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Drilling, bonding, and forming conductive path in the hole by laser percussion drilling
Precision Engineering 61 (2020) 147–151
Contents lists available at ScienceDirect
Precision Engineering journal homepage: http://www.elsevier.com/locate/precision
Drilling, bonding, and forming conductive path in the hole by laser percussion drilling Shun Sato, Hirofumi Hidai *, Souta Matsusaka, Akira Chiba, Noboru Morita Department of Mechanical Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser drilling Glass Deposition Copper Bonding
Several studies have focused on the electrical connections between the front and rear surfaces of stacked sub strates in order to improve device performance. The fabrication and mounting process of the substrates involves three steps: (1) through-hole drilling, (2) formation of a conductive path inside the hole, and (3) physical bonding and electrical wiring connection of the substrates. In this paper, we demonstrate a technique for per forming the process above simultaneously by laser percussion drilling. A borosilicate glass sample was used as the substrate, while copper was used as the wiring material. The substrate was drilled to a diameter of ~30 μm by laser radiation, while the copper was evaporated and deposited in a ~12-μm-thick layer on the inner surface of a glass-copper hole. Hence, a conductive path was formed inside the glass hole, facilitating bonding and con duction between the glass substrate and the copper sheet. The conductivity and bonding strength per 100 points between the glass surface and the copper sheet were ~5 Ω and ~1 N respectively. Furthermore, gaps were observed between the glass substrate and the copper sheet by energy dispersive X-ray analysis using a scanning electron microscope. However, the glass substrate and the copper sheet were bonded by the formation of a redeposited layer on the inner surface of the hole and in the gap between the glass and copper surfaces.
1. Introduction In recent times, several studies have focused on the electrical con nections between the front and rear surfaces of stacked substrates in electronic circuit packaging. These connections enable threedimensional wiring and shorten the wiring length, thereby facilitating high performance, low power consumption, and small packaging size [1]. Hence, they are used in various devices such as printed circuit boards [2], glass interposers [3], and through-silicon vias (TSV) [4]. The fabrication process of the wiring connections involves three steps [5]: (1) drilling of through-holes into the substrates; (2) plating of metal into the holes to form a conductive path; and (3) bonding of the substrates into stacks and electrical wiring connections. In most cases, electronic substrates are drilled by sandblasting, reactive ion etching (RIE), and lasers. Sandblasting and RIE are achieved by drilling the required number of holes at once using mask patterns. However, laser drilling is generally performed individually; therefore, its processing speed is faster when the number of holes produced is smaller, and the position of the holes can be changed easily because a mask pattern is not required.
Copper is generally used as an electroplating material to fill the through-holes. However, this process is complicated for fine holes because voids are often formed. To prevent the formation of these voids, various process parameters need to be adjusted during electroplating [6, 7]. These parameters include solution composition, wafer circulation speed, applied current waveform, current pulse duration, and current density. To achieve three-dimensional wiring, the substrates are stacked, bonded, and electrically connected via through-wiring. The bonding and connections are performed using solder bumps placed on the wiring of a substrate, while another substrate is perfectly stacked on it. The sub strates are then heated at high pressures such that electrical connections are made and bonding occurs through the solder bumps. However, the junction misalignment between the two substrates leads to connection failure such as open or short circuits. Hence, it is very difficult to connect many electrodes without some electrical failures [8]. In our previous research, we proposed a technique for performing the two processes above, where substrate drilling and through-hole wiring are performed simultaneously by laser percussion drilling, which is a method of making a hole with irradiating a plurality of shots of pulsed
* Corresponding author. E-mail address: [email protected] (H. Hidai). https://doi.org/10.1016/j.precisioneng.2019.10.007 Received 13 February 2019; Received in revised form 12 September 2019; Accepted 27 September 2019 Available online 23 October 2019 0141-6359/© 2019 Elsevier Inc. All rights reserved.
