Daniel Seligson

Soft X-ray dosimetry and its application on the lithography beamline at SSRL

Abstract We describe simple methods of soft X-ray dosimetry (1000–3000 eV) for synchrotron based X-ray lithography. These methods are generally applicable to other synchrotron installations and possibly to so-called “stand alone” X-ray sources. A double crystal monochromator of synthetic multilayer crystals was used to obtain narrow band X-rays for energy-dependent measurements. The quantum efficiency of a stainless steel vacuum photocathode is shown to be well modeled by a simple power law, and its use in measuring the quantum efficiency of semiconductor diodes is described. The photocathode modeling is used in conjunction with the monochromator to demonstrate that resist sensitivity depends uniquely on absorbed dose density. The validity of the modeling is discussed, and the limitations of each type of detector are outlined. Suggestions are made for the application of these inexpensive detectors to real time source calibration in practical X-ray lithography equipment.

Synchrotron-based X-ray lithography at Stanford University

Abstract In this paper we describe present and planned synchrotron-based X-ray lithography facilities at Stanford. We also present standard procedures used at the Stanford Synchrotron Radiation Laboratory (SSRL) for conducting experiments in lithography mask damage, device damage, and X-ray resist characterization.

Radiation damage in boron nitride x-ray lithography masks

The optical and mechanical properties of boron nitride vapor deposited at 400 °C are shown to degrade when exposed to synchrotron radiation. The extent of the damage and the rate at which the damage occurs are similar to that first reported by Johnson et al. Transmission through membranes of boron nitride was measured in situ during exposure to x rays. Membranes darkened considerably with the transmission through a typical membrane falling from 50% to 20% after absorbing ∼250 kJ/cm3 of x rays. Changes in local film stress were measured with a simple cantilever technique. Films originally in tension (∼5E8 dyne/cm2) were found to become compressive after absorbing 300 kJ/cm3 of x rays. Both forms of damage responded well to annealing. Fourier transform infrared (FTIR) and near‐edge x‐ray absorption measurements were made to discern the structural differences between degraded and unexposed films. No significant structural changes were observed. Boron nitride films deposited at higher temperatures (600 °C) pr...

Process control with chemical amplification resists using deep ultraviolet and x‐ray radiation

Resists for optical electron beam, and x‐ray lithographies that operate on the principles of chemical amplification are entering into widespread use in laboratories. They offer an attractive alternative to conventional positive Novolac photoresists and we may soon see similar use in manufacturing environments. One class of resists within this family is based on acid‐hardening chemistry. We have characterized one such resist (Shipley ECX‐1033) for illumination by excimer laser deep ultraviolet sources and also by x‐ray radiation. A matrix of postexposure bake (PEB) conditions and development conditions was used to examine resist sensitivity and contrast. For all exposure sources we found that contrast is independent of PEB processing and that sensitivity obeys an Arrhenius dependence. Contrast increases with increasing development time while exposed resist loss is minor. A simple kinetic model was developed to explain the observed variations of apparent resist sensitivity with PEB time and temperature. The generality of this model suggests that it is widely applicable to other chemical amplification resists that require a postexposure bake. Together with aerial image calculations for different light sources, the model makes it possible to predict the dependence of linewidth on PEB conditions. In other words, we can anticipate the PEB process control required to achieve a specified critical dimension control. Electrical linewidth measurements of submicron features and their temperature dependence are compared with the predictions of the model.

The impact of high‐sensitivity resist materials on x‐ray lithography

Major research and development efforts are underway throughout the world that are directed toward establishment of very large scale integrated manufacturing with x‐ray lithography. Point‐source x‐ray lithography was once the favored approach to this technology but point sources have been all but abandoned in favor of synchrotron sources. This paper reviews the reasons for the emphasis on synchrotron sources and reexamines the reasons for this emphasis. Recent developments in point‐source technology and in resist chemistry warrant this reexamination. We conclude that point‐source x‐ray lithography is feasible and renewed effort is warranted in point‐source and high‐resistivity resist research and development.

New Energy-Dependent Soft X-Rav Damage In MOS Devices

An energy-dependent soft x-ray-induced device damage has been discovered in MOS devices fabricated using standard CMOS process. MOS devices were irradiated by monochromatic x-rays in energy range just above and below the silicon K-edge (1.84 keV). Photons below the K-edge is found to create more damage in the oxide and oxide/silicon interface than photons above the K-edge. This energy-dependent damage effect is believed to be due to charge traps generated during device fabrication. It is found that data for both n- and p-type devices lie along a universal curve if normalized threshold voltage shifts are plotted against absorbed dose in the oxide. The threshold voltage shift saturates when the absorbed dose in the oxide exceeds 1.4X105 mJ/cm3, corresponding to 6 Mrad in the oxide. Using isochronal anneals, the trapped charge damage is found to recover with an activation energy of 0.38 eV. A discrete radiation-induced damage state appears in the low frequency C-V curve in a temperature range from 1750C to 325°C.

Computing with a difference neuron

The need for efficient implementation of neural networks in silicon is used to motivate the investigation of alternatives to the McCulloch-Pitts neuron; in particular, one which computes the norm of a difference rather than an inner product. Earlier work is reviewed briefly and formal relationships between the two types are provided. The two types are shown to be equivalent under certain circumstances. Hopfield-like networks and multilayer feedforward networks of the difference neuron are simulated and analysed by comparing them to conventional types. Outside the domain of equivalence, the difference networks are found to perform less well than networks of identical architecture but which incorporate the McCulloch-Pitts neuron.

Aqueous Developable, Negative Working Resist Made Of Chlorinated Novolac Resin

Recently, three approaches to sub-micron imaging using negative working, aqueous developable, novolac resin or poly(hydroxystyrene)-based resists have been reported: (1) Image reversal of positive working photoresists (2) Acid hardenable resist with post-bake process, and (3) Contrast enhanced-resist with deep uv flood exposure. Negative working resists with high resolution are important for fabrication of advanced devices. In the present study we examine a simple two component system consisting of a chlorinated novolac resin and an aromatic bisazide suitable for uv, electron and x-ray exposure. The resist is conventionally processed in aqueous developer, and provides negative working resist images with 0.25 μm space and line resolution without swelling or scum.

X-Ray Induced Damage In Boron Nitride, Silicon, And Silicon Nitride Lithography Masks

Boron nitride membranes (produced through chemical vapor deposition of diborane and ammonia) have been exposed to synchrotron radiation and have showed severe degradation in optical properties after absorbing doses on the order of 200kJ/cm3. Damage kinetics are described as well as measurements made to identify the damage mechanism. Preliminary results on associated mechanical damage are also presented. Boron nitride membranes (produced through the pyrolysis of borazine), silicon nitride and silicon membranes exposed and tested in the same manner showed no such degradation.