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Cu bonds during thermosonic wire bonding process
Microelectronics Reliability 46 (2006) 449–458 www.elsevier.com/locate/microrel
Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process Cheng-Li Chuang a, Jong-Ning Aoh
b,*
, Rong-Fong Din
b
a
b
Department of Occupational Safety and Health, Chung Shan Medical University, No. 110, Sec. 1, Chien-Kuo N. Road, Taichung 402, Taiwan, ROC Department of Mechanical Engineering, National Chung Cheng University, 160 San-Hsing, Minhsiung, Chiayi 621, Taiwan, ROC Received 17 September 2003; received in revised form 11 January 2005 Available online 23 March 2005
Abstract To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad. Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)2 forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects. 2005 Elsevier Ltd. All rights reserved.
1. Introduction As semiconductor devices are continuing to be rapidly scaled down, the interconnect dimensions are *
Corresponding author: Tel.: +886 5 272 1429; fax: +886 5 272 0589. E-mail addresses: [email protected] (C.-L. Chuang), [email protected] (J.-N. Aoh).
shrinking into the deep sub-micron regime. The resistance–capacitance (RC) time constant (RC) has become a major total delay concern. A possible way to reduce the delay time is to replace the conventional SiO2 dielectric material with materials with lower dielectric constant (k) or to replace the aluminum interconnects with lower resistive metals such as copper. The integration of low resistive metal inter-connections with a low k dielectric substrate will dramatically reduce chip
0026-2714/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2005.01.010
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resistivity (R) and capacitance (C) [1]. Since copper has lower resistivity (1.7 lX m) and higher electro-migration (EM) resistance than aluminum, it is a potential interconnection material for future ULSI devices. In general, chips with copper interconnects are often referred to as copper chips or copper wafer. However, it is well known that copper oxidizes easily in air and humid atmospheres. Unlike aluminum oxidation behavior, the copper oxidation rate is high with no self-passivation layer forming to prevent the underlying copper from further oxidation [2]. Therefore, oxide layer formation on a copper pad could be a serious concern in thermosonic wire bonding processes. Mechanical and electrical properties of the copper film bonding interface will also deteriorate due to oxide layer formation on the surface [3]. Several passivation schemes for preventing copper interconnects from oxidation have been studied [4–8]. The oxidation behavior of copper leadframes has been investigated [9,10]. The efforts in preventing copper pads and interconnects from oxidizing involve applying three categories of passivation layers: (1) Thin film deposition onto the copper film to prevent the copper from oxidizing [2,4,6–8]. Thin films such as Cr, TiN, Ta, TaN and Al could be used as passivation layer on copper. Chuang and Chen [6,7] reported that a Ta-passivated Ta/Cu/SiO2/Si structure could provide copper film with effective oxidation protection. No oxidation film was found after the copper film was annealed in an O2 atmosphere at 400 C for 50 min. Using a sputter deposited Tantalum-nitride passivation layer under appropriate conditions further improved the passivation capability. It was proposed that the amorphous structure of TaN thin film was the main effect for this improvement. Moreover, annealing a TaN layer in an N2 atmosphere at elevated temperatures (500–700 C) could heal the sputtering defects to further improve the copper film oxidation resistance. (2) Adding minor amounts of alloying elements such as Al, Mg and Cr to copper film during the specimen preparation could reduce oxidation. The pure copper film becomes a copper alloy film. An impurity-segregated layer is formed on the surface of the copper film that reduces the copper film oxidation rate. However, the addition of alloy elements to copper film results in an increase in the initial resistance [5]. This increase in resistance may deteriorate the electrical performance of copper film. (3) A copper-silicide protective layer was formed by reacting silane with copper film [11]. Copper thin film reacts with 2% SiH4/N2 at various temperatures and holding times to form metal-silicides (Cu5Si3 or Cu3Si). It is well known that layer of metal-silicides on the copper surface is an oxidation resistant layer for the underlying metal. Both metal-silicides could decompose when exposed to an oxygen atmosphere at high temperature,
forming an oxide layer on the surface that inhibits further oxidation. Several passivation layers were deposited on copper pad to prevent the copper pad from oxidizing in the past years and obtain sufficient bondability and strength in gold wire thermosonic bonding to chips with copper pads. These passivation layers included a thin ceramic/ glass layer [12], a self-assembled monolayer [13] and noble metal layer [14,15]. Our previous work [16] found that depositing a thin Ti passivation layer onto a copper pad was effective in preventing the copper pad from oxidizing. The thin titanium passivation layer could be removed using ultrasonic power during the thermosonic wire bonding process allowing the gold wire to bond onto the fresh copper pad. Perfect bondability and sufficient bonding strength were thus achieved. The oxide layer stacking sequence on the copper leadframe is CuO/Cu2O/Cu after the copper leadframe was heated from 150 C to 400 C in air atmosphere according to ChongÕs results [9]. Slightly different results were obtained in BerricheÕs work [10]. It was found in [10] that the oxide layer stacking structure was Cu(OH)2/CuO/Cu2O/Cu if leadframe was heated in an oven from 175 C to 200 C in an air atmosphere. Because the copper film oxidation mechanism is complicated, no single oxidation theory can explain all of the oxide layer results at different temperatures and oxygen partial pressure. The power law for the Cu2O growth rate depends strongly on the temperature range and oxygen partial pressure [17]. However, the oxidation rate becomes independent of the oxygen partial pressure when Cu2O further oxidizes into CuO [5]. Summarizing the related studies, most were focused on preventing copper interconnects from oxidation at elevated oxidation temperatures between 300 C and 600 C, or preventing the copper leadframe from oxidation to improve the second leadframe bond [18]. In this work, a copper pad oxidation investigation was conducted in the thermosonic wire bonding temperature range between 90 C and 200 C. The copper oxide effect on the quality of the first copper pad ball bonds was studied. This result is essential to further thermosonic wire bonding process development for chips with copper interconnects.
2. Experimental method A 3-gun radio-frequency (RF) magnetron sputtering system was used to deposit titanium and copper film ˚ in thickness, onto a silicon wafer. A Ti-film, 1000 A was first deposited onto a P type [1 1 1] bare silicon wafer prior to depositing the copper film to improve the adhesion between the copper film and silicon wafer. Copper film 1.2 lm in thickness was then sputter-deposited onto
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copper pad 3 mm in diameter using a Perkin–Elmer TAC-7/DX analyzer.
