Microstructure Characterization

Dektak surface profilometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and ellipsometry are four techniques commonly used to assess the size, morphology and surface topology of microstructure devices. These techniques will find their use in the characterization of fabricated devices in Section 5 and are therefore shortly introduced in the following sections.

3.3.1. Dektak Surface Profilometry

Dektak surface profilometry is a method for assessing surface roughness or step height across devices on a substrate. A stylus in contact with the substrate scans along a straight line while the vertical stylus movement is being recorded and visualized.

The assessment of step heights is performed on a set of specific test structures

especially incorporated in between fabricated devices. When assessing step heights, the values are usually averaged over a set of scanned structures. Depending on the scan speed and step height, the typical vertical resolution in an averaged measurement is 5 nm.

While Dektak surface profilometry provides a relatively fast and accurate method for acquiring information about the surface topology of a sample (albeit only in a single direction), determining the thickness of a generic thin film implies the need for lithographic patterning of the required test structures. Section 3.3.4 discusses an alternate method for acquiring layer thickness information through an optical measurement which does not require layer patterning.

3.3.2. Scanning Electron Microscopy

Scanning electron microscopy (SEM) imaging relies on the collection of secondary (low energy) or backscattered (high energy) electrons resulting from the exposure of a sample to a focused, high energy electron beam. The electrons in this high energy beam are usually generated through thermally assisted field emission from a tungsten or LaB6 filament and are subsequently accelerated by an electric potential and focused onto the sample by magnetic lenses. The scanning nature of SEM is expressed in the beam being deflected by sets of magnetic coils and enables scanning of a square area.

SEM delivers high resolution (1 to 5 nm) images that can be used to assess surface morphology and nanometer scale features present on a device, such as the point contacts that will be etched into an insulating SiO2 layer.

3.3.3. Atomic Force Microscopy

Atomic force microscopy (AFM) is a scanning-probe technique capable of imaging the surface topology of a substrate with the tip of a micrometer scale cantilever as shown in Figure 3-6. Interaction of the tip with the surface leads to a deflection of the cantilever which can be recorded with the use of laser optics. Possible interaction mechanisms between tip and substrate surface include mechanical contact force, Van der Waals forces, capillary forces, chemical bonding forces, electrostatic forces, magnetic forces (exploited in the MFM variant of AFM) and Casimir forces. Instead of moving the tip to scan the substrate surface, the substrate itself is fitted on a piezo-electric scan stage that moves the sample underneath the tip.

A typical AFM scanner can operate in a number of different modes, such as a static (contact) mode and a dynamic (tapping) mode. In the static mode, the interaction of the cantilever tip with the sample is directly recorded through the absolute movement of the tip induced by the substrate topology. In tapping mode, the cantilever tip is

made to oscillate at or near its eigen-frequency. The force acting on the tip can then be deduced from the change in swing amplitude or frequency. AFM offers some specific advantages over SEM, such as sub-nanometer depth resolution (it is in fact a three-dimensional technique) while it does not require a vacuum. On the other hand, an AFM scan of an extended area proceeds slowly and may suffer from associated (thermal) drift. Moreover, in some instances, hysteresis effects occurring in the piezo-electric scanning stage can induce artifacts in the measurements.

Figure 3-6: Simplified overview of an atomic force microscope. A substrate is fitted on a piezo-electric scanning stage so that a micrometer scale cantilever tip can scan the sample surface. The interaction of the cantilever tip with the surface results in a movement of the cantilever, which is recorded using a laser beam and a photodiode.

3.3.4. Ellipsometry

When linearly polarized light reflects from a thin film surface, its polarization changes and becomes elliptic. The induced ellipticity reflects the optical properties of the material being probed. By measuring the ratio of the reflected intensities of plane polarized incident light along with the occurring phase differences, optical constants (refractive index and extinction coefficient) can be extracted. Moreover, the thickness of the layer can be estimated through an iterative regression fitting procedure of the data according to a model based on assumed values of the physical material parameters. This is usually performed through a Levenberg-Marquardt procedure that adjusts the model parameters until an acceptable fit with the recorded data is achieved, after which the model delivers the quantities of interest, such as film

Detector and Feedback Electronics

Photodiode

LASER

PZT Scanner

Sample Surface Cantilever and Tip

thickness and refractive index. For accurate fitting, the exact optical properties of the substrate and deposited layers are extracted from detailed libraries of material constants.

An interesting application of ellipsometry is found in the calibration of the sputter rates for SiO2. Although the thickness of the SiO2 layer can be calibrated in various other ways (e.g. through a selective buffered HF wet etch that stops on a platinum sub-layer or a lift-off procedure in combination with a lithographic mask), an ellipsometric measurement can provide, in addition to the layer thickness, complementary physical parameters such as the refractive index and index of extinction (together making up the complex refractive index) of the material. When they are compared with known values of completely stoichiometric SiO2, the obtained values give an indication of the quality of the oxide, making ellipsometry an interesting method to assess both the quality and thickness of uniform layers of SiO2. For example, an increased refractive index may indicate larger absorption, relating to inferior quality of the SiO2 layer.