True color white-light interferometry: Fine details image of test sample surface Roughness measurement

Significance 

Fabrication and manipulation of various materials at both microscale and nanoscale levels have given birth to several advanced new technologies as compared to the traditional ones. For instance, recent advancement in the field of imaging systems is attributed to the development of efficient white-light interferometers. This has further attracted significant attention of scientists owing to its excellent properties such as a true color display. Consequently, white-light interferometry has been used in sample testing of surface conditions. Presently, among the available types of white-light interferometry, scanning white-light interferometry is widely used in surface characterization. Unfortunately, the techniques generally based on the piezoelectric transducer that requires a significant amount of time to set up and operate.

To this note, Nanjing University of Science and Technology researchers led by Professor Tao ChunKanand from the Department of Optics in China identified a static state-based imaging method as a promising solution for characterization of finer surface details. Fundamentally, they developed an efficient and true color imaging method that is less time-consuming. Their research work is published in the research journal, Optics Communications.

In brief, the developed technique was based on the sub-dark field illumination in the white-light interferometry. Also, the imaging method was presented in the principal interference fringe produced by the white light interference. Next, the surface profiles of the test samples for both smooth and rough surfaces were analyzed using the newly developed method to validate its effectiveness. Furthermore, the relationship between the optical path difference and the desired colors were determined using the true color sequence band. Lastly, the properties and characteristics associated with the white-light interference images were analyzed.

Based on the experimental results, the research team observed that the developed method was much fast as compared to the traditional convention ones. Consequently, both the smooth and rough surface profiles were efficiently resolved. On the other hand, the relationship between the optical path difference and the surface colors were successfully analyzed. Also, a maximum value for the image contrast was obtained in principal interference fringe which was attributed to coinciding between the tested surface and the interference reference surface. Furthermore, sub-dark field illumination was noted to depend mainly on the equal thickness of the interference fringes and the principal interference fringe shifts due to the optical path difference regulation. This requires keenness in selecting the sub-dark field illumination region for illuminating the surface under test.

In summary, the Tao ChunKanand and his colleagues at Nanjing University of Science and Technology successfully developed a relatively fast imaging method based on the sub-dark field illumination. To actualize their work, they applied it in determining the fine detail image and three-dimensional profile of a test surface sample. It was worth noting that the obtained fine detailed images existed in pairs while all the images observed were of true colors. Altogether, the study provides vital information that will further pave way for developing more advanced white-light interferometers technologies suitable for numerous applications in different fields.

 Advances in Engineering
Fig.1 FDI for the HVPE GaN sample. The surface roughness RMS = 8.35 nm. This figure clearly shows that these prolate-spheroidal structures incline toward a common direction, with their tops appearing.

- Advances in Engineering

Fig.2 FDI also for the HVPE GaN sample above.

- Advances in Engineering
Fig.3 FDI for the GaN-on sapphire sample. The surface roughness RMS = 1.76 nm. The figure clearly shows individual.
- Advances in Engineering
Fig.4 3D image of the FDI for the GaN-on sapphire sample above.
- Advances in Engineering
Fig.5 FDI for the GaN-on Si sample. The surface roughness RMS = 1.50 nm.
 - Advances in Engineering
Fig. 6 3D image of the FDI for the GaN-on Si sample above.
 - Advances in Engineering
Fig.7 FDI for the CdZnO/ZnO MQW LED sample. The surface roughness RMS = 3.39 nm.
- Advances in Engineering
Fig. 8 3D image of the FDI for the CdZnO/ZnO MQW LED sample above.
 -  Advances in Engineering
Fig.9 FDI for another kind of GaN-on sapphire sample. The surface roughness RMS =6.8nm.
- Advances in Engineering
Fig. 10 3D image of the FDI for another kind of GaN-on sapphire sample above.
- Advances in Engineering
Fig.11 FDI for the edge of a piece of single layer graphene. The graphene is put on a glass plane. When regulating OPD carefully, the bedding images of graphene are displayed step by step. The bedded images are rich. The surface roughness RMS =1.5nm
- Advances in Engineering
Fig.12 3D image of the FDI for the edge of a piece of graphene above.
-  Advances in Engineering
Fig.13 FDI also for the edge of a piece of single layer graphene. The graphene is put on a glass plane. When regulating OPD carefully, the bedding images of graphene are displayed step by step. The bedded images are rich. The surface roughness RMS =1.5nm
- Advances in Engineering
Fig.14 3D image of the FDI for the edge of a piece of graphene above.

About the author

Tao ChunKan  Professor, Department of Optics, Nanjing University of Science and Technology. He received B.S degree in Nanjing University of Science and Technology.

He researchs zoom design, optical range finder, optical pattern recognition, laser confocal scanning microscope, digital Michelson interferometer, white-light interferometry, etc.

He has monographs ”Zoom Design”, “Theory of Optical Information”. In 1995, SPIE 40 anniversary, he published an invited paper. He has published a series of research papers in “Optical Society of America”, ”Optica Acta”, “Applied Optics”, ”Optics Letters”, “Optics Communications”, etc.

About the author

WU Yujing. She received her bachelor and master degree from Department of Optics, Nanjing University of Science and Technology, China. She is committed to researching optical measurement, including Phase Shift Interferometry, Scanning White Light Interferometry and image processing.

.

About the author

Wang Weiyi He is a master of mechanical engineering. He received his MS in Mechanical Engineering and BS in Material Forming and Control Engineering from Nanjing University of Science and Technology in 2015 and 2018, respectively. His research area is white light interferometry and optical-mechanical structure design.

Current research has been focused on optical equipment structure design and optimization.

.

About the author

Qian Yunsheng. He received the PhD degree from Nanjing University of Science & Technology, China, majoring in optical Engineering. He has been at NUST since 1995 where he is currently a professor of School of Electronic Engineering & Photoelectric Technology. His research interests include photoemission materials, photoelectric imaging technology and related testing techniques. He is also a SPIE member.

.

About the author

Tao Rui. Manager, The Nanjing BMODPEC Photo-Electronic Company in China.

She graduated from Nanjing University of Science and Technology, Dept. of enterprise management. She researches, manufactures and experimentizes the white-light interferometer Types MM200 and MM300.

.

About the author

Kang tianyou. he graduated from China Metrology University and now he is a master’s degree student in Nanjing University of Science and Technology. During his master’s degree, he researches the surface 3D topography detection system based on white light interference, and high-performance photomultiplier tubes, etc.

.

Reference

Tao, C., Wu, Y., Wang, W., Qian, Y., Tao, R., & Kang, T. (2019). Experimental investigation of white-light interferometry based on sub-dark-field illumination. Optics Communications435, 108-117.

Go To Optics Communications

Check Also

Backscattering polarimetric images derive structural information on scattering media - Advances in Engineering

Backscattering polarimetric images derive structural information on scattering media