3. Results and discussion 3.1. Thermosonic bonding of Au wire to Cu pad The percentage of gold wire adhered to the copper pad at various bonding temperatures under air atmosphere is shown in Fig. 1. This percentage is derived from the number of gold wires bonded onto copper pads divided by the total number of bonding actions. There were 128 bond pads forming a daisy chain on each chip. The percentage of gold wire adhered to the pad increased with increasing bonding temperature from 90 C to 180 C, and then decreased with further increasing bonding temperature to 200 C. Only 53% of the total wire bonds actually bonded at 180 C. Fig. 1 also shows that the bonding strength increased with increasing stage temperature from 90 C to 150 C, and then dropped with further increasing stage temperature to 180 C and above. The apparent diameter of the ball bonds varied in a narrow range with an average diameter of 3.5 mil (90 lm). Therefore, the bonding strength could be obtained from the ball-shear force divided by the ball bond area. However, all of the measured bonding strengths were far lower than the minimal requirements based on JEDEC JESD22-B116 [19] specifications. A histogram of the ball-shear force for gold balls bonded at 180 C reveals a random ball shear force distribution in a range from 8 g to 24 g (Fig. 2). This implies unreliable bonding between gold balls and copper pads at 180 C under atmospheric conditions.
100
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Percentage of stick on pad Bonding strength
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70
4.0
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50
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40
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30
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Percentage of stick on pad
the titanium film without breaking the vacuum. The titanium film sputter parameters were: 200 W in power, 1.9 · 106 Torr basic pressure in a vacuum chamber, ˚ /s approxi24 S.C.C.M. argon gas flow rate, and 0.35 A mate deposition rate. The copper film sputtered parameters were same as those for the titanium film, except that the deposition power was 250 W, and the deposi˚ /s. The deposition tion rate was approximately 1.39 A film thickness was calibrated with a quartz crystal monitor during the sputtering. After the sputtering process was completed, the wafer was diced into 6 · 6 mm2. square chips. The chips were mounted onto the copper based leadframes with adhesive. The assemblies were then cured at 175 C/5 min under an N2 shielding atmosphere. All specimens were stored in an N2 chest prior to the thermosonic wire bonding process. A TOSHIBA HN-932-FAB automatic wire bonder was employed for thermosonic wire bonding experiment. Gold wire 25 lm in diameter was used. The thermosonic wire bonding parameters were: bonding time 20 ms, ultrasonic power 0.15 W (100 unit), bonding force 0.5 N, and bonding temperature 90–200 C. The gold ball on copper pad bonding strength obtained via the ball-shear test according to the EIA/JEDEC JESD22-B116 [19] specifications. A Royce 552 ballshear tester was used to determine the ball-shear force. The edge of the shear tool was set 3.5 lm above the bond pad surface. The sample size for each experimental condition consisted of at least 15 ball bonds. The copper pads were placed on the heat stage of the thermosonic wire bonder at 90–200 C for 2 min under air atmospheric conditions to investigate the copper pad oxidation status during thermosonic bonding. The heat stage conditions were equivalent to those during thermosonic wire bonding. After heating, an electron spectroscopy for chemical analysis (ESCA) was used to identify the compositions and depth profiles of the copper oxide formed on the copper pad. A Mg target was chosen for the X-ray source during the ESCA operation since the Mg Ka line is narrower than that for Al, thus achieving better resolution [20]. The copper oxide phase could be identified according to the shifted binding energy spectrum of Cu 2p. The copper oxide was successively removed along the thickness using an argon ion sputtering device to determine the copper pad and copper oxide concentration depth profiles. Calibrating the copper oxide removal rate was performed on a Ta2O5 layer 100 nm in thickness. The argon ion etching depth was derived from this removal rate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to investigate the gold ball residue on copper pads after the ballshear test. The copper pad surface morphology after heating was also examined using AFM and SEM. A thermogravimetric analysis (TGA) was conducted on a
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Fig. 1. Influence of the stage temperature on the percentage of stick on pad and on the bonding strength. Bonding parameters are: bonding force 0.5 N, ultrasonic power 0.15 W (100 unit), and bonding time 20 ms.
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Counts
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Ball-shear force (g) Fig. 2. Histogram showing ball-shear force distribution for gold ball bonded at 180 C.
The gold ball residue on the copper pad after ballshear test show a ‘‘type 1’’ failure according to [19]. This failure involves ball bond separation from the interface between the gold ball and copper pad, as shown in Fig. 3. This implies that the gold ball bonding strength is weaker than the strength of the gold ball itself. Only several discrete gold residues on copper pad were observed. In general, no reliable bond can be achieved on a copper pad if thermosonic wire bonding is carried out in an air atmosphere. It is thus essential in the following study to understand to what extent the bonding strength was deteriorated by the copper oxide formed on the copper pads under an air atmosphere. 3.2. Surface morphology of Cu pad Fig. 4 shows the surface morphology of the copper pad before and after heating on a thermosonic wire bonder heat stage. The as-deposited copper pad reveals a
smooth surface revealing a fine grain structure, as shown in Fig. 4(a) and (b). Fig. 4(c) and (d) show the surface morphology of a copper pad after heating at 200 C for 2 min. under an air atmosphere. In contrast to the as-deposited specimen, the surface of copper pad reveals extensive voids. The surface roughness of as-deposited copper pads and copper pads after heating at 90 C, 200 C for 2 min were investigated using atomic force microscopy (AFM), as shown in Fig. 5. The surface morphologies of bare copper pads after heat treatment at 90 C and 200 C were generally rougher than those of as-deposited pads. The copper pad roughness increased as the heating temperature increased. Extensive voids formed with increasing roughness with increasing temperature. This implies that the oxide formation on the copper pad surface increased at elevated temperature. This will be characterized in the following paragraphs. 3.3. Depth profiles of copper and oxygen atoms ESCA provides precise analysis of the oxide depth profile determination and phase identification. Fig. 6 shows the depth profiles of copper and oxygen atoms along the copper pad thickness after heating. The per˚ thickness centage of oxygen atoms was 4.23 at.% at 10 A after the copper pad was heated at 90 C for 2 min. in an air atmosphere. The oxygen atom distribution was lim˚ depth from the surface, as shown in Fig. ited to a 20 A 6(a). No oxygen atoms were detected at depths beyond ˚ . Similarly, the oxygen atom distribution was lim20 A ˚ and 150 A ˚ in depth from the surface after ited to 30 A the copper pad was heated at 150 C and 200 C, respectively, as shown in Fig. 6(b) and (c). The penetration depth of the oxygen atoms generally increased with increasing bonding temperature and thus could be used as an oxide thickness index on the copper pad under the same heating duration.
Fig. 3. Scanning electron micrographs showing the residues of Au ball on Cu pad after ball-shear test, which were bonded at different stage temperatures under air atmosphere: (a) 150 C and (b) 200 C.
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Fig. 4. Scanning electron micrographs showing the surface morphology of the as-deposited Cu pad (a) [(b) larger magnification] and the Cu pad after heated at 200 C/2 min under air atmosphere (c) [(d) larger magnification].
