Specific Process Knowledge/Characterization/Sample imaging

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Sample imaging

In the cleanroom at DTU Nanolab a number of instruments are available for sample imaging, including several optical microscopes, and optical profiler, a number of SEMs (scanning electron microscopes), an AFM (atomic force microscope) and two stylus profilers (Dektak).

The optical microscopes provide fast and easy information about most samples without sample preparation. The resolution is limited by the objectives and wavelength of the light. Also the depth of focus is limited, especially for higher magnifications.

The main purpose of the optical profiler is to obtain 3D images of different samples and to measure surface roughness or step heights, also for structures with high aspect ratio. Two different types of measurements can be done - confocal and interference (phase shift and vertical scanning interference) measurements. It is possible to measurement height aspect ratio structures. The resolution is limited by the objectives and the pixel size on the screen.

The SEMs are used for inspection of different sample. The resolution is very good - It is possible to obtaion good images of structures smaller then 100 nm with all SEMs in the cleanroom. Samples can be either flat or tilted.

The optical profiler provides standard microscope imaging, confocal imaging, confocal profiling, PSI (Phase Shift Interferometry), VSI (Vertical Scanning Interferometry) and high resolution thin film thickness measurement on a single instrument. The main purpose is 3D topographic imaging of surfaces, step height measurements in smaller trenches/holes than can be obtained with standard stylus method, roughness measurements with larger FOV than the AFM, but less horisontal resolution.

The AFM is used for inspection of nanoscale structures and for surface roughness measurements. The vertical resolution is very good. The horizontal resolution is limited by the tip, but it is possible to buy special tips for high aspect ratio structures. The scan speed is slow, and the field of view is very limited, so it is only possible to get information about a small area of the sample.

The Dektak is is stylus profiler. A step height measurement can be done very fast in a line scan. The vertical resolution is very good, but the horizontal resolution and the aspect ratio you can measure are limited by the tip. Stress measurements can also be done with the Dektak.

Imaging Equipment:

• Optical Microscopes

• Optical Profiler (e.g. Sensofar S Neox)

• Scanning Electron Microscopy (SEM)

• Atomic Force Microscopy (AFM)

• Stylus Profiler (e.g Dektak XTA)

Comparison of optical microscope, optical profiler, SEM, AFM and stylus profiler for sample imaging

	Optical microscopes	Optical profiler	SEM	AFM	Stylus profiler
Operation priciple	Light	Light	Detection of Secondary electrons Backscattered electrons	Atomic forces between tip and sample surface	Contact
Sample information		3D surface topography Step height Surface roughness Film thickness	Structure dimensions Surface topography Material composition	3D surface topography	3D surface topography (if you make a map scan)
Vertical resolution		Confocal measurements: 10x objective: <50 nm 50x objective, NA 0.95: <1 nm Interference measurements: PSI: 0.01 nm VSI: 1 nm	1-100 nm (lowest resolution for the SEM Tabletop 1) Depends on what SEM you use	< 1Å
Horizontal resolution		Three times pixel-to-pixel distance: 10x objective: 4.95 μm 100x objective: 0.495 μm	1-100 nm Depends on what SEM you use	Down to 1.4 nm
Resolution limitations	Objectives and wavelength of light	Objectives and pixel size on the screen	Interaction volume	Tip shape (standard tips: width 10 nm, angle 45o). It is possible to buy Super Sharp tips and High Aspect Ratio tips	Stylus shape (width 5 μm, angle 45o)
Magnification
XY range/field of view		10x magnification: 1.27 × 0.995 mm 100x magnification: 0.127 × 0.095 mm	Depending on detector, working distance and magnification	Camera: 150 × 675 μm X-Y scan range: Up to 90 × 90 μm
Z range				1 µm (can go up to 6µm with special settings)
Working distance	Few mm Depends on what objective you use	10x objective: 17.5 mm 50x objective, NA 0.95: 0.3 mm	3 mm - 20 mm	Contact/tapping measurement	Contact measurement
Sampling speed				Depends on FOV and image resolution: FOV: 10-90 µm ~8:30 min (256x256 pixels) FOV: -10 µm ~4:15 min (256x256 pixels)
Sample requirements	None		Samples have to be (semi)conducting, but may have a thin (> ~ 5 µm) layers of non-conducting materials on top. Non-conducting samples can be studied using VP (variable pressure) mode in the SEM Suora 1, 2 and 3	Sample dimensions have to be smaller than stylus dimensions	Sample dimensions have to be smaller than tip dimensions
Batch size	Small samples One 50 mm wafer One 100 mm wafer One 150 mm wafer One 200 mm wafer Stage size depends on what microscope you use	Small samples One 50 mm wafer One 100 mm wafer One 150 mm wafer	Small samples One 50 mm wafer One 100 mm wafer One 150 mm wafer (not possible to inspect entire wafer in SEM Supra 1 and 3) One 200 mm wafer (only SEM Supra 2, not possible to inspect entire wafer)	One small sample One 50 mm wafer One 100 mm wafer One 150 mm wafer	One small sample One 50 mm wafer One 100 mm wafer One 150 mm wafer
Allowed materials	All standard cleanroom materials (optical measurement)	All standard cleanroom materials (optical measurement)	All sample materials, expect: Samples that may disintegrate, produce dust/particles or degas (e.g. wet polymers and powders). Samples with resist or polymer should be properly baked and outgassed before SEM inspection	All standard cleanroom materials, except samples that might damage or stick to the tip.	All standard cleanroom materials, expect samples that might damage or stick to the tip.

