Wire Bonding as dynamic process of Hardening and

Wire Bonding as Dynamic Process of Hardening and softening

Ute Geißler and Martin Schneider-Ramelow

Ute Geißler, Fraunhofer IZM/Universität Rostock, Gustav-Meyer-Allee 25, 13355 Berlin, Germany Email: [email protected] Martin Schneider-Ramelow, Fraunhofer IZM Berlin

Abstract Ultrasonic wedge-wedge bonding of AlSi1 wires is characterised as a dynamic process of hardening and softening. The bonding parameters ultrasonic energy and bond force are acting in the opposite direction: While the bond force causes wire hardening ultrasonic energy causes softening and a better plasticity of the wedge. Increasing ultrasonic power is resulting in reduced wedge-hardness, i.e. the Aluminium wedge is more softened than using lower ultrasonic power. In the first phase of wire bonding the wedge is predeformed and cold worked by the bond force. After ultrasonic energy has been switched on, recrystallisation starts at the interface. During the bonding time hardening and softening processes alternate and a maximum in hardness is measured after 15 ms. Hardening and softening processes correlate well with the grain structure, the measured grain sizes and a typical plateau in the z-deformation curve of the contact. At the end of the wire bonding process the wedge is recrystallised and softer than the predeformated wedge, but harder than the as-received wire. Key words: Ultrasonic Wedge/Wedge Bonding, Aluminium, Wedge-Hardness, Softening and Hardening, Dynamic Recrystallisation, Interface

1. INTRODUCTION For more than 45 years, wire bonding has been the most important micro joining technique for semiconductor devices. It has reached a very high level of process quality and is used for a wide range of applications in diverse fields for electronic systems (automotive, medical, communication and information, etc.). At certain fields of system integration like COB, power electronics assembly or for specific sensor structures, wire bonding with aluminum is in a dominant position. While practical application of ultrasonic (US) wedge/wedge bonding is very well understood, the metallurgical and microstructural processes during interconnection of wire and substrate metallization are fragmentary investigated. However, these processes directly affect bonding quality and reliability especially because the trend in wire bonding goes to fine pitch bonding with very small wires and narrow tools. To understand wire bonding completely, fundamental research of microstructure and mechanical properties is required. The conditions, as well as procedures to optimize this interaction are not clear in all points [1]. Fig. 1 illustrates the characteristics of the different phases in the wire bonding process. In the first phase of wire bonding, the as-received wire is predeformed only by the acting bonding force. When ultrasonic power is turned on, the fibre texture of the wire initially remains. Only the vertical grain size has been decreased, resulting from work hardening of the wire material in the pre-deformation phase [2]. In the second phase (surface activation phase), after turning on ultrasonic power, the surfaces of wire and pad are cleaned through a relative motion between wire and the metallization (friction) [3]. After a few milliseconds of ultrasonic action first spots of wire and metallization material are welded. Further wedge deformation occurs in this third phase of the bonding process. The last phase is characterised by a decreasing deformation of the wedge and an interdiffusion process until a quality-like wedge is created [4]. This paper describes the micro structural changes especially in the wire/wedge material progressing with increasing ultrasonic power and in the different phases of the bonding process and the effect of ultrasonic energy. Micro-hardness measurements in addition of Focused Ion Beam (FIB) investigations and deformation measurements serve to illustrate the dynamic of hardening and softening during the bonding time.

Fig. 1

Characteristics of the different phases in wire bonding process

2. EXPERIMENTS The Ultrasonic wedge/wedge technique was used to bond 25 µm AlSi1 wire on Cu/Ni/Au pads of a printed circuit boards. The wire characteristics 14.8 cN breaking load and 1.7 - 3.1% elongation were provided by the manufacturer. The frequency of the automatic bonding machine was 106 kHz and the foot length of the bonding tool was 50 µm. Different samples were prepared to investigate the effect of ultrasonic energy and the effect of bondig time on the grain structure of the bonded wires and the microstructure of the interface between the wire and the Cu/Ni/Au metallization at the final stage of the bonding process (Tables 1 and 2). Bonding parameters were optimized by pull tests [5]. Investigation of different phases of the bonding process Different types of samples were prepared to investigate the time-dependent microstructure, hardness and interface formation. First the bonding parameters were optimized [5], a bonding force of 27 cN was used, the US energy value was 65 arbitrary units, resulting in a tool vibration amplitude of 0.85 µm. The complete bonding time for welding wire and metallization was 50 ms. In Figure 2 the wire deformation in z-direction of an optimized wedge is illustrated schematically. The wire bonding process was interrupted after 7 ms (deformation phase), 15 ms (end of deformation phase) and 22 ms (interdiffusion phase). After 7 ms, the bonding wire (wedge) stuck on the bond pad and first investigations of bonded wedges were carried out. Furthermore, in order to characterise hardness and the microstructural changes, as-received wires and predeformed wires (bonding time 0 ms) were investigated, as well as wedges after 15, 22 and 50 ms. The contact height z of the wedges was measured in FIB cross sections, the degree of deformation in the different bonding states (Table 1 and 2) has been calculated by: (d0-z)/d0 * 100 %. d0 - wire diameter z – height of wedge after bonding