Fig. 7 shows the weight gain versus holding time relationship during thermogravimetric analysis. We found that the mathematic model expressing the copper oxidation rate obeys a parabolic law in the temperatures from 90 C to 200 C. This is revealed by the fitted curved in Fig. 8. The parabolic law can be expressed as: DW 2 ¼ kt þ C; where DW is the weight gain per area, t is the holding time, k is the reaction rate constant and C is a constant. The reaction rate constant k is determined from the slope of the fitted curves. It is obvious that the reaction rate constant increases with increasing heating temperature. Fig. 9 depicts a linear relationship between ln(k) and 1/T and indicates that the same mechanism governs copper pad oxidation in the 90–200 C range. The activation energy can be calculated from the straight line slope in Fig. 9 and is found to be 35 kJ/mol, which is much lower than the activation energy for bulk copper (80 kJ/mol) [21]. The different activation energies for bulk copper and copper pad imply different oxidation mechanisms. 3.4. Copper oxide phase identification The copper oxide phases can be identified by detecting the copper shifted binding energy spectrum using
ESCA. Figs. 10–12 show the binding energy spectra of Cu 2p and O 1s after the copper pads were heated at 90 C, 150 C and 200 C, respectively. After heating at 90 C, the Cu 2p3/2 region has a peak at 934.8 eV and the peak value of O 1s is 533.2 eV as shown in Fig. 10. Similarly, the copper pad after heating at 150 C and 200 C, the peak values for Cu 2p3/2 and O 1s were 934.9 eV, 935.0 eV, 533.6 eV and 531.5 eV, as can be observed in Figs. 11 and 12, respectively. It is apparent that shake-up satellites appear in the Cu 2p of the binding energy spectra, as shown in Figs. 10–12. This implies that the chemical nature of the copper ions at the outer surface of these samples is cupric (Cu2+) [20]. The peak values for Cu 2p3/2 and O 1s binding energy fall in the Cu(OH)2 domain according to the reference binding energy listed in [10]. Thus, the top surface region layer is identified as Cu(OH)2. It is presumed that moisture is partially adsorbed by the copper pad surface during storage. This moisture interacted with copper atoms to form Cu(OH)2 when the copper pad was heated on the stage. Berriche et al. [10] obtained similar results while analyzing the oxide layer on leadframes after heating in an oven from 175 C to 200 C under an air atmosphere. The binding energy of the copper oxides was detected when the oxide layer was etched using argon ions to identify the copper oxide phases at different heating
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temperatures and various depths. The oxide layer phase was identified using the shifted binding energy (DE). The pure copper film (as deposited) has a Cu 2p3/2 peak centered at 932.5 eV with a full width at half maximum (FWHM) of 1.19 eV. Thus, the shifted binding energy can be obtained by subtracting the binding energy of pure copper from that of copper oxide, as shown in Fig. 13 according to [22]. It is also listed in [22] that the shifted binding energy of CuO is approximately 1.0 eV higher than that of pure copper. The shifted binding energy of Cu2O is about 0.2 eV less than that of the
pure copper. Moreover, the shifted binding energy of Cu(OH)2 distributes in the range from +2.3 eV to ˚ beneath the surface of +2.9 eV [10]. At a depth of 5 A copper pad, the shifted binding energy values are 1.17 eV, 1.37 eV and 1.46 eV after the copper pads were heated at 90 C, 150 C and 200 C for 2 min under an air atmosphere, respectively. These values are larger than those for CuO and smaller than those for Cu(OH)2.
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(c) Fig. 5. AFM micrographs showing the surface roughness of copper pad after heated at different temperatures under air atmosphere.
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Sputtered depth (Å)
Fig. 6. ESCA depth profiles of copper pad after heating at (a) 90 C for 2 min in air, (b) 150 C for 2 min in air and (c) 200 C for 2 min in air.
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Therefore, a mixture of CuO and Cu(OH)2 species was identified based on the reference shifted binding energy in the literature [10,20].
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Furthermore, the shifted binding energy was ˚ beneath the copper oxide +0.78 eV at a depth of 10 A surface after heating at 90 C. A mixture of CuO and pure copper species was found at this depth according to the shifted binding energy. The shifted binding energy
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Weight gain (mg/cm2)
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90°C
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Holding time (min)
Fig. 7. A plot of weight gain versus holding time at various heating temperatures.
Square of weight gain (mg2/cm4)
0.004 2 -5 -6 ∆W =4.5x10 t-4.5x10
0.003
200°C
0.002
Fig. 10. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 90 C/2 min under air atmosphere. 0.001
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∆W 2=1.3X10-5 t+2x10-5
90°C ∆W2=4.5x10-6t-1.0x10-4
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ln(k)
Fig. 8. The linear relationship of the square of weight gain versus holding time and the curve fitting equations at various heating temperatures.
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1/T (x10 )
Fig. 9. A plot of ln(k) versus reciprocal temperature (1/T).
Fig. 11. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 150 C/2 min under air atmosphere.
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Cu0 Cu+2
Concentration (%)
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I : Cu(OH)2 II : CuO III : Cu
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I
III
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Sputter depth (Å)
Fig. 14. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heated at 90 C/2 min.
Fig. 12. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 200 C/2 min under air atmosphere.
Shifted binding energy ∆E (eV)
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Cu(OH)2 zone 2.0 1.5
Cu (OH)2 +CuO zone
90°C 150°C 200°C
1.0 0.5
CuO zone
Similar results were obtained for the copper pads heated at 150 C. The shifted binding energy values vary in a narrow range between 0.81 eV and 0.78 eV from a ˚ to 20 A ˚ . The shifted binding ensputter depth of 10 A ergy decreases to zero when sputter depth is larger than ˚ as shown in Fig. 13. The phase distribution in 30 A oxide layer after heated at 150 C is shown in Fig. 15. Cu(OH)2 was found to be the main phase existed on the top copper pad surface based on the energy spectrum of Cu 2p shown in Fig. 11. CuO exists at a sputter depth ˚ to 30 A ˚ beneath the surface. For the copper from 5 A pad heated at 200 C, the main phase on the top surface is Cu(OH)2 based on energy spectrum of Cu 2p shown in ˚ to 140 A ˚ beFig. 12. CuO exists in the range from 5 A neath the copper pad surface, as shown in Fig. 16. The pure copper concentration was determined from the area under the pure copper spectrum divided by that
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Sputter depth (Å)
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˚ was zero when the sputter depth was larger than 20 A after heating at 90 C. No copper oxide formed at a ˚ . Thus, two kinds of sputter depth greater than 20 A hydroxides or oxides can be identified along the sputter depth on the copper pad heated at 90 C. Cu(OH)2 is mainly formed on the top copper pad surface according to the energy spectrum of Cu 2p shown in Fig. 10. The ˚ to 20 A ˚ along underlying layer in the range from 5 A the sputter depth consists mainly of CuO. The phase distribution in the oxides along sputter depth is shown in Fig. 14.
