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Corrosion-induced degradation of microelectronic devices

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Semicond. Sci. Technol. 11 (1996) 155–162. Printed in the UK

TOPICAL REVIEW

Corrosion-induced degradation of microelectronic devices John W Osenbach AT&T Bell Laboratories, 9999 Hamilton Blvd, Brenigsville, PA 18031-9359, USA Received 12 July 1995, accepted for publication 20 October 1995 Abstract. Corrosion of the metallization used for interconnecting the various circuit elements on a microelectronic device continues to to cause concern regarding reliability despite the fact over 500 papers have been published on this subject representing 30 years of effort by industry. Controversy still remains in a number of technologically important areas, including the functional form and fitting parameters of the acceleration functions. In this paper we briefly summarize the current understanding of corrosion-induced degradation and its effect on the reliability of microelectronic circuits

1. Basic principles A simplified version of a typical microelectronic circuit is schematically shown in figure 1. The electrical interconnections between the different circuit elements (transistors, capacitors and resistors) are made from a variety of different metals. The most widely used metal for silicon-based microelectronic circuits is thin film aluminium, usually containing 0.5 to 4% copper and 0.5 to 2% silicon. Tungsten is used as a ‘plug’ to connect silicon areas to the first level aluminium. Tungsten plugs may also be used as connections between aluminium layers at different levels in the structure. Titanium and titanium nitride are sometimes used as adhesion/barrier layers above and below the aluminium interconnection layer. Thin film gold in a trimetal combination with a base metal such as titanium as the adhesion layer, platinum or palladium as a barrier metal, and gold as the current-carrying layer is used for high-speed and high-drive current circuits. The thin film metallization is typically connected to the package leads, allowing the circuit to be connected to the outside world with a gold wire. The leads are then attached to printed circuit boards or hybrid circuit boards, typically with solder. A variety of other metals are commonly used for the interconnections on circuit boards and hybrid circuits, including silver, copper, tin and lead. In the presence of adsorbed moisture, corrosion of one or more of these metals can occur. Metal corrosion occurs as a result of chemical reactions between the metal, M, and moisture. A simple anodic oxidation, corrosion, reaction is of the form yM + xH2 O → My Ox + aH2 ↑ +bH+ + be−

(1)

where a + b = x. Anodic oxidation is accompanied by an equivalent cathodic reaction, usually including the c 1996 IOP Publishing Ltd 0268-1242/96/020155+08$19.50

Figure 1. Schematic diagram of the typical elements that are integrated to form a microelectronic device.

reduction of H2 O or O2 2H2 O + 2e− → H2 ↑ +2(OH)−

(2)

O2 + 2H2 O + 4e− → 4(OH)− .

(3)

As shown in these three equations, transfer of charge in the form of electrons is required for these reactions to proceed to the right (i.e. for corrosion to proceed). Preventing the charge transfer by minimizing current flow between the anode and cathode is therefore an important consideration in prevention of corrosion. Finally, these equations show that the creation of H+ ions leads to a more acidic (pH < 7) environment near the anode, while the creation of (OH)− leads to a more basic environment (pH > 7) near the cathode. The local pH, as is discussed next, strongly influences the corrosion behaviour of metals. The chemical reactions that are possible are limited by the thermodynamics of the system. The thermodynamically possible reactions for a metal exposed to moisture under a variety of electric potential and solution pH conditions are elegantly and conveniently portrayed in pH–potential diagrams, usually referred to as Pourbaix diagrams [1]. Figure 2 shows the Pourbaix diagrams for aluminium and copper. The regions of immunity to corrosion, the 155

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Figure 2. Pourbaix diagrams for (a ) aluminium metallization and (b ) copper metallization.

