Laser dicing of silicon and electronics substrates

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Laser dicing of silicon and electronics substrates

H. Y. Z h e n g, X. C. W a n g and Z. K. W a n g, Singapore Institute of Manufacturing Technology (SIMTech), Singapore

Abstract: The chapter is intended to provide an overview and the recent research and development in laser dicing of silicon and laser cutting of printed circuit boards (PCBs). As consumer electronic products demand smaller, faster, more capacity and higher performance, thinner silicon wafers below 100 microns and low k dielectric materials are increasingly being used, which creates challenges for the diamond sawing process due to high tool breakage and delamination of the material. This creates an opportunity for high speed laser machining of silicon. Similarly, laser profile cutting of thin PCBs offers many advantages over mechanical routing. Key words: laser cutting, laser dicing, silicon wafer, printed circuit boards, plasma measurement, in situ temperature distribution.

5.1

Introduction

In the semiconductor industry, conventional electronic device structures are inadequate to satisfy the steadily growing demands for higher performances. The development of electronic devices is moving toward higher speed, more integrated functions and compact volume. Low and ultra-low package height in the case of chips cards, miniaturized size with increased density of integrated circuits, the requirement for enhanced electrical performance of power semiconductors and high frequency devices are driving the development of thin wafer technology. Thin silicon wafer offers a variety of new possibilities in micro-electronic, solar and micromechanical industries, e.g. for three-dimensional integration of stacked dies, thin micro-electro mechanical packages or thin single crystalline solar cells. Furthermore, the mechanical flexibility of thin wafers is ideal for bendable systems, such as smart cards, chip-in-paper and contactless labels. By reducing the thickness of silicon substrate, the device is moved closer to the metal heat sink so that it conducts away from the active area more effectively, which is critical for high-frequency operation. Today, silicon wafer thickness is getting thinner, down to 50 microns or less.1,2 Therefore, micromachining of silicon wafer has been one of the crucial issues in miniaturized device manufacture. 88 © Woodhead Publishing Limited, 2010

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Traditionally, a metallized or resin-bonded diamond saw blade is used to singulate the wafer with the blade rotating at a linear speed generally below 100 mm/s.3 Sawing the wafers with a diamond-edged blade has been a wellestablished process used in the cutting of thick silicon, which guarantees good and reliable cuts for wafers of more than 300 mm thickness.4 However, damage to the parts during mechanical dicing has emerged as a significant issue as wafers become thinner. Saw dicing faces serious challenges, including chipping, die breaking and delamination of the bonded layers due to the application of mechanical force. Furthermore, because of the contact nature of blade sawing, mechanical damage cannot be reduced without great sacrifice in the dicing speed. In the case of thin silicon wafers, the cutting speed in the blade dicing of thin silicon wafers should be below 10 mm/s to avoid the partial breaking of the silicon wafer.5 Therefore, productivity is compromised as the cutting speed significantly reduces with the wafer thickness. As the thickness reduces, wafer handling becomes extremely difficult. Even minute force will cause wafer breakage or tear; the worst scenario in semiconductor manufacturing. To avoid the disastrous damage, contact-free dicing tools are preferred. Also, processes that allow for round corners on individual dies are desired in order to enhance the mechanical strength of dies. Moreover, separation processes that allow adjacent circuits to be closer together allow a greater amount of devices to be fabricated on a single silicon wafer and thereby reduce the cost per device. It is necessary, however, to have a sufficient amount of separation between adjacent circuits for blade sawing. Along with the reduced wafer thickness of silicon, innovation devices mandate the introduction of new materials and structures so as to achieve the desired high performance, including low-k interlevel dielectric, polyimide and FR4 and BT/epoxy-based substrates. In addition, FR4 and BT/epoxybased substrates are extensively used in electric printed circuit boards (PCB) and will be discussed later. These new materials/structures in return restrain manufacturers from introducing these new developments into their product lines. As a matter of fact, dicing is a cutting operation to separate the thin wafer into chips. These separated chips are further tested, packaged and so on as finished products, which are widely used in electric appliances. In the past couple of years, several new wafer-dicing techniques have been developed with the attempt to meet the stringent requirements and overcome the challenges imposed by the technical progress in electronic devices, such as scribe-and-break,6 dicingbefore-grinding7 and dicing by thinning with dry-etched trenches.8 However, dicing methods using laser are one of the remarkably efficient methods. As a non-contact tool, laser technology offers a flexibly controllable micromachining process which is more competitive than diamond saw. Moreover, the laser cutting speed is increased with decreasing wafer thickness. Laser processing is

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suitable for handling thin wafers, reducing contamination, tool wear and the risk of wafer breakage. The laser’s tight focus results in reduced kerf width below 30 mm, which in turns increases the chip yield. While a typical mechanical dicing street width is 125 mm or more.9 The use of laser beam cutting reduces front- and back-side chipping and cracking. Furthermore, contour can be cut into a wafer, providing flexibility in die arrangement on a silicon wafer. The dicing at curvilinear profiling can be processed. Dies of various sizes can be placed on the same wafer. Moreover, laser dicing can be easily integrated with other processes in the production environment. Nonetheless, laser dicing is not a flawless technique. In order to produce good cut quality, many issues need to be well understood. This includes the selection of appropriate laser type, processing parameters, analysis of metallurgical effects of the cutting process, minimization of micro-cracks, heat affected zone, surface roughness, redeposit/debris around the cutting area, etc. Generally, most of the drawbacks are associated with the thermal effects of laser ablation: the heat affected zone reduces the die strength and decolours non-silicon layers on top of the silicon wafer; recast molten material contaminates the active area. In the following sections, the main discussion focus will be on laser dicing of silicon wafer. Current diversified laser dicing silicon techniques will be reviewed, including the remaining technical issues and developing trends. The cutting of thin rigid and flexible PCB substrates is critical in the manufacturing of electronic products. Lasers are preferred as an efficient cutting tool for thin PCB substrates of less than 300 mm, since mechanical cutting methods encounter many problems such as delamination, deformation, as well as frequent changes in the complex tooling and fixtures to meet the change in board design and thinner PCBs. Similar to laser cutting of silicon wafers, laser-induced heat affected zone, charring, redeposition and residue are challenging issues for laser cutting of PCB substrates.10 Another difficulty stems from the fact that the fibre reinforced PCB substrate is inhomogeneous, as the glass fibre has very different thermal-physical properties from the epoxy resin. This leads to different ablation threshold energy density for glass fibre and epoxy resin. In the following section, we first review current developments on laser cutting of PCB substrates. Then, we discuss how charring/carbonization occur during laser cutting of PCB and how to minimize and even eliminate charring. Lastly, a systematic investigation and analysis of 355 nm UV DPSS laser cutting FR4 and BT/epoxy-based PCB substrates are reported.

5.2

Laser machining of silicon: an overview of the cutting process

For more than three decades the tool ‘laser’ has been used for cutting various materials. The usual material removal process in laser dicing of silicon © Woodhead Publishing Limited, 2010

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wafer is a laser beam ablation process under pulse operation conditions. The material removal from the laser irradiation zone can occur by both thermal and non-thermal mechanisms, depending on laser wavelength and pulse duration. So far, a wide range of lasers from 10.6 mm CO2 laser, q-switched 1.064 mm Nd:YAG laser and its harmonics, to ultrashort pulse near-infrared (IR) lasers have attempted to cut silicon wafers. Laser beams focus on the surface of the target object in a very short time and release energy simultaneously. The grooving or cutting can be achieved by the traversing laser beam or the motion of working platform to produce the desired shape. As the removal depth per laser scanning pass is usually less than 10 mm, for example around 1 mm per pulse at a laser fluence of around 100 J/cm2 of a 355 nm Nd:YVO4 laser,11 multiple scans over the cutting line are usually necessary to cut through a wafer substrate by pulsed laser ablation evaporation. The scan pass can be significantly reduced if 1.064 mm Nd:YAG or 10.6 mm CO2 lasers are used due to the higher laser power. A full picture of recent research status is discussed below on specific features of silicon wafer dicing/cutting using the various lasers.

5.3

Conventional laser dicing of silicon wafer

5.3.1 Cutting with CO2 lasers For many years far- and near-IR lasers have been used for machining applications. Their high power output produces high machining speeds but they also produce very high thermal damage in semiconductor materials. The heat affected zones (HAZ) can cause shifts in device characteristics and micro-fractures that severely weaken the single crystal silicon. For the subsequent integration of microdevices, the additional processing must be applied to completely remove the deeply damaged regions and eliminate the mass of ejected material adjacent to the machined surface. In principle, the absorption coefficient of the CO2 laser energy by silicon is practically zero, it is not suitable to use CO2 laser for silicon wafer cutting. However, the demonstration done in SIMTech of Singapore several years ago shows great possibility, although the cut edge did not meet industrial requirements. The best parameter settings achieved with CO2 laser (Coherent Diamond CO2 laser) for the selected wafers are summarized in Table 5.1. As can be seen, the kerf widths obtained for all four wafers are generally below 50 mm. Because of the lateral damage resulting from spatter deposition and/ or lateral heating, the total cut width may increase significantly up to 70 mm. Only the bare and SiN coated wafers contain total cut widths of 50 mm and below. Figure 5.1 shows the cutting morphologies under the different laser operating conditions. The coating material significantly influences the kerf morphology, a cleaner cutting is achieved for non-coated silicon wafer. Here

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Table 5.1 Best parameter settings used for the CO2 laser cutting of selected wafers Parameters

280 mm thick wafer

87–100 mm thick wafer

Assist gas type Assist gas pressure (near nozzle) Cutting speed

He > 14 bar 33 mm/s

He 6–8 bar 52 mm/s

Kerf width obtained 40–50 mm

Bare wafer: 30–39 mm SiO2 coated: 40 mm SiN coated: 23 mm

Total cut width including 70 mm damage obtained

Bare wafer: 30–39 mm SiO2 coated: 87 mm SiN coated: 53 mm

(a)

