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Generation and applications of finely focused beams of low-energy electrons
Classified abstracts
5679-5667
experiments such as those related to beam propagation in gases, opening switches, and laser phenomenon. B Fell et al, J Appl Phys, 53 (4), 1982,2818-2824. 36 5679. Catwatina aad appliatioos of fbely foctwd beans of bwaergy dectrooa (USA) In electron-beam lithography, two well-known problems are proximity effects due to electron scattering and slow throughput because of limited exposure rate and resist sensitivity. We have investigated the generation of finely focused electron beams of low landing energy (g 5 keV) and their application to materials processing. Lenses employing magnetic and retarding electric fields are found to have very low spherical and chromatic aberrations, thus facilitating the production of beams of low-energy electrons without seriously sacrificing current density. With such beams the power dissipated per unit volume at the surface of a bulk target is higher than with high-energy electrons, thus increasing the speed to exposure of very thin resists and dramatically reducing proximity effects. This compact volume of high-power dissipation is also attractive for selected area annealing. Y W Yau et al, J Voc Sci Technol, 19 (4), 1981, 1048-1052. 36 56tIO.Curre~ diatrihution in ahaped abeanu (USA) The current distribution in electron beams shaped by using either KShler or critical illumination of apertures has been determined experimentally. The experimental technique is to scan a square image across sharp edgesof a Faraday cage aperture while the first and second differentials of the current are displayed on an oscilloscope. The measurements show that the type of illumination is not an important factor in determining the ultimate edge sharpness. All measurements were made with 20 kV accelerating voltage and it was found that for a square of side 3.5 pm imaged at an angular aperture 3.7 mrad the current density edge deteriorated to about 0.7 p at 3 pA for both types of illumination. Results of imaging a menu of variable shapes in resist are used to demonstrate uniform current density distribution in the shaped beam. (GB) V R M Bao et al, J Vat Sci Tech&, 19 (S), 1981, 1098-1102. 36 5681. A lOO-kV ion probe microfahriitioa system with a tetrode pa. (USA) In the design of probe-forming systems incorporating liquid-metal ion sources, it is necessary lo maxim& the current into a probe of controlled diameter and to employ a configuration that is compatible with the incorporation of a filter for the selection of ion species. A variablemagnification two-lens system has been adopted as the basis of a microfabrication machine for direct selective ion implantation of wafers, and a lOO-kVsystem has been designed. Its tetrode gun acts as the first lens and is followed by an einzel lens as the probe-forming objective. The system has been analysed and operating characteristics have been computed, predicting a current of 0.1 nA in a 0.1 pm diameter probe and 10 nA into 1 pm dua. (GB) J R A Cleaver and H Ahmed, J Vat Sci Tech1~1.19 (4), 1981,1145-l 148. 36 5682. Liquid metal alloy ion soorces for B, Sb, and Si. (USA) B, Sb and Si ion sources using liquid metal alloys have been fabricated and basic characteristics such as current-voltage relations, angular current intensity, energy distribution and mass spectra have been measured. Singly charged B ions which amount to 33% of the total ions and have an angular current intensity of -40 pA sr-I at a total energy spread (fwhm) of -20 eV were obtained using B-Ni-Pt alloys. Singly charged Si and Sb ions with an angular current intensity of 5 11 pA sr - ’ and - 1.4 PA sr - ‘ at an energy spread of z 20 eV were obtained by using Si-Au and SbPb-Au alloys, respectively. These ion sources could be operated for more than 10 b without any changes in mass spectra. K Gamo et PI. J Vat Sci Tech&, 19 (4). 1981, 1182-1185. 36 5683. Energy spreading in the hydrogen field ionization source. (USA) The application of field ionization as a tool for routine submicron fabrication will depend critically upon the characteristics of the source; the angular intensity and the energy spread of the ions. The chromatic aberration coef%ient of the ion optical system, the angular aperture, and the energy spread combine to set the limit of spot size and current density for very-high-resolution probes with sources of energy widths even as low as 1 eV. It is known that the various molecular species produced by hydrogen field ionization are identifiable by the energy distribution, and because the mass of the ion will determine the particle range in resists at any given energy, energy measurements carry further importance in the
