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

Publisher Summary This chapter describes the advanced evaluation techniques in interferometry The chapter discusses the methods that rely on the adjustment of a fringe pattern The chapter examines the historically older interpolation methods where the phase information is extracted from the positions of the extrema The chapter also discusses more general methods that deliver the phase in a more continuous form A comparison of the different methods for phase evaluations necessitates a consideration of the error sources and limitations These error sources for the case of phase sampling interferometry (PSI) have been discussed The error causes are classified into statistical errors (measuring errors that can be diminished in their influence by averaging) and systematic errors (errors that cannot be quenched) The interferometers used in the assessment of test data are rather complex, thus, there is some necessity for calibration The interferometer aberrations, even with ideal test samples, are caused by deviations of optical elements and to also by the state of adjustment The chapter identifies the types of error sources—namely, environmental errors, errors that are dependent on the method, and errors typically encountered in interferometry

IV Advanced Evaluation Techniques in Interferometry

Digital speckle-pattern interferometry systems for automatic measurement of deformations of a diffuse object are presented, which are based on a fringe scanning method with phase-shifted speckle interferograms. A digital speckle pattern before deformation of an object is recorded in the mass storage device of a computer facility. After deformation, four digital speckle patterns are recorded as changing the phase of reference light such as 0, π/2, π, and 3π/2, respectively. Four speckle interferograms, whose phases are shifted by 0, π/2, π, and 3π/2, are generated by calculating the square of the differences between speckle patterns before and after deformation. These interferograms are low-pass filtered to reduce speckle noise. The calculation of the arctangent with four phase-shifted speckle interferograms gives the optical path difference which is proportional to the deformation. A correction of the discontinuity of the calculated phase gives the numerical data of the deformation in the whole object area. Some experimental results for the measurement of out-of-plane, in-plane, and 3-D deformations are presented.

Fringe scanning speckle-pattern interferometry.

A system is described which uses a diode array TV camera to view the real-time fringes and digital electronics to measure and store the irradiance levels at a 100 × 100 array of points. A microcomputer calculates the phase at each point from the irradiance values obtained from three successive scans of the array made with the phase of the reference beam shifted in steps of 120° by means of a mirror mounted on a piezoelectric translator. The optical system permits four holograms to be recorded in quick succession, using a photothermoplastic camera, with the object illuminated from four different directions. Phase data from these holograms are processed in a microcomputer to determine the components of the vector displacement at each of these points; these figures can then be transferred to a fast computer for further processing to evaluate the stress distribution.

Real-time holographic interferometry: a microcomputer system for the measurement of vector displacements.

A projection moire profilometer is presented in which both projection and optical demodulation are realized with liquid crystal light modulators. The computer generated grids, realized on thin film transistor matrices, allow phase-stepping and discrete grid averaging without the need for any mechanically moving component. Spatial line pitch and phase steps can thus be readily adjusted to suit the measurement precision and object geometry. The device is able to perform topographic measurements with a height resolution of 15 microm on every pixel of the recording device.

Moiré profilometry using liquid crystals for projection and demodulation.

A method for automatically measuring the three-dimensional (3-D) deformations of a diffuse surface is studied, using a real-time holographic interferometer and a television-computer system. The object surface is illuminated from three different directions. The scattered waves are recorded on a thermoplastic hologram. Real-time holographic interference patterns are observed through the hologram in a fixed direction by sequentially applying the three illuminating beams. The three fringe patterns are put into the computer using a television (TV) camera. The relative fringe order numbers are determined over the whole surface. Calculations for obtaining 3-D deformation distributions are performed, and the results are displayed on a cathode-ray tube (CRT) monitor.

Hybrid Holographic Interferometer For Measuring Three-Dimensional Deformations

In wafer steppers, the alignment of an exposure field to the reticle being imaged is known to affect the success or failure of that field's circuit(s) Because of this relationship between alignment accuracy and device yield, much emphasis is placed on obtaining and consistently maintaining an alignment accuracy within tight design rules In current wafer steppers, alignment options can be placed in two primary categories: Global Alignment techniques and Site-by-Site Alignment techniques During Global Alignment, selected areas of the wafer undergo alignment The grid of all exposure field positions is generated according to the information obtained during the selected alignments Step-and-Repeat exposure of all fields is then performed according to this grid In Site-by-Site Alignment (aka Field-by-Field Alignment), the stepper performs a step/align/expose sequence on each exposure field until all fields on the wafer are printed While Site-by-Site alignment can result in greater overlay accuracy, Global Alignment is primarily used on all but the most critical of alignment levels This is due to the much higher throughput that Global Alignment allows Nikon has developed and implemented an original technique that achieves a throughput nearing that of most standard Global Alignment methods, while maintaining an overlay accuracy consistent with a Site-by-Site alignment scheme This method, called Enhanced Global Alignment (EGA), utilizes the Laser Step Alignment (LSA) system of a Nikon Step-and-Repeat (NSR) system to measure the offsets of several selected fields The NSR's computer constructs a mathematical model of the wafer's exposure field grid according to the measured offsets The model includes six components that may contribute to overlay error: 1)Translation in X 2)Translation in Y 3) Scaling in X 4)Scaling in Y 5) Wafer Rotation 6) Pattern Orthogonality Error All exposure field addresses are determined during the EGA Thus, there is no further alignment once the Step-and-Repeat exposure sequence begins This sequence is performed on each wafer The wafer model constructed by EGA relies heavily on the offsets measured by LSA While the LSA system has a wide dynamic range (over which no variation in offset measurement accuracy is seen), the alignment mark topography must be constructed such that it can be detected by the LSA system Since different processes may yield differing alignment topographies, an alignment mark size found to be optimum for one device manufacturer may not be best suited to another manufacturer with a different process Optimization studies have focused on mark design variables that affect the LSA, while monitoring the process variables encountered and their effects Discussions of these variables, including current recommendations, will be detailed

Enhanced Global Alignment For Production Optical Lithography

Comments on speckle interferometry with regard to displacements of the camera during an exposure

We develop a new double exposure Moire method for an optical registration metrology system in photolithography. Our method enables us to achieve at least a factor of 10 improvements in precise displace- ment metrology using a conventional optical sensor. We utilize a new registration mark printed to the photoresist on a bare silicon wafer using a double exposure of the gratings. The mark consists of two types of Moire with opposite phases. The two types of Moire are oriented in alternate directions. Displacement is measured from the distance between the positions of the two types of Moire in analogy with the conventional registration method. This concept is called alternating direction Moire. Performance is experimentally confirmed using an i-line wafer exposure apparatus. Precision is improved by up to 32 times as compared with the conventional method and can be applied to other Moire metrologies. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.OE.53.8 .084101)

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-53/issue-8/084101/Optical-displacement-metrology-using-alternating-direction-Moire/10.1117/1.OE.53.8.084101.pdf

Nobutaka Magome

Papers

Hybrid Holographic Interferometer For Measuring Three-Dimensional Deformations

Enhanced Global Alignment For Production Optical Lithography

Comments on speckle interferometry with regard to displacements of the camera during an exposure

Optical displacement metrology using alternating direction Moire

Acoustical interferometric method for measuring the internal stress distribution in a solid