# Experimental Research on Discharge Forming Cutting-Electrochemical Machining of Single-Crystal Silicon

Authors: Bin Xin; Wei Liu
Journal: Mathematical Problems in Engineering
 (2021)

---

## Abstract

During the wire electrical discharge machining (WEDM) process, a large number of discharge pits and a recast layer are easily generated on the workpiece surface, resulting in high surface roughness. A discharge forming cutting-electrochemical machining method for fabricating single-crystal silicon is proposed in this study to solve this problem. On the same processing equipment, single-crystal silicon is first cut using the discharge forming cutting method. Second, electrochemical anodic reaction technology is used to dissolve the discharge pits and recast layer on the single-crystal silicon surface. The machining mechanism of this process, the surface elements of the processed single-crystal silicon and a comparison of the kerf width are analyzed through experiments. On this basis, the influence of the movement speed of the copper foil electrode during electrochemical anodic dissolution on the final surface roughness is qualitatively analyzed. The experimental results show that discharge forming cutting-electrochemical machining can effectively eliminate the electrical discharge pits and recast layer, which are caused by electric discharge cutting, on the surface of single-crystal silicon, thereby reducing the surface roughness of the workpiece.


---

## Body

## 1. Introduction

Silicon is widely used in integrated circuits (ICs) and ultralarge-scale integrated circuits (ULSIs) due to its special physical, chemical, and electrical properties [1]. The current manufacturing process of a single-crystalline silicon chip mainly includes cutting (rough), grinding (fine), and grinding (polishing). Different processing stages require surfaces with varying roughness and smoothness. Cutting is the key process for processing a single-crystal silicon chip, and the cutting cost accounts for more than 50% of the entire processing cost of the single-crystal silicon chip. The surface of the silicon wafer, which is a high-quality substrate for the chip, should have extremely high flatness and low surface roughness, and there should be no deterioration layer or lattice structure defects on the surface. However, single-crystal silicon is a typical brittle and rigid material that is difficult to process. At present, the primary machining method for silicon chip cutting is wire saw cutting [2–4], which is based on indentation fracture theory for brittle materials [5]. The processing of the chip by the wire saw is realized by cutting the ingot with a high-speed wire saw. As the fixed-abrasive wire saw is a flexible tool, vibration is unavoidable during the cutting process [6]. Severe vibration affects the surface quality of the workpiece, causing surface defects and shedding of abrasive particles and affects the life of the wire saw. The lateral vibration of the wire saw is the main factor affecting the cutting and wafer surface quality [7]. The Mohs hardness of single-crystal silicon is 7, with extremely low fracture toughness and a very low critical cutting depth. Although the cost of wire saw processing equipment is lower, its machining efficiency is also lower, especially during high-speed cutting and high-frequency swing process. The strong vibration caused by the very large inertial force affects the cutting accuracy when producing the chip; thus, there is an approximately 10 µm damaged layer on the chip surface and warping. More seriously, the vibration may cause the chip to collapse or break.Wire electrical discharge machining (WEDM) is a unique noncontact machining technology [8]. WEDM can continuously erode the workpiece material using the heat generated by a pulse discharge between the electrode wire and the workpiece [9]. Due to its unique noncontact removal mechanism, machining is not limited by the brittleness of the material, and it can process conductive material of any hardness, strength, and brittleness, which has nothing to do with the mechanical properties of the material. Thus, it is an ideal choice for brittle and hard materials that are difficult to process [10–14]. In particular, it is suitable for processing brittle and hard single-crystal silicon [15–18] and polycrystalline silicon [19, 20]. Although WEDM has certain advantages in cutting brittle and hard single-crystal silicon (no contact and high machining efficiency), there are still two main problems: (1) the vibration of the electrode wire can easily produce a workpiece with poor surface quality and cause wire breakage [21, 22]. The vibration of the electrode wire affects the cutting speed and surface finish and causes secondary discharge, resulting in an unstable discharge state [23]. During WEDM, the wire electrode is always exhibiting rapid reciprocating motion. Due to the low stiffness of the wire, vibration and deformation easily occur during the machining process, which significantly affects the machining quality (mainly reflected in increased surface waviness and wider cutting of the workpiece) [10]. Puri and Bhattacharyya [24] analyzed the influence of wire electrode tension, pulse power frequency, working height, and guidewire spacing on wire electrode vibration. When the wire electrode tension is high, the amplitude of the wire electrode can be reduced, but the risk of wire breakage increases. (2) The problem of the surface metamorphic layer.WEDM integrates fluid dynamics and thermal explosive forces. During machining, discharge pits and metamorphic layers are formed on the surface of single-crystal silicon. The appearance of the metamorphic layer dramatically reduces the accuracy and service life of the silicon crystal element. Yeo and Nachiappan [25] pointed out that the surface structure produced by WEDM is very complicated, with pits and solidified metamorphic layers. The metamorphic layer is produced by the deposition of recrystallized material on the surface after single-crystal silicon is melted. Therefore, it is necessary to reduce the deposition of the metamorphic layer by rotating the electrode or increasing the amount of flushing liquid during processing to improve the surface quality after processing. Murray et al. [26] found that in the process of WEDM, the electrode material (tungsten grains) was deposited on the surface of single-crystal silicon, thereby forming a metamorphic layer, which further affects the surface quality of the processed material (single-crystal silicon). Yeh et al. [27] found that when a phosphorous dielectric was used as the electrolyte for WEDM, the surface modification layer depth of single-crystal silicon after cutting was 20 μm, and the content of phosphorous reached 0.49 wt%. Furthermore, their experiment showed that when the discharge current was larger, the metamorphic layer was thicker, and the material contained more phosphorus. Ge et al. [28] used the X-ray diffraction rocking curve method to measure the thickness of the metamorphic layer on the surface of single-crystal silicon after using different wire cutting parameters. The measurement results showed that the thickness of the metamorphic layer increased with increasing pulse width.Solving the problems of WEDM is a prerequisite for cutting high-quality wafers. (1) Solving the problem of wire vibration and broken wire: Flaño et al. [29] proposed cutting silicon carbide with metal foil electrodes. As the thickness of the metal foil electrode used in this method was smaller than the diameter of the electrode wire, the cutting width can be reduced, thereby reducing material loss. Using tooling to tighten the metal foil electrode can effectively prevent the impact of electrode vibration on the processing accuracy and decrease the risk of wire breakage. Zhao et al. [30] proposed cutting silicon carbide with multilayer metal foil electrodes. This method reduced the cutting width and prevented electrode vibration (the surface waviness error was below 10 μm). Furthermore, cutting multilayer metal foil electrodes can greatly improve the cutting efficiency (reaching 5.23 mm2/min). (2) Solving the problem of the surface metamorphic layer: electrochemical machining (ECM) does not need to consider the strength and hardness of the processed material. It removes the surface layer of the material, in this case, single-crystal silicon, by anodic dissolution [31, 32]. Allongue et al. [33] confirmed the feasibility of electrochemical processing for P-type single-crystal silicon. At a sufficiently high voltage, the silicon surface potential locally reached the electropolished state, and machining could be realized. The ECM accuracy is related to pulse voltage, pulse duration, and solution composition. Lyubimov et al. [34] plated three metals, namely, copper, nickel, and platinum, on plastic electrodes. The electrochemical machining of P-type single-crystal silicon by this new type of metallized plastic electrode tool was studied, and the process parameters during machining were experimentally optimized. Wang et al. [35] used wire electrodes to electrochemically process single-crystal silicon. This method can process single-crystal silicon with a resistivity of 0.5 to 3 Ω cm. Compared with WEDM, this method can effectively remove the metamorphic layer on the surface of single-crystal silicon.On the basis of the aforementioned research, combining the advantages of metal foil electrodes in electrical discharge cutting and electrochemical machining, a discharge forming cutting-electrochemical machining method of single-crystal silicon is proposed in this paper. Electrolytic anodic dissolution is used to remove the discharge pits and recast layer on the surface of single-crystal silicon to improve the surface quality of the resulting chip.


