Microelectrochemical Deposition

11.19

Microelectrochemical Deposition

MA Habib, Islamic University of Technology, Gazipur, Bangladesh Ó 2014 Elsevier Ltd. All rights reserved.

11.19.1 11.19.2 11.19.2.1 11.19.2.2 11.19.3 11.19.3.1 11.19.3.1.1 11.19.3.1.2 11.19.3.1.3 11.19.3.1.4 11.19.3.2 11.19.3.2.1 11.19.3.2.2 11.19.3.2.3 11.19.3.2.4 11.19.3.2.5 11.19.3.2.6 11.19.4 References

11.19.1

Introduction Process Mechanism Basic Concept of ECD ECD Using Pulse Voltage Micromanufacturing Using ECD 3D Microstructure Fabrication Using the Anode as a Counter Electrode Principle of Operation LECD with Conventional Analog Feedback Control LECD with Adaptive Tip Withdrawal Control ECD Structures Using the Anode as a Counter Electrode 3D Microstructure Fabrication Using Anode as Counter Electrode and Micro-EDM Principle of Operation Open-Loop Control for LECD Closed-Loop Control for LECD Effect of Different Operating LECD Parameters LECD Structures Using the Anode as a Counter Electrode Microholes Fabricated by LECD Electrodes Conclusions

523 524 524 525 527 527 527 528 528 529 531 531 532 533 533 539 541 544 545

Introduction

Today, fabrication of products and its miniaturization with a broad range of materials enable microsystem technology to enhance health care and quality of life, to attain new technological breakthroughs, and to cover engineering applications with environmentally friendly and energy-saving practices. Currently, state-of-the-art fabrication techniques refer to the fabrication of components and parts for microelectromechanical systems (MEMS), subminiature actuators and sensors, components for biomedical devices, high-precision equipment, components for advanced communication technology, long microchannels for lab-on-chips, shape memory alloy ‘stents,’ fluidic graphite channels for fuel cell applications, and many more (1–4). The more recent trends have shown that the drive has gone beyond the earlier challenge of precision and minuteness in dimension to a new level where components of the same precision and invisible dimensions are demanded to be machined on tough materials with lower cost. Semiconductor processing technologies such as photolithography on a silicon substrate are used for fabricating MEMS components (5,6). The material properties of silicon often do not meet the requirement of recent applications of these microparts because they require high-quality structure and capability to withstand high strength. Such applications are in microsurgery, biotechnology, fluidics, and high-temperature environments (7). Moreover, the photolithography technique is not capable of fabricating high-aspect ratio microstructures (8,9). On the other hand, the LIGA process (from the German Lithographie Galvanformung und Abformung – a combination of lithography, electroplating, and molding) can fabricate highaspect ratio components with submicron structure using the synchrotron radiation process and the focused ion beam machining process. However, LIGA requires special and extremely expensive facilities such as a synchrotron system and requires fabrication of expensive masks, which are not economical for microparts fabrication on a laboratory scale and fabrication industries (8,10). Nonconventional micromachining technology such as microturning, microgrinding, micro electro-discharge machining (micro-EDM), and microelectrochemical machining (ECM) have many advantages in productivity, efficiency, flexibility, and cost effectiveness; consequently, these nonconventional methods have been applied to a variety of substrates and materials to fabricate microstructures (6,11–14). Among the nonconventional micromachining techniques, micro-EDM has provided an efficient solution for machining hard conductive materials and fabricating complex cross-sectional structures. In order to fabricate these complex cross-sectional structures effectively, the noncircular electrode is required, which is one of the challenges in the micro-EDM area. As an alternative, localized electrochemical deposition (LECD) is a fashionable method in the fabrication of small and shaped electrodes directly. People have been using electrochemical deposition for centuries. The term LECD means electrochemical deposition (ECD) in a predetermined and controlled area. By using this method, three-dimensional microstructures can be made easily on high-strength metals. This process has advantages over any other microfabrication method in terms of fabrication time and cost.

Comprehensive Materials Processing, Volume 11

http://dx.doi.org/10.1016/B978-0-08-096532-1.01109-2

523

524

11.19.2

Microelectrochemical Deposition

Process Mechanism

11.19.2.1 Basic Concept of ECD ECD is a process whereby the metallic ion can become solid metal and deposited on the cathode surface if sufficient amounts of electric current go through an electrolyte or plating solution, which is known as a solution that contains charged ions. These charged ions, especially positively charged ions, can be achieved by dissolving metallic salt into water. Although traditionally this process has been used for depositing coatings, it can also be used to fabricate 3D microstructure as well as both metallic and semiconducting nanowires. If a metal M of valency n, then n electrons would be required to reduce the cation into its metallic form. A more realistic equation is Mnþ þ ne 0M

[1]

where M metal will be deposited on the cathode by receiving n number of electrons. The total cathodic charge used in the deposition (Q) (coulomb) is the product of the number of g mol of the metal deposited (m), the number of electrons taking part in the reduction (n), Avogadro’s number (NA) (the number of atoms in a mole), and the electrical charge per electron (Qe) (coulomb). To reduce 1 mol of a given metal, (n) moles of electrons are therefore required. Q0m n NA Qe

[2]

The product of the last two terms in the equation above is the Faraday constant (F). Therefore, the number of moles of metal reduced by charge (Q) can be obtained as: m¼

Q nF

[3]

The total charge used in the deposition can be obtained as the product of the current (I) (ampere) and the time of deposition (t) (second) if the deposition current is held constant. If, however, the current varies during ECD, we arrive at the following equation. Z Q ¼ I dt [4] And so the number of moles deposited can be calculated as: m¼

1 nF

Z I dt

[5]

The weight of the deposit (w) (gram) can now be obtained by multiplying the equation above by the atomic weight (WM) of the deposited metal. To calculate the height of the deposit, we have to use the density of the metal DM (g cm3), where Vol is the volume of the deposited metal in cm3, AD is the area of the deposit in cm2, and Ht is its height in cm. DM ¼

w w ¼ Vol AD Ht

[6]

Solving the height of deposition for the unit cross-sectional area and the mole volume of deposited metal (VM) (cm3g1) is the ratio of WM and DM, we arrive at the following equation (15,16). Z Z w WM VM I dt [7] I dt ¼ Ht ¼ ¼ nF AD DM nFAD DM Figure 1 shows a schematic of an electrolytic cell for the ECD of metal M from an aqueous solution of metal salt MA. However, in order to localize the ECD, pulse voltage is very helpful. In the next section, the mechanism of pulse ECD is described.

e–

e–

Cathode Anode Deposition M metal MA Solution

Cathode

Deposition M metal

Mask

MA Solution

Anode

e–

e– (a)

(b)

Figure 1 Schematic diagram of ECD setup: (a) anode as a counter electrode and cathode works as a metal substrate, (b) cathode as a counter electrode. Here mask is used in order to localize the deposited metal on the cathode.

