Patent Application
20080044647
The above object and other objects, features, and advantages of the present invention will become apparent more easily from the following detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings.
10 . . . diamond substrate; 11, 16 . . . resist; 12 . . . hole; 13 . . . metal; 14 . . . deposition; 15 . . . silicon nitride film or silicon nitride oxide film
The teaching of the present invention can be readily understood in view of the following detailed description with reference to the accompanying drawings presented by way of illustration only. Subsequently, embodiments of the present invention will be described with reference to the accompanying drawings. When possible, parts identical to each other will be referred to with symbols identical to each other.
A method of forming a carbonaceous material projection according to an embodiment of the present invention will be described. As shown in part (a) of
Thereafter, as shown in part (b) of
After that, diamond is etched using a mask consisting of the deposition of the circular cone or circular truncated cone (see part (a) of
For example, the mask material can be made of Au, Mo, W, and so on. However, the etching of diamond needs to use a gas containing oxygen. In the use of a gas containing a large amount of Ar or N2, the content of Ar or N2 (flow rate) should preferably be less than 10% (e.g., oxygen: 100 sccm, Ar or N2: 10 sccm) because the mask material is etched to become small soon, the selectivity cannot be large, and the angle of the projection cannot be a small value of not more than 45 degrees.
A resist for lift-off can be formed directly on the diamond, but a layer of metal or the like for undercoat may be formed between them. Use of this undercoat layer is effective in avoiding charge-up because of low conductivity of the diamond. It is important to avoid charge-up in order to obtaining high accuracy of microprocessing. Use of the undercoat layer might improve adhesion between the diamond and the projection material for the mask. However, the undercoat metal has to be removed after the formation of the projections.
Accordingly, it is needed to meet the condition that the undercoat material has to be removed without removing the mask material. This condition is satisfied by use of Au for the mask material and Ti or Mo for the undercoat material. It is preferable to use a good chemical resistant material like Au or Pt for the mask material. Since the mask material is formed by lift-off, it can be basically made of usual materials.
The mask material can also be a silicon oxide (e.g., SiO2 or SiOx), a silicon nitride oxide (e.g., SiOxNy), or a silicon nitride (e.g., SiNx). A preferred method of forming these mask materials is an EB evaporation method or the like. Preferred ranges for the subscripts are shown as follows: x in SiOx is 0<x<2; x and y in SiOxNy are x=z×x1 and y=(1−z)×y1, 0<x1<2, 0<y1<1.33, and 0<z<1; x in SiNx is 0<x<1.33.
A method of forming the present projection mask will be described below with specific examples of the mask material. As shown in part (a) of
There is no precedent study on using the metal 13 as a mask for etching of diamond or the like. The mask including an extremely pointed projection(s) is suitably applicable for forming a diamond projection(s) below it. In addition, it is important that the angle of the projection of the diamond be controllable through adjustment of the selectivity even if the projection angle is small.
The diamond was etched in oxygen gas containing a small amount of CF4 and using a mask with an array of microprocessed projections, to form projections each having a tip of a size smaller than the pattern size of the mask (see part (b) of
The size of the present diamond projections is dependent on the pattern size of the mask, and the size of projections can be reduced as small as a few micrometers. However, it should be noted that a projection having its maximum diameter equal to or less than 300 nm can be formed. The reason will be described below.
Supposing a mask with its tip of not more than 100 nm should be formed by etching a mask of 1 μm size, the pattern size accuracy would be degraded gradually during the etching of the mask. Even if the etching accuracy were better than 10%, there would arise variation equivalent to that of the tip diameter. For obtaining the accuracy better than 5%, it is necessary to achieve the etching accuracy better than 0.5%. This is difficult in practice.
On the other hand, when the pattern size of the mask is equal to or less than 300 nm, the variation is 30% for formation of the tip of 100 nm even if the etching variation is 10%; accordingly, the variation of 3% can be realized when the etching variation is 1%. A desired projection can be realized. Therefore, the pattern of the mask for the present emitter is equal to or less than 300 nm.
When diamond is etched using the mask having pattern of a projection shape as described above to form a diamond projection, its pointed tip can be extremely thin, and its location accuracy is extremely high even though the total width thereof is smaller than 300 nm.
Since the aspect ratio (height/diameter) of the projection mask relates to the etching selectivity, it can be used as a mask even if the aspect ratio thereof is low. The aspect ratio of the diamond projection formed is preferably equal to or more than 2.
Projections of small size were actually formed as follows: diamond projections were formed with a mask in the diameter of 150 nm, and the projections formed had the thickness of 150 nm and the tip diameter of not more than 10 nm. These fine projections were formed in the density of 4 projections per 1 pm square.
