2.1.

BR-LIFT Method

Figure 1 shows the BR-LIFT process. When a ring-shaped laser beam irradiates the donor layer from a transparent substrate side, a ring-shaped vapor bubble (hereinafter, BR) is generated in the boundary area between the transparent substrate and the frontside of the donor substrate, as shown in Fig. 1(a). In addition, low viscosity regions are formed near the BR in the boundary region and on the backside of the donor layer. This BR creates an inward pressure toward the axis. This phenomenon is consistent with the lateral flow observed in the central zone when two expanding bubbles are slightly separated.^19–21 Then, BR rapidly expands at an extremely high pressure, which drives the wrapped donor volume. The low-viscosity liquid envelops a high-viscosity liquid. Finally, the donor is ejected from the backside of the donor substrate and lands on the receiver, as shown in Fig. 1(b).

Fig. 1

BR–LIFT process: (a) the bubble ring is generated and (b) the donor is pushed out.

Figure 2 shows the BR-LIFT optical system. The light source used in the BR-LIFT system must exhibit a high-quality beam profile and high power. Thus, a nanosecond-pulsed fiber laser is used as the laser light source with a wavelength of 1064 nm. The maximum energy of the pulsed laser is 0.2 mJ when the repetition rate is 50 kHz. The pulsed laser passes through a spatial isolator, quarter-wave plate, collimator lens, and acousto-optic modulator (AOM). The repetition rate of the pulsed laser is controlled using AOM. The laser then passes through a nonlinear optical crystal, inducing second-harmonic generation (SHG). The SHG converts a wavelength of 1064 nm into that of 532 nm.

Fig. 2

Schematic diagram of the BR-LIFT system: AOM, acousto-optic modulator; SHG, second-harmonic generation element; HS, harmonic separator; ND, neutral density filter. SHG converts a wavelength of 1064 nm into that of 532 nm. The pulse duration is 8 ns.

Next, the laser beam passes through a wave front converter to make corrections, such as astigmatism.²² Astigmatism is one of the most important factors that must be eliminated for a successful implementation of the BR-LIFT. The laser beam with a wavelength of 532 nm is converted into an annular beam by passing it through a beam converter. The beam converter consists of a reflective spatial light modulator (SLM) and a prism mirror.¹⁸ The laser beam is reflected by the prism mirror and reaches the SLM. When the laser beam is reflected by the SLM, it is converted into the annular beam. The laser beam is then reflected again by the prism mirror and travels in the same direction as the incident beam.

Then, the laser beam passes through the mirror and other optics and is deflected using a galvano scanner. Finally, the laser beam focuses near the donor layer applied to the transparent substrate using the $f\theta$ lens. The incident beam diameter to the $f\theta$ lens is 3 mm, and the focal length of the $f\theta$ lens is 100 mm. The receiver is placed parallel to the donor layer and mounted on an adjustable stage. The annular beam maintains a constant profile at a distance of a few millimeters from the focal position. Figure 3 shows the annular beam with a ring diameter of $84\mu \mathrm{m}$ placed on the overfocus side by 0.5 mm from the focal position.

Fig. 3

Beam profile: the ring diameter was $84\mu \mathrm{m}$.

The light intensity distribution in the donor layer can be obtained using the Fresnel diffraction equation.²³ Figure 4 shows the light intensity distribution near the focal plane. The $x_i\text{–}{y}_i$ plane represents the frontside of the donor. The Fresnel diffraction equation is represented as

Fig. 4

Light intensity distribution near the focal plane.

Fraunhofer diffraction occurs when the $x_i\text{–}{y}_i$ plane coincides with the focal length $f$, and the light intensity distribution is represented using the Fourier transform of $g(x_0,y_0)$. Next, the light intensity distribution near the focal plane is derived. When $\alpha$ is defined as

3.1.

Relationship Between Viscoelastic Properties and Fluence Threshold

Clarifying the effect of the ink property is important to achieve high-quality printing. The donor viscosity is an important property involved in the interaction with light and significantly affects the droplet quality. The donor materials used in the experiment are NNFs and contain light-absorbing pigments and dyes. NNFs greatly vary in viscosity based on a shear rate.²⁷ It is unclear under which conditions viscosity is an appropriate as an evaluation index to explain the LIFT phenomenon.