region of annodic dissolution and the region of passivity are shown on these diagrams. Thus, Pourbaix diagrams can be an invaluable source of information. It should be noted, however, that in general Pourbaix diagrams have been derived assuming that formation of complexes does not occur between the metal and various common environmental ionic contaminants such as sulphur or chlorine. Therefore, many Pourbaix diagrams do not reveal all of the important information required for predicting the susceptibility to corrosion of the interconnection metallization. Electrolytic corrosion processes in microelectronic devices proceed at a rate that is determined by the electrochemical kinetics at the corroding metal or by the rate of charge transfer between the two electrodes. Experience has indicated that if charge transport, which usually occurs in the surface-adsorbed water film on the surface of the insulator, is not the limiting factor, then corrosion of the thin film interconnect metallization occurs 156

rapidly in minutes to hours. Because we are interested in devices that last for more than ten years under normal field conditions, we do not consider the chemical kinetics of the corrosion reactions in this review. Charge transport can be limited by ensuring that substantially all of the potential drop between the anode and cathode interconnections appears as a current–resistance (I –R) drop across the surface or through the bulk of an insulator. In most cases, transport of charge through the bulk of an insulator is so slow that corrosion effects are dominated by transport on insulator surfaces. Thus, corrosion effects are most likely to be observed at openings in the protective layers overlaying the metal such as are found at defects such as pinholes or bond pad openings. The I –R drop results from the transport of small currents, surface leakage, between the electrodes [2]. For a given applied voltage and electrode spacing, the leakage current across the surface of an insulator depends on the thickness of the adsorbed water film, which is a function of relative humidity, and the concentration of ionic contamination on that surface. These effects are illustrated in figure 3. Figure 3 is a plot of the dependence of the surface leakage current on relative humidity for three different commonly used thin film insulators, silicon dioxide, silicon nitride and polyimide. Also shown is the effect that ionic contamination, in this case sodium acetate, has on the leakage current. As shown, the surface leakage current is approximately exponentially dependent upon ambient relative humidity for relative humidities above 40 to 50%. In addition, the surface leakage current is strongly dependent upon the level of ionic contamination. In this case contaminating the silicon dioxide surface with a 10−6 M solution of sodium acetate causes an increase in the leakage current at any humidity of more than three orders of magnitude. Assuming that the corrosion process is 100% efficient and that the efficiency is not affected by the ionic contaminant (we know this is not the case), then the expected lifetime (i.e. the inverse of the corrosion rate) of microelectronic device will decrease substantially with increasing humidity and level of ionic contamination (i.e. device lifetime is proportional to 1/surface leakage current). In addition to having a significant influence on the surface leakage current, ionic contamination can prevent the formation of a bond between the polymer encapsulant and the component surface. If a contaminant is on the surface of the component prior to encapsulation, then the encapsulant can form a bond with it preventing the formation of a bond between the component surface and the encapsulant. This may result in an interface leakage path. Ionic contaminants can also destroy the protective oxide film that forms on the positively biased metallization and/or lead to a change in the local pH. Ionic contaminants can also form soluble complexes with metals resulting in dendrite formation. Therefore, ionic contamination can have a profound influence on the susceptibility of thin film metallization to corrosion [3–5]. 2. Influence of packaging There are two fundamental microelectronics packaging technologies; hermetic packages, that in principle are not


Figure 4. Schematic diagram showing the different parts of a polymer-encapsulated microelectronic device.

Figure 3. Surface leakage current versus relative humidity for metallized dielectric films of silicon dioxide, silicon nitride, polyimide and silicon dioxide, the surface of which has been intentionally contaminated with a 10−6 M solution of sodium acetate in water.

influenced by the environment external to the package, and non-hermetic packages that are by design influenced by the external environment. Because the environment inside a hermetic package is controlled by the manufacturing process, corrosion-related failure mechanisms of components in such packages are usually not taken into consideration in reliability evaluations. Unfortunately, many current hermetic package specifications may not be sufficient to ensure that hermetically packaged devices are immune from corrosion [6–8]. This is especially true for those applications where the environmental conditions are demanding, such as the tropical regions of the world. For example, Sinnadurai et al [8] have reported that more than 20% of the field failures found in telecommunications equipment installed in the tropical regions of India were a result of interconnect corrosion. These failures were not a result of the hermetic seal failing such that the leak rate of the hermetic package increased to an unacceptably high rate, but rather they occurred as a result of the fact that even when the leak rate into the package is at or below the hermetic package specification level, it can take less than one year for the moisture content in the package to increase to levels at which corrosion can occur [6]. Since additional corrosion protection such as an organic coating is not usually provided in most hermetically packaged components, corrosion will occur rapidly when sufficient moisture enters the package. Therefore, it is essential to carefully consider the leak rate specification for a specific hermetic package before deploying it in environmentally demanding regions of the world.

