Parameters for pulse electrodeposition of Au–Cu alloy films.
Strengthening of electrodeposited gold-based materials is achieved by alloying with copper according to the solid solution strengthening mechanism. Composition of the Au–Cu alloys is affected by the applied current density. The mechanical properties are evaluated by micro-compression tests to evaluate the mechanical properties in microscale to take consideration of the sample size effect for applications as microcomponents in MEMS devices. The yield strength reaches 1.15 GPa for the micropillar fabricated from constant current electrodeposited Au–Cu film, and the film is composed of 30.3 at% Cu with an average grain size of 5.3 nm. The yield strength further increases to 1.50 GPa when pulse current electrodeposition method is applied, and the Cu concentration is 36.9 at% with the average grain size at 4.4 nm.
- gold-based alloys
- mechanical property
- microcompression test
- Hall-Petch relationship
- solid solution strengthening
1.1 Application of Au materials in MEMS devices
In recent years, microelectromechanical system (MEMS) capacitive accelerometers have been developed and used in a variety of consumer electronics for acceleration detection in a range of 1–5 G (1 G = 9.8 m/s2) [1, 2, 3]. For applications in medical and health care fields, accurate sensing with sub-1 G detection is necessary to monitor hardly detectable body motions [4, 5]. To detect such low acceleration in a compact sensor module, various types of MEMS accelerometers based on silicon (Si) bulk micromachining have been reported [6, 7]. In order to suppress the thermal-mechanical noise (i.e., Brownian noise (BN) ) for the highly sensitive detection, a large proof mass is required. Limited choices of materials for the proof mass and other movable components in a CMOS-MEMS accelerometer have been a major challenge to reduce the BN, which becomes more critical when the parasitic capacitance is reduced in miniaturized devices. Yamane et al. [9, 10, 11] propose a miniaturized MEMS accelerometer by using a post-CMOS process with electrodeposited Au in the main components, which enables further size reduction of the proof mass and the device footprint without compromising the sensitivity. With the application of electrodeposited Au in MEMS accelerometers [9, 10, 11], a wide range of acceleration from 1 mG to 20 G can be achieved and are expected to be used in monitoring of hardly detectable body motions.
However, mechanical strengths of Au are much lower than the values of other commonly used materials in electronic devices. For instance, the yield strength of Au is 50–200 MPa in its bulk state , and the fracture strength of Si is 1–3 GPa , which is one order larger than the strength of Au. The low mechanical strength of Au raises concerns on the structure stability when employed as movable microcomponents. In a study on long-term vibration test of microcantilever made of electrodeposited Au, an obvious tip defection is reported after 107 cycles of the vibration . Therefore, strengthening of the Au-based material is necessary to ensure high structure stability for applications in MEMS devices.
1.2 Strengthening mechanisms in electrodeposits
There are four strengthening mechanisms in metallic materials, including work (strain) hardening, grain boundary strengthening, precipitation strengthening, and solid solution strengthening. Except for the work hardening, the other strengthening methods are plausible in the electrodeposits by controlling the electrodeposition conditions. For example, Rashidi et al. [15, 16] report that a finer crystalline grain structure is obtained in the electrodeposited Ni by controlling the electrodeposition parameters such as current density, bath temperature, and additive amount in the aqueous electrolyte. Grain boundary strengthening of the electrodeposited gold is therefore applicable by the grain refinement effect. Classically, the mechanical strength is proportional to inverse square root of the average grain size according to the Hall-Petch Equation  given by.
Alloying is also one of the commonly applied methods to increase the mechanical strength in electrodeposits. Solid solution strengthening results from the interaction between dislocation and solute atoms can take place. The solute atoms affect the elastic energy of a dislocation due to both local size and modulus changes and act as obstacles to dislocation motions. The alloys could be electrodeposited from a mixed electrolyte containing different metal salts. Schuh et al.  reported that the hardness of Ni increased from 1 to 8 GPa by forming Ni–W alloys. Similar strengthening was also reported in Ni–Co [19, 20], Ni–P , and Ni–Mn  alloys. In addition, alloying of elements having a large difference in the atomic masses would exhibit pronounced strengthening as demonstrated in Cu-based alloys .
