Method and device for producing nanotips

Abstract

A method for producing a nanotip from a tip material provides a substrate which consists of the tip material or has the material in the form of a coating, produces a mask from a mask material selected so that, in a predefined reactive ion etching process, the mask material is removed at a lower etching rate than the tip material, and carries out the reactive ion etching process in an etching chamber. The mask material is additionally selected so that a gaseous component is released therefrom during the reactive ion etching process, the gaseous component not being released from the tip material. The method further comprises detecting the gaseous component while the ion etching process is being carried out, repeatedly determining whether an amount of the gaseous component in the etching chamber reaches a predefined lower threshold, and stopping the reactive ion etching process when the lower threshold is reached.

Claims

1. A method for producing at least one nanotip that comprises a tip having a radius of less than 10 nm from a material, hereinafter called a tip material, said method comprising the steps of: providing a substrate which consists of the tip material or comprises said tip material in the form of a coating; producing a mask from a mask material, wherein the mask material is selected so that, in a predefined reactive ion etching process, the mask material is removed at a lower etching rate than the tip material, carrying out the reactive ion etching process in an etching chamber, additionally selecting the mask material such that a gaseous component is released from the mask material during the reactive ion etching process, said gaseous component not being released from the tip material during the reactive ion etching process, detecting the gaseous component while the ion etching process is being carried out, repeatedly determining during the ion etching process whether an amount of the gaseous component in the etching chamber reaches a predefined lower threshold, said predefined lower threshold set to substantially no detection of the gaseous component, and stopping the reactive ion etching process as soon as the lower predefined threshold is reached.

2. The method according to claim 1, wherein the gaseous component is detected by means of a detection method which uses a gas chromatograph or a spectrometer.

3. The method according to claim 1, wherein the predefined lower threshold is a detection limit of the gaseous component in the detection method being used.

4. The method according to claim 1, wherein for a given tip material and mask material, the ratio of the etching rate acting on the mask material to the etching rate acting on the tip material is set to 1:5.

5. The method according to claim 1, wherein the predefined lower limit set to substantially no detection of the gaseous component is a value greater than a lower detection limit of the gaseous component in the detection method used, so that ions that are still available in the etching chamber for a short period time delay after stopping the reactive ion etching process are accounted for in the method.

6. The method according to claim 1, wherein producing the mask comprises the following steps: applying a layer of the mask material directly on the tip material forming the substrate; applying an anti-reflection layer on the mask material; applying a photoresist layer on the anti-reflection layer; carrying out a photolithography process to structure the photoresist such that the photoresist remains only at those places where at least one nanotip having a radius of less than 10 nm is to be formed later in the process; carrying out a selective etching process to expose the substrate only at those places that are not covered by photoresist; and carrying out a selective etching process to remove all the layers except for the mask material and the substrate.

7. The method according to claim 6, wherein the layer of the mask material is applied directly onto the tip material by means of plasma-enhanced chemical vapor deposition.

8. The method according to claim 1, wherein the mask material is an oxide and that the gaseous component is oxygen.

9. The method according to claim 8, wherein the mask material is SiO.sub.2 and the tip material is Si.

10. The method according to claim 8, wherein the mask material is SiO.sub.2 and the tip material is a metal, and wherein the SiO.sub.2 is produced at a temperature lower than the melting point of the metal.

11. The method according to claim 1, wherein the tip material is selected from the group Si, W, Ta, Nb, Mo.

12. The method according to claim 11, characterized in that the mask material is an oxide and that the gaseous component is oxygen.

13. The method according to claim 12, wherein the mask material is an SiO.sub.2 and the tip material is Si.

14. The method according to claim 12, wherein the mask material is SiO.sub.2 and the tip material is a metal, wherein the SiO.sub.2 is produced at a temperature lower than the melting point of the metal.

15. The method according to claim 14, wherein the gaseous component is detected by means of a detection method which uses a gas chromatograph or a spectrometer.

16. The method according to claim 15, wherein the predefined lower threshold is a detection limit of the gaseous component in the detection method being used.

17. The method according to claim 16, wherein for a given tip material and mask material, the ratio of the etching rate acting on the mask material to the etching rate acting on the tip material is set to 1:5.

18. The method according to claim 17, wherein producing the mask comprises the following steps: applying a layer of the mask material directly on the tip material forming the substrate; applying an anti-reflection layer on the mask material; applying a photoresist layer on the anti-reflection layer; carrying out a photolithography process to structure the photoresist such that the photoresist remains only at those places where at least one nanotip that comprises a tip having a radius of less than 10 nm is to be formed later in the process; carrying out a selective etching process to expose the substrate only at those places that are not covered by photoresist; and carrying out a selective etching process to remove all the layers except for the mask material and the substrate.

19. The method according to claim 18, wherein the layer of the mask material is applied directly onto the tip material by means of plasma-enhanced chemical vapor deposition.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Further variants shall now be explained with reference to the drawings, in which:

(2) FIG. 1 shows an example, already described in the foregoing, of a prior art method for producing nanotips;

(3) FIGS. 2a-2d show a schematic view of one variant of the method according to the invention for producing nanotips;

(4) FIGS. 3a-3d show a schematic view of a method for producing a mask on a silicon wafer, to be performed before the steps in FIGS. 2a-2d;

(5) FIG. 4 shows a schematic block diagram of a variant of a method according to the invention.

