I. C. Edmond Turcu

Repeller field debris mitigation approach for EUV sources

We describe studies of the debris produced from a high-repetition-rate laser plasma EUVL source based on the mass-limited target concept. In particular, we are developing mass-limited target designs based on complex targets containing tin. Comprehensive analysis of witness-plate detection techniques can reveal many interesting details of the interaction regime, and the impact of the debris. These techniques include Optical Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, Auger Electron Spectroscopy, and Auger Electron Microscopy. We also describe developments of the repeller field concept of debris inhibition. This technique uses electrostatic fields to reduce the flux of plasma ions impinging on the EUV collimating optics. Here, the first measurements of debris mitigation of a tin-doped target are described, and comparisons with earlier measurements of the impact of repeller fields on ion emission from a mass-limited water-droplet target are made.

High-power x-ray point source for next-generation lithography

An x-ray power of 2.8 Watts at the 1 nm x-ray lithography wavelength was generated by a copper plasma formed by a single laser beam focused to an intensity of greater than 1014 W/cm2 on a copper tape target. The all solid state BritelightTM YAG laser has 700 ps pulse duration, 300 Hz pulse repetition rate, average power of 75 Watts, and less than 2 times diffraction limited beam quality at the fundamental 1.064 micrometer wavelength. The single beam laser system has a master oscillator, a preamplifier and one power amplifier, all diode pumped. Measurements confirmed negligible copper vapor debris at 8 cm from the laser-plasma source with atmospheric pressure He gas and modest gas flow. The point source x-ray radiation was collimated with either a polycapillary or grazing mirror collimator. The near-parallel beam of x-rays has good divergence both globally (0.5 mrad) and locally (less than 3 mrad), good uniformity (2% achievable goal) and large uniform field size (20 mm X 20 mm full field and 25 mm X 36 mm scanning system). High-resolution lithography was performed for the first time with collimated 1 nm point source x-rays. A power scaling system is being built with eight amplified beams in parallel on the x-ray target, and is expected to achieve 24 - 30 Watts of x-rays. A 16 beam laser plasma x-ray lithography system could achieve a throughput of 24 wafer levels per hour using 300 mm diameter wafers.

Focused ion beams and life science applications: cell tomography and biomachining at ultrahigh resolution

A new technique of Focused Ion Beam (FIB) microscopy- nanomachining is proposed for life sciences. Its performances are compared with those of currently available ultramicroscopy apparatuses. Ultra-high resolution tridimensional tomography can be performed on whole cells without preparation. This can be achieved by sequentially etching layers of material and subsequently viewing the result of the operation under a different perspective. Very fast imaging times (minutes) allow quasi real time microscopy. The complementary technique of nano-biology can be performed on the same apparatus. The use of the ion beam allows to imaging both the surface and the inner part of the sample along any desired plane that can be chosen while the observation is on.

L-shell x-ray spectroscopy of laser-produced plasmas in the 1-keV region

Laser based x-ray sources at h(upsilon) approximately equals 0.9 - 1.2 keV (Fe to Cu L-shell) developed for applications are studied from the point of view of x-ray emission spectra. X-ray spectra obtained with different laser intensities and wavelengths using Nd and KrF lasers are described. Results are qualitatively compared with the predictions of simple analytical models for plasma temperature Te and average ionization Z*. When using KrF lasers it is possible to introduce a He atmosphere in the interaction chamber to reduce the debris problem. X-ray spectra at different He pressures were recorded and x-ray L-shell emission of Fe and Cu were shown to be largely independent of the pressure.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Atmospheric pressure operation of a repetitive KrF laser-plasma x-ray source at hv = 1.1 keV

A repetitively pulsed (5Hz) KrF laser-based X-ray source producing photons at i-ru 1.1 keV (copper, L-shell) from a copper coated rotating target has been used to study soft X-ray induced DNA damage effects in Chinese hamster cells. The source was computer controlled for accurate delivery to the biological material of pre-set doses. DNA damage was induced by exposures lasting 7s for V79 cells and 40s for AA8 cells. To minimise the debris from the laser-plasma source and for convenient handling of biological specimens, the target chamber contained helium at 1 atmosphere with a slow flow. The X-ray yield of the source decreased by only at most 10-20% compared to vacuum operation and a further 16% of X-rays were absorbed in helium between target and the biological material placed outside the target chamber behind a beryllium filter. The measured spectral and spatial distribution of the copper X-ray emission was found to be largely independent of the ambient helium pressure. The time resolved X-ray signal lasted for only 3 ns starting at the beginning of the 2lns laser pulse and its shape was independent of helium pressure in the target chamber.

