The advent of integrated metrology (IM) for lithography critical dimension (CD) control has been
widely discussed and debated. A number of factors are pushing chip makers in the direction of
IM implementation, including shrinking line widths and decreasing CD budgets, higher throughput
Litho cells, escalating cost and impracticality of stand-alone CD metrology, and reducing
overhead (or non value-add) time. These factors combine to make the question of IM for CD
control "when" rather than "if".
Scatterometry can provide a wealth of information about structures on a wafer including CD,
sidewall angle, and film thickness for various layers. Although this information unquestioningly
provides additional insight into the lithography process, in the end, the rate of IM implementation
depends on its return on investment (ROI). In this paper, we discuss the implementation of
integrated Optical Digital Profiling (iODPTM) on an advanced lithography track (Tokyo Electron
CLEAN TRACK LITHIUSTM). Included are discussions of lithography trends, metrology
requirements, and IM data flow and analysis. Various strategies for IM implementation are
presented along with their associated ROIs.
The Grating Light Valve (GLV) is a diffractive MOEMS spatial light modulator capable of very high-speed modulation of light combined with fine gray-scale attenuation. GLV-based products are field-proven in a variety of applications. In this paper, we describe the GLV device, its structure, theory of operation, and optical performance. The versatility and speed of the GLV device are described. We explain how the GLV device is integrated into an optical write engine to create a complete digital imaging system. In addition to the MOEMS die and drive electronics, the light engine also comprises illumination optics, Fourier filter, and imaging optics. We present current applications of the GLV device for high-resolution displays, and computer-to-plate printing, as well as future plans for digital imaging applications opened up by the unique properties of this diffractive MOEMS technology.
The Grating Light Valve diffractive MOEMS device has been successfully used in imaging applications (lithography and display) requiring image data-rates of 1-5 giga-bits per second (Gb/s). However, new applications such as maskless photolithography and high performance displays require larger pixel counts and finer control of gray-scale. This paper discusses the suitability of the GLV device for high data-rate applications. It discusses the factors governing GLV device switching speed and illustrates how these properties are optimized relative to other requirements of the imaging system.
This paper describes a high performance dynamic gain equalizer (DGE) based on a diffractive MEMS structure, called the Grating Light ValveTM (GLVTM) device. The precise attenuation of the GLV-based DGE provides very low spectral ripple after equalization. Additionally, the GLV-based DGE has fine spectral resolution and high dynamic range. This paper will discuss applications of a DGE in optical networks, and experimental results showing performance characteristics of the GLV-based DGE are presented.
The Grating Light ValveTM (GLVtm) spatial light modulator is a unique and proven CMOS process-compatible optical MEMS device. The modulator employs a dynamically adjustable diffraction grating to manipulate an optical signal. Today, the GLVTM technology is successfully used in high-resolution display and imaging systems, where its high efficiency, large dynamic range, precise analog attenuation, fast switching speed, high reliability, high yield, and the ability to integrate thousands of channels into a single device are fundamental advantages. These same properties make the GLVTM device desirable for optical telecommunication applications. The optical properties, functionality, device design, and CMOS processing of the GLVTM will be presented. Challenges and solutions that arise from adapting the current GLVTM technology to optical telecommunications wavelengths will be discussed. Measured results will be presented that describe GLVTM performance parameters, including insertion loss, dynamic range, polarization dependent loss, and spectral attenuation accuracy.
Overlay budgets for advanced products are in the range of 100 to 150 nm; mean plus 3 sigma. It is crucial that the metrology tool contribute a minimal amount of error since the majority of the overlay specification is used up by the stepper errors. Most of the newer tools available today have no problem in achieving this criterion, even at these tight specifications. However, certain semiconductor processes produce overlay targets that are very difficult to measure such as grainy metal, tungsten plugged vias, and well planarized substrates. These substrates can be, roughly, grouped into two categories of `low contrast' and `distorted targets.' In the case of the low contrast targets, the signal to noise ratio is very low and the metrology tool has difficulty in identifying the actual target. Distorted targets can be measured but appear different to the metrology tool from one location to the other due to random distortions of the target edges thus producing erroneous measurements.
Coherence probe metrology (CPM) is a unique optical imaging technology which allows non- contact three dimensional measurements of sub-micron features. This technology has been applied to CD and overlay metrology on semiconductor wafers. CPM technology employs a Linnik interferometer to collect 3-D information, measuring both the amplitude and phase of an image. This makes CPM a promising technology for phase shift mask metrology. This paper gives a technical description covering the theory of the coherence probe microscope. Cross-sectional CPM images of phase shift mask features are presented, showing the ability to image quartz and PMMA shifter structures. Initial metrology performances for feature width and shifter thickness are presented. Linearity of CD is examined. Results are shown for shifter only and rim shifter masks. Shifter thickness measurement precision is compared for envelope-only and phase based signal measurement algorithms.
IC fabrication problems grow as nominal feature sizes shrink, due in large part to fundamental optical diffraction limits. Currently, one of the most pressing needs is robust critical dimension measurement. However, optical methods must be refined for this scale of submicron metrology, particularly in the case of thick features. This paper examines the problem of reflected light microscopy for nominal 1 micron high lines on silicon using 2-D, time-domain finite element simulations. The experimental basis is a prototype line width standard that is characterized using optical, contact, and SEM measurements. Microscope and simulated images are compared for 1 and 3 micron wide lines. Good first-order correlation is found between real and synthetic images but model uncertainties need to be reduced and microscope aberrations need to be quantified before second-order differences can be eliminated. Numerical experiments are used to relate images to resonance patterns in the feature; determine the strength of evanescent waves near the line; and contrast isolated and periodic line images as a function of pitch.
Linearity of response is one of the most important features of a measurement system. Linearity implies that accurate linewidths can be obtained from measured values knowing only the slope and offset of the data with respect to reference data taken with another, presumably more accurate, instrument. A first-order linear regression of the data yields the slope, offset, and estimate of the goodness of fit. Ideally, the slope is near unity, so that the magnification scales of the two instruments agree. The offset is considerably less important since, in IC process control, absolute changes in linewidth are often of more concern than the linewidths themselves. This paper demonstrates by simulation and experiment that the linearity (R-Squared) of an optical microscope depends not only upon the characteristics of the tool but also upon the characteristics of the object being measured. In virtually all optical microscopes, transparent structures support waveguide resonant eigenmodes which are strongly affected by geometry and contribute substantially to non- linearities in response. For isolated lines, nonlinearities are found to occur especially at certain widths where the eigenfunctions change rapidly with small change in width. The theory of these singular points is presented. The authors demonstrate that the coherence microscope, which uses both phase and amplitude information, has a potential advantage over brightfield and confocal microscopes in dealing with these problems. The introduction of a 'complex phase filter' in the measurement algorithm greatly reduces unwanted phase noise and its concomitant contribution to non-linearity. The ability to simulate the optical images and resulting measurement non-linearities offers promise in improving understanding and accuracy of optical metrology.
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