Advances in Thermocompression Bonding

Thermocompression bonding is widely used to form interconnections at first-level package interfaces. Wire bonding, flip chip bonding, and wafer bonding technologies are examples of the thermocompression bonding process. The formation of joint is enabled by diffusion which needs the bonding interfaces to be in intimate contact. This allows the metallic atomic interaction across the bonding interfaces when force and heat are applied. The earliest approach of bonding wires to silicon semiconductor devices is by thermocompression bonding. This is followed by the development of ultrasonic and thermosonic modes of wire bonding technologies. In this section, thermocompression bonding related to flip chip bonding and wafer bonding technologies will be discussed.

Flip Chip Bonding

A flip chip is a wafer-scale operation whereby the interconnects are formed on an entire wafer. The wafer is then diced and the individual dice is picked and placed on the substrate. Flip chip bonding takes place by the thermocompression process as schematically presented in Fig. 4. In the case of solid-state diffusion bonding, it usually involves the direct metal bonding of Au or Cu interconnects by thermocompression. The Au and Cu interconnects are either plated or formed as stud bumps using the wire bonder as shown in Fig. 5.
In a study by Ang et al. (2006), the interfacial temperature and bonding pressure dependences in thermocompression gold stud bonding were established. Results revealed that there is a critical bonding temperature below which no bonding will take place. However, above this temperature, the metal surface becomes thermally activated, and the shear strength improves with bonding temperature due to the increase in the true bonded area. This critical temperature can be interpreted to be the onset of the breakup of organic barrier films. Furthermore, pressure dependence was demonstrated when the tensile strength of the joint exhibited a maximum beyond the critical temperature.
With continual miniaturization of feature size to meet the ever-increasing performance demands on integrated circuits, three-dimensional (3-D) integration is the technology which offers the flexibility in system design, placement, and routing. A 3-D integrated circuit (IC) is made up of a stack of vertically bonded and interconnected chips. The inter-chip connection is done using the through-silicon-via (TSV) technology. Cu–Cu bonding is favored by the industry for TSV integration as it concurrently forms the necessary mechanical and electrical connections.  For 3-D interconnects, the Cu–Cu thermocompression bonding technology is the most prominent direct metal bonding method. Cu–Cu joints are desired compared to joints formed using solder materials as:

(i) Cu–Cu bond is more scalable and ultra-fine pitch can be achieved.
(ii) Cu–Cu joints exhibit better electrical, mechanical, and thermal properties as no intermetallic compound is formed, unlike for the joints formed by solder materials.
(iii) Cu has better electromigration resistance and can withstand higher current density in the future.

In recent years, much research effort is also placed to achieve low-temperature Cu–Cu bonding for (i) thermal budget consideration, (ii) lower thermal–mechanical stress, and (iii) alignment control.
In existing literature, several promising approaches such as the utilization of a self-assembled monolayer and a nanoporous metal layer have successfully demonstrated the formation of metal–metal joints by thermocompression.

Use of Self-Assembled Monolayer (SAM)

Several researchers have reported that by coating a SAM monolayer on metallic surfaces such as gold or copper prior to bonding, the bonding temperature required for forming joints can be significantly reduced (Li et al. 2009, 2010; Ang et al. 2007, 2008a, b, 2009; Chin et al. 2006; Tan et al. 2009).
Typically, for the case of direct Cu–Cu bonding, a high bonding temperature (more than 300 C) is necessary to forge a reliable joint, due to the oxidative nature of copper. In a study by Ang et al. (2009), direct Cu bonding with the aid of self-assembled monolayer (SAM) layer was demonstrated under ambient condition, at low bonding temperature below 140o C. The joint formed has a reliable mechanical joint integrity of 50 MPa. In this study, electroplated Cu bumps were acid cleaned and immersed into ethanolic thiol solution followed by the deposition of a layer of undecanethiol. It was reported that the SAM monolayer protects the Cu surface from further oxidation after acid clean. In comparison to the bulk Cu oxide layer, the SAM monolayer is easily displaced by mechanical deformation at the bonding interface.
In another study by Chin et al. (2006), SAMs are also applied to improve the bondability of Au-Au thermocompression bonding at low temperatures. Au stud bumps were successfully bonded to sputtered Au surface with a layer of dodecanethiol (DDT) SAMs. Low-temperature bonding ranging from 80o C to 180o C and bonding pressures ranging from 225 to 566 MPa are achieved. The presence of the SAM layer was found to passivate the bonding surface after the removal of the organic contaminant layer by chemical cleaning.

Nanoporous Au Bump Joints

Oppermann and co-workers from IZM (Oppermann and Dietrich 2012) formed joints of nanoporous Au bumps. Firstly, nanoporous metal layers are electroplated on copper strips and then transferred to Si wafers using a standard wafer bumping equipment. The nanoporous gold bumps were deposited by electroplating silver–gold alloy. The silver is then etched away to result in a nanoporous gold layer as an open porous sponge with porosity of 70–80 %. The porous joints are formed by thermocompression flip chip bonding, and a significant reduction in bonding temperature and pressure can be achieved with the nanoporous bump structure (refer to Fig. 6). The lowest bonding temperature of 150o C is achieved at a bonding pressure of 10 MPa. The bumps formed are reported to have a reduction in height without an increase in the bump’s diameter due to the compressible interface.

