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Atmospheric pressure plasma cleaning of gold flip chip bump for ultrasonic flip chip bonding

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2008 J. Phys.: Conf. Ser. 100 012034

(http://iopscience.iop.org/1742-6596/100/1/012034)

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Atmospheric Pressure Plasma Cleaning of Gold Flip Chip Bump for Ultrasonic Flip Chip Bonding

J-M Koo1, J-B Lee1, YJ Moon2, W-C Moon3, S-B Jung1 1School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea 2Corporate Technology Operations, Samsung Electronics Co. Ltd., 416 Maetan-3dong, Yeongtong-gu, Suwon 443-742, Republic of Korea 3Micro Electronic Packaging Consortium, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, Republic of Korea E-mail: sbjung@skku.ac.kr

Abstract. In this study, the optimization of gas and processing time of atmospheric pressure plasma cleaning was performed for successful ultrasonic direct bonding of electroplated Au flip chip bump. The plasma cleaning conditions strongly affected bondability of the Au flip chip bumps. The plasma cleaning with Ar gas for 1 s effectively removed contaminants from the surface without the surface oxidation, thereby improving the joint strength.

1. Introduction The needs for novel, miniaturized, high performance and multi-functional electronics have necessitated high density and precise assembles between the IC chip and substrate, which is achieved by flip chip bonding (FCB) technology [1]. Recently, interest in ultrasonic FCB technology has been growing, due to the potential advantages such as fast bonding time, low bonding temperature, low bonding pressure, environment-friendly process and good reliability [1,2]. However, the joint strength is very sensitive to cleanness of the bonding surface. Wet cleaning effectively cleans the surface, but the use of wet chemicals raises environmental problems and maintenance costs. Dry cleaning is an environment-friendly process, but has disadvantages such as high cost of the vacuum chamber and low productivity. The atmospheric pressure plasma cleaning technique can be an effective method to clean the bump surface because of its simple structure, high productivity and applicability throughout the in-line process [3]. However, plasma cleaning without the optimized processing conditions can do more harm than good [4]. Au has been used as a good bump material for ultrasonic bonding [2]. In this study, therefore, the atmospheric pressure plasma cleaning and ultrasonic bonding of the Au flip chip bump were investigated with different gases and processing times to achieve the quick, traceless, effective and environment-friendly surface cleaning for successfully ultrasonic flip chip bonding.

2. Experimental Procedure Two different samples were prepared to investigate atmospheric pressure plasma cleaning and ultrasonic FCB, as shown in Fig. 1: a practical complementary metal oxide semiconductor (CMOS) image sensor chip (hereafter, FC), and an Au-plated Si wafer (hereafter, LS). In the FC, SiO2, Al and W-20wt.%Ti layers acted as a passivation layer, interconnection line and adhesion layer between the

IVC-17/ICSS-13 and ICN+T2007 IOP PublishingJournal of Physics: Conference Series 100 (2008) 012034 doi:10.1088/1742-6596/100/1/012034

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Al line and Au bump, respectively. Forty-eight electroplated Au bumps, sized 110 (W) x 90 (D) x 18 (H) μm3, were arrayed at the periphery of the chip die. For the LS, a 4-inch-diameter silicon wafer was prepared. The wafer was sputtered with Ti (100 nm) and Cu (300 nm), which acted as the adhesion layer and interconnection, respectively. The Au was electroplated on the Cu surface at a thickness of 10 μm. FC and LS were diced to sizes of 6 x 6 mm2 and 10 x 10 mm2, respectively.

Both FC and LS were cleaned using the atmospheric pressure plasma cleaning system without any wet etching after Au plating. Table 1 shows the atmospheric pressure cleaning conditions used in this study.

The contents and structures of the elements existing on the surface after plasma cleaning were analyzed with Auger electron spectroscopy (AES: Model 660, Perkin-Elmer Physical Electronics Inc.) and X-ray photoelectron spectroscopy (XPS: ESCA2000, VG Microtech Co. Inc., England), respectively. The gold, carbon and oxygen contents were analyzed with the depth profile using Ar sputter of the AES with an etching rate of 1.12 nm/min. All the samples were stored in vacuum after the cleaning and the surface analysis carried out within 1 hr after the cleaning to minimize the surface oxidation of the samples.

The cleaned samples ultrasonically bonded within 3 min after the cleaning. The frequency, amplitude, bonding pressure and bonding time were 40 kHz, 4 μm, 15 N (about 31.25 MPa) and 0.4 s, respectively. After bonding, the samples were die-sheared using a bonding tester (PTR-1000, Rhesca Co., Japan). The displacement rate, probe height and probe width were 200 μm/s, 10 μm and 1 mm, respectively.

Figure 1. Schematic diagrams of the flip chip (a) and Au-electroplated Si wafer (b) used in this study.

Table 1. Conditions of atmospheric pressure plasma cleaning. Constant Variable

Power Gap distance Gas Processing time 300 W 3 mm Ar (8 slm) / Ar + O2 (40 sccm) 0.1, 0.3, 0.5, 1 and 10 s

3. Results and Discussion To investigate the effect of atmospheric pressure plasma cleaning on the chemical compositions of the Au surface, the surface elements were analyzed with AES. Based on the EDS analysis results of the AES, gold, carbon, oxygen, nitrogen, nickel and silver were found on the Au surface before plasma cleaning, whereas the nickel and silver disappeared and the amount of the carbon and oxygen was reduced after cleaning. This suggests that the atmospheric pressure plasma cleaning was effective to remove the contaminants from the Au surface.

