WO2008104394A1

WO2008104394A1 - Method for producing a particle radiation source

Info

Publication number: WO2008104394A1
Application number: PCT/EP2008/001584
Authority: WO
Inventors: Josef Sellmair
Original assignee: Josef Sellmair
Priority date: 2007-03-01
Filing date: 2008-02-28
Publication date: 2008-09-04
Also published as: DE102007010462B4; DE102007010462A1

Abstract

The invention relates to a method for producing a micromechanical particle radiation source that comprises at least one field emission tip for emitting particles, said method comprising the following steps: at least one surface layer of an electrically semi-conductive or electrically conductive material is applied to the surface (2) of a substrate; the at least one surface layer applied to the substrate (1) is textured in such a way that a path is produced which is as narrow as possible and has a low height; uncovered areas of the electrically insulating substrate surface, which are disposed between the paths, are partially removed such that the covering of the areas of the newly formed electrically insulating substrate surface is prevented by the superimposed surface layers (3, 4); a further conductive layer (6) is applied to the thus obtained surface; and the field emission tip is applied to the narrow path so that the field emission tip (8) is exactly positioned with an accuracy of a few nanometers in the center of the optical axis of the micromechanical particle radiation source.

Description

Method for producing a particle beam source

The invention relates to a method for producing a particle beam source according to the preamble of claim 1. In particular, the invention relates to a particle beam source for emitting an electron beam for electron-optical devices, such as scanning electron microscopes for the investigation of the surface structure or micro-probes for the material investigation by electron beam excitation where the diameter of the electron beam on the sample surface (probe current) in combination with the probe current strength is a parameter that determines the resolution of the device.

In particular, such a particle beam source has a very small particle emission surface ("virtual source size") and is produced by means of MEMS (Micro-Electro-Mechanical Systems) and nanotechnological methods, in particular the emitter tip is determined by particle beam induced deposition (also called particle beam deposition). For example, electron beam-induced deposition, which allows controlled generation of three-dimensional structures with a precision of a few nanometers, is a problem in the production of emitter tips by means of particle beam deposition in that the particles (eg, electrons or ions ) are scattered (changed in their direction of flight) upon impact with material during the deposition phase and cause further unintentional particle beam deposition on secondary surfaces (eg insulator surfaces) .Particularly in the case of electron beam deposition, there is a problem that the electrons Due to their high speed, the resulting emitter tip and other material layers penetrate and are also scattered in the material. These scattered electrons are found in an area around the emitter tip thus produced, which extends several microns around the tip. In this Therefore, further unintentional Teilchenstrahld- eposition with a lower dose. As a result, a thin, slightly conductive layer of further deposited material is formed in this area. This situation has the effect that the insulator surfaces in this area have insufficient insulation capability for the electrical voltages to be applied to the operation of the particle beam source. The insufficient dielectric strength leads to the destruction of the structures in the course of emitter operation. There are known methods to increase the dielectric strength of the insulation gap. This includes thermal annealing (K. Murakami, N. Yamasaki, S. Abo, F. Wakaya, and M. Takai: "Effect of thermal annealing on emission characteristics of nanoelectron source fabrication using beam-assisted process", J. Vac Technol. B23, no. 2 (2005), 759-761), or the resetting of the insulator surface to a greater distance from the location of the electron beam deposition (S. Bauerdick, C. Burkhardt, DP Kern, and W. Nisch: " Addressable field emitter ar- ray: A tool for designing field emitters and a multibeam electronon source ", J. Vac.So., Technol. B22, no. 6 (2004), 3539-3542). Both previously known methods have disadvantages, such. Additional process steps, insufficient efficiency of the additional process steps, need for a more complex multilayer starting substrate, or a mechanical weakening of the structure by the relocation of the insulator layer.

The present invention has for its object to provide a simpler method for producing a particle beam source, which allows the production of particle-beam-deposited tips, without producing this low-conductivity layer on the surrounding insulator surfaces, and without having the disadvantages just described.

