Ga-Based Alloys in Microelectronic Interconnects: A Review

Gallium (Ga) and some of its alloys have a range of properties that make them an attractive option for microelectronic interconnects, including low melting point, non-toxicity, and the ability to wet without fluxing most materials—including oxides—found in microelectronics. Some of these properties result from their ability to form stable high melting temperature solid solutions and intermetallic compounds with other metals, such as copper, nickel, and aluminium. Ga and Ga-based alloys have already received significant attention in the scientific literature given their potential for use in the liquid state. Their potential for enabling the miniaturisation and deformability of microelectronic devices has also been demonstrated. The low process temperatures, made possible by their low melting points, produce significant energy savings. However, there are still some issues that need to be addressed before their potential can be fully realised. Characterising Ga and Ga-based alloys, and their reactions with materials commonly used in the microelectronic industry, are thus a priority for the electronics industry. This review provides a summary of research related to the applications and characterisation of Ga-based alloys. If the potential of Ga-based alloys for low temperature bonding in microelectronics manufacturing is to be realised, more work needs to be done on their interactions with the wide range of substrate materials now being used in electronic circuitry.


Introduction
A noteworthy trend is being observed with microelectronic products becoming smaller and more energy efficient [1,2], and performing well while having minimal impact on human health and the environment [3]. Also, both academic and industrial interest has been expressed in flexible or deformable electronics. To meet these challenges, new technologies for manufacturing advanced microelectronic components and packaging of electronic devices are required.
Alloys that are liquid at room temperature or that have a low melting point have the potential to create unique combinations of electrical and thermal conductivity. They offer the additional benefit of enabling the production of flexible electronic circuitry. Much research has been completed on the microelectronic components and circuit innovations that use liquid metals, and the efforts are continuing. These alloys also have the potential of enabling electronic assembly at lower process temperatures than those required by conventional alloys. As well as saving energy, low process temperatures allow the assembly of components and materials that can be damaged by the process temperatures required for assembly with conventional solder alloys, such as the traditional Sn-37Pb solder with a melting point of 183 • C and the lead-free solders required by the European Union's Table 1. Physical properties of gallium (Ga), Ga-based alloys, and mercury (Hg) [5][6][7][8][10][11][12]. Ga has a low vapour pressure at high temperatures, especially when compared to mercury, which is currently the most commonly used room temperature liquid metal. As well as its low toxicity, its low vapour pressure also makes it a safe alternative to Hg and Pb. Liquid Ga can wet many kinds of materials, including most metals and glasses. It has low viscosity and good conductivity (Table 1). Ga exhibits significant undercooling on solidification [13]. It expands by 3.1% on solidification, which is different from most other metals. The thermal expansion coefficient of Ga (18.3 × 10 −6 K −1 ) is closer to that of Cu (16.7 × 10 −6 K −1 ) than Sn (20.5 × 10 −6 K −1 ) or Pb (29.0 × 10 −6 K −1 ) at 20 • C [9]. When contained in flexible tubing at room temperature, liquid Ga-based alloys have been shown to flow and maintain metallic conductivity while being stretched [14].
Although semiconductor applications such as GaAs and GaN dominate the commercial demand for Ga [6,[15][16][17], Ga and Ga-based alloys have attracted significant attention as possible replacements for liquid metals in various applications because of their remarkable fluidity and metallic properties at low temperatures, as well as their lack of toxicity. Ga and its alloys have been proposed as replacements for Hg in dental filling materials [18][19][20] and high-temperature thermometers [21][22][23][24]. Ga and Ga-based alloys have also been studied as potential coolants in nuclear power plants as an alternative to Na [10,[25][26][27][28][29], and in microelectronic devices, such as computers and smart phones [30][31][32][33][34][35]. Eutectic Ga-In (EGaIn) alloys and Ga-In-Sn alloys (such as Galinstan) have received the most attention in the literature in terms of microelectronic applications. The inherent properties of Ga and Ga-based alloys enable their use in microelectronics where deformability, miniaturisation, low process temperature during fabrication, and low toxicity are required. The growing demand for wearable devices and artificial body parts, including prosthetics and implantable device, has inspired research and the production of deformable electronics [36,37]. Many exciting and promising circuits [14,[38][39][40][41][42] and electronic components [43][44][45][46][47][48][49][50][51] that can be bent (flexible) or elongated (stretchable) have been developed in recent years, although these applications are mainly at the laboratory stage. Figures 1 and 2 are examples of flexible or stretchable applications based on Ga alloys. In order to realise these potential applications, basic processing techniques with liquid Ga based alloys have been studied [52]. These efforts include injecting [7] and printing [53][54][55][56] the liquid alloy into or onto various microfluidic channels or substrates. The substrates tested are mainly polymers, such as polydimethylsiloxane (PDMS) and polyvinyl chloride (PVC). Liquid metal patterning is emerging as a major area in room temperature liquid metal research.
Materials 2018, 11, x FOR PEER REVIEW 3 of 19 research and the production of deformable electronics [36,37]. Many exciting and promising circuits [14,[38][39][40][41][42] and electronic components [43][44][45][46][47][48][49][50][51] that can be bent (flexible) or elongated (stretchable) have been developed in recent years, although these applications are mainly at the laboratory stage. Figures 1 and 2 are examples of flexible or stretchable applications based on Ga alloys. In order to realise these potential applications, basic processing techniques with liquid Ga based alloys have been studied [52]. These efforts include injecting [7] and printing [53][54][55][56] the liquid alloy into or onto various microfluidic channels or substrates. The substrates tested are mainly polymers, such as polydimethylsiloxane (PDMS) and polyvinyl chloride (PVC). Liquid metal patterning is emerging as a major area in room temperature liquid metal research. The fiber is stretched to 20 cm and the metal appears to uniformly fill the stretched fiber [14].

