- Email: [email protected]
Investigation of CuAlO2 composite dielectric properties and selective metallization by laser direct structure technology
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Investigation of CuAlO2 composite dielectric properties and selective metallization by laser direct structure technology Tae-Ho Lee, Soon-Mi Hwang, Myong-Jae Yoo* Reliability Research Center, , Korea Electronics Technology Institute, #25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, 13509, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser direct structuring (LDS) technology CuAlO2 ceramic composite Selective metallization Dielectric properties Electroless copper plating
In this work, we investigated CuAlO2 ceramic composite dielectric properties and potential application to laser direct structuring (LDS) technology. To sinter at low temperature various amounts of glass was added to CuAlO2 and their microstructure, dielectric properties were systematically investigated. The Glass +30 wt% CuAlO2 composite sintered at 800 °C for 1 h was measured to have a high relative density of 84.5 %, low dielectric constant and loss of 4.6 and 0.08 at 9.1 GHz. Using this composite after laser irradiation well-defined copper metal patterns were fabricated on ceramic composite surface. Two important factors which enabled metallization was observed to be morphology of the ceramic composite surface and reduction of Cu+, Cu2+ in the CuAlO2 composite to Cu0 after laser irradiation.
1. Introduction Recently developments of the Molded Interconnect Device (3-MID) technology has made significant advances in interconnecting device manufacturing methods, including substrates materials, structuring metering, and various connectivity devices. One of the common processes for the 3D-MID technology is the Laser Direct Structuring (LDS) technology. It is emerging as a very convenient and cost-effective approach for the fabrication of metallized patterns on nonconductive substrates [1–4]. In the process of LDS, injection-molded substrates with activation additives are structured by laser, and subsequently metallized by using electroless plating process [2,4,5]. The polymer substrates fabricated using LDS technology commonly show a low thermal stability, conductivity and temperature resistance [6]. For example, representative MID materials such as polyether ether ketone (PEEK) has thermal stability up to 240 °C [6]. However, applications ranging from automobiles to aerospace engineering require materials with high thermal resistance, such as ceramics [7]. In order to metallize on substrate using a ceramic material, DCB (Direct Copper Bonding) [8,9], AMB (Active Metal Brazing) [10,11] and screen printing on LTCC (Low Temperature Cofired Ceramics) [12,13] is commonly used. These typical metallization processes require heat treatment at high temperatures and intensive chemical etching and cleaning [9,11]. Moreover, the geometric complexity of the substrate during metallization is severely limited [11,13]. Compared to these conventional metallization processes for ceramic substrates LDS technology can realize
⁎
very fine and complex structures with few process steps, less time and smaller energy consumption [14]. For forming metals on surfaces by LDS technology, the porous structure of surface formed in the laser machining process is considered to be a key element. However, it also requires that the surface of the ceramic substrate be seeded with a metallic catalyst [15,16]. For this, selective metallization composites are doped with additives. The LDS additives are important for forming conductive patterns on surface of composite. The additive are excited by laser irradiation and are separated into metal and organic residuals [17]. This process is called ‘activation’. In electroless plating, the activated filler acts as a plating catalyst [18,19]. Examples of activation additive include Al2O3, CeO2, ZnO, AlN, SIC, CuO and a Cu-Cr complex [20,21]. When a laser is irradiated on the composite surface, the ceramic matrix is melted (photochemical ablation) or evaporated, and the activation additive is exposed and activated [22]. The activated additive donates electrons in electroless plating [20,23]. During the electroless plating process, the porous structure could retain the copper plating solution, but in the untreated region the copper plating solution is washed out. It explains why plating is done only in the laser irradiation area [24]. LDS additive materials have been extensively studied, among which Cu-Cr complex have been reported to show good copper plating quality [21]. However, it is expensive to use as LDS additive material. To overcome this problem, we have studied CuAl2O4 material as an additive material for LDS technology, which has lower cost by substituting Cr with aluminum [25]. From our previous work, we have confirmed
Corresponding author. E-mail address: [email protected] (M.-J. Yoo).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.036 Received 8 July 2019; Received in revised form 8 November 2019; Accepted 11 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tae-Ho Lee, Soon-Mi Hwang and Myong-Jae Yoo, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.036
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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electroless plating, the solution containing copper sulfate (8 g/L) as a source of Cu2+ ions, formaldehyde (12 mL/L) as the reducing agent, potassium sodium tartrate (30 g/L), ethylene diamine tetra-acetic acid (3 g/L), sodium citrate (3 g/L) as the complexing agent, and methanol (150 mL/L) as the stabilizer was used. The crystal structures of the powder and specimens were examined via X-ray diffraction (XRD: Rigaku D/max-RC, Tokyo, Japan) using CuKα radiation and operating at 40 kV and 40 mA. The specimens were scanned in a 2θ range of 10-80° at a scanning rate of 8° per min. The surface and cross-sectional microstructures of specimens were studied using field-emission scanning electron microscopy (FE-SEM: Hitachi S-4800, Osaka, Japan). The true density of glass powder was determined by helium pycnometry using an Ultra PYC 1200e pycnometer (Quantachrome, USA). The bulk density of the composite specimens was measured using the Archimedes method and the relative density was calculated using the theoretical density. The dielectric constants and dielectric losses of the composite specimens were measured at 9.4 GHz with an PNA-L network analyzer (Agilent Technologies N5230A, Santa Clara, CA, USA). The surface chemical elements were characterized by X-ray photoelectron spectroscopy (XPS: ULVACPHI X-tool, Japan) operating at the AL Ka achromatic X-ray source (1486.6 eV) and an X-ray beam of around 1 mm. The surface pattern formed on the CuAlO2 composite by laser irradiation was observed using an optical microscope (VK-9710, Keyence Co., LTd.).
