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Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar plating of IC substrates
Accepted Manuscript Title: Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar plating of IC substrates Author: Jing Xiang Chong Wang Yuanming Chen Shouxu Wang Yan Hong Huaiwu Zhang Lijun Gong Wei He PII: DOI: Reference:
S0169-4332(17)30596-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.223 APSUSC 35332
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-12-2016 15-2-2017 26-2-2017
Please cite this article as: J. Xiang, C. Wang, Y. Chen, S. Wang, Y. Hong, H. Zhang, L. Gong, W. He, Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar plating of IC substrates, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.02.223 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar
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plating of IC substrates
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Jing Xianga, Chong Wang a, Yuanming Chena , Shouxu Wanga , Yan Honga , Huaiwu Zhang a , Lijun Gongb, Wei He a,c a
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State Key Laboratory of Electronic Thin Films and Integrated Devices, University of
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Electronic Science and Technology of China, Chengdu 610054, China b
Research and development department, Guangzhou Fastprint Circuit Tech Co., Ltd,
Research and development department, Guangdong Guanghua Sci-Tech Co., Ltd.,
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Shantou 515000, China
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Guangzhou 510663, China
Highlights
Air atmosphere plasma could generate hydrophilic groups of photo-resistive film.
Better wettability of photo-resistive film led to higher plating uniformity of copper pillars.
Abstract
The wettability of the photo-resistive film (PF) surfaces undergoing different pretreatments including the O2-CF4 low-pressure plasma (OCLP) and air plasma (AP), is investigated by water contact angle measurement instrument (WCAMI) before the bottom-up copper pillar plating. Chemical groups analysis performed by
Corresponding author. Tel.: +86 28 83203218
E-mail address: [email protected] (W. He).
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attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectra (XPS) shows that after the OCLP and wash treatment, the wettability of PF surface is attenuated, because embedded fluorine and decreased oxygen content both enhance hydrophobicity. Compared with OCLP treatment，the PF surface treatment by non-toxic air plasma displays features of C-O, O-C=O, C=O and -NO2 by AIR-FTIR and XPS, and a promoted wettability by WCAM. Under the identical electroplating condition, the surface with a better wettability allows electrolyte to spontaneously soak all the places of vias, resulting in improved copper pillar uniformity. Statistical analysis of metallographic data shows that more coplanar and flat copper pillars are achieved with the PF treatment of air plasma. Such modified copper-pillar-plating technology meets the requirement of accurate impedance, the high density interconnection for IC substrates.
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Keywords: IC Substrates, Copper pillar plating, Plasma treatment, Wettability
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1. Introduction
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Integrated circuit (IC) substrates are special printed circuit board (PCB) for packaging semiconductor chips to serve as multi-functional carriers for protecting chips from potential damage, providing rigid support, heat dissipation, electrical response, signal and power distribution [1]. Chips and IC substrates can be packaged by several methods, such as wire bonding (WB) and ball grid array (BGA) [2-4]. IC substrates are generally used to interconnect chips by tin soldering of copper pad. With the trend of enhancing performance and down-sizing for electronic products, BGA package is spreading rapidly because of its advantage for larger pin counts and higher reliability than WB [5]. IC substrates are manufactured in accordance with layer-by-layer additive process. The circuits in different layers are interconnected by copper plated vias and through holes (THs). Conventionally, THs are drilled mechanically, penetrating through layers which are glass fiber cloth reinforced resin or ceramics with thicknesses of 200 μm to 800 μm and with minimum hole diameters of about 100 μm. However, the pursuit of smaller electronic devices has driven a desire for finer interconnection and higher density of the IC package [6, 7]. Laser drill promises more precise performance that burns hole approximately as small as 30 μm [8], but the weakness of laser drill is obvious that it is unable to punch through ceramic nor glass fiber cloth. Coreless technology employs resin with no glass-fiber clothes as middle layer which can be penetrated by laser drilling, and has been developed to produce smaller IC substrate. Recently, bottom-up copper plating, an advanced coreless technology, combining build-up and photo-lithography, has been proposed to achieve more exquisite IC substrates [9]. Finer and smaller patterns and vias are made on PF by lithography instead of drilling, subsequently fulfilled by bottom-up copper plating.
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After stripping PF, copper pillars and lines are completed. It is distinguished that the advantage of this technique is capable of producing more miniaturized IC substrates. In addition, the smooth copper surface indicates low skin effect, which is beneficial for high frequency portable and wearable devices [10-14]. And the height consistency of copper pillars is also a crucial factor for the quality and production yield, because it limits the flatness of whole plate and the joints strength between copper pillars and upper layer lines.
