Abstract:
The invention relates to a particle source, particularly an ion source for the production of excited particles in gaseous media. A dielectric, e.g., Kapton foil, is coated electrically conductively on both sides, and a voltage, preferably pulsed, is applied between the two coatings. A gas discharge is ignited in the gas through-flow by the voltage. Due to a pressure difference between the two sides of the foil, the gas expands from the high pressure side to the low pressure side, preferably in an ultrasonic expansion, whereby a directed, cold beam of excited particles or ions is produced.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     A particle source in general, and in particular to a particle source is provided for the production of excited particles in gaseous media. 
     Ion sources play an important part today in many regions of physics and in industrial application (plasma deposition, implantation, ion etching of microstructures, etc.). The requirements on such ion sources are most manifold, e.g., a given kind or charge state of ions, high intensity, high brilliance, pulsed operation, etc. In general the aim is, however, to combine high intensity with good brilliance in ion sources. 
     TECHNICAL FIELD 
     In known ion sources, the ions are produced in plasmas, which as a rule are ignited and operated in the sub-millibar pressure region. It is then found that because of this limited gas and plasma density, only ion beams with surface densities of up to about 0.5 Ampere/cm 2  can be attained. A detailed description of such known ion sources is to be found, e.g., in B. Wolf,  Handbook of Ion Sources , CRC Press, Boca Raton (1995) or I. G. Brown,  The Physics and Technology of Ion Sources , John Wiley &amp; Sons, New York (1989), which are wholly incorporated herein by reference. 
     The brilliance or emittance of the ion beam is limited by the temperature of the ions in the source. This temperature is typically several thousand degrees Celsius for the known ion sources, which corresponds to an energy uncertainty in all three spatial directions of about 0.1-1 eV (electron volt). In order to produce ion beams with high current, usually large plasma volumes are required. The same holds for particle sources for the production of particles, e.g., atoms or molecules with electrons in bound excited states, as for ion sources. 
     Thus, for example, beams with atoms in bound excited states are used for lithography. This also is a field of application for the present invention. 
     SUMMARY OF THE INVENTION 
     Therefore an object of the invention is to provide a particle source for excited particles, the particle source having a very small volume, a high particle current, a low emittance and/or a high brilliance, in particular a low energy uncertainty. 
     A further object of the invention is to provide a cost-efficient and compact particle source for excited particles. 
     A further object is to make available a particle source for large-area excited particle beams. 
     The object of the invention is achieved in a surprisingly simple manner by a particle source having a partition with at least one opening. The opening connects a first volume on a first side of the partition with a second volume on a second side of the partition. First particles move from the first volume through the opening into the second volume. Energy is transmitted to the first particles and at least some of the first particles transform to excited states. 
     In the sense of the invention, the concept “excited particle” includes both particles with electrons in excited bound states and also particles with electrons in excited continuum states, i.e., ions. The concept “particle source for the production of excited particles” thus includes in particular an ion source and also a source for particles, e.g., atoms or molecules, in bound excited states. The latter can additionally also be ionized. Furthermore the concept “excited particle” in the sense of the invention also includes chemical radicals, e.g., by means of a dissociation, particularly of molecules. The particles are thus in particular to be carriers of potential energy. The particles are excited in a manner such that potential energy is stored and can be transferred in a reaction, e.g., to other particles. The particles can however also be carriers of kinetic energy. 
     The particle source according to the invention produces in an advantageous manner a directed and cold beam of, or at least with, excited microscopic particles. 
     As a development of the invention, the particle source or ion source includes a first and a second gas volume on a first or second side of a partition, wherein a pressure difference exists between the first and second gas volumes, and gas flows out of the first into the second gas volume through at least one opening in the partition and when flowing through is ionized or excited in a gas discharge. In particular, the particles, atoms or molecules of gas are electronically excited or dissociated. Thus, by means of the particle source according to the invention, e.g. helium ions or electronically excited metastable states, in particular of helium atoms, can be produced, or radicals, e.g., oxygen radicals, can be produced by dissociation of O 2  molecules. 
     As a development of the invention, the particle source uses a partition comprising a dielectric or electrically insulating base layer, an electrically conductive first layer on the first side of the base layer, and an electrically conductive second layer on the second side of the base layer. 
