Variable intensity ablation with the SMI system

A unique property of SMI, when compared to other mask imaging systems discussed in Sections 2.4.2 and 2.5.1, is the ability to have some degree of control over the time between consecutive shots delivered to a given part of the mask and substrate. The relatively high repetition rate DPSS laser, and the accurate and rapid scanning of the mask allowed by the galvanometer scan head, allows the user to change the mask scanning regime to change the period between shots at a given point on the substrate. The mask is scanned with a galvanometer scanner with a certain mark speed, which determines shots in x direction delivered at the repetition rate of the laser. The pitch determines shots delivered in y direction, and the number of bursts of pulses at the repetition rate of the laser. The mask can be scanned a number of times to build up the required number of shots. These 3 parameters determine the intensity profile. The graphic in Figure 5.7 illustrates how energy would be delivered to the substrate over time with 4 Shots Per Area (SPA) per scan, and the start of a 2nd scan of the mask. The mark speed is set so that there are 2 shots in the x direction, such that the scanner

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mirrors have deflected the beam by a distance equal to half the spot size in the period between pulses. The pitch of the lines in the illumination pattern is then set to half the spot size, such that there are 2 shots in the y direction. If the mask were scanned twice at 4 SPA to deliver a total of 8 SPA, this would be denoted (2, 2, 2) such that the product of the numbers gives the total SPA and each number denotes (shots in x, shots in y, number of mask scans). The time between shots in a given area also depends on other factors such as the size of the area being scanned at the mask, and whether the area is being scanned with a serpentine or raster scan illumination pattern. However, in general, the higher the first 2 numbers, the more thermal effects become a factor in the laser ablation process because this indicates a large number of shots per area occur per scan of the mask giving the substrate less time to cool down between successive scans.

Figure 5.7 A graphic illustrating the intensity control possible with the SMI system. The cartoon graph shows the energy delivered to area A, a given area of the mask or substrate. The mark speed determines how many shots are delivered in the x direction, and the number of pulses in the burst, P1. The pitch determines the shots delivered in

Legend Area A

Square beam size

Time 1: Area A receives a burst of pulses at the repetition rate of the laser

Time 2: Area A receives a second burst of pulses as the laser is scanned along line 2.

Time 3: the laser completes a full scan of the mask and the mask is rescanned, at which point area A receives another burst of pulses t1 t2 t3 Energy Time P1 P2 P3 Simulating 2 shots in x, 2 shots in y, and the beginning of a 2nd scan of the mask

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the y direction, and the number of bursts of pulses in P2. The total number of times the mask is scanned with this mark speed and pitch determines how many times P2 is repeated in segment P3.

To investigate the effect of different mask scanning regimes on the ablation quality, a resolution chart was scanned and ablated with the same fluence but different mask scanning regimes into an ABF type build up substrate common in the chip packaging industry. The ABF type substrates are epoxy with small glass filler particles. The ablation of the resolution chart can be seen in Figure 5.8, where the finest scale features at the top of the image are 2 µm L/S. Both charts were scanned with 25 SPA, however the chart on the left was scanned with (5, 5, 1) and the chart on the right with (1, 1, 25). The impact of delivering the 25 shots in a single scan is clear, particularly in the fine scale features. The walls between the channels have completely collapsed for all channels at 6 µm L/S and below. Comparing this with the 25 scans at 1 SPA, despite the 25 SPA delivered to the substrate, there is only trace collapse in some areas of the 2 µm L/S features. By simulating the scanning of the illumination pattern used in the (1, 1, 25) mask scanning regime, the effective repetition rate as seen by a given area of the substrate is estimated to be <100 Hz. This is in contrast to the (5, 5, 1) scan which delivers shots in the x direction at the frequency of the laser, 5 kHz. This lower intensity ablation gives the substrate time to cool between shots, and in this substrate, gives a much better result in the smaller features.

Figure 5.8 Image of ablation of a resolution chart in an ABF type substrate at 2 J cm-2 and 25 SPA. In the image on the left, all 25 shots per area were put down in a single scan of the mask, with 5 shots in the x direction and 5 shots in the y direction. In the image on the right, the mask was scanned 25 times at 1 SPA.

Therefore, based on the images in Figure 5.8, one can conclude that for this particular substrate, changing the mask scanning regime to increase the time between consecutive shots in a given part of the substrate reduces the probability of wall collapse of the side walls of the fine line space channels for a given number of shots. Alternatively, it

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allows a higher SPA to be delivered without wall collapse compared to higher intensity mask scanning regimes.

