RU2574527C1 - Method of forming polymer templates of nanostructures of different geometry - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: method of forming polymer templates of nanostructures of different geometry comprises forming a digital template of nanostructures, transfer of this template to the surface of positive resist deposited on a substrate, the resist development, the substrate, together with the semiconductors, are used as metal-plated substrates, at that the templates in the form of nanoscale rings are formed with single-point exposure of the positive resist by an electron beam with a diameter of 2 nm and a dose in the range of 0.2 pC to 100 pC per the point, and the templates of nanostructures of complex shape and high resolution are formed by sequential point exposure of the positive resist at a pitch of 5 to 30 nm with increase of the average speed of exposure to 10 times.

EFFECT: provision of opportunity of obtaining the polymer nanostructures of different geometry, having sharp outlines of faces at sub-20 nm resolution at various semiconductor and metallic substrates.

4 cl, 11 dwg

Description

The invention relates to the field of nanotechnology, and in particular to a method for forming nanoscale polymer patterns with controlled geometric parameters, and can be used in micro- and nanoelectronics to obtain functional nanostructures and devices based on them.

The continuous development of the nanotechnology and microelectronic industries requires the development of new approaches and methods for the formation of nanoscale structures of different geometries with a resolution of less than 100 nm. One of the most advanced technologies is maskless lithography based on the interaction of a polymer resist with an electron beam.

The main purpose of lithography is to create a template in a resist layer with the aim of subsequent transfer of this template to the surface or into the underlying substrate [1]. Electron beam lithography (ELL) allows you to obtain structures with high resolution due to the small wavelength of the electron and the small size of the electron beam, while the resolution of optical lithography is limited by the wavelength of light that is used for exposure. In addition, ELL is a very flexible template production technique that can work with various resistes and substrates. The main disadvantage of ELL is the low exposure speed (usually no more than 10 ⁷ pixels per second) [2] and the high cost of the mass production process.

A known method for producing polymer patterns of nanostructures by exposure of a resist to a large number of parallel electron beams. [3]. This method allows you to increase the performance of the lithographic system and reduce the cost of integrated circuits. The disadvantage of this method is the high cost of the lithographic system, a complex and unreliable beam control system, low resolution due to the impossibility of simultaneously focusing all the beams of the array.

The known electron-beam system and method that allows you to set the planar distribution of current density in the electron beam focused on the surface of the sample, in order to prevent the effect of proximity and the accumulation of space charge deflecting the electron beam [4]. The method allows you to adjust the radiation dose to obtain high-contrast patterns. The disadvantages of the method are the slow rendering of the template, the inability to use it for mass production.

There is also a method of producing micro rings based on photoresist with a thickness of 800 nm [5]. The method requires the use of photomasks with sprayed arrays of metal nanodisks, which is placed on the surface of the photoresist. When the photomask is illuminated with ultraviolet light, a phenomenon called the “Poisson spot” occurs. The essence of the phenomenon is the appearance of a bright spot on the photoresist behind an illuminated directional light beam by an opaque body (in this case, a metal disk) in the region of its geometric shadow. As a result, the central part is illuminated under the disk in the photoresist, which, after development, results in micron size rings. The disadvantage of this method is the low resolution (worse than 200 nm) due to the use of a light wave to expose the polymer layer and the use of an expensive photomask.

The closest to the claimed technical solution for the essential features and the achieved result is the development described in article [6]. The prototype method for the formation of polymer patterns of nanostructures consists in transferring the template to a PMMA resist (polymethylmethacrylate) 950 K 128 nm thick, which is preliminarily applied by centrifugation on a cleaned substrate of naturally oxidized silicon, held for 60 s on a plate at a temperature of 180 ° C, after exposure in a solution of MIBK (methyl isobutyl ketone): IPA (isopropyl alcohol) in a ratio of 1 to 3 (1: 3) and incubated in isopropyl alcohol for 60 s. Then, a high-dose electron beam (several tens of mCl / cm ² ) is exposed to the surface surface macromolecules, as a result of which the hydrogen bonds of the macromolecules are broken and the resist is carbonized.

