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Patterning on glass substrates has been a promising technology for many applications, but it is difficult to make a high contrast pattern without chemical pollution. Glass substrates are basically transparent to 1064 nm laser irradiation and cannot be patterned directly by a 1064 nm pulsed laser. In this article, we report a new technique for laser patterning on glass substrates without damage. This technique has several advantages such as high contrast, high durability and low cost without rigorous experiment environment. By contacting the glass substrate with a graphite plate and using 1064 nm laser to irradiate at the interface, patterning is formed on the rear glass substrate. This method is based on the heating of graphite plate by laser. The confinement of glass substrates and strong absorption of graphite for an incident laser radiation provide the ambient of high local pressure and temperature, which make the graphite particles bond to the surface of glass, thus the purpose of making black color patterning of glass substrates can be achieved. Furthermore, various intricate patterns can be patterned on glass substrates.
© 2017 Optical Society of America
1. Introduction
Glass is a solid, transparent and homogeneous material which is breakable. Generally it consists of silicon dioxide [1]. Glass has various properties, such as transparency to light of a wide spectrum of frequencies, and being durable and inert to many chemicals. These properties make glass an excellent candidate for a wide range of applications, from display panels to biomedical detection [2].
Accordingly, glass patterning is playing a more and more important role among these applications, the patterning on glass substrate provide wealth information. It presents two principal aspects, namely the purely decorative side and the aspect trace ability or identification of products. Indeed, it makes it possible to include in a permanent way of the job numbers, matrix code of data or any other important information on parts and articles [1]. Parker proposed a method and apparatus for patterning egg with an advertisement and a freshness date. In this present method, by using laser irradiate to the shell of an egg to cause discoloration forming the date and traceability code of an eggshell [3]. Qi used a Q-switched Nd:YAG laser patterning process of stainless, in the calibration that utilize charged-couple-device (CCD) to measure the pattern contrast and discuss the relationship of patterning parameters that including average laser power, peck power, pulse frequency, patterning depth, width and contrast [4]. Haferkamp proposed the increasing product imitation and request for plagiarism protection by using laser patterning and infrared technology [5].
However, glass is a fragile material, which is easily crack under excessive mechanical and thermal stresses [6, 7]. Compared with other materials, patterning glass is very difficult and many methods are not suitable. Therefore, it is of great significance to make a high contrast pattern on the glass substrates.
Various glass patterning techniques have been developed and investigated. Mechanical patterning method patterns glass through printing needle to do high frequency impact motion in the effects of compressed air or electromagnetic, but this method can only be used for dot matrix patterning, and its accuracy is poor and the glass substrates are easily to crack [8]. Glass screen printing is using printing plate and frit to print on the glass substrates. However this process needs to use a screen and special ink under high temperature baking to integrated on glass substrates, and the ink jet volume cannot be controlled, so the persistence and quality of the label are poor [9–11]. UV printing technique presses the desired pattern by print head, the method needs to spray the curing agent, but the patterning will fade with the passage of time as before [12,13]. In general, the traditional ink printing has several drawbacks such as chemical disposal into the environment, non-permanent pattern, curing time for ink to dry and untidy working conditions.
Laser patterning is a technology which uses the heating effect of the laser ablation off the object surface materials in order to leave a lasting pattern. Compared with the traditional patterning technologies, it has many advantages such as no pollution, high speed, high quality, flexibility. And it does not contact with the work piece compared with traditional electrochemical or mechanical patterning method. Those advantages make laser patterning technology be applicated widely in many fields of industry, national defense, scientific research and etc [14–16]. In general, the most common lasers used in industrial application include CO2 laser, green laser and ultraviolet laser [16].
However, only white and frosted or slightly darker trace can be patterned on the glass surface by direct laser irradiation, and because of the transparency of glass material, the pattern contrast is poor and hard to identify. In addition, a lot of cracks and debris appear easily under the heat stress because of the hard and brittle materials properties. It will lead to the poor quality on the patterning glass.
Laser induced backward transfer technology can also be applied for transfer of substance towards the target. This is a method for additive patterning, which allows deposition of diverse material microstructures onto different substrates. Kuznetsov discussed the mechanism of the process of the laser-induced backward transfer of gold nanodroplets towards a glass substrate [17]. Feinaeugle studied the transfer regimes and dynamics of polymer flyers from laser-induced backward transfer via time-resolved shadowgraph [18]. Sakata performed patterning of Bi2O3 films by laser-induced and backward transfer techniques using the SHG beam of a Nd:YAG pulsed laser [19]. This technology produced patterning with high repeatability which may approve to be an advantageous for the patterning in glass substrate.