Microelectrochemical Deposition

525

11.19.2.2 ECD Using Pulse Voltage To make a microstructure by ECD, the deposition area should be localized. Ultrashort pulses can be applied for localizing the deposition region because the double layer has the property of the electric condenser. Capacitance value multiplied by electrolyte resistance value equals the double-layer charging time constant in the electrochemical cell. This charging time constant varies according to location on the electrode surface. The distance between tip end and substrate is closer than that of tip side and substrate. Therefore, the double layer between the former one is charged first and current flows in advance. The pulse is cut off before charging the latter double layer, so the current flow through the electrode side is negligible. This means that localization can be achieved without insulation of the electrode. If we choose proper pulse duration considering the charging time constant of double layer, we can deposit copper on a substrate beneath the electrode. When a cathode and anode electrodes are immersed in a solution, an interface consists of two equal and opposite layers of charge, one on the metal (4m) and the other in solution (4s). This pair of charged layers, called the double layer, is equivalent to a parallelplate capacitor (Figure 2). The variation of potential in the double layer with the distance from the electrode is linear. The capacitance of the double layer is a function of potential. On both the electrode surfaces, the electrochemical double layer forms a capacitor. This double layer is charged when a potential is applied between the two electrodes. The charging time constant (sc) for the double layer is the product of resistance (R) and capacitance (cDL). The charging current has to flow through the electrolyte, whose resistance is proportional to the length of the current path; that is, the distance between the electrodes (dgap). Therefore, resistance is the product of the gap distance between the electrodes (dgap) and the specific electrolyte resistivity (r). Finally, the time constant: sc ¼ R  C ¼ r$cDL $dgap

[8]

In this process, pulse potential is applied to deposit the metal ions. The charging time (tc) of the double layer should be at least four times the time constant, that is, 98% of the pulse on time. If the duration of the pulse on time (ton) is longer than the charging time (tc), the double layer will be charged properly for metal deposition. On the other hand, if the charging time (tc) is longer than the pulse on time (ton), the double layer will not be charged sufficiently for metal deposition. Since the chemical reaction rate is exponentially proportional to the potential drop in the double layer, metal deposition can be controlled by controlling the pulse duration. The charging time of the double layer is the time before the charged pulse potential (4c) reaches the value corresponding to the applied pulse potential (40) (Figure 3(b)). If the charging time is longer than the duration of the pulse on time (ton), the double layer is not completely charged and 4c never reaches to 40 (Figures 3(c) and 3(d)). A similar phenomenon occurs after the end of the pulse. The double layer must be discharged, and it takes some time before the potential drops to the value corresponding to zero value. Therefore, it takes some time before 4c drops to zero. If this time is longer than the off time (toff), the double layer is not completely discharged and 4c never decreases to zero. Therefore, the charged potential of a double layer at any time (t):   t t 4c ¼ 40 1  esc z40 [9] sc When an electrode is made a part of an electrochemical cell through which current is flowing, its potential will differ from the equilibrium potential. If the equilibrium potential of the electrode is E and the potential of the same electrode as a result of external current flowing is E(I), then its difference is known as overpotential (h). h ¼ EðIÞ  E m

d

[10]

s

Helmholtz Plane, HP

– Metal (Cathode)

Solution

+

– – –

+

–

Hydrated ions

Double layer capacitance, cDL m

– +

– (a)

–

–

cDL R

s

+ + +

– – –

–

–

Cathode

d

– –

+ + +

(b)

cDL

Anode

(c)

Figure 2 (a) HP model of double layer: 4m, excess charge density on metal; 4s, excess charge density in solution, (b) HP double layer: a parallel-plate capacitor, and (c) electrochemical cell upon application of a voltage pulse. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

526

Microelectrochemical Deposition

t period 0

t on

0

t off

C

0

0

time

time (c)

(a)

Applied potential ( 0)

0

Charged potential (

C)

0

C

C

0

time

0

(b)

time (d)

Figure 3 Applied pulse voltage in LECD and double layer (DL) time constant effect (a) tc >> ton no damping, (b) tc < ton small damping, and (c, d) tc > ton, tc >> ton strong damping. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

The overpotential (h) is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control: charge transfer, diffusion, chemical reaction, and crystallization. Thus, four different kinds of overpotential are distinguished, and the total overpotential (h) can be considered to consist of four components h ¼ hct þ hd þ hr þ hc

[11]

Here, hct, hd, hr, and hc are charge transfer, diffusion, chemical reaction, and crystallization overpotential, respectively. In order to complete the deposition model, and simplify the formulation of determining the deposition rate and height, the following assumptions are considered (18): l

In the deposition reaction, only copper ions are deposited. In the electrolyte, there are no concentration gradients; therefore the solution is well stirred. l The diffusivity of the reacting species is constant during the deposition, and the rate of change of shape of the deposit is slow compared with the establishment of the concentration field, and l LECD current efficiency is unity. l

Since there is no electrochemical reaction or metal deposition during the pulse off time, pulse off time voltage is comparatively less than pulse on time voltage. For this reason, charged potential (4c) can be judged as overpotential (h). From the Butler–Volmer equation, during the pulse on time, reaction current density (i) is:           ð1  aÞnF anF ð1  aÞnF anF h  exp h ¼ i0 exp 4c  exp 4c i ¼ i0 exp RT RT RT RT

[12]

For large negative values of overpotential, the Butler–Volmer equation can be simplified. As the first exponential term in the equation (corresponding to the anodic partial current) decreases, the second exponential term (corresponding to the cathodic partial current) increases and the second exponential term can be neglected.  exp

   ð1  aÞnF anF 4c [exp 4c RT RT

[13]

Therefore, reaction current density (i) during the pulse on time is:     ð1  aÞnF ð1  aÞnF t 4c zi0 exp 40 i ¼ i0 exp RT RT sc

[14]

Here, i0, exchange current density; a, leakage factor; F, Faraday constant; R, gas constant; T, temperature; n, the number of electrons taking part in the reduction. Since the reaction rate is proportional to the reaction current density, that is, z(t)Ni. This can be represented as i nF

[15]

    i0 ð1  aÞF t i0 ð1  aÞF 40 t exp exp 40 ¼ nF nF RT sc RTrcDL dgap

[16]

zðtÞ ¼ zðtÞ ¼

Microelectrochemical Deposition

527

The electrochemical reaction or deposition occurs only during on time of pulse. For this reason, the deposition rate can be calculated by integrating the reaction rate during pulse on time. Therefore, the LECD rate, Z, is:  Z 40 ; ton ; tperiod ; dgap ¼

1 tperiod

Zton zðtÞdt ¼ 0

1 tperiod

Zton 0

  i0 ð1  aÞF 40 t exp dt nF RTrcDL dgap

    dgap f i0 RTrcDL ð1  aÞF 40 ton ¼ exp  1 40 tperiod ð1  aÞzF 2 RTrcDL dgap

[17]

In order to control the LECD rate and height of deposited electrodes, four experimental parameters are used. These four variable parameters are pulse potential amplitude (40), pulse frequency ( f ), pulse duty ratio (D), and effective gap distance between two 1 ton ton D electrodes (dgap). We know that frequency f ¼ and duty ratio D ¼ ¼ ¼ ton  f 0ton ¼ ton þ toff tperiod tperiod f Therefore, LECD rate, Z, is:     dgap f i0 RTrcDL  ð1  aÞF 40 D  1 [18] exp Z 40 ; f ; D; dgap ¼ 40 ð1  aÞzF 2 RTrcDL dgap f So the deposition height (Ht) at time t ¼ 0, 1, 2, 3 . will be: H0 ¼ 0



    dgap  H0 f i0 RTrcDL ð1  aÞF 40 D  exp  1 40 ð1  aÞzF 2 RTrcDL dgap  H0 f      dgap  H1 f i0 RTrcDL ð1  aÞF 40 D  exp H2 ¼ H1 þ Z2 ¼ H0 þ  1 40 ð1  aÞzF 2 RTrcDL dgap  H1 f .. .. .. .. ..

