WO2018218217A2

WO2018218217A2 - Mechanistic investigation and prevention of al bond pad corrosion in cu wire-bonded device assembly

Info

Publication number: WO2018218217A2
Application number: PCT/US2018/034757
Authority: WO
Inventors: Oliver Ming-Ren Chyan; Nick Ross ROSS
Original assignee: University Of North Texas
Priority date: 2017-05-26
Filing date: 2018-05-25
Publication date: 2018-11-29
Also published as: WO2018218217A3; CN110891616A; JP2020521878A

Abstract

Heavy corrosion on aluminum (Al) bond pads, with a "mud-crack" appearance, in a copper (Cu) wire-bonded assembly can be a critical failure mode, especially under harsh conditions such as automotive environments. This corrosion can be associated with the presence of contaminants such as chloride ions (CI-). However, the exact corrosion activation mechanism remains unclear and hence the effective corrosion prevention cannot be achieved. A novel immersion corrosion screening metrology was utilized as an in situ characterization tool to establish the corrosion mechanism directly relevant to Cu wire-bonded devices. With this improved understanding of Al bond pad corrosion mechanism, significant progress has been made toward developing effective corrosion prevention strategies to improve and ensure the overall packaging reliability.

Description

MECHANISTIC INVESTIGATION AND PREVENTION OF AL BOND PAD CORROSION IN CU WIRE-BONDED DEVICE ASSEMBLY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/511,863, filed May 26, 2017 and entitled, "MECHANISTIC INVESTIGATION AND PREVENTION OF AL BOND PAD CORROSION IN CU WIRE- BONDED DEVICE ASSEMBLY," the disclosure of which is incorporated here by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure generally relates to bi-metallic devices, and more particularly to techniques for reducing corrosion of bi-metallic devices comprising copper and aluminum.

BACKGROUND

[0003] Copper (Cu) has rapidly replaced gold (Au) as the preferred wire-bonding material in microelectronic packaging due to its higher electrical conductivity, lower cost, and better mechanical strength, which leads to reduced pad size and pad pitch. However, corrosion- related failures need to be minimized to ensure packaging reliability. Specifically, Aluminum (Al) bond pads are particularly susceptible to corrosion with a characteristic "mud-crack" appearance. Such corrosion can be a critical failure mode for Cu wire-bonded assemblies, especially under harsh conditions such as automotive environments. The emerging trend of wearable electronics also imposes new, more stringent packaging reliability requirements to ensure corrosion protection from sweat/mud/rain in all-terrain non-stop usage conditions. In addition, the recent transition to non-lead SAC (Sn-Ag-Cu) solder necessitates the use of stronger flux (2-3%) and higher reflow temperatures (~ 260 °C) that can leave flux residues and ion contamination to initiate corrosion. While this type of Al bond pad corrosion is often associated with the presence of contaminants such as halides (CI^", Br^", F^"), the fundamental corrosion mechanism remains unclear and hence the effective corrosion prevention cannot be achieved. SUMMARY

[0004] The present disclosure provides corrosion prevention strategies that improve packaging reliability and reduce the Halides-induced corrosion mechanism of a Cu wire-bonded Al bond pad. Previous corrosion prevention strategies focused on interfacial layers of intermetallic compounds like CU9AU, CuAl and CuAl₂ from the failed wire-bonded devices after Autoclave testing. However, the real causes of corrosion vulnerability are best revealed by careful monitoring of the active corrosion progression. In aspects of the present disclosure a novel immersion corrosion screening metrology as an in situ characterization tool, combined with SEM, optical microscopy, and other characterization techniques is proposed to establish a corrosion mechanism directly relevant to Cu wire-bonded devices. The disclosed immersion corrosion screening may be considered as an accelerated extreme water humidity stress test. In a properly molded package, extent of absorbed water and ionic impurities at the mold compound/die interface will be less severe.

[0005] By monitoring the active corrosion in real time, surface A10_x dendrite formation/propagation and active hydrogen (H₂) gas evolution may be observed. In aspects, immersion corrosion screening according to aspects of the present disclosure has identified that H₂ evolution plays a key role in extracting electrons from Al bond pads to fuel the corrosion cycle. These observations provide mechanistic insights, upon which a new corrosion inhibition strategy configured to eliminate this H₂ evolution half-reaction was developed. In aspects, the corrosion inhibition strategy may apply a selected surface treatment to greatly raise the activation energy barrier of the H⁺ H₂ cathodic reaction. Testing of the corrosion inhibition strategy proposed by aspects of the present disclosure has confirmed that the surface treatments may be highly effective in eliminating the severe Al pad mud-crack corrosion in acidic chloride testing solution with an un-molded Cu wire-bonded assembly. The new surface treatments according to aspects of the present disclosure have been demonstrated to exhibit good corrosion protection after annealing at temperatures around 175° C. In aspects, the selected surface coating may be applied at the pre-molding step in a manufacturing-compatible wire-bonding and molding assembly line.

