Nikon and Synopsys Deliver on Advanced OPC Promise

It was at the Photomask Technology conference last year that litho toolmaker Nikon Corp. (Belmont, Calif.) and EDA provider Synopsys Inc. (Mountain View, Calif.) announced that they were collaborating on advanced lithography models and DFM-enabled lithography manufacturing solutions for 45 nm and below.¹ A year later, again at Photomask, they were showing the fruits of their teamwork.

The latest release of Synopsys's Proteus optical proximity correction (OPC) software now incorporates proprietary data from Nikon's lithography exposure tools. The two companies developed an embedded scanner parameter module that delivers the “manufacturing-aware” OPC and resolution enhancement technology (RET) lithography simulation models needed for 45 nm and below IC manufacturing.

Without more information about specific scanner parameters, Synopsys instead typically starts with an idealized set of parameters, explained Tracy Weed, director of the manufacturing products group at Synopsys. “We then do a bunch of calibration by actually running test wafers and measuring the features and the differences from ideal on the wafer,” he said. “We fit those into the OPC model, and then we give that model to the customer as part of our OPC Proteus capability. And then they work to actually match the pattern fidelity that they're trying to get on the wafers.”

As a result of the partnership, however, Nikon tool users will now have access to a host of more specific tool parameters that will enable improved OPC model accuracy and reduced time to silicon. The Nikon Scanner Signature File (NSSF) provides information about such tool pa rameters as the illuminator, projection lens, laser spectrum, wavelength, refractive index medium, numerical aperture (NA), etc. Presently, the parameters are available on a network and accessible by the Proteus modeling software. Beginning with the December 2007 release, the NSSF will be fully embedded into the software, making it easier to use.

At 45 nm and below, allowable CD error is in the 2-3 nm range, Weed said. “As you move below 65 to 45 and 32, you really have to have a better handle on the specifics of the equipment that you're working with.” Some examples of helpful parameters that haven't previously been considered for modeling purposes include illuminator pupil fill or illuminator polarization, Weed noted, adding that immersion lithography and the introduction of hyper-NA “really changes how we look at the RET end of the business.”

Using an ideal model for OPC and a scanner-based model for verification helps to identify pinching and bridging errors on the real silicon. In the scenario shown at the bottom, the OPC results match the ideal case very well. (Source: Nikon, Synopsys)

A key result of this is the ability to describe mathematically the polarization effects within the lens by what's called a Jones matrix.² The importance of this comes as a result of hyper-NA imaging. “If that is altered and the polarization is affected and it's not quite as you expect, you're going to get different levels of contrast, which is going to impact the image ultimately that you're creating at the bottom,” Weed said. The Jones matrix captures non-idealities in the lens. “The lens pupil's non-ideality is a major contributor to the error that's found in the optical proximity errors. That's why we're focusing on this. Out of all the things we could look at, this has one of the bigger contributors.”

Other EDA schemes using hardware acceleration, for example, are forced to use more computationally intensive routes. A model using the NSSF and only 50 kernels is more accurate than an OPC model without Jones pupil using 256 kernels. Using fewer kernels speeds computations and requires a lot less engine power, Weed explained. The NSSF-based model accuracy could be improved with the use of 128 kernels — still offering faster computation than 256 kernels without the NSSF and an accuracy improvement of 40%.

The new interface is available for Nikon's NSR-S610C, NSR-S609B and NSR-S308F exposure systems. Two parameters that are not yet modeled with the new release are flare and stage synchronization issues, Weed said. “So there's still some things that we're working on that are not in this version, but I would say the bulk of what is required to get improved accuracy…are taken care of.”

The timing is right for the partnership between Nikon and Synopsys, according to Bernie Wood, director of marketing at Nikon Precision Inc. “These elements become very important at 65 nm, especially because polarization is a key element that's used more for sub-65 nm, and with the range you're getting into when you get to 45 nm,” he said. “I think the timing right now, the technology, the industry needs this, and Synopsys is the first to offer it.”