H1 ¼ H0 þ Z1 ¼ H0 þ

..

[19]

..

.. .. ..      dgap  Ht1 f i0 RTrcDL ð1  aÞF 40 D  exp Ht ¼ Ht1 þ Zt ¼ Ht1 þ  1 40 ð1  aÞzF 2 RTrcDL dgap  Ht1 f      dgap  Ht f i0 RTrcDL ð1  aÞF 40 D  Htþ1 ¼ Ht þ Ztþ1 ¼ Ht þ exp  1 40 ð1  aÞzF 2 RTrcDL dgap  Ht f The deposition rate can be obtained by eqn [18] over deposition time. The change of the deposition height can be calculated from the deposition rate. Equation [19] shows that the deposition rate is not constant for every unit time. This is why, in order to calculate the deposition height, the deposition rate needs to be calculated in every unit time (19). As discussed earlier, to control the deposition rate and quality, the four variable parameters need to be optimized and properly controlled.

11.19.3

Micromanufacturing Using ECD

ECD was introduced by Madden and Hunter about a decade ago as a realistic technique for the inexpensive free-form microfabrication method. ECD has a huge prospective to afford solutions to a variety of challenges for the microfabrication of threedimensional metal structures (20,21). Jansson et al. had deposited a nickel structure from a different kind of nickel plating solutions (22). El-Giar and Thomson and El-Giar et al. deposited long, thin micrometer-size copper columns, copper electrical interconnects, and tips for scanning probe microscopy applications (23,24). Yeo et al. had used opened-loop (without analog feedback) and closed loop (with analog feedback) conditions in order to investigate the deposition phenomena of the Ni microcolumn structure in LECD (25). Afterward, they studied the effects of ultrasonic vibration on the rate of deposition, concentration, and porosity of the nickel microcolumns and the rotation of the electrode on the growth of nickel microcolumn structure (26,27). Park et al. fabricated microstructures such as micropatterns, microcolumns, and microsprings by applying ultrashort pulses with LECD (28). In order to form noncircular shaped deposition, a nonconductive mask is used, which is shaped by a micromilling operation. The substrate is fixed on the machine z-axis, which is over the anode electrode because it eases the next operation subsequent to the deposition. The electrode clamping error can be minimized, and the production rate can be increased with this fabrication method.

11.19.3.1 3D Microstructure Fabrication Using the Anode as a Counter Electrode 11.19.3.1.1

Principle of Operation

The schematic drawing in Figure 4 illustrates a general arrangement of a typical setup used for LECD. A microelectrode is placed very close to a conducting substrate, while both are immersed in an ionically conducting electrolyte that contains ions of the material to be deposited. The microelectrode is usually insulated from all sides except for an exposed tip region with micrometer-scale

528

Microelectrochemical Deposition

Figure 4 An illustration of a typical LECD setup arrangement, including reaction current monitoring for feedback and probe positioning purposes. Reproduced from Said, R. A. Microfabrication by Localized Electrochemical Deposition: Experimental Investigation and Theoretical Modeling. Nanotechnology 2003, 14, 523–531.

dimensions. An electric potential is applied between the microelectrode and the substrate, thus causing a faradic current to flow through the electrolyte between the microelectrode and the substrate. Since the electrolyte contains reducible metal ions (e.g., Cu2þ ions in the present work) and the substrate is connected to a negative potential with respect to the microelectrode, then the flow of faradic current results in an oxidation process at the microelectrode tip and a deposition of metal ions at the substrate. Unlike typical electroplating methods, where deposition occurs at a uniform rate on all exposed regions of the substrate, the deposition process outlined in Figure 4 is much localized to the region beneath the electrode tip. This is due to the highly localized electric field in the space between the microelectrode tip and the substrate region directly below the tip. The result is highly localized growth with an extent of approximately the dimensions of the microelectrode tip, as illustrated by the lower circled drawing in Figure 4. At the start of fabrication, however, the extent of deposition is usually less confined and extends outside the region beneath the tip due to a fringing electric field emanating from the tip boundaries and terminating on the substrate, which will be demonstrated in the next section. As the deposition end moves farther away from the substrate, the geometry of the deposit becomes more confined to a region of the same extent as the tip boundaries. Copper columns were formed from CuSO4$5H2O (250 g L1) and H2SO4 (75 g L1), with thiourea (0.04 g L1) onto copper or steel substrates. Microelectrode tips were prepared by sealing Pt, Pt–Ir, or Pt–Rh wires of 25 mm diameter. When a satisfactory seal is obtained, the disk is exposed by polishing with successive grades of 600, 1200, and 1500 silicon carbide paper. Final polishing was with 6.0 and 1.0 mm diamond polish on nylon cloth followed by 0.05 mm alumina on another cloth to provide a smooth surface with nanometer scale roughness. In order to increase the aspect ratio of the deposited structure, it is very important to withdraw the tip of the microelectrode with proper speed and control. For this reason, analog feedback control and adaptive feedback control are discussed in the next sections.

11.19.3.1.2

LECD with Conventional Analog Feedback Control

In the conventional feedback arrangement of Figure 5, the output of a buffer current amplifier is connected to the electrode contact pin and serves as constant voltage source. As deposition starts, a difference amplifier samples the deposition current, in the form of an electric voltage as indicated by node a in Figure 5, and compares it with a reference value, Vref, that is usually set to represent high current amplitudes. The addition of a current limiter at the buffer output can be of an advantage to cut the deposition current to a limited value for the cases where the contact area between the tip and deposition end is large enough to sink a current beyond the electrode capabilities. The difference amplifier output is cascaded to an inverter that controls an analog switch used to couple a pulse generator to the driving circuitry of a microstepping positioner holding the electrode. Every pulse passed to the positioner circuitry triggers the micropositioner, causing an incremental step withdrawal. Thus, the repetition rate of the pulses, ftrig, determines the withdrawal speed of the tip and is a user controllable parameter utilized by the experimenter for adjustment relative to the deposition rate. Successful deposition of structures with desired geometry and characteristics is critically dependent on the proper selection of this parameter (29).

11.19.3.1.3

LECD with Adaptive Tip Withdrawal Control

Figure 6 shows a block diagram of a possible arrangement of the proposed adaptive tip withdrawal control. The output of the differential stage is connected to an integrator, thus yielding the total instantaneous change in the deposition current. The output of the integrator is used to tune a voltage-controlled oscillator that generates pulses at a rate proportional to the tuning voltage amplitude and is also used to trigger positioner movement at a speed corresponding to the generated pulse rate. The pulse generation rate would thus be modulated by the total change in the deposition current resulting from either a decrease in the tipdeposit spacing due to deposition growth, or from an increase in the tip-deposit spacing due to tip withdrawal away from the deposit end (29).

Microelectrochemical Deposition

529

Figure 5 A block diagram of a conventional analog feedback implemented in an LECD. Reproduced from Said, R. A. Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition. J. Microelectromech. Syst. 2004, 13 (5), 822–832.

