Young-Sik Ghim

Reflectometry-based wavelength scanning interferometry for thickness measurements of very thin wafers

With the development of microelectronics, the demand for silicon wafers is greatly increased for various purposes, especially the use of thin wafers for smart cards, cellular phones and stacked packages. In this paper, we describe an innovative scheme of combining wavelength scanning interferometry (4 nm tuning range centered at 1550 nm) with spectroscopic reflectometry that enables us to measure the thickness profile of thin wafers below 100 microm with high thickness resolution. The performance of this method is compared with that of an existing technique and verified by measuring several thin wafers.

Micro-optic reflection and transmission interferometer for complete microlens characterization

A combined Twyman–Green and Mach–Zehnder interferometer especially designed for the characterization of refractive microlenses is presented. This instrument allows for the quantitative characterization of the microlens form, the transmitted wavefront errors, the radius of curvature and the front focal length without removing the sample under test. All of these microlens properties are important when benchmarking different microlens fabrication technologies (Ottevaere et al 2006 J. Opt. A: Pure Appl. Opt. 8 S407–29). The interferometer was calibrated by the random ball test method. This paper describes the optical design and demonstrates the performance with the characterization of the instrument bias and measurements of a typical microlens. The performance is also compared with that of a semi-commercial instrument.

Field-Curvature Correction According to the Curvature of a CMOS Image-Sensor Using Air-Gap Optimization

Lens designers generally refer to flat image fields and attempt to minimize the field curvature. Present-day CMOS image sensors for mobile phone cameras, however, are not flat, but curved. Sometimes it is necessary to generate an intentional field curvature according to the degree and direction of the CMOS image-sensor’s curvature. This paper presents the degree of curvature of a CMOS image sensor measured using an interferometer, and proposes an effective compensation method that minimizes the net field curvature through optimizing the air gap between lens elements, which is demonstrated using simulations and experiments.

Uncertainty analysis on the absolute thickness of a cavity using a commercial wavelength scanning interferometer

Wavelength scanning interferometry offers many advantages over traditional phase shifting interferometry, most significantly the elimination of mechanical movement of the part/s for phase modulation by implementing a tunable light source. Further, Fourier analysis on the interference time history enables this technique to accurately measure distances, treating the distance between two optical surfaces as an interferometric cavity. We propose to evaluate the uncertainty in the thickness measurement of a transparent cavity using a commercial Fizeau wavelength scanning interferometer. This work follows the theory and measurement performed in a previous manuscript of measuring absolute distances of opaque objects using a commercial wavelength scanning interferometer. The limits in measuring a cavity using the commercial wavelength scanning interferometer depend on many factors such as temperature variations that affect the test and reference cavity, uncertainty in the reference cavity calibration, tuning rate non-linearities, etc. In addition to an analytical approach, a simulation is described to better understand the measurement process and the uncertainty associated in measuring absolute distances (thickness) of cavities. Preliminary experimental results on the absolute thickness of a transparent cavity are reported along with uncertainty sources.

Modeling of Various Tool Influence Functions in Computer Controlled Optical Surfacing

The computer controlled optical surfacing (CCOS) technique provides superior fabrication performance for optical mirrors when compared to the conventional method, which relies heavily on the skill of the optician. The CCOS technique provides improvements in terms of mass production, low cost, and short polishing time, and are achieved by estimating and controlling the moving speed of the tool and toolpath through a numerical analysis of the tool influence function (TIF). Hence, the exact estimation of various TIFs is critical for high convergence rates and high form accuracy in the CCOS process. In this paper, we suggest a new model for TIFs, which can be applied for various tool shapes, different velocity distributions, and non-uniform tool pressure distributions. Our proposed TIFs were also verified by comparisons with experimental results. We anticipate that these new TIFs will have a major role in improving the form accuracy and shortening the polishing time by increasing the accuracy of the material removal rate.

Wavelength Scanning Lateral Shearing Interferometer for Freeform Surface Measurement

We propose a new variant of lateral shearing interferometer with a tunable laser source that enables 3D surface profile measurements of freeform optics with high speed, high vertical resolution, large departure, and large field-of-view. We have verified the proposed technique by comparing our measurement result with that of an existing technique and measuring a representative sample of freeform optics. Moreover, we propose a new algorithm that is able to compensate the rotational inaccuracy.