Chalcogenide phase-change thin films used as grayscale photolithography materials

Chalcogenide phase-change thin films are used in many fields, such as optical information storage and solid-state memory. In this work, we present another application of chalcogenide phase-change thin films, i.e., as grayscale photolithgraphy materials. The grayscale patterns can be directly inscribed on the chalcogenide phase-change thin films by a single process through direct laser writing method. In grayscale photolithography, the laser pulse can induce the formation of bump structure, and the bump height and size can be precisely controlled by changing laser energy. Bumps with different height and size present different optical reflection and transmission spectra, leading to the different gray levels. For example, the continuous-tone grayscale images of lifelike bird and cat are successfully inscribed onto Sb2Te3 chalcogenide phase-change thin films using a homebuilt laser direct writer, where the expression and appearance of the lifelike bird and cat are fully presented. This work provides a way to fabricate complicated grayscale patterns using laser-induced bump structures onto chalcogenide phase-change thin films, different from current techniques such as photolithography, electron beam lithography, and focused ion beam lithography. The ability to form grayscale patterns of chalcogenide phasechange thin films reveals many potential applications in high-resolution optical images for micro/nano image storage, microartworks, and grayscale photomasks. ©2014 Optical Society of America OCIS codes: (160.2900) Optical storage materials; (210.4810) Optical storage-recording materials; (220.3740) Lithography. References and links 1. S. R. Ovshinsky, “Reversible electrical switching phenomena in disordered structures,” Phys. Rev. Lett. 21(20), 1450–1453 (1968). 2. A. Kolobov and J. Tominaga, Chalcogenides: Metastability and Phase Change Phenomena (Springer, 2012). 3. W. Wełnic, S. Botti, L. Reining, and M. 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Introduction
S. R. Ovshinsky in the early 1960s proposed that chalcogenide phase-change thin films can be used in memory applications, thereby marking the beginning of vast and extremely practical fields [1,2].Chalcogenide phase-change thin films have been successfully used as optical storage media because of the optical contrast between crystalline and amorphous states [3][4][5][6][7][8] and are being developed into recording materials of solid-state memory because of the large electrical resistance difference between crystalline and amorphous phases [9][10][11].The optical and/or electronic multi-levels of chalcogenide phase-change structures can be obtained through careful modulation from the input energy, which can provide a simple and efficient foundation for patterning future phase-change devices [12][13][14].Chalcogenide phase-change thin films have been experimentally verified to be good inorganic resists through the etching selectivity of crystalline and amorphous states [15][16][17][18].The optical nonlinear absorption characteristics of chalcogenide phase-change thin films can break through the diffraction limit and realize nanolithography and super-resolution nano-optical memory [19][20][21][22].High electric conductivity and low thermal conductivity make chalcogenide phase-change thin films be good thermoelectric materials [23,24].The characteristic of a single Dirac cone on the surface causes the chalcogenide phase-change materials to be strong topological insulators [25,26].In the present work, we present another application of chalcogenide phase-change thin films, i.e., as grayscale lithography materials.The grayscale patterns can be directly inscribed on the chalcogenide phase-change thin films by a single process through direct laser writing method.
Grayscale lithography is an efficient technique for the fabrication of three-dimensional (3D) microstructures [27], microfluidic device [28], and diffractive and refractive microlens arrays [29,30].The grayscale patterns are required to have more qualifications than common ones.Generally, a good grayscale pattern should meet the following requirements: (1) continuous-tone grayscale level, (2) high resolution, (3) simple process, (4) simple and low-cost material system, and (5) good photo-thermal stability.Currently, the mainstream grayscale patterns are based on chrome on glass and high energy-beam-sensitive glass.For the former, specifically designed hole arrays with different sizes and densities in the Cr film can offer grayscale patterns with a lower resolution, but complicated fabrication processes involving film deposition, lithography, etching, resist striping, etc. are required.For the latter, grayscale patterns need to be manufactured by expensive electron beam writing on very complicated material systems [31].The two techniques discussed are often too costly for practical applications; thus, the creation of a grayscale pattern with low cost is of great significance.However, designing simple and cost-effective fabrication methods remain big challenge.In this work, the grayscale patterns are directly inscribed on the chalcogenide phase-change thin films by a single process through direct laser writing method, which is a simple and cost-effective fabrication method.