that CuAl2O4 can be used in polymer composite for LDS technology with good selective metallization properties [25]. In this work we synthesized CuAlO2, a new material using aluminum by conventional solid state process. The synthesized CuAlO2 with glass frit was used to make CuAlO2 ceramic composite and their dielectric properties and microstructures were studied. Furthermore, we studied conditions under which selective metallization was enabled on the Glass + CuAlO2 composite surfaces using LDS technology. 2. Material and methods Using conventional solid-state synthesis, CuAlO2 ceramics were prepared from high purity (> 99.99) oxides for laser activation additive. Cu2O (99.9 %), Al2O3 (Alfa aesar, 99.99 %) powders were ballmilled in a nylon jar with zirconia balls for 24 h in ethanol. After drying, the mixed Cu2O + Al2O3 powders were calcined at 1100 °C for 24 h at neutral atmosphere [26]. The CuAlO2 powders with various amounts of commercial glass powder (Temen tech, L3SH, Korea) were mixed by ball milling with zirconia balls for 24 h. The dried mixtures were pressed into pellets (Ф 16 mm × 2 mm) at 10 MPa and sintered at various sintering temperatures ranging from 750 °C to 900 °C for 1 h at neutral atmosphere. Fig. 1 shows a schematic of the laser direct structuring (LDS) technology based on CuAlO2 composite. The specified square area (0.5 cm × 0.5 cm) on the CuAlO2 composite surface was first activated using 1064 nm pulsed Nd:YAG laser, and then electroless copper plating at 70 °C for 10 min was performed. The laser irradiations were performed with an Nd:YAG pulse laser (1064 nm, WLF-2000) with a maximum output power of 20 W, repetition of 1000 kHz and spot size of 25 μm. The laser variables were power (1−7 W), repetition 60 kHz and scan speed 2 m/s, respectively. An electroless copper plating solution (ECP-90) was used as supplied from MSC company. For the
3. Results and discussion Fig. 2(a) shows the XRD patterns of the CuAlO2 powder that was calcined at 1100 °C for 24 h. All the reflections (003, 006, 101, 012, 104, 009, 107, 018, 110, 0012, 1010, 116 and 202) could be indexed on the basis of pure phase of homogeneous rhombohedral structure with
Fig. 1. Schematic of the laser direct structuring (LDS) technology based on the Glass + CuAlO2 composite. 2
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Fig. 2. Powder (a) X-ray diffraction patterns and (b) FE-SEM image of CuAlO2 by calcined 1100 °C for 24 h.
Fig. 3. XRD patterns of (a) the Glass + x wt% CuAlO2 composites sintered at 800 °C for 1 h and (b) the Glass +30 wt% CuAlO2 composite sintered at various temperature for 1 h.
¯
space group of R 3 -CuAlO2 (JCPDS File card no. 35-1401). Fig. 2(b) shows the FE-SEM image of synthesized CuAlO2 particles. The synthesized particle size ranged from 500 ∼ 800 nm with irregular shapes. Fig. 3(a) shows the XRD patterns of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 sintered at 800 °C for 1 h. A small amount of CuO phase was detected in the specimen with x = 10, and CuAl2O4 phase was observed in the specimen with x = 70. The detection of CuO and CuAl2O4 is thought to be due to decomposition behavior of the CuAlO2 at 800 °C in air which is explained as follows: 2CuAlO2 + ½O2 → CuAl2O4 + CuO [26]. Fig. 3(b) shows the XRD patterns of the Glass +30 wt% CuAlO2 composites sintered at various temperature for 1 h. A small amount of CuO phase appeared in the specimen sintered at 800 and 850 °C. The CuAlO2 phase decreased with increasing sintering temperature and CuAl2O4 and CuO phase was observed in the specimen sintered at 900 °C. This is as explained before due to the decomposition of CuAlO2 in air at high temperature. Fig. 4(a)-(d) show the SEM images of the fractured surface of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 wt% sintered at 800 °C for 1 h. The specimen with x = 10 shows a large amount of the glass matrix, as shown in Fig. 4(a). A dense microstructure was obtained when CuAlO2 was added to the specimen with x =30 wt% as shown in Fig. 4(b). The specimens with 50 = x show a porous microstructure having a large number of pores, as shown in Fig. 4(c). This was because the viscose glass could not completely wet the CuAlO2 particles, as shown in the inset of Fig. 4(c). When x was 70 wt%, the amount of the viscose glass was insufficient for sintering of the Glass + CuAlO2 composites and a porous microstructure was developed as show in Fig. 4(d). Therefore, dense microstructure formed in the specimen with x ≤ 30 wt%, but the microstructure was not fully densified when the amount of CuAlO2 exceeded with 50 wt%.