(b)

(c)

5.1 Kerf morphology of silicon wafer cut using CO2 laser. (a) 85 mm thick uncoated Si wafer; cutting speed: 52 mm/s; typical kerf width: 30–39 mm; minimal HAZ, (b) 85 mm SiN thick coated Si wafer; cutting speed: 52 mm/s; typical kerf width: 23 mm; total width: 53 mm, (c) 280 mm thick IC wafer; cutting speed: 33 mm/s; typical kerf width: 40–50 mm; total width including HAZ: 70 mm.

the absorptivity of the coating layer did not make a contribution to improve the wafer cutting quality. The thermal melting effect often appears which may lead to a smooth kerf. This is attributed to the long wavelength of the CO 2 laser that causes thermal heating. By this thermal heating, higher absorption of the laser energy by the silicon becomes possible because of its temperature dependent-absorption characteristic.12,13 As in Figure 5.1(a), a better cutting quality can be achieved with an industry acceptable cutting speed under

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optimized laser conditions and with a helium assist gas. But the use of CO2 laser, especially CW source, mostly results in the thermal interaction that leads to cracking and serious melting rather than wafer cutting.14 Effective cutting is rarely achieved and less reported in literature although CO2 laser is a low cost laser. CO2 lasers have ever been investigated to etch silicon in gaseous atmospheres such as XeF2, NF3 and SF6 and in aqueous solutions such as HF, NaOH and KOH, etc. Unfortunately these etching processes could not be applied to cut silicon wafers efficiently.15 Recently, a possible approach to silicon cutting has been proposed using CO2 laser with glass attached to the silicon wafer back-side, by which to change the silicon absorption behavior in 10.6 mm CO2 laser.16 It is supposed that the change of absorption behaviour in the silicon interior may be due to glass absorbing CO2 laser at the bottom to heat silicon. The laser scanning pass number influences the thermal interaction, consequently determining the change of silicon absorption. Multiple passes lead to more heating than single pass, producing a higher absorption. Commercially available air-cooled CO2 laser (VL-200, Universal Laser system Inc., USA) was operated at 15–30 W with a focused beam spot of 76 mm, and the scanning speed was 2.3–11.4 mm/s. A plate of Pyrex glass was placed below a silicon chip with a thickness of 525 mm16,17 the experimental results show that the cutting depth increases with the pass number; however, the roughness is larger at greater passes machining as well. The redeposits from resolidification and accumulation of the silicon vapour around the kerf associated with thermal heating are the remaining challenges.

5.3.2 Cutting with Nd:YAG lasers High quality cutting and drilling of silicon can be achieved with diode pumped Q-switched Nd:YAG lasers because they are capable of high peak power and excellent beam quality to achieve efficient material removal. Note that dicing of silicon wafer by the material removal process using laser ablation requires high laser fluence to vaporize the material from the wafer substrate and high laser absorption to localize the volume where the laser energy is deposited. Therefore, absorption coefficient a of silicon is considered to be an important factor during laser cutting operations. As seen in Fig. 5.2, the light beam absorptivity of silicon is a function of the wavelength. The penetration depth of the laser beam into the material is usually quantified by a decay length, which is the inverse of the absorption coefficient a. It can be derived from Fig. 5.2 that the optical penetration depth of the Nd:YAG laser radiation is 60 mm at 1064 nm, 0.5 mm at 532 nm or 1.5 mm at 533 nm and 10 nm at 355 nm, respectively.18,19 In the case of a small penetration depth there is a high concentration of deposited laser power per pulse so that essentially the entire heated layer can be removed while the amount of melt © Woodhead Publishing Limited, 2010

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Absorption coefficient [1/cm]

Silcon, single crystal 1 000 000

1 000 000

100 000

100 000

10 000

10 000

200 300 400 500 600 700 800 900 1000 1100 Wavelength [nm]

5.2 Absorption coefficient of silicon as a function of the wavelength.18,19

present can be negligibly small. In the case of a larger penetration depth and correspondingly lower concentration of deposited laser power per pulse, a larger amount of melt is present but the rate of removal may be low. The shorter penetration depth at the shorter wavelength means more localized absorption which concentrates the laser energy in a smaller volume of material and therefore leads to a smoother surface. Optical absorption depth decreases dramatically with decreasing wavelength, thus the surface absorption of an ultraviolet (UV) laser creates a clean cut with much reduced thermal damage. The process is more direct ablation and less thermal melting. Furthermore, a reduction of the plasma temperature and a reduced plasma heat influence on the kerf walls can be realized because of the low plasma absorption to a small laser wavelength. Therefore, the cut quality using infrared lasers such as 1.064 mm Nd:YAG is typically marred by redeposition of molten silicon along the wafer surface and along the walls of the cut.20 Essentially improved accuracy of silicon wafer cutting is expected from using harmonics of Nd:YAG lasers with their shorter wavelengths such as tripled harmonics of UV laser at 355 nm.21 Actually, the accuracy of silicon wafer cutting with pulse laser is a result of a convolution of multi-interactive parameters. It depends on not only laser wavelength, but also pulse width, pulse repetition rate, laser power/pulse energy, beam spot size, and intensity distribution on the beam spot, as well as on the beam guiding technique. The pulse repetition rate may have direct influence on the cutting speed. This could be the reason that excimer UV lasers are not often used to cut silicon wafers because of the low frequency of one to a few tens of hertz. Furthermore, it is also believed that a smaller

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focusing spot and higher energy density are preferable for high-speed cutting. Plenty of research work under widely varied cutting conditions including the influence the gas atmosphere on cutting speed and achievable quality has been carried out in the last few years. The narrowest kerf width with smallest HAZ and free of deposited material are generally considered as the evaluation criteria of laser cutting silicon wafer. Figure 5.3 shows a typical cutting result using 1.064 mm laser beam in multimode operated at an average power of 75 W with a repetition rate of 2 kHz. There was significant melting during the cutting process because of the thermal heating nature of IR laser wavelength. Furthermore, the high gas flow rate did not play an effective role to remove the re-solidified particles and to reduce the HAZ by convective cooling. As a result, a rough kerf is obtained with surrounding debris. Figure 5.4 shows the results obtained using the 355 nm laser cutting in

100 µm

50 µm

100 µm

50 µm

(a)

(b)

100 µm (c)

(d)

5.3 400 mm thick silicon wafer cutting with a single pass of 1.064 mm laser beam scan. Cutting speed at 8.33 mm/s; N2 assist gas at 9 bar from a nozzle diameter of 1 mm. (a) straight linear cutting; (b) curvilinear arc cutting; (c) rectangular cutting; (d) view of cut edge from the top.

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(a)

(b)

(c)

5.4 355 nm laser beam dicing of 100 mm thick silicon wafer (both sides SiO2-coated) at an 80 mm/s single scan. Kerf width: 37 mm; (a) top view as cut; (b) after cleaning with DI water using cotton bud; (c) sidewall of the cutting kerf.

SIMTech of Singapore. The laser output beam was first expanded to eight times then focused down to the wafer surface by a 50 mm focusing lens. Consequently, a smaller beam spot was obtained, leading to a reduced kerf width. The average laser average power was 8 W, pulse duration at 50 ns, and a pulse repetition rate of 30 kHz. As can be seen, the cut kerf width is below 40 mm, which is significantly narrower than that achieved in Fig. 5.3. Furthermore, the ejected molten droplets adhered to the kerf edge are markedly reduced, though the surface of the kerf sidewall is not smooth (Fig. 5.4(c)). As the absorption efficiency of silicon is higher for the shorter wavelength, a 4th harmonic Nd:YAG laser was used to cut silicon. The quadruple harmonics of Nd:YAG laser produces a wavelength of 266 nm that has an absorption coefficient of approximately 0.212 nm–1, which is equal to an optical penetration depth of 4.7 nm.22 and is half that of the 355 nm wavelength. The lower absorption depth at 266 nm means a stronger surface absorption, less thermal effect, and less ablation depth. Therefore, a cleaner cut is expected. In fact, smaller particle size distribution has been produced by the shorter UV wavelengths.23 Experiments comparing the two wavelengths of 355 nm and 266 nm have been performed by M. Li of Spectra-Physics, Inc. No significant difference was observed between the grooves scribed on silicon substrate using the two wavelengths under the same laser conditions. Laser power was

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set at 1.6 W with a pulse frequency of 50 kHz, pulse duration of 11–12 ns, and a scanning speed of 150 mm/s for both wavelengths. Considering the laser fluence, this would be understandable. With a shorter wavelength and a smaller focal spot using the same optical setup, the 266 nm laser generates higher fluence and irradiance on the material surface and thus initiates more multi-photon absorption. This might improve material removal efficiency and thus produce comparable groove dimensions to those obtained with the 355 nm laser. Furthermore, a sharper groove bottom compared to the 355 nm laser may also indicate a stronger laser-material interaction, as shown in Fig. 5.5. Therefore, a short wavelength laser is often employed for efficient and clean cutting. Laser repetition rate, beam scan rate and pulse width also play significant roles in determining the throughput and quality of the resultant cut feature.3,22 A slow repetition rate will create ragged edges, poor structural integrity and low throughput. A fast repetition rate and surface scan rate will produce clean-cut features. A frequency tripled diode-pumped solid-state Nd:Vanadate laser (355 nm wavelength, 25 ns pulse width) capable of delivering average power levels exceeding 15 W was used to cut silicon wafers up to ~200 mm thick at speeds higher than 20 mm/s.21 It showed that at a given wafer thickness, increasing the pulse energy appeared to be less effective than increasing pulse frequency to increase the effective cutting speed. As for the pulse width, although longer laser pulse is more effective in material removal than shorter laser pulse, shorter pulse has the better cutting quality in terms of recast build-up and bottom-edge roughness under the same pulse repetition rate, average power and scanning speed.18,22 Shorter pulse durations are necessary to reduce the heat affected depth due to heat flow from the ablation area into the bulk material. The duration and intensity of plasma

