Study of bonding parameters with Al and Au wires on Pd and Ag coatings

The process of electrical connection with gold and aluminium wires to palladium and silver electrodes is studied. Parameters of the wire bonding, such as bond force, ultrasonic power, bond time and temperature of the layers, controlled by heating the substrate are varied to explore the reliability of the bonds in terms of mechanical strength, electrical conductivity and interdiffusion of particles between the electrode and functional material at different bonding conditions. The importance of this study is due to the need of electrical connectivity at the stage of wiring and packaging of gas sensors with a novel organic nanomaterial (carbyne), requiring different types of electrodes – Ohmic or Schottky - according to the measurement principle and sensor architecture. Pd and Ag are identified as the most suitable for Ohmic and Schottky contacts, respectively, because of their favourable energy level alignment at the organic/metal interface. The chosen method for bonds testing is measurement of the pull force causing bond failure. The results show that Pd coating was bondable by Au wire, and Ag coating by Al wire, as well as Au on Pd bonds have 4 times higher strength. It is found that the bonding procedure doesn’t affect negatively on the Au/Pd/carbyne interface in terms of diffusion and redistribution of metal particles, but Al/Ag/carbyne is characterized by poor adhesion and the bonds are peeled-off. The results give new knowledge for the fabrication of advanced gas sensors and for the interaction of the carbyne with different metals, which will serve as a base for further optimization of the manufacturing technology toward their commercialization.


Introduction
Wire bonding is a technique used to connect thin films to other components or substrates in the electronic devices.It involves the use of fine metal wires with a diameter of a micrometer, to make electrical connections between different parts of a device [1].In the process flow first the thin films to be bonded are cleaned to ensure surfaces free from contaminants.Typically, oxide ultrathin coatings are formed on the top of some metals and ultrasonic cleaning, generated from the bonding tool, is applied [2].After the preparation of the thin films the appropriate wire material and diameter are chosen based on the specific requirements of the application.Gold wires are used due to their excellent conductivity and corrosion resistance, however, it is expensive and may diffuse.Therefore, an alternative bonding with the aluminium wire exists [3].Next is the setup of the important parameters of the wire bonder, which are substrate temperature, bonding force, ultrasonic power, and bonding time.These parameters may vary depending on the materials being bonded and the desired bond strength [4].After that is the wire bonding process, where the wire bonding machine uses a combination of heat, pressure, and ultrasonic energy to create a bond between the thin film and the component.The wire is first guided to the desired bonding location, and then the bonding tool applies pressure and ultrasonic energy to create a solid bond.The heat generated during the process helps to soften the wire and promote adhesion [5].Lastly is the bond inspection, where the quality of the bond is inspected by using specialized equipment to check for proper alignment, bond strength, and electrical continuity [6].
As new nanomaterials have been used for advanced electronic devices, new metallization systems, and respectively bonding technologies, have to be developed and explored.Recently, the 1D nanomaterial called carbyne, which is a carbon allotrope, has been intensively studied for its potential gas sensor properties toward organic volatile compounds [7].It consists of one-dimensional linear chains of carbon atoms with alternating single and triple bonds, which is also called polyyne, or chain with only double bonds between the carbon atoms, also called cumulene [8].Depending on the types of carbon bonds -cumulene or polyyne -the carbyne can have different electrical, mechanical and chemical properties.It has been proven that polyynic structure is more stable than the cumulenic and its energy levels has been determined [9].A layer from polyyne carbyne has been successfully incorporated in sensing structures, using different gas detecting principles -capacitive and surface acoustic wavebased [10], however, they were tested without package.Therefore, wire bonding to the electrode pads has not been realized, thus the need of development of technology flow for a bonding process becomes crucial for the device reliability and for future commercialization of the carbyne sensors.The different sensing mechanisms require different types of electrical contacts -Ohmic for the SAW detection and Schottky for the capacitive detection.It was found that the palladium (Pd) forms an Ohmic contact to carbyne and the silver (Ag) forms a Schottky contact [11].
In this study, the effect of the substrate heating on the bond quality at the metal interface Pd/Au and Ag/Al is investigated.To the best of the author's knowledge, this is the first time exploring the bonding process on the metallization systems intended for carbyne-based device.An overview of the literature shows that results for metallization carbyne-based devices are seldom and reports for their bonding miss.Thus, the achieved results are crucial for the reliability of the carbyne device.The aim of the work is to propose a bonding technology flow that will pave the path toward packaging of a variety of carbynebased nanoelectronic devices, which currently hinders their commercialization.