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Photo-resistive film is mainly poly (3-aldehydecinnamate acid vinyl ester), a hydrophobic material, which is hardly wetted for plating electrolyte. And, there are more or less PF residues on copper surface after development, which may deteriorate plating results, such as the undercut of copper pillars or void. Chemical desmear is an effective solution in PCB industry to clean copper surface and improve wettability of resin. However, it is impossible to be used with PF which solutes in the desmear solution because chemical solution isn’t easy to soak all the places of micro-via and pollutes the environment [15-18].
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Plasma treatment is a dry etching approach which can be operated under gaseous condition and only causes minor geometrical change. Plasma treatment has been used in PCB desmear and descum process for its excellent high-aspect-ratio cleaning ability and all of the unwanted organic PF residues from the copper surface have been turned to solid ashes and removed to decrease the undercut of copper pillars [19-21]. However, the PF surface remains hydrophobic after OCLP [22-24]. Insufficient wetting of via surface results in dissatisfied copper plating, such as rough surface, void and uneven copper pillars [25-27]. After plasma treatment, the generation of particulate needs cleaning process and makes process complex. Furthermore, the CF4 is a gaseous pollutant emitted from the OCLP treatment and causes a high cost for scrubbing by-products such as HF [28, 29]. A simple, low-cost, environmentalfriendly and high-efficiency method or process is desirable and has been explored in many studies, such as air plasma (AP) and the modification of Ultraviolet (UV). Compared with the modification of Ultraviolet, AP is high efficient, free of hydrophobic group (-CH3) and an ideal choice to meet requirements of environmental protection, simplified manufacturing flow and high efficiency [30-36]. We developed ambient AP which highly improves hydrophilicity of the PF surface, after OCLP, to achieve coplanar copper pillar plating. Compared with double OCLP, the chemical and physical properties of PF surfaces were characterized by scanning electron microscope (SEM), contact angle measurement, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR). The heights of plated copper pillars were measured by metallographic microscope and counted. 2. Experiment
2.1 Preparation Process
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Figure 1 is the flow scheme of one layer process on copper plating. First, 30 μm thickness copper foil laminates PF whose thickness were 50±3 µm and resolution was ±0.0254 mm (by ACCESS Co. Ltd.). Subsequent exposures and developments in transfer patterns were microvias with the diameter of 60 μm. Two prepared routes followed: A is double OCLP, B is OCLP and AP. In route A, the sample was washed after each OCLP step to remove plastic slags and ashes. As for the case for B, the sample was treated instead of AP for the second-time plasma treatment and was plated directly without any wash step.
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OCLP operated at vacuum pressure of 110 Pa and power of 3.96 kW. Gas flow of O2 and CF4 were 9.6×10-2 m3/h and 1.38 m3/h, respectively. Treating time was 1800 s. AP operated at atmospheric pressure and power of 5 kW for 300 s.
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Copper plating electrolyte contained 75 g/L CuSO4 ·5H2O, 240 g/L H 2SO4 and 70 mg/L Cl- and commercial 3 components additives which were supplied by Dow chemical company. The temperature of electrolyte bath was kept at 25 ℃and the current density was controlled at 1.8 A/dm2 for 1×104 s. 2.2 Characterization
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The morphology of PF surface was examined by SEM (JEOL JSM-6060). The samples were sputtered with a gold coating for imaging. The wettability of PF surface before and after plasma treatment was examined by WCAMI (SINDIN SDC-100). The chemical conditions of PF surface were characterized by XPS (Thermo ESCALAB 250XI) and ATR FTIR (Thermo Fisher Nicolet 6700). The heights of 60 copper pillars were measured by metallography microscope (Olympus MX61L) to count growth uniformity of bottom-up copper pillars. 2.3 Data analysis
To make a quantitative analysis of the uniformity of plating copper pillar, the uniformity of deposition and Coefficient of Variation (COV) are defined as:
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where μis the average height of 60 copper pillars evenly distributed on the test board, and σis the standard deviation of 60 copper pillars. Smaller COV implies smaller fluctuation, and therefore indicates a better uniformity.
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3. Results and discussion
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The holes with different pre-treatment are showed in Figure 2. After the “development” process, some big PF residues lead to the undercut of copper pillars at the bottom of the hole in Figure 2a. OCLP treatment removes the residues at the bottom of the holes, but damaged orifice (Figure 2b). As shown in Figure 2c, AP treatment makes hole no residue and intact. It is clear that AP treatment obtains better and more complete holes than OCLP. The effect of different pretreatment procedures is studied by the surface morphology images of PF, as illustrated in Figure 2. In contrast to the blank sample which is almost smooth surface (Figure 2a), an uneven and shaggy surface obtained by OCLP treatment is evidently observed in Figure 2b, but a relatively even surface treated with AP is displayed in Figure 2d. Some particles are generated on PF surface after the high energy OCLP treatment and needed wash to remove, as shown in Figure 2b, 2c. The absence of generated particles makes low energy air plasma a simplified route compared with OCLP because the former skips the wash step. The water contact angle test for PF surface of different treatment including no treatment, OCLP and AP pretreatment, are illustrated in Figure 4 and Figure 5, respectively. The surface treatment by OCLP has an initial angle of 58.4° which is a little higher than the control group with the initial angle of 57.3°, corresponding to enhanced surface hydrophobicity. After the air plasma treatment, surface with an initial angle of 38.3° is drastically reduced by 19.9° compared to the one from OCLP treatment, demonstrating that the AP immensely improves the hydrophilicity of PF surface. Comparison between air plasma and the OCLP treatment denotes that air plasma dramatically increases the wettability of PF surface but the OCLP reduces it.
