Specific Process Knowledge/Characterization/Sample imaging

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Sample imaging

A Danchip 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 wavelenght 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 hight 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. Stres measurements can also be done with the Dektak.



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

Optical microscopes Optical profiler SEM AFM Stylus profiler
Generel description Optical microscope

(several)

Optical profiler

(Sensofar)

Scanning electron microscope

(SEM Supra 60VP, SEM Supra 40VP, SEM-LEO, SEM-FEI, SEM-JEOL)

Atomic force microscope

(NanoMan)

Stylus profiler

(Dektak 8, Dektak XTA)

Operation priciple Light Light
  • Secondary electrons
  • Backscattered electrons
 Atomic forces between tip and sample surface Contact
Sample information 3D surface topgraphy
  • Step height
  • Surface roughness
  • Film thickness
 3D surface topography 3D surface topography (if you make a map scan)
Vertical resolution Cofocal measurements:
  • 10x objective: <50 nm
  • 50x objective, NA 0.95: <1 nm

Interference measurements:

  • PSI: 0.01 nm
  • VSI: 1 nm
 1-20 nm

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
 1-20 nm

Depends on what SEM you use

Down to 1.4 nm
Resolution limitations Objectives and wavelenght 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 (thick polymer, qaurtz or glass samples) can be inspected in the FEI-SEM or one of the Supra-SEMs.
 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
(not possible to inspect entire wafer in JEOL-SEM)
  • One 150 mm wafer
(not JEOL-SEM)
  • One 2000 mm wafer
(only Supra 60VP, 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 measument)
  • 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

  • Graphene and carbon nanotubes
(Only FEI, use special sample holder)
  • Non-conducting samples
(Use low vacuum detector in SEM-FEI or VPSE detector in SEM-Zeiss)
  • 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.





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
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