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Characterization of copper precipitates on aluminum copper bond pads formed after plasma clean and de-ionized water exposure
Microelectronics Reliability 55 (2015) 1101–1108
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Characterization of copper precipitates on aluminum copper bond pads formed after plasma clean and de-ionized water exposure Raj Sekar Sethu ⇑, Hong Seng Ng, Alvin Chan, Cheng Nee Ong, Sieng Fong Chan Quality, X-FAB Sarawak Sdn. Bhd., 1 Silicon Drive, Sama Jaya Free Industrial Zone, 93350 Kuching, Sarawak, Malaysia
a r t i c l e
i n f o
Article history: Received 28 May 2014 Received in revised form 24 March 2015 Accepted 30 March 2015 Available online 23 April 2015 Keywords: Bond pad Corrosion Ball shear Plasma clean
a b s t r a c t Semiconductor bond pads made from aluminum and small percentages of copper is susceptible to galvanic corrosion. In galvanic corrosion, the cathode (copper precipitate) is usually protected by the aluminum oxide that covers the surface of aluminum which acts as the anode. However, when the aluminum oxide thickness is reduced by plasma cleaning, the precipitates can be exposed. When exposed precipitates come in contact with de-ionized water, galvanic corrosion takes place. Therefore, though plasma cleaning in general is supposed to improve semiconductor bond pad surface in preparation for package level interconnection, adding the plasma clean step just before a process with de-ionized water can cause bond pad corrosion through the galvanic reaction between the exposed precipitate (cathode) and the surrounding aluminum (anode). This paper aims to investigate the mechanism of corrosion and characterize corroded bond pads by using wire bond ball shear method. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Copper doped aluminum interconnects help reduce electromigration [1]. This however introduces copper precipitation which enhances the risk of galvanic corrosion [2,3]. The wire bonding process has many issues which are well documented [4]. In order to overcome most of these issues, plasma cleaning is used to remove contaminants [5,6]. However plasma cleaning can also cause the top native aluminum oxide to become thin and thus expose copper precipitates that can enhance galvanic corrosion when in contact with an electrolyte such as ionized water. In typical assembly process flows, after plasma cleaning, the bond pad does not get in contact with an electrolyte. 2. Materials and methods The experiment outlines the methods used to examine bond pads (99.5% Al and 0.5% Cu) with galvanic corrosion followed by a risk assessment using ball bond shear test per EIA/JESD22B116: Wire Bond Shear Test Method by JEDEC [7]. The simplified flow is per Fig. 1. Completed bond pad samples were first examined using a JEOL JAMP-9500F Auger Electron Spectroscopy (AES) before being ⇑ Corresponding author. Tel.: +60 12 331 6041. E-mail address: [email protected] (R.S. Sethu). http://dx.doi.org/10.1016/j.microrel.2015.03.018 0026-2714/Ó 2015 Elsevier Ltd. All rights reserved.
subjected to the first plasma clean (see parameters in Table 1) then examined again through AES to determine how much of top surface material is removed by the first plasma clean process. Subsequently, the samples were submerged for twelve hours in DIW with resistance of 18.1 MOhm/cm after wafer saw. This is to simulate a worse case scenario of the bond pad being improperly dried or stuck inside a wet equipment/chamber for a long period of time due to maintenance. The samples had ten bond pads (labelled Pad1 to Pad10) in each die. Four visually good dies (labelled G1, G2, G3 & G4) and four visually bad dies (labelled B1, B2, B3 & B4) were chosen based on their bond pads’ surface appearance. Two dies from each group underwent the second plasma clean prior to wire bonding; whereas the other two dies were kept as control samples i.e. without second plasma clean. The Good–Bad criteria was decided by a single engineer. Automated visual inspection was not employed as the cost to setup such a system for the evaluation could not be justified. Optical images of all ten bond pads were taken for the visually bad dies and a sample each was taken for the visually good dies before and after second plasma cleaning. This is to determine whether the second plasma clean can reduce the surface contamination that can be detected through optical microscopy. Ball shear testing is a well known method to assess the adhesion strength between the wire and the bond pad. This method is preferred over a cross section over the precipitate, as the precipitate size is very small compared to the ball diameter which would be covering all precipitates. Therefore all bond pads