Overexposure of the resist allowed the authors to obtain irregularly shaped nanoscale columns. The authors explain the formation of nanostubes by the carbonization of a positive resist PMMA, as a result of which it acquires the properties of a negative resist.

The task posed and solved by the present invention is the creation of polymer nanostructures of different geometries on semiconductor and conductive surfaces to obtain complex lithographic templates having sub-20 nm resolution and consisting of both individual elements and arrays of nanostructures.

Technical results - obtaining polymer nanostructures of different geometries with sharp edges at sub-20 nm resolution on various semiconductor and metallized (coated with a thin metal film) substrates.

To achieve the above technical results, a method for manufacturing polymer nanostructures is proposed, including applying a thin layer of positive resist to a cleaned semiconductor substrate or a substrate pre-coated with a thin metal film (e.g., Au, Pt, Al, Cu, Ti, Cr, etc.), heat treatment of the resist to obtain a solid polymer film and ensure high adhesion to the surface of the substrate, the subsequent exposure of the polymer film by an electron beam in the spot mode according to a given digital template with a dose exposure, which allows changing the chemical properties of the resist and to switch it from a negative to a positive resist, development with a substrate solution in MIBK: IPA to remove the resist from the areas exposed normal dose until a predetermined template nanostructures. The proposed method can significantly increase the resolution of the resulting nanostructures compared to ELL on most negative resistes (for example, NEB-31, ma-N, etc.) and significantly reduce the time it takes to draw a template compared to traditional ELL.

Semiconductor substrates, for example, naturally oxidized silicon substrates, both with intrinsic charge carriers and doped with impurity atoms, for example, B, P, In, etc., and substrates coated with a continuous thin film from the working surface, can be selected as a substrate for resisting (thickness from 5 nanometers to several micrometers) of a metal, such as Au, Pt, Al, Cu, Ti, Cr, etc. The use of substrates with different crystal structures (single crystal, polycrystalline) and grain size, as well as roughness surface, allows you to control the resolution of the resulting nanostructures.

The use of point (single-pixel) exposure of the polymer film by an electron beam with a dose of 0.2 pCl to 100 pCl per point and a distance between the nearest points of 5 to 30 nm allows re-exposure of selected areas in accordance with a digital image in order to form solid carbonized areas that after exposure in the developer MIBK: IPA (1: 3) or in acetone, form the basis of polymer patterns.

The templates of nanostructures of different geometries formed by the claimed method have high resistance to acid and plasma etching due to their carbonization, which makes them attractive for creating solid forms for nanoimprint lithography, mask for etching, nanoelectronic, nanophotonic and nano- and microelectromechanical devices.

The method of forming polymer patterns of nanostructures allows you to:

- to form polymer patterns of nanostructures of complex geometry with high resolution;

- expand the class of substrate materials on which polymer patterns of nanostructures can be formed;

- increase the accuracy of reproduction during the formation of polymer patterns of nanostructures of complex geometry;

- ensure the stability of the process of forming polymer patterns of nanostructures on substrates with a diameter of more than 1 cm;

- to simplify and reduce the cost of the method of forming carbonized nanostructures and reduce the time of the technological process.

Thus, the hallmarks of the proposed technical solution are:

- the use along with naturally oxidized silicon substrates of semiconductor substrates coated with thin metal films with different crystalline structures (single crystal or polycrystalline) and grain size, and, accordingly, with different surface roughness;

- the use of point (single-pixel) exposure of the polymer film by an electron beam with a high dose of more than 0.2 pC per point and a distance between the exposure points from 5 to 30 nm;

- the possibility of forming defect-free polymer patterns of nanostructures of various geometric shapes - ovals, squares, triangles, polygons, etc. with sub-20 nanometer resolution.

A comparative analysis of the set of essential features of the proposed method and the set of essential features of analogues and prototype indicates the presence of the criterion of "novelty" in the claimed technical solution.