Basic principle
In this work, grayscale patterns are produced on chalcogenide phase-change thin films by a single process through direct laser writing method.The physical basis is as follows.Chalcogenide phase-change thin films possess low thermal conductivity, deform easily under pressure, and can be crystallized and melt under focused laser pulse irradiation.According to these characteristics, Fig. 1 gives the formation illustration.A collimated laser beam is focused onto "chalcogenide phase-change thin film/glass substrate" sample, and the chalcogenide phase-change thin film is heated by absorbing the laser energy, as shown in Fig. 1(a).The temperature inside the thin film can be increased up to the melting point by controlling the heat flow direction.Hence, the inner part of the thin film expands because of melting or gasification, which causes the formation of bump-like relief structures on the surface, as shown in Fig. 1(b).The bump may be hollow or looser than the thin film itself.
The bump height and size can be controlled by changing the laser energy.Generally, bumps with different height and size lead to different optical reflection and transmission.Hence, high-resolving continuous-tone grayscale patterns can be realized on the chalcogenide phasechange thin films by accurately changing the laser energy.

Experimental and simulation methods
The photolithography was carried out by a homemade setup of the direct laser writing system, as shown in Fig. 2. A blue-violet laser diode emitted laser beam with a light wavelength of 405 nm.The laser beam was filtered and expanded, and then was reflected into the objective lens (numerical aperture 0.8 ~0.9 NA = ) and focused into a diffraction-limited spot onto the sample surface.The sample of chalcogenide phase change thin film was placed onto a 2dimensional piezoelectric transducer (PZT) movement stage.The blue-violet laser diode can be modulated into arbitrary pulse light through connecting a signal generator to the high frequency TTL analog/digital modulation port.In the process of direct laser writing, the servo tracking was used to keep the sample at the focal plane of the objective lens, where the objective lens was mounted onto PZT with z-direction movement.For serve tracking, astigmatism method was used, where a linearly polarized red light laser beam passed through the polarized beam splitter and 1/4 wave plate, and then entered the objective lens and was focused onto the sample surface.The red light was reflected and passed through 1/4 wave plate, again, and became circularly polarized light.The circularly polarized light was refracted into cylindrical lens and formed focal spot onto the fourquadrant detector.Based to the principle of astigmatism, the shape of focal spot by the cylindrical lens can be tuned among circular spot, the first and third quadrant ellipse spot, and the second and quadrant ellipse spot according to the sample position at the focal plane of objective lens.The objective lens was moved in the z-direction by a PZT according to the signal from four-quadrant detector.The arbitrary patterns can be formed by combination operation of the 2-dimensional PZT movement and signal generator modulating laser pulse.
The samples were prepared by radio frequency magnetron controlling sputtering system.The microscopic structures were observed by scanning electronic microscopy (SEM) (Zeiss Auriga with focusing ion beam milling system, Carl Zeiss, Germany) and atomic force microscopy (AFM) (Multi-mode 5, Vecco).The optical images were obtained by a optical microscopy (Ergolux, Leitz, Germany).The reflection spectra were tested by micro-reflection spectrum (PG2000-Pro, Idea Optics Company, China).
The temperature field profiles were calculated through the analytical and numerical procedures in a multilayer stack illuminated by a laser beam, as previously reported in Refs [32,33], where the step distances in z direction and radial direction are , and 5 r nm Δ = , respectively.The time step is