11.19.3.1.4

ECD Structures Using the Anode as a Counter Electrode

11.19.3.1.4.1 Microcolumn Figure 7 shows a scanning electron microscope (SEM) image of an example of a successfully deposited copper column, which has a diameter of about 25 mm and a length of about 6.5 mm, thus giving an aspect ratio of 1:282. The deposition rate of the deposited column is estimated to be around 8 mm s1. After deposition, the tip shape was inspected, for possible effects such as retraction due to erosion, and was found to have been preserved. In Figure 8 the SEM images of a column microstructure were deposited at a tip withdrawal speed almost equal to the estimated deposition growth rate, along with the deposition current signal (Va) and control signal (Vb) monitored during the deposition session, where Va and Vb are voltage signals at the nodes indicated in Figures 5 and 6.

Figure 6 A block diagram of the adaptive tip withdrawal control feedback. Reproduced from Said, R. A. Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition. J. Microelectromech. Syst. 2004, 13 (5), 822–832.

530

Microelectrochemical Deposition

Figure 7 An SEM image of a copper microcolumn deposited by LECD using conventional feedback control. Reproduced from Said, R. A. Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition. J. Microelectromech. Syst. 2004, 13 (5), 822–832.

The quality of deposition shown in Figure 8(b) in terms of geometry confinement, reduced porosity, and enhanced uniformity has clearly improved, even when compared to the results of equal tip withdrawal speed and deposit growth rate using the conventional feedback control illustrated in Figure 8(a) (29). 11.19.3.1.4.2 Microspring If an electrode moves circularly on an XY plane and upward simultaneously, microspring can be fabricated by LECD. An insulated tip was used because spring was easy to be affected by the exposed tip side. The experimental condition was fixed with applied voltage 2.5 V and duty ratio 0.45. Motion was controlled to make a spring with a 100 mm diameter and a 350 mm pitch. The final shape of the deposited microspring is shown in Figure 9. The diameter of the spring coil was maintained with 10–12 mm, and the deposited shape was uniform (28).

Figure 8 SEM images of a column microstructure deposited using (a) conventional feedback control and (b) adaptive tip withdrawal control under same operating condition. Reproduced from Said, R. A. Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition. J. Microelectromech. Syst. 2004, 13 (5), 822–832.

Microelectrochemical Deposition

531

Figure 9 Spring made with applied voltage 2.5 V and duty ratio 0.45. Reproduced from Park, J. W.; Ryu, S. H.; Chu, C. N. Pulsed Electrochemical Deposition for 3D Micro Structuring. Int. J. Precis. Eng. Manuf. 2005, 6 (4), 49–54.

11.19.3.1.4.3 Micropatterning LECD using ultrashort pulses can be applied for micropatterning. Micropatterns can be made by moving the electrode on an XY plane sustaining gap of a few micrometers from the substrate. In this research, micropatterns such as alphabets and a spiral were written as shown in Figures 10 and 11. The growth direction of structure was perpendicular to the feed direction of electrode. So, deposition could occur discontinuously. The width of pattern line was 10–15 m (28).

11.19.3.2 3D Microstructure Fabrication Using Anode as Counter Electrode and Micro-EDM 11.19.3.2.1

Principle of Operation

The schematic diagram of the LECD experimental setup is shown in Figure 12. The acidic super sulfate is used as an electrolyte, and an anode is immersed in this electrolyte. A cathode is placed above the anode and between the anode and cathode; a nonconductive mask is located to create the noncircular shape of the deposition. A small constant gap is maintained between the anode and mask during deposition time. When both of the electrodes are conducted electrically, current will pass through the plating solution. The positively charged metal ions get (Cu2þ) deposited as solid metal on the cathode through the nonconductive mask. In this way, metal can be deposited on the cathode surface and electrodes can be fabricated. In this process, the cross section of the electrode will be same as the mask. Finally, the deposited electrode can be used directly in the micro-EDM process without changing its orientation. The LECD subsetup consists of two main parts: a cathode–electrode holder and a deposition tank (Figure 13). The electrode holder is attached to a voice coil motor, which has a resolution of 0.1 mm fixed on the z-axis of the machine. The voice coil motor is capable of sensing the mask and maintaining a constant distance between the anode and cathode. It does so by giving the feedback motion after measuring the current of the system through the pico ammeter. The voice coil motor and pico ammeter are connected with PC by RS232 serial communication. The cathode electrode where deposition will take place is attached to this voice coil motor. The deposition tank consists of the anode electrode and the mask. In the deposition tank, a micrometer screw and two wedges are

Figure 10 Image of letters made with applied voltage 2.5 V and duty ratio 0.45 by micropatterning. Reproduced from Park, J. W.; Ryu, S. H.; Chu, C. N. Pulsed Electrochemical Deposition for 3D Micro Structuring. Int. J. Precis. Eng. Manuf. 2005, 6 (4), 49–54.

532

Microelectrochemical Deposition

Figure 11 Image of a spiral with applied voltage 2.5 V and duty ratio 0.35 by micropatterning. Reproduced from Park, J. W.; Ryu, S. H.; Chu, C. N. Pulsed Electrochemical Deposition for 3D Micro Structuring. Int. J. Precis. Eng. Manuf. 2005, 6 (4), 49–54.

used to adjust the gap between the mask and anode. The mask is made from a nonconductive material like PMMA (poly methyl methacrylate) because of its advantages over other materials; it has greater transparency, ease of fabrication, excellent alkaline, and good acidic chemical resistance. The masks are machined in different kinds of cross-sectional shapes by using the micromilling process such as ‘X,’ ‘Y,’ and ‘NUS.’ The thickness of the mask was 250 mm. In order to increase the aspect ratio of the deposited structure, it is very important to withdraw the cathode with proper control. For this reason, both open-loop control and closed-loop control are discussed in the next sections.

11.19.3.2.2

Open-Loop Control for LECD

An open-loop controller, also called a nonfeedback controller, is a type of controller that computes its input into a system using only the current state and its model of the system. It is often used in simple processes because of its simplicity and low cost, especially in systems where feedback is not critical. A characteristic of the open-loop controller is that it does not use feedback to determine whether its input has achieved the desired goal. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system cannot engage in machine learning and cannot correct any errors that it could make. It also may not compensate for disturbances in the system. In this process, two height values input is required: one is for the

Figure 12 (a) A simple illustration of a typical LECD setup arrangement and (b) concept of the LECD and EDM setup. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

Microelectrochemical Deposition

533

Figure 13 Schematic diagram of LECD EDM combined process. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

initial growth height (Hi), and the other is for the final desired height (Ht). The controller feedback system will start when the deposition reaches Hi height. When the controller receives the interrupt, then it will shift the motor by (Ht–Hi) distance. In this way, it is possible to increase the aspect ratio by open-loop control. The detailed algorithm of the open-loop control system is shown in Figure 14 (19).

