Non-contact Thickness Measurement Research Articles

Experiment on Non-Uniformity of Material Removal on Wafer Surface in Wafer CMP

In the manufacturing process of ultra large scale integrated circuits (ULSI), chemical mechanical polishing (CMP) has already become a practical and main planarization technology because of its global and local planarization ability. The non-uniformity of material removal on wafer surface is one of important indicators of surface precision in wafer CMP process. In this paper, According to the σ calculation method, the calculation equation of the non-uniformity of material removal was put forward. Then, the CMP experiments on the non-uniformity of material removal were conducted under different rotational speed on a CMP machine CP-4. The WaferCheck-7200 equipment, with the theory of non-contact capacitive thickness measurement, was used to detect the thickness of each point on silicon wafer surface. After calculation, the non-uniformity of material removal on the silicon surface was obtained under different polishing speed. The test results can provide some theoretical guide for further understanding the form mechanism of the non-uniformity of material removal and the material removal mechanism in wafer CMP.

Real-time thickness measurement of biological tissues using a microfabricated magnetically-driven lens actuator

A fiber optic confocal catheter with a micro scanning lens was developed for real-time and non-contact thickness measurement of biological tissue. The catheter has an outer diameter and rigid length of 4.75 mm and 30 mm respectively and is suitable for endoscopic applications. The catheter incorporates a lens actuator that is fabricated using microelectromechanical systems (MEMS) technology. The lens is mounted on a folded flexure made of nickel and is actuated by magnetic field. Thickness measurements are performed by positioning the catheter in front of the tissue and actuating the lens scanner in the out-of-plane direction. A single-mode optical fiber (SMF) is used to deliver a 785 nm laser beam to the tissue and relay back the reflected light from the tissue to a photomultiplier tube (PMT). When the focal point of the scanning lens passes tissue boundaries, intensity peaks are detected in the reflecting signal. Tissue thickness is calculated using its index of refraction and the lens displacement between intensity peaks. The utility of the confocal catheter was demonstrated by measuring the cornea and skin thicknesses of a mouse. Measurement uncertainty of 8.86 µm within 95% confidence interval has been achieved.

A micro-fabricated optical scanner for rapid non-contact thickness measurement of transparent films

In this work a fiber optic scanner for non-contact thickness measurement of transparent and semi-transparent films is presented. Thickness measurement is performed by scanning the focal point of a microlens across the thickness of a film and logging the intensity signal of the single scattering light reflected back from the sample. When the focal point of the scanning microlens passes the surface boundaries of the sample, intensity peaks are detected in the reflecting signal. Thickness is calculated using the index of refraction of the sample and the displacement of the lens between intensity peaks. The measurement is performed in a fraction of a second to eliminate signal artifacts that are due to the external vibrations. A magnetically driven micro-fabricated actuator is developed for actuating the microlens. Scanning range of 204 μm at resonant frequency of 544 Hz is achieved when an AC magnetic field of 14 mT is applied. To evaluate the scanner, thickness of SU-8 negative photoresist layers deposited on silicon wafers was measured.

Axial-scanning low-coherence interferometer method for noncontact thickness measurement of biological samples

We investigated a high-precision optical method for measuring the thickness of biological samples regardless of their transparency. The method is based on the precise measurement of optical path length difference of the end surfaces of objects, using a dual-arm axial-scanning low-coherence interferometer. This removes any consideration of the shape, thickness, or transparency of testing objects when performing the measurement. Scanning the reference simplifies the measurement setup, resulting in unambiguous measurement. Using a 1310 nm wavelength superluminescent diode, with a 65 nm bandwidth, the measurement accuracy was as high as 11.6 μm. We tested the method by measuring the thickness of both transparent samples and nontransparent soft biological tissues.

Simple confocal thickness gauge based on fibre-optic confocal sensor for non-contact measurement

A simple non-contact thickness measurement method based on a dual-confocal microscope has been developed and tested. A reference comparison method was adapted for calibration of the system. The high accuracy potential of the method was demonstrated by measuring the thickness of transparent samples and a non-transparent soft biological sample. Lensed-tip fibre-optic confocal sensors were tested for the method.