[0006] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0008] FIG. 1 A is an image illustrating dendrite progression due to aluminum (Al) corrosion;

[0009] FIG. IB is an image illustrating a mud-cracking morphology formed at the surface of an Al wafer due to chloride (CI^") induced corrosion;

[0010] FIG. 1C is a scanning electron microscope image illustrating islands formed on a surface of a corroded Al wafer;

[0011] FIG. 2 is a diagram illustrating aspects of surface elemental composition of a corroded Al wafer;

[0012] FIG. 3A is a series of images illustrating the progression of corrosion in a copper (Cu)/Al micro-pattern and Al wafer;

[0013] FIG. 3B is a plot diagram illustrating results of energy dispersive x-ray spectrography (EDX) analysis of corrosion in Cu/Al micro-pattern and Al wafer; [0014] FIG. 4 A is a diagram illustrating results of gas chromatography mass spectrometry (GC-MS) analysis for gas generated from a blank sample without active corrosion;

[0015] FIG. 4B is a diagram illustrating results of GC-MS analysis for gas generated due to active corrosion of Al in a Cu/Al system;

[0016] FIG. 5 is a sequence of SEM images illustrating aspects of hydrogen evolution during corrosion of Al in Cu/Al bimetallic system;

[0017] FIG. 6A is an image illustrating a device used to study the corrosion of the Al pads;

[0018] FIG. 6B is an image illustrating aspects of corrosion of Al pads;

[0019] FIG. 6C is an image illustrating aspects of wire bond lift off due to corrosion of Al pads;

[0020] FIG. 6D is an image illustrating additional aspects of wire bond lift off due to corrosion of Al pads;

[0021] FIG. 7 is a diagram illustrating aspects of a Cu wire bonded to an Al pad;

[0022] FIG. 8A is an image illustrating a Cu wire-bonded device;

[0023] FIG. 8B is an image illustrating corrosion of a Cu wire-bonded device;

[0024] FIG. 8C is an image illustrating utilization of an inhibitor compound to reduce corrosion and protect a Cu wire-bonded device in accordance with aspects of the present disclosure;

[0025] FIG. 9A is an image illustrating corrosion of untreated Cu dot samples on an AL (0.5% Cu) substrate;

[0026] FIG. 9B is an image illustrating improved thermal stability and corrosion protection of Cu dot/ AL (0.5% Cu) samples treated with an inhibitor in accordance with aspects of the present disclosure; [0027] FIG. 1 OA is a series of images illustrating the results of corrosion screening of Cu microdots without an inhibitor coating;

[0028] FIG. 10B is a series of images illustrating the results of corrosion screening of Cu microdots without an inhibitor coating and annealed without the inhibitor coating;

[0029] FIG. IOC is a series of images illustrating the results of corrosion screening of Cu microdots after application of a spray coated inhibitor and annealing in accordance with aspects of the present disclosure;

[0030] FIG. 10D is a series of images illustrating the results of corrosion screening of Cu microdots after application of a vapor deposited inhibitor and annealing in accordance with aspects of the present disclosure;

[0031] FIG. 11 is a flow diagram illustrating an internal chloride contamination loading and testing scheme for Cu wire bonded devices after a molding process;

[0032] FIG. 12 is a chart illustrating electrical testing data for samples of molded Cu wire bonded devices after autoclave stress testing; and

[0033] FIG. 13 is a flow diagram of a method for reducing corrosion of Cu wire bonded devices in accordance with aspects of the present disclosure.

DETAILED DESCRIPTION

[0034] Various features and advantageous details are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known starting materials, processing techniques, components, and equipment are omitted so as not to unnecessarily obscure the invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the invention, are given by way of illustration only, and not by way of limitation. Various substitutions, modifications, additions, and/or rearrangements within the spirit and/or scope of the underlying inventive concept will become apparent to those skilled in the art from this disclosure.

[0035] FIGs. 1A-1C illustrate aspects of aluminum (Al) corrosion observed during a study of real time corrosion in an Cu/Al bimetallic system, which was performed using a novel metrology called micro-pattern corrosion screening. The micro-pattern corrosion screening technique used for the study was prepared by sputtering Cu dots (50 nm thick, 130 μηι diameter) onto an 1.2 μιη thick Aluminum wafer (99.5% Al + 0.5% Cu ) substrate. All the micro-patterns were prepared through a microdots mask using a Denton Vacuum Desktop Pro sputtering unit. The real-time inspection of the corrosion process was carried out by a Nikon Eclipse LV150 microscope. The samples (both Cu/Al micro-pattern and Cu wire-bonded device were immersed in the test environment solution (pH 5, 0-20 ppm CI^" solution) and the sample surface was constantly monitored for indications of surface corrosion processes. A FEI Nova 200 NanoLab scanning electron microscope (SEM) provided the high resolution imaging of corroded samples. Surface elemental composition information was obtained by an energy dispersive X-ray spectroscopy (EDX) system. The hydrogen gas (H₂) evolution was detected by a gas chromatography-mass spectrometer (GC-MS) using a Finnigan Trace GC Ultra equipped with a quadrupole mass filter. The inhibitor surface treatments of the micro-patterns and Cu wire- bonded devices were carried out by spray coating and chemical vapor deposition processes. The spray coating was carried out by spraying a solution of corrosion inhibitor compound onto the sample surface at -30 centimeter (cm) distance using a pneumatic sprayer at 70 psi pressure. The chemical vapor deposition was carried out by heating the micro-pattern sample and corrosion inhibitors in situ near the melting point of the compound for 6-10 min. After additional thermal annealing (> 175 °C), the corrosion inhibitor treated micro-pattern samples were tested in acidic chloride solution to evaluate the corrosion inhibition effect.