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R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108 Table 2 Ball Shear tester input parameters. Parameter
Units
Setting
Test speed Test load Land speed Shear height Over travel
lm/s
250 50 50 3.4 150
gf lm/s lm lm
3.2. Optical/SEM review after submerge in de-ionized water (DIW) Samples that were submerged in DIW for twelve hours after the first plasma clean showed signs of corrosion as seen in a sample image of Fig. 3, which were consistent with findings by [9]. The corrosion was random across the bond pads and had no selectivity as observed by [10]. However the control sample without the first plasma clean showed no signs of corrosion even when submerged in de-ionized water for twelve hours. Further analysis through SEM (see Fig. 4) showed that the dark surface irregularities are actually the theta phase Al2Cu precipitates with debris scattered around the cavity. In an electrolyte, the theta phase Al2Cu would act as a cathode whereas the alpha phase anodic aluminum around it can dissolve [11]. The EDX spectroscopy seen in Fig. 5 also shows a small silicon peak which may been deposited on the bond pad surface during wafer sawing. The presence of silicon may have been the catalyst for the galvanic corrosion that would have exposed the aluminum surrounding the Al2Cu seed [12]. The EDX showed no signs of fluorine.
Fig. 1. Process flow for the experiment.
3.3. Effect of second plasma clean were wire bonded with 1 mil diameter gold wire detailed in [8] and underwent ball shear testing [7] using a Dage 4000 shear test equipment per settings in Table 2. The worst shear values for the visually bad bond pads were recorded and analyzed against the bond pad images prior to wire bonding. 3. Results 3.1. Auger Electron Spectroscopy – before & after first plasma clean As seen in Fig. 2(a) the percentage of pure aluminum on the surface increased after undergoing the first plasma cleaning, indicating that the native oxide is thinner after the first plasma clean. By examining the Atom% at the 50.0% line, for both aluminum and oxygen, the oxide removal depth is estimated to be about 3.3 nm. The AES elemental depth profile seen in Fig. 2(c) for the bond pad shows a significant decrease in carbon content at the top surface after the first plasma cleaning. Though the metalization is known to have about 0.5% copper, AES did not detect any copper up to the depth of 31 nm. This result is expected as the AES spot size is only 35 lm2. Furthermore, since no surface anomaly was observed on the bond pad before and after the first plasma clean, a random location within the bond pad was selected for this purpose.
Table 1 Parameters used for both plasma cleaning processes. Parameter
Units
Setting
RF power Time Gas mixture Vacuum (pressure)
W min % m bar
195 3 Ar – 95, H – 5 0.6
As seen in Table 3, the second plasma clean process has very little effect on improving the visual appearance of the bond pads with contamination. In fact, for the good bond pad sample i.e. G1-Pad1, the pitting (dark spots on the bond pad surface which do not reflect light back) on the surface actually increased. This may be due to the nature of plasma cleaning itself which is destructive [13]. 3.4. Ball shear analysis The wire bonded samples were measured and had an average wire ball diameter of 67.92 lm and an average wire ball height of 14.20 lm; before being sheared using a Dage 4000 shear test equipment with parameters in Table 2. The resultant ball shear strength values were stratified into four groups i.e. Visually Good with Second Plasma Clean, Visually Good without Second Plasma Clean, Visually Bad with Second Plasma Clean and Visually Bad without Second Plasma Clean. Each group has two dies with ten bond pads each (total 20 samples per group). Table 4 shows that the worst shear reading is still above the lower specification limit (15.6 gf) by JEDEC. As seen in Fig. 6, though the bonding did take place generally along the periphery of the ball [14] the surface under the ball of the B4-Pad1 seems to have less contamination compared to the rest of the surface. This indicates that the corrosion is surface phenomenon that may not impact bond shear strength if the wire material goes deep enough into the bond pad during wire bonding. 4. Discussion As seen in the AES depth profiling of Fig. 2, the first plasma clean can reduce the amount of native oxide on the bond pad.
R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108
Fig. 2. AES depth profile for various elements on the top surface of the bond pad before and after first plasma clean. (a) Aluminum, (b) oxygen, and (c) carbon.