The solution of the problem is achieved by the method specified in claim 1. The method according to the invention for producing a micromechanical particle beam source which has a field emission tip for the emission of particles comprises the following steps:

Applying at least one first surface layer of an electrically semiconductive or conductive material to the surface of a substrate, wherein the substrate is electrically insulating at least on the surface,

Patterning the at least one first surface layer applied to the substrate in such a way that one or more, as narrow as possible, i. narrower than about 1 micron, low profile electrically conductive paths, i. again less than about 1 micron, the webs serving as a support for later depositing a field emitter tip, and one or more further electroconductive structures electrically insulated from each other and from these webs also arise on the surface layer .

partial removal of exposed areas of the electrically insulating substrate surface lying between the webs and surface structures in such a way that the resulting new electrically insulating substrate surfaces are not completely covered during the subsequent application of a further conductive layer, but the coverage of substantial areas of the newly formed electrically insulating substrate - Surface is prevented by the overlying surface layers,

Applying a further conductive layer to the resulting surface such that essentially only the surfaces parallel to the first surface layer are covered with it,

Applying the field emitter tip to the narrow path and thereby exact positioning of the field emitter tip with a Accuracy of a few nanometers in the center of the optical axis of the micromechanical particle beam source.

Following the principle of the invention it can be provided that the first conductive layers are undercut, in particular by means of an isotropic, preferably wet-chemical etching step.

In a further step, the processing of the substrate thus produced by means of a reactive isotropic process, z. Example, by the use of an oxygen plasma, for the partial or total removal and / or modification of the composition remaining material from unintentional Teilchenstrahldeposition.

These method steps avoid the cause of the formation of electrically low-conductivity layers during the particle beam deposition of the field emitter tip, which is due to the scattering of electrons on insulator surfaces during the particle beam deposition and the "parasitic" particle beam deposition there.

With the method according to the invention, it is possible to produce field emitter tips by means of particle beam deposition, without causing the weakly conductive layers customary in particle beam deposition, in particular electron beam deposition, between the electrodes which are insulated from one another. At the same time, the particle beam emitted by these field emitter tips has only minimal beam aberrations because of the high positioning accuracy of the field emitter tip.

With a modified method in which instead of the field emitter tip carrying web serves a much easier to produce edge of a suitably structured metal surface as a carrier of the field emitter tip succeeds the simple

Production of particle emitters in which particle beam rations and radiation angles are of secondary importance.

An advantage of the described manufacturing method is the maintenance of the dielectric strength of the structures after electron beam deposition. From each other electrically isolated structures z. B. serve for applying the extraction voltage, even after the electron beam deposition of the emitter titer unaltered high dielectric strength. This is essential for the operation of the particle emitter.

Another significant advantage of the particle beam sources produced by the method according to the invention is that no insulator surfaces in the direction of the vacuum space, in which the particle beams are present. This increases the stability of the emission direction of the particle beam, since the insulator surfaces can not charge and thus the electric fields do not change unintentionally.

The invention makes it possible to produce a particle beam source in miniaturized form, which contains an emitter tip produced with particle radiation deposition. Such emitter tips enable electron microscopy with the highest resolution. Miniaturization allows these particle beam sources to operate at low voltage (eg, as 100V electron beam sources, as opposed to conventional ones operating at several 1000V), which offers a variety of other advantages.

The production method of the field emission tip with particle radiation deposition has the further advantage that the field emission peak can be exactly positioned down to a few nanometers. This makes it possible to position the field emitter tip exactly in the center of the electron-optical axis and to avoid aberrations resulting therefrom. This distinguishes this method described here from other methods (eg deposition of carbon nanotubes as a field emitter), the because of the lack of positioning ability produce large aberrations in the electron-optical design.

In a continuation of the invention, it is provided that the astigmatism of the beam induced by the emitter tracks is minimized or even eliminated by suitable emitter area geometry. The emitted particle beam has astigmatism due to the emitter conductor track and the feed track. A corresponding layout of the actual extractor surfaces, which can correct this geometry-induced astigmatism, remedies this problem. This is possible because the

Known astigmatism-inducing geometries and their effects are predictable and constant over time. This allows an adaptation of the extraction geometry in the layout already taken into account. If magnetic elements are used, they cause a rotation of the emitted beam about the beam axis. This rotation is to be included in the calculations.

If, in a continuation of the invention with a further lower etching step, the extraction structure is undercut, a leakage current can be prevented which would be generated by primary electrons scattered on the extraction structure in the gap.

In a further preferred embodiment, provision is made for different electrical voltages to be applied to the mutually electrically insulated layers during particle beam deposition. In this way, it is possible to favorably influence scattered primary electrons or to control them in their direction.