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3.2. Chemical analysis
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The depth of surface modification by plasma treatment is confined only to a few nanometers below the surface. This study employs FTIR spectroscopy in attenuated total reflection mode which is one of the methods used to capture the trace spectral features to characterize the PF surface in Figure 6. With the OCLP treatment, the spectra of the surface contain signals associated with the C-F (1162 cm-1), indicating that C-F as a hydrophobic group is generated on PF surface by OCLP, as compared to the sample without pretreatment. Upon treatment with air plasma, the new stretch at 1715 cm-1 can be ascribed to C=O and the peak at 1386 cm-1 is assigned to –NO 2. Compared with C=O and –NO 2, C-F is a more hydrophobic group that only found after the treatment by OCLP. Fluorine atom weakens the hydrophilicity of PF surface. Contrarily, the oxygen atom and the nitrogen atom can contribute to a higher wettability [40-44]. Therefore, the hydrophobic group (C-F) is evoked by the atmosphere of CF4 to enhance hydrophobicity while AP improves wettability of PF surfaces.
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Figure 7a shows the XPS results of the PF surfaces with different treatments. F element is obtained on surface treatment by OCLP in Figure 7b, the -CF and -C-CF2 are denoted after OCLP treatment (Figure 7d) and the -NO 2 (407.2 eV) is observed on PF with air plasma treatment in Figure 7c. The results of XPS are in agreement with the results of ATR-FTIR. In Figure 8, the C-C (284.5 eV), C=C (285.2 eV), C-O (286.2 eV), C=O (286.7 eV), and O-C=O (288.9 eV) are found. The C-O (533.2 eV), C=O (532.3 eV), and O-C=O (531.7 eV) are explicitly shown in Figure 9. As shown in table 1, after OCLP treatment, the amount of C-O and C=O on surface are reduced while the C-C and C=C are increased. And therefore the OCLP increases the C: O ratio of the PF surface and generates the highly hydrophobic group (R-F) with the content of 0.51% to a more hydrophobic surface of PF. On the contrary, the percentage of C-O, C=O and O-C=O are all raised and a low content (about 0.15%) of weak hydrophobic group (-NO2) is formed on surface by AP treatment. In short, OCLP generates more hydrophobic group (R-F) to enhance hydrophobicity while the AP reduces the C: O ratio to gain higher wettability. The results of ATR-FTIR and XPS both indicate that the air plasma can enhance oxygen content and hydrophilic groups (C-O, C=O and O-C=O) to improve wettability of surface. In the OCLP case, the numbers of these hydrophilic groups are reduced. In OCLP, the free electrons accelerate on PF surface and the electrons’ kinetic energy is high enough for CF4 excitation, ionization and generating F atoms [40, 45]. F atoms give rise to the hydrogen abstraction reaction [46]: RH+2F·→ RF +HF
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The main chemical reactions of the PF surface with different pretreatments are illustrated in Figure 10. Figure 10 demonstrates the reactions that the F atoms are embedded on the PF surface after OCLP treatment while the C=O and weak hydrophobic groups (–NO 2) are generated on the surface by air plasma treatment. Compared with AP, OCLP generates a more hydrophobic group (R-F) to weaken the wettability of PF.
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3.3. Plating uniformity
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The heights of 60 plating copper pillars are measured by metallographic microscope. The lower values of the standard deviation (μ) and COV denote a better uniformity of plating copper pillars. The uniformity of 60 copper pillars with different treatments is shown in Figure 11, the values of σand COV after AP are 1.9 μm and 3.36% while the values after OCLP are 2.25μm and 3.97%. With lower values, AP gets a better uniformity than OCLP. As shown in Figure 12, air plasma modification gives rise to a better uniformity of copper pillar plating than the OCLP and wash. 4. Conclusions
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In this paper, comparative studies of air plasma and OCLP treatments for modification of PF surface are investigated to analyze the formation of plating copper pillar. Environment-friendly air plasma is an effective tool to enhance the wettability of PF by producing hydrophilic group, thereby leading to improve the plating uniformity of copper pillars. Thus, air plasma is a promising approach in plating pretreatment regarding the perspective of advanced quality requirements in IC substrate manufacturing. Acknowledgements
The authors gratefully acknowledge the support of Guangdong Innovative Research Team Program (No. 201301C0105324342), National Natural Science Foundation of China (No. 61474019) and the Project of Science and Technology Planning of the Guangzhou City China (No. 201604010086). References [1] C.T. Ko, S. Chen, C.W. Chiang, T.Y. Kuo, Y.C. Shih, Y.H. Chen, Embedded active device packaging technology for next-generation chip-in-substrate package, CiSP, in: Electronic Components & Technology Conference, 2006. [2] Z.W. Zhong, Wire bonding using copper wire, Microelectronics International, 26 (2009) 10-16. [3] R. Kisiel, Z. Szczepański, Trends in assembling of advanced IC packages, Journal of Telecommunications & Information Technology, (2005).
