Image imbalance refers to the phenomenon that printed image sizes are different between etched and unetched openings if the effective phase and transmission of an alternating phase shifting mask (PSM) are not ideal. The root cause of the phenomenon is the presence of a 0-th diffraction order. When the 0-th order interacts with plus or minus 1 orders, depending on location, it constructively or destructively interferes with the ideal sinusoidal fields generated by the plus or minus 1 orders, causing aerial image difference between the etched and unetched openings. Furthermore, the image imbalance affect changes through focus because optical path length of the 0-th order differs from that of the plus or minus order. An analytical solution of the aerial image has been derived to quantitatively understand the phenomenon. It can be shown that the contributions of phase error and transmission error are orthogonal, thus both errors must be eliminated to eliminate the 0-th diffraction order. The biased etchback approach proposed by Ferguson et al. has been shown to be effective in correcting transmission errors. Detailed implementation of the technique are explored in this presentation. The technique consists of multiple steps of uniform wet etch coupled with aerial image measurements after each etch step. After transmission error is corrected, a phase trimming process may be necessary to improve through-focus image balance.
The manufacturing implementation of alternating aperture PSM's (AltPSM) has been gated by the impacts these techniques have on reticle manufacturing, specifically reticle defect inspection and repair. Die-to-die inspection techniques have been achieved for some clearfield multiphase alternate phase reticles, but the required die-to-database solutions are not currently available with defect inspection systems. In response to these mask manufacturing issues and IC design layout issues, resolution enhancing techniques based on Darkfield Alternate Phase (DAP) reticle designs are now of growing importance. A DAP Programmed Evaluation Reticle, DAPPER, was fabricated and inspected on a new high numerical aperture ultraviolet reticle inspection system. The results show reasonable defect sensitivity performance in the presence of both reticle geometry and quartz etch topography characteristic of 130-nm node advanced logic circuit DAP reticles.
Accurate phase measurement is required to fabricate manufacturing-quality phase shifting masks. Phase error can severely limit the total process window as the k1 factor becomes smaller and smaller. Existing phase measurement techniques use either a specialized phase interferometer or an aerial image measurement system (AIMS) with image-plane analysis. In this presentation we report a direct interferometric phase measurement technique that can be implemented on an AIMS tool with a few inexpensive accessories. The method is based on analyzing far-field interference patterns of two double-slit features on the mask. In one double-slit feature, the left slit is phase- shifted; in the other feature the right slit is phase- shifted. By measuring a relative shift of position between the interference patterns one can calculate phase shift directly. Advantages of the method over other published methods based on image plane measurements include: 1) significantly better accuracy, and 2) no aerial image simulation is required, thus eliminating a source of measurement error. Compared wit lateral shearing interferometry used in dedicated phase interferometers, the current method can be applied to smaller geometries without adding complexity. In addition, results obtained with current method are effective phase shifts that may correlate directly with lithographic performance of the mask. We will present the experiment setup, phase extracting algorithm, and experimental result on an alternating phase shifting mask. Result will be compared with that of a step height measurement.
Attenuated phase-shifting masks have gained wide acceptance in the manufacturing environment during the last few years. Etching attenuated films remains a challenging process step that affects several critical mask parameters including critical dimension (CD) and phase angle. This paper reports the result of etching MoSi attenuated phase-shifting materials using an inductively coupled plasma system. CD and phase-control performance is presented as well as a performance comparison between ICP and reactive ion etching. Attenuated PSMs have typically been used primarily for contact-type patterns. However, recent lithographic simulation result show significant benefits of attenuated PSM with off-axis illumination for gate-type patterns. Fabrication of gate-type attenuated PSMs introduces new challenges for the etch process. Initial etch performance result are also presented.
Resonant grating filters based on 2D gratings are investigated experimentally. A two-layer structure, which consists of a uniform guiding layer and a grating layer, is used to achieve a symmetric, low-sideband resonance that is suitable for narrow-band filter applications.
Computationally efficient and stable implementations of rigorous coupled-wave analysis for 1D and 2D surface-relief gratings are presented. The eigenvalue problem for a 1D grating in a conical mounting is reduced to two eigenvalue problems in the corresponding nonconical mounting. The matrix in the eigenvalue problem of 2D gratings is reduced in size by a factor of two. These simplifications reduce the computation time for the eigenvalue problem by 8 to 32 times compared to the original computation time. The required computer memory is also decreased thus complicated grating diffraction problems can be solved efficiently.
The control of optical distortion is useful for the design of a variety of optical systems including those used for laser scanning. A lens used for focusing a scanned laser beam onto a flat image field with constant intensity profile must also satisfy the f-(theta) condition, i.e., the image height is proportional to the input field angle itself, so that the scan velocity across the image plane remains constant. The lens needs to be free from coma, astigmatism, and field curvature and must have a prescribed amount of distortion. We describe the design and development of a diffractive f-(theta) lens and present experimental verification of the theoretical predictions.
Described in this paper is a compact phase measuring ESPI system that incorporates single mode fiber and diode laser. The single mode fiber along with diode laser provides ease of system arrangement but also causes wavelength fluctuation and instability. The wavelength instability was reduced to an acceptable level with an optical isolator. Experimental studies on wavelength stability of the diode laser, the back-reflection intensity from the fiber ends, and their influence on the system are presented.
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