Noncontact thickness measurement of plane-parallel transparent plates with a lateral shearing interferometer

A technique for determination of the thickness of a plane-parallel transparent plate using a lateral shearing interferometer (LSI) is discussed. With this technique, the parallel plate whose thickness is to be determined is used to introduce a change in the collimation of the retro-reflected beam from an optical setup in which a corrected lens focuses an expanded collimated laser beam on the surface of a plane mirror placed at the back focal plane of the lens. The thickness of the plate is calculated by measuring the defocusing caused by the plate, which is inserted in the beam path between the collimating lens and the plane mirror, with an LSI. Results obtained for a parallel plate are presented.

Optical substrate thickness measurement system using hybrid fiber-freespace optics and selective wavelength interferometry

Proposed and demonstrated is a simple few components non-contact thickness measurement system for optical quality semi-transparent samples such as Silicon (Si) and 6H Silicon Carbide (SiC) optical chips used for designing sensors. The instrument exploits a hybrid fiber-freespace optical design that enables self-calibrating measurements via the use of confocal imaging via single mode fiber-optics and a self-imaging type optical fiber collimating lens. Data acquisition for fault-tolerant measurements is accomplished via a sufficiently broadband optical source and a tunable laser and relevant wavelength discriminating optics. Accurate sample thickness processing is achieved using the known material dispersion data for the sample and the few (e.g., 5) accurately measured optical power null wavelengths produced via the sample etalon effect. Thicknesses of 281.1μm and 296μm are measured for given SiC and Si optical chips, respectively.

Fibre-optic focus-detection system for non-contact, high-resolution thickness measurement of transparent tissues

A low-cost, high-resolution, fibre optic system for non-contact thickness measurement of ocular tissues in cadaver eyes was developed. The optical system uses direct modulation at 100 kHz of a 670 nm laser diode delivered to a single-mode fibre coupler. The output of the fibre coupler is focused onto the tissue using a high numerical aperture aspheric lens mounted on a motorized translation stage. Light reflected from the sample is collected by the fibre coupler and sent to a photoreceiver. Peaks in the power signal are detected when the focal point of the aspheric lens coincides with the tissue boundaries. The thickness is proportional to the distance between successive peaks. The optical system has a sensitivity of 52 dB, which corresponds to a detectable change in the refractive index of 0.005 at a boundary-assuming specular reflection. With a focusing numerical aperture of 0.68, the resolution is 4.0 µm and the precision is ± 0.2 µm. The optical system was used to measure the corneal layers and lens capsule thickness of cadaver monkeys eyes. The system was able to measure the thickness of the epithelium, stroma and Descemet's membrane of the cornea, as well as the lens capsule and lens epithelial cell layer.

Application of pulse compression signal processing techniques to electromagnetic acoustic transducers for noncontact thickness measurements and imaging

A pair of noncontact Electromagnetic Acoustic Transducers (EMATs) has been used for thickness measurements and imaging of metallic plates. This was performed using wide bandwidth EMATs and pulse-compression signal processing techniques, using chirp excitation. This gives a greatly improved signal-to-noise ratio for air-coupled experiments, increasing the speed of data acquisition. A numerical simulation of the technique has confirmed the performance. Experimental results indicate that it is possible to perform noncontact ultrasonic imaging and thickness gauging in a wide range of metal plates. An accuracy of up to 99% has been obtained for aluminum, brass, and copper samples. The resolution of the image obtained using the pulse compression approach was also improved compared to a transient pulse signal from conventional pulser/receiver. It is thus suggested that the combination of EMATs and pulse compression can lead to a wide range of online applications where fast time acquisition is required.

Physical modeling of an eddy current sensor designed for real time distance and thickness measurement in galvanization industry

An analytical model of an eddy current sensor designed for noncontact thickness and distance measurement is presented. A dipolar model allows the external physical parameters (conductivity and permeability of alloys) to be taken into account. Analysis of an iron sheet coated with a conductive material gives an accurate interpretation of the detected signals, useful for optimization of the sensor design. Experimental and theoretical data are compared for a galvanization line, demonstrating the potential of this system for nondestructive continuous evaluation. >

THICKNESS TOUCHLESS MEASUREMENTS USING EDDY CURRENT SENSORS

ABSTRACT Analytic expressions for the reluctance and the losses measured using an eddy current sensor placed in front of a conductive and magnetic metal sheet are derived. The computation is made both in the simple case and also when the sheet is coated with a small thickness of conductive material. The results can be easily applied to the non-contact distance and thickness measurement, for the continuous control of a coating process (galvanization for example).