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General Laser MicroMachining Principles

Emre Teoman, Dana Lee Church Version 2 January 2003

TeoSys Engineering LLC, Laser Micromachining Principles

GENERAL LASER MICROMACHINING PRINCIPLES A laser micromachining system is generally comprised of a laser, a protective structure, a beam delivery system, a visible monitoring system, a motion system, and a parts handling system. Micromachining systems come in many different configurations for many different applications. The following graphic ( Figure 1 ) details some of the variety available. Lasers, such as excimer lasers and frequency-tripled or ­ quadrupled Nd:YAG lasers are LMT-5000 Micromachining System used for a variety of precision machining operations and are particularly well-suited for micromachining of polymer materials or the marking of difficult materials such as diamonds. Infra Red lasers such as CO2 and YAG tend to be used for high-speed metal cutting applications. 1. Overview of laser sources Excimer lasers generate a light beam through production of an excited dimer, or two atom gas molecule. The dimers utilized in excimer lasers are composed of rare gas and halogen atoms, e.g., krypton fluoride, xenon fluoride, argon fluoride, xenon chloride. The choice of gas mixture used in the laser determines the dimer that is formed and wavelength of the ultraviolet laser light. Other wavelengths can be derived from solid and gas state lasers. Table 1 shows the output wavelength generated by commercially available micromachining lasers. Figure 2 details the electromagnetic spectrum of interest for most laser micromachining applications. Laser species CO2 YAG (Nd3+) XeF XeCl KrF KrCl ArF Wavelength (nm) 10.6 m 1.064 m 351 nm 308 nm 248 nm 222 nm 193 nm

Table 1. Emission wavelengths for micromachining lasers

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TeoSys Engineering LLC, Laser Micromachining Principles

Laser Spectrum

.1m .2m .3m .4m .5m .6m .7m 1 m 10 m 100 m

Ultra Violet

(.1m - .4m)

Visible

(.4m - .7m)

Infra Red

(.7m - 100m)

Figure 2. Commercially available industrial micromachining lasers Wavelength conversion of many near infrared lasers can be accomplished by passing their light through appropriate nonlinear optical crystals like lithium niobate or beta barium borate. The third and fourth harmonic wavelengths of lasers in the neodymium family ­ Nd:YAG, Nd:YLF, and Nd:YVO4 ­ are in the ultraviolet, near 355 nm and 266 nm. Wavelength conversion is a two-step process in which infrared light is converted to green wavelengths followed by conversion of green light to ultraviolet wavelengths. Since wavelength conversion requires additional optical components and conversion efficiency is typically 10 ­ 20%, ultraviolet light produced by wavelength conversion is significantly more expensive to generate than light at the fundamental wavelength. On a cost per watt basis, excimer light also is substantially more expensive than CO2 or Nd:YAG light. Consequently, industrially viable UV laser cutting applications tend to be those in which in which UV light produces a better quality of cut or allows production of finer detail. 2. Materials properties and cutting principles Most materials absorb light more strongly as the optical wavelength decreases. For example, at the ArF laser wavelength deep in the ultraviolet, oxygen absorbs the laser photons and even air becomes weakly attenuating. Many polymers, crystals and metals that are either highly transmissive or highly reflective at infrared and visible wavelengths ­ for example, borosilicate glass, polyurethane, or silver ­ absorb strongly in the ultraviolet. This allows ultraviolet lasers to be used in cutting applications that are unsuitable for longer wavelength sources. Some materials show dramatic changes in absorption even within the ultraviolet. Figure 2 shows the absorption depth of PMMA at three excimer laser wavelengths. Between the ArF and XeF wavelengths the absorption depth of PMMA changes by a factor of 100. At the longer 351 nm wavelength this material requires high laser power for cutting, and the quality of the cut is poor. At 193 nm the same material cuts cleanly with significantly reduced power requirements.

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TeoSys Engineering LLC, Laser Micromachining Principles

1000

Absorption depth (µm)

100 PMMA 10

1

.1

ArF

KrF

XeF

0.1

0.2

0.3

0.4

Wavelength (µm)

Figure 2. Absorption depth of PMMA at three excimer laser wavelengths. Ultraviolet lasers also benefit from the fact that a high-quality laser beam can be focused with high-quality optics into a spot of dimensions comparable to its wavelength. A suitable ultraviolet source can be used to machine features that are more than an order of magnitude smaller than those available with similar CO2 laser equipment. Since many laser micromachining applications require production of feature dimensions in the micron range, short laser wavelength can be essential to the feasibility of a process. When polymers absorb ultraviolet light of sufficiently short wavelength, molecular bonds are broken and the material is converted to small particles and gaseous fragments. In UV laser cutting of plastics, this "cold cutting" effect minimizes damage to surrounding material. All ultraviolet lasers used for cutting produce pulsed light with individual pulse duration in the tens of nanosecond range. The short pulse duration of UV laser sources also limits the time available for heat to spread from the cutting zone to adjacent material. The net result of short wavelength and short pulse duration is a higher quality cut in most materials. Figure 4 shows the essential processes involved in machining with pulsed UV light and the effect produced on a worksurface by a single laser pulse. When the worksurface strongly absorbs the light, the penetration depth often is less than one micron and there is little opportunity for lateral or axial diffusion of the absorbed energy. The excited volume is very well defined, and material within it is ejected in a vapor plume. The shallow pit produced by the single pulse has a predictable depth and a lateral profile that mimics the optical energy distribution in the focused spot. If the laser light is uniformly distributed in the focal spot, the ablated area will have a "top hat" profile, a Gaussian beam will produce a Gaussian profile, and more complex distributions will tend to produce more complex profiles.