Fig. 5(a) shows the bulk densities of the specimens sintered at various temperatures, with x ranging from 10 to 70. At x = 10, the composites showed a bulk density range of 2.12–2.25 g/cm3, with the composite sintered at 750 °C having a bulk density of 2.25 g/cm3. The composite with x = 30 and sintered at 800 °C showed bulk density of 2.52 g/cm3. The specimen with x =50 wt% increased bulk density as sintering temperature increased because the amount of CuAlO2 with large true density (5.04 g/cm3) has increased. However, the composite with x = 70 exhibited a low bulk density even though it contained a large amount of CuAlO2, and this can be explained by the formation of a porous microstructure. Fig. 5(b) shows the relative densities of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 wt% sintered at various temperatures. The relative density was calculated using the following equation:
D=
W1 + W2
( )+( W1 D1
W2 ) D2
Where W1 and W2 are the weight percentages of Glass and CuAlO2 ceramics in the composite, respectively, D1 and D2 are the theoretical density of Glass and CuAlO2, respectively. The relative densities decreased as x increased up to 70 because the true density of CuAlO2 (5.04 g/cm3) is larger than the true density of glass, which was measured to be 2.14 g/cm3. The variation in the relative density was closely related to the microstructure, as shown in Fig. 4(a)-(d). The variation of dielectric constant and loss of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 sintered at various temperature for 1 h were measured at 9.4 GHz as shown in Fig. 5(c). In this study, good correlation between dielectric properties and sintered density of 3
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Fig. 4. SEM image of the fractured surface of the Glass + x wt% CuAlO2 composites sintered at 800 °C 1 h : (a) 10; (b) 30; (c) 50; (d) 70.
Fig. 5. Variation of (a) bulk density; (b) relative density; (c) dielectric constant and (d) loss of the Glass + x wt% CuAlO2 sintered at various temperature for 1 h.
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dense composite specimens. It is thought the wetting property of the glass with synthesized CuAlO2 particles need further modification. Further study in this regard is on going. Fig. 6 shows surface metallization state of each specimens according to x wt% CuAlO2 addition and sintering temperature. Inside yellow circle frame in Fig. 6, specimen with 30 wt% CuAlO2 sintered at 800 °C can be plated successfully. On the right side of Fig. 6, a schematic diagram of the specimen and an enlarged image of the yellow circle were inserted. The enlarged image shows that the Cu electroless plating proceeded well at 5, 7 W laser power. The specimens on the right side show increase in the amount of CuAlO2, with the color of the specimens became increasingly dark. This is because, CuO was formed as the additive amount of CuAlO2 increases as shown in Fig. 3(a). As the additive amount of CuAlO2 increased, composite specimen with good selective metallization was not observed. Also, specimens on the bottom side show increase sintering temperature, the color of the specimens changed increasingly brown and black. This is because, CuO and CuAl2O4 were formed as the sintering temperature increases as shown in Fig. 3(b). The specimens with high sintering temperatures and large additive amount did not obtain selective metallization properties. The chemical state of Copper(Cu) element on the Glass + 30wt% CuAlO2 composites were confirmed using high-resolution XPS measurements. Fig. 7(a) shows a schematic of the process of laser irradiation on the ceramic surface. Also it shows a concept diagraph of the laser irradiated surface of the ceramic compsite. Two factors are shown the first is the formation of roughness and secondly the activated additive material. As show in Fig. 7(b) and(c) the asymmetric summation function of Lorentzian-Gaussian is applied to curve fitting, and good fitting results are obtained. For Cu element, the curve fitting results of before laser irradiation (the top of Fig. 7(b)) demonstrate that the Cu 2p3/2 and Cu 2p1/2 characteristic peaks appear at 934.67 and 954.56 eV, which are attributed to Cu2+ in CuAlO2 according to the literature, and their corresponding baseline satellite peaks are also observed in the regions of 937–947 eV [27]. For the Glass/CuAlO2 composite after laser activation (the bottom of Fig. 7(b)), it can be observed that, besides the Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+ 934.67 and 954.56 eV, two new major peaks occur at 932.25 and 952.3 eV. According to the literature, these two new peaks are assigned to the Cu 2p3/2 and Cu 2p1/2 peaks of Cu0. In addition, through the fitting results of the Cu LMM peak occur at 568.1 eV(the bottom of Fig. 7(c)) [27]. As is known, the active metal element (Cu0) can be an effective catalytic active center during
Fig. 6. Surface metallization state of each specimens according to x wt% CuAlO2 addition and sintering temperature. The most well plated specimen is highlighted by the yellow circle frame (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
the specimens was not observed, as generally the dielectric constant increases with increasing density. The decreasing trend of dielectric constant up to 900 °C can be correlated to the presence of the glass as a main phase in the Glass + CuAlO2 composites system. The dielectric constant and loss value of the specimen with x = 30 sintered at 800 °C was at approximately 4.6 and 0.08, respectively. As x increased to 70, dielectric constant and loss increased to 6.3 and 0.27, respectively. Composite specimens with relative density near 95 % was obtained with 10 wt% ≤ x specimens, but increasing x amount resulted in less
Fig. 7. (a) Schematic of the ceramic surface with laser irradiation. High-resolution XPS patterns of copper (Cu) elements. (b) Cu 2p1/2 and Cu 2p3/2 of the Glass +30 wt% CuAlO2 composite specimen without laser irradiation (top of Fig. 7(b)) and after laser irradiation (bottom of Fig. 7(b)). (c) Cu LMM of the Glass +30 wt% CuAlO2 composite specimen without laser irradiation (top of Fig. 7(c)) and after laser irradiation (bottom of Fig. 7(c)). 5
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Fig. 8. SEM images of untreated area and laser irradiated areas with repetition rate of 60 kHz; scanning speed of 2 ms−1 and laser power of 7 W : The Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h: (a) 750 °C; (b) 800 °C; (c) 850 °C; and (d) 900 °C; and sintered at 800 °C with (e) 50 wt% and (f) 70 wt% of CuAlO2. The SEM images with single quote (a’)-(f’) are enlarged images of a quadrangular zone of SEM images (a)-(f).
