Center For X-ray Lithography Research Articles

Evaluation of Image Shortening for Rectangular Array Patterns in X-Ray Lithography

The image shortening of rectangular array patterns replicated by proximity X-ray lithography was investigated. The shorter pattern width was 100 to 200 nm, and shortening on the longer side was studied. Either a negative or positive tone of the mask was used. The experimental results were compared with the absorbed dose distribution calculated using ToolSet developed at the Center for X-ray Lithography (CXrL). The geometrical features of mask patterns, such as absorber side-wall angle, corner rounding, and pattern resize were taken into account. It was found that image shortening is greater with negative tone masks, while negative tone masks provide higher resolution than positive tone masks. It was also found that the slope of the absorber side wall plays an important role in image shortening. The results suggest that the use of negative tone masks, careful tailoring of the absorber side wall, and proper pattern resizing will provide required dimensions for rectangular array patterns.

Installation and initial operation of the Suss Advanced Lithography Model 4 X-ray Stepper (abstract)

A Suss Advanced Lithography X-ray Stepper designed as a production tool for high throughput in the sub-quarter-micron device range has been installed and is being commissioned at the University of Wisconsin’s Center for X-ray Lithography (CXrL). Illumination for the stepper is provided by a scanning beamline designed and constructed at CXrL. The beamline optical components are a gold-coated plane mirror, a 1-micron-thick silicon carbide window, and a 25-micron-thick beryllium exit window. Beamline features include synchronized scanning of the mirror and exit window, variable scan velocity to compensate for reflectivity changes as a function of incident angle, and a horizontal oscillation of the beryllium window during vertical scanning to average the effects of nonuniform beryllium window transmission. A helium purged snout transports the x-rays from the beamline exit window, to the exposure plane in the stepper. This snout is retractable to allow for the loading and unloading of masks into the stepper. The motions of the mirror, exit window, and snout are computer controlled by a LABVIEW program that communicates with the stepper control software. The design of the beamline and initial operating experiences with the beamline and stepper will be discussed.

Performance of the Modified Suss XRS 200/2M X-Ray Stepper at CXrL

An XRS 200 model 2M X-ray stepper has been installed at the Center for X-ray Lithography. This paper describes the capability of the machine and its installation. Of greater interest is the performance the stepper gives with respect to the dose control and the mask/wafer alignment. How robust a process is depends on the latitude of each process parameter and the control tolerance accorded the machines and materials that affect each one. The dose repeatabillty and uniformity is determined by the optical elements in the beamline, the helium delivery system, and the accuracy of the scanning stage. Test results are reported for dose control using three separate methods. The scan speed is measured using Suss supplied equipment and is verified separately by CXrL's own technique. The dose delivered is analysed by the response of radiachromic film. Also important is the behavior and performance of the ALX-100 alignment system. The optical set-up is described as a model that includes the ALX hardware and illumination as well as the mask and wafer. The signal and contrast from the mask and wafer are compared to data for several substrates. A figure of merit is discussed for determining which membranes will give the best overlay.

The center for X-ray lithography facility status and beamlines development

The University of Wisconsin-Madison Center for X-ray Lithography (CXrL) is a national facility for basic and applied research in the field of X-ray lithography operating five beamlines dedicated to X-ray lithography at the University of Wisconsin-Madison Aladdin storage ring. In addition to the beamlines, support facilities for lithographic processing are available. A recent development program with ARPA and Motorola has led to a large increase in the processing and methological facilities available at CXrL. CXrL is in the process of upgrading several key aspects of the facility to accommodate the new equipment and research initiatives. A description of the facility upgrades and a summary of beamlines capabilities, research activities and support facilities is provided.

Linear-Fresnel-Zone-Plate-Based Two-State Alignment Method for Sub-0.25 µm X-Ray Lithography System

In this paper, we present a linear-fresnel-zone-plate-based two-state alignment method developed at the Center for X-ray Lithography. The alignment system uses a linear Fresnel zone plate (LFZP) as a mask alignment mark and an array of dots as a wafer alignment mark. The alignment error signal extraction is based on a two-state modulation of the incident light. The optical system is arranged outside of the exposure X-ray path and alignment can be performed during X-ray exposures. In the experiment, we obtained an alignment signal depth of focus larger than 4 µm and the gap change between the mask and wafer did not affect the alignment position. The X-ray double-exposure experiment on the system demonstrated an alignment accuracy better than 0.035 µm (3σ) on both Al and silicon nitride marks.

Analysis and optimization of x-ray masks using finite element methods

The development of stable, low-distortion masks is one key to the success of x-ray lithography. The 0.25 μm critical dimensions desired require that the membrane distortions due to mounting be minimized. This paper presents some of the analyses, optimizations, and modelling techniques developed at the Center for X-ray Lithography in the area of mask mounting.

Synchrotron-produced Ultrasoft X-rays: A Tool for Testing Biophysical Models of Radiation Action

Ultrasoft X-rays are useful for mechanistic studies of ionizing radiation damage in living cells due to the localized nature of their energy depositions. To date radiobiology experiments in this energy region have relied on characteristic X-rays (mainly Alk and Ck) from X-ray tubes. However, limitations in the photon intensity and the available energies from X-ray tube sources prevent a definitive characterization of the relationship between photon energy and biological damage. Synchrotron radiation has the potential to avoid these limitations, since it produces X-rays with high intensity over a continuous spectrum. We have established a synchrotron-based system for radiation biology studies using the ES-0 exposure station of the Center for X-ray Lithography at the University of Wisconsin Synchrotron Radiation Center storage ring, Aladdin. A characterization of the system including spectral and intensity properties of the photon beam is presented. The first mammalian cell survival curve for synchrotron-produced ultrasoft X-rays was generated and is presented. Cell survival curves of C3H/10T 1/2 cells using synchrotron radiation of 1.48 keV agree with previous data using Alk X-rays (1.49 keV). An RBE of 1.47 +/- 0.30 at the 10% survival level was measured with reference to 250 kVp X-rays.

Synchrotron radiation x-ray lithography fabrication of 0.35 μm gate-length n-type metal–oxide–semiconductor transistors

Synchrotron radiation x-ray lithography, using commercial x-ray masks, has been used in a mix-and-match scheme with an optical stepper to fabricate functional 0.3 μm gate-length n-type metal–oxide–semiconductor devices. The x-ray exposures were done at the Center for X-ray Lithography (CXrL) at the University of Wisconsin-Madison. The gate-level mask was fabricated to our specifications by Perkin-Elmer Advanced Lithography Operations, Danbury, CT. All other processing and the optical stepper exposures were carried out at the McDonnell Douglas Electronics Systems Company—Microelectronics Center, St. Louis, MO. Processing results and device characteristics are discussed.

Wisconsin x-ray lithography program (invited)

The facilities and research activities of the University of Wisconsin’s Center for X-ray Lithography (CXrL) are outlined. We discuss recent modifications to the two x-ray lithography beamlines operated by CXrL at the Aladdin storage ring. Ongoing research in various aspects of x-ray lithography, including photoresist characterization, mask stability and distortion studies, and modeling of the x-ray lithography process are summarized. We present plans for the expansion of the CXrL research facilities.