Trong quá trình mạ, nồng độ của tất cả các chất trong dung dịch đều bị thay dổi theo thời gian dẫn đến tốc độ mạ cũng bị thay đổi theo chiều hướng giảm dần. Mối tương quan tỉ lệ số mol Cu2+/Ni2+ ảnh hưởng rất lớn đến tốc độ mạ đồng hóa học. Nickel được thêm vào nhằm tăng tốc độ mạ hóa học và duy trì tính liên tục của quá trình mạ. Tuy nhiên do ion Cu2+ bị khử thành đồng nguyên tử nên tỉ lệ mol Cu2+/Ni2+ giảm dần hay nói cách khác số mol ion Ni2+ tăng lên dần so với số mol ion đồng Cu2+ và đến một lúc nào đó thì tốc độ mạ giảm đi nhanh chóng.
Hình 3.11. Biểu đồ biến thiên tốc độ mạ theo thời gian
0 1 2 3 4 5 6 7 0 20 40 60 80 T ốc độ m ạ (µm /hr) Thời gian mạ (phút)
Tốc độ mạ chỉ duy trì được tốc độ cao ở thời gian 15-20 phút đầu, sau đó giảm nhanh chóng là một nhược điểm cần phải khắc phục của phương pháp mạ hóa học, do đó cần phải bổ sung thêm hóa chất để đảm bảo tỉ lệ mol ion Cu2+/Ni2+tối ưu cũng như các chất thành phần khác.
KẾT LUẬN VÀ HƯỚNG PHÁT TRIỂN
Phương pháp mạ không điện cực sử dụng hệ chất khử non-formaldehyde là một xu hướng mới trong ngành mạ hóa học. Tuy nhiên việc phát triển hệ dung dịch hiệu quả thì đòi hỏi phải có một thời gian nghiên cứu nghiêm túc trong một thời gian dài. Tốc độ và hiệu suất mạ còn khá thấp khi so sánh với dung dịch mạ hệ chất khử formaldehyde hay khi so sánh với phương pháp mạ có sư dụng điện cực
Với yêu cầu sử dụng phương pháp mạ hóa học để ứng dụng chế tạo ăng-ten cho thẻ RFID, hệ dung dịch non-formaldehyde đã được lựa chon để nghiên cứu vì tính an toàn và quy trình mạ đơn giản của hệ này đồng thời nếu được sử dụng trên quy mô công nghiệp sẽ hứa hẹn cho giá sản xuất thấp hơn phương pháp mạ có sử dụng điện cực.
Những kết quả nghiên cứu đạt được
Trong quá trình thực hiện luận văn này chúng tôi đã đạt được những kết quả:
- Xác định được các chất thành phần trong dung dịch mạ đồng hóa học với nồng độ tối ưu và mối tương quan của chúng đến tốc độ mạ đồng cũng như mối quan hệ giữa một số thành phần này với nhau.
- Đưa ra được quy trình mạ để ứng dụng chế tạo ăng-ten cho thẻ nhận dạng siêu cao tần RFID, đồng thời thỏa mãn những yêu cầu kĩ thuật của việc chế tạo ăng-ten, cụ thể là độ dày phải đạt từ 6-10 µm và điện trở suất phải đạt tối thiểu 3.10-6
Ω.
- Xác định được thời điểm tốc độ phản ứng mạ đồng hóa học giảm nhanh khi tỉ lệ mol ion Cu2+/Ni2+ giảm xuống 1/20, trong khi đó tốc độ đạt giá trị cao nhất khi tỉ lệ này dao động vào khoảng 1/10 cho đến 1/5.
-Trong quá trình thực hiện luận văn, chúng tôi đã công bố một bài báo tham dự Hội nghị Quốc tế về Công nghệ Nano và ứng dụng (IWNA 2013). Tên bài báo tham gia Hội nghị: “Effects of Polyethylene glycol and ion Nickel on non-formaldehyde electroless copper plating” (Phụ lục 2).
Do đó trong tương lai, một hướng nghiên cứu tiếp theo cần được thực hiện là:
- Xác định giá trị nồng độ mol của các chất thành phần bể mạ ở thời điểm tốc độ mạ sụt giảm nhanh.