     Such partitions, particularly in the form of a flexible foil, can be produced easily and at low cost. A voltage can be applied between the two electrically conductive layers providing extremely high electric field strengths within the small opening due to the small geometry. The electric field strengths in the region of the opening are at least about 10 4 , 10 5 , 10 6 , 10 7 , or even 10 8  V/cm. For this purpose, only relatively low voltages are required, of the order of about 1-1,000 volt. Because of the high field strengths, the particle source can be operated at high pressures of up to 10 −3 , 10 −2 , 10 −1 , 10, or 10 2  bar on the first side of the partition. The pressure on the second side of the partition is preferably 10 −4 , 10 −5 , 10 −6 , 10 −7  or 10 −8  bar. 
     As a development of the invention, the pressure difference between the first and second side of the partition is at least one, two, three, four, five or six powers of ten. Thereby the gas expands substantially adiabatic isochorically on flowing through the opening. Thereby the whole enthalpy of the gas in converted into directed motion, so that the gas atoms receive an average speed of v=(5 kT/m) 1/2 , where k is Boltzmann&#39;s constant, T is the gas temperature, and m is the particle mass. The gas then cools to temperatures in the milliKelvin region. An ultrasonic gas jet arises. Ultrasonic gas jets are basically known to a skilled person. The ultrasonic gas jet is now ionized by electron impact ionization in the region of the opening, according to the invention, so that an extremely cold and directed particle beam or ion beam arises. 
     As a development of the invention, the coldest inner portion of the particle beam is stripped out by a diaphragm, an aperture or a skimmer, so that an even lower energy uncertainty is produced. In order to achieve particularly low particle temperatures, the gas in the first volume is preferably cooled to below 100, 70, 30, 20 or 10 Kelvin. 
     As a development of the invention, the operation employs a mixed gas of a carrier gas and a working gas, where preferably only the atoms or molecules of the working gas are ionized. The carrier gas substantially determines the thermodynamic properties of the gas expansion. For example, helium is particularly well suited as the carrier gas because of its low atomic weight and its high excitation potential and ionization potential; it cools during the expansion of the working gas. Furthermore, in helium, because of its high electronic excitation energy, the electrons arising in the gas discharge and thereafter accelerated by the electric field assume a high kinetic energy in spite of the high gas pressure. The working gas has a substantially lower excitation potential and ionization potential than helium, so that substantially only the working gas is excited and/or ionized. By selection of the mixing ratio of the carrier and working gases, the average kinetic energy of the electrons, and hence the excitation and/or ionization of the working gas, can be adjusted in a targeted manner. 
     The transverse momentum uncertainty of the gas, and thereby of the particle beam, is further reduced by cooling the carrier gas, so that the particle beam has an extremely good transverse brilliance. 
     A development of the invention uses so-called microstructure electrode foils. A microstructure electrode (MSE) comprises one, plural, or many micro-openings. In the case of plural or many openings, these are preferably arranged as a regular, two-dimensional matrix. This can be produced cost-efficiently, over large surfaces, with a small distance between the openings and very small openings. In this embodiment, a large-surface plasma is produced by means of a great number of pores. Ion current densities can thereby be produced of at least 10 −3 , 10 −2 , 10 −1 , 10, 100 or 1,000 Ampere/cm 2  in a continuous current or in pulsed operation. 
     The invention is described hereinbelow with the aid of preferred embodiments and with reference to the accompanying drawings. 
     Ion sources according to the invention are presented by way of example in what follows. It is however evident to the skilled person that particles, particularly atoms or molecules with electrons in bound excited states which arise, e.g., by electron impact excitation or electron capture can also be produced with the ion sources shown. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 a  shows a sectional diagram through a first embodiment of a single pore, 
     FIG. 1 b  shows a sectional diagram through a second embodiment of a single pore, 
     FIG. 2 shows a perspective view of a cut-out portion of an MSE foil, 
     FIG. 3 shows a sectional diagram of the potential distribution in a pore, according to a computer simulation, 
     FIG. 4 shows a schematic sectional diagram of a first embodiment of the ion source according to the invention, 
     FIG. 5 shows a schematic sectional diagram of an embodiment of the multi-electrode pore according to the invention, 
     FIG. 6 shows a schematic sectional diagram through a second embodiment of the invention, 
     FIG. 7 shows a plan view of two different pore forms, before (upper row) and after operation (lower row). 