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Fig 2. SEM images for vias of different pre-treatment: (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment. Fig 3. SEM images for comparison of surface morphology acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment, (c) O2-CF4 low-pressure plasma treatment and wash and (d) air plasma treatment. Fig 4. Water contact angles of the PF with different pretreatment. Fig 5. The water contact angles photographs of different flow: (a) before pretreatment, (b) OCLP treatment and wash and (c) air plasma treatment. Fig 6. ATR-FTIR spectra of different treatment for PF Fig 7. XPS results of the PHOTO-RESISTIVE FILM surface with different pretreatments are (a) Full spectrum diagrams, (b) F1s spectra and (c) N1s spectra, (d) O2-CF4 low-pressure plasma treatment and wash. Fig 8. XPS results for C1s spectra are (a) before the pretreatment, (b) O2-CF4 lowpressure plasma treatment and wash and (c) air plasma treatment. Fig 9. XPS results for O1s spectra are (a) before the pretreatment, (b) O2-CF4 lowpressure plasma treatment and wash and (c) air plasma treatment. Fig 10. The two routes of reaction mechanism of the PF treated by different plasma pretreatments, O2-CF4 low-pressure plasma is PF treated by O2-CF4 low-pressure plasma and wash, air plasma is PF only treated by air plasma. Fig 11. The uniformity of 60 copper pillars with different treatments included O2-CF4 low-pressure plasma and air plasma. Fig 12. The uniformity of copper pillar photographs by metallographic microscope and SEM are generated from process flow A with two-time O2-CF4 low-pressure plasma treatment (a and c) and process flow B with O2-CF4 low-pressure plasma treatment and air plasma treatment (b and d). Table1. XPS results of the PF surface with different pretreatment. Item
C:O ratio C-C C=C C1s C-O C=O O-C=O C-O O1s C=O O-C=O -NO2 N1s C-N Nitrogen-content Fluoride-content
Before pretreatment 2.72:1 39.99% 22.90% 16.49% 14.28% 6.33% 40.14% 38.41% 21.44% 0 100% 0.46% 0
The O2-CF4 lowpressure plasma 2.87:1 42.57% 22.72% 14.13% 13.65% 6.92% 39.06% 39.42% 21.53% 0 100% 0.6% 0.51%
Air plasma 2.42:1 40.42% 18.99% 17.00% 15.79% 7.81% 38.34% 39.95% 21.72% 30% 70% 0.5% 0
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Figure1. The process flow of copper pillar production, A is the conventional flow and B is the new flow.
Figure2. SEM images for hole of different pre-treatment acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment.
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Figure3. SEM images for comparison of surface morphology acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment, (c) O2-CF4 low-pressure plasma treatment and wash and (d) air plasma treatment.
Figure4. Water contact angles of the PF with different pretreatment.
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Figure5. The water contact angles photographs of different flow: (a) before pretreatment, (b) OCLP treatment and wash and (c) air plasma treatment.
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Figure6. ATR-FTIR spectra of different treatment for PF
Figure7. XPS results of the PHOTO-RESISTIVE FILM surface with different pretreatments are (a) Full spectrum diagrams, (b) F1s spectra and (c) N1s spectra, (d)
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O2-CF4 low-pressure plasma treatment and wash.
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Figure8. XPS results for C1s spectra are (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment.
Figure9. XPS results for O1s spectra are (a) before the pretreatment, (b) O2-CF4
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low-pressure plasma treatment and wash and (c) air plasma treatment.
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Figure10. The two routes of reaction mechanism of the PF treated by different plasma pretreatments, O2-CF4 low-pressure plasma is PF treated by O2-CF4 low-pressure plasma and wash, air plasma is PF only treated by air plasma.
Figure11. The uniformity of 60 copper pillars with different treatments included O2-CF4 low-pressure plasma and air plasma.
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Figure12. The uniformity of copper pillar photographs by metallographic microscope and SEM are generated from process flow A with two-time O2-CF4 low-pressure plasma treatment (a and c) and process flow B with O2-CF4 low-pressure plasma treatment and air plasma treatment (b and d).