-Khảo sát để có thể đưa ra các giá trị bổ sung nồng độ các chất thành phần nhằm đảm bảo tốc độ ổn định trong thời gian dài.
-Ngoài ra cần nghiên cứu sử dụng hệ nhiều chất tạo phức với ion đồng nhằm bảo vệ tốt hơn ion đồng khỏi các phản ứng phụ trong quá trình mạ, đồng thời tránh hiện tượng xuất hiện các tâm hoạt động làm kết tủa đồng nhằm nâng cao tốc độ và hiệu suất mạ.
TÀI LIỆU THAM KHẢO
[1] Mai Thanh Tùng, Mạ hóa học Ni-P trong dung dịch hypophosphite, ảnh hưởng của các thông số đến tốc độ mạ”. Tạp chí hóa học và ứng dụng, 4, 32-35, 2005
[2] Trần Minh Hoàng, Công nghệ mạ điện, nhà xuất bản khoa học và kỹ thuật, Hà nội, 1998
[3]. W. T. Tseng, C. H. Lo, and S. C. Lee, J. Electrochem. Soc., 148 (2001) 327-332 [4]. M. Matsuoka, J. Murai, and C. Iwakura, J. Electrochem. Soc., 139 (1992) 2466- 2470
[5]. S. Nakahara, Y. Okinaka, and H. Straschil, J. Electrochem. Soc., 136 (1989) 1120-1124
[6]. A. Hung and K. M. Chen, J. Electrochem. Soc., 136 (1989) 72-75
[7]. L. D. Burke, G. M. Bruton, and J. A. Collins, Electrochim. Acta, 44 (1998) 1467- 1479
[8]. A. Hung, Plat. Surf. Finish. 75 (1988) 62-65
[9]. Xueping Gan, Yating Wu, Lei Liu, Wenbin Hu, J.Appl. Electrochem, 37 (2007) 899-904
[10]. J. Li and P. A. Kohl, J. Electrochem. Soc., 149 (2002) 631-636 [11]. J. Li, P.A. Kohl, J.Electrochem. Soc. 150 (2003) 558-562
[10] D.H. Cheng, W.Y. Xu, Z.Y. Zhang, Z.H. Yiao, Met. Finish. 95 (1997) 34-37 [12]. L. G. Sillen, Stability Constants of Metal-Ion Complexes, Suppl. no. 1, Chemical Society, London (1971)
[13]. Li J, Hayden H, Kohl PA, Electrochim Acta 49 (2004) 1789-1795
[14]. J. E. A. M. Van Den Meerakker, J. Appl. Electrochem., 11 (1981) 395-400 [15]. W. T. Tseng, C. H. Lo, and S. C. Lee, J. Electrochem. Soc., 148 (2001) 327-332 [16]. Valery M Dubin, Yoshi Shacham-Diamand, Bin Zhao, P.K. Vasudev, and Chiu H. Ting, J. Electrochem. Soc., 144 (1997) 898-908
PHỤ LỤC 1
Phổ XRD chuẩn của của Nickel:
(Nguồn: http://chemistry.beloit.edu/Edetc/nanolab/nickel2/index.html )
PHỤ LỤC 2
NMD-073-P Effects of Polyethylene glycol and ion Nickel on non-formaldehyde
electroless copper plating
Dinh Tran An, Thi My Dung Dang and Chien Mau Dang
Laboratory for Nanotechnology (LNT), Vietnam National University in Ho Chi Minh City, Community 6, Linh Trung Ward, Thu Duc District, Ho Chi Minh, Vietnam.
Email: tranandinh88@gmail.com
ABSTRACT
High electroless copper deposition rate can be achieved using sodium hydrophosphite as reducing agent. However, the high deposition rate can cause the dark deposits. In the hypophosphite bath, nickel ions ( 0.0057M with Ni2+/Cu2+ mole ratio 0.1) were used to catalyze hypophosphite oxidation. In this study, additives (PEG) were investigated to improve the microstructure and properties of the copper deposits in the hypophosphite (non- formaldehyde) bath. On the other hand, thiourea has been shown to the accelerate the deposition rate of the electroless plating just as it does with electroless plating solution using ethylene-diaminetriacetic acid trisodium salt hydrate (HEDTA). PEG also increases the growth colony size of copper deposits and improves its conductivity.