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Fig. 3. Sample optical microscopy of bond pads. Left Image – before being submerged. Center Image – with first plasma clean, after being submerged in DIW. Right Image – no first plasma clean, after being submerged in DIW.
Fig. 4. SEM image of bond pad that have undergone DIW submergence after first plasma clean. Left image – copper precipitate overview; right image – tilted view of another copper precipitate showing sidewall erosion. The copper precipitate sampled had a length of 986.7 nm and a width of 1859 nm.
Fig. 5. EDX spectroscopy of the copper precipitate show that it is made of primarily aluminum and copper.
This in turn, exposes the bottom layer which may contain the Al2Cu precipitate seed. Once this Al2Cu seed is exposed (see Fig. 7) to an electrolyte such as de-ionized water, and with the help of a catalyst such as a silicon particle from wafer saw, galvanic corrosion can take place. A two-way analysis of variance (ANOVA) was done using a statistical software (Minitab 16) to determine effect of the input factors i.e. surface contamination (Visual) and the 2nd Plasma Clean on the output response (i.e. ball shear strength).
The p-value results [15] seen in Table 5 indicate that the factors Visual (Good vs Bad), 2nd Plasma Clean (Yes vs No) and its two way interaction are significant in changing the bond shear strength. Table 6 explains the components of the ANOVA of Table 5. Further analysis is done through Minitab’s Individual Value Plot which provides a graphical description. Based on the shear readings seen in Fig. 8, the second plasma cleaning can increase the ball shear strength of visually bad bond pads, but has no discernable effect on the visually good bond pad.
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Table 3 Visually, there is no discernible effect on the bond pads by doing the second plasma clean. The bond pad samples chosen for before and after optical analysis are considered the worst from their respective die. Die-Pad ID
Before second plasma clean
After second plasma clean
G1-Pad1
B1-Pad7
B2-Pad3
Table 4 Result summary. All individual shear values passed the JEDEC bond shear lower specification limit (15.6 gf). The shear specification limit is derived from the ball diameter [7]. Die ID
2nd Plasma Clean (Y/N)
Average shear reading (gf)
Worst shear reading (gf)
Worst pad ID
Visually worst pad image before wire bonding
G1 G2 G3 G4 B1
Y Y N N Y
28.385 28.063 27.295 28.670 27.454
26.750 26.840 25.963 26.485 26.450
Pad Pad Pad Pad Pad
None None None None
7 1 3 5 7
(continued on next page)
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R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108
Table 4 (continued) Die ID
2nd Plasma Clean (Y/N)
Average shear reading (gf)
Worst shear reading (gf)
Worst pad ID
B2
Y
28.799
27.113
Pad 9
B3
N
25.752
23.553
Pad 1
B4
N
25.434
21.735
Pad 8
G2-Pad2
Visually worst pad image before wire bonding
B4-Pad1
Fig. 6. All bond pads tested had a shear mode Type 6 per EIA/ JESD22-B116: Wire Bond Shear Test Method by JEDEC [7] as above. None failed the specification limit. The above are examples from a visually good (left) and a visually bad (right) bond pad.
R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108
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Fig. 7. Loss of top layer native aluminum oxide (Al2O3) would cause the copper precipitate seed (Al2Cu) to be exposed.
Table 5 Extract from Minitab 16 showing that the input factors Visual (Good vs Bad), 2nd Plasma Clean (Yes vs No) and its two way interaction are significant in changing the output response (bond shear strength). The p-values indicate the statistical significance (typically below 0.05) [15]. Source
MS
F
P
Visual (Good vs Bad) 2nd Plasma Clean (Yes vs No) Interaction (Visual & 2nd Plasma Clean) Error
DF 1 1 1
SS 31.288 38.912 25.926
31.2875 38.9121 25.9259
10.36 12.88 8.58
0.002 0.001 0.004
76
229.603
3.0211
Total
79
325.729
5. Conclusion By itself, the aluminum copper metalization is robust to DIW as seen in the control sample of Fig. 3. Though DIW is typically known to inhibit galvanic corrosion, it appears to provide no protection when the bond pad surface has undergone prior plasma cleaning that reduces the native oxide thickness, which can expose copper precipitates underneath (see Fig. 7). It is possible that silicon/ metal debris from wafer sawing may the conduit [12] for this galvanic reaction. Bond pad corrosion issues are usually never found by the wafer fabrication site’s automated visual inspection done through
Fig. 8. Individual value plot of the samples. Per JEDEC [7], the Minimum Individual Shear Reading is 15.6 gf whereas the Minimum Shear Average is 24.5 gf.