[0036] The presence of chloride ions may cause corrosion in the aluminum (with 0.5% Cu), simply described as Al sample hereafter. Immersion corrosion screening showed the progression of the corrosion in Al started with the formation of dendrites on the Al surface and corrosion continued leading to the formation of mud-crack like morphology. The concentration of the chloride ions in the solution was observed to have a direct impact on the progression of the corrosion of Al (w/0.5% Cu). The corrosion of the Al sample in 20 ppm CI^" concentration at a pH of 4 was observed and the images are taken after 96 hours of immersion, as shown in FIGs. 1A-1C. For example, FIG. 1A is an image illustrating the observed progression of dendrite due to Al corrosion. FIG. IB is an image illustrating a closer look at the dendrite formation and reveals a mud-cracking morphology formed at the surface of the Al wafer due to severe chloride ion (CI^") induced corrosion. FIG. 1C is a SEM image of the corroded Al wafer and shows islands of the corroded Aluminum surface with severe, developed mud-cracks. [0037] As part of the analysis, the surface elemental composition of the corroded Al samples was examined using EDX. The results of this analysis are graphically illustrated in FIG. 2 and Tables 1 and 2 below. Table 1 illustrates the surface elemental composition of the corroded sample for a mudcrack (e.g., mudcrack 202 of FIG. 2) and Table 2 illustrates the surface elemental composition of the corroded sample for an island (e.g., mudcrack 204 of FIG. 2).

Table 1

[0038] As briefly mentioned above, the data associated with the corroded sample illustrated in FIG. 2 was obtained using EDX for an Al sample exposed to 20 ppm CI^" at 4 pH for 96 hours. In the inset of FIG. 2, a SEM image of the corroded samples is shown. As shown in FIG. 2 and Tables 1 and 2, the corroded samples showed a high oxygen signal in almost a 1 : 1 ratio to Al, suggesting severe oxidation occurred at the region where the mud-cracks occurred, compared to a slightly lower 0:A1 ratio of 1:2 at the island region. For comparison, the surface of a pristine Al (with 0.5% Cu) sample exhibits an 0:A1 composition ratio of 1:70, suggesting more aggressive Al corrosion in the crack regions, which leads to more erosion in the crack regions as compared to the Al island plateau area. [0039] The progression of corrosion of a Cu/Al micro-pattern and Al wafer was studied side by side using the immersion corrosion screening technique described above. In FIG. 3A, images comparing observed corrosion progression in Al (e.g., Al region 302) and Cu/Al bimetallic systems (e.g., Cu/Al region 304) are shown, and in FIG. 3B, a plot representing the observed EDX of a corroded Cu/Al micro-pattern is shown. It is noted that even with a more dilute 2 ppm CI^", pH 5 solution, the active Al corrosion may be observed along the Cu/Al bimetallic contacts in as little as 10 minutes. Similar to FIGs. 1A-1C, the severity of Al corrosion driven by Cu/Al bimetallic contact may be seen by the intense dendrite formation and mud-crack developing on Al surface around the Cu dots. In contrast, the Al wafer did not undergo such severe corrosion under the same conditions and exhibited only minor surface roughening at the same time mark.

[0040] This difference in corrosion severity illustrates the important effects of Cu/Al bimetallic contact. Cu and Al, being apart in their position in galvanic series, form a galvanic couple where the less noble metal Al becomes the anode and nobler metal Cu becomes the cathode. In the presence of CI^" and water moisture, the electrochemical cell is completed and the corrosion takes place rapidly. On the other hand, Al does not have such a powerful driving force to attain a rapidly corroding state. It takes approximately double the time (e.g., 96 hours) while being submerged in more concentrated 20 ppm CI^" for an Al wafer to heavily corrode. The screening data also showed a different corrosion progression in Cu/Al and Al systems. In a Cu/Al system, the corrosion starts at the Cu/Al interface and proceeds with dendrite formation. Later, as expected, the Al corrosion, when submerged in acidic chloride solution, was greatly accelerated in micro-pattern sample (e.g., as shown in Cu/Al region 304) with Cu/Al bimetallic as compared to the Al wafer (e.g., as shown in Al region 302) submerged in the same solution. As shown in FIG. 3A, even with the more dilute 2 ppm CI^", pH 5 solution, the active Al corrosion can be observed along the Cu/Al bimetallic contacts in as little as 10 minutes. Similar to the corrosion illustrated in FIGs. 1A-1C, the severity of Al corrosion driven by Cu/Al bimetallic contact may be seen by the intense dendrite formation and mud-crack developing on Al surface around the Cu dots. In contrast, the Al wafer did not undergo such severe corrosion under same conditions and exhibited only minor surface roughening at the same time mark, as shown in (e.g., as shown in Al region 302). This difference in corrosion severity illustrates the important effect of Cu/Al bimetallic on the Al in Cu/Al systems, which also ended up developing mud-cracking morphology at about 44 hrs, as illustrated in FIG. 3A. An 0:A1 ratio of more than 2:1 was observed using EDX, as illustrated in FIG. 3B.