Cu0 Cu+2 I :Cu(OH)2 II :CuO III :Cu
80
Concentration (%)
Fig. 13. The relationship between sputter depth and shifted binding energies of Cu 2p3/2 spectra after the copper pads were heated at 90 C, 150 C and 200 C.
70 60 50 40
II
III
30 20 10 0
0
I 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sputter depth (Å)
Fig. 15. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heating at 150 C/2 min.
C.-L. Chuang et al. / Microelectronics Reliability 46 (2006) 449–458 100 90
Cu0 Cu+2 I :GCu(OH) 2 II :GCuO III :GCu
Concentration (%)
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III
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0
90 C. The thermal energy is an important driving force for atomic inter-diffusion between the gold ball and copper pad during thermosonic bonding [23]. Therefore, lower bondability and insufficient bonding strength at lower bonding temperatures are mainly attributed to insufficient thermal energy applied during thermosonic bonding. 3.5. Problems to be overcome for thermosonic wire bonding on Cu chips
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10
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I 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sputter depth (Å) Fig. 16. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heating at 200 C/2 min.
of copper oxide after curve fitting. The remainder is the hydroxide or copper oxide concentration. The Cu0 and Cu2+ concentrations on the surface and at various sputter depths after heating at different temperatures are shown in Figs. 14–16. For the copper pad heated at 90 C/2 min, the Cu2+ concentration on the surface was approximately 10%, and decreased with increasing depth. A similar result was obtained with the copper pad heated at 150 C/2 min. The Cu2+ concentration was approximately 33%, and decreased with increasing sputter depth. These results indicate that the hydroxide or oxide concentrations on the copper pad surface was far below than that of pure copper because the Cu2+ values were much lower than that for Cu0. In contrast, the Cu2+ concentration on copper pad surface after heating at 90 C/2 min and 150 C/2 min, was approximately 65% for the copper pad after heating at 150 C/2 min. This was much higher than that for Cu0. This implies that the copper pad did not form a dense hydroxide layer or film within the ESCA analyzed area, because the Cu2+ concentration was lower than 100% for the copper pad after heating at 90 C, 150 C and 200 C. The bonding temperature exhibits a significant influence on the hydroxide concentration distribution on the copper pad surface. The Cu2+ concentration on the copper pad surface heated at 200 C was approximately 6 times higher than that heated at lower bonding temperature 90 C. The hydroxide or oxide layer at the bonding interface inhibits or hinders atomic diffusion between the gold ball and copper pad. Thus, the percentage of gold ball adhered to the pads and the bonding strength decrease drastically if hydroxide or copper oxide is formed on the copper pad surface at higher bonding temperatures during thermosonic bonding. Yet, hydroxide or copper oxide has only a minor effect on the percentage of gold adhered to the pad and bonding strength if the bonding temperature remains low, e.g.,
For bonding temperatures higher than 200 C, the hydroxide layer or copper oxide on copper pads gradually become a serious concern. The hydroxide concentration on the copper pad surface increases drastically to 65% as indicated in Fig. 16. The low percentage of gold sticking to the pad was attributed to the enhanced hydroxide or copper oxide formation on the copper pad surface at elevated temperatures. These hydroxide and copper oxides cannot be broken or removed by ultrasonic power during thermosonic wire bonding and are thus detrimental to the bondability and bonding strength in Au/Cu ball bonds. The dilemma is that thermosonic wire bonding is widely used in the electronic packaging industry. The typical bonding temperature from 125 C to 220 C would result in serious bonding strength issues. The drawbacks of copper pads for thermosonic wire bonding make protective measures such as deposition of a passivation layer or using shielding gas inevitable for thermosonic wire bonding on chips with copper interconnects. A separate study [24] has shown the feasibility of using appropriate argon shielding to provide sufficient protection for the copper pad against oxidation.
4. Conclusions The bondability and bonding strength of Au/Cu bonds were investigated. At the elevated bonding temperature of 200 C, both bondability and bonding strength were deteriorated by hydroxide and copper oxide formation on the copper pad if no appropriate measures were applied during thermosonic wire bonding. At the lower bonding temperature of 90 C, very low bondability and insufficient bonding strength originated from insufficient thermal energy for atomic diffusion between the gold ball and copper pad. ESCA analysis found that the oxygen atom depth distribution increased with increasing bonding temperature. Extensive voids were found on the copper pad hydroxide surface. The different activation energies of bulk copper and copper pads imply different oxidation mechanisms. Hydroxide and copper oxides were identified based on the shifted binding energy spectrum of Cu 2p. Cu(OH)2 formed mainly on the top surface of the
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copper pad. The underlying layer consisted mainly of CuO after heating under an air atmosphere. However, hydroxide and copper oxide did not form a dense layer within analyzed ESCA area. To prevent the copper pad from oxidizing, protective measures such as passivation layer deposition or using shielding gas should be considered for thermosonic wire bonding on chips with copper interconnects.
Acknowledgements This study was supported by the National Science Council, Republic of China, under grant number NSC-92-2212-E-194-005. The authors are grateful to MIRL and ERSO of ITRI (Taiwan) for their assistance in providing experimental facilities and in thin film deposition. Thanks are also given to semiconductor group of Oriental Semiconductor Corp. (OSE) for their assistance in die saw and die mount process.
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[9] Chong CT, Leslie A. Beng LT. Investigation on effect of copper leadframe oxidation on package delamination. In: 45th IEEE electronic components and technology conference, 1995. p. 463–9. [10] Berriche R, Lowery R, Rosenfield MI. An oxidation study of Cu leadframe. In: IEEE international symposium on advance materials conference, 1999. p. 275–81. [11] Hymes S, Murarka SP. Passivation of copper by silicide formation in dilute silane. J Appl Phys 1992;71:4623–5. [12] Lee CC, Tran TA, Yong L, Harun F, Yong CC. A thermal aging study on both AuACu and AuAAl wire-bonded interface. In: 25th electronic components and technology conference, 2003. p. 1761–6. [13] Whelan CM, Kinsella M, Carbonell L, Ho HM, Maex K. Corrosion inhibition self-assembled monolayers for enhanced wire bonding on Cu surface. Microelectron Eng 2003;70:551–7. [14] Strandjord JG, Popelar S, Jauerning C. Interconnecting to aluminum- and copper-based semiconductors (electrolessnickel/gold for solder bumping and wire bonding). Microelectron Reliab 2002;42:265–83. [15] Rohan JF, OÕriordan G, Boardman J. Selective electroless nickel deposition on copper pad as a final barrier/bonding layer material for microelectronics applications. Appl Surf Sci 2002;185:289–97. [16] Aoh JN, Chuang CL. Thermosonic bonding of gold wire onto copper pad with titanium thin film deposition. J Electron Mater 2004;33:290–9. [17] Hauffe K. Oxidation of metals. New York: Plenum; 1965. [18] Lin TY, Leong WS, Chua KH, Oh R, Miao Y, Pan JS, et al. The impact of contamination on quality of the second wire bonding process using X-ray photoelectron spectroscopy method. Microelectron Reliab 2002;42: 375–380. [19] JEDEC (EIA) solid state technology product engineering council. Arlington, 1998. [20] OÕConnor DJ, Sexton BA, Smart RSC. Surface analysis method in material science. Berlin: Spring-Verlag; 1992. [21] Scully JC. The corrosion and oxidation of metal. New York: Pergamon Press; 1976. [22] Wanger CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE. Hand book of X-ray photoelectron spectroscopy. Eden Prairie: Perkin–Elmer; 1992. [23] Jeng YR, Aoh JN, Wang CM. Thermosonic wire bonding of gold wire onto copper pad using the saturated interfacial phenomena. J Phys D: Appl Phys 2001;34:1–7. [24] Aoh JN, Chuang CL. Development of a thermosonic wire bonding process for gold wire bonding to copper pads using argon shielding. J Electron Mater 2004;33:300–11.
Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process Cheng-Li Chuang a, Jong-Ning Aoh
b,*
, Rong-Fong Din
b
a
b
Department of Occupational Safety and Health, Chung Shan Medical University, No. 110, Sec. 1, Chien-Kuo N. Road, Taichung 402, Taiwan, ROC Department of Mechanical Engineering, National Chung Cheng University, 160 San-Hsing, Minhsiung, Chiayi 621, Taiwan, ROC Received 17 September 2003; received in revised form 11 January 2005 Available online 23 March 2005
Abstract To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad. Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)2 forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects. 2005 Elsevier Ltd. All rights reserved.
1. Introduction As semiconductor devices are continuing to be rapidly scaled down, the interconnect dimensions are *
Corresponding author: Tel.: +886 5 272 1429; fax: +886 5 272 0589. E-mail addresses: [email protected] (C.-L. Chuang), [email protected] (J.-N. Aoh).
shrinking into the deep sub-micron regime. The resistance–capacitance (RC) time constant (RC) has become a major total delay concern. A possible way to reduce the delay time is to replace the conventional SiO2 dielectric material with materials with lower dielectric constant (k) or to replace the aluminum interconnects with lower resistive metals such as copper. The integration of low resistive metal inter-connections with a low k dielectric substrate will dramatically reduce chip
0026-2714/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2005.01.010
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resistivity (R) and capacitance (C) [1]. Since copper has lower resistivity (1.7 lX m) and higher electro-migration (EM) resistance than aluminum, it is a potential interconnection material for future ULSI devices. In general, chips with copper interconnects are often referred to as copper chips or copper wafer. However, it is well known that copper oxidizes easily in air and humid atmospheres. Unlike aluminum oxidation behavior, the copper oxidation rate is high with no self-passivation layer forming to prevent the underlying copper from further oxidation [2]. Therefore, oxide layer formation on a copper pad could be a serious concern in thermosonic wire bonding processes. Mechanical and electrical properties of the copper film bonding interface will also deteriorate due to oxide layer formation on the surface [3]. Several passivation schemes for preventing copper interconnects from oxidation have been studied [4–8]. The oxidation behavior of copper leadframes has been investigated [9,10]. The efforts in preventing copper pads and interconnects from oxidizing involve applying three categories of passivation layers: (1) Thin film deposition onto the copper film to prevent the copper from oxidizing [2,4,6–8]. Thin films such as Cr, TiN, Ta, TaN and Al could be used as passivation layer on copper. Chuang and Chen [6,7] reported that a Ta-passivated Ta/Cu/SiO2/Si structure could provide copper film with effective oxidation protection. No oxidation film was found after the copper film was annealed in an O2 atmosphere at 400 C for 50 min. Using a sputter deposited Tantalum-nitride passivation layer under appropriate conditions further improved the passivation capability. It was proposed that the amorphous structure of TaN thin film was the main effect for this improvement. Moreover, annealing a TaN layer in an N2 atmosphere at elevated temperatures (500–700 C) could heal the sputtering defects to further improve the copper film oxidation resistance. (2) Adding minor amounts of alloying elements such as Al, Mg and Cr to copper film during the specimen preparation could reduce oxidation. The pure copper film becomes a copper alloy film. An impurity-segregated layer is formed on the surface of the copper film that reduces the copper film oxidation rate. However, the addition of alloy elements to copper film results in an increase in the initial resistance [5]. This increase in resistance may deteriorate the electrical performance of copper film. (3) A copper-silicide protective layer was formed by reacting silane with copper film [11]. Copper thin film reacts with 2% SiH4/N2 at various temperatures and holding times to form metal-silicides (Cu5Si3 or Cu3Si). It is well known that layer of metal-silicides on the copper surface is an oxidation resistant layer for the underlying metal. Both metal-silicides could decompose when exposed to an oxygen atmosphere at high temperature,
forming an oxide layer on the surface that inhibits further oxidation. Several passivation layers were deposited on copper pad to prevent the copper pad from oxidizing in the past years and obtain sufficient bondability and strength in gold wire thermosonic bonding to chips with copper pads. These passivation layers included a thin ceramic/ glass layer [12], a self-assembled monolayer [13] and noble metal layer [14,15]. Our previous work [16] found that depositing a thin Ti passivation layer onto a copper pad was effective in preventing the copper pad from oxidizing. The thin titanium passivation layer could be removed using ultrasonic power during the thermosonic wire bonding process allowing the gold wire to bond onto the fresh copper pad. Perfect bondability and sufficient bonding strength were thus achieved. The oxide layer stacking sequence on the copper leadframe is CuO/Cu2O/Cu after the copper leadframe was heated from 150 C to 400 C in air atmosphere according to ChongÕs results [9]. Slightly different results were obtained in BerricheÕs work [10]. It was found in [10] that the oxide layer stacking structure was Cu(OH)2/CuO/Cu2O/Cu if leadframe was heated in an oven from 175 C to 200 C in an air atmosphere. Because the copper film oxidation mechanism is complicated, no single oxidation theory can explain all of the oxide layer results at different temperatures and oxygen partial pressure. The power law for the Cu2O growth rate depends strongly on the temperature range and oxygen partial pressure [17]. However, the oxidation rate becomes independent of the oxygen partial pressure when Cu2O further oxidizes into CuO [5]. Summarizing the related studies, most were focused on preventing copper interconnects from oxidation at elevated oxidation temperatures between 300 C and 600 C, or preventing the copper leadframe from oxidation to improve the second leadframe bond [18]. In this work, a copper pad oxidation investigation was conducted in the thermosonic wire bonding temperature range between 90 C and 200 C. The copper oxide effect on the quality of the first copper pad ball bonds was studied. This result is essential to further thermosonic wire bonding process development for chips with copper interconnects.