## 2. Principle of Single-Crystal Silicon Discharge Forming Cutting-Electrochemical Machining

No processing equipment should be replaced for the single-crystal silicon discharge forming cutting-electrochemical machining proposed in this research, and the cutting and electrolytic processing of single-crystal silicon can be completed on one piece of equipment. To meet this requirement, the working fluid must simultaneously have both dielectric (insulation) and electrolytic (conductivity) properties. Considering environmental protection, this paper uses deionized water with low resistivity [9, 11] as the working fluid medium. The overall processing principle of the discharge forming cutting-electrochemical machining of single-crystal silicon is shown in Figure 1, and the processing steps are shown in Figure 2.Figure 1
General machining principle.Figure 2
Processing steps.Figure2 shows that in deionized water with low resistivity, the positive and negative poles of the pulsed power supply are placed between the single-crystal silicon and copper foil electrode. As the copper foil electrode moves downward, the distance between the electrodes reaches the spark discharge distance; thus, spark discharge is generated. The removal mechanism of single-crystal silicon discharged forming cutting is shown in Figure 2(a). After the single-crystal silicon is cut, the distance from the copper foil electrode to the cut surface of the single-crystal silicon is already greater than the spark discharge distance as the discharge is removed. The spark discharge disappears completely. The surface of the workpiece is covered with discharge pits and a recast layer, resulting in single-crystal silicon with poor surface quality, as shown in Figure 2(b).To improve the surface quality of single-crystal silicon and reduce its surface roughness, it is necessary to remove the discharge pits and recast layer. Therefore, electrochemical anodic reaction technology is introduced to remove the discharge pits and recast layer. As the distance between the copper foil electrode and single-crystal silicon is already greater than the spark discharge distance, there is no spark discharge between them. At this stage, the copper foil electrode is repeatedly fed at a certain speedv along the route of discharge cutting. As deionized water is used as the working fluid, which has a weak electrochemical effect, the removal of single-crystal silicon material at this stage is realized by electrochemical machining, as shown in Figure 2(c). The discharge pits and recast layer on the surface of single-crystal silicon are dissolved and removed due to the principle of anodic dissolution. Thus, the surface quality of the workpiece is improved, and the surface roughness is reduced, as shown in Figure 2(d). According to the aforementioned principle of single-crystal silicon discharge forming cutting-electrochemical machining, the entire machining process is carried out on the same equipment. No other processing equipment is required, and no machining parameters need to be changed.