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TeoSys Engineering LLC, Laser Micromachining Principles

LASER BEAM

PLUME

ABLATION DEPTH

ABLATION REGION

(a)

(b)

Figure 4. (a) During the ablation process, a pulse of UV laser light is focused onto a highly-absorbing substrate. Vaporization of material in the illuminated region produces a plume of ejected molecules and particles. (b) After the optical pulse, a thin layer of material has been removed with little damage to unilluminated adjacent areas. The depth of material removed by a single pulse strongly depends on the composition of the worksurface, the wavelength of the laser, and the energy per unit area in the focal spot. Table 2 and Figure 5 show the depth of material removed from a polyimide surface by single pulses of 248 nm krypton fluoride laser light at various energy densities. Vaporization does not occur when the focused energy density, or fluence, is less than approximately 10 millijoules/cm2. As the fluence increases above this threshold level, the ablation depth rapidly increases and then begins to plateau. This type of threshold behavior at low fluence and plateau behavior at high fluence is typical of most materials. Highest cutting efficiency is attained when the fluence is near the onset of the plateau region. Table 2 shows the ablation depth and associated fluence range for polymers, metals, and ceramics.

Material Polymer Metal Ceramic

Ablation depth (microns) 0.3 ­ 1.0 0.1 ­ 0.3 0.1 ­ 0.3

Fluence () 1-2 5 - 10 3 - 10

Table 2 Single pulse ablation depth and associated fluence (energy density) for three types of materials.

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TeoSys Engineering LLC, Laser Micromachining Principles

0.6 0.5

Etch depth (µm)

0.4 0.3 0.2 0.1 0.0 0.0

Kapton

0.5

1.0

1.5

2.0

2.5

3.0

Fluence (J/cm2)

Fig. 5. Ablation (etch) depth as a function of energy density (fluence) for ablation of Kapton (polyimide) by a 248 nm KrF excimer laser.

3 Beam delivery and cutting speed Reasonably accurate estimates of cutting speed, S, and required laser pulse energy, E, and laser average power, P, can be made from the single pulse ablation depth, and the associated fluence. S = Rdw/t E = (Fw2)/4 x 10-8 P = RE Where: S = cutting speed (microns/sec) P = laser average power (watts) E = pulse energy (Joules) R = pulse repetition rate (Hz) d = single pulse ablation depth (microns) t = material thickness (microns) w = laser focal spot diameter (microns) F = fluence (J/cm2)

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TeoSys Engineering LLC, Laser Micromachining Principles

In comparison with CO2 and Nd:YAG lasers, ultraviolet lasers have relatively low power. The largest commercially available excimer lasers produce average powers of a few hundred watts. With only a few exceptions, excimer lasers produce 5 ­ 500 mJ pulses at relatively low repetition rates of (10 ­ 500 Hz). These energy levels are sufficient to ablate thin layers of material from areas of several square millimeters on most worksurfaces. However, the pulse low repetition rate of excimer lasers limits the rate of penetration of the beam into the workpiece. As a result, excimer lasers are seldom used for cutting through materials greater than 1 mm thick. Many excimer laser micromachining systems are used for projection patterning of thin surface layers. Efficient energy utilization usually requires use of special beam delivery optics to either shape the beam or project an image onto the worksurface. With image projection optics like that shown in Fig. 6, complex patterns can be etched into most materials with high spatial resolution and little damage to unilluminated regions. This optical configuration resembles a lithography system with a very high power light source. The spatial resolution and fidelity of image transfer are strongly influenced by the quality of the beam delivery and imaging optics.

