Specific Process Knowledge/Characterization/Sample imaging

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.

What kind of instrument to use

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.

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.
• 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

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 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 image that one gets from a SEM

	Optical microscopes	SEM	AFM
Magnification range	25x to 1000x	100x to 500.000x	Maximum scanned area 90x90 µm
Depth of focus	Low	Very high