Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    The ultrasonic wedge/wedge bonding process investigated using in situ real-time amplitudes from laser vibrometer and integrated force sensor
    ( 2010)
    Gaul, H.
    ;
    Shah, A.
    ;
    Mayer, M.
    ;
    Zhou, Y.
    ;
    Schneider-Ramelow, M.
    ;
    Reichl, H.
    The ultrasonic transversal force transmitted to a chip during ultrasonic bonding is derived from measurements of the vibration amplitude at the tool tip and the die edge. To proof the derivation, the transversal force is measured as well by means of a microsensor, which is sensitive to the stress field in the silicon die. The force measured by the microsensor is further referred to as "y-force" To Al-metalized test pads with the integrated microsensors, AlSi1 wire of 25 mu m diameter was bonded using a wedge/wedge auto-bonder. Measurements of the vibration amplitudes and the y-force during bonding were conducted for nine different bonding parameter settings of force and ultrasound (us) amplitude. They confirm a theory for the friction cleaning phase as it was described earlier and will be partially presented here. Compared to earlier measurements of Au-ball-bonds, the results largely show the same behavior and imply that us wedge bonding and thermosonic ball bonding are similar processes. Furthermore, the data approves former interpretations of the bonding process starting with a stiction phase. A clear break off point was found in all pad amplitude measurements, which is followed by a friction plateau that implicates the need of a minimum friction cleaning power. The discussion made in this paper is interesting fora bond process control system. The transversal force reflects the important stages of the bond process and contains the information to Suit as a control signal. But it is impractical to measure the transversal force in situ under the wedge in industrial production, where chip, Substrate and bonding table create a complex setup with a high geometric variety. An indirect measurement of the transversal force via the tool tip amplitude opens up new possibilities for gaining an efficient control variable, because the geometry and the properties of the bonding machine are well defined. As a first step it is shown by correlating vibration measurements with microsensor signals, that the tool tip amplitude measured by laser vibrometer contains all of the necessary information needed to control the bond process. From that point, process integrable measurement systems - which are cheaper, more handy and more fail safe than the laser vibrometer - might be developed.
  • Publication
    Analytic model verification of the interfacial friction power in Al us w/w bonding on Au pads
    ( 2010)
    Gaul, H.
    ;
    Schneider-Ramelow, M.
    ;
    Reichl, H.
    To close the gap between the development of wire bonding equipment and the knowledge about the welding process itself, a friction power model is presented. The model can be used to calculate the welding quality of a wedge in dependence to the bonding and material parameters. Based on theories for Au ball/wedge bonding, the model is enhanced for us wedge/wedge (us w/w) bonding with aluminum wire. Therefore, the deformation of the wire is described by the von Mises stress, and the geometry changes due to the decreasing wedge height during bonding are taken into consideration. Furthermore, the paper introduces the minimum friction amplitude needed for any frictional cleaning of the interface and takes the precleaning during touchdown into consideration. To prove the model, four equations are highlighted to theoretically predict the shear force, the tool tip and pad amplitude, and the characteristic of the deformation during the us stage of w/w bonding. These magnitudes are measured for different bonding parameters while bonding 25 ?m AlSi-1 wire to Au metalized Si test structures. The model parameters were then fit to the experimental results for all bonding parameters.
  • Publication
    Ultrasonic friction power during Al wire wedge-wedge bonding
    ( 2009)
    Shah, A.
    ;
    Gaul, H.
    ;
    Schneider-Ramelow, M.
    ;
    Reichl, H.
    ;
    Mayer, M.
    ;
    Zhou, Y.