Temperature distribution by laser pulse heating
According to the description in Fig. 1, a higher temperature inside the sample than the thin film surface is one of the critical factors.Sb 2 Te 3 thin films are used as examples to analyze temperature distribution inside the sample.The sample structure is denoted as "Sb 2 Te 3 /glass substrate".The temperature is calculated through the analytical and numerical procedures in a multilayer stack illuminated by a laser beam, as previously reported [32,33].In the calculation, the incident laser wavelength is (Cρ) 2 10 /( ) . n is a complex refractive index; C and k are heat capacity and thermal conductivity, respectively; ρ is density, and Cρ is the product of C and ρ .
The calculated temperature profiles are given in Fig. 3 for different laser pulses, where the sample consists of a two-layer structure: one is a chalcogenide phase-change thin film, and the other is a glass substrate.The coordinates of thin film thickness direction and distance from the spot center are defined as z and r, respectively.0 r = and 0 z = are defined as spot center and the surface of the phase-change thin film, respectively.
Figure 3(a) shows the temperature profile along the r z − cross-section plane at 5 , where the highest temperature (about 282 C °) is near the sample surface, which exceeds the crystallization temperature of Sb 2 Te 3 of about 150 C ° [36].Consequently, crystallization begins to occur in the spot center.After increasing the laser pulse width to 10   Figure 3 shows that the temperature reaches the maximum at p t t = and then quickly diffuses into the glass substrate.After the laser pulse, heat continues to diffuse toward the glass substrate and the sample temperature quickly decreases during cooling.Figure 4 shows the temperature change for a laser pulse of 60 , and is about 650 C °.The temperature change in the cooling process is analyzed in Fig. 4(c), where the temperature along the thin film thickness direction at 0 r = is presented for different times after the laser pulse.The maximum temperature quickly decreases, and the position of the highest temperature moves toward the glass substrate.Figure 4(d) is the position dependence of the highest temperature on time.During heating, the highest temperature increases with time, and the position of the highest temperature moves toward the glass substrate with heating time.After the laser pulse, the sample begins to cool, and the cooling speed is fast.