In the present method, since the selectivity of the diamond and the projection material can be large, the projection with an extremely high aspect ratio can be formed. This method is a technology capable of forming the extremely small pattern size of the mask of not more than 100 nm, and thus, even if the etching is stopped prior to removing the mask by the etching and the mask is left at the tip of the diamond projection formed, a diamond column as the diamond projection can be formed and has an extremely small tip, the tip is not more than the size of the mask pattern, and the variation of height can be made as small as the surface roughness of the substrate.
Next, the reason why it is important to make the emitter density higher will be described below. When projections having the diameter of 1 μm are formed, the emitter height is at least 2 μm or more, and the emitter density is at most 250,000 emitters/mm2 in consideration of the projection spacing. Supposing an electric current stably emitted from each emitter is 0.1 μA, the current density is estimated to be 25 mA/mm2. This value is small, when compared with the emission current density of approximately 100 mA/mm2 from the conventional emitters using a hot-cathode material.
However, where the projections of the diameter of not more than 300 nm are formed, the projection spacing can be made as small as about 500 nm and the emitter density can be raised to 1,000,000 emitters/mm2 or more. Then, the current density is estimated to be 2 equal to or more than 100 mA/mm , and it becomes feasible to form devices the current density in which is not less than that in the conventional emitters. Accordingly, forming the emitters in the density of four or more in 1 μm square leads to capability of providing significant devices on reaching the above density. There was nothing proposed heretofore about the method of forming the emitters in such a density. Furthermore, if the diameter of the projections is not more than 200 nm, the density is estimated to be not less than 6.25 projections in 1 μm square and it is clearly effective.
Here the diamond emitters refer to projections with a high aspect ratio arrayed regularly. The reason is as follows. In the case of emitters of a structure using gate electrodes, projections with a low aspect ratio of not more than one have a large capacitance between the projections and the substrate and the large capacitance hinders high-frequency operation of the emitters. In the case of the emitters the gates of which are fabricated by self-alignment, it becomes difficult to make holes around the emitters if the aspect ratio of the projections is low. It is also undesired to use emitters where the projections are located at random because these random emitters cannot avoid local concentration of current (does not allow the design of uniform current distribution), and this is a hindrance to providing emitters with emitting of a large amount of current.
The emitters as described above can be obtained by processing diamond by dry etching using oxygen with the mask of the present embodiment.
In order to realize the fine projections, the substrate needs to have extremely good flatness. The desired flatness can never be achieved by as-deposited polycrystalline diamond. It is understood from the size, 1 μm, of projections to be formed that the flatness needs to be smaller than 100 nm. Since the projections having the size of 200 nm can be formed, the flatness preferably needs to be smaller than 20 nm. Since it is possible to obtain a substrate which has the flatness of several nm, use of this substrate is useful therefor. The substrate with such flatness can be more readily formed from a single crystal, but it may be made of polycrystalline.
Furthermore, each projection itself is preferably made of a single crystal because carriers need to reach the tip of the projection column without annihilation. The single crystal is preferable rather than polycrystals for the projections of the sizes heretofore, and this reason is that grain boundaries of crystal should not be included in the projections. However, since the projections of the present invention are very small, the substrate may be made of a polycrystalline one. For the above reason, grain sizes of the poly crystal are preferably larger than sizes of the projections. For the same reason, a highly oriented polycrystalline diamond is more desirable than a randomly oriented polycrystal except for the single crystal substrate, and a heteroepitaxial substrate is much more preferable than the highly oriented polycrystalline diamond.
The above description can also apply to carbonaceous materials with extremely high flatness (flatness of not less than 100 nm, preferably, not less than 20 nm), e.g., DLC, a-CN film, a-C film, CNT/SiC, and graphite/SiC, in addition to the diamond substrates.
The usefulness of forming the fine projections in the size of 300 nm and in the high density of not less than 4 projections per 1 μm square has been described with reference to a forming method using the lift-off. Subsequently, we will describe a method of forming a carbonaceous material projection in which a mask is formed by etching process without the use of a lift-off process.
First, as shown in part (a) (plan view) of
After that, as shown in part (c) (plan view) of
Thereafter, the diamond substrate 10 is etched using a mask including the dots formed by etching the film 15. This results in forming conical small projections of diamond (carbonaceous material projections) (see part (f) of
The Inventors have further found that a fine, acutely pointed projection is formed if the following two conditions are met: an extremely fine mask can be formed with high accuracy; etching of the carbonaceous material can be controlled in the vertical lateral directions of the mask material in etching the carbonaceous material (diamond in the present embodiment). Then the Inventors have found that silicon nitride film (SiNx film) or silicon nitride oxide film (SiOxNy film) can be used for diamond as material of the mask satisfying the above two conditions. According to experiments by the Inventors, the silicon oxide film (SiOx film) failed to satisfy the two conditions.