2. Experimental method A 3-gun radio-frequency (RF) magnetron sputtering system was used to deposit titanium and copper film ˚ in thickness, onto a silicon wafer. A Ti-film, 1000 A was first deposited onto a P type [1 1 1] bare silicon wafer prior to depositing the copper film to improve the adhesion between the copper film and silicon wafer. Copper film 1.2 lm in thickness was then sputter-deposited onto
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copper pad 3 mm in diameter using a Perkin–Elmer TAC-7/DX analyzer.
3. Results and discussion 3.1. Thermosonic bonding of Au wire to Cu pad The percentage of gold wire adhered to the copper pad at various bonding temperatures under air atmosphere is shown in Fig. 1. This percentage is derived from the number of gold wires bonded onto copper pads divided by the total number of bonding actions. There were 128 bond pads forming a daisy chain on each chip. The percentage of gold wire adhered to the pad increased with increasing bonding temperature from 90 C to 180 C, and then decreased with further increasing bonding temperature to 200 C. Only 53% of the total wire bonds actually bonded at 180 C. Fig. 1 also shows that the bonding strength increased with increasing stage temperature from 90 C to 150 C, and then dropped with further increasing stage temperature to 180 C and above. The apparent diameter of the ball bonds varied in a narrow range with an average diameter of 3.5 mil (90 lm). Therefore, the bonding strength could be obtained from the ball-shear force divided by the ball bond area. However, all of the measured bonding strengths were far lower than the minimal requirements based on JEDEC JESD22-B116 [19] specifications. A histogram of the ball-shear force for gold balls bonded at 180 C reveals a random ball shear force distribution in a range from 8 g to 24 g (Fig. 2). This implies unreliable bonding between gold balls and copper pads at 180 C under atmospheric conditions.
100
5.5
90
5.0
Percentage of stick on pad Bonding strength
80
4.5
70
4.0
60
3.5
50
3.0
40
2.5
30
2.0
20
1.5
10
1.0
Bonding strength (g/mil2)
Percentage of stick on pad
the titanium film without breaking the vacuum. The titanium film sputter parameters were: 200 W in power, 1.9 · 106 Torr basic pressure in a vacuum chamber, ˚ /s approxi24 S.C.C.M. argon gas flow rate, and 0.35 A mate deposition rate. The copper film sputtered parameters were same as those for the titanium film, except that the deposition power was 250 W, and the deposi˚ /s. The deposition tion rate was approximately 1.39 A film thickness was calibrated with a quartz crystal monitor during the sputtering. After the sputtering process was completed, the wafer was diced into 6 · 6 mm2. square chips. The chips were mounted onto the copper based leadframes with adhesive. The assemblies were then cured at 175 C/5 min under an N2 shielding atmosphere. All specimens were stored in an N2 chest prior to the thermosonic wire bonding process. A TOSHIBA HN-932-FAB automatic wire bonder was employed for thermosonic wire bonding experiment. Gold wire 25 lm in diameter was used. The thermosonic wire bonding parameters were: bonding time 20 ms, ultrasonic power 0.15 W (100 unit), bonding force 0.5 N, and bonding temperature 90–200 C. The gold ball on copper pad bonding strength obtained via the ball-shear test according to the EIA/JEDEC JESD22-B116 [19] specifications. A Royce 552 ballshear tester was used to determine the ball-shear force. The edge of the shear tool was set 3.5 lm above the bond pad surface. The sample size for each experimental condition consisted of at least 15 ball bonds. The copper pads were placed on the heat stage of the thermosonic wire bonder at 90–200 C for 2 min under air atmospheric conditions to investigate the copper pad oxidation status during thermosonic bonding. The heat stage conditions were equivalent to those during thermosonic wire bonding. After heating, an electron spectroscopy for chemical analysis (ESCA) was used to identify the compositions and depth profiles of the copper oxide formed on the copper pad. A Mg target was chosen for the X-ray source during the ESCA operation since the Mg Ka line is narrower than that for Al, thus achieving better resolution [20]. The copper oxide phase could be identified according to the shifted binding energy spectrum of Cu 2p. The copper oxide was successively removed along the thickness using an argon ion sputtering device to determine the copper pad and copper oxide concentration depth profiles. Calibrating the copper oxide removal rate was performed on a Ta2O5 layer 100 nm in thickness. The argon ion etching depth was derived from this removal rate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to investigate the gold ball residue on copper pads after the ballshear test. The copper pad surface morphology after heating was also examined using AFM and SEM. A thermogravimetric analysis (TGA) was conducted on a
451
0.5
0
0.0 90
100
110
120
130
140
150
160
170
180
190
200
Stage temperature (°C)
Fig. 1. Influence of the stage temperature on the percentage of stick on pad and on the bonding strength. Bonding parameters are: bonding force 0.5 N, ultrasonic power 0.15 W (100 unit), and bonding time 20 ms.
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Counts
4
3
2
1
0 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Ball-shear force (g) Fig. 2. Histogram showing ball-shear force distribution for gold ball bonded at 180 C.
The gold ball residue on the copper pad after ballshear test show a ‘‘type 1’’ failure according to [19]. This failure involves ball bond separation from the interface between the gold ball and copper pad, as shown in Fig. 3. This implies that the gold ball bonding strength is weaker than the strength of the gold ball itself. Only several discrete gold residues on copper pad were observed. In general, no reliable bond can be achieved on a copper pad if thermosonic wire bonding is carried out in an air atmosphere. It is thus essential in the following study to understand to what extent the bonding strength was deteriorated by the copper oxide formed on the copper pads under an air atmosphere. 3.2. Surface morphology of Cu pad Fig. 4 shows the surface morphology of the copper pad before and after heating on a thermosonic wire bonder heat stage. The as-deposited copper pad reveals a
smooth surface revealing a fine grain structure, as shown in Fig. 4(a) and (b). Fig. 4(c) and (d) show the surface morphology of a copper pad after heating at 200 C for 2 min. under an air atmosphere. In contrast to the as-deposited specimen, the surface of copper pad reveals extensive voids. The surface roughness of as-deposited copper pads and copper pads after heating at 90 C, 200 C for 2 min were investigated using atomic force microscopy (AFM), as shown in Fig. 5. The surface morphologies of bare copper pads after heat treatment at 90 C and 200 C were generally rougher than those of as-deposited pads. The copper pad roughness increased as the heating temperature increased. Extensive voids formed with increasing roughness with increasing temperature. This implies that the oxide formation on the copper pad surface increased at elevated temperature. This will be characterized in the following paragraphs. 3.3. Depth profiles of copper and oxygen atoms ESCA provides precise analysis of the oxide depth profile determination and phase identification. Fig. 6 shows the depth profiles of copper and oxygen atoms along the copper pad thickness after heating. The per˚ thickness centage of oxygen atoms was 4.23 at.% at 10 A after the copper pad was heated at 90 C for 2 min. in an air atmosphere. The oxygen atom distribution was lim˚ depth from the surface, as shown in Fig. ited to a 20 A 6(a). No oxygen atoms were detected at depths beyond ˚ . Similarly, the oxygen atom distribution was lim20 A ˚ and 150 A ˚ in depth from the surface after ited to 30 A the copper pad was heated at 150 C and 200 C, respectively, as shown in Fig. 6(b) and (c). The penetration depth of the oxygen atoms generally increased with increasing bonding temperature and thus could be used as an oxide thickness index on the copper pad under the same heating duration.
