Semiconductor Raman spectroscopy

Undeniably, one of the most important teclmological achievements in the last half of this century is the microelectronics industry, the computer being one of its outstanding products. Essential to current and fiiture advances is the quality of the semiconductor materials used to construct vital electronic components. For example, ultra-clean silicon wafers are needed. Raman spectroscopy contributes to this task as a monitor, in real time, of the composition of the standard SC-1 cleaning solution (a mixture of water, H2O2 and NH OH) [175] that is essential to preparing the ultra-clean wafers. 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

The use of organic polymers as conductors and semiconductors in the electronics industry has led to a huge research effort in poly(thiophenes), with a focus on the modification of their electronic properties so that they can behave as both hole and electron conductors. Casado and co-workers [60] have performed combined experimental and theoretical research using Raman spectroscopy on a variety of fluorinated molecules based on oligomers of thiophene, an example of one is shown in Figure 7. 

Wright of Advanced Micro Devices discusses the use of Raman microspectroscopy to measure the integrity of a film on semiconductor wafers during manufacture in US patent 6,509,201 and combined the results with other data for feed-forward process control [181]. Yield is improved by providing a tailored repair for each part. Hitachi has filed a Japanese patent application disclosing the use of Raman spectroscopy to determine the strain in silicon semiconductor substrates to aid manufacturing [182]. Raman spectroscopy has a well established place in the semiconductor industry for this and other applications [183]. 

Wright of Advanced Micro Devices, Inc. discusses the use of Raman spectroscopy to measure the integrity of a film on semiconductor wafers during manufacture in US patent 6,50 9,201.87 The Raman measurements are made during the manufacturing process and can be considered an on-line system. Unlike many process Raman installations, this one is based on micro-Raman, where a microscope is used to focus the laser beam to a spot only a few micrometers in diameter. The Raman data is combined with other measurements, such as scatterometry, to calculate a stress level and compare it to... 

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. 

A similar result has been found using Raman spectroscopy of core-shell nanocrystals. Like XRD, Raman spectroscopy has also been widely employed to study doping of bulk semiconductors (110-112) but so far has only rarely been applied to doped semiconductor nanocrystals (70). Analogous to Vegard s law, shifts in lattice Raman vibrational energies have been found to occur with increasing dopant concentration in both the bulk and nanocrystalline materials. 

In recent years, charge-coupled devices (CCDs) have been used increasingly in Raman spectroscopy (13, 14). A CCD is a silicon-based semiconductor arranged as an array of photosensitive elements, each one of which generates photoelectrons and stores them as a small charge. An example format of a 512 x 512 array is shown in Fig. 2-12. Charges are stored on each individual... 

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