Wednesday was a day full of papers, culminating with the poster session.  Bright and early at 8:00 am Alain Moussa of imec reviewed progress towards in-line AFM.  While not diving into the specifics of individual AFM products, he showed results indicating that the main AFM weaknesses (probe stability and lifetime, especially for thin and narrow probes, and scan speed) have shown great improvement in recent years.  A particular difficulty is high aspect ratio holes and trenches.  He showed decent results for holes with an aspect ratio of 3 and trenches with an aspect ratio of 8, which is quite reasonable.  Later, an AFM manufacturer told me that even higher aspect ratio holes can be measured reliably.

The roadmap for implementing high-NA EUV lithography includes many difficult and time-consuming steps, most of which are outside of the control of the user.  But one step that every chipmaker would like to shorten as they prepare for those first high-NA wafers is OPC model calibration.  Good printing results for real device patterns require good OPC, which at the highest resolutions must be tuned to the specific aberrations and other non-idealities of the scanner.  A simple approach would wait until the high-NA EUV scanner was installed and accepted (with final system adjustments completed) before printing the wafers that begin the OPC calibration cycle.  Can (pronounced “John”) Guven of Intel described a novel solution that takes advantage of how good rigorous simulations have become.  Before the EUV scanner is shipped, aberrations are measured, then modified for expected improvements in final optics adjustments.  Those aberrations and other information are used in simulations to predict printing differences between early wafer results and what they expect final results to be, thus enabling better OPC calibration at an earlier stage in the scanner installation cycle.  Of course, there are lot details (and probably a few major concepts) that I am ignoring and/or getting wrong, but this seems like a good idea that works.

Wataru Yamane of Hitachi, along with coauthors at NIST, gave the talk that takes the prize (so far) for the most rigorously scientific and well-executed work at this conference.  With the goal of improving the accuracy of CD-SEM imaging simulations, they systematically explored various options for modeling low energy electrons as they travel and scatter through a sample, then compared simulations to very carefully measured SEM and transmission SEM data.  Moving the needle on SEM simulation accuracy is not easy; it is good to see valuable progress such as this.

Directed Self Assembly (DSA) was originally thought to be a technique to improve resolution through pitch division:  conventionally print a pattern at pitch P, then use that pattern to direct the self-assembly of block copolymers at pitch P/N, with N = 2, 3, 4, or even 5.  The promise of “resolution in a bottle” from DSA has never been fully realized for a variety of reasons.  But along the way another use for DSA become appreciated:  rectification of EUV-printed patterns by letting N = 1.  The idea is not to improve the resolution of the patterns, but rather to improve their quality, that is, their roughness.  DSA rectification has been shown to enable EUV dose reduction by a factor of 2 while simultaneously reducing stochastics effects compared to the full dose.  For low-NA EUV, with pitches as low as 24 nm, the best DSA material is PS-b-PMMA, a material that is well known, well tested, and ready for use in manufacturing (at least so claimed by people more knowledgeable than me).  To use DSA rectification below this pitch, however, requires both high-NA EUV patterning and a new class of DSA materials called high-Chi block copolymers. Victor Monreal of EMD showed good progress in the development of high-Chi materials, though more work is still needed. 

Brian Watson of Micron described experimental techniques for answering an important but difficult question:  where does line-edge roughness (LER) come from?  This breaking down of LER into components requires both good experiments and clever analysis.  The basic idea is to take one component of LER that can be individually manipulated and look for changes in LER with that parameter.  For Micron’s analysis of a 193i process, they began with speckle, randomness in exposure dose caused by unwanted coherent interactions from the ArF laser.  Speckle is proportional to one over the square root of the number of laser pulses, and that can be manipulated without affecting any other LER component.  The experiment showed the expected result:  LER is reduced as the number of pulses is increased (keeping dose constant).  But the LER change was quite small, so extremely good metrology (and a lot of it) was an enabler for this approach.  The next step was harder: separating the influence of the image from that of the resist by extrapolating LER versus 1/NILS (or its exposure latitude equivalent) to the case of a perfect image.  I’m going to have to study the paper when it comes out before rendering judgement on the details, but I commend Micron for taking on such an important and ambitious project.

One of the coolest ideas in stochastics is the Conservation of Roughness principle:  the LWR of an infinitely long line partitions into both roughness and local CD variations for a shorter line in specific way that is controlled by the correlation length of the roughness.  We tend to measure roughness for long lines, but the impact of stochastics on devices tends to occur on a short length scale – a device-relevant length that depends on the layer and the device.  It was great to see Gopal Kenath if IBM apply these concepts in a very practical way when comparing three photoresists.  It was also very interesting to see how different illuminators affected roughness by changing PSD(0), the low-frequency component of roughness, rather than the correlation length or roughness exponent.  I’m looking forward to his future work, where after-etch results will be analyzed in this same way.

I finished out the night (at least the technical portion of the night) by hanging out in front of my poster.  Thanks to SPIE for providing way-above-average quality beer, and to Bill Usry (one of my coauthors on the poster) for relieving me halfway through the session so I could enjoy that beer.