20 um (a)

20 um (b)

5.5 Cross-sectioned micrographs showing the grooves scribed using q-switched diode-pumped Nd:YVO4 lasers at 50 kHz, 150 mm/s. (a) 355 nm laser and (b) 266 nm laser.22

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heating is also reduced. In order to achieve a higher cutting rate, a 355 nm laser beam has been further shaped to be elliptical by inserting a cylinder lens into the optical path before the scan lens. The results showed that the cutting rate was doubled for 100 mm thick wafer through-cutting.24 The gas atmosphere influences the heat transport and material cooling, and therefore affects the cutting speed and achievable cutting quality. A comparison was made by laser cutting of a 210 mm thick silicon wafer in helium, argon, air and in a vacuum of 8 Pa, respectively, at Fraunhofer Institute for Material and Beam Technology IWS of Germany.25 A q-switched and frequency-tripled 355 nm Nd:YAG laser (Gator UV from Lambda Physik AG) was operated at 10 kHz with a pulse duration of 15 ns and with an average power of 3 W. The cutting performance shows that the number of passes needed is 65 at a scan speed of 50 mm/s in helium, which is three times faster than cutting in air. The pass number required for through-cutting increases in order of helium, vacuum, argon, air, respectively. Owing to the thermal characteristics of gases used, the kerf width is significantly affected by ablation plasma and thermal melting of the materials in different gas atmosphere. The observed kerf width of entrance is 25.8 mm in helium, then in order of vacuum, air, argon increasing from 34 to 39.2 mm. The best result is obtained when cutting in vacuum. It is shown that the main advantage of inert gases is the prevention of oxidation of silicon during laser cutting.

5.4

Other laser dicing techniques

5.4.1 Multiple laser beam dicing of thin silicon wafer Although laser dicing may overcome most of the issues related to saw blade dicing, such as a narrow kerf width, cracks and fractures, etc., the dicing throughput remains below current industrial requirements. For example, to separate a 80 mm thick silicon wafer, the effective dicing speed is less than 5 mm/s, far below the throughput required by the industry. The laser source was a q-switched diode-pumped solid-state UV laser, which delivered a tripled 355 nm wavelength at 15 ns pulse width and with an average laser power of 11 watt.26 In order to improve the cutting speed, two laser beams with different wavelengths have been applied in silicon wafer cutting as disclosed in US Patent No. 6562698. A CO2 laser is first applied to form the scribe lines in the wafer surface, followed by through-cutting of the wafer substrate along the scribe lines with an Nd:YAG laser or its harmonics.27 Generally speaking, the average laser power has to be increased in order to dice through the wafer at a high speed. However, the amount of energy per laser pulse irradiating the target material must not exceed a certain material specific value in order to prevent damage and excessive plasma shielding. The damage and plasma shielding cause not only widening of the dicing kerf

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but also a substantial decrease of the material removal efficiency. On the other hand, simply increasing the pulse repetition frequency is not a good solution as the dicing quality is compromised. Recently, an innovative technique of thin silicon wafer cutting with a dual-focused laser beam has been developed to improve the cutting speed in Ryerson University of Canada.28 A collimated laser beam is incident on a modified Newton’s ring set-up, which consists of a polarization plate beam splitter and a convex mirror placed in near contact. The output of this set-up is two co-linear beams. One of them has altered divergence with respect to the other, as shown in Fig. 5.6. The generated two foci have nearly equivalent spot size and both fall on the optical axis of the focusing optics, but at different focal lengths. The dual-focus optics allows for variations of the laser power of each focal point and the distance between the two focal points by rotating the half waveplate placed before the polarizing plate beam splitter. The extended focal length of dual focus makes it more capable of penetrating through a thicker substrate than conventional single focus. Using this dual-focus method, the cutting speed is increased 2-to 4-fold. Furthermore, two-focus always provides a wider exit dimension due to the enhanced laser power at the bottom. The notorious feature of taper formed by laser cutting, i.e. a larger entrance with a smaller exit, is remarkably improved. Meanwhile, the kerf width can be kept smaller. The achieved kerf width is 25 mm for a 250 mm thick silicon wafer, and only 10 mm for a 80 mm thick wafer.26 Figure 5.7 shows an example of a 250 mm thick wafer where multiple singulated dies are stacked above each other. The effective cutting speed was about 20 mm/s, which was improved by a factor of 4. The feature of higher cutting speed with a smooth kerf is a promising development in laser cutting of silicon wafer. Multiple beam technology developed by Advanced Laser Separation International of Netherlands also provides an efficient combination of high dicing speed with good dicing quality. A single mode diode pumped solidHalf waveplate Plate beam splitter Quarter waveplate

Convex mirror

Front focus

Focusing lens

5.6 Optical configuration for generating dual-focus.28

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5.7 Sidewall of the die cut from silicon wafer by dual laser focus. Laser wavelength: 532 nm, laser power: 20 W and repetition rate: 85 kHz. The effective cutting speed was about 20 mm/s.26

state laser beam is split into a number of beams using a diffractive optical element.29 All parallel beams fall under a slightly different angle on one focus lens, which images the beams on a series of adjacent focus spots. A number of focused beams (for instance, up to 14) arranged in a line interact with the target material simultaneously. In this way, the energy fluence per position can be limited, resulting in a high quality cut. The stationary foci from multiple laser beams are aligned along the cutting direction. The vertical displacement of the foci thus forms consecutive laser pulses, which can be synchronized with the wafer motion in a way that each spot exactly hits the material at a position where earlier pulses have already removed material. By increasing the number of beams, the material removal rate per pass can be increased. The effective dicing speed at 150 mm/s was achieved for 100 mm thick wafer.29

5.4.2 Femtosecond laser pulse dicing of thin silicon wafer It has been shown that the quality of the resultant cut in silicon wafer is dependent on the pulse width. Dahm et al.’s research discloses that the primary case for poor cut quality resulting from redeposition and thermal melting when employing near infrared lasers, is the use of laser pulse width

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exceeding 1 ns.30 It is proposed that the use of near infrared lasers with short pulse width of less than 1 ns to solve the deep absorption depth. Such short pulse widths may produce surface plasmas which can act as highly absorbing layers. Near infrared lasers, such as Nd:YAG lasers, are used for high-speed applications because of their ability to produce greater power than UV lasers. Note that the optical penetration depth of silicon is a relevant quantity which depends on the comparison with the thermal penetration depth (thermal diffusion length, LD) related to the thermal diffusivity (diffusion coefficient, D) of silicon during laser irradiation. LD = (Dt)1/2, where t is the pulse width of the laser. Here, D = kT/(rCP), where kT, r and CP are heat conductivity, density and heat capacity, respectively. Thus, since the heat diffusion length LD is proportional to the square root of the pulse width t, ultra-short pulse will result in a greatly reduced heat diffusion length as compared with the aforementioned nanosecond (ns) pulses during laser irradiation. It is shown when the pulse width becomes less than picoseconds (ps), heat diffusion can be almost entirely neglected. Note that the energy duration of femtosecond (fs) laser pulses is short enough to limit the thermal diffusion in very narrow regions in the materials to prevent the generation of molten layer.31 Therefore, ablation induced by fs laser irradiation is very attractive for photomachining of Si substrates such as cutting and drilling. Much effort has been put into the application of ultra-short pulse laser for silicon wafer cutting.32–36 Figure 5.8 shows cutting result using a commercial fs laser workstation with a 800 mW regeneratively amplified Ti:sapphire laser (Clark MXR) at 775 nm wavelength with a pulse duration of 150 fs and a repetition rate of

50 µm

5.8 Top view of the through-cut slots in a 400 mm thick silicon wafer using a fs laser.

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1 kHz. A 400 mm thick silicon wafer was cut with pulse energy of 78 mJ at a scanning speed of 100 mm/s. Seven scanning passes were overlapped for a through-cut. It shows that thermal effect from the laser pulse heating is significantly reduced and consequently a clean cut with fewer ejected droplets around the kerf edges is achieved. On the other hand, low cutting speed leads to a significant low throughput. Even though two scan passes can cut through a 200 mm thick silicon wafer with a first scan at 250 mm/s followed by a second scan at 1 mm/s, as demonstrated in SIMTech of Singapore, this is still too slow for practical use. Tönshoff et al. demonstrated that when fs laser cutting with line foci instead of spot foci, cutting speeds can be significantly improved and kerf widths are reduced at the same time.36 The use of line foci provides an approach to increase the cutting speed for practical application of fs laser cutting of silicon wafer. As the effective speed is in the millimetre range per second, it is not yet high enough for practical wafer cutting. Although the ultra-short pulse width considerably reduces the thermal heating compared to ns laser, continuous plasma from subsequent ablation pulses may play a role in the thermal melting of the cutting kerf. In addition, the polarization of the laser beam relative to the translation direction has a significant effect on the cross-sectional profile of deep grooves, with significant branching in the case of the polarization parallel to the translation direction. The walls of cutting kerf exhibit a great deal of unevenness.37 Recently, double-pulse irradiation by fs laser has been proposed to cut silicon wafer. 38 The first pulse is irradiated to the silicon wafer in order to induce the changes of the physical properties in the substrate positively, and the second pulse is used to drill a hole. Adequate pulse separation time and energy ratio of two pulses could reduce thermal melting resulting from the continuous plasma of subsequent ablation pulses. In this way, the laser processing area is also able to limit strictly the double-pulsed fs laser irradiation. Femtosecond lasers are known to produce high quality cutting but the throughput is very low and considerable technical obstacles must be overcome before they can be economically viable. With the development of new fs laser systems with higher pulse repetition rates, which lead to higher average powers even at lower pulse energies; cutting of thin silicon can be expected to become even faster and more precise in the near future.

