Quality control in materials inspection requires the ability to detect both minute surface imperfections as well as those that are beneath the surface. Close examination of samples and specimens is therefore often conducted with a digital imaging microscopy setup. These images are sometimes captured such that they appear true to life, however, to reveal hidden details and to generate images suitable for quantitative analysis, the illumination and contrast must be appropriately set.
This is important across wide-ranging industries, where samples and specimens can vary greatly. For example, glossy, transparent or mirrored surfaces make precise identification of features difficult. A single setup cannot be successfully applied to all these different surfaces and so it is essential to select the best contrast method for each sample and application.
This enables the sensor to precisely detect features for post-capture evaluation and quantification. Ultimately this results in fewer downstream failures of materials and components and an overall improvement in the efficacy of quality control.
The ideal contrast method
Although the use of the ideal illumination and contrast method can reveal more features with greater detail, it can be difficult to determine the best method or combination of methods for each sample and application.
Generally speaking, as most materials inspection samples and specimens are opaque, epi-illumination setups are prevalent. However, the choice of contrast method from the several available can greatly influence image detail and clarity.
These methods include brightfield, darkfield, differential interference contrast (DIC) and polarised light microscopy. Each has its own advantages and, therefore, applications (Figure 1).
Using the common brightfield setup, it is possible to capture images with adequate detail for the examination of samples and specimens for flaws in certain applications. These images appear bright and more true to life than other contrast methods and therefore tend to suit human preferences. They reveal detail in samples that contain materials with different light absorption levels, such as the variety of electronic components on a PCB. However, in samples which contain materials of similar light absorption levels, many flaws could be missed.
This is due to the direct illumination of samples and specimens resulting in much of the light being reflected back up the light path through the objective. As a result, images tend to be bright, but lack contrast due to the minimal shadow produced from shallow features and edges. Minute features, such as nanometre scratches and small contaminants, may not be visible at all, especially if the sample contains a transparent material, such as glass.
Related to this, brightfield images may also suffer from an apparent low optical resolution due to the blur introduced from material outside of the depth of field.
A darkfield microscopy setup can be achieved relatively easily. Many existing microscopes include the ability to switch between brightfield and darkfield while many others can be adapted. Switching is achieved through partially blocking the light path ahead of the condenser using an opaque disk or with specific darkfield mirror assemblies. The result is circular oblique illumination of the sample or specimen.
Rather than reflecting directly back into the objective, with this type of setup light can only enter the objective if scattered from surface features in opaque materials and both surface and subsurface features in transparent materials.
The image produced therefore consists of highlighted features on a dark background. This is especially useful for identifying fine scratches and contaminating particles on smooth surfaces, such as silicon wafers. It can also be used to clearly define raised features to identify any imperfections, for example, conductive lines on a PCB. As such, darkfield is a useful and straightforward method for enhancing contrast on samples and specimens for quality control purposes.
Further enhancement of the contrast can be achieved by setting oblique illumination from specific a direction, rather than from all sides. A similar effect to this directional darkfield (Figure 2) can also be achieved using ringlight illumination systems, creating a pseudo-darkfield image. This can further highlight features by enhancing shadows to create a relief image.
Polarised light microscopy
Polarised light microscopy can be used to amplify signal from features that affect the polarisation of incident light as it is reflected off the surface or when passing through a transparent sample. It is also useful in removing glare from glossy surfaces, known as anti-halation.
A typical polarised microscope setup consists of an initial polarising filter and an analyser, a second polarising filter at the end of the light path, set at a perpendicular angle to the first. Light is polarised before it enters the condenser and then blocked by the analyser if the polarisation is unchanged after reflecting off the sample. This effectively cuts glare and enhances light that has been depolarised by features in the sample.
Polarised light microscopy, therefore, can provide a substantial amount of information on the composition of samples and specimens that may be unobservable using brightfield alone, including thermal history and stress as well as manufacturing inconsistencies. For example, it can reveal distortions or tensions caused by rapid cooling in injection moulded plastics. Polarisation is also essential in differential interference contrast (DIC) microscopy.
Differential interference contrast
Differential interference contrast (DIC) images can highlight minute surface features similar to darkfield, however, the physical principle is different.
A number of additional components produce pairs of light waves, each polarised perpendicular to the other (0º and 90º), by passing light through a polariser and then a Nomarski prism before allowing it to enter the condenser.
In a reflective DIC setup, the objective and condenser are the same element. Angular splitting (or shear) of the light waves from passing through the prism results in adjacent positions being hit on the sample by each light wave of the pair. This allows for a potential phase shift in one light wave relative to the other due to any slight differences in the surface elevation.
If the sample or specimen allows light to penetrate the surface, differences in total travel distance create the phase shift. As both waves return through the objective and pass through the prism, they are recombined, generating a single light wave polarised at 135º ahead of passing through the analyser. This recombination results in interference, producing a brighter or darker point in the image that is proportional to the difference in light path lengths.
DIC can be used as an alternative to specialised scanning electron microscopy (SEM) to generate relief images. It is suited to revealing nanometre imperfections on a surface, such as the polishing direction on a mirrored surface or imperfections and contaminants on transparent materials and the surfaces need not be conductive (Figure 3). Additionally, as the signal is proportional to the phase difference, DIC has the potential to be standardised for use as a measurement tool for nanometre differences in elevation.
Simplifying the decision making process
The selection and use of different contrast methods normally requires specific knowledge and the physical insertion of different components, dependent on the microscopy system that is in place. To combat the issues encountered with selecting the optimum contrast method, and to remove the need to physically modify the microscopy setup every time the contrast method is changed, Olympus has developed the DSX range of opto-digital microscopes.
Consisting of the DSX100, DSX500 and DSX500i (inverted), these complete reflected microscopy systems enable an automated capture of images with different contrast methods using fully integrated hardware. This ‘best image’ function allows for immediate image comparison and selection of the optimal method for the application as well as post-capture processing options for further enhancement. The user interface allows operators at every level of expertise to capture images and select the optimal method, or combination of methods, for their application (Figure 5).
The DSX500 and DSX500i feature brightfield, darkfield, DIC and polarisation modes. Directional darkfield is achieved through individually selectable quadrants in the dedicated darkfield illumination system. These also introduce a ‘MIX’ mode that enables both brightfield and darkfield imaging to be combined, resulting in images with the brightness and true to life characteristics of brightfield and feature detection capabilities of darkfield (Figure 6). The DSX100 features a ringlight brightfield epi-illumination system that can also be used to provide pseudo-darkfield functionality using individually lit quadrants.
The future, with opto-digital microscopes
Optimal image capture and anmation gained from imaging but identifying the most appropriate method for a given sample can be difficult.
With the most important contrast malysis is essential to maintain quality and reliability within industrial inspection. Contrast methods greatly influence the quality and quantity of inforethods integrated and immediately available, the Olympus DSX series provides a complete easy-to-use high-resolution imaging solution that is ideal for industrial applications. These microscopes can also offer a convenient alternative to more specialised systems, such as SEM, which require experience and expertise to generate optimal data. As such, opto-digital microscopes with these types of capabilities are becoming the standard in quality control within industrial inspection.