Ed’s Threads 080317
Musings by Ed Korczynski on March 17, 2008

There is no more noise...
There is only signal. In controlling the manufacturing processes used for advanced nano-scale IC, the aspects of metrology which we used to be able to ignore as “just noise” are now essential signal we must control. Where to draw the line, and how close is close are just some of the challenges in ensuring that data streams become productive information for fabs. Metrology sessions at SPIE this year shone fractional wavelengths of light into the darkness of controlling accuracy, too.

When IC features were greater than the wavelength of light used in photolithography—and likewise much greater than a countable number of physical atoms—there were many aspects of manufacturing which we could simply ignore. With the smallest IC feature, typically defined by the minimum half-pitch spacing between lines, now reaching ~45nm (which is less than one-quarter of the 193nm wavelength used in litho) we now experience “second-order” and “third-order” effects which must be controlled.

Vladimir Ukrainstev of Veeco Instruments co-led a panel discussion at SPIE 2008 on the need for CD-SEMs to be accurately calibrated with 3D-AFMs. Researchers have reportedly seen a mere 1° change in the sidewall angle of a device structure result in a 2nm change in the CD measured by a standard 2D SEM. With the allowable budget for CD variation shrunk down to 3nm-4nm, this sidewall angle dependence must be controlled. The greatest risk is in process drift in an etch chamber, where sidewall angle can change spacially (e.g., from the center to the edge of wafers) or temporally (from wafer to wafer over time), which can induce substantial error in the CD-SEM measurement.

With tight feedback loops in advanced fabs, erroneous CD-SEM data can be mistakenly used to set the wrong etch parameters for following lots, which can degrade yield. “Instead of changing CD etch time by the week, we’re changing by the lot or the wafer as part of APC,” explained Kevin Heidrich, Nanometrics’ senior director of new business development, in an exclusive interview with WaferNEWS. Total CD control is ~4nm for all variability; a normal rule of thumb for precision over tolerance is 0.1, so the total budget for metrology is 0.4nm.

All measurement techniques are subject to some error, and even the best 3D-AFM is still subject to tip-wear and calibration. Veeco has been working with 3^rd-party specialists to optimize AFM tips for different applications, with great results reported for various shapes nano-machined from single-crystal silicon for strength and then coated with some manner of a carbon coating for wear-resistance. NIST showed SPIE attendees this year that even with a slow, expensive, and destructive technique like TEM, there is still 0.33nm (standard deviation, 1σ) of the sidewall angle uncertainty. Everything else adds up to 0.63nm of total uncertainty. Calibration is vital to minimize the propagation of uncertainties.

One of the issues in determining the side-wall angle is what portion of the sidewall to include in the analysis. For features with corner rounding, this could be challenging even with ideal 90 degree sidewalls. Just considering 2nm radii of curvature on the top corners of etched polysilicon lines of 32nm to 45nm widths, and ~10% of the linewidth varies with where a CD-SEM draws the line for the edge.

To help control APC in all manner of deposition and removal processes, Nanometrics recently announced the delivery of the company’s 1000^th integrated metrology sub-system; the milestone system was integrated into an advanced plasma etch system used to control gate CD in advanced logic devices.

At SPIE, IBM (Daniel Fischer et al.) showed OPC requirements for 32nm and the metrology tool calibrations need to support this advanced node. Modeling calibration sites per mask level has increased dramatically: normalized to the 90nm node, 65nm had 10×, and 32nm is 100×. There are now multiple CDs per contour, which results in a reduced number of measurement sites per wafer. For tool calibration, fundamental parameters of magnification, rotation, etc. each must be properly considered in modeling. The researchers showed that scanning a line array in orthogonal directions in a CD-SEM induced up to 2% variation in measurement due to the beam’s oval shape. It’s not noise anymore. “The users must understand the measurement techniques and have them constant or have a consistent offset to be able to use the data,” said Fischer. He added that with real device structures, 144nm was seen by a 2D tool while 160nm was measured by a 3D tool, so some manner of rigorous automated edge-detection is essential.

OCD looks very extendable to finFETs, too. SEMATECH and KLA-Tencor presented a paper on metrology for high-k finFETs at SPIE. Using high-k HfSiO thicknesses of 1.5nm and 3nm over Si₃N₄, and using TiN as the metal gate, a thorough DOE of depositions over fins was done. Then using KLA-Tencor's next-generation spectroscopic ellipsometer (measuring 225nm and up) for OCD, and CD-SEM from AMAT and also HR-TEM, cross-checks between the OCD and standard thin-film measurements showed that the offset was ~1nm. For the metal gate measurements, it was found that the TiN optical properties varied due to what is suspected to be some manner of slight oxide formation. Data from dense arrays showed serious offset from the pad areas, so correlations must be considered. Measuring in the fin area seems to provide sufficient resolution for process control for both the high-k and metal-gate depositions. OCD measurement precision was at the 1% level or better, and in good agreement with reference measurements. OCD looks very promising for finFET gate stack characterization.

n&k Technologies has modified the optical path of their spectroscopic ellipsometer tool to add a pinhole lens which narrows the transmitted beam spot size from 400μm to 50μm. Since real-world ICs and photomasks tend to have designed areas with regular 50μm arrays, this opens up the ability to measure many more real structures. Collecting the reflectance and transmission in both s- and p-polarizations using 50μm spots provides four separate signals to be used in determining all the layer thicknesses on the mask, including quart etch dimensions for phase-shift masks.

In pushing the limits of signals, IBM and Hitachi recently announced a unique, two-year joint semiconductor metrology research agreement for 32-nm and beyond characterization and measurement of transistor variations. Engineers from the two companies and Hitachi's subsidiary, Hitachi High-Technologies, will conduct joint research at IBM's Thomas J. Watson Research Center in Yorktown Heights, NY and at the College of Nanoscale Science and Engineering's Albany NanoTech Complex. Combining individual research strengths and IP will help "reduce the significant costs associated with research needed to advance the next generation of chip technology," said Bernie Meyerson, VP of strategic alliances and CTO for IBM's systems & technology group, in a statement.

Rudolph Technologies has become the first OEM to join SEMATECH's Metrology Program headquartered at the College of Nanoscale Science and Engineering (CNSE) of the University at Albany. The initial program addresses a range of issues, including the metrology of thin films and metal gate stacks; wafer front, back, and edge macro defect inspection; and inspection and metrology for through silicon vias (TSV) and three-dimensional integrated circuits (3DIC).

-- E.K.

Labels: accuracy, CD, fab, manufacturing, metrology, noise, SEM, semiconductor, SPIE

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080317: There is no more noise...

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