Figure 4 depicts a microelectronic device in a simple non-hermetic package. Typically, the microelectronic circuit is bonded to the lead frame with conductive epoxy. The connections to the lead frame are made with gold wire. The entire assembly is encapsulated with a polymer, usually epoxy, phenolic or silicone. All polymeric materials that are used to protect microelectronic devices are permeable to moisture. Since the interface between the lead frame and the polymer encapsulant is typically mechanical and not chemical in nature, moisture can also enter the package by that route. The best polymers available for electronic applications, polyimides and Novolac epoxies, typically absorb 0.1 to 3% by weight of moisture. Although the equilibrium saturation level is typically attained in days to weeks, if adsorption of a few monolayers of moisture on the active surfaces of the microelectronic circuit can be prevented, then corrosion can be avoided for much longer. If, however, there are interfacial gaps between the polymer encapsulant and the active surface of the microelectronic circuit, then corrosion is possible. Because many materials, including polymers, do not adhere well to gold, gaps can form near the bond between the gold wire and the circuit thin film metallization. If the thin film metallization is different from gold (i.e. dissimilar metals are in contact), then a built-in potential develops across the gold wire–thin film metal interface. Therefore, even in the absence of an externally applied potential, electrolytic corrosion can occur if sufficient surface leakage (i.e. corrosion current resulting from ionic motion) is present. Similar effects can occur for other dissimilar metal joints. Dissimilar metal interfaces can also result from the precipitation of an alloying element in a particular thin film metallization system. For example, small concentrations of Al2 Cu precipitates form at grain boundaries in thin film copper-doped aluminium films. Because there is a builtin potential between these precipitates and the surrounding metal grains, corrosion can occur at this location of the metal interconnection. All of these specific mechanisms related to dissimilar metal corrosion can lead to device 157

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failure [9]. Dissimilar metal corrosion is usually called galvanic corrosion. Semiconductor junctions are another source of builtin potential. This potential across the junction will be transmitted to any metal interconnects that contact the junction. Because of this, corrosion of integrated circuit metallization in the absence of metal precipitates (as described above) can and sometimes does occur while the circuits are being fabricated. It is possible for corrosion of this type to cause device failure even before fabrication is completed. In addition, it is possible for a small amount of corrosion to occur, typically called pitting corrosion, leading to a reduction in the cross-sectional area of the interconnection line. Since current-driven electromigration of aluminium atoms within the interconnection line scales as current density squared, pitting corrosion can lead to premature electromigration induced field failure [10]. The illumination of a junction during device fabrication can increase the potential on the metal connected to the junction. As a result of thermal expansion coefficient differences between stiff polymer encapsulants such as epoxy and the microelectronic component, large stresses can develop after encapsulation [11]. These stresses can cause the thin film dielectric used to protect the metallization of many microelectronic components to crack, exposing the metallization to surface and ambient contaminants. Openings, intentional or unintentional, in the thin film dielectric generally increase the corrosion susceptibility of the metallization [9]. Figure 5 is an example of this effect for silicon nitride (SiN:H) passivated aluminium metallized devices. The two test structures were made at the same time with the same process. In one set of devices holes were intentionally placed in the SiN:H passivation, but not in the other set of devices. Both device types were encapsulated with a silicone-based polymer prior to ageing. The failure distributions of two different test structures versus ageing time at 85 ◦ C/85% relative humidity at an electric field of 30 V µm−1 is shown in figure 5. As shown, the defects increased the susceptibility of the devices to corrosioninduced failure by a factor of approximately five. This effect results from a combination of factors, including but not limited to contamination, interfacial delamination and possible condensation in capillaries. The type and concentration of ionic contamination in the polymeric packaging material also influences the corrosion of device metallization [12]. Ionic contaminants can be leached out of the packaging material by atmospheric moisture at elevated humidity and be deposited onto the surface of the microelectronic device. This ionic contamination can then act to accelerate corrosion in a similar manner to that of surface contamination. Therefore it is important to control the level of ionic contamination (particularly free halides) in the polymer to < 5 ppm, to minimize the risk of corrosion-induced failure. In addition, bromine-containing fire retardants that are added to many polymer packaging materials can decompose at high temperatures and influence the corrosion susceptibility of the device to corrosion-induced failure. Finally, atmospheric pollutants, such as SO2 and NOx , can enter 158