Table I: Optimised Wedge – Effect of the Bonding Time on Wedge Geometry, a0= 0.85 µm Bonding Time Degree of Contact Height z [ms] Deformation [%] [µm] As-received wire 25 Pre-deformed wire 0 18 20.5 7 35.6 16.1 15 38.8 15.3 22 44.4 13.9 50 62 9.5

Table II Effect of Ultrasonic Power on Wedge Geometry, Bondforce 27 cN, Time 50 ms US – Power US – Power Degree of Contact Height a0 Deformation z (Arb. Units) [µm] [%] [µm] As-received wire 25 Pre-deformed wire 0 0 18 20.5 Underbonded wedge 40 0.45 28 18 Optimised wedge 65 0.85 62 9.5 Overbonded wedge 90 1.1 72 7

Micro-Hardness Measurements Micro-hardness measurements were taken at cross sections of wedges that had been grinded and polished parallel to the wire axis. To remove the interfering layer, that might be work-hardened by mechanical polishing, the surface was etched by an Argon beam. The micro-hardness measurement device was a Shimadzu DUH 202 with Vickers indentor. The selected test force was 5 mN, the sample rate was 20 values/s and the loading speed was 0.028 mN/s. Micro-Structural Investigations Focused Ion Beam technique (FIB) was used to visualize the very fine grain structure [6] of the AlSi1 wedges (grain diameter 150 - 400 nm) and the interface between the wire and the pad metallization. The samples were bonded using the same bond force and the same ultrasonic energy, but different bonding time. For a quantitative analysis of the grain size variations, FIB images of cross sections parallel to the wire axis of the different bonded wedges were used. In the wedges three regions were investigated: 1. the interface region, 2. the wedge centre and 3. the tool region. Within these three regions areas of 20 µm² were analyzed. The vertical dimension of the grains perpendicular to the wire axis was measured and in each selected region the number of grains was counted.

3. RESULTS AND DISCUSSION 3.1. Effect of Ultrasonic Power As reported in previous works ultrasonic power causes recrystallised grains [7], softening of the bonded wedges´ microstructure [8] and a decrease in microhardness compared to the as-received and predeformed state of the bonding wire (Figure 2). The degree of softening depends on the level of US power. Increasing ultrasonic power results in a lower micro-hardness (Figure 2) and a completely recrystallised grain structure in the wedge (Figure 3a-d). Applying different amounts of ultrasonic power alters the dimensions of the bonded wedges, resulting in changes in the wire bond grain structure. The most strongly deformed wires, bonded with the highest ultrasonic energy (Table 2), show a completely recrystallised wedge, characterised by very small grains near the AlSi1 wire/metallisation interface and larger grains in the centre of the wedges (Fig. 3d). Although the ultrasonic wedge/wedge process is conducted at room temperature, recrystallisation appears to take place as a temperature controlled process. Earlier studies have described recovery, recrystallisation and grain growth of zinc resulting from ultrasonic power treatment for a period of 30 minutes [9, 10]. Contrary to these observations the bonding process in this study only requires a very short time, i.e. 50 ms, to transform the equiaxed grains obtained

