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Laser Transfer
Laser Transfer of Metals and Metal Alloys for Digital Microfabrication of 3D Objects Michael Zenou,* Amir Sa’ar, and Zvi Kotler Digital and 3D functional printing technology is of growing interest in science and technology. Recently, it was referred as “the print revolution”[1] and “the third industrial revolution”[2] due to the novel capabilities offered by these methods such as rapid fabrication/prototyping,[3,4] the ability to combine multimaterials,[5–8] tunable material properties in all three dimensions at the voxel scale,[9,10] high resolutions,[11,12] as well as the prospective potential to replace the current complex fabrication of devices by a single machine, the socalled “3D printer.” Typically, digital functional printing has evolved from well-known methods in the graphic arts industry including screen printing,[13–15] inkjet printing,[16–21] and flexography and gravure[22–24] with advances made in developing new functional materials and adapting them to well-known printing methods. This has motivated a major effort in developing new functional materials, which can fit traditional printing techniques. However, a parallel effort has been undertaken to develop novel digital printing techniques[25–38] with improved performance, which can better fit the growing demand for printed functional structures. The capability to print various metals plays a major role in making this transformative technology real, as most contemporary 3D printing methods rely on polymeric materials which lack the electrical, thermal, and mechanical properties required for functional device manufacturing. Metal deposition is carried by one of the three main methods. The first is an atomic deposition process, i.e., thermal/plasma evaporation in vacuum or an electrolytic reduction process of ionic metal in solution to grow metal layers. Such nonlocal deposition methods require complex pre- and post-treatment steps in order to define a 2D pattern.[12,16] Unlike digital printing, these methods also require specific environmental conditions, receiver substrates, and patterning steps. A second approach relies on various phases of nano- or microparticles, namely colloids,[16–21,25–27,31–38] aerosols,[30] and
M. Zenou, Dr. Z. Kotler Additive Manufacturing Group Orbotech Ltd. P.O. Box 215, Yavne 81101, Israel E-mail: [email protected] M. Zenou, Prof. A. Sa’ar Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and Nanotechnology The Hebrew University of Jerusalem Jerusalem 91904, Israel DOI: 10.1002/smll.201500612
4082 www.small-journal.com
powders.[39–42] “Metal inks” can be prepared from the colloid phase and can be printed using digital dispensing systems. The metal inks[27] can be dispensed drop-by-drop[16–21,34,37,38] or as a continuous filament.[25–27,33] There are several known limitations associated with the current printing methods. For one, while several sources for noble metal inks based on silver and gold can be found, and recently nanocopper inks have started to emerge as well,[32,43,44] there is a rather limited range of metals that can be rendered printable as inks or pastes. There is typically a thermal post-treatment step, which involves sintering the metal particles in order to make the print track conductive. This last step often limits the type of substrates that can be used and also impairs the line conductivity. The third category of metal printing, which is the focus of this article, consists of transferring molten metal from the bulk phase. One way of doing this is similar to the previously mentioned dispensing methods, but here the dispenser is made of a thermally resistant material, which must be chosen according to the melting temperature of the metal to be transferred.[29,45–50] These methods clearly favor the transfer of metals with low melting temperature such as gallium alloys,[29] solder,[45–48] and aluminum,[49,50] and they become more complex for metals with high melting temperature. Another limitation is droplet size due to the high viscosity of the molten metal and dispenser engineering. This results in droplets in the picoliter range or larger. A different approach to printing from the bulk metal phase is laser-induced forward transfer (LIFT).[51] This method relies on a so-called “donor,” which consists of a transparent substrate coated by a thin layer of the material to be printed (Figure 1a). Typically, the layer thickness is on the order of few tens of nanometers. Metal donor layers are prepared by various standard methods such as evaporation or sputtering. To date, LIFT printing of various solid materials has been demonstrated, e.g., metals,[52–57] semiconductors,[58] dielectrics,[59,60] organics,[61–64] and more. With the LIFT technique, metal microdroplets[65–67] and sub-micrometer droplets[68–70] can be printed directly from the bulk solid phase, thereby avoiding the complexity involved in preparing metal ink formulations. LIFT printing of 3D microstructures was first proposed by Piqué and co-workers.[36] In this technique, the printed pixels are made of materials with special rheological properties as needed for such “decal” transfer.[36] Also the printed pixel shape is identical to the laser pulse spatial shape, which defines the printable pattern shape. Chichkov and co-workers
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. a) A scheme of the LIFT printing process, the supplier material. The so-called donor is composed of a transparent substrate coated with a thin layer of metal, M1. a1) A focused laser beam liquefies the metal donor layer M1; a2) the molten material is transferred to the receiver; a3) The droplet solidifies on the substrate; b) A scheme of the LISAT process which follows the same steps as in (a), however with a multilayer metal donor stack (M1/M2) instead of a single metal donor layer.