[0041] More importantly, the in situ corrosion screening described above showed, for the first time, a rapid gas bubbling associated with the active corrosion of Al in Cu/Al systems. Gas chromatography mass spectrometry (GC-MS) analysis was performed on the evolving gas and the results are shown in FIGs. 4A and 4B. FIG. 4A illustrates the results of performing GC-MS of a blank sample and gas generated by the corrosion process is illustrated in FIG. 4B. The results of the GC-MS analysis provide a clear signal for hydrogen gas.

[0042] The hydrogen evolution reaction plays a major role in the severe Al corrosion. Once the hydrogen evolution takes place, it propels Al oxidation to early dendrite formation, as shown above in FIGs. 3A and 3B. Once the dendrite forms, the inter-granular corrosion progress proceeds both laterally and downward to result in mud-crack corrosion. The hydrogen evolution can also lead to internal stress build up, causing violent eruptions, as shown in FIG. 5, which illustrates a SEM image of hydrogen evolution during corrosion of Al in Cu/Al bimetallic system (left), a SEM image of hydrogen evolution site (middle), and a SEM image of hydrogen evolution at Cu/Al interfacial site (right).

[0043] Exposing the underlying unprotected Al to hydrogen evolution furthers corrosion. As shown in FIG. 3, hydrogen evolution is always followed by severe corrosion, forming dendrites and eventually mud-crack formations. As will be described in more detail below, the newly discovered hydrogen evolution accompanying the severe Al corrosion provides an important insight to aid in establishing corrosion mechanisms and creating effective prevention strategies.

[0044] The immersion screening metrology described above was also applied to study the corrosion taking place in the industrial Cu wire-bonded devices. A device with a palladium (Pd)-coated Cu wire-bonded to an Al pad was used to study the corrosion of the Al pad. This study is shown in FIGs. 6A-6D, which illustrates a process for immersion screening of the Pd-coated Cu wire-bonded device.

[0045] FIG. 6A is an image illustrating the Pd-coated Cu wire-bonded device before immersion. FIG. 6B is an image illustrating the corrosion starting with hydrogen evolution, which occurred within approximately 6 minutes of the immersion screening process, and FIG. 6C is an image illustrating that the first wire bond lift off, which occurred within approximately 7 minutes of the immersion screening process. FIG. 6D is an image illustrating that a 10% wire lift off occurred within 43 minutes of the immersion screening process. As shown in FIG. 6, the corrosion progression of the wire-bonded devices was similar to that of the lab made Cu/Al bimetallic micro-pattern system. Corrosion always started at the Cu wire-bonded Al pad and proceeded along Al contact leads accompanied by the hydrogen evolution. This suggests that the additional Pd coating on Cu wires bonded to Al pad was not effective at preventing Al corrosion, which, in the studies described above, was induced using acidic chloride conditions. This also suggests that the bimetallic contact between a Cu ball and Al pad plays a major role in the Al pad corrosion in Cu wire-bonded devices.

[0046] The severity of this CI^" induced Al corrosion may be seen from the time frame of corrosion screening events. The first sign of corrosion, with hydrogen evolution, was noticed after as few as 6 minutes of immersion in 5 ppm CI^", 5 pH solution, as shown in FIG. 6B. The first wire-bond lift-off took place at 7 minutes, as shown in FIG. 6C, and it just took 43 minutes for more than 10% of all the Cu wires to lift off from the Al pads, as shown in FIG. 6D.

[0047] The mechanism of galvanic corrosion, which occurs when dissimilar metal contacts are exposed to an electrolyte, such as chloride ions, has been previously reported. In the current context of Cu wire-bonded devices, large electrochemical potential differences (> +2.0V) between Al and Cu, in the presence of CI^" ions, cause the observed severe Al bond pad corrosion. However, without the presence of CI^" ions, the surface oxide coating on Al metal provides good passivation protection to prevent continuous Al corrosion.

