Advanced packaging is inseparable from it: die bonding technology analysis

Bonding process technology is an important technical method in semiconductor packaging, and the failure of bonding system also directly affects the interconnection reliability of electronic components. Although they are all failures in the bonding region, the failure mechanisms are very different. The common failure modes based on material properties and processes in three bonding systems, Au-Al, Cu-Al, and Al-Al, are discussed. Combined with the relevant practical cases, the failure mechanisms of bonding process cracking, bimetallic bond degradation, contact corrosion and bond wire degradation of power devices were studied and analyzed by physical and chemical analysis methods such as scanning electron microscopy (SEM), X-ray energy dispersive dispersive (EDX) and ion grinding (CP). Through the listed detection methods, the failure mode of the device can be accurately identified, and the corresponding improvement strategies can be proposed, which provides guidance for improving the reliability of the bonding system. At the same time, the bonding degradation of power devices can be observed and identified through power cycling tests.

Among the failures related to the packaging of microelectronic devices and the failures that occur during use, the failures related to the bonding system account for a considerable proportion. The failure of the bonding system is not only due to the method of the process, but also to the influence of materials, packaging defects, process parameters, contamination, application scenarios, external mechanical stresses and electrical stresses. Therefore, it is important to correctly identify the failure modes and use appropriate methods to analyze them to confirm the causes and mechanisms of failures. By accurately identifying failure modes and taking the right approach to improve their root causes, it saves time and costs for improving product reliability.

01

Bonding process basics and bonding systems

1.1 Bonding process

The process of connecting the electrode pads on the surface of a chip to a substrate or lead frame using a metal wire is called bonding. Taking a typical gold wire bonding process as an example, the process steps mainly include:

a) Molten the tail wire by discharging the lighter;

b) Exercise the melt ball crimp in the bonding area under the capillary knife, applying ultrasonic power;

c) After the spherical bonding point (usually called the first bonding point) is formed, the upward capillary with the bonding wire travels to the corresponding area of the lead frame in a certain arc;

d) The downward crimp is applied to the corresponding bonding area on the lead frame, the ultrasonic power is applied, the action time is over, the wedge bonding point is cut off, the gold wire is cut to form a wedge-shaped bonding point (commonly referred to as the second bonding point), and the upward capillary retains the exposed tail wire. Repeat the above steps to complete the bonding process in the chip package.

The bonding process steps of copper wire are basically the same as those of gold wire, and it should be noted that nitrogen and hydrogen mixed gas needs to be injected into the copper process to reduce the risk of oxidation. The aluminum wire is generally carried out at room temperature by ultrasonic bonding method, without additional heating, and ultrasonic power is applied to the capillary to break the oxide layer on the surface of the ring bonding surface while loading, so that the metal is in close contact. The capillary used for aluminum wire is different from gold wire and copper wire, but the steps are similar.

Tables 1~2 are the requirements for shear and bond tension at bond points in standard AEC-Q100 and MIL-STD 883, respectively. For example, the shear force of a 3 mil diameter gold ball is generally required to be greater than 30.8 g, and the tensile force of a 3 mil gold wire is greater than 15 g.

1.2 Characteristics of each wire material

As an ideal bonding lead material, it should have the characteristics of low contact resistance, good electrical conductivity, high chemical stability, good mechanical properties and stable shape. Gold wire has good chemical stability and strong tensile performance, and the process of gold wire bonding is relatively complete and mature, while the disadvantage of gold wire as bonding is that its intermetallic compound (IMC) is easy to overgenerate and reduces its mechanical strength.

Copper bonding is often used in modern consumer products to reduce costs. Generally speaking, the hardness of copper wire is large, and greater bonding pressure and ultrasonic power are required for bonding, and the shear and tensile force of copper bonding is slightly higher than that of gold wire for bonding points of the same size. But at the same time, the application of large stress parameters will also increase the risk of cracking in the chip itself. For devices or modules that require high currents, Cu bonding has higher fatigue resistance and can be tested for longer power cycles, which can ensure a longer product life.