    Al wire bonding, also called ultrasonic wedge-wedge bonding, is a microwelding process used extensively in the microelectronics industry for interconnections to integrated circuits. The bonding wire used is a 25 mu m diameter AlSi1 wire. A friction power model is used to derive the ultrasonic friction power during Al wire bonding. Auxiliary measurements include the current delivered to the ultrasonic transducer, the vibration amplitude of the bonding tool tip in free air, and the ultrasonic force acting on the bonding pad during the bond process. The ultrasonic force measurement is like a signature of the bond as it allows for a detailed insight into mechanisms during various phases of the process. It is measured using piezoresistive force microsensors integrated close to the Al bonding pad (Al-Al process) on a custom made test chip. A clear break-off in the force signal is observed, which is followed by a relatively constant force for a short duration. A large second harmonic content is observed, describing a nonsymmetric deviation of the signal wave form from the sinusoidal shape. This deviation might be due to the reduced geometrical symmetry of the wedge tool. For bonds made with typical process parameters, several characteristic values used in the friction power model are determined. The ultrasonic compliance of the bonding system is 2.66 mu m/N. A typical maximum value of the relative interfacial amplitude of ultrasonic friction is at least 222 nm. The maximum interfacial friction power is at least 11.5 mW, which is only about 4.8% of the total electrical power delivered to the ultrasonic generator.
  • Publication
    Analysis of the friction processes in ultrasonic wedge/wedge-bonding
    ( 2009)
    Gaul, H.
    ;
    Schneider-Ramelow, M.
    ;
    Reichl, H.
    Research on wire bonding, the most common chip interconnection technology, has been done for over 40 years by now. The most common description is a model of three phases: friction, approach and interdiffusion (Lang 1988; Osterwald 1999). To quantify this model, with the main aspect at the duration of the friction phase, ultrasonic oscillation was filmed with a high speed camera. A pulsed LED light-source allowed very short exposure times of 2 mu s. For the first time it was possible to make the movement of the wire visible in a reflected light exposure. After the friction phase, the end of friction (EOF) occurs. Laser vibration measurements of the tool and the pad amplitude turned out that the EOF correlates with a second, characteristic plateau of the pad amplitude. At this time, the tool amplitude has already shown its characteristic maximum and was attenuated to approximately 80% of the maximum value. After the EOF, when the wedge is bonded to the pad, a bending of the wire towards the wedge in the heel region was observed. This is a possible reason for a weakening of the heel by the US-power. While a change of the wire texture could be proved in recent investigations (GeiAler et al. 2006), the exposure of the heel to reverse bending cycle was never verified before.
  • Publication
    Hochgeschwindigkeitsaufnahmen der Werkzeug- und Drahtschwingung beim US-Wedge/Wedge-Bonden
    ( 2007)
    Gaul, H.
    ;
    Schneider-Ramelow, M.
    ;
    Reichl, H.
    Mit einer Hochgeschwindigkeitskamera konnte bei starker Vergrößerung die Bewegung von Werkzeug und Draht während des Ultrasonic(US)-Wedge/Wedge(W/W)-Bondens gefilmt werden. Hintergrund der Untersuchung waren Fragen, die bei der Quantifizierung des Drahtbondprozesses durch Modelle der Reinigung durch Reibung bisher ungeklärt waren [1, 2]. Damit eine Reinigung im Interface von Draht und Bondpadmetallisierung durch Reibung erfolgen kann, muss davon ausgegangen werden, dass Bondwerkzeug (Tool) und Draht - zumindest zu Beginn des Bondprozesses - formschlüssig, also fest miteinander verbunden sind und sich synchron bewegen. Dieser Sachverhalt konnte nun durch die Videoaufnahmen nachgewiesen werden.
  • Publication
    Hochgeschwindigkeitsaufnahmen der Wedgebewegung beim US-Bonden/Wedge-Bonden
    ( 2007)
    Schneider-Ramelow, M.
    ;
    Gaul, H.
    ;
    Schmitz, S.
    ;
    Reichl, H.
    Durch verbesserte Technik konnte die Bewegung von Werkzeug, Draht und Chip beim Ultraschallbonden sichtbar gemacht werden. Dadurch konnte die Phase der Reibung (Draht an Pad) näher charakterisiert werden