## 3. Experimental Processing Equipment and Processing Objects

The experimental equipment used in this paper mainly includes the machining tools shown in Figure3. The tool electrode in Figure 3 is a copper foil electrode, and the single-crystal silicon is under the copper foil electrode. The single-crystal silicon is completely immersed in deionized water. The copper foil electrode is connected to the negative electrode of the pulsed power supply, and the single-crystal silicon is connected to the positive electrode of the pulsed power supply. The acquisition and control system is shown in Figure 4. The acquisition and control system mainly includes the main control computer, data acquisition card (PXle-5172 of NI) and motion controller (PXle-7342 of NI). The data acquisition card simultaneously collects the voltage and current, the motion controller controls the motor movement, and the motor drives the feeding movement of the copper foil electrode.Figure 3
Processing equipment.Figure 4
Acquisition and control system.The discharge forming cutting-electrochemical machining of single-crystal silicon is realized by processing equipment. The acquisition and control system mainly completes the electrical signal acquisition during processing and the feed movement control of the copper foil electrode. The surface roughness is measured using the Leica dual-core DCM 3D laser confocal microscope from Sensofar Technology, USA. The resolution of the Leica dual-core DCM 3D laser confocal microscope is as high as 0.1 nm. Scanning electron microscopy (SEM) was used to observe the surface morphology of the workpiece. A HITACHI SU-1510 tungsten filament scanning electron microscope was adopted, with a magnification of 5–300,000 times and a resolution of 3 nm. The maximum size of the detected workpiece is 153 mm.The experimental object was single-crystal silicon, and the properties of the single-crystal silicon are shown in Table1. The electrode is made of copper foil, and the material properties of the copper foil are shown in Table 2.Table 1
Parameters of single-crystal silicon.
TypeP-type single-crystal siliconMethod of growthCZCrystal orientation<111>Doping typePhosphorus dopedMelting point1410°CResistivity0.5 Ω cmDensity2.33 g/cm³Boiling point2355°CTable 2
Physical properties of copper foil electrode.
Density (g/cm3)Thermal conductivity (W/m °C)Conductivity (ms/m)Melting point (°C)8.9639459.91085


## 4. Results and Discussion

The discharge forming cutting-electrochemical machining of single-crystal silicon requires two stages of operations: in the first stage, copper foil electrodes are used in place of filaments to conduct the forming cutting of single-crystal silicon. Using copper foil electrodes can prevent the electrode wire vibration and reduce the risk of wire breakage. In the second stage, the copper foil electrode is again fed along the cutting route at a certain speedv. The discharge pits and recast layer left after the first stage are removed with electrochemical anodic dissolution, thereby reducing the surface waviness of the single-crystal silicon and improving its surface quality.