development of the probe-forming system. In order to characterize the source-energy widths and mass species as a function of heam current, angular intensity and surface characteristics of the emitter tip, we have designed and operated a unitized, positionable intermediate image filter lens with a demonstrated resolution of 0.1 eV at 3.5 kV and acceptance half-angles of less than 9 mrad. At angular intensities less than 10 pA sr - I, 90% of the field ions from bright site emission of fields of 1.5 V A-' occur within an energy width of 1.2-1.9 eV. This narrow spread shows that only Hi is being produced. The beam current and angular intensity from a single bright site can be increased by raising the source gas pressure and the applied field (5% increase in voltage). That energy broadening occurs at high intensities is demonstrated by the fact that at 3.5 pA sr-’ CO.5nA emitted through 5Opm aperture subtending a half-angle (0,,2) of 0.0067 radf. 90X of ions annear within 1.9 eV 10.94eV fwhml. while at 18 pa sr-’ (2.5 nk: B,,, of 0%67 rad), 90% of iins appear within 2.2 eV (0.98 eV fwhm) where a tail develops on the low-energy side causing a broadening of the energy distribution. The impact of field emitter surface configuration on source angular intensity and reliability are discussed. G R Hamoa aad B M Siegel, J Vat Sci Techd, 19 (4), 1981.11761181. 36 5684. A mass-seprating focosed-iin-beam system for masklcss ion implantetion. (USA) The use of a focused ion beam for direct implantation of dopants into a semiconductor substrate results in appreciable simplification in the processing of semiconductor devices. We have demonstrated that liquid metal (LM) field-ionization sources (based upon the electrostatic formation of an emitting cusp of liquid metal) offer the necessary high brightness to make focused ion beam microfabrication economically feasible. This paper reports upon two developments: (1) the development of eutectic-alloy LM ion sources for the production of boron and arsenic for direct implantation of silicon devices, and (2) the development of a three-lens variable-energy focusing column that incorporates a massseparator of low aberration. Mass spectra of the ion emission of these sources show that the stoichiometric fraction of boron and arsenic is emitted. We have also demonstrated that the high vapour pressure of arsenic can be suppressed in the eutectic liquid metal, and that boron is predominantly emitted as a singly ionized species, while arsenic is emitted as both singly and doubly ionized species. A new focusing column has heen developed that incorporates the new ion sources. It has the capability of focusing to submicrometer dimensions with mass-separation, a variable beam voltage of up to 150 kV, and a spot current of near 1 A cm-*. A high-speed electrostatic-deflection system with microprocessor control allows this machine to perform simple pattern exposuies. Examples of the operation of this microfabrication system with eutectic alloy sources will be presented. V Wang et al, J Vat Sci Tech&, 19 (4), 1981, 1158-1163. 36 5685. Ion sowee performance in a foeusing column with large defbction fields (USA) The theoretical performance of liquid metal and field ionization ion sources in systems with large deflection fields is discussed. It is shown that simple postlens electrostatic deflection with dynamic focusing has the capability of achieving large ddlection fields at reasonable beam currents. A deflection field of 3 x lo3 spot diameters on a side is theoretically achievable for liquid gallium ion sources at beam currents of 1 nA at 100 kV and a spot size of 50 nm. Recent data from Hanson and Siegel for liquid helium cooled hydrogen field ionization indicate that even better performance may be achieved with this source. I L Berry, J Vat Sci Tech&, 19 (4), 1981, 1153-1157. 36 5686. Cc&m supersonic jet for D- production by double electron capture. (USA) A steady-state, supersonic cesium jet generator has been developed for us in an intense negative deuterium ion source, based on double electron capture. The caesium target thickness is 2.5 x lOI atoms cm-* for an oven temperature of 6oo”C, with an average geometrical thickness of 3.0 cm. The caesium flux, determined from calorimetric measurements, is found to be in agreement with a simple calculation of supersonic flow through a slit. (France) M Bacal et al, Reu Sci Instrum, 53 (2). 1982, 159-167. 36 WI7. Scanning microberm usbg a liquid metal ion source. (USA) A scanning ion microprobe system using a Iiquid-Ga ion source and a voltage-asymmetric threczlectrode lens is developed. It produces 2-20 keV Ga’ beams of 0.1-5 pm dia with 20 pA-8 nA current. Beam 423