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S0169-4332(17)30596-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.223 APSUSC 35332
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-12-2016 15-2-2017 26-2-2017
Please cite this article as: J. Xiang, C. Wang, Y. Chen, S. Wang, Y. Hong, H. Zhang, L. Gong, W. He, Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar plating of IC substrates, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.02.223 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improving wettability of photo-resistive film surface with plasma surface modification for coplanar copper pillar
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plating of IC substrates
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Jing Xianga, Chong Wang a, Yuanming Chena , Shouxu Wanga , Yan Honga , Huaiwu Zhang a , Lijun Gongb, Wei He a,c a
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State Key Laboratory of Electronic Thin Films and Integrated Devices, University of
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Electronic Science and Technology of China, Chengdu 610054, China b
Research and development department, Guangzhou Fastprint Circuit Tech Co., Ltd,
Research and development department, Guangdong Guanghua Sci-Tech Co., Ltd.,
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Shantou 515000, China
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Guangzhou 510663, China
Highlights
Air atmosphere plasma could generate hydrophilic groups of photo-resistive film.
Better wettability of photo-resistive film led to higher plating uniformity of copper pillars.
Abstract
The wettability of the photo-resistive film (PF) surfaces undergoing different pretreatments including the O2-CF4 low-pressure plasma (OCLP) and air plasma (AP), is investigated by water contact angle measurement instrument (WCAMI) before the bottom-up copper pillar plating. Chemical groups analysis performed by
Corresponding author. Tel.: +86 28 83203218
E-mail address: [email protected] (W. He).
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attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectra (XPS) shows that after the OCLP and wash treatment, the wettability of PF surface is attenuated, because embedded fluorine and decreased oxygen content both enhance hydrophobicity. Compared with OCLP treatment，the PF surface treatment by non-toxic air plasma displays features of C-O, O-C=O, C=O and -NO2 by AIR-FTIR and XPS, and a promoted wettability by WCAM. Under the identical electroplating condition, the surface with a better wettability allows electrolyte to spontaneously soak all the places of vias, resulting in improved copper pillar uniformity. Statistical analysis of metallographic data shows that more coplanar and flat copper pillars are achieved with the PF treatment of air plasma. Such modified copper-pillar-plating technology meets the requirement of accurate impedance, the high density interconnection for IC substrates.
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Keywords: IC Substrates, Copper pillar plating, Plasma treatment, Wettability
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1. Introduction
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Integrated circuit (IC) substrates are special printed circuit board (PCB) for packaging semiconductor chips to serve as multi-functional carriers for protecting chips from potential damage, providing rigid support, heat dissipation, electrical response, signal and power distribution [1]. Chips and IC substrates can be packaged by several methods, such as wire bonding (WB) and ball grid array (BGA) [2-4]. IC substrates are generally used to interconnect chips by tin soldering of copper pad. With the trend of enhancing performance and down-sizing for electronic products, BGA package is spreading rapidly because of its advantage for larger pin counts and higher reliability than WB [5]. IC substrates are manufactured in accordance with layer-by-layer additive process. The circuits in different layers are interconnected by copper plated vias and through holes (THs). Conventionally, THs are drilled mechanically, penetrating through layers which are glass fiber cloth reinforced resin or ceramics with thicknesses of 200 μm to 800 μm and with minimum hole diameters of about 100 μm. However, the pursuit of smaller electronic devices has driven a desire for finer interconnection and higher density of the IC package [6, 7]. Laser drill promises more precise performance that burns hole approximately as small as 30 μm [8], but the weakness of laser drill is obvious that it is unable to punch through ceramic nor glass fiber cloth. Coreless technology employs resin with no glass-fiber clothes as middle layer which can be penetrated by laser drilling, and has been developed to produce smaller IC substrate. Recently, bottom-up copper plating, an advanced coreless technology, combining build-up and photo-lithography, has been proposed to achieve more exquisite IC substrates [9]. Finer and smaller patterns and vias are made on PF by lithography instead of drilling, subsequently fulfilled by bottom-up copper plating.
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After stripping PF, copper pillars and lines are completed. It is distinguished that the advantage of this technique is capable of producing more miniaturized IC substrates. In addition, the smooth copper surface indicates low skin effect, which is beneficial for high frequency portable and wearable devices [10-14]. And the height consistency of copper pillars is also a crucial factor for the quality and production yield, because it limits the flatness of whole plate and the joints strength between copper pillars and upper layer lines.
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Photo-resistive film is mainly poly (3-aldehydecinnamate acid vinyl ester), a hydrophobic material, which is hardly wetted for plating electrolyte. And, there are more or less PF residues on copper surface after development, which may deteriorate plating results, such as the undercut of copper pillars or void. Chemical desmear is an effective solution in PCB industry to clean copper surface and improve wettability of resin. However, it is impossible to be used with PF which solutes in the desmear solution because chemical solution isn’t easy to soak all the places of micro-via and pollutes the environment [15-18].