Figure 6. Laser and visible imaging optics Projection imaging compensates for the relatively low pulse repetition rate of large excimer lasers by allowing each laser pulse to process a relatively large worksurface area. It also allows the laser beam to be used as a shaped tool at the workpiece. For example, imaging of a square aperture allows drilling of a square hole. By shaping the beam, and moving the workpiece during exposure, excimer lasers can be used to make channels with rectangular, triangular, or other complex cross-sections. In this manner they provide a degree of control over the cutting process that is often difficult to achieve with other laser sources. The shape of the projection pattern determines the relative fractions of light that are absorbed by the mask and transmitted to the worksurface. Efficient utilization of the laser light is one of the challenges to the excimer laser machinist, but the use of a

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TeoSys Engineering LLC, Laser Micromachining Principles

beam that is easily shaped and strongly absorbed by most materials provides new options in precision cutting and drilling. Small excimer lasers and wavelength-converted lasers in the neodymium family produce lower energy pulses at multikilohertz rates to generate average powers in the .05 - 1.5 watt range. Figure 7 shows the energy density or fluence that can be produced by directing laser pulses with energies between 100 nanojoules and 100 microjoules into focal spots of diameter between 1 and 100 microns. The graph demonstrates that even these very low pulse energies are adequate to ablate most materials if the focal spot size is small and the material absorbs strongly at the laser wavelength. The submillijoule pulse energies and high pulse repetition rates of wavelength-converted Nd:YAG, Nd:YLF, and Nd:YVO4 lasers are well-suited to precision cutting of polymers like polyimide that are used extensively in the electronics industry.

100

Ceramic or metal ablation 100 µJ 10 µJ

10 Fluence (J/cm2)

1 µJ

1

100 nJ Polymer ablation

.

.01 1 10 Focal Spot Diameter (µm)

Fig. 7. Fluence produced by focusing laser energies of 0.1, 1, 10, and 100 microjoules into focal spot diameters between 1 and 100 micron. Threshold fluence ranges for ablation of most metals, ceramics, and polymers are also shown. High-repetition-rate UV lasers are normally used in "direct-write" configuration in which the sharply focused beam is rapidly moved over the worksurface to generate hole arrays or cut complex patterns. Beam delivery and motion system configurations are often similar to those used with CO2 or Nd:YAG lasers. Moving optics, moving workpiece, and hybrid systems all can be used to generate relative motion between the beam and the workpiece. UV-transmitting optical fibers are seldom used for transmitting the light from the laser source to the focus head because of their tendency toward color center generation and optical damage.

10

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TeoSys Engineering LLC, Laser Micromachining Principles

Table 3 summarizes typical speeds for polyimide cutting using frequency-tripled Nd:YLF and Nd:YVO4 lasers. Since the cutting process relies more on vaporization than melt removal in most UV applications, assist gases have only a weak influence on cutting speed and edge quality. However, gases such as nitrogen, argon, or helium are often directed at the worksurface during cutting to minimize the accumulation of particulates deposited from the ablation plume onto nearby areas of the worksurface. These gases are sometimes delivered from auxiliary nozzles or jets that are configured to bathe the ablation region in a nonreactive atmosphere and sweep ablation products away from protected areas. Thickness (microns) 25 50 100 125 175 250 Focal spot diameter (microns) 25 25 25 60 60 60 Pulse repetition rate (Hz) 30,000 30,000 30,000 15000 15000 15000 Average power (watts) 375 375 375 1.0 1.0 1.0 Cutting speed (mm/sec) 12 6 3 2.8 2.0 1.5

Table 3. Polyimide cutting speeds for wavelength-converted Nd:YLF and Nd:YVO4 lasers.

4 Depth control and 3-D machining Since each pulse removes only a very thin layer of material from the worksurface, good depth control is usually achievable with pulsed UV laser micromachining. Depth control in the micron range can be maintained when drilling blind holes or machining channels like those in Fig. 9. With careful choice of materials, three-dimensional structures with very smooth surfaces also can be fabricated. An approach that is quite effective involves scanning the laser beam over the substrate surface to remove a thin layer of material in selected areas, as shown in Fig. 8. By repetitively scanning a selected area and removing sequential layers a three dimensional structure can be produced. Best results are obtained when the material is strongly absorbing at the laser wavelength and vaporizes without going through a melt phase.

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UV BEAM

Scan 1 Scan 2 Scan 3

SUBSTRATE

Fig. 8. Production of three-dimensional shapes on a substrate surface by repetitive scan patterning with a UV laser.

Fig. 9. Blind hole in aluminum oxide drilled with a KrF excimer laser beam with uniform energy distribution. Diameter of the hole is approximately 30 microns.

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5 Cutting and patterning of layered structures Threshold fluence for UV laser ablation can differ dramatically among polymers, metals, ceramics, and glass materials. This allows easily vaporized overlayers to be removed without damaging underlying materials that are more difficult to ablate. This capability is utilized in applications like stripping insulation from small wires (Fig. 10), patterning flat panel display materials, or failure analysis of integrated circuits.

Fig. 10. Stripping of insulation from miniature cable of 50 micron diameter copper wires using a KrF excimer laser Other applications of laser cutting and drilling include eye surgery (photorefractive keratectomy) Figure 11, drilling of the inkjet holes for inkjet printers, drilling of precision orifices for gas flow control, and cutting of STENTs just to name a few.

Stent like pattern cut from solid 1 mm OD stainless steel tube. Width of the thin struts is 50 microns.

Fig. 11. Other laser micromachining applications

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