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Fig. 9. The surface microscope images and roughness of the Glass +30 wt% CuAlO2 sintered at various temperature for 1 h: (a) 750 °C; (b) 800 °C; (c) 850 °C; and (d) 900 °C after laser irradiation (7 W).
specimens sintered above 850 °C, Ra value decreased to 0.94 μm (Fig. 9(c) and (d)). Therefore, to achieve metallization of CuAlO2 composite it is observed that sufficient roughness of surface Ra value is also needed beside the formation of Cu0 catalytic sites by laser irradiation.
electroless plating. In this CuAlO2 ceramic composite by laser irradiation, it is thought that the newly formed Cu0 is a cause for the selective metallization on composite specimens metallization. Fig. 8(a)-(f) show SEM images of surface of untreated area and laser irradiated areas with repetition rate of 60 kHz, scanning speed of 2 m/s and laser power of 7 W : The Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h. The SEM images with single quote (a’)-(f’) are enlarged images of a yellow quadrangular zone of SEM images Fig. 8(a)-(f). As can be seen in, surface of specimen with 30 wt% CuAlO2 sintered at 750 °C show porous microstructures because the viscous sintering did not proceed completely and the relative density was low at approximately 73.7 % as shown in Fig. 8(a). Fig. 8(b)-(d) show denser microstructures and formation of rough microstructure of the ceramic specimen surface after laser irradiation. Nevertheless, the Cu layer plating was not created under the condition Fig. 8(c) and (d) as shown in Fig. 6. The cause of this is thought to be that the Cu metal seed on ceramic surfaces could not be produced due to the production of CuO at high sintering temperature. In the Fig. 8(e) and (f), as the amount of CuAlO2 is increased the amount of viscose glass matrix is relatively decreased, and the relative density is low due to lack of viscous sintering of the specimen. These samples as shown in Fig. 6 also could not obtain selective metallization of the irradiated areas. Fig. 9 (a)-(d) show the surface microscope images and roughness of the Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h after laser irradiation (7 W). The average roughness (Ra) of surface on the Glass +30 wt% CuAlO2 composites sintered at 750 °C after the laser irradiation was relatively low at approximately 0.66 μm, as shown in Fig. 9(a). However, the Ra value of the specimen sintered at 800 °C was relatively large at approximately 1.27 μm (Fig. 9(b)). For the
4. Conclusions In this study, it was confirmed that CuAlO2 is a good activation additive for fabricating ceramic composite with selective metallization performance by LDS technology. A porous microstructure formed when a large amount of CuAlO2 (≥ 50 wt%) was added into the glass because the amount of the viscose glass is not sufficient to completely wet the CuAlO2 particles. The glass with 30 wt% CuAlO2 sintered at 800 °C showed a dense microstructure, and this composite showed a low dielectric constant and loss of 4.6 and 0.08 at 9.1 GHz. These results demonstrated the Glass +30 wt% CuAlO2 composite to be good candidate materials for microwave substrate. Moreover it was found that, after 1064 nm Nd:YAG pulsed laser activation, 30 wt % CuAlO2 in the glass composite sintered at 800 °C for 1 h could endow the ceramic substrate with a good capacity of selective metallization. Both the surface chemistry and morphology of the Glass + CuAlO2 composites after laser activation and selective metallization were systematically investigated. XPS results of the surface of the laser irradiated the Glass +30 wt% CuAlO2 composite sintered at 800 °C for 1 h showed that a small part of Cu2+, Cu1+ was reduced to Cu0. Moreover, Ra of surface of the laser irradiated this composite showed 1.27 μm. In short, the strategy of selective metallization on the Glass +30 wt% CuAlO2 composite proposed here is very suitable for manufacturing 3D metallic 7
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Declaration of Competing Interest
patterns, especially 3D circuits. During the laser activation, the required 3D patterns can be conveniently inscribed on the ceramic surface through an advanced 3D laser machining system. After electroless copper plating, the 3D metallic patterns or circuits can be realized.
No conflict of interest exists. Acknowledgement
Funding This work was supported by a grant from the Ministry of Trade, Industry and Energy (MOTIE). (Grant 10063453)
No funding was received for this work.