Materials and methods
Palladium thin films with a thickness of 500 nm were sputtered on a cleaned glass substrate by at a partial argon pressure of 2.5x10 -2 Torr and sputtering voltage of 1.5 kV.Silver films with similar thickness was thermally evaporated at a base pressure of 1x10 -6 Torr and current of 1 A. Vacuum deposition system Laybold A400VL was used for both films.Their sheet resistance was measured by four-point prober Veeco FPP-5000.Bonding process was conducted by a semi-automatic ultrasonic wire bonder F&S Bondtec.At the standard wire bonding ball-wedge, the second bond is low-strength.For this reason, "stitch-ball" method is used for it, which means preliminary formation of a ball for better stabilization of the bond.The first bond is marked with B1 and the second one (the stitch-ball), with B2.It should be also noted that the Pd film is easy bondable by Au wire only and the Ag film is easy bondable by Al wire.The bonding strength test was conducted by the automatic pull and shear tester function of the bonder.The accuracy of the measurement is +/-0.25% of max.value, when the test is performed with a 90° hook positioned under the wire (pull angle of 90°).

Results and discussion
The target pull-off load for reliable bond is minimum 10 cN [12].In order to find the optimal substrate temperature, at which the adhesive strength of the gold bonds to the metal films is maximal, and the pull force tearing the Au bond is as close as possible to 10 cN, minimal stitch-ball loading of 30 g was set first and the temperature varied in the range 40 -100 °C.The bonds were made at constant bonding time of 50 ms, ultrasonic power of 150 W and bond pressing force of 70 g (pressure of the needle with the wire to the metal contact pad) for the two bonds.The bond time and ultrasonic power values are recommended by the manufacturer [13].It was observed that at temperatures of substrate higher than 150 °C, independent of the bond force, time and ultrasonic (US) power, bonding of Au wire on Pd coating could not be achieved.Also bonding of Au wire on Ag coating was not possible, independent of the temperature, bond time, force and US power.
It was found that a substrate temperature of 100 °C is optimal for the Au bond strength, as it tears at the highest bond strength of 8.49 cN, as compared to the Au bonds realized at lower temperatures.The lower bond strength at a lower bond temperature during gold wire bonding of palladium can be explained by a few factors, such as slow diffusion rate, lack of intermetallic compound formation, or lack of suitable surface conditions promoting adhesion.At lower temperatures, the diffusion rate of atoms is slower [14].During the gold wire bonding process, diffusion is necessary for the formation of strong intermetallic bonds between the gold wire and the palladium substrate.When the bond temperature is lower, the diffusion is impaired, leading to weaker bonds and lower bond strength.Gold and palladium can form intermetallic compounds, which contribute to the bond strength.However, the formation of these compounds is temperature dependent.At lower bond temperatures, the reaction kinetics for intermetallic compound formation are reduced, leading to incomplete or weaker bonding.Gold wire bonding relies on different adhesion mechanisms, including mechanical interlocking and chemical bonding.At lower temperatures, the mechanical interlocking between the wire and substrate may be less effective, leading to weaker bonds.Additionally, the formation of chemical bonds can be less favorable at lower temperatures, further contributing to the lower bond strength.Therefore, the experiment was conducted at 100 °C, keeping the rest parameters constant, and setting higher pull-off loads of 50 g and 70 g.The repeatability of the results for each of the cases was checked by testing of 5 bonds, fabricated at one and same conditions.The measured values of the bond strength are presented in table 2. Graphical representation of the Au bond tearing process in a time sweep is shown in figure 1 (a), together with a histogram of the values for the 10 samples.The results for the bonding process of Al wire to Ag film are summarized in table 3 and the graphical representation of the Al bond tearing process in a time sweep is shown in figure 1 (b) (with a histogram).For bonding of Al wire on Ag coating it was observed that at bond time of 30 ms, bond force between 26 and 50 g, and US power higher than 120 W, the wires broke, and lower US power should be used.It can be noted almost 4 times lower bond strength of the aluminum bond on the silver film, as compared to the gold bond on Pd, despite the ease of bonding.The lower bond strength between aluminium and silver, regardless of the bonding parameters, can be attributed to a few factors, such as shorter bond time, lower ultrasonic power and lower bond force.However, at higher values of these parameters' bonds were unable to form due to complete peeling off the silver film.The reason is the overheating of the bond spot considering the high thermal conductivity of the silver.Thus, despite the high bonding temperature of 100 °C, the quality of bonding is not satisfactory.If the formation of intermetallic compound Ag-Al is incomplete or weak, it can result in lower bond strength.The time sweep was chosen arbitrarily as a demonstration, the histograms were chosen to represent the pull of force with statistical significance.Repeatability for 5 samples with different pull-off loads is done to be able to track the trends (table 1 and 2), and for the optimally obtained results, additional 10 measurements were done for statistical significance of the reproducibility of bond quality (table 3).Table 3. Al bonds strength at pull-off load of 30 g (samples 1-5), 50 g (6-10) and 70 g (11)(12)(13)(14)(15)  The sheet resistance of the Pd film before Au wire bonding was 0.472 Ω/sq (averaged from 5 points).After bonding at low temperature, the sheet resistance was almost unchanged (0.475 Ω/sq), which is an evidence for lack of gold diffusion into the palladium.After bonding at 100 o C the sheet resistance increased slightly to 0.563 Ω/sq, which can be ascribed to the Au-Pd intermetallic compound.Sheet resistance of Ag before bonding was 1.21 Ω/sq and after bonding was 1.08 Ω/sq, which is negligible change and cannot be an indication for intermetallic compound formation.
To explore the effect of the bonding process on the microstructure and metal atoms redistribution at the interface electrode/carbyne, it was observed the cross-section of the interface by SEM and a depth profile of the elemental analysis was conducted.The results are shown in figure 2 (a) for Pd bonding case and in figure 2 (b) for Ag bonding.Figure 2 (a) supposes that there is a sharp transition between the layers without formation of zone, containing intermixed particles.This is visible also from the sharp difference between the concentration of the dominant Pd particles during the elemental analysis scan (below).To avoid overcharging of the sample due to the gold bond wire, it was removed and the corresponding hole is visible from the cross-section.It is reflected as a Pd concentration drops at the detach place.In contrast to this stable behavior, the interface between the silver and carbyne tends to delaminate and form a gap.A large amount of aluminium material remained on the top of the silver during detachment, thus lacking hole, but measured as an equal in quantity, like the silver one, as can be seen in figure 2 (b).The background chemical elements having concentrations of lower than 2-4 wt% can be ascribed to contamination particles due to the common technology cycle of multilayer deposition of all metal films in a single chamber by switching the cathode plasma without devacuuming.