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Plasma treatment is a dry etching approach which can be operated under gaseous condition and only causes minor geometrical change. Plasma treatment has been used in PCB desmear and descum process for its excellent high-aspect-ratio cleaning ability and all of the unwanted organic PF residues from the copper surface have been turned to solid ashes and removed to decrease the undercut of copper pillars [19-21]. However, the PF surface remains hydrophobic after OCLP [22-24]. Insufficient wetting of via surface results in dissatisfied copper plating, such as rough surface, void and uneven copper pillars [25-27]. After plasma treatment, the generation of particulate needs cleaning process and makes process complex. Furthermore, the CF4 is a gaseous pollutant emitted from the OCLP treatment and causes a high cost for scrubbing by-products such as HF [28, 29]. A simple, low-cost, environmentalfriendly and high-efficiency method or process is desirable and has been explored in many studies, such as air plasma (AP) and the modification of Ultraviolet (UV). Compared with the modification of Ultraviolet, AP is high efficient, free of hydrophobic group (-CH3) and an ideal choice to meet requirements of environmental protection, simplified manufacturing flow and high efficiency [30-36]. We developed ambient AP which highly improves hydrophilicity of the PF surface, after OCLP, to achieve coplanar copper pillar plating. Compared with double OCLP, the chemical and physical properties of PF surfaces were characterized by scanning electron microscope (SEM), contact angle measurement, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR). The heights of plated copper pillars were measured by metallographic microscope and counted. 2. Experiment
2.1 Preparation Process
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Figure 1 is the flow scheme of one layer process on copper plating. First, 30 μm thickness copper foil laminates PF whose thickness were 50±3 µm and resolution was ±0.0254 mm (by ACCESS Co. Ltd.). Subsequent exposures and developments in transfer patterns were microvias with the diameter of 60 μm. Two prepared routes followed: A is double OCLP, B is OCLP and AP. In route A, the sample was washed after each OCLP step to remove plastic slags and ashes. As for the case for B, the sample was treated instead of AP for the second-time plasma treatment and was plated directly without any wash step.
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OCLP operated at vacuum pressure of 110 Pa and power of 3.96 kW. Gas flow of O2 and CF4 were 9.6×10-2 m3/h and 1.38 m3/h, respectively. Treating time was 1800 s. AP operated at atmospheric pressure and power of 5 kW for 300 s.
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Copper plating electrolyte contained 75 g/L CuSO4 ·5H2O, 240 g/L H 2SO4 and 70 mg/L Cl- and commercial 3 components additives which were supplied by Dow chemical company. The temperature of electrolyte bath was kept at 25 ℃and the current density was controlled at 1.8 A/dm2 for 1×104 s. 2.2 Characterization
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The morphology of PF surface was examined by SEM (JEOL JSM-6060). The samples were sputtered with a gold coating for imaging. The wettability of PF surface before and after plasma treatment was examined by WCAMI (SINDIN SDC-100). The chemical conditions of PF surface were characterized by XPS (Thermo ESCALAB 250XI) and ATR FTIR (Thermo Fisher Nicolet 6700). The heights of 60 copper pillars were measured by metallography microscope (Olympus MX61L) to count growth uniformity of bottom-up copper pillars. 2.3 Data analysis
To make a quantitative analysis of the uniformity of plating copper pillar, the uniformity of deposition and Coefficient of Variation (COV) are defined as:
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where μis the average height of 60 copper pillars evenly distributed on the test board, and σis the standard deviation of 60 copper pillars. Smaller COV implies smaller fluctuation, and therefore indicates a better uniformity.
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3. Results and discussion
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The holes with different pre-treatment are showed in Figure 2. After the “development” process, some big PF residues lead to the undercut of copper pillars at the bottom of the hole in Figure 2a. OCLP treatment removes the residues at the bottom of the holes, but damaged orifice (Figure 2b). As shown in Figure 2c, AP treatment makes hole no residue and intact. It is clear that AP treatment obtains better and more complete holes than OCLP. The effect of different pretreatment procedures is studied by the surface morphology images of PF, as illustrated in Figure 2. In contrast to the blank sample which is almost smooth surface (Figure 2a), an uneven and shaggy surface obtained by OCLP treatment is evidently observed in Figure 2b, but a relatively even surface treated with AP is displayed in Figure 2d. Some particles are generated on PF surface after the high energy OCLP treatment and needed wash to remove, as shown in Figure 2b, 2c. The absence of generated particles makes low energy air plasma a simplified route compared with OCLP because the former skips the wash step. The water contact angle test for PF surface of different treatment including no treatment, OCLP and AP pretreatment, are illustrated in Figure 4 and Figure 5, respectively. The surface treatment by OCLP has an initial angle of 58.4° which is a little higher than the control group with the initial angle of 57.3°, corresponding to enhanced surface hydrophobicity. After the air plasma treatment, surface with an initial angle of 38.3° is drastically reduced by 19.9° compared to the one from OCLP treatment, demonstrating that the AP immensely improves the hydrophilicity of PF surface. Comparison between air plasma and the OCLP treatment denotes that air plasma dramatically increases the wettability of PF surface but the OCLP reduces it.