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laser at a fixed point [9]. Fig. 1 shows the schematic of the experimental process. The sample consists of a substrate to be drilled and a metal plate, which is the electroplating material. The substrate is drilled by laser illumination, while the metal plate is evaporated and deposited on the sides of the substrate hole. In this study, we demonstrated that laser percussion drilling by 4000 shots facilitates drilling, film formation, and simultaneous connection and bonding of the substrate and the wiring material. In addition, the conductivity and bonding strength were evaluated. Finally, the bonding area was analyzed cross-sectionally by energy dispersive X-ray analysis (EDX) using a scanning electron microscope (SEM). 2. Materials and methods Fig. 2 shows the experiment sample. The sample consists of boro silicate glass substrates B and A and oxygen-free copper sheets A and B, which are stacked on top of each other by a jig. After the laser irradia tion, glass substrate A and copper sheet A were bonded, while glass substrate B and copper sheet B were removed using tweezers. The cop per is scarcely deposited near the glass hole surface on the laserirradiated side; therefore, to expose the redeposited copper on the inner surface of the hole, glass substrate B was removed after the laser irradiation. Conductivity was achieved between the glass surface and the copper sheet [9]. In addition, copper sheet B produced sufficient amounts of redeposited copper to form a layer in the hole. The thickness of the borosilicate glass substrates A and B and the oxygen-free copper sheets A and B were 0.15, 0.15, 0.1, and 1 mm, respectively, and their shape was square, with side of 15–20 mm. The sample was processed by the fourth harmonic of an Nd: YVO4 laser (DS 20H-266, Photonics Industries International Inc., Bohemia, NY, USA) with a wavelength of 266 nm, the same as that described in the literature [9]. The laser was operated at an energy of 100 μJ, a repetition frequency of 10 kHz, total pulse number of 4000, and pulse width of 8 ns, irradiated onto the top surface of glass substrate B and focused by a lens with a focal length of 30 mm. Holes were drilled by shifting the focal position by 100 μm; their depths were measured, and the focal position was set at the point at which the deepest hole was obtained. The bonding strength was measured when glass substrate A and copper sheet A were pulled at a speed of 250 μm/s by the load cell attached to the glass surface of the sample. The conductivity was measured with a digital multimeter. The glass surface of the bonded sample was put in contact with a copper foil, and the electric resistance between the foil and the copper sheet was measured. Additionally, the cross-section of the hole of the bonded sample was observed from the direction perpendicular to the optical axis. The bonded sample was solidified by a thermosetting resin to prevent disjunction while exposing the cross section by wire saw cutting and cross-sectional ion polishing. The bonded sample was observed after sectioning without removing the resin. In addition, the contact surfaces of the glass substrate and copper sheet were examined after separation.
Fig. 2. Schematic of the experiment sample.
values of ~5 Ω and ~1 N per 100 points. Fig. 3 shows the SEM-EDX results of the entire hole section and magnified parts around the interface between (i) the glass hole and the redeposited layer of the copper and (ii) the glass substrate and the copper sheet. The upper and lower materials are glass substrate A and copper sheet A, respectively, as shown in Fig. 3(a). A hole was pene trated through the glass substrate and the copper sheet, while the laser was illuminated from the top of the samples. Furthermore, a redeposited copper layer formed on the inner surface of the hole became thicker as the hole depth increased. Fig. 3(c) shows an enlarged view of the interface between the glass hole and the redeposited copper, and an enlarged view of the area is indicated by a square shown in Fig. 3(b). Nanoscale voids were observed in the redeposited copper layer as shown in Fig. 3(b), while a region of mixed glass and copper components was identified at the interface between the glass hole and the redeposited copper layer, as shown in Fig. 3(c). At the mixed region, nanoscale particles (diameter < 1 μm) of copper were observed in the glass. Fig. 3 (d)–(g) are enlarged views of the area marked by a square in Fig. 3(a). Fig. 3(d) shows a SEM image, while Fig. 3(e)–(g) show the elemental maps of Cu, Si, and C obtained by EDX. The bonded sample is solidified with resin, and the gap is also filled with resin. Fig. 3(g) shows the space where C was detected, i.e., the gap between the glass substrate and the copper sheet. The gap near the hole was filled with glass and copper because Si and Cu were detected, as shown in Fig. 3(e) and (f). The interface between the glass hole and the redeposited copper layer at the glass hole depth of 100–150 μm is trumpet-shaped and the trumpetshaped redeposited copper has a thickness of ~12 μm. However, some cracks were observed at the interface between the redeposited copper and the glass hole. Fig. 3(h)–(j) show enlarged views of the area marked by a square in Fig. 3(d). Fig. 3(h) shows an SEM image, while Fig. 3(i) and (j) show the elemental maps of Cu and Si. The interface between the glass and the redeposited copper layer is not flat, and no significant gap (<1 μm) was observed between them; hence, they are in close contact. Furthermore, the redeposited layers of copper and glass observed in the gap were found to have a convoluted shape as shown in Fig. 3(i) and (j), and the glass layer was observed like an anchor. Fig. 4 shows the junction surfaces of the glass substrate and copper sheet after separation. Fig. 4(a)–(c) show the SEM image, Cu X-ray map,
3. Results The electric resistance and bonding strength were measured at 100 points (10 � 10 points at 300 μm intervals) as one set, with respective
Fig. 1. Schematic of the experimental process. 148
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Fig. 3. SEM-EDX results of hole cross-section: (a) complete view; (b)–(c) enlarged view of the glass hole; (d)–(g) enlarged views of the region of the interface between the glass substrate and the copper plate; (h)–(j) enlarged views of the area marked by a square in Fig. 3d; (e) and (i) X-ray map of Cu; (f) and (j) X-ray map of Si; (g) Xray map of C.