[0048] Based on the data obtained during the above-described experiments, an anti- corrosion mechanism is proposed, as illustrated in FIG. 7. The anti-corrosion mechanism illustrated in FIG. 7 is configured to account for the acidic chloride induced Al bond pad corrosion described above. In FIG. 7, a Cu wire 702, Al surface 704, Cu ball 706, and an Al bond pad 708 are shown. The Cu ball 706 and the Al bond pad 708 may form a Cu/Al bimetallic interface. In aspects, the Al surface 704 may be covered by a mixed layer of aluminum oxide/aluminum hydroxide when exposed to water moisture. Under acidic conditions, the surface charge of aluminum oxide/hydroxide may be positive, which leads to the attraction of negative ions such as CI^". Adsorbed CI^" ions may cause the dissolution of the passivating oxide by converting exposed aluminum oxide/hydroxide to aluminum chloride (AlCb), which has very high solubility in water (46 g/100 mL). Without the passivating oxide, the exposed Al bond pads 708 that are bonded to the Cu wires (e.g., Cu wire 702) may be driven by the large galvanic potential difference to be rapidly oxidized. Hydrolysis of aluminum chloride on the surface generates localized concentrated protons to effectively capture electrons from Al oxidation, producing the observed hydrogen gas formation. As shown in FIG. 5, this hydrogen evolution can build up large physical stress, causing "volcanic eruption"-like damage, which further exposes the underlying and unprotected Al to CI^" induced corrosion. As H₂ gas continues to leave the system, the Al pad corrosion becomes non-self-limiting and self-propels rapidly to form the observed "mud-crack" morphology that severely corrodes the Al bond pad.

[0049] In any context, active corrosion requires efficient electron transfer between both anodes and cathodes in the localized corrosion cells. The corrosion prevention strategy according to aspects of the present disclosure is aimed at stopping this newly discovered cathodic half reaction of hydrogen evolution using selected surface treatments to modify Al/Cu bimetallic systems. According to aspects, a surface of the Cu/Al micro-pattern may be treated with a thin (e.g., less than 3 μιη) coating of a selected corrosion inhibitor, which may significantly increase the energy barrier for the H₂ evolution and prevent the active Al corrosion cycle.

[0050] Referring to FIGs. 8A-8C, various images illustrating the improvements provided by applying treatments of inhibitor coatings in accordance with aspects of the present disclosure are shown. FIG. 8A is an image illustrating a Cu wire-bonded device; FIG. 8B is an image illustrating a Cu wire-bonded device that has not been treated with an inhibitor compound in accordance with the present disclosure after immersion in a 5 ppm 5 pH CI^" solution for approximately 17 hours; and FIG. 8C is an image illustrating a Cu wire-bonded device treated with an inhibitor compound in accordance with the present disclosure after immersion in a 5 ppm 5 pH CI^" solution for approximately 12 days. As shown in FIGs. 8B and 8C, the selected inhibitor compound may completely stop the hydrogen evolution and provide an extended corrosion protection of Cu wire-bonded device.

[0051] In the absence of the inhibitor compound, the hydrogen evolution and Al pad corrosion started rapidly (e.g., less than 10 minutes) and reached complete lift-off of all Cu wires at 17 hours of immersion, as illustrated in FIG. 8B. However, with the inclusion of the inhibitor compound, the corrosion was stopped completely and the Al pads and Cu wire-bonds remained intact even after 12 days. This may be due to the covalently bonded inhibitor raising the potential barrier for the hydrogen evolution reaction thereby preventing Al corrosion. In aspects of the present disclosure, the inhibitor compound may comprise fluorescein, cetyl trimethyl ammonium bromide, 3 -hydroxy flavone, cerium dibutyl phosphate, 8-hydroxyquinoline, sodium benzodate, tolyltriazole, benzotriazole, amino-azole-thiol, glutaric acid, polyvinyl alcohol, thiourea, phenyl thiourea, 4-carboxy phenyl thiourea, 1,4-naphtoquinone, indole, tryptamine, tryptophane, monoethanolamine, thioacetamide, quinaldic acid, a-benzoionoxime, 2-(2- hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, cupferron, 2-butine-l,4-diol,benzamide, 4-aminobenzenesulfonamide, and thioacetamide.

[0052] In aspects, packaging-friendly application methods may be used to apply the lab-proven corrosion inhibiting coatings to Cu wire-bonded devices in an assembly line production environment. In an industrial environment, the inhibitor coating may go through a series of harsh temperature conditions after the application of the inhibitor coating but before the packing process may be complete. For example, a wire-bonded device coated with a selected inhibitor may be exposed to temperatures of approximately 175 °C for about 5-10 minutes during the molding process. It may also experience temperatures of approximately 220-260 °C for 10- 15 minutes during wire-bonding and solder reflow processes. The applied surface treatment (e.g., the inhibitor compound) should be configured to withstand these higher temperatures. Preliminary thermal stress testing of an inhibitor coating in accordance with aspects of the present disclosure was carried out at Autoclave testing conditions to evaluate its thermal stability and resulting corrosion protection. FIGs. 9A and 9B are images illustrating the inhibitor-treated Cu dot samples tested at approximately 121 °C, at 2 atm pressure and at 100% relative humidity inside a pressure cooker. In these harsh conditions, the samples treated with an inhibitor coating resisted corrosion for almost 4-8 days.

