1 Introduction

Lasers have widely been employed in the micromachining of various materials due to their high processing resolution and speed. The material removal mechanism of the process is generally subjected to the melting and vaporization of material irradiated by a high intensity laser beam. However, thermal damage, such as cracks and microstructural changes of work material, is a major concern of the process, and it has to be minimized particularly in the fabrication of micro-components. This is because the damage can apply to the entire micro-part or structures fabricated by laser, so that its functionality and quality are poor and unable to meet the required characteristics. Although a proper selection of laser processing condition can offer a high material removal rate with minimum thermal damage, the damage still exists in and around the laser-irradiated area at a significant level. To minimize the damage, the workpiece has to be cooled down during the process in order to reduce the excessive heat induced by laser.

Underwater laser micromachining process has been developed and applied for reducing thermal damage and promoting material removal rate in the ablation of difficult-to-machine materials [1]. Two major roles of water involved in the process are (1) the cooling of workpiece and (2) the increase in ablation rate through the mechanical impingement of microjets towards the work surface [2,3,4]. The cooling effect can be enhanced by reducing water temperature [5] and/or increasing water flow velocity over the workpiece surface during the ablation [6]. Using low water temperature can significantly reduce the thermal damage and heat-affected zone (HAZ) occurred along the cut in the underwater laser micromachining of silicon [5]. However, when the water temperature is lower than a certain degree, the cooling effect turns to dominate the ablation performance. At too low temperature, the laser-irradiated material is hardly vaporized from the work surface, and this in turn decreases the material removal rate. The microjets formed during the laser ablation in water can cause the mechanical shock pressure in the order of 1.7 ± 0.3 GPa [7], and this can assist the removal of molten and cut debris from the laser-ablated area. In addition to the microjet formation, the use of ultrasound can energize water during the laser ablation. This technique is able to enhance the material removal performance and diminish the deposition of cut debris as well as cavitation bubbles on the work surface through the ultrasonic cleaning mechanism [8,9,10,11].

A crucial limitation of underwater laser ablation is the presence of gas bubbles generated during the process. The bubbles are induced by the vaporization of water and work material, and the bubble size is subject to laser energy and thermal properties of work material [2, 3]. These bubbles significantly cause optical disturbances to the incident laser beam by means of reflection and refraction [12], thus deteriorating the laser beam quality for the ablation. By flowing water across the workpiece surface in the underwater laser ablation, the bubbles adhering to the work surface can be flushed away [6, 13, 14]. However, new bubbles are repeatedly formed after each laser pulse and some can coalesce to be a larger bubble [3]. This makes it difficult to blow all of them at all times throughout the laser micromachining process in water. Although using a higher water flow rate is possible to blow the bubbles away from the workpiece surface, the significant heat loss due to the forced convection importantly reduces the material removal rate, hence requiring a higher laser power to compromise this tradeoff. Instead of blowing off the bubbles, they can be atomized by ultrasound in the ultrasonic-assisted underwater laser ablation [10]. However, the amount of tiny bubbles suspending in water turns to increase, and this can adversely scatter the incident laser beam in water to some extent.

Another technique to minimize the bubble size and to diminish the laser beam disturbance in water is to increase the static pressure of water around the bubbles [15]. The higher the pressure, the smaller the bubble is formed [16]. Li et al. [17] noted that the bubble size in water can be reduced by 50.3% when the positive pressure of 3.5 bar is applied. The reduction of bubble size reduces the chance of bubbles to interfere with the incident laser beam in water, thus minimizing the optical disturbances. By applying high water pressure in the underwater laser ablation, the gas bubbles caused by the vaporization could be smaller and/or of less amount than the typical underwater technique. According to the best of our knowledge and literature search, there has been no study investigating the effect of water pressure on the ablation performance in the underwater laser micromachining process. This paper therefore aims at this investigation to better the understanding of laser-water-work material interactions, roles of water pressure, and removal mechanism in the underwater laser micromachining process.