5679-5667
experiments such as those related to beam propagation in gases, opening switches, and laser phenomenon. B Fell et al, J Appl Phys, 53 (4), 1982,2818-2824. 36 5679. Catwatina aad appliatioos of fbely foctwd beans of bwaergy dectrooa (USA) In electron-beam lithography, two well-known problems are proximity effects due to electron scattering and slow throughput because of limited exposure rate and resist sensitivity. We have investigated the generation of finely focused electron beams of low landing energy (g 5 keV) and their application to materials processing. Lenses employing magnetic and retarding electric fields are found to have very low spherical and chromatic aberrations, thus facilitating the production of beams of low-energy electrons without seriously sacrificing current density. With such beams the power dissipated per unit volume at the surface of a bulk target is higher than with high-energy electrons, thus increasing the speed to exposure of very thin resists and dramatically reducing proximity effects. This compact volume of high-power dissipation is also attractive for selected area annealing. Y W Yau et al, J Voc Sci Technol, 19 (4), 1981, 1048-1052. 36 56tIO.Curre~ diatrihution in ahaped abeanu (USA) The current distribution in electron beams shaped by using either KShler or critical illumination of apertures has been determined experimentally. The experimental technique is to scan a square image across sharp edgesof a Faraday cage aperture while the first and second differentials of the current are displayed on an oscilloscope. The measurements show that the type of illumination is not an important factor in determining the ultimate edge sharpness. All measurements were made with 20 kV accelerating voltage and it was found that for a square of side 3.5 pm imaged at an angular aperture 3.7 mrad the current density edge deteriorated to about 0.7 p at 3 pA for both types of illumination. Results of imaging a menu of variable shapes in resist are used to demonstrate uniform current density distribution in the shaped beam. (GB) V R M Bao et al, J Vat Sci Tech&, 19 (S), 1981, 1098-1102. 36 5681. A lOO-kV ion probe microfahriitioa system with a tetrode pa. (USA) In the design of probe-forming systems incorporating liquid-metal ion sources, it is necessary lo maxim& the current into a probe of controlled diameter and to employ a configuration that is compatible with the incorporation of a filter for the selection of ion species. A variablemagnification two-lens system has been adopted as the basis of a microfabrication machine for direct selective ion implantation of wafers, and a lOO-kVsystem has been designed. Its tetrode gun acts as the first lens and is followed by an einzel lens as the probe-forming objective. The system has been analysed and operating characteristics have been computed, predicting a current of 0.1 nA in a 0.1 pm diameter probe and 10 nA into 1 pm dua. (GB) J R A Cleaver and H Ahmed, J Vat Sci Tech1~1.19 (4), 1981,1145-l 148. 36 5682. Liquid metal alloy ion soorces for B, Sb, and Si. (USA) B, Sb and Si ion sources using liquid metal alloys have been fabricated and basic characteristics such as current-voltage relations, angular current intensity, energy distribution and mass spectra have been measured. Singly charged B ions which amount to 33% of the total ions and have an angular current intensity of -40 pA sr-I at a total energy spread (fwhm) of -20 eV were obtained using B-Ni-Pt alloys. Singly charged Si and Sb ions with an angular current intensity of 5 11 pA sr - ’ and - 1.4 PA sr - ‘ at an energy spread of z 20 eV were obtained by using Si-Au and SbPb-Au alloys, respectively. These ion sources could be operated for more than 10 b without any changes in mass spectra. K Gamo et PI. J Vat Sci Tech&, 19 (4). 1981, 1182-1185. 36 5683. Energy spreading in the hydrogen field ionization source. (USA) The application of field ionization as a tool for routine submicron fabrication will depend critically upon the characteristics of the source; the angular intensity and the energy spread of the ions. The chromatic aberration coef%ient of the ion optical system, the angular aperture, and the energy spread combine to set the limit of spot size and current density for very-high-resolution probes with sources of energy widths even as low as 1 eV. It is known that the various molecular species produced by hydrogen field ionization are identifiable by the energy distribution, and because the mass of the ion will determine the particle range in resists at any given energy, energy measurements carry further importance in the
