Richard A. Allen

Traceable Calibration of Critical-Dimension Atomic Force Microscope Linewidth Measurements with Nanometer Uncertainty

The use of critical dimension atomic force microscopes (CD AFMs) in semiconductor manufacturing, both for process control and as a reference metrology tool, is increasing. If the tip width is calibrated consistently between measurements, a CD AFM can function as an excellent width comparator. Relative widths can be measured with uncertainties of 1 nm or less. However, to perform accurate measurements, the absolute tip width must be accurately calibrated. Until recently, conventional methods for accomplishing this had standard uncertainties on the order of 5 nm. Recently developed CD reference materials now make it possible to calibrate absolute tip width with uncertainties at the 1 nm level. The highlights of our method are: (1) the use of single-crystal silicon and preferential etching to pattern well-defined and highly uniform features; (2) the use of high resolution transmission electron microscopy (HRTEM) to access the Si lattice spacing directly as a source of traceable width information, and (3) the...

A new test structure for the electrical measurement of the width of short features with arbitrarily wide voltage taps

Accurate determination of the linewidth of a narrow conducting film for VLSI applications using electrical test structure metrology has required that the length of the line be many times its width to minimize geometric error due to the finite width of the voltage taps. However, long lines obscure important local effects such as nonuniformities in the film. Shorter lines highlight such effects. This work describes a method of measuring the width of a short line having taps of arbitrary width. The effect of the taps is measured and used in the extraction of the linewidth allowing the determination of local linewidth variations with confidence.<<ETX>>

Modeling and Simulation of Resistivity of Nanometer Scale Copper

Abstract A highly versatile simulation program was developed and used to examine how the resistivity of thin metal films and lines is increased as their dimensions approach and become smaller than the mean free path of electrons in metals such as copper and aluminum. The simulation program is flexible in that it can include the effects of surface and grain-boundary scattering on resistivity either separately or together, and it can simulate the effect on resistivity where each surface of a film or line has a different value for the scattering parameter. The simulation program (1) provides a more accurate calculation of surface scattering effects than that obtained from the usual formulation of Fuchs’ theory, (2) calculates grain-boundary effects that are consistent with the theory of Mayadas and Shatzkes, (3) shows that surface and grain-boundary scattering effects are interdependent, and (4) shows that the change in resistivity with temperature begins to increase as dimensions approach the bulk mean free path of the electrons in the metal.

RM 8111: Development of a Prototype Linewidth Standard.

Staffs of the Semiconductor Electronics Division, the Information Technology Laboratory, and the Precision Engineering Laboratory at NIST, have developed a new generation of prototype Single-Crystal CD (Critical Dimension) Reference (SCCDRM) Materials with the designation RM 8111. Their intended use is calibrating metrology instruments that are used in semiconductor manufacturing. Each reference material is configured as a 10 mm × 11 mm silicon test-structure chip that is mounted in a 200 mm silicon carrier wafer. The fabrication of both the chip and the carrier wafer uses the type of lattice-plane-selective etching that is commonly employed in the fabrication of micro electro-mechanical systems devices. The certified CDs of the reference features are determined from Atomic Force Microscope (AFM) measurements that are referenced to high-resolution transmission-electron microscopy images that reveal the cross-section counts of lattice planes having a pitch whose value is traceable to the SI meter.

CD-AFM reference metrology at NIST and SEMATECH

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.

Intercomparison of SEM, AFM, and electrical linewidths

Uncertainty in the locations of line edges dominates the uncertainty budget for high quality sub-micrometer linewidth measurements. For microscopic techniques like scanning electron microscopy (SEM) and atomic force microscopy (AFM), the image of the sharp edge is broadened due to the instruments non-ideal response. Localizing the true edge position within its broadened image requires a model for the instrument-sample interaction. Ideal left and right edges are mirror images of one another, so any modeling error in the position assignment will have opposite signs for the two types of edges. Linewidth measurements inherently involves such opposite edges and consequent doubling of model errors. Similar considerations apply to electrical critical dimension (ECD) measurement. Although ECD is a non-imaging technique, one must still model the offset between the position of the physical edge and the effective edge of the conducting part of the line. One approach to estimating the reliability of existing models is to compare result when fundamentally different instruments measure the same line. We have begun a project to perform such an intercomparison, and we report here initial results for SEM, AFM, and ECD measurements of sub-micrometer lines in single crystal Si. Edge positions are determined from SEM images using Monte Carlo tracing of electron trajectories to predict the edge shape.In the AFM, we estimate and correct for tip geometry using tools from mathematical morphology. ECD measurements are corrected for band bending in the neighborhood of the edges.

Recent Developments in Electrical Linewidth and Overlay Metrology for Integrated Circuit Fabrication Processes

Electrical linewidth measurements have been extracted from test structures replicated in planar films of monocrystalline silicon that were electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide formed by separation by the implantation of oxygen (SIMOX) processing. Appropriate selection of the surface orientation of the starting material, the design and orientation of the structures features, and patterning by a lattice-plane selective etch provide features with planar, atomically smooth sidewalls and rectangular cross sections. The primary motivation for this approach is to attempt to overcome the serious challenge posed by methods divergence to the certification of linewidth reference-materials for critical-dimension (CD) instrument calibration and related tasks. To enhance the physical robustness of reference features with deep submicrometer linewidths, the new test structure embodies short reference-segment lengths and arbitrarily wide voltage taps. Facilities for reconciliation of measurements extracted from the same feature by all normally practiced techniques are also implemented. In overlay metrology, electrical inspection of two types of hybrid overlay targets allows pixel calibration of, and shift extraction from, the overlay instruments. The overall strategic focus of this research is to resolve methods-divergence issues and possibly to develop universal deep-submicrometer linewidth reference materials for CD instruments and techniques for instrument- and process-specific shift extraction for optical overlay metrology.

Traceable atomic force microscope dimensional metrology at NIST

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.

Test structures for referencing electrical linewidth measurements to silicon lattice parameters using HRTEM

A technique has been developed to determine the linewidths of the features of a prototype reference material for the calibration of critical-dimension (CD) metrology instruments. The reference features are fabricated in mono-crystalline-silicon with the sidewalls aligned to the (111) lattice planes. A two-step measurement procedure is used to determine the CDs. The primary measurement is via lattice-plane counting of selected samples using high-resolution transmission electron microscopy (HRTEM); the transfer calibration is via electrical CD (ECD) test-structure metrology. Samples of these prototype reference materials were measured and provided, as the National Institute of Standards and Technology (NIST) Reference Material RM8110, to International SEMATECH for evaluation by its member companies. In this paper, we will describe the measurement procedure and show how the combined uncertainty of less than 15 nm was derived. Additionally, we demonstrate a technique to automate the analysis of the phase-contrast images in order to both minimize the cost and reduce the uncertainty of the calibration of the standards.

High-resolution transmission electron microscopy calibration of critical dimension (CD) reference materials

The National Institute of Standards and Technology and Sandia National Laboratories have developed a procedure for producing and calibrating critical dimension (CD), or linewidth, reference materials. These reference materials will be used to calibrate metrology instruments used in semiconductor manufacturing. The reference features, with widths down to 100 nm, are produced in monocrystalline silicon with all feature edges aligned to specific crystal planes. A two-part calibration of these linewidths is used: the primary calibration, with accuracy to within a few lattice plane thicknesses, is accomplished by counting the lattice planes across the sample as-imaged through use of high-resolution transmission electron microscopy. The secondary calibration is the high-precision electrical CD technique. In this paper, we describe the calibration procedure for these reference materials and estimate the related uncertainties.