Formation of different bump structures by laser pulse heating
Both Figs. 3 and 4 indicate that the position of highest temperature is not at the sample surface but inside the sample, which can cause the formation of bump structures on the chalcogenide phase-change thin films by laser pulse heating.
The chalcogenide phase-change thin films are directly deposited onto glass substrate by magnetron-controlling sputtering method.A focused laser spot is irradiated onto the chalcogenide phase-change thin films.The chalcogenide phase-change thin films are heated by absorbing laser energy, and a series of structural transformation occurs with the volume change.Figure 5  Δ is small and can be neglected in real applications, but the optical contrast (including reflectivity and transmittivity) between point A and point B is large, which can be used in the formation of grayscale patterns.Given that the volume thermal expansion coefficient of the molten phase is almost three orders larger than that of the crystalline state, heating the molten phase leads to large volume expansion.Figure 5  When the temperature exceeds g T , chalcogenide phase-change thin films gasifies and undergoes a very large volume change.For example, when the temperature reaches point E , the volume change is because the volume expansion of the gas phase is far larger than those of the liquid and solid phases.Given that the inner temperature of the sample is higher than that of the sample surface, a hollow cavity forms inside the sample.When the temperature increases, the cavity expands and pushes the sample surface into a cavity-bump structure.The cavity-bump height and size can be precisely adjusted by changing the laser energy.Cavity bumps with different height and size possess different optical reflectivity and transmittivity, which can be used to produce grayscale patterns.When the laser energy exceeds a certain threshold, the cavity bump collapses and ruptures because of the extreme driving force from the gasification effect, which is marked as ruptured bump.The ruptured bump presents different optical reflectivity and transmittivity values with the cavity bump and solid bump, and can be used to reach a grayscale level.
Taking Sb 2 Te 3 thin films as examples, different bumps are formed by changing the laser pulse energy.Figure 6 gives the experimental results, where the laser wavelength and power density are 405nm and 6 2 6.24 10 / W cm × , respectively.Similar to report in Ref [37], at low laser pulse energy, the material never melts, and there is no quenching and reamorphization.Only a shallow crystalline mark is formed.At moderate laser pulse energy, there is a consolidated amorphous bump region generation.At high levels of laser pulse energy, the center of the laser pulse irradiation region evaporates, a hollow core is surrounded by quenched amorphous eruption ring.→ of Fig. 5.A small volume reduction in the crystallization mark region occurs compared with the initial state because of the higher density of crystallization marks than the initial deposited state.
When the laser pulse width increases to ns 10 , small solid-bump structures are produced, as shown in Fig. 6(b), where the inset is the cross-section of solid bumps obtained by focused ion beam milling.The inset indicates that the bump is solid, and without hollow structures occur inside the bumps.The generation process of solid bumps corresponds to stage C D → in Fig. 5.The volume of solid bumps becomes larger than that of the initial deposited state because the solid bumps result from the quenching of the molten phase, and the large volume of the molten state is inherited by the solid bumps.The solid bump should be amorphous state, which is because the formation of solid bump is from quenching-cooling of the molten phase.
By changing the laser pulse width to ns 15 , cavity bumps occur because gasification takes place inside the sample.Figure 6(c) shows the experimental results of cavity bumps, where the inset is the cross-section of cavity bumps obtained by focused ion beam milling.The inset indicates that bumps are hollow because of gasification inside the sample due to the temperature being higher than the gasification point g T .The production of cavity bumps corresponds to stage of D E → in Fig. 5. Gasification inside the sample results in a larger volume than the solid bumps.
Figure 6(d) shows the ruptured bumps obtained with a laser pulse width of ns 30 .The inset is the cross-section of ruptured bumps obtained by focused ion beam milling.According to Ref [37], the ruptured bump can be actually considered to be a hollow core being surrounded by quenched amorphous eruption ring.The generation of ruptured bumps results from the extreme driving force of the gasification effect.