Herein, we evaluated the correlation between irradiation energy and ink viscosity for each stress obtained in dynamic viscoelasticity measurements. In the viscoelasticity measurement, the storage elastic modulus ($G^{\prime }$) and loss elastic modulus ($G^{″}$) can be obtained by applying stress to the liquid and detecting the strain and phase difference caused by the stress. The viscosity ($\eta =\sqrt{(G^{\prime 2}+G^{″2})}/2\pi f$) is derived using $G^{\prime }$ and $G^{″}$. The dynamic viscoelasticity was measured using a rheometer (HAAKE RheoStress600, Thermo Fisher Scientific).

Seven donor materials (types A to G) were fabricated using three ultraviolet (UV) curable inks (UV CORE type A [pigment red 57.1], UV CORE type A [pigment yellow 13], and BEST CURE UV flexo500 process red by T&K TOKA) and two viscosity modifiers (UV DG reducer and BEST CURE No. 2 UV contex by T&K TOKA).

Figure 5 shows the relationship between shear stress and viscosity measured using the rheometer. The seven donor materials are all NNFs. Types A and B inks are pseudoplastic fluids whose viscosity decreases by two orders of magnitude in the range from 1 to 1000 Pa. The viscosities of types C to G are adjusted to an almost constant viscosity that does not depend on a shear stress.

Fig. 5

Viscoelastic properties.

Table 1 shows the measurement results of different ink types and viscosities. Considering the four ink types A to D, at 1 Pa, types A and B exhibit approximately the same viscosity, and types C and D exhibit approximately the same viscosity. Additionally, at 1000 Pa, types A and D exhibit approximately the same viscosity, and types B and C exhibit approximately the same viscosity.

Table 1

Ink type and viscosity measurement results.

Figure 6 shows the relationship between laser fluence and the droplet diameter using type D donor materials as a typical sample. The ink donor substrate used in the experiment was $20\mu \mathrm{m}$ thick, and the transmittance of the donor layer was a few percentages or less. The transmittance was measured with a UV–Vis–NIR spectrophotometer UV3600 manufactured by Shimadzu. The receiver substrate was a photo paper, and the gap was set to $500\mu \mathrm{m}$. One-shot exposure was performed at a scanning speed of $100\mathrm{mm}/\mathrm{s}$.

Fig. 6

Relationship between fluence and droplet diameter using type D donor materials.

Experimental results show that the diameter of the droplet remains constant regardless of the irradiation energy. Moreover, no droplets are generated when the fluence energy is reduced below the threshold. The fluence threshold is the optimal fluence for achieving high quality droplets. This fluence threshold is defined as $F_{\mathrm{th}}$. In the result of Fig. 6, $F_{\mathrm{th}}$ is derived as $0.42\mathrm{J}/{\mathrm{cm}}^2$. The results show the same tendency in other donor materials.

Figure 7 shows the experimental results of landing droplets. Figures 7(a)–7(g) correspond to types A to G donor materials, respectively. Experiments were basically performed under the same conditions as in Fig. 6. BR-LIFT enables the printing of high-quality droplets for any materials listed in Table 1.

Fig. 7

Experimental results of landing droplets. (a)–(g) The experimental results of types A to G donor materials, respectively. Printing results obtained in a line at an interval of $200\mu \mathrm{m}$ using a galvano scanner. The gap was set to $500\mu \mathrm{m}$. The bright spots on the left and right sides of droplets are artifacts owing to the illumination light from diagonally left and right.

The experiment was performed by exposing a range of 10 mm at an interval of $200\mu \mathrm{m}$. Multiple experiments were then performed while changing the exposure energy, and the minimum exposure energy required to print droplets was determined as the optimum exposure condition. Type F is different color from other types because it contains yellow ink in addition to magenta. The droplet diameters are slightly different depending on the different materials, but the reproducibility of the droplet diameters is high in the same donor materials. It was possible to print high-quality droplets for materials with a viscosity of 0.6 to 197 Pa·s under a shear stress of 1 Pa. These facts are reliable evidence of this method.

Figures 8(a)–8(d) show the relationship between the shear-stress viscosities of types A to G donor materials and the measured fluence thresholds to clarify the effective viscosity in the LIFT phenomenon. The dotted lines represent the logarithmic approximation lines. $R^2$ represents the correlation coefficient. $F_{\mathrm{th}}$ had a weak correlation with the viscosity at low shear stresses, i.e., 1 to 100 Pa. $F_{\mathrm{th}}$ had the highest correlation with the viscosity at 1000 Pa. It is presumed that the behavior of the ink when stress is applied is related to the flight state. The donor layer receives the gas pressure generated by the increasing heat in the high-viscosity ink owing to laser irradiation, as shown in Fig. 1. It is suggested that this ink was subjected to a stress equivalent to at least 1000 Pa in the flight process.