In this paper, patterning on glass substrates with high contrast and good permanence has been fabricated successfully by contacting the glass substrate with a graphite plate and using 1064 nm laser to irradiate at the interface. Through composition analysis and high speed camera observation, we have studied the processing mechanism and carried out the experiment, and some patterns were obtained by this novel method.
2. Experimental
Figure 1 shows the experimental setup of the pulsed laser patterning of glass substrates. A MOPA pulsed fiber laser system (IPG, YLPM series of ytterbium doped fiber laser) at a wavelength of 1064nm with tunable pulse duration (4~200ns) and max output power of 20 watts is used as a light source. Laser beam is reflected by a galvanometer, which can scan the laser beam on the target surface at a high speed by the fast rotating of the attached X &Y mirrors. Then an f-theta lens focuses the laser beam to the contact area between the glass substrates and the graphite plate. The focal length of the F-theta lens is 100mm, the focal spot size of the laser is 0.034mm and the confocal parameter is 2.36mm. The glass substrate is in close contact with the graphite plate pressed by external force, therefore, the confocal parameter of the beam is bigger than the gap between glass substrate and graphite plate. The glass substrate and graphite plate are fixed on an X/Y stage. The galvanometer scanner and the processing parameters are controlled by a PC. It is important to note that all the glass plates are almost transparent (Internal transmission τ ~0.96) for the incident radiation and the absorptance of the extended graphite in a wide range of wavelengths is closed to 1 [20].
The soda-lime glass substrates with a dimension of 25.4 × 76.2 × 1mm (length × width × thickness) are used as samples for the laser patterning. The graphite plate and glass substrate are separated easily by hand after laser pattering. These two materials do not stick together tightly. All the samples are cleaned by ultrasonic cleaning in deionized water for 5minutes to remove the debris after laser patterning. Several characterization tools are employed to analyze the patterning material. Scanning electron microscopy (Philips Nova NanoSEM 230) and white light interferometer (BRUKE) are used to observe the surface morphology of the sample, EDS (Energy-dispersive spectroscopy) was used to analyze the chemical composition of the patterning materials on the glass substrates. X ray diffract meter (Rigaku Corporation, D/MAX-ULTIMA IV) is used to analyze the material phase of the patterning material.
3. Results and discussion
3.1 Morphology analysis
Figure 2 shows the SEM photographs of QR code patterned on the glass substrate at the laser fluence of 88.113 J/cm2, the scanning speed of 150mm/s, the pulse duration of 14ns and the repetition frequency of 50 kHz. It is found that graphite particles are adhered to the glass substrate. The dark color area is the glass substrate, while the white color area is the patterning layer. From the enlarged photo of the patterning edge area, the granular material is attached to the glass with the diameter of about 200nm.
Graphite particles are expected to be deposited onto the reverse side of glass substrates by observing the morphology. EDS analysis are carried out at those specific patterning surfaces to detect whether carbon is deposited. Prior to the EDS analysis, the samples have been washed in deionized water in an ultrasonic bath for 5 minutes to dissolve and remove any debris and contaminations. Figure 3(a) shows the EDS result of the unpatterned area of glass substrate. It is found out that the unpatterned glass surface mainly contains the element of oxygen and silicon, and the sample used is soda lime glass, therefore, the element of sodium, magnesium and calcium have been detected. Figure 3(b) shows the EDS result of the patterned area of glass substrates. It is found out that the patterned glass surface mainly contains the element of carbon, oxygen and silicon, and the carbon content is as high as 62.119%, which means the main ingredient of the black patterning are carbon particles. In addition, it is found that the elements of the pattern contain sodium, calcium which is the main elements of the soda-lime glass. Therefore, we can conclude that the glass melts and splashes during the laser processing.
However, there are a variety of carbon allotropes. To further determine the phase components of the particles, the deposited particles on the glass substrates are collected to analyze the phase of the particles by X-ray diffract meter, shown in Fig. 4. It is found that the diffraction pattern of the particles is in agreement with the graphite phase. The X ray diffraction images are not clearly enough, that’s because the glass melts and splashes during the laser processing, which means the other elements might mix with the main graphite substance. Therefore the deduction of the graphite phase making up the deposited layer is reasonable. Therefore, the composition of the pattern can be identified as graphite particles.