Grayscale patterns written on Sb 2 Te 3 phase-change thin films
According to the above mentioned analysis, different laser pulse energy can produce different bump structures on the chalcogenide phase-change thin films.The height and size of bump structures can influence optical reflectivity and transmittivity and form grayscale patterns, accordingly., respectively.Gray level 1 is the lightest, and the grayscale becomes dark with increasing gray level, and gray level 8 is the darkest.Thus, the patterns can be used as grayscale photomasks for fabricating typical micro-optical elements.In addition, some complex arbitrarily shaped grayscale images can also be formed.
Figure 7(b) is the optical reflection spectra of the gray levels of Fig. 7(a), with the Sb 2 Te 3 base reflectivity being set to 100%.The light wavelength ranges from 450nm to 600nm , which are the common imaging light wavelengths for optical microscopy.The reflectivity decreases with increasing gray level, which may result from the light scattering or trapping effect of micro-structures, which are usually used in anti-reflection films [38] and solar modules [39].The micro-structures generated by low laser power density present low bump height size, thus the light scattering or trap effect is smaller than those formed by high laser power density due to large bump height size, and the weak scattering or trapping effect makes the reflected light be strong, thus the reflectivity increases with decreasing the laser power density.The detailed origin of the different reflectivity of the phase change nanobump microstructues needs to be further considered in future investigations.Figure 7(b) also indicates that the reflectivity difference between two adjacent levels is obvious, indicating that the Sb 2 Te 3 thin film can form continuous-tone gray levels by precisely controlling the laser power density and energy.The formation mechanism of the gray levels is further analyzed by the microscopic structures formed on the surface of Sb 2 Te 3 thin films.
Figure 7(c) is the SEM images of gray level 1 to gray level 8.The microscopic structure size increases with increasing gray level.Gray level 1 is very fine structure with a size of about 0.3 um because the low laser power density induces small solid-bump structures.With increasing laser power density, the cavity-bump structure begins to form, and the grayscale becomes dark.With increased bump size, the grayscale becomes darker and the gray level consequently becomes higher.When the laser power density is increased to 6 2 ~4.98 10 / I W cm × , the microscopic cavity-bump structures become coarse, and the size is about 1um , where the grayscale is the darkest.Thus, bump structures with different height and size, as well as continuous-tone grayscale patterns can be obtained by precisely adjusting the laser power density.
The continuous-tone grayscale images are written through using a home-built laser direct writer of Fig. 2, where a GaN semiconductor laser is used as beam source, and the "Sb 2 Te 3 (100 nm )/glass substrate" samples were placed on the focal plane of the objective lens for raster scanning with power density ranging from 0 to , typically 150 nm ).Continuous-tone grayscale patterns (also see Fig. 8) are obtained through direct laser writing.Different gray levels are realized by adjusting power density, where a high power density yields a low reflectance.Each pixel of the image is assigned to a laser power density to obtain a certain reflectance.The high-resolution optical image shown in Fig. 8(b) is lifelike bird with fine structures.For example, the eyes can be seen in the inset observed by the AFM image.The AFM image is very consistent with the corresponding optical image marked with red circle in Fig. 8(b).The black eyeball has larger microscopic structure than the white eyeball.Figure 8(a) is the original image of the bird, and a comparison between Figs. 8(a) and 8(b) demonstrates that the appearance and expression of the bird have been fully written on the Sb2Te3 thin films.Figure 8(c) is the original image of a lifelike cat.As observed, the cat's eyes show that the cat is concentrating its attention on something.The lifelike cat is written on the Sb2Te3 thin films, as shown in Fig. 8(d).By comparing Fig. 8(d) with Fig. 8(c), the expression and appearance of the lifelike cat are fully inscribed onto Sb2Te3 thin films by precisely controlling the laser power density, which can be seen from fire-like eyes and some fine structures like furs and whiskers.

Conclusion
The bump relief structures can be produced on chalcogenide phase-change thin films by direct laser writing because of the low thermal conductivity and moderate melting temperature.The bump height and size can be precisely controlled by changing the laser energy.Bumps with different height and size cause different optical reflection spectra; thus, the chalcogenide phase-change thin films can be used as grayscale photolithography materials.By using a home-built laser direct writer wherein a GaN semiconductor laser serves as a beam source, the samples are placed onto the focal plane of the objective lens for raster scanning.The continuous-tone grayscale images of lifelike bird and cat are successfully written onto Sb 2 Te 3 phase-change thin films through a single process.The expression and appearance of the lifelike bird and cat are fully presented.The current work provides a way to fabricate complicated grayscale patterns using laser-induced bump relief structures on chalcogenide phase-change thin films, differing from current techniques such as photolithography, electron beam lithography, and focused ion beam lithography.The optical performance of the bump relief structures reveals many potential applications in highresolution optical images for micro/nano-image storage, micro-artworks, and grayscale photomasks.

Fig. 2 .
Fig. 2. Setup of the direct laser writing system.
dissipation, a constant defined by controlling the heat-flow rate from the sample surface, is 7 γ 1 10 = × .The thermo-physical properties used in the calculation are listed as follows [34, 35]: the melting point of Sb 2 Te 3 thin film of 620 C °. Consequently, melting occurs inside the Sb 2 Te 3 thin film, and the heat begins to diffuse into the glass substrate, as shown in Fig. 3(b).