For power devices, bonding leads or connections through high currents is most commonly done with aluminum. Typically, IGBT power modules use Al wires with a diameter of 300 μm or more as a connection, corresponding to a transmission current of more than 18 A. Ordinary metal wires will form a similar effect to a fuse under the limit current, causing a similar arcing phenomenon to cause serious system failure, reasonably increase the diameter and number of bonding wires to optimize the contact requirements at the top of the chip, in order to ensure safety, so it is not difficult to find that the wire diameter of the bonding aluminum wire is usually thicker, and the power device will also choose multiple bonding wires or even aluminum to carry away the high current. In addition, Al-coated Cu wire is also suitable for power devices, which aims to combine the superior electrical properties of copper wire with the bonding process of aluminum wire to improve the coefficient of thermal expansion (CTE) of the assembled connection, reduce the resistivity, and improve the thermal conductivity.

1.3 Bonding system

In a complete bonding system, in addition to bonding leads, there is another important contact surface to consider – the pad. Conventional bonding systems typically include: gold-to-gold, aluminum-aluminum, gold-aluminum, and copper-aluminum.

The schematic diagram of the chip pad profile structure is shown in Figure 1, and the top-down structure mainly includes:

a) Passivation layer

It is mainly used to protect the surrounding pad area, and the middle part is opened through a window for bonding;

b) Top layer metal

Aluminum metal is commonly used for direct bonding;

c) Through-holes

It is used to connect the top metal with the bottom metal;

d) Dielectrics

Fill the area between the upper and lower layers of interconnected metals except for the through holes;

5) Sublayer metal

The key factors that can be adjusted in the structure of the PAD are the thickness of the top layer of metal and the arrangement of the through-hole array, with a thicker topmetal providing better stress relief and a high-density through-hole array reducing the risk of dielectric cracking.

02 Mechanism study of different failure modes in bonding system and detection methods

2.1 Cracking of the bonding process

Bond cracks, also known as "craters", are most likely to occur in Cu-Al bonding systems. The gold bonding process was developed relatively early, and the process is relatively mature, but with the expansion of scale and the intensification of market competition, cost reduction has become the focus of attention in the packaging industry. Copper bonding stands out due to its low cost advantage and superior physical and electrical properties, and the process platform is not much different from gold bonding, so it is easy to be transplanted, and it has gradually become a trend in the consumer market bonding process. However, due to the improvement of the parameters of the bonding process (contact, pre-bonding and bonding), this method does not match the characteristic parameters of other materials, resulting in bond cracking.

The detection of bond cracking can generally be observed by chemical methods after opening with light or electron microscopy, using highly corrosive acids (sulfuric acid and fuming nitric acid) to remove the molding compound and expose the chip bonding area, especially when the bonding power is too high, delamination or even cracking can be directly seen on the surface of the PAD, as shown in Figure 2. In cases where adequate observation is not possible, it is possible to observe the PAD region after delamination (debonding point and top layer Al) treatment with strong acids.

There are several key influencing factors in the whole copper bonding process, if the bonding pressure is high enough, the copper ball will be fully deformed at this stage, and then the ultrasonic power during the bonding process can be evenly applied to the contact interface to form a good bond. However, if a large amount of ultrasonic energy has already begun to be applied to the copper ball without sufficient deformation, it will be directly applied to the copper-aluminum interface, which will cause damage to the dielectric layer below and then cause cracks.

In addition to adjusting the process parameters of the process, there is also the option of structurally optimizing the copper-aluminum bonding system. The risk of cracking can also be significantly reduced by increasing the thickness of the aluminium layer. The top layer of aluminum on the PAD has good ductility, and increasing the thickness of the metal can effectively buffer the force of the dielectric. Alternatively, by changing the through-hole array (commonly tungsten through-hole), the density of the through-hole array can be increased, the strength of the dielectric layer can be increased, and the cracking condition can be improved.

2.2 Degradation of bimetallic intermetallic bonds

In the bonding system, excessive diffusion between the two different metals will lead to a large accumulation of fragile intermetallic compounds, which will reduce the bonding strength of the interface, increase the contact resistance, and cause product failure.