11.19.3.2.3

Closed-Loop Control for LECD

This section provides the design of a controller for the LECD process in order to increase the height of the deposited structure, which was one of the main objectives of this project. To increase the height of the deposited structure, it is necessary to lift up the cathode from the mask. The reason is, if the cathode is kept in the same initial place, then after a certain time the deposited structure will come out from the mask and the actual structure of the deposited electrode will be affected. This is why, to keep the deposited structure inside the mask, it is required to lift up the cathode from its original position. However, in order to lift up the cathode, it is very necessary to know the deposition height at the particular time. This is why two different controllers are designed: One is an open-loop controller, and the other is a closed-loop controller in order to control the lifting mechanism of the cathode. For the two controllers, two input heights are needed to be given for control simulation. One is the initial growth height, and the other is the final desired height. The next subsections present the determination of the initial growth height, as well as the design of an openloop and closed-loop controller. Generally, to obtain a more accurate or more adaptive control, it is necessary to feed the output of the system back to the inputs of the controller. This type of system is called a closed-loop system. A closed-loop system utilizes feedback to measure the actual system operating parameter being controlled such as temperature, pressure, flow, level, or speed. This feedback signal is sent back to the controller where it is compared with the desired system set point. In order to design such a controller, a system model has been derived from Faraday’s basic law of electrochemistry (19). The detailed algorithm of the closed-loop control that is implemented in the process is given in Figure 15.

11.19.3.2.4

Effect of Different Operating LECD Parameters

Figures 17–24 present the simulation of deposition height and rate with respect to deposition time at different levels of voltage amplitudes, frequencies, duty ratios, and gap distance based on eqns [18] and [19]. The details LECD parameters are tabulated in Table 1. In order to study the effect of pulse amplitude (40)f, D and dgap are kept constant on 100 kHz, 0.33, and 350 mm. Similarly, to study the effect of pulse frequency (f)40, D and dgap are kept constant on 1.5 V, 0.33, and 350 mm. Likewise, to study the effect of the pulse duty ratio (D)40, f and dgap are kept constant on 1.5 V, 100 kHz, and 350 mm. Lastly, to study the effect of the electrode gap distance (dgap)40, f and D are kept constant on 1.5 V, 100 kHz, and 0.33. In order to verify the mathematical model, experimental results are also shown in the figures. Results show that in case of a higher deposition rate the experimental results are not properly matched with the simulation results. The simulation results show that the deposition height and rate are higher than those of experimental results (19). At the beginning of the deposition process, the anode electrode touches the mask. The anode electrode surface is properly polished, but still there is some horizontal misalignment. This is why the electrolyte leaks through the gap (Figure 16). This causes

534

Microelectrochemical Deposition

START

Electrode Positioning

Starting of Deposition

Calculating the cumulative initial growth height from equation (4.12)

Total initial growth height

Initial growth Height (Hi)

Hi

False True Feeding backward of (Ht-Hi)µm in every t sec interval according to equation (4.12)

Count of total height = Total backward height + Hi

Total Height

Final required Height(H )

H

False True END Figure 14 Algorithm for open-loop control. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

the metal deposited on the places the electrolyte reaches. For this reason, in the experimental results the initial rate is not matched with simulation results. However, this phenomenon has also occurred in a low deposition rate, but the effect is comparatively less. After the above leakage, the gap seals within a very short time. This is why the effect is not that great. Once the deposited metal enters into the mask, the gap is sealed. After the above conditions, the results show that the deposition height of experimental data is higher than simulation data. One probable occurrence may be that when the deposition starts the top surface is not perfectly flat all the time. Some peaks and valleys can be visible on the top surface (Figure 16). For measuring the height of the deposition, only the peaks are taken into account. This is why the total height becomes higher. However, the actual equivalent height is less than the measurement height. The effects of different parameters are given in the following (19). 11.19.3.2.4.1 Effect of Pulse Voltage amplitude Figures 17 and 18 indicate that the deposition height and rate increase with the increase of deposition time at all voltage amplitudes. Moreover, at a certain point, the deposition height and rate increase almost suddenly and the system becomes uncontrollable. This point of transition is different for different voltages. For lower voltage, the transition point arrives at a higher deposition time. On the other hand, for higher voltage, the position is the reverse. For a voltage value lower than 1.5 V, the deposition rate is comparatively lower than the higher voltages. The results indicate that below 1.5 V, the supplied energy is not sufficient to deposit the material at a higher rate (17). On the other hand, for a voltage value higher than 1.5 V, the deposition rate is comparatively higher than the lower voltages. Due to the higher deposition rate after the transition point, the deposition structures become tree and powder type. One possible explanation is that at a certain point of high-voltage deposition, the corresponding current value exceeds the limit value of the electrolyte, which causes the deposit to become powdery. It is probable that the area surrounding the anode becomes depleted of ions for discharging the anode. At the same time, a high volume of hydrogen gas is produced at the anode (17).

Microelectrochemical Deposition

535

START

Electrode Positioning

Starting of Deposition

Calculating and summing the initial growth height from equation (4.12)

Total initial growth height

Initial growth Height (Hi)

Hi

False True

Reference Current (I ref)

Measurement of initial current

Measure of current value (I) of present position

I > I ref True

False

Feeding backward with the minimum resolution of motor (0.1µm)

Feeding forwardward with the minimum resolution of motor (0.1µm)

Count of total height = Hi + Total backward height

False

Count of total height = Hi - Total forward height

Total Feedback height

H

Input of Desired Height(H )

True END

Figure 15 Algorithm for closed-loop control. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

Gap

Extra deposited metal

Anode Electrode

Mask

Mask Electrolyte

Anode Electrode Mask

Deposited electrode Peaks and valleys Mask

Electrolyte (a)

(b)

Figure 16 (a) Showing the gap between the electrode and mask and (b) SEM image showing the extradeposited material through the gap. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

536

Microelectrochemical Deposition

180

Deposition height (µm)

160 140 120 100 80 60 40 20 0 0

500

1000

1500

2000

2500

Deposition time (s) 1.2V (sim) 1.2V (exp)

1.5V (sim) 1.5V (exp)

1.6V (sim) 1.6V (exp)

1.8V (sim) 1.8V (exp)

2.0V (sim) 2.0V (exp)

Figure 17 Effect of pulse voltage amplitude on deposition height (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

11.19.3.2.4.2 Effect of Pulse Voltage Frequency The simulation and experimental results of Figures 19 and 20 show that the deposition height and rate increase with the increase of deposition time at any value of frequency. For a frequency value higher than 100 kHz, the deposition rate is comparatively lower than the lower frequencies. In contrast, for frequency values lower than 100 kHz, the deposition rate is comparatively higher than the lower frequencies (17). It is well known that the frequency has an inverse relationship with the pulse period. This is why the pulse period or duration increases with the decrease of the pulse frequency. With the increase of the pulse period time, the amount of energy per pulse also increases. Owing to the increased deposition energy, the number of ions also increases for deposition, which causes the deposition rate to increase. Alternatively, when the frequency increases, the condition will be the reverse (17). 0.3

Deposition rate (µm s–1)

0.25

0.2

0.15

0.1

0.05

0 0

500

1000

1500

2000

2500

Deposition time (s) 1.2V (sim) 1.2V (exp)

1.5V (sim) 1.5V (exp)

1.6V (sim) 1.6V (exp)

1.8V (sim) 1.8V (exp)

2.0V (sim) 2.0V (exp)

Figure 18 Effect of pulse voltage amplitude on deposition rate (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

Microelectrochemical Deposition

537

180

Deposition height (µm)

160 140 120 100 80 60 40 20 0 0

500

1000

1500

2000

2500

Deposition time (s) 70kHz (sim) 70kHz (exp)

85kHz (sim) 85kHz (exp)

100kHz (sim) 100kHz (exp)

115kHz (sim) 115kHz (exp)

130kHz (sim) 130kHz (exp)

Figure 19 Effect of pulse voltage frequency on deposition height (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