2 Materials and methods

Single-crystalline silicon was selected as a work sample in this study since it has widely been used in micro-components and semiconductor devices. A nanosecond pulse laser (IPG YLP-1–100-30–30, Germany) emitting a wavelength of 1064 nm was applied for grooving a silicon wafer. The laser machine used in this work provided the maximum average power of 30 W, pulse duration of 100 ns, and pulse repetition rate of 30 kHz. A collimated laser beam with Gaussian distribution was focused by a 254-mm f-theta lens. The focused laser beam was scanned on the workpiece surface with a single straight line at a traverse speed of 1 mm/s for producing a groove. The whole sample was clamped inside a water chamber as shown in Fig. 1. Water was pumped into the chamber and directionally flowed across the workpiece surface at a constant rate of 4 l/min. The top side of chamber had a 10-mm-thick fused silica window, allowing the incident laser beam to reach the work surface for ablation. The distance between the back side of window and workpiece surface was 1.0 mm, where the heat convection and flushing of cut debris took place during the ablation. The focused beam diameter after passing through the glass window and water layer was 68.81 μm.

Fig. 1
figure 1

Schematic of experimental setup for the underwater laser micromachining process

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Four different levels of water pressure, i.e., 10, 20, 30, and 40 bar were tested in this study to understand its influence on the groove width, groove depth, width of heat-affected zone (HAZ), and cut surface morphology. The width of HAZ was taken from the edge of groove to the boundary of recast or spatter deposition zone. In addition to the water pressure, three levels of average laser power were examined in this work as listed in Table 1. The selection of these processing conditions was based on a number of preliminary tests and our previous studies [5, 6]. The experiment was designed with regard to the full-factorial scheme, and the number of experimental runs was 12 with three replications. Each cut sample was observed by a scanning electron microscope (Hitachi SU3900 Series, Japan). The groove width, groove depth, and size of HAZ were measured by a confocal laser microscope (Olympus LEXT OLS5000, Japan).

Table 1 Parameters considered in the experiment
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3 Results and discussion

The surface morphology of grooves produced by the underwater laser micromachining process is depicted in Fig. 2. The groove surface was substantially cleaner and smoother with almost no recast deposition when the ablation was performed under a higher water pressure. It is anticipated that the high static pressure of water limits the expansion of gas bubbles during the ablation in water. The bubbles are the results of the vaporization of work material and the optical breakdown of water subjected to a high laser intensity. The formation of these bubbles is not evitable in the laser materials processing in liquids. As for the observation in this study, the bubble size tended to be small under the high water pressure environment. This thereby reduces the chance of the laser beam to be interfered by the bubbles previously created by preceding laser pulses as shown in Fig. 3. Thus, the refraction of laser beam at the bubble surface unlikely takes place, and this allows the beam to maintain its focusing quality and position for a precise ablation throughout its processing in water. In addition, the small bubbles adhering to the workpiece surface occupy small footprint on the surface where water can flush away the recast and cool down the workpiece temperature more efficient than the large bubble formation. This could be a reason for a clean groove surface with negligible recast when ablating silicon under the high water pressure.

Fig. 2
figure 2

Grooves obtained after the laser ablation in water under the pressure of a, c 10 and b, d 40 bar and average laser power of a, b 20 and c, d 30 W

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Fig. 3
figure 3

Optical interaction between laser and gas bubble formed under the different levels of water pressure

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The use of low water pressure resulted in the formation of large gas bubbles at which the laser beam underwent the dynamic refraction and reflection during the ablation in water. The former effect adversely changes the focusing position of laser beam, whereas the latter is conducive to the loss of laser intensity in the ablation. The groove surface obtained at low water pressure was also found to be rougher than the ablation at high water pressure as shown in Fig. 2. This is a result of the dynamic refraction and change of focusing position during the process. The refracted distance (L) of laser beam is subject to the incident angle of laser at the bubble surface (θw) and bubble radius (Rb) as shown in Fig. 4, and L is calculated by using [18]

$$L = R_{b} \sin \theta_{w} \left( {1 - \frac{{n_{w} }}{{\cos \theta_{w} \sqrt {n_{b}^{2} - n_{w}^{2} \sin^{2} \theta_{w} } + n_{w} \sin^{2} \theta_{w} }}} \right)$$
(1)

where nw and nb are refractive index of water and refractive index of gas in the bubble, respectively. By assuming the hemisphere shape of gas bubble, the vertical distance of laser beam path in the bubble (h) and the incident angle (θw) are calculated by using:

