Method and Device for Producing Photons in the Ultraviolet-Wavelength Range

This invention relates to a method for producing photons in the UV-wavelength range, which comprises planting in a solid matrix, ions from a gas which is inert or insoluble relative to the matrix, excitating the gas being retained captive as extended faults of the solid matrix, and radiating said photons by the excitated gas, as well as to a device for the working of this method.

The usual light sources for the near, far and extreme UV-wavelengths, are generally discharge sources where the light is produced by passing an electric discharge through a capillary element containing noble gases or similar under pressures lying between 10 and a few ten-thousands Pa. For He, a commonly-used gas, a continuous emission spectrum results from the forming in the discharge and the subsequent radiative decrease, of excitated helium molecules He.sub.2 *.

Another main radiation source in this spectrum range is the synchrotron which is a complex and costly equipment, available in a few locations only in the world.

It has also been considered to use the fluorescence of liquid helium as radiation source in the UV range, but this method requires a cryogenic cooling, thus at very low temperatures, as well as a differential pumping, and the working thereof is thus very costly (see C. M. Surko, R. E. Packard, G. J. Dick and F. Reif, Spectroscopic Study of the luminescence of liquid helium in the vacuum ultraviolet, in Physical Review Letters, Vol. 24, Nr. 12 (1970), p. 657 and ff.).

It is known on the other hand to prepare solid matrixes having extended faults, such as gap agglomerates, or bubbles containing a gas which is inert or insoluble relative to the matrix. There is for example obtained such a matrix with helium microbubbles by bombardment by means of energizing He ions. The Al/He matrix has been the object of spectroscopy studies for optic absorption and electronic power loss (see J. C. Rife, S. E. Donnelly, A. A. Lucas and J. J. Ritsko, Optical absorption and electron-energy-loss spectra of helium microbubbles in Aluminum, Physical Review Letters, Vol. 46, Nr.18 (1981), p. 1220 and ff.).

It has finally been noticed that a solid matrix bombed by means of high-energy helium ions (200-600 keV) emits, from some He proportion on, a continuous spectrum in the UV range (see R. S. Bhattacharya, K. G. Lang, A. Scharmann and K. H. Schartner, Continuous emission in the vacuum ultraviolet under energetic inert gas ion bombardment of aluminum, in J. Phsy.D, vol. 11 (1978), p. 1935 and ff.). It is to be noted that the planting of high-energy helium ions is a relatively costly method and that as the helium ions are planted at a deep depth inside the matrix, the intensity of the UV emission is low.

The present invention has for object to develop a method and a device for producing photons in the UV-wavelength range, which are simple and of low-cost while giving comparable results to the discharge sources.

For this purpose according to the invention, one performs an ion bombardment of the surface from the solid matrix with low-energy ions from at least one gas such as mentioned above, so as to obtain a low-depth planting of gas in the solid matrix, and thereafter a low-energy electronic bombardment of the solid matrix, with an excitating of the captive gas and emission of said photons.

According to one embodiment of the invention, the method comprises the planting at low depth of gas ions through one side of a mass solid matrix, and the electronic bombardment of this same side with emission of said induced photons from this side.

According to another embodiment of the invention, the method comprises the planting at low depth of gas ions inside a laminated solid matrix with a thickness smaller than 1 .mu.m, and said electronic bombardment of the one side of this matrix with emission of said induced photons from the other side of the matrix.

There is also provided according to the invention, a device for the working of the method according to the invention, this device comprising a vacuum enclosure, a solid matrix, into which are planted at low depth ions from at least one gas which is inert or insoluble relative to the matrix, this matrix being mounted on a support inside the vacuum enclosure, an electron-producing apparatus capable of subjecting the matrix to a low-energy electronic bombardment, and an electric connection connecting the matrix to the outside, as well as an outlet for the resulting photons, provided in the enclosure.