     FIG. 8 shows a plan view of a pore with integrated passive resistance, 
     FIG. 9 shows a plan view of a MSE foil with 16 pores, and 
     FIG. 10 shows a plan view of the MSE foil of FIG. 9 with glowing micro glow discharges. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 a shows a first preferred embodiment of a single pore  1  with a base layer  11  of Kapton(R) (Polyimide) 50 μm thick or ceramic about 300 μm thick, with electrically conductive electrode layers  12 ,  13  on both sides of the base layer  11 . The thickness of the electrode layers  12 ,  13  is 40-200 μm of copper or copper-nickel. The electrically conductive layer  12  on the high pressure side  21  is operated as the anode, and the layer  13  on the low pressure side  22  of the pore  1  as the cathode  13 . The hole diameter d at the narrowest place, which in this pore is situated in the middle of the insulation layer, is between about 10 and 100 μm. The diameter D at the boundary between the electrode layer and base layer is 70 μm up to 150 μm. The hole spacing when many such pores are used is about 10 μm up to about 1 mm. 
     FIG. 1 b  shows a second embodiment of a pore  1 , which differs from the first embodiment of the pore in FIG. 1 a  in that the smallest diameter d is situated at the boundary between the base layer  11  and the cathode layer  13 . 
     Each individual micro ion source is formed by a micropore  1  in a thin foil. This has a volume of only less than about 10 −5  cm 3  and can be operated at pressures on the high pressure side of a few millibar up to a few bar. The electrical voltages required for the production of the ions in a gas discharge  14  are here substantially below 1,000 V and are preferably 200-450 V. Because of the special geometry of the micro ion source system, e.g. with a pore  1  about 100 μm in diameter and about 250 μm in length, with very sharp electrode edges, such high field strengths are attained that the discharge ignites immediately and automatically on applying the voltage. Very short delay times of about &lt;1 μsec are thereby attained. The measured power density per micro-discharge (per pore  1 ) can be from milliWatt up to several hundred Watt in continuous current operation. Power densities of more than 1, 10, or 100 kW/cm 2  can thereby be attained. Even higher powers are possible in pulse operation. 
     FIG. 2 shows a portion cut from a MSE foil  100 , with 16 pores  1 . 
     FIG. 3 shows, by way of example, the simulated potential distribution in a pore  1  of a MSE foil. 
     FIG. 4 shows an ultrasonic gas jet with the micro gas discharge according to the invention. An ion source  99  with high intensity and excellent emittance is thereby provided. Gas flows from the high pressure side  21  through a pore  1  to the low pressure side  22 , with adiabatic isochoric expansion. The then arising ultrasonic gas jet  15  passes in part through the skimmer  25  into the volume  23 . 
     As shown in FIG. 4, the pressure on the anode side, i.e., the high pressure side  21  (vacuum stage  21 ) of the pore  1  in this embodiment of the invention is about 1 bar; flowing through the pore  1 , a very cold ultrasonic beam or jet  15  is formed, with an internal gas temperature of less than 1° K. On the cathode side, i.e., the low pressure side, pressures between 10 −3  up to a few 10 −1  mbar occur in the volume  22 , depending on the pump performance. The electrical discharge  14  takes place in the pore  1  and produces ions in a gas discharge  14  by electron impact ionization. The gas to be ionized, i.e., the working gas, is in this embodiment, e.g., O 2  or Kr, and is mixed with the carrier gas, here about ninety volume percent helium. However, basically any carrier gas and any working gas can be used. The gas mixture is precooled to about 20° K in the high pressure stage  21 . The ionization energies of the exemplary working gases O 2  and Kr are substantially lower than that of helium, and hence substantially only the working gas is ionized. In the collisions in the pore  1  during the formation of the ultrasonic jet  15 , the ions are cooled by the He, and because of the great difference in ionization potentials, hardly any charge exchange with the helium atoms takes place. Transversely, the ions are cooled down substantially to the internal temperature of the ultrasonic jet  15  and thus attain transverse temperatures of less than 1° K. Longitudinally, the ion beam temperature chiefly depends on the point where the ions arise in the pore, since they are at a slightly different potential depending on the point where they arise. By making the pore  1  longer, or by constructing a multi-electrode pore  80  (FIG.  5 ), a further improvement in cooling with the carrier gas and in electrical focusing of the ions can be attained in the longitudinal direction also. The longitudinal emittance is thereby also reduced. According to the pressure in the stage  22 , the distance between the outlet of the pore  1  on the cathode side  22  and the ion beam stripper  25  or skimmer is optimally set, so that the ultrasonic beam  15  is not destroyed. The distance is about a few mm up to about 1.5 cm. The ultrasonic beam  15  exits the pore  1  directed but not appreciably focused. According to the size of the skimmer  25 , about 1 mm diameter or smaller in this embodiment, only a miniscule fraction of the carrier gas  31  will pass through the pore, whereas a larger fraction of the ions passes through the skimmer  25  because of their excellent transverse emittance and focusing. With the aid of a focusing lens  26  between the pore  1  and the skimmer  25 , a particularly large fraction of the ion current  32  is conducted through the skimmer. 