The Inventors presumes that the above silicon nitride film and silicon nitride oxide film satisfy the two conditions because advantageously acting factors are as follows: they have a low transmittance for ultraviolet light; and they are amorphous. Namely, the photolithography technology utilizes sensitivity to ultraviolet light to perform the fine patterning, and if the mask material is transparent to ultraviolet light, the ultraviolet light will leak to adjoining regions, to degrade the accuracy of patterning. In contrast to it, the silicon nitride film and silicon nitride oxide film have a low transmittance for ultraviolet light, so that the patterning can be performed with high accuracy. Since the silicon oxide film is transparent to ultraviolet light, it is difficult to implement a fine patterning.
In addition to the above, other important conditions for the mask material are as follows: an etching selectivity to the diamond is obtained (i.e., the etching rate of the mask material is smaller than that of diamond); a high etching selectivity to the resist is obtained (i.e., the etching rate of the mask material is sufficiently larger than that of the resist). SiO2 as a mask material for diamond satisfies the former condition but fails to satisfy the latter condition. On the other hand, the Inventors have discovered that the silicon nitride film and silicon nitride oxide film of the aforementioned compositions satisfy these conditions and can be used as very good masks.
A further condition is that amorphous material is used for the film for the mask material. When the mask material is crystalline, the etching rates for the mask become anisotropic, and it becomes difficult to uniformly form the size and shape of the mask pattern. Another reason therefor is that crystalline materials contain grain boundaries therein. When the mask material is amorphous, the mask can be patterned in uniform size and shape because of no grain boundaries. The silicon nitride film and silicon nitride oxide film of the aforementioned compositions satisfy the above condition.
As described above, the silicon nitride film and silicon nitride oxide film of the foregoing compositions are advantageous in that they can be dry-etched using a resist mask. The Inventors have found the condition for enabling the etching of the silicon nitride film and silicon nitride oxide film of the foregoing compositions in the oxygen gas containing CF4, and these silicon nitride film and silicon nitride oxide film satisfy the condition that the film can be etched using the resist mask. The patterning of resist may be performed by photolithography technology or electron beam lithography technology. With either of these technologies, it is feasible to make the pattern size of the resist mask uniform and to form a desired array of patterns, and thus the silicon nitride film and silicon nitride oxide film are useful. On the other hand, the SiO2 film does not provide a large selectivity of the resist and SiO2 film even by use of dry etching (i.e., the etching rate of the SiO2 film is not sufficiently larger than that of the resist). Therefore, it is infeasible to pattern it except for very thin films. In addition, it is infeasible to form a mask of circular cylinder shape. According to the Inventors' experiment, such masks cannot be used to form no acute carbonaceous material projections.
Most of metals can be etched using a resist mask, and thus satisfy the above condition (a large selectivity of the metal and resist is secured), but these metals fail to satisfy the condition described below.
In the etching process using a mask consisting of the silicon nitride film and silicon nitride oxide film of the foregoing compositions, the mask is laterally etched in etching the carbonaceous material such as diamond, and the shape of the mask is changed into circular cone shape or circular truncated cone shape. Therefore, the projections of diamond can be processed into circular cone shape or circular truncated cone shape. Since the etching rate of the mask is not larger than that of diamond, the diamond projections can be formed in the conical shape with an acute angle at its tip. The apex angle and the height of the projection can be controlled through adjustment of etching conditions. As described above, the Inventors' experiment has found that with the use of the silicon nitride film or silicon nitride oxide film, even if an initial shape of the mask is of a circular cylinder shape, the mask itself is laterally etched during the etching of the diamond and the diamond projection is formed in conical shape. In contrast to it, masks of many metal materials are not etched laterally during the etching of diamond, and the diamond projections have a circular cylinder shape by the production process shown in
When the mask of SiN was used in practice in formation of diamond projections of small size, the following diamond projections were formed as described later in Examples: the tip diameter is not more than 40 nm; the vertex angle is not more than 32 degrees; the height uniformity of projections is not more than ±3%. Then such small projections were formed in the density of four projections in 1 μm square. When the mask of SiON is used, the following diamond projections were formed as described later in Examples: the tip diameter is not more than 50 nm; the apex angle is not more than 39 degrees; the height uniformity of projections is not more than ±5%. Such small projections were formed in the density of 4 projections in 1 μm square.
