT introduces a phase-step miscalibration at the other wavelengths. This problem can be overcome in various ways [8]. In the current work, first the phase shifts (αi) at individual wavelengths are calculated and then used in the 5-step phase equation to calculate the error free phase [13] as explained in Section 3. For comparison we have also used the higher order (8-step) phase shifting algorithm, which has a higher tolerance of ±20% for phase-shift miscalibration error and is well suited for RGB wavelength analysis [8], [14]. Use of this approach combining white light interferometer with colour CCD camera increases the unambiguous range for surface profile measurement. We have also applied the two weavelength procedure (equivalent wavelength) to get unambiguous result, which is limited to half the equivalent length. Experimental results on a micro-specimen are presented.

Section snippets

Optical interferometry

The schematic of the microscopic white light interferomeric arrangement as shown in Fig. 1 is used for the measurements. The white light beam is collimated using a collimating lens (CL). The collimated beam illuminates the micro-specimen via the beam splitter (BS) and the Mirau objective. The specimen is mounted on a 3-axis stage for alignment. The microscopic imaging system consists of a 5X, 10X, 20X microscopic objectives. The Mirau interference pattern is stored using a high resolution JAI

Theory of measurement of large discontinuities

The intensity distribution of the phase shifted frames corresponding to any one wavelength of the white light interferogram can be expressed asIin=Io(1+Vcos(ϕi(z)+(n1)αi))where Io is the bias intensity, V, the visibility ϕi, the phase; n, the frame number, and αi, the phase shift for wavelength λi with i=1, 2, 3.

To evaluate the phase, we use the 5-step (n=5) algorithm [13], which gives the phase asϕi5=arctan{2sinα(Ii2Ii42Ii3Ii5Ii1)}where αi=arccos{1/2(Ii5Ii1/Ii4Ii2)}, the unknown phase

Experimental results

Fig. 2(a) shows the white light tilt fringes on reflective sample obtained with the system shown in Fig. 1. The dimensions of the stored RGB image are 2456×2058×3 pixels. Each pixel contains the information regarding the red, green, blue wavelengths. It is then separated into its monochrome R, G, and B components of dimensions 2456×2058. The decomposed components at Red (λ1=620 nm), Green (λ2=540 nm), and Blue (λ3=460 nm) are shown in Fig. 2(b). Multiple frames are recorded to use phase shifting

Conclusions

We have demonstrated a white light interferometer system using a single-chip colour CCD for 3-D surface profiling on micro-samples. White light phase shifted frames are stored using the colour CCD and then decomposed in to individual R, G, B images. In the proposed method the data acquisition and phase evaluation procedures are as simple as in case of single wavelength method. The phase subtraction method can increase the measurement range but the resolution of the profile will be reduced. On

Acknowledgement

This paper is based on a presentation made at XXXVI OSI Symposium On Frontiers in Optics and Photonics (FOP11), IIT Delhi, December 3–5, 2011. This work is supported by Defence Research and Development Organisation (DRDO).

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