DETAILED DESCRIPTION

(6) FIGS. 2a-2d show a schematic view of one variant of the method according to the invention.

(7) FIG. 2a shows a schematic view of a silicon wafer 210 with a mask 220 made of SiO.sub.2 applied thereon. Silicon wafer 210 with applied mask 220 made of SiO.sub.2 is used as a starting point for an RIE process described below. In one variant of the method according to the invention, silicon wafer 210 is highly p-doped.

(8) Metallic nanotips can also be produced with the aid of the method according to the invention, for example metallic nanotips made of W, Ta, Nb or Mo. The starting point in that case is generally a silicon substrate with a coating of the respective metal, the layer thickness of the coating being equal at least to the intended final height of the nanotips. Masks of silicon oxide or SiO.sub.2 can likewise be used to produce metallic nanotips, but are preferably deposited at lower temperatures than when producing silicon nanotips, in order to take account of the respective melting points. Oxide can be successfully deposited at relatively lower temperatures by using a PECVD (plasma-enhanced chemical vapor deposition) method, for example.

(9) Wafer 210 is placed into an etching chamber and subjected to an RIE process in which, in the variant of the method shown here, a gas mixture consisting of SiCl.sub.4+Cl.sub.2+N.sub.2 is used.

(10) As shown in FIG. 2b, both silicon wafer 210 and SiO.sub.2 mask 220 are etched in this case. By setting an appropriate gas mixture, the etching rate ratio of silicon dioxide to silicon is set to 1:5. This results in a truncated cone with the desired pitch being formed. When etching SiO.sub.2 mask 220, oxygen 230 is released. The oxygen 230 is detected by an oxygen sensor 240. The reactive ion etching process is carried out in a high-vacuum environment. This is achieved by continuously evacuating the etching chamber, which means that reaction products that ensue remain in the etching chamber for only a very short time. By detecting the oxygen concentration, information is thus obtained about the amount of oxygen being released just before the time of measurement. The sensor passes its measurement result to a controller (not shown here) which is configured to stop the inflow of the etching gas mixture when a predefined threshold for the oxygen concentration has been reached.

(11) FIG. 2c shows an advanced stage of the etching process. The pitch of the cone becomes steeper the longer the etching process continues. Mask layer 220 has become narrower in the course of the etching process, but is still present. As oxygen is still being released, and detected by oxygen sensor 240, etching gas continues to flow into the etching chamber.

(12) FIG. 2d shows the end of the etching process. What can be seen is silicon wafer 210 with nanotips 251 and 252 formed thereon. The mask has been completely removed, and since no oxygen is released from the Si substrate during etching, no oxygen is detected by oxygen detector 240. The predefined lower threshold has thus been reached, or the concentration has fallen below it, and the flow of etching gas into the etching chamber is stopped.

(13) FIGS. 3a to 3d show schematic views of a known sequence of production steps, as an example of a method for producing mask 220 on silicon substrate 210 as shown in FIG. 2a. As shown in FIG. 3a, a first layer 320 of thermal silicon dioxide and a second layer 360 of thermal Si.sub.3N.sub.4 are deposited on a highly p-doped silicon wafer 310. An organic antireflection coating (ARC) 370 is applied to the second layer 360. A lacquer film structured by means of photolithography is applied to this organic ARC layer 370, thus producing a positive mask, i.e., the mask is applied in the places where the nanotips are later to be produced. An SiO.sub.2 mask is likewise used to produce metallic nanotips. In this case, the SiO.sub.2 layer is applied using a low-temperature vapor deposition method, such as plasma-enhanced chemical vapor deposition (PECVD), so that the process temperatures are significantly lower than the melting point of the respective metal being used.

(14) FIG. 3b shows the layer structure after the photolithographic process for producing resist mask 380. In the next step, an etching process is carried out which results in the silicon wafer 310 being exposed wherever there is no resist mask. The result of the etching process is shown in FIG. 3c.

(15) In the last step, which results in the structure shown in FIG. 3d, comprising silicon wafer 310 and SiO.sub.2 mask 320, the remaining resist mask 380, the organic ARC layer 370 and the thermal Si.sub.3N.sub.4 360 are removed by selective etching.

(16) FIG. 4 shows a schematic block diagram of one variant of a method according to the invention, comprising a system for reactive ion etching. An etching chamber 410 of the system allows a reactive ion etching process to be carried out. A detection device 420 for a gaseous component, which is an oxygen sensor in the variant shown, is configured to detect any oxygen present in the etching chamber and to generate a signal containing information about the amount of oxygen in the etching chamber. Detection device 420 outputs this signal to a controller 430. Controller 430 is configured to start, maintain and stop a reactive ion etching process running in etching chamber 140. Controller 430 compares the information outputted from detection device 420 about the current amount of the gaseous component in the etching chamber with a predefined lower threshold, and stops the reactive ion etching process in the etching chamber when the lower threshold is reached.