8
Contents lists available at ScienceDirect
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Original Article
Investigation of CuAlO2 composite dielectric properties and selective metallization by laser direct structure technology Tae-Ho Lee, Soon-Mi Hwang, Myong-Jae Yoo* Reliability Research Center, , Korea Electronics Technology Institute, #25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, 13509, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser direct structuring (LDS) technology CuAlO2 ceramic composite Selective metallization Dielectric properties Electroless copper plating
In this work, we investigated CuAlO2 ceramic composite dielectric properties and potential application to laser direct structuring (LDS) technology. To sinter at low temperature various amounts of glass was added to CuAlO2 and their microstructure, dielectric properties were systematically investigated. The Glass +30 wt% CuAlO2 composite sintered at 800 °C for 1 h was measured to have a high relative density of 84.5 %, low dielectric constant and loss of 4.6 and 0.08 at 9.1 GHz. Using this composite after laser irradiation well-defined copper metal patterns were fabricated on ceramic composite surface. Two important factors which enabled metallization was observed to be morphology of the ceramic composite surface and reduction of Cu+, Cu2+ in the CuAlO2 composite to Cu0 after laser irradiation.
1. Introduction Recently developments of the Molded Interconnect Device (3-MID) technology has made significant advances in interconnecting device manufacturing methods, including substrates materials, structuring metering, and various connectivity devices. One of the common processes for the 3D-MID technology is the Laser Direct Structuring (LDS) technology. It is emerging as a very convenient and cost-effective approach for the fabrication of metallized patterns on nonconductive substrates [1–4]. In the process of LDS, injection-molded substrates with activation additives are structured by laser, and subsequently metallized by using electroless plating process [2,4,5]. The polymer substrates fabricated using LDS technology commonly show a low thermal stability, conductivity and temperature resistance [6]. For example, representative MID materials such as polyether ether ketone (PEEK) has thermal stability up to 240 °C [6]. However, applications ranging from automobiles to aerospace engineering require materials with high thermal resistance, such as ceramics [7]. In order to metallize on substrate using a ceramic material, DCB (Direct Copper Bonding) [8,9], AMB (Active Metal Brazing) [10,11] and screen printing on LTCC (Low Temperature Cofired Ceramics) [12,13] is commonly used. These typical metallization processes require heat treatment at high temperatures and intensive chemical etching and cleaning [9,11]. Moreover, the geometric complexity of the substrate during metallization is severely limited [11,13]. Compared to these conventional metallization processes for ceramic substrates LDS technology can realize
⁎
very fine and complex structures with few process steps, less time and smaller energy consumption [14]. For forming metals on surfaces by LDS technology, the porous structure of surface formed in the laser machining process is considered to be a key element. However, it also requires that the surface of the ceramic substrate be seeded with a metallic catalyst [15,16]. For this, selective metallization composites are doped with additives. The LDS additives are important for forming conductive patterns on surface of composite. The additive are excited by laser irradiation and are separated into metal and organic residuals [17]. This process is called ‘activation’. In electroless plating, the activated filler acts as a plating catalyst [18,19]. Examples of activation additive include Al2O3, CeO2, ZnO, AlN, SIC, CuO and a Cu-Cr complex [20,21]. When a laser is irradiated on the composite surface, the ceramic matrix is melted (photochemical ablation) or evaporated, and the activation additive is exposed and activated [22]. The activated additive donates electrons in electroless plating [20,23]. During the electroless plating process, the porous structure could retain the copper plating solution, but in the untreated region the copper plating solution is washed out. It explains why plating is done only in the laser irradiation area [24]. LDS additive materials have been extensively studied, among which Cu-Cr complex have been reported to show good copper plating quality [21]. However, it is expensive to use as LDS additive material. To overcome this problem, we have studied CuAl2O4 material as an additive material for LDS technology, which has lower cost by substituting Cr with aluminum [25]. From our previous work, we have confirmed
Corresponding author. E-mail address: [email protected] (M.-J. Yoo).
https://doi.org/10.1016/j.jeurceramsoc.2019.11.036 Received 8 July 2019; Received in revised form 8 November 2019; Accepted 11 November 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tae-Ho Lee, Soon-Mi Hwang and Myong-Jae Yoo, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2019.11.036
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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electroless plating, the solution containing copper sulfate (8 g/L) as a source of Cu2+ ions, formaldehyde (12 mL/L) as the reducing agent, potassium sodium tartrate (30 g/L), ethylene diamine tetra-acetic acid (3 g/L), sodium citrate (3 g/L) as the complexing agent, and methanol (150 mL/L) as the stabilizer was used. The crystal structures of the powder and specimens were examined via X-ray diffraction (XRD: Rigaku D/max-RC, Tokyo, Japan) using CuKα radiation and operating at 40 kV and 40 mA. The specimens were scanned in a 2θ range of 10-80° at a scanning rate of 8° per min. The surface and cross-sectional microstructures of specimens were studied using field-emission scanning electron microscopy (FE-SEM: Hitachi S-4800, Osaka, Japan). The true density of glass powder was determined by helium pycnometry using an Ultra PYC 1200e pycnometer (Quantachrome, USA). The bulk density of the composite specimens was measured using the Archimedes method and the relative density was calculated using the theoretical density. The dielectric constants and dielectric losses of the composite specimens were measured at 9.4 GHz with an PNA-L network analyzer (Agilent Technologies N5230A, Santa Clara, CA, USA). The surface chemical elements were characterized by X-ray photoelectron spectroscopy (XPS: ULVACPHI X-tool, Japan) operating at the AL Ka achromatic X-ray source (1486.6 eV) and an X-ray beam of around 1 mm. The surface pattern formed on the CuAlO2 composite by laser irradiation was observed using an optical microscope (VK-9710, Keyence Co., LTd.).