5.4.3 Silicon wafer laser dicing under an active gas or with a protective coating When a laser beam is applied to the front surface of a silicon wafer, heat energy is concentrated on the exposed area to produce debris. Redeposition of molten silicon adheres to the front surface of silicon wafer, thereby greatly

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reducing the quality of the device. A chemical etching process often needs to be used to clean the debris to avoid failures in subsequent packaging operations. So far, various methods have been developed to achieve debrisfree-cutting minimization of debris during wafer dicing. Inert gas such as argon is commonly used to assist laser cutting; however, it could not effectively eliminate the debris during laser cutting of silicon wafer. It has been shown that when the reagent gas such as sulfur hexafluoride (SF6) is introduced locally to the cutting region, it reacts with the high temperature vapour ejected from the cutting kerf and produces gaseous compounds, thereby reducing the redeposit on the silicon wafer.26,39 However, it is found that the assist gas reduces the overall dicing speed. The reagent gas does not allow laser cutting at a rate to enable sufficiently high throughput for manufacturing. Note that in many cutting applications, a top-hat intensity distribution is preferable. A quasi top-hat distribution can be obtained simply by blocking of the outer region of the focused beam at a position not far from its focus. The ‘melting’ side effect and ultimately the burrs and debris formation, could be significantly reduced. This has been demonstrated on a multiplepass cutting of a 280 mm thick silicon wafer using a lamp-pumped pulsed 532 nm green laser.40 So far, soluble protective coating on the surface of the silicon wafer to trap the debris has become an accepted method in the wafer laser dicing industry. The soluble coating may be a film, a tape, a polymer or a liquid resin.40–42 Among them, the most popular and most commonly used method is water soluble coating, which can be applied by spraying, brushing, flow coat, or screen printing. After laser cutting, the coating is then washed away using standard pressurized water cleaning techniques. Industrial coatings that have been used in recent years are water soluble resist such as PVA (polyvinyl alcohol), PVB (Polyvinyl butyral), PEG (polyethylene glycol), PEO (polyethylene oxide), TPF8000™ etc., as well as hairspray and other soap-based solutions. The thickness of this protective coating film is about 0.05 to 10 mm. For example, the TPF8000 supplied by Tokyo Ohka Kogyo Co., Ltd can be favourably used. According to the experiments performed by Disco Corporation, Tokyo, when 30 ml of an aqueous solution 10% of PVA is dripped and spun on the surface of a silicon wafer, a 0.2 mm thick protective film can be formed in an area of diameter of 200 mm.42 Advanced Laser Separation International of Netherlands demonstrated that an efficiently separating layer between the debris and the product surface could be minimized down to only 100 nm thick.29 Figure 5.9 shows the result of 355 nm laser cutting of a protectively coated silicon wafer. The coating product is dispensed and spin-coated in an integrated chamber prior to dicing. For a 300 mm thin silicon wafer, the volume of coating used will be 50 ± 2 ml, which corresponds to a coating

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(a)

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5.9 Top view of thin patterned wafer after coating and dicing, before wash (a) and after wash (b).43

thickness of 2.6 ± 0.3 mm. Coating uniformity is better than 5%.43 The wafer was coated first then, after drying, laser cutting was performed (Fig. 5.9(a)). A clean debris-free wafer cut was achieved after washing (Fig. 5.9(b)). A smooth kerf internal wall can be obtained if successive chemical wet etching is further applied to the cut wafer. This obviously shows that employing a non-ionic, water-soluble surfactant as a coating medium can provide a cost-effective and reliable solution. A cleaning step washes away the coating and any debris generated during laser cutting, ensuring a clean surface with excellent quality and devices free of contamination.

5.4.4 Water-assisted laser dicing silicon wafer It has been shown that no debris is deposited and the etch depth increases when the metal is placed under water or covered with a water film during laser ablation. It is considered that there are several reasons that affect the ablation mechanism with the presence of water.44–46 The plasma generated by the high power laser that is confined in the water induces a recoil pressure that gives a mechanical impulse to the workpiece. The plasma size and lifetime is smaller in water due to this confinement so that the plasma shielding effect is reduced. Water acts as a cooling medium to dissipate the excessive laser heat into the water and carries away the debris and eliminates the redeposit of molten material. Tuan A. Mai40 used a small syringe-type nozzle to generate a hair-thin laminar flowing water film that covers the entire laser irradiated area for silicon wafer cutting. The nozzle was fixed to the laser head and a high flow rate greater than 1 m/s was applied. Figure 5.10 shows a comparison of the cutting results between air and water. Through-cutting of a 300 mm thick silicon wafer with the same number of laser scanning passes; a significant high level of kerf cleanliness is observed in flowing water compared to in air. Furthermore, a less tapered kerf can be achieved because the water

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5.10 Through-cutting of a 300 mm silicon wafer with a diode-pumped 355 nm UV laser, 50 passes: conventional dry process (a) and with a flowing water film (b).40

carries the laser-generated debris away, and prevents any debris in the kerf that could potentially block the incoming laser light.47 It is further shown that this technique can be applied for different laser wavelengths from 355 to 1064 nm. Choo et al.46 investigated machining under water and showed that the molten silicon rapidly solidifies to form a dendritic structure. The ablated surface is smooth as in the case of cutting in air, while it is somewhat rougher in the case of under water cutting. It is considered that the nature of the material removal in the two cases may be different. Besides the effect of the water carrying away the debris, the effect of rapid cooling is also important in laser cutting under flowing water. Since water can be an efficient coolant, laser-induced breaking of singlecrystal silicon wafers with the backside in contact with water has been demonstrated and is expected to be an effective method for silicon wafer dicing by the mechanism of chilling crack propagation. A Nd:YAG was used at powers up to 80 W and feed rates of 0.4–20 mm/s: The use of water was reported to result in nearly half the crack deviation, damage depth, and branching crack length compared to that without water.48 However, an initial crack is necessary in the laser breaking method, and crack propagation does not always follow the desirable line. In recent years, the water jet guided laser technique has been developed in order to reduce the heat damaged zone near the cut. In fact, many other advantages, such as absence of beam divergence and efficient melt expulsion, are observed. In 1993, scientists at the Institute of Applied Optics of the Swiss Federal Institute of Technology of Lausanne in Switzerland succeeded, for the first time, in creating a laser guided in a stable water jet, called Laser-Microjet® by its inventors.49–51 Now the Laser-Microjet developed by Synova in Switzerland, has become increasingly popular in the semiconductor industry over the past years. The laser is coupled in a fine stable water jet and conducted to the workpiece by means of total internal reflection like through an optical fibre. The difficulty of laser-water jet lies

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in the geometry of the water chamber where the water jet and the laser are coupled. A sketch of the water-jet guided laser cutting unit is shown in Fig. 5.11. The nozzles are made out of sapphire or diamond in order to generate a long stable water jet. The tiny jet nozzle ranges from 25 mm to 100 mm in diameter. The pure de-ionized and filtered water ranges from 5 to 50 MPa in pressure. The laser beam is focused through a quartz window into the nozzle, and is thereafter reflected in the water jet at the air-water interface. The lasers used are either flash lamp pumped pulsed YAG lasers with pulse durations of less than 120 ms, or multimode q-switched lasers (pulse duration 150 ns) operating at 1064 nm, 532 nm, and 355 nm. Since the laser light transmits in the water guide, it can reach a deeper area of an ablated hole without energy loss. The deep groove can be formed faster than the normal laser ablation. In addition, the pressurized water sweeps away the debris and reduces the heat affected zone.50 The achieved kerf width can be less than 50 mm.52,53 Figure 5.12 shows an example obtained with the water guided laser cutting of a 75 mm thick silicon wafer. A frequency-doubled Nd:YAG laser at 532 nm with an average power of 60 W was used, and the water jet nozzle diameter

Laser

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Diamond nozzle Laser water jet Sample to be cut

5.11 Schematic view of the laser beam coupling with a water microjet.49

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200 µm

5.12 Laser water-jet dicing of a 75 mm thick silicon wafer at single pass of 50 mm/s.53

was 45 mm. A 49 mm wide cut has been achieved at a speed of 50 mm/s. The wafer is undamaged by the dicing process and free of contamination.

5.4.5 Laser stealth dicing silicon wafer by internal deteriorated layer formation In laser dicing of silicon wafer by the ablation mechanism (sometimes called evaporation cutting) to remove the material, debris pollution and thermal effect on the device domain essentially happen. In order to solve these problems, a non-ablation dicing process based on the laser internal modification of silicon, named as the ‘Stealth Dicing’ (SD) method, has been proposed by Hamamatsu Photonics K. K., Japan.54–56 A nanosecond pulse infrared laser beam which is a transmissible wavelength for the silicon wafers is applied to the front surface of silicon wafer upon focusing on the interior of the wafer. The focused laser beam power is absorbed at this sharply focused region inside the silicon wafer; however, the beam does not damage the surface. The peak power density is higher than the process threshold at the focal point and it is lower than the ablation threshold at the wafer surface. The absorbed laser power at the focused region in the interior of the silicon wafer brings a continuously deteriorated layer by laser beam scanning, consisting of recrystallization, polycrystalline, dislocations, and microcracks, etc.57 Since the laser beam power is not absorbed on the wafer surface, modified region and cracks are generated in the intermediate plane inside the wafer. Subsequently, the wafers can be easily separated with a small external tensile stress applied perpendicularly to the modified layer. Therefore, the