Figure 5. Plot of the time to fail versus cumulative per cent failure for devices passivated with silicon nitride and aged at 85 ◦ C and 85% relative humidity. This figure shows the effects of intentionally placing holes in the passivation above the metallization on the time dependence of failure. This shows how passivation defects influence device reliability.

the package by the same mechanisms that allow moisture to enter. These pollutants can increase the susceptibility of the device to corrosion. 3. Failure mechanisms Corrosion can occur when there is water adsorption or condensation on the surface of the microelectronic device. Thus the loss or absence of polymer adhesion is usually a prerequisite for failure. Delamination can occur as a result of ionic contamination, encapsulationinduced insulator cracking, severe topology such that the viscous polymer encapsulant completely covers the exposed surfaces, and long-term degradation of the polymer– microelectronic interface as a result of the exposure to temperature and humidity and bias. Once delamination occurs, moisture can adsorb or condense onto the surface of the microelectronic device. The presence of water and electrical bias is sufficient to initiate electrolytic corrosion. Figure 6 schematically represents the surface of a microelectronic component where delamination has occurred. The delamination creates surface leakage paths in an area of the device where two metal interconnection lines reside at two different potentials. The surface leakage current leads to metal oxidation at the anode in the form of M+ ions and to reduction of water at the cathode in the form of hydroxyl ions (OH)− . The metal ions can oxidize near the anode and form a passive oxide film that protects the underlying metal from future corrosion [3–5]. However, if the pH is low or halide ionic contaminants are present then this passive oxide can break down, leading


Figure 6. (a ) Schematic diagram of a polymer-encapsulated microelectronic device where the polymer/passivation interface has delaminated. As shown, water adsorption and possible condensation can occur if there is delamination. (b ) Schematic diagram of a variety of the most common corrosion-induced reactions that can lead to device failure. Cathode reactions: (i) Reduction of water-forming [OH]− ions that can migrate towards the anode. (ii) Chemically formed metal oxides and/or hydroxides followed by dissolution of these species. This would eventually lead to erosion of the cathode causing an open circuit failure. (iii) Reduction of dissolved metal ions leading to nucleation and growth of metal dendrites. This would eventually lead to the formation of an anode–cathode short failure. (iv) Reduction of dissolved metal ion complexes leading to nucleation and growth of metal dendrites. This would eventually lead to the formation of an anode/cathode short failure. When reduced, the complexing ion will be released, migrated back toward the anode where it can react with the metal and form additional metal complexes. Anode reactions: (i) Metal oxidation creating metal ions that migrate towards the cathode. When the metal ions reach the cathode dendritic growth can occur. The oxidation can eventually lead to an open circuit failure. (ii) Metal oxidation and passive oxide growth protects the metal from additional corrosion. (iii) Halide ion breakdown of the passive oxide creating a freshly exposed metal surface that can now oxidize. As this process continues, an open circuit failure will occur. (iv) Metal oxidation creating metal ion complexes that migrate towards the cathode. When the metal ions reach the cathode dendritic growth can occur. The oxidation can eventually lead to an open circuit failure.

to further corrosion. The device will continue to function until the corrosion process consumes a sufficient amount of metal to cause an open circuit. If oxidation of the metal ion formed at the anode does not occur, then the metal ion can migrate towards the cathode and form a soluble salt. An open circuit will occur when sufficient metal has been corroded to cause a break in the line. Similar effects related to the formation of hydrolyzed metal ions can also lead to device failure.

The negatively biased interconnection line (i.e. the cathode) is also susceptible to corrosion. This effect is usually referred to as cathodic corrosion. The reduction of water at the cathode increases the local pH of the solution. At high pH, many metals and their oxides and hydroxides chemically react with (OH)- to form soluble salts. Failure occurs as a result of an open circuit. Alkaline contaminants lead to a higher solution pH. They also tend to migrate to the cathode. Therefore, they increase the susceptibility of 159

