Fabrication of Fresnel zone plates for the hard x-ray spectral region combines the challenge of high lateral resolution (~100 nm) with a large thickness requirement for the phase-shifting material (0.5-3 μm). For achieving a high resolution, the initial mask was fabricated by e-beam lithography and gold electroforming. To prevent the collapse of the structures between the developing and electroforming processes, drying was completely eliminated.
Fabrication errors, such as nonuniform gold electroplating and collapse of structures, were analyzed and systematically eliminated. We optimized the exposure and developing processes for 950k and 2200k polymethylmethacrylate of different thicknesses and various adhesion promoters. We discuss the effects of these fabrication steps on the zone plate's resolution and aspect ratio. Fresnel zone plates with 110 nm outermost zone width, 150 μm diameter, and 1.3 μm gold thickness were fabricated. Preliminary evaluation of the FZPs was done by scanning electron microscopy and atomic force microscopy. The FZP focusing performance was characterized at the Advanced Photon Source at Argonne National Laboratory.
The ability to produce fine features using X-ray proximity lithography is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature 'bias.' The classical approach is to attempt to minimize the bias; that is, to print features which are 1:1 images of those on the mask. However, bias can also be exploited to print features smaller than those on the mask. This demagnification-by-bias technique can be optimized with respect to mask-wafer gap and resist processing, and can provide reductions of 3X to 6X. Demagnification offers many of the same advantages as projection optical lithography in terms of critical dimension control: relaxed mask features CD. In addition, it provides a very large 'depth of focus' and wide dose latitude. In consequence proximity X-ray lithography is extendible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 0.1 nm) and large gaps (10 -25 micrometer). The method was demonstrated for demagnification values down to X3.5. To produce DRAM half- pitch fine features techniques such as multiple exposures with a single development step are proposed.
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.
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.
In this paper we report the results of simulations and experiments on application of phase-shifting mask to x-ray lithography (XPM). We have built an XPM with PMMA as a phase shifter; we printed patterns using the XPM and characterized the linewidth dependence on gap and dose. Small lines around 70 nm were printed at 25 micrometer gap. The resist lines have good uniformity, and aspect ratio as high as 4. The characterization experiment was performed on the Karl Suss X- ray Stepper installed at the Center for X-ray Lithography. The results show that the exposure-gap latitude window is obviously increased compared with traditional x-ray mask. XLITH has been used to analyze the aerial image under the XPM and predict linewidth. Different phase shift materials, such as PMMA, Si, SiC and Al have been simulated and characterized with respect to blur, dose and gap. The model predicts a mask contrast 6 - 7, and resolution of 50 nm using gaps around 5 micrometer. XPM is a path to nanostructures with more reasonable process latitudes. We printed sub-0.1 micrometer dots using XPM. The XPM can also be used for gate-level lithography of ultrafast MOS devices to share its high resolution and large process latitude. It is possible to form both of a fine gate line and a contact pad at the same time with the combination of phase and transmission in a single mask. An experimental demonstration of printing unclosed lines is given in this paper. The further combination of phase-edge effects and phase-shifter size can also make it possible to form a wide range of patterns, ranging from line-space to isolated bright or dark areas with varied sizes, in a single exposure.
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.
We report on an IC lithographic resolution study in which APEX-E resist on polysilicon coated wafers was exposed to synchrotron x radiation through a high-resolution mask that contained Au-electroplated features ranging in size from 0.5 down to 0.15-micrometers . Exposures were made at mask-to-substrate gaps ranging from 20 to 35 micrometers and at doses from 100 to 134 mJ/cm2. We probed the wafers with an SEM, both before and after etch, and electrically. From the electrical linewidth probing, we found that for isolated lines and 1:2 L:S patterns the feature widths were linear down to 0.18-micrometers . For the 1:1 and 2:1 L:S arrays, the widths were linear down to 0.25-micrometers . Dense and isolated lines down to 0.25-micrometers exhibited +/- 15% dose latitude over a 10-micrometers gap range. Contact holes were examined only by SEM. The smallest size that printed was nominally 0.225 micrometers , but was measured to be 0.20-micrometers after etch. Critical dimension uniformity, calculated with each feature type allowed its own mean value, was approximately equals 40 nm (3(sigma) ), including intrafield and across wafer variation. The mask CD uniformity was approximately equals 30 nm (mean + 3(sigma) ). The wafer-to-wafer CD variation was found to be 6 nm (3(sigma) ) and the electrical test-to-test CD variation was 3 nm (3(sigma) ). We use regression analysis to separate the component of CD variation that is assignable to intrafield form that assignable to interfield. The regression analysis to separate the component of CD variation that is assignable to intrafield from that assignable to interfield. The regression analysis indicates that these components of CD variation are systematic rather than random. The main contributor to the interfield component may be polysilicon etch. The intrafield error is believed to be caused predominantly by beamline nonuniformity and not by errors on the 1x mask.
The establishment of standard resist processes are a primary requirement for the X-ray Lithography National Test Bed at the Center for X-ray Lithography. For this, experimental design is a necessary component of the methodology given the large parameter space associated with chemically amplified resists (CARs). The process development is carried out in three phases. DOX is applied to several steps in the development. The vacuum hot plate pre-bake, post-bake time, temperature and the exposure dose have the greatest effect on controlling the performance of the resist. Constraints are placed on the contrast, develop time and unexposed resist loss. The exposure dose needed to meet these requirements is obtained from the modified response surface of the bulk behavior. The final optimization is based on the CD control and side wall angle for quarter-micron features in resist. The process is run on a sampled basis in order to determine the control issues. Control limits are set from these data, and the process performance is determined.
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