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3.2. Chemical analysis
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The depth of surface modification by plasma treatment is confined only to a few nanometers below the surface. This study employs FTIR spectroscopy in attenuated total reflection mode which is one of the methods used to capture the trace spectral features to characterize the PF surface in Figure 6. With the OCLP treatment, the spectra of the surface contain signals associated with the C-F (1162 cm-1), indicating that C-F as a hydrophobic group is generated on PF surface by OCLP, as compared to the sample without pretreatment. Upon treatment with air plasma, the new stretch at 1715 cm-1 can be ascribed to C=O and the peak at 1386 cm-1 is assigned to –NO 2. Compared with C=O and –NO 2, C-F is a more hydrophobic group that only found after the treatment by OCLP. Fluorine atom weakens the hydrophilicity of PF surface. Contrarily, the oxygen atom and the nitrogen atom can contribute to a higher wettability [40-44]. Therefore, the hydrophobic group (C-F) is evoked by the atmosphere of CF4 to enhance hydrophobicity while AP improves wettability of PF surfaces.
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Figure 7a shows the XPS results of the PF surfaces with different treatments. F element is obtained on surface treatment by OCLP in Figure 7b, the -CF and -C-CF2 are denoted after OCLP treatment (Figure 7d) and the -NO 2 (407.2 eV) is observed on PF with air plasma treatment in Figure 7c. The results of XPS are in agreement with the results of ATR-FTIR. In Figure 8, the C-C (284.5 eV), C=C (285.2 eV), C-O (286.2 eV), C=O (286.7 eV), and O-C=O (288.9 eV) are found. The C-O (533.2 eV), C=O (532.3 eV), and O-C=O (531.7 eV) are explicitly shown in Figure 9. As shown in table 1, after OCLP treatment, the amount of C-O and C=O on surface are reduced while the C-C and C=C are increased. And therefore the OCLP increases the C: O ratio of the PF surface and generates the highly hydrophobic group (R-F) with the content of 0.51% to a more hydrophobic surface of PF. On the contrary, the percentage of C-O, C=O and O-C=O are all raised and a low content (about 0.15%) of weak hydrophobic group (-NO2) is formed on surface by AP treatment. In short, OCLP generates more hydrophobic group (R-F) to enhance hydrophobicity while the AP reduces the C: O ratio to gain higher wettability. The results of ATR-FTIR and XPS both indicate that the air plasma can enhance oxygen content and hydrophilic groups (C-O, C=O and O-C=O) to improve wettability of surface. In the OCLP case, the numbers of these hydrophilic groups are reduced. In OCLP, the free electrons accelerate on PF surface and the electrons’ kinetic energy is high enough for CF4 excitation, ionization and generating F atoms [40, 45]. F atoms give rise to the hydrogen abstraction reaction [46]: RH+2F·→ RF +HF
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The main chemical reactions of the PF surface with different pretreatments are illustrated in Figure 10. Figure 10 demonstrates the reactions that the F atoms are embedded on the PF surface after OCLP treatment while the C=O and weak hydrophobic groups (–NO 2) are generated on the surface by air plasma treatment. Compared with AP, OCLP generates a more hydrophobic group (R-F) to weaken the wettability of PF.
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The heights of 60 plating copper pillars are measured by metallographic microscope. The lower values of the standard deviation (μ) and COV denote a better uniformity of plating copper pillars. The uniformity of 60 copper pillars with different treatments is shown in Figure 11, the values of σand COV after AP are 1.9 μm and 3.36% while the values after OCLP are 2.25μm and 3.97%. With lower values, AP gets a better uniformity than OCLP. As shown in Figure 12, air plasma modification gives rise to a better uniformity of copper pillar plating than the OCLP and wash. 4. Conclusions
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In this paper, comparative studies of air plasma and OCLP treatments for modification of PF surface are investigated to analyze the formation of plating copper pillar. Environment-friendly air plasma is an effective tool to enhance the wettability of PF by producing hydrophilic group, thereby leading to improve the plating uniformity of copper pillars. Thus, air plasma is a promising approach in plating pretreatment regarding the perspective of advanced quality requirements in IC substrate manufacturing. Acknowledgements
The authors gratefully acknowledge the support of Guangdong Innovative Research Team Program (No. 201301C0105324342), National Natural Science Foundation of China (No. 61474019) and the Project of Science and Technology Planning of the Guangzhou City China (No. 201604010086). References [1] C.T. Ko, S. Chen, C.W. Chiang, T.Y. Kuo, Y.C. Shih, Y.H. Chen, Embedded active device packaging technology for next-generation chip-in-substrate package, CiSP, in: Electronic Components & Technology Conference, 2006. [2] Z.W. Zhong, Wire bonding using copper wire, Microelectronics International, 26 (2009) 10-16. [3] R. Kisiel, Z. Szczepański, Trends in assembling of advanced IC packages, Journal of Telecommunications & Information Technology, (2005).
