Patent Number: 
Section: claims

1. A method of producing a microscopic structure, comprising:directing a precursor gas toward a substrate; anddirecting an particle beam toward the substrate to initiate a reaction with the precursor gas;controlling the beam current during processing by controlling the duty cycle of the beam by blanking and unblanking the beam to control the depletion of the adsorbed precursor gas to etch or to deposit a materialin which the particle beam is a beam of electrons or a beam of ions; andin which the particle beam is focused on the substrate using a lens, and the substrate is biased with respect to said lens so that the energy of the particle beam is reduced to less than 1000 eV as the beam approaches the substrate. 2. The method of claim 1 in which controlling the beam current during processing by controlling the duty cycle of the beam by blanking and unblanking the beam includes reducing the beam current by at least one order of magnitude. 3. The method of claim 2 in which the time during which the electron beam is unblanked per blank/unblank cycle is smaller than or equal to 1 ns. 4. The method of claim 1 in which rapidly blanking and unblanking the electron beam includes changing the duty cycle to change the beam current and the dimension of the microscopic structure while it is being formed, including reducing the duty cycle to increase the ratio of vertical growth to horizontal grown of the microscopic structure or increasing the duty cycle to decrease the ratio of vertical growth to horizontal growth of the microscopic structure. 5. The method of claim 1 in which the substrate is cooled to increase the precursor adsorbate residence time on the substrate surface. 6. A particle-optical apparatus for forming fine structures on a substrate, the particle-optical apparatus comprising:a particle source for producing a beam of particles, in which the particle source is an electron source and the beam of particles is a beam of electrons or the particle source is an ion source and the beam of particles is a beam of ions;one or more particle-optical lenses for focusing the beam of particles on a substrate;a deflector unit for positioning the beam of particles on the substrate;a beam blanker for blanking the particle beam; anda programmable controller for controlling at least the deflector unit and the beam blanker;the programmable controller is programmed to reduce the average current of the particle beam by blanking/unblanking the particle beam with a cycle time less than the time that the beam is positioned at one position on the substrate; andthe programmable controller is further programmed to bias the substrate with respect to said lens so that the energy of the particle beam is reduced to less than 1000 eV as the beam approaches the substrate. 7. The particle-optical apparatus of claim 6 in which the controller controls the blanker such, that the time during which the beam is unblanked per blank/unblank cycle is smaller than or equal to 1 ns. 8. Software carrier having stored thereon program code for programming the programmable controller of the apparatus of claim 6 to reduced the average current of the particle beam by blanking/unblanking the particle beam with a cycle time less than the time that the beam is positioned at one position on the substrate.