Optimizing VSB Shot Count For Curvilinear Masks

The increased photomask write time using a variable-shape e-beam (VSB) writer has been a barrier to the adoption of inverse lithography technology (ILT) beyond the limited usage for hot spots. The second installment of this video blog looked at the challenge in depth. In this five-minute panel video with industry luminaries, Ezequiel Russell describes the collaborative study between his company, Micron Technology, and D2S to optimize VSB shot count for curvilinear masks.

The purpose of the joint study was to find the most efficient way to reduce the VSB write time for curvilinear photomasks without losing the curvilinear benefit of improved wafer performance. Blindly optimizing the VSB mask writer shot count (a proxy for the write time) won’t result in the same wafer performance benefits. The proposal resulting from the joint study is to co-optimize the shot count reduction with the ILT algorithm using mask and wafer simulation (MWCO). The result is the best possible write time and wafer performance. Looking at the bar chart on the left of figure 1, the conventional number of shots for curvilinear shapes explodes in the bar on the left. In the third bar, MWCO results in shot count that is only slightly higher than Micron’s plan-of-record OPC solution but with far superior wafer performance.  In fact, the wafer performance from MWCO is comparable to the full curvilinear ILT solution written on multi-beam writers. MWCO is not suitable for EUV masks, however, because of the larger geometry count and the higher energy required to expose the slower resists to achieve the required precision.

Fig. 1: Results from a study on optimizing VSB shot count for curvilinear photomasks.

The MWCO solution depends on the VSB mask writer having the capability of writing overlapping shots. Noriaki Nakayamada from NuFlare Technology confirms that their VSB machines have a feature to write overlapping shots without difficulty. He cautions that to achieve the results of MWCO as proposed requires sophisticated collaboration between the mask shop and the wafer fab. Danping Peng from TSMC sees the value of the MWCO work as many customers are still using VSB mask writers. Using multi-beam mask writers to write curvilinear masks would be easier in his opinion but does require investment in new mask writers.

To learn more about the topic of MWCO, you can read the entire paper on the collaborative study here. You can watch the full 90-minute panel discussion for the eBeam Initiative’s virtual event at SPIE Advanced Lithography here.