Recording of Micro/Nanosized Elements on Thin Films of Glassy Chalcogenide Semiconductors by Optical Radiation

Abstract

Inorganic resists based on chalcogenide glassy semiconductor (CGS) films can be effectively used in the creation of micro- and nanoelements of optoelectronic devices, micro- and nanoelectromechanical systems, and diffractive optical elements. The use of these materials is based mainly on their sensitivity to different types of radiation, which causes phase and structural changes in CGS films, and transparency in the infrared range. A number of photoinduced changes are observed in CGS, which are associated with structural transformations, phase transitions, defect formation, and atomic diffusion. It is important to determine technologies for the formation of micro- and nanoscale structures on CGS films, which can be used in the creation of diffractive optical elements for optoelectronic devices. Increasing the resolution of recording media based on vitreous chalcogenide semiconductors can be achieved by choosing the recording modes and composition of glasses, in which the strongest nonlinearity of the exposure characteristics of photosensitive material, as well as the introduction into the structure of recording media nanoparticles of noble metals for excitation of plasmonic resonance.

1. Introduction

Lithography is one of the key technological processes in the production of semiconductor integrated circuits, storage devices, and precision devices for optics and micromechanics. Laser lithography, near-field, and probe technologies, and radiation exposure methods are widely used to create nanoscale structures [1, 2]. Among the methods of obtaining nanoscale structures, it is necessary to note laser lithography, which allows to carry out mask less image formation in the photoresist layer on the surface of the substrate with a laser beam. Research in the development and manufacture of nanostructures for various purposes is largely determined by the level of development of technologies that allow atomic accuracy to obtain nanostructures of the required configuration and dimension, as well as a comprehensive diagnosis of the properties of nanostructures. Modern methods of nanoscale optical recording are based on the use of various methods, including photoinitiation with high beam intensity. A significant limitation on the size of the elements is due to the ability to focus optical radiation to sizes smaller than the diffraction limit. Many technical solutions are proposed and developed to realize the possibilities of optical nanolithography [1, 2, 3]. Special photosensitive materials can be used to form nanosized structures with focused laser radiation. Direct optical lithography can be created without the use of organic photoresists of functional inorganic nanomaterials. The ability to directly pattern completely inorganic layers using a radiation dose comparable to that of organic photoresists provides an alternative method for producing thin-film devices [4, 5]. Among these materials should be noted inorganic resist based on films of chalcogenide glassy semiconductors (CVS). Inorganic resists based on CVS films can be effectively used in the creation of micro- and nanoelements of optoelectronic devices, micro- and nanoelectromechanical systems (MEMS/NEMS), and diffractive optical elements. The use of these materials is based mainly on their sensitivity to different types of radiation, which cause phase and structural changes in CVS films, and transparency in the infrared range [4, 6, 7, 8, 9]. Numerous studies have been conducted aimed at studying the processes of formation of nanostructures on CVS films [7, 9, 10, 11, 12]. A number of photoinduced changes are observed in CVS, which are associated with structural transformations, phase transitions, defect formation, and atomic diffusion [6, 7, 8]. It is important to determine technologies for the formation of micro- and nanoscale structures on CVS films, which can be used in the creation of diffractive optical elements for optoelectronic devices.

Photoinduced transformations in CVS have been widely used in the optical recording of information to create optical media of various types. This is largely because the properties of thin chalcogenide films allow recording at high speed and do not impose restrictions on the minimum size of the recorded elements and, consequently, on the density of information recording [7, 12]. Numerous theoretical and experimental studies of the interaction of electromagnetic radiation and particle fluxes with CVS have shown the possibility of achieving ultra-high resolution when exposed to optical radiation or electron beam [10, 11, 13, 14]. It is shown that the resolution of an inorganic resist based on CVS is determined by the size of the structural units that form the matrix of films and is 1–2 nm [10, 13, 14]. In this regard, CVS are promising materials for the formation of nanoscale structures on their surface and the creation of ultra-dense information recording devices [1]. Numerous experiments on the exposure of thin films of CVS and their subsequent selective chemical etching have shown that when using immersion optical systems, it is possible to record microrelief structures with submicron dimensions (120–150 nm). With electron beam exposure of chalcogenide films, it is possible to form structures with sizes of 50–70 nm [1, 7]. Particular attention was paid to the choice of CVS that do not contain highly toxic elements, modes of their heat treatment and selective chemical etching [15].