Table 6 Components of an ANOVA table extracted from Minitab help menu. Component
Description
Source DF
Indicates the source of variation, either from the factor, the interaction, or the error. The total is a sum of all the sources Degrees of freedom from each source. If a factor has three levels, the degrees of freedom is 2 (n 1). If you have a total of 30 observations, the degrees of freedom total is 29 (n 1) Sum of squares between groups (factor) and the sum of squares within groups (error) Mean squares are found by dividing the sum of squares by the degrees of freedom Calculate by dividing the factor MS by the error MS; you can compare this ratio against a critical F found in a table or you can use the p-value to determine whether a factor is significant Use to determine whether a factor is significant; typically compare against an alpha value of 0.05. If the p-value is lower than 0.05, then the factor is significant
SS MS F P
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sampling on selected mission critical products. In wafer fabrication process steps [16], the bond pads are exposed only at the last steps where there is little or almost no contact with DIW. However subsequent process steps that involve changes to the bond surface such as plasma cleaning and processes that involve prolonged submergence in DIW such as wafer saw and other process steps can enhance bond pad corrosion [17,18]. Most devices do not undergo automated visual inspection at assembly process steps and there is always a possibility of an escape. Detection through wire pull test method is not effective as this usually results in break at neck failures. Ball shear is more effective in determining the adhesive strength of the ball to the bond pad. However, as seen in Fig. 8, ball shear strength for all samples were within the specification limits by JEDEC [7]. Subsequent plasma cleaning does help in improving the ball shear strength of the visually bad bond pads. The time dependence of galvanic corrosion is something worth investigating. In order to assess long term reliability of the interconnection, it is recommended that a sample of these corroded bonds undergo reliability tests through thermal annealing [19] as well as higher humidity. Acknowledgements The authors would like to thank all parties including X-FAB Semiconductors suppliers and customers in supporting this research. References [1] Ames I, d’Heurle FM, Horstmann RE. Reduction of electromigration in aluminum films by copper doping. IBM J Res Dev 1970;14(4):461–3. [2] Galvele JR, de De Micheli SM. Mechanism of intergranular corrosion of Al–Cu alloys. Corros Sci 1970;10(11):795–807.
[3] Askeland, Donald R, Phulé Pradeep Prabhakar. Science & engineering of materials. Cengage Learning; 2006. [4] Sethu RS. Reducing non-stick on pad for wire bond: a review. Aust J Mech Eng 2012;9(2):147. [5] Chong YF, Gopalakrishnan R, Tsang CF, Sarkar G, Lim S, Tatti S. An investigation on the plasma treatment of integrated circuit bond pads. Microelectron Reliab 2000;40(7):1199–206. [6] Chong YF, Gopalakrishnan R, Tsang CF, Sarkar G, Lim S, Tatti S. Chemical and morphological studies of plasma-treated integrated circuit bond pads. J Electron Mater 2001;30(3):275–82. [7] Electronic Industries Alliance (EIA), 1998. EIA/ JESD22-B116: Wire bond shear test method; July. [8] Ng Hong Seng. Outsource bond pad reliability methodology setup for foundry process changes. In: IEEE symposium on Industrial electronics and applications (ISIEA), 2011 IEEE; 2011. p. 696–9. [9] Simon Thomas, Berg H. Micro-corrosion of Al–Cu bonding pads. IEEE Trans Compon Hybr Manuf Technol 1987;10(2):252–7. [10] Younan Hua, Ping Lee Yuan, Rao Nistala R, Qinghua Tian. Studies on galvanic corrosion on floating and grounded bondpads in wafer fabrication. ECS Trans 2009;25(3):219–23. [11] Xu Chongchen. Corrosion in microelectronics. Partial Fulfillment of MatE 2003;234. [12] Eve, Tai, Hua Younan, Rao Ramesh, Zhao Siping. Failure analysis of pitting problem on microchip