Preferred developments of the invention are specified in the further subclaims. Further advantages, features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the drawing. It shows:

Figures Ia to If the process steps of the invention for producing a particle beam source according to a preferred embodiment of the invention;

FIGS. 2 a to 2 c show a further preferred exemplary embodiment produced by the method according to the invention.

A preferred embodiment for a particle beam source produced by the described method is shown in Figs. Ia to If. A preferably of an insulator material such as glass, SiO ₂ , quartz, ceramic, or the like, or of a semiconductor material such as Si, GaAs, InP or the like existing and at least on its surface 2 electrically insulating substrate 1 is electrically conductive surface layers. 3 and 4, which are electrically isolated from each other, covered (Figure la, not to scale). The surface layers 3, 4 may consist of one or more superimposed layers. One of these surface layers, preferably a narrow (preferably narrower than 1 μm) metallic or other conductive web 4 of small height (preferably less than 1 μm) serves as a base for depositing one or more field emitter tips 8 and is of suitable geometry for this purpose (Fig. Ib: greatly enlarged section of the surface of Fig. Ia). Further adjacent, separated by an insulator surface paths or other geometric shapes of metal serve as additional electrodes for extraction, focusing, or shaping of the particle beam. These electrodes are preferably by means of known lithographic methods such. B. electron beam lithography produced. The conductive surface layers 3 and 4 or other conductive tracks Structures are on a flat insulator, e.g. As silica, applied.

In a further production step (FIG. 1 c), the insulator surface 2 exposed between the structures of the surface layer 3 and / or the webs 4 is removed, for B. by selective chemical etching in the liquid or gas phase or another suitable method. In this case, the process parameters are to be selected such that the resulting new insulator surfaces are not completely covered during the subsequent application of a further conductive layer, but that the covering of the regions 5 of the newly formed insulator surface is prevented by the overlying surface layers 3 and / or webs 4 becomes. The first conductive surface layers 3 and / or webs 4 are preferably undercut (eg by isotropic etching).

In the following step (Fig. Id) becomes another conductive

Layer 6, which may also consist of several layers, applied to the surface thus formed (eg, by means of directional vapor deposition) that substantially only the surfaces parallel to the first surface layer are covered with it.

Optionally, a further process step (for example, isotropic etching) can be carried out for further removal of the insulator surface and thus for increasing the insulation resistance (FIG. The newly formed insulator surfaces 7 have an increased dielectric strength in relation to the original insulator surfaces 5.

An advantageous feature of this structure is that the regions 5 which lie closest to the web 4 can for the most part not be struck by the scattered particle beam 10 from the particle beam deposition of the field emitter tip 8 with the primary particle beam 9 and thus the formation a conductive layer that would destroy the insulating power does not take place. The Teilchenstrahldeposition can also be supported by suitable electric fields which can be generated via the conductive surface layers 3 and / or webs 4, and the slow secondary particles from the aforementioned process from the areas of the insulator surface 5 or 7 keep away. This is also the essential difference from the method shown in DE 69116859T2, in which the field emitter tip, if it were to be produced by electron beam deposition, would significantly reduce the insulation strength of the insulator regions.

A further advantageous feature of the structure according to FIG. If is that the remaining insulator regions 5, 7, viewed from the direction perpendicular to the surface of the substrate, are concealed and therefore do not charge by secondary processes during operation of the particle beam source, or a charge thereof Insulator areas does not interfere with the pitting emission of the particle beam source.

The method of producing the field emission tip 8 with particle beam deposition has the further advantage that the field emitter tip 8 can be positioned exactly up to a few nanometers. Only then is it possible to position the field emitter tip 8 exactly in the center of the electron-optical axis and to avoid aberrations caused by mispositioning. This distinguishes this method from other methods (eg deposition of carbon nanotubes as field emitters), which, due to the lack of positioning capability, generate large aberrations in the electron-optical design.

All conductive layers are carried on to macroscopic flat geometries (eg contact pads) of the substrate surface 2, via which electrical voltage is applied in a simple manner to said layers.