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Fig 2. SEM images for vias of different pre-treatment: (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment. Fig 3. SEM images for comparison of surface morphology acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment, (c) O2-CF4 low-pressure plasma treatment and wash and (d) air plasma treatment. Fig 4. Water contact angles of the PF with different pretreatment. Fig 5. The water contact angles photographs of different flow: (a) before pretreatment, (b) OCLP treatment and wash and (c) air plasma treatment. Fig 6. ATR-FTIR spectra of different treatment for PF Fig 7. XPS results of the PHOTO-RESISTIVE FILM surface with different pretreatments are (a) Full spectrum diagrams, (b) F1s spectra and (c) N1s spectra, (d) O2-CF4 low-pressure plasma treatment and wash. Fig 8. XPS results for C1s spectra are (a) before the pretreatment, (b) O2-CF4 lowpressure plasma treatment and wash and (c) air plasma treatment. Fig 9. XPS results for O1s spectra are (a) before the pretreatment, (b) O2-CF4 lowpressure plasma treatment and wash and (c) air plasma treatment. Fig 10. The two routes of reaction mechanism of the PF treated by different plasma pretreatments, O2-CF4 low-pressure plasma is PF treated by O2-CF4 low-pressure plasma and wash, air plasma is PF only treated by air plasma. Fig 11. The uniformity of 60 copper pillars with different treatments included O2-CF4 low-pressure plasma and air plasma. Fig 12. The uniformity of copper pillar photographs by metallographic microscope and SEM are generated from process flow A with two-time O2-CF4 low-pressure plasma treatment (a and c) and process flow B with O2-CF4 low-pressure plasma treatment and air plasma treatment (b and d). Table1. XPS results of the PF surface with different pretreatment. Item
C:O ratio C-C C=C C1s C-O C=O O-C=O C-O O1s C=O O-C=O -NO2 N1s C-N Nitrogen-content Fluoride-content
Before pretreatment 2.72:1 39.99% 22.90% 16.49% 14.28% 6.33% 40.14% 38.41% 21.44% 0 100% 0.46% 0
The O2-CF4 lowpressure plasma 2.87:1 42.57% 22.72% 14.13% 13.65% 6.92% 39.06% 39.42% 21.53% 0 100% 0.6% 0.51%
Air plasma 2.42:1 40.42% 18.99% 17.00% 15.79% 7.81% 38.34% 39.95% 21.72% 30% 70% 0.5% 0
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Figure1. The process flow of copper pillar production, A is the conventional flow and B is the new flow.
Figure2. SEM images for hole of different pre-treatment acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment.
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Figure3. SEM images for comparison of surface morphology acquired from (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment, (c) O2-CF4 low-pressure plasma treatment and wash and (d) air plasma treatment.
Figure4. Water contact angles of the PF with different pretreatment.
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Figure5. The water contact angles photographs of different flow: (a) before pretreatment, (b) OCLP treatment and wash and (c) air plasma treatment.
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Figure6. ATR-FTIR spectra of different treatment for PF
Figure7. XPS results of the PHOTO-RESISTIVE FILM surface with different pretreatments are (a) Full spectrum diagrams, (b) F1s spectra and (c) N1s spectra, (d)
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O2-CF4 low-pressure plasma treatment and wash.
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Figure8. XPS results for C1s spectra are (a) before the pretreatment, (b) O2-CF4 low-pressure plasma treatment and wash and (c) air plasma treatment.
Figure9. XPS results for O1s spectra are (a) before the pretreatment, (b) O2-CF4
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low-pressure plasma treatment and wash and (c) air plasma treatment.
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Figure10. The two routes of reaction mechanism of the PF treated by different plasma pretreatments, O2-CF4 low-pressure plasma is PF treated by O2-CF4 low-pressure plasma and wash, air plasma is PF only treated by air plasma.
Figure11. The uniformity of 60 copper pillars with different treatments included O2-CF4 low-pressure plasma and air plasma.
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Figure12. The uniformity of copper pillar photographs by metallographic microscope and SEM are generated from process flow A with two-time O2-CF4 low-pressure plasma treatment (a and c) and process flow B with O2-CF4 low-pressure plasma treatment and air plasma treatment (b and d).
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