G. Owen

An exact algorithm for self-calibration of two-dimensional precision metrology stages

We describe an algorithm that can achieve exact self-calibration for high-precision two-dimensional (2-D) metrology stages. Previous attempts to solve this problem have often given nonexact or impractical solutions. Self-calibration is the procedure of calibrating a metrology stage by an artifact plate whose mark positions are not precisely known. By assuming rigidness of the artifact plate, this algorithm extracts the stage error map from comparison of three different measurement views of the plate. The algorithm employs the orthogonal Fourier series to expand the stage error map, which allows fast numerical computation. When there is no random measurement noise, this algorithm exactly calibrates the stage error at those sites sampled by the mark array. In the presence of random measurement noise, the algorithm introduces a calibration error of about the same size as the random measurement noise itself, which is the limit to be achieved by any self-calibration algorithm. The algorithm has been verified by computer simulation with and without random measurement noise. Other possible applications of this algorithm are also discussed.

Low voltage alternative for electron beam lithography

The current trend in electron beam lithography for patterning submicron features is towards the use of higher beam voltages (20–100 keV). Among the problems often perceived to be associated with the use of low voltages are the poorer resolution, the lower brightness, and the greater sensitivity to electric and magnetic interference. Both by simulation and by experiment at 2 kV it is shown: (1) features of less than 100 nm are clearly resolved in resist of about the same thickness; (2) such features are clearly resolved in both sparse and dense pattern; (3) such features in sparse and dense areas are clearly resolved over a twofold range of exposure doses; (4) such delineation is largely independent of substrate material; (5) there is no evidence of alternating‐current magnetic interference; (6) the lower beam brightness at low voltages is compensated by the increased sensitivity of resists to lower energy electrons. The remaining concerns about low voltage lithography are the reliability of resist with an...

Self-calibration in two dimensions: the experiment

A two-dimensional self-calibration experiment obtains Cartesian traceability for high-precision tools. The calibration procedure incorporates group theory principles to solve our industrys two-dimensional calibration problem. With group theory, a Cartesian system is obtainable through mathematics; thus, eliminating the need for any certified standards. The calibration algorithm was developed by Jun Ye at Stanford University and funded by the Semiconductor Research Corporation (SRC) with collaboration from Hewlett Packard (HP) and IBM. The data was collected from Leicas LMS2000 and LMS2020 systems.

Depth of focus enhancement in optical lithography

Apodization, or pupil plane filtering, can be used to enhance the depth of focus of lithographic projection systems. However, this improvement comes at the expense of decreasing radial concentration of energy in the point spread function. To examine this tradeoff quantitatively, we first define the average encircled energy as a criterion of the energy concentration over a nonzero axial distance. We then derive and numerically solve an integral equation for the apodization function that maximizes the average encircled energy. Finally, we use the eigenfunctions of this equation to construct pupil functions producing an on‐axis intensity profile that is approximately constant over a nonzero axial distance, while maintaining a large value of the average encircled energy within that distance.

Workpiece charging in electron beam lithography

A major contribution to total overlay error can be pattern placement imprecision due to charging of the workpiece in electron beam lithography for mask manufacture. A first‐generation, worst‐case model is presented which indicates that an electron can experience quite a large placement error for a modest workpiece surface potential (100 nm/V). This model also predicts that the amount of error is proportional to the working distance. A novel method which measures the surface potential, to within 50 mV, is also presented. Results indicate that when exposed with 10 kV electrons the surface potential of 3000 A PMMA on silicon is 1.5 V while that of 4000 A SAL‐601 on chrome on quartz is 0.5 V. The discharging time for both samples was found to be of the same order as the write time for a typical mask.

Effect of central obscuration on image formation in projection lithography

Central obscuration of the pupil is a prominent feature of many high performance reflective designs being considered for sub-200nm lithography. The performance of centrally-obscured designs were investigated using computer simulations of projected image intensity and major features from simulation were experimentally confirmed. The effect of partially-coherent illumination on centrally-obscured systems was studied and an optimized annular illumination system is proposed. 1.

Markle-Dyson optics for 0.25 μm lithography and beyond

Using a modified Dyson design working at 0.7 NA with 248 nm illumination, reflective 1X masks, and conventional single‐layer resists, 0.25 μm lithographic resolution has been obtained over a semicircular field 4 mm in diameter. 0.19 μm lines and spaces have been printed using an experimental bilayer resist. To offset the small depth of focus, two precision autofocus schemes have been demonstrated and FLEX has been shown by computer simulation to triple the depth of focus for isolated features. A full size system has been designed with a 20×40 mm2 field sufficiently large for patterning 256 Mbit DRAM chips. This concept can be extended to shorter wavelengths limited only by availability of one refractive material and a suitable light source. Thus optical projection lithography appears feasible for features at least as small as 0.15 μm.

Analytic optimization of Dyson optics

A prototype of a 248-nm optical projection printer based on the unit magnification Dyson configuration was designed and built [J. Vac. Sci. Technol. 9, 3108 (1991)]. Iterative computer methods were used to optimize the geometry of the system to provide minimum aberrations over a given field. Here we present an alternative optimization procedure and use it to design a 193-nm system. This new procedure yields useful insights into the design process and allows us to see why modifications in geometry are required for optimum performance. An understanding of the trade-offs involved in the optimization allows us to suggest and evaluate future design changes.

Field distortion characterization using linewidth or pitch measurement

Field distortion in steppers and mask pattern generators can be a serious source of placement errors, which subsequently translate to overlay errors if different steppers/masks (with different distortion) are used in different layers of wafer printing. Such distortion is often determined using an expensive coordinate measuring system (e.g., Leica LMS2020 optical metrology system) which may not be readily available. We describe a simple and inexpensive technique for measuring field distortion using readily available instruments. The essence of the technique is to build up a pattern using multiple exposures with different portions of the field overlapped in the complete pattern, so that the resulting linewidth or pitch variation across the field gives the difference between the field distortion and a shifted field distortion. We can then reconstruct the field distortion by measuring the linewidth/pitch. We have applied this technique to two optical steppers, and have shown that the reconstructed field disto...

Optical projection system for gigabit dynamic random access memories

Containing the cost of new generations of optical lithography, while maintaining improvements in performance, is a major challenge because every three years tends to see a requirement for a three‐fold increase in the number of resolvable pixels and a substantial improvement in resolution. Here an optical approach is reported that promises to ameliorate the rapidly increasing cost of optical systems through the means of 1×, mostly reflecting optics. Only three projective components are required and the spectral range is 1000× larger than that of corresponding refractive systems. Such a system appears capable of meeting the lithography requirements of the 256 Mbit dynamic random access memory (DRAM) and, with evolutionary improvements, the 1 Gbit DRAM.