Metrology

Metrology is the "science of measurement and its application".^[1]

Metrology and characterization of micro/nanostructures and materials requires a broad range of technologies and techniques. The diagram and outline below can be used to help guide you to the technology best suited to provide the measurement or information you desire. Metrology and characterization can be used to explore the following three functional areas:

Surface Topography and Physical Dimensions
Material Composition
Material Physical Properties

Technologies/Techniques by Function

Surface Topography and Physical Dimensions

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is a method for high-resolution imaging of surfaces. The SEM uses electrons for imaging, much as a light microscope uses visible light. The advantages of SEM over light microscopy include much higher magnification (~1,000,000X) and greater depth of field up to 100 times that of light microscopy. Qualitative and quantitative chemical analysis information is also obtained using an energy dispersive x-ray spectrometer (EDS) with the SEM.^[2]. SEM is often used to observe the cross sectional detail of a structure by cleaving the sample and mounting it on edge.

Atomic Force Microscopy

An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip cause the cantilever to deflect towards the surface. However, if the cantilever is brought even closer to the surface, such as the tip makes contact with it, increasingly repulsive force takes over and causes the cantilever to deflect away from the surface. A laser beam is used to detect the cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes which generates the surface map.

Atomic force microscopy (AFM) is a high resolution scanning probe technology capable of nanometer resolution. It is most commonly used for mapping fine surface topographic features in tapping or AC mode, but there are several other modes such as, electrical, piezo, magnetic, Kelvin Probe, force; where resistivity, piezoelectric charge coefficient d₃₃, magnetic features, relative work-function, adhesion forces or cantilever displacement, can be measured respectively.

Stylus profilometry

Stylus profilometry brings a stylus tip in contact with a sample surface to trace out the height of surface topography to allow for measurement of feature heights and lateral dimensions. This technique is less sensitive than Atomic Force Microscopy and is capable of accurately measure feature step heights greater than ~50 nm. It is limited by the physical dimensions of the stylus and can only be used for feature heights less than 100 μm and maximum trench feature aspect ratios of ~1. For taller features or higher aspect ratio trench features, see Optical 3D profilometry below.

Optical 3D profilometry

Optical profilometry requires the surface under analysis to be reflective. If your sample or layers on your sample are transparent, this may cause issues with using these techniques. In that case, Stylus Profilometry or Atomic Force Microscopy may be the better method to use.

Scanning white light interferometry (SWLI)

SWLI is used for non-contact optical profiling. It utilizes interference generated when white light reflected from a sample surface recombines with a reference beam. The interference pattern is analyzed to allow for mapping of a sample's surface topography. This technique is capable of trench feature aspect ratios > 1 and can be used to measure feature heights that are larger than the 150 μm limit for stylus profilometry. This measurement technique is most frequently used for measuring the depth of high aspect ratio trenches etched using the Deep Reactive Ion Etch (DRIE) tools.

Confocal laser optical profilometry

This is also a non-contact optical profiling method, however, this technique is based on a confocal microscope configuration using high numerical aperture objectives with narrow depth of focus to map surface topography. It provides similar data as scanning white light interferometry (SWLI) (see above). The LNF confocal laser profiler tool is the Olympus OLS 4000 LEXT. It offers better lateral resolution than the LNF SWLI tool (the Zygo), however the Zygo is capable of better vertical resolution. LEXT lateral resolution is 0.12 μm versus 0.64 μm at best for the Zygo. The Zygo has vertical resolution down to 0.1 nm versus 10 nm resolution for the LEXT.

Optical microscopy

Standard optical microscopy

The LNF offers a number of standard optical microscopes for inspection of samples and to take measurements of lateral features in the μm range. In addition, there are also the advanced, specialized microscopes described below.

Fluorescent microscopy

This versatile fluorescent microscope has transmission, reflection, and metal halide light sources. It is capable of darkfield, brightfield, DIC, polarized light, and fluorescence microscopy. This microscope is perfectly suited for fluorescent work in microfluidic channels.