### 4.1. Removal Mechanism of Discharge Forming Cutting-Electrochemical Machining

First, the machining equipment shown in Figure3 is used to complete the discharge forming cutting of the single-crystal silicon. The voltage and current signals in the cutting process are shown in Figure 5. When cutting was finished, a scanning electron microscope was used to observe the surface morphology of the single-crystal silicon, which is shown in Figure 6. Scanning electron microscopy was used to measure the profile of the single-crystal silicon, and the measurement results are shown in Figure 7.Figure 5
Voltage and current signal of discharge forming cutting.
(a)(b)Figure 6
Surface morphology of single-crystal silicon after discharge forming cutting.Figure 7
Measurement result of recast layer thickness.Figure5 shows that the current and voltage signals in the discharge forming cutting process of the copper foil electrode are similar to those of WEMD and EDM [16], the current signal is not synchronized with the voltage signal, and the voltage signal has a noticeable breakdown delay [16]. After the breakdown delay, the voltage signal drops rapidly (drops from 100 V to approximately 24 V and remains unchanged), the current signal rises rapidly (rises up to approximately 17 A and remains unchanged), and there is obvious negative resistance between the electrodes [18]. Figures 5 and 6 show that the discharge forming cutting machining mechanism of the copper foil electrode is similar to that of WEMD. After discharge removal, discharge pits and a recast layer are left on the surface of the single-crystal silicon [25]. Since the material on the surface of the single-crystal silicon is removed by melting and vaporization, the removed material is carried out in the machining area by the moving electrode and flowing working fluid. However, some of the melted and vaporized materials that remain in the machining area are cooled by the working fluid and resolidify on the surface of the single-crystal silicon, thus forming the recast layer [26]. As shown in Figure 7, a recast layer with a thickness of approximately 15 μm is formed on the surface of monocrystalline silicon after discharge forming cutting. The thickness of the recast layer in Yeh et al.’s study [27] is 20 μm. The reason for this phenomenon is that the processing voltage in this paper is lower than the processing voltage in Yeh et al.’s study [27], so the thickness of the recast layer is also smaller. The discharge pits and recast layer on the surface of single-crystal silicon reduce the surface quality of the workpiece. The surface roughness of the single-crystal silicon shown in Figure 6 is 2.92 μm.Second, the machining equipment shown in Figure3 is used to conduct the discharge forming cutting of single-crystal silicon. The copper foil electrode is fed again along the cutting route at a speed of 120 μm/s. The voltage and current signals during the second feeding process are shown in Figure 8. A scanning electron microscope was used to observe the surface morphology of single-crystal silicon, as shown in Figure 9.Figure 8
Voltage and current signal during the second feeding of the copper foil electrode.
(a)(b)Figure 9
The surface morphology of single-crystal silicon after discharge forming cutting-electrochemical machining.Figure8 shows that the voltage and current signal of the discharge during the second feeding of the copper foil electrode is completely different from that of WEMD, and the current signal is completely synchronized with the voltage signal. This signal is similar to the signal characteristics in the electrochemical machining process by Wang et al. [35]. Figure 9 shows the surface morphology of single-crystal silicon processed by discharge forming cutting-electrochemical machining. As electrochemical anodic dissolution technology is introduced, the discharge pits and recast layer on the surface of single-crystal silicon basically disappear, the surface morphology tends to be flat, and the surface quality is greatly improved. The surface roughness of the single-crystal silicon is 0.76 μm. The processing parameters of the above two machining methods are shown in Table 3.Table 3
Processing parameters.
Processing parametersParameter valueVoltage,U (V)100Pulse width,Ton (μs)100Pulse interval,Tof (μs)400First-stage feeding rate of the copper foil electrode,v (mm)0.2Second-stage feeding rate of the copper foil electrode,v (μm)120From the above analysis, it can be seen that the material removal mechanism of single-crystal silicon discharge forming cutting-electrochemical machining includes electric spark melting, vaporization, and electrolytic dissolution corrosion. It can flatten the surface morphology of single-crystal silicon and greatly reduce its surface roughness.


### 4.2. Analysis of the Elemental Composition of the Single-Crystal Silicon Surface after Discharge Forming Cutting-Electrochemical Machining

EDS was used to perform an elemental energy spectrum analysis on the workpiece surface (Figure9), and the analysis results are shown in Figure 10.Figure 10
Energy spectrum analysis result.It can be seen from the energy spectrum analysis results in Figure10 that after the discharge forming cutting-electrochemical machining of single-crystal silicon, the elemental composition of the workpiece surface minimally changes, which is determined by the characteristics of electrochemical machining. During the electrolysis process, the anode undergoes an oxidation reaction, the anodic silicon atoms release electrons, which enter the working fluid medium in a free state; thus, the anodic single-crystal silicon is dissolved and removed during electrolysis. The electrode reaction formula of the anode is as follows [33]:(1)Si⇌Si4++4e4OH−⇌4H2O+O↑+4e.A reduction reaction occurs at the cathode and releases hydrogen gas. The electrode reaction formula of the cathode is(2)2H++2e⇌H2↑.As the working liquid medium is deionized water, the elemental composition of the workpiece surface after discharge forming cutting-electrochemical machining of single-crystal silicon minimally changes, and no other new chemical elements are introduced on the surface of single-crystal silicon.


### 4.3. Comparative Analysis of the Kerf Width after Discharge Forming Cutting-Electrochemical Machining

Figure11 shows the kerf morphology after discharge forming cutting of single-crystal silicon, and the kerf width is 314 μm. Figure 12 shows the kerf morphology of single-crystal silicon after discharge forming cutting-electrochemical machining, and the kerf width is 342 μm.Figure 11
Kerf morphology after electric discharge cutting.Figure 12
Kerf morphology after discharge forming cutting-electrochemical machining.Comparing Figure11 with Figure 12, it can be seen that the kerf width of single-crystal silicon obtained by discharge forming cutting-electrochemical machining is larger than that of single-crystal silicon obtained by discharge forming cutting. The reason can be summarized as follows: the kerf width of single-crystal silicon by discharge forming cutting is mainly determined by discharge removal. The kerf width of single-crystal silicon by discharge forming cutting-electrochemical machining is mainly determined by discharge removal and electrolytic dissolution. Since electrochemical anodic dissolution technology is introduced, the kerf obtained by discharge forming cutting is further dissolved, and the actual kerf width is larger than the width formed by only discharge removal.


