It began under the Laboratory directorship of David A. Shirley. It was completed under the directorship of Charles V. Shank. Jay Marx oversaw its construction. Brian Kincaid is overseeing its operation. It has been the most talked about and eagerly awaited project at LBL since it was first proposed nearly 10 years ago. The wait is finally over. Now it is here.
"It" is the Advanced Light Source (ALS)-a $100 million synchrotron radiation source capable of boosting the energies of electrons to 1.5 billion electron volts (GeV) and extracting from them laserlike beams of x-ray and ultraviolet light that will outshine any in the world. Its premier beams, in the soft (low-energy) x-ray and extreme ultraviolet region of the electromagnetic spectrum, will be from 20 to 100 times brighter than beams from other synchrotron radiation sources and a hundred million times brighter than the light from the most powerful x-ray tubes.
The x-ray and ultraviolet light from the ALS, collectively known as XUV radiation, will serve scientists from across the nation and around the globe in a broad number of fields. Materials scientists are getting a spectroscopic probe that can focus on surface areas only a few hundred atoms in diameter, identifying which elements are present and how their electrons are arranged or how they could be rearranged for new applications. Chemists get a camera that can freeze-frame lightning- fast chemical reactions, such as combustion, at less than a billionth of a second for step-by-step observations of what is taking place. Biologists and other life scientists will have an x-ray microscope that can peer deep inside living cells in their natural state and perhaps provide the first truly three- dimensional images of the structures within.
The ALS could also play a major role in the development of one of the hottest new technologies on the horizon-nanotechnology. From supercomputers that fit in the palm of the hand to self-assembling, molecular-sized machines that clean up our environment and conserve energy, major opportunities lie with nanostructures-devices less than a millionth of a meter in size.
A key to achieving this future is x-ray lithography (see sidebar, page 12). Through x-ray lithography, patterns with features as tiny as a hundred nanometers (about 1/1000th the thickness of a human hair) can be transferred onto materials to make such exotic creations as a gigabit memory chip-one that contains 64 times more information than today's most advanced microchips. ALS x-rays are ideal for studying and testing the optical systems and coatings used in x-ray lithography and other nanostructure research.
The versatility of the ALS stems from the quality of its XUV radiation beams. These beams have properties similar to laser light-they are exceptionally bright and highly monochromatic, and they are coherent, meaning that individual light waves are all the same frequency and phase, and travel in the same direction. ALS radiation is also tunable: its wavelengths can be varied, and pulsed: it flashes on and off like a strobe light. The pulse rates of ALS beams can be as high as 500 million pulses per second, with each lasting only 30 to 40 picoseconds (trillionths of a second). This allows the ALS to function as a highly effective stop-action camera.
In addition to their unique combination of spectral features and high brightness, the soft x-ray beams of the ALS will match the primary atomic resonances of many important elements, including carbon, oxygen and silicon. Other types of ALS beams extend this match to almost every element on the periodic table. What this means, among other things, is that ALS photons are readily absorbed by the atoms of these elements. When atoms absorb photons, they eject electrons, with the energy and number of electrons ejected dependent upon the frequency and brightness of the light. Analysis of absorption spectra or of the associated photoelectron spectra yields information about the electronic configurations of the atoms. These configurations are, in general, responsible for the physical properties of substances, including appearance, texture, freezing and melting temperatures, electrical conductivity, and so on.
The ALS is located on the site where LBL founder Ernest 0. Lawrence built the 184-inch cyclotron during World War II. (The 184-inch took its name from the diameter of the magnet's pole faces. ) Although much of Lawrence's `'184" had to be demolished to make room for the ALS, the dome that crowned the old accelerator and became a Berkeley landmark was preserved. The building underneath the dome, however, with a floor space of Beneath the historic dome that approximately two acres, is nearly four now tops The Advanced Light times larger than the previous structure. Source, girder assemblies for
Housed inside this building are a pair the booster synchotron are of electron accelerators and a state-of- the- lifted by crane and lowered into art electron storage ring. The accelerator the concrete shielding tunnel. complex, which serves to inject the electrons into the storage ring, consists of a linear accelerator, or `'linac," that can lift source-one of the first members of a forthcoming international family of such machines.