that CuAl2O4 can be used in polymer composite for LDS technology with good selective metallization properties [25]. In this work we synthesized CuAlO2, a new material using aluminum by conventional solid state process. The synthesized CuAlO2 with glass frit was used to make CuAlO2 ceramic composite and their dielectric properties and microstructures were studied. Furthermore, we studied conditions under which selective metallization was enabled on the Glass + CuAlO2 composite surfaces using LDS technology. 2. Material and methods Using conventional solid-state synthesis, CuAlO2 ceramics were prepared from high purity (> 99.99) oxides for laser activation additive. Cu2O (99.9 %), Al2O3 (Alfa aesar, 99.99 %) powders were ballmilled in a nylon jar with zirconia balls for 24 h in ethanol. After drying, the mixed Cu2O + Al2O3 powders were calcined at 1100 °C for 24 h at neutral atmosphere [26]. The CuAlO2 powders with various amounts of commercial glass powder (Temen tech, L3SH, Korea) were mixed by ball milling with zirconia balls for 24 h. The dried mixtures were pressed into pellets (Ф 16 mm × 2 mm) at 10 MPa and sintered at various sintering temperatures ranging from 750 °C to 900 °C for 1 h at neutral atmosphere. Fig. 1 shows a schematic of the laser direct structuring (LDS) technology based on CuAlO2 composite. The specified square area (0.5 cm × 0.5 cm) on the CuAlO2 composite surface was first activated using 1064 nm pulsed Nd:YAG laser, and then electroless copper plating at 70 °C for 10 min was performed. The laser irradiations were performed with an Nd:YAG pulse laser (1064 nm, WLF-2000) with a maximum output power of 20 W, repetition of 1000 kHz and spot size of 25 μm. The laser variables were power (1−7 W), repetition 60 kHz and scan speed 2 m/s, respectively. An electroless copper plating solution (ECP-90) was used as supplied from MSC company. For the
3. Results and discussion Fig. 2(a) shows the XRD patterns of the CuAlO2 powder that was calcined at 1100 °C for 24 h. All the reflections (003, 006, 101, 012, 104, 009, 107, 018, 110, 0012, 1010, 116 and 202) could be indexed on the basis of pure phase of homogeneous rhombohedral structure with
Fig. 1. Schematic of the laser direct structuring (LDS) technology based on the Glass + CuAlO2 composite. 2
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Fig. 2. Powder (a) X-ray diffraction patterns and (b) FE-SEM image of CuAlO2 by calcined 1100 °C for 24 h.
Fig. 3. XRD patterns of (a) the Glass + x wt% CuAlO2 composites sintered at 800 °C for 1 h and (b) the Glass +30 wt% CuAlO2 composite sintered at various temperature for 1 h.
¯
space group of R 3 -CuAlO2 (JCPDS File card no. 35-1401). Fig. 2(b) shows the FE-SEM image of synthesized CuAlO2 particles. The synthesized particle size ranged from 500 ∼ 800 nm with irregular shapes. Fig. 3(a) shows the XRD patterns of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 sintered at 800 °C for 1 h. A small amount of CuO phase was detected in the specimen with x = 10, and CuAl2O4 phase was observed in the specimen with x = 70. The detection of CuO and CuAl2O4 is thought to be due to decomposition behavior of the CuAlO2 at 800 °C in air which is explained as follows: 2CuAlO2 + ½O2 → CuAl2O4 + CuO [26]. Fig. 3(b) shows the XRD patterns of the Glass +30 wt% CuAlO2 composites sintered at various temperature for 1 h. A small amount of CuO phase appeared in the specimen sintered at 800 and 850 °C. The CuAlO2 phase decreased with increasing sintering temperature and CuAl2O4 and CuO phase was observed in the specimen sintered at 900 °C. This is as explained before due to the decomposition of CuAlO2 in air at high temperature. Fig. 4(a)-(d) show the SEM images of the fractured surface of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 wt% sintered at 800 °C for 1 h. The specimen with x = 10 shows a large amount of the glass matrix, as shown in Fig. 4(a). A dense microstructure was obtained when CuAlO2 was added to the specimen with x =30 wt% as shown in Fig. 4(b). The specimens with 50 = x show a porous microstructure having a large number of pores, as shown in Fig. 4(c). This was because the viscose glass could not completely wet the CuAlO2 particles, as shown in the inset of Fig. 4(c). When x was 70 wt%, the amount of the viscose glass was insufficient for sintering of the Glass + CuAlO2 composites and a porous microstructure was developed as show in Fig. 4(d). Therefore, dense microstructure formed in the specimen with x ≤ 30 wt%, but the microstructure was not fully densified when the amount of CuAlO2 exceeded with 50 wt%.