1.3 Electrodeposition of metallic materials
In metal electrodeposition, current density is often used to control the characteristics in electrodeposits, in particular, grain size. Metal electrodeposition generally follows Butler-Volmer Equation , which indicates the current density applied to the electrode is interrelated to the overpotential
Pulse electrodeposition is a versatile method that has been proven to produce nanocrystalline materials [28, 29]. Pulse electrodeposition parameters (current on-time, current off-time, and pulse current density) play important roles in controlling the electrodeposition process and hence the microstructure and properties of the electrodeposits [30, 31].
1.4 Mechanical properties of small-scale materials
Microcomponents used in MEMS such as microsprings, cantilevers, and structural support could suffer mechanical straining during employment, and mechanical property evaluation of the specimen in microscale is needed. Conventional indentation or wear tests are widely used to characterize mechanical properties of the electrodeposited metallic materials . However, the obtained results are often affected by the substrate, which may not represent the real information of the microcomponents. Moreover, mechanical properties of materials in microscale are much different from those of bulk materials due to the sample size effect . Since Uchic et al.  firstly introduced the uniaxial compression testing of micropillars, a new wave of studies of small-scale plasticity has been explored in numerous materials [35, 36, 37, 38, 39, 40, 41]. Therefore, micromechanical tests using specimens (i.e., micropillars , microcantilevers ) in microscale are recognized as the most reliable method to provide reliable information on the mechanical properties for design of MEMS microcomponents.
2. Electrodeposition of Au–Cu alloys from noncyanide electrolyte
Electrodeposition of Au-based alloys is reported for the uses of decorative jewelry, conductive materials in electronic devices, magnetic materials, or catalysts. For applications in MEMS accelerometers, it is particularly important to have properties such as high mechanical strength, high electrical conductivity, and high density. Au–Ni [43, 44] and Au–Co  alloys are reported to show improved mechanical strength, but their magnetic properties may cause the undesired effects in the MEMS devices. Au–Sn alloys are reported to be soft materials and mainly used for soldering . Among these solute elements, Cu has high electrical conductivity and is widely used in electronic devices. Besides the difference of atomic masses between Au and Cu is large, a pronounced effect of solid solution strengthening is expected. The Au–Cu alloys are usually electrodeposited from the alkaline cyanide electrolyte due to the electrolyte stability [47, 48]. However, such strong alkaline electrolyte cannot be used in the lithography process for fabrication of MEMS components, which would cause damage of the photoresists. In this chapter, we utilize the noncyanide electrolyte to electrodeposit Au–Cu alloys and characterize their properties.
2.1 Fabrication of Au–Cu alloys by constant current electrodeposition
The Au–Cu electrolyte used in this work is a commercially available electrolyte provided by MATEX Co., Japan, which contained 17.3 g/L of X3Au(SO3)2 (X = Na, K), 1.26 g/L of CuSO4, and EDTA as the additive with pH of 7.5. A potentiostat (Solartron SI1287) is served for applying the constant current. The electrodeposition is carried out at 50 °C, and the current density is varied from 2 to 9 mA/cm2. A piece of Pt plate and Cu plate with the same dimensions of 1 × 2 cm2 is used as the anode and the cathode, respectively. Two thicknesses of the films are prepared for the characterization. Thin films with a thickness of ~3 μm are used for surface characterization, and thick films with a thickness of ~50 μm are used only for fabrication of the microcompression specimens.
Figure 1 shows surface morphology of the Au–Cu alloy films electrodeposited at various current density. The films deposited at lower current densities exhibited nodular-like structures as shown in Figure 1(a) and (b). When a higher current density is used (4–7 mA/cm2), the surface morphology gradually changes to smooth surface condition as shown in Figure 1(c)–(f). Large agglomerates of bump clusters are observed on the surface when the current density is higher than 8 mA/cm2, as shown in Figure 1(g) and (h). Similar morphology is reported for the Au-based alloys electrodeposited at high current density .