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SD process consists of two steps: laser irradiation to form a modified layer in the interior of the silicon wafer, followed by wafer separation by applying an external force. It is considered that there is no influence to the active area in the devices, and there is no grinding sludge and debris pollution in the SD process. Thus, SD is a completely dry process, without water contamination or oxidation, and high quality and high reliability can be expected. It is already being used in the manufacturing process of the device. Figure 5.13 shows a schematic illustration of the formation of the modified internal layer by SD method.58 The laser beam is designed to be focused into the interior of the wafer. The modified layers, also called SD layers by the inventors, are formed along the scanned line in the wafer. The SD layers act as crack initiation sites and guides for the separation. Tape expansion is a typical method used to apply tensile stress in the wafer separation process. When a cylindrical stage pushes up the dicing tape, the wafer on the dicing tape is separated into individual chips. The SD layers are in the interior of the wafer and are not visible from the top before the separation in most cases, but is visible in some cases depending on the depth of the modified layer below the wafer surface. The crack extends to the top and the bottom of the wafer during tape expansion. Usually, one modified layer is enough for a 50 mm thick silicon wafer, or even 100 mm.56,57 For a thicker silicon wafer, sometimes multi-modified layers are necessary to achieve a good wafer separation. For example, 200 mm thick can be divided by inducing three modified layers from near bottom to near top surface inside the wafer under different scanning speeds up to 600 mm/s, and laser pulse repetition rate at 100 kHz and laser output at 20 mJ/pulse.55 The laser used in SD can be a 150 ns pulse or less Nd:YAG laser with a wavelength of 1064 nm, tightly focused to a spot diameter of 1.0 mm with an extremely high peak power density,59 for example 1.3 or 3.2 × 1010 W/ cm2.60–62 Figure 5.14 shows the result of a SD processed 50 mm thick wafer under a pulse energy of 4.4 mJ and scanning speed of 180 mm/s. The modified layers appear as belt-shaped area. The pulsed laser was irradiated from the upside of the photograph and was scanned to the right (Fig. 5.14(b)). Voids Laser beam Scanning direction Wafer Tape

5.13 Illustration of the laser beam design used to form modified layer in the SD method.58

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SD layer

50 µm

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5.14 Scanning electron microcopy images of a divided 50 mm thick silicon wafer. (a) a chip corner after laser processing and external force separation;56 (b) cross-sectional view of internal modified layer generated in side of silicon wafer.59

can be found under the modified layer, and cracks are seen up and down. The SD layer behaves as a guide for separation when tensile stress is applied to the wafer. Ohmura et al. explained the formation mechanism of a SD layer by a thermal simulation which applied the temperature dependence of optical absorption.59,63 When a sudden laser absorption takes place at the focal point, a void is generated. Then a thermal shock wave propagates toward the upper surface, and the high dislocation density layer is formed. When the thermal shock wave pushes up the high dislocation density layer which occurred in the previous pulse irradiation, a crack, whose initiation is a dislocation, is produced.63 In order to achieve this modified layer, the laser process condition is chosen such that it is higher than the internal process threshold and lower than the surface process threshold, that is 590 MW/cm2 and 17 MW/cm2 respectively. For example, for a 400 mm thick silicon wafer, the modified layer width is about 3.5 mm. The external force for separation may have a value between 1.4 and 2.4 N.57 Recent investigation also shows the potential that a CO2 laser thermal-induced crack propagation can be used to take the role of external mechanical force to divide the wafer in SD method. 64 In the SD method, the quality of the cut edges and the stability of the separation are issues that need to be taken care of. The 1.064 mm wavelength corresponds to an intermediate range from a wavelength having absorptivity to a wavelength having transmissivity for silicon wafer. Therefore, when a 1.064 mm pulse laser beam is applied along the dividing lines of silicon wafer to form a deteriorated layer, multi-photon absorption is not completely carried out inside the wafer and therefore, a satisfactory deteriorated layer is not always formed, thereby making it difficult to divide the silicon wafer along the dividing lines smoothly. It has been proposed that the preferred

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wavelength of the laser beam may be set to 1.3 mm to 1.6 mm.65 For example, a laser diode excited q-switch Nd:YVO4 laser can be operated at wavelength of 1.064 mm, 1.340 mm or 1.550 mm but at the same repetition frequency of 40 kHz with a pulse width of 20 ns and average power of 4 W. If the laser beam is focused by a condenser lens down to a spot size of wavelength value, one deteriorated layer has a thickness of about 50 mm, 12 layers require to be formed so that they are exposed to both surface of a 600 mm silicon wafer at a processing speed of 40 mm/s.65 Furthermore, the width of the deteriorated layer formed by the above laser processing is around 1 mm or larger, this width is not always satisfactory as the width required for division. Therefore, when an external force is applied to the wafer to divide it along the designed lines, the chips may be damaged.61 It is further proposed that a beam splitter is applied to form a twin beam from a single laser source, thus a plurality of pulse laser beams are obtained, with a predetermined space there-between, in the width direction of a dividing line to form a plurality of parallel deteriorated layers along the dividing line. A predetermined space inside the wafer can be set to 1 to 5 mm so as to achieve damaged free separation.61

5.4.6 Die strength of silicon wafer by laser dicing A number of investigations have been performed to compare the die strength between laser dicing and mechanical blade sawing. A comparative analysis for the same die thickness of 50 mm shows that the load at break for laserdiced die is significantly reduced.66 Similar trends are observed in the extension at break during there-point bent test. Laser cutting experiments were carried out with a frequency tripled (355 nm) q-switched Nd:YAG laser. It is further found that the die strength between the cutting of the frontside and the backside is different. Prior to laser cutting, a polymer solution was coated onto the wafer to protect it from contamination during the laser dicing process. Parameters were carefully tuned to achieve the best cutting quality in laser dicing. The test results show that the frontside die fracture strength is less than the bottom side die fracture strength, around 200 MPa, which is only 30% of the value obtained by blade sawing.66 Die strength can be determined by a number of factors such as defect density, location and size in the wafer front and bottom surface and the kerf edge. Generally, the surface fracture strength is improved by stress relief processes such as chemical mechanical polishing, spin etching and dry polishing, etc., to achieve a good surface roughness and relieve the stress in the dies during wafer manufacturing. Therefore, for a given surface treatment, kerf sidewall defects are the most important factor in determining die strength. The differences in the sidewall quality should be taken into account when comparing dies cut with different singulation methods. Laser

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dicing is supposed to achieve a high die strength. However, the die strength obtained is lower than blade sawing, which indicates the generation of severe thermal damaged layers in the laser cut kerf than expected. The re-solidified debris, as well as the oxidation and dislocation produced in the laser cutting process are the main contributions that reduce the die strength.66 Furthermore, because of the tapered kerf shape induced in laser ablation, the thermal damage from laser heating is more serious at the upper part of the kerf than that at the bottom of the kerf. As a result, the frontside die strength is lower than bottom die strength. Die fracture strength using laser processing depends on the thermal damage and debris. These defects are directly related to the applied pulse energy level used for the ablative dicing. Higher energy will cause higher degree of thermal damage, therefore, lower die strength. With a reduced pulse energy, the extent of heat-affected zone will reduce. It has been shown that to dice a given wafer at the same speed, the energy required for dual focus is much less than that for single focus.26 Dual focus offers a better die strength as it needs less pulse energy for the same productivity. The average die strength of dual focus dicing technique is nearly twice the die strength of conventional single-focus dicing for a 80 mm thick wafer.26 When laser dicing with water-assistance such as water jet guided laser technique, a significant reduction of thermal heating is achieved due to the pressurized water cooling. Consequently, the die strength is greater than non-water cooling laser dicing.67 It has been further shown that when a stress-release method is applied prior to dicing, the die fracture strength is up to 1.5 times higher with laser water jet than with an abrasive saw.67 Note that the recent laser SD technique offers the narrowest kerf width, approaching zero, and significantly less thermal effect, resulting in a high kerf quality with significantly increased die fracture strength. A three-point bent test shows that the achieved die strength by SD is 70% higher than that achieved by blade sawing for a 50 mm thick wafer.68 Whether laser dicing or blade sawing, cracks resulting from the wafer singulation will propagate into the active area of the die, rendering it useless. Maximizing the silicon wafer fracture strength is important as it improves the ability of the wafers to survive the mechanical and thermal stresses. The die is subjected to stress during wafer handling and further processing as in packaging and during service operation. When the wafer is thick enough, there is no mechanical problem in the packaging processes, because the die is robust enough to sustain the loading force. However, the die is easy to crack as the wafer thickness gets thinner. The die strength, especially for thin wafer, is affected by the level of defects induced by wafer thinning and cutting processes. The level of flaws and defects on the die sidewall will determine the die strength. When subjected to the stress loading, there exists a high potential that the cracks would propagate and result in fracture of

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the die. Therefore, increasing the die strength is essential to ensure the chip stability during the assembly process. In the last few years, several techniques have been developed to remove the damaged portion inside the cut kerf in order to increase the diced die strength. One is a chemical wet-etching method, where the silicon dies are etched by an etchant such as KOH solution until the damaged layer is removed, followed by cleaning in deionised water and drying in an inert gas such as N2. Dry-etching process can be plasma etching using radio frequency power to drive chemical reaction, such as chemical reaction of silicon with etching gas CF4 and O2, etc.66 The other commonly used dry etching is performed by direct reaction of silicon with spontaneous etchants. This process is characterized with high selectivity to masking layers, without the need for plasma processing equipment, highly controllable via temperature and partial pressure of the reactants. Gaseous spontaneous etchant such as xenon difluoride are commonly used. Halide or hydrogen compound such as F2, Cl2, HCl or HBr is also possible.69 The test performance after the etching process has shown a significant increase in die strength which is two times higher than before etching, and increases with the etching time. 43

5.5

Laser-silicon interaction

5.5.1 Optimal ablation fluence for silicon Lasers ranging from near infrared (1.064 mm) to ultraviolet (266 nm) have been studied in silicon wafer cutting, the interaction between the laser beam and the material being ablated depends not only on the thermo-optical properties of the silicon matrix for a given laser (wavelength) irradiation but also on the appropriate choice of the other laser parameters used (pulse energy, pulse duration, energy density and its profile). The literature on laser ablation of silicon materials is quite extensive. Here, the discussion will focus on the general trends in laser ablation of silicon, including under water ablation. The mechanisms responsible for material ejection are strongly dependent on the laser fluence and in particular, the pulse duration. It has been argued that homogeneous nucleation is the principal ablation mechanism at laser fluence just above the ablation threshold, and plasma formation occurs only at fluences above 1 J/cm2 (F > 5Fabl).70 Yoo et al.71 investigated various material removal mechanisms involved in single pulse laser ablation on a single crystal silicon by high irradiance (109 to 1011 W/cm2). They pointed out that an understanding of the material removal mechanisms requires the identification of the dominant energy transport mechanism. According to them, material removal can occur by both thermal and athermal mechanisms. The incident laser radiation on silicon creates a large population of highly