11.19.3.2.4.3 Effect of Pulse Voltage Duty Ratio For any particular duty ratio, the deposition height and rate increase with the increase of deposition time, and the results are shown in Figures 21 and 22. For the 0.2 and 0.25 duty ratio, however, the deposition rate is relatively lower than the higher duty ratio. Conversely, for the 0.4 and 0.5 duty ratio the deposition rate is relatively higher than the lower duty ratio, and after a certain point it increases rapidly. A reasonable explanation is that this study is conducted on a fixed pulse frequency where the pulse period is fixed and the duty ratio is the ratio of pulse on time and pulse period. This is why, due to the decrease of duty ratio, the pulse on time also decreases (17). If the pulse on time is much less than the double-layer time constant, then there will be strong damping in the system. This incidence causes the insufficient charging and discharging of the double layer. As a final point, the deposition rate decreases owing 0.45

Deposition rate (µm s–1)

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0

500

1000

1500

2000

2500

Deposition time (s) 70kHz (sim) 70kHz (exp)

85kHz (sim) 85kHz (exp)

100kHz (sim) 100kHz (exp)

115kHz (sim) 115kHz (exp)

130kHz (sim) 130kHz (exp)

Figure 20 Effect of pulse voltage frequency on deposition rate (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

538

Microelectrochemical Deposition

180

Deposition height (µm)

160 140 120 100 80 60 40 20 0 0

500

1000

1500

2000

2500

Deposition time (s) 0.20 (sim) 0.20 (exp)

0.25 (sim) 0.25 (exp)

0.33 (sim) 0.33 (exp)

0.40 (sim) 0.40 (exp)

0.50 (sim) 0.50 (exp)

Figure 21 Effect of pulse voltage duty ratio on deposition height (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

to this occurrence. Inversely, for a higher duty ratio the pulse on time is much higher than the double-layer time constant, causing an increase in deposition energy. Due to the increase of deposition energy, the number of ions also increases for deposition, which causes the deposition rate to increase (17). 11.19.3.2.4.4 Effect of Electrode Effective Gap Distance As can be seen from the simulation as well as experimental results, the deposition height and rate increase with the increase of deposition time at any electrode effective gap distance (Figures 23 and 24). In addition, with the increase of gap distance, the overall deposition rate decreases. These results can be adequately explained by the double-layer time constant characteristics, which is 0.6

Deposition rate (µm s–1)

0.5

0.4

0.3

0.2

0.1

0 0

500

1000

1500

2000

2500

Deposition time (s) 0.20 (sim) 0.20 (exp)

0.25 (sim) 0.25 (exp)

0.33 (sim) 0.33 (exp)

0.40 (sim) 0.40 (exp)

0.50 (sim) 0.50 (exp)

Figure 22 Effect of pulse voltage duty ratio on deposition rate (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

Microelectrochemical Deposition

539

180

Deposition height (µm)

160 140 120 100 80 60 40 20 0 0

500

1000

1500

2000

2500

Deposition time (s) 350 µm (sim) 350 µm (exp)

400 µm (sim) 400 µm (exp)

450 µm (sim) 450 µm (exp)

500 µm (sim) 500 µm (exp)

600 µm (sim) 600 µm (exp)

Figure 23 Effect of gap distance on deposition height (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

a product of resistivity, capacitance, and gap distance. If the gap distance increases, then the time constant also will increase. When the time constant increases, it takes more time to charge the double layer. This will lead to a strong damping condition. Due to this insufficient charging and discharging of the double layer, the deposition rate decreases (17).

11.19.3.2.5

LECD Structures Using the Anode as a Counter Electrode

LECD is an extraordinary method for easily making noncircular cross-sectional electrodes. This process has advantages in terms of fabrication time and cost compared with any other microfabrication methods. To fabricate an electrode, a plating solution of acidic CuSO4$5H2O with a Cu2þ ion concentration of 1.0–1.25 M and some organic additives such as thiourea is used to 0.25

Deposition rate (µm s–1)

0.2

0.15

0.1

0.05

0 0

500

1000

1500

2000

2500

Deposition time (s) 350 µm (sim) 350 µm (exp)

400 µm (sim) 400 µm (exp)

450 µm (sim) 450 µm (exp)

500 µm (sim) 500 µm (exp)

600 µm (sim) 600 µm (exp)

Figure 24 Effect of gap distance on deposition rate (simulation and experimental). Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755.

540

Microelectrochemical Deposition

Table 1

LECD parameter for simulation and experiments

Parameters

Value

Pulse amplitude, 40 Pulse frequency, f Pulse duty ratio, D Electrode gap distance, dgap Exchange current density, i0 Specific electrolyte resistivity, r Leak factor, a Stoichiometric number, n Specific capacitance, cDL Temperature, T Copper mole volume Faraday constant, F Gas constant, R

1.2, 1.5, 1.6, 1.8, and 2.0 V 70, 85, 100, 115, and 130 kHz 0.20, 0.25, 0.33, 0.40, 0.50 350, 400, 450, 500, and 600 mm 1.5 mA cm2 10 U cm 0.5 2 10 mF cm2 298.15 K 7.11 cm3 mol1 96485 C mol1 8.314 J mol1 K1

improve the microstructure of the copper deposit. Based on the parametric study of the previous section, an optimized electrode gap of 350 mm, voltage amplitude of 1.6 V, voltage frequency of 100 kHz, and duty ratio of 0.33 is used to fabricate a good structure of copper. These optimum conditions produce a smooth, fine-grained, and low-porosity copper electrode suitable for use in EDM. Figures 25(a) and 25(b) show the SEM images of deposited electrodes before and after the EDM process. Figures 25(c) and 25(d) show that the structural accuracy is better for a closed-loop than for an open-loop control. However, in the top side of the structure the deposited structure is not uniform. This may be due to the change of copper concentration during the deposition process. When the concentration becomes less than a certain limit, then the number of ions available for discharging is low when the concentration is low, creating a depletion layer just beneath the electrode. There is no such effect on the deposition rate due to this occurrence, but the deposited structures are irregular and highly porous and it will become tree type. Figure 26 represents the EDX spectrum analysis of the deposited electrode before and after EDM. The mass percentages of copper (Cu), carbon (C), and oxygen (O) obtained in the EDX spectrum in Figure 26(a) were 81.37%, 12.31%, and 2.73%, respectively. On the other hand, in Figure 26(b) the relative masses of Cu, C, and O were 55.94%, 39.87%, and 4.09%, respectively. This analysis shows that after EDM there is more carbon and oxygen content than before EDM. This excess amount of carbon came from the dielectric oil in the form of debris and burrs that remain resolidified over the surface of the electrode. Sometimes oxidization of debris occurred during resolidification at the edge; hence, the oxygen content (19).

Figure 25 X shape deposited electrode (a) before EDM and (b) after EDM. Deposited structure for (c) open loop control and (d) close loop control. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

Microelectrochemical Deposition

541

Figure 26 EDX analysis of a deposited electrode (a) before EDM and (b) after EDM. Reproduced from Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

11.19.3.2.6

Microholes Fabricated by LECD Electrodes

Figures 27(a) and 27(b) shows that, at almost all levels of discharge energy the microholes obtained were free of burrs and recast layers, and improved circularity was achieved. By using a very low value of capacitance, the discharge energy can be minimized, which can give good surface finish and edge linearity. Therefore, as the energy per pulse is smaller in the RC circuit, smaller craters are generated, which means smaller amount of material is removed per cycle. For this reason, it is easy to wash away the debris from the machining zone by the low-pressured side dielectric flushing.

