D. G. Boyers

Beryllium foil transition‐radiation source for x‐ray lithography

We have measured the total soft‐x‐ray power from a transition radiator composed of a stack of 25 beryllium foils each 1.0 μm thick which were penetrated by a relativistic electron beam whose maximum power was approximately 7 kW. The maximum total soft‐x‐ray power was measured to be 15.2 mW for a 245 MeV, 37 μA electron beam. The bandwith of the radiation at the full width half maximum points was calculated to be between 0.6 and 1.6 keV. In addition, we have exposed photoresist‐coated silicon wafers at a distance of 3 m from the radiator. Exposure times of the bare resist were as short as 120 s for 5 cm2 of wafer are (resist sensitivity is 55.6 mJ/cm2). The shortest time for mask/wafer exposure was 180 s for 5 cm2.

Tests of variable-band multilayers designed for investigating optimal signal-to-noise versus artifact signal ratios in dual-energy digital subtraction angiography (DDSA) imaging systems

Abstract In recent work, various design techniques were applied to investigate the feasibility of controlling the bandwidth and bandshape profiles of tungsten/boron-carbon (W/B 4 C) and tungsten/silicon (W/Si) multilayers for optimizing their performance in synchrotron radiation based angiographical imaging systems at 33 keV. Varied parameters included alternative spacing geometries, material thickness ratios, and numbers of layer pairs. Planar optics with nominal design reflectivities of 30–94% and bandwidths ranging from 0.6–10% were designed at the Stanford Synchrotron Radiation Laboratory, fabricated by the Ovonic Synthetic Materials Company, and characterized on Beam Line 4-3 at the Stanford Synchrotron Radiation Laboratory. In this paper we report selected results of these tests and review the possible use of the multilayers for determining optimal signal to noise vs artifact signal ratios in practical dual-energy digital subtraction angiography systems.

Increased x‐ray power from a transition‐radiation source

Soft x rays were produced by a stack of 12 aluminum foils, each 1.0 μm thick, which were bombarded by a relativistic electron beam whose maximum power was ∼10 kW. The maximum total soft x‐ray flux was measured to be 8 mW, a factor of 10 over the previous value. The maximum average power per unit area was ∼0.3 mW/cm2 over 8.5 cm2 area at 3 m from the transition radiator. Using an Intel mask, we obtained lithographs with features of 0.5 μm. Exposure times of the bare resist were as short as 150 s for 4.3 cm2 of wafer area (the resist had a 230 mJ/cm2 energy dose per unit area). The shortest time for mask/wafer exposure was 300 s for 8.5 cm2. We reduced the exposure time of the photoresist by a factor of 9 from the previous value.

Observation of the focusing of x-ray transition radiation using cylindrical optics

We have measured the profile of x rays generated by a transition radiator and focused by simple cylindrical optics. Soft x rays with photon energies between 1 and 4 keV were generated by a 93‐MeV electron beam striking a stack of eight foils of 3.5‐μm‐thick mylar. These x rays were emitted in an annular cone and were collected by a quartz cylinder which focused the x rays to a 0.5‐mm‐diam spot at a distance of 1.35 m from the transition radiator.

Remarks on the Role of Multilayer Optics and Short Period Insertion Devices for Medical Imaging Sources and Applications

Over approximately the last ten years, research and development has been conducted at the Stanford Synchrotron Radiation Laboratory (SSRL) in two areas critical to the status of Dual-energy Digital Subtraction Angiography (DDSA) and related medical imaging techniques: 1) multilayer optics, and 2) short-period insertion device technology. In this paper selected results of these experimental and theoretical studies have been reviewed, emphasizing their relevance to future DDSA systems design. A basic perspective predicated on our work is that continued development and implementation of these two technologies can favorably impact the performance and economy of existing or planned medical imaging facilities.

High-power x-ray generation using transition radiation

In experiments using targets consisting of many thin metal foils, we have demonstrated that a narrow, forward-directed cone of transition radiation in the 0.8 to 60 keV spectral range can be generated by electron beams with moderate energies (between 17 and 500 MeV). We have measured the spectral and spatial photon densities of these radiators using low current electron beams. Using high currents, we have measured the total power of these radiators. A total power of 15.2 mW was measured from a beryllium radiator, 8.1 mW from the aluminum radiator, 4.8 mW from the titanium radiator, and 3.6 mW from the copper radiator. These values matched calculated predictions within experimental error. Both x-ray lithography for the production of integrated circuits and Laue diffraction for the study of biological materials are possible applications of this radiation. In particular, using the Al and Be radiators we have exposed photoresist-coated silicon wafers. Exposure times of the bare resist were as short as 120 s for 5 cm2 of wafer area (this resist had a 230 mj/cm2 energy dose per unit area). The shortest time for mask/wafer exposure was 180 seconds for 5 cm2. Using an Intel mask, we obtained lithographs with features of 0.5 micrometers . In addition, we have calculated that the quasi-monochromatic bandwidth of transition radiation would be feasible for Laue diffraction of biological crystals. By designing transition radiators to emit x rays at the foil materials K-, L-, or M-shell photo-absorption edge, the x-ray spectrum is narrowed. The sources is then quasi-monochromatic and uses an electron beam whose energy is considerably lower than that needed for synchrotron sources.