Al bondpads in wafer fabrication. In: IEEE international conference on Semiconductor electronics, 2006. ICSE’06. IEEE; 2006. p. 876–80. [13] Liston Edward M. Plasma treatment for improved bonding: a review. J Adhes 1989;30(1–4):199–218. [14] Xu Hui, Liu Changqing, Silberschmidt Vadim V, Chen Zhong, Wei Jun. Initial bond formation in thermosonic gold ball bonding on aluminium metallization pads. J Mater Process Technol 2010;210(8):1035–42. [15] Montgomery, Douglas C, Runger, George C. Applied statistics and probability for engineers. Wiley.com; 2010. [16] Doering Robert, Nishi Yoshio. Handbook of semiconductor manufacturing technology. CRC Press Llc; 2008. [17] Ueng HY, Liu CY. The aluminum bond-pad corrosion in small outline packaged devices. Mater Chem Phys 1997;48(1):27–35. [18] Ragay FW, Avd Pol J, Naderman J. In-situ monitoring of dry corrosion degradation of au ball bonds to al bond pads in plastic packages during HTSL. Microelectron Reliab 1996;36(11):1931–4. [19] Xu H, Liu C, Silberschmidt VV, Pramana SS, White TJ, Chen Z, et al. Intermetallic phase transformations in Au–Al wire bonds. Intermetallics 2011;19(12): 1808–16.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Characterization of copper precipitates on aluminum copper bond pads formed after plasma clean and de-ionized water exposure Raj Sekar Sethu ⇑, Hong Seng Ng, Alvin Chan, Cheng Nee Ong, Sieng Fong Chan Quality, X-FAB Sarawak Sdn. Bhd., 1 Silicon Drive, Sama Jaya Free Industrial Zone, 93350 Kuching, Sarawak, Malaysia
a r t i c l e
i n f o
Article history: Received 28 May 2014 Received in revised form 24 March 2015 Accepted 30 March 2015 Available online 23 April 2015 Keywords: Bond pad Corrosion Ball shear Plasma clean
a b s t r a c t Semiconductor bond pads made from aluminum and small percentages of copper is susceptible to galvanic corrosion. In galvanic corrosion, the cathode (copper precipitate) is usually protected by the aluminum oxide that covers the surface of aluminum which acts as the anode. However, when the aluminum oxide thickness is reduced by plasma cleaning, the precipitates can be exposed. When exposed precipitates come in contact with de-ionized water, galvanic corrosion takes place. Therefore, though plasma cleaning in general is supposed to improve semiconductor bond pad surface in preparation for package level interconnection, adding the plasma clean step just before a process with de-ionized water can cause bond pad corrosion through the galvanic reaction between the exposed precipitate (cathode) and the surrounding aluminum (anode). This paper aims to investigate the mechanism of corrosion and characterize corroded bond pads by using wire bond ball shear method. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Copper doped aluminum interconnects help reduce electromigration [1]. This however introduces copper precipitation which enhances the risk of galvanic corrosion [2,3]. The wire bonding process has many issues which are well documented [4]. In order to overcome most of these issues, plasma cleaning is used to remove contaminants [5,6]. However plasma cleaning can also cause the top native aluminum oxide to become thin and thus expose copper precipitates that can enhance galvanic corrosion when in contact with an electrolyte such as ionized water. In typical assembly process flows, after plasma cleaning, the bond pad does not get in contact with an electrolyte. 2. Materials and methods The experiment outlines the methods used to examine bond pads (99.5% Al and 0.5% Cu) with galvanic corrosion followed by a risk assessment using ball bond shear test per EIA/JESD22B116: Wire Bond Shear Test Method by JEDEC [7]. The simplified flow is per Fig. 1. Completed bond pad samples were first examined using a JEOL JAMP-9500F Auger Electron Spectroscopy (AES) before being ⇑ Corresponding author. Tel.: +60 12 331 6041. E-mail address: [email protected] (R.S. Sethu). http://dx.doi.org/10.1016/j.microrel.2015.03.018 0026-2714/Ó 2015 Elsevier Ltd. All rights reserved.