     The ion source  99  is enclosed by a vacuum chamber (not shown), which is evacuated by plural pumps (not shown). Preferably the gas volume  21  is cooled by means of a cryostat (not shown). 
     FIG. 5 shows a further embodiment of a pore  1  according to the invention, namely a multi-electrode pore  80 . The electrical field is shaped in manifold ways within the pore  80  by means of plural electrodes, six  111 ,  112 ,  113 ,  114 ,  115 ,  116  in this embodiment, separated by base layers or insulator layers  117 ,  118 ,  119 ,  120 ,  121  and arranged one behind the other and control-label independently of each other. Each individual stage, comprising an insulator layer and the two adjacent conductive layers or electrodes, represents a micropore as described hereinabove. 
     The diameter is narrowed in the flow direction from the electrode  111  as far as the electrode  113 , is narrowed with a smaller slope from the electrode  113  as far as the electrode  115 , and is widened from the electrode  115  as far as the electrode  116 , the diameter at the electrode  116  being smaller than that at the electrode  111 . Due to this preferred geometry of the opening, the pressure falls by about an order of magnitude within the region between the electrodes  111  and  113 . The ultrasonic jet proper is formed between the electrodes  113  and  116 . The ions are cooled here by elastic collisions. The ion beam is optimally transported by the application of suitable voltage to the electrodes. The cooling behavior in the expansion is affected in a predetermined manner by the inner geometry of the pore  80 . 
     FIG. 6 shows a multi-pore ion source  100 . Here each pore  1  can be individually controlled. In each pore, independently of each other, a discharge can be switched on and off again in the sub-microsecond region. Such a multipore ion source is particularly suitable for surface cleaning and surface coating of a substrate  90  with an ion current of more than 10 15  ions/sec per pore which can be produced. Because of the very good emittance of the ion beam, a macroscopic mask can for example be reduced to the nanometer range on the substrate  90 , and ion beam serigraphy or lithography is possible in the atomic region. 
     Systematic investigations with single pores  1  have shown that, per pore  1 , a discharge of about 3-5 Watt can be operated for hours at 200 V discharge voltage and 15-25 mA current. These values were attained with foils using Kapton(R) as the base layer  13 . Ceramic-based foils should give even longer life. 
     These discharges can be switched on and off extremely rapidly in pulse operation. Switching times of below 10 or 1 μsec are attained. On-off switching times are longer in known ion sources by more than a factor 100, according to the plasma geometry. 
     FIG. 7 shows a plan view of two different MSE pore shapes, before (upper row) and after (lower row) operation for a few hours. 
     The individually controllable micro-discharges  14  can be closely integrat-ed with one another in two dimensions, so that more than 10 3  pores/cm 2  are attained. In principle, the size of the surface is nearly unlimited. The limitation is substantially determined solely by the performance of the pumps in the vacuum stage  22  in order to pump out the recyclable He carrier gas, and also by the thermal loading per unit surface, which can lead to destruction of the pore foil  100  by the thermal loading. 
     The volume of a micro ion source with about 1,000 pores measures only about a surface of about 4×4 mm 2 , and has a thickness of about 0.3 mm. The geometric volume of the cooled high pressure stage  21  is matched to the desired temperature; it preferably lies in the range of a few cm 3 . Ion currents of a few hundred mA up to 1 A can be attained with the ion source  99  at transverse temperatures of about 1° K or less. Furthermore, the applied voltage is only a few hundred V, 200-450 V in this embodiment. 
     An ion source  99  is thus presented which offers very high power densities at high brilliance and very fast control times, based on microstructure electrodes with ultrasonic expansion and ultrasonic ion cooling, and furthermore, because of its size, is to be considered as a microsystem. A decisive physical difference from conventional ion sources results from the extremely high field strength based on the microstructure geometry, so that discharges (glow discharges) can be ignited at a high pressure of about 1 bar and with relatively low voltages.