have first demonstrated LIFT-printed 3D microstructures in the transfer regime of sub-micrometer droplets.[70] He proposed a method to print 2D and 3D microstructures with high lateral resolution. In this work, we rely on recent LIFT printing work carried out by our group[71] in which stable jetting from a relatively large donor-to-acceptor distance (e.g., the printing gap) was demonstrated by using subnanosecond pulses and micrometer-scale thick metal layers. We demonstrated for the first time, printing of an arbitrary, micrometer scale, copper object digitally printed under ambient atmospheric conditions. The printed structure consists of overlapped molten copper metal droplets. Scanning electron microscope (SEM) images of an array of single voxels are provided in Figure 1 (Supporting Information). The voxel is a microdroplet of average volume of 20 fL. The single droplet average diameter is 6.5 µm and the average height is 600 nm. This result does not reflect the highest potential resolution of LIFT-printed 2D and 3D metal structures, since printing of sub-micrometer metal voxels was already demonstrated when using a femtosecond laser source.[70] The same method allows for printing of the support material, an essential part of complex shape 3D structures. For printing the support material, we used a novel method that we have termed laser-induced selfalloying transfer (LISAT) (Figure 1). Our method amounts to LIFT printing droplets of a slightly alloyed metal alongside the pure metal, e.g., copper and its alloy. The chemical potential difference between the pure LIFT printed metal and the printed LISAT droplets allows for selective etching of the more anodic metal in a standard copper etchant, due to the galvanic effect. Figure 1a shows the standard LIFT transfer of molten metal. The donor is composed of a transparent substrate (in small 2015, 11, No. 33, 4082–4089
our case, a 1 mm thick soda-lime glass) coated with a thin layer of the material to be transferred. In our study, we used a copper donor with a thickness of 500 nm, which was obtained by physical thermal evaporation from a target composed of 99.9% copper. We can identify three basic steps in the droplet formation and transfer: laser-induced heating of the metal layer by a focused pulse and the thermal propagation toward the free metal–air interface (Figure 1a1,b1), mechanical ejection of the molten material (Figure 1a2,b2), and finally, droplet spreading and cooling on the receiver substrate (Figure 1a3,b3). The donor for the LISAT method is a multilayer structure composed of different metals. Figure 1b shows a donor composed of only two metals but the method is not necessarily limited to two layers. During the printing process, the metals melt and mix to form an alloy or a composite droplet on the receiver. The introduction of an additional thin metal layer as part of the donor can affect the printing conditions since the optical properties of a multilayer stack are typically different. It can also result in an increase in the pulse energy needed to melt the additional layer (silver has a higher melting temperature). However, if a thin layer of the doping material is placed within the donor layer at a distance larger than the absorption depth of the laser beam (>≈17 nm in copper), then the laser reflectivity will still be the same. Given the small percentage of silver (≈1%) also the energy required for jetting will essentially be the same. Finally, the geometrical/morphological properties of the printed structure will also appear very similar. As will be discussed later, the slight compositional change that keeps the printing and physical properties almost unchanged is of great importance in providing us with a favorable metallic support material. The latter is an essential ingredient in 3D printing of free standing objects. We have used a metal donor composed of three layers: 100 nm of copper on glass, then a 7 nm silver layer, and an additional 400 nm copper layer on top. We specifically chose silver for doping the copper for its similarity in thermal properties to copper. Figure 2 demonstrates the high aspect ratio one can achieve when printing such metal structures. We have printed the logo (“Small”) of this journal with copper “letters” each with a width of 50 µm and with gradually increased height: “s” = 40 µm, “m” = 70 µm, “a” = 120 µm, and “l” = 190 µm. Since there is only negligible broadening a larger aspect ratio than the one shown in the above example can practically be achieved. For example, we previously managed to print copper structures with an aspect ratio >20. We note that the results shown in Figure 2 represent a considerable advancement in the particular field of LIFT printing and, more generally, in the field of digital, functional printing. In fact, it is the first printing demonstration of an arbitrary 2D pattern by LIFT at such a high accuracy. Achieving high accuracy droplet positioning when jetting from a large donor to receiver distance, in the above case from a 200 µm gap, and in general from a larger gap, is a necessary precondition for arbitrary 2D pattern printing. Usually, the printing of solid metals by the LIFT technique is limited to a small gap,[51–70] typically less than 30 µm. Low positional
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Figure 2. a) SEM image of LIFT-printed copper logo of the “Small” journal on an epoxy glass laminate. The width of the printed letters is 50 µm and the letters are gradually printed each to a higher height as follows: the height of “s” is 40 µm, then, “m” = 70 µm, “a” = 120 µm, “l” 190 µm. d) A zoom in on (a) (image taken at the middle of the letter “m”); b) SEM image taken at a tilt angle of 55°; e) a zoom in on (b) (on the letters “all”); c) 3D profile image; f) 1D height profiles for each of the letters in “small.” The scan direction is indicated in the inset with corresponding colors coding.
