Patent Number: 
Section: claims

1. A phase plate for use in a particle-optical apparatus, said phase plate to be irradiated by a beam of particles, said phase plate comprising:a central structure that is non-transparent to particles,said central structure surrounded by an area transparent to particles;said central structure surrounding a foil transparent to particles, said foil surrounding a central through-hole for passing a part of the beam, said foil equipped to cause a phase shift between the part of the beam passing through the through-hole and the part of the beam passing through the foil; andsaid central structure equipped to cause a phase shift between the part of the beam passing through the foil and the through-hole and the part of the beam passing outside the central structure. 2. The phase plate of claim 1 in which the foil is a carbon foil. 3. The phase plate of claim 1 in which for at least one line in the plane of the phase plate and passing through the centre of the through-hole, said line thus intersecting the central structure at two opposite sides, the at least one line intersects the central structure from a distance R1 to a distance of R2 from the through-hole in one direction, and a distance from R3 to a distance of R4 from the through-hole in the other direction, and in which R3≧R2. 4. The phase plate of claim 3 in which the central structure is formed from two half-annuli, one half-annulus with inner radius R1 and outer radius R2, and the other half-annulus with an inner radius of R3 and an outer radius of R4, and in which R3≧R2, and the through-hole is located at the centre points of the two annuli from which the two half-annuli are formed. 5. Particle-optical apparatus equipped with a phase plate and equipped with an objective lens, said particle-optical apparatus illuminating a sample with a beam of particles the phase plate placed substantially in a plane where the beam illuminating the sample is focused, in which the phase plate is the phase plate according to claim 1. 6. The particle-optical apparatus of claim 5 in which, in working, the combined phase shift caused by the foil and the electric potential in the central structure results in a phase shift θ of substantially θ=n·π for the particles transmitted through the foil, with n an integer. 7. The particle-optical apparatus of claim 6 in which, in working, the combined phase shift caused by the foil and the electric potential in the central structure results in a phase shift θ of substantially θ=0. 8. The particle-optical apparatus according to claim 5 in which the apparatus is equipped to image the back-focal plane of the objective lens on the phase plate with a variable magnification. 9. Method of forming an image using a particle-optical apparatus equipped with an objective lens for illuminating a sample and a phase plate, the particle-optical apparatus equipped to image the plane where the objective lens forms a focus onto the phase plate, characterized in that:the particle-optical apparatus is equipped with the phase plate according to claim 1, andthe particle-optical apparatus is equipped to image the plane where the objective lens forms a focus onto the phase plate with a variable magnification,and the method comprises;determining a desired lower spatial frequency range of the image, said lower spatial frequency range caused by the interference of particles transmitted through the foil with particles passing through the through-hole;determining a desired higher spatial frequency range of the image, said higher spatial frequency range caused by the interference of particles transmitted around the central structure with particles passing through the through-hole; andadjusting the magnification with which the back-focal plane is imaged on the phase plate so that particles corresponding with said lower and the higher spatial frequency range are not intercepted by the central structure. 10. The method of claim 9 in which determining a lower spatial frequency range and determining a higher spatial frequency range takes the form of determining a central spatial frequency around which said lower spatial frequency range and said higher spatial frequency range are centred. 11. A particle-optical apparatus for forming an image of a sample comprising:an objective lens for illuminating a sample with charged particles; anda phase plate defining a first region in which the phase of the charged particles passing through the first region is shiftable by an adjustable amount;a second region within the first region, in which the phase of the charged particles passing through the second region is shifted by a fixed amount in addition to the adjustable amount; anda third region, outside of the first region, through which pass charged particles that did not pass though the first region, the charged particles passing through the three regions combining to form an image of a sample. 12. The particle-optical apparatus of claim 11 in which the phase plate include electrodes to provide in the first region an electric field that is adjustable to adjust the phase shift of charged particles passing through the first region and in which the second region includes a film that shifts the phase of the charged particles passing through the second region. 13. The particle-optical apparatus of claim 11 in which the phase of the charged particles passing through the third region is not shifted. 14. The particle-optical apparatus of claim 11 in which the sum of the fixed amount of phase shift in the second region and the adjustable phase shift in the first region results in a phase shift such that particles passing through the second region differ in phase from particles passing though the third region by n·π radians, with n being an integer. 15. The particle-optical apparatus of claim 11 in which the fixed amount of phase shift in the second region is equal in magnitude and opposite in sign to the adjustable phase shift in the first region. 16. The particle-optical apparatus of claim 11 in which the second region is defined by a film through which the charged particles pass and in which the portion of the first region that is not included in the second region is a hole in the film defining second region. 17. A particle-optical apparatus for forming an image of a sample comprising:an objective lens for illuminating a sample; anda phase plate for shifting the phase of some particles in a beam relative to other particles in the beam, the phase plate defining three regions through which charged particles pass:a foil region defined by a foil that shifts the phase of charged particles passing through the foil by a fixed amount;a through-hole region defined by a hole in the foil; andan exterior region outside the outer diameter of the foil,the charged particles passing through the fixed shift region, the through-hole region, and the exterior region combining to form an image of the sample. 18. The particle-optical apparatus of claim 17 in which the phase plate comprises two partial annuluses having different inner diameters so that if a particle scattered at an angle of α impacts one of the annuluses, a second particle scattered at an angle −α will not impact the second annulus, except at the areas at which the annuluses connect. 19. The particle-optical apparatus of claim 18 further comprising electrode to provide an electric field to shift the phase of the charged particles passes through the foil region and the through-hole region. 20. The particle-optical apparatus of claim 18 in which the shape of the foil region is defined by two partial annuluses having different inner diameters such that if a particle scattered at an angle of α impacts one of the annuluses, a second particle scattered at an angle −α will not impact the second annulus, except at the areas at which the annuluses connect. 21. Method of forming an image using a particle-optical apparatus equipped with an objective lens for illuminating a sample and a phase plate, the particle-optical apparatus equipped to image the plane where the objective lens forms a focus onto the phase plate, comprising:passing a first set of charged particles through a first region in which the phase of the charged particles is shifted by a first amount;passing a subset of the first set of charged particles through a second region, which is a subset of the first region and in which the charged particles in the subset are phase shifted by second amount;passing a second set of charged particles though a third region in which the charged particles are phase shifted by a third amount;combining the charged particles in the first and second sets of charged particles to form an image. 22. The particle-optical apparatus of claim 21 in which the second amount and the third amount differ by n·π, n being an integer.