WO1993005530A1

WO1993005530A1 - Production of fine points on a substrate

Info

Publication number: WO1993005530A1
Application number: PCT/GB1992/001596
Authority: WO
Inventors: Pippa Howard
Original assignee: Era Patents Ltd.
Priority date: 1991-09-02
Filing date: 1992-09-01
Publication date: 1993-03-18
Also published as: GB9118721D0

Abstract

A method of forming a fine point or spike or array of fine points or spikes on a substrate (5) in which an electron beam (6) is focussed onto the substrate (5) at each point that a fine point or spike is required. The substrate (5) is positioned within a low pressure atmosphere, the atmosphere containing a precursor of the material for example a hydrocarbon or organo-metallic gas which forms the fine point or spike. The focussed electron beam (6) causes the deposition of material onto the substrate (5) to form the fine point or spike. An array of such fine points or spikes can be used as a field-emitter cathode surface in vacuum electronics.

Description

Production of fine points on a substrate

TECHNICAL FIELD

This invention relates to a method for producing a fine point or sharp spike on a substrate. Such points or spikes are usually arranged in an array on a substrate and can be used as a field-emitter cathode surface in vacuum electronics. Such points or spikes also have applications in lamps, displays, regulators and switching devices.

BACKGROUND ART

Conventionally, arrays of fine points are produced by etching methods such as wet-etching, ion-beam etching or selective etching of metal oxide-metal or metal carbide- metal composites. Wet-etching and ion-beam etching require the preliminary steps of mask production and are therefore relatively expensive. Wet-etching also has inherent problems with undercutting where material is etched away from under the protective mask, thereby reducing the potential packing density of the points. Selective etching methods are complex because they require the substrate to be made from special materials leading to increased cost.

SUMMARY OF THE INVENTION According to this invention, a method of producing a fine point or spike on a substrate comprises placing the substrate in a chamber containing an atmosphere at low pressure including a precursor of a species to be deposited to form the point or spike, irradiating a sharply focused electron beam on a predetermined location on the substrate to cause the deposition of the species from the atmosphere in the chamber onto the surface of the substrate, and

•maintaining the irradiation of the electron beam at the predetermined location for a period of time to build up a point or spike on the surface.

The method of the present invention makes it possible to produce spikes in a process involving fewer steps and offering far greater flexibility by comparison wit conventional techniques. It is possible with the method o the present invention to produce structures typically 1 t 3 μm tall with tips of radius 20 to 60 nm. Such structures are particularly suitable for use as field-emission (FE) needles.

Preferably the process is repeated with relative movement between the electron beam and the surface to produce an array of points or spikes, the centres of which may be separated by as little as 0.3 μm. Preferably the electron beam is blanked out whilst the relative movement occurs between the production of each point or spike. This ensures that the species is only deposited at the location where the points or spikes are required. A plurality of electron beams may be used simultaneously to produce a plurality of points or spikes.

The method of the present invention is particularly suitable for producing an array of points. The method of production means that the configuration of the array is not limited to that of a predetermined mask, as with the prior art techniques. At the same time, it is found that the method produces spikes with a particularly uniform tip radius.

Preferably the precursor of the species to be deposited is either a hydrocarbon or an organo-metallic gas, and the substrate is either a metal or semiconductor. More preferably the substrate is a metal of relatively low conductivity such as Mo or Cr. The height of the points or spikes may be increased by increasing the period of time of the irradiation of the electron beam at the predetermined location on the substrate.

Where the points or spikes have a considerable extent the focus point of the electron beam may be moved away from the surface as the point or spike is built-up so that the electron beam is always in sharp focus on the surface on which the species is being deposited. The electrical conductivity of the points or sharp spikes may be improved by depositing a thin film of metal such as platinum or zinc aluminium onto them once formed, and they may be made more durable by depositing a thin film of a durable material. Ion-beam or wet-etching can be used to improve the sharpness of the points after they have been formed in accordance with the present invention.

A suitable device for forming a point or sharp spike in accordance with the present invention is a scanning electron microscope, which has the advantages of being able to monitor the degree of focus and the beam current and can also image the final array of points or sharp spikes.

DESCRIPTION OF THE DRAWINGS

A method for producing a fine point or sharp spike on a substrate in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows an apparatus for producing a point or spike; Figure 2 shows a plan view of an array of points or spikes;

Figure 3 shows the profiles of points grown on different substrates;

Figures 4a and 4b are TEM micrographs; and Figure 5 shows a profile of a needle grown for a field emission device.

