Silicon film-thickness profiles

Clean substrates of silicon were used to entrain liquid films of various compositions in a dip-coating geometry, as in Figure 1 with a = 0. Interference images were obtained in reflection with monochromatic light. The liquid index of refraction and the angle of incidence were known, so the film thickness profiles were obtained from the position of the interference fringes. 

Zink et al. used a blend of polystyrene (hPS) and its deuterated counterpart (dPS), both of molecular weight 1.95 x 106 (abbreviated 1.95 M). The average volume fraction (4>dPS) of deuterated polystyrene was 30%. The polymers were dissolved in toluene and spin cast on thin silicon wafers (about 10 x 10 mm), the resulting film thickness being about 300 nm. The samples were annealed at 245°C for 8 days, and the measurement of the resulting depth profiles was conducted by NRA using a monoenergetic 700 keV 3He beam. The nuclear reaction employed can be written ... 

Fig. 9.9 The calculated mode profiles for (a) a surface plasmon mode propagating along a 50 nm thick Au film, with an excitation wavelength of X 800 nm (note the plasmon field amplitude within the Au film is multiplied by 10 for clarity), and (b) a silicon PWEF waveguide with silicon core thickness of 220 nm, for an input wavelength of X 1550 nm...

For these conditions, Armaou and Christofides [4] determine the thickness profile, in Fig. 10.4-3, for the amorphous silicon film after 60 s, when the average thickness reaches 500 A. When characterizing the non-uniformity of the film, the sharp increase in thickness calculated near the outer edge of the wafer is assumed to be due to the boundary conditions, which assume step changes to zero concentrations at the edge. Brass and Lee (2003) disregard the profile from r = 3.6 to 4 cm, and compute the non-uniformity as ... 

A typical surface profile is shown on the video display. Vertical resolution of 5 A and horizontal resolution of 400 A is claimed. As long as the deposited film can be etched off the substrate without etching the substrate, this technique can be used for any thin film. Its primary utility is for R D studies, as it is clearly not a production technique. The only film for which it is not suited is an epi silicon film on a single-crystal silicon substrate. A technique for measuring the thickness of these films will be described in the section on Infrared Spectroscopy. 

Whether or not a chemical process step has been successful is difficult to measure, since there are few on-line measurable electrical properties. For example, film thickness and grain structure of polycrystalline silicon can be measured after a deposition step. However, their effect on device performance might not show up until subsequent doping or patterning steps fail. Similarly, it is possible to measure etch rates on-line by laser interferometry, but the etch profiles must be checked by electron microscopy. Unexpected mask undercutting or undiscovered etch residues can result in subsequent contact and device lifetime problems. 

Figure 14.33 Depth profile analysis of a Nichrome film on a silicon substrate. The outermost layer shows both Cr and oxygen, probably as a stable chromium oxide layer. The Ni and Cr (the Nichrome film) signals drop off at about 175 A while the Si substrate signal starts to appear, indicating the approximate film thickness. (From Weber.)...

Figure 3 A calculated reflectivity profile for a perdeuterated polystyrene film with a thickness of SO nm on a silicon substrate. The calculation was for a specimen where the interfaces between the specimen and air and the specimen and the substrate were sharp. This causes the reflectivity on average (shown by the dashed line) to decrease in proportion to k or 9. The separation distance between the minima of the oscillations diractly yields the thickness of the specimen, as shown.

If a sample of polycrystalline material is rotated during the sputtering process, the individual grains will be sputtered from multiple directions and nonuniform removal of material can be prevented. This technique has been successfully used in AES analysis to characterize several materials, including metal films. Figure 9 indicates the improvement in depth resolution obtained in an AES profile of five cycles of nickel and chromium layers on silicon. Each layer is about 50 nm thick, except for a thinner nickel layer at the surface, and the total structure thickness is about 0.5 pm. There can be a problem if the surface is rough and the analysis area is small (less than 0.1-pm diameter), as is typical for AES. In this case the area of interest can rotate on and off of a specific feature and the profile will be jagged. 

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