accuracy is obtained when jetting very small droplets, typically less than 10 fL. On the contrary, much larger large droplets, >50 fL, typically suffer from deformation during the jetting process, and this gives rise to broadening of the printed structure. In contrast, droplets of intermediate volume show rather good positional accuracy and limited deformation. In order to generate such droplets, it is necessary to use a thicker donor (≈100 nm thick) and tune the pulse duration to match the LIFT printing condition to such a layer thickness.[71] Monolithic passive Q-switch laser provides such subnanosecond pulse regime and more recently, also higher power fiber lasers in Master Oscillator Power Amplifier (MOPA) or Master Oscillator Fiber Amplifier (MOFA) configurations are able to serve the same purpose. The electrical resistivity of the printed structure was measured to give ρ = 6.09 µΩ·cm. Note that the resistivity is ×3.6 times the bulk copper value and is due to the multicrystalline structure (measured grain size is ≈
Laser Transfer
Laser Transfer of Metals and Metal Alloys for Digital Microfabrication of 3D Objects Michael Zenou,* Amir Sa’ar, and Zvi Kotler Digital and 3D functional printing technology is of growing interest in science and technology. Recently, it was referred as “the print revolution”[1] and “the third industrial revolution”[2] due to the novel capabilities offered by these methods such as rapid fabrication/prototyping,[3,4] the ability to combine multimaterials,[5–8] tunable material properties in all three dimensions at the voxel scale,[9,10] high resolutions,[11,12] as well as the prospective potential to replace the current complex fabrication of devices by a single machine, the socalled “3D printer.” Typically, digital functional printing has evolved from well-known methods in the graphic arts industry including screen printing,[13–15] inkjet printing,[16–21] and flexography and gravure[22–24] with advances made in developing new functional materials and adapting them to well-known printing methods. This has motivated a major effort in developing new functional materials, which can fit traditional printing techniques. However, a parallel effort has been undertaken to develop novel digital printing techniques[25–38] with improved performance, which can better fit the growing demand for printed functional structures. The capability to print various metals plays a major role in making this transformative technology real, as most contemporary 3D printing methods rely on polymeric materials which lack the electrical, thermal, and mechanical properties required for functional device manufacturing. Metal deposition is carried by one of the three main methods. The first is an atomic deposition process, i.e., thermal/plasma evaporation in vacuum or an electrolytic reduction process of ionic metal in solution to grow metal layers. Such nonlocal deposition methods require complex pre- and post-treatment steps in order to define a 2D pattern.[12,16] Unlike digital printing, these methods also require specific environmental conditions, receiver substrates, and patterning steps. A second approach relies on various phases of nano- or microparticles, namely colloids,[16–21,25–27,31–38] aerosols,[30] and
M. Zenou, Dr. Z. Kotler Additive Manufacturing Group Orbotech Ltd. P.O. Box 215, Yavne 81101, Israel E-mail: [email protected] M. Zenou, Prof. A. Sa’ar Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and Nanotechnology The Hebrew University of Jerusalem Jerusalem 91904, Israel DOI: 10.1002/smll.201500612
4082 www.small-journal.com
powders.[39–42] “Metal inks” can be prepared from the colloid phase and can be printed using digital dispensing systems. The metal inks[27] can be dispensed drop-by-drop[16–21,34,37,38] or as a continuous filament.[25–27,33] There are several known limitations associated with the current printing methods. For one, while several sources for noble metal inks based on silver and gold can be found, and recently nanocopper inks have started to emerge as well,[32,43,44] there is a rather limited range of metals that can be rendered printable as inks or pastes. There is typically a thermal post-treatment step, which involves sintering the metal particles in order to make the print track conductive. This last step often limits the type of substrates that can be used and also impairs the line conductivity. The third category of metal printing, which is the focus of this article, consists of transferring molten metal from the bulk phase. One way of doing this is similar to the previously mentioned dispensing methods, but here the dispenser is made of a thermally resistant material, which must be chosen according to the melting temperature of the metal to be transferred.