[0053] In aspects, various inhibitor application methods may be utilized for industrial implementation. For example, in aspects, the sample may be spray coated with solutions containing different inhibitor concentrations using a pneumatic sprayer at 70 psi pressure. The spray coated samples may then be annealed at approximately 175 °C for around 4 minutes and then subjected to corrosion screening in 5 ppm CI^" in 5 pH testing solution. Testing has shown that spray coated samples may resist corrosion for more time than the untreated samples, however, Al corrosion may be observed at around 4 hr mark. In additional aspects, chemical vapor deposition techniques may be used to apply the inhibitor. Chemical vapor deposition techniques for applying an inhibitor compound to a Cu microdot on an Al wafer were conducted, which included heating in situ the Cu microdot on the Al wafer after the application of the inhibitor compound with a selected temperature and duration. Infrared spectroscopy was performed on the samples and showed the inhibitor compound formed strong covalent bonds with the Cu/Al microdot samples. The infrared absorption peak height also revealed that the chemical vapor deposition application method may provide a thinner and more uniform inhibitor coating. During the testing, the sample was again annealed at 175 °C for 4 min and tested for corrosion in 5 ppm CI^" in 5 pH solution. The samples coated using the chemical deposition method showed excellent corrosion protection for around 10 hours and no sign of Al corrosion was observed.

[0054] Referring to FIGs. 10A-10D, images illustrating the effect of inhibitor coatings, applied through different methods, on the corrosion protection of Cu/Al samples are shown. In FIG. 10A, images illustrating the results of corrosion screening of Cu microdots on Al in 5 ppm 5 pH CI^" solution without an inhibitor coating are shown at different periods of time during the testing, with the left image corresponding to 0 minutes, the right image corresponding to 2 hrs, and the right image corresponding to 4 hrs. In FIG. 10B images illustrating the results of corrosion screening of Cu microdots on Al in 5 ppm 5 pH CI^" solution without an inhibitor coating and annealed at 175 °C for 4 minutes without the inhibitor coating are shown at different periods of time during the testing, with the left image corresponding to 0 minutes, the right image corresponding to 2 hrs, and the right image corresponding to 8 hrs. In FIG. IOC, images illustrating the results of corrosion screening of Cu microdots on Al in 5 ppm 5 pH CI^" solution after application of a spray coated inhibitor and annealed at 175 °C for 4 minutes in accordance with aspects of the present disclosure are shown at different periods of time during the testing, with the left image corresponding to 0 minutes, the right image corresponding to 2 hrs, and the right image corresponding to 6 hrs. In FIG. 10D images illustrating the results of corrosion screening of Cu microdots on Al in 5 ppm 5 pH CI^" solution after application of a vapor deposited inhibitor coating and annealed at 175 °C for 4 minutes in accordance with aspects of the present disclosure are shown at different periods of time during the testing, with the left image corresponding to 0 minutes, the right image corresponding to 4 hrs, and the right image corresponding to 10 hrs. As shown in the images, the samples treated with inhibitor coatings in accordance with the present disclosure exhibited less corrosion than the untreated samples.

[0055] Referring to FIG. 11, a flow diagram illustrating an internal chloride contamination loading and testing scheme for Cu wire bonded devices after a molding process is shown. As shown in FIG. 11, a Cu wire bonded device 1112 is provided/obtained, at step 1102. At step 1104, the Cu wire bonded device 1112 is loaded with sodium chloride (NaCl(aq)), as shown at 1114. In an aspect, the Cu wire bonded device may be loaded with approximately 1.25 of NaCl(aq). It is noted that utilization of NaCl to load the Cu wire bonded device has been provided for purposes of illustration, rather than limitation and that other compounds and/or solutions may be utilized depending on the particular test performed. Additionally, it is noted that volumes below or in excess of 1.25 \iL may be utilized to load the Cu wire bonded device depending on the compound/solution utilized, the particular test being conducted, and/or other testing considerations. Following the loading step 1104, the loading solution (e.g., NaCl(aq)) may be allowed to dry, at step 1106. As the loading solution dries, crystals 1116 (e.g., NaCl crystals) may form on the Cu wire bonded device 1112. Once dried, the Cu wire bonded device 1112 may be molded and subjected to a stress test, at step 1108. In an aspect, the stress test may be configured according to a set of testing parameters. The set of testing parameters may include a temperature parameter, a time parameter, and the like. For example, testing of aspects of the present disclosure were performed using a temperature parameter of 130° C (e.g., a temperature parameter) for a duration of forty eight hours (e.g., a time parameter). Following molding and stress testing, a defect analysis of the Cu wire bonded device may be performed at step 1110.