and Si X-ray map, respectively, of the separated glass substrate surface. The glass hole had a diameter of 30 μm, and some flake marks were observed around the redeposited copper layer. A redeposited layer of copper was observed in the hole at a depth of 20 μm from the separation surface of the glass substrate. Fig. 4(d) shows an SEM image of the separated copper sheet surface. A pipe-shaped copper formation with a height and outer diameter of 20 μm and 30 μm, respectively, is observed on the copper sheet. The height of this pipe-shaped redeposition was found to be equal to the distance from the separation surface to the redeposited copper in the hole formed in the separated glass substrate. In addition, the outer diameter of the deposition on the copper sheet was matched with the diameter of the hole formed in a separate glass sub strate. Flake marks were also observed at a diameter of ~50 μm, and the
area where they were observed on the copper sheet was matched with the area where they were observed on the glass substrate. The lines on the copper sheet were already formed by rolling at the time of production. 4. Discussion In order to evaluate the redeposited copper, the electrical resistance value taken in this experiment was calculated under the assumption that the dimensions of the pure and redeposited copper were the same. The electrical resistance value can be calculated using the following equation: R¼ρ
L A
(1)
where R is the electrical resistance [Ω], ρ is the electrical resistivity [Ω・ m], L is the length [m], and A is the cross-sectional area of the rede posited copper in the hole [m2 ]. The value of electrical resistivity ρ ¼ 1:69 � 10 8 [Ω・m] of pure copper at 20 � C [10] was used in the calculation. To estimate the area of cross-section easily, the area of the redeposited copper was evaluated by dividing by hole depth of 30 μm. The cross-sectional area was considered to be constant in one division, and the cross-sectional area at the center depth position of each division was used as the representative value. Specifically, the cross-sectional area at a depth of 15 μm represented that between 0 and 30 μm, and so on. The cross-sectional area of the redeposited copper was calculated by measuring its thickness from the cross-sectional view of the hole, as shown in Fig. 3. The calculated cross-sectional areas of each of the divided hole depths were as follows: 0–30 μm: 33.6 μm2 , 30–60 μm: 41.3 μm2 , 60–90 μm: 105.9 μm2 , 90–120 μm: 132.1 μm2 , and 120–150 μm: 340.8 μm2 . The calculated electrical resistance per 100 holes was 3:7 � 10 4 Ω, which was much smaller than the measured result. The reason was considered to be copper oxidation, and contact resistance at the measurement probe and the region of mixed glass and copper inside the
Fig. 4. SEM-EDX results of the bonding surface after separation: (a)–(c) glass surface; (d) copper surface; (b) X-ray map of Cu; (c) X-ray map of Si. 149
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Fig. 5. Schematic of the bonded state: (a) before separation; and (b) after separation.