Soldering
Soldering has been a fundamental interconnection technology in microelectronic packaging since the commencement of the electronic age. During the soldering process, the gap between the surfaces to be joined (usually metals) is filled with a molten alloy that has the ability to wet the substrates, usually with the assistance of a flux that removes oxides from the substrates and the molten alloy. Traditional Sn-37Pb solder has a melting point of 183 °C. Pb-free solders that are now widely used in electronic assembly have even higher melting points. Sn-3Cu-0.5Cu ("SAC305"), has a melting range of 217-220 °C and Sn-0.7Cu-0.05Ni (SN100C ® ), has a melting point of 227 °C. The temperature required in processes based on those solders has the potential to damage the heatsensitive materials used in electronic devices, including modern microchip components and flexible substrates such as polyester film. There have been ongoing efforts to study various alloying elements The fiber is stretched to 20 cm and the metal appears to uniformly fill the stretched fiber [14]. research and the production of deformable electronics [36,37]. Many exciting and promising circuits [14,[38][39][40][41][42] and electronic components [43][44][45][46][47][48][49][50][51] that can be bent (flexible) or elongated (stretchable) have been developed in recent years, although these applications are mainly at the laboratory stage. Figures 1 and 2 are examples of flexible or stretchable applications based on Ga alloys. In order to realise these potential applications, basic processing techniques with liquid Ga based alloys have been studied [52]. These efforts include injecting [7] and printing [53][54][55][56] the liquid alloy into or onto various microfluidic channels or substrates. The substrates tested are mainly polymers, such as polydimethylsiloxane (PDMS) and polyvinyl chloride (PVC). Liquid metal patterning is emerging as a major area in room temperature liquid metal research. The fiber is stretched to 20 cm and the metal appears to uniformly fill the stretched fiber [14].

Soldering
Soldering has been a fundamental interconnection technology in microelectronic packaging since the commencement of the electronic age. During the soldering process, the gap between the surfaces to be joined (usually metals) is filled with a molten alloy that has the ability to wet the substrates, usually with the assistance of a flux that removes oxides from the substrates and the molten alloy. Traditional Sn-37Pb solder has a melting point of 183 °C. Pb-free solders that are now widely used in electronic assembly have even higher melting points. Sn-3Cu-0.5Cu ("SAC305"), has a melting range of 217-220 °C and Sn-0.7Cu-0.05Ni (SN100C ® ), has a melting point of 227 °C. The temperature required in processes based on those solders has the potential to damage the heatsensitive materials used in electronic devices, including modern microchip components and flexible substrates such as polyester film. There have been ongoing efforts to study various alloying elements