[0056] Referring to FIG. 12, a chart illustrating results of electrical testing data for samples of molded Cu wire bonded devices after autoclave stress testing is shown. As illustrated in FIG. 12, the results of the electrical failure analyses demonstrate the inhibitor coating was highly effective at preventing Al bond pad corrosion, which was expected due to the massive internal CI^" ion contamination caused by loading of the Cu wire bonded device with NaCl(_aq), as described above with respect to FIG. 11. The electrical conductivity test, based on application of a 2 volt input/output of the surface treated molded wire bonded device was carried out after autoclave stress test. As shown in FIG. 12, excellent electrical conductivity was observed even though the device was subjected to massive contamination by internal chloride ions. This confirms the selected inhibitor treatment was effective at preventing corrosion of the Cu wires and Al pads exposed that were subjected to the presence of the chloride ions. It is noted that during testing, unprotected control devices experienced extensive Cu ball lift-off and open- circuit fatal failures.

[0057] Referring to FIG. 13, a flow diagram illustrating of a method for reducing corrosion of Cu wire bonded devices in accordance with aspects of the present disclosure is shown as a method 1300. At 1310, the method 1300 includes providing at least one aluminum substrate and, at 1320, disposing one or more copper elements on the at least one aluminum substrate. In an aspect, disposing the one or more copper elements on the at least one aluminum substrate may include coupling the one or more coper elements to a bonding pad of the at least one aluminum substrate. At 1330, the method 1300 includes applying a coating to the at least one aluminum substrate and the one or more copper elements. In an aspect, the coating may be applied to the aluminum substrate prior to disposing the one or more copper elements on the at least one aluminum substrate. For example, the aluminum substrate may be coated and then one or more copper elements, such as Cu wires, may be disposed on or coupled to one or more areas of the aluminum substrate, such as bonding the Cu wires to Al bonding pads, as described above with reference to FIGs. 1-12. As described above, the coating may be configured to inhibit corrosion of the at least one aluminum substrate and the one or more copper elements. In an aspect, the coating may include at least one of fluorescein, cetyl trimethyl ammonium bromide, 3-hydroxy flavone, cerium dibutyl phosphate, 8-hydroxyquinoline, sodium benzodate, tolyltriazole, benzotriazole, amino-azole-thiol, glutaric acid, polyvinyl alcohol, thiourea, phenyl thiourea, 4-carboxy phenyl thiourea, 1,4-naphtoquinone, indole, tryptamine, tryptophane, monoethanolamine, thioacetamide, quinaldic acid, a-benzoionoxime, 2-(2-hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, cupferron, 2-butine-l,4-diol,benzamide, 4- aminobenzenesulfonamide, and thioacetamide. In an aspect, the coating may be applied using a spray coating technique. In an aspect, the coating may be applied using a chemical vapor deposition technique. It is noted that the particular techniques described herein for applying the coating have been provided for purposes of illustration, rather than by way of limitation and that other techniques for applying the coating may be utilized in accordance with the present disclosure to reduce corrosion of Cu/Al bimetallic systems.

[0058] In an aspect, the method 1300 may include subjecting the at least one aluminum substrate and the one or more copper elements to a molding process and/or an annealing process. The annealing process may be performed after the coating is applied. In an aspect, the method 1300 may include immersing the at least one aluminum substrate and the one or more copper elements in a solution for a period of time and generating observation data associated with the corrosion of the at least one aluminum substrate and the one or more copper elements over the period of time. As described above, the solution may be configured to induce corrosion of the at least one aluminum substrate and the one or more copper elements. The observation data may include data representative of an ability of the coating to inhibit the corrosion of the at least one aluminum substrate and the one or more copper elements. For example, the observation data may include images, plots, graphs, and/or other data generated using at least one of a scanning electron microscope (SEM), an energy dispersive X-ray spectroscopy (EDX) system, and a gas chromatography-mass spectrometer (GC-MS), as described above with reference to FIGs. 1A-12.

[0059] As shown above, corrosion mechanisms according to aspects of the present disclosure may provide excellent corrosion protection by blocking the cathodic hydrogen evolution. The selected inhibitor coating may be compatible at Autoclave Test conditions and remain active to prevent the Al corrosion for at least 4-8 days. Additionally, aspects of the present disclosure provide packaging-friendly application methods, such as spray coating, chemical vapor deposition and the like, that provide good thermal stability and corrosion inhibition performance. It is noted that the particular techniques disclosed herein may also be applied to industry prepared wire-bonded device samples and testing and optimizing process parameters for the application of inhibitors to the wire-bonded devices in assembly line conditions.

[0060] Although embodiments of the present application and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification.

Claims

1. A system comprising:

at least one aluminum substrate;

one or more copper elements disposed on the at least one aluminum substrate; and a coating applied to the at least one aluminum substrate and the one or more copper elements and configured to inhibit corrosion of the at least one aluminum substrate and the one or more copper elements.

2. The system of claim 1, wherein the coating comprises at least one of fluorescein, cetyl trimethyl ammonium bromide, 3 -hydroxy flavone, cerium dibutyl phosphate, 8- hydroxyquinoline, sodium benzodate, tolyltriazole, benzotriazole, amino-azole-thiol, glutaric acid, polyvinyl alcohol, thiourea, phenyl thiourea, 4-carboxy phenyl thiourea, 1,4- naphtoquinone, indole, tryptamine, tryptophane, monoethanolamine, thioacetamide, quinaldic acid, a-benzoionoxime, 2-(2-hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, cupferron, 2-butine-l,4-diol,benzamide, 4-aminobenzenesulfonamide, and thioacetamide.