Keywords: electroless plating, PEG, non-formaldehye.
INTRODUCTION
Electroless copper plating involves the reduction of Cu2+ ions to copper metal and the surface catalyzed by a reducing agent. These processes have advanced rapidly in recent years including widespread application in areas such as through-hole plating in printed circuit boards and other electric devices because of the their conformal deposition, low cost, high conductive of copper and simple
equipmental setup[1]. Commercial
electroless copper plating solutions often use formaldehyde or derivatives as reducing agent and are operated at pH above 11 [2],[3]. Furthermore, it has been found that this bath may release hazardous gases during operation. Therefore, previous paper has investigated electroless copper solutions using nonformaldehyde reducing agents such as dimethylamin borane (DMBA), sodium hypophosphite, and cobalt(II)-enthylenediamine
complex [4],[5]. Among these, sodium hypophosphite is especially attractive because of its low pH, low cost and relative safety [11] . However, the hypophosphite-based electroless copper plating process is complicated because copper is good catalyst for the oxidation of hypophosphite resulting in little or no plating on a pure copper surface. One approach to catalyze the oxidation of the reducing agents is to add nickel ions to the bath, resulting in a very small amount of co-deposited nickel in the copper deposit. The nickel serves to catalyze the oxidation of hypophosphite enabling continuous copper deposition[6]. When the mole ratio of (Ni2+/Cu2+) in the bath was low, the deposition rate of the copper plating decreased with time and finally stop because the surface catalytic activity was not replenished. Thus, it was necessary to maintain the mole ratio of (Ni2+/Cu2+) above a critical value to sustain the
deposition rate. However, the copper deposit properties were degraded and deposit appearance became darker with increased the mole ratio of (Ni2+/Cu2+). Consequently, it is important to improve the micro structure and properties of copper deposit while maintaining the nickel (II) concentration in the bath.
In this study, the influence of PEG on the hypophoshite-based electroless copper process has been investigated. PEG has been used as stabilizer or brightener in baths to improve the physical properties of the copper deposit. The composition, microstructure and properties of copper deposits obtained at different PEG concentrations in bath are reported along with voltametric analysis of its role.
EXPERIMENTAL
The electroless copper plating bath contained 0.04M CuSO4, 0.004M NiSO4, 0.015M HEDTA, 0.051M Na3C6H5O7, 0.485M H3PO3, 0.17M NaH2PO2 [9,10], 400ppm PEG. All chemicals used in the present work were of analytical purity. HEDTA was added to accelerate the electroless copper plating [8]. A layer copper about 200 nm were sputtered onto PET substrates, this layer Cu was used as the substrates to the copper atoms crystallize when they are released from the reduction reaction ion Cu2+. Deionized water was used to prepare the solution. The PH was adjusted using NaOH or H2SO4 to a final value 9.0-9.2. the temperature was held at 700C ± 0.50C.
Plating was performed in a 200ml electroless copper solution with continuous stirring. The electroless plating was carried out many multi-step processes The layer Cu substrates was cleaned and degreased with ethanol (C2H5OH) for a minute. Then the specimens was rinsed in a large volume of deionized water for 5 minutes to prevent contamination of the plating bath. The specimens were then immersed in the electroless copper plating bath.
The most important reaction, occurring in the electroless copper plating when sodium hypophosphite (NaH2PO2) was used as reducing agent, is as follows:
Cu2+ + 2 H2PO2¯ + 2OH¯ Cu + 2H2PO3¯ + H2 [15]
The deposition rate is defined as the mass gain of unit mass substrate and unit time. The deposition rate was calculated by the following equation:
ν = (m2 – m1)/(m1.t)
Where m1 and m2 are the masses of the specimen before and after electroless plating procedure, respectively; t is the stabilizing time of electroless plating. Copper deposition rate were determined by the change in weight of the PET boards after plating assuming uniform plating in 30 minutes and the mass density of copper to be 8.92g cm-3. The copper thickness was determined using DekTak profilometer. The resistivity of the copper deposits were measured using the four point probe method as describe in ASTM F390.