hole. Generation of oxygen by the atmosphere and decomposition of the glass substrate could be causes of the copper oxidation. Sunohara et al. reported that the value of resistance per TSV was 17 mΩ when they fabricated a silicon interposer of 144 Cu TSVs (silicon thickness: 200 μm, TSV diameter: 60 μm) [11]. The TSV resistance value was much smaller than the electrical resistance value obtained in this experiment. The copper on the inner surface of the hole obtained in this experiment is thin film; hence filling by plating after this process is preferable to improve the conductivity of the hole. Fig. 5(a) shows the schematic of the bonding state. The bonding of the glass substrate and the copper sheet was due to the formation of the redeposited layer on the inner surface of the hole, and the gap between the glass substrate and the copper sheet. The hole was drilled through the glass substrate and the copper sheet as shown in Fig. 3(a). The redeposited copper layer was in close contact with the inner surface of the glass hole, as shown in Fig. 3(b). An anchoring effect and interaction force (e.g., intermolecular force) occur when a small gap, in the sub micron range, exists between the materials [12]. Some research has demonstrated that a metal and polymer can be chemically bonded using base containing oxygen [13,14]. In this study, copper was deposited on the inner surface of the glass containing oxygen at a high temperature. Hence, the glass and copper could be chemically bonded via oxygen, and the redeposited layer of the glass hole formed on the inner surface of the hole and served as an adhesive layer. Meanwhile, a gap was observed between the glass surface and the copper surface, and redeposited layers of glass and copper were formed in the gap. Flake marks were also observed on the glass and copper surfaces after separation. Hence, the collection of redeposited layers in the gap served as the adhesive layer. In addition, these redeposited layers had a rough shape, and the bonding strength increased. Furthermore, a redeposited copper layer was formed in the glass substrate, and a pipe-shaped copper formation was observed on the copper sheet after separation. Therefore, the bonded sample of the glass substrate and the copper sheet is separated by tensile test as shown in Fig. 5(b). The separation of the bonded sample was caused by the cracks that were observed in the trumpet-shaped redeposited copper and on the interface between the glass hole and the redeposited copper. In addition, the positions of the cracks and the breaking point in the redeposited layer were approximately the same. A stronger bonding effect is ach ieved when the cracks are reduced. The cracks were not noticeable on the copper sheet side but were observed on the glass substrate side, as shown in Fig. 3(e) and (g). The occurrence of cracks was related to the heat shrinkage of the redeposited copper, which expanded during the redeposition but cooled and then
shrank after laser irradiation. Thus, the cracks were caused by the dif ference in the thermal expansion coefficients of the glass and redepos ited copper, and they occurred at the interface between the redeposited copper and the glass hole on the glass substrate. No cracks occurred on the copper sheet because the thermal expansion coefficients of the copper sheet and the redeposited copper were equal. On the other hand, even on the glass substrate side, concentrated cracks were observed in the trumpet-shaped redeposited copper. Hence, a greater tensile force was applied at the trumpet-shaped redeposited copper region, because the quantity of the deposited copper was larger there than at the glass hole depth of 0–100 μm. However, the cracks could also have formed when the jig was detached and the sample was handled. Furthermore, voids were observed in the redeposited copper that could have helped the development and occurrence of the cracks. The mechanism of the crack formation and development will be considered in future research. 5. Conclusion In this study, a simultaneous process involving the drilling of glass, deposition of copper into the glass hole, and the bonding of glass and copper was performed. In addition, the properties of the bonded sample and observed samples were analyzed using SEM-EDX. The conclusions are as follows: 1) The electrical resistivity between the laser-irradiated surface of the glass hole and the copper sheet was ~5 Ω per 100 points. 2) The bonding strength between the glass substrate and the copper sheet was ~1 N per 100 points. 3) A redeposited copper layer was formed on the inner surface of the glass hole without a gap. 4) A gap was observed between the glass substrate and the copper sheet, while the redeposited layers of the glass and copper were filled in this gap. 5) Cracks were observed in the trumpet-shaped redeposited copper layer close to the rear of the glass hole; the redeposited copper was detached from the trumpet-shaped part of the layer. Funding This work was supported by the Japan Science and Technology Agency (JST) [grant number AS2124025B]. Acknowledgment The authors acknowledge the support of the Japan Science and 150
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Technology Agency (JST) under the Collaborative Development of Innovative Seeds, Potentiality Verification Stage Program.
[7] Dixit P, Miao J. Fabrication of high aspect ratio 35 μm pitch interconnects for next generation 3-D wafer level packaging by through-wafer copper electroplating. In: 2006 Electr components Technol Conf; 2006. p. 388–93. [8] Aoyagi M, Tung BT, Suzuki M, Watanabe N, Kato F, Na ML, Nemoto S. Method for producing semiconductor device and device for producing semiconductor. 2014. Patent WO2014/045828. [9] Hidai H, Matsusaka S, Chiba A, Morita N. Laser drilling and conducting film formation of vias in silicon. J Electron Mater 2015;44:4928–32. [10] National Astronomical Observatory of Japan, editor. Chronological scientific tables 2014. Tokyo: Maruzen; 2014. p. 420. [11] Sunohara M, Tokunaga T, Kurihara T, Higashi M. Silicon interposer with TSVs (through silicon vias) and fine multilayer wiring. In: 2008 58th electr components and technol conf; 2008. p. 847–52. [12] Katayama S, Kawahito Y, Niwa Y, Tange A, Kubota S. Laser direct joining between stainless steel and polyethylene terephthalate plastic -laser assisted metal-plastic (LAMP) joining-. Q J Jpn Weld Soc 2007;25:316–22 [In Japanese]. [13] Ho PS, Hahn PO, Bartha JW, Rubloff GW, LeGoues FK, Silverman BD. Chemical bonding and reaction at mtal/polymer interfaces. J Vac Sci Technol A 1985;3: 739–45. [14] Jordan JL, Kovac CA, Morar JF, Pollak RA. High-resolution photoemission study of the interfacial reaction of Cr with polyamide and model polymers. Phy Rev B 1987; 36:1369–77.
