Facilities

EUV Source Radiation Development Facility

This facility is equipped for studies on laser plasma, EUV radiation, high-resolution spectroscopy, broadband spectroscopy, sub-nanosecond interferometry, and so on, for the purpose of developing a high-efficiency source for EUV lithography. The lab space (below) for this facility houses a radiation vacuum chamber in which the EUV source is generated and with which several diagnostics can be coupled to it to make experimental observations.

Combining two spectrometers, the Transmission Grating Spectrometer (TGS) and the Flat Field Spectrometer (FFS), enables high-resolution spectroscopy for wavelengths from 1nm to 20 nm and larger. Sub-nanosecond interferometry allows high speed measurement of the electron density and electron temperature of the plasma during laser-plasma interaction to determine the plasma conditions for optimum EUV radiation.

The FFS uses a 1200 lines/mm variable spaced reflective grating. Its spatial dispersion is linear on the image plane, where the surface of an x-ray CCD camera (PI-SX, Roper Scientific) is located to record the dispersed light. Light from the EUV source is collimated onto the grating by an entrance slit whose size determines the spectral resolution of the spectrometer. A unique feature of this FFS is that the distances of slit-to-grating and grating-to-image-plane must be 23.7 cm and 23.5 cm respectively to achieve minimum aberration on the spectra’s image. Free standing metal filters can be inserted in a holder to select the wavelengths in the region of interests. The FFS is equipped with slits of sizes ranging from 10 μm to 80 μm, which correspond to a range of spectral resolutions 0.006 nm to 0.01 nm. In addition, this FFS has a configuration that allows it to be precisely aligned independently, making it an effective, portable EUV diagnostic.

The TGS (below) utilizes a grating 5000 or 10000 lines/mm periodicity and is designed to work with light incident normal to the grating. Its overall structure consists of an entrance arm, a grating, and an observation arm. The grating is held in place by a holder inside the TGS to intercept the incident light coming from the entrance arm. The observation arm, which can be swung about a pivot to different angles of observation, is connected to an x-ray CCD camera to record the dispersed spectra. Spectral resolution of the TGS depends on the size of the slit used for collimating the light onto the grating. For slit sizes ranging from 10 μm to 80 μm, our TGS has spectral resolutions 0.05 nm to 0.15 nm (with the 5000 lines/mm grating). The working wavelengths in the region of interest are selected by using free standing metal filter(s) placed between the slit and the grating.

Spectrographs recorded by the CCD camera are analyzed by using computer programs that can perform de-convolution of the recorded spectrum to extract the true spectrum. Shown below is an example of a true spectrum of a tin-doped EUV source.

High-Repetition-Rate EUV Source Development Facility

This facility is a dedicated system for the study of debris detection and mitigation of a Laser-Produced-Plasma EUV source. The system consists of a laser system, vacuum chamber, target dispensing system, and diagnostics. A commercial Nd:YAG laser produces pulses of 300mJ max. energy, pulse width of 10ns, and a repetition rate of 100Hz. The vacuum chamber is the vessel for the laser-material interaction, generating EUV emission when the background pressure goes as low as 10^-4 Torr order. The target dispensing system generates microscopic droplets, ~30μm in diameter, with a high repetition rate; these are the ideal targets (so called Mass-Limited Targets) for a Laser-Produced-Plasma EUV source. Several methods of debris detection are available, such as witness plate post-shot analysis, Faraday cup charge collector, and a Thomson parabola ion spectrometer (now under construction). The post-shot analysis is integrated with the on-site surface and material characterization facilities such as Scanning Electron Microscopes, Atomic Force Microscopes, an X-ray Photoelectron Spectrometer, and an Auger Electron Spectrometer. The study of debris mitigation is ongoing with these means of detection and analysis.

Precision 2J Nd:YAG Laser Facility

The precision laser was designed and built by visiting professor Dr. Etsuo Fujiwara. It is a Q-switched Nd:YAG laser system (λ = 1064 nm) that operates at 1Hz, 2 J per pulse, and 10 ns pulse duration. It consists of a master oscillator and three amplifier modules. The laser output from the oscillator makes two passes through the first amplifier, followed by a single pass through the second and the third amplifier.

The beam profile at far-field was measured, utilizing a Spiricon CCD, to be Gaussian and as having M² ~ 1.3, which is close to the diffraction limit. This allows the beam to be focused to a spot size of 20 μm diameter with a f = 10 cm lens. The temporal pulse shape is also Gaussian in shape of width ~ 10 ns, having slight modulations on its envelope. The laser is currently being used for developing a laser plasma source of high conversion efficiency for EUV lithography. The following are pictures related to the facility (click any to enlarge).

Laser system	Layout	Beam Profile and Performance

10 Terawatt Laser Facility

The Terawatt laser facility located at CREOL is a custom-built 10 Terawatt CPA laser system in combination with a large 60 port vacuum chamber and two laser lab bays. Experiments on this system range from x-ray production to ablation analysis with the flexibility to be configured to perform almost any task quickly. The large chamber provides easy installation of equipment, of which we have a large base of various instruments and diagnostic sensors for most requirements.