2. Thermolithographic recording on chalcogenide films

The diffraction nature of light prevents us from achieving sub-diffraction or nanometer resolution in an optical beam lithography system. It is necessary to develop special recording methods based on nonlinear interaction with photosensitive materials and conversion of the energy of incident radiation. The size of the elements formed on the films of CVS was determined mainly by the resolution of the optical focusing system and the accuracy of the automatic focusing system. Previous analysis showed that microrelief structures on CVS films, the width of which is less than the resolution of diffraction-limited optics, can be created using the thermolithographic recording mode. Local heating of the film in the recording area by radiation with a non-uniform intensity distribution allows to reduce the size of the elements by selecting the recording mode [1, 7, 16, 17].

The recording of elements on photosensitive materials is carried out by a focused beam with a non-uniform intensity distribution (usually with a Gaussian intensity distribution) and this allows on a photosensitive material with a nonlinear exposure characteristic to record elements smaller than the diameter of the focused beam. One of the first experiments on recording on a semiconductor–metal material with a nonlinear exposure characteristic showed the possibility of reducing the size of the elements recorded by laser radiation by 2-3 times compared to the diameter of the laser beam measured at 1/e [18] formed on the films of chalcogenide vitreous semiconductors was determined mainly by the resolution of the optical focusing system and the accuracy of the automatic focusing system. Previous analysis showed that microrelief structures on CVS films, the width of which is less than the resolution of diffraction-limited optics, can be created using the thermolithographic recording mode. The local heating of the film in the recording area by radiation with a non-uniform intensity distribution allows to reduce the size of the elements by selecting the recording mode [19, 20, 21].

The minimum size of elements recorded on films of chalcogenide semiconductors with phase transitions by thermolithography was 100 nm [22]. The use of this technology was of interest for optical disc recording systems in CD, DVD, BD [7, 16], and the creation of diffractive optical elements [1]. Examples of thermolithographic recording on chalcogenide semiconductor films are shown in Figure 1. The recording beam diameter was 1.0 mm. Elements 0.7-0.8 μm wide were recorded [7, 15].

Figure 1.

The principle of the thermolithographic recording [21].

Limitations of the thermolithographic recording mode are associated with a significant effect of fluctuations in the power of laser radiation on the size of the elements recorded on the nonlinear photosensitive material (Figure 2). The formation of nanoscale relief structures on thin films of CVS by diffraction-limited optical systems is problematic.

Figure 2.

(a) the relief microstructure on the positive inorganic photoresist GeSe_2. [15]. (b) the relief microstructure on the negative inorganic resist As₂S₃ [7].

3. Use of near-field optical radiation focusing systems for the formation of submicron structures

For the formation of nanoscale elements and structures, it was proposed to use near-field systems for focusing optical radiation. Lines with a width of 100 nm and a depth of 23 nm on the As₂S₃ film were recorded with a near-field probe with an aperture of 120 nm [23]. The main disadvantage of recording by this method is the low scanning speed (100 μm/s), which is due to the low efficiency of near-field probes based on conical optical fibers [2, 23]. The situation with the use of near-field probes for recording nanoscale elements on inorganic resistors may change with the creation of new more efficient probes for focusing laser radiation, in particular, microstrip pyramidal probes [2]. Images of different types of probes are shown in Figure 3.

Figure 3.

(a) Most commonly used near-field tapered fiber probe, (b) near-field probe based on optical plasmon microstrip line (optical microstrip probe) [2].

The pyramid-type microstrip probe (PTMP) has a transparent pyramid-like core with a truncated corner. Metal strips coat two opposite sidewalls of the pyramid. The transparent body and two metal strips form a tapering microstrip line, similar to an ordinary microstrip line where two opposite sides of a dielectric rectangular slab are coated with metal films, as shown in Figure 3. The incident beam (either a focused beam or a dielectric waveguide mode) couples to the probe through its wide end, and propagates along the probe, reaching the narrow end that forms the aperture. The light passing through the narrow end interacts with the scanned sample. In far-infrared band metal strips can be represented with high accuracy as perfect conductors which can support quasi-TEM wave which has no cut-off size. The incident light should have electric field polarization orthogonal to the metal strips in order to excite the quasi-TEM mode that has no cut-off size. A microstrip probe has a significant advantage over a conventional near-field probe in far-field transmission coefficient, especially for the small aperture size (a < 100 nm) since it decreases with a decrease of the aperture size as a square of the aperture diameter.

