Patent Application
20060233309
This invention relates to an apparatus and a method for emitting x-rays, and in particular to an apparatus for generating pulsed x-ray emission by creation of hot electrons in a continuous target.
X-rays are used for various analytical techniques, e.g. for x-ray photoelectron spectroscopy, electron spectroscopy for chemical analysis, and extended x-ray absorption fine structure. Intensive pulsed x-rays with high brilliance of the x-ray source, and high pulse repetition rate are necessary for some applications.
Focusing of x-rays to achieve beam cross-sections in the micrometer range enables space resolved techniques for micro structures. Pulsed x-ray sources enable time resolved measurements. The time resolution is given by the pulse duration of the x-ray source. Higher pulse repetition rates enable a faster data acquisition. X-ray sources with high repetition rates can enhance the x-ray photon flux. Higher x-ray pulse repetition rates may also enable measurements of complete processes with an improved time resolution of the process. The combination of improvements in time resolution, for a single data acquisition and for the complete process, and space resolution enables time resolved measurements of processes on a micrometer and nanometer scale.
The current state of the art x-ray source for high end analytical applications, as described above, is a synchrotron. X-ray generation by synchrotron is currently the preferred technique to produce high brilliance x-rays for analytical measurements. The problem with synchrotrons is that they are bulky, not portable, and the pulse duration is usually limited to several 10 picoseconds.
Compact x-ray source apparatuses are known as well. However, there is a need for the improvement of such compact x-ray source apparatuses. Measurements, such as those described above, could be performed with an improved compact x-ray source apparatus in a standard laboratory.
To achieve compact pulsed x-ray sources the following method is currently used: A pulsed laser beam hits target material in a target and creates a plasma. The interaction of the plasma with the laser beam excites the electrons and creates hot electrons. The interaction of the hot electrons with the target material yields the x-ray emission.
The target material is deteriorated by the high energy of the laser beam, usually the target material is evaporated at the position where the laser beam hits the target material. The target can be moved to cope with the problem of the deterioration of the target material and accordingly each pulse of the laser beam hits new target material. The movement can be achieved by rotating targets, wire targets or by so-called band targets.
U.S. Pat. No. 5,151,928 describes a x-ray source with a band target. The '928 patent describes a method and an apparatus for generating x-rays by laser impingement on a target in a vacuum enclosure to generate a plasma. The band target comprises a first film made of a suitable metal and a second film made of an x-ray transmitting material. The second film is superimposed on one surface of the first metal film, and a space A is formed between opposite parts of the first and the second film. A laser beam is projected onto the metal film so that plasma is generated by laser pulses and confined in the space between the first and the second film to increase the efficiency of x-ray generation.
The '928 patent teaches a transmission geometry in which the laser beam hits the first film on a first side, and the x-ray emission is extracted from the x-ray source apparatus from a second, opposite side. The patent describes the problem that the pressure of the highdensity plasma generated in the space A by the laser impingement causes a hole to be formed in the second film. The particles, so called debris, of the plasma pass through the hole and are likely to fly as far as the x-ray optics. The patent describes a high speed shutter system to solve this problem. Other solutions described in the patent are to optimize the pulse length or to modify the band target by addition of a third film.
The efficiency of x-ray generation by the method and the apparatus taught by the '928 patent is restricted by the limitations in generating the plasma, e.g., the thickness of the first metal film. Limitations posed on the x-ray source apparatus by the aim to prevent particle emission on the x-ray optics further restrict efficiency. The x-ray intensity is increased by extension of the interaction time of the laser and the generated plasma. Therefore, the time for each single x-ray pulse is increased and thus the time resolution for measurements is reduced.
U.S. patent application publication No. US-A-2002/0 141 536 introduces liquid droplets as a “moving” target to enable an efficient source for radiation in the extreme ultra violet (EUV) and x-ray wavelength regions. The '536 application describes a method in which liquid droplet targets are irradiated by a high power laser and are plasmarized to form a point source for EUV and x-ray emission. The described liquid droplet targets include metallic solutions and solutions of nano particles of different types of metals and non-metal materials. No damaging debris is emitted from the particle solution, according to the '536 application. The use of nano particles as an efficient droplet point source is taught in the '536 application as a preferred embodiment.
By adjusting the size of the droplet, the size of the x-ray source can be adjusted and thus the brilliance of the x-ray source can be influenced. It is difficult to achieve a constant drop rate and droplet size. However, for precise time and space resolved measurements it is essential to maintain a constant drop rate as well as droplet size.
