Olga Vladimirsky

Demagnification in proximity x-ray lithography and extensibility to 25 nm by optimizing Fresnel diffraction

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature bias. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local demagnification. This demagnification ( ? 3- ? 6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a ~ 50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 100 nm) and large gaps (30-15 ? m). The method is demonstrated for demagnification values down to ? 3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.

Overlay budget analysis for the 100 nm device generation

In order to improve overlay capabilities for future exposure tools, it is necessary to understand the contributions from various sources. The most important contributions come from well-known sources like mask pattern placement accuracy, alignment system accuracy and from exposure tool stage performance. As the allowances for overlay budget decrease, and improvements in mask fabrication and stage performance are made, a number of previously less significant contributions need to be considered. These contributions include resolution, optical interference, focusing accuracy of the alignment system, and wafer processing deviations. These contributions are characterized in detail in this paper. The investigation was focused on a proven optical alignment system (ALX^T^M) contributions from wafer processing (resist coating and metallization).

X-ray microfabrication of multi-level structures and 3-D patterning

Abstract Preliminary results of a novel techinique for producing true three-dimensional patterns in thick resits are presented in this work.

Heating of x-ray masks during e-beam writing

In this work we characterized the temperature increase in SiHN mask membrane during e-beam writing. We observed an exponential decay with a decay length in the order of 1mm-1, and absolute temperature raises of 8 degrees K. This is the first time that direct measurement have been obtained. By fitting the observed data, we have extracted the thermal conductivity and emissivity of the film. These experimental values are essential in the modeling of the response of the masks.

Surface chemistry of GaAs wafers and reaction with chemically amplified resists during resist processing

The surface preparation of wafers used with chemically- amplified resist is critical for successful resist processing. GaAs wafers provide an additional complexity because the composition of the surface can be greatly affected by the chemical treatment and subsequent resist processing conditions. In order to get consistent rust with GaAs wafers, we have found that the surface composition of the wafer has to be determined. Secondary ion mass spectrometry (SIMS) and electron spectroscopy for chemical analysis were used to determine the surface composition before and after the treatment with HCl. A non-destructive and simple method of contact angle measurement was used to provide advance warning of difficulties with adhesion between GaAs and Shipley SAL605 negative-tone chemically- amplified resist. An observed correlation between the contact angel of water suggested that, when it was in the range of 60 degrees to 75 degrees, the wafers generally showed good adhesion when using aqueous developer, Shipley MF312. From the work of direct measurement of the wafer surface before and after treatment, we have found conditions that permit patterning linewidths on the order of 0.15 micron in 0.5 micron thick resist. The effects of each of the surface treatments found to influence the adhesion will be described in terms of the chemical changes on the wafer surface and their effect on the resist chemistry.

Analysis and identification of factors contributing to the overlay budget

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.

Method for planarizing rigid graphite for use as an x-ray mask substrate

The usefulness of thin (< 250 micrometers ) rigid graphite plates as x-ray mask substrates for micromachining and LIGA applications has been demonstrated. Rigid graphite offers unique properties, such as moderate x-ray absorption and optimal filtration of synchrotron radiation, relatively low cost, compatibility with additive (electroplating) and subtractive (etching, micromachining) processes for absorber patterning. The surface roughness of these substrates is associated with the inherent porosity of a commercially available rigid graphite material (typical Ra values are in the range of 1 - 2 micrometers ). The surface roughness of this rigid graphite sheet is reduced down to a 0.1 - 0.2 micrometers Ra value by polishing. To reduce surface roughness further and make the substrate usable for fine e-beam or optical absorber imaging, additional smoothing is required. In this paper, the surface characteristics of rigid graphite sheets are analyzed and a glazing technique developed to smooth the graphite surface is described. This technique employs hard baking process of novolac-based resins. An average Ra roughness value of approximately 5 nm was obtained after 5 coating using novolac-based AZ type resist.

X-ray mask fabrication at CXrL

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.

X-ray mask replication using a Suss stepper at CXrL

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.