### 4.4. Influence of the Feeding Speed of the Electrode on the Machining Effect in Discharge Forming Cutting-Electrochemical Machining

In this section, experimental methods will be used to verify the removal effect of the discharge forming cutting-electrochemical machining method on the discharge pits and recast layer on the surface of single-crystal silicon. Since the recast layer is the product of the discharge forming cutting stage, the recast layer can only be removed by dissolution in the electrolysis stage. According to Faraday’s law of electrolysis, the volume of dissolved material on the anode surface can be expressed as(3)V=ω⋅I⋅t,where V is the volume dissolved on the surface of the anode workpiece, ω is the electrochemical volume equivalent, I is the effective working current in the electrolysis stage, and t is the electrochemical machining time. As I is related to the voltage of the pulsed power supply and the voltage of the pulsed power supply is always constant, this section controls the amount of anodic dissolution by adjusting the electrochemical machining time t. The time t of electrochemical machining is realized by controlling the movement speed of the copper foil electrode. First, the processing parameters in Table 3 are used to cut the single-crystal silicon, and then the copper foil electrode is refed along the cutting route to realize the electrolytic dissolution of the anode. At the electrolysis stage, the second movement velocities of the copper foil electrode are 160, 140, and 120 μm/s. The other processing parameters are shown in Table 3. The original surface morphology of single-crystal silicon after discharge forming cutting is shown in Figure 13(a). The surface morphology of single-crystal silicon after discharge forming cutting-electrochemical machining is shown in Figures 14–16(a).Figure 13
Original surface morphology of single-crystal silicon after discharge forming cutting.
(a)(b)Figure 14
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 160μm/s.
(a)(b)Figure 15
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 140μm/s.
(a)(b)Figure 16
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 120μm/s.
(a)(b)Figure13(a) shows that after the single-crystal silicon is cut by discharge forming, there are a large number of discharge pits and a recast layer on the surface of the single-crystal silicon. This is caused by the machining mechanism of discharge cutting, leading to an uneven surface. The surface roughness of single-crystal silicon is 2.92 μm.When single-crystal silicon is processed with discharge forming cutting-electrochemical machining, due to the electrochemical dissolution that occurs during the second feeding process of the copper foil electrode, the discharge pits and recast layer on the surface of the single-crystal silicon are dissolved to varying degrees, as shown in Figures14–16(a). The continuous decrease in the second feeding speed of the copper foil electrode implies that the electrochemical machining time t increases. Thus, more resolidified substances on the surface of the single-crystal silicon are dissolved, and the surface quality improves.When the second feeding speed of the copper foil electrode is 160μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon cannot be fully dissolved, and there are still some discharge pits and recast layer material on the surface of the single-crystal silicon, resulting in poor surface quality (Figure 14(a)). Under the aforementioned machining condition, the two-dimensional morphology of the single-crystal silicon surface is shown in Figure 14(b). The surface roughness Ra of the single-crystal silicon is 1.68 μm.When the second feeding speed of the copper foil electrode is 140μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon can be fully dissolved, and there are basically no discharge pits or recast layer materials on the surface of the single-crystal silicon. Therefore, the surface quality of single-crystal silicon is greatly improved, as shown in Figure 15(a). The two-dimensional morphology of the single-crystal silicon surface under the aforementioned machining condition is shown in Figure 15(b). The surface roughness Ra of the single-crystal silicon is 0.86 μm.When the second feeding speed of the copper foil electrode is reduced to 120μm/s, the copper foil electrode has sufficient time to dissolve the discharge pits and recast layer. The discharge pits and recast layer on the surface of the single-crystal silicon are fully dissolved, and a small amount of the surface substrate of the single-crystal silicon is also dissolved. There are no discharge pits or recast layer material on the surface of the single-crystal silicon, and the surface of the single-crystal silicon is very flat (Figure 16(a)). The two-dimensional morphology of the single-crystal silicon surface under the above machining condition is shown in Figure 16(b), and the surface roughness Ra of the single-crystal silicon is 0.72 μm. If the substrate on the surface of single-crystal silicon undergoes electrolytic dissolution, grain boundaries can be found on the surface of single-crystal silicon. Figure 17(b) shows the diagram of the red area in Figure 17(a) enlarged by 1000 times, and the grain boundary can be clearly identified. When the second feeding speed of the copper foil electrode is less than 120 μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon can be completely dissolved, and the substrate on the surface of the single-crystal silicon also undergoes electrolytic dissolution.Figure 17
Processing effect when the second feeding speed of the copper foil electrode is 120μm/s: (a) 200 times magnification and (b) 1000 times magnification.
(a)(b)It can be concluded that discharge forming cutting-electrochemical machining can effectively remove the discharge pits and recast layer on the surface of single-crystal silicon, effectively improving the surface quality of the workpiece. As the feeding speed of the copper foil electrode is decreased, the surface roughness of the workpiece decreases. Thus, the feed speed of the copper foil electrode should be optimized to lower speeds.