Based on the EDS analysis results, the gold, oxygen and carbon contents were analyzed with increasing sputtered depth, as shown in Fig. 2. The depth profiles indicated that the contaminants, such as carbides and oxides, existed only within a 2.5 nm-deep surface layer. The Au content increased with increasing depth, while the oxygen and carbon contents decreased, which showed that contaminants concentrated on the topmost Au surface.

The Au content peaked at a processing time of 1 s, while the carbon and oxygen contents decreased to minima. Plasma cleaning was performed in air atmosphere, not in vacuum. The active radicals in

IVC-17/ICSS-13 and ICN+T2007 IOP PublishingJournal of Physics: Conference Series 100 (2008) 012034 doi:10.1088/1742-6596/100/1/012034

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plasma can react with carbon and oxygen in air to form carbides and oxides on the Au surface during extended plasma cleaning. Therefore, it is likely that the cleaning time of 10 s was so long that contaminants were redeposited on the Au surface.

The small addition of oxygen in Ar gas apparently affected the characteristics of plasma cleaning. The pure Ar and mixed gases were effective to reduce the oxides and carbides, respectively. However, the oxygen content increased on the topmost surface after cleaning with the mixed gas, marked as A in Fig. 2(f). This phenomenon was ascribed to the strong oxidation ability of oxygen plasma.

Figure 2. AES analysis results of the Au surface after atmospheric pressure plasma cleaning with Ar gas (a,c,e) and mixed gas (Ar+O2) (b,d,f), for different cleaning times: (a,b) Gold, (c,d) carbon and (e,f) oxygen.

The structure of the oxide formed on the topmost Au surface after atmospheric pressure plasma cleaning was analyzed with XPS. Figure 3 shows the Au spectra analysis results after cleaning with different gases for 10 s. The Au4f peaks appeared before and after plasma cleaning with pure Ar gas. However, a new compound, Au2O3, was found on the surface after cleaning with the mix gas. Generally, Au is not oxidized easily, due to its nobleness. Recently, several reports have indicated that the Au surface was oxidized easily to form an instable compound, Au2O3, during exposure to O2-plasma [5,6]. It is likely that the Au2O3 formation increased the oxygen content on the Au surface after plasma cleaning with the mixed gas.

Figure 3. XPS spectra of Au4f of the Au surface before (a) and after atmospheric pressure plasma cleaning with Ar gas (b) and mixed gas of Ar and O2 (c) for 10 s. The letters a, b, c and d indicate Au4f7/2, Au4f5/2, Au2O34f7/2 and Au2O34f5/2 peaks, respectively.

IVC-17/ICSS-13 and ICN+T2007 IOP PublishingJournal of Physics: Conference Series 100 (2008) 012034 doi:10.1088/1742-6596/100/1/012034

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To investigate the effect of atmospheric pressure plasma cleaning on ultrasonic Au FCB, die shear testing was carried out with different gases and processing times. Figure 4 shows the shear strength of the Au bumps with different gases and processing times. The shear strength was sensitive to the cleaning conditions, indicating that the optimization of the plasma cleaning conditions was essential to improve the joint strength of the Au bumps.

The Ar gas was more effective to improve the shear strength. Many researchers demonstrated that the addition of oxygen in an inert gas, such as He and Ar, enhanced the adhesion between the metal/polymer and polymer/polymer [7]. However, the oxygen plasma was ineffective for ultrasonic Au FCB, due to the oxidation of the gold surface. Suga et al. reported that the surface oxide layer degraded the bonding strength of the Cu interconnection directly bonded using the surface activated bonding [8]. This suggests that the oxide layer, formed on the surface by the oxygen plasma, was unsuitable for direct metal-to-metal bonding without adhesive and polymer.

Generally, a plasma cleaning time of over 60 s was required for the surface activation and cleaning of polymers and metals [5]. In this work, however, the shear strength significantly increasing with increasing processing times up to 1 s, due to the high sputtering rate of the Au and redeposition of carbides and oxides [7].

Based on the die shear testing results, the optimum gas and processing time of atmospheric pressure plasma cleaning were pure Ar gas and 1 s, to improve the joint bonding strength, respectively.

Figure 4. Die shear strength of the Au flip chip bumps ultrasonically bonded on the Au-electroplated Si wafer after atmospheric pressure plasma cleaning with different gases and processing times.

4. Conclusions Atmospheric pressure plasma cleaning and ultrasonic bonding of Au flip chip bumps were investigated with different gases and processing times. The results are summarized as follows.

The plasma cleaning for 1 s with pure Ar gas was effective for the ultrasonic bonding. An addition of O2 to Ar decreased the bonding strength between the Au bump and Au substrate.

The plasma cleaning with mixed gas (Ar + O2) effectively decreased the carbon content on the surface, but increased the gold oxide content. The use of pure Ar gas effectively decreased the contaminants, such as carbides and oxides, on the surface without the gold oxide formation, thereby improving the bonding strength.

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913-923. [5] Park ED and Lee JS 1999 J. Catalysis 186 1-11 [6] Tsai H, Hu E, Perng K, Chen M, Wu JC and Chang YS 2003 Surf. Sci. Letters 537 L447-L450 [7] Yi CH, Jeong CH, Lee YH, Ko YW and Yeom GY 2004 Surf. Coat. Tech. 177-178 711-715 [8] Wang Q, Hosoda N, Itoh T and Suga T 2003 Microelectron. Relia. 43 751-756

IVC-17/ICSS-13 and ICN+T2007 IOP PublishingJournal of Physics: Conference Series 100 (2008) 012034 doi:10.1088/1742-6596/100/1/012034

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