after pre-deformation [2] to recrystallised grains at the end of the bonding process. Although the arrangement and the nature of dislocation structures inside the very small recrystallised grains could not yet be resolved, it is assumed that ultrasonic wire bonding of AlSi1 wires is accompanied by dynamic recrystallisation. This assumption is based on the very short bonding time (30-50 ms) and the stacking fault energy (SFE) of aluminium. The high SFE of Al of 200 mJ / m² at 300 K [11] is very advantageous for dynamic softening processes such as dynamic recrystallisation. In earlier TEM studies of aluminum wires bonded to pure aluminum substrates Murdeshwar and Krzanowski also reported on recrystallised grains in aluminum substrates at the end of the ultrasonic wire bonding process [12]. In the Al wires near the interfacial layer equiaxed grains with a defect structure indicating dynamic recovery were observed [12]. In contrary to Murdeshwar and Krzanowski the results in this work demonstrated dynamic recrystallised grains inside the bonded AlSi1 wires (Figs. 3 a-d). The recrystallisation is induced by ultrasonic wire deformation. Defect generation induced by plastic wire deformation resulting from the bonding force and recrystallisation proceed concurrently. Dynamic recrystallisation as a result of ultrasonic treatment during bonding takes place. The optimized wedges exhibit a fully interconnected interface, characterised by very small grains near the interface (Figure 3c). Compared with cold-worked pre-deformed wedges (Figure 3a), ultrasonic power causes the softening of the microstructure of the bonded wedges and a decrease in micro-hardness. The amount of softening depends on the level of ultrasonic power. Increasing ultrasonic power causes lower micro-hardness and a completely recrystallised grain structure inside the wedge (Figure 3d). It activates defect movement and diffusion processes in and between the bond partners and results in a closed interface without voids. The interface between the wire and the metallisation layer of the optimised and overbonded wedges are completely closed (Fig. 3c and d), whereas in the underbonded wedges, some sections without welded areas exist. These voids are shown in Figure 3b. In such only partially interconnected sections of the interface, the appearance of long, non-recrystallised grains was observed.

Fig. 2: Effect of US power on wedge hardness

a) pre-deformed bond

b) underbonded wedge

d) overbonded wedge

c) optimised wedge Fig. 3 Increasing ultrasonic power causes recrystallisation of the wedge´s microstructure

3.2. Bonding Time Now the effect of bonding time on micro-hardness, wire deformation and the contact height z of the wedges is demonstrated. Over a period of 50 ms bonding time, the acting bond force and ultrasonic energy reduce the contact height z of the wedge (Fig. 4) due to wire deformation (Figs. 5 and 6). The deformation processes are associated with changes in micro-hardness (Fig. 7), resulting from changes in the wedges grain microstructure of the AlSi1 wire (Fig. 8 and 9) and microstructural rearrangement of the wire.

Fig. 4

Contact height z of the wedge

Fig. 5

z-deformation curve of an optimised wedge [(d0-z)/d0] * 100%

Fig. 6

z-deformation, laservibrometric measured

Fig. 7

Fig. 8

Progress of wedge hardness

Variations of grain sizes during the wire bonding process

a) 7 ms b) 15 ms

c) 22 ms

d) 50 ms Fig. 9

Microstructural rearrangement during the wire bonding time

So the progress in wire deformation under ultrasonic power has to be considered in the context of with micro-hardness and the microstructural variations in the wedge. The wire deformation is characterised as a dynamic process of softening and hardening processes in the wedge. At the end of the bonding time, the measured hardness of the wedge is higher than the hardness of the as-received wire (Fig. 7) even though it is a generally known fact that ultrasonic energy softens metals [7]. Hardening of the wedges compared with the as-received wires at the end of the wire bonding process for Al heavy wires and 50 µm