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excited non-equilibrium electrons near the sample surface. This can lead to bond breaking, which is an athermal mechanism. Alternately, the excited electrons transfer energy to phonons during electron–phonon relaxation. The energy is redistributed through lattice vibrations and consequently heat is conducted into the sample. This heat may melt or vaporize the sample. Material removal in the form of large micrometer sized droplets can also result from hydrodynamic instability of the molten liquid layer. Craciun et al.72 on the contrary reported nanosecond 1.064 mm laser-induced explosive boiling of silicon based on heterogeneous boiling. Craciun et al.72 further investigated the surface morphology of single crystal silicon irradiated by 266 and 1.064 mm laser pulses emitted by Nd:YAG laser. They found that the morphology at the bottom of the crater with 266 nm wavelength, which is well absorbed by silicon, remained flat and featureless. The rims of the craters show signs of radial liquid flow but the vaporization is confined to the surface region. Ren et al.73 used multiple laser shot irradiation at 355 nm but attributed the observed sudden increase in etch rate to secondary plasma heating effects. This has also been suggested before for high intensity laser ablation of metals.74 It was shown that following plasma ignition, the electron thermal flux irradiated towards the surface from the hot plasma outweighs the photon flux above few tens of GW/cm2. Similar development could be applicable to silicon at presumably different threshold intensity. The influence of laser-induced plasma on the phase explosion process and the role of thermal diffusion in subsurface heating have been highlighted by Lu75 in a theoretical study. It demonstrates that phase explosion occurs after the completion of the incident laser pulse. Another mechanism that might be responsible for the violent ejection of a thick pool of material at high irradiance conditions has been described by Bulkakova et al.76 and is based on a boiling crisis threshold process. In such cases, significant subsurface heating deep inside the material could drive a transformation from nucleation to film boiling. The superheated vapour nucleates deep inside the material, coalescing into a large vapour volume that eventually bursts. The exact mechanism for the high ablation rate observed in high power nanosecond laser ablation of silicon remains unclear, although various models have been developed to study this effect. However, a laser-induced explosive boiling mechanism with secondary plasma heating contribution can be identified to be the two distinct irradiation regimes based on laser intensity.77 The unusually high etch depths are reached above a threshold intensity of about 11.5 GW/cm2 77 or experience a transition influence until 23 GM/cm2 78 for a nanosecond Nd:YAG 355 nm laser. Figure 5.15 is a plot of silicon ablation rate to laser ablation fluence by using a lightwave electronics diode-pumped Nd-YVO4 laser operating at a wavelength of 355 nm and a repetition rate of 10 kHz, where the ablation behaviour is typical of many materials.24,79 There is no ablation until a threshold fluence is reached. The ablation rate

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Etch rate (µm/pulse)

1.2 1.0 0.8 0.6 0.4 0.2 0

0

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50 60 70 80 Fluence (J/cm2)

90 100 110 120

Ablation efficiency (mm3/J *1000)

5.15 Etch rate vs. fluence for nanosecond ablation of silicon at 355 nm.24 10 9 8 7 6 5 4 3 2 1 0

0

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40 50 60 Fluence (J/cm2)

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5.16 Ablation volume vs. fluence for nanosecond ablation of silicon at 355 nm.24

then rises rapidly until the fluence reaches about 6 J/cm2. The curve then flattens at higher fluences. Migliore24 converted these results to material removal efficiency by dividing the etch rate by the fluence. It shows that the most efficient material removal is achieved at a fluence of about 4 J/cm2 (Fig. 5.16). This suggests that whatever the ablation mechanism, the way to get the highest cutting speed from any given laser is to adjust the fluence to optimal material removal level. When a femtosecond laser is employed to ablate a silicon substrate, the laser fluence for the single-pulse melting and ablation thresholds of silicon is ~150 mJ/cm2 and ~300 mJ/cm2 respectively.80 However, there is a great

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variation in the ablation depth for fs laser ablation in the literature. For 150 fs pulses at a wavelength of 267 nm, Zhao et al.81 obtained an ablation depth of ~2 nm for a single pulse at a fluence of 6.4 J/cm2. Kautek and Krüger82 used 300 fs pulses at a wavelength of 612 nm on Si(111). Ablation depths are approximately 22 nm and 45 nm at fluences of approximately 500 mJ/cm2 and 1600 mJ/cm2, respectively. Kim et al.83 used 150 fs pulses of wavelengths of 780–800 nm. Ablation rate is only 0.48 nm per pulse at a fluence of 210 mJ/cm2. This fluence is only slightly above their reported threshold fluence of 200 mJ/cm2.83 Bärsch et al.84 used 150 fs pulses with 780 nm centre wavelength. Ablation rate is up to 5 nm per pulse at fluences below 2.3 J/ cm2, corresponding to pulse energies of 450 mJ. For fluences above 2.3 J/ cm2 corresponding to pulse energy of 1 mJ, they determined an ablation rate of 35 nm per pulse. Yokotani et al.85 concluded that the energy around (0.18–0.20 mJ/pulse) should be the optimal machining condition for dicing very thin Si substrates with fs lasers. Crawford et al.37 revealed that the depth of a single pass groove was consistent with a logarithmic dependence on the incident pulse energy. Two ablation regimes were observed when using 150 fs pulses at a wavelength of 800 nm and at a repetition rate of 1 kHz. The characteristic ablation depths per pulse in the low-fluence ablation regime are 25–43 nm for translation speeds ranging from 100 to 500 mm/s respectively. In the high-fluence ablation regime the analogous ablation rates range from 270 to 590 nm for these same speeds. The differences between various groups’ results are likely due in part to the different ablation atmospheres and the spot sizes employed, as well as the wide range of wavelengths and pulse energies used. A single model could not unify the ablation features. Underwater machining is one method to prevent the debris from redepositing on the finished surface. Kruusing45 and Choo et al.46 recently published an extensive review of both underwater and water-assisted laser processing of materials. Some of the advantages of water-assisted laser processing include light transmission, development of higher plasma pressure due to confinement, water carrying away the debris, more effective cooling, useful chemical reactions, reduced pollution by waste gases and aerosols, higher optical breakdown threshold than in air and smaller focal spot size. Disadvantages of laser ablation under water include light absorption by water, light scattering by the water surface, suspensions, and bubbles, and furthermore, laser power lose due to water cooling. In addition, water photolysis and possible corrosion of materials may happen and water vapour may make electronics hazardous. It has been further shown that the laser ablation rate is dependent on the materials, which can be higher or lower under water than in air. The etching rate of silicon was reported to be two times higher in water than in air at laser fluences of up to 5 J/cm2.86 This is attributed to the ablated material washout by the water. However, the maximum critical laser fluence seems

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to be less in air (1.7 J/cm2) than under water (2.0 J/cm2) for a 248 nm KrF laser ablation because of the water effect.46

5.5.2 Characterization of laser-silicon interaction Investigations of the fundamental mechanisms of laser ablation have been receiving growing attention because of the laser ablation based micromachining in the past decades. With the advances in femto- and picosecond laser technology, it is becoming possible to study the initial stages leading to ablation in real time and without the added problem of dealing with laser-plume interactions. The interaction of laser to plasma complicates the interpretation of nanosecond laser ablation. In the early pioneering work by Downer et al.,87 pump-probe measurements of the ablation process in silicon were carried out. Similar investigations were performed by Von Der Linde and Schüler88 for several materials, including dielectrics. The temporal development of the reflectivity of the surface after strong excitation was investigated. An increase in the reflectivity was observed on the timescale of the laser pulse due to plasma formation at the surface. After sufficient energy is delivered to the material, it typically takes another 50 to 100 ps to melt a 20 nm surface layer of silicon using picosecond laser pulse ablation.89 However, many details concerning the energy transfer into the lattice after short pulse laser excitation remain poorly understood. Recently, in situ direct measurement of the temperature fields induced by the deposited thermal energy during femtosecond laser processing has been carried out using infrared thermography technique in SIMTech of Singapore.90 A Ti:sapphire fs laser (Clark-MXR, Inc., CPA-2001) has a central wavelength of 775 nm. The pulse duration of ~240 fs measured at 1 kHz repetition rate was used to irradiate the top of the silicon specimen. The specimen had dimensions of ~1.0 mm by ~0.7 mm (cross-section) by ~12 mm (length) and was cut from a silicon wafer. The femtosecond laser irradiated the specimen through a focusing lens (f = 50 mm) but at an out-of-focus position, with an irradiated spot size of ~0.3 mm and ~0.6 mm in diameter. Different spot sizes were chosen to control the laser fluences at the appropriate levels. The temperature field along the silicon specimen was captured by an infrared camera (AGEMA Thermovision® 900). The spectral response of the infrared camera was in the long wavelength (LW) band (8–12 mm) with a temperature measurement range from –30 °C to 1500 °C. The accuracy of the infrared camera is ± 1 °C. The maximum frequency to capture the thermal images was ~16 Hz. The typical time sequences of thermal images for silicon during fs laser heating for spot sizes of 0.3 mm (incident average laser power ~210 mW) and of 0.6 mm (~300 mW) are shown in Plate II (between pages 428 and 429). The temperature rise with time and the temperature gradient along the specimen are observed. Note that for the thermal image of silicon at the spot size of © Woodhead Publishing Limited, 2010