Infrared microscopy

An IR microscope uses infrared light to examine wafers or samples. This is particularly useful for examining samples (like silicon) which are opaque in the visible light spectrum (~390 nm to ~700 nm) but have some level of transmission in the infrared spectrum. This allows for non-destructive through wafer inspection.

Thin Film Thickness Measurement

Ellipsometry

Ellipsometry uses elliptically polarized light reflected off of the measurement sample to allow for extraction of both thickness and optical constants of transparent and semi-transparent thin films. This technique is more powerful than spectroscopic reflectometry (described below) where only film thicknesses can be extracted using fixed sets of optical constants vs. wavelength saved in material files. Our ellipsometer has a relatively large spot size of 3 mm by 1.2 cm standard and 300 μm by 1.2 mm with focusing optics.

Spectroscopic reflectometry

Spectroscopic reflectometry analyzes light intensity vs. wavelength for light reflected off of the measurement sample to allow extraction of thin film thicknesses of transparent and semi-transparent thin films. It is a fast and easy to use technique. With an automated mapping stage, this is the method of choice for collecting multiple site data sets to analyze within sample thickness variation. Our NanoSpec 6100 offers a minimum spot size of 25 μm diameter which allows for measurements on patterned samples. The optical constants of the thin film(s) can not be measured with this technique. Fixed optical constants vs. wavelength are stored in material files and are used in modeling to extract thin film thicknesses. If measurement of optical constants is needed, see ellipsometry above.

Material Composition

Energy Dispersive X-ray Spectroscopy

The LNF scanning electron microscope (SEM) is equipped with an EDS (aka EDX or XEDS) (energy dispersive x-ray spectroscopy) unit which allows for elemental composition analysis of the sample's surface.

FTIR - Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) is used to analyze absorption behavior of samples in the IR wavelength range. This information can be used to understand sample composition and bonding configuration. The FTIR available at LNF has spectral range from 9000 to 700 cm^-1 wave number (1111 to 14285 nm wavelength) and can be used to collect reflection or transmission spectra.

Material Physical Properties

Ellipsometry for Optical Constants

Ellipsometry can be used to extract the optical constants (n,k or e1,e2) for materials. It uses elliptically polarized light reflected off of the measurement sample to allow for extraction of both thickness and optical constants (n,k) of transparent and semi-transparent thin films.

Atomic Force Microscopy

Our atomic force microscope (AFM) offers a number of advanced sensing modes including: TUNA tunneling AFM (to analyze conductivity), Piezo-response microscopy (PFM), Torsion resonance (TR-TUNA), Electrostatic Force Mode (EFM), Magnetic Force Mode (MMF), and Surface Potential Microscopy (Kelvin Probe).

Four Point Probe for Sheet Resistance

A four point probe measures resistive properties of thin films (typically sheet resistance). It utilizes two probes to force a current through the film and the other two probes to measure voltage with a high impedance volt meter to avoid including contact resistance in the measurement. This technique is for relatively large samples as the probe spacing for our instrument is 1.55 mm, making the probe head width 4.65 mm.

Thin Film Stress Measurement

Thin film stress is calculated based on the change in radius of curvature of a circular substrate caused by the deposition of a thin film. This requires measurement of the radius of curvature before and after film deposition. The Flexus measures radius of curvature using a laser and detector. This requires a reflective substrate. For transparent or non-reflective substrates/materials, the Dektak XT can also be used for this measurement.

Contact Angle Measurement

The contact angle is the angle measured where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation.

Additional metrology resources at University of Michigan

Additional characterization capabilities (TEM, FIB, XPS, etc) are available at the U-M Michigan Center for Materials Characterization (MC2).

Figures of merit

Precision - closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions^[1] (repeatability)
Accuracy - closeness of agreement between a measured quantity value and a true quantity value^[1]
Resolution - smallest change in a quantity being measured that causes a perceptible change in the corresponding indication^[1]