3.2 Endurance of the pattern
Figure 5 shows the sample before cleaning, washed by deionized water and sulphuric acid solution respectively. Durability is an important indicator of the patterning on the glass substrate. In order to prove its good persistence under the corrosion environment, the patterned sample is washed with deionized water and sulphuric acid solution respectively in ultrasonic cleaning, which can verify the pattern still has good quality under the effect of ultrasonic shock and chemical corrosion. During ultrasonic washing by deionized water, the ultrasonic vibration can exert the mechanical force on the deposited graphite layer. It still remains good surface quality of patterning after ultrasonic cleaning. Therefore, the black patterning has a very good durability and mechanical stability under harsh environment. In addition, the pattern has good abrasion resistance, it is difficult to remove using mechanical, and this pattern can exist for a long time exposed in air, which means the graphite is incorporated into the glass at its surface permanently.
Fig. 5 (a) Before cleaning (b) Washed by deionized water (c) Washed by sulphuric acid solution Endurance test of samples.
The main reasons are explained as follows: (1) Because the graphite particles adhering to the glass surface is in black color, the pattern contrast is intensely high; (2) Due to the stable physical and chemical properties of the graphite itself, and the strong corrosion resistance, it makes the prepared pattern with good corrosion resistance;(3) During the formation of the patterning, graphite sheet is boiled to graphite particles with quite small particles size about 200nm, such a small particles size can ensure a tight binding of the patterning.
3.3 Mechanism of Pattern formation
3.3.1 Theoretical calculation
The process of black patterning formation on the glass substrates is determined not only by laser processing parameters but also the physical thermal characteristics of glass. This implies that the main glass physical-thermal characteristics (Table 1) are such as its transparency, optical homogeneity, thermal stability, density and melting temperature.
Table 1. The main physical-thermal characteristics of glasses [21,22]
To explain the mechanism, it is necessary to take into account the conductive heating of glass due to a heat contact with graphite plate. The maximum temperature at the graphite surface in the center of irradiation can be estimated as [23]:
Where Ac-absorbability of extended graphite (Ac~1.0), ɑc-thermal conductivity of extended graphite (ɑc = 1.24∙10−4 m2/s), kc-heat diffusion of the graphite (kc = 2000W/(m∙K)), τ-pulse duration (14ns) and q0-the laser power density in the focal spot (q0~5.97∙106W/cm2 at the average laser power 12W and the pulse repetition rate 70kHz). TH is the room temperature. The calculated Tc, the maximum surface temperature values of cellular graphite, can reach up to 4.0∙103 K and even more, which is bigger than the boiling point of graphite of 3700K. Therefore, the graphite plate will be evaporated to graphite particles.
Gasification occurs when the temperature of the target surface exceeds the boiling point. As the vapor particles continue to absorb the laser energy, the temperature of the vapor continues to rise, then the vapour ionization and formed the plasma with high temperature and density, the plasma absorbs the remaining laser energy rapidly, and expands rapidly, forming the plasma shock wave [24].
It brings us to the conclusion that the part of the absorption energy was spent to kinetic energy of graphite particles flying with finite rate during plasma formation. It was suggested to calculate the particles separation speed through the energy conservation law. The suggestion was made that the absorbed radiation energy spent at graphite heating up to the evaporation temperature and the total kinetic energy of all flying particles removed from the surface with average speed. Taking into account the pulse duration we supposed that the material mass in the area of radiation broke away in form of the graphite particles during the ablation process. Having done the assumption, we could calculate the graphite particles average speed from the area of radiation [20]:
Where ρ-graphite density (ρ = 2265kg/m3), d0-radiation area (d0 = 17∙10−6m), h-the removable depth in graphite plate (h = 6μm) and Lu-graphite vaporization heat (Lu = 50∙103J/kg) .Having the average power of Pav = 12W, the graphite particles average speed could reach 7∙103m/s. And by the calculation of formula (1), the temperature of the irradiation area is above the glass softening temperature. Therefore, the change of surface topography is censured by mechanical treatment of the graphite particles flying with high speed and high temperature.
3.3.2 High speed camera observation
In order to study the formation mechanism of the pattern, it is necessary to observe the interaction of laser with material more directly. Therefore, we removed the confining plate, using high-speed camera (FASTEC IMAGING company, America) to observe the corresponding phenomenon under different laser fluence. The integration time of the high speed camera is 17239μs, which means it needs 86.2ms to take five consecutive photos, and 172.4ms to ten photos. Figure 6 shows the effect of different laser fluence on the laser patterning under the condition of the scanning speed of 400mm/s, the repetition rate of 70 kHz and the pulse width of 14ns. The laser fluence is 62.938J/cm2, 75.526J/cm2, and 88.113J/cm2 respectively. Due to the limitation of the equipment, it is difficult to take the photos at narrower pulse duration in the nanosecond range, however, from the graph we find that dazzling flash of plasma were produced at the moment of laser ablation on the graphite plate, then a lot of granular material fell off from the target surface. The reason for explaining the result is that the plasma shock wave is produced during the process. Under the effect of plasma shock wave, the graphite particles are separated from the graphite surface quickly, and contacted with the melting glass surface, and finally combined with the surface of the glass substrate. Higher energy absorbed by the graphite target will result in more graphite particles bombarded. Therefore, more graphite particles combined with the glass substrates.