This set of distinctive features allows us to solve the problem and eliminate the disadvantages of the prototype method, providing higher resolution and efficiency of lithography, making it possible to obtain polymer nanostructures of any shape on semiconductor and metal surfaces for their further use as a nanoimprint lithography stamp, mask for etching the material substrates or for spraying a wide class of materials on a substrate followed by explosive lithography ( lift-off process) to remove residual resist and deposited material. The inventive method provides stability and high productivity of the process of forming defect-free polymer patterns of nanostructures on substrates with a diameter of more than 1 cm

The method of forming polymer patterns of nanostructures of different geometries is illustrated by pictures, diagrams and graphs shown in FIG. 1-11:

in FIG. Figure 1 shows photographs of a scanning electron microscope, demonstrating the effect of the substrate on ring formation at an irradiation dose of 2.4 pC per dot and resist thickness of 150 nm: (a) a silicon substrate; (b) a silicon substrate coated with an Au film 200 nm thick;

in FIG. Figure 2 schematically shows the process of formation of a column and a halo around it depending on the type of substrate: stage 1 — interaction of electrons with a resist and a substrate during exposure, stage 2 — interaction leads to exposure of the selected resist regions; stage 3 - the development of the resist and the formed profile;

in FIG. Figure 3 shows the effect of exposure dose and type of substrate on column growth and halo diameter;

in FIG. Figure 4 shows snapshots of a nanoscale in the form of a ring formed by the method of spot lithography on a silicon substrate coated with a 120 nm thick PMMA A2 resist layer: (a) top view of the ring, (b) three-dimensional image of the ring, (c) transverse profile of the ring. Images obtained using atomic force microscopy;

in FIG. 5 shows the dependence of the halo diameter d _out on the radiation dose at a fixed resist thickness of 150 nm and an accelerating voltage at the cathode of 10 kV for different types of substrates;

in FIG. Figure 6 shows the dependence of the diameter of the column d _in on the radiation dose for a fixed resist thickness of 150 nm and an accelerating voltage at the cathode of 10 kV for different types of substrates;

in FIG. 7 shows the effect of the distance between the exposure points on the formation of nanoscale patterns;

in FIG. Figure 8 shows images of polymer nanostructures obtained by spot lithography on a RMMA A2 resistor with a thickness of 75 nm and an accelerating voltage at the cathode of 10 kV: (a) a polygon with a line width of 20 nm, (b) an oval with a line width of 22 nm, (c) a triangle with a line width of 33 nm;

in FIG. 9 shows three-dimensional images of polymer nanostructures obtained by spot lithography on a resist PMMA A2 75 nm thick with an accelerating voltage at the cathode of 10 kV; (a) a ring with a line width of 20 nm; (b) a square with a line width of 25 nm;

FIG. 10 shows the effect of merging halos of nanostructures as they move relative to each other;

in FIG. 11. An image of a complex polymer nanoscale with a minimum line width of 15 nm is presented.

The examples below confirm, but do not limit, the invention.

Example 1. The creation of a polymer nanostructure in the form of a ring on a silicon substrate

A thin layer of PMMA 150 nm thick was deposited by centrifugation on a preliminarily purified naturally oxidized silicon semiconductor substrate. Immediately after this, the substrate was placed on a hot surface with a temperature of 180 ° C for 90 seconds to solidify the polymer layer and remove the solvent. Then the sample was placed in the chamber of an electron beam lithograph (E-line, Raith) or scanning electron microscope (Supra, Carl Zeiss), which has an electron beam cut-off mechanism. To obtain the highest possible resolution of lithography, the smallest cross section of the electron beam was chosen equal to 2 nm (e-Line Raith nanolithograph).

To create a nanostructure, point exposure of the selected region with a certain dose was performed, while the accelerating voltage of the electron beam was 10 kV. The magnitude of the accelerating voltage determines the energy of the electron beam and, therefore, the depth of its penetration into the resist and substrate. The dose indicates the number of electrons interacting with the resist during exposure. It was experimentally established that at doses less than 0.2 pC per point, the PMMA resist behaves as positive, which, after developing in MIBK: IPA solution (1: 3) for 30 s under normal conditions, leads to leaching of the resist at the exposure sites. Exposure in acetone for 30 s leads to the complete removal of the resist. It was established experimentally that at doses ranging from 0.2 pCl to 100 pCl per point, a centered column is formed due to the irreversible process of polymer carbonization at the site of direct electron beam impact. This is due to the switching nature of the resist from positive to negative. After developing the sample in a MIBK: IPA solution (1: 3) for 30 s under normal conditions, the resist around the column is washed out, forming a round halo (Fig. 1a). An exposure in acetone for 30 s leads to the complete removal of the resist except for the column. The diameter of the halo, as well as the outer diameter of the carbon column, depends not only on the exposure dose, but also on the thickness of the resist, the accelerating voltage of the electron beam, the roughness and crystal structure of the surface of the substrate.