Wafer Bonding

Wafer Bonding with Metallic Interlayer

Wafer bonding technologies can be broadly classified into (i) direct wafer bonding (DWB) and (ii) mediated wafer bonding (MWB). For DWB, there is no need for any intermediate layer between two plain wafers which are to be bonded, while for MWB, as the name suggests, there is a presence of an intermediate layer between the two wafers to facilitate the formation of permanent joint. Thermocompression bonding is one of the techniques categorized under the MWB approach. Figure 7 shows the schematic diagram of a thermocompression bonding setup of two wafers with a metal interlayer such as gold or copper (Tsau et al. 2002, 2004; Taklo et al. 2004; Tsau 2003; Kim et al. 1995). Firstly, the two wafer surfaces are brought to close contact. Heat and pressure are then applied to allow interdiffusion to take place. Any oxide present on the surface of the metal interlayer will impede the formation of good joint; thus, it must be removed prior to bonding.
Tsau et al. (2004) demonstrated the thermocompression bonding of silicon substrates using a bonding layer of gold thin film with modest application of temperature and pressure. The following summarizes the findings:

(1) There is a need for a diffusion barrier layer (such as silicon dioxide, SiO2) as the presence of Si at the Au bonding layer will hinder the thermocompression bonding. The Au film reacts with the underlying Si (Kim et al. 1995).
(2) In order to facilitate interdiffusion at the bonding interfaces, the mating surfaces must be brought into close contact when pressure is applied. Thus, a smooth Au film surface is desired so as to utilize low bonding temperatures.
(3) UV-ozone treatment is recommended to aid the removal of organics present on the surface of the wafers.
(4) Based on the study (Tsau et al. 2004) on the effects of bonding pressure, bonding temperature, and bonding time on the bond toughness, it is recommended that a bonding temperature of 260o C and bonding pressure of 120 MPa can meet the repeatability and yield requirements of the industry. Furthermore, bonding time is found to have the least influence on the bond quality, and owing to the preference of faster throughput, the bonding duration can be as short as 2 min.

In another wafer-level thermocompression bonding with gold study by Taklo et al. (2004), a bond yield of 89 % and bond strength of 10.7 ± 4.5 MPa are achieved when the wafers are bonded at 298o C, with an applied pressure of 4 MPa for 45 min. The wafer pair consists of a piece of Corning #7740 (Pyrex) glass wafer with a 2,000 A° thick Au layer bonded to a Si wafer. The joints are reported to be sufficiently strong and dense for MEMS application.

Cu Nanorod Array

In a study by Wang et al. (2009; Ko and Chen 2012), an array of Cu nanorods with a diameter of 10–20 nm grown by an oblique angle deposition technique is used to achieve low-temperature wafer bonding. It is reported that the surface-melting disintegration of the nanorod arrays took place at a temperature significantly lower than that of the bulk Cu melting point. With the aid of a wafer bonder, at a bonding temperature of 200o C and an applied pressure of 0.32 MPa, the Cu nanorod arrays undergo coalescence. As the bonding temperature increases to 400o C, a fully dense Cu bonding layer with homogeneous structure is achieved (refer to Fig. 8). This is attributed to the higher-mobility nature of Cu species associated with Cu nanorods in comparison with the use of a Cu blanket film. It is observed that the phase transformation temperature of Cu nanorod is considerably lower than its bulk melting point of 1,085o C. Based on the Gibbs–Thomson theory, it is well known that the temperature to activate atomic transport decreases with decreasing particle size.

Surface-Activated Wafer Bonding

Conventionally for direct wafer bonding, high annealing temperature is necessary. This often results in thermal stress and the generation of defects. In order to overcome the use of high annealing temperature, the surface-activated bonding approach is developed to achieve room temperature without any annealing steps. As schematically shown in Fig. 9, the contaminant layers and surface-absorbed molecules are removed from the bonding surfaces by intentionally bombarding the surfaces with ion or fast atom beam. When the two surfaces are brought into close promixity, the content area propagates spontaneously by surface attractive forces between the wafers. This is due to the resultant dangling bonds on the surface atoms after the surface activation step. The atoms being in the unstable state will form strong chemical bonds between the two surfaces.
Room temperature Cu–Cu direct wafer bonding is reported by Kim et al. (2003) using the surface-activated bonding method. Thin Cu films are firstly deposited on a diffusion barrier layered 8 in. silicon wafer. It is a requisite for direct bonding to have clean bonding interfaces. The presence of any oxides and carbon-based contaminants on the wafer surfaces will hinder effective wafer bonding. Thus, prior to bonding, the Cu surface is activated using a low-energy Ar ion beam of 40–100 eV. Auger analysis revealed that the strong carbon and oxygen peaks detected due to organic contaminants and native metal oxides are mostly removed after the cleaning step using Ar ion beam bombardment for 60 s. Furthermore, although no thermal annealing is needed to increase the bond strength after bonding, the bonding is carried out under an ultrahigh vacuum environment (~10-8 Torr). Tensile test results revealed that despite bonding at room temperature, high bonding strength equivalent to that of bulk material is achieved. The bonded Cu interfaces also showed no presence of voids from the transmission electron microscopy (TEM) analysis.