U.S. patent application publication No. US-A-2002/0 094 063 describes a laser plasma source apparatus and target for the generation of extreme ultra violet (EUV) light. The '063 application combines the utilisation of nano particles and a band target. The '063 application teaches the generation of an electromagnetic wave in the EUV area by the use of a repeatedly irradiating laser beam. The described laser plasma EUV light source apparatus comprises a vacuum chamber, a target disposed in the vacuum chamber, an input optical system for directing the beam to the target, an output optical system for extraction of EUV light emission, a shield device for protecting the input optical system and the output optical system from debris. The generation of the debris can be restricted, and generated debris is shielded in the '063 application by the shield device.
The '063 application teaches the use of a band target comprising a polymer film with a thickness of 10 μm to 100 μm and a target material. The target material of the band target can be contained in the film or laminated on a surface of the polymer film. The '063 application teaches the use of metals or metal alloys as target materials, preferably those formed from the metals aluminium, copper, tin or silicon. The particle size is chosen to be in the range of 0.1 μm to 80 μm for the length of the particles and 5 to 10 μm thickness of the particles. The lower limits of the size ranges are determined by decreasing efficiency of the EUV light generation. The upper limits of the size ranges are determined by an increasing amount of emitted debris. The reason to use particles in the '063 application is to reduce the debris formation.
The '063 application does not teach the generation of x-rays. The size of the particles in the '063 application is mainly determined by the desire to suppress debris emission. The '063 application does not teach a transmission geometry which could enable a debris reduction on its own. The transmission geometry is not applicable to the invention of the '063 application because the polymer films are not transparent in the EUV wavelength regime.
It is therefore an object of the invention to provide a compact x-ray source apparatus.
It is a further object of the invention to provide a x-ray source apparatus with an efficient generation of x-ray emission.
It is a further object of the invention to provide a x-ray source apparatus with a high brilliance.
It is a further object of the invention to provide a x-ray source apparatus with a high pulse repetition rate and/or a short pulse length.
It is a further object of the invention to provide a x-ray source apparatus with a reduced debris emission.
It is a further object of the invention to provide a x-ray source apparatus with a reduced jitter for pump probe experiments.
It is a further object of the invention to provide a x-ray source apparatus with a short distance between a place at which the x-rays are emitted and a sample and/or a x-ray optics.
These and other objects are solved by providing a x-ray source apparatus with a continuous target. The continuous target is hit on a first side of the continuous target by a photon beam from a photon source. X-rays are emitted from the continuous target. The emitted x-rays are extracted from the x-ray source apparatus from a second side of the continuous target. The first side of the continuous target and the second side of the continuous target are opposite sides of the continuous target. The used continuous target comprises nano particles.
In this context, continuous target means any target which can be moved with respect to the photon beam. Thus, the photon beam will not forever hit the same part of the continuous target.
The x-ray source apparatus with the continuous target can be realized as an item of laboratory equipment. The use of nano particles increases the efficiency of the x-ray source apparatus.
The transmission geometry which is employed, whereby the continuous target is irradiated from one side and the x-rays which are emitted on the other side of the continuous target are used, reduces debris emission in the used direction of the x-ray emission.
The transmission geometry enables a close distance between a sample and a place at which the x-rays are emitted. Thereby a large solid angle of the x-ray emission can be used.
The transmission geometry reduces jitter in pump probe experiments. Pump probe experiments are generally experiments in which a first part of a photon beam is used to irradiate a sample and a second part of the same photon beam is used to detect changes caused by the irradiation. Time variations between the pulse run time of photon pulses of the first part of the beam and photon pulses of the second part of the beam are called jitter. In the context of this invention the second part of the photon beam is not used directly. The second part of the photon beam is used to generate x-rays. The generated x-rays are used to detect changes caused by the first part of the photon beam. The length of a path comprising the distance from the photon source to the continuous target and the distance from the continuous target to the sample is not substantially changed by vibrations of the continuous target. On the other hand in the so called reflection geometry the length of the path comprising the distance from the photon source to the continuous target and the distance from the continuous target to the sample can be changed by vibrations of the continuous target. The variation of the length of the path creates jitter.
A laser can be used as photon source and the photon beam can be focused on a small spot on the continuous target. Thereby a high radiance of the x-rays can be achieved. The laser as photon source can be pulsed with short pulses yielding also short x-ray pulses.
The objects of the invention are further solved by using a continuous target which comprises oxide nano particles and which generates x-rays after photon irradiation. In a preferred embodiment of the invention, the oxide nano particles are metal oxide nano particles.
The oxide nano particles can be shaped in a way to enhance electron emission and plasma generation. In a preferred embodiment of the invention the oxide nano particles are needle shaped. The spikes of the needle-shaped particles can enhance the electron emission by deforming the electrical field at the spikes.
In a further preferred embodiment of the invention, the continuous target comprises at least a first layer and a second layer, whereby the first layer is a support layer. The second layer comprises the oxide nano particles and, on the photon irradiation, x-rays are emitted from the second layer.