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the metal to cathodic corrosion induced failure. Another type of failure involves the formation of metal bridges between two metal lines at different potentials. Typically this type of failure mechanism is associated with noble metals, such as silver and gold, although in principle it could happen in less noble metal systems especially if complexing ions are present. In the presence of a complexing contaminant, such as chloride residues from the package encapsulation polymer, soluble gold complexes form at the anode. These complexes diffuse from the anode to the cathode. At the cathode, the gold complex is reduced to the metallic state and precipitates out of solution, liberating the complexing ion. The ion will then, under the influence of the electric field, migrate toward the anode. When it reaches the anode it can participate once again in complex formation. When the gold ion precipitates out at the cathode, there is a small but real decrease in the line to line spacing, and therefore the local electric field which drives the process increases there. Thus the next gold complex that forms will be more likely to migrate to this location than to some random position along the cathode. As this process continues, a dendrite forms. When the dendrite bridges the gap between the two metal lines, a short circuit is formed leading to a permanent failure or temporary device malfunction. The temporary malfunction process involves the melting of the dendrite by the short circuit current, effectively removing the short. Dendritric type shorts can also occur when metal ions or complexes form, followed by metal precipitation at locations of specific solution pH. An example of this is the formation of cathodic anodic filaments (CAF) in the copper system [13]. In this system, copper ions are created at the anode, which is at a low pH, and migrate in the field toward the cathode, which is at a high pH. As the ions migrate across the gap, they traverse regions of increasing solution pH. At a pH greater than 6, the ions precipitate out of solution as metallic copper. This process continues, leading to a local increase in copper precipitates. These copper precipitates can diffuse toward the cathode, under the influence of a concentration gradient. This process continues until the gap between the precipitate nucleation position and the cathode is bridged. At this time the cathode potential appears at the initial copper nucleation site. The pH at this site then increases due to the reduction of H2 O. Copper ions that dissolved at the anode then begin to precipitate at a point in the new narrowed gap. The process continues until the gap is totally bridged with copper, and the device failure occurs due to a short circuit. This failure mechanism has been observed on printed circuit boards. Corrosion-induced metal loss can act to initiate other apparently unrelated failure mechanisms. A failure seemingly related to a melted dielectric was traced back to gold dendrite formation [14]. In this case, water was adsorbed on and in the silicon-doped silicon dioxide insulator. Under bias, the adsorbed moisture and chlorine contamination led to the formation of gold dendrites. As the dendrites spanned the insulator surface, the high field at the leading edges of the dendrite caused local heating. When the local temperature exceeded 360 ◦ C, the gold dendrite reacted with the excess silicon in the insulator. This caused 160

local melting of the insulator and eventual device failure. Another possible corrosion-initiated failure is related to decreases in line width. The loss of interconnection crosssectional area leads to an increased local current density. Since the electromigration resistance (i.e. current-induced metal diffusion) decreases as the current density squared, it is possible that partial corrosion of a metal line will eventually lead to electromigration-induced device failure. Other failure mechanisms have been related to partial dendrite growth that effectively decrease the line spacing and increases surface leakage. The increased leakage could cause the device to fail. Finally, local corrosion of small sections of the metallization can initiate arcing that leads to device failure [15]. Finally, we review two possible modes of failure due to corrosion that are related to corrosion of the dielectric protective films [15]. Interfacial voids or delamination, present as a result of topology or local ionic contamination, can be nucleation sites where capillary condensation can occur. This leads to device failure due to chemical reactions between the condensed moisture and the dielectric. Figure 7 shows an example of this effect. Condensation of water occurs at a void between the dielectric and the polymer encapsulant. Silicon-based dielectrics, such as silicon nitride, silicon dioxide and alloys of the two, are hydrolyzed when exposed to moisture. These hydrolysed reaction products dissolve slowly in condensed water. Thus if the solubility limit of the dissolved silicon-based reaction product is not reached before the dielectric is breached, then water can come in contact with the metallization and/or any underlying dielectric thin films such as phosphorus-doped silica . In such a way the susceptibility of the device to corrosion-induced failure is increased. It is also possible for moisture-induced electrochemical degradation of the dielectric to occur [16]. If the current flowing through the dielectric is large enough, then the dielectric can corrode, changing its physical size and properties. This could lead to device failure. Finally, moisture can interact with phosphoruscontaining passivations and form phosphoric acid which significantly increases the susceptibility of aluminium metallization to corrosion [9]. 4. Acceleration models Estimates of microelectronic device reliability in the field are usually based on accelerated aging studies where the time for device failure is on the order of days or months as opposed to years in the field. This accelerated aging strategy is used because failure mechanisms are usually not reversible and because devices are usually expected to last at least 10 years in the field. Accelerated aging strategies begin with two fundamental assumptions: (i) All data that define a failure can be measured at some highly accelerated stress condition, where degradation occurs in days or months, and then extrapolated to the field stress condition, where degradation occurs after years. (ii) The devices that are subjected to accelerated aging are representative of the entire population.