As described above, even with the mask of cylindrical shape, the carbonaceous material projection production method by use of the mask consisting of the silicon nitride film or silicon nitride oxide film is able to form the fine diamond projections satisfying the following: excellent locational and dimensional accuracy; extremely acute tip. Since an etching rate ratio between the mask and diamond can be controlled through the adjustment of etching conditions in the etching of diamond, the vertex angles of cones of diamond projections can be controlled using a mask having a side inclined with an arbitrary angle. According to the Inventors' experiment, it was not easy to etch SiO2 with the use of the resist mask and when it was forcedly etched, the inclination of the mask side of SiO2 became too large. It is hard to use such diamond projections for emitters and this reasons are as follows: the lateral etching rate to diamond is too large; because of the large etching rate, large-height and small-diameter diamond projections cannot be formed; the vertex angles of the diamond projections formed are very large (e.g., 44 degrees or more). For these reasons, the silicon nitride film and silicon nitride oxide film permitting formation of diamond projections with a small vertex angle are suitable as mask materials.
A high-pressure-synthesized single-crystal diamond substrate (100) and a CVD polycrystalline diamond wafer substrate were planarized to not more than several nm by polishing to prepare substrates, the prepared substrates were coated with resist for electron beam lithography, electron beam exposure was conducted, and holes of inversely tapered shape were formed (cf. part (a) of
Since projections of the masks were formed in similar shape, the diamond projections were also formed with little variation in accuracy. When etching was stopped midway with the mask being left, the projections were formed with very small variation in height (equivalent to the level corresponding to roughness of the substrate) though the tip diameter was large.
There is no difference of projections due to the difference between the substrates, the single-crystal substrate and polycrystalline wafer substrate, except that the etched surface was flatter in the single-crystal substrate.
A high-pressure-synthesized single-crystal diamond substrate (100), a CVD polycrystalline diamond wafer substrate, and a SiC substrate were planarized to not more than several nm by polishing to prepare substrates. Each of these prepared substrates was implanted with ions of inert gas or nitrogen to prepare a sample. Some of these samples were treated by high-temperature anneal (1500° C. or 1800° C.) in vacuum, and the rest of these samples and samples not subjected to the ion implantation were processed at high temperature. On a surface of each of these samples, a black layer of electrically conductivity, which may be assumed to be graphite, was formed. A patterned mask of a projection shape was formed on the samples by the same method as in Example 1. The carbonaceous material was etched using them as masks, whereby extremely acute projections of the carbonaceous material were formed as shown in part (a) of
A high-pressure-synthesized single-crystal diamond substrate (100) and a CVD polycrystalline diamond wafer substrate were planarized to not more than several nm by polishing to prepare substrates, they were coated with resist for electron beam lithography, electron beam exposure was conducted, and holes of inversely tapered shape were formed (cf. part (a) of
A high-pressure-synthesized single-crystal diamond substrate (100) and a CVD polycrystalline diamond wafer substrate were planarized to not more than several nm by polishing to prepare substrates, and a silicon nitride oxide film (SiON film) or silicon nitride film (SiN film) was deposited (in the thickness of 0.2-1.0 μm) on each of the substrates by CVD or by sputtering. Then a resist was formed on the film and patterned by photolithography or by electron beam exposure. As a sample for comparison, a silicon oxide film (SiO2 film) was formed on a substrate and a resist was formed thereon and patterned to prepare a sample. Another substrate prepared was a carbonaceous material substrate such as a carbon nano tube (CNT)/SiC substrate different from the above substrates, a SiON film was also formed on this substrate, and a resist was formed on the film and patterned. Next, using the resist mask, each of the SiON film, SiN film, and SiO2 film was etched with CF4, CHF3, C2F6, or SF6 gas to form a mask for etching of the substrate. With any one of these gases, masks having circular cylinder dots were formed well with good accuracy from the SiO film and SiN film. However, it was difficult to form circular cylinder dots with accuracy from the SiO2 film.
Then the diamond wafer substrate and CNT/SiC substrate were etched using the mask of circular cylinder dots. Using the mask made of the SiN film or SiON film, the substrate was etched by RIE (power density: 0.04-0.05 W/mm2, pressure: 1-5 Pa) with O2 and CF4 gas (CF4/O2=1%), whereby carbonaceous material projections of extremely fine, acute (the tip diameter≦50 nm) conical shape were formed as shown in parts (a) to (c) of
On the other hand, where SiO2 was used as a mask, not so sharp diamond projections were formed as shown in
Each of rows (J) to (U) in
Diamond projections were formed on a diamond substrate by a method similar to that in Example 4. In the formation, etching conditions (1) to (4) shown in
Having described and illustrated the principle of the invention in a preferred embodiment thereof, it is appreciated by those having skill in the art that the invention can be modified in arrangement and detail without departing from such principles. We therefore claim all modifications and variations coming within the spirit and scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-096008 | Mar 2004 | JP | national |
| 2004-253159 | Aug 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/05408 | 3/24/2005 | WO | 00 | 7/20/2007 |