Soldering
Soldering has been a fundamental interconnection technology in microelectronic packaging since the commencement of the electronic age. During the soldering process, the gap between the surfaces to be joined (usually metals) is filled with a molten alloy that has the ability to wet the substrates, usually with the assistance of a flux that removes oxides from the substrates and the molten alloy. Traditional Sn-37Pb solder has a melting point of 183 • C. Pb-free solders that are now widely used in electronic assembly have even higher melting points. Sn-3Cu-0.5Cu ("SAC305"), has a melting range of 217-220 • C and Sn-0.7Cu-0.05Ni (SN100C ® ), has a melting point of 227 • C. The temperature required in processes based on those solders has the potential to damage the heat-sensitive materials used in electronic devices, including modern microchip components and flexible substrates such as polyester film. There have been ongoing efforts to study various alloying elements including Ag, Bi, Cd, Cu, In, Sb, Zn, and Al to create alloys that make possible low temperature soldering processes [58], to reduce energy consumption, and to reduce the risk of damaging components. Their low melting points, together with the possibility of forming intermetallic compounds (IMCs) with other metals meant that Ga and Ga-based alloys are potential joining materials in microelectronics.
The feasibility of using pure Ga paste to realise transient liquid phase bonding with a Cu substrate (with and without Pt coating) at processing temperatures ranging from 160 to 300 • C has been examined [59,60]. In the studies, Cu/Ga/Cu and Cu/Pt/Ga/Pt/Ga sandwich couples have been fabricated at 160 and 300 • C, respectively, to gain knowledge for developing practical industrial Ga-based solder materials ( Figure 3). including Ag, Bi, Cd, Cu, In, Sb, Zn, and Al to create alloys that make possible low temperature soldering processes [58], to reduce energy consumption, and to reduce the risk of damaging components. Their low melting points, together with the possibility of forming intermetallic compounds (IMCs) with other metals meant that Ga and Ga-based alloys are potential joining materials in microelectronics. The feasibility of using pure Ga paste to realise transient liquid phase bonding with a Cu substrate (with and without Pt coating) at processing temperatures ranging from 160 to 300 °C has been examined [59,60]. In the studies, Cu/Ga/Cu and Cu/Pt/Ga/Pt/Ga sandwich couples have been fabricated at 160 and 300 °C, respectively, to gain knowledge for developing practical industrial Gabased solder materials ( Figure 3). The bonding between a magnetron sputtered Cu layer and an electroplated Ga layer on a 50 nm Au seed layer has been investigated [61]. A schematic cross section of the interface to be joined is illustrated in Figure 4a  In addition to the cases in which pure Ga was used as a joint material, some Ga-based alloys have also been studied.
A ternary Sn-32 wt % Bi-6 wt % Ga alloy with a melting point of 128 °C was developed as a solder alloy [62]. The Ga content in this alloy is the maximum amount possible without forming a liquid phase or creating the possibility of Ga segregation. This new solder was reflowed on Cu substrates at 158 °C and then annealed at 70, 90, and 110 °C for 24-720 h to determine the IMCs that The bonding between a magnetron sputtered Cu layer and an electroplated Ga layer on a 50 nm Au seed layer has been investigated [61]. A schematic cross section of the interface to be joined is illustrated in Figure 4a. The bonding of these samples was performed at 25 • C in commercial wafer bonding equipment with a pressure of 2.5 MPa and holding time of 10 min. Subsequently, the wafers were annealed at different temperatures up to 200 • C for 80 h. After annealing at 90 • C, a shear strength of up to 90 MPa was achieved. The scanning electron microscopy (SEM) image of the bond cross section is shown in Figure 4b. including Ag, Bi, Cd, Cu, In, Sb, Zn, and Al to create alloys that make possible low temperature soldering processes [58], to reduce energy consumption, and to reduce the risk of damaging components. Their low melting points, together with the possibility of forming intermetallic compounds (IMCs) with other metals meant that Ga and Ga-based alloys are potential joining materials in microelectronics. The feasibility of using pure Ga paste to realise transient liquid phase bonding with a Cu substrate (with and without Pt coating) at processing temperatures ranging from 160 to 300 °C has been examined [59,60]. In the studies, Cu/Ga/Cu and Cu/Pt/Ga/Pt/Ga sandwich couples have been fabricated at 160 and 300 °C, respectively, to gain knowledge for developing practical industrial Gabased solder materials ( Figure 3). The bonding between a magnetron sputtered Cu layer and an electroplated Ga layer on a 50 nm Au seed layer has been investigated [61]. A schematic cross section of the interface to be joined is illustrated in Figure 4a. The bonding of these samples was performed at 25 °C in commercial wafer bonding equipment with a pressure of 2.5 MPa and holding time of 10 min. Subsequently, the wafers were annealed at different temperatures up to 200 °C for 80 h. After annealing at 90 °C, a shear strength of up to 90 MPa was achieved. The scanning electron microscopy (SEM) image of the bond cross section is shown in Figure 4b. In addition to the cases in which pure Ga was used as a joint material, some Ga-based alloys have also been studied.
A ternary Sn-32 wt % Bi-6 wt % Ga alloy with a melting point of 128 °C was developed as a solder alloy [62]. The Ga content in this alloy is the maximum amount possible without forming a liquid phase or creating the possibility of Ga segregation. This new solder was reflowed on Cu substrates at 158 °C and then annealed at 70, 90, and 110 °C for 24-720 h to determine the IMCs that In addition to the cases in which pure Ga was used as a joint material, some Ga-based alloys have also been studied.
A ternary Sn-32 wt % Bi-6 wt % Ga alloy with a melting point of 128 • C was developed as a solder alloy [62]. The Ga content in this alloy is the maximum amount possible without forming a liquid phase or creating the possibility of Ga segregation. This new solder was reflowed on Cu substrates at 158 • C and then annealed at 70, 90, and 110 • C for 24-720 h to determine the IMCs that formed and the evolution of the interfacial morphology as a function of time. The only IMC phase observed at the interface area was CuGa 2 . The shear strength of the joints formed by reflowing this solder alloy on Cu substrate with an organic solderability preservative (OSP) coating was measured and the authors concluded that although the CuGa 2 IMC is brittle, this solder is still a potential candidate for low-temperature microelectronics packaging.
As well as the Sn-Bi-Ga alloy, a eutectic Ga-Zn (Ga-10 wt % Zn, melting point 24.7 • C) solder paste has been proposed, and the interactions with single-phase brasses at 150 and 200 • C have been investigated during the soldering process [63]. This Zn-containing Ga alloy has advantages in both the manufacturing process and joint quality, and is regarded as a promising material for low-temperature diffusion soldering of materials used in microelectronics.
The solderability of the eutectic Ga-Sn (EGaSn, Ga-13.5 wt % Sn, melting point 20.5 • C) liquid alloy on Au-coated Cu substrates was observed by forming sandwich joints and holding them at room temperature or 100 • C for seven days [64]. Pressure (10 kPa) was required during the process in order to form the bond. This study showed the possibility of using EGaSn in low temperature bonding of Cu substrates.
The bonding of Ga and Ga-based alloys with substrates other than Cu has also been investigated. A composite soldering paste for cermet sections has been developed using Ga as the low-melting base and a two-part powder mixture (Cu-20 wt % Sn alloy, and eutectic Ag-28.1 wt % Cu alloy) as the high temperature filler [65]. Several materials, including titanium wrought alloy (OT4-VK94-1), aluminum alloy (M1-SK-1) and bronze (BrB2-SK-1), were soldered using this paste. This solder needs a process temperature of 200 • C, a holding time of two hours and a pressure of 3-4 MPa to make a joint with adequate strength.
A metallic paste created by mixing Ga, Al, and Ni powders (45 wt % Ga, 15 wt % Al, and 40 wt % Ni) has been reported [66]. The diffusion soldering process was conducted at a temperature of 700 • C for 20 min. Pb-free joints were produced between Cu and Ni substrates by forming a solid solution and IMCs that are stable even at temperatures of about 1200 • C. As shown in Figure 5, the phases detected in the joint interface area were: α'-Ni 3 Ga (labelled 'A' in the image), β-Cu 3 Ga ('B') and a Cu solid solution ('C') of the Ga-Cu system. formed and the evolution of the interfacial morphology as a function of time. The only IMC phase observed at the interface area was CuGa2. The shear strength of the joints formed by reflowing this solder alloy on Cu substrate with an organic solderability preservative (OSP) coating was measured and the authors concluded that although the CuGa2 IMC is brittle, this solder is still a potential candidate for low-temperature microelectronics packaging.
As well as the Sn-Bi-Ga alloy, a eutectic Ga-Zn (Ga-10 wt % Zn, melting point 24.7 °C) solder paste has been proposed, and the interactions with single-phase brasses at 150 and 200 °C have been investigated during the soldering process [63]. This Zn-containing Ga alloy has advantages in both the manufacturing process and joint quality, and is regarded as a promising material for lowtemperature diffusion soldering of materials used in microelectronics.
The solderability of the eutectic Ga-Sn (EGaSn, Ga-13.5 wt % Sn, melting point 20.5 °C) liquid alloy on Au-coated Cu substrates was observed by forming sandwich joints and holding them at room temperature or 100 °C for seven days [64]. Pressure (10 kPa) was required during the process in order to form the bond. This study showed the possibility of using EGaSn in low temperature bonding of Cu substrates.
The bonding of Ga and Ga-based alloys with substrates other than Cu has also been investigated. A composite soldering paste for cermet sections has been developed using Ga as the low-melting base and a two-part powder mixture (Cu-20 wt % Sn alloy, and eutectic Ag-28.1 wt % Cu alloy) as the high temperature filler [65]. Several materials, including titanium wrought alloy (OT4-VK94-1), aluminum alloy (M1-SK-1) and bronze (BrB2-SK-1), were soldered using this paste. This solder needs a process temperature of 200 °C, a holding time of two hours and a pressure of 3-4 MPa to make a joint with adequate strength.
A metallic paste created by mixing Ga, Al, and Ni powders (45 wt % Ga, 15 wt % Al, and 40 wt % Ni) has been reported [66]. The diffusion soldering process was conducted at a temperature of 700 °C for 20 min. Pb-free joints were produced between Cu and Ni substrates by forming a solid solution and IMCs that are stable even at temperatures of about 1200 °C. As shown in Figure 5, the phases detected in the joint interface area were: α'-Ni3Ga (labelled 'A' in the image), β-Cu3Ga ('B') and a Cu solid solution ('C') of the Ga-Cu system. The flip-chip interconnect applications of Ga-based alloys have also been studied [67]. Ga-Cu-Ni ternary alloys (Ga-30 wt % Cu-5 wt % Ni and Ga-21.4 wt % Cu-3.6 wt % Ni) have been used as a micro-miniature interconnect to join bare silicon chips and printed circuit boards. The bonding process began with pressure being applied for one hour, after which the test vehicles were placed in a convection oven at 150 °C for one hour to complete the Ga alloy and underfill cure.
The processability of Ga, 30 wt % Cu, 5 wt % Ni alloy for room temperature via filling applications has also been demonstrated using stencil printing onto stainless steel substrates [68]. After filling, a 16 h cure at 130 °C was required to achieve interconnections. The research results The flip-chip interconnect applications of Ga-based alloys have also been studied [67]. Ga-Cu-Ni ternary alloys (Ga-30 wt % Cu-5 wt % Ni and Ga-21.4 wt % Cu-3.6 wt % Ni) have been used as a micro-miniature interconnect to join bare silicon chips and printed circuit boards. The bonding process began with pressure being applied for one hour, after which the test vehicles were placed in a convection oven at 150 • C for one hour to complete the Ga alloy and underfill cure.
The processability of Ga, 30 wt % Cu, 5 wt % Ni alloy for room temperature via filling applications has also been demonstrated using stencil printing onto stainless steel substrates [68]. After filling, a 16 h cure at 130 • C was required to achieve interconnections. The research results suggest that this alloy has potential as a low temperature interconnect material.

