Michael T. Reilly

X-ray damage in low temperature ultrathin silicon dioxide

The electrical characteristics of ultrathin oxides used in an x‐ray lithography n‐channel metal‐oxide‐silicon process grown at 700 and 950 °C were studied. The breakdown field exceeded 15 MV/cm for both low and high temperature oxides and the interface trap density of the fresh oxide was of order of 1010 cm−2 eV−1. Oxides grown at 950 °C had a lower interface trap density than 700 °C oxides, but 950 °C oxides are more sensitive to x‐ray radiation damage. After 350 °C hydrogen annealing about 80% of the radiation damage in the form of interface traps was recovered.

CXrL aligner: An experimental x‐ray lithography system for quarter‐micron feature devices

This article describes the CXrL aligner, an experimental x‐ray proximity lithography system developed at the University of Wisconsin Center for X‐Ray Lithography. The main features of the aligner are (1) exposure in an atmospheric He environment; (2) mask to wafer alignment error detection and correction during exposure; and (3) mask to wafer continuous gap setting based on capacitance gauges. The aligner consists of a three‐axes two‐state alignment system for continuous alignment error detection and a piezobased precision mechanical stage for alignment error correction. The wafer is held by a flat vacuum chuck and the mask is held by three vacuum suction cups located around the glass ring. Since the optical system is located outside of the synchrotron radiation path, alignment can be performed during the exposure. We have obtained a noise equivalent misalignment of 2 nm with an alignment signal response time less than 10 ms. An alignment signal repeatability (3σ) better than 0.06 μm has been achieved. In...

Implementation of two-state alignment system into CXrL aligner (Poster Paper)

We describe the implementation of the two-state alignment system into the CXrL aligner, which is developed at our Center for X-ray Lithography. The CXrL aligner is designed to expose sub 0.25 μm feature size integrated circuits. The aligner consists of a three-axes two-state alignment system for alignment error detection and a piezo based precision mechanical stage for alignment error correction. The wafer is held by a precision vacuum chuck, while the mask is held by three vacuum suction cups located around the glass ring. In the prototype, the mask to wafer relative positioning is achieved by 3 motorized stages (for gap setting) and 3 piezo-actuators (for lateral alignment). Since the optical system is designed to be located outside of the synchrotron radiation path, alignment can be performed during exposure. We present the results of the alignment system performance, such as noise equivalent displacement and alignment signal response time. An alignment signal repeatibility of much better than 3σ = 0.07μm is achieved. We also briefly describe the future evaluation of the system, such as overlay measurement of the system using verniers and SEM inspection of some specially designed patterns.

Design and development of plasma enhanced chemical vapor deposition universal antireflective layer films for deep subquarter micron deep ultraviolet applications

A dual-layer, oxynitride film stack functions as a universal antireflective layer (UARL) for patterning deep subquarter micron features on transparent dielectric films. The UARL optical constants at 248 nm are n=1.96 and k=0.3 for top layer and n=2.24 and k=1.04 for bottom layer with film thickness 360 and 650 A, respectively. The bottom UARL layer absorbs most of the incoming light, minimizing the light reflected from the underlying substrate. Therefore, the reflectance back into the photoresist is a few tenths of one percent and is independent of the substrate material’s optical properties and structures. Results of patterning 0.18 μm photoresist lines with the UARL on various Damascene film stacks show very tight critical dimension control.

X-ray mask fabrication process

The Center for X-ray Lithography (CXrL) has developed an x-ray mask fabrication process based on silicon nitride membranes and gold absorber. The LPCVD conditions for the growth of the nitride film produce 2 micrometers thick films with low tensile stress and an optical transmission sufficient for optical alignment. The membranes are formed with an reactive ion etch of the membrane window on the backside nitride, followed by a KOH etch of the silicon wafer. A plating base of 100 angstrom chrome followed by 200 angstrom gold is evaporated on the wafers. The wafer is then mounted on a glass ring using either adhesive or anodic bonding. The absorber pattern is delineated via e-beam lithography into either PMMA or SAL 601. Following resist development and an oxygen plasma cleaning, gold plating is used to produce features of the desired thickness.

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

Abstract 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.

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 CXrLs 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.

Interferometric investigation of x-ray mask fabrication distortions

The investigation of x-ray mask fabrication distortions was initiated in an effort to identify the fabrication parameters responsible for the final x-ray mask shape and configuration. The investigation has identified the sources of fabrication-induced distortion in x-ray mask blank manufacture. The extraction of distortions at each process step allows for mask flatness control via distortion compensation as the mask fabrication process evolves. Interferometric characterization of the final mask blank configuration guarantees the mask flatness. Mask blanks with alignment windows are mapped to determine the locations of the alignment windows relative to the membrane. An additional interferometric wedge test is performed to determine the membrane tilt magnitude and orientation relative to the backside of the mask ring. With proper selection of mask blank materials and control of membrane material deposition and bonding parameters, x-ray masks up to 100 mm in diameter have been fabricated routinely with less than 5 micrometers of bow. Fine-tuning of the x-ray mask configuration may be controlled by variations in the anodic bonding process parameters. Optimization of the anodic bonding process is currently in progress.

X-ray mask replication using square synchrotron radiation illumination

X‐ray masks are an expensive part of the x‐ray lithographic technology. The overall mask making process can be simplified and the cost reduced by a judiciously applied replication process. It is desirable that the copy has negligible feature bias and minimal placement distortion. It should therefore be produced by a process that has the greatest possible latitude with respect to dose and gap. In this article, it is described how these conditions are met. The latitude requirement is met by engineering the exposure system so that the source has a prescribed amount of spatial incoherence. This technique is an extension of the zero magnification method used by Wells et al. [J. Vac. Sci. Technol. B 10, 3221 (1992)]. This approach is particularly well suited for synchrotron‐based x‐ray lithography.

Optimization design program for chemically amplified resist process

In this article, a novel optimization program applicable to chemically amplified resist (CAR) process development, which is based on the method named as chemically amplified resist process optimization design developed in CXrL, is described. This program is separated as three subprograms. The first program called vertices is used to find most efficient sampling space. The second program called OPTIM1 is used to find the optimal process conditions. The third program called OPTIM2 is used to find the design center (maximum process tolerance space) of a CAR process with minimum experimental runs. In this article, the process optimization of AZ PF‐514 has been used as an example to show that the program can identify the optimal process condition as well as the maximum tolerable parameter space with minimum experimental runs.