Other details and features of the invention will stand out from the description given hereinafter in a non-limitative way and with reference to the accompanying drawings.

FIG. 1 shows diagrammatically a device for the working of a method for producing photons according to the invention.

FIG. 2 shows diagrammatically a device for the working of a variation of embodiment of the invention.

FIG. 3 shows with more details a view partly in axial section through a device according to the invention.

FIG. 4 shows the fluorescence spectrum obtained when using a device according to the invention. The units marked in abcissae show the wavelengths of the fluorescence spectrum, and in ordinates are marked arbitrary units for the emission strength.

In the figures, identical or similar elements are marked with the same references.

The device shown in FIG. 1 comprises a solid matrix 1 which is prepared by planting low-energy He ions into an Al sheet with a thickness smaller than .mu.m. A bombardment with low-energy He ions, about 5 keV, allows planting a high He concentration (locally more than 10 atoms) over a thickness from a few tens to some thousands .ANG.. The helium does agglomerate naturally in the matrix gaps obtained with the bombardment and forms extended faults, such as gap agglomerates or microbubbles 2 which remain stable at room temperature and may resist to temperature increases up to a few hundreds .degree.C., for example up to 300.degree. C.

The device shown in FIG. 1 also comprises an apparatus for producing low-energy electrons 3, such as an electron gun, said apparatus casting an electron beam 4 on the one side 5, the so-called back side, of matrix 1. The electrons 1 from the electron beam 4 advantageously have an energy smaller than or equal to 20 keV, preferably lying between 1 and 5 keV. A fluorescence of the target is induced in such a case from the front side 6 of the matrix, the photon emission being shown by wavy-line arrows 7.

There has been shown in this FIG. 1, in broken lines, a variation of embodiment in which the laminated matrix 1 is in the shape of a continous strip, wound at the one end into a supply roll 20 and at the other end thereof in a discharge roll 21. In this way, when that part of the matrix being subjected to the electronic bombardment is accidendtally affected after some operating time, it is possible to move said matrix in the direction of arrow 22, by rotating said rolls 20 and 21, and to bring in front of the electron beam a new matrix part not yet subjected to the electronic bombardment. This moving may be performed manually or automatically, and it may be continuous or intermittent during the operation of the device according to the invention.

In the device shown in FIG. 2, the matrix is a mass substrate 8. The planting of the He ions is performed in the same way as for the laminated matrix 1, by proceeding in such a way as to obtain a maximum concentration of He microbubbles 2 at a depth preferably less than 5,000 .ANG.. In this case, the electron gun 3 casts a low-energy electron beam on the same surface as the one through which the He ions have been planted, and a fluorescence of the target is then induced through this surface 9, the photon emission being shown by wavy-line arrows 10. One thus has in this latter case what will be called an electronic bombardment of the matrix front side, as opposed to the electronic bombardment of the matrix back side shown in FIG. 1.

The device shown in FIG. 3 shows with more details a device performing an electronic bombardment of the matrix front side.

This device according to FIG. 3 comprises an enclosure 11 retained under vacuum, inside which the matrix 8 is mounted on a support 12 which may be cooled by a cooling circuit 13, for example with water, in the possible case of using the device under high intensity. A electron gun 3 emitting an electron beam with low energy and adjustable intensity, is mounted on the enclosure so as to direct this beam on the matrix. The angle of incidence between the beam and the matrix plane is computed in such a way that the emitted photons may propagate through the outlet opening 14 formed at the one end of the enclosure 11. This end is provided with a flange 15 which is used for the connection of the device according to the invention to an apparatus in which the UV light will be used.

An electronic connection 16 notably allows to measure the electric current in the matrix. It is possible to provide in the outlet opening 14, an electronic screen 17 intended to avoid any escape of electrons through this opening, this electronic screen 17 then also being connected to the outside through an electric connection 18.

The enclosure 11 is retained under vacuum either by a pumping device not shown, connected to the enclosure through the flanged fitting 19, or by the pumping device retaining under vacuum the apparatus not shown connected to the flange 15.

