Application Of Raman Spectroscopy In Measuring Residual Stress Of Silicon Wafer
In the semiconductor production process, annealing, cutting, photolithography, wiring, packaging and other production processes will introduce stress, and stress is divided into tensile stress and compressive stress; Stress can also be good or bad.
Strain Si (Si or sSi) refers to the effect of stress on Silicon single crystal, its lattice structure and lattice constant are different from that of unalloyed Silicon crystal. The existence of strain changes Si crystal structure from cubic crystal to tetragonal crystal, resulting in the change of its band structure and ultimately the change of its carrier mobility. The results show that introducing tensile strain and compressive strain into Si single crystal can significantly improve the electron mobility and hole mobility respectively. Therefore, starting from the 90nm process of Si CMOS IC, introducing strain into Si device channel and wafer material can improve the device channel mobility or material carrier mobility. Thus improving the high-speed performance of the device and current.
Polysilicon thin film is an important structural material in MEMS (Micro-electro-mechanical Systems) devices. It is usually formed on single crystal silicon substrate by deposition method. Due to the comprehensive action of many factors, such as different thermal expansion coefficient, deposition temperature, deposition mode and environmental conditions, polycrystalline silicon film generally has different tensile stress or compressive stress. As a structural material, the mechanical properties of polysilicon film largely determine the reliability and stability of MEMS devices. The residual stress of polysilicon film has a significant effect on its mechanical properties such as fracture strength and fatigue strength. Surface and subsurface damage will also cause residual stress, which will affect the strength of the wafer and lead to wafer warpage as shown in Figure 1. Therefore, accurate measurement and characterization of residual stress of polysilicon film is of great significance for the production of mature MEMS devices.
Stress testing is very difficult. It is not easy to accurately measure the residual stress of polysilicon film because of the obvious small-scale characteristics of polysilicon film in MEMS. At present, there are two main methods for measuring the residual stress of thin films:
One is X-ray diffraction, which calculates the residual stress of the film by measuring the change of the lattice constant in the film crystal. This method can accurately measure the residual stress in the micro-area of the film, but the measurement range is small, and the preparation of the sample has high requirements, and basically can not achieve online measurement of the residual stress of the film.
The other is the microscopic Raman spectrum measurement method, which has the advantages of non-contact, non-destructive, wide spectrum range and high spatial resolution. The distribution of residual stress can be deduced by measuring the movement of Raman spectrum peak of the film under the action of residual stress. This method can realize the on-line monitoring of the stress state of thin film specimens, and is an important method to characterize the residual stress of thin film materials, especially in MEMS devices.
For mechanical measurement, an excitation source of ULTRAVIOLET or visible light with a high level of wavelength stability and a microscopic Raman spectrum system with a high spectral resolution (less than 1cm-1) is generally required.
1 Measurement principle
Relationship between film residual stress and Raman peak shift
The diagram of measuring the residual stress of the film by Raman spectroscopy is shown in Figure 2. After passing through bandpass filter and beam separator, the monochromatic laser emitted by the laser (with solid arrows) irradiates on the sample surface through the objective lens and collides with the thin film atoms, resulting in the scattering of laser photons. The beam with inelastic collision (dotted arrow line) passes through the beam separator and reflection filter and converges to the sound spectrometer to form the Raman spectrum peak of the film. The generation of Raman scattering spectrum is related to the vibration of the atom of the thin film. Only when the vibration of the atom of the thin film is accompanied by the change of polarizability can the photon of laser interact with the atom of the thin film to form Raman spectrum. When there is residual stress of tension or compression in the film, the bond length of the atom will be extended or shortened accordingly, so that the force constant of the film will decrease or increase, so that the vibration frequency of the atom will decrease or increase, and the peak of the Raman spectrum will move to low frequency or high frequency. At this point, there is a linear relationship between the shift of Raman peak frequency and the residual stress in the film.
2. Residual stress calculation of polysilicon film
For monocrystalline silicon, there are three optical vibration modes when the laser photon interacts with it, one in the vertical direction in two planes, which is closely related to the crystal structure.
3. Raman scanning imaging of stress
In a semiconductor wafer manufacturer, Optosky 'ATR8800 was used to test the stress distribution of the wafer. After data processing, the stress distribution of the entire wafer was measured.
4. Summary and discussion
Raman spectroscopy has the characteristics of non-destructive, non-contact, fast and strong characterization ability, which can clearly characterize the stress and stress distribution of the wafer, providing a very good measurement tool for semiconductor production, annealing, packaging and testing processes.
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