Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
  • H01J37/10—Lenses
  • H01J37/12—Lenses electrostatic
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
  • H01J37/05—Electron or ion-optical arrangements for separating electrons or ions according to their energy or mass
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
  • H01J37/153—Electron-optical or ion-optical arrangements for the correction of image defects, e.g. stigmators

Definitions

• the present invention relates to an electron optical device. Also, the present invention relates to an electron lens system comprising such an electron optical device. Further, 5 the present invention relates an electron microscope comprising such electron optical device. Finally, the present invention relates an electron lithography system comprising such electron optical device.
• these correctors are operated at a beam energy at the foil of 100 keV and larger.
• the pressure in an electron microscope is usually 10 " mbar at best and mobile adsorbants will be present on the foils surface. Electrons impinging on the surface with an energy larger than ⁇ 5 eV will crack these adsorbants and create a carbon species that locally sticks to the surface. As the adsorbants move over the surface, new adsorbants move into the beam and are cracked continuously. So the electron beam will cause a carbon build up on the foils surface. This contamination and its associated scattering and charging is a major problem for the use of phase plates in electron microscopes (see for example: R. Danev, K.
• an electron optical device for, in use, creating negative spherical and chromatic aberration and reducing the energy spread in an electron beam travelling on an optical axis, comprising:
• the very low voltage operation advantageously makes the electron optical device according to the present invention, or foil corrector, more attractive for low voltage SEM and it greatly reduces the contamination problem.
• the foil of the device may act as a high-pass energy filter: electrons with insufficient forward energy are reflected at the foil.
• the present invention relates to an electron lens system, using an electron optical device as described above for a compensation, in use, of a positive spherical aberration of other lenses in the electron lens system.
• the present invention relates to an electron lens system, using an electron optical device as described above for a compensation, in use, of a positive chromatic aberration of other lenses in the electron lens system.
• the present invention relates to an electron microscope, using an electron optical device as described above. Also, the present invention relates to an electron microscope, using an electron lens system as described above.
• the present invention relates to an electron lithography system, using an electron optical device as described above. Moreover, the present invention relates to an electron lithography system, using an electron lens system as described above.
• Figure 1 shows schematically a basic design of a foil corrector according to the present invention (not to scale) wherein D denotes a diameter of an aperture and s denotes a gap between a foil and the aperture;
• Figure 2 shows schematically positive spherical aberration for a positive and a negative lens, by means of two rays entering the lens at different radii, r / and r 2 . In both cases, the intercept with the z-axis shifts in the negative z direction for increasing radius of incidence;
• Figure 3 shows a schematic sketch of electric field lines in the foil corrector according to the present invention
• Figure 5 shows schematically a foil corrector according to the present invention with equi-potentials as calculated by Elens;
• Figure 8 shows a plot of coefficient of 2 n order chromatic aberration C c2 , normalized to the aperture diameter! ) , versus the corrector gap, for a voltage on aperture
• Figure 10 shows schematically an electron microscope column with the foil corrector according to the present invention.
• FIG. 1 shows schematically a basic design of a foil corrector according to the present invention (not to scale) wherein D denotes a diameter of an aperture and 5 denotes a gap between a foil and the aperture. It consists of a flat free-standing foil of nanometer size thickness with apertures on both sides.
• the foil is put on a retarding potential, such that the electrons (e-) have almost 0 eV kinetic energy when they enter the foil (and also when they have just left the foil at the other side).
• Figure 2 shows schematically positive spherical aberration for a positive (left) and a negative (right) lens, by means of two rays entering the lens at different radii, r; and r 2 .
• positive spherical aberration is the effect that the focussing power of the lens increases for increasing radius of incidence.
• a negative lens with positive spherical aberration the defocusing power of the lens decreases for increasing radius of incidence. This is illustrated in figure 2.
• a spherical aberration corrector the opposite effect is desired.
• an approximative description of its properties will be obtained by a simple analysis of the radial momentum the electron obtains in the electric field.
• the calculation can be limited to one half part of the corrector: a flat surface (representing the foil) with an aperture in front of it. First, the calculation will be done for a conventional foil corrector operating at high beam energy. Thereafter the calculation for the low voltage foil corrector will be done according to the same reasoning and the difference between both correctors will be pointed out.
• E r (z,r) is the radial component of the electric field
• v z (z,r) is the electron's axial velocity component.
• a cylindrical coordinate system is adopted here in which the positive z-direction is perpendicular to the foil and directed towards the aperture, and the radial coordinate r is perpendicular to the z-axis. Close to the axis the potential ⁇ (z,r) can be expanded as
• Vz (6) p z is the axial momentum of the electron leaving the corrector. If the velocity of the electron is assumed to be constant and the radial velocity component is negligible, the following substitution is allowed:
• the first term l nd term proportional to r is the 3 r order focal strength and is a measure for the 3 r order spherical aberration.
• the deflection angle can be expressed in terms of the z-field at the foil (neglecting the 0(r") term):
• Figure 3 shows a schematic sketch of electric field lines in the foil corrector according to the present invention.
• the side at the aperture with the lower potential has a positive spherical aberration. Because the electron velocity is lower at this side, its contribution is larger than the side with the negative spherical aberration and the net result is always positive.
• E t is the kinetic energy and m is the electron mass.
• m is the electron mass.
• a new expression for the radial momentum change is obtained.
• the l/v z term in the integral can be expanded into a series of r using equation 2.
• the deflection angle is ⁇ p0p z (equation 6).