Heat-Free Bonding
Stable undercooled liquid nano-and/or micro-particles have been considered for use as a low temperature solder [69]. Since Ga and some of its alloys are liquid near or at room temperature and exhibit significant undercooling on solidification, a few attempts have been made to eliminate the heat required in conventional soldering.
A new kind of metallic glue termed "MesoGlue Eutectic" has been developed [70]. The bond is achieved by first planting Ga and In coated rods alternately along a substrate. The teeth-on-comb rods on each substrate are interlaced. When they contact the In and Ga form a liquid. The eutectic liquid eventually turns into a solid and forms a metal bond. The gluing can be performed at room temperature and in air, although it requires some pressure (<100 psi). Figure 6 is a schematic diagram of this metallic gluing bond. The bond is about as strong as a metallurgical bond between two parent materials. This metallic glue is proposed for use as a joining method in the microelectronic packaging industry, and may replace thermal greases as it transfers heat more efficiently. At this point, this metallic glue has only been applied in a laboratory.

Heat-Free Bonding
Stable undercooled liquid nano-and/or micro-particles have been considered for use as a low temperature solder [69]. Since Ga and some of its alloys are liquid near or at room temperature and exhibit significant undercooling on solidification, a few attempts have been made to eliminate the heat required in conventional soldering.
A new kind of metallic glue termed "MesoGlue Eutectic" has been developed [70]. The bond is achieved by first planting Ga and In coated rods alternately along a substrate. The teeth-on-comb rods on each substrate are interlaced. When they contact the In and Ga form a liquid. The eutectic liquid eventually turns into a solid and forms a metal bond. The gluing can be performed at room temperature and in air, although it requires some pressure (<100 psi). Figure 6 is a schematic diagram of this metallic gluing bond. The bond is about as strong as a metallurgical bond between two parent materials. This metallic glue is proposed for use as a joining method in the microelectronic packaging industry, and may replace thermal greases as it transfers heat more efficiently. At this point, this metallic glue has only been applied in a laboratory. Another type of metallic cement (or metallic glue) is obtained by the chemical interaction of liquid metals with metallic alloy powders. This type of cement has high adhesion to materials of different types and presents good heat and electric conductivity. This type of bond can be widely applied to joining ceramics, metals, quartz, graphite, and other thermostable materials in aviation and electronics industries and in instrument engineering. Grigor'eva et al. [71] investigated the structure of metallic cements formed by the interaction of Cu/Bi mechanocomposites with liquid Ga at room temperature. The Cu/Bi mechanocomposites are mechanically activated powders obtained by mixing Cu and Bi powders in planetary ball mill in an argon atmosphere. The application of the Cu-Bi mechanocomposite, instead of Cu, as a filler for obtaining Ga-based glues not only reduces the curing time of the cement from two days to 14 h, but also increases the compressive strength of joints from 30 to 70 MPa.
Ye et al. [72] took advantage of the low melting temperature of Ga and found that Ga layers can act as reversible and switchable adhesives. The adhesion status can be controlled by slight temperature changes. The switch is sensitive and the joint has a similar strength to a conventional glue. The temporary adhesion is electrically conductive, repeatable, and leaves the surface clean after lifting. Applications of this adhesion can be found in areas such as industrial pick-and-place processes and temporary wafer bonding. Adhesive switchability is proposed as an enabling technology for the feet of climbing robots.

