Lattice-selective etching of silicon is used in a number of applications, but it is particularly valuable in those for which the lattice-defined sidewall angle can be beneficial to the functional goals. A relatively small but important niche application is the fabrication of tip characterization standards for critical dimension atomic force microscopes (CD-AFMs). CD-AFMs are commonly used as reference tools for linewidth metrology in semiconductor manufacturing. Accurate linewidth metrology using CD-AFM, however, is critically dependent upon calibration of the tip width. Two national metrology institutes and at least two commercial vendors have explored the development of tip calibration standards using lattice-selective etching of crystalline silicon. The National Institute of Standards and Technology standard of this type is called the single crystal critical dimension reference material. These specimens, which are fabricated using a lattice-plane-selective etch on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate CD-AFM tip width to a standard uncertainty of less than 1 nm. During the different generations of this project, we evaluated variations of the starting material and process conditions. Some of our starting materials required a large etch bias to achieve the desired linewidths. During the optimization experiment described in this paper, we found that for potassium hydroxide etching of the silicon features, it was possible to independently tune the target linewidth and minimize the linewidth nonuniformity. Consequently, this process is particularly well suited for small-batch fabrication of CD-AFM linewidth standards.
Critical dimension atomic force microscopes (CD-AFMs) are rapidly gaining acceptance in semiconductor
manufacturing metrology. These instruments offer non-destructive three dimensional imaging of structures and can
provide a valuable complement to critical dimension scanning electron microscope (CD-SEM) and optical metrology.
Accurate CD-AFM metrology, however, is critically dependent upon calibration of the tip width. In response to this
need, NIST has developed prototype single crystal critical dimension reference materials (SCCDRMs).
In 2004, a new generation of SCCDRMs was released to the Member Companies of SEMATECH - a result of the
fruitful partnership between several organizations. These specimens, which are fabricated using a lattice-plane-selective
etch on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate CD-AFM tip
width to a standard uncertainty of about ± 1 nm.
Following the 2004 release, NIST began work on the "next generation" of SCCDRM standards. A major goal of this
thrust was to improve upon the SCCDRM characteristics that impact user-friendliness: the linewidth uniformity and
cleanliness. Toward this end, an experiment was designed to further optimize the process conditions. The first round of
this experiment was recently completed, and the results show great promise for further improvement of the SCCDRM
manufacturing process.
Among other observations, we found that the minimum linewidth and linewidth uniformity were primarily sensitive to
different factors - and can thus be independently tuned to meet our future goals - which include linewidths as small as
20 nm and a standard uncertainty due to non-uniformity at the ± 0.5 nm level. Our future work will include a new
refining experiment to further optimize the important factors that we have identified, and extension of the methodology
to a monolithic 200 mm implementation.
The National Institute of Standards and Technology (NIST) has a multifaceted program in atomic force microscope
(AFM) dimensional metrology. There are two major instruments being used for traceable AFM measurements at NIST.
The first is a custom in-house metrology AFM, called the calibrated AFM (C-AFM), and the second instrument is a
commercial critical dimension AFM (CD-AFM). The C-AFM has displacement metrology for all three axes traceable
to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. In the current generation of this system, the relative
standard uncertainty of pitch and step height measurements is approximately 1.0 x 10-3 for pitches at the micrometer
scale and step heights at the 100 nm scale, as supported by several international comparisons. We expect to surpass this
performance level soon. Since the CD-AFM has the capability of measuring vertical sidewalls, it complements the
C-AFM. Although it does not have intrinsic traceability, it can be calibrated using standards measured on other
instruments - such as the C-AFM, and we have developed uncertainty budgets for pitch, height, and linewidth
measurements using this instrument. We use the CD-AFM primarily for linewidth measurements of near-vertical
structures. At present, the relative standard uncertainties are approximately 0.2% for pitch measurements and 0.4% for
step height measurements. As a result of the NIST single crystal critical dimension reference material (SCCDRM)
project, it is possible to calibrate CD-AFM tip width with a 1 nm standard uncertainty. We are now using the CD-AFM
to support the next generation of the SCCDRM project. In prototypes, we have observed features with widths as low as
20 nm and having uniformity at the 1 nm level.
The National Institute of Standards and Technology (NIST) and SEMATECH have been working together to improve the traceability of critical dimension atomic force microscope (CD-AFM) dimensional metrology in semiconductor manufacturing. A major component of this collaboration has been the implementation of a Reference Measurement System (RMS) at SEMATECH using a current generation CD-AFM. An earlier tool, originally used at SEMATECH, has now been installed at NIST. Uncertainty budgets were developed for pitch, height, and CD measurements using both tools. At present, the standard uncertainties are approximately 0.2 % for pitch measurements and 0.4% for step height measurements. Prior to the current work, CD AFM linewidth measurements were limited to a standard uncertainty of about 5 nm. However, this limit can now be significantly reduced. This reduction results from the completion of the NIST/SEMATECH collaboration on the development of single crystal critical dimension reference materials (SCDDRM). A new generation of these reference materials was released to SEMATECH Member Companies during late 2004. The SEMATECH RMS was used to measure the linewidths of selected features on the distributed specimens. To reduce the uncertainty in tip width calibration, a separate transfer experiment was performed in which samples were measured by CD-AFM and then sent for high resolution transmission electron microscopy (HRTEM). In this manner, CD-AFM could be used to transfer the HRTEM width information to the distributed samples. Consequently, we are now able to reduce the limit on the standard uncertainty (k = 1) of CD-AFM width measurements to 1 nm.
The implementation of a new test structure for HRTEM (High-Resolution Transmission Electron Microscopy) imaging, and the use of CD AFM (CD Atomic Force Microscopy) to serve as the transfer metrology, have resulted in reductions in the uncertainties attributed to critical dimension (CD) reference-material features, having calibrated CDs less than 100 nm. The previous generation of reference materials, which was field-tested in 2001, used electrical CD as the transfer metrology. Calibrated CD values were in the range 80 nm to 150 nm and expanded uncertainties were approximately ± 14 nm. The second-generation units, which have now been distributed to selected industry users for evaluation, have uncertainties as low as ±1.5 nm and calibrated CDs as low as 43 nm.
Prototype linewidth reference materials with Critical Dimensions (CDs) as narrow as 70 nm have been patterned in silicon-on-insulator films. The sidewalls of the reference features are parallel, normal to the substrate surface, and have almost atomically smooth surfaces. Linewidth calibration begins with the measurement of the electrical CDs of multiple reference features located at a selection of die sites on a wafer. The absolute widths of the cross sections of a sub-set of reference features on several chips that are diced from the wafer are then subjected to high resolution transmission electron microscopy (HRTEM) imaging to determine their physical CDs by lattice-plane counting. Sample preparation for lattice-plane counting by HRTEM Is destructive, and other reference features on the same chip become unusable for reference-material purposes. However, a calibration curve for converting the measured electrical CDs of reference features on other chips on the wafer, known as 'product reference features', to their physical values is obtained. The uncertainty attributed to the physical CD values of the product reference features generally varies inversely with the linear correlation between the cross- section lattice-plane counts and the corresponding electrical CD measurements of the sub-set of reference features that were selected for HRTEM imaging. A linear correlation value of approximately 0.97 has been obtained from a sub-set of 12 HRTEM measurements. In this case, the uncertainty attributed to the physical CD values of the product reference features is believed to be responsible for most of the product reference feature uncertainty. However, it has now been found that a forming-gas annealing treatment appears to prevent the referenced time dependence and thus has the potential for reducing the uncertainty level.
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