References [1] Kuo TY, Chang SM, Shih YC, Chiang CW, Hsu CK, Lee CK, Lin CT, Chen YH, Lo WC. Reliability tests for a three dimensional chip stacking structure with through silicon via connections and low cost. In: 2008 electr components technol conf; 2008. p. 853–8. [2] Takahashi A. Recent trend and future prospect on printed wiring boards. J Soc Rubber Sci Technol Jpn 2011;84:301–5 [In Japanese]. [3] Wei T, Wang Q, Cai J, Chen L, Huang J, Wang L, Zhang L, Li C. Performance and reliability study of TGV interposer in 3D integration. In: 2014 IEEE 16th electr packaging technol conf (EPTC); 2014. p. 601–5. [4] Makoto M. Through-Silicon via (TSV). Proc IEEE 2009;97:43–8. [5] Pavlidis VF, Friedman EG. Three-dimensional integrated circuit design. Imprint of Elsevier; 2009. p. 48–9. [6] Kim B, Sharbono C, Ritzdorf T, Schmauch D. Factors affecting copper filling process within high aspect ratio deep vias for 3D chip stacking. In: 56th electr components technol conf 2006; 2006. p. 838–43.
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Contents lists available at ScienceDirect
Precision Engineering journal homepage: http://www.elsevier.com/locate/precision
Drilling, bonding, and forming conductive path in the hole by laser percussion drilling Shun Sato, Hirofumi Hidai *, Souta Matsusaka, Akira Chiba, Noboru Morita Department of Mechanical Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser drilling Glass Deposition Copper Bonding
Several studies have focused on the electrical connections between the front and rear surfaces of stacked sub strates in order to improve device performance. The fabrication and mounting process of the substrates involves three steps: (1) through-hole drilling, (2) formation of a conductive path inside the hole, and (3) physical bonding and electrical wiring connection of the substrates. In this paper, we demonstrate a technique for per forming the process above simultaneously by laser percussion drilling. A borosilicate glass sample was used as the substrate, while copper was used as the wiring material. The substrate was drilled to a diameter of ~30 μm by laser radiation, while the copper was evaporated and deposited in a ~12-μm-thick layer on the inner surface of a glass-copper hole. Hence, a conductive path was formed inside the glass hole, facilitating bonding and con duction between the glass substrate and the copper sheet. The conductivity and bonding strength per 100 points between the glass surface and the copper sheet were ~5 Ω and ~1 N respectively. Furthermore, gaps were observed between the glass substrate and the copper sheet by energy dispersive X-ray analysis using a scanning electron microscope. However, the glass substrate and the copper sheet were bonded by the formation of a redeposited layer on the inner surface of the hole and in the gap between the glass and copper surfaces.
1. Introduction In recent times, several studies have focused on the electrical con nections between the front and rear surfaces of stacked substrates in electronic circuit packaging. These connections enable threedimensional wiring and shorten the wiring length, thereby facilitating high performance, low power consumption, and small packaging size [1]. Hence, they are used in various devices such as printed circuit boards [2], glass interposers [3], and through-silicon vias (TSV) [4]. The fabrication process of the wiring connections involves three steps [5]: (1) drilling of through-holes into the substrates; (2) plating of metal into the holes to form a conductive path; and (3) bonding of the substrates into stacks and electrical wiring connections. In most cases, electronic substrates are drilled by sandblasting, reactive ion etching (RIE), and lasers. Sandblasting and RIE are achieved by drilling the required number of holes at once using mask patterns. However, laser drilling is generally performed individually; therefore, its processing speed is faster when the number of holes produced is smaller, and the position of the holes can be changed easily because a mask pattern is not required.
