Nanoscale Components Require Specialized Microscopy Techniques
The development of new materials and components requires new characterization techniques not only in the research and development stage but also in quality assurance. Two areas of technology that are growing in importance are thin film coatings and microelectromechanical systems (MEMS), each of which present unique challenges to achieve fast, accurate and reliable characterization. Carl Zeiss offers solutions in both of these applications, allowing fast, precise and reliable analysis.
MEMS are microscopic mechanical devices used as sensors, actuators and switches, such as airbag sensors, digital displays, optical switches and inkjet printheads. MEMS fabrication technologies include surface micromachining (structural layers on the surface), bulk micromachining (machining through the substrate) and LIGA (lithography, electroplating and molding) deposition of material into a mold.
Device complexity (MEMS devices can have more than 7 layers of metal on advanced IC technologies) challenges the capabilities of optical inspection due to the limited depth of focus of optical microscopes. Vertical surfaces with extended dimensions and moving components further complicate inspection.
Three technologies that have been offered as solutions are: (1) true high depth of focus imaging using a scanning electron microscope (SEM), (2) laser scanning confocal microscopy, and (3) software-based systems. High depth of focus imaging using SEM exposes the MEMS device to high vacuum and destructive electron beams. Laser scanning confocal microscopy extends the depth of focus by producing a stack of many different images, which are then processed. Similarly, software-based systems also require post processing of images and both result in time-consuming delays.
These limitations and lack of tilt capability and the desire for images having SEM depth of focus led Carl Zeiss to develop DeepView, an optical real-time inspection technique. DeepView is a technique that increases the depth of focus by a factor of about 18. It addresses the technological challenges posed for optical microscopy of MEMS community giving real-time SEM-like depth of focus without the drawbacks of electron microscopy.
Extending the depth of field makes it possible to look at difficult specimens, such as fracture surfaces and uneven samples. The technique also allows users to tilt samples while viewing them at high magnifications. The DeepView principle involves placing a wave front modulator close to the back focal plane of the objective lens, which makes the optical system invariable to focus (Fig 1). The primary intermediate image now contains raw optical information from structures in conventional focus as well as from structures from above and below. After spatial filtering (deconvolution), a clearly defined microscopic image is created on the monitor in quasi-real time and with almost diffraction-limited quality (Fig. 2). DeepView is an ideal solution for real-time inspection of components in development and production because the entire depth of structures is visualized as a single image in quasi-real time, eliminating tiring refocusing of the microscope. An increase of up to 18 times better depth of focus than that using conventional optical focus is possible.
The combination of a Zeiss Axioskop 2 MAT microscope fitted with DeepView provides an optical microscope having a high depth of focus, long working distance objectives, achievable video rate imaging, achieving optimum results with little operator knowledge and intuitive operation (not significantly different than using conventional optical microscopy methods such as BF, DF and DIC).
Thin-film technology also presents characterization and analysis challenges. One of the latest thin-film coating techniques is pulsed laser deposition (PLD), a physical vapor deposition (PVD) technique. Also called laser ablation, high-power laser pulses are used to evaporate matter from a target surface such that the stoichiometry of the material is preserved in the interaction. The resulting supersonic plume of particles is ejected and deposited onto the target surface. PLD is a very flexible technique, which also allows multiconstituent and oxide ceramic materials to be deposited very easily.
The development of new thin-film coatings, particularly on new target materials, requires accurate determination of the deposition rate, which directly equates to layer thickness. Measurements of layer thickness during the process are the prerequisite for successful layer development. This is especially important in the case of optical applications (e.g., interference filters), where the optical properties of the construction element are mainly determined by the film thickness of the deposited material. At the same time, it is important to maintain a homogeneous layer thickness over the total area to guarantee proper manufacturing and operation.
Thin-film characterization requires:
- Quick determination of layer thickness (from nm to several microns)
- Easy evaluation
- Examination of layer thickness over large areas
- No complicated structuring steps
- Measurement by optical transmission
- Additional information on the surface
Carl Zeiss developed the total interference contrast (TIC) technique for thickness measurement to address the measurement issues of PLD, which has advantages over conventional methods. Contrary to traditional polarization interferometers, work is carried out in circular-polarized rather than in linear-polarized light, which enables rotation of the TIC prism without alteration of the contrast of the interference pattern (Fig. 3). Stage rotation, which is necessary in conventional techniques, is not necessary. This is of particular advantage for the imaging and measurement of specimen structures found in various directions; i.e., differently aligned specimen structures can be presented successively in high contrast and measured. Differences in height in specimen structures are determined by measuring the shift in the interference pattern.
TIC is very easy to use and, unlike traditional polarization interferometers, is not sensitive to vibration. However, the major benefits of this technique over conventional layer thickness measuring devices, such as profilometers or scanning force microscopes, are fast evaluation times, easier specimen preparation and the analysis of large-area specimens. The effectiveness of TIC as a combined analysis system and layer thickness measuring device has been shown in the development of highly refractive, thin layers of TiO2 and ZrO2.
During the development of TiO2 and ZrO2 layers by means of PLD, researchers used Axioskop 2 MAT with TIC for layer analysis. This microscope is used to investigate the surface morphology of the layers and also to determine layer thickness. TIC images in reflected light mode with evaluated layer thickness is shown in Fig. 4a (TiO2, 107 nm) and Fig. 4b (ZrO2, 154 nm). Supplementary investigations were carried out on these layers with a profilometer and a scanning force microscope (AFM), which verified the TIC measurements and proved the high precision of the TIC process.
Figs. 4a and b clearly show there is no need for precise specimen structuring to determine layer thickness; a random edge course is sufficient. In addition to layer thickness, the material-dependent phase jump in light-reflection applications can be determined. This is a key property in optical applications. Measurements on the amorphous TiO2 and ZrO2 layer systems show there are phase jumps in the case of reflection of 136¿ for TiO2 and 159¿ for ZrO2.
The Axioskop 2 MAT with TIC has great potential as a combined analysis system (optical microscope and layer thickness measurement device), particularly in thin-film engineering.