Fig. 6 High speed photos of laser ablation of graphite target and the pattern with different laser fluence under the condition of the scanning speed of 400mm/s, the repetition rate of 70kHz and the pulse width of 14ns.
When the laser fluence is 75.526J/cm2, the best quality of the pattern is obtained. Subsequently, with the increasing of laser fluence, the surface of the glass is easily broken due to high laser energy, which leads to the poor quality of the pattering. The reason for this phenomenon is that the energy absorbed by the target increases with the increasing of laser fluence, and the number of particles released from the target surface increase per unit time, and the particles attached to the glass surface also increase, which will improve the contrast of the pattern. Large laser fluence means large absorption, which results in the damage to the glass substrate and the make the pattern quality worse.
3.4 Patterning process
In conclusion, the physical process can be described in four stages [25–28], as illustrated in Fig. 7(a)–7(d). In the first stage, the graphite target is ablated by pulsed laser irradiation (Fig. 7(a)), graphite particles vaporize immediately and create a dense plasma plume. In the second stage, the plasma explodes violently in the limited space and continues absorbing the laser energy as the laser pulse is applied. The heating and condensation of the plasma result in local high temperature and pressure [25].The third stage is the rapid quenching of the high-temperature high-pressure plasma (Fig. 7(b)), as the continuous laser ablation, surface of graphite is removed and result in the formation of graphite particles with high kinetic energies, more number of energetic carbon species are activated [27], and glass surface begins to soften. Under the effect of local high temperature and high pressure, the graphite particles are separated from the graphite surface quickly, and contacted with the melting glass surface, and finally combined with the surface of the glass, forming a black pattern (Fig. 7(c)-(d)). The laser-induced shock pressure in this process is advantageous for the formation of pattern. As laser fluence increases, higher local pressure is generated. Thus more graphite particles are separated from the graphite plate and adhered to the rear side of the soda lime glass substrate, which increases the contrast of the image and enhances the efficiency and quality of the patterning.
3.5 Application
During the glass substrates production process, pattern are used to decorate, track progress, and identify components, the pattern on glass substrate provide wealth information. We used this method to prepare some patterns on the glass substrate. Figure 9 shows the dot array prepared under the condition of scanning once, Fig. 8(a) and Fig. 8(b) represents the surface topography and three dimensional topography. The laser fluence is 75.526J/cm2, the scanning speed of 400mm/s, the repetition frequency of 70 kHz and the pulse width of 14ns. The dot array with a minimum diameter of 0.5mm can be obtained by this method.
Fig. 9 (a) Surface topography of the stripe structure, (b)Three-dimensional topography of the stripe structure.
Figure 9 shows the stripe structure prepared under the condition of scanning once, Fig. 8(a) and Fig. 8(b) represents the surface topography and three-dimensional topography. The laser fluence is 75.526J/cm2, the scanning speed of 400mm/s, the repetition frequency of 70 kHz and the pulse width of 14ns. The stripe width is 0.2mm with separation distance of 0.5mm can be obtained by this method. Due to the electrical conductivity of graphite, this method may be used to make some conductive structures on glass substrates.
4. Conclusions
In this paper, we develop a new technique to make high contrast and durability patterning on glass substrates. Graphite plate is used as the absorbing layer for the patterning of the glass substrate, 1064 nm light is irradiated onto the sample from the top and etching occurs on the surface in contact with the graphite. It is found that transparency and confine of glass and strong absorption of graphite for an incident laser radiation provides the ambient of high local pressure and temperature, which make the graphite particles bond to the surface of the glass. EDS and XRD analysis confirm the graphite deposition after 1064 nm laser irradiation. Using this technique, it offers potential to replace the ink printing on the related field.
Funding
National Nature Science Foundation of China (No. 51575114); Ordinary University Characteristics Innovation Project of Guangdong Province (Natural Science, No. 2014KTSCX059); Guangzhou Science and Technology Project (No. 201607010156); Guangdong Natural Science Foundation (No. S2013010014070) and Open Fund of Jiangsu Key Laboratory of Precision and Micro-manufacturing Technology.
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