RU2784410C1 - Method for manufacturing a matrix of field-emission tubular cathodes based on doped nanocrystalline diamond films - Google Patents

Abstract

FIELD: cathodes production.

SUBSTANCE: invention relates to a technology for the manufacture of pointy field emission cathodes based on nanocrystalline (NCD) diamond films with high emission characteristics, which are widely used in vacuum microelectronic devices. In the method for manufacturing a matrix of field-emission point cathodes based on NCD films, each cathode is made in the form of a thin-walled tube with walls made of a doped NCD film by forming vertical cylindrical columns on the surface of a conductive silicon substrate using photolithography and plasma-chemical etching, then seeding the resulting structure with nanodiamond particles with a size less than 5 nm in an ultrasonic bath and deposition of a doped conductive NCD film by the NCD method in an H₂/CH₄ gas mixture with the addition of a dopant gas, further maskless anisotropic etching of the NCD film at the top of the columns using inductively coupled plasma in a mixture of O₂+Ar+He and etching of silicon in SF₆ plasma inside the pillars.

EFFECT: high degree of accuracy in the reproduction of geometric characteristics in the manufacture of pointed tubular field emission cathodes with high emission characteristics, current stability and a long service life.

7 cl, 3 dwg

Description

The invention relates to a technology for manufacturing pointed field emission cathodes based on nanocrystalline diamond films with high emission characteristics, which are widely used in vacuum microelectronic devices, in particular, diodes and triodes, as well as devices based on them: field emission displays, vacuum microelectronic current switches, devices vacuum microwave electronics, etc.

The principle of operation of cold cathodes is based on the phenomenon of field emission; that is, the emission of electrons from the surface of a solid under the influence of an applied electric field. To increase the emission current density, the cathode surface is made rough, and materials with a low work function are used as the cathode material (in particular, coatings based on diamond films). Near the tops of the micropoints, a local increase in the electric field strength is realized, which makes it possible to obtain high field emission current densities at a relatively low voltage applied to the electrodes.

Electron field emission from microtip arrays //Vacuum. V. 82. P. 1062-1068. (2008)] детально описывается последовательность всех технологических операций, позволяющих структурировать кремниевую подложку и получить на исходной подложке регулярные массивы кремниевых микроострий, пригодных для использования в качестве автоэмиссионного катода. Недостатком микроострийных катодов на основе кремния или металла является то, что при большом эмиссионном токе бомбардировка ионами остаточного газа приводит к разрушению острия и. тем самым, к уменьшению локальной напряженности электрического поля и падению тока эмиссии. Одним из наиболее перспективных и активно исследуемых материалов для автоэмиссионных катодов являются покрытия на основе, алмазных и алмазоподобных пленок [Kang W.P., Davidson J.L.. Wisitsora-at A., Kerns D.V., Kerns S. Recent development of diamond microtip field emitter cathodes and devices // J. Vac. Sci. Technol. B. V. 19. №3. P. 936-941, (2001): O.A. Ivanov, S.A. Bogdanov, A.L. Vikharev. et. al., Emission properties of undoped and boron-doped nanocrystalline diamond films coated silicon carbide field emitter arrays. J. Vac. Sci. Technol. В 36(2), 021204 (2018)].Cathodes for field emission electronics, as a rule, are regular arrays of micropoints (pillars, pyramids, cones; on silicon) created by well-known and widely used in microelectronics methods of photolithography, etching, deposition through a mask, etc. in [Takeuti DF. Tirolli MN, Danieli CL. de Faria PHL Fabrication of silicon field-emission arrays using masks of amorphous hydrogenated carbon films electronics // Microelectron. JV 38. P. 31-34, (1996): Cichy B. ,

Electron field emission from microtip arrays //Vacuum. V. 82. P. 1062-1068. (2008)] describes in detail the sequence of all technological operations that make it possible to structure a silicon substrate and obtain regular arrays of silicon micropoints suitable for use as a field emission cathode on the original substrate. The disadvantage of microtip cathodes based on silicon or metal is that at a high emission current, bombardment with residual gas ions leads to the destruction of the tip and. thereby, to a decrease in the local electric field strength and a drop in the emission current. One of the most promising and actively studied materials for field emission cathodes are coatings based on diamond and diamond-like films [Kang WP, Davidson JL. Wisitsora-at A., Kerns DV, Kerns S. Recent development of diamond microtip field emitter cathodes and devices // J. Vac. sci. Technol. BV 19. No. 3. P. 936-941, (2001): OA Ivanov, SA Bogdanov, AL Vikharev. et. al., Emission properties of undoped and boron-doped nanocrystalline diamond films coated silicon carbide field emitter arrays. J. Vac. sci. Technol. B 36(2), 021204 (2018)].

