Wire bonds have been shown to be a highly reliable, flexible, interconnection scheme for decades. In fact, wirebonded products have been in continuous use in space and other domains for over 25 years. Automated bonding has intro- duced a new level of control and precision bond placement that even further
7 7.5 8 8.5 9 9.5 10 10.5 11
5
0 10 15 20 25 30 35 40 45 50 55 60 65
Pull Strength (grams(force))
Time (Days)
Baseline Period Upper Control Limit
Process Average
Lower Control Limit
Specification Limit
Run Chart
Out-of-Control Limit Condition (Replace Capillary) New Operator
(exclude data point)
Fig. 4.21 Run chart for destructive wire bond pull test for thermosonically bonded 25.4mm diameter gold wire on gold metallization (on ceramic). Process monitoring was used to develop control limits for the process during the baseline time limit. These limits (2 in this case) are much tighter than the specification limit of 7.5 g (force) generated from grams (force) (Reference ASTM Standard Test Method: F459-06 (2006)). Here the run chart allowed the identification of capillary wear before the product fell below the specification limit
improves the reliability and reproducibility of microelectronic products. These automated bonders coupled with improvements in bond pad metallurgy, reduc- tion in bonding wire unwanted impurity content, more effective pad cleaning processes, high purity and stable die attach adhesives, and reduced temperature bonding processes (ultrasonic and thermosonic) have contributed to the current widespread use and reliability. In fact, wirebond defect rates, for single chip packages, are typically in the very low parts per million (ppm) range (e.g. 3 ppm or less).
Wirebonds, like all complex physical and chemical processes, can be fraught with reliability detractors if proper cautions and controls are not exercised and if phenomenological factors are not well understood. Some of the typical problems include:mechanical wire fatigue due to conditions of thermal or power cycling; interactions both chemical and mechanical with encapsulation during molding and after cure; corrosion induced by the die attach material, the atmosphere, and/or process-related contamination; and wire structural changes due to bonding parameters, such as uncontrolled grain growth associated with the heat-affected zone. An entire book by Harman [38] has been devoted to reliability issues and yield problems. Of all the issues two particular ones deserve further discussion: intermetalics and cratering.
4.7.1 Intermetallics
The most widely studied and publicized wirebond reliability probability is associated with the alloying reactions that occur at the gold wire-aluminum alloy bonding pad interface (and, to a much lesser degree, aluminum wire-gold bonding pad interface). Aluminum-gold intermetallic formation occurs natu- rally during the bonding process and contributes significantly to the integrity of the gold-aluminum interface. Intermetallics (in particular, AuAl2 or purple plague and Au5Al2 or white plague) are generally brittle; and, under conditions of vibration or flexing (either mechanically or thermally induced due to coeffi- cient of thermal expansion mismatches), may break due to metal fatigue or stress cracking, resulting in bond failure [67].
At elevated temperatures, aluminum rapidly diffuses into the gold forming the AuAl2 phase, leaving behind Kirkendall voids [67] at the aluminum-AuAl2 interface. Figure 4.22 shows views of extensive intermetallic growth around and under various thermosonic wirebonds (both ball and tail bonds). Kirkendall voiding has also been observed at gold-Au5Al2 interfaces. Excessive interme- tallic growth can lead to the coalescence of voids, which can lead to a bond crack or lift and an open circuit. Impurities in the bonding wire, on the pad metallization, or at the wirebond-pad interface have been shown to cause rapid intermetallic growth and Kirkendall voiding at temperatures below those asso- ciated with normal intermetallic formation [9]. Table 4.9 gives the formation temperature, activation energies, and some notes for the five aluminum-gold
Fig. 4.22 Scanning electron photomicrographs of advanced intermetallic growth:(a)underside of ball bond with regions of intermetallic voiding (Kirkendall);(b)residual intermetallic left on bonding pad corresponding to the voided regions of the ball in view (a) ; and(c)tail bond with extensive intermetallic formation under the bond edge and consuming part of the flattened bond region.
Magnification approximately 75X
intermetallics. The deleterious effects of intermetallics can be controlled if the time of exposure to high temperature is minimized and if proper materials and cleaning procedures are used [86]. See also Section 4.4 Pad Cleaning above.
Design rules have been developed for minimizing intermetallic void failures by controlling film layer composition and thickness [24]. In addition, proper optimization of the wire bonding process has a significant influence on inter- metallic growth (e.g. [25]).
4.7.2 Cratering
As mentioned previously, cratering can be a significant problem associated with the bonding and subsequent shearing of ball bonds from silicon integrated circuits. Intermetallic formation, induced stress, metallization thickness, bond- ing parameters, and underlying dielectric layers have all been noted to have an effect. [25] (Clatterbaugh and Charles, 1990). To help separate these phenom- ena, a series of cratering-related experiments (bonding, etching, metallizations, ect.) and finite elements analysis (FEM) have been performed. The results of these studies show:
1. the effects of gold-aluminum diffusion-induced strains within the weld region are negligible compared to those introduced by shear testing
2. the smaller the weld region, the more likely the underlying silicon will crater when shear tested;
3. the taller the ball bond, the more likely the underlying silicon will crater when shear tested; and
4. the stress field for an angular type weld is similar to that for a continuous circular type of the same radius.
