Joseph Fu

Hybrid bilayer membranes in air and water: infrared spectroscopy and neutron reflectivity studies.

In this report we describe the fabrication and characterization of a phospholipid/alkanethiol hybrid bilayer membrane in air. The bilayer is formed by the interaction of phospholipid with the hydrophobic surface of a self-assembled alkanethiol monolayer on gold. We have characterized the resulting hybrid bilayer membrane in air using atomic force microscopy, spectroscopic ellipsometry, and reflection-absorption infrared spectroscopy. These analyses indicate that the phospholipid added is one monolayer thick, is continuous, and exhibits molecular order which is similar to that observed for phospholipid/phospholipid model membranes. The hybrid bilayer prepared in air has also been re-introduced to water and characterized using neutron reflectivity and impedance spectroscopy. Impedance data indicate that when moved from air to water, hybrid bilayers exhibit a dielectric constant and thickness that is essentially equivalent to hybrid bilayers prepared in situ by adding phospholipid vesicles to alkanethiol monolayers in water. Neutron scattering from these samples was collected out to a wave vector transfer of 0.25 A(-1), and provided a sensitivity to changes in total layer thickness on the order of 1-2 A. The data confirm that the acyl chain region of the phospholipid layer is consistent with that observed for phospholipid-phospholipid bilayers, but suggest greater hydration of the phospholipid headgroups of HBMs than has been reported in studies of lipid multilayers.

Accurate dimensional metrology with atomic force microscopy

Atomic force microscopes (AFMs) generate three dimensional images with nanometer level resolution and, consequently, are used in the semiconductor industry as tools for sub-micrometer dimensional metrology. Measurements commonly performed with AFMs are feature spacing (pitch), feature height (or depth), feature width (critical dimension), and surface roughness. To perform accurate measurements, the scales of an AFM must be calibrated. We have designed and developed the calibrated AFM (C-AFM) to calibrate physical standards for other AFMs. The C- AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a z- axis piezoelectric actuator with an integrated capacitance sensor. This capacitance sensor is calibrated with a third interferometer. We have performed both pitch and height measurements for external customers. Recently, we performed pitch measurements on holographic gratings as part of an ongoing international comparison driven by BIPM (Bureau International des Poids et Measures). We have also completed a preliminary design of a prototype pitch/height standard and are evaluating preliminary test samples. Additionally, we are working toward the development of linewidth standards through the comparison of C-AFM width measurements with values obtained from other methods. Our step height and linewidth measurements are in good agreement with those obtained by other methods, and we are working to improve the lateral resolution and hence the uncertainty of our probe-based linewidth measurements by studying the use of nanotubes and other types of sharp tips as linewidth probes.

Algorithms for calculating single-atom step heights

Recently, our work on the measurement of Si(111) single atomic steps has prompted us to investigate the algorithm for the calculation of a one-sided step height. We compared the results of a two-point subtraction and a histogram technique under different conditions of surface tilt with respect to the measuring frame. By evaluating a simulated Si(111) atomic step, we found its calculated height could deviate from the true value as high as 2% due to a misalignment of the measuring axis and sample axis of 0.1°.

Line edge roughness metrology using atomic force microscopes

Line edge roughness (LER) measurements using two types of atomic force microscopes and three types of tips are compared. Measurements were made on specially prepared samples with inscribed edge roughness of different amplitudes and wavelengths. The spatial wavelengths and amplitudes each instrument was able to measure are compared. Techniques on checking the noise level of LER measuring instruments are highlighted.

Dimensional metrology with the NIST calibrated atomic force microscope

AFMs are increasingly used in the semiconductor industry as tools for sub-micrometer dimensional metrology. The scale of an AFM must be calibrated in order to perform accurate measurements. We have designed and developed the calibrated AFM (C-AFM) at the NIST to calibrate standards. Specifically, our primary calibrations are expected to be of combined pitch/height, or 3D magnification standards for AFM. THe C-AFM has metrology traceable to the International System of Units meter for all three axes. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a z-axis piezoelectric actuator with an integrated capacitance sensor. Our first pitch measurements for an outside customer were recently compete, in which we were able to report relative expanded uncertainties as low as 1 percent on sub- micrometer pitches. The uncertainty budget for these measurements includes the effect of sample non-uniformity, which is the dominant contribution for some of the reported uncertainties. Four samples were measured - two with grid patterns and two with grid recently made considerable improvements in our uncertainty budget for step height measurements. For example, we recently achieved 0.2 nm expanded uncertainty on a 20 nm step, and achieved 0.008 nm expanded uncertainty in the measurement of the approximately 0.3 nm single atom step on Si. We also participated in the recently competed first round of the NIST linewidth correlation project, in which linewidht measurements obtained from different methods are compared. In this paper, we will report on the current status of the C-AFM, and on our plans for further development.