$$h = \sqrt {R_{b}^{2} - x^{2} }$$
(2)
$$\theta_{w} = \tan^{ - 1} \left( { - \frac{x}{h}} \right)$$
(3)

where x is a distance between the consecutive laser pulses, and it is given as

$$x_{n} = \left\{ \begin{gathered} \frac{v}{f} - L_{n - 1} {\text{; laser moving to }} + x{\text{ and }}\left| \frac{v}{f} \right| > \left| {L_{n - 1} } \right| \hfill \\ - \frac{v}{f} - L_{n - 1} {\text{; laser moving to }} - x{\text{ and }}\left| \frac{v}{f} \right| > \left| {L_{n - 1} } \right| \hfill \\ \end{gathered} \right.$$
(4)

n is laser pulse number. The angle of refraction in bubble (θb) can be calculated with regard to Snell’s law, which is

$$\theta_{b} = \sin^{ - 1} \left( {\frac{{n_{w} \sin \theta_{w} }}{{n_{b} }}} \right)$$
(5)
Fig. 4
figure 4

Laser beam refraction caused by gas bubble in water: a 1st bubble formation; b laser beam refraction induced by the 1st bubble; c 2nd bubble formation

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According to Eq. (1), the greater the bubble size, the more tendency of the laser beam is divergent from the designated grooving path and in turn produces a rough or wavy groove surface as a result. Such poor groove quality is normally found in the underwater laser ablation. However, it is apparent from Fig. 2 that the groove quality can significantly be improved by performing the underwater laser ablation under the high-pressure environment.

According to our previous study [3], the bubble size can range from submillimeter to a few millimeters depending on the applied laser energy. The use of high laser power generally increases the bubble size, which thereby increases the angle of refraction (θb) and refracted distance (L). However, the size of bubbles induced by the laser ablation in water is dynamically changed with time, and a dense plasma plume is also apparent inside the bubble [19]. These could exaggerate the optical disturbances and add variances to the dimensions of groove and its surface quality. Besides the single bubble produced by each laser shot as depicted in Fig. 3, the collapsing of these bubbles can introduce many small bubbles in water [3], and these can also reflect, refract, and absorb the incident laser beam in a random manner. Since the collapsing is very dynamic, the size, shape, and location of bubbles are difficult to be predicted by the analytical approach. However, the bubble size and lifetime are anticipated to be decreased under the high-pressure environment [17], and this is affirmed by our findings through the analyses of optical refraction and groove surface quality. The interference of bubbles to the laser beam in water can therefore be kept at minimum, and a better cut surface quality is achievable compared to the typical underwater laser micromachining process.

In addition to the quality of groove surface, the width and depth of groove as well as the width of heat-affected zone (HAZ) were quantified in this study. Analysis of variance (ANOVA) was applied to statistically determine the significance of the tested parameters towards the groove dimensions and size of HAZ, and the ANOVA results for these responses are listed in Table 2. The results indicated that the change of average laser power significantly changed the sizes of groove and HAZ (p-value < 0.05), whereas only the groove depth and width of HAZ were affected by the change of water pressure. This implies that using high water pressure can facilitate the increased groove depth and reduced HAZ without significantly enlarging the groove width. A high aspect ratio of groove was thereby achievable when ablating silicon in the pressurized water.