3. The system of claim 1, wherein the one or more copper elements comprise at least one of a copper ball and a copper wire.

4. The system of claim 1, wherein the coating is applied to the at least one aluminum substrate and the one or more copper elements prior to annealing the at least one aluminum substrate and the one or more copper elements.

5. A method comprising:

providing at least one aluminum substrate;

disposing one or more copper elements on the at least one aluminum substrate; and applying a coating to the at least one aluminum substrate and the one or more copper elements, the coating configured to inhibit corrosion of the at least one aluminum substrate and the one or more copper elements.

6. The method of claim 5, wherein the coating is applied to the at least one aluminum substrate using a spray coating technique.

7. The method of claim 5, wherein the coating is applied to the at least one aluminum substrate using a chemical vapor deposition technique.

8. The method of claim 5, wherein the coating comprises at least one of fluorescein, cetyl trimethyl ammonium bromide, 3 -hydroxy flavone, cerium dibutyl phosphate, 8- hydroxyquinoline, sodium benzodate, tolyltriazole, benzotriazole, amino-azole-thiol, glutaric acid, polyvinyl alcohol, thiourea, phenyl thiourea, 4-carboxy phenyl thiourea, 1,4- naphtoquinone, indole, tryptamine, tryptophane, monoethanolamine, thioacetamide, quinaldic acid, a-benzoionoxime, 2-(2-hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, cupferron, 2-butine-l,4-diol,benzamide, 4-aminobenzenesulfonamide, and thioacetamide.

9. The method of claim 5, wherein the one or more copper elements comprise at least one of a copper ball and a copper wire.

10. The method of claim 5, wherein the method comprises subjecting the at least one aluminum substrate and the one or more copper elements to a molding process.

11. The method of claim 5, wherein the method comprises subjecting the at least one aluminum substrate and the one or more copper elements to an annealing process.

12. The method of claim 5, further comprising immersing the at least one aluminum substrate and the one or more copper elements in a solution for a period of time and generating observation data associated with the corrosion of the at least one aluminum substrate and the one or more copper elements over the period of time, wherein the solution is configured to induce corrosion of the at least one aluminum substrate and the one or more copper elements, and wherein the observation data comprises data representative of an ability of the coating to inhibit the corrosion of the at least one aluminum substrate and the one or more copper elements.

13. The method of claim 12, wherein the observation data is generated using at least one of a scanning electron microscope (SEM), an energy dispersive X-ray spectroscopy (EDX) system, and a gas chromatography-mass spectrometer (GC-MS).

14. A method comprising:

providing at least one aluminum substrate;

disposing one or more copper elements on the at least one aluminum substrate;

applying a coating to the at least one aluminum substrate and the one or more copper elements, the coating configured to inhibit corrosion of the at least one aluminum substrate and the one or more copper elements; immersing the at least one aluminum substrate and the one or more copper elements in a solution for a period of time, wherein the solution is configured to induce corrosion of the at least one aluminum substrate and the one or more copper elements; and

generating observation data associated with the corrosion of the at least one aluminum substrate and the one or more copper elements over the period of time, wherein the observation data comprises data representative of an ability of the coating to inhibit the corrosion of the at least one aluminum substrate and the one or more copper elements.

15. The method of claim 14, wherein the coating is applied to the at least one aluminum substrate using a spray coating technique.

16. The method of claim 14, wherein the coating is applied to the at least one aluminum substrate using a chemical vapor deposition technique.

17. The method of claim 14, wherein the coating comprises at least one of fluorescein, cetyl trimethyl ammonium bromide, 3 -hydroxy flavone, cerium dibutyl phosphate, 8-hydroxyquinoline, sodium benzodate, tolyltriazole, benzotriazole, amino-azole-thiol, glutaric acid, polyvinyl alcohol, thiourea, phenyl thiourea, 4-carboxy phenyl thiourea, 1,4- naphtoquinone, indole, tryptamine, tryptophane, monoethanolamine, thioacetamide, quinaldic acid, a-benzoionoxime, 2-(2-hydroxyphenyl) benzoxazole, dithiooxamide, cuprizone, cupferron, 2-butine-l,4-diol,benzamide, 4-aminobenzenesulfonamide, and thioacetamide.

18. The method of claim 14, wherein the one or more copper elements comprise at least one of a copper ball and a copper wire.

19. The method of claim 14, wherein the method comprises subjecting the at least one aluminum substrate and the one or more copper elements to a molding process.

20. The method of claim 14, wherein the method comprises subjecting the at least one aluminum substrate and the one or more copper elements to an annealing process prior to immersing the at least one aluminum substrate and the one or more copper elements in the solution.