The introduction of lithographic systems with NA=1.35 has enabled the extension of optical lithography to 45 nm and
below. At the same time, despite the larger NA, k1-factors have dropped to 0.3 and below. Defining the appropriate
strategies for these high-end lithographic processes requires the integration and co-optimization of the design, mask and
imaging parameters. This requires an in-depth understanding of the relevant parameters for imaging performance during
high volume manufacturing.
Besides the Critical Dimension Uniformity (CDU) budget for the baseline lithographic system, it is crucial to realize that
system performance may vary over time in volume manufacturing.
In this paper the CDU budget will be restated, with all the well-known contributors, and extended with some new terms,
such as volume manufacturing effects.
Experimental low-k1 results will be shown from NA=1.35 lithographic tools and compared to model-based predictions
under realistic volume manufacturing circumstances.
The combination of extreme NA and low k1 makes it necessary to introduce computational lithography for scanner
optimization. The potential of using LithoCruiserTM and TachyonTM for optimising scanner source and OPC will be
described. Also, using the fast scanner correction mechanisms to compensate for reticle, track and etch fingerprints and
variations will be discussed.
The exponential increase in areal density of magnetic hard disk drives during the last years has led to incredibly tighter manufacturing tolerances. For the lithography process that is being used to make the principal component in a hard disk drive, the read-write head, this means that improved process control is required with respect to imaging and positioning performance. Beside the overlay performance from layer to layer, the relative placement of images in a single critical layer is being looked at as an important performance requirement on state-of-the-art DUV and I-line steppers. The terms stitching and co-linearity characterize the relative placement of images in a single layer. In this paper verification tests for stitching and co-linearity are presented, as they are developed by ASML. Since these tests require an understanding of the terms stitching and co-linearity, the definitions of these terms are outlined. The results for the two tests on an ASML PAS 5500/300 DUV stepper are presented and discussed. Also the effects of certain error sources and some ideas for future optimization are shown.
This paper analyzes and demonstrates the possibility of producing lithographic images at or below the 'diffraction limit' for synchrotron radiation-based x-ray proximity lithography. It is shown that at reasonable mask/wafer gaps of 15-30 micrometers , for feature sizes down to approximately 100 nm, a 30-40 nm uniform positive bias is observed. In proximity lithography, masks with clear features on a dark background demonstrate better linewidth control and more stable process optimization in terms of achieving smaller features: Sub-100 nm imaging requires positive bias for mask features: clear features have to be increased in sizes and the proper bias will depend on the mask/wafer gap. Features down to 43-46 nm have been formed in negative resists, and down to 60 nm in positive resist. The extendibility of synchrotron radiation-based x-ray proximity lithography into the sub-50 nm region at reasonable mask/wafer gaps of 20-30 micrometers was demonstrated.
Availability of production-worthy x-ray masks is of great concern to the lithographic community in anticipation of insertion of x-ray lithography as the leading contender among the next generation lithographies.
Despite growing expectations of significant progress in projection lithography using shorter wavelengths, x-ray lithography is still the most developed and production ready technology compared with the other NGL approaches. For the timely introduction of this technology into the manufacturing environment the development of fully integrated x-ray lithography systems becomes very important. Reflecting manufacturing and R and D demands, the x-ray technology integration has been pursued for goth synchrotron radiation and x-ray point source based approaches. While the synchrotron-based approach provides the high volume platform, the point source will provide the platform for low volume production and R and D efforts. SAL recognizes the needs for both, a synchrotron based stepper as well as a point source stepper and is focused on meeting those needs. This paper will present the status of integration efforts at SAL utilizing a point source system.
The most important contributions to overlay inaccuracy are coming from well-known sources like mask pattern placement accuracy, alignment system accuracy and stage performance of the exposure tools. As the allowances for overlay budget decrease, and improvements in mask fabrication and stage performance are made, a number of previously less significant contributions, such as resolution, optical interference, and focusing accuracy of alignment system, as well as from wafer processing, have to be considered. These contributions are characterized in detail in this paper. The investigation was focused on a proven optical alignment system and overlay contribution as they apply to x-ray and optical lithography. Special emphasis was made on contributions form wafer processing.
Crucial to any viable lithographic mask technology is the requirement that a given mask pattern be usable for the hundreds of thousands of exposures in a production environment. In a conventional approach this would be accomplished by making robust masks. A better strategy to ensure the longevity of the pattern itself, is realized by producing many defect-free copies of master masks. This approach is especially important in the case of x-ray masks, although the optical masks also have a limited usable lifetime. X-ray mask generation is accomplished today via e- beam lithography, which as a replication method has several inherent disadvantages, including low speed and high cost. X- ray replication is the best solution. In this paper, we describe the development of a mask replication method realized on a Suss x-ray stepper. The approach is based on supporting parent mask and the daughter blank in fully kinematic fixtures during replication, ensuring a minimum of distortion, excellent gap control and optimized exposure conditions. Minor modifications of the mask mounting fixtures, the replication setup, and details of processing are presented. Preliminary results of mask replication are also shown.
The prototype of a high volume production stepper for x-ray lithography, designed and built by Suss Advanced Lithography, Inc. (SAL), has been installed for beta testing at the Center for X-ray Lithography (CXrL) at the University of Wisconsin. SAL has put substantial efforts into the design and construction of this third generation x-ray stepper in order to meet industry requirements, mainly throughput, CD-control and overlay. Several design changes -- compared to the second generation XRS200 -- have been implemented to assure those goals. This includes a scanning beamline end to achieve exposure times of 1 sec as well as a high speed xy-stage for combined step end expose times below 2 seconds. Together with an improved optical alignment system (ALX70) for overlay accuracy below 50 nm (3(sigma) ) and a state-of-the-art loading/unloading wafer handler the measured throughput is currently between 26 WLPH (32 fields/200 mm wafer). Proximity gap is controlled to an accuracy of plus or minus 0.4 micrometer while minimum proximity gap is currently at 20 micrometer. Mask and wafer chuck are mounted kinematically to avoid mechanical stress to either mask or wafer for improved distortion control. A 3-axis-state-of-the-art HP laser interferometric system with a resolution of 2.5 nm controls mask to wafer movement to better than 20 nm. While this prototype is designed for wafer sizes of 75 to 200 mm, design changes are on the way to enable 300 mm wafer handling with increased throughput. Results obtained during a GaAs based MMIC and QWIP device fabrication program with 0.2 micrometer CD, demonstrate consistency with the prototype specification. Areas have been defined to further enhance the capabilities of the production stepper XRS 2000.
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