The near-field recording mode can be realized using a nanoscale diaphragm on the surface of the photosensitive material. To record nanosized elements by diffraction-limited optical systems, it was previously proposed to place an additional masking layer on the photosensitive layer, which changes the refractive index at elevated temperatures, such as a semiconductor film with a large band gap [24]. For the formation of nanosized elements on a thin film of chalcogenide semiconductor, the technology of exposure through the diaphragm (mask) was implemented, created in a material with a nonlinear exposure characteristic (technology of high-resolution near-field storage Super-RENS). One of the main elements of the Super-RENS disk is a mask, which is used to form a light beam of minimum size to expose the photosensitive layer of chalcogenide semiconductor and separated from it by a protective layer of fixed thickness. The size of the optical spot created by the mask ultimately determines the size of the elements that are recorded in the media. The scheme of the Super-RENS recording method is shown in Figure 4b [25].

Figure 4.

(a) Conventional recording by NSOM, (b) super-RENS recording method [25].

As the material of the diaphragm in the first experiments, thin antimony (Sb) films with a thickness of 15 nm were used, located between the protective layers of SiN (Figure 4) [25]. Significant optical nonlinearity of the thin antimony film located between the dielectric layers was detected. The change in transmittance was relatively significant and stable over time in the region with submicron dimensions. This recording technology made it possible to obtain fingerprints with linear dimensions of less than 100 nm on GeSbTe (GST) films [25]. In this structure, a thin film mask made of Sb was placed at a distance of the near field to the recording layer. It was found that the Sb₂Te₃ material can also be used as a masking layer. Using it, fingerprints with linear dimensions of 60 nm were recorded on a phase-transition material (GeSbTe). The dielectric material ZnS-SiO2 was used as a protective layer. The possibility of recording and reproducing elements with submicron dimensions is explained by the fact that in the process of recording and reading in the masking layer a small hole is formed, which functions as a local solid-state near-field lens [26].

4. The use of nanocomposite photosensitive materials, the formation of images on which is carried out using the methods of nanoplasmonics

Plasmonic nanolithography, which uses surface plasmons to create submicron elements, is a promising technology for producing nanoscale structures. Plasmonics can focus light into zones smaller than the diffraction limit, due to the connection of light with the surface collective vibrations of free electrons at the metal-dielectric interface. Surface plasmon resonances (SPP) have been used to create nanoscale structures [27]. The method of plasmon nanolithography is being developed in which metal lattice masks are used to excite SPP and structural nanoscale elements. The mask is in close contact with the photoresist applied to the substrate. Typically, the incident light passes through the mask through the SPP and is directed to the photoresist [27]. It was demonstrated that the use of surface plasmons in the optical near field of a metallic mask can produce fine patterns with a subwavelength resolution. Using a silver grating mask with 300 nm periodicity, lithography with 100 nm pitch has been demonstrated by using the interference of surface plasmon waves within the grating area [28].

The method of plasmon nanolithography was used for the alternative design of the Super-RENS recording method. In this method, a layer of noble metal oxides (AgO_x, PtO_x, and PdO_x) was used instead of the Sb layer. Using surface plasmons has greater possibilities for the creation of super-dense recording systems. Irradiation of the oxide layer led to the decomposition of the oxide and the formation of a layer of metal nanoparticles. The process of chemical decomposition occurs in the temperature range from 400°C to ~500°C. Surface plasmons, excited by light on the formed nanoparticles of precious metals, generate optical near-field radiation, which is exposed to the photosensitive layer. The structure of such a medium is shown in Figure 5 [26]. The media with the Ag₂O layer were studied in the most detail. The Ag₂O layer in the Super-RENS carrier acts as a center of strong light scattering in the local region of the multilayer carrier. The optical near field, which is created around the scattering center with Ag₂O, is 40 times stronger than the field created by the antimony layer [29, 30]. Studies have shown that the higher efficiency of high-resolution super-RENS disks with an AgO_x layer is associated with the formation of localized surface plasmons by silver clusters dissociated from the AgO_x layer. The diameter of the silver nanoparticles was approximately 4 nm. The density and distribution of dissociated silver nanoparticles are affected by the intensity of focused laser radiation. Localized surface plasmons improve the reading efficiency in such media [31].

Figure 5.

The structure of the carrier made by technology super-RENS using oxides of noble metals [26].

One of the possible ways to overcome the diffraction barrier can be the use of the near light field of metal nanoparticles (NPs) integrated into chalcogenide films, i.e., the formation of a kind of plasmonic nanostructures [5]. This field arises upon irradiation with light with a certain wavelength due to the excitation of collective oscillations of free electrons in NPs (surface plasmon resonance (SPR)). The spatial distribution of this field can be changed in a controlled manner due to appropriate changes in the size and geometry of the woofer. The technology of excitation of metal nanoparticles and the use of optical near-field radiation for the exposure of photosensitive layers has proved to be quite effective and continues to develop in the creation of new types of media for recording nanoscale structures. A schematic representation of the information carrier with a layer of nanoparticles of precious metals is shown in Figure 6 [32].