AlSi1 wires was also detected by Schneider-Ramelow et al. [13]. This hardening was discussed as a result of coldworking during wire deformation [13]. The new microstructural investigations and hardness measurements prove that dynamic softening processes cause the increased micro-hardness. During the wire deformation in the four phases the dislocation densitiy rises, that means hardening. However the simultaneous action of ultrasonic power starting with the beginning of the activation phase rearranges the dislocations into cells and small grains so that the wire is softening [7]. These softening processes reduce the yield strength for further wire deformation so that the wire is hardened again. Although wedge/wedge bonding is carried out at room temperature, softening processes like recrystallisation occur. During the pre-deformation phase, the wire hardness increases only through the acting bond force. This is a hardening effect in a sense of cold working [2]. But the measured hardness at the end of the bonding process after 50 ms is on a lower level than the hardness in the pre-deformated state before switching on ultrasonic power. This effect means a softening of the wedge within the bonding time in the sense of recrystallisation. As seen in Figure 7, softening of the pre-deformed wire does not proceed continuously. The maximum of wedge hardness within the whole bonding time after 15 ms indicates the dynamic of competing softening and hardening mechanisms and correlates with the temporal progress of deformation. After 15 ms the deformation is characterised by an approximate plateau, measured by a laser vibrometer (Fig. 8) and by the contact height z (Fig. 4). Changes in the wire´s microstructure illustrate the dynamic of hardening resulting from the acting bondforce and softening caused by ultrasonic power (Fig.9). The microstructural changes are quantified by measurements of the vertical grain diameters (Fig.8) and put into a correlation to wire deformation (Figs. 5 and 6) and hardness (Fig. 7). Interrupting the wire bonding process after only 7 ms leads to wedges sticking on the metallization. At the interface of these wedges the grains are already recrystallised, but in the remaining wedge material the fibre texture dominates (Fig. 9a). Continuing the wire bonding process, a progress of recrystallisation in the wedge can be observed (Fig. 9 a-d). As a result of pre-deformation the wire is cold worked and its hardness by far exceeds that of the as-received wire. Softening of the wedge already starts within the first 7 ms of ultrasonic deformation. The softening process is characterised by a reduced hardness compared to the hardness value of the pre-deformed wire and by the appearance of small recrystallised grains at the interface (Figs. 7, 8 and 9 a). Because of the friction amplitude during surface activation in the first milliseconds of ultrasonic wire bonding the recrystallisation starts at the interface. Fig. 9 a shows that recrystallised grains in the wedge centre and in the tool region are not visible yet after 7 ms bonding time. Elongated deformed grains with reduced diameters in vertical direction appear even though a decrease in hardness is measured at this time. A possible mechanism for the measured softening in the elongated grains of the wedge centre is dynamic recovery. Due to the ongoing deformation under ultrasonic load the dislocations generated by the bond force are rearranged in cell structures [14] and/or subgrains with low disorientation inside of the elongated grains so that the existing grain structure (large-angle grain boundaries) does not change yet. The strongest wire deformation (velocity) of the bond process takes place in the first 7 ms. Wire deformation at this time is accompanied by a reduction in vertical grain diameters and a rise in dislocation density. Nevertheless, while the ultrasonic power is active the wedge material is softened at the interface as well as in the tool region and at the wedge centre. Dynamic recrystallisation is the mechanism for softening especially at the interface, where small recrystallised grains develop. The other areas of the wedge are softened by dynamic recovery, keeping the existing grain structure. In the further progress of wire deformation up to a bonding time of 15 ms the wire texture disappears. The dynamic recrystallisation, after 7 ms only observed at the interface, now continues into other wedge areas. Small recrystallised grains develop at the tool region and partially at the wedge centre (Fig. 9 b). Driving power for the proceeding recrystallisation is the deformation energy, stored in the dislocations. Compared to the bonding state at 7 ms bonding time, the continuing recrystallisation during the further wire deformation up to 15 ms bonding time, is accompanied by increasing hardness and increasing grain diameters. The increase of hardness can be explained by the recurrent increase of the dislocation density in the dynamically recovered/recrystallised grains resulting from further wire deformation. After 15 ms bonding time, the maximum in hardness (Fig. 7) correlates well with the measured plateau of the deformation curve (Fig. 6) and reduces the wire deformation rate in z-direction. The maximum hardness measured coincides with the plateau in wire deformation demonstrated by laservibromertric measurments of the wire deformation. During the following milliseconds of the wire bonding process the bonding parameters ultrasonic power and bondforce cause softening of the wedge, despite of further wire deformation, measured in decreased hardness values (Fig. 7). At the end of the wire bonding process the grain structure is finely recrystallised (Fig. 9 d).

Hardening and softening processes proceed competitively and pass a maximum of hardness during ultrasonic wire bonding of AlSi1. This maximum hardness results from dislocation transactions and dislocation rearrangement during dynamic recovery and dynamic recrystallisation during the ultrasonic wedge deformation. Fundamental to the joining process is the formation of recrystallised grains in the interface region. A multidude of small recrystallised globular grains develop during ultrasonic wire deformation. So the yield strength for further wire deformation is reduced and a good adaption of wire and metallization is possible. Finally the dynamic softening of wire is essential for a nearly completely closed interface.

4. CONCLUSIONS Ultrasonic wedge/wedge bonding of AlSi1 wires is carried out at room temperature, but the wire is recystallising during the bonding time of only a few milliseconds. The recrystallisation starts at the interface region after switching on ultrasonic power. At this time the wire surface is highly activated by deformation and friction of the pre-deformation and the surface activation phase. The softening is measured as a decrease in hardness. The softening in the first milliseconds of the wire bonding process reduces the yield strength so that further wire deformation is possible. An increase in hardness is measured because of the defect generation inside the recrystallised grains during further plastic wire deformation. In the last milliseconds the wire is softening again, the recrystallisation continues also in the wedge centre, the hardness at the end of the wire bonding process decreases. The action of bond force and ultrasonic power causes alternating hardening and softening. The deformation of the softened wire enables the formation of a nearly completely closed interface between wire and substrate metallisation.

ACKNOWLEDGEMENTS The authors would like to thank Prof. Herbert Reichl for his encouragement. The work was funded by German Research Foundation-DFG.

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