Copper is generally used as an electroplating material to fill the through-holes. However, this process is complicated for fine holes because voids are often formed. To prevent the formation of these voids, various process parameters need to be adjusted during electroplating [6, 7]. These parameters include solution composition, wafer circulation speed, applied current waveform, current pulse duration, and current density. To achieve three-dimensional wiring, the substrates are stacked, bonded, and electrically connected via through-wiring. The bonding and connections are performed using solder bumps placed on the wiring of a substrate, while another substrate is perfectly stacked on it. The sub strates are then heated at high pressures such that electrical connections are made and bonding occurs through the solder bumps. However, the junction misalignment between the two substrates leads to connection failure such as open or short circuits. Hence, it is very difficult to connect many electrodes without some electrical failures [8]. In our previous research, we proposed a technique for performing the two processes above, where substrate drilling and through-hole wiring are performed simultaneously by laser percussion drilling, which is a method of making a hole with irradiating a plurality of shots of pulsed
* Corresponding author. E-mail address: [email protected] (H. Hidai). https://doi.org/10.1016/j.precisioneng.2019.10.007 Received 13 February 2019; Received in revised form 12 September 2019; Accepted 27 September 2019 Available online 23 October 2019 0141-6359/© 2019 Elsevier Inc. All rights reserved.
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Precision Engineering 61 (2020) 147–151
laser at a fixed point [9]. Fig. 1 shows the schematic of the experimental process. The sample consists of a substrate to be drilled and a metal plate, which is the electroplating material. The substrate is drilled by laser illumination, while the metal plate is evaporated and deposited on the sides of the substrate hole. In this study, we demonstrated that laser percussion drilling by 4000 shots facilitates drilling, film formation, and simultaneous connection and bonding of the substrate and the wiring material. In addition, the conductivity and bonding strength were evaluated. Finally, the bonding area was analyzed cross-sectionally by energy dispersive X-ray analysis (EDX) using a scanning electron microscope (SEM). 2. Materials and methods Fig. 2 shows the experiment sample. The sample consists of boro silicate glass substrates B and A and oxygen-free copper sheets A and B, which are stacked on top of each other by a jig. After the laser irradia tion, glass substrate A and copper sheet A were bonded, while glass substrate B and copper sheet B were removed using tweezers. The cop per is scarcely deposited near the glass hole surface on the laserirradiated side; therefore, to expose the redeposited copper on the inner surface of the hole, glass substrate B was removed after the laser irradiation. Conductivity was achieved between the glass surface and the copper sheet [9]. In addition, copper sheet B produced sufficient amounts of redeposited copper to form a layer in the hole. The thickness of the borosilicate glass substrates A and B and the oxygen-free copper sheets A and B were 0.15, 0.15, 0.1, and 1 mm, respectively, and their shape was square, with side of 15–20 mm. The sample was processed by the fourth harmonic of an Nd: YVO4 laser (DS 20H-266, Photonics Industries International Inc., Bohemia, NY, USA) with a wavelength of 266 nm, the same as that described in the literature [9]. The laser was operated at an energy of 100 μJ, a repetition frequency of 10 kHz, total pulse number of 4000, and pulse width of 8 ns, irradiated onto the top surface of glass substrate B and focused by a lens with a focal length of 30 mm. Holes were drilled by shifting the focal position by 100 μm; their depths were measured, and the focal position was set at the point at which the deepest hole was obtained. The bonding strength was measured when glass substrate A and copper sheet A were pulled at a speed of 250 μm/s by the load cell attached to the glass surface of the sample. The conductivity was measured with a digital multimeter. The glass surface of the bonded sample was put in contact with a copper foil, and the electric resistance between the foil and the copper sheet was measured. Additionally, the cross-section of the hole of the bonded sample was observed from the direction perpendicular to the optical axis. The bonded sample was solidified by a thermosetting resin to prevent disjunction while exposing the cross section by wire saw cutting and cross-sectional ion polishing. The bonded sample was observed after sectioning without removing the resin. In addition, the contact surfaces of the glass substrate and copper sheet were examined after separation.
Fig. 2. Schematic of the experiment sample.