## 4.1. Removal Mechanism of Discharge Forming Cutting-Electrochemical Machining

First, the machining equipment shown in Figure3 is used to complete the discharge forming cutting of the single-crystal silicon. The voltage and current signals in the cutting process are shown in Figure 5. When cutting was finished, a scanning electron microscope was used to observe the surface morphology of the single-crystal silicon, which is shown in Figure 6. Scanning electron microscopy was used to measure the profile of the single-crystal silicon, and the measurement results are shown in Figure 7.Figure 5
Voltage and current signal of discharge forming cutting.
(a)(b)Figure 6
Surface morphology of single-crystal silicon after discharge forming cutting.Figure 7
Measurement result of recast layer thickness.Figure5 shows that the current and voltage signals in the discharge forming cutting process of the copper foil electrode are similar to those of WEMD and EDM [16], the current signal is not synchronized with the voltage signal, and the voltage signal has a noticeable breakdown delay [16]. After the breakdown delay, the voltage signal drops rapidly (drops from 100 V to approximately 24 V and remains unchanged), the current signal rises rapidly (rises up to approximately 17 A and remains unchanged), and there is obvious negative resistance between the electrodes [18]. Figures 5 and 6 show that the discharge forming cutting machining mechanism of the copper foil electrode is similar to that of WEMD. After discharge removal, discharge pits and a recast layer are left on the surface of the single-crystal silicon [25]. Since the material on the surface of the single-crystal silicon is removed by melting and vaporization, the removed material is carried out in the machining area by the moving electrode and flowing working fluid. However, some of the melted and vaporized materials that remain in the machining area are cooled by the working fluid and resolidify on the surface of the single-crystal silicon, thus forming the recast layer [26]. As shown in Figure 7, a recast layer with a thickness of approximately 15 μm is formed on the surface of monocrystalline silicon after discharge forming cutting. The thickness of the recast layer in Yeh et al.’s study [27] is 20 μm. The reason for this phenomenon is that the processing voltage in this paper is lower than the processing voltage in Yeh et al.’s study [27], so the thickness of the recast layer is also smaller. The discharge pits and recast layer on the surface of single-crystal silicon reduce the surface quality of the workpiece. The surface roughness of the single-crystal silicon shown in Figure 6 is 2.92 μm.Second, the machining equipment shown in Figure3 is used to conduct the discharge forming cutting of single-crystal silicon. The copper foil electrode is fed again along the cutting route at a speed of 120 μm/s. The voltage and current signals during the second feeding process are shown in Figure 8. A scanning electron microscope was used to observe the surface morphology of single-crystal silicon, as shown in Figure 9.Figure 8
Voltage and current signal during the second feeding of the copper foil electrode.
(a)(b)Figure 9
The surface morphology of single-crystal silicon after discharge forming cutting-electrochemical machining.Figure8 shows that the voltage and current signal of the discharge during the second feeding of the copper foil electrode is completely different from that of WEMD, and the current signal is completely synchronized with the voltage signal. This signal is similar to the signal characteristics in the electrochemical machining process by Wang et al. [35]. Figure 9 shows the surface morphology of single-crystal silicon processed by discharge forming cutting-electrochemical machining. As electrochemical anodic dissolution technology is introduced, the discharge pits and recast layer on the surface of single-crystal silicon basically disappear, the surface morphology tends to be flat, and the surface quality is greatly improved. The surface roughness of the single-crystal silicon is 0.76 μm. The processing parameters of the above two machining methods are shown in Table 3.Table 3
Processing parameters.
Processing parametersParameter valueVoltage,U (V)100Pulse width,Ton (μs)100Pulse interval,Tof (μs)400First-stage feeding rate of the copper foil electrode,v (mm)0.2Second-stage feeding rate of the copper foil electrode,v (μm)120From the above analysis, it can be seen that the material removal mechanism of single-crystal silicon discharge forming cutting-electrochemical machining includes electric spark melting, vaporization, and electrolytic dissolution corrosion. It can flatten the surface morphology of single-crystal silicon and greatly reduce its surface roughness.