subjected to the first plasma clean (see parameters in Table 1) then examined again through AES to determine how much of top surface material is removed by the first plasma clean process. Subsequently, the samples were submerged for twelve hours in DIW with resistance of 18.1 MOhm/cm after wafer saw. This is to simulate a worse case scenario of the bond pad being improperly dried or stuck inside a wet equipment/chamber for a long period of time due to maintenance. The samples had ten bond pads (labelled Pad1 to Pad10) in each die. Four visually good dies (labelled G1, G2, G3 & G4) and four visually bad dies (labelled B1, B2, B3 & B4) were chosen based on their bond pads’ surface appearance. Two dies from each group underwent the second plasma clean prior to wire bonding; whereas the other two dies were kept as control samples i.e. without second plasma clean. The Good–Bad criteria was decided by a single engineer. Automated visual inspection was not employed as the cost to setup such a system for the evaluation could not be justified. Optical images of all ten bond pads were taken for the visually bad dies and a sample each was taken for the visually good dies before and after second plasma cleaning. This is to determine whether the second plasma clean can reduce the surface contamination that can be detected through optical microscopy. Ball shear testing is a well known method to assess the adhesion strength between the wire and the bond pad. This method is preferred over a cross section over the precipitate, as the precipitate size is very small compared to the ball diameter which would be covering all precipitates. Therefore all bond pads
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R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108 Table 2 Ball Shear tester input parameters. Parameter
Units
Setting
Test speed Test load Land speed Shear height Over travel
lm/s
250 50 50 3.4 150
gf lm/s lm lm
3.2. Optical/SEM review after submerge in de-ionized water (DIW) Samples that were submerged in DIW for twelve hours after the first plasma clean showed signs of corrosion as seen in a sample image of Fig. 3, which were consistent with findings by [9]. The corrosion was random across the bond pads and had no selectivity as observed by [10]. However the control sample without the first plasma clean showed no signs of corrosion even when submerged in de-ionized water for twelve hours. Further analysis through SEM (see Fig. 4) showed that the dark surface irregularities are actually the theta phase Al2Cu precipitates with debris scattered around the cavity. In an electrolyte, the theta phase Al2Cu would act as a cathode whereas the alpha phase anodic aluminum around it can dissolve [11]. The EDX spectroscopy seen in Fig. 5 also shows a small silicon peak which may been deposited on the bond pad surface during wafer sawing. The presence of silicon may have been the catalyst for the galvanic corrosion that would have exposed the aluminum surrounding the Al2Cu seed [12]. The EDX showed no signs of fluorine.
Fig. 1. Process flow for the experiment.
3.3. Effect of second plasma clean were wire bonded with 1 mil diameter gold wire detailed in [8] and underwent ball shear testing [7] using a Dage 4000 shear test equipment per settings in Table 2. The worst shear values for the visually bad bond pads were recorded and analyzed against the bond pad images prior to wire bonding. 3. Results 3.1. Auger Electron Spectroscopy – before & after first plasma clean As seen in Fig. 2(a) the percentage of pure aluminum on the surface increased after undergoing the first plasma cleaning, indicating that the native oxide is thinner after the first plasma clean. By examining the Atom% at the 50.0% line, for both aluminum and oxygen, the oxide removal depth is estimated to be about 3.3 nm. The AES elemental depth profile seen in Fig. 2(c) for the bond pad shows a significant decrease in carbon content at the top surface after the first plasma cleaning. Though the metalization is known to have about 0.5% copper, AES did not detect any copper up to the depth of 31 nm. This result is expected as the AES spot size is only 35 lm2. Furthermore, since no surface anomaly was observed on the bond pad before and after the first plasma clean, a random location within the bond pad was selected for this purpose.