values of ~5 Ω and ~1 N per 100 points. Fig. 3 shows the SEM-EDX results of the entire hole section and magnified parts around the interface between (i) the glass hole and the redeposited layer of the copper and (ii) the glass substrate and the copper sheet. The upper and lower materials are glass substrate A and copper sheet A, respectively, as shown in Fig. 3(a). A hole was pene trated through the glass substrate and the copper sheet, while the laser was illuminated from the top of the samples. Furthermore, a redeposited copper layer formed on the inner surface of the hole became thicker as the hole depth increased. Fig. 3(c) shows an enlarged view of the interface between the glass hole and the redeposited copper, and an enlarged view of the area is indicated by a square shown in Fig. 3(b). Nanoscale voids were observed in the redeposited copper layer as shown in Fig. 3(b), while a region of mixed glass and copper components was identified at the interface between the glass hole and the redeposited copper layer, as shown in Fig. 3(c). At the mixed region, nanoscale particles (diameter < 1 μm) of copper were observed in the glass. Fig. 3 (d)–(g) are enlarged views of the area marked by a square in Fig. 3(a). Fig. 3(d) shows a SEM image, while Fig. 3(e)–(g) show the elemental maps of Cu, Si, and C obtained by EDX. The bonded sample is solidified with resin, and the gap is also filled with resin. Fig. 3(g) shows the space where C was detected, i.e., the gap between the glass substrate and the copper sheet. The gap near the hole was filled with glass and copper because Si and Cu were detected, as shown in Fig. 3(e) and (f). The interface between the glass hole and the redeposited copper layer at the glass hole depth of 100–150 μm is trumpet-shaped and the trumpetshaped redeposited copper has a thickness of ~12 μm. However, some cracks were observed at the interface between the redeposited copper and the glass hole. Fig. 3(h)–(j) show enlarged views of the area marked by a square in Fig. 3(d). Fig. 3(h) shows an SEM image, while Fig. 3(i) and (j) show the elemental maps of Cu and Si. The interface between the glass and the redeposited copper layer is not flat, and no significant gap (<1 μm) was observed between them; hence, they are in close contact. Furthermore, the redeposited layers of copper and glass observed in the gap were found to have a convoluted shape as shown in Fig. 3(i) and (j), and the glass layer was observed like an anchor. Fig. 4 shows the junction surfaces of the glass substrate and copper sheet after separation. Fig. 4(a)–(c) show the SEM image, Cu X-ray map,
3. Results The electric resistance and bonding strength were measured at 100 points (10 � 10 points at 300 μm intervals) as one set, with respective
Fig. 1. Schematic of the experimental process. 148
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Fig. 3. SEM-EDX results of hole cross-section: (a) complete view; (b)–(c) enlarged view of the glass hole; (d)–(g) enlarged views of the region of the interface between the glass substrate and the copper plate; (h)–(j) enlarged views of the area marked by a square in Fig. 3d; (e) and (i) X-ray map of Cu; (f) and (j) X-ray map of Si; (g) Xray map of C.
and Si X-ray map, respectively, of the separated glass substrate surface. The glass hole had a diameter of 30 μm, and some flake marks were observed around the redeposited copper layer. A redeposited layer of copper was observed in the hole at a depth of 20 μm from the separation surface of the glass substrate. Fig. 4(d) shows an SEM image of the separated copper sheet surface. A pipe-shaped copper formation with a height and outer diameter of 20 μm and 30 μm, respectively, is observed on the copper sheet. The height of this pipe-shaped redeposition was found to be equal to the distance from the separation surface to the redeposited copper in the hole formed in the separated glass substrate. In addition, the outer diameter of the deposition on the copper sheet was matched with the diameter of the hole formed in a separate glass sub strate. Flake marks were also observed at a diameter of ~50 μm, and the
area where they were observed on the copper sheet was matched with the area where they were observed on the glass substrate. The lines on the copper sheet were already formed by rolling at the time of production. 4. Discussion In order to evaluate the redeposited copper, the electrical resistance value taken in this experiment was calculated under the assumption that the dimensions of the pure and redeposited copper were the same. The electrical resistance value can be calculated using the following equation: R¼ρ
L A
(1)
where R is the electrical resistance [Ω], ρ is the electrical resistivity [Ω・ m], L is the length [m], and A is the cross-sectional area of the rede posited copper in the hole [m2 ]. The value of electrical resistivity ρ ¼ 1:69 � 10 8 [Ω・m] of pure copper at 20 � C [10] was used in the calculation. To estimate the area of cross-section easily, the area of the redeposited copper was evaluated by dividing by hole depth of 30 μm. The cross-sectional area was considered to be constant in one division, and the cross-sectional area at the center depth position of each division was used as the representative value. Specifically, the cross-sectional area at a depth of 15 μm represented that between 0 and 30 μm, and so on. The cross-sectional area of the redeposited copper was calculated by measuring its thickness from the cross-sectional view of the hole, as shown in Fig. 3. The calculated cross-sectional areas of each of the divided hole depths were as follows: 0–30 μm: 33.6 μm2 , 30–60 μm: 41.3 μm2 , 60–90 μm: 105.9 μm2 , 90–120 μm: 132.1 μm2 , and 120–150 μm: 340.8 μm2 . The calculated electrical resistance per 100 holes was 3:7 � 10 4 Ω, which was much smaller than the measured result. The reason was considered to be copper oxidation, and contact resistance at the measurement probe and the region of mixed glass and copper inside the
Fig. 4. SEM-EDX results of the bonding surface after separation: (a)–(c) glass surface; (d) copper surface; (b) X-ray map of Cu; (c) X-ray map of Si. 149
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Fig. 5. Schematic of the bonded state: (a) before separation; and (b) after separation.