Characterisation of Reactions between Liquid Ga-Based Alloys and Solid Metals
Various characteristics of pure liquid Ga and liquid Ga-based alloys (mainly EGaIn and Galinstan) have been observed. These includes basic thermophysical [11,21,34,44,45,73,74], electrochemical [46,[75][76][77], electromagnetic [78,79], fluidity and wettability properties [80][81][82], and self-fueled actuation, in which chemical energy spontaneously converts into mechanical activity to Another type of metallic cement (or metallic glue) is obtained by the chemical interaction of liquid metals with metallic alloy powders. This type of cement has high adhesion to materials of different types and presents good heat and electric conductivity. This type of bond can be widely applied to joining ceramics, metals, quartz, graphite, and other thermostable materials in aviation and electronics industries and in instrument engineering. Grigor'eva et al. [71] investigated the structure of metallic cements formed by the interaction of Cu/Bi mechanocomposites with liquid Ga at room temperature. The Cu/Bi mechanocomposites are mechanically activated powders obtained by mixing Cu and Bi powders in planetary ball mill in an argon atmosphere. The application of the Cu-Bi mechanocomposite, instead of Cu, as a filler for obtaining Ga-based glues not only reduces the curing time of the cement from two days to 14 h, but also increases the compressive strength of joints from 30 to 70 MPa.
Ye et al. [72] took advantage of the low melting temperature of Ga and found that Ga layers can act as reversible and switchable adhesives. The adhesion status can be controlled by slight temperature changes. The switch is sensitive and the joint has a similar strength to a conventional glue. The temporary adhesion is electrically conductive, repeatable, and leaves the surface clean after lifting. Applications of this adhesion can be found in areas such as industrial pick-and-place processes and temporary wafer bonding. Adhesive switchability is proposed as an enabling technology for the feet of climbing robots.
Notably, the interfacial reactions between the liquid Ga-based alloy and solid materials are important for making better use of these materials, since liquid alloy/solid state contact is common in all the applications mentioned in this study. More specifically, the interfacial reactions could be divided into microstructure development in the interfacial layer and wettability. In some situations, these two processes can influence one another, so understanding the mechanisms in these two aspects can be complex. These fundamental but important research achievements help in the selection of reliable materials for these applications and thus enable the further use of Ga and Ga-based alloys.

Microstructure Development between Liquid Ga-Based Alloy and Solid Materials
To understand microstructure development, the reaction products between the liquid and solid layers need to be identified and their morphologies observed. These affect the contact quality and stability as measured by thermal and electrical conductivity and/or joint strength.
During the interaction between the liquid Ga alloy and metal substrate or powder, interfacial IMCs usually form. Thus, dissolution of the materials, IMCs composition, epitaxial growth of the IMC grains, formation of solid/liquid or even new solid/solid interfaces, and their evolution with time or surrounding environment (temperature, atmosphere, etc.) may all require consideration.

Liquid Ga-Based Alloy Reactions with Metal Powder
The interaction between liquid Ga (or Ga-based alloys) and Cu or single phase Cu alloy powder has been the focus of several research papers [90][91][92][93][94][95][96]. The researchers mainly used Cu, Cu-based solid solution, or IMC powders and studied their reaction with liquid Ga or liquid Ga-based alloy, and investigated the formation of IMCs. This group of researchers used the mechanochemical method (planetary ball-milling in an argon atmosphere) to obtain the solid solution or IMC powder, which was then mixed with the liquid Ga (or Ga-based alloys). The in-situ X-ray diffraction (XRD) observations were recorded at the Siberian Center of Synchrotron Radiation, Russia. Their results showed that, when pure Cu powder, Cu + 20 wt % Ga solid solution, or Cu 9 Ga 4 , are mixed with liquid Ga in proportions approximately corresponding to the stoichiometry of the main reaction of Cu with Ga at a small Cu excess, CuGa 2 is the single product of the interaction at room temperature. In the Cu solid solution (with Sn, In, or Bi) Ga liquid eutectic systems, the first phase to form is a microcrystalline IMC. Then, after an induction period, the IMC resulting from the reaction between Cu and Ga appears. The metallic phase consists of much coarser grains in comparison with the starting powder and primary (IMC) phase. All the reactions are summarized in Table 2.
The increasing Sn concentration in the Cu-Sn solid with liquid Ga or EGaSn system shortens the induction period for the crystallisation of the Sn phase. For Cu(Sn) powders reacting with liquid Ga, Sn segregation into an autonomous phase occurred 30 h after the components were mixed. When using EGaSn as the liquid phase, Sn was segregated into an autonomous phase four hours after the components were mixed. When using mechanochemical synthesized Cu 3 Sn or Cu 6 Sn 5 , CuGa 2 and Sn appeared almost immediately after mixing, and all Ga from the eutectic melt was consumed by CuGa 2 formation in 10 and 2 h, respectively. The IMC phase decreased in size as the Sn concentration increased. For the Cu 3 Sn + EGaSn, the IMC CuGa 2 had particle sizes of at most 1 µm; Sn had far coarser particles. For the Cu 6 Sn 5 + EGaSn, the CuGa 2 crystal sizes were less than 0.1 µm. All the reactions were analysed at room temperature (~20 • C). Compositions are in weight percent; 1 Pure Ga was brought to a liquid state at~37 • C before the mixture; 2 Sn(Ga) stands for Ga solid solution in Sn.
Besides the characterisation of the solid solution powder/ liquid Ga system for metallic bonding material fabrication, research on another series of Ag-Sn-Cu-based alloy powders mixed with Ga-In-Sn liquid has been completed for dental use [20,97,98]. Several techniques were employed to investigate the reaction mechanisms during the setting process and the microstructural morphology evolution of these dental alloys, including SEM, XRD [97], transmission electron microscopy (TEM) [98], and differential scanning calorimetry (DSC) [20]. When these Ag-Sn-Cu-based alloy powders are mixed with the Ga-In-Sn liquid at room temperature, the materials transform to a solid. θ-CuGa 2 , Ga 28 Ag 72 , β-Sn, Ag 3 Sn, γ-Cu 9 Ga 4 , Ag 9 In 4 , and hexagonal Ag 2 Ga were reported to be observed in the system, as listed in Table 2 [99]. The reaction phase composition and the setting reaction kinetics influenced the clinically relevant properties, including the mechanical strength, corrosion resistance, and biocompatibility of these dental amalgams [99].