Synchrotron light is the electromagnetic radiation emitted by electrons when their speed or direction of motion is changed. This radiation can be in the XUV region of the spectrum when the electrons are forced to travel along a curved path at speeds approaching the speed of light. Synchrotron accelerators, which use dipole `'bending" magnets to curve the paths of accelerating electrons into circular orbits, produce copious amounts of synchrotron light in a sweeping beam that is similar to that of a beacon.
Synchrotron light was once an unwanted by-product in accelerator storage rings. High-energy physicists trying to maintain the boosted energy levels of electrons were not happy that some of this precious energy was being lost through emitted photons. Eventually, however, scientists in other disciplines learned to put the light to good use, which led to the demand for much greater, more intense quantities of photons. The result was a recognition of the usefulness of undulators and wigglers and the subsequent advancement of insertion-device technology.
An insertion device is an array of dipole magnets with alternating north and south poles. The reversing polarity of an insertion device forces a beam of electrons speeding through it to oscillate sideways, causing the electrons to emit light in a direction tangential to the path of the beam. The wavelength of light produced depends on the wavelength and amplitude of the oscillations and energy of the electrons.
Undulators generate laserlike beams as a result of the constructive interference in the emission of light from each oscillation of the electron beam. This interference compresses the radiation into a few narrow spectral peaks. The wavelength of the light produced by an undulator can be tuned by mechanically expanding or reducing the gap between the undulator's array of magnetic poles. Wigglers, on the other hand, emit light in a nearly continuous spectrum, just as bending magnets. However, unlike time sweeping beacon of light from bending magnets, the light from a wiggler is directed, like that of a flashlight.
The ALS storage ring has room for 10 insertion devices and will probably contain eight undulators and two wigglers. The undulators can be up to 4.6 meters in length (nearly twice as long as most existing synchrotron insertion devices) and can feature a variety of periods, which is the distance in the array from one north pole to the next. Smallest is an undulator with a period of 3.9 centimeters-dubbed `'U3.9"-which provides soft x-rays at energies from less than 200 electron volts (eV) up to 2500 eV, a range which covers the so-called "water-window" for penetrating and imaging biological objects. The undulator with the longest period will be U10 ( 10 centimeter period), which will cover the energies around 6 eV and will be used in studies of chemical dynamics. In between these are U5, which will provide the photons in the 52 to 1900 eV range for studies of materials, surfaces and interfaces, and U8, with photons ranging from 8 to 1100 eV, which will shed light on atoms, ions, and molecules.
One wiggler in the ALS will be 2.9 meters long, with a period of 16 centimeters (W16). Light from the W16 wiggler will extend beyond 10,000 eV, which is the realm of hard x-rays, and will be useful in x- ray crystallography studies and spectroscopy research. Such light will also serve as a highly sensitive x-ray microprobe with a spatial resolution approaching one micron.
The second wiggler-not yet designed-will probably be what is known as an `'elliptical wiggler." This type produces circularly polarized W radiation, which is particularly useful for probing both magnetic materials and `'chiral" molecules-those that have a left-handed or right-handed character to their structure.
The development at LBL of permanent-magnet wigglers and undulators was a major factor in making it possible for the ALS and other third-generation synchrotron radiation facilities to produce their bright beams. Unlike electromagnets, permanent-magnets require neither expensive electrical power nor space-consuming coils to generate their magnetic fields. Consequently, their poles can be spaced close enough together to obtain x-ray wavelengths of light.
In addition to the light from its insertion devices, the ALS can also generate light, including hard x- rays, through the bending magnets in the arc sections of its storage ring. Although the synchrotron light from the ALS bending magnets is not as bright as that from the ALS insertion devices, it compares very favorably with the light from other synchrotron bending magnet sources because of the high quality of the ALS' electron beam.