2.2 Crystalline structure and chemical composition of electrodeposited Au–Cu alloys
Figure 2 shows XRD patterns of the Au–Cu alloys electrodeposited at current densities ranging from 2 to 9 mA/cm2. Both the (111) and (200) peaks shift continuously to a higher diffraction angle as the current density increases. For instance, the (111) peak shifts from 2
On the other hand, an increased in the grain size is observed when the current density increases beyond 6 mA/cm2. Increasing the current density also promotes side reaction(s), such as hydrogen evolution. Because of this, overpotential of the main reactions, which are reduction of Au and Cu in this study, would be lowered when the side reaction(s) is promoted . This should be the cause of the grain coarsening observed when the current density is higher than 6 mA/cm2. Meanwhile, a sustained increase of the Cu concentration from 12.2 to 46.7 at% is observed when the current density is increased from 2 to 9 mA/cm2. The results can be interpreted by the difference in the standard reduction potential between Au and Cu . The standard reduction potential of Cu is more negative than that of Au. An increase in the cathodic current density would make the applied potential to be more negative; hence, reduction of Cu is gradually favored and leads to an increase in the Cu concentration.
2.3 Fabrication of Au–Cu micropillars and micromechanical properties
Micromechanical properties of the Au–Cu alloys are evaluated using micropillars fabricated from the thick Au–Cu films by focus ion beam (FIB, Hitachi FB2100). Fabrication process of micropillar is shown in Figure 4. The Au–Cu micropillars have square cross section of 10 × 10 μm2 and height of 20 μm. The microcompression tests are conducted with a testing machine specially designed for microspecimens. The compression is conducted at a constant displacement rate of 0.1 μm/s using a piezoelectric actuator.
Figure 5 shows SIM images of the Au–Cu alloy micropillars fabricated from the thick Au–Cu alloy films before and after the microcompression tests. Barrel-shape deformations are observed in the micropillars fabricated from the films electrodeposited at current density 3, 5, and 6 mA/cm2, which are typical deformation behaviors for polycrystalline metallic materials [52, 53]. When the current density is further increased to 8 mA/cm2, brittle fractures indicated by the cracks along boundaries of the agglomerates are observed after the compression test. The Au–Cu alloy film electrodeposited at 8 mA/cm2 is composed of nano-grains, which is similar to the film electrodeposited at lower current density of 3 mA/cm2; however, formation of the bump-clustered agglomerates at high current density might be the main cause of the brittle deformation. Au–Cu alloys are known to be highly ductile materials. To the best of our knowledge, this is the first report on brittle fracture of Au–Cu alloys, and this information is essential for the design of components used in MEMS devices.
Engineering strain-stress (
The enhanced yield stress in the Au–Cu alloys is mainly attributed by the following two mechanisms: (i) grain boundary strengthening  and (ii) solid solution strengthening [26, 27]. As shown in Figure 3, the grain refinement effect goes along with an increase in the Cu concentration as the current density increases. According to the grain boundary strengthening mechanism, the strength of metallic materials increases as the total amount of grain boundary in a specimen increases, which is also understood as a decrease in the average grain size. Moreover, the solid solution strengthening mechanism could restrict the dislocation movement due to interaction of the dislocations with the strained lattice surrounding the solute atoms, which then leads to a stacked strengthening beyond the grain boundary strengthening mechanism.