Due to the different chemical potentials of Au-Al bonding, a variety of intermetallic compounds can be produced at high temperatures, and five intermetallic compounds are easily formed at conditions higher than 200 °C, namely: Au2Al, AuAl, AuAl2, Au4Al and Au5Al2, among which Au5Al2 is the main one. Due to the different lattice constants and CTEs of various IMCs, coupled with the inconsistent volume fraction caused by the formation stage, the different formation and applicable environmental conditions make the differences greater. After high temperature or long-term use, components are prone to brittle bonding surfaces and decreased bond strength, which will seriously cause failure modes such as increased power consumption and even open circuits. The phenomenon of "purple spots" or "white spots", i.e. the color of AuAl2 or Au2Al, can be observed by light microscopy after chemical or mechanical opening. Slice the bond point to see the thickness change of the IMC.

Another failure mode related to Au-Al is the Kirkendal effect, which mainly occurs on the gold surface, because the diffusion rate of gold is faster than that of aluminum at high temperatures, and the rapid diffusion of gold to aluminum produces a large amount of Au2Al, and produces fine voids and cracks on the gold surface, which gradually accumulate and expand in subsequent applications, resulting in eventual shedding. Korkendal voids rarely occur at the Cu-Al interface, largely because of the relatively slow generation of intermetallic compounds of copper and aluminum. Also at 200 °C, it takes less than 0.3 s for Au-Al to form a 100 A alloy layer, while Cu-Al takes about 20 s.

For Cu-Al bonding systems, metal-to-metal phases diffuse to form intermetallic compounds, namely CuAl2, CuAl, and Cu9Al4, at high temperatures. In general, the IMC at the copper-aluminum interface will continue to thicken with continuous use and time, which will reduce the bonding strength between metals, increase the brittleness of the bonding system, and eventually increase the contact resistance or even open circuit. In the reliability test of components, the high-temperature storage test (HTSL) is generally used to accelerate the growth of IMC to evaluate the reliability and service life of the device.

Yang Jianwei et al. observed the changes of IMC growth with time (as shown in Fig. 4) of bonding wires composed of bonding wires of different materials, and it can be seen that the thickness of IMC increased significantly after 500 h of high temperature test, and reached nearly 3.5 μm after 1 000 h. In addition, the doping of palladium in copper wires can effectively inhibit the growth of IMC.

2.3 Contact corrosion

When different kinds of metals make electrical contact in an electrolyte or electrolyte-like environment, an electric current (electron transport) is generated due to the potential difference, and a working mechanism similar to that of a galvanic cell emerges. The low-potential metal is the cathode, and the high-potential metal is the anode, and the anode metal is gradually depleted and corroded. Generally speaking, the greater the potential difference, the greater the probability of contact corrosion, and the more severe the corrosion phenomenon. In the case of a Cu-Al bonding system, copper is the cathode and aluminum is the anode in the copper-aluminum contact, and the cathode aluminum is slowly depleted in the process and cracks occur at the copper-aluminum interface

The failure mode of contact corrosion is mostly manifested as abnormal contact resistance or even open circuit at the bonding point, and this failure mechanism generally does not directly cause the risk of leakage or short circuit. Observed with chemical opening, the bond points can be easily peeled off, and Figure 5 shows that the copper bond points are peeled off, leaving the aluminum metal around the bond point and the dielectric layer underneath the partially exposed. Different from the above-mentioned cracking phenomenon in the bonding process, contact corrosion does not cause physical damage such as cracks to the dielectric layer below the aluminum layer, so it can be distinguished by delayering the bonding area and observing whether there is a crack morphology in the dielectric layer below. In order to better observe the IMC and corrosion morphology, the bond can be ionized and then observed under scanning electron microscopy (SEM), and the copper bond is obviously cracked in Figure 6, and there is obvious corrosion at the surrounding aluminum pad connection. Halogens can also be detected at severely corroded bond points with EDX.

Why is contact corrosion more likely to occur between Cu-Al, which has a relatively small potential difference, when the electrode potential of the metal Au is +1.498 V, the electrode potential of Cu is +0.337 V, and the electrode potential of Al is -1.662 V? As mentioned above, the reaction speed between Au-Al is faster, which will directly lead to a thicker IMC, while in the mechanism of contact corrosion, the thick IMC acts as a better buffer to smooth the potential difference, on the contrary, there is a lack of such an effective buffer between Cu-Al. In addition, because Cu is more prone to oxidation reaction in a humid environment than Au, the resulting Cu2+ robs the electrons of aluminum to undergo a reduction reaction, which eventually leads to the oxidation of Al, which causes cracks at the Cu interface and corrosion and depletion of Al metal.