## 4.2. Analysis of the Elemental Composition of the Single-Crystal Silicon Surface after Discharge Forming Cutting-Electrochemical Machining

EDS was used to perform an elemental energy spectrum analysis on the workpiece surface (Figure9), and the analysis results are shown in Figure 10.Figure 10
Energy spectrum analysis result.It can be seen from the energy spectrum analysis results in Figure10 that after the discharge forming cutting-electrochemical machining of single-crystal silicon, the elemental composition of the workpiece surface minimally changes, which is determined by the characteristics of electrochemical machining. During the electrolysis process, the anode undergoes an oxidation reaction, the anodic silicon atoms release electrons, which enter the working fluid medium in a free state; thus, the anodic single-crystal silicon is dissolved and removed during electrolysis. The electrode reaction formula of the anode is as follows [33]:(1)Si⇌Si4++4e4OH−⇌4H2O+O↑+4e.A reduction reaction occurs at the cathode and releases hydrogen gas. The electrode reaction formula of the cathode is(2)2H++2e⇌H2↑.As the working liquid medium is deionized water, the elemental composition of the workpiece surface after discharge forming cutting-electrochemical machining of single-crystal silicon minimally changes, and no other new chemical elements are introduced on the surface of single-crystal silicon.


## 4.3. Comparative Analysis of the Kerf Width after Discharge Forming Cutting-Electrochemical Machining

Figure11 shows the kerf morphology after discharge forming cutting of single-crystal silicon, and the kerf width is 314 μm. Figure 12 shows the kerf morphology of single-crystal silicon after discharge forming cutting-electrochemical machining, and the kerf width is 342 μm.Figure 11
Kerf morphology after electric discharge cutting.Figure 12
Kerf morphology after discharge forming cutting-electrochemical machining.Comparing Figure11 with Figure 12, it can be seen that the kerf width of single-crystal silicon obtained by discharge forming cutting-electrochemical machining is larger than that of single-crystal silicon obtained by discharge forming cutting. The reason can be summarized as follows: the kerf width of single-crystal silicon by discharge forming cutting is mainly determined by discharge removal. The kerf width of single-crystal silicon by discharge forming cutting-electrochemical machining is mainly determined by discharge removal and electrolytic dissolution. Since electrochemical anodic dissolution technology is introduced, the kerf obtained by discharge forming cutting is further dissolved, and the actual kerf width is larger than the width formed by only discharge removal.


## 4.4. Influence of the Feeding Speed of the Electrode on the Machining Effect in Discharge Forming Cutting-Electrochemical Machining