Crucial to getting optimal performances from the ALS insertion devices is the quality of the storage ring's electron beam. Rather than a continuous stream of particles, the electron beam in the ALS' storage ring is formed from discrete bunches of electrons. These discrete bunches are what makes the ALS light pulsed, giving it a strobed effect and enabling it to initiate and track the progress of i'me- dependent processes such as chemical reactions.
ALS electrons are created by a high-intensity electron gun that can generate single or multiple bunches of 120,000 eV electrons in pulses of 2.5 nanoseconds ( billionths of a second). As electrons emerge from the gun, they pass through two `'subharmonic bunchers" (frequencies of 125 and 500 megahertz, respectively) that compress the beam first into pulses of 800 picoseconds, then into 200-picosecond pulses: The beam is then fed into the accelerating structures of the 50 MeV linac. The 1.5 GeV booster synchrotron into which the injection system feeds its 50 MeV electrons is composed of 24 dipole magnets, 32 quadrupole magnets, and 20 sextupoles, distributed over 12 girders. Every second, the booster sends pulses of 1.5 GeV electrons into the storage ring, filling the ring in about five minutes. It is in the storage ring where electron beam quality is maintained at an unprecedented level of excellence.
Another precisely positioned armada of magnets and a complex vacuum system are the keys. Each of the ring's 12 arc-shaped sections contain three dipole bending magnets, six quadrupole magnets that focus the electron beam, and four sextupole magnets that correct for the spread of electron energies. This arrangement, called a `'triple bend achromat," stabilizes the beam and keeps emittance-a product of beam size and divergence-exceptionally low, while the electrons orbit around the ring for six hours, covering a distance equivalent to that from the Earth to the planet Neptune (about 2.7 billion miles).
The ALS storage ring's vacuum chamber is a worthy complement to its unique lattice of magnets. Nothing must impede the flight of electrons in a storage ring, so they travel through a chamber-encased channel that has been evacuated. Maintaining the vacuum of this channel is complicated by the fact that the circling electrons are continually emitting photons that can desorb gas from the chamber walls. The ALS vacuum chamber is designed to permit most of these photons to escape the channel through a slot into an antechamber, where they are absorbed by photon stops. Any gas that is produced is removed through a series of pumps.
After light is extracted from the ALS storage ring, it is sent through beam lines, branched off and directed into experimental areas by means of special mirrors and optical devices. Light from the undulators can appear over a range of W wavelengths, depending on the magnetic field, and light from the wigglers and bending magnets covers a broad, continuous spectrum. To customize this light for their own specific purposes, scientists at the different experimental areas can select specific wavelengths, using a monochromator.
Monochromators employ a combination of slits and gratings to disperse photons and collect the wavelengths of interest. Because conventional gratings cannot handle the intense beam of energetic photons produced in the ALS insertion devices, LBL scientists and engineers designed and fabricated a prototype for a new type of high-resolution monochromator. This new monochromator features a unique water-cooled spherical grating that rotates about an axis under computer control to scan the wavelengths of incoming photons. The grating is made from a nickel-coated copper alloy that can withstand the power of the ALS' light, and its spherical surface is super-smooth and precisely shaped to preserve the light's high brightness.
During its first year, the ALS will have up to six beamlines, including five for scientific research and one that is used for beam diagnostic purposes. Three of the scientific beamlines generate their light from undulators. One of the two lines built at LBL is equipped with a photoelectron spectrometer and two microscopes that can provide a resolution of 500 angstroms or better for spectromicroscopy research. The other will be used to investigate atomic and molecular photoprocesses. The third undulator beamline, built at IBM's Almaden Research Center, will be used for photoelectron spectroscopy and for soft x-ray emission and absorption spectroscopy.
The other two scientific beamlines at the ALS obtain their light from bending magnets. One of these beamlines was formerly operated at the Stanford Synchrotron Radiation Laboratory. Relocated and modified to take advantage of the ALS' brighter light, this beamline will be used to study problems in materials and chemical sciences. The other bending magnet beamline, built by researchers at LBL's Center for X-ray Optics, will serve as a `'white-light" microprobe to detect and measure the presence of trace amounts of elements-amounts as small as a millionth of a billionth of a gram.
Five more scientific beamlines will be added to the ALS complement within the next two years.
