A projection exposure apparatus including an irradiation optical system including a light source and irradiating a mask with irradiation light beams, a projection optical system for projecting an image of a pattern of the mask on a substrate, a plurality of first fly-eye type optical integrators each having an emission side focal plane disposed on a Fourier transformed surface with respect to the pattern of the mask in the irradiation optical system or on a plane adjacent to the same and having a center located at a plurality of positions which are eccentric from the optical axis of the irradiation optical system, a plurality of second fly-eye type optical integrators each having an emission side focal plane disposed on a Fourier transformed plane with respect to the incidental end of each of a plurality of the first fly-eye type optical integrators or on a plane adjacent to the same and being disposed to correspond to a plurality of the first fly-eye type optical integrators, and a light divider for dividing and causing the irradiation light beams from the light source to be incident on each of a plurality of the second fly-eye type optical integrators.

Projection exposure apparatus

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10× tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.

Improving resolution in photolithography with a phase-shifting mask

Lithography is a field in which advances proceed at a swift pace. This book was written to address several needs, and the revisions for the second edition were made with those original objectives in mind. Many new topics have been included in this text commensurate with the progress that has taken place during the past few years, and several subjects are discussed in more detail. This book is intended to serve as an introduction to the science of microlithography for people who are unfamiliar with the subject. Topics directly related to the tools used to manufacture integrated circuits are addressed in depth, including such topics as overlay, the stages of exposure, tools, and light sources. This text also contains numerous references for students who want to investigate particular topics in more detail, and they provide the experienced lithographer with lists of references by topic as well. It is expected that the reader of this book will have a foundation in basic physics and chemistry. No topics will require knowledge of mathematics beyond elementary calculus.

Principles of Lithography

Miniaturization is the central theme in modern fabrication technology. Many of the components used in modern products are getting smaller and smaller. In this paper, the recent development of the electron beam lithography technique is reviewed with an emphasis on fabricating devices at the nanometer scale. Because of its very short wavelength and reasonable energy density characteristics, e-beam lithography has the ability to fabricate patterns having nanometer feature sizes. As a result, many nanoscale devices have been successfully fabricated by this technique. Following an introduction of this technique, recent developments in processing, tooling, resist, and pattern controlling are separately examined and discussed. Examples of nanodevices made by several different e-beam lithographic schemes are given, to illustrate the versatility and advancement of the e-beam lithography technique. Finally, future trends in this technique are discussed.

Electron beam lithography in nanoscale fabrication: recent development

WHILE the introduction of statistical methods into the analysis of aeronautical experimental data, whether for quality control in production, for the interpretation of the results of structural and aerodynamic laboratory experiments, or for airline operation, has been brought about only in recent years, it may by now be fair to assert that their advantages and even their indispensability are no longer in dispute. Hitherto, investigations on these lines have usually involved, explicitly or implicitly, only the ‘normal curve of error’ and allied considerations; owing, it may be thought, to the controllability of the various manufacturing or laboratory techniques, but also perhaps to the scarcity of data hitherto available. It may well be, however, that with the accumulation of information arising out of investigations planned with particular reference to the statistical analysis of their results the whole range of the apparatus for statistical analysis, usually confined to such fields as those of biology or economics, will be called into full play.

The Statistical Analysis, of Experimental Data: A Fresh Approach Employing a Graphical Method for Investigating Non‐Normal Frequency Distributions

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.

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

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 imaging layer less than 100 nm thick and the extent and effect of charging of such a resist.

Low voltage alternative for electron beam lithography

A two-dimensional self-calibration experiment obtains Cartesian traceability for high-precision tools. The calibration procedure incorporates group theory principles to solve our industry's 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 Leica's LMS2000 and LMS2020 systems.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Self-calibration in two dimensions: the experiment

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.

Depth of focus enhancement in optical 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.

G. Owen

Papers

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

Low voltage alternative for electron beam lithography

Self-calibration in two dimensions: the experiment

Depth of focus enhancement in optical lithography

Workpiece charging in electron beam lithography