3. Pulse current electrodeposition of ultrahigh strength nanocrystalline Au–Cu alloys
3.1 Fabrication of Au–Cu alloys by pulse current electrodeposition
The Au–Cu alloys are electrodeposited on cold-rolled Cu substrates with a commercially available electrolyte (see Section 2.1). Temperature of the electrolyte is maintained at 50 ± 1°C using a water bath. The pulse current electrodeposition is carried out using a pulse power supply (plating electronic GmbH, type pe86CB-20-5-25-S/GD). For all experiments, the current on-time (
|Pulse current density (mA/cm2)||5–60|
|Current on-time (ms)||10|
|Current off-time (ms)||5–600|
|Electrolyte temperature (°C)||50|
3.2 Effects of the pulse current density
Figure 7(a) shows XRD patterns of the Au–Cu alloys electrodeposited at the
3.3 Effects of current off-time
Effects of the
Dependence of the Cu concentration and grain size on the
Effects of the
As a result, a wide Cu concentration ranging from 10.1 to 53.0 at% is attained by adjusting either or both the
3.4 Morphology of pulse electrodeposited Au–Cu alloys
Effects of the pulse current electrodeposition parameters on morphology of the Au–Cu films are observed by the SEM as shown in Figure 11. The overview of the Au–Cu alloys electrodeposited at the
|20||20||46.4||4.8||Colony-like clusters and dull surface|
|50||33.9||4.7||Pebble structure and bright surface|
|15||20||36.9||4.7||Pebble structure and bright surface|
|30||34.2||4.8||Pebble structure and bright surface|
|50||29.5||4.9||Pebble structure and bright surface|
|100||21.2||6.2||Pebble structure and dull surface|
|5||30||18.0||7.0||Pebble structure and dull surface|
|100||12.1||9.1||Pebble structure and dull surface|
Effects of the
3.5 Micromechanical properties of pulse electrodeposited Au–Cu alloys
Micromechanical properties of the pulse current electrodeposited Au–Cu alloys are evaluated by microcompression tests to demonstrate the potential for applications in microelectronic devices. The micropillars with the same dimensions of 10 × 10 × 20 μm3 are fabricated from the thick Au–Cu films by FIB. Figure 13 shows SIM images of 6 Au–Cu micropillars with different alloy compositions after the microcompression tests. Typical polycrystalline deformation (barrel-shape) is observed in the micropillars at the Cu concentration below ~35 at% (Figure 13(a)–(d)). As the Cu concentration increases to ~37 at%, the deformation behaviors change into brittle fracture (Figure 13(e)). For the Cu concentration of 46.4 at% pillar (Figure 13(f)), the brittle fracture occurs from the crack boundaries originating from the large agglomerates as observed in Figure 11(g). The large agglomerates and the brittle fracture are also observed in the constant current Au–Cu alloys electrodeposited using a high current density, in which the brittle fracture is observed when the Cu concentration is higher than 37 at% (Figure 5(g) and (h)).
Engineering strain–stress (SS) curves obtained from the microcompression tests are shown in Figure 14. The
3.6 Strengthening mechanisms in electrodeposited Au–Cu alloys
Overall, the values reported in the literature all follow the Hall-Petch relationship. However, softening caused by the inverse Hall-Petch effect occurs when the grain size scales down to ~6 nm. The results presented in this work also follow the Hall-Petch relationship well. Most importantly, the results obtained in this study are much more reliable than those of Vicker microhardness tests since the hardness results are often affected by the substrate, which cannot reflect real strength of the electrodeposited films. A number of theories for solid solution strengthening have proposed that the strength is proportional to the solute concentration with order of 1/2 [Fleischer (1963)] or 2/3 [Labusch (1970)], which depends on the solute concentration. It is worth noticing that the highest
In the present study, high-strength Au–Cu alloys with nanocrystalline structure are successfully fabricated by electrodeposition techniques in order to be applied in fabrication of movable microcomponents in MEMS devices. The Au–Cu alloys are first fabricated by constant current electrodeposition. Surface morphology of the Au–Cu alloy films shows a wide variation from smooth surface to bump-clustered agglomerates as the current density varies from 2 to 9 mA/cm2. A reduction in the grain size and an increase in the Cu content are observed with an increase in the current density. The film with the finest grain size at 5.3 nm is obtained when current density 6 mA/cm2 is used. For the microcompression tests, the specimens evaluated are micropillars with dimensions of 10 × 10 × 20 μm3 fabricated from the electrodeposited Au–Cu alloys. The highest
Furthermore, effects of the pulse current parameters on the alloy composition, grain size, surface morphology, and micromechanical property of the Au–Cu alloys are investigated. A wide Cu concentration in the Au–Cu alloys ranging from 10 to 54 at% is obtained. An increase in the Cu concentration is observed by using either or both of a high pulsed current density and a short current off-time. The smallest grain size of ca. 4.4 nm is achieved in films having the Cu concentration ranging from 30 to 40 at%. Grain refinement is achieved with a high
This work was supported by JST CREST Grant Number JPMJCR1433 and by the Grant-in-Aid for Scientific Research (S) (JSPS KAKENHI Grant number 26220907).
Conflict of interest
I confirm there are no conflicts of interest.