Figure 6.

Information carrier with a layer of noble metal nanoparticles: (a) schematic diagram (cross-section) of the created structures with gold nanoparticles (GNPs) and chalcogenide layer; (b) SEM picture of the created GNPs [32].

Nanoparticles of precious metals with sizes of the order of tens of nanometers can have a significant impact on the processes of recording information in different types of optical and magnetic media. The technology of using nanoparticles is one of the ways to overcome the diffraction limit in the process of recording nano-sized structures. The generation of localized plasmons in noble metal nanoparticles is widely used to enhance the interaction of light with the matrix surrounding these plasmon nanostructures. The incident light, which is absorbed by the nanoparticles and transforms into collective oscillations of free electrons in them, leads to a strong amplification of the local electric field [31]. Metal nanoparticles effectively absorb light. Their ability to focus light in small volumes has led to the use of woofer in a variety of areas, including as light concentrators for the solar cells. The light-concentrating properties of metal nanostructures are a consequence of the amplification of electromagnetic fields due to the generation of localized plasmons [31, 33, 34, 35]. Light-induced plasmon heating of a magnetic medium in the process of magnetic recording (with a built-in plasmon antenna) can be used to implement the mode of thermal assistance and, ultimately, to increase the density of information recording [31, 33]. The surface plasmon interference nanolithography (SPIN) allows to obtained uniform interference patterns far beyond the free-space diffraction limit of the light. This technique provides a new alternative fabrication method for nanodevices [28].

5. Formation of microrelief structures on the surface of chalcogenide films

One of the most promising uses of optical and electron beam recording on CVS is a direct one-step process of microrelief formation on the surface of films, which is closely related to induced mass transfer (vertical or lateral directions) in amorphous material under illumination [4, 36, 37, 38, 39]. The formation of relief on the surface of ChS films is possible by a purely optical method due to photoinduced mass transfer (FM) under the action of the light from the spectral region of the absorption edge even at relatively low intensities of the light wave. However, due to light diffraction, the lateral scale of such topographic structures is limited, which blocks the formation of nanoscale information elements [3]. In direct one-stage laser or electron-beam recording, there is an irreversible amplitude-phase optical and geometric structuring of the surface. This effect can be used for the manufacture of microlenses, amplitude-phase optical elements. The process of direct photoinduced fabrication of microrelief structures on CVS films by lateral mass transfer was studied on films of different compositions and with different irradiation methods. As a result, the observed process models were proposed and areas of possible application were identified [4, 36, 37, 38]. The possibility of creating planar diffraction optical elements during electron-beam exposure with a local change in the refractive index was experimentally demonstrated. The lens is created in the form of electron beam-recorded annular zones with a stepwise decrease in refractive index. The image of the Fresnel lens obtained by this method is shown in Figure 7. The minimum width of the elements in the recorded image is ~0.6–1.0 μm [31, 39].

Figure 7.

Image of a Fresnel lens obtained by the method of direct one-stage process forming microrelief (a), e-beam fabricated scattering lens on Se/As2S3 NML and its profile measured by AFM (b) [6].

Direct single-stage laser or electron beam recording is more efficient in nanosized layered structures Se/As₂S₃ and Sb/As₂S₃ than in homogeneous layers of As₂S₃ [6, 39]. The effect of photoinduced mass transfer allows to obtain holographic gratings, integral optical elements by a purely optical method at relatively low intensities of light fluxes [4, 13, 38, 39]. The image of the diffraction grating obtained by the direct (optical) method due to photoinduced mass transfer of the substance of the CVS film is shown in Figure 8.

Figure 8.

AFM image of diffraction gratings recorded at temperatures of 77 K (a) and 300 K (b) on As₂₀Se₈₀ thin films [38].

The relief shape and diffraction efficiency can be changed by the ratio of the polarization of the recording rays and the beam of additional illumination.

The photoinduced changes in amorphous Ge-based chalcogenide layers deposited on gold nanoparticles change significantly. The rate and final magnitude of the volume change is higher in a structure with localized plasmon fields, mainly because the latter affects the processes of charge generation and the movement of atoms, initiated by illumination. The results showed that the superposition of the localized plasmon field of nanoparticles with the electromagnetic field of incident photons during irradiation enhance light-induced transformations (Figure 9) [32].