Table 1 Parameters used for both plasma cleaning processes. Parameter
Units
Setting
RF power Time Gas mixture Vacuum (pressure)
W min % m bar
195 3 Ar – 95, H – 5 0.6
As seen in Table 3, the second plasma clean process has very little effect on improving the visual appearance of the bond pads with contamination. In fact, for the good bond pad sample i.e. G1-Pad1, the pitting (dark spots on the bond pad surface which do not reflect light back) on the surface actually increased. This may be due to the nature of plasma cleaning itself which is destructive [13]. 3.4. Ball shear analysis The wire bonded samples were measured and had an average wire ball diameter of 67.92 lm and an average wire ball height of 14.20 lm; before being sheared using a Dage 4000 shear test equipment with parameters in Table 2. The resultant ball shear strength values were stratified into four groups i.e. Visually Good with Second Plasma Clean, Visually Good without Second Plasma Clean, Visually Bad with Second Plasma Clean and Visually Bad without Second Plasma Clean. Each group has two dies with ten bond pads each (total 20 samples per group). Table 4 shows that the worst shear reading is still above the lower specification limit (15.6 gf) by JEDEC. As seen in Fig. 6, though the bonding did take place generally along the periphery of the ball [14] the surface under the ball of the B4-Pad1 seems to have less contamination compared to the rest of the surface. This indicates that the corrosion is surface phenomenon that may not impact bond shear strength if the wire material goes deep enough into the bond pad during wire bonding. 4. Discussion As seen in the AES depth profiling of Fig. 2, the first plasma clean can reduce the amount of native oxide on the bond pad.
R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108
Fig. 2. AES depth profile for various elements on the top surface of the bond pad before and after first plasma clean. (a) Aluminum, (b) oxygen, and (c) carbon.
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Fig. 3. Sample optical microscopy of bond pads. Left Image – before being submerged. Center Image – with first plasma clean, after being submerged in DIW. Right Image – no first plasma clean, after being submerged in DIW.
Fig. 4. SEM image of bond pad that have undergone DIW submergence after first plasma clean. Left image – copper precipitate overview; right image – tilted view of another copper precipitate showing sidewall erosion. The copper precipitate sampled had a length of 986.7 nm and a width of 1859 nm.
Fig. 5. EDX spectroscopy of the copper precipitate show that it is made of primarily aluminum and copper.
This in turn, exposes the bottom layer which may contain the Al2Cu precipitate seed. Once this Al2Cu seed is exposed (see Fig. 7) to an electrolyte such as de-ionized water, and with the help of a catalyst such as a silicon particle from wafer saw, galvanic corrosion can take place. A two-way analysis of variance (ANOVA) was done using a statistical software (Minitab 16) to determine effect of the input factors i.e. surface contamination (Visual) and the 2nd Plasma Clean on the output response (i.e. ball shear strength).
The p-value results [15] seen in Table 5 indicate that the factors Visual (Good vs Bad), 2nd Plasma Clean (Yes vs No) and its two way interaction are significant in changing the bond shear strength. Table 6 explains the components of the ANOVA of Table 5. Further analysis is done through Minitab’s Individual Value Plot which provides a graphical description. Based on the shear readings seen in Fig. 8, the second plasma cleaning can increase the ball shear strength of visually bad bond pads, but has no discernable effect on the visually good bond pad.
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Table 3 Visually, there is no discernible effect on the bond pads by doing the second plasma clean. The bond pad samples chosen for before and after optical analysis are considered the worst from their respective die. Die-Pad ID
Before second plasma clean
After second plasma clean
G1-Pad1
B1-Pad7
B2-Pad3
Table 4 Result summary. All individual shear values passed the JEDEC bond shear lower specification limit (15.6 gf). The shear specification limit is derived from the ball diameter [7]. Die ID
2nd Plasma Clean (Y/N)
Average shear reading (gf)
Worst shear reading (gf)
Worst pad ID
Visually worst pad image before wire bonding
G1 G2 G3 G4 B1
Y Y N N Y
28.385 28.063 27.295 28.670 27.454
26.750 26.840 25.963 26.485 26.450
Pad Pad Pad Pad Pad
None None None None
7 1 3 5 7
(continued on next page)
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Table 4 (continued) Die ID
2nd Plasma Clean (Y/N)
Average shear reading (gf)
Worst shear reading (gf)
Worst pad ID
B2
Y
28.799
27.113
Pad 9
B3
N
25.752
23.553
Pad 1
B4
N
25.434
21.735
Pad 8
G2-Pad2
Visually worst pad image before wire bonding
B4-Pad1
Fig. 6. All bond pads tested had a shear mode Type 6 per EIA/ JESD22-B116: Wire Bond Shear Test Method by JEDEC [7] as above. None failed the specification limit. The above are examples from a visually good (left) and a visually bad (right) bond pad.