• » z is the axial momentum of the electron when leaving the corrector, it is (under the same approximation for v z as above)
• a positive electrostatic lens will, in general, have a positive chromatic aberration: electrons with a larger velocity will spend less time in the lens field and are less deflected. So the focussing power is weaker for higher energies. In a negative lens, the higher energy electrons are less deflected as well and the defocusing power is weaker. This results in a negative chromatic aberration for negative electrostatic lenses. Therefore a foil corrector with the foil on a retarding potential is expected to have a negative chromatic aberration.
• any optical element can also be determined, when the electron trajectories in the exit plane are known as function of their radius and angle of incidence. Therefore, the spherical and chromatic aberration can be determined from ray tracing results. This will be discussed below.
• the symmetry around the foil is used. The electrons start just in front of the foil, perpendicular to its surface with almost zero kinetic energy.
• Coefficient C s is obtained from ray tracing results in two steps (see figure 2 for visual illustration): 1. Evaluate the deflection angle as function of the radius of incidence:
• this angle is equal to the deflection angle due to the 1 st order lens effect:
• E start is the starting energy of the electron
• U en d the potential in the exit plane
• C c i and C c2 the coefficients of 1 st and 2 nd order chromatic aberration.
• Figure 5 shows schematically a foil corrector according to the present invention with equi-potentials as calculated by Elens.
• Elens divides this geometry into a fine mesh of maximum 100,000 points. The potential on every mesh point is determined such that the total energy is minimized.
• the electron trajectories are calculated with Trasys (B. Lencova, G.Wisselink, "Electron Optical Design Program Package Trasys 3.7", 2002).
• Trasys uses a high accuracy interpolation in z and r to determine the potential in between the mesh points, as described by J. Chmelik and J.E. Barth, "An interpolation method for ray tracing in electrostatic fields calculated by the finite element method", SPIE Charged-Part. Opt. 2014 (1993) 133. With this information, it can calculate the electric force on the electron at any point and thus trace its trajectory.
• the corrector has 3 independent parameters: the gap s between foil and aperture, the diameter D of the aperture (both as shown in Figure 1) and the voltage V applied to the aperture.
• An important result of the calculations is that the magnitude of the voltage on the aperture has little influence on the trajectories.
• The/ C S 3, C s s and C c j are hardly affected by the voltage, as is illustrated in figure 6, only the C c2 is.
• the results scale linearly with the size of the corrector. Therefore the corrector properties as function of the gap are normalized to the diameter of the aperture, see figure 7.
• Figures 8 and 9 show that the coefficient of second order chromatic aberration is independent of the gap and increases for increasing voltage respectively.
• Figure 10 shows schematically an electron microscope column with an electron optical device or foil corrector according to the present invention.
• the corrector can be used in an electron lithography system for the reduction of the spherical or chromatic aberration.
• the foil will act as a high pass energy filter because the electrons in the lower part of the energy distribution do not have sufficient energy to pass the foil.
• a quantum mechanical effect due to the wave character of the electrons has to be taken into account.
• the electrons enter the foil their kinetic energy is increased with around 10 eV, being the difference between the vacuum level and the bottom of the conduction band. As a consequence the wavelength is decreased. Opposed to its high voltage counterpart, this effect is not negligible for the low energy foil corrector.
• the electrons form a standing wave in the foil and a quantum mechanical reflection at the foils surfaces can occur. Calculations of the transmission as function of the electron kinetic energy show an oscillating behaviour, causing a high cut-off as well.
• a corrector geometry is assumed of identical apertures on both sides of the foil, gap sizes 30 ⁇ m, aperture diameters 200 ⁇ m and a voltage between foil and apertures of 300 V, which is based on the practical limit of 10 kV/mm. It has been taken into account that in this case the field on the optical axis is considerably lower, see fig. 4. Then for an electron beam with a reduced brightness of 10 Am " sr " V " , a diameter of 40 ⁇ m at the foil (20% of the aperture diameter) and current 1 nA (assuming no current loss in the foil), the illumination angle is 5.8x10 " rad and the Coulomb interactions angular deflection is 2.2x10 " rad. This causes an increase of the spot size of about 7%.
• the fifth order C s and the second order C c can limit the spot size.
• the calculations show that the C s s of the corrector can be zero, giving no extra contribution to the spot size due to aberrations. Preliminary calculations indicate that the spot size due to the second order chromatic aberration can be kept sufficiently small.
• the transmission should be measured for sub 10 nm foils and with a sub 0.1 eV energy resolution because of the quantum mechanical reflection.
• the principal scattering mechanism at the energy of interest is electron-electron scattering. See: P. Wolff, "Theory of secondary electron cascade in Metals", Physical Review 95 (1954) 56. Therefore, a semiconductor foil may be favourable over a metal foil. Because of its lower density of conduction electrons, scattering is expected to be much less and the transmission will be better or such a foil needs to be less thin.
• Another option to improve the transmission is the use of a gauze or perforated foil. The gauze mazes or perforations should be sufficiently small, that they do not significantly disturb the electric field.
• a low voltage spherical and chromatic aberration corrector is proposed based on a thin transparent foil sandwiched between two apertures.
• the electrons are retarded to almost zero energy at the foil, at which energies the electrons may travel ballistically through the foil.
• From an approximate analytical model the feasibility to correct spherical and chromatic aberrations was shown.
• the third and fifth order spherical aberration coefficients as well as the first and second order chromatic aberration coefficients were obtained from electric field simulations and ray tracing.
• a schematic design of a corrector for potential use in a low- voltage scanning electron microscope or in the gun-section of a scanning transmission electron microscope has been described.

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• 2003
  • 2003-09-01 WO PCT/NL2003/000612 patent/WO2004021391A1/en not_active Ceased
  • 2003-09-01 EP EP03791500A patent/EP1547119A1/de not_active Withdrawn
  • 2003-09-01 AU AU2003261023A patent/AU2003261023A1/en not_active Abandoned

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