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0.6 mm (Plate II(b)), the region where the femtosecond laser irradiated the top of the specimen can be clearly seen because the dimension of the spot size (~0.6 mm) is nearly the same as that of the cross-section of the silicon specimen (~0.7 mm). Furthermore, from the temperature profiles and the boundary condition, the total power for heat flow is calculated and it is found that the power for heat flow is dominated by the power for conduction deposited into the specimens compared to the heat losses to the environment by convection and radiation. It has been shown that a significant amount of the incident laser power (two-thirds or more) was deposited into the bulk silicon substrate instead of being carried away by the ablated materials irrespective of the laser fluence below, between, and above the thresholds of the silicon. The percentage of the laser power deposited into the solids as thermal energy was substantial, ranging from 66.7 to 87.8%. It indicates that the thermal effect could be one of the major intrinsic features during laser ablation even if ultrashort pulse is employed. One powerful technique to investigate the physical mechanism of laser ablation is emission spectroscopy. Fundamental plasma parameters such as the electron temperature (Te) and electron number density (ne) can be obtained from optical emission spectra. Material removal with the fs laser is a very fast and complex process. Plasma temperature and profile are closely related to thermal-related quality issues such as heat affected zone as plasma is a hot source. Plasma can also be related to material redepositions as the ablated materials fall back onto the substrate surface. Plasma analysis will help understand its profile, composition and intensity, which has significant influence on the machined quality issues. Therefore, time-averaged electron temperature and electron density of the silicon sample have been investigated by measuring the optical emission during fs laser silicon processing. The optical emission of the plasma was collected by a UV/VIS spectrometer (Ocean Optics USB2000) through a multimode fibre during a similar fs ablation silicon wafer as in Deng et al.91 The spectra range of the spectrometer is 200–850 nm with a resolution of 1.33 nm. The detector of the spectrometer is a Sony ILX511 linear CCD array, which has 2048 pixels with a pixel size of 14 × 200 mm. The time-integrated spectra of the silicon sample under different laser power are presented in Fig. 5.17. The emission lines detected are Si I 243.9 nm, 252.1 nm, 263.3 nm, 288.5 nm, 298.8 nm and 390.1 nm. In addition, emission lines for Si II are also observed at 385.4 nm and 412.5 nm. Under the assumption of Local Thermodynamic Equilibrium (LTE), the plasma temperature can be estimated using relative intensities of the emission lines from the same atomic or ionic species.92,93 The electron temperature Te can be estimated using the Boltzmann plot under the LTE condition. Under the limited energy spread, such as less than 2 eV for the emission lines of Si (I) in Fig. 5.17, it is difficult to estimate the excitation

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temperature accurately using the Boltzmann plot. A modified approach to combine the Saha equation with the Boltzmann equation can be employed. By adopting both emission lines of atoms and ions, a wider range of the upper level energies is obtained, thus increasing the accuracy of temperature estimation.94,95 For example, based on the emission line at 288.2 nm in Fig. 5.17, the time-integrated electron density of silicon under a laser power of 385 mW is estimated to be 1.8 ¥ 1019cm–3. Under the laser power of 385 mW (corresponding to an intensity of ~1.3 TW / cm2), the estimated temperature is 21616 K. These results may serve as a necessary basis of further research into the development of on-line monitoring techniques for the optimization of the fs laser silicon dicing or other material processing.

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Laser processing of printed circuit board (PCB) substrates

5.6.1 Current laser cutting methods used in cutting PCB substrates It is well known that the primary use of laser system in PCB production is for drilling microvias. Laser drilling of microvias has been studied extensively and has worked very well for years. Laser cutting application is growing in a price sensitive environment. The choice of laser for cutting PCB is dependent on pulse duration, wavelength, and also PCB material properties.

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When PCB substrate is cut by laser, the laser beam energy must be absorbed by the material. If the light absorption is high enough, the local temperature increases to a level where the material “evaporates”. The extent to which the light is reflected or absorbed depends on the properties of the material and the wavelength of the laser beam. The processing speed and the resulting quality depend both on the characteristics of the PCB material being processed and the nature of the laser emission (wavelength, fluence, peak power, pulse width, and pulse rate). It is important to know the absorption characteristics of the material to be cut. Routing, de-panelization and most PCB cutting processes involve the machining of organic materials like polyimide, epoxy, acrylic adhesives, PTFE and LCP. The absorption characteristics of these materials show strong similarities. They are almost transparent in the visible and near infrared range, so that solid-state lasers with a wavelength around 1 mm cannot be used for machining them. The most commonly used laser in cutting PCB substrate is CO2 laser. The CO2 laser is one of the oldest laser types and has a wavelength of 10.6 mm which is strongly absorbed by PCB material. This laser technology is commonly available and provides high laser power at reasonable costs. But long wavelength, lower photon energy radiation turns the cutting process into a thermal one. In such a process, the material is mainly burned away, leaving residues and melted material around the cutting area. With PCBs, reinforcements like glass fibres often have to be taken into account, which is opaque to CO2 radiation. Compared to organic materials, the threshold of glass is high for infrared radiation. Therefore, with reinforced materials, it is the reinforcement that determines the level of infrared laser energy that must be applied for the cutting process. With standard CO2 lasers, this level can only be reached by extending the time of interaction, which in turn increases the extent of the melting of the surrounding organic materials. So the CO 2 laser is largely used to cut through plain PCB material and can reach cutting speeds of up to 500 mm/s for thin boards (0.2 mm thick FR4).96 However, IR lasers remove material by intense local heating leaving carbonization and residue that must be cleaned during post-processing. Furthermore, far-IR radiation is completely reflected by copper. As a result, CO2 lasers are not suitable for cutting flex or flex-rigid circuits. When the PCB material is cut by the UV laser, the incident photons are absorbed in a thin layer of the material up to the optical penetration depth. This is the depth at which the intensity of the absorbed radiation has decreased by a factor of 1/e. At the common wavelength of UV lasers (355 nm) these materials absorb the laser radiation, but not completely. The high photon energy of short wavelength radiation and short pulses with high peak power lead to an almost cold ablation process and so lead to excellent cutting results in homogeneous organic materials.

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In any case, an improved cutting quality can be achieved by reducing the thermal influence on the material, i.e. by keeping the heat-affected zone and charring/carbonization to a minimum. The optical and thermal penetration depth parameters provide a good estimate of how much thermal influence is involved in the cutting process: the smaller the thermal penetration depth compared to the optical penetration depth, the more the energy is confined to a small volume, increasing the thermal gradient and thus the cutting quality. Q-switch frequency-tripled diode-pumped Nd:YOV4 lasers (UVDPSS) emitting a wavelength of 355 nm are the correct choice in the UV region, producing short optical laser pulses in the range of a few tens of nanoseconds in length with a peak power in the range of a few kilowatts.

5.6.2 Charring/carbonization during laser processing of polymer Owing to the composition and the characteristics of the polymer material used in PCB substrates, such as epoxy resin, and the nature of laser polymer material interaction process, and the inhomogeneous properties of FR4, which means that glass fibre and epoxy resin have quite different properties (lead to different ablation threshold energy density), there exist a number of challenging issues such as heat affected zone, charring, redeposition and residue during laser cutting of PCB substrates. Among them, the most critical issue is charring since carbonized charring is conductive which must be avoided, otherwise it will lead to short-circuit in PCB circuits. Srinivasan et al. investigated UV laser radiation induced decomposition of a monomer unit in Kapton polyimide.97 It was found that the UV laser-induced decomposition resulted in 51% of the polymer weight being converted to gaseous products consisting mostly of CO (67%), HCN (15%), C2H2 (12%), and some (<5%) CO2. The deficit can be attributed to the minor products such as methane, ethylene and ammonia as well as water vapour. The remainder is solid carbon. The nature of the carbon was determined to be ‘glassy’ carbon and highly crystalline but is made up of small crystallites. It is also found97 that the important difference in the chemistry of the products under ablative and non-ablative conditions during laser processing of polymer is the remaining mass (~50%) of the original material. It has been shown that under non-ablative conditions of this material, which is 95% carbon, is left behind as glassy carbon. Under ablative conditions, this fraction is ejected as part of the ablated material. The black soot-like material is a major product. It has been taken to be ‘carbon’, which is probably an accurate description of its elemental content. It was suggested that small species containing only a few carbon atoms are directly formed in the ablation process, which subsequently react with each other to form soot.

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O2 assisting gas can minimize or even eliminate carbon residue through reacting with carbon to form volatile CO2. Singleton et al. systematically investigated effects of O2 on the carbonization during 308 nm excimer laser ablation of Kapton.98 It was found that with an increase of oxygen concentration the yields of carbon-related particulates decreased whereas the yield of the volatile CO2 increased. It is clear from Singleton’s work98 that with increasing oxygen content of the atmosphere above the sample, the yield of CO2 increases at the expense of carbon particulates and C2H2. In pure oxygen, nearly all the recovered carbon is in the form of CO2. So, the oxygen assisting gas can be used to effectively scavenge the carbon debris.

