Originally Published MDDI May 2003

SURFACE CHARACTERIZATION

A variety of scanning electron microscopy techniques are available to medical manufacturers to assist in analyzing and characterizing material surfaces.

by John Humenansky

Scanning electron microscopy (SEM) is a valuable technique for characterizing the surface morphology of all kinds of samples and materials. The well-known expression “a picture is worth a thousand words” describes the value of electron microscopy to the investigation of a number of applications and materials. Among the most notable benefits of SEM are two things: the minimal or nonexistent sample preparation that is required, and the ability to tilt and translate samples while they are under observation. 

Instruments of Electron Microscopy

Scanning electron microscopes come in many forms featuring different types of electron guns and vacuum systems. Each type of SEM has its own unique advantages and disadvantages and requires different instrumentation (see Table I).

Electron microscopes operate in a vacuum. For conventional SEM, which uses either tungsten or lanthanum hexaboride (LaB6) filaments, the main sample requirements are only that it be vacuum compatible and electrically conductive. If a material is not vacuum compatible, however, either critical-point drying or a cold stage can be used to observe the sample. Nonconducting samples can be coated with a thin layer of carbon or precious metal in a thermal vacuum evaporator or plasma sputter-coater. 

Another type of scanning electron microscope, the environmental scanning electron microscope (ESEM), was developed in the late 1980s. The ESEM uses a patented secondary electron detector that makes possible the imaging of wet samples in up to 20 Torr of chamber pressure without any sample preparation at all. The only sample requirement is that it fit inside the specimen chamber.

In an ESEM it is also possible to observe a sample under different conditions, such as during the introduction of gas or water vapor. Additionally, the engineer can use temperature stages for heating or cooling the sample to observe and record reactions or changes in the material under study––this is one example of SEM's usefulness in observing materials under dynamic conditions. Most SEMs in this class sacrifice some resolution in favor of versatility, however.

After ESEM's debut, a low-vacuum (or variable-pressure) microscope (LVSEM), using back-scattered electron signals for imaging, was developed. Using the LVSEM, an engineer can characterize wet samples at pressures as high as 2–3 Torr. These microscopes are ideal for examining all types of samples––especially organic materials––and are usually operated with tungsten filaments.

Creating an Image and Analyzing the Material

In a scanning electron microscope, several types of signals, created simultaneously, can be used for creating an image or determining the chemical composition of the sample material. Detectors convert secondary or back-scattered electrons into electrical signals that are used to form the image. Secondary electrons are used in most types of surface imaging and are sensitive to such changes in surface morphology as pits, cracks, steps, and edges. Back-scattered electrons are an important imaging signal in the SEM because their generation provides atomic number contrast. This means that areas on the sample with materials of a higher atomic number will be brighter.

X-rays are important signals that provide chemical information from a sample by measuring the characteristic energies that are generated wherever the beam strikes the sample. Energy dispersive spectroscopy (EDS) is used to measure the characteristic energy of the x-rays and identify the chemical's composition.

The type of electron gun used largely determines the resolution of an SEM image. The main types of electron guns use tungsten, LaB6, or field-emission sources that contain an etched tungsten crystal.

The images in Figure 1 show the physical characteristics of each type of electron source along with its approximate first-crossover diameter. From these images alone, it is easy to see why the field-emission tip has the smallest first-crossover diameter and, therefore, the best resolution. The first crossover is important in determining what the spot size will be when the electron beam reaches the sample. 

The electron gun is followed by one or more condenser lenses, which demagnify the spot. Scanning coils then systematically scan the spot across the surface of a sample. The final spot is focused with an objective lens that has separate astigmatism correction to ensure that the spot is round when it hits the sample. As the focused spot is scanned across the sample surface, the number of electrons that are generated from that surface will depend on the surface morphology for secondary electrons, or compositional changes for back-scattered electrons. These signals are processed and displayed on a viewing monitor. The magnification is a ratio of the area scanned on the sample to the area displayed on the viewing monitor. 

Obtaining the highest resolution possible is often necessary; this requires the use of a field-emission scanning electron microscope (FESEM). Field- emission guns provide the smallest spot size with high current density, which results in a very high signal-to-noise ratio. The resolution of an SEM image is partially dependent upon the accelerating voltage—resolution is improved at higher accelerating voltages. Because of the small first crossover of an FESEM, lower accelerating voltages can be used, thereby making it possible to look at nonconducting samples without any sample preparation or coating.

Fine surface detail can unwittingly be overlooked in a conventional SEM image because of the large spot size in the thermal emitter. The images in Figure 2 show the difference between the effects of a tungsten thermal emitter source and a field-emission source on the same sample. Many fine details are clearly shown on the field-emission image, which are not seen with the thermal emitter. 
Microstructural defects such as cracks, pits, or steps can easily be overlooked using a thermal emitter source. Higher resolution sometimes can reveal fine surface details that could be important in evaluating a material or characterizing a defect. 