The matrix material has to fulfill two main conditions; the gas insolubility in the matrix, and a relatively low absorption by the matrix of the continuous gas emission continuum. The matrix material should preferably have such optical properties that the escape depth of the photons being produced be compatible with the planting depth. Consequently, it is not necessary, that the penetration depth of the electron beam be deeper than this penetration depth of the photons, and electron energies in a range from 0.1 to 20 keV are sufficient, with a relatively razing incidence. This property of the photon source allows to avoid the high costs required for obtaining high-energy ionic and electronic bombardments, such as the ones used in the known methods and devices. The excitating may notably be produced by using a low-energy electron gun.

As matrix material use may be made advantageously of the materials selected among the group comprising metals, such as Sn, Al, Mg, semiconductros, such as Si, or some insulating materials.

As the emission depth of the source is very short, the source is substantially flat, and the surface and the shape of the source may be simply adjusted by structuring the electron beam. By focalizing the beam, it is possible to obtain an effective pin-point source; by scanning or spreading the beam, it is possible to obtain an extended source compatible for example, with the geometry of the slit used in some spectroscopic works. Moreover, a fluorescence intensity varying in time may easily be obtained by modulating the electron beam with pulses, which allows the use of the source in servo techniques ("lock-in type"). The life duration of the fluorescence is less than 10 sec.

According to a non limitative example of working the method according to the invention, a photon source on the basis of Al/He is prepared in the above-described way and operated according to the invention under an electronic bombardment having an energy of 3800 V. This source generates at it appears from the FIG. 4, a continuous fluorescence spectrum which extends from 580 to 900 .ANG., which is similar to what is obtained with the conventional discharge source. The photon production from the matrix has been compared to the synchrotron source SURF II and this comparison shows an efficiency higher than or equal to 10.sup.-4 photons per electron. With a sufficient electric current, the brightness may reach the one obtained with discharge lamps.

To the difference of the conventional discharge sources, there is the possibility by means of the method according to the invention, of designing a multi-gas composite source with a view to extending the width of the spectrum band from 580 to 3000 .ANG.. Indeed it is clear that besides helium, other inert gases relative to the matrix may be used, for example rare gases or other gases, such as Ne, Ar, . . . , H.sub.2, N.sub.2, . . . . When ions from a plurality of such gases are planted in the matrix, it is thus possible to obtain said multi-gas composite source.

The applications of a photon source according to the invention are numerous. It is possible to use same in any application where the discharge sources are presently used, for example for the photon-electronic spectroscopy in the ultraviolet, for reflectivity, adsorption and photoconductivity studies, etc.

Besides the use of the device according to the invention as simple means for producing photons in the near, far- and extreme UV range, the source according to the invention may be incorporated in a large number of new more intricate systems. It is notably possible to mention:

(a) particle sensors, by detecting the fluorescence being induced in the device due to the passage of charged particles;

(b) contact lithography in the ultraviolet, by planting in an integrated circuit by means of an inert gas ion beam on a mask in the shape of a thin film and by later printing the circuit, by locating the film in contact with a photoresist coating which is sensitive to the UV photons, and by bombardment with an electron beam to activate the UV fluorescence;

(c) stimulated emission source for extreme ultraviolets (excimer laser), by means of the choice of a suitable pumping mechanism and a suitable resonant cavity.

The methods and devices according to the invention have the advantage of not requiring a differential pumping, a gas replacement and a cryogenic cooling. Moreover they are very easy to operate and have a flexible working. The source according to the invention has the advantage of a brightness which may vary by six magnitude orders or more, by modifying the electron beam intensity. It does allow a pin-point geometry by focalizing. Finally, the working thereof is relatively cheap.

It must be understood that the present invention is in no way limited to the above-described embodiments and that many changes may be brought thereto without departing from the scope of the present patent.