hole. Generation of oxygen by the atmosphere and decomposition of the glass substrate could be causes of the copper oxidation. Sunohara et al. reported that the value of resistance per TSV was 17 mΩ when they fabricated a silicon interposer of 144 Cu TSVs (silicon thickness: 200 μm, TSV diameter: 60 μm) [11]. The TSV resistance value was much smaller than the electrical resistance value obtained in this experiment. The copper on the inner surface of the hole obtained in this experiment is thin film; hence filling by plating after this process is preferable to improve the conductivity of the hole. Fig. 5(a) shows the schematic of the bonding state. The bonding of the glass substrate and the copper sheet was due to the formation of the redeposited layer on the inner surface of the hole, and the gap between the glass substrate and the copper sheet. The hole was drilled through the glass substrate and the copper sheet as shown in Fig. 3(a). The redeposited copper layer was in close contact with the inner surface of the glass hole, as shown in Fig. 3(b). An anchoring effect and interaction force (e.g., intermolecular force) occur when a small gap, in the sub micron range, exists between the materials [12]. Some research has demonstrated that a metal and polymer can be chemically bonded using base containing oxygen [13,14]. In this study, copper was deposited on the inner surface of the glass containing oxygen at a high temperature. Hence, the glass and copper could be chemically bonded via oxygen, and the redeposited layer of the glass hole formed on the inner surface of the hole and served as an adhesive layer. Meanwhile, a gap was observed between the glass surface and the copper surface, and redeposited layers of glass and copper were formed in the gap. Flake marks were also observed on the glass and copper surfaces after separation. Hence, the collection of redeposited layers in the gap served as the adhesive layer. In addition, these redeposited layers had a rough shape, and the bonding strength increased. Furthermore, a redeposited copper layer was formed in the glass substrate, and a pipe-shaped copper formation was observed on the copper sheet after separation. Therefore, the bonded sample of the glass substrate and the copper sheet is separated by tensile test as shown in Fig. 5(b). The separation of the bonded sample was caused by the cracks that were observed in the trumpet-shaped redeposited copper and on the interface between the glass hole and the redeposited copper. In addition, the positions of the cracks and the breaking point in the redeposited layer were approximately the same. A stronger bonding effect is ach ieved when the cracks are reduced. The cracks were not noticeable on the copper sheet side but were observed on the glass substrate side, as shown in Fig. 3(e) and (g). The occurrence of cracks was related to the heat shrinkage of the redeposited copper, which expanded during the redeposition but cooled and then
shrank after laser irradiation. Thus, the cracks were caused by the dif ference in the thermal expansion coefficients of the glass and redepos ited copper, and they occurred at the interface between the redeposited copper and the glass hole on the glass substrate. No cracks occurred on the copper sheet because the thermal expansion coefficients of the copper sheet and the redeposited copper were equal. On the other hand, even on the glass substrate side, concentrated cracks were observed in the trumpet-shaped redeposited copper. Hence, a greater tensile force was applied at the trumpet-shaped redeposited copper region, because the quantity of the deposited copper was larger there than at the glass hole depth of 0–100 μm. However, the cracks could also have formed when the jig was detached and the sample was handled. Furthermore, voids were observed in the redeposited copper that could have helped the development and occurrence of the cracks. The mechanism of the crack formation and development will be considered in future research. 5. Conclusion In this study, a simultaneous process involving the drilling of glass, deposition of copper into the glass hole, and the bonding of glass and copper was performed. In addition, the properties of the bonded sample and observed samples were analyzed using SEM-EDX. The conclusions are as follows: 1) The electrical resistivity between the laser-irradiated surface of the glass hole and the copper sheet was ~5 Ω per 100 points. 2) The bonding strength between the glass substrate and the copper sheet was ~1 N per 100 points. 3) A redeposited copper layer was formed on the inner surface of the glass hole without a gap. 4) A gap was observed between the glass substrate and the copper sheet, while the redeposited layers of the glass and copper were filled in this gap. 5) Cracks were observed in the trumpet-shaped redeposited copper layer close to the rear of the glass hole; the redeposited copper was detached from the trumpet-shaped part of the layer. Funding This work was supported by the Japan Science and Technology Agency (JST) [grant number AS2124025B]. Acknowledgment The authors acknowledge the support of the Japan Science and 150
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Technology Agency (JST) under the Collaborative Development of Innovative Seeds, Potentiality Verification Stage Program.
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