The ALS has been designated by the U.S. Department of Energy as a national user facility and will be available for research purposes to qualified scientists and engineers. Hundreds of researchers from throughout the world are expected to visit the facility each year. i.
LBL's Advanced Light Source has been designated by the U.S. Department of Energy (DOE) as a national user facility. This means that its powerful beams of laserlike X-rays are available for use by qualified scientists and engineers from throughout the country-not only researchers from other national laboratories and universities but private industry as well.
Glen Dahlbacka is in charge of industrial program development for LBL's Technology Transfer Department. A physicist with extensive experience in x-ray science, he came to LBL from Maxwell- Brobeck Laboratories, builders of synchrotron light sources and equipment, where he was vice-president of business development.
Dahlbacka believes that private industry could eventually account for as much as 30 percent of ALS beam use. He sees four principal areas of opportunity for companies - microscopic machines, structural biology, analytical services (metrology, spectroscopy, etc.) and x-ray microscopy.
Microscopic machines, also known as micro-electromechanical systems or MEMS, are considered to be the hottest things to hit the high-technology market since integrated circuits. Though they were once thought to be the stuff of science fiction, the giobal market for MEMS has already reached $500 million and is expected to soar to $8 billion by the turn of the century.
The key to the fabrication of future MEMS is lithography techniques (similar to what is used in the production of integrated circuits), using the types of x-rays that the ALS will generate in abundance.
Explains Dahlbacka, "The MEMS being made with visible light lithography are essentially two dimensional devices. The penetration obtained with light beams from the ALS will make possible the fabrication of three-dimensional MEMS."
In the area of structural biology, one of the most promising applications of the ALS is x-ray protein crystallography. Among other things, this would help pharmaceutical companies design more effective drugs.
In the past, the designing of new pharmaceutical drugs has been a discovery process because the structure of a given protein could only be estimated, " says Dahlbacka. "Knowing the precise structure of a protein would enable the intelligent design of drugs to fit or modify an enzyme for special chemical jobs. There is also strong industrial interest in x-ray microscopy, another very useful tool for studying chemically sensitive biological and surface structures."
The x-ray microscope, a microprobe beamline, and a spectroscopy facility will form the core instruments for the analytic services offered at the ALS.
Private industry research groups seeking to work on the ALS can do so in a number of ways, Dahlbacka says. The quickest and easiest is for the group to become what is cailed an ALS `'participating research team." Such teams contribute to the costs of constructing and operating an ALS experimental facility for either an insertion device beamline or a bending magnet beamline. In return, participating research teams are guaranteed a percentage of beamtime, depending on the extent of their contribution to the project.
The commitment of a participating research team is six years for an insertion device beamline and four years for a bending magnet beamiine. Companies wanting time on the ALS but unable to make such long-term commitments might hire LBL to do the research under a work- for-others contract. They might also negotiate a Cooperative Research and Development:Agreement (CRADA) with LBL through DOE.
"There is a growing awarene is of the capability of the national laboratories to quickly contribute value back into the U.S. economy," Dahlbacka says. "Through user facilities such as the ALS, and new contractual tools and funds provided by the legislation, LBL has accepted this added mission and is ready to act."
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Housed in the Advanced Materials Laboratory, adjacent to LBL's Advanced Light Source, is the Center for X-ray Optics (CXRO), the. world's first research center: devoted exclusively to the study and utilization of x-ray light. During the past year, scientists at this center won a contract from the Advanced Research Projects Agency (ARPA) to develop a program in one of the prime new critical technologies, x-ray lithography.
Lithography is the process by which a beam of light is used to transfer intricate patterns from a mask onto the surface of a material in order to make a device, such as an integrated circuit. Today's microchips are fashioned with visible light. As tiny and powerful as these devices are, they could be made even tinier and more powerful - if fashioned from the much shorter wavelength of x-rays. It is like carving with a scalpel rather than a sword.
The ARPA program at the CXRO is being supported by nearly $60 million, spread over a five-year period. A large portion of this money will go to the design and construction of a pair of beamlines at the Advanced Light Source - an undulator beamline for metrology.