Figure 7. Schematic diagram of a failure due to water condensation, via capillarity, and its effect on device stability: (i) Water condensation on the passivation of the device in those areas where a gap or void exists between the encapsulant and the passivation surface. (ii) Formation of passivation hydroxides. (iii) Dissolution of the passivation hydroxides exposing a ‘new’ passivation surface. (iv) Formation of passivation hydroxides on the ‘new’ passivation surface. (v) Continuation of the process until the passivation is breached. (vi) If the breach occurs such that metals such as aluminium or copper are exposed to the basic condensed water, then the metal will corrode as depicted in the Pourbaix diagrams in figure 2.

The validity of assumption (i) for humidity-induced failure is somewhat questionable when data taken at high humidities are extrapolated to low humidities. This is especially true if surface leakage controls the rate of corrosion. As a case in point, let us consider the data shown in figure 3. At relative humidities below approximately 40%, within experimental error, the surface leakage current does not appear to depend on relative humidity. However, above 50% relative humidity, the leakage current increases approximately exponentially with relative humidity. In this case if corrosion were occurring as a result of the surface leakage current, then below 40% relative humidity the corrosion rate would be invariant with humidity. However, above 50% relative humidity, the corrosion rate would increase exponentially with humidity. Unfortunately, the rate of corrosion is sufficiently low below 40% relative humidity, so that lifetime measurements cannot be made in real time. Therefore, the validity of assumption (i) has not be verified. In addition, there are a rather large number of potential failure mechanisms that can lead to device failure. It is possible that different mechanisms are operating at different relative humidities. In spite of these problems, reliability projections based upon the highly accelerated stress conditions have in fact provided reasonable field reliability predictions, especially if the device operates above 40% relative humidity.

The goal of the accelerated aging strategy is to determine the dependence of lifetime of the the device on environmental factors such as temperature and humidity. The functional dependences are referred to as the acceleration functions. Typically, acceleration functions are determined empirically. Randomly selected devices are subjected to at least two different stress conditions. The usual acceleration stresses are temperature, humidity, bias or current. The magnitude of the individual stress is chosen so that failure occurs relatively quickly. The devices are aged at the two stresses, and the time for device failure is determined. The ratio of the time it takes for device failure at stress condition 1 to that at stress condition 2 is the acceleration factor. The acceleration functions can be determined by subjecting similar devices to at least two different settings of all important accelerants. Formally, the kinetics of wear-out of the device due to corrosion can be written in terms of an Eyring type reaction rate for the particular chemical reaction. The environmental driving forces for the reaction can be taken into account thermodynamically. Note that there may be multiple chemical reactions involved, each of which has its own unique time dependence. Assuming no interaction between temperature T , relative humidity RH , current I and bias V , the Eyring type rate equation describing the time to fail, tf , can be written as tf = 1/reaction rate ∝ 1/f (T )f (RH )f (I )f (V ).

(4)

The object of the accelerated aging strategy is to determine explicitly f (T ), f (RH ), f (I ), and f (V ). It is generally found that corrosive aging mechanisms depend on temperature through an Arrhenius relationship tf ∝ exp(E/kT )

(5)

where E is the activation energy and k is Boltzmann’s constant. For aluminium metallization, E typically is reported to fall between 0.6 and 1.0 eV [17]. The functional dependence of the device lifetime on humidity in non-hermetic silicon-based devices is somewhat controversial [17]. This controversy is most likely a result of the fact that the range of humidities used in the accelerated aging tests is rather small, 50 to 90%. In addition, it is possible that more than one mechanism controls the degradation in this humidity range. Also, specific experiments may have involved contaminated materials that affected the reaction type or rate. Because of these difficulties, there are at least four functions which have been used to fit the empirical data tf ∝ exp(B/RH )

(6)

tf ∝ exp(−B RH )

(7)

tf ∝ exp[−B(RH )2 ]

(8)

tf ∝ RH

−B

(9)

where B is the fitting parameter. The function that best fits the available data continues to be a source of discussion. However, equation (8) has been shown to fit data taken from 161