RESULT AND DISCUSSION
The effect of polyethyleneglycol (PEG) on the deposition rate and copper properties were investigated. The change in deposition rate ò the electroless copper plating as a function of PEG concentration is shown in Fig. 1. The nickel ion concentration in the bath was 0.002M and the Ni2+/Cu2+ mole ratio was 0.1. The deposition rate of the electroless plating in the absence of PEG is very low and the deposits were dark. The deposition rate increased from 0.4 to 6 µm/h ( as measured after 15 min plating) as the concentration of PEG was increased from 0 to 800 ppm. The increase in deposition rate with further additions of PEG occurred more gradually but with the concentration of PEG from 600 ppm and higher caused some dark point appeared on the deposits. In the other hand, when the concentration of Ni was changed from 0.001 to 0.008M, the deposition rate increased from 0.5µm to 6.5µm.
Fig. 1. The change in deposition rate of electroless copper plating as function of PEG concentration.
Fig. 2. Effect of Nickel ions concentration on the deposition rate of electroless copper plating
Nickel ions are required in the hypophosphite-based electroless copper plating bath to maintain the deposition rate [11]. Nickel catalytically participates in the oxidation of hypophosphite. The standard redox potential of Cu/Cu2+ (0.34V vs NHE) is more positive than that of Ni/Ni2+ (-0.25V vs NHE). The complexing agent in the electroless copper solution shifts the redox potentials of Cu/Cu2+ and Ni/Ni2+ toward more negative values as given by the Nernst equation:
= - log = - log Where and are the standard redox potentials of Cu/Cu2+ and
Ni/Ni2+ ; and are the formation of constants of copper(II) and nickel(II) complexes, respectively. Table I list formation constants of copper(II) nickel ions with sodium citrate and HEDTA [12]. Although the formation constant of copper(II) complexes are higher than that of nickel(II) complexes, the redox potential of Cu/CuL(II) is more positive than that of Ni/NiL(II) in solution. Furthermore, the concentration of copper(II) ions in the solution is higher than that of nickel ions (Cu2+/Ni2+ mole ratio is 10), further favoring the deposition of copper over nickel. In addition, the concentration of deposited nickel is constrained be the displacement reaction with copper(II):
Ni + Cu2+ Ni2+ + Cu
The appearance of nickel ions caused the higher electroless copper deposition rate is shown in Fig. 2. Nickel ions were required in solution to maintain the deposition rates because nickel ions ha catalytic activity for the oxidation of hypophosphite. If the overall rate of the deposition reaction was limited by the half reaction for the oxidation of hypophosphite on the exposed nickel region, then one would expect the deposition rate to be controlled by the surface content of nickel.
The deposition rate of the electroless plating increased with the initial addition of nickel ions in the solution and was constant with nickel ion concentration [13] . When the concentration of nickel ions increased further, the deposition rate increased dramatically. 0 1 2 3 4 5 6 7 0 200 400 600 800 1000 D eposi ti on rat e (µm /hr) PEG concerntration (ppm) 0 2 4 6 8 10 0 0.002 0.004 0.006 0.008 R es ist ivi ty (10 -6 ohm.c m ) Nickel concentration (M)
* Table I. Complex formation equilibrium constants and redox potentials of copper (II) and nickel (II) complexes (room temperature). Complexi -ng agent Complex formatio n Formatio n constant (logβ) Redox potentia l (V vs NHE) Cu2+ sodium citrate CuL 18.0 -0.191 Ni2+ sodium citrate NiL 14.3 -0.672 Cu2+- HEDTA CuL 17.4 -0.173 Ni2+- HEDTA NiL 17.0 -0.752
Table II lists the atomic concentration of nickel in the deposits obtained at different Cu2+/ Ni2+ mole ratio in the solutions, as measured by XPS
*Table II. The effect of the Cu2+/Ni2+ mole ratio in the solution on the nickel content in the deposit. Ni(II) concentration in the solution (10- 4 M) Cu2+/Ni2+ mole ratio in the solution Nickel content in the deposit (atom %) 5.56 71.94 0 10.04 39.84 0 40.05 10 2.3 50.06 7.99 6.2 66.57 6.01 8.1
The resistivity of the as-plated copper deposits versus PEG concentration is shown in Fig. 3. The resistivity of the
copper deposits did not change with the addition of PEG to the plating bath, even though the deposit microstructure improved with addition of PEG.