According to CXRO leader David Attwood, the light generated by an ALS undulator (see main story), with its high spectral brightness and its partial coherence at wavelengths ranging from 45 to 300 angstroms, is a "perfect match" for studying and testing the materials used in x-ray lithography.
Initial plans for the metrology project, which is led by CXRO physicist James Underwood, call for the beamline to be divided into two branches-one for extreme ultraviolet light (25 to 250 angstroms) and the other for soft x-rays (6 to 30 angstroms). Together, these two branches will enable researchers to do a number of metrological activities, such as measuring the reflectivity of x-ray mirror coatings or testing the sensitivity of various lithography pattern recording materials.
Most of the remainder of the ARPA grant will go towards building a nanostructure fabrication facility. The main component; of this facility will be an electron ; beam writing tool capable of generating a beam 25 angstroms in diameter that will accurately carve patterns over an area of 65 microns. These 65-micron areas can then be stitched together to form patterned areas that approach one square centimeter.
Says Attwood, "We will have full in-house capabilities for nanostructure patterning, synthesis, and processing. We will also be able to provide a very powerful tool-set for scientists who wish to pursue the novel properties of new materials in structures of near-atomic dimensions."
Managing LBL's x-ray lithography program is Keith Jackson, a physicist who came to the CXRO this year from the Rocketdyne Corporation, where he had been developing diamond films and optical elements for synchrotrons and free electron lasers. In addition to his program manager duties, Jackson is also overseeing the design and construction of the project's undulator beamline.
One of Jackson's first objectives as program manager will be to demonstrate the feasibility of proJection lithography for making circuit patterns. Current x-ray lithography programs use proximity or shadow printing, where light is beamed through a mask that is essentially in direct contact with the target material. Projection printing offers a number of advantages over this technique, Jackson says, but none more important than the size of the patterns that can be made.
"In projection printing, the patterns that are transfered can be as much as five times smaller than the features on the mask," he says. "This is because imaging optics demagnify or reduce the mask pattern when they project it onto the surface of a semiconductor substrate." In proximity printing, the mask features and the printed pattens are the same size.
Jackson is also developing a proposal for a new program in what os called deep x-ray projection lithography, using a technique known as LIGA (a German acronym for Lithographic Galvanoformung Abformung, or lithography electroplating molding).
Unlike the lithography processes discussed earlier where the action takes place on the surface of a material, deep x-ray lithography, as its name implies, is used to sculpt out material several hundred microns bejow the surface. As a result, instead of producing devices such as computer chips, which are essentially two-dimensional, truly three-dimensional devices, such as the microscopic machines known as MEMS (for micro-electromechanical systems), can be made.
"Using x-rays at about 5 kilovolts of energy (the kind produced by ALS bending magnets), we get deep exposures into a target material," Jackson says. "If we then tilt the target through a series of such exposures, we can create highly complex structures."
In the LIGA technique, high-energy x-rays penetrate the transparent portion of a lithography mask, the exposed material is removed, and a metal is electroplated into the open space. This produces either a working device or a master mold for use in injection molding.
Currently, there are only two LIGA programs under development in the United States, one at the University of Wisconsin, and one that is just getting started at Louisiana Tech. The LIGA program that Jackson and his CXRO colleagues are proposing will have the advantage of the ALS as its~source of x-rays.
"Light from an ALS bending magnet, with its high flux at 2- to 3- angstrom wavelengths, will enable us to get the exposures needed for a LIGA operation in a matter of minutes, rather than the bours it now takes using x-rays from other synchrotron facilities, " says Jackson. Also, unlike other facilities, at the ALS we can operate under atmosphenc pressure and with fewer optical elements."
The combination of beams from an ALS bending magnet with the LIGA technique can provide mechanical devices that, according to Jackson, "wi11 enjoy all the manufacturing advantages associated with integrated circuits."
On the LIGA proposal, Jackson and his LBL colleagues are working in close collaboration with Richard Muller, Dick White, and Roger Howe, researchers with the University of California at Berkeley, who are considered pioneers in the field.
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