Specific Process Knowledge/Characterization/Sample imaging

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Sample imaging

Deposition of silicon nitride can be done with either LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition). Stiochiometric nitride or silicon rich (low stress) LPCVD nitride is deposited on a batch of wafers in a LPCVD nitride furnace, and PECVD nitride (or oxynitride) is deposited on a few samples at a time in a PECVD system. LPCVD nitride has a good step coverage and a very good uniformity. Using PECVD it is possible to deposit a thicker layer of nitride on different types of samples, but the nitride does not cover sidewalls very well.

• Sample imaging using optical microscopes

• Sample imaging using optical profiler

• Sample imaging using SEM

• Sample imaging using AFM

Comparison of optical microscopes, optical profiler, SEM and AFM for sample imaging

	Optical microscopes	Optical profiler	SEM	AFM
Generel description	Optical microscopes (several)	Optical profiler (Sensofar)	Scanning electron microscope (Zeiss, LEO, FEI, JEOL)	Atomic force microscope (NanoMan)
Stoichiometry	Si3N4 SRN (only 4" nitride furnace) Si3N4: Stoichiometric nitride SRN: Silicon rich (low stress) nitride	SixNyHz SixOyNzHv Silicon nitride can be doped with boron, phosphorus or germanium
Film thickness	Si3N4: ~50 Å - ~1400 Å SRN: ~50 Å - ~2800 Å Thicker nitride layers can be deposited over more runs	~40 nm - 10 µm
Process temperature	780 oC - 845 oC	300 oC
Step coverage	Good	Less good
Film quality	Deposition on both sides og the substrate Dense film Few defects	Deposition on one side of the substrate Less dense film Incorporation of hydrogen in the film
KOH etch rate (80 oC)	Expected <1 Å/min	Dependent on recipe: ~1-10 Å/min
BHF etch rate	Very low	Very high compared the LPCVD nitride
Batch size	1-25 100 mm wafers 1-25 150 mm wafers (only 6" furnace) Depending on what furnace you use	Several smaller samples 1-several 50 mm wafers 1-3 100 mm wafers 1 150 mm wafer Depending on what PECVD you use
Allowed materials	Silicon Silicon oxide Silicon nitride Pure quartz (fused silica) Processed wafers have to be RCA cleaned	Silicon Silicon oxide (with boron, phosphorous and germanium) Silicon nitrides (with boron, phosphorous and germanium) Pure quartz (fused silica)

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