Samples can be observed in many different ways to obtain important information about them. Figure 3 shows an image of an organic coating on a metal substrate, which is observed in cross section. Coating thickness, uniformity, and adhesion to the substrate material are easily characterized. Figure 4 shows an image of an organic coating on a coronary stent. A focused-ion-beam device was used to make a cutout in the coating, making evaluation of the coating thickness and uniformity possible.

Taking Measurements

In addition to observing and characterizing the structure of a sample, SEM can also be used to take precision measurements of very small features. Figure 5 shows an example of a multilayered power cell that is used in portable medical devices. The layers of the rechargeable device undergo physical changes during normal cycling, which SEM can help engineers measure and explain.

In SEM, samples can be prepared and observed in many different ways to aid in the interpretation of the resultant images. In Figure 6, the electron micrographs of a plating defect on a medical device show the utility of tilting a sample. They also show how additional sample preparation methods can be used to provide another view of a defect, which can help in understanding what has happened to cause the failure. More importantly, a structured or rough surface can be better understood when it is viewed at a tilted angle.

Medical Applications

Scanning electron microscopes are routinely used in several applications. The following represent some examples of their uses:

• Failure analysis.
• Material comparison.
• Process evaluation.
• Fatigue and corrosion.
• Surface preparations.
• Contamination identification.
• Product or process marketing. 
• Quality control.
• Characterization of defects.

Medical device engineers can use SEM to characterize surface finish, defects, or chemical composition. Figure 7 shows an example of an orthopedic device made from a polymer material that undergoes a number of surface-finishing steps. SEM is used to evaluate the materials and the processes used in production of the device.

At the very least, SEM is a good choice for a serious first look at a problem or issue following conventional optical light microscopy (OLM) techniques, because of its superior depth of field. The two micrographs in Figure 8 compare an SEM image with an OLM image of the same coronary stent sample. 

Using SEM as a problem-solving tool offers several advantages to manufacturers. For many types of samples, little or no sample preparation is required and large samples can often be accommodated. Surface contamination is easily characterized, as shown in the micrographs taken from a precision medical guidewire and presented in Figure 9.

Among the biggest advantages of sample observation by SEM is the ability to examine relatively large samples and to rotate, tilt, and translate them to obtain near-three-dimensional images. The great depth of field that SEM offers, even at high magnification, makes this possible. It is almost like having a highly magnified piece of the sample in one's hands.

Even if imaging at high magnification and great depth of field were the only feats one could accomplish using SEM, it would still be a useful tool for characterizing surface morphology and various defects. With the addition of an energy dispersive spectrometer (EDS) system, however, SEM becomes a complete analytical tool linking images with chemical composition. EDS systems measure characteristic x-rays generated when an electron beam strikes a sample, and they provide a fingerprint of the material. 

Qualitative analysis is performed to reveal what the material or composition of a sample is, and quantitative analysis is used to tell how much of the material there is. X-ray maps can also be created to show the distribution of elements in a multielement field of view. Each material has its own unique chemical signature, and these spectra demonstrate some typical examples of various materials. 

Because the individual peak heights generated in a multielement material are related to the amount of that material in the sample, quantitative analysis is easily accomplished with modern software programs that take into account accelerating voltage and corrections for atomic number, x-ray fluorescence, absorbance, and others.

Secondary and back-scattered electrons come from each point on the sample where the beam is positioned at any given time. Characteristic x-rays also are generated this way. This makes it possible to take an image of a field of view on a sample, and then, by using x-ray mapping, show the distribution of elements within that field of view.

The images in Figure 10 show the distribution of elements by x-ray mapping from a cross section of an electrode.

Getting the Job Done

SEM and EDS are important enough analytical tools to make it advisable for a manufacturer to perform such analyses in-house. Of course, some companies cannot afford or may not want to dedicate personnel or space to this equipment. But nearly all medical manufacturing companies involved in subminiature assembly, quality control, or failure analysis already routinely use optical light microscopes. An SEM/EDS system is a natural extension of such microscropy, and employees familiar with existing systems can easily learn to operate SEM or EDS equipment. Moreover, used equipment can be purchased for less than $100,000.

For those companies that prefer to contract out laboratory analysis, it's a good idea to establish a relationship with a vendor of laboratory services by being present when the analysis is performed. In doing so, everyone understands the problem areas and the best approach for reaching a solution. 

Often, a contract lab will offer to perform analysis at no charge, to demonstrate the value of using a particular technique. One thing to keep in mind, however, is that the client usually knows a lot more about a sample or particular problem than the analyst who does the work. It is a great benefit to both parties to have the client present when the work is done. A good contract laboratory will encourage this practice.

Conclusion

SEM and EDS are very useful tools for characterizing surface morphology, coatings, defects, and corrosion in just about any type of sample or material. Whether it's evaluating a process engineering step that removes burrs on an orthopedic device, identifying contamination on a precision lead wire, measuring critical dimensions on a small scale, or understanding a failure on a coronary stent, SEM is an ideal imaging tool. 

Copyright ©2003 Medical Device & Diagnostic Industry