Reactions between Liquid Ga-Based Alloys and Cu Substrates
When using these alloys in microelectronics, liquid Ga-based alloys and solid substrates interact. Since Ga has a high solubility in the face centered cubic (FCC) Cu-rich phase, and there are several IMCs that could form over a large temperature range [100]. Ga-based solders attract attention when the materials for connecting Cu substrates in microelectronic packaging have to be chosen.

Microstructure Evolution
Bulk Cu/Ga couples and Cu/Ga/Cu sandwich couples with reactions at moderate processing temperatures ranging from 160 to 300 • C have been studied [59,60]. Room-temperature IMC formation and the relevant interdiffusion behaviour in thin-film Cu/Ga [101] couples have been analysed. Reaction conditions and products are summarised in Table 3.  [64] Compositions are in weight percent. Figure 7 shows the interfacial microstructure of Ga/Cu couples reacted at 200 • C from 3 to 48 h. Diagrammatic sketches in Figure 8 interpret the microstructure evolution when liquid Ga is exposed to Cu substrates (Figure 8a), Cu dissolves into the liquid at certain localised sites and forms a "basin-type" depression ( Figure 8b). Then, the θ-CuGa 2 phase nucleates at sites within the basins (Figure 8c). As the reaction progresses, the θ-CuGa 2 phase is also found in the remaining uncorroded areas (Figure 8d). During this stage, a very thin layer of γ 3 -Cu 9 Ga 4 phase can be detected in the basins/depressions, between the θ-CuGa 2 crystals and the Cu substrate. As the reaction time increases, a planar IMC layer is formed and thickens, and extends to cover the substrate (Figure 8e). Finally, the γ 3 -Cu 9 Ga 4 phase thickens and forms a continuous layer that covers all the interface between the Cu and the CuGa 2 (Figure 8f). As a result of substrate dissolution in liquid Ga and consumption in IMC growth (also referred to as a type of erosion), the Cu interface retreats. Compositions are in weight percent. Figure 7 shows the interfacial microstructure of Ga/Cu couples reacted at 200 °C from 3 to 48 h. Diagrammatic sketches in Figure 8 interpret the microstructure evolution when liquid Ga is exposed to Cu substrates (Figure 8a), Cu dissolves into the liquid at certain localised sites and forms a "basintype" depression ( Figure 8b). Then, the θ-CuGa2 phase nucleates at sites within the basins ( Figure  8c). As the reaction progresses, the θ-CuGa2 phase is also found in the remaining uncorroded areas (Figure 8d). During this stage, a very thin layer of γ3-Cu9Ga4 phase can be detected in the basins/depressions, between the θ-CuGa2 crystals and the Cu substrate. As the reaction time increases, a planar IMC layer is formed and thickens, and extends to cover the substrate (Figure 8e). Finally, the γ3-Cu9Ga4 phase thickens and forms a continuous layer that covers all the interface between the Cu and the CuGa2 (Figure 8f). As a result of substrate dissolution in liquid Ga and consumption in IMC growth (also referred to as a type of erosion), the Cu interface retreats.    The basin-type morphology in the interface is caused by non-uniform reactions and is referred to as liquid metal embrittlement. This kind of interface has a negative effect on the joint strength and efforts have been made to inhibit this process. Methods that have been tried include changing the substrate from polycrystalline to a single-crystal to eliminate grain boundary effects [59], improving the wettability of liquid Ga on substrates by the addition of interlayers [59], and controlling the amount of liquid Ga [60]. The results in Figure 9 show that the non-uniform morphology forms at The basin-type morphology in the interface is caused by non-uniform reactions and is referred to as liquid metal embrittlement. This kind of interface has a negative effect on the joint strength and efforts have been made to inhibit this process. Methods that have been tried include changing the substrate from polycrystalline to a single-crystal to eliminate grain boundary effects [59], improving the wettability of liquid Ga on substrates by the addition of interlayers [59], and controlling the amount of liquid Ga [60]. The results in Figure 9 show that the non-uniform morphology forms at both the Ga/polycrystalline Cu and the Ga/single-crystal Cu interface, though the degree of "basin" formation in the Ga/polycrystalline Cu interface is greater. This suggests that grain boundaries are not the only reason for the basin-type interface formation. By sputtering Pt under bump metallization on polycrystalline Cu substrates, the wettability of the Cu substrate improved and the IMC layer grew uniformly along the interface. both the Ga/polycrystalline Cu and the Ga/single-crystal Cu interface, though the degree of "basin" formation in the Ga/polycrystalline Cu interface is greater. This suggests that grain boundaries are not the only reason for the basin-type interface formation. By sputtering Pt under bump metallization on polycrystalline Cu substrates, the wettability of the Cu substrate improved and the IMC layer grew uniformly along the interface. There are drawbacks to these low temperature bonding methods using Ga and Ga-based alloys. A lack of hermeticity is one of the common problems that occurs during the bonding process as a consequence of the microstructure evolution, shown in Figure 3. Notably, cracks occur in the θ-CuGa2 phase regions of the Cu/Ga/Cu joints [59,60]. Similar cracks were found within the γ1-Cu9Ga4 phase in the Cu/Pt/Ga/Pt/Ga joints [59] and the Cu/Au/EGaSn/Au/Cu joints [64], as shown in Figure 10a,b. There are drawbacks to these low temperature bonding methods using Ga and Ga-based alloys. A lack of hermeticity is one of the common problems that occurs during the bonding process as a consequence of the microstructure evolution, shown in Figure 3. Notably, cracks occur in the θ-CuGa 2 phase regions of the Cu/Ga/Cu joints [59,60]. Similar cracks were found within the γ 1 -Cu 9 Ga 4 phase in the Cu/Pt/Ga/Pt/Ga joints [59] and the Cu/Au/EGaSn/Au/Cu joints [64], as shown in Figure 10a,b.
There are drawbacks to these low temperature bonding methods using Ga and Ga-based alloys. A lack of hermeticity is one of the common problems that occurs during the bonding process as a consequence of the microstructure evolution, shown in Figure 3. Notably, cracks occur in the θ-CuGa2 phase regions of the Cu/Ga/Cu joints [59,60]. Similar cracks were found within the γ1-Cu9Ga4 phase in the Cu/Pt/Ga/Pt/Ga joints [59] and the Cu/Au/EGaSn/Au/Cu joints [64], as shown in Figure 10a,b.  One explanation for the cracks and voids in the Cu/Ga/Cu sandwich couples is the brittle nature of the θ-CuGa 2 phase formed at the joint interface. Based on this possibility, several methods have been proposed to improve the joint reliability. One such method involves using a higher bonding temperature to mitigate the formation of the θ-CuGa 2 phase. Froemel et al. [61] found that by increasing the annealing temperature (from 90 to 200 • C), the amount of CuGa 2 decreased, favouring the formation of Cu 9 Ga 4 , and the shear strength increased correspondingly. Another hypothesis considered that the cracks were formed during the liquid-solid reaction instead of the physical vibration [64]. The different morphologies of the Cu/Au/EGaSn/Au/Cu joint interface are likely to arise from different temperatures of Sn precipitation, as demonstrated in Figure 10c. The two layers of Sn shown in Figure 10b demarcate the liquid/solid interface location when the Sn(Ga) precipitation occurred.
Another kind of defect formed in the interface are Kirkendall voids, which are caused by the fast diffusion of the substrate materials into the molten Ga alloy [61,102]. Thus, the interdiffusion coefficient of the liquid/solid couple is an important consideration when selecting the joining alloy.