Conclusions
In summary, wire bonding is a widely used technique for connecting thin films to other components in electronic devices.The lower bond strength at lower bond temperatures during gold wire bonding of palladium can be attributed to slower diffusion rates, reduced intermetallic compound formation, and weakened adhesion mechanisms.The low bond strength between aluminum and silver, regardless of the bonding parameters, can be attributed to limited intermetallic compound formation.While the palladium can be easily bonded and the bond strength can reach near 10 cN, the silver needs additional ultrathin film for improving the adhesive strength of the material before bonding.Elemental analysis of the interface Pd/Au and Ag/Al, and observation of the cross-sections of the interfaces between the materials by scanning electronic microscope confirmed that in the former case the mechanism of bonding doesn't affect the position of the palladium atoms due to the temperature, force and US power of the bonding tool, while in the latter case, due to poor adhesion, there is detachment at the aluminum bond zone and in addition easy tearing of the silver from the carbyne.The results are confirmed by the bond strength test, showing lower value of the pooling force for the aluminum wire on silver electrode, as compared to the gold wire on palladium electrode (4 ties better values for Pd contacts).The presents results should be considered as preliminary with respect to the technology flow development of Ohmic and Schottky contacts bonding for carbyne-based nanoelectronic devices.

Figure 1 .
Figure 1.Time sweep of the applied force (y axis is force in cN, x axis is time in ms) for bond strength test: (a) moments of tearing of gold bond to Pd and histogram of the bond strengths for 10 bonded samples; (b) moments of tearing of aluminum bond to Ag and histogram of the bond strengths for 10 bonded samples.

Figure 2 .
Figure 2. SEM and EDX of a) Pd/carbyne after bonding (Au bond was removed to avoid interference of the electron beam); b) Ag/carbyne after bonding (Al bond was removed to avoid interference of the electron beam).

Table 1 .
Effect of the substrate temperature on the Au bonds strength at pull-off load of 30 g.