R.S. Sethu et al. / Microelectronics Reliability 55 (2015) 1101–1108
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Fig. 7. Loss of top layer native aluminum oxide (Al2O3) would cause the copper precipitate seed (Al2Cu) to be exposed.
Table 5 Extract from Minitab 16 showing that the input factors Visual (Good vs Bad), 2nd Plasma Clean (Yes vs No) and its two way interaction are significant in changing the output response (bond shear strength). The p-values indicate the statistical significance (typically below 0.05) [15]. Source
MS
F
P
Visual (Good vs Bad) 2nd Plasma Clean (Yes vs No) Interaction (Visual & 2nd Plasma Clean) Error
DF 1 1 1
SS 31.288 38.912 25.926
31.2875 38.9121 25.9259
10.36 12.88 8.58
0.002 0.001 0.004
76
229.603
3.0211
Total
79
325.729
5. Conclusion By itself, the aluminum copper metalization is robust to DIW as seen in the control sample of Fig. 3. Though DIW is typically known to inhibit galvanic corrosion, it appears to provide no protection when the bond pad surface has undergone prior plasma cleaning that reduces the native oxide thickness, which can expose copper precipitates underneath (see Fig. 7). It is possible that silicon/ metal debris from wafer sawing may the conduit [12] for this galvanic reaction. Bond pad corrosion issues are usually never found by the wafer fabrication site’s automated visual inspection done through
Fig. 8. Individual value plot of the samples. Per JEDEC [7], the Minimum Individual Shear Reading is 15.6 gf whereas the Minimum Shear Average is 24.5 gf.
Table 6 Components of an ANOVA table extracted from Minitab help menu. Component
Description
Source DF
Indicates the source of variation, either from the factor, the interaction, or the error. The total is a sum of all the sources Degrees of freedom from each source. If a factor has three levels, the degrees of freedom is 2 (n 1). If you have a total of 30 observations, the degrees of freedom total is 29 (n 1) Sum of squares between groups (factor) and the sum of squares within groups (error) Mean squares are found by dividing the sum of squares by the degrees of freedom Calculate by dividing the factor MS by the error MS; you can compare this ratio against a critical F found in a table or you can use the p-value to determine whether a factor is significant Use to determine whether a factor is significant; typically compare against an alpha value of 0.05. If the p-value is lower than 0.05, then the factor is significant
SS MS F P
1108
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sampling on selected mission critical products. In wafer fabrication process steps [16], the bond pads are exposed only at the last steps where there is little or almost no contact with DIW. However subsequent process steps that involve changes to the bond surface such as plasma cleaning and processes that involve prolonged submergence in DIW such as wafer saw and other process steps can enhance bond pad corrosion [17,18]. Most devices do not undergo automated visual inspection at assembly process steps and there is always a possibility of an escape. Detection through wire pull test method is not effective as this usually results in break at neck failures. Ball shear is more effective in determining the adhesive strength of the ball to the bond pad. However, as seen in Fig. 8, ball shear strength for all samples were within the specification limits by JEDEC [7]. Subsequent plasma cleaning does help in improving the ball shear strength of the visually bad bond pads. The time dependence of galvanic corrosion is something worth investigating. In order to assess long term reliability of the interconnection, it is recommended that a sample of these corroded bonds undergo reliability tests through thermal annealing [19] as well as higher humidity. Acknowledgements The authors would like to thank all parties including X-FAB Semiconductors suppliers and customers in supporting this research. References [1] Ames I, d’Heurle FM, Horstmann RE. Reduction of electromigration in aluminum films by copper doping. IBM J Res Dev 1970;14(4):461–3. [2] Galvele JR, de De Micheli SM. Mechanism of intergranular corrosion of Al–Cu alloys. Corros Sci 1970;10(11):795–807.
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