5.6.3 355 nm DPSS UV laser cutting of FR4 and BT/ epoxy based PCB substrates The PCB substrates used in the laser cutting process were FR4 and BT/ epoxy-based PCB substrates. The sample thickness ranges from 0.1 to 0.5 mm. A Coherent Avia X 10 W q-switched diode pumped solid state (DPSS) ultraviolet (UV) laser system was used for the experiments. The laser wavelength was 355 nm. The laser pulse frequency ranged from 10 kHz to 100 kHz. The pulse duration of the laser beam was 20 to 35 ns depending on the laser pulse frequency used. The output beam profile was Gaussian shape. The spatial mode is TEM00 (M 2<1.3). The beam divergence is less than 0.3 mrad. The laser beam with a diameter of 3.5 mm (@ 1/e2) was introduced to the PCB substrate using a galvanometric scanner with an f-theta flat field lens achieving a spot size of 25 mm (1/e2). As discussed in Section 5.6.2, in order to achieve good quality cutting in terms of charring and HAZ, an O2 side jet was used in the cutting process. The morphology and cutting quality of the laser cut PCB substrates were analyzed using optical microscope and SEM. Figure 5.18 shows the optical images of a 0.3 mm thick FR4 substrate laser cut at different scanning speeds with different pass numbers. In all cases, the cumulative speed was the same to be at 100 mm/s. Here, the cumulative speed was defined as the scanning speed divided by the pass number. As shown in Fig. 5.18(a), there is significant melting, charring and HAZ along the cutting line. However, with an increase in the scanning speed but maintaining the same cumulative cutting speed of 100 mm/s, the charring and HAZ were reduced gradually as shown in Fig. 5.18 (b)–(e). The kerf width was also reduced from 43 to 26 mm. It is known that high speed multi-pass cutting has the potential to minimize thermal input to the substrate by spatially separating each pulse on the surface.99 It can also improve energy coupling by displacing each pulse to

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avoid impinging on ‘defocusing-plasma’ generated by the previous pulses. As a result, the lateral heat conduction is minimized for the high speed multi-pulse cutting so that the HAZ, charring and kerf width are reduced with increasing scanning speed. In order to understand how the charring occurred during the laser cutting process, we also studied the effects of interval time during scanning on the HAZ and charring. The cutting was done under two conditions, one was at 1000 mm/s for 100 continuous passes, another was at 1000 mm/s for 100 passes with 2 seconds pause at every 10 passes. Both cases had the same cumulative cutting speed of 10 mm/s. As shown in Fig. 5.19, more HAZ and charring are observed for continuous non-stop scanning than intermittent scanning. It is known that the FR4 substrate has a low glass transition temperature of 130–170 °C. When the UV laser beam irradiates the FR4 substrate, the laser photons are absorbed and cause the FR4 surface

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5.19 Effect of interval time between scanning of the line on HAZ and charring. (a) continuous non-stop scanning for 100 passes, and (b) scanning for 100 passes with 2 seconds pause at every 10 passes.

temperature to increase, causing melting and vaporization of the FR4 material. If the scanning was conducted in non-stop continuous mode, the excessive heat cannot be dissipated away effectively due to the low thermal diffusivity of the epoxy resin and the heat effect of subsequent scanning is overlapped with previous scanning. As a result, these accumulated excessive heat effects cause the sample temperature to increase and then cause more melting and burning, HAZ and charring as shown in Fig. 5.19 (a). When the cutting was done in an intermittent way where the cutting was carried out at 1000 mm/s for 100 passes with a 2 s pause between every 10 passes, the excessive heat could be dissipated away during the pause time before next scanning. Consequently, the HAZ and charring was reduced significantly as shown in Fig. 5.19 (b). The results show that there is a certain amount of heat generated during laser cutting of FR4 substrate so that a certain amount of cooling time is needed before subsequent laser passes is scanned during high speed multi-pass laser cutting process. When a big sheet of FR4 substrate is cut, the time for each pass is usually more than 2 s, so the intermittent stop is not needed during practical PCB cutting process. High speed multi-pass cutting was demonstrated to be a viable method to cut FR4 PCB substrate with high quality. Figure 5.20 shows the effects of an O2 assist side jet on charring and HAZ. It can be seen that with O2 assist gas, the HAZ and charring was reduced considerably. Just as stated in Section 5.6.2, when the laser beam irradiates on a PCB substrate, the laser induced decomposition of polymer material resulted in 51% of the polymer weight being converted to gaseous products consisting mostly of CO (67%), HCN (15%), C2H2 (12%), and some (<5%) CO2 and the remaining major solid product was “glassy” carbon].97,98,100 It is also known that small species containing only a few carbon atoms which are directly formed in the ablation process, react with each other to form carbon soot or react with the O2 or O based radicals to form CO2 in competing reactions.98 This suggests that smaller carbon species formed

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during laser ablation are more rapidly oxidized to CO2 with O2 assist gas. So, it is expected that O2 assist gas can effectively scavenge the carbon debris. Also the O2 gas helps to blow debris away from the ablation site. Furthermore, the assisting O2 gas helps to dissipate the heat and cool the substrate during the cutting process to give less HAZ and charring. Figure 5.21 shows the images of laser cut FR4 samples at different repetition rates. In the graph, the corresponding average power and pulse energy were also indicated. As shown in Fig. 5.21, with increasing repetition rate, the HAZ and charring were reduced and almost no HAZ and charring were observed at 80 kHz. As shown in Fig. 5.21, with an increase in the repetition rate, the average power increased up to a maximum of 60 kHz, and then decreased with further increasing the repetition rate whereas the

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corresponding pulse energy decreased gradually. In addition, the laser pulse duration increased with the increase of the repetition rate. As a result, the laser intensity of the laser beam decreased with an increase in the repetition rate. So, the laser beam had a higher laser intensity at lower repetition rate so as to induce more HAZ and charring whereas the laser beam at higher repetition rate had a lower laser intensity so as to produce less HAZ and charring as shown in Fig. 5.21. The laser cutting was also done at different repetition rates by keeping the average power constant. As shown in Fig. 5.22, the HAZ and charring decreased with an increase in the repetition rate. It is believed that laser cutting of FR4 PCB substrate at higher repetition rate can achieve high quality cutting in terms of HAZ and charring as long as the laser beam intensity is high enough to ablate the reinforced glass fibre. Figure 5.23 shows the laser cut 0.3 mm thick FR4 substrate at optimized conditions. The cutting speed can be achieved at 60 mm/s for 0.1 mm thick, 40 mm/s for 0.2 mm thick and 20 mm/s for 0.3 mm thick. It can be seen from Fig. 5.24 that the cutting quality is good, with no debris, almost no charring, and minimum thermal damage. The result demonstrated the suitability of UV laser to cut thin PCB substrates with heat-sensitive embedded components. The optimized laser cutting process was also applied to cut a laminar CuBT/epoxy-Cu PCB substrate. BT/epoxy has enhanced thermal, mechanical and electrical properties over standard epoxy systems such as high glass transition temperature (180 °C), low coefficient of thermal expansion and excellent electrical insulation in high humidity and high temperatures. Figure 5.24 shows the schematic structure of the laminar Cu-BT/epoxy-Cu PCB 320

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substrate. The copper foil is 15 mm thick and the central BT/epoxy is 100 mm thick. Figure 5.25 shows the SEM images of UV laser cut Cu/BT-epoxy/ Cu PCB substrate. It can be seen that the cutting edge is quite clean, with no burr, no distortion, minimum HAZ and the contour is quite straight. Figure 5.26 shows the cross-sectional view of the laser cut BT/epoxy-based PCB substrate. It is apparent that the cutting quality is very good. There is no evidence of delamination and deformation, no epoxy recession, no fibre pulling out and the cutting surface is clean and uniform. Finally, based on the achieved optimum cutting conditions, high quality laser cutting of the real multi-layered PCB substrates was demonstrated. Figure 5.27 shows the results of the laser cut multi-layered FR4 PCB substrates. It can be clearly seen that the cutting quality is very good. Almost no charring, no debris was observed. From Fig. 5.27(c), the cutting surface is very clean

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and consistent, with no delamination, no fibre pulling out, and no epoxy recession. Also, it was found as shown in Fig. 5.27 (d, e) that the cutting edge is sharp and uniform. In summary, 355 nm UV DPSS laser was demonstrated to be a promising tool to cut PCB substrates with high quality. The effects of selected laser processing parameters on the cutting quality of thin PCB substrates in terms of HAZ and charring were investigated. Multi-pass laser cutting with high cutting speed was shown to give good quality cutting of FR4 or BT-epoxy based PCB substrates. It was also found that a certain amount of interval time between scans, assisting O2 gas, and laser pulse repetition rate had significant effects on the cutting quality.

5.7

Conclusions

Not only because of its high degree of flexibility but also the high cutting speed for thin silicon wafer, significantly reduced kerf width, and reduced

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5.26 Cross-sectional image of laser cut BT-epoxy based PCB substrate. (a) overview, and (b) enlarged view.

front- and back-side chipping and cracking, laser cutting has now become a real competitor to present silicon wafer separating methods like grinding with dicing saws in the semiconductor industry. The material removal process in laser cutting silicon wafer is mainly a laser induced-material evaporation process under the pulse operation conditions. The harmonics of Nd:YAG lasers make them the best lasers to achieve good cutting quality. Meanwhile, fs laser cutting has niche applications due to the high precision micro-cutting and markedly reduced thermal effect; however, the speed is too slow when compared with ns laser. So far, many innovative methods to achieve high quality cutting have been developed. Besides the use of a cylinder lens, the cutting speed can be significantly increased by applying a dual-beam or multiple beam technique. Concerning the debris produced during silicon wafer cutting, laser dicing with a protective coating on wafer surface has been an efficient method used in the present laser dicing market. Particularly in recent years, the creative water-jet-guided laser technique is becoming widely known and accepted in the semiconductor market. Currently,

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5.27 Images of laser cut multi-layered PCB substrate (a) entrance, (b) exit, (c) cutting surface, (d) and (e) SEM images of the cut.

the method of laser stealth dicing of silicon wafer has become another representative innovative technique because of the near-zero-kerf width dicing, which has already moved into the silicon wafer dicing market. It is believed that with the development of laser optics and new laser systems, a significantly increased wafer dicing market will be taken by laser techniques. For example, high power fibre laser can be another good candidate and has shown great possibility to achieve an efficient cutting of silicon wafers in SPI Lasers UK Limited and SIMTech of Singapore. The same can be said of thinner electronic PCB substrate cutting by lasers. Mechanical cutting is commonly used for singulating rigid and thick PCBs. When PCB substrates are thinner, it encounters issues of delamination and deformation. Furthermore, mechanical cutting faces frequent changes in the complex tooling and fixtures. Also, the decreasing space between components makes accessibility of mechanical routing difficult. Such issues create opportunities for laser cutting as a viable alternative for thin PCB substrates. Diode pumped solid state UV 355 nm laser has been demonstrated to be a promising tool to cut PCB substrates with high quality.

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