Fig. 5(a) shows the bulk densities of the specimens sintered at various temperatures, with x ranging from 10 to 70. At x = 10, the composites showed a bulk density range of 2.12–2.25 g/cm3, with the composite sintered at 750 °C having a bulk density of 2.25 g/cm3. The composite with x = 30 and sintered at 800 °C showed bulk density of 2.52 g/cm3. The specimen with x =50 wt% increased bulk density as sintering temperature increased because the amount of CuAlO2 with large true density (5.04 g/cm3) has increased. However, the composite with x = 70 exhibited a low bulk density even though it contained a large amount of CuAlO2, and this can be explained by the formation of a porous microstructure. Fig. 5(b) shows the relative densities of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 wt% sintered at various temperatures. The relative density was calculated using the following equation:
D=
W1 + W2
( )+( W1 D1
W2 ) D2
Where W1 and W2 are the weight percentages of Glass and CuAlO2 ceramics in the composite, respectively, D1 and D2 are the theoretical density of Glass and CuAlO2, respectively. The relative densities decreased as x increased up to 70 because the true density of CuAlO2 (5.04 g/cm3) is larger than the true density of glass, which was measured to be 2.14 g/cm3. The variation in the relative density was closely related to the microstructure, as shown in Fig. 4(a)-(d). The variation of dielectric constant and loss of the Glass + x wt% CuAlO2 composites with 10 ≤ x ≤ 70 sintered at various temperature for 1 h were measured at 9.4 GHz as shown in Fig. 5(c). In this study, good correlation between dielectric properties and sintered density of 3
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Fig. 4. SEM image of the fractured surface of the Glass + x wt% CuAlO2 composites sintered at 800 °C 1 h : (a) 10; (b) 30; (c) 50; (d) 70.
Fig. 5. Variation of (a) bulk density; (b) relative density; (c) dielectric constant and (d) loss of the Glass + x wt% CuAlO2 sintered at various temperature for 1 h.
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dense composite specimens. It is thought the wetting property of the glass with synthesized CuAlO2 particles need further modification. Further study in this regard is on going. Fig. 6 shows surface metallization state of each specimens according to x wt% CuAlO2 addition and sintering temperature. Inside yellow circle frame in Fig. 6, specimen with 30 wt% CuAlO2 sintered at 800 °C can be plated successfully. On the right side of Fig. 6, a schematic diagram of the specimen and an enlarged image of the yellow circle were inserted. The enlarged image shows that the Cu electroless plating proceeded well at 5, 7 W laser power. The specimens on the right side show increase in the amount of CuAlO2, with the color of the specimens became increasingly dark. This is because, CuO was formed as the additive amount of CuAlO2 increases as shown in Fig. 3(a). As the additive amount of CuAlO2 increased, composite specimen with good selective metallization was not observed. Also, specimens on the bottom side show increase sintering temperature, the color of the specimens changed increasingly brown and black. This is because, CuO and CuAl2O4 were formed as the sintering temperature increases as shown in Fig. 3(b). The specimens with high sintering temperatures and large additive amount did not obtain selective metallization properties. The chemical state of Copper(Cu) element on the Glass + 30wt% CuAlO2 composites were confirmed using high-resolution XPS measurements. Fig. 7(a) shows a schematic of the process of laser irradiation on the ceramic surface. Also it shows a concept diagraph of the laser irradiated surface of the ceramic compsite. Two factors are shown the first is the formation of roughness and secondly the activated additive material. As show in Fig. 7(b) and(c) the asymmetric summation function of Lorentzian-Gaussian is applied to curve fitting, and good fitting results are obtained. For Cu element, the curve fitting results of before laser irradiation (the top of Fig. 7(b)) demonstrate that the Cu 2p3/2 and Cu 2p1/2 characteristic peaks appear at 934.67 and 954.56 eV, which are attributed to Cu2+ in CuAlO2 according to the literature, and their corresponding baseline satellite peaks are also observed in the regions of 937–947 eV [27]. For the Glass/CuAlO2 composite after laser activation (the bottom of Fig. 7(b)), it can be observed that, besides the Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+ 934.67 and 954.56 eV, two new major peaks occur at 932.25 and 952.3 eV. According to the literature, these two new peaks are assigned to the Cu 2p3/2 and Cu 2p1/2 peaks of Cu0. In addition, through the fitting results of the Cu LMM peak occur at 568.1 eV(the bottom of Fig. 7(c)) [27]. As is known, the active metal element (Cu0) can be an effective catalytic active center during
Fig. 6. Surface metallization state of each specimens according to x wt% CuAlO2 addition and sintering temperature. The most well plated specimen is highlighted by the yellow circle frame (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
the specimens was not observed, as generally the dielectric constant increases with increasing density. The decreasing trend of dielectric constant up to 900 °C can be correlated to the presence of the glass as a main phase in the Glass + CuAlO2 composites system. The dielectric constant and loss value of the specimen with x = 30 sintered at 800 °C was at approximately 4.6 and 0.08, respectively. As x increased to 70, dielectric constant and loss increased to 6.3 and 0.27, respectively. Composite specimens with relative density near 95 % was obtained with 10 wt% ≤ x specimens, but increasing x amount resulted in less
Fig. 7. (a) Schematic of the ceramic surface with laser irradiation. High-resolution XPS patterns of copper (Cu) elements. (b) Cu 2p1/2 and Cu 2p3/2 of the Glass +30 wt% CuAlO2 composite specimen without laser irradiation (top of Fig. 7(b)) and after laser irradiation (bottom of Fig. 7(b)). (c) Cu LMM of the Glass +30 wt% CuAlO2 composite specimen without laser irradiation (top of Fig. 7(c)) and after laser irradiation (bottom of Fig. 7(c)). 5
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Fig. 8. SEM images of untreated area and laser irradiated areas with repetition rate of 60 kHz; scanning speed of 2 ms−1 and laser power of 7 W : The Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h: (a) 750 °C; (b) 800 °C; (c) 850 °C; and (d) 900 °C; and sintered at 800 °C with (e) 50 wt% and (f) 70 wt% of CuAlO2. The SEM images with single quote (a’)-(f’) are enlarged images of a quadrangular zone of SEM images (a)-(f).