Figure 27 (a) Entrance and (b) exit side SEM image of microhole with LECD electrode at different energy level of discharge energy on stainless steel. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755 and Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

542

Microelectrochemical Deposition

11.19.3.2.6.1 Parameters Influencing the Micro-EDM of LECD Electrode The RC-type pulse generator is mainly determined by the gap voltage and the capacitance. In RC-type pulse generator, the maximum discharge energy per pulse that can be obtained from RC circuit is: Eds ¼

1 CV 2 2

[20]

where, C ¼ capacitance and V ¼ gap voltage. So in the RC-type generator, the performance of the micro-EDM process can be more precisely controlled by knowing the effect of only the gap voltage and the capacitance. For the fine-finish micro-EDM, the discharge energy should be minimized, which can be more easily done in the RC generator by using low values of voltage and capacitance. To determine the optimum conditions for quality microholes in different workpiece materials with LECD electrodes, a series of experiments were conducted by varying the major operating parameters. The machining conditions for this study are listed in Table 2. Therefore, to assess the optimum conditions and dimensional accuracy of the microholes the material removal rate (MRR), relative wear ratio (RWR), average spark gaps (ASG), and average taper angel (ATA) were also measured. Equations [21] to [24] were used to calculate the MRR, RWR, ASG, and ATA, respectively. Figures 28(a) and 28(b) shows the ASG and ATA measuring procedure. MRR ¼

Amount of material removed from workpiece ðvolm Þ Unit time

[21]

RWR ¼

Amount of material removed from electrode ðvolm Þ Amount of material removed from workpiece ðvolm Þ

[22]

  1 g1 þ g2 þ g3 þ g4 a  4 2

[23]

ASG ¼

 1

ATA; q ¼ tan

dtop  dbottom 2h

[24]

Here, a is actual dimension (mask), g1, g2, g3, g4 are the machined dimensions (hole), dtop and dbottom are the top and bottom diameter of the hole, and h is the height of the workpiece. 11.19.3.2.6.2 Effect of Gap Voltage The gap voltage plays an important role in micro-EDM applications. Figures 29(a), 29(e), and 29(g) shows that with the increase of voltage, the MRR, ASG, and ATA increase for all values of capacitance. This is because; from eqn [20], it is clear that when the gap voltage increases the discharge energy as well as the spark gap also increase. Taper angle depends on the spark gap and material Table 2 Machining parameters of RC pulse generator micro-EDM for microholes machining of LECD electrode Parameters

Values

EDM circuit Supply voltage (V) Capacitance (pf) Resistance (kU) Dielectric coolant

R–C 60, 80, 100, 120, 140 100, 220, 470, 1000, 2200 Fixed to 1 kU EDM oil 3

Figure 28 Measurement of (a) average spark gap and (b) taper angle q. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755 and Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

0.0035

0.0035

0.0030

0.0030

MRR (mm3 min–1)

MRR (mm3 min–1)

Microelectrochemical Deposition

0.0025 0.0020 0.0015 0.0010 0.0005

0.0025 0.0020 0.0015 0.0010 0.0005

0.0000 60

80

c = 100 pf

c = 220 pf

100 120 Gap Voltage (V) c = 470 pf

c = 1000 pf

140

0.0000 100

c = 2200 pf

220 470 1000 Capacitance (pf) 60 Volt 100 Volt 140 Volt

60

50

50

40

40

30 20

30 20 10

10

0

0 60

80

c = 100 pf

c = 220 pf

100 Gap Voltage (V) c = 470 pf

120

c = 1000 pf

100

140

220

470 1000 Capacitance (pf)

60 Volt

c = 2200 pf

Average spark gap (µm)

20 18 16 14 12 10 8 6 4 2 0 60

c = 100 pf

80 c = 220 pf

100 120 Gap Voltage (V) c = 470 pf c = 1000 pf

100

140 c = 2200 pf

220

470 1000 Capacitance (pf)

60 Volt

100 Volt

2200

140 Volt

(f)

12

10

10

Taper angle (degree)

12

8 6 4 2

c = 100 pf

140 Volt

20 18 16 14 12 10 8 6 4 2 0

(e)

0 60

100 Volt

2200

(d)

(c)

Average spark gap (µm)

2200

(b)

60

RWR (%)

RWR (%)

(a)

Taper angle (degree)

543

8 6 4 2 0

80 c = 220 pf

100 120 Gap Voltage (V) c = 470 pf c = 1000 pf (g)

140 c = 2200 pf

100

220 470 1000 Capacitance (pf) 60 Volt

100 Volt

2200

140 Volt

(h)

Figure 29 Effect of gap voltage on (a) MRR, (c) RWR, (e) average spark gap, (g) taper angle; effect of capacitance on (b) MRR, (d) RWR, (f) average spark gap and (h) taper angle. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755 and Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010.

544

Microelectrochemical Deposition

Figure 30 (a) NUS shape deposited electrode; and (b) NUS shape hole was machined by NUS shape electrode with EDM discharge energy of 2.35 mJ. Reproduced from Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755; Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010 and Habib, M. A.; Rahman, M. Performance Analysis of EDM Electrode Fabricated by Localized Electrochemical Deposition for Micro-Machining of Stainless Steel. Int. J. Adv. Manuf. Technol. 2010, 49 (9–12), 975–986.

removal rate. This is why when the spark gap increases the taper angle also increases. However, in the lower voltage, the taper angle is higher than the higher voltage. This is due to the material removal rate is low in lower voltage that means the machining time longer which cause the taper angle larger. In an RC-type pulse generator, relatively smaller craters are generated due to its lower energy per pulse, and debris created by machining is flushed away from the machining zone by the dielectric. Figure 29(c) shows that the relative wear ratio increases with the increase of gap voltage due to increased discharge energy. In the case of dimensional accuracy, Figure 29(e) shows that the dimensional accuracy decreases as the average spark gap and taper angle increase due to the increase of gap voltage. In order to improve the dimensional accuracy, appropriate gap voltage, and capacitance value can be used to achieve around a 2 mm average spark gap and a 2 of taper angle (17,19, 30). 11.19.3.2.6.3 Effect of Capacitance The capacitor serves a very important role in micro-EDM applications. In the RC-type pulse generator, the capacitor controls the charging and discharging pulse frequency. Therefore, the nano pulse can be generated in the RC type with very short pulse duration. For this purpose, by using a very low value of capacitor, the pulse energy minimization can be easily fulfilled. This is why, by changing the capacitor value, good dimensional accuracy can be achieved. Figure 29(b) shows that with the increase of capacitance the MRR increases as the discharge energy increases. Therefore, the larger capacitance results in deeper craters, which increase the material removal. The RWR also increases with the increase of capacitance. However, Figure 29(d) shows that at a very high value of capacitance the RWR decreases. Actually, this is not investigative of lower electrode wear. In this study, the electrode wear was measured as a ratio of the volume of electrode material eroded to the volume of material removed from the workpiece. In the case of lower capacitance, RWR is high for lower voltage, because in this condition MRR is very low, which causes the electrode to erode more. For this reason, the ratio shows a decreased value, although more material is removed from the electrode compared to that of lower capacitance. Therefore, at a very high value of capacitor, the average spark gap increases, which causes more material to be removed with respect to electrode erosion (Figure 29(f)). Moreover, the dimensional accuracy is also reduced as the average spark gap and taper angle (Figure 29(f) and 29(h)) increase with respect to gap voltage (17,19, 30). Figure 30 shows the deposited electrode with NUS shape, and the NUS shape microhole is fabricated on an austenitic stainless steel (SUS 304) workpiece.