Fig. 8

Relationship between shear stress and fluence threshold.

It is considered that the viscosity at 1000 Pa has a high correlation with $F_{\mathrm{th}}$ and accurately represents the material properties in the BR-LIFT system. Hereinafter, the viscosity at 1000 Pa is defined as the effective viscosity. Figure 8(d) shows that there is a logarithmically high correlation between an effective viscosity and $F_{\mathrm{th}}$.

3.2.

Relationship Between Donor Layer Transmittance and Droplet Quality

The transmittance of a donor layer is known to depend on the droplet quality; however, this phenomenon is not yet fully understood quantitatively. For quantitative evaluation, a sample with a constant viscosity must be prepared. Therefore, we produced samples with different transmittances and a constant effective viscosity by changing the mixing ratio of two ink types with similar viscosities at 1000 Pa.

The donor material with the effective viscosity of $\sim 4\mathrm{Pa}·\mathrm{s}$ was used in the experiment. To fabricate the samples, a high-transmittance yellow ink (UV CORE type A yellow) and low-transmittance magenta ink (UV CORE type A red) were blended at ratios of 0.5%, 1%, 2%, and 5%. 5% magenta ink is the same prescription as type F in Table 1. All ink materials are obtained from T&K TOKA.

Table 2 shows the formulation of various inks. Four types of mixed inks were used. Multiple levels of film thickness were prepared. The film thickness was a value calculated from the coating weight and coating area, assuming that the specific gravity of ink was 1.1. Different experimental samples were prepared with a film thickness of 8 to $27\mu \mathrm{m}$ and transmittance of 4% to 51%. The transmittance was measured with the spectrophotometer UV3600.

Table 2

Formulation of various inks.

Figure 9 shows the quality evaluation of landing droplets. The evaluation result “good” represents a state in which almost circular droplets are formed without satellites or with few satellites, as shown in Fig. 10(a). The evaluation result “not good” represents a state in which satellites and mists are generated in addition to droplets, as shown in Fig. 10(b). The judgment criteria were determined by the rate of satellite occurrence. If the number of satellites is 1 or less per dot, it is accepted.

Fig. 9

Relationship between film thickness, transmittance, and droplet quality.

Fig. 10

Sample of the guideline for judgment criteria: (a) good sample and (b) not good sample. The judgment criteria are determined by the rate of satellite occurrence. If the number of satellites is one or less per dot, it is accepted. (a) Although there is the satellite, it is judged as good because it is a single satellite in five dots. (b) It is judged as not good because it is five dots and there are five or more satellites.

The droplet quality under the condition of transmittance of 10% or less was good, as shown in Fig. 9. When the transmittance was 10% or more, the droplet quality gradually deteriorated. The droplet quality under the condition of transmittance of 10% or more and 20% or less is now mixed with good and not good. Furthermore, when the transmittance exceeded 20%, the droplet quality deteriorated overall.

The BsF was calculated to analyze the results of these experiments. We believe that BsF on the incident beam is an important parameter for achieving droplet quality. Moreover, the fluence of the ink surface on the incident side is called the frontside fluence (FsF) to distinguish between it and BsF. BsF does not affect reflectance. BsF is represented as

Figure 11 shows the relationship between the transmittance of the donor layer and fluence. The FsF shows no correlation with transmittance, as shown in Fig. 11(a). In contrast, the BsF showed a high correlation with the transmittance regardless of good or not good, as shown in Fig. 11(b). These results indicate that BsF is an important parameter for achieving high-quality droplets.

Fig. 11

Relationship between the transmittance of the donor layer and fluence: (a) front side fluence and (b) backside fluence.

The viscosity of the donor used in the experiment decreases as the temperature increases. The increasing temperature of the backside was calculated to understand the reason why the transmittance and BsF were highly correlated. The calculation of a backside temperature does not consider the effects of heat conduction. The donor expands and forms filaments in only $\sim 5\mu \mathrm{s}$ after laser irradiation. When the laser irradiates the front surface of the donor layer with a film thickness of $20\mu \mathrm{m}$ and a thermal conductivity of $0.27\mathrm{W}/(\mathrm{m}·\mathrm{K})$, the increasing temperature on the back surface after $5\mu \mathrm{s}$ is estimated to be only 1 deg or less. Thus, the effect of heat conduction can be ignored.

