Strain measurements of a Si cap.pdf
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1、 1/3 Semiconductor 01 Strain measurements of a Si cap layer deposited on a SiGe substrate determination of Ge content Introduction Raman spectroscopy is one of the most popular tools for investigating the basic properties of semiconductors. It is particularly efficient in establishing the characteri
2、stics of microelectronic devices because the performance of SiGe based devices highly depend on the knowledge of the composition, Ge content and the strain of the layers. Indeed, the ability to measure alloy composition and strain in semiconductor structures is essential for the calibration of growt
3、h processes and control of the electrical and optical behaviour of these materials. The first requirement for the study of mechanical stress with Raman spectroscopy is that the investigated material exhibits Raman active modes, ie that there is a well defined Raman band in the spectrum. These includ
4、e materials such as diamond, silicon, SiGe, InGaAs, GaAs, GaN, to name a few. Mechanical strain directly affects the frequency positions of the Raman modes and can lift their degeneracy. Typically, it is possible to observe and detect strain by analysing the shift in the band position, but it can al
5、so affect the band shape and induce broadening and deformation of peaks. These effects depend upon the material characteristics and on the stress and strain geometry. The typical structure of the sample (named sample 1) studied here is shown in figure 1. In this study, we will focus on the choice of
6、 the laser excitation and measurement conditions. Results obtained in the visible and in the UV range will be presented Influence of the laser The choice of the laser excitation will influence different parameters : - The signal collection, because the Raman intensity is directly proportional to (1/
7、)4 . - The probing volume that depends on the laser spot size and the beam penetration. - The level of fluorescence that may overwhelm Raman signal. - The resonance effect where there is strong wavelength dependence of some Raman bands. - The coverage and resolution since the grating dispersion vari
8、es along the spectral range. For strongly absorbing materials such as many semiconductors, the Raman signal originates from a volume defined by the penetration depth and the diameter of the laser beam. A shorter laser wavelength gives rise to lower penetration and therefore provides information on t
9、he strain closer to the surface. Different penetration depths in silicon and germanium are given for different wavelengths in the table below. Laser wavelength (nm) Penetration depth in Si (nm) Penetration depth in Ge (nm) 633 3000 32 514 762 19.2 488 569 19 457 313 18.7 325 10 9 244 6 7 Figure 2 :
10、Table of penetration depths in Si and Ge influence of the excitation wavelength Figure 1 : Typical structure of Si/SiGe/Si sample Strained Si cap layer Uniform SiGe layer Graded SiGe layers Silicon substrate 10 to 20 nm Few m Few m 2/3 Semiconductor 01 The commonly used visible excitations limit the
11、 spatial discrimination in both lateral and axial dimensions. UV micro Raman can greatly enhance the spatial resolution by taking advantage of the shorter wavelength and much smaller optical penetration depth. For example, one can reduce the focused laser spot size from 0.7 m to 0.45 m by changing t
12、he laser line from 514 nm to 325 nm using an X100 objective with a NA of 0.9. Moreover, the much shorter optical penetration depth in the UV (10 nm at 325 nm in Si) allows probing shallow active layers. The effect of the laser selection is illustrated in figure 3 that shows the spectra recorded at d
13、ifferent wavelengths on the sample described in figure 1. Figure 3 : Spectra recorded at 325, 488, 633 and 785 nm on sample 1 Interest of combining visible and UV investigations : Because of their different penetration depths, combining visible and UV measurements can be of great interest for sample
14、s with a structure such as described in figure 1. At the end of this note, the technique used to separate the effects of composition and strain will be discussed Measurement conditions In terms of experimental definition and instrumental set up, several parameters have to be considered in order to o
15、ptimise the accuracy and reliability of the information. To start with, the determination of the peak position can be improved by using a reference band. Indeed, this will allow one to remove the influence of external parameters such as environmental temperature that can also affect the frequency of
16、 the monitored Raman peak. Ideally this reference should be acquired simultaneously with the Raman band. This is why the plasma lines from the laser source itself are usually very well suited as they provide sharp emission of well known frequency that are collinear with the Raman spectrra. Spectra w
17、ill then be corrected with respect to the plasma position. The use of an autofocus device is also of great importance. Optimising the focus manually can induce an artificial variation of the position of the plasma lines, due to differences in illumination of the entrance slit of the spectrograph. Th
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