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silicon integrated circuits, printed circuit boards and InPbased optoelectronic devices [18]. Furthermore, the fitting parameter B for all three technologies falls between 4 and 5 × 10−4 %−1 [2], indicating the utility of this function. Finally, we discuss the effects of device drive current on corrosion susceptibility. If the rate-limiting step for degradation is the supply of current, an increase in current will increase the degradation rate. However, increased currents also cause increases in the local temperature (Tr ). The increase in local temperature leads to a decrease in relative humidity at the device through %RHdevice = %RHenvir × exp{−5235[1/(Tr − Tenvir ) − 1/Tenvir ]}

(10)

Since the device lifetime is strongly influenced by the local humidity (equations (6)–(9)), an increase in drive current can and often does lead to a reduction in the degradation rate. Because of this, it is difficult to assess the stability of high-power microelectronics in humid environments. 5. Conclusion We have reviewed corrosion-induced failure in microelectronic devices, including a review of the basics of corrosion and a review of packaging influences. The general failure mechanisms responsible for corrosion-induced failure, including those related to dielectric degradation, were given. Finally, the acceleration factors were described, including the combined and sometimes conflicting effects of local temperature rises on local humidity. Acknowledgment I thank R B Comizzoli for the insight and knowledge I have gained as a result of many discussions on corrosion in microelectronics and also for critically reading this manuscript. References [1] Pourbiax M 1966 Atlas of Electrochemical Equilibria in Aqueous Solutions (New York: Pergamon) [2] Comizzoli R B, Frankenthal R P, Milner P C and Sinclai J D 1986 Corrosion of electronic materials and devices Science 234 340.

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[3] Iannuzzi M 1981 Development and evolution of a pre-encapsulation cleaning process to improve reliability of HIC’s with aluminium metallized chips 19th Ann. Proc. Reliability Physics (New York: IEEE) p 228 [4] Iannuzzi M 1983 Bias humidity performance and failure mechanisms of non-hermetic SIC’s in an environment contaminated with Cl2 IEEE Trans. Components Hybrids Manuf. Technol. 2 191 [5] Paulson W M and Lorigan R P 1976 The effect of impurities on corrosion of aluminium metallization 14th Ann. Proc. Reliability Phys. (New York: IEEE) p 112 [6] Gordan Davy J 1975 Calculations for leak rates of hermetic packages IEEE Trans. Parts Hybrids Packaging 11 177 [7] Stroehle D 1977 On the penetration of gases and water vapour Int. Reliability Physics Symp. (New York: IEEE) p 101 [8] Sinnadurai N, Kuppuswamy T S, Chandramouli R, and Rao B K N 1992 Environmental testing component reliability observations of telecommunications equipment operated in tropical climatic conditions Quality Reliab. Eng. Int. 8 189 [9] Schnable G L, Comizzoli B L, Kern W, and White L K 1979 A survey of corrosion failure mechanisms in microelectronic devices RCA Rev. 40 416 [10] d’Heurle F M and Ho P S 1978 Thin Films Interdiffusion and Reactions ed J M Poate, K N Tu and J W Mayer (New York: Wiley) p 243 [11] Inayoshi H, Nishi K, Okikawa S, and Washima Y 1987 Moisture-induced aluminium corrosion and stress on the chip in plastic-encapsulated LSI’s Int. Reliability Physics Symp. (New York: IEEE) p 113 [12] Feinstein L C 1979 Failure mechanisms in molded microelectronic packages Semicond. Int. 51 [13] Lando D J, Mitchell J P, and Welsher T L 1979 Conductive anodic filaments in reinforced polymeric dielectrics: formation and prevention Int. Reliability Physics Symp. p 51 [14] Osenbach J W 1992 Corrosion-induced SIPOS reaction Mater. Lett. 13 80 [15] Osenbach J W and Zell J L 1993 Corrosion of thin film aluminium metallization: conformal coating materials IEEE Trans. Components Hybrids Manuf. Technol. 16 350 [16] Osenbach J W 1993 Water-induced corrosion of materials used for semiconductor passivation J. Electrochem. Soc. 140 3667 [17] Hallberge O and Peck D S 1990 Recent accelerations, a base for testing standards Quality Reliab. Eng. Int. 7 169 [18] Sutherland R R, Stokol J C D, Skrimshire C P, MacDonald B M, and Sloan N F 1989 The reliability of planar InGaAs/InP PIN photodiodes with organic coatings for use in low cost receivers Proc. SPIE 1774 226