Another embodiment is shown in Figs. 2a to 2c. On the insulator surface 2 of a substrate 1 becomes produced by means of optical lithographic, electron beam lithographic, chemical and / or other suitable methods of a first conductive surface layer 3. This surface covers only a portion of the insulator surface 2 of the substrate 1. Preferably, said surface is created with a geometry having regions surrounded by as much of the surface not covered by the conductor surface as possible (eg, longer sharp corners). FIG. 2b shows a greatly enlarged top view of a possible preferred embodiment. Preferably, a bulge or sharp corner of the conductive surface with a length preferably greater than 1 μm serves as foot point 11 for the particle beam deposition of the field emitter tip 8. Analogously to FIG Ib to If, the undercut and production of the further layer is carried out. The field emitter tip 8 is excited to emit via a voltage difference between the conductive layers. This embodiment has the advantage of not requiring high-resolution structuring measures (eg electron beam lithography).

Of course, a variety of geometries are possible, sharing only the feature of initially having a conductive surface with a rim towards the insulator and being mechanically strong enough to allow sufficient undercutting of the insulator at that edge as well as being thick enough to support the insulator Block primary electron beam 9. Likewise, various embodiments of the particle beam-deposited field emitter tip 8 are possible. In particular, the field emitter tip may be generated at angles other than the angles shown in the drawings relative to the surface 2. In particular, the track 4 carrying the field emitter tip may also have a higher symmetry in order to counteract the astigmatism of the emitted particle beam.

Claims

claims

1. A method for producing a micromechanical particle beam source, which has at least one field emitter tip for the emission of particles, characterized in that the method comprises the following steps:

Applying at least one surface layer made of an electrically semiconductive or conductive material to the surface (2) of a substrate (1), the substrate (1) being electrically insulating at least on the surface,

- structuring the at least one surface layer applied to the substrate (1) in such a way that as narrow as possible, i. narrower than about 1 micron, low profile electrically conductive web, i. again less than about 1 micron is made, the web serves as a support for the later depositing a field emitter tips,

Such partial removal of lying between the webs, exposed portions of the electrically insulating substrate surface, that the resulting new electrically insulating substrate surfaces are not completely covered in the subsequent application of a further conductive layer, but that the coverage of areas of the newly formed electrically ^'insulating substrate surface by: - the overlying surface layers (3, 4) is prevented

Applying a further conductive layer (6) to the surface thus formed such that the sides of the electrically insulating substrate surfaces facing away from the substrate surface are covered, and

Application of the field emitter tip to the narrow path and exact positioning of the field emitter tip (8) with an ner accuracy of a few nm in the center of the optical axis of the micromechanical particle beam source.

2. The method according to claim 1, characterized in that the first conductive layers (3, 4) are undercut, in particular by means of an isotropic etching step.

3. The method according to claim 1 or 2, characterized in that the removal of lying between the tracks, exposed areas of the electrically insulating substrate surface by selective chemical etching in the liquid or gas phase or another comparable etching method is carried out.

4. The method according to any one of the preceding claims, characterized in that the surface layers consist of one or more superimposed layers.

5. The method according to any one of the preceding claims, character- ized in that additional adjacent, separated by an insulator surface tracks or other geometric shapes of metal are produced, which serve as additional electrodes for extraction, for focusing, or for particle beam shaping.

6. The method according to any one of the preceding claims, characterized in that the webs by means of lithographic methods such. B. electron beam lithography are produced.

7. The method according to any one of the preceding claims, characterized in that the conductive tracks (3, 4) as predetermined geometric shapes on a planar insulator, in particular one made of silicon dioxide, are applied.

8. The method according to any one of the preceding claims, characterized in that a further process step for further removal of the insulator surface and thus to increase the insulation is carried out insulation. (Fig. Ie).

9. The method according to any one of the preceding claims, characterized in that a further conductive layer (6), which may consist in particular of several layers, is applied by directional vapor deposition.

10. The method according to any one of the preceding claims, characterized in that the conductive structures cover all insulator surfaces from the vacuum side and thereby charges of these insulator surfaces by the web (4) outgoing particle beam (9) or caused by the particle beam secondary processes are prevented.

11. The method according to any one of the preceding claims, characterized in that the extraction structure is undercut with a further undercut step.

12. The method according to any one of the preceding claims, character- ized in that the induced by the emitter tracks astigmatism of the beam is minimized by suitable Emitterflächengeometrie or even eliminated.

13. The method according to any one of the preceding claims, characterized in that the emitter comprises carbon, and further a Verasch-step is performed, with which a thin parasitic film is removed.

14. The method according to any one of the preceding claims, characterized in that during the Teilchenstrahldeposition to the mutually electrically isolated layers (3, 4 and 6) different electrical voltages are applied.