Fig. 3. Scanning electron micrographs showing the residues of Au ball on Cu pad after ball-shear test, which were bonded at different stage temperatures under air atmosphere: (a) 150 C and (b) 200 C.
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Fig. 4. Scanning electron micrographs showing the surface morphology of the as-deposited Cu pad (a) [(b) larger magnification] and the Cu pad after heated at 200 C/2 min under air atmosphere (c) [(d) larger magnification].
Fig. 7 shows the weight gain versus holding time relationship during thermogravimetric analysis. We found that the mathematic model expressing the copper oxidation rate obeys a parabolic law in the temperatures from 90 C to 200 C. This is revealed by the fitted curved in Fig. 8. The parabolic law can be expressed as: DW 2 ¼ kt þ C; where DW is the weight gain per area, t is the holding time, k is the reaction rate constant and C is a constant. The reaction rate constant k is determined from the slope of the fitted curves. It is obvious that the reaction rate constant increases with increasing heating temperature. Fig. 9 depicts a linear relationship between ln(k) and 1/T and indicates that the same mechanism governs copper pad oxidation in the 90–200 C range. The activation energy can be calculated from the straight line slope in Fig. 9 and is found to be 35 kJ/mol, which is much lower than the activation energy for bulk copper (80 kJ/mol) [21]. The different activation energies for bulk copper and copper pad imply different oxidation mechanisms. 3.4. Copper oxide phase identification The copper oxide phases can be identified by detecting the copper shifted binding energy spectrum using
ESCA. Figs. 10–12 show the binding energy spectra of Cu 2p and O 1s after the copper pads were heated at 90 C, 150 C and 200 C, respectively. After heating at 90 C, the Cu 2p3/2 region has a peak at 934.8 eV and the peak value of O 1s is 533.2 eV as shown in Fig. 10. Similarly, the copper pad after heating at 150 C and 200 C, the peak values for Cu 2p3/2 and O 1s were 934.9 eV, 935.0 eV, 533.6 eV and 531.5 eV, as can be observed in Figs. 11 and 12, respectively. It is apparent that shake-up satellites appear in the Cu 2p of the binding energy spectra, as shown in Figs. 10–12. This implies that the chemical nature of the copper ions at the outer surface of these samples is cupric (Cu2+) [20]. The peak values for Cu 2p3/2 and O 1s binding energy fall in the Cu(OH)2 domain according to the reference binding energy listed in [10]. Thus, the top surface region layer is identified as Cu(OH)2. It is presumed that moisture is partially adsorbed by the copper pad surface during storage. This moisture interacted with copper atoms to form Cu(OH)2 when the copper pad was heated on the stage. Berriche et al. [10] obtained similar results while analyzing the oxide layer on leadframes after heating in an oven from 175 C to 200 C under an air atmosphere. The binding energy of the copper oxides was detected when the oxide layer was etched using argon ions to identify the copper oxide phases at different heating
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temperatures and various depths. The oxide layer phase was identified using the shifted binding energy (DE). The pure copper film (as deposited) has a Cu 2p3/2 peak centered at 932.5 eV with a full width at half maximum (FWHM) of 1.19 eV. Thus, the shifted binding energy can be obtained by subtracting the binding energy of pure copper from that of copper oxide, as shown in Fig. 13 according to [22]. It is also listed in [22] that the shifted binding energy of CuO is approximately 1.0 eV higher than that of pure copper. The shifted binding energy of Cu2O is about 0.2 eV less than that of the
pure copper. Moreover, the shifted binding energy of Cu(OH)2 distributes in the range from +2.3 eV to ˚ beneath the surface of +2.9 eV [10]. At a depth of 5 A copper pad, the shifted binding energy values are 1.17 eV, 1.37 eV and 1.46 eV after the copper pads were heated at 90 C, 150 C and 200 C for 2 min under an air atmosphere, respectively. These values are larger than those for CuO and smaller than those for Cu(OH)2.
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Sputtered depth (Å) 100
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(c) Fig. 5. AFM micrographs showing the surface roughness of copper pad after heated at different temperatures under air atmosphere.
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Sputtered depth (Å)
Fig. 6. ESCA depth profiles of copper pad after heating at (a) 90 C for 2 min in air, (b) 150 C for 2 min in air and (c) 200 C for 2 min in air.
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Therefore, a mixture of CuO and Cu(OH)2 species was identified based on the reference shifted binding energy in the literature [10,20].
455
Furthermore, the shifted binding energy was ˚ beneath the copper oxide +0.78 eV at a depth of 10 A surface after heating at 90 C. A mixture of CuO and pure copper species was found at this depth according to the shifted binding energy. The shifted binding energy
0.10
Weight gain (mg/cm2)
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200°C
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150°C
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90°C
0.01 0.00 0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
Holding time (min)
Fig. 7. A plot of weight gain versus holding time at various heating temperatures.
Square of weight gain (mg2/cm4)
0.004 2 -5 -6 ∆W =4.5x10 t-4.5x10
0.003
200°C
0.002
Fig. 10. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 90 C/2 min under air atmosphere. 0.001
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∆W 2=1.3X10-5 t+2x10-5
90°C ∆W2=4.5x10-6t-1.0x10-4
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40
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Holding time (min)
ln(k)
Fig. 8. The linear relationship of the square of weight gain versus holding time and the curve fitting equations at various heating temperatures.
-9.8 -10.0 -10.2 -10.4 -10.6 -10.8 -11.0 -11.2 -11.4 -11.6 -11.8 -12.0 -12.2 -12.4 -12.6 2.0
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2.2
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2.4
2.5
2.6
2.7
2.8
3
1/T (x10 )
Fig. 9. A plot of ln(k) versus reciprocal temperature (1/T).
Fig. 11. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 150 C/2 min under air atmosphere.
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Cu0 Cu+2
Concentration (%)
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I : Cu(OH)2 II : CuO III : Cu
60 50 40 30
I
III
II
20 10 0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sputter depth (Å)
Fig. 14. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heated at 90 C/2 min.
Fig. 12. The ESCA spectra of Cu 2p and O 1s on the surface of copper pad after heating at 200 C/2 min under air atmosphere.
