Specific Process Knowledge/Characterization/Sample imaging

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Sample imaging

A Danchip a number of instruments are available for sample imaging, including a several optical microscopes, an optical profiler, three SEMs (scanning electron micrscopes), an AFM (atomic force microscope) and two stylus profilers (Dektak). These instruments cover wide range of applications.

One of the advantages of the optical microscopes is that they provide fast and easy accessible information about any sample without any sample preparation. They do, however, also have some limitations. Since the depth of focus is quite limited, especially at high magnifications. Another disadvantage is the physical limit to the resolution that makes it impossible to image structures below 1 µm

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.

• Sample imaging using optical microscopes

• Sample imaging using optical profiler (Sensofar)

• Sample imaging using SEM

• Sample imaging using AFM (NanoMan)

• Sample imaging using stylus profiler (Dektak)

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

	Optical microscopes	Optical profiler	SEM	AFM	Stylus profiler (Detkak)
Generel description	Optical microscopes (several)	Optical profiler (Sensofar)	Scanning electron microscope (Zeiss, LEO, FEI, JEOL)	Atomic force microscope (NanoMan)	Stylus profiler (Dektak 8, Dektak XTA)
Vertical resolution
Horizontal resolution
Resolution limited by	Wavelenght of light		Interaction volume	Tip shape	Stylus shape
Field of view
Probe	Light	Light	Electrons	Forces	Contacts
xxx
xxx
xxx
Batch size	Several small samples One 50 mm wafer One 100 mm wafer One 150 mm wafer Stage size depends on what microscope you use	Several small samples One 50 mm wafer One 100 mm wafer One 150 mm wafer	Several small samples One 50 mm wafer One 100 mm wafer (not possible to inspect entire wafer in JEOL SEM) One 150 mm wafer only Zeiss, LEO and FEI SEM, 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	Silicon,silion oxide, silicon nitride Quartz, polymers and photoresist Metals (except type IV) III-V materials Graphene and carbon nanotubes	Silicon,silion oxide, silicon nitride Quartz, glass Polymers and photoresist Metals (except type IV) III-V materials Graphene and carbon nanotubes	Silicon,silion oxide, silicon nitride Quartz, glass polymers and photoresist (outbaked) Metals (except type IV) III-V materials Graphene and carbon nanotubes (Only FEI, use special sample holder)	Silicon,silion oxide, silicon nitride Quartz, glass Polymers and photoresist Metals (except type IV) III-V materials Graphene and carbon nanotubes	Silicon,silion oxide, silicon nitride Quartz, glass Polymers and photoresist Metals (except type IV) III-V materials Graphene and carbon nanotubes

The list of instruments for sample imaging available at Danchip includes 6 optical microscopes , three scanning electron microscopes (SEM's) and an atomic force microscope (AFM). These instruments cover a wide range of applications.

The 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 strucutures 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.

The optical profiler (Sensofar)

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 scanning electron microscopes

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 milimeters.
• 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.

The atomic force microscope

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