There is a known method for manufacturing pointed diamond cathodes (patents US 6132278 and US 7256535) by a two-stage method, when a matrix of depressions in the form of inverted pyramids is created on a silicon wafer, and then they are overgrown in a microwave gas discharge plasma by deposition of a diamond film. Then silicon is etched and cathodes are formed on the substrate in the form of pointed diamond pyramids several microns high. The disadvantage of such cathodes is the high requirements for the pyramid tip (the radius of its curvature), which determines the field amplification factor, which are not always possible to withstand in such a manufacturing method, the dull area of the emitting surface (sharp tip) and the relatively low quality of the film on the surface of the pyramids, associated with The fact that the surface actually represents a transition layer from silicon to diamond can therefore contain silicon carbide, which is formed at the initial stage of film growth.

A known method for the manufacture of multipoint field emission cathodes (patent RU 2486625 C2, publ. 27.06.2013, bull. No. 18), in which when performing the proposed sequence of technological processes, including polishing, cleaning, the surface of the carbon substrate, using photolithography and thermochemical etching, creates a monolithic carbon structure with a given height of microsized columns, which, in turn, are subjected to group nano-sized sharpening in a low-temperature plasma of an RF discharge in oxygen or in an oxygen-inert gas medium, to obtain a periodic matrix of equally high tips of monolithic carbon. The disadvantage of such cathodes is their low degradation resistance compared to cathodes based on diamond-like materials.

To increase the electric field amplification factor and thereby reduce the operating voltage, diamond films are deposited on pointed structures, which are obtained by anisotropic etching of silicon wafers using a mask coating.

There are known methods (patents RU 2653843, publ. 05/15/2018, Bull. No. 14, and RU 2654522, publ. 05.21.2018, Bull. No. 15) for manufacturing field-cathodes based on a silicon microstructured columnar structure with a relatively low aspect ratio formed by the reactive method. ion etching of a planar substrate coated with a field emission film of nanocrystalline carbon (nanodiamond) or nanodiamond powder. At the same time, the manufacture of a matrix of a multipoint field emission cathode is carried out on single-crystal silicon plates in a microwave gas discharge plasma by deposition from ethanol vapor, using the phenomena of self-organization and structuring of submonolayer carbon coatings into nanoisland formations.

In the article [T.K. Ku. S.H. Chen. CD. Yang, et. al., Enhanced electron emission from phosphorus- and boron-doped diamond-clad Si field emitter arrays. Thin Solid Films 290-291, 176-180, (1996)] describes a diamond-coated pointed cathode and a method for manufacturing the same. The cathode had the shape of a pyramid with dimensions of 3×3 μm (the height and length of the base of the pyramid) and was made by etching a single-crystal silicon wafer (100) through a preformed mask. Then, a polycrystalline diamond film was deposited on the cathode surface using a plasma-chemical microwave reactor by the CVD method. For deposition, a gas mixture of H ₂ /CO ₂ =18/30 was used at a pressure of 25 Torr. To dope the films with phosphorus or boron, additions of trimethyl phosphate or trimethyl borate were introduced into the mixture. The deposited films completely covered the pyramid, covering the tip and changing its shape.

Closest to the proposed technical solution, a method for manufacturing a field emission cathode is proposed in the article [J. Vacuum Science & Technology B 28, 3 016 (2010)] - (prototype). The article describes a field emission cathode based on silicon pillars of square cross section (10 × 10 μm) and up to 60 μm in height, formed by reactive ion etching of a planar substrate preliminarily coated with a nanocrystalline diamond (NCD) film using the CVD method. At the same time, at the first stage, by applying a thin aluminum layer and etching it, a microstructure is formed on the NCA film, which, in turn, serves as a mask for etching the surface of the silicon substrate. As a result, the resulting structure is an array of columns with a high (more than 5) aspect ratio, and NCA with a film covering only the flat top of the column, which is not enough to obtain a field cathode with high emission characteristics.

The technical result of the proposed solution is a high degree of accuracy in the reproduction of geometric characteristics in the manufacture of pointed tubular field emission cathodes with high emission characteristics, current stability with a long service life, suitable for use in vacuum electronics devices.

