The deposited copper thickness was measured by x-ray fluorescence (XRF) at 20 reference points of the pattern. A suitable calibration of the XRF, based on standard copper foils was performed to ensure maximum accuracy. Before deposition, the copper seed layer thickness was measured to be able to calculate the actual deposited value accurately. The reference points were selected to ensure that the x-ray beam was no wider than the structures to be measured to avoid errors in the measurements.
In order to assess the benefits of the new tooling approach, experiments with the main anode alone and with the segmented anode alone were performed. The measured deposit thickness was compared with simulations for both cases.
Figure 9 shows a comparison between the simulated and measured deposit thickness for Pattern 1, using the main anode configuration. An excellent agreement between the simulations and the experimental values is observed, again showing the validity of simulations in optimization the plating thickness distribution.
Figure 10 compares the measured deposit in the 20 predefined sample points for the two configurations of Pattern 2. The variation of the deposit thickness is clearly reduced by the segmented anode. The standard variation is reduced from 36% for the main anode configuration to 23% for the segmented anode configuration. Note that both the lowest and the highest deposition values are improved. If another target value for the optimization is selected, it is possible to focus the improvement more towards increasing the lowest values or decreasing the highest values. This choice depends on the requirements of the customer and is not investigated further here. From these measurements, the improvement of the deposition uniformity is evident. Different experiments on different patterns show a very similar behavior. Within the range examined in the experiments, no significant influence of the distance between the segmented anode and the substrate was detected. This is mainly due to the very high conductivity of the commercial copper plating electrolyte which ensures that the ohmic drop in the electrolyte is not the determining factor in the distribution of the current density on the patterned substrate.
Experiments performed with an in-house standard copper plating electrolyte consisting of copper sulfate and sulfuric acid show the influence of the electrolyte conductivity on the deposition distribution. A limited amount of PEG + Cl- was added to ensure the adhesion and improve the quality of the electrodeposited copper layer. The conductivity of this electrolyte was 25 S/m, half of the value of the commercial electrolyte. From the experiments, as shown in Fig. 11, it is clear that the reduced conductivity of the electrolyte yields a significant improvement in deposition uniformity. The standard deviation is reduced from above 17% for the commercial electrolyte to below 11%. This means that for this electrolyte, the copper thickness at all measured points is within 20% of the average value. Note also the very significant improvement when comparing the distribution using the segmented anode and the optimized electrolyte (Fig. 11) with the distribution from only the main anode and the standard electrolyte (Fig. 9). Based on these results, it can be concluded that it would be very beneficial to tune the electrolyte and the segmented anode approach together to achieve maximum uniformity in the plating distribution.
Additional experiments using the Pattern 2 design show that the improvement of the uniformity achievable with new tooling depends strongly on the electrolyte conductivity. Table 1 shows the standard deviation as measured over the 20 sample points for the two electrolytes, each time with the main anode and with the segmented anodes. It is clear that the best results are achieved with a less conductive electrolyte combined with the segmented anodes.
Table 1 - Standard deviation of the distribution for the different configurations.
From the results as presented above it can be concluded that the validity of the new tooling approach has been clearly demonstrated for industrially relevant printed circuit boards. The gain in uniformity of the deposition as achieved in the experiments is significant.
The proposed tooling concept yields a significant increase in the overall control of the deposit distribution. The electroplating cell actively compensates the non-uniformity in the deposit based on upfront simulations and optimization.
Different targets and specification for the optimizations are possible depending on the user requirements:
· More uniformity with the same current density
· Higher plating speed/throughput with same uniformity
· Increasing the minimum deposit or decreasing the maximum deposit with limited influence on the rest of the distribution
· Combinations of the above
It was established that the control of the deposit distribution can be further improved when the electrolyte is tuned together with the hardware.
The resulting increase in plating control is important to a production plant for PCBs for several reasons:
· It allows plating at higher speed, thus improving throughput.
· It reduces the consumption of copper.
· It relaxes the design rules (e.g., the width and distance between single tracks, isolated vias, etc.), so more complex designs can be produced reliably.
This work is supported by the IWT Flanders in project KMO 090316.
* Corresponding authors:
Dr. Gert Nelissen
Phone: +32 16 47 49 60
Dr. Alan Rose
Business Manager, North American Operations
176 Millard Farmer Ind. Blvd.
Newnan, GA 30263, USA
Tel: (770) 328 1346
Elsyca Intellitool®, Elsyca NV, Vaartdijk 3/603, B3018 Wijgmaal, Belgium.
*** Rohm&Haas Coppergleam 125 T-2 A, Rohm & Haas, Subsidiary of the Dow Chemical Co., Midland, MI, USA.