development of the probe-forming system. In order to characterize the source-energy widths and mass species as a function of heam current, angular intensity and surface characteristics of the emitter tip, we have designed and operated a unitized, positionable intermediate image filter lens with a demonstrated resolution of 0.1 eV at 3.5 kV and acceptance half-angles of less than 9 mrad. At angular intensities less than 10 pA sr - I, 90% of the field ions from bright site emission of fields of 1.5 V A-' occur within an energy width of 1.2-1.9 eV. This narrow spread shows that only Hi is being produced. The beam current and angular intensity from a single bright site can be increased by raising the source gas pressure and the applied field (5% increase in voltage). That energy broadening occurs at high intensities is demonstrated by the fact that at 3.5 pA sr-’ CO.5nA emitted through 5Opm aperture subtending a half-angle (0,,2) of 0.0067 radf. 90X of ions annear within 1.9 eV 10.94eV fwhml. while at 18 pa sr-’ (2.5 nk: B,,, of 0%67 rad), 90% of iins appear within 2.2 eV (0.98 eV fwhm) where a tail develops on the low-energy side causing a broadening of the energy distribution. The impact of field emitter surface configuration on source angular intensity and reliability are discussed. G R Hamoa aad B M Siegel, J Vat Sci Techd, 19 (4), 1981.11761181. 36 5684. A mass-seprating focosed-iin-beam system for masklcss ion implantetion. (USA) The use of a focused ion beam for direct implantation of dopants into a semiconductor substrate results in appreciable simplification in the processing of semiconductor devices. We have demonstrated that liquid metal (LM) field-ionization sources (based upon the electrostatic formation of an emitting cusp of liquid metal) offer the necessary high brightness to make focused ion beam microfabrication economically feasible. This paper reports upon two developments: (1) the development of eutectic-alloy LM ion sources for the production of boron and arsenic for direct implantation of silicon devices, and (2) the development of a three-lens variable-energy focusing column that incorporates a massseparator of low aberration. Mass spectra of the ion emission of these sources show that the stoichiometric fraction of boron and arsenic is emitted. We have also demonstrated that the high vapour pressure of arsenic can be suppressed in the eutectic liquid metal, and that boron is predominantly emitted as a singly ionized species, while arsenic is emitted as both singly and doubly ionized species. A new focusing column has heen developed that incorporates the new ion sources. It has the capability of focusing to submicrometer dimensions with mass-separation, a variable beam voltage of up to 150 kV, and a spot current of near 1 A cm-*. A high-speed electrostatic-deflection system with microprocessor control allows this machine to perform simple pattern exposuies. Examples of the operation of this microfabrication system with eutectic alloy sources will be presented. V Wang et al, J Vat Sci Tech&, 19 (4), 1981, 1158-1163. 36 5685. Ion sowee performance in a foeusing column with large defbction fields (USA) The theoretical performance of liquid metal and field ionization ion sources in systems with large deflection fields is discussed. It is shown that simple postlens electrostatic deflection with dynamic focusing has the capability of achieving large ddlection fields at reasonable beam currents. A deflection field of 3 x lo3 spot diameters on a side is theoretically achievable for liquid gallium ion sources at beam currents of 1 nA at 100 kV and a spot size of 50 nm. Recent data from Hanson and Siegel for liquid helium cooled hydrogen field ionization indicate that even better performance may be achieved with this source. I L Berry, J Vat Sci Tech&, 19 (4), 1981, 1153-1157. 36 5686. Cc&m supersonic jet for D- production by double electron capture. (USA) A steady-state, supersonic cesium jet generator has been developed for us in an intense negative deuterium ion source, based on double electron capture. The caesium target thickness is 2.5 x lOI atoms cm-* for an oven temperature of 6oo”C, with an average geometrical thickness of 3.0 cm. The caesium flux, determined from calorimetric measurements, is found to be in agreement with a simple calculation of supersonic flow through a slit. (France) M Bacal et al, Reu Sci Instrum, 53 (2). 1982, 159-167. 36 WI7. Scanning microberm usbg a liquid metal ion source. (USA) A scanning ion microprobe system using a Iiquid-Ga ion source and a voltage-asymmetric threczlectrode lens is developed. It produces 2-20 keV Ga’ beams of 0.1-5 pm dia with 20 pA-8 nA current. Beam 423