The main effect of electrons in the irradiation process falls on the central region (column), while the backward-reflected and secondary electrons are responsible for the formation of the halo around the column (Fig. 2a).

The formation of the column begins at an irradiation dose of ~ 0.2 pC, which, in terms of the area of the incident beam with a diameter of 2 nm, is 6.37 C / cm ² , while the average dose of 110 μC / cm ^{2 is} used when drawing a nanodot in the traditional way. A significant increase in the radiation dose (10 ⁵ times) leads to overexposure of the central region and the formation of a carbonized column. This fact is confirmed by the high chemical resistance of the column to the developer (MIBK solution: IPA) and solvents (acetone, EBR remover).

At a low radiation dose, the shape of the column strongly depends on the crystal structure of the substrate, however, with increasing dose, the effect of the substrate becomes negligible (Fig. 3), due to the deeper penetration of electrons deep into the resist film and the substrate.

In FIG. Figure 4 shows the atomic force microscopy images obtained on an Ntegra Aura (NT-MDT) instrument, from which the spatial structure and profile of the resulting template are visible. It is important to note that after development, the upper part of the column is washed off, and at the same time, a characteristic peak is formed on the surface, indicating the point of interaction of the electron beam with the resist.

Example 2. The creation of a polymer nanostructure in the form of a ring on a silicon substrate coated with gold

The templates in the form of nanosized rings were prepared according to Example 1, but a naturally oxidized silicon substrate coated with a thin polycrystalline gold film 200 nm thick was used as a substrate. The average grain size was 10 nm. The Au film was deposited by thermal spraying of a gold sample on a VUP-5 automated vacuum unit at a base pressure of 10 ^-5 bar. The gold deposition rate was 100 nm per minute.

In FIG. Figure 1b shows the effect of the polycrystalline (granular) structure of an Au film 200 nm thick deposited on the surface of naturally oxidized silicon on the quality of the halo. Due to the equiprobable backward reflection of electrons from the grain plane, the halo boundary becomes blurred, while reflection from a single-crystal silicon substrate occurs with a smaller scatter, which increases the sharpness of the halo and column boundaries (Fig. 1a). In the case of a polycrystalline metal substrate closer to the edge of the halo, an area of an underexposed overhanging resist is formed. In FIG. 2b schematically shows the process of forming patterns of nanostructures on a silicon substrate coated with a 200 nm thick gold film.

In FIG. 3b shows the effect of dose on the formation of nanorings. Increasing the dose results in a regular column shape and clearer halo boundaries. This is due to the smaller effect of the graininess of the gold film on the formation of larger rings.

In FIG. 5-6 show the experimental dependence of the diameter of the halo d _out and column d _in on the radiation dose for gold and silicon. As you can see, increasing the dose leads to an increase in diameters. Moreover, on a lower dose of silicon, rings of much larger diameter can be formed. It was experimentally established that the thickness of the resist also makes a significant contribution to the formation of nanostructures. So, with increasing thickness of the resist, the diameter of the halo increases. This is primarily due to the peculiarity of the interaction of direct and retroreflected electrons with a resist and a substrate.

Example 3. The formation of nanostructures of complex geometry

A polymer film was prepared according to Example 1. Previously, a digital template of the nanostructure was created in the form of a set of single-pixel points, according to which exposure was made by a point electron beam with a dose above 0.2 pC. In the process of forming complex geometric shapes with a minimum distance (not more than 30 nm) between adjacent irradiation points, intergrowth of carbonized columns is observed, with a repetition of the geometric contour of the control pattern (Fig. 7a and c). As the distance between the points increases, the interaction weakens, which leads to the "collapse" of the figure (Fig. 7b and d). It was established experimentally that the optimal distance between the irradiation points should lie in the range from 5 to 30 nm. At a shorter distance, the resolution of the resulting elements is higher. At a distance of more than 30 nm, the integrity of the elements after development is violated. In FIG. Figure 8 shows examples of complex geometric shapes with a line width in the range from 10 to 30 nm. FIG. 9 shows three-dimensional images of round and square rings. When the structures are displaced relative to each other, their halos also shift (Fig. 10).