Optical Microscopes

There is a lot of optical microscopes scattered around in the cleanroom because they are in great need. They are useful if, for instance, you need to

• inspect the quality of UV exposed photoresist when doing photolithography,
• check for particles on wafers that have been processed in the furnaces or the PECVD's,
• check the quality of KOH etched structures or
• generally verify any in batch process.

Using the different options such as bright/dark field, polarizer or transmitted/reflected light one can find a better signal for a specific need. Some of them have a camera that allows you to capture and store images.

One of the advantages of the optical microscopes is that they provide fast and easy accessible information about any sample without any kind of sample preparation. They do, however, also have some limitations. Since the depth of focus is quite limited, especially at high magnifications, one will experience problems when trying to image structures that have been etched more than some 10 µm: One cannot focus on both the top and the bottom at the same time. Another disadvantage is the physical limit to the resolution that makes it impossible to image structures below 1 µm.

Optical Profilers

An optical profiler, f.ex. Sensofar S Neox, provides standard microscope imaging, confocal imaging, confocal profiling, PSI (Phase Shift Interferometry), VSI (Vertical Scanning Interferometry) and high resolution thin film thickness measurement on a single instrument.

The main purpose is 3D topographic imaging of surfaces, Step height measurements in smaller trenches/holes than can be obtained with standard stylus method, roughness measurements with larger FOV than the AFM, but less horisontal resolution.

Scanning Electron Microscopes (SEM)

Both shortcomings of the optical microscopes mentioned above are addressed by the use of a beam of electrons (as you do in a SEM) instead of light. The depth of focus and the resolution of a scanning electron microscope are at least one order of magnitude better. The list of advantages of a SEM compared to an optical microscope includes:

• Much better depth of focus: Depending on the image setup it may be on the order of millimeters.
• Much better resolution: Down to a few nanometers.
• Much higher magnifications possible: Up to 500.000 times on some samples.
• Quantification: As a metrology instrument the SEM is absolutely necessary.
• The stage: It allows you to image your sample from almost any angle.
• Tunability: One can tune the image in a number of ways in order to enhance topography or material contrast.
• Elemental analysis: The EDX detector allows you to make detailed investigation of the sample composition.

The SEM is, however, much more complicated in terms of

• Operation: You need training and it takes some experience and skill to obtain good images.
• Hardware: In order to work the SEM needs a chamber under vacuum and sophisticated electronics.
• Sample preparation and mounting: You may have to prep your sample in several ways, either coating, cleaving or mounting on specific sample holders.

Atomic Force Microscope (AFM)

The atomic force microscope has limited use as a sample imaging instrument. In some cases the resolution of the SEM is not enough:

• Nanometer sized particles on a surface
• If you need to know the exact height (z) of some surface structures. The SEM only measures lateral (x,y) distances precisely.
• Surface roughness