The probability of bond point contact corrosion can be reduced by: first, the reliability of the bond point itself can be enhanced by increasing the bond temperature to increase the IMC thickness of Cu-Al, for example; Secondly, encapsulation materials with low water absorption and lower halogen content can be selected to reduce the possibility of oxidation and corrosion reactions. Thirdly, contact corrosion can be better prevented by employing palladium-plated copper wire (PCC).

2.4 Degradation of bonding wires in power devices

The degradation of bonding wires in power devices is often difficult to observe individually, because there is no significant structural failure in the stage of parametric degradation, and this aging is a combination of degeneration of various materials and geometries within the device. Once the bonding wire is disconnected and disconnected, due to the loading of high current, the separation interface will instantaneously produce arcing and ablation or even damage to the device, and also destroy the original topography characteristics.

Reliability testing is commonly used to monitor and detect device degradation. Reliability verification for some of the bond failures described above is achieved by simulating environmental changes through experiments such as temperature cycling or temperature shock, which is equivalent to applying an external environmental stress to the component. Of course, in some cases, the device will also be energized, but because the test is mainly to simulate the ambient temperature change, the current and voltage have little influence on it, and the main purpose is to stimulate the stress excitation and change caused by the difference in thermal expansion coefficient and cyclic changes at the interface of various materials. For the reliability test for power devices described in this section, it is also necessary to select the power cycle test, which is to actively heat the self-generated consumption of the device with a certain current and then passively cool down after power-off, so that the device junction temperature change (ΔTj) is kept at a constant value (usually 100, 125 or 150 °C) in each cycle. During each temperature fluctuation, the temperature gradient in different directions between the differences in the CTEs of different materials and the geometry of the device itself creates stresses that cause fatigue of the material and its connections. Under normal rules, the thermal resistance of the device begins to increase slowly after a certain cycle period, and the electrical parameter VCE begins to increase gradually after a longer period of time (generally greater than 5 000 cycles), which usually indicates that the bonding wire has been degraded, and cracking or even detachment of the bonding wire will occur if the test continues. The degradation of the bond point will also cause a chain reaction, and the contact resistance of the bond point will gradually increase in the process of continuous degradation, which will increase the power consumption of the device and the temperature will continue to rise, and will also cause the degradation of the solder and lead to the degradation of heat dissipation, which will not only affect the reliability of the chip soldering but also affect the bond point. The state of the bond can be determined by detecting changes in VCE, e.g. when the increase exceeds 5%.

A schematic diagram of the degradation of the aluminum bond can be observed by the power cycling test (shown in Figure 7), and it can be seen from Figure 7 that the bond wire has undergone a certain displacement and detachment at the bonding point. Another degradation is cracking at the root of the bond point (as shown in Figure 8). For MOS tubes, surface molding is generally removed by mechanical methods, and for IGBTs, the outer shell is removed by mechanical opening, and then the silicone gel or epoxy potting resin is chemically removed, and then the bonding point is observed.

2.5 Summary

Table 3 summarizes the bonding systems, causes and improvement methods of the above-mentioned failure modes.

Of course, the failure modes of the bonding system are much more than those mentioned above, and there are also mechanical stress cracking, such as the cracking of the bond neck caused by the significant thermal process of the damp device in the "popcorn effect"; Corrosion open circuits at external bonding points due to corrosion contamination; The presence of high levels of halogens on the surface of the chip leads to dendrite migration and leakage in the bonding point region. General mechanistic modes such as bond wire fusion are not discussed here.

Failed or severely degraded products can be analysed by means of corresponding detection and analysis methods, which usually occur at the end of the product stage, i.e. at the failure stage. Correspondingly, we can also carry out quality control of products using known failure modes, and the evaluation of engineering batch or product process quality usually occurs after the initial state or reliability test, and the bonding process is controlled by observing the plane and cross-section of the bonding connection. Planar observation is the observation of crack anomalies in the area, area and dielectric layer of residual IMC on the bonded spherical surface after chemically and directionally removing the aluminum pads; The cross-section is to check whether the IMC interface voids, cracks or abnormal deformation at the bottom of the solder ball are checked after the connection system is sliced, and these methods can be used as process control to determine the effective area of the bond, identify whether the quality of the bonding process has reached the expected level, so as to ensure the reliability of the product batch.

