Burn Jeng Lin

The Paths To Subhalf-Micrometer Optical Lithography

The limitations of subhalf-micrometer optical lithography in depth of focus, field size, and overlay accuracy, are discussed. The existing depth of focus budget as well as those for the near and the ultimate future are given. The optical depth of focus is explored in the point of view of microlithographers and means to overcome the depth of focus limit are suggested. Field size requirement of projected IC products is compared with the capability of known and projected optical systems followed with a discussion on methods to overcome the field size limit. The configuration of imaging optics is characterized and discussed by 1X versus reduction systems, refractive versus catadioptric and reflective systems, step-and-repeat versus step-and-scan systems. Overlay accuracy is discussed in terms of off-axis versus through-the-lens alignment, bright-field versus dark-field, actinic versus nonactinic alignment, followed with an overlay budget outlining the specification of the contributors to alignment errors. If all speculations are fulfilled, it is feasible to consider 0.18 μm resolution with 0.07 μm overlay.

Phase-shifting and other challenges in optical mask technology

Optical lithography has benefitted from progresses in wavelength reduction, lenses, resist systems, alignment, focussing, table accuracy, and insute metrology. As a result, the minimum feature size of integrated circuits has been reduced through many generations from 2 micrometers to 0.5 micrometers in manufacturing. There are many opportunities in improving the mask to help continue the progress. The image contrast can be restored by reducing stray light with mask anti-reflection coating at the absorber-air interface. Pre-distorting the mask against the distortion of the imaging lens can drastically improve the overlay performance. Adjusting the gray level or the feature size individually according to the pattern proximity environment can create a larger common exposure-defocus window for different feature shapes. Introducing phase shifts in the mask can simultaneously improve resolution and depth of focus, with the potential of a two-generation improvement with any given projection imaging equipment, provided the overlay capability is upgraded accordingly. In addition to describing and comparing these opportunities, the phase-shifting technology is given a special focus on the working principle, the different approaches to phase shifting, their imaging characteristics in terms of exposure-defocus diagrams, a systems view, and the scope of its development.

The future of subhalf-micrometer optical lithography

Abstract The limitations of subhalf-micrometer optical lithography in depth of focus, field size, and overlay accuracy, are discussed. The existing depth of focus budget as well as those for the near and the ultimate future are given. The optical depth of focus is explored in the point of view of microlithographers and means to overcome the depth of focus limit are suggested. Field size requirement of projected IC products is compared with the capability of known and projected optical systems followed with a discussion on methods to overcome the field size limit. The configuration of imaging optics is characterized and discussed by 1X versus reduction systems, refractive versus catadioptric and reflective systems, step-and-repeat versus step-and-scan systems. Overlay accuracy is discussed in terms of off-axis versus through-the-lens alignment, bright-field versus dark-field, actinic versus non-actinic alignment, followed with an overlay budget outlining the specification of the contributors to alignment errors. If all speculations are fulfilled, it is feasible to consider 0.18 μm resolution with 0.07 μm overlay.

Methods to print optical images at low-k1 factors

The minimum half pitch for optical projection lithography in manufacturing has been held to 0. 8 A/NA or larger. This is supported by the experience gained by semiconductor manufacturers worldwide yet the reasons are not clearly understood. Three main causes for the high-k1 requirement are identified in this paper. First imperfections in the imaging system can reduce the processing margin. Subsequently the k1 factor has to be raised to compensate for the loss. Vibration and stray light are two examples. Secondly even a perfect imaging system can be limited by an unoptimized coherence factor and by optical proximity effects which are increasingly dominant as k1 is reduced. These are basic limitations imposed by the diffraction phenomenon. Thirdly the conditions at the recording media i. e. the photoresist and the substrate can require unsuspectedly large processing margins which necessitate a high k1. Experimental and theoretical results are given to substantiate the above and to lead to methods for low-k1 manufacturing. 1.

