Formas de pago

Given the proposed scanned mask imaging concept described above, the following section reviews the literature related to the frequency tripled solid state laser processing of materials relevant to high density interconnect fabrication, namely polymers and thin metal films.

2.6.1 Nanosecond 355nm laser ablation

Working at UV wavelengths offer a number of advantages in laser micromachining, including better resolution of features due to the shorter wavelength, and usually better absorption or a shorter optical penetration depth on account of the higher photon energy [39]. However this comes at a cost. In frequency tripled solid state laser systems, the cost is a reduced power due to the efficiency of the frequency conversion in non-linear crystals. The capital cost of excimer laser system, as well as the cost of ownership, is also high compared to lasers at longer wavelengths (as discussed in Section 6.1.3).

In both the case of frequency tripled solid state laser systems and excimer laser systems, the highest power, lowest cost per watt systems are multimode pulsed lasers with nanosecond pulse lengths. In relation to scanned mask imaging, it is worth noting that multimode lasers are better than single mode lasers in mask imaging applications. This

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is because single mode lasers with a long optical coherence length when used to image an aperture or mask show strong interference fringes in the ablation causing undesired depth variation [39]. These diffraction fringes are not present when using multimode lasers due to their short optical coherence length.

2.6.2 Ablation mechanisms in pulsed nanosecond UV laser ablation

Nanosecond pulsed laser ablation can be analysed using thermal, mechanical, photophysical, photochemical and defect models. However, consideration of a single ablation mechanism severely restricts the experimental parameter space over which the model is applicable. A brief description of the relevant ablation mechanisms is given below [39]:

Thermal ablation: incident radiation causes a temperature rise which can cause direct

material vaporisation or induce stresses within a substrate which can lead to explosion type ablation or in the case of thin films on a thick substrate, detachment of the film.

Photochemical ablation: if the photon energy of the incident radiation exceeds the bond

energy of bonds within the substrate, molecules or fragments of the substrate can desorb from the substrate surface. The breaking of bonds within a substrate can also lead to the build-up of stresses causing mechanical photochemical ablation. These processes need not affect the substrate temperature.

Photophysical ablation: processes resulting from a combination of thermal and

photochemical affects, such as enhanced desorption of photochemically broken bonds through a rise in surface temperature.

2.6.3 Ablation mechanism of polymers

The exact ablation mechanism of polymers such as polyimide is still under debate, and depends on the polymer [61]; however they undoubtedly have a larger photochemical contribution than metals. When fluences reach values >10 J/cm2, photochemical models alone can no longer accurately predict the shape and depth of the craters formed during laser ablation so thermal affects should also be considered [62]. The SEM micrographs in Figure 2.20 show the increased thermal effects seen at higher fluences.

The field is relatively young with UV solid state lasers only being commercially available fairly recently, but even the earliest studies on the ablation of polymers at 355nm concluded that thermal effects cannot be ignored [63]. The paper also reports the significance of an absorption coefficient an order of magnitude smaller at 355nm

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compared to 248nm, which leads to a larger optical penetration depth and more severe thermal effects, but can also improve the ablation rate.

Ablation of polymers at 355nm depends on a vast parameter space, and hence is complicated to model. Some of the parameters which can affect the ablation include spot size, beam energy profile, pulse length, power distribution of the pulse as a function of time, pulse energy, fluence, repetition rate and shots per area determined by the repetition rate and the beam deflection speed [64]. Material properties also come into play, as well as the proximity of neighbouring features. Balogh et al. report that even with a constant set of controllable parameters, differences in ablation quality are observed depending on the underlying conductive pattern in multilayer boards [65].

Yung et al. report a strong dependence of ablation rate and the composition of Heat Affected Zones (HAZ) on the repetition rate of solid state lasers when ablating Upilex, a proprietary polyimide formula [66]. They report that the ablation rate of Upilex can be increased at higher laser repetition rates due to the accumulation of heat at the substrate.