IMC Properties
As can be seen from the Cu-Ga phase diagram and microstructure analysis, the main IMCs formed in the Cu-Ga system at low temperature are CuGa 2 and Cu 9 Ga 4 . A study showed that effective packaging of EGaIn with CuGa 2 would remarkably enhance the electrical conductivity (6 × 10 6 S m −1 , ∼80% increase) and thermal conductivity (50 W m −1 K −1 , ∼100% increase) compared to EGaIn [103]. The result further indicated the possibility of utilizing Ga-based alloys in microelectronic interconnections to provide good thermal and electrical conductivity as well as mechanical support.
The thermal stability of the CuGa 2 phase was studied in an inert atmosphere [104]. In this study, the authors found that the CuGa 2 transforms to Cu 9 Ga 4 and liquid Ga at 258 • C, rather than the 225 • C predicted from modelling. Thermal diffusivity and thermal conductivity of CuGa 2 at temperatures between 25 and 227 • C were measured.
Of the commonly used joining materials listed in Table 4, the Cu/Ga system shows the smallest volume change. This means that the strain introduced by the reaction that occurs during the bonding process is the smallest, which could have implications for subsequent reliability of the joint. Table 4. Volume changes associated with interfacial reactions in soldering process [60].

Reactions between Liquid Ga-Based Alloys and Other Substrates
Reactions between Ga-based alloys and substrates other than Cu are less well characterized than those with Cu substrates. The dissolution and diffusion-reaction processes between Ga pastes (Ga-40 wt % Ni-15 wt % Al, or Ga-45 wt % Al) and two substrates (Cu and Ni) at 700 • C were analysed [66]. The IMCs and solid solution layers formed at the interface are shown in Figure 4. Room-temperature IMC formation and the relevant interdiffusion behavior in thin film Au/Ga [105] and Pd/Ga [101] couples has been analysed, and Au has been identified as a rapidly diffusing species. The suitability of Ga-In-Sn alloy solder as a detachable contact material with thermoelectric materials CoSb 3 , Mg 2 Si, and FeSi 2 has been studied [106]. W, Ni, Cr, and Ti were tested as protective coatings between the thermoelectric material and liquid metal solder. W was recognized as a long-term stable coating material to protect materials from Ga-In-Sn solder. Although Cr and Ni react with Galinstan, they showed promising results as effective protective coatings for short-term applications. Reaction conditions and products are summarized in Table 5. Ga-40% Ni-15% Al a Cu foil and a Ni foil 700 • C 20 min Ni 3 Ga, Cu 3 Ga, Cu solid solution [66] Compositions are in weight percent.
One of the problems for Ga in coolant applications is the serious corrosion that occurs when Ga comes into contact with Al alloys, which are the construction materials most commonly used in nuclear power plant cooling systems. Several studies have focused on the reaction mechanisms of Ga and Ga-based alloys with the candidate structural materials in nuclear power plant cooling systems. Therefore, we could gain some knowledge about the reactions between liquid Ga-based alloys and other substrates from these corrosion observations, as summarized in Table 5.
For atomic reactor cooling purposes, Fe, Ni, and Cr react with Ga quickly, whereas Nb 5 Mo 1 Zr alloy and 316 L stainless steel have better resistance to corrosion by Ga [107,108]. Further mathematical analysis of liquid Ga and liquid Ga-Sn-Zn alloy corrosion of austenitic stainless steels has been carried out [109]. The compatibility of Ga with four typical substrates, including two Al alloys, a Cu alloy, and a stainless steel, was observed in the temperature range relevant to the cooling of computer chips [110].