[29,45–50] These methods clearly favor the transfer of metals with low melting temperature such as gallium alloys,[29] solder,[45–48] and aluminum,[49,50] and they become more complex for metals with high melting temperature. Another limitation is droplet size due to the high viscosity of the molten metal and dispenser engineering. This results in droplets in the picoliter range or larger. A different approach to printing from the bulk metal phase is laser-induced forward transfer (LIFT).[51] This method relies on a so-called “donor,” which consists of a transparent substrate coated by a thin layer of the material to be printed (Figure 1a). Typically, the layer thickness is on the order of few tens of nanometers. Metal donor layers are prepared by various standard methods such as evaporation or sputtering. To date, LIFT printing of various solid materials has been demonstrated, e.g., metals,[52–57] semiconductors,[58] dielectrics,[59,60] organics,[61–64] and more. With the LIFT technique, metal microdroplets[65–67] and sub-micrometer droplets[68–70] can be printed directly from the bulk solid phase, thereby avoiding the complexity involved in preparing metal ink formulations. LIFT printing of 3D microstructures was first proposed by Piqué and co-workers.[36] In this technique, the printed pixels are made of materials with special rheological properties as needed for such “decal” transfer.[36] Also the printed pixel shape is identical to the laser pulse spatial shape, which defines the printable pattern shape. Chichkov and co-workers
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2015, 11, No. 33, 4082–4089
www.MaterialsViews.com
Figure 1. a) A scheme of the LIFT printing process, the supplier material. The so-called donor is composed of a transparent substrate coated with a thin layer of metal, M1. a1) A focused laser beam liquefies the metal donor layer M1; a2) the molten material is transferred to the receiver; a3) The droplet solidifies on the substrate; b) A scheme of the LISAT process which follows the same steps as in (a), however with a multilayer metal donor stack (M1/M2) instead of a single metal donor layer.
have first demonstrated LIFT-printed 3D microstructures in the transfer regime of sub-micrometer droplets.[70] He proposed a method to print 2D and 3D microstructures with high lateral resolution. In this work, we rely on recent LIFT printing work carried out by our group[71] in which stable jetting from a relatively large donor-to-acceptor distance (e.g., the printing gap) was demonstrated by using subnanosecond pulses and micrometer-scale thick metal layers. We demonstrated for the first time, printing of an arbitrary, micrometer scale, copper object digitally printed under ambient atmospheric conditions. The printed structure consists of overlapped molten copper metal droplets. Scanning electron microscope (SEM) images of an array of single voxels are provided in Figure 1 (Supporting Information). The voxel is a microdroplet of average volume of 20 fL. The single droplet average diameter is 6.5 µm and the average height is 600 nm. This result does not reflect the highest potential resolution of LIFT-printed 2D and 3D metal structures, since printing of sub-micrometer metal voxels was already demonstrated when using a femtosecond laser source.[70] The same method allows for printing of the support material, an essential part of complex shape 3D structures. For printing the support material, we used a novel method that we have termed laser-induced selfalloying transfer (LISAT) (Figure 1). Our method amounts to LIFT printing droplets of a slightly alloyed metal alongside the pure metal, e.g., copper and its alloy. The chemical potential difference between the pure LIFT printed metal and the printed LISAT droplets allows for selective etching of the more anodic metal in a standard copper etchant, due to the galvanic effect. Figure 1a shows the standard LIFT transfer of molten metal. The donor is composed of a transparent substrate (in small 2015, 11, No. 33, 4082–4089
