Raman spectroscopy crystal thickness

Adem of Advanced Micro Devices, Inc. was granted a patent on the use of Raman spectroscopy to monitor the thickness, crystal grain size, and crystal orientation of polysilicon or other films as they are deposited on semiconductor wafers via low-pressure chemical vapor deposition (CVD).89 The spectra are acquired with a non-contact probe through a suitably transparent window in the loading door. A feedback scheme is discussed. When the thickness has achieved the targeted value, the deposition is stopped. If the crystal grain size or orientation is deemed unsuitable, the deposition temperature is adjusted accordingly. 

Thus, the optimum sample arrangement for Raman spectroscopy of crystal pow ders with a low absorption coefficient is a forward-scattering (0°) arrangement of coarse crystallites with an optimum thickness in a multiple scattering arrangement. These are the conditions for the investigation of colorless samples in the visible range of the spectrum. 

The value of the pre-exponential factor is strongly dependent on the relative orientation of the two vectors. If they are parallel, cos = 1, the scattering strength reaches its maximum value but if they are perpendicular, cos = 0, the intensity in this transition disappears. (Except that, due to the finite thickness of samples, the observed intensity never falls to zero. This is similar to the effects of leakage in single crystal Raman spectroscopy.) In favourable cases the angular dependence of the intensity variation can be used to examine some vibrations in isolation from others, or alternatively to determine the relative orientations of different vibrational displacements. 

Polymer Structure. In addition to visualization, profiling, thickness measurements and chemistry of polymer wear it is frequently desirable to know whether the polymer is in the amorphous or crystalline state because other properties relate to state. Raman spectroscopy is very useful in studying very low frequency modes associated with vibrations of polymer chain backbones and the lattice modes of polymer crystals. It complements infrared spectroscopy. 

Small-angle X-ray scattering (SAXS) yields information about the long period (crystal plus amorphous thickness) distribution. The presence of two coexisting periodicities indicates segregation. Similar information can be obtained by Raman spectroscopy (see Chapter 7). 

One of the main uses of Raman spectroscopy in morphological analysis is the measurement of the longitudinal acoustic mode (LAM) in polyethylene which goes back to relatively early work (1949) on paraffins. The LAM frequency (u(LAM- )) can be converted to all-tmns chain length or crystal thickness (Lc) according to the following formula ... 

In Figure 2.6, the most important aspects for the implementation of the individual spectroscopies as process-monitoring tools are addressed. The very small, representative sample volume or thickness in Raman and MIR/ATR spectroscopy may certainly lead to problems if special care is not taken to avoid the formation of a stationary layer on the reactor window or on the ATR crystal. In this respect, NIR spectroscopy is the method of choice in view of the comparatively large sample volume/thickness involved in these measurements. The ability to separate the spectrometer from the point of sampling is certainly a great advantage for Raman and NIR spectroscopy. 

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