The technical result is achieved that in the method of manufacturing a matrix of field-emission point cathodes based on nanocrystalline diamond films, each cathode is made in the form of a thin-walled tube with walls made of a doped nanocrystalline diamond film by forming vertical cylindrical columns on the surface of a conductive silicon substrate using photolithography and plasma-chemical etching, then seeding the resulting structure with nanodiamond particles with a size of less than 5 nm in an ultrasonic bath and deposition of a doped conducting nanocrystalline diamond film by the CVD method in an H ₂ /CH ₄ gas mixture with the addition of a dopant gas, further maskless anisotropic etching of the nanocrystalline diamond film at the top of the columns using an inductively coupled plasma in a mixture of O ₂ +Ar+He and silicon etching in SF ₆ - plasma inside the columns.

In addition, the cathode walls are made with a thickness of 10 nm to 500 nm.

In addition, after seeding the resulting structure with nanodiamond particles, a phosphorus-doped conductive nanocrystalline diamond film is deposited.

In addition, after seeding the resulting structure with nanodiamond particles, a boron-doped conducting nanocrystalline diamond film is deposited.

In addition, after seeding the resulting structure with nanodiamond particles, a nitrogen-doped conductive nanocrystalline diamond film is deposited.

In addition, the doped conductive nanocrystalline diamond film is deposited by the CVD method in the H ₂ /CH ₄ gas mixture with the addition of a dopant gas at a gas mixture pressure of 10-100 Torr and a temperature of 400-800°C.

In addition, silicon etching in SF ₆ - plasma inside the columns is carried out to a depth of 2-4 μm.

The invention is illustrated by the following drawings.

In FIG. Figure 1 shows the technological route for manufacturing a matrix of tubular cathodes with walls from a doped NCA film: a - formation of a mask from a photoresist on a conductive silicon substrate; b - formation of a matrix of vertical columns on a silicon substrate; c - deposition of a doped NCA film on the resulting silicon structure; d - plasma-chemical etching of NCA film; (f) silicon etching inside the columns and the formation of a tubular cathode from an NCA film.

In FIG. Figure 2 shows a view of tubular field emission cathodes at various stages of the technological route: (a) - silicon columns formed using photolithography and subsequent etching (stage 1): (b) - silicon columns with a CVD-doped NCA film deposited on them by the CVD method (stage 3) ; (c) - tubular cathodes with NCA film walls formed from columns after plasma-chemical etching processes (stage 5).

In FIG. 3 shows a view of a part of the array of field emission tubular cathodes,

In FIG. 1 adopted the following designations:

1 - conductive silicon plate;

2 - photoresist mask;

3 - NCA film.

The method for manufacturing a matrix of field emission tubular cathodes (Fig. 1, 2) based on NCA film, in the form of cups with walls made of thin doped NCA film with high conductivity and thermal conductivity, includes five main stages: 1 - formation of a matrix of vertical columns from a conductive silicon wafer by photolithography and etching (Fig. 1a, b); 2 - seeding of the resulting structure with detonation diamond nanoparticles in an ultrasonic bath: 3 - deposition of a nanocrystalline diamond film with a thickness of 10-500 nm by the CVD method in a mode that allows coating with high uniformity and low (less than 20 nm) roughness (Fig. 1c); 4 - anisotropic maskless ICP etching of a diamond deposited film in an oxygen-containing plasma (Fig. 1d): 5 - etching of silicon inside cylindrical columns, the side walls of which are protected by a diamond film and the formation of thin-walled tubular cathodes in the form of glasses of a given depth (Fig. 1f).

At the first stage, vertical columns are formed from a plate of single-crystal conductive silicon 1 under a mask of photoresist or metal (Ni, Al) 2 (Fig. 1a, b). For this, the SF ₆ etchant is used in combination with CHF ₃ or C ₄ F ₈ passivating agents. By alternating cycles of etching and passivation, it is possible to form vertical columns of Si with a height of 2–20 μm.

At the second stage, the substrate is processed in an ultrasonic bath to create diamond nucleation centers on its surface using a suspension of detonation nanodiamond powder with a size of less than 5 nm. The duration of this process is 20-30 minutes, which makes it possible to achieve uniform seeding of a silicon structure with vertical columns of nanodiamond particles at a nucleation density of up to 10 m ¹¹ cm ^-2 .