In summary, bonding systems play a crucial role in semiconductor packaging. Starting from the bonding process, different wire materials such as gold, copper, and aluminum have their own characteristics and application scenarios in the bonding process, and there are certain differences in the bonding process steps. In the bonding system, in addition to the bonding leads, the pad is also a non-negligible part, and its structural factors such as the thickness of the top layer of metal and the arrangement of the through-hole array have an important impact on the entire bonding system.

In terms of the failure mode of the bonding system, the cracking of the bonding process mostly occurs in the Cu-Al bonding system, which is caused by the mismatch of process parameters. The degradation of bimetallic intermetallic bonding exists at both the Au-Al and Cu-Al interfaces, and the Au-Al interface is more common, mainly due to the fast growth rate of IMC and the rapid diffusion of gold to aluminum to form Kirkendal holes, which can be detected by sectioning. Contact corrosion mainly occurs at the Cu-Al interface, which is due to the principle of galvanic cells formed in an electrolyte-like environment due to the potential difference between different metals, which can be improved by methods such as opening and slicing observation, and increasing the IMC thickness. The degradation of bonding wires of power devices is not easy to observe, but the life can be predicted by monitoring parameter changes by power cycling test, and there is detachment or root fracture after degradation, which can be detected by mechanical and chemical opening. In addition, there are other failure modes, such as mechanical stress cracking, which are not detailed but also indicate the complexity of the failure modes of the bonding system.

The research and detection methods of these failure modes can accurately identify the failure modes and determine the failure causes, and then propose improvement strategies. This not only helps to improve the reliability of the bonding system and ensure the quality of semiconductor devices, but also in terms of product quality control, whether it is the initial state or after the reliability test, the bonding process can be effectively controlled by observing the plane and cross-section of the bonding connection, so as to ensure the reliability of the product batch.

Bonding has always been an important process in semiconductor packaging technology, and a good bonding system is also a quality guarantee for the high reliability of semiconductor devices. In this paper, the bonding process and background of semiconductor devices are first introduced, and the different characteristics of commonly used gold, copper, and aluminum are discussed, and the main differences in the bonding process are discussed. In addition, another important component of the bonding system, the pad, is described, and the bond cracking can be improved by improving the thickness of the aluminum layer of the pad to increase the bonding buffer, and the density of the through-hole array under the metal layer can be increased to optimize the overall strength of the dielectric layer and thus improve the bond cracking.

The main failure modes of the bonding system are analyzed, and the corresponding improvement measures are proposed.

a) The cracking of the bonding process mainly occurs at the copper-aluminum interface, which can be observed by chemical opening and demetallic layer, and the main factor of failure lies in the incompatibility of process parameters, which can be improved by adjusting the process parameters and pad structure;

b) The degradation of bimetallic bonds is present at both gold-aluminum and copper-aluminum interfaces, but is more common at gold-aluminum interfaces, mainly by slicing for cross-sectional detection, because the IMC between gold and aluminum grows fast under the same conditions, and gold-aluminum is also prone to form Kirkendal voids, mainly because the diffusion rate of gold to aluminum is fast under high temperature conditions, and the gold layer at the contact surface cracks and continuously consumes aluminum to become IMC;

c) Contact corrosion mainly occurs at the Cu-Al interface, because there is an electric potential difference between different metals, which will form a working principle similar to that of galvanic cells in an electrolyte-like environment, usually the bond point can be observed to fall off by the opening method, and the cracking and corrosion phenomenon of the bond point section can be observed by slicing, which can be improved by increasing the thickness of IMC, improving the metal resistance (palladium and copper), controlling the halogen ion concentration of the package and reducing the water absorption of the material;

d) The degradation of the bonding wire of the power device is not easy to observe, but the life can be predicted by monitoring the parameter changes of the power cycle test, and the phenomenon of detachment or root fracture will occur after degradation, which can be detected after mechanical and chemical opening.