Electromagnetic Near-Field Diffraction of a Medium Slit*

Investigation of electromagnetic diffraction at any distance from an infinite slit is presented for slit widths 0.5 wavelength and wider. Plane-wave incidence of arbitrary two-dimensional angle in a plane perpendicular to the edges of the slit is used. Infinite conductivity is assumed. The contribution of this work is in the near-field and medium-slit-width regions, where no numerical results have been given. One obvious application is high-resolution, contact or near-contact, printing.

A comparison of projection and proximity printings - from UV to X-ray

Abstract Projection and proximity printings are compared in terms of linewidth tolerance. Computer simulated Exposure-Defocus and Gap-Exposure diagrams are used to characterize the two techniques respectively. The depth of focus for projection printing in terms of the universal depth parameter k 2 is evaluated as a function of the universal width parameter k 1 , for five long and short representative lithographic features, then three long features. Similarly, the depth of focus and working distance for proximity printing are evaluated for a 0.25-μm system in x-ray and 2.5-μm system in uv. The image contours and depth of focus of the two imaging techniques are compared. Proximity effects are present for both techniques. The distinction of working distance and depth of focus for proximity printing is observed for the first time. Against intuition, a shorter mask-to-wafer gap is not always better.

Single-level electric testsites for phase-shifting masks

The phase shifting mask technology has quickly progressed from the exploratory phase to a serious development phase. This requires high resolution measurement techniques to quantify experimental results to optimize the designs. This paper describes a set of electrical linewidth measurement testsites which covers all five representative lithographic features in combination of dark-field and light-field patterns, positive and negative resists. The testsites can investigate binary intensity mask, attenuated, alternating, subresolution-assisted, rim, unattenuated, edge, and covered edge phase shifting masks. All testsites can be used with a single-level wafer exposure. There is no need to remove extra shorts or opens induced by uncovered phase shifters.

A study of projected optical images for typical IC mask patterns illuminated by partially coherent light

Optical projection printing using partially coherent illumination is simulated for one micrometer and half micrometer objects representative of typical mask patterns such as contact holes, rectangular bars and openings, intersections of perpendicular lines, and adjacent lines of unequal lengths. The image intensity distributions in absorptionless photoresists on nonreflective substrates are plotted as sets of constant intensity contours. For each pattern and illumination, an exposure-defocus (E-D) diagram is generated by evaluating the combined exposure and defocus tolerance yielding linewidths within ±2.5 percent of the mask linewidth. Besides comparing the image and ED margins of different object shapes and sizes, the effects of high versus low degrees of coherence, single versus dual wavelength, as well as long-wavelength high NA versus short-wavelength low NA were studied using the 1-µm rectangular opening.

Where Is The Lost Resolution

In addition to raising the numerical aperture of the imaging lens or reducing the exposing wavelength to improve resolution in optical lithography, a third direction is pursued, namely restoration of the potential resolution capability that is lost due to incorrect practice. Unlike the former two methods, a gain in depth of focus can accompany improvement in resolution. In this paper, elimination of vibration between the mask and the wafer, resist contrast improvement, and multilayer resist systems are cited as possible means for improvement. Simulation study are given on the effects of vibration and resist contrast improvement to quantitatively assess the improvements.

The optimum numerical aperture for attenuated phase-shifting masks

Abstract The imaging behavior of the attenuated phase shifting mask system is studied in terms of the figure of merit of depth of focus for single layer resist, single layer resist with anti reflection coating, and multi-layer resist systems using a ±10% linewidth tolerance and the exposure-defocus window methodology. Significant imaging improvements over conventional intensity masks for contact holes and line openings are identified. A small positive mask bias pushes the improvements towards higher resolution and practical exposure dosages. The imaging performance of contact holes will be compared with that of rim phase shifting masks. The attenuated phase shifting mask produces larger depth of focus, requires lower exposure times, and takes up less mask areas.