The present invention eliminates the disadvantages of the prototype method, providing the possibility of forming complex polymer nanostructures of any shape with a line thickness of 10 nm not only on semiconductor substrates, but also on films of various metals, such as Au, Pt, Al, Cu, Ti, Cr and etc. The minimum line width at an electron beam diameter of 2 nm is limited only by the length of the polymer chain (~ 5 nm) and the thickness of the resist. It was experimentally established that the thinner the resist, the higher the resolution.

Another advantage is the fact that the lines of the resulting nanostructures have a practically defect-free shape (Fig. 11). This allows you to get carbonized patterns of high quality with sub-20 nm resolution. The transfer of patterns into the substrate by wet chemical or plasma-chemical etching is possible due to the high chemical resistance of the carbonized template.

An additional important advantage of the proposed method for the formation of polymer nanostructures is the short drawing time of the template compared to the exposure time of the same template in the traditional way, the essence of which is the sequential scanning of the selected area. To create a template in the form of a square array of 25 by 25 (625 elements in total) equilateral triangles with a side of 210 nm and a line thickness of 40 nm requires 17.6 s, while drawing the same template in the traditional way requires 152 s, which is 8.6 times longer. By selecting the parameters, the average speed of spot exposure can be increased up to 10 times compared with the traditional method.

The prior art confirms that in scientific and technical sources there is no data indicating the influence of the distinguishing features of the claimed invention on the achievement of technical results, which confirms the inventive step of the proposed method.

Literature.

1. S.P. Li, D. Peyrade, M. Natali, A. Lebib, Y. Chen, U. Ebels, L.D. Buda, and K. Ounadjela, "Flux Closure Structures in Cobalt Rings", Phys. Rev. Lett. 86 (2001) 1102.

2. Broers, A.N .; 1995; Fabrication limits of electron beam lithography and of UV, X-ray and ion-beam lithographies; Phil. Trans. Roy. Soc., Ser. A (Phys. Sci. & Eng.), 353 (1703) 291-311.

3. Parallel multi-electron beam lithography for IC fabrication with precise X-Y translation, patent US 7075093 B2, publ. 07/11/2011.

4. Electron beam lithography system and method, patent US 5334282 (A), publ. 08/02/1994.

5. Shibing Tian, Xiaoxiang Xia, Wangning Sun, Wuxia Li, Junjie Li and Changzhi Gu, Large-scale ordered silicon microtube arrays fabricated by Poisson spot lithography, 2011 Nanotechnology 22395301).

6. Huigao Duan, Jianguo Zhao, Yongzhe Zhang, Erqing Xie and Li Han, Preparing patterned carbonaceous nanostructures directly by overexposure of PMMA using electron-beam lithography, Nanotechnology 20 (2009) 135306.

Claims (4)

1. The method of forming polymer patterns of nanostructures of different geometries, including the formation of a digital pattern of nanostructures, transferring this pattern to the surface of a positive resist deposited on a substrate, the manifestation of a resist, characterized in that metal-coated substrates are used as a substrate, the patterns being in the form of nanoscale rings are formed by single-point exposure of a positive resist by an electron beam with a diameter of 2 nm and a dose in the range from 0.2 pC to 100 pC per point, and the template S nanostructures of complex shape and high resolution are formed by sequential point exposure of a positive resist in increments of 5 to 30 nm with an increase in the average exposure speed up to 10 times. 2. The method according to p. 1, characterized in that as a metal for coating a silicon substrate can be selected metals from a number of Au, Pt, Al, Cu, Ti, Cr, etc. 3. The method according to p. 1, characterized in that as substrates for forming defect-free polymer patterns of nanostructures, metallized substrates with a diameter of more than 1 cm can be used. 4. The method according to p. 1, characterized in that the digital template is presented in the form of a set of single-pixel dots.

Images