Table 2 Analysis of variance results for groove width, groove depth, and width of heat-affected zone
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The average value and standard deviation of groove width, groove depth, and HAZ width obtained under the different processing conditions are plotted in Fig. 5a–c. The aspect ratio of groove was calculated by dividing the width into the depth, and it is plotted in Fig. 5d. The results showed that a large and deep groove was produced by using high laser power. The width of HAZ also increased with the average laser power applied in the ablation. This is associated with the high heat energy that melts and vaporizes the laser-irradiated material in a large area. A large groove with large HAZ is thereby obtained as a result. When a higher water pressure was employed, the size of gas bubbles and laser scattering in water was reduced. This allows the laser beam to travel in water and reach the workpiece surface with less scattering by the bubbles. The contribution of water pressure towards the material removal rate was discernable when the applied pressure was equal to or greater than 20 bar at which the groove depth substantially increased. Based on the compression of gas bubbles under the high static pressure and the analysis of optical refraction described earlier, the water pressure of 20 bar could be a threshold where the bubble size is shrunk and small enough to not interfere with or impede the incident laser beam in water. This thus allows the laser beam to maintain its focusing position and quality for deepening the groove. However, the threshold of 20 bar is only valid for the laser traverse speed of 1 mm/s and laser pulse repetition rate of 30 kHz tested in this study. Since the incident angle of laser at the bubble surface (θw) is a function of v and f as noted in Eqs. (3 and 4), the threshold could be lower than 20 bar when using higher laser traverse speed or lower pulse repetition rate at which the distance between the consecutive laser pulses (xn) is larger than the bubble radius (Rb). When the water pressure was greater than the threshold of 20 bar, there was no significant improvement for the groove depth as shown in Fig. 5b. This is plausibly because the process already reaches the status of no bubble disturbance, so that the laser can perform its maximum capability for the ablation in water.

Fig. 5
figure 5

Measures of groove and heat-affected zone caused by using the different combinations of laser power and water pressure: a groove width; b groove depth; c width of heat-affected zone; d aspect ratio of groove

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As per the results shown in Fig. 5d, the highest aspect ratio of groove is 1.7 when using the average laser power of 30 W and water pressure of 40 bar. Under this condition, the width of HAZ was only 26 μm which was smaller than that of 10-bar condition by 36%. Some samples of groove profile caused by using the different processing conditions are depicted in Fig. 6, noting that the groove in silicon is substantially deeper when applying higher water pressure in the underwater laser micromachining process.

Fig. 6
figure 6

Groove profiles obtained after using the different processing conditions: a 20-W laser power with 10-bar water pressure; b 20-W laser power with 40-bar water pressure; c 30-W laser power with 10-bar water pressure; d 30-W laser power with 40-bar water pressure

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Energy-dispersive X-ray spectroscopy (EDS) was also used in this study to detect and compare chemical composition on the groove surface produced by using the 30-W laser ablation at 10- and 40-bar water pressures. The EDS spectrums are shown in Fig. 7, noting that the peak of oxygen is low when ablating silicon under the high water pressure. A greater amount of oxygen detected on the laser-ablated surface could imply a more amount of oxidized recast on the surface. According to these results, the groove surface quality and removal rate of silicon were significantly improved with negligible damage when the laser micromachining process was performed in the pressurized water.

Fig. 7
figure 7

EDS spectrum of grooves produced by using the average laser power of 30 W and water pressure of (1) 10 bar and (2) 40 bar

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4 Conclusions

This study investigated the influence of water pressure and average laser power in the underwater laser micromachining of single-crystalline silicon. The width and depth of grooves as well as the size of heat-affected zone were measured and discussed together with the analysis of optical refraction in water. The findings and significance of this work can be drawn as follows:

  1. 1.

    A clean and deep groove with the maximum aspect ratio of 1.7 was made by performing the laser ablation in pressurized water. Using the water pressure of 20 bar was able to compress the gas bubbles whose size was small enough to not interfere with the incident laser beam in water. This can avoid the dynamic refraction and reflection of laser beam at the bubble surface, so that the laser beam can maintain its focusing position and quality for a precise ablation.

  2. 2.

    When the water pressure was greater than a certain threshold, the groove depth as well as material removal rate was not substantially improved unless applying a higher laser power. According to the analysis of optical refraction at the bubble surface, the threshold could further be reduced by using high laser traverse speed and/or low laser pulse repetition rate.

  3. 3.

    Recast depositing on and nearby the groove was found to be minimum when the ablation was performed in the pressurized water. EDS spectrums revealed low oxygen peak on the groove surface produced by the aid of 40-bar water pressure. This could be implied that the oxidized recast was less when the high water pressure was applied.

  4. 4.

    The findings of this work have brought another new micromachining technique and its potential to improve the cut quality and material removal rate by applying high water pressure to the underwater laser micromachining process. The proposed technique could be a promising method for scribing silicon and possibly other materials at high removal rate with high cut quality and less damage.