At the third stage, the process of deposition of the doped NCA film 3 by the CVD method (using a microwave discharge or an incandescent filament) in an H ₂ /CH ₄ gas mixture with the addition of doping gas (B ₂ H ₆ , B (OCH ₃ ) ₃ , N ₂ , PH ₃ , etc.) (Fig. 1c). The process is carried out at a gas mixture pressure of 10–100 Torr and a substrate temperature of 400–800°C, which makes it possible to deposit homogeneous NCA films 10–500 nm thick with high uniformity and low (less than 20 nm) roughness. The required film thickness is controlled by the deposition time, and the degree of doping is controlled by the proportion of the doping gas in the mixture. The mode makes it possible to obtain NCA films with p-type or n-type conductivity, depending on the type of impurity used and the impurity concentration in the film in the range of 10 ¹⁷ -10 ²¹ cm ^-3 . It is also possible to deposit multilayer films with different types and levels of doping in one process by rapidly replacing the gas mixture entering the reactor.

At the fourth stage, the maskless anisotropic removal of the diamond film 3 from the surface of the pillars occurs without affecting its side walls (Fig. 1d). This process is possible due to the use of inductively coupled oxygen-containing plasma O ₂ +Ar-He. By selecting the ratio of gases in the mixture, the total flow of reagents, the operating pressure in the reactor, and the power of the capacitive and inductive discharges, high etching anisotropy is ensured. In this case, the required process time varies depending on the thickness of the diamond film.

At the final stage, silicon is etched in SF ₆ plasma inside cylindrical columns, the side walls of which are protected by a diamond film, and thin-walled tubular field emission cathodes are formed in the form of cups of a given depth (Fig. 1f). In this case, the depth of the glasses is determined by the etching time. It should be noted that the manufacturing method proposed above ensures high reproducibility of the geometric characteristics of the cathodes.

In FIG. Figure 2 shows the evolution of the cathode structure at various stages of the implementation of the method for manufacturing thin-walled tubular field emission cathodes from a nanocrystalline diamond film. A micrograph of a part of a matrix with tubular cathodes based on NCA films, produced by the proposed method, is shown in Fig. 3.

Thus, the distinguishing features of the invention in comparison with analogues and prototype are:

- a method for manufacturing field-emission cathodes of an original shape, which allows, with a high degree of reproduction of geometric characteristics, to manufacture tubular cathodes in the form of a glass with sharp edges formed by thin doped NCA films, on which the electric field is amplified, which makes it possible to combine a high aspect ratio with a developed emission surface of the emitter tip .

- the method allows you to adjust the depth of the glass and the thickness of its walls (the radius of curvature of the end wall) and thus change the aspect ratio of the cathode, which largely determine the emission threshold.

- the method makes it possible to deposit multilayer doped NCA films with high uniformity and low roughness, different types of conductivity and work function, which affect the emission characteristics of the emitter, in one technological cycle without opening the reactor.

- in contrast to the prototype, in which the electron-emitting NCA film covers only the top of the column. The NCA film, which is the glass wall and has a high thermal conductivity inherent in diamond and has extensive contact with the substrate, provides better heat removal, which increases the lifetime of the emitter.

Claims (7)

1. A method for manufacturing a matrix of field-emission point cathodes based on nanocrystalline diamond films, characterized in that each cathode is made in the form of a thin-walled tube with walls made of a doped nanocrystalline diamond film by forming vertical cylindrical columns on the surface of a conductive silicon substrate using photolithography and plasma-chemical etching, then seeding the resulting structure with nanodiamond particles with a size of less than 5 nm in an ultrasonic bath and deposition of a doped conducting nanocrystalline diamond film by the CVD method in an H ₂ /CH ₄ gas mixture with the addition of a dopant gas, further maskless anisotropic etching of the nanocrystalline diamond film at the top of the columns using an inductively coupled plasma in a mixture of O ₂ +Ar+He and silicon etching in SF ₆ plasma inside the pillars. 2. The method according to p. 1, characterized in that the walls of the cathode are made with a thickness of 10 to 500 nm. 3. The method according to claim 1, characterized in that after seeding the resulting structure with nanodiamond particles, a phosphorus-doped conductive nanocrystalline diamond film is deposited. 4. The method according to claim 1, characterized in that after seeding the resulting structure with nanodiamond particles, a boron-doped conducting nanocrystalline diamond film is deposited. 5. The method according to claim 1, characterized in that after seeding the resulting structure with nanodiamond particles, a nitrogen-doped conductive nanocrystalline diamond film is deposited. 6. The method according to claim 1, characterized in that the doped conductive nanocrystalline diamond film is deposited by the CVD method in an H ₂ /CH ₄ gas mixture with the addition of a doping gas at a gas mixture pressure of 10-100 Torr and a temperature of 400-800 ° C. 7. The method according to p. 1, characterized in that the etching of silicon in SF ₆ plasma inside the columns is carried out to a depth of 2-4 microns.

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