The laser consists of a Ti:Sapphire oscillator at 90 MHz and λ=850 nm. This acts as the seed laser for a regenerative amplifier using a gain medium of Cr:LiSAF and operating at 5 Hz. Typical output is 3 mJ per pulse. For experiments requiring higher energies, multiple Cr:LiSAF amplifiers are configured downstream to provide up to 10 terawatts on target at .5 Hz. Laser pulses are 100 femtoseconds in duration and have an M² of approximately 1.5. See system schematic below.

The target chamber is XX in diameter and has ample ports for instrument mounting. There is a dedicated motorized three-axis precision translation stage with GPIB controllability in the chamber. The vacuum system is located in a chase and is currently capable of achieving 10^-6 torr for clean experiments.

Current experiments include femtosecond laser propagation through the atmosphere, burst-mode ablation studies, elemental ablation studies, and in the near future, droplet target research.

Facility images are shown below along with the laser system layout. Click on the thumbnail for a larger image.

		(Image coming soon)
System Schematic	Laser System	Target Chamber

Biological X-Ray Microscopy

Here in LPL we think that microscopy and imaging techniques will be highlighted, followed by x-ray applications in microbiology, cellular differentiation, and cancer research. Soft X-rays have a photon energy of 100-1000 eV, or a wavelength of about 1-10 nm. The wavelength gives the potential for high-spatial-resolution imaging. The photon energy is well-matched to the inner-shell electron binding energy in low-Z elements. This provides very good intrinsic contrast between organic material and water in the "water window" between the K edges of carbon and oxygen, and good penetration in micrometer-thick specimens. Between the Carbon and Oxygen absorption edges at 284 and 543 eV, X-rays provide very good contrast between protein and water and therefore for organic materials in their natural environment.

In our laboratory the soft x-rays that are generated from (Nd:YAG) laser-produced plasma. This laser energy is focused onto an yttrium target within the chamber. This chamber must be always under vacuum because the X-rays cannot travel any substantial distance in air; thus the biological sample is placed in a hydrated cell in the specially designed biological holder that can be manipulated from outside the chamber for better positioning. Before the sample can be exposed to C-rays, it needs a preparation. Each sample has a specifically preparation but the main factor this preparation is to find the proper medium that does not allow the sample to become dehydrated. The sample is then affixed to the photo resist, and placed between PMMA and the coated photo resist and a silicon nitrate window used to protect the sample from the conditions within the vacuum as well as plasma debris. Therefore the PMMA acts as negative photographic film where the image corresponds to the X-rays' shadow produced by the specimen placed on it. This technique allows novel study of biomedical structures.

 

After exposure the photo resist must be developed; we wash, rinse and treat the sample with different chemical substances in the clean room. After development the resist is scanned using an Atomic Force Microscope to produce an image of the structure.

The Atomic Force Microscope (AFM) facility is composed of a TopoMetrix 2000 Discoverer Scanning Probe Microscope, a clean room, computer room, and specimen preparation room. In addition we have an optical microscope (Nikon Epi-illuminator 10). The strength of optical imaging techniques is their ability to view living organisms in pure, untreated states. Its main limitation is spatial resolution.

 

Principles of operation

Several techniques are available for magnifying the detailed features of a surface. Methods for magnifying surface feature originated with magnifying lenses and optical microscopy in the late 18th century. During the 20th century, methods for magnification based on electron and ion beams were developed. The atomic force microscope (AFM) is a recent innovation that relies on a mechanical probe for generation of magnified images: it provides pictures of atoms on or in surfaces. A system that uses variations of the principles used by an AFM to image surfaces is often called a scanning probe microscope (SPM). The AFM works by scanning a fine ceramic or semiconductor tip over a surface much the same way as a phonograph needle scans a record. The tip is positioned at the end of a cantilever beam shaped much like a diving board. As the tip is repelled by or attracted to the surface, the cantilever beam deflects. The magnitude of the deflection is captured by a laser that reflects at an oblique angle from the very end of the cantilever. A plot of the laser deflection versus tip position on the sample surface provides the resolution of the hills and valleys that constitute the topography of the surface. The AFM can work with the tip touching the sample (contact mode), or the tip can tap across the surface (tapping mode) much like a blind person uses a cane.

Another mode can be used, called the electrochemical mode. This process can be studied with a scanning probe microscope by placing the sample, which is the working electrode, in a liquid holder fitted with a reference electrode and counter electrode. Additionally, characterization of electrochemical processes requires a potentiostat for controlling the potentials of the sample, the counter electrode, and the scanning probe.

 

Nanoscope/Nanodetector

The Nanodetector uses EUV radiation with wavelengths between about 3 nm and 20 nm to probe structures with features as small as 10 nm. We can call also this device an "EUV conversion microscope," that uses very short wavelengths to get a very high resolution. This microscope is designed to do transmission microscopy. As shown in the figure below, the EUV light is collected, filtered, and focused onto the sample. The varying EUV light transmitted through the sample caused by the different absorption of various structures in the sample is converted to low energy electrons which are magnified and imaged using an electrostatic lens. The magnified electron image is converted into a visible photon image using a readout system. The visible photon energy is displayed using a charge-coupled device camera.

In LPL we are trying to improve the system and to use it to visualize biomedical structures and phenomena.