Brian J. Grenon

Optical proximity correction: a first look at manufacturability

The feasibility of large scale optical proximity correction with a focus on mask manufacturability is demonstrated on the support and logic gates of a leading edge 64 Mb DRAM chip. Analysis of post reactive ion etch SEM data of the 500 - 600 nm, DUV exposed gates indicates two major contributors to across chip line width variation: first order proximity, that is, the minimum spacing to the nearest neighboring structure, and local area density or pattern loading. Data presented show a very long range (approximately equals 1 mm) impact of pattern density on post reactive ion etch line widths, favoring optical proximity correction approaches that are not based on biasing patterns to compensate for these effects. In this project, pattern density induced effects were alleviated by homogenizing the pattern loading across the chip to approximately 50% instead of biasing the gate structures to compensate for pattern density differences. Proximity induced effects were compensated for with a one- dimensional, single parameter (distance to nearest neighbor), four bucket proximity correction routine with a strong focus on mask manufacturability. Even though the unbiased 64 Mb DRAM gate level challenges mask makers with 480 MB of MEBES data, the optical proximity corrected mask posed no substantial post-processing, writing, or inspection problems in IBMs Burlington, Vermont maskhouse. A very significant 80% reduction in post reactive ion etch across chip line width variation was achieved with this corrected mask.

Comparison of wet and dry chrome etching with the CORE-2564

Chrome masks have traditionally been wet etched in an acidic solution of cerric ammonium nitrate. The etchant is commonly sprayed on the mask while the mask is slowly rotated, using an APT-914 or equivalent processor. While this process is well-understood, relatively trouble- free and inexpensive, the isotropic nature of wet etching results in an undercut of the chrome relative to the resist etch mask of approximately equals 150 nm per edge. Compensation for the undercut, in order to maintain control of the mean critical dimension (CD), is done by adjusting the printed feature size such that the undercut grows the printed feature to the desired final size. This sizing can be performed by manipulating the computer aided design database, which can be expensive and time consuming. In this paper, we present a comparison of wet and dry chrome etch processes using plates printed with the CORE-2564 in OCG-895 i resist. The differences in CD performance and resolution are illustrated.

Impact of attenuated mask topography on lithographic performance

Experimental evaluations were used in conjunction with rigorous electromagnetic simulations to evaluate the affect of attenuated phase-shifting mask (PSM) fabrication processes on lithographic performance. Three attenuated PSMs were fabricated including a normal leaky- chrome reticle and two novel approaches: a recessed leaky-chrome reticle for reduction of edge scattering and a single-layer reticle employing a hydrogenated amorphous carbon film. Direct aerial image measurements with the Aerial Image Measurement System (AIMSTM), exposures on an SVGL Micrascan 92 deep-UV stepper, and TEMPEST simulations were used to explore the effects of edge-scattering phenomena for the different mask topographies. For each reticle, the process window at a feature size of 0.25 micrometers was evaluated for four basic feature types: nested lines, isolated lines, isolated spaces, and contact holes. Further evaluation of the sidewall profiles and the image size on the mask are required to address these discrepancies.

Advanced X-ray mask inspection system (AXIS) using scanning electron microscopy for sub- 70nm die- to- database inspections

The concept of using Scanning Electron Microscopy and Die-to-Database techniques to rigorously inspect advanced lithography products such as X-ray Lithography, Imprint, and Stencil masks as well as other Next Generation Lithography (NGL) is compelling. Current optical capabilities reach down to 0.2μm and do so by interpolating pixilated optical data. Applications at 4x magnifications, such as Chrome on Glass or Phase Shift Photomasks mesh with this resolution of inspection and have been able to migrate down the lithography nodes hand in hand. As the demands for resolution progress, optical lithography has been increasing the requirements upon inspection systems presently available through the addition of assist features and serifs, which are difficult to directly verify. These assist features are effectively approaching 1x dimensions. A printed feature that is slightly out-of-tolerance for CD, shape, or position relative to other structures, may still yield acceptable performance. This added resolution challenge of working closer to a 1x Magnification with ever decreasing structure sizes is easily achieved with Scanning Electron Microscope technology. The Die-to-Database inspection technique utilizes the CAD image, which defines the designers original intended structure, as the reference image. In this paper, we will introduce a revolutionary approach for utilizing the full potential of Scanning Electron Microscope images for inspection purposes. The technique incorporates an aggressive but reliable interpretation of the image data to recreate GDS data files which can then be validated against the desired GDS data for hard defects, abbreviated / missing features, and even shifts or placement errors.

Manufacturing performance of the ALTA 3000 mask laser writer

This paper describes the manufacturing performance of the ALTA 3000 laser writer at the IBM mask fabrication facility in Essex Junction, Vermont. Current mask parametric performance for feature size control (x-bar and 3-sigma), registration and defect density of 4x and 5x reticles is presented. In addition, reliability data and write-time data for typical 64 Mb and 256 Mb reticles are provided.

Investigation of a negative i-line resist with the CORE-2564 laser writer

The process of record for mask fabrication on CORE pattern generators has been OCG-895i positive resist. This process has demonstrated excellent performance in a production environment, however it is recognized that having both negative and positive resist process capability would significantly improve the flexibility of most mask fabricators. The CORE exposure wavelength of 363.8 nm is close enough to i-line (365 nm) to provide the opportunity to use many of the i-line resists developed for wafer lithography. Negative acting resists sensitive at i-line are now becoming available. The most popular chemistry for this application has been acid catalyzed chemical amplification. These formulations typically contain a novolak resin, an acid generator, and a melamine crosslinking agent. The chemistry of such formulations has been previously described. An evaluation of OCG LMB-7011, an acid catalyzed chemically amplified negative i-line resist, has been conducted with the CORE-2564. This resist can be processed similarly to OCG-895i in standard mask process equipment, except that a post exposure bake is required to crosslink the exposed resist. With wet chrome etching, this resist exhibits exposure latitude similar or better than OCG-895i, benefiting from the fact that over-exposure is required to compensate for etch undercut when using a negative resist. Sub-micron resolution has been achieved with good linearity. CD control is marginal, due to a strong CD sensitivity to PEB temperature. A modified PEB process demonstrates improved CD control.

Reticle defect sizing and printability

As reticle defect specifications become more stringent and maximum allowable defect sizes approach the resolution of current defect inspection systems, the mask fabricator1s ability to accurately determine the size of these defects becomes limited. We have evaluated several techniques by which the size of reticle defects can be measured and have found significant discrepancies between techniques. Using a standard defect inspection system, we measured defects at or below 1.0 ^m and found that the results varied with each technique. Defects were sized using both reflected and transmitted light and the results were compared to SEM measurements. The reflected-light measurements best correlated with the SEM measurements, which are considered more accurate. The results of this study indicated that current sizing techniques are inaccurate. A printability evaluation was also performed using a mask with accurately sized defects. Defect printability results were evaluated in terms of their effect on linewidth.