Design And Fabrication Of A Light Box Interferometry

ABSTRACT

 Light box interferometry is a well-developed and very old technique for optical measurements. The thesis describes the design of a vertical scan interferometer system to study the surface topography of surfaces down to nanometers. The desired properties of the system are its simplicity, portability and compact size, making it suitable for use in general labs and for educational purposes. By acquiring a sequence of images of the deformed fringe pattern, the surface topography can be observed, giving greater understanding of the surface roughness.

The principle behind the system is coherence peak sensing where the resulting fringe pattern of the object gets changed in accordance with its surface topography. To accomplish this, individual components of the interferometer were studied and a prototype was built in the lab. A series of experiments were performed which validate the working of the system. The results of the validation which are produced in the report give the accuracy of the system. The output from the prototype interferometer is processed by MATLAB to decode the surface topography of the object under measurement.  The design of the prototype is also discussed. Possible application of this device for sensing the surface topography of a cylindrical object is also put forward.

ABBREVIATIONS

CCD – Charge Coupled Device OPD – Optical Path Difference PZT – Piezoelectric Transducer LED – Light Emitting Diode GUI – Graphical User Interface

AVI – Audio Video Interleave

CHAPTER ONE

INTRODUCTION

Light box interferometry is a widely used technique for measurement of surface topography of objects over large areas. Systems which use this technique can measure areas which are equal to the field of view of the instrument. Interferometric optical topographers are widely used to study surface topography due to the high measurement accuracy, non-contact, rapid data acquisition and analysis.

Even though the conventional interferometers are extensively used they are built for ruggedness and have many features and different techniques of measurement integrated which are not effectively used by all. Moreover they have features for automated adjustment which can be done manually to some extent.

In this project we try to replicate the light box interferometer by constructing a proto type in the lab based on the light interferometry measurement technique. With measurement results from the constructed interferometer the feasibility for practical usage is discussed.

The report also presents the use of the technology on surfaces and analyzes the accuracy of the prototype compared with the commercial interferometer.

1.2                           AIM AND OBJECTIVE OF THE STUDY

Aim of this project is to construct a coherence peak light box interferometer and define its accuracy. To achieve this goal this project includes the design, construction, experiments and validation of the interferometer system.

1.3                                  ADVANTAGES OF THE STUDY

1. Capable of measuring a wide field of view.
2. Measurement in sub-nanometer range is possible.

iii. Quick measurement

1.4                                      PROBLEM OF THE STUDY

• No or limited angular characteristic
• Use is limited on certain objectslight box interferometers can only measure when there is a good reflection. There, it does not support measurement capabilities on a wide variety of objects.

  Measurements may also not be possible when there is a significant difference between the light reflected from the reference mirror and light reflected from the measurement area. (light box interferometers handle mirrored surfaces well, but cannot measure spiky or bumpy samples or nonreflective parts.

• Requires tilt correctionPrior to measuring, sample tilt correction must be performed using a goniometric stage. Tilted samples can cause closely-spaced interference patters, which hinder measurement accuracy. Some light box interferometry systems are equipped with a tilt mechanism that automatically corrects the sample tilt.
• Low resolution for XY stage measurements. The resolution for XY stage measurements are low due to the low number of sampling data sets (approximately 300,000). Some light box interferometry systems can scale up to use approximately 980,000 data sets.
• Sensitive to vibrationsPlace of installation is limited due to the equipment’s high sensitivity to vibrations. Shock-absorbing tables are necessary for installation.
• No or limited angular characteristic
• Use is limited on certain objectsLight box interferometers can only measure when there is a good reflection. There, it does not support measurement capabilities on a wide variety of objects.

  Measurements may also not be possible when there is a significant difference between the light reflected from the reference mirror and light reflected from the measurement area. (White light interferometers handle mirrored surfaces well, but cannot measure spiky or bumpy samples or nonreflective parts.

• Requires tilt correctionPrior to measuring, sample tilt correction must be performed using a goniometric stage. Tilted samples can cause closely-spaced interference patters, which hinder measurement accuracy. Some light box interferometry systems are equipped with a tilt mechanism that automatically corrects the sample tilt.
• Low resolution for XY stage measurementsThe resolution for XY stage measurements are low due to the low number of sampling data sets (approximately 300,000). Some light box interferometry systems can scale up to use approximately 980,000 data sets.
• Sensitive to vibrationsPlace of installation is limited due to the equipment’s high sensitivity to vibrations. Shock-absorbing tables are necessary for installation.

1.5                                         SCOPE OF THE STUDY

Light interference occurs when there is a difference in distance traveled by the light (light path) from the surface of a target object to a certain point; the light box interferometer uses this phenomenon to measure the surface roughness of a sample. The figure on the right is a structural diagram of an interferometer. The light emitted from the source (semiconductor laser, etc.) is separated into reference and measurement beams. While the reference beam is passed through the reference mirror through a half mriror, the measurement beam is reflected and guided to the sample surface. The passed beam is reflected by the reference mirror to the CCD image sensor and forms an interference pattern. The other beam is reflected off the sample surface, passes the half mirror, and forms an image through the CCD image sensor.

1.6                                    PURPOSE OF THE PROJECT

The light box interferometer is designed so that the optical path length from the CCD element to the reference mirror and that from the CCD element to the sample surface are the same. The asperity on the sample surface causes these path lengths to be unequal, which results in forming an interference pattern at the CCD element. The number of lines in the interference pattern is translated to peaks and troughs (heights) on the sample surface.

1.7                                  APPLICATION OF THE STUDY

Light box interferometry is used for different purposes. The main applications are:

• The measurement of chromatic dispersion. Here, the dispersive optical element is placed in one interferometer arm, and the detector signal is monitored while scanning the relative arm length through some range. Around zero arm length difference, interferometric wiggles occur, whereas the signal is about constant for large arm length differences. With strong dispersion, the recorded interferogram becomes broader. By applying a Fourier transform algorithm to the recorded interferogram, it is possible to retrieve the complex reflection or transmission coefficient of the device under test, and numerical differentiation reveals the wavelength-dependent group delay and chromatic dispersion.
• The measurement of distances. Compared with interferometers based on narrow-linewidth laser sources, the typical ambiguity issues are avoided. A special case is the measurement of surface profiles. For example, a Michelson interferometer with a CCD camera as detector may be used. Again, images are recorded for different arm length differences. Each pixel displays the interferometric wiggles around the point of zero arm length difference at the given transverse location. Unlike the situation in a narrow-band interferometer, no phase-unwrapping procedure is required, so that even rough surfaces can be easily handled.