In this section, experimental methods will be used to verify the removal effect of the discharge forming cutting-electrochemical machining method on the discharge pits and recast layer on the surface of single-crystal silicon. Since the recast layer is the product of the discharge forming cutting stage, the recast layer can only be removed by dissolution in the electrolysis stage. According to Faraday’s law of electrolysis, the volume of dissolved material on the anode surface can be expressed as(3)V=ω⋅I⋅t,where V is the volume dissolved on the surface of the anode workpiece, ω is the electrochemical volume equivalent, I is the effective working current in the electrolysis stage, and t is the electrochemical machining time. As I is related to the voltage of the pulsed power supply and the voltage of the pulsed power supply is always constant, this section controls the amount of anodic dissolution by adjusting the electrochemical machining time t. The time t of electrochemical machining is realized by controlling the movement speed of the copper foil electrode. First, the processing parameters in Table 3 are used to cut the single-crystal silicon, and then the copper foil electrode is refed along the cutting route to realize the electrolytic dissolution of the anode. At the electrolysis stage, the second movement velocities of the copper foil electrode are 160, 140, and 120 μm/s. The other processing parameters are shown in Table 3. The original surface morphology of single-crystal silicon after discharge forming cutting is shown in Figure 13(a). The surface morphology of single-crystal silicon after discharge forming cutting-electrochemical machining is shown in Figures 14–16(a).Figure 13
Original surface morphology of single-crystal silicon after discharge forming cutting.
(a)(b)Figure 14
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 160μm/s.
(a)(b)Figure 15
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 140μm/s.
(a)(b)Figure 16
Original surface morphology of single-crystal silicon when the second feeding speed of the copper foil electrode is 120μm/s.
(a)(b)Figure13(a) shows that after the single-crystal silicon is cut by discharge forming, there are a large number of discharge pits and a recast layer on the surface of the single-crystal silicon. This is caused by the machining mechanism of discharge cutting, leading to an uneven surface. The surface roughness of single-crystal silicon is 2.92 μm.When single-crystal silicon is processed with discharge forming cutting-electrochemical machining, due to the electrochemical dissolution that occurs during the second feeding process of the copper foil electrode, the discharge pits and recast layer on the surface of the single-crystal silicon are dissolved to varying degrees, as shown in Figures14–16(a). The continuous decrease in the second feeding speed of the copper foil electrode implies that the electrochemical machining time t increases. Thus, more resolidified substances on the surface of the single-crystal silicon are dissolved, and the surface quality improves.When the second feeding speed of the copper foil electrode is 160μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon cannot be fully dissolved, and there are still some discharge pits and recast layer material on the surface of the single-crystal silicon, resulting in poor surface quality (Figure 14(a)). Under the aforementioned machining condition, the two-dimensional morphology of the single-crystal silicon surface is shown in Figure 14(b). The surface roughness Ra of the single-crystal silicon is 1.68 μm.When the second feeding speed of the copper foil electrode is 140μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon can be fully dissolved, and there are basically no discharge pits or recast layer materials on the surface of the single-crystal silicon. Therefore, the surface quality of single-crystal silicon is greatly improved, as shown in Figure 15(a). The two-dimensional morphology of the single-crystal silicon surface under the aforementioned machining condition is shown in Figure 15(b). The surface roughness Ra of the single-crystal silicon is 0.86 μm.When the second feeding speed of the copper foil electrode is reduced to 120μm/s, the copper foil electrode has sufficient time to dissolve the discharge pits and recast layer. The discharge pits and recast layer on the surface of the single-crystal silicon are fully dissolved, and a small amount of the surface substrate of the single-crystal silicon is also dissolved. There are no discharge pits or recast layer material on the surface of the single-crystal silicon, and the surface of the single-crystal silicon is very flat (Figure 16(a)). The two-dimensional morphology of the single-crystal silicon surface under the above machining condition is shown in Figure 16(b), and the surface roughness Ra of the single-crystal silicon is 0.72 μm. If the substrate on the surface of single-crystal silicon undergoes electrolytic dissolution, grain boundaries can be found on the surface of single-crystal silicon. Figure 17(b) shows the diagram of the red area in Figure 17(a) enlarged by 1000 times, and the grain boundary can be clearly identified. When the second feeding speed of the copper foil electrode is less than 120 μm/s, the discharge pits and recast layer on the surface of the single-crystal silicon can be completely dissolved, and the substrate on the surface of the single-crystal silicon also undergoes electrolytic dissolution.Figure 17
Processing effect when the second feeding speed of the copper foil electrode is 120μm/s: (a) 200 times magnification and (b) 1000 times magnification.
(a)(b)It can be concluded that discharge forming cutting-electrochemical machining can effectively remove the discharge pits and recast layer on the surface of single-crystal silicon, effectively improving the surface quality of the workpiece. As the feeding speed of the copper foil electrode is decreased, the surface roughness of the workpiece decreases. Thus, the feed speed of the copper foil electrode should be optimized to lower speeds.


## 5. Conclusion

In this paper, a discharge forming cutting-electrochemical machining method for single-crystal silicon is proposed. This method can effectively eliminate the discharge pits and recast layer that form on the surface of single-crystal silicon during discharge cutting; thus, the surface quality of the workpiece is improved. The following conclusions can be drawn from this paper:(1)
The interpole voltage and current signal analysis during the machining process show that the discharge process for the discharge forming cutting-electrochemical machining of single-crystal silicon contains two kinds of discharge signals. The voltage and current signals during the discharge forming cutting stage are identical to those of WEDM. At the second feeding stage of the copper foil electrode, the voltage and current signals are similar to those of electrochemical machining. Therefore, the material removal mechanism during processing includes electric spark melting, vaporization, and electrolytic dissolution corrosion. The material removal mechanism can flatten the surface morphology of single-crystal silicon and greatly reduce its surface roughness.(2)
Since the working fluid medium is deionized water, the elemental composition on the surface of the workpiece after discharge forming cutting-electrochemical machining of single-crystal silicon hardly changes, and no other new chemical elements are introduced on its surface.(3)
During the processing, the copper foil electrode is required to be fed twice. Anodic dissolution occurs during the second feeding, which is shown in Figure9. Anodic dissolution removes the recast layer, which was caused by discharge cutting, on the surface of the single-crystal silicon. Therefore, the cutting width is 28 μm larger than that caused by discharge cutting.(4)
When the second feeding speed of the copper foil electrode is 140μm/s, the discharge pits and recast layer on the surface of single-crystal silicon can be fully dissolved. The surface roughness Ra of the single-crystal silicon is 0.86 μm. When the second feeding speed of the copper foil electrode is reduced to 120 μm/s, a small amount of the surface substrate of the single-crystal silicon is also dissolved. Grain boundaries can be found on the surface of the single-crystal silicon. Furthermore, the surface of the single-crystal silicon is very flat, and its surface roughness Ra is 0.72 μm. Therefore, with the decrease in the second feeding speed of the copper foil electrode, the discharge pits and recast layer on the surface of single-crystal silicon fully dissolve, and the surface quality improves.



---

*Source: 6024662-2021-08-05.xml*