Wettability
Wettability properties are vital for liquid alloys in nearly all applications, including interconnection fabrication. Taking soldering as an example, the success of the joining process depends on the development of a liquid interlayer that has the ability to wet the substrate materials. Factors identified as influencing wetting include liquid metal oxidation behavior, liquid-solid alloying, or corrosion reactions with substrates, and the substrate surface conditions including roughness, contamination, segregation, and oxides [111].
The wetting behavior of room temperature liquid Ga-based alloys on metal substrates, based on microelectronic interconnect applications, have not been well observed. In other applications, such as three-dimensional (3D) structure patterning, microfluidic and nano-particle fabrication, extensive research has been completed to study the wettability of liquid Ga-based alloys in order to maintain the required geometries. The studies include surface tension of alloy droplets in different atmosphere [112] and the substrate conditions [113,114]. Here, we briefly discuss the progress of wettability characterisation of Ga-based alloys on substrates.

Wetting Characteristics of Liquid Ga and Ga-Based Alloys
The surface tension of pure Ga and its temperature dependence have been measured by the sessile drop technique [80,115] and the pendant-drop method [116].
For Galinstan and eutectic Ga-In, several measurements have been recorded at room temperature in different atmospheres. Liu et al. [82] obtained the contact angle of Galinstan on several materials commonly used in micro-electromechanical systems devices (including tungsten, silicon nitride, glass, parylene, Teflon, phlogopite, and muscovite) using the sessile drop method. The surface tension of Galinstan on these substrates was measured using the pendant-drop method under a nitrogen atmosphere. There were no chemical reactions between any of the substrates tested and Galinstan, and the surface tension of Galinstan was measured to be around 534.6 mN/m. The authors found that the oxidation state had an obvious effect on the Galinstan drop morphology and fluidity. During the wettability tests, only when the oxygen level was precisely controlled to be below 1 ppm, would Galinstan droplets behave like a normal liquid. The results confirmed the effect of the oxide skin on the Galinstan wettability on solid substrates. Xu et al. [117] measured the wettability including viscosity, surface tension, and contact angles of pure Ga and eutectic Ga-In alloy, while controlling the oxidation level by immersing the liquid metal drops in a hydrochloric acid bath with different HCl concentrations. The dynamic and static contact angles of eutectic Ga-In alloy on cellulose paper and double-sided adhesive were also obtained by Han et al. [85]. All these measurements were taken on substrates that do not chemically react with or are penetrated by these liquid Ga-based alloys.
As well as these wettability measurements at room temperature, the surface tension of liquid Ga-Bi, Ga-In, Ga-Bi-Sn, and Ga-Bi-In alloys at 600 • C were also measured using the sessile drop method on graphite substrates in a hydrogen atmosphere [118].

Effect of Liquid-Solid Interaction on Wettability
For the Ga/solid thin film couples, interdiffusion was found to occur heterogeneously over the film surface, and blisters formed due to the diffusion-induced stresses [105].
The spreading and penetration phenomenon between Ga and thin film Ag was investigated at temperatures ranging from −78 to 60 • C [119]. The activation energies of solid and liquid Ga in linear spreading on silver films were obtained. The effects of IMCs or solid solution formation, grain boundary diffusion, and grooving were examined, and models were produced for the spreading rates. This kind of investigation and associated mathematical models may be helpful in many liquid metal applications, including, for example, low temperature soldering.

Summary
The application of Ga and Ga-based alloys in microelectronics has already received significant attention. The most attractive properties of Ga and Ga-based alloys arise from their combination of thermal and electrical conductivity, with fluidity at low temperatures or even at room temperature. In addition, they exhibit other desirable properties, such as low toxicity, an ability to wet almost all materials used in electronics, high boiling points, low vapour pressure, and the possibility of forming stable high temperature solid solutions and IMCs with other metals (Al, Cu, Ni, etc.).
The properties of Ga and Ga-based alloys make them very suitable for use in microelectronics. Their deformability creates opportunities for further applications, providing a potential pathway to meet the demands for deformability and miniaturisation of products, low fabrication temperatures, and minimising the impact on both human health and the environment. Demonstrations of Ga and liquid Ga-based alloy applications in microelectronics have been reported in the literature, including deformable electronics fabrication, thermal management or heat transfer in integrated circuit systems, and low temperature bonding in electronic packaging.
Various characteristics of Ga and Ga-based alloys (mainly eutectic Ga-In and Galinstan) have been observed. This research has laid the foundation for some techniques for liquid alloy control and utilisation. For example, methods for "printing" or "writing" liquid Ga metals is emerging as a popular research area. However, there are relatively few studies relating to the interfacial reactions between liquid Ga-based alloys and common substrates in microelectronics.
Though Ga and Ga-based alloys hold some appeal for the electronics industry, a fundamental understanding about the interfacial reactions between liquid Ga-based alloys and other components and substrates at low temperatures needs to be developed to fully utilise these interesting materials and expedite their use in industry. Their long-term reliability in real devices where they have been used for low temperature bonding or their stretchability remains to be verified.