Figure 3 (
c) shows the temperature profile at 15 p t t ns = = , and the highest temperature is about 860 C °, which can be larger than the gasification point of Sb 2 Te 3 , and thus gasification occurs.When the laser pulse width is increased to 30 quickly diffuses into the glass substrate, as shown in Fig. 3(d).Comparison among Figs.3(a)-3(d) reveals that with increased laser pulse width, the highest temperature quickly increases and moves toward thin film thickness direction.
4(a) is the temperature distribution along the r z − cross-section plane, and the highest temperature occurs diffuses toward the glass substrate after the laser pulse.Figure 4(b) is the temperature profile at 70

Fig. 4 .
Fig. 4. Temperature profile for laser pulse width 60 p t n s = at the r z − cross-section: (a) at

1 V
illustrates the volume change with temperature, where the initial sample is an amorphous state at room temperature R T , which is marked as point A. The sample is heated to point B at crystallization temperature c T and becomes crystalline state, which is accompanied with a volume change 1 the density of crystalline state is larger than that of the amorphous phase.The volume change Δ is irreversible with #203161 -$15.00USD Received 17 Dec 2013; revised 10 Feb 2014; accepted 10 Feb 2014; published 24 Feb 2014 (C) 2014 OSA 10 March 2014 | Vol.22, No. 5 | DOI:10.1364/OE.22.004973| OPTICS EXPRESS 4979 decreased temperature from point B to point A .Notably, 1 V

Fig. 5 . 2 V 3 V 2 V
Fig. 5. Volume change with temperature of chalcogenide phase-change thin films.A little volume change

3 V Δ is larger than 2 V
demonstrates that when heating the sample from point C of melting point to point D of the gasification temperature, the accompanying volume change Δ of the crystalline state thermal expansion volume.The volume change 3 V Δ causes the deformation of the sample surface and produces solid-bump morphology on chalcogenide phase-change thin films because of the Gaussian profile of the focused laser spot intensity.The volume change is irreversible when the temperature returns from point D to point C .The optical reflectivity and transmittivity of solid bumps differ from those of the initial state, which can be thus used to form grayscale patterns.

Fig. 6 .
Fig. 6.Laser pulse induced bump structure on chalcogenide phase-change thin film with laser power density of

Figure 6 (
Figure 6(a) presents the crystallization marks formed by 5 ns pulse width.The formation process corresponds to stage A B→ of Fig.5.A small volume reduction in the crystallization mark region occurs compared with the initial state because of the higher density of crystallization marks than the initial deposited state.When the laser pulse width increases to ns 10 , small solid-bump structures are produced, as shown in Fig.6(b), where the inset is the cross-section of solid bumps obtained by focused ion beam milling.The inset indicates that the bump is solid, and without hollow structures occur inside the bumps.The generation process of solid bumps corresponds to stage C D → in Fig.5.The volume of solid bumps becomes larger than that of the initial deposited state because the solid bumps result from the quenching of the molten phase, and the large volume of the molten state is inherited by the solid bumps.The solid bump should be amorphous state, which is because the formation of solid bump is from quenching-cooling of the molten phase.By changing the laser pulse width to ns 15 , cavity bumps occur because gasification takes place inside the sample.Figure6(c) shows the experimental results of cavity bumps, where the inset is the cross-section of cavity bumps obtained by focused ion beam milling.The inset

Fig. 7 .
Fig. 7. Grayscale levels obtained at different laser power densities, (a) optical microscopy reflection images, (b) reflection spectrum, and (c) microscopic structures observed by scanning electronic microscopy.

Figure 7 (
Figure 7(a) shows a set of grayscale patterns with discrete gray levels fabricated on Sb 2 Te 3 phase-change thin film through direct laser writing, thereby displaying good grayscale features.The laser power densities from gray level 1 to gray level 8 are of tens to hundreds of nanoseconds.A bitmap file automatically transformed from an original picture defines the laser power density of each pixel, writing path, and pixel stepping ( 50 200 nm nm to

Fig. 8 .
Fig. 8. Grayscale images written on Sb 2 Te 3 phase-change thin films: (a) original and (b) grayscale lifelike bird images written onto Sb 2 Te 3 thin film with scanning time of 30 minutes, and (c) original (d) grayscale lifelike cat images inscribed onto Sb 2 Te 3 thin film with scanning time of 45 minutes.