Minimum Spot Size

Using the above two equations, the minimum spot size 8 can be written as 

for micromachining small spot size, we should have short wavelength, short focal length, and = 1. The laser type influences the beam quality. The He-Ne (A = 0.63 pm), Nd Yag A = 1.06 pm), andC02 laser (10.6 pm) have beam quality equal to 0.98,10, and 1.5, respectively. 

---

Focusing Laser Light. One of the most important properties of laser radiation is the abiHty to coUect all of the radiation using a simple lens and to focus it to a spot. It is not possible to focus the laser beam down to a mathematical point there is always a minimum spot size, set by the physical phenomenon of diffraction. A convenient equation is... 

The ratio F/d is the F number of the lens. For F numbers much less than unity, spherical aberration precludes reaching the ultimate diffraction-limited spot size. Therefore a practical limit for the minimum spot size obtainable is approximately the wavelength of the light. Commonly this is expressed as the statement that laser light may be focused to a spot with dimensions equal to its wavelength. 

Figure 16.3. (a) Scatter plot illustrating the fluorescent intensity measurements in two channels plotted against each other. Note that most measurements fall on the diagonal. Deviation from this pattern at low intensity indicates that the data are skewed due to nonspecific fluorescence in one channel. For analysis, the data should be filtered to exclude these points, (b) After data are filtered for intensity and a minimum spot size (to exclude spurious measurements), an outlier list can be displayed. A spreadsheet containing the data from the entire microarray can also be saved. 

Figure 11.10 A plot of w(z) and the corresponding spatial dependence of the two-photon transition probability for an excitation wavelength of 800 nm and a minimum spot size of 1 pm. Approximately 82% of the two-photon absorption takes place within the Rayleigh range Zr about the focal point at Z = 0, leading to an effective focal volume of ca. 32 x 10 litres (32 fl)...

That being said, the spatial dimensions over which a charged particle beam can be focused (the minimum spot size) is also a function of the space charge effects and the aberrations (primarily spherical and chromatic) suffered. This explains why ... 

As the current of an ion beam is increased, so too is the minimum spot size (this explains why imaging in the microprobe mode, whether in SIMS, SEM, or HIM, must be carried out at very low ion currents). 

Temperature probes are portable spot-measuring instruments that can be a simple combination of an IR thermocouple and a readout device. They are characterized by a low price (from under 100 to 1,500), pocket portability, and a wide collecting angle. They are battery powered and are generally optically preadjusted for minimum spot size at a short working distance (a 1/4-in. spot at a 3/4-in. working distance is typical). Some models are designed to operate in a conventional... 

This instrument has a resolution of 0.6 nm at 5keV. Correction decreases the minimum spot size at IkeV from 18 nm to 2nm, at the same time increasing the beam current by 30x... 

The attainable resolution of the technique is limited by the minimum spot size that can be obtained with the incident electron beam, and ultimately by the scattering of this beam as it interacts with the substrate. With modern instruments, a resolution of better than 5 nm is achievable. This is more than adequate for imaging semiconductor device structures, for example, but is insufficient to enable many supported metal catalysts to be studied in any detail. 

The minimum spot size of a laser beam determines the size of the structure that can be micromachined. Figure 10.32 shows the schematic of a laser beam being focused by a... 

---