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.

Long‐range scanning for scanning tunneling microscopy

We report a scanning tunneling microscope (STM) with 500 μm×500 μm field of view. It departs from past designs in that a long‐range X‐Y stage carries the specimen and scans while the STM head is held stationary. The STM head is capable of scanning with a range of 8 μm. Combining the capability of tip scanning and X‐Y stage scanning yields a wide dynamic range and has useful applications for measuring optical surfaces.

In Situ Testing and Calibrating of Z-Piezo of an Atomic Force Microscope

By scanning a slightly tilted, smooth surface with an atomic force microscope (AFM), it is possible to obtain hysteresis loops which contain information on the nonlinearity and hysteresis in the z axis of the AFM’s piezoelectric actuator. A 15% variation in vertical sensitivity was revealed by this PZT tube during vertical scans ranging in amplitude between 0.4 and 2.5 μm, which could result in a high level of uncertainty in a vertical measurement. Therefore a separate vertical measuring system or a correction scheme is required for a precise and accurate measurement.

Multilaboratory comparison of traceable atomic force microscope measurements of a 70-nm grating pitch standard

The National Institute of Standards and Technology (NIST), Advanced Surface Microscopy (ASM), and the National Metrology Centre (NMC) of the Agency for Science, Technology, and Research (A*STAR) in Singapore have completed a three-way interlaboratory comparison of traceable pitch measurements using atomic force microscopy (AFM). The specimen being used for this comparison is provided by ASM and consists of SiO2 lines having a 70-nm pitch patterned on a silicon substrate. For this comparison, NIST used its calibrated atomic force microscope (C-AFM), an AFM with incorporated displacement interferometry, to participate in this comparison. ASM used a commercially available AFM with an open-loop scanner, calibrated with a 144-nm pitch transfer standard. NMC/A*STAR used a large scanning range metrological atomic force microscope with He-Ne laser displacement interferometry incorporated. The three participants have independently established traceability to the SI (International System of Units) meter. The results obtained by the three organizations are in agreement within their expanded uncertainties and at the level of a few parts in 104.

Measurement of pitch and width samples with the NIST calibrated atomic force microscope

Because atomic force microscopes (AFMs) are capable of generating three dimensional images with nanometer level resolution, these instruments are being increasingly used in many industries as tools for dimensional metrology at sub- micrometer length scales. To achieve high accuracy, the scales of an AFM must be calibrated. Presently available standards for this purpose are commonly calibrated using stylus instruments and optical techniques. We have developed the calibrated AFM (C-AFM) in order to calibrate pitch and height standards using an AFM. Our instrument has metrology traceable to the wavelength of light for all three axes. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a digital-signal-processor based closed-loop feedback system to control the x-y scan motion. The z-axis translation is accomplished using a piezoelectric actuator with an integrated capacitance sensor, which is calibrated using a heterodyne laser interferometer. When fully developed, this instrument will be a calibration tool for pitch and height standards for scanning probe microscopes. We have recently completed a reevaluation of the titling motions of the C-AFM scanner. This has allowed a refinement in our estimate of the Abbe error contribution to our measurement uncertainty. Our most recent pitch measurements are consistent with this new estimate and thus support our refined uncertainty budget. We have recently completed measurements of pitch on several samples, including both grid type and linear scale patterns, for an industrial user. We are also working toward the development of linewidth standards through the comparison of C-AFM width measurements with values obtained from other methods, including an electrical resistance techniques. In this paper, we will describe the current status of the C-AFM, discuss the use of the instrument for measurements of pitch and width, and describe our plans for future measurements.