Fig. 3. The resistivity of copper deposits dependence on PEG concentration.
A reaction mechanism has been proposed to account for the electroless process by Van Den Meerakker [14] .In this mechanism, the dehy-drogenation of the reducing agent in the first step in the reaction:
Anodic process
Dehydrogenation H2PO2¯ ( PO2¯ )ads + ads
Oxidation ( PO2¯) + OH¯ H2PO3¯ + e Recombination ads + ads H2
Oxidation ads + OH¯ H2O + e Cathodic process Metal deposition
[CuII(HEDTA)] + 2e Cu + HEDTA [NiII(HEDTA)] + 2e Ni + HEDTA (*) Hydrogenation 2H2O + 2e H2
Codeposition H2PO2¯ + e P + 2 OH¯ The codeposited nickel in Eq. (*) can be replaced be cupric ions in the solution:
Ni + [CuII(HEDTA)] Cu + [NiII(HEDTA)] 0 2 4 6 8 10 0 0.005 0.01 R e si sti vi ty (10 -6 o h m .cm ) PEG concentration (ppm)
CONCLUSION
PEG improves the microstructure and properties of copper deposits obtained from electroless copper plating using hypophosphite as reducing agent. With the addition of PEG to the plating bath containing 0.004M nickel ions, the color of the deposits became uniform and compact. The electroless deposition rate and resistivity of the copper deposits decreased significantly with addition of PEG. However, the resistivity of the deposits was higher than that of deposits from formaldehyde-based electroless copper plating due to the presence of nickel. The nickel and phosphorus content in the deposits decreased slightly with addition of PEG.
Acknowledgment
The authors highly appreciate the financial support of the Ministry of Sciences and Technology of Vietnam.
References
[1]. W. T. Tseng, C. H. Lo, and S. C. Lee,
J. Electrochem. Soc., 148 (2001) 327-332 [2]. M. Matsuoka, J. Murai, and C. Iwakura, J. Electrochem. Soc., 139 (1992) 2466-2470
[3]. S. Nakahara, Y. Okinaka, and H. Straschil, J. Electrochem. Soc., 136 (1989) 1120-1124
[4]. A. Hung and K. M. Chen, J. Electrochem. Soc., 136 (1989) 72-75
[5]. L. D. Burke, G. M. Bruton, and J. A. Collins, Electrochim. Acta, 44 (1998) 1467-1479
[6]. A. Hung, Plat. Surf. Finish. 75 (1988) 62-65
[7]. E. Chassaing, M. Cherkaoui, and A. Srhiri, J. Appl. Electrochem., 23 (1993) 1169-1174
[8]. J. Li and P. A. Kohl, J. Electrochem. Soc., 149 (2002) 631-636
[9]. J. Li, P.A. Kohl, J. Electrochem. Soc.
150 (2003) 558-562
[10] D.H. Cheng, W.Y. Xu, Z.Y. Zhang, Z.H. Yiao, Met. Finish. 95 (1997) 34-37 [11]. L. G. Sillen, Stability Constants of
Metal-Ion Complexes, Suppl. no. 1,
Chemical
Society, London (1971)
[12]. Li J, Hayden H, Kohl PA,
Electrochim Acta 49 (2004) 1789-1795 [13]. J. E. A. M. Van Den Meerakker, J. Appl. Electrochem., 11 (1981) 395-400 [14]. W. T. Tseng, C. H. Lo, and S. C. Lee,
J. Electrochem. Soc., 148 (2001) 327-332 [15]. Valery M Dubin, Yoshi Shacham- Diamand, Bin Zhao, P.K. Vasudev, and Chiu H. Ting, J. Electrochem. Soc., 144 (1997) 898-908