Figure 9.

AFM surface morphology of the holographic grating recorded in the pure chalcogenide layer (a) and in the sample with GNPs (b), and the cross-section of the created surface structures (c): 1—pure chalcogenide layer, 2—sample with GNP [32].

The use of a layer of gold nanoparticles allows a higher level of mass transfer and more efficient modification of the surface of a chalcogenide semiconductor film to be realized.

6. The formation of nanosized relief structures on films of chalcogenide vitreous semiconductors by the methods of probe microscopy

Scanning probe lithography (SPL) is a direct-write nanolithography technique in which elements on thin films of chalcogenide semiconductors are created by scanning a sample with a sharp nanometer tip to create localized modifications. The interactions of the tip with the sample are varied and can include mechanical, electrical, diffusion, and thermal effects. SPL techniques are being studied intensively. SPL methods provide nanometer resolution, however, they are characterized by a low write speed, which is 0.1–50 μm/s [40]. The technology of nano-heating of thin-film materials with a phase transition has been studied in detail and it has been shown that it makes it possible to register ultra-small spots with sizes less than 50 nm [41].

With the use of technologies based on atomic force microscopy, the recording of data and the formation of nanosized structures on films of CVS with phase transitions were successfully executed [42]. The information recording is based on the formation of local (crystalline/amorphous) sections with different structures and, respectively, conductivities in the nanosized layer of a chalcogenide material with the help of electric pulses [42]. It was shown that, due to the applied pulses of the voltage between a probe and a conducting electrode, the conductance of a chalcogenide amorphous GeSb₂Te₄ film increases by at least two orders. An increase in the conductance is caused by the phase transition of a chalcogenide film from the amorphous state into a crystalline one. The recorded data are read with the probe of an AFM by the measurement of changes in the conductance of a chalcogenide film. The simultaneous measurement of the conductance and topographic images with the help of an AFM showed that the surface relief of recorded zones is invariable in the process of recording. The least recorded imprints were down to 10 nm in diameter [42].

Such technology of recording allows one to form elements with sizes of a recording zone of ~30-70 nm, which is 3-5 times less than the real diameter of focused exposing rays of laser sources of violet and ultraviolet emissions.

The creation of a relevant relief in a recording zone occurs directly during the simultaneous exposure and indentation, which presents the essential advantage over the available lithographic methods, which require the additional treatment of a carrier with selective chemical etchants or the ion-beam or plasma chemical etching. The profiles of imprints obtained at the nanoindentation of an As₂₀Se₈₀ film in dark and light at various loads are given in Figure 10 [1].

Figure 10.

General view of imprints (upper row) and their profiles (lower row) obtained at the nanoindentation of an As₂₀Se₈₀ film in dark and in light at loads of 120-240 mN (a) and 700-900 mN (b). Scheme of application of a load (c) and the relevant curves of nanoindentation in dark (d) and in light (e) [1, 43].

One of the advantages of the given method is the absence of a thermal heating source used for the softening of the surface area that is modified and, respectively, the absence of shortcomings related to such heating (power losses, the complexity of a micromechanical system, loads, etc).

An extension of scanning probe lithography (SPL) is plasmonic nanolithography with a focused beam. This technology is promising due to its sub-diffraction resolution. In this method, the resist is scanned and illuminated by a focused light spot created by a plasmonic lens.

7. Conclusions

1. The physicochemical properties of glassy chalcogenide semiconductors make it possible to record nanosized elements on them under the action of actinic radiation. When recording nanoscale elements, the main problem is to focus the radiation to the required size. Increasing the resolution of recording media based on vitreous chalcogenide semiconductors can be achieved by choosing recording modes and glass composition, in which the strongest nonlinearity of the exposure characteristics of photosensitive material.

2. Among the optical methods of formation of nano-sized structures, the method, in which the near light field of nanoparticles of noble metals integrated within a thin film of CVS is used, is of high meaning. The main limitations in the use of near-field recording methods are associated with the low recording speed of nanosized elements and the need to maintain the distance between the focusing element and the photosensitive material with high accuracy.

3. The unique properties of CVS allow the formation of microrelief images without selective chemical etching due to the irradiation of thin films with rays of different polarization. One of the main tasks when using this method is the choice of recording modes, which ensures a high rate of formation of microrelief structures.

4. As a promising method of formation of nanosized structures in films of CVSs, may be utilize the optic mechanical method based on the photo plastic effect. The formation of nanosized elements (30-70 nm) in the recording zone occurs during the simultaneous exposure and indentation of the surface of a film.