Polycapillary collimator for laser-generated plasma source x-ray lithography

Collimating of the x-ray beam is essential to point source proximity x-ray lithography for controlling radial magnification and increasing the beam intensity. Polycapillary optic collimators were developed to meet the challenges of point source proximity x-ray lithography. Sophisticated modeling software was developed for design and optimization of polycapillary collimators to meet specific requirements. Using this software, a highly efficient collimator was designed to deliver a well-collimated beam centered at 1.1 keV for a 20 mm X 20 mm field. The collimator was constructed and was tested with both an electron bombardment source and a laser generated plasma source. The design goals of intensity gain and divergence controls have been achieved. The intensity variation within the printing field can be less than 2%.

Film calibration for soft x-ray wavelengths

The response of photographic film to X-rays from laser- plasma is of practical interest. Film is often used for the ultimate detection of x-rays in crystal and grating spectrometers and in imaging instruments such as pinhole cameras largely because of its high spatial resolution (approximately 1 - 10 microns). Characteristic curves for wavelengths--3 nm and 23 nm are presented for eight x-ray films (Kodak 101-01, 101-07, 104-02, Kodak Industrex CX, Russian UF-SH4, UF-VR2, Ilford Q plates and Shanghai 5F film). The calibrations were obtained from the emission of laser-produced carbon plasmas and a Ne-like Ge X-ray laser.

Atmospheric-pressure soft x-ray source for contact microscopy and radiobiology applications

A large volume non-conventional XeCl excimer laser (HERCULES) emitting long pulses (from 10 ns up to 120 ns at a wavelength of 308 nm) has been used to drive a soft x-ray plasma source. The x-rays pulse duration and the energy conversion efficiency in different spectral regions have been measured; x-rays emission lasting up to 100 ns has been obtained in the 70 eV region. The dependence of x-ray pulse duration on the size of the laser spot is discussed. The x- ray source can be operated both in vacuum and in helium at atmospheric pressure. This allows irradiating over a large area both for contact microscopy of living specimens (up to 1 mm2 windows) and for radiobiology (up to some cm2 windows). The experimental results obtained for these two applications as well as for radiographic images of living insects are discussed.

X-ray irradiation of yeast cells

Saccharomyces Cerevisiae yeast cells were irradiated using the soft X-ray laser-plasma source at Rutherford Laboratory. The aim was to produce a selective damage of enzyme metabolic activity at the wall and membrane level (responsible for fermentation) without interfering with respiration (taking place in mitochondria) and with nuclear and DNA activity. The source was calibrated by PIN diodes and X-ray spectrometers. Teflon stripes were chosen as targets for the UV laser, emitting X-rays at about 0.9 keV, characterized by a very large decay exponent in biological matter. X-ray doses to the different cell compartments were calculated following a Lambert-Bouguet-Beer law. After irradiation, the selective damage to metabolic activity at the membrane level was measured by monitoring CO2 production with pressure silicon detectors. Preliminary results gave evidence of pressure reduction for irradiated samples and non-linear response to doses. Also metabolic oscillations were evidenced in cell suspensions and it was shown that X-ray irradiation changed the oscillation frequency.

High-power laser-plasma x-ray source for lithography

A compact x-ray source radiates 24 Watts average power of 1nm x-rays in 2 (pi) steradians. The laser produced plasma x-ray source has a 300 W laser driver which is a compact, diode-pumped solid-state Nd:YAG laser system. The x-ray conversion efficiency is 9 percent of the laser power delivered on target. The x-ray source was used to demonstrate x-ray lithography of 75 nm lines. The x-ray source is optimized for integration with a x-ray stepper to provide a complete x-ray lithography exposure tool for the manufacture of high-speed GaAs devices.