It isn’t always obvious when a NanoScan has a damaged slit. There’s no loud sound and often no obvious problem with the equipment or the results. Since the damage is often confined to a small location on the slit, a NanoScan with a damaged slit will sometimes still generate good results. The problem is, if the beam wanders onto a damaged area of a slit, the measurements are suddenly no longer valid. These changes can be subtle but still can cause erroneous conclusions.
Slit damage is caused by a variety of factors. The most obvious cause is from objects inadvertently inserted into the NanoScan aperture which then come in contact with the spinning slits, causing damage. Many customers continue to ship NanoScans without covering the aperture with a front dust cover; this is a great way to damage slits in shipment.
If the dust cover is lost, a business card or piece of stiff plastic taped to front plate of the NanoScan to protect the slits usually does the job. Of course, exposing the NanoScan to a beam which exceeds the power or energy density threshold of the slit causes thermal damage. Finally, large airborne particulates can lodge into the NanoScan slits, clogging the slits and cause errors in the readings.
To show the subtlety of the issue, note that the profile below was taken with a NanoScan with a damaged slit.
Figure 1. NanoScan with damaged slit.
At first glance, nothing seems to be wrong. The beam is a little elliptical. Along the x-direction, the beam measures 905 microns, along the y-direction 1040 microns. (All beam measurements in this report are defined as 1/e-squared.) However, the customer had good reason to believe the beam was nearly perfectly circular and, sure enough, when the beam was translated only a millimeter within the NanoScan aperture, it measured between 900-905 microns along both axes.
Figure 2. Translating the beam a mm within the NanoScan aperture produced measurements of 900-905 microns along both axes.
Moving the Beam Across the Aperture
This shows the most straightforward way to tell if a NanoScan has a bad slit. Just move the beam across the aperture and see if how the measured beam widths change. They always change a little bit, as the 1/e-squared beam size tolerance is +/- 2% across the aperture. However, if the measured beam width changes by more than this amount as the beam moves through the NanoScan aperture, it’s likely there’s a problem with the slits. Usually at a local area of damage, the beam size becomes larger. That’s because at the damaged area, the local slit width becomes larger and due to the slit convolution effect, the measured beam width increases.
It’s important to note that smaller beam sizes used in this test work better in finding slit damage. A large beam of 3-4 mm will only uncover very large damage, around a millimeter in size. A 100-micron imperfection will largely be missed with a beam larger than 500 microns.
Rotating the NanoScan
Sometimes it’s difficult to translate the beam in the aperture so another simple test of slit damage is to rotate the NanoScan 90 degrees using the rotation mount. If the beam is truly elliptical, the long axis and short axis should “flip” to the other slit after this rotation. However, if after rotation the same slit measures as large a beam as it did before the 90-degree rotation, it usually means the slit is damaged.
In the example above, we found that when rotating the NanoScan 90 degrees, the yaxis continued to measure 1040 microns; if the beam was truly elliptical, it should have measured 905 microns instead. The x-axis continued to measure around 900 microns after this rotation, owing to the fact this slit was not damaged.
A final signature of slit damage is to look at sensor amplifier gain set by the NanoScan Gain Tracking feature. Usually, the gain value for Axis X and Axis Y are about the same. If one of the NanoScan slits is damaged, the gain values become greatly dissimilar, usually differing by more than 20 due to the fact that one slit is allowing much more light through onto the detector than the other.
Figure 3. An example where the sensor amplifier gain values are nearly equal, 47 and 43.
Can I still use a NanoScan if the slit is damaged?
We don’t recommend using a NanoScan if the slit is damaged and those who do live dangerously. However, it is possible with care to move the beam to an unaffected area and perform measurements that way. Obviously, this work-around is fraught with difficulties. However, if you are in a pinch and need results immediately after damaging a slit, directing a beam away from the damaged area normally gives valid results.