Greyscale lithography is applied to manufacture complex 2.5D and freeform microstructures in photoresists which serve as master for the pattern transfer into materials for permanent applications, often used in micro-optics. We present the results and the challenges in reproducible generation of deep greyscale patterns in a highly sensitive greyscale positive photoresist, mr-P 22G_XP, when using photomask-based mask aligner greyscale lithography in contrast to laser direct writing on which resist development had been focused. Furthermore, we show the influence of resist aging on the resist response, and ways to correct it by process adaption, as well as we conclude requirements to greyscale photomasks suitable to make use of the full potential of the mr-P 22G_XP resist dedicated for >100μm deep greyscale patterns.
In this paper, a patterning approach via i-line grayscale exposure is presented. The i-line wafer stepper (NIKON NSR2205i11D) together with specialized grayscale reticles from Benchmark Technologies (USA), manufactured with half-toning technique, are used. The positive tone and low contrast grayscale ma-P 1275G photoresist is manufactured by Micro Resist Technology (Berlin, Germany) and used in this work; it is part of the ma-P 1200G grayscale resist series and can cover a thickness range of 5 μm to 14 μm. The lithographical pattering process is performed on 6-inch wafers. Essential parameters like the contrast curve measured in the resist as well as after the dry etching are evaluated. Different 2.5D structures like micro lens arrays, blazed gratings, frustums and Fresnel lenses are fabricated by i-line stepper gray scale lithography and ma-P 1275G demonstrating its excellent behavior to generate 2.5D grayscale patterns. For the characterization of the generated 2.5D grayscale patterns a stylus profilometer, atomic force microscopy (AFM), scanning electron microscopy (SEM) and confocal microscope are used. In this paper the process of setting up a grayscale exposure with an i-line stepper and a grayscale reticle and the need to adapt the grayscale reticle in different iterations is presented and discussed.
Recent work has shown that bimetallic films, such as Bi/In and Sn/In, can create laser direct-write grayscale
photomasks. Using a laser-induced oxidation process; bimetallic films turn transparent with variations in optical
transparency that are a function of the laser power. The films exhibit transmittances <0.1% when unexposed and >60%
when full laser exposed. A novel grayscale photolithography technique is presented that utilizes conventional chrome
photomasks as the high resolution pattern-defining layer with a bimetallic thin film layer deposited on top as the
grayscale-defining layer. Having the grayscale layer on top of the chrome, grayscale patterns can be aligned to the
underlying chrome patterns. Laser power and bimetallic thin film thickness are carefully calibrated such that no chrome
ablation or conversion occurs. The calibration ensures that during laser scanning, the bottom chrome layer defines the
fine features of the underlying patterns and remains unchanged, while the bimetallic thin film layer is converted to
provide grayscale tones. To further investigate the optical density (OD) properties of this type of mask, we measured the
transient time response for pure chrome mask and Bi/In coated chrome mask to help fine tune the laser writing
parameters. Using bimetallic Bi/In/Cr photomasks, we have successfully created continuous tone 3D structures with
superimposed binary structures in SU-8 photoresist. By introducing this novel combined chrome-bimetallic mask, the
fine detail features found in binary lithography may be combined with smoothly-varying 3D microstructures best suited
to grayscale methods.
KEYWORDS: Electron beam lithography, Electron beams, Line edge roughness, Semiconducting wafers, Photomasks, Lithography, Systems modeling, Standards development, Line width roughness, Tolerancing
Proximity effects during electron beam exposure have been kept under control by using sophisticated correction algorithms and software, combined with a strategy which aims at increasing the electron beam energy to 50 keV and 100 keV. At these energies, the proximity effects are more uniform and provide a situation where they are easier to correct. However, as feature sizes shrink, and the pattern density increases, this task becomes extremely complex, since tolerances to pattern definition errors are becoming more restricted. An alternate approach is to move to lower electron energies where proximity effects become negligible. Several programs are underway to develop massively parallel electron beam (MPEB) writer systems that have greatly reduced energy in the ≤5keV regime. Selection of the electron beam energy becomes critical below 10 keV, since the tolerance window where proximity effects are indeed negligible is very small. A shot noise model has been elaborated providing minimum exposure doses required for resists at technology nodes of 45 nm and below. These doses increase rapidly with reducing linewidth and impose a minimum number of electron beams for MPEB systems in order to be able to pattern a surface corresponding to a standard full field 6 inch reticle in a reasonable time, and to directly pattern 300 mm wafers at rates of 5, 50 and 100 wafers per hour. An overall set of results is obtained indicating minimum number of electron beams and electron beam current.
Nano-Imprint lithography has garnered much interest in the microlithography and nano-fabrication communities, and appears on the ITRS as a possible future lithography solution. The promise of this approach includes realization of ultimately finer features than might be possible through optical lithography and simplification of mask pattern complexity through the elimination of optical enhancements such as phase shifting and optical proximity correction. Imprinting approaches have demonstrated that sub-50nm lithography of 2D structures is possible using this approach. A potentially enabling method to enhancing imprint lithography is to add additional structures to a 2D template to form 3D profiles, thereby realizing additional benefits that can be achieved through imprinting 3D structures. In this paper we discuss fabrication of such a template, which has the potential to eliminate masking layers by allowing for two or more layers to be imprinted with a single template. A 2D template is formed on a fused silica substrate using Quantiscript's QSR-5 sterol-based vapor deposited electron beam resist, low energy e-beam lithography and reactive ion etching of the underlying substrate. Vapor deposition is especially conducive for patterning ultra thin (<50nm) layers of resist on imprint templates where high resolution structures on the order of 50-100nm are desired. After complete fabrication of the 2D template, a second resist vapor deposition, lithography and etch sequence is performed to add the 3D structures. Since the QSR-5 vapor deposited resist exhibits substrate conformal properties, uniformly thin coatings can be achieved on both 2D surfaces, allowing for high resolution trench-bottom or ridge/mesa-top lithography and processing while simultaneous protection of the initial 2D structures is realized.
An ultra-high resolution measurement technique makes the assessment of the period uniformity in a long grating used as a phase mask for Fibre Bragg Gratings (FBG). It comprizes a pair of two identical displacement sensors placed close to each other at a strictly constant spacing flying over the grating under test at an essentially constant velocity. The phase difference between the two sensors is a simple function of the local spatial frequency of the grating. A proper solution of an inverse problem provides the period variation along the grating. The technique is used here to characterize phase masks fabricated by means of a MEBES 4500 electron beam pattern generator. The period does not deviate by more than 3 pm from the prescribed period over a length by more than 100 mm.
It is now a widely held opinion that the current and projected reticle CD (critical dimension) uniformity specification demands will continue to task the ability of reticle manufacturers. Similarly wafer lithography tool suppliers are being challenged to demonstrate next generation printing capability in advance of the development of the required reticle manufacturing tools and processes. Although reticle manufacturers are continuing their diligent work in improving CD uniformity, there exists a window of time in which these specification needs for demonstration purposes by the wafer lithography tool developers exceeds those current deans of the broader semiconductor industry. This paper presents an approach to reticle manufacturing for the specific purpose of qualifying an advanced 4X reduction scanner. Typical e-beam reticles written on current generation tools in PBS [Poly(butene-1-sulfone)] resist and wet etched demonstrate CD uniformity of 65 nm - 80 nm expressed as total measured range in both the horizontal and vertical axis across the scanner field of 110 mm by 133 mm. This paper examines methodology employed using PBS and wet etching to generate a demonstration reticle which exhibits such CD range properties. Both reticle properties and wafer results are examined.
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