Table 4.9 Aluminum-gold intermetallic alloy properties
Alloya
Formation Activation Energyb temperature
(8C)
eV kJ/
mo11 k cal/
mo11 Comments Au5A12 23–100 0.62 59.4 14.3 Tan in color
Au2A1 50–80 1.02 98.3 23.5 Metallic gray in color (orthorhombic, randomly oriented monocrystals AuA12 150 1.20 115.8 27.7 Deep purple in color (purple plague-
resistivity 8m0cm)
Au4A1 150 Tan in color
AuA1 250 White in color
aThe intermetallic alloys typically form in the order listed (Au5A12,. . .AuA1) consistent with their temperature of formation.
bA range of activation energies from 0.2 to 1.2 eV, have been observed for the aluminum-gold system depending upon growth, testing, and contamination conditions
Thus, a flatter bond with a larger weld area (or large annulus) is less prone to produce silicon cratering when shear tested.
The results of the etching experiments indicated that the occurrence of cracks in the bare silicon or in the silicon dioxide (SiO2) on silicon due to improper bonding parameters did not occur, if the bonding machine parameters fell within the bonding window determined for the experimental configuration..
This rules out the requirement that initial substrate damage due to improper bonding parameters is necessary for ball shear-induced cratering to occur.
For the case of the borophosphosilicate glass (BPSG) on SiO2, damage of the thin glass coating was prevalent for almost all bonding conditions. The best set of bonding conditions to resist cracking in BPSG films was found to be the combination of low power, short dwell, high stage temperature, and high force.
This is consistent with the results reported by Koch and his co-investigators (Koch, et al. 1986) who studied the effects of bonding parameters for thermo- sonic gold ball bonding on aluminum pads over phosphosilicate glass (PSG).
The results from the metallization thickness experiment do indicate a sig- nificant reduction in the incidence of ball shear-induced cratering as the metal- lization thickness is increased. However, since the etching analysis of untreated wire bonded samples showed no initial substrate damage as mentioned above, a cushioning effect of the additional metallization was ruled out. A more plau- sible explanation would be that the additional metal would prevent the alloying of the gold ball and aluminum to the underlying silicon substrate during the thermosonic scrub. This would prevent a rigid link to the substrate available to transfer shear energy to the underlying silicon substrate.
Results from the experiment conducted to study the effect of bonding con- ditions on ball shear-induced cratering indicated that the greater the force and the power parameters or, equivalently, the stronger the bond, the less likely the substrate was to crater when ball shear tested. This is equivalent to stating that the larger the weld (a flatter bond), the less likely that cratering will occur. Once again, this is consistent with the results from finite element modeling and previous data indicating that the manufacture of larger, more robust bonds is less likely to cause cratering [25].
Some conclusions can be drawn from the information above concerning the effects of the gold-aluminum intermetallic on the cratering effect in silicon and thermally grown SiO2reported in Weiner, et al. (1983). As stated above, the effect of strains introduced due to structural misfit of intermetallic phases is of second order compared to those introduced by the ball shear ram. Also, no damage was observed in either the silicon or SiO2for the full range of bonding parameters used in this study. Therefore, it must be concluded that the cratering effect observed here is not a result of initial substrate damage. Several factors point to the rigid intermetallic bond between the ball and the substrate as the sufficient cause for call shear-induced catering. These factors include:
1. the absence of cratering in samples which had not been thermally annealed (i.e., there is little intermetallic formation, and, thus, the aluminum
metallization will yield before much energy can be transmitted to the under- lying substrate);
2. the delay in the onset of cratering for thicker metallizations (a longer period of time would be required to penetrate the thicker metallization and alloy with the underlying silicon substrate);
3. the effect is significantly less for pads over SiO2(Au-A1 intermetallic does not form as good a bond to SiO2as it will to bare silicon); and
4. as the weld area increases with additional intermetallic formation, the esti- mated stress concentration factors become significantly reduced, thus explaining the reduction in cratering in strong bonds after prolonged thermal aging.
In summarizing this cratering effect, a thicker bond pad metallization, a larger weld area (high ultrasonic energy and stage temperatures), and flatter bonds (high force) will produce bonds that are more resistant to any form of shear-induced cratering for bond pads on SiO2. Weak bonds are to be avoided since they can crater at much lower shear values. Preliminary information suggests that for bonding above multilevel oxides such as PSG, BPSG, and other low-strength chemical vapor (CVD) deposited oxides, thicker pad metal- lization and the combination of low power, short dwell, hot stage temperature, and high force will be required to produce reasonably strong bonds and reduce damage to the underlying dielectric layers.