Christine E. Murabito

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.

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.

CD reference features with sub-five nanometer uncertainty

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.

Test chip for electrical linewidth of copper-interconnect features and related parameters

This paper reports a new electrical test structure for measuring the barrier-layer thickness and total physical linewidth of copper-cored interconnect features. The test structure has four critical dimension (CD) reference segments of different drawn linewidths. A new linewidth-extraction algorithm has been developed and extensively tested with and sheet-resistance measurement emulations. It has also been applied to measurements extracted from scaled-up physical structures. A second test structure for measuring conducting feature and interlayer-dielectric (ILD) thickness by use of the charge-based capacitance method (CBCM) is located on the test chip. Test-chips featuring both of these structures have been patterned in aluminum using a standard 0.18 /spl mu/m CMOS process and preliminary results are reported here.

Critical Dimension Calibration Standards for ULSI Metrology

NIST and International SEMATECH are developing single‐crystal reference materials for use in evaluating and calibrating critical dimension (CD) metrology tools. Primary calibration of these reference materials uses a high‐resolution transmission electron microscopy (HRTEM) image of the cross section of the feature at sufficient magnification to resolve and count the individual lattice planes; the transfer calibration is provided by complementary metrology techniques. In previous work, electrical test structure metrology served as the transfer metrology. Recent work has centered on evaluating the performance of these CD reference materials in the metrology tools which we expect will comprise the bulk of their usage: the critical dimension scanning electron microscope (CD‐SEM) and atomic force microscope (AFM). In particular, a critical dimension AFM (CD‐AFM) is particularly useful. This technique uses flared tips and two‐dimensional feedback to allow scanning of near‐vertical sidewalls. It is currently exp...

Comparison of SEM and HRTEM CD Measurements Extracted From Test Structures Having Feature Linewidths From 40 to 240 nm

Critical dimension (CD) measurements have been extracted from SEM and high-resolution transmission electron microscopy (HRTEM) images of the same set of monocrystalline silicon features having linewidths between 40 and 240 nm. The silicon features are incorporated into a new test structure that has been designed to facilitate this type of CD metrology study. Major improvements to previously reported HRTEM sample-preparation and fringe-counting procedures have been implemented. The purpose of this paper is to make a preliminary assessment of the calibration statistics of SEM transfer metrology when HRTEM is used as the primary metrology in CD reference material calibration. The linearity and the correlation of the regression between HRTEM and SEM measurements were very encouraging. However, further study of the calibration statistics, from which uncertainty estimates of the SEM CD measurements were obtained, revealed small but significant test-chip-to-test-chip variability of the SEM-to-HRTEM offset at the low single-digit nanometer level. Further measurements made the case that this unanticipated variability originated in the differences in the amounts of hydrocarbon deposition that were made by the SEM tool during the measurement cycle. This is considered to be a very useful finding because modern SEM tools, which can reduce hydrocarbon deposition below levels that were encountered here by almost an order of magnitude, are now becoming available. The results reported here provide a strong indication that HRTEM-SEM-based calibration approaches offer great promise for single-digit nanometer uncertainty.

Extraction of critical dimension reference feature CDs from new test structure using HRTEM imaging

The National Institute of Standards and Technology (NIST) is completing a project to provide the semiconductor industry with critical dimension CD reference materials, using the silicon (111) lattice spacing as a reference to establish the linewidth. Recent developments include both a new test structure design as well as changes to the high-resolution transmission electron microscopy (HRTEM) sample preparation and fringe counting procedures. These changes contribute to an improvement over earlier work, in which overall uncertainties of 10 nm to 15 nm were observed for approximately 100 nm wide features; in the current work, overall uncertainties of less than 2 nm have been observed for features as narrow as 40 nm.

Measurement of the linewidth of electrical test-structure reference features by automated phase-contrast image analysis

NIST, Sandia National Laboratories, and International SEMATECH are developing a new type of linewidth standard for calibrating Critical Dimension (CD) metrology instruments for lithographic process control. The standard reference feature is the bridge of an electrical linewidth test structure that is patterned in a monocrystalline silicon film. Phase-contrast images of the cross sections of a sample of the bridge features on each wafer, produced by High-Resolution Transmission-Electron Microscopy (HRTEM), are used to trace the measured electrical linewidths of the standard reference feature to the lattice constant of silicon. This paper describes the automated analysis of the phase-contrast images that was developed in order to minimize the cost and uncertainty of the linewidths of the standards.

Process optimization for lattice-selective wet etching of crystalline silicon structures

Abstract. 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.

A new critical dimension metrology for chrome-on-glass substrates based on s-parameter measurements extracted from coplanar waveguide test structures

The technical objective of the work reported here is to assess whether radio-frequency (RF) measurements made on coplanar waveguide (CPW) test structures, which are replicated in conducting material on insulating substrates, could be employed to extract the critical dimension (CD) of the signal line using its center-to-center separation from the groundlines as a reference. The specific near-term objective is to assess whether this CPW-based CD-metrology has sensitivity and repeatability competitive with the other metrology techniques that are now used for chrome-on-glass (COG) photomasks. An affirmative answer is encouraging because advancing to a non-contact and non-vacuum implementation would then seem possible for this application. Our modeling of specific cases shows that, when the pitch of the replicated lines of the CPW is maintained constant, the sensitivity of its characteristic impedance to the CDs of the signal and ground lines is approximately 60 Ω/μm. This is a potentially useful result. For the same implementation, the quantity ∂C/∂w has a value of approximately 45 (pF/m)/μm, which appears to be large enough to provide acceptable accuracy.