Figure 5.9 shows the ablation of the largest, 15 µm L/S, features in the same two resolution charts. Examining the images of the top of the channels, there is little difference between the high intensity (5, 5, 1) mask scanning regime and the low intensity (1, 1, 25) regime. The high intensity ablation has caused the feature to broaden by 0.6 ± 0.2 µm, and there are more debris particles on the sample surface. The depth of the features was measured by focusing on the top and bottom surfaces of the channels and taking readings from the z axis encoder on the optical microscope, and it was found that the ablation depth of the channels scanned with the high intensity regime was significantly greater. The bottoms of the channels in the high intensity regime are narrower than in the (1, 1, 25) regime, but the taper angle of both channels is the same within error, 79±1°, such that the additional narrowing can be attributed to the increased depth of the channel. Therefore for the ablation of features larger than 10 µm L/S in this particular substrate, a higher intensity mask scanning regime is beneficial to the process. It increases the ablation rate (µm/shot) reducing the number of shots required to reach a target depth, and therefore the process time required to pattern a sample. In the example shown, the ablation rate is increased by a factor of ~1.5 from 0.44 µm/shot to 0.64 µm/shot

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Figure 5.9 Comparison of nominally 15 µm L/S features ablated with the same fluence and total number of shots per area, 2 J cm-2 and 25 SPA respectively, but with different mask scanning regimes. The depth, as measured by focusing on the top and bottom of the channels and taking readings from the encoder on the z-axis of the microscope, is significantly greater when delivering all shots in a single scan of the mask and giving the substrate less time to cool.

The response of a material to a given mask scanning regime will vary from material to material. The response of a material depends on its ability to dissipate the incident energy, determined by its thermal properties such as its thermal diffusivity, and the threshold energies in the material such as the melting point. For each substrate the mask scanning regime, in combination with the fluence, needs to be tuned to determine the optimum intensity for the ablation of features of a given size, to a given depth. Figure 5.10 shows the ablation of 2 resolution charts with identical laser parameters and mask scanning regimes in 2 different ABF type substrates made by different manufacturers. Despite the similarities in the composition of the 2 substrates, the substrate in the image on the right shows significantly more erosion to the side walls of fine line space channels.

(5, 5, 1) (1, 1, 25) Top Bottom 15.64µm 15.06µm Depth: 16±1 µm Depth: 11±1 µm

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Figure 5.10 Ablation of 2 proprietary ABF type substrates from different manufacturers ablated with the same fluence (2 J cm-2) and mask scanning regime (1, 1, 25) but showing a different response to the laser. Despite the similar composition of the substrates, the side walls of the channels in the substrate on the right are clearly more prone to collapse.

The intensity (W/unit area) can also be tuned by modifying the fluence as well as the mask scanning regime. Figure 5.11 shows how reducing the fluence to lower the intensity of the laser ablation can also help to prevent wall collapse in the laser ablation of fine scale features. Each chart was ablated with the same (1, 1, 100) mask scanning regime to deliver 100 SPA to the same ABF type substrate shown in Figure 5.8 at 2 J cm-2. At 2 J cm-2 and 100 SPA, there is now some side wall erosion in all features below 6 µm L/S. Reducing the fluence to 1.5 J cm-2, and therefore reducing the intensity of the laser irradiation, significant side wall erosion now begins to occur for the 3 µm L/S features and below. Stepping down the fluence again to 1 J cm-2, there is no longer any side wall collapse for any of the features down to and including 2 µm L/S despite the high SPA. Despite being able to deliver 100 SPA to the substrate without eroding the side walls of the channels, the ablation depth has become limited by the taper angle of the ablation, such that the fine line space features are very shallow, less than 2 µm deep. This trade-off between depth, fluence and side wall erosion is examined more closely in Section 5.4.

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Figure 5.11 Image showing laser ablation of a resolution chart in the same substrate with the same (1, 1, 100) mask scanning regime with different fluences. The fluence was varied by tuning the laser diode current. At 1 J cm-2, there is no erosion of the side walls of the 2 µm L/S features, but the ablated depth is <2 µm.

It has been shown that by changing the intensity, that is the power delivered to a unit area, it is possible to influence the thermal loading of the substrate. Reducing the intensity by changing the mask scanning regime, to increase the time between consecutive shots in a given part of the substrate, it was possible to reduce the probability of the side wall collapse of fine L/S features. Conversely, increasing the intensity by changing the mask scanning regime was shown to increase the ablation depth and ablation rate by 50% in the larger channels spaced further apart, which were not prone to suffering from side wall collapse.

Thus by simply changing the vector file sent to the scanner, it was possible to significantly reduce thermal degradation or significantly improve the ablation rate by controlling the thermal loading of the substrate. Simulation of marking the vector files used by the scanner to illuminate the mask showed the effective repetition rate of the laser as seen by a given part of the substrate could be tuned between <100 Hz and 5 kHz by changing the mask scanning regime. This is something not possible with an excimer laser mask imaging system due to the comparatively low maximum repetition rate of high average power excimer lasers.