The support layer can be manufactured independently from the second layer. Therefore, the mechanical properties of the support layer can be improved independently from the properties to generate free electrons of the second layer. Thereby the efficiency of the continuous target for x-ray generation can be improved.
The continuous target can have at least the first layer, a third layer and a fourth layer. The third layer comprises the oxide nano particles and, on the photon irradiation, hot electrons can be generated in the third layer. X-rays are emitted from the fourth layer.
The third and fourth layers enable a separate improvement of the plasma generation and the x-ray generation which allows an improved overall process.
a shows a measured spectrum of generated x-rays.
b shows a comparison of x-ray spectra generated according to the invention and generated conventionally is depicted.
a shows a two layered band target.
b shows a three layered band target.
A continuous target 10 is placed in the focus point of the first optic 40. In this embodiment of the invention, the continuous target 10 is made of a band or a tape. The continuous target 10 comprises nano particles, preferably it comprises metal oxide nano particles 350 such as chromium oxide nano particles with a length of 500 nm and a diameter of 50 nm. The preferred embodiments of the continuous target 10 will be described in more detail below.
The laser beam 30 is focused on a first side 60 of the continuous target 10. The energy of the focused laser beam 30 creates a plasma at the position at which the focused laser beam 30 hits the band target 10. The interaction of the plasma with the laser beam 30 excites the electrons in the plasma and creates hot electrons. The interaction of the hot electrons with the material of the continuous target 10, in particular with the chromium oxide nano particles 350, yields emitted x-rays 80. Characteristic x-ray emission 80 of the chromium metal in the nano particles and bremsstrahlung continuum x-ray emission 80 is generated by this process. A part of the x-ray emission 80, which has photon energies exceeding approximately 1 keV, passes through a carrier layer 320. The chromium oxide nano particles 350 are preferably shaped in a way to enhance the process of generating the plasma. The x-rays 80 are emitted in a “reflection geometry” on the first side 60 of the band target 10 and in a “transmission geometry” on a second side 70 of the band target 10.
The continuous target 10 may be moved in a way such that two consecutive pulses from the laser 10 hit two different spots on the continuous target 10. The movement of the continuous target 10 can be realized by a spooling system (not shown) in which one end of the continuous target 10 is taken up by a first spool whilst further continuous target 10 is provided from a second spool. The speed of the continuous target 10 is in a range of 1.5 cm/s to 20 cm/s. Other forms of continuous target 10, such as movable sheets, are conceivable.
The continuous target 10 may be moved in a way such that two consecutive pulses from the laser 10 hit two overlapping spots on the continuous target 10. It can be advantageous that a spot is hit at least partly by the laser beam of a following pulse. A roughening of the surface by a first pulse can enable a better incoupling or absorption of a successive pulse.
A part of the x-ray source apparatus 50 comprising at least a part of the continuous target 10 where the x-rays 80 are generated is enclosed in a vacuum chamber 90. The pressure in the vacuum chamber 90 is approximately 5×10−2 mbar. The vacuum chamber 90 has an out-coupling window 100 for the generated x-ray emission 80. In one example of the invention the out-coupling window 100 is made of a beryllium window. The out-coupling window 100 is mounted in the transmission geometry, facing the second side 70 of the band target 10. The transmission geometry reduces debris hitting the out-coupling window 100. In the transmission geometry the reflected parts of the laser beam 30 (i.e., the reflection geometry described above) are not reflected to the out-coupling window 100.
In
In
A comparison of x-ray spectra which have been measured in a reflection geometry and in the transmission geometry is shown in
A preferred embodiment of the continuous target 10 is shown in
The continuous target 10 of
In a preferred embodiment of the invention, the nano particles 350 have spikes. The generation of free electrons is enhanced at spikes. Sharp spikes facilitate the generation of free electrons by the electrical field of the photon beam 30. A high density of nano particles 350 can further enhance the efficiency of the emission of x-rays 80.
In a further preferred embodiment, the nano particles 350 are oriented with respect to each other. The nano particles, e.g., can all be oriented in plane of the continuous target 10 and vertical to the direction of movement of the continuous target 10. In a further embodiment, the nano particles are oriented in plane of the continuous target 10 and parallel to the direction of the movement of the continuous target 10.
b shows a different preferred embodiment of the continuous target 10. The continuous target 10 comprises the carrier layer 320 as the first layer. The continuous target 10 comprises a third layer 330 and a fourth layer 340 instead of the second layer 310 of the embodiment shown in
The foregoing is considered illustrative of the principles of the invention and since numerous modifications will occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation described. All suitable modifications and equivalents fall within the scope of the claims.