our case, a 1 mm thick soda-lime glass) coated with a thin layer of the material to be transferred. In our study, we used a copper donor with a thickness of 500 nm, which was obtained by physical thermal evaporation from a target composed of 99.9% copper. We can identify three basic steps in the droplet formation and transfer: laser-induced heating of the metal layer by a focused pulse and the thermal propagation toward the free metal–air interface (Figure 1a1,b1), mechanical ejection of the molten material (Figure 1a2,b2), and finally, droplet spreading and cooling on the receiver substrate (Figure 1a3,b3). The donor for the LISAT method is a multilayer structure composed of different metals. Figure 1b shows a donor composed of only two metals but the method is not necessarily limited to two layers. During the printing process, the metals melt and mix to form an alloy or a composite droplet on the receiver. The introduction of an additional thin metal layer as part of the donor can affect the printing conditions since the optical properties of a multilayer stack are typically different. It can also result in an increase in the pulse energy needed to melt the additional layer (silver has a higher melting temperature). However, if a thin layer of the doping material is placed within the donor layer at a distance larger than the absorption depth of the laser beam (>≈17 nm in copper), then the laser reflectivity will still be the same. Given the small percentage of silver (≈1%) also the energy required for jetting will essentially be the same. Finally, the geometrical/morphological properties of the printed structure will also appear very similar. As will be discussed later, the slight compositional change that keeps the printing and physical properties almost unchanged is of great importance in providing us with a favorable metallic support material. The latter is an essential ingredient in 3D printing of free standing objects. We have used a metal donor composed of three layers: 100 nm of copper on glass, then a 7 nm silver layer, and an additional 400 nm copper layer on top. We specifically chose silver for doping the copper for its similarity in thermal properties to copper. Figure 2 demonstrates the high aspect ratio one can achieve when printing such metal structures. We have printed the logo (“Small”) of this journal with copper “letters” each with a width of 50 µm and with gradually increased height: “s” = 40 µm, “m” = 70 µm, “a” = 120 µm, and “l” = 190 µm. Since there is only negligible broadening a larger aspect ratio than the one shown in the above example can practically be achieved. For example, we previously managed to print copper structures with an aspect ratio >20. We note that the results shown in Figure 2 represent a considerable advancement in the particular field of LIFT printing and, more generally, in the field of digital, functional printing. In fact, it is the first printing demonstration of an arbitrary 2D pattern by LIFT at such a high accuracy. Achieving high accuracy droplet positioning when jetting from a large donor to receiver distance, in the above case from a 200 µm gap, and in general from a larger gap, is a necessary precondition for arbitrary 2D pattern printing. Usually, the printing of solid metals by the LIFT technique is limited to a small gap,[51–70] typically less than 30 µm. Low positional
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.small-journal.com
4083
communications www.MaterialsViews.com
Figure 2. a) SEM image of LIFT-printed copper logo of the “Small” journal on an epoxy glass laminate. The width of the printed letters is 50 µm and the letters are gradually printed each to a higher height as follows: the height of “s” is 40 µm, then, “m” = 70 µm, “a” = 120 µm, “l” 190 µm. d) A zoom in on (a) (image taken at the middle of the letter “m”); b) SEM image taken at a tilt angle of 55°; e) a zoom in on (b) (on the letters “all”); c) 3D profile image; f) 1D height profiles for each of the letters in “small.” The scan direction is indicated in the inset with corresponding colors coding.
accuracy is obtained when jetting very small droplets, typically less than 10 fL. On the contrary, much larger large droplets, >50 fL, typically suffer from deformation during the jetting process, and this gives rise to broadening of the printed structure. In contrast, droplets of intermediate volume show rather good positional accuracy and limited deformation. In order to generate such droplets, it is necessary to use a thicker donor (≈100 nm thick) and tune the pulse duration to match the LIFT printing condition to such a layer thickness.[71] Monolithic passive Q-switch laser provides such subnanosecond pulse regime and more recently, also higher power fiber lasers in Master Oscillator Power Amplifier (MOPA) or Master Oscillator Fiber Amplifier (MOFA) configurations are able to serve the same purpose. The electrical resistivity of the printed structure was measured to give ρ = 6.09 µΩ·cm. Note that the resistivity is ×3.6 times the bulk copper value and is due to the multicrystalline structure (measured grain size is ≈