11.19.4

Conclusions

LECD is a remarkable method for easily making a 3D microstructure. This process has advantages in a variety of materials and cost efficiency compared with other microfabrication methods. The LECD process is capable of fabricating on-machine noncircular microelectrodes. These microelectrodes can be used directly in the micro-EDM process. This fabrication process will be a good solution in MEMS and bio-MEMS industries, where concircular-shaped holes and cavities are required to fabricate. Moreover, this fabrication process is very effective for industrial applications, where production time and cost can be minimized.

See also: Introduction to Advanced Machining Technologies; Compound and Hybrid Micromachining Processes; Compound and Hybrid Micromachining; Micromilling; Micro-Electrical Discharge Machining (Micro-EDM): Processes, Varieties, and Applications; Electrochemical Micromachining.

Microelectrochemical Deposition

545

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Corbett, J.; McKeown, P. A.; Peggs, G. N.; Whatmore, R. Nanotechnology: International Developments and Emerging Products. CIRP Ann. 2000, 49, 523–546. Lang, W. Reflexions on the Future of Microsystems. Sens. Actuators 1999, 72, 1–15. Madou, M. J. Fundamentals of Microfabrication; CRC Press: Boca Raton, 1997. Weck, M.; Fischer, S.; Vos, M. Fabrication of Micro Components Using Ultra Precision Machine Tools. Nanotechnology 1997, 8, 145–148. Meeusen, W.; Clijnen, J.; Reynaerts, D.; Van, H. Micro-Electro-Discharge Machining as Microsensor Fabrication Technology. IEEE Sens. J. 2003, 3 (5), 632–639. Schoth, A.; Förster, A.; Menz, W. Micro Wire EDM for High Aspect Ratio 3D Microstructuring of Ceramics and Metals. Microsyst. Technol. 2005, 11, 250–253. Kuo, C. L.; Huang, J. D.; Liang, H. Y.; Huang, J. D. Fabrication of 3D Metal Microstructures Using a Hybrid Process of Micro-EDM and Laser Assembly. Int. J. Adv. Manuf. Technol. 2003, 21, 796–800. Okuyama, H.; Takada, H. Micromachining with SR and FEL. Nucl. Instrum. Meth. Phys. Res. B 1998, 144, 58–65. Rajurkar, K. P.; Yu, Z. Y. 3D Micro-EDM Using CAD/CAM. CIRP Ann. 2000, 49 (1), 127–130. Ananthakrishnan, A.; Sarma, R.; Ananthasuresh, G. K. Systematic Mask Synthesis for Surface Micromachined Microelectromechanical Systems. J. Micromech. Microeng. 2003, 13, 927–941. Fang, F. Z.; Liu, K.; Kurfess, T. R.; Lim, G. C. Tool-Based Micromachining and Applications in MEMS. In MEMS/NEMS Handbook Techniques and Applications, Vol. 3; Springer: US, 2006; pp 678–740. Li, T.; Gianchandani, Y. B. A Micromachining Process for Die-Scale Pattern Transfers in Ceramics and Its Application to Bulk Piezoelectric Actuators. J. Microelectromech. Syst. 2006, 15 (3), 605–612. Yu, Z. Y.; Masuzawa, T.; Fujino, M. Micro-EDM for Three-Dimensional Cavities–Development of Uniform Wear Method. CIRP Ann. 1998, 47 (1), 169–172. Zhao, W.; Yang, Y.; Wang, Z.; Zhang, Y. A CAD/CAM System for Micro-ED-Milling of Small 3D Freeform Cavity. J. Mater. Process. Technol. 2004, 149, 573–578. Habib, M. A.; Gan, S. W.; Rahman, M. Fabrication of Complex Shape Electrodes by Localized Electrochemical Deposition. J. Mater. Process. Technol. 2009, 209 (2), 4453–4458. Habib, M. A.; Gan, S. W.; Lim, H. S.; Rahman, M. Fabrication of EDM Electrodes by Localized Electrochemical Deposition. Int. J. Precis. Eng. Manuf. 2008, 9 (2), 75–80. Habib, M. A.; Shaleh, T.; Rahman, M. Modeling for Fabrication of Micro Electrodes by Localized Electrochemical Deposition for Micro-EDM. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2010, 224 (11), 1741–1755. Said, R. A. Microfabrication by Localized Electrochemical Deposition: Experimental Investigation and Theoretical Modeling. Nanotechnology 2003, 14, 523–531. Habib, M. A. Development of Localized Electrochemical Deposition Process for the Fabrication of On-Machine Micro-EDM Electrode. Ph.D. Thesis, National University of Singapore (NUS), 2010. Hunter, I. W.; Lafontaine, S. R.; Madden, J. D. Three-Dimensional Microfabrication by Localized Electrochemical Deposition and Etching. U.S. Patent Specification 5641391, 1997. Madden, J. D.; Hunter, I. W. Three-Dimensional Microfabrication by Localized Electrochemical deposition. J. Microelectromech. Syst. 1996, 5 (1), 24–32. Jansson, A.; Thornell, G.; Johansson, S. High Resolution 3D Microstructures Made by Localized Electrodeposition of Nickel. J. Electrochem. Soc. 2000, 147, 1810–1827. El-Giar, E. M.; Cairo, U.; Thomson, J. D. Localized Electrochemical Plating of Interconnectors for Microelectronics. In Proc. IEEE Conf. Communications, Power and Computing, WESCANEX 97, 1997, pp 327–332. El-Giar, E. M.; Said, R. A.; Bridges, G. E.; Thomson, D. J. Localized Electrochemical Deposition of Copper Microstructures. J. Electrochem. Soc. 2000, 147, 586–591. Yeo, S. H.; Choo, J. H.; Yip, K. S. Localized Electrochemical Deposition – The Growth Behaviour of Nickel Micro-Columns. Proc. SPIE 2000, 4174, 30–39. Yeo, S. H.; Choo, J. H. Effects of Rotor Electrode in the Fabrication of High Aspect Ratio Microstructures by Localized Electrochemical Deposition. J. Micromech. Microeng. 2001, 11, 435–442. Yeo, S. H.; Choo, J. H.; Sim, K. H. A. On the Effects of Ultrasonic Vibrations on Localized Electrochemical Deposition. J. Micromech. Microeng. 2002, 12, 271–279. Park, J. W.; Ryu, S. H.; Chu, C. N. Pulsed Electrochemical Deposition for 3D Micro Structuring. Int. J. Precis. Eng. Manuf. 2005, 6 (4), 49–54. Said, R. A. Adaptive Tip-Withdrawal Control for Reliable Microfabrication by Localized Electrodeposition. J. Microelectromech. Syst. 2004, 13 (5), 822–832. Habib, M. A.; Rahman, M. Performance Analysis of EDM Electrode Fabricated by Localized Electrochemical Deposition for Micro-Machining of Stainless Steel. Int. J. Adv. Manuf. Technol. 2010, 49 (9–12), 975–986.