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Fig. 9. The surface microscope images and roughness of the Glass +30 wt% CuAlO2 sintered at various temperature for 1 h: (a) 750 °C; (b) 800 °C; (c) 850 °C; and (d) 900 °C after laser irradiation (7 W).
specimens sintered above 850 °C, Ra value decreased to 0.94 μm (Fig. 9(c) and (d)). Therefore, to achieve metallization of CuAlO2 composite it is observed that sufficient roughness of surface Ra value is also needed beside the formation of Cu0 catalytic sites by laser irradiation.
electroless plating. In this CuAlO2 ceramic composite by laser irradiation, it is thought that the newly formed Cu0 is a cause for the selective metallization on composite specimens metallization. Fig. 8(a)-(f) show SEM images of surface of untreated area and laser irradiated areas with repetition rate of 60 kHz, scanning speed of 2 m/s and laser power of 7 W : The Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h. The SEM images with single quote (a’)-(f’) are enlarged images of a yellow quadrangular zone of SEM images Fig. 8(a)-(f). As can be seen in, surface of specimen with 30 wt% CuAlO2 sintered at 750 °C show porous microstructures because the viscous sintering did not proceed completely and the relative density was low at approximately 73.7 % as shown in Fig. 8(a). Fig. 8(b)-(d) show denser microstructures and formation of rough microstructure of the ceramic specimen surface after laser irradiation. Nevertheless, the Cu layer plating was not created under the condition Fig. 8(c) and (d) as shown in Fig. 6. The cause of this is thought to be that the Cu metal seed on ceramic surfaces could not be produced due to the production of CuO at high sintering temperature. In the Fig. 8(e) and (f), as the amount of CuAlO2 is increased the amount of viscose glass matrix is relatively decreased, and the relative density is low due to lack of viscous sintering of the specimen. These samples as shown in Fig. 6 also could not obtain selective metallization of the irradiated areas. Fig. 9 (a)-(d) show the surface microscope images and roughness of the Glass +30 wt% CuAlO2 composites sintered at various temperatures for 1 h after laser irradiation (7 W). The average roughness (Ra) of surface on the Glass +30 wt% CuAlO2 composites sintered at 750 °C after the laser irradiation was relatively low at approximately 0.66 μm, as shown in Fig. 9(a). However, the Ra value of the specimen sintered at 800 °C was relatively large at approximately 1.27 μm (Fig. 9(b)). For the
4. Conclusions In this study, it was confirmed that CuAlO2 is a good activation additive for fabricating ceramic composite with selective metallization performance by LDS technology. A porous microstructure formed when a large amount of CuAlO2 (≥ 50 wt%) was added into the glass because the amount of the viscose glass is not sufficient to completely wet the CuAlO2 particles. The glass with 30 wt% CuAlO2 sintered at 800 °C showed a dense microstructure, and this composite showed a low dielectric constant and loss of 4.6 and 0.08 at 9.1 GHz. These results demonstrated the Glass +30 wt% CuAlO2 composite to be good candidate materials for microwave substrate. Moreover it was found that, after 1064 nm Nd:YAG pulsed laser activation, 30 wt % CuAlO2 in the glass composite sintered at 800 °C for 1 h could endow the ceramic substrate with a good capacity of selective metallization. Both the surface chemistry and morphology of the Glass + CuAlO2 composites after laser activation and selective metallization were systematically investigated. XPS results of the surface of the laser irradiated the Glass +30 wt% CuAlO2 composite sintered at 800 °C for 1 h showed that a small part of Cu2+, Cu1+ was reduced to Cu0. Moreover, Ra of surface of the laser irradiated this composite showed 1.27 μm. In short, the strategy of selective metallization on the Glass +30 wt% CuAlO2 composite proposed here is very suitable for manufacturing 3D metallic 7
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Declaration of Competing Interest
patterns, especially 3D circuits. During the laser activation, the required 3D patterns can be conveniently inscribed on the ceramic surface through an advanced 3D laser machining system. After electroless copper plating, the 3D metallic patterns or circuits can be realized.
No conflict of interest exists. Acknowledgement
Funding This work was supported by a grant from the Ministry of Trade, Industry and Energy (MOTIE). (Grant 10063453)
No funding was received for this work.
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