DESCRIPTION OF AN EXAMPLE

Figure 1 shows an apparatus which is used for producing points or sharp spikes on a substrate. The substrate which is a metal or semi-conductor mounted on a stage in a chamber containing a precursor of a species to be deposited in a low pressure atmosphere. In the present example, the precursor is a residual gas left after evacuation of the chamber. The precursor may be for example an organo-metallic or hydrocarbon gas. An electron beam 6 is emitted from an electron source 1 towards the substrate 5. The beam is focused by a lens focusing system 2a, 2b positioned by a deflecting means 3 to pass through an aperture 4 and onto the substrate 5. By focusing a beam for a predetermined period on a stationary position on the substrate a point or sharp spike is produced. The height of the point or spike may be increased by increasing the time that the beam is incident upon the substrate. The precise operating conditions of the apparatus will determine the aspect ratio (diameter of base/height) of the tip. When one point has been formed, the beam or substrate is moved so that the electron beam irradiates a different location on the surface of the substrate and therefore an array of points or sharp spikes is produced as shown in Figure 2. The relative position of the beam and substrate is varied either by deflecting the electron beam although alternatively this may be done by moving the stage on which the substrate is mounted. This can be achieved by a computer control system, which is also able to blank the beam to prevent the whole surface of the substrate from being irradiated, either by a deflection means or a blocking means, when the beam is being repositioned. When using a scanning electron microscope to irradiate the substrate, the array of points or sharp spikes produced can be inspected by scanning the substrate with the microscope in the secondary electron or imaging mode.

In a first example in which points or sharp spikes are formed on an aluminium substrate, the substrate is first degreased in acetone and placed on a stage in a scanning electron microscope. The scanning electron microscope is set to have an electron beam voltage of 20 kv, the chamber is evacuated to 10 -4 to 10-6 Torr, the final aperture has a diameter of 50 μm and the working distance is 27 mm. By irradiating the surface of the substrate for about 50 seconds, a point is formed having a diameter of about 400 nm. Optimum results have been found to be obtained using such a 20 kv electron beam with a 3 picoamp probe current (that is the current flow to the substrate) , a spot diameter of 200 A and a beam divergence having a semi-angle of 3 milliradians. Acceptable results are obtained with probe currents of 1 to 30 pA, beam diameters of 100 to 400 k and a beam divergence of 2.7 - 4 milliradians. The time taken to grow a point is varied according to the size required, but is typically in the range 20 to 100 seconds. Prolonging this up to, say, 400 seconds results in further growth of the point. Other metal substrates have been tested and it has been found that the best spikes having the highest ratio of height to tip radius are produced on substrates of molybdenum or chromium. The tip profiles obtained with a number of different substrates are shown in Figure 3 where it will be seen that in tips of acceptable size are produced using Ti, W, Zr, or Fe. Copper and gold produce the worst results. The height of the tip grown on Mo is 760 nm, and the other tips are shown on the same scale.

The present inventor has found that cleaning of the substrate surface prior to deposition has a marked effect on the quality of the structure produced. In the example shown in Figure 3, acetone cleaning was used but still better results are to be obtained if the substrate is plasma-cleaned prior to the deposition of the spike. It is found, for example, that after cleaning in an argon glow discharge, copper is a better substrate than aluminium.

Figures 4a and 4b are TEM micrographs showing spikes deposited on the edge of an Aluminium TEM grid. Figure 4b shows the structures in profile. Examination of the needles under a field-emission SEM show that the needle is not entirely homogenous but has a fine structure of the order of 3-5 nm.

The present invention is well suited to the production of field emission devices. As shown in Figure 5, an insulating layer 10, typically 1 to 2 μm thick, is provided on a substrate, and a thin metal layer 11 is provided on the insulator. A window, typically of 2 μm diameter, is provided through the insulator, preferably undercutting the thin metal layer 11. A needle is grown by the method of the present invention on the substrate within the window.

Claims

1. A method of producing a fine point on a substrate (5) comprising placing the substrate (5) in a chamber containing an atmosphere at low pressure including a precursor of a species to be deposited to form the point, directing a sharply focused electron beam (6) on a predetermined location on the substrate thereby causing the deposition of the species from the atmosphere onto the surface of the substrate, and maintaining the irradiation of the electron beam (6) at the predetermined location for a period of time to build up a point on the surface.

2. A method according to claim 1, further comprising moving the electron beam (6) to a second position in the vicinity of the first position to form a second point.

3. A method according to claim 2, further comprising blanking the electron beam (6) as it is moved between the first and second locations.

4. A method according to claim 2 or 3, further comprising forming a plurality of such points in a regular array on the substrate (5) .

5. A method according to any one of the preceding claims, in which the precursor of the species to be deposited is a hydrocarbon or an organo-metallic gas.

6. A method according to any one of the preceding claims, in which the substrate is a metal.

7. A method according to claim 6, in which the metal is copper, chromium or molybdenum.

8. A method of manufacturing a field-emission device including the step of forming a fine point for use as a field-emitter by a method according to any one of the preceding claims.

9. A method according to claim 8, further comprising providing an insulator layer (10) on the substrate and a thin metal layer (11) on top of the insulator, etching a window through the insulator (10) to the substrate, and subsequently forming the point in the window.

10. A method according to any preceding claim, in which the precursor species comprises one or more residual gases left in the chamber after evacuation of the chamber to a pressure of 10 -4 Torr or less.

11. A method according to any preceding claim, in which the chamber forms part of an electron microscope.

12. A method according to any of the preceding claims, in which the probe current is in the range 1 to 30 pA.

13. A method according to any of the preceding claims, in which the beam diameter is in the range 11 to 40 nm.

14. A method according to any of the preceding claims, in which the beam divergence is in the range 2.7 to 4 milliradians.

15. A method according to any of the preceding claims, in which the beam irradiates the substrate (5) for 20 to 100 seconds.