We describe a system for performing high-accuracy, noncontact rms roughness measurements of flat and curved machined parts in the industrially relevant range of ∼0.05 to 0.35 μm. The system uses a near infrared (NIR) super-continuum laser to measure the intensity of specular reflection versus wavelength, at relatively long (∼1 m) stand-off distances and has the potential to be used in high speed, in-line manufacturing applications. The surface roughness value is extracted from the slope of the normalized specular intensity using the Beckmann-Kirchhoff (BK) model. According to the BK model, the normalized specular intensity in the NIR mostly depends on the surface roughness parameter alone and is independent of the absolute reflectance due to the normalization process. We discuss the benefits of performing the reflectance measurements in the NIR versus the commonly used visible spectrum. These include measurements at lower angles of incidence and the lack of need for a reference of the same metal composition. The roughness measurements performed by this system are in very good agreement with comparative data from a stylus profilometer and a white light interferometer. A potential industrial application is also demonstrated where the system is used to detect polishing defects in automotive engine crankshaft journals.
We demonstrate an optical probe for detection of porosity inside spool bores of a transmission valve body with diameters down to 5 mm. The probe consists of a graded-index relay rod that focuses a laser beam spot onto the inner surface of the bore. Detectors, placed in the specular and grazing directions with respect to the incident beam, measure the change in scattered intensity when a surface defect is encountered. Based on the scattering signatures in the two directions, the system can also validate the depth of the defect and distinguish porosity from bump-type defects coming out of the metal surface. The system can detect porosity down to a 50-µm lateral dimension and ~40 µm in depth with >3-dB contrast over the background intensity fluctuations. Porosity detection systems currently use manual inspection techniques on the plant floor, and the demonstrated probe provides a noncontact technique that can help automotive manufacturers meet high-quality standards during production.
A Mid-InfraRed FIber Laser (MIRFIL) has been developed that generates super-continuum covering the spectral range
from 0.8 to 4.5 microns with a time-averaged power as high as 10.5W. The MIRFIL is an all-fiber integrated laser with
no moving parts and no mode-locked lasers that uses commercial off-the-shelf parts and leverages the mature
telecom/fiber optics platform. The MIRFIL power can be easily scaled by changing the repetition rate and modifying
the erbium-doped fiber amplifier. Some of the applications using the super-continuum laser will be described in defense,
homeland security and healthcare. For example, the MIRFIL is being applied to a catheter-based medical diagnostic
system to detect vulnerable plaque, which is responsible for most heart attacks resulting from hardening-of-the-arteries
or atherosclerosis. More generally, the MIRFIL can be a platform for selective ablation of lipids without damaging
normal protein or smooth muscle tissue.
Specular-mode spectroscopic scatterometry is currently being used as an in-line metrology tool for wafer-to-wafer process monitoring and control in lithography and etch processes. Experimental real-time, in situ demonstrations of critical dimension monitoring and control have been made for reactive ion etching. There have been no similar demonstrations of real-time control in the critical step of resist development. In this paper, we will show the results of a simulation study on the use of scatterometry in an immersion mode both to improve resolution and to act as a real-time monitor for photoresist topography evolution during development. We have performed realistic simulations of the experimental performance by using Prolith to generate developing resist profiles vs. time and a rigorous couple wave algorithm (RCWA) simulator (modified to include the immersion ambient) to generate simulated scatterometry data. We have examined several modes of operation of the proposed measurement including specular and 1st order scattered modes using spectroscopic ellipsometry and reflectometry. For our simulations, we have used pure water to approximate the developer refractive index. We have created realistic simulation data by adding appropriate amounts of random noise to perfect simulations, and then used regression analysis to extract profiles from these data. Water immersion increases feature shape resolution for small period gratings by increasing the scattering into real diffracted modes.
Specular-mode spectroscopic ellipsometry (SE) or reflectometry (SR) from periodic gratings have been successfully demonstrated as accurate methods for extracting detailed topography of integrated circuit structures. However, as critical dimensions become very much less than the current minimum measurement wavelengths and as film thicknesses are simultaneously reduced, there are significant questions regarding the long-term usefulness of this method. In this paper, I will attempt to address some major aspects of this problem.
Absorption measurements of HCl during plasma etching of poly-silicon are made using the P(4) transition in the first vibrational overtone band near 1.79 μm. Single path absorption provides a real-time HCl monitor during etching of six-inch wafers in a commercial Lam Research 9400SE reactor at the University of Michigan. Wavelength modulation at 10.7 MHz is used to distinguish the absorption signal from the strong plasma emission. The laser center frequency is ramp-tuned at 500 Hz providing an HCl measurement every 2ms. Direct absorption measurements without the plasma are used to calibrate the wavelength modulation signal. The minimum detectable absorbance was 5x(10)-6 with 50 ms averaging, leading to an HCl detection limit of ~(10)12cm-3. For a given ratio of the feedstock HBr/Cl2, the measured HCl concentration tracks the average etch rate. These measurements demonstrate the feasibility of a real-time diode laser-based etch rate sensor.
In this article, we report the use of ultraviolet absorption spectroscopy for CF2 detection in a large area parallel plate capacitively coupled reactive ion etching system and correlation of data from this and other plasma sensors to the etch rate of SiO2 and a-Si in CF4/CHF3 plasmas. We present statistical models for estimation of a- Si etch rate in the operational regime in which the CF2 concentration is in the range of 0.4 approximately 1.6 volume % of total gas in the etch chamber. A small change in CF2 concentration translates into quite a large variation in terms of SiO2/a-Si etch selectivity, and this makes CF2 concentration a useful variable in process control. We will show statistically that silicon etch rates can be very well estimated by using sensors for CF2 and fluorine.
This paper explores the application of modern feedback control technology to the regulation of the reactive ion etching process. Currently, this process is run open-loop, except for the PID controller to regulate pressure. We investigate the utility of additional measurements for the purpose of feedback control to improve process performance and robustness. First, we compare a feedback controller that regulates Vbias and chamber pressure to one that regulates Vbias and fluorine. We show that the fluorine controller yields better control of etch rate; this result is to be expected since fluorine is more closely related to the chemical etching process than is pressure. Our second study compares various controllers that regulate Vbias and fluorine using the conductance throttle and applied RF power. We show that multivariable feedback controllers that can compensate for process coupling by coordinating control inputs have advantages over decentralized controllers consisting of two independent feedback loops.
Real-time process monitoring has been a primary concern of the semiconductor industry for a number of years. In an attempt to provide higher yield and performance it has become accepted that the monitoring of the etch step is critical. This is the primary motivation for the development of a real time process monitor with particular attention paid to wafer monitoring at the surface. To this end, the use of the results of diffraction, in particular, diffraction from 3-dimensional gratings is proving to be a viable technique for real time, process monitoring. Thus, the primary focus of this paper is to present the progress made toward the development of a non destructive, real time, optical metrology based system for direct wafer monitoring, utilizing the results of diffraction from a grating structure. The results of experiment and simulation will also be discussed.
This paper reports the theoretical and experimental sensitivity of Fourier imaging (Fl) and the Fourier imaging system (FIS)'' respectively. The theory based on scalar diffraction which specifies these sensitivities is presented and discussed. Specifically the theoretical sensitivity of the Fl technique at a wavelength of 633 nm is determined to be 0. 003 nm while the experimental sensitivity of the FIS is approximately 15 nm. Both of these figures utilize a one-dimensional 1-pm rectangular diffraction structure for their calculation.