Mask Metrology Challenges Grow

Solutions are required amid a shift toward new mask shapes.


Photomasks are becoming more complex at each node. In fact, masks are moving from traditional shapes to non-orthogonal patterns and complex shapes, such as curvilinear mask patterns.

To measure patterns and shapes on the mask, photomask makers use traditional critical-dimension scanning electron microscopes (CD-SEMs). In general, the CD-SEM, the workhorse metrology tool in the mask shop, uses electron beams to form images of tiny structures at high magnifications.

The CD-SEM is sufficient to measure traditional mask shapes. But the CD-SEM could struggle, or may not work, amid the shift to these more complex patterns and shapes on the mask. Most complex mask patterns do not have uniform CDs after optical proximity correction (OPC) or inverse lithography technology (ILT).

Indeed, there are several challenges in mask metrology, particularly with the CD-SEM. For one thing, mask makers must take more measurements than ever before with the CD-SEM. In addition, the number of mask defect issues flagged during mask inspection increases, but not all of these will actually result in yield problems on the wafer.

All told, the CD-SEM needs some help. Looking to solve the problem, D2S has partnered with Advantest to integrate D2S’ Wafer Plane Analysis engine into Advantest’s E3640 line of CD-SEMs.

“For the CD-SEM itself, it won’t run out of steam,” said Linyong (Leo) Pang, chief product officer and executive vice president of D2S. “Even for 10nm and 7nm, the CD-SEM will still be the mainstream metrology tool.”

For traditional mask patterns, mask makers can simply use traditional CD-SEM techniques. “In metrology, you basically measure the CDs. The CDs are well defined. It’s either a square, a contact or a line/space,” Pang said.

But for complex masks, the CD-SEM requires new capabilities. “Computational techniques are needed,” Pang said. “A computational layer on top of the CD-SEM machine will be required.”

And as mask makers move to more complex shapes and patterns, photomask makers will require two types of technologies for CD metrology–mask plane metrology and wafer plane metrology. “In mask plane metrology, instead of measuring CDs, you have to extract the contours that measure EPE (edge placement error). So you are doing 2D metrology,” he said. “In wafer plane metrology, you basically convert this back to 1D metrology, so you can measure the CD.”

In fact, wafer plane analysis has emerged as the solution of choice for identifying mask-level CDU issues that will impact the wafer. However, optical-based wafer plane analysis solutions are sometimes slow and expensive.

In contrast, D2S’ Wafer Plane Analysis engine uses GPU-based acceleration technologies to provide aerial image simulation of 2D contours extracted from the CD-SEM. It enables CD metrology for complex mask shapes. And it promises to speed up wafer‐level CD metrology.

“Combining D2S’ expertise in GPU-accelerated simulation technologies with our CD-SEM tools allows us to provide a cost-effective platform for extremely fast lithography simulation,” added Takayuki Nakamura, executive officer and general manager of Nanotechnology Business Division at Advantest.