The increasing temperature of the liquid $T_r$ is obtained as

When the light intensity incident on the donor layer is $I_0$, the light intensity $I$ on the backside of the donor layer is defined as $I=I_0\exp (-\alpha t)$ using Beer–Lambert’s law. Here, $\alpha$ is the absorption coefficient, $t$ is the film thickness, and $I/I_0$ is the absorption rate at film thickness $t$. This implies that the amount of heat input differs in the depth direction and that the increasing temperature varies. Figure 12 shows the relationship between the transmittance of the donor layer and the increasing temperature $T_r$. $T_r$ is obtained by substituting $Q$ derived from Eq. (6) and Beer–Lambert’s law into Eq. (7).

Fig. 12

Relationship between the transmittance of the donor layer and the increasing temperature on the backside.

The experimental results were divided into two categories at an increasing temperature of $\sim 55\mathrm{K}$ (hereinafter, $T_b$), as shown in Fig. 12. As accurately measuring the temperature is difficult, $T_b$ is just a reference value. It was revealed that the temperature of the backside affects the high-quality printing.

We consider the causal relationship between the increasing temperature on the backside of the donor layer and the generation of satellites and mist. The temperature of the donor layer increases owing to heat accumulation under laser irradiation.

When the donor material complies with Andrade’s equation, the viscosity of the donor layer decreases because of the temperature increase associated with laser irradiation.³⁰ Andrade’s equation has been proposed to describe the temperature dependence on the viscosity

Fig. 13

Temperature dependence on effective viscosity.

The viscosity of high-viscosity ink shown in Fig. 13 decreases as temperature increases, i.e., the ink viscosity decreases to 0.13 Pa at a temperature of 75°C. When the temperature increases above $T_b$, the ink viscosity decreases, and mist is likely to form under low-energy conditions. In other words, if the transmittance is excessively high, the temperature of the backside of the donor layer becomes excessively high and owing to a decrease in the viscosity, mists will be easily generated. For a high-transmittance material, BsF can be reduced by increasing the film thickness. Thus, the film thickness must preferably be set according to the absorption characteristics of the donor material.

It is estimated that the donor material partially begins to undergo thermal pyrolysis at 200°C and reaches thermal pyrolysis completion at 500°C or higher. BR is caused by the gas pressure generated by the increase in heat in the high-viscosity ink associated with laser irradiation.⁴ It can be estimated that when the frontside temperature rises by 500 K due to laser irradiation, the backside temperature with a transmittance of 10% rises by 50 K. In the case of a donor layer with a transmittance of 10% or less, BR-LIFT experiment can be carried out under the favorable conditions of $T_b$ or less. In contrast, when the transmittance exceeds 10%, the increasing temperature of the backside becomes higher than $T_b$, resulting mist generation. Alternatively, if the experiment is conducted under the energy condition of $T_b$ or less, BR cannot be formed on the frontside due to insufficient energy. Therefore, the conditions for achieving both frontside and backside temperatures of the donor layer are set within a limited range.

3.3.

Discussion

In this study, we clarified that a shear stress of 1000 Pa should be set as the effective viscosity as an evaluation index for explaining the LIFT phenomenon. We also clarified that BsF for a transparent material is an important parameter that affects print quality. Table 3 shows a comparison between BR-LIFT and typical LIFT.

Table 3

Method comparison.

These findings suggest that BR-LIFT can be used in various practical applications. Specifically, a thick coating layer will be feasible for printed electronics and digital deposition by adopting a high-viscosity material. Digital full-color printing utilizes cyan, magenta, yellow, and black inks. By adjusting the film thickness to optimize the BsF, it should be possible to print any color with high quality, even if the transmission of each ink is different.

BR-LIFT can contribute to the mass production process due to the advantage of its long flight distance. Using a diffractive optical element instead of an SLM, a simple and low-cost optical system can be developed. The donor material is limited to an effective viscosity of 6 Pa·s or less. If BR-LIFT can be applied with effective viscosity materials of tens of Pa·s, screen printing ink and letterpress ink can be used. Thus, our proposed method should be a promising alternative to the conventional method.