Gordon et al. attempt to take into account the cumulative build-up of heat after consecutive shots in a given area which occurs with the high repetition rates typical of frequency tripled Nd:YAG lasers [64]. They provide experimental evidence for an increase in ablation rate when raising the substrate temperature before ablation. They introduce a temperature dependent ablation threshold and use a finite elements model to simulate the residual heat dissipation after a laser pulse through 3 modes: conduction, radiation and transfer. They simulate the surface temperature of the polyimide as a function of time and report a 100 °C difference in temperature at a time of 0.02 ms and 1 ms after the laser pulse, which is the time between consecutive shots for repetition rates of 50 kHz and 1 kHz respectively.

They use this analysis to qualitatively explain the difference they observe in ablation rates at 1 kHz and 50 kHz. Quantitative predictions were not possible since the homogenous heating of a sample before laser ablation to a maximum temperature of 186°C cannot be extrapolated to the inhomogeneous, highly localised heating of the substrate by the laser spot to temperatures of the order of thousands of degrees.

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Figure 2.20 Laser ablation of polyimide at (a) a laser fluence of 0.28J/cm2 (b) a laser fluence of 13.56J/cm2 [62].

Gordon et al.’s model does not consider many factors such as plume shielding [67, 68], energy transfer by acoustic waves formed in explosion like processes, the effect of scanning the beam across the substrate and more complex heat transfer mechanisms; however it does highlight the importance of the heating of the substrate during 355nm Nd:YAG laser processing. The paper neglects to mention the impact of repetition rate on the HAZ or crater shape, for example re-solidification height, but it is clear that a higher repetition rate would further raise the surface temperature of the polymer and not only increase the ablation rate but possibly cause thermal degradation to areas surrounding the ablation.

Sinkovics in collaboration with Gordon et al. then elaborated on this phenomenological model to include change in morphology of the ablated region after each consecutive pulse, and was able to model the heat dissipation of an ablated region over time [69]. By modelling the temperature increase of one shot, a threshold temperature is used to determine which material is ablated. Knowledge of the ablated volume allows calculation of the new surface topography from which the new thermal simulation based on convection, conduction and radiation is run for the period of time preceding the next laser pulse. The process is then cycled to model a number of laser pulses. The model was successfully corroborated with experimental results by comparison of the depth and shape of ablated vias. An insightful addition to the model would be a material dependent threshold temperature defining the heat affected zone, such that substrate temperatures exceeding this value but below the ablation threshold temperature value could be highlighted as a heat affected zone. This would allow the model to predict the extent of thermal damage around ablated regions as a function of repetition rate, fluence and a number of other easily changeable processing parameters.

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Although the low fluence ablation in Figure 2.20(a) is clearly a less thermal process, it would need considerably more shots than the ablation shown in Figure 2.20(b) to reach a given target depth. It is unclear whether the build-up of heat after many consecutive shots at short time intervals would compare favourably to the large thermal effects generated from a relatively small number of high fluence pulses. In terms of implementing the scanned mask imaging concept to reach feature resolutions down to 2μm, the literature is clear in so much as localised heating of the substrate will occur if a high repetition rate 355nm solid state laser fires consecutive shots in a given region of the substrate. The optimum laser parameters to minimise thermal damage in a scanned mask imaging system require investigation and novel mask scanning regimes may have to be employed to increase the time between consecutive shots on a given area of the substrate.

2.6.4 Ablation of metal films

In metals, because the thermal relaxation time of the electrons is short in comparison with a nanosecond pulse length, the predominant ablation mechanism is evaporation from the metal surface and the laser can be considered a heat source [70]. Nanosecond metal ablation usually results in the formation of a burr around the ablated region. This typically has the morphology of recast molten metal. The thermal properties of a thin metal film substrate can also significantly impact on the size of the burr or heat affected zones [62]. With very thin metal films, <1 µm in thickness, the film removal is a single shot process and the threshold fluence, crater diameter and the size of burrs or heat defects become strongly dependent on the film thickness [70].