Shifted binding energy ∆E (eV)
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Cu(OH)2 zone 2.0 1.5
Cu (OH)2 +CuO zone
90°C 150°C 200°C
1.0 0.5
CuO zone
Similar results were obtained for the copper pads heated at 150 C. The shifted binding energy values vary in a narrow range between 0.81 eV and 0.78 eV from a ˚ to 20 A ˚ . The shifted binding ensputter depth of 10 A ergy decreases to zero when sputter depth is larger than ˚ as shown in Fig. 13. The phase distribution in 30 A oxide layer after heated at 150 C is shown in Fig. 15. Cu(OH)2 was found to be the main phase existed on the top copper pad surface based on the energy spectrum of Cu 2p shown in Fig. 11. CuO exists at a sputter depth ˚ to 30 A ˚ beneath the surface. For the copper from 5 A pad heated at 200 C, the main phase on the top surface is Cu(OH)2 based on energy spectrum of Cu 2p shown in ˚ to 140 A ˚ beFig. 12. CuO exists in the range from 5 A neath the copper pad surface, as shown in Fig. 16. The pure copper concentration was determined from the area under the pure copper spectrum divided by that
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Sputter depth (Å)
90
˚ was zero when the sputter depth was larger than 20 A after heating at 90 C. No copper oxide formed at a ˚ . Thus, two kinds of sputter depth greater than 20 A hydroxides or oxides can be identified along the sputter depth on the copper pad heated at 90 C. Cu(OH)2 is mainly formed on the top copper pad surface according to the energy spectrum of Cu 2p shown in Fig. 10. The ˚ to 20 A ˚ along underlying layer in the range from 5 A the sputter depth consists mainly of CuO. The phase distribution in the oxides along sputter depth is shown in Fig. 14.
Cu0 Cu+2 I :Cu(OH)2 II :CuO III :Cu
80
Concentration (%)
Fig. 13. The relationship between sputter depth and shifted binding energies of Cu 2p3/2 spectra after the copper pads were heated at 90 C, 150 C and 200 C.
70 60 50 40
II
III
30 20 10 0
0
I 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sputter depth (Å)
Fig. 15. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heating at 150 C/2 min.
C.-L. Chuang et al. / Microelectronics Reliability 46 (2006) 449–458 100 90
Cu0 Cu+2 I :GCu(OH) 2 II :GCuO III :GCu
Concentration (%)
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II
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0
90 C. The thermal energy is an important driving force for atomic inter-diffusion between the gold ball and copper pad during thermosonic bonding [23]. Therefore, lower bondability and insufficient bonding strength at lower bonding temperatures are mainly attributed to insufficient thermal energy applied during thermosonic bonding. 3.5. Problems to be overcome for thermosonic wire bonding on Cu chips
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I 0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Sputter depth (Å) Fig. 16. The concentration distribution of copper together with the CuO and Cu(OH)2 along sputter depth after the copper pad heating at 200 C/2 min.
of copper oxide after curve fitting. The remainder is the hydroxide or copper oxide concentration. The Cu0 and Cu2+ concentrations on the surface and at various sputter depths after heating at different temperatures are shown in Figs. 14–16. For the copper pad heated at 90 C/2 min, the Cu2+ concentration on the surface was approximately 10%, and decreased with increasing depth. A similar result was obtained with the copper pad heated at 150 C/2 min. The Cu2+ concentration was approximately 33%, and decreased with increasing sputter depth. These results indicate that the hydroxide or oxide concentrations on the copper pad surface was far below than that of pure copper because the Cu2+ values were much lower than that for Cu0. In contrast, the Cu2+ concentration on copper pad surface after heating at 90 C/2 min and 150 C/2 min, was approximately 65% for the copper pad after heating at 150 C/2 min. This was much higher than that for Cu0. This implies that the copper pad did not form a dense hydroxide layer or film within the ESCA analyzed area, because the Cu2+ concentration was lower than 100% for the copper pad after heating at 90 C, 150 C and 200 C. The bonding temperature exhibits a significant influence on the hydroxide concentration distribution on the copper pad surface. The Cu2+ concentration on the copper pad surface heated at 200 C was approximately 6 times higher than that heated at lower bonding temperature 90 C. The hydroxide or oxide layer at the bonding interface inhibits or hinders atomic diffusion between the gold ball and copper pad. Thus, the percentage of gold ball adhered to the pads and the bonding strength decrease drastically if hydroxide or copper oxide is formed on the copper pad surface at higher bonding temperatures during thermosonic bonding. Yet, hydroxide or copper oxide has only a minor effect on the percentage of gold adhered to the pad and bonding strength if the bonding temperature remains low, e.g.,
For bonding temperatures higher than 200 C, the hydroxide layer or copper oxide on copper pads gradually become a serious concern. The hydroxide concentration on the copper pad surface increases drastically to 65% as indicated in Fig. 16. The low percentage of gold sticking to the pad was attributed to the enhanced hydroxide or copper oxide formation on the copper pad surface at elevated temperatures. These hydroxide and copper oxides cannot be broken or removed by ultrasonic power during thermosonic wire bonding and are thus detrimental to the bondability and bonding strength in Au/Cu ball bonds. The dilemma is that thermosonic wire bonding is widely used in the electronic packaging industry. The typical bonding temperature from 125 C to 220 C would result in serious bonding strength issues. The drawbacks of copper pads for thermosonic wire bonding make protective measures such as deposition of a passivation layer or using shielding gas inevitable for thermosonic wire bonding on chips with copper interconnects. A separate study [24] has shown the feasibility of using appropriate argon shielding to provide sufficient protection for the copper pad against oxidation.
4. Conclusions The bondability and bonding strength of Au/Cu bonds were investigated. At the elevated bonding temperature of 200 C, both bondability and bonding strength were deteriorated by hydroxide and copper oxide formation on the copper pad if no appropriate measures were applied during thermosonic wire bonding. At the lower bonding temperature of 90 C, very low bondability and insufficient bonding strength originated from insufficient thermal energy for atomic diffusion between the gold ball and copper pad. ESCA analysis found that the oxygen atom depth distribution increased with increasing bonding temperature. Extensive voids were found on the copper pad hydroxide surface. The different activation energies of bulk copper and copper pads imply different oxidation mechanisms. Hydroxide and copper oxides were identified based on the shifted binding energy spectrum of Cu 2p. Cu(OH)2 formed mainly on the top surface of the
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copper pad. The underlying layer consisted mainly of CuO after heating under an air atmosphere. However, hydroxide and copper oxide did not form a dense layer within analyzed ESCA area. To prevent the copper pad from oxidizing, protective measures such as passivation layer deposition or using shielding gas should be considered for thermosonic wire bonding on chips with copper interconnects.
Acknowledgements This study was supported by the National Science Council, Republic of China, under grant number NSC-92-2212-E-194-005. The authors are grateful to MIRL and ERSO of ITRI (Taiwan) for their assistance in providing experimental facilities and in thin film deposition. Thanks are also given to semiconductor group of Oriental Semiconductor Corp. (OSE) for their assistance in die saw and die mount process.
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